Abstract:
Diffusion of water or oxygen present in the dielectric protection film is restrained by eliminating free oxygen in an oxide thin film while maintaining the characteristics of a dielectric protection film. As the dielectric material for forming a dielectric protection film for an optical disc or the like, an oxide mixture thin film of a niobium oxide and one of a silicon oxide and a titanium oxide is used. In a preferable example, a target made of a niobium oxide as the main component with 1 to 30% by weight of a silicon oxide added is used for formation of an oxide thin film by sputtering. Moreover, the oxide thin film is formed preferably in a nitrogen atmosphere. A nitrogen containing oxide thin film is produced by carrying out sputtering using a target with the oxygen lacked and a minute amount of nitrogen added. Thereby, a thin film having little reducing function and a high barrier property while having the characteristics comparable to a complete oxide can be produced.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a technique for forming an oxide thin film. 
       BACKGROUND TECHNIQUE 
       [0002]    In a recording type optical disc such as an inorganic additional recording type disc, a phase change disc and an optical magnetic disc, since a recording film of an inorganic material reacts with oxygen or water so as to be modified to an oxide or a hydroxide, it becomes unusable as time passes by. For preventing the phenomenon, a protection film of a transparent dielectric substance is provided to the optical discs. In the same manner, a display element of a flat panel display such as a plasma display or an electron light emitting display is provided with a transparent protection film for preventing corrosion by reaction with oxygen or water. The recording films and the display element films as mentioned above are called a medium film as the general term. 
         [0003]    The oxide thin film is a thin film effective as a protection film for a dielectric protection layer for a recording disc or a protection layer for a flat panel display. As the film forming method, various methods are adopted. For the reasons of the film forming time reduction and the simple film forming device, the sputtering method is frequently used. At the time, for reduction of the film forming time, a method of producing the thin film by preliminarily producing a target having a high sputtering efficiency by partially lacking the oxygen from the oxide, instead of an oxide having a low sputtering rate, and introducing the oxygen at the time of forming a film thereby to form the thin file by reactive sputtering has conventionally been used frequently. Moreover, according to the target with the lack of the oxygen, since the conductivity as a metal can also be improved, the DC sputtering can be performed so that the equipment can be provided further inexpensively, and thus it is advantageous. 
         [0004]    Conventionally, at the time of forming an oxide thin film, any of the following methods has been adopted commonly. 
         [0005]    (A) A method of forming an oxide thin film by deposition using a metal target while reacting in an atmosphere including oxygen 
         [0006]    (B) A method of forming a thin film as it is or while making up for the lacked oxygen by adding a minute amount of oxygen to such a degree that the oxygen lack can be compensated, using an oxide target 
         [0007]    However, according to both the methods (A) and (B), since the oxygen amount of the oxide thin film is excessive and unstable, in the applied fields of the dielectric protection layer for an optical disc, the flat panel display, the semiconductor, or the like, in which the oxide thin film is used frequently, deterioration by aging and the characteristic fluctuation of the formed oxide film with the excessive or unstable oxygen are problematic. 
         [0008]    For example, according to an oxide thin film with the excessive oxygen, since excessive oxygen becomes free, in the case the oxide thin film is active to the metal layer or other layers to be formed adjacently, it is reacted at the interface with the layers so as to form a product, and phenomena of deteriorating the effective inter-layer characteristics are generated as a result. 
         [0009]    For avoiding the phenomena, a technique of forming a film with lacking the nitrogen in a nitride or the oxygen in an oxide in a reducing atmosphere has been known (see the patent document 1). However, if the nitrogen or the oxygen is lacked until a sufficient effect can be obtained in the technique, the optical characteristics of the protection layer are changed. Specifically, a problem of deteriorating the effect as a transparent protection film due to an increase of the light absorption by the protection film has been involved. 
         [0010]    Moreover, in the case of an oxygen lacking state, since the reducing function works further strongly, increase of the oxygen lacking amount according to time passage does not provide or continue an effect suitable for the purpose of the oxide film formation, or the like, and thus it is problematic. 
