C-axis 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.

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.

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).

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.

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.

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.

Influence of the magnetron on the growth of aluminum nitride thin films deposited by reactive sputtering

Aluminum nitride (AlN) thin films deposited on high-vacuum systems without substrate heating generally exhibit a poor degree of c-axis orientation. This is due to the nonequilibrium conditions existing between the energy of the sputtered particles and the energy at the substrate surface. The application of substrate bias or substrate temperature is known to improve the adatom mobility by delivering energy to the substrate; both are hence well-established crystal growth promoting factors. It is well known that low sputtering pressures can be used as a parameter improving the growth of highly c-axis oriented aluminum nitride films at room temperature even without applying bias voltage to the substrate. Generally, the use of high pressures implies thermalization of particles within the gas phase and is considered to increase the energy gap between these and the substrate surface. However, in later experiments we have learned that the use of high processing pressures does not necessarily implies a detriment of crystallographic orientation in the films. By measuring (for the first time to the author’s knowledge) the full width at half maximum value of the rocking curve of the 0002-AlN peak at several positions along the 100mm diameter (100)-silicon wafers on which aluminum nitride thin films were deposited by reactive sputtering, a new effect was observed. Under certain processing conditions, the growth of the AlN thin films is influenced by the target magnetron. More precisely, their degree of c-axis orientation varies at wafer areas locally coincident under the target magnetron. This effect should be considered, especially where large area substrates are employed such as in silicon wafer foundry manufacturing processes.

Temperature dependence of piezoelectric properties of sputtered AlN on silicon substrate

We report for the first time the temperature dependence of the piezoelectric coefficient d 33 of the aluminum nitride (AlN) film measured at temperatures up to 300 °C. A highly c-axis oriented AlN film was successfully deposited on a polycrystalline silicon/silicon dioxide/silicon wafer by a reactive sputtering technique to form a piezoelectric uni-morph actuator. A scanning laser Doppler vibrometer was used to measure the picometer level displacement of the actuator at 20 kHz at different temperatures. The piezoelectric coefficient d 33 has a constant value at temperatures ranging between 20 °C and 300 °C.

Temperature Dependence of Piezoelectric Properties of Sputtered Aluminum Nitride on Silicon Substrate

We report for the first time the temperature dependence of the piezoelectric coefficient d33 of the aluminum nitride (AlN) film measured at temperatures up to 300 °C. A highly c-axis oriented AlN film was successfully deposited on a polycrystalline silicon / silicon dioxide / silicon wafer by a reactive sputtering technique to form a piezoelectric uni-morph actuator. A scanning laser Doppler vibrometer was used to measure the picometer level displacement of the actuator at 20 kHz at different temperatures. The piezoelectric coefficient d33 has a constant value of 5 pC/N at temperatures ranging between 20 °C and 300 °C.

Silicon Bulk Micromachined High Q Film Bulk Acoustic Resonator Devices with Mo/AlN/Mo Structures

ABSTRACTFully integrated film bulk acoustic resonators and band pass filters using a bulk micromachining method have been designed, fabricated, and characterized. Aluminum nitride (AlN) has been used for a piezoelectric material and Molybdenum (Mo) was employed as top and bottom electrodes. Also, Ti seed layers were used in order to obtain highly c-axis oriented AlN piezoelectric films. The film bulk acoustic resonators fabricated with highly c-axis oriented AlN films showed good performances such as an effective mechanical coupling coefficient of 7.5% and a quality factor of 1530. The band pass filter consisted of seven film bulk acoustic resonators with a high quality factor exhibited a transmission loss of −3.2 dB, a ripple of 1.2 dB in pass band, and attenuation values of −38 dB and −49 dB at both band boundaries, respectively.

High velocity SAW using aluminum nitride film on unpolished nucleation side of free-standing CVD diamond.

High performances surface acoustic wave (SAW) filters based on aluminium nitride (AlN)/diamond layered structure have been fabricated. The C-axis oriented aluminum nitride films with various thicknesses were sputtered on unpolished nucleation side of free-standing polycrystalline chemical vapor deposition (CVD) diamond obtained by silicon substrate etching. Experimental results show that high order modes as well as Rayleigh waves are excited. Experimental results are in good agreement with the theoretical dispersion curves determined by software simulation with Green's function formalism. We demonstrate that high phase velocity first mode wave (so-called Sezawa wave) with high electromechanical coupling coefficient are obtained on AlN/diamond structure. This structure also has a low temperature coefficient of frequency (TCF), and preliminary results suggest that a zero TCF could be expected.

C-Axis oriented AlN films prepared on diamond film substrate by electron cyclotron resonance plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition of aluminum nitride (AlN) films was performed on diamond film substrates using electron cyclotron resonance which have an attractive potential for application such as surface acoustic wave devices operating at high frequency. N 2 and AlCl 3 were used as reactant precursors. The dependences of the film morphology, film orientation, deposition rate on the diamond crystal orientation, substrate temperature and microwave power were investigated by scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction. c-Axis oriented pure AlN films have been achieved.

Growth of c -axis oriented aluminum nitride films on GaAs substrates by reactive rf magnetron sputtering

c-axis oriented aluminum nitride (AlN) films deposited on GaAs substrates by reactive rf magnetron sputtering were fabricated. The structural and morphological characterizations were found to be sensitive to the deposition conditions including rf power, N2 concentration, sputtering pressure and substrate temperature. A highly c-axis oriented AlN film was identified as low as 350 °C by x-ray diffraction. The crystallization of the AlN film with (002) preferred orientation tended to improve with increased rf power and N2 concentration within the experimental range. A densely pebble-like surface texture of the c-axis oriented AlN films with an average grain size of about 80 nm was observed by scanning electron microscopy (SEM). The film showed a high degree of alignment of the columnar structure as was indicated by the cross-sectional SEM photograph. The selected area diffraction pattern showed that the deposited films exhibited a polycrystalline microstructure.