Abstract:
Provided is an apparatus and method for cleaning and drying a semiconductor wafer. Isopropyl alcohol and deionized water are premixed in a desired ratio before a cleaning solution containing isopropyl alcohol and deionized water is supplied into a treating bath. Accordingly, a chemical compound remaining due to deionized water can be effectively removed and the creation of water marks due to isopropyl alcohol can be effectively prevented. As a result, cleaning and drying effects can be increased and a cleaning solution can be reused.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims the priority of Korean Patent Application Nos. 2002-10969, 2002-59765, and 2002-59764 filed on February 28, October 1, and Oct. 1, 2002, respectively, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a method and apparatus for cleaning and drying a semiconductor wafer, and more particularly, to a method and apparatus for cleaning and drying a semiconductor wafer using a mixture of isopropyl alcohol and deionized water as a cleaning solution by which cleaning and drying effects can be increased due to premixing isopropyl alcohol and deionized water in an accurate ratio before supplying them into a treating bath in which a cleaning process is performed and the cleaning solution used in the cleaning process can be circulated and reused.  
           [0004]    2. Description of the Related Art  
           [0005]    Generally, most semiconductor wafer cleaning methods were developed in the 1970s, and use large amounts of chemical solutions. A typical cleaning method of removing pollutants remaining on a semiconductor wafer includes a standard clean-1 (SC1) process of removing particles and organic pollutants at an ambient temperature or a high temperature using a mixture of 1:4:20 of ammonia, hydrogen peroxide, and water and a standard clean-2 (SC2) process of removing transient metal pollutants at a high temperature using a mixture of 1:1:5 of hydrochloric acid, hydrogen peroxide, and water.  
           [0006]    In another cleaning method, a piranha cleaning process is first performed to remove an organic pollutant such as a sensitizer or a surfactant at a high temperature using a mixture of sulphuric acid and hydrogen peroxide and an HF cleaning process is lastly performed to effectively remove a natural oxide layer from the surface of a wafer, and simultaneously, a metal pollutant from the natural oxide layer.  
           [0007]    The above-described conventional cleaning methods have problems in that a chemical solution in a large amount is used during a cleaning process, and the chemical solution is mainly a mixture of an acid and a base, a base material of the mixture being a hydrogen peroxide, and thus wastewater cannot be easily treated. The use of a large amount of chemical solution and a large amount of cleaning water results in increasing cleaning costs and is environmentally regulated.  
           [0008]    Since most cleaning processes are performed at a high temperature, a chemical solution used for cleaning decomposes and evaporates, which decreases the life span of the cleaning solution and a cleaning effect. In addition, the reuse rate of the cleaning solution is reduced due to the excessive use of deionized water (DIW). Also, since the above-described conventional cleaning process is composed of multi-steps, a huge piece of equipment is necessary for performing the cleaning process.  
           [0009]    After such a cleaning process, a process of drying the wafer is performed. The drying process includes spin drying, isopropyl alcohol (IPA) vapor drying, IPA layer drying, and so forth. Spin drying is performed to remove a cleaning solution remaining on the wafer using a centrifugal force by spinning the wafer at a high speed. IPA vapor drying is performed so that IPA vapors contact the surface of the wafer, and a residual liquid such as moisture remaining on the surface of the wafer is substituted for IPA, and then removed. IPA drying is performed so that a wafer is dipped into a cleaning solution having a DIW layer and an IPA layer, DIW and IPA sequentially contact the wafer to clean residual liquid with DIW, and the residual liquid is substituted for IPA. IPA layer drying is also called marangoni drying.  
           [0010]    However, during spin drying, dust generated in the spinning apparatus may be adsorbed onto the semiconductor wafer due to the charging of the semiconductor wafer. In particular, moisture remains in the notch lines for dicing the semiconductor wafer into individual chips, trenches or contact holes, and creates water marks.  
           [0011]    During IPA vapor drying, since a vapor area is instable, IPA vapors cannot uniformly contact the semiconductor wafer. Thus, the possibility that water marks will be created is high. Also, since flammable IPA vapors are used, accidental fires may break out.  
           [0012]    Furthermore, marangoni drying can somewhat solve problems occurring during spin drying or IPA vapor drying. However, marangoni drying is not effective in removing various types of pollutants, such as particles, which may remain on the surface of the wafer.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, the present invention provides an apparatus and method for cleaning and drying a semiconductor wafer using a mixture of IPA and DIW as a cleaning solution by which cleaning and drying effects can be increased due to premixing IPA and DIW in an accurate ratio before supplying them into a treating bath in which a cleaning process is performed and the cleaning solution used in the cleaning process can be circulated and reused to prevent waste and reduce environmental pollution.  
