Abstract:
A structure of an optical recording medium that mainly includes a first dielectric layer, a dye layer, a second dielectric layer, and a reflective layer formed on a substrate in sequence, and a method for fabricating an optical recording medium of the foregoing structure. The additional dielectric layers of the optical recording medium, instead of a quencher used in a conventional optical recording medium, increase the lifetime of a dye layer and reduce the fabrication cost. Furthermore, the additional dielectric layers are capable of isolating the dye layer from oxygen and moisture to enhance the lightfastness of the dye layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 87113547, filed Aug. 18, 1998, the full disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical recording medium, and more particularly, to a recordable optical recording medium. 
     2. Description of Related Art 
     Since the growth of multi-media applications for computers is raised according to the increasingly advanced computer hardware, recording media that are capable of storing a mass of data are needed. An optical compact disc (CD) has been currently utilized as a medium for storing a mass of data, such as multi-media programs and files because of its capacity and data-retrieving rate. 
     CDs can be divided into three categories, read-only CD, recordable CDs, and rewritable CDs mainly. The read-only CD, or a ca-called CD read-only memory (CD-ROM), is developed based on CD digital audio (CD-DA) and is capable of storing 650-Megabyte (MB) data. Because the data stored on a CD-ROM is burned onto a surface of the CD-ROM during the fabrication process, users are not allowed to modify and edit the stored data. Hence, a recordable optical recording medium, CD-recordable or CD-R, was introduced into the market in 1988. 
     The structure of a CD-R is similar to what of a traditional CD as shown in FIG.  1 . Referring to FIG. 1, a conventional CD-R consists of a 1.2-mm grooved polycarbonate (PC) substrate  12 , a 0.2-μm dye layer  14 , a metallic reflective layer  16 , and a protective layer  18 . The metallic reflective layer  16  is gold or silver formed by a vacuum sputtering process, and the protective layer  18  is an ultraviolet-cured resin formed by spin-coating. The dye layer  14 , which is not present in a conventional CD-ROM, reacts with laser beams to record data on the CD-R. Compared with a conventional CD-R, besides the lack of a dye layer  14 , a conventional CD-ROM has pre-formed data pits on the substrate  12  before the formation of other components. 
     The dye layer  14  of a CD-R has certain requirements including: 
     1. capability of absorbing the laser beam and converting the laser beam into heat, wherein the heat is then used to heat the dye layer  14 , melt and dissolve the dye layer, and form data pits on the substrate  12 ; 
     2. property of a excellent process resolvability for a spin-coating process; and 
     3. stability on environment to keep the stored data for a long period of time. 
     The dye layer in most currently available CD-Rs include cyanine, phthalocyanine, and azo-metal complex, wherein cyanine is the most common material used to form a dye layer in a CD-R because of its advantages include non-toxicity, metal-likeness, film-formability, and excellent resolvability. In the fabrication process of a conventional CD-R with a cyanine dye layer, a quencher is normally added to enhance the poor lightfastness of cyanine. Even though the method, which is provided in U.S. Pat. No. 5,328,741, U.S. Pat. No. 5,328,802 and U.S. Pat. No. 5,336,584, is able to last the lifetime of dye layers of CD-R by adding a quencher, there are still drawbacks. The added quencher costs several times than the cyanine, so that the fabrication cost is increased. Since the volatility and the resolvability of quencher are relatively poor, that makes it difficult to processed. In addition, the crystallization of quencher occurring in the dye layer due to an improper process degrades the recording capability of the dye layer. 
     According to the foregoing, a conventional method that increases the lightfastness of a dye layer by adding quencher is not cost effective and cost competitive, and tends to damaging the dye layer. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide a method and a structure for fabricating an optical recording medium that includes forming an additional dielectric layer, instead of adding a quencher, to increase the lifetime of a dye layer. The additional dielectric layer is capable of preventing the dye layer from oxygen and moisture to enhance the lightfastness of the dye layer. The method of the invention is also able to reduce the fabrication cost by not adding expensive quencher. 
     In accordance with the foregoing and other objectives of the present invention, the invention provides a structure of an optical recording medium that includes a dielectric layer, a dye layer, and a reflective layer formed on a substrate in sequence. The dielectric layer isolates the dye layer from oxygen and moisture to improve the lightfastness and the thermostability of the dye layer. Furthermore, the dielectric layer further increases the lifetime of the dye layer. 
