Positioning mechanism and method

Abstract: A positioning device includes a movable table supported by a rectilinear guide mechanism or rotary mechanism, the movable table being moved by a driving mechanism so as to position a member, to be conveyed, placed on the movable table, with respect to a predetermined site. After the member being conveyed is conveyed to the predetermined site, the conveyed member is contacted intimately to a stationary table which provides a reference with respect to the attitude of the conveyed member, such that the attitude correction of the conveyed member is attained.

Description: FIELD OF THE INVENTION AND RELATED ART This invention relates to a positioning mechanism and method for an alignment unit, e.g. for moving, by a suitable driving means, a movable table which is supported by a rectilinear guide mechanism or rotary mechanism, to position a member, to be conveyed, placed on the movable table. Such a mechanism for moving and positioning a member to be conveyed is used for example in semiconductor microcircuit manufacturing apparatuses. One representative example of such a mechanism may include a structure provided by a combination of a cross roller guide, a linear encoder and a motor. The cross roller guide is a guide mechanism for moving an optical component or a recording member in one direction. The motor is a drive source for driving a movable table of the cross roller guide, the drive being transmitted through a transmission mechanism such as a gear means or ballscrew means. The linear encoder functions as a means for detecting the position of a member to be conveyed, an output signal therefrom being applied by way of a servo circuit to the motor to control the same to thereby position the member being conveyed. An example of such a positioning system is shown in Figure 1. This positioning system comprises a combination of a cross roller guide, a linear encoder and a motor and, in this example, the positioning system is used with an alignment unit which is operable to align a reticle and a wafer in a process of manufacture of semiconductor microcircuits. In Figure 1, denoted at 31 is a movable guide which is movable in an X direction. Denoted at 32 is a stationary guide which is fixed to a frame 33. These two guides provides a cross roller guide 34. Another stationary guide 35 and another movable guide 36 provide a Y-direction cross roller guide 37 which is mounted on the movable guide 31 so that it extends in a direction perpendicular to the cross roller guide 34. These two cross roller guides 34 and 37 cooperate to provide an X-Y stage. Motors for driving these movable guides as well as associated linear encoders (position detecting means) are provided, but they are not illustrated in Figure 1. Mounted on the movable guide 36 is an alignment unit which comprises a laser tube (light source) 38, an alignment optical system 39 and an alignment signal detecting sensor 40. Reticle 41 has a pattern to be printed on a wafer 42, which pattern may be formed on the reticle by an electron beam drawing apparatus. The reticle 41 is supported by a reticle holder 43 so that a gap g is defined between it and the wafer 42. The wafer 42 is held by a wafer chuck 44 by attraction, and can be moved and positioned in the X and Y directions by means of a wafer stage 45. An alignment mark such as shown at 46 in Figure 2A is formed on the wafer 42, while an alignment mark such as shown at 47 in Figure 2B is formed on the reticle 41. These marks can be detected by the alignment unit, by using a laser beam, for example. The wafer stage 45 is displaced so that these marks 46 and 47 as detected are brought into a positional relationship such as illustrated in Figure 2C. After completion of reticle-to-wafer alignment, the exposure 16 of the wafer to the pattern of the reticle is made by using a radiation energy supplied from a suitable source, not shown. By means of the cross roller guide means of the X-Y stage as described, the alignment unit can be moved and positioned at a desired site with respect to the reticle 41. As a result, irrespective of the position of an alignment mark which is changeable with the size of a pattern, a process to be made, etc., the reticle-to-wafer alignment using the alignment unit is attainable. However, such a positioning system involves some inconveniences such as follows: Referring to Figures 3A and 3B which are schematic representations of the mark detecting operation using the alignment unit shown in Figure 1. Figure 3A illustrates an occasion where the axis 49 of an alignment beam is parallel to the axis 48 of an exposure beam (light used for the photoprinting). Figure 3B illustrates an occasion where the axis 49 of the alignment beam is inclined by  with respect to the axis 48 of the exposure beam. In both cases of the Figure 3A example and the Figure 3B example, patterns of alignment completion, such as illustrated in Figure 2C, can be sensed by the alignment detecting sensor 40. Since in the Figure 3A example the axis 48 of the exposure beam and the axis 49 of the alignment beam are parallel to each other, a pattern 50 of the reticle 41 can be superposed, by exposure, upon a pattern 51 having been formed on the wafer 42, without an overlay error. In the Figure 3B example, however, since the axis 49 of the alignment beam is inclined by  with respect to the axis 48 of the exposure beam, the completion of the reticle-to-wafer alignment as sensed by the alignment detecting sensor 40 causes an overlay error ε between the patterns 50 and 51. Thus, if the exposure is made in this state, there is produced an overlay error ε between the pattern 51 having been formed on the wafer 42 and a pattern just transferred onto the