           [0014]    According to an aspect of the present invention, there is provided an apparatus for cleaning and drying a semiconductor wafer. The apparatus includes a treating bath, a cleaning solution mixing unit, a cleaning solution supplying unit, and a return line. The treating bath is supplied with a cleaning solution from an external source and in which the semiconductor wafer is cleaned and dried. The cleaning solution mixing unit mixes cleaning solutions to be supplied into the treating bath in a predetermined ratio and includes isopropyl alcohol tanks that are supplied with isopropyl alcohol from an external source, mixing tanks that are supplied with deionized water from an external source, are connected to the isopropyl alcohol tanks to be supplied with isopropyl alcohol contained in the isopropyl alcohol tanks and mix isopropyl alcohol and deionized water, and level sensing means that sense the amount of isopropyl alcohol supplied into the IPA tanks and the amount of deionized water supplied into the mixing tanks. The cleaning solution supplying unit connects the mixing tanks to the treating bath and supplies the cleaning solution mixed in the mixing tanks into the treating bath. The return line returns the cleaning solution that was used in the cleaning of the semiconductor wafer back to the mixing tanks.  
           [0015]    Two or more isopropyl alcohol tanks of the cleaning solution mixing unit are interconnected in parallel, the mixing tanks are connected to the isopropyl alcohol tanks, respectively, and disposed in parallel so that each of the isopropyl alcohol tanks and each of the mixing tanks make a group, and the cleaning solution circulates through a mixing tank in a group selected from two or more groups.  
           [0016]    The level sensing means are a plurality of level sensors that are installed at the isopropyl alcohol tanks and a plurality of level sensors that are installed at the mixing tanks.  
           [0017]    The cleaning solution mixing unit further includes a nitrogen bubble generator that injects a nitrogen gas into the mixing tanks so as to completely mix deionized water and isopropyl alcohol.  
           [0018]    Isopropyl alcohol supplementing pumps additionally supply isopropyl alcohol in the isopropyl alcohol tanks into the mixing tanks by a desired amount are further installed between the isopropyl alcohol tanks and the mixing tanks.  
           [0019]    The cleaning solution supplying unit includes a circulating pump, a filter, an isopropyl alcohol concentration measurer, and a liquid particle counter. The circulating pump pumps the cleaning solution in the mixing tanks into the treating bath. The filter filters the cleaning solution flowing via the circulating pump off impurities. The isopropyl alcohol concentration measurer checks the concentration of isopropyl alcohol contained in the cleaning solution that has passed through the filter. The liquid particle counter detects the amount of liquid particle contained in the cleaning solution flowing into the treating bath.  
           [0020]    The return line includes a liquid particle counter that detects liquid particle contained in the cleaning solution returning from the treating bath back to the mixing tanks.  
           [0021]    The apparatus further includes a heating tank, an isopropyl alcohol filter, an isopropyl alcohol collecting tank, a deionized water filter, and a deionized water collecting tank. The heating tank is connected to the cleaning solution mixing unit, is supplied the cleaning solution from the mixing tanks, and heats the cleaning solution, and evaporates isopropyl alcohol from the cleaning solution using a difference between boiling points of isopropyl alcohol and deionized water to decompose the cleaning solution into pure isopropyl alcohol and deionized water. The isopropyl alcohol filter is connected to the heating tank and filters vapor isopropyl alcohol off impurities. The isopropyl alcohol collecting tank is supplied with isopropyl alcohol filtered through the isopropyl alcohol filter and condenses isopropyl alcohol. The deionized water filter is connected to the heating tank and passes deionized water remaining in the heating tank to filter impurities of deionized water. The deionized water collecting tank is supplied with deionized water filtered through the deionized water filter and temporarily stores deionized water.  
           [0022]    A sink that is supplied with deionized water from an external source and sinks a lower portion of the treating bath under deionized water is further installed beneath the treating bath. The sink includes an ultrasonic generator that generates ultrasonic waves, passes the ultrasonic waves through deionized water and the treating bath to the semiconductor wafer that is being cleaned in the treating bath.  
           [0023]    According to another aspect of the present invention, there is also provided a method of cleaning and drying a wafer. Dionized water and isopropyl alcohol are mixed to make a cleaning solution. The cleaning solution is supplied into a treating bath so that the cleaning solution contacts the semiconductor wafer in the treating bath to clean the semiconductor wafer. After the semiconductor wafer is cleaned, the semiconductor wafer is separated from the cleaning solution. The semiconductor wafer separated from the cleaning solution is dried to remove the cleaning solution remaining on the semiconductor wafer.  
           [0024]    Before the cleaning solution is supplied into the treating bath, the semiconductor wafer may be put into the treating bath.  
           [0025]    After the supply of the cleaning solution into the cleaning bath is completed, the semiconductor wafer may be dipped into the cleaning solution.  
