Abstract:
A cassette position control device controls the position of a cassette in which wafers are stored, when the wafer cassette is loaded on a cassette support. In addition to the cassette support, the device has a cassette moving section and a control section. The control section senses the state of the wafer cassette on the cassette support, and generates and outputs a motor drive signal to move the cassette to a designated relative position on the cassette support. The cassette moving section moves the wafer cassette to the designated position in response to the motor drive signal. Therefore, wafers can be loaded/unloaded to and from the same designated position after the wafer cassette has been placed on a cassette support.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to semiconductor manufacturing equipment. More particularly, the present invention relates to a device for controlling the position of a cassette in which wafers are supported, after the cassette has been loaded on a cassette support. 
   2. Description of the Related Art 
   Semiconductor manufacturing equipment typically includes a robot for automatically transferring wafers one-by-one between a wafer cassette and a process chamber in which the wafers are processed. More specifically, the wafer cassette has slots for accommodating the wafers, respectively, and the wafer cassette itself is loaded onto a wafer cassette support. Each wafer is taken out of a respective slot of the wafer cassette and is transferred to the process chamber by the robot. Once the wafer is processed, the wafer is removed from the process chamber and then is returned to a slot of a wafer cassette by the robot. A detector installed in the wafer cassette support detects whether the wafer cassette is at a position which ensures that the wafers will be smoothly transferred by the robot. The detector uses a mechanical device, such as a contact switch, to physically detect for the presence of the wafer cassette at a predetermined location on the wafer cassette support. 
   However, a detector relying on a mechanical elements has a very limited operability. Accordingly, a wafer cassette support having a detector that can detect for the presence of a wafer cassette with a higher degree of reliability has been developed. Such a prior art wafer cassette support and detector, as well as the wafer cassette itself, will be described with reference to  FIGS. 1 to 3 . 
   The wafer cassette  2  has the general form of a box of which one side, e.g., the front, has an opening  2   a  through which wafers W are loaded into or are taken out of the cassette. The wafer cassette  2  also has a pair of opposing inner walls. Each of the inner walls is made up of a plurality of wafer support projections  3  spaced vertically one above the other. The wafer support projections  3  define a plurality of slots  4  sized to accommodate the wafers W, respectively. An outer peripheral part of a wafer W is supported by a pair of the wafer support projections  3  while the wafer is accommodated in one of the slots  4 . The back of the wafer cassette  2  includes a pair of protrusions  5  that extend rearwardly of the cassette  2 . The bottom of the wafer cassette  2  includes a horizontal protrusion  6  extending laterally (in a direction between the sides of the cassette having the protrusions  3 ). 
   The wafer cassette support  1  has a base plate  7  formed of stainless steel, aluminum or the like. Four position fixation guides  8  disposed on the base plate  7  guide the wafer cassette  2  to a certain position on the base plate  7 . To this end, the upper portion of each position fixation guide  8  has a bevel  8   a  that easily guides the wafer cassette  2  as the cassette  2  is lowered onto the base plate  7 . The bevels  8   a  also serve to prevent the wafer cassette  2  from being damaged as the wafer cassette  2  is placed on the base plate  7 . 
   As shown best in  FIG. 3 , the position fixation guides  8  are adapted to respectively guide an inner wall surface F 1  of a front left end part of the wafer cassette  2 , an inner wall surface Fr of a front right end part of the wafer cassette  2 , an inner wall surface R 1  of a back left end part (left side protrusion  5 ) of the wafer cassette  2 , and an inner wall surface Rr of a back right end part (right side protrusion  5 ) of the wafer cassette  2 . As is clear form the figure, this arrangement of the position fixation guides  8  does not compromise the space efficiency of the base plate  7 . 
   A cassette retainer  9  is disposed at the center of the base plate  7 . The cassette retainer  9  defines a guide recess  10  sized to receive the horizontal protrusion  6  ( FIG. 2 ) at the bottom of the wafer cassette  2 . Movement of the wafer cassette  2  in forward and backward directions is restrained by the cassette retainer  9  when a wafer cassette  2  rests on the base plate  7  with the protrusion  6  received in the recess  10 . On the other hand, the wafer cassette  2  can slide laterally as guided by the position fixation guides  8  while the protrusion  6  is received in the recess  10 . 
