This paper contributes a novel design and the corresponding fabrication process to research on the unique topic of micro-electro-mechanical systems (MEMS) deformable convex micromirror used for focusing-power control. In this design, the shape of a thin planar metal-coated polymer-membrane mirror is controlled electromagnetically by using the repulsive force between two magnets, a permanent magnet and a coil solenoid, installed in an actuator system. The 5 mm effective aperture of a large-stroke micromirror showed a maximum center displacement of 30.08 µm, which enabled control of optical power across a wide range that could extend up to around 20 diopters. Specifically, utilizing the maximum optical power of 20 diopter by applying a maximum controlling current of 0.8 A yielded consumption of at most 2 W of electrical power. It was also demonstrated that this micromirror could easily be integrated in miniature tunable optical imaging systems.
© 2015 Optical Society of America
Reflective optics, as a part of adaptive optics (AO), is now becoming a rising interest in laser physics, nonlinear optics, and biomedical applications [1, 2 ] owing to their relative simplicity and low cost, usability during high-power operation, and more detailed chromatic aberration-free multispectral quality imaging compared to traditional refractive optical systems. The designs and performances of refractive optical systems initially rely on the refractive properties of the lenses involved, as do most of lens-based systems . These systems have substantial disadvantages when they are used for multispectral imaging applications that involve extensive and dynamic wavelength ranges.
With the rapid progress in micro-electro-mechanical system (MEMS) miniaturization technology and the growing demands of several other applications, including high-resolution in vivo human retinal imaging [4, 5 ], micro-optics researchers have been under pressure to develop smaller and less expensive portable micro-optical components that can improve the performances of optical systems by reducing wavefront aberration. The key components of AO systems, such as deformable mirrors (DMs), have primarily been designed for wavefront aberration correction, although some have been intended for optical-focus control . AO systems designed for wavefront aberration correction consist of two essential parts: a wavefront sensor and a DM, which acts as wavefront corrector. These components are connected to real-time control systems. The DM deflection is usually performed at almost the same speed as that of the dynamic wave aberration caused by atmospheric turbulence in order to compensate for phase error and generate high-quality images for astronomic observations. On the other hand, in focus-control mirrors, multiple functioning electrodes precisely change the mirror shape to control its focal length and minimize wavefront aberration in order to enable more accurate imaging and laser-beam shaping. Very thin but frequently shape-changing reflective membranes possessing good optical and mechanical properties are essential components of focus-control DMs. Initially, DM films were composed of inorganic materials, such as silicon nitride. Although such materials exhibit stress uniformity and minimal aberrations, limited surface stroking, and consequently restricted focal-length control, result from their high mechanical stiffness [7, 8 ]. To increase the focal-length variation, DMs have recently been fabricated on polymer membranes utilizing polyimide [9, 10 ], CYTOP , and SU-8 [7, 12 ]. These polymer-based membranes have low residual stresses and effective elastic properties, which increase their stroke ranges and reduce their voltage requirements compared to those of inorganic DM films. However, these MEMS DMs still require very large voltages for actuation, as the actuation force generally depends on electrostatic, piezoelectric, thermal, or electromagnetic, including magnetic-composite-based, principles [13–16 ]. Moreover, in the complex arrays that are used in actuation-controlling circuitry, the high-voltage power supply requirement, the long response time, and sometimes the magnetic composite surface that limits the DM image sharpness are the main obstacles that prevent rapid adaptive-control installation and widespread use.
All of the DMs investigated in the above-mentioned studies were designed either as a mirror for wave-front aberration correction or as a concave shape mirror for focal-length control. A small number of reports exist on convex DMs including electrostatic actuation for some specific system integrations [17, 18 ], although convex mirrors are essential parts of Cassegrain reflectors and Schwarzschild objective-type optics, such as those used in reflective objective spectroscopy applications . Therefore, research on realizable varifocal convex DM fabrication and characterization based on variety of applications may promote more efficient optical device designs.
