An extended structure zone diagram is proposed that includes energetic deposition, characterized by a large flux of ions typical for deposition by filtered cathodic arcs and high power impulse magnetron sputtering. The axes are comprised of a generalized homologous temperature, the normalized kinetic energy flux, and the net film thickness, which can be negative due to ion etching. It is stressed that the number of primary physical parameters affecting growth by far exceeds the number of available axes in such a diagram and therefore it can only provide an approximate and simplified illustration of the growth condition?structure relationships.

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A structure zone diagram including plasma based deposition and ion etching - eScholarship

With the rapid commercialization of fifth generation (5G) technology in the world, the market demand for radio frequency (RF) filters continues to grow Acoustic wave technology has been attracting great attention as one of the effective solutions for achieving high-performance RF filter operations while offering low cost and small device size Compared with surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators have more potential in fabricating high- quality RF filters because of their lower insertion loss and better selectivity in the middle and high frequency bands above 25 GHz Here, we provide a comprehensive review about BAW resonator researches, including materials, structure designs, and characteristics The basic principles and details of recently proposed BAW resonators are carefully investigated The materials of poly-crystalline aluminum nitride (AlN), single crystal AlN, doped AlN, and electrode are also analyzed and compared Common approaches to enhance the performance of BAW resonators, suppression of spurious mode, low temperature sensitivity, and tuning ability are introduced with discussions and suggestions for further improvement Finally, by looking into the challenges of high frequency, wide bandwidth, miniaturization, and high power level, we provide clues to specific materials, structure designs, and RF integration technologies for BAW resonators

https://www.mdpi.com/2072-666X/11/7/630/pdf

Materials, Design, and Characteristics of Bulk Acoustic Wave Resonator: A Review

AlN piezoelectric thin films for energy harvesting and acoustic devices

The ferroelectricity of (Al1−xScx)N (x = 0–0.34) thin films with various thicknesses was investigated. (Al1−xScx)N films were prepared at 400 °C on (111)Pt/TiOx/SiO2/(001)Si substrates by the radio frequency dual-source reactive magnetron sputtering method using Al and Sc targets under pure N2 gas or a mixture of N2 and Ar gases. The film deposited under N2 gas showed larger remanent polarization than those under N2 + Ar mixture. Ferroelectricity was observed for films with x = 0.10–0.34 for about 140-nm-thick films deposited under N2 gas. The x = 0.22 films showed ferroelectricity down to 48 nm in thickness from the polarization–electric field curves and the positive-up-negative-down measurement. The ferroelectricity of the 9 nm-thick film also was ascertained from scanning nonlinear dielectric microscopy measurement. These results reveal that ferroelectric polarization can switch for films with much broader compositions and thicknesses than those in the previous study.

Effects of deposition conditions on the ferroelectric properties of (Al1−xScx)N thin films

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

Al x Sc 1 − x N films were deposited by reactive pulse magnetron co-sputtering from aluminum and scandium targets without additional substrate heating at deposition rates between 100 and 150 nm/min. With increasing incorporation of scandium into the hexagonal wurtzite structure, the piezoelectric properties are drastically improved. The piezoelectric charge coefficient d 33 is increased from 8.4 pC/N for AlN up to 23.6 pC/N for Al x Sc 1 − x N with 33% scandium. Between 35 and 43% scandium content a relative broad maximum with high piezoelectric coefficients between 26.9 and 27.3 pC/N was detected. A further increase of scandium concentration above 50% results in the formation of the cubic and centrosymmetric rock salt structure and therefore the complete loss of piezoelectric properties. By FE-SEM, XRD and TEM investigations it was shown that up to up to 43% scandium concentration the wurtzite structure becomes more and more disordered and the c/a ratio is decreased from 1.6 to 1.27. Nevertheless, no aluminum or scandium segregation could be detected by high resolution TEM investigations. The Young's modulus of the wurtzite phase is reduced with increasing scandium concentration from 340 GPa to 185 GPa. The drastic improvement of the piezoelectric properties can be explained by weakening of the chemical bonding and by high distortion of the wurtzite structure.

