Patent Publication Number: US-2005126474-A1

Title: Coating film forming method and coating film forming apparatus

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a coating film forming method and a coating film forming apparatus for forming a coating film by coating a target substrate such as a semiconductor wafer with coating liquid such as a resist solution.  
      2. Description of the Related Art  
      In the manufacturing process of, for example, a semiconductor device, a resist pattern is formed as a mask for forming a prescribed pattern by a so-called photolithography technology in which a resist film is formed by supplying a resist solution onto the surface of a semiconductor wafer (hereinafter referred to simply as “wafer”), followed by applying a light exposure treatment to the wafer after the resist coating in conformity with a prescribed pattern and subsequently developing the exposed pattern formed in the resist film on the wafer. In the resist coating process included in the photolithography technology noted above, a spin coating method is employed in many cases as a method for uniformly coating the wafer surface with the resist solution.  
      In the spin coating method, a wafer fixed on, for example, a spin chuck by the vacuum suction is rotated together with the spin chuck, and a resist solution is allowed to drip from a resist nozzle arranged above the wafer onto substantially the central portion of the wafer. The resist solution dripping onto the wafer surface is centrifugally expanded outward in the radial direction of the wafer, with the result that a resist film is formed on the entire surface of the wafer. Then, the dripping of the resist solution is stopped, and the wafer is kept rotated so as to remove the excess resist solution on the surface of the wafer W so as to control the thickness of the resist film and to dry the resist film.  
      It should be noted that, in the conventional spin coating method, a resist solution is allowed to drip onto substantially the central portion of the wafer, and the resist solution is expanded by the centrifugal force generated by the rotation of the wafer, as described above. What should be noted is that the peripheral speed in the outer peripheral portion of the wafer is markedly higher than that in the central portion, with the result that a considerably large amount of the resist solution is scattered from the outer peripheral portion of the wafer. It follows that only about 10 to 20% of the supplied resist solution is actually used for forming the resist film, leading to a markedly large consumption of the resist solution required for forming the resist film. Under the circumstances, it is of high importance nowadays to decrease the amount of the resist consumption for the resist coating step, i.e., to decrease the dripping amount of the resist solution onto the wafer, in view of the saving of the manufacturing cost.  
      As a method for decreasing the resist solution consumption for forming the resist film, proposed in, for example, JP 7-320999 A is a method (pre-wet system) of allowing a solvent such as a thinner to drip onto the substrate prior to the dripping of the resist solution so as to facilitate the diffusion of the resist solution and, thus, to decrease the supply amount of the resist solution.  
      However, the effect produced by the prior art quoted above differs depending on the kind of the solvent. In other words, a sufficient effect is not necessarily obtained depending on the solvent used. It is certainly possible to overcome this difficulty by selecting an effective solvent. However, the solvent used by the user is limited and, thus, required is a coating film forming method that permits stably decreasing the amount of the coating liquid used such as a resist solution regardless of the kind of the solvent used.  
     BRIEF SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a coating film forming method and a coating film forming apparatus that are based on a pre-wet system and permit stably decreasing the consumption of the coating liquid regardless of the kind of the solvent used.  
      According to a first aspect of the present invention, there is provided a coating film forming method for forming a coating film by coating the surface of a target substrate to be processed with a coating liquid, comprising the steps of supplying a mixture of a solvent for dissolving said coating liquid and a volatilization suppressing substance for suppressing the volatilization of the solvent onto the surface of said target substrate; expanding said mixture onto the entire surface of said target substrate; and supplying a coating liquid onto substantially the central portion of said target substrate while rotating said target substrate that has supplied said mixture thereby expanding the coating liquid outward in the radial direction of the target substrate thereby forming a coating film.  
      According to a second aspect of the present invention, there is provided a coating film forming method for forming a coating film by coating the surface of a target substrate with a coating liquid, comprising the steps of supplying a mixture of a solvent dissolving a resist solution and water onto the surface of said target substrate; rotating said target substrate thereby expanding said mixture onto the entire surface of the target substrate; supplying a coating liquid onto substantially the central portion of said target substrate that has supplied said mixture while rotating the target substrate thereby expanding the coating liquid outward in the radial direction of the target substrate, thereby forming a coating film; and centrifugally removing the excess coating liquid after formation of said coating film thereby controlling the thickness of the coating film.  
      Further, according to a third aspect of the present invention, there is provided a coating film forming apparatus for forming a coating film by supplying a coating liquid onto a rotating target substrate, comprising a substrate holding member for holding a target substrate substantially horizontal; a rotating mechanism for rotating said substrate holding member; a mixture supply mechanism for supplying a mixture of a solvent dissolving a coating liquid and a volatilization suppressing substance for suppressing the volatilization of the solvent onto the target substrate held by said substrate holding member; and a coating liquid supply mechanism for supplying a coating liquid onto substantially the central portion of the target substrate held by said substrate holding member, wherein said mixture is supplied from said mixture supply mechanism onto the target substrate before formation of said coating film, and the target substrate is rotated by said rotating mechanism thereby permitting the mixture to be diffused onto the entire surface of the target substrate.  
