Arash Hajjam

Individual Air-Borne Particle Mass Measurement Using High-Frequency Micromechanical Resonators

This work demonstrates mass measurement of individual submicron air-borne particles using resonant micromechanical nano-balances. Thermally actuated high-frequency single crystalline silicon resonators fabricated using a single mask process have been used as mass sensors. Mass sensitivity of the resonators have been characterized using artificially generated airborne particles of known size and composition. Mass sensitivities as high as 1.6 kHz/pg have been demonstrated for devices with resonant frequencies in the tens of MHz range. The measured mass sensitivities are in good agreement with the calculated values based on the resonator physical dimensions. Due to the high mass sensitivities, the shift in the resonator frequencies caused by individual particles as small as ~200 nm in diameter is distinguishable. Counting and individual mass measurement of single arbitrary particles in air samples from a cleanroom have also been demonstrated. The results in this work present the possibility of implementation of low-cost and small-size instruments for airborne particle mass and size distribution analysis in highly controlled environments (e.g., for cleanroom classification) or for environmental applications.

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

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 resonators physical dimensions. Finally, measurement of mass density of arbitrary airborne particles in the surrounding lab environment has been demonstrated.

Thermal actuation, a suitable mechanism for high frequency electromechanical resonators

This work presents high-frequency thermally actuated micromechanical resonators and demonstrates potential suitability of thermal actuation for high frequency applications. Thermally actuated single crystal silicon resonators with frequencies up 61 MHz have been successfully fabricated and characterized. It is shown both theoretically and experimentally that as opposed to the general perception, thermal actuation is a more efficient actuation mechanism for higher frequency rather than lower frequency applications. Thermal actuation can become a viable and competitive approach as the electromechanical device dimensions reach the lower micron and nanometer range.

Sub-100ppb/°C temperature stability in thermally actuated high frequency silicon resonators via degenerate phosphorous doping and bias current optimization

In this work, we study temperature drift behavior of thermally actuated high frequency single crystalline silicon resonators and demonstrate temperature compensation of such via a combination of N-type degenerate doping and adjustment of the operating bias current. Significant suppression of the large negative temperature coefficient of frequency (TCF) for the resonators (−40ppm/°C) has been demonstrated using phosphorous degenerate doping resulting in even slightly positive TCF for some of the devices. Furthermore, it is shown that the TCF for such resonators can be fine tuned by changing the operating bias current enabling very close to zero TCF to be realized. Temperature compensation results for several resonators with different frequencies ranging from 3MHz to 60MHz are presented. TCF as low as −0.05ppm/°C (−50ppb/°C) has been demonstrated for an 8.2MHz resonator, which to the best of our knowledge is the lowest reported value for silicon-based micromechanical resonators.

Fabrication and Characterization of MEMS-Based Resonant Organic Gas Sensors

Polymer coated thermal-piezoresistive micromechanical resonant silicon nanobalances have been utilized for detection and concentration measurement of volatile organic compounds in gas phase. Polyglycolic acid, which is the main polymer ingredient of the shipley-1813 photoresist, was used as the absorbent coating layer. Experiments have shown that polymer thickness determines the achievable sensitivity towards the gas molecules. The main challenge is to coat the devices with a thick layer of polymer while still maintaining their high-Q resonance. Multiple polymer coating approaches have been demonstrated. Following polymer deposition, in order to find the sensitivity of the devices, they were tested by exposure to regular gasoline and toluene vapor. The best measurement results were obtained by utilizing the photoresist already present on the resonant structures from the fabrication process. A maximum frequency shift of 3600 ppm (55 kHz) was obtained from a 15.5 MHz resonator with a ~ 1.5 μm thick coating upon exposure to nitrogen saturated with toluene vapor at room temperature. Based on the measurement results, minimum detectable concentration of toluene in the gas phase for such devices is in the range of a few ppm.

Thermally actuated MEMS resonant sensors for mass measurement of micro/nanoscale aerosol particles

This work presents thermally actuated Micro-electromechanical resonant sensors capable of measuring the mass of micro/nanoscale aerosol particles deposited on them. The resonators were specifically designed for maximum tolerance of air viscous damping and deposited particles and were fabricated using a single-mask fabrication process on SOI substrates. The fabricated resonators were placed in a custom-made aerosol particle collection and deposition system. We have successfully shown up to 6kHz (0.33%) shift in resonance frequency resulting from the particle mass loading. Sensor sensitivities are calculated to be in the order of hundreds of Hz/ng. The sensors presented in this work offer the prospect of implementing miniaturized and low-cost instruments for air quality monitoring and environmental research.

Doping-Induced Temperature Compensation of Thermally Actuated High-Frequency Silicon Micromechanical Resonators

Temperature compensation of thermally actuated high-frequency single crystalline silicon micromechanical resonant structures via high concentration n-type doping has been demonstrated in this paper. The effect of doping level, structural dimensions, and bias current on temperature coefficient of frequency (TCF) for such resonators has also been investigated. It has been shown that the large negative TCF of the silicon resonators (-38 ppm/°C) can be highly suppressed by doping the devices with a high concentration of phosphorous. The TCF can also be fine tuned by changing the operating bias current of the resonators. Temperature drift characteristics for several high- frequency I-shaped resonators thermally doped under different conditions have been measured and compared. For an ideal doping level, an overall linear temperature drift of -3.6 ppm over the range of 25°C to 100°C, which is equivalent to a TCF as low as -50 ppb/°C, has been demonstrated for one of the resonators. The results in this paper imply the possibility of having low-cost high-frequency thermally actuated resonators with a near-zero TCF.

Thin-film piezoelectric-on-silicon particle mass sensors

In this paper, high quality factor (Qair>18000), high frequency (∼27MHz and ∼54MHz), lateral-extensional mode film piezoelectric-on-silicon resonators are used as aerosol particle mass sensors. Using an aerosol particle generator, mass sensitivities of −4.2Hz/pg and −42Hz/pg are measured respectively which confirms the benefits of employing higher frequency resonators given the quality factor is not deteriorated. These results are in good agreement with the theoretical and simulated values. Our work suggests the TPoS resonators as an easily integrated, low-loss platform for particle sensing applications.

Temperature compensated single-device electromechanical oscillators

This work presents temperature compensated single-device fully micromechanical (circuit-less) oscillators. Thermal-piezoresistive interactions in certain micromechanical resonant structures can lead to self-sustained mechanical vibrations. Self-sustained single crystalline silicon oscillators with frequencies in the few MHz range have been fabricated and their temperature drift behavior is characterized. Temperature drifts as high as - 38ppm/°C measured for the fabricated devices were sharply reduced to less than ±1.5ppm/°C by high concentration phosphorous doping of the structures. TCF as small as 0.4ppm/°C has been demonstrated for a 2.4MHz MEMS oscillator. Furthermore, the TCF of the oscillators has been found to be dependent on the resonator bias current making very close to zero TCF achievable for such devices.

Self-sustained micromechanical resonant particulate microbalance/counters

This work presents the technological basis for implementation of MEMS-based air-borne particulate counters capable of simultaneous mass measurement of individual particles. Self-sustained thermal-piezoresistive micromechanical oscillators with frequencies in the few MHz range have been used as high-precision microbalances eliminating the need for supporting analog circuitry. Mass sensitivity of the sensors were measured using artificially generated aerosol particles to be in the 10–30 Hz/pg range allowing detection and mass measurement of individual particles with submicron diameters.