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Integration of grating-image-type encoder using Si micromachining
Sensors and Actuators A 97±98 (2002) 139±146
Integration of grating-image-type encoder using Si micromachining K. Hane*, T. Endo, M. Ishimori, Y. Ito1, M. Sasaki Department Mechatronics and Precision Engineering, Tohoku University, Sendai, 980-8579, Japan Received 4 July 2001; received in revised form 16 October 2001; accepted 12 November 2001
Abstract Integration of a grating-image-type encoder is proposed and the sensor for the encoder has been fabricated using Si micromachining technology. The sensor consists of the Si grids with line photodiodes, light emitting diode (LED) and signal ampli®er. The Si grids works as the photodetectors as well as the object and index gratings. The incoherent light emission through the Si grids and the light detection with the grids make the optical system compact. Phase change of the signal is caused by the translation of the grating image on the Si grids. Two phaseshifted signals are generated by installing two sets of photodiodes on the Si substrate with a corresponding spatial displacement. Two types of grating structures are fabricated for the practical sensors. The signal contrast is high enough for the displacement measurements at a large air gap. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Si micromachining; Encoder; Optical sensor; Displacement sensing; Grating
1. Introduction Optical encoders are indispensable devices for sensing precise displacement in several mechanical systems [1]. Recently, with the requirement for high precision in the displacement measurements, the encoders using the optical interference of light diffracted from the scale grating were commercialized [2]. In general, however, the encoders using the optical interference are composed of the bulky components. On the other hand, compact encoders with low cost are preferable for installing them in the industrial systems. Lately, an advanced micro-encoder using optical interference has been fabricated [3]. Likewise, since the optical con®guration is very simple, Moire encoders with two gratings superimposed with an air gap are still important in many practical applications. In these simple Moire encoders, recently, the modulated-pitch gratings were proposed for suppressing the harmonic noise of the encoder signal [4,5]. In the optical system similar to the Moire encoder, the grating imaging effect was used previously [6,7], in which the encoder signal was insensitive to the change of the air gap between the two gratings under incoherent illumination. The grating imaging effect was further investigated on the basis of the optical transfer function for the displacement measurement [7,8]. More recently, we carried out a preliminary experiment for the grating-image-type encoder *
Corresponding author. Tel.: 81-22-217-6962; fax: 81-22-217-6963. E-mail address: [email protected] (K. Hane). 1 Harmonic Drive Systems Inc., Hodaka-cho, Nagano 399-8305, Japan.
using micromachined grating, in which the transmission grating was fabricated by silicon micromachining [9]. However, the integration and optimization for practical sensors have not been carried out. In this paper, the integration of the grating-image-type encoder is demonstrated. New version of micromachined gratings is presented. A light emitting diode (LED) has been specially fabricated for the incoherent light source. Two gratings, photodetectors (two line photodiodes on each grid), LEDs, and preampli®er-circuit-chip are integrated by stacking them. The signal characteristics have been investigated for practical sensors. 2. Design for integration Fig. 1 shows the conventional grating-image-type encoder. It consists of ®ve components for encoder sensor: light source; object grating; scale grating; index grating; photodetector. Since the grating imaging effect is used in the measurement principle, three gratings are needed as shown in Fig. 1. The incoherent light passing through the object grating impinges on the scale grating. The index grating is illuminated with the light re¯ected by the scale grating. The light passing through the index grating is detected by photodetector. Although the optical con®guration is in re¯ection, the optical system consists of three grating in tandem with the same gaps between the gratings. The optical system can be analyzed using the optical transfer function of the second grating, i.e. scale grating. The object grating is
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 8 0 0 - 7
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Fig. 1. Conventional grating-image-type encoder.
Fig. 2. Integrated sensor as index grating.
imaged by the scale grating onto the plane of the index grating. The image of the object grating is superimposed on the index grating to generate Moire effect, which is used for displacement sensing in a manner similar to the conventional Moire encoder. Fig. 2 shows the proposed integration of the encoder. The ®ve components of the conventional encoder are stacked. The proposed encoder consists of the two gratings, an incoherent light source and the photodetectors installed in the grating. The scale grating is assumed to be an amplitude re¯ection grating. The optical con®guration in re¯ection makes the encoder system compact as used in the conventional Moire encoders. The light source used in the proposed encoder is assumed to be a polychromatic incoherent light source such as LED. The index grating is fabricated from a silicon wafer and consists of the transmission grating. In each grating line, which is a thin silicon beam, photodiodes are installed using semiconductor microfabrication technique.
Fig. 3(a) and (b) shows the schematic front views of the designed index gratings (denoted by Type-1 and Type-2, respectively). The Si substrate is etched through to form the index gratings. In Fig. 3(a), two kinds of phase-shifted line photodiodes are installed in each grating line which is a thin Si beam. Totally, four 908-phase-shifted photodiodes, in which the spatial phase differences are 908, respectively, are needed to obtain four sinusoidal signals. They are used for eliminating dc offset of signals, for interpolating the signal, and for sensing the direction. As shown in Fig. 3(a), since the two kinds of 908-phase-shifted photodiodes are installed in each Si beam alternately and closely in space, the photodiode sensitivities are nearly equal and the average intensities of signals are not affected by the light intensity distribution in a large area, especially in Type-1. The index grating is illuminated from backside and the light passes through the slits between the gratings. In the case of Fig. 3(b), the object grating which is a transmission Si grating is fabricated on the optical axis, while the index grating which consists of line photodiodes are installed at the off-axis positions. The two kinds of phase-shifted gratings are located above and below the transmission object grating as shown in Fig. 3(b). The light emitted through the object grating is re¯ected by the scale grating and detected with the photodiodes located at the off-axis positions. Although an ideal on-axis con®guration is obtained in Fig. 3(a), the design shown in Fig. 3(b) is somewhat easier than that in Fig. 3(a) for the fabrication. In principle, the grating period, which can be used in the proposed encoder, should be large enough to generate light diffraction (i.e. P > l), because the grating image is generated by the diffraction. Therefore, the general binary gratings used for the conventional encoders also suit the proposed encoder. In the actual fabrication, however, since the two line photodiodes were installed in a single Si grid, the grating period was limited by the lithographic precision used in the fabrication. We used the periods of 80 and 40 mm, considering the alignment accuracy, minimum transferable pattern, and diffusion length of the doped impurity.
