IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. …...interferometricmodulator(IMod)technologywillbecoveredin Section III. An IMod chip is composed of MEMS Fabry–Pérot etalons

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009 1057

The Evolution of MEMS DisplaysChun-da Liao and Jui-che Tsai, Member, IEEE

Abstract—Due to the advancement of microoptoelectromechan-ical systems and microelectromechanical systems (MEMS) tech-nologies, novel display architectures have emerged. One of themost successful and well-known examples is the Digital Micromir-ror Device from Texas Instruments, a 2-D array of bistable MEMSmirrors, which function as spatial light modulators for the projec-tion display. This concept of employing an array of modulatorsis also seen in the grating light valve and the interferometricmodulator display, where the modulation mechanism is based onoptical diffraction and interference, respectively. Along with thistrend comes the laser scanning display, which requires a singlescanning device with a large scan angle and a high scan frequency.A special example in this category is the retinal scanning display,which is a head-up wearable module that laser-scans the imagedirectly onto the retina. MEMS technologies are also found inother display-related research, such as stereoscopic (3-D) displaysand plastic thin-film displays.

Index Terms—Grating light valve (GLV), microelectromechan-ical systems (MEMS) display, retinal scanning display (RSD),scanning mirror.

I. INTRODUCTION

THE WIDESPREAD application of microelectromechan-ical systems (MEMS) technologies to optical fiber

communication has been one of the major driving forcesof the development of microoptoelectromechanical systems(MOEMS) devices. Optical MEMS components have beenwidely seen in variable optical attenuators [1]–[7], wavelength-selective switches [8]–[10], 2-D and 3-D optical cross connects[11], [12], dynamic gain equalizers [13], [14], and many othermodules in fiber optic communication. Other MOEMS appli-cations include endoscopes for medical imaging [15]–[18] andadaptive optics [19], [20].

MEMS displays form another circle of growing interest.The most well-known product is probably the Digital LightProcessing (DLP) projector based on the Digital Micromir-ror Device (DMD), developed by Texas Instruments in 1987[21]–[23]. The DMD consists of hinge-mounted electrosta-tically actuated bistable micromirrors suspended on a chipas shown in Fig. 1 [24]. In a DLP projection system, eachmicromirror corresponds to an image pixel and can control theprojected pixel brightness by switching between two tilt states.

Manuscript received April 14, 2008; revised August 20, 2008. First publishedOctober 31, 2008; current version published April 1, 2009. This work wassupported in part by the National Science Council of Taiwan under Grant NSC96-2221-E-002-198-MY2 and in part by the Excellent Research Projects ofNational Taiwan University, 95R0062-AE00-06 and 97R0062-07.

The authors are with the Graduate Institute of Photonics and Optoelectronicsand the Department of Electrical Engineering, National Taiwan University,Taipei 10617, Taiwan (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2008.2005684

Fig. 1. Schematic sketch of the DMD shows that it consists of micromirrors,springs, hinges, yoke, and CMOS substrate, and the deflection angle of themicromirror is ±10◦. (Picture courtesy of L. J. Hornbeck. Reprinted from [24]with permission. © 1998 IEEE)

Fig. 2. Schematic system of DLP projector. (Picture courtesy ofL. J. Hornbeck. Reprinted from [24] with permission. © 1998 IEEE)

In the ON state, the light is guided through the projection lensto create a bright pixel, whereas in the OFF state, it is directedinto a light absorber for a dark pixel. This working principle isshown in Fig. 2 [24].

The DMD chip consists of millions of micromirrors. There-fore, a high-yield requirement in the fabrication process hasto be met to ensure a minimum number of “dead pixels.” Thearchitecture of the laser scanning display (LSD) addresses thisissue by using a single MEMS device to scan a color laser beam

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1058 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 4, APRIL 2009

on the screen. The LSD working principle resembles that of thetraditional cathode-ray tube (CRT) monitor, except that a laserbeam is scanned instead of an electron beam. Section II willcover the use of MEMS scanning mirrors for this purpose.

