DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be described in more detail.

[0020] FIG. 1 is a cross sectional view of a multilevel phase change optical recording medium according to the present invention. As shown in FIG. 1, a lower protective layer 2 is formed on a substrate 1. On the lower protective layer 2, there are alternately provided, from the lower side where a laser beam is incident on, first to N-th (N≧2) recording layers R₁, R₂, . . . , R_N-1 and R_N, and intermediate layers S₁, . . . , and S_N-1. On the uppermost recording layer R_N, an upper protective layer 3 and a reflective layer 4 are formed. For recording data on the phase change optical recording medium, a plurality of recording laser beams having different power levels is used. The phase change optical recording layers are located within the depth of focus of each of the laser beams.

[0021] Now, let us study two recording layers, referred to as the i-th layer and the j-th layer (1≦i, 2≦j, and i≠j), which are arbitrarily selected from the first to N-th recording layers R₁ to R_N constituting the phase change optical recording medium of the present invention. FIGS. 2A and 2B show the thermal responses of the i-th and j-th layers, respectively, when these layers are irradiated with the recording laser beams. In these diagrams, two profiles denoted by the letters “A” and “B” represent thermal responses corresponding to two recording laser beams different in power level, respectively. The power levels of the two laser beams are in a relationship of A<B.

[0022] The important factors in phase change optical recording are the maximum temperature and the time response during cooling of the recording layer. Now, assume that T is a maximum temperature of the recording layer in a recording operation, T_m is the melting point of the recording layer, T_x is the crystallizing temperature of the recording layer, τ_w is a time required for the recording layer to be cooled from T_m to T_x after the laser beam irradiation, and τ_x is the crystallizing time of the recording layer. The above values for the i-th and j-th layers are represented by subscripts “i” and “j”, respectively.

[0023] First, the thermal responses when the recording layers are irradiated with the recording laser beam “A” are described. The i-th layer is heated up to higher than its melting point T_mi and takes a time τ_wiA to be cooled down from the melting point T_mi to the crystallizing temperature T_xi. Amorphous recording marks are formed in the i-th layer if the crystallizing time t_xi of the i-th layer is longer than the time τ_wiA, whereas no recording marks are formed in the contrary case. Here, assume that the recording marks are formed in the i-th layer. The j-th layer is restrained its temperature raise by making use of a material having higher melting point T_mj or by the effect of the intermediate layer. In the j-th layer, the maximum temperature T_j is lower than the melting point T_mj. Accordingly, no recording marks are formed in the j-th layer regardless of the cooling speed. Therefore, with respect to the recording laser beam “A”, the conditions of T_i>T_mi and τ_wi>τ_xi for i-th layer, and T_j<T_mj for j-th layer are established simultaneously. This permits the recording laser beam “A” to form recording marks in the i-th layer selectively.

[0024] Next, the thermal responses when the recording layers are irradiated with the recording laser beam “B” are described. The laser beam “B” is higher in intensity compared with the laser beam “A” and causes the i-th layer to rise up far over the melting point T_mi. The cooling time τ_wiB of the i-th layer is longer than the cooling time τ_wiA. Amorphous recording marks are formed in the i-th layer if the crystallizing time τ_xi (>τ_wiA) of the i-th layer is longer than the time τ_wiB, whereas no recording marks are formed in the contrary case. Any one of the two cases may be selected depending on involved conditions. Here, assume that the recording marks are also formed in the i-th layer under the power level “B”. When the laser beam “B” is used, the j-th layer is heated up to higher temperature than the melting point T_mj with the cooling time being τ_wjB. Recording marks are formed in the j-th layer if the crystallizing time τ_xj of the j-th layer is greater than τ_wjB, whereas no recording marks are formed in the contrary case. Here, assume that the recording marks are also formed in the j-th layer.

[0025] Although not shown in FIGS. 2A and 2B in order to avoid complexity, when a recording laser beam “C” having a higher power level than that of the laser beam “B” is used, thermal responses are as follows. In this case, both the i-th and j-th layers are heated up to higher temperatures than their melting points. When the cooling time τ_wiC of the i-th layer is longer than τ_xi (>τ_wiB), no recording marks are formed in the i-th layer. If the cooling time τ_wjC of the j-th layer is shorter than τ_xj (>τ_wjB), recording marks are formed in the j-th layer.

