Abstract:
To heat an object, a first solid heat transfer medium is supplied with heat. The heat is transmitted from the first solid heat transfer medium to a fluid heat transfer medium which is partitioned into an interconnected plurality of evaporation cavities each containing a liquid. The heat causes the liquid to evaporate into a plurality of vapor parts in the respective plurality of evaporation cavities, and the plurality of vapor parts are guided in parallel in an upward direction towards the object. The vapor parts contact a second solid heat transfer medium to heat the second solid heat transfer medium, thereby transmitting the heat to the second solid heat transfer medium. The second solid heat transfer medium is thermally contacted with the object to transmit the heat from the second solid heat transfer medium to the object.

Description:
CROSS-REFERENCE OF RELATED APPLICATIONS  
       [0001]    This application is a Continuation-in-part application of “WAFER HEATING APPARATUS HAVING FLUID HEAT TRANSFER MEDIUM AND METHOD OF HEATING A WAFER USING THE SAME”, by the present inventor, Ser. No. 09/484,051, filed on Jan. 18, 2000, the contents of which are herein incorporated by reference in their entirety. This application also relies for priority upon Korean Patent Application No. 99-30350 filed on Jul. 26, 1999, the contents of which are herein incorporated by reference in their entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method for heating a wafer, a method for baking a photoresist film on a wafer, an apparatus for heating a wafer, and an apparatus for baking a photoresist film on the wafer. More particularly, the present invention relates to a method for uniformly heating a wafer during a photolithography process for forming a photoresist pattern, a method for uniformly baking a photoresist film on a wafer for forming a photoresist pattern, an apparatus for performing this heating method, and an apparatus for performing this baking method.  
           [0004]    2. Description of the Related Art  
           [0005]    The fabricating of semiconductor devices typically includes a photolithography process in which a wafer is coated with liquid photoresist (PR) to form a PR film. The PR film is patterned by being exposed to light produced by an optical source and passed through a mask or reticle. The pattern is then developed, and the wafer is heated to a predetermined temperature several times throughout the course of these steps.  
           [0006]    Apparatus for performing this photolithography process thus requires a PR coater, an exposure device, a developer, and a baking unit. The current trend in such technology is the use of a system in which the PR coater, the developer and the baking unit are clustered in one place, whereby the distance required to move the wafer between the devices and hence, the time required to move the wafer therebetween, is minimized. In other words, the clustered system is capable of performing the conventional photolithography process with a high degree of efficiency.  
           [0007]    The PR coater is typically of a type which performs a spin coating method in which the wafer is rotated at a predetermined speed, and photoresist solution is sprayed onto the rotating wafer. As a result, the photoresist is uniformly spread over the wafer by centrifugal force.  
           [0008]    The heating of the wafer during the fabricating of a semiconductor device is generally considered to include four steps. The first step is a pre-baking step of heating a wafer at a predetermined temperature to evaporate organic materials or foreign materials from the surface of the wafer. The second step is a soft-baking step of heating the wafer just after the wafer is coated with the photoresist in order to dry the photoresist and to strongly attach the film of photoresist to the surface of the wafer. The third step is a post-exposure-baking (PEB) step (described below) of heating the photoresist which has been exposed. The fourth step is a hard-baking step of heating a wafer just after the photoresist film has been developed so as to strongly attach the resultant photoresist pattern to the wafer surface.  
           [0009]    When the exposure device comprises a source of ultra violet (UV) and deep ultra violet (DUV) light, the light diffracts and produces interference according to the reflectivity and refractive index of the substrate, such as a wafer, and the optical absorptivity of the photoresist film, which is irradiated with the light. The phenomena of interference, in turn, causes both the profile of the pattern of the photoresist to be abnormal, and the critical dimensions of the pattern to be non-uniform. The PEB step is performed to compensate for these problems. In the PEB step, the exposed photoresist film is heated at a predetermined temperature to rearrange the resins which were optically decomposed due to thermal diffusion, thereby cleaning the cross section of a profile of the exposed pattern. When the exposure light is a DUV light, a chemically-amplified resist is used as the photoresist. A portion of the chemically-amplified resist, which is exposed by thermal treatment, changes into an acid which is soluble in a developing solution. Also, the alteration of the chemically-amplified resist occurs due to a chain reaction, so that the balance of heat applied to the entire wafer in the PEB step has the greatest effect on the uniformity of the critical dimensions of the photoresist pattern.  
           [0010]    Hence, a uniform heating of the entire surface of the wafer is very important in increasing the yield. A heating device of a conventional baking unit, as shown in FIG. 1, includes a lower plate  2  in which an electrical heat source, that is, a heater  21  is installed. The heater  21  is situated just below the lower surface of an upper plate  1  on which a wafer  100  is supported. Referring to FIGS. 2 and 3, a spiral groove  22  is formed in the upper surface of the lower plate  2 , and the heater  21  is seated in the groove  22 . In this structure, heat generated by the heater  21  is transferred from the lower plate  2  to the upper plate  1  to heat the wafer  100  on the upper plate  1 . Also, the power of the heater  21  is feedback-controlled, by detecting the temperature of the upper plate  1  using a temperature sensor (not shown) which is installed on the lower plate  2 , so that the temperature is kept within a predetermined range. In the conventional heating device, heat is conducted via the bodies of the upper and lower plates  1  and  2 , respectively. Consequently, as discussed below, an uneven thermal distribution occurs at the surface of the upper plate  1 .  
           [0011]    [0011]FIG. 4 is a temperature distribution diagram illustrating the temperature at the surface of a wafer heated by the conventional heating device, wherein the temperature difference between adjacent isotherms is 0.02° C. As shown in FIG. 4, the temperature distribution is irregular and abnormally distorted, and the difference in the temperature between the coolest and warmest regions is about 1.76° C. In this figure, bold isotherm A crossing the center of a wafer indicates a temperature of 145.31° C., isotherm B indicates a temperature of 146.28° C., and isotherm C indicates a temperature of 144.32° C. As can be seen from this temperature distribution, the temperature of the surface of the wafer gradually increases on one side of the bold isotherm A and reaches 146.28° C. at one peripheral portion of the wafer, gradually decreases on the other side of the bold isotherm A, and reaches 144.32° C. at another peripheral portion of the wafer. This irregular temperature distribution and wide temperature difference greatly affects the yield as described above. Therefore, the temperature distribution produced by heating the wafer must be improved by all means.  
           [0012]    [0012]FIG. 5 is a temperature-time graph showing variations in temperature of regions of a wafer while the wafer is being heated by the conventional heating apparatus, and FIG. 6 shows the locations at which the temperature of the wafer surface are measured. These locations include the center of the wafer surface, and various points on two circles, which are concentric with the center of the wafer surface.  
