JPH10208299A - Optical information recording medium and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve both of the small variation in the recording marks occurring in the state of a recording layer and the durability to many times of repetitive overwriting by forming a second protective layer of a thin film which consists of a compsn. contg. carbon and hydrogen and has an absorption coefft. within a specific range. SOLUTION: This optical information recording medium is constituted by successively laminating a first protective layer 2, the recording layer 3, the second protective layer 4 and a refractive index layer 5 on a transparent substrate 1. The recording layer 3 reversibly performs the phase transition from the crystalline state to the amorphous state according to the intensity of photoirradiation. The second protective layer 4 is a thin film consisting of a compsn. contg. carbon and hydrogen and the absorption coefft. thereof is specified to a range of 0.05 to 2.5. As a result, the light transmitted through the amorphous part of the recording layer 3 is absorbed in the second protective layer 4. Consequently, the light absorption quantity of the recording layer 3 is higher in the crystalline parts than in the amorphous part. Oxygen is contained in the thin film constituting the second protective layer 4, by which the fluidization of the thin film constituting the recording layer 3 is prevented even if overwriting is repeated many times.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording medium capable of recording, erasing, and reproducing information by irradiating light, and more particularly, to a recording layer, reversibly between a crystalline state and an amorphous state. The present invention relates to a so-called phase-change optical information recording medium using a phase-change material, and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, optical information recording media capable of recording and reproducing information with light have been attracting attention as playing a central role in recording media in an advanced information society, and have been actively studied. I have. Among them, an optical disk using a circular thin plate as a substrate has attracted attention as the most influential recording medium with the spread of multimedia in the future.

There are three types of such optical disks: a read-only type typified by a compact disk or a laser disk, a write-once type in which a user can write information, and a rewritable type in which information can be rewritten. Among them, the rewritable type includes a magneto-optical type and a phase change type. In the phase change type, a material in which a phase change between crystal and amorphous reversibly occurs is used as a recording layer.

Recording of information by the phase change method is performed by irradiating a recording surface with a laser beam whose intensity is modulated according to a recording signal (for example, as shown in FIG. 4). At the same time, it is easy to record new information (so-called "overwrite"). That is, a portion of the recording layer irradiated with a laser beam having a high power (recording power) for a short time is rapidly heated to a temperature equal to or higher than its melting point, melted, and then rapidly cooled to form an amorphous state. Is formed, and the portion irradiated with the laser beam having a weaker power (erasing power) is heated to a crystallizable temperature range lower than the melting point to be in a crystalline state.

[0005] Conventional phase-change type optical discs have a purpose of increasing signal amplitude and improving repetition characteristics.
It has a multilayer structure in which a plurality of layers are provided in addition to the recording layer. For example, there is a four-layer structure in which a first protective layer, a recording layer, a second protective layer, and a reflective layer are provided in this order on a transparent substrate formed of a polycarbonate resin or the like.

As the first protective layer and the second protective layer, a mixed film of ZnS and SiO ₂ having excellent protection characteristics of the recording layer and sufficient mechanical characteristics for repetition of recording / erasing is used. I have. The recording layer is mainly composed of an Sb-Te-Ge alloy or the like having a high crystallization rate, and the reflective layer is composed of an alloy mainly composed of Al which is inexpensive and has excellent environmental resistance. Is widely used.

Here, the recording layer of the phase change optical disk is
At the time of recording / erasing by overwriting (except for the first recording), it is partially in an amorphous state or a crystalline state according to the immediately preceding recording / erasing. The recording layer material as described above generally has a higher thermal conductivity in the crystalline state than in the amorphous state, and the heat dissipated in the recording layer is larger in the crystalline state than in the crystalline state. Further, in the phase change type optical disk having the conventional layer configuration as described above, the light absorption in the recording layer is higher in the amorphous state than in the crystalline state. Furthermore, latent heat is required to melt the crystalline portion.

From these facts, in order to form a recording mark equivalent to a portion which has become amorphous by the immediately preceding recording on a portion of the recording layer which has been brought into a crystalline state by the immediately preceding recording,
Despite the fact that the crystalline state portion requires more energy than the amorphous state portion, the amount of heat absorption required for forming a recording mark is larger in the crystalline state portion than in the amorphous state portion. Also less.

