Abstract:
A method and apparatus for providing integrated bake and chill thermal cycling applied to a material substrate is provided. The apparatus is a module comprising an integrated bake and chill plate with one or more fluid channels in a generally spiraling arrangement. The fluid channels have inlets and outlets near the center and perimeter of the integrated bake and chill plate. Additionally, the fluid channels can have microchannels through portions thereof. The module can also comprise one or more thermoelectric devices, a thermally conductive plate on which the substrate directly or indirectly rests, a printed circuit board, and a variable power source. The integrated bake and chill plate, the thermally conductive plate and the thermoelectric devices are all in thermal contact with each other. The thermoelectric devices are also in electrical contact with the variable power source and comprise a top plate, a bottom plate, a support positioned between the top and bottom plates, and one or more copper pads beneath the bottom plate for establishing electrical contact. The module bakes and chills a substrate by flowing fluids of different temperatures through the generally spiraling fluid channels and adjusting the temperature of the thermoelectric devices according to a preprogrammed cycle. Different fluid flow patterns can be chosen where fluid enters near the center of the integrated bake and chill plate and drains near the perimeter, where fluid enters near the perimeter and drains near the center, or where the two patterns are combined.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to methods and apparatuses for providing integrated bake and chill thermal cycling applied to material substrates. More particularly, it relates to an integrated bake and chill thermal cycling module having improved fluid channels and structural design for providing controlled thermal cycling of material substrates such as semiconductor wafers and flat panel displays.  
         BACKGROUND OF THE INVENTION  
         [0002]    Certain stages of semiconductor manufacturing require baking the semiconductor substrate material, such as a wafer, and subsequently chilling it. For example, the photoresist processing state of semiconductor manufacturing requires such baking and chilling, or thermal cycling. In order to produce high quality wafers suitable for present integrated circuit applications, the temperature of the wafer during this thermal cycling must be precisely controlled with respect to both the temporal temperature profile of the baking and chilling cycles and to the uniformity of the temperature across the substrate.  
           [0003]    The conventional method for baking and chilling wafers involves first baking the wafer at a temperature ranging typically between 70° C. and 250° C. for a period of time ranging typically between 30 seconds and 90 seconds. After baking the wafer, the wafer is mechanically moved to a cold plate where it is chilled to a temperature ranging typically between 0° C. and 30° C.  
           [0004]    There are several disadvantages of the above method. First, moving a wafer through the air between the hot and cold plates subjects the wafer to uncontrolled temperature variations during the bake and chill cycles. Moreover, the time required to move the wafer between the bake and chill plates prevents the realization of very short thermal transition times between thermal cycles. Finally, mechanically moving the wafer from the hot plate to the cold plate can contaminate or otherwise damage the wafer.  
           [0005]    Attempts have been made to overcome the disadvantages of separate bake and chill plates. One apparatus places the hot plate upside down and directly above the cold plate. Because the wafer moves only a short distance from the cold plate directly upward to the hot plate, the apparatus reduces the uncontrolled and nonuniform temperature fluctuations normally present during the transition from the baking step to the chilling step. Nevertheless, because the wafer must be moved between separate bake and chill plates, the wafer is still subjected to uncontrolled and nonuniform temperature fluctuations during thermal cycling. Moreover, physical movement inhibits short thermal transition times. Finally, the wafer may still be exposed to contaminates or otherwise damaged during the physical movements from the hot plate to the cold plate.  
           [0006]    Accordingly, the present invention provides an improved apparatus for the thermal cycling of material substrates such as wafers used in the manufactures of semiconductors. In particular, the present invention provides an improved apparatus for thermal cycling that eliminates the need to move the substrate between distinct bake or chill plates and that provides improved continuous control of substrate temperature throughout the entire baking and chilling cycle. Further features and advantages of the invention will be apparent from the following description and drawings.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a single thermally conductive plate for baking and chilling a substrate such as a wafer. Because the substrate is not moved during the entire baking and chilling cycle, the invention avoids problems associated with the transfer of the substrate between separate bake and chill plates. Another feature of the present invention is the use of a thinner plate for baking and chilling the substrate. A thinner plate provides an advantage of relatively thicker plates used previously due to the thinner plates lower thermal mass. Accordingly, the thinner plate provides for more efficient heat transfer.  
