Oriented Aluminum Nitride Films Research Articles

Simultaneously realizing reversal of piezoelectric coefficient and enhancement of piezoresponse by chromium-doping in aluminum nitride films

Polarity inversion is an interesting phenomenon in non-centrosymmetric wurtzite-structured aluminum nitride (AlN), which offers an important platform to establish acoustic devices with heteropolar junctions. However, previous studies showed that switching polarity generally resulted in only reversing the piezoelectric coefficient (d33) in AlN films. Here, we discovered that appropriate Cr-doping would not only allow to reverse d33 but also improve piezoelectric response in the c-axis oriented AlN films by co-sputtering dual targets of Cr and Al. Specifically, the d33 was reversed from +2.6 pC/N for the undoped AlN to −2.6 pC/N for the 6.2 at. % Cr-doped CrxAl(1−x)N films. As the Cr-doping ratio increased to 9.3 at. %, d33 was −7.0 pC/N, which was 1.7 times higher than that of the undoped AlN films. Independent PFM phase image measurement offered further evidence of the polarity inversion by comparing the undoped and 9.3 at. % Cr-doped CrxAl(1−x)N films. This work offers a simple doping strategy that allows for simultaneous reversal of piezoelectric coefficient and enhancement of piezoresponse. As a result, it establishes a promising foundation for the design and development of acoustic wave devices featuring heteropolar junctions.

Effect of RF power on AlN film crystallinity in low pressure range using Ar-20%N2 gases by magnetic mirror-type magnetron cathode

Effect of radio frequency (RF) power on aluminum nitride (AlN) film crystallinity has been studied under low gas pressure of 0.1 Pa and low RF power of 3 W with Ar-20%N2 gas by a proposed balanced magnetron cathode with a high plasma production efficiency. The gas pressure and the RF power dependences of AlN film crystallinity show that the c-axis oriented AlN films can be obtained only at 50 W and 100 W in 0.1 Pa. The AlN film crystallinity was not promoted in the low RF power range of <50 W. An Ar-rich sputtering condition at 0.1 Pa induced the Al–Al bonds in the deposited films in a high RF power range of 15 W. The control of the total flux balance between Al and nitrogen atoms at the substrate surface is important to design the optimum sputtering condition at 0.1 Pa.

Growth of aluminum nitride on a silicon nitride substrate for hybrid photonic circuits

Aluminum nitride (AlN) is an emerging material for integrated quantum photonics with its excellent linear and nonlinear optical properties. In particular, its second-order nonlinear susceptibility χ (2) allows single-photon generation. We have grown AlN thin films on silicon nitride (Si3N4) via reactive DC magnetron sputtering. The thin films have been characterized using x-ray diffraction (XRD), optical reflectometry, atomic force microscopy (AFM), and scanning electron microscopy. The crystalline properties of the thin films have been improved by optimizing the nitrogen to argon ratio and the magnetron DC power of the deposition process. XRD measurements confirm the fabrication of high-quality c-axis oriented AlN films with a full width at half maximum of the rocking curves of 3.9° for 300 nm-thick films. AFM measurements reveal a root mean square surface roughness below 1 nm. The AlN deposition on SiN allows us to fabricate hybrid photonic circuits with a new approach that avoids the challenging patterning of AlN.

Realization of preferential (100) oriented AlN thin films on Mo coated Si substrate using reactive RF magnetron sputtering

a-axis (100) oriented Aluminium Nitride (AlN) favours transverse acoustic wave applications. We have been able to realize preferential (100) oriented AlN films on Molybdenum (Mo) coated Si substrate using reactive RF magnetron sputtering. X-ray diffraction and Fourier-transform infrared spectroscopy studies reveal the crystallinity, preferred orientation and bond formation between Al and N2. Al 2p subpeak at 73.8 eV and N 1s subpeak at 396.6 eV from X-ray photoelectron spectroscopy studies confirm the Al-N and N-Al formation in wurtzite hexagonal phase. From the fabricated Mo-AlN-Mo capacitors, the measured dielectric constant and electrical resistivity were (~18) and (~7.5 × 106Ω.m) respectively. The piezoelectric coefficient d33(eff) of AlN film was (~4.5) pmV−1. These excellent properties of preferential (100) AlN/Mo films will be very useful for high frequency resonator and energy harvesting applications.

