Detection Of Surface Acoustic Waves Research Articles

Rapid measurement of surface acoustic wave velocity on single crystals using an all-optical adaptive scanning acoustic microscope

An experimental method has been developed to measure the phase velocity of laser-generated and detected surface acoustic waves. An optical grating produced by a spatial light modulator was imaged onto the sample surface to generate the ultrasound whose frequency and wave front were controlled electrically by tailoring the grating. When the grating period matched the surface acoustic wavelength, strong excition of the surface wave was observed. Thus, the wavelength and, thereby, the phase velocity were determined. We present results with this method that allow the phase velocity and the angular dispersion of the generalized surface wave as well as the pseudosurface wave on the (100) nickel and (111) silicon single crystals to be measured, with the precision of approximately 0.2%. Those factors affecting the measurement precision are discussed.

Performance evaluation of a surface acoustic wave analyzer to measure vocs in air and water

AbstractThe performance of a fast gas chromatograph equipped with a Surface Acoustic Wave (SAW) detector is evaluated in this paper. The instrument was developed for the rapid analysis of volatile organic compounds in air or water. The SAW detector, the first and only known integrating detector for gas chromatographs (GCs), operates by responding to the mass of material exiting the GC column. Material adhering to the SAW surface changes its operating frequency, which is detected by the system electronics and converted into a chromatogram. The system requires no high voltages or radioactive ionization sources, and is compact and portable. The millisecond response of the detector allows high speed analysis—measured in seconds—to take place. The performance of the GC/SAW system is comparable to laboratory GCs in terms of limit of detection, precision, accuracy, and interference while screening environments in essentially real time (10 seconds). The results of the instrument evaluation for 10 common organic compounds in air and water matrices are presented.

Fullerene C60-cryptand coated surface acoustic wave quartz crystal sensor for organic vapors

A fullerene C60-cryptand 22 (C60-di-propylamine-cryptand 22) coated surface acoustic wave (SAW) quartz crystal gas detection system with 250 MHz oscillator and a computer interface for signal acquisition and data processing was prepared to detect various organic vapors. The C60-cryptand 22 coated SAW quartz crystal sensor exhibited sensitivity to both polar and nonpolar organic molecules. The C60-cryptand 22 was demonstrated as a better and more sensitive coating material than fullerene C60 or cryptand 22 for polar organic molecules. Comparison of C60-cryptand 22 coated SAW crystal and quartz crystal microbalance (QCM) sensors was also made and the SAW sensor exhibited much better response than the piezoelectric sensor for organic molecules with the same concentrations. The C60-cryptand 22 coated SAW crystal sensor could be repeatedly reused for the detection of most of organic vapors, e.g. alcohols, aldehydes, ketones, ethers, alkanes and alkenes. The selectivity of the C60-cryptand 22 coated SAW crystal sensor for various polar organic vapors seem to be in the order: alcohols>aldehydes>ketones>ethers. For nonpolar organic molecules, the C60-cryptand 22 coated SAW crystal sensor showed sensitivity for alkenes, e.g. hexene and benzene than for alkanes, 1-hexane and cyclohexane. Effects of molar mass, steric structures and isomers of organic molecules on the SAW frequency responses were also investigated. The SAW detection system showed the good detection limit of 0.1–3.0 mg/ml for these organic molecules. The C60-cryptand 22 coated SAW crystal sensor was successfully applied as a quite sensitive gas chromatography (GC) detector for various organic compounds. The C60-cryptand 22 coated surface acoustic wave GC detector compared well with the commercial thermal conductivity detector (TCD) for various organic molecules, e.g. organic halides, alcohols, alkenes and ketones.

Laser detection of surface acoustic waves as a method of measuring an Ar ion beam modification of carbon thin film

Pulsed energy impacts are expected to generate the elastic waves reflecting the elastic properties of impacted substances. Among the generated waves, the surface acoustic waves (SAWs) can be more specific to the surface structures and can be used effectively in the characterization of thin films in a non-destructive way. In the present study, pulsed laser impacts were employed to generate SAWs on a thin film and the SAWs are detected with a laser reflection technique. To test the effectiveness of this method, an amorphous carbon films were irradiated with 13 keV Ar ions at room temperature in a broad range and the SAW propagation velocity was evaluated as a function of Ar ion dose. The present preliminary experiment using pulsed laser suggests the successful detection of SAW from typical substances with different modulus of elasticity such as fcc metals, oxides and semi conducting materials.