           [0008]    The integrated bake and chill plate of the present invention contains one or more fluid channels that wind around a center point, gradually receding from the center point until reaching the perimeter in a square-like, rectangular-like, triangular-like, circular or other geometric pattern. The fluid channels permit thermally conductive fluids of pre-selected temperatures to be introduced at the perimeter of the integrated bake and chill plate and drained at the center or introduced at the center of the integrated bake and chill plate and drained at the perimeter. This design facilitates different flows and counterflows, which compensates for different heat transfer at the perimeter of the integrated bake and chill plate versus the center of the integrated bake and chill plate. In addition, the integrated bake and chill plate also can contain microchannels throughout portions of the fluid channels. The microchannels provide a larger pressure drop within the fluid channels thereby increasing the velocity of the thermally conductive fluid. The greater velocity inhibits the development of areas of nonuniform heat transfer in the integrated bake and chill plate. Additionally, the microchannels add stiffness to the integrated bake and chill plate, thereby reducing the potential for warping of the plate.  
           [0009]    To improve the precision and uniformity of the plate&#39;s temperature, the present invention further comprises one or more thermoelectric devices (TEDs) in thermal contact with the thermally conductive plate. Because the TEDs may be quickly and precisely adjusted to heat the plate, they improve the control of the plate temperature and provide for shortened plate temperature transition times. Moreover, independently controlling an array of TEDs allows for compensation of spatial nonuniformities in substrate temperature that might arise. Alternatively, such independent control can provide intentional temperature nonuniformities if desired for special processing purposes.  
           [0010]    The present invention also comprises a design facilitating easy replacement of individually damaged TEDs. The TEDs are attached with a connector, such as a screw or bolt, through a hole located in the center of the TED. This design also improves the electrical connection between the TED and a variable power source and the thermal connection between the thermally conductive plate and each TED.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The advantages, features and design of the invention will become apparent from the following detailed description of the invention and the accompanying drawings in which like reference numerals refer to like elements and in which:  
         [0012]    [0012]FIG. 1 is a perspective view of a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a top view of an arrangement of fluid channels in a preferred embodiment of the invention;  
         [0014]    [0014]FIG. 3 is a cross-sectional view of a preferred embodiment of the invention; and  
         [0015]    [0015]FIG. 4 is a top view of an arrangement of fluid channels and holes in the integrated bake and chill plate in a preferred embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The present invention provides a single thermally conductive plate for baking and chilling a substrate such as a wafer. Because the substrate is not moved during the entire baking and chilling cycle, the invention avoids problems associated with the transfer of the substrate between separate bake and chill plates. Another feature of the present invention is the use of a thinner plate for baking and chilling the substrate. A thinner plate provides an advantage over relatively thicker plates used previously due to the thinner plate&#39;s lower thermal mass. Accordingly, the thinner plate provides for more efficient heat transfer.  
         [0017]    Referring to FIG. 1, a substrate  38 , such as a semiconductor wafer or flat panel display, is baked and chilled through thermal contact with a thermally conductive plate  30 . Thermal contact includes physical proximity or direct physical contact sufficient to permit the transfer of heat. Both methods of establishing thermal contact are well known in the art and include positioning the substrate approximately 0.005-0.006 inches from the plate, or holding the substrate directly against the plate with a vacuum line, electrostatic clamp or gravity. It will be appreciated by anyone skilled in the art that although the present description uses physical proximity thermal contact for purposes of definiteness, any of the known methods of thermal contact may be used. In another preferred embodiment, the thermally conductive plate  30  has vertical extensions from its bottom surface that extend to make thermal contact with an integrated bake and chill plate  34 .  
