Abstract:
There is provided a phase-change type optical recording medium, including: a substrate; a recording layer formed on the substrate, phase condition of the recording layer being changed when a laser beam is radiated thereonto, to thereby record, erase or reproduce data into or from the recording layer; and a reflection layer formed on the recording layer for reflecting a laser beam having been radiated onto the recording layer. The recording layer is formed so that the following equation is established: 
     
       
           R c&gt; R m&gt; R a 
       
     
     wherein Rc indicates a reflectivity to be obtained when the recording layer is in crystal condition, Ra indicates a reflectivity to be obtained when the recording layer is in amorphous condition, and Rm indicates a reflectivity to be obtained when the recording layer is in mixed condition of molten condition and crystal or amorphous condition. For instance, the substrate, recording layer and reflection layer may be designed to have a thickness and/or made of particular material so that the equation is established. The phase-change type optical recording medium makes it possible to accurately accomplish verification by which it is confirmed whether data is properly recorded at both initial recording and over-recording.

Description:
This is a divisional of Application Ser. No. 08/925,534 filed Sep. 8, 1997, now U.S. Pat. No. 6,252,844 issued Jun. 26, 2001, which claims the benefit of Japanese Application No. 8-242949, filed Sep. 13, 1996 and Japanese Application No. 8-254620, filed Sep. 26, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an optical recording medium capable of recording data thereinto or reproducing data therefrom, and more particularly to an optical recording medium suitable for carrying out so-called “simultaneous recording and verification” where confirmation as to whether data is properly recorded is carried out concurrently with recording data into a medium. The invention also relates to a method of optically recording data into an optical recording medium and reproducing data therefrom. 
     2. Description of the Related Art 
     As one of optical disc mediums capable of recording data thereinto and reproducing data therefrom both by radiating laser beam spot thereto is known a phase-change type optical disc. The phase-change type optical disc is capable of carrying out over-recording by a single beam, which is difficult to carry out by a magneto-optical (MO) disc. An optical head system for the phase-change optical disc is simpler than that for the MO disc. 
     A method of carrying out simultaneous recording and verification in such a phase-change type recording medium has been suggested in Japanese Unexamined Patent Publications Nos. 60-145537, 63-183624, and 6-349067. According to these Publications, when pulse beams are radiated onto the phase-change type recording medium, an intensity of reflected beams is varied almost at the same with the radiation. The suggested methods make it possible to confirm whether data is properly recorded by detecting the variation in the reflected beams by means of either the reflected beams itself or RF signals. 
     Roughly speaking, data is recorded into a phase-change type optical disc medium by the following steps: radiating laser beams onto a non-recorded region, which is in crystal condition, to thereby heat a recording layer; a temperature of the recording layer being raised; melting of the recording layer; and cooling down the recording layer to thereby reduce the recording layer amorphous. Hence, non-recorded and recorded regions have different phase conditions, and as a result, non-recorded and recorded regions have different reflectivity. Accordingly, an intensity of reflected laser beams is varied in accordance with the phase condition. A phase-change type optical disc medium utilizes such variation in an intensity of reflected laser beams for recording data thereinto. 
     It is known that a recording layer becomes as illustrated in FIG. 1, when a recording layer is molten by a laser beam spot. Thus, all regions of a recording layer are not always molten in a beam spot, even if a laser beam is radiated onto the recording layer. Herein, the molten region  60  corresponds to a region in which data is recorded, and the non-molten region  62  corresponds to a region in which data is not recorded yet. As mentioned earlier, the molten region  60  becomes amorphous when cooled down, whereas the non-molten region  62  remains crystal. If a recording layer is not molten, the recording layer is not reduced amorphous. In the beam spot, reflectivity of the recorded or amorphous region  60  coexists together with reflectivity of the non-recorded or crystal region  62 , and hence it would be quite difficult to clearly distinguish the recorded region  60  from the non-recorded region  62 . As a result, it would be difficult to accurately carry out simultaneous recording and verification only based on variation in the reflectivity. Hence, verification has to be carried out again after data has been recorded into a phase-change type optical disc medium in the above mentioned conventional mediums. As a result, it is substantially impossible to carry out simultaneous recording and verification in the conventional mediums. 
     In addition, in the conventional mediums where the recorded region has to have reflectivity quite different from that of the non-recorded region, there would be generated a large fluctuation in RF signals in over-recording, resulting in unstable verification together with reduction in reliability. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems of the conventional recording mediums, it is an object of the present invention to provide a phase-change type optical recording medium which is capable of accurately carrying out simultaneous recording and verification, specifically carrying out verification by which it is confirmed whether data is properly recorded both in initial recording and over-recording. 
     Another object of the present invention is to provide a phase-change type optical recording medium which is capable of carrying out verification with high accuracy in smaller period of time to thereby make it possible to record data with higher reliability. 
     It is also an object of the present invention to provide a method of optically recording data by employing the above mentioned phase-change type optical recording medium. 
     In one aspect, there is provided a phase-change type optical recording medium, including (a) a substrate, (b) a recording layer formed on the substrate, phase condition of the recording layer being changed when a laser beam is radiated thereonto, to thereby record, erase or reproduce data into or from the recording layer, and (c) a reflection layer formed on the recording layer for reflecting a laser beam having been radiated onto the recording layer. The recording layer is formed so that the following equation (A) is established: 
     
       
           Rc&gt;Rm&gt;Ra   (A) 
       
     
     wherein Rc indicates a reflectivity to be obtained when the recording layer is in crystal condition, Ra indicates a reflectivity to be obtained when the recording layer is in amorphous condition, and Rm indicates a reflectivity to be obtained when the recording layer is in mixed condition of molten condition and crystal or amorphous condition. 
     By arranging the recording layer to have a thickness such that the above mentioned equation (A) is established, it is possible to have recording sensitivity suitable for recording data into and reproduce data from an optical recording medium, and accomplish simultaneous recording and verification. In addition, the phase-change type optical recording medium is capable of providing high reproducing output, being accessible at a higher speed, and recording data at a higher density. 
     There is further provided a phase-change type optical recording medium, including (a) a substrate, (b) a recording layer formed on the substrate, phase condition of the recording layer being changed when a laser beam is radiated thereonto, to thereby record, erase or reproduce data into or from the recording layer, and (c) a reflection layer formed on the recording layer for reflecting a laser beam having been radiated onto the recording layer. The recording layer is formed so that the following equation (B) is established: 
     
       
         | Rc−Ra |≦5% 
       
     
     