Applications of laser-generated surface acoustic waves for copper film process monitoring in integrated circuit industry (abstract)

We describe applications of a compact commercial instrument for laser generation and detection of surface acoustic waves (SAWs) to problems in metal film process control for the integrated circuit (IC) industry. [M. Gostein, M. Banet, M. Joffe, A. A. Maznev, R. Sacco, J. A. Rogers, and K. A. Nelson, in Handbook of Silicon Semiconductor Metrology, edited by A. C. Diebold (Marcel Dekker, New York, 2001)] The IC industry is undergoing dramatic changes with the continued drive to reduce feature size and increase circuit speed. One of the most important of these changes is the industry-wide move to replace circuit interconnect processes based on aluminum metallization with copper-based processes. The unique process challenges of copper metallization, coupled with the increasing cost of IC manufacturing in general, have resulted in an increased need for metal film thickness measurement for process control. Laser-generated and detected surface acoustic waves provide an ideal method for nondestructively measuring film thickness on product wafers as they move through an IC factory. A patented version of the technique has been incorporated into a commercial high-throughput measurement station. The measurement station analyzes specialized test structures in the scribe lines in between IC chips on a product wafer. Here, we describe application of the technique to all stages of the copper metallization process, including measurement of seed-layer copper and its associated underlying barrier metals, measurement of electroplated copper deposited atop the seed layer, and measurement of remaining copper film thickness following a chemical–mechanical polishing step. We highlight special capabilities to measure test arrays of submicron metal lines that closely resemble actual circuit elements. In addition, we discuss characterization of the elastic properties of typical and emerging thin film materials used in the semiconductor industry, which is a necessary step in setting up the SAW instrument for measurements on IC film stacks.

Laser-based surface acoustic wave spectrometer for industrial applications

A compact, robust, and fully automated optical system for laser generation and heterodyne detection of surface acoustic waves (SAWs) in the frequency range 0.1–1.3 GHz is presented. SAWs are generated by two short laser pulses forming an interference pattern at the sample surface, and detected via diffraction of a quasi-cw probe beam off the surface ripples produced by the SAWs. The detection is enhanced by optical heterodyning achieved through the introduction of a fourth (reference) beam. Both excitation and probe/reference beam pairs are produced by a phase grating optimized to provide maximum efficiency for the desirable diffraction orders at the two different optical wavelengths used for excitation and probing. The heterodyne arrangement has an excellent phase stability and provides a number of advantages compared to a conventional transient grating setup. These include enhanced signal level and suppression of “parasitic heterodyning” due to scattered light. The system provides reproducibility of SAW frequency measurements of ∼0.01% with measurement time of ∼1 s. Measurements can be performed on small test structures such as 50×50 μm boxes, with an integrated vision system helping position the measurement spot on the sample.

Dynamic higher-order correction of acoustic aberration due to material microstructure

Material microstructure, such as grains in metals, can perturb ultrasound as it propagates through—or on the surface of—the material. This acoustic aberration affects the accuracy and reliability of ultrasound measurements and is a fundamental limit to resolution for many materials. Using an all-optical approach to generation and detection of surface acoustic waves, we detect the acoustic wave front aberrations, and correct for them by calculating a different generation profile, which is imaged onto the material using a spatial light modulator.

A portable, high-speed, vacuum-outlet GC vapor analyzer employing air as carrier gas and surface acoustic wave detection.

Vacuum-outlet GC with atmospheric-pressure air as the carrier gas is implemented at outlet pressures up to 0.8 atm using a low-dead-volume polymer-coated surface acoustic wave (SAW) detector. Increases in the system outlet pressure from 0.1 to 0.8 atm lead to proportional increases in detector sensitivity and significant increases in column efficiency. The latter effect arises from the fact that optimal carrier gas velocities are lower in air than in more conventional carrier gases such as helium or hydrogen due to the smaller binary diffusion coefficients of vapors in air. A 12-m-long, 0.25-mm-i.d. tandem column ensemble consisting of 4.5-m dimethyl polysiloxane and 7.5-m trifluoropropylmethyl polysiloxane operated at an outlet pressure of 0.5 atm provides up to 4 x 10(4) theoretical plates and a peak capacity of 65 (resolution, 1.5) for a 3-min isothermal analysis. At 30 degrees C, mixtures of vapors ranging in vapor pressure from 8.6 to 76 Torr are separated in this time frame. The SAW detector cell has an internal volume of < 2 microL, which allows the use of higher column outlet pressures with minimal dead time. The sensor response is linear with solute mass over at least 2-3 decades and provides detection limits of 20-50 ng for the vapors tested. The combination of atmospheric-pressure air as carrier gas, modest operating pressures, and SAW sensor detection is well-suited for field instrumentation since it eliminates the need for support gases, permits smaller, low-power pumps to be used, and provides sensitivity to a wide range of vapor analytes.