Besides using MEMS mirrors which are reflective-type de-vices, diffractive-type optical components can also performlight modulation. Section III will describe the grating lightvalve (GLV) display technology, which belongs to this category.GLV was proposed by Solgaard et al. [25] and is currentlyowned by Silicon Light Machines. It consists of fixed andmovable ribbons for each pixel and functions as a tunablegrating. Modulation based on optical interference known asinterferometric modulator (IMod) technology will be covered inSection III. An IMod chip is composed of MEMS Fabry–Pérotetalons where the role of each element is a subpixel of red,green, and blue color [26]. The technology was developed inthe company of Qualcomm (originally in Iridigm Display Cor-poration, by Miles, Mark W.). Another interferometric cavity-based MEMS display that will be discussed in Section IIIis proposed by Taii et al. of the University of Tokyo, Japan[27], [28]. The device employs a flexible plastic thin filmpolyethylene naphthalate (PEN) to adjust the resonant lengthof the cavity in order to switch the light in each pixel. Thebasic working principle is also based on the same Fabry–Pérotinterferometer as the IMod technology. Section IV will discussanother interesting matter, which is the MEMS stereoscopicdisplay, also known as the MEMS 3-D display [29], [30].

The MEMS-based retinal scanning display (RSD), alsoknown as virtual retinal display, will be included in Section V.Strictly speaking, RSD is considered as a type of LSD. How-ever, due to its uniqueness, we have allocated a separate sectionfor it. Microvision Inc. has been devoted to the development ofMEMS RSD [31]–[35]. The RSD is a head-worn display, whichdirectly laser-scans the image onto the retina of the human eyewith a biaxial MEMS mirror.

The paper is organized as follows. Section II will discuss theLSDs; displays using arrays of spatial light modulators will becovered in Section III; MEMS stereoscopic display and RSDwill be introduced in Sections IV and V, respectively.

II. LSDs

In comparison to the DMD, the LSD requires a relativelylower yield rate and is easy to integrate with the IC fabricationprocess. However, the LSD requires a micromirror scannerwhich operates at a high scanning frequency and a large scanangle. The scanning frequency of a micromirror scanner mustachieve a nominal frame rate (e.g., 60 Hz) for the proper images[36]. Also, a large scanning angle is required to provide a wide-range projection. Hence, more and more micromirror scannersfor LSD were proposed and usually equipped with electrostaticor electromagnetic actuators [24], [37]–[43]. Moreover, theconsideration of the surface roughness of scanning micromir-rors is crucial so as not to distort the wavefront of the reflectedlaser beam [44], [45].

Utilizing a comb-drive actuator is a way to realize a largerscanning angle, fast scanning rate, and voltage-controlled prop-erty. A gimbal-less two-axis micromirror scanner driven by

Fig. 3. Optical image of gimbal-less two-axis micromirror scanner.(Picture courtesy of V. Milanovic et al. Reprinted from [46] with permission.© 2007 IEEE)

electrostatic comb-drive actuators is shown in Fig. 3 [46]–[50].This novel structure was proposed by Milanovic et al. of theAdriatic Research Institute, Berkeley. The schematic workingprinciple is shown in Fig. 4 [50]. The central plate, which isa piston-tip-tilt platform with 3 DOF, can serve as the mirroritself [50], or a separate mirror can be bonded onto the platformto achieve a high fill-factor scanner array [47], [50]. The tip-tiltplatform is actuated by four rotators which are operated in pairsand powered by electrostatic comb-drive actuators. The outsidelinkage is propelled by the rotator and connected with theinside linkage by the rotation transformer. The optical scanningangle of the micromirror scanner with a diameter of 0.8 mmis 16◦ for each rotation axis [46]. In this display system, themicromirror scanner can be driven from dc to about 4 kHzwithout resonance.

