Abstract:
An apparatus, system, and method for uniformly and controllably heating the active surface of a semiconductor wafer during processing. The present invention includes a scanner assembly, which is operable to scan over a single semiconductor wafer. A radiation energy source is provided enclosed within the main body of the scanner assembly. The radiation energy source may be surrounded by a reflective/absorptive surface, which reflects and absorbs the emitted radiation, such that the resultant energy output is substantially free of non-uniformities. The reflected energy is directed through a slit in the scanner assembly to the wafer. The narrow wavelength band of energy allowed to escape the scanner assembly is uniformly scanned over the wafer to heat only the active layer of the wafer surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to semiconductor manufacturing equipment and, more particularly, to equipment for rapid thermal processing of a semiconductor wafer. 
     2. Description of Related Art 
     To make semiconductor devices of decreased dimensions, new processing and manufacturing techniques have had to be developed. One important requirement for the new techniques is to be able to reduce the amount of time that a semiconductor wafer is exposed to high temperatures during processing. One such processing technique designed to address this requirement is know as Rapid Thermal Processing (RTP). The rapid thermal processing technique, typically includes quickly raising the temperature of the wafer and holding it at that temperature for a time long enough to successfully perform a fabrication process, while avoiding such problems as unwanted dopant diffusion that would otherwise occur at the high processing temperatures. 
     Generally, conventional RTP systems use a light source and reflectors to heat the bulk of the semiconductor wafer. The light source is usually a bank of Halogen lamps that emit radiation energy that is focused on the wafer by the reflectors. 
     Conventional Halogen lamp-based RTP systems have considerable drawbacks with regard to achieving and maintaining a uniform temperature distribution across the active layer of the wafer surface. For example, the Halogen lamp has a filament, which generates broadband radiation. By applying more power to the filament, the intensity of the lamp can be increased. However, silicon wafers are heated using a useable band of short wavelengths, and are otherwise transparent to wavelengths outside of this band. The peak intensity of the lamp tends to increase the wavelengths outside of the useable wavelength band. As a consequence, much of the applied power is wasted. 
     Another drawback to filament type lamps is that they generally create a wavelength distribution that is non-uniform and independently uncontrollable. Consequently, temperature fluctuations occur on the surface of the wafer which may cause crystal defects and slip dislocations in the wafer at high temperatures (e.g. ˜1000° C.). 
     One particular solution to the drawbacks of Halogen lamp-based systems is disclosed in U.S. Pat. No. 5,893,952. In the 952 patent, an apparatus is described for rapid thermal processing of a wafer using a narrow band beam of electromagnetic radiation generated by a high wattage laser. The beam is directed at the wafer, through a thin absorption film, which absorbs substantially all the energy from the beam, which, in turn, radiates heat to the wafer. Unfortunately, the apparatus described above has some limitations and drawbacks. For example, the thickness of the thin film must be accurately determined. If the thin film is too thin, energy from the beam may be transmitted directly to the wafer, or if the thin film is too thick the film may not heat up fast enough for rapid thermal processing. A film must be used that does not degrade over time, and must not sputter, bubble, or degas when heated, otherwise non-uniform absorption will result. Because of the requirements placed on the thin absorption film, the materials for this film are limited. As a result, the same RTP apparatus may heat wafers differently and unpredictably, which wastes both time and materials. 
     For the above reasons, what is needed is an apparatus, system, and method for uniformly and controllably heating the surface of a semiconductor wafer during rapid thermal processing. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus, system, and method for uniformly and controllably heating the active surface of a semiconductor wafer during processing. The present invention may include a scanner assembly which is operable to scan over a single semiconductor wafer. As described in greater detail below, a radiation energy source is provided enclosed within the main body of the scanner assembly. The radiation energy source may be surrounded by a reflective/absorptive surface, which both reflects and absorbs the radiation, emitted from the energy source such that the resultant energy output as seen by the wafer is substantially free of non-uniformities. The reflected energy is directed through a slit in the scanner assembly. Advantageously, the narrow band of energy allowed to escape the scanner assembly is uniformly scanned over the wafer to heat only the active layer of the wafer surface. Because the beam is uniform over the diameter of the wafer there is no heating overlap. 
