Abstract:
A nozzle that extends telescopically upward is used to elevate a wafer without contacting the wafer. The nozzle includes a stationary hollow cylinder, closed at its lower end and open at its upper end, within which a spool is disposed to slide vertically. In its lowest position the spool is spaced from the closed bottom of the cylinder, and purified water is supplied under pressure to the space. A passage extends vertically through the spool, and the water is discharged at the upper end of the spool from the passage. As a wafer is lowered toward the upper end of the spool, the wafer partially impedes the discharge, increasing the pressure in the space below the spool. The increased pressure drives the spool and the wafer upward, but the wafer never comes into contact with the wafer because the discharge of water creates a protective cushion between them.

Description:
This application is a continuation of Ser. No. 09/247,109 filed Feb. 9, 1998 now U.S. Pat. No. 6,102,057, which claims benefit of No. 60/075,129 filed Feb. 14, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is in the field of semiconductor manufacturing and more specifically relates to improvements in apparatus for handling semiconductor wafers as they are carried through a planarizing machine. 
     As supplied to the planarizing machine, the surface of a wafer may exhibit departures from flatness. Typically, 25 wafers are loaded into a cassette which is hand carried to a complex machine called a planarizer. The purpose of the planarizer is to render the front surface of the wafer flat to within a fraction of a micron. After each wafer has been rendered flat, the planarizer inserts it into a cassette. When the cassette is full, it may be removed from the planarizer and transported to any of a number of other processing machines. 
     The present invention cannot be fully appreciated without an understanding of the operation of the planarizer. Once the cassette containing typically 25 pre-processed wafers has been inserted into the planarizer, the operation of the planarizer in processing the wafers is completely autonomous. The operations are carried out under control of a computer that controls the application of power to various actuators in response to sensed inputs in a time-coordinated manner. In order for its lengthy program of operations to be carried out without mishap, it is important that means be provided to interrupt the accumulation of positional errors. It can also be appreciated that the wafers must be handled carefully to avoid scratching the processed surfaces which are so fragile that they are never again touched by human hands. 
     The present invention can best be understood by following a typical wafer as it is moved through the planarizer. The wafer is extracted from the cassette by a portion of the planarizer called a robot. The robot carries the wafer to a fixed location known as the load station. After the robot has deposited the wafer at the load station and has moved out of the way, a different part (called a spindle carrier) moves into position above the wafer and acquires it. The spindle carrier transports the wafer to a rotating polishing platen and rotates the wafer while holding it in contact with the rotating polishing platen until the wafer has been rendered flat. Thereafter, the spindle lifts the wafer from the polishing platen and transports it back to the load station where the wafer is rinsed. After the spindle carrier has deposited the wafer at the load station and has moved out of the way, the robot picks up the polished wafer and carries it to a cassette and deposits the wafer in the cassette. Throughout these operations, the wafer is maintained in a horizontal position with the side that is to be polished facing downward. That downward facing side of the wafer is also referred to as the front side of the wafer. The path of an individual wafer through the planarizer will be depicted in greater detail below. 
     Upon reflection it will be seen that the main purpose of the load station is to accurately center the wafer into the carrier notwithstanding the errors in their relative positions as they enter the load station. 
     In addition to centering the wafer with respect to the carrier, the load station performs a number of useful ancillary functions. The load station includes provision for sensing the presence of the wafer at the load station, for rinsing the wafer, for lifting the wafer into the carrier, and for supporting the processed wafer on cushions of water to avoid direct contact. Thus, the load station serves several other functions in addition to centering the wafer with respect to the carrier. 
     SUMMARY OF THE INVENTION 
     The process of centering the wafer with respect to the spindle carrier must be completed before the spindle carrier acquires the wafer; once acquired, the wafer is drawn against the vacuum pad of the spindle carrier and cannot be shifted laterally. To guard against premature acquisition, at the end of the centering process, when the wafer is at its lowest position, a space is left between the upper surface of the wafer and the vacuum pad of the spindle carrier. Some way is needed to elevate the centered wafer into contact with the vacuum pad without contacting the fragile lower surface of the wafer. 
