Patent Publication Number: US-2022230893-A1

Title: Substrate processing method and substrate processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority from Japanese Patent Application No. 2021-005966 filed on Jan. 18, 2021 with the Japan Patent Office, the disclosure of which is incorporated herein in its entirety by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a substrate processing method and a substrate processing apparatus. 
     BACKGROUND 
     A process for manufacturing a semiconductor device includes a liquid processing in which a processing liquid such as a chemical liquid for etching or cleaning is supplied to a substrate to perform a predetermined liquid processing on the substrate. In order to reduce the particles remaining on the substrate after the liquid processing, it is required to reduce the particles contained in the processing liquid supplied to the substrate. The particles contained in the processing liquid include the particles generated from a control valve that adjusts the flow rate of the processing liquid supplied to a liquid processing unit, and Japanese Patent Laid-Open Publication No. 2015-041751 discloses a substrate processing apparatus equipped with means for reducing such particles. 
     The substrate processing apparatus disclosed in Japanese Patent Laid-Open Publication No. 2015-041751 includes a first line connected to a processing liquid supply source, a pump that sends the processing liquid from the processing liquid supply source to the first line, a plurality of second lines that is connected to the first line and into which the processing liquid flowing through the first line flows, a branch line connected to a branch point on each second line, a liquid processing unit that performs a processing on a substrate with the processing liquid supplied via each branch line, an orifice provided on the upstream side of the branch point on each second line, and a first control valve provided on the downstream side of the branch point on each second line. The first control valve changes the amount of processing liquid that flows to the downstream side of the first control valve to control the pressure of the processing liquid in the section of the corresponding second line between the orifice and the first control valve, and control the flow rate of the processing liquid supplied to the corresponding liquid processing unit via the corresponding branch line. According to the above configuration, since it is not necessary to interpose a control valve for adjusting the flow rate in the branch line, particles that may be generated from such a control valve do not flow to the liquid processing unit. 
     SUMMARY 
     A substrate processing method according to an embodiment includes: supplying a pre-wet liquid to a substrate while the substrate is being rotated, and forming a liquid film of the pre-wet liquid on a surface of the substrate; after the forming of the liquid film of the pre-wet liquid, supplying a chemical liquid to the substrate at a first flow rate while the substrate is being rotated at a first rotation speed, and performing a first chemical liquid processing on the substrate with the chemical liquid to form a liquid film of the chemical liquid having a first thickness on the surface of the substrate; and after the first chemical liquid processing, supplying the chemical liquid to the substrate at a second flow rate to the substrate while the substrate is being rotated at a second rotation speed, and performing a second chemical liquid processing on the substrate with the chemical liquid to form a liquid film of the chemical liquid having a second thickness on the surface of the substrate. The pre-wet liquid is a liquid different from the chemical liquid and having a higher cleanliness than the chemical liquid used at the processings of the substrate with the chemical liquid. The first flow rate is larger than the second flow rate, and the first thickness is thicker than the second thickness. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating a configuration of a substrate processing system according to an embodiment of a substrate processing apparatus. 
         FIG. 2  is a schematic cross-sectional view illustrating a configuration of a processing liquid supply mechanism of the substrate processing apparatus illustrated in  FIG. 1 . 
         FIG. 3  is a schematic piping system view illustrating a configuration of the processing liquid supply mechanism related to one processing unit illustrated in  FIG. 2 . 
         FIG. 4  illustrates time charts illustrating a rotation speed of a substrate and a discharge flow rate of a processing liquid in each step of the liquid processing in an embodiment. 
         FIGS. 5A to 5H  are schematic views illustrating a discharge situation of the processing liquid in each step of the liquid processing. 
         FIG. 6  is a schematic view illustrating an initial state of an initial thick film forming step. 
         FIG. 7  is a schematic view illustrating a final state of the initial thick film forming step. 
         FIG. 8  is a schematic view illustrating a flow velocity distribution of a chemical liquid in a liquid film of the chemical liquid. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawing, which form a part hereof. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made without departing from the spirit or scope of the subject matter presented here. 
     An embodiment of a substrate processing apparatus will be described with reference to the accompanying drawings. 
