Frequency SAW Research Articles

A Bi-grating laser acoustic spectrum method for small surface-breaking crack imaging and depth evaluation

The Laser ultrasonic testing method is effective for the location and evaluation of small surface-breaking cracks. This paper presents a bi-grating laser acoustic spectrum method for precise evaluation and imaging of surface cracks. With a bi-grating laser source, the reflection and transmission coefficient of the SAWs at the crack can be directly obtained with a single measurement by monitoring the amplitude ratio of the two frequency SAW in the frequency domain. It provides a self-reference method with good reliability, high SNR, and high resolution. Simulation results show that the surface crack can be precisely positioned from the ultrasonic reflection/transmission coefficient, and the characteristic parameter increases linearly with the surface crack depth when crack depth is smaller than the wavelength of the surface wave. Moreover, the feasibility and capability of the method are verified by experiment validation. The ultrasonic reflection/transmission coefficient is taken to obtain the C-scan imaging. The experimental results show that the proposed method is effective to locate and evaluate the small surface crack depth, and promising in the in-situ detection and imaging for surface cracks.

2Pb3-2 Systematic Design of Layered Structures for Wideband and High Frequency SAW Resonators

2Pb3-2 Systematic Design of Layered Structures for Wideband and High Frequency SAW Resonators

Research of micro area piezoelectric properties of AlN films and fabrication of high frequency SAW devices

Aluminum nitride (AlN) thin films were optimized on Pt/Ti/SiO2/Si (100) substrates by magnetron sputtering at different substrate temperatures (room temperature, 400 °C, 500 °C, and 600 °C). The analysis of X-ray diffraction (XRD) and atomic force microscopy (AFM) indicated that the optimal deposition temperature for AlN films with high (002) orientation, smallest value of rocking curve FWHM and superior grain distribution is 500 °C. Micro area piezoresponse force microscopy (PFM) results show that AlN samples deposited under 500 °C have the highest relative piezoelectric coefficient (d⁎33(max) = 0.79) and uniform piezoelectric performance. Moreover, interdigital transducers (IDT) of submicron scale (finger width 300 nm) were designed and fabricated by electron beam lithography (EBL) and lift-off process. Finally, the Pt/AlN/Pt two-port SAW device cell was constructed, with a device frequency up to 4.47 GHz, and an acoustic velocity of deposited AlN of approximately 5300 m/s.

Microstructure and piezoelectric properties of reactively sputtered highly C-axis ScxAl1-xN thin films on diamond-like carbon/Si substrate

We report deposition of pure AlN and ScxAl1-xN thin films with different Sc concentration on DLC/Si substrate by RF and DC reactive magnetron co-sputtering method by using Al and Sc as targets. The X-ray diffraction (XRD) results showed that the films have high c-axis-orientation. Electron probe microanalyzer (EPMA) analysis reveals the presence of scandium atoms reduce the space occupied by aluminum atoms, leading to lattice distortion during the phase transition. The SEM cross-section results showed that the ScxAl1-xN films are highly aligned along c-axis and have columnar like morphology. The top view of SEM results indicated that the new phase formation above Sc 30.33%, however no new peak appeared in the XRD pattern. The piezoelectric coefficient (d33) of ScxAl1-xN thin films are measured and the highest value (11.54 pC/N) is achieved at x=30.33% increasing 14 times, as that with AlN/DLC/Si, 0.73 pC/N. ScxAl1-xN films on DLC/Si substrate have a great potential to be applied on high frequency SAW devices in the future.

