Ultra-high-purity steam, also called the 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 the growth of oxide layers on silicon wafers for the semiconductor industry, originally described by the Deal-Grove model, and for the formation of 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.
Characteristics 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 composed only of H2O molecules. However, steam may also contain other molecules such as metals, urea, volatiles, chlorine, particles, microdroplets, and organics in reality. 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.
Metals: Metals can be in the source water or gases and can migrate into the steam from components in the steam generation and delivery path. Metallic systems corrode and impart metallic ions. Stainless steel, for example, can slough off molecules into the steam path. Limiting or eliminating metals from the water, gas and steam delivery paths reduce the risk of metallic contamination but do not affect the presence of metals in the source water and gas. Metal ions degrade electrical performance in semiconductors and metal ions in solar cells can be recombination centers that reduce the efficiency of the photovoltaic device. Urea: Fertilizers, auto emissions, and human and animal sources contribute to the presence of urea and ammonia. Normally stable at room temperature, Urea has a high conversion rate to Ammonia when boiled. This contaminant is difficult to control, varies with the water supply and has large seasonal fluctuations. Urea is not easily rejected by reverse osmosis membranes. It is non-polar, so not removed by de-ionized water processes, and chemically stable so not easily destroyed by UV sterilization processes. Controlling Ammonia levels makes the difference between conformity and uncontrolled variation. For example, "T-Topping" in lithography is a real danger, resulting from chemically amplified resists. Other structural defects caused by ammonia include incorrectly imprinted line width and short circuits, and ammonia can also deposit on optical surfaces, causing equipment downtime. Residual urea on the wafer surface can react when wafers are processed at higher temperatures leading to the injection of nitrogen atoms into the layer being grown. Silica: Colloidal silica is typically found in surface waters and has created problems for water treatment because of its stability as an un-ionized compound, making it difficult to remove using ion exchange processes. Particle size is often 1 to 5 nm, but can form chains if concentrations increase. Silica is at the lower end of selectivity for anion resins, creating a scenario where silica breakthrough is one of the first to occur. As a result, silica can be effectively removed only if the ion exchange resins are completely and properly regenerated. Oxygen: If oxygen makes up part of the process recipe during wet oxidation, it will reduce the water vapor partial pressure, slowing the overall growth rate. Because the oxidation rate of silicon with oxygen molecules to water molecules is almost ten times slower, variability in the oxygen to water vapor pressure can lead to process variability. This is frequently a problem when the operating pressure is kept at ambient pressure. Water vapor pressure is a function of the temperature of the water source while the overall process pressure is a function of atmosphere. As the atmosphere varies the oxygen pressure will increase or decrease relative to the water vapor pressure, leading to changes in the overall oxidation growth rate of the film. Microdroplets: Water vapor with entrained water microdroplets can cause deformities or irregularities in wafers as the water settles on hot surfaces. Microdroplets are the result of incomplete vaporization of the water source. This is common with boiling and vaporizers where it is difficult to get sufficient heat into the boiling liquid. These microdroplets can cause contamination and problems in uniformity. The boiling action of water creates microdroplets, which act as liquid capsules that entrain particles and molecular contaminants such as ions, organics, and pyrogens. Microdroplets are known to carry particulate and ionic impurities that cannot be carried by pure vapor alone. Furthermore, cold spots occur where microdroplets land leading to non-uniformity and warpage. In order for oxide films to work properly, the film thickness and uniformity are critical.
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