METHOD OF MANUFACTURING GLASS SUBSTRATE FOR USE IN INFORMATION RECORDING MEDIUM FIELD

Example embodiments relate generally to the consistent and reliable production of substrates for use in high capacity information recording and reproducing, and substrates, information recording mediums and information recording devices thereof. BACKGROUND

Information storage technology using substrates continue to evolve with the growing demands for reducing the physical size of information storage devices, increasing information storage capacity, producing larger quantities in cost-efficient ways, and improving operable reliability of information storage devices.

In respect to reductions in physical size, today's applications and demands continue to drive substrate sizes to become smaller, and now include those that are specified at 3.5 inches (89 mm), 2.5 inches (65 mm), 1.8 inches (48 mm), 1 inch (27.4 mm), 0.8 inches (21.6 mm) and smaller.

To cooperatively affect greater information storage capacity, various inter-related aspects throughout an information recording device have been developed and utilized by substrate, medium and device manufacturers. From the standpoint of medium and substrate manufacturers, information storage capacity can be affected in several ways, including increasing recording densities through improvements to the substrate. In this regard, a highly effective method of consistently producing large quantities of substrates to achieve high recording densities was taught in US Patent No. 6,277,465. In the said patent, small and uniform sized crystal grains of the magnetic layer of a medium are formable through controlling specific combinations of the parameters Ra, Rq, Rmax and ratio Rmax/Ra of the surface roughness of a glass substrate. From the standpoint of device manufacturers, information storage capacity can be affected in several ways, including appropriately selecting the type of information recording device to produce, and making improvements to the head, flying height, and rotational speed. For instance, the employing of a load/unload type device in replacement of a conventional contact start-stop type device (CSS device) can effectively increase the information storage capacity (or total useable storage area) by including those inherently non-recordable areas designated in a CSS device for a head to contact with, or land on, the medium.

With increases in information recording capacities, heads have also evolved from conventional thin-film heads to magneto-resistive heads (MR heads) and to today's giant magneto-resistive heads (GMR heads). The flying height, which is generally known as the distance between a flying head and a medium surface, has also correspondingly decreased to about 10 nm or less. Furthermore, increases in rotational speeds have also been correspondingly achieved to affect an increase in recording and reproducing rates. In this regard, devices have seen rotational speeds increase from relatively low speeds of about 4200 rpm to 7200 rpm, and to rotational speeds greater than 10,000 rpm.

Various substrate manufacturing developments have enabled the continuous production of larger quantities of substrates. For example, typical planetary gear-type polishing machines in use today include those having capabilities to polish one hundred substrates in a single polishing step. Operable reliability of high capacity information recording devices obtained from such producing of large quantities of substrates can be affected in many ways, such as altering substrate polishing conditions to achieve improved flatness, waviness and micro-waviness of the useable recording area and, more recently, improving the maximum variation in height of the surface of a peripheral portion of the substrate. In general, the flying path of a head can be disturbed when the head undesirably comes into contact with any part of the medium surface. Such undesirable occurrence, which may cause irregular reproduction output, is generally known as a flying stiction failure. In this respect, when a flying stiction failure occurs, there is a high probability of a potentially fatal problem called a head crash failure.

It has been recently taught in US Laid-Open Patent Publication No. 2010/0040907 that an improvement in the maximum variation of the height of an outer peripheral portion can be achieved so as to effectively eliminate the occurrence of a flying stiction failure. Specifically, a method is taught for controlling the variation in the amount of rising (ski -jump) and lowering (roll-off), and the variation between a maximum ski-jump and maximum roll-off thereof, of the surface height of the outer peripheral portion, as compared to the substantially flat useable recording areas. In general, the outer peripheral portion is the annular non-recordable area circumferential ly surrounding the useable recording area of a substrate. That is, assuming the distance from the center of a substrate to its outer diameter is 100%, the area of the outer peripheral portion is generally targeted by substrate manufacturers in the production of large quantities of substrates as a uniform-sized annular area definable by two concentric circles, wherein the inner circle has targeted radius of Rl and the outer larger circle has a targeted radius of R2. For example, a typical 2.5 inch (or 65 mm) glass substrate having Rl and R2 to be targeted at a constant radius of 92% and 97%, respectively, will have a targeted value of Rl and R2 to be about 29.9 mm and 31.5 mm, respectively. In controlling the maximum height variation of the said targeted outer peripheral portion, and more particularly, controlling the variation between the maximum ski-jump and the maximum roll-off within the targeted outer peripheral portion, the problem of flying stiction failure becomes eliminated in the said patent.

Despite such advances in technology, manufacturers of substrates, mediums and devices alike continue to face problems with meeting the ever-growing demands for even more reliability of even more information storage requirements within even smaller sizes.

SUMMARY

Under conventional means, substrate manufacturers continue to face problems in reliably producing large quantities of substrates that achieve consistently greater sized useable recordable areas. Specifically, a problem arises due to the significant variation in the achieved recordable area of substrates produced in large quantities, which is directly affected by the significant variation in the achieved non-recordable area of the peripheral portions of the substrates.

