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Chemistry and electronics
1997 has seen some significant anniversaries in the area of electronics: it is the 100th. Anniversary of the discovery of the electron by J.J. Thomson; the 75th. Anniversary of the introduction of curly arrows by Robert Robinson; the 50th. Anniversary of the invention of the transistor; and the 25th. Anniversary of the invention of the microchip. These last two events underpin much of modern civilisation and its dependence on the computer. This issue thus focuses on these important anniversaries, firstly with a historical perspective and secondly, in an account of the chemistry involved in the manufacture of the microchip. We rarely think that chemistry has anything to do with electronics and that there are jobs for chemists in the electronics industry. This is not true as there is a lot of chemistry involved from the production of the pure silicon crystals from which the wafers are cut, to the final production of an encapsulated microchip. The electronics industry is a major customer for speciality gases and for organic solvents and polymers. A number of our Industrial Chemistry and Environmental Science students from the University of Limerick got co-op placements at Intel in Leixlip, Co. Kildare this year and nearly every team in the different areas of the plant involved at least one chemist.
Another chemical anniversary in 1997 is the centenary of the introduction of aspirin, acetylsalicylic acid, by Bayer. This common drug is still made and consumed in enormous quantities as an analgesic, but the discovery in recent years of unexpected medicinal uses has led to it being termed a 'wonder drug'.
ChemEd-Ireland 1998
The last two issues of Chemistry in Action! (51 and 52) contained the Proceedings of ChemEd-Ireland 1996 on "Analytical Chemistry". Issue #54 will contain the Proceedings of ChemEd-Ireland 1997 on "Industrial Chemistry". There is still no sign of the new LC syllabus coming in and it will now be into the new millennium when the first students sit the new examination. It may be 1999 before it is introduced into schools.
As usual at this year's ChemEd-Ireland we asked the teachers present to identify the topics they would like see covered in subsequent years: the clear favourite for 1998 was "Practical work in chemistry" and for 1999 "Information technology in chemistry".
Chemistry and society
Chemistry has always been a discipline that was involved in everyday life, from its beginnings in the simple technologies of metal smelting, brewing, soap-making, dyeing etc., not to mention the discovery of fire and cooking. Combustion and cooking are, of course, chemical processes. In this issue we have an article from Martin Knox (well-known through the Study Tours he ran while at Athlone R.T.C.) on "Chemistry and Society". I would welcome other contributions for this series to show how chemistry interacts with society and modern life. 'Chemistry and the chip' is another example of this.
Keeping up-to-date
It is vital for chemistry/science teachers to keep up-to-date and have fresh material and ideas to infuse into their lessons. This has been one of my continuing themes over the years and one which Chemistry in Action!, in its small way, tries to address. I always recommend to my students that they read New Scientist regularly, as this weekly magazine is the best single way of keeping up-to-date in science. You can access New Scientist at its website (URL: www.nsplus.com), but there is nothing to beat the printed version and the special subscription rate for ASE members is the cheapest way to get it. I remember getting copies of the first issues at school, at a special bulk student's rate of 1/- (I believe), when it started publication in the late 1950s, with a distinctive blue cover. I have been reading it ever since, on and off, and now it is in full colour. It is a great source of material to enliven lessons and help create a "Current Awareness Board", along with clippings from newspapers. I encourage our science education students to start a cuttings file to provide them with a source of curriculum-enrichment material. Why don't you start one yourself and use it to supply a "Current Awareness Board" in your school?
Peter E. Childs
Hon. Editor
Disclaimer
he views expressed in Chemistry in Action! are the views of the authors and the Editor is not responsible for any views expressed. Chemistry in Action! does not represent the official views of any institution, organisation or body. Any unsigned articles or items are the responsibility of the Editor and if reprinted the Editor should be credited. If any errors of fact are published or anyone's views are misrepresented, then the Editor will be glad to publish a correction or a reply.
The Editor is not responsible for any actions taken as a result of material published in Chemistry in Action!. Any experiments or demonstrations are done at your own risk and you should take all necessary precautions, including eye protection.
Teachers may copy materials from Chemistry in Action! freely, without permission, for use in their schools. Articles and other material in Chemistry in Action!, except those originating in other publications, may be used freely in other educational publications without prior permission. Please acknowledge the source and author and send a copy of the publication to the Editor. Prior permission is needed if material is being used in commercial publications.
Contributions on any matter of interest to second-level chemistry teachers is welcome. Normally the results of research are not published.
The electron has always been with us, of course, but in 1997 we celebrated its identification and characterisation as a fundamental particle by J.J. Thomson (1856-1940). The electron was named by the Irish physicist George Johnstone Stoney in 1891. It was named after rubbing amber with a dry cloth produced the Greek word elektron for amber, since it was known that static electricity.
The American inventor Thomas Edison discovered that a hot wire in a vacuum emitted electrons in 1883. J.J. Thomson investigated these negative rays in 1897 and came to the conclusion that they consisted of very light, negatively-charged particles. Thomson was awarded the Nobel prize in Physics in 1906. His son Sir George P. Thomson (1892-1975) was later to get the Nobel Prize in Physics in 1937 for demonstrating the wave nature of the electron. A case of keeping it in the family! Thomson's work was published in 'Cathode Rays' Phil.Mag. 44, 293 (1897) and was announced at the Royal Institution on April 30th. of that year. The idea of particles (corpuscles) smaller than atoms was greeted with disbelief.
"Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?"
J.J. Thomson
Thomson's experiments are the foundation of atomic structure and are covered in most introductory chemistry and physics courses. He did his work at the Cavendish Laboratory in Cambridge. He performed a series of three experiments to show that the cathode rays were negatively charged, were the same from all metals, and using the combined effects of electrical and magnetic fields he could measure the e/m ratio and show that they were either very light particles or had an enormous charge. When e was determined two years later, the hypothesis that electrons were very light was confirmed.
".. we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than the ordinary gaseous state: a state in which all matter .. is of one and the same kind; this matter being the substance from which all the chemical elements are built up."
J.J. Thomson
Although Thomson got the model of the atom wrong - he plumped for the 'plum pudding' model, his discovery of electrons was to be the crucial discovery that opened up the atom and led eventually to Rutherford's work which established the nuclear model, and the existence of protons and neutrons. We still use cathode ray tubes, which led Thomson to the electron, in our TVs and computers, and harnessing the electron was to lead to many devices, not least the transistor.
RESOURCES:
There have been a number of special conferences, books, articles and websites to commemorate the centenary of J.J. Thomson's discovery of the electron. Some are listed below.
The discovery of the electron
This web site is an illustrated history of the discovery of the electron illustrated by diagrams and photographs and it also features clips of J.J.Thomson describing the electron. You can read the original paper and J.J. Thomson's Nobel prize lecture at Carmen Giunta's site of Selected Scientific Papers:
http://maple.lemoyne.edu/~giunta/thomson1897.html
Life, the universe and the electron:
An on-line exhibition mounted by the Science Museum and the Institute of Physics: www.nmsi.ac.uk/on-line/electron/
The electron centennial page at www.xmission.com/~dparker/electron.html has a series of very readable articles on the history of atomic physics from the electron onwards.
******
Quotable quotes:
"I think there is a world market for maybe five computers" Thomas Watson (IBM) 1943
"Computers in the future may weigh no more than 1.5 tons" Popular Mechanics 1949
"640k ought to be enough for anyone" Bill Gates
"But what is it good for?" IBM engineer on the microchip, 1968
"There is no reason anyone would want a computer in their home" Ken Olsen (Digital) 1977
On the 16th. Dec. 1947 history was made by two American physicists, John Bardeen and Walter Brittain, working at Bell Laboratories (now Lucent Technologies), Murray Hill, New Jersey.
"Cautiously they glued a fragment of gold foil to a wedge of plastic, pressed the wedge on to a sliver of germanium and secured the contact with a paperclip "spring". Then they clamped their clumsy handiwork into a plastic vice, and wired up an electrical circuit; copper wire linked the device, on one side, to a battery to supply power, and on the other, to an oscilloscope to record the device's response. When all was ready, Brattain switched on the battery, sending a trickle of power through the circuit. As the two men watched in silence, the faint signal leapt to 100 times its strength, a sudden powerful glow on the oscilloscope."
Joanna Bawa
On the 23rd. Dec. 1947 they showed the device to their boss William Shockley and some of Bell's 'top brass'. At the time electronics was dominated by the vacuum tube (valve) and some of you may remember radios (wirelesses) in the pre-transistor age: bulky, slow to warm up and fragile. The transistor worked but they didn't really understand why - the theory still had to be worked out fully. The transistor effect they discovered using a piece of germanium crystal was that small changes in input could produce large changes in output, and amplification of the signal. Fifty years later there are around 200 million billion transistors in use around the world, around 40 for every person! Each Pentium II chip contains 7 million transistors!
Transistors are switches and can be used to switch a current on and off (0 and 1 in binary) or to amplify a signal. A modern microchip contains up to 7 million transistors, which combine to enable complex instructions to be carried out as part of an integrated circuit.
The point-contact transistor was announced to the world in 1948, after being patented (in Bardeen and Brattain's names only), and Schockley, Bardeen and Brattain shared the 1956 Nobel Prize for Physics. Semiconductors, of which germanium was an example, offered a smaller, faster, cheaper and more robust alternative to relays and valves then in use for switching and amplification. Much development of the transistor was needed to turn it into a useful commercial device. The point-contact was replaced with the more robust junction transistor, and germanium (which is very expensive) was replaced by the much cheaper silicon. The importance of purity in the starting crystal and the introduction of controlled impurities to produce electronic defects (doping) was discovered and the age of solid state electronics was born.
Many technologies had to be developed, not least how to produce large single crystal ingots of pure silicon. "Once these techniques were perfected, the transistor shrank in size and grew in importance." in 1954 IBM decided to switch from valves to transistors for its giant room-sized computers, which dominated the computing market at that time. The transistor radio followed and transistors shrank in size and cost and increased in usefulness. Next came the integrated circuit and 25 years ago the microprocessor and the future was born.
Gordon Moore co-founded the Intel Corporation and managed to compress 13 different integrated circuits on to a silicon slice (wafer) and the microchip was born in 1972. Together with Robert Noyce he founded Intel in 1968. This December's Time magazine named Andrew Groves 'Man of the year' and gave a useful account of his life from rags to riches. A recent book, Crystal Fire (by) recounts the history of the transistor in a very readable way.
"Nothing compares with the scale and rate of development of the transistor, except maybe an epidemic."
Ian Ross
In 1965 Moore proposed what came to be known as Moore's law: the density of microcircuitry doubles every two years (see diagram below). It still holds true and it is expected to do so until at least 2020! By then microminiaturisation may have reduced the transistor, the heart of the microcircuit, to less than 100 atoms across! The transistor has been called the most significant scientific and technological development of the century.
Bell Labs (now part of Lucent Technologies) has just announced the nanotransistor: four times smaller, five times faster and needing a 100 times less power than a normal transistor. Its size is measured in nanometres (10-9m) rather than microns (10-6m) and it could lead to the next generation of microchips with billions rather than millions of transistors.
(Based on the article by Joanna Bawa, Guardian Online 3/12/97; available on-line at http://go2.guardian.co.uk/)
RESOURCES:
Bell Laboratories is now called Lucent Technologies. They have set up a website on the history of the transistor at:
www/lucent.com/ideas2/heritage/transistor/ and in November 1997 they launched an interactive exhibit called LIVES ON which is linked in to displays at five leading science museums around the world. This can be accessed at: www.lucent.com/liveson/ Each museum illustrates a different aspect of the transistor and its impact on everyday life.
1880s - semiconductors first identified
1920 - invention of the vacuum tube amplifier by Irving Langmuir and Lee De Forest.
"Nature abhors the vacuum tube"
J.R. Pierce (Bell labs - who coined the word transistor)
1936 - Mervin Kelly at Bell Labs recruits solid-state physicists to form a research group at Murray Hill, N.J.
