DESCRIPTION OPTICAL INFORMATION RECORDING MEDIUM Technical Field 5 [00011 The present technology relates to an optical information recording medium. More specifically, the present technology relates to an optical information recording medium that includes a substrate having a recessed 10 portion on a surface thereof, a recording layer, and a reflective layer. Background Art [00021 I I i As a recordable optical information recording medium, 1 15 there are a rewritable optical information recording medium represented by a compact disc-rewritable (CD-RW) or a digital versatile disc f rewritable (DVDkRW) and a writeonce optical information recording medium represented by a compact disc-recordable (CD-R) or a digital versatile disc- 20 recordable (DVD-R), but in particular, the latter has contributed greatly to the expansion of the market as lowcost media. [00031 As a recording material used for the write-once optical 25 information recording medium, there are an inorganic - 2 - SP347112 material and an organic dye material, but conventionally, the organic dye material has been mainly examined as the recording material. As an optical information recording medium using the organic dye material, an optical 5 information recording medium having a configuration in which a reflective layer is provided on a surface of a recording layer including the organic dye material has been adopted. As a material of the reflective layer, silver (Ag) is widely used. As a material other than silver (Ag), an example in 10 which aluminum (Al) is used is disclosed (for example, see PTLs 1 to 4). Citation List Patent Literature [0004] 15 PTL 1: Japanese Unexamined Patent Application Publication No. 5-54431 PTL 2: Japanese Unexamined Patent Application Publication No. 5-62245 PTL 3: Japanese Unexamined Patent Application Publication No. 20 6-195746 PTL 4: Japanese Unexamined Patent Application Publication No. 6-282870 Disclosure of Invention Technical Problem 25 [0005] In recent years, a further reduction in costs of the medium has been desired, and as a technology to meet this demand, a case in which aluminum (Al) is used instead of silver (Ag) as the material of the reflective layer has been examined. However, when using aluminum (Al) as the material of the reflective layer, the reflectance is degraded compared to a case when using silver (Ag) as the material of the reflective layer. [0006] The present technology provides an optical information recording medium that can suppress degradation of the reflectance. Technical Solution [0007] In order to solve the above described problem, the present technology provides an optical information recording medium including: a substrate that has a recessed portion on a surface thereof; a recording layer; and a reflective layer, in which degradation of the reflectance is suppressed by combination of ranges of an optical density of the recording layer and a depth of the recessed portion of the substrate. Advantageous Effects [0008] As described above, According to the present technology, degradation of the reflectance can be suppressed. Brief Description of Drawings [0009] Fig. 1A is a schematic cross-sectional view showing a 5 configuration example of an optical information recording medium according to an embodiment of the present technology, and Fig. 1B is an enlarged cross-sectional view showing a part of Fig. 1A. Fig. 2A is a view showing changes in a maximum 10 reflectance Rtop with respect to an effective groove depth d and a push-pull signal PPb, and Fig. 2B is an enlarged view showing an area A of Fig. 2A. Fig. 3A is an enlarged cross-sectional view showing a part of an optical information recording medium having a 15 deep groove, Fig. 3B is an enlarged cross-sectional view showing a part of an optical information recording medium having a groove with a specified depth, and Fig. 3C is an enlarged cross-sectional view showing a part of an optical information recording medium having a shallow groove. 20 Fig. 4A is a view showing an evaluation result of a maximum reflectance Rt,, of a write-once optical information recording medium in samples 1-1 to 1-8 and 2-1, and Fig. 4B is a view showing an evaluation result of a push-pull signal PPb at an unrecorded time of a write-once optical 25 information recording medium in samples 1-1 to 1-8 and 2-1. Fig. 5A is a view showing an evaluation result of a push-pull signal PPb at an unrecorded time of a write-once optical information recording medium in samples 3-1 to 6-2, and Fig. 5B is a view showing an evaluation result of a 5 maximum reflectance R,,, of a write-once optical information recording medium in samples 3-1 to 6-2. Best Modes for Carrying Out the Invention 10 Embodiments of the present technology will be described with reference to the accompanying drawings in the following order. 