Oriented Aluminum Nitride Research Articles

A strong dependence of sputtering power on c‐axis oriented aluminium nitride on Si (111): A structural and electrical study

AbstractGrowing and controlling the c‐axis orientation of the aluminium nitride (AlN) thin film on unheated Si (111) substrates using reactive magnetron sputtering are challenging. Sputtering parameters such as nitrogen concentration and sputtering power were effectively tuned to grow the c‐axis oriented AlN thin film. The results show that a low concentration of (25%) N2 is enough for forming AlN at a reduced flow rate, whereas a higher flow rate requires a higher concentration of N2. Low concentration with a low flow rate is preferred to grow AlN at low temperature and power. The poor crystallinity of AlN with (100) orientation was improved by varying the power from 75 to 175 W. The X‐ray diffraction results confirmed the improvisation of crystallinity of grown AlN films and indicated the strong dependence of preferred c‐axis orientation on sputtering power. Also, the dependence of sputtering power on microstrain and stress was analysed. The surface morphology study by field emission scanning electron microscopy and topography measured by an atomic force microscope shows a dependence on sputtering power. The high c‐axis orientation was observed at 175 W with low surface roughness, low leakage current density (2 × 10−9 A/cm2) and low dielectric constant (6.8).

Structural and electrical correlation in aluminum nitride thin films grown by plasma enhanced atomic layer deposition as interface insulating layers on silicon carbide (4H-SiC)

(0001) oriented aluminum nitride (AlN) thin films have been grown by plasma enhanced atomic layer deposition (PE-ALD) on silicon carbide (4H-SiC) substrates. During different PE-ALD processes, the ammonia (NH3) plasma pulsing time has been varied and its effect on the microstructure and on the orientation degree of the AlN layers has been monitored. Structural characterization by Transmission Electron Microscopy (TEM) showed that the crystalline structure of the deposited films was strongly dependent on the NH3-plasma pulsing, so that different polymorphic structures were observed. In particular, both processes resulted in wurtzite AlN structure for few nanometers at the interface with the 4H-SiC substrate, while upon increasing thickness a poly-crystalline wurtzite phase was obtained by short-pulse NH3-plasma, whereas longer plasma exposure resulted in a mixture of wurtzite and zincblende defective phases. Phase formation mechanism were discussed and electrical nanoscopic characterization by conductive atomic force microscopy showed a clear correlation between the different AlN crystalline phases and the insulating properties.

Characterization of Structure, Morphology, Optical and Electrical Properties of AlN–Al–V Multilayer Thin Films Fabricated by Reactive DC Magnetron Sputtering

Composite thin films of the AlN–Al–V type, grown by magnetron sputtering, were analyzed by several complementary diagnostic methods. The power of the magnetron was used as a variable parameter, while gas flows, chamber pressure, and substrate temperature remained unchanged during the film fabrication. According to grazing incidence X-ray diffraction (GIXRD) results, in most cases, it was possible to obtain an (002)-oriented aluminum nitride (AlN) layer in the films, although, with an increase in the magnetron power to 800 W, the formation of X-ray amorphous AlN was observed. Similarly, according to the Raman results, the width of the peak of the vibrational mode E1, which characterizes the correlation length of optical phonons, also significantly increased in the case of the sample obtained at 800 W, which may indicate a deterioration in the crystallinity of the film. A study of the surface morphology by atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed that the AlN film grows in the form of vertically oriented hexagons, and crystallites emerge on the surface in the form of dendritic structures. During the analysis of the AFM roughness power spectral density (PSD-x) functions, it was found that the type of substrate material does not significantly affect the surface roughness of the AlN films. According to the energy–dispersive X-ray spectroscopy (SEM-EDS) elemental analysis, an excess of aluminum was observed in all fabricated samples. The study of the current-voltage characteristics of the films showed that the resistance of aluminum nitride layers in such composites correlates with both the aluminum content and the structural imperfection of crystallites.