           [0013]    Referring to the variations in the temperature obtained by taking temperature readings at the above-described points, as shown in FIG. 5, the temperature differs greatly amongst the measuring points at any given time. Moreover, after a predetermined time passes, the temperature drops sharply (zone D in figure). Such great differences in temperature imparts a serious thermal shock not only to the wafer but also to the photoresist film formed on the wafer. Such a thermal shock adversely affects the physicochemical properties of the photoresist film.  
           [0014]    Therefore, the conventional heating apparatus described above impedes the success of the photolithography process in forming a photoresist having a normal profile and uniform critical dimension on a wafer. This problem becomes particularly acute as the design rule of patterns becomes finer and finer (for example, 0.25 μm, 0.18 μm, and 0.15 μm) in response to the demand for increased levels of circuit integration. Thus, the conventional heating apparatus is an impediment to enhancing yield.  
         SUMMARY OF THE INVENTION  
         [0015]    In view of the foregoing, it is a first object of the present invention to provide a method and apparatus for uniformly heating an object, such as a wafer.  
           [0016]    It is another object of the present invention to provides a method and apparatus for uniformly heating a wafer in order to avoid or at least minimize the application of thermal shock to the wafer and to a photoresist film formed on the wafer.  
           [0017]    It is yet another object of the present invention is to provide a method and apparatus for uniformly baking a photoresist film on a wafer in order to avoid or at least minimize the application of thermal shock so as to reduce critical dimension variance induced therefrom.  
           [0018]    According to one aspect of the present invention, in a method for uniformly heating an object, a first solid heat transfer medium is supplied with heat. The heat is transmitted from the first solid heat transfer medium to a fluid heat transfer medium which is partitioned into an interconnected plurality of evaporation cavities each containing a liquid. The heat causes the liquid to evaporate into a plurality of vapor parts in the respective plurality of evaporation cavities, and the plurality of vapor parts are guided in parallel in an upward direction towards the object. The vapor parts contact a second solid heat transfer medium to heat the second solid heat transfer medium, thereby transmitting the heat to the second solid heat transfer medium. The second solid heat transfer medium is thermally contacted with the object to transmit the heat from the second solid heat transfer medium to the object.  
           [0019]    According to another aspect of the present invention, in a method for baking a photoresist film on a wafer, a photoresist solution is coated on a wafer to form the photoresist film. After the photoresist film is exposed to a light, the wafer is transported onto a hot plate. In the hot plate, a first solid heat transfer medium is supplied with heat. The heat is transmitted from the first solid heat transfer medium to a fluid heat transfer medium which is partitioned into an interconnected plurality of evaporation cavities each containing a liquid. The heat causes the liquid to evaporate into a plurality of vapor parts in the respective plurality of evaporation cavities, and the plurality of vapor parts are guided in parallel in an upward direction towards the wafer. The vapor parts contact a second solid heat transfer medium to heat the second solid heat transfer medium, thereby transmitting the heat to the second solid heat transfer medium. The second solid heat transfer medium is thermally contacted with the object to transmit the heat from the second solid heat transfer medium to the wafer.  
           [0020]    According to another aspect of the present invention, the above-described method for heating a wafer may be used for forming a photoresist pattern. After coating a photoresist solution on a wafer to form a photoresist film, the photoresist film is exposed to light, such as deep ultra violet light. The exposed photoresist film is developed to form a first photoresist pattern having a first opening of a first size. The wafer is heated by the above-describe heating method at a predetermined temperature, so as to reflow the first photoresist pattern to form a second photoresist pattern having a second opening of a second size which is smaller than the first size.  
           [0021]    According to still another aspect of the present invention, an apparatus for heating an object includes a first solid heat transfer medium and a fluid heat transfer medium, thermally coupled to the first solid heat transfer medium, which is partitioned into an interconnected plurality evaporation cavities. The apparatus further includes a second solid heat transfer medium, thermally coupled to the fluid heat transfer medium, for making thermal contact with the object. The plurality of evaporation cavities extend in a same plane between the first and second solid heat transfer mediums.  
           [0022]    According to still another aspect of the present invention, an apparatus for heating a wafer includes a heating element, a lower solid heat transfer medium thermally coupled to the heating element, a first solid heat transfer medium thermally coupled to an upper surface of the lower solid heat medium, and a second solid heat transfer medium having a wafer mounting surface and thermally coupled to said first solid heat transfer medium opposite said wafer mounting surface. The apparatus further includes a fluid heat transfer medium defined by a plurality of interconnected evaporation cavities interposed between said first and second solid heat transfer media. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The above objects and other advantages of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which:  
         [0024]    [0024]FIG. 1 is a schematic cross-sectional view of a wafer heating apparatus of a conventional baking unit;  
         [0025]    [0025]FIG. 2 is a plan view of the heat source of the conventional wafer heating apparatus;  
         [0026]    [0026]FIG. 3 is an enlarged view of part of the heat source of the conventional wafer heating apparatus;  
         [0027]    [0027]FIG. 4 is a temperature distribution diagram of a wafer surface heated by the conventional wafer heating apparatus;  
         [0028]    [0028]FIG. 5 is a graph showing variations in the temperatures of regions of a wafer with respect to time while the wafer is being heated by the conventional wafer heating apparatus;  
         [0029]    [0029]FIG. 6 show the locations where the surface temperature of a wafer heated by the conventional wafer heating apparatus were measured to yield the temperature distribution diagram shown in FIG. 5;  
         [0030]    [0030]FIG. 7 is a schematic side view of a first embodiment of the wafer heating apparatus according to the present invention;  
         [0031]    [0031]FIG. 8 is a schematic cross-sectional view of the heat source of the wafer heating apparatus according to the present invention;  
         [0032]    [0032]FIG. 9 is an enlarged view of part of the heat source;  
         [0033]    [0033]FIG. 10 is a schematic side view of a second embodiment of the wafer heating apparatus according to the present invention;  
         [0034]    [0034]FIG. 11A is a schematic perspective view of a lattice, which may be employed in the second embodiment of the wafer heating apparatus according to the present invention;  
         [0035]    [0035]FIG. 11B is a schematic perspective view of another form of the lattice suitable for use in the second embodiment of the wafer heating apparatus according to the present invention;  
         [0036]    [0036]FIG. 12 is a schematic side view of a third embodiment of the wafer heating apparatus according to the present invention;  
