Patent Publication Number: US-2009239384-A1

Title: Substrate processing apparatus and substrate processing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a substrate processing apparatus and a substrate processing method for performing process steps including cleaning, etching and the like upon a substrate such as a semiconductor substrate, a glass substrate for a liquid crystal display, a glass substrate for a photomask and others by dipping the substrate into a processing solution. 
     2. Description of the Background Art 
     Process steps of manufacturing semiconductors employ what is called a batch type substrate processing apparatus in which a plurality of substrates are dipped into a processing solution stored in a processing bath to collectively process the substrates.  FIG. 19  is a longitudinal sectional view showing an example of a conventional substrate processing apparatus. As shown in  FIG. 19 , a substrate processing apparatus  100  conventionally used comprises a processing bath  110  for storing therein a processing solution, and a lifter  120  for holding a plurality of substrates W in the processing bath  110 . In the substrate processing apparatus  100 , a processing solution is discharged from a pair of nozzles  114  provided at the bottom of the processing bath  110  to cause the processing solution to flow over the upper edge of the processing bath  110 , thereby supplying surfaces of the substrates W held by the lifter  120  with the processing solution to process the substrates W. 
     Such a batch type substrate processing apparatus is disclosed for example in Japanese Patent Application Laid-Open No. 2007-36189 or 2007-266360. 
     In the conventional substrate processing apparatus  100 , processing solutions discharged from the pair of nozzles  114  meet at the center or its vicinity of the processing bath  110  to form a liquid flow F 1  that moves up in the processing bath  110 . However, all the processing solution forming the liquid flow F 1  does not reach the upper edge of the processing bath  110 . Some of the processing solution forms liquid flows F 2  which move sideways and then downward to return to the bottom of the processing bath  110 . As a result, an area CA in which the processing solution circulates (circulation area) is formed in the processing bath  110 . 
     If the circulation area CA expands widely, the processing solution may not be drained out of the processing bath  110  efficiently. This may result in the retention of particles or components to be removed for a long time in the processing bath  110 . 
     Meanwhile, the batch type substrate processing apparatus  100  is required to smoothly pour the processing solution into gaps between the plurality of substrates W arranged on and held by the lifter  120 . 
     SUMMARY OF THE INVENTION 
     The present invention is intended for a substrate processing apparatus for processing a plurality of substrates by dipping the plurality of substrates into a processing solution. 
     According to the present invention, the substrate processing apparatus comprises: a processing bath for storing therein a processing solution; a holding part for holding a plurality of substrates in the processing bath; a pair of nozzles arranged near the bottom of the processing bath, for discharging a processing solution onto the upper surface of a bottom plate of the processing bath through a plurality of discharge holes arranged in a direction in which the plurality of substrates held by the holding part are arranged, discharges through the plurality of discharge holes being directed at an angle of 5 degrees to 40 degrees slanting inward with respect to a normal to the upper surface of the bottom plate; and a processing solution drainage part for draining a processing solution flowing over the upper edge of the processing bath. 
     Thus, a circulation area of the processing solution formed in the processing bath is reduced to enhance the efficiency of displacement of the processing solution in the processing bath. Further, the processing solution smoothly flows into gaps between the plurality of substrates. 
     Preferably, the pair of nozzles are tubular members each provided with the plurality of discharge holes, and the plurality of discharge holes each have an opening diameter within a range of 0.5 mm to 1.5 mm. 
     Thus, an excessively large difference is not generated between the pressure at which the processing solution is discharged through the discharge holes on the upstream side and the pressure at which the processing solution is discharged through the discharge holes on the downstream side. Further, there will be no high pressure loss of the processing solution at each discharge hole. 
     Preferably, the pair of nozzles extend along recesses defined in side walls of the processing bath. 
     This prevents the processing solution from staying between the nozzles and the side walls of the processing bath. 
     Preferably, the plurality of discharge holes are arranged at positions corresponding to the positions of gaps between the plurality of substrates held by the holding part, and the positions outside the substrates at the opposite ends. 
     Thus, the processing solution is smoothly supplied to each of the plurality of substrates. 
     Preferably, the substrate processing apparatus further comprises an additional pair of nozzles for discharging a processing solution toward contact points between the holding part and the plurality of substrates. 
     This prevents the retention of particles or components to be removed at the contact points between the holding part and the plurality of substrates. 
     The present invention is also intended for a substrate processing method of processing a plurality of substrates by dipping the plurality of substrates into a processing solution. 
