The nanogap Pirani—a pressure sensor with superior linearity in an atmospheric pressure range

Abstract

We have designed and fabricated a surface micromachined Pirani pressure sensor with an extremely narrow gap between its heater and heatsink (substrate) with superior output linearity in the atmospheric pressure range. The gap size of the device has been reduced to 50 nm by using a layer of PECVD amorphous silicon as a sacrificial layer and a xenon difluoride (XeF 2 ) gas phase etching technique. Such a narrow gap pushes the transition from molecular to continuum heat conduction to pressures beyond 200 kPa. The higher transition pressure increases the measurement range and sensitivity of the gauge in atmospheric pressures. The gas phase etching of the sacrificial layer eliminates stiction problems related to a wet etching process. The active area of the sensor is only a 6 × 50 μm 2 microbridge anchored to the substrate at both ends. An innovative fabrication technique was developed which resulted in a virtually flat microbridge with improved mechanical robustness. This process enabled us to have a very well-controlled gap between the microbridge and the substrate. The device was tested in a constant heater temperature mode with pressure ranges from 0.1 to 720 kPa. The heater power was only 3 mW at 101 kPa (atmospheric pressure), which increased to about 8 mW at 720 kPa. The output sensitivity and nonlinearity of the device were 0.55% per kPa at 101 kPa and ±13% of the output full scale, respectively.

Nanogap Pirani Sensor Operating in Constant Temperature Mode for Near Atmospheric Pressure Measurements

“…In 1906, Marcello Pirani invented it using a temperature-dependent resistor in vacuum tube. With the development of microelectronics and micro-electromechanical systems techniques, the Pirani gauges were subject to miniaturization, as reported in numerous papers [1][2][3]. Micromachined Pirani sensors consist in a hot-wire, the heater, suspended over a heatsink, the substrate, by a gap.…”

“…This upper pressure limit is conditioned by the gap size in Pirani sensors: the pressure-dependence of thermal conductivity occurs when the gap is near the mean free path of the gas molecules. For sensing pressure near the atmospheric pressure, the gap has to be decreased to a nanoscale [2,4]. However, when the gap size reduces to sub-micron scale, the fabrication process is more complex: the size of the wire has to be decreased as well to avoid collapsing of the structure onto the substrate [3].…”

A micro-machined Pirani gauge for vacuum measurement of ultra-small sized vacuum packaging

“…In addition to these devices, a new surface micro-machined Pirani pressure sensor with an extremely narrow gap between heater and heatsink (substrate) was also designed and fabricated in reference [16].…”

A bulk micromachined vertical nano-gap Pirani wide-range pressure test structure for packaged MEMS performance monitoring

“…high pressure: 1013Pa ∼ 101,300 Pa), Pirani gauge requires a nano-meter-scale gap between a heater and a heat sink to realize molecular flow regime in the gap for successful operation in such low vacuum range [3]. A vertical narrow gap is much more advantageous for integration with modern bulk micromachined MEMS.…”