1939 - Jack Scaff and Henry Theurer discover p-type and n-type regions in silicon at Bell Labs, Holmdel, N.J.
29/12/1938 - William Schockley theorises that semiconductors can be used to make an amplifier: "It has occurred to me that an amplifier using semiconductors rather than a vacuum is in principle possible" William Schockley
1945 - Mervin Kelly establishes a solid-state group headed by Schockley, who recruits John Bardeen and Walter Brattain joins the group
16/12/19 47 - Walter Brattain and John Bardeen demonstrate the point-contact transistor using germanium
23/12/1947 - device is demonstrated as an amplifier to Bell Labs management. Sparks a huge R&D effort to make practical devices. "The information age has begun."
26/6/1948 - transistor announced and demonstrated at a press conference in New York
1/7/1948 - New York Times mentions the transistor on p. 46: "A device called a transistor which has several applications in radio where a vacuum tube ordinarily is employed, was demonstrated for the first time yesterday at Bell telephone Laboratories..Its action is instantaneous, there being no warm-up delay since no heat is developed as in a vacuum tube."
1951 - Gordon Teal and J.B. Little grow large crystals of germanium at Bell Labs to make junction transistors feasible
- William G. Pfann develops zone refining as a method of purifying crystals, a development essential for making ultra-pure silicon
- team at bell labs develop the junction transistor, which is more reliable and effective than the point-contact transistor
- first commercial production of transistors at the Western Electric Co., Allentown
1952 - Bell Labs license the transistor to companies including GE, Texas Instruments, IBM and Sony
1953 - techniques for producing pure, single crystal wafers of silicon are refined at Bell Labs, paving the way for the development of the microchip
1954 - Carl J. Frosch and L. Derrick of Bell Labs discover that a layer of SiO2 can be used to mask the surface of silicon allowing the precise placement of impurities for creating p-n junctions
1956 - Bardeen, Brattain and Schockley receive the Nobel prize in Physics for their work in developing the transistor
"Much of my good fortune comes from being in the right place at the right time and having the right sort of people to work with" Walter Brattain
1958 - Integrated circuit invented by Jack Kilby: "I perceived that a method for low-cost production of electronic circuits was in hand .. that instead of merely being able to build things smaller, we could fabricate entire networks in one sequence, and that we had extended the transistor's capability as a fundamental electronics tool." Jack Kilby 1958
1965 - Gordon Moore proposes Moore's law: chip capacity doubles every 18-24 months.1968 Robert Noyce and Jack Kilby founded Intel
1971 - Intel introduces their first chip, the 4004 (2,300 transistors)
1982 - 286 chip (134.000 transistors)
1985 - Intel 386 chip (275,000 transistors)
1989 - Intel 486 chip (1.2 million transistors)
1993 - Pentium chip (3.1 million transistors)
1995 - Pentium Pro chip (5.5 million transistors)
1997 - Pentium II (7.5 million transistors)
1997 - Bell Labs now Lucent Technologies continues fundamental research in solid-state electronics: 5 million transistors can be packed into a single chip, with 200 chips on an 8 inch silicon wafer.
Clare Grealis, Michael McEnery, Michelle Minihan and Niall O'Reilly*
Introduction
It is commonly perceived that Chemistry has little or no relevance in the Semiconductor Industry. We too shared the same opinion even after we received our Coop assignments with Intel Ireland Ltd. in Leixlip, Co. Kildare, the world's largest manufacturer of microprocessors. However, this is not true as we soon found out when we began our eight month contracts with Intel. Once we began comparing our knowledge of the various different parts of the process, we discovered that Chemistry does in fact play a very large role in the Microprocessor Industry. Intel employs a significant number of chemists, both at technician and graduate level.
In this article, we will outline the Chemistry involved in the various parts of the process in an attempt to illustrate its relevance in the Semiconductor Industry. Areas of the process will be outlined as follows:
Microprocessors are manufactured on eight-inch (200mm) silicon wafers (99.99999% pure) and a wafer may have up to 300 such devices on its surface. Microprocessors are made essentially of transistors (devices which control current flow) and any one microprocessor has millions of transistors. Microprocessors are currently manufactured by a 0.6æm process i.e. the width of the circuits is 0.6 microns but Intel will soon be upgrading to a 0.25æm process!
Figure 1 shows an overview of the steps in the process from silicon wafer to the final packaged chip.
Diffusion
Main functions of the diffusion area are as follows:
Figure 1 Flow diagram of the production process
Growth of Oxide
The starting material of the process is always a blank silicon wafer (8" in diameter) and the first operation performed on the wafer is the growth of an oxide. There are of course, many more stages in the process which involve the growth of oxides and the purpose of these oxides vary widely but the reaction conditions are always similar. Conditions are:
The silicon oxide is grown in a Vertical Diffusion Furnace (VDF) whose reaction chamber is made of quartz (SiO2). The other main features of the VDF are control valves, which control the flow of gas into and out of the furnace and a thermocouple to control the reaction temperature. The control mode employed is PID (Proportional, Integral, Derivative), which allows for very tight control over the furnace temperature. The reaction is a simple one:
Si(s) + O2(g) SiO2(s)
It is always silicon dioxide that is grown and the wafers can be masked in order to prevent oxide growth on those areas of the chip where it is not required. Figure 2 depicts an atmospheric VDF.
Figure 2 An atmospheric Vapour Diffusion Furnace
Gases react in the torch and enter the chamber at the top of the inner tube. Excess and unwanted gases are disposed of via the exhaust.
Functions of the oxides
The oxide layers grown on the silicon have the following functions:
Nitride Deposition
This operation also takes place in a VDF but not in an atmospheric VDF. Nitride deposition takes place in an LPCVD VDF - Low Pressure Chemical Vapour Deposition VDF.
There are a few essential differences between the Atmospheric and Vacuum VDF's. Gases react in the cap of the Nitride Furnace, not in a torch and there is also a pump to create the vacuum environment. There are two different nitride operations but the reaction is the same:
?,vac.
3SiH2Cl2(g) + 4NH3(g) Si3N4(s) + 6HCl(g) + 6H2(g)
Ammonia and dichlorosilane (SiH2Cl2) are reacted at a lower temperature than in the atmospheric furnace and at a very low pressure to form a deposit of silicon nitride (Si3N4, 'nitride'). Nitride has two functions:
Figure 3 is an illustration of an LPCVD VDF.
Figure 3 A diagram of an LPCVD VDF
A pump creates the vacuum environment necessary for the deposition. There is another type of deposition that takes place in the LPCVD - Polysilicon Deposition.
Polysilicon Deposition
Polysilicon or amorphous silicon is also deposited in an LPCVD. Reaction conditions are similar, i.e. low pressure and high temperature. The reaction is again quite simple and involves the decomposition of silane (SiH4) into amorphous silicon and hydrogen gas.
SiH4(g) Si(s) + 2H2(g)
Polysilicon forms the gates for transistors and also the transistor connections. Its formation is therefore a very sensitive operation and it is essential that it is carried out correctly. Any discrepancies at this stage could mean disposing of semi-processed wafers.
Anneals
Anneals are essentially a repair operation which are carried out in an atmospheric furnace. Wafers are exposed to a flow of inert gas, such as N2, at a very high temperature. This flow of gas repairs surfaces which have been damaged by heavy ion implantation but it also activates the implanted dopants.
The only anneal which is not carried out with N2 is Polyimide Cure (PiCure). Polyimide is a polymer is used as a passivation or protection layer on microprocessors. It is deposited using a solvent and the deposit is then cured with an H2 anneal, which is essentially solvent removal. PiCure is a 'back-end' operation which means that it occurs near the end of the process. All other operations carried out in the Diffusion Area are termed 'front-end' operations as they are at the start of the process flow.
Drives
Drives are similar to anneals in the method by which they are carried out. Once ions have been implanted into Silicon, they need to be 'driven' deeper into the silicon lattice. The implant operation itself does not drive the dopants to a sufficient depth, so the dopants are driven in an atmospheric furnace at high temperature using an inert gas such as N2 or Ar. As the temperature increases, so does the diffusion coefficient of the dopants and they are then able to diffuse deeper into the silicon lattice (Figure 4).
Figure 4 Driving dopant atoms into the silicon lattice
Implant
The main operations in the implant area are as follows:
Ion Implantation
This is a process more commonly referred to as 'doping'. Its purpose is to alter silicon's ability to conduct electricity. Dopants such as boron, phosphorus and arsenic increase the conductivity of silicon by providing charge carriers. The charge carriers can be positive (holes) or negative (electrons). Positive charge carriers are called 'free holes' while negative charge carriers are called 'free electrons'.
Every Si atom in the silicon lattice is surrounded by four neighbouring Si atoms with each of the Si atoms providing one electron for the central atom. In this way, silicon's valence shell achieves a noble gas structure, it is relatively inert and because the bonds have no dipole moment, it will not conduct electricity - that is without the presence of defects. But the basic principle of all semiconductors is that they do conduct because their lattices contain defects, i.e. free holes or free electrons. This introduces carriers into the conduction band or into the band gap. Silicon, like any other semiconductor, will have a variable quantity of these such defects (due to impurities or its thermal history). Doping simply increases the concentration of these defects, in a controlled way, so as to make the silicon more conductive.
When silicon is doped with boron, a silicon atom is replaced with a boron atom. Because boron (group III) has only three valence electrons, it can only be surrounded by three silicon atoms which leaves a positive hole in the lattice. This 'free hole' is filled by an electron from somewhere else in the lattice therefore leaving a positive charge in the lattice.
When silicon is doped with group V elements such as phosphorous and arsenic, the converse is true. Arsenic will replace a Si atom but this time, the result will be a 'free electron' because it can only donate four electrons to form bonds with Si therefore leaving an extra electron in the lattice. Silicon which has been doped with boron is referred to as p-type silicon while Si doped with arsenic or phosphorous is referred to as n-type silicon. (This material is part of the LC Physics course.)
The following diagram (Figure 5) illustrates the basic features of an Implanter.
Figure 5 An ion implanter
Ions from the ion source are accelerated by an electric field (15-170KeV) to a very high velocity and are fired into the semiconductor material. Selection of a particular size of ion is done by means of a Mass Separation Magnet, which filters out unwanted isotopes of the impurity ion. (This is essentially mass spectrometer.)
The ion beam can then selectively implant different parts of a transistor by means of the multiple target changer. The ion beam is adjustable and parameters such as the initial lodgement depth and dopant concentration can be varied by means of the acceleration voltage and the ion dose respectively.
The more doped the semiconductor is, the less its resistivity and conductivity is inversely proportional to resistivity.
The Implant Operations are:
The above are all transistor components and it is imperative that they receive the correct treatment. The Varian E1000 is the Implanter used at Intel. After Varian processing, the wafer will go for an anneal as implantation is heavy on the wafer surface (see Diffusion).
BPSG Deposition
This is a more complex deposition process because of the nature of the deposit. BPSG stands for BoroPhosphoSilicate Glass and its purpose is to separate transistors from the metal layer which will be the next event in the process. BPSG deposition is carried out in a Watkins-Johnson (WJ) machine using a mixture of SiH4 (silane), B2H6, PH3 and O2.
Figure 6 BPSG deposition
The hydrides are mixed before entering the injectors. On exiting the injectors, the gas mixture meets a N2 curtain which keeps them separated from the oxygen below until just above the wafer surface where deposition occurs. In the Diffusion section, we discussed LPCVD - Low Pressure Chemical Vapour Deposition, here deposition takes place at vacuum pressures. BPSG deposition is an example of APCVD - Atmospheric Pressure Chemical Vapour Deposition, where deposition takes place at 1 atm. pressure.
BPSG also aids in the planarization of the wafer surface. However, the deposit itself is not smooth at first. The surface topography of the deposit is quite poor and therefore it must be 'flowed'. Flowing is a high temperature 'smoothing' operation and therefore is done in a VDF with a steam atmosphere and 1 atm. pressure, a shown in Figure 7.
Figure 7 Smoothing the surface by flowing
The wafer is now ready for further processing.