     According to the present invention, the substrate processing method comprises the steps of: a) dipping a plurality of substrates into a processing solution stored in a processing bath; and b) discharging a processing solution through a plurality of discharge holes defined near the bottom of the processing bath and arranged in a direction in which the plurality of substrates are arranged onto the upper surface of a bottom plate of the processing bath, at an angle of 5 degrees to 40 degrees slanting inward with respect to a normal to the upper surface of the bottom plate. 
     Thus, a circulation area of the processing solution formed in the processing bath is reduced to enhance the efficiency of displacement of the processing solution in the processing bath. Further, the processing solution smoothly flows into gaps between the plurality of substrates. 
     Preferably, the plurality of discharge holes are formed in each of a pair of tubular nozzles arranged near the bottom of the processing bath, and the plurality of discharge holes each have an opening diameter within a range of 0.5 mm to 1.5 mm. 
     Thus, an excessively large difference is not generated between the pressure at which the processing solution is discharged through the discharge holes on the upstream side and the pressure at which the processing solution is discharged through the discharge holes on the downstream side. Further, there will be no high pressure loss of the processing solution at each discharge hole. 
     Preferably, the plurality of discharge holes are arranged along a recess defined in a side wall of the processing bath. 
     This prevents the processing solution from staying between the plurality of discharge holes and the side wall of the processing bath. 
     Preferably, the plurality of discharge holes are arranged at positions corresponding to the positions of gaps between the plurality of substrates, and the positions outside the substrates at the opposite ends. 
     Thus, the processing solution is smoothly supplied to each of the plurality of substrates. 
     Preferably, in the step b), a processing solution is further discharged toward contact points between the plurality of substrates and a holding part for holding the plurality of substrates. 
     This prevents the retention of particles or components to be removed at the contact points between the holding part and the plurality of substrates. 
     It is therefore an object of the present invention to provide a substrate processing apparatus and a substrate processing method of reducing a circulation area of a processing solution formed in a processing bath to enhance the efficiency of displacement of the processing solution, while smoothly pouring the processing solution into gaps between a plurality of substrates. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of a substrate processing apparatus, taken along a plane parallel to main surfaces of substrates. 
         FIG. 2  is a longitudinal sectional view of the substrate processing apparatus, taken along a plane perpendicular to the main surfaces of the substrates. 
         FIG. 3  is a longitudinal sectional view showing one lower nozzle and its vicinity in enlarged manner. 
         FIG. 4  is a longitudinal sectional view showing another lower nozzle and its vicinity in enlarged manner. 
         FIG. 5  shows the flow of a processing solution in an inner bath when the angle of a discharge hole of the lower nozzle is set at −50 degrees. 
         FIG. 6  shows the flow of a processing solution in the inner bath when the angle of the discharge hole of the lower nozzle is set at 0 degrees. 
         FIG. 7  shows the flow of a processing solution in the inner bath when the angle of the discharge hole of the lower nozzle is set at 20 degrees. 
         FIG. 8  shows the flow of a processing solution in the inner bath when the angle of the discharge hole of the lower nozzle is set at 50 degrees. 
         FIG. 9  shows the flow of a processing solution in the inner bath when the angle of the discharge hole of the lower nozzle is set at 70 degrees. 
         FIG. 10  shows the speed at which a processing solution flows in each region defined in the inner bath when the angle of the discharge hole of the lower nozzle is set at −50 degrees. 
         FIG. 11  shows the speed at which a processing solution flows in each region defined in the inner bath when the angle of the discharge hole of the lower nozzle is set at 0 degrees. 
         FIG. 12  shows the speed at which a processing solution flows in each region defined in the inner bath when the angle of the discharge hole of the lower nozzle is set at 20 degrees. 
         FIG. 13  shows the speed at which a processing solution flows in each region defined in the inner bath when the angle of the discharge hole of the lower nozzle is set at 50 degrees. 
         FIG. 14  shows the speed at which a processing solution flows in each region defined in the inner bath when the angle of the discharge hole of the lower nozzle is set at 70 degrees. 
         FIG. 15  shows the pressure of a processing solution discharged through a plurality of discharge holes when the opening diameter of the discharge hole of the lower nozzle is set at 1.6 mm. 
         FIG. 16  shows the pressure of a processing solution discharged through the plurality of discharge holes when the opening diameter of the discharge hole of the lower nozzle is set at 1.2 mm. 
