Abstract:
A flattening method, by utilizing the advantages of the CARE method and making up for the disadvantages, can perform removal processing of a surface of a workpiece at a sufficient processing rate and can provide a processed surface having enhanced flatness without leaving damage in the processed surface. A flattening method comprises at least two surface removal steps and at least two cleaning steps, the final surface removal step being a catalyst-referred etching step comprising immersing a workpiece in a processing solution containing at least one of hydrohalic acid, hydrogen peroxide water and ozone water, and bringing a surface of a catalyst platen into contact with or close proximity to a surface to be processed of the workpiece to process the surface, said catalyst platen having in a surface a catalyst selected from the group consisting of platinum, gold, a ceramic solid catalyst, a transition metal, glass, and an acidic or basic solid catalyst.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a flattening method and a flattening apparatus, and more particularly to a flattening method and a flattening apparatus for flatly processing a surface (surface to be processed) of a workpiece, such as a simple substrate consisting of SiC or GaN, a bonded substrate (epitaxial substrate) having a layer of SiC or GaN, or a substrate for use in MEMS (micro electro mechanical system). 
         [0003]    2. Description of the Related Art 
         [0004]    With the recent progress toward higher integration of semiconductor devices, the circuit interconnects are becoming finer and the distance between adjacent interconnects is becoming smaller. Especially when forming a circuit pattern by optical lithography with a line width of not more than 0.5 μm, a stepper requires a high flatness of imaging surface because of the small depth of focus. A CMP apparatus for carrying out chemical mechanical polishing (CMP) is known as a means for flattening a surface of such a semiconductor substrate. 
         [0005]    SiC, GaN, etc. are becoming increasingly important as a material for semiconductor devices. As a new processing method for flatly processing with high precision a surface of SiC or GaN, a CARE (catalyst-referred etching) method has been proposed which utilizes a catalytic action, capable of a chemical reaction, to etch a surface of a workpiece. The CARE method comprises immersing a workpiece in a processing solution containing hydrohalic acid, such as hydrofluoric acid; placing a platinum, gold or ceramic solid catalyst in contact with or close proximity to a surface (surface to be processed) of the workpiece so as to cause molecular dissociation of hydrogen halide at the surface of the catalyst, thereby generating a halogen radical; and dissolving a halogen compound, which has been produced by chemical reaction between the halogen radical and a surface atom of the workpiece, in the processing solution. The surface of the workpiece is processed (etched) in this manner (see e.g., Japanese Patent Laid-Open Publication No. 2006-114632). 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The CARE method, which carries out removal processing (etching) of a surface of a workpiece, e.g., composed of SiC or GaN, has the advantageous features that it can process a surface (surface to be processed) of a workpiece with higher precision as compared to conventional processing methods such as CMP and can obtain a processed surface having high flatness and, in addition, can process the workpiece without leaving damage in the processed surface. However, the CARE method, in which removal processing (etching) of a surface of a workpiece progresses through dissolution in a processing solution of a halogen compound produced by chemical reaction between a halogen radical and a surface atom of the workpiece, has the drawback that the processing rate is significantly low as compared to grinding, CMP, etc. which are commonly used in the manufacturing of a semiconductor device. Thus, high-throughput processing is difficult with the CARE method. 
         [0007]    The present invention has been made in view of the above situation in the related art. It is therefore an object of the present invention to provide a flattening method and a flattening apparatus which, by utilizing the advantages of the CARE method and making up for the disadvantages, can perform removal processing of a surface of a workpiece at a sufficient processing rate and can provide a processed surface having enhanced flatness without leaving damage in the processed surface. 
         [0008]    In order to achieve the object, the present invention provides a flattening method comprising at least two surface removal steps and at least two cleaning steps, the final surface removal step being a catalyst-referred etching step comprising immersing a workpiece in a processing solution containing at least one of hydrohalic acid, hydrogen peroxide water and ozone water, and bringing a surface of a catalyst platen into contact with or close proximity to a surface to be processed of the workpiece to process the surface to be processed, said catalyst platen having in a surface a catalyst selected from the group consisting of platinum, gold, a ceramic solid catalyst, a transition metal, glass, and an acidic or basic solid catalyst. 
         [0009]    The “surface removal step” is the step of processing and removing a surface portion of a workpiece, such as a substrate, and the “cleaning step” is the step of removing particulate contaminants, organic contaminants, metal contaminants, etc. from the surface of the workpiece before or after the surface removal step. The cleaning step also includes the step of removing an oxide film remaining on the surface of the workpiece after the surface removal step. 
         [0010]    According to this flattening method, after carrying out at least one surface removal step to enhance the flatness of a surface to be processed of a workpiece, CARE processing, which can flatly process the surface (surface to be processed) of the workpiece with high precision, is carried out in the final surface removal step. This makes it possible to process the workpiece at a sufficient processing rate while gradually enhancing the flatness of the surface to be processed and finally create a damage-free, very flat processed surface. 
         [0011]    The processing solution may also contain a wetting improver for improving wetting of the catalyst platen. Nitric acid or ethanol may be used as the wetting improver. 
         [0012]    When processing is carried out using hydrofluoric acid as a processing solution and platinum as a catalyst, because of poor wetting of a surface of platinum with hydrofluoric acid, it is difficult to efficiently supply the processing solution (hydrofluoric acid) to the catalyst surface which serves as a processing reference surface. In such a case, efficient supply of the hydrofluoric acid processing solution to the catalyst surface becomes possible by adding a wetting improver to the processing solution. Preferable examples of the wetting improver include nitric acid and ethanol. 
         [0013]    The processing solution may also contain a buffering agent for pH adjustment. 
         [0014]    The processing solution may also contain an organic alcohol or an inorganic acid. Methanol or ethanol may be used as the organic alcohol, and sulfuric acid or nitric acid may be used as the inorganic acid. 
         [0015]    The processing solution may also contain a buffering agent. 
         [0016]    A buffering agent is a solution which prevents a change in the pH of the processing solution even when a small amount of an acid or base is added to the processing solution. Especially when processing GaN, because of the formation of its oxide Ga 2 O 3  which is soluble in an acid or a base, it is necessary to keep the processing solution at a pH around 7 so as to prevent dissolution of Ga 2 O 3  in the processing solution. The pH of the processing solution can be adjusted by adding a pH-adjusted buffering agent to the processing solution. Further, a buffering agent has the property of consuming H +  ions or OH +  ions generated, and therefore can shorten the active distance of H +  ions or OH +  ions in the vicinity of an acidic or basic solid catalyst. Thus, the use of a buffering agent can cause H +  ions or OH +  ions to be present only in close proximity to a solid catalyst, making it possible to more flatly process a workpiece. 
         [0017]    The present invention also provides a flattening method comprising at least two surface removal steps and at least two cleaning steps, the final surface removal step being a catalyst-referred etching step comprising immersing a workpiece in a processing solution containing a buffering agent, and bringing a surface of a catalyst platen into contact with or close proximity to a surface to be processed of the workpiece to process the surface to be processed, said catalyst platen having in the surface a catalyst selected from the group consisting of platinum, gold, a ceramic solid catalyst, a transition metal, glass, and an acidic or basic solid catalyst. 
         [0018]    At least one of the surface removal steps other than the final surface removal step may be grinding, lapping, CMP or light irradiation catalyst-referred etching of the surface to be processed of the workpiece, the light irradiation catalyst-referred etching corresponding to said catalyst-referred etching step as carried out under light irradiation. 
         [0019]    For example, it is possible to first carry out a surface removal step by grinding or lapping which, though the processing accuracy is not so high, can be carried out at a relatively high processing rate, and also carry out, according to necessity, CMP or light irradiation catalyst-referred etching, i.e., catalyst-referred etching carried out under light irradiation, and lastly carry out the catalyst-referred etching step as the final surface removal step. This flattening process can flatten a surface to be processed, having relatively large initial irregularities, of a workpiece into a processed surface having a desired flatness in a shorter time. The grinding may be carried out by, for example, electrolytic in-process dressing (ELID) mirror grinding. 