1. Summary 2. Configuraticn of optical information recording medium 15 3. Optical characteristics of optical information recording medium 4. Principle of compatibility between push-pull signal PPb and maximum reflectance R,,, 5. Method of manufacturing optical information recording 20 medium [OOll] [I. Summary] When using aluminum (Al) as a material of a reflective layer, the reflectance is degraded compared to when using 25 silver (Ag) as the material of the reflective layer, and therefore it is difficult to achieve a good balance between an excellent push-pull signal and reflectance. According to the findings of the present inventors, the push-pull signal and the reflectance are in a conflicting relation. That is, 5 when increasing a depth of a recessed portion (for example, groove) of a substrate, the push-pull signal can be improved, but the reflectance can be degraded. Thus, in order to achieve the good balance between the excellent push-pull signal and the reflectance, keen examination has been made 10 by the present inventors. As a result, the present inventors have found that variation of the reflectance with respect to a change in the depth of the recessed portion is suppressed in a case in which optical density of a recording layer of a predetermined range and the depth of the recessed 15 portion of the substrate of a predetermined range are I combined, contrary to prediction. I [00121 [2. Configuration of optical information recording medium] I Fig. 1A is a schematic cross-sectional view showing a I I 20 configuration example of an optical information recording medium according to an embodiment of the present technology, and Fig. 1B is an enlarged cross-sectional view showing a part of Fig. 1A. The optical information recording medium 10 is a so-called write-once optical information recording 25 medium in a single layer, and as shown in Fig. lA, a recording layer 2, a reflective layer 3, and a protection layer 4 are laminated on one main surface of the substrate 1 in this stated order. [0013] 5 In the optical information recording medium 10 according to the present embodiment, by irradiating the recording layer 2 with a laser beam L from a surface C of the substrate 1 side, recording or reproduction of information signals is performed. For example, by 10 condensing the laser beam L having a wavelength in a range of 770 nm to 790 nm using an objective lens having a numerical aperture in a range of 0.44 to 0.46 and irradiating the recording layer 2 with the condensed laser beam from the surface C of the substrate 1 side, recording 15 or reproduction of the information signals is performed. As such an optical information recording medium 10, for example, a CD-R in a single layer can be used. [0014] Hereinafter, the substrate 1, the recording layer 2, 20 the reflective layer 3, and the protection layer 4 which constitute the optical information recording medium 10 will be sequentially described. [0015] (Substrate) 25 The substrate 1 has an annular shape while providing an opening (hereinafter, referred to as center hole) at a center. One main surface of the substrate 1 is an uneven surface, and the recording layer 2 is provided on the uneven surface. Hereinafter, a recessed portion of the uneven 5 surface is referred to as a groove la and a projection portion thereof is referred to as a land lb. [0016] As a shape of the groove la and the land lb, for I I example, various shapes such as a spiral shape, a concentric 10 shape, and the like can be used. In addition, the groove la and/or the land lb is wobbled (meanders) for the purpose of, for example, stabilization of linear velocity, address information addition, or the like. [00171 15 A diameter of the substrate 1 is selected as, for example, 120 mm. A thickness of the substrate 1 is selected in consideration of rigidity, preferably 0.3 mm to 1.3 mm, more preferably 0.6 mm to 1.3 mm, and is selected as, for example, 1.2 mm. In addition, a diameter of the center hole 20 is selected as, for example, 15 mm. A thickness D of the groove la of the substrate 1 is in, for example, a range of 198 nm to 220 nm. [0018] As a material of the substrate 1, for example, a 25 plastic material or glass can be used, and in view of cost, the plastic material is preferably used. As the plastic material, for example, a polycarbonate resin, a polyolefin resin, an acrylic resin, and the like can be used. . [0019] 5 (Recording layer) The recording layer 2 is a recording layer capable of recording information signals by irradiation of the laser beam L. The recording layer 2 includes an organic dye as a main component. As the organic dye, at least one of, for 10 example, phthalocyanine dyes, naphthalocyanine dyes, porphyrin dyes, cyanine dyes, merocyanine dyes, styryl dyes, squarylium dyes, and azo dyes can be used. [0020] (Reflective layer) 15 The reflective layer 4 preferably includes aluminum (Al) as a main component. This is because the cost of the optical information recording medium 10 can be reduced compared to a case in which