Fig. 3. Designed index gratings: (a) Type-1; (b) Type-2.
K. Hane et al. / Sensors and Actuators A 97±98 (2002) 139±146
Fig. 4. Cross-sectional view of the designed encoder.
Fig. 4 shows the cross-sectional diagram of the encoder for Type-1 (in which scale grating is not shown). The index grating fabricated from Si substrate is ®xed to the LED holder to encapsulate the LED. The electrodes for the LED are patterned on the surface of LED holder. The light is emitted from the LED through the Si index grating. The light re¯ected from the scale grating is detected by the photodiode fabricated on the Si grating. A chip for electronic circuits is attached to the LED holder with polymer spacer. The electronic circuits may be fabricated on the side area of the index grating if the fabrication facility can accept both processes. From Figs. 2 and 4, an equivalent optical system is described by the three gratings placed at the same distance between the gratings along the optical axis. When the ®rst grating (object grating) is irradiated with spatially incoherent light, the object grating pattern is transferred by the center grating (re¯ection scale grating) on to the image plane (which is equal to the plane of the object and index gratings). The center grating of the three gratings works as a pupil for imaging. The optical principle of the encoder system can be explained theoretically by the grating imaging [6,7]. The theoretical analysis of the encoder optics has been carried out using optical transfer function. Based on the analytical results [7], the grating period and distance between the gratings have been decided. Since the principle of the grating imaging has been described in detail, a brief theoretical approach for the displacement measurement in our proposed system is given below. The essential optical system of the encoder is described by the three gratings placed in tandem at the same distances z between the gratings as shown in Fig. 1. The grating-like image is formed by the slit array of the pupil grating under Fresnel diffraction. For understanding the grating imaging, it may be easier to consider that each slit of the pupil grating images the object grating on the plane of the index grating and the superposition of the images generated by the respective slits of the grating produces a grating-like image
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if they are in phase. The optical transfer function of the pupil grating is given under our experimental conditions as follows [7]: mp lz F s P 2sp exp ip2smp 1 s lz 2e mp lz sinc 4es 1 s lz 2e mp mp 2e s< (1) for lz lz lz mp lz F s P 2sp exp ip2smp 1 s lz 2e mp lz sinc 4es 1 s lz 2e mp 2e mp s< (2) for lz lz lz for mp/lz 2e=lz s < mp/lz here, s is the image frequency, p and 2e are the pitch and the slit width of the pupil grating, respectively, z is the distance between the gratings, l is the wavelength of light and m is an integer. The function P(x) represents a comb function which becomes unity when x is equal to integers. Since the image frequency is equal to the object frequency (s 1=p) and the slit width is assumed to be half of the pitch (e p=4), then Eqs. (1) and (2) are simpli®ed to 1 1 2x 2m F s 1m 1 2x 2m sinc p 2 for m x < m 12 1 F s 1m 1 2x p for
m
1 2
1 2x 2m sinc 2
(3) 2m
x
(4) 2
where x is the distance z normalized by p /(2l) (x 2zl=p2 ). Therefore, the grating condition that the pitches of the three gratings are equal to each other corresponds to the second imaging condition (x 2). The image contrast calculated as a function of the normalized distance x is shown in Fig. 3. As shown in Fig. 5, the image contrast is always positive under the optical conditions, although the contrast varies periodically with increasing x. The period of the image contrast as a function of z is equal to p2/(2l) (i.e. x 1), which is dependent on the wavelength l of light. The positive contrast means that the image is located at the position of the geometrical shadow. Therefore, in the case of polychromatic illumination, the images generated by the respective wavelengths are superimposed constructively, and thus the image contrast is not degraded by the polychromaticity of the light source under these conditions. Moreover, the maximum value of the image contrast does not decrease with increasing the distance between the gratings, and thus the displacement can be measured at a distance z much larger than that used in the conventional Moire encoder.
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Since imaging effect of the grating is used in the encoder, the relative displacement d of the object grating generates that of image on the plane of the index grating in the opposite direction. Therefore, the encoder signal varies by two periods for the relative displacement of the grating equal to a single grating pitch. The sensitivity of the displacement detection in this encoder is improved by a factor of two by this phenomenon when compared with the conventional Moire encoder. Although the contrast of the encoder signal is nearly independent of the distance between the gratings over a wide range under the polychromatic illumination, the magnitude of the light intensity decreases with the increase of the separation z. Therefore, at a large separation, the signal to noise ratio decreases. If a line light source perpendicular to the grating lines is used in the encoder, the light intensity decreases approximately as a function of 1/z 1/z2, where z is the separation between the gratings.