       
           Rc&gt;Rm, Ra&gt;Rm   (B) 
       
     
     wherein Rc indicates a reflectivity to be obtained when the recording layer is in crystal condition, Ra indicates a reflectivity to be obtained when the recording layer is in amorphous condition, and Rm indicates a reflectivity to be obtained when the recording layer is in mixed condition of molten condition and crystal or amorphous condition. 
     Since the above mentioned equation (B) includes reflectivity in the laser beam spot as one of standards for judgement, it is surely confirmed as to whether the recording layer has a molten or recorded region in the laser beam spot by means of the phase-change type optical recording medium meeting the equation (B). Thus, it is possible to carry out verification while data is being recorded into the phase-change type optical recording medium. 
     For instance, if verification is to be carried out by employing RF signals obtained from reflected laser beams, it would be possible to obtain clearly distinguishable RF waveforms between when data is properly recorded and when data is not properly recorded. In addition, since RF waveforms obtained when data is properly recorded is stable, precise verification can be carried out. Furthermore, since a difference between Rc and Ra is arranged within 5% and Rm is arranged smaller than Rc and Ra, it would be possible to have the same RF waveforms even in over-recording as the waveforms obtained in initial recording, which ensures that accurate verification can be carried out similarly to verification in initial recording. 
     It would be possible to accurately carry out verification also by analyzing RF waveforms obtained immediately after a certain region of a recording layer in the laser beam spot commences to be molten. According to this mode of verification, a period of time necessary for verification can be shortened, and data can be recorded in initial recording or over-recording without reduction in transfer speed with higher reliability. 
     The above-mentioned equations (A) and (B) may be established in various ways. For instance, the substrate and recording layer may be designed to have a thickness so that the equation (A) or (B) is established. As an alternative, the substrate and recording layer may be made of particular material so that the equation (A) or (B) is established. 
     The phase-change type optical recording medium may further include a lower protection layer formed between the substrate and the recording layer, and an upper protection layer formed between the recording layer and the reflection layer, in which case the substrate, recording layer, lower protection layer and upper protection layer may be designed to have a thickness so that the equation (A) or (B) is established. As an alternative, the substrate, recording layer, lower protection layer and upper protection layer may be made of particular material so that the equation (A) or (B) is established. 
     A laser beam may be radiated to the phase-change type optical recording medium in any directions. However, it is preferable that a laser beam is radiated through the substrate for the purpose of simplification of a driver optical system and reduction of a recording apparatus in size. 
     The recording layer may be made of at least germanium (Ge), antimony (Sb) or tellurium (Te). The lower and upper protection layers may be made of any material suitable for optically recording medium. For instance, it is preferable that the lower and upper protection layers include ZnSSiO 2 , SiN, SiO, SiO 2 , TaO 2  or SiAlON. The reflecting layer may be made of any material suitable for optically recording medium. For instance, it is preferable that the reflecting layer includes Al, Ti, Ta, Cu, Au or Si as principal ingredient. 
     The phase-change type optical recording medium may further include additional layers. For instance, the phase-change type optical recording medium may further include an UV resin protection layer formed on the reflection layer. As an alternative, the phase-change type optical recording medium may further include an intermediate layer between the IN resin protection layer and the reflection layer, which intermediate layer is made of the same material as that of the lower protection layer. In the phase-change type optical recording medium, the reflection layer is required to have a thickness through which a laser beam can pass. The provision of the intermediate layer enhances designability in the phase-change type optical recording medium. 
     In the present invention, verification is carried out with higher accuracy by arranging a difference between Rc and Ra at a wavelength of a laser beam used for recording data to be within 5%. Hence, if a laser beam having a certain wavelength were commonly employed for both recording and reproducing data, it would not be possible to have a preferred C/N ratio by a difference between Rc and Ra. Thus, in order to have a preferred C/N ratio, the optical recording medium may be constructed so that a difference between Rc and Ra necessary for reproducing data having been recorded in the medium is able to be obtained by a wavelength for reproducing data. However, it is preferable that crystal and amorphous regions formed in the recording layer have a phase difference, in order that a common wavelength can be employed for both recording and reproducing data. It is preferable that a phase difference φ between crystal and amorphous regions both formed in the recording layer at a laser wavelength used for recording data meet the following equation (C). 
     
       
         30°≦|φ|≦180°  (C) 
       
     
     The equation (C) may be established in various ways. For instance, the layers constituting the phase-change type optical recording medium may be designed to have a thickness or be made of particular material such that the equation (C) is established. 
     The phase-change type optical recording medium may be fabricated by means of conventional processes such as sputtering. 
     In another aspect, there is provided a method of optically recording data, including the steps of (a) preparing a phase-change type optical recording medium, the phase-change type optical recording medium comprising: a substrate; a recording layer formed on the substrate, phase condition of the recording layer being changed when a laser beam is radiated thereonto, to thereby record, erase or reproduce data into or from the recording layer; and a reflection layer formed on the recording layer for reflecting a laser beam having been radiated onto the recording layer, the recording layer being formed so that the following equation (A) is established: 
     
       
           Rc&gt;Rm&gt;Ra   (A) 
       
     
     wherein Rc indicates a reflectivity to be obtained when the recording layer is in crystal condition, Ra indicates a reflectivity to be obtained when the recording layer is in amorphous condition, and Rm indicates a reflectivity to be obtained when the recording layer is in mixed condition of molten condition and crystal or amorphous condition, (b) radiating laser beam onto the recording layer, (c) monitoring laser beam reflected from the recording layer, and (d) judging whether data is properly recorded, based on how the reflected laser beam varies in an amount. 
     When laser beam having a predetermined intensity is radiated onto the recording layer, data may be judged to be properly recorded, if the reflected beam is suddenly reduced in an amount immediately after radiation of the laser beam. 
     There is further provided a method of optically recording data, including the steps of (a) preparing a phase-change type optical recording medium, the phase-change type optical recording medium comprising: a substrate; a recording layer formed on the substrate, phase condition of the recording layer being changed when a laser beam is radiated thereonto, to thereby record, erase or reproduce data into or from the recording layer; and a reflection layer formed on the recording layer for reflecting a laser beam having been radiated onto the recording layer, the recording layer being formed so that the following equation (B) is established: 
     
       
         | Rc−Ra |≦5% 
       
     
     
