Kenjiro Higaki

Theoretical study on SAW characteristics of layered structures including a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAWs with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO/sub 3//diamond/Si and LiTaO/sub 3//diamond/Si structures. It was also found that a SiO/sub 2//ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%.<<ETX>>

High Frequency Surface Acoustic Wave Filter Using ZnO/Diamond/Si Structure

Diamond films grown by chemical vapor deposition (CVD) method is receiving much attention as surface acoustic wave (SAW) substrate material, due to its remarkable characteristics of large SAW velocity to offer much attraction in fabrication of high frequency SAW devices. The basic properties of SAW propagating in a ZnO/diamond/Si structure were studied utilizing thin film poly-crystalline diamond. Phase velocity, group velocity, electro-mechanical coupling coefficient (K2), and temperature coefficient of frequency ( TCF) were examined by experiments and theoretical calculations. And it was confirmed that this structure provides high phase velocity up to 10000 m/s, high K2 up to 4.8% and pratical TCF value of -28 ppm/° C. An 1.3 GHz SAW filter of a ZnO/diamond/Si structure was fabricated for the first time.

HETEROEPITAXIAL GROWTH OF ZNO FILMS ON DIAMOND (111) PLANE BY MAGNETRON SPUTTERING

ZnO thin film has been epitaxially grown on the (111) plane of the diamond substrate by rf magnetron sputtering at substrate temperature as low as 260 °C. The crystallinity was examined by x‐ray diffraction and reflection high‐energy electron diffraction. It was found that the smallest standard deviation angle estimated from the x‐ray rocking curve of the ZnO(0002) peak was 0.27° whereas the mismatch of the lattice parameter between film and substrate is 28.8%. The epitaxial relationship between ZnO film and the diamond is determined as [1120] ZnO//[101] diamond.

SAW devices on diamond

SAW characteristics in a ZnO/diamond/Si structure were investigated by theoretical calculations and experiments using poly-crystalline diamond films. It was found that the 1st mode in the ZnO/IDT/diamond/Si structure has the velocity of 10,500 m/s and K/sup 2/ value of 1.5%, and the 2nd mode in the IDT/ZnO/diamond/Si structure has the velocity of 11,600 m/s and K/sup 2/ value of 1.1%. The experimental results agreed well with the calculated data. Utilizing this high velocity, a 2.5 GHz filter was fabricated with 1 /spl mu/m line-and-space IDTs with the ZnO/IDT/diamond/Si structure. Propagation loss was confirmed to be much lower than ever reported for a ZnO/glass structure. It was also found by calculations that a SiO/sub 2//ZnO/diamond/Si structure realizes a zero TCD value with the velocity of 10,700 m/s and the K/sup 2/ value of 0.78% and that a LiNbO/sub 3//diamond/Si structure has a high velocity of 12,000 m/s and a large K/sup 2/ value of 9%. These structures including a diamond layer can be effectively used for a variety of applications in GHz range with much higher velocities than those of conventional SAW materials such as LiTaO/sub 3/, LiNbO/sub 3/ or quartz.

Study on Surface Acoustic Wave Characteristics of SiO2/Interdigital-Transducer/ZnO/Diamond Structure and Fabrication of 2.5 GHz Narrow Band Filter

Surface acoustic wave (SAW) characteristics of the SiO2/interdigital-transducer(IDT)/ZnO/diamond structure were studied theoretically and experimentally. It was found that the 2nd mode in this structure can provide a zero temperature coefficient of frequency (TCF) as well as high velocity of 10,000 m/s and a large electromechanical coupling coefficient of 1.4%. Utilizing this wave, a 2.5 GHz narrow-band SAW filter was successfully fabricated with IDTs of 0.9 µm finger width, which resulted in superior characteristics for a retiming filter with a small insertion loss of 6.7 dB, Q value of 660 and small frequency deviation of 100 ppm in the temperature range from -40 to 85°C. The insertion loss is smaller than any other retiming filters which have ever been reported with quartz, a conventional SAW material, and also the frequency deviation with temperature is smaller than that in the case of ST-cut quartz.

Diamond-based surface acoustic wave devices

Research and development have been carried out to apply the CVD diamond film to surface acoustic wave (SAW) devices. Several kinds of layered structures including a diamond layer have been investigated by the calculations and experiments, and it has been found that the diamond SAW has great advantages for the application of high-frequency SAW devices with high SAW velocity, small temperature coefficient and high power durability. Practical SAW devices have been successfully fabricated with ZnO/diamond/Si and SiO2/ZnO/diamond/Si structures whose characteristics are superior to those with conventional SAW materials.

High power durability of diamond surface acoustic wave filter

The power durability of the ZnO/diamond surface acoustic wave (SAW) filter was investigated and compared with the LiTaO/sub 3/ SAW filter. The ZnO/diamond SAW filter maintained linearity of the input-output relation up to the input power of 36 dBm at 2.90 GHz, whereas the LiTaO/sub 3/ SAW filter, which operated at 822 MHz, was nonlinear above the input power of 23 dBm and degraded significantly at 27.7 dBm. The input power dependence of the time to failure was also investigated for both SAW filters. It was found that the ZnO/diamond SAW filter was durable for 8 dB higher input power than that of the LiTaO/sub 3/ SAW filter even at 3.5 times higher frequency. These results indicate the extremely high power durability of the ZnO/diamond SAW filter.

Epitaxial Growth of MgO Layer on Y1Ba2Cu3O7-y Thin Film

A MgO/Y1Ba2Cu3O7-y (YBCO) structure on a MgO(100) substrate was prepared by a reactive vacuum evaporation method. The MgO layer with (100) orientation was obtained on each of the (001) and (100) YBCO thin films at the substrate temperature of 400°C. The crystallinity of the MgO layer as a function of the MgO thickness was investigated. The randomness of the crystallinity became smaller with a decrease in the thickness. The (100) epitaxial MgO layer without other orientations was obtained at a thickness of 5 nm. Structures of both YBCO/MgO/YBCO(001) and YBCO/MgO/YBCO(100) were studied. From the result of the cross-sectional TEM observation, it was confirmed that for both the (001) and (100) base YBCO layers, top YBCO layers with the same orientation were epitaxially grown via the intermediate MgO layer with a thickness of 5 nm.

Fabrication of 2.5 GHz SAW retiming filter with SiO/sub 2//ZnO/diamond structure

The variety of layered structures with a diamond film are expected to be applied for high-frequency SAW devices due to their high velocities. The SiO/sub 2//ZnO/IDT/diamond structure is one of the promising, especially for high-frequency and narrow-band filters. In this paper, the 1st mode SAW in the SiO/sub 2//ZnO/IDT/diamond structure was experimentally studied, and confirmed that it can provide zero temperature coefficient of frequency (TCF) as well as high velocity of 9,000 m/s. A 2.5 GHz narrow-band SAW filter was successfully fabricated with this structure with IDTs of 0.9 /spl mu/m finger width, and it was shown that this filter has practical and superior characteristics for the 2.5 GHz retiming filter. Its insertion loss was 10 dB that is smaller than ever reported with quartz.

Crystal Orientation of YBa2Cu3O7-y Thin Films Prepared by RF Sputtering

The effect of growth temperature on the crystallinity and morphology of YBa2Cu3O7-y thin films was investigated. Thin films were grown on MgO (100) single-crystal substrates by RF magnetron sputtering. Preferred orientations of thin films can be controlled by adjusting the substrate temperature, Ts; (100) a-axis oriented films were deposited at 540°C<Ts<550°C, (110) oriented films were deposited at 565°C<Ts<600°C and (001) c-axis oriented films were deposited above 610°C.