         [0018]    The thermally conductive plate  30  is preferably made of a ⅛or {fraction (3/16)}inch thick piece of aluminum, aluminum nitride or other suitable ceramic or metal. The thinner plate provides an advantage over relatively thicker plates used previously due to the thinner plates lower thermal mass, which provides for more efficient heat transfer. Positioned on the other side of plate  30  is an array of thermoelectric devices (TEDs)  32  capable of heating. The TEDs  32  can be controlled to increase or decrease the amount of heat applied to the thermally conductive plate. Furthermore, when multiple TEDs  32  are used and individually controlled, one region of the plate  30  and substrate  38  can be heated or cooled in a different manner from another region. Thermal grease can be placed between the plate  30  and TEDs  32  to act as a conductive interface and improve the thermal contact. In a preferred embodiment, there are no spaces between the TEDs  32 . In another preferred embodiment there are varying spaces between the TEDs to allow vertical extensions of the thermally conductive plate  30  to rest between the TEDs and make direct contact with an integrated bake and chill plate  34 .  
         [0019]    Beneath the layer of TEDs  32  is an integrated bake and chill plate  34  in thermal contact with the TEDs  32  and with the thermally conductive plate  30 . While the integrated bake and chill plate  34  shown in FIG. 1 has a square shape across a horizontal plane, the plate  34  could also have a circular, oval or other shape. In a preferred embodiment, the integrated bake and chill plate  34  also has receiving ribs that are integrally connected to the top surface of the integrated bake and chill plate to receive the vertical extensions of the thermally conductive plate  30  thereby improving thermal conductivity between the integrated bake and chill plate  34  and thermally conductive plate  30  and reducing the potential for the plate  30  to warp. Preferably, thermal grease is also placed between the TEDs  32  and integrated bake and chill plate  34  to act as a conductive interface.  
         [0020]    Beneath the integrated bake and chill plate  34 , in a preferred embodiment, is a layer of insulation  35 . Then, beneath the insulation layer  35  is a printed circuit board (PCB)  36 . Any variable power source can be connected to the TEDs  32 , but the preferred embodiment is to connect the TEDs  32  to a variable power source through the PCB  36 . The TEDs  32  can be individually controlled by the PCB  36 .  
         [0021]    The integrated bake and chill plate of the present invention contains one or more fluid channels that wind around a center point, gradually receding from the center point until reaching the perimeter in a square-like, rectangular-like, triangular-like, circular or other geometric pattern. The fluid channels permit thermally conductive fluids of pre-selected temperatures to be introduced at the perimeter of the integrated bake and chill plate and drained at the center or introduced at the center of the integrated bake and chill plate and drained at the perimeter. This design facilitates different flows and counterflows, which compensates for different heat transfer at the perimeter of the integrated bake and chill plate versus the center of the integrated bake and chill plate. In addition, the integrated bake and chill plate also can contain microchannels throughout portions of the fluid channels. The microchannels provide a larger pressure drop within the fluid channels thereby increasing the velocity of the thermally conductive fluid. The greater velocity inhibits the development of areas of nonuniform heat transfer in the integrated bake and chill plate. Additionally, the microchannels add stiffness to the integrated bake and chill plate, thereby reducing the potential for warping of the plate.  
