Keishin Koh

Fabrication of Surface Acoustic Wave-Semiconductor Coupled Devices Using Epitaxial Lift-Off Technology

In this paper, we report the results of a study on the fabrication process of surface acoustic wave-semiconductor coupled devices, using epitaxial lift-off (ELO) and thin-film bonding technology. In order to realize a rugged bonding interface between the semiconductor film and piezoelectric substrate, we studied the 1) optimum conditions controlling stress in GaAs films, 2) reduction of releasing time for GaAs films, 3) and enhancement of bonding force between the film and LiNbO3 substrate. The experimental results clarified that these process improvements were effective for fabricating functional devices with the AlGaAs/LiNbO3 structure.

Design on semiconductor coupled SAW convolver

This paper presents results of a design study on a semiconductor coupled surface acoustic wave (SAW) convolver in which bi-directionally propagating SAWs, on a piezoelectric substrate with a high coupling coefficient, couple with bonded semiconductor diodes through multistrips. To obtain convolution signals with a high efficiency, we adopted a diode-balanced bridge structure for the nonlinear operation. We also found that the tapping pitches of the multi-strip electrodes have robustness against operation frequency variation and temperature-dependent variation on the delay of the SAW. We verified the effectiveness of the device in. a circuit simulation and an experiment on a test circuit, which was fabricated by using an epitaxial lift-off film-bonding process.

Photoresponse on Surface Acoustic Wave Devices with Compound Semiconductor and LiNbO3 Structures

We studied the photoresponse on a surface acoustic wave (SAW) device coupled with AlGaAs film. The experimental results indicate that the transferred carriers by SAW are optically generated electrons rather than optically generated holes. The electric field due to SAW propagation reduces the recombination of the optically generated electrons and holes. We were able to observed a modulated SAW signal due to the nonlinear interaction between SAW and optically induced carriers. The photoresponse due to the interaction between SAW and optically induced carriers shows a high sensitivity for the light intensity. The main results are (1) the control of SAW by optically induced carriers, and (2) the control of the optically induced carriers by SAW. We expect that the characteristics of the photoresponse will be applicable to the fabrication of functional optical-SAW coupled devices.

Fabrication of Si/LiNbO3 Structure by Using Film Bonding Process

In this work, we studied a fabrication process of the surface acoustic wave (SAW)-semiconductor coupled device with a Si/LiNbO3 structure by a film bonding process. The aim of this process is to realize a coupled device with a high performance and mass productivity for cost reduction. In order to realize bonding silicon films on a LiNbO3 substrate, we proposed the process of releasing a Si thin film from a silicon-on-insulator (SOI) wafer and examined the etching conditions of a SiO2 (box) layer. We also tried to apply local heating treatment using a microwave and an infrared laser to enhance the bonding force between the Si film and LiNbO3 substrate, and successfully bonded Si films released from a SOI wafer onto the LiNbO3 substrate. Based on these results, a test device with a Si/LiNbO3 structure was fabricated and the interaction between SAW and the optically induced carrier in the Si film semiconductor was discussed.

Piezoelectric sensor using a porous alumina film covered with a catalytic material

This paper reports results of a basic study on SAW sensor with a porous alumina film whose surface is covered with a catalytic materials of Pt and Co. We investigated basic adhesive characteristics for CO, NO, and NH/sub 3/ gas. In the sensor, we used a fluorinated polyimide passivation film for protecting physical and chemical attack which may cause in a fabrication process and sensing operation. The result verified the high sensitive performance of the sensor and the effectiveness of of the passivation film. We also discussed requirement for applying the mean to investigate catalytic phenomena.

Study on photo-induced acoustic charge transport effect in GaN film

In this paper, we investigated transport characteristic of photo-induced carriers by the potential well caused by travelling acoustic wave. We have carried out a basic experiment using delay lines consisting of GaN film on Al 2 O 3 substrate. As acoustic waves, we used SAW and guide wave layer mode in GaN thin film and employed MSM detector having the same structure as that of input IDT. The results have shown that the DC out put signals are obtained at the output diode for both modes. However, we have observed a relatively complicated phenomena, such as change on DC output signal polarity depending on the intensity of UV, trapping effect of carriers. We clarified that excess carrier either electron or hole in transported carrier would reasonably explain these effects. We also discuss device structures suitable for UV sensors.

Bonding Technology of Semiconductor Film on Piezoelectric Substrate Using Epitaxial Lift-Off Technology

In this paper, we report the results of a study on the basic problems of the mass production of epitaxial liftoff (ELO) film bonding technology. We propose a new releasing method for a large number of semiconductor films using polyimide to protect the semiconductor films. We investigated the basic process conditions and successfully released a large number of stripe shaped GaAs films. We also studied the heating method for enhancing the bonding strength between a small-sized semiconductor film and a piezoelectric substrate. We investigated the effect of migration of water molecules from the bonding interfaces using microwave and laser irradiation. We estimated the crystallinity of semiconductor films by X-ray diffraction. The results clarified that these processes were effective in improving mass productivity on the fabrication technology of surface acoustic wave (SAW)-semiconductor coupled devices.

P2I-1 Surface Acoustic Wave Ultraviolet Sensor Using Epitaxial AlGaN/(Al,Ga)N Film

In order to realize SAW ultraviolet sensor with short variable wavelength selectivity and high sensitivity, we study a SAW ultraviolet sensor using the AlGaN/(Al, Ga)N/Al2O3 system. When irradiating UV light on the surface of propagating path, the shift of frequency and change of insertion loss can be measured. Experimental results indicate that this sensor have good wavelength selectivity and could detect the weaker UV light power with several microwatt

Design consideration on ultra-wideband-SAW devices operating at GHz frequency range

This paper studies a feasibility of ultra-wideband SAW device for the matched filter device useful for the sensing device and local area CDMA communication systems with the pulse position spread spectrum signal modulation scheme at GHz frequency range. We propose an excellent coded device suited for realizing using SAW device. We clarify difference of the effect of so-called second order effect on these devices from that of narrow bandwidth devices and discuss device design methods. We discuss basic circuits of programmable device coupled with CMOS devices.

Simulation Study on Semiconductor Coupled Surface Acoustic Wave Convolver through a Multi-Strip Electrodes

This paper presents results of simulation study on a semiconductor coupled surface acoustic wave (SAW) convolver, in which the propagating SAW on a highly coupling coefficient piezoelectric substrate, couples with a bonded semiconductor diodes through multi-strip electrodes. We focus our study on a relatively wide band device which is the main feature of a highly efficiency device. By using a simple analysis and circuit simulator, based on the simulation program with integrated circuit emphasis (SPICE), we clarified the effect of device parameters, such as the shape of multi-strip tapping electrodes, characteristics of diode, impedance matching condition, kinds of transmission code and electro-mechanical coupling coefficient of SAW, on the device performances. We discussed the phenomenon, which cause the degradation, focusing on the frequency domain. We also clarified the essential problems of second order effect on the wide bandwidth device, which should be solved.