Alireza Ramezany

Amplitude modulated Lorentz force MEMS magnetometer with picotesla sensitivity

This paper demonstrates ultra-high sensitivities for a Lorentz force resonant MEMS magnetometer enabled by internal-thermal piezoresistive vibration amplification. A detailed model of the magneto-thermo-electro-mechanical internal amplification is described and is in good agreement with the experimental results. Internal amplification factors up to ~1620 times have been demonstrated by artificially boosting the effective quality factor of the resonator from 680 to 1.14 × 106 by tuning the bias current. The increase in the resonator bias current in addition to the improvement in the quality factor of the device led to a sensitivity enhancement by ~2400 times. For a bias current of 7.245 mA, where the effective quality factor of the device and consequently the sensitivity is maximum (2.107 mV nT−1), the noise floor is measured to be as low as 2.8 pT (√Hz)−1. This is by far the most sensitive Lorentz force MEMS magnetometer demonstrated to date.

Ultrahigh Frequency Nanomechanical Piezoresistive Amplifiers for Direct Channel-Selective Receiver Front-Ends

Channel-selective filtering and amplification in ultrahigh frequency (UHF) receiver front-ends are crucial for realization of cognitive radio systems and the future of wireless communication. In the past decade, there have been significant advances in the performance of microscale electromechanical resonant devices. However, such devices have not yet been able to meet the requirements for direct channel selection at RF. They also occupy a relatively large area on the chip making implementation of large arrays to cover several frequency bands challenging. On the other hand, electromechanical piezoresistive resonant devices are active devices that have recently shown the possibility of simultaneous signal amplification and channel-select filtering at lower frequencies. It has been theoretically predicted that if scaled down into the nanoscale, they can operate in the UHF range with a very low power consumption. Here, for the first time nanomechanical piezoresistive amplifiers with active element dimensions as small as 50 nm × 200 nm are demonstrated. With a device area of less than 1.5 μm2 a piezoresistive amplifier operating at 730 MHz shows effective quality factor ( Q) of 89,000 for a 50Ω load and gains as high as 10 dB and Q of 330,000 for a 250Ω load while consuming 189 μW of power. On the basis of the measurement results, it is shown that for piezoresistor dimensions of 30 nm × 100 nm it is possible to get a similar performance at 2.4 GHz with device footprint of less than 0.2 μm2.

Micromachined Frequency-Output Force Probes

This letter presents a new class of microelectromechanical systems-based frequency output force and displacement probes with sub-nanometer displacement resolution. The force probe is a single layer monolithic crystalline silicon micro-structure comprised of a thermal-piezoresistive resonator embedded within a micro-cantilever. Deflection of the cantilever due to the applied force mechanically distorts the resonator modulating its resonance frequency. Such devices can be used as atomic force microscope probes or high-resolution surface profilometers with fully electrical operation eliminating the bulky and complex optical detectors typically used in such systems. A prototype sensor, operating at 7.5MHz, shows a displacement sensitivity of 2.5 Hz/nm. With a typical Allen deviation of 0.1-0.2 ppm for similar resonators operated as self-sustained oscillators, frequency resolution in the order of 1 Hz is expected to be achievable. This translates to 0.4 nm of displacement and 11 nN of force resolution for such sensors.

Sail-shaped piezoelectric micro-resonators for high resolution gas flowmetry

This work presents sail-shaped thin film aluminum nitride resonators operating as high resolution gas flow meters. Deformation of the sail-like structure of the resonator due to the gas flow changes the effective stiffness and consequently the resonant frequency of the resonator. For a 10.2 MHz resonator engaged in a simple oscillator configuration Allan deviation as small as 10-8 (df/f0) was achieved for measurement periods less than one minute. Sensitivity of frequency to flow velocity was measured to be 0.5Hz/mm/s leading to minimum detectable velocity of 0.2mm/s. In addition, transient behavior of the sensor was investigated showing a rise time of approximately 20 ms. The presented sensors with frequency modulated output are much less susceptible to noise compared to the conventional sensors with amplitude modulated output. Furthermore, the output of such sensors can be directly fed into a digital readout/control system without the need for analog to digital conversion.

