double-layered vibratory grating scanners for high-speed high-resolution laser scanning

1186 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 19, NO. 5, OCTOBER 2010

Double-Layered Vibratory Grating Scanners forHigh-Speed High-Resolution Laser Scanning

Yu Du, Guangya Zhou, Koon Lin Cheo, Qingxin Zhang, Hanhua Feng, and Fook Siong Chau

Abstract—A novel micromachined electrostatic double-layeredvibratory grating scanner has been successfully developed forhigh-speed high-resolution laser scanning applications. This paperpresents its design, modeling, fabrication, and measurement re-sults. A comprehensive dynamic model considering the geometricnonlinearity of the platform suspension flexures is also proposed topredict the dynamic performance of the device at large scanningamplitudes. Compared with previously reported single-layered vi-bratory grating scanners, double-layered scanners—in which thediffraction grating and its driving actuator are located in differentlayers—have the potential to scan at large amplitudes and at highscanning speeds with large aperture sizes. We have demonstrateda prototype with a 2-mm-diameter diffraction grating which iscapable of scanning at 23.391 kHz with an optical scan angle ofaround 33◦ and a resulting θopticalD product (product of theoptical scan angle and diameter of the diffraction grating) of66 deg · mm. [2010-0095]

Index Terms—Diffraction grating, microresonators, microscan-ners, microoptoelectromechanical systems (MOEMS).

I. INTRODUCTION

NUMEROUS applications, such as raster scanning for au-tomotive head-up displays, head-worn displays [1]–[4],

and other displays for personal electronic devices or mobilecomputing, utilize and require high-speed high-resolution laserscanning technology. In recent years, microelectromechanical-systems (MEMS)-based micromirror laser scanners [5], [6],which utilize the out-of-plane deflection of a reflective surfaceto scan the laser beam, have been developed primarily due totheir outstanding advantages, such as having a miniaturizeddevice size, a high scanning speed, low power consumption,and a low per unit cost compared with macrolaser scanners.However, the mirror plate in a micromirror scanner is usuallyvery thin due to the nature of the micromachining process, re-sulting in a dynamic nonrigid body deformation of the reflectivesurface and, consequently, a significant aberration to the opticalsystem during high-speed scanning [7]–[9]. In contrast, MEMSvibratory grating scanners [10]–[15], which utilize the in-plane

Manuscript received April 5, 2010; revised July 14, 2010; accepted July 19,2010. Date of publication September 7, 2010; date of current version October 1,2010. Subject Editor O. Solgaard.

Y. Du is with the National University of Singapore, Singapore 119077, andalso with the Institute of Microelectronics, Agency for Science, Technology,and Research, Singapore 117685.

G. Zhou, K. L. Cheo, and F. S. Chau are with the National University ofSingapore, Singapore 119077 (e-mail: [email protected]).

Q. Zhang and H. Feng are with the Institute of Microelectronics, Agency forScience, Technology, and Research, Singapore 117685.

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

Digital Object Identifier 10.1109/JMEMS.2010.2067440

Fig. 1. Working principle of the MEMS vibratory grating scanner.

rotation of a diffraction grating, have the potential to scan at ahigh scanning speed with an enhanced mechanical stability andare subjected to less dynamic deformation.

A brief working principle of the vibratory grating scanneris outlined as follows. As illustrated in Fig. 1, the diffractiongrating lies in the XOY plane, and the grating lines areorientated parallel to the Y -axis. A laser beam, which lies in theXOZ plane, illuminates the diffraction grating with an incidentangle of θi. The incident laser beam coupled with the rotationof the diffraction grating about the Z-axis causes the diffractedlaser beam (except the zeroth order diffraction beam) to scanaccordingly. The wavelength and incident angle of the illumi-nation laser beam, as well as the pitch of the diffraction grating,need to obey certain scanning conditions [12] to achieve abow-free scanning trajectory. A high diffraction efficiency ofmore than 75% can be obtained when a linearly TM-polarizedlaser beam is utilized [13]. Since the diffraction grating is adispersive optical element, vibratory grating scanners with asingle grating are only suitable for narrow-band laser scanningapplications such as monochromatic laser scanning display.Nevertheless, vibratory grating scanners can also be used inmultiwavelength collinear scanning applications, such as colordisplays, by configuring multiple diffraction grating elementson a common platform [12].

High-speed laser scanning was successfully demonstratedwith prototype electrostatic single-layered vibratory gratingscanners in which the diffraction grating and its driving ac-tuator are located in a single layer [10]–[15]. The opticalscan angle of these scanners is limited by the maximum al-lowable deformation and internal stiffness of the suspensionflexures that support the grating platform [15]. Since there is an

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DU et al.: DOUBLE-LAYERED VIBRATORY GRATING SCANNERS 1187

Fig. 2. Schematic illustration of electrostatic (a) single-layered and(b) double-layered vibratory grating scanners.

inversely proportional relationship between the diameter of thediffraction grating platform and the maximum optical scanangle, the optical resolution, the product of the two quantitiesθoptical (optical scan angle) and D (diameter of the diffractiongrating), cannot be further improved by increasing the aperturesize. Thus, to further enhance the optical resolution, a novelconfiguration where the size of the diffraction grating and themaximum optical scan angle are independent is required.

