Abstract:
In a multilayer optical recording medium having three or more recording layers, when data is recorded on a recording layer far away from an incident surface of laser light, and especially on a recording layer farthest away from the incident surface of the laser light, by applying the laser light, the recording power margin of the farthest recording layer allows for a variation in the optimal recording sensitivity even if the transmittance of the recording layers located between the laser light incident surface side and the target recording layer is varied due to existing recordings and the amount of the laser light passing therethrough is also varied. In this case, the recording layer which is the farthest from an incident surface of laser light has a phase change recording film, and the recording layer which is the nearest from the incident surface of the laser light has a write-once read-multiple recording film.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multilayer optical recording medium having three or more recording layers and a method for recording information on the multilayer optical recording medium. 
     2. Description of the Related Art 
     Optical recording media such as CDs (Compact Discs) and DVDs (Digital Versatile Discs) are widely utilized as information recording media. Furthermore, some types of optical recording media which utilize a blue or blue-violet laser as the irradiation light to store more information than ever receives attention in recent years. 
     The optical recording media are generally classified into three types: a ROM (Read Only Memory) type, which can neither record nor rewrite data, a RW (Rewritable) type, which can rewrite data, and an R (Recordable) type, which can record data only once. 
     In the R-type and RW-type optical recording media, a recording layer is irradiated with laser light and a recording mark which has a reflectivity different from that of surrounding spaces is formed, thereby allowing for the recording of data. Generally in the optical recording medium, the recording layer is irradiated with laser light and a photodetector then measures the difference in reflectance between the recording mark and the space section to reproduce data. 
     Such optical recording media can have an increased storage capacity if a plurality of recording layers is utilized. For example, in R-type or RW-type optical recording media that have a plurality of recording layers, data can be selectively recorded on a target recording layer when a recording laser beam is focused onto that target recording layer. The data can therefore be selectively reproduced from that target recording layer alone when the reproducing laser beam is focused onto that target recording layer. Furthermore, in a multilayer recording medium having three or more recording layers, it is preferable that the extinction coefficient of each recording layer except for the L 0  layer, through which laser light does not need to pass because the L 0  layer is the layer farthest away from the recording layer, be 0.5 or less for the wavelength of the laser light used with regard to both transmittance and recording sensitivity. 
     As described above, in a multilayer optical recording medium, the focus of the laser light is adjusted onto the target recording layer when recording and reproducing data. In such cases, the laser light passes through the recording layers lying on the side of the incident surface of the laser light and onto the target recording layer. It is preferable that transmittance should be constant irrespective of the recording conditions of the recording layers lying between the incident surface and the target recording layer. 
     In actual fact, however, when data is stored on the recording layer on the incident surface side of the target recording layer, the recording mark area formed varies in both reflectivity, as described above, and transmittance. The inventors discovered that since the amount of laser light passing through a recording mark area of varying transmittance was different from the amount of laser light passing through an area other than the recording mark area, the amount of laser light (being the amount of transmittance) reaching the target recording layer varied and hence the optimal recording sensitivity of the target recording layer also varied. This problem is especially noticeable in a multilayer optical recording medium having three or more recording layers. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, various exemplary embodiments of this invention provide a multilayer optical recording medium having three or more recording layers and to provide a method for recording information on the multilayer optical recording medium in which, when data is recorded on a recording layer far away from an incident surface of laser light, and especially on a recording layer farthest away from the incident surface of the laser light, by applying the laser light, the recording power margin of the farthest recording layer allows for a variation in the optimal recording sensitivity even if the transmittance of the recording layers located between the laser light incident surface side and the target recording layer is varied due to existing recordings and the amount of the laser light passing therethrough is also varied. 
     As a result of thorough investigations, the inventors have found that, in a multilayer optical recording medium having three or more recording layers, the number of recording layers through which light needs to pass through is not proportional to the ratio of variation in the transmittance of laser light before and after recording on the recording layer through which light needs to pass. Specifically, when the number of recording layers of a multilayer optical recording medium increases from 2 to 3 or to 5, the number of recording layers through which light needs to pass increases from 1 to 2 or 4, but the ratio of variation in transmittance before and after recording becomes greater than two or four times. 
     The inventors have therefore manufactured a multilayer optical recording medium having five recording layers in total using Si/Cu as the recording film material for the L 0  layer and Bi—Ge—O as the recording film material for the L 1  to L 4  layers. The L 0  layer has a phase change recording film and the L 1  to L 4  layers have a write-once read-multiple recording film. 
     A description of how to calculate the ratio of variation in the transmittance of each recording layer will now be described. 
     Pwa represents the optimal recording power at which jitter occurring during reproduction is minimized when information is recorded on a target recording layer where data has been recorded on all recording layers located between the laser light incident surface side and the target recording layer (the recording layer on which an attempt is made to record and reproduce data) of the multilayer optical recording medium, and Ta represents the rate of an amount of light passing through from the incident surface of the laser light to just before the target recording layer. Pwb represents the optimal recording power at which jitter occurring during reproduction is minimized when information is recorded on the target recording layer where data has not been recorded on any recording layer located between the laser light incident surface side and the target recording layer, and Tb represents the rate of an amount of light passing through from the incident surface of the laser light to just before the target recording layer. 
     Tb/Ta represents the ratio of variation in cumulative transmittance between the case where information has been recorded on all recording layers located between the laser light incident surface side and the target recording layer and the case where information has not been recorded at all. 
     In the case where information has not been recorded on any recording layer which is not adjacent to a target recording layer on the laser light incident surface side, when information is recorded on a recording layer adjacent to the target recording layer on the laser light incident surface side, Ta′ represents the rate of an amount of light passing through the recording layer. When information is not recorded on the recording layer adjacent to the target recording layer on the laser light incident surface side, Tb′ represents the rate of an amount of light passing through the recording layer. Tb′/Ta′ represents the ratio of variation in transmittance of only the recording layer adjacent to the target recording layer in the case where information is recorded on the recording layer adjacent to the target recording layer on the laser light incident surface side and in the case where information is not recorded on the recording layer adjacent to the target recording layer on the laser light incident surface side. 
     The different in optimal recording power between Pwa and Pwb is caused by a difference in the amount of light reaching the target recording layer. When Pw represents the optimal recording power in the case where the transmittance of light reaching the target recording layer is set at 100%, Tb/Ta can be calculated by the following equation.
 
