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Integrated VCSEL-Microlens Scanner With Large Scan Range Jeffrey B. Chou, Niels Quack, and Ming C. Wu, Fellow, IEEE

Abstract— We present an integrated beam steering system with a microlens scanner and a vertical cavity surface-emitting laser (VCSEL) source. The scanner and the VCSEL are assembled with an alignment sphere process and with an accuracy of 121 ± 7 µm defined by precision microspheres. The microlens scanner employs an electrostatic comb-drive actuator with prebent springs to extend the maximum displacement to 83 µm; 60% larger than that with standard folded springs. This translates to an angular scan range of ±9.6° with the double sided comb-drive scanner. The mechanical resonance with lens is measured to be 236 Hz. [2013-0186] Index Terms— Beam steering, large displacement, optics, packaging.

I. I NTRODUCTION

O

PTICAL BEAM steering with precise control has many important applications ranging from industrial, military, medical, and consumer applications. The ability to condense the optical source and steering systems to sub-millimeter scales opens up a new range of technological applications. Microlens scanners [1] have been used for Shack-Hartmann sensors [2], confocal microscopy [3], and free-space optical interconnect applications [4]–[8]. Compared with micromirrorbased scanners [9], [10], the microlens scanners offer a number of advantages: first, the microlens scanner is a transmission type device. The resulting optical system is usually more compact, requiring a fewer number of optical beam reflections. Second, microlens scanners can be easily integrated with a semiconductor laser source by simply stacking them along the beam path. Third, the microlens also collimates the output beam, eliminating the need for a separate collimating lens. The optical loss from either a micromirror or a microlens heavily depends on the design parameters, such as metal thickness, surface roughness, lens material, and clipping losses. Previous comparisons have shown both methods are able to reflect or transmit light at >90%, and are thus comparable [11]. Spherical aberrations may play a major role in microlens systems where the beam spot size on the lens is typically a few factors smaller than the diameter of the lens [4]. Manuscript received December 9, 2013; revised March 20, 2014; accepted March 26, 2014. Subject Editor S. Merlo. J. B. Chou was with the University of California at Berkeley, Berkeley, CA, 94704 USA. He is now with the Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: [email protected]). N. Quack and M. C. Wu are with the University of California at Berkeley, Berkeley, CA 94704 USA (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2014.2315810

Fig. 1. Schematic of MEMS scanner and alignment chip. The VCSEL is aligned to the center of the lens. The red spheres are used to align and accurately separate the MEMS chip from the VCSEL to be at the desired focal length for beam collimation. Wire bond pads for the VCSEL are routed out and away from the center of the MEMS chip for external probing.

The displacement of the lens relative to the incoming optical beam serves to heighten this issue as the beam becomes closer to the edge of the lens. The undesired spherical aberration effects in microlens based systems will affect the overall collimation and focusing quality. However, by using a larger lens, the spherical aberration effects can be mitigated. Thus, depending on the application, a tradeoff with device size and optical collimation quality exists in microlens systems. In this paper, we present an alignment process using precision microspheres to align the MEMS lens scanner to a vertical cavity surface emitting laser (VCSEL). Previously demonstrated methods used steel balls and grooves for mechanical coupling [5], micro-spheres and pits [12], and flip-chip bonding [8], [11]. The overall alignment accuracy in these systems is typically in the range of micrometers, which is comparable to the accuracy of our present method. To increase the maximum displacement and the optical scan range, we employ a combdrive actuator with, dual sided prebent, spring structure [13]. A large displacement (83μm) and large scan range (±9.6°) are achieved. Thus a compact and integrated VCSEL-MEMS device with enhanced steering angle is presented. II. D ESIGN A. Large Range Scanner The schematic of the integrated microlens scanner is shown in Fig. 1. The VCSEL is integrated at the center of the focal plane of the microlens. The design goal for the MEMS lens

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Fig. 3. Simulated spring constants to determine maximum displacement before pull-in. The spring constant k pre−bent and kstraight are calculated from FEM simulations of the entire MEMS shuttle using pre-bent and straight springs, respectively. Dotted lines A and B correspond to the experimentally observed maximum displacements for the straight and pre-bent springs. TABLE I D ESIGN PARAMETERS FOR L ENS S CANNER

Fig. 2. Simplified drawing of the MEMS lens scanner with to-scale bending of the pre-bent spring structures. The lens shuttle is shown displacing to the (a) left, (b) center, and (c) right. Note how certain springs condense and straighten to increase the stiffness in the y-direction.