This paper presents the design, modeling, fabricationprocess, and measurement results of a micromachined vibra-tory grating scanner which overcomes the aforementioned con-straint by employing a double-layered design. The prototypedevice, which has a 2-mm-diameter diffraction grating, canachieve an optical scan angle of around 33◦. The θopticalDproduct is obtained as 66 deg · mm for a 632.8-nm wavelengthlaser beam at a vibratory resonant frequency of 23.391 kHz.

Section II gives a brief introduction to the electrostaticdouble-layered vibratory grating scanner with its structuraldesign and the dynamic modeling of the prototype devicepresented in Section III. Section IV describes the customizedmicromachining processes developed, and Section V reportsand discusses the experimental results obtained with the pro-totype device. Finally, Section VI provides some concludingremarks.

II. ELECTROSTATIC DOUBLE-LAYERED

VIBRATORY GRATING SCANNER

Depending on the location of the diffraction grating platform,MEMS vibratory grating scanners can be classified as beingsingle layered or double layered, as schematically illustratedin Fig. 2(a) and (b), respectively. In a single-layered vibratorygrating scanner, the grating platform is suspended by severalsuspension flexures at its edge and located in the same plane

as its driving actuator [shown in Fig. 2(a)]. Although adoptingthe single-layered configuration makes the fabrication of thedevice very simple, it makes further improvement of the opticalperformance very difficult. Since the rotation angle of thegrating platform is inversely proportional to the diameter of thediffraction grating platform, the aperture size and the opticalscan angle cannot be increased simultaneously for a givenmaximum allowable deformation of the platform suspensionflexures. In addition, increasing the diameter of the gratingplatform will increase its rotational inertia, and the stiffnessof the suspension flexures would have to be increased byeither widening or shortening the flexures to maintain the samescanning frequency. This will induce an excessive stress in thesuspension flexures and reduce the scanning amplitude.

Double-layered vibratory grating scanners, in which the grat-ing platform and its driving actuator are located in different lay-ers, are proposed to improve the optical performance instead.As shown in Fig. 2(b), the grating platform is located in thetop layer and connected to a connection platform through around pillar at its center. The connection platform and drivingactuator are located in a separate layer below the grating andare supported by several flexures. Under this configuration, therotation angle of the grating platform is no longer determinedby the size of the diffraction grating but by the diameter ofthe connection platform. Therefore, the size of the diffractiongrating can be increased, and the size of the connection platformis reduced to the minimum, thus increasing the aperture sizeand the scanning amplitude simultaneously. This configurationsignificantly boosts its optical resolution. Furthermore, thedouble-layered configuration makes the thickness of the gratingplatform and suspension flexures independent of each other.This is highly advantageous for high-speed scanning. It is wellknown that increasing the diameter of the grating platform willincrease its rotational inertia. However, high-speed resonantscanning can be maintained by either thinning the gratingplatform or thickening the suspension flexures, together withthe driving comb drives. The former is not likely to result in thedynamic deformation of the grating platform, and the latter willreduce the stress level in the suspension flexures. Consequently,double-layered vibratory grating scanners are capable of scan-ning at high speeds with large scanning amplitudes and whichhave large grating aperture sizes.

III. MECHANICAL DESIGN AND MODELING

There are many different ways to fabricate the electrostaticdouble-layered vibratory grating scanner, such as the multi-layer surface micromachining process [16], the wafer bondingprocess [17], and the postassembly process. In this paper,we prefer to use a combination of the bulk micromachiningprocess and the postassembly process to fabricate the device,which not only reduces the complexities and difficulties of thedevice fabrication but also supplies enough flexibility to thedevice design. The grating platform, which has the diffractiongrating on its top and the connection pillar on its back, and thedriving actuator are designed and fabricated separately usingthe bulk micromachining process. They are assembled togethermanually by using a specially designed manual aligner and


Fig. 3. Schematic illustration of the assembled electrostatic double-layeredvibratory grating scanner.

Fig. 4. Schematic illustration of the grating platform with connection pillar.

bonded together using the epoxide resin. The manual assemblyprocess is mainly used in the stage of concept proving, andit can be replaced by a customized semiauto- or automachin-ery assembly process in mass production, such as the dieattachment process in photonics packaging. The fabrication andassembly processes will be discussed in detail in Section IV.The schematic illustration of the assembled double-layeredvibratory grating scanner is shown in Fig. 3.

A. Structure Design

The grating platform with the connection pillar is schemat-ically shown in Fig. 4. The diffraction grating, with a 400-nmgrating pitch and a 2-mm diameter, is patterned on the frontside of the circular grating platform, which has a radius andthickness of 1010 and 10 μm, respectively. The connectionpillar is attached to the back side of the grating platformat its center. Its radius and thickness are 150 and 200 μm,respectively.

The schematic illustration of the 2-DOF electrostatic comb-driven circular resonator, which acts as the driving actuator, isshown in Fig. 5. The circular connection platform, with themounting hole at its center, is connected to the outer comb-driven circular resonator through 28 single-beam flexures, eachof them having two pairs of perpendicularly connected stressalleviation beams. The radii of the connection platform and themounting hole are 250 and 150 μm, respectively. Among the28 single-beam flexures, 14 are 21 μm wide and 1300 μm long,and the rest are 23 μm wide and 1500 μm long. The outercircular resonator is suspended by 32 sets of circular folded

Fig. 5. Schematic illustration of the 2-DOF electrostatic comb-driven circularresonator.

beam suspensions with beams that have a 17-μm width anda 340-μm length. Each stress alleviation beam has a width of6 μm and a length of 255 μm. There are 280 movable circularfingers for one-side driving with a finger width of 7 μm, afinger gap of 4 μm, and an initial finger overlap angle of 0.7◦.The thickness of all the structures in the driving actuator layeris 80 μm.

