Vacuum Encapsulated Resonators for Humidity Measurement

Abstract

This paper demonstrates using charge-biased vacuum encapsulated single-anchored double-ended tuning fork (DETF) resonators for humidity sensing. We charge-bias the DETF resonators and measure the change in the stored charge by observing the resonant frequency. Environmental humidity modifies the discharge characteristic of the resonator by varying the resistance between the bias bondpad and the surface silicon dioxide. Using this variation, we demonstrate a sensor that can determine relative humidity from <1 % to 60% with an absolute measurement uncertainty of less than ±0.8% RH.

Vacuum encapsulated resonators for humidity measurement

“…3). For this example, the resonant frequency increases 4 kHz when the charge decays from 35 pC to 0 pC, or, equivalently, from 15 V to 0 V [9].…”

“…We demonstrate here an indirect measurement technique to measure large surface resistances [8,9]. This technique utilizes three functional components: charge decay through the surface resistance, a MEMS resonator, and an oscillator (Fig.…”

Stable charge-biased capacitive resonators with encapsulated switches

“…As such, it is desired to increase the charge leakage time constant τ = RC, and some demonstrated methods include: 1) increasing the stored charge by inserting a large capacitor [2]; 2) increasing the leakage resistance by controlling the environment, e.g. decreasing the temperature or humidity [6]; 3) trapping charge at the surface with an oxide-nitride layer [3,5] or within an electrically floating electrode surrounded by oxide [4]. Of these methods, charge trapping is seen to work the most reliably, with 19 months of no observed charge leakage on an electrically floating electrode surrounded by a thick 3 µm layer of oxide for a capacitive micro-machined ultrasonic transducer in [4].…”