Patent Publication Number: US-2023150094-A1

Title: Method for producing high-porosity vitrified grinding stone

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing a vitrified grinding stone that has a high porosity due to presence of pores communicating with each other. 
     BACKGROUND ART 
     In general, for grinding a semiconductor wafer, there is proposed a high-porosity vitrified grinding stone in which abrasive grains are bound by a vitrified bond to increase an abrasive-grain coercive force and in which a high porosity such as a porosity of 75-95 volume% is established to advantageously provide a self-sharpening effect of the abrasive grains. A vitrified grinding stone disclosed in each of Patent Documents 1 and 2 is an example of such a vitrified grinding stone. 
     In such a vitrified grinding stone having the high porosity, the high porosity provides the self-sharpening effect of the abrasive grains for thereby increasing a grinding performance, and the high porosity is established by independent pores that provide the grinding stone with a high strength, so that it is possible to advantageously perform a grinding operation with sufficient grinding pressure. 
     By the way, the high-porosity vitrified grinding stone as disclosed in the Patent Document 1 is produced by pressing a grinding-stone raw material, which is made by kneading and mixing an organic pore-forming agent such as polystyrene particles into abrasive grains and vitrified bond, to form a molded body, and then burning the organic pore-forming agent by firing the molded body. Therefore, in the high-porosity vitrified grinding stone obtained by firing the molded body, most of the pores are the independent pores, i.e, closed pores. Thus, there is a case in which swarf generated in the grinding operation is accumulated in the independent pores thereby making it impossible to satisfactorily obtain the grinding performance. 
     On the other hand, in Patent Document 3, there is proposed a method of producing a high-porosity vitrified grinding stone having a high porosity of 50-98 volume%, by obtaining a meringue-like foam material by mixing abrasive grains, a vitrified bond, a solidifying agent (gelling agent), a water and a surface-active agent that is used in place of the pore-forming agent, then preparing a molded body by cooling the foam material in a molding mold, then firing the molded body that has been dried, and then immersing the molded boy into a liquid resin so as to cover bond bridges serving as shells surrounding pores, with resin coating layers, for thereby increasing the strength. 
     Prior Art Documents 
     Patent Documents 
     
         
         Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-001007 
         Patent Document 2: Japanese Unexamined Patent Application Publication No. 2017-080847 
         Patent Document 3: Japanese Unexamined Patent Application Publication No. 2007-290101 
       
