Patent Document

BACKGROUND 
   1. Field of the Invention 
   This invention generally relates to semiconductor manufacturing equipment and, more particularly, to an apparatus and method used for the processing of semiconductor wafers. 
   2. Description of the Related Art 
   During the processing of semiconductor devices, it is highly desirable to accurately control the thermal treatment to which the devices are exposed during processing. 
   In the semiconductor industry, advancements in the development of semiconductor devices of decreased dimensions require the development of new processing and manufacturing techniques. One such processing technique is known as Rapid Thermal Processing (RTP). The RTP technique reduces the amount of time that a semiconductor device is exposed to high temperatures during processing. The RTP technique, typically includes irradiating the semiconductor device or wafer with sufficient power to rapidly raise the temperature of the wafer and maintaining the temperature for a time period long enough to successfully perform a fabrication process, but which avoids such problems as unwanted dopant diffusion that would otherwise occur during longer exposure to high processing temperatures. 
   For the above reasons, what is needed is an apparatus and method for isothermally distributing a temperature across the surface of a semiconductor device during rapid thermal processing. 
   SUMMARY 
   The present invention provides a heating apparatus and method for isothermally distributing a temperature across the surface of a semiconductor device or wafer during processing under a varying range of processing temperatures and pressures. The invention provides a potentially slip-free RTP process. 
   A furnace is provided including a process chamber defining a cavity, which is configured to house a processing tube. The furnace also includes a plurality of resistive heating elements advantageously arranged therein. The heating elements can be disposed across the furnace and aligned in close proximity to one another so as to provide an even heating temperature distribution. The resistive heating elements may be positioned to surround the processing tube to provide dual-sided heating during processing. Advantageously, the heating elements may be covered with a heat diffusing material, which provides uniform temperature dissipation of the heat energy provided by the resistive heating elements. 
   In one aspect of the invention, an apparatus is provided for heating a wafer during processing. The apparatus includes a process chamber enclosing a processing tube defining a processing area. The processing tube includes a first wall and a second wall which define a hollow cavity or passageway therebetween. The second wall includes a plurality of holes or outlets formed thereon which allow environmental communication between the hollow cavity and the processing area. The apparatus also includes a plurality of resistive heating elements positioned adjacent to the processing tube. A thermal energy output from the resistive heating elements is configured to heat a gas flowing through the hollow cavity. The gas flowing through the hollow cavity exits the hollow cavity through the plurality of holes and convectively change the temperature of the wafer disposed in the processing tube. 
   In another aspect of the invention, a method is provided for processing a semiconductor wafer, including flowing a gas through a hollow cavity defined by the walls of a processing tube; generating a thermal output from a plurality of resistive heating elements to change the temperature of the gas while the gas is resident in the hollow cavity; and flowing the heated gas out from the hollow cavity into a wafer processing area to change the temperature of a wafer disposed therein. 
   Fortunately, in the present invention rapid thermal processing can be a potentially slip-free processing technique for a wide range of temperatures and time domains, particularly near the wafer edge. 
   No moving parts, such as lift pins or wafer spinners, are required within the processing area to load the wafer, nor are other complex and costly components required, such as reflectors, actuators, and complex power transformers and controllers. Since the furnace does not require large lamps for heating nor moving parts, the size of the furnace, as well as the volume of the processing area, may be substantially reduced relative to other furnaces. The reduced volume and size are of particular advantage for reasons that are made apparent below. 
   These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a schematic illustration of a side view of one embodiment of a semiconductor wafer processing system that establishes a representative environment of the present invention; 
       FIG. 2  is a simplified cross-sectional view of a furnace in accordance with one embodiment of the present invention; 
       FIG. 3  is a simplified illustration of a processing chamber including a processing tube in accordance with an embodiment of the present invention; 
       FIG. 4  is a simplified illustration of a portion of the processing tube of  FIG. 3 ; 
       FIG. 5  is a graph illustrating the effect of the present invention on wafer heating; and 
       FIG. 6  is a graph illustrating the effect of the present invention on wafer cooling. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic illustration of a side view of one embodiment of a semiconductor wafer processing system  100  that establishes a representative environment of the present invention. It should be understood that the present invention is in no way limited to use with or in any particular wafer processing system. 
