Surface acoustic wave sensors to detect volatile gases by measuring output phase shift

This paper presents properties of saw acoustic wave (SAW) gas sensors to detect volatile gases such as acetone, methanol, and ethanol by measuring phase shift. A dual-delay-line saw sensors with a center frequency of 100 MHz were fabricated on 128^∘ Y-Z LiNbO₃ piezoelectric substrate. In order to improve sensitivity of SAW sensors, a thin titanium (Ti) film as mass sensitive layer was deposited using e-beam evaporation on the surface of the SAW sensors. In our investigation the response time and sensitivity of SAW sensors were measured. The response time and sensitivity of SAW sensor with thin Ti film were strongly improved because of changing electrical and mechanical properties in the mass sensitive layer. As a result, high sensitivity and fast response time could be achieved by deposition of thin Ti film as mass sensitive layer on the surface of SAW sensor. It can be applied for high performance electronic nose system by assembling an array of different sensors.     

Characteristics of lithium niobate based capacitors and transistors

Abstract

The electrical properties of thin film (<1000 Å) capacitor devices of lithium niobate grown on silicon and platinum and of thicker film metal-ferroelectric-semiconductor field effect transistors (MFSFET) with lithium niobate as the gate material were measured. Dielectric constants of the thin films on silicon were as high as 27, while those for films on platinum were as high as 49. The MFSFET structures showed good FET properties, and demonstrated a channel current modulation consistent with switching of the ferroelectric gate by pulsing.     

Optical Waveguiding Properties of (Pb,La)TiO3/Al2O3 Planar Structures

Thin films of lead lanthanum titanate (Pb,La)TiO₃ have been grown by radio-frequency magnetron sputtering on (0001) Al₂O₃ substrates. The structure, the microstructure and the optical properties of the films have been investigated as a function of the postdeposition annealing. Films deposited at low temperatures crystallize to a perovskite phase after the annealing treatment from 500°C to 650°C. X-ray (–2) diffraction studies have shown that films are crystallized with a strong (111) orientation and the best crystalline structure is reported at 600°C. The optical properties were both demonstrated by spectrophotometry and prism coupling. PLT thin films with a transparency of 80% in the wavelength range 300–2000nm have exhibited a refractive index of 2.38 @ 632.8nm representing 97% of the bulk corresponding material. Investigation of optical propagation has been accomplished in a 10mm long planar optical waveguide using a butt-coupling configuration.     

Electrical switching in lithium niobate thin films

The ferroelectric switching properties of thin films of lithium niobate (LiNbO3) on silicon have been investigated. The polarization is shown to be partially reversible with an applied electric field at room temperature. The samples were films of LiNbO3 which were magnetron sputter deposited on silicon substrates. A double pulse method was used to investigate the ferroelectric switching properties of the films. Evidence for partial stable switching of the LiNbO3 film was observed at a field of roughly 500 kV/cm.     

ZnO based surface acoustic wave ultraviolet photo sensor

Ultraviolet (UV) sensor based on ZnO thin film surface acoustic wave (SAW) device is reported. ZnO films were grown using an RF magnetron sputtering technique. SAW devices were made using such ZnO films exhibiting a central frequency at ～41.2 MHz. The SAW UV sensor was fabricated by depositing a 70 nm thin photoconducting ZnO overlayer on the fabricated SAW device. The SAW UV sensor was found to exhibit interesting photoresponse behavior to UV illumination, and a downshift in frequency of ～45 kHz, and a change in insertion loss ～1.1 dB were observed under UV illumination intensity of 19 mW/cm². The changes in the frequency of operation and the insertion loss have been attributed to the acoustoelectric interaction between the photogenerated charge carriers and the potential associated with the acoustic waves. Results show the promise of ZnO for the fabrication of low cost wireless SAW UV sensors.     