Si substrate (1), a SiO2 ®lm is formed by wet oxidation (2). The Si substrate is n-type and 200 mm thick and the resistivity is 1±10 O cm. The SiO2 ®lm is 500 nm thick. From the rear surface, the wafer is etched with TMAH (3). The etched area is 3:5 mm 5:7 mm and 40 mm thick. Next, the line photodiode is fabricated on the etched area by implanting B ions (4). The dose density of B ion is 2 1014 /cm3 at 120 keV implantation voltage. After annealing (5) at the temperature of 1000 8C, the Al electrode is patterned (6). The gratings with the line photodiodes is then fabricated by etching through with inductively coupled reactive plasma (7). The pitches of the gratings fabricated in this study are 40 and 80 mm, respectively. For Type-2, the fabrication procedures are almost the same as those for Type-1 except for the mask patterning. Polychromatic spatially incoherent light source is needed for the encoder. Two spatially incoherent sources are used for Type-1 and one for Type-2. The LEDs are to be wide enough to illuminate the grating region. We have designed and fabricated the LED by ourselves in this experiment. A GaAlAs wafer is used for the substrate. The p2, p1, and n layers of the double-hetero-structure are 12, 0.35, and 160 mm thick, respectively. The LED is 3 mm long and 1mm wide with 50 mm wide AuZn comb electrode for pregion and AuSn for n-region. The comb electrode is formed perpendicularly to the lines of object grating. For further integrating the proposed encoder, we designed an preampli®er for obtaining the two phase-shifted signals without dc offset from the four channel photodiodes, which consisted of eight operational ampli®ers designed by ourselves. Since all the operational ampli®ers were fabricated on one chip, and the photodiodes for sensing the light intensities were located closely in space, the signal intensity nearly equal to each other was obtained, which was effective for a high precision interpolation.
3. Fabrication processes and integration
4. Results and discussion
Fig. 6 shows the sequence of the lithographic processes for fabricating the index grating of Type-1. Starting from a n-type
The fabricated index grating for Type-1 with the period of 80 mm is shown in Fig. 7. As shown in Fig. 7, the duty ratio
Fig. 5. Image contrast calculated as a function of normalized distance for a single wavelength.
Fig. 6. Fabrication process for encoder index grating.
K. Hane et al. / Sensors and Actuators A 97±98 (2002) 139±146
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Fig. 7. Fabricated index grating (Type-1).
between the Si beam and slit widths is nearly unity. The two line photodiodes are installed as clearly shown in the magni®ed image of the grating in Fig. 7. The photo-current of the diode was measured to be 500 with 10 nA dark current using standard light source of 12.5 lx. Although a little larger dark current may be caused by a contamination in the fabrication, it has been con®rmed that the sensitivity of the fabricated photodiode is comparable to that of the commercialized one. The frequency response of the fabricated photodiode was examined. The cut-off frequency was around 200 kHz, which was high enough to encoder applications. The properties of the index grating were investigated by combining with scale grating. In this experiment, the index grating shown in Fig. 7 was illuminated with the white light from a halogen lamp through a ®ber bundle. The ®ber bundle with the diameter of about 10 mm was used for the illumination of the scale grating through the index grating. Since hundreds of ®bers were bound in the ®ber bundle, the grating was illuminated incoherently. The distance between the index grating and the ®ber-end was about 5 mm. The signals from the photodiodes were ampli®ed and subtracted to
compensate the dc components with the operational ampli®ers. The ampli®ed signals were fed to a computer through the digital-analog converters. The scale grating ®xed to a table was translated by stepping motor controlled by the computer. Fig. 8 shows the encoder signals measured as a function of displacement and the Lissajour ®gure of the two 908-phaseshifted signals. The gap between the index grating and scale grating is 3 mm under the experimental condition. The light re¯ected from the scale grating is detected with the photodiodes installed on the grids of the index grating. Therefore, the areas of light emission and detection are nearly same. Two line photodiodes are installed on one grating line to obtain the 908-phase-shifted signals. Another set of the phase-shifted signals is obtained from the index grating located below as shown in Fig. 7. In Fig. 8, the two sinusoidal signals are obtained and the phase difference between the signals is nearly 908. Since the Lissajour ®gure is close to a circle, the signal includes little harmonic noise. The signal contrast was measured as a function of the gap between the index and scale gratings. As investigated previously [9], the signal contrast was kept constant at
Fig. 8. Encoder signal (Type-1) measured as a function of displacement and its Lissajour figure.
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Fig. 9. Fabricated index grating (Type-2).
the large air gaps from 1 to 30 mm. In the case of the index grating shown in Fig. 7, the contrast decreased somewhat faster than the previous measurement since the two index gratings were placed vertically as shown in Fig. 7 and thus the light emitted from the respective gratings merged in the detection at a large gap. Light power of about 1 mW, which was a typical output of LED was enough to obtain a good S/N ratio (>10) at the separation around 2 mm. Increasing the separation between the gratings, the signal intensity from the photodiode decreased due to the diffusion of the incoherent light although the signal contrast was nearly constant. At the distance of 10 mm, S/N ratio was still high enough to measure the displacement. The light power needed in the proposed encoder was not much different from that of the conventional Moire encoder under our experimental conditions. The measurement accuracy was linearly dependent on the optical power of the light source when the separation of the gratings was kept constant.
