Abstract:
An apparatus for processing semiconductor wafers includes a single imaging stepper for exposing wafers processed on a first track and a second track. A method for processing semiconductor wafers includes selecting one of a first coater and a second coater for coating a first wafer with a photoresist. The coated wafer is exposed in a single stepper to form an exposed wafer. An operator selects one of a first developer and second developer to develop the exposed wafer.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method and apparatus for manufacturing a semiconductor integrated circuit (“IC”). More specifically, this invention relates to an improved method and apparatus for processing semiconductor wafers with a dual track and stepper. 
     BACKGROUND OF THE INVENTION 
     The present invention applies particularly to the fabrication of semiconductor integrated circuits. Some examples of these semiconductor integrated circuits comprise non-volatile memory integrated circuits. Non-volatile memory integrated circuits include an EPROM, an EEPROM, a flash memory device, and a complementary metal oxide silicon (“CMOS”) type device. Exemplary devices may comprise field-effect transistors (“FET”) containing a gate over gate insulator over silicon (“MOSFET”), as well as other ultra-large-scale integrated-circuit (“ULSI”) systems. 
     Integrated circuits are utilized in a wide variety of commercial and military electronic devices, including, e.g., hand held telephones, radios and digital cameras. The market for these electronic devices continues to demand devices with a lower voltage, lower power consumption and a decreased chip size. Also, the demand for greater functionality is driving the “design rule” lower, for example, into the sub-half micron range. 
     These integrated circuit devices are generally fabricated in groups on a semiconductor wafer. During fabrication, a photolithographic process is utilized to form various components and structures. The components and structures are formed according to a photolithographic pattern provided on the semiconductor wafer. This photolithographic process is conventionally utilized in the front end portion of a semiconductor wafer production. There are three basic steps involved in the photolithography processing of each semiconductor wafer. First, a photoresist is applied to each wafer in a coater. Each wafer is then exposed to a radiation source in a stepper, and finally each exposed wafer is developed in a photoresist developer. Since the IC are typically multilayered, this process is repeated a number of times. 
     More specifically, in a portion of the photolithography of these wafers, a photoresist coater and developer system is utilized in the patterning of various layers of the wafer that will form the circuit device. The photoresist coater and developer system applies, or coats, a light-sensitive resin, i.e., a photoresist layer, to wafers by depositing a pre-selected amount of the photoresist solution. Next, the system spins the wafers at a relatively high rate of speed to distribute the photoresist into a relatively even coating over the wafer. Then, the wafers are baked to induce a volatization of a casting solvent in the photoresist. Next, the wafers are exposed to a light source in a stepper, e.g., a deep ultraviolet (“DUV”) light source, for patterning. The exposed wafers are then developed by a chemical treatment, and are again baked to dry the wafers. 
     Conventional examples of resist coater and developer systems, e.g., are the Tokyo Electron Limited (TEL) sub-half micron compatible Coater/Developer Clean Track systems. Conventional systems may include a chemically amplified resist (“CAR”) in the deep ultraviolet (“DUV”) process that has been adopted for the sub-half micron design rule type of circuit devices. The combination of the coater and developer is typically referred to as a “track.” 
     As to the development of the photoresist that has been formed on the wafer, conventionally, a chemical developer is utilized to remove areas defined in the steps of masking and exposure of the photoresist layer that has been deposited on the wafer. The development of the photoresist is an important part of the wafer fabrication. 
     For example, in sub-half micron semiconductor processing, one of the most important parameters in the photolithography area is the critical dimension (“CD”). The above described relatively complex integrated circuits will only function as designed if the critical dimensions are within specification. There are many parameters that control the critical dimension. One of these parameters comprises the time interval between the application of the photoresist in the coater and the exposure of the wafer in the stepper. 
     Wafers may be processed in either a batch or interface mode. In the batch mode a number (25 for example) wafers may be processed together such that a photoresist is applied to each of the wafers individually. Depending on the process, the time between the application of the photoresist to the first wafer to be processed and the last wafer may be 45 minutes. Once the photoresist has been applied to all of the wafers, each wafer is then individually exposed in the stepper. After all of the wafers have been exposed, then each wafer is developed in the developer. 