A mapping between input voltage and output scanning anglefunctions as a point-to-point control. Utilizing a fisheye lens infront of the scanning micromirror can improve the projectionangle to more than 100◦ [46] and expand the viewing scope.

In addition to the electrostatic actuator, the electromagneticactuator is also suitable for the LSD and can exert relativelygreater torque than an electrostatic one at small voltage. How-ever, the system volume is increased due to the external per-manent magnet. In order to minimize the system size, Ji et al.of LG Electronics Institute of Technology proposed a 2-Delectromagnetic scanner which is a relatively compact structureas shown in Fig. 5 [51]. This is a balanced gimbal structure ac-tuated by a radial magnetic field with a single turn electroplatedCu coil in each frame. The scanning micromirror is mounted onthe inner circular frame; the outer circular frame is connectedwith the inner circular frame. Under the micromirror, two con-centric permanent magnets produce the radial magnetic field toproduce a Lorentz’s force while the current passes through themetal coil. The schematic of the working principle is shown inFig. 6 [51]. The maximum optical scanning angles are 8.8◦ inhorizontal and 8.3◦ in vertical directions. The scanning ratesare 60 Hz in the vertical axis (slow scan) and 19.1 kHz inthe horizontal axis (fast scan). Therefore, the device is suitablefor a raster scanning display system where the input sinusoidalsignal and the sawtooth signals are sensed by the horizontal andvertical axes, respectively.

In addition, the biaxial electromagnetic actuator is alsodesigned to use a magnetic field laterally oriented 45◦ to the


LIAO AND TSAI: EVOLUTION OF MEMS DISPLAYS 1059

Fig. 4. Working principle of micromirror rotation. (Picture courtesy of V. Milanovic et al. Reprinted from [50] with permission. © 2004 IEEE)

vertical and horizontal scan axes [52]–[55]. This actuator alsohas the advantages of large torque and compact size. The RSDuses this architecture to construct a head-worn microdisplay. InSection V, these electromagnetic actuators will be discussed indetail.

III. DISPLAYS USING ARRAYS OF

SPATIAL LIGHT MODULATORS

A. GLV Display

The GLV display and the MEMS cavity array display willbe covered in this section. The key idea of the GLV displayis the use of movable ribbons to modulate the phase of lightso that it can be regarded as a MEMS tunable phase grating.Three pairs of ribbons form one pixel of a GLV. Each pair of

ribbons consists of a fixed and a moveable strip. The schematicstructure is shown in Fig. 7 [56], [57]. The GLV ribbons arefabricated of silicon nitride; the aluminum thin film is coatedon the surface of ribbons. Moreover, the device is built on asilicon substrate so that it is feasible to integrate with electroniccircuits and is capable of containing high-density arrays ofpixels [58]. Furthermore, only two masks are needed in thefabrication process of a basic GLV pixel, reducing the cost ofmanufacturing. The GLV device provides advantages such aseasy fabrication by using the mainstream IC process and lowpower consumption.

The mechanism of the GLV employs diffraction and reflec-tion modes to produce bright and dark states, respectively [59].The air gap between ribbon and substrate is 130 nm (λ/4 ofthe green light). When the applied control voltage is turned



Fig. 5. Structure of 2-D electromagnetic micromirror scanner. (Picture courtesy of C. H. Ji et al. Reprinted from [51] with permission. © 2007 IEEE)

Fig. 6. Illustration of the rotation principle of micromirror. (Picture courtesyof C. H. Ji et al. Reprinted from [51] with permission. © 2007 IEEE)

Fig. 7. GLV device with six ribbons and an air gap between the bottom ofribbon and substrate is about one-quarter of the wavelength. (Reprinted from[56] with permission. © 2004 IEEE)

on, the movable ribbons are attracted down, and the ribbonarray (i.e., the pairs of ribbons) acts as a phase grating, whichcreates a phase modulation depth of π after a round-trip pass.In this state (bright state), the incident light is diffracted.Otherwise, all ribbons remain at their original positions in the

dark state and function as reflective mirrors when the appliedvoltage is removed. With no applied voltage, the first-orderdiffracted light from the GLV is nearly zero. Therefore, a GLVdisplay system performs a high contrast ratio (bright/dark statelight intensity) up to over 2600 : 1 [60]. On the other hand, thecolor/brightness of a pixel is tuned by modulating the movableribbons which move up and down rapidly to switch between thebright and dark states.