Fig. 9 shows the fabricated index grating for Type-2 with the period of 80 mm. The transmission grating region is clearly etched through as shown in Fig. 9. In Type-2, the line-photodiodes are located above and below the transmission object grating. The characters on paper are seen through the object grating. The characteristics of the fabricated index grating were examined in the manor similar to Type-1. The sensitivities of the photodiodes were the same as those of Type-1. Fig. 10 shows the encoder signals measured as a function of displacement and the Lissajour ®gure at the gap of 7 mm when the white light from the halogen lamp is used as a light source through ®ber bundle. The measured signals are almost sinusoidal. The harmonic noises in the encoder signals in Fig. 10 are smaller than those in Fig. 8 judging from the deviation from a circle. In the optical con®guration of Type-2, the two sets of photodiodes are located separately by the width of the transmission grating, and thus the light re¯ected from the scale grating merges little on the plane of index grating. This was con®rmed by the fact that the
Fig. 10. Encoder signal (Type-2) measured as a function of displacement and its Lissajour figure.
K. Hane et al. / Sensors and Actuators A 97±98 (2002) 139±146
contrast of the encoder signal in Type-1 was kept high at a large gap. After testing the fabricated index grating as described above, we fabricated the integrated encoder sensor. The integration of the index grating, LED, LED holder and the IC chip was carried out as shown in Fig. 4 by stacking them with epoxy resin. The integrated encoder sensor is 1.2 mm thick and is shown in Fig. 11. In the LED holder, two electrodes for LED are patterned. The square LED emitting light is seen bright behind the upper index grating in Fig. 11. The emitting area of the LED is as wide as the index grating. Fig. 12 shows the optical micrograph of the light emission from the fabricated encoder sensor. As shown in Fig. 12, the grating-like emission through the Si grating is clearly obtained. Therefore, the Si grating on which photodiodes are installed works simultaneously as a transmission object grating. placing the integrated encoder sensor close to the scale grating, the signals were measured. Fig. 13 shows the encoder signals from the two channels. Although some noise is superimposed on the signals, the sinusoidal encoder signals are clearly seen in Fig. 13. The low signal to noise ratio is mainly due to the low intensity of the fabricated LED.
Fig. 11. Micrograph of the fabricated integrated sensor.
Fig. 12. Light emission from the integrated sensor.
Fig. 13. Encoder signals (displacement signal and Lissajour figure) from the integrated sensor.
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We have not observed a signi®cant in¯uence of the heat generated by the integrated electronics on the encoder signal in the experiments. The heart effect was considered to be small since the integrated electronics consisted of only eight operational ampli®ers, which consumed little energy, and the LED consumed less than 20 mW. It may need further investigation to discuss the heat effect in detail.
[6] R.M. Pettigrew, Analysis of grating imaging and its application to displacement metrology, Proc. Soc. Photo-Opt. Instrum. Eng. 136 (1977) 325±331. [7] K. Hane, C.P. Grover, Magnified grating images used in displacement sensing, Appl. Opt. 26 (1987) 2355±2359. [8] K. Hane, C.P. Grover, Imaging with rectangular transmission gratings, J. Opt. Soc. Am. A4 (1987) 706±711. [9] K. Hane, T. Endo, Y. Ito, M. Sasaki, A compact optical encoder with micromachined photodetector, J. Opt. A: Pure Appl. Opt. 3 (2001), 191±195.
5. Conclusions We proposed an integration of the grating-image-type encoder. The sensor for the encoder was fabricated using Si micromachining technology. The sensor was fabricated by stacking the Si grating with line photodiodes, LED and signal ampli®ers. The Si grating worked as the photodetectors as well as the object and index gratings. The incoherent light emission through the Si grating and the light detection with the grating made the optical system compact. The two types of grating structures were fabricated for practical sensors. The signal contrasts were high enough for the displacement measurements. References [1] J. Guild, Diffraction Gratings as Measuring Scales, Oxford University Press, London, 1960. [2] K. Ishizuka, H. Watanabe, S. Ishii, M. Tsukiji, T. Nishimura, DENSHI, Vol. 30, Tokyo, 1991, p. 67 (in Japanese). [3] R. Sawada, E. Higurashi, O. Ohguchi, Y. Jin, Long-lift micro-laser encoder, in: Proceedings of the MEMS, 2000, pp. 491±495. [4] A. Ieki, K. Hane, T. Yoshizawa, K. Matsui, M. Nashiki, Optical encoder using a slit-width-modulated grating, J. Mod. Opt. 46 (1999) 1±14. [5] A. Ieki, K. Matsui, M. Nashiki, K. Hane, Pitch-modulated phase grating and its application to displacement encoder, J. Mod. Opt. 47 (2000) 1213±1225.
Biographies Tetsuo Endo received ME degree from Tohoku University in 2000. He is now belonging to NEC Communication Ltd. Masashiro Ishimori received BE degree at Tohoku University in 2000. He is studying the light emitting diode in the master course of Tohoku University. Yoshinori Ito graduated from Nagano Technical College in 1974. Currently he is a member of Future Business Division of Harmonic Drive Systems Inc., Nagano Japan. He is engaged in development of encoder and other sensors. Kazuhiro Hane received the MS and doctorate degree in engineering from Nagoya university in 1980 and in 1983, respectively. From 1983 to 1994, he worked as a member of Department of Electrical Engineering in Nagoya University. From 1985 to 1986, he was a visiting researcher of National Research Council of Canada. Since 1994, he has been a Professor of Graduate School of Mechanical Engineering, Tohoku University, and is currently engaged in the research and development of optical microsensors and optical MEMS. Minoru Sasaki received the MS and doctorate degree in engineering from Nagoya University in 1993 and 1995, respectively. In 1996, he is a Research Fellow of Japan Society for the Promotion of Science. From 1996 to 2000, he was a Research Associate of Graduate School of Mechanical Engineering, Tohoku University Since 2000, he has been a Assistant Professor, and is currently engaged in the research and development of semiconductor micro sensors and optical MEMS.
