Oriented Aluminum Nitride Thin Films Research Articles

Effect of silicon-based substrates and deposition type on sputtered AlN thin films: Physical & chemical properties and suitability for piezoelectric device integration

The synergistic effect of silicon-based substrates on the physical and chemical properties of aluminum nitride (AlN) thin films was investigated. AlN thin films were deposited by RF magnetron sputtering on Low Resistivity (LR) Si, Silicon On Insulator (SOI) and Si/SiO2 substrates at room temperature. The morphological and structural properties were investigated by X-ray diffraction (XRD), Raman spectroscopy, Atomic Force microscopy (AFM). XRD analyses evidenced the co-presence of (002) and (101) orientations. The substrate influence on films morphology, crystalline order, intrinsic stress and grain size is well evidenced, as shown by Raman and AFM analyses. These surface characterization techniques represent a valid support to select the suitable Si-substrate/piezoelectric thin film combination for the fabrication of a piezoelectric device. AlN sputtered on Si-LR substrate showed an enhancement of structural arrangement along (002) planes while the sample sputtered on Si/SiO2 resulted mainly oriented along (101) planes. For these reasons, further characterization was done: (002)-oriented AlN thin films were characterized in terms of piezoelectric response by piezometer and Piezoresponse Force Microscopy (PFM) measurements, while cytotoxicity and biocompatibility were investigated for (101)-oriented AlN thin films. This further investigation helped to assess the suitable film to integrate into piezoelectric devices operating in air or in liquid, respectively.

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.

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.

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).

(1 0 1) and (0 0 2) oriented AlN thin films deposited by sputtering

Reactively sputtered aluminum nitride (AlN) is a piezoelectric material, largely used for technological applications, ranging from sensors to energy harvesting. The structural properties of AlN are crucial for its integration in piezoelectric devices. The preferential orientation of the film discriminates its applicability in devices, where the excitation of the longitudinal rather than the shear mode is required. In order to tune the film crystallographic orientation, a fine control of the sputtering parameters is required. In this paper, a structural switch, from (002) to (101) preferential orientation was obtained by increasing the total pressure in the deposition chamber with the only use of the throttle valve, while keeping constant all the other deposition parameters. This structural change was interpreted on the basis of the growth kinetics, which explains how deposition parameters control the energy of the species impinging on the substrate.

Synthesis of c-axis oriented AlN thin films at room temperature

In this study, aluminium nitride (AlN) thin films were synthesised by plasma-enhanced reactive DC magnetron sputtering on glass and Si (100) substrates and the effect of bias voltages on the structural, optical and morphological properties of the coatings has been studied. According to the grazing angle XRD studies, AlN in hexagonal (wurtzite) structure has been obtained for all the coatings. (002) plane c-axis oriented films have been achieved at 150 and 175 V bias voltages. AFM analyses demonstrated that with the increase in the bias voltages, a transformation from coarse to fine granular morphology has been occurred and smoother surfaces with a decrease in the surface roughness from 3.40 nm to about 1.90 nm were obtained. The results also demonstrated that the high transmittance values of AlN films were not affected by the change in the bias voltages and about 80% of transmittance obtained for all AlN films deposited in this study.

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.

Impact of the substrate dependent polarity distribution in c-axis oriented AlN thin films on the etching behaviour and the piezoelectric properties

In this work, the influence of substrate properties on the polarization of highly c-axis oriented aluminium nitride (AlN) thin films and as a consequence, on the piezoelectric properties and the wet-chemical etching behaviour is investigated. Therefore, 620 nm thin AlN layers are simultaneously sputter-deposited under nominal unheated substrate conditions on silicon (Si) substrates or on those covered with a sputter-deposited titanium (Ti) film. After wet-chemically etching in a phosphorous acid based solution at 80 °C different residues of AlN remain. Wet-chemical etching of AlN films deposited on Ti results in a high film porosity. In contrast, AlN layers on Si are either hardly attacked or the complete thin film is removed except some remaining conical shaped residues. Furthermore, we demonstrate a change in the measured electro-mechanical properties with changing maximum deposition temperature caused by a self-heating effect of the substrate during the AlN deposition process. The change in piezoelectric properties and the differing etching behaviour is caused by a change in polarity within the AlN layer. These domains are visualized by piezoresponse force microscopy measurements, and are in good agreement with the observed etching results. For layers with mixed polarization, the absolute values of the piezoelectric constant d33 are reduced due to the counteraction of piezoelectric domains with opposite polarization.