scanner is a displacement of ±80 μm with a voltage of 100 V. To achieve this, we employ a slight modification of the pre-bent spring structures introduced by Grade et. al. [13]. Eight pairs of pre-bent springs connect the shuttle to the anchors so that at maximum displacement half of springs are “straightened”, as shown in Fig. 2. This increases the stiffness of the spring in the y-direction. As a result, we are able to achieve large displacements in both the left and right directions without lateral pull-in, which limits the displacement of standard combdrive actuators [14]. We have analyzed the transverse stiffness of the springs by the finite element method (FEM). Fig. 3 shows the spring constants, k y , of the scanner in the y-direction for both prebent (k pre−bent ) and straight beams (kstraight ) as a function of

the displacement in the x-direction. As expected, the kstraight stiffness falls off exponentially as it moves in the positive x-direction [13]. Also plotted in Fig. 3 is the electrical stiffness, ke , which represents the electrostatic force to tilt the combdrive teeth in the y-direction. The electrical stiffness is described by following expression [15]: ke =

2x 2 k x g2

(1)

where x and k x are the displacement and the spring constant in the x-direction, respectively, and g is the comb gap spacing. The analytical expression for k x = 4Et Ws3 /L 3s is used, where E is the Young’s modulus for silicon (160 GPa), and the geometrical terms are described in Table 1. The dimensions of the comb teeth and springs are also summarized in Table 1. The transverse stability condition is given by k y >ke [16]. The displacement at which ke intersect kstrraight is therefore the maximum stable displacement allowed without lateral pull-in. Figure 3 shows that the maximum displacement for the scanner with straight spring beams is 57 μm. In contrast, the spring constant k pre−bent decreases much more slowly with x because half of the springs become more straight with displacement. As a result, k pre−bent intersects ke at a higher x-displacement of 87 μm, which denotes the maximum theoretical displacement for the pre-bent springs.

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Fig. 4. Cross sectional schematic of the assembly. Alignment spheres are used to align the MEMS to alignment chip in the X, Y, and Z directions.

The dotted lines marked A and B in Fig. 3 represent the experimentally observed maximum displacement for the straight and pre-bent springs, respectively. B. Assembly Fig. 4 shows the cross-section design schematic of the fully assembled MEMS/VCSEL chip. The alignment chip employs lithographically patterned silicon blocks for the mechanical positioning of the VCSEL chip and alignment spheres. To position the top of the VCSEL chip at the back focal length, first we calculate the BFD via ray transfer matrix method to be   t (n − 1) R 1− (2) BFD = (n − 1) nR where t is the center lens thickness, n is the refractive index, and R is the radius of curvature. Given dimensions of the wafer and VCSEL chip, we calculate the separation between the MEMS and the alignment chip, where h ball is determined by h ball = B F D + L − h device1 − h handle − h device2

(3)

To achieve the desired h ball, the width of the alignment wells holding the microsphere is determined by    2  D h ball 2 − (4) wwell = 2 2 2 With the above calculations, exact values can be determined for any design. With four alignment spheres, we can accurately align the scanner in all 3 axes. Positioning along the z-axis is important to ensure that the VCSEL is at the focal point of the lens, and positioning along the x-y plane is important to ensure that the beam is at the center of the lens. For designs where the BFD is significantly less than the MEMS chip thickness, designs without the microspheres can easily be determined. However, such designs may employ lenses with small radii of curvature which will cause spherical aberrations (e.g., paraxial approximation not valid). Thus designs must be tailored depending on the end applications and their optical needs. The alignment chip also serves as an electrical contact pad for the VCSEL. To ensure low resistance contacts with the silicon pads, the resistivity of the device layer is chosen to be 0.01 -cm.

Fig. 5. Mask layout for the (a) alignment chip, (b) backside MEMS throughwafer etching, and (c) MEMS scanner. The fully overlapped layout is shown in (d).

