Along with the progress of image sensors in recent years, fix-focus cameras on mobile electronic devices do not fulfill consumer needs. With the size of mobile devices getting smaller and smaller, the displacement-to-thickness ratio is getting larger, and that makes mechanical motor systems difficult to be packaged inside cameras to achieve autofocus function. We propose a design using micromachined fluoropolymer deformable mirrors rather than traditional mechanical motor. With low color dispersion and adjustable power range of 20-diopter, deformable mirrors can be integrated into optical module and are well suitable for miniature optical auto-focus camera.
©2009 Optical Society of America
Mobile electronic devices are playing an important role in communication for daily life and have become necessities for people. Along with the progress of image sensors in recent years, cameras with auto-focus function on mobile electronic devices have entered the market; however, the do not fulfill consumer needs. Traditionally, the focusing function is achieved by moving lenses in most cameras. The moving displacement is required from 0.25mm to 2mm for auto-focusing . With the size of mobile devices getting smaller and smaller, the displacement-to-thickness ratio is getting larger, and that therefore makes mechanical motor system difficult to be placed inside a mobile devices, such as cellular phone. Besides, miniaturization of those systems is challenging due to assembling small components. Furthermore, with increasing surface-to-volume ratio, movement gets harder because of mechanical friction. With those constraints, conventional motor systems might reach a bottleneck soon in the near future.
There are two solutions for changing the focusing power without using mechanical motors. One is refractive-type design using liquid lenses  that change their interface shape between two immiscible liquids by electro wetting method. But this approach may suffer from some physical problems such as density mismatching, temperature, shaking, and tilt. The other is a reflective-type design by adopting deformable mirrors, which has been proposed by other research groups [3-4]. Reflective optics is attracting more attention as a means of achieving long light path within thin form factor because of folded light-path design, as shown in Fig. 1. In addition, a deformable mirror can be easily incorporated into reflective optical systems.
Deformable mirrors have been used for a long time as wave front correctors for astronomy . While deformable mirrors have the advantages of low color dispersion and high reflectivity, typically the optical power range of commercially available devices is relatively small because deformable mirrors are made with brittle inorganic materials, such as silicon. In this paper, we present a micromachined fluoropolymer deformable mirror that can operate over a larger power range of 20-diopter, which is two orders of magnitude higher than silicon/silicon nitride devices. It can be integrated into an optical lens module. The membrane is mechanically supported at its periphery and suspended at a fixed distance above an actuating electrode plane. We will first describe a micromachined deformable mirror after the introduction. To implement this micromachined device into a reflective optical system, optical design is explained in Section 3. Finally, experimental results are presented to prove that a micromachined fluoropolymer deformable mirror may be used as a compact auto-focus camera.
2. Micromachined Fluoropolymer Deformable Mirrors
It is reported that the yield strain of an organic polymer is around 5%, which far exceeds the breaking limit of silicon-based materials, and the Young’s modulus is about one order of magnitude lower than that of most inorganic materials. Moreover, some polymers have excellent chemical resistance during wet etching and may be coated at room temperature either by spin coating or injection printing, which requires less complicated fabrication processes as compared with standard semiconductor production lines. Based on those characters, an organic polymer is an ideal candidate for deformable membrane applications. We selected amorphous fluoropolymer, CYTOP™, from Asahi Glass to fabricate the organic membrane for the deformable mirrors. CYTOP™ lies in the field of fluoropolymers that have excellent chemical resistance to TMAH (tetra-methyl ammonium hydroxide) wet etching.
The fabrication process of a micromachined fluoropolymer deformable mirror is illustrated in Fig. 2. Starting with a 4-inch (100) silicon wafer, 300-nm silicon dioxide was thermally grown on a wafer as a wet etching protection mask. A 6 mm × 6 mm square opening was patterned on thermal oxide and etched by buffer HF at the backside of the silicon wafer, as shown in Fig. 2(b). The silicon wafer was then anisotropically etched in the solution of Tetramethyl Ammonium Hydroxide (TMAH) at 90 °C, that the silicon etching rate and surface roughness are sensitive to the temperature of TMAH. The etching depth of the silicon was time-controlled to left about 30 μm silicon membranes before etching through the wafer.