         [0020]    CMP or light irradiation catalyst-referred etching, i.e., catalyst-referred etching carried out under light irradiation, though inferior to catalyst-referred etching in terms of the flatness of a processed surface, can perform surface removal processing at a higher processing rate as compared to catalyst-referred etching. Accordingly, by carrying out catalyst-referred etching after carrying out CMP or light irradiation catalyst-referred etching, the overall processing rate can be increased. 
         [0021]    At least one of the cleaning steps may be SPM (sulfuric acid-hydrogen peroxide mixture) cleaning, aqua regia cleaning or hydrofluoric acid cleaning. 
         [0022]    A surface to be processed of a workpiece can be cleaned by aqua regia cleaning when the surface to be processed is contaminated with platinum, by SPM cleaning when the surface to be processed is contaminated with organic matter or with a metal other than noble metals, or by hydrofluoric acid cleaning when an oxide film is formed in the surface to be processed. 
         [0023]    At least one of the cleaning steps may be rinsing of the workpiece in a cleaning unit. 
         [0024]    At least one of the cleaning steps may be rinsing of the workpiece carried out around the catalyst platen. 
         [0025]    The present invention also provides a flattening apparatus comprising at least two surface removal processing units and at least two cleaning units, the surface removal processing unit for carrying out the final surface removal processing being a catalyst-referred etching unit comprising: a catalyst platen having in a surface a catalyst selected from the group consisting of platinum, gold, a ceramic solid catalyst, a transition metal, glass, and an acidic or basic solid catalyst; a holder for holding a workpiece and bringing a surface to be processed of the workpiece into contact with or close proximity to the surface of the catalyst platen; a processing solution supply section for supplying a processing solution, containing at least one of hydrohalic acid, hydrogen peroxide water and ozone water, to between the catalyst platen and the workpiece held by the holder and kept in contact with or close proximity to the surface of the catalyst platen; and a drive section for moving the catalyst platen and the workpiece, held by the holder and kept in contact with or close proximity to the surface of the catalyst platen, relative to each other. 
         [0026]    The present invention also provides a flattening apparatus comprising at least two surface removal processing units and at least two cleaning units, the surface removal processing unit for carrying out the final surface removal processing being a catalyst-referred etching unit comprising: a catalyst platen having in a surface a catalyst selected from the group consisting of platinum, gold, a ceramic solid catalyst, a transition metal, glass, and an acidic or basic solid catalyst; a holder for holding a workpiece and bringing a surface to be processed of the workpiece into contact with or close proximity to the surface of the catalyst platen; a processing solution supply section for supplying a processing solution containing a buffering agent to between the catalyst platen and the workpiece held by the holder and kept in contact with or close proximity to the surface of the catalyst platen; and a drive section for moving the catalyst platen and the workpiece, held by the holder and kept in contact with or close proximity to the surface of the catalyst platen, relative to each other. 
         [0027]    Preferably, a plurality of through-holes is formed in the catalyst platen. 
         [0028]    A processing solution can be uniformly supplied through the through-holes to the surface of the catalyst platen, whereby an entire surface to be processed of a workpiece can be processed more uniformly. 
         [0029]    Preferably, a plurality of concentric grooves is formed in the surface of the catalyst platen. 
         [0030]    A processing solution can be held in the grooves provided in the surface of the catalyst platen, thus allowing the processing solution to be present between a workpiece and the catalyst platen. 
         [0031]    The holder preferably has a retainer ring, made of the same material as the surface of the catalyst platen, for preventing escape of the workpiece. 
         [0032]    When the holder having the retainer ring is used, there is a case where the retainer ring contacts the catalyst platen and is chipped off, and the retainer ring chip adheres to the surface of the catalyst platen. Even in such a case, the retainer ring chip, because of the same material as the surface of the catalyst platen, will not adversely affect the removal processing reaction. 
         [0033]    In a preferred aspect of the present invention, the flattening apparatus further comprises a conditioning section for carrying out conditioning of the surface of the catalyst platen. 
         [0034]    Specific examples of the conditioning section include: a pure water jet nozzle for jetting pure water toward the catalyst platen while causing cavitation or applying ultrasonic waves as necessary; alight irradiator for applying light to the catalyst platen to remove a broken piece or an organic contaminant from the surface of the catalyst platen by photoelectrochemical etching; and an electrolytic removal apparatus which includes an electrode disposed opposite the catalyst platen and removes a broken piece or an organic contaminant from the surface of the catalyst platen by applying a voltage between the catalyst platen and the electrode. A broken piece which has been separated from the catalyst platen can be removed, e.g., by filtration. 
         [0035]    At least one of the surface removal processing units other than the surface removal processing unit for carrying out the final surface removal processing may be a grinding unit, a lapping unit, a CMP unit, or a light irradiation catalyst-referred etching unit having the same construction as said catalyst-referred etching unit but additionally comprising a light source. 
         [0036]    According to the present invention, after carrying out at least one surface removal step to enhance the flatness of a surface to be processed of a workpiece, CARE (catalyst-referred etching) processing, which can flatly process the surface (surface to be processed) of the workpiece with high precision, is carried out in the final surface removal step. This makes it possible to process the workpiece at a sufficient processing rate while gradually enhancing the flatness of the surface to be processed and finally create a damage-free, very flat processed surface. The present invention thus makes it possible to increase the throughput while fully enjoying the merits of CARE. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a plan view showing the overall construction of a flattening apparatus according to an embodiment of the present invention; 
           [0038]      FIG. 2  is a vertical sectional front view schematically showing a CARE unit used as a final surface removal processing unit in the flattening apparatus of  FIG. 1 ; 
           [0039]      FIG. 3  is a plan view of a catalyst platen for use in the CARE unit of  FIG. 2 ; 
           [0040]      FIG. 4  is an enlarged view of a portion of  FIG. 3 ; 
           [0041]      FIG. 5  is a plan view of another catalyst platen; 
           [0042]      FIG. 6  is a cross-sectional view showing the catalyst platen shown in  FIG. 5  as mounted to a rotating body; 
           [0043]      FIG. 7  is a graph showing the results of total reflection X-ray fluorescence analysis of platinum contamination as measured on a sample after CARE processing and on the processed sample after cleaning with aqua regia; 
           [0044]      FIG. 8  is a graph showing the results of total reflection X-ray fluorescence analysis of platinum contamination as measured on a sample after CARE processing, on the processed sample after cleaning with SPA and on the processed sample after additional cleaning with hydrofluoric acid (HF); 
           [0045]      FIGS. 9A and 9B  are graphs showing the results of X-ray photoelectron spectroscopy of iron contamination as measured on a sample after CARE processing and on the processed sample after cleaning with SPM; 
           [0046]      FIG. 10  is a graph showing the results of X-ray photoelectron spectroscopy of a surface of an SiC sample after light irradiation CARE processing; 
           [0047]      FIG. 11  is a graph showing the results of X-ray photoelectron spectroscopy of the surface of the processed SiC sample of  FIG. 10  as measured after cleaning the sample with hydrofluoric acid; 
           [0048]      FIGS. 12A through 12D  are diagrams illustrating the concept of CARE processing of a workpiece with the use of a processing solution containing a buffer solution (buffering agent); 
           [0049]      FIG. 13  is a diagram schematically showing a CARE unit (light irradiation CARE unit) provided with a light source; 
           [0050]      FIG. 14A  shows a phase-shifting interference microscopic image of a surface of a sample (GaN wafer) before CARE processing,  FIG. 14B  shows a phase-shifting interference microscopic image of the surface of the sample after carrying out CARE processing using a processing liquid (ultrapure water) not containing a buffer solution (buffering agent), and  FIG. 14C  shows a phase-shifting interference microscopic image of the surface of the sample after carrying out CARE processing using a processing solution containing a buffer solution (buffering agent); 
           [0051]      FIG. 15  shows XPC O1s spectra for a GaN substrate having a vapor-deposited platinum film on a back surface and for a GaN substrate having no platinum film on a back surface, as measured before and after carrying out light irradiation of the substrates while immersing the substrates in ultrapure water; and 
