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
With the development in the SOI technology and MEMS technology [1–3], research works have been extensively conducted on micro optical and mechanical components for optical heads [4–6]. On the demands of a new-generation optical data storage system, a micro-fabricated optical head with smaller volume, faster response speed is required . A lens actuator is a basic and indispensable optical component for optical head applications . 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 . Wu et al. fabricated an electrothemally-actuated micro-lens vertical scanner which has a large tunable range and a low driving voltage . Bu et al. reported an electrostatic actuator for optical focusing . Especially, Wu et al. demonstrated an out-of-plane bidirectional focusing actuator using the self-aligned vertical comb drive structure . 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 . 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 . 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.
Figures 2(a) and 2(b) show the schematic diagrams of the integrated tracking and focusing comb structures at the initial position, respectively, and the actuated situations are illustrated in Figs. 2(c) and 2(d), respectively. The resulting electrostatic fringe field causes related displacements in tracking direction (x) and focusing direction (z). A relatively simple analytic approximation for the capacitance between the fixed parts and movable parts is given by the parallel plate capacitances between the overlapped sides of adjacent combs. Thus for a single movable comb, the stored energy can be divided into Wx and Wz which are given byFig. 2(a)), l is the overlapping length of the shared movable combs and the fixed combs of the focusing actuator, and U1 and U2 are the actuation voltages in tracking and focusing directions, respectively.
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
The resulting two-dimensional displacements are
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
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 . The approximate displacements without the coupling are estimated by
The stiffness of the spring for supporting the comb actuator is estimated byTable 1 .
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) 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.
A two-mask process is employed to define the proposed device. The high aspect ratio comb structures are achieved by deep reactive ion etching (DRIE) and the movable parts are released by a buffered hydrogen fluoride (BHF) solution. The relief of the residual stress in silicon device layer may cause fracture-related issues when separating the top device layer from the silicon handle layer. Thus a thick silicon layer based process is proposed to reduce the fracture risk for fabricating a large-area lens holder. Figure 4 illustrates schematically the fabrication process. The starting substrates are 2 cm × 2 cm SOI cut wafers with a 50μm silicon device layer, a 2μm buried oxide layer, and a 150μm silicon handle layer. The silicon device layer is photo-lithographically patterned and subsequently etched down to the buried oxide layer to create the lens holder, springs, tracking actuator and movable combs of the focusing actuator (steps a and b). Then the microstructures on the silicon device layer are protected by thick photoresist (step c). The fixed combs of the focusing actuator are defined on the silicon handle layer by backside alignment technology and then etched down to the buried oxide layer by DRIE (steps d and e). The buried oxide layer is removed in the BHF solution (step f), generating the holder. Finally, the micro ball lens is manually assembled and then the integrated comb-drive tracking and focusing actuator is fabricated (step g).
3. Experimental results and discussion
Figure 5(a) shows the scanning electron microscope (SEM) images of the fabricated device having a footprint of 1.65mm × 3.4mm. The integrated actuating structures and the lens holder are fabricated and no fracture-related issues are found during the fabrication. Figure 5(b) shows the device assembled with a BK7 ball lens with a diameter of 1mm. The integrated comb structures are illustrated in Figs. 5(c) and 5(d). The movable combs are shared by the tracking and focusing actuators, and the fixed combs of the tracking and focusing actuators are insulated by the silicon oxide layer. Thus the tracking actuator and focusing actuator are integrated.
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) 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 .
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 . 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.
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.
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