Tomonobu Hata

Optimization of zinc oxide thin film for surface acoustic wave filters by radio frequency sputtering

Abstract Electrical characteristics of zinc oxide (ZnO)/glass surface acoustic wave (SAW) filters, the structure of which is ZnO thin film on a glass substrate with aluminum inter digital transducers, are greatly influenced by deposition parameters of a radio frequency sputtering for making ZnO thin films. The deposition conditions for making the ZnO thin film are also considered to obtain good piezoelectricity for SAW devices. Oxygen concentration in the radio frequency sputtering greatly affects the properties of ZnO thin films. The interface microstructure of ZnO thin films is investigated by cross-section transmission electron spectroscopy. The growth figures of ZnO on glass and ZnO on Al are similar. The average crystal size of ZnO on glass is larger than that of ZnO on Al.

Effect of substrate surface morphology and interface microstructure in ZnO thin films formed on various substrates

Abstract ZnO thin films have been grown by radio frequency sputtering on Al, Au, Ni, Cu and glass substrates. Crystallinity of ZnO thin films has been estimated by X-ray diffraction (XRD) and reflected high-energy electron diffraction (RHEED). ZnO/substrates interfaces have been observed by cross-section transmission electron microscopy (TEM). ZnO thin films are classified into two groups depending on the type of substrate. ZnO thin films on glass, Au and Al show good c -axis orientation, but orientation of ZnO thin films on Ni and Cu is rather disordered. Considering the relationship between surface morphology of substrates and crystallinity of ZnO, the orientation of ZnO thin film is disordered, when the substrate surface is rough. Crystallinity of ZnO is greatly influenced by surface morphology as well as substrate crystallinity.

High rate deposition of thick piezoelectric ZnO films using a new magnetron sputtering technique

ZnO films were prepared on glass substrates with a new reactive magnetron sputtering technique using a zinc target. A solenoid coil around a bell jar was used to cause an increase in the parallel component of the leakage lines to the target surface. Highly oriented ZnO films (c‐axis orientation) were fabricated on the glass substrates. The deposition rate increases with increasing discharge current, and it attains 15 μm/h for (O2 50%:Ar50%), 11 μm/h for (81.5%:18.5%), and 9 μm/h for 100% oxygen. These films were obtained at a 500‐mA discharge current and a 0.2‐Torr total gas pressure. Resistivities of the films were 106–109 Ω cm. A piezoelectric coupling constant of thick ZnO films (∼30 μm) was obtained by measuring a frequency response of transducer impedance near the fundamental resonance frequency of the interdigital electrode. Experimental results were about 80% of the theoretical values.

Material properties of ZrN film on silicon prepared by low-energy ion-assisted deposition

Abstract We investigated the crystalline quality and the resistivity of a ZrN film on an Si(100) substrate, prepared by an ion-assisted deposition method. With this method, metallic Zr is deposited by electron beam evaporation, while simultaneously bombarding the film with nitrogen ions of low ion energy E i (100–700 eV). The crystalline quality was improved and the resistivity was decreased by lowering the ion energy to 150 eV. However, the quality of the film prepared at E i = 100 eV became poorer than those at E i = 150 and 200 eV. The optimum arrival rate ratio N/Zr for the film quality depends slightly on the ion energy, and is about 1.2–1.5 for E i = 150 and 200 eV. Increasing the substrate temperature to 820 °C improves the crystalline quality of the ZrN film and decreases the resistivity. However, at 850 °C, a solid phase reaction between Si and Zr occured, and Zr silicide was sometimes formed. With the optimum deposition conditions, we can obtain a (100)-oriented ZrN film on Si with a resistivity as low as 11.4 μΩ cm.

Improvement of the crystalline quality of an yttria-stabilized zirconia film on silicon by a new deposition process in reactive sputtering

Abstract A heteroepitaxial yttria-stabilized zirconia (YSZ; (ZrO2)1−x(Y2O3)x) film is difficult to grow on Si by reactive sputtering with Ar+O2 gases, since the bare Si surface is easily oxidized by plasma radiation or excited oxygen. In order to overcome this difficulty, a very thin metallic Zr1−xYx film is deposited on the Si substrate by sputtering with Ar gas before depositing the YSZ film. This new deposition process can improve the crystalline quality of the YSZ film and produce a heteroepitaxial YSZ film on Si even in reactive sputtering. It was also found that the thickness and deposition temperature of the metallic film strongly influenced the crystalline quality of the YSZ film on Si, their optimum values being about 0.4 nm and 800 °C respectively.

