Abstract:
A method and apparatus for minimizing the surface contamination of semiconductor wafers ( 11 ) during the semiconductor device manufacturing process. Semiconductor wafers ( 11 ) are stored in a storage cassette ( 12 ) with their face sides ( 17 ) facing downward and their back sides ( 16 ) facing upward. Particulate contamination present on the back sides of the wafers is thereby secured to the wafers by the force of gravity, and the faces of the wafers are shielded from falling debris. An automated wafer handling device ( 19 ) is provided with a rotary joint ( 22 ) to accomplish the wafer flipping motion before inserting a wafer into a cassette and after removing the wafer from the cassette.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to semiconductor processing methods and apparatus and, more particularly, to a method and apparatus for handling of semiconductor wafers during processing. 
     One of the primary concerns in the manufacture of semiconductor devices, such as integrated circuit chips, is avoiding contamination from particulate matter which may fall onto or otherwise be deposited on a top surface or face of a semiconductor device. The manufacture of semiconductor devices includes multiple fabrication steps such as masking, etching, electroplating, chemical mechanical planarization and polishing, oxidation and dielectric layer deposition on a semiconductor wafer. Between these various process steps, the wafer may be picked up by a mechanical handling apparatus and moved to temporary storage. In some applications, a plurality of wafers may be collected and stored in a storage cassette between processing steps. 
     As shown in FIG. 1, a semiconductor wafer storage cassette includes a plurality of wafer support elements arranged to receive and support a plurality of stacked wafers. In one form, support elements engage wafers at their outer edges and support the wafers in horizontal, spaced planes. A robotic wand may be used to insert the wafers into and remove the wafers from the cassettes. To remove a wafer from a processing station, the wand engages the wafer using vacuum or other known means, then lifts and moves the wafer to the cassette where the wafer is stored until it is needed for a next processing step. 
     It is known that semiconductor-manufacturing areas must be maintained in an extremely clean condition in order to minimize the contamination of the wafers being processed. In spite of such efforts, particulate debris cannot be completely eliminated, and some semiconductor devices on some wafers must be scrapped as a result of particulate contamination. Accordingly, it is advantageous to provide apparatus and/or methods for reducing opportunities for contamination of semiconductor wafers during the manufacturing process. 
     SUMMARY OF THE INVENTION 
     Applicants have discovered that a possible source of contamination is debris that may accumulate on a backside of a semiconductor wafer during processing and which can then fall onto a face of another wafer when the wafers are stored in a stacked position between processing steps. The present invention addresses this potential source of contamination by changing how such semiconductor wafers are stored within storage cassettes between processing steps. In one form of the invention, wafers are stored in conventional cassettes but in an inverted position so that the wafer face is oriented downward. Accordingly, any debris falling under the force of gravity will land on a backside of the wafer where such debris will not affect the circuit formed on the face of the wafer. The invention further includes adapting the wafer handling apparatus, such as a vacuum wand, with a rotatable section to enable inversion of the wafer between processing equipment and storage. More particularly, the wand is adapted to rotate the wafer through 180 degrees in order to position the wafer in an upside down orientation in the storage cassette whereby, if contaminants do drop from a wafer in a cassette, the particles will fall on the backside of a lower stacked wafer. In one form, a rotary coupling enables the wand head of the handling device to rotate through 180 degrees, and/or rotate to a plurality of positions between a position of the wafer facing upward and upside down. 
     It is also contemplated that the wafers could be on one edge in a different form of cassette such that the wafers would only be rotated 90 degrees. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevational view of a cassette with semiconductor wafers stored in a conventional face up position as is known in the prior art. 
     FIG. 2 is a top view of a semiconductor wafer 
     FIG. 3 is a side elevational view of a semiconductor wafer 
     FIG. 4 is a partial front elevational view of a cassette with semiconductor wafers stored in a face down position. 
