Substrate holding device

Abstract: A device for holding a substrate includes a holding system for holding a substrate by attraction and a guard system for preventing drop of the substrate held by the holding system, the guard system being out of contact with the substrate as the same is held by the holding system.

Description: FIELD OF THE INVENTION AND RELATED ART This invention relates to a substrate holding device, and more particularly, to a substrate holding device which is suitably usable, e.g., in an exposure apparatus for transferring and printing an image of an original such as a mask onto a substrate, to be exposed, such as a semiconductor wafer very precisely, as a wafer hand mechanism for transporting the semiconductor wafer or as a wafer chuck mechanism for holding the semiconductor wafer, for example. With recent increasing degree of integration of semiconductor integrated circuits; in an exposure apparatus (aligner) for manufacture of the same, further enhancement of transfer precision is required. As an example, for an integrated circuit of 256 megabit DRAM, an exposure apparatus capable of printing a pattern of a linewidth in the order of 0.25 micron is necessary. As such super-fine pattern printing exposure apparatus, an exposure apparatus which uses orbit radiation light (SOR X-rays) has been proposed. The orbit radiation light has a sheet beam shape, uniform in a horizontal direction. Thus, for exposure of a plane of certain area, many proposals have been made, such as follows: (1) Scan exposure method wherein a mask and a wafer are moved in a vertical direction whereby the surface is scanned with X-rays of sheet beam shape in a horizontal direction; (2) Scan mirror exposure method wherein X-rays of sheet beam shape are reflected by an oscillating mirror whereby a mask and a wafer are scanned in a vertical direction; and (3) Simultaneous exposure method wherein X-rays of sheet beam shape in a horizontal direction are diverged in a vertical direction by an X-ray mirror having a reflection surface machined into a convex shape, whereby an exposure region as a whole is irradiated simultaneously. The inventors of the subject application have cooperated to devise such a simultaneous exposure type X-ray exposure apparatus, which is disclosed in Japanese Laid-Open Patent Application No. 243519/1989. In such SOR X-ray exposure apparatus, a SOR X-ray bundle (SOR beam) emanates from a SOR device substantially horizontally and, to meet this, usually an original and a substrate to be exposed are held substantially vertically during the exposure period. Because a substrate or the like is held substantially vertically during the exposure period, at many stages during the transportation it is held substantially vertically. This causes such inconvenience that a substrate or the like held by vacuum attraction, for example, during the normal operation period falls off and is damaged due to failure in attraction caused by electric surface interruption, malfunction of a sensor for wafer transfer, degraded flatness of the substrate or the like, and scattered fractions of the substrate damage various units constituting the exposure apparatus. EP-A-0134621 discloses apparatus for the transport of semiconductor wafers including an engagement element with a plurality of openings, suction means to provide a vacuum at the openings for the purpose of adhering the object to the engagement element, transport means to enable the engagement element to be transported relative to the object to bring the openings close enough to an engagement surface of the object to enable suction engagement of the object at the openings, position detection means to sense the relative position of the object and the engagement element, sensing means to sense suction engagement at the openings, processing means including engagement-failure-detection means responsive to the sensing and position detection means for detecting when suction engagement has not occurred despite the engagement element having been transported toward the object a distance sufficient to enable such engagement, and control means respond to the engagement-failure-detection means to provide a control signal to the transport means to adjust the position of the engagement element relative to the object to facilitate the suction engagement. EP-A-0357424 discloses a chuck for securing a wafer by vacuum. An object of the present invention is to provide an arrangement, in a substrate holding device usable, e.g. in an exposure apparatus wherein exposure or transportation of a substrate to be exposed is effected while holding the substrate upright, which arrangement is effective to prevent fall-off of the substrate which otherwise results from electric surface interruption, malfunction of a sensor or insufficient flatness of the substrate, to thereby prevent contamination by foreign particles by the fall-off or damage of the exposure apparatus by pieces of the substrate. According to the invention there is provided a substrate support device comprising holding means for attracting the rear surface of the substrate, characterised by guard means for preventing falling of the substrate from said holding means movable between a first retracted position at which the latter is out of contact with the substrate, and a second position to catch and prevent the substrate from falling, without the guard means engaging the front surface of the substrate. Furthermore, according to the invention, an exposure apparatus according to claim 7 and a semiconductor wafer support device according to claim 11 are provided. