Patent Publication Number: US-2015068680-A1

Title: Polishing method, polishing apparatus and polishing tool

Description:
TECHNICAL FIELD 
     The present invention relates to a polishing method and a polishing apparatus, and more particularly to a polishing method and a polishing apparatus for performing the planarization polishing of a surface (surface to be processed) of a substrate such as a single substrate made of a compound semiconductor containing an element such as Ga, Al, In, or the like, or a bonded substrate (epitaxial substrate) on which a compound semiconductor containing an element such as Ga, Al, In, or the like is mounted. The present invention is also related to a polishing method and a polishing apparatus, such as a catalyst-referred chemical processing system, for performing the planarization polishing of a surface (surface to be processed) of a substrate while monitoring the progress of a polishing process in order to determine the timing of an endpoint of the polishing process. (e.g., stoppage of polishing process or change of polishing conditions). Furthermore, the present invention relates to a polishing tool provided in such as the catalyst-referred chemical processing system. 
     BACKGROUND ART 
     The applicant has proposed a catalyst-referred chemical processing system for processing a surface (surface to be processed) of a substrate by placing the substrate in an acid processing solution, placing an acid or basic solid catalyst in contact with or in close proximity with the surface of the substrate, and eluting surface molecules of the surface which is held in contact with or in close proximity with the solid catalyst into the acid processing solution (see Japanese laid-open patent publication No. 2008-121099). According to the catalyst-referred chemical processing system, the surface of the substrate (workpiece) is irradiated with light, preferably ultraviolet light, or a voltage is applied between the substrate and the solid catalyst, to promote oxidization of the surface for an increasing machining rate. The catalyst-referred chemical processing system makes it possible to planarize the surface of the substrate with only a chemical action while causing little damage to the surface of the substrate. However, it is generally difficult to planarize a surface of a single substrate made of a compound semiconductor containing an element Ga, for example, to a sufficiently highly accurate surface roughness within a practical given period of time. 
     Demands for finer and multi-layered interconnects for highly integrated semiconductor devices in recent years require that the surface of films such as metal films on the surface of substrates such as semiconductor wafers or the like be planarized to a high degree. To meet such demands, it has widely been customary to planarize the surface of films on the surface of substrates by removing surface irregularities therefrom according to chemical mechanical polishing (CMP). In a chemical mechanical polishing process, it is necessary while a film is being polished to monitor the polished state of the film in order to stop polishing the film at a desired position. Therefore, it has been the general practice to monitor the polished state of an electrically conductive metal film formed on a surface of a substrate by applying an inductive magnetic field to the metal film and detecting the rate of attenuation of an eddy current generated in the surface of the metal film with an eddy current sensor for thereby measuring the thickness of the metal film. 
     The applicant has proposed an apparatus for monitoring the polished state of a film formed on a surface of a substrate by applying light to a surface (surface to be polished) of a film on a surface of a substrate, breaking up the light reflected from the surface into spectral data with a spectrometer, and measuring the thickness of the film based on the spectral data (see Japanese laid-open patent publication No. 2004-154928). 
     The applicant has also proposed a polishing method of polishing a surface (surface to be polished) of a semiconductor substrate by holding the surface of the semiconductor substrate and a contact platen (polishing tool), at least the surface of which is made of a catalyst, in contact with each other and moving them relative to each other within a processing liquid, as a polishing method of precisely and efficiently polishing a semiconductor substrate of a hard-to-process material such as Sic, GaN, or the like (see Japanese laid-open patent publication No. 2009-117782). The contact platen comprises, for example, a base of molybdenum or the like and a catalyst of platinum or the like which is attached to a surface of the base. 
     SUMMARY OF INVENTION 
     Technical Problem 
     The applicant has found that when a substrate of a compound semiconductor containing an element Ga is immersed in a processing liquid comprising a pH buffer solution of a neutral pH which contains Ga ions, and light is applied to the surface of the substrate or a bias potential is applied to the substrate, a Ga oxide is formed on the surface of the substrate, and the formed Ga oxide can be polished away by holding the Ga oxide and a polishing platen (polishing tool) in contact with each other and moving them relative to each other (see Japanese patent application No. 2009-78234). 
     However, it has been discovered that if light is applied to the surface of the substrate or a bias potential is applied to the substrate to form a Ga oxide, and the Ga oxide is polished away to flat the surface of the substrate of the compound semiconductor containing the element Ga, then it takes a long time to polish the surface of the substrate until the polished surface achieves a desired level of flatness or the polished surface fails to achieve a sufficient level of flatness. 
     For example, in a process of fabricating a substrate made of a semiconductor alone, called a bare substrate, a mass of semiconductor material known as an ingot is cut into substrates, and thereafter each surface of the substrate is polished for removing damage therefrom. 
     Heretofore, however, although the progress of the polishing process can be monitored for the purposes of removing a film formed on a surface of a substrate and planarizing the surface of the film based on the measuring principles, the progress of the polishing process cannot be monitored for the purpose of removing damage from a surface of a substrate free of a film, such as a bare substrate. 
     It has been found that if a contact platen (polishing tool) is constructed of a base cut to a certain shape and a catalyst attached to a surface of the base, for example, then a polished surface of a workpiece after it has been polished by the contact platen tends to be scratched by buffs formed on the surface of the base when it was cut, and the surface irregularities of the surface of the base tend to be transferred to the polished workpiece. The catalyst of the contact platen serves as a catalyst in the reception of electrons between the surface of the catalyst and the surface of the workpiece. The catalyst needs to be periodically replaced because it is mechanically worn when the surface of the workpiece is processed while the catalyst and the workpiece are pressed against each other and moved relative to each other. It has also been found that if a contact platen is constructed of a base such as of molybdenum or the like and a catalyst such as of platinum or the like attached to a surface of the base, then it is necessary to replace the contact platen as a whole when the catalyst is to be replaced, and hence the contact platen is highly costly. 
     The present invention has been made in view of the above situation. It is therefore a first object of the present invention to provide a polishing method and a polishing apparatus which are particularly suitable for finishing a surface of a substrate of a compound semiconductor containing an element such as Ga or the like to a desired level of flatness, so that a surface of a substrate of a compound semiconductor containing an element of Ga can be flattened with high surface accuracy within a practical processing time. 
     It is a second object of the present invention to provide a polishing method and a polishing apparatus which are capable of polishing a surface of a substrate such as a bare substrate made of a semiconductor alone while monitoring the progress of the polishing process by measuring a level of damage to the surface of the substrate at the same time that the surface of the substrate is polished. 
     It is a third object of the present invention to provide a polishing tool which is capable of producing a polished surface that is flat and less scratched and which is made inexpensive and durable by basically allowing only a catalyst to be replaced, and a polishing apparatus incorporating such a polishing tool. 
     Solution to Problem 
     The present invention provides a polishing method of polishing a surface of a substrate made of a compound semiconductor containing either one of Ga, Al, and In by relatively moving the surface of the substrate and a surface of a polishing pad having an electrically conductive member in an area of the surface which is held in contact with the substrate, while holding the surface of the substrate and the surface of the polishing pad in contact with each other, in the presence of weak acid water, water with air dissolved therein, or electrolytic ion water. 
     The surface of the substrate, which is made of a compound semiconductor containing an element such as Ga or the like, can thus be finished to a high level of flatness. By combining the finishing process with a polishing process involving the application of light and/or the application of a bias potential, the time required to polish the surface of the substrate, which is made of a compound semiconductor containing an element such as Ga or the like, can greatly be reduced. 
     In a preferred aspect of the present invention, the weak acid water or the water with air dissolved therein has a pH in the range from 3.5 to 6.0. 
     The weak acid water, which has a pH in the range from 3.5 to 6.0, is manufactured by dissolving a gas of CO 2  into pure water or tap water, for example, without the addition of an acid, a pH buffer, or an oxidizer (H 2 O 2 , ozone water, persulfate, or the like). The water with air dissolved therein which has a pH in the range from 3.5 to 6.0 is manufactured by bringing pure water or tap water into contact with air and dissolving CO 2  in air into the water. The water with air dissolved therein may be manufactured by positively dissolving air with a gas dissolver or naturally dissolving air by exposing pure water or tap water to an air atmosphere. Preferably, the pH of the weak acid water or water with air dissolved therein should be in the range from 3.5 to 5.5. 
     The present invention also provides another polishing method of polishing a surface of a substrate at least partly made of a compound semiconductor containing either one of Ga, Al, and In by relatively moving the surface of the substrate and a surface of a polishing pad having an electrically conductive member in an area of the surface which is held in contact with the substrate, while holding the surface of the substrate and the surface of the polishing pad in contact with each other, in the presence of water, water with air dissolved therein, or electrolytic ion water. The water is preferably N 2 -purged water. 
     In a preferred aspect of the present invention, the water with air dissolved therein comprises water with CO 2  in air being dissolved therein while the surface of the substrate is being polished in an air atmosphere after the surface of the substrate has started being polished by the polishing pad which is supplied with pure water or tap water. 
     In a preferred aspect of the present invention, the electrically conductive member is made of a precious metal, a transition metal, graphite, electrically conductive resin, electrically conductive rubber, or electrically conductive organic matter. 
     The previous metal may be platinum or gold, and the transition metal may be Ag, Fe, Ni, or Co. The electrically conductive organic matter may be polyacetylene, polyparaphenyline, polyaniline, polythiofuran or polyparaphenyline-vinylene. 
     In a preferred aspect of the present invention, the compound semiconductor containing either one of Ga, Al, and In comprises GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, or InAs. 
     Since an oxide of GaN, GaP, GaAs, AN, AlP, AlAs, InN, InP, or InAs is dissolvable in a weak acid or weak alkali aqueous solution, they can be used in the polishing method according to the present invention. 
     In a preferred aspect of the present invention, the electrolytic ion water has a pH in the range from 3.5 to 6.0 or of 8.0 or higher. 
     In a preferred aspect of the present invention, the surface of the substrate is polished while applying excitation light to the surface of the substrate via through holes provided in the polishing pad. 
     Alternatively, the surface of the substrate may be polished while applying voltage between the polishing pad and the substrate. 
     Preferably, the surface of the substrate is polished while controlling at least one of the temperature of the water or electrolytic ion water, the temperature of the substrate, and the temperature of the polishing pad. 
     In a preferred aspect of the present invention, a first polishing process for polishing the surface of the substrate while applying excitation light to the surface of the substrate, and a second polishing process for polishing the surface of the substrate without applying excitation light to the surface of the substrate are performed successively. 
     Alternatively, a first polishing process for polishing the surface of the substrate while applying excitation light to the surface of the substrate, and a second polishing process for polishing the surface of the substrate without applying excitation light to the surface of the substrate may be repeated alternately. 
     In a preferred aspect of the present invention, a third polishing process for polishing the surface of the substrate while applying voltage between the substrate and the polishing pad, and a forth polishing process for polishing the surface of the substrate without applying voltage between the substrate and the polishing pad are performed successively. 
     Alternatively, a third polishing process for polishing the surface of the substrate while applying voltage between the substrate and the polishing pad, and a forth polishing process for polishing the surface of the substrate without applying voltage between the substrate and the polishing pad are repeated alternately. 
     The present invention also provides a polishing apparatus for polishing a surface of a substrate made of a compound semiconductor containing either one of Ga, Al, and In, comprising a container for holding weak acid water, water with air dissolved therein, or electrolytic ion water therein, a polishing pad having an electrically conductive member in an area of a surface thereof which is held in contact with the substrate, the polishing pad being disposed in the container while being immersed in the water, a substrate holder for holding the substrate and binging the substrate into contact with the polishing pad while immersing the substrate in the water in the container, and a moving mechanism for relatively moving the polishing pad and the substrate held by the substrate holder while holding the polishing pad and the substrate in contact with each other. 
     The present invention provides another polishing apparatus for polishing a surface of a substrate made of a compound semiconductor containing either one of Ga, Al, and In, comprising a polishing pad having an electrically conductive member in an area of a surface thereof which is held in contact with the substrate, a substrate holder for holding the substrate and binging the substrate into contact with the polishing pad, a moving mechanism for relatively moving the polishing pad and the substrate held by the substrate holder while holding the polishing pad and the substrate in contact with each other, and a water supply section for supplying weak acid water, water with air dissolved therein, or electrolytic ion water to a contacting region of the polishing pad and the substrate held by the substrate holder. 
     In a preferred aspect of the present invention, the polishing pad has a number of through holes, and the polishing apparatus is configured to apply excitation light to the surface of the substrate held by the substrate holder via the through holes of the polishing pad. 
