Deep-UV sensors based on SAW oscillators using low-temperature-grown AlN films on sapphires

High-quality epitaxial AlN films were deposited on sapphire substrates at low growth temperature using a helicon sputtering system. SAW filters fabricated on the AlN films exhibited excellent characteristics, with center frequency of 354.2 MHz, which corresponds to a phase velocity of 5667 m/s. An oscillator fabricated using AlN-based SAW devices is presented and applied to deep-UV light detection. A frequency downshift of about 43 KHz was observed when the surface of SAW device was illuminated by a UV source with dominant wavelength of around 200 nm. The results indicate the feasibility of developing remote sensors for deep-UV measurement using AlN-based SAW oscillators.     

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO3 substrates

The c-axis-oriented aluminum nitride (AlN) films were deposited on z-cut lithium niobate (LiNbO₃) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AlN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N₂ concentration, and sputtering pressure. Highly c-axis-oriented AlN films to fabricate the AlN/LiNbO₃-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N₂ concentration of 60%, RF power of 165 W, and substrate temperature of 400°C. A dense pebble-like surface texture of c-axis-oriented AlN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K²) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm/°C     

SAW COM-parameter extraction in AlN/diamond layered structures

Highly c-axis oriented aluminum nitride (AlN) thin piezoelectric films have been grown on polycrystalline diamond substrates by pulsed direct current (DC) magnetron reactive sputter-deposition. The films were deposited at a substrate temperature below 50/spl deg/C (room temperature) and had a typical full width half maximum (FWHM) value of the rocking curve of the AlN-002-peak of 2.1 degrees. A variety of one-port surface acoustic wave (SAW) resonators have been designed and fabricated on top of the AlN films. The measurements indicate that various SAW modes are excited. The SAW phase velocities of up to 11.800 m/s have been measured. These results are in agreement with calculated dispersion curves of the AlN/diamond structure. Finally, the coupling of modes parameters have been extracted from S/sub 11/ measurements using curve fitting for the first SAW mode, which indicate an effective coupling K/sup 2/ of 0.91% and a Q factor of about 600 at a frequency of 1050 MHz.     

Surface acoustic wave characterization of PECVD films on galliumarsenide

Surface-acoustic-wave (SAW) measurement techniques can be effectively used to determine the acoustic properties of dielectric and piezoelectric films. Such films can be used for the development of semiconductor-integrated microwave-frequency surface and bulk acoustic wave devices. The acoustic properties of silicon nitride, silicon oxynitride, silicon carbide, and TEOS glass, deposited by plasma-enhanced chemical-vapor-deposition (PECVD) on GaAs, have been characterized using linear arrays of SAW interdigital electrodes operating in the harmonic mode over the frequency region from 30 MHz to above 1.0 GHz. The elastic constants of these amorphous films have been determined by fitting theoretical dispersion curves to the measured SAW velocity characteristics. Frequency-dependent SAW propagation-loss values have been determined from the observed linear change in loss as a function of transducer separation. Preliminary measurements of the temperature coefficient of frequency (TCF) for SAW propagation of the films on GaAs are also given     

Deposition and piezoelectric characteristics of ZnO films by usingan ECR sputtering system

Piezoelectric properties of ZnO films, were investigated by using an Electron Cyclotron Resonance (ECR) sputtering system. It was confirmed that this system was capable of depositing a ZnO film with a large specific resistance, and good c-axis orientation on an interdigital transducer (IDT)/glass substrate at a low temperature (less than 200°C) and in a low gas pressure (~10^-4 torr). Furthermore, these ZnO films exhibited excellent SAW characteristics (insertion losses) and effective electromechanical coupling factors (k _eff) compared with ZnO films deposited by a conventional RF magnetron sputtering system. Further, this ECR sputtering system was capable of depositing a ZnO film, without heating the substrate, that was capable of propagating a Rayleigh SAW at 700 MHz for the first time     

Effect of particle bombardment on the orientation and the residual stress of sputtered AlN films for SAW devices

