Patent ID: 7566673

Claim:
A chemically strengthened glass substrate for an information recording medium, wherein a strengthened layer formed by chemical strengthening exists on an outer edge surface and on an inner edge surface but substantially not on a surface on which an information recording layer is formed, wherein, on the surface on which the information recording layer is formed, the glass substrate comprises the following glass ingredients: 60 to 75% by weight of SiO 2 ; 3 to 20% by weight of Al 2 O 3 ; 0 to 8% by weight, zero inclusive, of B 2 O 3 ; a total of 5 to 15% by weight of R 2 O compounds, where R= Li, Na, and K; SiO 2 +Al 2 O 3 accounting for 63 to 90% by weight; a total of 0 to 20% by weight, zero inclusive, of R′O compounds, where R′═Mg, Ca, Sr, Ba, and Zn; and a total of 0 to 15% by weight, zero inclusive, of TiO 2 +ZrO 2 +Ln x O y , where Ln x O y represents at least one compound selected from the group consisting of lanthanoid metal oxides, Y 2 O 3 , Nb 2 O 5 , and Ta 2 O 5 , and wherein the following condition is fulfilled: 1.5<Al 2 O 3 /B 2 O 3 , or B 2 O 3 =0%.