Polycrystalline AlN Research Articles

The optical and structural properties of AlN thin films characterized by spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) was used to characterize AlN thin films deposited on SiO2/Si substrates by r.f. magnetron sputtering. Refractive index and thickness of each layer were extracted from the SE data analysis employing a multi-layer model and effective medium approximation. Non-destructive characterization of multi-layer films, with total thickness up to 2.5 μm, was successfully carried out, despite of the strong correlation between the film parameters such as thickness and surface roughness. The refractive indices of the AlN films were in the range of 1.98–2.15, which are typical values for polycrystalline AlN. The good agreement between the results of SE and those from X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM), regarding crystallinity, rms roughness, and thickness of the films, confirms that spectroscopic ellipsometry is a useful tool to extract the information on the film quality and uniformity.

Aluminum and aluminum nitride formation in sapphire by ion beam synthesis

Abstract The ion beam synthesis of Al layers in c-plane sapphire by a single high-dose, high-temperature Al implantation step is studied using various methods. At sufficiently high doses, buried single-crystalline Al layers are obtained. XRD pole-figure measurements and transmission electron diffraction reveal an epitaxial relationship with the sapphire matrix according to [1 1 0] Al ‖[1 1 0 0] Al 2 O 3 and (1 1 1) Al ‖(0 0 0 1) Al 2 O 3 . The layers exhibit smooth and atomically sharp interfaces in HREM studies. For low doses, indications are found that in situ ripening processes lead to self-organized arrangements of precipitates in chains parallel to the surface, and that the coalescence of precipitates is affected by the internal twin-structure of isolated Al precipitates. In addition, the transformation of the buried Al layer into a textured polycrystalline AlN layer by a subsequent high-dose nitrogen implantation is investigated.

Crystallinity and microstructures of aluminum nitride films deposited on Si(111) substrates

The crystallinity and microstructures of MOCVD AlN films deposited on Si(111) substrates with and without a buffer layer(s) were determined. The buffer layers were a thin 3C–SiC(111) layer produced via conversion of a Si(111) surface and a film stack consisting of graded-Al x Ga 1− x N/GaN/3C–SiC. A randomly oriented polycrystalline AlN film was obtained when this material was deposited directly on the Si(111). The use of a buffer layer led to the growth and coalescence of highly oriented AlN films produced by the coalescence of grains having average misalignments along the c-axis of 1.8° and that on the c-plane of 3.3°. The grains exhibited strongly faceted tips. The 2H–AlN(0001) films grown on a 3C–SiC(111) buffer layers showed adequate crystal perfection for use as a template for growth of single-crystal GaN and/or Al x Ga 1− x N films.

Preparation of Aluminum Nitride Thin Films by CVD

AlN films were prepared by CVD using aluminum halide (AlCl3) and aluminum alkyl ((CH3)3Al) precursors. The appropriate deposition conditions to obtain polycrystalline AlN films using A1C13 precursor were found at Tdep = 1173 K, Ptot = 66.6 Pa and N2/NH3 = 0.75. It was found that AlN films of different crystallinity can be obtained from (CH3)3Al precursor at Tde = 973-1023 K, Ptot = 1.99 kPa, only under a H2 atmosphere. AlN films can be grown highly oriented in the (210) direction on amorphous quartz substrates depending on their thicknesses. The 0.1 -0.2 μm thick AlN films were transparent and their refractive indexes were about 1.4-1.6.

Growth of AlN and TiN Structures by Plasma-Enhanced Pulsed Laser Deposition

AbstractNitride materials are of interest for a wide variety of applications, including wear-resistant coatings, insulating layers, high-temperature semiconductor devices, and short-wavelength emitters and detectors. TiN and AlN appear to be particularly amenable to crystalline thin film deposition, with stoichiometric material easily obtained even without the use of active nitrogen species. This paper describes the growth of crystalline AlN and TiN thin films on silicon and sapphire substrates using a KrF excimer laser (λ = 248 nm) to ablate elemental metallic targets, and an inductively-coupled RF plasma source to supply active nitrogen species. Growth was monitored in-situ using reflection high-energy electron diffraction (RHEED), and films were characterised using fourier-transform infrared spectroscopy (FTIR) and electron microscopy techniques. Optimised growth conditions led to single-crystal growth of TiN on both substrates, but only polycrystalline AlN was formed directly. Use of a TiN buffer layer on (0001) sapphire led to the successful growth of a single-crystal AlN layer as confirmed by RHEED and high-resolution transmission electron microscopy (HRTEM).