In this report, we present the details of a new design and fabrication process for a MEMS convex DM to be used for precise focusing-power control. The mirror in this design is composed of thin metalized SU-8 polymer film that acts as a flexible reflecting surface, and an electromagnetic force is used to generate larger strokes for the mirror surface and achieve greater back-focal-length variation. Because of the structural uniformity and low elastic modulus of SU-8 polymer film, the mirror possessed the expected properties, including the ability to be adjusted by large strokes, cost-efficient fabrication, and high-quality imaging. The experimental results obtained by investigating the actuated mirror surface shape are presented herein, as are the imaging profiles.
2. Device design and principles
Achieving optimal DM performance, such as large smooth strokes that consequently enable greater focal-length variation, depends on the proper utilization of the actuator force, which is usually applied to the back of the reflecting membrane. The proposed tunable varifocal convex micromirror device consists of a thin polymer membrane with a metal-coated reflecting surface and a distinctive electromagnetic actuator system to generate an effective actuation force. Figure 1 shows a 3D schematic of the mirror device attached to the electromagnetic actuator system, which would be fabricated separately. The DM consists of a very thin aluminum reflecting layer deposited on a thin polymer membrane that is supported by a silicon substrate frame. The electromagnetic actuator system includes a NdFeB permanent magnet (PM) and a copper coil solenoid installed in a poly(methyl methacrylate) (PMMA) microstructure and covered by an elastomeric poly-dimethylsiloxane (PDMS) membrane. This system is attached to the back surface of the mirror and contained within a silicon frame a sealed airtight chamber produced by using a specially prepared PDMS solution as glue. Finally, a base glass substrate is used to seal the NdFeB PM and coil solenoid and to hold them inside the PMMA microstructure.
2.1 Material selection
Traditionally, MEMS DMs are made of inorganic materials, such as silicon-based semiconductors. The disadvantages of using silicon-based materials for structures that are to be deformed are their high elastic moduli and high resistances to shape change. Therefore, achieving a certain stroke requires a significant actuation force, which in turn necessitates the use of a high voltage or current. For inorganic-material-based mirror surfaces, the stroke size is usually limited to less than 10 µm over a 10 mm aperture, which corresponds to an optical power of nearly 2 diopters .
As feasible alternatives to inorganic materials, organic polymers with high yield strains and low Young's moduli are frequently used as the functioning materials in MEMS devices. Polymers possess Young’s moduli about one order of magnitude lower than those of most inorganic materials, and also polymers have yield strains of 5% on average, which surpasses the fracturing limit of semiconductor materials . The low elastic moduli of these materials certainly enable deformation at actuation forces lower than those required to deform silicon devices with the same dimensions. Moreover, polymers can easily be deposited by spin-coating and can be patterned using UV exposure. Based on those characteristics, we selected an SU-8 polymer as a flexible membrane material to fabricate a DM. This selection was made because SU-8 exhibits an elastic modulus around 4.95 GPa , which is much smaller than that of silicon but is still robust enough to ensure mechanical stability in frequently deformation, as well as long-term usability in MEMS DM devices.
For the case of actuator frame, PMMA material was used to fabricate the microstructure, since it presents good moldability (melting point 130° C), is lighter than inorganic glass and silicon materials, is inexpensive, and is readily available. Most importantly, PMMA structures can be easily fabricated by computer numerical control (CNC) machining  instead of at semiconductor fabrication facilities that produce gaseous pollutants. In addition, due to their facile machinabilities, inorganic thin films, such as those containing SiO2, can be deposited onto PMMA structures to perform bonding with other micromachining devices.
2.2 Operational principles
As shown schematically in the cross-section of Fig. 1(b), the convex DM consists of an aluminum-coated thin SU-8 membrane suspended through a hollow silicon frame over a NdFeB PM and a copper-solenoid-based electromagnetic actuation system. The cavity inside the silicon frame is separated from the actuator system by an elastomeric PDMS membrane, ensuring that the cavity is an air-filled chamber. When an electrical signal is applied to the solenoid, a repulsive mechanical force, F, will be induced between the solenoid and permanent magnet according to Ampere's law and the following equations :24].