Low-temperature AlN film deposition using magnetic mirror-type magnetron cathode for low gas pressure operation

Low-temperature aluminum nitride (AlN) depositions were studied under low gas pressure conditions of 0.1 Pa. In order to operate under 0.1 Pa sputter-conditions, the magnetic mirror-type magnetron cathode (M3C) has been developed in the authors' studies. The plasma light-emission distributions generated by the M3C were observed at an input RF power of 5–100 W and an Ar gas pressure of 0.1–0.65 Pa. The M3C can operate effectively with both low gas pressure of 0.1 Pa (Ar) and low RF power of 5 W. The AlN films were deposited using a bare silicon substrate without external substrate heating. The c-axis oriented AlN film was obtained at 0.1 Pa (pure nitrogen), 100 W, and a target-to-substrate distance of 70 mm.

Transverse piezoelectric constant of aluminium nitride films deposited on aluminium substrate by reactive magnetron sputtering

In view of high temperature applications, c-axis oriented aluminium nitride films on aluminium substrate were produced by magnetron sputtering at low pressure (0.3 and 0.5 Pa) and different values of nitrogen concentration. XRD data show the highest intensity of (002) diffraction peak with nitrogen concentration of 0.4, and the peak value decreases when the nitrogen concentration moves away from 0.4. The transverse piezoelectric constant (absolute value) was determined for all conditions, the highest values observed with nitrogen concentration of 0.4 (in agreement with XRD data) and 0.8, with a slight preference for 0.4. These new experimental data and the presence of the two peaks of similar amplitude on the estimated transverse piezoelectric constant are useful information for the identification of good practical operative conditions for AlN films sputtered on aluminium, basic structure for the development of high temperature piezoelectric transducers.

Influence of Process Parameters and Formation of Highly c-Axis Oriented AlN Thin Films on Mo by Reactive Sputtering

It is challenging to grow aluminum nitride (AlN) in c-axis orientation on a metal electrode, primarily due to lattice mismatch and the difference between the coefficients of thermal expansion. In this work, we investigate and optimize the effects of several process parameters such as plasma power, N2 flow concentration, quality of the bottom electrode, and oxygen content. The optimized deposition parameters are necessary for the growth of highly c-axis textured AlN thin film having a thickness of 300 nm on a Mo/SiO2/Si substrate. To this end, c-axis oriented AlN thin films were sputtered on a molybdenum (Mo) electrode at the low substrate temperature (300°C). The quality of the Mo electrode was optimized in terms of the surface roughness, interfacial energy, and full width at half maximum (FWHM) of x-ray diffraction (XRD) peak for facilitating the c-axis growth of the AIN film. Results indicate that the growth of AlN film in (002) orientation on the Mo electrode strongly depends on plasma power, the optimum value of which is found to be 300 W. Also, an extremely low FWHM of AlN (002) peak 0.62° is achieved at ∼ 38% N2 concentration. The oxygen content is also found to be an influential parameter, with a threshold value of 28% by atomic weight, beyond which amorphous growth of the AIN film is observed. The fabricated Mo-AlN-Mo structure has a dielectric constant of 8.89 and a capacitance value of 42 pF measured across the top electrode area of 400 × 400 μm2. The grown c-axis oriented AlN film can be used in applications such as the piezoelectric energy harvester (PEH) and the thin-film bulk acoustic resonator (FBAR).

Indium hexagonal island as seed-layer to boost a-axis orientation of AlN thin films

Highly a-axis oriented aluminum nitride films have been grown on Indium coated (100) Si substrate by DC reactive magnetron sputtering. It is shown that In incorporated layer improve the extent of preferential growth along (100) axis and form dense AlN films with uniform surface and large grains, devoid of micro-cracks. As revealed by SEM cross section images, AlN structure consists of oriented columnar grains perpendicular to the Si surface, while AlN/In structure results in uniformely tilted column. SEM images also revealed the presence of In hexagonal islands persistent throughout the entire growth. Micro -Raman spectroscopy of the surface and the cross section of the AlN/In grown films evidenced their high degree of homogeneity and cristallinity.