In light of conventional inabilities to consistently achieve a targeted area of the peripheral portions when producing large quantities of substrates, it has been a practice for prudent device manufacturers to avoid the risk of using potentially non-recordable areas and correspondingly exclude those areas nearby the targeted peripheral portions from being used for information storage. This problem becomes much more significant as the size of the substrate becomes smaller since the size of the useable recordable area and the allowable errors of the dimensions of the useable recordable area also reduce.

In considering the above problems, it is recognized herein that the targeted area of the peripheral portions of substrates produced in large quantities, and correspondingly the useable recording area of substrates produced in large quantities, can in fact be achieved in a simple, cost-efficient, reliable and consistent manner.

Present example embodiments relate generally to a method of reliably and consistently suppressing variations in the peripheral portions among substrates produced in large quantities so as to achieve a consistently achievable increase in the useable recording areas, and more particularly to a method of reducing defects found in the surface of polishing pads used in the polishing of substrates, and more particularly to a process carried out to effectively remove polishing grains trapped in pores of a polishing pad, and more particularly to a pad dressing method.

An example embodiment relates generally to a method of manufacturing substrates for use in information recording mediums, said method comprising the steps of polishing a plurality of substrates using an upper polishing pad, a lower polishing pad, and a polishing liquid including a polishing grain; pad dressing said upper polishing pad and said lower polishing pad with a pad dresser after subjecting said upper polishing pad and said lower polishing pad to a plurality of polishing steps, wherein a force is applied onto said pad dresser, from said polishing pads, so as to affect a suppressed variation in outer peripheral portion size among the manufactured substrates; inspecting a surface of said polishing pads and calculating a variation in measurement of an outer peripheral portion size among the manufactured substrates so as to ensure said applied force is sufficient to cause said suppressed variation in outer peripheral portion size among the manufactured substrates; and making a decision to increase said applied force if a variation in the outer peripheral portion size is not suppressed among the manufactured substrates.

Another example embodiment relates generally to a method carried out on a polishing pad, said polishing pad having been subjected to a plurality of polishing processes in a manufacturing of substrates for use in information recording mediums, said method comprising the steps of placing a pad dresser in contact with said polishing pad; and cooperatively moving said polishing pad and said pad dresser relative to each other while applying a force onto said pad dresser, from said polishing pad, so as to cause a suppressed variation in outer peripheral portion size among the manufactured substrates.

Another example embodiment relates generally to a substrate for use in an information recording medium, said substrate manufactured according an example embodiment of a method of manufacturing a substrate.

Another example embodiment relates generally to a method of manufacturing an information recording medium, comprising the steps of chemical strengthening a substrate by immersing said substrate into a chemical strengthening solution including potassium nitrate and sodium nitrate, and depositing a thin film including at least a magnetic layer on said substrate so that said magnetic layer is composed of substantially uniform sized crystal grains.

Another example embodiment relates generally to an information recording medium for use in an information recording device, said information recording medium manufactured according to an example embodiment of a method of manufacturing an information recording medium.

Another example embodiment relates generally to a method of manufacturing an information recording device by mounting an information recording medium obtained from an example embodiment of a method of manufacturing an information recording medium.

Another example embodiment relates generally to an information recording device for use in information recording, said information recording device manufactured according to an example embodiment of a method of manufacturing an information recording device.

Although example embodiments may provide for an improved process for consistent, reliable and efficient mass production of high capacity magnetic disk substrates, with some references to example embodiments using glass substrates for magnetic recording media in hard disk drives for computing devices, it is to be understood by persons ordinarily skilled in the art that example embodiments can also be for use for other forms of substrates, storage media, devices and applications. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an example substrate for use in an information recording medium;

FIG. 2 is a cross-sectional side view of an example substrate having a ski-jump profile for use in an information recording medium;

FIG. 3 is a cross-sectional side view of an example substrate having a roll-off profile for use in an information recording medium;

FIG. 4 is a cross-sectional side view of an example substrate having substantially diminished or eliminated outer peripheral portion for use in an information recording medium;

FIG. 5 is an example of a first polishing machine having a plurality of first pad dressers in a plurality of first pad dressing carriers;

FIG. 6 is an example of a second polishing machine having a plurality of second pad dressers in a plurality of second pad dressing carriers; and

FIG. 7 is a side view of an example polishing pad including pores of the surface of the polishing pad.

Although similar reference numbers are used to refer to similar elements for convenience, it can be appreciated that each of the various example embodiments may be considered to be distinct variations.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention will now be described hereinafter with reference to the accompanying drawings, which form a part hereof, and which illustrate example embodiments by which the invention may be practiced. As used in the disclosures and the appended claims, the term "example embodiment" does not necessarily refer to a single embodiment, although it may, and various example embodiments may be readily combined and interchanged, without departing from the scope or spirit of the present invention. Furthermore, the terminology as used herein is for the purpose of describing example embodiments only and is not intended to be a limitation of the invention. In this respect, as used herein, the term "in" includes "in" and "on", and the terms "a", "an" and "the" include singular and plural references. Furthermore, as used herein, the term "by" may be construed to mean "from", depending on the context. Furthermore, as used herein, the term "if may be construed to mean "when" or "upon", depending on the context. Furthermore, as used herein, the words "and/or" refers to and encompasses any and all possible combinations of one or more of the associated listed items.