Integration of grating-image-type encoder using Si micromachining K. Hane*, T. Endo, M. Ishimori, Y. Ito1, M. Sasaki Department Mechatronics and Precision Engineering, Tohoku University, Sendai, 980-8579, Japan Received 4 July 2001; received in revised form 16 October 2001; accepted 12 November 2001
Abstract Integration of a grating-image-type encoder is proposed and the sensor for the encoder has been fabricated using Si micromachining technology. The sensor consists of the Si grids with line photodiodes, light emitting diode (LED) and signal ampli®er. The Si grids works as the photodetectors as well as the object and index gratings. The incoherent light emission through the Si grids and the light detection with the grids make the optical system compact. Phase change of the signal is caused by the translation of the grating image on the Si grids. Two phaseshifted signals are generated by installing two sets of photodiodes on the Si substrate with a corresponding spatial displacement. Two types of grating structures are fabricated for the practical sensors. The signal contrast is high enough for the displacement measurements at a large air gap. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Si micromachining; Encoder; Optical sensor; Displacement sensing; Grating
1. Introduction Optical encoders are indispensable devices for sensing precise displacement in several mechanical systems [1]. Recently, with the requirement for high precision in the displacement measurements, the encoders using the optical interference of light diffracted from the scale grating were commercialized [2]. In general, however, the encoders using the optical interference are composed of the bulky components. On the other hand, compact encoders with low cost are preferable for installing them in the industrial systems. Lately, an advanced micro-encoder using optical interference has been fabricated [3]. Likewise, since the optical con®guration is very simple, Moire encoders with two gratings superimposed with an air gap are still important in many practical applications. In these simple Moire encoders, recently, the modulated-pitch gratings were proposed for suppressing the harmonic noise of the encoder signal [4,5]. In the optical system similar to the Moire encoder, the grating imaging effect was used previously [6,7], in which the encoder signal was insensitive to the change of the air gap between the two gratings under incoherent illumination. The grating imaging effect was further investigated on the basis of the optical transfer function for the displacement measurement [7,8]. More recently, we carried out a preliminary experiment for the grating-image-type encoder *
Corresponding author. Tel.: 81-22-217-6962; fax: 81-22-217-6963. E-mail address: [email protected] (K. Hane). 1 Harmonic Drive Systems Inc., Hodaka-cho, Nagano 399-8305, Japan.
using micromachined grating, in which the transmission grating was fabricated by silicon micromachining [9]. However, the integration and optimization for practical sensors have not been carried out. In this paper, the integration of the grating-image-type encoder is demonstrated. New version of micromachined gratings is presented. A light emitting diode (LED) has been specially fabricated for the incoherent light source. Two gratings, photodetectors (two line photodiodes on each grid), LEDs, and preampli®er-circuit-chip are integrated by stacking them. The signal characteristics have been investigated for practical sensors. 2. Design for integration Fig. 1 shows the conventional grating-image-type encoder. It consists of ®ve components for encoder sensor: light source; object grating; scale grating; index grating; photodetector. Since the grating imaging effect is used in the measurement principle, three gratings are needed as shown in Fig. 1. The incoherent light passing through the object grating impinges on the scale grating. The index grating is illuminated with the light re¯ected by the scale grating. The light passing through the index grating is detected by photodetector. Although the optical con®guration is in re¯ection, the optical system consists of three grating in tandem with the same gaps between the gratings. The optical system can be analyzed using the optical transfer function of the second grating, i.e. scale grating. The object grating is
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 8 0 0 - 7
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Fig. 1. Conventional grating-image-type encoder.
Fig. 2. Integrated sensor as index grating.
imaged by the scale grating onto the plane of the index grating. The image of the object grating is superimposed on the index grating to generate Moire effect, which is used for displacement sensing in a manner similar to the conventional Moire encoder. Fig. 2 shows the proposed integration of the encoder. The ®ve components of the conventional encoder are stacked. The proposed encoder consists of the two gratings, an incoherent light source and the photodetectors installed in the grating. The scale grating is assumed to be an amplitude re¯ection grating. The optical con®guration in re¯ection makes the encoder system compact as used in the conventional Moire encoders. The light source used in the proposed encoder is assumed to be a polychromatic incoherent light source such as LED. The index grating is fabricated from a silicon wafer and consists of the transmission grating. In each grating line, which is a thin silicon beam, photodiodes are installed using semiconductor microfabrication technique.
Fig. 3(a) and (b) shows the schematic front views of the designed index gratings (denoted by Type-1 and Type-2, respectively). The Si substrate is etched through to form the index gratings. In Fig. 3(a), two kinds of phase-shifted line photodiodes are installed in each grating line which is a thin Si beam. Totally, four 908-phase-shifted photodiodes, in which the spatial phase differences are 908, respectively, are needed to obtain four sinusoidal signals. They are used for eliminating dc offset of signals, for interpolating the signal, and for sensing the direction. As shown in Fig. 3(a), since the two kinds of 908-phase-shifted photodiodes are installed in each Si beam alternately and closely in space, the photodiode sensitivities are nearly equal and the average intensities of signals are not affected by the light intensity distribution in a large area, especially in Type-1. The index grating is illuminated from backside and the light passes through the slits between the gratings. In the case of Fig. 3(b), the object grating which is a transmission Si grating is fabricated on the optical axis, while the index grating which consists of line photodiodes are installed at the off-axis positions. The two kinds of phase-shifted gratings are located above and below the transmission object grating as shown in Fig. 3(b). The light emitted through the object grating is re¯ected by the scale grating and detected with the photodiodes located at the off-axis positions. Although an ideal on-axis con®guration is obtained in Fig. 3(a), the design shown in Fig. 3(b) is somewhat easier than that in Fig. 3(a) for the fabrication. In principle, the grating period, which can be used in the proposed encoder, should be large enough to generate light diffraction (i.e. P > l), because the grating image is generated by the diffraction. Therefore, the general binary gratings used for the conventional encoders also suit the proposed encoder. In the actual fabrication, however, since the two line photodiodes were installed in a single Si grid, the grating period was limited by the lithographic precision used in the fabrication. We used the periods of 80 and 40 mm, considering the alignment accuracy, minimum transferable pattern, and diffusion length of the doped impurity.