Salicide Formation (RTP)
Salicide (Self-Aligned Silicide) is a layer which must be formed on all exposed silicon and polysilicon areas after the last implant has been carried out. Its purpose is to reduce the resistances of the transistor junctions and also to reduce the resistance of the polysilicon. Rapid-Thermal-Processing (RTP) is employed to carry out this operation. RTP uses radiant light to heat the wafers in a rapid and very uniform manner, but the main feature of RTP is that it is a non-conductive form of heating. Halogen lamps are the radiant light source and deposition takes place in a quartz reaction chamber. The non-conductive form of heating is a very attractive feature in a system where electrical effects are a major consideration. The titanium silicide is formed by exposing the wafer surface to a Ti layer at a high temperature in an N2 atmosphere. The N2 prevents growth of unwanted oxide, but at the same time it takes part in a reaction with the upper face of the Ti layer. This operation is therefore a double reaction and is self-stopping, as the Ti is being consumed from both top and bottom, as shown in Figure 8.
Figure 8 Salicide formation
The TiN phase can then be etched away leaving the Salicide (TiSi2) layer on top.
Lithography
Lithography is a means of patterning a wafer such that deposition of metal, oxide and nitride layers only occurs on certain parts of the wafer. This is achieved by spinning photoresist on to the wafer and then using a mask to pattern the photoresist with UV light. Areas of the resist which have been exposed to UV light are then soluble in a developer solution. Those areas which were unexposed remain, hence preventing deposition in those areas.
The three main functions of the Lithography area are:
(See Figures 9. 10 and 11).
a) Spin cycle:
The Spin cycle consists of Prebake/Prime, Spin and Softbake.
Prebake/Prime - prepares the surface of the wafer for the resist. A high temperature prebake is done to remove any moisture from the surface of the wafer. This will promote resist adhesion and reduce the possibility of the resist lifting. The wafer is baked on a hot plate and primed by HMDS (hexamethyldisilazane) vapour. The HMDS vapour is made to enter the oven by bubbling low pressure nitrogen through a canister filled with HMDS. Exposure to HMDS promotes even greater resist adhesion by providing better bonding sites for the resist.
Spin - in the spin chamber, photoresist is dispensed onto the centre of the wafer while the wafer spins at a low speed. The solvent level in the resist is high when first dispensed, allowing it to distribute easily and evenly. Centrifugal forces spread the resist uniformly until the desired thickness is achieved. Resist which builds up at the edge of the wafer during the spin cycle is removed by Edge Bead Removal (EBR). EBR prevents resist build up at the edge of the wafer from chipping off and contaminating equipment and other wafers. This edge bead is then removed by streaming a solvent at the edge of the wafer. (See Figure 9)
Softbake - removes some of the solvent in preparation for exposure. The wafer is again baked in an exhausted, enclosed proximity bake oven. The exposure to an elevated temperature helps to evaporate some of the solvent components in the resist.
Figure 9 The Spin step
Figure 10 The expose process using photoresist
Figure 11 Developing the image
Expose process
The Expose part of the process is used to expose the photoresist on the wafer with UV light through a chrome-patterned reticule, as shown in Figure 10. As the ultraviolet light exposes the resist, the polymers in the resist break down making it soluble in the developer solution. The resist that is not exposed to UV light is that which is shadowed by the chrome part of the reticule. The unexposed resist is not soluble in the developer solution. Reticules are a critical part of the exposure process. The reticule itself is a 6 inch piece of glass with the pattern made by a stack of Cr/Cr02. An electron beam system is used to expose the pattern on the reticule. As the reticules must 'fit' together on the wafer, registration between all the reticules must be confirmed. This is done by measuring each pattern position to a perfect grid and then comparing how each pattern fits, relative to one another.
Any defects on the wafer will be repeated at every exposure on the wafer. Therefore, all reticule defects are repaired before it is shipped into the fabrication plant. If the defect is a hole in the chrome, for example, selective deposition is used to cover it up. If the defect is an extra piece of chrome, a laser is used to trim it off.
Develop step
The Develop step in the process, removes the exposed resist from the wafer and leaves the pattern, which defines the active and isolation regions, i.e. where a deposit of a material will be made on the wafer. The develop cycle consists of:
Post Expose Bake, Develop and Post Develop Bake.
Post Expose Bake: the wafer is baked in an oven and then moved to a cool plate for temperature stabilisation. During exposure, the Photo-Active Compound (PAC) in the resist is changed from an inhibitor to an acid, where the resist has been exposed with light. The acid is soluble in developer solution. The final resist edge is determined by the location of a specific concentration of acid molecules in the concentration gradient. The tendency of both the inhibitor and the acid is to diffuse to a region of lower concentration. From the time of exposure until develop, the inhibitor and acid molecules are diffusing past one another in the direction of decreasing concentration into each others highly concentrated region. This movement is driven by both time and temperature. The PEB process is designed to speed up molecule diffusion by heating the resist. The acid reacts with the tetramethylammonium hydroxide (TMAH) developer, which is a base, and dissolves the resist in regions where the acid concentration is above the key concentration level. The resist that has either no acid in it, or at too small a concentration, dissolves only very slowly in the developer. It is therefore left behind after the develop process. and provides a mask for the subsequent steps. The pattern left behind is a replication of the chrome pattern on the reticule that is reduced five times and stepped across the wafer.
Develop: In the develop chamber, a TMAH-based developer is sprayed onto the wafer to develop away the exposed resist, leaving the unexposed resist intact. The develop step is done partly while the wafer is spinning and partly while it is stationary. A deionized water (DIW) rinse follows, to ensure that any remaining developer is removed along with the by-products of the developer and resist solution. This rinse step is done while the wafer is spinning. The final step is to spin the wafer dry.
Post Develop Bake: The wafer is baked again in this step. PDB reduces the solvent content of the resist, which increases the adhesion and reduces out-gassing in the subsequent process steps. The increased temperature also causes the resist to flow somewhat. The balance here is to remove as much solvent as possible without flowing the resist too much.
The wafer is now ready for a deposition process and the photoresist which remains on the wafer will prevent deposition in areas where it is not required hence defining the isolation regions of the wafer. A wafer will be patterned numerous times in the process as there are numerous deposition operations involved in the process. Once the photoresist has served its purpose on the wafer, it is etched away at a Wet Station (see Wet Etch).
Wet Stations
There are two primary functions at Automated Wet Stations (AWS):
Wet Etch
Wet Etch is the use of very concentrated acids to remove oxide, nitride, titanium and photoresist layers from wafers. Once the layer has served its purpose on the wafer, some or all of it may have to be removed and the use of concentrated acids such as hydrofluoric acid and phosphoric acid is one method employed to do this. The wafers are simply dipped into baths of these acids for periods of time. The time the wafers are left in the acid baths is dependent on the concentration of the acid and the rate at which the acid etches at that particular concentration (Angstroms/Min.).
Oxide Layers: oxide layers are removed from wafers surfaces using concentrated hydrofluoric acid (HF).
Nitride Layers: concentrated phosphoric acid (H3PO4)is used to etch nitride from the wafers. The wafers are then dipped in sulphuric acid to remove any traces of phosphoric acid.
Polymers: a mixture of DI water, ammonium fluoride and hydrofluoric acid are used to remove any unwanted polymers from the wafers Chlorine and other residues: a mixture of ethylene glycol and hydrofluoric acid is used to remove any residues remaining after Tungsten Hatchback to provide a clean surface for metal#1 deposition and also to remove any chlorine from the metal lines after Metal Etch.
Cleans
Cleans also use a variety of acids, bases and peroxides to remove resist and contaminants from the wafers.
Resist Removal: Sulphuric acid is used to remove resist from the wafer after various steps in the process. The effect of the acid is enhanced by the addition of peroxydisulphuric acid (H2S2O8), which helps to clean away photoresist effectively.
Organics: a mixture of DI water, hydrogen peroxide and ammonium hydroxide is used to remove any organic contaminants from the wafer. This operation is referred to as SC1.
Inorganics/Metallics: a mixture of DI water, hydrogen peroxide and hydrochloric acid removes any inorganic contaminants from the wafer (SC2).
SC1 and SC2 cleans are critical before the wafers go into any other machines for processing as the wafers need to be completely free from particulate and metallic contaminants. This super-clean surface is one of the main ingredients in a high quality CMOS device structure. Figure 12 is an illustration of how wafers move through an AWS.
SRD (Spin Rinse Dryer): this dries the wafer after it has been through the wet station.
QDR (Quick Dump Rinse): This tank fills up with DI wafer and then dumps and fills again. This process is repeated a no. of times until no hydrofluoric acid on the wafer. If there were any HF left on the wafer, it would be carried into the next tank thereby contaminating the solution in that tank. The number of times the tank is dumped and refilled depends on what is being cleaned off of the wafer. The QDR cycles are shortened in the back-end of the process (Thin Films, CVD/ILD) as excessive rinsing in DI here will cause corrosion of the metal deposits.
Monitoring at AWS
Etch Rates: regular checks are made to ensure that the acid solutions are etching at the correct rate. This can be determined using an Ellipsometer which measures deposition thickness (see Diffusion).
Particle Counts: excess particles are can be detected on wafers using a Tencor, which scans the surface using light rays. A particle on the surface will deflect a light ray at a different angle and this is the principle for particle detection.
Chemical Analysis: Two instruments/methods are used: A Kevex X-ray fluorescence analyser: gives the elemental content of an acid thereby showing up any contaminants in the acid.
ICPMS: Inductively-Coupled Plasma Mass Spectrometer: gives metal contamination in liquids by vapourising samples in a plasma and feeding the vapour into a mass spectrometer.
Figure 12 Movement of wafers through an AWS
Plasma Etching
Plasma Etching is a very common form of etching used in the semiconductor industry, mainly due to its ability to etch patterns and structures of very small dimensions. Some examples of materials, which are etched by plasmas include Oxide, Nitride, Tungsten, Aluminium, and Polysilicon. Plasma etching is said to be anisotropic (etches in one direction only) as opposed to a wet etch which is isotropic (etches evenly in all directions). Wet etches are faster but can only etch structures of greater than 5æ. A profile comparing a wet and a plasma etch is shown below in Figure 13:
Figure 13 Wet etching compared with dry (plasma) etching
The plasma, a partially-ionised but electrically neutral gas, which performs the etch is formed in a vacuum chamber by energising the reactant and carrier gases with radiowaves. The gases flow through holes in the upper electrode. The electric field attracts any free electrons in the gas towards the oppositely charged electrode (as the current applied is AC the polarity of the electrodes change at the frequency of the AC). These accelerating electrons bombard the reactant gasses causing dissociation and ionisation (positive ions are formed). Typically, only 1 in 1 x 106 of the molecules are ionised. The wafer sits on the lower electrode. When the lower electrode is negatively charged the plasma is attracted to the wafer on the lower electrode and the ions and radicals etch the pattern on the wafer. The reactant and carrier gasses are such that when ionised, they will be highly reactive to the substrate to be etched. Also the by products must be very volatile, as any solid particles on the wafer would cause a malfunction in the chip. An example of a plasma formation and etching Silicon Dioxide are shown below (dots indicate free radicals):
CH4(g) + e- CF3•(g) + F• (g) + 2e-
CH4(g) + e- CF3• (g) + F• (g) + e-
SiO2(s) + 4F• (g) SiF4(g) + O2(g)
Two types of etching occurs in plasma etch systems: Chemical reaction etching and reactive ion etching. Chemical reaction etching is as in the example above where the ions and radicals react chemically with the layer to be etched. Reactive ion etching occurs when radio frequency radiation (RF - basically AC at a very high frequency) power which is applied between two electrodes. Ions accelerating to the wafer surface physically remove or sputter the material from the wafer. Reactive ion etching is more directional than chemical etching and helps give an anisotropic etch. See Figure 14 below.
