Redeposition: a factor in ion-beam etching topography

The design of high performance SAW resonators and reflection pulse compression filters using grooves as the reflective arrays requires that the profiles of the grooves be rectangular and reproducible. The use of neutralized ion-beam technology has largely been adopted for etching these grooves in quartz and lithium niobate because of the precision and control which can be exercised in its use. The experimental work presented here asserts the major contribution that redeposition makes in the evolution of ion-beam etched topography of amorphous, polycrystalline and crystalline materials in a way which is directly useful to the determination of groove profiles. Data are also presented on the relationship between the incidence angle of the ion beam and the etch rate of some important SAW materials indicating the connection between this function and the etching characteristics of the materials.     

Acoustic wave-based sensors using mode conversion in periodic gratings

Using periodic gratings etched into the surface of a piezoelectric plate, surface acoustic waves (SAW) can be converted into bulk waves and vice versa with high efficiency. If parallel grating structures are fabricated on opposite surfaces of a piezoelectric plate, a SAW also can be directed from one surface to the other. Using such structures, acoustic wave-based sensors can be designed that utilize SAW for the detection of chemical analytes on an electrode-free surface, i.e., the back surface. As a result, spurious sensor response and electrode aging that may occur when a chemical analyte comes in contact with the transducers are minimized. The design principles of these grating-based SAW sensors are explained, and the mass sensitivity is investigated using chemical vapor deposited thin polymer films, a type of material used in many practical chemical sensor applications. Experimental results are presented for the detection of nitrogen dioxide (NO(2 )) in sub-ppm concentrations.     

A surface acoustic wave technique for monitoring the growth behavior of small surface fatigue cracks

The theory of Kino and Auld which relates the reflection coefficient of acoustic waves from a crack to its size is summarized. A scattering model is evaluated from this theory concerning the reflection of surface acoustic waves (SAW) from a small surface fatigue crack at a frequency such that the crack depth is much smaller than the acoustic wavelength. Acoustic predictions of crack depth are compared to postfracture measurements of depth for small surface cracks in Pyrex glass, 7075-T651 aluminum, and 4340 steel. Additionally, the minimum detectable crack depth as limited by the acoustic noise level is determined for several typical aluminum and steel alloys. The utility of SAW reflection coefficient measurements for inferring crack depth, crack growth, and crack opening behaviorin situ during fatigue cycling is discussed.     

声表面波换能器激励的有限元仿真

采用有限元法分析了声表面波换能器电极上的激励问题。从声场波动方程、麦克斯韦方程以及压电本构方程出发,利用哈密顿原理,推导了在压电介质中声表面波有限元方程,然后采用Newmark法对有限元方程进行时域变换。分析了换能器电极上的静态电荷分布和动态电荷分布。对压电介质中声表面波振动振幅进行计算并分析了质点振动振幅随深度的变化情况。     

Laser-induced breakdown spectroscopy for analysis of chemically etched polytetrafluoroethylene

Laser-induced breakdown spectroscopy (LIBS) is used to analyze chemically etched polytetrafluoroethylene (PTFE). The elements O, F, H, and Na are determined qualitatively in the depth dimension. It is shown that O, H, and Na signals are greatest at the surface and decrease as the laser burrows through the etched layer into the bulk. In contrast, the fluorine signal is a minimum at the surface and increases with depth. The average ablation rate for PTFE under the experimental conditions is found to be 1.9 microm per pulse. Using this value, the depth of the etched layer is determined to be 8 microm. A calibration curve produced by analysis of different polymers gives mole fractions of O, F, and H at the surface of the etched PTFE of 0.2, 0.3, and 0.1, respectively.     

A mathematical model for wet-chemical diffusion-controlled mask etching through a circular hole

Asymptotic solutions are presented for diffusion-controlled wet-chemical etching through a round hole in a mask. The three-dimensional diffusion field is assumed to be axisymmetric and fully developed. Two time regimes are considered. The first applies when the etched depth is small in comparison with the width of the mask opening. In the second, the depth of etching is much greater than the width of the mask opening. Explicit solutions are found for the shape of the etched surface as a function of the physical parameters. Among other things it is found that, as long as the etched pits are shallow, etching through small apertures is faster than through larger ones. The opposite is true for deep pits.     

