Abstract:
The present invention relates to an optical recording medium. The optical recording medium according to one embodiment of the present invention includes a substrate, a reflective layer located on the upper side of the substrate and reflecting an incident laser beam, and an information recording layer located on the reflective layer. The information recording layer includes a first recording layer containing a compound in the form of A X B 1-X  (0.1≦X≦0.9), wherein A is one element selected from the group consisting of Ni, Ag, W and Cr, and B consists of at least one element selected from the above group except for the one element corresponding to A, and a second recording layer containing at least one element selected from the group consisting of Si, Sn, Sb and Ge. The laser beam is irradiated on the first recording layer prior to the second recording layer. Therefore, the optical recording medium of the present invention may provide high recording density and transmittance velocity suitable for BD system by combining the recording layer materials.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical recording medium. In particular, the present invention relates to a recordable optical recording medium consisting of three recording layers.  
         [0003]     2. Description of the Related Art  
         [0004]     With the advent of multimedia age comprehensively dealing with video signal including moving picture and stop picture, audio signal, and computer data information, package media including all kinds of discs like CD and DVD have been widely used. Recently, there have been active efforts to apply an optical recording medium to the recording media of mobile phone, digital camera, broadcasting, and movie.  
         [0005]     The optical recording medium is divided into read-only memory (ROM) optical recording medium, recordable optical recording medium capable of recording information only once, and rewritable optical recording medium capable of writing, reading, and deleting information repeatedly.  
         [0006]     The recordable optical recording medium can be applied for data backup or storage of broadcasting, movie, etc. As recording layer material of the recordable optical recording medium, organic material like dye or inorganic material may be used. Only, in case of using the organic material as the recording layer material, a problem may be occurred in long term storage of data recorded in the optical recording medium.  
         [0007]     As recording mechanisms of the recordable optical recording medium, a) a pit is generated when the recording layer material is burned, b) a pit is formed from expansion of the volume as the recording layer material is decomposed, c) a new phase is formed by solidifing after the recording layer is melted, and d) a new material (for example, silicide, germanium compound, or antimony compound) is formed by reaction on the contact surface with foreign material.  
         [0008]     Also, these mechanisms may be occurred multiply. In case the recording mark is generated by multiple mechanisms, if a laser beam is irradiated onto the optical recording medium, first material and second material in the recording layer may be mixed and changed to form different material from the recording layer.  
         [0009]     In this case, data are recorded by optical property changed in the recording layer material, and the recorded data can be identified by change of reflexibility according to the changed optical property before and after recording.  
         [0010]      FIG. 1  is a sectional view illustrating the structure of a conventional optical recording medium.  FIG. 2  is a sectional view illustrating one form of generated record mark in case of performing land recording to the optical recording medium of  FIG. 1 . And,  FIG. 3  is a sectional view illustrating one form of generated record mark in case of performing groove recording to the optical recording medium of  FIG. 1 .  
         [0011]     In  FIG. 1 , the optical recording medium includes a substrate  60 , a reflective layer  50 , an information recording layer  100 , an optical transmittance layer  10 , and a dielectric layer  20 ,  30 .  
         [0012]     The substrate  60  plays a role to support physical form of the optical recording medium. As material of the substrate  60 , ceramic, glass, resin, etc. are generally used, but polycarbonate resin is preferable.  
         [0013]     The reflective layer  50  is deposited on the substrate  60 , reflects laser beam incident to the optical recording medium through the optical transmittance layer  10 , and sends back the beam to the direction of the optical transmittance layer  10 . Therefore, it is desirable that the optical transmittance layer  10  contains a material having high reflexibility or alloy to which highly reflexible material is added.  
         [0014]     The information recording layer  100  is deposited on the reflective layer  50 , and includes a first recording layer  110  and a second recording layer  120 .  
         [0015]     Each material contained in the first recording layer  110  and the second recording layer  120  forms a new material in mixture when the laser beam is irradiated thereon. The new material has totally different reflexibility from other materials around.  