The fact that much etching occurs is due to the chemical action of ions and radicals, it is necessary to have a mechanism to prevent isotropic etching into the sidewalls of the structure to be etched. Many plasma etch systems use polymer formation to block horizontal etching. Polymer formation usually occurs when a species in the plasma reacts with the photoresist of the pattern to form a polymer which condenses and coats the surface of the wafer. This polymer can be removed by ions accelerating towards the wafer and thus etching can occur in the vertical direction. However, the polymer coating the walls of the structure is not removed and so prevents the chemical reaction between ions and radicals with the sidewall. This is represented in Figure 15 below.
Figure 14 Reactive ion etching apparatus
Figure 15 Polymer coating to block horizontal etching
The extent of polymer formation depends on the concentration of the polymer-forming gas in the plasma and the wafer temperature: the cooler the wafer, the more polymer will condense.
Finally, an interesting use of analytical and quantum chemistry in Plasma Etching is using electron transitions to determine the end of the reaction. Given the high energy state of the plasma, many molecules, radicals and ions are present in excited states. Therefore when electrons relax from the excited states, light emission at a characteristic wavelength occurs. Many plasma etch systems take advantage of this phenomenon to detect the end of reactions, e.g. some systems search for light emissions at the wavelength of the product gases. When the intensity of the light at this wavelength decreases, it implies that the layer has been etched to completion. The plasma formation then stops and avoids excessive over-etching of undesirable parts of the wafer.
Thin Films
The main functions in the Thin Films section are:
Metal (M) deposition and Tungsten (W) Adhesion involve the 'sputtering' of a thin film of metal on to the surface of a wafer. Sputtering is a physical process and does not involve any heating. The wafer surface is bombarded with energetic ions in a vacuum. The material to be deposited onto the surface is called the 'target' and the target is ionized by an electrical power source. Hence, the target is 'sputtered' on to the wafer surface. The sputtering systems are similar for the etch, M deposition and W adhesion operations. The system is multi-chambered and each chamber has a different function. Figure 16 is an illustration of how the system works which will cover all three operations. An explanation of each operation will follow.
Figure 16 Schematic diagram for thin film production
Sputter Etch
Before a metal film can be deposited onto the wafer surface, the surface must be etched to remove any unwanted oxide, which has grown while the wafer is between operations. The etch is a sputtering process using an argon target to remove the residue. Because the argon is ionized using electrical energy, it will react with any oxide, which has formed on the surface removing it with ease. The wafer is now ready to undergo the deposition of a tungsten adhesion layer or a metal layer.
Tungsten (W) Adhesion
In a CMOS device, the basic transistor (gate, source, drain) is covered by a number of other layers and films, each serving a different purpose. Contacts and Vias connect the transistors to each other within a microprocessor and they enable the devices to be connected to external power sources. A Contact connects the basic transistor to the 1st metal layer whereas a Via makes the connections between the M2, M3 and M4 layers. Hence, all the transistors in the microprocessor become connected and M4 aids in the connection to the external power source.
Contacts and Vias, because they are electrical connectors, are therefore made of a metal - tungsten. But because they are lodged in the Insulating Layer Dielectric (ILD), which is a non-conductive material. An Adhesion layer is deposited as well because tungsten does not adhere well to the ILD. This is called the W Adhesion layer and it is composed simply of Titanium Nitride. Ti is sputtered from the target into a N2/Ar atmosphere. The Ti ions react with the less inert of the two gases, the N2, to form TiN and this forms the adhesion layer for the tungsten deposits to come. This operation is repeated three times in total in a four-metal layer device.
Metal Deposition
Once the tungsten has been deposited to form the Contacts and Vias, the wafer is now ready for the deposition of the metal layer which serves to connect the contacts and vias. The wafer first receives an Ar etch and is then lithographically patterned such that only those areas of the wafer which require metal deposits receive them. The Metal 1 & 2 layers are actually 'stacks' of sub-layers - first the metal which is aluminium, then TiN, then Ti and finally TiN again (see Figure 17(a))..
Figure 17
(a) Metal 1/2 stack:
b) Metal 3/4 stack:
The Al and Ti sub-layers are obviously the conductive components of the stack providing good current capability, electromigration and resistance. The first TiN layer serves to minimize interactions between the Al and the Ti and the top TiN layer acts as an anti-reflective coating so that lithographic processing which follows will not be affected by any reflectivity on the wafer surface.
However, the M3 and M4 stacks are slightly different. There is no Ar etch before deposition and they consist of only three sub layers: first Ti, then Al and finally a TiN layer (Figure 17(b)). The Ti layer is first this time because it acts as a buffer between the Al and the ILD2. The middle TiN layer has been left out only to simplify matters. Interactions between the Ti and Al layers are increased as a result but only by an acceptable order of magnitude and the final TiN layer still acts as the anti-reflective coating.
CVD/ILD
This area is concerned with the deposition of an Insulating Layer Dielectric (ILD) and filling the Contacts and Vias within the device with metal. The Metal and ILD properties vary as the process reaches its final steps but the operations can be classified under the following headings.
W deposition
Tungsten deposition is the operation concerned with filling Contacts and Vias with tungsten in order to facilitate electrical contact. This operation takes place in a Chemical Vapour Deposition Reactor (CVD Reactor) and is a three step process. Firstly, tungsten hexafluoride (WF6) is reduced using silane to leave a thin surface of tungsten.
SiH4(g) + 2WF6(g) 2W(s) + 3SiF4(g) + 6H2(g)
This reaction has a good deposition rate but the main content of the deposit forms by the nucleation of H2 on the tungsten deposit. Hydrogen reduces tungsten hexafluoride to leave a tungsten deposit and it is in this fashion that most of the tungsten filling is produced.
3H2(g) + WF6(g) 2W(s) + 3SiF4 + 6H2(g)
The latter reaction cannot take place on the Tungsten Adhesion Layer and this is why it is necessary for the former reaction to initiate tungsten deposition. The third and final step in the deposition process is an NF3 etch. This step removes any deposits from the backside of the wafer, not only tungsten, but any oxide, nitride or silicon which may have been deposited earlier in the process. The W2 and W3 deposits are deposited in the same fashion. The only difference is that they are thicker deposits than W1.
ILD
The main function of an ILD is to separate the metal layers and to provide the plug holes which form the Contacts and Vias. The ILD is PTEOS-based - Plasma TetraEthylOrthoSilicate and it is also deposited in a CVD system, but this CVD system is plasma-enhanced. The system has three chambers because the operation is also a three step process (Figure 18).
Figure 18 Deposition of the CVD ILD
The first step is a deposition of some PTEOS based ILD. Because the spaces between the metal lines are harder to fill evenly than the areas between the Vias and contacts, an argon sputter etch is employed as a second step. This etch effectively lifts the top of the ILD and redeposits it into the corners which the metal lines make with the BPSG, so as the deposit is more uniform and to ensure no holes remain in the ILD. The third step is the final deposit of ILD which completes the operation.
The reaction governing the operation is as follows:
(C2H5O)4Si/Plasma(g) SiO2(s) + By-Product
One of the advantages of this reaction is that it takes place at low temperatures (˜400oC) so this system is easier to run than a high temperature system. The ILD is finally polished so that it is as smooth as possible for the next operation. ILD2 is deposited in a similar fashion but it is not as thick as ILD1. ILD3 is thinner again and it also involves an extra step, a CF4/O2 etch back. It is this etch that makes ILD3 thinner than ILD1 & 2 but it also serves to smooth the deposit thereby making a polishing step at ILD3 unnecessary.
Passivation/Protection
The final stages of the process involve sealing and protecting the devices from the atmosphere, more commonly known as passivation. A polyimide layer is used next to M4 for internal passivation and the devices are then sealed completely using Chrome/Gold.
The devices are then electrically tested to make sure they function properly and any defective devices on the wafer are marked with ink and are disposed of once the wafer is cut up into individual microprocessors.
The finished microprocessors are packaged, and are then placed on to motherboards and finally into the computers that are used for such a wide range of applications in today's world.
Conclusion
This article has shown the amount of chemistry involved in the production of a microprocessor (microchip) at many stages in its production, but also the complexity and technical sophistication of the production process in going from a silicon wafer to a microchip. Intel is the largest producer of microchips and its plant in Leixlip, FAB 10 and the new FAB 14, are at the leading edge of microchip production. The Leixlip plant currently employs 5,000 people, including a substantial number of chemistry technicians and graduates. At least this article has given the lie to the belief that 'there is no chemistry in the electronics industry'!
It is our hope that this report will arouse an interest amongst chemists and chemistry students in the Semiconductor Industry, but more importantly, that it will inform them that there are careers in this industry and that this is an alternative to the traditional Chemical & Pharmaceutical Industries.
(*The authors of this article are currently 4th. year students in Industrial Chemistry in the Department of Chemical and Environmental Sciences at the University of Limerick. They did their 3rd. year coop assignments in Intel in 1997.)
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Intel Ireland Ltd. have a useful brochure describing the plant available from the address below and also have a website at http://www.intel.com/
Intel Ireland Ltd.,
Public Relations Officer,
Collinstown Industrial Park,
Leixlip,
Co. Kildare
(Tel. +353-1-606-7000/Fax. +353-1-606-7070)
Do all molecules have the same size?
Pasquale Fettow
Dipartimento di Chmica "G. Ciamician", Universit… di Bologna, via Selmi 2, 40126 BOLOGNA - ITALY
The experiment proposed by Robert C. Mebane and Thomas R. Rybold "Are some molecules small?"1 gives an interesting and simple approach to the concept of the size of molecules. The this experiment, addressed to pupils in primary school, the small size of a molecule was deduced from the smell of vanillin notices outside a rubber balloon, in which a small amount of vanillin was placed. Further insight into the concept of molecular size can be obtained using the experiment described below, which aims to show that molecules of different substances have different sizes. The experiment is based on the different rate of diffusion of the gases.
Materials:
Two balloons; Two 1000mL polypropylene flasks with screw caps and a side-arm (i.d. 6-7 mm); Two polyethylene/polyvinylidene fluoride (PVDF) three-way stopcocks for tubing i.d. 6-7 mm.; Four pieces rubber tube i.d. 9-10 mm.; Ten hose-clamps; Two pressure gauges; One timer/stop watch.
Hydrochloric acid (4M);Zinc (0.5-1.0mm pieces);Calcium carbonate; Funnel (top i.d. 80mm., stem o.d. 9mm.); Powder funnel (top i.d. 100 mm., stem 26 mm.)
Procedure
The apparatus is assembled as shown in Figure 1 below. Blow up the balloons more than once so that they will expand readily when filled with gas. Turn the stop-cocks connecting the balloons to the pressure gauges. Pour into each of the flasks 25mL 4M hydrochloric acid and then add, respectively, 1g zinc to one flask and 1 g calcium carbonate to the other. Hydrogen gas is produced in one flask (Zn + HCl) and carbon dioxide gas in the other (CaCO3 + HCl). Leave the flasks open for 30 seconds or so to flush out the air from the flasks. Close the flasks and turn the stop-cocks to connect the balloons to the flasks. When both balloons are fairly full and about the same size, close the stop-cocks and read the pressure gauges. If the pressures are too low more gas can be added. When the two pressures are similar start the timer and record the starting time and pressure. Record the pressures for each gas until each has dropped by the same amount (see table of results below).
The flasks should be opened and placed in a fume cupboard or outside for the reactions to finish. The surplus acid can be diluted with water and washed away.
Sample results:
| PA/mbar | t/m | pB/mbar | t/m |
| 44 | 0 | 44 | 0 |
| 42 | 0.31 | 42 | 3.17 |
| 40 | 1.15 | 40 | 6.00 |
| 38 | 2.05 | 38 | 8.45 |
| 36 | 3.20 | 36 | 11.20 |
| 34 | 4.25 | 34 | 14.05 |
| 32 | 5.30 | 32 | 17.25 |
| 30 | 6.40 | 30 | 20.05 |
Discussion:
The time taken for the pressure to fall the same amount is related to rates of diffusion and the size of molecules. The smaller molecules diffuse faster and the pressure falls faster for the gas with the smallest molecules. The ratio of diffusion rates for H2/CO2 should be 5:1. The experiment shows the qualitative difference in rate.