     In a preferred aspect of the present invention, the polishing pad has a light transmission area for applying excitation light to the surface of the substrate held by the substrate holder via a number of the through holes and a light non-transmission area, and the moving mechanism is configured to reciprocate the substrate holder between the light transmission area and the light non-transmission area on the polishing pad. 
     In a preferred aspect of the present invention, the polishing apparatus further comprises a conductive wire connecting the polishing pad and the substrate held by the substrate and interposing therein a power source. 
     The present invention provides another polishing method comprising: polishing a surface of a substrate by relatively moving a light-permeable polishing tool and the surface of the substrate while holding the light-permeable polishing tool and the surface of the substrate in contact with each other in the presence of a processing liquid; and measuring a damage level of the surface of the substrate with at least one of damage level measuring systems including a photocurrent-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring the value of a current flowing through a conductive line connecting the substrate and a metal wire on the polishing tool when excitation light is applied to the surface of the substrate, a photoluminescence-light-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring photoluminescence light emitted from the surface of the substrate when excitation light is applied to the surface of the substrate, and a Raman-light-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring Raman light included in reflected light from the surface of the substrate when visible monochromatic light is applied to the surface of the substrate, and monitoring the progress of a polishing process based on a reduction in the damage level of the surface of the substrate. 
     Since at least one of the photocurrent-type damage level measuring system, the photoluminescence-light-type damage level measuring system, and the Raman-light-type damage level measuring system is used, the damage level of the surface of the substrate can be measured at the same time that the surface of the substrate is polished, and the progress of the polishing process can be monitored from a reduction in the damage level of the surface of the substrate, even if the substrate is a bare substrate, for example, which is made of only a semiconductor with no film on its surface. 
     The present invention provides still another polishing method comprising: forming an oxide on a surface of a substrate by applying excitation light to the surface of the substrate and simultaneously applying a bias potential to the surface of the substrate in the presence of a processing liquid; polishing away the oxide formed in the surface of the substrate by relatively moving the oxide and a polishing tool while holding the oxide and the polishing tool in contact with each other; and measuring a damage level of the surface of the substrate with at least one of damage level measuring systems including a photoluminescence-light-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring photoluminescence light emitted from the surface of the substrate when excitation light is applied to the surface of the substrate, and a Raman-light-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring Raman light included in reflected light from the surface of the substrate when visible monochromatic light is applied to the surface of the substrate, and monitoring the progress of a polishing process based on a reduction in the damage level of the surface of the substrate. 
     When the surface of the substrate is polished while forming an oxide on the surface of the substrate by applying a bias potential to the substrate, the damage level of the surface of the substrate can be measured, while at the same time the surface of the substrate is being polished, by at least one of the photoluminescence-light-type damage level measuring system and the Raman-light-type damage level measuring system, and progress of the polishing process can be monitored based on a reduction in the damage level of the surface of the substrate. 
     The present invention provides yet another polishing method comprising: forming an oxide on a surface of a substrate by applying excitation light to the surface of the substrate in the presence of a processing liquid; polishing away the oxide formed on the surface of the substrate by relatively moving the oxide and a polishing tool while holding the oxide and the polishing tool in contact with each other; and measuring a damage level of the surface of the substrate with at least one of damage level measuring systems including a photocurrent-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring the value of a current flowing through a conductive line connecting the substrate and a metal wire on the polishing tool when excitation light is applied to the surface of the substrate, a photoluminescence-light-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring photoluminescence light emitted from the surface of the substrate when excitation light is applied to the surface of the substrate, and a Raman-light-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring Raman light included in reflected light from the surface of the substrate when visible monochromatic light is applied to the surface of the substrate, and monitoring the progress of a polishing process based on a reduction in the damage level of the surface of the substrate. 
     Preferably, the substrate is made of a semiconductor containing an element of Ga, and the processing liquid comprises a pH buffer solution of a neutral pH which contains Ga ions. 
     The present invention provides yet another polishing method comprising: polishing a surface of a substrate by relatively moving the surface of the substrate and a polishing tool while holding the surface of the substrate and the polishing tool in contact with each other in the presence of a processing liquid; and measuring a damage level of the surface of the substrate with a Raman-light-type damage level measuring system for measuring a damage level of the surface of the substrate by measuring Raman light included in reflected light from the surface of the substrate when visible monochromatic light is applied to the surface of the substrate, and monitoring the progress of a polishing process based on a reduction in the damage level of the surface of the substrate. 
     Because of the relationship to the polishing rate or the finished state of the surface such as a flatness level, there are instances where the surface of the substrate cannot be polished while excitation light is being applied to the surface of the substrate or a bias potential is being applied to the surface of the substrate. In such cases, the Raman-light-type damage level measuring system, which does not need to apply excitation light to the surface of the substrate or a bias potential to the surface of the substrate, is used to measure a damage level of the surface of the substrate while at the same time that the surface of the substrate is polished, and the progress of a polishing process can be monitored based on a reduction in the damage level of the surface of the substrate. 
     Preferably, the processing liquid comprises weak acid water, water with air dissolved therein, or electrolytic ion water, and an electrically conductive member is disposed on the polishing tool in an area which is held in contact with the surface of the substrate. 
     The present invention provides still another polishing apparatus comprising a container for holding a processing liquid therein, a light-permeable polishing tool disposed in the container while being immersed in the processing liquid, a substrate holder for holding the substrate and binging the substrate into contact with the polishing tool while immersing the substrate in the processing liquid in the container, a moving mechanism for relatively moving the polishing tool and the substrate held by the substrate holder while holding the polishing pad and the substrate in contact with each other, and at least one of damage level measuring devices including a photocurrent-type damage level measuring device for measuring a damage level of the surface of the substrate by measuring the value of a current flowing through a conductive line connecting the substrate and a metal wire on the polishing tool when excitation light is applied to the surface of the substrate, a photoluminescence-light-type damage level measuring device for measuring a damage level of the surface of the substrate by measuring photoluminescence light emitted from the surface of the substrate when excitation light is applied to the surface of the substrate, and a Raman-light-type damage level measuring device for measuring a damage level of the surface of the substrate by measuring Raman light included in reflected light from the surface of the substrate when visible monochromatic light is applied to the surface of the substrate. 
     The present invention provides yet another polishing apparatus comprising a light-permeable polishing tool, a substrate holder for holding the substrate and binging the substrate into contact with the polishing tool, a moving mechanism for relatively moving the polishing tool and the substrate held by the substrate holder while holding the polishing pad and the substrate in contact with each other, a processing liquid supply section for supplying a processing liquid to a contacting region of the polishing tool and the substrate held by the substrate holder, and at least one of damage level measuring devices including a photocurrent-type damage level measuring device for measuring a damage level of the surface of the substrate by measuring the value of a current flowing through a conductive line connecting the substrate and a metal wire on the polishing tool when excitation light is applied to the surface of the substrate, a photoluminescence-light-type damage level measuring device for measuring a damage level of the surface of the substrate by measuring photoluminescence light emitted from the surface of the substrate when excitation light is applied to the surface of the substrate, and a Raman-light-type damage level measuring device for measuring a damage level of the surface of the substrate by measuring Raman light included in reflected light from the surface of the substrate when visible monochromatic light is applied to the surface of the substrate. 
     The polishing apparatus preferably include at least one of a light source for applying excitation light to the surface of the substrate which is held by the substrate holder and immersed in the processing liquid in the container and a power source for applying a bias potential to the substrate. 
     The present invention provides a polishing tool comprising a light-permeable support platen with metal wires on a surface thereof, and a catalyst pad having a surface which includes at least a portion held in contact with the substrate and made of a catalyst, the catalyst pad having a plurality of holes defined therein for passing therethrough at least one of light and an ion current. 
     Since the support platen and the catalyst pad are separate from each other and the catalyst pad has a high level of surface flatness and does not tend to produce burrs, scratches, etc., the polishing tool is capable of producing a flat polished surface free of burrs, scratches, etc. The catalyst can be replaced with a new one by replacing the catalyst pad, while allowing the support platen to be reused. Accordingly, the polishing tool is inexpensive and highly durable. 
     The metal wires are provided by embedding metal wires in grooves defined in the surface of the support platen, forming a metal wire pattern on the surface of the support platen, or placing a wiring film with a metal wire pattern formed thereon on the surface of the support platen, for example. 
     Alternatively, the metal wires may be formed by a metal film pattern deposited on the surface of the support platen by vacuum evaporation. 
     The support platen is preferably made of glass or light-permeable resin. 
     The catalyst comprises at least one of precious metal, a transition metal, a ceramics-based solid catalyst, a base solid catalyst, an acid solid catalyst, graphite, electrically conductive resin, electrically conductive rubber, and electrically conductive organic matter, for example. 
     The catalyst pad is made of quartz glass, for example. 
     Alternatively, the catalyst pad may be formed by evaporating a precious metal, a transition metal, an acid or base metal oxide film, graphite, electrically conductive resin, electrically conductive rubber, or electrically conductive organic matter on a surface of a pad base which is made of glass, rubber, light-permeable resin, foamable resin, or non-woven fabric. 
     The acid or base metal oxide film, which is evaporated on the surface of the pad base, is not easily peeled off from the pad base. 
     The present invention provides yet another polishing apparatus comprising a container for holding a processing liquid therein, the above-described polishing tool disposed in the container while being immersed in the processing liquid, a substrate holder for holding the substrate and binging the substrate into contact with the polishing tool while immersing the substrate in the processing liquid in the container, a moving mechanism for relatively moving the polishing tool and the substrate held by the substrate holder while holding the polishing pad and the substrate in contact with each other, and at least one of a light source for applying excitation light to the surface of the substrate which is held by the substrate holder and immersed in the processing liquid in the container and a power source for applying a bias potential to the substrate. 
     The present invention provides yet another polishing apparatus comprising the above-described polishing tool, a substrate holder for holding a substrate and binging the substrate into contact with the polishing tool, a processing liquid supply section for supplying a processing liquid to a contacting region of the polishing tool and the substrate held by the substrate holder, and at least one of a light source for applying excitation light to the surface of the substrate which is held by the substrate holder and immersed in the processing liquid in the container and a power source for applying a bias potential to the substrate. 
     The present invention provides another polishing tool comprising an elastic base made of at least one of elastic materials including rubber, resin, foamable resin, and non-woven fabric, an electrically conductive member disposed on the elastic base in an area held in contact with at least the substrate, and an intermediate film made of carbon or chromium interposed between the elastic base and the electrically conductive member for increasing adhesion therebetween. 
     Since the elastic base of elastic material is elastically deformable along the surface (polished surface) of the substrate, even if the elastic base has surface irregularities, they are prevented from being transferred to the surface of the substrate. Inasmuch as the intermediate film made of carbon or chromium is interposed between the elastic base and the electrically conductive member for increasing adhesion therebetween, the adhesion between the elastic base and the electrically conductive member is increased by the intermediate film, so that the electrically conductive member is less liable to be peeled off from the elastic base. 
     The elastic base preferably has grooves defined in a surface thereof for efficiently supplying the processing liquid. 
     The elastic base preferably has a number of through holes for passing therethrough at least one of light and an ion current. 
     The present invention provides yet another polishing apparatus comprising a container for holding a processing liquid therein, the above-described polishing tool disposed in the container while being immersed in the processing liquid, a substrate holder for holding the substrate and binging the substrate into contact with the polishing tool while immersing the substrate in the processing liquid in the container, and a moving mechanism for relatively moving the polishing tool and the substrate held by the substrate holder while holding the polishing pad and the substrate in contact with each other. 
     The present invention provides a yet another polishing apparatus comprising the above-described polishing tool, a substrate holder for holding a substrate and binging the substrate into contact with the polishing tool, a moving mechanism for relatively moving the polishing tool and the substrate held by the substrate holder while holding the polishing pad and the substrate in contact with each other, and a processing liquid supply section for supplying a processing liquid to a contacting region of the polishing tool and the substrate held by the substrate holder. 
     Advantageous Effects of Invention 
     A polishing method and a polishing apparatus according to the present invention are capable of finishing a surface of a substrate, which is made of a compound semiconductor containing an element such as Ga or the like, to a high level of flatness. By combining the finishing process with a polishing process involving the application of light and/or the application of a bias potential, i.e., by polishing the surface of the substrate at a high polishing rate with the application of light and/or the application of a bias potential, and subsequently finishing the surface of the substrate to a high level of flatness, the time required to polish the surface of the substrate, which is made of a compound semiconductor containing an element such as Ga or the like, can greatly be reduced. 
