Atmospheric Gas-Phase Catalyst Etching of SiO2 for Deep Microfabrication Using HF Gas and Patterned Photoresist.

Micro/nanoscale structure fabrication is an important process for designing miniaturized devices. Recently, three-dimensional (3D) integrated circuits using SiO2 via-holes interlayer filling by copper have attracted attention to extend the lifetime of Moore's law. However, the fabrication of vertical and smooth-sidewall via-hole structures on SiO2 has not been achieved using the conventional dry etching method due to the limitation of the selective etching ratio of SiO2 and hard mask materials. In this study, we developed a unique method for the deep anisotropic dry etching of SiO2 using atmospheric gas-phase HF and a patterned photoresist. The hydroxyl groups in the photoresist catalyzed the HF gas-phase dry etching of SiO2 at high-temperature conditions. Therefore, fabrication of vertical with smooth-sidewall deep microstructures was demonstrated in the photoresist-covered area on SiO2 at a processing rate of 1.3 µm/min, which is 2-3 times faster than the conventional dry etching method. Additionally, the chemical reaction pathway in the photoresist-covered area on SiO2 with HF gas was revealed via density functional theory (DFT) calculations. This simple and high-speed microfabrication process will expand the commercial application scope of next-generation microfabricated SiO2-based devices.

ScAlN Thick-Film Ultrasonic Transducer in 40-80 MHz.

A medical ultrasound diagnostic system and an ultrasonic microscope are generally used in the frequency range of 1-20 MHz and 100 MHz-2 GHz, respectively. Ultrasonic transducers in the frequency range of 20-100 MHz are, therefore, not well developed because of less application into ultrasonic imaging or suitable piezoelectric materials with this frequency range. Polyvinylidene difluoride (PVDF) is usually used for ultrasonic transducers in the 10-50-MHz ranges. However, their electromechanical coupling coefficient of 4% is not enough for the practical uses. In order to excite the ultrasonic wave in the 20-100 MHz range, a 125-25- -thick piezoelectric film is required when the longitudinal velocity of the material is assumed to be 5000 m/s. However, it is difficult to grow such a thick piezoelectric film without a crack being caused by the internal stress during the dry deposition technique. We achieved a stress-free film growth by employing the unique hot target sputtering technique without heating the substrate. High-efficient 81- ( .5%) and 43-MHz ( %) ultrasonic generation by using the 43- and 90- extremely thick ScAlN (Sc: 39%) films were demonstrated, respectively. We discussed the advantage of ScAlN thick-film transducers by comparing them with the conventional PVDF transducer for the water medium.

Rapid Wave Velocity Measurement by Brillouin Scattering Using Coherent Phonons Induced by ScAlN Piezoelectric Thin-Film Transducer.

It is difficult to perform 2-D imaging of elastic properties using the Brillouin scattering technique because the weak thermal phonon signal in the sample leads to low measurement accuracy and long measurement times. To improve the phonon signal, we artificially induced acoustic phonons using a ScAlN thin-film piezoelectric transducer, which has a giant piezoelectricity. The film was grown using RF magnetron sputtering of a ScAl alloy target on a silica glass bar sample. Using a microwave probe, the electric power applied to the film was 1 mW at 875 MHz. We obtained the enhancement of the Brillouin scattering signal in the silica glass bar sample due to the induced phonons. Compared with and without the induced phonons from the ScAlN film transducer, the peak intensity improved by nearly 3 orders of magnitude. This technique can significantly shorten the time required for the Brillouin scattering measurements.

Electromechanical coupling coefficient k15 of polycrystalline ZnO films with the c-axes lie in the substrate plane.

The (1120) textured polycrystalline ZnO films with a high shear mode electromechanical coupling coefficient k15 are obtained by sputter deposition. An over-moded resonator, a layered structure of metal electrode film/(1120) textured ZnO piezoelectric film/metal electrode film/silica glass substrate was used to characterize k15 by a resonant spectrum method. The (1120) textured ZnO piezoelectric films with excellent crystallite c-axis alignment showed an electromechanical coupling coefficient k15 of 0.24. This value was 92% of k15 value in single-crystal (k15 = 0.26).

Characteristics of pure-shear mode BAW resonators consisting of (1120) textured ZnO films.

Thickness pure-shear mode film bulk acoustic wave resonators (FBARs) made of (1120) textured ZnO films have been fabricated. We also have fabricated FBAR structure consisting of two layers of the (1120) textured ZnO film with opposite polarization directions. This FBAR structure operated in second overtone pure-shear mode and allowed shear-mode FBARs at higher frequency. The effective electromechanical coupling coefficients k2 of pure-shear mode FBAR and second overtone pure-shear mode FBAR in this study were found to be 3.3% and 0.8%, respectively. The temperature coefficient of frequency (TCF) of thickness extensional mode FBAR, pure-shear mode FBAR, and second overtone pure-shear mode FBAR were measured in the temperature range of 10-60 degrees C. TCF values of -63.1 ppm/degrees C, -34.7 ppm/degrees C, and -35.6 ppm/degrees C were found for the thickness extensional mode FBAR, the pure-shear mode FBAR, and the second overtone pure-shear mode FBAR, respectively. These results demonstrated that pure-shear mode ZnO FBARs have more stable temperature characteristics than the conventional thickness extensional mode ZnO FBARs.