The physical layouts of the alignment and the MEMS chips are shown in Fig. 5. The alignment chip comprises alignment structures for the VCSEL in the center, interconnect lines for VCSEL biasing, and the microsphere alignment holes at four corners (Fig. 5(a)). The bottom side of the MEMS chip has four matching holes for the microspheres (Fig. 5(b)). Fig. 5(c) shows the top layer layout of MEMS lens scanner device. The entire layout overlaid on top of each other is shown in Fig. 5(d). III. FABRICATION A. Chip Fabrication The fabrication steps of both the MEMS and alignment chip are shown in Fig. 6(a) – 6(d) and Fig. 6(e) – 6(h), respectively. For both chips, we start with a silicon-on-insulator (SOI) wafer, with a 2 μm thick buried oxide layer, and a 20 μm thick device layer for the MEMS chip, and 100 μm thick device layer for the alignment chip, respectively, as shown in Fig. 6(a) and 6(e). The comb drive and spring structures are patterned and etched via deep reactive ion etching (DRIE), as shown in Fig. 6(b) and 6(f). On the MEMS chip, a throughwafer, backside etch is performed to singulate the dies and to create an optical path for the VCSEL. The alignment holes for microspheres are etched at the same time, but due to their smaller diameters they do not etch at the same rate, as shown in Fig. 6(c). This is desired as we do not want the alignment holes to drill through to the top device layer. Dicing separates the individual dies on the alignment platform wafer, as shown in Fig. 6(g). A final hydrofluoric (HF) vapor etch removes the sacrificial buried oxide for the MEMS chip release. The alignment chip requires the HF vapor etch to expose the bottom handle layer for electrical probing. B. Assembly The MEMS and the alignment chips are assembled using a custom built pick-and-place device. The VCSEL and the

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TABLE II FABRICATED D IMENSIONS OF A SSEMBLY

Fig. 6. Fabrication layout of the MEMS chip (a)-(d) and the alignment chip (e)–(h). Both chips start with SOI wafers (a), (e), then proceed with front side DRIE etch (b), (f), followed by backside through wafer etching (c), (g). Due to aspect ratio dependent etching, the smaller holes for the alignment spheres do not etch through the entire wafer. Finally an HF vapor release etch is done to release the silicon from the oxide (d), (h).

microspheres are passively aligned by the mechanical wells created by DRIE. The alignment spheres are commercially available, micro half-ball lenses made of BK7 glass. The manufactured diameter accuracy of the microspheres is less than 1 μm. The VCSEL chip is wire bonded to the silicon contact pads with conductive epoxy to improve contact resistance. Once the alignment spheres and VCSEL chip are fixed on the alignment chip, the MEMS chip is placed on top of the spheres. To ensure the spheres are in fact aligned in the corresponding MEMS chip wells, the MEMS chip is slid around until a visible “snap down” is observed. The MEMS chip is then mechanically fixed in the x-y plane. A commercially available half ball lens is placed on top of the MEMS scanner and thus completes our assembly process. IV. E XPERIMENT AND C HARACTERIZATION A. Assembly Accuracy Images of the completed device (lens, MEMS, and alignment chip) are shown in Fig. 7. The high parallelism between the two chips can be seen in Fig. 7(a). Fig. 7(b) shows the electrical probing pads for the VCSEL protruding from underneath the MEMS chip on the left and right. Fabricated dimensions of the entire assembly shown in Fig. 4 are listed in table 2. The h ball is measured across several assembled devices to be 121 ± 7 μm. The alignment chip with VCSEL and alignment spheres is shown in Fig. 7(c). To improve the accuracy of the gap, pressure may be applied to the MEMS and

alignment chips to decrease variation of the gaps. Typical high end commercial pick and place machines report an accuracy of ±25 μm. Due to the limited commercial availability of microlenses at these length scales, the fabricated dimensions hball and wwell do not correspond to the optimized dimensions as determined in Eq. (3) and (4). As a result the actual distance between the VCSEL and lens is approximately 490 μm, instead of the ideal 333 μm. This offset will cause for nonideal collimation of the VCSEL. Further generations of the device can easily be modified for the optimal dimensions, e.g. by using a custom lens design. The two corners of the alignment blocks fix the location of the VCSEL. Fig. 7(d) shows a close up of the alignment sphere settled into its corresponding well. Clearly the alignment spheres are precisely manufactured and ensure accurate alignment. B. Microlens Scanner To verify the displacement advantage of pre-bent springs, we fabricated devices with both pre-bent springs and traditional straight folded springs. The two sets of springs have identical width, thickness, and length. The pre-bent springs have a longer path length due to the curvature of the structure. We observe the straight spring devices have a maximum displacement of 52 μm before lateral pull-in, while the prebent spring devices exhibits displacements of 83 μm at 80 V, as shown in Fig. 8. These displacements are marked on Fig. 3 by two vertical dotted lines marked A and B for the straight and pre-bent springs, respectively. Clearly we see the measured results are in good agreement with the simulations, and show a 60% displacement increase over the traditional springs. Using the fully assembled devices, MEMS, VCSEL, and lens, we are able to obtain high quality displacement measurements using optical testing methods. We use a position sensing detector (PSD), which has a resolution of 1 nm and a response rate of up to 20 kHz, to monitor the steered beam out of the MEMS/lens system. The measured voltage versus displacement is shown in Fig. 9. The maximum displacement shown here is only 69 μm. We did not want to risk damaging the assembled device by going to the maximum voltage. The mechanical frequency response of the device, with lens, is shown in Fig. 10. We see a peak resonance at 236 Hz, which translates to an equivalent spring width of 1.83 μm.