 Pw=Pwa*Ta, Pw=Pwb*Tb  
 
 Tb/Ta=Pwa/Pwb  
 
     It should also be noted that when Ta 0 , Tb 0 , Pwa 0 , and Pwb 0  represent Ta, Tb, Pwa, and Pwb in an L 0  layer, respectively, Tb 0 /Ta 0 =Pwa 0 /Pwb 0  holds. Furthermore, the ratio of Tb 1 /Ta 1  in an L 1  layer is similarly Tb 1 /T 1 =Pwa 1 /Pwb 1 . In a medium having five layers of recording films, the same holds true for Ta 2  to Ta 4  and Tb 2  to Tb 4 . However, Ta 4 , Tb 4 , Ta 4 ′, and Tb 4 ′ become 1 (i.e., a transmittance of 100%) as there is no other recording layer on the light incident surface side upper than an L 4  layer. 
     Since Ta 0  and Tb 0  represent the rate of an amount of light passing through from the incident surface of the laser light to just before the L 0  layer, the following equations hold when there are five recording layers. In this instance, Ta 0 ′ to Ta 4 ′ represent Ta′ at the L 0  to L 4  layers, respectively, and similarly Tb 0 ′ to Tb 4 ′ represent Tb′. Therefore:
 
 Ta   0   =Ta   0   ′*Ta   1   ′ . . . *Ta   4 ′
 
 Tb   0   =Tb   0   ′*Tb   1   ′ . . . *Tb   4 ′
 
This also holds for Ta 1  and Tb 1 ,
 
 Ta   1   =Ta   1   ′*Ta   2 ′ . . . *Ta 4 ′
 
 Tb   1   =Tb   1   ′*Tb   2 ′ . . . *Tb 4 ′
 
Ta 2  to Ta 4  and Tb 2  to Tb 4  can be similarly expressed. Expanding these expressions gives:
 
 Tb   0   ′/Ta   0 ′=( Tb   0   /Ta   0 )/( Tb   1   /Ta   1 )
 
wherein Tb 0 /Ta 0  and Tb 1 /Ta 1  are equal to Pwa 0 /Pwb 0  and Pwa 1 /Pwb 1 , respectively, and therefore it is possible to derive Tb 0 ′/Ta 0 ′ from each optimal recording power.
 
     Accordingly, Tb′/Ta′ for all the recording layers except for the L 0  layer can be similarly calculated. Since Tb/Ta=Pwa/Pwb is able to be used as described above, it is possible to easily calculate the ratio of variation in the transmittance of each recording layer by measuring the optimal recording power. 
     By calculating the ratio of variation in transmittance like this, it became apparent that the transmittance of the L 0  layer did not vary significantly when data had been recorded on the L 2  to L 4  layers. When data had been recorded on the L 1  to L 4  layers, however, the transmittance of the L 0  layer varied significantly. Table 1 shows Pwa, Pwb, Tb/Ta, and Tb′/Ta′ for each recording layer. Since the L 4  layer is located at the position nearest to the incident surface of the laser light and the laser light which has passed through another layer is not applied thereto, Pwa is equal to Pwb and hence Tb/Ta and Tb′/Ta′ are “1.00.” 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Recording 
                 Recording 
                   
                   
                   
                   
               
               
                 layer 
                 material 
                 Pwa (mW) 
                 Pwb (mW) 
                 Tb/Ta 
                 Tb′/Ta′ 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 L 4   
                 Bi—Ge—O 
                 9 
                 9 
                 1.00 
                 1.00 
               
               
                 L 3   
                 Bi—Ge—O 
                 9.4 
                 9.5 
                 0.99 
                 0.99 
               
               
                 L 2   
                 Bi—Ge—O 
                 8.8 
                 9.1 
                 0.97 
                 0.98 
               
               
                 L 1   
                 Bi—Ge—O 
                 8.4 
                 8.9 
                 0.94 
                 0.98 
               
               
                 L 0   
                 Si/Cu 
                 8.8 
                 9.8 
                 0.90 
                 0.95 
               
               
                   
               
             
          
         
       
     
     As a comparative example, Table 2 shows Pwa, Pwb, Tb/Ta, and Tb′/Ta′ for each recording layer in an optical recording medium  100  having two recording layers, as shown in  FIG. 7 . As in the case of the L 4  layer of Table 1, an L 0  layer is located at the position nearest to the incident surface of the laser light and the laser light which has passed through the other layer is not applied thereto, so that Pwa is equal to Pwb and hence Tb/Ta and Tb′/Ta′ are “1.00.” 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Recording 
                 Recording 
                   
                   
                   
                   
               
               
                 layer 
                 material 
                 Pwa (mW) 
                 Pwb (mW) 
                 Tb/Ta 
                 Tb′/Ta′ 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 L 1   
                 Bi—Ge—O 
                 9 
                 9 
                 1.00 
                 1.00 
               
               
                 L 0   
                 Bi—Ge—O 
                 9.4 
                 9.5 
                 0.99 
                 0.99 
               
               
                   
               
             
          
         
       
     
