Thick aluminium nitride films deposited by room-temperature sputtering for ultrasonic applications

Materials in film form for electromechanical transduction have a number of potential applications in ultrasound. They are presently under investigation in flexural transducers for air-coupled ultrasound and underwater sonar operating at frequencies up to a few megahertz. At higher frequencies, they have the potential to be integrated with electronics for applications of ultrasound requiring high spatial resolution. However, a number of fabrication difficulties have arisen in studies of such films. These include the high temperatures required in many thick and thin film deposition processes, making them incompatible with other stages in transducer fabrication, and difficulties maintaining film quality when thin film--typically sub-1 microm--processes are extended to higher thicknesses. In this paper, we first outline a process which has allowed us to deposit aluminium nitride (AlN) films capable of electromechanical transduction at thicknesses up to more than 5 microm without substrate heating. As an ultrasonic transduction material, AlN has functional disadvantages, particularly a high acoustic velocity and weak electromechanical transduction. However, it also has a number of advantages relating to practicality of fabrication and functionality. These include the ability to be deposited on a variety of amorphous substrates, a very high Curie temperature, low permittivity, and low electrical and mechanical losses. Here, we present experimental results highlighting the transduction capabilities of AlN deposited on aluminium electrodes on glass and lithium niobate. We compare the results with those from standard simulation processes, highlighting the reasons for discrepancies and discussing the implications for incorporation of AlN into standard ultrasonic transducer design processes.     

Optical properties of aluminium nitride films obtained by pulsed laser deposition: an ellipsometric study

Aluminium nitride (AlN) films obtained by pulsed laser deposition (PLD) at different low ambient nitrogen pressure have been optically investigated by spectral ellipsometry within the 300–800 nm wavelength range, Sellmeier and Wemple–DiDomenico approximation approaches have been applied for the ellipsometric data analysis. Optical parameters such as refractive index and single oscillator energies of deposited films were estimated and their dependence on the N₂ pressure was studied. The obtained refractive index values of AlN films are around 2 in a wide wavelength region above 400 nm and suggest the growth of a polycrystalline structure. The relatively low (<3.4 eV) threshold energies indicate the formation of a defective and disordered structure, formed during the deposition process. PACS 78.20.-e; 78.20.Ci; 78.66.Fd     

Temperature dependent ultrasonic properties of aluminium nitride

Hexagonal wurtzite structured aluminium nitride has been characterized by the theoretical calculation of ultrasonic attenuation, ultrasonic velocity, higher order elastic constants, thermal relaxation time, acoustic coupling constants and other related parameters in temperature range 200-800 K for wave propagation along the unique axis of the crystal. Higher order elastic constants of AlN at different temperatures are calculated using Lennard-Jones potential for the determination of ultrasonic attenuation. A decrease in ultrasonic velocity with temperature has been predicted, which is caused by reduction in higher order elastic constants with temperature. The temperature dependent ultrasonic properties have been discussed in correlation with higher order elastic constants, thermal relaxation time, thermal conductivity, acoustic coupling constants and thermal energy density. Anomalous behaviour of the attenuation is found at 400 K. On the basis of attenuation, the ductility and performance of AlN have been studied.     

Blow-off of aluminium films

The blow-off of Al targets from a transparent support by single pulses of intense laser irradiation through the support is investigated for target thicknesses up to some m. The blow-off process is theoretically described by calculations of the phase front dynamics in the target, adopted from Harrach's analytical solutions for laser heating and burnthrough of opaque solid slabs. Experimentally two laser beam sources were used: a Nd:YAG laser with a Gaussian intensity profile and a Nd:Glass-laser system with a homogenized flat-top profile. Two types of removal mechanism could be identified. In the first mode (low laser intensities and/or thin targets) the target is completely molten and can easily be blown-off by the vapor originating at the target-support interface. In the second mode (high laser intensities and/or thick targets) superheating of the liquid Al or a burst of the remaining solid layer of the target occurs. In both cases in the second mode the blow-off process is characterized by higher vapor pressures.     