B. Restoring Torque of the Platform Suspension Flexure

As discussed in a previous report, the stress alleviation beamscan only partially release the axial tensile stress of the single-beam flexure, and the geometric nonlinearity cannot be fullyeliminated [15]. Moreover, the stress alleviation beams willslightly reduce the stiffness of the suspension flexure. A finite-element (FE) simulation (the type of analysis is static stressanalysis, the element type is C3D8R, the number of elementsis 19 350, and the number of DOFs is six) using ABAQUSwas conducted to estimate the restoring torques of the pro-posed platform suspension flexure at different half mechanicalrotational angles of the connection platform. Both linear andnonlinear analyses were conducted, and the results are shownin Fig. 6(a) and (b). The geometric nonlinearity due to the largedeformation of the flexures was considered in the nonlinearanalyses. The material properties of the single crystal siliconused in this simulation are shown in Table I. The simulationresults indicate that, as expected, when the rotational angle issmall, the restoring torque remains linear, and the geometricnonlinearity can be fully ignored. However, when the rotationalangle is larger than 0.1 arc, the nonlinearity of the restoringtorque appears, and the geometric nonlinearity can no longer beignored. The equations for both linear and nonlinear restoringtorques as a function of the rotation angle for the two types ofsuspension flexures are acquired by using fitting polynomials tothe simulated data points, as shown in (1) and (2), respectively,

{τ01 = 5.02 × 10−5θτ02 = 5.38 × 10−5θ

(1)

{τ̃01 = 5.02 × 10−5θ + 2.13 × 10−4θ3

τ̃02 = 5.38 × 10−5θ + 2.32 × 10−4θ3.(2)


Fig. 6. FE simulated torque–angle points and polynomial fitted curve for twotypes, (a) and (b), of main flexural beams.

TABLE IMATERIAL PROPERTIES USED IN THE CALCULATION

C. Linear Model of the Scanner Ignoring theGeometric Nonlinearity

The dynamic behavior of the scanner is simplified to thatof a 2-DOF undamped spring–mass vibration system, whichis schematically shown in Fig. 7. In this model, the geo-metric nonlinearity of the suspensions of the outer resonatorcan be ignored because the outer resonator is suspended bythe circular folded beam suspensions, which are assumed tohave a “perfect” stress releasing mechanism. Additionally, aspreviously discussed, the geometric nonlinearity of the platformsuspension flexure is negligible if its deflection is small.

According to our previous work [15], the linear model ofthe system, considering the influence of the beam mass and the

Fig. 7. Schematic illustration of the simplified 2-DOF undamped vibrationsystems.

stress alleviation beams, is as follows:{J1θ̈1 + ΔJcθ̈2 + K1(θ1 − θ2) = 0J2θ̈2 + ΔJcθ̈1 + K2θ2 + K1(θ2 − θ1) = 0

(3)

where

J1 =J10 + ΔJ10,

J2 =J20 + ΔJ20,

J10 =JG + JP + JC + JA

ΔJ10 = ρSiT

2∑i=1

niWiLi

(1335

R2C +

11105

RCLi +1

105L2

i

)

ΔJ20 = ρSiT

2∑i=1

niWiLi

(1335

R2C +

67105

RCLi +29105

L2i

)

+ ρSinfWfLfT

(8770

R22 +

4142

R2Lf +67210

L2f

)

ΔJc = ρSiT

2∑i=1

niWiLi

(970

R2C +

970

RCLi +142

L2i

)K1 = (n1τ01 + n2τ02)/θ,

K2 =nfEIf

L3f

(12R2

2 + 12R2Lf + 4L2f

).

J10 is the total rotary inertia of the grating platform (JG),the connection pillar (JP ), the connection platform (JC), andthe epoxide resin anchor (JA). The shape of the epoxide resinanchor is approximated by a half sphere with the same radiusas that of the connection platform. J20 is the rotary inertia ofthe outer resonator. ΔJ10, ΔJ20, and ΔJc represent the weightinfluence of the platform suspension flexures. The densitiesof the single crystal silicon (ρSi) and the epoxide resin arelisted in Table I. The amount, width, and length of each typeof platform suspension flexures are ni, Wi, and Li (i = 1, 2),respectively. nf , Wf , and Lf denote the total number, beamwidth, and length of the circular folded beam suspensions. Theradii of the connection platform and the part where the circularfolded beam suspension is connected are denoted by RC andR2, respectively. T is the thickness of all the structures in thedriving actuator.


Fig. 8. FE simulation results showing the resonant frequencies and modeshapes of (a) the first resonating vibration mode and (b) the second resonatingmode.

The linear spring constant of the platform suspension flex-ures is denoted by K1, which is obtained by using the FEsimulated linear restoring torque expressions for both types offlexures. The spring constant of the outer resonator circularfolded beam suspension is obtained through theoretical calcu-lations and represented by K2.