Fig. 3. Designed index gratings: (a) Type-1; (b) Type-2.
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Fig. 4. Cross-sectional view of the designed encoder.
Fig. 4 shows the cross-sectional diagram of the encoder for Type-1 (in which scale grating is not shown). The index grating fabricated from Si substrate is ®xed to the LED holder to encapsulate the LED. The electrodes for the LED are patterned on the surface of LED holder. The light is emitted from the LED through the Si index grating. The light re¯ected from the scale grating is detected by the photodiode fabricated on the Si grating. A chip for electronic circuits is attached to the LED holder with polymer spacer. The electronic circuits may be fabricated on the side area of the index grating if the fabrication facility can accept both processes. From Figs. 2 and 4, an equivalent optical system is described by the three gratings placed at the same distance between the gratings along the optical axis. When the ®rst grating (object grating) is irradiated with spatially incoherent light, the object grating pattern is transferred by the center grating (re¯ection scale grating) on to the image plane (which is equal to the plane of the object and index gratings). The center grating of the three gratings works as a pupil for imaging. The optical principle of the encoder system can be explained theoretically by the grating imaging [6,7]. The theoretical analysis of the encoder optics has been carried out using optical transfer function. Based on the analytical results [7], the grating period and distance between the gratings have been decided. Since the principle of the grating imaging has been described in detail, a brief theoretical approach for the displacement measurement in our proposed system is given below. The essential optical system of the encoder is described by the three gratings placed in tandem at the same distances z between the gratings as shown in Fig. 1. The grating-like image is formed by the slit array of the pupil grating under Fresnel diffraction. For understanding the grating imaging, it may be easier to consider that each slit of the pupil grating images the object grating on the plane of the index grating and the superposition of the images generated by the respective slits of the grating produces a grating-like image
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if they are in phase. The optical transfer function of the pupil grating is given under our experimental conditions as follows [7]: mp lz F s P 2sp exp ip2smp 1 s lz 2e mp lz sinc 4es 1 s lz 2e mp mp 2e s< (1) for lz lz lz mp lz F s P 2sp exp ip2smp 1 s lz 2e mp lz sinc 4es 1 s lz 2e mp 2e mp s< (2) for lz lz lz for mp/lz 2e=lz s < mp/lz here, s is the image frequency, p and 2e are the pitch and the slit width of the pupil grating, respectively, z is the distance between the gratings, l is the wavelength of light and m is an integer. The function P(x) represents a comb function which becomes unity when x is equal to integers. Since the image frequency is equal to the object frequency (s 1=p) and the slit width is assumed to be half of the pitch (e p=4), then Eqs. (1) and (2) are simpli®ed to 1 1 2x 2m F s 1m 1 2x 2m sinc p 2 for m x < m 12 1 F s 1m 1 2x p for
m
1 2
1 2x 2m sinc 2
(3) 2m
x
(4) 2
where x is the distance z normalized by p /(2l) (x 2zl=p2 ). Therefore, the grating condition that the pitches of the three gratings are equal to each other corresponds to the second imaging condition (x 2). The image contrast calculated as a function of the normalized distance x is shown in Fig. 3. As shown in Fig. 5, the image contrast is always positive under the optical conditions, although the contrast varies periodically with increasing x. The period of the image contrast as a function of z is equal to p2/(2l) (i.e. x 1), which is dependent on the wavelength l of light. The positive contrast means that the image is located at the position of the geometrical shadow. Therefore, in the case of polychromatic illumination, the images generated by the respective wavelengths are superimposed constructively, and thus the image contrast is not degraded by the polychromaticity of the light source under these conditions. Moreover, the maximum value of the image contrast does not decrease with increasing the distance between the gratings, and thus the displacement can be measured at a distance z much larger than that used in the conventional Moire encoder.
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Since imaging effect of the grating is used in the encoder, the relative displacement d of the object grating generates that of image on the plane of the index grating in the opposite direction. Therefore, the encoder signal varies by two periods for the relative displacement of the grating equal to a single grating pitch. The sensitivity of the displacement detection in this encoder is improved by a factor of two by this phenomenon when compared with the conventional Moire encoder. Although the contrast of the encoder signal is nearly independent of the distance between the gratings over a wide range under the polychromatic illumination, the magnitude of the light intensity decreases with the increase of the separation z. Therefore, at a large separation, the signal to noise ratio decreases. If a line light source perpendicular to the grating lines is used in the encoder, the light intensity decreases approximately as a function of 1/z 1/z2, where z is the separation between the gratings.
