Abstract:
An apparatus for processing semiconductor wafers has an automatic robot for independently and simultaneously handling two wafers (W) at the same time on two separate transfer planes. The robot comprises a first arm assembly having a left and a right arm each mounted at one end for independent rotation in a first horizontal plane about a center vertical axis, the other ends of the arms being movably joined together and holding a blade on which a wafer can be carried, the arms being horizontally bendable near their centers so they can be folded to retract the blade toward the center axis and rotated to a desired angular position, the arms being extendable along a radius from the center axis by moving the arms together near their centers to bring the arms nearly parallel to each other and to extend the blade from the center axis by a maximum amount; and a second arm assembly substantially identical to the first assembly and rotatable in a second horizontal plane closely spaced above the first plane, the operation of the second assembly being substantially identical to that of the first assembly but independent thereof.

Description:
FIELD OF THE INVENTION 
     This invention relates to an automatically controlled robot (mechanical mechanism) having substantially improved capacity for transferring semiconductor wafers between stations in processing equipment for the manufacture of semiconductors. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of semiconductors, such as integrated circuits (ICs), dynamic random access memories (DRAMs), etc., large thin wafers (typically of silicon) from which the semiconductors are fabricated must frequently be transferred from one processing chamber to another. This transfer of wafers must be carried out under conditions of absolute cleanliness and often at sub-atmospheric pressures. To this end various mechanical arrangements have been devised for transferring wafers to and from processing chambers in a piece of equipment or from one piece of equipment to another. 
     It is the usual practice to load wafers into a cassette so that a number of them can be carried under clean-room conditions safely and efficiently from one place to another. A cassette loaded with wafers is then inserted into an input/output (I/O) chamber (“load lock” chamber) where a desired gas pressure and atmosphere can be established. The wafers are fed one-by-one to or from their respective cassettes into or out of the I/O chamber. It is desirable from the standpoint of efficiency in handling of the wafers that the I/O chamber be located in close proximity to a number of processing chambers to permit more than one wafer to be processed nearby and at the same time. To this end two or more chambers are arranged at locations on the periphery of a transfer chamber which is hermetically sealable and which communicates with both the I/O chamber and the processing chambers. Located within the transfer chamber is an automatically controlled wafer handling mechanism, or robot, which takes wafers supplied from the I/O chamber and then transfers each wafer into a selected processing chamber. After processing in one chamber a wafer is withdrawn from it by the robot and inserted into another processing chamber, or returned to the I/O chamber and ultimately a respective cassette. 
     Semiconductor wafers are by their nature fragile and easily chipped or scratched. Therefore they are handled with great care to prevent damage. The robot mechanism which handles a wafer holds it securely, yet without scratching a surface or chipping an edge of the brittle wafers. The robot moves the wafer smoothly without vibration or sudden stops or jerks. Vibration of the robot can cause abrasion between a robot blade holding a wafer and a surface of the wafer. The “dust” or abraded particles of the wafer caused by such vibration can in turn cause surface contamination of other wafers, an undesirable condition. As a result the design of a robot requires careful measures to insure that the movable parts of the robot operate smoothly without lost motion or play, with the requisite gentleness in holding a wafer, yet be able to move the wafer quickly and accurately between locations. Because of these complex requirements, previous robot mechanisms have been unable to handle more than one wafer at a time in the limited space provided within a reasonably sized transfer chamber. It is desirable to provide a robot able to independently handle two wafers at the same time thereby increasing the through-put of a wafer-processing apparatus. It is also desirable to be able to place such a dual-capacity robot within substantially the same size of transfer chamber as used with previous robots. This also permits ease of fitting a dual-capacity robot into wafer processing apparatus of prior design and size. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, in one specific embodiment thereof, there is provided an improved robot for handling semiconductor wafers and having twice the wafer-moving and transfer capability of previous robots. This improved robot includes a first arm assembly having a left and a right arm each mounted at one end for independent rotation in a first horizontal plane about a center vertical axis, the other ends of the arms being movably joined together and holding a blade on which a wafer can be carried, the arms being horizontally bendable near their centers so they can be folded to retract the blade toward the center axis and rotated to a desired angular position, the arms being extendable from the center axis by moving the arms together near their centers to bring the arms toward each other and to extend the blade from the center axis by a desired amount. The robot further includes a second arm assembly substantially identical to the first assembly and rotatable in a second horizontal plane closely spaced above the first plane, the operation of the second assembly being substantially identical to that of the first assembly but independent thereof. 
