Method of determining the oxidation resistance of the recording film of an information storage medium

Abstract: An information storage medium is disclosed comprising a substrate (9), and a recording film (10), formed on the substrate (9), comprising tellurium or tellurium alloy, carbon, and hydrogen, the recording film (10) absorbing far-infrared rays in a wavelength range of 25 to 100 µm by a chemical bonding of tellurium and carbon having a carbon-hydrogen bond. This information storage medium has excellent oxidation resistance and its oxidation resistance can be quickly determined. A method is also disclosed for quickly determining the oxidation resistance of a tellurium film when the method used to manufacture the film is unknown.

Description: The present invention relates to a method of determining the oxidation resistance of the recording film of an information storage medium. Recording films in which information is recorded by forming pits using a semiconductor laser beam are known. For example, tellurium films are known as recording films having high sensitivity. However, tellurium recording films are susceptible to oxidation, and therefore their information write performance (a state wherein pits can be formed in the optical recording film) cannot be maintained for long periods of time. Attempts have been made to slightly modify the tellurium film while retaining its advantageous characteristics. For example, a recording film containing tellurium and carbon is formed by depositing tellurium and carbon together, thereinafter referred to as a codeposition Te/C film (W.Y. Lee et al, Proceedings of SPIE 382 , 282 (1983)). In addition, it is known that if a tellurium target is sputtered in an atmosphere of a hydrocarbon gas or mixture of a hydrocarbon gas and an inert gas, an optical recording film having a structure in which tellurium clusters are dispersed in a matrix containing carbon is obtained (U.S.P. 4,433,340). This is known as a Te/C film. The co-deposition Te/C film, however, does not have excellent oxidation resistance whereas the recording film containing tellurium and carbon formed by the above sputtering is excellent in oxidation resistance. If the manufacturing method used to produce the film is not known, it is difficult to discriminate between these two films based on their appearances. For this reason, oxidation resistance of a film must be determined by performing an acceleration test to check the decrease in reflectance of the film under high temperature and humidity. This test is very time-consuming, taking as long as 1,000 hours or more. A means for quickly determining oxidation resistance has not been proposed yet. Prior art document EP-A-186 467 discloses an optical data recording medium which includes a substrate and a recording layer thereon formed of crystalline material and including crystals that grow to record data in the layer in response to having a laser beam focused thereon. Examples of material suitable for the recording layer are tellurium including carbon of 10 atomic percent to 80 atomic percent, or chemical compounds of tellurium and carbon and tellurium including carbon of 10 atomic percent to 80 atomic percent and hydrogen of 50 atomic percent or less or chemical compounds of carbon and hydrogen or chemical compounds of tellurium, carbon and hydrogen. The recording layer can be formed by sputtering tellurium in an organic gas containing carbon and hydrogen. Further, prior art document Patent Abstracts of Japan, no. 282 (P-243)(1427) 16 December 1983, & JP-A-58158054 (Tokyo Shibaura Denki K.K.) 20 September 1983, describes an optical information recording medium having a high sensitivity and a high S/N ratio by composing a light absorbing layer in a three-layer structure of a film consisting of metal and elements such as N, C and H. A film excellent in oxidation resistance can be formed, if metallic Te is present together with -CH₃, -CH₂ and -CH. Finally, prior art document EP-A-238 095 describes a photosensitive member composed of a charge transporting layer and a charge generating layer. The charge transporting layer is a plasma-polymerized layer of an amorphous material comprising hydrogen and carbon wherein the infrared absorption spectrum has a specific peak absorptivity. Radials -CH₃ and -CH₂ can be detected by a far-infrared spectroscopy. It is an object of the present invention to provide a method of quickly and easily determining the oxidation resistance of an information recording film containing Te, C and H when the method used to manufacture the film is not known. To solve this object the present invention provides a method as specified in claim 1. The information recording film having a good oxidation resistance and in which information is read based on an intensity of light reflected from the recording film when reproducing light is applied to a recording film, is formed on a substrate that comprises tellurium or tellurium alloy, carbon, and hydrogen. The recording film comprises tellurium chemically-bonded to an organic carbon having a carbon-hydrogen bond, and absorbs far-infrared rays in a wavelength range of 25 to 100 µm. The recording film comprises preferably a -CH₂- bond of carbon and hydrogen. The method for determining the oxidation resistance of an information storage medium when the method of manufacturing the medium is unknown, comprises especially step of determining whether the carbon is organic or inorganic carbon by using X-ray photoelectron spectro-scopy or far-infrared absorption spectroscopy. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view of an information storage medium; Fig. 2 is a schematic view of an apparatus for manufacturing a recording film; Fig. 3 is a schematic view of an apparatus for manufacturing a co-deposition Te/C film; Fig. 4 is a graph showing a result of an oxidation resistance test; Fig. 5 is a graph of absorption spectra of recording films in a far-infrared wavelength range of 25 to 100 µm; Fig. 6 is a graph showing a correlation between an absorbance of far-infrared rays and a Q value; Fig. 7 is an XPS graph of the recording film; and Fig. 8 is an XPS graph of a co-deposition Te/C film. Preferred embodiments of the present invention are described with reference to the drawings. In an information storage medium, recorded information is read based on an intensity of light reflected from a recording film when reproducing light is applied to the recording film. Fig. 1 is a schematic sectional view showing a structure of the information storage medium which comprises a substrate 9 and a recording film 10 formed thereon. Substrate 9 is made of a material transparent to a laser beam which is radiated onto the information storage medium so as to record and reproduce information. When a laser beam having wavelength, for example, near the range of near-infrared rays is used, polycarbonate (PC), polymethyl methyacrylate (PMMA), polylefin, epoxy resin, and the like are used. Alternatively, when a write or reproducing laser beam is incident from the recording film side without passing through the substrate, the substrate may be opaque. Recording film 10 contains Te, C, and H, and has a structure in which Te clusters are dispersed in a C-H matrix where carbon and hydrogen are bonded to each other by chemical bonds. This film is an amorphous film in which no diffraction peak is recognized from any specific diffraction angle by an X-ray diffraction analysis. Information is recorded by forming pits in the recording film. The recording film preferably has a thickness of about 10 nm to 100 nm (100 to 1,000 Å). If the film thickness exceeds about 100 nm (1,000 Å), large energy is required to form pits, and hence write sensitivity is degraded since much energy is required to form pits. If it is smaller than about 10 nm (100 Å), the recording film becomes discontinuous, and probability of formation of pin holes is increased. These pin holes may be erroneously detected as pits in the reading operation. In addition, the pin holes become starting points for oxidation of the recording medium, and hence must be decreased as much as possible. The information storage medium can be obtained by, e.g., sputtering a Te target placed in a vacuum chamber by discharge while feeding a gas mixture of a hydrocarbon gas and an inert gas, and depositing a Te/C film composed of Te and C-H polymer on a substrate placed in the vacuum chamber. The recording film will hereinafter be referred to as a Te-C-H film. According to the present invention, the absorption spectrum of a recording film formed on a substrate in the far-infrared range (wavelength: 2.5 to 10 nm (25 to 100 µm)) is measured. When a recording film has a mixture of carbon and Te, i.e., when it has the structure of a co-deposition Te/C film, it does not absorb light in the far-infrared range. Only a recording film having a structure in which organic carbon in the form of -CH₃ bond, a -CH₂ bond or the like is chemically bonded to Te, i.e., a Te-C-H film, absorbs light in the farinfrared range. An absorption spectrum can be measured within an hour by IR spectroscopy. Therefore, according to the invention, discrimination of a Te-C-H film of the present invention from a co-deposition Te/C film, i.e., the determination of oxidation resistance of a film containing Te, C, and H when the method used to manufacture the film is not known can be quickly and easily performed. The recording films cannot be discriminated from each other by checking the carbon distributions of the recording films in the depth direction by performing Auger electron spectroscopy together with sputter etching. Binding electrons of carbon in a recording film escape outside the film as photoelectrons by X-ray radiation. Binding energy of electrons in carbon is changed depending on the state of chemical bonding of the carbon. The bonding state of C, i.e., whether the carbon is organic or inorganic carbon, can be determined by using an X-ray photoelectron spectroscopy (XPS). In the present invention, it was found that a Te-C-H film can be easily discriminated from a co-deposition Te/C film by checking an XPS peak at 284.7 eV which shows a -CH₂- bond. Example 1 A method of forming an information storage medium shown in Fig. 1 will be described below. Fig. 2 is a schematic view of a sputtering apparatus for forming the Te-C-H film. Chamber 1 is preliminarily evacuated to 26,66 Pa (0.2 Torr) by setting valve 2 to the rotary oil pump 3 side. Subsequently, valve 2 is set to cryopump 4 side so as to further evacuate chamber 1 to at least 6,666·10⁻³ Pa (5 × 10⁻⁵ Torr) or less. A CH₄ gas and an Ar gas are respectively fed into chamber 1 at 5 SCCM, respectively, through CH₄ gas line 5 and Ar gas line 6. Valve 2 is controlled so as to set the pressure in chamber at 6,666·10⁻³ Pa (5 × 10⁻³ Torr) by referring to an ion gauge (not shown), and it is confirmed that this pressure does not change for at least one minute. Sputtering is performed by applying an electric power of 100 W from power supply 8 to Te target 7 having a diameter of 12,7 cm (5 inches). PC substrate 9 is rotated at 60 rpm in advance by a substrate rotating unit (not shown), and recording film (Te-C-H film) 10 containing Te, C, and H is deposited on substrate 9 to a thickness of 30 nm (300 Å). In this Example, a CH₄ gas is used as a source gas for carbon and hydrogen in the recording film. However, other hydrocarbon gases such as