         FIG. 17  is a flow diagram showing the flow of process steps performed in the substrate processing apparatus. 
         FIG. 18  shows how a processing solution discharged through the discharge hole of the lower nozzle passes through a recess to flow upward. 
         FIG. 19  is a longitudinal sectional view of an example of a conventional substrate processing apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the below, a preferred embodiment of the present invention is described with reference to drawings. 
     &lt;1. Configuration of Substrate Processing Apparatus&gt; 
       FIG. 1  is a longitudinal sectional view of a substrate processing apparatus  1  according to a preferred embodiment of the present invention, taken along a plane parallel to main surfaces of substrates W.  FIG. 1  also schematically shows a control system, and solution supply and drainage systems of the substrate processing apparatus  1 .  FIG. 2  is a longitudinal sectional view taken along a plane perpendicular to the main surfaces of the substrates W.  FIGS. 1 and 2  include a common XYZ orthogonal coordinate system provided to provide clarity in relative positions of elements in the substrate processing apparatus  1 . The X-axis direction, the Y-axis direction and the Z-axis direction respectively correspond to a direction in which the substrates W are arranged, a horizontal direction along the main surfaces of the substrates W, and a vertical direction. 
     In a photolithography process of the substrates W that are semiconductor wafers, the substrate processing apparatus  1  is intended to remove photoresist films (organic films) formed on the main surfaces of the substrates W. The substrate processing apparatus  1  uses a processing solution containing sulfuric acid (H 2 SO 4 ) and a solution of hydrogen peroxide (H 2 O 2 ). By the action of Caro&#39;s acid (H 2 SO 5 ) generated by the reaction of sulfuric acid and a hydrogen peroxide solution in the processing solution, photoresist films formed on the main surfaces of the substrates W are dissolved and removed. 
     As shown in  FIGS. 1 and 2 , the substrate processing apparatus  1  mainly comprises a processing bath  10  for storing therein a processing solution, a lifter  20  for moving up and down a plurality of substrates (hereinafter simply referred to as “substrates”) W while holding the substrates W thereon, a processing solution supply part  30  for supplying the processing bath  10  with a processing solution containing sulfuric acid and a hydrogen peroxide solution, a processing solution drainage part  40  for draining the processing solution out of the processing bath  10 , and a control part  50  for controlling the operation of each element in the substrate processing apparatus  1 . 
     The processing bath  10  is a storage container made from quartz or chemical-resistant resin. The processing bath  10  includes an inner bath  11  storing therein a processing solution into which the substrates W are dipped, and an outer bath  12  provided at the outer periphery of the inner bath  11 . The inner bath  11  has a bottom plate  11   a  located under the substrates W when the substrates W are immersed in the processing solution, and side walls  11   b  to  11   e  located alongside the substrates W. The top of the inner bath  11  is open. The outer bath  12  is shaped into a gutter, and extends along the outer surfaces of the side walls  11   b  to  11   e  of the inner bath  11 . 
     Of the side walls  11   b  to  11   e  of the inner bath  11 , the pair of side walls  11   b  and  11   d  extending in parallel with the direction in which the substrates W are arranged project outward at their lower end portions (portions contacting the bottom plate  11   a ). These projections form a pair of recesses  13   b  and  13   d  in the inner surfaces of the lower end portions of the side walls  11   b  and  11   d,  and which extend in the direction in which the substrates W are arranged. The pair of recesses  13   b  and  13   d  are each a slot of substantially V-shape in cross section with an opening facing the inner side of the inner bath  11 . 
     A pair of tubular nozzles (hereinafter referred to as “lower nozzles”)  14   b  and  14   d  are provided near the pair of recesses  13   b  and  13   d.  The lower nozzles  14   b  and  14   d  horizontally extend along the recesses  13   b  and  13   d  (namely, in the direction in which the substrates W are arranged). The lower nozzles  14   b  and  14   d  are each provided with a plurality of discharge holes  141  evenly spaced in the direction in which the substrates W are arranged. 
     As shown in  FIG. 2 , the plurality of discharge holes  141  provided to the lower nozzle  14   b  are at positions defined in the X-axis direction that correspond to the positions of gaps between the substrates W held on the lifter  20 , and the positions outside the substrates W at the opposite ends. Although not shown in  FIG. 2 , the plurality of discharge holes  141  provided to the other lower nozzle  14   d  are also at positions defined in the X-axis direction that correspond to the positions of the gaps between the substrates W held on the lifter  20 , and the positions outside the substrates W at the opposite ends. 