Extended investigation on high velocity pseudo surface waves

Recent communication equipment such as mobile and cellular phones, radio systems, pagers, LANs, have demanded high performance components. Among these components, the last generation of SAW filters and signal processing devices, presenting low loss, flexible frequency and phase response characteristics, control of spurious, and so forth, have played a major role in designing new equipment and redesigning existing systems. The highest frequency obtainable with SAW technology in practical devices is limited to a couple of GHz, usually due to restrictions in the fabrication process involved and SAW propagation characteristics. The pseudo-SAW and the shear horizontal mode, presenting phase velocities circa 40% superior than the SAW and low attenuation along certain directions, have permitted the construction of devices operating at higher frequencies. The high velocity pseudo-SAW, with phase velocities about 100% higher than the SAW and low attenuation in many materials along certain directions, extends the high operating limit of SAW devices even further. In this paper the major characteristics of this new type of wave are reviewed. Extended topics such as: the boundary function magnitude behavior, the relationship between the "growing tilted bulk-like partial waves" and the bulk slowness, the number of roots (uncoupling of modes), and the Poynting vector behavior with depth are explored, enlightening the solution and behavior of this new type of high velocity pseudo-SAW.     

An analysis of a clear-view angelfish bubble-domain shift register

A clear-view display and printing device is proposed utilizing a new bubble-domain shift register which has triangular patterns etched into the surface of a magnetic crystal. The geometrical and materials requirements for the clear-view shift register are analyzed. The etched-out triangular pattern is shown to provide the unidirectionality of bubble motion under an oscillating bias field just as the triangular Permalloy overlay does. An etched depth of one-tenth of the plate thickness and an etched gradient of about unity are shown to be sufficient for the expected shift-register operation.     

PC software for SAW propagation in anisotropic multilayers

A software package that provides an interactive and graphical environment for surface acoustic wave (SAW) and plate-mode propagation studies in arbitrarily oriented anisotropic and piezoelectric multilayers is described. The software, which runs on an IBM PC with math coprocessor, is based on a transfer-matrix formulation for calculating the characteristics of SAW propagation in multilayers that was originally written for a mainframe computer. The menu-driven software will calculate wave velocities and field variable variations with depth for any desired propagation direction: the graphics capability provides a simultaneous display of slowness or velocity and of SAW Deltav/v coupling constant curves, and their corresponding field profiles in either polar or Cartesian coordinates, for propagation in a selected plane or as a function of one of the Euler angles. The program generates a numerical data file containing the calculated velocities and field profile data. Examples illustrating the usefulness of the software in the study of various SAW and plate structures are presented.     

Narrowband Bragg reflectors in Ti:LiNbO3 optical waveguides

Bragg grating reflectors etched in amorphous silicon overlay films have been integrated with Ti:LiNbO3 optical waveguides to obtain a narrow (0.05 nm) reflectance spectrum with a > 20 dB dip in the transmittance spectrum. These results were realized at a wavelength of 1542.7 nm for TE polarization on an x-cut, y-propagating substrate with gratings etched to a depth of approximately 93 nm in a 105 nm thick silicon film over a length of 12.5 mm. The reflectance in the channel waveguides is found to be strongly dependent on the depth of the etched grating. The effect of the Bragg waveguide loss factor on the transmittance and reflectance spectra is investigated by using a model for contradirectional coupling that includes an attenuation coefficient. The values for coupling constants kappa and amplitude attenuation constants alpha of samples etched for different time durations to control the grating depths are obtained from the model through the use of the depth of the dips in the transmittance spectra and the spectral widths of the reflectance peaks. It is concluded that the corrugated Si overlay film increases the insertion loss by approximately 2.7 dB, and the loss is not significantly affected by the grating depth.     

