Abstract:
A wafer positioning checking system used in a vertical furnace as found in a semiconductor manufacturing facility for manufacturing chips. The system utilizes a first sensor such as a photoelectric or laser sensor that checks the peripheral alignment of the wafers loaded in the boat. A second sensor is mounted on a robot having a wafer-handling arm for checking the position of a wafer that has just been loaded into the boat. An algorithm in a control unit responds to electrical signals generated by these two sensors to allow the loading operation to continue as long as the wafers are properly positioned and to controllable monitoring the wafers during a portion of the processing.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates in general to vertical semiconductor furnaces for manufacturing semiconductor devices, such as integrated circuits, and deals more particularly to checking the position of quartz boats in the furnace.  
         BACKGROUND OF THE INVENTION  
         [0002]    U.S. Pat. No. 6,223,096 entitled “Elevator System For Transferring a Wafer Boat With Automatic Horizontal Attitude Control” issued on Apr. 24, 2001 which is incorporated herein by reference, teaches an elevator system that automatically adjusts the inclination a wafer boat during the manufacturing of the semiconductor devices. This ensures that the wafers in the boat are maintained horizontal during their processing in the furnace. The elevator system includes the wafer boat, a base on which the boat is supported and an elevator for loading the boat into a processing chamber and removing it from the chamber. In addition there is a sensing unit for detecting the inclination of the boat relative to the horizontal. A horizontal control unit is interposed the base and the elevator and is drivable to maintain the boat in such a position to maintain the wafers in horizontal planes in the boat. A control unit receives the information from a sensing unit and, based on the information, outputs a control signal to the horizontal driving units. This system does not check the position or the condition of the wafer in the boat after the wafer is loaded.  
           [0003]    U.S. Pat. No. 6,002,516 entitled “Vertical Cassette Elevator and Load Lock Viewer” issued on Dec. 14, 1999 and assigned to a common assignee, a system for readily observing wafers when disposed in a vertical cassette elevator or in a load lock during transfer into and out of a fabrication processing system. This system provides an optical system allowing an operator to visually view the interior of the fabrication system. This prior art does not continuously monitor the positioning and the condition of the wafer during the processing. This patent is incorporated herein in its entirety by reference only.  
         SUMMARY OF THE INVENTION  
         [0004]    It is difficult to see by visual means the fine wafer position in a quartz boat in a vertical furnace system when calibrating the system. It is therefore a principal advantage of this system to utilize a plurality of sensors to check both vertical and horizontal alignment of the wafers automatically and continuously.  
           [0005]    It is yet another advantage of the present system to prevent the wafer from sliding out, being scratched, broken or dropped during transfer of the wafers within the system due its constant monitoring.  
           [0006]    It is another advantage of the present system to automatically check the wafer position in every run of the processing of the vertical furnace.  
           [0007]    It is yet still another advantage of the present system to guide the wafer as it is being loaded on the forks of the boat to prevent breaking of the wafers and the resulting cross contamination on the other wafers.  
           [0008]    These and other advantages will become apparent from the following wafer positioning checking system in a vertical semiconductor furnace, having a wafer storage stage for storing a plurality of wafers prior to be transferred to the furnace. At least one cassette receives the wafers from the storage stage by means of a cassette loader for transferring wafers from the wafer storage area to the cassette. A boat receives one or more wafers from the cassettes and loads and supports the wafers in the vertical furnace.  
           [0009]    A robot moves one or more wafers from the cassette to the boat and an elevator then moves the boat into the semiconductor furnace. A first sensor, for generating a first electrical signal, has both a receiver and a transmitter section with one of the sections mounted to the base of the boat and the other section mounted in line opposite the base. A second sensor, for generating a second electrical signal, is mounted to the robot and responds to the wafers for ensuring that the wafers are positioned properly in the boat by the robot.  
           [0010]    A control system includes a microprocessor having a memory and several input and output ports. The control system responds to the first and second electrical signals connected to different input ports and operable in response to an algorithm stored in the microprocessor for controlling through the output ports the robot, the elevator and the cassette loader to load the wafers in the furnace. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    These and other objects, features and advantages of the present invention will become apparent from the following detailed description and the appended drawings in which:  
         [0012]    [0012]FIG. 1 is a schematic of a portion of a semiconductor-manufacturing unit including a vertical furnace;  
         [0013]    [0013]FIG. 2 is a schematic illustrating the sensor placement in the semiconductor-manufacturing unit of FIG. 1; and  
         [0014]    [0014]FIG. 3 is bottom view of the quartz boat cap holder used in the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    Referring to the Figs by the characters of reference, there is illustrated in FIG. 1 a portion of vertical furnace used in a semiconductor-manufacturing unit  10 . In particular, there is illustrated a chamber  12  of the furnace, a boat  14  mounted to an elevator  16 , a robot  18 , a stack of cassettes  20 , a cassette loader  22 , a wafer storage area or stage  24 , a first sensor  26 , a second sensor  28  and a control unit  30 .  
         [0016]    In the processing of semiconductor wafers  32 , the wafers are processed through many, many steps from the fabrication of the initial substrate to the finished product. Illustrated in FIG. 1 and FIG. 2 is the portion of the process for loading the chamber  12  in the furnace. As illustrated in several patents, such as U.S. Pat. No. 5,846,073 entitle “Semiconductor Furnace Processing Vessel Base” issued on Dec. 8, 1998, which is incorporated herein by reference only. The chamber  12  is located so that the environmental conditions of the system are closely maintained. These include by way of illustration the cleanness of the chamber, the temperature of the chamber and various other conditions required for the fabrication of semiconductor chips. The chamber  12  is typically an enclosed tubular member opened at one end that is generally the bottom end.  
