Abstract:
Air bearings support a rotating wafer carrying base in an RTP system. The base in proximity to the air bearing is protected from warping due to absorption of radiation from the hot wafer being treated. The most preferred embodiment splits the base into an inner disk carrying the wafer and an outer ring, where the inner ring which absorbs the most energy contacts and is supported at three points by the outer disk which is supported by the air bearing.

Description:
FIELD OF THE INVENTION. 
     The present invention relates to a system, apparatus, and method for more uniformly heating objects in a Rapid Thermal Processing (RTP) system. More specifically, the present invention discloses a convenient, inexpensive way to rotate semiconductor wafers treated in such system. 
     BACKGROUND OF THE INVENTION 
     The major problem faced by the field of RTP has been the uniformity of heating of the semiconductor wafers treated in the RTP systems. RTP systems generally have a chamber with at least one wall transparent to radiation from sources of radiation such as lamps. The object to be processed is placed in the chamber and irradiated with radiation from the radiation source so that the object is heated. The chamber with the transparent wall is not strictly necessary in the system, provided that the system controls the atmosphere in which the object is placed during processing. The lamps could then be placed in proximity to the object without the intervening window. Much progress has been made in using batteries of lamps with individual control of each lamp to increase uniformity of the illuminating radiation. However, the uniformity of the resulting material is not sufficient for present and future demands from the industry. 
     One way to increase the uniformity of result in such systems is to rotate the substrate under the lamps. Many prior art systems have been published to effect this rotation. However, these many systems generally used only one bank of lamps on one side of the semiconductor wafer. The other side of the wafer could then be used for various shafts which penetrated through the chamber walls to mechanically rotate the wafer with respect to the lamps. The prior art is deficient in that the systems are expensive and difficult to seal. The prior art systems also allow contaminants scrubbed from the relatively moving parts to contaminate the chamber. The prior art systems can not be used with banks of lights on either side of the wafer since the shaft, the rotating base holding the wafer, and the fittings necessary to allow the shaft to rotate with respect to the chamber block or otherwise interfere with light from the bank on the same side of the wafer as the shaft, and the resulting light impinging on the wafer is no longer uniform. 
     RELATED APPLICATIONS 
     Reactors based on the RTP principle often have the entire cross section of one end of the reactor chamber open during the wafer handling process. This construction has been established because the various wafer holders, guard rings, and gas distribution plates, which have significantly greater dimensions and may be thicker than the wafers, must also be introduced into the chamber and must be easily and quickly changed when the process is changed or when different wafer sizes, for example, are used. The reaction chamber dimensions are designed with these ancillary pieces in mind. US Patent  5 , 580 , 830  teaches the importance of the gas flow and the use of an aperture in the door to regulate gas flow and control impurities in the process chamber. 
     The importance of measuring the temperature of the wafer using a pyrometer of very broad spectral response is taught in U.S. Pat. No. 5,628, 564. 
     A method and apparatus for improved temperature control is taught in U.S. Pat. No. 5,841,110. 
     The wafer to be heated in a conventional RTP system typically rests on a plurality of quartz pins which hold the wafer accurately parallel to the reflector walls of the system. Prior art systems have rested the wafer on an instrumented susceptor, typically a uniform silicon wafer. Patent application Ser. No. 08/537,409, now U.S. Pat. No. 5,841,110 teaches the importance susceptor plates separated from the wafer. 
     Rapid thermal processing of III-IV semiconductors has not been as successful as RTP of silicon. One reason for this is that the surface has a relatively high vapor pressure of, for example, arsenic (As) in the case of gallium arsenide (GaAs). The surface region becomes depleted of As, and the material quality suffers. Patent application Ser. No. 08/631,265, now U.S. Pat. No. 5,837,555, supplies a method and apparatus for overcoming this problem. 
     A method of raising the emissivity of a lightly doped, relatively low temperature wafer by locally heating the wafer with a pulse of light is disclosed in application Ser. No. 08/632,364, now U.S. Pat. No. 5,727,017. 
