Abstract:
In a system and method to reduce wafer breakages in a wafer handling system, the position of a wafer on a platen is monitored and closing of the platen on a vacuum chamber is prevented if a misaligned wafer is detected. In one embodiment the wafer position is monitored by monitoring the air pressure in vacuum channels of a platen faceplate.

Description:
BACK OF THE INVENTION 
   In the semiconductor industry, semiconductor chips are created by depositing layers on a semiconductor substrate, etching material, and growing and implanting certain regions. The circuits created in this way are formed on a semiconductor wafer which is moved to various workstations using wafer handling equipment. One common step in the fabrication process is the implanting of ions into the wafer using an ion implanter such as the Varian 350D Ion Implanter. 
   The Varian 350D, as illustrated in  FIG. 1 , includes a vacuum chamber  100  which constitutes the end station where the ions are implanted using an ion beam  102 . It also includes a wafer handling device in the form of a pick  104 , which sequentially moves wafers from a cassette  106  to a chuck  108  on a platen  110  for processing in the vacuum chamber  100 . 
   In practice the steps involved in processing the wafer are shown in  FIGS. 1-12 .  FIG. 1  shows the pick  104  in a home position before it picks up a wafer from the cassette  106 . The chuck  108  is retracted into the housing of the platen, and the vacuum chamber  100  is closed by means of an isolation valve  112 . It will be appreciated that although the ion beam  102  is shown for purposes of reference, no beam would typically be emitted at this stage since the vacuum chamber  100  is not processing any wafers at this stage. The beam is gated off electrostatically while wafer handling is in progress. 
   In  FIG. 2  the pick  104  picks up a wafer  120  and moves it to infront of the platen. Again, the ion beam is shown merely for reference purposes but is typically not present at this stage. 
   In  FIG. 3  the chuck  108  extends from the housing of the platen  110  and holds the wafer  120  by establishing a chuck vacuum between the chuck and the wafer, as will be discussed in greater detail below.  FIG. 4  shows the pick  104  retracted to its home position. In  FIG. 5  the chuck rotates to the appropriate angle depending on the particular process being used, and then retracts into the housing of the platen  110 , holding the wafer  120  against a faceplate by virtue of the chuck vacuum as will be discussed in more detail below. 
   In  FIG. 6 , the platen closes against the opening of the vacuum chamber. Since the vacuum chamber is at a reduced pressure, vent channels are provided on the surface of the platen to allow air pockets between the platen surface and the wafer to be eliminated. This will be discussed in greater detail below. Also, to avoid pressure differences on the two sides of the wafer once the isolation valve  112  over the chamber opening is opened, the chuck vacuum is switched off at this stage. 
   In  FIG. 7  the isolation valve  112  is opened and the platen  110  moves into abutment with the opening of the vacuum chamber  100 . The platen assembly and vacuum chamber are shown in greater detail in  FIG. 8  which shows the platen  110  moving into abutment with the vacuum chamber  100 . The platen includes a face plate  130  on which the wafer  120  is held. As can be seen in  FIG. 8 , the chuck  108  is retracted or extended by means of a chuck cylinder  132  housed in a gearbox housing  134 . By withdrawing the cylinder  132 , the chuck can be withdrawn to retract the wafer  120  against the platen faceplate  130 , as was discussed with respect to  FIG. 5  above. As is shown in  FIG. 8 , the faceplate  130  has a two-tier configuration.  FIG. 8  also shows an orientor motor  136  that turns gears in the gearbox housing  134 , to orient the wafer to a home position, as was also mentioned with respect to FIG.  5 . As can be seen from  FIG. 8 , the platen  110  moves against a clamp ring assembly  140 . An O-ring  142  mounted on the platen  110  sealingly engages a housing of the clamp ring assembly  140 , while the faceplate  130  with the wafer  120  mounted thereon moves into an opening of the assembly  140 . It will be appreciated that, at this point, the wafer  120  will be held between the faceplate  120  and a clamp ring  144  of the clamp ring assembly  140 . As will be discussed further below, the chuck vacuum will be disengaged at this point.  FIG. 8  also shows an isolation valve cylinder  150 , which serves to move the isolation valve  112  away from the chamber opening  152 . The isolation valve  112  seals against an O-ring  154  when closed. Once the implantation is complete, the isolation valve  112  closes and the chuck vacuum is reapplied, as shown in FIG.  9 . 
   In  FIG. 10  the platen  110  opens by moving away from the chamber  100 . 
   Thereafter, the wafer  120  is removed from the platen as shown in  FIGS. 11  to  12 . In  FIG. 10  the chuck  108  extends and rotates a home position. 
   In  FIG. 12  the pick  104  moves up to receive the wafer  120 , at which stage the chuck vacuum is switched off. 
   In  FIG. 13  the chuck  108  retracts and the pick  104  moves down with the wafer  120  and deposits it in the cassette  106 . 
