Ozone for IC industry
For more than 20 years, researchers in the semiconductor industry have been studying the application of ozone in wafer cleaning and anti stripping applications. In order to reduce chemical consumption and treatment costs and improve cleaning efficiency, ozone has been studied in the past decade as a substitute for traditional sulfuric acid peroxide and RCA cleaning using alkaline (SC-1) and acidic (SC-2) hydrogen. Peroxide mixing is effective due to the multiple effects of the disinfection activity of O3 and O3 derived oxidizing substances such as OH radicals.
In the process of chip manufacturing, ozone is mainly used to clean wafers; Elimination of organic matter, metals and particles; Remove photoresist; And disinfect deionized water facilities. Ozone cleaning always involves oxidation, and the process difference depends on the main purpose of the cleaning step.
Remove organic matter. Much information about the ability of ozone to remove organic matter comes from the study of drinking water and wastewater treatment. Ozonated deionized water (DIO3) has high oxidation potential and can degrade organic pollutants. The removal efficiency depends on the type of organic species, ozone concentration and reaction mode.
Ozone dissolved in ultrapure water produces Oh active radicals in the process of self decomposition. When ozone decomposes organics directly, active free radicals decompose them indirectly. Different reaction pathways lead to different oxidation products. The direct ozone reaction pathway is selective, and the reaction rate constant is usually slow. The indirect OH reaction is fast and non selective, but it must be activated by initiators such as high pH, hydrogen peroxide or UV radiation. Although rapid reactions are required, reactions using free radicals alone should be avoided. In many cases, active substances must act directly on the surface, because substances too far from the surface will inactivate and lose.
Removal of metals and particles. DIO3 alone cannot effectively remove metals such as iron, nickel, aluminum, magnesium and calcium, which are deposited on the surface of silicon, such as metal hydroxides or metal oxides. Depending on their properties, metals can be incorporated into the oxide layer or located at the Si-SiO2 interface. They can be removed with acids as ion exchangers, or oxides can be dissolved. Using hydrofluoric acid (HF), metals can be removed.
If the adhered particles are organic, DIO3 alone may be sufficient to remove the adhered particles. However, particles on silica are usually removed by etching the oxide below the particles with diluted hydrofluoric acid (DHF) and avoiding particle redeposition. If most of the particles are not dissolved by DHF, O3 as an oxidant can produce a new layer that can be etched by HF. This is true for both silicon particles and silicon surfaces.
The formation of oxide layer on silicon is a self limiting process. At room temperature, the oxidation of silicon surface produces an oxide layer with a thickness of about 1 nm. The quality of the thin oxide layer depends on other parameters, such as humidity. In the tests involving spray and impregnation tools, the initial oxide growth rate is a function of ozone concentration..2 in the immersion tool, the final oxide thickness depends on the initial ozone concentration and pH value, indicating the reaction limitation process. However, because of the electrostatic system used in these tests, the attenuation and consumption of ozone may affect the results.
Several studies have been published on clean processes that combine ozone with HF, hydrochloric acid (HCl), or both. In these studies, the chemicals were sequentially applied or used as a mixture in spray, immersion bath or wafer fabrication processes, using repeated ozone water and diluted fluoride (SCROD) methods for single wafer rotation cleaning, alternately distributing diluted dHF and DIO3 on rotating wafers. Depending on the desired final surface conditions, the process ends with DHF / flushing or DIO3 / rinsing, followed by rotary drying in nitrogen. One minute, three cycle process can remove 87% Al2O3 particles, 97% Si3N4 particles and 99.5% polystyrene latex particles. In contrast to the simultaneous application of DHF and DIO3, repeated scrod cleaning does not increase surface roughness.
The advanced cleaning and drying (ACD) method developed by astec (Berg, Germany) combines metal removal and drying in one process using a mixture of DHF and O3. Combined with the particle removal step using traditional SC-1 cleaner or surfactant, the ACD process consumes 60% less chemicals than the traditional RCA process. The result is that the hydrophobic wafer can be reoxidized directly in gaseous ozone above the DHF / O3 bath if necessary.
Photoresist removal. Conventional wet chemical processes for photoresist removal rely on the combination of concentrated sulfuric acid with hydrogen peroxide (SPM) or ozone (SOM). Alternative methods using ozone dissolved in deionized water provide environmental benefits and reduce costs.
The photoresist stripping rate in DIO3 increases with the increase of ozone concentration or temperature (at a constant ozone concentration). Unfortunately, with the increase of temperature, the saturated ozone concentration in water decreases and the rate of ozone attenuation increases. The ozone transport process must be carefully optimized to achieve maximum photoresist removal.
Several attempts to use ozone in resist stripping process are reported in the literature. For example, ozone has been mixed with hot DI water in order to achieve high ozone concentration, and scavengers have been added to prevent ozone decay. It has been found that the stripping rate has been affected. The mass transfer rate of dissolved ozone from a large amount of liquid to the boundary layer on the wafer surface. Diffusion limitation can be reduced by using megasonic stirring or reducing the thickness of the boundary layer – for example, by increasing the rotation speed of the wafer. In order to overcome the influence of the boundary layer barrier, the researchers mixed ozone gas and water with steam at high temperature. The addition of scavenger and the increase of temperature increased the stripping rate. However, the use of wet cleaning processes to remove photoresists remains a challenge, depending on the type of photoresist used and post exposure treatment.
disinfect About a century ago, ozone was introduced into water treatment systems to disinfect water polluted by microorganisms. In the semiconductor field, ozone is used in disinfection water purification systems. However, chemicals used to purify drinking water, such as chlorine or chlorine dioxide, are not acceptable in the IC industry. One advantage of ozone is that it decays back to oxygen. However, in a closed water purification system, the oxygen concentration may accumulate above the level specified in the international semiconductor technology roadmap
The disinfection report stated that the reduced dissolved oxygen concentration was achieved by combining the Gore Tex membrane contact system of WL Gore & Associates (Newark, Germany) with a high-capacity ozone generator from Astex (Berlin, Germany). 16 the oxygen concentration was ~ 240 ppb.
The ozone concentration required for water disinfection is much lower than that required for wafer cleaning. A key parameter is the free disinfectant concentration multiplied by the available exposure time t (CT value). 17 a CT value of 1.6-2.0 mg / L / min is considered sufficient for effective disinfection.