The induced repulsive force, F, pushes the magnet away, toward the air-filled chamber. Consequently, the mirror membrane experiences a force outwards due to the pressure of the compressed air. Since the compressed air pressure affects all points on the back SU-8 membrane of the mirror equally, the reflecting mirror surface should be deformed into a hemispherical shape, as shown in Fig. 1(c). Therefore, the mirror surface curvature can be varied to form a semi-hemispheric like reflector of any size. This variability may enable back-varifocal-length tunability by controlling the applied electrical signal, as shown in Fig. 1(d).
3. Device fabrication
The fabrication process used for the MEMS convex micromirror consisted of three main steps: mirror membrane fabrication, actuator microstructure construction, and mirror system component assembly, as illustrated in Fig. 2 . Mirror membrane fabrication (Fig. 2(a)) began with a polished (100) silicon (Si) wafer, on which 1 µm of thermal oxide was deposited on each side as an anisotropic etching protection mask. To open the freestanding membrane, thermal oxide was patterned and etched by a HF buffer solution on the back side to define the position for through-wafer etching. To obtain a symmetrically uniform membrane layer, air-bubble-free SU-8 polymer was deposited on the front side of the oxidized wafer by using a spin-coater. Underneath, thermal oxide was used as a buffer layer between the membrane layer and the Si wafer. Considering the metallization stress and mass and to ensure that the mirror surface would be flat in the absence of external forces, the thickness of the SU-8 layer was set to 20 µm. The SU-8 film layer was prebaked on a hotplate at 95 °C, flood-exposed to UV light to crosslink the membrane, and then hard-baked at 150 °C for 90 min. Later, the wafer was dipped into a 25% concentrated tetramethyl ammonium hydroxide (TMAH) solution at 90 °C with a wafer chuck to protect SU-8 membrane coated on the front side of the wafer until the thermal oxide underneath the SU-8 layer was exposed to the TMAH solution by through-wafer back etching. The membrane was released by removing the oxide layer by using a soft buffer oxide etchant solution. The released membrane was then taken to the evaporator to deposit 100 nm of aluminum film on the SU-8 layer, thus creating the optical reflecting surface of the mirror. The overall size of the back mirror opening was 5.5 mm × 5.5 mm, and the nature of anisotropic etching defined a 5-mm-diameter area for deformation of the mirror surface, which acted as an optical aperture.
For the actuator system (Fig. 2(b)), the PMMA microstructure was a 4-mm-high cylindrical shape with an outer diameter of 10 mm and an inner diameter of 2.2 mm, which was realized by using polymer micromachining technology. The PMMA structure frame was quickly fabricated by precision three-axis CNC commercial micromilling equipment with a high-aspect-ratio microprocessing tolerance of ± 10 μm. One side of the structure was covered by an elastomeric membrane composed of PDMS (Sylgard 184, Dow Corning, 10:1 ratio of base to curing agent) that essentially acted as an actuator membrane. For 100-µm-thick membrane preparation, the PDMS solution was spun onto a fluoropolymer layer (AF1600, Dupont)-coated hydrophobized flexible polyethylene terephthalate (PET) film, which was then cured using a hot plate. Utilization of hydrophobized film enabled easier peeling of the membrane from the PET transfer film during lamination onto the PMMA microstructure. The procedures described by Toh et al.  were followed to achieve a strong bond between the PDMS membrane and PMMA structure. Accordingly, air corona plasma treatment at room temperature was performed on both the PDMS and PMMA surfaces, and the surfaces were quickly brought into contact under pressure to ensure a permanent bond. Afterward, the PDMS-membrane-laminated PMMA microstructure was attached to the back of the separately fabricated silicon frame, which contains mirror membrane, with ensuring an air-tight chamber between two parts. A specially prepared PDMS solution (2:1 ratio of base to curing agent) has been used as glue to bond these two parts. Later on, a commercially available 2 mm × 2 mm (diameter × height) cylindrical NdFeB magnet with 0.61 T flux density as well as a solenoid with a paramagnetic copper alloy core (90 wt.% Cu, 7 wt.%Ag, and 3 wt.% Sn) and consisting of 200 turns of copper wire (Rwire = 28 Ω) were installed inside the PMMA structure from the back. Finally, a ordinary glass sheet was added by adhesive to form a base and a cover on the back side of the structure, as shown in Fig. 2(c). Photographs of the fabricated mirror membrane, actuator system, and assembled mirror device are shown in Fig. 3 .
