Abstract:
Chuck methods and apparatus for supporting a semiconductor substrate and maintaining it at a substantially constant background temperature even when subject to a spatially and temporally varying thermal load. Chuck includes a thermal compensating heater module having a sealed chamber containing heater elements, a wick, and an alkali metal liquid/vapor. The chamber employs heat pipe principles to equalize temperature differences in the module. The spatially varying thermal load is quickly made uniform by thermal conductivity of the heater module. Heatsinking a constant amount of heat from the bottom of the heater module accommodates large temporal variations in the thermal heat load. Constant heat loss is preferably made to be at least as large as the maximum variation in the input heat load, less heat lost through radiation and convection, thus requiring a heat input through electrical heating elements. This allows for temperature control of the chuck, and hence the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present patent application is related to U.S. patent application Ser. No. ______, entitled “Heated Chuck for Laser Thermal Processing,” filed on Dec. 1, 2004, the same day as the current application, and both are assigned to the present Assignee, Ultratech, Inc., of San Jose, Calif., which patent application is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to apparatus and methods for thermally processing semiconductor substrates in semiconductor manufacturing, and in particular relates to support members (“chucks”) for supporting a substrate (semiconductor wafer) during laser thermal processing (LTP).  
         [0004]     2. Description of the Prior Art  
         [0005]     The fabrication of integrated circuits (ICs) involves subjecting a semiconductor substrate to numerous processes, such as photoresist coating, photolithographic exposure, photoresist development, etching, polishing, and in some cases heating or “thermal processing”. Thermal processing is used, for example, to activate dopants in doped regions (e.g., source and drain regions) of the substrate for certain types of ICs. Thermal processing includes various heating (and cooling) techniques, such as rapid thermal annealing (RTA) and laser thermal processing (LTP).  
         [0006]     Various techniques and systems for performing LTP of semiconductor substrates (“wafers”) are known and are used in semiconductor device manufacturing. Example LTP systems and methods are described in U.S. Pat. No. 6,747,245 entitled “Laser Scanning Apparatus and Methods for Thermal Processing” (the &#39;245 patent), and in U.S. Pat. No. 6,366,308 B1, entitled “Laser Thermal Processing Apparatus and Method” (the &#39;308 patent), which patents are incorporated by reference herein.  
         [0007]     LTP involves rapidly bringing the temperature of the wafer up to the annealing temperature and then rapidly back down to the starting (e.g., ambient or background) temperature in a single cycle. Given of the relatively large sizes of the typical wafers used in semiconductor manufacturing (e.g., 300 mm in diameter), the heat is more efficiently applied to only a small region of the wafer at a given time.  
         [0008]     Using the &#39;245 patent and the &#39;308 patent as examples, a laser beam is formed into a narrow high-intensity image (e.g., a line image) that is scanned over the wafer surface, e.g., in a raster pattern. This process can involve a heat flux in excess of 1000 W/mm 2  over the narrow image. The peak temperature TP reached by the wafer surface at the region being irradiated during LTP is relatively high (e.g., ˜1,300° C.).  
         [0009]     The uniformity of the peak temperature TP determines the sheet resistance uniformity of activated doped regions formed therein, which in turn determines the performance of resulting semiconductor devices.  
         [0010]     Attaining a uniform peak temperature TP over the wafer depends on the stability of the laser power and on the temperature uniformity of the wafer surface (referred to hereinbelow as the “background substrate temperature”). Maintaining a constant background temperature of the substrate, however, is problematic when the LTP process utilizes a spatially varying thermal load such as a scanned laser beam.  
         [0011]     Accordingly, the art of LTP and related arts would benefit from apparatus and methods directed to maintaining the substrate being processed at a constant background temperature at the locations of the substrate not being directly subjected to the spatially varying thermal load.  