           [0052]      FIG. 16A  shows a phase-shifting interference microscopic image of a surface of a sample (GaN substrate), having no vapor-deposited platinum film on a back surface, before light irradiation CARE processing,  FIG. 16B  shows a phase-shifting interference microscopic image of the surface of the sample (GaN substrate) of  FIG. 16A  after light irradiation CARE processing, and  FIG. 16C  shows a phase-shifting interference microscopic image of a surface of a sample (GaN substrate), having a vapor-deposited platinum film on a back surface, after light irradiation CARE processing. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0053]    Preferred embodiments of the present invention will now be described with reference to the drawings. The following description illustrates the case of using hydrofluoric acid (HF) as a processing solution and platinum as a catalyst, and processing and removing a surface (surface to be processed) of a substrate, such as an SiC wafer, into a desired flatness. 
         [0054]      FIG. 1  is a plan view showing the overall construction of a flattening apparatus according to an embodiment of the present invention. As shown in  FIG. 1 , the flattening apparatus of this embodiment includes a generally rectangular housing  1  whose interior is divided by partition walls  1   a ,  1   b ,  1   c  into a loading/unloading section  2 , a surface removal processing section  3  and a cleaning section  4 . The loading/unloading section  2 , the surface removal processing section  3  and the cleaning section  4  are independently fabricated and independently ventilated. 
         [0055]    The loading/unloading section  2  includes at least two (three in this embodiment) front loading sections  20  each for placing a substrate cassette storing substrates (workpieces). The front loading sections  20  are arranged side by side in the width direction (perpendicular to the long direction) of the flattening apparatus. Each front loading section  20  can place an open cassette, a SMIF (standard manufacturing interface) pod or a FOUP (front opening unified pod). The SMIF and FOUP are closed containers which can house a substrate cassette and can keep an interior environment independent of the exterior environment. 
         [0056]    A traveling mechanism  21 , extending along the line of the front loading sections  20 , is provided in the loading/unloading section  2 . On the traveling mechanism  21  is provided a first transport robot  22  as a first transport mechanism, which is movable along the direction in which substrate cassettes are arranged. The first transport robot  22  can reach the substrate cassettes placed in the front loading sections  20  by moving on the traveling mechanism  21 . The first transport 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 transporting an unprocessed substrate. 
         [0057]    The loading/unloading section  2  is an area which 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 surface removal processing 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 traveling mechanism  21  for the first transport robot  22 . Clean air, from which particles, vapor and gas have been removed, continually blows off downwardly through the filter-fan unit. 
         [0058]    The surface removal processing section  3  is an area where removal processing of a surface (surface to be processed) of a substrate is carried out and, in this embodiment, includes therein a lapping unit  30 A as a first surface removal processing unit, a CMP unit  30 B as a second surface removal processing unit and two catalyst-referred etching (CARE) units  30 C,  30 D as third (final) surface removal processing units. As shown in  FIG. 1 , the lapping unit  30 A, the CMP unit  30 B and the CARE units  30 C,  30 D are arranged along the long direction of the flattening apparatus. 
         [0059]    The lapping unit  30 A includes a platen  300 A having a lapping surface, a top ring  301 A for detachably holding a substrate and pressing the substrate against the platen  300 A, a lapping liquid supply nozzle  302 A for supplying a lapping liquid, such as a diamond slurry or a colloidal silica slurry, to the platen  300 A, and a pure water supply nozzle  303 A for supplying pure water to the surface of the platen  300 A. During lapping in the lapping unit  30 A, the lapping liquid (slurry) is supplied from the lapping liquid supply nozzle  302 A onto the platen  300 A, and a substrate as a workpiece is held by the top ring  301 A and pressed against the platen  300 A to carry out lapping of the surface of the substrate. 
         [0060]    The lapping unit (first surface removal processing unit)  30 A is mainly directed to obtaining a large processing amount while enhancing the flatness of a substrate surface in the process of flattening, e.g., a substrate surface having relatively large initial irregularities into a desired flatness. The lapping unit  30 A can therefore be omitted when a surface to be processed of a substrate does not have large initial irregularities. Though in this embodiment the lapping unit  30 A is used as a first surface removal processing unit, it is also possible to use, instead of the lapping unit  30 A, a grinding unit, such as an electrolytic in-process dressing (ELID) mirror grinding unit, which can obtain a considerably larger processing amount than the CARE units  30 C,  30 D. 
         [0061]    The CMP unit (second surface removal processing unit)  30 B includes a polishing table  300 B having a polishing surface, a top ring  301 B for detachably holding a substrate and pressing the substrate against the polishing table  300 B to polish the substrate, a polishing liquid supply nozzle  302 B for supplying a polishing liquid or a dressing liquid (e.g., water) to the polishing table  300 B, a dresser  303 B for carrying out dressing of the polishing surface of the polishing table  300 B, and an atomizer  304 B for spraying a mixed fluid of a liquid (e.g., pure water) and a gas (e.g., nitrogen gas) in a mist form onto the polishing surface of the polishing table  300 B from one or a plurality of nozzles. 
         [0062]    A polishing cloth, abrasive grains (fixed abrasive grains), or the like, constituting a polishing surface for polishing a substrate surface, is attached to the upper surface of the polishing table  300 B of the CMP unit  30 B. During polishing in the polishing unit  30 B, a polishing liquid is supplied from the polishing liquid supply nozzle  302 B onto the polishing surface of the polishing table  300 B, and a substrate as a workpiece is held by the top ring  301 B and pressed against the polishing surface to carry out polishing of the surface of the substrate. 
         [0063]    The CMP unit (second surface removal unit)  30 B is to enhance the flatness of a substrate surface while processing the substrate at a higher processing rate, i.e., obtaining a larger processing amount, than the CARE units  30 C,  30 D. Thus, the CMP unit  30 B, when used in combination with the above-described lapping unit (first surface removal processing unit)  30 A, can effectively flatten a substrate surface having relatively large initial irregularities into a desired flatness. Depending on the degree of surface irregularities of the substrate to be processed, etc., however, the use of the CMP unit  30 B may be omitted. 
         [0064]    Though in this embodiment the CMP unit  30 B is used as a second surface removal processing unit, it is also possible to use, instead of the CMP unit  30 B, a light irradiation catalyst-referred etching unit (light irradiation CARE unit) which has the same construction as the CARE unit  30 C or  30 D shown in  FIGS. 2 through 4  but additionally includes a light source for applying light, preferably ultraviolet light, to a surface of a substrate held by a substrate holder and which, upon removal processing of a substrate surface, applies light to the substrate surface to activate it, so that the substrate surface can be processed at a higher processing rate than the CARE unit. It is also possible to use the light irradiation catalyst-referred etching unit (light irradiation CARE unit) shown in  FIG. 13 . 
         [0065]    As shown in detail in  FIG. 2 , the CARE units (third surface removal processing units)  30 C,  30 D each include a processing tank  124 , a catalyst platen  126  rotatably disposed in the processing tank  124 , and a substrate holder  130  for detachably holding a substrate (workpiece)  128  with its surface (surface to be processed) facing downwardly. The substrate holder  130  is made of, e.g., SiC which has excellent processability, chemical resistance and temperature resistance. It is also possible to use rigid PVC (polyvinyl chloride) or PEEK (polyether ether ketone) for the substrate holder  130 . The substrate holder  130  is coupled to the front end of a vertically-movable rotating shaft  132  which is disposed parallel to and eccentrically with respect to the axis of rotation of the catalyst platen  126 . The substrate holder  130  is supported (by a ball bearing) pivotably with respect to the rotating shaft  132 , so that the substrate holding surface of the substrate holder  130  can follow the surface of the catalyst platen  126  and the workpiece  128  can make plane contact with the catalyst platen  126 . 
         [0066]    In this embodiment, surface removal processing of the substrate  128 , such as an SiC wafer, in the CARE units  30 C,  30 D is carried out by using, e.g., hydrofluoric acid (50% HF) as a processing solution and using the catalyst platen  126 , e.g., composed of a 28 mm-thick molybdenum substrate  140  and a 1 mm-thick platinum sheet  142  as a catalyst attached to the substrate  140 . It is also possible to use a molybdenum or molybdenum alloy plate as the catalyst platen  126 , without providing a platinum sheet as a catalyst, and use the molybdenum or molybdenum alloy plate as a catalyst. Further, hydrohalic acid other than hydrofluoric acid, such as hydrochloric acid, may also be used as a processing solution. 