As a result, the recording mark formed by overwriting has a difference in shape and size between a portion that was in an amorphous state and a portion that was in a crystalline state before recording. That is, a recording mark formed in a portion in an amorphous state has a larger area to be melted than that formed in a portion in a crystalline state. Then, there is a problem that a recording mark different from the setting is formed in the amorphous portion.

In particular, in mark edge recording, which is the mainstream of high-density recording, since the recording signal is made to correspond to both ends of the mark, variations in the shape and size of the recording mark directly lead to errors.

Therefore, the amount of light absorption of the recording layer is made higher in the crystalline state than in the amorphous state, so that the same recording mark is formed regardless of whether the recording layer was in the amorphous state or in the crystalline state before recording. Various methods have been proposed to make this possible.

As one of the methods, the present applicant has first described
We have applied for an optical information recording medium in which a protective layer (second protective layer) on the upper side of the recording layer is formed of a light-absorbing material (having an absorption coefficient k in the range of 0.05 to 2.5) (International Publication). number
WO 96/17344). Among them, it is disclosed that carbon and a material obtained by adding a metal or a metalloid to carbon are suitable as a material for forming the protective layer.

[0013]

On the other hand, in an optical information recording medium of the phase change type, when overwriting is repeated many times, thermal damage is accumulated along with the number of repetitions, and the flow of the thin film forming the recording layer (non-flow) is increased. When making it crystalline,
The substance dissolved in the recording layer is extruded toward the lower temperature portion in the recording track direction), the film thickness fluctuates and the initial recording / reproduction characteristics cannot be maintained, and the jitter characteristics and error rate There is also a problem that deterioration of the data occurs. In the case of the optical information recording medium filed by the present applicant, there is still room for improvement in this respect.

The present invention has been made in view of such problems of the prior art, and has a small variation in the recording marks due to the state of the recording layer, and a durability against repeated overwriting many times. It is an object to provide a phase-change type optical information recording medium which is excellent in both of them.

[0015]

In order to solve the above-mentioned problems, the invention according to claim 1 comprises a transparent substrate 1 and a first protective layer formed on the substrate, as shown in FIG. 2 and
A recording layer 3 formed on the first protective layer and having a reversible phase change from a crystalline state to an amorphous state in accordance with the intensity of light irradiation; and a third layer formed on the recording layer. Two protective layers 4
And a reflection layer formed on the second protective layer, the second protective layer 4 is a thin film having a composition containing carbon and oxygen, and the absorption coefficient of the thin film is 0. .0
An optical information recording medium characterized by being in the range of 5-2.5.

According to this optical information recording medium, since the absorption coefficient of the thin film forming the second protective layer is in the range of 0.05 to 2.5, light transmitted through the amorphous portion of the recording layer is Absorbed by the second protective layer. As a result, the light absorption of the recording layer is
The crystalline part is higher than the amorphous part. The absorption coefficient of the thin film forming the second protective layer is more preferably 0.1 or more.

Further, since oxygen is contained in the thin film forming the second protective layer, even if overwriting is repeated many times, the flow of the thin film forming the recording layer is prevented. In the optical information recording medium of the present invention, the thickness of the second protective layer is 10 n
It is preferably in the range from m to 40 nm. 10
If the thickness is smaller than nm, the amount of light transmitted through the amorphous portion by the second protective layer is small, so that the above-mentioned effect cannot be obtained sufficiently and the sensitivity of recording / erasing becomes insufficient. On the other hand, 40
If the thickness is larger than nm, it becomes unfavorable because it becomes a slow cooling structure and the erased portion of the end of the recording mark is liable to be generated, the erasing ratio is lowered, and the mass transfer of the recording layer is increased by repeated overwriting. A more preferable range of the thickness of the second protective layer is a range of 15 nm or more and 40 nm or less.

In the optical information recording medium of the present invention, the thin film constituting the second protective layer contains oxygen in a range of 0.5 at% to 5.0 at% as a whole. Is preferred.

The oxygen content in the second protective layer is 0.5
At less than at%, the effect of preventing the flow of the thin film forming the recording layer cannot be substantially obtained. On the other hand, when the oxygen content in the second protective layer exceeds 5.0 at%, the recording sensitivity and the erasing ratio decrease.

