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Effects of Thermocompression Die Bond Residual Stress on MEMS Inertial Sensors
Keywords: Inertial sensor, Thermocompression Bond, Residual Stress
In inertial sensor applications a thermocompression bond is often used for die attachment in hermetic, vacuum packages. These bonds are made by heating the ceramic chip carrier and the MEMS sensor die to ~325C, bonding the components, and cooling to ambient conditions. Due to mismatch in thermal expansion coefficients between the ceramic package and the sensor die, there is a residual stress that results in deformation of the die. In applications where a capacitive sensor between a structure and the substrate is used for measurements this bowing may be a source of error, as in tuning fork gyroscopes. Additionally, as the temperature of the component changes the curvature in the die also changes, decreasing while approaching the bonding temperature. It has also been observed that over time there may be a relaxation effect, alleviating residual stresses and reducing surface curvature at ambient temperature. The changes in the shape topography of the MEMS die may adversely affect performance. By using several methodologies these effects are closely studied. Finite element analysis of a die bonded in a package is performed to determine the expected deformation. The degree of deformation (as a function of temperature) of components is measured using interferometric techniques to corroborate FEA findings. The performance of MEMS gyroscopes is measured using a Wuilfert EVO-10 rate table with custom thermal loading apparatus, allowing for investigation of the relation between sensing voltage with temperature and die curvature.
Ryan Marinis,
Worcester Polytechnic Institute - ME Dept.
Worcester, MA
USA


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