Abstract:
Semiconductor processing apparatus, including a chamber, into which a semiconductor wafer is introduced for processing thereof and a heater, which heats the wafer in the chamber. A radiation guide collects thermal radiation from a selected region of the wafer. A wafer support assembly supports the wafer and shields the radiation guide from radiation other than radiation from the region. A pyrometer, coupled to receive the radiation from the guide, analyzes the radiation to determine a temperature of the region, for use in controlling the processing.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to apparatus for thermal processing of materials, and specifically for processing semiconducting wafers at variable temperatures. 
     BACKGROUND OF THE INVENTION 
     Thermal processing of semiconducting wafers, such as silicon wafers, to produce semiconductor devices is well known in the art. The processing typically comprises maintaining a wafer at a different, known temperatures for predetermined times. During the processing, it is necessary to know the temperature of the wafer to a high accuracy in order to achieve repeatable results. Pyrometers, which measure temperature based on infrared emission and reflection from the wafer, are commonly used to monitor the temperature. Accurate pyrometric temperature measurement is dependent, however, on knowing the emissivity of the wafer, which varies with prior processing steps, temperature, and process steps performed while the measurement is being made. 
     Rapid Thermal Processing (RTP) is a state-of-the-art process used to manufacture semiconductor devices, wherein a wafer, or a region of a wafer, is cycled rapidly through a series of predetermined temperatures. Often rapid thermal processing is combined with chemical vapor deposition (RTCVD). Both processes rely on accurate, real-time determinations of the temperature of the region being processed. A review of RTP entitled  Rapid Thermal Processing Science and Technology  (ISBN 0-12-247690-5), Academic Press, California (1993), is herein incorporated by reference. Chapter 9 of the review indicates that processing efficiency is critically dependent on accurate temperature measurement of wafers during the processing. 
     In “Advances in Temperature Measurement and Control for RTP,” by Peuse et al., 5 th International Conference on Advanced Thermal Processing of Semiconductors—RTP  1997, which is incorporated herein by reference, the authors describe processing of semiconductor wafers using an RTP system known as the Centura system, produced by Applied Materials Inc., of Santa Clara, Calif. The system relies on placing the wafer in a highly reflective enclosure, so as to eliminate as nearly as possible the effects of variations in emissivity of the wafer, whereby the total enclosure closely approximates a black body. (The theory of pyrometric temperature determination by analysis of radiation from an object requires that an emissivity or an effective emissivity of the object be known; a black body has an emissivity of unity.) U.S. Pat. No. 5,490,728, to Schietinger et al., which is incorporated herein by reference, describes a non-contact pyrometric technique for measuring characteristics, including temperature, of a substrate. The technique estimates the emissivity of a wafer by comparing the amplitude of ripple flux in radiation reflected from the wafer with an incoming ripple flux amplitude due to an AC-powered heating lamp. 
     U.S. Pat. No. 5,255,286, to Moslehi et al., which is incorporated herein by reference, describes a method of optical pyrometry based on irradiating a semiconductor wafer with monochromatic coherent radiation at a wavelength of the order of 5 μm. Intensities of reflected coherent radiation and emitted incoherent radiation from the wafer are used to determine a value of the emissivity and of the temperature of the wafer. 
     In “Optical Pyrometry in RTP/RTCVD Systems: A New Approach,” by Glazman et al., 6 th International Conference on Advanced Thermal Processing of Semiconductors—RTP&#39; 98, which is incorporated herein by reference, the authors describe a technique for measurement of the temperature of a semiconducting wafer using optical pyrometry. The technique uses an active multi-spectral system to dynamically measure the true emissivity of the wafer. The measured emissivity is utilized together with measurements of radiation emitted by the wafer in order to evaluate the temperature of the wafer. 
     SUMMARY OF THE INVENTION 
     It is an object of some aspects of the present invention to provide methods and apparatus for improved thermal processing of materials. 
     It is a further object of some aspects of the present invention to provide methods and apparatus for improved temperature measurement of materials. 
     It is a further object of some aspects of the present invention to provide methods and apparatus for improved thermal processing of moving regions of materials. 
