Yukio Yoshino

Electrical properties of (Zr,Sn)TiO4 dielectric thin film prepared by pulsed laser deposition

We have been successful in obtaining temperature‐stable crystallized thin film of (Zr,Sn)TiO4. Preferential (111)‐oriented (Zr,Sn)TiO4 thin film was prepared by pulsed laser deposition. Effects of crystallization were elucidated based on a comparison of electric properties of crystallized and amorphous (Zr,Sn)TiO4 film. For crystallized film, the temperature coefficient of capacitance (TCC) was 20 ppm/°C at 3 MHz and the dielectric constant er=38 in the microwave range of 1–10 GHz. These values are superior to those for amorphous film (TCC=220 ppm/°C, er=27). The crystallization of this material was found quite effective for improving dielectrical properties. Atomic force microscope images showed the surface morphologies of crystallized and amorphous film of (Zr,Sn)TiO4 to differ.

Effects of buffer layers in epitaxial growth of SrTiO3 thin film on Si(100)

SrTiO3 thin film has been formed on Si(100) substrates with various single buffer layers such as SrO, CeO2, CaF2, CoSi2 and a multibuffer layer, YSZ/Y2O3/YBa2Cu3O7 by ArF excimer laser ablation. The relation of lattice orientation of buffer layers with SrTiO3 layer has been elucidated. The orientation of SrTiO3 film is influenced not only by lattice matching but by crystal structure and chemical bonding of the buffer layers. As well, the multibuffer layer more effectively forms preferential c‐axis oriented SrTiO3 film on Si(100), while CoSi2 buffer is more effective for improving the dielectric constant of SrTiO3 than other buffer layers.

Moisture-resistant ZnO transparent conductive films with Ga heavy doping

Moisture-resistant ZnO transparent conductive films were formed with Ga heavy doping by off-axis-type rf magnetron sputtering. The resistivity of 12.4wt% Ga-doped ZnO is 1.3×10−3Ωcm and changes by less than 3% over a 2000h reliability test at a temperature of 85°C and a humidity of 85%. The crystal structural analysis of the heavily Ga-doped ZnO films indicates that the c axis grows along various directions, which is quite different from the conventional c-axis oriented growth. The effect of heavy doping is discussed based on the crystal structural transformation and carrier compensation by excess Ga segregated in the film.

Effects of interface micro structure in crystallization of ZnO thin films prepared by radio frequency sputtering

ZnO thin films have been formed by radio frequency sputtering on glass, Al, Au and R-cut sapphire substrates. Micro structures of ZnO/substrate interfaces have been observed by transmission electron microscope and the surface morphology of the substrates has been measured by atomic force microscope. An amorphous layer is observed at the ZnO/glass interface and also a thicker amorphous layer at the ZnO/Al interface. No amorphous layers are observed at ZnO/Au and ZnO/sapphire interfaces, and direct orientation of ZnO thin films begins at the interface of both substrates. These results clearly demonstrate that crystallization of ZnO thin films prepared by radio frequency sputtering is strongly influenced by the surface crystallinity and morphology of substrates. Epitaxially grown ZnO thin films are confirmed, both on R-cut sapphire and on Au, by cross section transmission electron microscope.

Epitaxially Grown Aluminum Film on Rotated Y-Cut Lithium Niobate for High-Power Surface Acoustic Wave Devices

Epitaxially grown Al film was obtained on 36°-rotated Y-cut LiTaO3 substrate by an IBS system. The epitaxial relation was (111) Al//(012) LiTaO3. The Al film had very high resistance to stress migration. In SAW filters, time to failure caused by stress migration was improved by more than 100 times over an ordinary polycrystalline Al–Cu film.

