In the semiconductor industry, an insulating layer can be built onto silicon devices by oxidizing the silicon to form silica, SiO2. During processing, it is often necessary to remove the silica layer in selected areas by protecting other areas with a "resist" and then exposing the silicon wafer to dilute hydrofluoric acid, HF, to remove the silica in the exposed areas. HF dissolves the silica to form silicon tetrafluoride, SiF4.
a) Write down a balanced equation for the reaction between silica and HF.
b) Assuming that the rate controlling step for the removal of the silica layer is the mass transfer of HF to the silica surface, calculate how long it would take a 0.01 M solution of HF to remove a 100 nm thick layer of silica given the following data: mass transfer coefficient of HF = 2*10^-4m/s, density of silica = 2100 kg*m-3. (You may assume the silica layer is flat and that the HF solution is well stirred.)