Etched bonded silicon wafers as test objects for the resolution power of subsurface voids

Bonding of silicon wafers is a method that is widely used in microsystem technology. To quantify the quality of the bond only IR interference techniques which are restricted to vertical sizes of voids <EQ 250 nm have been applied up to now. The new idea is to use acoustic microscopy for the examination of these bonds. In order to be able to evaluate the possibilities and limitations of such a method we worked out a preparation technique to investigate bonded silicon wafers with defined etched structures. Etched wafers were bonded to nonetched wafers, chips of 10 X 15 mm2 size were sawed out of the wafer pair followed by grinding and polishing these structures under small angles of 34 feet to 3 degrees. With working frequencies of 200 MHz and 400 MHz we obtained good results with structures that have a height of 50 nm and a horizontal size of some micrometers. It was possible to show structures that were covered with a silicon layer that is 70 micrometers thick. Additionally wafer pairs with metallic interlayers were investigated. The results are compared with images taken with an IR transmission optical microscope.