Surface acoustic wave characterization of a thin, rough polymer film

Laser generated surface acoustic waves (SAW) in a heterodyne diffraction scheme is a powerful technique for elastic characterization of thin films and it is frequently used on samples of high optical quality. We show that the method can also be effectively used in difficult conditions, on rough samples. Measurements are presented on a 3 µm thick film of polymer, spin-coated on steel, and on the same sample after addition of an aluminum coating. The experimental data are interpreted using a model assuming a stack of perfect layers. The analyses show good consistency within the SAW results for both configurations, and consistency with nano-indentation results, cross-validating both approaches.     

Surface acoustic wave sensors incorporating Langmuir-Blodgett films

Acoustoelectric devices offer many attractive features for applications as physical and chemical sensors. Surface acoustic wave (SAW) oscillators are of particular importance owing to their high sensitivity. This paper describes the use of Langmuir-Blodgett (LB) films as gas absorbent layers on the surface of SAW devices.

Areal densities of standard LB film forming materials were measured and found to agree with those obtained from pressure-area isotherms. Sensors incorporating ω-tricosenoic acid and docosylamine overlayers were examined and their responses to alkanoic acids reported.

The room temperature chemiresponse of a SAW device coated with monolayers of tetra-4-tert butyl silicon phthalocyanine dichloride showed response and recovery times comparable with those reported for other phthalocyanine-based sensors operated at much higher temperatures. The detection limit of the LB film device was found to be 40 ppb NO₂ in dry air at an operating frequency of 98.6 MHz and an ambient temperature of 22°C. The frequency change was shown to be entirely due to the mass of gas absorbed by the film.     

Surface acoustic wave characteristics and electromechanical coupling coefficient of lead zirconate titanate thin films

Lead zirconate titanate (PZT) thin films were created on ST-X quartz using radio frequency magnetron sputtering deposition. PZT films deposited on quartz are used as a new piezoelectric substrate for surface acoustic wave (SAW) devices. Microelectromechanical technique was used to fabricate interdigital transducers on the surface of the substrate to be used as a SAW delay line device. The results show that the PZT film was successfully deposited on ST-X quartz, and that the PZT film on ST-X quartz can enhance the electromechanical coupling coefficients of SAW.     

Surface acoustic wave properties of freestanding diamond films

"Ideal" diamond has the highest acoustic velocity of any material known, and is of great interest as a substrate material for high frequency surface acoustic wave (SAW) device structures. However, little is known of the acoustic wave propagation properties of polycrystalline diamond grown by chemical vapour deposition (CVD) techniques, the commercially accessible form of this material. We report on propagation of laser-generated SAW on three forms of freestanding CVD diamond samples, "white" polycrystalline, "black" polycrystalline, and "highly oriented" diamond. Despite differing sample nature, SAW waves propagating along the smooth (nucleation) side of the diamond showed similar velocities in the range 10600-11900 ms(-1). These results are discussed in terms of the potential of each form of CVD diamond for SAW device fabrication.     

Vacuum deposition rate measurements on thin polymer films

The evaporation of polymer films is described by a simple theory, which indicates clearly that electronic rate measurement gives no correct result due to the evaporation mechanism. The theory is illustrated with experimenal results made on PTFE.     

Surface acoustic wave measurements of surface cracks in ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.     

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     

Surface impedance of NbN superconducting thin films by Josephson measurements

The microwave quality factorQ of NbN-Pb Josephson junctions has been determined via measurements of the amplitude of Fiske resonant modes in the junctions at different resonant frequencies in the range 30–300 GHz. It is proved that, for our samples, the main contribution to theQ comes from the surface impedance of the filmsR _s . Data show that, for NbN,R _s v^1.5±0.1 atT _c =4.2 K in the frequency range considered. The implications of our results for the possible applications of NbN films both in the context of Josephson microwave devices and as a coating material for rf cavities in high-energy physics are discussed.     

Surface acoustic wave properties of aluminum oxide films on lithium niobate

Aluminum oxide (Al₂O₃) films have been deposited on lithium niobate (LiNbO₃) substrates by electron beam evaporation without any interlayer between them to ensure a good adhesion of the Al₂O₃ films to LiNbO₃ substrates. As Al₂O₃ thin films can sufficiently increase the surface acoustic wave (SAW) velocity, they can be used to improve the performance of the SAW device. The SAW phase velocity in the Al₂O₃/LiNbO₃ structure was found to increase with the insertion of an Al₂O₃ film, which can be attributed to the stiffening effect of the Al₂O₃ layer. The velocity change ratio of SAW was about 4.39% (at 304 MHz) for the Al₂O₃ (9.7 μm)/LiNbO₃ sample. A comparison with other findings in literature reveals that this result is better than what is available from diamond-like carbon/SiC buffer layer/LiNbO₃ structure, whose the velocity change ratio is 2%.     

Viscoelastic properties of polymer films on surface acoustic wave organophosphorous vapor sensors

This work investigates the viscoelastic properties of the fluoropolyol (FPOL) polymer on the surface acoustic wave (SAW) organophosphorous vapor sensors. A complex shear modulus is used to express different polymer types (glassy, glassy-rubbery, and rubbery). The different polymer types leads to different propagating properties of SAW, such as attenuation change and velocity shift. Calculation results indicate that the glassy-rubbery film exhibits the highest sensitivity for detecting organophosphorous vapor. The thicker the glassy and glassy-rubbery film implies a higher sensitivity. Moreover, the SAW vapor sensor based on the rubbery film represents the response of acoustically thick layers which has a peak in attenuation with an increasing vapor adsorption. The selectivity factor between DMMP (10 ppm) and H₂O (40%RH) is so low that the selectivity of FPOL film towards water is ineffecient. However, the selectivity factor between ethanol (10 ppm) and DMMP (10 ppm) is as high as 2512, thus confirming that the selectivity of FPOL film towards ethanol is good. Therefore, a precise and dry humidity control in the sensors system with FPOL coating is required.     

