JPS6252368B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a storage medium in the field of optical storage.

Conventionally, various materials have been proposed as optical storage media, but among these, a method of writing information on a metal film adhered to a substrate using laser light or the like is attracting attention. This type of storage method is performed by forming pits (holes or recesses) in the film using the thermal energy of a processing beam such as a laser beam.

In order to miniaturize the entire optical storage system, the use of small lasers is desirable and a storage medium with low writing energy is required. The writing energy required to form pits in optical storage is determined by various parameters such as light absorption, surface reflectance, specific heat, thermal conductivity, and writing threshold temperature. Among these, the writing threshold temperature is particularly important. In the case of pit formation by metal film removal,
It is not clear whether the writing threshold temperature is the boiling point or melting point of the metal, and conventionally it was thought that the use of low melting point metals was effective in reducing the writing energy. For this reason, studies have focused on Bi (melting point: 271°C) films, which are low melting point metals and provide high quality films. Furthermore, In, which has an even lower melting point (melting point 157°C), has an island-like structure in its evaporated film, so it cannot be used as a single film, and it can be used as a co-evaporated film (cermet) with SiO ₂ etc.
was used. However, even if low melting point metals such as Bi or In were used, it was not possible to significantly reduce the writing energy.

In order to eliminate the above-mentioned drawbacks, the present invention pursues the mechanism of pit formation in the medium film, actively introduces a layer that acts as an initial trigger for pit formation, and lowers the writing energy, which will be described below.

In examining the pit formation mechanism, we first focused on the melting point of the media film. Assuming that writing occurs at the melting point of the metal, when the medium is heated with a heater or the like, the writing energy decreases as the temperature rises, and just below the melting point, the writing energy should become extremely small. The results of the experiment are shown in FIGS. 1 and 2. Figure 1 shows an In-SiO ₂ cermet film (SiO ₂ 50 volume percent, film thickness 1000 Å) heated at 126°C.
The threshold value was investigated by laser writing (using an A _r laser) at a temperature raised to 100°C, and is normalized to the value at room temperature. Figure 2 shows the Bi film (thickness 700 mm).
Å) was heated to 180°C and the write threshold was investigated. In Figure 2, white circles indicate pit formation.
Black circles are oxidation. In both cases, the write threshold is the same as the result at room temperature. However, the Bi thin film is easily oxidized, and oxidation occurs in the laser irradiated area, resulting in discoloration. From Figures 1 and 2, it is clear that even if low melting point metals such as Bi and In are used, laser writing is not done at the melting point, and the temperature is quite high, and the boiling point (In is ~2000℃, Bi is ~ The temperature may have been raised to 1500℃. In other words, at the time of writing, the temperature distribution is as shown in Figure 3, and the high-temperature part is considered to be in a molten state, and the formation of pits occurs when the center reaches the boiling point and evaporates, creating a minute hole. The tiny holes are enlarged by the surface tension of the liquid, and the part heated above the melting point becomes a pit. In this way, forming pits in a conventional single-layer metal film requires energy to raise the temperature of a portion of the metal film to its boiling point. The present invention is based on the idea that if a low boiling point substance coexists, its vaporization will act as a trigger, allowing pit formation at a relatively low temperature.

