Abstract:
An apparatus and method for thermally processing a substrate employs lift pin for supporting or contacting the substrate while conveying radiation from the substrate to a detector and/or processor through a hollow member. The lift pin comprises a contact member flexibly mounted on the hollow member to adjust to the angle of the substrate. By conforming the orientation of the contact member to the angle of the substrate, accurate detection and processing of the substrate may be performed.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to apparatus and methods of thermally processing a material such as a semiconductor substrate, and, in particular, to apparatus and methods for processing of a semiconductor substrate requiring precise measurement and control of the temperature over a wide range of temperatures. One example of such processing is rapid thermal processing (RTP), which is used for a number of fabrication processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). In the particular application of CMOS gate dialectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dialectrics are critical parameters that influence the overall device performance and fabrication yield. At least some of these processes require that temperature across the substrate vary by less than a few degrees Celsius. 
     During thermal processing, precise measurement of the substrate temperature may be required at various stages of the processing, including such stages as heating or preheating of the substrate. During the preheating stage, the substrate is typically held on one or more lift pins while it is heated by one or more radiative heat sources. During this heating process, the temperature of the substrate must be accurately determined. This determination may be provided to a feedback controller to adjust the power of the radiative heat source(s), thereby optimizing the heating process. 
     Measurement of the substrate temperature is conducted by various means. At temperatures above approximately 300-325EC, pyrometers may be employed to measure the substrate temperature based on the radiation which is emitted by the substrate. At lower temperatures, the emission intensity from the substrate (within the wavelength range to which pyrometers are most sensitive) generally is insufficient for the pyrometers to accurately measure substrate temperature. Contact probes (e.g., thermocouples) are therefore used to monitor substrate temperature, at lower temperatures. Such direct temperature measurement techniques, however, are difficult to reliably implement because of various problems, including degradation of the contact probe and maintenance of a stable thermal contact between the probe and the substrate. 
     One particular method of measuring the substrate temperature, including measurements below temperatures of approximately 300-325EC, which typically occur during preheating, involves measuring the temperature through the lift pins which support the substrate during heating. These lift pins typically comprise a hollow body through which radiation is transmitted. One end of the hollow body is in contact with the substrate, while the other end is connected to a detection and/or processing system. Radiation emitted from the substrate passes through the hollow body, and is conveyed to the detection system. The intensity of the radiation at certain wavelengths is then measured, and the temperature is derived from that intensity. U.S. Pat. No. 6,151,446, which is incorporated herein by reference, describes such a method. 
     Such systems, however, still suffer from a lack of accuracy. The determination of the substrate temperature is highly dependent on the intensity of the radiation received by the detector. If the radiation received by the detector does not accurately represent the light emitted by the substrate, the determination of the substrate temperature will likewise be inaccurate. That situation can arise, for instance, if the lift pins do not fully or closely seat against the substrate. Gaps between the ends of the lift pins and the surface of the substrate can allow radiation emitted by the substrate to escape, or can allow other radiation, which was not emitted by the substrate, to enter the lift pin. Either case may result in inaccurate measurement of the substrate temperature. 
     A need therefore exists for a more accurate method and apparatus for measuring substrate temperature. 
     SUMMARY 
     This invention is generally directed to method and apparatus for processing radiation emitted by a substrate. A lift pin is configured to convey radiation from the substrate to a measuring or processing device such as a pyrometer. One end of the lift pin comprises a contact surface configured to contact the substrate. The contact surface is flexibly connected so that the position and/or orientation of the contact surface may be adjusted to accommodate the surface of the substrate. 
     As used herein, the term substrate broadly refers to any object that is being processed in a thermal process chamber. Such substrates may include, for example, semiconductor wafers, flat panel displays, glass plates or disks, and plastic workpieces. 
     In one embodiment, the lift pin comprises a hollow member configured to convey radiation to a pyrometer or other receiving device. The lift pin further comprises a contact member flexibly connected to the hollow member. This connection may be a pivotal or rotatable connection, or may comprise any method of flexible connection known in the art. In one embodiment, the hollow member and the contact member are connected by a ball and socket joint. This flexible connection allows the contact member to seat against the adjoining portion of the substrate surface, improving the conveyance of radiation from the substrate to the pyrometer. One advantage of a ball and socket type connection is that such a connection can accommodate a central channel for transmitting radiation. Of course, the invention is not limited to lift pins that support the substrate; other types of pins or probes may be employed within the scope of the present invention. 
