PART II DEFORMABLE MIRROR
7.3 Device Fabrication and Experimental Results
The key component of the autofocus module is MEMS DM. The MEMS DM in this work consists of two parts. One is a deformable membrane and the other is a bottom electrode. A deformable membrane is a mirror made of a thin polyimide layer coated with aluminum reflecting layers and is actuated by electrostatic force. The deformable mirror was fabricated by MEMS technology. In deformable mirror part, we started from a (100) silicon wafer that is double side polished. A 6000 Å thermal oxide was grown for masking layer in wet etching process. Then, we patterned a square opening window on one side of oxide. We etched the non-PR covered region of oxide by BHF and remove PR after etching finished, as shown in Figure 7-3(b). The wafer was dipped into a 90 °C TMAH solution for about 8 hours wet etching tank and this resulted in a 25-μm residual silicon layer as Figure 7-3(c). The Al/Cr (1200 Å / 200 Å ) layer were evaporate on the flatten side. Aluminum layer will become reflection layer and chromium layer can solve the adhesion problem. The polyimide PI-2610 from Dupont® was spun at 4000 rpm for 60 seconds and was cured at 300 °C for 30 minutes in furnace to form a polymer layer on aluminum. We evaporated another Al (1200 Å ) layer for electrode and this formed a sandwich structure with polymer and previous Al layer as Figure 7-3(e).
We choose the polyimide because similar coefficient of thermal expansion to Si substrate results in low thermal strain and low residual stress. The sandwich configuration balances the residual stress on double side of polymer layer. A PR (photoresist) layer was spun and an elliptical opening was defined on it to form an elliptical outside frame of a DM as Figure 7-3(f). The opening of the photoresist can be changed accordingly to fit into optical system design. In our design, the silicon opening
opening is 20 μm and is used to define the deflection shape of the polyimide membrane.
Finally, we removed the residual Si layer by XeF2 and etched the oxide and Cr layer on reflection Al layer as shown in Figure 7-3(f). The remaining oxide on the membrane was then removed by Pad Etchant S (from AUECC, Taiwan.), which minimizes damage to metal layers unlike traditional buffered hydrogen fluoride (HF).
Regarding the bottom electrode part, a conducting Al/Cr layer was evaporated on a flatten wafer with an isolation oxide layer. In Figure 7-3(g), PR pillars were patterned to form air channels and short-circuit protection stoppers, which prevent DM from damage at snap down voltage. These air channels are important to improve the response time of the membrane because a sealed cavity underneath the membrane will slow down the moving speed of a membrane due to air damping effect. The thickness of the pillar is 12 μm so that the total gap between the polyimide membrane and the bottom Al electrode is 32 μm. This provides enough spacing for the membrane to deform to 20-diopter before snapping down caused by electrostatic force. Finally, top membrane and bottom electrode were bonded together by 100N force at 120C for 60 minutes. In order to connect the wires from the back side of DM, two small cavities were opened in TMAH etching step. Two conducting wires were attached on a DM by conductive epoxy. Due to the tensile residual stress of the polyimide membrane, the reflecting surface is very flat [4].
A cross-section schematic drawing in Figure 7-4(a) shows a flat membrane surface without applied voltage. One can see the polyimide membrane is supported by a silicon frame. The PR defines the circular opening of the reflecting mirror surface. When a voltage is applied between the deformable membrane and the bottom electrode, the membrane is pulled down by electrostatic force and forms a curved surface to focus light, as in Figure 7-4(b). The PR pillars allow the air to flow out the gap without damping the motion of the polyimide membrane. Besides, they also prevent short circuit damages when the membrane is in touch with the bottom Al electrode. We also purposely sandwiched the polyimide layer with two aluminum layers so that the
Figure 7-3 Device fabrication processes for a top polymer membrane and a bottom electrode.
addition, the polyimide is all covered with metal layers, preventing it from in contact with oxygen and moisture in air. We found this increases the membrane reliability compared with only one side aluminum coating. A fabricated device is shown in Figure 7-4(c) where the bottom electrode is underneath the membrane and cannot be seen from this angle of view. The diced chip is 6.5 mm in width with a 3.5 mm square opening.
The lower part of the diced chip is designed to be mounted on a package base later. The optical aperture of this MEMS DM is 3 mm because the photoresist define its deflection area as illustrated in Figure 7-3(f).
