Influence of Substrate Temperature on Structural Properties and Deposition Rate of AlN Thin Film Deposited by Reactive Magnetron Sputtering

Aluminum nitride (AlN) thin films with c-axis preferred orientation have been prepared by reactive direct-current (DC) magnetron sputtering. The degree of preferred crystal orientation, the cross-sectional structure, and the surface morphology of AlN thin films grown on Si (100) substrates at various substrate temperatures from 60°C to 520°C have been investigated by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. Results show that the substrate temperature has a significant effect on the structural properties, such as the degree of c-axis preferred orientation, the full-width at half-maximum (FWHM) of the rocking curve, the surface morphology, and the cross-sectional structure as well as the deposition rate of the AlN thin films. The optimal substrate temperature is 430°C, with corresponding root-mean-square surface roughness (R _rms) of 1.97?nm, FWHM of AlN (002) diffraction of 2.259°, and deposition rate of 20.86?nm/min. The mechanisms behind these phenomena are discussed. Finally, film bulk acoustic resonators based on AlN films were fabricated; the corresponding typical electromechanical coupling coefficient (k _t ² ) is 5.1% with series and parallel frequencies of 2.37?GHz and 2.42?GHz, respectively.     

Deposition Characteristics of AlN Thin Film Prepared by the Dual Ion Beam Sputtering System

In this experiment, a radio frequency dual ion beam sputtering (DIBS) system was used to prepare aluminum nitride (AlN) films with a bottom Al electrode on a Si (100) substrate. After systematic testing of the processing variables, a high-quality film with preferred c-axis orientation was grown successfully on the Si (100) substrate with an Al target under 700 eV energy flux, N₂/(N₂ + Ar) ratio of 55%, and 4 × 10⁻⁴ torr in vacuum. The characteristics of the deposited AlN thin films were studied by x-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), secondary ion mass spectrometry (SIMS), and electronic spectroscopy for chemical analysis (ESCA). The surface roughness was also measured. It was found that AlN films prepared by DIBS at room temperature are better than those prepared at 300°C, and those prepared with an Al target are better than those prepared with an AlN target. The inferiority of AlN films prepared with AlN targets is due to the AlN bond being broken down by the ion beam source.     

Effect of substrate biasing and temperature on AlN thin film deposited by cathodic arc ion

Aluminum nitride (AlN) films have been grown in pure N₂ plasma using cathodic arc ion deposition process. The films were prepared at different substrate bias voltages and temperatures. The aim was to investigate their influence on the Al macro-particles, structural and optical properties of deposited films. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Scanning electron microscope (SEM) and Rutherford backscattering spectrometry (RBS) were employed to characterize AlN thin films. XRD patterns indicated the formation of polycrystalline (hexagonal) films with preferential orientation of (002), which is suppressed at higher substrate bias voltage. FTIR and Raman spectroscopic analysis were used to assess the nature of chemical bonding and vibrational phonon modes of AlN thin films respectively. FTIR spectra depicted a dominant peak around 850 cm^?1 corresponding to the longitudinal optical (LO) mode of vibration. A shift in this LO mode peak towards higher wavenumbers was observed with the increase of substrate bias voltage and temperature, showing the upsurge of nitrogen concentration in the deposited film. Raman spectra illustrated a peak at 650 cm^?1 corresponding to E₂ (high) phonon mode depicting the c-axis oriented (perpendicular to substrate) AlN film. SEM analysis showed the AlN film deposited at higher substrate bias voltage contains fewer amounts of Al macro-particles.     

Influence of substrate temperature to prepare (1 0 3) oriented AlN films

(1 0 3) Oriented AlN films is an attractive piezoelectric material for the applications on surface acoustic wave (SAW) and film bulk acoustic wave (FBAR) devices. As regards the SAW properties of the (1 0 3) oriented AlN films, the electromechanical coupling constant (K²) is larger than the (0 0 2) oriented AlN films. As regards the bulk acoustic wave (BAW) properties of (1 0 3) oriented AlN films, it can excite a quasi-shear mode (velocity = 5957 m/s, K² = 3.8%) that can be used for FBAR liquid sensor. In this research, the (1 0 3) oriented AlN films were successfully prepared on the silicon substrate by rf magnetron sputtering. Different temperatures (100 °C, 200 °C, 300 °C, and 400 °C) were used in this experiment process. The crystalline structure of films was determined by X-ray diffraction (XRD) and the surface microstructure was investigated by the atomic force microscope (AFM). The result exhibited the optimal substrate temperature is 300 °C. The optimal (1 0 3) oriented AlN films have the strongest XRD intensity, the smallest full width at half maximum (FWHM) value (0.6°), the largest grain size (15.8 nm) and the smoothest surface (Ra = 3.259 nm).     