An example embodiment of an information recording device comprises one or more information recording mediums, a rotary unit in communication with the information recording mediums to affect a rotation of the information recording mediums at high speeds, and one or more heads to affect recording and reproducing of information. The information recording medium comprises a substrate, such as a glass substrate, having one or more layers formed thereon. As illustrated in FIGs. 1 to 3, a glass substrate 1 produced under conventional means is a substantially circular-shaped disk having a concentric circular hole 2 and comprising principal surfaces 3, wherein each principal surface 3 comprises a useable recording area 4 and a targeted ring-shaped outer peripheral portion area 5a, 5b, an outer edge surface 6 substantially perpendicular to the principal surfaces 3, an inner edge surface 7 substantially perpendicular to the principal surfaces 3, and a chamfered surface 8 formed between an outer peripheral portion 5a, 5b and an outer edge surface 6.

In conventional production of large quantities of glass substrates, manufacturers have been unable to achieve consistently suppressed variations in size and shape of the outer and inner peripheral portions among glass substrates. In general, manufacturers will desire the outer peripheral portion to be definable by two concentric circles having targeted radii Rl and R2, wherein R2 is greater than Rl. In conventional practices of producing large quantities of substrates, however, the actual achieved Rl and R2 among substrates will vary considerably. In considering the effects of not being able to reliably achieve a targeted peripheral portion in the production of large quantities of glass substrates, one can readily comprehend that any occurrence of reduction in the actual value of Rl, as compared to the targeted value of Rl, will cause a corresponding reduction in size of the useable recording area. In light of conventional inabilities to consistently and reliably suppress such variations in the achieved peripheral portions among substrates, information recording device manufacturers generally avoid using areas within and nearby such potentially non-recordable areas, and correspondingly produce devices having information recording areas that are substantially smaller than the said desired area. This practical limitation directly relates to the achievable recording capacity, and becomes much more significant as substrate sizes become smaller since the useable recordable areas and the allowable variations of the size of the useable recordable areas also reduce.

An example process of manufacturing large quantities of 2.5 inch (65 mm) glass substrates for use in high capacity information recording mediums will now be described hereinafter with references to some examples and comparative examples. In representing the production of large quantities of glass substrates, one hundred glass substrates were produced in each substrate production process, and each process was repeated for one hundred times so as to produce a total of 10,000 glass substrates for each of the examples and comparative examples. It is to be understood herein that example embodiments of said method are applicable to other types of substrates and those of other predetermined sizes and shapes, such as those that are 3.5 inches (89 mm), 2.5 inches (65 mm), 1.8 inches (48 mm), 1 inch (27.4 mm), 0.8 inches (21.6 mm), or smaller.

(1) Shaping and First Lapping Process

Plate-like glass members are obtained by any one of many known manufacturing methods using molten glass, such as a pressing method, a float method, a down-drawn method, a redrawing method, or a fusion method. It is recognized herein that the use of a pressing method can achieve the production of large quantities of glass substrates at low cost. The plate-like glass members may be any one of an amorphous glass, a ceramic or crystalline glass, an alumino-silicate glass, a soda-lime glass, a borosilicate glass, or the like. It is recognized herein that an alumino-silicate glass (Si02: 58 to 75 wt %; A1203: 5 to 23 wt %; Li20: 3 to 10 wt %; and Na20: 4 to 13 wt %) is preferably used for its capabilities of being chemically strengthened and having excellent flatness and strength.

Thereafter, a lapping process was applied to both principal surfaces of each of the plate-like glass members so as to obtain disk-shaped glass base members. The lapping process was carried out using a double-sided lapping machine employing a planetary gear-type mechanism and using alumina-based free abrasive grains. Specifically, the lapping process was carried out by pressing lapping surface plates onto both principal surfaces of each of the plate-like glass members from the upper and lower sides, supplying a grinding liquid containing said free abrasive grains onto the main surfaces of each of the plate-like glass members, and moving the plate-like glass members and the surface plates relative to each other. By performing said lapping process, glass base members having flat principal surfaces were obtained.

(2) Cutting, Coring, Forming and Chamfering Process

Glass base members obtained from the above process were then cut using a diamond cutter to form disk-shaped glass substrates. Thereafter, a coring process was carried out wherein a concentric inner hole was formed for each of the disk-shaped glass substrates using a cylindrical diamond drill so as to obtain annular glass substrates.

Thereafter, forming and chamfering processes were applied to an inner edge surface and an outer edge surface of each of the glass substrates using diamond grindstones.

