- Email: [email protected]
electrode coatings for flexural actuators
Sensors and Actuators 73 Ž1999. 267–274
Optical fibers with patterned ZnOrelectrode coatings for flexural actuators 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 ZnOrelectrode fiber coating on a standard telecommunications optical fiber. The actuator was composed of a concentric inner CrrAu electrode, a thick sputtered ZnO coating, and an outer CrrAu electrode. Using standard photolithography, 30-mm wide gaps in one of the electrodes were patterned along 2-cm lengths parallel with the fiber axis. This device can be driven in a bimorph mode. It was demonstrated that a split electrode actuator could be excited into electromechanical resonance to produce useful displacements at the end of the fiber. Such flexural fiber actuators could be used in scanning near field optical microscopes for fiber tip height adjustment. In addition, the actuator design can be extended to manufacture two-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 is attractive for a wide variety of passive and active devices. While there has been tremendous interest in the development of fiber-based sensors, less has been published in the area of fiber-based actuators. To date, much of the effort in actuated fibers has concentrated on devices in which the propagating optical signal can be modified in some way. In the area of electromechanical actuation, piezoelectric thin film coatings on optical fibers have been used in several types of telecommunications devices, including fast signal modulators. Several designs for optical phase modulators with modulation frequencies up to several hundred MHz w1–4x and tunable Bragg gratings w5x have been reported. In most of these applications, the piezoelectric fiber coating is used to generate a stress-induced optical path length change in the fiber, so that signals traveling along the fiber can be controlled. ZnO w2–4x, piezoelectric polymers w1,6x, and ferroelectric fiber coatings w7,8x have been investigated for this purpose. ) Corresponding author. 149 Materials Research Laboratory, Pennsylvania State University, University Park, PA, USA. Tel.: q1-814-8638348; Fax: q1-814-865-2326; E-mail: [email protected]
In addition to modulating the optical signal a piezoelectric coating can also be used to physically displace the fiber Ži.e., so that it acts as an actuator.. One potential application of such an actuator would be in near field optical microscopes, where the transducer used for either scanning or vibrating the tip could be integrated directly on the fiber. This latter function in particular would be useful since one of the difficulties in near field optical microscopy is the requirement to control the tip–sample distance at a value where high image resolution can be obtained while simultaneously avoiding collisions between the probe fiber tip and the sample w9x. One method by which this can be done is to set the tip into mechanical oscillation; as the tip approaches the sample surface, the resonance amplitude is decreased due to shear forces w10,11x. Many systems currently employ resonant amplitudes of ; 5–10 nm so that there is no loss in the microscope lateral resolution w9,12x. While optical detection of the amplitude change is possible, a recent report demonstrates that if the fiber is mounted on a piezoelectric resonator, the amplitude change can be detected electrically w9x. If a piezoelectric fiber coating, rather than a discrete piezoelectric element were utilized, additional miniaturization of the device should be possible. A second area where integrated fiber actuators are needed is in
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 2 7 3 - 8
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Fig. 1. Schematics for the fiber-based flexural actuators. Ža. Geometry I: split inner electrode, Žb. Geometry II: split outer electrode. In both cases, the electrodes are split on the opposite side of the fiber as well.
active fiber alignment devices for integrated optical networks, where fibers must be aligned within microns or tenths of microns w13x. This paper describes a means of patterning a ZnO piezoelectric coatingrelectrode system on a standard telecommunications optical fiber to prepare an integrated fiber actuator which could be attractive for these applications. Schematics for the fiber actuators considered in this paper are shown in Fig. 1. The actuator consists of a standard optical fiber coated with an inner electrode, a ZnO piezoelectric layer, and annular outer electrodes. In order to force the fiber to flex, two gaps were introduced along the fiber length in either set of electrodes. In principle, when a voltage is applied between the inner and outer electrodes for only half of the fiber, then the transverse component of the piezoelectric strain leads to a bimorph bending of the structure. The other half of the fiber could then be used to detect the motion via the direct piezoelectric effect. Similarly, externally applied mechanical excitation could be detected. Conversely, if the unpatterned electrode is driven at a voltage intermediate between the halves of the split electrode, then the strains from the two electrode segments should reinforce each other, producing larger displacements at the unclamped end of the fiber. The two designs shown are for actuators in which the inner and the outer electrode are patterned with gaps along the fiber length. Although in this work, the electrodes were patterned into only two segments, introduction of addi-
tional gaps around the diameter of the fiber could easily lead to x–y positioning of the fiber tip.