We present a study of the effect of particle bombardment on the preferred orientation and the residual stress of polycrystalline aluminum nitride (AlN) thin films for surface acoustic wave (SAW) applications. Films were deposited on silicon (100) substrates by radio frequency (RF) sputtering of an aluminum target in an argon and nitrogen gas mixture. The main deposition parameters were changed as follows: the total pressure from 4 mTorr to 11 mTorr, the N2 content in the gas mixture from 20% to 80%, and the substrate self-bias voltage from -10 V to -30 V. If a sufficiently high negative substrate self-bias voltage is induced, (00.2)-oriented films are obtained over the full ranges of pressure and N2 content. Such films have values of residual stress ranging from -3 GPa to +1 GPa, depending on the deposition conditions. Our results suggest that the energy of the Ar ions colliding with the substrate controls the preferred orientation of the films, whereas the directionality of the ions (for the same energy) is the main factor determining the residual stress. To demonstrate the suitability of our material for the intended application, SAW filters with good electroacoustic response have been fabricated using AlN thin films with optimized (00.2) orientation and controlled residual stress.     

Deposition and dielectric properties of CaCu3Ti4O12 thin films on Pt/Ti/SiO2/Si substrates using pulsed-laser deposition

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully deposited on Pt/Ti/SiO₂/Si(1 0 0) substrates using pulsed-laser deposition technique. The crystalline structure and the surface morphology of the CCTO thin films were greatly affected by the substrate temperature and oxygen pressure. Thin films with a (2 2 0) preferential orientation were obtained at the substrate temperature above 700 °C and oxygen pressure above 13.3 Pa. The 480-nm thin films deposited under 720 °C and 26.6 Pa have a fairly high dielectric constant of near 2000 at 10 kHz and room temperature. The values of the dielectric constant and loss and their temperature-dependence under different frequency are comparable with those obtained in the epitaxial CCTO films grown on oxide substrates.     

Epitaxial aluminum nitride films on sapphire formed by pulsed laser deposition

Highly oriented aluminum nitride thin films were grown on sapphire (0001)-substrate by pulsed laser deposition technique. Characterization was done by X-ray-diffraction, elastic recoil detection analysis and Rutherford backscattering/channeling measurements. The epitaxial properties were studied as function of the substrate temperature and the deposition rate. An epitaxial relation to the sapphire substrate is found to be AlN [0001] || Al₂O₃ [0001] and AlN [11 0] || Al₂O₃ [10 0]. XRD-texture-analysis on films deposited at 850°C shows a full width half maximum Δω of 0.13° (rocking curve) and Δ of 1.1° (in-plane).     

Effect of crystalline quality of diamond film to the propagation loss of surface acoustic wave devices

Diamond films with various crystal qualities were grown by chemical vapor deposition on silicon wafers. Their crystallinity was characterized by Raman scattering and electron backscattering diffraction. By fabricating a device structure for surface acoustic wave (SAW) using these diamond films, the propagation loss was measured at 1.8 GHz and compared with the crystallinity. It was found that the propagation loss was lowered in relatively degraded films having small crystallites, a narrow distribution in the diamond crystallite size, and preferential grain orientation. This experiment clarifies diamond film characteristics required for high-frequency applications in SAW filters.     

ZnO films on {001}-cut <110>-propagating GaAs substrates forsurface acoustic wave device applications