Multiplexed interferometer for ultrasonic imaging applications

A multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves from a number of points on the surface of a specimen. Phase gratings are used to create an array of laser beams that are directed to the specimen. The beams are collected using a lens system, combined with a single reference beam in a photorefractive crystal, and imaged on separate photodetector elements. Detection of surface acoustic waves scattered from a defect in an aluminum specimen is demonstrated using a nine-element receiver. Focusing of the array interferometer, by applying appropriate phase shifts between the detected signals, is also shown. The wide bandwidth and noncontact nature of this receiver make it well suited for phased array detection and ultrasonic imaging applications.

Laser-ultrasonic measurements of thin-film properties using surface acoustic waves

High-frequency, laser-ultrasonic methods to evaluate the mechanical properties of thin films have been developed. The approach is based on the optical generation and detection of surface acoustic waves (SAWs) to determine the frequency dependence of the phase velocity (dispersion relation). Broadband or variable-frequency, quasi-plane wave SAWs are generated by a line-focused, 200-ps pulsed laser. The out-of-plane SAW displacement amplitudes are measured using a path-stablized Michelson interferometer with line-focus detection. The apparatus incorporates differential photodiode detection with a −3-dB bandwidth of 40 kHz–800 MHz. With this system, several displacement waveforms over a range of propagation distances are acquired and used to determine the SAW dispersion. Dispersion relations up to 200 MHz are presented for a variety of specimens, including a series of Si wafers with TiN films 0.2–1.3 micrometers thick and containing residual stress. The results illustrate the applicability of this technique to various conditions such as stiffening and loading films and isotropic and anisotropic substrates. The experimental dispersion curves obtained with this method are compared to theoretical predictions to evaluate the film elastic properties.

Optical matched-filter and phased-array detection of surface acoustic waves

A method to optically measure surface acoustic displacements simultaneously over an array of detection points is described. Optical phase gratings are used to create a detection array of laser beams that is directed to the specimen. The scattered beams from the detection array are then collected and combined with a single reference beam in a photorefractive crystal in a multiplexed two-wave mixing configuration. The output beams of the resulting multiplexed photorefractive interferometer are imaged onto either a single photodetector (matched-filter scheme) or to separate photodetector elements (phased-array scheme). The optical matched filter requires less electronic overhead. By using different object beam footprints, it is possible to configure optical matched-filter receivers that selectively detect SAWs (broadband, narrow-band, chirped, etc.) with enhanced sensitivity. The responses of equispaced and chirped array receivers to propagating SAWs are discussed. The more general phased-array configuration, although requiring greater electronic overhead, provides time-domain signals from each element for postprocessing. This system can therefore be synthetically focused by applying appropriate phase shifts between the detected signals to create images of the acoustic scattering region. Both these multiplexed optical detection schemes, with their high spatial resolution and subnanometer displacement sensitivities, have applications in nondestructive testing and materials characterization.

Thin-Films for Saw Devices

Thin-films play a significant role in surface acoustic wave (SAW) technology. Thin metal films are essential for the generation and detection of SAWs on piezoelectric subtrates, can provide for directional control, influence the velocity and propagation loss, and can be configured to produce a wide variety of time and frequency signal processing functions for electronic systems. Dielectric films have been used to selectively modify the propagation properties of surface waves, protect the surfce, and improve the coupling efficiency and temperature characteristics of SAW devices. Piezoelectric films have extended SAW device developments to amorphous and nonpiezoelectric crystals for enhanced frequency and loss performance, lower cost, and microelectronic integration. Semiconductor, optical, and magnetic films have shown promise for active acoustoelectronic, acoustooptic, and magnetoacoustic devices. Chemically sensitive films are the basis for SAW sensor developments. Active research continues today in improving the properties and utilization of thin films on SAW substrates through advances in processing and characterization.

THIN-FILMS FOR SAW DEVICES

Thin-films play a significant role in surface acoustic wave (SAW) technology. Thin metal films are essential for the generation and detection of SAWs on piezoelectric subtrates, can provide for directional control, influence the velocity and propagation loss, and can be configured to produce a wide variety of time and frequency signal processing functions for electronic systems. Dielectric films have been used to selectively modify the propagation properties of surface waves, protect the surfce, and improve the coupling efficiency and temperature characteristics of SAW devices. Piezoelectric films have extended SAW device developments to amorphous and nonpiezoelectric crystals for enhanced frequency and loss performance, lower cost, and microelectronic integration. Semiconductor, optical, and magnetic films have shown promise for active acoustoelectronic, acoustooptic, and magnetoacoustic devices. Chemically sensitive films are the basis for SAW sensor developments. Active research continues today in improving the properties and utilization of thin films on SAW substrates through advances in processing and characterization.