Porosity development of RuO 2 filled glass thick films on aluminum nitride substrates at elevated temperatures

This study is focused on the evolution of the interfacial blisters appearing when a glass-filled resistor thick film is coated on polycrystalline AlN and fired over a temperature range of 600 to 800 °C. The AlN substrates used include those of the as-received form (hereafter termed the “bare” substrate) and thermally pre-oxidized form. Microstructural observations reveal that the film-substrate adhesion is critically influenced by the gas products formed at the interface during heating. A method is then proposed which involves a composite structure of the film to provide open porosity serving as a conduit for facilitating gas liberation at lower temperatures. This hypothesis is confirmed by the reduced blister occurrence at the interface when “rigid” (i.e., non-sintering) inclusions were blended into the resistor paste before coating on the bare substrate. The film with inclusion fractions less than 10 wt % densifies with improved film integrity as the firing temperature is raised above the gas-forming temperature, indicating that the porosity is transient in character.

Measurements of the bulk, C-axis electromechanical coupling constant as a function of AlN film quality

Piezoelectric thin film AlN has great potential for on-chip devices such as thin-film resonator (TFR)-based bandpass filters. The AlN electromechanical coupling constant, K(2), is an important material parameter that determines the maximum possible bandwidth for bandpass filters. Using a previously published extraction technique, the bulk c-axis electromechanical coupling constant was measured as a function of the AlN X-ray diffraction rocking curve [full width at half maximum (FWHM)]. For FWHM values of less than approximately 4 degrees , K (2) saturates at approximately 6.5%, equivalent to the value for epitaxial AlN. For FWHM values >4 degrees , K(2) gradually decreases to approximately 2.5% at a FWHM of 7.5 degrees . These results indicate that the maximum possible bandwidth for TFR-based bandpass filters using polycrystalline AlN is approximately 80 MHz and that, for 60-MHz bandwidth PCS applications, an AlN film quality of >5.5 degrees FWHM is required.

Chemical and structural transformation of sapphire (Al2O3) surface by plasma source nitridation

The chemical, morphological, and structural characteristics of nitrogen plasma treated c-plane sapphire substrate surfaces were studied by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The plasma treatment was carried out by exposing sapphire substrates (Ts=700 °C) to a flow of 35 sccm of activated nitrogen species generated by a constricted-plasma source for 5–60 min. The emergence of the N 1s peak in XPS indicates nitrogen incorporation in sapphire as soon as after 5 min of nitridation. AFM images show that the sapphire contained a high density of islands after 1 h of nitridation. A thin polycrystalline AlN layer was observed on the nitridated sapphire surface by TEM. Both the thickness of the AlN layer and the N 1s photoelectron peak intensity increase nonlinearly with respect to nitridation time. The nonlinear relationship between the thickness of the nitridated layer and the reaction time suggests the growth of the AlN layer follows a diffusion limited growth mechanism.

Low‐Temperature Deposition of Highly Textured Aluminum Nitride by Direct Current Magnetron Sputtering for Applications in Thin‐Film Resonators

Very large scale integration (VLSI), complementary metal on oxide semiconductor (CMOS) compatible piezoelectric materials such as AlN have great potential for applications as miniature bandpass filters. The AlN film quality strongly determines the figures of merit for such devices. For device integration into a VLSI CMOS process with Al electrodes, a low‐temperature deposition technique is necessary. A direct current (dc) magnetron sputtering system was reconstructed to deposit high‐quality, polycrystalline AlN films on Si and Al substrates at temperatures of approximately 150°C. X‐ray diffraction rocking curves of 2.3° on Si substrates and 5.7° on thin‐film Al substrates were achieved. Due to the relatively low deposition rates and the required low deposition temperature, the AlN film quality was found to be limited by the oxygen content within the film. For a deposition rate of 1 μm/h and a deposition temperature of 150°C, the partial pressure of oxygen‐containing species must be less than approximately Torr for such narrow rocking curve material. The amount of oxygen contamination is dependent on various system‐ and process‐related factors such as the deposition rate, deposition temperature, base pressure, oxygen and moisture partial pressure, target purity, and gas purity. Therefore, the AlN film quality is strongly system dependent. All films were tensile in nature, with stresses of 300–600 MPa for 1 μm films on either substrate. Minimal stress control was possible with the dc magnetron source. © 1999 The Electrochemical Society. All rights reserved.

