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.Holmes, R., Spiegelman, J. "Urea and Ammonia Removal from De-Ionized Water via Steam Purification". Technical White Paper. 2008. 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.Buomsellek, S., Spiegelman, J. "Water Vapor Delivery For Cigse And Other Thin Film Vacuum Processes". 35th IEEE Photovoltaic Specialists Conference, June, 2010.
The role of UHP steam in oxidation
Oxidation of silicon is a common and frequent step in the manufacture of integrated circuits (IC). The goal of oxidation is to grow a high quality, uniform oxide layer on a silicon substrate. During oxidation, a chemical reaction between the oxidants and the silicon atoms produces a layer of oxide on the silicon surface of the wafer. It is often the first step in wafer fabrication and will be repeated multiple times throughout the fabrication process.
Oxidation takes place in an oxidation tube. During the reaction, silicon reacts with oxidants to form silicon oxide layers. Typical operating temperature is between 800 °C and 1,200 °C. The oxide growth rate increases with temperature.
Growth rate, uniformity and yield are three important characteristics of the oxidation process. The faster the growth rate, the more wafers that can be manufactured in a given time. If the oxidation process is uniform across the tube—both horizontally and vertically—then the process will be complete at the same time across all wafers and the yield will be high. However, if the process is non-uniform, then intervention is required, increasing the process time and potentially reducing yield. In addition, fast growth rate can increase the defect rate, especially if contaminants are present.
During the first stage of oxidation, the chemical reaction results from the direct contact between silicon and oxidants at the wafer surface. The reaction is limited by the number of silicon atoms available. For approximately the first 500 Å, the oxide grows linearly with time. From that point on, the reaction rate slows down because the silicon dioxide layer covers the silicon atoms. As the silicon dioxide layer grows, it eventually prevents the oxidants from coming in direct contact with the silicon atoms.
The second stage begins when approximately 1,000 Å of silicon dioxide has been grown on the silicon substrate. At this point, the silicon atoms are no longer exposed to the oxidants and the oxidants begin to diffuse through the silicon dioxide in order to reach the silicon. The oxidation of silicon during this stage occurs at the silicon/silicon dioxide interface. As oxidation continues, the silicon dioxide layer thickens, and the distance the oxidants must travel to reach the silicon increases. The oxide growth rate is limited by the diffusion of the oxidants through silicon dioxide.
There are two methods of oxidation: dry oxidation and wet oxidation. During dry oxidation, dry oxygen is introduced into the process tube where it reacts with silicon. Dry oxidation is a slow process that grows films at a rate between 140 and 250 Å/hour. It is typically only used to grow thin oxides (<1000 Å).Salzman, J. "Microelectronic Processing Oxidation" by J. Salzman. Microelectronics Processing Course Presentation. January 2002. During wet oxidation, water vapor is introduced into the heated oxidation tube. Because water molecules form hydroxyl structures, they diffuse faster in silicon dioxide and the oxide growth rate increases. The wet oxidation growth rate is 1000 to 1200 Ǻ/hour, so wet oxidation is the preferred method to grow thick oxides. The growth rate slows with increasing thickness, according to the Deal-Grove model.Deal, B. E., Grove, A. S. "General Relationship for the Thermal Oxidation of Silicon, J. Appl. Phys., 36, 3770 (1965).
Water vapor is a key ingredient in the wet oxidation recipe. During a typical process, a furnace containing multiple wafers is flooded with water vapor. The oxygen in the water molecule reacts within the silicon wafer to form silicon dioxide. This is a sacrificial process, where the original substrate is reduced as the oxide forms on its surface.
When water vapor pressure is increased, the oxide growth rate is increased. According to the model of Deal and Grove, the growth rate of the oxide layer is directly related to the effective diffusion coefficient of the water molecules into the oxide layer and the equilibrium concentration in the immediate area. When a carrier gas is used to deliver water vapor, the carrier gas molecules generate a partial pressure. This partial pressure lowers the partial pressure of water vapor and slows the diffusion of water into the oxide film. The result is lower driving force and slower growth rate.
For a given temperature and process pressure, oxide growth rates are fixed if the gas ratio is also constant. However, for a given operating temperature, this growth rate is not maximized until the water vapor pressure is equal to 100% of the operating pressure. While increasing water vapor flow rate seems obvious to improve growth rates, technical difficulties interfere with increasing the actual quantity of delivered water vapor.
The role of UHP steam in annealing and passivation
Some deposition methods are faster than others, which saves process time but can lead to less dense film. To improve the density and heal the film, annealing can be applied. Annealing and passivation are techniques used to repair atomic defects within the crystal that propagate into the wafer macrostructure, reducing efficiencies in microelectronics and photovoltaic cells. High temperature annealing can increase carrier lifetimes by injecting H into the Si/SiO2 interface. Passivation, or thermal annealing, of the interface limits hole/electron recombination, removes dangling bonds, and reduces vacancies and dislocations at grain boundaries.