micromachined vibratory diffraction grating scanner for multiwavelength collinear laser scanning

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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006 1777 Micromachined Vibratory Diffraction Grating Scanner for Multiwavelength Collinear Laser Scanning Guangya Zhou and Fook Siong Chau Abstract—This paper presents an effective method to achieve multiwavelength collinear laser scanning using micromachined vi- bratory grating scanners, which have the potential to scan at high frequencies without the optical performance degradation resulting from dynamic nonrigid-body deformation. An optical simulation model has been developed to predict the scanning patterns of the vibratory grating scanners. The proposed multiwavelength collinear scanning method was studied both analytically with the optical simulation model and experimentally with a prototype device fabricated by the MUMPS polysilicon surface microma- chining process. The experimental results agree very well with the simulation data. The prototype scanner demonstrated collinear scanning of 532 nm (green) and 632.8 nm (red) wavelengths laser beams and achieved an optical scan angle of 12.7 degrees with virtually bow-free scan-line at a resonant frequency of 9.9 kHz when driven by electrostatic comb-drive resonators with 60 V dc bias and 32 ac voltages. [2006-0050] Index Terms—Diffraction grating, micromirrors, microscan- ners, optical microelectromechanical systems (MEMS). I. INTRODUCTION I N recent years, microelectromechanical systems (MEMS)- based optical scanners [1], [2] have attracted much attention because of their outstanding advantages compared to conven- tional macroscanners such as rotating polygons, galvanometric, and resonant optical scanners. These advantages include having a low mass, high scanning frequency, low power consumption, and potentially lower per unit cost through batch fabrication. MEMS optical scanners have the potential not only to provide significant performance enhancements such as small size, high speed, and low cost to existing applications such as barcode readers [3], laser printers, scanning laser confocal microscopy systems [4], and laser markers, but also to form the technolog- ical basis for a wide range of new applications in raster-scanning retinal projection displays [5], [6], endoscopic optical coher- ence tomography [7], and compact high-speed fiber optic com- ponents [8]. Majority of the research in this area has focused on microma- chined torsional scanning mirrors [9], [10], as shown schemat- Manuscript received March 23, 2006; revised June 8, 2006. Subject Editor O. Solgaard. The authors are with the Micro and Nano Systems Initiative, Department of Mechanical Engineering, National University of Singapore, 119260 Singapore (e-mail: [email protected]). Color versions of Figs. 1, 2, 5–7, 9–11, and 13–16 are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2006.886027 ically in Fig. 1(a). However, due to the nature of microfabrica- tion processes, MEMS mirrors are normally much thinner than conventional macroscanning mirrors, and therefore, at high scan frequencies, due to large out-of-plane acceleration, the mirror plate is no longer a rigid body and tends to deform dynam- ically during scanning [11]. This introduces dynamic aberra- tions into the optical system and seriously degrades the optical resolution [12]. Optical resolution or the number of resolvable spots/pixels per unidirectional scan is defined as the ratio of the optical scan angle and light beam divergence. For a given optical scan angle, there are three main factors that affect the optical resolution of a micromachined scanner, i.e., beam divergence caused by diffraction, static, and dynamic deformation of the micromirror. The static deformation of a micromirror, which is normally caused by residual stresses in the mirror material, can be minimized to a significant degree by improving the microfab- rication processes or introducing mirror curvature compensation optics to the scanner. However, the reduction of scanning beam divergence caused by beam diffraction and the micromirror’s dynamic deformation is still a major challenge for developers of high-speed scanning micromirrors. Both diffraction and dy- namic deformation are dependent on the mirror size. Increasing the mirror size decreases the beam divergence caused by diffrac- tion but simultaneously increases the dynamic deformation of the micromirror. A straightforward way to achieve both large mirror size and small dynamic deformation during scanning is to increase the micromirror thickness. However, this also in- creases the mass and, consequently, the stiffness of the mirror suspension in order to maintain a high scanning frequency. This results in a very high driving voltage and considerable power consumption. Efforts in the optical MEMS community are cur- rently under way to address this issue [13], [14]. In our previous work [15], [16], we reported a novel MEMS high-speed optical scanning technology based on MEMS in-plane vibratory diffraction gratings and demonstrated dy- namic-deformation-free scanning using a proof-of-concept device fabricated by SOI MUMPS process. As compared with other MEMS diffraction grating scanners [17]–[19], our grating scanners are based on freestanding vibratory micromachined structures, and hence are able to operate at high frequencies with negligible wear and tear. As shown in Fig. 1(b), the MEMS-based diffraction grating scanner utilizes in-plane rotational motion rather than rotational out-of-plane motion to scan a light beam. It is well known from elastodynamics that the dynamic nonrigid-body deformation of a plate under in-plane excitation is much smaller than that for rotational 1057-7157/$20.00 © IEEE