[0026] Table 1 shows the relationship between recording power levels and whether recording marks are formed or not. In the Table 1, the case where the recording marks are formed is expressed by “1”, and the case where no recording marks are formed is expressed by “0”. Note that “O” means a power level lower than the threshold for forming recording marks in the i-th layer. 1

[0027] As apparent from Table 1, the phase change optical recording medium of the present invention can attain four different recording states by irradiating two recording layers, which are different in thermal response, with recording laser beams having different power levels. On the other hand, conventional phase change recording medium permits only two recording states of “0” and “1”. Accordingly, the present invention can improve recording density to two times greater than that of the conventional recording medium. Even if the selection of power level corresponding to the recording laser beam “C” shown in Table 1 is not allowed, the recording density can be improved by 1.5 times greater than that of the conventional recording medium. Moreover, the two recording layers can be accessed simultaneously, so that the high-speed recording operation can be possible.

[0028] Furthermore, more than two recording layers may be used for multilevel recording, although design of the recording medium becomes difficult. In principle, with respect to a recording medium having N recording layers, recording density can be improved to 2^N times at maximum, to 2N times even if a large design margin is taken into account, and to (N+1)/2 times at least.

[0029] The phase change optical recording medium of the present invention is reproduced by continuously irradiating tracks on which recording patterns are formed with a laser beam having a read-out level. In this operation, a plurality of output levels is obtained depending on recording states. Intensity of output signals from the phase change optical recording medium is high enough, and a difference between output levels is also high enough. Therefore, a multilevel processing can be easily performed.

[0030] An example of the present invention will now be described referring to the accompanying drawings.

[0031] FIG. 3 is a cross sectional view of the multilayered multilevel phase change optical recording medium in this example. As shown in FIG. 3, a lower protective layer 2 is formed on a substrate 1. On the lower protective layer 2, there are provided a first recording layer (the i-th layer) R₁, an intermediate layer S₁, a second recording layer (the j-th layer) R₂, an upper protective layer 3, and a reflective layer 4. The thicknesses of these layers are so adjusted that the first and second recording layers are located within the depth of focus of recording laser beams.

[0032] FIG. 4 illustrates the construction of an optical disk drive used for recording and read-out operations. The optical disk 11 shown in FIG. 3 is mounted to a rotary shaft of a spindle motor 12. In recording operation, a laser 22 is operated by a light source controller 21 to emit a short-pulsed laser beam having a relatively high power level. The laser beam is passed through an objective lens 23, a half mirror 24 and a focusing lens 25 and then is incident onto the optical disk 11, thereby forming recording marks. In the read-out operation, a laser 22 is operated to emit a laser beam having a low power level. The laser beam is incident onto the optical disk 11 on which recording marks are formed. The laser beam reflected from the optical disk 11 is passed through the focusing lens 25 and reflected by the half mirror 24, and then is sent to the reproducing unit 26 where reflectance change between recording marks and non-recorded region is detected.

[0033] The optical disk drive has substantially similar arrangement to a conventional type, but the laser 22 can emit recording laser beams having different power levels by multilevel power modulation.

[0034] The operational conditions of the optical disk drive are as follows: linear velocity of the disk is 10 m/s, recording pulse frequency is 10 MHz, recording pulse width is 50 ns, laser beam wavelength is 650 nm, and numerical aperture (NA) of the objective lens is 0.6. When NA of the objective lens is 0.6, full width at half maximum (FWHM) of the beam spot becomes about 0.5 μm. The recording pulse width of 50 ns is substantially equal to duration within which FWHM of the beam spot passes the recording layer. This example uses recording laser beams having four different power levels of 6 mW, 9 mW, 12 mW and 15 mW corresponding to the power levels “O”, “A”, “B” and “C” shown in Table 1.

[0035] The thermal characteristics of the first and second recording layers are so adjusted that they meet suitable conditions for multilevel recording. The thermal conductivity and thickness of other layers are also designed in accordance with the thermal characteristics of the recording layers.

[0036] In this example, the first recording layer consists of 15 nm-thick Ge₂Sb₂Te₅, and the second recording layer consists of 15 nm-thick Ge₂Sb₂Te₅+5 at % -Sb. The first recording layer has a melting point T_m1 of 630° C. and a crystallizing time τ_x1 of 50 ns. The second recording layer has a melting point T_m2 of 630° C., equal to that of the first recording layer, and a crystallizing time τ_x2 of 70 ns. The lower protective layer 2 consists of about 150 nm-thick ZnS—SiO₂, the intermediate layer S₁ consists of polytetrafluoroethylene (PTFE) which is a good thermal insulator, the upper protective layer 3 consists of 20 nm-thick Si—N which is high in heat radiating effect, and the reflective layer 4 consists of 50-nm thick Al having a high thermal conductivity. These layers are deposited by conventional magnetron sputtering.