the reflective layer 3 includes silver (Ag) as the main component. (Protection layer) The protection layer 2 is a resin layer obtained by curing a photosensitive resin such as an ultraviolet curing resin or the like. As a material of the resin layer, for 25 example, an ultraviolet curing acrylic resin is used. SP347112 [00221 [3. Optical characteristics of optical information recording medium] By combination of ranges of optical density (OD) of the 5 recording layer 2 and the depth D of the groove la of the substrate 1, a reduction of a maximum reflectance Rtop is suppressed. By this suppression, the maximum reflectance Rtop is maintained in a range of, preferably, 0.6 or larger. When the maximum reflectance Rtop is 0.6 or larger, recording 10 or reproduction of the optical information recording medium 10 can be satisfactorily performed by a general consumer drive. [00231 By the combination of the ranges of the optical density 15 of the recording layer 2 and the depth D of the groove la of the substrate 1, it is preferable that a push-pull signal PPb at an unrecorded time be improved. By this improvement, it is preferable that the push-pull signal PPb at an unrecorded time be set in a range of 0.080 to 0.13. When 20 the push-pull signal PPb at an unrecorded time is in this range, recording of the optical information recording medium 10 can be satisfactorily performed by the general consumer drive. [00241 25 In a case in which recording or reproduction of - 11 - SP347112 information signals is performed by condensing, by the optical information recording medium 10, a laser beam L having a wavelength in a range of 770 nm to 790 nm using an objective lens having a numerical aperture in a range of 5 0.44 to 0.46 and irradiating the recording layer 2 with the condensed light from a surface C of the substrate 1 side, it is preferable that a range of the optical density of the recording layer 2 be a range larger than 0.560 and less than or equal to 0.700, and a range of the depth D of the groove 10 la of the substrate 1 be a range of 198 nm to 220 nm. This is because, when the depth D of the groove la is in the range of 198 nm to 220 nm, the push-pull signal PPb at an unrecorded time can be increased depending on an increase in the depth D of the groove la, and variation of the maximum 15 reflectance Rt,, with respect to a change in the depth of the groove la can be suppressed. [0025] In the case in which recording or reproduction of information signals is performed by condensing, by the 20 optical information recording medium 10, a laser beam L I having a wavelength in the range of 770 nm to 790 nm using I I I the objective lens having a numerical aperture in a range of I I 1 0.44 to 0.46 and irradiating the recording layer 2 with the I I condensed light from the surface C of the substrate 1 side, 25 it is more preferable that the range of the optical density of the recording layer 2 be a range larger than 0.560 and less than or equal to 0.710, and the range of the depth D of the groove la of the substrate 1 be a range of 203.5 nm to 220 nm. This is because, when the depth D of the groove la 5 is in the range of 203.5 nm to 220 nm, the push-pull signal PPb at an unrecorded time can be increased depending on the increase in the depth D of the groove la, and variation of the maximum reflectance Rtop with respect to the change in the depth of the groove la can be suppressed. 10 [0026] By the combination of the ranges of the optical density of the recording layer 2 and the depth D of the groove la of the substrate 1, it is preferable that the variation of the maximum reflectance Rtop in the range of the depth D of the 15 groove la be suppressed. More specifically, It is preferable that, by the combination of the ranges of the optical density of the recording layer 2 and the depth ~ D of 1 the groove la of the substrate 1, the variation of the maximum reflectance Rtop with respect to the increase in the I 20 depth D of the groove la be suppressed while the push-pull I signal PPb at an unrecorded time is increased depending on an increase in the depth D of the groove la in the range of the depth D of the groove la. By such suppression of the variation of the maximum reflectance RtOp, it is preferable 25 that the maximum reflectance Rtop be substantially constant. - 13 - SP347112 Here, being substantially constant means that the range of variation of the maximum reflectance Rtop is 0.015 or less. [0027] The maximum reflectance Rtop is a reflectance when a 5 reproduction signal corresponding to a 11T signal of an EFM modulation signal is at a maximum level, and is expressed by the following Equation (1) . Rtop = Ro x Itop/Io..(. 1 ) Here, Ro denotes a reflectance in a mirror surface 10 portion on the optical information recording medium 10, I. denotes a reproduction signal level in the mirror surface portion, and It,, denotes a maximum level of a reproduction signal corresponding to the 11T signal of the EFM modulation signal. 