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Page 1: Micromachined Vibratory Diffraction Grating Scanner for Multiwavelength Collinear Laser Scanning

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006 1777

Micromachined Vibratory Diffraction GratingScanner for Multiwavelength Collinear

Laser ScanningGuangya Zhou and Fook Siong Chau

Abstract—This paper presents an effective method to achievemultiwavelength collinear laser scanning using micromachined vi-bratory grating scanners, which have the potential to scan at highfrequencies without the optical performance degradation resultingfrom dynamic nonrigid-body deformation. An optical simulationmodel has been developed to predict the scanning patterns ofthe vibratory grating scanners. The proposed multiwavelengthcollinear scanning method was studied both analytically with theoptical simulation model and experimentally with a prototypedevice fabricated by the MUMPS polysilicon surface microma-chining process. The experimental results agree very well with thesimulation data. The prototype scanner demonstrated collinearscanning of 532 nm (green) and 632.8 nm (red) wavelengths laserbeams and achieved an optical scan angle of 12.7 degrees withvirtually bow-free scan-line at a resonant frequency of 9.9 kHzwhen driven by electrostatic comb-drive resonators with 60 V dcbias and 32 ac voltages. [2006-0050]

Index Terms—Diffraction grating, micromirrors, microscan-ners, optical microelectromechanical systems (MEMS).

I. INTRODUCTION

I N recent years, microelectromechanical systems (MEMS)-based optical scanners [1], [2] have attracted much attention

because of their outstanding advantages compared to conven-tional macroscanners such as rotating polygons, galvanometric,and resonant optical scanners. These advantages include havinga low mass, high scanning frequency, low power consumption,and potentially lower per unit cost through batch fabrication.MEMS optical scanners have the potential not only to providesignificant performance enhancements such as small size, highspeed, and low cost to existing applications such as barcodereaders [3], laser printers, scanning laser confocal microscopysystems [4], and laser markers, but also to form the technolog-ical basis for a wide range of new applications in raster-scanningretinal projection displays [5], [6], endoscopic optical coher-ence tomography [7], and compact high-speed fiber optic com-ponents [8].

Majority of the research in this area has focused on microma-chined torsional scanning mirrors [9], [10], as shown schemat-

Manuscript received March 23, 2006; revised June 8, 2006. Subject EditorO. Solgaard.

The authors are with the Micro and Nano Systems Initiative, Department ofMechanical Engineering, National University of Singapore, 119260 Singapore(e-mail: [email protected]).

Color versions of Figs. 1, 2, 5–7, 9–11, and 13–16 are available online athttp://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2006.886027

ically in Fig. 1(a). However, due to the nature of microfabrica-tion processes, MEMS mirrors are normally much thinner thanconventional macroscanning mirrors, and therefore, at high scanfrequencies, due to large out-of-plane acceleration, the mirrorplate is no longer a rigid body and tends to deform dynam-ically during scanning [11]. This introduces dynamic aberra-tions into the optical system and seriously degrades the opticalresolution [12]. Optical resolution or the number of resolvablespots/pixels per unidirectional scan is defined as the ratio of theoptical scan angle and light beam divergence. For a given opticalscan angle, there are three main factors that affect the opticalresolution of a micromachined scanner, i.e., beam divergencecaused by diffraction, static, and dynamic deformation of themicromirror. The static deformation of a micromirror, which isnormally caused by residual stresses in the mirror material, canbe minimized to a significant degree by improving the microfab-rication processes or introducing mirror curvature compensationoptics to the scanner. However, the reduction of scanning beamdivergence caused by beam diffraction and the micromirror’sdynamic deformation is still a major challenge for developersof high-speed scanning micromirrors. Both diffraction and dy-namic deformation are dependent on the mirror size. Increasingthe mirror size decreases the beam divergence caused by diffrac-tion but simultaneously increases the dynamic deformation ofthe micromirror. A straightforward way to achieve both largemirror size and small dynamic deformation during scanning isto increase the micromirror thickness. However, this also in-creases the mass and, consequently, the stiffness of the mirrorsuspension in order to maintain a high scanning frequency. Thisresults in a very high driving voltage and considerable powerconsumption. Efforts in the optical MEMS community are cur-rently under way to address this issue [13], [14].