       
           Rc&gt;Rm, Ra&gt;Rm   (B) 
       
     
     wherein Rc indicates a reflectivity to be obtained when the recording layer is in crystal condition, Ra indicates a reflectivity to be obtained when the recording layer is in amorphous condition, and Rm indicates a reflectivity to be obtained when the recording layer is in mixed condition of molten condition and crystal or amorphous condition, (b) radiating laser beam onto the recording layer, (c) monitoring laser beam reflected from the recording layer, and (d) judging whether data is properly recorded, based on how the reflected laser beam varies in an amount. 
     It is preferable that the laser beam is radiated in pulse. 
     The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view illustrating laser beam spot radiated while data is being recorded into an optical disc medium. 
     FIG. 2 is a cross-sectional view of an optical disc medium in accordance with the first to fourth embodiments of the present invention. 
     FIG. 3 is a schematic view illustrating waveforms of laser beams radiated while data is being recorded into an optical disc medium. 
     FIG. 4 is a graph showing relationship between recording laser power and carrier, noise and 2nd H/C levels in the first embodiment. 
     FIG. 5 is a schematic view illustrating RF signal waveforms in the first embodiment. 
     FIG. 6 is a cross-sectional view of an optical disc medium in the first to third reference example. 
     FIG. 7 is a graph showing relationship between recording laser power and carrier, noise and 2nd H/C levels in the first reference example. 
     FIG. 8 is a schematic view illustrating RF signal waveforms in the first reference example. 
     FIG. 9 is a graph showing relationship between recording laser power and carrier, noise and 2nd H/C levels in the second embodiment. 
     FIG. 10 is a schematic view illustrating RF signal waveforms in the second embodiment. 
     FIG. 11 is a graph showing relationship between recording laser power and carrier, noise and 2nd H/C levels in the third embodiment. 
     FIG. 12 is a schematic view illustrating RF signal waveforms obtained when data is being first recorded into the optical disc medium in accordance with the third embodiment. 
     FIG. 13 is a schematic view illustrating RF signal waveforms obtained when data is being over-recorded into the optical disc medium in accordance with the third embodiment. 
     FIG. 14 is a block diagram of a circuit for judging whether data is properly recorded into an optical disc medium. 
     FIG. 15 is a graph showing relationship between recording laser power and carrier, noise and 2nd H/C levels in the second reference example. 
     FIG. 16 is a schematic view illustrating RF signal waveforms obtained when data is being first recorded into the optical disc medium in accordance with the second reference example. 
     FIG. 17 is a schematic view illustrating RF signal waveforms obtained when data is being over-recorded into the optical disc medium in accordance with the second reference example. 
     FIG. 18 is a graph showing relationship between recording laser power and carrier, noise and 2nd H/C levels in the fourth embodiment. 
     FIG. 19 is a schematic view illustrating RF signal waveforms obtained when data is being first recorded into the optical disc medium in accordance with the fourth embodiment. 
     FIG. 20 is a schematic view illustrating RF signal waveforms obtained when data is being over-recorded into the optical disc medium in accordance with the fourth embodiment. 
     FIG. 21 is a graph showing relationship between recording laser power and carrier, noise and 2nd H/C levels in the third reference example. 
     FIG. 22 is a schematic view illustrating RF signal waveforms obtained when data is being first recorded into the optical disc medium in accordance with the third reference example. 
     FIG. 23 is a schematic view illustrating RF signal waveforms obtained when data is being over-recorded into the optical disc medium in accordance with the third reference example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 2 illustrates a phase-change type optical disc medium in accordance with the first embodiment of the invention. The illustrated phase-change type optical disc medium has a transparent disc substrate  11  formed with a guide groove in a spiral form or guide grooves coaxially about a rotation axis of the disc substrate  11 . There are formed on the disc substrate  11  on this order a lower protection layer  12  made of ZnSSiO 2  and having a thickness of 160 nm, a recording layer  13  made of GeSbTe and having a thickness of 15 nm, an upper protection layer  14  made of ZnSSiO 2  and having a thickness of 25 nm, a metal reflection layer  15  made of aluminum (Al) and having a thickness of 50 nm, and a resin protection layer  16 . The layers  12  to  15  are deposited by sputtering. 
     The lower protection layer  12 , the recording layer  13  and the upper protection layer  14  are designed to have a thickness so that the equation (A) is established: 
     
       
           Rc&gt;Rm&gt;Ra   (A) 
       
