Abstract:
A reduction in wafer processing cycle time is achieved by conducting wafer verification and slot randomization of a set of wafers as the wafers are moved through a singular processing location. In an example embodiment, a method of processing a set of wafers in a wafer processing system includes providing each of the wafers with a scribe code thereon. Each of the wafers is presented to a first processing location with the processing location having at least one processing chamber. The scribe code on each wafer is then read as the wafer is being placed into the processing chamber. Each wafer is processed and the set of wafers is slot randomized as they are removed from the first processing location and placed into a wafer cassette. An important advantage of the present invention is the reduced cycle times and reduced capital investment that this method and system bring to wafer processing.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to wafer handling in a semiconductor processing plant and, more particularly, to reducing cycle time in wafer processing by eliminating interim wafer sorting operations as wafers move through a multiple chamber processing tool. 
     BACKGROUND OF THE INVENTION 
     Some manufacturing processes require that the item being processed be rotated regularly in order to ensure that the item is properly processed, such as when painting an object or when applying a coating to a substrate. In the case of a mechanical process, the object is rotated to ensure that the tooling is being worn evenly or that the tooling is mechanically treating the object evenly. Even though some of these items may be individually processed, or processed in small lots, the items may form part of a larger lot being manufactured and it is difficult to distinguish the progress of the individual item as it moves through the processing line. As the number of processing steps increase tracking becomes even more difficult. This is particularly a problem in the processing of wafers in a semiconductor processing plant. 
     A conventional semiconductor fabrication plant typically includes multiple fabrication areas or bays interconnected by a path, such as a conveyor belt. Each bay generally includes the requisite fabrication tools (interconnected by a subpath) to process semiconductor wafers for a particular purpose, such as photolithography, chemical-mechanical polishing or chemical vapor deposition, for example. Material stockers or stocking tools generally lie about the plant and store semiconductor wafers waiting to be processed. Each material stocker typically services two or more bays and can hold hundreds of cassettes. The wafers are usually stored in cassettes in groups of about 25 wafers. The wafers are then disposed within a carrier and move from one process to another in the carrier. The carriers are usually tracked by their carrier code by a computer system as they move through the plant. 
     Once a lot has been retrieved, and the equipment has been set up, the operation on the wafers by a particular piece of equipment, or “tool,” can begin. At this point, the lot is “moved-in” to the operation. An operator on the line then communicates this information to the host computer. The lot remains in this state until the operation is completed. Once the operation is completed, the operator must perform tests and verifications on the wafers. When all tests and verifications have been performed, the host computer application program must be notified. Wafers may have moved from one cassette to another as a result of the operation; therefore the host application and computer has to be notified of these moves. The operator then places the cassette of “moved-out” wafers in the material stocker to await orders as to the location of the next piece of equipment that will perform operations on the wafers. 
     The semiconductor fabrication plant, including the bays, material stockers and the interconnecting path, typically operates under control of a distributed computer system running a factory management program. In this environment, the automated material handling system (AMHS) may conceptually include the cassettes, the transportation system (e.g., paths) and control system (e.g., the distributed computer system). An empty carrier management system as well as a separate test wafer management system may also form part of the AMHS. 
     Data gathered during the course of wafer processing is used to diagnose yield problems and forms the basis of yield improvement efforts. Such data includes parametric electrical test data gathered on individual circuits and test structures fabricated on the wafers, as well as wafer sort data which tests the suitability for use of the wafers once wafer processing is completed. One of the possible sources of yield variation is the order in which wafers in a lot are processed at a given processing step. When the processing is done one wafer at a time per step, a variation in yield may be caused by a build up of contaminants, uneven heating of a processing chamber or another physical aspect that changes during the processing of the lot. In a batch operation, the physical location of the wafer in the batch processing equipment may influence uniformity of the processing effects across the lot. In an example where wafers are moving through a contaminated chamber, if the order in which each wafer is processed is known then the final wafer yield may be plotted against the processing order in this step. For each wafer in a lot a drop-off in yield versus processing order would be observed due to the contamination problem. This data is used to make adjustments to the line to improve yield; however, this wafer tracking method lacks the level of precision in the data collected required by chip plants today. 