      In the present invention, the pre-wetting is performed by using a mixture of a solvent and a volatilization suppressing substance suppressing the volatilization of the solvent. Therefore, the volatilization of the solvent is suppressed even if the solvent used is highly volatile so as to make it possible to obtain a sufficient pre-wetting effect. It follows that the amount of the coating liquid used can be decreased stably regardless of the kind of the solvent used.  
      As a result of an extensive research, the present inventors have found that: 
          (1) In the case of employing the pre-wet system, the resist-saving effect differs depending on the kind of the solvent used because, in the case of using a solvent having a high volatility, the solvent is volatilized before the solvent exhibits a sufficient pre-wetting effect; and     (2) The difficulty can be prevented by using a substance capable of suppressing the volatilization of the solvent together with the solvent.        

      The present invention is based on the two findings pointed out above. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detail description of the preferred embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  is a plan view showing the entire construction of a resist coating-developing process system of a semiconductor wafer including a resist coating unit for working the coating film forming method of the present invention;  
       FIG. 2  is a front view of the coating-developing process system shown in  FIG. 1 ;  
       FIG. 3  is a back view of the coating-developing process system shown in  FIG. 1 ;  
       FIG. 4  is a cross sectional view showing the entire construction of the resist coating unit mounted to the resist coating-developing process system shown in FIGS.  1  to  3 ;  
       FIG. 5  is a plan view of the resist coating unit shown in  FIG. 4 ;  
       FIG. 6  is a flow chart showing the process steps in the resist coating unit;  
       FIG. 7  is a cross sectional view showing another example of a mixture supply means used in the resist coating unit;  
       FIG. 8  is a cross sectional view showing another example of a mixture supply means used in the resist coating unit; and  
       FIG. 9  is a cross sectional view showing still another example of a mixture supply means used in the resist coating unit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Some embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
       FIG. 1  is a plan view showing the entire construction of a resist coating-developing process system  1  including a resist coating unit for working the coating film forming method of the present invention, and  FIGS. 2 and 3  are a front view and a back view, respectively, of the resist coating-developing process system shown in  FIG. 1 .  
      As shown in the drawings, the resist coating-developing process system  1  comprises a cassette station  10 , which is a transfer station, a process station  11  including a plurality of process units, and an interface section  12  arranged adjacent to the process station  11  for the delivery of a wafer W between the process station  11  and a light exposure device (not shown).  
      The cassette station  10  is for transferring a wafer cassette CR housing a plurality of wafers W used as a target object to be processed, e.g., 25 wafers W, from another system to this system, for transferring the particular wafer cassette CR from this system to said another system, or for transferring the wafer W between the wafer cassette CR and the process station  11 .  
      As shown in  FIG. 1 , in the cassette station  10 , a plurality of positioning projections  20   a , i.e., four positioning projections  20   a  in the drawing, are arranged in the X-direction in the drawing on a table  20  on which the cassette CR is disposed. It is possible for the wafer cassettes CR to be disposed on the positions of the projections  20   a  in a manner to form a row such that the wafer entrances (exits at same time) of the wafer cassettes CR are allowed to face the process station  11 . In the wafer cassette CR, the wafers W are arranged in the vertical direction (Z-direction). Further, the cassette station  10  includes a wafer transfer mechanism  21  that is positioned between the table  20  and the process station  11 . The wafer transfer mechanism  21  includes a wafer transfer arm  21   a  movable in the cassette arranging direction (X-direction) and in the arranging direction of the wafers within the wafer cassette CR (Z-direction) such that the transfer arm  21   a  is capable of selectively gaining access to any of the wafer cassettes CR. Further, the wafer transfer arm  21   a  is swingable in the 0 direction so as to be capable of gaining access to the alignment unit (ALIM) and the extension unit (EXT) referred to herein later, which belong to a third process unit group G 3  on the side of the process station  11 .  
      The process station  11  includes a plurality of process units for a series of processes for coating and developing the resist to the wafer W. These process units are arranged in prescribed positions to form a multi-stage structure, and the wafers W are processed one by one in these process units. As shown in  FIG. 1 , a transfer path  22   a  is formed in the central portion of the process station  11 . A main wafer transfer mechanism  22  is arranged in the transfer path  22   a  and all the process units are arranged around the transfer path  22   a . These plural process units are divided into a plurality of process unit groups each consisting of a plurality of process units arranged in the vertical direction to form a multi-stage structure.  