2. Experimental procedure The processing outlined here utilizes a piezoelectric fiber coating system which has been described previously w14x. In brief, standard 125 mm diameter optical fibers ; 19 cm in length were first cleaned successively in dichloromethane and isopropanol to remove the protective organic coating. Five cleaned samples were then mounted on a holder which rotates the fibers continuously during deposition to enable uniform film coating thickness. Inner electrodes of approximately 13 nm of Cr Žto improve adhesion. and 130 nm of Au were thermally evaporated. A 6 mm thick, w0001x radially-oriented ZnO film was then deposited by reactive magnetron sputtering. The deposition conditions are given in Table 1. Additional details on the ZnO deposition as well as the characterization of the piezoelectric are given elsewhere Table 1 Deposition conditions for the ZnO piezoelectric coating Target-fiber distance Power to Zn source Gas pressure
9 cm 250 W 0.8 Pa Arr0.7 Pa O 2
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269
Fig. 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 can then be adjusted so that the mask is just in contact with the fiber.
w8,14x. To facilitate subsequent electrical measurements, a portion of the bottom electrode was masked during the deposition of the piezoelectric. Since the ZnO film surface was considerably rougher than that of the bare fiber, thicker CrrAu top electrodes were used Ž25 and 400 nm thick, respectively. to insure good electrical contact. All top electrodes were evaporated through another shadow mask to prepare 2 mm electrode sections separated by 2 mm gaps perpendicular to the fiber axis Žsee Fig. 1.. To prepare gaps in the inner electrode along the fiber length, patterning was done prior to deposition of the ZnO, while gaps in the outer electrode were made following all of the coating depositions. The patterning procedure used was as follows: fibers segments ; 9 cm long were glued into alignment screws machined with centered ; 150 mm holes. This facilitated subsequent handling. Fibers were then dip-coated in Shipley Microposit 1813 photoresist, using an automated dip-coater with a withdrawal rate of 1 mmrs. The resist was soft-baked for 30 min at 908C. Individual fibers were subsequently mounted into a special fixture Žsee Fig. 2. built to hold the fibers in contact with the mask on a Karl Suss MJB 21 double side mask aligner. The mask used was a 2 cm wide section cut from a standard Cr mask plate patterned with a 30 mm wide line. After being brought into contact and aligned with respect to the mask, the fiber was exposed twice at points ; 1808 apart around the fiber perimeter. The pattern was developed in a 1:5 developer w15x:H 2 O solution. To transfer the pattern to the electrode, the fibers were postbaked at 1208C for 30 min to harden the resist, and then the exposed Au
and Cr were removed in separate wet chemical etches. 1 Deionized water was used as the etch stop. The remaining photoresist 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 min, and the Cr etch to 1–2 min to completely clear the gap. Microstructural characterization of the coatings and the patterning quality was performed in a JEOL 6300F scanning electron microscope using a 5 kV accelerating voltage and 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 2 coated Si substrate. The remainder of the fiber was free to vibrate. Air dry silver paint was used to connect the fiber electrodes to larger contact pads on the substrate. The electrical impedance of the sample was characterized using an HP4194 Impedance Analyzer over the frequency range from 1 kHz–25 MHz. The same sample was used for the measurements of the electrically induced displacements at the end of a fiber segment using a Mach–Zehnder interferometer. To do this, connections were made with one side of the bottom electrode and a 2 mm long top electrode ring, and the device
1
The Au film was removed during a 45-s etch in a buffered iodine solution, while the Cr was dissolved during a 30-s etch in Ce ŽNH 4 . 2 ŽNO 3 .4 rCH 3 COOHrH 2 O Ž200 gr35 mlrwith enough H 2 O added to bring the total to 1000 ml..
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was driven at voltages Žbetween inner and outer electrodes. between 0.5 and 5 V rms over a frequency range of 25 Hz–25 kHz. A lock-in amplifier ŽSR-830. was used to measure the vibrational response at a single frequency and a dynamic signal analyzer ŽHP 3562A. was used to study the frequency dependence. Additional details on the interferometer set-up can be found elsewhere w16x.