           [0026]    The semiconductor wafer may be lifted up from the treating bath so that the semiconductor wafer is separated from the cleaning solution.  
           [0027]    The cleaning solution may be discharged from the treating bath so that the semiconductor wafer is separated from the cleaning solution.  
           [0028]    The concentration of isopropyl alcohol in the cleaning solution varies according to time to adjust the concentration of isopropyl alcohol to a target concentration.  
           [0029]    A hot nitrogen gas is sprayed on the surface of the semiconductor wafer, contacts, and dries the surface of the semiconductor wafer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The above features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0031]    [0031]FIG. 1 is a schematic view of an apparatus for cleaning and drying a semiconductor wafer according to an embodiment of the present invention;  
         [0032]    [0032]FIG. 2 is a view for explaining a step of dipping a semiconductor wafer into a treating bath during cleaning and drying a semiconductor wafer, according to an embodiment of the present invention;  
         [0033]    [0033]FIG. 3 is a view for explaining a step of dipping a semiconductor wafer into a treating bath during cleaning and drying a semiconductor wafer, according to another embodiment of the present invention  
         [0034]    [0034]FIGS. 4A through 4D are views illustrating examples of moving a semiconductor wafer within a cleaning solution to further effectively clean the semiconductor wafer put into a treating bath;  
         [0035]    [0035]FIG. 5 is a view for explaining a method of separating a semiconductor wafer from a cleaning solution after the semiconductor wafer is cleaned in a treating bath using a method of cleaning and drying a semiconductor wafer according to an embodiment of the present invention;  
         [0036]    [0036]FIG. 6 is a view for explaining a step of separating a semiconductor wafer from a cleaning solution after the semiconductor wafer is cleaned in a treating bath in a method of cleaning and drying a semiconductor wafer according to another embodiment of the present invention;  
         [0037]    [0037]FIG. 7 is a view for explaining a step of separating a semiconductor wafer from a cleaning solution after the semiconductor wafer is cleaned in a treating bath in a method of cleaning and drying a semiconductor wafer according to still another embodiment of the present invention;  
         [0038]    [0038]FIG. 8 is a view for explaining a step of spraying a nitrogen gas for drying a semiconductor wafer in a method of cleaning and drying a semiconductor wafer according to an embodiment of the present invention;  
         [0039]    [0039]FIG. 9 is a view for explaining a step of spraying a nitrogen gas for drying a semiconductor wafer in a method of cleaning and drying a semiconductor wafer according to another embodiment of the present invention;  
         [0040]    [0040]FIGS. 10 through 12 are views for explaining a step of changing the position of a semiconductor wafer with respect to a elevating support block when a nitrogen gas is sprayed on the semiconductor wafer; and  
         [0041]    [0041]FIGS. 13 through 20 are graphs illustrating various ways to adjust the concentration of IPA in a cleaning solution used for cleaning and drying a semiconductor wafer according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]    Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the attached drawings.  
         [0043]    [0043]FIG. 1 is a schematic view of an apparatus for cleaning and drying a semiconductor wafer according to an embodiment of the present invention. Referring to FIG. 1, the apparatus includes a treating bath  11 , a cleaning solution mixing unit  50 , a cleaning solution supplying unit  52 , a return line  54 , a drain line  90 , and a collecting unit  82 . A wafer W is cleaned and dried in the treating bath  11 . The cleaning solution mixing unit  50  premixes cleaning solutions to be supplied into the treating bath  11  in a predetermined ratio. The cleaning solution supplying unit  52  moves the cleaning solution mixed in the cleaning solution mixing unit  50  into the treating bath  11  and checks the state of the cleaning solution. The return line  54  returns the cleaning solution, which overflows after being used in a cleaning process in the treating bath  11 , back to the cleaning solution mixing unit  50 .  
         [0044]    If necessary, the drain line  90  returns the cleaning solution from the treating bath  11  back to the cleaning solution mixing unit  50 . The collecting unit  82  is connected to the cleaning solution mixing unit  50  and collects a polluted cleaning solution to decompose the polluted cleaning solution into IPA and DIW.  
         [0045]    The treating bath  11  may accommodate one or more wafers to be cleaned and dried and is connected to the cleaning solution supplying unit  52  and the return line  54 . The upper portion of the treating bath  11  is covered with a chamber cover  12  that is already known in the prior art.  
         [0046]    The wafer W is supported by an elevating support block  92  shown in FIG. 10 in the treating bath  11 . The elevating support block  92  is positioned inside the treating bath  11  when cleaning the wafer W so that the wafer W is sunk under the cleaning solution  5 . However, when drying the wafer W, the elevating support block  92  ascends over the treating bath  11  as shown in FIG. 11.  