    
     SUMMARY OF THE INVENTION 
     Object to Be Achieved by the Invention 
     However, in the vitrified grinding stone produced by the above-described producing method, the pores are not independent pores, so that there is a case in which the molded body shrinks due to a deformation phenomenon of the bond bridges that is progressed in process of drying of the molded body molded from the meringue-like foam material, so that it is difficult to produce a product having a stable shape. 
     The present invention was made in view of the background discussed above. It is therefore an object of the present invention to provide a method for producing a vitrified grinding stone having a high porosity due to presence of pores communicating with each other, wherein the method is capable of stably producing the vitrified grinding stone. 
     Measures for Solving the Problem 
     Having made various studies under the above-described situation, the present inventors and their colleagues found that it is possible to obtain a vitrified grinding stone which exhibits a sufficient strength in a grinding operation and which has a high porosity due to presence of pores communicating with each other, by obtaining a grinding-stone raw material slurry in which a gelling agent is dissolved and abrasive grains, a vitrified bond and a water are mixed, then preparing a molded body by gelling the grinding-stone raw material slurry in a molding mold, then generating a multiplicity of frozen particles inside the molded body by freezing the molded body, then sublimating the multiplicity of generated frozen particles under a vacuum state so as to form the communicating pores in the molded body, and then firing the porous molded body. The present invention was made based on this finding. 
     That is, the gist of the present invention is ( 1 ) a method of producing a high-porosity vitrified grinding stone that has a plurality of pores communicating with each other, wherein the method includes: ( 2 ) a grinding-stone-material preparing step of obtaining a grinding-stone raw material slurry that is a mixture fluid of abrasive grains, a vitrified bond and a water, such that a gellable water-soluble polymer is dissolved in the mixture fluid; ( 3 ) a molding step of obtaining a molded body, by gelling the grinding-stone raw material slurry with use of a molding mold; ( 4 ) a freeze vacuum drying step of generating a plurality of frozen particles inside the molded body by freezing the molded body after the molding step, and placing the molded body under a vacuum state, so as to sublimate the frozen particles generated inside the molded body for thereby drying the molded body; and ( 5 ) a firing step of obtaining the high-porosity vitrified grinding stone, by binding the abrasive grains with the vitrified bond by firing the molded body after the freeze vacuum drying step. 
     Effects of the Invention 
     According to the method of producing the high-porosity vitrified grinding stone of the present invention, at the freeze vacuum drying step, with the molded body being placed under the vacuum state, the frozen particles generated inside the molded body are sublimated whereby the molded body is dried so that the plurality of pores communicating with each other are formed after the frozen particles are sublimated. Thus, shrinkage of the molded body is suppressed, and it is possible to stably produce the vitrified grinding stone having the high porosity due to presence of the plurality of pores communicating with each other. 
     It is preferable that the pores are formed in places in which the frozen particles had been present in the grinding-stone raw material slurry, after the frozen particles have been sublimated at the freeze vacuum drying step. The pores thus formed are not eliminated so that it is possible to suppress the shrinkage of the molded body. 
     Further, it is preferable that, at the freeze vacuum drying step, the frozen particles are generated in the grinding-stone raw material slurry which is gelled and which constitutes the molded body, whereby the abrasive grains and the vitrified bond are gathered to base portions surrounding the frozen particles, and that, at the firing step, the base portions are fired whereby bond bridges, which are shells that surround the pores, are formed from the base portion. Thus, the strength of the bond bridges is increased even without the bond bridges being covered by reinforcing resin coating layers, so that the vitrified grinding stone can perform a grinding operation although having the high porosity. 
     Further, it is preferable that a pore volume ratio of the high-porosity vitrified grinding stone is 65-90 volume%. Thus, since the high-porosity vitrified grinding stone has the pore volume ratio of 65-90 volume%, it is possible to obtain both of a grinding efficiency and a grinding stone strength. 
     Further, it is preferable that a specific gravity of the high-porosity vitrified grinding stone is 0.34-1.48. Thus, the high-porosity vitrified grinding stone having the relatively small specific gravity of 0.34-1.48 can be obtained. 
     Further, it is preferable that the abrasive grains have a median grain diameter (median size) that is smaller than a thickness of the bond bridges constituting the shells of the pores. Thus, since the abrasive grains are significantly smaller than the thickness of the bond bridges corresponding to the shells of the pores, the bond bridges have a locally non-porous vitrified grinding stone structure so as to increase the strength and the grinding performance of the high-porosity vitrified grinding stone, and a surface roughness suitable for grinding a semiconductor wafer can be obtained. 