     The optical recording medium  100  is provided with two recording layers, being an L 0  layer  102  and an L 1  layer  104 . 
     The other structures of the multilayer optical recording medium shown in  FIG. 7  are denoted with the same reference numbers as detailed in the first exemplary embodiment (to be described later) and a description thereof has been omitted here. 
     In the L 0  layer  102 , Bi—Ge—O (22:10:68 mol %) was used as the recording film material. Dielectric layers, each being made of TiO 2  with a thickness of 15 nm, were provided on either side of the Bi—Ge—O layer, and the Bi—Ge—O layer had a thickness of 40 nm. In the L 1  layer  104 , Bi—Ge—O (22:11:67 mol %) was used as the recording film material. Dielectric layers, each being made of TiO 2  with a thickness of 15 nm, were provided on either side of the Bi—Ge—O layer, and the Bi—Ge—O layer had a thickness of 43 nm. The thickness of the spacer layer was 15 μm. The L 0  layer  102  and the L 1  layer  104  have write-once read-multiple recording films. 
     With reference to Table 1 and Table 2, it has been found that the transmittance hardly varies in the optical recording medium  100  that includes two recording layers, but the transmittance varies significantly in the multilayer optical recording medium having five recording layers. 
     Pwa and Pwb were measured using an optical disc evaluator (ODU-1000). The recording strategy was optimized on a recording layer basis, and a random signal was used as the recording signal. There were three recording powers used and the laser power was 0.5 mW. 
     In the measurement of the optimal recording power for calculating Pwa and Pwb, the same recording strategy was used and the ratio of intermediate recording power/recording power was kept constant. The recording and reproduction at this time were performed with a linear velocity of 4.9 m/s. 
     It should be noted that transmittance varies in a recording mark area even in the same recording layer, but transmittance does not vary in areas other than the recording mark area. When data is recorded on the L 0  layer, the amount of light reaching the L 0  layer varies between the case where data has not been recorded at all on an area on which an attempt has been made to form recording marks and which corresponds to the L 1  to L 4  layers, and the case where data has been recorded thereon. Thus, it is impossible to adequately record data using the same conditions in both cases, however, using a recording film which allows for variation in transmittance before and after recording can allow for a deviation in recording sensitivity caused by the presence or absence of recordings. 
     In a general write-once read-multiple recording film, a recording power margin is approximately 25% of the optimal recording power and jitter becomes 10% or less. In a rewritable recording film, however, the recording power margin is approximately 50% of the optimal recording power and jitter becomes 10% or less. Therefore, it is possible to record data even if there is large variation in transmittance. 
     In general write-once read-multiple recording films, recording is an irreversible reaction, and the recording power margin is narrow because the size of a recording mark increases in proportion to the strength of the recording power of the laser light used. In a phase change recording film, however, recording marks are amorphous. Since amorphousness is formed when a molten area of material is cooled at a rate that is faster than a critical cooling rate specific to that material, it is possible to form recording marks of the same size even if the size of the molten area or the cooling rate is varied. Specifically, if the same recording strategy is used when the recording power is relatively low, virtually all the molten area becomes amorphous so that a recording mark with a predetermined length is formed. When the recording power is relatively high, the cooling rate decreases in some parts of the molten area and only a part of the molten area becomes amorphous in order to form a recording mark with a predetermined length. 
     As described above, in the case of phase change recording where recording marks are amorphous and space sections are crystalline, it is possible to form recording marks with the same predetermined length even if the recording power is relatively high or low, and hence the recording power margin is wide. 
     It can be confirmed that the recording marks are amorphous and spaces (blanks) are crystalline from the fact that, for example, an electron diffraction image of a recording mark section produced by a TEM (transmission electron microscope) forms a halo pattern and an electron diffraction image of a space section (blank section) forms a spot or a ring which is derived from its crystal structure. The above matter can be also confirmed by use of a probe scanning electron microscope which can measure surface potential and the like by the fact that the electrical potential of the mark section is high and the electrical potential of the space section (being the blank section) is low. 
     In the present application, amorphousness and crystallinity are determined by an electron diffraction method using a TEM. 
     Also in the present application, the write-once read-multiple recording film is defined as a recording film on which data cannot be rewritten using the available recording linear velocity and laser light with its associated recording power. The phase change recording film is defined as a recording film in which a space section (blank section) is crystalline and an amorphous recording mark is formed using laser light with a particular recording power. 
     In the present application, the term “blank” should be understood to refer to a not-recorded area. The term “space” should be understood to refer to a section between recording marks when the recording marks are formed by recording laser light. 
     As a result of intensive investigations, the inventors have found that in a multilayer optical recording medium with three or more recording layers in which at least the recording layer nearest to an incident surface of laser light has a write-once read-multiple recording film, when data is recorded on the recording layer located farthest away from the incident surface of the laser light by applying the laser light, even if transmittance is varied by recordings occurring on the recording layers located between the farthest recording layer and the laser light incident surface side, using a phase change recording film in at least an L 0  layer can allow for a variation in recording sensitivity by a variation in transmittance according to the recording power margin of the farthest recording layer. 
     Accordingly, various exemplary embodiments of this invention provide the followings: 
     (1) A multilayer optical recording medium comprising three or more recording layers of an L 0  layer, . . . , an L n-2  layer, and an L n-1  layer which includes a write-once read-multiple recording film on a side farthest away from an incident surface of laser light to a side nearest to the incident surface of laser light, wherein each recording layer is irradiated with the laser light and a recording mark is formed in order to record information, wherein at least the L 0  layer has a phase change recording film. 
     (2) The multilayer optical recording medium according to (1), wherein the L 0  layer is rewritable at an available recording linear velocity. 
     (3) The multilayer optical recording medium according to (1), wherein the L 0  layer is a write-once read-multiple recording layer on which information is not rewritable at an available recording linear velocity. 
     (4) The multilayer optical recording medium according to (1), wherein a composition of a material forming the phase change recording film of the L 0  layer is adjusted so that information recorded on the L 0  layer is not rewritable at an available recording linear velocity. 
     (5) The multilayer optical recording medium according to (1), wherein a structure of the L 0  layer is adjusted so that information recorded on the L 0  layer is not rewritable at an available recording linear velocity. 
     (6) The multilayer optical recording medium according to one of (1) to (5), wherein the L 1  layer to the L n-2  layer each have a write-once read-multiple recording film. 
     (7) The multilayer optical recording medium according to one of (1) to (5), wherein an extinction coefficient of the recording film of the L n-1  layer at the wavelength of the laser light is 0.5 or less. 
     (8) The multilayer optical recording medium according to (6), wherein an extinction coefficient of the recording film of the L n-1  layer at the wavelength of the laser light is 0.5 or less. 
     (9) A method for recording information on a multilayer optical recording medium, the multilayer optical recording medium including three or more recording layers of an L 0  layer, . . . , an L n-2  layer, and an L n-1  layer which includes a write-once read-multiple recording film on a side farthest away from an incident surface of laser light to a side nearest to the incident surface of laser light, at least the L 0  layer having a phase change recording film, wherein each recording layer is irradiated with the laser light and a recording mark is formed in order to record information, wherein when information is recorded again on the L 0  layer on which a recording mark has already been formed, the L 0  layer is irradiated with the laser light having erasing power to erase or attenuate the recording mark and then a recording mark is newly formed. 
     (10) A method for recording information on a multilayer optical recording medium, the multilayer optical recording medium including three or more recording layers of an L 0  layer, . . . , an L n-2  layer, and an L n-1  layer which includes a write-once read-multiple recording film on a side farthest away from an incident surface of laser light to a side nearest to the incident surface of laser light, at least the L 0  layer having a phase change recording film, wherein each recording layer is irradiated with the laser light and a recording mark is formed in order to record information, wherein when information is recorded again on the L 0  layer on which a recording mark has already been formed, the recording mark is erased concurrently with a recording mark being newly formed. 
     (11) A method for recording information on a multilayer optical recording medium, the multilayer optical recording medium including three or more recording layers of an L 0  layer, . . . , an L n-2  layer, and an L n-1  layer which includes a write-once read-multiple recording film on a side farthest away from an incident surface of laser light to a side nearest to the incident surface of laser light, at least the L 0  layer having a phase change recording film, wherein each recording layer is irradiated with the laser light and a recording mark is formed in order to record information, wherein when information is recorded on the L 0  layer, a recording linear velocity different from that when recording on the recording layers other than the L 0  layer is used to form a not-rewritable recording mark on the L 0  layer. 
     (12) A method for recording information on a multilayer optical recording medium, the multilayer optical recording medium including three or more recording layers of an L 0  layer, . . . , an L n-2  layer, and an L n-1  layer which includes a write-once read-multiple recording film on a side farthest away from an incident surface of laser light to a side nearest to the incident surface of laser light, at least the L 0  layer having a phase change recording film, wherein each recording layer is irradiated with the laser light and a recording mark is formed in order to record information, wherein when information is recorded on the L 0  layer, a recording linear velocity different from that when recording on the recording layers other than the L 0  layer is used to form a rewritable recording mark on the L 0  layer. 
     According to the present invention, use of a phase change recording film in a recording layer can result in a multilayer optical recording medium becoming rewritable. Whether a multilayer optical recording medium is rewritable or not is determined by adjusting the data transfer rate of the system for recording information on the multilayer optical recording medium (i.e., adjusting the rotational speed and recording linear velocity of the multilayer optical recording medium), and the composition of the recording film material used for the phase change recording film or the structure of the recording film. 
     This is determined as follows. 
     By irradiating a phase change recording film with laser light, a change of phase between amorphousness and crystallinity occurs. The amorphous section becomes a recording mark and the crystalline section becomes a space. If the amorphous section is held at a predetermined temperature for a predetermined time, a phase change occurs and the amorphous section becomes crystalline. Based on this principle, in order to erase information which has been recorded on the phase change recording film, the amorphous section, being the recording mark, is irradiated with laser light of a predetermined recording power (corresponding to temperature) for a predetermined time and the amorphous section is turned into a crystalline region, being a space, by the phase change. Whether information is erasable or not is determined by the erasing power of the applied laser light, the irradiation time, and the crystalline temperature and speed of phase change of the amorphous section. Of these indexes, the irradiation time is determined by the recording linear velocity which is defined as an information transfer rate for recording information, and the crystalline temperature and speed of phase change of the amorphous section is determined by the composition of the recording film material and the structure of the recording film. Since the erasing power of the laser light and the recording linear velocity are determined by the specifications of the system, whether information having been recorded on a multilayer optical recording medium with a phase change recording film is erasable or not depends on the composition of the recording file material and the structure of the recording film. 
     Specifically, by adopting a recording film material with a composition having a low crystallization speed or a high crystallization temperature, a structure of the recording layer which prevents a sufficient crystallization temperature or time due to a high cooling effect from being obtained, or a recording film structure in which a plurality of recording film materials are mixed and the composition of this mixed section has a low crystallization speed or a high crystallization temperature, if the recording layer is designed in such a manner that the amorphous section cannot be crystallized with the erasing power of the laser light and the recording linear velocity determined by the specifications of the system, it becomes impossible to rewrite information. If a recording layer is instead designed opposite to that detailed above, it then becomes possible to rewrite information. 
     Conversely, from the viewpoint of a recording method, even if a recording film material with a high crystallization speed or a recording layer which has a structure which can obtain a sufficient crystallization temperature or time is adopted, for example, further increasing the recording linear velocity can realize a write-once read-multiple recording method which is not rewritable. Even if a recording film material with a low crystallization speed or a recording layer with a structure which cannot obtain a sufficient crystallization temperature or time is adopted, lowering the recording linear velocity or newly forming a recording mark after erasing or attenuating a recording mark by the irradiation of laser light with the erasing power can make the recording layer rewritable. 
     According to the present invention, as described above, it is possible to make a recording layer a distance away from the incident surface of the laser light, and especially the recording layer farthest away from the incident surface of the laser light either rewritable or a write-once read-multiple type. An optical recording medium in which all recording layers are the write-once read-multiple type is suitable for saving files requiring perfect accuracy because the information stored cannot be tampered with, and this type of optical recording medium is often used for saving medical files or government agencies files or the like. Using an optical recording medium with a rewritable recording layer introduces the possibility of tampering with stored information, but changing the composition of the recording film material or the dielectric material makes it impossible to tamper with or add information. Such an optical recording medium therefore becomes applicable for saving files that require perfect accuracy, so that it is possible to extend the application of such to the above listed fields and the like. 
     In the present application, the phrase “recording power margin” is expressed by the following equation in which Pw min  represents the lowest value and Pw max  represents the highest value of the recording power of laser light with jitter of 10% or less when information is recorded on the recording layer farthest away from the incident surface of the laser light in the case where data has been recorded on all recording layers except for the recording layer farthest away from the incident surface of the laser light or in the case where data has not been recorded on any of the recording layers.
 