   In order to understand the problems commonly experienced with the Varian 350D it is useful to consider a typical wafer handled by the Varian and the details of the platen.  FIG. 14  shows a typical wafer  1400 . In this embodiment, the wafer  1400  has an edge diameter  1410  of 150 mm. The flat surface  1402 , which serves to correctly orientate the angle of the wafer during handling and processing, defines a wafer flat diameter  1412  in this case. 
     FIG. 15  shows a platen  1500  with its two-tiered faceplate  1502 of the Varian 350D, on which the wafer (not shown) is held. The smaller tier  1504  of the faceplate  1502 , in this embodiment, has a diameter of 152 mm. It is connected to the larger tier, which, in turn, is secured to the platen  1500  by means of bolts (not shown) that extend through peripheral holes  1506  in the larger tier of the faceplate  1502 . The wafer used with this embodiment will have a diameter of 150 mm. Thus, the vent channels  1508  will extend beyond the wafer diameter to allow air pockets between the wafer and the smaller tier  1504  of the faceplate  1502  to be eliminated when the platen moves into abutment with the vacuum chamber as discussed above A second set of channels in the form of chuck vacuum channels  1510  extend from the chuck opening  1512  to a diameter of  100  mm as indicated by dotted line  1520 . By eliminating air from the chuck opening  1512  and chuck vacuum channels  1510 , to create a vacuum, the wafer (not shown) is sucked onto the faceplate  1502  of the platen  1500 . 
   Problems, however, arise when a wafer is misaligned on the platen faceplate  1502 . When this happens, the wafer may hit the side of the chamber opening  152  (see  FIG. 8 ) when the platen closes in the step shown in FIG.  6 . This may cause the wafer to break causing product wastage and significant downtime required for cleaning debris from the equipment. 
   The present invention seeks to address this problem for wafer processing equipment having platens similar to that described above. 
   SUMMARY OF THE INVENTION 
   The invention relates to a system and method of avoiding wafer breakages due to misalignment on a platen such as the Varian 350D. 
   According to the invention, the method includes retrofitting the platen of a wafer handling device to allow wafer misalignments that are large enough to cause wafer breakages when the platen closes on a vacuum chamber, to be detected, and providing an interlock circuit to stop the platen from closing in the event that such wafer misalignment is detected. The retrofitting includes providing a platen face that includes chuck vacuum channels that extend from a chuck orifice outwardly to a diameter that is only a little bit smaller than the diameter of the wafer being handled. The diameter to which the channels extend may be 2 to 4 mm less than the wafer diameter. The retrofitting typically includes monitoring the chuck vacuum to detect drops in the vacuum. The interlock circuit may be connected to a platen close signal to interrupt the close signal when a wafer misalignment is detected. The interlock circuit may include an override of the interrupt when the platen orientor is in the home position. The circuit may include an opto-isolator to switch on a transistor of the opto-isolator to permit the close signal to be transmitted. The opto-isolator may include two light emitting diodes, one being activated when no chuck vacuum drop is detected, and one being activated when the platen orientor is in its home position. 
   Further, according to the invention, there is provided a method of retrofitting a platen having chuck vacuum channels for holding a wafer, comprising extending the vacuum channels outwardly to have an outer diameter that is substantially the same as that of a wafer to be held. The outer diameter of the channels may be slightly smaller, e.g., 2 to 4 mm smaller than the diameter of the wafer. The method may also include monitoring the air pressure in the channels and issuing a signal if a reduced pressure is detected. The signal may be used to stop the platen from closing. The method may also include monitoring the position of the platen orientor and overriding the signal when the platen orientor is in the home position. 
   Still further according to the invention there is provided a wafer handling system for reducing wafer damage by ensuring that a platen of the system does not close unless the wafer is correctly aligned, the system comprising a platen in which the wafer is held by means of vacuum channels that extend almost to the periphery of the wafer. The system further includes means for monitoring the air pressure in the channel and generating a stop signal if the air pressure is below a define value. The system typically includes an interlock circuit for preventing closing of the platen if a stop signal is generated. The system may include an override to allow the platen to close when a platen orientor is in a home position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1  to  7 , and  9  to  13  show the steps in processing a wafer in a prior art ion implantation using the Varian 350D; 
       FIG. 8  shows a more detailed depiction of a typical platen assembly and vacuum chamber, 
       FIG. 14  is a plan view of a typical 150 mm wafer, 
       FIG. 15  is a front view of a Varian 350 D platen as known in the art; 
       FIG. 16  is a front view of one embodiment of a platen as proposed by the invention; 
       FIG. 17  is one embodiment of an interlock circuit of the invention, and 
       FIG. 18  is a depiction of a typical orientor switch assembly known in the art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention addresses the problems with the Varian 350 D implanter and any similar processing system that displays similar problems by checking for misalignment of wafers and including an interlock circuit that stops the platen from closing when a misalignment is detected. Referring again to  FIG. 15 , which shows the prior art platen faceplate  1502 . Chuck vacuum channels  1508  are shown extending from the chuck orifice  1512 . The channels  1508  serve to secure a wafer to the faceplate of the platen once the chuck has been retracted to bring the wafer flush with the faceplate  1502 . Also, the channels  1508  serve, to some extent, to detect when a wafer is misaligned by monitoring a chuck vacuum drop (since the chuck orifice  1512  is in flow communication with the channels  1508 ). However, it will be appreciated that since the channels  1508  extend outwardly only up to a 100 mm diameter, a chuck vacuum drop will not be detected unless there is a misalignment of more than 50 mm. This amount of misalignment would cause wafer breakage when the platen closes on the vacuum chamber. Furthermore, the Varian endstation does not even check the chuck vacuum after the chuck retracts, thus rendering the vacuum channels  1508  useless for purposes of monitoring misalignment of wafers. 
