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Optical fibers with patterned ZnO/electrode coatings for flexural actuators

Optical fibers with patterned ZnO/electrode coatings for flexural actuators
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  Ž . Sensors and Actuators 73 1999 267–274 Optical fibers with patterned ZnO r electrode coatings for flexuralactuators S. Trolier-McKinstry  ) , G.R. Fox, A. Kholkin, C.A.P. Muller, N. Setter  Laboratory of Ceramics, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland  Received 19 June 1997; accepted 3 November 1998 Abstract A fiber-based flexural actuator was developed using a patterned piezoelectric ZnO r electrode fiber coating on a standard telecommuni-cations optical fiber. The actuator was composed of a concentric inner Cr r Au electrode, a thick sputtered ZnO coating, and an outerCr r Au electrode. Using standard photolithography, 30- m m wide gaps in one of the electrodes were patterned along 2-cm lengths parallelwith the fiber axis. This device can be driven in a bimorph mode. It was demonstrated that a split electrode actuator could be excited intoelectromechanical resonance to produce useful displacements at the end of the fiber. Such flexural fiber actuators could be used inscanning near field optical microscopes for fiber tip height adjustment. In addition, the actuator design can be extended to manufacturetwo-axis integrated fiber alignment devices.  q 1999 Elsevier Science S.A. All rights reserved. Keywords:  ZnO; Piezoelectric fiber coating; Actuator; Photolithography 1. Introduction Direct integration of active elements with fiber optics isattractive for a wide variety of passive and active devices.While there has been tremendous interest in the develop-ment of fiber-based sensors, less has been published in thearea of fiber-based actuators. To date, much of the effort inactuated fibers has concentrated on devices in which thepropagating optical signal can be modified in some way. Inthe area of electromechanical actuation, piezoelectric thinfilm coatings on optical fibers have been used in severaltypes of telecommunications devices, including fast signalmodulators. Several designs for optical phase modulatorswith modulation frequencies up to several hundred MHz w x w x 1–4 and tunable Bragg gratings 5 have been reported. Inmost of these applications, the piezoelectric fiber coating isused to generate a stress-induced optical path length changein the fiber, so that signals traveling along the fiber can be w x w x controlled. ZnO 2–4 , piezoelectric polymers 1,6 , and w x ferroelectric fiber coatings 7,8 have been investigated forthis purpose. ) Corresponding author. 149 Materials Research Laboratory, Pennsyl-vania State University, University Park, PA, USA. Tel.:  q 1-814-863-8348; Fax:  q 1-814-865-2326; E-mail: In addition to modulating the optical signal a piezoelec-tric coating can also be used to physically displace the Ž . fiber i.e., so that it acts as an actuator . One potentialapplication of such an actuator would be in near fieldoptical microscopes, where the transducer used for eitherscanning or vibrating the tip could be integrated directlyon the fiber. This latter function in particular would beuseful since one of the difficulties in near field opticalmicroscopy is the requirement to control the tip–sampledistance at a value where high image resolution can beobtained while simultaneously avoiding collisions between w x the probe fiber tip and the sample 9 . One method bywhich this can be done is to set the tip into mechanicaloscillation; as the tip approaches the sample surface, theresonance amplitude is decreased due to shear forces w x 10,11 . Many systems currently employ resonant ampli-tudes of   ; 5–10 nm so that there is no loss in the w x microscope lateral resolution 9,12 . While optical detec-tion of the amplitude change is possible, a recent reportdemonstrates that if the fiber is mounted on a piezoelectricresonator, the amplitude change can be detected electri- w x cally 9 . If a piezoelectric fiber coating, rather than adiscrete piezoelectric element were utilized, additionalminiaturization of the device should be possible. A secondarea where integrated fiber actuators are needed is in 0924-4247 r 99 r $ - see front matter q  1999 Elsevier Science S.A. All rights reserved. Ž . PII: S0924-4247 98 00273-8  ( )S. Trolier-McKinstry et al. r Sensors and Actuators 73 1999 267–274 268 Ž . Ž . Fig. 1. Schematics for the fiber-based flexural actuators. a Geometry I: split inner electrode, b Geometry II: split outer electrode. In both cases, theelectrodes are split on the opposite side of the fiber as well. active fiber alignment devices for integrated optical net-works, where fibers must be aligned within microns or w x tenths of microns 13 .This paper describes a means of patterning a ZnOpiezoelectric coating r electrode system on a standardtelecommunications optical fiber to prepare an integratedfiber actuator which could be attractive for these applica-tions. Schematics