       FIG. 1  is a view illustrating a schematic configuration of a substrate processing system according to the embodiment. In the following, in order to clarify positional relationships, the X-axis, Y-axis and Z-axis are defined as being orthogonal to each other. The positive Z-axis direction is regarded as a vertically upward direction. 
     As illustrated in  FIG. 1 , a substrate processing system  1  includes a carry-in/out station  2  and a processing station  3 . The carry-in/out station  2  and the processing station  3  are provided adjacent to each other. 
     The carry-in/out station  2  is provided with a carrier placing section  11  and a transfer section  12 . In the carrier placing section  11 , a plurality of carriers C is placed to accommodate a plurality of substrates (e.g., substrates W such as semiconductor wafers in the present embodiment) in a horizontal state. 
     The transfer section  12  is provided adjacent to the carrier placing section  11 , and provided with a substrate transfer device  13  and a delivery unit  14  therein. The substrate transfer device  13  is provided with a substrate holding mechanism that holds the substrate W. Further, the substrate transfer device  13  is movable horizontally and vertically and pivotable around a vertical axis, and transfers the substrate W between the carrier C and the delivery unit  14  by using the substrate holding mechanism. 
     The processing station  3  is provided adjacent to the transfer section  12 . The processing station  3  is provided with a transfer section  15  and a plurality of processing units  16 . The plurality of processing units  16  are provided at both sides of the transfer section  15 . 
     The transfer section  15  is provided with a substrate transfer device  17  therein. The substrate transfer device  17  is provided with a substrate holding mechanism that holds the substrate W. Further, the substrate transfer device  17  is movable horizontally and vertically and pivotable around a vertical axis, and transfers the substrate W between the delivery unit  14  and the processing units  16  by using the substrate holding mechanism. 
     The processing unit  16  performs a predetermined substrate processing on the substrate W transferred by the substrate transfer device  17 . 
     Further, the substrate processing system  1  includes a control device  4 . The control device  4  is, for example, a computer, and includes a controller  18  and a storage unit  19 . The storage unit  19  stores a program that controls various processings performed in the substrate processing system  1 . The controller  18  controls the operation of the substrate processing system  1  by reading and executing the program stored in the storage unit  19 . 
     The program may be recorded in a computer-readable recording medium, and installed from the recording medium to the storage unit  19  of the control device  4 . The computer-readable recording medium may be, for example, a hard disk (HD), a flexible disk (FD), a compact disk (CD), a magnet optical disk (MO), or a memory card. 
     In the substrate processing apparatus  1  configured as described above, the substrate transfer device  13  of the carry-in/out station  2  first takes out the substrate W from the carrier C placed in the carrier placing section  11 , and then, places the taken-out substrate W on the delivery unit  14 . The substrate W placed on the delivery unit  14  is taken out from the delivery unit  14  by the substrate transfer device  17  of the processing station  3 , and carried into the processing unit  16 . 
     The substrate W carried into the processing unit  16  is processed by the processing unit  16 , and then, carried out from the processing unit  16  and placed on the delivery unit  14  by the substrate transfer device  17 . Then, the processed substrate W placed on the delivery unit  14  is returned to the carrier C of the carrier placing section  11  by the substrate transfer device  13 . 
     Next, a processing fluid supply system configured to supply a processing liquid to the plurality of processing units  16  will be described with reference to  FIGS. 2 and 3 . The processing liquid supply system includes a chemical liquid supply system, and a piping system of the chemical liquid supply system is illustrated in  FIG. 2 . 
     The chemical liquid supply system  30  includes a tank  32  that stores a chemical liquid, and a circulation line  34  that comes out of the tank  32  and returns to the tank  32 . A pump  36  is provided in the circulation line  34 . The pump  36  forms a circulating flow that comes out of the tank  32  and returns to the tank  32  through the circulation line  34 . The circulation line  34  is provided with a filter  38  that removes contaminants such as particles contained in the chemical liquid, and a temperature regulator  40  that regulates the temperature of the chemical liquid to a predetermined temperature. When the chemical liquid is supplied to the substrate W at room temperature, a temperature regulator  40  that performs cooling and heating using a Peltier device may be used. 