   The present invention proposes retrofitting the platen to provide for chuck vacuum channels that extend to a diameter that is not much smaller than the diameter of the wafer to be held by the platen faceplate. For instance, for a 150 mm diameter wafer, the chuck vacuum channels, in one embodiment, were made to extend from the chuck orifice outwardly to a diameter of 148 mm as shown in FIG.  16 . In  FIG. 16  the chuck vacuum channels  1600  are shown extending outwardly from a chuck orifice  1602  to a diameter of 148 mm to define a  148  mm detection diameter as indicated by doted line  1620 . The platen faceplate  1606  in this case has an inner or smaller tier  1622  of diameter 152 mm. In the embodiment of  FIG. 16 , a 150 mm wafer that is misaligned by more than 2 mm will therefore allow air to leak into the vacuum channels  1600 , thereby reducing the vacuum in the channels  1600  and also the chuck vacuum since the channels  1600  are in flow communication with the chuck orifice  1600 . The rest of the platen 1610  remains essentially unchanged. It includes peripheral holes  1612  for securing the faceplate  1606 , and an O-ring  1614  (which was depicted by reference numeral  142  in FIG.  8 ). 
   By monitoring the drop in vacuum, i.e, by monitoring the drop in air pressure in the chuck orifice  1602  or channels  1600 , a misalignment of as little as 2 mm can therefore be detected. This can be used by an interlock circuit such as the interlock circuit shown in FIG.  17 . 
   The circuit  1700  includes an opto-isolator that includes a transistor  1702  and a light emitting diode (LED)  1704 . The diode  1704  is connected to two inputs  1706 ,  1708 , that sink current from a SV source  1710  through a resistor  1712 . Input  1706  sinks current when the platen orient is in the home position. This occurs when processing is complete, the wafer is removed, and the platen wishes to close. Input  1708  sinks current when no wafer misalignment is detected due to a pressure drop in the chuck orifice of the platen. Since inputs  1706 ,  1708  are connected in parallel, current flows through the diode  1704  when either the platen orient is in the home position or when the wafer is correctly positioned on the platen. This turns on the transistor  1702  to close the drive signal circuit and allow the platen to close. When either of these conditions is not present, the transistor  1702  is turned off and the drive signal is not sent to the drive circuit. The inputs  1706 ,  1708  further include light emitting diodes  1720 ,  1722 , respectively, to indicated when current is flowing to the particular input. 
   The input signals  1706 ,  1708  can best be understood with reference to  FIG. 18 , which shows a representation of an orient switch assembly  1800 , which is also shown in FIG.  8 . As shown in  FIG. 8 , the assembly  1800  is connected to the chuck  108  by means of a belt  1802 . The assembly controls the rotational orientation of the wafer on the chuck  108 , by having an actuator  1804  which engages either a home switch  1806  or a orient position switch  1808 , to provide the input signals  1706 ,  1708 , respectively. This ensures that the wafer flat  1402  ( FIG. 14 ) is in the home position for loading and unloading of the wafer onto or from the platen, and is rotated to its preset angle prior to the chuck being retracted. The preset angle is process dependent and serves to avoid channeling by the ions when the wafer is subjected to the ion beam in the chamber. 
   While a specific embodiment has been described, it will be appreciated that the present invention is applicable to any wafer handlers that have similar chuck vacuum channel issues and an inability to properly monitor wafer alignment on the platen. The monitoring of the wafer position may even be performed using techniques other than monitoring air pressure in the vacuum channels. For instance, in another embodiment, light sensors were used to monitor the position of the wafer. Similarly, imaging techniques could be used to identify the position of the wafer, and have this relayed to an interlock circuit. Also, it will be appreciated that the circuit described in  FIG. 17  is only one example of an interlock circuit and that other circuits could be substituted to take a misalignment signal and use it to stop the platen from closing.