Information storage disk employing a reflective layer having good malleability
This application is a continuation of now abandoned application Ser. No. 07/396,214, filed on Aug. 21, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an information storage disk for recording, reproducing, and erasing information by irradiation of laser beam. 
     2. Description of Prior Art 
     Since the disclosure by S. R. Ovsinsky, et al. in U.S. Pat. No. 3,530,441 that the thin films of Te 85  EiGe 15 , Te 81 ,Ge 15  S 2  Sb 2 , etc. show reversible phase change by irradiation of laser beam and the like, there has been conducted development of an information storage disk using a chalcogenide alloy thin film as a recording layer by which recording, reproducing and erasing of information can be performed. 
     As one of the targets for development of the information storage disk of this type, there is a task to reduce the energy necessary for recording and erasing, i.e., to enhance sensitivity. British Patent Publication 2,079,031 specification discloses a tri-layer structure optical information recording medium made by inserting a spacer layer between the recording layer comprising a chalcogenide alloy thin film and a reflective layer comprising a metal thin film having large optical reflectivity such as Au, Al, etc. This is a means of enhancing optical absorptivity by utilizing the interference effect so as to obtain enhanced sensitivity. Further, Japanese Patent Publication (Unexamined) No. 63-9040/1988 specification discloses an optical information recording medium having the above-mentioned tri-layer structure, of which the reflective layer comprises Ni--Cr alloy thin film. This schemes, by using Ni--Cr alloy having small thermal diffusivity as a reflective layer, to reduce thermal diffusion to the surrounding at the laser beam irradiated part in recording and erasing and, by elevating the temperature of the recording layer in good efficiency, to enhance sensitivity. 
     However, there has been a problem that, when a Ni--Cr alloy is used as a reflective layer, the disk becomes weak to thermal shock. In the information storage disk for performing recording and erasing by phase change, in general, recording is performed by partial amorphization of recording layer, and erasing is performed by crystallization. This is because the amorphization is more suitable for forming a minute mark. However, as the recording layer has an amorphous phase in forming, it requires to be converted into a crystalline phase before use. This treatment is called an initialization. In order to effect good initialization in a short time, it is desirable to effect simultaneous initialization over a wide range by irradiating with a laser beam having a large spot diameter. However, when an information storage disk using Ni--Cr alloy as a reflective layer was initialized while rotating with an Ar laser having a spot diameter of about 40 μm and subjected to thermal shocks of charging in and taking out of a room with constant temperature kept at 90° C., cracks were generated on the reflective layer. It was due to the stress caused by the initialization and the expansion and shrinkage of each layer at the time of the exertion of the thermal shock, and is presumed to be attributed to the small malleability of the reflective layer. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an information storage disk which has high sensitivity and still is free from crack even by initialization with a laser beam having large spot diameter and thermal shock, and has good weather resistance. 
     In order to attain the above object, in the present invention, the reflective layer is constructed by an alloy thin film comprising as a main component a metal selected from the group consisting of Ni--Cr, Cr and Ti and a metal having malleability larger than said main component. 
     Ni--Cr, Cr and Ti have practically sufficient optical reflectivity and small thermal diffusivity, but have common property of small malleability and aptness to cause cracks. By adding a malleable metal to such a material, there can be produced an information storage disk which can be initialized with a laser beam having large spot diameter, which is strong against thermal shock, and has good weather resistance. It is preferred for a malleable metal to be selected from Au, Ag, Pd, Pt, AI and Cu. The metals having large malleability such as Au include many of those having large thermal diffusivity. Accordingly, if such a metal is added in a large amount, thermal diffusivity of reflective layer becomes large, and the sensitivity of the information storage disk is lowered. Accordingly, in order to keep the effect of use of Ni--Cr or the like which has small thermal diffusivity for obtaining high sensitivity, the amount of addition of Au or the like is preferably not to be large. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 and FIG. 2 are sectional views showing the structures of the information storage disk in the embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiment shown in FIG. 1 is constructed on a transparent substrate 1 comprising resin, glass, etc. a recording layer 2 comprising a substance which causes phase change such as a chalcogenide alloy thin film, a spacer layer 3 comprising a dielectric, and a reflective layer 4. The embodiment shown in FIG. 2 has more practical construction, in which there are provided on a disk form transparent substrate 1 having a central opening 8 an undercoat 5 comprising a dielectric, a recording layer 2, a spacer layer 3, and a reflective layer 4, and a protective substrate 7 is provided through an adhesive 6. The present invention is characterized by the material of the reflective layer, and the reflective layer 4 is composed of a thin layer of alloy comprising any of Ni--Cr, Cr, and Ti as a main component and a metal having a larger malleability than said main component. 
     In order to testify the significance of the reflective layer material in the present invention, samples having the construction of FIG. 2 with change of the material of reflective layer only were tested for their strength against thermal shock and their recording sensitivity. Each sample was made by forming on a transparent substrate of polycarbonate resin an undercoat comprising ZnS having a thickness of about 1000Å, a recording layer comprising Te--Ge--Sb alloy having a thickness of about 400Å, a spacer layer comprising having a thickness of about 1700Å, and a reflective layer comprising various alloys having a thickness of about 400Å, and providing a protective substrate of polycarbonate resin through an adhesive comprising an ultraviolet light cured resin. The sample, while being rotated at 1,200 r.p.m., was initialized with Ar laser beam whose spot diameter was condensed to about 40 μm, and was subjected to repeated charging in and taking out of a room with constant temperature maintained at 90° C., and its strength against thermal shock was evaluated according to the generation or not of cracks. Also, the initialized sample, while being rotated at 1,800 r.p.m., was exposed to a semiconductor laser beam having a wavelength of 830 nm to evaluate its recording sensitivity according to the intensity of the laser beam necessary for amorphization. The results are shown in Tables 1 to 6. In each table, the mark ◯ in the column of thermal shock resistance shows that there was no change in the aforementioned test, and the mark x shows that cracks were formed on the reflective layer. The mark ◯ in the recording sensitivity column shows that the sensitivity was equivalent to the case where nothing was added to each main component of the reflective layer, and the mark x shows that the power of the laser beams necessary for recording increased by more than 15%. 
     Table 1 shows the results of evaluation on the alloys of Ni--Cr, Cr, and Ti in the case of Au being added. Tables 2 to 6 show the results of evaluation in the case of Ag, Cu, AI, Pd, and Pt being added in place of Au. It can be seen from these results that, when Au is selected as a metal having large malleability, the range of the addition amount which satisfies both the strength to thermal shock and the high recording sensitivity is 2-30  at % (atomic %) when the main component is Ni--Cr, 10-30 at % when it is Cr, and 5-30 at % when it is Ti. When Ag and Cu were selected as the metals having large malleability, the results turned out to be the same as in the case of Au. Preferred range of addition amount when Al is selected is 2-40 at % when the main component is Ni--Cr, 10-40 at % when it is Cr, and 5-40 at % when it is Ti. Preferred range of addition amount when Pd is selected is 2-50 at % when the main component is Ni--Cr, 10-50 at % when it is Cr, and 5-50 at % when it is Ti. When Pt was selected, the results were the same as in the case of Pd. The undercoat is provided to prevent damage of the transparent substrate by heat generated in recording and erasing. Depending on the material quality of the transparent substrate, however, this undercoat may be omitted. As the undercoat and spacer layer, ZnS was used, but other dielectric (e.g., SiO 2 , Si 3  N 4 , a mixture of ZnS with SiO 2 ) may be used. 
     