Characteristics of ZnO thin films prepared by radio frequency magnetron sputtering

We investigated in this study structural and nanomechanical properties of zinc oxide (ZnO) thin films deposited onto Langasite substrates at 200 °C through radio frequency magnetron sputtering with an radio frequency power at 200 W in an O₂/Ar gas mixture for different deposition time at 1, 2, and 3 h. Surface morphologies and crystalline structural characteristics were examined using X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The deposited film featured a polycrystalline nature, with (1 0 0), (0 0 2), and (1 0 1) peaks of hexagonal zinc oxide at 31.75°, 34.35°, and 36.31°. As the deposition time increased, the ZnO film became predominantly oriented along the c-axis (0 0 2) and the surface roughness decreased. Through Berkovich nanoindentation following a continuous stiffness measurement technique, the hardness and Young’s modulus of ZnO thin films increased as the deposition time increased, with the best results being obtained for the deposition time of 3 h. In addition, surface acoustic wave properties of ZnO thin films were also presented.     

Surface acoustic wave characteristics of AlN thin films grown on a polycrystalline 3C-SiC buffer layer

In this study, AlN thin films were deposited on a polycrystalline (poly) 3C-SiC buffer layer for surface acoustic wave (SAW) applications using a pulsed reactive magnetron sputtering system. AFM, XRD and FT-IR were used to analyze structural properties and the morphology of the AlN/3C-SiC thin film. Suitability of the film in SAW applications was investigated by comparing the SAW characteristics of an interdigital transducer (IDT)/AlN/3C-SiC structure with the IDT/AlN/Si structure at 160 MHz in the temperature range 30-150 °C. These experimental results showed that AlN films on the poly (1 1 1) preferred 3C-SiC have dominant c-axis orientation. Furthermore, the film showed improved temperature stability for the SAW device, TCF = −18 ppm/°C. The change in resonance frequency according to temperature was nearly linear. The insertion loss decrease was about 0.033 dB/°C. However, some defects existed in the film, which caused a slight reduction in SAW velocity.     

Magnetron Sputter Epitaxy and Characterization of Wurtzite AlInN on Si(111) Substrates

AlInN films were grown on Si(111) substrates by the direct-current reactive sputtering method at temperatures from 150°C to 350°C. Growth of the AlInN films was found to be c-axis oriented, and the surfaces of the films were smooth. The bandgap structures and electron mobility of the AlInN films were studied using optical reflectance spectra and the Hall-effect method, respectively. The effects of an AlN buffer layer on the microstructure and optical and electrical properties of the AlInN films were investigated. It was found that the AlN buffer layer had a large influence on the crystalline quality and carrier concentration profiles of the AlInN films. This work suggests that reactive magnetron sputtering is a promising method for growing AlInN films at low temperatures.     

Properties of high sensitivity ZnO surface acoustic wave sensors on SiO2/(1 0 0) Si substrates

The properties of ZnO/SiO₂/Si surface acoustic wave (SAW) Love mode sensors were examined and optimized to achieve high mass sensitivity. SAW devices A and B, were designed and fabricated to operate at resonant frequencies around 0.7 and 1.5 GHz. The ZnO films grown by pulsed laser deposition on SiO₂/Si demonstrated c-axis growth and the fabricated devices showed guided shear horizontal surface acoustic wave (or Love mode) propagation. Acoustic phase velocity in the ZnO layer was measured in both devices A and B and theoretical and experimental evaluation of the mass sensitivity showed that the maximum sensitivity is obtained for devices with ZnO guiding layer thicknesses of 340 nm and 160 nm for devices A and B, respectively. The performance of the SAW sensors was validated by measuring the mass of a well-characterized polystyrene–polyacrylic acid diblock copolymer film. For the optimized sensors, maximum mass sensitivity values were as high as 4.309 μm²/pg for device A operating at 0.7477 GHz, and 8.643 μm²/pg for device B operating at 1.5860 GHz. The sensors demonstrated large frequency shifts per applied mass (0.1–4 MHz), excellent linearity, and extended range in the femto-gram region. The large frequency shifts indicated that these sensors have the potential to measure mass two to three orders of magnitude lower in the atto-gram range.     