     
     wherein Rc indicates a reflectivity to be obtained when the recording layer  13  is in crystal condition, Ra indicates a reflectivity to be obtained when the recording layer  13  is in amorphous condition, and Rm indicates a reflectivity to be obtained when the recording layer  13  is in mixed condition of molten condition and crystal or amorphous condition. A laser beam radiated through the disc substrate  11  passes through the lower protection layer  12 , the recording layer  13  and the upper protection layer  14 , and then is reflected at the metal reflection layer  15 . Thus, the laser beam is directed in the reverse direction. Apart of the radiated laser beam is reflected at the recording layer  13 , and thus directed in the reverse direction. Hence, a reflectivity of the recording layer  13  can be controlled by designing the layers  12 ,  13  and  14  to have a certain thickness. 
     After the recording layer  13  was crystallized or initialized by radiating a laser beam at a linear velocity of 6 m/s and at erasion power of 6 mW to the optical disc medium, data was recorded into and reproduced from the phase-change type optical disc medium. Waveforms of laser beams used for recording data into the disc medium are illustrated in FIG.  3 . The illustrated waveforms include reproduction power, erasion power, and recording power levels. 
     FIG. 4 shows the dependency of C/N ratios of carrier, noise and 2nd H/C levels on recording laser power, obtained when data was recorded into the disc medium in the following conditions: 
     Linear velocity; 6 m/s 
     Recording Frequency; 2 MHz 
     Duty Ratio; 50% 
     Reproduction Power; 1.0 mW 
     Erasion Power; 5.5 mW 
     Recording Power; 7.0-13.0 mW. 
     FIG. 5 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being recorded into the disc medium. 
     In view of FIG. 4, it has been found out that a high C/N ratio is obtained because the layers  12  to  14  are designed to have a thickness so that a difference between the reflectivity Rc of a non-recorded or crystal region and the reflectivity Ra of a recorded or amorphous region is maximized. 
     It is found in FIG. 5 that the waveforms are quite different between when the recording laser power is greater than 9.0 mW and when the recording laser power is smaller than 9.0 mW. In FIG. 5, the waveform found when the recording laser power is 13.0 mW indicates that data is properly recorded into the disc medium, whereas the waveform found when the recording laser power is 8.0 mW indicates that data is not properly recorded into the disc medium. 
     Taking the waveforms illustrated in FIG. 5 into consideration together with FIGS. 3 and 4, the recording layer  13  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in the case of the recording laser power being 13.0 mW. When the recording laser power is varied to the erasion power level, the waveform changes into a unique shape as if undershoot occurs, because the reflectivity Rm of the recording laser beam spot including the molten region is smaller than the reflectivity Rc obtained when the recording layer is in non-recorded condition. 
     On the other hand, in the case of the recording laser power being smaller than 9.0 mW, specifically equal to 8.0 mW, the reflected laser beam is not reduced in an amount, because the recording layer  13  is not molten even if the recording laser power is increased up to the recording power level. 
     Thus, it is possible to judge whether data is properly recorded into the disc medium by monitoring RF signals generated while data is being recorded into the disc medium. 
     [Reference 1] 
     FIG. 6 illustrates a phase-change type optical disc medium as a reference example. The illustrated phase-change type optical disc medium has a transparent disc substrate  21 , and there are formed on the disc substrate  21  on this order a lower protection layer  22  made of ZnSSiO 2  and having a thickness of 100 nm, a recording layer  23  made of GeSbTe and having a thickness of 10 nm, an upper protection layer  24  made of ZnSSiO 2  and having a thickness of 20 nm, a metal reflection layer  25  made of aluminum (Al) and having a thickness of 60 nm, and a UV resin protection layer  26 . The layers  22  to  25  are deposited by sputtering, similarly to the phase-change type optical disc medium in accordance with the first embodiment. 
     After the recording layer  23  was crystallized or initialized by radiating a laser beam at a linear velocity of 6 m/s and at erasion power of 6 mW to the optical disc medium, data was recorded into and reproduced from the phase-change type optical disc medium. Waveforms of laser beams used for recording data into the disc medium are the same as those illustrated in FIG.  3 . 
     FIG. 7 shows the dependency of C/N ratios on recording laser power, obtained when data was recorded into the disc medium in the following conditions: 
     Linear velocity; 6 m/s 
     Recording Frequency; 2 MHz 
     Duty Ratio; 50% 
     Reproduction Power; 1.0 mW 
     Erasion Power; 5.5 mW 
     Recording Power; 7.0-13.0 mW. 
     FIG. 8 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being recorded into the disc medium. 
     In this reference example, the layers  22  to  24  are designed to have a thickness so that the reflectivity Rc of a non-recorded or crystal region is almost equal to the reflectivity Ra of a recorded or amorphous region. However, since there exists a phase difference between the recorded and non-recorded regions, C/N ratios can be obtained for carrier, noise and 2nd H/C levels. 
     It is found in FIG. 8 that the waveform obtained when the recording laser power is greater than 9.0 mW is almost identical with the waveform obtained when the recording laser power is smaller than 9.0 mW. It is considered in view of FIG. 7 that data is properly recorded into the disc medium when the recording laser power is greater than 9.0 mW. However, the reflected laser beam is not reduced in an amount, even if the recording layer is molten immediately after radiation of the recording laser power, because the reflectivity Rm of the laser beam spot including the molten region is equal to both the reflectivity Rc of the non-recorded region and the reflectivity Ra of the recorded region. In addition, there cannot be found such a unique waveform where as if undershoot occurs as the waveform found in the optical disc medium in accordance with the first embodiment, even if the recording laser beam power is varied to the erasion power level, because the reflectivity Rm is equal to the reflectivity Rc. 
     On the other hand, in the case that the recording laser beam power is smaller than 9.0 mW where data is not properly recorded into the disc medium, the reflected laser beam is not reduced in an amount because the recording layer  23  is not molten, even if the laser beam power is increased up to the recording power level. 
     Thus, it is impossible to distinguish data being properly recorded into a disc medium from data being not properly recorded into a disc medium in the reference example, even if RF signals are monitored while data is being recorded into the disc medium. 
     Embodiment 2 
     Hereinbelow, a phase-change type optical disc medium in accordance with the second embodiment is explained. The optical disc medium in accordance with the second embodiment has the same layer structure as that of the optical disc medium in accordance with the first embodiment. With reference to FIG. 2 again, the phase-change type optical disc medium in accordance with the second embodiment has a transparent disc substrate  31 . There are formed on the disc substrate  31  on this order a lower protection layer  32  made of ZnSSiO 2  and having a thickness of 170 nm, a recording layer  33  made of GeSbTe and having a thickness of 15 nm, an upper protection layer  34  made of ZnSSiO 2  and having a thickness of 15 nm, a metal reflection layer  35  made of aluminum (Al) and having a thickness of 50 nm, and a UV resin protection layer  16 . The layers  32  to  35  are deposited by sputtering. 
     Similarly to the first embodiment, the lower protection layer  32 , the recording layer  33  and the upper protection layer  34  are designed to have a thickness so that the equation (A) is established: 
     
       
           Rc&gt;Rm&gt;Ra   (A) 
       