     In preferred embodiments of the present invention, an object to be thermally processed, typically a semiconducting wafer, is placed in a chamber wherein the object is heated to above the ambient temperature. The wafer is held within the chamber by a wafer support assembly, a section of which, generally opposite the wafer, is preferably cooled to a temperature substantially below that of the wafer. The wafer support assembly is constructed so that the assembly and a surface of the wafer together form a substantially closed cavity, so as to exclude therefrom radiation from sources other than the wafer surface. 
     The temperature of a region of the surface of the wafer is measured by collecting radiation from the region in a distal end of a radiation guide, preferably comprising a fiberoptic. The radiation guide preferably passes through and is thermally coupled to the cooled section of the assembly. The guide is shielded from radiation, other than radiation from the region, by the structure of the closed cavity. The radiation guide is radiatively coupled at its proximal end to a pyrometer, which determines the temperature of the region responsive to the collected thermal radiation. The shielding of the radiation guide from extraneous radiation and the reduction of thermal radiation entering the guide by virtue of cooling of the support structure substantially improve the accuracy of the temperature measurement of the particular region of the wafer at which the fiberoptic is directed. The ability to process the wafer at accurately known temperatures significantly improves the efficiency and speed of thermal processing, compared to methods and systems at present known in the art. Furthermore, thermally coupling the radiation guide to the cooled section of the assembly cools the distal portion of the guide, and thereby prolongs the life of the guide. 
     Preferably, the pyrometer measures the temperature of the region of the wafer based on a known emissivity of the region. Alternatively, the pyrometer is able to measure the temperature independently of prior knowledge of an emissivity of the region, as is described in the above-referenced patents and publications, for example, or as is otherwise known in the art. It is not necessary that the region be in thermal equilibrium with the chamber. 
     In some preferred embodiments of the present invention, the wafer undergoing thermal processing is mounted substantially horizontally inside a vacuum-tight chamber. The chamber comprises inlet and outlet tubing which are used to evacuate the chamber and to enable the ingress of gases used for the processing. Preferably, the wafer support assembly whereon the wafer is mounted is rotatable about a vertical axis, and rotates the wafer in the chamber in order to improve processing uniformity across the wafer. The chamber further comprises heaters, most preferably incandescent lamps positioned above the wafer, which are used to heat the wafer. The radiation guide, substantially transparent to radiation emitted from the heated wafer, is fixedly mounted generally along the axis of the wafer support assembly so as to rotate with the assembly and the wafer. An upper (distal) end of the rotating guide is positioned close to, but not touching, a region of the wafer whose temperature is to be measured, in order to collect thermal radiation from the region. 
     A lower (proximal) end of the rotating guide is optically coupled to a distal end of a fixed radiation guide, which is preferably aligned axially with the rotating guide. Most preferably, the diameter of the fixed guide is substantially equal to the diameter of the rotating guide, so that there is minimal loss when radiation transfers between the guides. The radiation collected by the upper end of the rotating guide is thus transferred to the fixed guide and is further transferred by the fixed guide to a radiation pyrometer, wherein a temperature of the region is evaluated responsive to measurements made on the radiation, by methods known in the art. 
     There is therefore provided, in accordance with a preferred embodiment of the present invention, semiconductor processing apparatus, including: 
     a chamber, into which a semiconductor wafer is introduced for processing thereof; 
     a heater, which heats the wafer in the chamber; 
     a radiation guide, which collects thermal radiation from a selected region of the wafer; 
     a wafer support assembly, which supports the wafer and shields the radiation guide from radiation other than radiation from the region; and 
     a pyrometer, coupled to receive the radiation from the guide, and which analyzes the radiation to determine a temperature of the region, for use in controlling the processing. 
     Preferably, at least a portion of the wafer support assembly is cooled to a temperature substantially below the temperature of the region. 
     Preferably, a portion of the radiation guide in proximity to the wafer is in thermal communication with the cooled portion of the wafer support assembly. 
     Preferably, the wafer and the wafer support assembly form a cavity that is substantially closed against entry of extraneous radiation. 
