Girish Rughoobur

Room temperature sputtering of inclined c-axis ZnO for shear mode solidly mounted resonators

ZnO films with a c-axis significantly inclined away from the surface normal were grown by a remote plasma sputtering technique at room temperature. The films were used to make solidly mounted resonators (SMRs) operating in shear mode at a resonant frequency of 1.35 GHz. Control of the ZnO microstructure was achieved using a polycrystalline AlN seed layer which can be added on top of a sputtered acoustic mirror to give a complete SMR device. The ZnO was reactively sputtered in an atmosphere of argon and oxygen from a zinc target. The c-axis of the ZnO was estimated to be at an angle of ∼45° to the surface normal. SMRs were measured to have quality factors (Q) of up to 140 and effective electromechanical coupling coefficients of up to 2.2% in air. Although an inclined c-axis can be achieved with direct growth onto the acoustic mirror, it is shown that the AlN seed layer provides higher coupling coefficients and narrower inclination angular distribution. The responses of the devices in liquids of different v...

Design and modelling of solidly mounted resonators for low-cost particle sensing

This work presents the design and fabrication of Solidly Mounted Resonator (SMR) devices for the detection of particulate matter (PM2.5 and PM10) in order to develop a smart low-cost particle sensor for air quality. These devices were designed to operate at a resonant frequency of either 870 MHz or 1.5 GHz, employing zinc oxide as the piezoelectric layer and an acoustic mirror made from molybdenum and silicon dioxide layers. Finite element analysis of the acoustic resonators was performed using COMSOL Multiphysics software in order to evaluate the frequency response of the devices and the performance of the acoustic mirror. The zinc oxide based acoustic resonators were fabricated on a silicon substrate using a five mask process. The mass sensitivity of the acoustic resonators was estimated using a 3D finite element model and preliminary testing has been performed. The theoretical and observed mass sensitivity were similar at ca. 145 kHz ng−1 for the 870 MHz resonator when detecting PM2.5 suggesting that SMR devices have potential to be used as part of a miniature smart sensor system for airborne particle detection.

Gravimetric sensors operating at 1.1 GHz based on inclined c-axis ZnO grown on textured Al electrodes.

Shear mode solidly mounted resonators (SMRs) are fabricated using an inclined c-axis ZnO grown on a rough Al electrode. The roughness of the Al surface is controlled by changing the substrate temperature during the deposition process to promote the growth of inclined ZnO microcrystals. The optimum substrate temperature to obtain homogeneously inclined c-axis grains in ZnO films is achieved by depositing Al at 100 °C with a surface roughness ~9.2 nm, which caused an inclination angle of ~25° of the ZnO c-axis with respect to the surface normal. Shear mode devices with quality-factors at resonance, Qr and effective electromechanical coupling factors,

Fabrication of nanostructured transmissive optical devices on ITO-glass with UV1116 photoresist using high-energy electron beam lithography

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Nanostructured plasmonic metapixels

keff2, as high as 180 and 3.4% are respectively measured. Mass sensitivities, Sm of (4.9 ± 0.1) kHz · cm2/ng and temperature coefficient of frequency (TCF) of ~−67 ppm/K are obtained using this shear mode. The performance of the devices as viscosity sensors and biosensors is demonstrated by determining the frequency shifts of water-ethanol mixtures and detection of Rabbit immunoglobin G (IgG) whole molecule (H&L) respectively.

Transparent thin film bulk acoustic wave resonators

For many applications, compensation of the temperature coefficient of the resonant frequency in solidly mounted resonators is needed. The influence of the different materials in such a complex structure upon this coefficient is not easily understandable. The material characteristics of each layer influence this coefficient but the relative position of each layer into the whole stack with respect to the piezoelectric material (AlN) can be even more influential. Study of this situation shows that the first two layers of the stacked reflector are most influential in the temperature coefficient. Several kinds of resonators with asymmetry (with layers of non-λ/4 thicknesses) are tested obtaining resonators with nearly zero temperature coefficient of the frequency and with optimized response.

A wide-range frequency tunable SMR-CMOS oscillator for gas sensing

High-energy electron beam lithography for patterning nanostructures on insulating substrates can be challenging. For high resolution, conventional resists require large exposure doses and for reasonable throughput, using typical beam currents leads to charge dissipation problems. Here, we use UV1116 photoresist (Dow Chemical Company), designed for photolithographic technologies, with a relatively low area dose at a standard operating current (80 kV, 40-50 μC cm-2, 1 nAs-1) to pattern over large areas on commercially coated ITO-glass cover slips. The minimum linewidth fabricated was ∼33 nm with 80 nm spacing; for isolated structures, ∼45 nm structural width with 50 nm separation. Due to the low beam dose, and nA current, throughput is high. This work highlights the use of UV1116 photoresist as an alternative to conventional e-beam resists on insulating substrates. To evaluate suitability, we fabricate a range of transmissive optical devices, that could find application for customized wire-grid polarisers and spectral filters for imaging, which operate based on the excitation of surface plasmon polaritons in nanosized geometries, with arrays encompassing areas ∼0.25 cm2.

Investigation of polymer deposition techniques on a Solidly Mounted Resonator arrays for vapour sensing

Annealing of cuprous oxide (Cu2O) thin films in vacuum without phase conversion for subsequent inclusion as the channel layer in p-type thin film transistors (TFTs) has been demonstrated. This is based on a systematic study of vacuum annealing effects on the sputtered p-type Cu2O as well as the performance of TFTs on the basis of the crystallographic, optical, and electrical characteristics. It was previously believed that high-temperature annealing of Cu2O thin films would lead to phase conversion. In this work, it was observed that an increase in vacuum annealing temperature leads to an improvement in film crystallinity and a reduction in band tail states based on the X-ray diffraction patterns and a reduction in the Urbach tail, respectively. This gave rise to a considerable increase in the Hall mobility from 0.14 cm2/V·s of an as-deposited film to 28 cm2/V·s. It was also observed that intrinsic carrier density reduces significantly from 1.8 × 1016 to 1.7 × 1013 cm−3 as annealing temperature increases....