Patent Publication Number: US-2022220637-A1

Title: Silicon carbide substrate

Description:
TECHNICAL FIELD 
     The present disclosure relates to a silicon carbide substrate. The present application claims priority to Japanese Patent Application No. 2019-093883 filed on May 17, 2019, the entire contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     WO 2016/063632 (PTL 1) describes a method for cleaning a silicon carbide substrate. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: WO 2016/063632 
       
    
     SUMMARY OF INVENTION 
     A silicon carbide substrate in accordance with the present disclosure includes a first main surface, a second main surface, and a chamfered portion. The second main surface is opposite to the first main surface. The chamfered portion is contiguous to each of the first main surface and the second main surface. The silicon carbide substrate has a maximum diameter of 150 mm or more. A surface manganese concentration in the chamfered portion is 1×10 11  atoms/cm 2  or less. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic plan view showing a configuration of a silicon carbide substrate in accordance with the present embodiment. 
         FIG. 2  is a schematic cross sectional view taken along a line II-II in  FIG. 1 . 
         FIG. 3  is an enlarged schematic view of a region III in  FIG. 2 . 
         FIG. 4  is a flowchart schematically showing a method for manufacturing the silicon carbide substrate in accordance with the present embodiment. 
         FIG. 5  is a schematic cross sectional view showing a first step of the method for manufacturing the silicon carbide substrate in accordance with the present embodiment. 
         FIG. 6  is a schematic cross sectional view showing a second step of the method for manufacturing the silicon carbide substrate in accordance with the present embodiment. 
         FIG. 7  is a schematic cross sectional view showing a third step of the method for manufacturing the silicon carbide substrate in accordance with the present embodiment. 
         FIG. 8  is a schematic cross sectional view showing a fourth step of the method for manufacturing the silicon carbide substrate in accordance with the present embodiment. 
         FIG. 9  is a schematic cross sectional view showing a fifth step of the method for manufacturing the silicon carbide substrate in accordance with the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Problem to be Solved by the Present Disclosure 
     An object of the present disclosure is to provide a silicon carbide substrate with a high cleanliness. 
     Advantageous Effect of the Present Disclosure 
     According to the present disclosure, a silicon carbide substrate with a high cleanliness can be provided. 
     Description of the Embodiment of the Present Disclosure 
     First, an embodiment of the present disclosure will be described in list form. 
     (1) A silicon carbide substrate  100  in accordance with the present disclosure includes a first main surface  1 , a second main surface  2 , and a chamfered portion  6 . Second main surface  2  is opposite to first main surface  1 . Chamfered portion  6  is contiguous to each of first main surface  1  and second main surface  2 . Silicon carbide substrate  100  has a maximum diameter of 150 mm or more. A surface manganese concentration in chamfered portion  6  is 1×10 11  atoms/cm 2  or less. 
     (2) In silicon carbide substrate  100  in accordance with (1) described above, an arithmetic mean roughness in chamfered portion  6  may be greater than an arithmetic mean roughness in each of first main surface  1  and second main surface  2 . 
     (3) In silicon carbide substrate  100  in accordance with (1) or (2) described above, a surface sodium concentration in chamfered portion  6  may be 1×10 8  atoms/cm 2  or more. 
     (4) In silicon carbide substrate  100  in accordance with any of (1) to (3) described above, a surface sodium concentration in chamfered portion  6  may be 3×10 11  atoms/cm 2  or less. 
     (5) In silicon carbide substrate  100  in accordance with any of (1) to (4) described above, a surface potassium concentration in chamfered portion  6  may be 1×10 11  atoms/cm 2  or less. 
     (6) In silicon carbide substrate  100  in accordance with any of (1) to (5) described above, a surface iron concentration in chamfered portion  6  may be 1×10 11  atoms/cm 2  or less. 
     Details of the Embodiment of the Present Disclosure 
     Hereinafter, the embodiment of the present disclosure will be described based on the drawings. It should be noted that, in the drawings below, identical or corresponding parts will be designated by the same reference numerals, and the description thereof will not be repeated. Regarding crystallographic indications in the present specification, an individual orientation is represented by [ ], a group orientation is represented by &lt; &gt;, an individual plane is represented by ( ), and a group plane is represented by { }. Generally, a negative index is supposed to be crystallographically indicated by putting “−” (bar) above a numeral, but is indicated by putting the negative sign before the numeral in the present specification. 
