Enrico Mastropaolo

Electrothermally Actuated and Piezoelectrically Sensed Silicon Carbide Tunable MEMS Resonator

In this letter, we present the design, fabrication, and electrical testing of a silicon carbide microelectromechanical (MEMS) resonant device with electrothermal actuation and piezoelectric sensing. A doubly clamped flexural-mode beam resonator made of cubic silicon carbide has been fabricated with a top platinum electrothermal actuator and a top lead zirconium titanate piezoelectric sensor. Electrothermal transduction has been used to drive the device into resonance and tune its frequency. Piezoelectric transduction has been used as resonance sensing technique. Electrical measurements have shown that, by increasing the dc bias of the actuating voltage from 1 to 7 V, a tuning range of 171 kHz can be achieved with a device resonating at 1.766 MHz.

Dry etch release processes for micromachining applications

The authors report on the comparative study of two dry etch processes for polysilicon sacrificial layer release using vapor phase xenon difluoride (XeF2) continuous etching and inductively coupled plasma (ICP) etching with sulfur hexafluoride (SF6) gas. Test structures of 0.5μm thick polysilicon have been patterned and etch channels varying in widths from 1to500μm have been fabricated successfully for the purpose of comparison. The influence of etch pressure, aperture opening size, and ICP etch power on the undercut etching rate as well as selectivity between mask and substrate have been studied. It has been possible to achieve an undercut etch rate of up to 11.6μm∕min under a pressure of 3Torr in XeF2 etch gas, while for SF6 plasma, an undercut etch rate of 2.56μm∕min at 65mTorr is obtained. Moreover, the optimized process has been employed for the fabrication of silicon carbide (SiC) resonators.

Electrothermally Actuated Silicon Carbide Tunable MEMS Resonators

This paper presents the fabrication and characterization of silicon carbide (SiC) flexural-mode structures able to operate as electrothermomechanical tunable resonators. Single- and double-clamped beams, as well as circular structures, have been fabricated with aluminium and platinum (Pt) top electrodes. Electrothermal excitation has been used for device actuation and resonant-frequency tuning and mixing. Circular structures (i.e., disks) have been shown to possess higher resonant frequencies and Q factors (up to ~23 000) compared to beams having similar dimensions. The tuning of the resonant frequency has been performed by varying the dc and ac components of the actuating voltage on SiC beams with u-shaped and slab Pt electrodes. When increasing the dc bias, the frequency-shift rates of about -11 000 and -1100 ppm/V are measured for the u-shaped and slab electrodes, respectively. When increasing the amplitude of the ac input, shift rates of about -1800 and -800 ppm/V are measured. In addition, measurements have shown that the frequency-shift rate increases with the ambient temperature. Electrothermal mixing has been performed by applying two actuating voltages with the sum or difference of their frequencies matching the fundamental resonance of the SiC structure. The tuning of the electrothermally mixed output signal has been demonstrated on a disk resonator.

Electrothermal actuation studies on silicon carbide resonators

The electromechanical behavior of SiC clamped-clamped beam (bridge) resonators with u-shaped aluminium (Al) electrodes on top has been studied as a function of electrode length, width, and spacing. Negative and positive deflections have been observed, indicating a complex interplay exhibited by the combined single material and bimorph characteristics of the resonator structures. It has been found that, both experimentally and theoretically, devices with electrodes applied on the root of the beam have similar or higher displacement amplitudes compared to devices with electrodes covering the half or the entire beam. Moreover, the displacement and vibration amplitudes can be maximized by increasing the electrode width and/or decreasing the spacing.

Low frequency tantalum electromechanical systems for biomimetical applications

The integration of p-channel metal-oxide-semiconductor transistors and tantalum bridge structures for the fabrication of resonant gate transistors (RGTs) that operate in the audible frequency range has been developed. Resonant gate transistors with channel length of 15 μm and clamped-clamped tantalum bridges of 0.5 mm to 1.6 mm in length have been fabricated. The measured first modal frequency of the bridges has been found to be higher than the expected theoretical value. From the experimental and theoretical analysis of the first three modes, the stress in the bridges has been extracted and found to be tensile with values of 3 MPa – 10 MPa. Finite element simulation has validated the extracted stress and the mode shapes of the tantalum bridges. The modulation of conductance in the channel region between the source and drain by the tantalum bridge of the RGT has been demonstrated. The threshold voltage and transconductance of the fabricated p-channel RGT have been measured to be −37 V and 6.84 μS, respect...