A potential application for piezoelectric films on GaAs substrates is the monolithic integration of surface acoustic wave (SAW) devices with GaAs electronics. Knowledge of the SAW properties of the layered structure is critical for the optimum and accurate design of such devices. The acoustic properties of ZnO films sputtered on {001}-cut 〈110〉-propagating GaAs substrates are investigated in this article, including SAW velocity, effective piezoelectric coupling constant, propagation loss, diffraction, velocity surface, and reflectivity of shorted and open metallic gratings. The measurements of these essential SAW properties for the frequency range between 180 and 360 MHz have been performed using a knife-edge laser probe for film thicknesses over the range of 1.6-4 μm and with films of different grain sizes. The high quality of dc triode sputtered films was observed as evidenced by high K² and low attenuation. The measurements of the velocity surface, which directly affects the SAW diffraction, on the bare and metalized ZnO on SiO₂ or Si₃N₄ on {001}-cut GaAs samples are reported using two different techniques: 1) knife-edge laser probe, 2) line-focus-beam scanning acoustic microscope. It was found that near the 〈110〉 propagation direction, the focusing SAW property of the bare GaAs changes into a nonfocusing one for the layered structure, but a reversed phenomenon exists near the 〈100〉 direction. Furthermore, to some extent the diffraction of the substrate can be controlled with the film thickness. The reflectivity of shorted and open gratings are also analyzed and measured. Zero reflectivity is observed for a shorted grating. There is good agreement between the measured data and theoretical values     

Surface acoustic wave velocity in single-crystal AlN substrates

The surface acoustic wave velocity has been measured on a-plane (c-propagation) and c-plane oriented bulk aluminum nitride (AlN) single crystals using the S11-parameter method in the frequency range 160-360 MHz. The SAW velocity is 5760 m/s for both orientations. From comparison of this value with the simulations using various elastic constants of AlN available in literature, we estimated the elastic constant C44 to be 122 +/- 1 GPa.     

Optimization of the annealing process and nanoscale piezoelectric properties of (002) AlN thin films

In this paper, the effects of different annealing processes on the texture, surface morphology, and piezoelectric properties of aluminum nitride (AlN) thin films and the performance of AlN-based surface acoustic wave (SAW) devices were systematically investigated. Based on the crystallinity and the morphology results, it is evident that in-situ annealing method is superior to ex-situ annealing. For the AlN thin films, the crystallization and piezoelectricity were both enhanced and then receded as the annealing temperature increased from 300 to 600?°C. We demonstrated that good (002) orientation, excellent grain distribution and high relative piezoelectric coefficient of the AlN thin films were achieved via in-situ annealing at 500?°C. Meanwhile, the AlN thin films exhibited excellent polarization properties and polarization maintaining characteristics. Additionally, the uniform interdigital transducer (IDT) with 8 μm period (finger width?=?2 μm) were designed and the IDT/AlN/SiO₂/Si SAW devices with the center frequency f ₀ of 495 MHz and insert loss of ?24.1 dB were fabricated.     

等离子体辅助反应式脉冲激光熔蚀制备AlN薄膜的低温生长

使用等离子体辅助反应式脉冲激光溅射沉积薄膜的方法在Ｓｉ（１１１）和Ｓｉ（１００）基片上已经成功地低温制备出ＡｌＮ多晶膜。实验表明,当脉冲能量密度ＤＥ＝１．０Ｊ·ｃｍ－２,脉冲频率ｆ＝５Ｈｚ,氮气气压ＰＮ２＝１．３３×１０４Ｐａ,基底温度ｔｓｕｂ＝２００℃,放电电压Ｖ＝６５０Ｖ,基靶距离ｄＳ－Ｔ＝４ｃｍ时薄膜的生长速度等于６ｎｍ／ｍｉｎ。ＡｌＮ薄膜的折射率为２．０５,和基底的取向关系分别为：ＡｌＮ（１１０）∥Ｓｉ（１１１）和ＡｌＮ（１００）∥Ｓｉ（１００）。     

Growth of columnar aluminum nitride layers on Si(111) by molecular beam epitaxy

Single crystalline aluminum nitride (AlN) thin films are deposited by molecular beam epitaxy (MBE) using thermally evaporated aluminum and RF-plasma excited nitrogen gas. In this paper we report on films grown on Si(111) at substrate temperatures of 800° with growth rates between 65 and 350 nm h⁻¹. All layers consist of hexagonal and exactly c-axis oriented AlN crystals with column-like structure. For the smoothest layers surface roughness (rms) around 1 nm is obtained. In the XRD-spectra (ω-scan) we have achieved a minimum FWHM of 0.4° (=25′) for the AlN(00.2) reflex. At maximum growth rates (350 nm h⁻¹) for AlN a transition zone of about 200 nm is formed with high defect density compared to the subsequent growth. For lower growth rates (65 nm h⁻¹) no transition zone exists. Application of a substrate nitridation leads to a partial loss of epitaxial relation between AlN layer and Si(111)-substrate.     

