High temperature pressure sensor assembly and packaging for application up to 500 °C

High temperature stable (500 °C) pressure sensors are in high demand in fields like aerospace, automobile, and manufacturing industries. However, reliable sensors working at such elevated temperatures are still not sufficiently developed. Mainly, developing a high temperature stable package imposes new challenges due to thermal cross-sensitivity and temperature induced stress in the package. Important challenges include identifying stable materials at high temperatures and stress-tolerant sensor mounting techniques. This current research work will focus on the implementation of a high temperature (HT) packaging concept for a pressure sensor for applications up to 500 °C. The HT sensor package is fabricated out of a ceramic (Al2O3) substrate that is stable at very high temperatures. A membrane structure is fabricated on the ceramic package using ultrasonic machining. A micro-strain gauge deposited and patterned on a ‘Langasite’ La3Ga5SiO14 (LGS) crystal is then attached to the ceramic package by flip-chip interconnection and glass solder underfill. The deflection of the deforming membrane is transferred pointwise to the free end of the crystal inside the package using a quartz spacer. The strain produced on the cantilever is then measured by the change of resistance of the micro strain gauge. This special design concept aims at the elimination of thermal stress between membrane and the sensing element by allowing it to freely expand under temperature load. This concept is developed and fabricated methodically using Finite Element Method (FEM) and planar fabrication techniques. The final prototype is evaluated to demonstrate the high temperature stability of the pressure sensor package. Digital Image Correlation (DIC) is used to characterize the thermal expansion behavior of the flip-chip sensing cantilever. The sensing crystal expands up to ~25 µm along its length which proves the stress relieving behavior of this concept. Shear tests, performed on the flip-chip die attach shows strength of up to 34 MPa. Pressure tests were conducted at 200 °C on a low temperature variant of this sensor design including its temperature compensation concept. A measurement was performed at 500 °C to prove the operation of this sensor assembly at such elevated temperatures. This research work establishes a robust high temperature stable package for applications up to 500 °C for pressure sensing applications.