The invention according to claim 3 is the invention according to claim 1 or 2
3. The method for manufacturing an optical information recording medium according to 1., wherein the second protective layer comprises oxygen gas of 5 × 10 ⁻⁶ Torr or more and 1 × 10 ⁻⁴ T.
It is characterized by being formed by a sputtering method using a target containing carbon in an inert gas atmosphere containing a partial pressure of orr or less.

The oxygen content in the second protective layer formed by the sputtering method is controlled by changing the partial pressure of oxygen by changing the sputtering atmosphere to a mixed gas atmosphere of an inert gas (for example, Ar gas) and oxygen. can do.

The oxygen content in the second protective layer can be determined, for example, by subjecting the film formed on the substrate to X-ray photoelectron spectroscopy.
The spectral intensities of oxygen and carbon are detected, and can be calculated from the following equation using the detected values and the respective relative sensitivity coefficients.

[0023]

(Equation 1)

Here, Io: detected intensity of oxygen Ic: detected intensity of carbon RSFO: relative sensitivity coefficient of the device to oxygen RSFc: relative sensitivity coefficient of the device to carbon The graph of FIG. An example of the relationship between the content and the oxygen partial pressure of the sputtering atmosphere is shown.

As can be seen from this graph, by setting the oxygen partial pressure to 5 × 10 ⁻⁶ Torr or more and 1 × 10 ⁻⁴ Torr or less, the oxygen content becomes 0.5 at% or more and 5.0 at%.
% Or less.

The upper limit of the oxygen content of the second protective layer, 5.0 at%, is 5.0 at%, when the oxygen partial pressure is made larger than 1 × 10 ^-4 Torr in order to obtain an oxygen content exceeding 5.0 at%. It is also set from the viewpoint that the film formation rate is slowed and the productivity is reduced.

In general, the second protective layer is excellent in heat resistance, adhesion to the recording layer, and shielding from moisture, etc., for protecting the recording layer and stably storing data for a long period of time. It is necessary to be. Also, in order to increase the repetition durability of overwriting, high heat resistance and high mechanical strength are required. Therefore, in the optical information recording medium of the present invention, the composition of the thin film forming the second protective layer can include a metal or a metalloid in addition to carbon and oxygen.

In this case, usable metals or metalloids include, specifically, Al, Ag, W, Ti, Mo,
Ni, Pt, Cr, Pd, V, Tc, Nb, Ta, R
e, and at least one metal or metalloid selected from the group consisting of Ir.

In this case, the second protective layer is prepared by separately preparing a carbon target and a metal or metalloid target.
The film can be formed by a co-sputtering method in which sputtering is performed at the same time or a method in which one target made of a mixture of carbon and a metal or a metalloid is prepared and sputtering is performed.

Also in this case, it is preferable that the second protective layer contains oxygen in a range of 0.5 at% or more and 5.0 at% or less. In addition, the second protective layer has an oxygen partial pressure of 5 × 10 ⁻⁶ Torr or more and 1 × 1 in the sputtering atmosphere.
It is preferable to form it as 0 ^-4 Torr or less.

On the other hand, as described above, in the optical information recording medium of the present invention, the absorption coefficient of the thin film forming the second protective layer is 0.0
Since it is within the range of 5 to 2.5, light transmitted through the amorphous portion of the recording layer is absorbed by the second protective layer. As a result, the light absorption amount of the recording layer is more amorphous in the crystalline portion. It has the effect of being higher than the quality part. This effect increases as the amount of light passing through the amorphous recording layer increases. Therefore, the thickness of the first protective layer formed below the recording layer is preferably set to a value that reduces the reflectance when light is irradiated from the substrate side.

In general, the reflectance is uniquely determined by the refractive index and the absorption coefficient of the substrate and the thickness, refractive index, and absorption coefficient of each layer laminated on the substrate according to the wavelength of the incident light. Thus, for an optical information recording medium having a structure in which a first protective layer, a recording layer, a second protective layer, and a reflective layer are sequentially laminated on a substrate, the reflectance when the substrate is irradiated with light and the reflectance of the first protective layer. FIG. 3 is a graph showing an example of a simulation of the relationship with the thickness. Table 1 below shows the thicknesses and optical constants (refractive index, absorption coefficient) of each layer used in the simulation.