Piezoelectric thin films and their applications for electronics

ZnO and AlN piezoelectric thin films have been studied for applications in bulk acoustic wave (BAW) resonator. This article introduces methods of forming ZnO and AlN piezoelectric thin films by radio frequency sputtering and applications of BAW resonators considering the relationship between the crystallinity of piezoelectric thin films and the characteristics of the BAW resonators. Using ZnO thin films, BAW resonators were fabricated for a contour mode at 3.58 MHz and thickness modes from 200 MHz to 5 GHz. The ZnO thin films were combined with various materials, substrates, and thin films to minimize the temperature coefficient of frequency (TCF). The minimum TCF of BAW resonators was approximately 2 ppm/°C in the range −20 to 80 °C. The electromechanical coupling coefficient (k2) in a 1.9 GHz BAW resonator was 6.9%. Using AlN thin films, 5–20 GHz BAW resonators with an ultrathin membrane were realized. The membrane thickness of a 20 GHz BAW resonator was about 200 nm, k2 was 6.1%, and the quality factor...

Improvement of thickness distribution and crystallinity of ZnO thin films prepared by radio frequency planer magnetron sputtering

An improvement of thickness distribution and crystallinity in ZnO thin films prepared by radio frequency (rf) planer magnetron sputtering have been studied. Optimum thickness distribution of less than ±2.2% in a 3-inch wafer is obtained by changing the substrate angle to the ZnO target and is in accordance with cosine law. C-axis orientation perpendicular to the silicon substrate is confirmed by X-ray diffraction. The stress of ZnO thin films is more than 0.3 GPa and its distribution is independent of the substrate angle that is set at a slant to the optimum angle for thickness distribution. These results indicate that thickness distribution of ZnO thin films heavily depends on the substrate angle, while the c-axis orientation, the stress and its distribution are independent of the setting angle of the substrate. The stress distribution is attributed to the lattice constant distribution and the crystallinity distribution of ZnO thin films.

Fabrication of 5GHz band film bulk acoustic wave resonators using ZnO thin film

We successfully fabricated 5GHz band thin film bulk acoustic wave (BAW) resonators using ZnO thin film. Two types of BAW resonators utilizing fundamental resonance and secondary harmonics were examined. In the fundamental mode resonator, the quality factor (Q factor) and the effective electromechanical coupling coefficient (k/sub teff//sup 2/) were realized at 1000 and 6.7%, respectively. These values for the secondary mode resonator were 700 and 4.7%, respectively. Combining four fundamental mode resonators as a ladder type piezoelectric filter, the bandwidth at 3 dB was achieved to be 191 MHz at 4.9 GHz. Even with the filter using the secondary mode resonator, the bandwidth was 130 MHz at 5.2 GHz.

Internal Stress of ZnO thin Films Caused by Thickness Distribution and Crystallinity

The improvement of thickness distribution and crystallinity in ZnO thin films prepared by radio frequency (rf) planer magnetron sputtering has been studied. Optimum thickness distribution of less than ± 2.2% in a 3-inch wafer is obtained by changing the substrate angle to the ZnO target and is in accordance with cosine law. The c-axis orientation perpendicular to the silicon substrate is confirmed by x-ray diffraction. The stress of ZnO thin films is larger than 0.3GPa and its distribution is independent of the substrate angle that is set at a slant to the optimum angle for thickness distribution. These results indicate that thickness distribution of ZnO thin films heavily depends on the substrate angle, while the stress and its distribution are independent of the setting angle of the substrate. Stress distribution is attributed to the distribution of argon ions and sputtered molecules impinging a wafer.

Preparation and electrical properties of (Zr,Sn)TiO{sub 4} dielectric thin films by laser ablation

(Zr,Sn)TiO{sub 4} is considered as a promising dielectric material for microwave devices owing to the temperature stability of capacitance and excellent microwave properties. Preferential (111)-oriented (Zr,Sn)TiO{sub 4} thin film was obtained by an ArF laser ablation. Properties of the crystallized film were as follows; the temperature coefficient of capacitance TCC was 17.6 ppm/C at 3 MHz and the dielectric constant {var_epsilon}{sub r}, 38 in the microwave range of 1GHZ--10GHz. It has turned out that the crystallization of this material is quite effective for improving dielectrical properties. Surface morphologies were observed by atomic force microscope (AFM). Grains grew on the crystallized film at 1 {micro}m x 1 {micro}m size.