         [0037]    [0037]FIG. 13 is a schematic side view of a fourth embodiment of the wafer heating apparatus according to the present invention;  
         [0038]    [0038]FIG. 14 is a schematic cross-sectional view of a fifth embodiment of the wafer heating apparatus according to the present invention;  
         [0039]    [0039]FIG. 15 is a bottom view of a solid heating medium which may be employed in the fifth embodiment of the wafer heating apparatus according to the present invention;  
         [0040]    [0040]FIG. 16 is a schematic cross-sectional view of a sixth embodiment of the wafer heating apparatus according to the present invention;  
         [0041]    [0041]FIG. 17 is a cross-sectional view of part of a seventh embodiment of the wafer heating apparatus according to the present invention;  
         [0042]    [0042]FIG. 18 is a schematic cross-sectional view of a coronary body, which can be employed in the seventh embodiment of the wafer heating apparatus according to the present invention;  
         [0043]    [0043]FIG. 19 is a surface temperature distribution diagram of a wafer being heated by the wafer heating apparatus according to the present invention;  
         [0044]    [0044]FIG. 20 is a surface temperature distribution diagram of another wafer heated by the wafer heating apparatus according to the present invention;  
         [0045]    [0045]FIG. 21 is a graph showing variations in the temperatures of regions of a wafer with respect to time while the wafer is being heated by the wafer heating apparatus according to the present invention;  
         [0046]    [0046]FIG. 22 is a schematic perspective view showing an eighth embodiment of an apparatus for heating a wafer according to the present invention;  
         [0047]    [0047]FIG. 23 is a sectional view of the wafer heating apparatus taken along line E-E′ as shown in FIG. 22;  
         [0048]    [0048]FIG. 24 is an enlarged view of a portion F of FIG. 23;  
         [0049]    [0049]FIG. 25 is a sectional plan view of a layout of the inner partition walls of an embodiment of the main heat transfer body;  
         [0050]    [0050]FIG. 26 is a sectional plan view of a layout of the inner partition walls of another embodiment of the main heat transfer body;  
         [0051]    [0051]FIG. 27 is a perspective bottom view of the lower solid heat transfer medium;  
         [0052]    [0052]FIG. 28 is a sectional view of a conventional heat transfer medium to which a heat block is attached thereunder including isotherm diagrams for illustrating the temperature distribution;  
         [0053]    [0053]FIG. 29 is a sectional view of a heat transfer medium according to one embodiment of the present invention, to which a heat block is attached thereunder, including isotherm diagrams for illustrating the temperature distribution;  
         [0054]    [0054]FIG. 30 is a sectional view of a heat transfer medium according to another embodiment of the present invention, to which a heat block is attached thereunder including isotherm diagrams for illustrating the temperature distribution;  
         [0055]    [0055]FIG. 31 is a graph illustrating a top surface temperature of the main heat transfer body as shown in FIGS.  28  to  30 .  
         [0056]    [0056]FIGS. 32A to  32 D are sectional view s illustrating a method for forming a photoresist pattern in accordance with one embodiment of the present invention, utilizing the heating apparatus of the present invention;  
         [0057]    [0057]FIG. 33 is an isotherm diagram showing the distribution of the surface temperature of a wafer heated by using the main heat transfer as shown in FIG. 25;  
         [0058]    [0058]FIG. 34 is an isotherm diagram showing the distribution of the surface temperature of a wafer heated by using the main heat transfer as shown in FIG. 26;  
         [0059]    [0059]FIG. 35 is a critical dimension (CD) distribution diagram of a first opening obtained after developing the exposed photoresist film which has been post-baked using the main heat transfer medium as in FIGS. 22 and 26.  
         [0060]    [0060]FIG. 36 is a critical dimension distribution diagram of the second opening obtained by using the main heat transfer medium as in FIGS. 1 and 2; and  
         [0061]    [0061]FIG. 37 is a critical dimension distribution diagram of the second opening obtained by using the main heat transfer medium as in FIGS. 22 and 26. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0062]    The preferred embodiments of the present invention will now be described in detail below.  
         [0063]    Referring first to FIG. 7, the wafer heating apparatus according to a first embodiment of the present invention includes a sold heat transfer medium  10  which supports a wafer  100  in direct contact therewith, a heat source  20 , and a fluid heat transfer medium  30 , which is interposed between the solid medium  10  and heat source  20 . The state of the fluid medium  30  is changeable between a vapor and liquid state by heating the medium with the heat source  20  and allowing the medium to cool. Here, the arrows in the solid heat transfer medium  10  and the heat source  20  indicate the direction of movement of heat, and the arrows in the fluid heat transfer medium  30  indicate the direction of movement of the fluid medium. A portion of the fluid heat transfer medium  30  adjacent the solid heat transfer medium  10  is in a vapor state, and a portion of the fluid heat transfer medium  30  adjacent the heat source  20  is in a liquid state. The fluid heat transfer medium  30  absorbs heat from the heat source  20 , and moves toward the solid heat transfer medium  10  while being vaporized. When the vapor of the fluid heat transfer medium  30  contacts the solid heat transfer medium  10 , it transmits heat to the solid heat transfer medium  10 . The transfer of heat cools the vapor, causing it to condense, whereby the resultant liquid moves toward the heat source  20 . The absorption of heat from the heat source  20  by the fluid heat transfer medium  30 , and the transfer of heat to the solid medium  10  is a continuous cycle, during which a phase change of the fluid heat transfer medium  30  occurs continuously. The phase change of the fluid heat transfer medium is dependent upon the critical temperature and pressure of the fluid medium.  
         [0064]    The cycle of heat transfer occurs within a closed space according to the present invention, and is very fast as compared to the cycle of heat transfer which occurs in the conventional heating apparatus. The fluid medium of the present invention transfers the heat to the surface of the solid heat transfer medium  10  rapidly and evenly, whereupon the heat is uniformly transferred to the wafer  100 , which is supported on the solid medium  10 . Therefore, the surface of the wafer  100  is rapidly and uniformly heated by the heat, which is evenly distributed throughout the solid heat transfer medium  10 .  
         [0065]    As shown in FIGS. 8 and 9, the heat source  20  includes a heater  203  comprising an electrical heating coil, and upper and lower heater blocks  201  and  202  which contain the heater  203 . More specifically, the heater  203  is contained within a groove  204  formed in the lower surface of the upper heater block  201  or in the upper surface of the lower heater block  202 .  
         [0066]    According to a second embodiment of the present invention, the space in which the fluid heat transfer medium  30  is contained can be partitioned into a plurality of areas, as shown in FIG. 10.  
         [0067]    Referring now to FIG. 10, a plurality of partitions  301  are installed between the solid heat transfer medium  10  and the heat source  20 . Accordingly, the fluid heat transfer medium  30  exists within the area partitioned by the plurality of partitions  301 , and a phase change occurs in independent spaces delimited by the plurality of partitions  301 .  