(3) Second Lapping Process

A second lapping process was then performed to both principal surfaces of each of the glass substrates obtained from the above process in a similar manner to the first lapping process. In performing this second lapping process, fine irregularities on the principal surfaces of each of the glass substrates formed as a result of the above process are removable in advance.

(4) Edge Surface Polishing Process

A brush polishing process was then applied to the outer edge surface and the inner edge surface of each of the glass substrates so as to achieve a mirror surface state. In achieving a mirror surface state, precipitation of sodium and potassium becomes prevented. In the said process, a slurry of free abrasive grains containing cerium oxide abrasive grains were utilized as the polishing abrasive grains. Thereafter, the obtained glass substrates were thoroughly washed with water.

(5) First Polishing Process

A first polishing process was then carried out on the glass substrates obtained from the above process so as to remove cracks and/or strains remaining on the principal surfaces from the first and second lapping processes. In this first polishing process, the principal surfaces of each of the glass substrates were polished using a double-sided first polishing machine having a planetary gear-type mechanism and using upper and lower hard resin polishing pads. Cerium oxide abrasive grains used in the polishing liquid comprised a smaller diameter than the aperture diameter of the nap formation holes of the polishing pads.

The obtained glass substrates having been subjected to the said first polishing process were then cleaned by immersions in respective cleaning baths of neutral detergent, pure water and IPA (isopropyl alcohol).

(6) First Pad Dressing Process

A first pad dressing process was performed after 10 to 40 uses of the upper and lower polishing pads of the first polishing machine by placing a plurality of first pad dressers 51 and associated first pad dressing carrier 52 in the first polishing machine, as illustrated in FIG. 5. Although first pad dressers 51 comprising a circular plate-shape base material and having fine electrodeposited diamond abrasive grains formed on the surface were used in the process, it is to be understood that the pad dressers may also take other shapes, forms and compositions.

The first pad dressing process was performed for about 3 minutes with a specific weight (force) being applied onto the polishing pads 53 while the first pad dressing carrier 52 was rotated in the direction depicted by arrow 54, the first pad dressers were rotated in the direction depicted by arrow 55 and the first polishing pads were rotated in the direction depicted by arrow 56. The rotational speed of the polishing palate was set to about 15 to 30 rpm, the revolution speed of the pad dressing carrier was set to about 3 to 10 rpm and the rotation speed of the pad dressing carrier was set to about 3-10 rpm. It is recognized herein that the number of first polishing processes before performing the first pad dressing process may be more or less than twenty and the duration and rotation/revolution rates in which the first polishing machine performs the first pad dressing process may be longer or shorter in example embodiments. In this regard, it was observed that the same results were obtainable when performing the first pad dressing process for between 3 to 30 minutes, setting the rotational speed of the polishing palate to between 10 to 40 rpm, and setting the revolution speed of the pad dressing carrier and the rotation speed of the pad dressing carrier to between 3 to 20 rpm.

(7) Second Polishing Process A second polishing process was carried out so as to finish the principal surfaces to a mirror surface state. In this process, the principal surfaces were mirror-polished using a double-sided second polishing machine having a planetary gear-type mechanism and using a softer resin foam polishing pad. Smaller diameter silica oxide abrasive grains than those of the first polishing process were used in the polishing liquid of the second polishing process. In this regard, the diameter of the said abrasive grains used in the second polishing process was smaller than the aperture diameter of the nap formation holes of the polishing pad.

Glass substrates having been subjected to the second polishing process were then cleaned by immersions in respective cleaning baths of neutral detergent, pure water, and IPA (isopropyl alcohol). An ultrasonic wave was applied to each cleaning bath.

(8) Second Pad Dressing Process

As illustrated in FIG. 6, a second pad dressing process was then performed in a similar manner as the first pad dressing process, that is, after every twenty second polishing processes, by placing a plurality of second pad dressers 61 and associated second pad dressing carrier 62 in the second polishing machine and performing said second pad dressing process for about 5 to 10 minutes. A specific weight (force) was applied onto the second polishing pads 63 while the second pad dressing carrier 62 was rotated in the direction depicted by arrow 64, the first pad dressers were rotated in the direction depicted by arrow 65 and the first polishing pads were rotated in the direction depicted by arrow 66. The rotational speed of the polishing palate was set to 25 to 35 rpm, the revolution speed of the pad dressing carrier was set to 5 to 10 rpm and the rotation speed of the pad dressing carrier was set to 5 to 10 rpm. It is recognized herein that the number of second polishing processes before performing the second pad dressing process may be more or less than twenty and the duration and rotation/revolution rates in which the second polishing machine performs the second pad dressing process may be longer or shorter in example embodiments. In this regard, it was observed that the same results were obtainable when performing the second pad dressing process for between 3 to 30 minutes, setting the rotational speed of the polishing palate to between 10 to 40 rpm, and setting the revolution speed of the pad dressing carrier and the rotation speed of the pad dressing carrier to between 3 to 20 rpm. (9) Second Polishing Pad Inspection Process

After completing each second pad dressing process, a second polishing pad inspection process was performed on the polishing pads of the second polishing machine. Particularly, the second polishing pad inspection process was performed to determine the extent to which the repeated sixteen uses of the polishing pads and associated polishing liquid affected the surface state of the polishing pad, and thereby caused defects to the polishing pads. More particularly, as shown in FIG. 7, the inspection for defects included a determination of the accumulative extent by which individual abrasive grains of the polishing liquid became trapped in the pores 71 of the polishing pad 70, despite performing a second pad dressing process. In this regard, a finding of a defect of the polishing pad was concluded when the second polishing pad inspection process found trapped abrasive grains in the pores.

