Fabrication and characterization of thermally actuated micromechanical resonators for airborne particle mass sensing: II. Device fabrication and characterization

Abstract

This paper, the second of two parts, presents extensive measurement and characterization results on fabricated thermally actuated single-crystal silicon MEMS resonators analyzed in part I. The resonators have been fabricated using a single mask process on SOI substrates. Resonant frequencies in a few hundreds of kHz to a few MHz and equivalent motional conductances as high as 102 mA V −1 have been measured for the fabricated resonators. The measurement results have been compared to the resonator characteristics predicted by the model developed in part I showing a good agreement between the two. Despite the relatively low frequencies, high quality factors (Q) of the order of a few thousand have been measured for the resonators under atmospheric pressure. The mass sensitivities of some of the resonators were characterized by embedding them in a custom-made test setup and deposition of artificially generated aerosol particles with known size and composition. The resulting measured mass sensitivities are of the order of tens to hundreds of Hz ng −1 and are in agreement with the expected values based on the resonator's physical dimensions. Finally, measurement of mass density of arbitrary airborne particles in the surrounding lab environment has been demonstrated.

I-shaped thermally actuated VHF resonators with submicron components

“…When the material exhibits significant piezoresistivity, this expansion/contraction of the arms causes modulation of the DC current, leading to an AC current component called the motional current, which can be detected. In this geometry, because the longitudinal acoustic waves which propagate from the actuator arms are perpendicular to the support arm the amount of leakage through the support arms is reduced in comparison to geometries where the actuator also acts as a structural support to the substrate [7,8,12].…”

“…Recently an alternative approach to thermal-piezoresistive resonant devices has been considered with the development of in-plane extensional mode resonators fabricated entirely by patterning a single layer of uniformly doped single-crystal silicon (SCS) [6][7][8]. The concept of operation is similar to the aforementioned devices except the localized regions in which the heating occurs are explicitly controlled through the geometry of the devices.…”

Transport of airborne particles in straight and curved microchannels

“…Aerosols with diameters of less than half a micrometer have been sorted by size in microchannels using electrostatic forces, 12 and a micromachined virtual impactor was used to sort aerosols into two size bins with a fixed cutoff diameter around 1 μm. 13 Mass-measurements of particulate matter deposited onto thin-film bulk acoustic wave resonators 14 and thermally actuated silicon resonators 15 have also been presented. These approaches demonstrated high resolution and sensitivity to low concentration levels but do not inherently include a size-selective mechanism that would allow the device to characterize the concentration of particular sizes of aerosol.…”

Electrostatic frequency tuning of thermal piezoresistive MEMS resonators

“…A variety of microscale electromechanical resonators have been considered/used in various applications such as sensitive and highly integrated chemical [3] and environmental sensors [4], filters [5] and gyroscopes [6]. Arrays of such resonators can be batch fabricated at high quantities at a very low cost.…”