Pulsed direct-current reactive sputtering of high Young's modulus [002] oriented aluminum nitride thin films

This work reports a process-parameter optimization of [002] oriented high Young's modulus aluminum nitride (AlN) thin films deposited by pulsed direct current (DC) reactive sputtering method. Properties of deposited AlN films are mainly determined by the material's microstructure and morphology, which can be modulated by the process parameters. Commonly explored process parameters of reactive sputtering of AlN are substrate temperature, process pressure, gas composition, substrate type, etc. Process pressure and nitrogen concentration have the most significant influence on the topography and morphology of the film, deposited by the pulsed DC reactive sputtering method. DC pulse parameters such as pulse frequency and pulse duty cycle also influence the properties of deposited AlN films, which have been addressed in this work. The pulse frequency of 100 kHz and a duty cycle of 80% was found to be the most suitable pulse parameters. This paper also reports the voltage and current behavior and deposition rate dependence under varying process parameters and pulsed properties. Optimization of process parameters is commonly done by characterizing the deposited AlN films’ crystallinity, surface roughness, and residual stress. This work addresses the influences of process and pulse parameters on Young's modulus besides other characteristic properties of the film. The AlN films deposited with the optimized method and pulse parameters yield Young's modulus of up to 335 GPa while dropping the surface roughness to 1.2 nm.

Electrical Properties of Aluminum Nitride Thick Films Magnetron Sputtered on Aluminum Substrates.

The realization of a c-axis oriented aluminum nitride thick film on aluminum substrates is a promising step in the development of transducers for applications with a working temperature up to about 600 °C. The present paper deals with the realization of AlN thick films by means of reactive magnetron sputtering with a pulsed DC power supply, operating in continuous mode for 50 h. Two values (0.4 and 0.8) of nitrogen concentration were used; operative pressure and power were set at 0.3 Pa and 150 W, respectively. The thickness of the obtained aluminum nitride films on the aluminum substrate, assessed with a profilometer, varied from 20 to 30 µm. The preferential orientation of AlN crystals was verified by X-ray diffraction. Finally, as the main focus of the investigation, the films underwent electrical characterization by means of an LCR-meter used on a parallel plate capacitor set-up and a test system based on a cantilever beam configuration. AlN conductivity and ε33 permittivity were derived in the 100 Hz–300 kHz frequency range. Magnetron sputtering operation with nitrogen concentration equal to 0.4 resulted in the preferred operative condition, leading to a d31 piezoelectric coefficient, in magnitude, of 0.52 × 10−12 C/N.

Enhancement of c-Axis Oriented Aluminum Nitride Films via Low Temperature DC Sputtering

In this study, we successfully deposit c-axis oriented aluminum nitride (AlN) piezoelectric films at low temperature (100 °C) via the DC sputtering method with tilt gun. The X-ray diffraction (XRD) observations prove that the deposited films have a c-axis preferred orientation. Effective d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">33</sub> value of the proposed films is 5.92 pm/V, which is better than most of the reported data using DC sputtering or other processing methods. It is found that the gun placed at 25° helped the films to rearrange at low temperature and c-axis orientation AlN films were successfully grown at 100 °C. This temperature is much lower than the reported growing temperature. It means the piezoelectric films can be deposited at flexible substrate and the photoresist can be stable at this temperature. The cantilever beam type microelectromechanical systems (MEMS) piezoelectric accelerometers are then fabricated based on the proposed AlN films with a lift-off process. The results show that the responsivity of the proposed devices is 8.12 mV/g, and the resonance frequency is 460 Hz, which indicates they can be used for machine tools.

Structural and mechanical properties of a-axis AlN thin films growth using reactive RF magnetron sputtering plasma

Purpose This paper aims to report the influence of sputtering plasma deposition time on the structural and mechanical properties of the a-axis oriented aluminium nitride (AlN) thin films. Design/methodology/approach The AlN films were prepared using RF magnetron sputtering plasma on a silicon substrate without any external heating with various deposition times. The films were characterized using X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), atomic force microscope (AFM) and nanoindentation techniques. Findings The XRD results show that the AlN thin films are highly oriented along the (100) AlN plane at various deposition times indicating the a-axis preferred orientation. All the AlN thin films exhibit hexagonal AlN with a wurtzite structure. The hardness and Young’s modulus of AlN thin films with various deposition times were measured using a nanoindenter. The measured hardness of the AlN films on Si was in the range of 14.1 to 14.7 GPa. The surface roughness and the grain size measured using the AFM revealed that both are dependent on the deposition times. Originality/value The novelty of this work lies with a comparison of hardness and Young’s modulus result obtained at different sputtering deposition temperature. This study also provides the relation of AlN thin films’ crystallinity with the hardness of the deposited films.

