Strain gradients in poly-SiGe nanocantilevers: experimental and finite element modeling studies

T.B. Asafa
Ladoke Akintola University of Technology, NG

Keywords: N/MEMS, poly-SiGe, FEM, COMSOL


One of the fundamental structural requirements for successful performance of N/MEM devices such as nanoswitches, nanoresonators and biosensors is low strain gradient. Excessive strain gradient leads to unwanted deflection, which consequently alters the dynamic and reliability characteristics of the devices made therefrom. Following micromachining procedure discussed elsewhere [3], arrays of nanocantilevers are fabricated from ~100 nm thick poly-SiGe films. Subsequently, strain gradients were calculated from the tip deflections. Then, the cantilevers are modelled in COMSOL multiphysics as a superposition of smaller layers having varied thicknesses, with each later sustaining its corresponding intrinsic stress. The strain gradients obtained from the two approaches are similar for the two films. The experimental average strain gradient is -0.02±0.004 /µm while finite element approach gives -0.019±0.002 /µm for the film A. Similarly for the film B, the average strain gradients based on the experimental and finite element approaches are -0.083±0.009/µm and -0.078±0.007 /µm, respectively. This implies that the experimental average strain gradients differ from that of FEM by ~5% and ~6% for the films A and B, respectively. For M/NEM devices where accurate calculation of strain gradient is important, FEM can replace the expensive and time-consuming micromachining technique.