         [0022]    Referring to FIG. 2, the integrated bake and chill plate  34  is illustrated having fluid channels  40 A and  40 B and fluid inlets and fluid outlets through which thermally conductive fluid may flow for heating or cooling the thermally conductive plate. The temperature of the plate  30  is determined primarily by the temperature of the fluid flowing through channels  40 A and  40 B. The integrated bake and chill plate  34  is approximately 1 inch to 1½inch thick and is composed of a material of relatively high thermal conductivity, such as a metal composed of copper or aluminum. Copper has preferable thermal properties, but aluminum is less expensive and simpler to manufacture. The integrated bake and chill plate  34  can be comprised of one solid plate with channels  40 A and  40 B formed within it or of two plates that are braised together and have indentations formed on their respective surfaces so that when joined the two plates create fluid channels  40 A and  40 B. The temperature of the thermally conductive plate  30  is determined primarily by the temperature of the fluid following through the integrated bake and chill plate  34 . Any type of thermally conductive fluid can be used, but the preferred fluids are water for low temperatures or ethylene glycol, propylene glycol, or FLUORINERT™, which is manufactured by Minnesota Mining &amp; Manufacturing Corporation, for higher temperatures. The layer of TEDs  32  provide improved control of the temperature of thermally conductive plate  30  and the different fluid temperatures combined with the low thermal mass allow the single plate  30  to be used for both heating and cooling.  
         [0023]    Again referring to FIG. 2, channels  40 A and  40 B are illustrated having fluid-inlets and fluid-outlets near the center and near the perimeter of the integrated bake and chill plate. The fluid-inlets and fluid-outlets can be on the bottom surface of the integrated bake and chill plate or on the side surface. In a preferred embodiment, the perimeter fluid-inlets and fluid-outlets are on the side surface to avoid creating locations of concentrated heat and are on the bottom surface at the center. The parallel channels  40 A and  40 B form square pattern spirals as they extend gradually from the center of the integrated bake and chill plate  34  to the perimeter in a gradually receding manner. Other geometric spirals are appropriate as well. This design allows for several different patterns of fluid flow. First, fluid can flow from generally the perimeter of the integrated bake and chill plate  34  to generally the center in all channels or from generally the center of the integrated bake and chill plate  34  to generally the perimeter in all channels. Additionally, the fluid can flow in opposite directions (counterflow) in the different channels. These flow options improve the speed with which the integrated bake and chill plate&#39;s  34  temperature can change, thereby allowing for rapid heating and/or cooling, thereby improving the uniformity of heat transfer across the integrated bake and chill plate  34 .  
         [0024]    Channels  40 A and  40 B can also contain microchannels  42 . The microchannels extend throughout the fluid channels. Preferably, the microchannels  42  increase the area of heat transfer across the integrated bake and chill plate  34 , improve the rigidity of the integrated bake and chill plate  34  to reduce warping, and increase the velocity of the fluid through the channels for more rapid heating and cooling and overall better heat transfer. Ideally the microchannels will have a discontinuous phase at selected points in the fluid channels. In the preferred embodiment illustrated, the microchannels are discontinued at the corners to allow more uniform heat transfer.  
         [0025]    The thermal cycling module is controlled in a preferred embodiment by feedback control loop, which includes a multivariable feedback controller. The feedback control loop regulates the substrate temperature during the thermal cycle by monitoring the temperature of the substrate and various process parameters with sensors, e.g. thermocouple sensors or infrared (IR) sensors or a process sensor such as a scatterometer. Sensors can be positioned to sense particular temperatures and/or process parameters at specific regions of the substrate. For example, IR sensors may be positioned above the substrate to detect infrared radiation from particular substrate regions. Similarly, thermocouple sensors may be placed in thermal contact with the substrate to sense substrate temperatures at particular substrate regions. The thermocouples are preferably imbedded in the thermally conductive plate. Specific techniques for measuring substrate temperatures and process parameters are well known in the art.  
         [0026]    Based on electric signals sent by the sensors, a microprocessor in the controller calculates control signals for the TEDs and sends them to the variable power source, which is electrical contact with the TEDs  32  and controller. The variable power source changes flow of electric current through the TEDs in accordance with control signals received from the microprocessor. The microprocessor calculates and sends additional control signals to fluid supplies and to valves that control the flow of fluid through channels  40 A and  40 B of integrated bake and chill plate  34 . The fluid then flows through the integrated bake and chill plate  34  to roughly determine the temperature of the entire plate  30  over longer time periods, while the TEDs  32  precisely determine local variations in the temperature of the plate at specific locations and determine the plate  30  temperature over short time intervals.  