4. Results and discussion
After successfully fabricating the membrane-deformable mirror, various aspects of the device performance were tested. The aluminum-coated SU-8 polymer membrane that formed the reflective surface of the mirror was actuated by the electromagnetic force, and the membrane deflection could be adjusted by varying the current applied to the solenoid of the actuator.
A white-light interferometer (Zygo, NewView 8300) was used to measure the deformation of the reflecting membrane as a function of the current applied to the solenoid. Figure 4 shows the color-coded surface profile images. The DC input current was increased from 0 A to a maximum of 0.8 A, and the membrane deformation was measured. As shown in the height distributions of the mirror as a function of the applied currents with a current source (PAN250-2.5A, Kikusui, Japan) in Fig. 4, the center of the mirror surface deflects significantly for input currents between 0.5 A and 0.8 A, as is evident from the color patterns, where red indicates greater displacement. Furthermore, the membrane surface maintains a uniform deflection distribution, which is necessary to ensure precise optical-focus control. The relationship between the deflection of the center of the mirror membrane and the input current is plotted in Fig. 5 (a) . The maximum displacement of 30.08 µm between the center and the edge of the membrane is achieved with an applied current of 0.8 A. The surface of the metalized SU-8 membrane is seen smooth after returning to the plane level from the bulge shape by actuation. The membrane survives such shape changing deformation due to the high yield strain and low Young's modulus of the SU-8. Figure 5 (b) depicts that the surface roughness of the mirror plane is around 40 nm excluding some small area exceptions, which is measured by Atomic force microscopy (Park NX20, Korea). Clearly, as mentioned earlier, because the residual stress and elastic modulus of the SU-8 polymer film are lower than those of other inorganic materials, such as semiconductors and aluminum, this polymer-membrane convex mirror tolerates larger strokes without fatigue or breaking and does so more easily than could the expensive silicon-on-insulator (SOI) wafer mirrors reported by Hokari et al. .
Optical focal-length control is critical in any deformable system design. The mirror surface deformation changes the focus distance, consequently changing the optical power, which is the reciprocal of the focal length or half of the curvature of the deformed mirror. Using the first-order paraxial ray approximation , the optical power P(1/f) can be calculated by the equationFig. 6 (a) as functions of applied current for an effective 5-mm-diameter mirror aperture. The optical power varies with increasing input current, indicating that the focal length of the membrane mirror can be adjusted continuously. The maximum optical power of around 20 diopters, and hence theshortest focal length of 5 cm, corresponding to a 30.08 µm deflection of the center of the mirror (as shown in Fig. 5), is achieved at an applied current 0.8 A, at which about 2 W of DC electrical power is consumed. Over-heating due to electrical power consumption in solenoid causes to burn the solenoid. Therefore, it is important of measuring the maximum tolerable temperature with the applied current at which the solenoid remains undamaged. The solenoid temperature was measured by using a thermoelectric thermometer (HI 98701, Hanna Instruments, USA). Figure 6(b) shows the temperature relation with the applied current to the solenoid. It was observed that a noiseless and burning free operation is limited till the solenoid temperature raise of around 120 °C at the application current of 0.8 A. Furthermore, the fabricated mirror device required a threshold current of 0.3 A for actuation, which may have been present due to the mechanical stiffness and residual stress of the reflecting metallic aluminum layer as well as the use of a thick SU-8 membrane. The threshold current could be significantly reduced by decreasing the thicknesses of both the aluminum layer and the SU-8 membrane.
