HIGH SPEED RETICLE CHANGE SYSTEM

Abstract: Method and apparatus are disclosed for speeding up reticle changes during the production of semiconductor wafers under those circumstances weherein three reticles are employed, one (C) having a substantially longer exposure period than the others (A, B). The wafer is exposed to the reticles in the sequence ACB-BCA-ACB, etc...

Description: Technical Field This invention relates to the production of semiconductor wafers. More particularly, it relates to method and apparatus for rapidly changing the reticles during wafer production according to the generic clause of claims 1 and 2, respectively. Generally several reticles are required to expose a wafer in the semiconductor integrated circuit manufacturing process. Systems for handling and exchanging the reticles and conveying them to a exposing station are known, for example from Japanese patent application JP-59113624. Depending on the system of illumination used for exposing the wafers the respective masks may require different exposure times. This is the case for a system utilising an illuminated slit for exposing the wafers which this invention is referring to. Fast processing of the wafers at the exposure station, which includes exchange of reticles, is of critical importance for cost-efficient manufacturing. It is an object of the invention to provide a method and apparatus for fast reticle exchange. The problem is solved according to this invention with a method according to claim 1 and an apparatus according to claim 2. Reticles are rapidly transferred between a reticle library and a reticle transport stage by a delivery arm. The reticles comprise a circuit reticle, C, which takes relatively long to expose and A and B reticles having relatively short exposure times. A holding position is established closely adjacent the transport stage. After the circuit reticle is exposed, it is removed to this holding location. A "park" position for the delivery arm is established between the reticle library and the transport stage and relatively closer to the transport stage. While the circuit reticle is in its holding position, reticles A and B traverse a path from the transport stage to the library, from the library to the park position, and from the park position to the transport stage. The exposure cycle for successive wafers is in the order BCA-ACB-BCA-ACB. Brief Description of Drawings FIG. 1 is a schematic diagram of the reticle flow; FIG. 2 is a plan view of apparatus usable in the present invention; FIG. 3 is a front elevational view of the apparatus of FIG. 2; FIGS. 4 and 5 form a diagram illustrating successive steps in the practice of the invention; and FIG. 6 illustrates the relationship between FIGS. 4 and 5. Best Mode for Carrying out the Invention Semiconductor chips are formed by coating a semiconductor wafer disc with a photoresist and exposing it to a pattern carried by a glass "reticle". The process is repeated as many times as required. During the normal production of semiconductor wafers, only three reticles are required to expose the wafer. These reticles are referred to herein as reticles A, B, and C. Reticle A would normally consist of test patterns, reticle B might be clear in order to wipe out unexposed photoresist, and reticle C would contain the circuit to be applied to the wafer. The apparatus to which this invention relates utilizes an illuminated slit for exposing each wafer. Accordingly, a substantially longer period of time is required to expose reticle C than either of reticles A or B. In order to maximize wafer production, it is important that reticle changes be made as rapidly as possible. The reticle changes are made to and from a transport stage which grips them securely by means of a vacuum chuck and transports them to an exposure station which forms no part of this invention. FIG. 1 illustrates schematically a transport stage T at a loading location, a holding location H located closely adjacent thereto, a reticle library L for storing the reticles between exposures, and a park position P for temporarily supporting a reticle closely adjacent the transport stage prior to its being loaded thereon. The arrows indicate the travel of reticles in the load-unload cycle. FIGS. 2 and 3 illustrate apparatus for carrying out the invention. It comprises a reticle library 10 carrying reticles 12 supported therein on library pins 14. The reticle library 10 sits on an air bearing carriage 16 which is supported by air bearings 18 on bearing surfaces 20 and driven by a linear motor 22. Thus, the library 10 moves along the Z axis, the X, Y, and Z axes being indicated by arrows in FIGS. 2 and 3. Lifters 24 are positioned beneath the library and function to lift a selected one of the reticles 12 up and off of its library pins 14. Displaced from the library 10 is a reticle transport stage 26 in the form of a carriage 28 supported by three air bearings 30 on a bearing surface 32. Extending vertically upward from the carriage 28 is a vacuum chuck 34 having reticle support pins 36 for supporting a reticle against the vacuum chuck 34. Positioned below the edge of a reticle so supported are a pair of lifters 38. Positioned closely adjacent and parallel to the vacuum chuck 34 is a wafer load/unload holding arm 40. The holding arm 40 carries pliant rubber vacuum cups 56 of known type for grasping a wafer. The arm 40 is mounted upon a flexure assembly 42 which permits limited movement along the Z axis. This movement is under the control of a conventional air pot which is not shown. Somewhat further motion is provided by mounting the arm 40 and the flexure assembly 42 on an air bearing 44. Extending between the library 10 and the transport stage 26 is an air bar 46 supporting an air