     In addition, another dielectric layer can be added between the reflective layer and the dye layer to prevent the dye layer from being evaporated by a long-term illumination. 
     A method for fabricating the forgoing optical recording medium of the invention is also porvided. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
     FIG. 1 is a perspective cross-sectional view of an optical recording medium; 
     FIGS. 2A through 2G are fragmental cross-sectional views showing comparison between the structure of a conventional optical recording medium and the optical recording medium of a preferred embodiment according to the invention; 
     FIG. 3 is a graphic plot showing the relationship of optical densities of the structures shown in FIGS. 2A through 2C versus the exposure duration; 
     FIG. 4 is a reflection spectrum showing the reflectivity of the structure shown in FIG. 2D versus the exposure duration, where the exposure duration difference between two adjacent curves is 2 hours; 
     FIG. 5 is a reflection spectrum showing the reflectivity of the structure shown in FIG. 2E versus the exposure duration where the exposure duration difference between two adjacent curves is 2 hours; 
     FIG. 6 is a reflection spectrum showing the reflectivity of the structure shown in FIG. 2F versus the exposure duration where the exposure duration difference between two adjacent curves is 2 hours; and 
     FIG. 7 is a reflection spectrum showing the reflectivity of the structure shown in FIG. 2G versus the exposure duration where the exposure duration difference between two adjacent curves is 2 hours. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention provides a new structure of an optical recording medium and a method for fabricating the optical recording medium of the structure according to the invention. 
     Referring to FIG. 2F, a dielectric layer  32 , a dye layer  34 , and a reflective layer  38  are formed on a substrate  30  in sequence, wherein the dielectric layer  32  is one selected from ZnS, SiO 2 , Si 3 N 4 , Al 3 O 3 , Ta 2 O 3 . and other materials of the similar properties. The thickness of the dielectric layer  32  is about 100 Å to 1000 Å, and is preferably about 300 Å to 800 Å. The dye layer  34  is one selected from cyanine, phthalocyanine, azo-metal complex, and other materials of the similar properties, wherein the thickness of the dye layer  34  is about 1000 Å to 5000 Å, preferably about 1500 Å to 3000 Å. The reflective layer  38  is one selected from gold, silver, aluminum, nickel, titanium, and other materials of the similar properties, wherein the thickness of the reflective layer is about 500 Å to 2000 Å, preferably about 500 Å to 1000 Å. 
     The dielectric layer  32  between the substrate  30  and the dye layer  34  prevents the dye layer  34  from oxygen and moisture to improve the lightfastness and the thermal stability, and further to last the lifetime of the dye layer  34 . An additional dielectric layer  36  can be placed between the dye layer  34  and the reflective layer  38 , as shown in FIG. 2G, to prevent the dye layer from being evaporated by a long-time exposure. 
     Two examples are given as following to describe the processes and methods of fabricating the foregoing structures of the invention. 
     EXAMPLE ONE 
     FIG. 2C is a fragmental cross-sectional view showing the structure of an optical recording medium of the invention, wherein the dye layer  24  of the optical recording medium of the invention is between two dielectric layers  22  and  26 , and on a substrate  20 . The substrate  20  is placed in a radio frequency (RF) sputter for forming a dielectric layer  22 , wherein the preferred condition within the RF sputter includes 50-watt power and argon pressure about 3×10 −3  torr. The thickness of the dielectric layer  22  is about 500 Å preferably. The substrate  20  normally includes polycarbonate (PC), and the dielectric layer  22  includes ZnS—SiO 2 . 
     A dye layer  24  is then spin-coated on the dielectric layer  22  by spin coater, wherein the dye solution is 3 wt % 2,2,3,3-tetraflouropanol solution of cyanine. The dye layer  24  is formed by using a spin-coating method, wherein the preferable thickness of the dye layer  24  is about 1000 Å to 2500 Å thick at the end of drying. 
     The substrate  20  is place 0 d into a RF sputter again, wherein the preferred condition within the RF sputter includes 50-watt power and argon pressure about 3×10 −3  torr. The dielectric material (such as ZnS—SiO2) is vacuum deposited onto the dye layer  24  as another dielectric layer  26 . The preferable thickness of the dielectric layer  26  is about 500 Å. 
     EXAMPLE TWO 
     The structure of an optical recording medium of the invention is shown in FIG.  2 F. 