Underlayer Influence on AlN Deposition by Reactive Magnetron Sputtering

Sputter deposited aluminum nitride (AlN) thin film is the preferred piezoelectric material for microelectromechanical systems (MEMS) devices, such as film bulk acoustic resonator (FBAR) resonators and accelerometers. Silicon dioxide (SiO2) is usually used as a sacrificial layer which will be etched with BOE eventually to release the membrane. In the article, thermal oxide, as well as several types of SiO2 with different properties made by plasma enhanced chemical vapor deposition (PECVD) were selected as the underlying film. The influence of the surface morphology of silicon dioxide underlayer on the crystallinity of reactively sputtered c-axis oriented AlN thin films was investigated. The results show that the quality of AlN films is affected by the surface roughness of the underlying SiO2 film. The full width at half maximum (FWHM) of the x-ray rocking curve for 1μm AlN film ranged from 7.8¢ª to 1.5¢ª depending on the properties of PECVD SiO2 films. The rougher surface of underlayer worsen the crystallinity of AlN films, while adding chemical mechanical planarization (CMP) step was found to improve FWHM to 1.2¢ª. This paper discusses possible mechanisms for the influence of underlayer surface morphology on the properties of sputter deposited AlN films.

Memory and Electrical Properties of (100)‐Oriented AlN Thin Films Prepared by Radio Frequency Magnetron Sputtering

The (100)‐oriented aluminum nitride (AlN) thin films were well deposited onto p‐type Si substrate by radio frequency (RF) magnetron sputtering method. The optimal deposition parameters were the RF power of 350 W, chamber pressure of 9 mTorr, and nitrogen concentration of 50%. Regarding the physical properties, the microstructure of as‐deposited (002)‐ and (100)‐oriented AlN thin films were obtained and compared by XRD patterns and TEM images. For electrical properties analysis, we found that the memory windows of (100)‐oriented AlN thin films are better than those of (002)‐oriented thin films. Besides, the interface and interaction between the silicon and (100)‐oriented AlN thin films was serious important problem. Finally, the current transport models of the as‐deposited and annealed (100)‐oriented AlN thin films were also discussed. From the results, we suggested and investigated that large memory window of the annealed (100)‐oriented AlN thin films was induced by many dipoles and large electric field applied.

Structural and optical properties of c-axis oriented aluminum nitride thin films prepared at low temperature by reactive radio-frequency magnetron sputtering

Spectroscopic ellipsometry, X-ray diffraction and transmission electron microscopy experiments are employed to characterize aluminum nitride (AlN) thin films obtained by radio-frequency magnetron sputtering at low temperature (≈50°C). To understand the growth mechanism and to get in depth information of such films by using ex situ characterization techniques, the AlN thin film sample series were prepared for different sputtering times, while keeping constant all the other deposition conditions. The diffraction studies reveal a [002] oriented growth of the AlN thin films. The misorientation of this crystallographic axis to the normal to the surface reduces progressively with film growth. A nonmonotonic behavior of the AlN pseudo-refractive index versus deposition time indicates a complex depth profile of the AlN thin films optical properties. The difference in orientation dispersion of the [002] crystallite axis, the variation of defects concentration and each constituent atom density influence the refractive index evolution. Our interpretation validity was verified by producing and characterizing samples obtained at intermediate deposition time. The AlN thin films show also very good pull-out adherence values.

The influence of sputter deposition parameters on piezoelectric and mechanical properties of AlN thin films