         [0068]    The partitions  301  can constitute a lattice  302  having rectangular or honeycomb-shaped units, as shown in FIGS. 11A and 11B. Preferably, the cross sections of the units of the lattice  302  are designed so that the units will act as capillary tubes for the fluid heat transfer medium  30 .  
         [0069]    Referring to FIG. 12, according to a third embodiment of the present invention, a refractory porous body  303  having discrete sections is provided within the units of the lattice  302  in contact with the heat source  20 . The fluid heat transfer medium  30  thus fills the cavities of the porous body  303 . The fluid heat transfer medium  30  thus contained in the cavities of the refractory porous body  303  will be rapidly heated and evaporated. Also, the cavities act as capillary tubes, which promote the mobility of the fluid heat transfer medium  30 .  
         [0070]    Alternatively, as shown in FIG. 13, the refractory porous body  303  of a fourth embodiment of the present invention is a single body interposed between the solid heat transfer medium  10  and the heat source  20 . In this case, the refractory porous body  303  closely adheres to the inner surfaces of both the heat source  20  and the solid heat transfer medium  10 , or to the inner surface of either the heat source  20  or the solid heat transfer medium.  
         [0071]    [0071]FIGS. 14 and 15 show a fifth embodiment of the heating apparatus according to the present invention. In this embodiment, the solid heat transfer medium  10  adheres to the heat source  20 , and a groove  101  containing the fluid heat transfer medium  30  is formed at the interface between the solid heat transfer medium  10  and the heat source  20 .  
         [0072]    In particular, the groove  101  is formed in the bottom surface of the solid heat transfer medium  10 , but can be formed in the surface of the heat source  20  in some circumstances. The groove  101  forms a closed loop at the interface between the solid heat transfer medium  10  and the heat source  20 , through which the fluid heat transfer medium  30  can circulate. The end  101   a  of the groove  101  is open at the side surface of the solid heat transfer medium  10  or the heat source  20  so that the fluid heat transfer medium  30  can be placed in the groove  101 . A plug  10   a  closes the open end  101   a  of the groove  101 .  
         [0073]    In this structure, while the fluid heat transfer medium  30  circulates along the groove  101 , the phase of the fluid heat transfer medium  30  is changed due to heat absorption and heat transmission as described above. The loop of fluid heat transfer medium  30  leaves portions where the solid heat transfer medium  10  and the heat source  20  directly contact each other. Accordingly, heat is also transmitted from the heat source  20  to the solid heat transfer medium  10  via the contacting portions of the solid heat transfer medium  10  and the heat source  20 .  
         [0074]    However, heat transfer via the fluid medium  30  circulating in the groove  101  occurs at a faster rate than the direct heat transfer via the contacting portions of the solid medium  10  and the heat source  20 .  
         [0075]    Meanwhile, the groove  101  can have a shape other than that of a single closed loop. That is, a plurality of grooves  101  can be formed in the lower surface of the solid heat transfer medium  10  or in the surface of the heat source  20 . The plurality of grooves  101  are laid out at regular intervals across the interface between the solid heat transfer medium  10  and the heat source  20 . The independent grooves form discrete closed spaces in which the phase of the fluid heat transfer medium  30  is changed.  
         [0076]    [0076]FIG. 16 shows a sixth embodiment of the present invention in which the grooves  101  form a plurality of independent spaces as described above. Referring now to FIG. 16, a plurality of grooves  101  are formed in the upper surface of the heat source  20 . Walls  104 , which isolate the grooves  101  from each other, have triangular profiles. A vertex of each triangular wall  104  contacts the lower surface of the solid heat transfer medium  10 . This minimal contact between the wall  104  and the solid heat transfer medium  10  minimizes heat transfer from the former to the latter.  
         [0077]    [0077]FIG. 17 shows a seventh embodiment of the present invention in which a coronary (tubular) body  102  extends in the groove  101 . The fluid heat transfer medium is contained in the coronary body  102 . In this structure, the groove  101  extends in a closed loop at the interface between the solid heat transfer medium  10  and the heat source  20 .  
         [0078]    Referring to FIG. 18, the coronary body  102  includes fins  103  which contact the fluid heat transfer medium  30  in order to promote the phase change of the fluid heat transfer medium  30 . The fins  103  extend axially in the direction of movement of the fluid transfer medium  30  along the coronary body  102 . As an alternative to the fins  103 , a porous layer of a predetermined thickness can be formed on the inner wall of the coronary body  102 .  
         [0079]    According to the present invention as described above, the fluid heat transfer medium must be one whose phase can be changed between vapor and liquid within a predetermined range of temperatures targeted for heating a wafer during a semiconductor manufacturing process, e.g., in the photolithography process. When considering that the targeted temperature to which a wafer is heated is between 200° C. and 300° C., the fluid heat transfer medium can be, but it is not limited to, water, ethanol, methanol, acetone, ammonia, or Freon.  
         [0080]    In particular, the choice of liquid to be used in the various embodiments of the present invention will largely depend on the temperature ranges to which an object is to be heated. Although the invention is not so limited, the table below shows examples of various liquids which may be employed within the indicated temperature ranges.  
                                       −273° C. to −120° C.   −120° C. to 470° C.   450° C. to 2700° C.                   Helium   Water   Cesium       Argon   Ethanol   Sodium       Nitrogen   Methanol   Lithium           Acetone           Ammonia           Freon                  
 
         [0081]    Likewise, the choice of material for the solid heat transfer mediums will largely depend on the liquid employed. Although the invention is not so limited, the table that follows shows examples of materials which are recommended and not recommended for the indicated fluids.  
                                                                 Recommended   Not Recommended                                    Ammonia   Aluminum, Carbon steel,   Copper           Stainless steel, Nickel       Acetone   Aluminum, Copper,           Stainless steel, Silica       Methanol   Copper, Stainless steel,   Aluminum           Nickel, Silica       Water   Copper, 347 stainless   Aluminum, Stainless           steel   steel, Nickel, Carbon               steel, Inconel, Silica       Thermex   Copper, Silica, Stainless           steel                  
 
         [0082]    Surface Temperature Distribution I  
         [0083]    [0083]FIGS. 19 and 20 are isotherm diagrams showing the distribution of surface temperatures of a wafer heated by the heating apparatus according to an embodiment of the present invention. As can be seen from these figures, the isotherms are annular, the center of the wafer has the highest temperature, and the temperature decreases in a uniform pattern beginning from the center and moving out toward the periphery of the wafer. It is also clear that the isotherm distribution shown in FIG. 20 is preferable to that shown in FIG. 19.  