纯六方相氮化铝泡沫材料的合成

在15 mL的不锈钢反应釜中,利用无水三氯化铝与叠氮化钠在无溶剂的条件下直接反应,合成出了六方结构氮化铝泡沫材料,反应温度650 ℃,反应时间3 h.扫描电子显微镜测试结果显示,该试样呈现泡沫状外貌特征.X射线衍射结果表明该试样为六方结构.不同温度条件下的吸收谱表明在202 nm附近存在尖锐的吸收峰.红外吸收谱中存在1381 cm^-1和730 cm^-1两个吸收峰.同时,提出了六方结构氮化铝泡沫材料的合成机理. 关键词： 六方氮化铝泡沫材料 合成机理 X射线衍射     

First principle study of nitrogen vacancy in aluminium nitride

In the framework of density functional theory, using the plane-wave pseudopotential method, the nitrogen vacancy （VN） in both wurtzite and zinc-blende AlN is studied by the supercell approach. The atom configuration, density of states, and formation energies of various charge states are calculated. Two defect states are introduced by the defect, which are a doubly occupied single state above the valance band maximum （VBM） and a singly occupied triple state below the conduction band minimum （CBM） for wurtzite AlN and above the CBM for zinc-blende AlN. So VN acts as a deep donor in wurtzite AlN and a shallow donor in zinc-blende AlN. A thermodynamic transition level E（3＋/＋） with very low formation energy appears at 0.7 and 0,6eV above the VBM in wurtzite and zinc-blende structure respectively, which may have a wide shift to the low energy side if atoms surrounding the defect are not fully relaxed. Several other transition levels appear in the upper part of the bandgap. The number of these levels decreases with the structure relaxation. However, these levels are unimportant to AlN properties because of their high formation energy.     

Influence of substrate temperature and ion-beam energy on the syntheses of aluminium nitride thin films by nitrogen-ion-assisted pulsed-laser deposition

Aluminium nitride (AlN) thin films have been grown on Si(100), Si(111) and Sapphire Al₂O₃(001) substrates by pulsed KrF excimer laser (wavelength 248 nm, duration 30 ns) ablation of an AlN target with the assistance of nitrogen-ion-beam bombardment. The influence of process parameters such as substrate temperature and ion-beam energy has been investigated in order to obtain high-quality AlN films. The AlN films deposited by pulsed-laser deposition (PLD) have been characterized by X-ray diffraction (XRD) to determine the crystalline quality, grain size and growth orientation with respect to the substrate. The XRD spectra of AlN films on Si(100), Si(111) and Sapphire substrates yield full-width-half-maximum (FWHM) values of approximately 1.6. The bonding characteristics in the films have been evaluated by Raman spectroscopy. The chemical composition of the films has been characterized by X-ray photoelectron spectroscopy (XPS). The surface morphology of the films has been measured by atomic force microscopy (AFM). At a substrate temperature of at least 600 °C, polycrystalline AlN films with orientations of AlN(100) and AlN(101) have been synthesized. PACS 68.55.-a; 81.15.Fg; 77.84.Bw     

First principle study of nitrogen vacancy in aluminium nitride

In the framework of density functional theory, using the plane-wave pseudopotential method, the nitrogen vacancy ($V_{\rm N})$ in both wurtzite and zinc-blende AlN is studied by the supercell approach. The atom configuration, density of states, and formation energies of various charge states are calculated. Two defect states are introduced by the defect, which are a doubly occupied single state above the valance band maximum (VBM) and a singly occupied triple state below the conduction band minimum (CBM) for wurtzite AlN and above the CBM for zinc-blende AlN. So $V_{\rm N}$ acts as a deep donor in wurtzite AlN and a shallow donor in zinc-blende AlN. A thermodynamic transition level $E({3 + } \mathord{\left/ {\vphantom {{3 + } + }} \right. \kern-\nulldelimiterspace} + )$ with very low formation energy appears at 0.7 and 0.6eV above the VBM in wurtzite and zinc-blende structure respectively, which may have a wide shift to the low energy side if atoms surrounding the defect are not fully relaxed. Several other transition levels appear in the upper part of the bandgap. The number of these levels decreases with the structure relaxation. However, these levels are unimportant to AlN properties because of their high formation energy.     