An analog micro-electromechanical XOR

This work presents a passive resonant micro-electromechanical phase detector (PD). The PD structure is comprised of two thin film piezoelectric on silicon (TPoS) resonator plates coupled by a silicon beam in the middle. The device exhibits a width extensional resonance mode in which resonant plates vibrate out of phase. When actuated in this mode, the interference of acoustic waves generated from both plates determines the consequent displacement amplitude, and therefore the output signal. Such structure operates as an analog exclusive-OR gate, in which input signals with an identical phase lead to a destructive interference of acoustic waves and therefore a minimum output signal. Out-of-phase input signals (constructive interference), on the other hand, results in the maximum output voltage. Phase sensitivity as high as 1.3 mV/Deg was observed with minimum detectable phase error of 0.006° associated with 8μVrms output voltage standard deviation. Such PD can potentially be used in a MEMS-based analog phase locked loop (PLL). Noise reduction due to the pass-band filtering inherent in the operation of this PD, along with its minimal power consumption are the benefits of the resonant PD over its electronic counterparts.

MEMS resonant sensors for real-time thin film shear stress monitoring

This paper presents MEMS resonant stress sensors for real-time monitoring of induced shear stress during formation of a thin film on a micro-diaphragm. The device is comprised of a thin silicon membrane coupled to a thermal-piezoresistive resonator. Membrane deflection due to the stress resulting from deposition or growth of a thin film deforms the resonator, thus changing its resonance frequency. Such stress sensors can be used in different media without perturbing the resonator operation due to the isolation provided by the membrane. Response of the fabricated devices due to thermal stress induced in 400nm thick Ni and Al layers deposited on the membranes was measured demonstrating sensitivities as high as 1.7 Hz/Pa. Real-time characterization of the fabricated sensors was also performed demonstrating a sensitivity of ~1000ppm/MPa while etching a 2μm SiO2 layer underneath the membrane over the time. A finite element Simulation and a mathematical modeling were employed to calculate the stress in the center of the membrane resulting from different thicknesses of SiO2 layer. The modeling suggests the experimental frequency shift of 7kHz corresponds to the stress of ~7×106 N/m2 caused by formation of 2μm thick oxide layer underneath the membrane.

Frequency modulated electrostatically coupled resonators for sensing applications

This work presents a proof-of-concept for a frequency modulated output MEMS sensor based on electrostatic coupling of two resonators. Change in the gap between a single resonator and a movable capacitive electrode can modulate the electrostatic force acting on the resonator, and therefore, alter its resonance frequency. A second resonator is used as the movable electrode to facilitate displacement amplification in response to the external force at resonance. In this work, clamped-clamped beams are used for both resonators. Three devices are fabricated with different dimensions, and a DC magnetic field is used to actuate the second beam at resonance frequency, while the frequency of the first beam is monitored. It is shown that by actuating the movable electrode at its resonance, sensitivity is enhanced by more than 3 orders of magnitude compared to DC actuation, and sensitivity of 8Hz/mAmT is achieved for a device with 100μm × 5μm and 2μm × 3 μm beams and a capacitive gap of 1.5μm. While Lorentz force is used to actuate the beam in this work, the concept is not limited to magnetic field sensing.

SNR improvement in amplitude modulated resonant MEMS sensors via thermal-piezoresistive internal amplification

Effect of thermal-piezoresistive internal amplification on signal to noise ratio (SNR) of amplitude modulated resonant MEMS sensors (e.g. vibratory gyroscopes and Lorentz force magnetometers) has been studied in this work showing the possibility to significantly improve the detection limit. It has been shown that as the thermal-piezoresistive amplification sets in, noise rms value increases with a slower rate than the boost in vibration amplitude and output signal level, therefore the SNR increases. In addition to higher sensitivity due to internal amplification in such devices, improvement in SNR reduces the minimum detectable signal in presence of limiting Brownian and thermal noises. Preliminary measurement results show that increasing the DC bias current, which leads to a 3X increase in vibration amplitude, improves the SNR by a factor of 4.5 (6.6 dB).

An ultra high-Q micromechanical in-plane tuning fork

This work presents a micromechanical in-plane tuning fork with internal vibration amplification ability that can be used as an ultra-sensitive MEMS gyroscope. The structure is designed to incorporate a thermal-piezoresistive energy pump for one of the vibration modes leading to significant amplification of vibration amplitude with the same actuation force. Up to 473X vibration amplitude enhancement has been demonstrated leading to a measured effective quality factor (Q) of 10.4×106 for the vibration mode with intrinsic mechanical Q of 22,097. This was achieved by applying a bias current of only 1.607mA to the piezoresistor embedded within the structure. The internal thermal-piezoresistive amplification is caused by coupling of mechanical strain and Joules heating in the piezoresistor biased with a DC current. Measured effective Q values as a function of bias current agree with the theoretical predication. Theoretically there is no upper limit on the achievable effective Q provided that the bias current can be controlled with adequate precision.