     BRIEF DESCRIPTION OF DRAWINGS 
     [ FIG.  1   ] A perspective view showing a cup grinding stone including a base metal and high-porosity vitrified grinding stones fixed to the base metal, wherein each of the high-porosity vitrified grinding stones is produced by a method for producing the high-porosity vitrified grinding stone, which is an embodiment of the present invention. 
     [ FIG.  2   ] A process chart explaining a major part of the method of producing the high-porosity vitrified grinding stone shown in  FIG.  1   . 
     [ FIG.  3   ] A set of views schematically showing change of structure in a molded body of the high-porosity vitrified grinding stone in producing process shown in  FIG.  2   , wherein (a) shows a state in which a grinding-stone raw material slurry has been prepared at a grinding-stone-material preparing step, (b) shows a state in which frozen particles have been generated inside the molded body by freezing made at a freeze vacuum drying step, (c) shows a state in which the frozen particles have been sublimated inside the molded body by vacuum drying made at the freeze vacuum drying step, and (d) shows a state in which the molded body has been fired at a firing step. 
     [ FIG.  4   ] A view showing circular test pieces (molded bodies), wherein the left-side circular test piece is in a case in which drying has been made in substantially the same manner as in the freeze vacuum drying step of  FIG.  2   , and the right-side circular test piece is in a case in which a normal-pressure drying has been made. 
     [ FIG.  5   ] A view showing optical micrographs, wherein the left-side optical micrograph shows in enlargement a pore structure of the molded body before the firing step of  FIG.  2   , and the right-side optical micrograph shows in enlargement the pore structure of the molded body after the firing step of  FIG.  2   . 
     [ FIG.  6   ] An SEM photograph showing in enlargement the pore structure of the molded body after the firing step of  FIG.  2   . 
     [ FIG.  7   ] An SEM photograph showing in further enlargement the pore structure of the molded body after the firing step of  FIG.  2   . 
     [ FIG.  8   ] A view showing major data of a grinding stone structure and a grinding ratio of each of samples 11 and 12, so as to compare with samples 1-10 that have been produced substantially the same process of  FIG.  2   . 
     [ FIG.  9   ] A view showing samples 1-12 of  FIG.  8    in a two-dimensional coordinates having a horizontal axis representing Vg/Vb and a vertical axis representing a specific gravity ρ. 
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, there will be described an embodiment of the present invention, in detail with reference to the drawings. It is noted that figures of the drawings are simplified or deformed as needed, and each portion is not necessarily precisely depicted in terms of dimension ratio, shape, etc. 
     Embodiment 
       FIG.  1    is a perspective view showing a cup grinding stone  14  including a disk-shaped base metal  12  made of metal such as aluminum, and a plurality of segment grinding stones  10  fixed to a lower surface of the base metal  12  such that the segment grinding stones  10  are contiguous to each other and arranged in an annular manner along an outer periphery of the lower surface of the base metal  12 , wherein each of the segment grinding stones  10  is an example of a high-porosity vitrified grinding stone as an embodiment of the present invention. The segment grinding stones  10  have respective grinding surfaces  16  that are located on a lower side of an outer peripheral portion of the base metal  12  and contiguous to each other and arranged in an annular manner. 
     The base metal  12  is constituted by a disk-shaped thick plate made of the metal. With the base metal  12  being attached to a spindle of a grinding machine (not shown), the cup grinding stone  14  is to be driven and rotated. The cup grinding stone  14  has an outside diameter of about 250 mm. Each of the segment grinding stones  10  has a width of about 3 mm and a thickness of about 5 mm. When the base metal  12  is rotated, the segment grinding stones  10  are brought into sliding contact at the respective grinding surfaces  16  with a work material such as silicon wafer, so as to grind or polish the work material to a flat surface shape, for thereby machining a thickness of the work material. 
     The segment grinding stone  10  is produced, for example, in the producing process shown in  FIG.  2   . At a grinding-stone-material preparing step P 1 , a grinding-stone raw material slurry  18  for the segment grinding stone  10  is prepared. The grinding-stone raw material slurry  18  contains diamond abrasive grains  20  as examples of super abrasive grains, a vitrified bond  22 , a water-soluble polysaccharide gelling agent  24  functioning as a primary binder and a water  26  functioning as a pore-forming agent, which are mixed at respective mixing ratios of 17 mass% of the diamond abrasive grains, 15 mass% of the vitrified bond  22 , 3 mass% of the water-soluble polysaccharide gelling agent  24 , and 65 mass% of the water  26 , such that the grinding-stone raw material slurry  18  is prepared to have a liquidity, with the water-soluble polysaccharide gelling agent  24  being dissolved into the water  26  by heating at a temperature of, for example, about 90° C., which is higher than a melting temperature. It is noted that the water-soluble polysaccharide gelling agent  24  corresponds to “gellable water-soluble polymer” in the present embodiment. 
