(A) study on the pirani vacuum pressure sensor based on nanoporous anodic aluminum oxide (AAO) membrane for wide dynamic range

Abstract

In this thesis, the nanoporous Pirani vacuum pressure sensors based on anodic aluminum oxide (AAO) is proposed, and it is demonstrated to measure the high vacuum pressure. For the first time, nanoporous AAO membrane is used for a Pirani vacuum sensor to extend the measurement range. The quantitative relationship between the performance of the sensor and the porosity of the AAO membrane is characterized with a theoretical model. The proposed Pirani sensor is composed of a metallic resistor on a suspended nanoporous membrane, which simultaneously serves as the sensing area and the supporting structure. The AAO membrane has numerous vertically-tufted nanopores, resulting in a lower measurable pressure limit due to both the increased effective sensing area and the decreased effective thermal loss through the supporting structure. Additionally, the suspended AAO membrane structure, with its outer periphery anchored to the substrate, known as a closed-type design, is demonstrated using nanopores of AAO as an etch hole without a bulk micromachining process used on the substrate. In a CMOS-compatible process, a 200 $\mu$m × 200 $\mu$m nanoporous Pirani sensor with porosity of 25% was capable of measuring the pressure from 0.1 mTorr to 760 Torr. With adjustment of the porosity of the AAO, the measurable range could be extended toward lower pressures of more than one decade compared to a non-porous membrane with an identical footprint. To improve the detectability of the Pirani sensor toward the high vacuum pressure, micro-bridged sensor structure and enlarged membrane methods are presented in this thesis. By virtue of the conduction loss reduction through the sensor structure and effective surface increment for heat transfer to gas molecule, the detectable lower pressure is extended to about 1 decade in micro-bridged structure compared than closed-type structure and to 2×$10^{-7}$ Torr at the enlarged membrane dimension of 2000 u$\mu$ × 2000 u$\mu$. Since the gap spacing between the membrane and the substrate is kept at a few micro meter scale, there is no significant change in the detectable high pressure limit. Therefore, the remarkable enhancement in dynamic range of Pirani sensor is achieved. Given the merits of its CMOS compatible process, simple device structure, and excellent performance, the proposed nanoporous Pirani sensors can be a powerful candidate for the pressure monitoring of nano/microsystems