[0033]

[Table 1]

From the graph of FIG. 3, the thickness of the first protective layer at which the reflectivity is minimized is, for example, about 70 nm at point A and about 230 nm at point B. By setting the thickness at or near that value, the amount of light passing through the amorphous recording layer can be increased, and the light absorption of the recording layer can be higher in the crystalline portion than in the amorphous portion. it can. Further, when the thickness of the first protective layer is within a range of ± 30 nm with respect to the thickness at the points A and B, the light absorption amount of the recording layer is larger in the crystalline part than in the amorphous part. Can be higher.

As described above, the thickness of the first protective layer is set to a thickness of ± 30 nm at which the reflectance shows a minimum in order to make the light absorption of the recording layer higher in the crystalline portion than in the amorphous portion. It is preferably within the range. In addition, from the viewpoint that the heat generated by repeated overwriting is not exerted on the substrate,
It is preferably 100 nm or more. Further, in order to prevent the substrate from being deformed by the influence of heat generated during film formation, the thickness is preferably 400 nm or less.

Therefore, in the optical information recording medium of the present invention, the thickness of the first protective layer is 100 nm or more and 400 nm or more.
Within the following range, the thickness at which the above-mentioned reflectance shows a minimum is ± 30 n.
It is preferable to set the thickness to satisfy m.

As the material of the first protective layer, a conventionally known dielectric material can be used. For example, ZnS,
SiO ₂ , SiO, Al ₂ O ₃ , MgO, Si ₃ N ₄ ,
Examples thereof include sulfides such as AlN, MgF ₂ , and SiC, oxides, nitrides, fluorides, carbides, and mixtures thereof (for example, ZnS—SiO ₂ ).

As the material for the recording layer in the optical information recording medium of the present invention, conventionally known phase-change type recording layer materials can be used, for example, Sb-Te-Ge alloy, In-
Examples include an Sb alloy, an In-Sb-Te alloy, and an alloy containing an In-Se alloy as a main component.

The thickness of the recording layer is 10 nm or more and 50 n or more.
m or less is preferable. If the thickness of the recording layer is less than 10 nm, the difference between the reflectance in the crystalline state and the reflectance in the amorphous state becomes small, and the signal quality deteriorates. The recording layer has a thickness of 50 n.
When the thickness is larger than m, the amount of light absorption in the amorphous state increases,
The amount of light absorption in the crystalline portion cannot be greater than the amount of light absorption in the amorphous portion. A particularly preferred range of the thickness of the recording layer is from 20 nm to 35 nm.

As the material of the substrate in the optical information recording medium of the present invention, glass, polypropylene, acrylic resin,
Transparent materials such as a polycarbonate resin, a polystyrene resin, a vinyl chloride resin, an epoxy resin, and a polyolefin resin are exemplified. Among them, the polycarbonate resin and the acrylic resin are preferable in terms of optical and mechanical properties.

As the material of the reflection layer in the optical information recording medium of the present invention, Al, Ni, Cr, Au, Ti, Cu,
It is preferable to use metals such as Zr, Hf, Si, and Mg, and alloys thereof from the viewpoint of thermal conductivity and optical constant. Among them, Cr, Ti, Si, C
at least one selected from the group consisting of u, Zr, Hf, etc.
Seeds are added at 0.5 at. % At least 40 at. % Is particularly preferred.

The thickness of the reflective layer varies depending on the type of metal used and, in the case of an alloy, the mixing ratio of alloy components, but is preferably in the range of 25 nm to 200 nm. When the thickness is less than 25 nm, the heat capacity of the reflective layer is small and the structure becomes a slow cooling structure, so that the erasing ratio is lowered and the mass transfer of the recording layer is increased by repeated overwriting, which is not preferable. On the other hand, when the thickness is more than 200 nm, the sensitivity for recording and erasing becomes insufficient, which is not preferable.

As a method of forming each layer formed on the substrate, a conventionally known vacuum deposition method or sputtering method can be adopted, but from the viewpoint of productivity and the like, the sputtering method can be adopted. preferable. In particular, as a method for forming the second protective layer, as described above, it is preferable to employ a sputtering method in order to form a carbon film containing oxygen at a predetermined content.