An example is shown in FIG. 4, in which an organic dye phthalocyanine 1, which is a material with good stability and easy vapor deposition, was vapor-deposited on a glass substrate 2, and an In--SiO ₂ cermet film 3 was laminated thereon. When such an optical storage medium is irradiated with a laser beam, a boiling part 4 is generated in the phthalocyanine 1, which acts as a trigger, and at the same time, a melting part 5 is generated in the In--SiO ₂ cermet film 3, forming a pit. In addition, as another example, phthalocyanine and Bi films were laminated on a glass substrate. Figure 5 shows the effect of laminating a low-boiling point phthalocyanine as a trigger layer. In FIG. 5, the laser writing threshold was investigated by heating the medium with a heater. In Fig. 5, the white circles are In--SiO ₂ and the black circles are Bi. For both the In-SiO ₂ cermet film and the Bi film, the writing threshold value decreases as the medium temperature increases.
When the writing temperature is determined by extrapolation, it is around 400°C, and it is clear that writing is performed at the boiling point of phthalocyanine. Here, in FIG. 5, the measurement points for Bi at temperatures above 150° C. are lower than the extrapolated line, which indicates that Bi is oxidized as in FIG. 2. Observing the written pits, if the writing energy is too large, holes will be formed in both the metal film and the phthalocyanine film at the same time, but if the writing energy is appropriate, there will be no visible pits in the phthalocyanine film. However, pits with a size sufficient for readout (about 1 μm in diameter) are formed only in the metal film. Note that the threshold value at room temperature is In―SiO ₂
The values are almost the same for the cermet film and the Bi film, and
This is ~65% lower than when no trigger layer is laminated. The light energy required for writing is
In addition to the writing temperature mentioned above, it is determined by the surface reflectance, specific heat, thermal conductivity, etc., and it has been shown that the total value of these is almost the same for the In-SiO ₂ cermet film and the Bi film. There is. However, Bi films are extremely susceptible to oxidation and deteriorate easily even below 100°C, making it difficult to put them to practical use. There is no single-element metal film that has an appropriate melting point, but in order to further reduce the writing energy, we used a group of low-melting-point metal films containing Bi, Pb, and Sn as main components to further reduce the writing energy. Alloys can be used. Bi alone is easily oxidized, but alloyed with Pb, Sn, etc. is less likely to be oxidized.

As an example of this, Bi-Pb-Sn-
A cermet film made by co-evaporating an In eutectic alloy (melting point ~60°C) and SiO ₂ was used. In this case as well, if phthalocyanine is laminated, a similar effect can be obtained, and the write threshold is
The write threshold is ~50% lower than that of a SiO ₂ cermet film, and ~30% compared to a single layer Bi film.

In the above embodiments, phthalocyanine was used as the trigger layer, but the trigger layer of the present invention is not limited to only phthalocyanine, but may also include other low-boiling substances such as nitrocellulose. However, as is clear from the purpose of the invention,
The boiling point of the trigger layer needs to be higher than the melting point of the metal film forming the pits. Furthermore, the present invention is similarly applicable to the case where a substance that releases volatile substances through thermal decomposition is used as the trigger layer.

In addition, although the relationship between the media film on the glass plate and the trigger layer was described in the example, if a substrate with a relatively low boiling point or that is easily decomposed at low temperatures is used instead of the glass plate, the media film can be directly applied to the substrate. The same effect can be obtained by other methods of adhesion. In this case, the evaporation of the very surface of the substrate is utilized, and the medium manufacturing process can be simplified. Polymethyl methacrylate (PMMA) can be used as a substrate for this purpose, as it releases volatile substances at its decomposition temperature (350°C). However, if the writing energy is too large, the substrate itself will be seriously damaged, so it is necessary to use an appropriate writing energy that will only decompose the very surface layer.

As explained above, in optical storage by removing a metal film, there is no need to raise the temperature of the metal film to its boiling point, so the write threshold can be made extremely small. Further, when a low melting point metal or a low melting point alloy is used as the medium film, the characteristic of a low melting point can be effectively utilized.

Since the light absorption of a metal film has little wavelength dependence in the visible light region and near-infrared region, there is almost no limitation on the laser wavelength used. Furthermore, with regard to the substance used in the trigger layer, it is only necessary to pay attention to the boiling point, and there is no need to match the peak wavelength of light absorption with the wavelength of the laser used, unlike in the case of a single film of organic dye.

As described above, the optical storage medium of the present invention has many advantages.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing an example of the medium temperature dependence of the writing threshold value of the In-SiO ₂ co-deposited film.
The figure is a characteristic diagram showing an example of the medium temperature dependence of the writing threshold of a Bi film, FIG. 3 is a characteristic diagram showing an example of temperature distribution during writing, and FIG. 4 is an example of the present invention. FIG. 5 is a characteristic diagram showing an example of the dependence of the write threshold value on the medium temperature of the medium according to the embodiment of the present invention. 1...phthalocyanine, 2...substrate, 3...In-SiO ₂
Cermet membrane.

Claims (1)

[Claims] 1. An optical storage medium in which a layer made of a low boiling point substance or a low decomposition temperature substance and a metal film having a melting point lower than the boiling point or decomposition temperature of this layer and in which pits are formed are laminated on a substrate, An optical storage medium characterized in that a metal film on which pits are formed is made of a mixture of In and SiO2, or a mixture of a low melting point alloy containing Bi, Pb, and Sn as main components and SiO2.