Growth assessment of (002)-oriented AlN thin films on Ti bottom electrode deposited on silicon and kapton substrates

In the present work, the sputtering deposition conditions allowing the achievement of (002)-oriented aluminum nitride (AlN) thin films on titanium (Ti) bottom electrode were assessed on both silicon and kapton substrates. The AlN grain orientation was enhanced by tuning the conditions used for the Ti deposition, particularly the radiofrequency (RF) power applied to Ti target and the total pressure in the deposition chamber. Sputtered species energy and their flux towards the substrate were identified as key parameters to interpret Ti structure and morphology on both substrates, and to understand how they drive the AlN crystallization process. The evolution of morphology and structure of single Ti films and of the AlN/Ti system was characterized by X-ray Diffraction, Scanning Electron Microscopy and Transmission Electron Microscopy. Highly oriented AlN films were obtained on both substrates when the underneath Ti layer was mainly (002)-oriented; the presence in the Ti films of grains with (100) orientation produced a degree of misorientation in the following AlN growth, also resulting in a significant change of the surface morphology, on both substrates. The successful deposition of AlN/Ti on kapton substrate represents a promising result for the integration of this material in flexible piezoelectric electronics.

Growing c-axis oriented aluminum nitride films by Plasma-Enhanced Atomic Layer Deposition at low temperatures

The possibility of using plasma enhanced atomic layer deposition method for growing heteroepitaxial oriented AlN films on Si (100) and sapphire (001) substrates at temperatures less than 300°C was investigated. The resulting samples were studied by X-ray diffraction analysis and ellipsometry. It has been shown that, ceteris paribus, AlN films grown on sapphire substrates have higher crystallinity than the samples grown on silicon wafers. With duration of plasma exposure of more than 20s and at a temperature of 300°C synthesized heteroepitaxial film had refractive index equal to 2.03±0.03. The X-ray diffraction scans feature (002) and (004) reflections at 2Θ equal to 35.7° and 75.9°, which are characteristic of hexagonal polytype of AlN. For the best sample, (002) reflection had full width on the half maximum of 162±11″

Growth and Characterization of Polyimide-Supported AlN Films for Flexible Surface Acoustic Wave Devices

Highly c-axis oriented aluminum nitride (AlN) films, which can be used in flexible surface acoustic wave (SAW) devices, were successfully deposited on polyimide (PI) substrates by direct current reactive magnetron sputtering without heating. The sputtering power, film thickness, and deposition pressure were optimized. The characterization studies show that at the optimized conditions, the deposited AlN films are composed of columnar grains, which penetrate through the entire film thickness (~2 μm) and exhibit an excellent (0002) texture with a full width at half maximum value of the rocking curve equal to 2.96°. The film surface is smooth with a root mean square value of roughness of 3.79 nm. SAW prototype devices with a center frequency of about 520 MHz and a phase velocity of Rayleigh wave of about 4160 m/s were successfully fabricated using the AlN/PI composite structure. The obtained results demonstrate that the highly c-axis oriented AlN films with a smooth surface and low stress can be produced on relatively rough, flexible substrates, and this composite structure can be possibly used in flexible SAW devices.

Highly c-axis oriented AlN film grown by unbalanced magnetron reactive sputtering and its electrical properties

Highly c-axis oriented aluminum nitride films have been successfully grown by DC magnetron reactive sputtering with unbalanced magnetic fields at room temperature. It is shown that the application of unbalanced magnetic fields can improve the extent of preferential growth along the c-axis, and form AlN films with denser, larger grains and smaller surface roughness, compared to that grown with balanced magnetic fields. The AlN films prepared under optimized growth conditions have a dielectric constant of approximately 10.5 and a dielectric loss of 0.02 in the frequency range of 103–106 Hz, and a leakage current of <1 × 10−7 A/cm2 at 5 V bias voltage, all the dielectric and insulating properties being superior to the films synthesized with balanced magnetic fields. In this work, the unbalanced magnetic fields are induced by adding an electromagnetic coil on the magnetron cathode.

