Abstract:
An apparatus for holding and orienting a wafer having an alignment feature, and including a movable robot arm; and an end effector attached to an end of the robot arm, the end effector including a gripping mechanism which during operation both holds the wafer and rotates it about an axis that is perpendicular to the plane of the wafer and a sensing element for detecting the alignment feature on the wafer as the gripping mechanism rotates the wafer past the sensing element.

Description:
TECHNICAL FIELD 
     This invention relates to end effectors for robotic handlers such as might be used in materials processing, e.g. semiconductor wafer processing. 
     BACKGROUND 
     Robotic handlers are commonly used to move materials, e.g. semiconductor wafers, between different stages of a wafer fabrication process. For example, robotic handlers might be used to move the wafer, from a plasma etch station in a cluster tool to a deposition station or from a manufacturing station to a testing station. At some stages of the manufacturing process, the wafer that is delivered by the robotic handler must be in a known orientation. For example, if the stage involves a masking process, the orientation of the wafer is critical since the mask must be aligned with the previously formed patterns on the wafer. To achieve the proper alignment, the robotic handler typically moves the wafer to something referred to as a pre-aligning station. After the wafer is deposited at this station, the pre-aligner positions the wafer and rotates it to a predetermined orientation. Then, the robotic handler picks up the oriented wafer and moves it to the next processing stage. 
     A typical robotic handler includes an end effector and a robotic arm. The end effector is the part of the robotic handler that holds the wafer. The arm includes the mechanical mechanisms that are used to move the end effector and the wafer which it holds to the desired location. 
     SUMMARY 
     In general, in one aspect, the invention is an apparatus for holding and orienting a wafer having an alignment feature. The apparatus includes a movable robot arm; and an end effector attached to an end of the robot arm. The end effector includes a gripping mechanism which during operation both holds the wafer and rotates it about an axis that is perpendicular to the plane of the wafer and a sensing element for detecting the alignment feature on the wafer as the gripping mechanism rotates the wafer past the sensing element. 
     Other embodiments of the invention include one or more of the following features. The gripping mechanism includes a first contacting member, a second contacting member, and a drive element which increases and decreases the space between the first and second contacting members in response to a control signal. The first and second contacting members are arranged to grip opposing edges of the wafer. The first contacting member includes a first roller element and the second contacting member includes second and third roller elements separated in space from each other. The gripping mechanism also includes an mechanical actuator which is coupled to and moves the first roller element towards and away from the second and third roller elements. The first roller element has a cylindrically-shaped outer surface with a circumferential groove formed therein. The first, second and third roller elements are arrayed in a common plane and have parallel axes of rotation. The gripping mechanism further includes a drive motor which rotates the first roller element. 
     Other embodiments also include the following additional features. The sensing element includes a light emitter and a light detector. The light emitter and the light detector are positioned to lie on opposite sides of the wafer when the wafer is being held by the gripping mechanism. The sensor senses the orientation by detecting a feature on the perimeter of the wafer as the wafer is rotated. 
     The invention has one or more of the following advantages. The end effector allows a user to align the wafer without transferring the wafer to a separate pre-aligner. This eliminates the loss in alignment precision that might otherwise result from transferring the wafer from the pre-aligner to the end-effector. This also tends to reduce or eliminate possible contamination to the wafer that might tend to result from the additional contact with the pre-aligner. Eliminating the wafer transfers from the robot arm to the pre-aligner and back also saves time, thus increasing processing throughput. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1A shows a robotic handler holding a wafer; 
     FIG. 1B shows the robotic handler of FIG. 1A, without the wafer; 
     FIG. 2 shows details of the rollers shown in FIG. 1A; 
     FIG. 3 shows the robotic handler of FIG. 1A with the control printed circuit board removed to reveal inner components of the end effector mechanism; 
     FIG. 4 is a vertical cross section of the end effector of FIG. 1A; 
     FIG. 5 shows the details of the sensor of FIG. 1A; 
     FIG. 6 shows a wafer rack for holding the wafer of FIG. 1A; and 
     FIG. 7 is a flow chart of the operation of the end effector. 