     Alternatively, the wafer support assembly varies the position of the wafer and varies the position of the radiation guide in cooperation therewith. 
     Preferably, the apparatus includes a fixed radiation guide, which is coupled to the variably positioned radiation guide so as to transfer the radiation between the guide and the pyrometer. 
     Alternatively, the apparatus includes a connector coupling the fixed guide to the variably positioned guide in mutual axial alignment so as to enable radiation transfer therebetween. 
     Preferably, the radiation is transferred between the fixed and variably positioned guides substantially without optical elements intervening between the guides. 
     Preferably, the variably positioned guide rotates about a longitudinal axis common to the fixed and variably positioned guides. 
     Preferably, the pyrometer determines the temperature of the region substantially without prior knowledge of an emissivity of the region. 
     Alternatively, the pyrometer transmits radiation into the guide and measures a reflectance of the wafer responsive to the transmitted radiation, so as to estimate the emissivity of the wafer. 
     Preferably, the assembly includes a vertical drive mechanism, which drives the radiation guide to a position in proximity with the wafer. 
     Preferably, the assembly includes an axial drive mechanism, which rotates the radiation guide together with the wafer. 
     Preferably, the radiation guide is fixedly mounted in the wafer support assembly so as to rotate therewith. 
     Alternatively, the thermal radiation is substantially non-coherent. 
     There is further provided, in accordance with a preferred embodiment of the present invention, a method for thermal processing of an object, including: 
     heating the object; 
     bringing a thermal radiation collector into proximity with a selected region of the object; 
     shielding the radiation collector from radiation other than radiation from the region; 
     collecting thermal radiation from the region using the radiation collector; and 
     determining a temperature of the region responsive to the collected thermal radiation, whereby the processing is controlled responsive to the determined temperature. 
     Preferably, shielding the radiation collector includes cooling a structure in a vicinity of the object and the collector to a temperature substantially below the temperature of the region of the object. 
     Preferably, cooling the structure includes cooling a portion of the radiation collector in thermal communication with the structure. 
     Preferably, shielding the radiation collector includes supporting the object in such a manner as to shield an entrance to the radiation collector. 
     Preferably, bringing the radiation collector into proximity with the region includes forming a substantially closed cavity adjacent to the object and collecting radiation includes collecting radiation from within the cavity. 
     Alternatively, the method includes varying a position of the region and concomitantly varying a position of the radiation collector. 
     Preferably, varying the position of the region includes rotating the object, and varying the position of the radiation collector includes rotating the collector together with the wafer. 
     Preferably, collecting the radiation includes coupling a fixed radiation collector to the rotating collector in mutual axial alignment so as to enable radiation transfer therebetween, responsive to which the temperature is determined. 
     Preferably, the radiation is transferred between the fixed and rotating collectors substantially without optical elements intervening therebetween. 
     Preferably, determining the temperature includes determining a temperature of the region without prior knowledge of an emissivity thereof. 
     Alternatively, determining the temperature includes transmitting radiation to the region and measuring a reflectance of the region responsive to the transmitted radiation, so as to estimate the emissivity of the region. 
     There is further provided, in accordance with a preferred embodiment of the present invention, semiconductor processing apparatus, including: 
     a chamber, into which a semiconductor wafer is introduced for processing thereof; 
     a heater, which heats the wafer in the chamber; 
     a radiation guide, which collects thermal radiation from a predetermined region of the wafer; 
     a wafer support assembly, which supports the wafer, at least a portion of which assembly in proximity to the region and the radiation guide is cooled to a temperature substantially below a temperature of the region; and 
     a pyrometer, which is coupled to receive the radiation from the guide, and which analyzes the radiation to determine the temperature of the region, for use in controlling the processing. 
     There is further provided, in accordance with a preferred embodiment of the present invention, a method for thermal processing of a region of an object, including: 
     heating the region; 
     bringing a thermal radiation collector into proximity with the region; 
     cooling a structure in proximity to the region of the object and to the radiation collector to a temperature substantially below a temperature of the region of the object; 
     collecting thermal radiation from the region of the object using the radiation collector; and 
     determining the temperature of the region of the object responsive to the collected thermal radiation, whereby the processing is controlled responsive to the determined temperature. 