Acoustic Wave Velocities and Refractive Indices in an m-Plane GaN Single Crystal Plate and c-Axis Oriented ScAlN Films Measured by Brillouin Scattering Techniques.

We have experimentally investigated wave velocities and refractive indices in bulk and film samples [a GaN single crystal plate and c-axis-oriented ScxAl(1-x)N (x = 0.00-0.63) films] by Brillouin scattering. All of the piezoelectrically unstiffened elastic constants and the ordinary refractive index of the GaN single crystal plate were determined from the reflection induced A (RIA) scattering geometry and the combination of 90R and 180° scattering geometries. The uncertainties of the measured wave velocities were approximately 0.17% (RIA) and 2.5% (combination technique). In addition, the longitudinal wave velocities of ScxAl(1-x)N films propagating in the normal direction were obtained by the combination technique. The maximum uncertainty was approximately 3.3%. The shear wave velocities and refractive indices of ScxAl(1-x)N films were also investigated by the 90R scattering geometry using velocities measured by high-overtone bulk acoustic resonators. The softening trends of the elasticity were obtained from the measured longitudinal and shear wave velocities, although there were large uncertainties in the Brillouin measurement system owing to thermal instability.

Characteristics of (1010) and (1120) textured ZnO piezofilms for a shear mode resonator in the VHF-UHF frequency ranges.

This paper reports the fabrication and characterization of ZnO piezoelectric thin films in which the crystallite c-axis is unidirectionally aligned in the plane. The films were deposited by a conventional radio frequency (RF) magnetron sputtering apparatus without epitaxy. We have measured reflection coefficient S11 of the ZnO film/glass substrate composite shear mode resonator and confirmed that the resonator excites shear wave only in the very high frequency to ultra high frequency ranges (VHF-UHF). The crystallites c-axis orientation and alignment were determined by x-ray diffraction (XRD) patterns, phi-scan pole figure analysis, omega-scan rocking curves, and atomic force microscope (AFM) measurement. The transduction of the shear wave showed good agreement with properties of the crystallite alignment in the film.

Gigahertz acoustic wave velocity measurement in GaN single crystals considering acousto-electric effect.

The resistivity-frequency characteristics of longitudinal wave velocities propagating parallel to the c-axis in a GaN single crystal were theoretically estimated by considering the piezoelectric acousto-electric effect. The temperature and frequency dependences of longitudinal and shear wave velocities in conductive and semiconductive GaN single-crystal samples were experimentally investigated by Brillouin scattering. The temperature dependence of longitudinal and shear wave velocities had a linear tendency in the conductive sample, whereas in the semiconductive sample, those had a similar tendency to the predicted velocity changes resulting from the piezoelectric stiffening effect. However, the temperature dependence of shear wave velocity, which does not possess piezoelectric coupling, had a tendency similar to that of the longitudinal wave in the semiconductive sample, unexpectedly. The frequency dependence of longitudinal wave velocities in the semiconductive sample had a tendency similar to the predicted velocity changes resulting from the piezoelectric stiffening effect.

High-performance Brillouin spectroscopy of phonons induced by a piezoelectric thin film with a coaxial microwave resonator.

To overcome the low accuracy of acoustic velocity measurements based on Brillouin scattering from thermal phonons, we attempted to utilize induced coherent phonons, which cause intense Brillouin scattering. A ZnO piezoelectric film was used to induce gigahertz-range coherent phonons in a silica glass block sample. An evanescent electromagnetic wave leaked from a coaxial resonator was applied into the film to excite phonons. The scattered light obtained using this simple system was much more intense than that obtained from thermal phonons. This technique will improve the accuracy and reduce the measurement time.

Relationships between the anisotropy of longitudinal wave velocity and hydroxyapatite crystallite orientation in bovine cortical bone.

Quantitative ultrasound (QUS) is now widely used for evaluating bone in vivo, because obtained ultrasonic wave properties directly reflect the visco-elasticity. Bone tissue is composed of minerals like hydroxyapatite (HAp) and a collagen matrix. HAp crystallites orientation is thus one parameter of bone elasticity. In this study, we experimentally investigated the anisotropy of ultrasonic wave velocity and the HAp crystallites orientation in the axial-radial and axial-tangential planes in detail, using cylindrical specimens obtained from the cortical bone of three bovine femurs. Longitudinal bulk wave propagation was investigated by using a conventional ultrasonic pulse system. We used the one cycle of sinusoidal pulse which was emitted from wide band transmitter. The nominal frequency of the pulse was 1MHz. First, we investigated the anisotropy of longitudinal wave velocity, measuring the anisotropy of velocity in two planes using cylindrical specimens obtained from identical bone areas. The wave velocity changed due to the rotation angle, showing the maximum value in the direction a little off the bone axis. Moreover, X-ray pole figure measurements also indicated that there were small tilts in the HAp crystallites orientation from the bone axis. The tilt angles were similar to those of the highest velocity direction. There were good correlations between velocity and HAp crystallites orientation obtained in different directions. However, a comparatively low correlation was found in posterior bone areas, which shows the stronger effects of bone microstructure. In the radial-tangential plane, where the HAp crystallites hardly ever align, weak anisotropy of velocity was found which seemed to depend on the bone microstructure.

