In recent years, with the sophistication of information technology, there have been significant advancements in information recording techniques, in particular, magnetic recording techniques. Magnetic recording media, such as HDDs (hard disk drives), have rapidly decreased in size, decreased in thickness, increased in recording density, and increased in access speed. In a magnetic recording medium, a magnetic disk, which includes a magnetic layer disposed on a disk-shaped substrate, is rotated at high speed, and recording and reproduction are performed while making a magnetic head afloat over the magnetic disk.
With an increase in access speed, the rotation speed of substrates also increases. Therefore, higher strength is required for substrates. Furthermore, with an increase in recording density, there has been a transition in magnetic heads from thin-film heads to magnetoresistive heads (MR heads) or giant magnetoresistive heads (GMR heads). The flying height of a magnetic head above a substrate has decreased to about 8 nm. Under these circumstances, if an irregular shape is present on the surface of magnetic disks, in some cases, crash failures resulting from magnetic head crash or thermal asperity failures, in which read errors are caused by heating due to adiabatic compression of air or contact, may occur.
Consequently, nowadays, as a substrate for a magnetic recording medium, in place of conventional aluminum substrates, glass substrates have been used. The reason for this is that, compared with aluminum substrates composed of metal which is a soft material, glass substrates composed of glass which a hard material are excellent in flatness of the substrate surface, substrate strength, and rigidity.
However, in another aspect, glass substrates are composed of a brittle material. Therefore, various methods for strengthening glass substrates have been proposed. For example, Patent Document 1 (JP-A-2002-121051) describes a structure in which by immersing a glass substrate in a chemically strengthening salt solution heated to about 400° C., lithium ions and sodium ions in a surface layer of the glass substrate are ion-exchanged with sodium ions and potassium ions in the chemically strengthening salt solution, respectively, thereby forming a compressive stress layer on the surface layer of the glass substrate to perform strengthening.
Furthermore, Patent Document 2 (JP-A-2001-192239) describes a structure in which a chemically strengthening process is performed using a chemically strengthening salt solution that contains potassium nitrate and sodium nitrate as main components and less than 1% by weight of a cation component other than potassium and sodium and that has a completely freezing point of 130° C. or lower. According to Patent Document 2, although solidification of some components starts at about 230° C., the solution has sherbet-like flowability until it completely solidifies at about 110° C. Thus, stress/strain can be decreased, and warpage of the glass substrate in the chemically strengthening process can be prevented.
Patent Document 1: JP-A-2002-121051
Patent Document 2: JP-A-2001-192239