Effect of scandium content on structure and piezoelectric properties of AlScN films deposited by reactive pulse magnetron sputtering

This paper reports on the deposition of AlN and AlXSc1źXN films by pulse magnetron sputtering. The first part will focus on the AlXSc1źXN deposition process in comparison to the already established AlN process. The effect of doping AlN with Sc regarding piezoelectric and mechanical properties is presented. The films show the expected increase of piezoelectric properties as well as the softening of the material with higher Sc concentrations. Above a threshold concentration of around 40 % Sc in the AlXSc1źXN films, there exists a separation into two phases, an Al-rich and a Sc-rich wurtzite phase, which is shown by XRD. At Sc concentrations higher than 50 %, the films are not piezoelectric, as the films are composed primarily of the cubic ScN phase. The second main part of this paper evaluates the films for application in energy harvesting. Especially the Sc doping allows a significant increase in the energy generated in our test setup. Directly measuring the AC voltage at resonance depending on load resistance with base excitation of ±2.5 µm, 350 µW power have been generated under optimum conditions compared to 70 µW for pure AlN. For a more application oriented measuring setup, a standard and a SSHI-based ("Synchronised Switch Harvesting on Inductor") AC/DC converter circuit have been tested. The SSHI interface showed a significant improvement to 180 % compared to the standard interface.

Magnetron sputtering of piezoelectric AlN and AlScN thin films and their use in energy harvesting applications

This paper reports on the deposition and characterization of piezoelectric AlN and AlXSc1-XN layers. Characterization methods include XRD, SEM, active thermo probe, pulse echo, and piezometer measurements. A special focus is on the characterization of AlN regarding the mechanical stress in the films. The stress in the films changed between -2.2 GPa (compressive) and 0.2 GPa (tensile) and showed a significant dependence on film thickness. The cause of this behavior is presumed to be the different mean grain sizes at different film thicknesses, with bigger mean grain sizes at higher thicknesses. Other influences on film stress such as the sputter pressure or the pulse mode are presented. The deposition of gradient layers using those influences allowed the adjustment of film stress while retaining the piezoelectric properties.

Sputter deposition of stress-controlled piezoelectric AlN and AlScN films for ultrasonic and energy harvesting applications.

A new method of pulsed powering the magnetron discharge using a pulsed switching of the anode has been developed. Practically, this hybrid pulse mode is a combination of the conventional unipolar and bipolar pulsed powering, where the time slices of both pulse modes can be freely adjusted at a time scale smaller than 1 ms, i.e. much shorter than necessary for the deposition of one monolayer. This allows varying the average plasma parameters freely between the typical values of unipolar and bipolar pulse mode. During deposition of piezoelectric AlN, the film stress could be shifted in a wide range by changing the pulse mode ratio. In combination with the other known process parameters, it was possible to shift the film stress between compressive and tensile while maintaining piezoelectric properties. Hence, in addition to classical deposition parameters such as pressure or temperature, this new parameter gives an additional degree of freedom for optimization of film properties independent from sputtering power and deposition rate.

Adjustment of plasma properties in magnetron sputtering by pulsed powering in unipolar/bipolar hybrid pulse mode

Aluminum nitride (AlN) is a piezoelectric material often used as thin film in SAW/BAW devices. Furthermore, there is an increasing interest in its use for energy harvesting applications. Despite it has a relatively low piezoelectric coefficient, it is a suitable choice for energy harvesting applications and due to its low dielectric constant and good mechanical properties. In addition, it is a lead-free material. The films were deposited by reactive pulsed magnetron sputtering using the Double Ring Magnetron DRM 400. This sputter source together with suitable powering and process control allows depositing piezoelectric AlN very homogeneously on 8” substrates with deposition rates of up to 200 nm/min. With the developed technology, film thicknesses of several ten microns are technically and economically feasible. Moreover, by adjusting process parameters accordingly, it is possible to tune properties, like film stress, to application specific requirements. Additionally, it is known that the doping of AlN with Scandium results in a significantly increased piezoelectric coefficient. The influence of process parameters and Sc concentration on film properties were determined by piezometer, pulse echo, SEM, XRD, EDS and nanoindentation measurements. Energy harvesting measurements were done using an electromechanical shaker system for the excitation of defined vibrations and a laservibrometer for determination of the displacement of the samples. The generated power was measured as function of electric load at resonance. An rms power of up to 140μW using AlN films and of 350μW using AlScN films was generated on Si test pieces of 8x80mm2. Furthermore, energy harvesting measurements using manually bended steel strips of 75x25mm2 coated with AlScN were carried out as well. When using only a single actuation, energy of up to 8μJ could be measured. By letting the system vibrate freely, the damped vibration at resonance 50Hz resulted in a measured energy of 420μJ.

Stephan Barth

Papers

Effect of scandium content on structure and piezoelectric properties of AlScN films deposited by reactive pulse magnetron sputtering

Magnetron sputtering of piezoelectric AlN and AlScN thin films and their use in energy harvesting applications

Sputter deposition of stress-controlled piezoelectric AlN and AlScN films for ultrasonic and energy harvesting applications.

Adjustment of plasma properties in magnetron sputtering by pulsed powering in unipolar/bipolar hybrid pulse mode

Energy harvesting based on piezoelectric AlN and AlScN thin films deposited by high rate sputtering