Abstract:
Disclosed is a wafer chuck, which has protrusions for supporting a substrate, for attracting and holding the substrate by negative pressure while the substrate is being supported by the protrusions. The wafer chuck includes pin-shaped protrusions dispersed on a suction side of the chuck, and circular peripheral wall portions disposed in the vicinity of the rim of the supported substrate and in the vicinity of the outer peripheral portion of a lifting hole, respectively. The suction side of the wafer chuck is provided with a first area in which the pin-shaped protrusions are arrayed in a grid-line manner, and a second area in which the pin-shaped protrusions are arrayed in circumferential form. The second area is provided in the vicinity of the peripheral wall portion and peripheral wall portion, and the first area is provided elsewhere.

Description:
FIELD OF THE INVENTION 
   This invention relates to a substrate holding device for holding a substrate serving as a workpiece. More particularly, the invention relates to a substrate holding device used in a semiconductor manufacturing apparatus, a liquid-crystal substrate manufacturing apparatus, a magnetic-head manufacturing apparatus, a micromachine manufacturing apparatus, and the like. Also, the invention relates to an exposure apparatus and a device manufacturing method using such a substrate holding device. 
   BACKGROUND OF THE INVENTION 
   The growth of the sophisticated information-oriented society in recent years has been accompanied by rapid advances in the manufacture of finer and more highly integrated elements. Lenses of higher numerical apertures are being used in order to deal with the manufacture of finer elements in demagnifying projection exposure systems used in the manufacture of semiconductor devices. Though resolution rises owing to use of higher numerical apertures, however, effective focal depth decreases with an increase in numerical aperture and higher integration. In order to assure a sufficiently practical focal depth while maintaining resolving power, it is necessary to mitigate curvature of the image surface in the projection optical system, improve upon uniformity of substrate thickness and raise the precision of the chuck plane. 
   A method that minimizes the rate of contact between the underside of a wafer and the vacuum-retention side of a chuck is adopted conventionally as effective means for suppressing element defects ascribable to foreign matter. In particular, a pin-contact-type chuck that comes into point contact with the underside of the wafer is becoming increasingly in vogue. 
   The structure of an ordinary pin chuck is illustrated in  FIG. 2 . The ordinary pin chuck has an annular seal  14  on its outer circumference and a multiplicity of 0.2-mm circular or square pin-shaped contact portions (referred to also as “pin-shaped protrusions” below)  12  dispersed inwardly of the seal at a pin pitch of 2 mm. Further, in general, the circumferential seal  14  and a seal portion  13  of a hole  11  for a substrate lifting pin each define a continuous, embankment-like peripheral wall. When this pin chuck is used, three problems ascribable to vacuum-induced deformation arise. 
   The first problem is wafer flexure that occurs between the pin-shaped protrusions. In a case wherein a wafer is held by suction through pin-like contact, flexure occurs owing to the action of an external deforming force caused by the suction force of the vacuum with regard to the pin-to-pin intervals. For example, if pin spacing is 2 mm, flexure on the order of 5 nm occurs. This amount of flexure is not negligible in view of the specifications that will be demanded of exposure systems in the future owing to the use of higher numerical apertures and shorter wavelengths. 
   The second problem is doming deformation ascribable to wafer flexure that occurs at the portion of the lifting-pin hole  11 . As a result of such deformation, a large dome in excess of 100 nm is caused by wafer deformation between the peripheral wall portion of the seal for the lifting-pin hole and the adjoining pins, flexure of the pins themselves and digging of the pins into the wafer. 
   The third problem is lift-up, which occurs for reasons similar to those of the problems above, between the seal wall ( 14 ) at the outer circumference of the chuck and the adjacent pins. The circumferential portion undergoes a large amount of lift-up produced because wafer deformation acts on the free end. There are cases where this lift-up exceeds 300 nm. 