     A better understanding of the invention together with a fuller appreciation of its many advantages will best be gained from a study of the following description given in conjunction with the accompanying drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view partially broken away of a semiconductor wafer processing apparatus which includes a transfer chamber which houses an improved wafer-handling robot embodying features of the invention, together with an input/output (I/O) chamber and a plurality of processing chambers positioned around the periphery of the transfer chamber; 
     FIG. 2 is a perspective view partially broken away of a portion of the transfer chamber and of the improved robot embodying features of the invention; 
     FIG. 3 is a schematic side view of the improved robot of FIG. 2 with portions broken away and other portions shown in cross-section; and 
     FIG. 4 is a graph showing the improvement in wafer handling capability obtained by the improved robot embodying features of the present invention compared to the capability of a prior art robot having similar operating characteristics. 
     The drawings are not necessarily to scale. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is shown a schematic plan view, partially broken away, of a semiconductor wafer processing apparatus  10  including a transfer chamber  12 , an improved wafer-handling robot  14  embodying features of the invention and contained within the transfer chamber  12 , input/output (I/O) chambers  16  joined to the transfer chamber  12  at the periphery thereof, and four processing chambers  18  likewise joined to the transfer chamber  12  along its periphery. The I/O chambers  16  and the processing chambers  18  are well known in the art, as is the basic structure of the transfer chamber  12 . The robot  14  is not limited to use with particular kinds and numbers of such chambers. The robot  14  by way of example is attached to a floor or bottom wall  19  of the transfer chamber  12  and is sealed around an access opening (not shown here) in the floor  19  as will be explained hereinafter. A top wall or cover which covers the transfer chamber  12  is not shown. While shown here as circular, the transfer chamber  12  in certain applications may be elliptical. 
     The I/O chambers  16 , as illustrated here, are adapted to have attached to them respective wafer-holding cassettes  20 , two of which are shown, and each of which is capable of holding a number of wafers (not shown) on closely spaced vertical levels, or shelves, within the cassette. The cassettes  20 , as explained previously, provide a desirable way of carrying the wafers in clean-room condition from one piece of equipment, such as the apparatus  10 , to and from another location. Within each I/O chamber  16  is a mechanism (not shown and well known in the art) for moving a selected wafer on its respective shelf in a cassette  20  to a designated level at which the robot  14  can remove the wafer from the I/O chamber  16 . The robot  14  then brings the wafer into the transfer chamber  12  for subsequent insertion into a selected one of the processing chambers  20 . After processing, a wafer is removed by the robot  14  from a processing chamber  18  and returned to a selected I/O chamber  16  and thence to its respective level in a cassette  20 . Two wafers W, indicated in dotted outline in FIG. 1, are shown being held by the robot  14 . By way of example, a wafer W can be 300 millimeters (mm) in diameter, though the invention is not limited to use with any particular diameter of wafer. The I/O chambers  16  and the transfer chamber  12  are hermetically sealed off from each other by “slit valve” slots  22 , one for each cassette  20 , which slots  22  are located in a peripheral wall  23  of the transfer chamber  12  and are automatically opened and closed to permit the transfer of wafers to or from the chambers. Such slit valve slots  22  are well known in the art and are not further described herein. Similar slit valve slots  22  seal the transfer chamber  12  from the respective processing chambers  18 . The slit valve slots  22  are, by way of example, shown located at respective radii, indicated by dashed lines  24 , in the wall  23  at the respective entrances to the I/O chambers  16  and the processing chambers  18 . 
