MEMS SCANNERS AND EMERGING 3D AND INTERACTIVE AUGMENTED REALITY DISPLAY APPLICATIONS

H. Urey1, S. Holmstrom1, U. Baran1,2, K. Aksit1, M. K. Hedili1, O. Eldes1

1Ko University, Electrical Engineering, Istanbul, TURKEY 2University of Washington, Electrical Engineering, Seattle, USA

ABSTRACT

MEMS scanners have advanced rapidly during the last 20 years thanks to the excellent mechanical and optical properties offered by silicon. They have been used in various display and imaging products. The performance of high resolution and high frequency MEMS laser scanners is close to meeting the demands of full HD displays (~120 million pixels per second). Important performance metrics and the performance of various electromagnetic, electrostatic, and piezoelectric actuated MEMS scanners developed in our group are reviewed and recent improvements in the piezoelectric actuated resonant MEMS scanner is presented. Second part of the paper is about some of the emerging 3D and interactive Augmented-Reality display applications enabled by pico-projectors using MEMS scanners. Those technologies are expected to play an important role in the future of human-computer interface. KEYWORDS

MEMS scanners, actuators, pico-projectors, 3D displays, transparent displays, head-up displays. INTRODUCTION

Performance of MEMS laser scanners for displays has improved greatly over the last decade. For high resolution displays large mirror and displacement are required. Various approaches have been developed to match the requirements of the increasing demand for higher resolutions. Especially electrostatically (ES) actuated scanners have attracted much attention, but electromagnetic (EM) scanners are known to perform just as well. In recent years improvements in the PZT film processing technology have made actuation using piezoelectric films a viable competitor, but work on designs optimized for this mode of actuation has been lacking [1-3].

Our group investigated various EM [4], ES [5,6] and piezoelectric [7] actuated MEMS scanners. The MEMS scanners reviewed in this paper are limited to those developed in our group. The mechanical scanner structures in all cases were fabricated from crystalline silicon by standard silicon-on-insulator bulk processes. This paper also reports a new scanner using PZT thin film that has 40% larger scan angle (40% more pixels in a display application) than the highest performing MEMS scanner in the literature [7].

Moreover, 3D and interactive augmented-reality display applications using MEMS scanner powered pico-projectors are showing great promise as a new generation human-computer interface. Here, we also review the recently developed ideas in our group.

MEMS SCANNERS

For a convenient comparison between scanners, metrics that are independent of the actuation method are used. The two most commonly used metrics for comparing MEMS scanners are resonant frequency f (determines scan lines per frame assuming 60Hz refresh rate) and the optD-product (determines the number of resolvable pixels across the scan line) [5-7]. The best performing comparable microscanners in the literature are plotted according to these two metrics in Figure 1 along with the requirements for selected display resolutions assuming a 60 Hz refresh rate. The publications reviewed in this paper are encircled. In addition, the combined optDf product is used to compare scanners with a single number. This metric is proportional to the pixel rate enabled by the fast scanner in question. To make a comprehensive evaluation, it is also important to consider the static and dynamic flatness of the mirror surface and the mechanical reliability [8]. For high resolution display applications dynamic deformation of the mirror is often the limiting factor.

Figure 1: Comparison graph of the highest performing resonant scanners in the literature working over 15 kHz. The requirements for f and optD assume 60 Hz refresh rate and bidirectional scanning. The dotted curves indicate optDf-levels (in units of degmmkHz) to give a better understanding for how this metric relates to resolution requirements.

Figure 2 illustrates different scaners developed in our

group. Yalcinkaya et.al [4] presented in 2006 the first well-characterized and high performing EM 2D MEMS scanner with a 1.5 mm mirror that can rotate up to 26.5 and 33.5 optical scan angles for resonant horizontal

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scan at 21.3 kHz and 60 Hz the vertical linear scan, respectively. The scanner consists of two cascaded platforms and both axes are actuated by super positioned current signals in a coil on the outer frame. The outer frame scans with a linear saw-tooth motion at 60 Hz along the slow axis. The mechanical coupling combined with a very high Q factor creates a very large mirror motion along the fast axis from a small tilting of the outer frame. The dual axis performance level of the scanner enables SVGA resolution.

Figure 2: MEMS scanners with different actuators developed at Koc University, Optical Microsystems Laboratory in collaboration with Microvision Inc.

For ES resonant MEMS scanners, the performance is

typically limited by the small actuator capacitance of the structure due to limited perimeter area around the torsion mirror. Arslan et.al [5] reported an analytical solution for the mechanical-coupled configurations that provides a mechanical gain to eliminate this limitation. Moreover, by removing the fingers completely from the high velocity inner frame and transferring them to a lower velocity outer frame air damping was reduced. The 1 mm mirror of this 1D scanner has an optical scan angle of 76 at 21.8 kHz when actuated with 196 V peak-to-peak. Moreover, a 28 kHz scanner using the same actuation principle with added structures to limit dynamic deformation was published in 2011 [6]. Two separate measures were taken to reduce the dynamic deformation. Firstly a reinforcement rim was added to the backside of the mirror. Secondly an isolation frame is added between the inner and the outer frame. It suspends the mirror with two short flexures, which are orthogonal to the torsion bar. This structure is used to isolate the mirror caused by the large motion of the outer frame and the torsion bar. This mirror suspension was carried over to and improved in the PZT scanner, as can be seen in Figure 3.