The 100 nm aluminum was then deposited by e-beam evaporator on the front side of the silicon wafer with 20 nm chromium seed layer. A 2-μm organic thin film was then spin-coated and baked at 350°C in an oven to dry solvents out in the step of Fig. 2(d). Afterwards, the remaining silicon was etched by DRIE (deep reactive ion etching) and stopped at thermal oxide layer. Thermal oxide and chromium were removed by pad etchant (Pad Etch 4 from Ashland) and Cr etchant (Cr-7T). Pad etchant, which contains acetic acid, ammonium fluoride, surface tension and water, minimizes damages to metal layers. An aluminum reflection layer was undamaged during the etching process and has good reflectivity at visible light wavelength, as shown in Fig. 2(f). Finally, a dielectric tape was used as a spacer to bond a silicon frame and a bottom electrode. The polymer membrane was 6 mm wide and 2 μm thick. The gap between the polymer membrane and the bottom electrode was approximately 70 μm and a 4.5 mm circular opening of the dielectric tape spacer was punctured by a perforator so that the clear aperture of a micromachined fluoropolymer deformable mirror had a 4.5 mm circular aperture.
The schematic drawing of an organic deformable mirror is shown in Fig. 3(a). An aluminum-coated polymer membrane is supported by a silicon frame. A silicon bottom electrode coated with aluminum is attached to the silicon frame by a dielectric tape spacer. A spacer has a circular opening to define the deformation shape of the polymer membrane. When a voltage difference is applied between a silicon bottom electrode and an aluminum-coated polymer membrane, the electrostatic force will pull down the CYTOP™ membrane toward the bottom electrode due to its mechanical flexibility. A polymer membrane works as a reflective mirror surface to focus the light variably by adjusting the voltage difference. Figure 3(b) shows a photograph of the fabricated device.
The focusing power is critical for an optical system design. We conducted an experiment between optical power versus applied voltage. The polymer membrane was actuated by the electrostatic force and deformation of the polymer membrane can be adjusted by changing voltage difference continuously. The optical power is about reciprocal of half of the radius of curvature of the mirror surface. Using the first-order paraxial rays approximation, the optical power is related to the deformation of the mirror from the center to the edge. The optical power and deformation versus the applied voltage is plotted in Fig. 4(a). The maximum optical power achieved is around 20-diopter when 160 volts is applied, corresponding to 27-μm deformation at the center of the membrane. The optical power of an organic deformable mirror could be adjusted continuously. Even though the high voltage difference is required, the current through the device is almost negligible (~nA) so that the power consumption is quite low. Low power consumption makes it a potential candidate for mobile electronic devices. There is no permanent damage to the membrane when it is snapped down because the yield strength of a polyimide membrane survives such deflection. The surface of the polymer membrane is smooth due to the tensile residual stress of the polyimide and the surface roughness is measured to be around 20 nm by a white light interferometer, as in Fig. 4(b)
To observe how well a micromachined fluoropolymer deformable mirror could adjust its focusing power, we shined a collimated laser beam on it and recorded the results on the laser beam profiler. The laser beam profiler was located approximately at the focal distance when the deformable mirror was actuated to a concave shape to focus a light. Figure 5 shows the beam profiles when the mirror was not actuated and actuated, respectively. The laser beam was indeed converged by the concave deformable mirror when it was actuated, and the focus beam spot size is about 2.5 times smaller than its original size.
3. Optical Design with a Fluoropolymer Deformable Mirror and Experimental Results
To simplify the design, we would like to use only one single lens for the concept proof. The optical system design is shown in Fig. 6. The light is reflected by a prism and is focused through an off-the-shelf convex-plano lens. A micromachined fluoropolymer deformable mirror is located next to the convex-plano lens to reflect light back and form a clean image on an image sensor at the back of the convex-plano lens. The size of the plano-convex lens is 10 mm in diameter and the focal length is about 24 mm. From this design, we can see that the reflective optical system design can incorporate a deformable mirror and pack a long light path in a much compact form factor. We use a non-sequential system to simulate our optical imaging system in Fig. 6 because total internal reflection happens on the hypotenuse surface of the prism.