         [0016]     When the laser beam is irradiated, the materials of the first recording layer  110  and the second recording layer  120  make a recording mark in the mechanism that they react at the contact surface to form a new material.  
         [0017]     Also, the dielectric layer  20 ,  30  is laminated on at least one side of the information recording layer  100 .  
         [0018]     In  FIG. 2  and  FIG. 3 , the form of recording in an optical recording medium is explained in case of recording information by irradiating the laser beam to the optical recording medium.  
         [0019]     Groove and land are formed on the surface of the information recording layer  100 , and guide the laser beam irradiated onto the optical recording medium.  
         [0020]     Land recording means that data are recorded onto the convex part of the information recording layer  100  as shown in  FIG. 2 , and recording mark  90  is formed on the part that the laser beam first arrives at in the surface of the information recording layer  100 .  
         [0021]     Groove recording means that data are recorded onto the concave part of the information recording layer  100  as shown in  FIG. 3 , and recording mark  90  is formed on the part that the laser beam later arrives at in the surface of the information recording layer  100 .  
         [0022]     Therefore, both land recording and groove recording are possible in the optical recording medium.  
         [0023]     The optical recording medium in consisted of four or five layers like reflective layer  50 , information recording layer  100 , optical transmittance layer  10 , and dielectric layer  20 ,  30 . Thus, the optical recording medium is thick, and the manufacturing cost is increased. Especially, the reflective layer  50  contains very expensive substance like AgPdCu or AgNdCu, and its thickness is about 700 Å to 1000 Å. Therefore, the manufacturing cost of the optical recording medium is increased, and the thickness of the optical recording medium is thicker due to the reflective layer  50 . On the other hand, the recording medium for next generation requires very high recording density and transmittance velocity for data. The size of recording mark of the optical recording medium should be smaller than the present size to increase the recording density recorded in the optical recording medium. Thus, a laser wavelength irradiated onto the optical recording medium should be reduced to 450 nm or less, and numerical aperture should be increased to 0.7 or more. Also, the data transmittance velocity should be much higher than the present one of between 30 Mbps and 35 Mbps.  
         [0024]     In case of BD (Blu-ray Disc) that is one recording medium for next generation, the recording layer material having characteristic of jitter acceptable within the ranges of recording linear velocity of between 5.28 m/s and 10.56 m/s and a laser power of between 3 mW and 7 mW at 405 nm wavelength should be included in the optical recording medium.  
         [0025]     Particularly, the recordable optical recording medium having the above characteristic requires combination of the recording layer material in which □) the contrast between recording mark and space in the optical recording medium should be high, □) the recording sensibility should be high, □) the recorded mark should be stable, □) the recording characteristics including noise and jitter of the recording mark should be satisfied in BD system.  
         [0026]     Also, in case the recording mark  90  is made inside the optical recording medium by irradiated laser, combination of the recording layer material is required to prevent the laser power used for making the recording mark  90  from getting too high.  
       SUMMARY OF THE INVENTION  
       [0027]     An object of the present invention is to provide an optical recording medium in which the information layer can be consisted of three recording layers, the thickness can be reduced by eliminating a reflective layer and a dielectric layer, and the manufacturing cost can be saved.  
         [0028]     Another object of the present invention is to provide an optical recording medium that has high recording sensibility and big contrast between recording mark and space as well as satisfies characteristic of BD.  
         [0029]     Another object of the present invention is to provide an optical recording medium that has superior stability of recording mark and superior recording characteristic of jitter as well as satisfies characteristic of BD.  
         [0030]     Another object of the present invention is to provide an optical recording medium in which the laser power necessary for making recording mark is low.  