FEM simulation of AlN thin layers on diamond substrates for high frequency SAW devices

AlN/Diamond structure is very promising for high frequency acoustic wave devices. In this paper, a comprehensive investigation of AlN/Diamond structure is presented using finite element method. The phase velocity and the coupling coefficient of the first two modes of SAWs for the AlN/Diamond structure are numerically analyzed and compared to experimental data. Close agreement between experimental and numerical results is obtained. Results show that the mode 1 (Sezawa mode) exhibits the largest coupling coefficient of 1.28% associated with a phase velocity of 9500m/s. Additionally, the temperature coefficient of frequency (TCF) and the reflection coefficient (r) are studied. The simulation results show that for a zero TCF, a high phase velocity of 10,800m/s associated with a coupling coefficient of 0.5% can be obtained. These results demonstrate that the AlN/Diamond structure can be used to design wide-band and temperature-compensated SAW devices. The dependence of SAW devices performance with the electrode height, metallization ratio and mass-loading is also investigated.

HOLOGRAPHIC MEASUREMENT OF SURFACE ACOUSTIC WAVE CHARACTERISTICS IN CRYSTALS

Holographic interferometry allows visualizing SAW on the entire specimen surface simultaneously and measuring wave characteristics directly. A special method of SAW characteristics measurements by digital holographic interferometry was developed. The use of a picosecond laser allowed registering high frequency SAW two−exposure interferograms, and optical magnification of the CCD input image was used to increase spatial resolution.As a result we reach a sensitivity and resolution of visu-alization and SAW characteristics measurements that significantly exceeded the commonly known limitations of standard holographic methods.

AlN/Sapphire: Promising Structure for High Temperature and High Frequency SAW Devices

In this paper, the performance of AlN/sapphire structure in high frequencies is investigated. Several SAW devices are fabricated with various designs (free-space delay, wavelength, metallization ratio, etc.) to study simultaneously different parameters (acoustic velocity, electromechanical coupling (K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), acoustic propagation loss (α), and temperature coefficient of frequency) versus frequency and temperature. Experimental results show that, as expected, α increases with temperature while K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is enhanced at high temperatures. Because of the antagonistic evolution of these two parameters, insertion loss decreases or increases as function of the free-space delay. We also demonstrate that this structure allows fabrication of devices operating up to 1.5 GHz and that the frequency varies linearly with temperature.

Diamond underlayer microstructure effect on the orientation of AlN piezoelectric layers for high frequency SAW resonators by TEM

Due to its mechanical properties, diamond is very attractive as an active material for the fabrication of SAW resonators for high frequency applications. In this work, the synthesis of piezoelectric AlN films by reactive sputtering at room temperature has been optimized on thick diamond layers grown on Si and alumina substrates in order to process high frequency devices. The effect of diamond underlayer microstructure is evaluated by TEM. Two sets of samples are studied, AlN/NCD/Alumina and AlN/MCD/Si. The orientation of the AlN grains is shown to improve with the film thickness and the diamond grain size. For NCD underlayer, the AlN deposited on top is more oriented. Moreover, above 1μm from the AlN/diamond interface, a high degree of the c-axis orientation (perpendicular to the AlN/diamond interface) is demonstrated even though two different grain lattice orientations are shown to coexist: one with the (21¯1¯0) planes remaining parallel to the TEM-preparation lamella and the other with (011¯0) planes. The AlN/diamond interface is smooth down to the nm-scale.

Preparation of AlN/Diamond Film Structure for High Frequency SAW Devices

Freestanding diamond film was prepared by hot filament chemical vapor deposition (HFCVD) method. Inter-Digital Transducer (IDT) was formed on the nucleation side of freestanding diamond film by photolithography technique. Then piezoelectric AlN film was deposited by RF reactive magnetron sputtering to obtain the AlN/diamond film structure. Surface morphologies of the nucleation side and the IDT were characterized by atomic force microscopy (AFM) and optical microscopy. The results indicated that the surface of nucleation side was very smooth and the IDT was of high quality without discontinuity and short circuit phenomenon. Room temperature frequency dependence of dielectric constant and dielectric loss for freestanding diamond film suggest a good stability of dielectric properties in high frequency region for diamond film. X-ray diffraction (XRD) pattern of the structure of AlN /diamond film structure showed a strong diffraction peak of AlN (002), which indicated that as-sputtered AlN film was highly c-axis oriented.