The resonant frequencies (f01 and f02) and mode ratios,which are defined as the ratio of the rotational angle diffractiongrating and the outer resonator, are worked out from (3) forboth the first and the second resonant modes. FE simulationsusing ABAQUS (the type of analysis is natural frequencyextraction, the element type is C3D8R, the number of elementsis 194 850, and the number of DOFs is six) are conductedto investigate the resonant frequencies and mode ratios ofthe system and to verify the theoretical predictions. Fig. 8shows the simulation results, and the theoretically predicted andFE-simulated resonant frequencies and mode ratios are com-pared in Table II. The good agreement shows the validity ofthe linear model in predicting the dynamic performance of thesystem with small scanning amplitudes.

D. Comprehensive Model of the Scanner ConsideringGeometric Nonlinearity

As discussed in Section III-B, the geometric nonlinearityof the platform suspension flexure cannot be ignored at largescanning amplitudes, and the expressions for the restoringtorques for each type of the platform suspension flexures areshown in (2). The total nonlinear restoring torque τ̃ for a given

TABLE IISUMMARY OF THEORETICAL AND SIMULATION RESULTS

relative rotational angle θ between the diffraction grating andthe outer resonator is shown as follows:

τ̃ = n1τ̃01 + n2τ̃02 = K̃1θ + K̃1εθ3. (4)

The small parameter ε, the value of which is calculated to be4.2788, is defined to reflect the degree of nonlinearity in theflexures.

The comprehensive dynamic model can be derived from (3)and (4) by replacing the total linear restoring torque with thetotal nonlinear restoring torque, as shown in the following:{J1θ̈1 + ΔJcθ̈2 + K̃1(θ1 − θ2) + K̃1ε(θ1 − θ2)3 =0J2θ̈2 + ΔJcθ̈1 + K2θ2 + K̃1(θ2 − θ1) + K̃1ε(θ2 − θ1)3 =0.

(5)

Defining ω210 = K̃1/J1, ω2

20 = K2/J2, μ1 = ΔJc/J1, andμ2 = J2/J1, (5) can be expressed as{

θ̈1 + μ1θ̈2 + ω210(θ1 − θ2) + ω2

10ε(θ1 − θ2)3 = 0θ̈2 + μ1

μ2θ̈1 + ω2

20θ2 + ω210

μ2(θ2 − θ1) + ω2

10μ2

ε(θ2 − θ1)3 = 0.

(6)

The harmonic balance method [18] is used to solve (6). Theperiodic solutions are assumed to exist and are expanded using aFourier series with ω0 and only keeping the first order harmonic{

θ1(t) = A10 cos ω0tθ2(t) = A20 cos ω0t

(7)

where A10 and A20 denote the rotational amplitudes of thediffraction grating and the outer resonator, respectively. Thesymbol ϕ is used to represent the mode ratio of the system,which is shown in the following:

A10 = ϕA20. (8)

Then, (9) can be obtained by substituting (7) and (8) into (6)and ordering all the coefficients of the first harmonic to be zero{−ω2

0ϕ−ω20μ1+ω2

10(ϕ−1)+ 34ω2

10ε(ϕ−1)3A20 =0

−ω20− μ1

μ2ω2

0ϕ+ω220+ ω2

10μ2

(1−ϕ)+ 3ω210

4μ2ε(1−ϕ1)3A20 =0.

(9)


The solution of (9) yields the resonant frequencies (f), therotational amplitudes (A10 and A20) of the diffraction gratingand the outer circular resonator, and the optical scan angle(θopt.), as shown in the following:⎧⎪⎪⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎪⎪⎩

f = ω02π = ω20

2π

√μ2

ϕ+μ1+μ2+μ1ϕ

A10 = ϕA20

A20 =

√√√√ϕ2+(μ2−1)ω2

10−μ2ω2

20(1+μ1)ω2

10ϕ+

(μ1+μ2)ω210

+μ1μ2ω220

(1+μ1)ω210

3ε4 (ϕ+

μ1+μ21+μ1

)(1−ϕ)3

θopt. = 2CampA10.

(10)

In (10), Camp is the magnification coefficient from the me-chanical rotational angle of the diffraction grating to the opticalscan angle, which is calculated to be 1.5878 for the diffractiongrating that has a grating pitch of 400 nm and an incident laserbeam with a wavelength of 632.8 nm according to our previouswork [12].

The calculated resonant frequencies with different opticalscan angles are then normalized by their linear resonant fre-quencies f01 and f02 for both the first and the second resonatingmodes for easy comparison. The normalized resonant frequen-cies of both the resonating modes are denoted by S1 and S2,which are as follows: {

S1 = f/f01

S2 = f/f02.(11)

The theoretical model predicts the variations for the resonantfrequencies with different ε values of both the first and secondresonating modes, which are shown in Fig. 9. It is seen that theresonant frequencies increase with increasing optical scan anglefor both resonating modes, and the amount of variation of theresonant frequency with the same optical scan angle increaseswith increasing ε value due to the greater degree of nonlinearity.

IV. FABRICATION PROCESS

A. Diffraction Gating, Grating Platform, andConnection Pillar

The diffraction grating, grating platform, and connectionpillar were fabricated using silicon-on-insulator (SOI) micro-machining technology with a total of four photomasks that areused. A SOI wafer with a 10-μm-thick silicon device layer, a2-μm-thick buried oxide (BOX) layer, and a 750-μm-thicksilicon substrate was used in the fabrication process. Fig. 10shows a schematic of the fabrication process flow.