   As shown in  FIG. 1 , processing system  100  includes a loading station  102  which has multiple platforms  104  for supporting and moving a wafer cassette  106  up and into a loadlock  108 . Wafer cassette  106  may be a removable cassette which is loaded into a platform  104 , either manually or with automated guided vehicles (AGV). Wafer cassette  106  may also be a fixed cassette, in which case wafers are loaded onto cassette  106  using conventional atmospheric robots or loaders (not shown). Once wafer cassette  106  is inside loadlock  108 , loadlock  108  and transfer chamber  110  are maintained at atmospheric pressure or else are pumped down to a vacuum pressure using a pump  112 . A robot  114  within transfer chamber  110  rotates toward loadlock  108  and picks up a wafer  116  from cassette  106 . A furnace  120 , which may also be at atmospheric pressure or under vacuum pressure, accepts wafer  116  from robot  114  through a gate valve  118 . 
   Robot  114  then retracts and, subsequently, gate valve  118  closes to begin the processing of wafer  116 . After wafer  116  is processed, gate valve  118  opens to allow robot  114  to pick-up and remove wafer  116 . 
   Optionally, additional furnaces may be added to processing system  100 , for example furnace  122 . In accordance with the present invention, furnaces  120  and  122  are RTP reactors, such as those used in thermal anneals. In other embodiments, reactors  120  and  122  may also be other types of reactors, such as those used for dopant diffusion, thermal oxidation, nitridation, chemical vapor deposition, and similar processes. Reactors  120  and  122  are generally horizontally displaced, however in one embodiment, reactors  120  and  122  are vertically displaced (i.e. stacked one over another) to minimize floor space occupied by system  100 . 
   Reactors  120  and  122  are bolted onto transfer chamber  110  and are further supported by a support frame  124 . Process gases, coolant, and electrical connections may be provided through the rear end of the reactors using interfaces  126 . 
   As shown in  FIG. 2 , furnace  200  may generally include a closed-end processing chamber  208 , which defines an interior cavity  210 . Disposed within interior cavity  210  is a processing tube  212 . Externally, furnace  200  may be a metallic shell  202  made of aluminum or similar metal, having an opening provided on a face of shell  202 , configured to receive wafer  116  for processing. Furnace  200  may enclose a thermal insulation material, such as thermal insulation  204 , which substantially surrounds processing chamber  208  so as to minimize or eliminate the escape of heat energy through shell  202 . Insulation material  204  may include any suitable insulation material, such as ceramic fiber. 
   Optionally, to protect users and/or equipment near furnace  200 , the furnace may include a detachable water cooled jacket (not shown) or similar device, which may be used to externally surround furnace  200 . The water cooled jacket ensures that furnace  200  does not become too hot, so as to be a hazard to nearby equipment or personnel. 
   In one embodiment, a plurality of heating elements  220  are used to surround a top and a bottom portion of processing tube  212 . In this embodiment, resistive heating elements  220  may be disposed in parallel across and external to process chamber  208 . Each heating element  220  is in relative close proximity to each other element. For example, each resistive heating element  220  may be spaced between about 5 mm and about 50 mm, for example, between about 10 mm and about 20 mm. Accordingly, the close spacing of heating elements  220  provides for an even heating temperature distribution in processing tube  212 . 
   Resistive heating elements  220  may include a resistive heating element core surrounded by a filament wire. The core can be made of a ceramic material, but may be made of any high temperature rated, non-conductive material. The filament wire is conventionally wrapped around the core to allow for an optimal amount of radiated heat energy to emanate from the element. The filament wire may be any suitable resistively heatable wire, which is made from a high mass material for increased thermal response and high temperature stability, such as SiC, SiC coated graphite, graphite, NiCr, AlNi and other alloys. In one embodiment, resistive heating filament wire is made of a combination Al—Ni—Fe material, known commonly as Kantal A-1 or AF, available from Omega Corp. of Stamford, Conn. 
   Optionally, resistive heating elements  220  may be positioned in various configurations which may include, for example, circular, zigzag, cross-hatched patterns and the like. The variable patterns may be able to provide more optimal temperature distribution and further reduce the possibility of temperature fluctuations across the surface of the wafer. 
   In yet another embodiment, furnace  200  includes heat diffusing members  222 , which are positioned proximate to and between heating elements  220  and processing chamber  208 . Heat diffusing members  222  absorb the thermal energy output from heating elements  220  and dissipate the heat evenly across process chamber  208  and tube  212 . Heat diffusing members  222  may be any suitable heat diffusing material that has a sufficiently high thermal conductivity, preferably Silicon Carbide, Al 2 O 3 , or graphite. 