       FIGS. 3 and 4  are longitudinal sectional views, showing the lower nozzles  14   b  and  14   d  and their vicinities respectively in enlarged manner. As shown in  FIGS. 3 and 4 , discharges through the discharge holes  141  formed in the lower nozzles  14   b  and  14   d  are both directed downward and slightly inward of the inner bath  11 . Thus, a processing solution supplied to the lower nozzles  14   b  and  14   d  is discharged through the discharge holes  141  toward the upper surface of the bottom plate  11   a  of the inner bath  11 . The processing solution supplied onto the upper surface of the bottom plate  11   a  spreads across the upper surface of the bottom plate  11   a,  and thereafter flows upward toward the substrates W held on the lifter  20 . 
     While  FIGS. 3 and 4  each show only one discharge hole  141 , the other discharge holes  141  formed in the lower nozzles  14   b  and  14   d  are also directed downward and slightly inward of the inner bath  11 . 
     Turning back to  FIGS. 1 and 2 , a pair of tubular nozzles (hereinafter referred to as “upper nozzles”)  15   b  and  15   d  are provided above the pair of lower nozzles  14   b  and  14   d.  The pair of upper nozzles  15   b  and  15   d  are fixed to the pair of side walls  11   b  and  11   d  respectively, each in a position horizontally extending in the direction in which the substrates W are arranged. The pair of upper nozzles  15   b  and  15   d  are each provided with a plurality of discharge holes  151  evenly spaced in the direction in which the substrates W are arranged. 
     As shown in  FIG. 2 , the plurality of discharge holes  151  provided to the upper nozzle  15   b  are at positions defined in the X-axis direction that correspond to the positions of the gaps between the substrates W held on the lifter  20 , and the positions outside the substrates W at the opposite ends. Although not shown in  FIG. 2 , the plurality of discharge holes  151  provided to the other upper nozzle  15   d  are also at positions defined in the X-axis direction that correspond to the positions of the gaps between the substrates W held on the lifter  20 , and the positions outside the substrates W at the opposite ends. 
     As shown in  FIG. 1 , discharges through the discharge holes  151  formed in the upper nozzles  15   b  and  15   d  are both directed toward the contact points between holding bars  21  of the lifter  20  discussed next and the peripheries of the substrates W. 
     The lifter  20  is a transport mechanism for moving up and down the substrates W between positions in the inner bath  11  and positions above the inner bath  11  while holding thereon the substrates W. The lifter  20  has three holding bars  21  extending in the direction in which the substrates W are arranged, and a back plate  22  to which the holding bars  21  are fixed. With the substrates W engaged at their peripheries with a plurality of notches (not shown) defined in the three holding bars  21 , the lifter  20  holds the substrates W on the three holding bars  21  arranged in parallel with each other in upright positions. 
     With reference to  FIG. 2 , the lifter  20  has an up and down mechanism  23  connected to the back plate  22 . The up and down mechanism  23  is realized by a publicly known mechanism formed for example from a combination of a motor and a ball screw. When the up and down mechanism  23  is brought into operation, the back plate  22 , the three holding bars  21  and the substrates W held on the three holding bars  21  together move up and down. The substrates W are thereby transferred between positions immersed in the inner bath  11  (positions shown in  FIGS. 1 and 2 ), and positions raised above the inner bath  11 . 
     The processing solution supply part  30  is a solution supply system for supplying a processing solution containing sulfuric acid and a hydrogen peroxide solution to the lower nozzles  14   b,    14   d  and the upper nozzles  15   b,    15   d.  As shown in  FIG. 1 , the processing solution supply part  30  includes a sulfuric acid supply source  31 , a hydrogen peroxide solution supply source  32 , pipes  33   a  to  33   i,  and on-off valves  34  and  35 . 
     The sulfuric acid supply source  31  and the hydrogen peroxide solution supply source  32  are fluidly connected to the main pipe  33   c  through the pipes  33   a  and  33   b  respectively. The on-off valves  34  and  35  are interposed in the pipes  33   a  and  33   b  respectively. The end of the main pipe  33   c  on the downstream side is fluidly connected to the pipe  33   d  and  33   e.  The end of the pipe  33   d  on the downstream side is fluidly connected through the pipes  33   f  and  33   g  to the lower nozzle  14   b  and the upper nozzle  15   b  respectively. The end of the pipe  33   e  on the downstream side is fluidly connected through the pipes  33   h  and  33   i  to the lower nozzle  14   d  and the upper nozzle  15   d  respectively. 
