Patent Publication Number: US-2005136807-A1

Title: Polishing composition for magnetic disk

Description:
FIELD OF THE INVENTION  
      The present invention relates to a polishing composition for a magnetic disk, a polishing process with the polishing composition, and a method for manufacturing a substrate with the polishing composition.  
     BACKGROUND OF THE INVENTION  
      In order to have a smaller unit recording area and promote higher storage capacity for hard disk drives, it has been desired to lower the flying height of the magnetic head and prevent surface defects (surface stains). In order to lower the flying height of the magnetic head, there have been earnestly desired a reduction of short-wavelength waviness (waviness having a wavelength of 50 to 500 μm) and a reduction of long-wavelength waviness (waviness having a wavelength of 0.5 mm or more), and prevention of residual substances on the surface during the polishing step of the hard disk substrate. The “waviness” as used herein refers to dents and projections of a surface having a wavelength longer than that of roughness, and can be determined by using optical instruments represented by “Zygo” commercially available from Canon Sales Inc. In order to manufacture a substrate with reduced waviness and surface stains as described above, mechanical conditions such as the pore size control of a polishing pad, hardness control, polishing load during polishing and control of the number of rotations have been studied. However, although the mechanical conditions as mentioned above have some effects, they cannot be said to be satisfactory. On the other hand, an approach from a polishing composition has been studied. Japanese Patent Laid-Open No. 2003-14733 discloses a polishing composition containing an oxidizing agent, a phosphorus-containing inorganic acid, and other organic acid. Japanese Patent Laid-Open No. Hei 11-246849 discloses a slurry for a polishing step, containing abrasive particles, hydrogen peroxide, a metal nitrate and nitric acid. However, the approaches from these polishing compositions cannot be satisfactory from the viewpoint of satisfying both the polishing rate and reduction in waviness necessary for practical use, and also the prevention of surface stains.  
     SUMMARY OF THE INVENTION  
      The present invention relates to the followings: 
      [1] a polishing composition for a magnetic disk, containing alumina, water, a peroxide and an organic acid;     [2] a polishing process of a substrate to be polished, including the step of polishing the substrate to be polished with the polishing composition of the above [1]; and     [3] a method for manufacturing a substrate, including the step of polishing a substrate to be polished with the polishing composition of the above [1].   