      As shown in  FIG. 3 , the main wafer transfer mechanism  22  comprises a cylindrical support body  49  and a wafer transfer device  46  arranged movable in the vertical direction (Z-direction) inside the cylindrical support body  49 . The cylindrical support body  49  can be rotated by a not shown motor, and the wafer transfer device  46  can also be rotated integrally in accordance with the rotation of the cylindrical support body  49 .  
      The wafer transfer device  46  includes a plurality of holding members  48  movable back and forth along a transfer base  47  so as to achieve the wafer delivery among the process units.  
      In this embodiment, first to fourth process unit groups G 1 , G 2 , G 3  and G 4  are arranged around the transfer path  22   a , as shown in  FIG. 1 . Further, a fifth process unit group G 5  can be arranged, as required.  
      The first and second process unit groups G 1  and G 2  are arranged in a line on the front side of the system (lower side in  FIG. 1 ). The third process unit group G 3  is arranged adjacent to the cassette station  10 , and the fourth process unit group G 4  is arranged adjacent to the interface section  12 . Further, the fifth process unit group G 5  can be arranged in the back portion.  
      Arranged in the first process unit group G 1  is a resist coating process unit (COT) for coating a resist on the wafer W disposed on a not shown spin chuck within a cup CP. Further, stacked on the resist coating process unit (COT) is a developing process unit (DEV) for developing a pattern of the resist within the cup CP. Likewise, arranged in the second process unit group G 2  is a resist coating process unit (COT) as two spinner type process units, and a developing process unit (DEV) is stacked on the resist coating process unit (COT).  
      In the third process unit group G 3 , a plurality of oven type process units for applying a prescribed process to the wafer W disposed on a table SP are stacked one upon the other, as shown in  FIG. 3 . To be more specific, the third process unit group G 3  includes an adhesion unit (AD) for applying a so-called “hydrophobic processing” for improving the fixing properties of the resist, an alignment unit (ALIM) for aligning the position of the wafer W, an extension unit (EXT) for transferring the wafer W, a cooling unit (COL) for a cooling processing, and four hot plate units (HP) for a heat processing before or after the light exposure process and after the developing processing. These process units are stacked one upon the other in the order mentioned to form an eight-stage structure. Incidentally, the alignment unit (ALIM) may be replaced by the cooling unit (COL) which works also as an alignment unit.  
      The fourth process unit group G 4  also includes a plurality of oven type process units stacked one upon the other. To be more specific, the fourth process unit group G 4  includes a cooling unit (COL), an extension-cooling unit (EXTCOL) constituting a wafer delivery section equipped with a cooling plate, another cooling unit (COL), and four hot plate units (HP), which are stacked one upon the other in the order mentioned so as to form an eight-stage structure.  
      Where the fifth process unit group G 5  is installed at the back side of the main wafer transfer mechanism  22 , the fifth process unit group G 5  is movable sideward along a guide rail  25  as viewed from the main wafer transfer mechanism  22 . It follows that, even where the fifth process unit group G 5  is installed, the fifth process unit group G 5  can be slid along the guide rail  25  so as to ensure a free space behind the main wafer transfer mechanism  22 . As a result, a maintenance operation for the main wafer transfer mechanism  22  can be executed easily from behind the main wafer transfer mechanism  22 .  
      The interface section  12  is equal to the process station  11  in the length in the X-direction. As shown in  FIGS. 1 and 2 , a flexible pick-up cassette CR and a stationary buffer cassette BR are stacked one upon the other in a front portion of the interface section  12 . A peripheral light exposure device  23  is arranged in a back portion of the interface section  12 . Further, a wafer transfer mechanism  24  is arranged in the central portion of the interface section  12 . The wafer transfer mechanism  24  includes a wafer transfer arm  24   a , which is movable both in the X-direction and the Z-direction so as to gain access to the cassettes CR, BR and the peripheral light exposure device  23 . Also, the wafer transfer arm  24   a  is swingable in the θ direction so as to gain access to the extension unit (EXT) in the fourth process unit group G 4  of the process station  11  and to a not shown wafer delivery table adjacent to the light exposure device.  
