Ultra-sensitive, highly reproducible film stress characterization using flexible suspended thin silicon plates and local curvature measurements

Abstract

Film stress is crucial for stringent MEMS/NEMS design. In this paper, novel micromachined flexible suspended thin silicon plates combined with a sub-nanometer optical interferometry measuring setup have been developed to detect stresses in nanometer-scale films as well as ultra low stresses in thin films. By measuring the local out-of-plane curvature of a thin silicon substrate of 15 µm, a bending sensitivity two orders larger than that of the wafer curvature method has been achieved. Residual stresses in LPCVD Si3N4 thin films are measured, which are in good agreement with those extracted by other methods. Since non-contact optical instruments are used to detect all the parameters and the boundary conditions are well characterized, reproducibility better than 1% has been realized. By reusing the testing structures, residual stresses in sputtered metal films are also determined, which vary in a broad range. A stress difference as small as 1.5 MPa within a 30 nm film can be resolved with this metrology, which is among the best in the present reports.