Surface micromachined ring test structures to determine mechanical properties of compressive thin films

We report new robust micromachined buckled test structures to determine the in-plane residual strain eo, the coefficient of thermal expansion alpha (t h,film), and the biaxial modulus E/(1 - v) of compressively prestressed thi n films. The structures consist of rings with rigidly clamped outer edges a nd free inner edges. They were fabricated using sacrificial Al etching and are composed of a compressively prestressed PECVD SiN. The extraction of ep silon (0) and alpha (th,film) is supported by a finite element analysis of the buckling profiles of the structures. The biaxial modulus was obtained b y combining eo with the prestress extracted using the wafer curvature metho d. Results at 300 K are epsilon (0) = - (6.26 +/- 0.17) x 10(-4) and epsilo n (0) = - (9.09 +/- 0.25) x 10(-4), respectively, before and after an annea l up to 204 degreesC, alpha (th,film), = (2.7 +/- 0.3) x 10(-6)/K and E/(1 - v) = 150 +/- 7 GPa. Assuming a Poisson's ratio of v = 0.25 a value of You ng's modulus E = 112 +/- 5 GPa was found. Plane-strain modulus measurements rely on the load-deflection of the structures under point-like loads. (C) 2001 Elsevier Science B.V. All rights reserved.