   In order to deal with the first problem, it has been attempted to array the chuck protrusions in the form of a grid or concentric circles and reduce the spacing between the pins. However, if the pins are brought closer together by reducing pin spacing in order to reduce flexure between the pins, the amount of flexure decreases but the rate of contact with the underside of the wafer increases, thereby elevating the probability that foreign matter will intrude. 
   This type of deformation between pins does not exhibit positional correlation with respect to the exposure viewing angle of the exposure apparatus. As a consequence, pin contact position differs for every exposure viewing angle and deformed shape ascribable to deformation between the pins is not reproduced from one exposure shot to the next. As a result, the amount of variation in focus increases for every exposure viewing angle. Usually, in an exposure apparatus that performs exposure using a square or rectangular angle of view, the above-mentioned variation is more pronounced with the concentric circular array than with the grid-type array. 
   With regard to the second and third problems, the prior art is such that a decline in planarity at the time of vacuum-induced suction becomes conspicuous at the periphery of the hole for the substrate lifting pin near the center of the chuck. For this reason, it has been proposed to provide the seal portion with a difference in level (e.g., see the specification of Japanese Patent Application Laid-Open No. 10-233433), and a chuck in which all contacting portions employ point contact also has been proposed (e.g., see the specification of Japanese Patent Application Laid-Open No. 8-195428). These chucks are such that the peripheral wall portion of the seal for assuring vacuum is formed to be one level lower than the tops of the pins, and a plurality of protuberances are provided on the peripheral wall portion. In each of these examples of the prior art, however, leakage at the peripheral wall portion is a problem and various difficulties arise, such as a decline in vacuum pressure and loss of a plane correcting force at the rim of a wafer that exhibits a large amount of curvature. In addition, in order to lower the peripheral wall portion with stabilized dimensions, highly precise partial machining is required. This results in higher manufacturing cost. 
   Accordingly, in view of the state of the prior art set forth above, there is a need for the provision of a chuck in which excellent planarity is obtained between pins, in the vicinity of the lifting pin and at the rim of the chuck. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided a substrate holding chuck, which has protrusions for supporting a substrate, for attracting and holding the substrate by negative pressure while the substrate is being supported by the protrusions, the protrusions including: a plurality of pin-shaped protrusions provided on a vacuum surface; and a circular first peripheral wall portion disposed in the vicinity of an outer circumferential portion of the suction surface; the chuck further having: a first area in which the pin-shaped protrusions are arrayed in circumferential form in the vicinity of the first peripheral wall portion; and a second area, disposed inwardly of the first area, in which the pin-shaped protrusions are arrayed in a grid-like manner. 
   Preferably, the pitch at which the pin-shaped protrusions are placed in the grid-like array is a fraction of the size of the exposure viewing angle of an exposure apparatus. Flexure caused by an external deforming force ascribable to the suction force of the pin-shaped protrusions can be provided with reproducibility on a per-exposure-shot basis, and focusing accuracy between shots can be stabilized. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a diagram illustrating the external appearance of a wafer chuck according to an embodiment of the present invention; 
       FIG. 2  is a diagram illustrating the external appearance of an ordinary wafer chuck; 
       FIG. 3  is a sectional view illustrating the wafer chuck according to this embodiment; 
       FIG. 4  is a diagram showing the general structure of an exposure apparatus according to this embodiment; 
       FIG. 5  is a flowchart showing the manufacture of a microdevice; and 
       FIG. 6  is a flowchart showing the details of a wafer process. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. 
   Embodiment of an Exposure Apparatus 
   An embodiment of the invention will now be described in concrete terms using an example in which a substrate holding device according to the present invention is applied to a demagnifying projection exposure apparatus. 
     FIG. 4  is an overall schematic view of an exposure apparatus. As shown in  FIG. 4 , the exposure apparatus is such that a reticle  2 , which is an exposure master, is placed on a reticle stage  4  via a reticle chuck  3 . The reticle  2  is irradiated with exposing light guided to it from a light source (not shown) via an illuminating optical system  1 . The exposing light that has passed through the reticle  2  is demagnified to, e.g., one-fifth the size by a projection optical system  5  and illuminates a silicon wafer  8 , which is the workpiece. A so-called wafer chuck  9 , namely a substrate holding device serving as means for holding the silicon wafer  8 , is mounted on an XY stage  10  that is capable of moving the wafer in a horizontal plane. 