In our previous work [15], [16], we reported a novel MEMShigh-speed optical scanning technology based on MEMSin-plane vibratory diffraction gratings and demonstrated dy-namic-deformation-free scanning using a proof-of-conceptdevice fabricated by SOI MUMPS process. As compared withother MEMS diffraction grating scanners [17]–[19], our gratingscanners are based on freestanding vibratory micromachinedstructures, and hence are able to operate at high frequencieswith negligible wear and tear. As shown in Fig. 1(b), theMEMS-based diffraction grating scanner utilizes in-planerotational motion rather than rotational out-of-plane motionto scan a light beam. It is well known from elastodynamicsthat the dynamic nonrigid-body deformation of a plate underin-plane excitation is much smaller than that for rotational

1057-7157/$20.00 © IEEE

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1778 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006

Fig. 1. Schematic illustrations of (a) micromirror scanner and (b) vibratory diffraction grating scanner.

out-of-plane excitation. Consequently, the MEMS vibratorygrating scanner has the potential to scan at high frequencieswithout the optical performance degradation due to dynamicnonrigid-body deformation (dynamic aberration), which isprevalent in conventional high-speed out-of-plane torsionalmicromirror scanners.

Although our previously reported devices show promise formany narrow-band laser scanning applications, due to the dis-persive grating used, they are not suitable for applications thatrequire multiple-wavelength collinear scanning, such as minia-turized raster-scanning color displays, laser color printers, andlaser cameras. In miniaturized color displays, for instance, inorder to scan a color image, the scan lines of different wave-lengths (primary colors: red, green, and blue) have to be essen-tially collinear. In other words, the red, green, and blue laserspots must be always at the same location and scan at the samevelocity. However, since a diffraction grating is a dispersive ele-ment, different wavelengths leave the diffraction grating at dif-ferent diffraction angles and may have different scanning ve-locities. As a result, a MEMS vibratory structure having onediffraction grating cannot be used for multiwavelength collinearscanning applications such as color displays.

In this paper, we seek to overcome this problem by reportingan improved MEMS vibratory grating scanner capable ofcollinear multiwavelength scanning. This paper is organized asfollows. In Section II, a theoretical optical model is introducedto predict the scanning patterns of the MEMS vibratory gratingscanners. In Section III, multiwavelength collinear scanningusing multiple gratings on a common vibratory platform ispresented and simulated using the developed optical model.In Sections IV and V, preliminary experimental results on asurface micromachined MEMS grating scanner are reportedand discussed.

II. OPTICAL MODEL

In this section, we discuss a simulation model that relates theoptical scan angle of a chosen diffraction beam to the grating’s

in-plane rotation. As shown in Fig. 2(a), a grating with a spa-tial period of lies in the plane, and the grating linesare orientated parallel to the -axis. An incident laser beamwith a wavelength of lies in the plane and illuminatesthe grating at an angle of incident . Diffraction theory dic-tates that all the diffracted beams will lie in the plane.When the diffraction grating rotates about the -axis, the dif-fracted beams (except zeroth order) scan accordingly. The -and -components of the wave-vector of the chosen th orderdiffraction beam, i.e., and , are given by the well-knowngrating equation

(1)

(2)

where and are - and -components of the incidentwave-vector, and are the effective grating periods along

- and -directions, respectively, and is the diffractionorder. As depicted in Fig. 2(b), when the grating undergoesan in-plane rotation with an angle of , the effective gratingperiod and is

(3)

(4)

By inserting (3) and (4) into (1) and (2) and noticing thatand , the - and -components of the

wave-vector of the th order diffraction beam can be expressedas

(5)

(6)

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ZHOU AND CHAU: MICROMACHINED VIBRATORY DIFFRACTION GRATING SCANNER 1779

Fig. 2. (a) Optical simulation model of the vibratory grating scanner and (b) schematic showing the effective grating periods along X and Y -directions.

