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Two-dimensional optical scanner with monolithically integrated glass microlens

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2014 J. Micromech. Microeng. 24 055009

(http://iopscience.iop.org/0960-1317/24/5/055009)

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Journal of Micromechanics and Microengineering

J. Micromech. Microeng. 24 (2014) 055009 (8pp) doi:10.1088/0960-1317/24/5/055009

Two-dimensional optical scanner withmonolithically integrated glass microlens

Sunghyun Yoo1, Joo-Young Jin1, Joon-Geun Ha1, Chang-Hyeon Ji2

and Yong-Kweon Kim1

1 Department of Electrical Engineering and Computer Science, Seoul National University 301-1116(#007), Gwanak, PO Box 34, Seoul 151-600, Korea2 Department of Electronics Engineering, Ewha Womans University 52 Ewhayeodae-gil, Seodaemun-gu,Seoul 120-750, Korea

E-mail: yongkkim@snu.ac.kr

Received 28 October 2013, revised 26 February 2014Accepted for publication 28 February 2014Published 3 April 2014

AbstractA miniaturized two-dimensional forward optical scanner with a monolithically integrated glassmicrolens was developed for microendoscopic imaging applications. The fabricated devicemeasures 2.26 × 1.97 × 0.62 mm3 in size and a through-silicon microlens with a diameter of400 μm and numerical aperture of 0.37 has been successfully integrated within the siliconlayer. An XY stage structure with lens shuttle and comb actuators was designed, andproprietary glass isolation blocks were utilized in mechanical and electric isolation of X- andY-axis actuators. Resonant frequencies of the stage in X and Y directions were 3.238 and2.198 kHz and quality factors were 168 and 69.1, respectively, at atmospheric pressure.Optical scanning test has been performed and scan angles of ±4.7◦ and ±4.9◦ were achievedfor X and Y directions, respectively.

Keywords: two-dimensional, optical scanner, microlens, glass reflow

(Some figures may appear in colour only in the online journal)

1. Introduction

In recent years, high performance microscanners have beena major driving force for the evolution of microendoscopy.Diverse optical technologies have been combined withmicroendoscopy for applications such as optical coherencetomography [1–7], two-photon microscopy [8], and confocalmicroscopy [9]. In terms of the imaging direction,microscanners can be classified into three different categorieswhich are circumferential, side-viewing and forward-viewing.Circumferential and side-viewing scanners are relativelyadvantageous in sidewall inspection of tubular organswhile forward-viewing types are preferred in the image-guided surgical operation tools or inspection of halloworgans, which are rather straightforward consequence ofthe target directionality. Examples of the forward-viewingmicroscanners include vibrating optical fibers [1–3, 8],rotational GRIN lenses [6], micromirrors [7, 9] andmicrolens scanners [10–13]. Microlens scanners based onmicroelectromechanical systems (MEMS) are well suited for

forward-viewing microendoscopy owing to its transmissivescanning capability. However, given the fact that conventionalscanners with integrated microlens rely on manual assemblyof off-the-shelf miniature glass lens or liquid droplet, thesedevices are inherently vulnerable to compromised fabricationaccuracy and production yield.

Kwon et al and Park et al have realized two-dimensional (2D) microlens scanners by stacking one-dimensional scanners either vertically or laterally [10, 12, 14].Despite the successful demonstration of the device, stackingapproach inherently increases the complexity of the systemas well as that of the fabrication process. Moreover, accuratecontrol of the optical axis alignment between stages can be verychallenging. Several types of XY-stages have been introducedto resolve these issues, but rather expensive fabricationapproach utilizing SOI (silicon-on-insulator) substrate stillremains as an issue [15–17]. Previously, we have demonstrateda one-dimensional forward optical scanner based on a uniquewafer-level fabrication approach where thermal reflow ofanodically bonded glass substrate enables the monolithic

0960-1317/14/055009+08$33.00 1 © 2014 IOP Publishing Ltd Printed in the UK

J. Micromech. Microeng. 24 (2014) 055009 S Yoo et al

(a)

(b)

Figure 1. Schematic diagram of overall design and device features (a) and beam steering principle of the proposed scanner (b).

integration of glass microlens in the single crystalline siliconstructural layer [18].