In order to facilitate the use of the fabricated micromirror in varifocal optical systems, it should be determined how well the focusing power of the membrane mirror can be adjusted. To demonstrate the ability to control the focusing power, the mirror device was installed in a conventional microscope (SZM45, Ningbo Sunny Instru. Co. Ltd., China). The experimental setup and optical configuration are shown in Fig. 7 . The fabricated mirror was set at the microscope base and was slightly tilted (by ~10°) toward the light source. An object pattern coated onto transparent glass was placed between the objective lens of the microscope and the fabricated mirror. The object pattern, shown in Fig. 8(a) , was imaged with the light reflected from the focus-tunable mirror. Sharper images could only be obtained if the focused images have intensity stronger than that of its surroundings. The images of the pattern obtained through the microscope lens (WF10X/20) under different actuation conditions, which were caused by applying different currents to the actuator solenoid, are shown in Fig. 8. At the beginning of the mirror deformation and with a bias current of 0.4 A, the pattern in the image is defocused, as shown in Fig. 8(b). When the mirror surface curvature is around mid-level and the applied current is 0.6 A, a large area of the target pattern image is focused (Fig. 8(c)). Accordingly, when the convex mirror curvature is maximized, a small focused image is observed with an actuation current of 0.8 A, as shown in Fig. 8(c). Here, it should be mentioned that the fabricated mirror surface was not bounded by any transmissive refractive medium, such as the glass substrate that is served as the base for the actuator electrodes in the DM system reported by Huang et al. . Additional refractive media in front of the mirror surface would reflect and refract the ray away from the assigned light path and would also reduce the intensity of the incident light. Fortunately, our fabricated mirror surface is open so that light can be directly incident the surface or reflected from the surface. Hence, based on the study results, it can be concluded that this electromagnetic-actuation-based deformable convex micromirror can easily be integrated into miniature tunable optical systems.
In this study, we proposed a novel design for an MEMS-based deformable convex micromirror actuated by electromagnetic actuation for variable focus control. An aluminum-coated metalized SU-8 membrane is used as the reflecting surface of the mirror in this design, and a repulsive force generated by a permanent magnet and electromagnetic action are utilized to deform the membrane. A maximum stroke of around 30.08 µm at the center of the mirror, corresponding to a focusing power range of about 20 diopters, was achieved by applying a current of 0.8 A, yielding a maximum electrical power consumption of 2 W. Furthermore, the possibility of integrating the designed mirror miniature optical imaging systems was confirmed.
This study was supported by the BK21 Plus project funded by the Ministry of Education, Korea (21A20131600011).
References and links
1. T. Bifano, “Adaptive imaging: MEMS deformable mirrors,” Nat. Photonics 5(1), 21–23 (2011). [CrossRef]
2. N. Doble and D. R. Williams, “The application of MEMS technology for adaptive optics in vision science,” IEEE J. Sel. Top. Quantum Electron. 10(3), 629–635 (2004). [CrossRef]
3. R. Kingslake and R. B. Johnson, Lens Design Fundamentals, 2nd ed. (Academic, 2010).
4. N. Doble, G. Yoon, L. Chen, P. Bierden, B. Singer, S. Olivier, and D. R. Williams, “Use of a microelectromechanical mirror for adaptive optics in the human eye,” Opt. Lett. 27(17), 1537–1539 (2002). [CrossRef] [PubMed]
7. S. J. Lukes and D. L. Dickensheets, “SU-8 2002 surface micromachined deformable membrane mirrors,” J. Microelectromech. Syst. 22(1), 94–106 (2013). [CrossRef]
9. J. D. Mansell, B. G. Henderson, and G. Robertson, “Evaluation of polymer membrane deformable mirrors for high peak power laser machining applications,” Proc. SPIE 7861, 78160D (2010). [CrossRef]
10. P.-Y. Lin, H.-T. Hsieh, and G.-D. J. Su, “Design and fabrication of a large-stroke MEMS deformable mirror for wavefront control,” J. Opt. 13(5), 055404 (2011). [CrossRef]
11. T.-Y. Chen, C.-W. E. Chiu, and G.-D. J. Su, “A large-stroke MEMS deformable mirror fabricated by low-stress fluoropolymer membrane,” IEEE Photonics Technol. Lett. 20(10), 830–832 (2008). [CrossRef]
12. A. Liotard, F. Zamkotsian, V. Conedera, N. Fabre, P. Lanzoni, H. Camon, and F. Chazallet, “Polymer-based micro-deformable mirror for adaptive optics,” in SPIE MOEMS-MEMS 2006 Micro and Nanofabrication, (International Society for Optics and Photonics, 2006), paper 61130R.