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings in which:- Figure 1 is a side view of a wafer chuck according to the present invention. Figures 2A - 2D show a major part of an exposure apparatus according to another aspect of the present invention, wherein Figure 2A is a front view, Figures 2B and 2C are side views, and Figure 2D is a top plan view. Figure 3 is a schematic view of a wafer chuck used in the embodiment of Figures 2A - 2D. Figure 4 is a flow chart for explaining the operation of an exposure apparatus according to the present invention. Figure 1 shows a wafer chuck according to an aspect of the present invention. In this embodiment the presence/absence of a wafer is detected to control projection/retraction of pawls. Denoted in Figure 3 at 301 is an X stage; at 302 is a Y stage; at 303 is a  stage; at 304 is a wafer chuck; and at 305 is a wafer. Here, the X stage 301, the Y stage 302, the  stage 303 and the wafer chuck 304 constitutes a wafer stage 310 which itself is a known type. However, the wafer stage 310 is provided with an electrostatic sensor 306 for detecting presence/absence of a wafer 305 as well as a pawl 307 which is adapted to be projected/retracted in response to an output from the electrostatic sensor 306. Figure 3 shows a state in which the pawl 307 is retracted (solid linen position). When the pawl 307 is projected, it takes a position as depicted by a broken line. Denoted at 308 is a motor (actuator) for projecting/retracting the pawl 307. The result of detection by the electrostatic sensor 306 is monitored by a controller (not shown). If the absence (fall-off) of the wafer 305 on the wafer chuck 304 is detected by the sensor 306, the controller operates instantaneously to drive the motor 308 to project the pawl 307 to its fall-off preventing position (broken line position in Figure 3). Figures 2A - 2D show a wafer chuck and a wafer hand according to another embodiment of the present invention. Figure 2A is a front view and Figures 2B and 2C are side views. Figure 2D is a top plan view showing details of an exposing portion of an exposure apparatus into which the wafer chuck and the wafer hand of this embodiment are incorporated. Denoted in these drawings at 401 is a prealignment stage for relatively roughly positioning a wafer; at 402 is an exposure stage; at 402M is a wafer on which a pattern to be transferred to the wafer with X-rays is formed; at 412 is a mask chuck for holding the mask 402M; and at 403 is a wafer hand. The wafer hand 403 serves to convey the wafer 305, having been relatively roughly positioned at the prealignment stage 401, from this prealignment stage to the exposure stage 402 while holding the wafer 305 upright. Denoted at 404 are pawls of the wafer hand 403, and denoted at 405 is a guide for guiding the wafer hand 403 horizontally as viewed in Figure 2A. The prealignment stage 401 and the exposure stage 402 have a similar structure as that shown in Figure 1, except for that the electrostatic sensor 306 and the motor 308 are omitted and that the mechanism for driving the pawls 307 is modified such as described later. Denoted at 406 is a shutter for controlling irradiation of the mask 402M with X-rays, and denoted at 413 is a shutter controller for controlling the operation of the shutter 406. More specifically, the controller 413 serves to control the shutter 406 so as to start X-ray irradiation after the positioning of the wafer upon the exposure stage 402 is completed. Denoted at 407 is a window having a beryllium thin film. Denoted at 408 is a light emission point of a SOR ring which provides an X-ray light source. The synchrotron radiation light (X-rays) emanated from the light emission point 408 substantially horizontally. Here, the synchrotron radiation light has a uniform intensity distribution in a direction (X direction) parallel to the orbit plane of the electron beam, whereas it has a symmetric intensity distribution in a direction (Y direction) perpendicular to the orbit plane. Cylindrical mirror 409 having a convex reflection surface serves to expand the X-ray bundle from the light emission point 408, in a direction (Y direction) in which the symmetric intensity distribution appears. The cylindrical mirror 409 is used to obtain an irradiation region of a necessary size, at the position at which the mask 402M and the wafer 305 (one exposure zone) are disposed. As described, usually, the X-ray bundle from the SOR ring advances substantially horizontally. In consideration thereof, for irradiation of the wafer 305 with the X-rays, the exposure stage 402 is arranged to hold, by attraction, the wafer 305 with the wafer surface being placed vertically. In Figures 2A - 2C, the pawls 404 are fixed to the wafer hand 403 and, thus, they