         [0031]     The optical recording medium according to one embodiment of the present invention includes a substrate and an information recording layer located on the substrate. The information recording layer includes a first recording layer containing at least one element selected from the group consisting of Ag, In, Ge, Sb and Te, a second recording layer containing at least one element selected from the group consisting of Si, Sn, Sb and Ge, and a third recording layer containing at least one element selected form the group consisting of Ni, Ag, Au, W, Cr and Mo.  
         [0032]     The optical recording medium according to another embodiment of the present invention includes a substrate, at least two information recording layers located on the substrate and a separation layer formed between the information recording layers. The information recording layers includes a first recording layer containing at least one element selected from the group consisting of Ag, In, Ge, Sb and Te, a second recording layer containing at least one element selected from the group consisting of Si, Sn, Sb and Ge, and a third recording layer containing at least one element selected form the group consisting of Ni, Ag, Au, W, Cr and Mo.  
         [0033]     As described above, in the optical recording medium of the present invention, the information layer can be consisted of three recording layers, the thickness can be reduced by eliminating a reflective layer and a dielectric layer, and the manufacturing cost can be reduced.  
         [0034]     Also, the optical recording medium of the present invention may provide high recording sensibility and contrast between recording mark and space as well as record data in high density. Moreover, in the optical recording medium of the present invention, low laser power may be enough to make recording mark.  
         [0035]     Further, the optical recording medium of the present invention may provide very stable recording mark and superior recording characteristic. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]     The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the office upon request and payment of the necessary fee.  
         [0037]     The above and other features and advantages of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:  
         [0038]      FIG. 1  is a sectional view illustrating the structure of a conventional optical recording medium;  
         [0039]      FIG. 2  is a sectional view illustrating one form of generated record mark in case of land recording to the optical recording medium of  FIG. 1 ;  
         [0040]      FIG. 3  is a sectional view illustrating one form of generated record mark in case of groove recording to the optical recording medium of  FIG. 1 ;  
         [0041]      FIG. 4  is a sectional view illustrating the structure of the optical recording medium according to a first embodiment of the present invention;  
         [0042]      FIG. 5  is a sectional view illustrating one form of generated record mark in case of land recording to the optical recording medium of  FIG. 4 ;  
         [0043]      FIG. 6  is a sectional view illustrating one form of generated record mark in case of groove recording to the optical recording medium of  FIG. 4 ;  
         [0044]      FIG. 7  is a sectional view illustrating one form of generated record mark in case of land recording to the optical recording medium according to a second embodiment of the present invention;  
         [0045]      FIG. 8  is a sectional view illustrating one form of generated record mark in case of groove recording to the optical recording medium according to a second embodiment of the present invention;  
         [0046]      FIG. 9  is a sectional view illustrating the structure of the optical recording medium according to a third embodiment of the present invention;  
         [0047]      FIG. 10A  and  FIG. 10B  are views illustrating results of reflexibility before and after recording according to the optical recording mediums consisted of five recording layers and two recording layers;  
         [0048]      FIG. 11A  and  FIG. 11B  are views illustrating horizontal temperature distribution after recording of the recording layer containing Ge in the test results of  FIG. 10A  and  FIG. 10B ;  
         [0049]      FIG. 12A  and  FIG. 12B  are views illustrating vertical temperature distribution after recording of the recording layer containing Ge in the test results of  FIG. 10A  and  FIG. 10B ;  
         [0050]      FIG. 13  is a view illustrating result of reflexibility change of the optical recording medium according to one embodiment of the present invention in case combination of thickness of the third recording layer and the dielectric layer is changed;  
         [0051]      FIG. 14  is a view illustrating laser recording pulse waveform used for data recording of the optical recording medium for performance evaluation of the optical recording medium according to one embodiment of the present invention;  
         [0052]      FIG. 15  is a view illustrating temperature distribution of the optical recording medium according to recording time in case of recording in the recording pulse of  FIG. 14 ;  
         [0053]      FIG. 16  is a view illustrating temperature distribution depending on presence of the dielectric layer in the optical recording medium according to one embodiment of the present invention. 
