Abstract:
The present invention utilizes a combination of chemical and mechanical finishing processes to polish a disk substrate surface to near atomic smoothness. Broadly speaking, the surface of a disk substrate that has been machined (i.e., rough ground) to a predetermined surface roughness is subjected to attack by a chemical formulation (called an attacking agent). The chemical formulation is used to soften the substrate material. Then, the softened material is “wiped away” via mechanical action.

Description:
This is a divisional of application Ser. No. 08/184,718 filed on file on Jan 21, 1994 now U.S. Pat. No. 2,297,956. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to data processing systems. More particularly, the present invention relates to the polishing of substrate materials for magnetic media. 
     BACKGROUND OF THE INVENTION 
     The EDVAC computer system of 1948 is cited by many as the beginning of the computer era. However, well before the introduction of the world&#39;s first stored program computer, the concept of mass storage was well known to early technologists such as Herman Hollerith. In fact, without the advent of mass storage devices there may never have been a computer era. It is no surprise, then, that general advances in computer systems are often closely related to advances in mass storage technology. 
     Today, one of the most popular forms of mass storage technology is the magnetic disk drive. The disks used in these devices are typically constructed by coating a non magnetic material, such as aluminum, with a magnetic material, such as one of the cobalt alloys. The surface of the magnetic disk is then divided into tiny cells which are magnetically encoded to represent one of the two states of a binary digit (i.e., 1 or 0). The magnetic cells are encoded such that they collectively represent information that can be used by a computer system. 
     Many of today&#39;s magnetic disk devices further include at least one magneto resonance head which has a write element and a read element. The write element is used to magnetize the cells (i.e., encode the information), while the read element is used to retrieve the information from the magnetic disk. The head(s) is attached to an armature in much the same way as the needle of a record player is attached to a tone arm. Unlike the needle of a record player, however, the head of a conventional magnetic disk device is designed to be aerodynamic. This allows the head to literally “fly” across the surface of the disk on a cushion of air without actually contacting the disk surface itself. The altitude at which the head flies (i.e., the distance between the head and the disk) is called the “fly-height.” 
     For cost, space, and access speed reasons, the makers of magnetic disk devices have always been looking for ways to place more and more magnetic cells (i.e., information) on smaller and smaller disks. However, this increase in density hinders the ability of the head to discern one cell from another. This problem was initially resolved by reducing the head fly-height to a point where the head could once again distinguish between individual magnetic cells. Now, however, this constant effort to increase cell density is being hindered by the actual surface roughness of the disk itself. Achievable cell density relates to surface roughness in two key ways. The first and most straightforward relationship between achievable cell density and surface roughness is the aforementioned need to reduce head fly-height. It is easy to envision that a head can pass closer to a smooth disk than it can to a rough disk. The second relationship between achievable cell density and surface roughness has to do with how the magnetic cells are placed on the disk. Magnetic cells encoded on a rough surface require more space than magnetic cells which are encoded on a smooth surface. Therefore, the smoother the surface of the disk, the greater the achievable cell density. 
     This need to reduce surface roughness has caused increased focus on disk finishing and polishing processes. When a disk substrate is initially created, its surface is very rough. The disk substrate is then polished to reduce this surface roughness. Much like abrasive bathroom cleaners, conventional polishing processes commonly use abrasive particles to polish the disk substrate surface until a certain smoothness is achieved. However, the problem with these conventional processes is that the particles used are too large, too hard, and too sharp. It is easy to envision that at some point in the polishing process, particles of this type are causing surface roughness instead of reducing surface roughness. When this point is reached, the polishing process is merely removing additional material without actually reducing surface roughness. 
