Optical micro encoder with sub-micron resolution using a VCSEL

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Sensors and Actuators A 71 (1998) 213-218

PHYSICAL

Optical micro encoder with sub-micron resolution using a VCSEL Hiroshi Miyajima *, Eiji Yamamoto, Kazuhisa Yanagisawa Olympus Optical Co., Ltd., 2-3 Kuboyama-cho, Hachioji, Tokyo 192-8512, Japan

Received20 March 1998; accepted27 May 1998

Abstract An improved optical micro encoder using a vertical-cavity surface-emitting laser (VCSEL) is presented in this paper. A twin-beam VCSEL is used in order to obtain two quasi-sinusoidal signals in phase quadrature. Directional sensing and signal interpolation have become possible. Also, microlenses are monolithically integrated onto the VCSEL in order to improve resolution. The encoder chip size is approximately 1.5 × 2.0 × 0.6 mm3. Displacement measurement is performed using a scale with a pitch of 20 ~m, and resolution of 0.1 p,m is achieved as a result of signal interpolation, Additionally, the encoder is installed into a low-height linear stage and characterized, demonstrating a possible application. A prototype of the packaged encoder is also presented. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Opticalmicroencoder;VCSEL;Microlenses;Directionalsensing; Linearstage; Packaging

1. Introduction

Miniaturized sensors with high sensitivity are key components in precision instruments, medical instruments, and micromachines. Optical sensors have the advantage of high sensitivity and high electromagnetic interference (EMI) tolerance among various kinds of sensors. However, it has been difficult to reduce their size and cost, because precise alignment and assembly of several optical components are necessary. Sawada [ 1] previously presented the integrated optical encoder. Fabrication of this encoder is completely monolithic but challenging, because it relies on a dry etching process to fabricate optical mirror facets of an edge-emitting laser diode (LD). Hewlett-Packard has commercialized a small size optical encoder based on LED mounted on photodetector (PD) IC [2]. It consists of a clear plastic SO-8 package with two lenses located on its top and is probably the smallest commercialized encoder, but the resolution is approximately 170 ~m ( 150 lines per inch), because LED is used as an emitter. Our optical micro encoder consists of a VCSEL chip and a PD chip fabricated separately and assembled with tolerant alignment. Its fabrication can be relatively easy and reliable. Utilizing the beam with small divergence from the VCSEL, an index scale and a collimator lens are eliminated, and the size reduction is achieved. Basic characteristics of the encoder was reported, and the feasibility of improving its

resolution using an integrated microlens was described by fabricating and characterizing it on a separate substrate [ 3,4]. However, directional sensing was impossible, and the microlens was not integrated onto the VCSEL. In this paper, an improved optical micro encoder is presented [ 5 ]. Directional sensing and signal interpolation have become possible using a twin-beam VCSEL with integrated microlenses. Sub-micron resolution has been experimentally demonstrated. It should be noted that the improvement in the VCSEL presented in this paper is realized taking its advantage over edge-emitting LD, without adding any significant process complexity. As one of the future applications of the encoder, it is installed into a low-height linear stage and characterized. A new packaging method maintaining the advantage of the small size device is also presented. 2. Encoder with twin-beam VCSEL Fig. 1 shows a schematic drawing of the encoder. A twinbeam VCSEL (A = 980 nm) chip is assembled onto a PD

* Corresponding author. E-mail: [email protected] 0924-4247/98/$ - see front matter© 1998ElsevierScienceS.A. Atl rights reserved. PIIS0924-4247(98)00188-5

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Si PD chip twin-beamVCSEL Fig. 1. A schematicdrawingof the encoder.

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Fig. 2. A pair of optical spotsformedon the scale by the twin-beamVCSEL. chip with two detecting areas using eutectic bonding [6]. The VCSEL is designed to have two apertures separated at a distance of ( n + 1/4)p, where n is an integer and p is the scale pitch. The beams from the VCSEL illuminate the scale, making two optical spots on the scale at places with phase difference of p / 4 (i.e., 90 °) as shown in Fig. 2. The two reflected beams from the scale axe detected individually so that two periodic signals in phase quadrature are obtained, enabling directional sensing. The apertures of the VCSEL are rectangular with the sides parallel to the scale pitch direction longer than the other ones, realizing the different divergent angle of the optical beam in two orthogonal direction. Therefore, the spots on the scale are elliptical, making the encoder signals less susceptible to defects of the scale. Fabrication process of the twin-beam VCSEL is the same except for patterning a pair of mesas and apertures in one chip. This can be reatized only by slight modification of the photomasks without process modification. It is, as a matter of fact, one of the advantages of VCSELs over conventional edge-emitting LDs. Even two-dimensional arrays of VCSEL have been presented [7]. In our actual device, the aperture size is 30 × 10 )zm~, and the distance between the two apertures is 245 ~m (i.e., n - 1 2 ) . It is successfully operated under continuous wave at a room temperature.

