We hear stories of a not-so-distant future when we can wave our tricorder-like devices around and detect all kinds of substances that might be in the air. One of the ways sensors like this can work is by having a resonating body: when a substance adsorbs on the surface, it changes the mass, thereby changing the resonance frequency.
The problem is, however, that temperature also affects the frequency, and it’s actually pretty hard to calibrate that out of the system. Using a reference resonator or a complex software algorithm is possible, but, according to a team from Cambridge, Universities of Sheffield, Bolton, and Manchester in the UK, and Kyung Hee University in Korea, it makes things more complex and/or costly.
They’ve come up with a way of teasing the loading and temperature effects apart. It involves a two-layer structure: 2 µm of ZnO over 2 µm of SiO2. When they get this vibrating, they see two modes:
- One with a fundamental frequency at 754 MHz and harmonics at 2.26 and 3.77 GHz
- One with a fundamental frequency at 1.44 GHz, and the next harmonic at 4.34 GHz
The first mode comes from the resonance of the combined ZnO/SiO2 structure; its half-wavelength relates to the combined 4-µm thickness of the overall structure. The second mode results from the ZnO layer by itself, with a half-wavelength driven by the 2-µm thickness of this layer, although it’s also affected by the SiO2 load.
Both ZnO and SiO2 have positive coefficients of thermal expansion (CTE), so both layers get thicker as temperature goes up. But the longitudinal wave velocity goes up for SiO2 and down for ZnO. As a result, the frequencies move in opposite directions as temperature changes: roughly 79.5 ppm/K for SiO2 and -7 ppm/K for ZnO.
Given those as base numbers, it now becomes possible to deconvolve the temperature and loading effects of whatever it is you’re trying to sense.
This was, of course, a university project, although it looks like they will be open to commercializing it. You can get more details in the full paper, but it’s behind a paywall (actually, several; you can Google “Dual-mode thin film bulk acoustic wave resonators for parallel sensing of temperature and mass loading” and pick your favorite one).