Since the wavelength is not changed during the diffraction,the wave-number is preserved and the -component of thewave-vector can thus be obtained by

(7)

Consequently, the normalized wave-vector pointing to the di-rection of the outgoing th-order diffraction beam as a functionof the grating rotation is given by (8) as shown at the bottomof the page, where , , and are unit vectors along the -,

-, and -axes, respectively.

It is assumed that a projection screen is placed at adistance from the grating with its normal vector parallel to thedirection of the chosen th diffracted beam when the grating isat rest (initial position). The next step is to find the position ofthe projected laser spot on the screen when the grating rotatesabout the -axis with an angle of . As shown in Fig. 2(a),

since the vector is perpendicular to the vector , theirinner product is zero

(9)

(8)

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1780 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006

Fig. 3. Analytical scan-lines for the first-diffraction order with varying grating period (incident angle is 21.2 and screen to grating distance is 135 mm).

Fig. 4. Analytical scan-lines for the ninth-diffraction order with a grating period of 4 �m (angle of incidence is 21.2 and screen to grating distance is 135 mm).

where is the distance from to . Solving (9) for , we find

(10)

Hence, the projected laser spot on the screen is located at

(11)

In (11), the location of the laser spot is measured in the-coordinate system whose origin is located at the center

of the grating. In order to find the coordinates of the laser spoton the projection screen , a coordinate transform hasto be performed. As shown in Fig. 2(a), the coordinatesystem can be obtained by first rotating the system aboutthe -axis through an angle of followed by a translation of

a distance towards the positive -direction of the intermediatecoordinate system. The new coordinates of the spot measuredon the screen are then given by

(12)

where is the angle of diffraction of the th-order diffractedbeam when the grating is at rest, i.e., , which can bedetermined using

(13)

The components and of the vector from (12) arethen taken to locate the laser spot on the projection screen.

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ZHOU AND CHAU: MICROMACHINED VIBRATORY DIFFRACTION GRATING SCANNER 1781

Fig. 5. Schematic illustrations of vibratory grating scanners with (a) single grating and dual laser beams with the same incident angle, (b) single grating and duallaser beams with different incident angles, and (c) two diffraction grating and dual laser beams with the same incident angle.

The optical scan angle of the th-order diffraction beam canthen be easily calculated from the scan length of the laser beamon the projection screen.

Fig. 3 shows the simulated scan-lines of the first-order diffrac-tion beam on a projection screen located at a distance of

mm from the grating, whose period is varied from 400 nm

to 4 m. In our calculations, the wavelength of the incident laserbeam is assumed to be 532 nm (green), the angle of incidence

is set as 21.2 , and the grating rotation angle is variedfrom 5 to 5 . As shown in Fig. 3, it is observed that the op-tical scan angle increases when the grating period is reduced.It is also noted that the scan-lines on the screen demonstrate a

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1782 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006

TABLE IFEASIBLE DIFFRACTION ORDERS FOR BOW-FREE SCANNING AND THEIR RESPECTIVE ANGLE OF INCIDENCE

FOR GRATING PERIOD 4 �m AND WAVELENGTH 532 nm

Fig. 6. Simulated scan trajectories of two laser beams on a projection screen using the setup illustrated schematically in Fig. 5(b).

Fig. 7. Simulated scan trajectories of two laser beams on a projection screen using the setup illustrated schematically in Fig. 5(c).

“bow-like” pattern, which is not desirable for most scanningapplications such as laser project displays. However, from thefigure, it is also observed that the scan-bow (defined as deviationfrom the ideal straight scan-line) can be minimized. As reportedin [20], a diffraction grating scanner will produce a virtuallybow-free scan-line when the incidence and diffraction angles( and ) satisfy the following:

(14)

(15)

where is the chosen diffraction order for scanning. Hencefrom the parameters used ( , nm, and

), the optimal grating period to produce a bow-free scan-line is 444 nm. This optimal scan-line is shown by the solid linein Fig. 3.