Hardness and Elastic Modulus Measurements of AIN and TiN Sub-Micron Thin Films Using the Continuous Stiffness Measurement Technique with Fem Analysis

AbstractThe hardnesses and elastic moduli of aluminum nitride (AIN) and titanium nitride (TiN) sub-micron thin films pulsed laser deposited (PLD) on silicon (111) were measured using nanoindentation based on a continuous stiffness measurement (CSM) technique. Thin film thicknesses, based on profile measurements of simultaneously grown step samples, are 210 nm and 180 nm with surface roughnesses of 12 nm and 2 nm for AlN and TiN, respectively. X-ray diffraction showed AlN as a highly textured polycrystalline AlN wurzite structure with a (0001) orientation and TiN as a cubic structure with a (111) orientation. The CSM technique provided hardness and elastic modulus as a function of depth. Finite element modeling (FEM) aided in determining the optimum indenter contact depth at which the thin films behaved as a semi-infinite solid with negligible substrate induced artifacts. Hardnesses of these AlN and TiN thin films were, determined analytically, 25 GPa and 33 GPa, as compared to FEM results of 24 GPa and 30 GPa, respectively. The elastic moduli measured 320 GPa and 370 GPa for these AlN and TiN thin films, respectively.

Gas-dynamic effects in the laser-pulse sputtering of AlN: is there evidence for phase explosion?

The overall light intensity (fluorescence) of the sputtered atoms, ions, and molecules has been measured for polycrystalline AlN which was bombarded with 248 nm laser pulses in the presence of a background pressure of N 2. AlN is unusual in that, in spite of a ∼6 eV band gap, it is easily rendered highly absorptive of 248, 308, or 694 nm laser pulses. In fact, since it is well established in other work that ∼1.5 J/cm 2 (308 nm) brings AlN to the melting temperature, ∼3050 K, we will assume that the fluence used here (∼20 J/cm 2) was more than enough to bring the target surface first to a temperature sufficient for normal vaporization but finally to the vicinity of the thermodynamic critical temperature, T tc. As a result a significant quantity of particles can be assumed to have been expelled by phase explosion. The tentativeness in the argument rests in the problem that some part of the incident fluence beyond ∼1.5 J/cm 2 will have been consumed in laser–plume interaction. Nevertheless there is evidence in work by Pedraza et al. [A.J. Pedraza, J-Y. Zhang, H. Esrom, Mater. Res. Soc. Symp. Proc. 285 (1993) 209] that both AlN and Al respond linearly to the fluence up to at least 6 J/cm 2. It was found that the assumed phase-exploded particles decelerated rapidly, possibly due to their encounter with the normally vaporized particles, or possibly due to an electric field arising from positive charging of the target surface. The fluorescence maximum (which can be safely assumed to be also a density maximum) was then nearly stationary, a situation which characterized the lowest background pressures of N 2 (≤3.5 Pa). At higher pressures (≥3.5 Pa) a second fluorescence maximum appeared nearer the contact front and was found to move. Following the suggestion of Horwitz 1 J.S. Horwitz, Naval Research Lab., Washington, DC 20375, USA, personal communication, 1994. 1 we take this feature as being an artifact of electrons near the contact front diffusing (or scattering) backwards and causing fluorescence which is unrelated to the particle density. From the velocity of the contact front one obtains explicit information on the mean kinetic energies ( E 4) of the particles in the plume (1.5–2 eV). Another estimate of E 4 follows from the initial expansion observed from 0–200 ns (1.5–3 eV). Such energies suggest, independently of the fact that the fluence was high (∼20 J/cm 2), that a temperature near T tc was reached and that phase explosion may have occurred. We finally note that, however tentative is the claim for phase explosion, it is certain that a close relative of phase explosion, due to subsurface heating, was not involved. This is because the numerical demonstrations of subsurface heating have been flawed.

Excimer laser synthesis of thin AlN coatings

A new technique for the production of thin AlN films on Al samples by direct laser synthesis is reported. This has the advantages of good adhesion, low temperature deposition and localisation. A metallic, pure Al sample is irradiated by an excimer laser under a nitrogen containing atmosphere at high intensity levels such that a plasma is formed above the sample. The active nitrogen atoms and ions contained in this plasma react with the hot melted Al surface resulting in the synthesis of an AlN layer. The influence of the incident laser fluence, nature and pressure of the ambient atmosphere and of the number of laser pulses on the chemical composition and crystalline structure of the synthesised layers is reported. Under optimised conditions a few μm thick polycrystalline AlN layer, containing only a few percent oxygen and exhibiting high hardness and very good adhesion, has been prepared on Al.

Reactive partially ionized beam deposition of AlN thin films

Aluminum nitride films were synthesized on Si(111) wafer and quartz by the reactive partially ionized beam (RPIB) deposition technique. Under specific experimental conditions, polycrystalline and preferential crystalline AlN films of hexagonal structure were obtained by this method. The correlation between experimental parameters and the resulting structure as well as the stoichiometry of the AlN film are discussed.