         [0047]    The chamber cover  12  covers the treating bath  11  and induces a cleaning solution overflowing from the treating bath  11  to the return line  54 . The chamber cover  12  also includes a plurality of nitrogen spraying nozzles  14 . The nitrogen spraying nozzles  14  are supplied with a nitrogen gas from an external source, and further supply the nitrogen gas into the chamber cover  12  to fill the chamber cover  12  with the nitrogen gas. In particular, the nitrogen nozzles  14  spray the nitrogen gas on the wafer W when the wafer W rises onto the liquid surface of the cleaning solution in order to remove the cleaning solution attached to the surface of the wafer W.  
         [0048]    A heater  80  is further installed to heat the nitrogen gas proceeding toward the nitrogen spraying nozzles  14 . If necessary, the heater  80  can heat the nitrogen gas to further effectively remove the cleaning solution sticking to the surface of the wafer W.  
         [0049]    A sink  84  is installed under the treating bath  11 . The sink  84  contains DIW at a predetermined wafer level so that the lower portion of the treating bath  11  is sunk under the wafer surface of DIW. A wafer level sensor  88  is installed inside the sink  84 . The level sensor  88  senses the level of DIW so that the lower portion of the treating bath  11  is always positioned under the level of DIW.  
         [0050]    An ultrasonic generator  86  is installed beneath the sink  84 . The ultrasonic generator  86  may be a general ultrasonic generator. Ultrasonic waves generated by the ultrasonic generator  86  pass through DIW in the sink  84 , and then are transmitted to the wafer W through a case of the treating bath  11  and the cleaning solution  5 .  
         [0051]    In other words, when DIW is supplied into the sink  84  and the lower portion of the treating bath  11  is dipped into DIW, ultrasonic waves generated by the ultrasonic generator  86  are transmitted to the treating bath  11  through DIW during cleaning of the wafer W.  
         [0052]    The cleaning solution mixing unit  50  includes IPA tanks  18  and  19  and mixing tanks  24  and  25  respectively connected to the IPA tanks  18  and  19 . Since the IPA tanks  18  and  19  are disposed in parallel, IPA can be supplied simultaneously into the IPA tanks  18  and  19  from an external source. Also, since the mixing tanks  24  and  25  are disposed in parallel, DIW can be supplied simultaneously into the mixing tanks  24  and  25  from an external source.  
         [0053]    Each of the IPA tanks  18  and  19  has a plurality of level sensors  20 . The level sensors  20  sense the amount of IPA supplied into the IPA tanks  18  and  19  so that the desired amount of IPA is supplied. Each of the mixing tanks  24  and  25  also has a plurality of level sensors  21 . The level sensors  21  sense the amount of DIW supplied into the mixing tanks  24  and  25  so that a desired amount of DIW is supplied.  
         [0054]    Reference numeral  44  denotes an overflow drainpipe. As will be described later, the overflow drainpipe  44  discharges the cleaning solution outside the mixing tanks  24  and  25  when the cleaning solution returning back via the return line  54  or the drain line  90  rises above a predetermined level in the mixing tanks  24  and  25 .  
         [0055]    The IPA tanks  18  and  19  are respectively connected to the mixing tanks  24  and  25  via connecting pipes  56 . Thus, IPA in the IPA tanks  18  and  19  moves into the mixing tanks  24  and  25  through the connecting pipes  56 .  
         [0056]    The necessary amount of DIW is separately supplied into the mixing tanks  24  and  25 . Thus, in the mixing tanks  24  and  25 , IPA supplied from the IPA tanks  18  and  19  are mixed with DIW at a predetermined ratio. Here, the amount of IPA supplied into the IPA tanks  18  and  19  and the amount of DIW supplied into the mixing tanks  24  and  25  can be respectively measured by the level sensors  20  and  21 . Thus, the concentration of IPA to DIW can be adjusted, and thus cleaning solutions having a desired concentration can be obtained in the mixing tanks  24  and  25 . In other words, the concentration of IPA illustrated in graphs of FIGS. 12 through 19 can be obtained.  
         [0057]    Each of the connecting pipes  56  is split into two pipes. IPA supplementing pumps  22  and  23  are respectively installed at one of the two split connecting pipes  56 . The IPA supplementing pumps  22  and  23  are used to additionally and quickly supply the accurate amount of IPA from the IPA tanks  18  and  19  into the mixing tanks  24  and  25 .  
         [0058]    Since IPA is volatile, IPA may gradually volatilize during the circulation of the cleaning solution. When IPA volatilizes, the concentration of IPA in the cleaning solution becomes low. Thus, the IPA supplementing pumps  22  and  23  supplement IPA by a shortage of IPA.  