         [0067]    The processing solution used preferably contains as an additive a wetting improver for improving wetting of the surface of the catalyst platen. The wetting improver is a compound having a hydrophilic group and a hydrophobic group in the molecule and, when added to the processing solution, can improve wetting of the catalyst platen with the processing solution. This can increase the efficiency of supply of the processing solution to between a substrate (workpiece) and the catalyst platen, enabling stable surface removal processing. 
         [0068]    The processing solution may also contain as an additive a buffering agent for pH adjustment. By adjusting the pH of the processing solution with a buffering agent, a substrate (workpiece), in its area other than the removal processing reaction area, can be prevented from dissolving in the processing solution. Further, H +  ions or OH −  ions, which are active species generated at the surface of the catalyst platen, can be quickly deactivated, thereby limiting the removal processing reaction to the close vicinity of the surface of the catalyst platen. This can enhance a flatness of a processed surface of a substrate (workpiece). 
         [0069]    The processing solution may also contain an organic alcohol, such as methanol or ethanol, or an inorganic acid, such as sulfuric acid or nitric acid. F atoms or OH radicals generated at the surface of the catalyst platen are very active, and have a very short life, i.e., promptly lose their activity. There is therefore a case where a sufficient processing rate cannot be obtained. In such a case, by adding an alcohol such as ethanol or an inorganic acid such as nitric acid to the processing solution to cause the additive to react with F atoms or OH radicals, it becomes possible to secondarily generate organic radicals or nitrate radicals which have a longer life than F atoms or OH radicals, thereby increasing the processing rate. 
         [0070]    The above-described additives (wetting improver, pH adjuster, organic alcohol, inorganic acid) may be used either singly or in a combination of two or more. 
         [0071]    Further, the processing solution may also contain a buffering agent (buffer solution) capable of preventing a change in the pH of the processing solution. 
         [0072]    A heater  170 , embedded in the substrate holder  130  and extending into the rotating shaft  132 , is provided as a temperature control mechanism for controlling the temperature of the substrate  128  held by the substrate holder  130 . Above the processing tank  124  is disposed a processing solution supply nozzle  174  for supplying the processing solution (hydrofluoric acid), which is controlled at a predetermined temperature by a heat exchanger  172  as a temperature control mechanism, into the processing tank  124 . Furthermore, a fluid passage  176  as a temperature control mechanism for controlling the temperature of the catalyst platen  126  is provided in the interior of the catalyst platen  126 . 
         [0073]    Though the heater  170  as a temperature control mechanism for controlling the temperature of the substrate  128 , the heat exchanger  172  as a temperature control mechanism for controlling the temperature of the processing solution, and the fluid passage  176  as a temperature control mechanism for controlling the temperature of the catalyst platen  126  are used in this embodiment, it is possible to omit all of the temperature control mechanisms or to employ any one of them. 
         [0074]    As is known by the Arrhenius equation, with reference to a chemical reaction, the higher the reaction temperature, the higher is the reaction rate. Thus, by controlling at least one of the temperature of the workpiece  128 , the temperature of the processing solution and the temperature of the catalyst platen  126  so as to control the reaction temperature, the processing rate can be adjusted or changed in such a manner as to stabilize the processing. 
         [0075]    As shown in  FIGS. 3 and 4 , a plurality of concentric grooves  144  are formed over three groove formation areas  146   a ,  146   b ,  146   c  in the surface of the platinum (catalyst) layer  142  of the catalyst platen  126 . In this embodiment, the width of the groove formation area  146   b , lying in the middle of the three groove formation areas, is smaller than the diameter of the substrate  128  held by the substrate holder  130 . This allows two ends of the substrate  128 , held by the substrate holder  130 , to lie in the flat area between the groove formation areas  146   a  and  146   b  and in the flat area between the groove formation areas  146   b  and  146   c , respectively, so that the ends of the substrate  128 , held and being rotated by the substrate holder  130 , will not be caught in a groove  144 . When the use of a thin platinum (catalyst) layer  142  is intended, the depth of the grooves formed in the layer should necessarily be small accordingly. When the width, the depth and the pitch of the grooves are made small, however, the fluidity of a fluid will become poor and a large attraction (adsorption) power will be generated between a substrate and the catalyst. It is therefore desirable that the depth D of the grooves be made as small as possible within the acceptable range of adsorption power. 
         [0076]    As shown in phantom line in  FIG. 3 , when using a substrate holder provided with a retainer ring  148  surrounding the circumference of the substrate  128 , the overall width of the groove formation areas  146   a ,  146   b ,  146   c  is made smaller than the diameter of the retainer ring  148  so that two ends of the rotating retainer ring  148  are allowed to lie in the flat areas outside the groove formation areas  146   a ,  146   c  and will not be caught in a groove  144 . In this case, in order to prevent end portions, etc. of the substrate  128  from being caught in a groove  144 , the grooves  144  have not a rectangular but an r-shaped cross-section, as shown in  FIG. 4 . The grooves may have any tapered cross-section. 
         [0077]    The pitch (groove pitch) of the grooves  144  is preferably as small as possible in order to efficiently supply the processing solution to the area of contact (processing area) between the catalyst platen  126  and the substrate  128 . In this embodiment, as shown in  FIG. 4 , the groove pitch P is set about 3 to 5 mm, and the groove depth D is set about 0.5 to 1.0 mm. To make the platinum catalyst layer  142  thin, the groove depth D is preferably made small to such an extent as not cause unacceptable attraction between the platinum layer  142  and the substrate  128 . When the large number of concentric grooves  144  are provided in the surface of the platinum (catalyst) layer  142  of the catalyst platen  126  as in this embodiment, it is preferred to pivot the substrate  128  with the substrate holder  130  in order to enhance the uniformity of processing amount in the entire surface to be processed of the substrate  128 . Instead of the concentric grooves, eccentric grooves or a spiral groove may also be employed. 
         [0078]    When the processing solution (hydrofluoric acid) is supplied from the processing supply nozzle  174  to the catalyst platen  126 , the processing solution is held in the grooves  144  provided in the surface of the catalyst platen  126 . While pressing the substrate (workpiece)  128 , held by the substrate holder  130 , against the surface of the platinum (catalyst) layer  142  of the catalyst platen  126  at a predetermined pressure and allowing the processing solution to be present in the area of contact (processing area) between the substrate  128  and the platinum (catalyst) layer  142  of the catalyst platen  126 , the catalyst platen  126  and the substrate  128  are rotated, thereby flatly processing (etching) the surface (lower surface) of the substrate  128  such as an SiC wafer. Instead of pressing the substrate  128 , held by the substrate holder  130 , against the platinum (catalyst) layer  142  of the catalyst platen  126  at a predetermined pressure, it is also possible to keep the substrate  128  in close proximity to the platinum (catalyst) layer  142  in carrying out removal processing (etching) of the surface of the substrate  128 . 
         [0079]    When using a substrate holder having the retainer ring  148  for preventing escape of a substrate, it is preferred that at least that portion of the retainer ring  148 , which is to face the catalyst platen  126 , be made of the same material as the surface material of the catalyst platen  126 . For example, when platinum is used as the surface material (catalyst) of the catalyst platen  126 , that portion of the retainer ring  148 , which is to face or contact the catalyst platen  126 , is preferably made of platinum. Similarly, when iron is used as the surface material of a catalyst platen, that portion of the retainer ring, which is to face or contact the catalyst platen, is preferably made of iron. 
         [0080]    There is a case where the surface material of the retainer ring  148  contacts the catalyst platen  126  and is chipped off, and the retainer ring chip adheres to the surface of the catalyst platen  126 . Even in such a case, if the surface material of the retainer ring is made of the same material as the surface material of the catalyst platen  126 , processing can be continued without loss of the catalytic effect of the catalyst platen  126 . The surface material of the retainer ring  148  may also be an alloy rich in the same metal as the surface metal of the catalyst platen  126 . For example, when iron is used as the surface metal of the catalyst platen, the surface material of the retainer ring may be an iron-rich alloy, such as carbon steel or stainless steel. 