         [0011]    From these viewpoints, various skills have been applied for the production apparatus for forming an oxide thin film or the forming materials, however, a technique for the essential improvement has not been obtained, and thus the problem remains in the current circumstances. 
         [0000]    Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 1-133229 
       DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
       [0012]    As the problems that the present invention is to solve, the above-mentioned problems can be presented as an example. An object of the present invention is to restrain diffusion of water or oxygen present in the transparent dielectric protection film by eliminating free oxygen in an oxide thin film while maintaining the characteristics of a transparent dielectric protection film. 
       Means for Solving the Problem 
       [0013]    In a preferable form of the present invention, as the dielectric material for forming a dielectric protection film for an optical disc or the like, an oxide mixture thin film of a niobium oxide and one of a silicon oxide and a titanium oxide is used. As the dielectric material used for a protection film for an optical disc or the like, an oxide mixture of a zinc sulfide (ZnS) and a silicon oxide (SiO 2 ) or the like is known. In consideration of the environment, a niobium oxide is used as the substitute for the zinc sulfide, avoiding the use of zinc (Zn). In a preferable example, it is preferred to use a target made of a niobium oxide as the main component with 1 to 30% by weight of a silicon oxide added for formation of an oxide thin film by sputtering. 
         [0014]    Although addition of the silicon oxide provides the effect of improving the transparency required to a transparent dielectric protection film, as to the thin film refractive index observed with a 400 nm wavelength light beam, the silicon oxide is only 1.7 whereas Nb 2 O 5  is 2.4.  FIG. 1  shows the relationship between the ratio of the silicon oxide contained in the transparent dielectric protection film and the refractive index of the transparent dielectric protection film. Since a high refractive index of 2.0 or more is required in the optical disc application for ensuring the reflection ratio, it is preferable to add the silicon oxide to 30% by weight at most. However, as long as the optical characteristics required for a transparent dielectric protection film permit, addition of the silicon oxide to amount more than 30% by weight involves no problem. 
         [0015]    The oxide thin film functioning as a dielectric protection film is preferably a composite oxide containing a niobium oxide of a low electric resistance with the theoretical composition value of Nb 2 O 3  as the main component, and at least one kind of a silicon oxide and a titanium oxide. Since the Nb 2 O 3  having the conductivity is used, thin film formation by the DC sputtering can be performed. Moreover, since the silicon oxide or the titanium oxide is contained, the transparency required for a dielectric protection film can be obtained. 
         [0016]    It is preferable that the composite oxide is formed to a thin film in a nitrogen atmosphere. It is preferable that the oxide thin film is produced by using a target with the oxygen lacked and by the reactive sputtering introducing the oxygen at the time of forming a film, from the viewpoint of sputtering rate. However, in the case of the film formation by the reactive sputtering with the oxygen introduced, the oxide thin film contains free oxygen or it is in an excessively oxidized state. Since an oxide in the excessively oxidized state has a strong reducing function and the free oxygen also has the oxidizing function of the adjacent other metals or the like, in an optical disc or the like, the recording film or the reflection film adjacent to the dielectric protection layer is oxidized, or the adverse effect of the reaction with a metal comprising the oxide or the like is generated so as to remarkably deteriorate the storage durability. Therefore, a nitrogen containing oxide thin film is produced by sputtering using a target with the oxygen lacked and adding a minute amount of nitrogen. Thereby, it is possible to produce a thin film having little reducing function and a high barrier property while maintaining the characteristics comparable to a complete oxide. 
         [0017]    A composite oxide produced in this manner is preferable as a dielectric protection layer for an optical disc. In particular, in the case that a layer adjacent to the dielectric protection layer in an optical disc contains at least one metal of Al and Ag, or in the case that the layer has the nature of producing by reaction a metal to be easily oxidized such as Bi, Fe by use, the reaction with the metals can be prevented by using a nitrogen containing oxide thin film with little reducing function as mentioned above. Thus, the characteristics deterioration of the optical disc can be prevented, and thus it is extremely preferable. 
