1. Consider a case where you are evaporating aluminum (Al) on a silicon wafer in
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1. Consider a case where you are evaporating aluminum (Al) on a silicon wafer in the cleanroom. (a) The first treatm t thing you will do is to clean the wafer. Your cleaning process will also involve ent in a 1:50 HF:H2O solution to remove any native oxide from the surface of the silicon wafer. How would you find out that you have completely removed all oxide from the silicon? Give reasons for your answer. (3 pts) (b) After ensuring that all native oxide was removed by your cleaning processes, you take the wafer and walk over to the thermal evaporator. You place the wafer inside, close the chamber and start the pump to evacuate air from the chamber i.e. to create a vacuum inside. As soon as vacuum pressure is reached, you start the evaporation process and deposit Al. After the process is completed, subsequent tests show a thin oxide layer between silicon and the deposited Al layer. Give possible reasons to explain the presence of the oxide layer. 4 pts)Explanation / Answer
a) After dipping it in the diluted HF solution, a wetting test can be done to check whether there are any native oxides present. We know that silicon surface is hydrophobic whereas oxides are hydrophilic. So after wetting the surface, if water beads up and rolls off without wetting the surface we can ensure that the silicon wafer is completely devoid of oxides.
b) After cleaning the silicon wafer should be devoid of any oxides, so it is clean. The presence of thin layer after depositing aluminium is the presence of silicon dioxide layer. At high temperatures in the presence of oxygen or moisture, silicon will be reacted to give a thin stable layer of silicon dioxide. This is usually termed as thermal oxidation. This is why a thin layer is present between the silicon wafer and the deposited aluminium layer.
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