Cadmium tellutide etched surfaces

Studies of etched surfaces of single-crystal cadmium telluride (CdTe) are presented. A CdTe-electrolyte system is used for electrical measurements. The possibility of carrying out electrochemical processes to alter the etched surface is demonstrated. Auger electron spectroscopy and electron loss spectroscopy depth profiles are obtained for the etched and electrochemically treated CdTe surfaces. Disturbed layers whose stoichiometry varies with depth differently for p- and n-type CdTe were detected on the surfaces.

Precise interpretation of the data is quite complex, but the model for a “nonequilibrium metal/insulator/semiconductor” structure is adopted to explain the elimination of the “pinning” effect with a reduction in the surface layer thickness.     

Noise and dynamic range optimization of SAW transversal filters

A model for surface acoustic wave (SAW) transversal filters with special attention to the system and circuit designer's point of view is summarized. In the ideal situation the SAW transversal filter is driven with a voltage and the short-circuit current is sensed, which results in a minimization of the triple transit echo distortion. The aperture (width) of the SAW device is the only parameter that is not determined by the frequency dependence of the transfer and therefore it can be used to optimize the SAW device in relation to the electronic circuitry. Noise and dynamic range calculations on an amplifier-filter-amplifier configuration, are performed. It is shown that for a low noise floor at the input of the SAW device, the aperture of this device should be chosen large. However, the dynamic range of the amplifier-filter-amplifier configuration can be maximized by choosing a small aperture.     

Surface acoustic wave excitation on SF6 plasma-treated AlGaN/GaN heterostructure

In this work, we introduce a new modified approach to the formation of interdigital transducer (IDT) structures on an AlGaN/GaN heterostructure. The approach is based on a shallow recess-gate plasma etching of the AlGaN barrier layer in combination with “in-situ” SF₆ surface plasma treatment applied selectively under the Schottky gate fingers of IDTs. It enables one to modify the two-dimensional electron gas (2DEG) density and the surface field distribution in the region of the IDTs, as is needed for the excitation of a surface acoustic wave (SAW). The measured transfer characteristics of the plasma-treated SAW structures revealed the excitation of SAW at zero bias voltage due to fully depleted 2DEG in the region of the IDTs. High external bias voltages are not necessary for SAW excitation. SIMS depth distribution profiles of F atoms were measured to discuss the impact of SF₆ plasma treatment on the performance of the AlGaN/GaN-based IDTs.     

Optimal proportional relation between laser power and pulse number for the fabrication of surface-microstructured silicon

We experimentally demonstrate that, under the same laser fluence, there exists an optimal proportional relation between the laser power and pulse number for the fabrication of surface-microstructured silicon. During this fabrication process, the pulse number represents the interaction time between the laser and the silicon, which determines the depth of energy transferred into the inner part of the material, while the laser power determines the ablation and volatilization rate of the silicon. The proper combination of laser power and pulse number can ablate the material on the silicon surface effectively and have enough time to transfer the energy into the deep layer, which can produce microstructured silicon with a high spike. In addition, we compare the absorptance of samples etched by different combinations of laser power and pulse number; the corresponding results further prove the existence of an optimal proportional relation.     

1/f noise in etched groove surface acoustic wave (SAW) resonators

Measurements of 1/f (or flicker) frequency fluctuations in SAW resonators fabricated with etched groove reflectors on single crystal quartz have shown that the observed noise levels vary inversely with device size. These measurements were made on sixteen 450 MHz resonators of four different sizes. The 1/f noise levels were also evaluated on twenty-eight other SAW resonators ranging in frequency from 401 to 915 MHz. This additional data provides valuable information on the dependence of the flicker noise levels on resonator frequency. A model based an localized, independent velocity fluctuations in the quartz is proposed which correctly fits the observed size and frequency dependence of the measured 1/f noise levels. This model suggests that the velocity fluctuations originate in small regions (much less than ~5 mum in diameter) randomly distributed throughout the quartz with an average separation of about 5 mum between independent (incoherent) sources. The magnitude of the localized fractional velocity fluctuations, Deltav/v, averaged over a 5 micron cube is on the order of 1x10 (-9).     