The purpose of this work is to present a newly proposed 2Dmicrolens scanner for microendoscopic imaging applications.A plano-convex glass microlens is monolithically integratedwithin a silicon structural layer through process described in[18], while the numerical aperture (NA) of the microlensis improved in order to increase the beam tilting angle.Using the same fabrication procedure, a single-layer dual-axislateral translation stage is implemented without increasing thefabrication complexity. This is realized through embeddingglass isolation blocks into the structural layer resulting inmechanical and electrical separation of X and Y axes ofactuation. Proposed device can potentially be utilized inmicroendoscopic applications as well as other microphotonicdevices where steering of integrated microlens is required.Moreover, glass isolation block used in this work canbe utilized in various MEMS devices where electric orthermal isolation is required while maintaining the structuralconnectivity.

2. Design and working principle

Overall design and features of the proposed 2D microlensscanner are illustrated in figure 1(a). The overall size of themicrolens scanner is 2.26 × 1.97 × 0.56 mm3 which iswithin 3 mm in diameter for endoscopic application. Thescanner basically comprises a glass microlens, electrostatically

driven silicon comb-drives for each X- and Y-axis actuation,and glass isolation blocks. The plano-convex glass microlensis monolithically integrated within the inner stage throughthermal reflow process. The stage is mechanically supportedby two folded-beam flexures and connected with comb-driveelectrodes which enable the X-axis actuation. The outer stageis designed for Y-axis actuation, which is mechanically andelectrically isolated from X-axis actuation. Six glass blocksfunction as an electrical insulation layer between the drivingelectrodes, which is simultaneously fabricated with the glassmicrolens.

Figure 1(b) illustrates the optical layout relevant to thesystem. Incoming light comes from a light source located atthe focal length (fs) distance from the scanning microlens. Thedirection of the light beam is steered by linear translationalmotion of the microlens and the tilting angle is proportional tothe displacement of the scanning microlens (�d) at the centerof the XY stage. Corresponding tilting angle can be derived as,θ s = tan−1(�d/fs). Steered beam is focused on the imagingplane whose position is defined by the focal length of theobjective lens (fo). In a typical imaging system, NA of thescanning microlens has to be maximized in order to achievelarger maximum number of resolvable spots. The target NA ofthe microlens proposed in this work is over 0.35 consideringthe applicability to commercially available high NA singlemode fiber which has NA between 0.24 and 0.35. Radius andfocal length of the designed microlens are 200 and 490 μm,respectively. Relevant numerical aperture of the microlens is

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(a) (b)

Figure 2. Schematic diagram of the working principle of (a) X-axis and (b) Y-axis microlens translation. Colored sections are electrodeswhere the input signal is applied.

(a)

(e)

( f )

(g)

(b)

(c)

(d)

Figure 3. Schematic diagram of the fabrication process at full wafer level.

0.378. In terms of the misalignment between light source andscanning microlens, ±1 μm deviation yields tilt angle offsetof ±0.117◦ in typical configuration. Detailed features relatedto fabrication procedure of the microlens will be discussedin section 3. The XY-stage is designed to have as largedisplacement as possible within 3 mm diameter footprint withadditional constraints on driving voltage. Maximum voltagewas set to 50 V for both ac input and dc bias. The maximumlateral displacement of the stage is ±40 μm in both axes andresulting beam steering angle is ±4.7◦ in both directions.

Figure 2 shows the detailed working principle of dual-axis microlens translation. Cross mark indicates the fixedanchor while the movable stage in the center is suspendedabove the glass substrate. As all the anchors are connected toeither ground or driving electrodes, the anchors are designedto have a sufficient area (>150 × 150 μm2) in order towork as a wiring pad. The colored section in each figureindicates the electrode with applied electric potential. Whenan input voltage is applied, electrostatic force attracts the stagetowards the driving electrode. Due to the glass isolation blocksand appropriately designed flexures, Y-axis actuation can be

achieved without any interference with the X-axis drivingelectrodes.