The use of PZT film for actuation is gaining popularity since it can produce large forces at low actuation voltages and doesnt require any magnets or high voltages, which are the primary limitations of EM and ES actuated scanners, respectively. There has not been much focus on the optimization of the PZT film placement. Recently two types of PZT film actuated MEMS scanners with identical mechanical design were fabricated. Performance and the detailed fabrication

process of Type A were previously reported [7]. The Type B device, illustrated in Figure 3, is reported here for the first time. The device consists of two cascaded frames made out of 125 m thick silicon and is actuated by PZT thin film. The mechanical design is identical to that of Type A, but the PZT is tailored such that its static deflection profile conforms to the desired eigenmode shape, which is shown in Figures 3b and 3c. While the outer frame moves only a few microns, the inner frame makes a large rotation with the help of a large mechanical gain due to the particular shape of the mode. The ratio of the rotation angles between the two frames, i.e., the mechanical gain, is about 17 in both the model and the fabricated device. Compared to the full size PZT electrodes, 50% higher performance was estimated from the finite-element model. Moreover, by decreasing the PZT thin film area, the capacitance of the device and the power consumption were also reduced. The power consumption increases proportionally with the area, the 23% decrease of PZT electrode area therefore leads to an equally large decrease in power consumption for a given voltage.

Figure 3: (a) Device image of 125m thick Type B scanners. The total die size is 10mm x 6mm. (b) Desired resonant mode at 40kHz. The thick black lines delineate the PZT area. (c) The static deformation profile of Type B devices, which is engineered to coincide with the mode shape.

When actuated with a 25 V sine wave at 40 kHz, the

device produced an optical scan angle of opt=54.5 at ambient pressure, which is 40% higher than for the full size electrode device, as shown in Figures 1. The devices are operated at the resonant peak for each measurement. The mirror diameter of 1.4 mm, resonance frequency of 40 KHz leads to the following figure of merit values for the Type B scanner: OPTD=76.3 degmm and a OPTDf= 3052 degmmkHz. This level of performance is well above the fast scanner requirements for HD720p displays and very close to the requirements for full HD (1080p display) of 40kHz and 84degmm. This is the first demonstration of the dramatic performance improvement stemming from aligning the static deflection profile with that of the desired Eigen mode, done by only shaping the PZT film. The authors believe that the efficiency can be much further improved by further optimizing the mechanical structure for this purpose.

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PICO-PROJECTOR AND AUGMENTED REALITY SCREENS High performance MEMS scanners combined with red, green and blue lasers forms the basis of a color laser pico- projector. The intensity levels of the lasers are adjusted individually for every pixel to be displayed and the combined laser beam is scanned across the field, writing the 2D image pixel by pixel. Although only a single pixel is displayed at any given time, the persistence of vision allows to see a flicker-free image if the refresh rate is 60Hz or above.

As no imaging devices are used in the projector, i.e. lenses, the projected image is focused at all distances. Thus, the image can be projected onto any surface at any distance without the need for adjusting the focus. The use of RGB lasers provides a wide color gamut that enables projecting images with a greater range of colors. MEMS scanners and the lasers can be packed into a very small volume, making it possible for pico-projectors to be fitted into small devices, such as cell phones. As pico-projectors are hand-held consumer devices, the laser power is restricted to meet laser safety regulations. Low lumen output of the projector (typically less than 20 lumens) limits its use in bright conditions and the size of the projected image. Speckle noise due to coherent laser sources can degrade the image quality. If the screen is not a random diffuser, the speckle problem can be eliminated.

Figure 4: Pico-projector and a microlens array based transparent screen can be used for automotive HUD applications [9,11].

To overcome the power limitation and the speckle problem of the projector we designed a reflective microlens array (MLA) based see-through screen with high-gain. As the MLA is fabricated with good optical quality, there are no random variations across the surface and the image on the screen is speckle-free [9]. Moreover MLA based exit pupil expanders have been successfully demonstrated with color projectors, providing excellent color balance across the eyebox [10]. To make it a see-through screen, a partially reflective coated MLA is sandwiched between two epoxy layers that have the same refractive index. As a result the transmitted light does not get affected by the microlenses whereas the reflected light gets expanded towards the user, creating the eyebox. More detailed explanations of the MLA based see-though screens are discussed in ref. [9] and [11]. The resulting

screen can be used in any application where the information is desired to be superimposed into the users visual field of view, such as head-up displays, as seen in Figure 4. Instead of diffusing the light to a semi-sphere, as an ordinary screen would do, the MLA concentrates the incident light to a much smaller eyebox. As a result the user perceives a brighter image compared to an ordinary diffuser screen, making it possible to work with low power pico-projectors even in strong ambient light. INTERACTIVE AUGMENTED REALITY Pico-projectors equipped with MEMS scanner are promising candidates as the core of next generation stereoscopic display systems capable of augmented reality. We have recently demonstrated the capabilities of the pico-projectors in a novel stereoscopic display system. The system employs active polarization rotation in between the displayed frames and color multiplexing to present different stereo 3D perspectives to each eye in a time multiplexed manner [12]. Thus we named the technique as the mixed polarization technique. The proposed system uses single pico-projector, equipped with active polarization rotator that has 60 Hz refresh rate, as illustrated in Figure 5a. Perception of each frame by the user is depicted under Figure 5b. Note that the human visual system adopts and integrates the content into 3D full color images.