Synthesis of AlN by reactive sputtering

We present a systematic study of the sub-band gap optical absorption coefficients α(h ν) in the range 1.2–6 eV vs. deposition-temperature ( T s from 27 to 450°C) films deposited on silica by 13.6 MHz magnetron sputtering of an Al target with 53 and 72% N 2 in the reactive mixture. X-ray diffraction, infrared absorption and Raman diffusion are also presented, mainly on films deposited on Si in the same run to help in the characterisation of the films. All signals are specific of AlN polycrystalline films, which are of better quality when deposited with 72% N 2. The lowest sub-band gap optical absorption around 5×10 2 cm −1 is obtained for deposition on silica at T s=300°C with 72% N 2 and is close to that of heteroepitaxial films deposited on sapphire.

Materials science communication formation of aln films by al evaporation with nitrogen ion beam bombardment

Aluminum nitride films were synthesized by electron gun evaporation of aluminum on to Si (111) wafer with simultaneous bombardment by nitrogen ions. Under special conditions, polycrystalline AlN films of fine crystallinity were obtained.

Dislocations and extended defects in AlN deformed at high temperatures

Polycrystalline AlN could be deformed up to 10% strain in compression at elevated temperatures (1820–1920 K) under constant stress in the range 150–250 MPa or at a fixed strain rate of 5.6 × 10 −6 s −1. Stress-strain curves show a marked temperature dependence. TEM investigations point to cavitation at grain boundaries as the main source of damage. Intragranular plasticity is achieved mainly by a-type dislocations. A few dislocations with c + a Burgers vectors could also be observed.

Preparation of highly oriented polycrystalline AlN thin films deposited on glass at oblique-angle incidence

A new method to prepare polycrystalline AlN thin films biaxially oriented on amorphous substrates has been demonstrated. The films were deposited at different angles of incidence with a rf sputtering system. Population-1 crystallites, oriented with their c-axis pointing toward the incoming flux with random orientation in aximuthal directions, predominate at low angle of incidence (near normal). Population-2 crystallites result from non-normal incidence and have their c-axis tilted away from the incoming flux and predominate at high, near glancing, angle of incidence. The alignment of crystallites (population-2) increases with angle of incidence. Crystallites align along the [11*0] channeling direction, characterized by a low sputtering yield, while misoriented crystal grains suffer a higher resputtering and their growth is inhibited. For films deposited at 75°, the FWHM of an x-ray ϕ scan profile is 25°, indicating a high in-plane alignment.

Development of preferred orientation in polycrystalline AlN thin films deposited by rf sputtering system at low temperature

The development of preferred orientation in AlN thin films deposited on silica glass substrates by rf sputtering at low substrate temperature (<150 °C) has been studied. The main factors controlling the preferential orientation of the AlN thin films are the ion-bombardment energies, incidence angle of the arriving particles, and deposition rate. At low pressure, a perpendicular and highly directional energetic ion-bombardment induces an orientation of the crystallites with their c-axis perpendicular to the substrate surface. At higher pressure (>15 mTorr), a spreading in the incidence angle of the arriving particles, due to gas phase collisions, favors the formation of AlN crystal twinning. A change in the preferred orientation of the films from (0001) to (1011) for deposition rates above 1.8 Å/s is observed.

Electron Spin Resonance Analysis of Lattice Defects in Polycrystalline Aluminum Nitride

Electron spin resonance was used in the present study to detect lattice defects in an aluminum nitride lattice. The ESR spectra were obtained from polycrystalline AlN with various thermal conductivities. Measurement of the AlN g values clearly indicated that the obtained ESR signals arose from electrons trapped by nitrogen vacancies. The ESR study revealed that thermal conductivity increases with an increase in the number of electrons trapped by nitrogen vacancies. The explanation for that phenomenon is that the thermal conductivity of AlN increases with a decreasing concentration of oxygen impurities incorporated into the AlN lattice, and the concentration of nitrogen vacancies changes inversely with the concentration of oxygen impurities.

Structures and Electric Characteristics of Al–N Thin Films Prepared by RF Sputtering

A variety of Al-rich Al-N and stoichiometric AlN films with different structures, including Al+AlN, amorphous AlN, polycrystalline AlN, have been made by a sputter method at room temperature. AES, XPS, XRD and TEM have been used to investigate the composition, the ratio of metallic Al to nitrided Al, the structure and two-phase coexistence morphology for various Al-N films respectively, which have been found to be strongly dependent on the total sputtering pressure and content of nitrogen gas. The Al-N films obtained can be classified into three categories, that is, the Al-rich metallic films with a positive temperature coefficient of resistivity (TCR), the Al-rich semiconducting films with a negative TCR and the stoichiometric insulating films. The current-voltage curves of Al-rich Al-N films are presented. The metallic conduction and semiconductor conduction are two main electrical conduction mechanisms for the various Al-rich Al-N films.