       FIG.  3  ( a )  schematically shows the grinding-stone raw material slurry  18 . In  FIG.  3  ( a ) , the water-soluble polysaccharide gelling agent  24  dissolved into the water  26  constitutes fibrous polysaccharide molecules, and is schematically shown in  FIG.  3  ( a ) . These polysaccharide molecules (water-soluble polypeptides gelling agent  24 ) are entangled with each other and stick to each other to form a network structure, and lots of microscopic spaces for storing the water  26  are formed. Further, the polysaccharide molecule (water-soluble polysaccharide gelling agent  24 ) being entangled with each other, when the heated temperature of the water-soluble polysaccharide gelling agent  24  is lowered, the liquidity of the water-soluble polysaccharide gelling agent  24  is lost, and the water-soluble polysaccharide gelling agent  24  is gelled. It is noted that, as the water-soluble polysaccharide gelling agent  24 , it is possible to use, for example, curdlan, tamarind seed gum, kitan sang gum + locust bean gum, sodium alginate, gelangum, pectin, carrageenan, gelatin and agar. 
     The vitrified bond  22  has a composition of, for example, 50-54 weight% of SiO 2 , 13-15 weight% of Al 2 O 3 , 17.5-20.5 weight% of B 2 O 3 , 0.7-6.5 weight% of RO (that is at least one kind of oxide selected from CaO, MgO, BaO and ZnO), 0.0-9.0 weight% of R 2 O (that is at least one kind of oxide selected from Li 2 O, Na 2 O and K 2 O) and 0.7-1.3 weight% of P 2 O 5 . 
     At a subsequent molding step P 2 , the grinding-stone raw material slurry  18 , which is in a heated state and has the liquidity, is poured into a molding cavity inside the molding mold, wherein the molding cavity has a predetermined shape, e.g., a shape substantially the same as the segment grinding stone  10  and slightly larger than the segment grinding stone  10 . Then, with the temperature being reduced to a normal temperature or another temperature that is not higher than a melting temperature of the water-soluble polysaccharide gelling agent  24 , the grinding-stone raw material slurry  18   having the liquidity is gelled, namely, solidified, whereby a molded body  28  is obtained, and the molded boy  28  is taken out from the molding mold.  FIG.  3  ( a )  shows a state in which the grinding-stone raw material slurry  18  has been gelled. 
     Then, at a freeze vacuum drying step P 3 , the molded body  28  is put into a chamber of a vacuum freeze dryer. Thus, with the molded body  28  being frozen at a predetermined freezing temperature that is not higher than -15° C., for example, a plurality of frozen particles  30  each having a predetermined size is precipitated from the water  26  in the molded body  28 . The frozen particles  30  are maintained for a predetermined freezing time whereby each of the frozen particles  30  is caused to grow to a predetermined size.  FIG.  3  ( b )  schematically shows this frozen state. In this frozen state, the diamond abrasive grains  20  and particles of the vitrified bond  22  are driven to interfaces of the frozen particles  30 . That is, the diamond abrasive grains  20  and the vitrified bond  22  are gathered to base portions  34  that surround the frozen particles  30 . 
     Then, at the freeze vacuum drying step P 3 , the molded body  28  is vacuum-dried at 35° C., for example, such that the plurality of frozen particles  30  in the molded body  28  are slowly sublimated, with the molded body  28  being placed under a vacuum state with a predetermined vacuum value (e.g., 10 Pa) that is lower than 610 Pa, for a predetermined time. With the plurality of frozen particles  30  being sublimated, a plurality of pores  32  are formed in places in which the frozen particles  30  has been positioned.  FIG.  3  ( c )  schematically shows this state. In this state, the diamond abrasive grains  20  and the particles of the vitrified bond  22  are dispersed and incorporated in the fibrous polysaccharide molecule (water-soluble polysaccharide gelling agent  24 ) that surrounds the pores  32 . The plurality of frozen particles  30  inside the molded body  28  are brought into contact with each other in process of growth of the frozen particles  30 , so that most of the pores  32 , which are formed in place of the sublimated frozen particles  30 , are pores communicating with each other. 
       FIG.  4    is a photograph showing circular test pieces as molded bodies that were subjected to the same steps until each of the frozen particles was caused to grow to the predetermined size with the molded bodies being frozen at the predetermined freezing temperature for the predetermined freezing time, wherein a left-side molded body as one of the molded bodies shown in left side in  FIG.  4    was subjected to the vacuum drying in the frozen state, while a right-side molded body as the other of the molded bodies shown in right side in  FIG.  4    was subjected to the normal-pressure drying at 50° C. The left-side molded boy subjected to the freezing vacuum drying did not lose its shape and had a strength to be easily grasped by hand. On the other hand, the right-side molded body subjected to the normal-pressure drying had a large collapse in its shape, making it impossible to produce the high-porosity vitrified grinding stone. 
     At a firing step P 4 , the molded body  28  in which the plurality of pores  32  are formed is fired at a 500-1000° C. as an example of a firing temperature that is not lower than a softening temperature of the vitrified bond  22 . With the molded body  28  being fired, the fibrous polysaccharide molecule (water-soluble polysaccharide gelling agent  24 ) surrounding the pores  32  is burnt and at the same time the diamond abrasive grains  20  and the vitrified bond  22  are sintered to form bond bridges  36  surrounding the pores  32 , whereby the segment grinding stone  10  as the high-porosity vitrified grinding stone is obtained.  FIG.  3  ( d )  schematically shows this state. 