   Two of the position fixation guides  8 S disposed diagonally across from each other, namely the position fixation guides for guiding the front left end part of the wafer cassette  2  and the back right end part of the wafer cassette  2 , each have a slit  11  through which light can pass. The slit  11  is in the form of a tunnel extending through the position fixation guide  8 S and open at the lower surface of the position fixation guide  8 S. Optical detection switches  12  are provided adjacent the two position fixation guides  8 S, respectively. Each switch  12  is adapted to check whether a wafer cassette  2  is present at a certain position on the base plate  7 . 
   In particular, each optical detection switch  12  includes a light emitting part  12 A for emitting visible light, for example, and a light receiving part  12 B for receiving light emitted by the light emitting part  12 A. The light emitting part  12 A may comprise a light-emitting diode (LED) or a laser diode, and the light receiving part  12 B may comprise a photo diode (PD). The light emitting part  12 A and the light receiving part  12 B face each other across the slit  11  of a position fixation guide  8 S. Thus, if a wafer cassette  2  rests on the base plate  7  but is not present at a predetermined position relative to the base plate  7 , light from a light emitting part  12 A passes through the slit  11  of a position fixation guide  8 S and reaches the light receiving part  12 B. On the other hand, if a wafer cassette  2  is present at the predetermined position relative to the base plate  7 , the light from a light emitting part  12 A passes through the slit  11  but is blocked by the wafer cassette  2 , i.e., the light does not reach the light receiving part  12 B. Also, when a gap exists between the wafer cassette  2  and the base plate  7  after the wafer cassette  2  has been loaded onto the base plate  7 , light from a light emitting part  12 A passes through a slit  11 , under the wafer cassette  2  and then impinges the associated light receiving part  12 B. Thus, the wafer cassette  2  is detected as not being at a predetermined position on the base plate  7 . 
   Furthermore, beneficially, the light emitting part  12 A is disposed adjacent the side of the position fixation guide  8 S which is not used to guide the wafer cassette  2  onto the base plate  7 . Conversely, the light receiving part  12 B is disposed at the side of position fixation guide  8 S having the bevel  8   a , namely, the side used to guide the wafer cassette  2  onto the base plate  7 , as spaced therefrom. In the case in which the light emitting part  12 A comprises an LED, for example, light from the light emitting part  12 A is not scattered and passes through the slit  11  with good efficiency because the light emitting part  12 A is disposed close to the position fixation guide  8 S. 
   Regardless of the precision offered by the optical detectors  12 , the four position fixation guides  8  are fixed. Furthermore, the position fixation guides  8  do not abut the wafer cassette  2  when the cassette  2  is supported by the base plate  7 ; rather, gaps are present between the external walls of the cassette  2  and the position fixation guides  8  to allow for some latitude in the operation of setting the cassette  2  on the base plate  7 . Therefore, the position of the wafer cassette  2  can change or shift whenever the wafer cassette  2  is placed on the base plate  7 . Therefore, the wafer can be broken if the wafer cassette  2  is not positioned precisely relative to the base plate  7  when a cassette station (C/S) arm of the robot loads or unloads a wafer into or from the cassette  2 . 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a cassette position control device that always places a wafer cassette at a desired position whenever a wafer cassette is loaded onto a base plate in semiconductor manufacturing equipment. 
   According to one aspect of the invention, a cassette position control device for use in semiconductor manufacturing equipment comprises a cassette support, a control section, and a cassette moving section. The control section senses a state of the wafer cassette after the cassette has been loaded on the cassette support and outputs a motor drive signal once a certain state of the wafer cassette has been sensed. The cassette moving section moves the wafer cassette to a designated position when the motor drive signal has been outputted from the control section. 
