Stress control structures for MEMS and ultra thin devices

A novel method of bow and stress control in MEMS and ultra thin devices is presented that consists of creating counterbalancing structures to position stressed layers at the neutral plane of the device, eliminating the bending momentum acting on the device. As a result, curvature is kept under control and can be altered as desired by judicial design of the counterbalancing structures. In a particular case of a 3 mm thick, 800 mm diameter Si membrane, the counterbalancing structure is a 50 mm wide, 11 mm thick ring placed at the periphery of the membrane. Selective epitaxial silicon deposition was employed to grow counterbalancing structures. Upon metallization, the membrane without counterbalancing structure showed substantial bow under the stress developed as a result of the difference in the thermal expansion coefficients of a metal and silicon. The membrane with the counterbalancing structure remained flat in the entire temperature range (25-150 oC). The method gives a convenient, material independent solution to stress-induced curvature problems in a variety of thin membranes and devices.