   In one embodiment, furnace  200  may include up to any number of heating zones. In the embodiment shown in  FIG. 2 , furnace  200  includes three parallel heating zones, which include a central zone, referenced as zone  2 , and two adjacent outer zones, referenced as zones  1  and  3 . Each heating element  220  can be apportioned to a specific heating zone. 
   As described in more detail below, each heating zone has at least one temperature sensor  224 , which provides feedback to a controller  226 . As fluctuations in temperature within a heating zone are sensed by the temperature sensors, real-time controller  226  can cause the power from power supply  232  to increase or decrease, as necessary, to increase or decrease the energy output (heat) from each of resistive elements  220 . For example, if a drop in temperature is sensed in zone  1 , the thermal energy output from resistive heating elements  220  apportioned to zone  1 , increases until the temperature in zone  1  is returned to the desired level. In this manner, the temperature from zone-to-zone across the surface of wafer  116  may be kept substantially isothermal. 
   The number of zones and the number of resistive elements  220  apportioned to each zone may vary based on the energy output desired. The size of each zone (i.e. the heating volume) is also variable. Advantageously, the size of each zone can be scaled up or down as desired. For example, zone  2  can be scaled up for processing of larger wafers by re-apportioning heating elements  220  from zones  1  and zone  3  to zone  2 . This means that the number of heating elements  220  assigned to zone  2  is increased, while the number of heating elements assigned to zones  1  and  3  is decreased. The heating elements added to zone  2  are controlled by controller  226  to respond in the same manner as the heating elements already assigned to zone  2 . 
   In one embodiment, temperature sensors, such as thermocouples, are embedded within heat diffusing members  222 . For example, thermocouples  224   a,    224   b  and  224   c  can be strategically placed such that they can provide feedback via lines  230  as to the temperature conditions of heat diffusing members  222 . For example, a first and a second thermocouple  224   a  and  224   c  are placed at each end of heat diffusing member  222 . A third thermocouple, thermocouple  224   b,  is placed in the center of heat diffusing member  222 . In this configuration, the temperature of a zone (e.g. zone  1 , zone  2  and zone  3 ) can be monitored with feedback provided to controller  226 . By positioning the thermocouples  224   a - 224   c  at known positions on the heat diffusing members  222 , the temperature gradient can be determined with reference to a position within process chamber  208 . This data is used by controller  226  to control the temperature within each zone more precisely. Thermocouples  224   a,    224   b  and  224   c  can be conventional R-type or K-type thermocouples available from Omega Corporation of Stamford, Conn. 
   A microprocessor or process control computer  228 , generally controls the processing of a semiconductor wafer placed in the RTP reactor and may be used to monitor the status of the system for diagnostic purposes. In one embodiment, process computer  228  provides control signals to controller  226  in response to temperature data received from temperature sensors  224 . Process computer  228  may also direct pressure setpoints to pump assembly  112  ( FIG. 1 ) as well as gas and plasma inlet flow signals to mass-flow controllers in a gas network (not shown). In one embodiment, controller  226  is a real-time Proportional Integral Derivative (PID), multi-zone controller, available from Omega Corporation. Controller  226  provides control signals to a SCR-based phase controlled power supply  232 , which provides power to the resistive heating elements  220 . Advantageously, a direct line voltage of between about 100 volts and about 500 volts may be used to power resistive heating elements  220 . Thus, no complex power transformer is needed in the present invention for controlling the output of resistive heating elements  220 . 
   In operation, the multi-zone controller  226  receives temperature sensor outputs via sensing lines  230 , as well as the desired wafer temperature setpoint from computer  228  and delivers controlled power setpoints to the heating element power supply  232 . Heating elements  220  increase or decrease their energy output in response to the increase or decrease in power supplied from power supply  232 . 
     FIG. 3  is a simplified illustration of process chamber  208  including processing tube  212  in accordance with an embodiment of the present invention. In one embodiment, processing tube  212  may be constructed with a substantially rectangular cross-section, having a minimal internal volume surrounding wafer  116 . In one embodiment, the volume of processing tube  212  is usually no greater than about 5000 cm 3 ; preferably the volume is less than about 3000 cm 3 . One result of the small volume is that uniformity in temperature is more easily maintained. Additionally, the small tube volume allows furnace  200  ( FIG. 2 ) to be made smaller, and as a result, system  100  may be made smaller, requiring less clean room floor space. The smaller furnace size, in conjunction with the use of the robot loader, allows multiple furnaces to be used in system  100  by vertically stacking the reactors as shown in FIG.  1 . 