       SUMMARY OF THE INVENTION  
       [0012]     One aspect of the invention is a chuck apparatus for laser thermal processing a substrate. The apparatus includes a housing having a planar upper surface, a lower surface, and an enclosed interior chamber. The chamber has a peripheral interior surface, which generally consists of the inner surfaces of the outer portions of the housing, such as the bottom surface of a top plate, the upper surface of a bottom plate, and the inner surface of a cylindrical sidewall capped by the top and bottom plates. The chamber is adapted to contain a metal in liquid and vapor form (referred to herein simply as a “metal liquid/vapor”). The chuck apparatus includes one or more heating elements arranged within the chamber interior. The one or more heating elements are adapted to heat the housing and the metal liquid/vapor to a background temperature. The apparatus also includes one or more wicks arranged adjacent the chamber peripheral interior surface. The one or more wicks are adapted to supply liquid metal to most, or all, of the chamber peripheral interior surface. Portions of the metal liquid/vapor are redistributed within the chamber by vaporizing the liquid metal at a hot spot within the chamber formed by heat transferred from the substrate to the chamber, and condensing the metal vapor away from the hot spot. This redistribution of the metal liquid/vapor serves to quickly uniformize the temperature of the housing, so that the temperature of the housing can be maintained at the background temperature.  
         [0013]     Another aspect of the invention is a method of maintaining a substrate at a substantially constant background temperature while subjecting the substrate to a spatially varying thermal load, such as from a laser beam used to perform LTP of a semiconductor wafer as the substrate. The method includes transferring heat associated with the spatially varying thermal load from the substrate to a metal liquid/vapor held within a sealed chamber in thermal communication with the substrate. The transferred heat forms within the chamber a hot spot that is surrounded by cooler regions. The method also includes redistributing portions of the metal liquid/vapor by vaporizing the liquid metal at the hot spot, and condensing the metal vapor in the cooler regions to uniformize the temperature of the sealed chamber, and consequently, the substrate in good thermal communication therewith. The method may also include removing heat from the chamber using, for example, a heat sink in good thermal communication with the chamber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is an exploded side view of an example embodiment of the heater module of the present invention illustrating a short, hollow cylindrical center section capped by top and bottom plates that define the module chamber, and wherein a first cut-out (C 1 ) shows liquid and metal vapor within the chamber, a support brace along with openings in the braces that permit the circulation of metal throughout the chamber, and a second cut-out (C 2 ) shows a wick that lines most or all of the chamber interior surface, and liquid metal being conducted by the action of the wick to cover the chamber peripheral interior surface;  
         [0015]      FIG. 2  is a top-down view of an example embodiment of the heater module of the present invention, shown with the top plate removed and a portion of the wick removed to reveal the internal components within the module chamber, and showing a portion of the wick (dotted line) adjacent the inner surface of the sidewall;  
         [0016]      FIG. 3  is a cross-sectional view of a heated chuck for performing LTP of a substrate using an LTP laser beam, wherein the chuck includes the heater module of  FIG. 1 , and illustrating the hot spot ( 812 ) and the cooler regions ( 814 ) of the chamber interior that occur during LTP irradiation; and  
         [0017]      FIG. 4  is a close-up side view of the example embodiment of the top plate having a protective layer formed thereon that prevents contamination of the substrate. 
     
    
       [0018]     The various elements depicted in the drawings are merely representational and are not drawn to scale. Certain proportions thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various implementations of the invention, which can be understood and appropriately carried out by those of ordinary skill in the art.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     In the following detailed description of the embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
         [0020]     As mentioned above, achieving a uniform peak temperature over the substrate surface during LTP is critical in manufacturing semiconductor devices that require uniform sheet resistance of activated doped regions. Achieving peak temperature uniformity in LTP is facilitated by creating an environment wherein the substrate efficiently absorbs energy from the incident LTP laser beam. If the substrate is undoped or lightly doped, it is necessary to bring the substrate up to a constant background temperature TC prior to irradiating the substrate with the LTP laser beam in order to increase the absorption of the laser beam. Failure to do this can result in the beam passing through the substrate and to the chuck in some cases. Further, it involves maintaining the substrate at the constant background temperature TC even as the scanned LTP laser beam subjects the substrate to a spatially varying thermal load.  