13. O. Solgaard, A. A. Godil, R. T. Howe, L. P. Lee, Y.-A. Peter, and H. Zappe, “Optical MEMS: from micromirrors to complex systems,” J. Microelectromech. Syst. 23(3), 517–538 (2014). [CrossRef]
14. Y. Hishinuma and E. H. Yang, “Piezoelectric unimorph microactuator arrays for single-crystal silicon continuous-membrane deformable mirror,” J. Microelectromech. Syst. 15(2), 370–379 (2006). [CrossRef]
15. M. A. Helmbrecht, U. Srinivasan, C. Rembe, R. T. Howe, and R. S. Muller, “Micromirrors for adaptive-optics arrays,” in Technical Digest of the 11th International Conference on Solid-State Sensors and Actuators-Transducers ’01 Munich (IEEE, 2001), pp. 1290–1293. [CrossRef]
16. M. Pallapa and J. T. W. Yeow, “Design, fabrication and testing of a polymer composite based hard-magnetic mirror for biomedical scanning applications,” J. Electrochem. Soc. 161(2), B3006–B3013 (2013). [CrossRef]
17. 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]
18. Y.-H. Huang, H.-C. Wei, W.-Y. Hsu, Y.-C. Cheng, and G.-D. J. Su, “Optical zoom camera module using two poly-dimethylsiloxane deformable mirrors,” Appl. Opt. 53(29), H248–H256 (2014). [CrossRef] [PubMed]
19. P. R. Griffiths and E. V. Miseo, “Infrared and raman instrumentation for mapping and imaging,” in Infrared and Raman Spectroscopic Imaging, 2nd ed., R. Salzer and H. W. Siesler, eds. (John Wiley & Sons, 2014).
20. P.-H. I. Hsu, M. Huang, S. Wagner, Z. Suo, and J. Sturm, “Plastic deformation of thin foil substrates with amorphous silicon islands into spherical shapes,” in MRS Proceedings, (Cambridge University Press, 2000), paper Q8.6.1. [CrossRef]
21. L. Dellmann, S. Roth, C. Beuret, G. Racine, H. Lorenz, M. Despont, P. Renaud, P. Vettiger, and N. De Rooij, “Fabrication process of high aspect ratio elastic structures for piezoelectric motor applications,” in 1997 International Conference on Solid State Sensors and Actuators, (IEEE, 1997), 641–644. [CrossRef]
22. J. S. Mecomber, D. Hurd, and P. A. Limbach, “Enhanced machining of micron-scale features in microchip molding masters by CNC milling,” Int. J. Mach. Tools Manuf. 45(12–13), 1542–1550 (2005). [CrossRef]
23. J. R. Brauer, Magnetic Actuators and Sensors (John Wiley & Sons, 2006).
24. M. Shen, C. Yamahata, and M. A. M. Gijs, “Miniaturized PMMA ball-valve micropump with cylindrical electromagnetic actuator,” Microelectron. Eng. 85(5–6), 1104–1107 (2008). [CrossRef]
25. A. Toh, Z. Wang, and S. Ng, “Fabrication of embedded microvalve on PMMA microfluidic devices through surface functionalization,” in MEMS/MOEMS 2008 Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (IEEE, 2008), 267–272. [CrossRef]
26. G. D. J. Su, H. Toshiyoshi, and M. C. Wu, “Surface-micromachined 2-D optical scanners with high-performance single-crystalline silicon micromirrors,” IEEE Photonics Technol. Lett. 13(6), 606–608 (2001). [CrossRef]