Hideaki Nakahata

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>>

Properties of Boron-Doped Epitaxial Diamond Films

Boron-doped homo-epitaxial films were deposited by microwave plasma CVD on synthesized single-crystal diamonds. Boron concentration in the films was determined by the boron concentration in the reactant gases. The crystallinity of boron- doped films was found to be affected by the boron concentration in the reactant gases. The conductivity of boron doped films was determined by the concentration of boron, and the activation energy was nearly the same as for natural IIb diamonds. The effect of the surface orientation of substrates was found in the conductivity of highly doped films. The result of the Hall effect measurement indicated that the mobility of boron-doped diamond epitaxial films was 70 cm2/Vs even at 500°C.

Electrical Characteristics of Metal Contacts to Boron-Doped Diamond Epitaxial Film

Current-voltage characteristics have been obtained for various metal contacts formed on boron-doped diamond epitaxial film prepared on synthesized Ib diamond by the microwave plasma-assisted chemical vapor deposition method. Ti contacts and W contacts have exhibited good ohmic and Schottky properties, respectively. For the first time, we have fabricated Schottky diodes on boron-doped diamond epitaxial films using these contacts and investigated their properties.

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.

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.

High frequency bandpass filter using polycrystalline diamond

Abstract Because of its high Youngs modulus, diamond has the highest acoustic velocity of all materials, and thus for surface acoustic wave (SAW) devices diamond can be expected to be one of the candidates for passive component applications. Polycrystalline diamond 40 μm thick was deposited on a silicon substrate by hot-filament chemical vapour deposition. After the diamond surface had been polished, 100 nm of Al was deposited and fabricated into interdigital transducers, utilizing conventional photolithography and etching processes. This was followed by the deposition of a ZnO thin film, which served as a piezoelectric material to generate SAWs. Finally, an SAW device using a ZnO/diamond system was fabricated for the first time. With the high SAW velocity of the ZnO/diamond system being up to 8600 m s, −1 a 1.07 GHz bandpass filter was successfully demonstrated using 2 μm line and space interdigital transducers; additionally −25 dB insertion loss was observed. This preliminary result suggests that, by utilizing diamond, high frequency (2–5 GHz) bandpass filters can be made available, and broad applications in the optical and personal communication systems can be expected.

High frequency surface acoustic wave filter using ZnO/diamond/Si structure

The surface acoustic wave (SAW) characteristics in a ZnO/diamond/Si structure are studied. Using theoretical calculations, effective film thicknesses of ZnO and diamond providing high phase velocity and K2 are found. Based on these data, a SAW filter of a ZnO/diamond/Si structure is fabricated. It is shown that large velocities can be obtained as expected from theoretical calculation and that a high-frequency SAW filter can be fabricated with much larger scale interdigital transducers (IDTs) than those in conventional material systems. The results indicate that this material system will find broad application in high-frequency SAW filters and resonators.<<ETX>>