     
    
     DESCRIPTION OF EMBODIMENTS  
       [0054]     Hereinafter, the preferred embodiments of the present invention will be explained in more detail with reference to the accompanying drawings.  
         [0055]      FIG. 4  is a sectional view illustrating the structure of the optical recording medium according to a first embodiment of the present invention.  FIG. 5  is a sectional view illustrating one form of generated record mark in case of land recording to the optical recording medium of  FIG. 4 . And,  
         [0056]      FIG. 6  is a sectional view illustrating one form of generated record mark in case of groove recording to the optical recording medium of  FIG. 4 .  
         [0057]     In  FIG. 4 , the optical recording medium of the present invention includes a substrate  200 , an information recording layer  210 , and an optical transmittance layer  220 .  
         [0058]     The substrate  200  plays a role to support physical form of the optical recording medium. As material of the substrate  200 , ceramic, glass, resin, etc. are generally used, and polycarbonate resin is preferable.  
         [0059]     The optical transmittance layer  220  contains a material having high reflexibility or alloy to which highly reflexible material is added. A laser beam is incident to the optical recording medium through the optical transmittance layer  220 , and sends back the beam to the direction of the optical transmittance layer  220 .  
         [0060]     The information recording layer  210  is deposited on the substrate  200 , and includes a first recording layer  212 , a second recording layer  214  and a third recording layer  216 . Each material contained in the first recording layer  212 , the second recording layer  214 , and the third recording layer  216  forms a new material in mixture when the laser beam is irradiated thereon. The new material has totally different reflexibility from other materials around.  
         [0061]     The positions of the first recording layer  212 , the second recording layer  214  and the third recording layer  216  in the information recording layer  210  of  FIG. 4  may be changed each other. That is, it is not necessary that the first recording layer  212 , the second recording layer  214  and the third recording layer  216  are disposed in sequence.  
         [0062]     The first recording layer  212  contains at least one element selected from the group consisting of Ag, In, Ge, Sb and Te, preferably in the form of A W (Sb Z Te 1-Z ) 1-W (0≦W≦0.2, 0.5≦Z≦0.9), wherein A is one element selected from a first group consisting of Ge, Sn, Si, Cu, Au, Ag, Pd, V, Bi, Zr, Ti, Mn and Mo.  
         [0063]     The second recording layer  214  desirably contains at least one element selected from the group consisting of Si, Sn, Sb and Ge. It is also desirable that the second recording layer  214  contains one element selected from the group consisting of Si, Sn, Sb and Ge as main element in 50 atomic percents (atomic %) or more.  
         [0064]     The third recording layer  216  desirably contains at least one element selected from the group consisting of Ni, Ag, Au, W, Cr and Mo.  
         [0065]     When the laser beam is irradiated, the materials of the first recording layer  212 , the second recording layer  214  and the third recording layer  216  make a recording mark in the mechanism that they react at the contact surface to form a new material.  
         [0066]      FIG. 5  and  FIG. 6  illustrate forms recorded to the optical recording medium in case of recording information by irradiating laser beam to the optical recording medium according to the first embodiment of the present invention.  
         [0067]     Groove and land are formed on the surface of the information recording layer  210 , and guide the laser beam irradiated onto the optical recording medium.  
         [0068]     Land recording means that date are recorded onto the convex part of the information recording layer  210  as shown in  FIG. 5 , and recording mark  250  is formed on the part that the laser beam first arrives at in the surface of the information recording layer  210 .  
         [0069]     Groove recording means that date are recorded onto the concave part of the information recording layer  210  as shown in  FIG. 6 , and recording mark  250  is formed on the part that the laser beam later arrives at in the surface of the information recording layer  210 .  
         [0070]     Therefore, both land recording and groove recording are possible in the optical recording medium according to the present invention.  