Abstract Aluminium nitride (AlN) reactively sputter-deposited from an aluminium target is an interesting piezoelectric thin film material with high CMOS compatibility. A good c -axis orientation is essential for obtaining high piezoelectric coefficients. Therefore, the influence of different sputtering conditions on the microstructure of AlN thin films with a typical thickness of about 500 nm was investigated. In this study it is demonstrated that highly c -axis oriented AlN thin films can be deposited on nominally unheated (1 0 0) silicon substrates, most preferentially when using a pure nitrogen atmosphere. The degree of c -axis orientation increases with higher nitrogen concentration and with decreasing the sputtering pressure, whereas the influence of plasma power on the microstructure was found to be negligible. A low sputtering pressure is also useful for minimizing the amount of oxygen contaminations in the deposition chamber and hence for reducing the incorporation of impurities into the AlN films. Intrinsic stress values of AlN thin films were determined by wafer bow measurements and were found to be between −3.5 and 750 MPa depending on choice of deposition parameters. Finally, the piezoelectric coefficients d 33 and d 31 were determined experimentally by laser scanning vibrometry in conjunction with a theoretical model. Effective values in c -axis oriented 500 nm films with FWHM of 0.33° are 3.0 and −1.0 pm/V. For a film of 2.4 μm thickness, values of 5.0 and −1.8 pm/V were measured, which are near the bulk values.

OPTIMAL SYNTHESIS OF C-AXIS ORIENTED ALN THIN FILMS

ABSTRACT C-axis oriented AlN thin films have been deposited by reactive sputtering at room temperature. The crystallographic properties of layered film structures consisting of a piezoelectric layer, aluminum nitride (AlN), synthesized on fused silica as well as on single crystal silicon substrates, have been examined. Aluminum nitride thin films were deposited by reactive pulsed-DC magnetron sputtering using an aluminum target in an Ar/N2 gas mixture. The influence of the most critical deposition parameters on the AlN thin film crystallography has been investigated by means of X-ray diffraction (XRD) analysis of the Full Width at Half Maximum (FWHM) of the rocking curve of the AlN (0002) peak. The relationship between the deposition conditions and the crystallographic orientation of the films is discussed and a guide is given on how to optimize these conditions to obtain highly c-axis oriented AlN thin films without substrate heating.

Synthesis of c-axis oriented AlN thin films on different substrates: A review

Abstract Highly c-axis oriented AlN thin films have been deposited by reactive sputtering on different substrates. The crystallographic properties of layered film structures consisting of a piezoelectric layer, aluminum nitride (AlN), synthesized on a variety of substrates, have been examined. Aluminum nitride thin films have been deposited by reactive pulsed-DC magnetron sputtering using an aluminum target in an Ar/N2 gas mixture. The influence of the most critical deposition parameters on the AlN thin film crystallography has been investigated by means of X-ray diffraction (XRD) analysis of the rocking curve Full-Width at Half Maximum (FWHM) of the AlN-(0 0 0 2) peak. The relationship between the substrate, the synthesis parameters and the crystallographic orientation of the AlN thin films is discussed. A guide is provided showing how to optimize these conditions to obtain highly c-axis oriented AlN thin films on substrates of different nature.

Microstructure and chemical wet etching characteristics of AlN films deposited by ac reactive magnetron sputtering

The influence of the surface morphology of a molybdenum underlayer on the crystallinity and etchability of reactively sputtered c-axis oriented aluminum nitride thin films was investigated. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, high resolution x-ray diffraction, and defect selective chemical etching were used to characterize the microstructure of the Mo and AlN films. 1000nm thick films of AlN with a full width at half maximum (FWHM) of the x-ray rocking curve ranging from 1.1° to 1.9° were deposited on 300nm thick Mo underlayers with a FWHM of around 1.5°. The Ar pressure during the Mo deposition had a critical effect on the Mo film surface morphology, affecting the structure of the subsequently deposited AlN films and, hence, their wet etching characteristics. AlN films deposited on Mo sputtered at a relatively high pressure could not be etched completely, while AlN films deposited on low pressure Mo etched more easily. Postdeposition etching of the Mo surface in Ar rf discharge prior to deposition of the AlN film was found to influence the formation of AlN residuals that were difficult to etch. Optimal rf plasma etching conditions were found, which minimized the formation of these residuals.

Optimization of Deposition Process and Microscopic Characterization of Highly Oriented Aluminum Nitride Thin Films for Bimorph Structures of Piezoelectric Tunable Capacitors

Film deposition with RF sputtering to control residual stress in aluminum nitride (AlN) thin films has been investigated to fabricate the bimorph actuator for a piezoelectric tunable capacitor with low-voltage operation. The effects of conditions in sputtering and surface cleaning were studied both to obtain a preferable film orientation for piezoelectric actuation and to suppress structural deformation of the cantilever due to the residual stress in films. Microscopic analysis revealed that (0001)-oriented AlN and (111)-oriented Al films were epitaxially grown at each interface in the bimorph structure. The current–voltage (I–V) measurements showed a leakage current of less than 5 ×10-5 A m-2 in 500-nm thick AlN films up to 30 V. The effect of optimization of process parameters was demonstrated by the capacitance–voltage (C–V) characteristics of the folded-beam tunable capacitor, in which the suppression of residual stress in piezoelectric layers was shown to be effective to obtain low-voltage operation.