(10) Chemical Strengthening Process

A chemical strengthening process was then applied to the glass substrate by preparing a chemical strengthening solution in the form of a mixture of potassium nitrate (60%) and sodium nitrate (40%), heating this chemical strengthening solution to about 400C, preheating the cleaned glass substrate to about 300C, and immersing it in the chemical strengthening solution for about 3 hours. The immersion was carried out in a state where a plurality of glass substrates were placed in a holder so as to be held at their end faces, thereby enabling the entire surfaces of the glass substrate to be chemically strengthened.

In performing the immersion treatment in the chemical strengthening solution, lithium ions and sodium ions in a surface layer of the glass substrate are replaced by sodium ions and potassium ions in the chemical strengthening solution, respectively, so that the glass substrate becomes strengthened. The thickness of a compressive stress layer formed at the surface of the glass substrate was about 100 microns to 200 microns.

Glass substrates having been subjected to the chemical strengthening treatment were immersed in a water bath at about 20C so as to be rapidly cooled, and maintained for about 10 minutes. Thereafter, the rapidly cooled glass substrates were cleaned by immersing in concentrated sulfuric acid heated to about 40C. Thereafter, the glass substrates were immersed in respective cleaning baths of pure water and IPA (isopropyl alcohol) in turn.

In performing one glass substrate production process, which included the shaping process and first lapping process, the cutting out, coring, forming and chamfering process, the second lapping process, the edge surface polishing process, the first and second polishing processes, and the chemical strengthening process, as described above, a total of one hundred flat, smooth and high rigidity glass substrates were obtained.

(1 1) Final Glass Substrate Inspection Process

For the final glass substrate inspection process, one glass substrate was arbitrarily sampled after each glass substrate production process. This arbitrary sampling of one glass substrate was repeated for each of the one hundred glass substrate processes performed for each of the example embodiments and comparative examples.

The final glass substrate inspection process included an inspection of the outer and inner peripheral portions of each arbitrarily sampled glass substrate. The inspection process included a process of measuring the values of Rl and R2 of the outer peripheral portion at a plurality of positions equally spaced in the circumferential direction of each arbitrarily sampled glass substrate. After the conclusion of each example embodiment and comparative example, a calculation of the standard deviation of the difference between the measured values of Rl and R2 of the one hundred samples of glass substrates was obtained. A judgment process was then carried out to determine whether the variation of the measured outer peripheral portion of the sampled glass substrates was good (variation suppressed) or not good (variation not suppressed) based on the calculated standard deviation. This judgment process was then matched with the results of each corresponding second polishing pad surface inspection process, which inspected for defects of the polishing pad after performing each second pad dressing process.

(12) Information Recording Medium Manufacturing Process

On each principal surface of each glass substrate obtained through the above- mentioned glass substrate production process, an adhesive layer of a Cr alloy, a soft magnetic layer of a CoTaZr-group alloy, an underlayer of Ru, a perpendicular magnetic recording layer of a CoCrPt-group alloy, a protective layer of hydrocarbon, and a lubricating layer of perfluoropolyether were formed in this order, thereby manufacturing a perpendicular magnetic recording medium (PMR). More specifically, an adhesive layer of CrTi, a soft magnetic layer of CoTaZr/Ru/CoTaZr, an intermediate layer of Ru, a granular magnetic layer of CoCrPt-Si02, and a hydrogenated carbon protective layer were formed in this order on a glass substrate using an in-line type sputtering apparatus and further a perfluoropolyether lubricating layer was formed by a dipping method, thereby obtaining an information recording medium.

The above structure is an example embodiment of the structure of a perpendicular magnetic recording medium (PMR). It is to be understood herein that magnetic layers and underlayers may also be formed as a horizontal information recording medium, such as a Longitudinal Magnetic Recording medium (LMR) in example embodiments.