With reference to Fig. 10, a 200-nm low-pressure chemi-cal vapor deposition (LPCVD) silicon nitride layer was firstdeposited on both sides of the SOI wafer. The top layer wasthen removed by a reactive ion etching (RIE) process. Next,the diffraction grating with a 400-nm grating period and a50% duty cycle was patterned using deep-UV lithography andetched using timed plasma etching. The 10-μm-thick silicondevice layer was patterned and etched through to form thegrating platform by using a deep RIE (DRIE) process, whichstopped at the BOX layer. Then, the LPCVD silicon nitridelayer on the back side of the SOI wafer, which served as the

Fig. 9. Theoretical predictions of the variation of resonant frequency withthe optical scan angle of (a) the first and (b) the second resonating mode fordifferent ε values.

Fig. 10. Fabrication process flow for the diffraction grating, grating platform,and connection pillar.


Fig. 11. SEM image showing the fabricated grating platform and the connec-tion pillar.

hard mask for the following wet-etching process, was patternedusing RIE. Subsequently, a 550-μm-deep square cavity, whichthinned down the silicon substrate to 200 μm, was formed bywet anisotropic Si etch in a KOH solution (35 wt%, 75 ◦C).After that, a 1-μm plasma-enhanced chemical vapor deposition(PECVD) undoped silicon glass (USG) layer was depositedonto the SOI wafer’s back side and the bottom of the etchedcavity and patterned using the RIE process. The connectionpillar was then formed by another DRIE process, which stoppedat the BOX layer. Finally, the exposed BOX layer was etchedfrom the back side by using a buffered oxide etchant (BOE)solution (with six parts of 40% NH4F and one part of 49% HF).

The thickness of the grating platform was accurately definedby the thickness of the silicon device layer, and the thicknessof the connection pillar is defined by the depth of the back sidecavity, formed by the wet anisotropic Si etch which has excel-lent etching uniformity. The grating platforms and connectionpillars with different dimensions can be easily fabricated bysimply choosing the SOI wafers with the required specificationsand tuning process parameters. Fig. 11 shows a SEM image ofthe fabricated grating platform and the connection pillar.

B. 2-DOF Electrostatic Comb-Driven Circular Resonator

The 2-DOF electrostatic comb-driven circular resonator wasalso fabricated using the SOI micromachining technology;three photomasks were used in this process. The SOI wafer hasa 80-μm-thick heavily doped silicon device layer, a 2-μm-thickBOX layer, and a 650-μm-thick silicon substrate. The overalldie size is 6 mm × 6 mm. The fabrication process flow isillustrated in Fig. 12.

As shown in Fig. 12, a 1-μm PECVD USG layer was firstdeposited onto the SOI wafer’s front side and patterned byusing the RIE process. It was subsequently used as the hardmask for the following DRIE etching process. The SOI waferwas patterned on its back side, and the 650-μm-thick siliconsubstrate was etched through by the following DRIE processto expose the region encompassing all the structures. The BOXlayer was used as a stop layer for the DRIE etching process.Next, the 80-μm-thick silicon device layer was etched throughfrom the wafer’s front side by another DRIE process, which isalso stopped at the BOX layer. After this step, the connectionplatform with a mounting hole, the circular comb-drive actua-

Fig. 12. Fabrication process flow for the 2-DOF electrostatic comb-drivencircular resonator.

Fig. 13. Fabricated 2-DOF electrostatic comb-driven circular resonator shownby a microscopic image.

tor, and the suspension flexures were formed. The USG layer onthe wafer’s front side and the exposed BOX layer were etchedaway in the BOE solution so that all the structures formed inthe silicon device layer were released. Finally, the metal padsfor wire bonding were formed by evaporating a 1000 Å/5000 Åthick Ti/Au layer through a shadow mask.

Fig. 13 shows a microscopic image of the fabricated 2-DOFelectrostatic comb-driven circular resonator.

C. Manual Assembly Process

After the grating platform and the 2-DOF electrostatic comb-driven circular resonator have been fabricated, they were as-sembled manually, as schematically illustrated in Fig. 14.

As shown in Fig. 14(a), both the grating platform and 2-DOFelectrostatic comb-driven circular resonator were turned upsidedown before the assembly. A silicon substrate with a circularcavity located at its center, which is used as a mold during themanual assembly process, was designed and fabricated usingthe bulk micromachining process. The circular cavity with adiameter of 2050 μm and a depth of 90 μm was formed bya timed DRIE process, ensuring that the bottom of the cavityis parallel to the surface of the substrate. Then, the grating


Fig. 14. Schematic illustration of the manual assembly process.

platform was placed inside the circular cavity in the fixedsilicon substrate, and the driving resonator was held by a grip-per attached to a three-axes precision positioner. The 2-DOFcircular resonator was then aligned under a microscope so thatthe mounting hole and the connection pillar were concentric.Next, the 2-DOF circular resonator was pressed tightly until itcontacts the surface of the fixed silicon substrate. Meanwhile,the connection pillar was inserted into the mounting hole, asshown in Fig. 14(b). The gap between the grating platformand the 2-DOF circular resonator was determined by the depthof the circular cavity in the silicon substrate. The parallelismbetween the diffraction grating and its driving actuator wasensured by the accuracy of the fabricated silicon substrate.Finally, the connection pillar and the connection platform werebonded together by Araldite 2012 epoxy adhesive [as shown inFig. 14(c)] and cured in oven with a temperature of 40 ◦C for2 h to achieve a full strength of 25–30 MPa.