         [0021]     The chuck of the present invention is adapted to maintain a constant background substrate temperature TC significantly higher than room temperature even when the substrate is subject to the spatial varying thermal load from a scanned LTP laser beam. In an example embodiment, constant background temperature TC is in the range from about 350° C. to about 450° C. In one example embodiment, the constant background temperature TC is kept uniform across the substrate to +/−4° C., and in another example embodiment is kept uniform across the substrate to +/−6° C.  
         [0022]     In the description below, the phrase “spatially varying thermal load” is used to describe the delivery of heat to different locations (positions) on the substrate at different times, e.g., by scanning an LTP laser over the substrate surface to be processed. As discussed below, the spatially varying thermal load on the substrate is communicated to corresponding locations within the heater module chamber.  
         [0023]     Also, the phrase “constant background temperature” is understood to mean “constant or substantially constant,” wherein the variation in the background temperature is held to within a range that does not substantially affect the resultant LTP process. Likewise, the “constant background temperature” is assumed to be substantially uniform, i.e., is uniform over the substrate to the degree necessary to perform LTP of the substrate without substantial adverse results.  
         [0024]     Also, in a preferred embodiment, the “constant background temperature” is elevated, i.e., is significantly higher than room temperature, e.g., 350° C. to 450° C. Also, the “constant background temperature” refers to the temperature of a portion of an object (e.g., the substrate) other than that portion immediately surrounding the spatially varying thermal load at any given time.  
         [0025]     Further, the terms “gas” and “vapor” are used interchangeably herein. Also, as discussed below, the where term “metal” is intended to include both the liquid and vapor states, the phrase “metal liquid/vapor” is used for the sake of abbreviation and clarity.  
         [0000]     Heater Module  
         [0026]      FIG. 1  is an exploded side view of an example embodiment of a heater module  10  used in the chuck of the present invention. The chuck is described in detail below. Heater module  10  includes hollow cylindrical section (sidewall)  20  having respective upper and lower rims  22  and  24  and respective inner and outer surfaces  26  and  28  ( FIG. 2 ). Attached to upper rim  22  is a top plate  30 , and attached to lower rim  24  is a bottom plate  40 . Top plate  30  has an upper surface  32  and a lower surface  34 , and bottom plate  40  has an upper surface  42  and a lower surface  44 . Sidewall  20 , top plate  30  and bottom plate  40  constitute an enclosed, sealed housing having an enclosed interior chamber  50 . In an example embodiment, top plate  30  and bottom plate  40  are respectively sealed to upper and lower sidewall rims  22  and  24 , e.g., by welding.  
         [0027]     Chamber  50  contains a metal  51 , which may be solid at room temperature, and both a liquid and a gas at the elevated operating or background temperature. Note that the portion of metal  51  in the vapor state is shown as small circles in  FIG. 1  for the sake of illustration. In an example embodiment, metal  51  is or includes an alkali metal, such as one or more of potassium, cesium and sodium. In an example embodiment, metal  51  is introduced to interior chamber  50  during assembly and is permanently sealed therein during the operation of the heater module. In an example embodiment, the inner surface  26  of sidewall  20 , the bottom plate upper surface  42  and the top plate lower surface  34  define an example of a “chamber peripheral interior surface.” 
         [0028]     In an example embodiment, sidewall  20  is formed from Monel-metal. Also in an example embodiment, top plate  30  and bottom plate  40  are formed from or otherwise include Monel-metal in order to safely contain metal liquid/vapor  51 , which in the case of an alkali metal such as potassium is very reactive.  