Figure 7-4 Schematic drawings and photos of a fabricated MEMS deformable mirror device.
applied voltage is high, the power consuming is very low due to capacitor structure with negligible current (~nA statically). The optical power of the polymer DM is about reciprocal of half of the radius of curvature of the mirror surface. The diopter is calculated according to the first-order paraxial rays approximation [12], the radius of the curvature is calculated by the expression
2
1 6 x P
D
(1.12)
where P is the focusing power in unit of diopter (m−1), D is diameter of the polymer membrane, and x is the deformation of the mirror from the center to the edge. The clear aperture is 3 mm in diameter and maximum center displacement is around 12 μm so the diopter is approximately 20 m−1 at 150V applied voltage. The experimental results are shown in Figure 7-5.
Figure 7-6 illustrates the 3D layout of an autofocus module consisting a MEMS deformable mirror. Unlike traditional camera systems that lenses are mounted on tubes in sequence, optical components are mounted in shapes of lollipop (e.g. solid lenses) or pillar (e.g. fixed mirror and deformable mirrors) in order to be mounted on the base, as shown in Figure 7-6. A cover is then placed on top of the base, and the cover is also used to block out auxiliary lights. The whole system will thus be approximately 6.7 mm in thickness, 11.5mm in length, and 10mm in height. Because the advantage of reflective optics is that lights can be folded within the system, the long total-track-length system can be realized in a thinner module. We believe that the imaging system can be even further miniaturized in near future with careful optimization.
We used hot forming presses technology to manufacture solid lenses made of Arton, which is suitable for mass fabrication as well as MEMS deformable mirror. The free shape lens is manufactured by mechanical milling process. However, this part can be made by mode injection method in the future. Lens 1 and free shape mirror are mounted on the cover as in Figure 7-7(a), while lens 2, stop, and MEMS deformable mirror are plugged into the base shown in Figure 7-7(b). Traditional camera systems have lenses
Figure 7-6 Assembly 3D drawings of an autofocus module
Optical components are formed in lollipop shape fixtures, such as Lens 2 in Figure 7-6 A module is completed when the cover is place on top of the base. This system is only 5.4 mm thick in optical lays out and 6.7 mm in thickness after package. The final assembled module is shown in Figure 7-7(c). The image sensor is not shown in this optical module because we used off-the-shelf image sensor which is not customized to our package.
7.4 Sharpness Function and Experiment Result
In order to realize the auto-focus function, it is very important to quantify the degree of clarity. There are many sharpness functions to calculate the clarity called focal value. The one we adopt is called Tenegrade method. From the simulation of image analysis, we can calculate the focal value changes with the optical power in a particular position. Figure 7-8(a) shows the focal value of image analysis simulation calculated by
Figure 7-7 (a) Cover with a freeform mirror, (b) base with polymer MEMS DM and (c)final Assembly module.
it will focus on 550 mm with MEMS DM actuated and 44 mm without MEMS DM actuated. Combining focus-varying system and sharpness function, an auto-focus control system as shown in Figure 7-8(b) can be constructed with searching algorithm.
Figure 7-9 shows the experimental results of optical auto-focusing function in this module. The incident light is within ± 26 degree, and focus is on a 6-mm-diameter-circle in the image plane. However, the size of 2M pixel sensors is 3.52 mm x 2.64 mm, which is covered within a 4.4-mm-diameter-circular. In other words, the edge of image circular is chopped due to the size of sensor. When object was placed in the far distance, the clear image happened without actuating the MEMS DM. The far object (~160 mm in distance, 80 mm in height) shows few Chinese characters and English words written as “made from natura”.On the other hand, when object was at the near distance (~78 mm in distance, 40 mm in height), the clear image happened when the polymer MEMS DM was actuated. Numbers, 6 and 3 can be seen clearly. In principle, the autofocus function is achieved in this module. The images, however, were not as sharp as we originally predicted. This may be due to no anti-reflection coating on the lenses surfaces, misalignment of each component and/or partially imperfect solid
Figure 7-8 Pictures of near and far objects
Nevertheless, the concept of using MEMS deformable mirrors in a folded optical design can be realized in a thin package and demonstrated in the work.
7.5 Conclusion
In this work, we have presented a innovative auto-focusing system featuring a reflective design form and an organic polymer membrane deformable mirror. As the simulation results shown above, it can peform the optical autofocus function without stepping motors and moving lenses. The over size of the packaged module is 6.7 mm in thickness, 11.5 mm in length, and 10 mm in height. The power consumption is low becuase the polymer MEMS DM is actuated by elecctrotstatic force. We will also work on system improvement and add zooming function in the future. The MEMS deformable mirror indeed has focus-varying function, and the optical image system we designed also provide the ability to focus on object in different distances. This work demonstrates that a MEMS deformable mirror and folded optical system design can
Figure 7-9 Pictures of near and far objects
References
[1] Hsin-Ta Hsieh, Design and fabrication of compact optical devices: organic deformable mirror and microlens arrays, published Phd dissertation, National Taiwan University, Taipei, Taiwan (2010).