         [0071]      FIG. 7  is a sectional view illustrating one form of generated record mark in case of land recording to the optical recording medium according to a second embodiment of the present invention. And,  FIG. 8  is a sectional view illustrating one form of generated record mark in case of groove recording to the optical recording medium according to a second embodiment of the present invention.  
         [0072]     In  FIG. 7  and  FIG. 8 , the dielectric layer  260  may be Laminated around or inside the information recording layer  210  in the optical recording medium according to the second embodiment of the present invention  
         [0073]     In case the dielectric layer  260  does not exist, the temperature may rise a little at same recording power because the transmittance rate is high and the absorption rate is low at the time of irradiating the laser beam.  
         [0074]     It is desirable that the dielectric layer  260  contains at least one element selected from the group consisting of AN, GeN, SiN, Al 2 O 3 , ZnS—SiO 2 , TiO and SiO 2 .  
         [0075]     The elements added to the dielectric layer  260  has good absorption, and so the transmittance rate of laser beam is decreased at the time of irradiating the laser beam. Thus, in case the dielectric layer  260  is Laminated, the laser power necessary at the time of generating the recording mark  250  may be lowered.  
         [0076]     Also, in case the laser beam is irradiated to the information recording layer  210 , the dielectric layer  260  regulates temperature distribution of the information recording layer  210  by emitting heat generated in the information recording layer  210  in proper rate.  
         [0077]     The dielectric layer  260  may be disposed on at least one side of the information recording layer  210  or inside the information recording layer  210 .  
         [0078]     The other constitution is the same as the optical recording medium according to the first embodiment.  
         [0079]      FIG. 9  is a sectional view illustrating the structure of the optical recording medium according to a third embodiment of the present invention.  
         [0080]     In  FIG. 9 , the optical recording medium according to the third embodiment of the present invention includes a substrate  200 , a separation layer  240  located between two and more information recording layers  210 ,  230  and each information recording layer  210 ,  230 , and the optical transmittance layer  220 .  
         [0081]     Recording mark  250  may be formed in the information recording layers  210 ,  230  as described  FIG. 5  to  FIG. 8 .  
         [0082]     The information recording layers  210 ,  230  need not have the same structure. Thus, the dielectric layer  260  may be further included in only one of the information recording layers  210 ,  230 .  
         [0083]     Hereinafter, applicability of the optical recording medium of the present invention will be described, comparing reflexibility and heat distribution between optical recording medium consisting of five layers (hereinafter, referred to as “optical recording medium of five layers”) and optical recording medium consisting of two layers (hereinafter, referred to as “optical recording medium of two layers”).  
         [0084]      FIG. 10A  and  FIG. 10B  are views illustrating results of reflexibility before and after recording information according to the optical recording medium of five recording layers and that of two layers.  FIG. 11A  and  FIG. 11B  are views illustrating horizontal temperature distribution after recording the recording layer containing Ge in the test result of  FIG. 10A  and  FIG. 10B . And,  FIG. 12A  and  FIG. 12B  are views illustrating vertical temperature distribution after recording the recording layer containing Ge in the test results of  FIG. 10A  and  FIG. 10B .  
         [0085]     The optical recording medium of five layers includes the reflective layer  50  and the dielectric layers  20 ,  30  besides the two recording layers  110 ,  120 , like the other optical recording medium described above.  
         [0086]     The optical recording medium of two layers consists of two information recording layers.  
         [0087]     The optical recording mediums of five layers and two layers include information recording layers containing Ge and Au.  
         [0088]     In  FIG. 10A  and  FIG. 10B , the optical recording medium of two layers indicates satisfactory reflexibility difference before and after recording.  
         [0089]     In  FIG. 11A  and  FIG. 11B  showing the horizontal temperature distribution of the recording layer containing Ge, temperature is concentrated at the laser center in the optical recording medium of two layers, but diffused relatively widely to front and rear, and right and left in the optical recording medium of five layers. In case the optical recording medium of five layers, thermal interference may be occurred among recording marks, and so the optical recording medium of two layers is better in terms of mark control.  