   To conduct a process, processing tube  212  should be capable of being pressurized. Typically, processing tube  212  should be able to withstand internal pressures of about 0.001 Torr to 1000 Torr, preferably between about 0.1 Torr and about 760 Torr. In one embodiment, processing tube  212  can be made of quartz, but may also be made of silicon carbide, Al 2 O 3 , or other similarly suitable material. 
   A wafer support device  302  may be used to support a single wafer within processing tube  212 . Support device  302  may be made of any high temperature resistant material, such as quartz. Support device  302  can have any height necessary, for example, a height of between about 50 μm and about 20 mm. In one embodiment, support device  302  includes standoffs positioned within processing tube  212 . The standoffs will generally have a height of between about 50 μm and about 20 mm. The total contact area between the standoffs and wafer  116  can be less than about 350 mm 2 , preferably less than about 300 mm 2 . Standoffs  302  may be made of quartz or similar material. 
   An opening  304  is defined at one end of processing tube  212 , which provides access to processing area  310  for the loading and unloading of wafer  116  before and after processing. Opening  304  may be a relatively small opening, but with a height and width large enough to accommodate a wafer of between about 0.5 mm to about 2 mm thick and up to about 300 mm (˜12 in.) in diameter, and a robot arm of robot  114  ( FIG. 1 ) passing therethrough. The height of opening  304  is no greater than between about 18 mm and about 50 mm, and preferably, no greater than about 30 mm. The relatively small opening helps to reduce radiation heat loss from processing tube  212 . Also, the small opening keeps down the number of particles entering processing area  310  of processing tube  212  and allows for easier maintenance of the isothermal temperature environment. 
     FIG. 4  illustrates a magnified portion of processing tube  212  in accordance with an embodiment of the present invention. As shown, processing tube  212  can be formed having a hollow wall. For example, processing tube  212  can be formed having an outer wall  402  and an inner wall  404  which enclose an internal hollow cavity or passage way  406 . The thickness of outer wall  402  and inner wall  404  can be any thickness suitable to allow for high temperature processing of wafers in various pressure conditions. For example, the wall thickness can be between about 1 mm and about 5 mm. Hollow cavity  406  can also be defined with any volume necessary to facilitate wafer processing. For example, hollow cavity  406  can have a thickness d of between about 0.5 mm and about 5 mm. 
   Hollow cavity  406  has an inlet  311  (FIG.  3 ), which allows a gas to be fed from a gas reservoir (not shown) into hollow cavity  406 . The gas may include, for example, any suitable carrier gas, such as He, H 2 , O 2 , Ar, N 2  and the like and any processing gas, such as NH 3 , O 3 , SiH 4 , Si 2 H 6 , B 2 H 6  and other gases suitable for CVD applications, or a combination of both gases. Hereinafter, the carrier gas, the process gas and the combination of both shall be referred to generally as “the gas.” 
   In one embodiment, a plurality of holes or outlets  408  are formed through inner wall  404  to allow for environmental communication between hollow cavity  406  and processing area  310  (FIG.  3 ). Each outlet  408  can be sized to allow the various types of gases to move between hollow cavity  406  and processing area  310 . In one example, outlets  408  may be between about 0.1 mm to about 2 mm in diameter. 
   Outlets  408  can extend from substantially end  301  of processing tube  212  at opening  304  to a point  303  a fixed distance  305  from the gas entering end of processing tube  212 . Distance  305  is designed to allow the flowing gases to reach a minimum temperature at a given flow rate before exiting out from outlets  408 . 
   Processing tube  212  can be fabricated using many well known fabrication techniques. For example, processing tube  212  may be welded, braised, assembled or cast. 
   Heat transferred to the flowing gas is a function of the thermal mass of the heater, the flow rate of the gas and the diameter of the outlets, as well as the type of gas, the residence time of the gas in hollow cavity  406  and the nominal temperature of hollow cavity  406 . Each of these parameters can be adjusted until the exiting gas temperature is appropriate for a specific process. 