     FIG. 5 is an illustration of a robotic wafer-handling device moving a semiconductor wafer between a storage cassette and a processing location. 
     FIG. 6 is a front elevational view of a wafer skewed to a vacuum head. 
     FIG. 7 is a front view with the vacuum head adjusted with respect to the wafer. 
     FIG. 8 is a front elevational view of semiconductor wafers stored in alternating positions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With respect to FIGS. 2 and 3, a semiconductor wafer  11  is illustrated and includes a backside  16 , and a front side or face  17 . The terms front side and face are used herein to indicate the side of the wafer  11  upon which materials are deposited and electronic devices are formed. The term back or backside is used to indicate the reverse side of the wafer opposite the face. Normally, processing steps to create electronic devices or circuits are performed on the wafer backside. The electronic devices may be grouped into cells or chips  18  as illustrated in FIG.  2 . The cells  18  are illustrated as being rectangular in shape and they are eventually separated from one another and attached to a substrate to form an integrated circuit. The wafer  11  also has a notch  23 , or flat edge, which serves for alignment and orientation of the wafer  11  during the wafer processing steps. 
     The cells  18  are rectangular in shape, having linear edges as opposed to the curvilinear edge  14  of the wafer  11 . Areas disposed along the edge  14  of the wafer  11  are not entirely rectangular in shape and are not functional for forming useable dies. Accordingly, an interval  15  is formed between the edge  14  of the wafer  11  and the edge of the useable or functional cells  18 . 
     During the course of the manufacturing process, the wafer  11  may be placed into a processing position with the backside  16  of the substrate in contact with the tool chuck  25 , as illustrated in FIG.  5 . The backside  16  of the wafer  11  may collect particulate contamination by its contact with the various tool chucks used during these processes. 
     As shown in FIGS. 3 and 4, the wafers  11  are stored in a storage device such as a cassette  12  between the different manufacturing steps. A cassette  12  generally includes an outer frame  26  within which a plurality of wafer edge supports  13  are vertically spaced. Edge supports  13  are commonly referred to as “shelves” in the art. Other structures of storages devices may be envisioned by those skilled in the art, including other structures that may support a wafer  11  from an edge interval  15  containing no active devices or from the curvilinear edge  14  of the wafer. In the past, the wafers  11  would be placed on the shelves in a face-up position with the backside  16  of the wafer  11  contacting the shelves  13 . When the wafers  11  are stacked in the cassette  12 , the wafers  11  may then be jarred releasing any debris or contaminants from the backside  16  which debris could then fall onto the face  17  of a lower stacked wafer  11 . 
     In the present invention, as shown in FIG. 4, the wafers  11  are stacked in the cassette  12  in a face-down position. Prior to being inserted into the cassette  12 , the wafer  11  is rotated upside down with the face  17  of the wafer  11  directed downward. Consequently, the backside  16  of the wafer  11  is directed upward and the force of gravity serves to secure any particulate matter present on the backside  16 . The shelves  13  of the wafer  11  engage the face  17  of the wafer  11  along the interval  15 . The interval ranges from approximately one millimeter to two centimeters wide along the perimeter of the wafer  11 , and provides a sufficient area to support the wafer  11  on the shelf  13 . In this manner, the particulate contaminant that may collect on the backside  16  of the wafer  11  will remain there when the wafer  11  is inserted in the cassette  12 , and any particulate matter that does become loose will fall onto the backside of a lower stacked wafer rather than onto that wafer&#39;s face. 
