A Tube-Shaped Buried Pirani Gauge for Low Detection Limit With Small Footprint

We present a micromachined Pirani gauge that combines low detection limit and strongly reduced footprint. It consists of a tube-shaped resistor that is buried in the silicon substrate. The choice of the tube geometry gives the resistor a very high structural rigidity. This enables the fabrication of much longer resistors, thus shifting the detection limit toward lower pressures. In addition, since the resistor is buried under the silicon surface, its footprint is kept very small. The high stiffness allowed the fabrication of a 3-mm-long and 1.8-μm-thick poly-Si tube with a 1-μm gap without buckling and/or stiction problems. It shows a detection limit of 0.1 Pa for a noise level of 50 μV, and it has a footprint of only 0.012 mm². This is an improvement of at least 20 times compared with Pirani gauges with the same detection limit. Pirani tubes of 1.6- and 0.4-mm lengths have also been designed, fabricated, and tested. The 0.4-mm-long tube shows a low pressure limit of 2 Pa, whereas the tube of 1.6 mm shows a low pressure limit of 0.2 Pa. The measured transfer functions correspond very well to the 1-D analytical model.