     In one aspect of the present invention, an apparatus is provided for rapid thermal processing of a wafer. The apparatus includes a radiation energy source, preferably a filament-less lamp. The apparatus further includes a scanning assembly operable to scan a beam of the radiation energy across the surface of a wafer. The radiation energy is used to heat an active layer of the wafer. 
     In another aspect of the present invention, an apparatus for rapid thermal processing of a semiconductor wafer is provided. The apparatus includes a housing which defines a reflecting chamber. Within the reflecting chamber is disposed a radiation energy source. To allow at least a portion of the radiation energy to escape the reflecting chamber, a radiation outlet channel is also provided. Also provided is a scanner, which is operable to scan the radiation energy escaping from the reflecting chamber across the surface of a wafer to heat an active layer of the wafer. 
     In yet another aspect of the present invention, a method is provided for rapid thermal processing of a semiconductor wafer. The method includes providing a source of radiation energy and scanning a semiconductor wafer with a narrow band of the radiation energy to raise the temperature of an active layer of the semiconductor wafer. 
     Because the scanning RTP system of the present invention is designed to heat only the active region of the wafer surface, the process is advantageous for implant anneal applications, such as shallow junction, ultra shallow junction, and source drain anneal. The scanning RTP system may also be used effectively for thermal donor annihilation, recrystallization, and H 2  anneal. Moreover, since the bulk of the semiconductor wafer need not be heated during the heating process, the amount of power used by the RTP system can be reduced to less than 50 kWh, preferably, less than about 10 kWh. Similarly, scanning times, and therefore processing times, may be reduced since only the active surface of the wafer is being heated. 
     These and other features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIGS. 1A and 1B are schematic illustrations of a side view and top view, respectively, of one embodiment of a semiconductor wafer processing system that establishes a representative environment of the present invention; 
     FIG. 2A is a simplified illustration of an RTP reactor system in accordance with the principles of the present invention; 
     FIG. 2B is a simplified illustration of an RTP reactor system in accordance with an alternative embodiment of the present invention; 
     FIG. 2C is a simplified illustration of an RTP reactor system in cordance with an alternative embodiment of the present invention; 
     FIG. 2D is a simplified illustration of the active layer of a miconductor wafer in accordance with principles of the present invention; 
     FIG. 3 is a simplified illustration of an embodiment of a radiation amber in accordance with the present invention; and 
     FIG. 4 is a simplified illustration of another embodiment of the resent invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A and 1B are schematic illustrations of a side view and top view, respectively, of one embodiment of a semiconductor wafer processing system  10  that establishes a representative environment of the present invention. The representative system is fully disclosed in co-pending U.S. patent application Ser. No. 09/451,677, filed Nov. 30, 1999, which is herein incorporated by reference for all purposes. Processing system  10  includes a loading station  12  which has multiple platforms  14  for supporting and moving a wafer cassette  16  up and into a loadlock  18 . Wafer cassette  16  may be a removable cassette which is loaded into a platform  14 , either manually or with automated guided vehicles (AGV). Wafer cassette  16  may also be a fixed cassette, in which case wafers are loaded onto cassette  16  using conventional atmospheric robots or loaders (not shown). Once wafer cassette  16  is inside loadlock  18 , loadlock  18  and transfer chamber  20  are maintained at atmospheric pressure or else are pumped down to vacuum pressure using a pump  50 . A robot  22  within transfer chamber  20  rotates toward loadlock  18  and picks up a wafer  24  from cassette  16 . A reactor or thermal processing chamber  26 , which may also be at atmospheric pressure or under vacuum, accepts wafer  24  from robot  22  through a gate valve  30 . Optionally, additional reactors may be added to the system, for example reactor  28 . Robot  22  then retracts and, subsequently, gate valve  30  closes to begin the processing of wafer  24 . After wafer  24  is processed, gate valve  30  opens to allow robot  22  to pick-up and place wafer  24  into a cooling station  60 . Cooling station  60  allows the newly processed wafers, which may have temperatures upwards of 100° C., to cool before they are placed back into a wafer cassette in loadlock  18 . 