     The bond bridges  36  have a thickness of about 10 µm, and hold the diamond abrasive grains  20  having a median grain diameter of a few µm. Each of the bond bridges  36  is constituted by a dense structure like a non-porous vitrified grinding stone having no pore. Thus, the strength of the bond bridges  36  is increased even without the bond bridges  36  being covered by reinforcing resin coating layers. The median grain diameter of the diamond abrasive grains  20  corresponds to a median size (median diameter) defined by Japanese Industrial Standards (JIS Z 8825:2013), and is a value of D50 by volume conversion. 
       FIG.  5    is a view showing optical micrographs, wherein the left-side optical micrograph shows in enlargement a pore structure of the molded body  28  before the firing step P 4 , and the right-side optical micrograph shows in enlargement the pore structure of the molded body  28  after the firing step P 4 , i.e., the pore structure of the high-porosity vitrified grinding stone  10 . As is clear from  FIG.  5   , any change of the pore structure of the molded body  28  caused by the firing step P 4  is not seen, so that the pore structure before the firing step P 4  is maintained after the firing step P 4 . 
       FIG.  6    is an SEM photograph showing in further enlargement the pore structure of the molded body  28  after the firing step P 4  (i.e., the pore structure of the high-porosity vitrified grinding stone  10 ). In  FIG.  6   , white lines represent the bond bridges  36  surrounding the pores  32 .  FIG.  7    is an SEM photograph showing in further enlargement the bond bridges  36 . As shown in  FIG.  7   , the bond bridges  36  has the dense structure. Fine particles on the bond bridges  36  are the diamond abrasive grains  20 . The diamond abrasive grains  20  has the median grain diameter that is not larger than  1 /50 of the thickness of the bond bridges  36 . 
     Referring back to  FIG.  2   , at a bonding/finishing step P 5 , the plurality of segment grinding stones  10 , each of which has been fired at the firing step P 4 , are bonded to the base metal  12 , as shown in  FIG.  1   . Then, the segment grinding stones  10  bonded to the base metal  12  is finished by using a dresser. 
     Hereinafter, the median grain diameter of the diamond abrasive grains, a ratio (= Vg/Vb) of a volume ratio Vg (volume%) of the diamond abrasive grains to a volume ratio Vb (volume%) of the vitrified bond, a specific gravity ρ, a pore volume ratio Vp (volume%) and a grinding ratio GR of samples 1-10 (example products) that are chip-shaped vitrified grinding stones (40 mm×3 mm×5 mm) are shown in  FIG.  8   , wherein the samples 1-10 were obtained by the present inventors and their colleagues through a process substantially the same at the process shown in  FIG.  2   , with various ratios of the diamond abrasive grains, vitrified bond, water-soluble polysaccharide gelling agent and water. Further, samples 11 and 12 (comparative example products) that are chip-shaped vitrified grinding stones (40 mm×3 mm×5 mm) as well as the samples 1-10 are shown in  FIG.  8   , wherein the samples 11 and 12 were preprepared through a process in which the pores were formed with use of conventional pore-forming agents. Further,  FIG.  9    is a view showing the samples 1-12 of  FIG.  8    that are plotted in a two-dimensional coordinates having a horizontal axis representing the above-described ratio Vg/Vb and a vertical axis representing the specific gravity ρ. The specific gravity ρ of each of the grinding stones is a value obtained by dividing a mass of the grinding stone by a grinding stone volume that is obtained from dimensions of the grinding stone. The volume ratio Vg of the abrasive grains is a value obtained by dividing an abrasive grain volume, which is obtained by dividing a mass of the abrasive grains by a specific gravity of the abrasive grains, by the grinding stone volume. The volume ratio Vb of the bond is a value obtained by dividing a bond volume, which is obtained by dividing a mass of the bond by a specific gravity of the bond, by the grinding stone volume. The pore volume ratio Vp is a value obtained by dividing a volume of the pores by the grinding stone volume. 
     As shown in  FIGS.  8  and  9   , the samples 1-10 are clearly different from the samples11 and 12, particularly, in terms of the pore volume ratio Vp and the grinding ratio GR. The pore volume ratio Vp of the samples 1-10 is 65-90 volume%, so that the pore volume ratio Vp of the samples 1-10 is larger than that of the samples 11 and 12. The grinding ratio GR of the samples 1-10 is 11-750, so that the grinding ratio GR of the samples 1-10 is remarkably larger than that of the samples 11 and 12. 
     Hereinafter, grinding tests  1  and  2  will be described in details, wherein the grinding stones of the sample 9 (example product) and the sample 11 (comparative example product) were used in the grinding test  1 , and the grinding stones of the sample  3  (example product) and the sample 12 (comparative example product) were used in the grinding test  2 . The grinding test  2  was performed for an expected rough/finish machining. The grinding test  1  was performed for an expected finish machining. 
     Grinding Test  1   
     Composition of Vitrified Bond 
     Including SiO 2 :51.5 mass%, Al 2 O 3 :14.7 mass%, B 2 O 3 :18.9 mass%, Na 2 O:3.9 mass%, K 2 O:3.8 mass%, MgO:2.0 mass%, CaO:1.9 mass%, BaO:0.7 mass% and P 2 O 5 :0.9 mass%. 
     