         [0059]    As described above, in each of the mixing tanks  24  and  25 , IPA and DIW are mixed in a desired ratio. A nitrogen bubble generator  26  is additionally installed in order to completely mix IPA and DIW. The nitrogen bubble generator  26  is supplied with a nitrogen gas from an external source, sprays the nitrogen gas into the lower portions of the mixing tanks  24  and  25  so that DIW and IPA contained in the mixing tanks  24  and  25  are completely mixed.  
         [0060]    Also, in the present embodiment, the cleaning solution mixing unit  50  includes the IPA tanks  18  and  19  and the mixing tanks  24  and  25 . However, the number of IPA tanks and the number of mixing tanks may increase.  
         [0061]    The mixing tanks  24  and  25  are connected to a supplying pipe  58  via mixed solution supplying valves  28  and  29 .  
         [0062]    The mixed solution supplying valves  28  and  29  are opened alternately not simultaneously. In other words, when the mixed solution supplying valve  28  is opened, the mixed solution supplying valve  29  is closed. Alternatively, when the mixed solution supplying valve  28  is closed, the mixed solution supplying valve  29  is opened. This means that while the cleaning solution in the mixing tank  24  is circulated and supplied into the treating bath  11 , the cleaning solution in the mixing tank  25  is not circulated. In other words, in a case where only the mixed solution supplying valve  28  is opened to clean the wafer W using only the cleaning solution contained in the mixing tank  24 , and after a predetermined period of time, the cleaning solution is polluted more than the allowed pollution level, the mixed solution supplying valve  28  is closed, the mixed solution supplying valve  29  is opened, and the cleaning solution contained in the mixing tank  25  is circulated to continue cleaning of the wafer W.  
         [0063]    As described above, while the cleaning solution in the mixing tank  25  is supplied into the treating bath  11 , a mixed solution discharging valve  30  of the mixing tank  24  is opened to discharge the polluted cleaning solution in the mixing tank  24 . When the mixing tank  24  is completely emptied, the mixed solution discharging valve  30  is closed, IPA and DIW are newly supplied from external sources, IPA and DIW are mixed in a desired ratio, and the mixture is maintained in the mixing tank  24 .  
         [0064]    By alternately using the mixing tanks  24  and  25 , a pure cleaning solution can be continuously supplied without stopping the apparatus for cleaning and drying a semiconductor wafer. Thus, a wafer can be quickly and effectively cleaned and dried.  
         [0065]    Discharging pipes  64  and  65 , which are respectively installed beneath the mixing tanks  24  and  25 , discharge the cleaning solution in the mixing tanks  24  and  25  to the collecting unit  82  and are opened and closed by the mixed solution discharging valves  30  and  31 .  
         [0066]    The collecting unit  82  basically collects the cleaning solution that is polluted at an allowed pollution level or above due to the cleaning of a wafer and remakes the polluted cleaning solution into pure IPA and DIW.  
         [0067]    The collecting unit  82  includes a heating tank  70 , which is connected to the discharging pipes  64  and  65  and receives the polluted cleaning solution, an IPA collecting tank  74  and a DIW collecting tank  78 , which are connected to the heating tank  70 , an IPA filter  72 , which removes impurities contained in IPA flowing from the heating tank  70  to the IPA collecting tank  74 , and a DIW filter  76 , which removes impurities contained in DIW flowing from the heating tank  70  to the DIW collecting tank  78 .  
         [0068]    The heating tank  70  receives and heats a cleaning solution and evaporates IPA in the cleaning solution having a relatively low boiling point to decompose the cleaning solution into DIW and IPA. In other words, the heating tank  70  decomposes a cleaning solution into IPA and DIW using a difference between boiling points of IPA and DIW.  
         [0069]    IPA turning into a gaseous state in the heating tank  70  is filtered through the IPA filter  72 , moves into the IPA collecting tank  74 , is cooled, and is condensed into a liquid state in time.  
         [0070]    An additional cooler may be installed to condense IPA. IPA collected in the IPA collecting tank  74  is supplied into the IPA tanks  18  and  19  of the cleaning solution mixing unit  50  and reused.  
         [0071]    After IPA flows out of the heating tank  70 , polluted DIW remains in the heating tank  70 . Polluted DIW is filtered through the DIW filter  76 , and then is temporarily stored in the DIW collecting tank  78 . DIW collected in the DIW collecting tank  78  is supplied into the mixing tanks  24  and  25  of the cleaning solution mixing unit  50  and reused. Thus, the reuse of the cleaning solution can save considerably the cleaning solution.  
         [0072]    The cleaning solution supplying unit  52 , which moves the cleaning solution of a predetermined concentration supplied from the cleaning solution mixing unit  50  into the treating bath  11 , includes a circulating pump  36 , a filter  38 , an IPA concentration measurer  40 , and a liquid particle counter  42 .  