         [0027]    Typically, the present device is used through the specification of predetermined process parameters characteristic of the desired thermal cycle. For example, the controller may be programmed to start a substrate at 30° C. and quickly ramp it up to 80° C. for 60 seconds after which it is chilled to 10° C. for 30 seconds and then returned to 30° C. to complete the cycle. In this example, the controller sets fluid supplies to temperatures of 80° C., 30° C. and 10° C., respectively. Initially, the controller sets valves to permit only the 30° C. fluid to flow through the integrated bake and chill plate in a chosen flow pattern (either from the center of the integrated bake and chill plate  34  to the perimeter, from the perimeter of the integrated bake and chill plate  34  to the center, or alternating with some fluids flowing from the center to the perimeter and others flowing from the perimeter to the center). If any of the sensors indicate a temperature other than 30° C., then the controller sends a control signal to the variable power supply in order to appropriately heat or cool the appropriate plate region. In this manner, the temperature of the plate  30 , and the substrate  38 , is dynamically maintained at a uniform desired temperature.  
         [0028]    At a specified point in time, the controller begins a transition phase to ramp the temperature from 30° C. to 80° C. At this point, the controller causes valves to permit only the 80° C. fluid to flow through the integrated bake and chill plate in a chosen flow pattern. In order to achieve more rapid temperature response, the controller sends control signals to the variable power source  36  that will send currents through TEDs  32  to rapidly heat the plate. Once the sensors indicate temperatures near 80° C., the transition phase is completed and the controller begins a baking phase where a uniform temperature of 80° C. is maintained through feedback in the same manner as the temperature was maintained at 30° C.  
         [0029]    After baking the substrate for 60 seconds, the controller begins a second transition phase to ramp the temperature from 80° C. to 10° C. At this point, the controller causes valves to permit only 10° C. fluid to flow through the integrated bake and chill plate  34  in a desired flow pattern. The controller also sends control signals to the variable power supply that will send currents through the TEDs  32  to rapidly cool the plate. Once the sensors indicate temperatures of 10° C., the second transition phase is completed and the controller begins a chill phase where a uniform temperature of 10° C. is maintained through feedback.  
         [0030]    After the chill phase is completed, the controller begins a third transition phase to ramp the temperature from 10° C. to 30° C. in a manner analogous to the transition phases described above. When the third transition is completed, the thermal cycle is complete.  
         [0031]    Those skilled in the art can program the controller to execute many different thermal cycles involving any number of phases and transitions of different types. Additionally, the controller can be programmed by those skilled in the art to implement the described embodiments or any variations.  
         [0032]    To improve the precision and uniformity of the plate&#39;s temperature, the present invention further comprises one or more thermoelectric devices (TEDs) in thermal contact with the thermally conductive plate. Because the TEDs may be quickly and precisely adjusted to heat the plate, they improve the control of the plate temperature and provide for shortened plate temperature transition times. Moreover, independently controlling an array of TEDs allows for compensation of spatial nonuniformities in substrate temperature that might arise. Alternatively, such independent control can provide intentional temperature nonuniformities if desired for special processing purposes.  
         [0033]    The present invention also comprises a design facilitating easy replacement of individually damaged TEDs. The TEDs are attached with a connector, such as a screw or bolt, through a hole located in the center of the TED. This design also improves the electrical connection between the TED and a variable power source and the thermal connection between the thermally conductive plate and each TED.  