         [0018]    Moreover, in a preferable form of the present invention, in a production method of an optical disc, a process of forming a dielectric protection layer by sputtering, in which the oxygen lack in an oxide thin film is compensated by mixing nitrogen, is provided. Accordingly, by compensating the valence number lacking part of the oxide by adding nitrogen instead of adding a minute amount of oxygen in the sputtering process, a nitrogen containing oxide thin film is formed. Since the oxide thin film with the nitrogen atoms mixed in the valence-number lacking part of the oxide restrains the reducing function of the oxide with the contained nitrogen in an oxygen lacking state, the material stabilization can be realized in a state with the required oxide characteristics maintained. Therefore, by providing a dielectric protection layer of the nitrogen containing oxide thin film, reaction with the adjacent recording film or the reflection film can be prevented so that an optical disc with little deterioration even after repeated recording and reproduction can be produced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a graph showing the relationship between the silicon oxide ratio contained in a transparent dielectric protection film and the refractive index of the transparent dielectric protection film. 
           [0020]      FIG. 2  schematically shows a sputtering device according to an embodiment of the present invention. 
           [0021]      FIG. 3  schematically shows the structure of an optical disc according to the embodiment. 
           [0022]      FIG. 4(   a ) shows the thicknesses of each layer of the optical disc according to the embodiments and the comparative example, 
           [0023]      FIG. 4(   b ) shows the target compositions used in the film formation of the dielectric protection layers of the embodiments and the comparative example, the sputtering film forming atmosphere gases, and the optical constants of the produced oxide thin films, and 
           [0024]      FIG. 4(   c ) shows the characteristics (jitter) of the optical discs according to the embodiments and the comparative example. 
       
    
    
     BRIEF DESCRIPTION OF THE REFERENCE NUMBER 
       [0000]    
       
           10  chamber 
           11  cathode 
           12  anode 
           13  substrate 
           15  high voltage direct current power source 
           20  target 
           21  thin film 
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Hereinafter, preferable embodiments of the present invention will be explained with reference to the drawings. 
         [0033]      FIG. 2  schematically shows the sputtering device according to the embodiment of the present invention. The sputtering device  100  includes a cathode  11  and an anode  12  disposed facing with each other in a chamber  10 . A target  20  of a metal, an oxide, or the like is disposed on the cathode  11 , and a substrate  13  to be the subject of the thin film formation is disposed on the anode  12 . The chamber  10  is provided with an inlet opening  16  for introducing an inert gas such as argon, and a pipe  17  connected with an unshown vacuum pump. The cathode  11  and the anode  12  are connected with a high pressure direct current power source  15 . 
         [0034]    In the sputtering process, with keeping the inside of the chamber  10  in a vacuum state by the vacuum pump, a direct current high voltage is applied between the cathode  11  and the anode  12  while introducing argon gas. Thereby, the ionized argon (Ar + )  14  collides with the target  20 , and repelled and returned target substances  22  form a thin film  21  on the substrate  13  disposed on the anode  12 . In the case of carrying out the reactive sputtering, a minute amount of oxygen (O 2 ) or nitrogen (N 2 ) is introduced into the chamber  10  together with the argon gas. 
         [0035]      FIG. 3  schematically shows the structure of the optical disc according to the embodiment.  FIG. 3(   a ) shows the external appearance of the optical disc  50 , and  FIG. 3(   b ) shows the layer structure of the optical disc  50 . As shown in  FIG. 3(   b ), the optical disc  50  has a reflection film layer  52 , a dielectric protection layer  53 , a recording film layer  54 , a dielectric protection layer  55  and a cover layer  56  laminated and formed on a substrate  51 . The oxide thin film according to the present invention is formed on the optical disc  50  as the dielectric protection layers  53  and  55 . 
