Gases

A variety of gases are used in semiconductor manufacturing for process reactions in chemical vapor deposition, etching, ion implantation and many other processes. They are also used for such diverse purposes as chamber cleaning and purging. Generally speaking, gases are classified as processes gases for the first set of applications and bulk gases for the second. Bulk gases are hydrogen, helium and nitrogen, for example, which can be produced on-site through air separation plants. Process gases are typically supplied in the familiar gas tanks, or sometimes gas tanker trailers if the volume required is high enough. In some parts of the world, underground piping is used to supply multiple fabs

Aside for the gas type, the most common concern is the purity of the gas. Purity is often discussed in the “number of nines” level of purity. Gases that are 99.999% pure, for example, are “five nines” purity, gases that are 99.9999% pure are “six nines” purity and so on. Alternatively, trance contaminants are measured in parts per million (ppm), parts per billion (ppb) or even parts per trillion (ppt). Higher levels of purity are, of course, more difficult to produce and are therefore more costly. They are also more difficult to measure accurately. An ongoing challenge in the semiconductor industry is that gas users — IC manufacturers — tend to specify the highest level of purity available, but it’s often unknown if that higher level of purity actually provides any kind of benefit to device performance or yield. Indeed, sometimes trace impurities have proved to have some kind of beneficial effect and removing them can actually decrease performance.

Another important aspect — some would say the most important aspect — is safety. Gases can be toxic, carcinogenic, flammable, pyrophoric, corrosive and generally hazardous. A silane leak, for example, could result in a pocket of silane in a corner of the fab, which could explode. Arsine and phosphine, commonly used in ion implantation, are deadly in ppb and ppt, respectively. Fortunately, the semiconductor industry has an excellent safety record and danger to fab personnel is minimal — as long as established safety protocols are closely followed. This include storing gas cylinders in well monitored gas cabinets.

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