     
     After the recording layer  33  was crystallized or initialized by radiating a laser beam at a linear velocity of 6 m/s and at erasion power of 6 mW to the optical disc medium, data was recorded into and reproduced from the phase-change type optical disc medium. Waveforms of laser beams used for recording data into the disc medium are the same as those illustrated in FIG.  3 . 
     FIG. 9 shows the dependency of C/N ratios on recording laser power, obtained when data was recorded into the disc medium in the following conditions: 
     Linear velocity; 6 m/s 
     Recording Frequency; 4 MHz 
     Duty Ratio; 50% 
     Reproduction Power; 1.0 mW 
     Erasion Power; 5.5 mW 
     Recording Power; 9.0-14.0 mW. 
     FIG. 10 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being recorded into the disc medium. 
     In view of FIG. 9, it has been found out that a high C/N ratio is obtained because the layers  32  to  34  are designed to have a thickness so that a difference between the reflectivity Rc of a non-recorded or crystal region and the reflectivity Ra of a recorded or amorphous region is maximized. 
     It is found in FIG. 10 that the waveforms are quite different between when the recording laser power is greater than 10.0 mW and when the recording laser power is smaller than 10.0 mW. Similarly to the waveforms illustrated in FIG. 5, the waveform found when the recording power is 13.0 mW indicates that data is properly recorded into the disc medium, whereas the waveform found when the recording power is 9.0 mW indicates that data is not properly recorded into the disc medium. 
     Taking the waveforms illustrated in FIG. 10 into consideration together with FIGS. 3 and 9, the recording layer  33  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in an amount in the case that the recording laser power is greater than 10.0 mW. When the recording laser power is varied to the erasion power level, the waveform changes into a unique shape as if undershoot occurs, because the reflectivity Rm of the recording laser beam spot including the molten region is smaller than the reflectivity Rc obtained when the recording layer  33  is in non-recorded condition. 
     On the other hand, in the case that the recording laser power is smaller than 10.0 mW, specifically equal to 9.0 mW, the reflected laser beam is not reduced in an amount, because the recording layer  33  is not molten even if the recording laser power is increased up to the recording power level. 
     Thus, it is possible to judge whether data is properly recorded into the disc medium by monitoring RF signals generated while data is being recorded into the disc medium. 
     Embodiment 3 
     Hereinbelow, a phase-change type optical disc medium in accordance with the third embodiment is explained. The optical disc medium in accordance with the third embodiment has the same layer structure as that of the optical disc medium in accordance with the first embodiment. With reference to FIG. 2 again, the phase-change type optical disc medium in accordance with the third embodiment has a transparent disc substrate  41  made of polycarbonate and having a thickness of 1.2 mm. There are formed on the disc substrate  41  on this order a lower protection layer  42  made of ZnSSiO 2  and having a thickness of 110 nm, a recording layer  43  made of GeSbTe and having a thickness of 16 nm, an upper protection layer  44  made of ZnSSiO 2  and having a thickness of 80 nm, a metal reflection layer  45  made of aluminum (Al) and having a thickness of 50 nm, and a UV resin protection layer  16 . The layers  42  to  45  are deposited by sputtering. A laser beam employed herein for recording data into the optical disc medium has a wavelength of 685 nm±10 nm. 
     A designed reflectivity in accordance with phase conditions of the recording layer  43  and a phase difference between crystal and amorphous regions in the recording layer  43  are as shown in Table 1 described later. 
     The lower protection layer  42 , the recording layer  43  and the upper protection layer  44  are designed to have a thickness so that the equation (B) is established: 
     
       
         | Rc−Ra |≦5% 
       
     
     
       
           Rc&gt;Rm, Ra&gt;Rm   (B) 
       