     An inflatable seal for an RTP system is disclosed in copending allowed application Ser. No. 08/895,655, filed Jul. 17, 1997, by Aschner et al. 
     A method, apparatus, and system for RTP an object is disclosed in copending application Ser. No. 08/953,590, filed Oct. 17, 1997, by Lerch et al. 
     A method of RTP of a substrate where a small amount of a reactive gas is used to control the etching of oxides or semiconductor is disclosed in copending application Ser. No. 08/886,215, by Nenyei et al, filed Jul. 1, 1997. 
     A method of RTP of a substrate where evaporation of the silicon is controlled is disclosed in copending application Ser. No. 09/015,441, by Marcus et al. filed Jan. 29, 1998. 
     Methods of rotating the wafer in an RTP system are disclosed in applications Ser. Nos. 08/960,150 and 08/977,019 by Blersch et al. and Aschner et al. filed on Oct. 29. 1997 and Nov. 24, 1997 respectively. 
     The above identified patents and applications are assigned to the assignee of the present invention and are hereby incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     According to this invention, the object to be processed in an RTP system is placed on a rotating susceptor which is protected from warping due to uneven heating of the susceptor from radiation from the hot object. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art RTP processing system. 
     FIG. 2 shows a rotating base or susceptor  210  holding a wafer  110 . 
     FIG. 3 shows an alternative embodiment of the invention. 
     FIG. 4 shows a cross section of the most preferred embodiment of the invention. 
     FIG. 5 shows an alternative embodiment of the invention. 
     FIG. 6 shows an alternative embodiment of the invention. 
     FIG. 7 shows an expanded view of an enhanced version of the most preferred embodiment of the invention. 
     FIGS. 8A-E show detailed views of the most preferred embodiment of the invention. 
     FIGS. 9A-G show detailed views of the most preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a prior art RTP processing system. A semiconductor wafer  110  or other object to be processed is supported in a quartz RTP chamber  120  by quartz support pins  160  (only one shown). A guard ring  170  is used to lessen edge effects of radiation from the edge of the wafer  110 . An end plate  190  seals to the chamber  120 , and a door  180  allows entry of the wafer  110  and, when closed, allows the chamber to be sealed and a process gas  125  to be introduced into the chamber. Two banks of radiation sources  130  and  140  are shown on either side of the wafer  110 . A computer  175  or other control means as are known in the art is used to control the lamps  130  and  140 , and to control the gas flow controller  185 , the door  180 , and the temperature measuring system, denoted here as a pyrometer  165 . The gas flow may be an inert gas which does not react with the wafer, or it may be a reactive gas such as oxygen or nitrogen which reacts with the material of the semiconductor wafer to form a layer of on the semiconductor wafer, or the gas flow may be a gas which may contain a silicon compound which reacts at the heated surface of the object being processed to form a layer on the heated surface without consuming any material from the surface of the object. When the gas flow reacts to form a layer on the surface, the process is called rapid thermal—chemical vapor deposition (RT-CVD). An electrical current may be run through the atmosphere in the RTP system to produce ions which are reactive with or at the surface, and to impart extra energy to the surface by bombarding the surface with energetic ions. 