Nano-image profiles transferred by near field phase-shifting lithography precisely simulated by finite element method and fabricated

The operating frequency of the SAW filters is limited by the gap width but not the line width. The narrow gap width is required for the high operating frequency SAW filters. Therefore, in this study, high precision nano-image profiles transferred by near field phase-shifting mask (NFPSM) lithography at various exposure-energy-intensities (EEIs) are simulated by the finite element method (FEM). The transferred energy-intensity distribution (TEID) in the photoresist during the NFPSM process (at the wavelength of 248nm) can be accurately simulated by the FEM. The TEID at the interface between the mask and the photoresist is also simulated. The fabricated pattern widths clearly match the simulation results. The study of the simulated image profiles shows that they are dependent on the EEI. The greater the EEI is, the narrower the width and the shorter the height of the image profile. The nano-linewidth of 60nm is simulated and fabricated. The fabricated nano-imaging profile precisely fits the simulation results. Therefore, any expected nano-image profile can easily be fabricated by way of the simulation.

1J5-2 High Frequency SAW Resonator Filter composed of ZnO/high-densityelectrode/quartz Structure(Bulk wave devices/Surface wave devices)

1J5-2 High Frequency SAW Resonator Filter composed of ZnO/high-densityelectrode/quartz Structure(Bulk wave devices/Surface wave devices)

Investigation of High-Quality ZnO Film on Polycrystalline Diamond Wafer for Fabrication of High Frequency SAW Filter

ZnO films with piezoelectric properties were deposited on polycrystalline diamond wafers for high frequency surface acoustic wave filter applications by dc reactive magnetron sputtering. The influences of different sputtering pressures and substrate temperatures on the properties of ZnO films were discussed. Highly c-axis oriented, fine-grain ZnO films with excellent surface flatness (average surface roughness of 5.3 nm) and high resistivities (6.3×107 *·cm) have been obtained. Then Al films for interdigital-transducer were deposited on ZnO/diamond by electron evaporation, a 2.48 GHz longitudinally- coupled double-mode surface acoustic wave filter has been fabricated using the Al/ZnO film on diamond wafer.

High frequency SAW devices based on third harmonic generation

We demonstrate the third harmonic generation in a ZnO/Si layered structure to obtain high frequency SAW devices. This configuration eliminates the need of high lithography resolution and allows easy integration of such devices and electronics on the same wafer. A theoretical study was carried out for the determination of the phase velocity and the electromechanical coupling coefficient (K2) dispersion curves of the surface acoustic waves. These results are also in agreement with those measured on a SAW filter designed for the third harmonic generation and the operating frequency is up to 2468MHz.

Preparation of free-standing diamond films for high frequency SAW devices

Free-standing diamond films were prepared by hot filament chemical vapor deposition (HFCVD) method under different conditions. Inter-digital transducers (IDTs) were formed on the nucleation sides of free-standing diamond films by photolithography technique. Then piezoelectric ZnO films were deposited by radio-frequency(RF) reactive magnetron sputtering to obtain the ZnO/diamond film structures. Surface morphologies of the nucleation sides and the IDTs were characterized by means of scanning electron microscopy (SEM), atomic force microscope (AFM) and optical microscopy. The results indicate that the surfaces of nucleation sides are very smooth and the IDTs are of high quality without discontinuity and short circuit phenomenon. Raman spectra show the sharp diamond feature peak at about 1 334 cm −1 and the small amount of non-diamond carbon in the nucleation side. X-ray diffraction (XRD) patterns of the structure of ZnO/diamond films show a strong diffraction peak of ZnO (002), which indicates that as-sputtered ZnO films are highly c-axis oriented.