     &lt;Configuration of Silicon Carbide Substrate&gt; 
     First, a configuration of silicon carbide substrate  100  in accordance with the present embodiment will be described.  FIG. 1  is a schematic plan view showing a configuration of silicon carbide substrate  100  in accordance with the present embodiment.  FIG. 2  is a schematic cross sectional view taken along a line II-II in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , silicon carbide substrate  100  in accordance with the present embodiment mainly has first main surface  1 , second main surface  2 , and chamfered portion  6 . Second main surface  2  is opposite to first main surface  1 . Chamfered portion  6  is contiguous to each of first main surface  1  and second main surface  2 . Each of first main surface  1  and second main surface  2  is a flat surface. First main surface  1  is a surface on which an epitaxial layer (not shown) is to be formed. Silicon carbide substrate  100  is made of a hexagonal silicon carbide single crystal having a polytype of  4 H, for example. Silicon carbide substrate  100  contains an n type impurity such as nitrogen, for example. 
     First main surface  1  is a {0001} plane, or a plane angled off by less than or equal to 8° relative to the {0001} plane, for example. Specifically, first main surface  1  is a (0001) plane, or a plane angled off by less than or equal to 8° relative to the (0001) plane, for example. First main surface  1  may be a (000-1) plane, or a plane angled off by less than or equal to 8° relative to the (000-1) plane, for example. When first main surface  1  is the (0001) plane, second main surface  2  is the (000-1) plane. 
     As shown in  FIG. 2 , chamfered portion  6  has a first curved region  3 , an outer circumferential end portion  5 , and a second curved region  4 . First curved region  3  is contiguous to first main surface  1 . First curved region  3  is located more outward than first main surface  1 . Second curved region  4  is contiguous to second main surface  2 . Second curved region  4  is located more outward than second main surface  2 . As shown in  FIG. 2 , in a cross section perpendicular to first main surface  1 , each of first curved region  3  and second curved region  4  is arc-shaped. Each of first curved region  3  and second curved region  4  is curved to protrude outward. 
     Outer circumferential end portion  5  is a portion located most outward in a radial direction parallel to first main surface  1 . Outer circumferential end portion  5  is contiguous to each of first curved region  3  and second curved region  4 . In the radial direction, first curved region  3  is located between first main surface  1  and outer circumferential end portion  5 . Similarly, in the radial direction, second curved region  4  is located between second main surface  2  and outer circumferential end portion  5 . 
     As shown in  FIG. 1 , outer circumferential end portion  5  has an orientation flat portion  7  and an arc-shaped portion  8 . Arc-shaped portion  8  is contiguous to orientation flat portion  7 . Orientation flat portion  7  extends along a first direction  101 . Each of first direction  101  and a second direction  102  is parallel to first main surface  1 . Second direction  102  is a direction perpendicular to first direction  101 . First direction  101  is a &lt;11-20&gt; direction, for example. Second direction  102  is a &lt;1-100&gt; direction, for example. 
     When first main surface  1  is inclined relative to the {0001} plane, first direction  101  may be a direction in which the &lt;11-20&gt; direction is projected to first main surface  1 , for example. When first main surface  1  is inclined relative to the {0001} plane, second direction  102  may be a direction in which the &lt;1-100&gt; direction is projected to first main surface  1 , for example. 
     As shown in  FIG. 1 , silicon carbide substrate  100  has a maximum diameter (a first width W 1 ) of 150 mm or more. When viewed in a direction perpendicular to first main surface  1 , the maximum diameter of silicon carbide substrate  100  may be calculated as a diameter of a circle including arc-shaped portion  8 . First width W 1  may be 200 mm or more, or may be 250 mm or more. The upper limit of first width W 1  is not particularly limited, and may be 300 mm or less, for example. 
     As shown in  FIG. 1 , when viewed in the direction perpendicular to first main surface  1 , chamfered portion  6  has a width (a second width W 2 ) of 0.1 mm or more and 0.3 mm or less, for example. From another viewpoint, when viewed in the direction perpendicular to first main surface  1 , a distance from a boundary between first main surface  1  and chamfered portion  6  to outer circumferential end portion  5  is 0.1 mm or more and 0.3 mm or less, for example. 
       FIG. 3  is an enlarged schematic view of a region III in  FIG. 2 . As shown in  FIG. 3 , a plurality of level differences (grinding marks  9 ) are formed in chamfered portion  6 . Grinding marks  9  are formed along a circumferential direction, for example. 