In (14) and (15), the real solutions for and only existwhen

(16)

(17)

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ZHOU AND CHAU: MICROMACHINED VIBRATORY DIFFRACTION GRATING SCANNER 1783

For positive diffraction orders, the above inequalities lead to thefollowing condition for the grating period

(18)

Hence, in order for a grating scanner to produce a virtuallybow-free scan-line for the th-order diffraction beam, the valueof the grating period must reside within a specific range, i.e.,

m . From (18), it is clear that the grating periodmust be smaller than the wavelength of the incident laser beamsuch that the high-efficiency first-order beam can be utilizedto scan a bow-free line. For visible wave-lengths, this meansthat the ideal grating period for first-order scanners should beless than 1 m. In recent years, diffraction gratings with sub-micrometer periods can be formed using nanofabrication tech-nologies such as nanoimprinting and e-beam lithography [21].Thus, high efficiency first-order vibratory grating scanners maybe fabricated by customizing micromachining processes to in-clude a fine grating fabrication process capable of producingsubmicrometer period.

In this paper, the proposed scanning method is demon-strated using a prototype MEMS device fabricated usingthe poly-MUMPS surface micromachining process,1 a com-mercially available cost-effective foundry service offered byMEMSCAP. It is noted that the grating period of the currentprototype scanner is limited to 4 m by the poly-MUMPSdesign rules set by the commercial foundry. Hence, in the restpart of this paper, we will restrict our minimum grating periodto 4 m.

Let us consider a collimated laser beam with a wavelengthof 532 nm illuminating a vibratory diffraction grating with aperiod of 4 m. From (18), it is clear that the vibratory gratingcannot produce a bow-free scan-line for the first-diffractionorder, since the grating period is larger than the incident wave-length. However, on analyzing (18), the bow-free scan-linedoes in fact exist for the eighth-, ninth-, tenth-, eleventh-, ortwelfth-diffraction order provided that the angle of incidence isappropriately chosen for each of the above diffraction orders.Table I summarizes the analytical results for bow-free scanning.For brevity, only the scanning results for the ninth diffractionorder are presented herein. The corresponding angle of inci-dence for bow-free scanning is found to be 21.2 . Fig. 4 showsthe simulated scan trajectory of the ninth-order diffractionbeam on a projection screen located at a distance 135 mm fromthe grating. The grating rotation angle is assumed to varyfrom 5 to 5 . It is observed that the scan-bow is smaller than1 m, which is negligibly small as compared with the nearly30-mm-long scan-line.

III. MULTIWAVELENGTH SCANNING

In this section, we describe a method to achieve multiwave-length scanning using a MEMS vibratory grating scanner. Inmany applications, it is required that the scan lines of differentwavelengths be collinear; in other words, the laser spots of dif-ferent wavelengths must be always at the same location andscan at the same velocity. Such applications include, but are

1MEMSCAP, Inc.

Fig. 8. Schematic illustration of the MEMS vibratory grating scanner drivenby four comb-drive resonators.

not limited to, laser printers, laser projection displays, and lasercameras.

As shown in Fig. 5(a), two collimated laser beams withdifferent wavelengths ( and ) are directed at the sameangle of incidence onto a MEMS in-plane vibratory platformhaving a single diffraction grating. Since the diffraction gratingis a dispersive element, laser beams with different wavelengthsleave the grating at different diffraction angles and have dif-ferent scanning velocities, resulting in two separate scan lineswith different lengths on the projection screen. This problemcannot be solved by simply aligning the incident angles ofthe two laser beams. For example, as shown in Fig. 5(b), evenif we align the input laser beams such that the two outgoingdiffraction beams line up and coincide with each other whenthe grating is at rest (i.e., ), the two scan trajectories onthe projection screen will still differ significantly. To demon-strate this, we assume that a grating with a period of 4 m isilluminated by two laser beams at wavelengths 532 nm (green)and 632.8 nm (red) simultaneously. The green laser beam isaligned to produce bow-free scanning for the ninth-order beam.The red laser beam is aligned such that the two ninth-orderdiffraction beams, i.e., green and red, coincide with each otherwhen the grating rotation is zero. To achieve this, the incidentangle of the red laser beam is found to be 36.05 . Fig. 6 showsthe simulated scan trajectories of the two laser beams on aprojection screen located 135 mm from the grating. As shownin the figure, although the two laser spots may be close to eachother at the central region of the scan trajectories (i.e., whenis very small), they separate quickly as increases. The sep-aration between two spots can be as large as 2.7 mm when thegrating rotates about 5 . Clearly, the setup shown in Fig. 5(b)cannot be used for multiwavelength collinear scanning.