3. Results and discussion As has been described previously, the deposition conditions produced uniform, microstructurally homogeneous coatings of both the electrode and the piezoelectric layers. Fig. 3 shows a plan-view microstructure of the electroded ZnO fiber coating. For fibers with patterned inner electrodes, dip-coating the fibers produced a thin coating of photoresist ; 0.7 mm thick which was sufficiently uniform along the fiber length that the exposure conditions were consistent for the patterned length. This enabled clean patterning of the exposed region. As can be seen in Fig. 4, a well-defined electrode area was removed during the etching; the sidewalls showed ; 1 mm irregularities along the length. There was no difficulty in patterning both gaps in the electrode with equal clarity. For fibers with patterned outer electrodes, the rougher surface made alignment of the fiber with respect to the Cr mask more difficult. Once aligned, however, the photoresist was patterned using the same procedure described
above. It was found that the acidic etch used to pattern the Cr outer electrode was sufficiently corrosive that most of the exposed ZnO was also removed. A second AurCr etch cycle was then used to dissolve the exposed inner electrode, producing a decoupled structure. Not surprisingly, since the processing was not optimized for selectivity, the ZnO layer was undercut during the etching, so that the resulting electrode gaps were wider than the original 30-mm exposed region. It is expected that by improving either the adherence of the photoresist layer to the rough outer electrode, or by developing selective etches for the ZnO and the Cr layers, that this difficulty could be circumvented. The quality of the resulting patterning is shown in Fig. 5. The outer electrode was completely removed during the patterning; with the exception of some residual pyramidal bridges Žsee 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 the inner or the outer electrodes utilizing this straightforward processing approach. Although it was not attempted in this work, it should be possible to prepare narrower electrode gaps. Similarly, with proper registration marks on the fiber holder for the mask exposure, it should be possible to make equally spaced gaps at multiple intervals around the fiber. 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 electromechanical properties of the resulting fiber actuators. In
Fig. 3. Plan view SEM of fiber surface after deposition of the inner electroderZnOrouter electrode coatings. The microstructure is consistent with the columnar morphology of the thick magnetron sputtered ZnO coating observed previously w14x.
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Fig. 4. SEM image of a patterned inner electrode showing the gap with the exposed fiber surface.
impedance spectroscopy, both actuators with patterned inner and outer electrodes showed broad radial resonances between 21 and 25 MHz when a voltage was applied between the inner and outer electrodes for half of a patterned fiber. It was not possible to locate the low
frequency bending resonances due to large background signals associated with the sample holder. A better probe of the low frequency resonances was the measurement of displacements of the free end of the fiber by 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 gap correspond to incompletely etched ZnO bridges.
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Fig. 6. ZnO bridge in patterned section of fiber coating. The curled film at the top is an underetched section of the top electrode.
section of an 18-mm long fiber with a patterned inner electrode was excited with an alternating field applied between one section of the inner electrode and the annular outer electrode. The displacement was measured at a 15mm distance from the clamped end of the fiber as a function of the driving potential amplitude and frequency.
A light beam from a He–Ne laser was focused onto the free end of the fiber, which was coated by a thin Au layer to increase the reflectivity of the fiber surface. The incident spot size was comparable with the diameter of the fiber, therefore only part of the incident light was returned back to the focusing lens of the interferometer. A fringe
Fig. 7. SEM micrograph of the sidewall of a film patterned through all three coating layers. The columnar microstructure of the piezoelectric film is clearly visible. The loose material at the base of the sidewall is exposed bottom electrode. This electrode was removed completely through the majority of the gap.
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pattern compatible with that expected for the interference of two beams having planar and cylindrically distorted wave fronts was observed. The interference signal from the photodiode was calibrated by moving the flat mirror in the reference arm of the interferometer by a distance greater than lr4, where l is the wavelength for the He–Ne laser. Fig. 8 shows the resulting amplitude and phase responses. The first peak Žnear 50 Hz. was associated with the acoustical noise of near-by working machines and building vibrations since it was independent of the driving amplitude. All other peaks were due to resonances induced by the ZnO coating, each peak representing different resonant modes of the fiber. The resonances appeared at the same frequencies for both 0.5 and 1 V ac drive levels, with the amplitude of the response increasing with field, as expected. If the mechanical quality factor Q is taken as D frfr , where D f is the halfwidth of the amplitude response, and f r is the center resonance frequency, then Q of the fundamental resonance was ; 60. Fig. 9 shows the displacement and response as a function of the driving voltage at a low non-resonant frequency of 25 Hz and at the first observed resonance frequency of 110 Hz. A linear behavior is observed for both responses over most of the measurement range. For the highest amplitudes, the response is slightly non-linear due to the onset of the interferometer non-linearity. There is some question whether the obtained response represents solely the amplitude of the fiber motion in the horizontal plane comprising the fiber axis and the laser incident beam. Due to possible misalignment of the patterned gaps that ideally lie in a plane perpendicular to the probing beam, vertical Žout-ofplane. motion of the fiber is also possible. This vertical motion can result in a change of the interference signal due to the curvature of the fiber. The large laser spot size incident on the fiber and the complex shape of the interference fringes does not allow a straightforward interpretation of the measured response, though. The first approximation used for these measurements introduces an error that would
273
Fig. 9. Displacement as a function of excitation voltage for the same fiber as in Fig. 8 measured on Ž110 Hz. and off Ž25 Hz. resonance.