    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention relates to a polishing composition capable of having a high polishing rate and reduced waviness. Also, the present invention relates to a polishing process for, for instance, reducing waviness of a substrate to be polished with the polishing composition; and a method for manufacturing a substrate with the polishing composition.  
      These and other advantages of the present invention will be apparent from the following description.  
      By using the polishing composition of the present invention for the polishing of a magnetic disk, there are exhibited some effects that a high polishing rate and reduction in waviness of an object to be polished can be accomplished, and defects in a substrate, especially stains on a substrate can be reduced. Also, there are provided a polishing process for reducing waviness of a substrate to be polished with the polishing composition, and a method for manufacturing a substrate with the polishing composition.  
      One of the significant features of the polishing composition of the present invention resides in the combined use of an organic acid and a peroxide as polishing accelerators of the polishing composition, wherein an alumina is used as an abrasive. By using the polishing composition having the above features, a remarkable effect is exhibited such as achievement of a high polishing rate, reduction in waviness of a substrate to be polished, and sufficient reduction in surface defects such as scratches and pits, especially surface stains.  
      The details of the functional mechanism for polishing with the polishing composition of the present invention may not be certain. Although not wanting to be limited by theory, it is presumed that the lower the pH of the polishing composition during polishing, the higher the polishing rate, so that an etching effect on an object to be polished is caused by the hydrogen ion generated by the acid, whereby the polishing is accelerated. Accordingly, the higher the acid strength, the higher the polishing rate. In addition, the peroxide changes the surface of the object to be polished so that the polishing effect of the alumina is satisfactorily exhibited, thereby increasing the above-mentioned etching effect. Consequently, the waviness can be reduced at the processing speed on the actual manufacturing level. Although not wanting to be limited by theory, it is presumed that the amplifying effect by the combination of the organic acid and the peroxide is simultaneously connected to the reduction of the substrate defect, especially the reduction of substrate stains.  
      The polishing composition of the present invention contains an alumina as an abrasive. As the alumina preferable in the present invention, the alumina has a purity as alumina of preferably 95% or more, more preferably 97% or more, and even more preferably 99% of more, from the viewpoints of reducing the waviness, reducing the surface roughness, increasing the polishing rate and preventing the surface defects. The α-alumina is preferable, from the viewpoint of the polishing rate, and the intermediate alumina such as γ-alumina, δ-alumina, θ-alumina, η-alumina, and κ-alumina are preferable, from the viewpoints of the surface conditions and reduction in the waviness. The intermediate alumina in the present invention is a generic term referring to alumina particles other than the α-alumina particles, and concrete examples thereof include γ-alumina particles, δ-alumina particles, θ-alumina particles, η-alumina particles, κ-alumina particles, and mixtures thereof. Among these intermediate aluminas, γ-alumina, δ-alumina, θ-alumina, and mixtures thereof are preferable, from the viewpoints of increasing the polishing rate and reducing the waviness, and γ-alumina and θ-alumina are more preferable.  
      The alumina has an average primary particle size of preferably from 0.005 to 0.8 μm, more preferably from 0.01 to 0.4 μm, from the viewpoint of reducing the waviness, and an average secondary particle size of preferably from 0.01 to 2 μm, more preferably from 0.05 to 1.0 μm, and even more preferably from 0.1 to 0.5 μm. The average primary particle size of the abrasive may be obtained by subjecting the abrasive to an image analysis by observing with a scanning electron microscope (preferably from 3000 to 30000 times) or a transmission electron microscope (preferably from 10000 to 300000 times), and determining the particle size. In addition, the average secondary particle size may be determined as a volume-average particle size by using a laser diffraction method.  
      Especially, when the alumina is an intermediate alumina, the specific surface area determined by the BET method is preferably from 30 to 300 m 2 /g, more preferably from 50 to 200 m 2 /g.  