     
     wherein Rc indicates a reflectivity to be obtained when the recording layer  43  is in crystal condition, Ra indicates a reflectivity to be obtained when the recording layer  43  is in amorphous condition, and Rm indicates a reflectivity to be obtained when the recording layer  43  is in mixed condition of molten condition and crystal or amorphous condition. 
     After the recording layer  43  was crystallized or initialized by radiating a laser beam at a linear velocity of 6 m/s and at erasion power of 6 mW to the optical disc medium, data was recorded into and reproduced from the phase-change type optical disc medium. Waveforms of laser beams used for recording data into the disc medium are as illustrated in FIG. 3, similarly to the first and second embodiments. The waveforms include reproduction power, erasion power, and recording power levels. 
     FIG. 11 shows the dependency of carrier, noise and 2nd H/C levels on recording laser power, obtained when data was recorded into the optical disc medium in the following conditions: 
     Linear velocity; 6 m/s 
     Recording Frequency; 2 MHz 
     Duty Ratio; 50% 
     Reproduction Power; 1.0 mW 
     Erasion Power; 5.5 mW 
     Recording Power; 7.0-13.0 mW. 
     FIG. 12 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being first recorded into the disc medium. FIG. 13 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being over-recorded into the disc medium. 
     In view of FIG. 11, it has been found out that a high C/N ratio is obtained because there is selected an appropriate phase difference between crystal and amorphous regions, though the layers  12  to  14  are designed to have a thickness so that the reflectivity Rc of a non-recorded or crystal region is almost equal to the reflectivity Ra of a recorded or amorphous region. 
     It is found in FIG. 12 that the waveforms of RF signals are quite different between when the recording laser power is greater than 9.0 mW and when the recording laser power is smaller than 8.0 mW. In FIG. 12, the waveform found when the recording laser power is 13.0 mW indicates that data is properly recorded into the disc medium, whereas the waveform found when the recording laser power is 7.0 mW indicates that data is not properly recorded into the disc medium. 
     Taking the waveforms illustrated in FIG. 12 into consideration together with FIGS. 3 and 11, the recording layer  43  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in an amount in the case of the recording laser power being greater than 9.0 mW. When the recording laser power is varied to the erasion power level, the waveform changes into a unique shape as if undershoot occurs, because the reflectivity Rm of the recording laser beam spot including the molten region is smaller than the reflectivity Rc obtained when the recording layer  43  is in non-recorded condition. 
     On the other hand, in the case of the recording laser power being smaller than 8.0 mW, specifically equal to 7.0 mW, the reflected laser beam is not reduced in an amount, because the recording layer  43  is not molten, even if the recording laser power is increased up to the recording power level. 
     It is also found in FIG. 13 illustrating waveforms of RF signals in over-recording that the waveforms of RF signals are quite different between when the recording laser power is greater than 9.0 mW and when the recording laser power is smaller than 8.0 mW. The recording layer  43  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in an amount in the case that the recording laser power is greater than 9.0 mW. When the recording laser power is varied to the erasion power level, the waveform changes into a unique shape as if undershoot occurs, because the reflectivity Rm of the recording laser beam spot including the molten region is smaller than the reflectivity in the previous condition, namely, the reflectivity Ra for the recorded region or the reflectivity Rc for the non-recorded region. 
     On the other hand, in the case that the recording laser power is smaller than 9.0 mW, specifically equal to 7.0 mW, the reflected laser beam keeps unchanged in an amount, because the recording layer  43  is not molten, even if the recording laser power is increased up to the recording power level, and also because the reflectivity Rc of the crystal region is almost equal to the reflectivity Ra of the amorphous region. In addition, the waveforms (FIG. 13) of RF signals obtained in over-recording are almost identical with the waveforms (FIG. 12) of RF signals obtained when data is first recorded into the optical disc medium. 
     Thus, it is possible to judge whether data is properly recorded into the disc medium both in first recording and over-recording by monitoring RF signals generated while data is being recorded into the disc medium. 
     FIG. 14 is a block diagram of a circuit for judging whether data is properly recorded into the optical disc medium. The illustrated circuit includes a peak hold circuit  51 , an operational amplifier  52 , an integration circuit  53  and a comparator  54 . Synchronized signals are transmitted to the peak hold circuit  51 , the integration circuit  53  and the comparator  54 . In the illustrated circuit, an attention is paid to how reproduction outputs  55  or RF signals are varied immediately after rise-up of recording pulses. The reproduction outputs  55  are introduced into the operational amplifier  52  together with outputs from the peak hold circuit  51 , and then an output from the operational amplifier  52  is integrated in the integration circuit  53 . The integration is introduced into the comparator  54 , and then compared to a reference voltage in the comparator  54 . 
     If a recording layer in a laser beam spot were molten, the comparator  54  transmits an output, because there can be obtained a waveform which would be generated when the recording laser power is greater than 9.0 mW, as illustrated in FIGS. 12 and 13. Thus, verification can be accomplished while data is being recorded into an optical disc medium. If verification is accomplished based on variation of the reproduction output immediately after rise-up of recording pulses, it is possible to shorten a time necessary for verification, ensuring data recording with higher reliability and without reduction in data-recording speed. 
     [Reference 2] 
     Hereinbelow is explained a phase-change type optical disc medium as a reference example. The phase-change type optical disc medium as a reference example has the same structure except thicknesses of the substrate and layers. With reference to FIG. 6 again, the phase-change type optical disc medium as a reference example has a transparent disc substrate  71  made of polycarbonate and having a thickness of 1.2 mm. There are formed on the disc substrate  71  on this order a lower protection layer  72  made of ZnSSiO 2  and having a thickness of 160 nm, a recording layer  73  made of GeSbTe and having a thickness of 14 nm, an upper protection layer  74  made of ZnSSiO 2  and having a thickness of 20 nm, a metal reflection layer  75  made of aluminum (Al) and having a thickness of 60 nm, and a UV resin protection layer  76 . The layers  72  to  75  are deposited by sputtering. A laser beam employed herein for recording data into the optical disc medium has a wavelength of 685 nm±10 nm. 