   An exposure sequence in the exposure apparatus constructed as set forth above will now be described. 
   Once the silicon wafer  8  to be exposed has been set in the exposure apparatus automatically or manually by an operator, operation of the exposure apparatus starts in response to an exposure-start command. A first wafer  8  is fed into the wafer chuck  9 , which has been mounted on the stage  10 , by a conveyance system. Next, alignment marks inscribed on the wafer  8  are detected at a plurality of locations by an off-axis scope  7 , wafer magnification, rotation and amount of XY shift are determined and position is corrected. The stage  10  moves the wafer in such a manner that a first shot position of the mounted wafer will agree with the exposure position of the exposure apparatus. After focusing is achieved by surface measurement means  6 , exposure is carried out for about 0.2 s, the wafer is stepped to a second shot position on the wafer and exposure is repeated in succession. Processing for exposure of one wafer is completed by repeating a similar sequence up to the final shot. The wafer delivered from the wafer chuck to a recovery transport hand is returned to a wafer carrier. 
   Embodiment of a Wafer Chuck  
     FIGS. 1 and 3  illustrate the general features of the wafer chuck  9  according to this embodiment. The wafer chuck  9  comprises a sintered SiC ceramic that excels in thermal conductivity. The top side of the wafer chuck  9  on which a wafer is placed has the pin-shaped protrusions  12 , which are formed by etching, and embankment-like peripheral wall portions  13 ,  14 . The underside of the chuck is formed to have one or a plurality of vacuum suction holes  17  that pass through to the top side and communicate with a vacuum source. When a wafer is mounted on the chuck  9  and operated on, it is required that a lifting pin  15 , which is moved up and down in order to lift the wafer up from the chuck  9  temporarily, be caused to penetrate the chuck  9  at a point midway along its radius. For this reason, the chuck  9  has the through-hole (lifting-pin hole)  11  the diameter of which is greater than that of the lifting pin  15 . The peripheral wall portion  13 , which is formed to have a width that is substantially equal to the diameter of the pin-shaped protrusions, is provided surrounding the lifting-pin hole  11 . Similarly, the peripheral wall portion  14 , which has a diameter slightly smaller than the outer diameter of the wafer, is provided on the circumferential portion of the chuck  9 . 
   These peripheral wall portions should have a height the same as that of the pin-shaped protrusions  12 . Of course, the plane correcting ability will not decline even if the height of the peripheral wall portions is made slightly less, as in the prior art. It should be noted that an exhaust hole  17  is provided to attract the wafer by vacuum suction, as well as an exhaust pump  16  connected to the exhaust hole  17 . 
   Next, an area in which the pin-shaped protrusions  12  are arrayed in grid-like form will be described. 
   In the exposure apparatus according to this embodiment, a single exposure area is assumed to be 22×22 mm owing to lens limitations. The pin-shaped protrusions  12  are arranged in a grid array at a spacing that is a value obtained by dividing the exposure viewing angle by an integer ( 1/10 of the viewing angle, or 2.2 mm in this embodiment) in such a manner that even when the wafer is stepped to the neighboring shot and exposed, the relative positions of the pin-shaped protrusions  12  will coincide as seen from the lens. Thus, the shape of deformation caused by sandwiching the wafer between the pin-shaped protrusions is reproduced at every exposure shot and defocusing accuracy between shots becomes more stable. If the vacuum pressure is reduced further, a pitch larger than 2.2 mm can be selected and the contact rate can be reduced. In this embodiment, the pin-shaped protrusions  12  are arranged in a simple orthogonal grid-like array. However, if selection of a correlated arrangement with respect to the exposure viewing angle is taken into consideration, a staggered-type grid array may be adopted without departing from the gist of the present invention. 
