Side-by-Side Comparison of Passive MEMS Strain Test Structures under Residual Compression

Abstract

Knowledge and control of residual strain is critical for device design in MEMS, and therefore it is important to establish standards for residual strain measurement. In this study, pointer, microring, bent-beam, and fixed-fixed beam test structures are used to evaluate residual strain both theoretically and experimentally. An equation that enables easier evaluation of bentbeam structures is derived. Also, a finite difference model that incorporates the non-idealities of fixed-fixed beams and determines an optimum fit to the measured deflection curve is presented. The model allows accurate residual strain evaluation of each buckled fixed-fixed beam. Experimentally, pointer structures were found to be susceptible to adhesion. Microrings, intended for residual tension assessment, also could not be evaluated because the residual strain was compressive. Bent-beam and fixed-fixed beams could both be evaluated. The main criterion for test structure effectiveness was taken to be the repeatability of residual strain on structures in close proximity; each should exhibit the same value. Using optical microscopy, the residual strain of bent-beams was determined with ± 13 µε repeatability based on standard deviation of adjacent structures of similar design. Using optical interferometry, the residual strain of fixed-fixed beams was determined with ± 2 µε repeatability based on standard deviation for adjacent beams of different lengths. The strain values obtained from the two structures are in reasonably good agreement. Cantilevers were also evaluated to obtain film curvature values.

Mechanical Effects of Galvanic Corrosion on Structural Polysilicon

“…Microcantilever beams can be used to characterize the through-thickness-oriented strain gradient (TTSG), i.e., curvature, associated with the structural polySi layers [41]. The microcantilevers can also be made to actuate downwards, if a bias is applied between the beams and the Si substrate.…”

“…Pointer structures [ Fig. 2(b)] may be used to estimate the in-plane strain (IPS) present within a structural material layer [41], [42]. The resolution of the pointer structures used in this paper is 28 microstrain , i.e., 4.5 MPa.…”

Techniques in Residual Stress Measurement for MEMS and Their Applications

“…The goal of this section is to provide an introduction and overview on this field. Some excellent review articles have been published to address the related issues for gaining more detail insights [18], [20], [119], [120]. It is important to point out that the following MEMS stress measurement techniques usually do not measure stress directly and the mechanics presented in Section 3 are used to convert the measured quantities into the final stresses.…”

Ultra-lightweight telescope with MEMS adaptive optic for distortion correction.

“…Residual stress is a major factor in designing optically flat mirrors due to the highly curved surfaces that may result if residual stress and stress gradients are not small. There are several possible sources of residual stress including thermal cycling, dopants, impurities, grain growth and grain orientation [3]. Every layer in SUMMiT is annealed after deposition to reduce the amount of residual stress.…”

“…Although, the MEMS beam is not completely rigid on both ends; there is still a dependence on the compliance of the anchor. This study does not fully consider the effects of the beam take-off angle [43][44][45][46][47][48][49]. The out-of-plane displacement (see Fig.…”