The Effect of Stress on the Microwave Dielectric Properties of Ba0.5Sr0.5TiO3 Thin Films

Single phase, (1 0 0) epitaxial Ba_0.5Sr_0.5TiO₃ (BST) films have been deposited onto (1 0 0) LaAlO₃ and MgO substrates by pulsed laser deposition (PLD). The capacitance and dielectric losses of as-deposited and annealed films have been measured from 1–20 GHz as a function of electric field (0–80 kV/cm) at room temperature. The dielectric properties are strongly affected by the substrate type, post-deposition annealing time (6 h) and temperature (1200°C). For epitaxial BST films deposited onto MgO, it is observed that, after a post-deposition anneal the dielectric constant and the dielectric loss decreases. For epitaxial BST films deposited onto LAO, a post-deposition anneal (1000°C) results in an increase in the dielectric constant and an increase in the dielectric loss. The dc electric field induced change in the dielectric constant tends to increase with the dielectric constant and is largest for as-deposited films on MgO and post-deposited annealed films on LAO. In general, for epitaxial BST films, a large electric field effect is observed in films that have a large dielectric loss and a small electric field effect in films that have a low dielectric loss. High resolution X-ray diffraction measurements indicate that deposited film exhibit a significant tetragonal distortion which is strongly affected by a by a post deposition anneal. The observed differences in dielectric properties of the epitaxial BST films on MgO and LAO are attributed to the differences in film stress which arise as a consequence of the lattice mismatch between the film and the substrate and the differences in the thermal coefficient of expansion between the film and the substrate. A thin amorphous buffer layer of BST has been used to relieve stress induced by the lattice mismatch between the film and the substrate. Unlike epitaxial films, stress relieved films do not show an inverse relationship between dielectric tuning and Q (1/tan) and may be superior materials for tunable microwave devices.     

Ca Substituted PbTiO3 Thin Films for Infrared Detectors

Crack free Ca substituted PT thin films have been deposited on ITO coated 7059 glass substrates by sol gel technique and crystallized at 650C. Characterization of these films by X-ray diffraction show that the films exhibit tetragonal structure with perovskite phase. AFM, hysteresis, dielectric relaxation and pyroelectric studies have been carried out. The pyroelectric figures of merit of the films have been calculated. Our investigations show that these films are expected to give high infrared detector performance due to its high pyroelectric coefficient (43 nC/cm²K), high voltage responsivity (2340 Vcm²/J) and detectivity(3 × 10^{– 5} Pa^{– 1/2}) along with small value of dielectric constant (83) and loss tangent (0.04).     

Partial Discharge Signal Detection by Piezoelectric Ceramic Sensor and The Signal Processing

Partial discharge (PD) in an insulator or on surface of defective conductor emits acoustic wave transmitting through an air or an insulator. The acoustic wave between 20 kHz and several hundred of kHz can be detected by piezoelectric ceramic sensor that converts the acoustic wave into an electrical signal. Piezoelectric ceramic sensor has either the wide resonant band or the local resonant band depending on the ceramic material or the various combinations of each different component in the manufacturing process. This paper presents the piezoelectric ceramic sensor with 0.95 PZT–0.05 PMNS that yields the piezoelectric properties of high k_p, high Q_m. It has the frequency characteristics of local resonant band, such that it can be applied to PD detection. We have demonstrated the properties of the proposed piezoelectric ceramic sensor by comparing with the conventional electrical PD detector. Quantitative analysis is accomplished by comparing the -q_max from a PD detector and the -v_max from the proposed sensor while -n distributions are the same for both the conventional phase-resolved PD analysis method and the proposed one.     

Annealing Effects on Structural and Dielectric Properties of Tunable BZT Thin Films

Ba(Zr, Ti)O₃ thin films have attracted great attention in recent years for their potential use in DRAMs and MCMs due to their high dielectric constant and relatively low leakage current. However, their tunable dielectric properties were rarely investigated and the corresponding potential for tunable microwave applications was seldom reported. In this paper, we present the tunable dielectric behavior of BZT thin films deposited by RF magnetron sputtering from a Ba(Zr_0.3Ti_0.7)O₃ ceramic target on MgO single crystal substrates. The composition, thickness and crystallinity of the thin films were analyzed by Rutherford backscattering (RBS), scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The dielectric constant and loss tangent were measured as a function of electric field (0–7 kV/mm) and temperature (–140 to +160°C) at frequencies up to 1 MHz, using interdigital capacitors (IDC) with Au electrodes on thin films. By optimizing the preparation process, a tunability {defined as = [ (0) – (E_max)]/ (0)} of 76% at E_max = 7 kV/mm and a low loss tangent of 0.0078 can be achieved. In addition, the influence of annealing temperature on the dielectric properties of the thin films is also discussed.     