[2] R. C. Gutierrez, T. K. Tang, R. Calvet, and E. R. Fossum, “MEMS digital camera,”
Proc. SPIE 6502, paper36, 1–8 (2007).
[3] S. Kuiper, and B. H. W. Hendriks, “Variable-focus liquid lens for miniature cameras,” Appl. Phys. Lett. 85(7), 1128–1130 (2004).
[4] H. Ren, Y. Fan, S. Gauza, and S. Wu, “Tunable-Focus Cylindrical Liquid Crystal Lens,” Jpn. J. Appl. Phys. 43(2), 652–653 (2004).
[5] J. L. Wang, T. Y. Chen, Y. H. Chien, and G. D. Su, “Miniature optical autofocus camera by micromachined fluoropolymer deformable mirror,” Opt. Express 17(8), 6268–6274 (2009).
[6] D. Wick, “Active Optical Zoom System,” US patent 6,977,777 (2005).
[7] W. Smith, Modern Optical Engineering: the design of optical systems, 2nd, (McGraw-Hill, 1990), pp.436.
[8] E. Hecht, Optics, 4nd (Addison Wesley, 2001).
[9] Boston Micromachines Corporation, http://www.bostonmicromachines.com/.
[10] V. T. Srikar, and S. M. Spearing, “Materials selection for microfabricated electrostatic actuators,” Sens. Actuators A Phys. 102(3), 279–285 (2003).
[11] K. Seidl, J. Knobbe, and H. Grüger, “Design of an all-reflective unobscured optical-power zoom objective,” Appl. Opt. 48(21), 4097–4107 (2009).
[12] J. Wang, T. Chen, C. Liu, C. Chiu, and G. Su, “Polymer Deformable Mirror for
Chapter 8 A Low Voltage Deformable Mirror using Ionic-conductive Polymer Metal Composite
Deformable mirror is a crucial component of adaptive optics. It can be used to vary the optical power of an optical image system, such as auto-focus and optical zoom function. An electrostatic type MEMS deformable mirror used in an optical image system has been promoted recently. However, high voltage in the range of hundreds volts is a serious concern for electrostatic type MEMS deformable mirror.
Ionic-conductive polymer metal composite (IPMC) is a polymer actuator with the advantage of large deformation under low actuation voltage. It is a sandwich structure composed of two metal electrodes and a layer of polymer film. The hydrated cation inside the polymer film moves toward the cathode. Because of the migration of ions and water inside the film, volume expansion and contraction induce the deformation of IPMC. In this work, we design the IPMC type deformable mirror that is simulated by finite element method and then demonstrate its focus-varying function. It requires less than 5 volts to achieve 20 diopters.
8.1 Introduction
There are several methods to achieve auto focus function. These methods could be separated to two types, refractive and reflective types. Conventional camera module with moving lens sets is refractive type. It can change optical power using motors.
However, it is volume and power consuming. Liquid lens reported by B. Berge et al. [1]
is another solution of refractive type. It adopts a bi-liquid lens that can deform the shape of the interface between two immiscible liquids by the electro-wetting method to change its optical focusing power. However, the approach may suffer from some difficulties such as density variation with temperature, optical axis misalignment caused by gravity, and hand shaking. The other proposed solution is a reflective-type design, which adopts a MEMS deformable mirror (MEMS DM) to vary the optical power of the camera module. By changing the surface of a deformable mirror, the optical power of the module changes to designated degrees. MEMS DMs are made by rigid silicon-based materials [2-3] and their optical power adjustment capability is less than one diopter.
This results in large optical layout space and is not suitable for mobile devices. Besides, the actuating voltage is reported up to 300 volts.
On the other hand, Ionic Polymer-Metal Composite (IPMC) is a promising alternative material to be used in fabricating DMs because of its ability to exhibit large bidirectional actuation with low applied voltage. IPMC is a sandwich structure composed of two layers of metals as electrodes and a layer of Nafion® inside. It can be used as an actuator or a transducer which was discovered by Oguro et al. [4] form the ion-exchange film of fuel cell. IPMC shows great potential for many applications, such as artificial muscles, robotic actuators, MEMS actuators, pressure sensors, micro pumps
presence of low applied voltage. There are several models that have proposed to describe the principle of IPMC’s actuation: physical models and grey box models. Many researchers have built various physical models, or mathematical models, based on the fundamental mechanisms of IPMC which involved many deep physical parameters.