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.

Oxidation and Hot Corrosion Type I of Alternative Bond Coats for SiC/SiC Ceramic Matrix Composites

With the goal of increasing the combustion temperature and decreasing the weight of todays aero engines, the application of SiC/SiC ceramic matrix composites is widely investigated and first parts are in service. Besides the many advantages of SiC/SiC ceramic matrix composites, some challenges are encountered and have to be overcome to ensure a reliable operation. One of these challenges is the protection of the ceramic matrix composites from oxidation and volatilization. The oxidation of silicon carbide results in the formation of silicon dioxide, which is generally considered protective. The combustion atmosphere, however, contains water vapor and other contaminations. The water vapor results in the formation of volatile silicon hydroxide, while contaminations, like CMAS or Na2SO4, can result in spallation or an increase in oxidation rates. Also, the fiber matrix interface coatings are more prone to oxidation and need to be protected to avoid loss in mechanical properties. Therefore, a coatings system, consisting of an outer environmental barrier coating and an inner bond coat, is applied to SiC/SiC ceramic matrix composite when used in turbine engines.Within this study alternative bond coats are developed mainly based on aluminum oxide formers, which are in itself less prone to volatilization at the designated temperature of 1200°C. The coating process is realized via chemical vapor deposition, which can coat complex parts cost effectively and without the need for a line-of-sight. The coatings provide a reservoir of the oxide forming species and therefore the oxide scale is able to heal upon crack formation or spallation. One of the developed coatings is based on an oriented aluminum nitride layer, which is in contrast to bulk AlN resistant to high temperature oxidation in humid air. The influence of the orientation and the chlorine content, which is present due to the coating procedure, on the increased oxidation resistance is investigated. Cyclic exposures are done to investigated spallation and cracking behavior due to mismatches in thermal expansion. Further, the resistance to hot corrosion type I is compared to other materials used or investigated for the hottest sections of turbine engines. The microstructure of the coatings was examined before and after the exposures using X-ray diffraction, scanning electron microscopy and electron beam microanalysis.

Effect of growth and residual stress in AlN (0002) thin films on MEMS accelerometer design

This paper discusses the growth and evolution of residual stresses in (0002) preferentially oriented aluminum nitride (AlN) layers on Si (111) wafers by sputtering technique for the development of micro-electro-mechanical system (MEMS) accelerometer. The microstructure of the deposited films exhibited vertical columnar structures. Residual stresses in the sputtered AlN films are − 1.2 GPa, − 0.8 GPa, and − 0.25 GPa for the film thickness of 600 nm, 750 nm, and 900 nm respectively. The effect of the residual stress on the piezoelectric MEMS acceleration sensor structure is analyzed. The presence of residual stress reduced the resonant frequency (up to 14.72%) and bandwidth (up to 27.3%) of the accelerometer. The locations of the von-Mises stress maxima are shifted from the beam edges (adjoining to the proof-mass) to the whole proof-mass area. The normalized stress sensitivity (which defines the piezoelectric charge sensitivity) is reduced by two orders due to the residual stress in the AlN layers.

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.

The dependence of aluminum nitride thin-film microstructure on the number of low-temperature plasma-enhanced atomic layer deposition process cycles

Within the framework of the study, oriented aluminum nitride coatings were obtained on sapphire with orientation along the direction (0001) using the PEALD method at a temperature of 250°С and a number of deposition cycles from 80 to 500. The obtained samples were studied using ellipsometry, X-ray diffraction, and atomic force microscopy. It was shown that with an increase in the thickness of the coating, the refraction index increases to 2.02, and the root-mean-square (RMS) increases from 0.35 to 2.35 nm. All samples had (0002) and (0004) reflexes on X-ray diffractograms measured in the Bragg-Brentano geometry (θ/2θ-scan). On Δω-scans of the reflex (0002), the full width at half maximum (FWHM) amounted to 162 ± 7 arcsec.