     According to another polishing method and the polishing apparatus of the present invention, even if the substrate is a bare substrate, for example, which is made of only a semiconductor with no film on its surface, a damage level of the surface of the substrate can be measured by a photocurrent-type damage level measuring system, a photoluminescence-light-type damage level measuring system, or a Raman-light-type damage level measuring system while at the same time the surface of the substrate is being polished, and the progress of the polishing process can be monitored. 
     According to a polishing toll of the present invention, since the support platen and the catalyst pad are separate from each other and the catalyst pad has a high level of surface flatness and does not tend to produce burrs, scratches, etc., the polishing tool is capable of producing a flat polished surface free of burrs, scratches, etc. The catalyst can be replaced with a new one by replacing the catalyst pad, while allowing the support platen to be reused. Accordingly, the polishing tool is inexpensive and highly durable. 
     Further, since the elastic base of elastic material is elastically deformable along the surface (polished surface) of the substrate, even if the elastic base has surface irregularities, they are prevented from being transferred to the surface of the substrate. Inasmuch as the intermediate film made of carbon or chromium is interposed between the elastic base and the electrically conductive member for increasing adhesion therebetween, the adhesion between the elastic base and the electrically conductive member is increased by the intermediate film, so that the electrically conductive member is less liable to be peeled off from the elastic base. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing the concept of a photocurrent-type damage level measuring system; 
         FIG. 2  is a band diagram showing the concept of the photocurrent-type damage level measuring system; 
         FIG. 3  is a graph showing the relationship between polishing times and photocurrent values in the photocurrent-type damage level measuring system; 
         FIG. 4  is a schematic view showing the concept of a photoluminescence-light-type damage level measuring system; 
         FIG. 5  is a band diagram showing the concept of the photoluminescence-light-type damage level measuring system; 
         FIG. 6  is a graph showing the relationship between polishing times and photocurrent values in the photoluminescence-light-type damage level measuring system; 
         FIG. 7  is a schematic view showing the concept of a Raman-light-type damage level measuring system; 
         FIG. 8  is a graph showing Rayleigh light and Raman light before and after a polishing process in the Raman-light-type damage level measuring system; 
         FIG. 9  is a plan view showing the overall construction of a flattening system incorporating a polishing apparatus according to an embodiment of the present invention; 
         FIG. 10  is a schematic cross-sectional view of the polishing apparatus incorporated in the flattening system shown in  FIG. 9 ; 
         FIG. 11  is an enlarged cross-sectional view of a substrate holder of the polishing apparatus shown in  FIG. 10 ; 
         FIG. 12  is an enlarged cross-sectional view of a polishing tool of the polishing apparatus shown in  FIG. 10 ; 
         FIG. 13  is an enlarged cross-sectional view of another polishing tool; 
         FIG. 14  is a schematic cross-sectional view of the polishing apparatus incorporated in the flattening system shown in  FIG. 9 ; 
         FIG. 15  is an enlarged cross-sectional view of a polishing pad of the polishing apparatus shown in  FIG. 14 ; 
         FIG. 16  is a graph showing the relationship between polishing rates and polishing times when a GaN substrate is polished by the polishing apparatus shown in  FIG. 14 . 
         FIG. 17  is a graph showing excitation light and photoluminescence light before and after a polishing process in a photoluminescence-light-type damage level measuring device incorporated in the polishing apparatus shown in  FIG. 10 ; 
         FIG. 18  is a graph showing the relationship between polishing times and photocurrent values when a photocurrent is measured during a polishing process by a photocurrent-type damage level measuring device incorporated in the polishing apparatus shown in  FIG. 10 ; 
         FIG. 19A  is a diagram showing an interference microscope image of a GaN substrate surface before it is polished, and  FIG. 19B  is a diagram showing the relationship between distances and heights on the substrate surface; 
         FIG. 20A  is a diagram showing an interference microscope image of the GaN substrate surface after it is polished in a first stage, and  FIG. 20B  is a diagram showing the relationship between distances and heights on the substrate surface; 
         FIG. 21A  is a diagram showing an interference microscope image of the GaN substrate surface after it is polished in a second stage, and  FIG. 21B  is a diagram showing the relationship between distances and heights on the substrate surface; 
         FIG. 22A  is a diagram showing an interference microscope image of the GaN substrate surface after it is polished in a third stage, and  FIG. 22B  is a diagram showing the relationship between distances and heights on the substrate surface; 
         FIG. 23A  is a diagram showing an atomic force microscope image of a GaN substrate surface after it is polished according to Example 1, and  FIG. 23B  is a diagram showing the relationship between distances and heights on the substrate surface; 
         FIG. 24A  is a diagram showing an atomic force microscope image of a GaN substrate surface after it is polished according to Comparative Example, and  FIG. 24B  is a diagram showing the relationship between distances and heights on the substrate surface; 
         FIG. 25A  is a diagram showing an atomic force microscope image of a GaN substrate surface after it is polished according to Example 2, and  FIG. 25B  is a diagram showing the relationship between distances and heights on the substrate surface; 
         FIG. 26  is a schematic cross-sectional view of a polishing apparatus according to another embodiment of the present invention; 
         FIG. 27  is a diagram showing an atomic force microscope image of a GaN substrate surface after it is polished by the polishing apparatus shown in  FIG. 26  while applying excitation light to the GaN substrate surface; 
         FIG. 28  is a schematic cross-sectional view of a polishing apparatus according to yet another embodiment of the present invention; 
         FIG. 29  is a schematic cross-sectional view of a polishing apparatus according to yet another embodiment of the present invention; 
         FIG. 30  is an enlarged cross-sectional view of a polishing tool of the polishing apparatus shown in  FIG. 29 ; 
         FIG. 31  is an enlarged cross-sectional view of still another polishing tool for use in the polishing apparatus shown in  FIG. 10 ; 
         FIG. 32  is a plan view of the polishing tool shown in  FIG. 31 ; 
         FIG. 33  is a plan view of a support platen of the polishing tool shown in  FIG. 31 ; 
         FIG. 34  is a cross-sectional view taken along line O-A of  FIG. 33 ; 
         FIG. 35  is a cross-sectional view taken along line O-B of  FIG. 33 ; 
         FIG. 36  is an enlarged cross-sectional view of yet another polishing tool for use in the polishing apparatus shown in  FIG. 10 ; 
         FIG. 37  is a plan view of a support platen of the polishing tool shown in  FIG. 36 ; 
         FIG. 38  is a plan view of another support platen; 
         FIG. 39  is an enlarged cross-sectional view of a still further polishing tool for use in the polishing apparatus shown in  FIG. 10 ; and 
         FIG. 40  is an enlarged cross-sectional view of another polishing pad for use in the polishing apparatus shown in  FIG. 14 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. In examples below, a surface of a single substrate made of GaN (GaN substrate) is polished flatwise. The present invention is applicable to a bonded substrate (epitaxial substrate) having GaN mounted on a surface of a base of sapphire or SiC or the like, and a p-type or n-type substrate which is produced by introducing an impurity (dopant) into a GaN substrate or an epitaxial substrate. The present invention is also applicable to a substrate which is produced by forming an interconnect pattern in a surface of the substrate after introducing an impurity (dopant) into a GaN substrate or an epitaxial substrate, or an activated substrate by annealing after the formation of the interconnect pattern. Furthermore, the present invention is also applicable to a compound semiconductor of GaP, GaAs, AN, AlP, AlAs, InN, InP or InAs or the like other than GaN. 
     The concept of a photocurrent-type damage level measuring system for measuring a level of damage to a surface of a substrate by measuring the value of a current which flows through a conductive wire connecting the substrate and a metal wire provided on a polishing tool when the surface of the substrate is irradiated with excitation light, will be described below with reference to a schematic view of  FIG. 1  and a band diagram of  FIG. 2 . 
     As shown in  FIG. 1 , in the presence of a processing liquid (not shown), a light-permeable polishing tool  10  and a surface (surface to be polished) of a substrate W made of GaN, for example, which is held by a substrate holder  11  are held in contact with each other and moved relative to each other to polish the surface (e.g., GaN surface) of the substrate W. During the polishing process, a light source  12  applies light (excitation light) having an energy higher than the band gap of the substrate (e.g., GaN) W through the polishing tool  10  to the surface of the substrate W, and an ammeter  14  measures the value of a current flowing through a conductive wire  13  connecting the substrate W and a metal wire (not shown) provided on the polishing tool  10 . 
     The band gap of GaN, for example, is of 3.42 eV and the wavelength corresponding to the band gap is of 365 nm. Therefore, when the light source  12  applies light (excitation light) having an energy higher than the band gap of the substrate W to the surface of the substrate W, the light having a wavelength of 312 nm, for example, for polishing GaN, then as shown in  FIG. 2 , valence band electrons are excited to a conduction band, forming electron and hole pairs. If the level of damage to the surface of the substrate W is small, then some of the generated electrons move away from the surface of the substrate W, as indicated by the arrow  1  in  FIG. 2 , due to the curvature of the band on the surface of the substrate W, and flows as a photocurrent along the conductive wire  13 . If the level of damage to the surface of the substrate W is large, on the other hand, then the generated electrons move along a damage level formed by the damage to the surface of the substrate W, as indicated by the arrow  2  in  FIG. 2 , and disappear as they are reunited with holes. Therefore, if the level of damage to the surface of the substrate W is large, then the value of a photocurrent flowing along the conductive wire  13  is small. As the level of damage to the surface of the substrate W becomes smaller, the value of a photocurrent flowing along the conductive wire  13  becomes greater. In the polishing process, it is thus possible to measure the level of damage to the surface of the substrate W to monitor the progress of the polishing process by measuring the value of the photocurrent. 
     As the level of damage to the surface of the substrate W is reduced when the polishing process is in progress, the value of the current flowing through the ammeter  14  increases, as shown in  FIG. 3 . When the damage to the surface of the substrate W is removed, the value of the current flowing through the ammeter  14  stops increasing and becomes constant. Consequently, the value of the photocurrent is measured during the polishing process, and the time at which the value of the photocurrent stops increasing and becomes constant can be regarded as an endpoint of the polishing process. 
     The concept of a photoluminescence-light-type damage level measuring system for measuring a level of damage to a surface of a substrate by measuring photoluminescence light emitted from the surface of the substrate when the surface of the substrate is irradiated with excitation light, will be described below with reference to a schematic view of  FIG. 4  and a band diagram of  FIG. 5 . 
     As shown in  FIG. 4 , in the presence of a processing liquid (not shown), a light-permeable polishing tool  10  and a surface (surface to be polished) of a substrate W made of GaN, for example, which is held by a substrate holder  11  are held in contact with each other and moved relative to each other to polish the surface (e.g., GaN surface) of the substrate W. During the polishing process, a light source  12  applies light (excitation light) having an energy higher than the band gap of the substrate (e.g., GaN) W through the polishing tool  10  to the surface of the substrate W, and a spectrometer  16  performs a spectral analysis on photoluminescence light emitted from the surface of the substrate W and monitors the intensity thereof at a wavelength corresponding to the band gap of the substrate W, e.g., at the wavelength of 365 nm for GaN. 
     When the light source  12  applies light (excitation light) having an energy higher than the band gap of the substrate W to the surface of the substrate W, as shown in  FIG. 5 , valence band electrons are excited to a conduction band, forming electron and hole pairs. If the level of damage to the surface of the substrate W is small, then the excited electrons directly move to the valence band and are reunited with holes, bringing back a state of equilibrium, as indicated by the arrow  1  in  FIG. 5 . In this process, photoluminescence light having an energy corresponding to the band gap is observed. If the level of damage to the surface of the substrate W is large, on the other hand, then the excited electrons move via an energy level developed by a loss of the crystal periodicity to the surface of the substrate W and are reunited with holes, as indicated by the arrow  2  in  FIG. 5 . 
     In the above process, the wavelength of the observed photoluminescence light is shifted toward a longer wavelength than the wavelength corresponding to the band gap, or the electrons and holes are reunited without emitting light, so that the photoluminescence light itself will not be observed. In other words, if the level of damage to the surface of the substrate W is large, then the wavelength corresponding to the band gap of the photoluminescence light is small, and as the level of damage to the surface of the substrate W becomes smaller, the wavelength corresponding to the band gap of the photoluminescence light becomes greater. In the polishing process, it is thus possible to measure the level of damage to the surface of the substrate W to monitor the progress of the polishing process by measuring the intensity of the photoluminescence light at the wavelength corresponding to the band gap of the substrate W. 