         [0084]    In the isotherm diagram of FIG. 19, the difference between the highest and lowest temperatures is 0.73° C., the bold isotherms indicate a temperature of 1 55.63° C., the temperature of the center of the wafer is 156.00° C., and the lowest temperature of the wafer periphery is 155.26° C. In the isotherm diagram of FIG. 20, the difference between the highest and lowest temperatures is 0.72° C., the bold isotherm indicates a temperature of 155.63° C., the temperature of the center of the wafer is 155.96° C., and the lowest temperature of the wafer periphery is 155.32° C.  
         [0085]    As can be seen from FIGS. 19 and 20, the temperature of a wafer has an even distribution over the surface of the wafer, and particularly, the deviations between the highest and lowest temperatures of 0.73° C. and 0.72° C. are excellent results which cannot be obtained by the conventional wafer heating apparatus.  
         [0086]    [0086]FIG. 21 is a graph showing temperature-time variations obtained from a plurality of measuring points while a wafer was heated by the heating apparatus according to an embodiment of the present invention. As shown in FIG. 21, after heating starts, the temperature increases sharply, and thermal vibration, that is, a temperature variation with respect to the lapse of time, is gentle. In particular, a sudden drop in temperature, as occurs when using the conventional heating apparatus, does not occur when practicing the embodiment of the present invention. This tiny temperature variation over the wafer, and the small thermal vibration show that a very weak thermal shock is applied to the wafer and to the photoresist film formed on the wafer.  
         [0087]    According to the embodiments of the present invention as described above, stable heating of a wafer with a very small temperature deviation greatly reduces the intensity of the thermal shock on the wafer and the photoresist film formed on the wafer, in particular, the wafer can be heated with a regular and uniform temperature distribution. Accordingly, the present invention allows finer patterns to be formed successfully even when the design rule in the critical dimension is 0.25 μm, 0.18 μm, or 0.15 μm, with an increase in the level of the integration of circuits, thus greatly increasing the yield.  
         [0088]    [0088]FIG. 22 is a schematic perspective view showing an eighth embodiment of an apparatus for heating a wafer according to the present invention.  
         [0089]    Referring to FIG. 22, the hot plate  500  functioning as a wafer heating apparatus includes a main heat transfer body  510  and a lower solid heat transfer medium  520 , each configured as a circular plate of the same size and larger than a wafer to be heated. The lower solid heat transfer medium  520  is placed under the lower surface of the main heat transfer body  510 .  
         [0090]    At the upper surface portion of the main heat transfer body  510 , a round and shallow trench  512  is formed for receiving a wafer to be heated. Also, a plurality of wafer guides  513  are provided at a peripheral region of the upper surface portion. The wafer guides  513  guide a wafer when it is placed onto the trench  512 . The trench  512  reduces the introduction of the ambient air onto the wafer, thereby lowering the undesired influence of the ambient air.  
         [0091]    [0091]FIG. 23 is a sectional view of the hot plate  500  taken along line E-E′ as shown in FIG. 22. FIG. 24 is an enlarged view of a portion F of FIG. 23.  
         [0092]    Referring to FIG. 23, the main heat transfer body  510  includes a first solid heat transfer medium  514  and a second solid heat transfer medium  516 . Preferably, the media  514  and  516  are integrally formed and configured as a circular plate which is larger than a size of a wafer. The first solid heat transfer medium  514  is provided at a lower portion of the main heat transfer body  510  and the second solid heat transfer medium  516  is provided at an upper portion of the main heat transfer body  510 . As mentioned earlier, the trench  512  for receiving a wafer is formed at an upper portion of the second solid heat transfer medium  516 .  
         [0093]    As shown in the figures, an outer side wall  518  having a ring shape is formed at an outer periphery of the first solid heat transfer medium  514  and the second solid heat transfer medium  516 . That is, the first solid heat transfer medium  514  and the second solid heat transfer medium  516  are integrally formed by way of the outer side wall  518 . Also, a cavity  515  which forms a fluid heat transfer medium is defined between the first and second solid heat transfer media  514  and  516 .  
         [0094]    The cavity  515  is located below the trench  512  and an outer region thereof also has a generally circular configuration. When the diameter 2r of the cavity  515  is less than about 0.9 times the diameter 2R 0  of the main heat transfer body  510  (or first and second solid heat transfer mediums  514  and  516 ), undesirable heat transfer can occur when baking a photoresist film coated on a wafer. When the diameter 2r of the cavity  515  exceeds 0.98 times the diameter 2R 0  of the main heat transfer body  510 , manufacturing of the main heat transfer body  510  having the cavity  515  becomes difficult. Thus, the diameter 2r of the cavity  515  is preferably about 0.9 to 0.98 times, and more preferably about 0.94 to 0.98 times the diameter 2R 0  of the main heat transfer body  510 . Particularly, when the main heat transfer body  510  for heating an eight-inch wafer has a diameter 2R 0  of 240 mm, the diameter 2r of the cavity  515  is about 225 to 235 mm, and more specifically about 230 mm.  
         [0095]    In the cavity  515 , a plurality of inner partition walls  530  are provided for dividing the cavity  515  into an interconnected plurality of smaller evaporation cavities  515   a ,  515   b ,  515   c , etc., whereby a plurality of vapor parts are guided in parallel from the first solid heat transfer medium  514  to the second solid heat transfer medium  516 .  
         [0096]    As shown in FIG. 24, a liquid  540  is placed in the cavity  515 . Each of the evaporation cavities  515   a ,  515   b ,  515   c , etc. forming the cavity  515  have a curved cross-sectional shape at an upper portion thereof. The liquid  540  is evaporated upon receiving heat from the first solid medium  514 . The vaporized liquid, i.e. vapor  542  is guided in parallel in the evaporation cavities  515   a ,  515   b ,  515   c , etc. toward the second solid heat transfer medium  516 . At the top of each cavity, the vapor  542  comes in contact with the second solid heat transfer medium  516  to partially condense into a liquid state while transferring the latent heat of the vapor  542  to the second solid heat transfer medium  516 . The condensed liquid  544  returns to the first solid heat transfer medium  514  along a path formed on the inner surface (the curved ceiling and side wall) of the inner partition walls  530 .  
         [0097]    The heat transfer from the first solid heat transfer medium  514  to the second solid heat transfer medium  516  is continuously performed while the liquid  540  is evaporated and the vapor  542  is condensed, thereby uniformly transferring heat from the first solid heat transfer medium  514  to the second solid heat transfer medium  516 .  
         [0098]    As mentioned above, the cavity  515  is divided into a plurality of smaller evaporation cavities  515   a ,  515   b ,  515   c , etc. by a plurality of inner partition walls  530 , for guiding the vapor  542  in parallel toward the second solid heat transfer medium  516 .  
