Performance Study of MEMS Piezoresistive Pressure Sensors at Elevated Temperatures

Micro-Electro-Mechanical Systems (MEMS) technology has revolutionized the development of miniaturized sensors and actuators, with the associated electronic system on a single chip. Due to their compact size, high sensitivity, uniformity, and linearity, MEMS sensors are deployed in various industries, including oil & gas, automotive, defense, aerospace/avionics, and nuclear power plants. However, due to junction leakage, these sensors have an operating temperature limit of 125°C.

To mitigate this issue, the paper investigates two designs, i.e., single crystal silicon resistors over a silicon dioxide layer (SC) and polysilicon piezoresistors over a silicon dioxide layer (PS). Sensors with a conventional approach, i.e., doped piezoresistors in the silicon diaphragm (DS), were fabricated for comparison. The design of the proposed high-temperature pressure sensors complies with the requirement for process control instrumentation used in nuclear power plants.

The sensor design parameters, such as the dimensions of the diaphragm, maximum stress regions for the pizoresistors, the sensitivity of the sensors, etc., are simulated using Finite Element Modeling (FEM). The Single-crystal P-type silicon piezoresistors are located in the maximum stress region to maximize the change in resistance for an applied input pressure.

The design was carried out for a maximum pressure of 400 bar. The die size for the three types of sensors was 3 mm × 3 mm, and the diaphragms were 750 μm × 750 μm × 150 μm for SC and DS sensors and 1500 μm × 1000 μm × 140 μm for PS pressure sensors.

The main process steps for DS-type pressure sensor fabrication included thermal oxidation of silicon dioxide, bulk micromachining, front-side diffusion of boron, contact opening, metallization, and passivation. For SC pressure sensors, diaphragm etching was the same as for DS type sensors. The same process was followed for PS-type sensors, with polysilicon deposition performed on the front side using the Low Pressure Chemical Vapor Deposition (LPCVD) technique. The pressure sensor chips were packaged using anodic bonding and laser dicing.

The output response of pressure sensors at different temperatures was analyzed, focusing on the sensitivity. The sensors exhibited considerable offset voltage due to the non-uniformity of piezoresistors. The sensitivity of the pressure response gradually decreased with temperature increase due to the decrease in piezoresistive coefficients.

The DS type sensor could only be operated up to 100°C, while the other two types could operate without any failure up to 200°C. The degradation in sensitivities of DS, SC, and PS pressure sensors was observed to be 13% at 100°C, 19.5% at 200°C, and 9.0% at 200°C, respectively. Thus, Polysilicon piezoresistor-based sensors appear to be most suitable for elevated temperature applications, as they have the least sensitivity decrease compared to others.

The initial sensitivities of the three pressure sensor types differed due to different gage factors and geometrical arrangements of piezoresistors. The doping concentration of piezoresistors for SC pressure sensors was the lowest compared to the other two types of sensors.

The sensors were designed using various piezoresistors and compared at elevated temperatures. It was found that the PS pressure sensors' sensitivity degraded (< 10%) the least compared to other designs. Thus, the PS pressure sensor showed the best performance for elevated temperature.

considering a factor of safety of two to have a sensor to be used for a maximum pressure of 200 bar.