Membrane Mixes Materials to Measure Nanogram Masses, Temperature Simultaneously

The potential of thin film bulk acoustic wave resonators (FBARs—no sniggering from the back row, please) to measure mass at the nanogram level has marred by one persistent obstacle.

An FBAR membrane is electronically driven to vibrate at a characteristic resonant frequency. When a particle—a protein molecule, perhaps—adsorbs to membrane, the resonant frequency drops; the frequency change is proportional to the glued-on mass. So, measure the frequency drop, measure the mass. And because the frequency shift for a given mass is proportional to the square of the initial resonant frequency, the higher the initial resonant frequency, the more sensitive the measurement and the smaller the mass you can expect to measure.

And now for the obstacle. The FBAR’s acoustic characteristics change significantly with temperature. The speed of sound changes, and so does the thickness of the membrane. And this temperature sensitivity grows along with its measurement sensitivity as the resonant frequency climbs.

Up until now, solutions to this gravimetric problem have required two separate measurements—either measuring the frequency change in two distinct environments (an isolated test environment and the field environment) or building a second temperature sensor into the device.

But a collaboration among researchers from four British Universities (Cambridge, Manchester, Sheffield, and Bolton) and from Korea’s Kyung Hee University—has yielded a new twist on an old solution to the problem of temperature effects on precise measurement.

Temperature changes have dogged metrologists at least since John Harrison labored through much of the 18^th Century to construct the first accurate clocks. Balance springs sag as temperatures rise. Iron pendulum rods stretch.  Time seems to slow down as the weather warms up.  The solution is to combine materials to counteract these effects. Harrison devised a “compensation curb” (similar to the bimetallic spring at the heart of many home thermostats) to “rewind” the balance spring as temperatures rose. He also invented the gridiron pendulum, in which brass or zinc compression beams lift the pendulum weight to counteract the iron rod’s expansion. (See Dava Sobel’s Longitude or this Royal Society inventory of Harrison’s innovations.)

Though others had suggested a Harrisonian two-material solution to FBARs’ problems, this is the first time it’s been made to work over a broad range of real-world temperatures. To do this, Cambridge’s Luis Garcia-Gacendo and the team built a two-material device that resonates in two characteristic modes—arranged so that one of the composite's resonant modes increases in frequency as temperature rises while the other decreases in frequency. By measuring the shifts in both modes simultaneously, scientists can calculate both the temperature at the device and the mass of adsorbed particles.

The prototype FBAR device “for parallel sensing of temperature and mass loading” consists of a 2-micrometer-thick piezoelectric film of crystalline zinc oxide sputtered onto a 2 μm layer of silicon dioxide, which sits atop a silicon wafer sandwiched between chromium-gold electrodes. The prototype measurement tool showed native resonant modes at 754 megahertz and 1.44 gigahertz. Both silicon dioxide and zinc oxide expand with temperature (though at different rates), so both layers get thicker as the temperature rises. This would normally mean that the resonant frequencies of both modes would fall with temperature. In this case, though, rising temperatures cause the acoustic wave velocity to increase in the silicon dioxide layer and decrease in the zinc oxide layer. So a one-Kelvin change in temperature causes a 79.5 parts per million frequency increase in the SiO₂ and a 7 parts per million frequency drop in ZnO.

Thus, measuring frequency changes in both modes simultaneously indicates both temperature and mass.

The team successfully measured loads of human fibrinogen and bovine serum albumin in concentrations of about 1 to 1000 micrograms per milliliter (The total mass measured is unstated; the calibration tests measured masses on the nanogram level). Note, though, that the protein solutions were deposited on the membrane and then dried. The device as built relies on thickness longitudinal mode data, and cannot be applied to direct sensing of masses in liquids. The authors note, however, that repositioning the electrodes could enable thickness shear mode sensing in a successor device, opening up, for example, opportunities for direct sensing in biological or biotechnology applications. 

Images: Top: L. Garcia-Gacendo, Cambridge University. Bottom: Public Domain.