Patent ID: 7891212

Claim:
A method for manufacturing a magnetic disk glass substrate for use in a hard disk drive, the glass substrate comprising: compressive stress layers formed at the main surfaces; and a tensile stress layer formed between the compressive stress layers, the method comprising the steps of: chemically strengthening the glass substrate; bringing the glass substrate into contact with a melted mixture of at least three alkali metal nitrates in the chemical strengthening step; forming the compressive stress layers at both main surfaces of the glass substrate by an ion exchange; and forming the tensile stress layer between the compressive stress layers, wherein the melted mixture of at least three alkali metal nitrates contains 0.001% to 0.3% by volume of a nitrate of alkali metal having a smallest ion radius among the alkali metal nitrates, wherein the magnetic disk glass substrate has a disk thickness of less than 0.5 mm and mirror-finished main surfaces, wherein a product of a thickness of the tensile stress layer and a maximum tensile stress of the tensile stress layer is set at a predetermined value, so that the magnetic disk glass substrate has a impact resistance of 3000 G or more and the main surface of the magnetic disk glass substrate has a waviness (Wa) of 1.0 nm or less, and wherein the thickness of the tensile stress layer is measured by observing a longitudinal section of the magnetic disk glass substrate with a Babinet compensator.