wafer by the exposure. Where the gap between the wafer 42 and the reticle 41 is denoted by g, the error ε is equal to g · tan. The tilt of the axis 49 of the alignment beam may result from a change in attitude of the X-Y stage supporting the alignment unit, particularly, from the pitching and/or rolling of the X-Y stage. The problem of such an error ε resulting from the change in attitude of the X-Y stage, as described, is particularly disadvantageous in consideration of the requirement of highest precision alignment in the trend of further miniaturization and higher density of recent LSI microcircuit devices. The change in the attitude of an X-Y stage comprising a cross roller guide means may be caused by any waviness of a rolling surface along which rollers move, the straightness of a guide, irregularity in the thickness of an oil film, presence of foreign particles, etc. Particularly when foreign particles are present, it is difficult to remove them from the structural limitations, and, therefore, it is difficult to assure good precision reproducibility. There is known from US-A-4669867 an alignment and exposure apparatus in which a wafer is held against a movable wafer stage by vacuum suction, and a reticle is held against a movable reticle stage by vacuum suction. The reticle stage is driven relative to the wafer stage to align the reticle with the wafer. An alignment detection system, which detects alignment marks on the reticle and on the wafer, comprises various optical parts such as mirrors, stops and lenses, and these parts are movable to enable the alignment system to detect alignment marks at different positions. There is also known, from EP-A-0211292, a wafer transport system in which wafers are transported between cassettes and a rotatable support disc, and between the rotatable support disc and a substrate holder. Wafers are subjected to treatment while on the substrate holder. SUMMARY OF THE INVENTION According to the present invention there is provided a positioning apparatus according to claim 1, apparatus for printing pattern on a wafer according to claim 8, exposure apparatus according to claim 9, and a method according to claim 10. The remaining claims set out optional features. An embodiment of the present invention provides a positioning system usable for the positioning of an alignment unit of a semiconductor microcircuit manufacturing exposure apparatus, for example, wherein a member to be conveyed can be moved and positioned at a desired site without any change in its attitude which may be caused by the effect of a conveying means conveying the member to be conveyed. In an embodiment of the present invention, after movement by a conveying means, a member having been conveyed is contacted intimately to a predetermined reference surface. This allows that the member having been conveyed is held in a desired attitude with certainty and with high precision, without being affected by the precision of a moving mechanism, a guiding mechanism, etc., of the conveying means. Embodiments of the present invention, given by way of non-limiting example, will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a fragmentary perspective view of a known type reticle-to-wafer alignment system. Figures 2A - 2C are schematic illustrations of alignment marks and a state of them upon completion of alignment. Figures 3A and 3B are schematic illustrations for explicating inconveniences involved in the Figure 1 example. Figure 4 is a fragmentary perspective view of a reticle-to-wafer alignment system according to a first embodiment of the present invention. Figure 5 is a schematic and diagrammatic view, partially in section, showing a major portion of the Figure 4 embodiment. Figures 6A and 6B are representations for explicating the operation of the Figure 4 embodiment. Figure 7 is a flow chart showing the sequential operation made in the Figure 4 embodiment. Figure 8 is a fragmentary side view, partially in section, showing a major portion of a reticle-to-wafer alignment system according to a second embodiment of the present invention. Figures 9 - 11 and 12A and 12B are schematic views, respectively, showing modified forms according to the present invention. Figure 13 is a schematic view, partially in section, showing a reticle-to-wafer alignment system according to a third embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 4 is a perspective view of a positioning system according to an embodiment of the present invention, which embodiment is applied to a semiconductor microcircuit device manufacturing exposure apparatus. Figure 5 is a side view, partially in section, schematically and diagrammatically showing a major portion of the Figure 4 embodiment. In Figure 4, denoted at 61 is a surface plate (base member) on which a wafer stage 45, a cross roller guide means, etc. are placed. Denoted at 1 is a post which is fixedly secured to a movable guide 36 of a Y-direction cross roller guide 37. To this post 1, two parallel linkage plates 2a and 2b are coupled by means of resilient hinges 3. The other ends of the linkage plates 2a and 2b are coupled to an alignment unit 4 by means of similar resilient hinges 3 to resiliently support the alignment unit 4. Provided at the top of the post 1 is a frame 5 which extends up to the top of the alignment unit 4. The frame 5 supports thereon an actuator 6 such as an air-cylinder of short stroke, for moving the alignment unit 4 upwardly and downwardly. The air-cylinder 6 used in this embodiment not only functions to move the alignment unit 4 upwardly and downwardly, but also acts as a clamp means for pressing, with a certain pressure, the alignment unit 4 against a stage base table 7 having a surface whose flatness has been