         [0090]     Only, in the optical recording medium of two layers, temperature is less increased at same recording power because there is no thick layer at the time of irradiating the laser, and so the transmittance rate can be high and the absorption rate can be low. This can be complemented by coating the dielectric layer  260 .  
         [0091]     Referring to  FIG. 12A  and  FIG. 12B  showing the horizontal temperature distribution of the recording layer containing Ge, temperature is concentrated at the laser center, and heat loss to right and left is less, and thus mark control is more effective, in the optical recording medium of two layers. In case of the optical recording medium of five layers, more heat is exited to front and rear, and right and left since the thick reflective layer  50  has big thermal conductivity, and so transformation of the substrate by heat may be reduced. On the other hand, temperature is concentrated in the optical recording medium of two layers to cause transformation by heat. This can be complemented by coating the dielectric layer  260 .  
         [0092]     Therefore, the optical recording medium of two layers has satisfactory properties in reflexibility and heat.  
         [0093]     Hereinafter, test results to the optical recording medium according to the first to third embodiments will be described.  
         [0094]     First, the structure and recording layer material of the optical recording media used for the tests will be described.  
         [0095]     The optical recording media used in the tests include a substrate  200  of a doughnut form of 15 mm of internal diameter, 120 mm of external diameter, and 1.1 mm of thickness, and 0.32 μm of track pitch having land and groove is formed on the substrate  200 .  
         [0096]     The substrate  200  is made of polycarbonate, and multiple thin layers are laminated on the substrate  200  in the order of the third recording layer  216 , the dielectric layer  260  consisted of ZnS—SiO 2 , the second recording layer  214 , and the first recording layer  212 .  
         [0097]     Also, the optical transmittance layer  220  is adhered by a polycarbonate cover sheet of 80 μm with PSA glue of 20 μm onto the first recording layer  212 . In the lamination, the thickness of the dielectric layer  260  is 200 Å, that of the third recording layer  216  is 300 Å, that of the second recording layer  214  is 30 Å, and that of the first recording layer  212  is 90 Å.  
         [0098]     The conditions of the above tests according to the first to third embodiments of the present invention are described below.  
         [0099]     In the tests, the constant linear velocity of the optical recording media is 5.28 m/s, and the measuring position of the optical recording media was 30 mm from the inner circumference. The data recording to the optical recording medium was groove recording, the wavelength of laser beam used therein is 408 nm, and the reproduction power is set at 35 mW. Also, as test apparatus, ODU-1000 of Pulstec Co., Ltd was used.  
         [0100]     In Table 1, combination of materials of the information recording layer  210  and the dielectric layer  260  in the optical recording medium as shown above and test results thereof are explained below.  
                                                                                             TABLE 1                               The   The       8T       Start                   The first   second   third   The   modulation       DC test       Mark       Test   recording   recording   recording   dielectric   (I8 pp /I8H)       power       stability       NO.   layer   layer   layer   layer   (%)   determination   (mW)   determination   determination                                1   GeSbTe   Si   Ag   ZnS—SiO 2     45   ◯   3.0   □   ◯       2   GeSbTe   Ge   Ag   ZnS—SiO 2     40   □   1.8   ◯   □       3   GeSbTe   Sb   Ag       50   ◯   1.5   ◯   □       4   GeSbTe   Sn   Ag       50   ◯   2.7   □   ◯       5   AgInSbTe   Si   Au   ZnS—SiO 2     55   ◯   1.0   ◯   □       6   AgInSbTe   Ge   Au   ZnS—SiO 2     52   ◯   1.4   ◯   □       7   AgInSbTe   Sb   Au       38   □   2.0   ◯   ◯       8   AgInSbTe   Sn   Au       40   □   4.0   □   □       9   GeSbTe   Si   Cr       54   ◯   3.5   □   ◯       10   GeSbTe   Si   W       50   ◯   4.0   □   ◯       11   GeSbTe   Si   Ni       40   □   4.0   □   ◯       12   GeSbTe   Sb   Mo       40   □   3.0   □   ◯                  
 
         [0101]     The first recording layer  212 , the second recording layer  214  and the third recording layer  216  were consisted of GeSbTe, Si and Ag, respectively, as main element in Test 1; GeSbTe, Ge and Ag in Test 2; GeSbTe, Sb and Ag in Test 3; GeSbTe, Sn and Ag in Test 4; AgInSbTe, Si and Au in Test 5; AgInSbTe, Ge and Au in Test 6; AgInSbTe, Sb and Au in Test 7; AgInSbTe, Sn and Au in Test 8; GeSbTe, Si and Cr in Test 9; GeSbTe, Si and W in Test 10; GeSbTe, Si and Ni in Test 11; and GeSbTe, Sb and Mo in Test 12.  