Effect of sputtering power on surface characteristics and crystal quality of AlN films deposited by pulsed DC reactive sputtering

Aluminum nitride (AlN) is a promising material for use as a substrate for AlGaN‐based ultraviolet LEDs and piezoelectric devices. Among the sputtering methods, pulsed DC sputtering provides a superior growth rate and dense AlN films because of the promotion of two‐dimensional growth. In this study, the effect of sputtering power on surface characteristics, crystal quality, and residual stress of AlN films deposited by pulsed DC reactive sputtering on a nitrided sapphire substrate was investigated. The homoepitaxial growth of AlN films on this substrate gives high crystal quality compared with the use of other substrates such as sapphire or Si for heteroepitaxial growth. Surface damage on the AlN films increased with an increase in sputtering power, leading to a rough surface. At all sputtering powers, c‐axis oriented AlN films were obtained in this study. From the difference in lattice constant compared with the AlN bulk, the inplane compressive stress of the AlN sputtered films was confirmed as residual stress. Residual stress in the AlN films was mostly caused by a peening effect rather than a thermal expansion coefficient difference.

Synthesis and characterization of 10 nm thick piezoelectric AlN films with high c-axis orientation for miniaturized nanoelectromechanical devices

The scaling of piezoelectric nanoelectromechanical systems (NEMS) is challenged by the synthesis of ultrathin and high quality piezoelectric films on very thin electrodes. We report the synthesis and characterization of the thinnest piezoelectric aluminum nitride (AlN) films (10 nm) ever deposited on ultrathin platinum layers (2–5 nm) using reactive sputtering. X-ray diffraction, high-resolution transmission electron microscopy, and fast Fourier transform analyses confirmed the proper crystal orientation, fine columnar texture, and the continuous lattice structure within individual grains in the deposited AlN nanometer thick films. The average extracted d31 piezoelectric coefficient for the synthesized films is −1.73 pC/N, which is comparable to the reported values for micron thick and highly c-axis oriented AlN films. The 10 nm AlN films were employed to demonstrate two different types of optimized piezoelectric nanoactuators. The unimorph actuators exhibit vertical displacements as large as 1.1 μm at 0.7 V for 25 μm long and 30 nm thick beams. These results have a great potential to realize miniaturized NEMS relays with extremely low voltage, high frequency resonators, and ultrasensitive sensors.

Pyroelectric aluminum nitride micro electromechanical systems infrared sensor with wavelength-selective infrared absorber

This paper describes a micro electromechanical systems type wavelength-selective pyroelectric sensor, with highly c-axis oriented Aluminum nitride film as the pyroelectric material. Wavelength-selective infrared absorption is realized via periodic structures of holes patterned into the top metal electrode that also collects pyroelectric charge signal. The periodic hole array results in optical absorption resonances whose wavelength is determined by the hole pitch, demonstrated experimentally using a Fourier transform infrared spectrometer and numerically calculated using the finite difference time domain method. A significant difference in infrared absorption between patterned and unpatterned detectors is demonstrated through optical experiments comparing the pyroelectric responses.

Growth of AlN Films on Silicon Substrates by Radio Frequency Magnetron Sputtering

Polycrystalline aluminum nitride (AlN) films were deposited on Si (111) substrates by radio frequency (RF) magnetron sputtering method in an nitrogen (N2) + argon (Ar) gas mixture. The effect of the preparation conditions- sputtering pressure (p), sputtering power (w), gas mixture (Ar/N2) and post-deposition annealing treatment -on the properties of AlN films were investigated by means of X-ray diffraction (XRD). Highly c-axis oriented AlN films were obtained with optimized growth parameters: p=0.3Pa, w=400w and Ar/N2=2.