Pb(Mg1/3Nb2/3)O3 and (1 − x)Pb(Mg1/3Nb2/3)O3 − xPbTiO3 Relaxor Ferroelectric Thick Films: Processing and Electrical Characterization

The lead magnesium niobate [Pb(Mg_1/3Nb_2/3)O₃ or PMN], and its solid solutions with lead titanate (PbTiO₃ or PT), are of great interest because of their high electromechanical properties. At large PMN content, these materials exhibit relaxor characteristics with large electrostrictive strains and a large permittivity, while compositions near the morphotropic phase boundary present very interesting piezoelectric properties. So far, properties of these materials in ceramic, thin film and single-crystal form have been investigated. In this paper, we report on preparation and properties of pyrochlore free PMN and 0.65PMN-0.35PT thick films (thickness = 10 to 20 m). The films were prepared from ethyl cellulose ink by screen printing on alumina substrate. The influence of various parameters, such as powder characteristics, inks formulation and films sintering conditions, on films densification are discussed. The dielectric and electromechanical properties of the films were examined. Relaxor-like behaviour was clearly demonstrated in PMN films. The maximum relative permittivity for PMN film was 10000 (at 0.1 kHz), which is lower than in bulk ceramics (17800 at 0.1 kHz) prepared under the same conditions. For 0.65PMN-0.35PT, the maximum relative permittivity was around 15500 against 24000 in the bulk. Several parameters, which might be responsible for the lower permittivity, are discussed. Poled 0.65PMN-0.35PT thick films exhibit relatively large piezoelectric response (d ₃₃ up to 200 pm/V) and unipolar strains approaching 0.1%, making these films of interest for various actuator and transducer applications.     

Sol-gel Synthesis of Lithium Niobate Powder and Thin Films Using Lithium 2,4-Pentanedionate as Lithium Source

Lithium niobate powder was prepared at 500°C by a sol-gel method using lithium 2,4-pentanedionate as the lithium source. This method offers an advantage over the double alkoxide method as lithium 2,4-pentanedionate is less prone to moisture as compared to lithium ethoxide precursor and also it is easily soluble in 2-methoxyethanol. Microstructural investigation revealed sub- micrometer gel powder heat treated at 600°C. Thin films of lithium niobate with no preferred orientation were obtained at 350°C by spin coating of the above precursor solution onto platinum coated silicon substrates.     

Temperature Stability Analysis of CMOS-SAW Devices by Embedded Heater Design

The design, finite element (FE) modeling, and electrical characterization of an embedded heater in complementary metal–oxide–semiconductor (CMOS) are presented. The heater is used to analyze the temperature stability behavior of CMOS-surface acoustic wave (SAW) devices. The heater employs n-well layer of standard CMOS technology to provide high efficiency resistive heating without physically perturbing the SAW architectures and performances. A detailed 3-D model and FE investigation is laid out to characterize the heat, current, temperature, and thermal energy distributions within the substrate and the piezoelectric material of interest ZnO. Electrical characterization based on Wheatstone configuration is presented to analyze the temperature stability of the sputtered ZnO and the CMOS-SAW delay lines. A temperature coefficient of frequency of $-hbox{48.815} hbox{ppm}/^{circ}hbox{C}$ for the fabricated SAW devices with operating frequency of 322.5 MHz is obtained. The experimental results show close agreement with the FE simulations. The results demonstrate that the embedded heater design can be used as a robust analytical tool to investigate temperature stability of CMOS-SAW devices and potentially be utilized as an on-chip element for chemical, biological, and temperature sensor applications.      