     With respect to FIG. 5, a robotic wand  19  is shown loading and unloading a wafer  11  into a cassette  12 . Position A represents the wafer  11  being removed by the wand  19  from a processing workstation tool chuck  25 . Position B shows the wafer  11  having been rotated by the wand  19  to an upside down position for loading the wafer  11  in the cassette  12 . The wand  19  includes a vacuum head  20  attached to an arm  21 . The robotic wand  19  and head  20  are equipped with vacuum capabilities to secure the wafer  11  against the head  20 . A means for rotating the wafer  11  such as rotary coupler or joint  22  allows the wand  19  to rotate or pivot 180 degrees about the axis of the arm  21 . Conventional robotics parts and software can be used and programmed as known to those skilled in the art to position and orient the wafer  11  with respect to the cassette shelves  13  and the wafer tool chuck. One may appreciate that other means for rotating the wafer may be used to flip the wafer face side down and back, such as any sort of gear/shaft mechanism, edge grasping and turning mechanism, etc. 
     The wand  19  is preferably equipped with components such as a gimbal and/or force sensor, which are known in the art. Such components assure proper orientation of the wafer with the wand  19 , detect when the wand  19  contacts the wafer  11 , maintain sufficient vacuum between the wand  19  and wafer  11  and prevent unnecessary forces that can fracture the wafer  11 . In addition, the wand  19  may be equipped with a scanning system that determines which slots of a cassette are populated with wafers, and the approximate location of the wafers with respect to wafer height, which scanning systems are also well known in the art. 
     When a wafer  11  is positioned normally, the vacuum head  20  on the wafer secures the wafer  11  against the wand. In some cases, the wafer  11  may be slightly skewed at an angle theta with respect to a normal position of the vacuum head  20  as shown in FIG. 6. A gimble can be operated to rotate the head  20  and remove any such skew angles so the vacuum head approaches the wafer  11  in a normal position with respect to the wafer  11 , as shown in FIG.  7 . 
     In operation, the wand  19  engages and removes the wafer  11  from the chuck  25 . The wand  19  rotates 180 degrees so the wafer is turned upside down with the backside  16  of the wafer  11  facing upward. The wand  19  then inserts the wafer  11  into the cassette  12  and onto a shelf  13 . The interval  15  on the top surface of the wafer  11  engages the shelf  13  thereby avoiding contact between the shelf  13  and any functional die or chip  18 . 
     When the wafer  11  must be placed on a processing station, the wand  19  engages the wafer  11  and removes the wafer  11  from the cassette  12 . The wand  19  rotates 180 degrees so the face  17  of the wafer  11  faces upward and the backside  16  of the wafer  11  faces downward. Thus, the wafer can be placed in a processing station in a face upward orientation for further processing. 
     Although the invention is illustrated as being implemented by automated equipment, one skilled in the art may appreciate that manual handling and storage of a semiconductor wafer may be accomplished in a similar manner. In any storage location for wafer  11  where it may be exposed to falling particulate matter, the wafer  11  may be stored in a face-down position to protect its face  17 . Similarly, in any location where a wafer  11  may be handled above another wafer, the wafer  11  may be rotated to a face-down position in order to secure any particulate matter on its back side  16 . 
     An additional embodiment of the invention alternates the position of the wafers  11 . As shown in FIG. 8, a first wafer  11 A is in an upside down position, with the face  16  of the wafer facing downward. The next wafer  11 B is positioned with the face  16  of the wafer facing upward. The wafers are alternated as such within the cassette. 
     In yet another embodiment, the wafers are vertically disposed within a cassette. The cassette includes shelves that are horizontally spaced apart, and support the wafers in a vertical position, so that particulate falling from the backside of a wafer  11  will not touch or collect on other wafers in the cassette. In this embodiment, the wafers may be disposed at angles with respect to the cassette. In the previous embodiment, where the wafers are horizontally disposed there is little if any difference in skew angle between the wafers. The only variable is the skew angle between the wafer  11  and the vacuum head  20 . 
     The vertical disposition of wafers imposes two variable angles including the angle between the wafer and the cassette, and the angle between respective wafers. Thus a device is necessary to bias the wafers to remove the variable angle between the wafers, so the wand  19  and head  20  may be rotated as necessary to engage a wafer  11 . In this embodiment, it may also be desirable to orient wafers in a back-to-back and face-to-face arrangement. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.