Safety:
No flames should be present when hydrogen is being produced. This experiment is done with plastic flask for safety, but could be done with glass filter flasks.
Acknowledgement:
The author wishes to thank Professor Giuseppe Inorta, University of Bologna, for his helpful support.
References:
1. Robert C. Mebane and Thomas R. Rybolt, Adventures with atoms and molecules, Chemistry experiments for young people, Hillside, New Jersey: Enslow Publishers.
Figure 1 Gas preparation apparatus for hydrogen
(Second set-up is identical except CaCO3 used instead of Zn)
RSC North-West Industry Study Tour
26-29 October 1997
John Daly
Blackrock College, Dublin
Notification of this event arrived with my Autumn delivery of the RSCs Schools Publications Service (SPS) package. This bimonthly package of teaching materials and magazines is excellent value and is available to any teacher of chemistry through their school. You don't need to be an RSC member to get it.
The industries mentioned in the North-West Industry Study Tour included Anglesey Aluminium (aluminium production), Ilford (photographic materials), Salt Union Ltd. (salt mining)and Associated Octel (bromine extraction and bromine products). These are located in Cheshire and North Wales. I applied immediately as demand for places on these courses is very high. The tours are excellent value as they are highly subsidised by the RSC: the cost to RSC members is only œ30 (œ90 to non-members). The cost included all meals, accommodation at a comfortable hotel in Chester and transport to and from all the industrial sites. The proximity of Chester to Dublin and easy access via Ryanair to Liverpool was also an attraction. I was prepared to fund the trip myself but I would like to thank Blackrock College for funding the travel costs and course fee.
Early on the Monday morning of mid-term break I and twenty other chemistry teachers headed out in our coach from Chester along the North Wales coast to Anglesey. The sun struggled up out of the mist and the coastal scenery was beautiful. Having seen the video in the Chemistry In Action TV series on the production of aluminium at Anglesey Aluminium, I was anxious to see how the video compared with the real thing.
Anglesey Aluminium, Holyhead
Anyone travelling to the UK via Holyhead by train or car passes by Anglesey Aluminium and its tall chimney is visible far out to sea. First impressions are interesting. The scale of the plant is enormous. Much of the work is done in well-separated buildings, often open to the outdoors. While not denigrating the efforts of the company, it is not an area of delicacy and refinement as you would find in a pharmaceutical plant. This is indeed a heavy chemical industry. You get the clear impression of winning a metal from the earth's crust by brute force. (See Chemistry in Action!. #15, Spring 1985.)It brought to mind the writings of Primo Levi in his book The Periodic Table. While the chemistry happening before our eyes was basic electrolysis, it still seemed miles away from theoretical discussions on atomic theory in the classroom. There was something quite visceral about the conversion from white inert powder to bright shiny molten metal. You almost wanted to run your hand through the molten metal, neglecting the fact that it was running into the sows at very high temperature. (The aluminium industry uses the sow and pigs terminology originally used in the steel industry for the pouring of large and small ingots from molten metal.)
The size and fragility of the carbon anodes was another surprise. Acres of the plant are taken up with their manufacture and on the day we visited there were a few hundred rejects stacked outside one building waiting their turn for recycling. The specification of the pitch from which they are made had changed and their corners were crumbling. This is the sort of day to day problem you don't realise unless you visit a plant and they can be serious for an operation which runs 24 hours a day, and must have a continuous supply of anodes to function.
Another interesting thing was that we had to leave all our credit cards, watches etc. in a safe in the visitor centre as there was a serious risk of them being wiped by the magnetic fields created by the enormous flow of current through the bus-bars in the pot rooms. Fortunately nobody in our group had a heart pacemaker or they would not have been able to visit parts of the plant. Great care is taken in controlling and monitoring any fluorine emissions, which are liberated by the decomposition of cryolite in the cells. The employees are regularly screened for fluorine levels in the blood. Anyone with raised levels is removed from the pot room and relocated elsewhere in the plant.
A chemist told us an interesting anecdote about an employee whose fluorine levels were rising. Before action was taken the person went off sick for three weeks with a back injury unconnected with work. On returning to work his fluorine levels were found to be higher than when he left! Further investigations showed an addiction to tea drinking - he drank 20-30 cups a day. Once this habit was modified the fluorine levels dropped. Apparently tea and stout have high fluorine levels. is there a young scientist's project here? Tea consumption versus tooth decay or tea consumption versus brittle bones?
Associated Octel, Amlwch, Anglesey
After lunch we headed a few miles across Anglesey to Amlwch, where Associated Octel have a plant winning bromine from sea-water. (The extraction of bromine on Anglesey was described in Chemistry in Action! #21, Spring 1987.) The hazardous nature of this element and of many other substances in the plant was brought home to us when we were all issued with gas-masks before setting off on our tour of the headland. Bromide ion concentration is sea-water is around 65ppm, compared to a chloride ion concentration of around 19,000ppm.
Chlorine produced by the electrolysis of brine in Cheshire is transported to Amlwch by road or rail. It is used to oxides the bromide ions in sea-water to bromine. Sulphuric acid and hydrochloric acid are produced on site and used to lower the pH of the sea-water to minimise hydrolysis of the bromine and chlorine during the redox reaction. The bromine is stripped from the treated sea-water by blowing a strong current of air through it. The air stream carrying the bromine and excess chlorine is too low in concentration to be separated economically without further chemical processing. It is therefore reduced to hydrobromic acid in order to concentrate it and then re-oxidised to bromine by a stream of chlorine gas. The efficiency of the extraction process is largely controlled by the temperature of the sea-water as the vapour pressure of bromine increases with temperature. The sea-water temperature varies from 16oC in summer to 5oC in winter and the efficiency of the process drops correspondingly from 77% to 60%.
Around 30,000 tonnes of bromine are manufactured annually at Amlwch. Some of this is converted to 1,2-dibromoethane for use as a scavenger for tetraethyl lead in leaded petrols. This market is obviously decreasing rapidly and a diversification programme was essential to keep the company viable. Bromine compounds play a role in flame retardants, water sanitation, photography, pharmaceuticals and dyestuffs, to mention but a few areas. A batch production facility has recently been completed which can switch rapidly from making one brominated organic compound to another. New products made at the plant since 1985 include hydrogen bromide, hydrobromic acid, dibromomethane and bromochloromethane.
The sea-water outflow contains no free chlorine or bromine. It has a pH of 3.5 but this has recovered to pH 8 within 100m of the outfall.
Our guide was Gwyn Owen, training officer for the company and a local man. He remembers as a child in the 50s when effluent from the factory turned the washing on the line yellow. Things have come on a long way from then and the air was cold, fresh and invigorating as we walked around the headland site. He also had an interesting comment about the Welsh language, of which he is a fluent speaker. Much of the work in the plant is conducted through Welsh. However, this can present a difficulty to prospective employees coming from outside the area, particularly as regards schooling for their children and mastering a new language.
A long trip back to Chester was followed by a very pleasant evening: a first-class meal and local jazz session in the bar rounded off a great day.
Catalyst: the Chemical Industries Museum, Widnes
Tuesday morning brought a short journey to Widnes on the banks of the Mersey. Catalyst is located there and this museum and interactive science centre is a wonderful introduction, for children and adults, to the history of the development of the chemical industry in the north-west of England. The panoramic view from the top floor pin-points many of the chemical plants in the area. I can recommend a visit to Catalyst to any chemistry teacher, school group or family visiting this part of England.
Salt Union Ltd., Winsford
After a quick ploughman's lunch it was off to the largest salt mine in the U.K., hidden away in the beautiful Cheshire countryside near Northwich. The only access to the hundreds of miles of tunnels which lie 150m below the surface is by lift. We donned the usual haute couture one-piece suits, helmets. lamps, emergency battery packs and safety glasses. Casting aside all incipient thoughts of claustrophobia, it was off to the salt-face.
Our first surprise was the dryness and cleanliness of the mine. An impervious layer supports the ground water above and the lift shafts are sealed with concrete against the salt. The salt deposits are 95% pure but are coloured pink/orange by marl grit. The salt from this mine is not used for food preservation and human consumption, but is used for de-icing roads in the winter. It is blasted, crushed underground to reduce noise and dust pollution on the surface, and raised to the surface in skips. We watched the drilling of the blasting face and sampled the texture of the explosive used: a mixture of fusel oil and ammonium nitrate pellets, until my accent seemed to give rise to slightly more careful counting of the containers!
The mine produces 2.5 million tonnes of salt each year. We were allowed to collect as many samples of crystal as we could carry and I now have a huge lump of pink rock salt in my laboratory, which i find very useful when trying to introduce ionic bonding and crystals structures to my second year pupils. We had a wonderfully enthusiastic guide in the person of Linda Danischewsky, who probably ruined their day's quota by stopping an enormous 20 tonne Caterpillar Loader, so that some of us could ride with the driver. We were collecting desk-sized pieces from the 1300 tonnes of fragmented rock-salt which is produced in each blast. We were told that the salt for human consumption and electrolysis is mined by solution mining and pumped as brine to Merseyside for processing.
We had just enough time to pay a quick visit to the salt museum near-by to view the development of the industry from the time when brine-rich springs were discovered in the area.
I could have visited United Glass instead of the salt mine, as the other half of the party did. They also reported an interesting afternoon.
Ilford, Mobberley
On the final morning we set off towards Manchester to find this very large, ultra-modern plant making black and white photographic paper and film. It was tucked away discreetly behind a period house in a country town in Cheshire. A factory with virtually no windows seems a little odd at first sight until the obvious hits you. The product is light-senstive! As an organiser of the school camera club and consumer of Ilford paper and film, I had a special interest. There was so much to ask about: the nature and particle size in the 'emulsion', the control of crystal size and shape of the silver halides, the methods used to produce multi-grade paper. The guides were very patient in answering our detailed questions, particularly when we spotted an unusual chemical and started asking commercially-sensitive and therefore unanswerable questions, about its role in the process.
An excellent lunch and some welcome free films and mugs completed our tour on a suitably high note. This is the second RSC tour have attended, the first one being based in Drogheda a few years ago (organised by Jim McCarthy and Peter Childs). The companies approached by the RSC invariably give an excellent tour, usually much more detailed than a school visit and often including areas which are not normally open to the site visitor. I would like to thank our organiser Mike Tingle, called in at short notice, who did an excellent job.
In encouraging others to watch out and apply for these tours I must say that my experience is that they do perk up your classroom work. They allow you to quote from the real world of today's chemical industry, not just the formal textbook stuff but also the anecdotal material which, to the pupil, has the ring of authenticity about it. Another important consideration is that meeting and eating with twenty strangers, from the same subject but teaching different syllabi in different types of school, is very beneficial. In a teaching career, noteworthy for its isolation in the classroom, it is essential to make the effort to move outside our own circle. The Royal Society of Chemistry funds these tours up the cost of £300 per teacher. Their enlightened self-interest is wonderful. I sincerely hope they continue to run Industry Study Tours in the future.
Background
Ilford Ltd.:, The Ilford factory at Mobberley was opened in 1903, and was originally called Rajar Ltd. before becoming part of Ilford Ltd.. Ilford was started in 1879 by Alfred Harman in Ilford, Essex, making dry photorgaphic plates. The history of Ilford is recorded in the book Silver by the ton: A history of Ilford Ltd. 1879-1979, R.J. Hercock and G.A. Jones, McGraw-Hill 1979. Over that period Ilford's use of silver rose from a few thousand ounces a year to 4 million ounces a year (126 tons).
Salt:
There is useful material on salt on the Internet at the following sites:
www.northwich.com/industry.htm (salt in and around Northwich)
www.salt.org.il/index.html (a massive archive on all aspects of salt)
www.lionsaltworkstrust.co.uk (The Lion Salt Works were the last open pan salt works in the UK and closed in 1986. The site is being restored and their is an exhibition open daily from 13.30-16.30 hrs. It is located at Marston on the Trent and Mersey Canal.)