     Preferably, the invention is employed in a system for measuring substrate temperatures during thermal processing. As a substrate is radiatively heated, a contact member is in contact with the surface of the substrate. The contact member is flexibly mounted, allowing it to seat closely or snugly against the surface of the substrate. The contact member is connected to a hollow member. Radiation emitted from the substrate is conveyed past the contact member, through the hollow member, and to the pyrometer or other receiving device. The contact member and the hollow member are configured to provide the most efficient and accurate transmittal of radiation to the pyrometer. Further, gaps between the contact member and the substrate are reduced or eliminated by the flexible connection between the contact member and the hollow member, allowing the contact member to adjust to the angle of the substrate surface. In this way, radiation received by the pyrometer is conveyed directly from the substrate; losses or leakage outside of the lift pin are minimized or eliminated. 
     For a better understanding of these and other aspects of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which are not necessarily to scale: 
     FIG. 1 is a side schematic view of an apparatus for thermally processing a wafer in accordance with one aspect of the present invention; 
     FIG. 2 is a side view of a lift pin in accordance with one aspect of the present invention; and 
     FIG. 3 is a side view of the lift pin shown in FIG. 2 when seated against an uneven substrate surface, in accordance with one aspect of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to a method and apparatus for processing radiation emitted by a substrate. Although the present invention may be used in any application requiring processing of such radiation, the present invention has particular application in a system for measuring substrate temperature. 
     Generally, processing of the substrate requires precise analysis of the radiation emitted by the substrate. For example, if the substrate is a semiconductor wafer (e.g., a silicon wafer), it may be characterized by a bandgap energy (direct or indirect) that decreases as the wafer temperature increases. As a general rule of thumb, radiant energy that is greater than the bandgap energy is absorbed by the substrate, while lower energy radiation is transmitted through the substrate. The intensity of radiation emitted by the substrate at given bandgap energies must therefore be precisely measured to accurately determine the temperature of the substrate  20 . 
     Preferably, the lift pin of the present invention is employed in a substrate processing system such as is shown in FIG.  1 . As shown therein, a thermal processing apparatus  10  includes a chamber  12  that contains a substrate support  14  and a radiant heat source  16 . The substrate support  14  includes a support ring  18  which contacts the peripheral edge of a substrate  20  (e.g., a semiconductor wafer characterized by a temperature-responsive bandgap energy). Typically, the support ring  18  contacts only a smaller fraction of the bottom surface of the substrate  20 , leaving a larger fraction of the substrate surface exposed to emit radiation to a reflector  21 . The underside of the substrate  20  and the top surface of the reflector  21  form a reflecting cavity that enhances the effective emissivity of the substrate  20 . The support ring  18  is mounted on a support tube  26  that is rotatably supported by a bearing assembly  28 . Magnets  30  mounted on the bearing assembly  28  magnetically couple with magnets  32  mounted on a drive ring  34 . As the drive ring  34  rotates, the magnetic coupling causes the support tube  26  and support ring  18  to rotate. Alternatively, the bearing assembly  28  and magnets  30 ,  32  may be replaced by a sealed drive assembly, or other drive mechanism known in the art. 
     In operation, the radiant heat source  16  heats the interior of the chamber  12  to a desired preheat temperature (e.g., about 300-400 degrees Celsius). A robot arm then moves the substrate  20  into the chamber  12  through an opening in the wall of the chamber  12 . A lift pin assembly  36  rises up from beneath the substrate  20 , lifts the substrate  20  off of the robot arm (at which point the robot arm may be withdrawn from the chamber  12 ), and lowers the substrate  20  onto the support ring  18 . To avoid problems (e.g., substrate warping or other substrate degradation) that might result from the rapid increase in temperature by contact between the substrate  20  and the preheated substrate support  14 , the lift pin assembly  36  holds substrate  20  in a fixed position adjacent to the radiant heat source  16  until the temperature of the substrate  20  is within a desired range (e.g., 300-350 degrees Celsius). At this point, the substrate is lowered onto the support ring  18  and the substrate  20  is processed. One or more lift pin assemblies, rather than a single lift pin assembly, may be employed within the scope of the present invention. Further, the various components of the thermal processing system and method are merely exemplary, and other variations known in the art may be employed within the scope of the present invention. 
