https://www.repository.cam.ac.uk/bitstream/1810/264501/1/Fu_et_al-2017-Progress_in_Material_Science-VoR.pdf

Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

This paper discusses piezoelectric acoustic devices based on widely used piezoelectric materials. Commonly used piezoelectric thin film deposition techniques and the influence of process parameters on the growth, texture and orientation of the films are discussed. Etching techniques are also outlined. A comparative study of different devices developed previously is given. Also, applications of developed devices in aero-acoustic and medical fields have been briefly discussed. Flow charts of various techniques of deposition along with a combined one for full acoustic device fabrication are given. Various techniques, frequently used for thin film characterization have been discussed. The testing and measurement techniques to determine the responses of acoustic devices such as sensitivity, resonance frequency, frequency response, piezoelectric co-efficient etc. have been briefly illustrated. This paper discusses common failure modes with respect to the field of use of acoustic devices. It also concisely discusses various reliability tests done in industries to assess the quality of the developed devices. This review has also suggested directions for future development of thin film acoustic sensors.

Piezoelectric MEMS based acoustic sensors: A review

Sputter erosion of materials is among the most important techniques for fabricating advanced thin film coatings. Sputter processes are also of considerable relevance for surface polishing down to an atomic scale, nano-structuring of surfaces as dot and ripple patterns and micro-machining of materials using focused ion beams or reactive ion etching. We present a new, versatile sputter technique utilizing the steady state coverage of a substrate surface with up to 1016 cm−2 of foreign or self atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These surfactant atoms (surface active agents) strongly modify the substrate sputter yield on atomic to macroscopic length scales. Depending on the surfactant–substrate combination, the novel technique allows enhanced smoothing of surfaces, the generation of novel surface patterns and nanostructures and the controlled shaping of surfaces on the nanometer scale. We present selected examples of surface morphology evolution, smoothing of surfaces and shaping of surfaces to demonstrate the capabilities of the new surfactant sputtering technique.

/pdf/surfactant-sputtering-3fk2318iku.pdf

Surfactant sputtering

Aluminum nitride (AlN) thin films with c-axis preferred orientation have been prepared by reactive direct-current (DC) magnetron sputtering. The degree of preferred crystal orientation, the cross-sectional structure, and the surface morphology of AlN thin films grown on Si (100) substrates at various substrate temperatures from 60°C to 520°C have been investigated by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Results show that the substrate temperature has a significant effect on the structural properties, such as the degree of c-axis preferred orientation, the full-width at half-maximum (FWHM) of the rocking curve, the surface morphology, and the cross-sectional structure as well as the deposition rate of the AlN thin films. The optimal substrate temperature is 430°C, with corresponding root-mean-square surface roughness (R
 rms) of 1.97 nm, FWHM of AlN (002) diffraction of 2.259°, and deposition rate of 20.86 nm/min. The mechanisms behind these phenomena are discussed. Finally, film bulk acoustic resonators based on AlN films were fabricated; the corresponding typical electromechanical coupling coefficient (k
 t
 2
 ) is 5.1% with series and parallel frequencies of 2.37 GHz and 2.42 GHz, respectively.

Influence of Substrate Temperature on Structural Properties and Deposition Rate of AlN Thin Film Deposited by Reactive Magnetron Sputtering

The structural and optical properties of aluminum nitride (AlN) films deposited on glass and silicon substrates by single ion beam sputtering technique have been investigated. The X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) study revealed the formation of the amorphous phase of AlN. The optical characteristics of films, such as refractive index, extinction coefficient, and average thickness, were calculated by Swanepoel's method using transmittance measurements. The refractive index and average roughness values of the films increased with film thickness. Moreover, it was found that thickness augmentation leads to a decrease in optical band gap energy calculated using Tauc's relation.

https://iopscience.iop.org/article/10.1143/JJAP.49.095802/pdf

Optical Properties of Amorphous AlN Thin Films on Glass and Silicon Substrates Grown by Single Ion Beam Sputtering

In this experiment, a radio frequency dual ion beam sputtering (DIBS) system was used to prepare aluminum nitride (AlN) films with a bottom Al electrode on a Si (100) substrate. After systematic testing of the processing variables, a high-quality film with preferred c-axis orientation was grown successfully on the Si (100) substrate with an Al target under 700 eV energy flux, N2/(N2 + Ar) ratio of 55%, and 4 × 10−4 torr in vacuum. The characteristics of the deposited AlN thin films were studied by x-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), secondary ion mass spectrometry (SIMS), and electronic spectroscopy for chemical analysis (ESCA). The surface roughness was also measured. It was found that AlN films prepared by DIBS at room temperature are better than those prepared at 300°C, and those prepared with an Al target are better than those prepared with an AlN target. The inferiority of AlN films prepared with AlN targets is due to the AlN bond being broken down by the ion beam source.

M. F. Wu

Papers

Deposition Characteristics of AlN Thin Film Prepared by the Dual Ion Beam Sputtering System