         [0081]    It is also possible to use a catalyst platen  200  which, as shown in  FIGS. 5 and 6 , has a large number of through-holes  200   a  in its area which is to contact the substrate (workpiece)  128  held by the substrate holder  130  shown in  FIG. 2 . As with the above-described catalyst platen  126 , the catalyst platen  200  may be composed of a molybdenum substrate and a platinum sheet as a catalyst attached to the substrate. 
         [0082]    In this embodiment, the through-holes  200   a  are formed in equally-spaced lattice positions in the catalyst platen  200 . The lattice spacing is, for example, 5 mm and the diameter of the through-holes  200   a  is, for example, 1 mm. The lattice spacing and the hole diameter of the through-holes  200   a  are preferably as small as possible. The provision of the uniformly distributed though-holes  200   a  in the catalyst platen  200  enables uniform processing of the entire surface of the substrate (workpiece)  128 . 
         [0083]    A recess  200   b  is formed in the lower surface of the catalyst platen  200  and, when the catalyst platen  200  is mounted on the upper surface of a rotating body  202 , a processing solution reservoir  204  is formed between the catalyst platen  200  and the rotating body  202 . The catalyst platen  200  has a vertical through-hole in the center, and a rotary joint  206  is mounted to the through-hole. The rotary joint  206  is connected to the processing solution supply nozzle  174  shown in  FIG. 2 , for example. 
         [0084]    In this embodiment, processing of the substrate (workpiece)  128  is carried out in the following manner: While rotating the catalyst platen  200  together with the rotating body  202 , the processing solution is supplied through the rotary joint  206  into the processing solution reservoir  204  between the catalyst platen  200  and the rotating body  202 , and the processing solution is spouted from the through-holes  200   a . This manner of processing can prevent the substrate (workpiece)  128  from sticking to the catalyst platen  200  and, at the same time, can replace the processing solution remaining in the through-holes  200   a.    
         [0085]    The pressure of the processing solution spouting from the through-holes  200   a  is preferably much lower than the pressure of the substrate (workpiece)  128  on the catalyst platen  200 . In consideration of the strength of the catalyst platen, it is also possible to form radial grooves extending from the center of the catalyst platen so that the processing solution will be supplied through the grooves to below the through-holes. 
         [0086]    As shown in  FIG. 1 , each of the CARE units  30 C,  30 D is provided with a pure water jet nozzle  150  as a conditioning section for carrying out conditioning of the surface of the catalyst platen  126  by jetting pure water toward the surface platinum (catalyst) layer  142  of the catalyst platen  126  while causing cavitation or applying ultrasonic waves, as necessary. It is also possible to use as a conditioning section a light irradiator for applying light to the catalyst platen to remove a broken piece or an organic contaminant from the surface of the catalyst platen by photoelectrochemical etching, or an electrolytic removal apparatus which includes an electrode disposed opposite the catalyst platen and electrolytically removes a broken piece or an organic contaminant from the surface of the catalyst platen by applying a voltage between the catalyst platen and the electrode. A broken piece, which has been separated from the catalyst platen  126 , can be removed, e.g., by filtration. 
         [0087]    Between the lapping unit  30 A, the CMP unit  30 B and the cleaning unit  4  is disposed a first linear transporter  5  as a second (translatory) transport mechanism for transporting a substrate between four transport positions (first transport position TP1, second transport position TP2, third transport position TP3 and fourth transport position TP4 in order of distance from the loading/unloading section  2 ) along the long direction of the apparatus. A reversing machine  31  for reversing a substrate received from the first transport robot  22  is disposed above the first transport position TP1 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 transport position TP2, a vertically-movable pusher  34  is disposed below the third transport position TP3, and a vertically-movable lifter  35  is disposed below the fourth transport position TP4. 
         [0088]    Beside the CARE units  30 C,  30 D and adjacent to the first linear transporter  5  is disposed a second linear transporter  6  as a second (translatory) transport mechanism for transporting a substrate between three transport positions (fifth transport position TP5, sixth transport position TP6 and seventh transport position TP7 in order of distance from the loading/unloading section  2 ) along the long direction of the apparatus. A vertically-movable lifter  36  is disposed below the fifth transport position TP5, a pusher  37  is disposed below the sixth transport position TP6, and a pusher  38  is disposed below the seventh transport position TP7. Further, a pure water replacement section  160  including a tub and a pure water nozzle is disposed between the CARE unit  30 C and the pusher  37 , and a pure water replacement section  162  including a tub and a pure water nozzle is also disposed between the CARE unit  30 D and the pusher  38 . 
         [0089]    As will be understood from the use of a slurry or the like in surface removal processing, the surface removal processing section  3  is the dirtiest area. In this embodiment, therefore, discharge of air is carried out around a removal processing site, such as a platen, so as to prevent particles in the surface removal processing section  3  from flying to the outside. Further, the internal pressure of the surface removal processing section  3  is made lower than the external pressure of the apparatus and the internal pressures of the neighboring cleaning section  4  and loading/unloading section  2 , thereby preventing particles from flying out. A ventilation 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 through the ventilation duct and the filter. 
         [0090]    The cleaning section  4 , which is an area for cleaning a substrate, includes a second transport robot  40 , a reversing machine  41  for reversing a substrate received from the second transport robot  40 , three cleaning units  42 ,  43 ,  44  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 transport robot  46  for transporting the substrate between the reversing machine  41 , the cleaning units  42 ,  43 ,  44  and the drying unit  45 . The second transport robot  40 , the reversing machine  41 , the cleaning units  42  to  44  and the drying unit  45  are arranged in a line along the long direction of the apparatus, and the third transport robot  46  is movably disposed between the first linear transporter  5  and the line of the second transport robot  40 , the reversing machine  41 , the cleaning units  42  to  44  and the drying unit  45 . A filter-fan unit (not shown) having a clean air filter is provided above the cleaning units  42  to  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 unit  4  is constantly kept at a higher pressure than the surface removal processing section  3  to prevent inflow of particles from the surface removal processing section  3 . 
         [0091]    In this embodiment, the first and second cleaning units  42 ,  43  are chemical cleaning units which cleans a substrate by immersing the substrate in a chemical solution, such as aqua regia, SPM (sulfuric acid-hydrogen peroxide mixture) or hydrofluoric acid, and the third cleaning unit  44  is a pure water cleaning unit which cleans a substrate by supplying pure water to the substrate. In particular, the first cleaning unit  42  uses hydrofluoric acid as a cleaning chemical and the second cleaning unit  43  uses aqua regia as a cleaning chemical. 
         [0092]    As shown in  FIG. 1 , a shutter  10 , located between the reversing machine  31  and the first transport robot  22 , is provided in the partition wall  1   a  surrounding the surface removal processing section  3 . The shutter  10  is opened when transferring a substrate between the first transport robot  22  and the reversing machine  31 . Further, a shutter  13  located at a position facing the CMP unit  30 B and a shutter  14  located at a position facing the CARE unit  30 C are provided in the partition wall  1   b  surrounding the surface removal processing section  3 . 
         [0093]    Processing for flattening a surface of a substrate by the flattening apparatus having the above construction will now be described. 
         [0094]    One substrate is taken by the first transport robot  22  out of a substrate cassette mounted in one of the front loading sections  20 , and the substrate is transported to the reversing machine  31 . The reversing machine  31  180° reverses the substrate and then places the substrate on the lifter  32  at the first transport position TP1. The top ring  301 A of the lapping unit  30 A receives the substrate from the lifter  32 , and the lapping unit  30 A carries out lapping of the surface of the substrate. In particular, in the lapping unit  30 A, lapping of the substrate surface is carried out, e.g., at a processing rate of not more than several tens of μm/h while supplying a lapping liquid, such as a diamond slurry or a colloidal silica slurry, to the platen  301 A, thereby removing the substrate surface in an amount corresponding to a thickness of about 10 μm and flattening the substrate surface. In this case, the depth of damage in the substrate surface after processing is about 1 μm. The substrate surface is then rinsed with pure water, as necessary. 