     Referring now to FIG. 2, there is shown partially broken away a perspective view of the transfer chamber  12  and of the robot  14  embodying features of the invention. The I/O chambers  16  and the processing chambers  18  are not shown. The robot  14  is aligned along a vertical center axis  25  and comprises a hub  26 , an upper or first pair of extendable arms  28  and  29  and a lower or second pair of substantially identical arms  31  and  32  which are inverted or turned upside down relative to the first pair of arms. This permits the pairs of arms to be spaced closely together in the vertical direction in parallel, horizontal planes (see also FIG.  3 ). The inner ends of the upper arms  28  and  29  are rigidly fixed respectively to an upper pair of separately rotatable narrow ring-like bodies  34  and  36  (see also FIG.  3 ), and the outer ends of these arms  28  and  29  are geared together by a wrist mechanism  38  which supports horizontally an upper wafer-holding blade  40  and holds it radially aligned. The lower pair of extendable arms  31  and  32  similarly have inner ends fixed respectively to a second, lower pair of narrow ring-like bodies  42  and  44  which are rotatable on bearings around the hub  26  and are spaced by a narrow-diameter vertical gap, indicated at  46  (see also FIG.  3 ), a short distance below the upper pair of ring-like bodies  34  and  36 . The outer ends of the lower arms  31  and  32  are geared together by a wrist mechanism  48  which supports horizontally a lower wafer-holding blade  50  and holds it radially aligned. The two wafer-holding blades  40  and  50  are aligned radially with the hub  26 , though each blade is extendable or retractable, and also rotatable, independently of the other blade. Each blade  40  and  50  has a front lip  52  and a rear shoulder  54  which engage the rim or edge of a wafer W (see FIG. 1) and position it on a respective blade. A retractable detent or finger mechanism (not shown) at each rear shoulder  54  of each blade  40  or  50 , and contained respectively within the wrist mechanism  38  or  48 , automatically engages the edge of a wafer W to help hold it in place when the wafer is being moved into or out of a chamber. Each detent mechanism is automatically disengaged by its wrist mechanism  38  or  48  to free the wafer to be lifted off of, or placed onto a respective blade  40  or  50  by further mechanism (not shown) when a blade is fully inserted into a chamber  16  or  18 . The upper and lower pairs of arms  28 ,  29  and  31 ,  32  are shown in FIG. 2 folded with their respective wrist mechanisms  38  and  48  partially retracted into the vertical hub gap  46 , thereby minimizing the inside diameter necessary for the transfer chamber  12 . Each of the arms  28 ,  29  and  31 ,  32  is provided near its center with a respective one of four elbow bearings  58  which permit the arms to bend easily in their respective horizontal planes to a folded position as shown, but these bearing  58  resist vertical play or up-and-down lost motion of the outer portions of the respective arms  28 ,  29  and  31 ,  32 . 
     The upper pair of arms  28 ,  29  can be extended (for example along one of the radii  24 ) to move its blade  40  through a selected slit valve slot  22  (and into one of the chambers  16 , and  18 ) by rotating the ring-like bodies  34  and  36 , by which the arms are supported, incrementally in opposite directions relative to each other and by the same amount (see also FIG.  3 ). Thus, rotating the body  34  counterclockwise about the axis  25  and at the same time rotating the body  36  clockwise will straighten out the arms  28  and  29  until they are nearly parallel to each other. This fully extends the wafer-holding blade  40  outward from the hub  26  along a radius  24  and through a selected slit valve slot  22 . Thereafter the arms  28  and  29  by opposite action of the ring-like bodies  34 ,  36  are folded back to the position shown to retract the blade  40  and permit it to be rotated to a different angular position for insertion into a selected chamber  16  or  18 . The wafer-holding blade  50  and the arms  31 ,  32  are similarly controlled by selective rotation of their respective ring-like bodies  42  and  44 . Because the separate horizontal blades  40  and  50  are so closely spaced vertically, each blade (and a wafer being held by it) can easily pass horizontally without interference through any slit valve slot  22 . The ordered sequencing in operations of the robot  14 , and its arms  28 ,  29  and  31 ,  32  and the slit valve slot  22  is controlled by a computer (not shown) and is well known in the art. 
     Referring now to FIG. 3, there is shown a schematic side view of the robot  14 , provided according to the invention, with portions broken away, other portions in cross-section, and still other portions schematically shown. The bottom of the hub  26  of the robot  14  is sealed around a circular access opening, indicated at  59 , in the floor  19  of the transfer chamber  12  (not otherwise shown here). The hub  26  near its top has first a thin, cylindrical vertical wall  60  of non-magnetic material such as aluminum, on top of which is fixed a sealing plate  62 . The bottom end of the thin wall  60  is fixed (and sealed to) an annular member  64  which forms the hub gap  46  (see also FIG.  2 ). The bottom end of the annular member  64  is sealed to the upper end of a second, thin cylindrical vertical wall  66  (also non-magnetic), axially aligned with the first wall  60 . The lower end of the second wall  66  is fixed and sealed to an annular disc  68  which in turn is sealed to the chamber floor  19  around the opening  58 . 