Si substrate (1), a SiO2 ®lm is formed by wet oxidation (2). The Si substrate is n-type and 200 mm thick and the resistivity is 1±10 O cm. The SiO2 ®lm is 500 nm thick. From the rear surface, the wafer is etched with TMAH (3). The etched area is 3:5 mm 5:7 mm and 40 mm thick. Next, the line photodiode is fabricated on the etched area by implanting B ions (4). The dose density of B ion is 2 1014 /cm3 at 120 keV implantation voltage. After annealing (5) at the temperature of 1000 8C, the Al electrode is patterned (6). The gratings with the line photodiodes is then fabricated by etching through with inductively coupled reactive plasma (7). The pitches of the gratings fabricated in this study are 40 and 80 mm, respectively. For Type-2, the fabrication procedures are almost the same as those for Type-1 except for the mask patterning. Polychromatic spatially incoherent light source is needed for the encoder. Two spatially incoherent sources are used for Type-1 and one for Type-2. The LEDs are to be wide enough to illuminate the grating region. We have designed and fabricated the LED by ourselves in this experiment. A GaAlAs wafer is used for the substrate. The p2, p1, and n layers of the double-hetero-structure are 12, 0.35, and 160 mm thick, respectively. The LED is 3 mm long and 1mm wide with 50 mm wide AuZn comb electrode for pregion and AuSn for n-region. The comb electrode is formed perpendicularly to the lines of object grating. For further integrating the proposed encoder, we designed an preampli®er for obtaining the two phase-shifted signals without dc offset from the four channel photodiodes, which consisted of eight operational ampli®ers designed by ourselves. Since all the operational ampli®ers were fabricated on one chip, and the photodiodes for sensing the light intensities were located closely in space, the signal intensity nearly equal to each other was obtained, which was effective for a high precision interpolation.
3. Fabrication processes and integration
4. Results and discussion
Fig. 6 shows the sequence of the lithographic processes for fabricating the index grating of Type-1. Starting from a n-type
The fabricated index grating for Type-1 with the period of 80 mm is shown in Fig. 7. As shown in Fig. 7, the duty ratio
Fig. 5. Image contrast calculated as a function of normalized distance for a single wavelength.
Fig. 6. Fabrication process for encoder index grating.
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Fig. 7. Fabricated index grating (Type-1).
between the Si beam and slit widths is nearly unity. The two line photodiodes are installed as clearly shown in the magni®ed image of the grating in Fig. 7. The photo-current of the diode was measured to be 500 with 10 nA dark current using standard light source of 12.5 lx. Although a little larger dark current may be caused by a contamination in the fabrication, it has been con®rmed that the sensitivity of the fabricated photodiode is comparable to that of the commercialized one. The frequency response of the fabricated photodiode was examined. The cut-off frequency was around 200 kHz, which was high enough to encoder applications. The properties of the index grating were investigated by combining with scale grating. In this experiment, the index grating shown in Fig. 7 was illuminated with the white light from a halogen lamp through a ®ber bundle. The ®ber bundle with the diameter of about 10 mm was used for the illumination of the scale grating through the index grating. Since hundreds of ®bers were bound in the ®ber bundle, the grating was illuminated incoherently. The distance between the index grating and the ®ber-end was about 5 mm. The signals from the photodiodes were ampli®ed and subtracted to
compensate the dc components with the operational ampli®ers. The ampli®ed signals were fed to a computer through the digital-analog converters. The scale grating ®xed to a table was translated by stepping motor controlled by the computer. Fig. 8 shows the encoder signals measured as a function of displacement and the Lissajour ®gure of the two 908-phaseshifted signals. The gap between the index grating and scale grating is 3 mm under the experimental condition. The light re¯ected from the scale grating is detected with the photodiodes installed on the grids of the index grating. Therefore, the areas of light emission and detection are nearly same. Two line photodiodes are installed on one grating line to obtain the 908-phase-shifted signals. Another set of the phase-shifted signals is obtained from the index grating located below as shown in Fig. 7. In Fig. 8, the two sinusoidal signals are obtained and the phase difference between the signals is nearly 908. Since the Lissajour ®gure is close to a circle, the signal includes little harmonic noise. The signal contrast was measured as a function of the gap between the index and scale gratings. As investigated previously [9], the signal contrast was kept constant at
Fig. 8. Encoder signal (Type-1) measured as a function of displacement and its Lissajour figure.
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Fig. 9. Fabricated index grating (Type-2).
the large air gaps from 1 to 30 mm. In the case of the index grating shown in Fig. 7, the contrast decreased somewhat faster than the previous measurement since the two index gratings were placed vertically as shown in Fig. 7 and thus the light emitted from the respective gratings merged in the detection at a large gap. Light power of about 1 mW, which was a typical output of LED was enough to obtain a good S/N ratio (>10) at the separation around 2 mm. Increasing the separation between the gratings, the signal intensity from the photodiode decreased due to the diffusion of the incoherent light although the signal contrast was nearly constant. At the distance of 10 mm, S/N ratio was still high enough to measure the displacement. The light power needed in the proposed encoder was not much different from that of the conventional Moire encoder under our experimental conditions. The measurement accuracy was linearly dependent on the optical power of the light source when the separation of the gratings was kept constant.