( Pw   max   −Pw   min )/( Pw   max   +Pw   min )×2×100 (%)
 
     Therefore, the greater the difference between the highest value and the lowest value of the recording power of the laser light with jitter of 10% or less, the greater the recording power margin becomes. 
     The allowable recording power margin is defined as 22% or more within the confines of jitter of 10% or less irrespective of whether transmittance varies or not. 
     According to the present invention, it is possible to provide a multilayer optical recording medium having three or more recording layers and a method for recording data on the multilayer optical recording medium where fine recording marks can be formed on a recording layer farthest away from the incident surface of laser light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view schematically showing the whole structure of a multilayer optical recording medium according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a cross-sectional side view schematically showing the whole structure of a multilayer optical recording medium according to a fifth exemplary embodiment of the present invention; 
         FIG. 3  is a cross-sectional side view schematically showing the whole structure of a multilayer optical recording medium according to a sixth exemplary embodiment of the present invention; 
         FIG. 4  is a graph showing the relationship between the recording power of laser light and jitter when data is recorded on an L 0  layer of the multilayer optical recording medium according to the first exemplary embodiment of the present invention; 
         FIG. 5  is a graph showing the relationship between the recording power of laser light and jitter when data is recorded on an L 0  layer of the multilayer optical recording medium according to a second exemplary embodiment of the present invention; 
         FIG. 6  is a cross-sectional side view schematically showing the whole structure of a multilayer optical recording medium according to a comparative example 1 of the present invention; 
         FIG. 7  is a cross-sectional side view which schematically shows the whole structure of a conventional multilayer optical recording medium having two recording layers; and 
         FIG. 8  is a graph showing the relationship between the recording power of laser light and jitter when data is recorded on an L 0  layer of the multilayer optical recording medium according to the comparative example 1 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A multilayer optical recording medium according to the best mode of the present invention has three or more recording layers such as an L 0  layer, an L 1  layer, . . . , and an L n-1  layer which includes a write-once read-multiple recording film on the side farthest away from the laser light incident surface side to the side nearest to the laser light incident surface side. Each recording layer is irradiated with the laser light and recording marks are formed in order to record information. The L 0  layer at least has a phase change recording film in which amorphous recording marks and crystalline spaces are formed by irradiation of the laser light. 
     First Exemplary Embodiment 
     A multilayer optical recording medium  10  according to the first exemplary embodiment of the present invention will now be described in detail with reference to  FIG. 1 . In the recording layers of the multilayer optical recording medium according to the various exemplary embodiments of the present application, all deposition of materials is carried out using sputtering. 
     The multilayer optical recording medium  10  is in the shape of a disc with an outside diameter of 120 mm and a thickness of approximately 1.2 mm. The multilayer optical recording medium  10  comprises a substrate  32 , a first recording layer (an L 0  layer)  12  disposed on the substrate  32  on the side (the upper side in  FIG. 1 ) of an incident surface  22  of laser light (indicated with an arrow in  FIG. 1 ), a second recording layer (an L 1  layer)  14  disposed nearer to the incident surface  22  of the laser light than the L 0  layer  12 , a third recording layer (an L 2  layer)  16  disposed closer to the incident surface  22  of the laser light than the L 1  layer  14 , a fourth recording layer (an L 3  layer)  18  disposed closer again to the incident surface  22  of the laser light than the L 2  layer  16 , and a fifth recording layer (an L 4  layer)  20  disposed yet closer again to the incident surface  22  of the laser light than the L 3  layer  18 . Spacer layers  36  are provided between the L 0  layer  12  and the L 1  layer  14 , between the L 1  layer  14  and the L 2  layer  16 , between the L 2  layer  16  and the L 3  layer  18 , and between the L 3  layer  18  and the L 4  layer  20 . A light transmission layer  34  is provided on the L 4  layer  20  on the side closest to the incident surface  22  of the laser light. 
     The substrate  32  has a thickness of approximately 1.1 mm. A concavo-convex pattern for composing grooves is formed in the surface of the substrate  32  and on the side of the light transmission layer  34 . The term “groove” generally refers to a concave portion used for the recording and reproducing of data. In the present application, however, the term “groove” is used even if the section used for the recording or reproducing of data is a convex portion that protrudes toward the side of the light transmission layer  34 . In the first exemplary embodiment, the convex section protruding toward the side of the light transmission layer  34  corresponds to a groove. The substrate  32  may be made of, for example, a polycarbonate resin, an acrylic resin, an epoxy resin, a polystyrene resin, a polypropylene resin, a silicon resin, a fluorocarbon resin, an ABS resin, an urethane resin, glass, and the like, or various kinds of metal in the instance where light does not need to pass through the substrate. 
     The light transmission layer  34  has a thickness in the range of, for example, 30 to 150 μm. The light transmission layer  34  may be made of, for example, a transparent energy beam-curable resin such as an acrylic UV-curable resin or an epoxy UV-curable resin. The term “energy beam” is used to collectively refer to an electromagnetic wave such as, for example, ultraviolet rays, electron beams or corpuscular rays that have the characteristic of curing a specific liquid resin. The light transmission layer  34  may be formed in such a manner that either the liquid resin is applied to the substrate and is then cured by an energy beam, or a previously manufactured transparent film may be adhered onto the substrate. 
     The spacer layer  36  has a thickness in the range of, for example, approximately 5 to 90 μm. Both surfaces of the spacer layer  36  have a concavo-convex pattern with grooves which correspond to those of the substrate  32 . The spacer layer  36  is made of, for example, a transparent energy beam-curable resin such as an acrylic UV-curable resin or an epoxy UV-curable resin, as for the light transmission layer  34 . 
     The L 0  layer  12  is formed into a concavo-convex pattern that corresponds to the concavo-convex pattern of the substrate  32 . The L 1  layer  14 , the L 2  layer  16 , the L 3  layer  18 , and the L 4  layer  20  are formed into a concavo-convex pattern also corresponding to the concavo-convex pattern of the spacer layer  36 . 
     The structure of each recording layer will now be described. 
     In the L 0  layer  12 , SbTeGe (75:20:5 mol %) was used as the recording film material. Dielectric layers made of a mixture of ZnS and SiO 2  were provided on both sides of the SbTeGe layer, where the SbTeGe layer had a thickness of 12 nm. The dielectric layer on the light transmission layer  34  side of the L 0  layer  12  is composed of a ZnS:SiO 2  (80:20 mol %) film having a thickness of 35 nm, and the dielectric layer on the substrate  32  side of the L 0  layer  12  is composed of a ZnS:SiO 2  (50:50 mol %) film having a thickness of 13 nm. A reflective layer made of an alloy of Ag, Pd, and Cu (98:1:1 mol %) having a thickness of 100 nm was provided between the dielectric layer located on the substrate  32  side of the L 0  layer  12  and the substrate  32  itself. 
     In the L 1  layer  14 , the L 2  layer  16 , the L 3  layer  18 , and the L 4  layer  20 , Bi—Ge—O was used as the recording film material. Dielectric layers made of TiO 2  were provided on both sides of the Bi—Ge—O layer of each recording layer. 
     Table 3 shows the thicknesses of the TiO 2 /Bi—Ge—O/TiO 2  layers in the L 1  layer  14  to the L 4  layers  20  and the composition of Bi—Ge—O (recording film). 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Composition (mol %) 
               
             
          
           
               
                   
                 Thickness (nm) 
                 Bi 
                 Ge 
                 O 
               
               
                   
                   
               
             
          
           
               
                   
                 L 4   
                 TiO 2   
                 15 
                 20 
                 13 
                 67 
               
               
                   
                   
                 Bi—Ge—O 
                 43 
               
               
                   
                   
                 TiO 2   
                 15 
               
               
                   
                 L 3   
                 TiO 2   
                 15 
                 22 
                 11 
                 67 
               
               
                   
                   
                 Bi—Ge—O 
                 40 
               
               
                   
                   
                 TiO 2   
                 15 
               
               
                   
                 L 2   
                 TiO 2   
                 14 
                 22 
                 10 
                 68 
               
               
                   
                   
                 Bi—Ge—O 
                 38 
               
               
                   
                   
                 TiO 2   
                 14 
               
               
                   
                 L 1   
                 TiO 2   
                 10 
                 28 
                 2 
                 70 
               
               
                   
                   
                 Bi—Ge—O 
                 34 
               
               
                   
                   
                 TiO 2   
                 10 
               
               
                   
                   
               
             
          
         
       
     
     The thicknesses of the spacer layers  36  were 15 μm, 21 μm, 13 μm, and 15 μm on the substrate  32  side of each recording layer, and the thickness of the light transmission layer  34  was 40 μm. The L 0  layer  12  has a phase change recording film and the L 1  to L 4  layers  14  to  20  have a write-once read-multiple recording film. 
     In the present embodiment, the recording power Pwa 0  of the laser light used when data was recorded once on the L 0  layer  12  where data had been recorded on all recording layers from the L 4  layer  20  to the L 1  layer  14  in the multilayer optical recording medium  10 , the recording power Pwb 0  of the laser light when data was recorded once on the L 0  layer  12  where data had not been recorded on any of the recording layer from the L 4  layer  20  to the L 1  layer  14 , and jitter were evaluated. The wavelength λ of the recording laser light was 405 nm. The NA (numerical aperture) of the objective lens was 0.85, and the recording was performed with a linear velocity of 4.9 m/s. 
     Table 4 and  FIG. 4  show results. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Pwa 0   
                 Jitter 
                 Pwb 0   
                 Jitter 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 7.5 
                 20.0 
                 8.5 
                 18.5 
               
               
                   