at most give a factor of two variance in the measured values. Further work is needed to identify the vibrational modes that correspond with the observed resonance peaks. It is expected that the electric field-induced motion could be amplified if both halves of the fiber were actuated in antiphase or if the drive voltage were increased further. The resonant displacements measured here are large enough for some scanning near field optical microscope applications, even for drive voltages as low as 1 V rms. Previous work on similar devices has shown that drive levels up to 6 V rms can be utilized without electrical breakdown. This, combined with the presence of multiple resonances should enable the frequency to be tailored at least over the range between 100 and 10,000 Hz. Changes in the actuator geometry Žespecially the length and the clamping conditions. will also facilitate control of the resonant frequencies and amplitudes. The mechanical quality factor Q of the resonance observed should also be high enough for SNOM applications, particularly given the fact that the piezoelectric coefficients of ZnO are higher than those of the quartz piezoelectric reported for electrical detection of vertical tip position in SNOM applications w17x. Fiber alignment systems would require larger static motions than were observed here. The core of an optical fiber is typically several microns in diameter, and the alignment of two cores with respect to each other would probably require motions on the same order. To approach this field, either larger segments could be actuated, or a stronger piezoelectric, such as lead zirconate titanate, could be employed as the piezoelectric coating.
4. Conclusions
Fig. 8. Displacement and phase angle measured as a function of frequency for fiber actuator with a split inner electrode. One side of the split electrode was excited over a 2-mm length. The first resonance occurs at 110 Hz. This resonance probably corresponds to forced oscillations at one of the natural frequencies of the fiber.
A processing procedure was developed to photolithographically pattern the electrodes for piezoelectric fiber coatings along the fiber length. The introduction of 30-mm gaps allowed the two halves of the fiber to be driven independently, which enabled a flexural motion of the fiber. This type of structure should be useful as an inte-
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grated actuator for near field optical microscope tip positioning, and potentially in optical fiber alignment.
Acknowledgements The assistance of Markus Kohli is gratefully acknowledged. Helpful discussions with Genaro Zavala are also appreciated. This work was partially funded through a DARPA contract DABT63-95-C-0053 and an NSF award DMR 9502431. This work was also supported by the Optical Sciences, Applications, and Technology Priority Program of the Board of the Swiss Federal Institute of Technology.
References w1x J. Jarzynski, Frequency response of single-mode optical fiber phase modulator utilizing a piezoelectric plastic jacket, J. Appl. Phys. 55 Ž1984. 3243–3250. w2x N.H. Ky, H.G. Limberger, R.P. Salathe, G.R. Fox, Optical performance of miniature all-fiber phase modulators with ZnO coating, J. Lightwave Tech. 14 Ž1996. 23–26. w3x N.H. Ky, H.G. Limberger, R.P. Salathe, G.R. Fox, 400 MHz bandwidth all-fiber phase modulators with ZnO coating on standard telecommunication fiber, IEEE Phot. Tech. Lett. 8 Ž1996. 629–631. w4x D.S. Czaplak, J.F. Weller, L. Goldberg, F.S. Hickernell, H.D. Knuth, S.R. Young, AO phase modulator for single-mode fibers using cylindrical ZnO transducer, IEEE Ultrason. Symp. US 1 Ž1987. 491–493.