     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic, sectional drawing illustrating thermal processing apparatus, in accordance with a preferred embodiment of the present invention; 
     FIG. 1B is an enlargement showing details of an upper portion of the apparatus of FIG. 1A; 
     FIG.  2 A and FIG. 2B are schematic, sectional diagrams illustrating further details of an upper section of the apparatus of FIG. 1A, and showing a wafer in a loading position and in a processing position, respectively, in accordance with a preferred embodiment of the present invention; and 
     FIG. 3 is a schematic, sectional diagram showing details of a lower section of the apparatus of FIG. 1A, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1A, which is a schematic, sectional drawing illustrating thermal processing apparatus  10 , and to FIG. 1B, which is an enlargement of an upper portion  81  of FIG. 1A, in accordance with a preferred embodiment of the present invention. Apparatus  10  comprises an evacuable process chamber  12 , wherein a wafer  14  to be processed is placed by a robot arm  18 . For example, process chamber  12  may be generally similar to that provided in the thermal processing apparatus “IntegraPro,” manufactured by AG Associates (Israel) Ltd., of Migdal Ha&#39;Emek, Israel. Wafer  14  is processed by being heated in chamber  12  for one or more predetermined times to one or more predetermined temperatures in the presence of one or more selected gases. For example, wafer  14  may comprise a polysilicon wafer heated to 625° C. for 60 s in the presence of silane at 10 torr. Wafer  14  is most preferably heated by radiant heaters (not shown) irradiating the wafer via a radiation-transparent window  16 . Typically, the one or more of the gases introduced into chamber  12  cause chemical alteration of a surface of wafer  14  by chemical vapor deposition (CVD) thereon. 
     A pyrometer  90  is radiatively coupled to chamber  12  and is used to measure a temperature of wafer  14 , as described in detail hereinbelow. Preferably, prior or subsequent to wafer  14  being placed in chamber  12 , a plurality of test wafers having different, known reflectivities are sequentially placed by robot arm  18  in chamber  12 , without opening the chamber. Most preferably, the wafers having known reflectivities are maintained at room temperature within chamber  12 . The wafers are used for the purpose of calibrating pyrometer  90 , so that the pyrometer is capable of measuring the temperature of wafer  14  without advance knowledge of the wafer emissivity. 
     A wafer support assembly  24 , which is substantially axially symmetric about a vertical axis  26 , is disposed within chamber  12  in order to receive and retain wafer  14 . Wafer support assembly  24  is carried at the upper end of a vertical shaft  32 , which is driven in a vertical direction by a vertical drive  34 , and in a rotary direction by a rotary drive  36 . The vertical motion of shaft  32 , and of a lower section  80  of apparatus  10  to which shaft  32  is coupled, is accommodated by a bellows  82 . Thus shaft  32  is able to rotate within apparatus  10  while an outer housing  84  of apparatus  10  remains stationary. However, vertical drive  34  acts on both shaft  32  and outer housing  84 , so that both are raised or lowered simultaneously. 
     A base  25  of assembly  24  has a plurality of passages  27  passing therethrough, which passages are coupled to tubes  29  in shaft  32 . Cooling water is transferred via tubes  29  to passages  27 , in order to maintain base  25  at a temperature substantially below that of wafer  14  when the wafer is heated. 
     FIG.  2 A and FIG. 2B are schematic, sectional diagrams of a portion of process chamber  12 , showing wafer  14  in a loading position and in a processing position, respectively, in accordance with a preferred embodiment of the present invention. Wafer support assembly  24  comprises a plurality of supports  20 , preferably three supports, which are disposed symmetrically about axis  26  and which are able to move vertically in respective sleeves  22  contained in assembly  24 . During loading of wafer  14  into chamber  12 , wafer support assembly  24  is positioned in a lowered position (FIG. 2A) by drive  34 , so that supports  20  are vertically extended above assembly  24  in order to receive wafer  14  from robot arm  18 . Supports  20  are maintained in the extended loading position by a support stop  84 , which is preferably in the form of a ring. 
