Anthony S. Holland

A novel Love-mode device based on a ZnO/ST-cut quartz crystal structure for sensing applications

A novel Love-mode surface acoustic wave (SAW) sensor with ZnO guiding layer and 90° rotated ST-cut quartz crystal substrate will be presented. Analysis of this device with different thicknesses of ZnO films was undertaken. It will be shown that this novel structure offers distinct advantages over previously fabricated Love-mode structures. These advantages include significantly high sensitivity, small temperature coefficient of frequency and high electromechanical coupling coefficient.

Enhanced tunability of magnetron sputtered Ba0.5Sr0.5TiO3 thin films on c-plane sapphire substrates

Thin films of Ba0.5Sr0.5TiO3 (BST) were deposited on c-plane (0001) sapphire by rf magnetron sputtering and investigated by complementary materials analysis methods. Microwave properties of the films, including tunability and Q factor were measured from 1to20GHz by patterning interdigital capacitors (IDCs) on the film surface. The tunability is correlated with texture, strain, and grain size in the deposited films. An enhanced capacitance tunability of 56% at a bias field of 200kV∕cm and total device Q of more than 15 (up to 20GHz) were achieved following postdeposition annealing at 900°C.

Measurement of high piezoelectric response of strontium-doped lead zirconate titanate thin films using a nanoindenter

Strontium-doped lead zirconate titanate (PSZT) is reported to have a high piezoelectric coefficient (d33) in the range of 200−600 pm∕V, when in the form of ceramic disks or pellets. This article reports piezoelectric response results for PSZT thin films deposited by rf magnetron sputtering on gold-coated silicon substrates. The compositions of the deposited thin films have been found to be uniform with depth, using secondary ion mass spectroscopy. The surfaces of the deposited thin films have been studied using an atomic force microscope and observed to be regular and nanostructured in nature. The piezoelectric response of the thin films, using the inverse piezoelectric effect, has been measured using a nanoindenter. Values of thin film d33 up to 608 pm∕V were obtained, which is much higher than previously reported values of d33 for any thin film. The high values can be attributed to optimized deposition conditions and the low stress measured for the thin film arrangement on the substrate. The technique h...

A varactor tuned branch-line hybrid coupler

This paper introduces a novel branch-line 90/spl deg/ hybrid coupler incorporating varactor diodes which allow tuning of the frequency response. A design covering the DCS, PCS and IMT2000 cellular frequency bands (1710-2170 MHz) is presented. Given a varactor tunability of 2.5:1, simulations suggest 20 dB return loss and 3/spl plusmn/1 dB coupling is achievable across each transmit and receive sub-band by tuning the varactors. These results offer an improvement over a conventional single-section branch-line hybrid centred at 1950 MHz, and some miniaturisation is also achieved due to the capacitive loading. A prototype is constructed using commercially available varactor diodes, and reasonable agreement between the measured and simulated results is achieved.

The effect of post-deposition cooling rate on the orientation of piezoelectric (Pb0.92Sr0.08)(Zr0.65Ti0.35)O3thin films deposited by RF magnetron sputtering

Strontium-doped lead zirconate titanate (PSZT) has been reported for its high piezoelectric and ferroelectric properties. The deposition of piezoelectric thin films (from a few nanometres to a few microns) is an essential part of microsystem devices for sensing and actuating. For PSZT to exhibit pronounced piezoelectric behaviour it must have a crystalline grain structure (perovskite orientation). This paper is a study of the deposition of PSZT thin films by RF magnetron sputtering and the effect of cooling rate, after deposition at temperatures between 500 °C and 700 °C. X-ray diffraction (XRD) results are used to show how a cooling rate of 5 °C min−1 increases the degree of perovskite orientation in sputtered films, when compared to a cooling rate of 15 °C min−1. X-ray photoelectron spectroscopy and energy dispersive x-ray analyses were used to determine the composition of the thin films. Results from deposition on silicon membranes and the position of diffraction peak patterns in XRD results are used to demonstrate low stress in the deposited films. Atomic force microscope imaging is used to show the crystalline nature of the PSZT thin films.