w5x G.R. Fox, C.A.P. Muller, C.R. Wuthrich, A. Kholkine, D.M. Costantini, H.G. Limberger, Applications of active thin film coatings on optical fibers, MRS Proc. 459 Materials for Smart Systems II, 1996, pp. 25–34. w6x M. Imai, H. Tanizawa, Y. Ohtsuka, Y. Takase, A. Odajima, Piezoelectric copolymer jacketed single-mode fibers for electric-field sensor application, J. Appl. Phys. 60 Ž1986. 1916–1918. w7x D.A. Barrow, O. Lisboa, C.K. Jen, M. Sayer, In-line phase modulators using coaxial thick lead zirconate titanate coated optical fibers, J. Appl. Phys. 79 Ž1996. 3323–3329. w8x G.R. Fox, C.A.P. Muller, N. Setter, N.H. Ky, H.G. Limberger, Sputter deposited piezoelectric fiber coatings for acousto-optic modulators, J. Vac. Sci. Technol. A 14 Ž1996. 800–805. w9x E. Betzig, P.L. Finn, J.S. Weiner, Combined shear force and nearfield scanning optical microscopy, Appl. Phys. Lett. 60 Ž2. Ž1992. 484–2486. w10x K. Karrai, R.D. Grober, Piezoelectric tip–sample distance control for near field optical microscopes, Appl. Phys. Lett. 66 Ž1995. 1842–1844. w11x P.C. Yang, Y. Chen, M. Vaez-Iravani, Attractive-mode atomic force microscopy with optical detection in an orthogonal cantileverrsample configuration, J. Appl. Phys. 71 Ž1992. 2499–2502. w12x R.D. Grober, T.D. Harris, J.K. Trautman, E. Betzig, Design and implementation of a low temperature near-field scanning optical microscope, Rev. Sci. Instr. 65 Ž1994. 626–631. w13x R. Jebens, W. Trimmer, J. Walker, Microactuators for aligning optical fibers, Sensors and Actuators 20 Ž1989. 65–73. w14x G.R. Fox, N. Setter, H.G. Limberger, Fabrication and structural analysis of ZnO coated fiber optic phase modulators, J. Mater. Res. 11 Ž1996. 2051–2061. w15x Shipley Microposit 351 Developer. w16x A.L. Kholkin, Ch. Wutchrich, D.V. Taylor, N. Setter, Interferometric measurements of electric field-induced displacements in piezoelectric thin films, Rev. Sci. Instr. 67 Ž1996. 1935–1941. w17x K. Karrai, R.D. Grober, Piezo-electric tuning fork tip–sample distance control for near field optical microscopes, Ultramicroscopy 61 Ž1995. 197–205.
Optical fibers with patterned ZnOrelectrode coatings for flexural actuators 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 ZnOrelectrode fiber coating on a standard telecommunications optical fiber. The actuator was composed of a concentric inner CrrAu electrode, a thick sputtered ZnO coating, and an outer CrrAu electrode. Using standard photolithography, 30-mm wide gaps in one of the electrodes were patterned along 2-cm lengths parallel with the fiber axis. This device can be driven in a bimorph mode. It was demonstrated that a split electrode actuator could be excited into electromechanical resonance to produce useful displacements at the end of the fiber. Such flexural fiber actuators could be used in scanning near field optical microscopes for fiber tip height adjustment. In addition, the actuator design can be extended to manufacture two-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 is attractive for a wide variety of passive and active devices. While there has been tremendous interest in the development of fiber-based sensors, less has been published in the area of fiber-based actuators. To date, much of the effort in actuated fibers has concentrated on devices in which the propagating optical signal can be modified in some way. In the area of electromechanical actuation, piezoelectric thin film coatings on optical fibers have been used in several types of telecommunications devices, including fast signal modulators. Several designs for optical phase modulators with modulation frequencies up to several hundred MHz w1–4x and tunable Bragg gratings w5x have been reported. In most of these applications, the piezoelectric fiber coating is used to generate a stress-induced optical path length change in the fiber, so that signals traveling along the fiber can be controlled. ZnO w2–4x, piezoelectric polymers w1,6x, and ferroelectric fiber coatings w7,8x have been investigated for this purpose. ) Corresponding author. 149 Materials Research Laboratory, Pennsylvania State University, University Park, PA, USA. Tel.: q1-814-8638348; Fax: q1-814-865-2326; E-mail: [email protected]
In addition to modulating the optical signal a piezoelectric coating can also be used to physically displace the fiber Ži.e., so that it acts as an actuator.. One potential application of such an actuator would be in near field optical microscopes, where the transducer used for either scanning or vibrating the tip could be integrated directly on the fiber. This latter function in particular would be useful since one of the difficulties in near field optical microscopy is the requirement to control the tip–sample distance at a value where high image resolution can be obtained while simultaneously avoiding collisions between the probe fiber tip and the sample w9x. One method by which this can be done is to set the tip into mechanical oscillation; as the tip approaches the sample surface, the resonance amplitude is decreased due to shear forces w10,11x. Many systems currently employ resonant amplitudes of ; 5–10 nm so that there is no loss in the microscope lateral resolution w9,12x. While optical detection of the amplitude change is possible, a recent report demonstrates that if the fiber is mounted on a piezoelectric resonator, the amplitude change can be detected electrically w9x. If a piezoelectric fiber coating, rather than a discrete piezoelectric element were utilized, additional miniaturization of the device should be possible. A second area where integrated fiber actuators are needed is in
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 2 7 3 - 8
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Fig. 1. Schematics for the fiber-based flexural actuators. Ža. Geometry I: split inner electrode, Žb. Geometry II: split outer electrode. In both cases, the electrodes are split on the opposite side of the fiber as well.