Fig. 15 shows the whole view and the part of the assem-bled electrostatic double-layered vibratory grating scanner. Theback-side view of the assembled device and the magnified viewof the epoxide resin anchor are shown in Fig. 16.

Since the MEMS vibratory grating scanner utilizes the in-plane rotational motion of a diffraction grating to scan the laserbeam, the parallelism between the diffraction grating and therotational plane is a very important factor that influences thequality of the laser scanning trajectory. To evaluate the accuracyof the proposed manual assembly process, the parallelism of the

Fig. 15. Whole view and the part of the assembled electrostatic double-layered vibratory grating scanner.

Fig. 16. Microscopic images showing the back side of the assembled elec-trostatic double-layered grating scanner and the magnified view of the epoxideresin anchor.

diffraction grating and its driving actuator was measured usinga white-light interferometer. The measurement result shows thatthe measured slope angle of the diffraction grating relative toits driving actuator is only 0.0297◦, a value so small that ithas a negligible effect on the quality of the laser scanningtrajectory. This indicates that the proposed manual assemblyprocess is sufficiently accurate for the proposed electrostaticdouble-layered vibratory grating scanner.

V. EXPERIMENTAL RESULTS

The performance of the prototype electrostatic double-layered vibratory grating scanner was tested using a lin-early TM-polarized He–Ne laser beam (with a wavelength of632.8 nm), and the experimental setup is schematically il-lustrated in Fig. 17. Since the grating pitch of the diffrac-tion grating is 400 nm, the incident angle of the illuminated


Fig. 17. Schematic illustration of experimental setup for prototype double-layered vibratory grating scanner.

TABLE IIISUMMARY OF THEORETICAL ANALYSES AND EXPERIMENTAL RESULTS

laser beam was determined to be 71.8◦ to achieve bow-freescanning [12].

The dynamic performance of the prototype device was testedin a vacuum environment (1.2 mtorr). The circular comb-driveresonator was driven by a push–pull mechanism [12] with adc bias and an ac signal following a sinusoidal pattern. Theoptical scan angle was determined by measuring the length ofthe laser scanning trajectory on a projection screen which wasaligned perpendicular to the first order diffracted laser beamwhen the diffraction grating is at a standstill. The frequencyresponse of the prototype scanner was obtained by sweepingthe frequencies of the driving signal. As expected, the double-layered vibratory grating scanner has two resonating modes.The resonant frequencies of the first and the second resonatingmodes were determined to be around 22.987 and 34.941 kHz,respectively. The driving voltages were fixed at 5-V dc bias and10-V ac peak-to-peak voltages to avoid the shift of the resonantfrequencies due to large deflection nonlinearities. The gener-ally good agreement between the theoretical estimations, FEsimulations, and measurement results (see Table III) indicatesthe validity of the theoretical models. The deviations betweenthe theoretical and experimental values are mainly attributedto the imperfections of the fabrication process [20], the uncer-tainty in the material properties, and the imperfect boundaryconditions for the suspension flexures.

The electrostatic double-layered vibratory grating scanner isdesigned to work at frequencies near the first resonant mode,as its higher mode ratio results in larger scanning amplitudeswith a less risk of brittle fracture of the platform suspensionflexures as compared with the second resonant mode. Due to thesmall travel range of the circular comb-drive actuator as well asthe low mode ratio at the second resonant mode, the scanning

Fig. 18. Measured frequency response of the prototype scanner in vacuum atfrequency regions near the resonant frequencies of the first resonating mode.

Fig. 19. Photograph of the laser scanning trajectory of the prototype scanneron a projection screen.

amplitude obtainable is very small, and further increasing thedriving power may cause the instability of the circular comb-drive actuator. Therefore, only the frequency response near theregion of the first resonant mode with a high driving powerwas measured. Fig. 18 shows the measured frequency responseof the prototype grating scanner near the region of the firstresonating mode. With fixed driving voltages of 65-V dc biasand 130-V ac peak-to-peak voltages, the prototype scannerwith a 2-mm-diameter diffraction grating was able to scan ata frequency of 23.391 kHz with an optical scan angle of 33◦,resulting in a θopticalD product of 66 deg · mm. High-speedlaser scanning is thus experimentally demonstrated, and Fig. 19shows a photograph of the projected laser scanning trajectoryon a projection screen located at a distance of 100 mm away.

As expected, the dynamic performance of the proto-type scanner demonstrated some level of the large-deflection


Fig. 20. Comparison of theoretical and experimental measured variations ofnormalized resonant frequencies with optical scan angle.

nonlinearity. For example, the scanning amplitudes were dif-ferent during the forward and backward frequency sweeps, theamplitude jumping phenomena was observed, and the resonantfrequency depended on the scanning amplitude. By graduallyincreasing the driving voltages from 5-V dc bias and 10-V acpeak-to-peak voltages to 65 V-dc bias and 130-V ac peak-to-peak voltages, the resonant frequencies at different scanningamplitudes were determined by recording the frequencies atwhich the amplitude jumping occurred and then normalized bythe smallest resonant frequency. Fig. 20 shows the comparisonbetween the theoretical predictions and experimental measure-ments of the variations of the normalized resonant frequencieswith the optical scan angle. It can be seen that there is agood agreement when the scanning amplitude is small, withthe error increasing as the scanning amplitude increases. Thetheoretical model could be improved by increasing the orderused to approximate the solution, which could be obtainedusing a numerical method.