Adaptive optical array receivers for detection of surface acoustic waves

Adaptive interferometric detection systems based on two-wave mixing in photorefractive crystals have been configured as distributed optical receivers. The spatial distribution of the detection laser power on the sample surface is controlled by use of phase gratings and amplitude masks. The responses of point, line, array, and chirped optical receivers to propagating surface acoustic waves (SAW's) are discussed theoretically and demonstrated experimentally. It is shown that by use of different object beam footprints it is possible to configure adaptive holographic SAW receivers that are either broadband or narrow band and that are preferentially sensitive to SAW's propagating in given directions. The receivers also allow for the distribution of laser power over the sample, eliminating the excessive heating or surface damage that can occur in some materials when high power, point-focused, detection lasers are used.

Laser ultrasonic surface wave inspection of alumina ceramics of varying density

In this paper, the surface acoustic wave velocity results acquired from the inspection of specially manufactured and characterised alumina ceramic materials are presented. Ultrasonic velocity data of alumina-based ceramics in the range 60–100% theoretical density was generated utilising non-contacting laser-ultrasonic measurements based on laser generation and detection of surface acoustic waves with the objective of creating a routine technique for industrial advanced alumina inspection. With linear fitting the surface acoustic wave velocity data serves as a calibration graph for using laser ultrasonics for routine monitoring of alumina. A second laser ultrasonic technique based on the laser generation and foil transducer detection of surface acoustic waves was used to validate the surface acoustic wave velocities measured by the laser generation/detection technique.

Imprinting with sensor development - On the way to synthetic antibodies

Molecular imprinting is an attractive tool for the development of artificial recognition systems. Even non-covalent imprinting provides universal interaction centers for sensoric applications. The coated chemical sensors have high stabilities under harsh conditions in both the gas and liquid phases. With adequate efforts optical and mass-sensitive sensors (quartz crystal microbalance, QCM, surface acoustic wave detector, SAW) are suitable for analysis down to the ppb (nL/L) range. PAHs, isomer VOCs as well as complex oil mixtures are appropriate analytes.

Depth profiling of residual stress in steel by laser ultrasonics

Depth profiling of residual stress in steel is of great interest in many practical applications. Conventional techniques are either destructive (x-ray diffraction and hole drilling) or with obvious restriction (Barkhausen noise). This paper presents a noncontact and nondestructive laser ultrasonics method for determining the depth profile of the residual stress layer in steel samples. It is known that surface acoustic waves penetrate into solids to a depth proportional to the wavelength. They are expected to be dispersive in the presence of gradients in physical properties such as residual stress. Based on the dispersion theory, the depth dependence of an effective velocity v(z), which is related to the stress profile S(z), can be fitted from the frequency dependence of the phase velocity v(w). Due to the advantage of wide bandwidth, laser-generated and -detected surface acoustic waves (SAW) offer a promising nondestructive stress depth profiling method. The profiles obtained by SAW are compared with the stress data obtained by other techniques. a)Also at Service de Métallurgie Physique, Université Libre de Bruxelles, Belgique.

Precise Determination of Thin Metal Film Thickness With Laser-Induced Acoustic Grating Technique

ABSTRACTAn all-optical technique based on laser excitation and detection of surface acoustic waves is capable of precise, fast, noncontact and nondestructive measurements of metal film thickness. The technique is integrated into a compact and robust instrument. Applications to metal interconnect process control in integrated circuit technology are presented.

Transverse surface acoustic wave detection by scanning acoustic force microscopy

We present a scanning acoustic force microscope (SAFM) for the study of surface acoustic wave (SAW) phenomena on the submicron lateral scale. Until now, SAWs with in-plane oscillation components could only be studied effectively via nonvanishing out-of-plane oscillation contributions. By operating the microscope in lateral force mode, where both bending and torsion of the cantilever are detected, additional amplitude-dependent signals are found, which are due to the interaction with purely in-plane polarized surface oscillations. To demonstrate the capabilities of this type of SAFM, Love waves were studied on the surface of layers deposited on ST-cut quartz with SAW propagation perpendicular to the crystal X-axis. The phase velocity of the wave as well as the amplitude of a standing wave field was measured and compared to calculated values.

SAW gas detection using Langmuir–Blodgett polypyrrole films

Surface acoustic wave (SAW) devices are very attractive as gas sensors due to high sensitivity and low cost. Polypyrrole, obtained by a solid reaction from a ferric stearate Langmuir–Blodgett (LB) film, has been deposited onto 128° YX-LiNbO 3 substrate in order to implement a SAW chemical sensor. The formation of the conducting polymer has been confirmed by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM) measurements. The SAW delay line, equipped by interdigital transducers operating at a frequency of 42 MHz and coated by a sensitive polypyrrole LB film, has been tested towards NH 3 gas in air. Due to the absorption process, the various loadings cause a variation of acoustic wave velocity measured by the phase shift technique at the output of SAW device. Partially reversible SAW acoustic response towards NH 3 in air has showed high sensitivity. The calibration curve towards NH 3, its detection limit and degradation phenomena of the coating have been performed. Our LB polypyrrole SAW detector exhibits excellent selectivity towards NH 3.