     For example, the spectrometer  16  performs a spectral analysis on photoluminescence light emitted from the substrate W and monitors the intensity of the photoluminescence light at the wavelength (e.g., 365 nm) corresponding to the band gap of the substrate W. As the level of damage to the surface of the substrate W is reduced when the polishing process is in progress, the intensity of the light increases, as shown in  FIG. 6 . When the damage to the surface of the substrate W is removed, the intensity of the light stops increasing and becomes constant. Consequently, the time at which the intensity of the light stops increasing and becomes constant can be regarded as an endpoint of the polishing process. 
     The concept of a Raman-light-type damage level measuring system for measuring a level of damage to a surface of a substrate by measuring Raman light included in light reflected from the surface of the substrate, which is irradiated with a visible monochromatic beam, will be described below with reference to a schematic view of  FIG. 7 . 
     As shown in  FIG. 7 , in the presence of a processing liquid (not shown), a light-permeable polishing tool  10  and a surface (surface to be polished) of a substrate W made of GaN, for example, which is held by a substrate holder  11  are held in contact with each other and moved relative to each other to polish the surface (e.g., GaN surface) of the substrate W. During the polishing process, a laser beam source  17  applies a visible monochromatic beam through the polishing tool  10  to the surface of the substrate W, and a spectrometer  18  performs a spectral analysis on Raman light included in the light reflected from the surface of the substrate W. 
     It is known that when light having an appropriate wavelength is applied to a semiconductor substrate, the light reflected from a surface of the semiconductor substrate includes dispersed light (Rayleigh light) which has the same wavelength as the applied light and also light (Raman light) which has a wavelength slightly shifted from the wavelength of the applied light. Since the width of the shift depends on the periodicity of the crystal structure, it is known that if the level of damage to the surface of the substrate is small and the crystal structure is in order, then the width of the wavelength shift is of a value inherent with the material of the surface of the semiconductor substrate. If the level of damage to the surface of the substrate is large, then the width of the wavelength shift is varied by defects of the crystal structure. Consequently, it is possible to measure the level of damage to the surface of the substrate to monitor the progress of the polishing process by measuring the spectrum of the Raman light. 
     For example, when the laser beam source  17  applies a visible monochromatic beam to the surface of the substrate W during the polishing process, and the spectrometer  18  performs a spectral analysis the light reflected from the surface of the substrate, Rayleigh light having the same wavelength and half bandwidth as the applied light and Raman light whose wavelength is shifted toward a longer wavelength than the applied light are measured, as shown in  FIG. 8 . If the level of damage to the surface of the substrate W is large, then the spectral intensity of the Raman light is reduced and the half bandwidth is increased. If the level of damage to the surface of the substrate W is small, then the spectral intensity of the Raman light is increased and the half bandwidth is reduced. It is possible to monitor the progress of the polishing process by monitoring the spectrum of the Raman light during the polishing process. The time at which the intensity or half bandwidth of the Raman light stops changing can be regarded as an endpoint of the polishing process. 
       FIG. 9  is a plan view showing the overall construction of a flattening system incorporating a polishing apparatus according to an embodiment of the present invention. This flattening system is used for polishing a surface of a GaN substrate to flatten the surface. As shown in  FIG. 9 , this flattening system includes a substantially rectangular housing  1  whose interior is divided by partition walls  1   a,    1   b,    1   c  into a loading/unloading section  2 , a polishing section  3  and a cleaning section  4 . The loading/unloading section  2 , the polishing section  3  and the cleaning section  4  are independently fabricated and independently evacuated. 
     The loading/unloading section  2  includes at least one (e.g., three as shown) front loading section  200  on which a substrate cassette, storing a number of substrates (objects to be polished) therein, is placed. The front loading sections  200  are arranged side by side in the width direction (perpendicular to the long direction) of the flattening system. Each front loading section  200  can receive thereon an open cassette, a SMIF (standard manufacturing interface) pod or a FOUP (front opening unified pod). The SMIF and FOUP are a hermetically sealed container which can house a substrate cassette therein and can keep the interior environment independent of the exterior environment by covering with a partition. 
     A moving mechanism  21 , extending along the line of the front loading sections  200 , is provided in the loading/unloading section  2 . On the moving mechanism  21  is provided a first transfer robot  22  as a first transfer mechanism, which is movable along the direction in which substrate cassettes are arranged. The first transfer robot  22  can reach the substrate cassettes placed in the front loading sections  200  by moving on the moving mechanism  21 . The first transfer robot  22  has two hands, an upper hand and a lower hand, and can use the two hands differently, for example, by using the upper hand when returning a processed substrate to a substrate cassette and using the lower hand when transferring an unprocessed substrate. 
     The loading/unloading section  2  is an area that needs to be kept in the cleanest environment. Accordingly, the interior of the loading/unloading section  2  is constantly kept at a higher pressure than any of the outside of the apparatus, the polishing section  3  and the cleaning section  4 . Furthermore, a filter-fan unit (not shown) having an air filter, such as an HEPA filter or a ULPA filter, is provided above the moving mechanism  21  for the first transfer robot  22 . Clean air, from which particles, vapor and gas have been removed, continually blows off downwardly through the filter-fan unit. 
     The polishing section  3  is an area where polishing of a surface (surface to be processed) of a substrate is carried out. The polishing  3  includes two polishing apparatuses  30 A,  30 B which carry out polishing at a first stage and polishing at a second stage successively and two polishing apparatuses  30 C,  30 D which carry out polishing at a third stage. The polishing apparatuses  30 A through  30 D are arranged along the long direction of the flattening system. 
     As shown in  FIG. 10 , the polishing apparatuses  30 A,  30 B each includes a container  132  for holding therein a processing liquid (polishing solution)  130  comprising a neutral pH buffer solution containing Ga ions. Above the container  132  is disposed a processing liquid supply nozzle  133  for supplying the processing liquid  130  into the container  132 . As the processing liquid  130  may be used a solution which is prepared by adding Ga ions, e.g., in an amount of not less than 10 ppm, to a phosphate buffer solution, e.g., having a pH of 6.86 to bring Ga ions in the processing liquid  130  near to saturation. The pH of the neutral pH buffer solution (at 25° C.) is, for example, 6.0 to 8.0. 
     A polishing tool (polishing platen)  134  having light permeability is mounted on the bottom of the container  132 , so that the polishing tool  134  becomes immersed in the processing liquid  130  when the container  132  is filled with the processing liquid  130 . The polishing tool  134  is, for example, composed of quartz glass which is an acid solid catalyst having excellent light permeability. It is possible to use a basic solid catalyst for the polishing tool  134 . Further, it is possible to use one having an acidic or basic solid catalyst layer only in a surface of the polishing tool  134 . 
     The container  132  is coupled to an upper end of a rotatable rotating shaft  136 . The bottom plate of the container  132  has a ring-shaped opening  132   a  formed around the rotating shaft  136  and closed by the polishing tool  134 . A light source  140  for emitting excitation light, preferably ultraviolet light, is disposed right below the opening  132   a.  Thus, the excitation light, preferably ultraviolet light, emitted from the light source  140 , passes through the opening  132   a  of the container  132 , permeates through the interior of the polishing tool  134  and reaches above the polishing tool  134 . 
     Right above the container  132  is disposed a substrate holder  144  for detachably holding a substrate  142 , e.g., a GaN substrate, with a front surface facing downwardly. The substrate holder  144  is coupled to a lower end of a main shaft  146  that is rotatable and vertically movable. In this embodiment, the rotating shaft  136  for rotating the container  132  and the main shaft  146  for rotating the substrate holder  144  constitute a movement mechanism for moving the polishing tool  134  and the substrate (GaN substrate)  142 , held by the substrate holder  144 , relative to each other. However, it is also possible to provide only one of them. 
     The polishing apparatus of this embodiment is also provided with a power source  148  for applying a voltage between the substrate  142 , held by the substrate holder  144 , and the polishing tool  134 . A switch  150  is interposed in a conductive wire  152   a  extending from the positive pole of the power source  148 . 
     In this embodiment, processing of the substrate  142  is carried out in an immersion manner: the container  132  is filled with the processing liquid  130  and the polishing tool  134  and the substrate  142  held by the substrate holder  144  are kept immersed in the processing liquid  130  during processing. It is also possible to employ a dripping manner in which the processing liquid  130  is supplied between the substrate  142  and the polishing tool  134  by dripping the processing liquid  130  from the processing liquid supply nozzle  133  onto the surface of the polishing tool  134 , so that processing of the substrate  142  is carried out in the presence of the processing liquid  130 . 
     As shown in  FIG. 11 , the substrate holder  144  has a cover  160  for preventing intrusion of the processing liquid  130 . Inside the cover  160 , a metal holder body  170  is coupled, via a rotation transmission section  168  including a universal joint  164  and a spring  166 , to a drive flange  162  that is coupled to the lower end of the main shaft  146 . 
     A retainer ring  172  is vertically movably disposed around the lower portion of the holder body  170 . A conductive rubber  174  is mounted to a lower surface (substrate holding surface) of the holder body  170  such that a pressure space  176  can be formed between the lower surface of the holder body  170  and the conductive rubber  174 . An air introduction pipe  178  is connected to the pressure space  176  via an air introduction passage extending in the holder body  170 . The flange portion of the metal holder body  170  is provided with an extraction electrode  180  to which is connected the conductive wire  152   a  extending from the positive pole of the power source  148 . 
     In order to prevent wear of the surface of the retainer ring  172  by making into contact with the polishing tool  134  to thereby prevent the surface material of the retainer ring  172  from adhering to the surface of the polishing tool  134 , at least the surface portion of the retainer ring  172 , which comes into contact with the polishing tool  134 , is preferably made of a glass material, such as quartz, sapphire or zirconia, or a ceramic material such as alumina, zirconia or silicon carbide. The conductive rubber  174  is, for example, a conductive chloroprene rubber, a conductive silicone rubber or a conductive fluororubber. 
     When the back surface of the substrate  142  is held, e.g., by attraction, on the lower surface (substrate holding surface) of the holder body  170  of the substrate holder  144 , the conductive rubber  174  comes into contact with the back surface of the substrate  142  to feed electricity to the back surface. While maintaining the electricity feeding to the back surface of the substrate  142 , air can be introduced into the pressure space  176  so as to press the substrate  142  against the polishing tool  134 . 
     The substrate holder  144  can thus hold the substrate  142  while feeding electricity to the substrate  142  in a simple manner at a low resistance. The substrate holder  144  is preferably configured to be capable of filling a polar conductive grease between the conductive rubber  174  and the substrate  142  when bringing the substrate  142  into contact with the conductive rubber  174  upon holding of the substrate  142  on the substrate holder  144 . 
     As shown in  FIG. 12 , a large number of grooves  134   a  are provided in an upper surface of the polishing tool  134  in an area corresponding to the opening  132   a  of the container  132 . Vapor-deposited metal films  154  comprising metal wires are formed in bottoms of the grooves  134   a.  To the metal films  154  is connected a conductive wire  152   b  extending from the negative pole of the power source  148 . The metal films  154  are preferably made of platinum or gold, which is corrosion-resistant. Though the grooves  134   a  provided in the upper surface of the polishing tool  134  are preferably arranged in concentric circles, it is also possible to arrange the grooves in a spiral, radial or lattice-shaped configuration. As shown in  FIG. 13 , it is also possible to provide metal wires (metal wires)  156  of, e.g., gold or platinum in the bottoms of the grooves  134   a  provided in the upper surface of the polishing tool  134 . 
     A heater  158  (see  FIG. 10 ), embedded in the substrate holder  144  and extending into the main shaft  146 , is provided as a temperature control mechanism for controlling the temperature of the substrate  142  held by the substrate holder  144 . A heat exchanger as a temperature control mechanism for controlling the processing liquid  130  to be supplied into the container  132  at a predetermined temperature is provided to the processing liquid supply nozzle  133  disposed above the container  132 , as necessary. Furthermore, a fluid passage (not shown) as a temperature control mechanism for controlling the temperature of the polishing tool  134  is provided in the interior of the polishing tool  134 . 
     As is known by the Arrhenius equation, the higher the reaction temperature of a chemical reaction is, the higher is the reaction rate. Thus, by controlling at least one of the temperature of the substrate  142 , the temperature of the processing liquid  130  and the temperature of the polishing tool  134  so as to control the reaction temperature, the processing rate can be adjusted and the stability of the processing rate can be enhanced. 