   Next, the arrangement of the pin-shaped protrusions in the vicinity of the peripheral wall portion  14  at the rim of the chuck  9  will be described. 
   If the grid-like array were adopted even in the vicinity of the rim of the chuck  9 , portions with many of the pin-shaped protrusions and portions with few of the pin-shaped protrusions would occur adjacent to the peripheral wall portion  14 . As a result, this would foster vacuum deformation of the wafer and bring about the lift-up phenomenon. Accordingly, the pin-shaped protrusions are arranged in a plurality of concentric circles in the vicinity of the peripheral wall portion  14 . According to this embodiment, two concentric circles of the pin-shaped protrusions are provided. Further, a transition area is provided inwardly of the innermost circle of pin-shaped protrusions to achieve a smooth transition from the grid area to the area of concentric circles. 
   Preferably, the arrangement of the concentric circles, the arrangement of the pin-shaped protrusions on the concentric circles, the arrangement of the transition area and the arrangement of the pin-shaped protrusions inside the transition area are decided based upon a pitch P of the pin-shaped protrusions arrayed in the grid pattern on the inner side. The inventors have discovered that if the pins are arranged in this manner, a major improvement in flatness can be achieved. 
   First, it is preferred that the position of a first circle  21  nearest to the peripheral wall portion  14  be selected within a range that satisfies the relation 0.2P≦A≦1.2P, where A represents distance from the peripheral wall portion  14 . In this embodiment, 2.2 mm, which is equivalent to 1.0×P, is adopted. Accordingly, spacing D of the pin-shaped protrusions disposed on the first circle  21  is made 2.2 mm, which is the same as P. It should be noted that D preferably falls within the range 0.8P≦D≦1.2P. 
   Next, a second circle  22  is placed at a position located a distance B inwardly of the first circle  21 . Here B is selected within a range that satisfies the relation 0.8P≦B≦1.2P. In this embodiment, 2.2 mm (=1.0P) is adopted. Further, it is preferred that a spacing D′ of pin-shaped protrusions disposed on the second circle  22  be selected within a range that satisfies the relation 0.8P≦D′≦1.2P. In this embodiment, D′=2.2 mm is adopted. 
   Furthermore, a third circle  23  is placed at a position located a distance P inwardly of the second circle  22 . An area  24  between the second circle  22  and third circle  23  is adopted as the transition area. The pin-shaped protrusions  12  are formed into a grid array of pitch 2.2. mm on the inner side of the third circle  23 . In the transition area  24 , the pin-shaped protrusions  12  are arranged in accordance with an array rule described below. 
   First, the area S of the transition area  24  is found and a value (S/P 2 ), which is the result of dividing the area S by the grid area P 2  of grid pitch P, is adopted as the optimum number of pins in the transition area  24 . In this embodiment, an integral value close to S/P 2  is adopted as the number of pins and this is the number of pin-shaped protrusions provided. It is preferred that the pin-shaped protrusions  12  in the transition area  24  be arranged in such a manner that a distance E between mutually adjacent pin-shaped protrusions satisfies the relation 0.7P≦E≦1.2P. 
   A rule similar to the above-described pin arrangement rule regarding the peripheral wall portion  14  at the circumference of the chuck can be applied also to the vicinity of the lifting-pin hole  11  provided at the center of the chuck. 
   First, a first circle  25  (a circle that is concentric with respect to the peripheral wall portion  13 ) nearest to the peripheral wall portion  13  encircling the lifting-pin hole  11  is provided. If a represents the distance from the peripheral wall portion  13 , the position of the lifting pin  15  preferably is selected within a range that satisfies the relation 0.3P≦a≦0.6P. In this embodiment, 1.1 mm, which is equivalent to 0.5P, is adopted. Spacing d of the pin-shaped protrusions disposed on the first circle  25  preferably is selected within a range that satisfies the relation 0.8P≦d≦1.2P. In this embodiment, it is assumed that the spacing d is 2.2 mm, which is the same as P. 