Growth of GaN and AlN thin films by laser induced molecular beam epitaxy

Hexagonal GaN and AlN thin films were grown by laser induced molecular beam epitaxy using Al or Ga metal as target material and N₂as nitrogen source. The films were deposited on sapphire (0001) and SiC (0001) substrates. Epitaxial growth of GaN has been achieved at 730°C and 10⁻³ mbar N₂ pressure. The AlN films were polycrystalline with predominant (0001) orientation.     

Analysis of SAW properties of epitaxial ZnO films grown on R-Al2O3 substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112¯0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al₂O₃ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h/λ). The acoustic wave properties of the material system were calculated using Adler's matrix method, and the devices were simulated using the quasi-static approximation based on Green's function analysis     

Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications

This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K(2) and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.     

Circuital Model for the Analysis of the Piezoelectric Response of AlN Films Using SAW Filters

In this paper we describe a method to assess the piezoelectric response of a piezoelectric thin film deposited on a conductive substrate. It is based on analyzing the frequency response of a surface acoustic wave (SAW) filter made on the piezoelectric thin film. For this analysis, we use a circuital model that takes into account the theoretical response of the ideal filter along with all the external and internal parasitic effects that deteriorate the response. Using this model, we can obtain the electromechanical coupling factor of the piezoelectric material (k² _m) with good accuracy. If parasitic effects are not considered, k² _m can be underestimated by a factor of up to 20. We have tested our model using SAW filters made on AlN thin films sputtered on substrates with different conductivities. A discussion on the relation between the different circuital elements and the physical properties of the filters also is provided.     

Epitaxial growth of AlN on single crystal Mo substrates

We have grown AlN films on single-crystalline Mo(110), (100), and (111) substrates using a low temperature pulsed laser deposition (PLD) growth technique and investigated their structural properties. Although c-axis oriented AlN films grow on Mo(100), the films contain 30° rotated domains due to the difference in the rotational symmetry between AlN(0001) and Mo(100). AlN films with only poor crystalline quality grow on Mo(111) substrates, probably due to the poor surface morphology and high reactivity of the substrates. On the other hand, single crystal AlN films grow epitaxially on Mo(110) substrates with an in-plane relationship of AlN[11-20] // Mo[001]. Reflection high-energy electron diffraction or electron backscattered diffraction analysis has revealed that neither in-plane 30° rotated domains nor cubic phase domains exist in the AlN films. X-ray reflectivity measurements have revealed that the heterointerface between AlN and Mo prepared by PLD at 450 °C is quite abrupt. These results indicate that PLD epitaxial growth of AlN on single crystal Mo substrates is quite promising for the fabrication of future high frequency filter devices.     

Growth of SrTiO3 thin epitaxial films by aerosol MOCVD

SrTiO₃ thin films were prepared by aerosol metal-organic chemical vapour deposition on (001) MgO, R-plane Al₂O₃ and (001) Si single-crystal substrates. Strontium tetramethyl heptadionate and titanium n-butoxide dissolved in diethyleneglycol dimethyl ether were used as precursors. The structure of the films was investigated by X-ray diffraction and transmission electron microscopy. Epitaxial films with [001] and [111] orientation perpendicular to the substrate surface were obtained on MgO and Al₂O₃, respectively. The epitaxial films on the MgO substrate were found to be in a relaxed state with lattice parameters corresponding to the bulk values. SrTiO₃ films on the Si substrate were grown as highly textured in the [011]direction and randomly oriented in the plane parrallel to the substrate surface.