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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 1 — Jan. 2, 2012
  • pp: 627–634
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Comb-drive tracking and focusing lens actuators integrated on a silicon-on-insulator wafer

P. Li, T. Sasaki, L.F. Pan, and K. Hane  »View Author Affiliations


Optics Express, Vol. 20, Issue 1, pp. 627-634 (2012)
http://dx.doi.org/10.1364/OE.20.000627


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Abstract

This paper reports integrated tracking (horizontal) and focusing (vertical) actuators using microelectromechanical (MEMS) technology for optical storage. The design, fabrication and characterization of the integrated tracking and focusing actuators are demonstrated. The integrated tracking and focusing lens actuators that consist of springs, a lens holder and comb-drive actuators are implemented to obtain the two-dimensional tuning effect of an objective lens. A two-mask process is used to define the actuators on a silicon-on-insulator (SOI) wafer. The DC displacements and the frequency responses are characterized by applying voltage to the micro lens actuators. Displacements are experimentally characterized with ± 24.6μm in tracking direction and 5.7μm in focusing direction. Moreover, the influences of the cross-axis coupling are measured and evaluated. This MEMS device which has a small form factor provides an excellent response time and size reduction. The minimization and integration of the lens actuators can offer low costs in assembly and expand the application area of the optical head.

© 2011 OSA

1. Introduction

With the development in the SOI technology and MEMS technology [1

1. M. Wu, “Micromachining for optical and optoelectronic systems,” Proc. IEEE 85(11), 1833–1856 (1997). [CrossRef]

3

3. K. Hane, “MEMS technologies for optical storage application,” in Proceedings of International Symposium on Optical Memory (The Japan Society of Applied Physics, Nagasaki, Japan, 2009), pp. 14–15.

], research works have been extensively conducted on micro optical and mechanical components for optical heads [4

4. M. Sasaki, F. Bono, and K. Hane, “Large-displacement micro-XY-stage with paired moving plates,” Jpn. J. Appl. Phys. 47(4), 3226–3231 (2008). [CrossRef]

6

6. R. Hokari and K. Hane, “A varifocal convex micromirror driven by a bending moment,” IEEE J. Sel. Top. Quantum Electron. 15(5), 1310–1316 (2009). [CrossRef]

]. On the demands of a new-generation optical data storage system, a micro-fabricated optical head with smaller volume, faster response speed is required [7

7. H. Cheng, S. Hsiao, M. Wu, and W. Fang, “Integrated tracking and focusing systems of MEMS optical pickup head,” IEEE Trans. Magn. 43(2), 805–807 (2007). [CrossRef]

]. A lens actuator is a basic and indispensable optical component for optical head applications [8

8. B. Zhang, J. Ma, L. Pan, X. Cheng, H. Hu, and Y. Tang, “High-sensitivity actuator with new magnetic circuit in optical pickup,” Jpn. J. Appl. Phys. 47(7), 5809–5811 (2008). [CrossRef]

]. The horizontal motion and the vertical motion of the actuators are called the tracking and the focusing, respectively. For instance, Fang et al. reported a V-beam thermal actuator for lens tracking [9

9. M. Wu, S. Hsiao, C. Peng, and W. Fang, “Development of tracking and focusing micro actuators for dual-stage optical pick-up head,” J. Opt. A, Pure Appl. Opt. 8(7), S323–S329 (2006). [CrossRef]

]. Wu et al. fabricated an electrothemally-actuated micro-lens vertical scanner which has a large tunable range and a low driving voltage [10

10. L. Wu and H. Xie, “A millimeter-tunable-range microlens for endoscopic biomedical imaging applications,” IEEE J. Quantum Electron. 46(9), 1237–1244 (2010). [CrossRef]

]. Bu et al. reported an electrostatic actuator for optical focusing [11

11. S. Kim, J. Park, G. Park, J. Lee, J. Lee, H. Jung, J.-Y. Kim, S.- Kim, Y. Yee, J. H. Kim, J. H. Kim, and J. U. Bu, “An optical flying head assembly for a small-form-factor plastic disk in PCMCIA-like drive,” Jpn. J. Appl. Phys. 43(7B), 4752–4758 (2004). [CrossRef]

]. Especially, Wu et al. demonstrated an out-of-plane bidirectional focusing actuator using the self-aligned vertical comb drive structure [12

12. Y. Chiu, J. Chiou, W. Fang, Y. Lin, and M. Wu, “Design, fabrication, and control of components in MEMS-based optical pickups,” IEEE Trans. Magn. 43(2), 780–784 (2007). [CrossRef]

]. However, there are few reports on the integrated tracking and focusing actuators. Fang et al. proposed integrated tracking and focusing systems driven by thermal, as well as magnetic means [7

7. H. Cheng, S. Hsiao, M. Wu, and W. Fang, “Integrated tracking and focusing systems of MEMS optical pickup head,” IEEE Trans. Magn. 43(2), 805–807 (2007). [CrossRef]

]. The minimization and location precision of these systems are limited by the separated permanent magnet for the focusing actuation. It is still a challenge to integrate all the tracking and focusing parts on one chip with MEMS technology to achieve two dimensional displacements of the lens.

