High-stability piezoresistive pressure sensors

Abstract

Piezoresistive pressure sensors have been in use for decades and are among the most successful micromachined sensors. From their introduction in the 1960s, they have experienced a dramatic improvement on performance and stability, as new microfabrication techniques have been developed. For high-accuracy avionic applications, there is a strong focus on the long-term stability of the sensor output-signal. Higher stability makes increased service-intervals possible and open the doors to new applications areas. This thesis focuses on possible root causes influencing the stability for piezoresistive pressure sensors. Although the main focus has been on the SP82-design from MEMSCAP, the results are relevant also for other sensor designs using similar packaging technologies. It has been found that even small amounts of excess glass frit material in the die lamination process may cause a major influence on the sensor zero point and temperature sensitivity. High residual stress in the excess glass frit volume will also result in increased risk for mechanical stress release in the form of microcrack formation. By optimizing the glass frit distribution, the influence from the die lamination on the sensor output-signal has been reduced and the stability has been improved. Another major finding has been that the use of a thermoset polymer as a die-attach material might result in an unwanted signal drift when the sensors are stored for weeks under ambient environmental conditions. This was found to be caused by diffusion of water into the polymer, leading to spatially localized swelling with resulting mechanical stress changes, combined with physical aging. A new support die design has been designed and tested, reducing this effect to neglectable levels. The sensitivity of a sealed sensor structure to changes in the sensor cap pressure has been modelled and experimentally verified. Examples of resulting influence from various storage conditions on signal drift as well as characteristic over temperature are demonstrated, and resulting hermeticity requirements are discussed. A new metallization pattern for in situ observation of metal properties in a piezoresistive pressure sensor has been developed and initial testing has been performed. By limiting the design change to the metal mask only, this provides a cost effective approach for characterizing of metal properties where the test structure can be combined with functional devices on the same wafer.