         [0102]     The test standards according to each test combination are difference of reflexibility of the optical recording medium according to 8T modulation, start DC test power, and mark stability. The results according to each standard of Table 1 are marked as ‘∘’, ‘□’ or ‘x’ in the right column of each test standard (own column for mark stability) in the order of test combination showing favorable result.  
         [0103]     First, 8T modulation indicates difference of reflexibility between space and mark.  
         [0104]     That is, the difference (I8 pp ) between minimum reflexibility value and maximum reflexibility value to the 8T modulation pulse of the laser beam divided by maximum reflexibility value (I8H) is expressed in percent (%). In the 8T modulation test of laser beam, the difference of reflexibility is the first standard of the optical recording medium. Thus, as its difference of reflexibility is bigger, the optical recording medium is determined more appropriate.  
         [0105]     In the test to the optical recording medium according to the present invention, the difference of reflexibility between space and mark of Test 5 was the best as 55%.  
         [0106]     Start DC test power is a determination standard that can indirectly confirm whether the optical recording medium having optimized structure can be recorded as prescribed recording power (Pw).  
         [0107]     BD should generate the recording mark  250  at the laser recording power within the range of less than 7 mW to 1×, 2× velocity of the optical recording medium. For indirect confirmation thereof, by measuring power that any change in the information recording layer  210  begins, if the measured power is between 1.5 mW and 2.5 mW, the optical recording medium is determined to have proper recording sensibility to the prescribed laser power.  
         [0108]     Hereinafter, a specific method of measuring start DC test power as determination standard will be described.  
         [0109]     First, laser beam of space power (Ps(mW)) is irradiated to the optical recording medium. Pulse of the laser beam is single pulse type, not multiple pulse type. And, in case the laser beam is irradiated, the original size of reflexibility of the optical recording medium is measured as laser power to initiate change in the oscilloscope. The size of the measured laser power is the value of start DC test power.  
         [0110]     In the test, the optical recording medium is determined to be optimized if the start DC test power is less than 2.5 mW. In the test, the value of the start DC test power was shown the best in Test 5, second best in Test 6, and third best in Test 3.  
         [0111]     As a determination standard of the test, mark stability is a standard to determine whether the recording mark  250  can be maintained for a long time without any change in time. The size of recording mark  250  should be maintained without growing bigger or smaller in time under effect of the laser beam of regenerative power or at room temperature when the recording mark  250  was formed to the optical recording medium by the laser pulse of 8T modulation.  
         [0112]     In the test, the recording mark  250  in the recording layers was changed in Test 2, Test 3, Test 5, Test 6 and Test 8. Thus, the material of the recording layers  212 ,  214 ,  216  may be improved by material ratio of the recording layer  212 ,  214 ,  216  and structure of the optical recording medium.  
         [0113]     According to the three test results, the combinations of Ag, In, Ge, Sb, Te, and Ag, In, Ge, Sb, Te, and Ag, Au, Cr, Mo as the first recording layer  212 , the second recording layer  214  and the third recording layer  216 , respectively, were evaluated as the best materials for the information recording layer  210 . However, combination of most of the above materials has a little large start DC test power, and so they may be more desirable for the information recording layer  210  if there is a way to lower the recording laser power a little bit more.  