Fig. 9 shows the fabricated index grating for Type-2 with the period of 80 mm. The transmission grating region is clearly etched through as shown in Fig. 9. In Type-2, the line-photodiodes are located above and below the transmission object grating. The characters on paper are seen through the object grating. The characteristics of the fabricated index grating were examined in the manor similar to Type-1. The sensitivities of the photodiodes were the same as those of Type-1. Fig. 10 shows the encoder signals measured as a function of displacement and the Lissajour ®gure at the gap of 7 mm when the white light from the halogen lamp is used as a light source through ®ber bundle. The measured signals are almost sinusoidal. The harmonic noises in the encoder signals in Fig. 10 are smaller than those in Fig. 8 judging from the deviation from a circle. In the optical con®guration of Type-2, the two sets of photodiodes are located separately by the width of the transmission grating, and thus the light re¯ected from the scale grating merges little on the plane of index grating. This was con®rmed by the fact that the
Fig. 10. Encoder signal (Type-2) measured as a function of displacement and its Lissajour figure.
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contrast of the encoder signal in Type-1 was kept high at a large gap. After testing the fabricated index grating as described above, we fabricated the integrated encoder sensor. The integration of the index grating, LED, LED holder and the IC chip was carried out as shown in Fig. 4 by stacking them with epoxy resin. The integrated encoder sensor is 1.2 mm thick and is shown in Fig. 11. In the LED holder, two electrodes for LED are patterned. The square LED emitting light is seen bright behind the upper index grating in Fig. 11. The emitting area of the LED is as wide as the index grating. Fig. 12 shows the optical micrograph of the light emission from the fabricated encoder sensor. As shown in Fig. 12, the grating-like emission through the Si grating is clearly obtained. Therefore, the Si grating on which photodiodes are installed works simultaneously as a transmission object grating. placing the integrated encoder sensor close to the scale grating, the signals were measured. Fig. 13 shows the encoder signals from the two channels. Although some noise is superimposed on the signals, the sinusoidal encoder signals are clearly seen in Fig. 13. The low signal to noise ratio is mainly due to the low intensity of the fabricated LED.
Fig. 11. Micrograph of the fabricated integrated sensor.
Fig. 12. Light emission from the integrated sensor.
Fig. 13. Encoder signals (displacement signal and Lissajour figure) from the integrated sensor.
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We have not observed a signi®cant in¯uence of the heat generated by the integrated electronics on the encoder signal in the experiments. The heart effect was considered to be small since the integrated electronics consisted of only eight operational ampli®ers, which consumed little energy, and the LED consumed less than 20 mW. It may need further investigation to discuss the heat effect in detail.
[6] R.M. Pettigrew, Analysis of grating imaging and its application to displacement metrology, Proc. Soc. Photo-Opt. Instrum. Eng. 136 (1977) 325±331. [7] K. Hane, C.P. Grover, Magnified grating images used in displacement sensing, Appl. Opt. 26 (1987) 2355±2359. [8] K. Hane, C.P. Grover, Imaging with rectangular transmission gratings, J. Opt. Soc. Am. A4 (1987) 706±711. [9] K. Hane, T. Endo, Y. Ito, M. Sasaki, A compact optical encoder with micromachined photodetector, J. Opt. A: Pure Appl. Opt. 3 (2001), 191±195.
5. Conclusions We proposed an integration of the grating-image-type encoder. The sensor for the encoder was fabricated using Si micromachining technology. The sensor was fabricated by stacking the Si grating with line photodiodes, LED and signal ampli®ers. The Si grating worked as the photodetectors as well as the object and index gratings. The incoherent light emission through the Si grating and the light detection with the grating made the optical system compact. The two types of grating structures were fabricated for practical sensors. The signal contrasts were high enough for the displacement measurements. References [1] J. Guild, Diffraction Gratings as Measuring Scales, Oxford University Press, London, 1960. [2] K. Ishizuka, H. Watanabe, S. Ishii, M. Tsukiji, T. Nishimura, DENSHI, Vol. 30, Tokyo, 1991, p. 67 (in Japanese). [3] R. Sawada, E. Higurashi, O. Ohguchi, Y. Jin, Long-lift micro-laser encoder, in: Proceedings of the MEMS, 2000, pp. 491±495. [4] A. Ieki, K. Hane, T. Yoshizawa, K. Matsui, M. Nashiki, Optical encoder using a slit-width-modulated grating, J. Mod. Opt. 46 (1999) 1±14. [5] A. Ieki, K. Matsui, M. Nashiki, K. Hane, Pitch-modulated phase grating and its application to displacement encoder, J. Mod. Opt. 47 (2000) 1213±1225.
Biographies Tetsuo Endo received ME degree from Tohoku University in 2000. He is now belonging to NEC Communication Ltd. Masashiro Ishimori received BE degree at Tohoku University in 2000. He is studying the light emitting diode in the master course of Tohoku University. Yoshinori Ito graduated from Nagano Technical College in 1974. Currently he is a member of Future Business Division of Harmonic Drive Systems Inc., Nagano Japan. He is engaged in development of encoder and other sensors. Kazuhiro Hane received the MS and doctorate degree in engineering from Nagoya university in 1980 and in 1983, respectively. From 1983 to 1994, he worked as a member of Department of Electrical Engineering in Nagoya University. From 1985 to 1986, he was a visiting researcher of National Research Council of Canada. Since 1994, he has been a Professor of Graduate School of Mechanical Engineering, Tohoku University, and is currently engaged in the research and development of optical microsensors and optical MEMS. Minoru Sasaki received the MS and doctorate degree in engineering from Nagoya University in 1993 and 1995, respectively. In 1996, he is a Research Fellow of Japan Society for the Promotion of Science. From 1996 to 2000, he was a Research Associate of Graduate School of Mechanical Engineering, Tohoku University Since 2000, he has been a Assistant Professor, and is currently engaged in the research and development of semiconductor micro sensors and optical MEMS.