Observation of induced longitudinal and shear acoustic phonons by Brillouin scattering.

To improve the accuracy of velocity measurements in the Brillouin scattering technique using weak thermal phonons, we have used induced coherent phonons, which intensify the scattering. To induce phonons in the gigahertz range, we used a c-axis tilted ZnO film transducer that was developed in our laboratory. This allowed us to induce longitudinal and shear acoustic phonons effectively at hypersonic frequencies. As a result, we obtained scattered light in the silica glass sample that was much more intense than that obtained from the thermal phonons. Because the Brillouin scattering from induced phonons was measured, the shift frequency was that of the electric signal applied to the ZnO transducer. Strong peaks lead to a reduction of the measurement time. This is useful for two-dimensional mapping of thin film elasticity using Brillouin scattering. Additionally, Brillouin scattering enables the simultaneous measurement of longitudinal and shear phonon velocities in the sample plane. This opens up a potential new technique for non-destructive elasticity measurements of various materials.

c-Axis zig-zag ZnO film ultrasonic transducers for designing longitudinal and shear wave resonant frequencies and modes.

A method for designing frequencies and modes in ultrasonic transducers above the very-high-frequency (VHF) range is required for ultrasonic non-destructive evaluation and acoustic mass sensors. To obtain the desired longitudinal and shear wave conversion loss characteristics in the transducer, we propose the use of a c-axis zig-zag structure consisting of multilayered c-axis 23° tilted ZnO piezoelectric films. In this structure, every layer has the same thickness, and the c-axis tilt directions in odd and even layers are symmetric with respect to the film surface normal. c-axis zig-zag crystal growth was achieved by using a SiO(2) low-temperature buffer layer. The frequency characteristics of the multilayered transducer were predicted using a transmission line model based on Mason's equivalent circuit. We experimentally demonstrated two types of transducers: those exciting longitudinal and shear waves simultaneously at the same frequency, and those exciting shear waves with suppressed longitudinal waves.

Distribution of hydroxyapatite crystallite orientation and ultrasonic wave velocity in ring-shaped cortical bone of bovine femur.

At the nanoscopic level, bone consists of calcium phosphate, which forms incomplete hydroxyapatite (HAp) crystals. The preferred orientation of the c-axis of HAp crystallites induces anisotropy and inhomogeneity of elastic properties in bone. In this study, the effect of the preferred orientation of HAp crystallites on the spatial distribution of ultrasonic wave velocity was experimentally investigated, considering bone mineral density (BMD) and microstructure. Three ring-shaped cortical bone samples were made from a 36-month-old bovine femur. Longitudinal wave velocity was measured by a conventional ultrasonic pulse system, using self-made polyvinylidene fluoride transducers. The integrated intensity of the (0002) peak obtained using X-ray diffraction was estimated to evaluate the amount of preferred orientation. The velocity distribution pattern was similar to the distribution of integrated intensity of (0002). The effect of the preferred orientation of HAp crystallites on velocity was clearly observed in the plexiform structure, despite the fact that the BMD value was almost independent of the preferred orientation of HAp crystallites. Velocity measurement of cortical bone can reveal information about HAp crystallite orientation.

Correlation between hydroxyapatite crystallite orientation and ultrasonic wave velocities in bovine cortical bone.

The mineral component of bone is mainly composed of calcium phosphate, constituting 70% of total bone mass almost entirely in the form of hydroxyapatite (HAp) crystals. HAp crystals have a hexagonal system and uniaxial elastic anisotropy. The objective of this study was to investigate the effect of HAp crystallite preference on macroscopic elasticity. Ultrasonic longitudinal wave velocity and the orientation of HAp crystallites in bovine cortical bone are discussed, considering microstructure, density, and bone mineral density (BMD). Eighty cube samples of cortical bone were made from two bovine femurs. The orientation of HAp crystallites was evaluated by integrated intensity ratio of (0002) peak using an X-ray diffractometer. Ultrasonic longitudinal wave velocity was investigated with a conventional pulse system. The intensity ratio of HAp crystallites and velocity were measured in three orthogonal directions; most HAp crystallites aligned in the axial direction of the femurs. Our results demonstrate a linear correlation between velocity and intensity ratio in the axial direction. Significant correlation between velocity and BMD values was observed; however, the correlation disappeared if we focused on the identical type of microstructure. In conclusion, differences in microstructure type have an impact on density and BMD, which clearly affects the velocity. In addition, at the nanoscopic level, HAp crystallites aligned in the axial direction also affected the velocity and anisotropy.