     As shown in  FIG. 9 , the polishing apparatuses  30 A,  30 B are each provided with a conditioning mechanism (conditioner)  190 , e.g., comprised of a polishing pad, for conditioning the surface (upper surface) of the polishing tool  134  so that it has desired flatness and appropriate roughness. The surface (upper surface) of the polishing tool  134  is conditioned by the conditioning mechanism (conditioner)  190  so that it has a PV (peak-to-valley) roughness of the surface of about 0.1 to 1 μm. During the conditioning of the polishing tool  134 , an abrasive-containing slurry may be supplied to the surface of the polishing tool  134 , as necessary. 
     As shown in  FIG. 10 , the polishing apparatus  30 A,  30 B each includes a photocurrent-type damage level measuring device  201  having the light source  140  as a component thereof and a photoluminescence-light-type damage level measuring device  202  also having the light source  140  as a component thereof The photocurrent-type damage level measuring device  201  comprises a conductive wire  152   c  which connects the conductive wire  152   a  extending from the power source  148  and connected to the substrate  142  held by the substrate holder  144  and the conductive wire  152   b  extending from the power source  148  and connected to the metal films (metal wires)  154  of the polishing tool  134 , an ammeter  204  connected to the conductive wire  152   c,  and the light source  140 . The conductive wire  152   c  has a switch  206 . The photoluminescence-light-type damage level measuring device  202  comprises a spectrometer  208  for performing a spectral analysis on photoluminescence light reflected from the surface of the substrate  142  to measure the intensity of the photoluminescence light having a wavelength (e.g., 365 nm) corresponding to the band gap of the substrate W, and the light source  140 . 
     In this embodiment, the light source  140  for applying excitation light to the substrate  142  to oxidize the surface of the substrate  142  is used as respective light sources of the photocurrent-type damage level measuring device  201  and the photoluminescence-light-type damage level measuring device  202 . However, the photocurrent-type damage level measuring device  201  and the photoluminescence-light-type damage level measuring device  202  may have respective dedicated light sources separately from the light source  140 . 
     As shown in  FIG. 14 , the polishing apparatus  30 C,  30 D each includes a rotatable turntable  230  having a flat upper surface and a ring-shaped dam member  236  mounted on the peripheral edge of the turntable  230  and providing a container  234  for holding water  232  therein on the upper surface of the turntable  230 . The water  232  may comprise weak acid water, water with air dissolved therein, or electrolytic ion water. The weak acid water or water with air dissolved therein has a pH in the range from 3.5 to 6.0 or preferably from 3.5 to 5.5. The electrolytic ion water has a pH in the range from 3.5 to 6.0 or preferably from 3.5 to 5.5 or of 8.0 or higher. 
     The weak acid water, which has a pH in the range from 3.5 to 6.0, is manufactured by dissolving a CO 2  gas or CO 2  in air into pure water or tap water, for example, without the addition of an acid, a pH buffer, or an oxidizer (H 2 O 2 , ozone water, persulfate, or the like). When the weak acid water, which has a pH in the range from 3.5 to 6.0 that is produced without the addition of an acid, a pH buffer, or an oxidizer, is used, the substrate  142  can be polished at a polishing rate that is kept against being lowered, while the surface thereof (surface to be polished) is prevented from developing etch pits. Since this process does not involve chemicals, such as an acid, a pH buffer, and an oxidizer, which need to be processed by a waste liquid treatment, the process is advantageous in that the cost of the waste liquid treatment is dispensed with and a cleaning process subsequent to the polishing process is simplified. 
     In this embodiment, a water supply line (water supply section)  241  extending from a pure water supply source (not shown) and having a gas dissolver  238  and a heat exchanger  240  is disposed above the turntable  230 . The gas dissolver  238  dissolves a gas of CO 2  or the like or air into pure water delivered from the pure water supply source to produce water  232  whose pH is adjusted to 3.5 to 6.0, or preferably 3.5 to 5.5, which is introduced into the container  234  provided by the dam member  236 . If necessary, the water  232  to be introduced into the container  234  is adjusted to a prescribed temperature by the heat exchanger  238 . In an initial stage of the polishing process, pure water or tap water may be introduced, and the substrate  142  may be polished in an air atmosphere, so that CO 2  in air may be dissolved into pure water or tap water at the same time that the substrate  142  is polished, adjusting the pH of the water to 3.5 to 6.0, or preferably 3.5 to 5.5. 
     Though in this embodiment the water  232  having a pH of 3.5-6.0, preferably 3.5-5.5, by dissolving a gas of CO 2  or the like or air into pure water is used, neutral N 2 -purged water that is produced by purging N 2  into tap water or pure water may be used instead of the water  232 . When the neutral N 2 -purged water is used, the processing rate decrease while obtaining an equal processed surface as compared with the case where the water  232  having a pH of 3.5-6.0, preferably 3.5-5.5, is used. 
     A polishing pad  242  serving as a polishing member is mounted on the upper surface of the turntable  230  at a position within the container  234  whose periphery is defined by the dam member  236 . When the water  232  is introduced into the container  234 , the space above the polishing pad  242  is filled with the water  232 . The turntable  230  has a fluid passage  230   a  defined therein for controlling the temperature of the turntable  230 . 
     The substrate  142 , such as a GaN substrate or the like, is detachably held, with its surface facing downward, by a substrate holder  244  disposed above the container  234  whose periphery is defined by the dam member  236 . The substrate holder  244  is connected to the lower end of a main shaft  246  which is vertically movable and rotatable about its own axis. The substrate holder  244  and the main shaft  246  are identical in structure to the substrate holder  144  and the main shaft  146  shown in  FIG. 10 , and will not be described in detail below. 
     As shown in  FIG. 15 , the polishing pad  242  comprises an elastic base  260  made of an elastic material, e.g., having a Shure hardness in the range from 50 to 90 and including concentric or spiral grooves  260   a  defined in a surface thereof for effectively supplying the processing liquid (water)  232 , an intermediate layer  262  deposited on the surface of the elastic base  260  by vacuum evaporation or the like, and an electrically conductive member  264  deposited on a surface of the intermediate layer  262  by vacuum evaporation or the like. The electrically conductive member  264  that is disposed on the surface of the elastic base  260  with the intermediate layer  262  interposed therebetween finds it easy to follow irregularities in long/single periods on the surface (surface to be polished) of the substrate  142 . The elastic base  260  may have holes for effectively supplying the processing liquid (water), rather than the grooves  260   a.    
     The elastic base  260  has a thickness in the range from 0.5 to 5 mm, for example. The elastic material of the elastic base  260  may be rubber, resin, foamable resin, non-woven fabric or the like, for example. The elastic base  260  may comprise two or more superposed layers of elastic material which have different moduli of elasticity. 
     The intermediate layer  262  has a thickness in the range from 1 to 10 nm, for example. The intermediate layer  262  is interposed between the elastic base  260  and the electrically conductive member  264  in order to increase the adhesion between the elastic base  260  and the electrically conductive member  264 , and is made of chromium or graphite (SP2-bonded) carbon, for example, which is characterized by better adhesion to both the elastic base  260  and the electrically conductive member  264 . When the intermediate layer  262  is formed on the surface of the elastic base  260  by vacuum evaporation, it is preferable to employ ion sputtering deposition in order to suppress expansion and modification of the elastic base  260  due to high temperatures. This holds true also when the electrically conductive member  264  is formed on the surface of the intermediate layer  262  by vacuum evaporation. 
     The electrically conductive member  264  has a thickness in the range from 100 to 1000 nm, for example. If the thickness is smaller than 100 nm, then the electrically conductive member  264  will be unduly worn when the polishing process is carried out for about one hour, and hence is not practical. If the thickness is greater than 1000 nm, then the surface of the electrically conductive member  264  will tend to crack when the polishing process is carried out. The electrically conductive member  264  is preferably made of platinum, but may be made of any of precious metals such as gold, transition metals (Ag, Fe, Ni, Co, etc.), graphite, electrically conductive resin, electrically conductive rubber, electrically conductive organic matter etc. which are insoluble or slightly soluble (at a solving rate of 10 nm/h. or lower) in the water  232 . If the electrically conductive member is made of a transition metal, then it is preferable to use, as the supplied water, water (hydrogen water) in which there is dissolved hydrogen generated at a cathode when pure water or tap water is electrolyzed, thereby preventing the electrically conductive member from being corroded by oxygen dissolved in the water. 
     As is known from the Arrhenius equation, a chemical reaction has a higher reaction rate as the reaction temperature is higher. Therefore, it is possible to increase the stability of the reaction rate while varying the reaction rate, by controlling at least one of the temperatures of the substrate  142 , the water  232 , and the turntable  230  to control the reaction temperature. 
     As shown in  FIG. 9 , the polishing apparatus  30 C,  30 D each includes the conditioning mechanism (conditioner)  190 , which may comprise a polishing pad, for example, for conditioning the surface (upper surface) of the polishing tool  134  to a good level of flatness and an appropriate level of roughness. 
     The polishing apparatuses  30 C,  30 D are each capable of polishing the surface of the substrate  142  by holding the surface of the substrate  142  and the electrically conductive member  264 , such as of platinum or the like, of the polishing pad  242  in contact with each other and moving them relative to each other while the substrate  142  held by the substrate holder  244  and the polishing pad  242  are being immersed in the water  232 . The surface of the substrate  142  may be polished while the water  232  is being interposed between the substrate  142  held by the substrate holder  244  and the polishing pad  242  by supplying the water  232  from the water supply line  241  onto the upper surface of the polishing pad  242 . 
     The mechanism of the polishing process is considered as follows: When the surface of the substrate  142  and the electrically conductive member  264 , such as of platinum or the like, of the polishing pad  242  are held in contact with each other and moved relative to each other, the contacting region is strained to excite valence band electrons to a conduction band, generating electron and hole pairs. The electrons excited to the conduction band are moved to the electrically conductive member  264 , such as of platinum or the like, which has a large work function, leaving holes on the substrate surface. Off ions or H 2 O molecules in the water  232  act on the left holes, resulting the oxidization of only the contacting region. Since oxides of Ga, Al, and In are dissolvable in a weak acid such as a carbon dioxide solution or the like or a weak alkali, the oxides produced in the contacting region are dissolved into the water  232  and removed from the surface of the substrate  142 . 
     The pressure under which the surface of the substrate  142  and the electrically conductive member  264 , such as of platinum or the like, of the polishing pad  242  are held in contact with each other should preferably be in the range from 0.1 to 1.0 kgf/cm 2 , for example, and particularly should preferably be of about 0.4 kgf/cm 2 , in order to eliminate any warpage of the substrate  142  and prevent the substrate  142  from being slipped out and the electrically conductive member  264  of the polishing pad  242  from being peeled off from the base  260 . 
       FIG. 16  shows the relationship between polishing rates and polishing times when a GaN substrate is polished by the polishing apparatus shown in  FIG. 14 , which employs pure water with air dissolved therein as the water  232  and a polishing pad evaporated platinum thereon as the polishing pad  242 . As can be seen from  FIG. 16  that though the polishing rate is about 22 nm/h at the start of the polishing process, the polishing rate gradually decrease as the polishing process progresses, and the polishing rate becomes almost 0 nm/h at the end of the polishing process. In this regard, it is conceivable that the polishing rate is high when there are large volume of material to be removed in such a state that a surface is roughen at the start of the polishing process, and the polishing rate becomes lower as the surface is flatten and the material to be removed by polishing process is decrease. 
     Both the elastic base  260  and the intermediate layer  262  of the polishing pad  242  may be made of an electrically conductive material, and, as indicated by the imaginary lines in  FIG. 14 , a conductive wire  272   a  extending from the anode of a power source  270  and a conductive wire  272   b  extending from the cathode thereof may be connected respectively to the substrate  142  held by the substrate holder  244  and the elastic base  260  of the polishing pad  242 . Then, the surface of the substrate  142  may be polished while a voltage is being applied between the electrically conductive member  264  of the polishing pad  242  and the substrate  142  held by the substrate holder  244 . In this manner, the polishing rate can be increased. 