However, it is too difficult to be applied on arbitrary shapes or patterns. So, the grey box models provide an easier way to reach the purposes. Grey box model consist of simple physical laws and use some parameters experimentally measured by curve fitting. There are several grey box models have been proposed. [5-6]
In this work, we discuss about reflective-type deformable mirror designs using IPMC, which has the advantage of high deformation with low actuation voltage, by Finite Element Method (FEM) simulations. The principle of IPMC would be introduced at first. Then the FEM model of IPMC in cantilever beam shape would be discussed.
The grey box model is simplified to a double-virtual layers Nafion® cantilever beam with metal layers outside. One of the beams is contractive and another is expansive. The actuation force is equalized to normal surface compressive and tensile pressures on each surface of elements. This model is combined with important physical properties of IPMC: Young’s modulus and poisson’s ratio. Although the physical properties are quite difficult to determine experimentally due to the complexity of the morphology of IPMC, for the needs of further designs, we chose the physical properties of metal and Nafion®
separately for which was reported by Jiang Yu Li et al. [7] The voltage-deformation experiment data shows the agreement with the FEM model. Finally, we propose a deformable mirror design in simulation which would manipulate the optical power to
8.2 Principle of IPMC
Ionic polymer-metal composite (IPMC) is a sandwich structure composed of two layer of metals as electrodes and a layer of Nafion® inside. Figure 8-1(a) shows the schematics of the electro-osmotic migration of hydrated counter-ions within the IPMC network. Figure 8-1(b) shows the molecular formula of Nafion® corresponding to Figure 8-1(a). Nafion® can be separated into two chains. The main chain builds the backbones to determine the mechanical strength and the side chain terminated by ionic groups, e.g., SO3- for cation exchange. Main chain is hydrophobic and side chain is hydrophilic, the chains form cluster networks (or nano-channels) with side chain inside.
So, the hydrated cations, or hydrated alkali metal molecular typically sodium, can transport in the hydrophilic cluster networks. The underlying principle of actuation for IPMC is that when the electric field is applied, the hydrated cations move through clusters toward cathode so the volume will expand near cathode side and contract near anode side. Finally the IPMC will bend toward anode.
8.3 FEM simulation model of cantilever beam
According to the actuation mechanisms, the real actuation force should be symmetric distributed inside IPMC in the manner of positive stress linear increasing
(a)
(b)
Figure 8-1 The schematics of the IPMC network and (b) the molecular formula of Nafion.
positive, or compressive, and another is negative, or tensile. Figure 8-2 shows this simplified stress field distributed model which would be implied to the FEM model. The total thickness is 2h and Me denotes the bending moment caused by the internal stress.
Figure 8-3 shows the structural model in ANSYS® . The IPMC could be divided to four parts, two layers of metallic electrodes, a layer of Nafion® with constant compressive surface force, and a layer of Nafion® with constant tensile surface force. Figure 8-4 shows the element model in ANSYS® . The red arrow depict that the compressive surface force on upper Nafion® elements, and blue arrow depict that the tensile surface force on lower Nafion® elements. All the stresses are applied normal to the element surface. The element type used in ANSYS® is SOLID45. The physical properties are as follows: Young’s modulus for platinum is 168GPa, and for Nafion® is 275MPa.
Poisson’s ratio for platinum is 0.38, and for Nafion® is 0.487. All the physical parameters are listed in Table 8-1.
Figure 8-2 The simplified model with constant stress field which would be implied to the FEM model.
Figure 8-4 ANSYS® element model of IPMC Figure 8-3 ANSYS® structure model of IPMC
From the FEM simulation which varied the length (L) and width (w) of IPMC and the half thickness (h) of Nafion® with constant surface force (P = 100 Pa), as shown in Figure 8-5, we could express the relation of surface force with tip displacement as
Where Sansys is the simulated tip displacement, L is length, h is half thickness of Nafion, and P is the surface force, C1 is a constant number. Then, compare with the result from Sia Nemat-Nasser et al., we could derive the relation of the surface force, which is the assumed virtual constant stress in our simplified FEM model, with small applied
Where φ0 is the applied voltage and C2 is constant which depends on the characteristic of Nafion® . This reveals that the surface force just depends on the thickness with applied voltage. For different process conditions, the characteristic of Nafion® is different, and then the constant C2 is different too. According to equation (1.14), we can simulate the deformation profile of arbitrary shaped IPMC with given boundary
Table 8-1 Physical parameters of different materials
Table 8-1 Physical parameters of different materials