     In accordance with the present invention, reactors  26  and  28  are RTP reactors, such as those used in thermal anneals, dopant diffusion, thermal oxidation, nitridation, chemical vapor deposition, and similar processes. Reactors  26  and  28  are generally horizontally displaced, however in a preferred embodiment, reactors  26  and  28  are vertically displaced; (i.e. stacked one over another) to minimize floor space occupied by system  10 . Reactors  26  and  28  are bolted onto transfer chamber  20  and are further supported by a support frame  32 . Process gases, coolant, and electrical connections may be provided through the rear face of the reactors using interfaces  34 . 
     FIG. 2A a illustrates of an embodiment of RTP reactor system  40  in accordance with the principles of the present invention. In this embodiment, reactor system  40  includes a process chamber  102  and a scanner assembly  200 . Scanner assembly  200  may be positioned proximate to process chamber  102 , such that in operation, the scanner assembly can be made to adequately scan the wafer disposed in the chamber. 
     In a preferred embodiment, process chamber  102  may include a closed-end tube  103 , defining an interior cavity  104 . Within tube  103  are wafer support posts  106 , typically three (of which two are shown), to support a single wafer  108 . An opening or aperture (not shown) on one end of tube  103 , provides access for the loading and unloading of wafer  108  before and after processing. The aperture may be a relatively small opening, but large enough to accommodate a wafer of between about 0.5 to 0.8 mm thick and up to 300 mm (˜12 in.) in diameter, and the arm and end effector of robot  22 . Preferably, the aperture is no greater than between about 18 mm and 22 mm, preferably 20 mm. The relatively small aperture size helps to reduce radiation heat loss from tube  103 . 
     Because wafer  108  is loaded and un-loaded using robot  22 , tube  103  requires no internal moving parts to position wafer  108 , such as lift pins, actuators, and the like. Thus, tube  103  may be constructed with a minimal internal volume-surrounding wafer  108 . In a preferred embodiment, the volume of interior cavity  104  is usually no greater than about 5000 cm 3 , and preferably the volume is no greater than about 3000 cm 3 . Accordingly, the small tube volume allows reactor system  40  to be made smaller, and as a result, system  10  may be made smaller, requiring less floor space. Preferably, tube  103  is made of a transparent quartz or similar material. 
     FIG. 2A also illustrates scanner assembly  200 , which may be used in conjunction with a radiation energy source  202 , to provide rapid thermal processing of semiconductor wafer  108 . Scanner assembly  200  includes a housing  216  which supports an actuator  204 , a reflecting chamber  212 , and a radiation outlet channel  214 . The external dimensions of housing  216  are determined by the application. For example, the length of housing  216  may be at least as great, or greater than the diameter of wafer  108 . 
     Actuator  204  provides a conventional means for making scanner assembly  200  operable to scan wafer  108 . Actuator  204  may be configured to provide a back and forth scanning motion, as indicated in FIG. 2A by arrows  206  and  208 , along a scanning length of tube  103 . Actuator  204  may include, but is not limited to, conventional drivers and motion translation mechanisms, such as linear motors, stepper motors, hydraulic drives, and the like, and gears, pulleys, chains, and the like. 
     In the embodiment shown in FIG. 2A, scanner assembly  200  may be mounted external to both process chamber  102  and tube  103 . Scanner assembly  200  is positioned above an optical window  210 , which is provided along the scanning length of chamber  102  (i.e. at least as great as the diameter of wafer  108 ) to allow the radiation energy emitted from housing  216  to enter tube  103  and impinge on wafer  108 . In an alternative embodiment shown in FIG. 2B, the scanning motion of scanner assembly  200   a  may take place internal to process chamber  102   a , but external to tube  103   a . Scanner assembly  200   a  is positioned above optical window  210   a , formed on tube  103   a  along the scanning length (i.e. at least as great as the diameter of wafer  108 ) to allow the radiation energy emitted from housing  216   a  to enter tube  103   a  and impinge on wafer  108 . 