active fiber alignment devices for integrated optical networks, where fibers must be aligned within microns or tenths of microns w13x. This paper describes a means of patterning a ZnO piezoelectric coatingrelectrode system on a standard telecommunications optical fiber to prepare an integrated fiber actuator which could be attractive for these applications. Schematics for the fiber actuators considered in this paper are shown in Fig. 1. The actuator consists of a standard optical fiber coated with an inner electrode, a ZnO piezoelectric layer, and annular outer electrodes. In order to force the fiber to flex, two gaps were introduced along the fiber length in either set of electrodes. In principle, when a voltage is applied between the inner and outer electrodes for only half of the fiber, then the transverse component of the piezoelectric strain leads to a bimorph bending of the structure. The other half of the fiber could then be used to detect the motion via the direct piezoelectric effect. Similarly, externally applied mechanical excitation could be detected. Conversely, if the unpatterned electrode is driven at a voltage intermediate between the halves of the split electrode, then the strains from the two electrode segments should reinforce each other, producing larger displacements at the unclamped end of the fiber. The two designs shown are for actuators in which the inner and the outer electrode are patterned with gaps along the fiber length. Although in this work, the electrodes were patterned into only two segments, introduction of addi-
tional gaps around the diameter of the fiber could easily lead to x–y positioning of the fiber tip.
2. Experimental procedure The processing outlined here utilizes a piezoelectric fiber coating system which has been described previously w14x. In brief, standard 125 mm diameter optical fibers ; 19 cm in length were first cleaned successively in dichloromethane and isopropanol to remove the protective organic coating. Five cleaned samples were then mounted on a holder which rotates the fibers continuously during deposition to enable uniform film coating thickness. Inner electrodes of approximately 13 nm of Cr Žto improve adhesion. and 130 nm of Au were thermally evaporated. A 6 mm thick, w0001x radially-oriented ZnO film was then deposited by reactive magnetron sputtering. The deposition conditions are given in Table 1. Additional details on the ZnO deposition as well as the characterization of the piezoelectric are given elsewhere Table 1 Deposition conditions for the ZnO piezoelectric coating Target-fiber distance Power to Zn source Gas pressure
9 cm 250 W 0.8 Pa Arr0.7 Pa O 2
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Fig. 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 can then be adjusted so that the mask is just in contact with the fiber.
w8,14x. To facilitate subsequent electrical measurements, a portion of the bottom electrode was masked during the deposition of the piezoelectric. Since the ZnO film surface was considerably rougher than that of the bare fiber, thicker CrrAu top electrodes were used Ž25 and 400 nm thick, respectively. to insure good electrical contact. All top electrodes were evaporated through another shadow mask to prepare 2 mm electrode sections separated by 2 mm gaps perpendicular to the fiber axis Žsee Fig. 1.. To prepare gaps in the inner electrode along the fiber length, patterning was done prior to deposition of the ZnO, while gaps in the outer electrode were made following all of the coating depositions. The patterning procedure used was as follows: fibers segments ; 9 cm long were glued into alignment screws machined with centered ; 150 mm holes. This facilitated subsequent handling. Fibers were then dip-coated in Shipley Microposit 1813 photoresist, using an automated dip-coater with a withdrawal rate of 1 mmrs. The resist was soft-baked for 30 min at 908C. Individual fibers were subsequently mounted into a special fixture Žsee Fig. 2. built to hold the fibers in contact with the mask on a Karl Suss MJB 21 double side mask aligner. The mask used was a 2 cm wide section cut from a standard Cr mask plate patterned with a 30 mm wide line. After being brought into contact and aligned with respect to the mask, the fiber was exposed twice at points ; 1808 apart around the fiber perimeter. The pattern was developed in a 1:5 developer w15x:H 2 O solution. To transfer the pattern to the electrode, the fibers were postbaked at 1208C for 30 min to harden the resist, and then the exposed Au
and Cr were removed in separate wet chemical etches. 1 Deionized water was used as the etch stop. The remaining photoresist 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 min, and the Cr etch to 1–2 min to completely clear the gap. Microstructural characterization of the coatings and the patterning quality was performed in a JEOL 6300F scanning electron microscope using a 5 kV accelerating voltage and 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 2 coated Si substrate. The remainder of the fiber was free to vibrate. Air dry silver paint was used to connect the fiber electrodes to larger contact pads on the substrate. The electrical impedance of the sample was characterized using an HP4194 Impedance Analyzer over the frequency range from 1 kHz–25 MHz. The same sample was used for the measurements of the electrically induced displacements at the end of a fiber segment using a Mach–Zehnder interferometer. To do this, connections were made with one side of the bottom electrode and a 2 mm long top electrode ring, and the device
1
The Au film was removed during a 45-s etch in a buffered iodine solution, while the Cr was dissolved during a 30-s etch in Ce ŽNH 4 . 2 ŽNO 3 .4 rCH 3 COOHrH 2 O Ž200 gr35 mlrwith enough H 2 O added to bring the total to 1000 ml..