can not be retracted. On the other hand, pawls 307 of the stages 401 and 402 can be protruded and retracted in accordance with the presence/absence of a wafer 305 on the wafer chuck 304, as in the Figure 1 embodiment. These pawls are arranged to be held out of contact with a wafer unless it falls off. In this embodiment, an electrostatic sensor 306 of the preceding embodiment for wafer detection is not used. In place thereof, the pawls 307 are protruded by using the phenomenon that the back pressure of the wafer attraction vacuum increases if no wafer is placed on the wafer chuck 304. The actuator means effective to protrude the pawls 307 by using the phenomenon of increasing back pressure of the wafer attracting vacuum, may be structured such as shown in Figure 3, for example. Denoted in Figure 3 at 501 is a vacuum source such as a vacuum pump, for example; at 502 is a spring for urging the pawl 307 toward the wafer fall-off preventing position 307'; at 503 is a bellows; at 504 is an attracting bore; and at 505 is a vacuum supplying passageway. When the wafer 305 is held by the wafer chuck 304, the attracting bore 504 is covered by the wafer 305. Therefore, the pressure in the vacuum supplying passageway 505 is low. As a result, the bellows 504 inside of which is communicated with the vacuum supplying passageway 505 is contracted and, consequently, the pawl 307 is retracted by the bellows to its retracted position as depicted by a solid line. If the wafer 305 falls off the wafer chuck 304 due to electric service interruption, malfunction in the vacuum system, insufficient flatness of the wafer or for any other reason, the attracting bore 504 is uncovered. As a result, the pressure in the vacuum supplying passageway 505 increases suddenly. This causes extension of the bellows 504 and, as a consequence, the pawl 307 is urged by the biasing force of the spring 502 and is protruded to the wafer fall-off preventing position 307′. As shown in Figure 2C, the wafer 305 which is just going to fall downwardly off the wafer chuck 304 slides downwardly by a small distance with its lower edge abutting against the slant portion 307a of the pawl 307, and it is stopped by a bent 307b of the pawl 307. In this manner, the lower edge of the wafer 305 is supported by the pawl 307, with the upper portion of the wafer leaning against the wafer chuck 304, whereby the fall-off of the wafer from the wafer chuck 304 is prevented. The bent 307b of the pawl 307 has a set angle, not to contact the surface of the wafer 305 as it supports the slid wafer. This effectively avoids damage of patterns on the wafer surface as fallen off the wafer chuck 304. Therefore, the wafer can still be used for manufacture of semiconductor devices. A further embodiment will now be explained. This embodiment corresponds to a modified form of the embodiment shown in Figures 2A - 2D, and the actuator for driving the pawl 307 comprises a motor such as at 308 in Figure 1. In an arrangement such as shown in Figures 2A - 2C wherein a wafer is loaded in a direction facing to the front, there is a possibility that the pawl 307 of the wafer chuck 304 side interferes with the wafer loading. If it actually interferes the loading, the pawl 307 should be moved back to the retracted position during the loading operation. Figure 4 shows the sequence of a controller (not shown) for controlling the motor 308 for moving the pawl. More specifically, at the alignment stage 401 a wafer is attracted to the wafer hand 403 (step 601) and, immediately after this, the pawl 307 of the wafer chuck 304 at the exposure stage 402 side is moved back to its retracted position (step 602). During this period, the pawl of the alignment stage side is kept at its retracted position. After the retraction of the pawl 307 of the exposure stage side to the position not interfering with the loading is completed (step 603), the wafer hand 403 is moved to be faced to the wafer chuck 304 of the exposure stage 402 (step 604), and the wafer 305 is transferred from the wafer hand 403 to the wafer chuck 304 (step 605). Thereafter, the wafer hand 403 is moved away from the position of the wafer chuck 304 (step 606), and, after the wafer hand 403 reaches a position not interfering with the exposure process or the like (step 607), the pawl 307 of the wafer chuck 304 is projected to the fall-off preventing position. The pawl 307 of the wafer chuck is so formed that it does not contact to the wafer unless the same falls off. Subsequently, the shutter controller 413 operates to control the shutter 406 to start the X-ray exposure. A similar sequence as described above will be effected for the wafer loading from a different position to the alignment stage 401. With the embodiments described hereinbefore, even if electric surface interruption or malfunction of a pump or a sensor occurs, damage of a substrate is effectively prevented. Further, damage of the pump or any other units in the apparatus by fractions of the substrate is effectively prevented. It is to be noted here that the attraction of the substrate such as a wafer is not limited to vacuum attraction, and electrostatic attraction may be used as an example.