   The control section includes a position sensor situated relative to the base plate so as to sense whether a wafer cassette is at a stably supported position on the wafer cassette, and a controller operatively connected to the position sensor so as to generate and output the motor drive signal when the position sensor senses the presence of a wafer cassette at a stably supported position on the base plate. The cassette moving section includes a motor operatively connected to the controller so as output a driving force when the motor drive signal is received from the controller, and a movable first wafer cassette guide connected to the motor so as to be driven by the driving force output by the motor. 
   According to another aspect of the invention, in addition to the aforementioned first cassette guide, the cassette moving section of the cassette position control device comprises a stepping motor, a ball screw connected with the stepping motor, and an LM (Linear Movement) guide which is engaged with the ball screw so as to convert rotational movement of the ball screw into linear movement. The first cassette guide is fixed to the LM guide so as to move therewith. 
   In this case, the cassette support also includes a fixed second cassette guide disposed spaced from the first cassette guide in a forward direction of movement thereof. The second cassette guide serves to establish the designated position to which the wafer cassette is moved. 
   In addition, the control section may comprise a photo sensor for sensing whether the wafer cassette is stably positioned on the cassette support. The controller of the control section is operatively connected to the photo sensor for driving the stepping motor in a forward direction when the photo sensor senses that the wafer cassette is stably positioned on the cassette support. The control section preferably also includes a bracket that is fixed to the LM guide, first and second limit position sensors for respectively detecting forward and limit positions of the first cassette guide through the movement of the bracket, and at least one guide sensor integrated with the second cassette guide to detect when the wafer cassette arrives at the second cassette guide and contacts the same. 
   According to another exemplary aspect of the invention, a method of positioning a cassette for use in semiconductor manufacturing equipment comprises loading the wafer cassette onto the base plate of a wafer cassette support, and subsequently moving a first cassette guide in a forward direction relative to the base plate while the first cassette guide is engaged with the wafer cassette to thereby move the wafer cassette along the base plate to a designated position. 
   Preferably, the wafer cassette is moved along the base plate only after the wafer cassette is sensed as being stably positioned on the base plate. The cassette is moved by operating a stepping motor in a forward direction. The operation of the stepping motor is stopped once the presence of the wafer cassette at the designated position has been sensed. Subsequently, a counting operation is initiated and the stepping motor is operated in a backward direction after a predetermined period of time has elapsed to thereby return the cassette guide to an initial position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description of the preferred embodiments thereof that follows as made with reference to the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a prior art wafer cassette support and of a typical wafer cassette supported thereby; 
       FIG. 2  is a bottom view of the wafer cassette shown in  FIG. 1 ; 
       FIG. 3  is a plan view the wafer cassette support shown in  FIG. 1  in a state in which a wafer cassette is disposed thereon; 
       FIG. 4  is a perspective view of a wafer cassette, and of a wafer cassette position control device for use in aligning the cassette in semiconductor manufacturing equipment according to the present invention; 
       FIG. 5  is a schematic diagram of a cassette position control section of the wafer cassette position control device according to the present invention; and 
       FIG. 6  is a flow chart of a method of controlling the positioning of a cassette according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described in detail with reference to  FIGS. 4 to 6 . For purposes of clarity, though, a detailed description of known functions and systems of the semiconductor manufacturing equipment to which the present invention relates has been omitted. 
   Referring first to  FIG. 4 , a wafer cassette  20  can accommodate a plurality of wafers. To this end, the wafer cassette  20  has essentially the same structure as that described in connection with  FIG. 1 . Briefly, then, the wafer cassette  20  has the general form of a box. One side of the cassette, e.g., the front, defines an opening  20   a . Wafers W are inserted through or taken out of the cassette  20  through the opening  20   a . The wafer cassette  20  also has a pair of inner side walls comprising a plurality of wafer support projections  22  spaced vertically from one another along each wall by uniform intervals. Thus, the wafer support projection parts  22  define a plurality of slots  24  therebetween. The outer peripheral edge of a wafer W is supported by an opposing pair of the wafer support projections  22  while the wafer W is accommodated in a respective one of the slots  24 . The back of the wafer cassette  20 , on the other hand, comprises a pair of protrusions  26 . the bottom of the wafer cassette  20  includes a horizontal protrusion ( 60  in  FIG. 5 ) extending laterally, i.e., in a direction between the sides of the cassette  20  having the projections  22 . 