     Referring to FIG. 2, the lift pin assembly  36  includes a lift pin  100  that is formed of an outer sheath  102  and an inner optical transmission channel  37 . The outer sheath may be formed from suitable materials known in the art, such as silicon having a thickness of about 2 mm. The optical transmission channel may be formed of a light pipe of suitable material, such as quartz or sapphire. The optical transmission channel  37  is coupled to a detector (not shown), through means known in the art. The ends of the optical transmission channel are preferably polished to improve transmission through the channel. The outer sheath  102  may extend beyond the end  112  of the optical transmission channel  37  closest to the substrate so that the optical transmission channel  37  does not contact the substrate. Suitable variation in the structure, materials, and other aspects of the lift pin will be apparent to those of skill in the art. 
     To evaluate the substrate temperature, a detection system  24  (shown in FIG. 1) receives radiation emitted by the substrate through the optical transmission channel  37  and provides signals to a processor  38 . The processor is preferably configured to compute from the detection system signals a measure of the substrate temperature. Of course, the processor  38  may be configured to perform other functions, such as computation of other temperature information such as an indication of the relative accuracy of the computed measure of substrate temperature, and/or computation of a measure of the rate at which the substrate  20  is heated inside the chamber  12 . The processor  38  preferably uses this information to provide a signal that controls subsequent processing, such as the timing of when the substrate is lowered onto the support ring. In this way, indirect pyrometric temperature measurements may be used to control the preheating stage of the substrate processing. The lift pin assembly of the present invention, however, is not limited to systems for determining the temperature of the substrate. Rather, the present invention may be employed in any system wherein precise analysis of radiation emitted from a substrate or other object is desired. 
     The detector and detection system may employ various means known in the art. For example, the detector may be a silicon photodiode and/or an indium gallium arsenide (InGaAs) photodetector. Multiple detectors may be employed, for example, to measure intensity at different bandgap energies. Further, the detection system may employ one or more filters. U.S. Pat. No. 6,151,446 describes examples of such detection systems. 
     The lift pin assembly  36  of the current invention is configured so that the radiation transmitted through the optical transmission channel  37  most accurately reflects the radiation emitted by the substrate  20 . In the embodiment shown in FIG. 2, the lift pin assembly  36  comprises a contact member  120  that is flexibly mounted with respect to the outer sheath  102 . This flexible connection between the contact member  120  and the outer sheath  102  allows the contact member  120  to seat more closely against the underside of the substrate  20 . Preferably, the contact member  120  is connected to the distal end  122  of the outer sheath  102  such that the contact member  120  is rotatable about at least two axes with respect to the outer sheath. For instance, the contact member  120  may include a concave surface  124  which seats against a convex surface  126  of the distal end  122  of the outer sheath  102 , allowing the contact member to rotate about the top of outer sheath, as in a ball and socket connection. Alternatively, the distal end  122  may have a concave surface and the contact member  120  a convex surface. Further, the contact member  120  may be connected to one or more other members (not shown), which are in turn connected to the outer sheath  102 . Other configurations known in the art for providing a flexible connection may also be used within the scope of the present invention. 
     This flexible connection between the. contact member  120  and the outer sheath  102  allows the detection system  24  to more accurately detect the radiation emitted by the substrate  20 . If the underside of the substrate, or the portion of the surface of the substrate  20  adjacent to the lift pin, is oriented at an angle to the lift pin, there is the possibility that radiation emitted from the substrate will not be captured in the optical transmission channel. Similarly, there is the possibility that background radiation, which is not emitted by the substrate, will be captured in the optical transmission channel. These possibilities are minimized by the flexible connection, which allows the portion of the lift pin that is in contact with the substrate to adjust to the orientation of the substrate, reducing the risk of a gap between the contact member and the substrate through which radiation can enter or escape. 
     The advantage of this flexibility can be seen by comparing FIGS. 2 and 3. In FIG. 2, the underside of the substrate  20  is relatively flat and level with respect to the contact member  120 . In FIG. 3, the underside  202  of the substrate  20  is at an angle. If the contact portion of the lift pin were not flexible, there would be a gap between part of the lift pin and the substrate. Through that gap, radiation could enter or escape the optical transmission channel  37 , reducing the accuracy of the temperature measurement. With the lift pin of the present invention, the contact member  120  adjusts to the angle of the substrate, reducing or eliminating the possibility of a gap between the lift pin and the substrate. Accuracy of the measurement by the detector of the intensity of the emitted radiation is thereby enhanced. 
     Those skilled in the art to which the invention pertains may make modifications and other embodiments employing the principles of this invention without departing from its spirit or essential characteristics particularly upon considering the foregoing teachings. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description Consequently, while the invention has been described with reference to particular embodiments, modifications of structure, sequence, materials and the like would be apparent to those skilled in the art, yet still fall within the scope of the invention.