         [0029]      FIG. 2  is a top-down view of heater module  10  of  FIG. 1 , shown with top plate  30  removed to reveal the internal components of the module that reside within chamber  50 . In an example embodiment, heater module  10  includes one or more thin, rectangularly shaped braces  100  that span chamber  50  from one portion of sidewall inner surface  26  to another, and that extend upward from bottom plate upper surface  42  up to the plane defined by upper rim  22 . Braces  100  serve to define sub-chambers, such as sub-chambers  50 A,  50 B,  50 C and  50 D, within chamber  50 . Braces  100  preferably include openings  110  ( FIG. 1 ) sized to allow for metal liquid/vapor  51  to flow between the sub-chambers and throughout the entire chamber  50 , as described below. In an example embodiment, braces  100  are arranged at equal angles relative to one another and divide chamber  50  into equal-sized sub-chambers, such as the four sub-chambers  50 A- 50 D, as illustrated. In an example embodiment, heater module  10  also includes support members  55 , arranged in chamber  50  and mechanically coupled to top plate  30  and to bottom plate  40  to add stiffness to the heater module.  
         [0030]     Heater module  10  further includes one or more heating elements  150 , such as heater cartridges, arranged to heat chamber  50 . Heating elements  150  serve to heat chamber  50 , and to convert some of the liquid metal to vapor. In an example embodiment, a number of heating elements (e.g., eight, as show in  FIG. 2 ) are arranged adjacent inner surface  26  of sidewall  20  and extend inwardly toward the center of the chamber. In an example embodiment, a heating element  150  is arranged on either side of each brace  100  so that each sub-chamber  50 A- 50 D contains two heater elements.  
         [0031]     Each heating element  150  is connected to a lead  190  (e.g., wires) that connects the heating element to a power supply  200 . Power supply  200  is adapted to provide select amounts of power to the heating elements, as described in greater detail below. Power supply  200  is operably connected to a heater module controller  220  that controls the operation of heater module  10 , as described in detail below. Each heating element  150  generates heat by dissipating electrical power provided to it by power supply  200 .  
         [0032]     Heater module  10  also includes one or more temperature probes  300  at corresponding one or more positions within chamber  50 . Temperature probes  300  measure the temperature of chamber  50  at each of the one or more locations and generate corresponding temperature signals ST in response thereto. Temperature probes  300  are operably coupled to heater module controller  220 , which is adapted to receive and process the temperature signals.  
         [0033]     With reference to  FIGS. 1 and 2 , heater module  10  includes one or more wicking elements (“wicks”)  360  that cover most or all of chamber  50  peripheral interior surface. Wicks  360  serve to transport by capillary action liquid metal to most or all of the chamber peripheral interior surface. This process is illustrated in  FIG. 1  in cut-out C 1 , which illustrates liquid metal  51  being conducted up wick  360  toward top plate lower surface  34  so that the chamber  50  peripheral interior surface is covered with a thin coating or film of liquid metal.  
         [0034]     In an example embodiment, one or more wicks  360  are supported by or are fixed to bottom plate upper surface  42  and extend upward along inner wall surface  26  of sidewall  20  and extend across top plate lower surface  34 . In an example embodiment, one or more wicks  360  also cover heater elements  150  to facilitate the heating of liquid metal  51 .  
         [0035]     In  FIG. 1 , the wicks  360 , shown adjacent top and bottom plates  30  and  40 , have respective folded ends  361  that extend downward and upward along the inner surface  26  of sidewall  20 . This wick arrangement illustrates example embodiments wherein the folded ends either establish contact with an existing wick arranged along inner surface  26 , or meet up with one another to cover some or all of the sidewall inner surface.  
         [0036]     In respective example embodiments, each wick  360  is in the form of a screen or fiber bundle made of metal, ceramic or glass compatible with the metal liquid/vapor. The material used in wicks  360  is preferably readily “wet” by the liquid metal. Wick  360  has interstices  362  sized to support capillary transfer of liquid metal  51  to those portions of the chamber peripheral interior surface not otherwise accessible by the liquid metal at rest within the chamber. The term “wet” as used herein refers to the requirement for a small contact angle between the liquid metal and the wick material. The wicking action of one or more wicks  360  serves to maintain a film of liquid metal on those portions of chamber peripheral interior surface that play a significant role in heat transport to and from the chamber, as described below. In an example embodiment, the entire chamber peripheral interior surface is covered with a film of liquid metal using one or more of wicks  360 .  