       
         
           
               
               
             
               
                 Mixing for sample 9 
               
             
            
               
                 Diamond abrasive grains of median grain diameter of 0.2 µm 
                 :17 mass% 
               
               
                 Vitrified bond 
                 :15 mass% 
               
               
                 Water-soluble polysaccharide gelling agent (agar) 
                 : 3 mass% 
               
               
                 Water 
                 : 65 mass% 
               
            
           
         
       
     
     Method of Producing Sample 9 
     The chip-shaped vitrified grinding stone (sample 9), which has substantially the same shape as the segment grinding stone  10 , was prepared from the above-described mixing through the producing process of  FIG.  2   , such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of the diamond abrasive grains is 7.0 volume%, the volume ratio Vb of the vitrified bond is 8.6 volume%, the ratio Vg/Vb is 0.8, the pore volume ratio Vp is 84.4 volume%, and the specific gravity is 0.46 g/cm 3 . 
     Method of Producing Sample 11 
     The chip-shaped vitrified grinding stone (sample 11), which has substantially the same shape as the segment grinding stone  10 , was prepared through a conventional process forming pores using a pore-forming agent, such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of diamond abrasive grains having a median grain diameter of 0.2 µm is 27.2 volume%, the volume ratio Vb of the vitrified bond substantially the same as that in the sample 9 is 17.3 volume%, the ratio Vg/Vb is 1.6, the pore volume ratio Vp is 55.5 volume%, and the specific gravity is 1.55 g/cm 3 . 
     Grinding Test Method 
     The grinding tests were performed to a silicon wafer having a diameter of 12 inches under a grinding condition specified below by using grinding wheels each of which was attached to a vertical plane grinding machine, wherein the vitrified grinding stones of a corresponding one of the samples 9 and 11 were bonded, as shown in  FIG.  1   , to a lower surface of a base metal made of aluminum and having an outside diameter of 300 mm in each of the grinding wheels. 
     
       
         
           
               
               
             
               
                 Machining condition in grinding test 
               
             
            
               
                 Rotational speed of grinding stone 
                 :3000 rpm 
               
               
                 Rotational speed of table (wafer) 
                 :395 rpm 
               
               
                 Axial feed rate of grinding stone 
                 :0.5 µm/sec 
               
               
                 Machining allowance of wafer 
                 :thickness 20 µm 
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 Result of grinding test of sample 9 
               
             
            
               
                 Amount of wear of grinding stone of sample 9 
                 :1.7 µm 
               
               
                 Current value during machining 
                 :19.0A 
               
               
                 Surface roughness Ra (JIS B 0601:2013) 
                 :1.2 nm 
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 Result of grinding test of sample 11 
               
             
            
               
                 Amount of wear of grinding stone of sample 11 
                 :35.3 µm 
               
               
                 Current value during machining 
                 :18.9A 
               
               
                 Surface roughness Ra (JIS B 0601:2013) 
                 :1.2 nm 
               
            
           
         
       
     
     Evaluation of Results of Grinding Tests 
     The grinding using the sample 9 is not so different from the grinding using the sample 11 in terms of the current value during machining and the surface roughness Ra. However, the amount of wear of grinding stone was remarkably reduced when the sample 9 was used, since the grinding ratio was 11.8 (= 20 µm/1.7 µm) with use of the sample 9 while the grinding ratio was 0.57 (= 20 µm/35.3 µm) with use of the sample 11. 
     Grinding Test  2   
     Composition of Vitrified Bond 
     Including SiO 2 :51.5 mass%, Al 2 O 3 :14.7 mass%, B 2 O 3 :18.9 mass%, Na 2 O:3.9 mass%, K 2 O:3.8 mass%, MgO:2.0 mass%, CaO:1.9 mass%, BaO:0.7 mass% and P 2 O 5 :0.9 mass%. 
     
       
         
           
               
               
             
               
                 Mixing for sample 3 
               
             
            
               
                 Diamond abrasive grains of median grain diameter of 6 µm 
                 :21 mass% 
               
               
                 Vitrified bond 
                 :12 mass% 
               
               
                 Water-soluble polysaccharide gelling agent (agar) 
                 : 3 mass% 
               
               
                 Water 
                 : 64 mass% 
               
            
           
         
       
     
     Method of Producing Sample 3 
     The chip-shaped vitrified grinding stone (sample 3), which has substantially the same shape as the segment grinding stone  10 , was prepared from the above-described mixing through the producing process of  FIG.  2   , such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of the diamond abrasive grains is 17.3 volume%, the volume ratio Vb of the vitrified bond is 14.2 volume%, the ratio Vg/Vb is 1.2, the pore volume ratio Vp is 68.5 volume%, and the specific gravity is 0.96 g/cm 3 . 
     Method of Producing Sample 12 
     The chip-shaped vitrified grinding stone (sample 12), which has substantially the same shape as the segment grinding stone  10 , was prepared through a conventional process forming pores using a pore-forming agent, such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of diamond abrasive grains having a median grain diameter of 6 µm is 40.9 volume%, the volume ratio Vb of the vitrified bond substantially the same as that in the sample 3 is 12.3 volume%, the ratio Vg/Vb is 3.3, the pore volume ratio Vp is 47.8 volume%, and the specific gravity is 1.95 g/cm 3 . 
     Grinding Test Method 
     The grinding tests were performed to a silicon wafer having a diameter of 12 inches under a grinding condition specified below by using grinding wheels each of which was attached to a vertical plane grinding machine, wherein the vitrified grinding stones of a corresponding one of the samples 3 and 12 were bonded, as shown in  FIG.  1   , to a lower surface of a base metal made of aluminum and having an outside diameter of 300 mm in each of the grinding wheels. 
     