     In yet another embodiment, shown in FIG. 2C, scanner assembly  200   b  may be mounted external to process chamber  102   b , with no process tube. In this embodiment, scanner assembly  200   b  is positioned above optical window  210   b , which is provided along the scanning length of chamber  102   b  (i.e. at least as great as the diameter of wafer  108 ) to allow the radiation energy emitted from housing  216   b  to impinge on wafer  108 . 
     Optical window  210  (or  210   a ) may be made of any material that allows for the transmission of the radiation energy, preferably quartz. Window  210  may have a thickness of between about 1 and about 5 mm and a diameter that is at least as great or greater than wafer  108 . 
     Whether the scanner assembly is positioned inside or outside of the tube, the distance between the surface of the wafer and the scanner assembly, indicated in FIG. 2A as gap  213 , should be no greater than about 50 mm, preferably between about 10 mm and 25 mm. The relatively small gap  213  ensures that adequate control of the temperature distribution across wafer  108  is maintainable. A larger gap  213  may cause some of the radiation energy to be escape before it impinges on wafer  108 . 
     As further illustrated in FIG. 2A, reflective chamber  212  and radiation outlet channel  214  are disposed within housing  216 . Radiation source  202  is disposed within reflective chamber  212 , typically positioned such that substantially all of the broadband radiation is allowed to impinge on an internal surface  218  of the chamber. In one embodiment, radiation energy source  202  may be a high-intensity lamp of the type conventionally used in lamp heating operations. In a preferred embodiment, radiation energy source  202  is a filament-less lamp, such as a Xe arc lamp. Typical, power requirements for the preferred lamp  202  of the present invention are between about 500 Watts and about 50 kWatts. 
     The energy emitted from lamp  202  impinges inner surface  218  of chamber  212 , which is highly reflective of certain wavelengths and absorptive or non-reflective of others. In one embodiment, surface  218  is coated with a material which has the reflecting/absorbing characteristic. For example, surface  218  may be coated with gold or silver, where the silver is further coated with a protection coating, such as SiN or any transparent coating, which prohibits oxidation of the silver. Preferably, the coating efficiently reflects wavelengths of less than 900 nm, to produce an average wavelength of between about 900 nm and about 200 nm. 
     Chamber  212 , which may be formed into any suitable geometric shape. For example, as shown in FIG. 2A, chamber  212  may be a round chamber. In a round chamber  212  light energy can be focused at the center of chamber  212  and directed toward radiation outlet channel  214 , described below. In this example, radiation energy source  202  can be off-center in chamber  212  to ensure that the focused light energy does not over heat energy source  202 . FIG. 3 shows an alternative example of chamber  212 , which may be formed into an elliptical chamber. Elliptical chamber  212  can have two focal points. Energy source  202  can be positioned at a first focal point  203 , such that the light energy is focused at the second focal point  205  and directed to radiation outlet channel  214 . 
     Referring again to FIG. 2A, the narrow-band energy escapes from chamber  212  through radiation outlet channel  214 . Radiation outlet channel  214  can be about 5 mm to 20 mm long; preferably about 10 mm long, to adequately direct the radiation energy along the desired path. Radiation outlet channel  214  has an opening or slit  222  formed on the end of the channel which allows a beam  220  of the radiation energy to escape housing  216 . Slit  222  is designed to shape beam  220  as desired, such that an optimal amount of energy may be focused on wafer  108 . In a preferred embodiment, slit  222  may be a rectangular opening, which extends the length of scanner assembly  200 , and is as great, or greater than the diameter of wafer  108 . The size of the opening should be small enough to minimize the amount of energy, which will naturally disperse at the slit opening. Thus, slit  222  may have a width of between about 1 mm and 10 mm, preferably 2 mm. As beam  222  is scanned over wafer  108 , a uniform temperature distribution is created across the surface of wafer  108 , which heats an active layer  224  of the wafer. 
