Optical Fibers with Patterned ZnO/Electrode Coatings for Flexural Actuators
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Mat. Res. Soc. Symp. Proc. Vol. 459 0 1997 Materials Research Society
130nm of Au were thermally evaporated. A 6 pm thick, [0001] radially-oriented ZnO film was then deposited by reactive magnetron sputtering in an Ar/0 2 atmosphere with a target-fiber distance of 9 cm. Additional details on the ZnO deposition are given elsewhere [8]. To facilitate subsequent electrical measurements, a portion of the bottom electrode was masked during the deposition of the piezoelectric. The Cr/Au top electrodes were made thick (25 and 400nm thick, respectively) to insure good electrical contact on the rougher fiber surface following the ZnO deposition. All top electrodes were evaporated through another shadow mask to prepare 2mm electrode sections separated by 2mm gaps perpendicular to the fiber axis (See Fig. 1). To create gaps in the inner electrode, patterning was done prior to deposition of the ZnO, while gaps in the outer electrode were made following all of the coating depositions. Patterning the electrodes was accomplished by dip-coating the fiber in Shipley Microposit 1813 photoresist, using a withdrawal rate of lmm/sec. After soft-baking for 30min at 90 °C, -30%tjm gaps in the electrode were exposed using a contact aligner and a special fixture in which the fiber could be held in contact with a Cr mask. The patterned region is 2 cm long. The fiber was then rotated 1800 about its axis and the exposure was repeated. The pattern was developed using the recommended
developer solution. Fibers were subsequently postbaked at 120'C for 30 min to harden the resist. For the bottom electrode, the pattern was then transferred into the electrode by etching for 45 seconds in an iodine-based Au etch, and 30 seconds in a Ce(NH 4)2(NO3)4 / CH 3COOH / H20 bath to remove the Cr. Deionized water was used as the etch stop. The protective resist layer was then removed in acetone and the fiber was rinsed in isopropanol. The procedure was similar for the top electrode, with the exception that Au etch was extended to 2 - 4 minutes, and the Cr etch to 1 - 2 minutes to completely clear the gap. Scanning electron microscopy was used to inspect the pattern quality. For electrical and optical measurements, the fiber was then placed into a slotted Si substrate, and glued into place at one end. Electrical connections were made with silver paint. Impedance measurements were made using an HP4194 Impedance Analyzer. The displacement of the fiber was then measured as a function of the driving frequency using a single beam MachZehnder interferometer. Optical Fiber
ZnO Piezoelectric Film
(a) Split Inner Electrode
/ Outer Ring Electrodes
ZnO Piezoelectric Film Optical Fiber
/ (b) )
Inner Electrode Split Outer Electrodes
Figure 1: Schematics for the fiber-based flexural actuators. (a) Geometry I: Split inner electrode, (b) Geometry II: Split outer electrode. In both cases, the electrodes are split on the opposite side of the fiber as well. 190
RESULTS AND DISCUSSION Figure 2 shows an example of the photolithographic patterning of an inner electrode.
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