Method for producing chemically strengthened glass, method for producing chemically strengthened glass substrate, method for producing glass substrate for information recording medium, and glass substrate for information recording medium obtained by the method

#The present invention relates to a method for producing a chemically strengthened glass, a method for producing a chemically strengthened glass substrate, a method for producing a glass substrate for information recording medium, and a glass substrate for information recording medium obtained by the method.

#Strengthened glass used for electrical appliances represented by information processing instruments calls for high cleanliness. In particular, a chemically strengthened glass substrate for a magnetic disk requires a substrate excellent in smoothness. With an increase of the information packing density of a magnetic disk memory device, decrease in the flying of a magnetic head has been intended. In order to decrease the flying height of the magnetic head, a chemically strengthened glass substrate for a magnetic disk excellent in smoothness on a surface of which no projected protrusion exists is required. A product obtained by forming an information recording film having magnetic characteristics on the chemically strengthened glass substrate for a magnetic disk are used as an information recording medium. In order to increase the density of information recording, it is necessary to decrease the spacing (flying height) between the magnetic head and the information recording medium.

#In a hard disk, the presence of protrusions existing on a surface of a glass substrate contributes to the collision between the glass substrate and the magnetic head. The protrusions are formed due to foreign matter adhering to the surface of the glass substrate. As for methods for removing foreign matter on the glass substrate, investigations have hitherto been made. However, with the progress of a decrease in flying height, requirements for smoothness have become severe, and minute foreign matter which has hitherto counted for nothing has also come to be considered as the cause of a defect.

#Smoothing treatment was carried out to a substrate obtained by carrying out chamfering and lapping to doughnut-shaped aminosilicate-based glass (SiO2: 6.0 mol %, Al2O3: 11.0 mol %, Li2O: 8.0 mol %, Na2O: 9.1 mol %, MgO: 2.4 mol %, CaO: 3.6 mol %) having a thickness of 0.6 mm, an outer diameter of 65 mm and an inner diameter of 20 mm. The smoothing treatment was carried out by polishing both faces using a cerium oxide-containing abrasive and polishing pads having an Asker C (according to the standard of Japan Rubber Association) hardness of 70. Abrasive adhered to a surface of the substrate was removed by scrubbing with sponge made from polyvinyl alcohol and ultrasonic cleaning using an strong alkali, followed by rinsing with pure water. The substrate was dried in IPA vapor for 1 minute.

#The resulting glass substrate was exposed in an atmosphere containing 20.2 μg/m3 of suspended iron particulates for 20 days. Using a defect detector (RS7000 manufactured by Hitachi DECO), the average number of defects per μm square on the surface of the glass substrate exposed was examined. As a result, it was 12. The number of particles per μm square on the surface of the glass substrate not exposed was 0 to 1 on the average. Further analysis of the exposed glass substrate with a total reflection X-ray fluorescence apparatus gave the result that the amount of Fe was evidently rich. It was therefore presumed that all particles detected with the defect detector were iron particles.

#After the exposed glass substrate was heated at 80° C. for 30 minutes, the substrate was immersed for 4 hours in a mixed molten salt of potassium nitrate (80% by weight) and sodium nitrate (20% by weight) heated at 350° C., thereby conducting the chemical strengthening treatment of ion exchanging lithium ions and sodium ions in the glass substrate for potassium ions having an ion radius smaller than their ion radius. After the chemical strengthening treatment, the substrate was cleaned with pure water to remove the molten salt. Cleaning was then conducted with a mixed acid of nitric acid and sulfuric acid, pure water, an alkali cleaning solution and pure water in this order.