They\\$quot;re everywhere - in our phones, TVs, DVD players, kettles, toasters, computers- -the list is endless. Suffice to say, without them I wouldn\\$quot;t have been able to write this article. What are they? The answer is obvious - microchips. But how did they get here?
The semiconductor industry dates back to the mid \\$quot;˜50s when the first integrated circuits contained only a few transistors. Called \\$quot;˜Small-Scale Integration\\$quot; (SSI), they used circuits containing transistors numbering in the tens.
Today the process is more advanced: chips are developed in fabrication units known as \\$quot;˜tools\\$quot;, a complex network of deposition chambers built to cater for
the five-hundred plus development stages that every silicon wafer must now go through before the finished product is cut for distribution. The tens of transistors have now increased to millions of tiny transistors but what role has gas played in this technological revolution?
A reliable supply of high purity gases is critical to advanced semiconductor manufacturing. Producing an integrated circuit requires over 30 different process gases for etching, deposition, oxidation, doping, and inerting applications. The extremely precise requirements require that impurities are measured down to ppb levels and control of impurities is absolutely essential.
Semiconductor process gases are classifi ed as: atmospherics (generally supplied in bulk) and silicon precursors, reactants, dopants and etchants mainly supplied in cylinders, (often referred to as \\$quot;˜special gases\\$quot;). The semiconductor industry uses by far the greatest variety of gases of any industry and the increasing complexity of chip and memory manufacturing mean that they also have the highest quality requirements.
Pure gases (generally atmospherics) supplied from bulk storage are used in a variety of different applications. Examples include nitrogen used as an inerting
atmosphere or as a reactive gas purge, oxygen used in the oxidation of silicon, argon used as an inert environment in sputtering where nitrogen would form unacceptable metal nitrides and hydrogen which provides a reducing atmosphere in the annealing of metal films.
At other stages of the manufacturing process a wide range of special gases are used. These gases may be either pure or supplied as mixtures and are often extremely hazardous and are generally supplied in cylinders contained in \\$quot;˜gas cabinets\\$quot; to ensure safe delivery to the tool.
Silicon precursor gases such as silane, dichlorosilane and trichlorosilane are used in a processes known as epitaxy and chemical vapour deposition(CVD) to deposit layers of silicon directly onto the silicon substrate.
Dopant gases such as arsine, phosphine and boron trifluoride mixed with inerts at precise levels are introduced as controlled impurities to modify the local
electrical properties of the semiconductor material. These dopants modify the molecule\\$quot;s crystal structure by either creating an electron surplus (n dopant) or deficiency (p dopant) to alter the conductivity of the material.
During the etching process inert fluorocarbons such as carbon tetra fluoride *Halocarbon R14) , trifluoromethane (R23) and hexafluoroethane (R116) and highly reactive products such as nitrogen trifluoride are used to react with silicon and its oxides and nitrides. These gases are shot at the metal layers in a method known as plasma etching which modifies the layer properties.
The difference between pure and special
Atul Athalye, director of Technology at BOC Edwards, explains some of the key differences between speciality and bulk gases.
" gases are highly reactive and sometimes more hazardous," he says. " is also a huge difference in the quantity in which they are used. Most speciality gases are used in small amounts, as they are more expensive to produce and can only be used for select procedures - bulk gases on the other hand can be used to fulfil any number of standard procedures."
One recent development within deposition has been the emergence of liquid chemicals with some manufacturers now favouring this process over gas deposition.
" wet chemical is definitely more accurate in the deposition process," says Andreas Janotta, marketing manager at Linde Nippon Sanso.
She adds, " you may have to evaporate it before the liquid chemical enters the deposition chamber, that\\$quot;s no problem for a gas supplier."
Colin Overton, gases sales manager at Epichem, disagrees. " have distinct advantages over liquids. They can be pumped around by simply opening a valve and they don\\$quot;t leave any residue, whereas if you use a liquid, you have the problem that it takes time to evaporate. People are looking for process times of a few seconds - in this day-and-age you can\\$quot;t afford to wait minutes."
That\\$quot;s because we\\$quot;re living in a world dominated by technology, where the call for faster, smaller, and more power-efficient microchips is growing stronger.
" economic cycles are not as unpredictable as they were ten years ago," suggests Janotta." in multimedia formats, as well as communication and computing, has been rising by around ten per cent per year in Europe, and by an even greater margin in Asia."
The need to mass-produce as many chips as possible has resulted in semiconductor manufacturers contemplating a possible change in direction.
" chips become smaller, you have to deposit transistors on to them more and more densely," says Overton. " are vertical layers, and by making them thinner in order to improve both output and production, the basic properties become subject to a change in requirements."
He adds: " can design them to a higher standard, you can make them purer, but there is a limit to that at which the simple laws of physics are being challenged."
Atomic layer desposition
One of the answers to this is a wide range of new materials such as Epichem\\$quot;s trimethyl aluminium (TMA), and other products including compounds of hafnium, which are used in a process known as atomic layer deposition (ALD) to deposit ultra thin layers of aluminium and hafnium onto the chip surface.
" from TMA, there are a whole variety of new chemicals that could be used to impart specific properties," says Overton. " oxide, zirconium
oxide, and certain esoteric metals could all be used in this way. Because of the way chips are evolving,silicon is no longer considered to be a very good insulator, but one of these products could provide the correct properties."
Atul Athalye further points out, it\\$quot;s not just microchips that could gain from these experiments.
" device that uses transistors could benefit from these new products. LCD screens, for example, are processed in a similar way to chips, using many of the
same gases that can be found in the semiconductor industry."
One thing\\$quot;s for sure: with a host of new gases and new production techniques raising expectations even higher, the next few years promise to be the most
intriguing yet for an industry the world simply couldn\\$quot;t live without.
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