Patent Publication Number: US-6222991-B1

Title: Method for rotationally aligning and degassing semiconductor substrate within single vacuum chamber

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 08/383,112 filed Feb. 3, 1995 now U.S. Pat. No. 5,982,986 on Nov. 9, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor substrate processing. More particularly, this invention relates to apparatus and method for rotationally aligning and degassing a semiconductor substrate in the same vacuum chamber. 
     2. Description of the Related Art 
     In the processing of semiconductor substrates or wafers in the formation of integrated circuit structures thereon, it is important that the wafer be thoroughly degassed to remove adsorbed gases, moisture, etc. from the wafer prior to, for example, performing a physical vapor deposition (PVD) process to deposit materials on the wafer by sputtering from a target in a vacuum processing chamber. Other processes such as advanced chemical vapor deposition (CVD) processing, may also require degassing of the wafer. Degassing prior to PVD processing conventionally is carried out at temperatures exceeding 350° C. for time periods of from about 40 seconds to about 2 minutes to remove sufficient gases from the wafer to assure a satisfactory deposition by sputtering. Outgassing of substrates during aluminum PVD is more severe than during the prior CVD steps because the PVD process is performed at much higher vacuums and somewhat higher substrate temperatures, both of which induce greater outgassing. Therefore, to avoid outgassing from contaminating the PVD process, the de-gassing of the wafer before the first PVD step must be more extensive than the de-gassing performed before the CVD steps. 
     Degassing of a wafer is conventionally carried out in one of two ways. One method used to degas a wafer comprises a radiant heating of the wafer, using heat lamps located external to the vacuum chamber containing the wafer, and positioned adjacent transparent windows through which the heat is radiated from the lamps to the wafer. This method is relatively low in cost, is fairly rapid, and does not require clamping the wafer to the wafer support within the vacuum chamber. However, the radiant heating method is unsatisfactory for temperatures in excess of 350° C., because the temperature of the wafer is not easily controlled, and the heating is usually not uniform across the entire wafer. Typical temperature nonuniformity across the wafer at 350° C. is greater than ±30° C. Furthermore, alignment of the rotational orientation of the wafer, during the degassing step, is usually not possible because the radiation from the heat lamps interferes with operation of the optical means conventionally used for such rotational alignment. 
     The other method conventionally used to degas a wafer, particularly when subsequent PVD processing will be carried out which requires degassing at temperatures in excess of about 350° C., comprises physically (mechanically) clamping the wafer to a wafer support in a vacuum chamber and then heating the wafer using a resistive heater located in the wafer support adjacent the undersurface of the wafer resting on the wafer support. However, since the wafer normally only physically touches the wafer support at the physically clamped periphery or edges of the wafer, and the transmission of heat from the heater in the wafer support to the underside of the wafer via conduction through a vacuum is very poor, a thermally-conductive gas is normally admitted into the space between the wafer support and the underside of the wafer, with the clamped edge of the wafer serving to at least partially retain the gas in this space. This heating method permits degassification temperatures of as high as about 500-600° C. to be achieved. 
     This method thus permits the use of degassing temperatures in excess of 350° C., and permits measurement and reasonable control of the temperature of the wafer. However, alignment of the rotational orientation usually cannot be carried out during the degassing step because the conduit for the thermally conductive gas inhibits rotation of the chuck. The clamping ring also inhibits rotation due to its weight. Rotation of a clamped wafer could also cause wafer breakage and particles. The alignment of the rotational orientation of the wafer must, therefore, be carried out in a separate chamber prior to the degassing step. Furthermore, because this form of degassing must be preformed in a chamber very similar to a PVD chamber (i.e., it must include a cryopump, heated chuck, wafer lift assembly, cryo isolation valve, transfer chamber, isolation valve, clamp ring, etc.), it is a very expensive solution. Also typical temperature uniformities across the wafer achieved with this type of degassing apparatus are approximately ±10 to 15° C. Temperature uniformities of ±5° C. are required for advanced devices. 
     Furthermore, regardless of which heating method is used, because of the extended time period needed for degassing prior to PVD processing, the degassing step can reduce process throughput. One prior art approach which has been considered for solving this particular problem is to provide parallel degassing chambers, i.e., two degassing chambers are provided in a semiconductor wafer processing apparatus for each PVD processing chamber. However, this adds considerable extra cost to the apparatus. In addition, when the rotational orientation of the wafer must also be carried out in a separate chamber, either three or four preprocessing chambers must be utilized (depending whether or not each of the two parallel degassing chamber is coupled to its own separate rotational orientation chamber), which greatly adds to the overall expense of the apparatus. 