     Once wafer  14  is in position, wafer support assembly  24  is raised by drive  34 , so that supports  20  and wafer  14  move vertically downward relative to assembly  24  due to their own weight. Wafer support assembly  24  comprises an annular slip-free ring  28  and a ring retainer  30  which fixedly and symmetrically holds ring  28  in place relative to base  25 . Assembly  24  is raised until wafer  14  contacts ring  28 . Raising the head assembly further causes supports  20  to retract into assembly  24  (FIG.  2 B). Preferably, ring  28  is constructed from aluminum nitri-decoated graphite or polysilicon in order to provide a slip-free, heat-resisting support for wafer  14 . Preferably, base  25 , supports  20 , and retainer  30  are made of aluminum nitride-coated Inconel or stainless steel. Other materials may also be used, as will be clear to those skilled in the art. 
     Once assembly  24  is in its raised position, wafer  14  is processed by introduction of gases into thermal contact with wafer  14 , and purging of the gases, via ports  31  (FIG. 1B) in chamber  12 , as is known in the art. During processing, assembly  24  and wafer  14  are most preferably rotated about axis  26  by means of drive  36 , thus varying the position, including the angular orientation, of regions within the wafer over time, in order to enhance the uniformity of the temperature of wafer  14 , and in order to enhance the uniformity of chemical deposition thereon. 
     The temperature of a lower surface  38 , substantially centered on axis  26 , of wafer  14 , is evaluated by measurements made of thermal radiation from the surface as described hereinbelow. A radiation guide  40 , which is substantially transparent to thermal radiation from region  38 , is axially disposed within shaft  32 . Guide  40  is most preferably made from quartz or sapphire, and is fixedly connected to shaft  32  so that guide  40  is in good thermal and mechanical contact with base  25 , and so that the guide rotates as shaft  32  rotates. Guide  40  preferably has a diameter of the order of 3 mm, and is positioned so that an upper end  42  thereof is of the order of 7-10 mm from lower surface  38 , so that thermal radiation from surface  38  is collected by guide  40 . Guide  40  transfers thermal radiation from upper end  42  to a lower end  44  of guide  40 . 
     Upper end  42  of guide  40  is shielded from radiation other than that produced by lower surface  38  by being part of a substantially closed cavity  33  defined by wafer  14 , base  25 , ring  28 , and retainer  30 . Furthermore, since base  25  is cooled to a temperature substantially below the temperature of surface  38 , intrinsic thermal radiation from base  25  and objects in thermal contact with the base will be significantly reduced. Thus, the fraction of radiation received by guide  40 , other than radiation from surface  38 , is reduced, compared to systems where shielding and/or cooling are not utilized, so that the accuracy of temperature measurements of surface  38  based on the thermal radiation received at lower end  44  of guide  40  is significantly enhanced. Furthermore, unlike systems known in the art, since cavity  33  is not required to act as a black-body cavity, there is no requirement for the cavity to reach thermal equilibrium or for the surface of base  25  to have a particular, known reflectance. 
     FIG. 3 is a schematic, sectional diagram of a lower section of apparatus  10 , showing details of coupling of guide  40  to pyrometer  90 , in accordance with a preferred embodiment of the present invention. Guide  40  protrudes beneath a lower end  46  of shaft  32 , and a lower region  48  of guide  40  is clamped by a guide holding assembly  50 . Assembly  50  comprises a guide adapter  52 , an O-ring  54 , and a guide housing  56 , which are all axially symmetric, and through all of which guide  40  passes substantially axially. Holding assembly  50  is fixedly attached to guide  40  by tightening a plurality of screws  60  between adapter  52  and housing  56 , in such a way as to compress O-ring  54 , so that lower end  44  of guide  40  is substantially aligned with a base  64  of housing  56 . 