Fig. 5(c) shows the proposed method to achieve multiwave-length collinear scanning. In this method, multiple diffractiongrating elements are formed on a common MEMS vibratingplatform, as shown in the inset of Fig. 5(c), each having a dif-ferent grating period. The grating elements all have the same

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1784 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006

Fig. 9. SEM image of the MEMS vibrating grating scanner.

line orientation and (wavelength-to-grating period) ratio.Since the elements are formed on the same platform, they allhave the same rotation angle when the platform oscillates.According to (8), the beams of different wavelengths diffractedfrom their corresponding grating elements have the same nor-malized wave-vector and point to the same direction. As a re-sult, the laser spots are always at the same location and scan atthe same velocity. Thus, the scanner shown in Fig. 5(c) is ca-pable of achieving collinear scanning for different wavelengths.It is noted that a laser beam of a certain wavelength diffractedfrom its unrelated grating element has a different normalizedwave-vector and points to a different direction. As shown inFig. 5(c), by using an optical slit, these unwanted diffractionbeams can be filtered out.

Fig. 7 shows the simulated results for multiwavelengthcollinear scanning using the proposed method. In our simula-tion work, the MEMS in-plane vibratory platform is assumedto contain two diffraction gratings, one having a period of 4 mdesignated for green laser beam scanning at wavelength 532 nmand the other having a period of 4.758 m designated for redlaser beam scanning at wavelength 632.8 nm. The green andred laser beams are aligned collinearly and collimated to thevibratory platform at an incident angle of 21.2 (fulfilling thebow-free scanning condition for both green and red ninth-orderdiffraction beams). The distance between the projection screenand the platform is assumed to be 135 mm. As shown in Fig. 7,the two scan trajectories on the screen are almost coincidentalwith each other with the separation between the two laser spotsfound to be smaller than 7 m throughout the whole scanningrange. From the above simulation, it is clear that the proposedmethod is capable of achieving multiwavelength collinearscanning.

IV. DEVICE DESIGN AND FABRICATION

To demonstrate the proposed multiwavelength collinear scan-ning principle, a prototype MEMS vibratory grating scanner

Fig. 10. Closeup view showing the diffraction gratings and platform suspen-sion beams.

was designed and fabricated using the MUMPS polysiliconsurface micromachining process. Fig. 8 illustrates a simplifiedschematic, and Fig. 9 shows an annotated scanning electronmicroscope (SEM) image of the device. As shown in Fig. 9,a vibratory platform with a diameter of 518 m and thicknessof 2 m is made out of the Poly1 layer and contains twodiffraction gratings. The grating structures are composed ofpolysilicon beams separated by air gaps (through-hole open-ings). The grating having a period of 4 m is placed on theleft-hand side of the platform for green laser beam scanning atwavelength 532 nm, and the grating having a period about4.76 m is placed on the right-hand side of the platform for redlaser beam scanning at wavelength 632.8 nm. The platform issuspended about 2 m above the substrate and driven by foursets of identical comb drive resonators [22], each having a totalnumber of 172 movable fingers with finger length of 50 m,

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ZHOU AND CHAU: MICROMACHINED VIBRATORY DIFFRACTION GRATING SCANNER 1785

width of 3 m, finger gap of 2 m, and overlap length of 25 m.The comb-drives are made of the stacked Poly1-Poly2 layerswith a total thickness of 3.5 m. Each comb-drive resonatoris suspended by four sets of folded-beam flexures with beamsof length 200 m, width 3 m, and thickness 3.5 m. FourPoly2 suspension beams are used to connect the comb-driveresonators to the center of the vibratory platform. A closeupview showing the details of the diffraction gratings and theplatform suspension beams is given in Fig. 10. The length,width, and thickness of the platform suspension beams are260 m, 5.6 m, and 1.5 m, respectively. The comb-driveresonators are placed symmetrically about the center of theplatform and synchronized such that the translational vibrationsof the resonators, coupled through the suspension flexures tothe platform, produce a pure in-plane rotational vibration of theplatform about its geometric center. The relationship betweenthe platform rotation and translational motions of the fourcomb-drive resonators is illustrated schematically in Fig. 8.

