Multilevel phase change optical recording medium

Abstract

A multilevel phase change optical recording medium comprises first to N-th (N≧2) phase change optical recording layers, wherein an i-th recording layer and a j-th recording layer, which are two recording layers arbitrarily selected from the first to N-th recording layers, meet the conditions of T i >T mi and τ wi <τ xi , and T j <T mj or τ wj >τ xj, with respect to a particular recording laser beam selected from recording laser beams having different power levels, where T is the maximum temperature of the recording layer in a recording operation, T m is the melting point of the recording layer, T x is the crystallizing temperature of the recording layer, τ w is a time required for the recording layer to be cooled down from T m to T x after the laser beam irradiation, and τ x is the crystallizing time of the recording layer.

1 . A combination comprising: 
 (a) a recording device adapted to generate recording laser beams having different power levels during a recording operations; and    (b) a multilevel phase change optical recording medium received in said recording device so as to enable said recording laser beams having said different power levels to record data thereon during said recording operation, said recording laser beams each irradiating the same side of said recording medium, said medium comprising: 
 first to Nth rewritable phase change optical recording layers, N being greater than or equal to two, wherein said recording layers have different compositions and are separated by intermediate layers and are arranged within a depth of focus of a laser beam;  
 said recording layers including an i-th recording layer and a j-th recording layer each being arbitrarily selected from said recording layers and having the following characteristics for a recording beam selected from the aforementioned recording laser beams having different power levels: 
 T i >T mi  and τ wi <τ xi , and  
 T j <T mj  or τ wj >τ xj ,  
 wherein T i  and T j  are the maximum temperatures to which the selected laser beam respectively heats said i-th and j-th recording layers during the recording operation, T mi  and T mj  are the temperatures at which said i-th and j-th recording layers will respectively melt, τ wi  and τ wj  are the times required for said i-th and j-th recording layers to respectively cool from T mi  and T mj  to respective crystallizing temperatures T xi  and T wj  for each of said i-th and j-th layers, and τ xi  and τ xj  are the times required for said i-th and j-th layers to respectively crystallize.  
 
   
     
     
         2 . The combination according to  claim 1 , wherien the j-th layer is lower in the maximum temperature and shorter in the cooling time compared with the i-th layer.  
     
     
         3 . The combination according to  claim 1 , wherein said medium further comprises an intermediate layer having lower thermal conductivity than that of the recording layer and provided on the j-th layers.  
     
     
         4 . The combination according to  claim 1 , wherein said medium further comprises an upper protective layer and a reflective layer on the j-th layer, wherein the upper protective layer has a higher thermal conductivity than that of the recording layer.  
     
     
         5 . The combination according to  claim 1 , wherein the i-th layer consists of Ge 2 Sb 2 Te 5  and the j-th layer consists of Ge 2 Sb 2 Te 5  doped with 5 at % of Sb.  
     
     
         6 . The combination according to  claim 1 , wherein the i-th layer is closer to the substrate than the j-th layer.