     When the on-off valves  34  and  35  are opened in the processing solution supply part  30 , sulfuric acid supplied from the sulfuric acid supply source  31  and a hydrogen peroxide solution supplied from the hydrogen peroxide solution supply source  32  are mixed in the main pipe  33   c  to generate a processing solution. The processing solution thereby formed is supplied through the pipes  33   d  to  33   i  to the lower nozzles  14   b,    14   d  and the upper nozzles  15   b,    15   d.  Then, the processing solution is discharged to the inside of the inner bath  11  through the plurality of discharge holes  141  formed in the lower nozzles  14   b  and  14   d,  and through the plurality of discharge holes  151  formed in the upper nozzles  15   d  and  15   d.    
     The processing solution discharged from the lower nozzles  14   b,    14   d  and the upper nozzles  15   b,    15   d  is stored in the inner bath  11 . With the processing solution reaching the upper edge of the inner bath  11 , the processing solution may be further discharged from the lower nozzles  14   b,    14   d  and the upper nozzles  15   b,    15   d.  In this case, the processing solution flows over the upper edge of the inner bath  11 , and an overflow of the processing solution is collected by the outer bath  12 . 
     A pressure at which a processing solution is supplied to each of the lower nozzles  14   b,    14   d  and the upper nozzles  15   b,    15   d  is about 0.05 to 0.1 MPa, for example. 
     The processing solution drainage part  40  is a solution drainage system for releasing a processing solution stored in the outer bath  12  to a drainage line in a factory. As shown in  FIG. 1 , the processing solution drainage part  40  has a pipe  41  for making the connection between the outer bath  12  and the drainage line, and an on-off valve  42  interposed in the pipe  41 . When the on-off valve  42  is opened, a processing solution is released from the outer bath  12 , passes through the pipe  41 , and is then poured into the drainage line. 
     The control part  50  is an information processing part for controlling the operation of each element of the substrate processing apparatus  1 . The control part  50  is realized by a computer for example with a CPU and a memory. As shown in  FIGS. 1  and the  2 , the control part  50  is electrically connected to the up and down mechanism  23 , and to the on-off valves  34 ,  35  and  42 . The control part  50  gives instructions to the up and down mechanism  23  and to the on-off valves  34 ,  35  and  42  according a previously installed program and various input signals to control the operations thereof, thereby encouraging the processing of the substrates W. 
     &lt;2. Discharge Holes of Lower Nozzles&gt; 
     Next, the plurality of discharge holes  141  formed in the lower nozzles  14   b  and  14   d  are discussed in more detail. 
     As shown in  FIGS. 3 and 4 , the plurality of discharge holes  141  formed in the lower nozzles  14   b  and  14   d  are both directed downward and slightly inward of the inner bath  11 . If an angle θ formed by the direction of discharge through the discharge holes  141  and a normal N to the upper surface of the bottom plate  11   a  is too small, a processing solution discharged through the discharge holes  141  reaches the upper surface of the bottom plate  11  a at an angle nearly perpendicular to the upper surface of the bottom plate  11   a.  This considerably reduces the flow pressure of the processing solution, causing difficulty in pouring the processing solution into the gaps between the substrates W. Meanwhile, if the angle θ formed by the direction of discharge through the discharge holes  141  and the normal N to the upper surface of the bottom plate  11   a  is too large, a processing solution discharged through the discharge holes  141  reaches the upper surface of the bottom plate  11   a  at an angle nearly parallel to the upper surface of the bottom plate  11   a.  This does not reduce the flow pressure of the processing solution much, thereby generating the flow of the processing solution of relatively high speed in the inner bath  11 . In this case, part of the processing solution forming the high-speed flow circulates in the inner bath  11 , thereby generating a relatively wide circulation area CA of the processing solution (see  FIGS. 8 and 9 ) in the inner bath  11 . 
     From these points of view, in the substrate processing apparatus  1  of the present preferred embodiment, the direction of the discharge holes  141  is so controlled that it forms the angle θ of 5 degrees to 40 degrees slanting inward with respect to the normal N to the upper surface of the bottom plate  11   a.  Thus, the flow pressure of a processing solution discharged through the discharge holes  141  is not excessively reduced. Further, the circulation area CA does not expand widely in the inner bath  11 . As a result, the substrate processing apparatus  1  of the present preferred embodiment is capable of effectively displacing the processing solution in the inner bath  11  while smoothly pouring the processing solution into the gaps between the substrates W. 