     The upper arms  28 ,  29  and the lower arms  31 ,  32  (shown partially broken away in FIG. 3) are substantially identical but are inverted relative to each other. These arms along their outer portions have horizontal flat surfaces which as shown in FIG. 3 face each other and are separated by a small vertical space indicated at  69 . This close spacing  69  permits the arms (and their respective wrists  38  and  48 ) to partially recess in the hub gap  46  (see also FIG.  2 ). The wafer-holding blades  40  and  50  (not shown in FIG. 3) are also close enough together, with respect to the vertical direction, that both of these blades easily fit through the slit valve slot  22  (see FIG.  2 ), as was mentioned previously. 
     The topmost rotatable ring-like body  34 , to which is fixed the arm  28  (see also FIG.  2 ), is rotatably supported on the hub  26  by a bearing assembly  70  which in turn is supported by an upper portion of the ring-like body  36  (to which the arm  29  is attached). The body  36  is rotatably supported by a bearing assembly  72  in turn supported by a fixed portion of the hub  26  just above the annular member  64 . The ring-like bodies  34  and  36  are thus able to rotate independently and opposite of each other, or in unison together, as was described previously. The two lower ring-like bodies  42  and  44  (attached to the arms  31 ,  32 ) are rotatably supported in a substantially identical way by a bearing assembly  74  and a bearing assembly  76 , and are similarly operable. 
     The topmost ring-like body  34  is rotatably coupled through the thin hub wall  60  (transparent to a magnetic field) via a magnetic coupling assembly  80  to the upper end of a vertical rotatable drive shaft  84 , aligned with the axis  25 , and extending downward through the hub  26  and through the opening  59  in the chamber floor  19  to a first servo motor  86 , The motor  86  is held within a vertical support frame  90  attached to the floor  19 ; a rotatable part of the motor  86  drives the shaft  84  in either direction and positions it (and the ring-like body  34 ) with great angular precision. The magnetic coupling assembly  80  (well known in the art) tightly couples the rotation of the shaft  84  to the ring-like body  34  so that there is no angular play or error in the rotation of the body  34 . The thin wall  60 , and the thin wall  66 , provide an hermetic seal between the rotating members inside and outside of the hub  26 . In substantially identical fashion described above with respect to the ring-like body  34 , the ring-like body  36  is rotationally coupled by a magnetic coupling assembly  92  to the upper end of a shaft  94  which extends downward to a servo motor  96 ; the ring-like body  42  is rotationally coupled via a magnetic coupling assembly  98  to the upper end of a shaft  100  which extends downward to a servo motor  102 , and the ring-like body  44  is rotationally coupled via a magnetic coupling assembly  104  to a short vertical shaft  106  which extends downward to a servo motor  108 . The magnetic coupling assemblies  80 ,  92 ,  98  and  104  are substantially identical to each other. The shafts  84 ,  94 ,  100  and  106  are concentric with each other, are aligned with the axis  25 , and are independently rotatable. The servo motors  86 ,  96 ,  102  and  108  are identical to each other, are independently rotatable, have hollow cores through which certain of the shafts  84 ,  94 ,  100  and  106  can pass, and are supported on the frame  90 . These servo motors are commercially available. Bearings associated with the respective motors and shafts are not shown. The operation of these motors is controlled by a computer (not shown) and is well known in the art. 
     Referring now to FIG. 4, there is shown a graph  120  with a vertical axis showing values of wafer throughput per hour (Wph) and a horizontal axis showing “chamber busy” time in seconds for a wafer processing apparatus having four processing chambers. The values of “Wph” are calculated using a combination of “chamber busy” times and times needed to transfer wafers between chambers. The graph  120  has a first curve  122  showing the improved productivity of an apparatus (such as the apparatus  10 ) utilizing an improved dual-wafer-handling robot (able to handle two wafers simultaneously such as the robot  14 ) provided by the invention, and a second curve  124  showing “Wph” of a similar apparatus but with a single-wafer-handling robot (able to handle only one wafer at a time). Below about 100 seconds of chamber busy time, the wafer throughput (Wph) provided by a robot embodying the invention, as indicated by the curve  122 , is markedly superior to the wafer throughput of a robot without the invention, as indicated by the curve  124 . And below about 50 seconds busy time, the wafer throughput provided by the invention is more than twice as great as throughput without the invention. 
     The above description of the invention is intended in illustration and not in limitation thereof. Various changes or modifications in the embodiment set forth may occur to those skilled in the art and may be made without departing from the spirit and scope of the invention as set forth in the accompanying claims.