Effect of Oxygen Plasma on Growth, Structure and Ferroelectric Properties of SrBi2Ta2O9 Thin Films Formed by Pulsed Laser Ablation Technique

Growth of SrBi₂Ta₂O₉ (SBT) thin films has been carried out in the presence of O₂-plasma created by applying a potential at an auxiliary ring electrode placed near the substrate. Effect of plasma excitation potential and polarity, especially negative polarity, on the formation of a proper SBT phase at 700°C and in modifying crystallite orientation and microstructure of SBT films over (1 1 1) oriented Pt film coated over TiO₂/SiO₂/Si(1 0 0) substrates has been demonstrated. Preferred c-axis orientation of SBT films changes to (a–b) orientation with decrease in plasma excitation potential from –700 to –350 V and eliminates secondary Bi₂Pt phase formation even at 600°C Microstructural study show a 2-dimensional large flat c-oriented crystallites formed at –700 V change to small crystallites in conformity with the changed aspect ratio for crystallites in (a–b) plane parallel to film plane. Spectroscopic ellipsometric results are in agreement with the microstructural data. These affects are attributed to O₂-ion bombardment during film growth which reduces nucleation barrier for growth of crystallites in (a–b) plane. O₂-plasma sustains the cationic species formed by laser ablation, which along with O ₂ ⁺ ions, provide necessary activation energy and enhance the oxidation rates required for SBT phase formation even at 700°C. SBT films grown in O₂-plasma show enhancement in remnant polarization value from 1.2 to 6.6 C/cm² and display ferroelectric properties superior to those formed without plasma. Further O₂-plasma eliminates post deposition annealing step for observance of enhanced polarization values. This study shows O₂-plasma excitation potential could be exploited as a new process parameter in laser ablation growth of ferroelectric oxide thin films.     

BaTiO3−δ: Defect Structure, Electrical Conductivity, Chemical Diffusivity, Thermoelectric Power, and Oxygen Nonstoichiometry

Electrical conductivity, thermoelectric power, and chemical diffusivity are the most typical, charge-and-mass transport properties of a mixed ionic electronic conductor oxide which are essentially governed by its defect structure, and the oxygen nonstoichiometry is a direct measure of its overall defect concentration. For the system of BaTiO_3–, the total electrical conductivity has been the most extensively and systematically studied as a function of oxygen partial pressure at elevated temperatures. The other properties have also been studied, but much less extensively and systematically. The electrical conductivity and thermopower were occasionally measured together on the same specimens so that mutual compatibility or consistency might be secured. But, the rest were all determined separately on the specimens of differing quality, consequently lacking in mutual consistency. It, thus, has remained hard to evaluate the canonical, defect-chemical parameters which are consistent with each and every of these defect structure-sensitive properties that were observed. Very recently the authors have determined the total conductivity, chemical diffusivity and thermoelectric power altogether on the same specimens of BaTiO_3–, and the nonstoichiometry on the same-quality specimens at temperatures of 1073 T/K 1373 over wide enough a range of oxygen partial pressure (normally, 10^–16 Po₂/atm 1) that encloses an electron/hole/ion mixed regime. In this article, we will compile all the literature data on these defect-structure-sensitive properties and extract from the authors' own, without using any ad hoc assumptions regarding, e.g., the electronic carrier mobilities and effective density of states, the basic defect-chemical parameters including defect-equilibrium constants, carrier mobilities and densities, and electronic heats of transport, which are the most consistent with the properties observed. Compared to the conventional picture of the defect structure of undoped BaTiO₃, thus, some new insights into the defect chemical nature of BaTiO_3– are provided.     

Optimization of Non-Fluorine Sol-Gel Derived YBCO Thin Films

Sol-gel deposition of thin film YBa₂Cu₃O_7– (YBCO) is widely seen as the most cost effective means to manufacture long length HTS wires (A.P. Malozemoff, D.T. Verebelyi, S. Fleshler, D. Aized, and D. Yu, HTS Wire: Status and Prospects, Physica C, 386, 424 (2003)). We present a sol-gel technique for YBCO deposition using low cost starting materials and forming only benign by-products during processing. Optimization has concentrated on producing the correct stoichiometry in the final films. RBS measurements show that copper diffuses into the substrate due to its high mobility at the temperatures required to form the YBCO phase. Therefore a starting stoichiometry of 1:2:3 can produce a film highly copper deficient. In order to compensate for this we have made films with different excesses of copper in the precursor solution. The surface morphologies of these films have been examined by SEM and AFM, and the stoichiometry and cation depth profiles characterized by ion beam analysis. The effect of the varying copper stoichiometry is correlated with the superconducting properties and the surface morphology for films on lanthanum aluminate (LAO) and magnesium oxide (MgO) substrates. The residual carbon concentration in the films is measured by nuclear reaction analysis.     

