Combined Pressure and Temperature Sensor for Hot Harsh Environments

In conjunction with academia and industry, NASA's Glenn Research Center has developed a range of microelectromechanical systems (MEMS)-based and Silicon Carbide (SiC)-based microsensor technologies that are well-suited for many applications. The suite of technologies includes hydrogen and hydrocarbon leak detection sensors; emissions sensor arrays; and high-temperature contact pads for wire bond connections. Currently used to protect astronauts on the International Space Station, the hydrogen and leak detection sensors have many Earth-based applications as well. They can function as a single-sensor unit or as part of a complete smart sensor system that includes multiple sensors, signal conditioning, power, and telemetry. The system can comprise sensors for hydrogen, hydrocarbons, oxygen, temperature, and pressure. The emissions sensor array features a gas-sensing structure that detects various combustion emission species (carbon monoxide, carbon dioxide, oxygen, hydrocarbons, and nitrogen oxides) over a wide range of concentrations. In addition, the emissions sensor array remains highly sensitive and stable while providing gas detection at temperatures ranging from 450 to 600°C. These new sensors provide a combination of responsiveness and durability that offers great value for a wide range of applications and industries.

Pressure sensors play an important role in engine maintenance and monitoring systems by diagnosing problems before they happen. To capture the most accurate data, however, these sensors must be placed directly on an engine. In order to withstand extreme temperature and vibration, traditional pressure sensor technologies are bulky and complex, lacking the on-board control of microsystem technologies. Glenn's new capacitive pressure sensor system and packaging is the first of its kind to achieve high-temperature capability while maintaining miniaturization. This novel system consists of a Clapp-type oscillator that is fabricated on a high temperature alumina substrate. It comprises a silicon carbide (SiC) nitride pressure sensor, a metal-semiconductor field-effect transistor, and one or more chip resistors, wire-wound inductors, and SiC metal-insulator-metal (MIM) capacitors. The pressure sensor is located in the tank circuit of the oscillator so that a variation in pressure causes a change in capacitance, thus altering the resonant frequency of the sensing system. The chip resistors, inductors, and MIM capacitors have been characterized at temperature and operational frequency, and exhibit less than 5% variance in electrical performance. The system, which can be installed with a borescope plug adaptor in an on-wing operating engine, has been extensively tested and proven to operate reliably under extreme conditions. Its compact size, wireless capability, and ability to provide real-time in-situ data acquisition make this technology a game-changer in next-generation maintenance and monitoring systems.

Prior approaches to bonding a SiC sensor and a SiC cover member relied on either electrostatic bonding or direct bonding using glass frits. The problem with the former is that its relatively weak bond strength may lead to debonding during thermal cycling, while the latter requires the creation of apertures that can allow sealant to leak. Glenn's innovation uses NASA's microelectromechanical system direct chip attach (MEMS-DCA) technology that can be bulk-manufactured to reduce sensor costs. The MEMS-DCA process allows a direct connection to be made between chip and pins, thereby eliminating wire bonding. Sensors and electronics are attached in a single-stage process to a multifunctional package, which, unlike previous systems, can be directly inserted into the housing. Additional thick pins within the electrical outlet allow the package to be connected to external circuitry. Furthermore, because the top and bottom substrates' thermomechanical properties are similar to that of the sensors, the problem of mismatch in the coefficient of thermal expansion is significantly reduced, minimizing thermal cycling and component fatigue. By protecting sensors and electronics in temperatures up to 600°C, approximately twice what has previously been achievable, Glenn's innovation enables SiC components to realize one of their most exciting possibilities - direct placement within high-temperature environments.

Glenn's invention is game-changing in its ability to produce ultra-thin SiC-based microstructures and diaphragms that are essential for high-sensitivity pressure sensors that not only monitor engines but can also act as biosensors (monitoring bone density or brain pressure). DSRIE offers selective etching that reliably isolates conductive microstructures from the bulk material and has precision-etch control that minimizes yield loss due to manufacturing defects. Therefore, the thickness of structures, such as diaphragms, can be ultra-thin and selectively realized during dopant reactive ion etching. The ultra-thin diaphragm is a key enabler for Glenn's novel pressure sensor that can measure at low pressures in the sub-psi range. Until now, batch fabrication of SiC sensors has been hindered by the fact that only one type of sensor per wafer could be produced at a time. Given the expense of fabrication, this limitation has greatly reduced the commercial viability of SiC sensors and electronics. Glenn's batch fabrication offers manufacturers the opportunity to simultaneously produce multiple multifunctional MEMS/NEMS products on a single SiC wafer. Such products include flow sensors, pressure sensors, biosensors, accelerometers, inertial sensors, angular rate sensors, and yaw rate sensors. By simplifying production, reducing capital equipment, and lowering production costs, Glenn's novel process makes the use of SiC-based MEMS/NEMS in sensors and electronics much more practical and attainable for countless industries.

The technology is part of a new generation of NASA Glenn SiC integrated circuits with unprecedented durability in the field of high-temperature electronics. For robust operational amplifiers based on SiC Junction Field Effect Transistors (JFETs), this novel compensation method mitigates issues with threshold voltage variations that are an effect of die location on the wafer. Modern high-temperature op amps on the market fall short due to temperature limits (only 225C for silicon-based devices). Previously, researchers noted that multiple op amps on a single SiC wafer had different amplification properties due to different threshold voltages that varied spatially as much as 18&#37 depending on the circuit's distance from the SiC wafer center. While 18&#37 is okay for some applications, other important system applications demand better precision. By applying this technology to the amplifier circuit design process, the op amp will provide the same signal gain no matter its position on the wafer. The compensation approach enables practical signal conditioning that works from 25C up to 500C.