Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Utility Patent
1999-12-08
2001-01-02
Dinh, Tan (Department: 2752)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C430S270130
Utility Patent
active
06169722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical recording medium of phase change type wherein small record marks are formed.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change in reflectance by the crystallographic change is detected for reading of the information. The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit is simple as compared to magnetooptical recording media.
Most optical recording media of phase change type used chalcogenide systems such as Ge—Te system and Ge—Sb—Te system which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state. It was also recently proposed to use new compounds known as chalcopyrites. Chalcopyrite compounds have been investigated as compound semiconductor materials and have been applied to solar batteries and the like. The chalcopyrite compounds are composed of Ib-IIIb-VIb
2
or IIb-IVb-Vb
2
as expressed in terms of the Groups of the Periodic Table and have two stacked diamond structures. The structure of chalcopyrite compounds can be readily determined by X-ray structural analysis and their basic characteristics are described, for example, in Physics, Vol. 8, No. 8 (1987), pp. 441 and Denki Kagaku (Electrochemistry), Vol. 56, No. 4 (1988), pp. 228. Among the chalcopyrite compounds, AgInTe
2
is known to be applicable as a recording material by diluting it with Sb or Bi. The resulting optical recording media are generally operated at a linear velocity of about 7 m/s.
See Japanese Patent Application Kokai Nos. (JP-A) 240590/1991, 99884/1991, 82593/1991, 73384/1991, and 151286/1992. In addition to the optical recording media of phase change type wherein chalcopyrite compounds are used, optical recording media of phase change type wherein
AgSbTe
2
phase is formed with the crystallization of the recording layer is disclosed in JP-A 267192/1992, 232779/1992, and 166268/1994.
When information is recorded on the optical recording medium of phase change type, the entire recording layer is first brought into crystalline state, and then, a laser beam of high power (recording power) is applied so that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of relatively low power (erasing power) is applied so that the recording layer is heated to a temperature higher than the crystallization temperature and lower than the melting temperature. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single light beam.
In general, recording density of optical recording media including optical recording media of phase change type can be increased to a level higher than that of magnetic recording media. Today, further increase in the recording density is demanded for processing an enormous amount of information as in the case of image processing. Recording density per unit area can be increased by reducing the record mark length.
The inventors of the present invention conducted an experiment wherein record marks of different sizes are formed in the recording layer comprising a Ge—Sb—Te—based material which is a common phase change material, and the recording layer is observed by transmission electron microscope. It was then found that coarse crystal grains are formed near the rear end of the record marks, and formation of such coarse crystal grains results in significant deformation of the record marks as well as change in the position of the record mark rear end. The shape of the crystal grains in the rear region of the recorded marks are random, and therefore, the shape of the deformed record marks and the shift of the rear end position differ from mark to mark. The strategy of adjusting the record marks in their reading can not be adopted. When variation in shape and size of the record marks is large in relation to the record mark length, the recording medium will exhibit markedly increased jitter.
On the bases of such results, the inventors of the present invention have found that critical increase in jitter is induced by the variation in shape and size of the record marks by the formation of coarse crystal grains in the Ge—Sb—Te—based material when the record mark length is reduced to a length below a particular value, more specifically, to 350 nm or less, and in particular to 300 nm or less.
The inventors of the present invention also found that thermal stability of the record marks formed in the recording layer of phase change type critically reduces when the length of the record mark is reduced beyond the above-specified value, and the resulting recording medium will be less reliable since the record marks are more likely to become crystallized during its storage under high temperature conditions.
SUMMARY OF THE INVENTION
In view of the situation as described above, an object of the present invention is to enable formation of small record marks at high shape and dimensional precision in an optical recording medium of phase change type adapted for formation of small record marks. Another object of the present invention is to improve thermal stability of such small record marks with high shape and dimensional precision to thereby provide a highly reliable optical recording medium of phase change type.
Such objects are attained by the present invention as described in (1) and (2), below.
(1) An optical recording medium having a recording layer of phase change type, wherein record marks with minimum length of up to 350 nm are formed in the recording layer, and the recording layer contains Ag, In, Sb and Te as its main components.
(2) An optical recording medium according to the above (1) wherein the recording layer contains Ge and/or N as its sub-component.
REFERENCES:
patent: 5708651 (1998-01-01), Sugaya et al.
patent: 5838657 (1998-11-01), Tsuchiya et al.
patent: 5914926 (1999-06-01), Maeno et al.
patent: 5946287 (1999-08-01), Nakayama et al.
patent: 6016302 (2000-01-01), Yamatsu et al.
patent: 6046968 (2000-04-01), Abramovitch et al.
Kikukawa Takashi
Utsunomiya Hajime
Dinh Tan
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
TDK Corporation
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