                                           TABLE 1
__________________________________________________________________________
Main component
Ni--Cr           Cr        Ti
      Thermal
           Recording
                 Thermal
                      Recording
                            Thermal
                                 Recording
Au addition
      Shock
           sensi-
                 Shock
                      sensi-
                            shock
                                 sensi-
amount
      resist.
           tivity
                 resist.
                      tivity
                            resist.
                                 tivity
__________________________________________________________________________
 1 at %
      X    ◯
                 X    ◯
                            X    ◯
 2 at %
      ◯
           ◯
                 X    ◯
                            X    ◯
 5 at %
      ◯
           ◯
                 X    ◯
                            ◯
                                 ◯
10 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
20 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
30 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
40 at %
      ◯
           X     ◯
                      X     ◯
                                 X
__________________________________________________________________________
 
    
     
                                           TABLE 2
__________________________________________________________________________
Main component
Ni--Cr           Cr        Ti
      Thermal
           Recording
                 Thermal
                      Recording
                            Thermal
                                 Recording
Ag addition
      Shock
           sensi-
                 Shock
                      sensi-
                            shock
                                 sensi-
amount
      resist.
           tivity
                 resist.
                      tivity
                            resist.
                                 tivity
__________________________________________________________________________
 1 at %
      X    ◯
                 X    ◯
                            X    ◯
 2 at %
      ◯
           ◯
                 X    ◯
                            X    ◯
 5 at %
      ◯
           ◯
                 X    ◯
                            ◯
                                 ◯
10 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
20 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
30 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
40 at %
      ◯
           X     ◯
                      X     ◯
                                 X
__________________________________________________________________________
 
    
     
                                           TABLE 3
__________________________________________________________________________
Main component
Ni--Cr           Cr        Ti
      Thermal
           Recording
                 Thermal
                      Recording
                            Thermal
                                 Recording
Cu addition
      Shock
           sensi-
                 Shock
                      sensi-
                            shock
                                 sensi-
amount
      resist.
           tivity
                 resist.
                      tivity
                            resist.
                                 tivity
__________________________________________________________________________
 1 at %
      X    ◯
                 X    ◯
                            X    ◯
 2 at %
      ◯
           ◯
                 X    ◯
                            X    ◯
 5 at %
      ◯
           ◯
                 X    ◯
                            ◯
                                 ◯
10 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
20 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
30 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
40 at %
      ◯
           X     ◯
                      X     ◯
                                 X
__________________________________________________________________________
 
    
     
                                           TABLE 4
__________________________________________________________________________
Main component
Ni--Cr           Cr        Ti
      Thermal
           Recording
                 Thermal
                      Recording
                            Thermal
                                 Recording
Al addition
      Shock
           sensi-
                 Shock
                      sensi-
                            shock
                                 sensi-
amount
      resist.
           tivity
                 resist.
                      tivity
                            resist.
                                 tivity
__________________________________________________________________________
 1 at %
      X    ◯
                 X    ◯
                            X    ◯
 2 at %
      ◯
           ◯
                 X    ◯
                            X    ◯
 5 at %
      ◯
           ◯
                 X    ◯
                            ◯
                                 ◯
10 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
20 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
30 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
40 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
50 at %
      ◯
           X     ◯
                      X     ◯
                                 X
__________________________________________________________________________
 
    
     
                                           TABLE 5
__________________________________________________________________________
Main component
Ni--Cr           Cr        Ti
      Thermal
           Recording
                 Thermal
                      Recording
                            Thermal
                                 Recording
Pd addition
      Shock
           sensi-
                 Shock
                      sensi-
                            shock
                                 sensi-
amount
      resist.
           tivity
                 resist.
                      tivity
                            resist.
                                 tivity
__________________________________________________________________________
 1 at %
      X    ◯
                 X    ◯
                            X    ◯
 2 at %
      ◯
           ◯
                 X    ◯
                            X    ◯
 5 at %
      ◯
           ◯
                 X    ◯
                            ◯
                                 ◯
10 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
20 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
30 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
40 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
50 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
60 at %
      ◯
           X     ◯
                      X     ◯
                                 X
__________________________________________________________________________
 
    
     
                                           TABLE 6
__________________________________________________________________________
Main component
Ni--Cr           Cr        Ti
      Thermal
           Recording
                 Thermal
                      Recording
                            Thermal
                                 Recording
Pt addition
      Shock
           sensi-
                 Shock
                      sensi-
                            shock
                                 sensi-
amount
      resist.
           tivity
                 resist.
                      tivity
                            resist.
                                 tivity
__________________________________________________________________________
 1 at %
      X    ◯
                 X    ◯
                            X    ◯
 2 at %
      ◯
           ◯
                 X    ◯
                            X    ◯
 5 at %
      ◯
           ◯
                 X    ◯
                            ◯
                                 ◯
10 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
20 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
30 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
40 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
50 at %
      ◯
           ◯
                 ◯
                      ◯
                            ◯
                                 ◯
60 at %
      ◯
           X     ◯
                      X     ◯
                                 X
__________________________________________________________________________