Flip-chip Die Attachment for High-temperature Pressure Sensor Packages up to 500 °C

Abstract

Pressure sensors working at temperatures of about 500 °C impose new challenges in packaging due to thermal cross- sensitivity and temperature induced stresses on the package. One of the major issues is the identification of stress-tolerant sensor mounting technique with stable materials at high temperature. This research-based work will mainly focus on developing a flip-chip die attachment technique for pressure sensor assembly for applications up to 500 °C. A concept for this application was developed based on a deforming ceramic membrane. A micro- strain gauge is patterned onto a Langasite (LGS) crystal. It is attached to a ceramic substrate with a membrane (Al2O3) like a cantilever by flip-chip interconnection and glass. The deforming membrane induces a pressure dependent displacement at the free end of the cantilever. The strain produced on the cantilever is measured by the change of resistance of the microstrain gauge. This special design concept aims for the elimination of thermal stresses by having no constraints for thermal expansion at the free end of the cantilever. LGS is a well-established material for Surface Acoustic Wave (SAW) based applications. Later this resistive strain gauge could be replaced by a SAW delay line. In order to mount the sensing element like a cantilever, one side of the LGS strain gauge chip is fabricated with gold stud bumps on its contact pads. Additionally, the flip-chip attachment is underfilled with glass solder and cured at 780 °C. Due to the high process temperature and anisotropic Thermal Coefficient of Expansion (TCE) of the LGS crystal it will tend to expand. By allowing it to expand freely at one end, the potential thermal stresses developed in the package is reduced. In this paper, processes to develop high temperature stable flip-chip die attachment using stud bumps and glass solder underfill is presented. The free expansion of the LGS crystal at its free end is determined using Digital Image Correlation (DIC) technique for temperatures up to 500 °C. With the same construction, a Lithiumniobate (LN) crystal is also introduced for applications up to 300 °C. The thermal expansion behavior of the die attachment is characterized using DIC. Strength of the cantilever die attachment is measured using shear tests and results are presented.