The GLV device can be utilized to construct color displaysystems in various formats [59]. Fig. 8(a) shows a GLV thatemploys different ribbon pitches to create the color-orientedsubpixels. The system shown in Fig. 8(b) uses a red–green–bluecolor wheel filter to build a color-sequential projection system.The third one in Fig. 8(c) is composed of three LED lightsources and a GLV device to modulate the incident light to formthe color image.

B. IMod Array Display

Another novel approach for the MEMS display is the IModdeveloped by the Iridigm Display Corporation (now mergedinto Qualcomm). The display system based on the IMod is alsoknown as the Mirasol display. The IMod display consists oftwo conductive plates. One is the thin film stack on the glasssubstrate; the other is the membrane suspended under the thinfilm stack. A 1-μm air gap exists between the thin film stack andthe reflected membrane. The IMod is simply a tunable opticalcavity. Fig. 9 shows a simplified example of the IMod operationfor a single color [61]. When the voltage is off in the open state,the light will be reflected off the membrane due to constructinginterference. While the voltage is on, the reflected membranewill be attracted up and the dark state formed. The ambientlight enters the cavity and interferes with itself. For a certaincolor, the constructive interference leads to a bright pixel, while



Fig. 8. Use of GLV device to construct the color display systems. (Redrawnafter [59])

the destructive interference results in a dark one. Whetherconstructive or destructive interference occurs depends on boththe optical wavelength and the gap spacing. Therefore, tuningthe air gap through the applied voltage not only selects the colorfrom the ambient white light but also determines the brightnessof the selected color.

The IMod device exhibits bistability which originates fromthe inherent hysteresis of MEMS actuators. This means that thedeformation of the reflective membrane can be held when thevoltage is moved. For this reason, the IMod display implementslow power consumption in comparison to the liquid crystaldisplay (LCD). This also implies that the IMod element hasmemory of the applied voltage [62].

C. Plastic MEMS Flexible Display

A MEMS flexible display was proposed by Taii et al. ofthe University of Tokyo, Japan. The schematic structure isshown in Fig. 10 [63]–[65]. It consists of tunable cavity-based

interferometers similar to the IMod. The device is mainlycomposed of a bottom PEN substrate (200 μm) and a suspendedupper PEN (16 μm). The PEN thin film is a flexible material;hence, flexible displays are feasible with this architecture.Semitransparent aluminum layers with a thickness of 12 nm aredeposited on the substrate and the upper film to serve as thecavity reflectors as well as electrodes. The color-determininglayer on the bottom reflector is made of silicon oxide, thethickness of which is between 240 and 370 nm based on thedesired color. The photoresist spacers with a thickness 0.6 μmare positioned between the upper plate and the bottom substrate.When the voltage is applied to the bottom electrode, the topPEN thin film with the aluminum reflector is attracted down,and the cavity length is shortened, allowing the backlight topass through. While the applied voltage is removed, the cavityblocks the light and forms a dark pixel.

IV. MEMS STEREOSCOPIC DISPLAY

Another interesting field of display is the stereoscopic dis-play, known as the 3-D display. Traditionally, stereoscopicdisplays are implemented by holography. With the growth ofMEMS technologies, an alternative solution has been provided.The planner displays have been accomplished by CRTs andthin-film-transistor LCDs while the 3-D display may be createdby using the display hologram [29], [30]. The use of themicromirror scanner is not only applied to the 2-D displaysystem but also to 3-D virtual displays.