     FIG. 2 shows a rotating base or susceptor  210  holding a wafer  110 . Such a rotating base driven by a gas flow has been described in great detail in application Ser. No. 08/977,019 by Aschner et al. filed on Nov. 24, 1997. The base  210  is supported by air bearings  220 . A gas flow  230  impinging on the rotating base causes the base to rotate about axis  240 . A means for centering the base  210  is not shown in FIG.  2 . When the device described in application Ser. No. 08/977,019 is used for heating wafer  110  to high temperatures and relatively long times, the infra red radiation from the hot wafer  110  is partially absorbed by the base which is made of quartz or other material transparent to the radiation from the lamps  140  and may cause warping of the base so that the flat surfaces of the base needed to ride on the air bearings  220 , and the rotation may stop. The present invention details apparatus and methods to prevent such warping. One such method of preventing absorption and warping is shown in FIG. 2, where a layer  250  is shown deposited on or part of base  210 . The layer  250  may be a reflective layer which reflects the infrared radiation from the wafer, but transmits the visible and near infrared radiation from the lamps  140 . Such a reflective layer may be uniform over the base as shown, or it may be non uniformly applied to counteract the non-uniformity of the infrared radiation from the wafer impinging on the base. The layer  250  may also be an absorbing layer which absorbs radiation in a pattern to counteract the non uniform radiation from the wafer  110 . Another preferred embodiment of the invention is to dope the quartz glass of the base  210  with atoms or molecules which absorb radiation from the wafer, so that a radial gradient in concentration of the molecules or atoms, preferably increasing from the inner to the outer portions of the base  210 , is provided. The doping will result in a more uniform radial temperature profile of the base  210 , if the base is non uniformly irradiated mainly in the center region. Due to the more uniform radial temperature distribution of the base  210 , buckling of the wafer and the base is prevented. 
     FIG. 3 shows an alternative embodiment to prevent the infra red radiation from the wafer  110  heating and warping the base  210 . A plate  310  is interposed between wafer  110  and base  210  which absorbs radiation from the wafer  110  and prevents the infra red radiation from heating the base  210 . The plate  310  is preferably made of quartz, so that the heating radiation from the lamps  140  will be transmitted, while the longer wavelength radiation from the wafer  110  will be absorbed. The plate  310  may also be coated with a reflective or absorptive layer to control the temperature distribution of the plate  310  and the base  210 . A further solution is to dope the quartz glass of the plate  310  with atoms or molecules to get a radial gradient in concentration of the molecules or atoms which absorb radiation from the wafer, preferably increasing from the inner to the outer portions of the plate  310 . The doping will result in a more uniform radial temperature profile of the susceptor  310 , if the susceptor is non uniformly irradiated mainly in the center region. Due to the more uniform radial temperature distribution of the plate  310 , buckling of the wafer is prevented. The diameter of plate  310  is preferably approximately the same as the diameter of the wafer  110 . 
     FIG. 4 shows a cross section of the most preferred embodiment of the invention. The rotating base  410  of the invention is a ring which is supported by the air bearings  220 . The ring supports a plate  420  which is supported at a plurality of points  430 . The plate  420  is shown having a plurality of projections  440  for support (only one shown). Now, when plate  420  is heated by radiation from wafer  110 , it may expand within the ring of base  410 , and base  410  which receives relatively little radiation from wafer  110  will not be under so much stress to warp and cause problems riding on air bearings  220 . While projections  440  are shown attached to plate  410 , such projections could equally well be attached to base  410  to support plate  420  from the bottom. Once again, a centering post or detent arrangement which forces the base  410  and plate  420  to rotate about axis  240  is not shown. 
     FIG. 4 also shows a method of determining the angular position of base  410 . A light beam  450  shines through the base  410  and is detected by a detector  460 . Features  470  are placed on base  410  which change the light beam and thus may be detected by detector  460 . The preferred features are sandblasted features, which scatter the light beam  450  but do not otherwise interfere with the radiation from the lamps  140 . The most preferred features are the teeth of application Ser. No. 08/977,019 which have been sandblasted to interrupt light from a laser. As a convenience, there are 360 teeth arranged equidistant around the circumference of base  410 . An extra tooth is additionally used inserted in between two of the 360 teeth to produce an extra reference signal. Other preferred features may be absorptive features or reflective features. Features  470  may also be magnetic features which may be detected by a magnetic detector in place of an optical detector. 
     In order to prevent plate  420  rotating with respect to base  410 , plate  420  may engage base  410  with a tooth projecting from plate  420  into a detent in base  410 , or with the projections  440  engaged in detents in base  410 , or any suitable combination or other means as would be obvious to one skilled in the art. 