     An intuitive solution to this problem is the use of free abrasive particle mixtures (called slurries) that utilize smaller abrasive particles. There are, however, two prohibitive problems associated with the use of very small particles. First, particles that are very small (i.e., less that one micron in size) tend to clump together (i.e., congeal, coagulate, flocculate, and/or agglomerate) such that the benefit of using small particles is lost. Second, small, soft particles lack abrasiveness to the extent that they are practically useless in conventional polishing processes. 
     At present, the magnetic disk industry is desperately searching for a finishing process that can further reduce the surface roughness of magnetic disk surfaces. Without such a process, increased cell densities are not achievable. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a principal object of this invention to provide an enhanced finishing process for disk surfaces. 
     It is another object of this invention to provide an enhanced Chemical-Mechanical, disk substrate polishing process which uses a slurry that includes soft colloidal particles. 
     It is still another object of this invention to provide an enhanced slurry which can be used to polish the disk substrate surface by both chemical and mechanical means. 
     It is yet another object of this invention to provide a hard disk device which has at least one disk substrate with a surface of near atomic smoothness. 
     It is yet another object of this invention to provide a disk substrate which has a surface of near atomic smoothness. 
     It is yet another object of this invention to provide a disk substrate which has an enhanced recording surface of near atomic smoothness. 
     These and other objects of the present invention are accomplished by the substrate independent polishing process and slurry disclosed herein. 
     The present invention utilizes a combination of chemical and mechanical finishing processes to polish a disk substrate surface to near atomic smoothness. Broadly speaking, the surface of a disk substrate that has been machined (i.e., rough ground) to a predetermined surface roughness is subjected to attack by a chemical formulation (called an attacking agent). The chemical formulation is used to soften the substrate material. Then, the softened material is “wiped away” via mechanical action. As discussed in the background section, mechanical polishing of disk substrates is well known in the magnetic disk art. Damage free chemical polishing of disk substrates, however, is yet to be accomplished because the process parameters have, heretofore, been too difficult to control. Leaving an attacking agent on the substrate for too long causes unacceptable damage to the substrate surface, while leaving an attacking agent on the substrate for too short a period of time defeats the purpose (i.e., material removal). 
     To overcome this problem, and the inherent problems associated with the use of hard abrasive particles in mechanical polishing, the finishing process of the present invention involves tight control over process parameters and a slurry that comprises soft colloidal particles. When selected appropriately, the relative softness of the colloidal particles combined with their spherical shape and small size make them ideal for damage free removal of the chemically softened material. 
     In addition to the soft colloidal particles, the slurry used in the present invention is comprised of a polymeric matrix which includes: an attacking agent to soften and loosen the disk substrate material, organic and inorganic stabilizing agents for particle dispersion, and an aqueous carrier that is pH adjusted to be synergistic with the attacking agent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a magnetic disk drive utilizing a disk substrate that has been polished in accordance with the present invention; 
     FIG. 2A shows a disk substrate that has been polished in accordance with the present invention. 
     FIG. 2B shows a cross section view of the disk substrate shown in FIG.  2 A. 
     FIG. 2C shows a microscopic view of a disk substrate surface that has been rough ground. 
     FIG. 2D shows a graphical view of the surface roughness of the disk substrate surface of FIG.  2 C. 
     FIG. 2E shows a microscopic view of a disk substrate surface after the disk substrate has been polished in accordance with the present invention. 
     FIG. 2F shows a graphical view of the surface roughness of the disk substrate surface of FIG.  2 E. 
     FIG. 3 shows how the polishing pads of the preferred embodiment operate to generate polishing action. 
     FIG. 4 shows how the disk substrate carriers of the preferred embodiment operate in relation to polishing pads to generate polishing action. 