bearing 48 movable therealong by a linear motor 50. A flexure assembly 52 is carried by the air bearing 48 and from it depends a wafer delivery arm 54 which is also provided with pliant vacuum cups 56. Cups of this type are described in U. S. Patent 4,530,635 of Engelbrecht and Laganza. The function of the flexure assembly 52 is to permit limited movement of the load/unload arm 54 in the Z direction for the purpose of acquiring and depositing reticles 12. It will be understood that the reticles 12 which are likely to be employed in wafer production, are positioned on the pins 14 in the reticle library 10. The library is movable along the Z axis to position a reticle such that the load arm 54 can enter the library without hitting the reticle or the library 10. The airbearing 48 is then moved along the X axis to position the delivery arm 54 adjacent the selected reticle in the library. The library then makes another move along the Z axis to put the arm 54 in the "slot" which holds the library. The lifters 24 raise the selected reticle up off the pins 14 and the delivery arm 54 is translated toward it along the Z axis by means of the flexure assembly 52. The vacuum cups 56 carried by the arm 54 are forced up against the selected reticle, vacuum switched on, and arm 54 thereby acquires the reticle. The lifters 24 thereupon are lowered and the arm 54 retracts along the Z axis. The reticle is now no longer touching the library structure and is free to be moved out of the library. Delivery arm 54 is then moved along the X axis to the transport stage 26 which is in the load/unload position to receive the reticle. The air driven flexure assembly 52 translates the arm 54 along the Z axis until the reticle contacts the vacuum chuck 34. The vacuum chuck 34 includes both vacuum and pressurized air ports which hold the reticle closely adjacent the chuck 34 while permitting it to move substantially without friction along the face of the chuck. The lifters 38 are raised into contact with the reticle, the vacuum to the delivery arm 54 is switched off, and the arm is retracted. The lifters 38 lower the reticle onto the pins 36 and the vacuum chuck 34 is changed to vacuum alone, thereby causing the reticle to firmly adhere to the chuck. The next step is to expose a wafer at an exposure station with the reticle. The transfer stage 26 moves away from the load/unload position to the exposure station, which is not illustrated herein. When the exposure is complete, the transfer stage 26 returns to the load/unload position. The selected reticle may be returned to the reticle library 10 by reversing the foregoing procedure. Alternatively, it may be loaded onto the holding arm 40. If this is to be done, the delivery arm 54 must be moved along the X axis toward the library to a position such that it will not interfere with holding arm 40. Holding arm 40 is then translated along the Z axis on the air bearing 44 until it is relatively close to the reticle 12. It is is then advanced further by the air actuated flexure assembly 42 and air pressure is restored to the vacuum chuck 34, thereby permitting the reticle 12 to move along its surface in substantially friction-free fashion. The lifters 38 push the reticle up into its load/unload position and the arm 40 is further advanced,permitting the vacuum cups 56 to contact the reticle. Vacuum is then switched on, the lifters 38 are retracted, and the vacuum maintaining the reticle in contact with the vacuum chuck 34 is switched off. The holding arm 40 is then retracted along the Z axis by the flexure assembly 42 and the holding arm is then translated further along the Z axis on the air bearing 44 to a holding position which permits a new reticle to be loaded by the delivery arm 54. As explained above, during normal production of wafers, only three reticles are required to expose the wafer. These are referred to herein as the reticles A and B, both of which have short exposure times, and the circuit reticle C which has a substantially longer exposure time. It is essential that reticles be interchanged as quickly as possible in order to maximize wafer production. The manner in which this is accomplished will be described with reference to the sequence illustrated beginning at the top of FIG. 4. Initially, circuit reticle C is on the transport stage 26 which, in return, is at the exposure station where the wafer is used to expose wafer I. Reticle A is on the delivery arm 54 which is in the park position. The holding arm 40 is empty. Reticle B has already been used to expose wafer I and is currently stored in the reticle library. Given these initial conditions, the following steps occur: Step 1. The transport stage 26 exposes the wafer I with the circuit reticle C and returns to the load/unload position as illustrated. Step 2. The holding arm 40 translates along the Z axis and acquires the circuit reticle C. Step 3. The holding arm 40 retracts to the holding position along the Z axis. Step 4. Delivery arm 54 translates along the X axis to the load/unload position with reticle A. Step 5. The delivery arm 54 translates along the Z axis and loads reticle A onto transport stage 26. Step 6. The delivery arm 54 retracts along the Z axis until it just clears reticle A. In actual embodiment of this invention, the total time to exchange reticles by completing steps 1-6 was no more than 3 seconds. Step 7. The transport stage 26 moves away to expose reticle A. Step 8. The exposure of wafer I with reticle A, reticle B, and reticle C has been completed and wafer I is replaced by wafer II. Step 9. Transport stage 26 exposes wafer II with reticle A and returns to the load/unload position. Step 10. The delivery arm 54 translates along the Z axis and acquires reticle A. Step 11. The