V. EXPERIMENTAL RESULTS

The optical performances of the scanner were tested using aHeNe laser beam at wavelength 632.8 nm and a diode-pumpedsolid-state laser beam at wavelength 532 nm. A schematicshowing the setup of the experiment is given in Fig. 11(a).The two laser beams were aligned collinearly using a mirrorand a beam-splitter and directed onto the MEMS gratingscanner at an angle of incidence of around 21.2 . This fulfillsthe bow-free-scanning condition for both ninth-order red andgreen laser beams diffracted from their corresponding gratingelements. An optical slit was used to filter out the unwanteddiffraction orders such that only the selected scanning laserbeams pass through and reach a projection screen located at adistance about 135 mm from the scanner with its normal vectorparallel to the direction of the chosen diffraction beams. In ourexperiment, the MEMS grating scanner was operated in air anddriven by a push–pull mechanism with 60 V dc bias and 32

ac voltages at a driving frequency of 9.9 kHz. Fig. 11(b)shows a recorded scan image on the projection screens. Thecollinear scanning of the green and red laser beams was ob-served and the optical scan angle was found to be around 12.7 .It is also noticed that the scan-lines in Fig. 11(b) demonstraterelatively larger scan-bows as compared with the theoreticalresults shown in Fig. 7. This might be due to the alignmenterrors in the optical experimental setup—for example, theactual angle of incidence might deviate slightly from the idealbow-free-scanning condition.

In order to compare the experimental results with analyticaldata obtained in Section III, the actual mechanical rotation angleof the platform has to be found. To achieve this, the methodof stroboscopic video microscopy was used. A strobed imageshowing an extreme position of the vibrating platform under theabove-mentioned driving condition is presented in Fig. 12. Fromthe figure, we estimated that the rotational vibration amplitudeof the platform was around 5–6 . Comparing the experimentalresults shown in Fig. 11(b) and the analytical scan-lines shownin Fig. 7 with the grating rotation angle varying from 5to 5 , it is found that the experimental results are in good agree-ment with the analytical predictions.

Fig. 11. (a) Optical setup to demonstrate multiwavelength collinear scanningand (b) recorded scan trajectories of the ninth-order diffracted green and redlaser beams on a projection screen.

Fig. 12. Strobed microscopic image showing an extreme position of the MEMSgrating scanner.

The optical scan angle as a function of driving frequency wasobtained by measuring the length of the scan-line on the pro-jection screen while sweeping the driving frequency around theresonant frequency of the device. In this paper, the dc bias ofthe device was fixed at 60 V. The experimental results are givenin Fig. 13. The diamond symbols in the figure show the mea-sured frequency response of the device when a 4 ac drivingsignal is applied. Under such a small ac driving voltage, thevibration amplitude of the platform is small, and the scannerdemonstrates a linear performance. The resonant frequency andQ factor of the scanner were found to be around 9.7 kHz and13.8, respectively. However, as the ac driving voltage increases,the vibration amplitude of the platform becomes large and non-linear effects (such as stress-stiffening and frequency jumping)start to appear. The circles and squares in the Fig. 13 show thefrequency response of the scanner when the ac driving signalis large. As can be seen, the scanner demonstrated high-am-plitude nonlinearity. The scanner has different optical scan an-gles depending on whether the ac driving signal approaches the

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1786 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006

Fig. 13. Measured frequency responses of the MEMS grating scanner.