         [0114]     Hereinafter, the combination of GeSbTe, Si and Ag was selected as main element of the first recording layer  212 , the second recording layer  214  and the third recording layer  216  among the combinations of Ag, In, Ge, Sb, Te, and Ag, In, Ge, Sb, Te, and Ag, Au, Cr, Mo selected as proper materials through the above tests, and the test results to improve reflexibility and recording sensibility to the optical recording medium giving the above combination will be described in detail.  
         [0115]      FIG. 13  is a view illustrating result of reflexibility change of the optical recording medium according to one embodiment of the present invention in case the combination of thickness of the third recording layer and the dielectric layer is changed.  
         [0116]     In  FIG. 13 , the horizontal axis indicates the thickness of the third recording layer  216  as □ unit, the vertical axis indicates the thickness of the dielectric layer  260  as □ unit, and the value in  FIG. 13  indicates reflexibility according to thickness of the third recording layer  216  and the dielectric layer  260 .  
         [0117]     The optical recording medium used in the above test designed HTL (High to Low) reflexibility, as can be seen by reflexibility difference in the part of mark and space.  
         [0118]     The test was conducted for two cases that the dielectric layer  260  consisted of ZnS—Si0 2 (number two of  FIG. 13 ) was not included when the third recording layer  216  is 300□ and that the dielectric layer  260  is laminated by the thickness of 200□ (number one of  FIG. 13 ), to consider heat variation of the substrate  260  by selecting two areas of high contrast.  
         [0119]      FIG. 14  is a view illustrating laser recording pulse waveform used for data recording of the optical recording medium for performance evaluation of the optical recording medium according to one embodiment of the present invention.  
         [0120]     In  FIG. 14 , the recording power (Pw) of the laser pulse used for recording of the optical recording medium was 5.7 mW, the space power (Ps) was 1.5 mW, and the bottom power (Pb) was 0.1 mW. The laser recording pulse has random multi pulse from 2T to 6T, and recording was done by modulating the pulse to make N-1 partition pulse to each recording pulse.  
         [0121]      FIG. 15  is a view illustrating temperature distribution of the optical recording medium according to recording time in case of recording by the recording pulse of  FIG. 14 . And,  FIG. 16  is a view illustrating temperature distribution depending on presence of the dielectric layer in the optical recording medium according to one embodiment of the present invention.  
         [0122]     In  FIG. 15 , one, two and three figures indicate temperature distribution in vertical section of the track center of the optical recording medium at each time of one, two and three figures of  FIG. 14 .  
         [0123]     The temperature distribution of  FIG. 15  shows that the optical recording medium according to the present invention forms a middle form of temperature distribution of the optical recording mediums of two layers and five layers.  
         [0124]     That is, in case the reflective layer and the dielectric layer do not exist like the optical recording medium of the present invention, satisfactory property can be achieved. In  FIG. 16 , temperature distribution of the front edge and rear edge of 2T shown  FIG. 14  is similar regardless of presence of the dielectric layer  260 . Therefore, the dielectric layer  260  need not be laminated in the optical recording medium according to the present invention.  
         [0125]     The superiority of performance of the optical recording medium according to the present invention is shown in a case that the information recording layer  100  is one (1). However, it is same when there are two or more information recording layers  100 ,  200 . One or more information recording layers in two or more information recording layers  100 ,  200  may be combined by constitutional element, thickness ratio, sum of thickness, etc., as shown above.  
         [0126]     From the preferred embodiments for the present invention, it is noted that modifications and variations can be made by a person skilled in the art in light of the above teachings. Therefore, it should be understood that changes may be made for a particular embodiment of the present invention within the scope and the spirit of the present invention outlined by the appended claims.