         [0000]     Chuck With Heater Module  
         [0037]      FIG. 3  is a side view of a heated chuck  500  for laser thermal processing according to the present invention, and that includes the heater module  10  discussed above. Chuck  500  includes a thermal insulator layer  520  having an upper surface  522  and a lower surface  524 . In an example embodiment, insulator layer  520  is arranged with its upper surface  522  immediately adjacent lower surface  44  of bottom plate  40  so that the insulator layer and the heater module are in good thermal communication. In an example embodiment, insulator layer  520  is in direct contact with bottom plate  40 , while in another example embodiment a thin layer of flexible graphite (not shown), such as GRAFOIL® (available from American Seal and Packing Co., Fountain Valley, Calif.), is arranged between the insulator layer and the bottom plate. In an example embodiment, insulator layer  520  is a plate of fused silica or quartz. In an example embodiment, insulator layer  520  includes LD-80, available from Pyromatics Corporation of Willoughby, Ohio.  
         [0038]     Chuck  500  also includes a heat sink  600  arranged to be in good thermal communication with the insulator layer  520  through lower surface  524 . In an example embodiment, heat sink  600  is in the form of a cooled plate made from a material with a high thermal conductivity. In an example embodiment, the cooled plate of the heat sink is made of aluminum. In an example embodiment, heat sink  600  includes a cooling channel  602  (partially shown in  FIG. 3 ) fluidly coupled to a cooling unit  540  adapted to flow a cooling fluid through the cooling channel to remove heat from the heat sink. In an example embodiment, cooling channel  602  is formed in the cooled plate.  
         [0039]     Insulator layer  520  is arranged between heater module  10  and heat sink  600  and is adapted to maintain a substantially constant thermal gradient between the two. In an example embodiment, heater module  10  is at a temperature of about 400° C., while heat sink  600  is at a temperature of about 20° C.  
         [0040]     Upper surface  32  of top plate  30  is adapted to support a substrate (semiconductor wafer)  700  having an upper surface  702 , a lower surface  704  and an outer edge  706 . With reference to  FIG. 4 , in an example embodiment, top plate  30  includes, atop upper surface  32 , a layer  710  of material (e.g., a coating or a plate) having an upper surface  712  upon which substrate  700  is supported. The material making up layer  710  is one that does not contaminate substrate  700 . Example materials for layer  710  include silicon, silicon oxide or silicon nitride, or any combination thereof.  
         [0041]     With reference again to  FIG. 3 , chuck  500  also includes a chuck controller  720  operably coupled to heater module controller  220 . Chuck controller  720  controls the operation of the chuck, including the heater module, as described below. Chuck controller  720  is also operably coupled to cooling unit  540  to control the flow of a cooling fluid (e.g., water) through cooling channel  602  of heat sink  600 .  
         [0000]     Method of Operation  
         [0042]     With continuing reference to  FIG. 3 , there is also shown a LTP laser beam  880  incident upon substrate upper surface  702 . LTP laser beam  880  is moved (“scanned”) over substrate surface  702  as part of performing LTP of substrate  700 , e.g., to activate dopants in the substrate at or near the substrate upper surface.  
         [0043]     LTP laser beam  880  presents a spatially varying thermal load to the substrate that will ultimately end up increasing the substrate&#39;s background temperature if the heat it creates in the substrate is not properly dissipated. Any change in the substrate background temperature creates undesirable variations in the LTP process, and in particular affects the activation of dopants in the substrate during LTP.  
         [0044]     Accordingly, prior to irradiating substrate  700  with LTP laser beam  880 , chuck controller  720  instructs heater module controller  220  via a signal S 1  to activate power supply  200  via signal S 2 . In response thereto, power supply  200  provides electrical power (shown schematically as arrow  810 ) to heating units  150  via a power signal SP, which heats up heater module  10  by introducing heat into chamber  50 . In an example embodiment, the power input from power supply  200  is about 3.5 kW steady state to maintain heater module  10  at about 400° C.  