In situ micro-Raman analysis and X-ray diffraction of nickel silicide thin films on silicon.

This article reports on the in situ analysis of nickel silicide (NiSi) thin films formed by thermal processing of nickel thin films deposited on silicon substrates. The in situ techniques employed for this study include micro-Raman spectroscopy (microRS) and X-ray diffraction (XRD); in both cases the variations for temperatures up to 350 degrees C has been studied. Nickel silicide thin films formed by vacuum annealing of nickel on silicon were used as a reference for these measurements. In situ analysis was carried out on nickel thin films on silicon, while the samples were heated from room temperature to 350 degrees C. Data was gathered at regular temperature intervals and other specific points of interest (such as 250 degrees C, where the reaction between nickel and silicon to form Ni(2)Si is expected). The transformations from the metallic state, through the intermediate reaction states, until the desired metal-silicon reaction product is attained, are discussed. The evolution of nickel silicide from the nickel film can be observed from both the microRS and XRD in situ studies. Variations in the evolution of silicide from metal for different silicon substrates are discussed, and these include (100) n-type, (100) p-type, and (110) p-type silicon substrates.

Polycrystalline Ba0.6Sr0.4TiO3 thin films on r-plane sapphire : Effect of film thickness on strain and dielectric properties

Polycrystalline Ba0.6Sr0.4TiO3 (BST) films grown on r-plane sapphire exhibit strong variation of in-plane strain over the thickness range of 25–400nm. At a critical thickness of ∼200nm, the films are strain relieved; in thinner films, the strain is tensile, while compressive strain was observed in the 400nm film. Microwave properties of the films were measured from 1to20GHz by the interdigital capacitor method. A capacitance tunability of 64% was observed in the 200nm film, while thinner films showed improved Q factor. These results demonstrate the possibility of incorporating frequency agile BST-based devices into the silicon on sapphire process.

Ion beam etching of CVD diamond film in Ar, Ar/O2 and Ar/CF4 gas mixtures

The Ar + ion beam etching of diamond in the presence of O 2 or CF 4 gases was examined as a function of ion energy and gas composition. The details of etch rate and the structural features of the etched surfaces have been interpreted in terms of the mechanism of etching. In all cases, the etch rate has shown a square root dependence on ion energy, E 1/2 i . The etching of the diamond film in Ar/O 2 gases was characterised by a moderate etch rate (12-20 nm/min) dependent on the percentage of O 2 and has been attributed to ion-enhanced chemical etching. In comparison, the etching with Ar + ions or Ar + in the presence of CF 4 has shown a low etch rate (8-10 nm/min) which was insensitive to variation in CF 4 percentage indicative of sputter etching.

Electrical characterization and hydrogen gas sensing properties of a n-ZnO∕p-SiC Pt-gate metal semiconductor field effect transistor

A new hydrogen gas sensitive n-ZnO∕p-SiC Pt-gate metal semiconductor field effect transistor (MESFET) is reported. The observed current-voltage curves for the source to drain region indicate that this MESFET operates in enhancement mode. A change in gate potential, due to different ambient atmospheres caused a change in the width of the depletion region, hence modulating the current in the n channel (ZnO layer). The H2 gas sensing mechanism of the presented MESFET structure is discussed using energy band diagrams.

Comparison between conductometric and layered surface acoustic wave hydrogen gas sensors

A comparison between the performance of conductometric and layered surface acoustic wave (SAW) hydrogen sensors is presented. Both sensor structures employ an RF magnetron sputtered tungsten trioxide (WO3) thin film as a selective layer for hydrogen (H2) sensing applications. The conductometric device is based on an alumina substrate, while the layered SAW device structure is fabricated on a 36° Y-cut, X-propagating LiTaO3 substrate with a zinc oxide (ZnO) guiding layer. The sensors were investigated for different operational temperatures and concentrations of H2 ranging between 0.06 and 1% in synthetic air. Approximately 1.4 µA current and 27 kHz frequency shifts were observed towards 1% H2 in air for the conductometric and layer SAW device, respectively.