for the fiber actuators considered in thispaper are shown in Fig. 1. The actuator consists of astandard optical fiber coated with an inner electrode, aZnO piezoelectric layer, and annular outer electrodes. Inorder to force the fiber to flex, two gaps were introducedalong the fiber length in either set of electrodes. In princi-ple, when a voltage is applied between the inner and outerelectrodes for only half of the fiber, then the transversecomponent of the piezoelectric strain leads to a bimorphbending of the structure. The other half of the fiber couldthen be used to detect the motion via the direct piezoelec-tric effect. Similarly, externally applied mechanical excita-tion could be detected. Conversely, if the unpatternedelectrode is driven at a voltage intermediate between thehalves of the split electrode, then the strains from the twoelectrode segments should reinforce each other, producinglarger displacements at the unclamped end of the fiber.The two designs shown are for actuators in which the innerand the outer electrode are patterned with gaps along thefiber length. Although in this work, the electrodes werepatterned into only two segments, introduction of addi-tional gaps around the diameter of the fiber could easilylead to  x –  y  positioning of the fiber tip. 2. Experimental procedure The processing outlined here utilizes a piezoelectricfiber coating system which has been described previously w x 14 . In brief, standard 125  m m diameter optical fibers ; 19 cm in length were first cleaned successively indichloromethane and isopropanol to remove the protectiveorganic coating. Five cleaned samples were then mountedon a holder which rotates the fibers continuously duringdeposition to enable uniform film coating thickness. Inner Ž electrodes of approximately 13 nm of Cr to improve . adhesion and 130 nm of Au were thermally evaporated. A w x 6  m m thick, 0001 radially-oriented ZnO film was thendeposited by reactive magnetron sputtering. The depositionconditions are given in Table 1.Additional details on the ZnO deposition as well as thecharacterization of the piezoelectric are given elsewhere Table 1Deposition conditions for the ZnO piezoelectric coatingTarget-fiber distance 9 cmPower to Zn source 250 WGas pressure 0.8 Pa Ar r 0.7 Pa O 2  ( )S. Trolier-McKinstry et al. r Sensors and Actuators 73 1999 267–274  269Fig. 2. Sample holder for exposure of photoresist on fiber. The top piece is suspended upside down from the aligner. The mask holder on the bottom canthen be adjusted so that the mask is just in contact with the fiber. w x 8,14 . To facilitate subsequent electrical measurements, aportion of the bottom electrode was masked during thedeposition of the piezoelectric. Since the ZnO film surfacewas considerably rougher than that of the bare fiber, Ž thicker Cr r Au top electrodes were used 25 and 400 nm . thick, respectively to insure good electrical contact. Alltop electrodes were evaporated through another shadowmask to prepare 2 mm electrode sections separated by 2 Ž . mm gaps perpendicular to the fiber axis see Fig. 1 .To prepare gaps in the inner electrode along the fiberlength, patterning was done prior to deposition of the ZnO,while gaps in the outer electrode were made following allof the coating depositions. The patterning procedure usedwas as follows: fibers segments  ; 9 cm long were gluedinto alignment screws machined with centered  ; 150  m mholes. This facilitated subsequent handling. Fibers werethen dip-coated in Shipley Microposit 1813 photoresist,using an automated dip-coater with a withdrawal rate of 1mm r s. The resist was soft-baked for 30 min at 90 8 C.Individual fibers were subsequently mounted into a special Ž . fixture see Fig. 2 built to hold the fibers in contact withthe mask on a Karl Suss MJB 21 double side mask aligner.The mask used was a 2 cm wide section cut from astandard Cr mask plate patterned with a 30  m m wide line.After being brought into contact and aligned with respectto the mask, the fiber was exposed twice at points  ; 180 8 apart around the fiber perimeter. The pattern was devel- w x oped in a 1:5 developer 15 :H O solution. To transfer the 2 pattern to the electrode, the fibers were postbaked at 120 8 Cfor 30 min to harden the resist, and then the exposed Auand Cr were removed in separate wet chemical etches.  