      The content of the alumina is preferably 0.05% by weight or more, more preferably 0.1% by weight or more, even more preferably 0.5% by weight or more, and even more preferably 1% by weight or more, of the polishing composition, from the viewpoints of increasing the polishing rate and reducing the waviness. Also, the content is preferably 40% by weight or less, more preferably 30% by weight or less, even more preferably 25% by weight or less, and even more preferably 20% by weight or less, of the polishing composition, from the viewpoints of the surface quality and economic advantages. Specifically, the content of the alumina is preferably from 0.05 to 40% by weight, more preferably from 0.1 to 30% by weight, even more preferably from 0.5 to 25% by weight, and even more preferably from 1 to 20% by weight, of the polishing composition.  
      The polishing composition of the present invention contains a peroxide, from the viewpoints of benefits such as increasing the polishing rate and reducing the waviness. The peroxide of the present invention is roughly classified into inorganic peroxides and organic peroxides according to their structures. Concrete examples of the peroxides are as follows. As the inorganic peroxides, hydrogen peroxide, peroxides of alkali metals or the alkaline earth metals, such as sodium peroxide, potassium peroxide, calcium peroxide, barium peroxide and magnesium peroxide; peroxocarbonates, such as sodium peroxocarbonate and potassium peroxocarbonate; peroxosulfuric acids or salts thereof, such as ammonium peroxodisulfate, sodium peroxodisulfate, potassium peroxodisulfate and peroxomonosulfuric acid; peroxonitric acids and salts thereof, such as peroxonitric acid, sodium peroxonitrate and potassium peroxonitrate; peroxophosphoric acids or salts thereof, such as sodium peroxophosphate, potassium peroxophosphate and ammonium peroxophosphate; peroxoboric acids and salts thereof, such as sodium peroxoborate and potassium peroxoborate; peroxochromic acids and salts thereof, such as potassium peroxochromate and sodium peroxochromate; permanganates, such as potassium permanganate and sodium permanganate; halogen-containing oxyacids and derivatives thereof, such as sodium perchlorate, potassium perchlorate, chloric acid, sodium hypochlorite, sodium periodate, potassium periodate, iodic acid and sodium iodate may be used. As the organic peroxides, percarboxylic acids, such as peracetic acid, performic acid and perbenzoic acid; peroxides, such as t-butyl peroxide and cumene peroxide may be used. Among them, the inorganic peroxide is preferable, because of increase in the polishing rate and easy handling such as availability and water-solubility. Moreover, in consideration of the environmental problems, an inorganic peroxide which does not contain a heavy metal is preferable. From the above viewpoint, the inorganic peroxide is more preferable, and hydrogen peroxide, peroxosulfates, halogen-containing oxyacids and salts thereof are even more preferable, and hydrogen peroxide is even more preferable. These peroxides can be used alone or in admixture of two or more kinds.  
      The content of the peroxide is preferably 0.002% by weight or more, more preferably 0.005% by weight or more, even more preferably 0.007% by weight or more, and even more preferably 0.01% by weight or more, of the polishing composition, from the viewpoints of increasing the polishing rate and reducing the waviness. The content is preferably 20% by weight or less, more preferably 15% by weight or less, even more preferably 10% by weight or less, and even more preferably 5% by weight or less, of the polishing composition, from the viewpoints of the surface quality and economic advantages. Specifically, the content of the peroxide is preferably from 0.002 to 20% by weight, more preferably from 0.005 to 15% by weight, even more preferably 0.007 to 10% by weight, and even more preferably from 0.01 to 5% by weight, of the polishing composition.  
      The polishing composition of the present invention contains an organic acid in addition to the alumina and the peroxide, from the viewpoints of benefits such as increasing the polishing rate and reducing the waviness. The organic acid usable in the present invention has a pK1 of preferably 7 or less, more preferably 5 or less, even more preferably 4 or less, and even more preferably 2 or less, from the viewpoints of increasing the polishing rate and reducing the waviness. Here, the pK1 is expressed as a logarithmic value of an inverse of a first acid dissociation constant at 25° C. The pK1 of each compound is listed in Kagaku Binran (Kiso-hen) II, Fourth Revision, pp. 316-325 (Edit. by Nippon Kagakukai), and the like.  