[0044]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to specific examples. A four-layer structure phase-change optical disk (one side) shown in FIG. 1 was produced as follows.

That is, first, on a clean polycarbonate substrate 1 provided with a guide groove, a 230 nm-thick ZnS
Consisting -SiO ₂ first protective layer 2 formed of a thin film, a recording layer 3 made of SbTeGe alloy thin film having a thickness of 25 nm, a thin film containing carbon and oxygen (oxygen content is the value shown in Table 2 below) A second protective layer 4 having a thickness of 20 nm,
The reflective layer 5 made of an Al alloy thin film having a thickness of 150 nm was sequentially formed by a sputtering method and laminated.

Here, the film thickness of each layer is determined in advance by forming a single layer for each layer, and measuring the film thickness of the obtained thin film with a stylus type surface step meter. The relationship between the formation speed and the film formation time was derived for each layer, and the relationship was controlled by forming a film at the thin film formation speed and the film formation time having a target film thickness using the relationship.

When the second protective layer 4 is formed, N
Except for o.1, oxygen gas was introduced into the sputtering apparatus at the same time as Ar gas, and the flow rate of oxygen gas was adjusted for each sample so that the predetermined oxygen partial pressure shown in Table 2 below was obtained. In addition, about No.1, sputtering was performed by introducing only Ar gas.

For the formed second protective layer 4, the oxygen content in the film was measured according to the method described above using an X-ray photoelectron spectrometer. The absorption coefficient of the second protective layer 4 was measured for each sample. The results of these measurements are also shown in Table 2 below.

Next, the surface of the reflective layer 5 is coated with an ultraviolet curable resin to complete a single-sided disk. Two samples of the same single-sided disk manufactured in this manner were bonded with a hot melt adhesive with the reflection layer side facing inward, thereby producing each sample of a phase-change optical disk having a full-contact structure.

Each of the obtained samples is driven by a driving device (laser light
Wavelength: λ = 680 nm, numerical aperture of objective lens: NA =
0.6), and rotate at a linear velocity of 15 m / s.
Number f _Two(Here, 16MHz) signal with peak power
And overwrite 10 times at each peak power.
And measure the C / N (carrier to noise) ratio of the recorded signal.
Was. Then, for each sample, the C / N ratio exceeds 40 dB
The peak power required to obtain the recording sensitivity was defined as the recording sensitivity.

For each sample, a signal of the frequency f ₁ (here, 6 MHz) is recorded in the same manner as described above, and another signal f ₂ (here, 16 MHz) is recorded thereon.
Was overwritten once. At this time, the peak power was set to 1.3 times the recording sensitivity, and the bias power was set to about 43% of the peak power.

[0052] Here, in advance by measuring the signal intensity I _B of a frequency f ₁ before overwriting was measured residual signal intensity I _A of the frequency f ₁ after overwriting. Then, each sample was calculated overwriting difference in signal intensity of the frequency f ₁ before and after the (I _B -I _A) as an erase ratio.

The measurement of the C / N ratio and the measurement of the signal intensity for calculating the erasure ratio were performed using a spectrum analyzer. Further, each sample was rotated at a linear velocity of 8.4 m / s by the driving device, and a signal in which 3T to 8T of 2-7 RLL modulation appeared at random was set to a peak power of 1.3 times the recording sensitivity and a bias power of 1.3 times. Set to about 43% of the peak power and set 10% for the same track.
Overwritten 10,000 times. Thereafter, each sample was subjected to a jitter analyzer to measure a jitter value corresponding to a bit error rate of 10 ^−5, and to calculate a “window occupancy” obtained by normalizing this value with a window width. The lower the window occupancy, the higher the durability against repeated overwriting.

The results are shown in Table 2 below.

[0055]

[Table 2]

As can be seen from these results, No. 2 to No. 7 in which the second protective layer 4 was formed in a mixed gas atmosphere of Ar gas and oxygen gas were substantially used as the second protective layer 4. Since the thin film containing carbon and oxygen was formed as a whole, the window occupancy was lower than that of No. 1 in which the second protective layer 4 was formed in an atmosphere containing only Ar gas, and the The durability was high.