   A wafer cassette position control device for aligning the cassette, e.g., moving the cassette to a predetermined position relative to a transfer robot or the like, includes a cassette support  30  used to support the wafer cassette  20 . The cassette support  30  has a base plate  32  formed of stainless, aluminum or the like. First and second cassette guides  34  and  36  are disposed on the base plate  32  to position a cassette  20  thereon. More specifically, the first cassette guide  34  is part of a cassette moving section of the device and is movable to guide a wafer cassette  20  against the second cassette guide  36  once the wafer cassette  20  has been placed on the cassette support  30 . To this end, the base plate  30  has a guide slot  41  therein and along which the first cassette guide  34  can be moved toward and way from the second cassette guide  36 . A third cassette guide  38  is disposed at a central portion of the base plate  32  to fix the wafer cassette  20  relative to the base plate  32 . The center of the third cassette guide  38  defines a recess  40  into which the protrusion at the bottom of the wafer cassette  20  is inserted. Also, a photo sensor ( 62  in  FIG. 5 ) is installed in a central portion of the guide flute  40  to detect whether the wafer cassette  20  is stably mounted to the cassette support  30 . That is, the photo sensor senses whether the protrusion  60  of the wafer cassette  20  is properly received in the recess  40  of the third cassette guide such that the cassette  20  is stably supported by the base plate  32 . 
   Referring to  FIG. 5 , in addition to the first cassette guide  34 , the cassette moving section of the wafer cassette position control device also includes a stepping motor  44 , a ball screw  56  connected to the stepping motor  44  so as to be rotated by the stepping motor  44 , and an LM (Linear Movement) guide  54  through which the ball screw  56  extends as meshed therewith so as to convert rotation of the ball screw  56  into linear (horizontal) movement. The ball screw  56  extends in the direction of the guide slot  41  ( FIG. 4 ) of the base plate  32  so that the LM guide  54  is driven by the ball screw  56  in the direction of the guide slot  41  when the stepping motor  44  is operating. The LM guide  54  is integrated with the first cassette guide  34  so that the first cassette guide  34  is moved in conformity therewith. The wafer cassette position control device also includes a bracket  58  that is fixed to the LM guide  54 , and first and second limit position sensors  46  and  48  for detecting limit positions of the first cassette guide  34  through contact with the bracket  58 . Furthermore, the device includes a controller  42 , and first and second guide sensors  50  and  52  that are disposed on the second cassette guide  36  to detect whether the wafer cassette  20  is disposed against the second cassette guide  36 . The first and second guide sensors  50  and  52  may comprise proximity sensors, pressure sensors or limit switches. 
   Basically, the controller  42  controls the stepping motor  44  to rotate the ball screw  56  in one (a forward) direction once the photo sensor  62  senses the presence of the guide protrusion  60  of the wafer cassette in the guide recess  40  of the third cassette guide  38 . The controller  42  also stops the stepping motor  44  when the second limit position sensor  48  senses the arrival of the bracket  58  at a forward limit position or the first and second guide sensors  50  and  52  sense the presence of the wafer cassette  20  adjacent the second cassette guide  36  (indicative of the transfer of the cassette  20  to a designated position). Then, the controller  42  controls the stepping motor  44  to rotate the ball screw  56  in the other (reverse) direction, and stops the stepping motor  44  when the first limit position sensor  46  senses the arrival of the bracket  58  at a backward limit position. 
   Hence, the cassette moving section of the wafer cassette position control device comprises the stepping motor  44 , the ball screw  56 , the LM guide  54  that is integrated with the ball screw  56 , and the first cassette guide  34  that is connected to the LM guide  54  and is moved by the LM guide  54  to guide the wafer cassette  20  to a designated position relative to the base plate  32 . On the other hand, a control section of the wafer cassette position control device comprises the controller  40 , the sensors/detectors operatively connected thereto, and the bracket  58 . 
   A more detailed operation of the present invention will now be described referring to  FIGS. 4 to 6 . 