     In accordance with the present invention this is accomplished by the use of an unusual type of nozzle located beneath the center of the wafer. The nozzle extends telescopically upward to provide an upwardly-moving cushion of purified water that lifts the centered wafer up into contact with the vacuum pad of the spindle carrier, while preventing contact between the nozzle and the fragile lower surface of the wafer. 
     The nozzle includes a stationary hollow cylinder within which a vertically movable spool slides in a loose sliding fit. In its lowest position, the spool is spaced from the closed bottom of the cylinder, and the purified water is supplied under pressure to the space. A passage extends vertically through the spool. In the absence of a wafer, the spool remains in its lowest position and the water is discharged at the upper end of the spool from the passage. 
     As the wafer reaches its lowest position at the conclusion of the centering process, the wafer closely approaches the upper end of the spool but does not make contact with it. However, the wafer partially impedes the discharge of the water, thereby increasing the pressure in the space below the spool. This increased pressure, acting on the lower end of the spool, overcomes the weight of the spool and the weight of the wafer, which is supported on the liquid cushion that prevents the nozzle from making contact with the wafer. As the spool moves upward it lifts the wafer on the liquid cushion until the upper surface of the wafer makes contact with the vacuum pad on the underside of the spindle carrier. In the process of elevating the wafer, the spool also rinses the lower side of the wafer, the rinse water flowing outwardly from the center of the wafer. 
     The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which a preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the use of a robot to remove a wafer from a cassette; 
     FIG. 2 is a diagram showing the robot placing a wafer on the load station; 
     FIG. 3 is a diagram showing the spindle carrier after it has moved to a position directly above the load station; 
     FIG. 4 is a diagram showing the spindle carrier descending onto the load station and acquiring the wafer; 
     FIG. 5 is a diagram showing the spindle carrier transporting the wafer to a polishing platen; 
     FIG. 6 is a diagram showing the spindle carrier holding the wafer on the polishing platen; 
     FIG. 7 is a diagram showing the spindle carrier transporting the polished wafer to the load station; 
     FIG. 8 is a diagram showing the spindle carrier depositing the polished wafer on the load station; 
     FIG. 9 is a diagram showing the robot removing the polished wafer from the load station after the spindle carrier has returned to its home station above the polishing platen; 
     FIG. 10 is a diagram showing the robot placing the polished wafer into a cassette; 
     FIG. 11 is a top plan view of the load station of FIGS. 1-4; 
     FIG. 12 is a side elevational view in cross section through the load station of FIG. 11, in the direction  12 — 12  indicated in FIG. 11, showing the configuration of the parts when the carrier first makes contact with the centering apparatus of the load station; 
     FIG. 13 is a side elevational cross sectional view through the load station of FIG. 11 showing the positions of the parts at the conclusion of the centering operation; 
     FIG. 14 is a side elevational cross sectional view through the load station of FIG. 11 showing the center nozzle actuated to lift the wafer into the carrier; 
     FIG. 15 is a side elevational cross sectional view through the load station of FIG. 11 showing the center nozzle rinsing the polished wafer; 
     FIG. 16 is a hydraulic diagram of the hydraulic system used for hovering of the wafer and sensing its presence; 
     FIG. 17 is a side elevational view of an alternative embodiment of the centering apparatus showing its configuration at the instant the carrier portion of the spindle carrier first makes contact with the centering apparatus; and, 
     FIG. 18 is a side elevational view of the alternative embodiment of the centering apparatus of FIG. 17 showing its configuration when the spindle carrier is in the lowest position with the wafer centered in the carrier. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention includes a series of operations carried out by a planarizing machine on a wafer that is to be planarized. Generally, the front surface, on which the electronic circuitry will be deposited is not as flat as required. The overall purpose of the planarizing machine is to render the front surface of each wafer as flat as possible. 
     The wafers are carried to the machine in an incoming cassette  16  in which the wafers are temporarily stored in a stacked configuration. The incoming cassette is inserted into the planarizing machine, and thereafter the machine processes the wafers autonomously, presenting the processed wafers in an outgoing cassette  24  of FIG. 10, which the operator removes from the machine. A typical cassette has a capacity of 25 wafers. 