However, in No. 7, since the oxygen content of the second protective layer 4 was as high as 5.9 at%, the erasing ratio was extremely low at 24.7 dB. Therefore, in order to achieve both the durability against repeated overwriting and the erasing ratio, No. 2 to N
As shown in o.6, the oxygen content of the second protective layer 4 was set to 0.65a
It is preferable to set it to t% or more and 4.8 at% or less.

Further, as shown in No. 3 to No. 5, the second protective layer 4
In particular, when the oxygen content is in the range of 0.95 at% to 3.3 at%, the window occupancy is reduced to less than 60%, the durability against repeated overwriting is remarkably increased, and the erasing ratio and the recording sensitivity are also increased. Will be retained. Therefore, the oxygen content of the second protective layer 4 is more preferably 0.95 at% or more and 3.3 at% or less.

In this embodiment, the oxygen partial pressure in the sputtering atmosphere gas at the time of forming the second protective layer 4 is set to 1 × 10 ⁻⁵ Torr or more and 1 × 10 ⁻⁴ Torr or less. The oxygen content of the protective layer 4 is preferably set to 0.65 at% or more and 4.8 at% or less, because a phase change optical disc having excellent durability against repeated overwriting and excellent erasing ratio can be obtained. Also, 2 × 10 ^-5 To
If it is rr or more and 6 × 10 ⁻⁵ Torr or less, the oxygen content of the second protective layer 4 is 0.95 at% or more and 3.3 at% or less, and the durability against repetitive overwriting is remarkably high. This is more preferable because a phase-change optical disk held at a high level can be obtained.

Since the absorption coefficient of the second protective layer 4 is in the range of 0.05 to 2.5 for all samples, the light absorption of the recording layer in the crystalline state is higher than that in the amorphous state. Since the height is higher, the same recording mark is formed irrespective of the state before recording (whether amorphous or crystalline).

In this embodiment, the second protective layer 4 having a composition containing carbon and oxygen has an oxygen concentration of 0.65 at.
It has been shown that 95 at%, 2.1 at%, 3.3 at%, and 4.8 at% are preferable in terms of both the durability against repeated overwriting and the erasing ratio, but the oxygen concentration is 0.1%. If it is 5 at% or more and 5.0 at% or less, the same effect can be obtained.

[0062]

As described above, claims 1 and 2
The optical information recording medium according to (1), wherein the second protective layer is a thin film having a composition containing carbon and oxygen and having an absorption coefficient in the range of 0.05 to 2.5, so that the recording mark caused by the state of the recording layer is obtained. Is reduced, and the durability against repeated overwriting many times is also excellent.

In particular, according to the second aspect, the effect can be enhanced while improving the recording / erasing characteristics. According to the method of claim 3, the optical information recording medium of the present invention can be easily obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a layer structure of an optical information recording medium of the present invention.

FIG. 2 is a graph showing the relationship between the oxygen partial pressure of a sputtering atmosphere when forming a second protective layer and the oxygen content of the obtained second protective layer.

FIG. 3 is a graph showing the relationship between the reflectance of light incident from the substrate side and the thickness of a first protective layer in an optical information recording medium having a four-layer structure.

FIG. 4 is a waveform diagram showing intensity modulation of a laser beam performed during overwriting on an optical information recording medium whose recording method is phase change.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 First protective layer 3 Recording layer 4 Second protective layer 5 Reflective layer

Claims (3)

[Claims] 1. A transparent substrate, a first protective layer formed on the substrate, a crystalline state-amorphous state formed on the first protective layer and depending on the intensity of light irradiation. In a optical information recording medium comprising a recording layer in which the phase change is reversibly performed, a second protective layer formed on the recording layer, and a reflective layer formed on the second protective layer. An optical information recording medium, wherein the second protective layer is a thin film having a composition containing carbon and oxygen, and the thin film has an absorption coefficient in a range of 0.05 to 2.5. 2. The optical information recording medium according to claim 1, wherein the thin film forming the second protective layer contains oxygen in a range of 0.5 at% or more and 5.0 at% or less. 3. The second protective layer comprises oxygen gas of 5 × 10^-6T
1 × 10 or more ^-FourInert including partial pressure below Torr
In a gas atmosphere, use a carbon-containing target to
2. The method as claimed in claim 1, wherein the step is formed by a taring method.
3. The method for manufacturing an optical information recording medium according to item 2.