     In the interface mode, each wafer is coated with a photoresist and then sent to the stepper to be exposed. Since the time interval required to expose each wafer in the stepper is less than the time required to apply the photoresist in the coater, no queue develops between the coater and the stepper. However, the time required to develop each wafer is longer than the time required to expose the wafer in the stepper. As a result a queue develops between the stepper and the developer. 
     As discussed above, IC&#39;s are often multi-layered. Accordingly, once the lot of wafers has been processed, the process of photoresist application, exposure, and developing is repeated on the lot until the IC&#39;s are completed. The type of photoresist that is applied for each layer of the IC may be different than the previous photoresists that have been applied. Additionally, a different type of photoresist may require a different developer material. For example, a track may include the ability to apply three different photoresists in a single coater, and two different developer materials in the developer. The number of photoresists and developer materials available is a function of the number of nozzles designed into the system. Additionally, a different image is used to expose the wafers for each layer of the IC. The image exposed on the wafer is formed on a reticle used in the imaging process. 
     Some different photoresists react with one another and coagulate. This can result in clogging the drainage of the coater station. As a result, either the drainage system must be cleaned periodically to prohibit the system from backing up, or the wafers must be processed on a separate track and stepper line. 
     A typical setup includes a track having a coater and a developer that is connected to a single stepper. The various components communicate via software that coordinates the processing of the wafers in the coater, stepper and developer. A conventional wafer transfer system includes a plurality of cassettes used to store the wafers both before and after they are processed in the coater, stepper or developer. In the batch mode, a group of wafers is stored in a cassette. Automated equipment moves each wafer from the cassette for processing in the given station and then replaced to the cassette when complete. The entire cassette may then be moved manually by an operator to the next station for processing. The entire system may be automated utilizing lateral feeders and conveyor belts, thereby permitting the wafers to be processed in the interface mode. 
     Alternatively, as described in U.S. Pat. No. 5,455,894, assigned to a common assignee, the wafer transfer system may include a computer controlled robotic interface unit having a robotic arm that moves the wafers without the need for conveyor belts. The &#39;894 patent is included herein by reference. 
     The cost of the stepper may be six time the cost of the coater and developer track unit. Since the stepper processes the wafers more quickly than the coater and developer, the stepper is often idle. Additionally, maintenance is required on the coater and developer more often than the stepper. Since the stepper is dedicated to a specific track, the stepper will be idled during the periodic maintenance for the track. 
     Thus, a problem exists in that the most expensive portion of the track and stepper system is idle for a period of time, both during the fabrication of the wafers and during periodic maintenance on the coater and developer units. Additionally, the number of photoresists and developers available are limited due both to the limited number of photoresist nozzles and the coagulation problem. Accordingly, more than one track and stepper line may be required for the processing of the wafers. 
     Accordingly, it would be desirable to develop a track and stepper system that would permit the timely processing of the wafers, that minimizes the downtime of the stepper, and that provides for a greater number of possible photoresist and developer combinations. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention relates to an apparatus for processing semiconductor wafers. The apparatus includes a first track having a first photoresist coater to apply a photoresist a first wafer. The first track also includes a first developer. A second track includes a second photoresist coater to apply a photoresist to a second wafer. The second track includes a second developer. A stepper integrated with the first and second tracks exposes the first and second wafers. 
     Another embodiment relates to a method for processing semiconductor wafers including coating a first set of wafers with a photoresist material in a first coater station. The first set of wafers is exposed to a light source in a stepper. The method also includes developing the first set of wafers with a developer in a first developer station. A second set of wafers is coated with a second photoresist material in a second coater station. The second set of wafers is exposed to the light source in the stepper. The method also includes developing the second set of wafers with the developer in the second developer station. 
     Still a further embodiment includes a method for processing semiconductor wafers comprising first selecting one of a first coater and a second coater. A first wafer is coated with a photoresist from the one of the selected first and second coaters to form a coated wafer. The coated wafer is exposed with an image in a single stepper to form an exposed wafer. One of a first developer and second developer is selected, and the exposed wafer is developed in the selected developer. 