     A plurality of branch lines (also referred to as “dispense lines”)  44  is connected in parallel to connection areas  42  set in the circulation line  34 . Each branch line  44  supplies the chemical liquid flowing in the circulation line  34  to the corresponding processing unit  16 . In order to stabilize the pressure in the connection area  42  of the circulation line  34 , a constant pressure valve  43  is provided on the downstream side of the connection area  42 . 
     The chemical liquid supply system  30  further includes a tank liquid replenishing unit  46  that replenishes the chemical liquid or the chemical liquid constituent component in the tank  32 . A drain unit  48  is provided in the tank  32  to discard the chemical liquid within the tank  32 . 
     As illustrated in  FIG. 3 , one branch line  44  is provided with a flow meter  50 , a constant pressure valve  52  as a flow control valve, and an opening/closing valve  54  in this order from the upstream side. A chemical liquid nozzle  56  is provided at the downstream end of the branch line  44 . A drain line  58  is branched from the branch line  44  between the opening/closing valve  54  and the chemical liquid nozzle  56 . An opening/closing valve  60  is provided in the drain line  58 . 
     The flow meter  50  and the constant pressure valve  52  constitute a flow rate adjusting unit that adjusts the flow rate of the chemical liquid flowing through the branch line  44 . The constant pressure valve  52  operates such that a secondary-side pressure is implemented according to an operation pressure (air pressure) supplied to a pilot port of the constant pressure valve  52  from an electropneumatic regulator (not illustrated). The operation pressure supplied to the pilot port of the constant pressure valve  52  is feedback-controlled by a control device (the control device  4  in  FIG. 1  or its subordinate controller) such that a detected flow rate of the flow meter  50  becomes a desired value (set value). 
     The chemical liquid nozzle  56  is supported by the tip of a nozzle arm  62  schematically illustrated in  FIG. 3 . A rinse nozzle  64  and an IPA nozzle  66  are also supported by the tip of the nozzle arm  62 . At least one of the rinse nozzle  64  and the IPA nozzle  66  may be supported by a nozzle arm (not illustrated) other than the nozzle arm  62 . 
     A rinse liquid (e.g., DIW) may be supplied at a controlled flow rate from a rinse liquid supply source  65 A to the rinse nozzle  64  via a rinse liquid line  65 C provided with a rinse liquid supply controller  65 B. The rinse liquid supply controller  65 B includes, for example, an opening/closing valve and a flow rate control valve (both not illustrated) configured to switch between the supply of the rinse liquid and stop of the supply and to control the supply flow rate of the rinse liquid. 
     IPA (isopropyl alcohol) may be supplied from an IPA supply source  67 A to the IPA nozzle  66  via an IPA line  67 C provided with an IPA supply controller  67 B at a controlled flow rate. The IPA supply controller  67 B includes, for example, an opening/closing valve and a flow rate control valve (both not illustrated) configured to switch between the supply of the IPA and stop of the supply and to control the supply flow rate of the IPA. 
     The processing unit  16  includes a spin chuck (substrate holding and rotating mechanism)  70 . The spin chuck  70  includes a substrate holder (chuck)  72  that holds the substrate W in a horizontal posture and a rotation driver  74  that rotates the substrate holder  72  and the substrate W held by the substrate holder  72  around a vertical axis. 
     The substrate holder  72  may be of a type called a mechanical chuck that mechanically holds the peripheral edge of the substrate W by a holding member such as a gripping claw, and of a type called a vacuum chuck that vacuum-adsorbs the central portion of the rear surface of the substrate W. The rotation driver  74  may be constituted by, for example, an electric motor. 
     A liquid receiving cup  76  is provided around the substrate holder to collect the processing liquid scattered from the rotating substrate W. The processing liquid collected by the liquid receiving cup  76  is discharged to the outside of the processing unit  16  through a drain port  78  provided in the bottom portion of the liquid receiving cup  76 . An exhaust port  80  is also provided in the bottom portion of the liquid receiving cup  76 , and the inside of the liquid receiving cup  76  is sucked through the exhaust port  80 . 