       
         
           
               
               
             
               
                 Machining condition in grinding test 
               
             
            
               
                 Rotational speed of grinding stone 
                 :2000 rpm 
               
               
                 Rotational speed of table (wafer) 
                 :300 rpm 
               
               
                 Axial feed rate of grinding stone 
                 :40.00 µm/sec 
               
               
                 Machining allowance of wafer 
                 :thickness 150 µm 
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 Result of grinding test of sample 3 
               
             
            
               
                 Amount of wear of grinding stone of sample 3 
                 :0.2 µm 
               
               
                 Current value during machining 
                 :23.8A 
               
               
                 Surface roughness Ra (JIS B 0601:2013) 
                 :44.4 nm 
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 Result of grinding test of sample 12 
               
             
            
               
                 Amount of wear of grinding stone of sample 12 
                 :164 µm 
               
               
                 Current value during machining 
                 :14.2A 
               
               
                 Surface roughness Ra (JIS B 0601:2013) 
                 :39.1 nm 
               
            
           
         
       
     
     Evaluation of Results of Grinding Tests 
     The grinding using the sample 3 is not so different from the grinding using the sample 12 in terms of the surface roughness Ra, although the current value during machining using the sample 3 is increased as compared with machining using the sample 12. However, the amount of wear of grinding stone was remarkably reduced when the sample 3 was used, since the grinding ratio was 750 (= 150 µm/0.2 µm) with use of the sample 3 while the grinding ratio was 0.91 (= 150 µm/164 µm) with use of the sample 12. 
     As described above, the method of producing the high-porosity vitrified grinding stone  10  (segment grinding stone  10 ) of the present embodiment is a method of producing the high-porosity vitrified grinding stone  10  that includes the plurality of pores  32  communicating with each other. The method includes: the grinding-stone-material preparing step P 1  of obtaining the grinding-stone raw material slurry  18  that is a mixture fluid of the diamond abrasive grains (abrasive grains)  20 , the vitrified bond  22  and the water  26 , such that the water-soluble polysaccharide gelling agent  24  is dissolved in the mixture fluid; the molding step P 2  of obtaining the molded body  28 , by gelling the grinding-stone raw material slurry  18  with use of the molding mold; the freeze vacuum drying step P 3  of generating the plurality of frozen particles  30  inside the molded body  28  by freezing the molded body  28  after the molding step P 2 , and placing the molded body  28  under the vacuum state, so as to sublimate the frozen particles  30  generated inside the molded body  28  for thereby drying the molded body  28 ; and the firing step P 4  of obtaining the high-porosity vitrified grinding stone  10 , by binding the abrasive grains  20  with the vitrified bond  22  by firing the molded body  28  after the freeze vacuum drying step P 3 . Thus, at the freeze vacuum drying step P 3 , with the molded body  28  being placed under the vacuum state, the frozen particles  30  generated inside the molded body  28  are sublimated whereby the molded body  28  is dried so that the plurality of pores  32  communicating with each other are formed after the frozen particles  30  are sublimated. Thus, shrinkage of the molded body  28  due to elimination of bubbles is not caused, so that it is possible to stably produce the vitrified grinding stone  10  having the high porosity due to presence of the plurality of pores  32  communicating with each other. 
     Further, in the method for producing the high-porosity vitrified grinding stone  10  according to the present embodiment, the pores  32  communicating with each other are formed in places in which the frozen particles  30  had been present inside the molded body  28 , after the frozen particles  30  have been sublimated at the freeze vacuum drying step P 3 . The pores  32  thus formed are not eliminated so that it is possible to suppress the shrinkage of the molded body  28 . 
     Further, in the method for producing the high-porosity vitrified grinding stone  10  according to the present embodiment, at the freeze vacuum drying step P 3 , the frozen particles  30  are generated inside the molded body  28  in which the grinding-stone raw material slurry  18  that is solidified into a gel, whereby the diamond abrasive grains  20  and the vitrified bond  22  are gathered to the base portions  34  surrounding the frozen particles  30 , and, at the firing step P 4 , the base portions  34  are fired whereby the bond bridges  36  are formed as the shells that surround the pores  32 . Each of the bond bridges  36  is constituted by a structure like a non-porous vitrified grinding stone having no pore. Thus, the strength of the bond bridges  36  is increased even without the bond bridges  36  being covered by reinforcing resin coating layers, so that the vitrified grinding stone  10  can perform a grinding operation although having the high porosity. 
