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The use of gases in integrated circuit fabrication is increasing, particularly with VLSI technology. This technology is demanding more and more dry processing in order to achieve sub-micrometre geometries. These dry processes are, for the most part, based on the use of gases as reactants. Thus, there is an increasing dependence upon clean, dry, particle-free gases. Gases are used at various stages of fabrication, including the growth of polysilicon and epitaxial layers, the generation of a variety of chemical vapour deposited (CVD) thin films, the development of oxide films, and the etching of thin films with plasmas.
Particulates in gases can be generated from the movement of mechanical parts such as regulators, valves and flow controllers, or from copper plumbing, corroded pipes, solder debris, rust in gas lines, fibrous filters and foreign powder precipitates in cryogenic or ambient gas storage cylinders. Another source of particulates in gases is inherent in the manufacture or separation of the gases. Throughout the processes that utilize gases, the semi-conductor devices are exposed to these particulates. They often can be seen as bright speckles of ‘ moon dust ’ on the wafer surfaces under high intensity reflected light.
The elimination of particulate impurities in gases is critical in order to prevent the introduction of these impurities into vapour-deposited thin films, silicon substrate surfaces and silicon crystal lattices. Incorporation of gas-derived particulates
and metallic contaminants often leads to crystallographic defects. These defects can be manifested in stacking faults, dislocations and metal precipitates, all of which could alter diffusion profiles in silicon devices.
Processes that utilize gases are typically highly energetic, requiring either high temperatures or high radiation levels. Therefore, particles in gases are able to bombard wafer surfaces with sufficient energy to drive mobile impurities from the surface into the film or crystal, creating material and electrical anomalies.
For the final filtration of gases, the best type of filter is a membrane filter, which exhibits the characteristic of being able to retain particles that are actually smaller than the rated pore size of the filter.