The Salt Museum is located on the London Road (A556) into Northwich and tells the story of salt mining in Cheshire since Roman times. It is Britain's only salt museum and it is open Tuesday-Friday 10.00-17.00 hrs. and Saturdays and Sundays 14.00-17.00 hrs.
www.maldonsalt.kemc.co.uk/ (This site describes the history and production of salt at the Maldon crystal Salt Company from the sea at England's only remaining sea salt producer. The site is well illustrated and the company has been making salt at Maldon, Essex for over 100 years.)
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Many stories and novels, particularly detective stories, involve chemistry as an element of the plot. Some take place in chemistry departments or have a scientific theme e.g. Isaac Asimov's murder mystery A whiff of death; Carl Djerassi's novels e.g. Cantor's Dilemma, ; Agatha Christie's novel The Pale Horse involves thallium poisoning (see Chemistry in Action! #34 1991) - this was on TV over Christmas; Arthur Conan Doyle's famous detective Sherlock Holmes was an amateur chemist; R. Austin Freeman wrote a series of detective stories featuring Dr. Thorndyke (see below). Several TV series have featured forensic scientists (Quincy, McCallum) and MacGyver also uses science to escape from some impossible situation in every episode. There must be many more. If you know of any please send in details of stories where chemistry or chemists are a key part of the plot. Discussing these books or TV programmes is another way of getting students interested in science and getting them to learn some science along the way, I'd be interested to hear of anyone who has used this sort of approach in their teaching and how successful it was. Agatha Christie used poison to despatch more than 30 victims in 66 books. She had served as a hospital pharamcy dispenser in World War I.
The following extract is from a detective story by R. Austin Freeman, which were published at the beginning of this century and are now out of print. Science is an essntial part of the plot.
The Red Thumb Mark (1911)
R. Austin Freeman
In the book the plot hinges around the authenticity of a thumb-print and Dr. Thorndyke is called by the judge to give evidence on the possibility of forging such a print. The extract stars with the judge asking Dr. Thorndyke to explain how this can be done. So if you want to learn how to forge a finger-print, read on.
"'..To return to the possibility of forging a finger-print, can you explain to us, without being too technical, by what methods it would be possible to produce such a stamp as you have referred to?'
'There are two principal methods that suggest themselves to me. The first, which is rather crude though easy to carry out, consists of taking a actual cast of the end of the finger. A mould would be made by pressing the finger into some plastic material, such as fine modelling clay or hot sealing wax, and then, by pouring a warm solution of gelatine into the mould, and allowing it to cool and solidify, a cast would be produced which would yield very perfect finger-prints. But this method would, as a rule, be useless for the purpose of the forger, as it could not, ordinarily, be carried out without the knowledge of the victim.; though in the case of dead bodies and persons asleep or unconscious or under an anaesthetic, it could be practised with success, and would offer the advantage of requiring practically no technical skill or knowledge and no special appliances. The second method, which is much more efficient, and is the one, I have no doubt, that has been used in the present instance, requires more knowledge and skill.
'In the first place it is necessary to obtain possession of, or access to, a genuine finger-print. Of this finger-print a photograph is taken, or rather, a photographic negative, which for this purpose requires to be taken on a reversed plate, and the negative is put into a special printing frame, with a plate of gelatine which has been treated with potassium bichromate, and the frame is exposed to light.
'Now gelatine treated in this way - chromicized gelatine, as it is called - has a very peculiar property. Ordinary gelatine, as is well known, is easily dissolved in hot water, and chromicized gelatine is also soluble in hot water as long as it is not exposed to light; but on being exposed to light, it undergoes a change and is no longer capable of being dissolved in hot water. Now the plate of chromicized gelatine under the negative is protected from the light by the opaque parts of the negative, whereas the light passes freely through the transparent parts; but the transparent parts of the negative correspond to the black marks on the finger-print, and these correspond to the ridges on the finger. Hence it follows that the gelatine plate is acted upon by light only on the parts corresponding to the ridges; and in these parts the gelatine is rendered insoluble, while all the rest of the gelatine is soluble. The gelatine plate, which is cemented to a thin plate of metal for support, is now carefully washed with hot water, by which the soluble part of the gelatine is dissolved away leaving the insoluble part (corresponding to the ridges) standing up from the surface. Thus there is produced a facsimile in relief of the finger-print having actual ridges and furrows identical in character with the ridges and furrows of the finger-tip. If an inked roller is passed over this relief, or if the relief is pressed lightly on a sheet of paper. a finger-print will be produced which will be absolutely identical with the original, even to the little white spots which mark the orifices of the sweat glands. It will be impossible to discover any difference between the real finger-print and the counterfeit because, in fact, no difference exists.'"
Dr. Thorndyke (a lecturer in Medical Jurisprudence and Toxicology) goes on to produce a sheet of mixed real and counterfeit prints which totally confound the expert witnesses in the affair of The Red Thumb Mark.
Solvents and their uses
Dr. Martin Knox
Roche Ireland Ltd., Clarecastle, Co. Clare
It is difficult to imagine what society would be like today without the powerful and all pervading influence of chemistry in our lives - at least to those of us who live in the West. The influence of chemistry is to be found in such diverse endeavours as motor car manufacture, the manufacture of fabrics, dyes, insecticides, herbicides, paints, drugs, detergents, soaps, explosives, packaging, fertilisers, floor coverings, computers, and many other items. Key ingredients in the manufacture of these materials are SOLVENTS.
The vast range of useful products derived from chemical processes which are available in society today would not be possible without the use of organic and non-organic solvents. Most of these materials were developed in the West during the last 100 years or so. To exemplify the impact of chemistry in our lives one would cite one familiar item in use in virtually every family in the western world, namely the motor car.
A significant portion of the materials associated with today's vehicles owe their origins to the study of chemistry. The upholstery, the dashboard materials, the brake and clutch fluids, the engine oil, the paint work, the fuel which propels the car, the tyres, the hoses, the cable insulation, the pigments and dyes used in colouring the paints, the material used in the cooling fans, etc., would not exist without a knowledge of chemistry and the use of solvents.
As well as industrial use, solvents are in use in very many household materials as well. Take the familiar items such as paints, paint strippers (Nitromors), insecticide and herbicide sprays of all sorts, nail varnish remover, stain removers, polishes, lacquers, varnishes, adhesives (wood adhesives, nut-lock adhesives, Super Glue, etc., all contain solvents either as a major component as in the case of a paint stripper such as Nitromors, or as a minor ingredient as in touch-up car spray such as DUPLI-COLORR).
My DUPLI-COLORR spray can carries the following hazard warning signs: FLAMMABLE and HARMFUL, and it also says that the can contains xylene. Here the xylene is the solvent which evaporates as the paint dries and leaves the solid polymeric coat a couple of molecules thick behind, covering that nasty piece of rust I'm trying to conceal before selling my treasured and trusty possession to an unsuspecting customer!
The chemist classifies solvents into two broad categories: ORGANIC, i.e. containing carbon with other elements; and NON-ORGANIC. The most important solvent of all of course is water, which belongs to the latter category. Water as you all know is an invaluable solvent for all kinds of things, especially whiskey - though there are those who do not hold with this practice! What would some of us do without the aid of this giver and sustainer of life? Think of the many cups of tea and coffee that are consumed each day throughout the world with the assistance of water. The general principle that a hot solvent has more power than a cold one is well illustrated here; the hot water enhancing the solubility of caffeine which is the drug whose effects as a stimulant many of us crave. We rely on water too to purge the body of unwanted wastes and to keep the salt balance within the bounds of what the properly functioning body demands.
There are 2 to 3 million known organic chemicals many of which can be used as solvents. The expression "as a solvent" which we use when describing many of the uses for chemical substances covers a multitude of applications. Basically, a solvent is a medium used to dissolve other substances, as water dissolves sugar for example. The solvent can also be used as a medium in which one or more substance is converted into others, as would be the case if two raw materials react to form a drug substance.
It is usual to classify organic compounds into families; these families or classes will have members useful as solvents. The most common categories of compound are given in the table below together with the members of the family that find a use as a solvent. For a family member to be useful as a solvent this member must evaporate fairly readily, but not too readily. Usually the solvent is a liquid with a relatively low boiling point, <2000C or so. Some familiar solutions not involving liquids, however, are brass (zinc dissolved in copper), bronze (tin dissolved in copper) and steel. These are known as 'solid solutions' or alloys.
Another issue in relation to solvents which is a fairly recent phenomenon is the problem of solvent abuse. The abuser inhales whatever solvents and gases are most readily available. These could be any of those listed in the table, but the hydrocarbons appear to be the most commonly used in this way. The solvents found in glues, lighter fluid and petrol are most often used.
| FAMILY | FAMILY MEMBER IN COMMON USE | EXAMPLES OF THE USES OF SOLVENTS |
| HYDROCARBONS1
CHLORINATED HYDROCARBONS2 |
Benzene
Toluene Xylenes
Methylene chloride Chloroform Carbon tetrachloride ‘Westrosol’, ‘Genklene’ |
Paints and
paint thinners,
General solvent, Constituent of Nitromors, Formerly used as anaesthetic Dry cleaning fluids |
| ALCOHOLS3 | Methanol
Ethanol Diethylene glycol, ethylene glycol |
Paint
solvent,
Ballpoint stain remover Antifreeze |
| KETONES4 | Acetone
Methylethylketone (MEK) |
Nail
Varnish remover
Paint solvent Paint solvent |
| ACIDS5 | Formic and Acetic Acids | Industrial solvents |
1These materials consist of carbon and hydrogen only. They are very important economically being an important source of energy as fuels for transport and industry. They are also used as raw materials for many important compounds.
2This family constitutes the most controversial group because of the toxicity of the compounds: they can cause serious liver damage. Carbon tetrachloride is now banned as a dry cleaning solvent. Chloroform, once used as an anaesthetic, is suspected of causing cancer. Methylene chloride continues to be used in industry but is gradually being phased out. Members of this family are also known as chlorinated hydrocarbons or halogenated hydrocarbons. DDT and Lindane are but two members of this family used as insecticides. Polychlorinated biphenyls are widely used in electricity transformers.
3 The alcohols all contain -OH groups are familiar because of the widespread consumption of 'alcohol' = ethanol. The other alcohols are harmful and poisoning cases have occurred where methanol ('wood alcohol') or anti-freeze have been drunk deliberately or accidentally.
4 The ketones contain the C=O group.
5 The acids contain the carboxylic acid group -COOH and are classed as weak acids and when dilute are fairly harmless. Vinegar is, of course, dilute acetic (ethanoic) acid. However, when concentrated they are quite lachrymatory and corrosive.
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Potatoes and paint
What use are potatoes in making paint? Chemists at Dulux (the ICI paint manufacturer) have shown that up to a quarter of the structure-forming materials in paint can be replaced by starch from potatoes or maize. Starch is cheap and biodegradable unlike the vinyls or acrylics used in paints to form the structural film. Exposed to air they harden to form a hard surface. If the process is speeded up using catalysts then starch can be incorporated into the resultant polymer film. The new films aren't as water-resistant as conventional paints and the chemists are still working on this.
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Chemical Caricatures
Professor William B. Jensen of the University of Cincinnati is well-known for a series of caricatures of famous chemists which he drew in the early 70s. They have been published in several places. There are 34 in total from Robert Boyle to Linus Pauling, and they are based on paintings, drawings or photographs of the subjects. In this issue we start publication of this series, starting with Robert Boyle on p. 38, and we hope to publish them in subsequent issues, by kind permission of Professor Jensen. The series is called Chymists: that strange class of mortals and the cartoon below introduces the series.
Ryder's Rule, with apologies to ...?
Adrian Ryder
Colaiste Iognaid, Galway
The Octet Rule states that atoms, when bonding, gain, lose or share electrons so as to attain inert gas configuration in their outermost shell. (Apart from those atoms attaining the helium configuration all the others strive to attain 8 electrons in their outermost shell)
Examples abound, e.g. calcium loses 2 electrons to oxygen, giving argon configuration to the calcium Ion and neon configuration to the oxygen Ion.