         [0099]    When a volume occupied by the liquid  540  is less than about 15% of the volume of the cavity  515 , the generation of vapor may become insufficient. On the other hand, when the volume occupied by the liquid  540  exceeds about 25% of the volume of the cavity  515 , mixing of the generated vapor may be insufficient due to the short distance from the liquid  540  to the second solid heat transfer medium  516 , thus causing a non-uniform heat transfer. Thus, the volume occupied by the liquid  540  is preferably about 15 to 25%, but more preferably, 20% of the volume of the cavity  515 .  
         [0100]    As a liquid medium, a perfluorocarbon-type inert solvent is preferably used in the present embodiment. Examples of the perfluorocarbon-type inert solvent include FC- 72 , FC- 40 , FC- 43 , FC- 70  (trade names manufactured by 3M Korea Co. Ltd.) etc. Among them, a solvent is preferred which has a higher critical temperature (under an atmosphere) than the sum of the target temperature plus 100° C. For example, FC-40 solvent has a boiling point of 155° C. and a critical point of 270° C.  
         [0101]    The thickness of the main transfer body  500  is about 10 to 12 mm, preferably 11 mm. When the thickness of the main transfer body  500  is 11 mm, the evaporation cavities  515   a ,  515   b ,  515   c , etc. defined by the inner partition walls  530  have a width W of 5 to 7 mm, preferably, 6 mm, and a height H of 5 to 6 mm, preferably 5.5 mm.  
         [0102]    Due to the presence of the evaporation cavities  515   a ,  515   b ,  515   c , etc., the thickness of the first solid heat transfer medium  514  may vary within a range of 2 to 4 mm, and the thickness of the second solid heat transfer medium  516  may vary within a range of 1 to 2 mm, preferably 1.5 mm at a trench  512 . Also, the thickness Wp of the inner partition walls  530  may vary within a range of about 2 to 3 mm.  
         [0103]    In this embodiment, the thicknesses of the first and second solid heat media  514  and  516  are not limited to the above so long as the main heat transfer body  510  is capable of manufacture. The height H of the isolated space  515  is preferably 0.4 to 0.6 times of the thickness T of the main heat transfer body  510 .  
         [0104]    [0104]FIG. 25 is a sectional plan view of an embodiment of the main heat transfer body  510 , in particular showing a layout of the inner partition walls  530 .  
         [0105]    Referring to FIG. 25, the horizontal area of the cavity  515  is circular as defined by the outer sidewall  518 . Also, a plurality of inner partition walls  530  are provided in the cavity  515  so as to both radially and spirally (or circularly) divide the cavity  515  into a plurality of evaporation cavities  515   a ,  515   b ,  515   c , etc.  
         [0106]    In particular, the inner partition walls  530  are first formed in a spiral configuration within the cavity  515 . Then, the inner partition walls  530  are cut in a radial direction so as to form five radial mixing paths from the center to the periphery of the main heat transfer body  510 . Thus, as shown in FIG. 25, each spiral is divided into five radial sectors, each having an angle θ 1  of about 72 degrees.  
         [0107]    Reference number  505  denotes screw holes which may be provided for coupling the main heat transfer body  510  to the lower solid heat transfer medium  520 .  
         [0108]    [0108]FIG. 26 is a sectional plan view of another embodiment of the main heat transfer body  510 , particularly showing another layout of the inner partition walls  530 .  
         [0109]    Referring to FIG. 26, the inner partition walls  530  are arranged as concentric circles and are more densely provided in the cavity  515  than those of the embodiment shown in FIG. 25. That is, in this embodiment, each circle of the evaporation cavities  515   a ,  515   b ,  515   c , etc. is divided into twenty four radial sectors, each having an angle θ 2  of about 15 degrees.  
         [0110]    More particularly, the cavity  515  is divided in a circular direction into a plurality of concentric circle-shaped evaporation cavities  515   ca ,  515   cb ,  515   cc , etc. Further, each of the circle-shaped evaporation cavities  515   ca ,  515   cb ,  515   cc  are further divided in a radial direction into a plurality of arc-shaped evaporation cavities  515   ca   1 ,  515   ca   2 , . . .  515   cb   1 ,  515   cb   2 , . . .  515   cc   1 ,  515   cc   2  . . . etc.  
         [0111]    [0111]FIG. 27 is a perspective bottom view of the lower solid heat transfer medium  520 . As shown in the figure, a spiral groove  522  is formed at the lower surface of the lower solid heat transfer medium. In the spiral groove  522 , a heater  524  such as a heating coil is provided. The heater  524  is connected to an electrical source (not shown). When electric current is applied to the heater  524 , heat is generated to first heat the lower solid heat transfer medium  520 .  
         [0112]    At the peripheral region of the main heat transfer body  510 , a larger amount of heat loss occurs due to contact with ambient air. Thus, in a preferred embodiment of the present invention, the pitch Po at an outer peripheral region (where radius r is greater than about 0.75 Ro, and wherein Ro is the radius of the main heat transfer body  510 ) of the bottom surface of the lower solid heat transfer medium  520  is shorter than the pitch Pc at a central portion. This configuration compensates for the heat loss at the peripheral region. According to the experiments of the present inventor, the pitch Po at the outer peripheral region is preferably 0.1 to 0.5 times the pitch Pc at the central portion.  
         [0113]    Hereinafter, the heating mechanism of a wafer will be explained in detail.  
         [0114]    First, an electrical current is supplied to the heater  524  which is provided in the spiral groove  522  at the bottom surface of the lower solid heat transfer medium  520 , to thereby generate heat. The heat is transferred to the lower solid heat transfer medium  520  which is in contact with the first solid heat transfer medium  514 .  
         [0115]    Then, the heat is transferred from the lower solid heat transfer medium  510  to the first solid heat transfer medium  514 .  
         [0116]    On the first solid heat transfer medium  514 , a cavity  515  contain a liquid  540 , an outer sidewall  518 , and a plurality of inner partition walls  530  are provided.  
         [0117]    From the first solid heat transfer medium  514 , the heat may be transferred to the second solid medium  516  by conduction through the outer sidewall  518  and the inner partition walls  530 . However, this heat conduction is very small when compared to the heat transfer via the liquid  540  contained in the cavity  515 .  
         [0118]    That is, most of the heat of the first solid heat transfer medium  514  is used for heating the liquid  540 , thereby evaporating the liquid  540  into a vapor. The vapor is guided in parallel in an upward direction toward the second solid heat transfer medium  516 . thereby transferring the heat to the second solid heat transfer medium  516 , which has the trench  512  for receiving a wafer.  