                 8 
                 7.5 
                 9 
                 9.3 
               
               
                   
                 9 
                 5.9 
                 10 
                 6.5 
               
               
                   
                 10 
                 5.5 
                 11 
                 6.0 
               
               
                   
                 11 
                 5.9 
                 12 
                 6.3 
               
               
                   
                 12 
                 6.8 
                 13 
                 7.5 
               
               
                   
                 13 
                 8.3 
                 14 
                 8.9 
               
               
                   
                 14 
                 10.2 
                 15 
                 10.4 
               
               
                   
                 15 
                 12.5 
                 16 
                 11.5 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 4 and  FIG. 4 , when the recording power of the laser light is in the range of 8.8 mW to 13.8 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  layer  20  to the L 1  layer  14 . The recording power margin of the L 0  layer  12  of the present embodiment was 44% of the optimal recording power. 
     The L 4  layer  20  was deposited on a substrate without a groove using ETA-RA (made by STEAG ETA-Optik GmbH Co.) and had an extinction coefficient of 0.08 with a laser light having a wavelength of 405 nm. 
     Second Exemplary Embodiment 
     In the present exemplary embodiment, a multilayer optical recording medium being identical to that of the first exemplary embodiment was used. The recording power Pwa 0  of the laser light used when data was recorded ten times on an L 0  layer where data had been recorded on all recording layers of an L 4  layer to an L 1  layer in the multilayer optical recording medium, the recording power Pwb 0  of the laser light when data was recorded ten times on the L 0  layer where data had not been recorded on any recording layer of the L 4  layer to the L 1  layer, and jitter were evaluated as in the case of the first embodiment. The recording linear velocity was 4.9 m/s. When rewriting a recording mark, a new recording mark was formed concurrently with the erasing of an old recording mark. 
     Table 5 and  FIG. 5  show the results. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Pwa 0   
                 Jitter 
                 Pwb 0   
                 Jitter 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 7 
                 20.0 
                 8 
                 19.0 
               
               
                   
                 7.5 
                 11.0 
                 8.5 
                 9.8 
               
               
                   
                 8 
                 6.6 
                 9 
                 6.3 
               
               
                   
                 9 
                 5.5 
                 10 
                 5.7 
               
               
                   
                 10 
                 5.9 
                 11 
                 6.2 
               
               
                   
                 11 
                 7.3 
                 12 
                 7.5 
               
               
                   
                 12 
                 9.6 
                 13 
                 9.5 
               
               
                   
