Stress Hysteresis and Thermal-Mechanical Behavior of PECVD Silicon Nitride and Ebeam Aluminum Films for Microbolometer A

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Stress Hysteresis and Thermal-Mechanical Behavior of PECVD Silicon Nitride and Ebeam Aluminum Films for Microbolometer Applications Shusen Huang and Xin Zhang. Laboratory for Microsystems Technology, Department of Manufacturing Engineering, Boston University, Boston, Massachusetts 02215, USA. ABSTRACT Uncooled cantilever-based microbolometer arrays received more attention recently due to high sensitivity and low cost. The central idea is built on the deflection of a bilayer SiNx/Al material upon the temperature change. The thermal-mechanical behavior of the bilayer is significant for the performance of the microbolometers. In this paper, we perform thermal cyclings to aluminum and SiNx films. The CTEs and the stress evolutions were measured using a curvature measurement system. The curvature profile of a SiNx/Al/Si component was predicted using an extension of Stoney’s formula, well agreeing with the experimental results. This work demonstrates fundamental mechanics issues in bilayer SiNx/Al components, which have a great potential for the use in uncooled microbolometer applications. INTRODUCTION Recent advances in micro-electro-mechanical systems (MEMS) have led to the development of uncooled cantilever-based microbolometer arrays, whose function is based on the bending of a bimaterial beam upon absorption of infrared energy. The bimaterial beam is made of a SiNx layer and an Al layer. A large mismatch in thermal expansion coefficients (CTE) between these two materials ensures a detectable deflection upon the temperature change. The mechanical properties of theses materials are, therefore, important in determining the performance of microbolometers [1, 2]. In this work, the thermal-mechanical behaviors of single-layered plasma-enhanced chemical vapor deposited (PECVD) SiNx film and electron beam (e-beam) deposited Al film were studied individually to measure the CTE and the residual stress for each material. An analytical solution of the curvature change in the bimaterial structure was derived and compared to the experimental results. EXPERIMENTS In this work seven samples were studied to explore the thermal-mechanical behavior of a SiNx/Al bimaterial cantilever. Table I shows the detailed description of the samples, including film material, film thickness and wafer type. As shown in the table, two types of wafers were used, i.e., 525 µm (100) wafers and 380 µm (111) wafers. The samples were prepared as follow. For the aluminum films, an e-beam system was used to evaporate the films at 2 Ǻ/s. For the SiNx films, a STSTM PECVD unit was used. The operation parameters are given in Table II. A commercial curvature measurement tool (Tencor FLX-2320) was used to measure the wafer curvatures. The deposition stress was derived by comparing the curvatures of the wafers prior to and after the film deposition. To investigate the thermal-mechanical behavior and the evolution of the residual stress, the substrate/film component was subjected to four


heating-annealing- cooling cycles (Fig. 1). In each cycle