         [0073]    The circulating pump  36  is connected to the mixing tanks  24  and  25  via the mixed solution supplying valves  28  and  29 . The circulating pump  36  pumps the cleaning solutions in the mixing tanks  24  and  25  into the treating bath  11  through the cleaning solution supplying unit  52 .  
         [0074]    The cleaning solution moving into the treating bath  11  via the circulating pump  36  is purified through the filter  38 . The filter  38  passes the cleaning solution and filters off impurities contained in the cleaning solution so that the usable period of the cleaning solution is prolonged at its maximum.  
         [0075]    The IPA concentration measurer  40  connected to the filter  38  checks the concentration of IPA in the cleaning solution that is circulating. Since IPA is volatile, the concentration of IPA may gradually become low during the circulation of the cleaning solution. Due to this, the IPA concentration measurer  40  is installed to continuously check the concentration of IPA in the cleaning solution. When IPA measurer  40  perceives that the concentration of IPA is reduced, the IPA supplementing pumps  22  and  23  operate to supplement IPA contained in the IPA tanks  18  and  19  into the mixing tanks  24  and  25  so that the cleaning solution returns to an optimum concentration level.  
         [0076]    The liquid particle counter  42  detects the amount of liquid particle in the cleaning solution that is circulating to determine whether the cleaning solution has to be replaced. The liquid particle is a pollutant that is not filtered by the filter  38 .  
         [0077]    The cleaning solution supplied into the treating bath  11  through the cleaning solution supplying unit  52  cleans the wafer W in the treating bath  11 , overflows from the treating bath  11 , and flows to the return line  54 .  
         [0078]    The cleaning solution flowing through the return line  54  passes through the liquid particle counter  43  again. The liquid particle counter  42  detects liquid particle contained in the cleaning solution, which passed through the treating bath  11 , and rechecks the pollution level of the cleaning solution that underwent the cleaning of the wafer W. The cleaning solution passing through the return line  54  returns back into the mixing tanks  24  and  25  via the return valve  60  or  61 .  
         [0079]    Drying the wafer W is accomplished by lifting the wafer W up over the liquid surface of the cleaning solution. In other words, when the wafer W is lifted up over the liquid surface of the cleaning solution, drying the wafer W is firstly achieved due to the surface tension of the cleaning solution. Next, while the wafer W is rising, the nitrogen gas is sprayed on the wafer W through the nitrogen nozzles  14  to secondly achieve the drying of the wafer W.  
         [0080]    The procedure of lifting a wafer up over the liquid surface of a cleaning solution includes: moving the wafer over the liquid surface of the cleaning solution using an elevating support block  92  (shown in FIG. 10) while maintaining the level of the cleaning solution and reducing the level of the cleaning solution without moving the wafer.  
         [0081]    The drain line  90  is just an apparatus for reducing the level of the cleaning solution without moving the wafer. The drain line  90  is connected to the lower portion of the treating bath  11  and the mixing tank  24  or  25  and discharges the cleaning solution  5  in the treating bath  11  downward, so that the wafer is firstly dried.  
         [0082]    The drain line  90  controls the return values  60  and  61  to return the cleaning solution back to one of the mixing tanks  24  and  25  being used.  
         [0083]    As described above, an apparatus for cleaning and drying a semiconductor wafer according to the present invention includes the cleaning solution mixing unit  50 , which premixes cleaning solutions in a desired ratio, the cleaning solution supplying unit  52 , which is supplied with the cleaning solution from the cleaning solution mixing unit  50  and supplies the cleaning solution into the treating bath  11 , and the return line  54 , which returns the cleaning solution used in the cleaning of the semiconductor wafer in the treating bath  11  back to the cleaning solution mixing unit  50 . Thus, since the cleaning solution can be always maintained at an accurate concentration, effects of cleaning and drying the semiconductor wafer are good.  
         [0084]    Since the cleaning solution discharged from the cleaning solution mixing unit  50  returns back through the return line  50 , and then is reused to perform a cleaning process, the apparatus is economic. In particular, the collecting unit  82  decomposes a cleaning solution that is polluted at an allowed pollution level or above into pure IPA and DIW. Thus, the cleaning solution can be reused, the discarded amount of the cleaning solution can be remarkably reduced, and environmental pollution can be reduced.  
         [0085]    A method of cleaning and drying a semiconductor wafer using the cleaning and drying apparatus can be basically divided into four steps. The four steps will be described below.  
         [0086]    In the first step, DIW and IPA are mixed. As described above, the mixture of DIW and IPA is carried out by the cleaning solution mixing unit  50 . In other words, the mixing tanks  24  and  25  are supplied with IPA and DIW, and then IPA and DIW is completely mixed by the nitrogen bubble generator  26 .  
         [0087]    In the second step, a cleaning solution made by mixing IPA and DIW in the first step is supplied into the treating bath  11  so as to contact the wafer in the treating bath  11 .  