     Grinding marks  9  are formed in chamfered portion  6 . Accordingly, an arithmetic mean roughness (Ra) in chamfered portion  60  is greater than an arithmetic mean roughness in each of first main surface  1  and second main surface  2 . The arithmetic mean roughness in each of first main surface  1  and second main surface  2  is 0.056 nm, for example. The arithmetic mean roughness in each of first main surface  1  and second main surface  2  may be 0.03 nm or more and 0.2 nm or less, for example. The arithmetic mean roughness (Ra) in chamfered portion  6  is 0.1 μm, for example. The arithmetic mean roughness (Ra) in chamfered portion  6  may be 0.05 μm or more and 0.5 μm or less, for example. 
     Next, a method for measuring the arithmetic mean roughness (Ra) will be described. 
     An arithmetic surface roughness (Ra) can be measured using an atomic force microscope (AFM) or a stylus-type surface roughness measuring instrument. As the AFM, “Dimension 300” manufactured by Veeco can be used, for example. As a cantilever (probe), “NCHV-10V” manufactured by Bruker can be used, for example. As the stylus-type surface roughness measuring instrument, “SURFCOM 1800G” manufactured by Tokyo Seimitsu can be used, for example. 
     The range of measurement with the AFM is set to a region from a boundary  10  between first main surface  1  and chamfered portion  6  to outer circumferential end portion  5 . A straight line A which connects boundary  10  between first main surface  1  and chamfered portion  6  to outer circumferential end portion  5  serves as a reference height. As conditions for measurement with the AFM, for example, the measurement mode is set to a tapping mode, the pitch is set to 40 nm, and the measurement depth is set to 1.0 μm. For the sampling in the tapping mode, for example, the scanning speed is set to 5 seconds per cycle, the number of data per scan line is set to 512 points, and the number of scan lines is set to 512. Further, displacement control of the cantilever is set to 15.50 nm, for example. 
     Next, a concentration of a metal impurity in chamfered portion  6  will be described. 
     Sludge containing manganese tends to adhere to grinding marks  9 . In silicon carbide substrate  100  in accordance with the present embodiment, sludge containing manganese (Mn) in chamfered portion  6  is reduced. Specifically, a manganese concentration in chamfered portion  6  is 1×10 11  atoms/cm 2  or less. The manganese concentration in chamfered portion  6  is preferably 1×10 10  atoms/cm 2  or less, and more preferably 1×10 9  atoms/cm 2  or less. The lower limit of the manganese concentration in chamfered portion  6  is not particularly limited, and is 1×10 8  atoms/cm 2  or more, for example. 
     In silicon carbide substrate  100  in accordance with the present embodiment, a sodium (Na) concentration in chamfered portion  6  is 1×10 8  atoms/cm 2  or more, for example. The sodium concentration in chamfered portion  6  may be 1×10 9  atoms/cm 2  or more, or may be 1×10 10  atoms/cm 2  or more. The upper limit of the sodium concentration in chamfered portion  6  is not particularly limited, and may be 3×10 11  atoms/cm 2  or less, for example. 
     In silicon carbide substrate  100  in accordance with the present embodiment, a potassium (K) concentration in chamfered portion  6  is 1×10 11  atoms/cm 2  or less, for example. The potassium concentration in chamfered portion  6  may be 8×10 10  atoms/cm 2  or less, for example, or may be 6×10 11  atoms/cm 2  or less, for example. The lower limit of the potassium concentration in chamfered portion  6  is not particularly limited, and is 1×10 8  atoms/cm 2  or more, for example. 
     In silicon carbide substrate  100  in accordance with the present embodiment, an iron (Fe) concentration in chamfered portion  6  is 1×10 11  atoms/cm 2  or less, for example. The iron concentration in chamfered portion  6  may be 8×10 10  atoms/cm 2  or less, for example. The lower limit of the iron concentration in chamfered portion  6  is not particularly limited, and is 1×10 8  atoms/cm 2  or more, for example. 
     Next, a method for measuring the concentration of a metal impurity in chamfered portion  6  will be described. 
     First, an acid is dropped along the entire circumference of chamfered portion  6  of silicon carbide substrate  100  (except for orientation flat portion  7 ) using a horizontal substrate inspection device, and thereby a metal impurity on chamfered portion  6  is incorporated into the acid. Then, all the acid containing the metal impurity is collected from the entire circumference of chamfered portion  6 . It should be noted that sample preparation is performed within a class  10  clean draft placed within a class  100  clean room. 