This paper demonstrates an SOI technology for the design, fabrication and characterization of the integrated comb-drive tracking and focusing actuators. A double-side process is employed to define the integrated comb-drive tracking and focusing actuators to achieve 2D movements of the micro lens. The DC and the frequency responses of the integrated comb-drive tracking and focusing actuators are investigated. Both tracking and focusing displacements are experimentally characterized. Moreover, the influences of the cross-axis coupling are measured and analyzed. The integration of the tracking and focusing lens actuators is an important step for the further integration of the optical head, where all the components are monolithically fabricated and integrated in a single device.

2. Design and fabrication

The device consists of two folded-springs, a lens holder, shared movable combs, fixed combs for the tracking and focusing actuators, as shown in Fig. 1
Fig. 1 (a) Schematic structure of the micro lens actuator. (b) Structure of the micro lens actuator. (c) Magnified view of the integrated comb-drive area.
. The lens holder is anchored to the substrate through the springs. The shift of the movable combs results in a displacement of the springs and thus, the two-dimensional position tuning of the lens holder is achieved. The fixed combs of the tracking actuator are divided into two symmetric parts to allow for displacements in two opposite directions (along the tracking axis in Fig. 1(a)) from the equilibrium position.

Figure 1(c) is a magnified view of the integrated comb-drive area. The tracking actuator including the shared movable combs and the lens holder are created on the top silicon layer. The fixed combs of the focusing actuator are obtained on the silicon handle layer. The silicon substrate beneath the lens holder is removed to realize clearance for the lens holder motion.

The total stored energy is just the sum of the contributions from the individual combs. The fixed combs of the tracking actuator are insulated into the left set and the right set, thus only half of them are actuated while all of the fixed combs of the focusing actuator are actuated at the same time. Thus the stored energy produced by the integrated tracking and focusing comb structures is
W=nWx+2nWz=ε2g[nx(h1z)U12+4nl(zh3)U22]=ε2g(nh1U12xnU12xz+4nlU22z4nlh3U22),
(2)
where n is the number equal one half of the number of the entire fixed combs of tracking actuator. The electrostatic forces generated by these fringe fields and balanced by the mechanical restoring forces are given by

Fx=Wx=nε2g(h1U12zU12)=kxx,
(3.1)
Fz=Wz=nε2g(4lU22xU12)=kzz.
(3.2)

The resulting two-dimensional displacements are

x=2nεU12(gh1kz2nεlU22)4g2kxkzn2ε2U14,
(4.1)
z=nε(8glkxU22nεh1U14)4g2kxkzn2ε2U14.
(4.2)

The displacement in the opposite direction of the tracking actuator is obtained when U1 is applied to the right comb set as shown in Fig. 2. Thus the displacements in two opposite directions with respect to the tracking actuator are

x=±2nεU12(gh1kz2nεlU22)4g2kxkzn2ε2U14.
(4.3)

From Eq. (4.2) and Eq. (4.3), the parameters of the comb structures should be optimized to increase the motion range and decrease the coupling influence. The control method is complicated to achieve the desired displacement due to the complex formula. Hence an approximate equivalence method is used to simplify the actuation [13

13. S. Timpe, D. Hook, M. Dugger, and K. Komvopoulos, “Levitation compensation method for dynamic electrostatic comb-drive actuators,” Sens. Actuators A Phys. 143(2), 383–389 (2008). [CrossRef]

]. The approximate displacements without the coupling are estimated by

x=±nεh12gkxU12,
(5.1)
z=2nεlgkzU22.
(5.2)

The stiffness of the spring for supporting the comb actuator is estimated by
kx=Eh1wf3lf3,
(6.1)
kz=Ewfh13lf3,
(6.2)
where wfand lfare the width and length of the spring, respectively. The device parameters of the comb structures and spring are summarized in Table 1

Table 1. Main design parameters of the integrated comb-drive tracking and focusing actuators

table-icon
View This Table
.