     Like reference symbols in the various drawings indicate like elements. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1A and 1B, a robotic handler  7  for moving a wafer  1  has two primary components, namely, a robot arm  19  and an end effector  20  attached to one end of robotic arm  19 . End effector  20  is used to grab and hold wafer  1  and robotic arm  19 , which includes various motors and mechanical mechanisms not shown in the figures, moves end effector  20  and the wafer that it holds within its grasp. 
     Wafer  1  is typically a circular disk of semiconductor material, e.g. silicon. It generally is of uniform thickness and has an alignment feature  3  at one location on its circumference. Alignment feature  3  is typically a flat portion or it is a v-shaped notch, as depicted in FIG.  1 . The alignment feature serves as a reference that can be used to align the wafer to a known orientation. As will be described in greater detail below, end effector  20  has a drive mechanism for rotating wafer  1  as it is being held by the end effector and it has sensor circuitry for detecting the alignment feature and thereby determining and establishing the orientation of wafer  1 . 
     In the described embodiment, the drive mechanism includes two idler rollers  21   a  and  21   b  and a drive roller  21   c . Idler rollers  21   a  and  21   b  are mounted at the remote ends of corresponding supporting rods  50   a  and  50   b . Rollers  21   a  and  21   b  are supported by bearings (not shown) on corresponding support pins so that they freely rotate. Referring to FIG. 3, drive roller  21   c  is mounted on a drive roller housing  26   a . More specifically, drive roller  21   c  is mounted on a shaft that is itself supported by bearings in the housing. The two idler rollers  21   a  and  21   b  and the drive roller  21   c  are arrayed in a common plane and have parallel axes of rotation. 
     Referring to FIGS. 3 and 4, drive roller housing  26   a  pivots at one end about a pin  26   b . A gripper actuator cylinder  25  (e.g. a linear motor or a hydraulically operated device) includes a shaft  27 , which moves in and out of a cylinder  29  in response to a control signal. The far end of actuator shaft  27  is connected to housing  26   a  by means of a pin  25 . Thus, the in and out movement of shaft  27  of actuator  24  in response to the control signal causes housing  26   a  to rotate about pin  26   b  and, in turn, causes drive roller  21   c  to move, respectively, towards and away from the two idler rollers  21   a  and  21   b.    
     When actuator shaft  27  is retracted into cylinder  29 , the separation between drive roller  21   c  and the other two rollers  21   a  and  21   b  becomes larger enough to accept wafer  1 . Once wafer is located within an area defined by the three rollers  21   a-c , actuator shaft  27  is extended out of cylinder  24 , thereby pushing drive roller  21   c  toward the other two rollers until all three rollers contact the outer periphery of and hold wafer  1 . Rollers  21   a-c  are positioned so that they contact the periphery of wafer  1  at locations which are separated sufficiently from each other so that wafer readily slides into the grasp of the rollers and is held securely there. 
     The construction of roller  21   a  is shown in greater detail in FIG.  2 . The other two rollers  21   b  and  21   c  are constructed similarly. Roller  21   a  has a substantially cylindrical outer rim  26 , which includes a positioning groove  41  formed around its outer circumference. When the rim of the roller is brought into contact with the periphery of the wafer, positioning groove  41  receives and holds the edge of the wafer thereby preventing the wafer from sliding either up or down on the roller. Since all three rollers  21   a-c  have a similar positioning groove, when the rollers are contacting the periphery of the wafer and the wafer sits in the corresponding positioning grooves of the three rollers, the plane of the wafer is fixed and precisely determined. The outer surfaces of the rollers are made form a plastic that does not contaminate the wafer. 
     As is shown more clearly in FIG. 4, the mechanism for rotating the drive roller  21   c  includes a drive motor  30  that is also mounted on drive housing  26   a . Drive motor  30  is a servo-controlled motor that, has a drive shaft  61 , which extends down through housing  26   a . Attached to the other end of shaft  61 , below housing  26   a , there is a drive motor pulley  31 . Drive roller  21   c  is mounted on another shaft  63  that is rotatably supported in housing  26   a  by bearings  33 . At the other end of drive roller shaft  63  there is another pulley  32 . The two pulleys  31  and  32  are connected to each other by a belt (not shown). Thus, drive motor  30  causes drive roller  21   c  to rotate. And when drive roller  21   c  is contacting the periphery of wafer, it causes the wafer to rotate within the grasp of the three rollers. 
