Patent Publication Number: US-6213136-B1

Title: Robot end-effector cleaner and dryer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 08/757,698, filed Dec. 3, 1996, now U.S. Pat. No. 6,024,101 which is a division of application Ser. No. 08/680,739, filed Jul. 15, 1996 now U.S. Pat. No. 5,778,554. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a device for rinsing and drying flat substrates such as semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     In recent years it has become common in the semiconductor industry to polish the wafers after successive layers of conductive traces or other structures are formed on their surfaces. This produces very smooth surfaces for repeated photolithographic processes and significantly improves the yield. The polishing process, typically chemical-mechanical polishing (CMP), leaves grit and other debris on the surface of the wafer which must be removed before fabrication process can continue. This has led to the development of wafer cleaning devices, such as the device described in co-pending application Ser. No. 08/683,654, which is incorporated herein by reference in its entirety. Following cleaning, the wafers must be thoroughly dried before they can be returned to the fabrication line. 
     Wafer spin dryers use a combination of centrifugal force and air flow to remove all moisture from the surface of the wafer. Two known types of spin dryers are illustrated in FIGS. 1 and 2. In the version shown in FIG. 1, a wafer  10  is held by fingers  12  above a platen  13 . Platen  13  is rotated on a shaft  14  by a spin motor  15 . Rinse water is applied from above by one or more nozzles  16  to the “good” (active) side of wafer  10 . A problem with this type of dryer is that the drying chamber  17  is open to the atmosphere, which while normally quite clean still contains particulate matter. Particles which fall onto the wafer during and immediately following the drying operation will remain when the next process step begins. 
     In the prior art dryer shown in FIG. 2, the drying chamber  20  is not open to the atmosphere from above, and the wafer  10  is held by fingers  21  below the platen  22 . The platen is spun by a motor  23  which is mounted above the platen. The good side of the wafer faces downward, and one or more nozzles  24  rinse wafer  10  from below. A problem with this type of dryer is that water droplets which are thrown from the spinning wafer can strike the surface of the drying chamber and splash against the good side of the wafer. 
     Thus there is a need for a wafer dryer which avoids the above problems and reliably yields a clean, dry wafer suitable for further processing. 
     SUMMARY OF THE INVENTION 
     The wafer spin dryer of this invention includes a platen and a plurality of holding members or fingers which extend downward from the platen. The wafer is held with its good or active side facing upward. One or more nozzles are positioned so as to direct a rinse liquid (typically water) against the good side of the wafer. The rinsing liquid is applied to the good side of the wafer, and the wafer is rotated to create a centrifugal force which removes the liquid from the good side of the wafer. A surface laterally adjacent the edges of the spinning wafer is contoured and angled such that the liquid which flies from the wafer is directed downward to a portion of the drying chamber below the wafer. As a result, the used rinse liquid cannot come into contact with the good side of the wafer. 
     The wafer is preferably placed in the spin dryer by a robot. In the preferred embodiment, the wafer is held in the spin dryer by three fingers which extend downward from the platen and which are spaced at equal (120°) angles around the axis of rotation. The fingers contain notches or other concave surfaces which grip the edge of the wafer. One of the fingers is movable to allow the robot to place the wafer in a position where it can be held by the three fingers. The wafer is positioned slightly eccentric to the axis of rotation such that it is forced against the two fixed fingers as it is rotated. The rotating mass (wafer and platen) is balanced as a whole so that undue vibrations do not occur as the wafer is rotated. 
     In many situations, the same robot arm places the wafer into the cleaner, transfers the wafer from the cleaner to the dryer, and transfers the wafer from the dryer to the finished wafer cassette. The end-effector of the robot arm can thus become contaminated with grit and chemicals. As another aspect of this invention, the wafer dryer contains a separate chamber which is used to clean and dry the end-effector while the wafer is being dried. Thus, when the wafer is withdrawn from the spin dryer, it does not become re-contaminated with grit and/or chemicals from the end-effector. 
     In the preferred embodiment the end-effector cleaning chamber contains one or more nozzles which spray a rinse liquid onto the end-effector and one or more nozzles which direct a flow of nitrogen against the end-effector and sweep the rinse liquid from the end-effector as it is being withdrawn from the end-effector cleaning chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a prior art wafer spin dryer in which the wafer is positioned above the platen with its good side facing up. 
     FIG. 2 illustrates a prior art wafer spin dryer in which the wafer is positioned below the platen with its good side facing down. 