The displacements of the movable comb calculated from Eq. (4.1) and Eq. (4.2) are simulated for investigating the coupling influence as a function of the applied voltage, as shown in red in Figs. 3(a)
Fig. 3 (a) Simulated displacement of the tracking actuator; (b) simulated displacement of the focusing actuator.
and 3(b), respectively. The approximate displacements without coupling calculated from Eq. (5.1) and Eq. (5.2) are shown in blue in Fig. 3. The error caused by the neglected coupling is illustrated from the difference of the two surfaces.

The coupling displacement occurs in response to a change of the voltage in the orthogonal direction. As the change of U2 is small, the tracking displacement is mainly determined by the value of U1. In the focusing direction, the coupling displacement does not significantly change in response to an increase in U1 when the value of U1 is small. Therefore, the actuated displacement is slightly affected by a small change of the voltage in the orthogonal direction in the design.

3. Experimental results and discussion

The dynamic motion of the lens is evaluated by the dynamic change of the middle point of the spring which is connected to the lens holder. Voltages are applied on the fixed comb structures and the moving lens holder is grounded. The electrostatic force is thus generated by the fringing field between two sets of the comb fingers. The two-dimensional movement of the movable comb fingers in response to the change in the driving force results in a motion of the lens holder. Figure 6(a)
Fig. 6 Displacements versus the square of the applied voltage at different coupling voltages: (a) displacements in tracking direction; (b) displacements in focusing direction.
shows the measured and simulated tracking displacements as functions of the square of the driving voltage. About a displacement of 24.6μm is obtained at 400V in the tracking direction before the sticking occurs. Thus the total horizontal displacement of about 49.2μm is generated by the symmetric tracking actuator. Structures generated in reality deviate from the ideal element, and the fabrication deviations cause the differences between the experimental and simulated results. Moreover, the gravity-induced deflection in the focusing direction also influences the displacement in the tracking direction. These can be improved by optimizing the structural parameters and decreasing the fabrication errors.

Figure 6(b) shows the measured and calculated results of the focusing actuator versus the squared actuation voltage. At 300V, the focusing actuator produces a displacement of 5.7μm along the focusing direction. The results indicate that the tracking actuator has a good linearity between the square of the applied voltage and the actuated displacement. Compared with those in Fig. 6(a), the experimental results are in good agreement with the calculated displacements.

High driving voltage is required for the actuation due to the big size of the lens holder and the large mass of the movable part. Thick insulation layer also increases the threshold of driving voltage in the focusing actuator. Under the circumstance of fulfilling the spring and the practical requirement of micro ball lens, structural parameters and material system can be optimized to decrease the power consumption. For example, when the gap is decreased from 5μm to 3μm and n is increased from 206 to 300, both the tracking and focusing driving voltages can be decreased by 35.8% in our case. Moreover, another effective method has been reported to improve the travelling distance and driving voltage by controlling the thickness and location of the vertical comb electrodes [12

12. Y. Chiu, J. Chiou, W. Fang, Y. Lin, and M. Wu, “Design, fabrication, and control of components in MEMS-based optical pickups,” IEEE Trans. Magn. 43(2), 780–784 (2007). [CrossRef]

].

When the driving voltage exceeds a side instability voltage, VSI, the comb drive becomes unstable, which leads to side sticking of the movable and fixed comb fingers. And thus the maximum displacement of the proposed device is limited by the instability of the side-snap-over.

To quantify the decoupling of the integrated tracking and focusing lens actuators, displacements in the actuated directions caused by cross-axis coupling are measured and the results are also shown in Figs. 6(a) and 6(b). The side instability voltages VSI of the tracking actuator as shown in Fig. 6(a), are 400V (U2 = 0V), 330V (U2 = 50V) and 300V (U2 = 100V) respectively. Thus the motion range is decreased by the side instability that is susceptible to the coupling. When the U2 is 50V, the tracking displacement is mainly determined by the value of U1, as shown in Fig. 6(a). When U2 increases from 50V up to 100V, the tracking displacement decreases except at the side sticking situation (U1 = 300V, U2 = 100V).

The maximum focusing displacement and VSI of the focusing actuator also decrease in response to the increase of the coupling, as shown in Fig. 6(b). However, the value of the focusing displacement is not affected very much by a change of U1 before the side sticking. Moreover, the focusing displacement of the proposed actuators has good linearity versus the square of the voltage.