         [0017]    A boat  14  for carrying the wafers  32  into the chamber  12  is located on an elevator  16 . The elevator  16  reciprocally moves the boat  14  into and out of the chamber  12  under control of the control unit  30 .  
         [0018]    A robot  18  is position to transfer the wafers  32  to the boat  14  from the stack of cassettes  20 . U.S. Pat. No. 6,002,516 entitled “Vertical Cassette Elevator and Load Lock Viewer” issued Dec. 14, 1999 and U.S. Pat. No. 6,206,441 entitled “Apparatus and Method for Transferring Wafers by Robot” issued on Mar. 27, 2001 both teach the transferring of the wafers  32  to the boat  14 . Both of these patents are assigned to a common assignee and are herein incorporated by reference only.  
         [0019]    The wafers  32  are stored in a wafer storage area or stage  24  and by means of a cassette loader  22  are transferred to the cassettes  20 . This is a prior art system for loading the wafers  32 .  
         [0020]    By means of the present invention, the position of the wafers  32  in the quartz boat  14  has their alignment checked after the transfer is completed. The alignment of the wafer  32  is checked so that the wafer will be loaded into the chamber  12  with any failures. The present invention operates to prevent any wafer  32  from sliding out of the boat  14  or becoming scratched, broken or dropped due to misalignment. In addition the wafers  32  are continuously checked until it becomes either unnecessary because the boat  14  is in the chamber  12  or the environment of the boat precludes the operation of the sensors  26 ,  28 .  
         [0021]    To accomplish these advantages a first sensor  26  is mounted to the base  34  of the boat  14  as illustrated in FIG. 3. The base  34  of the boat has a ring  36  with a diameter that is almost the diameter of chamber  12 . In the preferred embodiment there is a five-millimeter (5 mm) gap  38  between the ring  36  and the chamber  12 . With such a small gap  38 , if a wafer  32  is not aligned properly, the chamber  12  walls may break or physically damage the wafer as the boat  14  is loaded into the chamber.  
         [0022]    As shown in FIG. 3, the base  34  has four equally and angularly spaced holes  40  that are aligned with the top plate  42  of the boat  14 . Through these holes  40  a photoelectric beam  45  from the first sensor  26  is transmitted from a transmitter  44  to a receiver  46  opposite the base  34  in the boat  14 . In the preferred embodiment the first sensor  26  has a transmitter  44  mounted in the base  34 . The transmitter  44  generates the photoelectric beam  45  from a light emitting diode, not shown. The receiver  46  is a reflective surface mounted on the top plate  42  wherein the reflective beam from the top plate  42  permits the first sensor  26  to generate a first electrical signal  48  when the wafers  32  are in correct alignment. This first electrical signal  48  is supplied to an input port  49  in the control unit  30 . Omron Electronics, Inc of Schaumburg, Ill., manufactures one such sensor, identified by their catalog number 3S-BR11.  
         [0023]    Another first sensor  26  may be a laser sensor with the transmitter in the base and the receiver in the top. The fine laser beam  45  can be guided along the peripheral surfaces of the wafers  32  so that if any wafer is not properly aligned or positioned, the receiver will generate a first electrical signal  48  indicating misalignment.  
         [0024]    Mounted to the base  34  and extending in a direction into the chamber  12  are a plurality of rails  50 , schematically shown in FIGS. 1 and 2, having means such as teeth, not shown, to support the wafers  32 . An example of such rails  50  may be found in U.S. Pat. No. 6,095,806 entitled “Semiconductor Wafer Boat and Vertical Heat Treating System” issued on Aug. 1, 2000, which is included herein by reference only.  
         [0025]    A second sensor  28  is mounted to the robot  18  for ensuring the position of the wafers  32  after transfer to the boat  14 . In particular, the second sensor  28  is an ultrasonic sensor that is physically mounted on the backside of the wafer-handling arm  52  of the robot  18  so as to check the wafer  32  after the robot has positioned it in the boat  14 . The second sensor  28  generates an ultrasonic signal of less than 300 kHz, typically approximately 270 kHz. If the wafer  32  is out of position, the reflective signal generates a second electrical signal  54  to an input port  55  of the control unit  30  to halt the operation of the robot  18 . Omron Electronics, Inc of Schaumburg, Ill. manufactures one such sensor, identified by their catalog number E4C-LS35 and has an amplifier identified as E4C-WHAL. If the wafer  32  is properly positioned the second electrical signal  54  is generated by the second sensor  28  and supplied to the control unit  30  for allowing continuing operation of the robot  18 . The algorithm  58  that is stored in the memory, as is well known in the art, functions to respond to the first  48  and second  54  electrical signals to allow the transferring and storing of the wafers  32 . In addition, the algorithm  58  in conjunction with the microprocessor  56  controls one or more output ports  60 ,  62 ,  64  to control the elevator  16 , the robot  18  and the cassette loader  22 .  
         [0026]    Since this is typically a digital electronic control system, the first and second electrical signals  48 ,  54  are pulsed signals and depending on the characteristic of the microprocessor  56  in the control unit  30 , the one or zero level is defined by the system designer.  
         [0027]    While the present invention has been described in an illustrative manner, it should be understood that the terminology used is intended to be in a nature of words of description rather than of limitation.  
         [0028]    Accordingly, various changes and modifications may be made to the illustrative embodiment without departing from the spirit or scope of the invention. It is to be appreciated that those skilled in the art will readily apply these teachings to other possible variations of the inventions. However, it is intended that the scope of the invention not be limited in any way to the illustrative embodiment shown and described but that the invention be limited only by claims appended hereto.