delivery arm 54 retracts along the Z axis with reticle A. Step 12. The delivery arm 54 with reticle A translates along the X axis to the park position. Step 13. The holding arm 40 loads reticle C onto transport stage 26. Step 14. The holding arm 40 retracts along the Z axis. The total time required to exchange reticles by completing steps 9-14 is no greater than 3 seconds. Step 15. The transport stage 26 moves away to expose the wafer II with reticle C. Step 16. While the transport stage 26 is exposing wafer II with reticle C, the delivery arm 54 has translated along the X axis, exchanged reticle A for reticle B at the library 10, and has now translated back along the X axis from the library to the park position. The time required to complete step 16 is no greater than 20 seconds. Since, during this period, the wafer II is being exposed by reticle C, this trip to and from the library does not add to the cycle time. Step 17. The transport stage 26 exposes wafer II with the reticle C and returns to the load/unload position. Transport stage 26 prepares to unload reticle C as previously explained. Step 18. The holding arm 40 translates along the Z axis and acquires reticle C. Step 19. The holding arm 40 retracts along the Z axis to its holding position. Step 20. The delivery arm 54 moves along the X axis from its park position to the load/unload position with reticle B. Step 21. The delivery arm 54 translates along the Z axis and loads reticle B onto transport stage 26. Step 22. The delivery arm 54 retracts along the Z axis until it just clears reticle B. The total time required to exchange reticles by completing step 17-22 is no more than 3 seconds. Step 23. Transport stage 26 moves away to begin the exposure of wafer II with reticle B. Step 24. The exposure of wafer II with reticle B is completed. Wafer II has now been exposed by reticle A, reticle B, and reticle C and the wafer is replaced by wafer III. Step 25. The cycle now returns to step 9. Transport stage 26 exposes wafer III with reticle B and returns to the load/unload position. The cycle continues, exposing as many wafers as are required to the reticles A, B, and C. It will be noted that the exposure cycle is in the order BCA-ACB-BCA-ACB, i.e., (a) the reticle having the longest exposure time is preceded and followed by the reticles having the shortest exposure time, and (b) the reticle last employed to expose a wafer is the first reticle employed to expose the succeeding wafer. By means of this technique, the time required to change reticles is reduced to a minimum. It is believed that the many advantages of this invention will be apparent to those skilled in the art. It will also be apparent that a number of variations and modifications may be made in this invention without departing from its scope. Accordingly, the foregoing description is to be construed as illustrative only, rather than limiting. This invention is limited only by the scope of the following claims.

Claims: Method of sequentially loading three reticles (A,B,C) onto a transport stage (26) at a loading location for subsequently exposing each of a plurality of semiconductor wafers to each of said reticles, wherein one of said reticles is a circuit reticle (C) having a substantially longer exposure time than the remaining two reticles and wherein said remaining two reticles (A,B) are normally stored in a reticle library (10) relatively far removed from the loading location, characterised in that reticle C is positioned in a holding location closely adjacent the loading location; reticle A is positioned on the transport stage (26); a park position intermediate the library (10) and the loading location is established; a wafer is exposed with reticle A; reticle A is removed from the transport stage (26); reticle C is positioned on the transport stage (26); said wafer is exposed with reticle C; during the reticle C exposure period, reticle A is moved to the library (10) and reticle B is removed from the library (10) to the park position; reticle C is moved to its holding location; reticle B is positioned on the transport stage (26); said wafer is exposed with reticle B; the exposed wafer is replaced with an unexposed wafer and the exposure cycle is continued in the order BCA-ACB-BCA-ACB. Apparatus for serially exposing each of a plurality of semiconductor wafers to at least three reticles (A,B,C), a first of said reticles (C) having a substantially longer exposure period than the second (A) and third (B) of said reticles, wherein said reticles are moved from a reticle library (10) onto a transport stage (26) at a loading location for subsequent exposure, characterised by means (54) for positioning one of said second and third reticles on said transport stage (26) at said loading location and for positioning the other of said second and third reticles in said library (10); means (54, 26,40) for alternatively positioning said first reticle either onto said transport stage (26) at said loading location or at a holding location closely adjacent the loading location; means (54) for delivering a reticle from the transport stage (26) at said loading location to the library (10) during the period said first reticle is on the transport stage (26); means (54) for removing said other reticle from the library (10) to a parking location intermediate the library (10) and the loading location during the period said first reticle is on the transport stage (26); and means (54) for transferring a reticle from the parking location onto the transport stage (26) at the loading location while said first reticle is in the holding location.

Inventors: O'CONNOR, GEOFFREY
Applicants: SVG LITHOGRAPHY SYSTEMS, INC.
Application year: 1987
Main classification: G03B41
IPC codes: G03F7, H01L21