In general the number of electrons from a particular atom depends on the group in which it is found, being either the group number or eight minus the group number, whichever is the smaller. Where elements in a group have more than one valency the octet rule comes a cropper.
A different way of looking at the rule is given, by myself, as follows: Atoms bond together to form molecules/ion formulae such that the group numbers of the atoms, summed, is divisible by eight.
The groups are either the A or B groups as given in the mathematical tables. Let us examine a number of common chemicals...
CaO..group II + group VI; 2 + 6 = 8
CaSO4; 2 + 6 + (4 x 6) = 32
CS2; 4 + (2 x 6) = 16
Zn(NO3)2; 2 + (2 x 5) + (6 x 6) = 48
One could continue ad infinitum. But what about the exceptions? These are mainly two in number.. with hydrogen and with elements of variable valency. Take hydrogen: this element fits exactly into no one group having some group I and some group VII characteristics. This is because it tends to lose electron control to the electronegative elements and gain electron control from the electropositive elements. e.g. in CH4 one has group IV + 4 in group I, 4 + (4 x 1) = 8, but in LiH one has group I + group VII, 1 + 7 = 8. in methane hydrogen is acting as a group I element and in lithium hydride as an element in group VII.
If one examines PCl3 and PCl5 one gets
PCl3; 5 + (3 x 7) = 26
PCl5; 5 + (5 x 7) = 40
- the PCl5 is conforming and the PCl3 is non-conforming. One may explain the PCl3 by having the phosphorus pretending to be in group III for this compound. The other exceptions may be explained by use of the pretend valency.
A feature of Ryder's Rule is its use in determining the correct formula of a compound, e.g. is the formula for calcium nitrate, CaNO3 or Ca(NO3)2?
CaNO3; 2 + 5 + (3 x 6) = 25
Ca(NO3)2; 2 + (2 x 5) + (6 x 6) = 48
Thus, by the rule, the correct formula is Ca(NO3)2.
One may give any number of examples which conform to the rule and, being a rule, find examples where the pretend group comes into play. Why not try out the rule for yourself with your students?
_
Comment:
On the Octet Rule: the octet rule is a great over-simplification which only applies exactly to the elements of the first short period (Li to Ne) where they only have 4 orbitals in their outer shell and therefore a maximum of 8 electrons can be accommodated. All heavier elements have d orbitals available and can have more than 8 electrons in their outer shell. In fact, normally they can have oxidation numbers up to the group number and the non-metals can form up to N covalent bonds, where N is the group number. THus in group VII iodine can have oxidation states from -1 to +7 and can form a fluoride IF7. No rules are broken. The octet rule is useful in the initial presentation of the chemistry of the main group metals (Groups I and II, plus Al) and the non-metals of the first row (C to F). It is not really helpful for transition metals or the heavier p block elements.
In PCl3, the example above, P is obeying the octet rule (8 electrons around P) and in PCl5 it is breaking it (10 electrons around P).
The problem is the misuse and over-use of a 'rule' which really only applies to a few elements. It is a rule more honoured in its breach than its observance!
On numbering the groups: the use of A and B subgroups is now obsolete and since 1989 IUPAC has recommended a 1 to 18 system (Arabic numerals) for the s, p and d blocks. Previously the main group elements (s and p blocks) were given Roman numerals I, II, III, IV, V, VI, VII, VIII (with or without A or B designations). Thus group VIII is now group 18 and group III is group 11. This has removed the confusion between the A and B subgroups where the Americans and Europeans had opposite systems! One should either use the simple I to VIII system or the 1 to 18 system, but not A and B subgroups. They are both obsolete and confusing! One should NOT use e.g. group 7 to refer to the halogens (group VII or group 17), which is a mixture of bothe systems
The diagram below shows the different systems and is taken from the WebElements page at the University of Sheffield, maintained by Mark Winter, which is the best Periodic Table site on the Internet. (www.shef.ac.uk/chemistry/web-elements/main/welcome.html)
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Have you developed any learning systems for your students which you'd like to pass on to other teachers? If so, why not send them in to Chemistry in Action!
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Dear Dr. Childs,
In #50 of Chemistry in Action! I read Barry O'Brien's article to which I would like to react. First I should note that the current best value of Avogadro's Number is 6.0221367 x 1023 (The Green Book, 2nd ed., p.89) which differs from the one given by O'Brien.
The SI mole is not a number. In his article "Amount and the Mole" (Chemistry in Action! #50, p.30), Barry O'Brien treats dozen and gross as units of amount. In my opinion, dozen and gross are just names of particular numbers, and not units at all. There is no difference between a dozen eggs and twelve eggs.
Similarly, Avogadro's number, denoted by No, is the name of the (rounded) number 6.02 x 1023, whereas the mole has been defined as the unit of the SI base quantity "amount of substance". O'Brien's assertion that:
1 mole = No (1)
is incorrect, and his ensuing catechism of the mole is heretical.
The amount of substance n(X) of a sample consisting of particles of the type X is given by:
n(X) = N(X)/NA (2)
in which N(X) is the number of particles of the sample, and NA is the molar number of particles, also known as Avogadro's constant. In fact, equation (2) can be regarded as the definition of the concept of amount of substance.
Consider a sample of which the amount of substance n(X) is equal to exactly 1 mol. By definition, N(X) of this very special sample is equal to No. Substitution of these results into equation (2) yields:
1 mol = No/NA (3)
which turns out to be independent of X. Equation (3) differs markedly from O'Brien's view, expressed by equation (1). From equation (3) the relation between Avogadro's constant NA and Avogadro's number No can be found as:
NA = No mol-1
To perform chemical calculations in a logical way by the method of quantity calculus it is imperative to treat the mole as a unit, not as a number, however large. It should be used to measure the amount of particles that are the building stones of substances, not objects, such as O'Brien's potatoes. In all examples designed to demonstrate the enormous magnitude of the mole, it has been confused with Avogadro's number. One mole of water is not enormous. It amounts to the contents of one test-tube.
Marten J. ten Hoor,
Dr. Aletta Jacobscollege,
J.W. Frisolan 40,
9602 GJ Hoogezand
Nederland
The Editor welcomes comments, corrections, extra information or alternative views in response to material published in Chemistry in Action!. However, responses should be objective and not personal!
Chemical and Mining Industry News
Compiled by Marie Walsh, SICICI
Conroy hopes for Monaghan gold strike
Irish Independent 23/9/97
Conroy Diamonds and Gold has announced some high grade gold indications following the company's third bore hole in Co. Monaghan. The prospect, in the Clontibret area, is in the same region where mining took place in the last century and again in the 1950s. It is part of a rock formation called the Longford-Down massif, which extends into Scotland where it has been the base of gold mines in the past.
Conroy is very optimistic about the prospect, given findings of 3g, 5g, 10.5g and 16.24g per tonne in the test drillings. The average in many gold mines around the world is between 5g and 8g, so the double figure concentrations should ensure further exploration.
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Decision on mining lease welcomed
Irish Times 9/10/97
Ivernia West and Minorco Lisheen have welcomed the issuing of a state mining lease for their Lisheen lead/zinc mine as the completion of the regulatory process. The œ165 million project began construction in September and will employ up to 700 people on site work over the first two years, with up to 300 people on the next 14 year mining phase. The thirty year term of the lease is an indicator of confidence in further reserves being discovered during the lifetime of the mine.
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Du Pont expands Lycra business
ChemWeb 15/8/97
Du Pont is to invest $100 million to expand the Lycra manufacturing facility at Maydown near Derry. The capacity at the plant will be increased by 30% involving new production technology from polymer preparation to spinning and controls. The expansion is expected to be completed by 1999.
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Drugs, pills and stockmarket thrills
Sunday Business Post 9/11/97
A biotechnology company which was set up in 1992 and started operations from a portakabin is now worth more than œ3 million on paper, following a fund-raising which values the company at œ28 million. The company, Biotrin, was founded by Dr. Cormac Kilty, a graduate of UCD who was head of Baxter Healthcare's European division when he decided to set up his own company. Biotrin currently employs 57 staff, 40 of whom are graduates, in the development of diagnostic tests for niche markets. For example, Biotrin makes tests to detect signs of organ damage. The company discovered a protein in urine that appears when the kidneys have been damaged. This allows detection six weeks earlier than was previously possible. Tests like this are currently only sold to hospitals, but Biotrin hopes to develop a home self-diagnostic kit which will add immensely to the company's potential trading profits in the future.
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Irish scientists develop milk with ability to counteract heart disease
Irish Times 2/6/97
A research team based at Teagasc's Moorepark centre near Fermoy has developed a milk with an enhances ability to counteract heart disease and cancer. The researchers have succeeded in dramatically increasing the oleic acid content of the milk. Milk high in oleic acid is known as "soft milk" and manufactures spreadable butter at refrigeration temperatures. Its advantage in health terms is that it reduces low density lipoprotein cholesterol, while leaving high density lipoprotein cholesterol unchanged, i.e. it is good for the heart. Cows fed with crushed rapeseed or soybeans yield higher quantities of soft milk, which also contains enhanced levels of linoleic acid. The latter has been shown to have anticarcinogenic properties, antioxidant behaviour and inhibition of arteriosclerosis. The disadvantage is that such dietary supplements for cows would increase the shelf price of the milk product. What cost health?
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New healthcare company for Navan
Irish Times 16/12/97
An American healthcare company, Vivus Ireland Ltd., is the first tenant at the Navan Industrial Park. It will invest œ28 million building a 90,000 sq. ft. plant on the 30 acre site to produce 250 jobs by the end of 1999. The factory will produce MUSE, a transurethral delivery system for correcting erectile dysfunction.
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Glass old and new
Irish Times 18/12/97
Sean Quinn is due to open a new glass factory in Ballyconnell, Co. Cavan. Meanwhile archaeologists have discovered the remains of an old glass industry in Co. Offaly dating from the 17th. century. The remains of a forest-glass, wood-fired glass furnace is at Glasshouse, near Shinrone. The furnace has its barrel-vaulted roof intact and such furnaces from this period are rare. It used wood as a rwa material for fuel and to produce alkali, together with silica and lime. The glass industry was established in Ireland in 1586 by Capt. Woodhouse, who was granted a royal charter by Elizabeth I. In England the use of wood to make glass was banned in 1615 to conserve forests and glass-makers had to switch to coal. In 1618 glassmaking families moved to Ireland where wood was not banned and the industry flourished in Offaly until the manufacture and export of glass was banned in 1638. The find is reported in Archaeology Ireland. The Shinrone glasshouse may date from 1590-1640, and was probably associated with the Bigo and Hensey families. There is no surviving furnace of this type in England so this find is unique. Glass manufacture is one of the oldest chemical technologies going back to the ancient Egyptians.
(Archaeology Ireland 11(4) Winter 1997 pp21-23)
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IR£135 million expansion proceeds at Merck, Sharp & Dohme
HORIZONS No. 16 October 1997
Merck, Sharp & Dohme, the South Tipperary based pharmaceutical manufacturer, is currently proceeding with a £135 million expansion at its site at Ballydine near Clonmel. The expansion is the company's biggest single investment outside the US and aims to expand manufacturing capacity by 50%.
The construction phase of the expansion will employ up to 600 workers and is expected to be completed by early 1999 and full operational by June of that year. The permanent plant workforce will then increase by 50, which will bring the total number of plant employees to 400.
Merck, Sharp & Dohme started production at the Ballydine plant 21 years ago in 1976 with a workforce of 165. It is a subsidiary of Merck, Sharp & Dohme, New Jersey which has 23 facilities world-wide employing about 58,000 people. The Irish plant produces a range of ten diverse pharmaceutical products for treatment of ailments such as blood pressure problems, angina, asthma, migraine, etc. The company has consistently upgraded and invested in the plant and the area, including a £30 million environmental upgrade programme. It has also won a number of environmental management awards. More than 100 of the present workforce are graduates from Irish universities and RTCs.
