Authors: J. Liu, G.K. Fedder, S. Sassolini and N. Sarkar
Affilation: Carnegie Mellon University, United States
Pages: 546 - 549
Keywords: CMOS-MEMS, mechnical property, residual stress, stress gradient
A new Young’s modulus test structure has been designed and used in our lab. Simple models have been built in simulating lateral and vertical stress gradients. Test structures, measurements and FEA simulations are reported on the characterization of these four mechanical properties of high-aspect-ratio MEMS structures with different relative metal and dielectric content, formed by post-CMOS micromachining of two different 0.35-µm BiCMOS technologies provided by STMicroelecronics and Jazz Semiconductor. FEA simulation results match the experimental results, within 1% for Young’s modulus and axial residual stress, and 2% for lateral and vertical stress gradients. Characterization results on a die show that test structures with field oxide have a lower residual stress than the corresponding structures without field oxide. Lateral curl test structures show that naturally balanced beams (all the layers are of the same width and aligned in layout) are not straight after post-CMOS release due to residual stress gradients and process variations, and the in-plane radius of curvature increases when the beam width and/or the beam height increases. To keep the beam straight after post-CMOS release, we can increase the beam width or use tapered beam structures by making lower metal layer wider (0.1 or 0.2 µm in each side) than the adjacent upper metal layer.