Study of gradient stress in bimaterial cantilever structures for infrared applications

Abstract

Bimaterial SiNx/Al infrared cantilever structures are always initially curved because of the imbalanced residual stress in the two layers. Their performance and functionality are therefore significantly decreased. A thorough study of the residual stress (strain) has then become a key issue in the development of bimaterial SiNx/Al cantilever structures. In the curvature-based approach to the film stress, the residual strain is derived from the measured curvature based on certain assumptions on the distribution of the residual strain in the thickness direction. Previous models for a bimaterial cantilever structure, however, are not sufficient to characterize the residual strain in bimaterial SiNx/Al infrared structures. The main goal of this paper is to investigate gradient residual strain in bimaterial SiNx/Al infrared structures. To achieve this goal, the relationship between the residual strain and bending curvature is developed with the assumption that the residual strain in each layer is linearly distributed rather than uniform throughout the thickness. The profile of the gradient strain is then derived from the curvatures measured during the continuous etching of the top-most SiNx in the bimaterial cantilevers. The derived residual strain can then be inverted to predict curvature change further in the etching process. This paper demonstrates that a linear assumption of the residual strain yields a stronger agreement with the measured data in comparison to previously used models. In addition, several factors that may affect measurement accuracy are discussed at the end of the paper.