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Figure 5: (a) Sketch showing our novel stereoscopic display system using single pico projector, active polarization rotator, silver screen and polarized glasses. (b) Sketch showing how each frame is perceived by the user. Our benchmarking tests concluded that there is no color brake up or flicker problem in the system. The crosstalk is in the range of 3% for both eyes, which is acceptable. The system can work with both circular and linear polarizers. The 3D principle works also well with the HUD screen (Figure 3) as the screen maintains the polarization.

The interaction can be achieved using the pico-

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projector, AR screens, and retro-reflectors [13]. We have recently demonstrated an interaction interface that uses a pico-projector, a conventional web camera and a retro reflective foil, as illustrated in Figure 6a. Note that it is possible to integrate this interface to 2D/3D HUD screens. Our touch interface technique makes use of retro reflective property by observing the scene through a conjugate plane of the MEMS scanner. This conjugate plane is created by using a beam splitter. The same system can also be realized by placing a camera module close to the MEMS scanner as the retro-reflected beam expands very little due to diffraction and scattering. Figure 6b shows an interactive game demonstrated using a pico-projector, a webcam attached to the projector, and retroreflectors.

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Figure 6: (a) Sketch showing the touch interface using single pico projector, active polarization rotator, silver screen and polarized glasses. (b) Interactive game using retroreflector foils attached to shoes. ACKNOWLEDGEMENTS

This research is sponsored by Microvision Inc. (since 2002) and by TBTAK Grant 111E183 (since April 2012). We thank all of the current and former members of the Optical Microsystems Laboratory at Ko University (http://mems.ku.edu.tr) for their valuable contributions at different pars of this research. We thank Microvision and the researchers Wyatt Davis, Dean Brown, Mark Helsel, Mark Freeman, and Randall Sprague for valuable contributions and for being the main driver for this research during the past decade.

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Phys., 135 (2007), pp. 8691. [2] T. Iseki, M. Okumura, and T. Sugawara, IEEJ Trans.

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Hofmann, and W. Benecke, Piezoelectric resonant micromirror with high frequency and large deflection applying mechanical leverage amplification, in Proc. SPIE,2013, vol. 8612, pp. 86120I-1 86120I-8.

[4] A. D. Yalcinkaya, H. Urey, D. Brown, T. Montague, and R. Sprague, Two-axis electromagnetic microscanner for high resolution displays, J. Microelectromech. Syst., vol. 15, no. 4, pp. 786794, Aug. 2006.

[5] A. Arslan, D. Brown, W.O. Davis, S. Holmstrom, S.K. Gokce, and H. Urey, Comb-Actuated Resonant Torsional Microscanner With Mechanical Amplification, J. Microelectromech. Syst., vol. 19, pp. 936943, Aug. 2010.

[6] S. K. Gokce, S. Holmstrom, D. Brown, W. O. Davis, H. Urey, A high-frequency Comb-Actuated Resonant MEMS Scanner for Microdisplays, IEEE Optical MEMS and Nanophotonics, Istanbul, Turkey, Aug. 2011.

[7] U. Baran; D. Brown, S. Holmstrom, D. Balma, W. O. Davis, P. Muralt, and U. Urey, Resonant PZT MEMS Scanner for High-Resolution Displays, J. Microelectromech. Syst., vol. 21, pp. 1303-1310, Dec. 2012.

[8] H. Urey, D. W. Wine, and T. D. Osborn, Optical performance requirements for MEMS-scanner based microdisplays, in Proc. SPIE, Sep. 2000, vol. 4178, pp. 176185.

[9] M.K. Hedili, M. O. Freeman, H.Urey, Microstructured head-up display screen for automotive applications, Proc. SPIE 8428, (2012).

[10] H. Urey and K. D. Powell, "Microlens-array-based exit-pupil expander for full-color displays", Appl. Opt., vol. 44, pp.4930 - 4936 ,(2005).

[11] Hedili, M. Kivanc, Mark O. Freeman, and Hakan Urey. "Microlens array-based high-gain screen design for direct projection head-up displays." Applied Optics 52.6 (2013): 1351-1357.

[12] K. Akit, O. Eldes, S. Viswanathen, M. Freeman, and H. Urey, Portable 3D Laser Projector using Mixed Polarization Technique, Journal of Display Technology, vol. 8, pp. 582589, (2012).

[13] K. Akit, O. Elde, and H. Urey, Multiple Body Tracking for Interactive Mobile Projectors, in IMID2012 conference, 2012, SID/KIDS, 2012.

CONTACT *Hakan Urey, hurey@ku.edu.tr

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