      In the resist coating-developing process system  1  of the construction described above, unprocessed wafers W are taken out one by one from the wafer cassette CR so as to be transferred into the alignment unit (ALIM) of the process station  11 . Then, the wafer W whose position has been aligned is taken out by the main wafer transfer mechanism  22  so as to be transferred into the adhesion unit (AD) for the adhesion processing. After completion of the adhesion processing, the wafer W is taken out by the main wafer transfer mechanism  22  so as to be transferred into the cooling unit (COL) for the cooling processing. Further, the wafer W is transferred into the resist coating unit (COT) for the resist coating processing and, then, into the hot plate unit (HP) for the pre-bake treatment. Still further, the wafer W is transferred into the interface section  12  through the extension-cooling unit (EXTCOL) and, then, the wafer W is further transferred by the wafer transfer mechanism  24  into the adjacent light exposure device. The exposed wafer W is transferred by the wafer transfer mechanism  24  into the process station  11  through the interface section  12  and the extension unit (EXT). In the process station  11 , the wafer W is transferred by the main wafer transfer mechanism  22  into the hot plate unit (HP) for the post exposure processing and, then, transferred into the developing unit (DEV) for the developing processing. After the developing, the wafer W is post-baked in the hot plate unit (HP) and, then, cooled in the cooling unit (COL), followed by transferring the wafer W into the cassette station  10  through the extension unit (EXT). After completing a series of these processings, the wafer W is transferred by the wafer transfer mechanism  22  into the wafer cassette CR so as to be housed in the wafer cassette CR.  
      The resist coating unit (COT) for working the coating film forming method of the present invention will now be described with reference to  FIGS. 4 and 5 .  
      The resist coating unit (COT) includes a casing  50  provided with an opening  50   a  through which the holding member  48  of the main wafer transfer mechanism  22  is inserted into the casing  50 . A cup CP, which is a container for housing the wafer W, is arranged within the casing  50 , and a spin chuck  51  for holding the wafer W horizontal by vacuum suction is arranged inside the cup CP. The spin chuck  51  can be rotated by a driving motor  52  such as a pulse motor arranged below the cup CP, and the rotating speed of the spin chuck  51  can be controlled optionally. An exhaust pipe  53  is connected to that portion of the bottom of the cup CP which is positioned close to the central portion of the bottom of the cup CP, and a drain pipe  54  is connected to that portion of the bottom of the cup CP which is positioned close to the outer portion of the bottom of the cup CP. The gaseous material within the cup CP is discharged to the outside through the exhaust pipe  53 , and the resist solution and the solvent scattered during the coating processing are discharged to the outside through the drain pipe  54 . Incidentally, the spin chuck  51  can be vertically moved by a not shown lift mechanism such as an air cylinder.  
      A spurting head  60  movable between the position right above the spin chuck  51  and a retreat position is arranged above the spin chuck  51 . The spurting head  60  is connected to a driving mechanism  70  with an arm  61  interposed therebetween. The spurting head  60  can be moved in the X-direction by the driving mechanism  70 , the Y-direction and the Z-direction shown in  FIGS. 4 and 5 . Incidentally, the spurting head  60  is detachable from the arm  61 .  
      The spurting head  60  includes a base member  62 , a mixture supply nozzle  80  for supplying a mixture of a solvent capable of dissolving a coating liquid and a volatilization suppressing substance suppressing the volatilization of the solvent, and a resist solution supply nozzle  90  positioned close to the mixture supply nozzle  80  for supplying a resist solution, which is a coating liquid. As shown in the drawing, the spurting head  60  is constructed such that the mixture supply nozzle  80  and the resist solution supply nozzle  90  are mounted to the base member  61 . It should be noted that it is possible for the solvent capable of dissolving the coating liquid to be a solvent of the coating liquid. In addition, it is possible to use any solvent as far as the solvent is capable of dissolving the coating liquid.  
      The spurting head  60  is provided with tubes  65   a ,  65   b  for circulating a temperature adjusting fluid for the temperature adjustment such that the temperature of the resist solution spurted from the resist solution supply nozzle  90  is rendered constant and with tubes  66   a ,  66   b  for circulating a temperature adjusting fluid for the temperature adjustment such that the temperature of the solvent spurted from the mixture supply nozzle  80  is rendered constant. The tube  65   a  is arranged around a pipe contiguous to the resist solution supply nozzle  90  so as to constitute a forward passageway, and the tube  65   b  constitutes a return passageway. Also, the tube  66   a  is arranged around a pipe contiguous to the mixture supply nozzle  80  so as to constitute a forward passageway, and the tube  66   b  constitutes a return passageway.  
      The mixture supply nozzle  80  is connected to an intermediate tank  83  via a mixture supply pipe  81 , and a valve  82  is mounted to the mixture supply pipe  81 . A solvent supply pipe  84  for supplying a solvent into the intermediate tank  83  and a volatilization suppressing substance supply pipe  86  for supplying a volatilization suppressing substance into the intermediate tank  83  are connected to the intermediate tank  83 . Valves  85  and  87  are mounted to these pipes  84  and  86 , respectively. The solvent and the volatilization suppressing substance supplied into the intermediate tank  83  through the pipes  84  and  86 , respectively, are stirred by a not shown stirring mechanism so as to form a mixture, and the mixture thus formed is stored in the intermediate tank  83 . A compressed gas such as a compressed nitrogen gas (N 2 ) is supplied into the intermediate tank  83  so as to permit the mixture to be supplied onto the wafer W through the mixture supply pipe  81  and the mixture supply nozzle  80 . In this case, the flow rate of the mixture is controlled by controlling the pressurizing force of the N 2  gas.  