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was driven at voltages Žbetween inner and outer electrodes. between 0.5 and 5 V rms over a frequency range of 25 Hz–25 kHz. A lock-in amplifier ŽSR-830. was used to measure the vibrational response at a single frequency and a dynamic signal analyzer ŽHP 3562A. was used to study the frequency dependence. Additional details on the interferometer set-up can be found elsewhere w16x.
3. Results and discussion As has been described previously, the deposition conditions produced uniform, microstructurally homogeneous coatings of both the electrode and the piezoelectric layers. Fig. 3 shows a plan-view microstructure of the electroded ZnO fiber coating. For fibers with patterned inner electrodes, dip-coating the fibers produced a thin coating of photoresist ; 0.7 mm thick which was sufficiently uniform along the fiber length that the exposure conditions were consistent for the patterned length. This enabled clean patterning of the exposed region. As can be seen in Fig. 4, a well-defined electrode area was removed during the etching; the sidewalls showed ; 1 mm irregularities along the length. There was no difficulty in patterning both gaps in the electrode with equal clarity. For fibers with patterned outer electrodes, the rougher surface made alignment of the fiber with respect to the Cr mask more difficult. Once aligned, however, the photoresist was patterned using the same procedure described
above. It was found that the acidic etch used to pattern the Cr outer electrode was sufficiently corrosive that most of the exposed ZnO was also removed. A second AurCr etch cycle was then used to dissolve the exposed inner electrode, producing a decoupled structure. Not surprisingly, since the processing was not optimized for selectivity, the ZnO layer was undercut during the etching, so that the resulting electrode gaps were wider than the original 30-mm exposed region. It is expected that by improving either the adherence of the photoresist layer to the rough outer electrode, or by developing selective etches for the ZnO and the Cr layers, that this difficulty could be circumvented. The quality of the resulting patterning is shown in Fig. 5. The outer electrode was completely removed during the patterning; with the exception of some residual pyramidal bridges Žsee 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 the inner or the outer electrodes utilizing this straightforward processing approach. Although it was not attempted in this work, it should be possible to prepare narrower electrode gaps. Similarly, with proper registration marks on the fiber holder for the mask exposure, it should be possible to make equally spaced gaps at multiple intervals around the fiber. 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 electromechanical properties of the resulting fiber actuators. In
Fig. 3. Plan view SEM of fiber surface after deposition of the inner electroderZnOrouter electrode coatings. The microstructure is consistent with the columnar morphology of the thick magnetron sputtered ZnO coating observed previously w14x.
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Fig. 4. SEM image of a patterned inner electrode showing the gap with the exposed fiber surface.
impedance spectroscopy, both actuators with patterned inner and outer electrodes showed broad radial resonances between 21 and 25 MHz when a voltage was applied between the inner and outer electrodes for half of a patterned fiber. It was not possible to locate the low
frequency bending resonances due to large background signals associated with the sample holder. A better probe of the low frequency resonances was the measurement of displacements of the free end of the fiber by 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 gap correspond to incompletely etched ZnO bridges.
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Fig. 6. ZnO bridge in patterned section of fiber coating. The curled film at the top is an underetched section of the top electrode.
section of an 18-mm long fiber with a patterned inner electrode was excited with an alternating field applied between one section of the inner electrode and the annular outer electrode. The displacement was measured at a 15mm distance from the clamped end of the fiber as a function of the driving potential amplitude and frequency.