     The concentration of the metal impurity can be measured by ICP-MS (Inductively Coupled Plasma Mass Spectrometry). As a measuring device, ICP-MS8800 manufactured by Agillent can be used, for example. The mass (ng) of a metal impurity element obtained by measurement is divided by the atomic weight of the element to be converted into the number of moles, and then the number of moles is multiplied by the Avogadro number to be converted into the number of atoms. Then, the number of atoms is divided by the area of chamfered portion  6  to obtain the number of atoms per unit area. The concentration of the metal impurity is the number of atoms per unit area. It should be noted that, when silicon carbide substrate  100  has a maximum diameter of 150 mm, chamfered portion  6  has an area of about 3.78 cm 2 . The area of chamfered portion  6  is calculated as the area of a rectangle with a long side having the length of outer circumferential end portion  5  of silicon carbide substrate  100 , and a short side having the width (second width W 2 ) of chamfered portion  6 , when viewed in the direction perpendicular to first main surface  1  (see  FIG. 1 ). 
     Next, a concentration of a metal impurity in first main surface  1  will be described. 
     Manganese may be present in first main surface  1 . In first main surface  1 , a manganese concentration may be 1×10 9  atoms/cm 2  or less. The manganese concentration in first main surface  1  is lower than the manganese concentration in chamfered portion  6 . 
     Sodium may be present in first main surface  1 . In first main surface  1 , a sodium concentration may be 5×10 10  atoms/cm 2  or less. The sodium concentration in first main surface  1  is lower than the sodium concentration in chamfered portion  6 . 
     Potassium may be present in first main surface  1 . In first main surface  1 , a potassium concentration may be 2×10 10  atoms/cm 2  or less. The potassium concentration in first main surface  1  is lower than the potassium concentration in chamfered portion  6 . 
     Iron may be present in first main surface  1 . In first main surface  1 , an iron concentration may be 5×10 9  atoms/cm 2  or less. The iron concentration in first main surface  1  is lower than the iron concentration in chamfered portion  6 . 
     Sulfur (S) may be present in first main surface  1 . In first main surface  1 , a sulfur concentration may be 5×10 10  atoms/cm 2  or more. In first main surface  1 , the sulfur concentration may be 1×10 12  atoms/cm 2  or more. 
     Chlorine (Cl) may be present in first main surface  1 . In first main surface  1 , a chlorine concentration may be 5×10 10  atoms/cm 2  or more. In first main surface  1 , the chlorine concentration may be 1×10 12  atoms/cm 2  or more. 
     Next, a method for measuring the concentration of a metal impurity in first main surface  1  will be described. 
     The concentration of a metal impurity can be measured with a total reflection fluorescent X-ray analysis device. As the analysis device, TXRF-3760 manufactured by Rigaku can be used, for example. The analysis device has a plurality of excited X-ray sources, and can measure an element from a light element Na to a heavy element U using an excited X-ray optimal for the measured element. Specifically, an excited X-ray of W-Ma (1.78 keV) is used for Na, and an excited X-ray of W-Lb (9.67 keV) is used for K, S, Fe, Cl, and Mn. 
     The power of the X-ray is 35 kV-255 mA, for example. The incidence azimuth is 39°. The incident angle of W-Ma is 0.500°. The measurement time of W-Ma is 10 seconds/point. The incident angle of W-Lb is 0.100°. The measurement time of W-Lb is 10 seconds/point. Further, the analysis device has an XY drive stage, and can measure in-plane distribution of the measured element. For example, first main surface  1  is divided into 101 regions having an equal area, and the concentration of the measured element can be measured at each of the 101 regions. The concentration of the metal impurity in the first main surface is defined as an average value of the concentrations at the 101 regions. The concentration of the metal impurity is the number of atoms per unit area. 
     &lt;Method for Manufacturing Silicon Carbide Substrate&gt; 
     Next, a method for manufacturing silicon carbide substrate  100  in accordance with the present embodiment will be described. As shown in  FIG. 4 , the method for manufacturing silicon carbide substrate  100  in accordance with the present embodiment includes a crystal preparing step (S 10 ), a slicing step (S 20 ), a chamfering step (S 25 ), a double-sided mechanical polishing step (S 30 ), a chemical mechanical polishing step (S 40 ), a sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ), an ammonia-hydrogen peroxide water mixture cleaning step (S 60 ), a hydrochloric acid-hydrogen peroxide water mixture cleaning step (S 70 ), a hydrofluoric acid cleaning step (S 80 ), and a drying step (S 90 ). 
     First, the crystal preparing step (S 10 ) is performed. In the crystal preparing step (S 10 ), a silicon carbide ingot is formed by a sublimation method, for example. Then, the slicing step (S 20 ) is performed. In the slicing step (S 20 ), the silicon carbide ingot is cut into a plurality of silicon carbide substrates  100  by a saw wire. Silicon carbide substrate  100  is made of a hexagonal silicon carbide single crystal having a polytype of  4 H, for example. As shown in  FIG. 5 , silicon carbide substrate  100  has first main surface  1 , second main surface  2 , and outer circumferential end portion  5 . At this point of time, chamfered portion  6  is not formed. 