      The resist solution supply nozzle  90  communicates via a resist solution supply pipe  91  with a resist solution tank  92  housing a resist solution. Mounted to the resist solution supply pipe  91  are a suck back valve  93 , an air operation valve  94 , a bubble removing mechanism  95  for separating and removing the bubbles within the resist solution, a filter  96  and a bellows pump  97  in the order mentioned as viewed from the downstream side. The bellows pump  97  is shrinkable. By controlling the shrinkage of the bellows pump  97 , a prescribed amount of the resist solution is supplied onto the surface of the wafer W through the resist solution supply nozzle  90 . The bellows pump  97  makes it possible to control a very small supply amount of the resist solution. The driving section of the bellows pump  97  comprises a ball screw mechanism  98  including a screw  98   a  having one end mounted to one end of the bellows pump  97  and a nut  98   b  engaged with the screw  98   a , and a stepping motor  99  that rotates the nut  98   b  so as to permit the screw  98   a  to make a linear motion.  
      The suck back valve  93  mounted to the resist solution supply system noted above serves to bring the resist solution remaining by surface tension on the inner wall in the tip portion of the resist solution supply nozzle  90  after spurting of the resist solution from the resist solution supply nozzle  90  back into the resist solution supply nozzle  90 . As a result, the residual resist solution is prevented from being solidified.  
      As shown in  FIG. 5 , a holding section  55  capable of holding four spurting heads  60 , which are basically equal to each other in construction, is arranged in the outside portion of the cup CP within the casing  50 . In order to prevent the nozzle port of each nozzle from being dried and solidified, an not shown inserting section for placing the nozzle port of each nozzle under a solvent atmosphere is mounted to the holding section  55 . Each spurting head  60  can be mounted to the tip portion of the arm  61  by a mounting section  63  so as to permit the four spurting heads  60  to supply different kinds of resist solutions onto the surfaces of the wafers W. A selected one of the spurting heads  60  is mounted to the arm  61  so as to be taken out of the holding section  55 . As described previously, the arm  61  can be moved by the driving mechanism  70  in three dimensional directions, i.e., in the X-, Y- and Z directions, such that the spurting head  60  taken out of the holding section  55  and mounted to the arm  61  is moved to a prescribed position right above the wafer W in a coating processing. Incidentally, in this embodiment, the mixture supply nozzle  80  and the resist solution supply nozzle  90  are mounted to the spurting head  60 , and four spurting heads  60  of the particular construction are arranged in the holding section  55 . Alternatively, it is possible to fix a single or a plurality of mixture supply nozzles  80  directly to the arm  61  and to mount the resist solution supply nozzle  90  alone to the spurting head  60 .  
      The processing carried out in the resist coating unit (COT) of the particular construction described above will now be described in detail with reference to the flow chart shown in  FIG. 6 .  
      If the wafer W is transferred through the opening  50   a  of the casing  50  by the holding member  48  of the main wafer transfer mechanism  22  onto a position right above the cup CP within the resist coating unit (COT), the wafer W is held by vacuum suction by the spin chuck  51  moved upward by the not shown lift mechanism. After the wafer W is held by the spin chuck  51  by vacuum suction, the main wafer transfer mechanism  22  brings back the holding member  48  from within the resist coating unit (COT) so as to finish delivery of the wafer W into the resist coating unit (COT) (step ST  1 ).  
      Then, the spin chuck  51  is moved downward until the wafer W reaches a prescribed position within the cup CP, followed by allowing the driving motor  52  to rotate the spin chuck  51  at a rotating speed of about 1,000 rpm so as to make the temperature of the wafer W uniform (step ST  2 ).  
      In the next step, the rotation of the spin chuck  51  is stopped, and the spurting head  60  is moved by the driving mechanism  70  in the Y-direction so as to reach a position right above the wafer W. When spurting port of the mixture supply nozzle  80  included in the spurting head  60  has been moved to reach a position right above the center of the spin chuck  51 , i.e., above the center of the wafer W, the mixture of a prescribed solvent capable of dissolving the resist and a volatilization suppressing substance suppressing the volatilization of the solvent is supplied onto substantially the center on the surface of the stationary wafer W (step ST  3 ). In this step, the solvent and the volatilization suppressing substance are supplied at a prescribed mixing ratio into the intermediate tank  83  through the solvent supply pipe  84  and the volatilization suppressing substance supply pipe  86 , respectively. Further, the solvent and the volatilization suppressing substance are stirred by a not shown stirring mechanism within the intermediate tank  83 , with the valves  85  and  87  closed, so as to form a prescribed amount of the mixture. Further, the mixture formed and stored within the intermediate tank  83  is pressurized by a pressurizing gas such as a N 2  gas so as to be supplied onto the wafer W through the mixture supply pipe  81  and the mixture supply nozzle  80 . In this case, the flow rate of the mixture is controlled by controlling the pressurizing force of the N 2  gas.  
