The semiconductor manufacturing industry is responsible for most of the electrical and electronic appliances that we are fond of. It is the driving force behind such big companies like Samsung Electronics and Intel. Even though the final product is usually simple and easy for the user to operate, the main struggle is usually in the operations that take place behind the scenes before we get the finished product.
One of the aspects that the end-user may not be familiar with is silicon wafers. Silicon wafers are usually very delicate and should be handled with a lot of care. They can get contaminated simply by being exposed to air. They contain very strong electrostatic forces which don’t go along well with the high number of organic particles found in the air. Therefore, to avoid problems with all the concerned parties, silicon wafers have to undergo a thorough cleaning and purification process in order to be viable for further use.
The Silicon Wafer cleaning process in itself is not that easy. This is because, apart from being easily contaminated, silicon wafers are also very fragile. They need to be handled with the utmost care to ensure that there is no damage done to its surface.
Different manufacturers use different processes and methods. Some have a complicated series of processes that work specifically for them and their products. To avoid getting lost in the hullabaloo, industry experts have recommended a simpler and direct three-step method that ensures you clean your silicon wafers effectively. These steps are very effective as most of the other methods are just derivatives of it.
The best way to remove a contaminant from a silicon surface is by introducing a solvent. Solvent cleaning is the first and most important step. It is very effective at removing oils and other organic particles. However, it comes with only one downside. Solvents tend to leave their own residue on the surface. This is the reason as to why two solvents are usually used; acetone and methanol. While on is responsible for the removal of the contaminants, the other one is to ensure that the substrate returns to its normal state.
The process usually involves the preparation of two baths; one with acetone and the other with methanol. The acetone bath is then heated to a temperature of about 55 degrees Celsius and then the wafer is soaked in it for 10 minutes. After that, the wafer is then removed and placed in the methanol bath for about 5 minutes. It is then rinsed with DI water and blow-dried with nitrogen.
The next step that follows is the use of RCA. Just like solvents, it is also used to remove organic contaminants but using a different approach. The RCA oxidizes the wafers forming a thin oxide layer on the surface.
The first step involves preparing the RCA bath by mixing 1 part ammonium hydroxide (27%) with 5 parts DI water. The mixture is then heated to 70 degrees Celsius before being removed from the hot plate and mixed with 1 part hydrogen peroxide (30%). It will only take a minute or two for the solution to start bubbling and then the silicon wafer is soaked into it. After 15 minutes, it is removed and soaked in a bath full of DI water for rinsing. The water is changed regularly until the rinsing is proper. Finally, the wafer is removed from the bath under flowing water.
Hydrofluoric Acid Dip
The final step involves a HF dip to remove the silicon oxide layer. Hydrofluoric acid is a very dangerous chemical. A proper protective gear is required to handle it.
The dip is prepared by mixing 20 ml of the acid with 480 ml of water. This solution should not be put in a glass container as the reaction will be vigorous. The silicon wafer is soaked for 2 minutes and then rinsed in running DI water. After that, a wettability test is performed to confirm whether the cleaning was successful or not. If the water turns into little beads and roll off, then the wafers are good to go.
This is a standard procedure that all semiconductor businesses can implement to curb contamination challenges. The beauty of it is that it works for both large-scale and small-scale purposes.