     FIGS. 1-10 form a series showing schematically the path of a wafer as it progresses through the planarizing machine. 
     As shown in FIG. 1, a robot  14  removes the upper-most wafer  12  from the incoming cassette  16  and transports the wafer to a load station  18 . In a preferred embodiment, the robot holds onto the wafer by applying suction to the upper surface of the wafer, also called the back side of the wafer, which is not the surface that is to be rendered flat. 
     As shown in FIG. 2, the robot  14  deposits the wafer  12  onto the load station  18 . The robot then moves to an out-of-the-way station leaving the wafer reposing on a cushion of purified water provided by the load station, as will be described below. 
     Next, as shown in FIG. 3, the spindle  20  moves into a position directly above the load station  18 , and as indicated in FIG. 4, the spindle is lowered onto the load station  18 . As will be described below, the descending motion of the spindle  20  centers the load station  18  with respect to the spindle carrier  32  and simultaneously centers the wafer  12  with respect to the spindle carrier  32 . At this juncture, an upwardly-directed jet of purified water directed against the lower face of the wafer lifts the centered wafer into contact with the carrier, which then acquires the wafer through the use of a vacuum system of the type described in U.S. Pat. No. 5,423,716 issued Jun. 13, 1995 to Alan Strasbaugh for WAFER-HANDLING APPARATUS HAVING A RESILIENT MEMBRANE WHICH HOLDS WAFER WHEN A VACUUM IS APPLIED, in U.S. Pat. No. 5,449,316 issued Sep. 12, 1995 to Alan Strasbaugh for WAFER CARRIER FOR FILM PLANARIZATION, and in U.S. application Ser. No. 08/971,642 filed Nov. 17,1997 by Salman M. Kassir and Thomas A. Walsh for GRINDING PROCESS AND APPARATUS FOR PLANARIZING SAWED WAFERS. The disclosures of U.S. Pat. No. 5,423,716 and 5,449,316 are incorporated herein by reference. 
     Having thus acquired the wafer, the spindle  20  lifts the wafer from the load station  18  and carries it to the polishing platen  22 . The spindle  20  rotates the wafer  12  about a vertical axis that is displaced from the vertical axis about which the polishing platen rotates, as is known in the art. FIG. 6 shows the carrier holding the wafer  12  against the polishing platen  22 , whereby the lower surface of the wafer is polished flat. 
     After the polishing has been completed, the planarized wafer is transported by the spindle  20  to the load station  18  and deposited there as shown in FIGS. 7 and 8. Thereafter, the spindle moves to an out-of-the-way position and the wafer is rinsed by the load station  18 . Following this rinsing, the wafer is acquired by the robot  14  and transported to an outgoing cassette  24 , as shown in FIGS. 9 and 10, respectively. Thereafter, the planarizing machine repeats the process with the next wafer which is upper-most in the incoming cassette  16  of FIG.  1 . 
     More recent embodiments of the planarizing machine employ multiple polishing platens, robots, spindles, and load stations permitting two or more wafers to be processed simultaneously with the operations performed in parallel or staggered for maximum efficiency. 
     A portion of the spindle  20  called the carrier  32  is a ring-shaped part that encircles the wafer so as to retain the wafer in the spindle during the polishing process by preventing the wafer from slipping out sideways. The radial spacing between the circumference of the wafer and the inner edge of the carrier is typically a fraction of a millimeter. One of the most significant problems solved by the load station  18  is centering of the wafer with respect to the carrier ring to such a close tolerance. The initial placement of the wafer onto the load station, as indicated in FIG. 1, includes positional errors, and likewise, the positioning of the spindle  20  above the load station, as in FIG. 3, is also subject to position errors. Without some way of eliminating these various lateral position errors, it would be impossible for the wafer to be acquired within the carrier  32  of the spindle  20 . 