     Assembly  50  further comprises a bearing  62 , most preferably a tapered roller bearing, for example a 32004X bearing manufactured by SKF Inc. of Gothenburg, Sweden, fixedly connected by an inner ring  66  to an exterior surface of housing  56 . An outer ring  68  of bearing  62  is fixedly connected to a pyrometer guide holder  70 , wherein is positioned, as described in more detail below, a pyrometer radiation guide  72 , an inner section of which is substantially transparent to the thermal radiation from region  38 . Holder  70  is connected by a floating mechanism  7 land by a coupling bracket  76  to a housing  74 , in order to eliminate wobble between guide  40  and guide  72 . 
     Thus, as shaft  32  rotates under the action of drive  36 , guide  40  and guide holding assembly  50  also rotate at the same rate, while guide holder  70  and guide  72  remain fixed. As shaft  32  is moved vertically by drive  34 , lower section  80 , including guide holder  70  and guide  72 , moves vertically. 
     Guide  72  is adjustably fixed within holder  70  by floating mechanism  71 , which comprises a split hub clamp  86  and spring  88  and bearing  62 , so as to substantially align the axes of guide  72  and guide  40 , and so that an upper end  92  of guide  72  is of the order of 0.5 mm from lower end  44  of guide  40 . Most preferably, the inner section of guide  72  has a diameter substantially equal to the diameter of guide  40 , so that there is substantially no loss of thermal radiation as radiation transfers between guide  40  and guide  72 . Guide  72  is most preferably flexible, so that pyrometer  90  connected to a lower end of guide  72  may remain fixed during any vertical motion of guide holder  70 . For example, guide  72  comprises an EX524 optical conduit manufactured by Dolan-Jenner Industries Inc. of Lawrence, Mass., which has an external diameter of about 5 mm. 
     Guide  72  transfers thermal radiation received from guide  40  to pyrometer  90 , which evaluates the radiation in order to determine a value of the temperature of region  38 . The guides may also be used to transfer radiation from the pyrometer to wafer  14 , so as to measure reflection of the radiation from the wafer, in addition to thermal emission therefrom. Preferably, pyrometer  90  is able to evaluate the temperature of region  38  without prior knowledge of an emissivity of the region. Thus, unlike semiconductor processing chambers known in the art, apparatus  10  enables wafer  14  to be processed based on the temperature measured in a specific, predetermined region of the wafer, without the necessity of equilibrating the temperature or enclosing the wafer so as to bring the emissivity to an effective value close to 1. Consequently, wafer processing, particularly RTCVD-type processing, in apparatus  10  proceeds with greater speed and more accurate control of processing parameters than in currently-available systems. 
     Preferably, pyrometer  90  irradiates region  38  with a radiant source generating a radiation of a known wavelength and energy, and evaluates the temperature of the region responsive to reflected and intrinsic radiation from the region. Such pyrometers are described, for example, in the patent by Moslehi et al., and in the article by Glazman, et al., cited in the Background of the Invention. The pyrometer both transfers the irradiating radiation from guide  72  to guide  40 , and thence to region  38 , and receives the reflected and intrinsic radiation back through guide  40  and guide  72 . Alternatively, pyrometer  90  is able to evaluate the temperature of region  38  when an effective emissivity of the region is known, for example, by methods of calibration known in the art. Preferably, the temperature evaluated by pyrometer  90  is used as a parameter in a closed-loop control system used to maintain the temperature of wafer  14  at a predetermined value, by methods known in the art. 
     Although the preferred embodiment described hereinabove illustrates a certain configuration of processing apparatus  10 , those skilled in the art will appreciate that the principles of the present invention may be applied in a wide range of processing systems. Such systems may employ some or all of the elements of the present invention in different combinations and configurations. For example, the methods of shielding a radiation guide from extraneous radiation when transferring radiation for measuring the temperature of a wafer, and/or cooling regions close to the wafer to a temperature substantially below the temperature of the wafer, thereby increasing the accuracy of the temperature measurement, will be useful in many different processing contexts. All such alternative combinations, configurations and applications are considered to within the scope of the present invention. 
     It will thus be appreciated that the preferred embodiment described above is cited by way of example, and the full scope of the invention is limited only by the claims.