Growth of Highly C-Axis Oriented AlN Films at Water Cooling Condition

In this work, the aluminum nitride (AlN) thin film with highly c-axis orientation was prepared successfully at water cooling condition by RF sputtering. The influence of water cooling on the crystalline quality of AlN thin film is researched. The crystalline characteristics and microstructure of AlN thin films deposited on Si (111) and Mo/Si (111) were researched by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). The results indicated that highly c-axis AlN films can be synthetized at water cooling condition. The AlN film deposited on Mo thin film is titled to the surface, when that is perpendicular to the silicon substrate. Different models are proposed to explain the growth behaviors of AlN thin films on the two kinds of substrates.

Influence of Sputtering Pressure for Radio Frequency Magnetron Sputtering (103) Oriented AlN Films on (100) Silicon Substrate

The thin film of (103)-oriented aluminum nitride (AlN) is an attractive piezoelectric material for the applications in film bulk acoustic wave resonator (FBAR) devices. Due to the bulk acoustic wave (BAW) properties of (103) oriented AlN films, it can excite a quasi-shear mode (velocity = 5,957 m/s, K2 = 3.8%) that can be used for FBAR liquid sensor and even loss less than the FBAR device with (002) oriented AlN films. In this research, the (103) oriented AlN films were successfully deposited onto (100) silicon substrate by radio frequency (RF) magnetron sputtering. Different sputtering pressures (1m torr, 3m torr, 5m torr, and 7m torr) were discussed in this experiment process. Comparisons were made on their crystalline structures with X-ray diffraction (XRD) and the surface morphologies was investigated by the atomic force microscopy (AFM). The result exhibited the optimal sputtering pressure is 5m torr. The optimal (103) oriented AlN films have the strongest XRD intensity, the smallest full width at half maximum (FWHM) value (0.6°), the largest grain size (15.78nm) and the smooth surface roughness (Ra=3.259nm).

Investigation of thin-film ultrasonic transducers for high-temperature applications

Ultrasonic transducers based on aluminium nitride (AlN) piezoelectric thin films have been proposed for high-temperature (&gt;400°C) defect monitoring of industrial plants and components. To form the transducers, c-axis oriented AlN films, 6 μm thick, were grown on carbon steel substrates by RF sputter deposition. If the films are deposited directly onto components, then the problems of high-temperature couplant layers can be avoided. Two approaches were taken to testing the devices. In the first method, the devices were heated on a hot plate up to 400-500°C and tested by pulse-echo measurements at high temperature and on returning to room temperature. It was shown that AlN thin-film transducers could maintain their performance in pulse-echo mode at temperatures approaching 500°C. A deterioration in performance below 500°C was attributed mainly to degradation of the top electrode. In the second method, film and substrate were heated in an oven to 600°C and subsequently characterised by X-ray diffraction and scanning electron microscopy to identify any structural change in the films and to investigate the transducer failure mode. These tests showed no significant change in the AlN film after heating. The high temperature durability of the transducers, however, was limited by heat-induced surface oxidation of the carbon steel substrate at the elevated temperatures, which led to decoupling of the film from the substrate.

Deposition and characterization of c-axis oriented aluminum nitride films by radio frequency magnetron sputtering without external substrate heating

Aluminum nitride (AlN) films were deposited on a variety of substrates (glass, Si, oxidized Si, Al–SiO 2–Si, Cr–SiO 2–Si, and Au–Cr–SiO 2–Si) by radio frequency (RF) magnetron sputtering using an AlN target. The films were deposited without external substrate heating. The effect of RF power, ambient gas (Ar and Ar–N 2) and sputtering pressure on deposition rate and crystallinity were investigated. The structure and morphology of the films were studied by X-ray diffraction, scanning electron microscopy and atomic force microscopy techniques. These investigations revealed that the AlN films prepared in mixed gas ambient (Ar–N 2) were highly c-axis oriented with moderate surface roughness on all the substrate. A strong IR absorption band was observed around 670 cm − 1 which confirms the presence of Al–N bond in the film. The dc resistivity of the films was measured to be in the range of 10 11 to 10 12 Ω-cm at moderate electric fields. The application of these films in piezoelectric based micro-electro-mechanical systems is discussed.