     The chemical liquid nozzle  56 , the rinse nozzle  64 , and the IPA nozzle  66  may be moved between a position directly above the central portion of the substrate W held by the spin chuck  70  (processing position) and a position outside (radially outside) the liquid receiving cup  76  by the nozzle arm  62  (retracted position). A dummy dispense port  82  is provided below the chemical liquid nozzle  56 , the rinse nozzle  64 , and the IPA nozzle  66  to receive the liquid ejected from the nozzles  56 ,  65 , and  66  at the retracted position. 
     Next, a series of steps of the liquid processing performed on the substrate W in the processing unit  16  will be described with reference to time charts in  FIG. 4  and schematic views in  FIGS. 5A to 5H . As an example, the substrate W is a semiconductor wafer, the liquid processing is a chemical liquid processing performed in BEOL (wiring process), and the chemical liquid used in the chemical liquid processing may be an organic chemical liquid including, for example, buffered fluoride (BHF), dilute fluoride (DHF), and ammonium fluoride (NH 4 F). All the processing liquids (e.g., pre-wet liquid or chemical liquid) used in the following liquid processing are at room temperature. The types of the liquid processing, the chemical liquid used, and the temperature of the processing liquid described above are illustrative, and are not limited to those described above. 
     In the time charts in  FIG. 4 , the upper time chart illustrates the rotation speed of the substrate W, and the lower time chart illustrates the ejection flow rate of the processing liquid (the broken line indicates chemical liquid, the solid line indicates pre-wet liquid and rinse liquid, and the alternate long and short dash line indicates IPA). 
     It should be noted that the embodiment is to prevent or suppress not only particles having a size required to be reduced in the related art, but also particles having a smaller size (hereinafter, referred to as “small size” in the present specification) (e.g., particles having a size of approximately 13 nm) from adhering to the substrate W. The particles having a small size may be generated, for example, at the opening/closing valve  54 , as a result of dust generation due to interference (e.g., friction) between the valve body and the valve seat when the valve body is separated from the valve seat or when the valve body is sat on the valve seat. Although dust generation may occur theoretically in other valves (e.g., flow rate control valve  52 ) having a movable portion provided on the branch line  44 , but the possibility of dust generation at the flow rate control valve  52  used in the processing liquid supply system of the semiconductor manufacturing apparatus is much lower than the case of the opening/closing valve  54 , and thus, the flow rate control valve  52  is not considered in the present specification. 
     [Chemical Liquid Dummy Dispense Step (Step S 1 )] 
     The substrate W is carried into the processing unit  16 , and held by the spin chuck  70 . When the substrate W is carried into the processing unit  16 , the chemical liquid nozzle  56  is positioned at the retracted position, that is, directly above the dummy dispense port  82 . From this state, first, the opening/closing valve  54  is opened to perform a dummy dispense in which the chemical liquid is ejected from the chemical liquid nozzle  56  toward the dummy dispense port  82  for a predetermined time (see  FIG. 5A ). 
     The dummy dispense is performed at least until all the chemical liquid staying in the section of the dispense line  44  from the opening/closing valve  54  to the chemical liquid nozzle  56  is ejected from the chemical liquid nozzle  56 . Specifically, for example, the dummy dispense is performed for 30 seconds with the ejection flow rate of the chemical liquid from the chemical liquid nozzle  56  set to 200 mL/min. The ejection flow rate of the chemical liquid may be increased to shorten the ejection time. 
     The dummy dispense may be performed at least until all the chemical liquid staying in the entire area of the branch line  44  (i.e., the section from the connecting point with the circulation line  34  to the chemical liquid nozzle  56 ) is ejected from the chemical liquid nozzle  56 . 
     Contaminants such as particles staying in the flow path inside the chemical liquid nozzle  56  or the branch line  44  or adhering to the liquid contact surface may be discharged by performing the dummy dispense. Further, by discharging at least all the chemical liquid staying in the section of the branch line  44  from the opening/closing valve  54  to the chemical liquid nozzle  56  from the chemical liquid nozzle  56  during the dummy dispense, it is possible to prevent the dust (particles) generated at the opening/closing valve  54  from remaining in the chemical liquid temporarily staying in the branch line  44  immediately after the dummy dispense is stopped. Further, even when the inside of the nozzle is contaminated by the atmosphere around the chemical liquid nozzle  56  by performing the dummy dispense, the contaminants may be discharged. 