     FIG. 3 is a general view of a wafer cleaning/drying system, including the wafer spin dryer of this invention and a robot for transferring the wafers between units. 
     FIG. 4 is a broken away perspective view of a wafer spin dryer of this invention. 
     FIGS. 5A and 5B are broken away side and end views, respectively, of the wafer spin dryer in an open position allowing the introduction of a wafer. 
     FIGS. 6A and 6B are broken away side and end views, respectively, of the wafer spin dryer in a closed position. 
     FIG. 7 is a cross-sectional view of the platen and associated mechanisms. 
     FIG. 8 is a broken away view of the mechanism for pivoting one of the fingers used to grip the wafer. 
     FIG. 9 is an exploded view of the mechanism for lifting and lowering the hood unit of the spin dryer. 
     FIG. 10 is a top view of the end-effector cleaning section. 
     FIG. 11 is an exploded view of the end-effector cleaning section. 
    
    
     DESCRIPTION OF THE INVENTION 
     A wafer cleaning/drying system  30  is shown in the general perspective view of FIG.  3 . System  30  includes a wafer spin dryer  32  in accordance with this invention. Also shown are a buffer unit  34 , a wafer cleaning unit  36 , a robot  38 , and a finished wafer cassette  39 . Robot  38  grips the wafers with a vacuum actuated end-effector  38 A of a kind widely used in the semiconductor processing industry. In normal operation, the wafers are placed into buffer unit  34  from a CMP unit. Robot  38  takes the wafers from buffer unit  34  and inserts them into the cleaning unit  36 , where they are scrubbed and rinsed. Wafer cleaning unit  36  is preferably of the kind described in the above-referenced application Ser. No. 08/683,654. After the wafers have been cleaned, robot  38  removes them from the cleaning unit  36  and inserts them into the wafer spin dryer  32 . After further rinsing and drying, the wafers are placed into the finished wafer cassette  39 . 
     The operation of the various units in system  30  is controlled by a programmable logic controller (PLC) (not shown) which may, for example, be a Model 2600, manufactured by Control Technology Corp. of Hopkinton, Massachusetts. The PLC is controlled from a control panel  37 . 
     FIG. 4 shows a broken away perspective view of wafer spin dryer  32  taken from the other side as compared with the view of FIG.  3 . Wafer spin dryer  32  includes a hood unit  40 , which is movable vertically to allow wafers to be inserted into the spin dryer, and a base unit  41 . Hood unit  40  includes a slot  42  at the entrance of a section (described below) which is used to clean and dry the end-effector  38 A of robot  38  while a wafer is being dried in spin dryer  32 . 
     FIGS. 5A and 5B are broken away side and end views, respectively, of wafer spin dryer  32  in an open position, with hood unit  40  in a raised position, allowing a wafer to be inserted into wafer spin dryer  32 . Hood unit  40  preferably includes an external housing  50  of plastic, a top plate  51  also formed of plastic, and a horizontal mounting plate  52  formed of sheet metal. The housing of hood unit  40  is assembled in a conventional manner with plastic welding. 
     Base unit  41  includes a base plate  53  and a cylindrical side housing  54  for a drying chamber  55 . A bracket  56  is attached to the inside surface of side housing  54 . Extending upward from bracket  56  is a conical splash guard  57 , and extending downward from bracket  56  is a conical floor  58  of drying chamber  55 . Floor  58  slopes down to a drain  59  which is located at the lowest point in drying chamber  55  to remove any rinse water which accumulates in chamber  55  during the drying process. Drain  59  extends through a manifold block  60  which is mounted on base plate  53 . Manifold block  60  is used to supply water to a set of nozzles  61  which are used to direct rinse water upward at the back side of the wafer. A lower edge of floor  58  is welded to manifold block  60 . 
     A second set of nozzles  62  are mounted in a pair of manifold blocks  63 A and  63 B which are attached to side housing  54  as shown in FIG.  5 A. Nozzles  62  are positioned such that they direct a stream or spray of rinse water against the good side of a wafer when spin dryer  32  is in the closed position, with hood unit  40  lowered (see FIGS.  6 A and  6 B). As is evident from FIG. 5B, each of manifold blocks  63 A and  63 B supplies two nozzles  62 , and manifold blocks  63 A and  63 B are positioned on opposite sides of the centerline of drying chamber  55 . Nozzles  62  are oriented so as to direct a stream or spray of rinse water downward at an angle against the good side of a wafer. 