     Then, the chamfering step (S 25 ) is performed. In the chamfering step (S 25 ), a grinding device  25  is used. In the chamfering step, diamond abrasive grains are used, for example. As shown in  FIG. 6 , the vicinity of a boundary between first main surface  1  and outer circumferential end portion  5  of silicon carbide substrate  100  is pressed against rotating grinding device  25 . Similarly, as shown in  FIG. 7 , the vicinity of a boundary between first main surface  1  and outer circumferential end portion  5  of silicon carbide substrate  100  is pressed against rotating grinding device  25 . Thereby, chamfered portion  6  is formed in silicon carbide substrate  100  (see  FIG. 2 ). In the chamfering step (S 25 ), grinding marks may be formed in chamfered portion  6 . 
     Then, the double-sided mechanical polishing step (S 30 ) is performed. Specifically, silicon carbide substrate  100  is placed between a first surface plate (not shown) and a second surface plate (not shown) such that first main surface  1  faces the first surface plate and second main surface  2  corresponds to the second surface plate. Then, slurry is introduced between first main surface  1  and the first surface plate and between second main surface  2  and the second surface plate. The slurry contains diamond abrasive grains and water, for example. The diamond abrasive grains have diameters of 0.5 μm or more and 3 μm or less, for example. Mechanical polishing is performed on both sides of silicon carbide substrate  100  by applying a load to first main surface  1  by the first surface plate and applying a load to second main surface  2  by the second surface plate. 
     Then, the chemical mechanical polishing step (S 40 ) is performed. Specifically, first, CMP (Chemical Mechanical Polishing) is performed in first main surface  1  of silicon carbide substrate  100 . As abrasive grains, colloidal silica is used, for example. A polishing solution containing a permanganate is used. A polishing cloth is attached to a surface plate. The polishing cloth is a nonwoven fabric, for example. The processing pressure is 300 g/cm 2 , for example. The flow rate of the polishing solution is 50 cc/minute, for example. The number of rotations of the surface plate is 40 rpm, for example. The processing time is two hours, for example. 
     Then, the sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ) is performed. In the sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ), an ultrasonic wave cleaning device is used. As shown in  FIG. 8 , an ultrasonic wave cleaning device  20  mainly has an ultrasonic wave generation source  19 , a first cleaning tank  12 , and a second cleaning tank  13 . Second cleaning tank  13  is placed on first cleaning tank  12 . Second cleaning tank  13  is hung on an opening of first cleaning tank  12 . A first cleaning solution  14  (specifically, water) is put in first cleaning tank  12 . A second cleaning solution  15  (specifically, a sulfuric acid-hydrogen peroxide water mixture) is put in second cleaning tank  13 . Silicon carbide substrate  100  is immersed in the sulfuric acid-hydrogen peroxide water mixture. Ultrasonic wave generation source  19  is placed at the bottom of second cleaning tank  13 . Second cleaning tank  13  is placed on ultrasonic wave generation source  19 . 
     In the sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ), silicon carbide substrate  100  is cleaned with the sulfuric acid-hydrogen peroxide water mixture being irradiated with ultrasonic waves, in order to improve the effect of removing a metal impurity. The ultrasonic waves have a frequency of 450 kHz or more and 2 MHz or less, for example. The ultrasonic waves promote a chemical reaction. This enhances the reactivity of the metal impurity to the sulfuric acid-hydrogen peroxide water mixture. Further, the sludge containing manganese which has entered the grinding marks in chamfered portion  6  can be effectively removed by a cavitation effect caused by irradiation of the ultrasonic waves. 
     In the sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ), an organic substance, a metal impurity, and the like are mainly removed. The sulfuric acid-hydrogen peroxide water mixture is a solution in which sulfuric acid, hydrogen peroxide water, and ultrapure water are mixed. As the sulfuric acid, concentrated sulfuric acid having a mass percentage concentration of 96% can be used, for example. As the hydrogen peroxide water, hydrogen peroxide water having a mass percentage concentration of 30% can be used, for example. The same applies to the hydrogen peroxide water used in the subsequent steps. 
     The volume ratio among the sulfuric acid, the hydrogen peroxide water, and the ultrapure water contained in the sulfuric acid-hydrogen peroxide water mixture is from 10 (sulfuric acid): 1 (hydrogen peroxide water): 1 (ultrapure water) to 10 (sulfuric acid): 3 (hydrogen peroxide water): 1 (ultrapure water), for example. In other words, the volume of the sulfuric acid is 10 times the volume of the ultrapure water. The volume of the hydrogen peroxide water is one time or more and three times or less of the volume of the ultrapure water. The immersion time of silicon carbide substrate  100  is five minutes or more, for example. The temperature of the sulfuric acid-hydrogen peroxide water mixture is room temperature, for example. 