(13) Information Recording Device Manufacturing Process

An information recording device was then manufactured by incorporating the above-mentioned information recording medium. The above glass substrate production process of steps 1 to 10 for producing one hundred glass substrates was repeatedly performed for a total of one hundred times so as to obtain one hundred arbitrarily sampled glass substrates for each example embodiment and comparative example. Remarkably, it is realized herein that example embodiments, such as those of Examples 1 and 2, enabled the consistent and reliable production of large quantities of glass substrates having suppressed variations of the outer and inner peripheral portions. More specifically, example embodiments were capable of consistently and reliably achieving larger useable recording areas. On the other hand, it was not possible to achieve glass substrates with suppressed variations of the outer and inner peripheral portions in the production of large quantities of glass substrates in comparative examples, such as those of Comparative Examples 1 to 3. These results will now be explained, along with a comparison explanation of the example embodiments of Examples 1 and 2 and comparative examples of Comparative Examples 1 to 3.

Comparative Example 1

One unit of 2.5 inch (65 mm) glass substrate was arbitrarily sampled after each production of one hundred units of glass substrates according to the above process. The above process was repeated for a total of one hundred times and a total of one hundred glass substrates were arbitrarily sampled for Comparative Example 1. In the comparative example, a conventional weight (force) of 28 gramforce / cm2 was applied in the first pad dressing process and the second pad dressing process. As expected, the final glass substrate inspection process for the one hundred arbitrarily sampled glass substrates of Comparative Example 1 resulted in a large standard deviation of 3.8, and therefore the variation in the outer peripheral portion was not suppressed. Furthermore, the second polishing pad inspection processes of Comparative Example 1 resulted in findings of significant defects to the surface state of the polishing pad after performing each second pad dressing process.

Comparative Example 2

A same number of 2.5 inch (65 mm) glass substrates were produced in the same manner as Comparative Example 1, except that the first pad dressing process and the second pad dressing process were varied. A total of one hundred glass substrates were arbitrarily sampled in the same manner as Comparative Example 1.

In this comparative example, an increased weight (force) of 38 gramforce / cm2 was applied in the first and second pad dressing processes, which is an increase over the conventionally applied weight of 28 gramforce / cm2 of Comparative Example 1. The final glass substrate inspection process for the one hundred arbitrarily sampled glass substrates of Comparative Example 2 resulted in a larger standard deviation of 3.9, as compared to the standard deviation of 3.8 of Comparative Example 1. That is, the variation in the outer peripheral portion among glass substrates produced in large quantities of Comparative Example 2 was also not suppressed, and in fact the variation increased. Furthermore, the second polishing pad inspection processes of Comparative Example 2 resulted in findings of significant defects to the surface state of the polishing pad after performing each second pad dressing process.

Comparative Example 3

A same number of 2.5 inch (65 mm) glass substrates were produced in the same manner as Comparative Examples 1 and 2, except that the first and second pad dressing processes were again varied. A total of one hundred glass substrates were arbitrarily sampled in the same manner as Comparative Examples 1 and 2.

In Comparative Example 3, an increased weight (force) of 47 gramforce / cm2 was applied in the first and second pad dressing processes. Not surprisingly, the final glass substrate inspection process for the one hundred arbitrarily sampled glass substrates of Comparative Example 3 resulted in a larger standard deviation of 4.0, as compared to the standard deviations of the other comparative examples. That is, the variation in the outer peripheral portion among glass substrates produced in large quantities of

Comparative Example 3 was also not suppressed, and in fact the variation increased. Furthermore, the second polishing pad inspection processes of Comparative Example 3 resulted in findings of significant defects to the surface state of the polishing pad after performing each second pad dressing process.

In comparing the conventionally applied weight of 28 gramforce / cm2 of Comparative Example 1 with the increased applied weight of 38 gramforce / cm2 of Comparative Example 2 and the further increased applied weight of 47 gramforce / cm2 of Comparative Example 3, it follows that increasing the applied weight fails to provide any improvements in suppressing variations of the outer peripheral portion of glass substrates produced in large quantities. In this regard, the achievable recordable area of the glass substrates produced in large quantities becomes reduced in Comparative Example 2 when compared to the conventional Comparative Example 1 and further reduced in Comparative Example 3 when compared to Comparative Example 2 and conventional Comparative Example 1.

Example 1

A same number of 2.5 inch (65 mm) glass substrates were produced in the same manner as the comparative examples, except that the first and second pad dressing processes were again varied. A total of one hundred glass substrates were arbitrarily sampled in the same manner as the comparative examples.

In the example embodiment, a substantially increased weight (force) of 66 gramforce / cm2 was applied in the first and second pad dressing processes, as compared to the comparative examples. It is recognized herein that the final glass substrate inspection process of the one hundred arbitrarily sampled glass substrates of Example 1 resulted in a substantially reduced standard deviation of 3.4, as compared to the comparative examples, which represents a significantly suppressed variation of the outer peripheral portion among glass substrates produced in large quantities. In this example embodiment, the second polishing pad inspection processes resulted in no findings of defects to the surface state of the polishing pad after performing each second pad dressing process. In this regard, the achievable recording area of glass substrates produced in large quantities according to example embodiments become substantially increased when compared with conventional substrates conventionally produced in large quantities.