Piezoelectrically driven silicon carbide resonators

Silicon carbide cantilever beam resonators have been designed with top electrodes made of piezoelectric lead zirconium titanate (PZT). The devices have been simulated, fabricated, and tested. Piezoelectric actuation has been performed by applying an alternating actuation voltage to the PZT electrodes, thus inducing vertical displacements. The devices have been fabricated with a beam length of 150 and 200 μm, and driven into resonance at frequencies in the kilohertz range. The devices’ resonance has been detected by monitoring the impedance of the actuating electrode. Simulations and measurements have shown that the electrode length on top of the beam influences the magnitude of the deflection and the resonant frequency of the devices. Furthermore, the electrical feedthrough capacitance presented by the piezoelectric electrode has been observed to strongly influence the output impedance of the resonators. The obtained results show the importance of the electrode design for the optimization of the performan...

Microelectromechanical systems for biomimetical applications

An etch release process capable of releasing long resonant gate transistor bridges from a sacrificial layer has been studied as a step towards developing a system to mimic the cochlear mechanism inside the human ear. The developed etch release process involves the use of a gentle etch tool that is capable of a clean and damage-free etch release. The influence of temperature and oxygen/nitrogen gas flow rates on the undercut etch rates and the profiles of photoresist and polyimide sacrificial layers have been investigated. An array of aluminum bridges of length 0.278–1.618 mm, which cover the frequencies from 1 to 33.86 kHz, has been designed and released from a sacrificial layer. The resonating beams have been measured.

Electrothermal actuation of silicon carbide ring resonators

Silicon carbide (SiC) ring resonators have been designed, simulated, and fabricated in order to achieve higher resonant frequency compared to beam resonators. The resonant frequency as a function of the ring radius and central hole radius, as well as the influence of the electrode design on the actuation efficiency have been investigated. Aluminum (Al) electrodes have been fabricated on top of the structures in order to study the electrothermal actuation of the structures. The bimorph Al/SiC ring resonators have been constructed by etching the SiC in inductively coupled plasma. The release of the Si sacrificial layer has been performed with a XeF2 chemical etching. The radial release and area release have been characterized as a function of the central hole dimension at chamber pressure of 1 and 2 Torr, whereby the release rates have been found to increase as the hole dimensions and the etching pressure increases. In addition, the release process has shown to be governed by aperture effects. The rings fabricated with different dimensions have been actuated mechanically and electrothermally, and the resonant frequency detected optically. The resonant frequency has been shown to increase as the ring radius decreases and the hole radius increases, both theoretically and experimentally.

Design of a spike event coded RGT microphone for neuromorphic auditory systems

This paper presents the design of a spike event coded resonant gate transistor microphone system for neuromorphic auditory applications. The microphone system employs an array of resonant gate transistors (RGT) to transduce acoustic input directly into bandpass filtered analog outputs. The bandpass filtered analog outputs are encoded as spike time events by a spike event coder and are then transmitted asynchronously by using the Address Event Representation (AER) protocol. The microphone system is designed to receive external inputs in the spike time domain to actively control the RGT response, a feature not present in other MEMS microphone systems implemented so far. System level simulations showing the response of the RGT sensor model and its spike event coded response are presented.

A Power-Efficient Capacitive Read-Out Circuit With Parasitic-Cancellation for MEMS Cochlea Sensors

This paper proposes a solution for signal read-out in the MEMS cochlea sensors that have very small sensing capacitance and do not have differential sensing structures. The key challenge in such sensors is the significant signal degradation caused by the parasitic capacitance at the MEMS-CMOS interface. Therefore, a novel capacitive read-out circuit with parasitic-cancellation mechanism is developed; the equivalent input capacitance of the circuit is negative and can be adjusted to cancel the parasitic capacitance. Chip results prove that the use of parasitic-cancellation is able to increase the sensor sensitivity by 35 dB without consuming any extra power. In general, the circuit follows a low-degradation low-amplification approach which is more power-efficient than the traditional high-degradation high-amplification approach; it employs parasitic-cancellation to reduce the signal degradation and therefore a lower gain is required in the amplification stage. Besides, the chopper-stabilization technique is employed to effectively reduce the low-frequency circuit noise and DC offsets. As a result of these design considerations, the prototype chip demonstrates the capability of converting a 7.5 fF capacitance change of a 1-Volt-biased 0.5 pF capacitive sensor pair into a 0.745 V signal-conditioned output at the cost of only 165.2 μW power consumption.