A 0.5 am Si02 on a Si substrate is to be etched away. It is known there is a 15% variation of the oxide thickness and a 15% variation in the oxide etch rate. (a) How much overetch (in % etch time) is required to ensure all oxide on Si is removed? (b)What minimum selectivity of the oxide etch rate to the Si etch rate is required so that a maximum of 5nm (= 0.005 ocm) of Si etched with the overetch calculated in part (a)