Fabrication of suspended thin film resonator for application of RF bandpass filter

Characteristics of AlN thin film and thin film resonator for RF bandpass filter have been studied. AlN thin films were deposited by RF magnetron sputter system. Deposition parameters such as N₂ contents, Ar and N₂ partial pressures, and the distance between metal target and substrate were found to affect the piezoelectric response. To fabricate the suspended thin film resonator (STFR) using the piezoelectric AlN thin film, the etching of AlN and the surface micromachining process were conducted. The thickness of AlN film and membrane for the STFR are 2 and 15 μm, respectively. This membrane was fabricated by SOI technology. The device with the dimension of 160 × 160 μm² has a resonant frequency of 1.653 GHz, a K_eff² of 2.4%, a bandwidth of 17 MHz, and a quality factor of 91.7. The device with the dimension of 200 × 200 μm² has a resonant frequency of 1.641 GHz, a K_eff² of 1.2%, and a bandwidth of 9 MHz, and a quality factor of 50.2.     

The influence of thickness and ammonia flow rate on the properties of AlN layers

Undoped AlN layers have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition in order to study the effects of ammonia (NH₃) flow rate and layer thickness on the structural quality and surface morphology of AlN layers by high-resolution X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Lower NH₃ flow rate improves crystallinity of the symmetric (0 0 0 2) plane in AlN layers. Ammonia flow rate is also correlated with surface quality; pit-free and smooth AlN surfaces have been obtained at a flow rate of 70 standard cm³ per minute. Thicker AlN films improve the crystallinity of the asymmetric (1 0 1̄ 2) plane.     

Power Factor Anisotropy of <Emphasis Type="Italic">p</Emphasis>-Type and <Emphasis Type="Italic">n</Emphasis>-Type Conductive Thermoelectric Bi-Sb-Te Thin Films

The best films for thermoelectric applications near room temperature are based on the compounds Bi₂Te₃, Sb₂Te₃, and Bi₂Se₃, which as single crystals have distinct anisotropy in their electrical conductivity σ regarding the trigonal c-axis, whereas the Seebeck coefficient S is nearly isotropic. For p- and n-type alloys, P _⊥c > P _||c, and the power factors P _⊥c of single crystals are always higher compared with polycrystalline films, where the power factor is defined as P = S ² σ, ⊥c and ||c are the direction perpendicular and parallel to the c-axis, respectively. For the first time in sputter-deposited p-type (Bi_0.15Sb_0.85)₂Te₃ and n-type Bi₂(Te_0.9Se_0.1)₃ thin films, the anisotropy of the electrical conductivity has been measured directly as it depends on the angle φ between the electrical current and the preferential orientation of the polycrystals (texture) using a standard four-probe method. The graphs of σ(φ) show the expected behavior, which can be described by a weighted mixture of σ _⊥c and σ _||c contributions. Because (σ _⊥c/σ _||c) _p  < (σ _⊥c/σ _||c) _n , the n-type films have stronger anisotropy than the p-type films. For this reason, the angular weighted contributions of P _||c lead to a larger drop in the power factor of polycrystalline n-type films compared with p-type films.     

Effect of rapid thermal annealing of sputtered aluminium nitride film in an oxygen ambient

Aluminium nitride (AlN) thin films were deposited by radio frequency (RF) magnetron sputtering on p-type silicon (Si) substrate of (1 0 0) orientation using only argon (Ar) gas at substrate temperature of 300 °C. In order to achieve improved electrical properties, we performed post-deposition rapid thermal annealing (RTA). Sputtered AlN films were annealed in an oxygen ambient at temperatures of 600, 700, and 800 °C using RTA for 30 min. The orientation of the AlN crystal in the film was investigated using X-ray diffraction (XRD). The characteristic spectra by functional group were analyzed by Fourier transformation infrared (FTIR) spectroscopy. The electrical properties of the AlN thin films were studied through capacitance–voltage (C–V) characteristics in metal–insulator–semiconductor (MIS) device using the films as insulating layers. The flatband voltages (V_FB) in C–V curves were found to depend on crystal orientations. Negative V_FB was found in the case when AlN (1 0 0) peak was found. Also, when AlN (1 0 3) peak was observed upon increasing the annealing temperature, the value of V_FB was positive and after annealing at 700 °C, AlN (1 0 3) peak intensity was found to be maximum and V_FB was as high as+6.5 V.     