3. Device fabrication

The proposed 2D microlens scanner is fabricated in a completewafer-level process without any manual assembly, as depictedin figure 3. The fabrication process and numerical modelingof the glass reflowed microlens are described in more detail in[18, 19]. The process starts with the definition of microlensgeometry and the position of the isolation blocks. Siliconcavities are formed using the deep reactive ion etching(DRIE, ICP-DRIE, Plasma-Therm) process (figure 3(a)). Aborosilicate glass (BSG, Borofloat R© 33, Schott) wafer isanodically bonded with the silicon wafer in a vacuum. The firstthermal reflow process (figure 3(b)) is done in a mini furnace(SMF-800, Seoul Electron Inc). The temperature is elevatedup to 850 ◦C in 3 h (4.6 ◦C min−1) and maintained for 5 h, thencooled down to room temperature in 6 h (−2.3 ◦C min−1). Aglass cylinder is formed with chemical-mechanical polishing(CMP) and subsequent DRIE process (figure 3(c)). The DRIEprocess is divided into several sub-steps considering the etch

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(a)

(b)

Figure 4. SEM images of the thermally reflowed glass microlensbefore (a) and after (b) the second thermal reflow process.

rate in order to precisely control the height of the cylinder.This procedure is important because the height of the cylinderprotruding over the top silicon layer eventually defines theradius of curvature of the microlens during the second thermalreflow process (figure 3(d)). The second thermal reflow processis also performed in the mini furnace while the process timeat 850 ◦C is reduced down to 30 min in order to preventthe surface devitrification. A BSG wafer with a chemicallyetched cavity, which will be utilized as a handling wafer inthe next stage, is anodically bonded with the silicon substrate(figure 3(e)). The alignment and wafer bonding processes aredone with EVG R© 620 and EVG R© 501, both manufactured byEV Group. Typical bonding condition includes temperatureof 380 ◦C, applied voltage bias of 800 V, contact forceof 400 N, and process period of 15 min. The thickness ofthe silicon structural layer is defined using CMP, and analuminum mask is patterned for masking during DRIE process(figure 3( f )). The structural silicon layer can be thinned downto a few tens of microns, whose thickness is 60 μm in theproposed device. A thin lens with a smaller mass is beneficialin implementing a microlens translating device in terms ofboth power consumption and operating speed. Finally, the

Figure 5. Comparison between the measured and calculated surfaceprofile of the microlens.

silicon structures including comb-drives are released throughthe DRIE process (figure 3(g)).

The fabrication results of the microlens are shown infigure 4 verifying successful shape transformation from a glasscylinder to a microlens through the thermal reflow process.The scanning electron microscope (SEM) images were takenbefore (figure 3(c)) and after (figure 3(d)) the second thermalreflow process and no visible defects were observed. Thedimensions of the fabricated cylinder were 396.9 μm indiameter and 61.0 μm in thickness (figure 4(a)). Given that thefocal length of the spherical lens can be derived from the radiusand the height of the cylinder [18], theoretically estimatedvalues of the focal length and NA of the microlens are 494 μmand 0.37, respectively. Comparison between the measuredand calculated surface profile of the microlens is shown infigure 5. Confocal microscopic surface profiler (Nanofocus,μsurf) and stylus profiler (Dektak 8, Veeco) were used tomeasure the precise geometry of the fabricated microlens.Note that the measured points with low height values withinthe inner part of the microlens are points immeasurable withconfocal microscope. Due to the inherent characteristics ofthe stylus profiler, scanning speed on a microlens surfacewas non-uniform and scanned profile was distorted slightly.However, partial profile measured with confocal microscopematched well with the calculation result and microlens sagand diameter measured using two different measurement toolswere in good agreement. The results verified that the radius ofthe fabricated microlens in XY plane, which was 198.4 μm, wasconsistent with that of the cylinder after the second thermalreflow process. The implication of this result is that the planarradius of the microlens is maintained during the thermal reflowprocess with the assistance of silicon cavity. Shrinkage of theviscous microlens during heat treatment due to wetting [20]can hinder the accurate design and fabrication of the microlens.Furthermore, the focal length of the microlens derived fromsagittal height was 498 μm, which corresponds well with thetheoretical prediction of 494 μm. The average roughness at the120 μm long scanned line crossing the center of the microlenswas 22 nm (figure 5). Given that the value is less than 1/15