         [0095]    The substrate after lapping is transferred to the pusher  33  at the second transport position TP2, and is then transported to the third transport position TP3 by horizontally movement of the first linear transporter  5 . The top ring  301 B of the CMP unit  30 B receives the substrate from the pusher  34  at the third transport position TP3, and the CMP unit  30 B carries out chemical mechanical polishing of the surface of the substrate. In particular, in the CMP unit  30 B, chemical mechanical polishing of the substrate surface is carried out, e.g., at a processing rate of not more than several μm/h while supplying a polishing liquid, e.g., containing colloidal silica, to the polishing table  300 B, thereby removing the substrate surface in an amount corresponding to a thickness of about several μm and further flattening the substrate surface. In this case, the depth of damage in the substrate surface after processing is about 10 nm. The substrate surface is then rinsed with pure water, as necessary. 
         [0096]    The substrate after CMP is transferred to the lifter  35  at the fourth transport position TP4. The second transport robot  40  receives the substrate from the lifter  35  and transports the substrate to the reversing machine  41 . The reversing machine  41  180° reverses the substrate and then transports it to the first cleaning unit (chemical cleaning unit)  42 . The first cleaning unit  42  carries out hydrofluoric acid cleaning of the substrate by immersing the substrate in hydrofluoric acid, for example. The substrate after the hydrofluoric acid cleaning is transported by the third transport robot  46  to the third cleaning unit (pure water cleaning unit)  45 , where the substrate is cleaned with pure water. Thereafter, the substrate is returned by the third transport robot  46  to the reversing machine  41 . When chemical cleaning of the substrate is not necessary, the substrate, which has been 180° reversed by the reversing machine  41 , is transported by the third transport robot  46  to the third cleaning unit (pure water cleaning unit)  45 , where the substrate is cleaned with pure water, and the substrate is then returned by the third transport robot  46  to the reversing machine  41 . The reversing machine  41  180° reverses the substrate. 
         [0097]    The second transport robot  40  receives the reversed substrate from the reversing machine  41  and places the substrate on the lifter  36  at the fifth transport position TP5. The second linear transporter  6  moves horizontally to transport the substrate on the lifter  36  to one of the sixth transport position TP6 and the seventh transport position TP7. The substrate holder  130  of the CARE unit  30 C or  30 D receives the substrate from the pusher  37  or  38 , and the CARE unit  30 C or  30 D carries out CARE (catalyst-referred etching) processing of the surface of the substrate. In particular, in the CARE unit  30 C or  30 D, CARE processing of the substrate surface, which utilizes the catalytic action on SiC by the surface platinum layer  142  of the catalyst platen  126 , is carried out, e.g., at a processing rate of not more than 100 nm/h while supplying a processing solution, e.g., comprising hydrofluoric acid, to the catalyst platen  126 , thereby removing the substrate surface in an amount corresponding to a thickness of about several tens of μm. In this case, the depth of damage in the substrate surface after processing is zero. 
         [0098]    For the substrate which has undergone CARE processing in the CARE unit  30 C, hydrofluoric acid remaining on the substrate surface after CARE processing is replaced with pure water in the pure water replacement section  160 , and the substrate is then returned to the sixth transport position TP6. For the substrate which has undergone CARE processing in the CARE unit  30 D, hydrofluoric acid remaining on the substrate surface after CARE processing is replaced with pure water in the pure water replacement section  162 , and the substrate is then returned to the seventh transport position TP7. The substrate after pure water replacement is then moved by the second linear transporter  6  to the fifth transport position TP5. There is a case where the surface of the substrate after CARE processing can be roughened when the surface with hydrofluoric acid attached is exposed to light of an excitation wavelength. In such a case, it is desirable to attach a UV blocking film to windows facing the transport path from the CARE unit  30 C to the pure water replacement section  160  and the transport path from the CARE unit  30 D to the pure water replacement section  162 . 
         [0099]    The second transport robot  40  takes the substrate out of the fifth transport position TP5 and transports the substrate to the reversing machine  41 . The reversing machine  41  180° reverses the substrate and then transports it to the first cleaning unit  42 . The third transport robot  46  transports the substrate form the first cleaning unit  42  to the second cleaning unit  43 , where the substrate is cleaned by immersing it in aqua regia. The aqua regia cleaning of the substrate is carried out a plurality of times, as necessary. 
         [0100]    The third transport robot  46  transports the substrate after aqua regia cleaning to the third cleaning unit (pure water cleaning unit)  44 , where the substrate is cleaned with pure water. The third transport robot  46  transports 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 transport 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  20 . 
         [0101]    Methods for detecting the end point of CARE processing in the CARE units  30 C,  30 D include (a) a method of detecting the end point from the processing time, (b) a method of detecting the end point from a change in the electric current of a drive motor for rotationally driving a catalyst platen, (c) a method of detecting the end point from a change in the concentration of a processing solution, (d) a method of detecting the end point by measuring the processing surface with an in-situ optical monitor (e.g., photoluminescence method), (e) a method comprising taking a substrate out of a processing solution, and detecting the end point (either in situ or ex situ) by using a device such as a TEM, (g) a method of detecting the end point from a change in the weight of a substrate (workpiece) before and after processing, etc. The end point of CARE processing can also be detected by an SEM for a patterned substrate such as an SiC wafer. When the crystallinity of a processed surface is of importance, it is preferred to use the photoluminescence method which can determine the crystallinity. The photoluminescence method is easy to operate because it can be carried out at atmospheric pressure. 
         [0102]    According to this embodiment, using the lapping unit  30 A as a first surface removal processing unit, lapping of a substrate surface to enhance the flatness of the surface is first carried out, e.g., at a processing rate of not more than several tens of μm/h, thereby removing the substrate surface in an amount corresponding to a thickness of about 10 μm (the depth of damage in the substrate surface after processing is about 1 μm); using the CMP unit  30 B as a second surface removal processing unit, CMP processing of the substrate surface to further enhance the flatness of the surface is carried out, e.g., at a processing rate of not more than several μm/h, thereby removing the substrate surface in an amount corresponding to a thickness of about several μm (the depth of damage in the substrate surface after processing is about 10 nm); and using the CARE unit  30 C or  30 D as a third (final) surface removal processing unit, CARE processing of the substrate surface to enhance the flatness of the surface is lastly carried out, e.g., at a processing rate of not more than 100 nm/h, thereby removing the substrate surface in an amount corresponding to a thickness of about several tens of nm (the depth of damage in the substrate surface after processing is zero). 
         [0103]    This processing method thus makes it possible to process a substrate (workpiece) at a sufficient processing rate while gradually enhancing the flatness of the surface (surface to be processed) of the substrate and finally create a damage-free, very flat processed surface. Depending on the type of the workpiece, the state of the processed surface, etc., one of lapping (first surface removal processing) and CMP (second surface removal processing) may be omitted. 
         [0104]    In the case of CARE processing of SiC, dissolved C (carbon) is present in a processing solution after processing and the carbon needs to be treated. A method for treating the carbon comprises irradiating the processing solution after use with UV light at a position opposite a substrate holder on a catalyst platen and/or in a circulation line to break the chemical bond, and allowing carbon to precipitate at a different place. The carbon concentration of the processing solution in a processing tank can thus be decreased. Further, it is preferred to provide a filter in a circulation line or piping for the processing solution so as to filter out carbon and other possible impurities that will adversely affect CARE processing. 
         [0105]    When flatly processing a surface of a substrate, an SiC wafer, by CARE using hydrofluoric acid as a processing solution and platinum as a catalyst as in this embodiment, the processed surface of the substrate will be contaminated with platinum. The platinum contaminant can be removed by aqua regia cleaning carried out by immersing the substrate in aqua regia. 
         [0106]      FIG. 7  shows the results of total reflection X-ray fluorescence (TRXF) analysis of platinum contamination as measured on an SiC wafer sample after CARE processing and on the processed sample after cleaning with aqua regia. The CARE processing was carried out by using hydrofluoric acid as a processing solution and platinum as a catalyst under the processing conditions shown in Table 1 below. The aqua regia cleaning was carried out by immersing the sample in a 3:1 mixed solution (aqua regia) of hydrochloric acid (60%) and nitric acid (60%) at 60° C. for 10 minutes, followed by rinsing of the sample with ultrapure water for one minute. The above cleaning operation was repeated a plurality of times. The TRXF measurement was carried out under the measurement conditions shown in Table 2 below. 
         [0000]    
       