                 13 
                 11.8 
                 14 
                 11.6 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 5 and  FIG. 5 , when the recording power of the laser light is in the range of 8.5 mW to 12.2 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  to L 1  layers. The recording power margin of the L 0  layer of the present exemplary embodiment was 36% of the optimal recording power. 
     According to the present exemplary embodiment, if information was rewritten, it was then clear that the recording power margin was large enough even. 
     Third Exemplary Embodiment 
     In the present exemplary embodiment, a multilayer optical recording medium being identical to that of the first exemplary embodiment was used. The recording power Pwa 0  of the laser light used when data was recorded a thousand times on an L 0  layer where data had been recorded on all recording layers from an L 4  layer to an L 1  layer in the multilayer optical recording medium, the recording power Pwb 0  of the laser light when data was recorded a thousand times on the L 0  layer where data had not been recorded on any recording layer from the L 4  layer to the L 1  layer, and jitter were evaluated as in the case of the first exemplary embodiment. The recording linear velocity was 4.9 m/s. When rewriting a recording mark, a new recording mark was formed concurrently with the erasing of an old recording mark. 
     When the recording power of the laser light is in the range of 8.5 mW to 12.0 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  to L 1  layers. The recording power margin of the L 0  layer of this embodiment was 34% of the optimal recording power. 
     According to the present exemplary embodiment, if information was rewritten, it was then clear that the recording power margin was large enough. 
     Fourth Exemplary Embodiment 
     In the present exemplary embodiment, a multilayer optical recording medium being identical to that of the first exemplary embodiment was used. The recording power Pwa 0  of the laser light when data was recorded once on an L 0  layer where data had been recorded on all recording layers of an L 4  layer to an L 1  layer in the multilayer optical recording medium, the recording power Pwb 0  of the laser light when data was recorded once on the L 0  layer where data had not been recorded on any recording layer of the L 4  layer to the L 1  layer, and jitter were evaluated as in the case of the first exemplary embodiment. The recording linear velocity was 14.7 m/s. 
     When the recording power of the laser light is in the range of 12.3 mW to 19.0 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  to L 1  layers. The recording power margin of the L 0  layer of the present exemplary embodiment was 43% of the optimal recording power. 
     When overwriting recorded data with a signal used for rewriting, jitter could not be measured. An 8T signal with a single frequency was recorded on this medium with a recording linear velocity of 14.7 m/s and a DC laser light of 8.4 mW was then applied to measure the rate of erasure, being the rate of crystallization. The rate of erasure was lower than 25 dB required in order to allow the medium to then be rewritable, and it was established that rewriting was impossible because erasure (crystallization) was insufficient when this composition was used with this recording linear velocity. 
     Fifth Exemplary Embodiment 
     A multilayer optical recording medium  40  according to a fifth exemplary embodiment will be now described in detail with reference to  FIG. 2 . 
     As with the multilayer optical recording medium  10  according to the first exemplary embodiment, the multilayer optical recording medium  40  is provided with five recording layers, that is, an L 0  layer  42 , an L 1  layer  44 , an L 2  layer  46 , an L 3  layer  48 , and an L 4  layer  50 . The same reference numbers as used in the first exemplary embodiment are used for the other structures of the multilayer optical recording medium, and an explanation thereof will be omitted here. 
     The L 0  layer  42 , the L 1  layer  44 , the L 2  layer  46 , the L 3  layer  48 , and the L 4  layer  50  are arranged in this order in a direction from a substrate  32  to an incident surface  22  of laser light. Spacer layers  36  are provided between the L 0  layer  42  and the L 1  layer  44 , between the L 1  layer  44  and the L 2  layer  46 , between the L 2  layer  46  and the L 3  layer  48 , and between the L 3  layer  48  and the L 4  layer  50 . A light transmission layer  34  is provided on the L 4  layer  50  on the side closest to the incident surface  22  of the laser light. 
     The multilayer optical recording medium  40  is different from the multilayer optical recording medium  10  according to the first embodiment only in the structure of the L 0  layer, and the structure of the L 1  to L 4  layers are the same. 
     In the L 0  layer  42 , SbTeGe (70:20:10 mol %) was used as the recording film material. Dielectric layers, each being made of a mixture of ZnS and SiO 2 , were provided on either side of the SbTeGe layer, and the SbTeGe layer had a thickness of 12 nm. The L 0  layer  42  has a phase change recording film and the L 1  layer  44  to the L 4  layer  50  have a write-once read-multiple recording film. 
     The dielectric layer on the light transmission layer  34  side is composed of a ZnS:SiO 2  (80:20 mol %) film having a thickness of 35 nm, and the dielectric layer on the substrate  32  side is composed of a ZnS:SiO 2  (50:50 mol %) film having a thickness of 13 nm. A reflective layer made of an alloy of Ag, Pd, and Cu (98:1:1 mol %) having a thickness of 100 nm was provided between the dielectric layer located on the substrate  32  side and the substrate  32  itself. 
     The recording power Pwa 0  of the laser light when data was recorded once on the L 0  layer  42  where data had been recorded on all recording layers of the L 4  layer  50  to the L 1  layer  44  in the multilayer optical recording medium  40 , the recording power Pwb 0  of the laser light when data was recorded once on the L 0  layer  42  where data had not been recorded on any recording layer of the L 4  layer  50  to the L 1  layer  44 , and jitter were evaluated. The recording linear velocity at this time was 4.9 m/s. 
     When the recording power of the laser light is in the range of 8.5 mW to 14.0 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  layer  50  to the L 1  layer  44 . The recording power margin of the L 0  layer  42  of this embodiment was 49% of the optimal recording power. 
     When overwriting the recorded data with a signal used for rewriting, jitter could not be measured. An 8T signal with a single frequency was recorded on this medium with a recording linear velocity of 4.9 m/s and then a DC laser light of 6.4 mW was applied to measure the rate of erasure, being the rate of crystallization. The rate of erasure was lower than 25 dB required in order to allow the medium to then be rewritable, and it was established that rewriting was impossible because erasure (crystallization) was insufficient when this composition was used with this recording linear velocity. 
     Sixth Exemplary Embodiment 
     A multilayer optical recording medium  60  according to a sixth exemplary embodiment will now be described in detail with reference to  FIG. 3 . 
     As with the multilayer optical recording medium  10  according to the first exemplary embodiment, the multilayer optical recording medium  60  is provided with five recording layers, that is, an L 0  layer  62 , an L 1  layer  64 , an L 2  layer  66 , an L 3  layer  68 , and an L 4  layer  70 . The same reference numbers as used in the first exemplary embodiment are used for the other structures of the multilayer optical recording medium, and an explanation thereof will be omitted here. 
     The L 0  layer  62 , the L 1  layer  64 , the L 2  layer  66 , the L 3  layer  68 , and the L 4  layer  70  are arranged in this order in the direction from the substrate  32  to the incident surface  22  of the laser light. Spacer layers  36  are provided between the L 0  layer  62  and the L 1  layer  64 , between the L 1  layer  64  and the L 2  layer  66 , between the L 2  layer  66  and the L 3  layer  68 , and between the L 3  layer  68  and the L 4  layer  70 . A light transmission layer  34  is provided on the L 4  layer  70  on the side closest to the incident surface  22  of the laser light. 