     These and other benefits and features of the present invention will be apparent upon consideration of the following detailed description of preferred embodiments thereof, presented in connection with the following drawings in which like reference numerals identify like elements throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of the prior art apparatus for coating, exposing, and developing wafers with a single track and stepper; and 
     FIG. 2 is a schematic illustration of the apparatus for coating, exposing, and developing wafers in a dual track and single stepper arrangement. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A schematic illustration of an exemplary prior art system  10  for imaging a semiconductor wafer is shown in FIG.  1 . The prior art system includes a single track  12  including a coater station  14  for applying a photoresist material to the wafer, and a developer station  16  for developing the wafer after it has been imaged in a stepper  1   8 . Typically, the wafers are processed in groups or lots. One lot size could include 25 wafers. In a batch mode, each wafer in the lot is coated individually with the photoresist and stored in an input/output port  20  until all of the wafers in the lot have been covered with the photoresist. Once the wafers have been covered with the photoresist they are moved as a lot to input/output port  22  of the stepper  18 . Each coated wafer is then individually exposed with an image formed on a reticle. After all of the wafers have been exposed in the stepper  18 , each of the exposed wafers are moved to the input/output port  24  to be individually developed in the developer  16 . Each input/output port  20 ,  22  and  24  can include a cassette for receiving each of the wafers. 
     The wafers are moved from station to station either by manually moving the cassette, or the wafers can be moved from station to station with an automated transfer system. 
     In an attempt to control the critical dimension of the sub micron circuit, the wafers are processed in an interface mode to minimize the time the photoresist sits on the wafers prior to exposure in the stepper  18  and subsequent developing in the developer  16 . 
     In the interface mode, once each wafer has been coated in the coater  14  it is immediately forwarded to the stepper  18  to be exposed. Once each wafer has been exposed they are stored in the input/output port  24  and subsequently developed in a developer station  16 . 
     In the prior art system shown in FIG. 1, a single stepper is interfaced physically with a single track including one coater and one developer. The stepper and track are controlled via a computer interface. Where a second set of wafers to be processed requires a different photoresist, or reticle, or developer, the prior art system does not permit the second set of wafers to be coated until all of the first set of wafers have been coated, stepped and developed. 
     An exemplary embodiment of the present invention is schematically illustrated in FIG.  2 . The apparatus  30  includes a first track  32 , a second track  34 , and a single stepper  36  interfaced with both the first and second tracks  32 ,  34 . The first track  32  includes a first coater  38  for applying a photoresist to the wafers and a first developer  40  for developing the wafers. The second track  34  includes a second coater  42  for applying a photoresist to wafers and a second developer  44  for developing the wafers once they have been imaged in the stepper  36 . 
     The single stepper  36  is used to expose the wafers that have been coated in both the first and second coaters  38 ,  42  on the first and second tracks  32 ,  34  respectively. 
     The single stepper  36  interfaces with both the first and second coaters  38 ,  42  as well as the first and second developers  40 ,  44 . A wafer transfer system  46  moves the wafers on the first track from a first input/output port  48  of the first coater  38  to an input/output port  50  of the stepper  36 , and from input/output port  50  to an input/output port  52  of the first developer  40 . Similarly, the wafer transfer system  46  moves the wafers on the second track  34  from a first input/output port  54  of the second coater  42  to the input/output port  50  of stepper  36  and from input/output port  50  to input/output port  56  of second developer  44 . As will be described below the wafer transfer system  46  may also be used to move wafers between the first and second tracks as required. While the wafer transfer system  46  is illustrated as a separate entity in FIG. 2, the wafer transfer system  46  interacts with the first and second tracks as well as the stepper  36 . The wafer transfer system  46  could also be an integrated part of the first and second tracks  32 ,  34  as well as the stepper  36 . The wafer transfer system could include a conveyor system to move the wafers from one station to another, or may include a robotic control unit as described in U.S. Pat. No. 5,455,894. 
     A computer controller and software controls the flow of the wafers to both maximize throughput of the system, permit more photoresist and developer combinations, and minimize the time that the stepper is idle during the interface mode. 
     There are a number of modes of operation in which the system can operate. One mode of operation includes processing a first lot of wafers on the first track  32  and a second lot of wafers on the second track  34 , where both of lots of wafers are processed using the same photoresist, the same reticle in the stepper, and the same developer material. 