     In tracking the wafer processing order, wafer sorters have been used to read scribed wafer identifiers, either immediately prior to or after critical processing steps. Sorters typically read the scribe marks to ensure that the wafers and the cassette are properly matched together as the wafers move through the semiconductor processing line. The scribe marks on the wafers usually comprise a series of characters formed in the wafer substrate that serve as an identifier. Since integrated circuits should not be formed on the scribe marks, the scribe marks are typically positioned near the unused outer edge of the wafer to maximize usable wafer surface area. 
     Randomizing the order of the wafers prior to such steps is often done to ensure certain processing effects are not compounded. The wafer positional data is fed into a computer system, the device performance metrics for a wafer lot of interest are manually entered, and then all possible graphs of the device metrics for that lot versus wafer processing order at each step are generated. The data is then reviewed to determine those steps at which the processing order may affect performance. This type of approach to tracking wafers can be costly in its implementation due to the amount of hardware and software needed to both randomize the wafer order and interface with the wafer processing system&#39;s main computer database. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to addressing the above and other needs in connection with reducing wafer processing cycle times as the wafers move through a multiple chamber processing tool. According to one aspect of the invention, it has been discovered that wafer sorting operations proximate to a processing location having a multiple chamber processing tool (or cluster tool) can be eliminated by conducting wafer verification and cassette slot randomization of a set of wafers while at the processing location. 
     According to one aspect of the invention, a method of processing a set of wafers in a wafer processing system includes providing each of the wafers with a scribe code thereon. Each of the wafers is then presented to a first processing location, the processing location having at least one processing chamber. The scribe code on each wafer is read as the wafer is being placed into the at least one processing chamber. Each of the wafers in the at least one processing chamber is processed and the set of wafers is randomized as they are removed from the first processing location and placed into a wafer cassette. 
     According to another aspect of the invention, a system for processing wafers in a wafer processing system includes a set of wafers having a scribe code thereon and an arrangement for presenting each of the wafers to a first processing location, the first processing location having at least one processing chamber. An arrangement for reading the scribe code on each wafer as the wafer is being placed into the at least one processing chamber is also included. An arrangement for randomizing the set of wafers as they are removed from the first processing location and placed into a wafer cassette is included. 
     In yet another aspect of the invention, an apparatus for processing a set of wafers in a wafer processing system includes an entry port for receiving a set of wafers having a scribe code thereon and at least one processing chamber for processing each of the wafers. An automated scribe code reading arrangement adapted to read the scribe code on each wafer as the wafer is being placed into the at least one processing chamber is included. The apparatus also includes a wafer handling arrangement adapted to place the wafers into the processing chamber and randomize the set of wafers as they are removed from the processing chamber and placed into a wafer cassette. In a related embodiment, an alarm and a manual scribe reading device are included where the automated scribe code reader is unable to read the scribe codes on the incoming wafers. 
     The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures in the detailed description that follow more particularly exemplify these embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: 
     FIG. 1 is a wafer cassette having a set of unaligned wafers; 
     FIG. 2 is a process flow diagram of a typical wafer processing line having wafer sorting devices located between processing locations; 
     FIG. 3A is a schematic of an example processing location having a multiple chamber processing tool in accordance with one embodiment of the invention; 
     FIG. 3B is a schematic of an example of a processing location having a multiple chamber processing tool in accordance with another embodiment of the invention; and 
     FIG. 4 is a flowchart of the manner in which wafers are verified and slot randomized in a wafer cassette while at a processing location in accordance with one embodiment of the invention. 
