Influence of environmental temperature on the dynamic properties of a die attached MEMS device

Die attach is one of the major processes that may induce unwanted stresses and deformations into micro-electro-mechanical systems (MEMS). The thermo-elastic coupling between the die and package may affect the performance of MEMS under various temperature loads, causing unreasonable effects of the output signal, such as zero offset, temperature coefficient of offset (TCO), nonlinearity, ununiformity and hysteresis, etc. A complete characterization of these effects is critical for a more reliable design. This work presents experimental studies of the temperature effects on the dynamic properties of MEMS. Microbridges and strain gauges with different dimensions were used as test structures. They were surface-micromachined on test chips and the chips were die attached on organic laminate substrates using epoxy bonding as well as tape adhering. The material and dimension of the substrate were specially defined to amplify the magnitude of the coupled deformation for the convenience of investigation. Modal frequencies of the microbridges under a set of controlled environmental temperature before and after die attach were measured using a laser Doppler vibrometer system. The average initial residual strain was also measured from the strain gauges to help analyze the dynamic behavior. Nonlinear TCO of the frequencies were observed to be as large as 2,500–5,000 ppm for the epoxy-bonded samples, in contrast with much smaller values for the tape-adhered and unpackaged ones. The frequencies recovered to their original values beyond the curing temperature of the epoxy. A distributed feature was also observed in frequencies of the microbridges with the same length but at different locations of the chip with a maximum relative difference of 20%. The process of thermal cycling and wire bonding was also applied to the samples and caused tender shifts of the frequencies. The experiments reveal major factors that are related to the temperature effects of die attached MEMS and the results are useful for improving the reliability of a package–device co-design.