     Referring now to FIGS. 2A and 2D, active layer or device layer  224  is a portion of wafer  108 , which extends from surface  223  of wafer  108  down to a depth a below surface  223 . The depth a is typically between about 0.05 μm and 1 mm, but will vary with the process and device feature size. Active layer  224  is well known in the semiconductor manufacturing industry as that portion of the wafer in which semiconductor devices are formed, such as transistors, diodes, resistors, and capacitors. 
     It should be understood that the temperature to which active layer  224  is heated is a function of the relationship between the speed at which scanner assembly  200  is moved across wafer  108  and the power supplied to lamp  202 . In an exemplary embodiment, the temperature of active layer  224  may range from between about 500° C. to about 1200° C. To achieve these temperatures, the scan rate may vary between about 1 mm/sec to about 10 mm/sec at 500 watts to 50 kwatts. The slower the scan rate, the less power is required. In one embodiment, wafer  108  can be pre-heated, for example, to about 300° C., such that the processing of active layer  224  begins at the higher temperature, which reduces processing time and saves energy. 
     Heating active layer  224  using reactor system  40  increases the diffusion rate and solubility of active layer  224 . Thus, a shallow doped region may be created in active layer  224 . Doping the active layer includes scanning active layer  224  to a process temperature, for example, from between about 500° C. to about 1200° C., in an environment of a doping compound, such as boron, phosphorus, nitrogen, arsenic, B 2 H 6 , PH 3 , N 2 O, NO, AsH 3 , and NH 3 . The concentration of the compound may range from about 0.1% to about 100% relative to a carrier gas, such as H 2 , N 2  and O 2  or a non-reactive gas, such as argon or helium. Higher concentrations of the compound can speed up the doping process and/or increase the dopant concentration within the active layer. 
     FIG. 4 is a simplified illustration of yet another embodiment of the present invention. In this embodiment, scanner assembly  300  includes a high intensity pulse or continuous wave laser  302  to provide rapid thermal processing of semiconductor wafer  304 . Scanner assembly  300  also includes a laser energy focusing assembly  306  and an actuator  308 . The components of scanning assembly  300  may be enclosed in a single housing, which is mountable on to a process chamber  320  in a manner similar to the embodiments described above in FIG.  2 A. 
     Laser focusing assembly  306  includes a first focusing lens  310 , a second focusing lens  312 , and mirror  314 . Focusing assembly operates in a well-known, conventional manner to focus the laser energy  301  from laser  302  onto wafer  304 . The laser energy  301  from laser  302  can have a wavelength of less than 1 μm. 
     Actuator  308  provides a conventional means for making scanner assembly  300  operable to scan wafer  304 . Actuator  308  may be configured to move laser  302  and focusing assembly  306  to provide a back and forth scanning motion across wafer  304 , as indicated in FIG. 4 by arrow  316 . Alternatively, only mirror  314  may be moved to cause the laser scanning of wafer  304 . In yet another alternative embodiment, wafer  304  may be made to move, such that a stationary beam  301  can be made to scan the wafer surface. Actuator  308  may include, but is not limited to, conventional drivers and motion translation mechanisms, such as linear motors, stepper motors, hydraulic drives, and the like, and gears, pulleys, chains, and the like. In one embodiment, scanner assembly  300  is positioned above an optical window  318 , which is provided along the scanning length of process chamber  320  to allow the laser energy to enter process chamber  320  and impinge on wafer  304 . Window  318  may be made of any material that allows for the transmission of laser energy  301 ; preferably transparent quartz. Window  318  may have a thickness of between about 1 and about 5 mm and a diameter that is at least as great or greater than wafer  304 . 
     The present invention overcomes many of the disadvantages of RTP systems using Halogen lamps. For example, filament-type Halogen lamps produce broadband energy, much of which cannot be used to heat the active layer of the wafer. To increase the amount of useable wavelengths in the filament-type lamp, power to the lamp is increased. Unfortunately, this increase in power shifts the peak intensity. The arc lamp used in the present invention does not shift peak intensity with an increase in power and thus can be made to perform at a peak intensity that is within the useable band of wavelengths. As a consequence, the added power is more efficiently consumed at the active layer. 
     Having thus described embodiments of the present invention, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Thus the invention is limited only by the following claims.