3.) Under a specific set of conditions, the etch rate of SiO2 in a buffered HF solution containing surfactants is observed to be 762A/min. The etchant selectivity of SiO2 to Si is 2500:1. Suppose an oxide layer is masked (partially covered) with a Silicon mask, and we wish to etch the exposed oxide to a depth of 0.46m. a.) What is the minimum thickness of the Si mask such that it would be completely etched away when the desired oxide etch is achieved? (Note: you'll get an impossibly thin value) b.) What if the wafer was left in the etch bath overnight (15 hr )? If the original oxide thickness was 1 m and the mask was 100nm (still unrealistically thin), explain (quantify) what would happen to the wafer (oxide and underlying Si substrate), as well as the Si mask. For the purpose of this exercise, neglect the effect of lateral etching (isotropic etching under the mask)