METHOD FOR PRODUCING HIGH-POROSITY VITRIFIED GRINDING STONE
A method for producing a high-porosity vitrified grinding stone that has a plurality of pores communicating with each other. The method includes: (a) 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, a gellable water-soluble polymer and a water; (b) a molding step of obtaining a molded body, by gelling the grinding-stone raw material slurry with use of a molding mold; (c) a freeze vacuum drying step of generating a plurality of frozen particles inside the molded body by freezing the molded body, 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 (d) 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.
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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 ARTIn 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
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 ProblemHaving 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 InventionAccording 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[
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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.
EmbodimentThe 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
The vitrified bond 22 has a composition of, for example, 50-54 weight% of SiO2, 13-15 weight% of Al2O3, 17.5-20.5 weight% of B2O3, 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 R2O (that is at least one kind of oxide selected from Li2O, Na2O and K2O) and 0.7-1.3 weight% of P2O5.
At a subsequent molding step P2, 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.
Then, at a freeze vacuum drying step P3, 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.
Then, at the freeze vacuum drying step P3, 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.
At a firing step P4, 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.
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.
Referring back to
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
As shown in
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 BondIncluding SiO2:51.5 mass%, Al2O3:14.7 mass%, B2O3:18.9 mass%, Na2O:3.9 mass%, K2O:3.8 mass%, MgO:2.0 mass%, CaO:1.9 mass%, BaO:0.7 mass% and P2O5:0.9 mass%.
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
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/cm3.
Grinding Test MethodThe 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
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 BondIncluding SiO2:51.5 mass%, Al2O3:14.7 mass%, B2O3:18.9 mass%, Na2O:3.9 mass%, K2O:3.8 mass%, MgO:2.0 mass%, CaO:1.9 mass%, BaO:0.7 mass% and P2O5:0.9 mass%.
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
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/cm3.
Grinding Test MethodThe 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
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 P1 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 P2 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 P3 of generating the plurality of frozen particles 30 inside the molded body 28 by freezing the molded body 28 after the molding step P2, 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 P4 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 P3. Thus, at the freeze vacuum drying step P3, 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 P3. 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 P3, 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 P4, 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 SiO2, 13-15 weight% of AI2O3, 17.5-20.5 weight% of B2O3, 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 R2O (that is at least one kind of oxide selected from Li2O, Na2O and K2O) and 0.7-1.3 weight% of P2O5. 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 SIGNS10: 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
Claims
1. A method for producing a high-porosity vitrified grinding stone that has a plurality of pores communicating with each other,
- the method comprising:
- 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;
- a molding step of obtaining a molded body, by gelling the grinding-stone raw material slurry with use of a molding mold;
- 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
- 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.
2. The method of the according to claim 1,
- wherein 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.
3. The method according to claim 1,
- wherein, 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
- wherein, 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.
4. The method according to claim 1,
- wherein a pore volume ratio of the high-porosity vitrified grinding stone is 65-90 volume%.
5. The method according to claim 1,
- wherein a specific gravity of the high-porosity vitrified grinding stone is 0.34-1.48.
6. The method according to claim 3,
- wherein the abrasive grains have a median grain diameter that is smaller than a thickness of the bond bridges constituting the shells of the pores.
Type: Application
Filed: Dec 11, 2020
Publication Date: May 18, 2023
Applicant: NORITAKE CO., LIMITED (Nagoya-shi, Aichi)
Inventors: Kouichi YOSHIMURA (Nagoya-shi), Tomoki KIMURA (Nagoya-shi)
Application Number: 17/916,354