      As the organic acid preferable in the present invention, sulfur-containing organic acids, carboxylic acids and phosphor-containing organic acids are preferable, from the viewpoints of increasing the polishing rate, reducing the waviness and preventing the surface stains. Concrete examples thereof are as follows: Monocarboxylic acids, such as formic acid, acetic acid, glycolic acid, lactic acid, propanoic acid, hydroxypropanoic acid, butyric acid, benzoic acid, and glycine; polycarboxylic acids, such as oxalic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, itaconic acid, malic acid, tartaric acid, citric acid, isocitric acid, phthalic acid, nitrilotriacetic acid and ethylenediaminetetraacetic acid; sulfur-containing organic acids, such as methanesulfonic acid and paratoluenesulfonic acid; phosphor-containing organic acids, such as ethylphosphoric acid, butylphosphoric acid, laurylphosphoric acid, phosphonohydroxyacetic acid, hydroxyethylidene-1,1-diphosphonic acid, phosphonobutane tricarboxylic acid and ethylenediaminetetramethylene phosphonic acid; and the like. Among them, the sulfur-containing organic acids or the phosphor-containing organic acids are preferable, more preferably organic sulfonic acids or organophosphonic acids, and even more preferably organic sulfonic acids, from the viewpoints of increasing the polishing rate and reducing the waviness. Additionally, the sulfur-containing organic acids and carboxylic acids are preferable, more preferably organic sulfonic acids and polycarboxylic acids, from the viewpoints of preventing the surface stains of the object to be polished.  
      These compounds can be used alone or in admixture.  
      The content of the organic acid is preferably 0.002% by weight or more, more preferably 0.005% by weight or more, even more preferably 0.007% by weight or more, and even more preferably 0.01% by weight or more, of the polishing composition, from the viewpoints of increasing the polishing rate and reducing the waviness. Also, the content is preferably 20% by weight or less, more preferably 15% by weight or less, even more preferably 10% by weight or less, and even more preferably 5% by weight or less, of the polishing composition, from the viewpoints of the surface quality and economic advantages. Specifically, the content of the acid is preferably from 0.002 to 20% by weight, more preferably from 0.005 to 15% by weight, even more preferably from 0.007 to 10% by weight, and even more preferably from 0.01 to 5% by weight, of the polishing composition.  
      Water in the polishing composition of the present invention is used as a medium. The content of water is preferably from 55 to 99% by weight, more preferably from 60 to 97% by weight, and even more preferably from 70 to 95% by weight, from the viewpoint of efficiently polishing the object to be polished.  
      It is preferable that an inorganic acid is used together in the polishing composition of the present invention, from the viewpoints of further increasing the polishing rate and reducing the waviness. The inorganic acid includes nitrogen-containing inorganic acids, such as nitric acid and nitrous acid; sulfur-containing inorganic acids, such as sulfuric acid, sulfurous acid and amide sulfuric acid; phosphor-containing inorganic acids, such as phosphoric acid, pyrophosphoric acid, polyphosphoric acid and phosphonic acid, and the like. Among them, nitric acid, nitrous acid, sulfuric acid, sulfurous acid and amide sulfuric acid are preferable, more preferably sulfuric acid, sulfurous acid and amide sulfuric acid, and even more preferably sulfuric acid, from the viewpoint of increasing the polishing rate. When the inorganic acid is used together, the content of the inorganic acid is preferably 0.002% by weight or more, more preferably 0.005% by weight or more, even more preferably 0.007% by weight or more, and even more preferably 0.01% by weight or more, of the polishing composition. The content is preferably 20% by weight or less, more preferably 15% by weight or less, even more preferably 10% by weight or less, and even more preferably 5% by weight or less, of the polishing composition, from the viewpoints of the surface quality and economic advantages. Specifically, the content of the inorganic acid is preferably from 0.002 to 20% by weight, more preferably from 0.005 to 15% by weight, even more preferably from 0.007 to 10% by weight, and even more preferably from 0.01 to 5% by weight, of the polishing composition.  
      In addition, there can be added one or more components such as an inorganic salt, a thickener, an anticorrosive agent or a basic substance to the polishing composition of the present invention, as occasion demands. Especially, the inorganic salt such as ammonium nitrate, ammonium sulfate, potassium sulfate, nickel sulfate, aluminum sulfate or ammonium sulfaminate has a subsidiary effect for increasing the polishing rate. These other components can be used alone or in admixture of two or more kinds. Also, the content of the other components is preferably from 0.05 to 20% by weight, more preferably from 0.05 to 10% by weight, and even more preferably from 0.05 to 5% by weight, of the polishing composition, from the viewpoint of economic advantages.  