     The end effector  20  has an optical sensing system  21  (shown in greater detail FIG. 5) for detecting the presence of the alignment feature  3  on the wafer  1  as it passes by while the wafer is being rotated. Sensing system  21  has an upper arm  65  that contains the light emitting components and a lower arm  67  that contains the light detecting components. When the wafer is being held by rollers  21   a-c , the edge of the wafer lies between upper and lower arms  65  and  67 . Upper arm  65  includes a light emitting diode  35  (shown in phantom) that is used to illuminate the edge of the wafer. The light from diode  35  passes to a collimator optic  36  that, in turn, directs the light down toward the wafer. The aperture of collimator optic  36  is narrow and long, with its longer dimension oriented perpendicular to the edge of the wafer. Lower arm  67  includes a silicon diode receiver  37  which has a detecting window that is also long and narrow, like collimator optic  36 , and is aligned with the aperture of the collimating optic  36 . The signal generated by diode receiver  37  is proportional to the amount of light from collimator optic  36  that reaches it. 
     When wafer  1  rotated within the grasp of end effector  20 , the edge of the wafer passes between the light emitting and light detecting components. Optical housing  22  is positioned so that the edge of the wafer prevents some of the light from collimator optic  36  from reaching diode receiver  37 . When the alignment feature passes between the light emitting and light detecting components, more light is allowed to reach diode receiver  37  and its output signal increases. And as the alignment feature moves past the sensor, the signal decreases to its previous value. Thus, by monitoring the output signal of the diode receiver, the electronics can detect the presence of the alignment feature, can determine its precise angular location as a function of the rotational position of the wafer, and can precisely align the angular orientation of the wafer. 
     The techniques for determining the angular location of the alignment feature and then aligning the wafer based on that information are well known to persons skilled in the art. Such techniques are typically used in connection with standalone pre-aligners of the type briefly mentioned earlier. An example of one such technique that can be used is described in U.S. Pat. No. 4,457,664, entitled “Wafer Alignment Station” and incorporated herein by reference. 
     Referring back to FIG. 1, end effector  20  also includes a control processor on a printed circuit board  23  which implements the electrical control functions that are necessary. For example, it generates the control signals for the drive motor and the actuator and it analyzes the sensing signal to determine and establish the orientation of the alignment feature of the wafer. 
     Referring to FIG. 6, a typical use of the end effector is to grab wafers from a wafer storage rack  70  and then transfer them to a masking station (not shown). Generally, rack  70  has a wafer holder  71  mounted on a platform  72  that can be displaced in a direction z. The wafer holder holds wafers  60   a-c , which are spaced apart by spaces  61   a ,  61   b.    
     Referring to FIG. 7, robotic handler  7  operates as follows to remove wafers from rack  70  and align them. Robotic handler inserts end effector  20  in the space below the wafer that will be grabbed. At some point in the operation, the control processor retracts the drive roller thereby opening up the space for receiving the wafer from the rack (block  101 ). When the end effector is in position and the roller is retracted enough to provide enough space for receiving the wafer, the platform holding the wafer lowers the wafer until it is within the plane of the three rollers on the end effector (block  102 ). When the wafer is aligned with the grooves of the rollers, the control processor then advances the driver roller towards idler rollers until the rollers snugly grip outside periphery (i.e., the edge) of the wafer (block  103 ). The robot arm can then move the wafer out of the wafer holder. 
     After the rollers have grabbed the wafer, the control processor uses the drive motor to rotate the wafer (block  104 ). As the end effector is rotating the wafer, it also monitors the diode receiver signal to detect the presence of the alignment feature and determine its precise angular orientation relative to the rotation of the wafer (block  105 ). The control processor uses the information obtained from the sensor to then orient the wafer such that the alignment feature is positioned in a predetermined angular position (block  106 ). 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, other kinds of sensors may be used to sense the orientation of the wafer. The sensors may detect the presence of the alignment feature by physical contact, magnetic fields, or capacitance, just to name a few possible ways. In addition, more than three rollers may be used to grasp the periphery of the wafer and the transport mechanism for rotating the wafer. Alternatively, other moving surfaces, such as a belt, may be used instead of rollers. Accordingly, other embodiments are within the scope of the following claims.