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
    
    
     DETAILED DESCRIPTION 
     The present invention is generally directed to a method and system for verifying and cassette slot randomizing a set of wafers in a wafer processing system. The invention is particularly suited for wafer processing lines that utilize a large quantity of wafer sorters or have a large quantity of wafer sorting operations incorporated therein. While the present invention is not necessarily limited to a wafer processing application the invention will be better appreciated using a discussion of exemplary embodiments in such a specific context. 
     In an example embodiment, a system for processing a set of wafers in a multiple chamber processing tool includes equipping the tool with scanning equipment that reads the scribe code as the wafer is being placed by a robotic arm into one of the processing chambers. After processing is completed at a particular processing location (for example, etching) the wafer is removed from the chamber and is returned to the original wafer cassette. However, instead of returning the wafer to its original slot location, the wafer is placed in a random slot location within the cassette that is different from the slot location when the wafer originally arrived to the processing location. As part of the wafer processing recipe, slot randomization sequence instructions are added to the recipe software directing the robotic arm to slot randomize the wafers as they are being removed from the cluster tool or one of the chambers. As the wafers leave this particular processing location they have been subjected to wafer verification and have been slot randomized, effectively eliminating a wafer sorting operation proximate to the processing location and reducing wafer processing cycle time. In addition, a substantial capital investment has been avoided or delayed in not having to purchase additional wafers sorters. 
     Referring now to the figures, FIG. 1 illustrates a set of wafers  10  disposed unaligned in a wafer cassette  12 . Cassette  12  has a series of slots  13  that hold a set of individual wafers  14  (the set can include up to twenty-five wafers) for movement through the wafer processing system. Wafers  14  have a notch  16  located along the circumference that serves as a point of reference and is used for aligning each wafer in a processing chamber. Each wafer also has a scribe code  18  located on the wafer surface and near the wafer&#39;s edge. The scribe mark is used for wafer verification in identifying the wafer to ensure that the wafer is matched correctly with a corresponding wafer cassette. In addition, each cassette in the wafer processing system is also identified and tracked by an identification tag, such as a bar code, which is read by a sensor along the processing path or material handling system. 
     Referring briefly to FIG. 2, a process flow diagram exemplifies a portion of a typical wafer processing line  20 . At location  22 , the set of wafers starts to move through wafer processing via a material handling system. At location  23 , the wafers are scanned for identification (via the scribe code) and are slot randomized by a wafer sorter  23  prior to moving to Stage one processing at location  24 . This information is then recorded in a computer arrangement  28  that is coupled to wafer processing line  20 . After the wafers exit location  24 , they are scanned and sorted by another wafer sorter  25  prior to moving into Stage two processing at location  26 . One of the disadvantages to this approach is the increase in wafer processing cycle time that occurs when there is a wafer sorter shortage and the system is forced to route the wafers to a material stocker until a sorter becomes available. One of the advantages of the present invention is ability to reduce processing cycle time by conducting wafer verification and cassette slot randomization while at a wafer processing location, thereby obviating the need for a wafer sorter. 
     Referring to FIG. 3A, a schematic diagram exemplifies a portion of a wafer processing system composed of a multiple chamber processing tool  30 . In this example, tool  30  has three processing chambers  32 A,  32 B and  32 C that perform subprocesses such as metrology, etching, wafer coating, and stepper exposure and film development. Wafer cassettes  12 A and  12 B are presented to tool  30  via a material handling track system  33  at an entry port  34  of tool  30 . Cassettes  12 A and  12 B are placed in a pair of loadlocks  36 A and  36 B, respectively. A wafer handling robotic arm  38  then aligns each set of wafers within cassettes  12 A and  12 B via notch  16  so that all of the wafers are aligned the same way as they move through the processing location. In this example, the wafers are removed from the cassettes individually and passed through automated scanning devices  40 A and  40 B, that are adapted to read scribe mark  18  located on the wafer&#39;s edge, for wafer verification. The scanning device includes an OCR (optical character reader); a laser or any other scribe or wafer marking reading device. Robotic arm  38  then moves the wafer into one of chambers  32 A- 32 C for processing. In a related application, a rotation table is used to move the wafer from the loadlocks to a particular chamber for processing. 