         [0119]    Referring to FIGS. 25 and 26, the inner partition walls  530  are formed so as to have an arc shape. The inner partition walls  530  divide the cavity  515  into a plurality of the evaporation cavities  515   a ,  515   b ,  515   c , etc. in both radial and circular (or spiral) directions. Thus, when the vapor moves in the upward direction, the vapor in the evaporation cavities  515   a ,  515   b ,  515   c , etc. is partially mixed with vapor from adjacent evaporation cavities, thus contributing to a uniform temperature distribution, and thereby uniformly transferring the heat to the second solid heat transfer medium  516 .  
         [0120]    Also, each of the evaporation cavities  515   a ,  515   b ,  515   c , etc. has an upper surface which is curved (or circular) in cross-section. When each vapor part guided by the inner partition wall  530  reaches the upper surface of the evaporation cavities  515   a ,  515   b ,  515   c , etc., and comes in contact with the second solid heat transfer medium  516 , the vapor partially condenses into liquid to generate latent heat to the second solid heat transfer medium  516 , thereby heating the second solid heat transfer medium  516 . Then, the condensed liquid  544  returns to the first solid heat transfer medium  514  and receives heat from the first solid heat transfer medium  514 .  
         [0121]    In the meantime, the vapor which is not condensed, but is simply cooled, is also circulated toward the first solid heat transfer medium  514 . Then, the returned vapor contacts the first solid heat transfer medium  514  to absorb heat again and is guided upward toward the second solid heat transfer medium  516 . That is, heat transfer is also performed by convection.  
         [0122]    As shown in FIGS. 25 and 26, the vapor mixing paths are formed in a radial pattern from the center to the periphery and in a circular direction. Since the direct mixing of the vapor in the radial direction may occur from the center out toward the periphery of the main heat transfer body  510 , the temperature difference of the vapor at the central portion of the main heat transfer body  510  and at the peripheral portion of the main heat transfer body  510  is greatly reduced.  
         [0123]    As described above, the second solid heat transfer medium  516  receives heat via the evaporation cavities from the first heat solid medium  514 . The thus heated solid heat transfer medium  516  is in contact with a wafer, which is located in the trench  512 . As such, the heat is transferred from the uniformly heated solid heat transfer medium  516  to the wafer to uniformly heat the wafer to a desired temperature.  
         [0124]    FIGS.  28  to  30  are sectional views of the heat transfer medium to which a heat block is attached thereunder, and in particular isotherm diagrams for illustrating temperature distributions.  
         [0125]    [0125]FIG. 28 illustrates a conventional heat transfer medium as shown in FIGS. 1 and 2. As shown in FIG. 28, the observed maximum temperature was 152.447° C. and the minimum temperature was 151.566° C.  
         [0126]    [0126]FIG. 29 illustrates one embodiment of the present invention wherein the cavity is formed under the trench, and the heating coil is provided in a groove which has a regular pitch. As shown in this figure, the observed maximum temperature was 152.769° C. and the minimum temperature was 151.259° C.  
         [0127]    [0127]FIG. 30 illustrates another embodiment of the present invention wherein the cavity is formed so as to have a diameter equal to about 0.96 times a diameter of the main heat transfer body, and heating coil provided in a groove in which the pitch thereof at a peripheral region is shorter than at a central region. As shown in this figure, the observed maximum temperature was 152.765° C. and the minimum temperature was 151.492° C.  
         [0128]    As can be seen in the figures, the temperature distribution of the hot plate of FIG. 30 is most uniform, followed by the hot plate of FIG. 29, and then by the hot plate of FIG. 28.  
         [0129]    [0129]FIG. 31 is a graph illustrating a top surface temperature distribution of the main heat transfer bodies of FIGS.  28  to  30 . In FIG. 31, the line interconnected by triangles was obtained from the main heat transfer body of FIG. 28. The line interconnected by circles was obtained from the main heat transfer body of FIG. 29. The line interconnected by rectangles was obtained from the main heat transfer body of FIG. 30.  
         [0130]    As can be seen in FIG. 31, a more uniform temperature distribution of the top surface of the main heat transfer body may be obtained in the present invention. Also, by increasing the cavity and by reducing the heating element pitch at a peripheral region, the temperature distribution of the top surface is further improved.  
         [0131]    Photoresist Pattern Formation  
         [0132]    [0132]FIGS. 32A to  32 D are sectional views illustrating a method for forming a photoresist pattern in accordance with one embodiment of the present invention, utilizing the above heating apparatus.  
         [0133]    Referring to FIG. 32A, a positive-type photoresist composition containing a novolak resin is coated on a silicon wafer  610  utilizing a spin coater to form a photoresist film  612 . Then, the photoresist film  612  is soft-baked utilizing a conventional hot plate at 90-120° C. for 60 seconds. The thickness of the photoresist film  612  is 0.8 to 0.9 μm.  
         [0134]    Referring to FIG. 32B, the photoresist film  612  is selectively exposed to deep ultra violet light  614  utilizing a stepper and a photo mask (not shown). Thereafter, the exposed photoresist film  612  is post-baked utilizing a heating method according to the present invention and a hot plate including a main heat transfer body  510  as shown in FIGS. 22 and 26. The post-baking is performed at a temperature of 140° C. to 150° C. for 30 to 90 seconds.  
         [0135]    Referring to FIG. 32C, the exposed photoresist film  612  is developed using a developer for one minute, then washed using water for about 30 seconds, and then dried to remove the exposed portion of the photoresist film. A first photoresist pattern  612   a  is formed which has an opening portion  616  of a first size to expose a portion of the silicon wafer  610 .  
         [0136]    Referring to FIG. 32D, the first photoresist pattern  612   a  is heated at a temperature of about 140 to about 160° C. for about one to three minutes. At this time, a heating method according to the present invention and a hot plate including a main heat transfer body  510  as shown in FIGS. 22 and 26 are also utilized. Then, the first photoresist pattern  612   a  is reflowed to form a final photoresist pattern  612   b  (as shown by a dotted line) having a second opening portion  616   a  of a second size which is smaller than the first opening size of the first photoresist pattern  612   a.    
         [0137]    Surface Temperature Measurement of a Wafer  
         [0138]    [0138]FIG. 33 is an isotherm diagram showing the distribution of the surface temperature of a wafer which was heated by using the main heat transfer body shown in FIG. 25. In FIG. 33, the temperature difference between one isotherm and its adjacent isotherm is 0.04° C. In this figure, the highest temperature is 155.02° C. at a central portion of the wafer and the lowest temperature is 153.91° C. at a peripheral region of the wafer. The temperature range (the temperature difference between the highest temperature and the lowest temperature) is 0.97° C. The mean temperature indicated by a bold isotherm is 154.65° C., and the standard deviation of the surface temperature is 0.31° C.  