     Referring to FIG. 2F, a substrate  30  is placed in a RF sputter wherein the condition is 50-watt power and the pressure is about 3×10 −3  torr. The dielectric material (such as ZnS—SiO2) is vacuum deposited onto the dye layer  30  as another dielectric layer  32 . The thickness of the dielectric layer  32  is about 500 Å preferably. The substrate  30  normally includes PC, and the dielectric layer  22  includes ZnS—SiO 2 . 
     A dye layer  34  is then formed on the dielectric layer  32 , wherein the dye solution 3 wt % 2,2,3,3-tetrafluoropropanol solution of cyanine. The dye layer  34  is formed by using a spin-coating method, wherein the preferable thickness of the dye layer  24  is about 1000 Å to 2500 Å. 
     A reflective layer  38  of about 800 Å thick is formed on the dye layer  34  by a sputtering deposition process, wherein the reflective layer  38  can be gold, silver, or other materials of the similar properties. The top surface of the optical medium is then covered with UV-cured resin as a protective layer  40 , wherein the protective layer  40  is ultraviolet curable and about 1 μm in thickness preferably. 
     Among FIGS. 2A through 2G, FIGS. 2C and 2F are cross-sectional views showing the structures of optical recording media according to the invention. FIGS. 2A and 2D, which are cross-sectional views showing the structures of conventional optical recording media, are listed for the comparison with the structures of the optical recording media of the invention. 
     The structure  2 A through are exposed to 7.4 mW/cm 2  Xe-lamp for several hours, the reflectivity (R), and the optical density (OD) of the structures shown in FIGS. 2A through 2G are measured every two hours. The measured data are then analyzed to signalize the improvement according the invention. 
     FIG. 3 is a graphic plot showing the normalized OD, OD (%), obtained from the structures shown in FIGS. 2A through 2C. Referring to FIG. 3, the Y axis represents OD in percentage (%), which is a normalized measurement, a ratio of OD i  to OD o . The OD i  is the measured OD under the foregoing exposure for i hours, and the OD o  is a measured OD without the presence of the foregoing exposure. As shown in FIG. 3, curve A, the OD value decreases with the increasing exposure time for structure  2 A (shown in FIG.  2 A). 
     There are two reasons causing the OD value to decrease rapidly: 
     1. Dye in the dye layer  24 ′ is vaporized by the exposure; and 
     2. Dye in the dye layer  24 ′ is degenerated because of reacting with oxygen after exposure. 
     Referring to FIG. 2B, a dielectric layer  26 ′ including ZnS—SiO 2  is deposited on the dye layer  24 ′ to prevent the vaporization of the dye in the dye layer  24 ′ in a long-time exposure. As a result, the measured OD, curve B in FIG. 3, still decreases with the increasing exposure time for structure, but in a smoother rate, that is the protection over the dye layer  24 ′, the lightfastness, is improved. Referring to FIG. 2C, which shows the structure of the invention, a dielectric layer  22 , a dye layer  24 , and another dielectric layer  26  are formed on a substrate  20  in sequence. The OD measured on the structure of the invention is represented by curve C in FIG. 3, wherein the curve C, the measured OD, remains almost unchanged after 12-hour exposure. 
     According the foregoing, the dielectric layer  22  of the structure according to the invention prevents the oxygen from osmosing to the dye layer  24  through the substrate  20 , that is, the invention actually improves the lightfastness of the dye layer  24 . 
     FIGS. 4 and 6 show the reflection spectra of a conventional optical recording medium (shown in FIG. 2D) and the structure according to this invention (shown in FIG. 2F) at different exposure time where the exposure duration difference between two adjacent curves is 2 hours. By comparing FIGS. 4 and 6, it is obvious that the reflectivity (R) of the structure of our invention (shown in FIG. 2F) remains almost unchanged after eight-hour exposure. It remains 65% reflectivity at wavelength 780 nm. Referring to the wavelengths covered in the graphic plot in FIG. 6, the optical recording medium of the invention can further used as a structure of a DVD-R. 
     According to the foregoing, the specificity of the invention is to add a dielectric layer between the substrate and the dye layer of an optical recording medium, instead of adding quencher, to improve the lightfastness of the dye layer and reduce the fabrication cost. 
     It is also a specificity of the invention to adding a dielectric layer between the substrate and the dye layer to ensure an excellent reflectivity of the dye layer after under a long-time exposure. 
     It is still a specificity of the invention to improve the stability of the dye layer for lasting the lifetime of the optical recording medium. 
     The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.