     A designed reflectivity in accordance with phase conditions of the recording layer  73  and a phase difference between crystal and amorphous regions in the recording layer  73  are as shown in Table 1 described later. 
     After the recording layer  73  was crystallized or initialized by radiating a laser beam at a linear velocity of 6 m/s and at erasion power of 6 mW to the optical disc medium, data was recorded into and reproduced from the phase-change type optical disc medium. Waveforms of laser beams used for recording data into the disc medium are as illustrated in FIG. 3, similarly to the first to third embodiments. The waveforms include reproduction power, erasion power, and recording power levels. 
     FIG. 15 shows the dependency of carrier, noise and 2nd H/C levels on recording laser power, obtained when data was recorded into the optical disc medium in the following conditions: 
     Linear velocity; 6 m/s 
     Recording Frequency; 2 MHz 
     Duty Ratio; 50% 
     Reproduction Power; 1.0 mW 
     Erasion Power; 5.5 mW 
     Recording Power; 8.0-13.0 mW. 
     FIG. 16 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being first recorded into the optical disc medium, and FIG. 17 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being over-recorded into the optical disc medium. 
     In this reference example, C/N ratios can be obtained for carrier, noise and 2nd H/C levels by designing the layers  72  to  74  to have a thickness so that a difference between the reflectivity Rc of a non-recorded or crystal region and the reflectivity Ra of a recorded or amorphous region is large. 
     It is found in FIG. 16 that the waveform of RF signals obtained when the recording laser power is greater than 10.0 mW is quite different from the waveform obtained when the recording laser power is smaller than 10.0 mW. Taking the waveforms illustrated in FIG. 16 into consideration together with FIGS. 3 and 15, the recording layer  73  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in an amount in the case that the recording laser power is greater than 10.0 mW where data is properly recorded into the optical disc medium. Even when the recording laser power is varied to the erasion power level, the waveform does not change into the unique shape like as if undershoot occurs, as found in the first to third embodiments, because the reflectivity Rm of the recording laser beam spot including the molten region is not so different from the reflectivity Rc obtained when the recording layer  73  is in non-recorded condition. 
     On the other hand, in the case of the recording laser power being smaller than 10.0 mW, the reflected laser beam is not reduced in an amount, because the recording layer  73  is not molten, even if the recording laser power is increased up to the recording power level. 
     It is also found in FIG. 17 illustrating waveforms of RF signals in over-recording that the reflected laser beam is changed in an amount in accordance with a difference prior to over-recording between the reflectivity Ra of a recorded region and the reflectivity Rc of a non-recorded region, because the recording layer  73  is not molten, even if the recording laser power is increased up to the recording power level. 
     When the recording laser power is greater than 10.0 mW where data is properly recorded into the optical disc medium, the reflectivity is reduced due to the recording layer being molten, resulting in that the reflected laser beam is reduced in an amount. However, there can be seen significant fluctuation in the waveform immediately after the rise-up of recording pulses. The fluctuation is caused by a great difference prior to over-recording between the reflectivity Rc of a crystal region and the reflectivity Ra of an amorphous region. The fluctuation is of almost the same degree as the fluctuation in reflectivity caused by the melting of the recording layer. 
     When the recording laser beam power is varied to the erasion power level, the waveform may be changed into the unique shape as if undershoot occurs, because the reflectivity Rm of a recording laser beam spot including a molten region may become larger or smaller than the reflectivity Ra for a recorded region or the reflectivity Rc for a non-recorded region. However, there can be still found significant fluctuation. In particular, paying attention to the fluctuation found immediately after rise-up of the recording pulses, there is not found clear and characteristic reduction in RF signals as found in the third embodiment, even though the recording layer  73  commences to be molten, in which case it is impossible to precisely judge whether data is properly recorded into the optical disc medium or not. As a result, it would be quite difficult or almost impossible to judge whether data is properly over-recorded into the optical disc medium, even if RF signals are monitored while data is being over-recorded into the medium. 
     Embodiment 4 
     Hereinbelow, a phase-change type optical disc medium in accordance with the fourth embodiment is explained. The optical disc medium in accordance with the fourth embodiment has the same layer structure as that of the optical disc medium in accordance with the first embodiment except thicknesses of the substrate and layers. With reference to FIG. 2 again, the phase-change type optical disc medium in accordance with the fourth embodiment has a transparent disc substrate  81  made of polycarbonate and having a thickness of 0.6 mm. There are formed on the disc substrate  81  on this order a lower protection layer  82  made of ZnSSiO 2  and having a thickness of 140 nm, a recording layer  83  made of GeSbTe and having a thickness of 12 nm, an upper protection layer  84  made of ZnSSiO 2  and having a thickness of 60 nm, a metal reflection layer  85  made of aluminum (Al) and having a thickness of 60 nm, and a UV resin protection layer  86 . The layers  82  to  85  are deposited by sputtering. A laser beam employed herein for recording data into the optical disc medium has a wavelength of 640 nm±10 nm. 
     A designed reflectivity in accordance with phase conditions of the recording layer  43  and a phase difference between crystal and amorphous regions in the recording layer  43  are as shown in Table 1 described later. 
     The lower protection layer  82 , the recording layer  83  and the upper protection layer  84  are designed to have a thickness so that the equation (B) is established: 
     
