editor's blog
Subscribe Now

Teasing Apart FBAR Loading and Temperature Effects

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).

Leave a Reply

featured blogs
Dec 12, 2018
For years chip designers have dealt with ECO'€™s when their source code was written in RTL. But the move to high-level synthesis (HLS) means that their source code is now one step further removed from... [[ Click on the title to access the full blog on the Cadence Communit...
Dec 12, 2018
A joint demonstration between Samtec and eSilicon — an eSilicon 7 nm 56 Gbps DSP SerDes over a Samtec 5 meter ExaMAX® backplane cable assembly — caught a lot of attention at SC18. The demo showed a true long-reach capability with a high-performance, flexible, eas...
Dec 10, 2018
We may think of prosthetics as devices created to enable basic functions, such as walking or grasping an object. Regarded as a necessity, but not an asset, prosthetics of the past......
Nov 14, 2018
  People of a certain age, who mindfully lived through the early microcomputer revolution during the first half of the 1970s, know about Bill Godbout. He was that guy who sent out crudely photocopied parts catalogs for all kinds of electronic components, sold from a Quon...