         
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Sample: n-type 4H-SiC 2 inch wafer 
               
               
                   
                 Plane direction: 8° off Si plane 
               
               
                   
                 Resistivity: 0.01-0.05 Ωcm 
               
               
                   
                 Catalyst platen: 300 mm Pt platen 
               
               
                   
                 Processing time: 30 hrs 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 Voltage applied: 40 kV 
               
               
                   
                 Electric current: 40 mA 
               
               
                   
                 Incidence angle of incident X-ray: 0.050 deg 
               
               
                   
                 Incidence time of incident X-ray: 100 sec 
               
               
                   
                   
               
             
          
         
       
     
         [0107]    As can be seen from  FIG. 7 , platinum contamination was observed on the sample after CARE processing; and the platinum contamination decreased with the repetition of the aqua regia cleaning of the sample. 
         [0108]    For comparison, the same SiC wafer sample was CARE-processed in the same manner as described above, and the processed sample was subjected to SPM cleaning and further to hydrofluoric acid cleaning. The SPM cleaning was carried out by immersing the sample in a 4:1 mixed solution of sulfuric acid (98%) and hydrogen peroxide water (30%) for 10 minutes, followed by rinsing of the sample with ultrapure water for one minute. The hydrofluoric acid cleaning was carried out by immersing the sample in hydrofluoric acid (50% HF) for 10 minutes, followed by rinsing of the sample with ultrapure water for one minute.  FIG. 8  shows the results of TRXF analysis of platinum contamination on the sample after CARE processing, on the processed sample after SPA cleaning and on the processed sample after the additional HF cleaning. As can be seen from  FIG. 8 , the platinum contaminant on the sample after CARE processing cannot be removed by SPM cleaning nor by hydrofluoric acid cleaning. 
         [0109]    In this embodiment, the surface of SiC is processed (etched) by CARE using hydrochloric acid as a processing solution and platinum as a surface catalyst of a catalyst platen. It is also possible to process (etch) a surface of Si, SiC, GaN, sapphire, ruby or diamond by CARE using hydrohalic acid, such as hydrofluoric acid or hydrochloric acid, as a processing solution and platinum, gold, a ceramic solid catalyst, molybdenum or a molybdenum alloy as a catalyst. 
         [0110]    Further, it is possible to process (etch) a surface of Si, SiC, GaN, sapphire, ruby or diamond by CARE using hydrogen peroxide water or ozone water as a processing solution and a transition element, such as Fe, Ni, Co, Cu, Cr or Ti, as a surface catalyst of a catalyst platen. 
         [0111]    For example, in the CARE units  30 C,  30 D shown in  FIGS. 2 through 4 , it is possible to process (etch) a surface of a substrate, such as an SiC wafer, by supplying hydrogen peroxide water as a processing solution from the processing solution supply nozzle  174  to a catalyst paten  126  wholly made of iron which itself has a catalytic action. When processing a surface of a substrate, such as an SiC wafer, by CARE using hydrogen peroxide water as a processing solution and iron as a catalyst, the processed surface of the substrate will be contaminated with iron and an oxide film will be formed in the processed surface of the substrate. 
         [0112]      FIGS. 9A and 9B  show the results of X-ray photoelectron spectroscopy (XPS) of iron contamination as measured on an SiC wafer sample after CARE processing and on the processed sample after cleaning with SPM,  FIG. 9A  showing the XPS data at the Fe2p core level for the sample after CARE processing and  FIG. 9B  showing the XPS data at the Fe2p core level for the sample after SMP cleaning. The CARE processing was carried out by using hydrogen peroxide water as a processing solution and iron as a catalyst under otherwise the same processing conditions as indicated in Table 1 above, and the SPM cleaning was carried out in the same manner as described above with reference to the TRXF analysis of platinum contamination. As clearly shown in  FIG. 9A , a signal at the Fe2p core level was detected for the sample after CARE processing, whereas no signal at the Fe2p core level was detected for the sample after SPM cleaning as will be appreciated from  FIG. 9B . This indicates that the iron contaminant on the sample after CARE processing was completely removed by the SPM cleaning. 
         [0113]    The surface removal process for SiC generally passes through the stage of oxidation of the surface and the stage of removal of the oxide film. Accordingly, an oxide film may remain on a surface of a workpiece after processing. Especially in a light irradiation CARE process in which processing of a workpiece is carried out while irradiating the surface to be processed of the workpiece with ultraviolet light, due to promoted oxidation of the surface to be processed by the UV irradiation, an oxide film is likely to remain on the processed surface. 
         [0114]      FIG. 10  shows the results of X-ray photoelectron spectroscopy of a surface of an SiC sample after light irradiation CARE processing, and  FIG. 11  shows the results of X-ray photoelectron spectroscopy of the surface of the processed SiC sample as measured after cleaning the sample with hydrofluoric acid. The light irradiation CARE processing was carried out for 6 hours by using hydrogen peroxide water as a processing solution and quartz glass as a catalyst and irradiating the surface to be processed of the sample with ultraviolet light during processing. The hydrofluoric acid cleaning was carried out by immersing the sample after the light irradiation CARE processing in 50% hydrofluoric acid solution for 10 minutes.  FIG. 10  shows the peak at 103. 6 eV, indicating the presence of SiO 2  oxide film, in addition to the peak (101.3 eV) indicating the Si—C bond of the sample material. On the other hand,  FIG. 11  shows no peak concerning SiO 2  and a larger peak value concerning Si—C bond. The comparative data clearly demonstrates the fact that the hydrofluoric acid cleaning can fully remove the oxide film remaining on the surface of the SiC sample after light irradiation CARE processing. 
         [0115]    Also in the case of CARE processing of an SiC workpiece as carried out by using hydrogen peroxide water as a processing solution and iron as a catalyst, it is considered, in view of the removal processing principle, that an oxide film will remain on the surface of the workpiece after processing. Thus, also in this case, it is preferred to carry out hydrofluoric acid cleaning of the workpiece after removal processing. 
         [0116]    As will be appreciated from the above, when using hydrogen peroxide water as the processing solution supplied from the processing solution supply nozzle  174  and a catalyst platen wholly made of iron having a catalytic activity as the catalyst platen  126  in the CARE units  30 C,  30 D shown in  FIGS. 2 through 4 , it is preferred to use a hydrofluoric acid cleaning unit, which cleans a substrate by immersing it in hydrofluoric acid, as the first cleaning unit  142  of the flattening apparatus shown in  FIG. 1  as in the above-described embodiment, and to use an SPM cleaning unit, which cleans a substrate by immersing it in SPM, as the second cleaning unit  43 . By subjecting a substrate after CARE processing to hydrofluoric acid cleaning in the first cleaning unit  42  and then to SPM cleaning in the second cleaning unit  43 , an iron contaminant on the surface of the substrate and an oxide film formed in the substrate surface can be removed. 
         [0117]    When hydrogen peroxide water or ozone water is used as a processing solution, it is possible to use as a surface catalyst of a catalyst platen, beside the above-described transition metals, a noble metal such as platinum or gold, a ceramic metal oxide or a glass-type metal oxide. The ceramic metal oxide is exemplified by alumina, and the glass-type metal oxide is exemplified by sapphire (Al 2 O 3 ), quartz (SiO 2 ) and zirconia (ZrO 2 ). 
         [0118]    Further, when hydrogen peroxide water or ozone water is used as a processing solution, it is also possible to use an acidic or basic solid catalyst as a surface catalyst of a catalyst platen. The acidic or basic solid catalyst is exemplified by a non-woven fabric, a resin or a metal having an ion exchange function. Preferable examples of such materials having an ion exchange function include a non-woven fabric composed of polyethylene fibers, resins such as fluororesin and PEEK, and oxidation-resistant metals such as Pt and Au. For example, an ion exchange function can be imparted to a non-woven fabric of polyethylene fibers by graft polymerization. In this case, the etching rate of a workpiece can be increased by increasing the ion exchange capacity of a solid catalyst. Examples of other usable solid acid catalysts include a solid acid comprising silica-alumina with H 2 SO 4 , H 3 PO 4 , or the like adsorbed on it, inorganic salts such as a metal sulfate and a metal phosphate, and oxides such as Al 2 O 3 , ThO 2 , Al 2 O 3 —SiO 2  and TiO 2 —SiO 2 . Examples of solid base catalysts include a solid base comprising silica gel with NaOH, KOH, Na, K or the like adsorbed on it, inorganic salts such as Na 2 CO 3 , Ba 2 CO 3  and Na 2 WO 4 , and oxides such as CaO, MgO, SrO, MgO—SiO 2  and MgO—Al 2 O 3 . 
         [0119]    Contamination (especially organic contamination) of a surface of a platen or the presence of an oxide film in the surface of the platen will affect wettability of the platen surface. In order to improve wettability of a surface of a catalyst platen, the following treatments of the catalyst platen can be carried out before or during processing: 
         [0120]    (1) Treatment to improve wettability of catalyst platen carried out before processing: 
         [0121]    A catalyst platen before processing is subjected to chemical cleaning (e.g., with SPM) or ultraviolet irradiation. The chemical cleaning can remove an organic contaminant from the platen surface, thereby improving wettability of the platen surface. The ultraviolet irradiation will oxidize the surface of the platen, when it is made of, e.g., iron, thereby improving wettability of the surface. The ultraviolet irradiation treatment is applicable to catalyst platens other than a platinum platen. 
         [0122]    (2) Treatment to improve wettability of catalyst platen carried out during processing: 
         [0123]    Ultraviolet light is applied to a surface portion, which is different from the portion facing a top ring, of a catalyst platen during processing to form an oxide film in the platen surface, thereby improving wettability of the platen surface. Owing to the improved wetting of the paten surface with a plating solution, organic matter which adheres to the platen surface during processing will be removed. This further enhances wettability of the platen surface and makes the surface more hydrophilic. If a surface oxide film of, e.g., an iron platen is removed during processing, the ultraviolet irradiation can form a surface oxide film again to improve wettability of the platen surface. 
         [0124]    In the above-described embodiments, CARE processing in the CARE unit  30 C or  30 D is carried out in a piece-by-piece manner by pressing one substrate, held by the substrate holder  130 , against the catalyst platen in each operating cycle. A catalyst platen is generally costly. Further, the surface conditions of a catalyst platen affect the processing performance. Accordingly, and also with a view to increasing the throughput, it is preferred to simultaneously process a plurality of substrates, e.g., four substrates, by simultaneously pressing the plurality of substrates, held by one substrate holder, against a catalyst platen. It is also possible to use a CARE unit which includes a plurality of substrate holders which each hold one substrate. 