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Microlenses are monolithically integrated onto the VCSEL in order to improve the encoder resolution. The process flow is shown in Fig. 3. After finishing the fabrication of the twinbeam VCSEL (Fig. 3a), a commercially available photoresist is patterned onto the apertures of the VCSELs using a photomask with circular patterns (Fig. 3b). The diameter of the circle is 70 tzm. Then the patterns are heated up to cause thermal flow, and nearly spherical profiles are obtained (Fig.

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5. Results and discussion Fig. 6. A top view picture of the VCSEL/PD unit. SEM picture of the twin-beam VCSEL with integrated microlenses, and Fig. 6 shows a top view of the encoder.

4. E x p e r i m e n t a l Fig. 7a shows the basic experimental set-up for encoder characterization. The encoder is mounted on a metal package (TO-5) and wire-bonded. Then a VCSEL controller and a signal amplifier are electrically connected to the package mounted on a goniometer stage. The scale is mounted on a motor driven stage and located above the encoder. The distance between the scale and the encoder (z.0 can be changed by manually adjusting the vertical position of the scale. The tilt angle (q~) can be changed by the goniometer stage. A close up view of the encoder and the scale is shown in Fig. 7b. When the VCSEL is driven and the scale is moved horizontally, encoder signals are obtained. The amplifier consists of current-to-voltage (I-V) conversion and the adjustment of the peak-to-peak ( p - p ) amplitude and the DC level of the signal. After being appropriately adjusted, the signal is digitally interpolated by the signal interpolator, which was orig-

5.1. Directional sensing and signal interpolation First of all, the encoder without microlenses are used, and the two signals are observed using an oscilloscope. A scale with the pitch of 20 txm is used throughout the characterization described below. Fig. 8a and b show two quasi-sinusoidal signals when the displacement direction of the scale changes. Lissajous' figure is obtained as in Fig. 8c from the two signals.

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Fig. 8. Two encoder signals in phase quadrature; (a) and (b) correspond to the opposite displacement direction, and (c) is the Lissajous' figure.

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One rotation of this figure (i.e., 0= 360 °) corresponds to the displacement of 20 ~m (i.e., scale pitch), and resolution smaller than the scale pitch can be obtained by interpolating the angle 0. In this experiment, the principle of the directional sensing and signal interpolation using the twin-beam VCSEL is verified. Interpolation by a factor of 64 is performed without observing any fluctuation of the interpolated encoder output displayed by the personal computer.

5.2. Resolution improvement using integrated microlenses Signals obtained from encoders with and without microlenses are compared in Fig. 9. Fig. 10 shows the p-p amplitude variation in both cases when the distance between the VCSEL and the scale is varied. The maximum p-p amplitude is larger and the p-p amplitude is more dependent on VCSELscale distance when the microlenses are integrated, because the laser beam becomes convergent and the spot size becomes smaller due to the effect of the microlenses. This experiment shows the effect of the microlenses and the possibility of improving resolution. The maximum p-p amplitude should be observed at a distance equal to the focal length, which is expected to be approximately 220 ~m in Fig. 10. This is a little less than the designed value (i.e., 280 txm). The profile and the refractive index of the microlens should be measured, and the intensity profile of the optical spot should be obtained for further characterization.

5.3. Characterization of signal interpolation

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pitch of 0.1 fxm using piezo-driven stage (instead of the motor driven stage shown in Fig. 7), monitoring the displacement using interferometric laser measurement system (HP 5528A). Interpolation by a factor of 256 is performed for this experiment. Fig. 11 shows the interpolated encoder output versus the interferometer output. This result shows that a displacement of 0.1 p,m is resolved by our encoder. Interpolation errors as welt as absolute position discrepancy can be seen in Fig. 11. Those are expected to be attributed to signal noise, signal profile discrepancy from perfect sinusoid, and misalignment of the direction between the scale and the stage. Further study is necessary for thorough characterization.

5.4. Application Since the size of this encoder is very small, it is possible to install it into applications which were impossible for previous encoders. Fig. 12 shows a small size linear stage with

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Fig. 12. Low-height stage with the optical micro encoder installed. (a) A perspective view. (b) A side view.