machined with high precision. While the exposure apparatus in which the described positioning system is employed includes a radiation energy source for exposure of a wafer 42 as well as other constitutional elements, they are not illustrated in the drawings. The alignment unit 4 comprises a light source 8 which may be a semiconductor laser, an alignment signal detecting sensor 40 and an alignment optical system (not shown). Similarly to the Figure 1 example, the alignment unit 4 is operable to detect an alignment mark formed on a reticle 41 (such as at 47 in Figure 2B) as well as an alignment mark formed on a wafer 42 (such as at 46 in Figure 2A). In accordance with the detection of these marks, the wafer stage 45 is displaced so that a pattern of the reticle (such as at 50 in Figure 3A) can be correctly superposed upon a pattern having been formed on the wafer (such as at 51 in Figure 3A). In Figure 5, the positioning system includes two actuator units 9X and 9Y for driving the X-Y stage in the X and Y directions. Each actuator unit comprises a motor rotatable in accordance with an instruction voltage supplied from a controller (central 26 processing unit) 28, a rotary encoder for detecting the amount of rotational motion of the motor, and a transmission mechanism for converting the rotational motion of the motor into a rectilinear motion and transmitting the same to the movable guide 36. The positioning system further includes a high-pressure air source 20 for supplying a high-pressure air to the air-cylinder 6; a solenoid valve 21 for adjusting the quantity of the high-pressure air to be supplied from the source 20 to the air-cylinder 6; a driver 22 for actuating the solenoid valve 21; an LD driver 23 for actuating the semiconductor laser 8 (alignment light source); an amplifier 24 for amplifying a signal as detected by the alignment signal detecting sensor 40; an analog-to-digital converter 25 for converting an analog output of the amplifier 24 into a digital signal; drive sources 26 and 27 for supplying drives to the actuator units 9X and 9Y, respectively; and a controller (central processing unit) 28 which is operable to control the drivers 22, 23, 26 and 27, for example, as well as to process an alignment detecting signal from the sensor, for example. The bottom surface 10 of the alignment unit 4 and the top surface 11 of the stage base table 7, opposed thereto, each has been finished by superflatness machining. Also, the stage base table 7 is fixedly secured to the surface plate of the exposure apparatus. The manner how to displace the alignment unit 4 to a predetermined site will now be explained, in conjunction with Figure 5 and the flow chart of Figure 7. Prior to the displacement, the air-cylinder 6 is brought into an extracted state so that the alignment unit 4 is lifted by a rod 12, with the result that the alignment unit 4 is disengaged from the stage base table 7 as illustrated in Figure 6A (Step 1). The amount of floating is restricted by the shape of each resilient hinge 3, the length of each linkage plate 2a or 2b as well as the materials of these components. In the state in which the alignment unit 4 is floated, the X-Y stage operates in response to an instruction signal from the controller to displace the alignment unit 4 to the predetermined site (Steps 2 - 5). Subsequently, the air-cylinder 6 is brought into an extended state so that the alignment unit 4 is pressed downwardly by the rod 12, whereby the alignment unit 4 is contacted intimately to the stage base table 7, as shown in Figure 6B (Step 6). At this time, since each of the bottom surface 10 of the alignment unit 4 and the top surface 11 of the stage base table 7 has been finished by super-flatness machining, the alignment unit can be brought into and held in a surely constant attitude and the alignment can be made in this attitude of the alignment unit. In an X-Y stage mechanism using an ordinary cross roller guide, the highest attitude precision is about 1.5 x 10 -4 rad because of the difficulty in avoiding pitching or rolling. As compared therewith, in accordance with the present invention, the attitude precision can be determined by the machining precision of these two surfaces, and a highest attitude precision of about 0.3 x 10- 4 rad is attainable. The stage base table 7 is formed with an opening, as best seen in Figure 5, to allow that the light from the alignment light source 8 of the alignment unit 4 as well as the light reflected from the marks of the reticle and the wafer pass therethrough toward the sensor 40, when the alignment unit 4 is contacted intimately to the stage base table 7. That is, when the alignment unit 4 is moved downwardly, the bottom surface 10 of the alignment unit 4 engages the top surface 11 of the stage base table 7 around the opening formed therein. The stage base table 7 is formed so as not interfere with the exposure region (path of the exposure light). For example, the stage base plate 7 may have an opening so that the exposure light can pass therethrough. In accordance with the present embodiment, as described hereinbefore, the alignment unit 4 can be positioned with high attitude precision. As a result, no tilt of the axis of alignment beam occurs and, therefore, high-precision alignment is attainable. In the present embodiment, the alignment unit itself which is a member to be conveyed by the cross roller guide means engages the reference surface formed on a stationary table for the attitude correction. However, the invention is not limited to such form. For example, as illustrated in Figure 13, the attitude correction of an alignment unit at a desired site can be made by specifically arranging a portion other than the alignment unit. This will be