The results demonstrate that the actuated displacement is not greatly affected by a small change of the voltage in the orthogonal direction. But the motion range decreases due to the coupling influence. Since the optical head is usually placed in a feedback loop to maintain focusing and tracking, the coupling of the proposed actuators can be adjusted and the stable travel range can be extended by the close-loop control system. Hence, the focusing actuator has better performance on the decoupling than the tracking actuator.

The typical frequency responses of the tracking and focusing actuators are illustrated in Fig. 7
Fig. 7 Frequency responses of the actuators before/after the micro ball lens is loaded: (a) frequency response of the tracking actuator; (b) frequency response of the focusing actuator.
. Regarding the tracking actuator, the resonance frequencies are 4.76kHz and 1.26kHz, respectively, before/after the micro ball lens is loaded. Correspondingly, the resonance frequencies are 7.55kHz and 4.35kHz, respectively, for the focusing actuator. The first resonance frequency of the actuators is higher than that of the conventional lens actuators (<200Hz) even though it decreases after being assembled with the micro ball lens. Thus the integrated tracking and focusing actuators have a higher sensitivity than the conventional ones.

4. Conclusions

We have demonstrated a double-sided process for the fabrication of the integrated comb-drive tracking and focusing actuators on an SOI wafer. The tracking actuator, lens holder and movable combs of the focusing actuator are fabricated on the thick silicon device layer which is used to create the uniform surface and obtain high intension for the lens holding. And the silicon handle layer is used not only to support the suspended movable parts, but also to realize the fixed comb structures of the focusing actuator. When the voltage is applied on the tracking actuator and/or the focusing actuator, the consequent electrostatic force generates a motion of the micro lens in the actuated direction. The optical movements are experimentally investigated and the influence of the coupling is analyzed. Large displacements of about ± 24.6μm in tracking direction and 5.7μm in focusing direction are demonstrated. The actuated displacement is not affected very evidently by a small change of coupling and the device has a high sensitivity. This work represents an important step towards fabricating optical lens actuator devices using MEMS technology and is meaningful for the further integration of the optical head.

References and links

1.

M. Wu, “Micromachining for optical and optoelectronic systems,” Proc. IEEE 85(11), 1833–1856 (1997). [CrossRef]

2.

L. Y. Lin, J. L. Shen, S. S. Lee, and M. C. Wu, “Realization of novel monolithic free-space optical disk pickup heads by surface micromachining,” Opt. Lett. 21(2), 155–157 (1996). [CrossRef] [PubMed]

3.

K. Hane, “MEMS technologies for optical storage application,” in Proceedings of International Symposium on Optical Memory (The Japan Society of Applied Physics, Nagasaki, Japan, 2009), pp. 14–15.

4.

M. Sasaki, F. Bono, and K. Hane, “Large-displacement micro-XY-stage with paired moving plates,” Jpn. J. Appl. Phys. 47(4), 3226–3231 (2008). [CrossRef]

5.

T. Yamasaki, R. Hokari, and K. Hane, “Spherical silicon micro-mirrors bent by anodic bonding,” in Proceedings of IEEE Conference on Optical MEMS and Nanophotonics (Institute of Electrical and Electronics Engineers, Clearwater, FL, USA, 2009), pp. 51–52.

6.

R. Hokari and K. Hane, “A varifocal convex micromirror driven by a bending moment,” IEEE J. Sel. Top. Quantum Electron. 15(5), 1310–1316 (2009). [CrossRef]

7.

H. Cheng, S. Hsiao, M. Wu, and W. Fang, “Integrated tracking and focusing systems of MEMS optical pickup head,” IEEE Trans. Magn. 43(2), 805–807 (2007). [CrossRef]

8.

B. Zhang, J. Ma, L. Pan, X. Cheng, H. Hu, and Y. Tang, “High-sensitivity actuator with new magnetic circuit in optical pickup,” Jpn. J. Appl. Phys. 47(7), 5809–5811 (2008). [CrossRef]

9.

M. Wu, S. Hsiao, C. Peng, and W. Fang, “Development of tracking and focusing micro actuators for dual-stage optical pick-up head,” J. Opt. A, Pure Appl. Opt. 8(7), S323–S329 (2006). [CrossRef]

10.

L. Wu and H. Xie, “A millimeter-tunable-range microlens for endoscopic biomedical imaging applications,” IEEE J. Quantum Electron. 46(9), 1237–1244 (2010). [CrossRef]

11.

