Optical recording medium and its substrate

Claim 1

An optical recording medium for recording information by irradiating the optical recording medium with a laser beam having a wavelength, comprising: a substrate having first and second opposite facing surfaces, and a spiral groove extending from said first surface towards said second surface, wherein said spiral groove is defined by (i) a bottom surface located between said first and second surfaces, (ii) a first side wall extending from said bottom surface to said first surface, and (iii) a second side wall extending from said bottom surface to said first surface, a light absorption layer formed by vacuum deposition on said first surface, said bottom surface, said first side wall and said second side wall, wherein said light absorption layer contains an organic pigment having an evaporation starting temperature and a decomposition temperature, with said evaporation starting temperature being within a range of 100° C. to 350° C. and a difference between said evaporation starting temperature and said decomposition temperature being within a range of 50° C. to 400° C.; and a reflective layer formed on said light absorption layer; wherein, (a) a radially extending distance from an intersection of said first side wall and said first surface of a first turn of said spiral groove to an intersection of said first side wall and said first surface of a second turn of said spiral groove corresponds to a track pitch; (b) said spiral groove has a radially extending width measured from said first side wall to said second side wall, such that a ratio of said radially extending width to said track pitch is within a range of 0.25 to 0.45; (c) said spiral groove has a depth extending orthogonally to said radially extending width and measured from said bottom surface to a plane containing said first surface, and said substrate has a refractive index, such that said depth is 20% to 80% of ¼×(the wavelength divided by said refractive index); and (d) said first side wall forms a first angle with a plane extending orthogonally to said bottom surface within a range of 45° to 80°, and said second side wall forms a second angle with a plane extending orthogonally to said bottom surface within a range of 45° to 80°, such that said spiral groove has a trapezoidal cross-section that increases in area from said bottom surface towards said first surface, and respective portions of said substrate between adjacent turns of said spiral groove have a trapezoidal cross-section that increases in area from said first surface towards said second surface, wherein a difference between said first angle and said second angle is not more than 10°.