Devices Employing Piezoelectric Boundary Waves for Integration with Semiconductor Circuitry

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The authors reported that there exists a boundary wave at the interface between the highly piezo-electric LiNbO_3 substrate and the Si substrate, and the wave enables us to realize high performance analog signal processing function comparable to the conventional surface acoustic wave (SAW) devices. This research intended to develop boundary wave devices through the development of to the substrate bonding technology. In order to develop very fine electrode pattern needed for this device structure, use of the electron-beam (EB) lithography was studied. For the purpose, the lift-off method is effective in general. However, since LiNbO_3 substrate is insulator, the direct EB exposure causes the charge-up problem. So as to circumvent this difficulty, the authors developed new lift-off process based on the water-solvable anti-static coating. This process enables us to realize Al IDT pattern of 0.2 μm line widths. Then the boundary wave device with Si/LiNbO_3 structure was fabricated in the f … More ollowing step : (1) very fine IDT was developed on the LiNbO_3 substrate by the EB-based lift-off process. (2) the SiO_2 intermediate layer was deposited as an insulation layer by the RF sputtering method, and (3) the Si top layer was deposited by the electron beam evaporation. The response by the boundary wave was clearly observed at 2.09 GHz, though the device insertion was 20 dB increased by the Si layer deposition. In addition, use of the adhesive was also investigated for the fabrication of the boundary wave devices. First use of the UV resin was studied. Although the wave response was observed at 991 MHz, the attained insertion loss was rather large (30.8 dB). It includes the ohmic loss of about 10 dB due to the electrode routing, whereas the remaining is due to the absorption with in the resin. Then the use of the kovar glass was studied, and the adhesion of Si with LiNbO_3 was succeeded. Although the response could not be detected at this moment due to the thickness of the glass layer, it could be used for the realization of high performance boundary wade devices. Less