     Returning to  FIG. 9 , between the polishing apparatuses  30 A,  30 B and the cleaning section  4  is disposed a first linear transporter  5  as a first (translatory) transfer mechanism for transferring a substrate between four transferring positions (a first transferring position TP 1 , a second transferring position TP 2 , a third transferring position TP 3 , and a fourth transferring position TP 4  in the order of distance from the loading/unloading section  2 ) along the long direction of the flattering system. A reversing machine  31  for reversing a substrate received from the first transfer robot  22  is disposed above the first transferring position TP 1  of the first linear transporter  5 , and a vertically-movable lifter  32  is disposed below the reversing machine  31 . Further, a vertically-movable pusher  33  is disposed below the second transferring position TP 2 , a vertically-movable pusher  34  is disposed below the third transferring position TP 3 , and a vertically-movable lifter  35  is disposed below the fourth transferring position TP 4 . 
     Beside the polishing apparatuses  30 C,  30 D and adjacent to the first linear transporter  5  is disposed a second linear transporter  6  as a second (translatory) transfer mechanism for transferring a substrate between three transferring positions (fifth transferring position TP 5 , sixth transferring position TP 6  and seventh transferring position TP 7  in order of distance from the loading/unloading section  2 ) along the long direction of the flattering system. A vertically-movable lifter  36  is disposed below the fifth transferring position TP 5 , a pusher  37  is disposed below the sixth transferring position TP 6 , and a pusher  38  is disposed below the seventh transferring position TP 7 . Further, a pure water replacement section  192  including a tub and a pure water nozzle is disposed between the polishing apparatus  30 C and the pusher  37 , and a pure water replacement section  194  including a tub and a pure water nozzle is also disposed between the polishing apparatus  30 D and the pusher  38 . 
     As will be understood from the use of a slurry or the like during polishing, the polishing section  3  is the dirtiest area. In this system, therefore, discharge of air is performed around a removal processing site, such as a platen, so as to prevent particles in the polishing section  3  from flying to the outside. Further, the internal pressure of the polishing section  3  is made lower than the external pressure of the system and the internal pressures of the neighboring cleaning section  4  and loading/unloading section  2 , thereby preventing particles from flying out. An exhaust duct (not shown) and a filter (not shown) are usually provided respectively below and above a removal processing site, such as a platen, so as to create a downward flow of cleaned air. 
     The cleaning section  4  is an area for cleaning a substrate. The cleaning section  4  includes a second transfer robot  40 , a reversing machine  41  for reversing a substrate received from the second transfer robot  40 , three cleaning units  42 - 44  each for cleaning the substrate, a drying unit  45  for rinsing the cleaned substrate with pure water and then spin-drying the substrate, and a movable third transfer robot  46  for transferring the substrate between the reversing machine  41 , the cleaning units  42 - 44  and the drying unit  45 . The second transfer robot  40 , the reversing machine  41 , the cleaning units  42 - 44  and the drying unit  45  are arranged in a line along the long direction of the flattering system, and the third transfer robot  46  is movably disposed between the first linear transporter  5  and the line of the second transfer robot  40 , the reversing machine  41 , the cleaning units  42 - 44  and the drying unit  45 . A filter-fan unit (not shown) having an air filter is provided above the cleaning units  42 - 44  and the drying unit  45 , and clean air, from which particles have been removed by the filter-fan unit, continually blows downward. The interior of the cleaning section  4  is constantly kept at a higher pressure than the polishing section  3  to prevent inflow of particles from the polishing section  3 . 
     A shutter  50 , located between the reversing machine  31  and the first transfer robot  22 , is provided in the partition wall  1  a surrounding the polishing section  3 . The shutter  50  is opened when transferring a substrate between the first transfer robot  22  and the reversing machine  31 . 
     Further, a shutter  53  located at a position facing the polishing apparatus  30 B and a shutter  54  located at a position facing the polishing apparatus  30 C are respectively provided in the partition wall lb surrounding the polishing section  3 . 
     Processing for flattening a surface of a substrate by the flattening system having the above construction will now be described. 
     The first transfer robot  22  carries a substrate out of the substrate cassette placed in the front loading section  200  and transfers the substrate to the reversing machine  31 . The reversing machine  31  reverses the substrate through 180° and then places the reversed substrate on the lifter  32  at the first transferring position TP 1 . The first linear transporter  5  moves to transfer the substrate on the lifter  32  to the first transferring position TP 1  or the third transferring position TP 3 . The substrate holder  144  of the polishing apparatus  30 A receives the substrate from the pusher  32  or the substrate holder  144  of the polishing apparatus  30 B receives the substrate from the pusher  34 . Then, the polishing apparatus  30 A or  30 B polishes the surface of the substrate in a first stage and a second stage. 
     Specifically, in the polishing apparatus  30 A, the substrate holder  144  which is holding the substrate  142  such as a GaN substrate or the like with its surface (surface to be processed) facing downward is moved to a position above the container  132 , and then is lowered to immerse the substrate  142  in the processing liquid  130  held in the container  132 . While the processing liquid  130  is present between the substrate  142  and the polishing tool  134 , the light source  140  applies excitation light, preferably ultraviolet light, to the surface (lower surface) of the substrate  142 . If the substrate  142  comprises a GaN substrate, then since the band gap of GaN is of 3.42 eV, the excitation light should preferably have a wavelength equal to or lower than the wavelength corresponding to the band gap of the workpiece, i.e., equal to or lower than 365 nm, e.g., should preferably have a wavelength of 312 nm. When the GaN substrate is machined, therefore, GaN is oxidized to form a Ga oxide (Ga 2 O 3 ) on the surface of the GaN substrate. 
     The substrate  142  may be polished while the processing liquid  130  is being supplied through the processing liquid supply nozzle  133  between the substrate  142  and the polishing tool  134 . 
     At this time, the switch  150  of the power source  148  is turned on to apply a voltage between the polishing tool  134  and the substrate  142  held by the substrate holder  144  such that the polishing tool  134  serves as a cathode. When the GaN substrate is processed, therefore, the formation of the Ga oxide (Ga 2 O 3 ) on the surface of the GaN substrate is promoted. 
     Then, while the light source  140  is emitting the excitation light, preferably ultraviolet light, and also while the voltage is being applied between the polishing tool  134  and the substrate  142 , the rotating shaft  136  is rotated to rotate the polishing tool  134 , and at the same time that the substrate holder  144  is rotated to rotate the substrate  142 , the substrate  142  is lowered to cause the surface of the polishing tool  134  to contact the surface of the substrate  142  under a surface pressure preferably in the range from 0.1 to 1.0 kgf/cm 2 . If the surface pressure is lower than 0.1 kgf/cm 2 , then any warpage of the substrate  142  may not possibly be corrected and the substrate  142  may not possibly be polished uniformly in its entirety. If the surface pressure is higher than 1.0 kgf/cm 2 , then the surface of the substrate  142  may possibly develop mechanical defects. In this manner, the surface of the substrate  142 , such as a GaN substrate, is polished in the first stage for selectively removing regions of an Ga oxide formed on the surface of the substrate  142  which are held in contact with the polishing tool  134 , i.e., for selectively removing the Ga oxide formed on the tip ends of convex regions the surface of the substrate  142  which has surface irregularities, primarily for removing damage to the surface of the substrate  142 . The surface of the substrate  142  is polished in the first stage at a polishing rate up to about 1000 nm/h., for a machining time of about 5 minutes, for example. 
     While the surface of the substrate  142  is being polished in the first stage, the spectrometer  208  of the photoluminescence-light-type damage level measuring device  202  performs a spectral analysis on photoluminescence light emitted from the surface of the substrate  142  and monitors the intensity of the photoluminescence light at a wavelength corresponding to the band gap of the substrate  142 , e.g., at the wavelength of 365 nm for GaN. As shown in  FIG. 17 , as the polishing process progresses, the intensity of the photoluminescence light at the wavelength of 365 nm increases. An endpoint of the polishing process in the first stage is detected by detecting when the intensity of the photoluminescence light at the wavelength of 365 nm reaches a predetermined value or becomes constant. 
     When the polishing process in the first stage is ended, the switch  150  is turned off to stop applying the voltage between the polishing tool  134  and the substrate  142  while the excitation light, preferably ultraviolet light, is being continuously applied from the light source  140 , thereby polishing the surface of the substrate  142  in the second stage for improving the surface roughness of the substrate  142 . The surface of the substrate  142  is polished in the second stage at a polishing rate up to about 200 nm/h., for a machining time of about 30 minutes, for example. While the surface of the substrate  142  is being polished in the second stage, the switch  206  is turned on to enable the ammeter  204  of the photocurrent-type damage level measuring device  201  to measure a current flowing along the conductive wire  152   c  which connects the substrate  142  and the metal films (metal wires)  154  of the polishing tool  134 . As shown in  FIG. 18 , as the polishing process progresses, the value of the current flowing through the ammeter  204  gradually increases until it becomes substantially constant in time. An endpoint of the polishing process in the second stage is detected by measuring the value of the current flowing through the ammeter  204  and detecting when the value of the current reaches a predetermined value or becomes constant. 
     After the polishing process in the second stage is ended, the excitation light, preferably the ultraviolet radiation, applied from the light source  140  is stopped, and the substrate holder  144  is raised, after which the rotation of the substrate  142  is stopped. If necessary, the surface of the substrate  142  is rinsed with pure water, and then the substrate  142  is transferred to the pusher  33  at the second transferring position TP 2 . The first linear transporter  5  transfers the substrate  142  on the pusher  33  to the lifter  35 . The polishing apparatus  30 B similarly receives the substrate  142  from the pusher  34  with the substrate holder  144 , and polishes the substrate  142  in the first stage and the second stage. Thereafter, the polished substrate  142  is transferred to the lifter  35  at the fourth transferring position TP 4 . 
     The second transfer robot  40  receives the substrate  142  from the lifter  35  and places the substrate  142  onto the lifter  36  at the fifth transferring position TP 5 . The second linear transporter  6  moves to transfer the substrate  142  on the lifter  36  to the sixth transferring position TP 6  or the seventh transferring position TP 7 . 
     The polishing apparatus  30 C receives the substrate  142  from the pusher  37  with the substrate holder  244 , and polishes the substrate  142  in the third stage. Specifically, the substrate holder  244 , which is holding the substrate  142 , such as a GaN substrate or the like, with its surface (surface to be processed) facing downwardly, is moved to a position above the container  234 , and then is rotated and lowered to immerse the substrate  142  in the water  232  held in the container  234 . The substrate holder  244  is further lowered to bring the surface of the substrate  142  into contact with the electrically conductive member  264 , such as of platinum, of the polishing pad  242 , which is being rotated, under a contact pressure (pressure force) in the range from 100 to 1000 hPa (from 0.1 to 1.0 kgf/cm 2 ), for example, preferably under a contact pressure of 400 hPa (0.4 kgf/cm 2 ). In this manner, the polishing process in the third stage is performed for finishing the surface of the substrate  142  by relatively moving the surface of the substrate  142  and the electrically conductive member  264  such as of platinum of the polishing pad  242  while holding them in contact with each other. The polishing process in the third stage may be performed while the water  232  is being supplied between the surface of the substrate  142  and the electrically conductive member  264 , such as of platinum, of the polishing pad  242 . 
     After the polishing process in the third stage is ended, the water  232  left on the surface of the substrate  142  is replaced with pure water by the pure water replacement section  192 , and then the substrate  142  is returned to the sixth transferring position TP 6 . The polishing apparatus  30 D similarly receives the substrate  142  from the pusher  38  with the substrate holder  244  and polishes (finishes) the surface of the substrate  142  in the third stage. The water  232  left on the surface of the substrate  142  after the polishing process is replaced with pure water by the pure water replacement section  194 , and then the substrate  142  is returned to the seventh transferring position TP 7 . Thereafter, the second linear transporter  6  moves to move the substrate  142 , on which the water  232  has been replaced with the pure water, to the fifth transferring position TP 5 . 
     The second transfer robot  40  takes the substrate out of the fifth transferring position TP 5  and transfers the substrate to the reversing machine  41 . The reversing machine  41  reverses the substrate through 180° and then transfers it to the first cleaning unit  42 , where the substrate is cleaned. The third transfer robot  46  transfers the substrate from the first cleaning unit  42  to the second cleaning unit  43 , where the substrate is cleaned. 
     The third transfer robot  46  transfers the substrate after cleaning to the third cleaning unit  44 , where the substrate is cleaned with pure water. The third transfer robot  46  transfers the substrate after pure water cleaning to the drying unit  45 , where the substrate is rinsed with pure water and then rotated at a high speed to spin-dry the substrate. The first transfer robot  22  receives the substrate after spin-drying from the drying unit  45  and returns the substrate to the substrate cassette mounted in the front loading section  200 . 