       FIGS. 5 to 9  show the flows of a processing solution in the inner bath  11  when the angle θ of the discharge holes  141  of the pair of lower nozzles  14   b  and  14   d  is set at −50 degrees, 0 degrees, 20 degrees, 50 degrees and 70 degrees respectively.  FIGS. 5 to 9  schematically provide simulation results obtained by using general-purpose thermo-fluid analysis software. With reference to  FIGS. 5 and 9 , the circulation area CA of a processing solution formed in the inner bath  11  is greater as the angle θ of the discharge holes  141  with respect to the normal N to the bottom plate  11   a  is larger. The circulation area CA is smaller as the angle θ of the discharge holes  141  with respect to the normal N to the bottom plate  11   a  is smaller. Especially when the angle θ of the discharge holes  141  is set at 50 degrees and 70 degrees as shown in  FIGS. 8 and 9 , the circulation area CA expands widely above the midpoint of the height of the inner bath  11 . Meanwhile, when the angle θ of the discharge holes  141  is set at 20 degrees as shown  FIG. 7 , the range of the circulation area CA is relatively small. 
     In the present preferred embodiment, the angle θ of the discharge holes  141  with respect to the normal N to the upper surface of the bottom plate  11   a  is set at 40 degrees or smaller according to the simulation results discussed above. This reduces the circulation area CA of a processing solution formed in the inner bath  11  to a relatively small range while efficiently draining the processing solution out of the inner bath  11  from its upper edge. As a result, a processing solution in the inner bath  11  is displaced efficiently. The angle θ of the discharge holes  141  is preferably as small as possible in terms of reducing the range of the circulation area CA. Accordingly, the angle θ of the discharge holes  141  with respect to the normal N to the upper surface of the bottom plate  11   a  is desirably 35 degrees or smaller, and is more desirably 30 degrees or smaller. 
       FIGS. 10 to 14  show the speed at which a processing solution flows in each region defined in the inner bath  11  when the angle θ of the discharge holes  141  of the pair of lower nozzles  14   b  and  14   d  is set at −50 degrees, 0 degrees, 20 degrees, 50 degrees and 70 degrees respectively. In  FIGS. 10 to 14 , the space in the inner bath  11  is divided into regions according to the speed at which a processing solution flows therein. The speed of the flow is A, B or C listed in order of decreasing speed.  FIGS. 10 to 14  also schematically provide simulation results obtained by using general-purpose thermo-fluid analysis software. 
     With reference to  FIGS. 10 to 14 , the speed at which a processing solution flows in a region in which the substrates W are arranged (region indicated as “WA” in  FIGS. 10 to 14 ), namely, the speed at which the processing solution flows through the gaps between the substrates W is lower as the angle θ of the discharge holes  141  with respect to the normal N to the bottom plate  11   a  is smaller. The speed is higher as the angle θ of the discharge holes  141  is larger. Especially when the angle θ of the discharge holes  141  is set at −50 degrees and 0 degrees as shown in  FIGS. 10 and 11 , a processing solution flows through the gaps between the substrates W at a speed that is substantially “C”. Meanwhile, when the angle θ of the discharge holes  141  is set at 20 degrees as shown  FIG. 12 , a region in which the processing solution flows through the gaps between the substrates W at the speed “B” covers a certain range. 
     With reference to  FIGS. 10 and 11 , regions in which a processing solution flows at the speed “A” extend outside the region in which the substrates W are arranged. Namely, in the cases of  FIGS. 10 and 11 , much of the processing solution discharged through the discharge holes  141  flows out to the regions outside the substrates W. Thus, the processing solution may have difficulty in flowing into the gaps between the substrates W. In contrast, as shown in  FIG. 12 , the processing solution flows at the speed “B” or “C” in regions outside the region in which the substrates W are arranged. Thus, in the case of  FIG. 12 , a smaller amount of processing solution flows out to the regions outside the substrates W, while a larger amount of processing solution flows into the gaps between the substrates W. 