VI. CONCLUSION

A novel electrostatic double-layered vibratory gratingscanner has been successfully developed, and high-speed high-resolution laser scanning has been experimentally demon-strated. Since the diffraction grating and its driving actuatorare located at different layers, increasing the diameter of thediffraction grating will not reduce the scanning amplitude.Hence, the double-layered vibratory grating scanner has thepotential to achieve large optical scan angles at high scanningspeeds with large aperture sizes. The optical performance wastested using a linearly polarized He–Ne laser beam, and theincident angle was determined to be 71.8◦ for a diffractiongrating with a grating pitch of 400 nm. The current prototypedevice is capable of scanning at a frequency of 23.391 kHz withan optical scan angle of 33◦ using a diffraction grating with a2-mm diameter, resulting in a θopticalD product of 66 deg · mm.The validity of the comprehensive dynamic model, which takesthe geometric nonlinearity of the platform suspension flexuresinto account, has also been demonstrated.

ACKNOWLEDGMENT

The authors would like to thank Dr. Y. Hongbin of theDepartment of Mechanical Engineering, National University ofSingapore, for his help on the measurements using a white-lightinterferometer.

REFERENCES

[1] H. Urey, D. Wine, and T. Osborn, “Optical performance requirements forMEMS-scanner based micro displays,” in Proc. SPIE MOEMS Miniatur-ized Syst., 2000, vol. 4178, pp. 176–185.

[2] H. Urey, D. Wine, and J. R. Lewis, “Scanner design and resolutiontradeoffs for miniature scanning displays,” in Proc. SPIE Flat PanelDisplay Technol. Display Metrology, San Jose, CA, Jan. 1998, vol. 3636,pp. 60–68.

[3] D. Wine, M. P. Helsel, L. Jenkins, H. Urey, and T. D. Osborn, “Perfor-mance of a biaxial MEMS-based scanner for microdisplay applications,”in Proc. SPIE Conf. MOEMS Miniaturized Syst., Santa Clara, CA, 2000,vol. 4178, pp. 186–196.

[4] R. B. Sprague, T. Montague, and D. Brown, “Bi-axial magnetic drive forscanned beam display mirrors,” in Proc. SPIE MOEMS Display ImagingSyst. III, 2005, vol. 5721, pp. 1–13.

[5] P. M. Hagelin and O. Solgaard, “Optical raster-scanning displays basedon surface-micromachined polysilicon mirrors,” IEEE J. Sel. TopicsQuantum Electron., vol. 5, no. 1, pp. 67–74, Jan. 1999.

[6] R. S. Muller and K. Y. Lau, “Surface-micromachined microoptical el-ements and systems,” Proc. IEEE, vol. 86, no. 8, pp. 1705–1720,Aug. 1998.

[7] R. A. Conant, J. T. Nee, K. Y. Lau, and R. S. Muller, “Dynamic defor-mation of scanning mirrors,” in Proc. IEEE/LEOS Int. Conf. Opt. MEMS,Kauai, HI, 2000, pp. 49–50.

[8] J. T. Nee, R. A. Conant, R. S. Muller, and K. Y. Lau, “Lightweightoptically flat micromirrors for fast beam steering,” in Proc. IEEE/LEOSInt. Conf. Opt. MEMS, Kauai, HI, 2000, pp. 9–10.

[9] M. R. Hart, R. A. Conant, K. Y. Lau, and R. S. Muller, “Stroboscopicinterferometer system for dynamic MEMS characterization,” J. Micro-electromech. Syst., vol. 9, no. 4, pp. 409–418, Dec. 2000.

[10] G. Zhou, L. Vj, F. S. Chau, and F. E. H. Tay, “Micromachined in-planevibrating diffraction grating laser scanner,” IEEE Photon. Technol. Lett.,vol. 16, no. 10, pp. 2293–2295, Oct. 2004.

[11] G. Zhou, L. Vj, and F. S. Chau, “Optical scanning using vibratory diffrac-tion gratings,” U. S. Patent 7 542 188, Jun. 2, 2009.

[12] G. Zhou and F. S. Chau, “Micromachined vibratory diffraction gratingscanner for multiwavelength collinear laser scanning,” J. Microelectro-mech. Syst., vol. 15, no. 6, pp. 1777–1788, Dec. 2006.

[13] G. Zhou, Y. Du, Q. Zhang, H. Feng, and F. S. Chau, “High-speed, high-optical-efficiency laser scanning using a MEMS-based in-plane vibratorysub-wavelength diffraction grating,” J. Micromech. Microeng., vol. 18,no. 8, p. 085 013, Aug. 2008.

[14] Y. Du, G. Zhou, K. L. Cheo, Q. Zhang, H. Feng, B. Yang, andF. S. Chau, “High speed laser scanning using MEMS driven in-planevibratory grating: Design, modeling and fabrication,” Sens. Actuators A,Phys., vol. 156, no. 1, pp. 134–144, Nov. 2009.