         [0125]    Prior to CARE processing, a substrate (workpiece) is preferably subjected to wet cleaning to clean a surface (surface to be processed). A surface of a substrate may be contaminated with particles, an organic substance and/or a metal. Non-scrub cleaning, such as water jet cleaning (hydrofluoric acid cleaning when using colloidal silica), is suited for particle contamination, SPM (sulfuric acid-hydrogen peroxide mixture) cleaning is suited for organic contamination, and SPM cleaning or hydrofluoric acid cleaning is suited for metal contamination. A surface of an SiC substrate per se does not change by such cleaning, and only a contaminant can be removed. 
         [0126]      FIGS. 12A through 12D  illustrate the concept of CARE processing of a workpiece with the use of a processing solution containing a buffer solution (buffering agent). As shown in  FIG. 12A , when an acidic solid catalyst  416  is immersed in a processing solution  412   b , hydrogen ions (H + )  416   a  are generated at a surface of the acidic solid catalyst  416 , and the hydrogen ions  416   a  leave the surface of the acidic solid catalyst  416 . The processing solution  412   b  contains a buffering agent  480  dissolved therein, and the hydrogen ions  416   a , which have left the surface of the acidic solid catalyst  416 , promptly react with the buffering agent  480  and become inactive, as shown in  FIG. 12B . Thus, the hydrogen ions  416   a  are present only on or in the close vicinity of the surface, serving as a processing reference surface, of the acidic solid catalyst  416 . When the acidic solid catalyst  416  is brought into contact with or close proximity to the surface to be processed of a workpiece  414  in the processing solution containing the buffering agent  480 , as shown in  FIG. 12C , surface atoms of the workpiece  414  at the contact portion are dissolved by chemical reaction in the processing solution  412   b . When the acidic solid catalyst  416  is separated from the surface to be processed of the workpiece  414 , as shown in  FIG. 12D , the hydrogen ions  416   a  generated at the surface of the acidic solid catalyst  416  do not act on the surface of the workpiece  414  any more whereby the dissolution reaction stops. Thus, the surface to be processed of the workpiece  414  is processed only when the acidic solid catalyst  416  is in contact with or in close proximity to the surface to be processed. 
         [0127]    Examples of solid acid catalysts include a solid acid comprising silica-alumina with H 2 SO 4 , H 3 PO 4 , or the like adsorbed on it, inorganic salts such as a metal sulfate and a metal phosphate, and oxides such as Al 2 O 3 , ThO 2 , Al 2 O 3 —SiO 2  and TiO 2 —SiO 2 . Examples of solid base catalysts include a solid base comprising silica gel with NaOH, KOH, Na, K or the like adsorbed on it, inorganic salts such as Na 2 CO 3 , Ba 2 CO 3  and Na 2 WO 4 , and oxides such as CaO, MgO, SrO, MgO—SiO 2  and MgO—Al 2 O 3 . 
         [0128]    When a processing solution does not contain a buffer solution (buffering agent), i.e., when a buffering agent is not present in the processing solution, hydrogen ions diffuse in the processing solution even after they leave a surface of an acidic solid catalyst. Hydrogen ions can therefore be present also in those places which lie at a distance from the surface of the acidic solid catalyst. Accordingly, when the acidic solid catalyst is brought into contact with or close proximity to the surface to be processed of a workpiece in the processing solution, surface atoms of the surface to be processed, not only those lying in the vicinity of the surface of the acidic solid catalyst but also those lying at a distance therefrom, will be dissolved by chemical reaction in the processing solution. Thus, hydrogen ions act not only on raised portions of the surface to be processed but on recessed portions as well, whereby the surface to be processed will be processed isotropically and flattening of the surface will not progress. 
         [0129]    A phosphate buffer solution is an exemplary buffer solution (buffering agent). Other buffer solutions, such as an acetate buffer solution and a citrate buffer solution, may also be used insofar as it reduces a change in the pH of the processing solution. When a buffer solution is used as an additive in a processing solution, the processing solution preferably is ultrapure water or an aqueous solution containing an oxidizing agent, such as ozone water, hydrogen peroxide water or an aqueous potassium persulfate solution. The concentration of buffering agent in a processing solution is preferably such as to adjust the pH of the processing solution to a predetermined value. When processing a GaN substrate, for example, the pH of a processing solution is preferably adjusted to around 7 (6.5 to 7.5). 
         [0130]      FIG. 13  schematically shows a CARE unit (light irradiation CARE unit) provided with a light source. The CARE unit (light irradiation CARE unit) includes a vessel  484  for filling it with a processing solution  482 , e.g., containing a buffer solution (buffering agent), a platen  488  coupled to the upper end of a rotating shaft  486  and rotatably disposed in the vessel  484 , and a holder  492  for detachably holding a workpiece  490 , such as a GaN substrate, with its surface to be processed facing downwardly. The holder  492  is secured to the lower end of a rotating shaft  494 . A light source  496  is provided below the platen  488 . The platen  488  is comprised of an acidic solid catalyst having high light transmissivity, such as quartz, so that light, e.g., ultraviolet light, from the light source  496  passes through the platen  488  and applied to the lower surface (surface to be processed) of the workpiece  490 . The platen  488  may also be comprised of sapphire, zirconia, or the like, having high light transmissivity. 
         [0131]    Using the CARE unit (light irradiation CARE unit) shown in  FIG. 13 , processing of a GaN wafer (sample) was carried out for 3 hours under ultraviolet irradiation.  FIG. 14A  shows a phase-shifting interference microscopic image of the surface of the sample before processing,  FIG. 14B  shows a phase-shifting interference microscopic image of the surface of the sample after carrying out processing using a processing liquid (ultrapure water) not containing a buffer solution (buffering agent), and  FIG. 14C  shows a phase-shifting interference microscopic image of the surface of the sample after carrying out processing using a processing solution (pure water) containing a buffer solution (phosphate buffer solution at a pH of 6.8). 
         [0132]      FIG. 14A  shows surface roughness as observed on the surface of the sample before processing. As will be appreciated from comparison between  FIG. 14B  and  FIG. 14C , the processing of the sample carried out using the processing solution containing the buffer solution (buffering agent) reduced the surface roughness to a much larger extent as compared to the processing carried out using the processing liquid containing no buffer solution (buffering agent). 
         [0133]    When a substrate, such as a semiconductor wafer, is irradiated with light having a larger energy than the band gap of the substrate material, electrons in the valence band are excited and electron-hole pairs are generated. Oxidation of the substrate surface occurs through the holes of the valence band generated by the light irradiation. When the electron-hole pairs generated by light adsorption recombine and disappear, an oxidation reaction will not occur at the substrate surface. The recombination should therefore be prevented in order to progress the surface oxidation. In this regard, when platinum (Pt) is allowed to be in electrical contact with the substrate, electrons in the conducting band are accumulated in the platinum, and electron exchange will take place between the platinum and a solution. This can prevent recombination of the electron-hole pairs generated. 
         [0134]    An experiment on photooxidation of a surface of a GaN substrate was conducted to confirm the effect of electrical contact of platinum with the substrate. A 500-nm thick platinum film was formed by electron beam deposition on the back (000-1) surface of an n-type GaN (0001) substrate, thereby preparing a test sample in which the substrate is in electrical contact with the platinum layer. The same GaN substrate but with no platinum film formed on the back surface was used as a comparative sample. 
         [0135]    Light (UV light) irradiation was carried out for 3 hours on the respective substrate which was kept immersed in ultrapure water.  FIG. 15  shows XPC O1s spectra for the GaN substrate sample and the comparative sample, as measured before and after the light irradiation. It is apparent from the data in  FIG. 15  that for the comparative sample having no platinum film, there is no change in the O1s peak intensity before and after the light irradiation, whereas the O1s peak intensity of the sample having the platinum film is larger after the light irradiation. The data thus demonstrates that surface photooxidation occurs only in the GaN substrate having the platinum film on the back surface. 
         [0136]    A light irradiation CARE processing experiment was conducted using, as a sample, a GaN substrate having a vapor-deposited platinum film on the back surface and using, as a comparative sample, a GaN substrate with no platinum film formed on the back surface. Processing was carried out for 3 hours by immersing the respective substrate in phosphate buffer solution at a pH of 6.86 while irradiating the substrate surface with ultraviolet light.  FIG. 16A  shows a phase-shifting interference microscopic image of the surface of the comparative sample (GaN substrate), having no vapor-deposited platinum film on the back surface, before the light irradiation CARE processing,  FIG. 16B  shows a phase-shifting interference microscopic image of the surface of the comparative sample after the light irradiation CARE processing, and  FIG. 16C  shows a phase-shifting interference microscopic image of the surface of the sample (GaN substrate), having the vapor-deposited platinum film on the back surface, after the light irradiation CARE processing. As can be seen from comparison between  FIGS. 16A and 16B , there is no significant change in the morphology and the micro roughness of the surface of the comparative sample, having no platinum film, before and after the processing, indicating no significant progress of processing. In contrast, as can be seen from comparison between  FIG. 16A  and  FIG. 16C , scratches present in the substrate surface before processing are not found in the surface of the sample, having the platinum film, after the processing, and the micro roughness value 6.497 nm rms before processing decreased to 0.804 nm rms after processing. The comparative data thus demonstrates significant flattening by the CARE processing of the surface of the sample having the platinum film. 
         [0137]    Besides platinum, it is possible to use other metals, such as Ti, Ni, Cr, etc. as a metal for contact with a substrate, though the use of platinum is preferred in the interface between a processing solution and a metal. A metal (platinum) film may be formed on a substrate by any known vapor deposition method, such as vacuum deposition, electron beam evaporation, sputtering, etc. While platinum is preferably vapor-deposited on substantially an entire back surface of a substrate, it is possible to vapor-deposit platinum on part of the back surface. Further, it is not always necessary to form a metal (platinum) film on a back surface of a substrate. Thus, any method that can ensure electrical contact between a substrate and a metal, such as the use of a substrate holder made of a metal, may be used. It is also possible to apply a platinum coating to an inner side of a retainer of a substrate holder so that the coating will contact the edge of a substrate. 
         [0138]    While the present invention has been described with reference to the embodiments thereof, it will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described above, but it is intended to cover modifications within the inventive concept.