     Then, the ammonia-hydrogen peroxide water mixture cleaning step (S 60 ) is performed. In the ammonia-hydrogen peroxide water mixture cleaning step (S 60 ), a polishing agent and dust are mainly removed. An ammonia-hydrogen peroxide water mixture is a solution in which an aqueous ammonia solution, hydrogen peroxide water, and ultrapure water are mixed. As the aqueous ammonia solution, an aqueous ammonia solution having a mass percentage concentration of 28% can be used, for example. In the ammonia-hydrogen peroxide water mixture cleaning step (S 60 ), silicon carbide substrate  100  may be cleaned with the ammonia-hydrogen peroxide water mixture being irradiated with ultrasonic waves. 
     The volume ratio among the aqueous ammonia solution, the hydrogen peroxide water, and the ultrapure water contained in the ammonia-hydrogen peroxide water mixture is from 1 (aqueous ammonia solution): 1 (hydrogen peroxide water): 5 (ultrapure water) to 1 (aqueous ammonia solution): 1 (hydrogen peroxide water): 10 (ultrapure water). In other words, the volume of the aqueous ammonia solution is 1/10 or more and ⅕ or less of the volume of the ultrapure water. Further, the volume of the hydrogen peroxide water is 1/10 or more and ⅕ or less of the volume of the ultrapure water. The immersion time of silicon carbide substrate  100  is five minutes or more, for example. The temperature of the sulfuric acid-hydrogen peroxide water mixture is room temperature, for example. 
     Then, the hydrochloric acid-hydrogen peroxide water mixture cleaning step (S 70 ) is performed. In the hydrochloric acid-hydrogen peroxide water mixture cleaning step (S 70 ), a heavy metal is mainly removed. A hydrochloric acid-hydrogen peroxide water mixture is a solution in which hydrochloric acid, hydrogen peroxide water, and ultrapure water are mixed. As the hydrochloric acid, concentrated hydrochloric acid having a mass percentage concentration of 98% can be used, for example. In the hydrochloric acid-hydrogen peroxide water mixture cleaning step (S 70 ), silicon carbide substrate  100  may be cleaned with the hydrochloric acid-hydrogen peroxide water mixture being irradiated with ultrasonic waves. 
     The volume ratio among the hydrochloric acid, the hydrogen peroxide water, and the ultrapure water contained in the hydrochloric acid-hydrogen peroxide water mixture is from 1 (hydrochloric acid): 1 (hydrogen peroxide water): 5 (ultrapure water) to 1 (hydrochloric acid): 1 (hydrogen peroxide water): 10 (ultrapure water), for example. In other words, the volume of the hydrochloric acid is 1/10 or more and ⅕ or less of the volume of the ultrapure water. Further, the volume of the hydrogen peroxide water is 1/10 or more and ⅕ or less of the volume of the ultrapure water. The immersion time of silicon carbide substrate  100  is five minutes or more, for example. The temperature of the sulfuric acid-hydrogen peroxide water mixture is room temperature, for example. 
     Then, the hydrofluoric acid cleaning step (S 80 ) is performed. In the hydrofluoric acid cleaning step (S 80 ), a silicon oxide film is removed by hydrofluoric acid, and a surface is terminated with fluorine. The hydrofluoric acid in a mixed solution in which the hydrofluoric acid and ultrapure water are mixed has a concentration of 10% or more and 40% or less, for example. The immersion time of silicon carbide substrate  100  is five minutes or more, for example. The temperature of the sulfuric acid-hydrogen peroxide water mixture is room temperature, for example. In the hydrofluoric acid cleaning step (S 80 ), silicon carbide substrate  100  may be cleaned with the hydrofluoric acid being irradiated with ultrasonic waves. 
     Then, the drying step (S 90 ) is performed. In the drying step (S 90 ), silicon carbide substrate  100  is dried using a spin drying machine  30 , for example. As shown in  FIG. 9 , spin drying machine  30  includes a main body portion  31 , a lid  32 , an opening  34 , and an exhaust port  33 . Spin drying machine  30  is placed in a class  100 -equivalent clean room. Before silicon carbide substrate  100  is introduced into spin drying machine  30 , air is passed from opening  34  toward exhaust port  33  of spin drying machine  30 , with lid  32  of spin drying machine  30  being opened. The main body portion has a volume of 127000 cm 3 , for example. Opening  34  has an area of 2700 cm 2 , for example. The amount of the passed air is 60 m 3 , for example. 
     Then, silicon carbide substrate  100  is placed in main body portion  31  of spin drying machine  30 , and lid  32  is closed. Silicon carbide substrate  100  rotates about a rotation axis substantially perpendicular to first main surface  1 , with pressure being reduced through exhaust port  33 . The number of rotations of silicon carbide substrate  100  is 800 rpm, for example. The rotation time is 300 seconds, for example. Thereby, the cleaning solution adhering to silicon carbide substrate  100  is removed by centrifugal force. 
     Next, the function and effect of the present embodiment will be described. 
