One of the most critical steps in manufacturing semiconductors and MEMS involves the cleaning process used for silicon wafers. The silicon wafer supplier makes sure that chemical and particle impurities are removed from each wafer without causing damage to its substrate or surface. One of the cleaning techniques they use is called the piranha etch cleaning method. Also known as piranha solution, this type of cleaning method uses a mixture of sulfuric acid and hydrogen peroxide to effectively eliminate organic matter from the surface of each wafer. Besides removing most organic photoresist residues, piranha etch also makes its surfaces compatible with water by adding OH groups or hydroxylating them.
It’s important to note that piranha etch is highly explosive. When the solution is mixed, it releases heat that can cause solution temperatures to rise up to 120°C. For this reason, manufacturers must allow the solution to cool before they apply heat. Violent boiling may also result from a sudden increase in temperature. Since the maximum concentration for peroxide in water solution is about 35%, explosions may occur if it exceeds 50%.
Manufacturers should use it soon after it’s prepared. It’s never a good idea to store piranha solutions because hydrogen peroxide self-decomposes. While it undergoes the process of self-decomposition, it gives off oxygen gas that causes the waste bottle to pressurize and break. Instead of accumulating waste piranha solution in containers, it’s best to neutralize it and dispose of it as soon as possible.
Since Piranha cleaning solutions are extremely dangerous to handle, extreme caution is required. The person who’s handling it must wear personal protective equipment which consists of the following:
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