[0037] Thermal responses of both recording layers calculated from the above conditions are as follows: When the recording laser beams having power levels of 6 mW, 9 mW, 12 mW and 15 mW are used, respectively, the maximum temperatures will be 500° C., 700° C., 900° C. and 1100° C. for the first recording layer, and 450° C., 600° C., 750° C. and 900° C. for the second recording layer.

[0038] Also, calculated values of duration within which the recording layers are retained below the melting point and above the crystallizing temperature in the cooling process after the irradiation of recording laser beams of various power levels (referred to as a cooling time hereinafter) are as follows. The cooling times for the first recording layer will be 30 ns, 45 ns and 60 ns corresponding to the power levels of “A”, “B” and “C”, respectively. The maximum temperature of the second recording layer is maintained below the melting point in the case where the power level “A”, so that there is no need to consider its cooling time. The cooling times for the second recording layer will be 30 ns and 50 ns corresponding to the power levels of “B” and “C”, respectively.

[0039] As described above, the second recording layer is lower in the maximum temperature and shorter in the cooling time compared with the first recording layer. This can be explained by the fact that the recording laser beam is incident onto the side of the first recording layer, temperature difference between the first and second recording layers is maintained because the intermediate layer having a low thermal conductivity, and heat is taken away from the second recording layer through the upper protective layer and the reflective layer having a high thermal conductivity.

[0040] Judging from the relationship between the maximum temperature and the cooling time of each recording layer, recording marks will be formed neither first nor second layers if the power level is “O”, will be formed only in the first recording layer if the power level is “A”, will be formed in both first and second recording layers if the power level is “B”, and will be formed only in the second recording layer if the power level is “C”.

[0041] The actual operations of recording and reproducing by the optical disk drive shown in FIG. 4 are as follows. While the spindle motor is operated to rotate the optical disk at a linear speed of 10 m/s, a laser beam spot having a read-out power level (for example, 1 mW) is focused on a desired track of the optical disk and then recording is performed by using laser beams having different recording power levels. Read-out is performed by continuously irradiating the recorded tracks with a laser beam having a read-out power level to detect read-out signals.

[0042] FIG. 5A shows an example of recording signal train. This diagram represents that recording is performed by irradiating a track with recording laser beams having three different power levels of “A”, “B” and “C” after the irradiation of laser beam having a read-out power level R. The recording frequency is set to 10 MHz. In FIG. 5A, the power level is set to “B” during the period of time t₁, t₅, and t₇, to “C” during the period of time t₂ and t₄, and to “A” during the period of time t₃ and t₆. In a sequence of t₁ to t₇, a recording pattern of “1010111” is formed in the first recording layer, and simultaneously, another recording pattern of “110110” is formed in the second recording layer.

[0043] The read-out signals are shown in FIG. 5B. The read-out signals include a peak level at a position where recording marks are formed in both first and second recording layers, a bottom level (without including the non-recording level) at a position where a recording mark is formed in only the second recording layer, and an intermediate level at a position where a recording mark is formed in only the first recording layer. This result can be explained by the fact that reflected laser beam from the first recording layer is directly incident upon the detecting system, whereas reflected laser beam from the second recording layer passes through the first recording layer and is then incident upon the detecting system.

[0044] The read-out signals shown in FIG. 5B can be digitized by setting windows with appropriate slice levels including the individual output levels, even if the output level is fluctuated to some extent. For example, the output levels of “A”, “B” and “C” may be digitized to “10”, “11”, and “01”. Assuming that the non-recording level is “00”, the recording and read-out can be performed at as a high density as two times that of the conventional medium.

[0045] The material of the recording layer is not limited to GeSbTe-based material employed in the above example and may be selected from other phase change optical recording materials including InSbTe-based and GeSbTe-based materials. Although the thermal responses of the recording layers in the above example are adjusted by varying the composition in the same material system, they may be controlled by using two or more materials having appropriate melting points and crystallizing times.

[0046] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.