Low Fatigue in Epitaxial Pb(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> on Si Substrates with LaNiO<Subscript>3</Subscript> Electrodes by RF Sputtering

Epitaxial PZT (001) thin films with a LaNiO₃ bottom electrode were deposited by radio-frequency (RF) sputtering onto Si(001) single-crystal substrates with SrTiO₃/TiN buffer layers. Pb(Zr_0.2Ti_0.8)O₃ (PZT) samples were shown to consist of a single perovskite phase and to have an (001) orientation. The orientation relationship was determined to be PZT(001)[110]∥LaNiO₃(001)[110]∥SrTiO₃ (001)[110]∥TiN(001)[110]∥Si(001)[110]. Atomic force microscope (AFM) measurements showed the PZT films to have smooth surfaces with a roughness of 1.15 nm. The microstructure of the multilayer was studied using transmission electron microscopy (TEM). Electrical measurements were conducted using both Pt and LaNiO₃ as top electrodes. The measured remanent polarization P _r and coercive field E _c of the PZT thin film with Pt top electrodes were 23 μC/cm² and 75 kV/cm, and were 25 μC/cm² and 60 kV/cm for the PZT film with LaNiO₃ top electrodes. No obvious fatigue after 10¹⁰ switching cycles indicated good electrical endurance of the PZT films using LaNiO₃ electrodes, compared with the PZT film with Pt top electrodes showing a significant polarization loss after 10⁸ cycles. These PZT films with LaNiO₃ electrodes could be potential recording media for probe-based high-density data storage.     

Structural and electrical characterization of AIN thin films obtained by nitridation of Al/Si substrate

AlN thin films have been grown on Al-covered Si(100) substrates using nitridation in high-purity nitrogen ambient, where the Al layer was previously deposited by ultrahigh vacuum (UHV) electron beam evaporation. The temperature of nitridation plays an important role in the formation of AlN films. The films have been characterized by x-ray diffraction, Fourier transform infrared spectroscopy, atomic force microscopy, transmission electron microscopy, and spreading resistance profile. Results indicate that the AlN films formed by nitridation at 1000°C for 30 min exhibit the preferred orientation of (002), with spreading resistance above 10⁸Ω.     

Microwave Dielectric Properties of ZnO-2TiO<Subscript>2</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramics with BaCu (B<Subscript>2</Subscript>O<Subscript>5</Subscript>) Addition

The influence of BaCu(B₂O₅) (BCB) addition on the sintering temperature and microwave dielectric properties of ZnO-2TiO₂-Nb₂O₅ (ZTN) ceramic has been investigated using dilatometry, x-ray diffraction, scanning electron microscopy, and microwave dielectric measurements. A small amount of BCB addition to ZTN can lower the sintering temperature from 1100°C to 900°C. The reduced sintering temperature was attributed to the formation of the BCB liquid phase. The ZTN ceramics containing 3.0 wt.% BCB sintered at 900°C for 2 h have good microwave dielectric properties of Q × f = 19,002 GHz (at 6.48 GHz), ε _r = 45.8 and τ _f  = 23.2 ppm/°C, which suggests that the ceramics can be applied in multilayer microwave devices, provided that Ag compatibility exists.     

The Influence of Film Thickness on the Transparency and Conductivity of Al-Doped ZnO Thin Films Fabricated by Ion-Beam Sputtering

To evaluate the influence of film thickness on the structural, electrical, and optical properties of Al-doped ZnO (AZO) films, a set of polycrystalline AZO samples with different thickness were deposited on glass substrates by ion-beam sputtering deposition (IBSD). X-ray diffraction (XRD), atomic force microscopy (AFM), energy-dispersive x-ray spectroscopy (EDS), four-point probe measurements, and spectrophotometry were used to characterize the films. XRD showed that all the AZO films had preferred c-axis orientation. The ZnO (110) peak appeared, and the intensity increased, with increasing thickness. All the samples exhibited compressive intrinsic stresses. AFM showed that the grain size along with the root-mean-square (RMS) roughness increased with increasing thickness. The decrease of resistivity is due to the corresponding change in grain size, surface morphology, and chemical composition. The average optical transmittance of the AZO films was over 80%, and a sharp fundamental absorption edge with red-shifting was observed in the visible region. The optical band gap decreased from 3.95 eV to 3.80 eV when the AZO film thickness increased from 100 nm to 500 nm.     