         [0088]    The cleaning solution may contact a wafer according to two ways. In one of the two ways, a wafer W is put into the treating bath  11  in advance, and then a cleaning solution is supplied into the treating bath  11  to gradually raise the liquid surface of the cleaning solution so that the wafer W is dipped into the cleaning solution. In the other way, when a cleaning solution is continuously supplied into the treating bath  11  to maintain the cleaning solution at a predetermined level, the wafer W descends to be dipped into the cleaning solution.  
         [0089]    [0089]FIGS. 2 and 3 illustrate a wafer W descending into a cleaning solution.  
         [0090]    [0090]FIG. 2 illustrates a wafer W that vertically descends, and FIG. 3 illustrates a wafer W that descends while swaying from side to side. When the wafer W descends while swaying from side to side, the wafer W rubs on a cleaning solution  5 , which results in an increase in a cleaning effect.  
         [0091]    Besides the two methods, the method of dipping a wafer into a cleaning solution may be modified into various forms.  
         [0092]    In order to supply a cleaning solution into the treating bath  11  with a wafer placed in the treating bath  11  in advance so that the wafer is dipped into the cleaning solution, the treating bath  11  has to be pre-emptied. To empty the treating bath  11 , the drain line  90  is opened to return the cleaning solution in the treating bath  11  back to the mixing tanks  24  and  25 .  
         [0093]    When wafer is dipped into a cleaning solution, it is preferable that the wafer moves so as to rub the wafer on the cleaning solution till the wafer is separated from the cleaning solution.  
         [0094]    [0094]FIGS. 4A through 4D illustrate a wafer that moves in the cleaning solution.  
         [0095]    [0095]FIG. 4A illustrates a wafer W that vertically sways, FIG. 4B illustrates a wafer W horizontally sways, FIG. 4C illustrates a wafer W that turns around the axis (Y-axis) of the vertical direction, FIG. 4D illustrates a wafer W that turns around the axis (X-axis) of the horizontal direction.  
         [0096]    As described above, impurities such as various types of particles can be effectively removed from the surface of a wafer by moving the wafer within a cleansing solution.  
         [0097]    In the third step, the wafer W is separated from a cleaning solution  5 . FIGS. 5 through 7 illustrate a method of separating the wafer W from the cleaning solution  5 .  
         [0098]    In FIGS. 5 and 6, the wafer W is lifted up with the liquid surface of the cleaning solution  5  maintained, and in FIG. 7, the cleaning solution  5  is discharged with the wafer W left in the treating bath  11 . Also, in FIG. 5, the wafer W is vertically lifted up, and in FIG. 6, the wafer W sways from side to side when being lifted up so that the friction between the wafer W and the cleaning solution  5  increases.  
         [0099]    As shown in FIGS. 5 and 6, paths for taking the wafer W out of the treating bath  11  may be changes in other various forms.  
         [0100]    The supply of the cleaning solution  5  into the treating bath  11  has to stop and the drain line  90  has to be opened in order to discharge the cleaning solution  5  as shown in FIG. 7. When the drain line  90  is opened, the cleaning solution  5  in the treating bath  11  returns back to mixing tanks that are operating.  
         [0101]    After the wafer W is completely separated from the cleaning solution  5 , the fourth step is performed. In the fourth step, the remaining cleaning solution is removed from the surface of the wafer W separated from the cleaning solution  5 . Also, the fourth step is carried out in the treating bath  11  from which the cleaning solution  5  is removed or over the treating bath  11  when the cleaning solution  5  remains in the treating bath  11 .  
         [0102]    In the fourth step, a nitrogen gas having a high temperature, preferably a temperature within a range of 50-150° C., is sprayed on the surface of the wafer W at a predetermined pressure so as to remove the cleaning solution remaining on the surface of the wafer W.  
         [0103]    [0103]FIG. 8 illustrates a state where when the wafer W is lifted up over the treating bath  11 , a hot nitrogen gas is concentratedly sprayed on the surface of the wafer W. In this state, the remaining cleaning solution is removed due to the pressure of spraying the nitrogen gas and heat transmitted from the nitrogen gas.  
         [0104]    [0104]FIG. 9 illustrates a state where a hot nitrogen gas is sprayed on the wafer W in the vertical laminar flow. Thereafter, the hot nitrogen gas contacts the surface of the wafer W to transmit heat to the surface of the wafer W so that a mixed solution is evaporated and dried.  
         [0105]    [0105]FIGS. 10 and 13 are views for explaining a step of changing the position of a wafer relative to an elevating support block and side support blocks when a nitrogen gas is sprayed on the wafer. As the elevating support block and the side support blocks, various types of elevating support blocks and side support blocks have been proposed.  