      There can be further added one or more components, such as a disinfectant or an antibacterial agent, as occasion demands. The content of the disinfectant or the antibacterial agent is preferably from 0.0001 to 0.1% by weight, more preferably from 0.001 to 0.05% by weight, and even more preferably from 0.002 to 0.02% by weight, of the polishing composition, from the viewpoint of exhibiting its function and from the viewpoints of the influence on polishing performance and economic advantages.  
      The concentration of each component of the polishing composition of the present invention is a concentration at which polishing is preferably carried out, and may be the concentration during the preparation of the composition. In many cases, the composition is usually prepared as a concentrate, which is diluted before use or upon use.  
      The polishing composition can be prepared by adding or mixing the intended components by an optional method.  
      It is preferable that the pH of the polishing composition is appropriately determined depending upon the kinds of the object to be polished and the required properties. The lower the pH, the more preferable, from the viewpoints of increasing the polishing rate and reducing the waviness, and the closer the pH of 7, the more preferable, from the viewpoints of prevention of the corrosion of the processing machine and safety for an operator. Accordingly, considering both viewpoints, the pH is preferably 0.1 or more and less than 6, more preferably 0.5 or more and less than 5, even more preferably 1 or more and less than 4, and even more preferably 1 or more and less than 3. The pH of the polishing composition can be adjusted by properly adding an inorganic acid, such as nitric acid or sulfuric acid; an organic acid, such as a hydroxycarboxylic acid or a polycarboxylic acid, an aminopolycarboxylic acid, an amino acid, a metal salt or an ammonium salt thereof; or a basic substance, such as, an aqueous ammonia, sodium hydroxide, potassium hydroxide or an amine, in a desired amount.  
      The method for manufacturing a substrate of the present invention includes the step of polishing a substrate to be polished with the above-mentioned polishing composition.  
      The magnetic disk substrate which is the substrate to be polished as a subject for the present invention is used as the substrate for magnetic recording media. Concrete examples of the magnetic disk substrates include representatively a substrate made of an aluminum alloy plated with Ni—P, and there are also included a substrate made of glass or glassy carbon, instead of aluminum alloy, and plated with Ni—P thereon; or a substrate coated with various metallic compounds by plating or deposition, instead of the substrate plated with Ni—P.  
      In the above-mentioned polishing process, the object to be polished may be polished by clamping a substrate with polishing platens to which a polishing cloth made of a porous organic polymer and the like is pasted; feeding the polishing composition of the present invention to a surface to be polished of the substrate; and moving the polishing platens or the substrate, while applying pressure. Accordingly, the present invention relates to a polishing process for a substrate to be polished, including the step of polishing the substrate with the above-mentioned polishing composition. Other conditions such as the kind of polishing machine, polishing temperature, polishing rate and feed amount of the polishing composition are not particularly limited.  
      The polishing composition of the present invention is especially effective in the polishing step, and the polishing composition can be similarly applied to other steps than this, for instance, the lapping step, and the like.  
     EXAMPLES  
      The following examples further describe and demonstrate embodiments of the present invention. The examples are given solely for the purposes of illustration and are not to be construed as limitations of the present invention.  
     Examples 1 to 6, Comparative Examples 1 and 2  
      1. Preparation of Polishing Composition  
      Given amounts of alumina (average secondary particle size: 0.2 μm, purity: about 99.9%), a peroxide, an organic acid, other additives as shown in Table 1, and balance ion-exchanged water were formulated while stirring, to give a polishing composition.  
      2. Polishing Process  
      Each of the polishing compositions obtained in the Examples and Comparative Examples was evaluated for its polishing properties by polishing a Ni—P plated, aluminum alloy substrate having a thickness of 1.27 mm, and a diameter of 3.5 inch (95 mm) (short-wavelength waviness: 3.8 nm and long-wavelength waviness: 1.6 nm, as determined by Zygo New-View 200), using a double-sided processing machine under the following setting conditions, to give an object to be polished made of Ni—P plated aluminum alloy substrate usable as a substrate for a magnetic recording medium.  
      The setting conditions for the double-sided processing machine are as follows.  
      Setting Conditions for Double-Sided Processing Machine] 
     