     When the chemical liquid nozzle  56  and the rinse nozzle  64  are supported by another nozzle arm, at least a part of the period during which the chemical liquid dummy dispense step is performed may be overlapped with a period during which a pre-wet step described later is performed. 
     [Pre-Wet Step (Step S 2 )] 
     After the chemical liquid dummy dispense step is completed, the rinse nozzle  64  is positioned at the position directly above the central portion of the substrate W, and the substrate W is rotated at a relatively low rotation speed (e.g., 100 to 150 rpm). In this state, a clean liquid serving as a pre-wet liquid (here, deionized water (DIW)) is ejected from the rinse nozzle  64  at a large flow rate (e.g., 1,500 mL/min) (see  FIG. 5B ). The pre-wet step is performed, for example, for 5 seconds. The pre-wet liquid adhering to the central portion of the substrate W flows to be spread toward the peripheral edge of the substrate W by centrifugal force, whereby the entire surface of the substrate W is covered with the liquid film of the pre-wet liquid. 
     At the initial stage of discharging the pre-wet liquid, dust due to dust generation according to the opening of the opening/closing valve as described later may be contained in the pre-wet liquid, but the pre-wet liquid is supplied at a large flow rate, and thus, dust (particle-causing substance) hardly adheres to the surface of the substrate W. 
     As long as a relatively thick pre-wet liquid (DIW) liquid film (i.e., liquid film that does not easily disappear by shaking off) is uniformly formed on the entire surface of the substrate W, the rotation speed of the substrate W and the ejection flow rate of the pre-wet liquid in the pre-wet step are not limited to those described above. As an example, in a case where a hydrophobic surface and a hydrophilic surface are mixed on the surface of the substrate W, it is desirable to set the rotation speed of the substrate to be high from the viewpoint of the uniformity of the liquid film. In this case, the ejection flow rate of the pre-wet liquid may be increased to form a relatively thick liquid film. 
     In order to confirm the effect of the pre-wet step, the processing was actually performed under the conditions based on the embodiment described in the present specification (with the pre-wet step), and under the conditions different only in that there is no pre-wet step. As a result, the number of particles in the case where there is the pre-wet step is approximately 50% of the number of particles in the case where there is no pre-wet step. 
     Further, the processing under the conditions based on the embodiment described in the present specification is performed while changing the rotation speed of the substrate in the pre-wet step (ejection flow rate is fixed at 1,500 mL/min). As a result, it is confirmed that, as the rotation speed decreases (that is, the film thickness increases), the number of particles decreases. 
     Dilute ammonia water may also be used as the pre-wet liquid. Further, the dilute ammonia water may be used as a rinse liquid in a rinse step described later. In general, it is known that, since a zeta potential on a solid surface (both the substrate surface and the particle surface) becomes negative in an alkaline liquid, particles are less likely to adhere. Even in the embodiment in which the adhesion of particles is required to be suppressed, it is beneficial to use the dilute ammonia water as at least one of the pre-wet liquid and the rinse liquid. In practice, as a result of using the dilute ammonia water as the pre-wet liquid and the rinse liquid, it is confirmed that particles are further reduced as compared with the case where DIW is used. 
     In the present specification, the expression “the nozzle is positioned directly above the central portion of the substrate W” is not limited to the fact that the nozzle is positioned directly above the rotation center point of the substrate W. As long as the liquid ejected from the nozzle and adhering to the surface of the substrate W is spread to such an extent that the rotation center point of the substrate W may be wet immediately after the liquid adheres thereto, the nozzle may be deviated from the rotation center point of the substrate W. 
     In the embodiment, the “liquid film” is formed on the surface of the substrate W by the liquid adhering to the central portion of the rotating substrate W and flowing to be spread toward the peripheral edge of the substrate W by centrifugal force regardless of the type of the liquid. Basically, the thickness of the liquid film is determined according to the rotation speed of the substrate W and the ejection flow rate of the liquid from the nozzle. 
     DIW and dilute ammonia water used as the pre-wet liquid have a higher cleanliness than the chemical liquid. The term “higher cleanliness” means that the content of particle-causing substance (particles such as dust, meaning substances that generate particles by precipitation or the like) is small. 