      After the mixture has been supplied onto the wafer W, the wafer W is rotated at a prescribed rotating speed, preferably at a rotating speed not higher than 1,000 rpm (step ST  4 ). As a result, the mixture supplied onto the surface of the wafer W is centrifugally diffused from the central portion toward the peripheral portion of the wafer W such that the mixture is uniformly spread over the entire surface of the wafer W. Incidentally, the mixture can be supplied onto the wafer W while rotating the wafer W. In other words, it is possible to carry out simultaneously the steps ST  3  and ST  4  described above. It is also possible to employ a spraying method for coating the entire surface of the wafer W with the mixture. In this case, the wafer W can be rotated or stopped during the spraying operation.  
      In the next step, the spurting head  60  is moved in the Y-direction by the driving mechanism  70  until the spurting port of the resist solution supply nozzle  90  is moved to reach a position right above the center of the spin chuck  51 , i.e., right above the center of the wafer W, and the rotating speed of the wafer W is increased to a prescribed level. Under this state, the resist solution is supplied from the spurting port of the resist solution supply nozzle  90  onto substantially the center on the surface of the rotating wafer W so as to permit the resist solution to be centrifugally diffused outward, thereby coating the surface of the wafer W with the resist solution (step ST  5 ). Where the wafer has a diameter of 200 mm, it is desirable to set the rotating speed of the wafer W at 2,000 to 6,000 rpm. Also, where the wafer has a diameter of 300 mm, it is desirable to set the rotating speed of the wafer W at 1,000 to 4,000 rpm.  
      After the resist solution has been supplied while rotating the wafer W, the supply of the resist solution is stopped, and the rotating speed of the wafer W is lowered (step ST  6 ). As a result, the function of adjusting the thickness of the resist solution film is produced so as to make the thickness of the resist solution film uniform over the entire region of the wafer W. The particular effect can be produced because, when the rotating speed of the wafer W is lowered, the force toward the center is exerted on the resist solution on the semiconductor wafer W by the deceleration. In addition, the resist solution is dried slowly because the target substrate is rotated at a low speed. It follows that the function of controlling the thickness of the resist film is produced. What should be noted is that the scattering of the resist from the wafer W is suppressed by the inward force produced by the deceleration so as to permit the resist to be retained in the outer peripheral portion as in the central portion of the wafer W, thereby making uniform the thickness of the resist film formed on the wafer W. In this case, it is desirable for the rotating speed of the wafer W to be 50 to 1,000 rpm. Particularly, if the rotating speed of the wafer W is not higher than 500 rpm, the drying of the resist scarcely proceeds, leading to a high degree of freedom in the thickness control. The holding time in this step is set appropriately so as not to exceed, for example, 3 seconds. Incidentally, the process in step ST  6  is not absolutely necessary and is carried out as required.  
      Then, the rotating speed of the wafer W is increased so as to centrifugally remove the residual resist solution (step ST  7 ). It is desirable for the rotating speed of the wafer W in this step to be 1,500 to 4,000 rpm in the case where the wafer W has a diameter of 200 mm and to be 1,000 to 3,000 rpm in the case where the wafer W has a diameter of 300 rpm.  
      After removal of the residual resist solution in step ST  7 , the rotation of the wafer W is continued so as to dry the resist film (step ST  8 ). It is desirable for the rotating speed of the wafer W in this step to be 1,000 to 2,000 rpm in the case where the wafer W has a diameter of 200 mm and to be 500 to 1,500 rpm in the case where the wafer W has a diameter of 300 rpm. The resist coating step is finished after the drying step is performed for a prescribed period of time in step ST  8 .  
      As described above, in the present invention, the entire surface of the wafer W is coated with a mixture of a solvent capable of dissolving the resist and a volatilization suppressing substance suppressing the volatilization of the solvent prior to the coating of the resist solution. Therefore, even if the solvent used is highly volatile, the volatilization of the solvent is suppressed by the volatilization suppressing substance so as to produce the effect of sufficiently diffusing the resist solution in the subsequent coating step of the resist solution. It follows that the amount of the resist solution used can be stably decreased regardless of the kind of the solvent used.  