Fig. 11 shows the structure proposed by Nakai et al. at theUniversity of Tokyo, Japan [66], [67]. This is an autostereo-scopic display with a scanning micromirror array. The systemconsists of a LED matrix used as the image source. Abovethe matrix, a microlens array is constructed to collimate thelight from the LEDs. As shown in Fig. 11(b) and (c), tiltingthe micromirrors in every other column generates two separateimages for the left and right eyes. Therefore, with appropriateartificial parallax, a stereoscopic effect is sensed by the viewer.The fast switches between the states of Fig. 11(b) and (c) resultin images with a full resolution determined by the size of theLED matrix. In this stereoscopic display system, the micromir-ror with Ni thin film is oriented at 45◦ in the initial state andvibrates at the amplitude of α degrees. In other words, the view-ing angle between the left and right eyes is, namely, 4α degrees[Fig. 11(c)]. The micromirrors are first coarsely tilted by an ex-ternal magnetic field [Fig. 12(a)]. They are then fine-tuned andmodulated by the Lorentz’s force, which is generated throughthe interaction between an injected current and the magneticfield [Fig. 12(b)]. To excite Lorentz’s force, the current isapplied to the outer coil attached on the micromirror plate.

V. RSD

In the LSDs, a laser beam is scanned onto a screen bya scanning micromirror, as discussed in Section II. Anothernovel head-mounted display technique, known as the RSD,laser-scans images directly onto the human retina instead of aprojection screen. This was demonstrated by Microvision Inc.[68]–[72]. The system is shown in Fig. 13 [70].



Fig. 9. IMod: The left one is in the open state, and the right is in the collapsed state. (Reprinted from [61] with permission. © 2006 IEEE)

Fig. 10. PEN film display. [1: Electrode/mirror (aluminum), 2: Air cavity spacer (photoresist), 3: Optical spacer (silicon oxide), 4: Electrode/mirror (aluminum)].(Picture courtesy of H. Toshiyoshi et al. Reprinted from [64] and [65] with permission. © 2005 and 2007 IEEE)

Yalcinkaya et al. proposed an RSD display using a biaxialmicromirror scanner which was able to achieve optical scanningangles of 53◦ and 65◦ [70]. The scanning mirror device is shownin Fig. 14. The entire device consists of a MEMS mirror, twosets of orthogonal torsion springs, and an outer frame withspiral coils. It is actuated by a Lorentz’s force with a fastscan of 21.3 kHz in the horizontal direction and slow scan of60 Hz in the vertical direction. The external magnetic field isoriented in-plane at 45◦ as shown in Fig. 15. This arrangementyields orthogonal driving torques; the horizontal component(21.3 kHz) is superimposed on the 60-Hz vertical drivingsignal. This combined current signal is sent through the coilsand interacts with the external magnetic field, generating theLorentz’s force (i.e., the driving torque).

Another two-axis micromirror scanner for an RSD wasdemonstrated [71]. The schematic structure is shown in Fig. 16.

The micromirror scanner is composed of a suspended micro-mirror and an outer frame with NiFe thin film. The coilturns, and the high-permeability core functions as a mag-netic flux generator. An alternating current is passed betweenports 1 and 2. The magnetic flux generator, together with apermanent magnet which induces a predefined magnetization(Fig. 17), creates the driving force. In the initial state, the micro-mirror plate is aligned with the permanent magnet. When thealternating current is applied, an alternating force is produced tooscillate the micromirror. It is worth mentioning that the centerof the micromirror plate is not aligned with that of the externalmagnetic generator. Instead, an offset is needed to guarantee anonzero torque at any angle.