     [Initial Thick Film Forming Step (Step S 3 )] 
     Next, the ejection of the pre-wet liquid (DIW) from the rinse nozzle  64  is stopped and the rinse nozzle  64  is retracted from the position directly above the central portion of the substrate W, so that the chemical liquid nozzle  56  is positioned at the position directly above the central portion of the substrate W. Then, the opening/closing valve  54  is opened to eject the chemical liquid from the chemical liquid nozzle  56  at an ejection flow rate (e.g., an intermediate flow rate of approximately 700 mL/min) larger than that of the main processing step to be subsequently performed (see  FIG. 5C ). The rotation speed of the substrate W is lowered to a value (e.g., extremely low speed of approximately 30 to 50 rpm) lower than that of the pre-wet step substantially at the same time when the liquid ejection is switched from the rinse nozzle  64  to the chemical liquid nozzle  56 . 
     An initial thick film forming step is performed at least until all the chemical liquid staying in the section of the dispense line  44  between the opening/closing valve  54  and the downstream side is ejected from the chemical liquid nozzle  56  until the time immediately before the initial thick film forming step starts. Since it is not desirable to increase the total ejection amount of the chemical liquid in the initial thick film forming step from viewpoint of saving the chemical liquid, the initial thick film forming step may be performed until all the chemical liquid staying in the section of the branch line  44  between the opening/closing valve  54  and the downstream side is exactly discharged. As an example, the initial thick film forming step is performed for approximately 10 seconds. 
     As schematically illustrated in  FIG. 6 , in the early period of the initial thick film forming step, the chemical liquid is supplied from the chemical liquid nozzle  56  onto the liquid film of the pre-wet liquid (DIW) formed by the pre-wet step and having a high cleanliness (containing substantially no contaminants). At this time, for example, since the valve body is moved with respect to the valve seat when the opening/closing valve  54  is opened, dust generation may occur, which may cause particles having a small size. The dust (particles) generated here is supplied to the substrate W together with the chemical liquid. The chemical liquid containing the dust falls on the liquid film of the pre-wet liquid that acts as a protective film. Although the chemical liquid and the pre-wet liquid are mutually diffused, but it is difficult for the dust in the chemical liquid to reach the surface of the substrate W, and the dust hardly adheres to the surface of the substrate W. That is, in the early period of the initial thick film forming step, the adhesion of the dust to the surface of the substrate is prevented by the protective effect of the liquid film of the pre-wet liquid. 
     With the lapse of time from the start of the initial thick film forming step, the pre-wet liquid is replaced with the chemical liquid, and finally, as schematically illustrated in  FIG. 7 , the chemical liquid is substantially only present on the surface of the substrate W. The initial thick film forming step is performed under the condition that all the chemical liquid staying in the section of the branch line  44  between the opening/closing valve  54  and the downstream side is completely ejected from the chemical liquid nozzle  56  at this time. At the time when all the chemical liquid staying in the section of the branch line  44  between the opening/closing valve  54  and the downstream side is completely ejected from the chemical liquid nozzle  56 , the dust derived from the valve is not contained or is hardly contained in the chemical liquid ejected from the chemical liquid nozzle  56  (see  FIG. 7 ). Even when the chemical liquid ejected from the chemical liquid nozzle  56  contains some dust, the chemical liquid is supplied at a relatively large flow rate (e.g., 700 mL/min) in the initial thick film forming step, so that the thickness of the film of the chemical liquid formed on the surface of the substrate W in the final period of the initial thick film forming step is relatively thick. As a result, the probability that the dust in the liquid film comes into contact with the surface of the substrate W is low, and the dust hardly adheres to the surface of the substrate W. 