S. Kim, J. Park, G. Park, J. Lee, J. Lee, H. Jung, J.-Y. Kim, S.- Kim, Y. Yee, J. H. Kim, J. H. Kim, and J. U. Bu, “An optical flying head assembly for a small-form-factor plastic disk in PCMCIA-like drive,” Jpn. J. Appl. Phys. 43(7B), 4752–4758 (2004). [CrossRef]

12.

Y. Chiu, J. Chiou, W. Fang, Y. Lin, and M. Wu, “Design, fabrication, and control of components in MEMS-based optical pickups,” IEEE Trans. Magn. 43(2), 780–784 (2007). [CrossRef]

13.

S. Timpe, D. Hook, M. Dugger, and K. Komvopoulos, “Levitation compensation method for dynamic electrostatic comb-drive actuators,” Sens. Actuators A Phys. 143(2), 383–389 (2008). [CrossRef]

OCIS Codes
(210.4590) Optical data storage : Optical disks
(230.3990) Optical devices : Micro-optical devices

ToC Category:
Optical Data Storage

History
Original Manuscript: September 30, 2011
Revised Manuscript: December 4, 2011
Manuscript Accepted: December 7, 2011
Published: December 23, 2011

Citation
P. Li, T. Sasaki, L.F. Pan, and K. Hane, "Comb-drive tracking and focusing lens actuators integrated on a silicon-on-insulator wafer," Opt. Express 20, 627-634 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-1-627


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References

  1. M. Wu, “Micromachining for optical and optoelectronic systems,” Proc. IEEE85(11), 1833–1856 (1997). [CrossRef]
  2. L. Y. Lin, J. L. Shen, S. S. Lee, and M. C. Wu, “Realization of novel monolithic free-space optical disk pickup heads by surface micromachining,” Opt. Lett.21(2), 155–157 (1996). [CrossRef] [PubMed]
  3. K. Hane, “MEMS technologies for optical storage application,” in Proceedings of International Symposium on Optical Memory (The Japan Society of Applied Physics, Nagasaki, Japan, 2009), pp. 14–15.
  4. M. Sasaki, F. Bono, and K. Hane, “Large-displacement micro-XY-stage with paired moving plates,” Jpn. J. Appl. Phys.47(4), 3226–3231 (2008). [CrossRef]
  5. T. Yamasaki, R. Hokari, and K. Hane, “Spherical silicon micro-mirrors bent by anodic bonding,” in Proceedings of IEEE Conference on Optical MEMS and Nanophotonics (Institute of Electrical and Electronics Engineers, Clearwater, FL, USA, 2009), pp. 51–52.
  6. R. Hokari and K. Hane, “A varifocal convex micromirror driven by a bending moment,” IEEE J. Sel. Top. Quantum Electron.15(5), 1310–1316 (2009). [CrossRef]
  7. H. Cheng, S. Hsiao, M. Wu, and W. Fang, “Integrated tracking and focusing systems of MEMS optical pickup head,” IEEE Trans. Magn.43(2), 805–807 (2007). [CrossRef]
  8. B. Zhang, J. Ma, L. Pan, X. Cheng, H. Hu, and Y. Tang, “High-sensitivity actuator with new magnetic circuit in optical pickup,” Jpn. J. Appl. Phys.47(7), 5809–5811 (2008). [CrossRef]
  9. M. Wu, S. Hsiao, C. Peng, and W. Fang, “Development of tracking and focusing micro actuators for dual-stage optical pick-up head,” J. Opt. A, Pure Appl. Opt.8(7), S323–S329 (2006). [CrossRef]
  10. L. Wu and H. Xie, “A millimeter-tunable-range microlens for endoscopic biomedical imaging applications,” IEEE J. Quantum Electron.46(9), 1237–1244 (2010). [CrossRef]
  11. S. Kim, J. Park, G. Park, J. Lee, J. Lee, H. Jung, J.-Y. Kim, S.- Kim, Y. Yee, J. H. Kim, J. H. Kim, and J. U. Bu, “An optical flying head assembly for a small-form-factor plastic disk in PCMCIA-like drive,” Jpn. J. Appl. Phys.43(7B), 4752–4758 (2004). [CrossRef]
  12. Y. Chiu, J. Chiou, W. Fang, Y. Lin, and M. Wu, “Design, fabrication, and control of components in MEMS-based optical pickups,” IEEE Trans. Magn.43(2), 780–784 (2007). [CrossRef]
  13. S. Timpe, D. Hook, M. Dugger, and K. Komvopoulos, “Levitation compensation method for dynamic electrostatic comb-drive actuators,” Sens. Actuators A Phys.143(2), 383–389 (2008). [CrossRef]

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