Microstructure analysis of c-axis oriented aluminum nitride thin films by high-resolution transmission electron microscopy

The microstructure, in particular, the surface and interface regions, of the c-axis orientated AlN films deposited on Si (100) substrates was studied. The films showed an evolutionary columnar growth process. In contrast to the previous reports, high-resolution transmission electron microscopy revealed that the AlN films grew directly on substrates without an amorphous interlayer, despite the large lattice mismatch between AlN and Si. The occurrence of misoriented and/or amorphous top layer suggested a subsurface growth/relaxation process of the AlN films by reactive sputtering.

Preparation of Oriented Aluminum Nitride Thin Films on Polyimide Films and Piezoelectric Response with High Thermal Stability and Flexibility

Abstractc‐Axis oriented aluminum nitride (AlN) thin films are successfully prepared on amorphous polyimide films by radiofrequency magnetron reactive sputtering at room temperature. Structural analysis shows that the AlN films have a wurtzite structure and consist of c‐axis oriented columnar grains about 100 nm wide. The full width at half maximum of the X‐ray diffraction rocking curves and piezoelectric coefficient d33 of the AlN films are 8.3° and 0.56 pC N–1, respectively. The AlN films exhibit a piezoelectric response over a wide temperature range, from –196 to 300 °C, and can measure pressure within a wide range, from pulse waves of hundreds of pascals to 40 MPa. Moreover, the sensitivity of the AlN films increases with the number of times it was folded, suggesting that we can control the sensitivity of the AlN films by changing the geometric form. These results were achieved by a combination of preparing the oriented AlN thin films on polyimide films, and sandwiching the AlN and polymer films between top and bottom electrodes, such as Pt/AlN/polyimide/Pt. They are thin (less than 10 μm), self powered, adaptable to complex contours, and available in a variety of configurations. Although AlN is a piezoelectric ceramic, the AlN films are flexible and excellent in mechanical shock resistance.

Growth of highly c-axis oriented aluminum nitride thin films on β-tantalum bottom electrodes

We have investigated the influence of tantalum (Ta) bottom electrodes on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films. AlN thin films and Ta electrodes were prepared by using rf magnetron sputtering method. The crystal structure of the Ta electrodes was tetragonal (β-Ta, a metastable phase) at room temperature. The crystallinity and orientation of the AlN thin films and Ta electrodes strongly depended on sputtering conditions. Especially, the crystallinity and crystal orientation of the Ta electrodes were influenced by their film thickness and the substrate temperature. When the thickness of the Ta bottom electrodes was 200 nm and the substrate temperature was 100 °C, the AlN thin films indicated high c-axis orientation (the full width at half maximum of rocking curve of 3.9°). The crystal orientation of the AlN film was comparable to that of AlN thin films deposited on face centered cubic (fcc) lattice structure metal, such as Au, Pt and Al, bottom electrodes.

A novel surface micromachining process to fabricate AlN unimorph suspensions and its application for RF resonators

A novel surface micromachining process is reported for aluminum nitride (AlN) thin films to fabricate piezoelectric unimorph suspension devices for micro actuator applications. Wet anisotropic etching of AlN thin film is used with a Cr metal mask layer in the microfabrication process. Tetra methyl ammonium hydroxide (TMAH) of 25 wt.% solution is used as an etching solution for the AlN thin films. Polysilicon is used as a structural layer. Highly c-axis oriented AlN thin films are deposited by RF reactive sputtering. Thin layers of chromium on either side of the AlN are used as top and bottom electrodes and also as a mask to etch the AlN and polysilicon layers. The fabricated suspended unimorph structures are tested for scattering parameters using a vector network analyzer. Results show resonant frequencies of devices above 1.7 GHz with an effective electromechanical coupling factor, K eff 2 ≈ 1.7 % .