squares 702a as shown in fig7 a the wafer is then etched from the top surface 703 until holes of each inverted pyramid shape 702 are etched out for each square 702a as shown in fig7 a and 7b with the sidewalls of the pyramid 702 formed by lt 111 gt planes thereafter the whole wafer is oxidized so that oxide is grown on the surface of the etched holes a large square window 704 which has the dimensions of the whole array of the etched holes 702a is then patterned photolithographically on the bottom side of the wafer the wafer is etched again from the back side as shown in fig7 b where the heavy lines represent the silicon surface covered by the etch mask for example thermally grown silicon oxide the etch is stopped when it reaches a predetermined depth as shown in fig7 c which leaves well defined holes in the silicon after the etch mask is removed this process can be used to advantage in the formation of accelerometer structures where typically a silicon mass is freed from the surrounding silicon substrate except for narrow or thin support beams as shown in fig8 a and 8b in fig8 a which is a top plan view of a wafer a silicon.