Sensors capable of operating in high-temperature harsh environments are required across various industries such as automotive, petrochemical, aerospace, nuclear energy, and materials engineering. Since the 21st century, developing such sensors has consistently posed significant challenges.

Although MEMS piezoresistive vacuum gauges based on silicon materials have been widely used in fields like automotive and semiconductors, their design temperature range typically does not exceed 150°C. This is because the mechanical stability of silicon-based materials in high-temperature applications (exceeding 500°C) remains a concern. However, there is growing interest in exploring the stability of MEMS sensors in high-temperature applications, as plastic deformation in silicon-based materials at high temperatures only occurs under stresses exceeding 3GPa; below this limit, the material remains in the elastic region.

↑ MEMS piezoresistive vacuum gauge from MIS Instruments (US)  ↑

This article presents a case study investigating the robustness of MEMS (micro-and nano-electromechanical systems) sensors in environments exceeding 500°C.

The device under evaluation was a MEMS barometer, featuring a circular diaphragm with a diameter of 60μm and thickness of 500nm, placed between external pressure and a low-pressure cavity. The diaphragm deforms under atmospheric pressure. This membrane integrates two nanogauges packaged within a 500nm-thick MEMS layer. A circular rigid piston is etched into a 20μm-thick MEMS layer and connected to the center of the diaphragm, transmitting its deformation to a lever arm. The shorter fixed lever arm compresses or stretches two underlying nanogauges.

↑ Physical image of the pressure sensor under scanning electrode microscopy ↑

↑ Schematic cross-section of the sensor ↑

Under vacuum conditions, the resistance of each nanogauge is 800 Ohms. The two nanogauges are rapidly connected in a half-bridge configuration. Each nanogauge is polarized with a 1mA DC current, and the voltage output across different nanogauges is measured using a four-point probe method. When pressure is applied to the diaphragm, one nanogauge is stretched while the other is compressed, causing bridge imbalance. This imbalance is converted into a voltage signal proportional to the pressure measurement.

↑ Circuit used for measuring the pressure sensor ↑

High-temperature measurements were conducted in a variable temperature probe station under vacuum conditions, with the resistance of the nanogauges measured during thermal cycling. Electrical measurements were performed using a multimeter.

↑ Probe station with four probes for temperature cycling and nanogauge resistance measurement ↑

↑ Temperature profile set for thermal cycling: ramp rate 20°C/min, total duration 1 hour. Values shown correspond to the actual stage temperature ↑

Experimental results show that the resistance of the two nanogauges remained highly stable during the first three cycles. Subsequently, damage to the AISI peak metallization due to thermal contact altered the contact resistance. Nevertheless, it can be concluded that up to 522°C, the nanogauges remain unaffected by high-temperature cycling.

↑ Resistance variation curves of two nanogauges at different temperatures ↑

This study is a preliminary investigation aimed at determining the potential of MEMS technology for high-temperature pressure sensors. Silicon has a melting point of approximately 1400°C. Current challenges related to damage caused by TAISI metallization could potentially be addressed in future research by replacing AISI alloy metallization with high-temperature resistant metals.

↑ GoGo Instruments Externally Adjustable Probe Heating/Cooling Stage  ↑

The GoGo Instruments Externally Adjustable Probe Heating/Cooling Stage is a scientific instrument specifically designed for electrical testing under variable temperatures during materials research. Based on an optical hot/cold stage, it incorporates an electrical module including probes and electrical interfaces. By adjusting external probes, the internal probe positions can be moved with high precision XYZ displacement, allowing the tips to contact any area on the sample surface.

During testing, electrical signals are transmitted through wires connected to the probes to electrical instruments (such as source meters, multimeters, etc.), thereby measuring relevant electrical data to analyze the electrical characteristics of materials at variable temperatures. This hot/cold stage utilizes liquid nitrogen dual cooling and resistance heating methods, achieving precise and stable temperature control across ranges of -190~400°C or RT~1000°C.