Surface modes in magnetic thin films and in spin wave resonance

Of late, the number of papers on magnetic surface states has greatly increased since the experimental detection of these states has been proven to open up a highly promising source of abundant data on the magnetic properties of materials. Spin wave resonance (SWR)-an effect observed in thin ferromagnetic films-is well adapted for such detection. In this paper, we are concerned with the theoretical conditions for surface mode detection in SWR. Since SWR depends essentially on the conditions existing on the surfaces of a film, we begin with a closer analysis of the concept of surface spin pinning. Next, resorting to the Surface Inhomogeneity Model (thin film, totally homogeneous within, but presenting arbitrary asymmetric conditions on its surfaces), we solve the problem of the existence of surface modes as a function of the pinning conditions, and discuss the essential features of these modes. Finally, we propose a criterion for identifying surface peaks in SWR spectra.     

Hardness measurements of thin films

A simple technique is presented to obtain the characteristic microhardness values of surface coatings that are too thin for the values to be directly measured. The application of the technique requires conventional microhardness measurements on both coated and uncoated substrates and a knowledge of the film thickness. The film hardness is obtained from these data by the use of a simple formula which is based on a physical model of film deformation during indentation. The model is verified for chromium films on four different substrate materials. Hardness values of the film material can be obtained from films with a thickness of more than 2000 Å with this technique.     

Conduction in thin polymer films

Thin polymer films have been made from a variety of starting monomers and their electrical conduction has been studied.The films are produced both by ultraviolet surface photolysis and by a cold-cathode glow-discharge technique. The samples are made in the form of a capacitor sandwich on a glass substrate with aluminium electrodes. The whole sample is made in a high-vacuum system, without breaking vacuum, using out-of-contact masking techniques.Generally, depending on the starting monomer, the polymer films are short-free down to a thickness of about 100 Å, and electric fields up to 10⁷ V/cm may be sustained.The current/voltage characteristics of the films have been investigated as a function of temperature and thickness. One theory to account for the conduction in these materials is presented and compared with other approaches to the problem.     

Viscoelastic properties of thin polymer films

Thin polymer films have been prepared by electron beam irradiation of adsorbed layers in vacuum. Electrodes have been applied so that capacitance measurements can be made in the presence of a d.c. bias. It is found that the polymer layer deforms under the influence of the electrostatic forces and that the deformation follows the extended Voigt model for viscoelastic deformation, indicating a combination of pure elastic and viscoelastic behaviour. Values of the elastic moduli and retardation time have been obtained.     

Surface acoustic waves in thin films of barium strontium titanate on magnesium oxide substrates

It is established that, using a thin barium strontium titanate (BST) film as the active element in a surface acoustic wave (SAW) device, it possible to double the working frequency of the converter due to the formation of a periodic domain structure in the BST film. A thin ferroelectric film device with a standard electrode structure is capable of effectively exciting the SAW second harmonic. Variation of the external polarizing voltage applied to the electrodes ensures field-controlled electromechanical feedback in the converter.     

Nondestructive acoustic microcharacterisation of heteropolysiloxane thin films

Heteroplysiloxanes (HPS) thin films, recognised as a new class of materials that can be used as very versatile coating systems, are nondestructively characterised by means of a scanning acoustic microscope. The acoustic material signatures were first measured, from which the characteristic surface velocities were determined. The influence of film thicknesses as well as operating frequencies on such velocities are reported. The results are then compared with theoretical velocity dispersion curves, giving good agreements. Rayleigh velocities were found to be 3080 m/s and 3420 m/s for HPS films and glass substrates respectively. Finally using an approximation method we were able to deduce Young modulus (E = 51 GPa) and shear modulus (G = 22 GPa) for HPS films; these constants were also determined for glass substrates: E = 86 GPa and 35 GPa.     

2.26 Nonlinearity measurements of thin films

Measurement of the generated third harmonic component of voltage has proved to be a most sensitive method for the detection of nonlinearities and disturbances in the conduction path of thin film resistors. Inhomogenities and contact failures of thin film capacitors are easily revealed by the same method.Tantalum thin film resistors and capacitors, today important for the realization of many hybrid circuits, are currently checked at the L M Ericsson Materials Laboratory by measurement in an equipment for determination of the third harmonic of voltage. Method and results with different material composition as well as relation to other thin film properties will be presented.     

Photothermal measurements on optical thin films

An overview of the application of the photothermal technique for optical as well as thermophysical characterizations of thin films is given. The peculiarities of this technique are discussed in some detail, and selected important results are pointed out. Emphasis is placed on the influence of both residual absorption and randomly distributed inhomogeneities in thin films on their laser-damage resistance.     

Spin wave resonance multilayer thin films

Spin wave resonance in thin ferromagnetic-dielectric-ferromagnetic multilayer films is considered. The proposed model includes the interaction between the ferromagnetic layers and also both the saturation magnetization distribution throughout the sample (VI model) and surface anisotropy (SI model). The model is applied for perpendicular resonance. The results indicate that the number of surface modes (none, one or two) depends on the thickness of the intermediate dielectric layer (through the interaction constant) and on the amplitude ΔM of the magnetization distribution. Experimental data are in agreement with the calculated numerical results.