C₂H₆, C₃H₈, C₂H₄, and C₂H₂ may be used. A co-deposition Te-C film was formed as a Comparative Example. In an apparatus shown in Fig. 3, valve 12 is set to the rotary oil pump 13 side so as to preliminarily evacuate chamber 11 to 26,66 Pa (0.2 Torr). Value 12 is then set to the cryopump 14 side so as to further evacuate chamber 11 to at least 6,666·10⁻³ Pa (5 × 10⁻⁵ Torr). Subsequently, an electric power of 100 W is applied from deposition heater power supply 29 to tungsten boat (deposition boat) 25 on which Te target 17 having a diameter of five inches is mounted, thereby heating boat 25 and evaporating Te. At the same time, an electric power of 100 W is applied from heating power supply 30 to electron beam heating stage 27 on which charcoal 28 is mounted, thereby evaporating carbon by an electron beam heating method. Simultaneously-evaporated Te and C were deposited on PC substrate 19 to form recording film (co-deposition Te/C film) 20. This recording film has the same thickness as that of the recording film in the Example. Fig. 4 shows a result obtained by checking changes in reflectance of the two recording films at a temperature of 60°C and a humidity of 90%. Reflectances are normalized assuming that a reflectance immediately after the formation of each film is 1. For the sake of comparison, Fig. 4 also shows the reflectance of a Te film having the same thickness. The reflectance of the Te film decreases most rapidly (oxidized). The reflectance of the co-deposition Te/C film decreases less rapidly than the Te film, but much more rapidly than the Te-C-H film. The reflectance of the Te-C-H film is not lowered even after 1,000 hours. Hence, the Te-C-H film has the best oxidation resistance. Fig. 5 shows far-infrared absorption spectra in the wavelength range of 25 to 100 µm of the Te-C-H film and the co-deposition Te/C film. As shown in Fig. 5, although C is present in both films, an absorption peak of 160 cm⁻¹ (62.5 µm), which represents a chemical bond of organic carbon C and Te, was found only in the Te-C-H film. Therefore, whether absorption in the far-infrared range is found or not is a criterion for judging oxidation resistance. When the results shown in Figs. 4 and 5 are compared, it is clear that determination of oxidation resistance of a recording film containing Te, C, and H can be performed faster by checking the magnitude of absorbance A at a wave number of 160 cm⁻¹, which is an index representing the amount of Te chemically bonded to organic carbon C in a recording film, than by an acceleration test requiring a period of time as long as 1,000 hours or more. Fig. 6 shows a relationship between ratio Q of flow rate X of a hydrocarbon gas (CH₄ gas) and flow rate Y of a rare gas (Ar gas) in a manufacturing process of a Te-C-H film (Q = {X/(X + Y)} × 100%) and absorbance A. It is apparent that absorbance A is substantially constant as long as Q is 20% or more. Constant absorbance A means that the number of chemical bonds of organic carbon C and Te is not changed in the film. Examples 2 - 9 A recording film containing a Te alloy, C, and H is formed for each Example under the same conditions as those in Example 1 except that the Te target in Example 1 is replaced with an alloy target of Te and Se, Ge, Bi, Pb, Sn, Ag, In or Ga. Absorption of far-infrared rays due to a chemical bond of a Te alloy and organic carbon C in each recording film fell within the range of 25 to 100 µm (400 to 100 cm⁻¹). Example 10 Fig. 7 shows X-ray photoelectron spectra (XPS) from the orbital C 1S of the Te-C film obtained in Example 1. As is apparent from Fig. 7, the main peak of -CH₂- bond appears at 284.7 eV, and small peaks of a C-O bond (286.2 eV), C = 0 bond (287.7 eV), and a bond (288.7 eV) are present. That is, carbon is mainly present as the -CH₂- bond in the Te-C-H film. Fig. 8 shows an XPS spectrum obtained from the co-deposition Te/C film (C, which is not bonded with H and O, is bonded with Te) obtained in the Comparative Example. In this spectrum, a peak at 284.7 eV representing a -CH₂- bond does not appear. It is apparent from both the spectra that if an XPS peak representing a -CH₂- bond is obtained from a given film, this film is a Te-C-H film. Since XPS measurement can be performed within an hour, discrimination of a Te-C-H film from a co-deposition Te-C film can be quickly performed by this method.

Claims: A method of determining the oxidation resistance of the recording film (10) of an information storage medium of the type having a substrate (9) and a recording film (10) supported on said substrate (9) and comprising tellurium or tellurium alloy, carbon, and hydrogen; the method being characterized by comprising the step of: determining whether the recording film comprises tellurium chemically bonded to an organic carbon having a carbon-hydrogen bond by using X-ray photoelectron spectroscopy or far-infrared absorption spectroscopy. A method of determining the oxidation resistance of the recording film (10) as claimed in claim 1, comprising the steps of: performing a far-infrared absorption scan of the recording film (10) between the wavelengths of about 25 to 100 µm, and determining the presence or absence of an absorption peak corresponding to a chemical bond of tellurium with an organic carbon having a carbon-hydrogen bond. A method of determining the oxidation resistance of the recording film (10) as claimed in claim 1, comprising the steps: producing an X-ray photoelectron spectra for the film, and determining the presence or absence of a peak (31) corresponding to a -CH₂- bond.

Inventors: OHKAWA HIDEKI C O PATENT DIVIS, OHKAWA, HIDEKI C/O PATENT DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: G11B7
IPC codes: B41M5, G11B7