      The solvent contained in the mixture should be capable of dissolving the resist. Typically, a thinner is contained as the solvent in the mixture. To be more specific, it is possible for the mixture to contain at least one kind of the solvent selected from the group consisting of, for example, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, butyl acetate, ethyl lactate, ethyl cellosolve acetate, and methyl methoxy propionate. Particularly, the effect of the present invention is improved in the case of using at least one kind of the solvent selected from the group consisting of propylene glycol monomethyl ether and propylene glycol monomethyl ether acetate as the solvent. Of course, it is also possible to use another solvent as far as the solvent is capable of dissolving the resist.  
      It is desirable for the volatilization suppressing substance used in the present invention to have a hydrogen bond. At least one kind of the substance selected from the group consisting of water, methyl alcohol, ethyl alcohol and isopropyl alcohol (IPA) is suitably used as the substance having a hydrogen bond. Among these substances, it is desirable to use water, particularly pure water, as the volatilization suppressing substance. Of course, it is also possible to use another substance as far as the substance is capable of effectively suppressing the volatilization of the solvent used.  
      It is desirable for the amount of the volatilization suppressing substance to be not smaller than 5% by mass and smaller than 50% by mass of the entire mixture. If the amount of the volatilization suppressing substance is smaller than 5% by mass, the volatilization suppressing effect tends to be rendered insufficient. On the other hand, if the amount of the volatilization suppressing substance is not smaller than 50% by mass, the resist solution diffusing effect produced by the solvent tends to be rendered insufficient. The supply amount of the mixture is, for example, 2 ml in the case where the wafer W has a diameter of 200 mm, and 3 ml in the case where the wafer W has a diameter of 300 mm.  
      The temperature control of the mixture is carried out at a suitable temperature falling within a range of between, for example, 18° C. and 24° C. Within this temperature range, the mixture is allowed to wet the wafer surface uniformly so as to make it possible to render uniform the thickness of the resist film. It should be noted that it is necessary to eliminate the temperature difference between the central portion and the outer peripheral portion of the wafer W as much as possible. Because of the particular requirement, the appropriate temperature is changed depending on the size of the wafer. For example, the appropriate temperature is 22 to 24° C. in the case of the wafer W having a diameter of 200 mm. However, where the diameter of the wafer W is increased to 300 mm, the peripheral speed in the outer peripheral portion of the wafer W is increased. As a result, the temperature difference tends to be generated between the central portion and the outer peripheral portion of the wafer W and, thus, the solvent contained in the mixture tends to be volatilized in the outer peripheral portion of the wafer W. Such being the situation, the temperature of the mixture is set a somewhat low level of 18 to 19° C. in order to eliminate the temperature difference between the central portion and the outer peripheral portion of the wafer W as much as possible.  
      Some experiments for actually confirming the effect of the present invention will now be described.  
      In the first experiment, the minimum amount of the resist that can be coated on the entire wafer surface was measured by using a wafer having a diameter of 200 mm. The experiment covered the case where butyl acetate, propylene glycol monomethyl ether (PGME), or ethyl lactate was used as a solvent for the pre-wetting and the case where the pre-wetting was not performed. A solution having a viscosity of about 7 CP, which was prepared by dissolving a KrF resist in an EL solvent, was used as the resist solution. The wafer was rotated at a speed of 3,000 to 6,000 rpm during dripping of the resist solution, and the supply amount of the solvent was 2 ml. It has been found that the minimum amount of the resist solution that can be coated on the entire wafer surface can be markedly decreased by applying the pre-wetting, as shown in Table 1, though the degree of reduction in the amount of the required resist solution differs depending on the kind of the solvent used. It is considered reasonable to understand that the difference in the degree of reduction noted above is caused by the difference in the volatilizing rate of the solvent. To be more specific, it is considered reasonable to understand that, if the solvent has a low volatilizing rate, a greater amount of the solvent remains on the wafer surface during dripping of the resist solution so as to improve the resist diffusion effect.  
               TABLE 1                       Minimum amount of resist solution on 200 mm wafer                                                no pre-wetting   0.85 ml           butyl acetate    0.6 ml           PGME   0.45 ml           ethyl lactate   0.25 ml                      
 
      Then, a second resist coating experiment was conducted similarly by using a wafer having a diameter of 300 mm. In the second experiment, the pre-wetting was performed by using the three kinds of the solvents equal to those used in the first experiment. The rotating speed of the wafer was set at 2,000 to 4,000 rpm during dripping of the resist solution, and the supply amount of the solvent was 3 ml. It has been found that the difference in the minimum amount of the resist solution required for the coating on the entire surface of the wafer depending on the kind of the solvent used is rendered greater than that in the case of using the wafer having a diameter of 200 mm, as shown in Table 2. Table 2 also shows that PGME constituting the main component of the PGME series thinner, which is most frequently used nowadays, is incapable of producing a sufficient resist saving effect.  