     An interference microscope image of a GaN substrate surface before it is polished is shown in  FIG. 19A , and the relationship between distances and heights on the substrate surface is shown in  FIG. 19B . An interference microscope image of the GaN substrate surface after it is polished in the first stage wherein the GaN substrate surface is polished at a polishing rate up to 1000 nm/h for 5 minutes with the excitation light and the voltage being applied is shown in  FIG. 20A , and the relationship between distances and heights on the substrate surface is shown in  FIG. 20B . An interference microscope image of the GaN substrate surface after it is polished in the second stage wherein the GaN substrate surface is polished at a polishing rate up to 200 nm/h for 30 minutes with only the excitation light being applied, after the polishing process in the first stage, is shown in  FIG. 21A , and the relationship between distances and heights on the substrate surface is shown in  FIG. 21B . An interference microscope image of the GaN substrate surface after it is polished in the third stage wherein the GaN substrate surface is polished at a polishing rate up to 10 nm/h for 180 minutes using platinum as the electrically conductive member and water with its pH adjusted to 5.5 by dissolving CO 2  in pure water, after the polishing process in the first stage and the polishing process in the second stage, is shown in  FIG. 22A , and the relationship between distances and heights on the substrate surface is shown in  FIG. 22B . 
     It can be seen from  FIGS. 19 through 22B  that the surface roughness Ra of the GaN substrate improved from Ra: 5.271 μm to Ra: 4.17 μm when it was polished in the first stage for 5 minutes, then improved to Ra: 0.363 μm when it was polished in the second stage for 30 minutes, and further improved to Ra: 0.083 μm when it was polished in the third stage for 180 minutes. It can thus be seen that a substrate having a sufficient level surface flatness can be obtained by polishing it for about 215 minutes. 
     Example 1  
     A GaN substrate was prepared by polishing a substrate surface in the first stage by applying excitation light and a voltage and then polishing the substrate surface in the second stage by applying only excitation light. The surface of the GaN substrate was polished (finished) in the third stage by the polishing apparatus shown in  FIG. 14 . In the polishing apparatus shown in  FIG. 14 , the electrically conductive member  264  was made of platinum, and water  232  with its pH adjusted to 5.5 by dissolving CO 2  in pure water with the gas dissolver  238  was poured into the container  234  provided by the dam member  236 . The surface of the GaN substrate was polished for 3 hours by rotating the turntable  230  at a rotational speed of 10 rpm and holding the substrate surface and the electrically conductive member (platinum)  264  in contact with each other under a contact pressure (pressing force) of 0.4 kgf/cm 2 . 
     An atomic force microscope (AFM) image of the GaN substrate surface after it is polished is shown in  FIG. 23A , the relationship between distances and heights on the substrate surface is shown in  FIG. 23B . Any etch pits depending on crystal defects are not observed from  FIG. 23A , and a clear terrace-step structure is observed from  FIG. 23B , indicating that the GaN substrate surface was polished in an atomic level to a flat damageless finish. 
     Comparative Example 1 
     A GaN substrate surface was polished under the same conditions as Example 1 except that an HF-dissolved solution was used instead of the water  232  with its pH adjusted to 5.5 by dissolving CO 2  in pure water with the gas dissolver  238 . 
     An atomic force microscope (AFM) image of the GaN substrate surface after it is polished is shown in  FIG. 24A , the relationship between distances and heights on the substrate surface is shown in  FIG. 24B . Although a clear terrace-step structure is observed from  FIG. 24B , strips of etch pits having a depth of 0.5 nm are observed on the terrace from  FIG. 24A . The etch pits are considered to result from selective etching of crystal defects, etc. with highly corrosive HF. 
     Example 2  
     A GaN substrate surface was polished under the same conditions as Example 1 except that a bonded substrate (epitaxial substrate) with GaN mounted on a surface of a sapphire substrate was used instead of the GaN substrate made of only GaN. 
     An atomic force microscope (AFM) image of the substrate surface after it is polished is shown in  FIG. 25A , the relationship between distances and heights on the substrate surface is shown in  FIG. 25B . Any etch pits depending on crystal defects are not observed from  FIG. 25A , and a clear terrace-step structure is observed from  FIG. 25B , indicating that the substrate surface was polished in an atomic level to a flat damageless finish. 
     In this Example 2, the bonded substrate with GaN mounted on the surface of the sapphire substrate. It is also possible to polish a bonded substrate with GaN mounted on a surface of a SiC substrate, a bonded substrate with GaN mounted on a surface of a Si substrate, or a bonded substrate with GaN mounted on a surface of a GaAs substrate. 
     In the above embodiment, the polishing process in the first stage and the polishing process in the second stage are performed by the same polishing apparatus. However, the polishing process in the first stage and the polishing process in the second stage may be performed by separate polishing apparatus. Though the third polishing process is performed on a substrate which has been polished in the first stage and the second stage in the above embodiment, the third polishing process ma be performed on a substrate which has been polished in the first stage or the second stage. 
     Furthermore, a substrate after it has been processed by CMP, ion implantation, activation annealing, or RIE may be finished (polished in the third stage) by the polishing apparatus shown in  FIG. 14 . 
       FIG. 26  is a schematic cross-sectional view of a polishing apparatus  30 E according to another embodiment of the present invention. The polishing apparatus  30 E is replaced with at least one of the polishing apparatuses  30 C,  30 D shown in  FIG. 9 , for example, and used for polishing process in the third stage. 
     As shown in  FIG. 26 , the polishing apparatus  30 E includes a container  132  for holding therein water  232 . Weak acid water, water with air dissolved therein, or electrolytic ion water is used as the water  232 . A turntable  302  having light permeability is mounted on the bottom of the container  300 , and a polishing pad  304  is mounted on an upper surface of the turntable  302 , so that the polishing pad  304  becomes immersed in the water  232  when the container  300  is filled with the water  232 . An electrically conductive member, for example, composed of platinum, is formed on at least a surface (upper surface) of the polishing pad  304  by vacuum evaporation or the like. 
     The container  300  is coupled to an upper end of a rotatable rotating shaft  306 . The bottom plate of the container  300  has a ring-shaped opening  300   a  formed around the rotating shaft  306  and closed by the turntable  302 . A light source  308  for emitting excitation light, preferably ultraviolet light, is disposed right below the opening  300   a.  The polishing pad  304  has a number of through holes  304  defined therein in a region corresponding the light source  308 . Thus, the excitation light, preferably ultraviolet light, emitted from the light source  308 , passes through the opening  300   a  of the container  300  and the through holes  304   a  of the polishing pad  304 , and reaches above the polishing pad  304 . 
     Right above the container  300  is disposed a substrate holder  310  for detachably holding a substrate  142 , e.g., a GaN substrate, with a front surface facing downwardly. The substrate holder  310  is coupled to a lower end of a main shaft  312  that is rotatable and vertically movable. The substrate holder  310  and the main shaft  312  are identical in structure to the substrate holder  144  and the main shaft  146  shown in  FIG. 10 , and will not be described in detail below. 
     The polishing apparatus  30 E of this embodiment is also provided with a power source  314  for applying voltage between the substrate  142  held by the substrate holder  310  and the polishing pad  304 . An ammeter  318  is interposed in a conductive wire  316   a  extending from the positive pole of the power source  314 . The conductive wire  316   a  extending from the positive pole of the power source  314  is connected to the substrate  142  held by the substrate holder  310 , and a conductive wire  316   b  extending from the negative pole of the power source  314  is connected to the polishing pad  304 . 
     When a substrate having deep scratches locally is polished, a large amount of shavings should be removed for removing the scratches. Since the polishing rate is lowered when most of a substrate is flattened as the polishing process progresses, it takes a long time to polish a substrate until the deep scratches locally present in the substrate are removed. In this case, the surface of the substrate  142  is polished by the polishing apparatus  30 E shown in  FIG. 26  while applying excitation light to the surface of the substrate  142  and voltage, as necessary, between the substrate  142  and the polishing pad  304 . By thus polishing the substrate  142 , the surface of the substrate  142  is polished while minute etch pits (1-2 nm) are being formed on the surface of the substrate  142  because of the influence of the excitation light, as shown in  FIG. 27 . The surface of the substrate  142  is polished at the polishing rate equal to the polishing rate when the surface having surface roughness is polished because of the present of the etch pits, and therefore a large amount of shavings can be removed without lowering the polishing rate. By performing finish polishing process after the application of the excitation light and the voltage are stopped, the etch pits formed on the surface of the substrate can be removed. 
     If the substrate comprises a GaN substrate, then since the band gap of GaN is of 3.42 eV, the excitation light should preferably have a wavelength equal to or lower than the wavelength corresponding to the band gap of the workpiece, i.e., equal to or lower than 365 nm, e.g., should preferably have a wavelength of 312 nm. 
     For example, as described above, the polishing apparatus  30 A or  30 B performs the polishing process in the first stage for five minutes while applying the excitation light, preferably ultraviolet light, from the light source  140  and the voltage between the polishing tool  134  and the substrate  142 . The polishing apparatus  30 A or  30 B performs the polishing process in the second stage for  25  minutes successively while applying the voltage between the polishing tool  134  and the substrate  142  is stopped and the excitation light, preferably ultraviolet light, is being continuously applied from the light source  140 . 
     Next, the polishing apparatus  30 E shown in  FIG. 26  performs a first step polishing process in the third stage for 15 minutes while applying the excitation light, preferably ultraviolet light, from the light source  308  and the voltage between the polishing pad  304  and the substrate  142 . Then, the polishing apparatus  30 E performs a second step polishing process in the third stage for 25 minutes while applying the excitation light from the light source  408  and the voltage between the polishing pad  304  and the substrate  142  are stopped. 
     By thus dividing the polishing process in the third stage into two polishing step and performing the polishing process, a substrate, e.g., having deep scratches locally can be polished effectively. 
     In the above embodiment, the surface of the substrate  142  is polished while applying the excitation light to the surface of the substrate  142 , and then finishing the surface of the substrate  142  without applying the excitation light to the surface of the substrate  142 . A polishing process for polishing the surface of the substrate  142  while applying excitation light to the surface of the substrate  142 , and a polishing process for polishing the surface of the substrate  142  without applying excitation light to the surface of the substrate  142  may be repeated alternately. In this case, the surface of the substrate  142  is finally polished for finishing without applying excitation light to the surface of the substrate  142 . 
     In the above embodiment, the substrate  142  is polished while applying the excitation light to the surface of the substrate  142  and the voltage, if necessary, between the substrate  142  and the polishing pad  304 . The substrate  142  may be polished while applying the voltage between the substrate  142  and the polishing pad  304  without applying the excitation light to the surface of the substrate  142 . This can form etch pits on the surface of the substrate  142  and enhance the polishing rate. 
       FIG. 28  is a schematic cross-sectional view of a polishing apparatus  30 F according to yet another embodiment of the present invention. The polishing apparatus  30 F shown in  FIG. 28  is different from the polishing apparatus  30 E shown in  FIG. 26  is that: a polishing pad  320  comprising a light transmission area E 1  having through holes  320   a  and a light non-transmission area E 2  is used instead of the polishing pad  304 ; and the substrate holder  320  is configured to reciprocate between the light transmission area E 1  and the light non-transmission area E 2  on the polishing pad  320 . 
     According to this embodiment, the application of the excitation light to the substrate  142  held by the substrate holder  310  can be stopped by moving the substrate holder  310  from the light transmission area El to the light non-transmission area E 2  on the polishing pad  320  while applying the excitation from the light source  308 . 
       FIG. 29  is a schematic cross-sectional view of a polishing apparatus  30 G according to yet another embodiment of the present invention. The polishing apparatus  30 G is replaced with at least one of the polishing apparatuses  30 C,  30 D shown in  FIG. 9 , for example, and used for polishing in the third stage. 
     As shown in  FIG. 29 , the polishing apparatus  30 G has a container  290  for holding therein water  232  such as weak acid water, water with air dissolved therein, or electrolytic ion water. A polishing pad  242   a  made of a light-permeable material is attached to the bottom of the container  290 . The polishing pad  242   a  is connected to an upper end of a rotatable rotating shaft  292 . The polishing apparatus  30 G also includes a Raman-light-type damage level measuring device  218  which comprises a laser beam source  214  for applying a visible monochromatic beam to a surface of a substrate  142  which is held by the substrate holder  244  and a spectrometer  216  for performing a spectral analysis on reflected light from the surface of the substrate  142  and measuring the intensity of Raman light. The other structural details are identical to those of the polishing apparatus shown in  FIG. 14 . 