     In recent years, a polishing solution containing a permanganate has been utilized in the step of polishing silicon carbide substrate  100  in order to improve a polishing effect. The polishing solution containing a permanganate has a high polishing efficiency for silicon carbide substrate  100 , whereas it has a property that it tends to be left on silicon carbide substrate  100  as an impurity. As is understood from the fact that potassium permanganate is used as a raw material for a coloring agent, for example, potassium permanganate salt turns into a mud-like sludge, and tends to adhere to silicon carbide substrate  100 . 
     In particular, grinding marks  9  are formed in chamfered portion  6  of silicon carbide substrate  100  due to grinding processing in the chamfering step (S 25 ). Sludge containing a permanganate tends to enter grinding marks  9  together with abrasive grains and the like. Once the sludge enters grinding marks  9 , it is difficult to remove the sludge by a general cleaning method. 
     In the sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ) for silicon carbide substrate  100  in accordance with the present embodiment, silicon carbide substrate  100  is cleaned with the sulfuric acid-hydrogen peroxide water mixture being irradiated with ultrasonic waves. The ultrasonic waves promote a chemical reaction. This enhances the reactivity of a metal impurity such as manganese to the sulfuric acid-hydrogen peroxide water mixture. Further, the sludge containing manganese which has entered grinding marks  9  in chamfered portion  6  can be effectively removed by a cavitation effect caused by irradiation of the ultrasonic waves. Accordingly, the manganese concentration in chamfered portion  6  can be reduced. Specifically, the manganese concentration in chamfered portion  6  can be set to 1×10 11  atoms/cm 2  or less. Thereby, silicon carbide substrate  100  with a high cleanliness is obtained. As a result, when silicon carbide substrate  100  is used to fabricate a silicon carbide semiconductor device, it is possible to suppress deterioration of electrical properties of the silicon carbide semiconductor device due to a metal impurity such as manganese. 
     Example 
     (Sample Preparation) 
     First, silicon carbide substrate  100  in accordance with a sample 1 and silicon carbide substrate  100  in accordance with a sample 2 were prepared. Silicon carbide substrate  100  in accordance with sample 1 was an example. Silicon carbide substrate  100  in accordance with sample 2 was a comparative example. Each silicon carbide substrate  100  had a maximum diameter (diameter) of 150 mm. 
     Silicon carbide substrate  100  in accordance with sample 1 was manufactured by the manufacturing method in accordance with the present embodiment. Specifically, in the sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ), the sulfuric acid-hydrogen peroxide water mixture was irradiated with ultrasonic waves. The ultrasonic waves had a frequency of 950 kHz. The volume ratio among the sulfuric acid, the hydrogen peroxide water, and the ultrapure water contained in the sulfuric acid-hydrogen peroxide water mixture was set to 10 (sulfuric acid): 1 (hydrogen peroxide water): 1 (ultrapure water). The immersion time of silicon carbide substrate  100  was set to 30 minutes. The temperature of the sulfuric acid-hydrogen peroxide water mixture was set to room temperature. On the other hand, according to a method for manufacturing silicon carbide substrate  100  in accordance with sample 2, in the sulfuric acid-hydrogen peroxide water mixture cleaning step (S 50 ), the sulfuric acid-hydrogen peroxide water mixture was not irradiated with ultrasonic waves. Other manufacturing conditions were the same as those of the method for manufacturing silicon carbide substrate  100  in accordance with sample 1. 
     (Measuring Method) 
     Then, an acid was dropped along the entire circumference of chamfered portion  6  of silicon carbide substrate  100  (except for orientation flat portion  7 ) using a horizontal substrate inspection device, and the acid containing a metal impurity was collected from the entire circumference of chamfered portion  6 . The concentration of the metal impurity was measured by ICP-MS. As a measuring device, ICP-MS8800 manufactured by Agillent was used, for example. The mass of a metal impurity element obtained by measurement was divided by the atomic weight of the element to be converted into the number of moles, and then the number of moles was multiplied by the Avogadro number to be converted into the number of atoms. Then, the number of atoms was divided by the area of chamfered portion  6  to obtain the number of atoms per unit area. It should be noted that chamfered portion  6  of silicon carbide substrate  100  had an area of 3.78 cm 2 . Further, an arithmetic mean roughness (Ra) in the chamfered portion was measured using a stylus-type surface roughness measuring instrument. As the measuring device, SURFCOM 1800G manufactured by Tokyo Seimitsu was used. 
     (Result of Measurement) 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Sample 1 
                 Sample 2 
               