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(a)

(b) (c)

Figure 6. SEM images of (a) the completely fabricated 2D forwardoptical scanner and magnified image of the glass isolation block,image taken from the (b) top and (c) bottom.

of the wavelength of visible spectrum, fabricated microlenscan potentially be used in endoscopic imaging applications.It should also be noted that the transmittance of the 250 μmthick reflowed glass plate is over 90% in the visible spectrum[21].

Figure 6 shows the SEM images of the completelyfabricated 2D microlens scanner and magnified images ofthe glass isolation block. The glass isolation block, whichis marked as a white circle in the general view, successfullyinterconnected the silicon structures as can be verified fromthe top-side image (figure 6(b)) and bottom-side image(figure 6(c)). The spherical-shaped part of the glass isolationblock was due to the fabrication procedure designed inorder to minimize the number of process steps. This can bemodified without difficulty by masking the glass isolationblock during the DRIE process (figure 3(c)). Compared toother reported XY-stages, the glass isolation block approachused in this work can be substantially advantageous in termsof structural rigidity and electrical stability [22]. Reflowedborosilicate glass which has the same thickness as the siliconstructural layer has adequate solidity and cementing powerto ensure a stable actuation of the scanner even in the highfrequency region [23]. Moreover, it was possible to realizea sufficiently wide isolation gap using glass isolation blocks,which greatly increases the design flexibility. In this work, thesilicon structures were electrically isolated from each otherwith a glass-filled gap of 70 μm. The image also demonstratessuccessful fabrication of the XY-stage including the springsand comb drive electrodes. Surface profile of the XY-stageneighboring the glass isolation block was measured in order

Figure 7. Surface profile measurement of the silicon structureneighboring glass isolation block.

Figure 8. Surface roughness measurement of the microlens usingAFM (XE-150, PSIA). The average roughness of the microlens onthe planar side was 11.98 nm.

to examine the possible influence of the block on the siliconstructure, e.g. deformation due to residual stress (figure 7). Theresult showed no significant deformation within the 800 μmlength silicon beam structure. The stepwise profile along theglass isolation block can be explained with the dishing effectduring CMP (figure 3(c)) and DRIE process (figure 3(g)).

In terms of the optical quality of a microlens, surfacequality including the roughness is an issue of great importancebecause it critically affects the performance of the opticaldevice. Although the planar side of the reflowed glassmicrolens looks hazy in figure 6(a), the average surfaceroughness of the 5 × 5 μm2 square at the center measuredwith AFM (XE-150, PSIA) was 11.98 nm which is low enoughfor the target application (figure 8).

4. Device characterization

Figure 9(a) shows the optical image of the fabricated scannerafter wire bonding on the anchors. The dynamic characteristicsof the 2D microlens scanner were measured at atmosphericpressure. Frequency response of the scanner was measured

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(b)

(a)

Figure 9. (a) Optical image of the wired scanner and (b) measuredmicrolens oscillation frequency. The resonant frequencies of thestage in X and Y direction were 3.238 and 2.198 kHz.

using a Laser Doppler Vibrometer. First mode resonance ofthe scanner in X and Y directions were 3.238 and 2.198 kHz,respectively, as shown in figure 9(b). The estimated qualityfactor for the X-axis and Y-axis resonance mode actuation were

168 and 69.1, respectively. As frequency responses of out-of-plane direction had no additional peak in the region near theresonance frequency of the in-plane actuation, it can be clearlyverified that there is no noticeable crosstalk between the twoactuation modes. Lateral displacement of the XY-scanner withintegrated microlens was measured by grabbed images througha microscope and a CCD image sensor, as a function of appliedsignal. Measured displacement of the microlens at resonancewas over ±40 μm in both axes, when an ac input of 21.0 V inamplitude with a dc offset of 21.0 V was applied.