Fig. 14. Schematic showing the experimental setup to “freeze” the motion of the scanning laser spots.

resonant frequency from below (forward sweep) or above (re-verse sweep). Forward sweeping from a frequency below res-onance results in large scan angles until a critical frequency isreached, after which the scan angle drops significantly (i.e., fre-quency jumping). In addition, the resonant frequency of the de-vice increases with increase in the amplitude of the ac drivingsignal. This nonlinearity can be explained by the well-knownDuffing’s effect [23], which has been extensively studied formany mechanical vibration systems [24]. At high vibration am-plitudes, the four platform suspension beams experience largeaxial stresses. This effectively stiffens the suspension flexure,causing the spring constant of the platform suspension to be de-pendent on the platform rotation, which in turn induces the non-linear Duffing’s effect.

In order to demonstrate that the laser spots of different wave-lengths are always at the same location during scanning, theincident laser beams were strobed using an acoustooptic mod-ulator (AOM) to produce 150 ns light pulses synchronized tothe device driving signal to “freeze” the motion of the scanningbeam. The experimental setup is shown schematically in Fig. 14.The green and red laser beams were directed to the Bragg cell atdifferent angles and aligned such that the two first-order beamsdiffracted from the Bragg cell were collinear with each other.The AOM was utilized to switch on and off the incoming lasersto produce short laser pulses. The pulsed collinear laser beamsfrom the AOM were then directed to the MEMS grating scannerwith an incident angle fulfilling the bow-free scanning condi-tion for the chosen ninth-order beams diffracted from the device,

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ZHOU AND CHAU: MICROMACHINED VIBRATORY DIFFRACTION GRATING SCANNER 1787

Fig. 15. Total intensity of the pulsed incident laser beams versus ac driving voltage.

Fig. 16. Strobed laser spots along the scan line showing the collinear scanning of the green and red laser beams.

i.e., . The total intensity of the pulsed incident laserbeams was also recorded using a photodetector. Fig. 15 showsfive laser pulses with a duration of 150 ns and a repetition fre-quency of 1 MHz (1 s between two successive laser pulses)synchronized to the device’s driving ac voltage. The scannerwas again driven by a push–pull mechanism with 60 V dc biasand 32 ac voltages at a driving frequency of 9.9 kHz. Thestrobed beam positions were controlled by adjusting the phasedifference between the strobe-pulses and the ac driving signal.The “frozen” laser spots were captured by a charge-coupled de-vice camera with a 150 mm focusing lens. Fig. 16 shows therecorded strobed laser spots along the scan line at three differentregions. It is clear that the green and red laser spots are alwaysat the same locations during the scanning and they scan at thesame velocity. The full-width half-maximum diameters of thegreen and red spots were determined to be about 230 and 200

m, respectively, representing an overall scanned optical reso-lution of roughly 145 pixels.

VI. CONCLUSION

We have successful demonstrated for the first time a mi-cromachined vibratory grating scanner for multiwavelength

collinear laser scanning. Due to the in-plane vibratory mo-tion, the MEMS grating scanner has unique properties thatallow high-speed optical scanning without optical performancedegradation due to dynamic nonrigid-body deformation. Theproposed multiwavelength collinear scanning method wasstudied both analytically with an optical simulation modeland experimentally with a prototype device developed usingthe MUMPS polysilicon surface micromachining process.The tested device is capable of collinear scanning of both532 and 632.8 nm wavelength incident laser beams with anoptical bow-free scan angle of 12.7 at a resonant frequencyof 9.9 kHz. It is also noted that, in the present embodimentof the vibratory grating scanner, the minimum grating periodwas constrained to 4 m by the MEMS foundry. As a result,the current prototype scanner has no doubt some known andexpected limitations, for example, bow-free scanning is onlypossible for laser beams with a high diffraction order, whichlimits the efficiency of the scanner. However, as indicated inthe optical simulation results, this problem can be overcome bymaking the grating period smaller than the wavelength of theincident laser beam such that the first-diffraction order can beutilized to produce a bow-free scanning. This can be achievedusing nanofabrication technologies such as nanoimprinting

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1788 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 6, DECEMBER 2006

and e-beam lithography. In conclusion, we believe that thereported MEMS grating scanners can be very useful in manyoptical applications particularly in miniaturized head-mounteddisplays, laser color printers, and scanning laser cameras.

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Guangya Zhou, photograph and biography not available at the time of publi-cation.

Fook Siong Chau, photograph and biography not available at the time of pub-lication.