         [0036]    Next, the embodiments and the comparative example of the optical disc  50  will be explained with reference to  FIG. 4 .  FIG. 4(   a ) shows the thicknesses of each layer of the optical disc according to the embodiments and the comparative example.  FIG. 4(   b ) shows the target compositions, the sputtering film forming atmosphere gases used in the film formation of the dielectric protection layers, and the refractive indices and the attenuation constants as the optical constants of the produced oxide thin films, according to the embodiments and the comparative example. The attenuation constant shows the light absorption degree by the thin film. A larger numerical value shows larger light absorption. For comparison, the same materials were used other than the dielectric protection layer. The disc-shaped substrate  51  made of a polycarbonate resin, having a 1.1 mm thickness and a 12 cm diameter, is provided with spiral grooves of a 0.320 μm pitch. The reflection film layer  52  made of a silver alloy, the dielectric protection layer  53 , and the recording film layer  54  made of Bi—Ge—N, and the dielectric protection layer  55  were laminated in this order on the substrate  51  by the sputtering method. Furthermore, a polycarbonate sheet was attached thereon using a ultraviolet ray curable resin as the adhesive so as to produce the 0.1 mm thickness light incident side substrate (cover layer)  56 . 
         [0037]    The embodiments 1-a to 1-c use a mixture of a niobium oxide (Nb 2 O 3 ) and a silicon oxide (SiO 2 ) as the target in the sputtering. In the Nb 2 O 3 —SiO 2  as the target, the SiO 2  amount was 18% by weight in either case. 
         [0038]    The atmosphere gas used for the sputtering process is argon to which 3% of oxygen is added by a 0.2 Pa gas pressure in the embodiment 1-a, argon to which 5% of oxygen is added by a 0.2 Pa gas pressure in the embodiment 1-b, and argon to which 5% of nitrogen is added by a 0.4 Pa gas pressure in the embodiment 1-c. 
         [0039]    In the embodiments 2-a and 2-b, a mixture of a niobium oxide (Nb 2 O 3 ) and a titanium oxide (TiO 2 ) is used as the target in the sputtering, and the TiO 2  amount is in both cases 96% by weight. The atmosphere gas used for the sputtering process is argon to which 3% by weight of oxygen is added by a 0.2 Pa gas pressure in the embodiment 2-a, and argon to which 5% of nitrogen is added by a 0.4 Pa gas pressure in the embodiment 2-b. 
         [0040]    In the comparative example 1-a, a mixture of a niobium oxide (Nb 2 O 3 ) and a silicon oxide (SiO 2 ) is used as the target in the sputtering, and the SiO 2  amount is 18% by weight. The atmosphere gas used for the sputtering process is argon to which 5% by weight of oxygen is added by a 0.4 Pa gas pressure. 
         [0041]      FIG. 4(   c ) shows the characteristics (jitter) of the optical discs according to the embodiments and the comparative example. As to the measurement conditions, a 1-7 modulation random pattern was recorded on the guide groove surface projecting to the light incident side on the optical disc with the oxide thin film formed as the dielectric protection layers  53  and  55 , by using an optical head with an objective lens having a 0.85 numerical aperture at a 4.92 m/s linear speed and with a 405 nm wavelength. For recording, a multi-pulse type strategy was used, and the window width was 15.15 nsec. 
         [0042]    As it is understood from  FIG. 4(   c ), in the all embodiments, preferable jitters were obtained without remarkable deterioration of the jitter even by the continuous reproduction. 
         [0043]    On the contrary, in the comparative example 1-a, the jitter value is deteriorated by the continuous reproduction. Deterioration with respect to the initial jitter is 20% as the initial jitter ratio, and a preferable value cannot be maintained. As it is learned from  FIG. 4(   b ), as to the comparative example 1-a and the embodiment 1-b, the pressure of the sputtering film forming atmosphere gas is higher in the comparative example (embodiment 1-b: 0.2 Pa, comparative example 1-a: 0.4 Pa). It is understood that the characteristic deterioration is generated because the oxide thin film in the comparative example 1-a is in an excessive oxygen state (excessively oxidized state). That is, in this recording disc, since a metal bismuth easily oxidized is generated in the recording film by recording, according to the optical disc produced so as to have the oxide in the excessively oxidized state by the film forming method of the comparative example, it is apparent that the free oxygen drastically reacts with the metal bismuth immediately after recording to produce a reaction product, thereby deteriorating the disc characteristics. 