     Manifold blocks  63 A and  63 B are preferably made of plastic and manifold block  60  is preferably made of stainless steel. Manifold blocks  60 ,  63 A and  63 B have channels and cavities formed therein in a customary manner to supply the rinse water to the respective nozzles. Rinse water is supplied to manifold blocks  60 ,  63 A and  63 B via fittings and tubes which are generally not shown in FIGS. 5A,  5 B,  6 A and  6 B. The flow of rinse water into the manifold blocks and nozzles is controlled by valves which are remote from the manifold blocks and which are in turn controlled by the PLC in wafer spin dryer  32 . The detailed structure of these elements has been omitted from the drawings for the sake of clarity but will be readily understood by persons skilled in the art. 
     Referring again to FIGS. 5A and 5B, a spin motor  64  is mounted on the upper side of horizontal mounting plate  52 . A platen  65  is positioned below spin motor  64  and is driven by a drive shaft  66 . The details of this structure are shown in the cross-sectional view of FIG.  7 . Motor  64  is mounted on a mounting block  67 , which is attached to the bottom side of horizontal mounting plate  52  with screws. Drive shaft  66  is connected to a hub  68  by means of a collet  69 . The top side of platen  65  contains a recess into which hub  68  fits, and platen  65  is attached with screws to hub  68 . Platen  65  is a circular plate of polypropylene which in the preferred embodiment is about 0.5 inches thick. To remove any airborne particles which might be generated by the bearings of motor  64 , a vacuum is applied through a tube  64 A and a fitting  64 B to a cavity  64 C which is formed in mounting block  67  (see FIG.  5 A). Cavity  64 C communicates with the space surrounding hub  68 , and thus any particles generated by the motor bearings are drawn through tube  64 A. 
     Referring again to FIG. 5B, fingers  69 A,  69 B and  69 C extend downward from platen  65  for gripping wafer  10 . Finger  69 A is located near the entrance  70  through which wafers are inserted into spin dryer  32 , and fingers  69 B and  69 C are located away from the entrance. As is evident from FIG. 7, finger  69 A can be pivoted so as to permit wafer  10  to be positioned between fingers  69 A,  69 B and  69 C. FIG. 7 also shows notches  69 D that are formed in fingers  69 A,  69 B and  69 C. When finger  69 A is in its normal, unpivoted position the edges of wafer  10  fit within notches  69 D, and wafer  10  is held in a fixed position below platen  65 . As described further below, the good or active side of wafer  10  (i.e., the side on which the electronic circuitry and components are formed) faces upward in the direction of platen  65  during the drying process. Fingers  69 A,  69 B and  69 C are positioned at equal angular intervals about the central axis of wafer  10  when wafer  10  is held in position for drying. 
     FIGS. 7 and 8 illustrate the mechanism used to pivot finger  69 A. Finger  69 A is mounted in a slot  71  in platen  65 , which permits finger  69 A to pivot about a horizontal axis on a pin  72  between a vertical position, where a wafer is held in place, and a tilted position, which allows a wafer to be inserted between fingers  69 A,  69 B and  69 C. 
     A spring plunger  73  is mounted in a cavity in platen  65  adjacent finger  69 A. Spring plunger  73  contains an actuator  74  which presses laterally against finger  69 A at a location above pin  72  and thereby urges finger  69 A into its vertical position. 
     An annular cavity  75  is formed in mounting block  67 , and a pneumatic cylinder  76  is mounted in the cavity. Pneumatic cylinder  76  has a club-shaped actuator  77  which extends radially outward. When pneumatic cylinder  76  is in its normal position, shown in FIG. 7, there is a small clearance between actuator  77  and a top end  78  of finger  69 A. When pneumatic cylinder  76  is actuated, actuator  77  is pulled to the right (in FIG. 7) and causes finger  69 A to pivot about shaft  72  to its tilted position. Spin motor  64  is preferably a servo motor and thus can be controlled to stop at a selected angular position with finger  69 A located radially inward from club-shaped actuator  77 . 