         [0139]    [0139]FIG. 34 is an isotherm diagram showing the distribution of the surface temperature of a wafer which was heated by using the main heat transfer body shown in FIG. 26. In FIG. 34, the temperature difference between one isotherm and its adjacent isotherm is 0.03° C. In this figure, the highest temperature is 137.97° C. at a central portion of the wafer and the lowest temperature is 137.42° C. at a peripheral region of the wafer. The temperature range (the temperature difference between the highest temperature and the lowest temperature) is 0.55° C. The mean temperature indicated by a bold isotherm is 137.68° C., and the standard deviation of the surface temperature is 0.15° C.  
         [0140]    As can been seen from a comparison of FIGS. 33 and 34, when the cavities  515  is more densely divided by the inner partition walls in a radial direction as in FIG. 26, a more uniform temperature distribution is obtained. As a result of many experiments, it has been determined that the evaporation cavities  515   a ,  515   b ,  515   c , etc. are radial divided into eighteen to thirty-six radial sectors each having an angle of 10 to 20 degrees, more preferably 15 degrees, the temperature range was less than 0.6° C., and thus a more uniform temperature distribution is obtained.  
         [0141]    Measurement of the Critical Dimension After Post-Exposure Baking of the Photoresist Pattern  
         [0142]    Referring again to FIG. 32A, a photoresist solution was coated on a wafer  610  to form a photoresist layer  612  and the thus obtained photoresist layer  612  was pre-baked at a temperature of 110° C. for about 60 seconds.  
         [0143]    Then, as shown in FIG. 32B, the photoresist layer  612  was exposed to deep ultra violet rays  614 . At this time, a mask having a pattern for forming a 135 nm (target dimension) contact hole was used. The exposed photoresist layer  612  was post-baked. At this time, a hot plate including a main heat transfer body  510  as shown in FIGS. 22 and 26 was used. For manufacturing a main heat transfer body  510 , the first and second solid heat transfer media  514  and  516 , the outer sidewall  518 , and the inner partition walls  530  were manufactured by using an aluminum alloy. As th liquid  540 , FC-40 (trade name purchased from 3M Korea LTD) was selected which has a boiling point and critical temperature of about 155° C. and 270° C., respectively. After forming the main heat transfer body  510 , the cavity  515  was evacuated to 10 7  Torr, and then about 20% of the volume of the cavity  515  was filled with the liquid  540 . The cavity  515  was then sealed.  
         [0144]    Thereafter, as shown in FIG. 32C, the exposed photoresist layer  612  was developed to form a first photoresist pattern  612   a  having a first opening portion  616 .  
         [0145]    [0145]FIG. 35 is a critical dimension (CD) distribution diagram of a first opening obtained after developing the exposed photoresist film which has been post-baked using the main heat transfer medium as in FIGS. 22 and 26.  
         [0146]    When the hot plate according to the present invention was used, the maximum and minimum CD was 140 nm and 129 nm, respectively. Also, the average CD was 135 nm and the dimension range was only 11 nm. When the acceptable dimension range is set at 120 to 150 nm, all measured contact holes had a size within an acceptable dimension range.  
         [0147]    Measurement of the Critical Dimension of the Photoresist Pattern After Reflowing the Photoresist Pattern  
         [0148]    Referring yet again to FIG. 32A, a photoresist solution was coated on a wafer  610  to form a photoresist layer  612 , and the thus obtained photoresist layer  612  was pre-baked at a temperature of 110° C. for about 60 seconds.  
         [0149]    Then, as shown in FIG. 32B, the photoresist layer  612  was exposed to deep ultra violet rays  614 . At this time, a mask having a pattern for forming a 185 nm contact hole was used. The exposed photoresist layer  612  was post-baked. At this time, a hot plate including a main heat transfer body as shown in FIGS. 22 and 26 was used. This hot plate was the same as that used for the above-described measurement of the CD after post-exposure baking of the photoresist pattern.  
         [0150]    The exposed photoresist layer  612  was developed to form a first photoresist pattern  612   a  having a first opening portion  616  as shown in FIG. 32C. Then, the first photoresist pattern  612   a  was heated at a temperature of 150° C. for two minutes. At this time, the same hot plate was used. As a result, as shown in FIG. 32D, a second photoresist pattern  612   b  was obtained having a second opening portion  616   a  which was smaller size than the first opening portion  616 .  
         [0151]    For comparison, the same procedure was performed using a conventional hot plate in both post-baking and reflowing steps. That is, the hot plate of FIGS. 1 and 2 was used instead of the hot plate according to the present invention.  
         [0152]    The critical dimension (size) of the second opening was measured per one map throughout the whole wafer.  
         [0153]    [0153]FIG. 36 is a critical dimension (CD) distribution diagram of the second opening obtained by using the conventional main heat transfer medium as in FIGS. 1 and 2. FIG. 37 is a critical dimension (CD) distribution diagram obtained by using the main heat transfer medium of the present invention as in FIGS. 22 and 26.  
         [0154]    As can be noted from FIG. 36, when the conventional hot plate was used, the maximum and minimum CD was 201 nm and 159 nm, respectively. Also, the average CD was 177 nm, and the dimension range was 42 nm.  
         [0155]    When the hot plate according to the present invention was used, the maximum and minimum CD was 205 nm and 182 nm, respectively. Also, the average CD was 194 nm, and the dimension range was 23 nm, as shown in FIG. 37.  
         [0156]    From the above, it can be noted that the critical dimension range was improved from 42 nm to 23 nm.  
         [0157]    As mentioned above, when the wafer is heated by the heating method of the present invention, the wafer can be uniformly heated with a temperature deviation of less than 1° C., and further less than 0.6° C.  
         [0158]    Thus, thermal shock which may be applied to the wafer and the photoresist film coated on the wafer is greatly reduced. Consequently, in a case where the heating method and apparatus of the present invention is used in the post-exposure baking step, a photoresist pattern having a uniform size may be formed on the wafer. Also, the heating method and apparatus of the present invention may be advantageously used for reflowing the photoresist pattern so as to form a finer photoresist pattern.  
         [0159]    The heating method and apparatus of the present invention may be used in other fields for uniformly heating an object preferably having a plate shape. Of course, in semiconductor fields which require the uniform heating of a wafer, the heating method and apparatus of the present invention may be advantageously used.  
         [0160]    While the present invention has been particularly shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be effected therein without departing from the spirit and scope of the invention as defined by the appended claims.