Effect of Thickness on the Electrical and Optical Properties of Sb Doped SnO2 (ATO) Thin Films

This work reports the preparation and characterization of (SnO₂) thin films doped with 7 mol% Sb₂O₃. The films were prepared by the polymeric precursor method, and deposited by spin-coating, all of them were deposited on amorphous silica substrate. Then, we have studied the thickness effect on the microstrutural, optical and electric properties of these samples. The microstructural characterization was carried out by X-ray diffraction (XRD) and scanning tunneling microscopy (STM). The electrical resistivity measurements were obtained by the van der Pauw four-probe method. UV-visible spectroscopy and ellipsometry were carried out for the optical characterization. The films present nanometric grains in the order of 13 nm, and low roughness. The electrical resistivity decreased with the increase of the film thickness and the smallest measured value was 6.5 × 10^{– 3} cm for the 988 nm thick film. The samples displayed a high transmittance value of 80% in the visible region. The obtained results show that the polymeric precursor method is effective for the TCOs manufacturing.     

BST Capacitor Based Tunable Matching Network for SAW Sensors

Surface acoustic wave (SAW) sensors are becoming extremely important in environmental health monitoring by wireless technologies. SAW devices need impedance matching networks at the input and output for efficient power transfer. In this paper, we are presenting the results of tunable matching networks for SAW devices with BST capacitors. We used simple L-matching network implemented with chip inductors and tunable BST capacitors. The characteristics of tunable matching network interfaced to SAW device is simulated using Agilent's ADS simulation tool. The return loss from the SAW devices can be increased and tuned significantly by applying a tuning voltage to the BST capacitors. The SAW device which has been studied is SAW filter.     

Coplanar Waveguide Using Ferroelectric Thin Oxide Film: Dielectric Constant

Coplanar waveguide (CPW) transmission lines were fabricated on thin ferroelectric Ba_{1 – x}Sr_xTiO₃ films for tunable microwave applications. The growth of the ferroelectric oxide films was accomplished by a pulsed laser deposition with a partial oxygen background. Microwave properties of the CPW phase shifter were measured using a HP 8510C vector network analyzer from 0.045–20 GHz with –40–40 V of dc bias. A large phase shift angle of 120 at 10 GHz was observed from the CPW (gap = 4m, length = 3 mm) with a 40 V of dc bias change. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameter by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film exhibits a gap dependency; dielectric constant (990–830) decreases with increasing gap size (4–19 m, respectively). By adjusting the filling factors of the film, a constant dielectric constant of BST film is found to be 810 ± 5.     

PIMOD processing and properties of a LiNbO3-based MFSFET for nonvolatile memories

Abstract

The photo-induced metallo-organic decomposition (PIMOD) process has been successfully used to deposit a lithium niobate thin film acting as the gate oxide of the conventional MFSFET structure. The use of the low-temperature PIMOD process for thin film deposition has increased the device yields of the molybdenum liftoff for small area isolation. The electronic alteration of the properties of the ferroelectric gate transistor was previously shown to be caused by charges in the semiconductor being injected into the ferroelectric film. To prevent this problem, a thin SiO₂ buffer layer was thermally grown on the silicon substrate immediately before lithium niobate deposition. The silicon-lithium niobate interface was stabilized and the charge injection effect was eliminated due to the formation of the buffer layer. The channel current was shown to be greatly altered by the application of voltage pulses between the gate of the device and the substrate. Upon switching, the change in surface conductivity of the semiconductor was the same as that expected for ferroelectric switching.     

A Silicon Modulator Enabling RF Over Fiber for 802.11 OFDM Signals

We investigate the linearity properties of silicon modulators and show that, contrary to the traditional lithium niobate Mach–Zehnder modulators (MZMs), the third-order intermodulation distortion (IMD3) for silicon modulators is a function of the modulator bias point. The bias point for silicon modulators can be chosen to reduce the IMD3 well below that of standard lithium niobate MZMs. Given the cost and integration advantages of the silicon photonics technology, silicon modulators offer significant advantages for emerging radio over fiber applications. As an example, we examine, for the first time to our knowledge, a silicon modulator for converting analog 802.11 RF signals to the optical domain, achieving an error vector magnitude of −30 dB.