Tailoring of Stress Development in MEMS Packaging Systems

Abstract

A better understanding of the origin and evolution of the stresses is a crucial step in improving reliability of packaging systems for microelectromechanical systems (MEMS). Given its importance, we examine the stresses developed in hermetically packaged MEMS inertial sensors. For this purpose, an optical surface profilometer is employed to assess the stresses by measuring the curvature of dummy silicon dies (3.5x3.5 mm 2 ) assembled in different types of packages and die attach adhesives. We also explore a temporal evolution of stresses during thermal exposure of the test packages in an effort to emulate actual packaging processes and device operation conditions. The result shows different levels of stresses generated from various adhesives and package types, and also a stress evolution during packaging processes. The mechanical stress data also show a good agreement with MEMS performance data obtained from actual accelerometers. Therefore, the stress data will not only be useful in better understanding performance of MEMS packages, but the testing protocol can also provide a diagnostic tool for very small packaging systems.

Evaluation of Die Stress in MEMS Packaging: Experimental and Theoretical Approaches

“…As a result, if more compliant adhesive is used, the stress will ultimately be at its maximum near the center of the die, resulting in uniform bending from the center of the die. In fact, the silicone adhesive (modulus of 10 MPa) confirmed this trend [17].…”

“…We used scanning white light interferometry (SWLI), in which an incoming light is split onto an internal reference surface and a silicon die surface. After reflection, the light beams recombine inside the interferometer, thereby undergoing constructive and destructive interference and producing light and dark fringe patterns [6].…”

“…for the individual adhesive types (silver glass 155 C; Our previous studies have indicated that most of the stresses result from the adhesive curing during the die-attach process [5], [6]. The numerical modeling considers a cooling cycle from the elevated temperatures to room temperature.…”

“…There have been several models developed to understand the thermal stress developed in the bilayer and trilayer systems due to the CTE mismatch [13]- [17]. In this study, a simple analytical model based on the Suhir's work [18] was taken into consideration to check the validity of numerical and experimental results.…”

“…The subsequent packaging processes and thermal cycles added minimal additional stresses to the die. The stress makes the silicon die warped, and more importantly, its development seems to influence the performance of the MEMS sensors [6]. It was therefore suggested that the die stress model be developed to predict the device performance of MEMS sensors and their packaging practices [6].…”