     FIG. 5 shows how slurry is introduced onto disk substrates in accordance with the preferred embodiment. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows magnetic disk drive  100  utilizing magnetic disks with disk substrates that have been polished in accordance with the preferred embodiment of the present invention. Each of disks  101  comprises a disk substrate that has been coated with a magnetic material. Disks  101 , which further comprise magnetic surface  102 , are rigidly attached to common hub or spindle  103 , which is mounted on base  104 . Spindle  103  and disks  101  are driven by a drive motor (not visible) at a constant rotational velocity. Comb-like actuator assembly  105  is situated to one side of disks  101 . Actuator  105  rotates through an arc about shaft  106  parallel to the axis of the spindle, driven by an electromagnet, to position the transducer heads. Cover  109  mates with base  104  to enclose and protect the disk and actuator assemblies. 
     Electronic modules for controlling the operation of the drive and communicating with another device, such as a host computer, are contained in circuit card  112 , typically mounted outside the enclosure. A plurality of head/suspension assemblies  107  are rigidly attached to the prongs of actuator  105 , one head/suspension assembly  107  corresponding to each disk recording surface  102 . Typically, data is recorded on both surfaces of the disk, making two opposed head/suspension assemblies for each disk. An aerodynamic transducer head  108  is located at the end of each head/suspension assembly  107  adjacent the disk surface. Head/suspension assembly  107  is essentially a beam spring tending to force transducer head  108  against the surface of the disk  102 . The aerodynamic characteristics of the head counteract the force of the beam spring, making the head “fly” a small distance from the surface of the disk due to air movement caused by the spinning disk. 
     FIG. 2A shows a disk substrate that has been polished in accordance with the present invention. Disk substrate  200 , which has disk substrate surface  205 , has a substrate material which comprises aluminum coated with a layer of Nickel Phosphorous (NiP). NiP is commonly used in the magnetic disk art to provide a hard, rigid, defect free surface to otherwise malleable or inclusioned substances like aluminum. A common substrate material has been chosen for the preferred embodiment to best illustrate the teachings of the present invention. However, it should be understood that the present invention is not limited to just NiP coated aluminum. Other substrate materials such as glass, titanium, carbon, zirconium, silicon carbide and NiP coated Beryllium could also be used. Therefore, “magnetic disk substrate” is defined herein to refer to disk substrates that are constructed using any one of the aforementioned substrate materials. It should further be understood that the present invention is not limited to magnetic disk substrates. The present invention is equally applicable to magnetic disks made entirely of magnetic material. 
     FIG. 2B shows a cross section view of disk substrate  200 . As shown, disk substrate  200  is comprised of top NiP layer  210  and bottom NiP layer  217  which are 10 micrometer (μm) thick and aluminum layer  215  which is 0.8 mm thick. 
     FIG. 2C shows a microscopic view of a disk substrate surface that was polished using a conventional, hard particle polishing process. FIGS. 2C through 2F and the measurements displayed thereon were produced by a NanoScope III microscope which is commercially available through Digital Instruments Corporation. The NanoScope III used for the measurements described herein has been calibrated in accordance with NIST 20 &amp; 40 nanometer step height standards. Atomic Force Microscopes, like the NanoScope III microscope, are capable of displaying and measuring surface roughness at the atomic level. 
     Graph  225  and table  230  of FIG. 2D show surface roughness measurements for the disk substrate surface shown in FIG.  2 C. Table  230  shows that the average surface roughness of the disk substrate surface shown in FIG. 2C is approximately 19 Angstroms (A) [i.e., shown as rms. value 19.27 A]. 
     FIG. 2E shows a microscopic view of disk substrate surface  205  which was polished in accordance with the present invention. Simple visual comparison of the disk substrate surface shown in FIG.  2 C and that of disk substrate surface  205  shows a marked difference in surface roughness. Graph  240  and table  245  of FIG. 2F shows surface roughness measurements for disk substrate  205 . Table  245  shows that the average surface roughness of disk substrate  205  is an order of magnitude less than that of the disk substrate surface shown in FIG. 2C [i.e., shown as rms. value 1.58 A]. 
     SuperPolishing Process 
     Polishing Machine and Process Parameters 
