On-Chip Structures for the Determination of the Dopant-Dependent Young’s Modulus of Heavily Phosphorus Doped Polysilicon With Stress Compensation

Accurate knowledge of thin-film material properties, such as Young’s modulus, is imperative in proper design and operation of MEMS devices. The use of on-chip devices allows direct access to the material properties as they are known to change with fabrication process, temperature as well as location within the wafer. Resonant and pull-in structures have been designed and modeled for the measurement of the Young’s modulus of heavily doped polysilicon thin films. The cantilever and clamped-clamped beams allow us to extract the Young’s modulus through observing the resonant frequency and pull-in voltage and cross-referencing the results. Mechanical actuation using a calibrated piezoelectric shaker for some devices and electrostatic actuation for others ensures that the structural effects, rather than the actuation technique, are responsible for the varying response at different temperatures. Optical readout will be used in order to reduce readout-associated errors, which can occur with purely electrical techniques at higher temperatures. However, electrical readout is also possible for some of the devices. The devices have been designed and fabricated using a customized 1-mask process. In this paper, we present the modeling and numerical simulations obtained for heavily doped polysilicon microstructures and will describe the method used for the determination of the Young’s modulus with stress compensation. Although the method described here has been used for heavily doped polysilicon thin films, it can be easily modified for the determination of Young’s modulus of other MEMS structural materials.