DC reactive magnetron sputtered ZnO films

Abstract This paper is concerned with a new reactive magnetron sputtering technique using a zinc target and 100% oxygen gas. A solenoid coil around a bell jar was used to cause an increase in the parallel component of the leakage magnetic lines to the target surface. Highly oriented ZnO films ( c -axis orientation) were fabricated on glass substrates at very high deposition rates (10 μm/h). The quality of the ZnO film was determined by X-ray diffraction, reflection electron diffraction, and scanning electron microscopy (SEM). Well-oriented ZnO films were obtained at relatively high oxygen pressures from 0.2 to 0.4 Torr. This sputtering system is more convenient than other existing sputtering systems.

Heteroepitaxial growth of YSZ films on Si(100) substrate by using new metallic mode of reactive sputtering

Abstract Yttria-stabilized zirconia (YSZ) thin film is deposited by reactive magnetron sputtering from a composite Zr–Y target in an Ar–O2 gas mixture. Various hysteresis curves, such as the deposition rate and Zr + , Y + , ZrO + , YO + ions, are observed as a function of the oxygen flow rate (O 2 /Ar). In the metallic mode, metallic films are deposited and in the oxide mode YSZ films are deposited. In order to control the reaction in reactive sputtering, we protected the metallic target from oxidation by using a target cover with a 20 mm-diameter aperture. It was successful in depositing YSZ films in the metallic mode which has ten times higher deposition rate than the oxide mode. In this mode it was successful to growth the YSZ film heteroepitaxially on p-(100)Si. The substrate temperature was 800°C, and full width at half maximum (FWHM) of the rocking curve was 1.08°. The initial stage of epitaxial formation was observed by using the X-ray diffraction pattern, the reflected high energy electron diffraction (RHEED) pattern, and the cross-sectional transmission electron microscope (XTEM) photograph.

High Rate Deposition of ZnO Film Using Improved DC Reactive Magnetron Sputtering Technique

This paper concerns with a new reactive magnetron sputtering technique using a zinc target and pure oxygen gas. A solenoid coil around a bell jar was used to improve an increase in the parallel component of the leakage magnetic lines to the target surface. Highly oriented ZnO films (c-axis orientation) were fabricated on glass and on (111) gold substrates. Deposition rate reached 10 µm/h at 500 mA. The quality of the ZnO film was determined by X-ray diffraction, reflection electron diffraction (RED), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Deviation σ on the glass substrate is 1.67°. Excellent ZnO films were obtained at relatively high oxygen gas pressures, 0.25 Torr on glass and 0.55 Torr on (111) gold substrate. Low temperature sputtering is possible for small discharge current.

Material properties of a ZrNx film on silicon prepared by ion‐assisted deposition method

The resistivity, chemical composition, and crystalline quality of ZrNx films deposited on Si(100) substrates by an ion‐assisted deposition (IAD) method were investigated as a function of deposition parameters. By this method, metallic Zr is deposited by electron‐beam evaporation while simultaneously bombarding the film with nitrogen ions having energies of 400–700 eV. It was found that the crystalline quality of the film was improved by increasing the substrate temperature Ts to 820 °C and decreasing the ion energy. When Ts was 850 °C, zirconium silicide was formed easily near the interface between the film and the Si substrate. At Ts=820 °C, the chemical composition ratio of the film was hardly influenced by the arrival rate ratio N/Zr, Ra, in the range of 1.5 to 3.0 and the deposited ZrN film was almost stoichiometric. However, when Ra was more than 1.9, the film quality was degraded by radiation damage due to excess nitrogen ions in the growing film. When Ra was less than 1.5, the surface of the film w...

Analysis of Sputter Process on a New ZrTi+PbO Target System and Its Application to Low-Temperature Deposition of Ferroelectric Pb(Zr,Ti)O3 Films

An rf reactive sputter deposition technique was adopted to deposit ferroelectric lead zirconate titanate (PZT) thin films from a ZrTi (50%/50%) alloy target combined with PbO pellets. Deposition characteristics including the effects of PbO area ratio were discussed. A new deposition mode called the near-metallic mode was observed. The crystallinity and the composition of the films prepared in the near-metallic mode were investigated as functions of PbO area ratio and substrate temperature. Excess Pb supply tended to enhance perovskite phase formation. Near-stoichiometric perovskite PZT films were obtained at a substrate temperature as low as 450° C. On the other hand, for a PZT+PbO target, perovskite PZT films were obtained only at temperatures higher than 540° C. Deposition rate of perovskite films from the ZrTi+PbO target was 2–3 times higher than that from the PZT+PbO target.