     In the preferred embodiment the centering action of the load station is accomplished in a manner which will now be described in connection with FIGS. 11,  12 , and  13 . The load ring assembly of FIG. 11 lies horizontally within a pan-like base  26 . The load ring  28  is not rigidly attached to the base  26  and, in fact is capable of limited lateral sliding motion with respect to the base  26 . A number, typically six, of centering tabs, of which the tab  30  is typical, are spaced around the circumference of the load ring  28 . These tabs are pivotably attached to the load ring, each pivoting about its own horizontal axis that is tangent to the load ring. The tabs pivot from an unloaded position shown in FIG. 12 to a loaded position shown in FIG.  13 . In the unloaded condition, the tabs are biased to the position shown in FIG. 12 by a helical spring  31 , and the tab bends at a living hinge  36 . 
     FIG. 12 shows the load station after the wafer has been deposited on it by the robot  14 , but before the spindle carrier  32  has descended. FIG. 13 shows the load station after the spindle has descended to its lowest extent. Note that FIG. 12 is a view in the directions  12 — 12  shown in FIG.  11 . 
     When first deposited onto the load station  18  the wafer  12  is unlikely to be concentric with the carrier  32  of the spindle  20 , and as seen in FIG. 12, clearance is provided for lateral movement of the wafer. The wafer is levitated on cushions of purified deionized water that emerge under low pressure from the nozzles  42 ,  44 ,  46 . In the preferred embodiment, there are three such nozzles, and the presence of the water flowing between the body of the nozzle  42 ,  44 ,  46  and the wafer prevents contact of the nozzle with the lower side of the wafer. 
     As the spindle carrier is lowered onto the load station  18  as shown in FIG. 4, the horizontal lower surface  34  of the carrier  32  pushes downwardly against the top horizontal surface of the load ring  28  overcoming the biasing force of the helical spring  31  causing a first inward-facing surface  33  of the centering tab to swing radially inward until the inward-facing surface makes contact with the edge of the wafer  12 . Concurrently, the pivoting of the centering tab brings a second inwardly facing surface  35  into contact with the circumference of the carrier  32 . The carrier  32  is not free to move laterally, and therefore the load ring shifts laterally in an effort to center itself with respect to the carrier. At the conclusion of the downward motion of the spindle, the load ring  28  is centered with respect to the carrier  32 , and the wafer is centered with respect to the load ring; therefore, the wafer is centered with respect to the carrier. Immediately thereafter, an upwardly-directed jet of water from the nozzle  40  elevates the wafer  12  into the carrier  32  and holds it in that position long enough for the vacuum system of the spindle carrier to acquire the wafer. Having thus acquired the wafer, the spindle then transports the wafer to the polishing platen  22  as shown in FIG.  5 . 
     To avoid the possibility of accidentally damaging the front (lower) face of the wafer  12 , both before and after it has been planarized, the wafer is always supported while at the load station  18  on three cushions of purified and deionized water. These cushions of water flow between the underside of the wafer and the upper end of the nozzles  42 ,  44  and  46  seen in FIG.  11 . As best seen in FIGS. 12 and 13, these nozzles are attached to the load ring  28 , so that as the load ring is pushed downwardly by the spindle, the nozzles  42 ,  44  and  46  move downward with the load ring, thereby continuing to support the wafer. 
     FIG. 16 is a hydraulic diagram showing how the nozzles are connected to a supply main  48 . It has been found that a relatively low volumetric flow rate is adequate to prevent the wafer  12  from contacting the nozzles. 
     The present inventors have discovered that the nozzles  42 ,  44  and  46  can perform another useful function, in addition to levitating the wafer. The presence of the wafer as it is being levitated above the nozzles increases the resistance of each nozzle to the outflow of water. Clearly, if each nozzle were completely closed off, the pressure in the branches  50 ,  52  and  54  would become equal to the supply pressure. At the other extreme, if the nozzles  42 ,  44  and  46  offered no resistance to the flow, then the pressure in the branches  50 ,  52  and  54  would be considerably less than the supply pressure. The present inventors saw that this effect can be used to sense the presence of the wafer as it rests on the cushion of water between the tops of the nozzles and the lower side of the wafer. To implement this, the inventors have inserted a pressure sensor  56  in the branch  54  as shown in FIG.  16 . The inventors have found that it is beneficial to insert a flow restrictor  58  in the branch  54  upstream of the pressure sensor  56 . Valves  60  and  62  in the branches  50  and  52  respectively permit equalization of the flow through the nozzles. 