     In the present preferred embodiment, the angle θ of the discharge holes  141  with respect to the normal N to the upper surface of the bottom plate  11   a  is set at 5 degrees or larger according to the simulation results discussed above. This smoothly pours a processing solution into the gaps between the substrates W to thereby efficiently process the substrates W. The angle θ of the discharge holes  141  is preferably as large as possible in terms of pouring the processing solution into the gaps between the substrates W. Accordingly, the angle θ of the discharge holes  141  with respect to the normal N to the upper surface of the bottom plate  11   a  is desirably 10 degrees or larger, and is more desirably 15 degrees or larger. 
     Next, the opening diameter of the discharge holes  141  is discussed. As shown in  FIG. 2 , the processing solution supply part  30  is connected to one end (the end on the −X side) of each of the lower nozzles  14   b  and  14   d.  A processing solution introduced from the ends of the lower nozzles  14   b  and  14   d  flows through the inside of each of the lower nozzles  14   b  and  14   d,  and is discharged through the plurality of discharge holes  141 . If the opening diameter of the discharge holes  141  is too large, a significant difference may be made between the pressure at which the processing solution is discharged through the discharge holes  141  on the upstream side and the pressure at which the processing solution is discharged through the discharge holes  141  on the downstream side. This causes difficulty in uniformly processing the substrates W. Meanwhile, if the opening diameter of the discharge holes  141  is too small, a high pressure loss of the processing solution may be generated at each discharge hole  141 . This causes difficulty in generating a desirable fluid flow in the inner bath  11 . 
     In light of the above, in the substrate processing apparatus  1  of the present preferred embodiment, the opening diameter (diameter) of the discharge holes  141  is set within a range of 0.5 mm to 1.5 mm. This does not generate an excessively large difference between the pressure at which a processing solution is discharged through the discharge holes  141  on the upstream side and the pressure at which the processing solution is discharged through the discharge holes  141  on the downstream side. Further, there will be no high pressure loss of the processing solution at each discharge hole  141 . Thus, the substrate processing apparatus  1  of the present preferred embodiment is capable of uniformly and smoothly processing the substrates W while generating a desirable fluid flow in the inner bath  11 . 
       FIGS. 15 and 16  show the pressure of a processing solution discharged through the plurality of discharge holes  141  of the lower nozzles  14   b  and  14   d  when the opening diameter of the discharge holes  141  are set at 1.6 mm and 1.2 mm respectively. With reference to  FIGS. 15 and 16 , the discharge pressure at the plurality of discharge holes  141  exhibits a relatively wide range of variation as shown in  FIG. 15  in which the opening diameter of the discharge holes  141  is set at 1.6 mm. Meanwhile, as shown in  FIG. 16  in which the opening diameter of the discharge holes  141  is set at 1.2 mm, the discharge pressure at the plurality of discharge holes  141  exhibits a relatively small range of variation. In the present preferred embodiment, the opening diameter of the plurality of discharge holes  141  of the lower nozzles  14   b  and  14   d  is set at 1.5 mm or smaller according to the results discussed above. 
     &lt;3. Operation of Substrate Processing Apparatus&gt; 
     Next, the operation of the above-mentioned substrate processing apparatus  1  for processing the substrates W is discussed with reference to the flow diagram of  FIG. 17 . The control part  50  controls each element of the substrate processing apparatus  1  according to a previously installed program and various input signals, thereby realizing a series of process steps discussed below. 
     In order to process the substrates W in the substrate processing apparatus  1 , the on-off valves  34 ,  35  and  42  are opened first. This initiates the supply of a processing solution containing sulfuric acid and a hydrogen peroxide solution to discharge the processing solution through the plurality of discharge holes  141  of the lower nozzles  14 b and  14   d,  and through the plurality of discharge holes  151  of the upper nozzles  15   b  and  15   d  to the inside of the inner bath  11  (step S 1 ). The processing solution thereby discharged is stored in the inner bath  11 , and will flow over the upper edge of the inner bath  11  to be collected by the outer bath  12  in due course. 
     Next, the substrates W transported to the substrate processing apparatus  1  by a certain transport mechanism from another device are transferred to the lifter  20  placed in standby at a position above the processing bath  10 . After the substrates W are placed on the three holding bars  21  of the lifter  20 , the substrate processing apparatus  1  brings the up and down mechanism  23  into operation to move the back plate  22  and the three holding bars  21  down, thereby dipping the substrates W into the processing solution stored in the inner bath  11  (step S 2 ). After the substrates W are dipped in the processing solution, photoresist films formed on the main surfaces of the substrates W are removed from the main surfaces of the substrates W by the action of Caro&#39;s acid in the processing solution. 