     The multilayer optical recording medium  60  is different from the multilayer optical recording medium  10  according to the first exemplary embodiment only in the structure of the L 0  layer, and the structure of the L 1  to L 4  layers are the same. 
     In the L 0  layer  62 , SbTeGe (75:20:5 mol %) was used as a recording film material. The L 0  layer  62  has a phase change recording film and the L 1  layer  64  to the L 4  layer  70  have a write-once read-multiple recording film. 
     The L 0  layer  62  comprises a dielectric layer made of a ZnS:SiO 2  (80:20 mol %) film having a thickness of 35 nm, a dielectric layer made of an Al 2 O 3  film having a thickness of 5 nm, a recording layer made of a SbTeGe layer (75:20:5 mol %) film having a thickness of 12 nm, a dielectric layer made of an Al 2 O 3  film having a thickness of 5 nm, a dielectric layer made of a ZnS:SiO 2  (50:50 mol %) film having a thickness of 13 nm, and a reflective layer made of an alloy of Ag, Pd, and Cu (98:1:1 mol %) having a thickness of 100 nm arranged in this order from the light transmission layer  34  side toward the incident surface  22  of the laser light. 
     The recording power Pwa 0  of the laser light when data was recorded once on the L 0  layer  62  where data had been recorded on all recording layers of the L 4  layer  70  to the L 1  layer  64  in the multilayer optical recording medium  60 , the recording power Pwb 0  of the laser light when data was recorded once on the L 0  layer  62  where data had not been recorded on any recording layer of the L 4  layer  70  to the L 1  layer  64 , and jitter were evaluated. The recording linear velocity at this time was 4.9 m/s. 
     When the recording power of the laser light is in the range of 9.5 mW to 15.2 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  layer  70  to the L 1  layer  64 . The recording power margin of the L 0  layer  62  of the present exemplary embodiment was 46% of the optimal recording power. 
     When overwriting the recorded data with a signal used for rewriting, jitter could not be measured. An 8T signal with a single frequency was recorded on this medium with a recording linear velocity of 4.9 m/s and a DC laser light of 6.4 mW was then applied to measure the rate of erasure, being the rate of crystallization. The rate of erasure was lower than 25 dB required in order to allow the medium to then be rewritable, and it was established that rewriting was impossible because erasure (crystallization) was insufficient when this composition was used with this recording linear velocity. 
     Seventh Exemplary Embodiment 
     In the present exemplary embodiment, a multilayer optical recording medium having exactly the same structure as that of the first exemplary embodiment was used. After data was recorded once on an L 0  layer of this multilayer optical recording medium with a recording linear velocity of 4.9 m/s where data had been recorded on all recording layers of L 4  to L 1  layers, the process where “recording marks are erased by DC laser light with a linear velocity of 4.9 m/s and power of 6.4 mW and new recording marks are formed” was repeated ten times with each recording power. Also, after data was recorded once on the L 0  layer with a recording linear velocity of 4.9 m/s where data had not been recorded on any of L 4  to L 1  layers, the process where “recording marks are erased by DC laser light with a linear velocity of 4.9 m/s and power of 6.4 mW and recording marks are newly formed” was repeated ten times with each recording power. The former recording power Pwa 0 , the latter recording power Pwb 0 , and jitter were then evaluated. 
     When the recording power of the laser light is in the range of 8.8 mW to 13.8 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  to L 1  layers. The recording power margin of the L 0  layer of this embodiment was 44% of the optimal recording power. The recording power margin of the present embodiment is exactly the same as that of the first embodiment. 
     This is because, since absorptance is different between a recording mark section and a space section, the difference in an optical recording condition narrows the recording power margin when rewriting is carried out without erasure being required first. However, if recording marks are erased by a laser light with an erasing power and then a new recording is carried out, the optical recording condition does not change, and it is possible to obtain the same recording power margin as that of the first recording undertaken without erasure being required first. 
     COMPARATIVE EXAMPLE 1 
     A multilayer optical recording medium  80  according to Comparative Example 1 will now be described in detail with reference to  FIG. 6 . 
     The multilayer optical recording medium  80 , as with the multilayer optical recording medium  10  according to the first exemplary embodiment, is provided with five recording layers, that is, an L 0  layer  82 , an L 1  layer  84 , an L 2  layer  86 , an L 3  layer  88 , and an L 4  layer  90 . The same reference numbers as used in the first exemplary embodiment are used for the other structures of the multilayer optical recording medium, and an explanation thereof will be omitted here. 
     The L 0  layer  82 , the L 1  layer  84 , the L 2  layer  86 , the L 3  layer  88 , and the L 4  layer  90  are arranged in this order in the direction from the substrate  32  to the incident surface  22  of the laser light. Spacer layers  36  are provided between the L 0  layer  82  and the L 1  layer  84 , between the L 1  layer  84  and the L 2  layer  86 , between the L 2  layer  86  and the L 3  layer  88 , and between the L 3  layer  88  and the L 4  layer  90 . A light transmission layer  34  is provided on the L 4  layer  90  on the side closest to the incident surface  22  of the laser light. 
     The multilayer optical recording medium  80  is different from the multilayer optical recording medium  10  according to the first exemplary embodiment only in the structure of the L 0  layer, and the structure of the L 1  to L 4  layers are the same. 
     In the L 0  layer  82 , Si and Cu were used as the recording film material, and an Si layer with a thickness of 6 nm and a Cu layer with a thickness of  6  nm were both deposited on the L 0  layer  82 . The Cu layer is disposed on the substrate  32  side of the L 0  layer  82  and the Si layer is disposed on the light transmission layer  34  side of the L 0  layer  82 . The L 0  layer  82  has a phase change recording film and the L 1  layer  84  to the L 4  layer  90  have a write-once read-multiple recording film. 
     Dielectric layers, each being made of a mixture of ZnS and SiO 2 , were provided on either side of the deposited Si layer and Cu layer. The dielectric layer on the light transmission layer  34  side is composed of a ZnS:SiO 2  (80:20 mol %) film having a thickness of 40 nm, and the dielectric layer on the substrate  32  side is composed of a ZnS:SiO 2  (80:20 mol %) film having a thickness of 40 nm. A reflective layer made of an alloy of Ag, Pd, and Cu (98:1:1 mol %) having a thickness of 100 nm was provided between the dielectric layer located on the substrate  32  side and the substrate  32  itself. 
     In the multilayer optical recording medium  80 , the recording power Pwa 0  of the laser light when data was recorded once on the L 0  layer  82  where data had been recorded on all recording layers of the L 4  layer  90  to the L 1  layer  84  in the multilayer optical recording medium  80 , the recording power Pwb 0  of the laser light when data was recorded once on the L 0  layer  82  where data had not been recorded on any of the L 4  layer  90  to the L 1  layer  84 , and jitter were evaluated. The recording linear velocity-at this time was 4.9 m/s. 
     Table 6 and  FIG. 8  show results. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Pwa 0   
                 Jitter 
                 Pwb 0   
                 Jitter 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 7.6 
                 11.8 
                 8.2 
                 13.0 
               