1 Deionized water was used as the etch stop. The remainingphotoresist was then removed in acetone and the fiber wasrinsed in isopropanol. The procedure was similar for thetop electrode, with the exception that Au etch was ex-tended to 2–4 min, and the Cr etch to 1–2 min tocompletely clear the gap.Microstructural characterization of the coatings and thepatterning quality was performed in a JEOL 6300F scan-ning electron microscope using a 5 kV accelerating voltageand a working distance of 11 or 12 mm.To make electrical connections to the fiber electrodes,the fiber was glued at one end to a slotted SiO coated Si 2 substrate. The remainder of the fiber was free to vibrate.Air dry silver paint was used to connect the fiber elec-trodes to larger contact pads on the substrate. The electri-cal impedance of the sample was characterized using anHP4194 Impedance Analyzer over the frequency rangefrom 1 kHz–25 MHz.The same sample was used for the measurements of theelectrically induced displacements at the end of a fibersegment using a Mach–Zehnder interferometer. To do this,connections were made with one side of the bottom elec-trode and a 2 mm long top electrode ring, and the device 1 The Au film was removed during a 45-s etch in a bufferediodine solution, while the Cr was dissolved during a 30-s etch in Ž . Ž . Ž Ce NH NO  r CH COOH r H O 200 g r 35 ml r with enough H O 4 2 3 4 3 2 2 . added to bring the total to 1000 ml .  ( )S. Trolier-McKinstry et al. r Sensors and Actuators 73 1999 267–274 270 Ž was driven at voltages between inner and outer elec- . trodes between 0.5 and 5 V rms over a frequency range of  Ž . 25 Hz–25 kHz. A lock-in amplifier SR-830 was used tomeasure the vibrational response at a single frequency and Ž . a dynamic signal analyzer HP 3562A was used to studythe frequency dependence. Additional details on the inter- w x ferometer set-up can be found elsewhere 16 . 3. Results and discussion As has been described previously, the deposition condi-tions produced uniform, microstructurally homogeneouscoatings of both the electrode and the piezoelectric layers.Fig. 3 shows a plan-view microstructure of the electrodedZnO fiber coating.For fibers with patterned inner electrodes, dip-coatingthe fibers produced a thin coating of photoresist ; 0.7 m mthick which was sufficiently uniform along the fiber lengththat the exposure conditions were consistent for the pat-terned length. This enabled clean patterning of the exposedregion. As can be seen in Fig. 4, a well-defined electrodearea was removed during the etching; the sidewalls showed ; 1  m m irregularities along the length. There was nodifficulty in patterning both gaps in the electrode withequal clarity.For fibers with patterned outer electrodes, the roughersurface made alignment of the fiber with respect to the Crmask more difficult. Once aligned, however, the photore-sist was patterned using the same procedure describedabove. It was found that the acidic etch used to pattern theCr outer electrode was sufficiently corrosive that most of the exposed ZnO was also removed. A second Au r Cr etchcycle was then used to dissolve the exposed inner elec-trode, producing a decoupled structure. Not surprisingly,since the processing was not optimized for selectivity, theZnO layer was undercut during the etching, so that theresulting electrode gaps were wider than the srcinal 30- m mexposed region. It is expected that by improving either theadherence of the photoresist layer to the rough outerelectrode, or by developing selective etches for the ZnOand the Cr layers, that this difficulty could be circum-vented. The quality of the resulting patterning is shown inFig. 5. The outer electrode was completely removed duringthe patterning; with the exception of some residual pyrami- Ž . dal bridges see Fig. 6 , so was the ZnO layer. In Fig. 7,the etch sidewall is shown. The columnar microstructure of the ZnO coating is apparent.It is clear that it is possible to pattern gaps in either theinner or the outer electrodes utilizing this straightforwardprocessing approach. Although it was not attempted in thiswork, it should be possible to prepare narrower electrodegaps. Similarly, with proper registration marks on the fiberholder for the mask exposure, it should be possible tomake equally spaced gaps at multiple intervals around thefiber. This should be particularly useful for preparation of resonant or positioning devices with more than one axis of control.Several procedures were used to characterize the elec-tromechanical properties of the resulting fiber actuators. In Fig. 3. Plan view SEM of fiber surface after deposition of the inner electrode r ZnO r outer electrode coatings. The microstructure is consistent with the w x columnar morphology of the thick magnetron sputtered ZnO coating observed previously 14 .  ( )S. Trolier-McKinstry et al. r Sensors and Actuators 73 1999 267–274  271Fig. 4. SEM image of a patterned inner electrode showing the gap with the exposed fiber surface. impedance spectroscopy, both actuators with patterned in-ner and outer electrodes showed broad radial resonancesbetween 21 and 25 MHz when a voltage was appliedbetween the inner and outer electrodes for half of apatterned fiber. It was not possible to locate the lowfrequency bending resonances due to large backgroundsignals associated with the sample holder.A better probe of the low frequency resonances was themeasurement of displacements of the free end of the fiberby optical interferometry. In this experiment, a 2-mm Fig. 5. SEM micrograph of a patterned outer electrode in which the ZnO in the gap has also been patterned. The bright irregular areas in the gapcorrespond to incompletely etched ZnO bridges.
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