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Fig. 8.

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Microscope image of the lens shuttle displaced 83 μm at 80V.

Fig. 9. Measured voltage-displacement of the shuttle with MEMS and VCSEL. Maximum possible displacement is not shown here to preserve device lifetime.

Fig. 7. Photographs and SEM images of the MEMS and alignment chip. A photograph of the fully assembled device is shown in (a). The VCSEL contact pads can be seen protruding from the device in (b). (c) Shows the alignment chip with alignment spheres and wire bonded VCSEL. (d) Is a close up view of the precise alignment sphere.

Fig. 10. Measured mechanical frequency response of the MEMS with lens. We observe a peak resonance at 236 Hz.

We observed with previous experience, that springs of these dimensions with wide clear areas tend to lose about 1 μm in width due to DRIE over etching. As a result, our actual mask

layout has a spring width of 3 μm. Our estimation leads to our actual device to be about 8.5% off from our desired width of 2 μm. This error is tolerable for our application needs.

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Two smaller, lower frequency peaks are observed at 78 Hz and 117 Hz. These may be due to out of plane motions, but are suppressed by the handle wafer floor. V. C ONCLUSION We demonstrate a packaging method to align a MEMS and VCSEL system in all 3 axis using precision microspheres and deep reactive ion etched silicon wells. The position accuracy is measured to be within ±7 μm of the desired distance of 120 μm. The VCSEL is wire bonded to the alignment chip and integrated with the MEMS chip. The MEMS lens scanner with pre-bent springs exhibits a displacement of 83 μm, which is 60% larger than the scanner with traditional folded springs (52 μm). The experimentally measured maximum displacements agree well with our FEM simulations. The large displacement corresponds to a beam steering angle of ±9.61°. Our fabrication method is compatible with standard MEMS processes, and the coarse pick-and-place accuracy is well within commercial machine capabilities. The applications of such a system can lend itself to mass manufactured compact optical systems, such as light detection and ranging (LADAR) systems, medical imaging devices, and free-space optical interconnects. R EFERENCES [1] K. Hedsten et al., “MEMS-based VCSEL beam steering using replicated polymer diffractive lens,” Sens. Actuators A, Phys., vol. 142, no. 1, pp. 336–345, Mar. 2008. [2] H. Choo and R. S. Muller, “Addressable microlens array to improve dynamic range of Shack-Hartmann sensors,” J. Microelectromech. Syst., vol. 15, no. 6, pp. 1555–1567, Dec. 2006. [3] S. Kwon and L. P. Lee, “Stacked two dimensional micro-lens scanner for micro confocal imaging array,” in Proc. 15th IEEE Int. Conf. Micro Electro Mech. Syst., Jan. 2002, pp. 483–486. [4] J. Chou et al., “Robust free space board-to-board optical interconnect with closed loop MEMS tracking,” Appl. Phys. A, vol. 95, pp. 973–982, Mar. 2009. [5] B. E. Yoxall, R. Walmsley, H.-P. Kuo, S.-Y. Wang, M. Tan, and D. A. Horsley, “Two-axis MEMS lens alignment system for free-space optical interconnect,” IEEE J. Sel. Topics Quantum Electron., vol. 17, no. 3, pp. 559–565, Jun. 2011. [6] J. B. Chou, K. Yu, and M. C. Wu, “Electrothermally actuated lens scanner and latching brake for free-space board-to-board optical interconnects,” J. Microelectromech. Syst., vol. 21, no. 5, pp. 1–10, Oct. 2012. [7] H. Toshiyoshi, G.-D. J. Su, J. LaCosse, and M. C. Wu, “A surface micromachined optical scanner array using photoresist lenses fabricated by a thermal reflow process,” J. Lightw. Technol., vol. 21, no. 7, pp. 1700–1708, Jul. 2003. [8] A. Tuantranont, V. M. Bright, J. Zhang, W. Zhang, J. A. Neff, and Y. C. Lee, “Optical beam steering using MEMS-controllable microlens array,” Sens. Actuators A, Phys., vol. 91, no. 3, pp. 363–372, Jul. 2001. [9] P. F. Van Kessel, L. J. Hornbeck, R. E. Meier, and M. R. Douglass, “A MEMS-based projection display,” Proc. IEEE, vol. 86, no. 8, pp. 1687–1704, Aug. 1998. [10] T.-W. Yeow, K. L. E. Law, and A. Goldenberg, “MEMS optical switches,” IEEE Commun. Mag., vol. 39, no. 11, pp. 158–163, Nov. 2001. [11] A. Tuantranont and V. M. Bright, “Segmented silicon-micromachined microelectromechanical deformable mirrors for adaptive optics,” IEEE J. Sel. Topics Quantum Electron., vol. 8, no. 1, pp. 33–45, Jan. 2002.