     The polishing machine of the preferred embodiment is a three motor, 9B-5P SpeedFam Double-Sided Polishing Machine made by SpeedFam Corporation. However, other conventional polishing machines could also be used. The double-sided polishing action of typical double sided polishing machines is shown on FIG.  3 . Individual disk substrates are held between polishing pads  310  and  315  by polishing plates  300  and  305 . The polishing pads used in the preferred embodiment are Ultra-Pol V polishing pads made by Rippey Corporation; however, other polishing pads with similar characteristics could also be used. While pressure is applied axially to shaft  320 , polishing plates  300  and  305  are rotated in opposite directions (shown by rotation arrows  330  and  335 ). The pressure applied to shaft  320  should be set to 0.052 kg/cm2 of disk area (for 48 mm disks). Lower polishing plate  305  and attached polishing pad  315  should be set to rotate at 60 RPM, while upper polishing plate  300  and attached polishing pad  310  should be set to rotate at 20.0 RPM. 
     Slurry supply ports, such as slurry supply port  325 , are used to introduce the slurry onto the disk substrates. As a result of this double-sided polishing action, both the top and bottom sides of the disk substrates are polished simultaneously. 
     FIG. 4 shows how the disk substrate carriers (such as disk substrate carrier  400 ) operate in relation to lower polishing pad  315  and upper polishing pad  310 . Disk substrate carrier  400  rotates in the same direction as polishing pad  315  (shown by rotation arrow  410 ) and in the direction opposite to polishing pad  310  [not shown] such that disk substrates (such as disk substrate  200 ) are polished on both sides. 
     FIG. 5 shows how slurry  500  is introduced onto disk substrates during polishing. As soon as the polishing process is begun, Slurry  500  is introduced onto disk substrates via slurry supply ports like slurry supply port  325 . Slurry  500  is introduced onto disk substrates at a rate of 30 ml per minute. Slurry  500  then attacks the surface of the disk substrates [not shown] located in disk substrate carrier  400 . Once slurry  500  has softened the disk substrate material, the excess material is removed from the disk substrate by the polishing action of polishing pads  310  and  315 . Under the above process parameters, the processing time is less than 10 minutes. Near the end of the process, rinse water should be introduced onto the disk substrates to dilute slurry  500  and increase the pH. The pH should be increased to a value at or above 3.0 before the rotation is stopped. Too short of a rinse time may cause pitting of the disk substrate surface which may lower the quality of the finish. In the preferred embodiment, a rinse time of 40 seconds is used. 
     Controlling Hard Particle Contamination 
     Hard particle contaminants which are inadvertently introduced into slurry  500  can cause scratches on the surface of disk substrate  200 . Since information cannot be stored in locations that have been scratched, the fewer the defects the better. Therefore, it is extremely important to minimize the extent to which hard particles are introduced into slurry  500 . In the preferred embodiment, fine filtration of slurry  500  is used to control hard particles that may be inadvertently introduced into slurry  500 . Slurry  500  is continuously circulated through a 3 micron filter by a circulating pump that is capable of operating at pressures up to 100 psi. [not shown]. 
     Composition of Slurry  500   
     Attacking Agents and pH 
     The chemical polishing portion of this chemical-mechanical process is achieved through the use of an attacking agent. Since disk substrate  200  is coated with NiP, aluminum nitrate is used as an attacking agent to soften the surface of disk substrate  200 . In addition, the pH of slurry  500  is adjusted to be acidic by adding nitric acid. However, it will be understood by those skilled in the art that the particular attacking agent used varies with the type of substrate involved. For example, if disk substrate  200  had been made of glass instead of being coated with NiP, cerium sulfate would have been used as an attacking agent and the pH of slurry  500  would have been adjusted to be acidic through the use of sulfuric acid. Accordingly, it should be understood that the present invention is not limited to a particular attacking agent or a particular substrate type. Additionally, it should be understood that there will be an optimum pH value that varies depending upon the particular combination of substrate type, attacking agent, and colloidal particle. When slurry  500  comes into contact with the surface of disk substrate  200 , it reacts with the NiP and softens the metal such that it can be easily removed through mechanical action. 
     Colloidal Particles for Substrate Material Removal 
     After the surface of disk substrate  200  has been softened, soft colloidal particles are used to mechanically remove the softened layer. Typically, particles are said to range in size from 1 millimeter in diameter to 0.001 μm in diameter. Particles can be broadly categorized into two groups: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Particle Category 
                 Particle Size 
               