氮化铝双端固支音叉谐振器的结构设计

基于氮化铝双端固支音叉（AlN DETF）的谐振式传感器具有尺寸小、稳定性和可靠性好、时间响应快等特点。为了提高灵敏度和分辨率,需要分析AlN DETF谐振器的振梁结构参数对灵敏度和信号功率的影响。在有限元仿真软件中建立AlN谐振器的多物理场模型,进行预应力特征频率分析,仿真验证单个振梁结构参数对灵敏度的影响。在振梁厚度保持恒定的情况下,对仿真结果的数据进行后处理,得到信号功率与振梁长度、宽度的关系。结果表明,相对灵敏度、信号功率随振梁长度、宽度的变化趋势相反。因此,需要根据工艺水平和结构强度等因素,综合考虑AlN谐振器的信号功率和相对灵敏度,对两者进行权衡。仿真分析了优化后AlN DETF谐振器的性能,10 N范围内的灵敏度为56 Hz/N,信号功率为6.810-4 nW,Q值为958。     

Annealing effects on zirconium nitride films

ZrN films were deposited by dc reactive magnetron sputtering on silicon substrates under optimized nitrogen partial pressure of 6×10⁻⁵ mbar. Structural, electrical and optical properties were systematically investigated. Films deposited at room temperature exhibited Schottky structure without any silicide interfacial layer. These films have electrical resistivity of 4.23×10⁻³ Ω cm, which were crystalline in nature, with cubic (1 1 1) orientation. Refractive index and extinction coefficient were found to be 1.95 and 0.43, respectively at a wavelength of 350 nm.

Samples were annealed for 1 h in air at two temperatures, 350 and 550 °C. Scanning electron microscopy (SEM) and energy dispersive analysis of X-rays (EDAX) showed alloy penetration pits. Extent of penetration was greater in the films, which were annealed at higher temperature (550 °C). Variation in refractive index was observed in the range of 1.95–1.80 at 350 nm, for the annealed films, with increase in grain size from 7.25 to 11.10 nm. Poly-crystalline nature has been observed with (1 1 1) and (2 0 1) orientations. Resistivity is found to increase from 4.23×10⁻³ to 6.21×10⁻³ Ω cm.     

Piezoresistive properties of aluminium thin films

The results of measurements of the longitudinal gauge factor of epitaxial aluminium films vacuum deposited onto {100} cleavage faces of rock salt crystals are discussed by extending the Fuchs-Sond-heimer theory to include piezoresistive phenomena.     

An XPS study of hafnium nitride films

XPS spectra are presented from films of different stoichiometry: HfN_1.0 made by CVD and reactive sputtering and HfN_1.09 made by ion plating. They support the existing theoretical calculations where the valence band comprises a strong Hf 5d band immediately below E_F and a band of mainly N 2p character some 6 eV below E_F. The core levels are shifted to larger binding energies by about 1.5 eV indicating electron transfer from hafnium to nitrogen. The effect of substoichiometry on the spectra is very small.     

Partial disorder in evaporated aluminium films

A simple model where it was assumed that the material near the grain boundaries constitute a disordered phase, has been used to explain the influence of deposition substrate temperature on the optical properties of aluminium.     

Vortex flow in granular aluminium films

The depression of vortex flow resistance R_f in granular Al films below H_c2 has been observed together with an electric field dependence of R_f. It might be attributed to thermal fluctuations in accordance with the theoretical prediction.     

Upper critical field of niobium nitride thin films

The temperature dependences of the superconducting transition of niobium nitride (NbN) thin films have been investigated via the first harmonic of the voltage in dc magnetic fields of up to 8 T. The temperature dependence of the second critical field of NbN has been determined. The parameter responsible for the effect of spin paramagnetism in this material and the temperature dependence of the upper critical field that describes well the experimental data have been found in terms of the Werthamer–Helfand–Hohenberg (WHH) theory. The key parameters of the superconductor have been estimated from the transport and optical measurements.     

Deposition of carbon nitride films for space application

Carbon nitride thin films were prepared by electron-beam evaporation assisted with nitrogen ion bombardment and TiN/CN_x composite films were by unbalanced dc magnetron sputtering, respectively. It was found that the sputtered films were better than the evaporated films in hardness and adhesion. The experiments of atomic oxygen action, cold welding, friction and wearing were emphasized, and the results proved that the sputtered TiN/CN_x composite films were suitable for space application.