Example 2

Again, a same number of 2.5 inch (65 mm) glass substrates were produced in the same manner as the comparative examples and Example 1 , except that the first and second pad dressing processes were again varied. One hundred glass substrates were arbitrarily sampled in the same manner as the comparative examples and Example 1.

In the example embodiment, an even greater weight (force) of 75 gramforce / cm2 was applied in the first and second pad dressing processes, which represents a greater applied weight over the 66 gramforce / cm2 of the example embodiment of Example 1 and a substantially greater applied weight over the conventionally applied weight of 28 gramforce / cm2 of Comparative Example 1. It is recognized herein that the final glass substrate inspection process for the one hundred arbitrarily sampled glass substrates of Example 2 also resulted in a substantially reduced standard deviation of 3.4, as compared to the comparative examples, which represents a significantly suppressed variation of the outer peripheral portion among glass substrates produced in large quantities. In this example embodiment, the second polishing pad inspection processes also resulted in no findings of defects to the surface state of the polishing pad after performing each second pad dressing process. In this regard, the achievable recordable area of the glass substrates produced in large quantities according to example embodiments become substantially increased when compared with conventional substrates conventionally produced in large quantities. Comparison between the Examples and Comparative Examples

In example embodiments, such as Examples 1 and 2, and comparative examples, such as Comparative Examples 1 to 3, the applied weight (force) in each of the first pad dressing process and the second pad dressing process were varied while keeping all other processes and process parameters the same. Conventionally, each of the first and second pad dressing processes utilized an applied weight of about 28 gramforce / cm2 to accomplish the refreshing of the polishing pads so as to prevent reductions in the polishing rate of the first and second polishing machines. Throughout each production of large quantities of glass substrates, as represented by one hundred production runs, wherein each production run produced one hundred glass substrates, for each of the example embodiments and comparative examples, two inspection processes were performed. The results of the second polishing pad inspection processes and the final glass substrate inspection processes for example embodiments and comparative examples, as representable by Examples 1 and 2 and Comparative Examples 1 to 3, respectively, are summarized in Table 1, followed by a discussion of the inspection processes and results.

TABLE 1

Weight (force) Second Standard Variation in applied during polishing pad deviation of outer first and second inspection outer peripheral pad dressing processes peripheral portion for processes results portion (from sample of 100

(gf/cm2) arbitrary units of glass sampling of substrates from 100 units of a large quantity glass of glass substrates) substrates

Comp. Ex. 1 28 Defect found 3.8 Not suppressed

Comp. Ex. 2 38 Defect found 3.9 Not suppressed

Comp. Ex. 3 47 Defect found 4.0 Not suppressed

Ex. 1 66 No defect 3.4 Suppressed

Ex. 2 75 No defect 3.4 Suppressed

(I) Measurement of Arbitrarily Sampled Glass Substrates

For each of the one hundred production runs of each of the example embodiments and the comparative examples, one unit of glass substrate was arbitrarily sampled for the final glass substrate inspection process, and its corresponding outer peripheral portions inspected.

In each final glass substrate inspection process, a measurement of the outer peripheral portion was performed for each arbitrarily sampled glass substrate of the example embodiments and comparative examples. In this respect, the actual achieved value of Rl for each sampled glass substrate was arbitrarily measured at several points as the radial distance from the center of the glass substrate to the commencement point of the actual observed surface height variation, that is, a ski-jump profile, as depicted in FIG. 2, or roll-off profile, as depicted in FIG. 3. As the value of R2 is recognized herein to not directly affect the actual useable recording area of the glass substrate, the value of R2 was fixed at 97% (or 31.5 mm for 2.5 inch substrates), which may represent the distance from the center of the glass substrate to a commencement point of a chamfered surface, as illustrated in FIGs. 2 and 3. Then, a calculation of the actual achieved difference (R2 - Rl) of each arbitrarily sampled glass substrate was performed. After the conclusion of measuring all one hundred arbitrarily sampled glass substrates of each example embodiment and comparative example, a calculation of the standard deviation of the difference between the measured values of Rl and R2 of the one hundred samples of glass substrates was obtained. A judgment process was then carried out to determine whether the variation of the measured values was suppressed or not suppressed based on the calculated standard deviation.

For Comparative Example 1, the conventionally applied weight of 28 gramforce / cm2 in the first and second pad dressing processes resulted in a poor standard deviation of 3.8. It is recognized herein that such a large standard deviation represents a large variation, or not suppressed variation, in the outer peripheral portion among glass substrates produced in large quantities.

Comparative Example 2 was varied from Comparative Example 1 by increasing the applied weight in the first and second pad dressing processes from 28 gramforce / cm2 to 38 gramforce / cm2. This variation resulted in a worse standard deviation of 3.9, which is recognized herein to represent an even larger variation, or not suppressed variation, in the outer peripheral portion among glass substrates produced in large quantities, as compared to Comparative Example 1. In Comparative Example 3, the applied weight was further varied from

Comparative Examples 1 and 2 by subjecting an increased applied weight of 47 gramforce / cm2 in the first and second pad dressing processes. The result was a further deterioration in the standard deviation to 4.0, which is recognized herein to represent an even larger variation, or not suppressed variation, in the outer peripheral portion among glass substrates produced in large quantities. It followed from observing the comparative examples, including Comparative Examples 1 to 3, that conventional expectations anticipate further increases to the standard deviation with further increases in applied weight in the first and second pad dressing processes.