100 new jobs for Warner Lambert in Cork
HORIZONS No. 16 October 1997
Warner Lambert, which currently has two manufacturing facilities in Dun Laoghaire employing some 260 people, has announced a £39 million investment programme (supported by IDA Ireland) for its newly acquired Cork properties. Earlier in 1997 Warner Lambert announced that it had acquired Hickson PharmaChem at Ringaskiddy, Plaistow at Little Island and Island Pharmaceuticals at Carrigtwohill. The company now proposes to reorganise and expand the facilities at Little Island and Ringaskiddy. Among the proposals are plans for a high grade finishing and tabletting plant alongside the existing production plant at Ringaskiddy. The investment programme will bring an additional 100 jobs to the Cork plants, which currently employ 250 people.
Japanese giant takes over Irish pharmaceutical company
Irish Times 23/10/97
Japan's biggest pharmaceutical corporation, Takeda Chemicals, has taken over the small Co. Wicklow-based Grelan Corporation. The company, in Bray Co. Wicklow, currently employs 40 people in what is viewed as basically a start-up operation. It is still awaiting licensing of its main product, a cholesterol-reducing drug.
The takeover is Takeda's first investment in Europe and there are high hopes that, as well as expanding the manufacturing capacity of the Grelan plant and increasing its workforce to 100 by 1999, it will become the central manufacturing base for Takeda in Europe.
New group to campaign on water quality
Irish Times 5/11/97
A new environmental campaigning organisation has been set up to fill the void left by the winding up of Greenpeace Ireland earlier this year. The new group Voice of Irish Concern for the Environment will be known as VOICE and its primary target will be water pollution. Many former members of Greenpeace Ireland are involved in the new group which can be contacted at its office 14 Upper Pembroke St., Dublin 2 (Tel.: 01 661 8123 or e-mail at avoice@iol.ie).
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The Green Government Guide
Environment Bulletin November 1997
The Department of Environment and local government has published a guide for public sector agencies and government departments as to how they can demonstrate good environmental practice. Perhaps some of their tips could be adopted in schools and classrooms!
*Use recycled paper wherever possible.
*Use discontinued paper stock and waste paper in draft documents, internal memos, etc.
*Use double sided photocopying/printing.
*Use E-mail where possible.
Use recycling bins wherever possible. Recycling of aluminium cans saves 95% of the energy required to make one from raw materials!
In relation to this, Forbairt has recently prepared an inventory of recycling firms in Ireland and has made it available on the internet. The list includes about 100 different companies involved in a number of areas of recycling, as well as the names of a number of specialist waste disposal contractors. The electronic database is on the Forbairt website and can be accessed at:
http://www.forbairt.ie/environment/waste/index.html
An exhibition by Self Help will take place at ENFO's Dublin offices from December 3rd - January 10th 1998. The Value of Rubbish shows how African people have reused and recycled what we regard as rubbish to create household implements, toys and agricultural tools.
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Refinery maybe prosecuted after leak
Irish Times 5/11/97
The EPA could be considering prosecution of the Irish Refining Company at Whitegate in Cork Harbour following the seepage of 700 gallons of heavy fuel oil into Cork Harbour waters. The refinery's emergency procedures were put into operation following discovery of the spillage from a ruptured pipeline. The worst affected area is at Graball Bay across the harbour from Whitegate, and unfortunately a relatively inaccessible part of the shore area. The oil which remained in the harbour was contained by the use of pollution control booms prior to recovery. An investigation will be carried out to determine the exact cause of the incident.
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Teagasc warns on phosphate pollution
Irish Times 23/9/97
Teagasc has warned farmers that fertiliser application may soon have to be licensed because of damage done to water quality by excessive phosphate use. Teagasc has found that 30,000 tonnes of excess phosphate are applied annually, at a waste of some œ25 million. The organisation is now mounting a œ5 million campaign aimed at curbing the waste and preventing water pollution.
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Road traffic may breach EU air quality
Irish Times 16/12/97
We've all heard of the traffic problems in Dublin but a side-effect is increased air pollution. The increasing traffic density will make it difficult to meet EU air pollution standards in the future. Cars are the biggest threat to air quality in towns, with emissions of hydrocarbons, nitrogen oxides, carbon monoxide and particles from diesel engines. In the summer the sunlight and car pollutants can produce photochemical smog, especially if air is trapped by a temperature inversion. Nitrogen dioxide levels at the curbside are already near the EU limits and the EPA is increasing its air monitoring programme. During 1996 the EPA started monitoring benzene, toluene and xylenes.
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EU to investigate phthalates in toys
Irish Times 17/12/97
The European Commission has reported to the EU Parliament that it has started an investigation into the use of phthalates in PVC used in soft toys like teething rings, where they might be ingested by small children. It is due to report next year. The decision has been welcomed by environmental groups who want to see all PVC products banned. Phthalates are used as plasticers to make the PVC more flexible.
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Sand Power
Grapevine 8/97
A German company BTC Biotechnik claims that blasting weeds with sand breaks down the leaf's waxy cuticle and dramatically reduces the amount of herbicides needed for control. Trials have shown that as little as 10% of the recommended dose can be effective when a sand blaster is fitted to the spray boom. BTC is currently seeking a partner to bring the technique to the UK.
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Summer 1997 - bad days for Irish water quality
It appears that Irish water quality is deteriorating at an accelerating rate. According to Patrick Buck, assistant manager of the South Western Region Fisheries Board, "it may take only 20 years to significantly alter our water quality to such a degree as to be seriously detrimental to our salmon stocks, fish which have swum quite happily in our rivers for the past 10,000 years". Summer 1997 saw a number of water pollution incidents, mostly as a result of human activity, and in 40% of incidents farming. Discharge of toxic substances into waterways resulted in fish kills, drinking water contamination and new restrictions on human bathing in some cases. The variety of the toxins was as frightening as was the extent of some of the devastation caused. Some of the major pollution incidents are summarised below.
By the end of August 1997 the running total of fish kills for the year to date was 35, lower than in recent years, but the extent of the devastation was immense. A major cause of eutrophication is phosphate pollution, with 80% traceable back to farming sources. Teagasc's new initiative to educate farmers in the proper application of phosphate fertilisers will surely be of great benefit in more ways than one. Farmers representatives are quick to defend themselves as only one of many sources of water pollutants, and there is no doubt that domestic and industrial sources also play a part in the problem. Most groups would agree that the best deterrent would be a "polluter pays" policy from the Government.
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| Location | Possible Cause | Implications |
| Cahore
Point
Co. Wexford |
Orgnaic combination of rotting vegetation and stagnant waters | Bathing/angling restrictions |
| Lakes of Killarney | Phosphate pollution | Eutrophication resulting in bathing, angling and drinking water restrictions |
| Mulkear
River
Co. Limerick |
Several possibilities:flood water/farming pollution/rotting vegetation | Wipe out of up to 100,000 salmon and trout |
| Moynalty
& Blackwater rivers,
Co. Meath |
Water run-off following fire at Wellman synthetic fibre plant. | More than 1,000 fish killed, Navan water supply cut off. |
| Martin & Shournagh Rivers, Co. Cork. | Discharge of up to 20,000 tonnes of toxic pig slurry | Killing of 100,000 young trout and salmon parr, Cork City water supply threatened. |
Exploring Chlorine (it's a gas)
This new booklet from the Chemical Industries Association is written for 8-12 year olds and colourfully tells the story of chlorine: its importance to human inventions, from bleach and fertilisers to medicines and plastics, and how by working with nature this green gas can be made greener. Single copies of the booklet are available free of charge from Chemical Industries Association, Kings Buildings, Smith Square, London, SW1P 3JJ.
Can White be Green?
A short (10 minutes) video accompanies a teaching pack developed for use with 11-16 year old students. The pack, made by the World wide Fund for Nature (WWF) in collaboration with Tioxide Europe looks at the environmental and social implications of the industrial manufacture of white pigment. Curriculum topics covered include pollution in waterways, oxidation and reduction processes, managing waste, location of industries, advertising and financial considerations, etc. This could be just what you are looking for for TY project work in science, with excellent photocopiable material for each unit and information technology in the form of a disc for statistical information. For more details and a full publications list contact WWF at Panda House, Weyside Park, Godalming, Surrey GU7 1XR. Please enclose a stamped addressed envelope of international reply coupon.
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Irish Times starts a science page
The Irish Times used to have a science page but it was dropped. The daily column Weather Watch by Brendan McWilliams and the Monday science article by William Reville from UCC have kept a regular science flag flying, together with occasional health and environment items and the weekly feature CompuTimes. At the end of 1997 the Irish Times launched Science on Mondays, including the column by William Reville and other items, taking up from half to a full page. This is very welcome and I hope teachers will use it by posting it on their "Current Awareness Board", and picking up items to discuss in class.
Considering the importance of science and technology in everyday life it is amazing that there is not more regular coverage in the Irish daily papers. If we consider the coverage for the arts and sport, the neglect of science becomes obvious. Health is news and so are environmental scare stories, but basic advances in science and technology, which will change our lives in the future in unimaginable ways, hardly get a mention. A weekly page is a start, but surely there are enough materials and enough science journalists, as well as academics, to fill a daily page?
Crossword Puzzles (Randel Henly)
2. For Sixth Year Chemists (including some physics, biology and general knowledge
Clues Across
1. The motion which supports kinetic theory
8. Technique involved in volumetric analysis
12. A small tailed amphibian
13. The basic chemicals of all matter
16. An acid is a proton...
17. A wide-mouthed water jug
19. Element of the rocks
20. Symbol for element named in memory of the discovere of dynamite.
21. Type of chemical bond
24. One mole per litre
27. American element(?)
28. A winged insect - may be black
30. Lawful
32. Relating to the nose
34. The bulking up principle
37. Its oxide is the basis of white paint
38. Carbon Chemistry
39. A group 1 symbol
40. This element is in the West
42. he discovered electron "shells"
43. A gas law is named after this scientist, who also discovered the existence of atoms.
47. Sub Atomic Particles
49. Visible fine particles often given off during combustion
50. Atmopspheric nitrogen is fixed in plants of this type
53. Change directly from solid to vapour
56. Reaction in which electrons are transferred
59. The positive one is called the anode
60. These substances have a lattice structure
62. Old unit of heat measurement - used by the Gas Company
63. Black liquid obtained from coal; Faraday extracted benzene from it
64. There is a hydroelectric power station on this Irish river
65. Process by which water from the soil enters plant roots.
66. Gaseous element
67. Number of carbon atoms in tetra methyl hexane
68. Chemical name for table sugar
Clues Down
1. German chemist of "burner" fame
2. The most common compound on Earth
3. Relating to the atom
4. Symbol for one of the coinage metal elements
5. A measure of the quantity of matter present
6. Symbol for an element discovered by M. Curie in honour of her native country
7. The most common element in the universe
9. Metal used to cover steel cans to prevent rusting
10. A soid halogen
11. Mass number of the lightest halogen
14. Symbol for element named in memory of the E=mc2 scientist
15. A street train
18. Austrailian marsupial - like a badger
22. The "combining power" of an atom
23. Group 6 element symbol 25. Different physical forms of the same element e.g. diamond and graphite
26. Transition element (3rd series) symbol
29. common beverage cpontaining tannic acid
31. 109 33. Pit or airtight structure in which green crops are pressed and kept for fodder
34. The opposite of a base
35. A substance whose use is to be burned to provide heat
36. Radioactive element used as a nuclear fuel
41. The protection weapon of a wasp or nettle
42. The liquid halogen
44. A blue dye
45. This type of calorimeter is used for measuring energy content of foods
46. Group 3 element symbol
48. Binary compound containing oxygen
51. This type of reaction gives out heat
52. Term used to describe the giving off of gaseous bubbles - Alka Seltzer does it!
54. Sub atomic pareticles
55. The universe as an ordered system
56. Symbol for a radioactive gas
57. Element vital for animal life
58. Glass containers used in the laboratory
60. Famous woman scientist - discoverer of radium
Original Page Design & Layout by Stephen Childs
Web Site Maintained By Darina Slattery,
Dept. of Computer Science & Information Systems,
University of Limerick.
(November 2000)