SAW focusing by circular-arc interdigital transducers on YZ-LiNbO(3)

Using the angular spectrum theory and experimental velocity data of surface acoustic waves (SAW) on YZ-LiNbO(3), the focusing characteristics of a circular-arc interdigital transducer have been demonstrated. The calculated results show that the depth of focus is long and the compressed acoustic beam width is very narrow. The concept of a caustic is shown to be an excellent way of characterizing SAW focusing by a circular-arc interdigital transducer on YZ-LiNbO(3). Comparison between theoretical and experimental results shows good agreement.     

Optimized condition for etching fused-silica phase gratings with inductively coupled plasma technology

Polymer deposition is a serious problem associated with the etching of fused silica by use of inductively coupled plasma (ICP) technology, and it usually prevents further etching. We report an optimized etching condition under which no polymer deposition will occur for etching fused silica with ICP technology. Under the optimized etching condition, surfaces of the fabricated fused silica gratings are smooth and clean. Etch rate of fused silica is relatively high, and it demonstrates a linear relation between etched depth and working time. Results of the diffraction of gratings fabricated under the optimized etching condition match theoretical results well.     

The experimental and theoretical characterization of the SAW propagation properties for zinc oxide films on silicon carbide

The surface acoustic wave (SAW) propagation properties of zinc oxide (ZnO) films on silicon carbide (SiC) have been theoretically and experimentally characterized in the film thickness-to-acoustic wavelength ratio range up to 0.12. The experimental characterization of the SAW propagation properties was performed with a linear array of interdigital transducer (IDT) structures. The measurements characterized the velocity and propagation loss of two surface modes, a generalized SAW (GSAW) mode with velocities between 6000 and 7000 m/s, and a high velocity Pseudo-SAW (HVPSAW) mode with velocities between 8500 and 12 500 m/s. The experimentally determined characteristics of the two waves have been compared with the results of calculations based on published data for SiC and ZnO. Simulation of wave characteristics was performed with various values of the elastic constant C(13), which is absent in the published set of material constants for SiC, within the interval permitted by the requirement of positive elastic energy in a hexagonal crystal. The best agreement between the measured and calculated propagation losses of the HVPSAW has been obtained for C(13) near zero. Although for the GSAW mode the calculated velocity dispersion has been found nearly insensitive to the value of C (13) and consistent with the experimental data, for the HVPSAW, some disagreement between measured and calculated velocities, which increased with ZnO film thickness, has been observed for any C(13 ) value. Theoretical analysis of HVPSAW has revealed the existence of a previously unknown high velocity SAW (HVSAW). The displacement components of this wave have been analyzed as functions of depth and confirmed its pure surface, one-partial character.     

A SEM (scanning electron microscopy)-based method to evaluate impurity segregation to prior austenite grain boundaries in high strength steels

The depth of grain boundary grooves on the etched surface of high strength Cr-Ni steels has been determined as a measure of P-segregation to prior austenite grain boundaries by a SEM (Scanning Electron Microscopy)-based grain boundary etching method (GEM). The mean depth and distribution of the depth can be decided by stereoscopy on the SEM micrographs of the replica of the etched surface. The results showed that the cast steel has less, but nonuniform P-segregation and the high temperature austenitization reduced the P-segregation on prior austenite grain boundaries in both steels.     

Wireless measurement of temperature using surface acoustic waves sensors

Surface acoustic wave (SAW) devices can be used as wireless sensor elements, called SAW transponders, for measuring physical quantities such as temperature that do not need any power supply and may be accessed wirelessly. A complete wireless sensor system consists of one or more such SAW transponders and a local radar transceiver. The SAW transponder receives an RF burst in the VHF/UHF band transmitted by the radar transceiver. The reader unit performs a radar measurement of the impulse response of the SAW transponder via a high-frequency electromagnetic radio link. A temperature variation changes the SAW velocity and thereby the response pattern of the SAW device. By analyzing the time delay between backscattered pulses with different time delays we get a rough estimation of the temperature of the SAW transponder. By using this information the ambiguity of +/-2pi in the phase differences between the pulses can be eliminated, which provides an overall and unambiguous temperature resolution of +/-0.2 degrees C.