An Ultra-Compact MEMS Pirani Sensor for In-Situ Pressure Distribution Monitoring

Abstract

In this study, we designed a microelectromechanical system (MEMS) Pirani vacuum sensor with a compact size. Specifically, the sensor was successfully fabricated based on the Pirani principle and using a commercial eight-inch MEMS foundry process. The sensor fabrication process was carried out using only four photomasks and the proposed sensor had an ultra-compact fabricated size (<2.2 × 2.2 mm2). A vacuum measurement system was set up to comprehensively evaluate the fabricated sensors. The results demonstrated that the MEMS Pirani vacuum sensor has a high responsivity in the low-pressure domain from 100 Pa. The proposed sensor with a 953.0-Ω heater exhibited an average responsivity of 11.9 mV/Pa in the preferred range of 100 to 7 Pa and 96.0 mV/Pa in the range of 7 to 1 Pa. The sensor may be potentially suitable in many applications, such as vacuum indicators for processing equipment, health monitoring systems for social infrastructure, and medical and health applications.

Keywords: MEMS; Pirani sensor; lift-off; simple process; vacuum test.

Figure 1

Conception and potential application of the MEMS Pirani vacuum sensor. Inserts are an optical image of the sensors fabricated on the eight-inch wafer and a package sensor for evaluation.

Figure 2

Calculation results of the thermal power of a typical sensor. The inset shows the simulation results of the temperature of a sensor with longest resistor design, indicating its temperature increase for an applied voltage.

Figure 3

Schematic view of the Pirani vacuum MEMS sensor design with surrounding circuit. (a) Pirani vacuum MEMS sensor design; and (b) schematic view of the sensor’s surrounding circuit.

Figure 4

Schematic fabrication sequence of the MEMS Pirani vacuum sensor. (a) Si₃N₄ layer generation; (b) metallic electrode sputtering; (c) lift-off processes with photoresist; (d) Au-pads generation and patterning; and (e) sensor structure release.

Figure 5

(a) MEMS Pirani vacuum sensor evaluation system and measurement board. (b) Optical photograph and configuration flow chart of the evaluation system. (c) Optical photograph of the packaged vacuum sensor set on the measurement board.

Figure 6

(a) SEM image of the proposed vacuum sensor. (b) The magnified SEM image of a reference heater with square-wave layout structures. (c) The sensor structure after the back side releasing process.

Figure 7

Measured output voltage of the fabricated sensor device with different resistors ranging from 420.5 to 953.0 Ω under different pressure conditions.

Figure 8

Measured surface temperature and heater resistance of the fabricated 953.0-Ω-heater sensor device for increasing the applied voltage from 0.3 to 3 V.

Figure 9

Measured surface temperature and heater resistance of the fabricated 953.0-Ω-heater sensor device for decreasing the applied pressure from 100,000 to 0.1 Pa.