         [0106]    The elevating support block  92  ascends and descends while vertically supporting a plurality of wafers. Side support blocks  94  temporarily separate a wafer W from the elevating support block  92  when the wafer W goes up, so that a lower edge Z of the wafer W is separated from the elevating support block  92  as shown in FIG. 12. When the elevating support block  92  ascends, the side support blocks  94  are fully apart from the wafer W so that the wafer W passes through a space between the side support blocks  94 .  
         [0107]    As can seen in FIG. 10, the wafer W is dipped under the cleaning solution  5  when being supported by the elevating support block  92 . The side support blocks  94  are positioned over the treating bath  11 . The side support blocks  94  are parallel and can move along directions indicated by arrows. If necessary, the side support blocks  94  support sides of the wafer W as shown in FIG. 12.  
         [0108]    As shown in FIG. 10, when the wafer W is completely cleaned under the cleaning solution  5 , the elevating support block  92  goes up in direction U indicated by arrow so that the wafer W is separated from the cleaning solution  5 . Thereafter, a nitrogen gas is sprayed on the wafer W.  
         [0109]    Referring to FIG. 11, the elevating support  92  completely moves up over the treating bath  11  and the wafer W is positioned between the-side support blocks  94 . Here, since the lower edge Z of the wafer W contacts the elevating support block  92 , the nitrogen gas does not reaches the lower edge Z, and thus the lower edge Z is not completely dried. Thus, the side support blocks  94  moves along direction A indicated by arrow so as to support the sides of the wafer W, while the elevating support block  92  temporarily descends as shown in FIG. 12.  
         [0110]    Referring to FIG. 12, the side support blocks  94  support the sides of the wafer W and the elevating support block  92  goes down, so that the nitrogen gas is sprayed on the lower edge Z. Here, the elevating support block  92  must not sink under the cleaning solution  5 .  
         [0111]    When the wafer W is completely dried, the elevating support  92  ascends and the side support blocks  94  are separated from the wafer W so that the elevating support block  92  re-supports the wafer W, and then the wafer W is taken out from the elevating support block  92 .  
         [0112]    Finally, a wafer is completely cleaned and dried through the first through fourth steps.  
         [0113]    [0113]FIGS. 13 through 20 are graphs illustrating ways to adjust the concentration of IPA with respect to variations in the time required for supplying a cleaning solution into a treating bath. When the concentration of IPA is properly adjusted to work conditions or other requirements, an improved cleaning effect can be achieved. In addition, the adjustment of the concentration of IPA can be realized by the IPA supplementing pumps  22  and  23  described with reference to FIG. 1.  
         [0114]    [0114]FIG. 13 illustrates a state where a cleaning solution containing IPA and DIW of a predetermined ratio is supplied into a treating bath without varying the concentration of IPA.  
         [0115]    [0115]FIG. 14 illustrates a state where when only DIW is first supplied into a treating bath, and then the addition of IPA to DIW is gradually increased and the concentration of IPA reaches a predetermined level, the supply of IPA stops.  
         [0116]    [0116]FIG. 15 illustrates a state where when the concentration of IPA of a cleaning solution supplied into a treating bath is gradually increased and reaches a predetermined level, the addition of IPA stops.  
         [0117]    [0117]FIG. 16 illustrates a state where the concentration of IPA to DIW is increased at regular intervals not gradually.  
         [0118]    [0118]FIG. 17 illustrates a state where only pure IPA is first supplied into a treating bath, and then DIW is supplied into the treating bath at predetermined time intervals, so that the concentration of IPA becomes low.  
         [0119]    [0119]FIG. 18 illustrates a state where a cleaning solution containing IPA of a predetermined concentration is supplied into a treating bath, and then the addition of IPA is reduced from a predetermined point of time, so that only DIW is supplied.  
         [0120]    [0120]FIG. 19 illustrates a state where a cleaning solution containing IPA of a predetermined concentration is first supplied into a treating bath, and then the addition of IPA is gradually increased, so that the concentration of IPA becomes higher.  
         [0121]    [0121]FIG. 20 illustrates a state where a cleaning solution containing IPA of a predetermined concentration is first supplied into a treating bath, and then the addition of IPA is gradually reduced, so that the concentration of IPA becomes lower.  
         [0122]    In an apparatus and method for cleaning and drying a semiconductor wafer according to the present invention, before DIW and IPA are supplied into a treating bath, IPA and DIW are completely mixed so that cleaning and drying works are effectively performed. Also, a cleaning solution can be reused to prevent waste of the cleaning solution.  
         [0123]    In addition, the semiconductor wafer is taken as an example of an object to be cleaned. However, the object to be cleaned is not limited to the semiconductor wafer, and the present invention may be applied to various objects including a substrate for a liquid crystal display device only, a substrate for a recording disc only, a substrate for a mask only, or the like.  
         [0124]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.