Multi-layer optical information recording medium

A method of manufacturing a multi-layer optical information recording medium comprising four or more information recording layers, the method comprising:

• forming four or more information recording layers successively in a direction of an optical axis on which signal information is recorded with any one of information pits whose concave-convex surfaces or phases change and recording marks whose reflectances or phases change the information recording layers are denoted by the m-th information recording layers, where m is any integer that is 0 or larger, an information recording layer that is farthest from an objective lens of a reproduction optical pickup being denoted by a 0th information recording layer;
• and
• forming a plurality of spacers each interposed between successive information recording layers, the spacers being made of a transparent material and having nearly constant thicknesses;
• and
• determining the thicknesses of the spacers so that Δtm and Δtm−sum satisfy formulas (1) and (2):
• Δtm>ACCT×Rm+1×Rm+2×Tm+12×nλ/(2NA2) (1)
• Δtm−sum>ACCT×Rm+a×Rm+b ×Tab2×nλ/(2NA2) (2)
• wherein when the thickness of the spacer interposed between the m-th information recording layer and an (m+1)th information recording layer is denoted by tm, the thickness of the spacer interposed between the (m+1)-th information recording layer and a (m+2)-th information recording layer is denoted by ttm+1, the difference between the thicknesses tm and tm+1 of the spacers is denoted by Δtm, the sum of the thicknesses of the spacers interposed between the m-th information recording layer and a (m+a)-th, where a is any integer that is 2 or larger, information recording layer is denoted by (tm +tm+1+. . . +m+a−1), the sum of the thicknesses of the spacers interposed between the (m+a)-th information recording layer and a (m+b)-th, where b is any integer that is 2 or larger, is denoted by (tm+a+tm+a+1+ . . . +tm+b−1), and the difference between the sum (tm+tm+1+ . . . +tm+a−1) and the sum (tm+a+tm+a+1+ . . . +tm+b−1) is denoted by Δtm−sum,
• where ACCT represents a crosstalk index, Rm+1, Rm+2, Rm+a, and Rm+b represent an intensity reflectances associated with reflective films formed on the (m+1)-th layer, (m+2)-th layer, (m+a)-th layer, and (m+b)-th layer, respectively, Tm+1 represents an intensity transmissivity of the spacer tm+1, Tab represents an intensity transmissivity from the (m+a)-th information recording layer to the (m+b)-th information recording layer, n represents a refractive index of the spacer material, and NA represents a numerical aperture of the objective lens, and
• wherein the intensity reflectances associated with reflective films are due to multiple reflections and decrease in a direction closer to an objective lens, and the differences in thicknesses between the spacers can be decreased in the direction closer to the objective lens based on the decreasing intensity reflectances associated with the reflective films.