described below in more detail. In Figure 13 which shows another embodiment, reference numerals the same as those used in the foregoing embodiment are assigned to similar or corresponding elements. Denoted at 62 is an alignment pickup head which is a member to be conveyed. Denoted at 63 are posts adapted to support clamp means 6. Denoted at 64 is a movable table on which the pickup head 62 is placed and supported. An X-Y stage mechanism having movable tables 34 and 37 is operable to displace the movable table 64 to move the pickup head 62, supported by the movable table 64, to position the pickup head 62 relatively to a reticle 41 and a wafer 42. The clamp means 6 is operable to selectively urge the movable table 64 so that a reference surface 10 of the movable table is pressed against a reference surface 11 of the stage base table 7, such that the movable table 64 is clamped. After the clamping is accomplished, the reticle-to-wafer alignment is made with the corrected attitude of the pickup head 62. In this embodiment, as described, the attitude correction of the alignment unit or head can be made by the engagement of the movable table 64 which may be considered as another member to be conveyed. In this embodiment, the floating action of the alignment unit or head is restricted by a parallel linkage using similar resilient hinge means 3. As a result, there is no play due to a bearing gap, which may be caused in a case where the linkage uses an ordinary hinge means. Also, for the floating, the alignment unit can float from and land on an opposing surface with a small attitude change and with high reproducibility. Additionally, there is no possibility of creation of foreign particles at the hinge portion. In the foregoing embodiments, the parallel linkage provided by resilient hinges and linkage plates may be replaced by a parallel leaf spring means comprising two or more leaf springs. Substantially the same effect is attainable. Where very high reproducibility or prevention of dust creation is not required, use may be made of a parallel linkage provided by hinge means 13 and linkage plates 2a and 2b such as shown in Figure 8. Also, in the above-described embodiments, an alignment unit 4 which is a member to be conveyed is moved while being floated from a stage base table 7. As a result, there is substantially no possibility of creation of dust at the portion adjacent the alignment unit. This is very desirable. In the foregoing embodiments, an air-cylinder is used as a means for floating the alignment unit 4. However, the present invention is not limited thereto. For example, as shown in Figure 9, a high-pressure air HA may be supplied to a member 14, to be conveyed, through a flexible tube 15, such that the high-pressure air HA may be discharged from an orifice 16 provided in the bottom of the member 14 to float the member 14. In that case, for engagement of the member 14 with a stage base table 7, the supply of the high-pressure air may be stopped and, additionally, a vacuum attracting means such as a pump, for example, may be used to produce a negative pressure so that the member 14 may be attracted to and contacted intimately to the stage base table 7. In an occasion where, in some embodiments described hereinbefore, an air-cylinder is used to press the member, being conveyed, against the stage base table, there is a possibility that the pressing force causes flexure of the stage base table. However, in this form of the present invention shown in Figure 9, no external force is directly applied to the stage base table, with an advantageous result of creation of no such flexure. Further, as illustrated in Figure 10, a stage base table 7 may be made of a magnetic material and a member 14, having been conveyed, may be contacted intimately to the stage base table 7 by means of an electromagnet 17 which may be provided in the neighborhood of the bottom of the member 14. Substantially the same advantageous effect is attainable in that case. While the foregoing description has been made to examples in which a member to be conveyed floats from a stage base table upon conveyance. However, the invention is not limited to this, and the system may be so arranged that the member to be conveyed does not float upon conveyance. In an example shown in Figure 11, a member 14 to be conveyed is contacted intimately to a stage base table 7 by the weight W thereof and, during conveyance, the member 14 displaces while sliding on the surface of the stage base table 7. Where a reproducibility in respect to the intimate contact of the member 14 to the stage base table 7 is required at the time of stoppage, the pressing by an air-cylinder, the attraction by a pump means, the attraction by an electromagnet, etc., as in the foregoing examples may be used. If, in the Figure 11 example, the friction resistance or creation of foreign particles as a result of the sliding motion is quite undesirable, rolling means such as rollers may be used for the displacement to solve the problem. In an example shown in Figures 12A and 12B, steel balls 19 each being biased by a spring 18 are embedded in the bottom of a member 14 to be conveyed. Upon movement, the balls 19 roll along the surface of a stage base table 7 (Figure 12A). At the time of positioning, on the other hand, a suitable pressing force P is applied to the member 14 from an air-cylinder or other means, so that the springs 18 are compressed and a flat surface portion 10 of the bottom of the member 14 is contacted intimately to the stage base table 7, whereby a desired attitude precision is attainable (Figure 12B). While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and modifications and changes will be apparent to those skilled in the art.