     As illustrated in  FIG. 8 , the flow velocity of the liquid flowing from the central portion of the substrate W toward the peripheral edge is lower as it is closer to the surface of the substrate W, and higher as it is closer to the surface of the liquid film (the length of the arrow with the reference numeral V indicates the velocity). In the vicinity of the surface of the substrate where the flow velocity is low, the dust (particle-causing substance) contained in the chemical liquid is likely to adhere to the surface of the substrate W. When the thickness of the film of the chemical liquid is thin, most of the dust contained in the chemical liquid moves in the vicinity of the surface of the substrate W at a low speed, and thus, the dust easily adheres to the surface of the substrate W. Further, once the dust (particularly, having the small size described above) adheres to the surface of the substrate W, it is difficult to be separated from the surface of the substrate W even when the dust is exposed to a liquid flow at a low flow velocity. When the thickness of the film of the chemical liquid is thick, most of the dust contained in the chemical liquid flows at a relatively high speed toward the peripheral edge of the substrate W at a position relatively distant from the surface of the substrate, and thus, the amount of the dust adhering to the surface of the substrate W is reduced. That is, when the chemical liquid containing a certain amount of dust is supplied to the substrate W, the adhesion of the dust (particle) may be suppressed by setting a large flow rate and a rotation speed of the substrate W that forms a thick film thickness. In consideration of this, the ejection flow rate of the chemical liquid and the rotation speed of the substrate W in the initial thick film forming step are determined. 
     An experiment is conducted to consider the conditions of the initial thick film forming step. The rotation speed of the substrate is fixed at 50 rpm, and the ejection flow rate of the chemical liquid is changed from 200 to 700 mL/min. As a result, it may be confirmed that, as the ejection flow rate of the chemical liquid increases, the particle level adhering to the substrate decreases. Further, the ejection flow rate of the chemical liquid is fixed at 200 mL/min, and the rotation speed of the substrate is changed from 50 to 300 rpm. As a result, it may be confirmed that, as the rotation speed of the substrate decreases, the particle level adhering to the substrate decreases. From this, it may be said that, as the thickness of the liquid film formed in the initial thick film forming step increases, the particle level decreases. 
     [Main Processing Step (Chemical Liquid Processing) (Step S 4 )] 
     Next, while maintaining the rotation speed of the substrate W at an extremely low speed (e.g., the same speed as the initial thick film forming step), the ejection flow rate of the chemical liquid from the chemical liquid nozzle  56  is lowered to a small flow rate (e.g., 200 mL/min) to perform a main processing step (see  FIG. 5D ). At this time, the thickness of the liquid film on the surface of the substrate W becomes thin by reducing the ejection flow rate of the chemical liquid. Of course, the ejection flow rate of the chemical liquid is maintained at a value equal to or higher than the value that ensures the entire surface of the substrate W to be seamlessly covered with the liquid film of the chemical liquid. As an example, the main processing step is performed for approximately 70 to 80 seconds. 
     At the time when the main processing step is started, substantially all the dust (particles) derived from the valve is ejected from the chemical liquid nozzle, and thus, the cleanliness of the chemical liquid supplied to the substrate W is high as compared with that in the initial thick film forming step. As a result, even when the thickness of the liquid film becomes thin, the adhesion of the dust to the surface of the substrate W is prevented or sufficiently suppressed. The main processing step is performed until a desired chemical liquid processing on the surface of the substrate W is completed (e.g., until a desired etching amount is obtained). Since the ejection flow rate of the chemical liquid is suppressed to a low level in the main processing step, the processing cost in a case of using a particularly expensive chemical liquid may be reduced. 
     [Switching Step (Transition from Main Processing Step to Rinse Step) (Step S 5 )] 
     Next, while maintaining the ejection flow rate of the chemical liquid from the chemical liquid nozzle  56 , the rinse nozzle  64  is positioned at the position directly above the central portion of the substrate W, DIW (pure water) serving as the rinse liquid is ejected from the rinse nozzle  64  at an intermediate flow rate (e.g., 700 mL/min), and the rotation speed of the substrate is increased to a low speed (e.g., 200 rpm) (see  FIG. 5E ). At this time, the liquid may be suppressed from splashing by suppressing the ejection flow rate of the rinse liquid to a low level as described above. When liquids are supplied from two nozzles  56  and  64  at the same time, liquid splashing may occur due to interference of different liquid flows on the surface of the substrate W. The splashed liquid may be splashed again in the cup and adhere again to the surface of the substrate W, which may cause particles. However, the liquid splashing may be prevented by suppressing the ejection flow rate from each of the nozzles  56  and  64  to a low level. Further, by simultaneously supplying liquids from two nozzles  56  and  64 , the liquid film is not torn finely, and the surface of the substrate W is not exposed even when the ejection flow rate of the liquid from each nozzle is suppressed to a low level. The period during which the ejection of the liquid from the chemical liquid nozzle  56  and the rinse nozzle  64  is performed simultaneously may be set to, for example, 10 seconds. 