Growth of ZnO thin film on SiO2/Si substrate by pulsed laser deposition and study of their physical properties

Zinc oxide films were grown on SiO2/Si substrate. The growths were performed using pulsed laser deposition at a moderate substrate temperature (500 °C). PLD ZnO/SiO2/Si indicated a very well c-axis orientation. Micro-Raman and FTIR measurements were indicated vibrations modes of wurtzite structure and no evidence of defect density. Brillouin light scattering (BLS) measurements were performed to study the acoustic properties of ZnO films. A laser light source (514.5 nm) probe revealed acoustic phonons of about 300 nm wavelength. The BLS spectra revealed Rayleigh (R) and a number of guided Sezawa modes (Si) in ZnO thin films. The Si modes have high velocities and are polarised in the sagittal plane. Consequently, radio frequency (GHz) SAW devices could be fabricated using ZnO thin films on SiO2/Si with great potential for use as SAW devices which can be readily integrated with Si-based semiconductor technology.

The rapid growth of thin transparent films of diamond

Abstract Thin transparent films of diamond have been grown only by chemical vapor deposition (CVD) methods at slow rates. Using a hot filament CVD reactor of large area ( 30 cm ×40 cm ), these films were deposited onto highly polished copper substrate at a rate of about 1 μm h−1. The cost of manufacturing thin transparent films of 20 μm thickness may be as low as $1 cm−2. At such a low cost, thin transparent films should have diversified applications, such as X-ray windows, high frequency (up to 10 GHz) SAW filters, and other advanced devices.

SAW Device Beyond 5 GHz

Here we describe recent high frequency SAW technology based on ultra-fine fabrication techniques using electron beam (EB) exposure direct writing and low-loss wide-band filter applications beyond 5 GHz. We especially consider the propagation property of SAWs in the 10 GHz range, which is important for low-loss characteristics, the fabrication technology, and some examples of low-loss filters using unidirectional transducers and ladder type SAW filters.

SAW DEVICE BEYOND 5 GHz

Here we describe recent high frequency SAW technology based on ultra-fine fabrication techniques using electron beam (EB) exposure direct writing and low-loss wide-band filter applications beyond 5 GHz. We especially consider the propagation property of SAWs in the 10 GHz range, which is important for low-loss characteristics, the fabrication technology, and some examples of low-loss filters using unidirectional transducers and ladder type SAW filters.

SAW device applications of longitudinal leaky surface waves on lithium tetraborate.

The possibility of high frequency SAW device applications of longitudinal leaky surface waves (LLSW) on lithium tetraborate (Li2 B4O7; LBO) is investigated in this paper. An electrical equivalent circuit model (ECM) is extended in order to consider effects of bulk wave scattering for the LLSWs. The equivalent circuit parameters used in the extended ECM for designing the LLSW devices are directly determined from numerically calculated dispersion curves. For applications of the LLSW, high frequency SAW filters on LBO with the Euler angles (0&deg;, 47.3&deg;, 90&deg;) are demonstrated. As examples of the high frequency devices, 1.5 GHz and 1.2 GHz SAW filters using the mode are designed by using the extended ECM, and fabricated by using conventional patterning processes. One is for the filter of the global positioning system (GPS), another is for the 1.2 GHz band data transmission radio system in Japan. As a result, low loss SAW filters can be obtained easily without submicron fabrication techniques by using the LLSWs on LBO. Furthermore, the frequency response calculated by the extended ECM are in a good agreement with the experiments.

Propagation of longitudinal leaky surface waves under periodic metal grating structure on lithium tetraborate

The dispersion properties of longitudinal leaky surface waves propagating under the periodic Al strip grating on lithium tetraborate (Li(2)B(4)O(7); LBO) are described theoretically and experimentally for applications of the mode to high frequency SAW devices. A theoretical method developed here is based on Floquet's theorem using space harmonics as an orthogonal function set and real boundary integral equations derived from the method of weighted residuals for a period of each region, i.e., substrate, metal, and free space. The boundary integral equations are solved by using the Galerkin procedure. The periodic strip gratings with both single-electrodes and double-electrodes are investigated, considering the convergency of the numerical computation for the number of the space harmonics. As a result, the propagation loss for shorted gratings was found to be relatively low in the thickness range of the Al strip below about 1% for the single-electrodes and 2% for the double-electrodes, although it greatly increases for a thickness over 2% for the single-electrodes and 3% for the double-electrodes.