Claims: Positioning apparatus for positioning an alignment unit (4), for detecting alignment information for use in aligning a mask (41) and a wafer (42), the apparatus comprising a movement means (34, 37) for moving the alignment unit (4), characterised by: a reference surface (11); and engaging means (5, 6) for moving the alignment unit (4) into engagement with the reference surface (11), so as to press the alignment unit (4) against the reference surface (11) and thereby align the alignment unit (4) in a desired orientation, following movement of the alignment unit (4) by the movement means (34, 37). Apparatus according to claim 1 in which the engaging means (5,6) retains the alignment unit (4) in engagement with the reference surface (11) without release of the alignment unit (4) from the movement means (34, 37). Apparatus according to claim 1 or claim 2 in which the alignment unit (4) is connected to the movement means (34, 37) by a flexible connector (2a, 2b). Apparatus according to claim 3 in which the flexible connector is deformable by movement of the alignment unit (4) into contact with the reference surface (11). Apparatus according to any one of the preceding claims comprising control means (28) arranged to control the movement means (34, 37) and engaging means (1, 2, 3, 6) so that the alignment unit (4) can be moved while being kept out of contact with the reference surface (11) and that, after the alignment unit has been moved to a desired site, the alignment unit can be contacted intimately to said reference surface. Apparatus according to any one of the preceding claims, wherein the movement means comprises a stage mechanism. Apparatus according to any one of the preceding claims in which the reference surface (11) is a surface of a stationary member (7). Apparatus for printing a pattern of a mask (41) on a wafer (42) comprising positioning apparatus according to any one of the preceding claims and means for using the alignment information to align the mask (41) and the wafer (42) before printing the pattern. An exposure apparatus for transferring a pattern of a mask (41) onto a wafer (42) placed on a wafer stage (44, 45), said apparatus comprising: an alignment unit (4) for detecting alignment information to be used for alignment of the mask (41) and the wafer (42); and positioning apparatus according to any one of claims 1 to 7 for positioning the alignment unit. A method of aligning a mask and a wafer and printing a mask pattern onto the wafer comprising: (a) positioning a first member (4), comprising an alignment unit for detecting alignment information about the alignment of a mask and a wafer, relative to a second member, comprising a mask (41) and a wafer (42), by a method in which the first member (4) is displaced to a desired position relative to a second member (41, 42) by a movement mechanism (34, 37) and during the displacement of the first member (4) it is kept out of contact with a reference surface (11) of a stationary member (7), and after the first member (4) has been displaced to the desired position it is subsequently moved into contact with the reference surface (11) thereby to align the first member in a desired orientation; (b) aligning the mask (41) with the wafer (42) on the basis of the alignment information; and (c) printing a mask pattern from the mask (41) onto the wafer (42) by an exposure process. A semiconductor microcircuit manufacturing method including a method of aligning a mask and a wafer and printing a mask pattern onto the wafer according to claim 10.

Inventors: EBINUMA RYUICHI, MIZUSAWA NOBUTOSHI, TANAKA YUTAKA, EBINUMA, RYUICHI, MIZUSAWA, NOBUTOSHI, TANAKA, YUTAKA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: H01L21
IPC codes: G12B5, H01L21, G03F7, B23Q1