            
           
           
               
               
               
            
               
                   
                 Unit 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Element 
                 10 10  atoms/cm 2   
                 10 10  atoms/cm 2   
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Na 
                 11.0 
                 46.0 
               
               
                   
                 K 
                 4.7 
                 4.1 
               
               
                   
                 Mn 
                 — 
                 12.0 
               
               
                   
                 Fe 
                 7.2 
                 21.0 
               
               
                   
                   
               
            
           
         
       
     
     Table 1 shows concentrations of metal impurities in chamfered portion  6 . As shown in Table 1, in chamfered portion  6  of silicon carbide substrate  100  in accordance with sample 1, the sodium (Na) concentration was 11.0×10 10  atoms/cm 2 . The potassium (K) concentration was 4.7×10 10  atoms/cm 2 . Manganese (Mn) was not detected. The iron (Fe) concentration was 7.2×10 10  atoms/cm 2 . 
     As shown in Table 1, in chamfered portion  6  of silicon carbide substrate  100  in accordance with sample 2, the sodium (Na) concentration was 46.0×10 10  atoms/cm 2 . The potassium (K) concentration was 4.1×10 10  atoms/cm 2 . Manganese (Mn) was 12.0×10 10  atoms/cm 2 . The iron (Fe) concentration was 21.0×10 10  atoms/cm 2 . 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Sample 1 
                 Sample 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Arithmetic Mean 
                 0.1 μm 
                 0.1 μm 
               
               
                   
                 Roughness (Ra) 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 shows an arithmetic mean roughness (Ra) in chamfered portion  6 . As shown in Table 2, chamfered portion  6  of silicon carbide substrate  100  in accordance with sample 1 had an arithmetic mean roughness (Ra) of 0.1 μm. On the other hand, chamfered portion  6  of silicon carbide substrate  100  in accordance with sample 2 had an arithmetic mean roughness (Ra) of 0.1 μm. 
     As described above, it was confirmed that sludge containing manganese (Mn) adhering to chamfered portion  6  was removed by cleaning silicon carbide substrate  100  with the sulfuric acid-hydrogen peroxide water mixture being irradiated with ultrasonic waves, and silicon carbide substrate  100  with a low manganese concentration in chamfered portion  6  can be manufactured. 
     It should be understood that the embodiment and the example disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the scope of the claims, rather than the above description, and is intended to include any modifications within the scope and meaning equivalent to the scope of the claims. 
     REFERENCE SIGNS LIST 
       1 : first main surface;  2 : second main surface;  3 : first curved region;  4 : second curved region;  5 : outer circumferential end portion;  6 : chamfered portion;  7 : orientation flat portion;  8 : arc-shaped portion;  9 : grinding marks;  10 : boundary;  12 : first cleaning tank;  13 : second cleaning tank;  14 : first cleaning solution;  15 : second cleaning solution;  19 : ultrasonic wave generation source;  20 : ultrasonic wave cleaning device;  25 : grinding device;  30 : spin drying machine;  31 : main body portion;  32 : lid;  33 : exhaust port;  34 : opening;  100 : silicon carbide substrate;  101 : first direction;  102 : second direction; A: straight line; W 1 : first width; W 2 : second width.