     Further, in the method for producing the high-porosity vitrified grinding stone  10  according to the present embodiment, the pore volume ratio Vp of the high-porosity vitrified grinding stone  10  is 65-90 volume%. Thus, since the high-porosity vitrified grinding stone  10  has the pore volume ratio Vp of 65-90 volume%, it is possible to obtain both of a grinding efficiency and a grinding stone strength. 
     Further, in the method for producing the high-porosity vitrified grinding stone  10  according to the present embodiment, the specific gravity of the high-porosity vitrified grinding stone  10  is 0.34-1.48. Thus, the high-porosity vitrified grinding stone  10  having the relatively small specific gravity of 0.34-1.48 can be obtained. 
     Further, in the method for producing the high-porosity vitrified grinding stone  10  according to the present embodiment, the grinding ratio of the high-porosity vitrified grinding stone  10  is 11-750. Thus, since the grinding ratio is a value as high as 11-750, the high-porosity vitrified grinding stone  10  having a durability can be obtained. 
     Further, in the method for producing the high-porosity vitrified grinding stone  10  according to the present embodiment, the diamond abrasive grains  20  have the median grain diameter that is smaller than the thickness of the bond bridges  36  constituting the shells of the pores  32 . Thus, since the diamond abrasive grains  20  are significantly smaller than the thickness of the bond bridges  36  corresponding to the shells of the pores  32 , the bond bridges  36  have a locally non-porous vitrified grinding stone structure so as to increase the strength and the grinding performance of the high-porosity vitrified grinding stone  10 , and a surface roughness suitable for grinding a semiconductor wafer can be obtained. 
     Further, in the method for producing the high-porosity vitrified grinding stone  10  according to the present embodiment, the vitrified bond  22  includes 50-54 weight% of SiO 2 , 13-15 weight% of AI 2 O 3 , 17.5-20.5 weight% of B 2 O 3 , 0.7-6.5 weight% of RO (that is at least one kind of oxide selected from CaO, MgO, BaO and ZnO), 0.0-9.0 weight% of R 2 O (that is at least one kind of oxide selected from Li 2 O, Na 2 O and K 2 O) and 0.7-1.3 weight% of P 2 O 5 . Thus, it is possible to obtain the vitrified bond  22  having a high strength and suitable for grinding a semiconductor wafer, and to increase the durability of the high-porosity vitrified grinding stone  10 . 
     While the embodiment of the present invention has been described in detail with reference to the drawings, the present invention is not limited to details of the embodiment but may be embodied also in other forms. 
     For example, in the above-described embodiment, the high-porosity vitrified grinding stone constitutes each of the arc-shaped segment grinding stones  10  that are fixed to the base metal  12 . However, the high-porosity vitrified grinding stone may be formed to have a disk or other shape. 
     Further, in the segment grinding stone  10 , a part to be involved in a grinding operation, for example, a grinding stone layer provided in a part including the grinding surface  16 , may be constituted by the high-porosity vitrified grinding stone. 
     Further, in the above-described embodiment, the diamond abrasive grains are employed as the abrasive grain. However, the abrasive grains do not necessarily have to be the diamond abrasive grains, and may be also abrasive grains other than super abrasive grains. Further, although the water-soluble polysaccharide gelling agent is used as the gellable water-soluble polymer in the above-described embodiment, the gellable water-soluble polymer does not necessarily have to be polysaccharide. 
     It is noted that what has been described above is merely an embodiment of the present invention, and that the present invention may be embodied with various modifications and improvements based on knowledges of those skilled in the art in a range without departing from the spirit of the invention, although the modifications and improvements have not been described by way of examples. 
     DESCRIPTION OF REFERENCE SIGNS 
       10 : segment grinding stone (high-porosity vitrified grinding stone)  12 : base metal  14 : cup grinding stone  16 : grinding surface  18 : grinding-stone raw material slurry  20 : diamond abrasive grains (grains)  22 : vitrified bond  24 : water-soluble polysaccharide gelling agent (gellable water-soluble polymer)  26 : water  28 : molded body  30 : frozen particles  32 : pores  34 : base portions  36 : bond bridges