     The details of the mechanism used to lift and lower hood unit  40  is shown in FIG. 9, although many alternative techniques for accomplishing this will be apparent to those skilled in the art. A rear support plate  90  and side support plates  91  and  92  are mounted on base plate  53 . A pneumatic lifting mechanism  93  is mounted inside the support plates  90 ,  91  and  92 , with a top member  94  and a bottom member  95  being bolted to rear support plate  90 . Air pressure tubes are connected to pneumatic lifting mechanism  93  and are controlled to cause a lifting member  96  to rise and fall. Lifting member  96  is bolted to a vertical interior wall within hood unit  40 , thereby enabling hood unit  40  to rise and fall with lifting member  96 . Lifting mechanism  93  is advantageously the rodless cylinder manufactured by Tol-o-matic of Minneapolis, Minn. 
     The operation of spin dryer  32  will now be described. Initially, lifting mechanism  93  causes hood unit  40  to move to its raised position, thereby opening entrance  70  to the interior of drying chamber  55 . Robot  38  removes wafer  10  from cleaning unit  36 , using its vacuum actuated end-effector  38 A, and inserts wafer  10  into drying chamber  55  with the good or active side of wafer  10  facing upward. Pneumatic cylinder  76  is actuated. Since platen  65  is in its “home” index position, with club-shaped actuator  77  adjacent upper end  78  of finger  69 A, this causes finger  69 A to pivot to its tilted position. Robot  38  lifts wafer  10  to a position between fingers  69 A,  69 B and  69 C, at the level of notches  69 D, and then adjusts the lateral position of wafer  10  until the edge of wafer  10  comes into contact with the notches  69 D of fingers  69 B and  69 C. Pneumatic cylinder  76  is then actuated so as to allow spring plunger  73  to force finger  69 A to its vertical position, firmly clamping wafer  10  in notches  69 D of fingers  69 A,  69 B and  69 C. The sides of notches  69 D are sloped at an angle (e.g., 45°) which allows for a small error in the vertical positioning of wafer  10  by robot  38  while insuring that wafer  10  is properly seated in notches  69 D. 
     From FIG. 7 it will be noted that the center X 1  of wafer  10  is slightly displaced in the direction of fixed fingers  69 B and  69 C from the axis of rotation X 2  of platen  65  and wafer  10 . For an eight-inch wafer, this displacement D is approximately ⅛″. This insures that as wafer  10  rotates it presses against the fixed fingers  69 B and  69 C rather than the movable finger  69 A. If wafer  10  were to press against movable finger  69 A, it might overcome the force of spring plunger  73  and cause finger  69  to pivot, releasing wafer  10  from the grip of fingers  69 A,  69 B and  69 C. Platen  65  is appropriately counter-balanced to compensate for the offset position of wafer  10  and avoid vibrations from occuring when wafer  10  is being rotated. 
     With wafer  10  gripped by fingers  69 A,  69 B and  69 C, the end-effector  38 A is withdrawn through entrance  70 . After the wafer spin dryer  32  has been closed to begin the drying process, end-effector  38 A is inserted through slot  42  into the end-effector cleaning section (described below). 
     Lifting mechanism  93  is pneumatically actuated so as to lower hood unit  40 , thereby closing off entrance  70 . As shown in FIGS. 6A and 6B, a resilient ring or bead  88  is fixed to the top edge of side housing  54 . When hood unit  40  is in its lowered position, bead  88  presses against the lower surface of mounting plate  52  to seal off drying chamber  55  from the outside environment. 
     Spin motor  64  is turned on to rotate platen  65  and wafer  10  at a relatively slow speed (e.g., 100 rpm) and rinse water (preferably deionized water) is sprayed through nozzles  62  onto the top (active) side of wafer  10 . At the same time, rinse water is sprayed through nozzles  61  against the back side of wafer  10 . This continues for approximately 10-15 seconds, at which time nozzles  61  and  62  are turned off and the rotational velocity of spin motor is increased to a much higher level (e.g., 5000 rpm). At this speed, centrifugal force causes the rinse water on the surfaces of wafer  10  to flow toward the edge of the wafer where it is thrown radially outward. 
     As shown in FIGS. 6A and 6B, when hood unit  40  is in its lowered position, wafer  10  is positioned well below the top edge of splash guard  57 . Therefore, as rinse water is thrown from the edges of wafer  10 , it strikes splash guard  10  and is deflected downward toward the portion of drying chamber  55  below wafer  10 . None of this rinse water can splash back to the top surface of wafer  10 , and thus the top (active) surface of wafer  10  is thoroughly and effectively dried. The rinse water runs down the sloping floor  58  and through drain  59 . 