Claims: A substrate support device comprising holding means (310) for attracting the rear surface of the substrate, characterised by guard means (307) for preventing falling of the substrate from said holding means, said guard means being movable between a first retracted position at which the latter is out of contact with the substrate, and a second position to catch and prevent the substrate from falling without the guard means engaging the front surface of the substrate. A device as claimed in claim 1, characterised in that said guard means comprises a pawl (307) for receiving the substrate. A device as claimed in claim 1 or 2, characterised in that said holding means holds the substrate in an upstanding position. A device as claimed in claim 1, 2 or 3, characterised by detecting means (306) for detecting presence or absence of the substrate at a holding position of said holding means, wherein said guard means is moved to said second position in accordance with the detection by said detecting means of absence of said substrate. A device as claimed in any one of claims 1-4, characterised in that said guard means (307) is positioned for preventing the substrate from falling in response to back pressure of vacuum attraction of said holding means (310). A device as claimed in any one of claims 1-5, in combination with a substrate transport mechanism, characterised in that means are provided to enable said guard means for positioning thereof to prevent the wafer falling, after said transport mechanism moves away from said holding means following delivery of a substrate. An exposure apparatus for exposing a substrate held by a device as claimed in any one of claims 1-6, comprising illumination means (408; 409) for irradiating the front surface of the substrate with radiation. An apparatus as claimed in claim 7, characterised in that said radiation includes X-rays. An apparatus as claimed in claim 7 or 8, characterised in that said illumination means irradiates a mask having a pattern, to transfer the pattern onto the front surface of the substrate. An apparatus as claimed in claim 7, 8 or 9, characterised in that said substrate is a semiconductor wafer. A semiconductor wafer support device comprising a vacuum chuck (310) for attracting the rear surface of the wafer and supporting the wafer in an upright position, characterised by a pawl (307) movable between a first position at which the latter is retracted to a non-effective position and a second position at which the pawl catches the wafer in the event of the latter falling off the chuck without engagement with the front surface of the substrate, an actuator (308) for said pawl (307), and pressure sensing means (306) coupled with said chuck for detecting the absence of the wafer and for controlling said actuator to move the pawl into the second position to catch the wafer in the event of said wafer falling off said chuck. The device of claim 11, wherein a spring (502) biases the pawl (307) towards its second position and a bellows (503) moves the pawl (307) to its first position when the chuck (310) is under vacuum. The device of claim 11 or 12, when incorporated into a wafer transporting or a wafer holding mechanism forming part of an exposure apparatus for transferring an image of a mask onto a semiconductor wafer. The device of any preceding claim, having a semiconductor wafer held by vacuum thereon.

Inventors: IWAMOTO KAZUNORI, KARIYA TAKAO, MARUMO MITSUJI, MIZUSAWA NOBUTOSHI, UZAWA SHUNICHI, IWAMOTO, KAZUNORI, KARIYA, TAKAO, MARUMO, MITSUJI, MIZUSAWA, NOBUTOSHI, UZAWA, SHUNICHI, IWAMOTO, KAZUNORI, CANON KABUSHIKI KAISHA, KARIYA, TAKAO, CANON KABUSHIKI KAISHA, MARUMO, MITSUJI, CANON KABUSHIKI KAISHA, MIZUSAWA, NOBUTOSHI, CANON KABUSHIKI KAISHA, UZAWA, SHUNICHI, CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H05K13
IPC codes: G03F7, H01L21