   First, a wafer cassette  20  is loaded onto the wafer cassette support  30  (step  101 ). In this respect, the wafer cassette  20  can transported by hand or by robot to the cassette support  30 . Next, the controller  42  checks whether the photo sensor  62  has sensed the wafer cassette  20 , i.e., has sensed the presence of the protrusion  62  of the cassette  20  within the guide recess  40  as an indicator of whether the cassette  20  is stably supported by the wafer cassette support  30  (step  102 ). If the wafer cassette  20  is not detected as being stably supported by the wafer cassette support  30 , the controller  42  generates an alarm and stops the operation of equipment (step  103 ). On the other hand, if the wafer cassette  20  is detected as being stably supported by the wafer cassette support  30 , the controller  42  drives the stepping motor  44  in a forward direction (step  104 ). As a result, the ball screw  56  rotates, thus moving the LM guide  54  and the first cassette guide  34  linearly in a first direction. At this time, the first cassette guide  34  guides the wafer cassette  20  towards the second cassette guide  36 . The first and second guide sensors  50  and  52  installed on the second cassette guide  36  detect when the wafer cassette  20  arrives at the second cassette guide  36 . Meanwhile, the controller  42  also monitors for the arrival of the bracket  58 , connected to the LM guide  54 , at a forward limit position as sensed by second limit position sensor  48  (step  105 ). 
   If the presence of the bracket  58  at the forward limit position is detected, the controller  42  generates an alarm and the stepping motor  44  is shut down (step  106 ). On the other hand, as long as the bracket  58  is not detected as being present at the forward limit position by the second limit position sensor  48 , the controller  42  checks whether the wafer cassette  20  is adjacent the second cassette guide  36  as detected by the first and second guide sensors  50  and  52  (step  107 ). The controller  42  continues to monitor for the arrival of the bracket  58  at the first limit position as long as the wafer cassette  20  has not arrived at the second cassette guide  36  (step  105 ). Once the arrival of the wafer cassette  20  at the second cassette guide  36  has been detected, the stepping motor  44  is stopped (step  108 ). Then, the controller  42  initiates a timing operation to allow for a predetermined period of time, e.g., 2 seconds, to elapse (step  109 ). Once the predetermined period of time has elapsed, the controller  42  drives the stepping motor  44  in a reverse direction (step  110 ). As a result, a ball screw  56  is rotated in reverse, the LM guide  54  is moved horizontally, and the first cassette guide  34  is moved horizontally away from the second cassette guide  36 . At this time, the controller  42  checks whether the bracket  58  connected to the first cassette guide  34  has arrived at a rearward position as sensed by the first limit position sensor  46  (step  111 ). Once the bracket  58  contacts the first limit position sensor  46 , the controller  42  stops the stepping motor  44  (step  112 ). At this time, the cassette position control device assumes a stand-by state in which the wafer cassette  20  can be removed from the wafer support  30  and the next wafer cassette can be loaded onto the wafer support  30 . 
   Though according to an exemplary embodiment of the invention, it was described above that a controller  42  sensed a loading of wafer cassette  20 , a stepping motor  44  was driven so that the wafer cassette  20  became adhesive to a second cassette guide  36  by moving a first cassette guide  34 , and a movement completion state was sensed by first and second guide sensors  50  and  52 , thus the stepping motor  44  was stopped; according to another exemplary embodiment of the invention, a stepping motor  44  can be driven for a predetermined time and then stopped after sensing a loading of wafer cassette  20 , thereby a position of wafer cassette  20  can be controlled, without deviating from a spirit of the invention. That is, instead of sensing a designated position of wafer cassette  20  through first and second guide sensors  50  and  52 , a stepping motor  44  can be driven only for a predetermined time and then stopped. 
   According to the present invention as described above, a wafer cassette is loaded on a cassette support, and then is moved to a designated position. Thus, the wafers can always be loaded/unloaded to and from the same relative position by a robot (C/S ARM) or the like. Hence, the wafers can be prevented from being broken. 
   Finally, modifications and variations of the preferred embodiments present invention will be apparent to those skilled in the art. Thus, these and other changes and modifications are seen to be within the true spirit and scope of the invention as defined by the appended claims.