Emad Mehdizadeh

Sensitivity Enhancement of Lorentz Force MEMS Resonant Magnetometers via Internal Thermal-Piezoresistive Amplification

This letter presents sensitivity enhancement of MEMS resonant magnetometers using the thermal-piezoresistive internal amplification effect in silicon microstructures. Preliminary results show up to ~15 X improvement in sensitivity per bias current for a resonator operated as a Lorentz force magnetometer. Magnetometer sensitivity figure-of-merit, defined as sensitivity (mV/T) over sensor dc bias current, has increased from 0.29 Ω/T(mV/Tesla/mA) to 4.22 Ω/T via internal thermal-piezoresistive amplification that also led to resonator effective quality factor (Q) increasing from its intrinsic value of 1140 to 16900 (in air). Previous work on the thermal-piezoresistive amplification effect suggests that amplification factors up to 3-4 orders of magnitude can be achieved using optimally designed structures, which can lead to ultra-high sensitivities for the presented sensors. It should be noted that the main focus of this letter is not to demonstrate a highly sensitive magnetometer, but rather to demonstrate the ability to improve magnetometer sensitivity as the resonator internal Q-amplification kicks-in. Although the resonant structure in this letter has not been optimized to operate as a magnetometer, sensitivities as high as 262 mV/T in air (minimum detectable field in the μT range) have been achieved.

A two-stage aerosol impactor with embedded MEMS resonant mass balances for particulate size segregation and mass concentration monitoring

This work reports on integration of MEMS resonant mass balances within a two-stage aerosol impactor capable of sampling and size separation of micro to nanoscale airborne particles. Two identical thin-film piezoelectric on substrate (TPoS) resonators acting as high resolution mass balances provide real-time information on mass concentration of size-segregated airborne particles adsorbed in each aerosol impactor stage. Tests performed on air samples from different environments with different particle mass concentrations ranging from 0.014-0.2μg/m3 (particles larger than 60nm) show a clear correlation between the resonator response and the expected particle concentrations. SEM images of the deposited particles clearly show a size difference between particles collected in the two stages with the cutoff sizes close to the values the impactor was originally designed for (230nm and 60nm for the first and second stages, respectively).

Inertial Impaction on MEMS Balance Chips for Real-Time Air Quality Monitoring

This paper reports on integration of microelectromechanical systems (MEMS) balance chips with aerosol inertial impactors for real-time monitoring of airborne particulate matter. Cascade inertial impactors have been extensively used for sampling and size separation of micro to nano-size airborne particles since they were first used in 1945. To introduce the real-time measurement capability to such tools, herein, MEMS resonator chips are employed as their impaction substrates. Dual-plate thermal-piezoresistive resonators (TPRs), that are shown to operate as promising mass balances, are integrated within a two-stage custom made aerosol impactor capable of size segregating particles down to a few tens of nanometers. Unlike cantilever-based microbalances or resonators operating in their bulk mode, TPRs provide uniform mass sensitivity over a big portion of their surface area. Upon deposition of airborne particles on the resonant sensors, the mass loading on each sensor and as a result the mass concentrations of size-segregated particles is measured in real-time. The air quality of different environments, including a class 10,000 cleanroom, was analyzed and monitored via the real-time impactor over a four day period. Comparison of the results with those of an optical particle counter demonstrates a clear correlation between the system response and the expected particle counts. Compared to existing optical particle counters, the proposed system offers the added advantage of detecting sub-100nm particles.

Ultra sensitive lorentz force MEMS magnetometer with pico-tesla limit of detection

This work presents ultra-high sensitivities for Lorentz Force resonant MEMS magnetometers enabled by internal thermal-piezoresistive vibration amplification. Up to 2400X increase in sensitivity has been demonstrated by tuning the resonator bias current to maximize its internal amplification factor boosting the effective Quality Factor (Q) from its intrinsic value of 680 to 1.14×106 (1675X amplification). For a bias current of 7.245mA, where the sensitivity of the device is maximum (2.107mV/nT), the noise floor is measured to be as low as 2.8 pT/√Hz. This is by far the most sensitive MEMS Lorentz force magnetometer demonstrated to date.