[12] A. V. Krishnamoorthy et al., “Optical proximity communication with passively aligned silicon photonic chips,” IEEE J. Quantum Electron., vol. 45, no. 4, pp. 409–414, Apr. 2009. [13] J. D. Grade, H. Jerman, and T. W. Kenny, “Design of large deflection electrostatic actuators,” J. Microelectromech. Syst., vol. 12, no. 3, pp. 335–343, Jun. 2003. [14] S. D. Senturia, Microsystem Design. Boston, MA, USA: Kluwer, 2001. [15] G. Zhou and P. Dowd, “Tilted folded-beam suspension for extending the stable travel range of comb-drive actuators,” J. Micromech. Microeng., vol. 13, no. 2, pp. 178–183, 2003. [16] R. Legtenberg, A. W. Groeneveld, and M. Elwenspoek, “Comb-drive actuators for large displacements,” J. Micromech. Microeng., vol. 6, no. 3, pp. 320–329, 1996.

Jeffrey B. Chou received the B.S., M.S., and Ph.D. degrees in electrical engineering and computer sciences from the University of California at Berkeley in 2007, 2010, and 2011, respectively. He is currently a Battelle Postdoctoral Fellow at the Massachusetts Institute of Technology (MIT) in the mechanical engineering department. His current research interests are in nanophotonics for solar energy conversion, photoelectrolysis, and optical communication devices.

Niels Quack received his MS from Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, and his PhD from Eidgenössische Technische Hochschule Zürich (ETH), 2005 and 2009, respectively. His research interests include MEMS, Optical MEMS, Tunable Optical Microsystems, Optomechanics, Silicon Photonics, and Heterogeneous Integration of Photonics and Electronics. He is currently Postdoctoral Fellow at the Integrated Photonics Laboratory at UC Berkeley.

Ming C. Wu (F’02) is a Nortel Distinguished Professor of Electrical Engineering and Computer Sciences at the University of California, Berkeley. He is also Co-Director of the Berkeley Sensor and Actuator Center (BSAC) and Faculty Director of the UC Berkeley Marvell Nanolab. Dr. Wu received his B.S. degree in Electrical Engineering from National Taiwan University, Taipei, Taiwan, and M.S. and Ph.D. degrees in Electrical Engineering and Computer Sciences from the University of California, Berkeley in 1986 and 1988, respectively. From 1988 to 1992, he was a Member of Technical Staff at AT&T Bell Laboratories, Murray Hill, New Jersey. From 1992 to 2004, he was a professor in the Electrical Engineering department at the University of California, Los Angeles. He has been a faculty member at Berkeley since 2004. His research interests include semiconductor optoelectronics, silicon photonics, MEMS (micro- electro-mechanical systems), MOEMS, nanophotonics, and biophotonics. He has published 8 book chapters, over 200 journal and 300 conference papers. He is the holder of 22 U.S. patents. Prof. Wu was a Packard Foundation Fellow (1992 - 1997), and received the 2007 Paul F. Forman Engineering Excellence Award from the Optical Society of America.