         
          Double-sided processing machine: Model 9B, commercially available from SPEEDFAM CO., LTD.  
          Processing pressure: 9.8 kPa  
          Polishing pad: “H9900” (trade name, commercially available from FUJIBO)  
          Rotational speed of a lower platen: 50 r/min.  
          Feeding amount for a polishing composition: 100 ml/min  
          Polishing time period: 4 min.  
          Number of substrates introduced: 10 
 
 3. Evaluation Methods 
 
 (1) Polishing Rate 
 
       
    
      Weights of each substrate before and after polishing were measured using a device commercially available from Sartorius under the trade name of BP-210S. Change in weight of each substrate was obtained, and an average of 10 substrates was referred to as an amount reduced, and a value obtained by dividing the amount reduced by the polishing time is referred to as a rate of weight reduced. The rate of weight reduced is introduced into the following equation and converted to a polishing rate (μm/min). A relative value of a polishing rate (relative rate) for each of the Examples and Comparative Examples was obtained, assuming that the polishing rate of Comparative Example 1 takes a standard value of 1. 
 
Rate of Weight Reduced ( g/min )=[Weight Before Polishing ( g )−Weight After Polishing ( g )]/Polishing Time ( min ) 
 
Polishing Rate (μ m/min )=Rate of Weight Reduced ( g/min )/Area of One Side of Substrate ( mm   2 )/Ni—P Plating Density ( g/cm   3 )×1000000 
 
 (2) Waviness 
 
      The waviness of each substrate after the polishing was determined for short-wavelength waviness and long-wavelength waviness at two points of 180° intervals (total of 4 points) under the conditions mentioned below.  
                                      Device:   Zygo New-View 200 commercially           available from Canon Sales, Inc.       Object Lens:   Magnification, 2.5 times, Michelson       Zooming Ratio:   0.5       Remove:   Cylinder       Filter type:   FFT Fixed Band Pass       Short-Wavelength Waviness:   Filter High Wavelength 50 μm           Filter Low Wavelength 500 μm       Long-Wavelength Waviness:   Filter High Wavelength 0.5 mm           Filter Low Wavelength 5 mm       Area:   4.33 mm × 5.77 mm                  
 
 (3) Surface Stains 
 
      The surface of each substrate after the polishing was observed with a polarization optical microscope at a magnification of 300-folds, and the following 5-rank evaluations were made. Here, those ranked in 1 and 2 are failures from the viewpoint of practical purposes. 
          5: no alumina residue and polishing debris are observed on the surface at all;     4: alumina residue and polishing debris are observed but in very little amounts;     3: alumina residue and polishing debris are observed but in slight amounts;     2: alumina residue and polishing debris are observed but in large amounts; and     1: alumina residue and polishing debris are observed but in very large amounts.        

      The results are shown in Table 1.  
                           TABLE 1                                      Composition of Polishing Composition (% by weight)                                                                 % by       % by       % by       % by               Alumina   wt.   Peroxide   wt.   Organic Acid   wt.   Other Additive   wt.   pH               Ex. No.       Ex. 1   α-Alumina   4   Hydrogen Peroxide   0.5   Citric Acid   3.3   —   —   2.0       Ex. 2   α-Alumina   4   Hydrogen Peroxide   0.5   Citric Acid   3.3   Sulfuric Acid   0.4   1.2       Ex. 3   α-Alumina   4   Hydrogen Peroxide   0.5   Citric Acid   1.2   Sulfuric Acid   0.2   1.4                                   Ammonium Sulfate   2.0       Ex. 4   α-Alumina   4   Hydrogen Peroxide   0.5   Tartaric Acid   3.3   —   —   2.0       Ex. 5   α-Alumina   4   Hydrogen Peroxide   0.5   Methanesulfonic Acid   1.5   —   —   0.7       Ex. 6   α-Alumina   4   Hydrogen Peroxide   0.5   Hydroxyethylidene   1.6   —   —   1.4                           Diphosphonic Acid       Comp.       Ex. No.       Comp.   α-Alumina   4   Hydrogen Peroxide   0.5   —   —   Phosphoric Acid   1.5   1.2       Ex. 1                           Nitric Acid   0.5       Comp.   α-Alumina   4   Hydrogen Peroxide   0.5   —   —   Nitric Acid   0.1   2.2       Ex. 2                           Aluminum Nitrate   1.2                                     Waviness                                                 Polishing Rate   Short-Wavelength,   Long-Wavelength,                   Relative Value   Relative Value   Relative Value   Surface Stains                       Ex. No.           Ex. 1   1.2   0.95   0.74   5           Ex. 2   1.6   0.90   0.69   5           Ex. 3   1.5   0.88   0.65   5           Ex. 4   1.1   0.97   0.76   4           Ex. 5   1.5   0.93   0.71   3           Ex. 6   1.2   0.89   0.68   3           Comp.           Ex. No.           Comp.   1.0   1.00   1.00   1           Ex. 1           Comp.   0.9   1.05   1.07   1           Ex. 2                      
 
      It can be seen from the results of Table 1 that the polishing compositions obtained in Examples 1 to 6 have high polishing rates, are capable of reducing both the short-wavelength waviness and the long-wavelength waviness, and further capable of remarkably reducing surface stains of a substrate.  
      The polishing composition of the present invention can be suitably used for the manufacture of a magnetic disk substrate for high-quality hard disks and the like.  
      The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.