With a root mean square of 100 mA, the optical scan anglesreach 1.8◦ for the fast scan axis and 88◦ for the slow scanaxis. The scan angle is a function of frequency. The device is



Fig. 11. Working principle of the MEMS autostereoscopy display. (Picturecourtesy of A. Nakai et al. Reprinted from [66] with permission. © 2006 IEEE)

Fig. 12. Micromirror is actuated by both the (a) external magnetic field andthe (b) Lorentz’s force. (Picture courtesy of A. Nakai et al. Reprinted from [66]with permission. © 2006 IEEE)

meant to operate at either or both of the mechanical resonantfrequencies of the orthogonal torsion modes, which are locatedat 367 Hz and 22.23 kHz for the slow and fast scan motions,respectively. The Q-factor of this biaxial electromagnetic scan-ner can achieve an order of 3000. In contrast with the previousone, no spiral coil is needed for the micromirror scanner,and the fabrication process is easier, owing to the reduced

Fig. 13. RSD system. (Picture courtesy of H. Urey et al. Reprinted from [70]with permission. © 2006 IEEE)

Fig. 14. Retinal scanning device. (Picture courtesy of H. Urey et al. Reprintedfrom [70] with permission. © 2006 IEEE)

Fig. 15. Schematic drawing of the scanning micromirror. (Courtesy ofH. Urey et al. Reprinted from [70] with permission. © 2006 IEEE)

Fig. 16. Scheme of micromirror scanner with an outer frame of NiFe thinfilm. (Picture courtesy of H. Urey et al. Reprinted from [71] with permission.© 2007 IEEE)



Fig. 17. Working principle of the micromirror scanner. (Picture courtesy ofH. Urey et al. Reprinted from [71] with permission. © 2007 IEEE)

mask number. Moreover, no electric current passes throughthe micromirror scanner; the thermal effects, which result infatigue, self-heating, stress gradient, and Q-factor variation, canbe avoided.

VI. CONCLUSION

MEMS display technologies with different architectures havebeen discussed. The DMDs, GLV, IMod, and PEN film displaysfall into the category of pixilated devices. The LSD and RSDsystems require a single mirror to scan the laser beam onto theprojection screen and human retina, respectively. The MEMSstereoscopic display offers the likelihood of generating imagesas vivid as life. The development of MEMS components fordisplay applications with low power consumption, simple fab-rication processes, large rotation angles or travel distances, andhigh scan rates will continue to be one of the major researchthemes in the future.

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Chun-da Liao received the B.S. degree in math-ematics from Fu Jen Catholic University, Taipei,Taiwan, in 2004, and the M.S. degree from the Grad-uate Institute of Electro-Optical Science and Tech-nology, National Taiwan Normal University, Taipei,in 2006. Currently, he is working toward the Ph.D.degree in the Graduate Institute of Photonics andOptoelectronics at National Taiwan University.

His research interests include optical micro-electromechanical systems (MEMS), bio-MEMS,and photonic crystals.

Jui-che Tsai (M’09) received the B.S. degree inelectrical engineering from National Taiwan Univer-sity (NTU), Taipei, Taiwan, in 1997, the M.S. degreein electro-optical engineering from the Graduate In-stitute of Electro-Optical Engineering [currently, theGraduate Institute of Photonics and Optoelectronics(GIPO)], NTU, in 1999, and the Ph.D. degree in elec-trical engineering from the University of California,Los Angeles, in 2005.

Between 1999 and 2001, he was in the militaryas a Second Lieutenant. Before joining the faculty

of NTU, he was a Postdoctoral Researcher with the Department of ElectricalEngineering and Computer Sciences and with the Berkeley Sensor and Actu-ator Center, University of California, Berkeley. He is currently an AssistantProfessor with GIPO and the Department of Electrical Engineering, NTU. Hisresearch interests include optical microelectromechanical systems (MEMS),MEMS technologies, optical fiber communication, and biophotonics.


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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. …...interferometricmodulator(IMod)technologywillbecoveredin Section III. An IMod chip is composed of MEMS Fabry–Pérot etalons