               TABLE 2                       Minimum amount of resist solution on 300 mm wafer                                                    butyl acetate   2.0   ml or more           PGME   1.7   ml           ethyl lactate   0.65   ml                      
 
      Further, a third experiment was conducted for measuring the minimum amount of the resist solution that can be coated on the entire surface of each of the wafer having a diameter of 200 mm and the wafer having a diameter of 300 mm, covering the cases where the pre-wetting was performed by using PGME and where the pre-wetting was performed by using a mixture specified in the present invention, i.e., a mixture of PGME and a pure water used as the volatilization suppressing substance. The mixing ratio of PGME to the pure water in the mixture was set at 5:1 (16.7% by mass of the pure water). The resist solution and the coating conditions were equal to those in the first and second experiments given above. As a result, it has been confirmed that, in the case of performing the pre-wetting by using a mixture of PGME and a pure water, the minimum amount of the resist solution required for the coating on the entire wafer surface is rendered smaller than half the amount in the case where the pre-wetting is performed by using PGME alone in any of the wafer having a diameter of 200 mm and the wafer having a diameter of 300 mm. It is considered reasonable to understand that, by the addition of a pure water, a hydrogen bond is formed between the solvent molecule and the water molecule so as to suppress the volatilization of the solvent, with the result that the amount of the solvent remaining on the wafer surface is increased in the resist dripping step. A similar result was exhibited in the case of using a mixture prepared by adding a pure water to a general purpose thinner of OK  73  (PGME:PGMEA (propylene glycol monomethyl ether acetate)=7:3) in the same mixing ratio. In conclusion, it has been confirmed that a prominent resist saving effect can be produced in the case of performing the pre-wetting by using a mixture specified in the present invention of a solvent and a pure water.  
                                   TABLE 3                                      Minimum amount of   PGME   0.45   ml           resist solution on   PGME + pure water   0.2   ml           200 mm wafer           Minimum amount of   PGME   1.7   ml           resist solution on   PGME + pure water   0.7   ml or less           300 mm wafer                      
 
      Other examples of the supply means of the mixture of a solvent and a volatilization suppressing substance will now be described. In the embodiment described above, a mixture is formed in the intermediate tank  83 , and the mixture formed is supplied from the intermediate tank  83  by utilizing a pressurizing gas. Alternatively, it is also possible to use a system shown in  FIG. 7 . In the system shown in  FIG. 7 , a solvent supplied through a solvent supply pipe  101  extending from a not shown intermediate tank and a volatilization suppressing substance supplied at a prescribed flow rate through a volatilization suppressing substance supply pipe  102  extending from a not shown intermediate tank are mixed in a static mixer  103  arranged in the vicinity of the mixture supply nozzle  80 , and the mixture is supplied from the static mixer  103  into the mixture supply nozzle  80  through the mixture supply pipe  81 .  
       FIG. 8  shows another example of the mixture supply means. As shown in the drawing, arranged is a mixture supply nozzle  110  provided with a mixing section  111 . In this case, a solvent supply pipe  112  extending from a not shown intermediate tank and a volatilization suppressing substance pipe  113  extending from a not shown intermediate tank are connected to the mixing section  111  so as to permit the solvent and the volatilization suppressing substance supplied through the pipes  112  and  113 , respectively, are mixed in the mixing section  111 .  
       FIG. 9  shows still another example of the mixture supply means. As shown in the drawing, arranged are a solvent supply nozzle  121  and a volatilization suppressing substance supply nozzle  122 . In this case, a solvent supply pipe  123  extending from a not shown intermediate tank and a volatilization suppressing substance supply pipe  124  extending from a not shown intermediate tank are connected to the solvent supply nozzle  121  and the volatilization suppressing substance supply nozzle  122 , respectively. What should be noted is that the solvent and the volatilization suppressing substance spurted from the solvent supply nozzle  121  and the volatilization suppressing substance supply nozzle  122 , respectively, are mixed on the wafer surface so as to form a mixture.  
      It should be noted that the embodiments described above are simply intended to clarify the technical idea of the present invention. Naturally, the technical scope of the present invention should not be construed solely on the basis of the specific embodiments described above. In other words, the present invention can be worked in variously modified fashions on the basis of the spirit of the present invention and within the scope defined in the accompanying claims.  
      For example, each of the embodiments described above covers the case where a resist solution is used as a coating liquid. However, the coating liquid is not limited to a resist solution. It is also possible to use another coating liquid such as a coating liquid for forming an antireflection film or an interlayer dielectric film by the rotary coating processing. Also, each of the embodiments described above covers the case where a semiconductor wafer is used as the target substrate. However, the target substrate used in the present invention is not limited to the semiconductor wafer. It is also possible to use another target substrate such as an LCD substrate or a reticle substrate for a mask.