Optical scanning experiment has been carried out in orderto examine the beam scanning angle of the fabricated scanner.The image of the optical setup and relevant optical layout forbeam scanning is demonstrated in figure 10.

The light beam from the laser (He-Ne, λ: 632.8 nm,JDSU) was expanded through an expander and focused onthe scanner through a converging lens with a diameter of25.4 mm and focal length of 40.0 mm (Thorlabs). Scannedbeam from the microlens scanner is directly projected on thescreen without an objective lens. Objective lenses with variousNAs can be utilized in order to meet the requirement of thetarget application. Figures 11 and 12 show measured beamscanning angle as a function of applied voltage and opticalscanning test result of the fabricated scanner, respectively.The screen was placed at 128 mm distance from the scannerand the minor tic label on the graph indicates 1 mm. The spotsize projected on the screen (figure 12(a)) was approximately2 mm in diameter. Although unclear boundary of the spotcan lead to measurement errors, rather reliable scan anglemeasurement can be achieved by maintaining the consistencyin beam spot and scanned line definition. The scan anglewas measured at mechanical resonance of the stage, andthe measured values were ±4.7◦ and ±4.9◦ in the X- andY-axis, respectively. The results are competitive comparedto previously reported miniaturized endoscopic microlensscanners which have scanning angle of ±4.6◦–±5.5◦ [10, 12].Driving voltages of these scanners, which are ac input of 5–10 V with dc bias of 5–20 V, are relatively lower compared

(a)

(b)

Figure 10. Optical setup (a) and layout (b) for beam scanning demonstration.

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J. Micromech. Microeng. 24 (2014) 055009 S Yoo et al

Figure 11. Measured beam scan angle as a function of appliedvoltage. In each axis, over ±4.7◦ tilting angle was achieved when acinput of 21.0 V and dc offset of 21.0 V was applied.

(a) (b)

(c) (d )

Figure 12. Optical scanning result of the fabricated 2D scanner:(a) beam spot, (b) X-axis scanning, (c) Y-axis scanning, and(d) scanning in both directions.

to the present work. However, the present scanner is ratherpreferable considering that both X and Y axes actuation isimplemented on single-layer substrate within similar planardimension. The scanning angle can be further enhanced whenthe numerical aperture of the microlens is improved, which isan ongoing work.

5. Conclusion

In the present work, a novel wafer-level fabrication methodof a two-dimensional forward optical scanner is proposed anddemonstrated successfully. A glass microlens with 396.9 μm

diameter and 109.0 μm sag has been fabricated, which werehighly consistent with the theoretical lens profile. The averagesurface roughness of 11–22 nm has been measured withthe fabricated microlens. Mechanical characteristics weremeasured in atmospheric pressure. Resonant frequency of thestage in X and Y axes are 3.238 and 2.198 kHz and relevantquality factors are 168 and 69.1, respectively. An opticalscanning test verified the applicability of the proposed scannerin microendoscopy. In each axis, over ±4.7◦ tilting angle wasachieved when an ac input of 21.0 V in amplitude with a dc off-set of 21.0 V was applied. The proposed device can potentiallybe utilized in various applications where dual-axis scanningof integrated microlens is required. Moreover, glass isolationblock enabled silicon structure can be an effective solution forapplications where electric or thermal isolation is required.

Acknowledgments

This research was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science andTechnology (2012R1A1A2009547).

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