               
                   
                 8.2 
                 7.8 
                 8.8 
                 8.9 
               
               
                   
                 8.7 
                 6.2 
                 9.4 
                 7.0 
               
               
                   
                 9.2 
                 6.1 
                 9.8 
                 6.1 
               
               
                   
                 9.7 
                 7.2 
                 10.4 
                 6.2 
               
               
                   
                 10.2 
                 8.5 
                 10.9 
                 7.0 
               
               
                   
                 10.7 
                 10.7 
                 11.4 
                 8.9 
               
               
                   
                 11.2 
                 13.0 
                 12.2 
                 12.2 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 6 and  FIG. 8 , when the recording power of the laser light is in the range of 8.7 mW to 10.5 mW, jitter becomes 10% or less, which is a desired value irrespective of the recording conditions of the L 4  layer  90  to the L 1  layer  84 . The recording power margin of the L 0  layer  82  of this comparative example was 19% of the optimal recording power. 
     The extinction coefficient of the recording film of the L 4  layer  90  with the laser light used was measured in the same way as the first exemplary embodiment, and a result of 0.08 was obtained. 
     The results showed that the recording power margin according to this comparative example was smaller than those of the various exemplary embodiments. 
     The inventors have discovered that, as described in the first to sixth exemplary embodiments, use of the phase change recording film in at least the L 0  layer makes it possible for the recording power margin of the L 0  layer to easily become at least 25% greater than the optical recording power. 
     In this case, if the extinction coefficient of the L 4  layer in the first to sixth exemplary embodiments is 0.5 or less, the high transmission reduces the reduction in reflectivity even in a state after deposition. Thus, it turns out that the deposition of many layers becomes easy as the desired reflectivity of each layer is reduced. 
     Furthermore, in this case, it was found that data was also easily recorded again on the L 0  layer in which recording marks had been formed, as detailed in the first and second exemplary embodiments, if the recording marks were newly formed after the erasure of the original recording marks. 
     As in the case of the seventh exemplary embodiment, it was found that the recording power margin of the L 0  layer in which recording marks had been formed easily became at least 25% greater than the optimal recording power, if, when data was recorded again on the L 0  layer, the recording marks were newly formed after the recording marks originally formed in the L 0  layer were erased or attenuated. 
     In the first to seventh exemplary embodiments, the multilayer optical recording mediums  10 ,  40 , and  60  are a one-sided recording disc which is provided with recording layers on one side only. The present invention, however, is applicable to a double-sided multilayer optical recording medium which is provided with recording layers on both sides. 
     In the first to seventh exemplary embodiments, the L 0  layer has a phase change recording film, and the L 1  to L 4  layers have a write-once read-multiple recording film. The L 1  to L 3  layers may have a phase change recording film, a write-once read-multiple recording film, or any recording film other than those already discussed herein. 
     In the first to seventh exemplary embodiments, the multilayer optical recording mediums  10 ,  40 , and  60  are a multilayer optical recording medium having five recording layers. The present invention, however, has the same effect on a multilayer optical recording medium having three or on four recording layers or on a multilayer optical recording medium having six or more recording layers. 
     According to the first to seventh exemplary embodiments, in the multilayer optical recording mediums  10 ,  40 , and  60 , the light transmission layer  34  is thinner than the substrate  32 . The present invention, however, is also applicable to an optical recording medium in which a substrate and a light transmission layer have the same thickness, like that of DVDs. 
     Furthermore, according to the first to seventh exemplary embodiments, the multilayer optical recording mediums  10 ,  40 , and  60  were evaluated using a Blu-ray (trademark) disc system with NA=0.85 and a wavelength of 405 nm. The present invention, however, is applicable to an NA and a wavelength other than these.