               
                   
                   
               
             
             
               
                   
                 Coarse 
                 Greater than 1 μm 
               
               
                   
                 Colloidal 
                 Between 1 μm and .001 μm 
               
               
                   
                   
               
             
          
         
       
     
     As discussed in the background section, use of large, hard particles for polishing causes scratches on the surface of the disk substrate. Hence, the smaller the particles the better. However, the smallest colloidal particles can be too small to remove the softened material. Hard colloidal particles are similarly ineffective because they leave a scratched or rougher surface. For these reasons, soft colloidal particles of intermediate size were chosen for slurry  500 . While the type of colloidal particle used in the preferred embodiment is colloidal silica, it will be appreciated by those skilled in the art that the present invention is not limited to the particular type of colloidal particle used in the preferred embodiment. Other types of colloidal particles with similar characteristics could also be used. 
     Stabilization Agent 
     Since colloids are thermodynamically unstable, the method for stabilizing slurry  500  is important. In a manufacturing environment, slurry  500  must remain stable while circulating through the 3 micron filter. In the preferred embodiment, this is accomplished by using organic or inorganic agents to enhance the electrical double layer charge and/or provide steric hindrance to maintain dispersion of the colloidal particles. 
     Preparation of Slurry  500   
     NiP Substrate Material 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Ingredient 
                 Amount 
               
               
                   
                   
               
             
             
               
                   
                 2360 Nalco Colloidal Silica 
                 10 liters 
               
               
                   
                 Deionized Water 
                 10 liters 
               
               
                   
                 50%/50% Nitric Acid &amp; 
                 250 ml. 
               
               
                   
                 Deionized Water 
               
               
                   
                 Aluminum Nitrate 
                 1 Kg. 
               
               
                   
                   
               
             
          
         
       
     
     For NiP substrates (such as disk substrate  200 ), the preparation of slurry  500  begins with 10 liters of Nalco 2360 Colloidal Silica made by Nalco Corporation and 10 liters of deionized water. While the colloidal silica used in the preferred embodiment is made by Nalco Corporation, it will be understood by those skilled in the art that the present invention is not limited to any particular brand of colloidal silica. Other commercially available colloidal silica products, such as Ludox from Dow Corning Corporation, could also be used. 1 Kg of aluminum nitrate is then dissolved into the deionized water. Once this is accomplished, nitric acid is added to the deionized water until the pH is close to 1.0. When the preparation of the deionized water is complete, the 10 liters of the Nalco 2360 Colloidal Silica is aggressively mixed with nitric acid until this solution&#39;s pH is also close to 1.0. The two solutions (i.e., the aluminum nitrate solution and the Nalco 2360 solution) are then mixed together to form slurry  500 . The pH of slurry  500  is then adjusted to be approximately 0.9. 
     Glass Substrate Material 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Ingredient 
                 Amount 
               
               
                   
                   
               
             
             
               
                   
                 2360 Nalco Colloidal Silica 
                 7 liters 
               
               
                   
                 Warm Deionized Water 
                 3 liters 
               
               
                   
                 Sulfuric Acid 
                 250 ml. 
               
               
                   
                 Cerium Sulfate 
                 7.5 grams. 
               
               
                   
                   
               
             
          
         
       
     
     As mentioned, the present invention is not limited to the type of substrate material involved. For example, if disk substrate  200  were a glass substrate instead of a NiP substrate, the preparation of slurry  500  would begin with 7 liters of the Nalco 2360 Colloidal Silica and 3 liters of warm deionized water. The de ionized water is adjusted to pH 2.0 with sulfuric acid. Then, 7.5 grams of cerium sulfate are dissolved in it. The pH of the sulfuric acid solution is then adjusted to be between 0.6 and 0.9. When the preparation of the sulfuric acid solution is complete, the 7 liters of the Nalco 2360 Colloidal Silica is aggressively mixed with sulfuric acid until this solution&#39;s pH is also between 0.6 and 0.9. The two solutions are then mixed together to form slurry  500 . The pH of slurry  500  is then adjusted such that it is between 0.6 and 0.9. 
     Regardless of the substrate material involved, it is important to note that the final pH and the concentrations of the ingredients are important in controlling the rate at which material is removed from disk substrate  200 . High levels of an attacking agent can cause pitting on the surface of disk substrate  200 , while lower levels of the attacking agent slow down the rate at which material is removed (i.e., the stock removal rate) from disk substrate  200 . This, of course, increases processing time. In addition, the stock removal rate also tends to decrease with higher pH values. 
     Although a specific embodiment along with some alternate embodiments have been disclosed, it will be understood by those skilled in the art that additional variations in form and detail may be made within the scope of the following claims.