     A wafer-processing recipe is downloaded and stored in a recipe module  42  that is adapted to direct the movement of arm  38  and the operation of processing tool  30 . Included in the recipe software are the slot randomization sequence instructions for each of the wafer sets in cassettes  12 A and  12 B. According to the slot randomization sequence, as the wafers are removed from one of chambers  32 A- 32 C the wafers are placed in slots that are different from the slot that the wafer originally arrive in. For instance, a wafer in slot  3  is first notch-aligned to the six o&#39;clock position and then placed into chamber  32 A. Once the processing is complete, the wafer is removed and then placed in slot  7  of the cassette or a slot other than slot  3  that is empty. 
     In another example embodiment, the processing tool is an etching chamber (such as an Alliance tool) that has a notch aligning arrangement coupled thereto for aligning incoming wafers before processing. The etching tool has an OCR coupled thereto that is adapted to read the scribe mark on the wafers as they are placed into the etching chamber. The processing recipe for the etching chamber that is loaded includes the slot randomization sequence for directing the robotic arm movement in its placement of the wafers into the slots of the cassette when the etching operation is complete. 
     FIG. 3B illustrates a related embodiment wherein processing tool  30  is further fitted with an alarm unit  44  and a manually operated scribe reading unit  46  (e.g. Barrel Lenz unit). Materials used to treat or coat wafers, particularly photoresist, can accumulate in the area of the scribe marks and obscure the mark so as to affect the optical properties of the wafer surface. When this occurs, automated scanning devices  40 A and  40 B have difficulty reading the scribe marks and will signal to module  42  that the operation is to be aborted for its inability to conduct wafer verification. To avoid aborting the current operation (and slowing down the processing of the wafers), alarm unit  44  is actuated to advise an operator to use scribe reading unit  46  to manually read the scribe code and input the code via a keyboard  48 . Upon alarm actuation, a robotic arm  50  removes the wafer from one of the loadlocks and places wafer  14 A under a magnifying lens  46 A in order to read scribe code  18 . Reading unit  46  includes an angled lens  46 B for reading the scribe code on smaller wafers. 
     Referring to FIG. 4, a flowchart  70  illustrates an example of the process flow of the method for processing a set of wafers in a wafer processing system having multiple-chamber processing tools in accordance with an embodiment of the present invention. At  72 , a set of wafers is provided each having a scribe code and a notch thereon. At  74 , the set of wafers is presented to a first processing location and loaded into the loadlock or chamber of the cluster tool. At  76 , the wafer processing recipe along with the slot randomization sequence instructions are loaded onto recipe module  42 . At  78 , the wafers are notch-aligned to a predetermined position dictated by the processing recipe or the chamber parameters. At  80 , the wafer&#39;s scribe code, cassette code and slot number (optional) are identified and recorded in a computer as part of the wafer verification procedure. 
     In a related embodiment, additional stages are added to the process flow where the automated scribe reading device is having difficulty reading the scribe code at location  80 A. In this case, an alarm is actuated at location  80 B advising an operator of a problem on the line with wafer verification. The operator then reads the scribe marks manually at location  80 C and inputs the information into the processing location. Once the wafer verification is made, either automatically or manually, the wafers proceed to processing at location  82 . At location  84 , the wafers are removed from the processing chambers by the robotic arm according to the slot randomization sequence instructions found in the processing recipe. At location  86 , the material handling system moves the wafer cassette to the next processing location where the process begins again at location  76 . In a related embodiment, the wafer verification information is forwarded to the next processing location, thereby eliminating the need to conduct this procedure at the following processing location and eliminating the cost of fitting that same tool with a scribe reader. 
     Accordingly, the present invention is not to be necessarily limited to the particular examples described above, but is intended to cover all aspects of the invention as fairly set out in the attached claims. For instance, while wafer verification and slot randomization in a cluster tool is illustrated, other processing tools having a single operation or chamber can benefit from the above mentioned teachings. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.