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our micro encoder installed. This stage is driven by an ultrasonic linear actuator [9] and has the advantage of its low height of 11 mm. However, the total height is almost doubled if a conventional encoder is u~_d. We installed our micro encoder into this stage with additional height of only 5 mm. Fig. 13a and b shows the enc0der signals and the driving sig-nal of the actuator. The enc0der signals are not affected by the driving signals of the ultrasonic actuator. Fig. 14 shows the frequency characteristics of the output signal amplitude. Approximately 400 mm/s is the maximum speed without observing significant change ( ~3 dB) of the output signal of this micro encoder when a scale with the pitch of 20 ~xm is used. It is good enough f~th/s stage, and it can be improved by changing the characteristiz~ of the low-pass filter in the signal amplifier if necessary. S~cessful operation of closed loop control using this encoder has also been performed. 5.5. P a c k a g i n g

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reliability of the microsensors, but the packaged size has to be as small as possible in order not to ruin their advantage. Fig. 15 a shows the encoder mounted on a print circuit board (PCB) made for installing it into the stage mentioned above. Although the height is reduced in comparison with the TO-5 package used for most of the characterization in this paper, the size of the PCB ( 15 × 5 mm 2) is much larger than the encoder chip itself. Furthermore, the encoder cannot be reliable with its bonding wires exposed. Fig. 15b shows a prototype of the small-sized packaging [ 10]. The encoder chip is flip chip bonded onto a thin ceramic PCB for electrical connection and mechanical protection. Then, a flexible printed circuit (FPC) is Connected to the PCB. Finally, the bonded parts including the surface of VCSEL and PD are covered with resin. The size of the packaged encoder is 3 × 3 × 1 mm 3. It is suitable for a limited space where this encoder should be installed. This packaged encoder has been operated between a room temperature and 50°C without any degradation of its performance.

6. Conclusions The improved optical micro encoder using a VCSEL has been presented. In comparison to our previous device, directional sensing and a resolution of 0.1 txm are demonstrated

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using a t w i n - b e a m V C S E L with integrated microlenses and signal interpolation. These features, indispensable to the encoder for practical use, are added without sacrificing the simple configuration, small size, and low cost. One of the possible applications is experimentally demonstrated, and a n e w small-sized packaging m e t h o d is presented. Future size reduction is possible b y monolithic integration of ¥ C S E L and PD.

[3]

[4]

[5]

Acknowledgements The authors wish to thank S. Hashimoto and S. Shinohara for V C S E L fabrication, I. K o m a z a k i and X. Shan for V C S E L characterization, M. Ito for the d e v e l o p m e n t of the bonding process, H. Y u k a w a and N. t-Iisata for providing the interpolation circuit, T. Tsubata and Y. T a n i g u c h i for characterizing the encoder with the linear stage, T. Hatakeyama for developing the prototype of the packaged encoder.

[6]

[7]

[8]

References [ 1] R. Sawada, Integrated optical encoder, Technical Digest, The 8th International Conference on Solid-State Sensors and Actuators: Eurosensors IX, Stockholm, Sweden, June 1995, pp. 281-284. [2] S. Krishnan, T.J. Lugaresi, R. Ruh, A miniature surface mount reflec-

[9]

[ 10]

five optical shaft encoder, Hewlett-Packard Journal, Dec. 1996, pp. 55-59. H. Miyajima, E. Yamamoto, M. tto, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa,Optical micro encoder using surface-emitting laser, Proceedings of the IEEE Micro Electro Mechanical Systems, San Diego, CA, USA, Feb. 1996, pp. 4i2--417. H. Miyajima, E. Yamamoto, M. Ito, S. Hashimoto, I. Komazaki, S. Shinohara, K. Yanagisawa, Optical micro encoder using vertical-cavity surface-emitting laser, Sensors and Actuators A 57 (1996) 127135, H. Miyajima, E. Yamamoto, K. Yanagisawa, Optical micro encoder using a twin-beam VCSEL with integrated microlenses, Technical Digest, The 9th LntemationalConference on Solid-State Sensors and Actuators (Transducers '97), Chicago, IL, USA, June, 1997, pp. 1233-t236.. M. Ito, E. Yamamoto, S. Hashimoto, I. Komazaki, H. Miyajima, S. Shinohara, K. Yanagisawa,Compound-cavity tactile sensor using surface-emitting laser, Proceedings of the International Symposium on Micro Machine and Human Science, Nagoya, Japan, Oct. 1995, pp. 83-88. D. Vakhshoori, J.D. Wynn, O.J, Zydzik, R.E. Leibenguth, 8 X 18 top emitting independentlyaddressable surface emitting laser arrays with uniform threshold and low threshold voltage, Applied Physics Letter 62 (15) (1993) 1718-1720. D. Daly, R.F. Stevens, M.C. Hurley, N. Davis, The manufacture of microIenses by melting photoresist, Measurement Science and Technology 1 (8) (1990) 759-766. T. Funakubo, T. Tsubata, Y. Taniguchi, K. Kumei, T. Fujimura, C. Abe, Ultrasonic linear motor using multilayer piezoelectric actuators, Japanese Journal of Applied Physics 34, Part t, No. 5B (1995), 27562759. E. Yamamoto, Micro encoder using a vertical-cavity surface-emitting laser, Japanese Journal of Optics fin Japanese), to be published.