15 [0028] The push-pull signal PPb at an unrecorded time is expressed by the following Equation (2). PPb = (11-12)pP / (I1+12)m ax... (2) Here, I, and I2 represent each output when dividing a 20 light receiving element of an optical detector into two in a radial direction, pp represents peak-to-peak, and max represents a maximum value. [00291 The optical density is a dimensionless quantity that 25 indicates how much intensity is weakened when light has passed through a certain object. When not including scattering and reflection, the optical density is simply called absorbance. In analytical chemistry, optical density Ah at a wavelength h is defined by the following Equation 5 ( 3 ) Ah = -loglo(I/Io)...(3) I: transmitted light intensity Io: incident light intensity Absorbance is represented by a logarithm while 10 transmittance is an exponential function of an optical path length, and therefore the absorbance, being proportional to the optical path length, is used to measure a film thickness of an organic dye. 15 [4. Principle of compatibility between push-pull signal PPb and maximum reflectance Rtopl Fig. 2A is a view showing changes in a maximum reflectance Rtop with respect to an effective groove depth d and a push-pull signal PPb, and Fig. 2B is an enlarged view 20 showing an area A of Fig. 2A. In Fig. 2B, each of a curved line Lo, a curved line LA, a curved line LBI and a curved line LC represents the following maximum reflectance Rtop of the optical information recording medium 10. Curved line Lo: maximum reflectance Rto, of the optical 25 information recording medium in which only the reflective layer and the protection layer are laminated without forming the recording layer on the substrate Curved line LA: maximum reflectance R,,, of the optical information recording medium 10 having a deep groove 5 Curved line LB: maximum reflectance R,,, of the optical information recording medium 10 having a groove with a specified depth Curved line LC: maximum reflectance Rt,, of the optical information recording medium 10 having a shallow groove 10 [00311 The effective groove depth d that is the horizontal axis of Figs. 2A and 2B is a groove depth at which a laser beam actually arrives, and particularly, a depth of the reflective layer 3. The depth of the reflective layer 3 15 depends on both the groove depth D of the substrate 1 and optical density OD (that is, a thickness t of the recording layer 2). Hereinafter, in a range of a specified optical density OD, the optical density OD which is a minimum value 1 is referred to as a minimum optical density ODmin, I and the 1 I I 20 optical density OD which is a maximum value is referred to I I as a maximum optical density OD,,,. Here, the thickness t of I the recording layer 2 is a thickness of the recording layer 2 in the groove la. [0032] 25 Fig. 3A to 3C are enlarged cross-sectional views showing a part of the optical information recdrding medium having a deep groove, a groove with a specified depth, and a shallow groove, respectively. Here, the groove with the specified depth denotes a groove in a range in which the groove depth D is a specified minimum depth Dmin or larger and a specified maximum depth Dm,, or less, the deep groove denotes a groove deeper than the groove with the specified depth, and the shallow groove denotes a groove shallower than the groove with the specified depth. [00331 Hereinafter, with reference to Figs. 2A to 3C, a principle of compatibility between a push-pull signal PPb and a maximum reflectance Rtop in the optical information recording medium 10 according to the embodiment will be described. [00341 Characteristics of the push-pull signal PPb do not rely on the groove depth D of the substrate 1, and therefore the following characteristics are shown regardless of the groove la being one of the deep groove (see Fig. 3A), the groove with the specified depth (see Fig. 3B), and the shallow groove (see Fig. 3C). That is, when the effective groove depth d is in a range of OIdIh/4, polarity of the push-pull signal becomes "positive". When the effective groove depth d is h / 8 , the push-pull signal becomes a maximum. LO0351 As shown in Fig. 2A, from the intersection between a straight line that defines upper and lower limits of a standard range of the push-pull signal PPb and a curved line 5 of the push-pull signal PPb, a range of the effective groove depth d that satisfies the standard range of the push-pull signal PPb is calculated. [0036] Meanwhile, characteristics of the maximum reflectance 10 Rto, do rely on the groove depth D of the substrate 1, and therefore the following characteristics are shown depending on whether the groove la is one of the deep groove (see Fig. 3A), the groove with the specified depth (see Fig. 3B), and the shallow groove (see Fig. 3C) . That is, when L\e 15 effective groove depth d is in a range of O