     At this time, the lower nozzles  14   b,    14   d  and the upper nozzles  15   b,    15   b  continue to discharge the processing solution in the inner bath  11 . In the present preferred embodiment, the lower nozzles  14   b  and  14   d  discharge the processing solution at the angle θ of 5 degrees to 40 degrees slanting inward with respect to the normal N to the upper surface of the bottom plate  11   a.  Thus, as discussed previously, the flow pressure of the discharged processing solution is not excessively reduced. Further, the circulation area CA of the processing solution does not expand widely in the inner bath  11 . As a result, the processing solution smoothly flows into the gaps between the substrates W while the processing solution in the inner bath  11  is effectively displaced. 
     In the present preferred embodiment, the lower nozzles  14   b  and  14   d  discharge the processing solution through the plurality of discharge holes  141  with the opening diameter within a range of 0.5 mm to 1.5 mm. As discussed previously, an excessively large difference is not generated between the pressure at which the processing solution is discharged through the discharge holes  141  on the upstream side and the pressure at which the processing solution is discharged through the discharge holes  141  on the downstream side. Further, there will be no high pressure loss of the processing solution at each discharge hole  141 . Thus, the substrates W are uniformly and smoothly processed while a desirable fluid flow is generated in the inner bath  11 . 
     Part of the processing solution discharged through the discharge holes  141  of the lower nozzle  14   b  diffuses toward the side wall  11   b  as shown in  FIG. 18 . In the present preferred embodiment, by the presence of the recess  13   b  of the side wall  11   b  defined near the lower nozzle  14   b,  the processing solution having diffused toward the side wall  11   b  passes through the recess  13   b  to easily flow up in the processing bath  11 . Thus, the processing solution does not stay between the lower nozzle  14   b  and the side wall  11   b.  Likewise, the recess  13   d  is defined in the side wall  11   d  near the other lower nozzle  14   d.  Thus, the processing solution does not stay between the lower nozzle  14   d  and the side wall  11   d.    
     In the present preferred embodiment, the upper nozzles  15   b  and  15   d  discharge the processing solution toward the contact points between the holding bars  21  of the lifter  20  and the peripheries of the substrates W. This prevents the retention of particles or components to be removed at the contact points between the holding bars  21  and the peripheries of the substrates W. 
     In the present preferred embodiment, the plurality of discharge holes  141  of the lower nozzles  14   b  and  14   d,  and the plurality of discharge holes  151  of the upper nozzles  15   b  and  15   d  are both at positions defined in the X-axis direction that correspond to the positions of the gaps between the substrates W held on the lifter  20 , and the positions outside the substrates W at the opposite ends. Thus, the processing solution is smoothly supplied to each of the substrates W. 
     After process steps in a certain period of time are completed, the substrate processing apparatus  1  brings the up and down mechanism  23  into operation to move the back plate  22  and the three holding bars  23  up, thereby raising the substrates W up from the processing solution stored in the inner bath  11  (step S 3 ). Thereafter the substrates W are transferred from the lifter  20  to a certain transport mechanism, and are transported to a device responsible for a subsequent process. The substrate processing apparatus  1  closes the on-off valves  34 ,  35  and  42 . As a result, the lower nozzles  14   b,    14   d  and the upper nozzles  15   b,    15   d  stop the discharge of the processing solution, and the processing solution drainage part  40  stops the drainage of the processing solution (step S 4 ). The series of process steps for a set of substrates W are thereby completed. 
     &lt;4. Modifications&gt; 
     The present invention is not limited to the preferred embodiment discussed above. By way of example, the substrate processing apparatus  1  of the above-discussed preferred embodiment includes the lower nozzles  14   b,    14   d  and the upper nozzles  15   d,    15   d.  Alternatively, the upper nozzles  15   b  and  15   d  may be omitted, and a processing solution may be discharged only from the lower nozzles  14   b  and  14   d.    
     The processing solution used in the preferred embodiment discussed above contains sulfuric acid and a hydrogen peroxide solution. The substrate processing apparatus of the present invention may use an alternative solution. By way of example, a processing solution may contain hydrofluoric acid, or may be an SC-1 solution or an SC-2 solution. Deionized water may also be employed as a processing solution. 
     In the preferred embodiment discussed above, the substrates W to be processed are semiconductor wafers. Alternatively, other types of substrates such as glass substrates for photomasks, glass substrates for liquid crystal displays and the like may also be processed in the substrate processing apparatus of the present invention. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.