     It would, therefore, be desirable to be able to consolidate the rotational alignment and degassing of the wafer into a single chamber which would avoid the expense of separate chambers, as well as the additional time consumed during transfer of the wafer from one chamber to the other. It would be of further advantage if the degassing could be carried out at high temperatures, i.e., temperatures in excess of about 350° C., without mechanically clamping the wafer to the wafer support, and while still maintaining an even and controllable heating of the wafer. It would be even more advantageous if both the degassing and the rotational orientation of the wafer could be carried out simultaneously in the same chamber at a high temperature and without mechanical clamping the wafer to the wafer support. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a semiconductor processing system is provided which is capable of degassing a semiconductor substrate at temperatures as high as 500° C. and also rotationally aligning the substrate in the same vacuum chamber, without the use of a mechanical clamping ring and thermally conductive gas. The apparatus of the semiconductor processing system includes a heated electrostatic clamping structure for supporting the semiconductor wafer and retaining the substrate in thermal communication therewith in the vacuum chamber, a heater within the electrostatic clamping structure for heating the electrostatically clamped substrate to degas it, a rotation mechanism for imparting rotation to the substrate in the same vacuum chamber, and a detector for detecting the rotational alignment of the substrate in the vacuum chamber in response to the rotation of the substrate. In a preferred embodiment, the substrate is rotated to rotationally align it as it is being heated to degas it without, however, using mechanical clamping apparatus to secure the substrate to a substrate support. In an alternate embodiment, the substrate may be rotated for alignment either prior to or after degassification, but in the same chamber, 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-sectional view in schematic form of a degasification and rotational alignment chamber of a semiconductor substrate processing apparatus comprising one embodiment of the invention wherein the substrate is rotationally aligned using a lift ring within the vacuum chamber to rotate the substrate. 
     FIG. 2 is an isometric view illustrating the optical orientation apparatus shown in FIG. 1 for rotationally aligning the substrate. 
     FIG. 3 is a vertical cross-sectional view in schematic form of a degasification and rotational alignment chamber of a semiconductor substrate processing apparatus comprising another embodiment of the invention wherein the substrate is rotationally aligned by rotation of the substrate support and electrostatic chuck therein. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention comprises a semiconductor processing system capable of degassing a semiconductor substrate or wafer at temperatures as high as 500° C. or higher, depending upon the temperature sensitivity of other materials already on the wafer, and also capable of aligning the rotational orientation of the wafer in the same vacuum chamber, without the use of a mechanical clamping ring and thermally conductive gas. The system utilizes an electrostatic clamping means for retaining the semiconductor wafer in thermal communication with a wafer support in the vacuum chamber while the wafer is heated by a heater within the wafer support to degas it. A rotation mechanism for imparting rotation to the wafer in the vacuum chamber and a detector for detecting the rotational alignment of the wafer in response to the rotation of the wafer are also provided. In a preferred embodiment, the wafer is simultaneously rotated to rotationally align it while it is being heated to degas it. 
     Turning now to FIG. 1, one embodiment of the system of the invention is generally illustrated wherein a vacuum chamber  2  is provided with a wafer support  10 , which may comprise a stainless steel material, mounted on a pedestal  12 . Forming the top surface of wafer support  10  is an electrostatic clamping means or chuck  20  which, in the illustrated embodiment, comprises an insulative material  24 , such as aluminum oxide, aluminum nitride, or other ceramic material, on the top surface of wafer support  10  and having embedded therein metallic electrodes  26  and  27  which are connected through leads  28  and  29  to a high voltage source (not shown) external to vacuum chamber  2 . Also embedded within chuck  20  is a heater  14 , such as a resistance heater, which may be connected through lead(s)  16  to a power source (not shown) external to vacuum chamber  2 . The inside of wafer support  10  and pedestal  12  are at atmospheric pressure. Wafer support  10  is brazed to ceramic chuck  20  to provide a vacuum seal. 
     A wafer  30 , to be degassed and rotationally aligned, may be placed on electrostatic chuck  20  and electrodes  26  and  27  energized with a high voltage, e.g., about 500-5000 volts DC, to thereby electrostatically clamp wafer  30  to the surface of electrostatic chuck  20 . Wafer  30  is removed from the system transfer robot (not shown) for placement on electrostatic chuck  20  (and later removal) by lift pins or fingers (not shown) on a ring (also not shown) attached to support plate  51  which, in turn, is connected to a pneumatic or motor-driven lift motor  48  and shaft  50  through a vacuum isolation bellows  54 . 
     Heater  14  is energized to thereby heat electrostatic chuck  20  which then heats wafer  30  through direct conduction. It should be noted that unlike prior art securement of the wafer to an upper surface of a wafer support during the heating of the wafer, not only is the periphery of the wafer in thermal contact with electrostatic chuck  20  (to provide thermal coupling therebetween), but all of the undersurface of wafer  30  is also in mechanical contact with electrostatic chuck  20  and therefore thermally coupled to electrostatic chuck  20  due to the uniformity of the electrostatic forces across the surface of electrostatic chuck  20 . Heater  10  advantageously is activated prior to the electrostatic clamping of wafer  30  to electrostatic chuck  20  to preheat electrostatic chuck  20  and thereby accelerate the heating process. Because of the intimate contact of the wafer to the heated electrostatic chuck, gas between the wafer and the chuck is not required. 