   Next, a second circle  26  is placed at a position located a distance b outwardly of the first circle  25 . Here b is selected within a range that satisfies the relation 0.8P≦b≦1.2P. In this embodiment, 2.2 mm is adopted. 
   Furthermore, a third circle  27  is placed at a position located a distance P outwardly of the second circle  26 . An area  28  between the second circle  26  and third circle  27  is adopted as a transition area  28 . 
   The pin-shaped protrusions in the transition area  28  are arranged as follows: The area s of the transition area  28  is found and a value (s/P 2 ), which is the result of dividing the area s by the grid area P 2  of grid pitch P, is adopted as the optimum number of pins in the transition area  28 . Accordingly, an integral number of the pin-shaped protrusions  12  close to this value is placed inside the transition area  28 . Preferably, the placement of the pin-shaped protrusions  12  in the transition area  28  is such that the distance e mutually adjacent pin-shaped protrusions will satisfy the relation 0.7P≦E≦1.2P. 
   By placing the pin-shaped protrusions based upon the rule described above, wafer flexure between pins when the wafer is attracted by suction can be made uniform, the supporting force offered by the individual pins can be made approximately the same value and it is possible to make uniform, over the entirety of the chuck, the amount of flexure of the pins themselves and the amount by which the pins dig into the underside of the wafer. 
   It should be noted that the present invention is not limited to the chuck of an exposure apparatus and is applicable also to an apparatus that applies and develops a resist. In particular, if the present invention is applied under identical conditions to a chuck, referred to as a “spin chuck”, of the spinning portion of a wafer, a high degree of flatness can be obtained. 
   &lt;Device Production Method&gt; 
   Described next will be an embodiment of a method of producing a device utilizing the exposure apparatus or exposure method set forth above. 
     FIG. 5  is a flowchart illustrating the manufacture of a microdevice (a semiconductor chip such as an IC or LSI chip, a liquid crystal panel, a CCD, a thin-film magnetic head, a micromachine, etc.). 
   The pattern for the device is designed at step  1  (circuit design). A mask on which the designed circuit pattern has been formed is fabricated at step  2  (mask fabrication). Meanwhile, a wafer is manufactured using a material such as silicon or glass at step  3  (wafer manufacture). The actual circuit is formed on the wafer by lithography, using the reticle and substrate that have been prepared, at step  4  (wafer process), which is also referred to as “pre-treatment”. A semiconductor chip is obtained, using the wafer fabricated at step  4 , at step  5  (assembly), which is also referred to as “post-treatment”. This step includes steps such as actual assembly (dicing and bonding) and packaging (chip encapsulation). The semiconductor device fabricated at step  5  is subjected to inspections such as an operation verification test and a durability test at step  6  (inspection). The semiconductor device is completed through these steps and then is shipped (step  7 ). 
     FIG. 6  is a flowchart illustrating the detailed flow of the wafer process mentioned above. The surface of the wafer is oxidized at step  11  (oxidation). An insulating film is formed on the wafer surface at step  12  (CVD), electrodes are formed on the wafer by vapor deposition at step  13  (electrode formation), and ions are implanted in the wafer at step  14  (ion implantation). The wafer is coated with a photoresist at step  15  (resist treatment), the wafer is exposed to the circuit pattern of the mask to print the pattern onto the wafer by the above-described exposure apparatus at step  16  (exposure), and the exposed wafer is developed at step  17  (development). Portions other than the developed photoresist are etched away at step  18  (etching), and unnecessary resist left after etching is performed is removed at step  19  (resist removal). Multiple circuit patterns are formed on the wafer by implementing these steps repeatedly. 
   If the manufacturing method of this embodiment is used, it will be possible to stably produce semiconductor devices having a high degree of integration. Such devices have been difficult to manufacture heretofore. 
   Thus, in accordance with the present invention as described above, highly precise flatness is obtained over the entire suction area of a chuck, and excellent vacuum-induced suction is produced even with regard to wafers exhibiting curvature. 
   As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.