Well-defined (0001)-oriented aluminum nitride polycrystalline films on amorphous glass substrates deposited by ion plating with direct-current arc discharge

500-nm-thick aluminum nitride (AlN) polycrystalline films were deposited on amorphous glass substrates at a substrate temperature of 200 °C by ion plating (IP) with direct-current (DC) arc discharge, radio frequency (RF) and DC reactive magnetron sputtering (MS). The IP technique enables a very high deposition rate of 113 nm/min compared with that of MS techniques. The structural properties of AlN films were investigated by X-ray diffraction (XRD), Raman spectroscopy, and electron probe micro analyzer (EPMA) measurements. XRD results show that AlN films with a columnar grain structure deposited by IP have a texture with a well-defined (0001) orientation between grains normal to the substrate, whereas AlN films grown by MS have a mixture of (0001) planes and planes with other orientations. Raman spectroscopy and XRD measurement results revealed high residual compressive stress in AlN films deposited by IP compared with that in AlN films grown by MS. The analysis of the data obtained by EPMA measurement showed that IP produces AlN films with a small amount of remaining oxygen species, whereas MS produces AlN films with a large amount of residual oxygen species.

Influence of pressure on the properties of AlN deposited by DC reactive magnetron sputtering on Si (100) substrate

Preferred (002)-oriented aluminium nitride (AlN) films by direct-current (DC) magnetron sputtering on Si (100) substrate were prepared at various deposition pressure (0.12-0.5 Pa). The influence of pressure on the surface morphology, crystal structure and optical properties of AlN thin films was discussed the with the methods of X-ray diffraction, atomic force microscopy and stress analyser, respectively. Optical properties were studied by ellipsometer and Fourier transform infrared spectrometer. It was observed that the AlN films tended to be preferred (002)-oriented when the pressure was decreased and the value of full width at half maximum is 0.325° when the pressure was kept at 0.12 Pa. the tensile stress, refractive index, and the density of Al-N bonds decreased with increasing the sputtering pressure from 0.12 to 0.5 Pa. This work demonstrated the influence of sputtering pressure on the structural and optical properties of the AlN films deposited on Si (100) substrate, the gallium nitride epitaxial film on Si (100) substrate was expected to be realised using sputtered AlN as the buffer layer.

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).

Electrical and structural characteristics of sputtered c-oriented AlN thin films on Si (100) and Si (110) substrates

This paper discussed about the morphological and electrical properties of c-axis oriented aluminium nitride (AlN) thin films grown on Si (100) and Si (110) substrates by direct current (DC) reactive sputtering technique. Both the films showed intense (002) peak with the texture coefficient (γ) of 3.1 and 2.8 corresponding to Si (100) and (110) substrates. The AlN/Si (110) have grain sizes (20–30 nm) compared to the relatively denser grains (30–40 nm) of the film on Si (100) substrate. Both the films exhibited slanted columnar structures with slanting angle of 15° (Si (100) substrate) and 20° (Si (110) substrate). The dielectric constant values of the films at low frequency are found to be 7.4 and 8.8 corresponding to the Si (100) and Si (110) substrates respectively. The variation in dielectric constant values over the low frequency regime seems to be due to the presence of trapped charges present in the AlN layer and AlN‑silicon interface (interface trap density~1012 cm-2 eV−1). The flat band voltages are found to be 0.85 V and − 0.35 V corresponding to the Si (100) and (110) substrates respectively. The AlN/Si (100) film showed poor leakage current density ~ 10−4 A/cm−2 as compared to AlN/Si (110) (~ 10−5 A/cm−2). The AlN films exhibited dielectric breakdown field of 4.4 MV/cm and 6.6 MV/cm corresponding to the Si (100) and Si (110) substrates respectively.

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.

Long-wave infrared selective pyroelectric detector using plasmonic near-perfect absorbers and highly oriented aluminum nitride

Pyroelectric detectors are desirable for infrared sensing because they operate at room temperature and are easier to fabricate than traditional detectors such as mercury-cadmium-telluride. Unfortunately, they cannot detect specific wavelengths or wavelength bands. Herein, a pyroelectric detector for selective detection in the long-wavelength infrared region will be presented. Pyroelectric aluminum nitride forms a dielectric layer separating a gold plasmonic hole array from a refractory metal, titanium nitride. Infrared radiation incident on the hole array forms a gap plasmon mode whose electric field is confined to the pyroelectric spacer layer. Relaxation of the mode heats the pyroelectric spacer layer, producing a pyroelectric voltage. Using this technique, 18.9 dB of spectral selectivity at λ=9 μm was achieved with a peak responsivity of 1.08 V/W.

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″