     The drying process normally lasts for about  45  seconds. Spin motor is then turned off, and lifting mechanism  93  is pneumatically actuated so as to raise hood unit  40 , thereby opening entrance  70 . Robot  38  inserts end-effector  38 A (which by now has been cleaned) into drying chamber  55 , and raises end-effector  38 A until it is in contact with the lower surface of wafer  10 . Pneumatic cylinder  76  is actuated to tilt finger  69 A, releasing wafer  10  from fingers  69 A,  69 B and  69 C, and robot  38  moves wafer  10  a short horizontal distance in the direction of finger  69 A, insuring that the edge of wafer  10  is clear of notches  69 D of fingers  69 B and  69 C. Robot  38  then lowers wafer  10 , withdraws wafer  10  through entrance  70 , and places wafer  10  in finished wafer cassette  39 . This completes the wafer drying process. 
     As described above, hood unit  40  also contains a end-effector cleaning section which is accessible through slot  42 . End-effector cleaning section  100  is shown in FIGS. 5A and 6A. FIG. 10 is a top view of cleaning section  100 , including end-effector  38 A, and FIG. 11 is an exploded view of cleaning section  100 . Included are a top plate  102  and a bottom plate  104 , preferably made of stainless steel, and spacers  106  and  108 , preferably made of plastic. Plates  102  and  104  and spacers  106  and  108  are fastened together with screws, creating an internal cavity which is shaped to fit end-effector  38 A. 
     A series of holes  110 A and  110 B are drilled in top plate  102 , and a corresponding series of holes  112 A and  112 B are drilled in bottom plate  104 . Holes  110 A,  110 B,  112 A and  112 B are used to supply jets of heated nitrogen into the internal cavity of cleaning section  100 , and they are preferably drilled at an angle of about 45° so that the flow of nitrogen is directed away from slot  42  and into the internal cavity of cleaning section  100 . The nitrogen is directed into holes  110 A and  110 B through a top manifold  114  and into holes  112 A and  112 B though a bottom manifold  116 . Manifolds  114  and  116  are clamped to top plate  102  and bottom plate  104 , respectively, with screws (not shown). Using bottom manifold  116  as an example, a cavity  116 A registers with holes  112 A, and a cavity  116 B registers with holes  112 B. Nitrogen flows through a tube (not shown) and a fitting  115  (FIG. 6A) into cavities  116 A and  116 B and is ejected through holes  112 A and  112 B into the internal cavity of cleaning section  100 . Top manifold  114  contains similar cavities with register with holes  110 A and  110 B and permit nitrogen to be ejected into the internal cavity of cleaning section  100  through holes  110 A and  110 B. Cavities  116 A and  116 B are connected with the corresponding cavities in top manifold  114  by means of a series of aligned holes that are formed in plates  102  and  104  and spacer  108 . The nitrogen is heated to a temperature of 140° F. by a heater (not shown) which is located in wafer cleaning/drying system  30 . 
     Top plate  102  also contains a pair of holes  118  which are used to spray water into the internal cavity of cleaning section  100  and bottom plate contains a similar pair of holes  120 . Holes  118  are angled similarly to holes  110 A and  110 B, and holes  120  are angled similarly to holes  112 A and  112 B. Bottom manifold  116  contains a cavity  116 C which registers with holes  120  and which is connected through holes in plates  102  and  104  and spacer  108  to a similar cavity in top manifold  114  which registers with holes  118 . Water flows through a tube (not shown) and a fitting  117  (FIG. 6A) into cavity  116 C. 
     When robot  38  has finished inserting wafer  10  into spin dryer  32 , it inserts end-effector  38 A through slot  42  into the internal cavity of cleaning section  100 . Rinse water is sprayed through holes  118  and  120  against the top and bottom surfaces of end-effector  38 A, removing grit and/or chemicals which have accumulated on the end-effector in the course of transferring the wafer from buffer unit  34  to cleaning unit  36  and to spin dryer  32 . The rinse water exits cleaning section  100  through a drain  122 . As the end-effector  38 A is withdrawn from cleaning section  100 , heated nitrogen is supplied through holes  110 A,  110 B,  112 A and  112 B. The jets of heated nitrogen “wipe” the rinse water from the surfaces of end-effector  38 A and dry the end-effector before it is called upon to remove wafer  10  from spin dryer  32 . 
     It is to be understood that the foregoing description is illustrative and not limiting. For example, while the embodiment described above is for drying a semiconductor wafer, the principles of this invention may be applied to drying other types of flat substrates. Many alternative embodiments in accordance with the broad principles of this invention will be apparent to persons skilled in the art.