     In this mode of operation, both tracks  32 ,  34  feed coated wafers to the stepper  36 . Since the time required to expose a coated wafer to light in the stepper  36  is less than the time required to develop the exposed wafer, the stepper  36  can image the wafers from the first and second tracks  32 ,  34  alternatively and forward them to the appropriate developer  40 ,  42 . As a result, the idle time of the stepper  36  is reduced. 
     In another mode of operation, both tracks  32 ,  34  are run concurrently and employ a different resist/developer combination and or a different imaging reticle. In this mode a first group of wafers are coated on the first track, while a second group of wafers are coated on the second track. Since the reticle used to image the wafers will be different for the first and second groups of wafers, the operator must program the system to recognize which group of wafers to expose first. Where the group of wafers on the first track  32  is to be exposed first, the second group of coated wafers will be held in the input/output port  50  of the stepper until all of the first group of wafers have been exposed, and the reticle is changed. 
     Additionally, since, the stepper  36  will image the entire first group of wafers one after another; a queue will develop for the first developer. While the wafers are being processed in the first developer the reticle can be changed in the stepper  36  and the second lot of wafers can be exposed and sent to the second developer  44 . This represents an improvement over the prior art system in which the stepper was idle during the period in which the developer ran through the queue and completed the developing of all of the wafers. 
     In yet another mode of operation the first track  32  can be down for periodic maintenance while the second track  34  can continue to process wafers in conjunction with the stepper  36 . Similarly, the second track  34  can be down for periodic maintenance while the first track  32  can continue to process wafers in conjunction with the stepper  36 . 
     In still another mode of operation a group of wafers can be coated with one of the three photoresists available in the first coater  38  and then developed in one of the two developer materials available in the second developer  44 . Similarly, the group of wafers may be coated with one of the three photoresists in the second coater  42  which may be different than the three photoresists available in the first coater  38 , and then developed with one of the two developer materials in the first developer  40 . In this manner the number of possible photoresist and developer combinations is greatly enhanced over the prior art single track/stepper configuration. If two coater stations are employed for each track the number of available photoresists may be doubled from six to twelve. 
     This improved system permits greater number of photoresist and developer combinations and therefore reduces the number of overall track and stepper systems that a manufacturer must employ to manufacture different types of integrated circuit chips. Additionally, as discussed above, since certain photoresist combinations will tend to clog the drainage system, it is possible to separate incompatible photoresist combinations by placing them in different tracks. As discussed above, the dual track with a common integrated stepper reduces the cost of the overall operation. 
     Further, it is possible to process a greater number of wafers utilizing a single stepper in this system and minimize the time that each coated wafer must wait to be exposed. The downtime required for periodic maintenance of the stepper is limited to the actual time required to service the stepper itself and not the time required to attend to the coater and developer stations. 
     Depending on the particular mode of operations, various wafer transfer systems may be utilized. The wafer transfer system as described in the &#39;894 patent may be modified to permit both the tracking of the individual wafers within a single coater/developer track and stepper as well as the ability to move the wafers from a first track to a second track for processing as required. The wafer transfer system will include an input device such as a keyboard to permit an operator to instruct the system on the mode of operation, including the photoresist to be applied, the reticle used for imaging and the type of developer to be used for each layer. A computer controller will direct a robotic interface to move the individual wafers to the appropriate first or second coater, and to the appropriate first or second developer based on the type of photoresist, and developer combination. The controller will also keep track of each wafer within a given lot and display the status and location of the lots being processed. 
     Based on the mode of operation selected, software will determine the order in which the lots of wafers will be processed. The controller will permit the automated processing of the first and second lots of wafers as discussed above, thereby minimizing the idle time of the stepper, maximizing the throughput of the wafers in the least amount of time, and providing for a greater number of photoresist and developer combinations. 
     The invention has been described in reference to particular embodiments as set forth above. However, only the preferred embodiment of the present invention, and but a few examples of its versatility are shown and described in the present disclosure. It is understood that the present invention is capable of use in various other combinations and environments, and is capable of changes or modifications within the scope of the inventive concept as expressed herein. Also, many modifications and alternatives will become apparent to one of skill in the art without departing from the principles of the invention as defined by the appended claims.