A light beam from a He–Ne laser was focused onto the free end of the fiber, which was coated by a thin Au layer to increase the reflectivity of the fiber surface. The incident spot size was comparable with the diameter of the fiber, therefore only part of the incident light was returned back to the focusing lens of the interferometer. A fringe
Fig. 7. SEM micrograph of the sidewall of a film patterned through all three coating layers. The columnar microstructure of the piezoelectric film is clearly visible. The loose material at the base of the sidewall is exposed bottom electrode. This electrode was removed completely through the majority of the gap.
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pattern compatible with that expected for the interference of two beams having planar and cylindrically distorted wave fronts was observed. The interference signal from the photodiode was calibrated by moving the flat mirror in the reference arm of the interferometer by a distance greater than lr4, where l is the wavelength for the He–Ne laser. Fig. 8 shows the resulting amplitude and phase responses. The first peak Žnear 50 Hz. was associated with the acoustical noise of near-by working machines and building vibrations since it was independent of the driving amplitude. All other peaks were due to resonances induced by the ZnO coating, each peak representing different resonant modes of the fiber. The resonances appeared at the same frequencies for both 0.5 and 1 V ac drive levels, with the amplitude of the response increasing with field, as expected. If the mechanical quality factor Q is taken as D frfr , where D f is the halfwidth of the amplitude response, and f r is the center resonance frequency, then Q of the fundamental resonance was ; 60. Fig. 9 shows the displacement and response as a function of the driving voltage at a low non-resonant frequency of 25 Hz and at the first observed resonance frequency of 110 Hz. A linear behavior is observed for both responses over most of the measurement range. For the highest amplitudes, the response is slightly non-linear due to the onset of the interferometer non-linearity. There is some question whether the obtained response represents solely the amplitude of the fiber motion in the horizontal plane comprising the fiber axis and the laser incident beam. Due to possible misalignment of the patterned gaps that ideally lie in a plane perpendicular to the probing beam, vertical Žout-ofplane. motion of the fiber is also possible. This vertical motion can result in a change of the interference signal due to the curvature of the fiber. The large laser spot size incident on the fiber and the complex shape of the interference fringes does not allow a straightforward interpretation of the measured response, though. The first approximation used for these measurements introduces an error that would
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Fig. 9. Displacement as a function of excitation voltage for the same fiber as in Fig. 8 measured on Ž110 Hz. and off Ž25 Hz. resonance.
at most give a factor of two variance in the measured values. Further work is needed to identify the vibrational modes that correspond with the observed resonance peaks. It is expected that the electric field-induced motion could be amplified if both halves of the fiber were actuated in antiphase or if the drive voltage were increased further. The resonant displacements measured here are large enough for some scanning near field optical microscope applications, even for drive voltages as low as 1 V rms. Previous work on similar devices has shown that drive levels up to 6 V rms can be utilized without electrical breakdown. This, combined with the presence of multiple resonances should enable the frequency to be tailored at least over the range between 100 and 10,000 Hz. Changes in the actuator geometry Žespecially the length and the clamping conditions. will also facilitate control of the resonant frequencies and amplitudes. The mechanical quality factor Q of the resonance observed should also be high enough for SNOM applications, particularly given the fact that the piezoelectric coefficients of ZnO are higher than those of the quartz piezoelectric reported for electrical detection of vertical tip position in SNOM applications w17x. Fiber alignment systems would require larger static motions than were observed here. The core of an optical fiber is typically several microns in diameter, and the alignment of two cores with respect to each other would probably require motions on the same order. To approach this field, either larger segments could be actuated, or a stronger piezoelectric, such as lead zirconate titanate, could be employed as the piezoelectric coating.
4. Conclusions
Fig. 8. Displacement and phase angle measured as a function of frequency for fiber actuator with a split inner electrode. One side of the split electrode was excited over a 2-mm length. The first resonance occurs at 110 Hz. This resonance probably corresponds to forced oscillations at one of the natural frequencies of the fiber.
A processing procedure was developed to photolithographically pattern the electrodes for piezoelectric fiber coatings along the fiber length. The introduction of 30-mm gaps allowed the two halves of the fiber to be driven independently, which enabled a flexural motion of the fiber. This type of structure should be useful as an inte-
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grated actuator for near field optical microscope tip positioning, and potentially in optical fiber alignment.
Acknowledgements The assistance of Markus Kohli is gratefully acknowledged. Helpful discussions with Genaro Zavala are also appreciated. This work was partially funded through a DARPA contract DABT63-95-C-0053 and an NSF award DMR 9502431. This work was also supported by the Optical Sciences, Applications, and Technology Priority Program of the Board of the Swiss Federal Institute of Technology.
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