To describe how well this system can perform auto-focus function quantitatively, we calculated the “focus value” of the image focused on sensors to describe the sharpness of an image taken by this optical system . Focused images can always be viewed as sharper images, which have stronger intensity. Thus, the focus value could be used to describe the sharpness of images. In the Tenengrad function, two operators are adopted to quantify the sharpness of each pixel in horizontal and vertical directions. We can judge the sharpness of an image in horizontal and vertical direction by performing multiplication on pixel intensity matrix element by the element. After multiplying, the strength gradients of horizontal and vertical can sum up to express the sharpness of each pixel. In other words, the higher a focus value means the sharper an image. With the sharpness of each pixel, we can derive the sharpness of the image. As shown in Fig. 7, we obtained the right diopter needed for this auto focusing system. From this simulation result, we find that the deformable mirror is not actuated too much when the object is place far away. If the object is placed closer to the optical system, the image tends to form behind the sensor plane. One needs to deform the micromachined fluoropolymer mirror more to provide additional optical power to bring the image back to the sensor plane.
The focal length of solid lens equals 24 mm. Therefore, the effective focal length of total system equals 12 mm because the rays passing through the lens twice. We placed an object 120 mm away from the lens representing infinity incident light. And a near object was located40 mm away from the lens, shown in Fig. 8.
The components in the experiment are shown in Fig. 8. On the right is our optical imaging system with a CMOS imaging sensor, the deformable mirror was attached to the solid lens and the prism is on top of the solid lens. In the middle is a name card as a near object with “Jen-Liang Wang” on it. On the left is a toy train with “HARIBO” shown on the head of the train. At the beginning, when the mirror was not actuated, the far object (the train) with “HARIBO” can be seen clearly while the near object, the name card with “Jen-Liang Wang”, is blurred as show in Fig. 9(a). After applying voltage to the deformable mirror, the effective focal length of the system changes. The focal point moves forward so that the near object can be clearly seen. We can see that there are still some aberrations at the edge of the images. This can be fixed by custom-designing the lenses. We demonstrate that the deformable mirror is feasible as vari-focus devices in camera lens module without moving component. This part is also shown in the video file submitted.
To evaluate the response time of a fluoropolymer deformable mirror, we made use of a pair of fiber collimators to transmit optical signals from one fiber collimator to the other via the reflection of a deformable mirror. The maximum optical signal was aligned while a deformable mirror was actuated to its largest stroke. When a deformable mirror was actuated under atmosphere, the 4.5 mm circular membrane deformed and optical signal reached its maximum within 0.4 msec, as shown in Fig. 10. The 0.4 msec response time was determined by its maximum deflection to focus optical signal in a fiber and we observed no resonance over the surface area. In order to operate at a lower voltage, decreasing spacer thickness is the simplest method, but it will restrict the mirror deflection range. Due to the snap down effect, a membrane actuated by electrostatic force cannot deflect over half of a spacer thickness. Spacer thickness should be carefully tailored for different applications. In this work, we chose a 70-μm acrylic adhesive spacer and achieved a deflection of 27 μm, which is considered large in comparison with most micromachined deformable mirrors. Meanwhile, we can keep the applied voltage within reasonable range.
In the paper, we propose a novel design concept for making a miniature camera module for mobile electronic devices with deformable mirrors. The properties of the deformable mirror are discussed. The operated power range of 20-diopter can be achieved by applied voltage of 160V. This range is two orders of magnitude higher than commercially available micromachined deformable mirrors. The surface roughness of the organic membrane is measured to be less than 20 nm, corresponding to less than λ/20 in the visible light range. We demonstrated the concept of auto-focusing camera with a micromachined fluoropolymer deformable mirror.
The authors would gratefully like to acknowledge National Science Council of Taiwan for their funding support under NSC 96-2221-E-002-165-MY3.
References and links
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