Ultra High Purity Steam for Oxidation and Annealing

Ultra-high-purity steam, also called clean steam, UHP steam or high purity water vapor, is used in a variety of industrial manufacturing processes that require oxidation or annealing. These processes include oxide layers grow on silicon wafers for the semiconductor industry and for passivation layers used to improve the light capture ability of crystalline photovoltaic cells. Several methods and technologies can be employed to generate ultra high purity steam, including pyrolysis, bubbling, direct liquid injection and purified steam generation. The level of purity, or the relative lack of contamination, affects the quality of the oxide layer or annealed surface. The method of delivery affects growth rate, uniformity and electrical performance. Oxidation and annealing are common steps in the manufacture of such devices as microelectronics and solar cells.

In simplest terms, steam is the gaseous state of water where the majority of the gas pressure is created by water molecules. This differs from humidified gas where water vapor is a minor component of the gas mixture. Ideally, steam is 100% H₂O molecules. In reality, steam may also contains other molecules such as metals, urea, volatiles, chlorine, particles, microdroplets, and organics. To be considered ultra high purity, steam must not have contaminants above a certain limit. Typical values for semiconductor are at part per billion (ppb) for any specific contaminant by volume. This is an arbitrary definition and is frequently set by the user.

Impurities in water are entrained into the steam as it is generated and more may migrate into the steam from process piping materials as it is conducted to the process. These impurities or contaminants can be quite harmful when the steam is an ingredient in industrial manufacturing processes. As microelectronic device size and geometry shrink, the susceptibility to damage from contaminants grows. This requires intervention through the use of filters, selective membranes or other techniques to clean the water or steam before delivery to process.

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