     As shown in  FIG. 30 , the polishing pad  242   a  comprises a base  250  made of transparent glass or the like, for example, an elastic body  252  having a Shure hardness in the range from 50 to 90 disposed on a surface of the base  250 , the elastic body  252  having light-permeable through holes  252   a  defined therein at grid pattern positions for contact with the substrate, an intermediate layer  254  deposited on a surface of the elastic body  252  by vacuum evaporation or the like, and an electrically conductive member  256  deposited on a surface of the intermediate layer  254  by vacuum evaporation or the like. The electrically conductive member  256  that is disposed on the surface of the elastic body  252  with the intermediate layer  254  interposed therebetween finds it easy to follow irregularities in long/single periods on the surface (surface to be polished) of the substrate  142 . 
     The elastic body  252  may be made of rubber, resin, foamable resin, non-woven fabric or the like, for example, and has a thickness in the range from 0.5 to 5 mm. The elastic body  252  may comprise two or more superposed layers of elastic material which have different modulus of elasticity. 
     The intermediate layer  254  has a thickness in the range from 1 to 10 nm, for example. The intermediate layer  254  is interposed between the elastic body  252  and the electrically conductive member  256  in order to increase the adhesion between the elastic body  252  and the electrically conductive member  256 , and is made of chromium or graphite (SP 2 -bonded) carbon, for example, which have good adhesion to both the elastic body  252  and the electrically conductive member  256 . When the intermediate layer  254  is formed on the surface of the elastic body  252  by vacuum evaporation, it is preferable to employ ion sputtering deposition in order to suppress expansion and modification of the elastic body  252  due to high temperatures. This holds true also when the electrically conductive member  256  is formed on the surface of the intermediate layer  254  by vacuum evaporation. 
     The electrically conductive member  256  has a thickness in the range from 100 to 1000 nm, for example. If the thickness is smaller than 100 nm, then the electrically conductive member  256  will be unduly worn when the polishing process is carried out for about one hour, and hence is not practical. If the thickness is greater than 1000 nm, then the surface of the electrically conductive member  256  will tend to crack when the polishing process is carried out. The electrically conductive member  256  is preferably made of platinum, but may be made of any of precious metals such as gold, transition metals (Ag, Fe, Ni, Co, etc.), graphite, electrically conductive resin, electrically conductive rubber, electrically conductive organic matter etc. which are insoluble or slightly soluble (at a solving rate of 10 nm/h. or lower) in the water  232 . 
     When the polishing apparatus  30 G shown in  FIG. 29  performs the polishing process in the third stage, the laser beam source  214  of the Raman-light-type damage level measuring device  218  applies a visible monochromatic beam to the surface of the substrate  142  held by the substrate holder  244  through the base  250  and the light-permeable through holes  252   a  in the elastic body  252  of the polishing pad  242   a,  and the spectrometer  216  performs a spectral analysis on the reflected light from the surface of the substrate  142  and measures the intensity of Raman light. As described above, as the polishing process progresses, the intensity of the Raman light increases, as shown in  FIG. 8 . Therefore, an endpoint of the polishing process in the third stage is detected by detecting when the intensity of the Raman light reaches a predetermined value or becomes constant. 
     In the above embodiment, the endpoint of the polishing process in the first stage is detected by the photoluminescence-light-type damage level measuring device  202 . However, the endpoint of the polishing process in the first stage may be detected by the Raman-light-type damage level measuring device  218  shown in  FIG. 29 . Alternatively, the endpoint of the polishing process in the first stage may be detected by a combination of the photoluminescence-light-type damage level measuring device  202  and the Raman-light-type damage level measuring apparatus  218  based on an average value of their output signals. 
     Though the endpoint of the polishing process in the second stage is detected by the photocurrent-type damage level measuring device  201 , it may be detected by the photoluminescence-light-type damage level measuring device  202  or the Raman-light-type damage level measuring apparatus  218  shown in  FIG. 29 , or two or more damage level measuring apparatus based on an average value of their output signals. The polishing process in the first stage and the polishing process in the second stage are performed using a pH buffer solution of a neutral pH which contains Ga ions. However, the detection of the endpoints is not limited to these polishing processes. 
     Since SiC and GaN are permeable to visible light and infrared rays, visible light or infrared rays may be applied to a substrate, and the thickness of the substrate may be measured based on light reflected from the face and reverse sides of the substrate. A film thickness measuring device based on such a light interference process and the above processes of monitoring the progress of the polishing process may be combined and used complementarily. 
       FIG. 31  is a cross-sectional of still another polishing tool  134   b  for use in the polishing apparatuses  30 A,  30 B shown in  FIG. 10 , and  FIG. 32  is a plan view of the polishing tool  134   b  shown in  FIG. 31 . As shown in  FIGS. 31 and 32 , the polishing tool  134   b  comprises a support platen  400  having a number of concentric grooves  400   a  defined therein in a region corresponding to the opening  132   a  of the container  132  and metal lines  402  disposed as metal wires in the grooves  400   a,  and a catalyst pad  408  including a pad base  404  disposed on the support platen  400  and having a plurality of through holes  404   a  defined therein in a scattered pattern for passing therethrough at least one of light and an ion current and a catalyst layer  406  deposited on the pad base  404  by vacuum evaporation or the like in areas except the through holes  404   a.    
     As shown in  FIG. 33 , the metal lines  402  extend fully diametrically across the substrate  142  which is held by the substrate holder  144 . As shown in  FIG. 32 , the through holes  404   a  are positioned in a region where the metal lines  402  are disposed. 
     The catalyst pad  408  is of a diameter which is substantially the same (same shape) as the diameter of the support platen  400  or smaller (small shape) than the diameter of the support platen  400 . The catalyst pad  408  is detachably mounted on the support platen  400  by screws or double-faced adhesive tapes at its outer circumferential edge. The polishing tool  134   b  itself is also detachably mounted on the bottom plate of the container  132  by mechanical fasteners such as screws or the like. 
     Since the support platen  400  and the catalyst pad  408  are separate from each other and the catalyst pad  408  has a high level of surface flatness and does not tend to produce burrs, scratches, etc., the polishing tool  134   b  is capable of producing a flat polished surface free of burrs, scratches, etc. Further, when the catalyst layer  406  is worn, the polishing tool  134   b  is replaced with a new one as follows: The polishing tool  134   b  is removed from the bottom plate of the container  132 , and then the catalyst pad  408  is detached from the support platen  400 . A new catalyst pad  408  is mounted on the support platen  400 , and the polishing tool  134   b  with the new catalyst pad  408  is mounted on the bottom plate of the container  132 . As the support platen  400  can be reused, the polishing tool  134   b  is inexpensive and highly durable. 
     The support platen  400  is made of a light-permeable material such as glass such as synthetic quartz or the like or light-permeable resin such as acrylic resin or the like. The metal lines  402  as metal wires are made of platinum, gold, silver, copper, aluminum, or the like. The pad base  404  is made of glass, rubber, light-permeable resin, foamable resin, or non-woven fabric. 
     The catalyst pad  408  is constructed of the catalyst layer  406  and the pad base  404 . The catalyst layer  406  is made of one or more of a precious metal, a transition metal, a ceramics-based solid catalyst, a base solid catalyst, and an acid solid catalyst, for example. The precious metal comprises one of platinum, gold, and silver or a combination thereof. The transition metal comprises at least one of Fe, Ni, Co, Cu, Cr, i, molybdenum, and a compound thereof, or a combination thereof The base solid catalyst or the acid solid catalyst comprises one of non-woven fabric, resin, and metal, an acid or base metal oxide, and glass with an ion exchange function or a combination thereof. 
     In this embodiment, the catalyst layer  406  is formed on the surface of the pad base  404  by depositing one of the above-described materials or a combination thereof by the way of vacuum evaporation. The catalyst layer  406  can be formed uniformly to a desired thickness on the surface of the pad base  404  by vacuum evaporation. 
     The catalyst pad  408  may be made of only quartz glass. If the catalyst pad  408  is made of quartz glass, it can be separate from and detachably mounted on the support platen  400 , so that the metal lines (metal wires)  402  in the support platen  400  can be reused. 
     The pad base  404  should preferably is made of an elastic material such as rubber or the like. In this case, since the pad base  404  is elastically deformable along the surface (surface to be processed) of the substrate  142 , even if the catalyst pad  408  has surface irregularities, they are prevented from being transferred to the surface of the substrate  142 . For example, if the pad base  404  is made of resin and the adhesion between the pad base  404  of resin and the catalyst layer  406  is weak, then carbon is evaporated on the surface of the pad base  404  of resin, and the catalyst layer  406  is formed on the carbon by evaporation, so that the adhesion between the pad base  404  of resin and the catalyst layer  406  is increased by the carbon. The catalyst pad  408  with increased adhesion as a whole can thus be produced. 
     In this embodiment, as shown in  FIG. 35 , the support platen  400  has a plurality of through holes  406   b  defined therein, and the metal wires  402  extend through the through holes  400   b  to the surface of the support platen  400 . The metal lines (metal wires)  402  are disposed along the grooves  400   a  defined concentrically in the surface of the support platen  400 , and the conductive wire  152   b  extending from the cathode of the power source  148  is connected to the metal lines  402 . 
       FIGS. 36 and 37  show yet another polishing tool  134   c  for use in the polishing apparatuses  30 A,  30 B shown in  FIG. 10 . The polishing tool  134   c  is different from the polishing tool  134   b  shown in  FIGS. 31 and 32  in that metal films  412  serving as metal wires are deposited on a flat surface of a support platen  410  by vacuum evaporation. The catalyst pad  408 , which is identical to the catalyst pad  408  described above, is detachably mounted on the surface of the support platen  410  by screws, double-faced adhesive tapes, or the like. The metal films (metal wires)  412  comprise a plurality of concentric metal films  412   a  and at least one straight connecting metal film  412   b  which connects the concentric metal films  412   a.  The connecting metal film  412   b  has an electrode  412   c  at a proximal end thereof to which the conductive wire  152   b  extending from the cathode of the power source  148  is connected. 
     In this embodiment, as shown in  FIG. 37 , the concentric metal films  412   a  extend fully diametrically across the substrate  142  which is held by the substrate holder  144 . The retainer ring  172  has opposite sides slightly projecting from the region where the concentric metal films  412   a  are disposed. 
     As shown in  FIG. 38 , the support platen  410  may be divided into a plurality of zones (five zones in  FIG. 38 ), and concentric metal films  412   a  disposed in the respective zones may be connected individually to a plurality of connecting metal films  412   b.  The connecting metal films  412   b  may have respective electrodes  412   c  connected individually to a plurality of conductive lines extending respectively from the cathodes of a plurality of power sources for controlling polishing rates in the respective zones. 
       FIG. 39  shows a still further polishing tool  134   d  for use in the polishing apparatuses  30 A,  30 B shown in  FIG. 10 . The polishing tool  134   d  is different from the polishing tool  134   b  shown in 
       FIGS. 31 and 32  in that a light-permeable wiring film  416  made of fluororesin, acrylic resin, or the like and having a thickness in the range from 10 to 500 μm, for example, is interposed between a flat surface of a support platen  410  and the catalyst pad  408 . A wiring pattern  418  serving as metal wires is evaporated on an upper surface of the wiring film  416 . The wiring pattern (metal wires)  418  comprises a plurality of concentric metal films as shown in  FIG. 37 , for example. The concentric metal films may be divided into a plurality of zones as shown in  FIG. 38 . 
       FIG. 40  shows another polishing pad  242   b  for use in the polishing apparatuses  30 C,  30 D shown in  FIG. 14 . The polishing pad  242   b  is different from the polishing pad  242  shown in FIG.  15  in that it includes a support platen  280  made of a light-permeable material such as glass such as synthetic quartz or the like or light-permeable resin such as acrylic resin or the like, an elastic base  282  made of an elastic material and having a number of through holes  282   a  defined therein, the elastic base  282  being disposed on an upper surface of the support platen  280 , an intermediate layer  284  deposited on a surface of the elastic base  282  in areas except the through holes  282   a  by vacuum evaporation or the like, and an electrically conductive member  286  deposited on an upper surface of the intermediate layer  284  by vacuum evaporation or the like. 
     INDUSTRIAL APPLICABILITY 
     The polishing method and the polishing apparatus according to the present invention are used to polish flatwise a surface (surface to be processed) of a substrate such as a single substrate made of a compound semiconductor (e.g., GaN) containing an element such as Ga, Al, In, or the like, or a bonded substrate (epitaxial substrate) on which a compound semiconductor containing an element such as Ga, Al, In, or the like is mounted.