Atomic Resolution Disk Resonant Force and Displacement Sensors for Measurements in Liquid

This letter presents resonant microelectromechanical systems capable of resolving subatomic features and/or measurement of atomic level forces in liquid media (in addition to air/vacuum). It has previously been shown that forces in the micro- and nano-Newton range applied to flexible extensional-mode resonant microstructures can cause significant resonant frequency shifts [1]. In this letter, the same principle has been applied to rotational mode disk resonators that are capable of maintaining relatively high quality factors in liquid. Preliminary results indicate about tenfold improvement in combined displacement-force resolution figure-of-merit compared with typical piezoresistive cantilevers when operating in liquid. Using integrated comb-drive electrostatic actuators, displacement and force resolutions as high as 200 fm and 1 nN, respectively, have been demonstrated. By application of the force through a levering mechanism, force sensitivities in the pN range can be achieved while maintaining sub-nm spatial resolution.

Chip-scale aerosol impactor with integrated resonant mass balances for real time monitoring of airborne particulate concentrations

This work presents chip-scale integration of MEMS resonant mass balances along with aerosol inertial impactors (airborne micro/nanoparticle collectors). A three mask microfabrication process has been developed to produce the main components; mass balance, impactor nozzle, and impaction micro-chamber on a single SOI substrate. In addition to extreme miniaturization of a conventionally bulky setup and allowing real-time particulate mass concentration data collection, this approach addresses assembly challenges for discrete versions of such systems, e.g. misalignment between MEMS resonators and nozzles. Furthermore, small nozzle diameters achievable through microfabrication, minimizes the air flow and therefore pump capacity requirements.

High-Q Lorentz force MEMS magnetometer with internal self-amplification

This work presents a MEMS resonant Lorentz force magnetometer with internal self-amplification using the thermal-piezoresistive quality factor (Q) enhancement effect in silicon microstructures. Close to three orders of magnitude (~720X) improvement in sensitivity of the sensor is demonstrated by increasing the resonator bias current from 1 mA to 28 mA. Magnetometer sensitivity figure-of-merit (FOM), defined as sensitivity (mV/T) over piezoresistor bias current, has increased from 1.3Ω/T (mV/Tesla/mA) to 36.3Ω/T by amplifying resonator Q from its intrinsic value of 494 to 13,571 in air. Further amplification up to 3-4 orders of magnitude sensitivity FOM and much higher sensitivities are expected to be achievable if narrower resonator actuator beams are used. Preliminary results on the presented magnetometer show sensitivities as high as 1.01V/T (minimum detectable field in the nT range) while operating in air.

Nano-precision force and displacement measurements using MEMS resonant structures

This work presents a new approach for measuring sub-nano-Newton forces and sub-picometer displacements using MEMS resonators. Different versions of thermally actuated dual plate micromechanical resonators coupled to electrostatic actuators are utilized as highly sensitive force/displacement sensors. The force generated by the actuator strains the associated resonator changing its resonant frequency. Upon thorough characterization, this approach can be used as a reliable and accurate solution for force and displacement measurements in micro and nano-electromechancial systems. Frequency-force and displacement sensitivities as high as 17Hz/nN and 540 Hz/pm have been measured for the presented structures, respectively, showing the potential of such devices for sub-nanoscale force and displacement measurement resolutions.

Direct detection of biomolecules in liquid media using piezoelectric rotational mode disk resonators

This work reports on direct detection of biomolecular adsorption events in liquid media using piezoelectric rotational mode disk resonators. Piezoelecrically transduced rotational mode silicon disk resonators with resonant frequencies in the 2.0-8.0MHz range capable of operating in liquid are utilized as direct real-time mass balances. The resonant frequency of the resonators was recorded in real-time while forming monolayers of 6-Mercapto-1-Hexanol (MCH) in aqueous solution. During one hour of exposure to the MCH solution, gradual frequency shift up to ~3600ppm was recorded which shows the capability of such resonators as highly sensitive biomolecular sensors. Detection of target ssDNA sequences has also been demonstrated using the same devices.

Self-controlled fabrication of single-crystalline silicon nanobeams using conventional micromachining

This paper reports on a low-cost top-down approach to the nano-precision fabrication of nanobeams on single-crystalline silicon using only conventional micromachining technology. The fabrication technique takes advantage of the crystalline structure of silicon for controllable feature size reduction of nanobeams with atomically smooth surfaces and sharp edges. Applying a deliberate rotational misalignment in a 2 μm resolution standard lithography process, followed by anisotropic wet etching of the silicon, nanobeams with well uniform widths as small as ∼85 nm are fabricated on thin SOI substrates. As a proof of concept for the incorporation of such nanobeams within electromechancial structures, we successfully demonstrate thermally actuated resonators that show very high frequencies (close to 50 MHz).