   Generally, the thermal mass and thermal energy output and capacity of the heating elements will be known. Accordingly, for a given thermal energy output the gas can be made to flow through hollow cavity  406  at any desired rate, for example, between about 10 sccm to about 100 slm. The flow rate of gases is selected to ensure that the wafer remains stable upon the standoffs and that the pressure difference between the ambient environment outside of the processing tube and inside the processing tube is relatively small. 
   Hollow cavity  406  provides for heat exchange, such that the gas can be heated as it travels from inlet  311  through to the exit points of outlets  408 . The gas entering inlet  311  can be at ambient temperature or may be pre-heated prior to entering hollow cavity  406 . Before the gas exits outlets  408 , the gas is made to flow through a distance  305  of hollow cavity  406 . The length of distance  305  is variable, but is at least long enough to provide the residence time for the gas to reach a desired minimum temperature before exiting outlets  408  into processing area  310 . 
   In one embodiment, the gas is made to move through hollow cavity  406  at a flow rate which allows the gas to be heated at a rate of between about 1° C./s and about 1000° C./s. to between approximately 100° C. and 1400° C. 
   As shown in  FIG. 3 , in one operational embodiment, wafer  116  is placed within processing tube  212  on standoffs  302 . A gas, such as a carrier gas combined with process gases, is allowed to flow through hollow cavity  406 . In one embodiment, the gas entering hollow cavity  406  can be pre-heated or, alternatively, can be at ambient temperature. In this example, the gas enters hollow cavity  406  as indicted by arrows  312  at approximately room temperature (˜25° C.). However, in either embodiment, the gas is heated to a processing temperature from heat transferred from heating elements  220  into heat diffusion material  222  and into process chamber  208  and finally, through outer wall  402  and inner wall  404 . Initially, the gas flows a distance  305  within hollow cavity  406  to reach a minimum desired processing temperature. The flowing gas then reaches outlets  408  to enter processing area  310 . The flowing gas contacts wafer  116  in processing area  310  to heat wafer  116  using the effect of forced convection. 
   The heated gas flows into hollow cavity  406  at a controlled rate. Thus, the ramp rate control for heating wafer  116  can be correlated to gas flow rate control. As described in detail below, the gas flow can be continuous through the processing of wafer  116 , pulsed, flown during temperature ramp up only, or flown during cool down, or a combination of both. 
   As illustrated in graph  500  of  FIG. 5 , wafers placed in furnace  200  and heated have different heating profiles and heating rates between a center portion and an edge portion of the wafer. For example, without gas flow through hollow cavity  406  of processing tube  212 , the wafer center  502  requires approximately 3.5 time units to reach a processing temperature of about 1000° C. The edge of the wafer requires about 2.5 time units to reach the same temperature. 
   A wafer heated using forced convection assistance in accordance with the present invention, created by flowing gas through hollow cavity  406  and into processing area  310 , is heated at a center portion and an edge portion of the wafer with almost identical heating profiles. For example, the wafer center  506  and the wafer edge  508  reach the processing temperature of about 1000° C. at about the same time, in less than 1 time unit. 
   A primary advantage of the present invention is the ability to conduct substantially slip-free RTP of a silicon wafer with lesser emissivity dependence and lesser pattern induced local heating effect. Further, by controlling ramp rate control using gas flow rate control, the wafer can be heated rapidly and uniformly as illustrated in FIG.  5 . 
     FIG. 6  illustrates the effect of another embodiment of the present invention in which forced convection enables forced cooling of the wafer while the wafer is within processing tube  212 . As shown in graph  600 , as the flow rate of gas through hollow cavity  406  ( FIG. 4 ) is increased, the wafer temperature ramp rate is increased. However, at a given thermal output and with a particular gas flow rate, as indicated at  602 , the wafer temperature ramp rate begins to decrease. At this juncture, the effect of the forced convection is to remove energy from the wafer causing the wafer to cool. The forced cooling reduces the post-processed wafer to a temperature below the critical slip formation temperature without requiring the cooling of the entire process chamber  208  or requiring a separate cooling chamber. 
   It should be understood that the wafer described above may be made of conventional materials commonly used in the industry, such as silicon, gallium arsenide, or other similar compound or the wafer may be a semiconductor wafer, made from quartz or glass. 
   Having thus described the preferred embodiments, 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.

Technology Category: 5