[15] Y. Du, G. Zhou, K. L. Cheo, Q. Zhang, H. Feng, and F. S. Chau,“A 2-DOF circular-resonator-driven in-plane vibratory grating laserscanner,” J. Microelectromech. Syst., vol. 18, no. 4, pp. 892–904,Aug. 2009.

[16] J. Tsai and M. C. Wu, “Design, fabrication, and characterization of a highfill-factor, large scan-angle, two-axis scanner array driven by a leveragemechanism,” J. Microelectromech. Syst., vol. 15, no. 5, pp. 1209–1213,Oct. 2006.

[17] I. W. Jung, U. Krishnamoorthy, and O. Solgaard, “High fill-factor two-axis gimbaled tip-tilt-piston micromirror array actuated by self-alignedvertical electrostatic combdrives,” J. Microelectromech. Syst., vol. 15,no. 3, pp. 563–570, Jun. 2006.

[18] N. Minorsky, Nonlinear Oscillations. Princeton, NJ: Van Nostrand,1962.

[19] W. C. Tang, T. C. H. Nguyen, M. W. Judy, and R. T. Howe, “Electrostatic-comb drive of lateral polysilicon resonators,” Sens. Actuators A, Phys.,vol. 21, no. 1–3, pp. 328–331, Feb. 1990.

[20] J. Li, Q. X. Zhang, A. Q. Liu, W. L. Goh, and J. Ahn, “Technique forpreventing stiction and notching effect on silicon-on-insulator microstruc-ture,” J. Vac. Sci. Technol. B, Microelectron. Process. Phenom., vol. 21,no. 6, pp. 2530–2539, 2003.


Yu Du received the B.S. degree in mechanical en-gineering from Xi’an Jiaotong University, Xi’an,China, in 2005. He is currently working toward thePh.D. degree in the Micro and Nano Systems Initia-tive, National University of Singapore, Singapore.

He is also currently an attached full-time ResearchStudent with the Semiconductor Process TechnologyLaboratory, Institute of Microelectronics, Agencyfor Science, Technology, and Research, Singapore.His research interests include optical microelectro-mechanical systems, and his current work focuses on

the design and fabrication of optical microdevices.

Guangya Zhou received the B.Eng. and Ph.D.degrees in optical engineering from Zhejiang Uni-versity, Hangzhou, China, in 1992 and 1997,respectively.

He is currently an Assistant Professor in theDepartment of Mechanical Engineering, NationalUniversity of Singapore, Singapore. His main re-search interests include bioimaging, microoptics,diffractive optics, microelectromechanical systemsdevices for optical applications, and nanophotonics.

Koon Lin Cheo received the B.S. and M.S. de-grees in mechanical engineering from the NationalUniversity of Singapore, Singapore, in 2003 and2005, respectively. He is currently working towardthe Ph.D. degree in the Micro and Nano SystemInitiative, National University of Singapore.

His research interests include optical micro-electromechanical systems, and his current workfocuses on system-level applications of opticalmicrodevices.

Qingxin Zhang received the B.S. and M.S. de-grees in semiconductor devices and physics fromHarbin Institute of Technology, Harbin, China, in1986 and 1989, respectively, and the Ph.D. degree inmicroelectronics from Tsinghua University, Beijing,China, in 1997.

For two years, he was a Research Fellow atNanyang Technological University, Singapore. Since1999, he has been with the Institute of Microelec-tronics, Agency for Science, Technology, and Re-search, Singapore, where he is currently a member

of the Technical Staff of the Semiconductor Process Technology Laboratory.His major research interests include microelectromechanical systems designand processes and systems on-package and platform technologies for integratedmicroelectromechanical systems, CMOS IC, and photonics.

Hanhua Feng received the B.Eng. degree in semi-conductor physics and devices and the M.Eng. andPh.D. degrees from Huazhong University of Scienceand Technology (HUST), Wuhan, China, in 1985,1988, and 1991, respectively.

For two years, she was a Lecturer and an Asso-ciate Professor of electronic materials and devicesat HUST. She was a Postdoctoral and Research Fel-low at the Northern Ireland BioEngineering Center,University of Ulster, Newtownabbey, U.K., and atthe School of Electrical and Electronics Engineering,

Nanyang Technological University, Singapore. Since January 1998, she hasbeen with the Institute of Microelectronics, Agency for Science, Technology,and Research, Singapore, where she is currently the MEMS Program Manager.She is familiar with aspects of wafer fabrication with expertise, particularly inCMOS/MEMS integration, plasma-enhanced chemical vapor deposition, andplasma etching, from her many years of work experience in the microelectron-ics and MEMS areas.

Fook Siong Chau received the B.Sc. (Eng.) andPh.D. degrees from the University of Nottingham,Nottingham, U.K., in 1974 and 1978, respectively.

He is currently an Associate Professor in theDepartment of Mechanical Engineering, NationalUniversity of Singapore, Singapore, where he isalso the Head of the Applied Mechanics AcademicGroup. His main research interests include devel-opment and applications of optical techniques forthe nondestructive evaluation of components and themodeling, simulation, and characterization of mi-

crosystems, particularly biomicroelectromechanical and microoptoelectro-mechanical systems.

double-layered vibratory grating scanners for high-speed high-resolution laser scanning

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