MEMS pressure sensors for low and highpressure applications with operation at elevated temperatures (100 – 200 ° C) have been aimed in this project. CSIR-CEERI has expertise in LPCVD polysilicon based MEMS sensor with a square diaphragm of 2 mm sides and thickness in a range of 50 – 100 micrometers. The sensing layer of polysilicon is deposited externally on the oxidized (100) silicon surface. The diffused version of silicon resistors which constitute a part of silicon diaphragm is an improved configuration in terms of thermal stability and noise. The sensitivity, however, remains an issue owing to reduced piezoresistive coefficients. Doping of the polysilicon thin film by ion implantation and by diffusion are two options which will be tried out for optimal response of the sensors. The e-beam based silicon polysilicon is another option where the institute has demonstrated improved response at elevated temperatures and the same will be tried out in this project. SOI based fabrication process has been identified as viable for high-temperature application of the sensor. This process is new to CEERI and efforts shall be put for its development. The bonding and packaging of the sensor chip for real-time characterization is a must for which CEERI has expertise. In 12th FYP Project of MicroSenSys, CEERI has already developed polysilicon piezoresistive MEMS sensors (Fig. 1).
The infrastructure is available at CEERI to meet all the requirements of user agency. Working prototypes of MEMS pressure sensors will be delivered to user agency. An in-house developed anodic bonding machine (Fig. 2) shall be employed for the glass to silicon bonding prior to dicing the dies. The design and mask layout of each configuration has been completed in association with user agency. Two fabrication runs of diffused version of the sensor have been completed. SOI based sensor fabrication process flow optimisation is underway. The activity has to be completed by August 2018.
Fig. 1: MEMS Pressure Sensor
Fig. 2: In-house developed Anodic Bonding Machine at CSIR-CEERI, Pilani