     Knowledge of whether a wafer is reposing on the load station  18 , obtained by the pressure sensor  56 , is useful in determining what the spindle  20  should do in the situation illustrated in FIG.  4  and in the situation illustrated in FIG.  8 . The presence of a wafer on the load station in FIG. 3 results in the spindle descending to the position shown in FIGS. 4 and 13 so as to acquire the wafer. In contrast, the absence of a wafer on the load station in FIG. 8 results in the spindle descending only far enough to deposit the wafer on the nozzles  42 ,  44  and  46  as shown in FIG.  12 . 
     FIGS. 12,  13 , and  14  show cross sectional views of nozzle  40 . It will be recalled that this nozzle provides a stream of water directed upward toward the lower face of the wafer  12  for the purpose of elevating the wafer into contact with a vacuum pad within the spindle carrier, whereby the carrier acquires the wafer from the load station. As best seen in FIG. 14, the nozzle  40  includes a cylinder  64  within which a spool  66  is disposed in a loose sliding fit. This permits the spool  66  to move upward from the position shown in FIG. 13 to the position shown in FIG. 14 wherein a shoulder  68  on the spool is stopped by a flange  70 . The spool  66  includes a central passage  72  through which the purified water is discharged. 
     After the carrier  32  is lowered into the load station to the position shown in FIG. 13, the upward flow of water out of the top of the spool  66  is partially impeded by the presence of the wafer. This causes an increase in pressure in the chamber immediately below the spool, thereby pushing the spool upwards within the cylinder  64  to the position shown in FIG.  14 . At no time does the spool  66  touch the wafer (because of the water cushion), but as the wafer moves upward the spool is able to keep up with it, thereby maintaining an upward force on the wafer that pushes the upper surface of the wafer against a vacuum pad within the carrier, and holding the wafer against the pad until the vacuum causes the wafer to be secured to the carrier. 
     The relatively strong upward stream discharged by the nozzle  40  rinses the wafer from the center outwardly seen in FIG.  15 . The spool never contacts the wafer after the wafer has been polished. 
     FIGS. 17 and 18 show an alternative embodiment of the pivoting fingers  30  of FIGS. 12 and 13 FIG. 17 shows the configuration after the wafer  12  has been deposited but before the centering process has begun. FIG. 18 shows the configuration at the conclusion of the centering process. 
     As the spindle  20  descends from the position shown in FIG. 17 to the position shown in FIG. 18, the pivoting body  76  is forced to rotate clockwise (as seen in the drawings) until the surface  76  contacts the cylindrical outer surface of the carrier  78  thereby centering the load ring with respect to the carrier, while simultaneously, the surface  80  contacts the edge of the wafer  12  so as to center the wafer with respect to the load ring. As shown in FIG. 18, at the conclusion of the centering process the load ring is centered with respect to the carrier and the wafer is centered with respect to the load ring, and therefore the wafer must be centered with respect to the carrier. 
     Thus, there has been described a very useful load station for handling wafers within a planarizing machine. The load station interrupts the accumulation of positional errors by centering the wafer with respect to the carrier that is acquiring the wafer. The acquisition is accomplished without touching the front side of the wafer by use of a novel nozzle that elevates the wafer into the carrier by directing a jet of water against the front side of the wafer. At no time does the wafer actually make solid contact with the load station. Instead, the wafer is always levitated on three fountains of water. Back pressure caused by the wafer impeding the flow of water from these fountains is sensed and the sensed signal is used to indicate the presence of a wafer on the load station. 
     The foregoing detailed description is illustrative of one embodiment of the invention, and it is to be understood that additional embodiments thereof will be obvious to those skilled in the art. The embodiments described herein together with those additional embodiments are considered to be within the scope of the invention.