         [0034]    Referring to FIG. 3, a cross-sectional view of a preferred embodiment of the is shown that illustrates how the integrated bake and chill plate  34  is in thermal and electric contact with the TED  32 , PCB  36  and thermally conductive plate  30 . The integrated bake and chill plate  34  and insulation layer  35  is illustrated having two holes or bores for each TED  32  for establishing an electrical connection with the PCB  36 . The holes extend through the integrated bake and chill plate  34  and insulation layer  35  and are adapted to receive a support  51  made of copper or other suitable metal. A preferred support  51  has a cylindrical structure with a section having a smaller diameter at the bottom for inserting into and soldering to the PCB  36  which is powered by a variable power source. Anything that can be soldered to the support  51  and create an electrical connection with a variable power source can be used instead of a PCB  36  as well. A spring loaded pin  53  is inserted in the support for establishing an electrical connection between the TED  32  and PCB  36  and support  51 . On the bottom surface of the TED  32  are copper pads  56 , which when the TED  32  is secured to the integrated bake and chill plate  34 , the pads  56  establish an electrical connection by depressing the spring loaded pin  53  into the support  51  soldered to the PCB  36 .  
         [0035]    The TEDs  32  comprise a lower plate  52 A and upper plate  52 B made of ceramic or other appropriate dielectric device with high electrical conductivity. The two plates are connected by a copper support structure  54  or other suitable metal structure and soldered together as shown in FIG. 5. Attached to the bottom of the lower plate  52 A, are copper pads  56 . Although the TED design shown in FIG. 5 is a preferred design, any type of TED can be used as long as the integrated bake and chill plate  34  and electrical connection means for connecting each TED to a variable power source are adapted accordingly.  
         [0036]    As shown in FIG. 3, the TEDs  32  preferably are secured to the integrated bake and chill plate  34  with a screw, bolt or other connector  58  through a hole in the center of each TED  32 . There are corresponding receiving holes on the integrated bake and chill plate  34  for receiving the connector  58 . In a preferred embodiment, a Belleville washer is used to facilitate support and allow for vertical movement. Additionally, the TEDs are secured by the thermally conductive plate  30  which rests above the TEDs  32 . Thermal grease can be placed on the top and bottom of the TEDs to improve the conductive interface between the thermally conductive plate and TEDs and between the TEDs and integrated bake and chill plate.  
         [0037]    In a preferred embodiment, the thermally conductive plate  30  is connected to the PCB  36  with a screw, bolt or other connector  55 . The connector  55  travels through a hole in the integrated bake and chill plate  34  to secure the top plate. Two Belleville washers  56  can be used, one upside down on top of the other, to handle vibration and thermal shock. Similarly, the PCB  36  can be connected to the integrated bake and chill plate  34  with a screw, bolt or other connector  57  as well. Again, in a preferred embodiment, two Belleville washers  59 , one upside down on the other, can be used to handle thermal shock and vibrations.  
         [0038]    [0038]FIG. 4 shows a top view of the integrated bake and chill plate and all of the locations of the mounting holes and TED support holes, as well as channels  40 A and  40 B for movement of fluids in a preferred embodiment. Hole  62  is a mounting hole location for the thermally conductive plate  30  and hole  63  is a mounting hole location for the PCB  36 . Holes  64  are locations of the TED supports  51  and hole  64  is the location for the connector  58  that secures the TEDs  32  to the integrated bake and chill plate  34 . Hole  65  accommodates a tooling pin to guide the integrated bake and chill plate  34  when placing it on the PCB  36 . In a preferred embodiment, the fluid channels  40 A and  40 B have sufficient space between them to accommodate the TEDs  32  and the supports  51  and to facilitate braising of the two integrated bake and chill plate plates if the integrated bake and chill plate  34  comprises two plates yet maximize the area of heat transfer between the integrated bake and chill plate  34  and the TEDs  32 . A preferred number of TEDs  32  to cover the surface of the integrated bake and chill plate  34  is 196 TEDs  32  arranged in a 14×14 matrix or 100 TEDs  32  arranged in a 10×10 matrix.