         [0045]     The liquid metal  51  contained in chamber  50  is heated by heating units  150 . This heat quickly and uniformly spreads over the entire inner surface of chamber  50  of heater module  10  via the wicking action of wicks  360  and the evaporation and condensation of the metal liquid/vapor within the chamber. Heat transport is highest at the chamber peripheral interior surface, which is mostly, or entirely, covered by wicks  360 . With substrate  700  in good thermal contact with heater module  10 , the substrate takes on the constant background temperature TC of the heater module.  
         [0046]     Heater module controller  220  also receives temperature signals ST from temperature probes  300  and uses these signals to regulate the temperature of heater module  10  by providing the temperature information to the heater module controller  220 . In response, heater module controller  220  regulates the amount of power  810  (via power signal SP) supplied by power supply  200  to heating units  150 . In this manner, the temperature of the heater module, as measured by temperature probes  300 , can be precisely controlled, e.g., to within 1° C.  
         [0047]     With continuing reference to  FIG. 3 , when substrate  700  is brought up to a desired constant background temperature TC, then LTP laser beam  880  is scanned over substrate upper surface  702 . This introduces a spatially varying thermal load on the substrate, which translates to a spatially varying temperature on the substrate. This, in turn, creates a corresponding spatially varying temperature within chamber  50  of heater module  10 . Chamber  50  has a “hot spot”  812  corresponding to the position of the LTP laser beam at substrate surface  702 , and “cooler regions”  814  surrounding the hot spot. Hot spot  812  moves around chamber  50  as LTP laser beam  880  scans over substrate surface  702 .  
         [0048]     The temperature variation in chamber  50  caused by the spatially varying thermal load is quickly ironed out by the evaporation and condensation of metal liquid/vapor  51  within the chamber, by the movement of the metal vapor throughout the chamber volume, and by the movement of the liquid metal via capillary action through the one or more wicks  360  covering the chamber peripheral interior surfaces. The transfer of heat and metal vapor from hot spot  812  out to cooler regions  814  is illustrated in  FIG. 3  by arrows  816 .  
         [0049]     Metal  51  in liquid form is capable of absorbing copious amounts of heat by evaporation because of its large latent heat of vaporization. The metal liquid turns to vapor in the “hot spots”  812  in the cavity corresponding to the location of the scanned LTP laser beam  880  at substrate  700 . The vaporized metal is then replaced by liquid metal via capillary action of one or more wicks  360 . The metal vapor then condenses to a liquid state in the cooler regions  814  of the chamber as the spatially varying thermal load moves to a different region of the chamber. The heat taken in by heater module  10  is transferred to heat sink  600  through insulator layer  520 , and is dissipated, as illustrated by power-out arrow  820 .  
         [0050]     Insulator layer  520  is adapted to maintain a substantially constant thermal gradient between the heater module and the heat sink, and therefore transfers heat from one to the other at a substantially constant rate. This rate is chosen so that the heater module can be electrically controlled at the constant background temperature, even when the laser is operated at maximum power.  
         [0051]     In an example embodiment, the amount of heat removed from chamber  50  is greater than that provided by the spatially varying thermal load associated with LTP laser beam  880 , less an amount of heat lost by radiation and convection from substrate  700  and the heater chamber. This ensures that the heating system (i.e., heating elements  150 , power supply  200 , heater module controller  220  and temperature probes  300 ) is required to provide some heat to maintain the heater module, and thus the substrate, at the substantially constant background temperature TC.  
         [0052]     The very high thermal conductivity effectively provided by heater module  10  ensures a high degree of temperature uniformity (e.g., to within +/−4° C.) except, of course, at or in close proximity to the position on the substrate being subject to the thermal load, e.g., LTP laser beam  880 . This in turn allows the substrate to have a uniform constant background temperature TC at those locations not being irradiated by LTP laser beam  880 . The maximum temperatures reached during the LTP process depend primarily on the substrate temperature at the beginning of the annealing cycle and the power stability in the laser beam. Keeping the substrate temperature uniform therefore assists in keeping the LTP annealing process uniform. This translates into consistent and reliable device performance.  
         [0053]     The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction and operation described herein. Accordingly, other embodiments are within the scope of the appended claims.