     Wafer  30  is also rotationally aligned in vacuum chamber  2 . Alignment of the rotational or angular orientation of a semiconductor wafer is necessary to provide the correct rotational alignment of a semiconductor wafer in a processing chamber, as is well known to those skilled in the art. Such rotational alignment is facilitated by the provision of some sort of alignment indicia on the wafer itself. A common alignment means is the provision of a flat or notch on one portion of the circumference of a normally circular wafer. A beam of light from a light source is then usually directed perpendicular to the plane of the wafer to intercept the wafer adjacent its edge. As the wafer is rotated, the light is reflected back to the source until the flat or notched portion is encountered, as which point the light beam is transmitted to a photo detector positioned on the other side of the wafer. 
     As shown in FIG. 1, a ring  40  may be provided to rotate wafer  30  to rotationally align wafer  30  in vacuum chamber  2 . When it is desired to rotate wafer  30 , the wafer is lowered onto rotatable ring  40 . Wafer  30  is lowered onto ring  40  by activation of fluid powered motor  90  to which is attached a shaft  92 , as shown in FIG. 1, which is centrally mounted within pedestal  12 . Shaft  92  is coupled to the upper portion of pedestal  12  by a cross bar  94 . Bellows  13  on pedestal  12  permit the upper portion of pedestal  12 , with support  10  and electrostatic chuck  20  secured thereto, to move up and down (vertically) while maintaining the vacuum within chamber  2 . This, in turn, permits the desired lowering of wafer  30  onto rotatable ring  40 , and subsequent raising of wafer  30  off ring  40  when the orientation step is complete. 
     Ring  40  is provided with arms  42  which are, in turn, connected to a central cylinder  44  to which are attached a first set of magnets  46  which form a part of magnetic coupling mechanism  52 . A hollow shaft  60  located within pedestal  12  has a second set of magnets  62  mounted thereon forming the other portion of magnetic coupling mechanism  52 . A motor  64  rotates shaft  60  via a belt  66  and this rotation is transmitted through magnetic coupling mechanism  52  to cylinder  44  and ring  40  to thereby rotate wafer  30 . 
     As wafer  30  is rotated on ring  40  by motor  64 , a light source  70 , external to vacuum chamber  2 , directs a light beam  72  through a first window  4  in the top wall of vacuum chamber  2  toward the top surface of wafer  30  adjacent the periphery thereof. When flat portion or notch  32  of wafer  30  is encountered, as shown in FIG. 2, light beam  72  passes through to a second window  6  located in the bottom wall of vacuum chamber  2  and is detected by photodetector  80 , signifying the rotational position of the flat or notched portion  32  of wafer  30 . 
     In the embodiment shown in FIGS. 1 and 2, the rotational alignment of wafer  30  is carried out in the same vacuum chamber as the degassification of wafer  30 . However, the rotational alignment and degassification are carried out sequentially, rather than simultaneously. The rotational alignment may be carried out either before or after the degassifying of wafer  30 . 
     It would, however, be even more advantageous if, in addition to using the same vacuum chamber for both rotational orientation and degassifying of the wafer, both steps could be carried out simultaneously. FIG. 3 illustrates another embodiment of the invention which permits such simultaneous rotational orientation and degassifying of a semiconductor wafer by rotating the wafer support and electrostatic chuck with the wafer clamped thereto so that the wafer continues to be heated and therefore degassified while the rotational orientation of the wafer is carried out by the light source and photodetector. 
     In FIG. 3, wherein like elements are identified with like numerals, the pedestal beneath wafer support  10  comprises a hollow cylinder  112  with its cylindrical wall magnetically coupled through magnetic coupling mechanism or clutch  152  to a hollow cylindrical shaft  160  external to vacuum chamber  2 . Cylindrical shaft  160  is, in turn, connected to a motor  164  which rotates cylindrical shaft  160  and this rotation is transmitted through magnetic coupling  152  to cylindrical pedestal  112  to thereby rotate wafer support  10 , electrostatic chuck  20 , and wafer  30  clamped thereto. 
     A flexible heater lead  116  connects heater lead  16  within vacuum chamber  2  to an external heater lead  118 ; while flexible high voltage leads  128  and  129  connect high voltage leads  28  and  29  with external high voltage lead  138  and  139 . This provision of such flexible leads permits rotation of wafer support  10 , for example, 180° in each direction while still maintaining electrical contact respectively to heater  14  and electrostatic chuck electrodes  26  and  27 . 
     As described in the previous embodiment, as wafer  30  is rotated by motor  164 , light source  70 , external to vacuum chamber  2 , directs light beam  72  through first window  4  in the top wall of vacuum chamber  2  toward the top surface of wafer  30  adjacent the periphery thereof. When flat portion  32  of wafer  30  is encountered, as previously shown and described in FIG. 2, light beam  72  passes to and through second window  6  located in the bottom wall of vacuum chamber  2  and is detected by photodetector  80 , signifying the rotational position of flat or notched portion  32  of wafer  30 . 
     Thus the semiconductor wafer processing system of the invention permits degassifying and rotational alignment of a semiconductor wafer to be carried out in the same vacuum chamber with temperatures above 350° C. being utilizable without, however, mechanical clamping the wafer to the wafer support. In a preferred embodiment, rotational alignment and degassification of the semiconductor wafer may be carried out simultaneously in the same chamber.