It is recognized herein that in sharp contrast to conventional expectations, the example embodiments, including Examples 1 and 2, achieved a substantially reduced standard deviation of 3.4 when applying a significantly increased weight of 66 gramforce / cm2 and 75 gramforce / cm2, respectively, in the first and second pad dressing processes. It is further recognized herein that other increases in the applied weight beyond 66 gramforce / cm2 for the first and second pad dressing processes yielded substantially the same improvements as in Examples 1 and 2. In other words, further increases in the applied weight beyond 66 gramforce / cm2 did not markedly improve the variation suppression over those achieved from the example embodiment of Example 1.

It is recognized herein that suppressed variations in the outer peripheral portions are achievable for example embodiments having targeted values of Rl other than 92% (or 29.9 mm for 2.5 inch substrates) and values of R2 other than 97% (or 31.5 mm for 2.5 inch substrates). In this regard, the targeted value of Rl may also be near or equal to R2 in example embodiments, which substantially reduces or effectively eliminates the outer peripheral portion altogether. Furthermore, it is to be understood herein that the height variation of the outer peripheral portion of a sampled glass substrate may take the shape of a substantially reduced ski-jump profile, as illustrated in FIG. 2, a substantially reduced roll-off profile, as illustrated in FIG. 3, a combination thereof, or may be eliminated altogether, as illustrated in FIG. 4, so as to utilize some, most or the entire outer peripheral portion as useable recording areas in example embodiments.

(II) Inspection of the Second Polishing Pad It is recognized herein that it is preferable to perform a pad dressing process after sixteen consecutive polishing processes for both the first and second polishing processes. After performing each second pad dressing process for each of the example

embodiments, including those of Examples 1 and 2, and the comparative examples, including those of Comparative Examples 1 to 3, a second polishing pad inspection process was performed to inspect the surface state of the polishing pad. In the second polishing pad inspection process, an inspection of the pores of each polishing pad was performed so as to determine whether or not there were significant amounts of abrasive grains trapped therein. A finding of such is indicative of defects to the surface state of the polishing pad, whereas a finding otherwise is indicative of no defects to the surface state of the polishing pad.

In Comparative Example 1 , defects to the surface of the polishing pad were observed in each of second polishing pad inspection processes after each of the second pad dressing processes. More particularly, a significant amount of individual abrasive grains were observed to be trapped in the pores.

For Comparative Examples 2 and 3, a significant amount of individual abrasive grains were also observed as being trapped, which represents defects to the surface of the polishing pad.

On the other hand, the second polishing pad inspection process of example embodiments, including those of Examples 1 and 2, yielded no defects to the surface state of the polishing pad. More particularly, no significant individual abrasive grains were observed to be trapped in the pores.

An equivalent inspection process was also performed after the first pad dressing process for each of the example embodiments, including those of Examples 1 and 2, and the comparative examples, including those of Comparative Examples 1 to 3. In this regard, it was observed that the first polishing pad inspection process also achieved the same results as those obtained from the above second polishing pad inspection process.

Information recording mediums comprising substrates produced in accordance with example embodiments are recognized herein to be capable of consistently and reliably achieving high information storage densities of about 200 GB/inch2 or more, and more preferably 250 GB/inch2 or more, in example embodiments. The said substrates are preferably glass substrates, such as those formed by plate-like glass, aluminosilicate glass, sodalime glass and borosilicate glass, or the like. It is to be understood herein that the material of example embodiments of the substrate need not be limited to glass, and thus example embodiments of the substrate may include any of those other materials suitably for use in one or more polishing steps utilized in the production of large quantities of substrates. It is also to be understood herein that an outer edge surface and an inner edge surface may be a partially, substantially or completely curved surface and thereby not necessarily be substantially perpendicular to a principal surface. It is also to be understood herein that a boundary between an edge surface and a chamfered surface can become unclear by, among other reasons, a polishing process and therefore an edge surface and a chamfered surface may cooperatively form one curved surface.

Although the aforesaid example embodiments provide for process improvements over those previously known in the art, with some references to example embodiments and applications using glass substrates for information recording mediums in information recording devices, such as hard disk drives (HDD), it is to be understood by persons ordinarily skilled in the art that example embodiments described herein are merely intended to facilitate understanding of the present invention, and implies no limitation thereof. Various modifications and improvements of the example embodiments are possible without departing from the spirit and scope thereof as recited in the appended claims, and these will naturally be included as equivalents in the present invention. Furthermore, various modifications and improvements of example embodiments are applicable in other applications and industries without departing from the spirit and scope thereof as recited in the appended claims, and these too will naturally be included as equivalents in the present invention.