     To prevent imbalance of the ring  410  and the plate  420 , the features of the apparatus such as the projections  440 , the pin holding means for holding pins  160 , the detent in base  410 , the extra tooth of plate  410  are arranged in a suitable way to balance the whole apparatus. 
     An alternative embodiment of the invention is shown in FIG. 5. A base  510  in the form of a ring is joined to a plate  520  by a plurality of rods  530 . The rods  530  are sufficiently elastic to ensure that little stress is placed on base  510  when plate  520  is heated by radiation from the wafer. 
     An alternative embodiment of the invention is shown in FIG. 6. A base  610  has a series of cuts  640  formed in the plate to ensure that stress will not be transmitted from the inner part  620  to the outer part  630 . 
     The advantage of the embodiments described in FIGS. 2,  5 , and  6  is that the distortion of the rotatable substrate is extremely reduced, since the inner part of the rotating system is mechanically decoupled from the outer part, but the outer part is the essential part of the rotation means regarding the functionality of the air bearings. As a result, the bearing surfaces of the outer parts remain very parallel to the surfaces of the air bearings, even if the diameter of the rotating system is large or if the temperature of the wafer and inner parts of the rotation means is very high. 
     FIG. 7 shows an expanded view of an enhanced version of the most preferred embodiment of the invention. A lower quartz plate  701  has gas lines  702  to deliver gas to gas bearings  220  A center bearing  703 , preferably made of sapphire, serves to center the apparatus with respect to the plate  701 . The base  410  and plate  420  of FIG. 4 ride on the air bearings and are rotated by gas blown from an external tube (not shown). A series of optional elements  707 - 716  are shown, which control the infra red radiation from the wafer  110 . Elements  707  are hollow cylinders which hold pins  160  to support wafer  110 . An additional holding means  708  holds a ring comprised of segments such as  710  and  711 . Holding means  708  and ring segments  710  and  711  do not rotate, but are held by plate  701 . The ring segments  710  and  711  are preferably made from quartz, and shield the rotating base  410  from radiation reflected and radiated from the wafer  110  and especially from the guard ring,  714 ,  715 , and  712 . The guard ring,  714 ,  715 , and  712  is shown made from segments. The ring  710  and  711  and the guard ring,  714 ,  715 , and  712  are made from segments for cost reasons, and for ease of replacement if one segment is broken. However, these rings could be made from single pieces of material. The guard ring,  714 ,  715 , and  712  is preferably made from silicon, and the silicon is preferably coated to make sure that the guard ring is stable and the reflectivity and absorption characteristics do not change with time. 
     The holding means  708  is engaged with the quartz plate  701  via pins  704  and  709 . The ring segments  710  and  711  are supported on the hollow cylindrical shaped pins  708 A of the holding means  708 . Pins  713  are inserted into the hollow pins  708 A, and project through the ring segments  710  and  711  to support the guard ring segments  712 ,  714 , and  715 . One segment  712  is shown displaced from the plane of the other segments  714  and  715  to show that a segment may optionally be placed out of the plane (either higher or lower) of the guard ring to allow withdrawal of a robot arm which has introduced wafer  110  into the system and lowered it so that wafer  110  is coplanar with guard ring segments  714  and  715 . 
     An additional quartz plate  716  resting on quartz pins  709  has the advantage that turbulence of the hot gas above the wafer is minimized. 
     FIGS. 8A-E and  9 A-G show detailed views of the most preferred embodiment of the invention. In particular, a notch  822  in ring  410  received a tooth  932  on plate  420  so that ring  410  may drive plate  420 . Also shown are the sandblasted teeth  450 , and an extra tooth  825  which gives the computer a calibration point from which to count the number of teeth rotating past the optical detection means. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, withing the scope of the appended claims, the invention may be practiced otherwise then as specifically described.