<Emphasis Type="Italic">In Situ</Emphasis> Stress Measurements During GaN Growth on Ion-Implanted AlN/Si Substrates

In situ wafer curvature measurements were used in combination with postgrowth structural characterization to study the evolution of film stress and microstructure in GaN layers grown by metalorganic chemical vapor deposition on N⁺ ion-implanted AlN/Si (111) substrates. The results were compared with growth on identical unimplanted substrates. In situ stress measurements revealed that, for the unimplanted sample, the GaN initiated growth under compressive stress of −1.41 GPa which arose due to lattice mismatch with the AlN buffer layer. In contrast, GaN growth on the ion-implanted sample began at lower compressive stress of −0.84 GPa, suggesting a reduction in epitaxial stress. In both cases, the compressive growth stress was fully relaxed after ~0.7 μm and minimal tensile stress was generated during growth. During post-growth cooling, tensile stress was introduced in the GaN layer of both samples due to thermal expansion mismatch. Post-growth optical microscopy characterization, however, demonstrated that the ion-implanted sample had lower density of channeling cracks compared with the unimplanted sample. Cross-sectional transmission electron microscopy images of the sample grown on ion-implanted Si with no post-implantation nitrogen annealing revealed the formation of horizontal cracks in the implanted region beneath the AlN buffer layer. The weakened layer acts to decouple the GaN film from the Si substrate and thereby reduces the density of channeling cracks in the film after growth.     

Structural Properties of AlN Grown on Sapphire at Plasma Self-Heating Conditions Using Reactive Magnetron Sputter Deposition

Aluminum nitride (AlN) films were grown on sapphire by reactive magnetron sputter deposition in N₂ discharges at plasma self-heating conditions. The growth temperature was as low as 94°C. The structural properties resulting from different substrate biases and growth pressures were investigated by atomic force microscopy, x-ray diffraction (XRD) measurements, and transmission electron microscopy (TEM). At 20 mTorr of N₂ with most sputtered species thermalized, films exhibited both AlN (0002) and (10[`1] 1) (10\overline{1} 1) XRD peaks, with the AlN (0002) intensity initially increasing with ion energy above 15 eV, showing enhanced film quality with an optimum of 25 eV. At a lower growth pressure of 5 mTorr with energetic sputtered species, the AlN (10[`1] 1) (10\overline{1} 1) peak disappeared and the crystallinity of AlN improved, exhibiting relaxed epitaxial AlN. The measured lattice parameter was 0.4975 nm, which was 0.10% smaller than that of bulk. The epitaxial relationship of a single-crystal AlN film was confirmed by pole figure and cross-sectional TEM. These results demonstrate that control of ion energy and energy of the sputter-deposited species is critical for film deposition at low temperature.     

Investigation into Texture,Preferential Orientation,and Optical Properties of Zinc Oxide Nanopolycrystalline Thin Films Deposited by the Sol–Gel Technique on Different Substrates

ZnO nanopolycrystalline thin films were deposited by the sol–gel technique on glass and silicon, and compared systematically via atomic force microscopy, scanning electron microscopy, x-ray diffraction, UV–Vis spectrophotometry, and fluorescence spectrophotometry. The thickness of the ZnO films was measured by ellipsometric microscopy. A higher preheating temperature was needed to obtain films with a strong preferential orientation. The optimal annealing temperatures for c-axis films on glass and silicon substrates were 525°C and 750°C, respectively. The relative intensity of the blue–green emission peak tends to increase with the annealing temperature. When the film is annealed in N₂, the transmittance of the film reduces while the intensity of the blue–green emission increases.     

The properties of annealed AlN films deposited by pulsed laser deposition

AlN films deposited on SiC or sapphire substrates by pulsed laser deposition were annealed at 1200°C, 1400°C, and 1600°C for 30 min in an inert atmosphere to examine how their structure, surface morphology, and substrate-film interface are altered during high temperature thermal processing. Shifts in the x-ray rocking curve peaks suggest that annealing increases the film density or relaxes the films and reduces the c-axis Poisson compression. Scanning electron micrographs show that the AlN begins to noticeably evaporate at 1600°C, and the evaporation rate is higher for the films grown on sapphire because the as-deposited film contained more pinholes. Rutherford backscattering spectroscopy shows that the interface between the film and substrate improves with annealing temperature for SiC substrates, but the interface quality for the 1600°C anneal is poorer than it is for the 1400°C anneal when the substrate is sapphire. Transmission electron micrographs show that the as-deposited films on SiC contain many stacking faults, while those annealed at 1600°C have a columnar structure with slightly misoriented grains. The as-deposited films on sapphire have an incoherent interface, and voids are formed at the interface when the samples are annealed at 1600°C. Auger electron spectroscopy shows that virtually no intermixing occurs across the interface, and that the annealed films contain less oxygen than the as-grown films.