     [Rinse Step (Step S 6 )] 
     Next, the ejection of the chemical liquid from the chemical liquid nozzle  56  is stopped, and the flow rate of the rinse liquid from the rinse nozzle  64  is increased to a large flow rate (e.g., 1,500 mL/min). In addition, the rotation speed of the substrate is increased to a high speed (e.g., 1,000 rpm) (see  FIG. 5F ). The rinse step may be performed for a time required to sufficiently remove the chemical liquid used in the main processing step and the generated reaction products, for example, approximately 30 seconds. 
     As long as a region that is not covered with the liquid is not generated on the surface of the substrate W, the main processing step may proceed directly to the rinse step without performing the switching step. That is, the ejection of the DIW from the rinse nozzle  64  may be started simultaneously with, or substantially simultaneously with the stop of the ejection of the chemical liquid from the chemical liquid nozzle  56 . 
     [IPA Replacing Step (Step S 7 )] 
     While maintaining the rotation speed of the substrate W at a high speed (e.g., 1,000 rpm), the ejection of the DIW from the rinse nozzle  64  is stopped. In addition, the IPA nozzle  66  is positioned at the position directly above the central portion of the substrate W, and IPA is ejected from the IPA nozzle  66  at a small flow rate of, for example, approximately 100 mL/min for 0.5 seconds (see  FIG. 5G ). Thereafter, while reducing the rotation speed of the substrate to an intermediate speed (e.g., 300 rpm), and maintaining the ejection flow rate of the IPA from the IPA nozzle  66 , the IPA nozzle  66  reciprocates between the position directly above the central portion of the substrate W and the position directly above the peripheral edge of the substrate W (see  FIG. 5H ). Therefore, the DIW on the surface (including the inside of a recess of a pattern) of the substrate W is replaced with the IPA. 
     [Dry Step] 
     Next, the rotation speed of the substrate W is maintained or increases, and the ejection of the IPA from the IPA nozzle  66  is stopped. Therefore, the IPA is removed from the surface of the substrate W, and the substrate W is dried. The dry step is not illustrated in the time charts in  FIG. 4 . When the substrate W is dried, the rotation of the substrate W is stopped. Thus, a series of processings for the substrate is completed. Thereafter, the processed substrate W is carried out from the processing unit  16 . 
     Effect of Embodiment 
     When the chemical liquid is supplied to the rotating substrate at a large flow rate during the chemical liquid processing, a thick liquid film is formed. Thus, particles are less likely to adhere to the surface of the substrate for the reasons described above. However, when the chemical liquid is expensive, it is required to reduce the consumption amount of the chemical liquid. Examples of the method for reducing the consumption amount of the chemical liquid may include a method in which the amount of the chemical liquid itself supplied to the substrate is reduced, or a method in which the used chemical liquid is collected and reused. In the latter case, it often does not meet recent particle reduction demands. 
     In the embodiment, even when the chemical liquid containing particle-causing substances having a small size is ejected onto the surface of the substrate, the chances of the particle-causing substances coming into contact with (i.e., adhering to) the surface of the substrate are greatly reduced by performing the pre-wet step and the initial thick film forming step. In the main processing step (chemical liquid processing), the chemical liquid containing the particle-causing substances suppressed to a sufficiently low amount is supplied to the substrate. As a result, even when the thickness of the liquid film becomes thin (i.e., even when the supply amount of the chemical liquid is reduced), there is no possibility that the particles having a small size adhere to the substrate. According to the embodiment, as compared with the case where the chemical liquid is constantly ejected at a large flow rate to perform the chemical liquid processing, it is possible to reduce the total consumption amount of the chemical liquid while suppressing the level of the particle adhering to the substrate to the same or lower. 
     The substrate is not limited to a semiconductor wafer, but may be an arbitrary type of substrate used in a technical field of manufacturing a semiconductor device such as a glass substrate or a ceramic substrate. 
     According to the present disclosure, it is possible to reduce the particles remaining on the substrate after the liquid processing while reducing the consumption amount of the chemical liquid. 
     From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various Modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.