         [0044]    On the contrary, according to the optical disc produced with an appropriate amount of oxygen added as in the embodiments 1-a and 1-b, as shown in  FIG. 4(   c ), it is confirmed that the characteristic deterioration can be restrained so that the performance as a sufficient dielectric protection film as the material can be provided. 
         [0045]    However, as it is understood by the comparison between the film forming atmosphere gases of the embodiments 1-a and 1-b and the comparative example 1-a, according to the film forming method by sputtering with the oxygen introduced, the oxygen introduction flow rate and the gas pressure should be adjusted strictly. Then, instead of the minute adjustment of the oxygen introduction amount and the gas pressure, in the embodiment 1-c, film formation was carried out with a minute amount of nitrogen added instead of the oxygen. The produced oxide thin film maintains the characteristics as the dielectric protection layer, and the characteristics deterioration is not observed. Such an oxide thin film formed by sputtering with the nitrogen introduction has the nitrogen atoms introduced into the valence number lacking part of the oxide so that the contained nitrogen restrains the reducing function of the oxide in the oxygen lacking state. Thereby, the reaction at the interface with the film to be contacted with the dielectric protection layer can effectively be restrained, and the material stabilization can be realized in a state with the required oxide characteristics maintained. 
         [0046]    In the embodiments 1-a to 1-c, a mixture of a niobium oxide (Nb 2 O 3 ) and a silicon oxide (SiO 2 ) is used as the target, and in the embodiments 2-a and 2-b, a mixture of a niobium oxide (Nb 2 O 3 ) and a titanium oxide (TiO 2 ) is used as the target. In the embodiment 2-a, a film is formed by sputtering with oxygen added, and in the embodiment 2-b, a film is formed by sputtering with nitrogen added. As shown in  FIG. 4(   c ), the optical discs of the embodiments 2-a and 2-b do not show the characteristics (jitter) deterioration by the continuous reproduction. Therefore, it is learned that the formed oxide thin film satisfies the performance as the dielectric protection layer also in the case a mixture of a titanium oxide is used as the target. 
       Modified Embodiment 
       [0047]    The present invention is characterized in that an oxide thin film functioning as a protection layer for a recording medium such as an optical disc is formed by the sputtering method using a target containing a niobium oxide as the main component and either one kind of a silicon oxide and a titanium oxide. Therefore, in the recording medium using an oxide thin film according to the present invention, the kinds of the other films are not limited. For example, the present invention can be applied to various kinds of optical discs having a protection layer, such as a re-writable type recording disc using a phase change material such as SbTe for a recording film and an organic pigment type recording disc using a pigment film for a recording film. It can also be applied for a disc using an Al alloy as a reflection film. 
         [0048]    Moreover, the layer structure of a recording medium using an oxide thin film of the present invention is not limited. For example, the present invention can be applied to various kinds of recording media such as a recording medium of a configuration without a light incident side substrate or a cover layer, a recording medium of a configuration having other material layers in addition to the protection layer, the recording film layer and the reflection layer, a recording medium of a configuration having the reflection film layer of two-layer structure, a recording medium of a configuration capable of multiple layer recording having one or more recording medium configuration further added to the position of the light incident side or light reflection side substrate, a recording medium such as a reproduction dedicated disc, and a recording medium of a shape other than a disc such as a card type. 
         [0049]    Moreover, although an oxide thin film according to the present invention is used for an optical disc in the above embodiments, the oxide thin film according to the present invention can also be used as a transparent protection film for a flat panel display or a semiconductor element because the performances required for a protection layer (transparency, oxidation prevention, reduction prevention, or the like) are substantially same. 
         [0050]    Moreover, although an example of sputtering using one target containing a niobium oxide and a silicon oxide or a titanium oxide is shown in the above embodiments, instead, a technique of forming films at the same time from a plurality of targets by co-sputtering using a niobium oxide and one of a silicon oxide and a titanium oxide as a target can also be adopted. 
       INDUSTRIAL APPLICABILITY 
       [0051]    The present invention can be utilized for the technology for forming an oxide protection film represented by a dielectric protection layer for a recording type optical disc such as an inorganic additional recording type disc, a phase change disc and an optical magnetic disc, and a protection film for a flat panel display.