Piezoresistive sensing with twin-beam structures in standard MEMS foundry processes

A technique for fabrication of piezoresistive sensors in standard MEMS processes is introduced. The basic twin-beam structure comprises of a pair of beams from different structural layers and the structure is designed such that when one beam is under tension, the other one is under compression. If at least one of the beams is made of a piezoresistive material, it is possible to measure the change in the resistance of beams as a result of the applied stress by properly routing an electrical current through the structure. The proposed method does not require electrical isolation of piezoresistors from structural layers, and as confirmed by experiments, can be practically used for small deflections. Sample structures were fabricated in the MUMPs process and were employed to prove the validity of the design principle. Using the Maxwell–Mohr method, an analytic model is developed for the proposed structure and is verified by finite element simulations. Using modeling and experimental results, the piezoresistive coefficient of the top polysilicon layer in MUMPs process was calculated to be −10.65×10−11−10.65×10−11  Pa−1. Having the proper structure, its model, and the piezoresistive coefficient of the material, it is now possible to design and optimize a wide variety of piezoresistive sensors such as accelerometers and magnetic field sensors in low-cost standard MEMS processes.