       
         | Rc−Ra |≦5% 
       
     
     
       
           Rc&gt;Rm, Ra&gt;Rm   (B) 
       
     
     After the recording layer  83  was crystallized or initialized by radiating a laser beam at a linear velocity of 6 m/s and at erasion power of 6 mW to the optical disc medium, data was recorded into and reproduced from the phase-change type optical disc medium. Waveforms of laser beams used for recording data into the disc medium are as illustrated in FIG. 3, similarly to the first and second embodiments. The waveforms include reproduction power, erasion power, and recording power levels. 
     FIG. 18 shows the dependency of carrier, noise and 2nd H/C levels on recording laser power, obtained when data was recorded into the optical disc medium in the following conditions: 
     Linear velocity; 6 m/s 
     Recording Frequency; 2 MHz 
     Duty Ratio; 50% 
     Reproduction Power; 1.0 mW 
     Erasion Power; 3.5 mW 
     Recording Power; 4.0-10.0 mW. 
     FIG. 19 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being first recorded into the disc medium. FIG. 20 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being over-recorded into the disc medium. 
     Though the layers  82  to  84  are designed to have a thickness so that the reflectivity Rc of a crystal or non-recorded region is almost equal to the reflectivity Ra of an amorphous or recorded region at a wavelength of a laser beam employed for recording and reproducing data, there can be obtained a preferred C/N ratio, since there is a phase difference between the crystal and amorphous regions. 
     It is found in FIG. 19 that the waveforms of RF signals are quite different between when the recording laser power is greater than 6.0 mW and when the recording laser power is smaller than 6.0 mW. Taking the waveforms illustrated in FIG. 19 into consideration together with FIGS. 3 and 18, it is considered that data is properly recorded into the disc medium when the recording laser power is greater than 6.0 mW. When the recording laser power is greater than 6.0 mW, the recording layer  83  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in an amount. When the recording laser power is varied to the erasion power level, the waveform changes into a unique shape as if undershoot occurs, because the reflectivity Rm of the recording laser beam spot including the molten region is smaller than both the reflectivity Rc obtained when the recording layer  83  is in non-recorded condition and the reflectivity Ra obtained when the recording layer  83  is in recorded condition. 
     On the other hand, in the case that the recording laser power is smaller than 6.0 mW where data is not properly recorded into the optical disc medium, the reflected laser beam is not reduced in an amount, because the recording layer  83  is not molten, even if the recording laser power is increased up to the recording power level. 
     It is also found in FIG. 20 illustrating waveforms of RF signals in over-recording that the waveforms of RF signals are quite different between when the recording laser power is greater than 6.0 mW and when the recording laser power is smaller than 6.0 mW. The recording layer  83  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in an amount in the case that the recording laser power is greater than 6.0 mW When the recording laser power is varied to the erasion power level, the waveform changes into a unique shape as if undershoot occurs, because the reflectivity Rm of the recording laser beam spot including the molten region is smaller than the reflectivity in the previous condition, namely, the reflectivity Ra for the recorded region or the reflectivity Rc for the non-recorded region. 
     On the other hand, in the case that the recording laser power is smaller than 6.0 mW, specifically equal to 5.0 mW, the reflected laser beam keeps unchanged in an amount, because the recording layer  83  is not molten, even if the recording laser power is increased up to the recording power level, and also because the reflectivity Rc of the crystal region is almost equal to the reflectivity Ra of the amorphous region. 
     Thus, it is possible to judge whether data is properly recorded into the disc medium both in first recording and over-recording by monitoring RF signals generated while data is being recorded into the disc medium. 
     [Reference 3] 
     Hereinbelow is explained a phase-change type optical disc medium as a reference example in comparison with the phase-change type optical disc medium in accordance with the embodiment 4. The phase-change type optical disc medium as a reference example has the same structure except thicknesses of the substrate and layers. With reference to FIG. 6 again, the phase-change type optical disc medium as a reference example has a transparent disc substrate  91  made of polycarbonate and having a thickness of 1.2 mm. There are formed on the disc substrate  91  on this order a lower protection layer  92  made of ZnSSiO 2  and having a thickness of 110 nm, a recording layer  93  made of GeSbTe and having a thickness of 14 nm, an upper protection layer  94  made of ZnSSiO 2  and having a thickness of 30 nm, a metal reflection layer  95  made of aluminum (Al) and having a thickness of 60 nm, and a UV resin protection layer  96 . The layers  92  to  95  are deposited by sputtering. A laser beam employed herein for recording data into the optical disc medium has a wavelength of 640 nm±10 nm. 
     A designed reflectivity in accordance with phase conditions of the recording layer  93  and a phase difference between crystal and amorphous regions in the recording layer  93  are as shown in Table 1 described later. 
     After the recording layer  73  was crystallized or initialized by radiating a laser beam at a linear velocity of 6 m/s and at erasion power of 6 mW to the optical disc medium, data was recorded into and reproduced from the phase-change type optical disc medium. Waveforms of laser beams used for recording data into the disc medium are as illustrated in FIG. 3, similarly to the first to fourth embodiments. The waveforms include reproduction power, erasion power, and recording power levels. 
     FIG. 21 shows the dependency of carrier, noise and 2nd H/C levels on recording laser power, obtained when data was recorded into the optical disc medium in the following conditions: 
     Linear velocity; 6 m/s 
     Recording Frequency; 2 MHz 
     Duty Ratio; 50% 
     Reproduction Power; 1.0 mW 
     Erasion Power; 3.5 mW 
     Recording Power; 4.0-10.0 mW. 
     FIG. 22 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being first recorded into the optical disc medium, and FIG. 23 illustrates waveforms of RF signals obtained from the reflected laser beams while data is being over-recorded into the optical disc medium. 
     In this reference example, C/N ratios can be obtained for carrier, noise and 2nd H/C levels by designing the layers  92  to  95  to have a thickness so that a difference between the reflectivity Rc of a non-recorded or crystal region and the reflectivity Ra of a recorded or amorphous region is large. 
     It is found in FIG. 22 that the waveform of RF signals obtained when the recording laser power is greater than 6.0 mW is quite different from the waveform obtained when the recording laser power is smaller than 6.0 mW. Taking the waveforms illustrated in FIG. 22 into consideration together with FIGS. 3 and 21, it is considered that data is properly recorded into the optical disc medium when the recording laser beam power is equal to or greater than 6.0 mW. In the case that the recording laser power is greater than 6.0 mW, the recording layer  93  is molten immediately after radiation of the recording laser power, and thus the reflectivity is reduced with the result of reduction in the reflected laser beam in an amount. Even when the recording laser power is varied to the erasion power level, the waveform does not change into the unique shape like as if undershoot occurs, as found in the first to fourth embodiments, because the reflectivity Rm of the recording laser beam spot including the molten region is almost identical to the reflectivity Rc obtained when the recording layer  93  is in non-recorded condition. 
     On the other hand, in the case that the recording laser power is smaller than 6.0 mW, the reflected laser beam is not reduced in an amount, because the recording layer  93  is not molten, even if the recording laser power is increased up to the recording power level. 
     It is found in FIG. 23 illustrating waveforms of RF signals in over-recording that the recording layer  93  is not molten, even if the recording laser beam power is increased up to the recording power level, in the case that the recording laser power is smaller than 6.0 mW, and hence, the reflected laser beam is slightly changed in an amount in accordance with a difference prior to over-recording between the reflectivity Ra of the recorded region and the reflectivity Rc of the non-recorded region. 
     When the recording laser power is greater than 6.0 mW where data is properly recorded into the optical disc medium, the reflectivity is reduced due to the recording layer being molten, resulting in that the reflected laser beam is reduced in an amount. However, there can be seen significant fluctuation in the waveform immediately after the rise-up of recording pulses. The fluctuation is caused by a great difference prior to over-recording between the reflectivity Rc of a crystal region and the reflectivity Ra of an amorphous region. The fluctuation is of almost the same degree as the fluctuation in reflectivity caused by the melting of the recording layer. 
     When the recording laser beam power is varied to the erasion power level, the waveform may be changed into the unique shape as if undershoot occurs, because the reflectivity Rm of a recording laser beam spot including a molten region may become larger or smaller than the reflectivity Ra for a recorded region or the reflectivity Rc for a non-recorded region. However, there can be still found significant fluctuation. As a result, similarly to the earlier mentioned reference 2, it would be quite difficult or almost impossible to judge whether data is properly over-recorded into the optical disc medium, even if RF signals are monitored while data is being over-recorded into the medium. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Phase Difference between 
               
               
                   
                   
                   
                   
                 Crystal and Amorphous 
               
               
                   
                 Rc [%] 
                 Ra [%] 
                 Rm [%] 
                 Regions [|φ|°] 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Embodiment 3 
                 22.5 
                 22.8 
                 16.5 
                 48.0 
               
               
                 Reference 2 
                 33.7 
                 8.4 
                 25.4 
                 29.3 
               
               
                 Embodiment 4 
                 23.3 
                 24.6 
                 18.3 
                 65.2 
               
               
                 Reference 3 
                 32.6 
                 0.4 
                 21.4 
                 6.4 
               
               
                   
               
             
          
         
       
     
     While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims. 
     The entire disclosure of Japanese Patent Application Nos. 8-252949 and 8-254620 filed on Sep. 13, 1997 and Sep. 26, 1997, respectively, including specification, claims, drawings and summary is incorporated herein by reference in its entirety.