The correct answer is: Aluminium.
Aluminium is sputtered on the whole wafer because it is a good conductor of heat and electricity. It also has a low coefficient of thermal expansion, which means that it does not expand or contract much when the temperature changes. This makes it ideal for use in semiconductor devices, where it is used to create the thin layers of material that make up the transistors and other components.
Silicon, calcium, and potassium are all also used in semiconductor devices, but they are not as good conductors of heat and electricity as aluminium. They also have higher coefficients of thermal expansion, which means that they expand and contract more when the temperature changes. This can cause problems in semiconductor devices, where the thin layers of material need to be very precise.
In addition, aluminium is a relatively inexpensive material, which makes it a cost-effective choice for sputtering on wafers.