Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-12-19
2003-12-02
Bell, Mark L. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C204S192100, C204S192260, C428S689000, C428S698000, C359S586000
Reexamination Certificate
active
06656260
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a ZnS-series sintered material and a method for producing the same, a sputtering target formed from the sintered material, a thin film, and an optical recording medium having the thin film.
A ZnS-series material has been well known not only as a fluorescent substance but also as an electroluminescent substance. In the field of photoelectronics, the ZnS-series material is used as a thin film that has light transmission properties and a high refractive index. For example, in a phase change-type optical recording medium comprising a recording layer composed of an alloy of tellurium (Te) or antimony (Sb), the ZnS-series thin film is used as a protecting layer for protecting the recording layer. This medium has been used for rewritable optical disks, such as rewritable compact disks (CD-RW), random access memory digital video disks (DVD-RAM), DVD-rewritable (DVD-RW), DVD+rewritable (DVD+RW), and the like.
FIG. 4
shows the basic structure of an optical disk that serves as an optical recording medium. A first protecting layer
2
(
21
) is formed on a disk substrate
10
. A recording layer
3
is formed on the first protecting layer
2
(
21
), and further, a second protecting layer
2
(
22
) is formed. A reflecting layer
4
is formed on the second protecting layer
2
(
22
). The reflecting layer
4
is composed of aluminum, gold, silver or an alloy containing these metals as a main component.
A laser beam is irradiated on the disk substrate
10
of the above-mentioned optical disk. The laser beam is transmitted through the protecting layers
2
(
21
,
22
) to the recording layer
3
and is reflected by the reflecting layer
4
. The reflected laser beam returns to the side of the irradiation source through the recording layer
3
and the protecting layers
2
(
21
,
22
). In the phase change-type optical recording medium, in recording, the laser beam, which is modulated according to the signal strength, is irradiated to the optical recording medium. The heat energy of the laser beams causes a phase change in the recording layer. For example, the alloy thin film in the recording layer undergoes an alternate change between the crystal phase and the amorphous phase. This phase change is recorded as a signal. In reproduction, a laser beam is irradiated, causing a phase change in the recording layer
3
. The change in reflection intensity of the laser beam in accordance with the phase change of the recording layer
3
is detected as a signal.
The protecting layers
2
transmit a laser beam and protect the recording layer
3
by contacting both surfaces of the recording layer
3
. The protecting layers
2
are composed of, for example, a ZnS element or a ZnS—SiO
2
composite.
In optical disks that are rewritable on demand, during recording/erasing of the signal information by laser beam irradiation, the above-mentioned protecting layers
2
(
21
,
22
) are heated in a temperature range of from 400 to 700° C. though for a very short time. Then, the protecting layers
2
(
21
,
22
) undergo a considerable temperature change. Therefore, ZnS, which has an excellent heat resistance, has been used in the protecting layers
2
(
21
,
22
). However, ZnS has a problem that a grain growth occurs due to repeated laser beam heating. A ZnS—SiO
2
composite is a material obtained by adding SiO
2
to ZnS. For example, a ZnS—SiO
2
composite having the composition of 80% by mole of ZnS and 20% by mole SiO
2
is known. The addition of SiO
2
suppresses grain growth caused by the repeated heating. Thus, the SiO
2
-series thin film of the composition of 80% by mole ZnS and 20% by mole SiO
2
has been prepared to have a fine structure such that the particle diameter of the crystal is small.
Further, due to the high power laser beam irradiation during writing, the recording layer
3
undergoes a temperature change. That is, the recording layer
3
is heated and cooled. The two protecting layers
2
(
21
,
22
) directly contact the recording layer
3
. For preventing the protecting layers
2
(
21
,
22
) from reacting with the recording layer
3
, they must have a low chemical reactivity to the alloy used for the recording layer
3
in a temperature range from room temperature up to a maximum of 700° C.
The protecting layers
2
(
21
,
22
) can be deposited in a form of thin film by a radio frequency (RF) sputtering process. According to this process, the disk substrate
10
and a target are arranged to face each other in an RF sputtering apparatus. A ZnS—SiO
2
sintered material is used as the target material. Then, a high frequency plasma is generated in a high vacuum and rare argon (Ar). Argon ions, which are generated, cause the target to release material, which forms a thin film (the protecting layer
2
) of the material on the disk substrate
10
. In addition, after the deposition of the recording layer
3
, the protecting layer
2
, which is a thin film, is deposited on the recording layer
3
in the RF sputtering apparatus. Examples of ZnS—SiO
2
sintered materials are disclosed in Japanese Unexamined Patent Publication Nos. Hei 11-278936 and 7-138071.
To produce a sintered material, for example, a method in which a mixed powder of ZnS and silica is subjected to hot pressing in an inert gas atmosphere at a specific high sintering temperature and a method in which a shaped body comprising a mixed powder is subjected to atmospheric sintering have been used. Further, a hot isostatic pressing (HIP) method for further rendering the sintered material dense has also been used.
Conventionally, to deposit the protecting layer
2
in the above-mentioned optical recording medium using these ZnS—SiO
2
sintered materials, only the RF sputtering process can be employed. This is because a direct current (DC) sputtering process cannot be employed due to the high electric resistance of the ZnS—SiO
2
sintered material. However, in the RF sputtering process, it is difficult to apply a high electric power to the target. For this reason, the sputtering rate and the efficiency of deposition are lowered, and the productivity of the thin film for the optical recording medium cannot be improved.
In RF sputtering, high frequency electrical heating (high frequency heating) is generated in the disk substrate
10
, which is made of a polymer such as polycarbonate. This may cause thermal damage to the disk substrate
10
. This is also disadvantageous in the productivity of the optical recording medium. In the production of large capacity optical disks, it is necessary that the deposition rate be increased to improve the productivity of the thin film. For this reason, it has been preferred that a deposition process other than RF sputtering be applied to the production of optical disks.
To control the uniformity and thickness of the protecting layer
2
, it is preferred to use a target having a size corresponding to the size of disk substrate
10
. However, it is difficult to obtain a large, dense ZnS—SiO
2
sintered material. Therefore, both the sintering strength and the production efficiency are low when a large piece of ZnS—SiO
2
is used.
The target sintered material is required to have a small number of internal pores and a high relative density. Since it is difficult to obtain a large, dense piece of ZnS—SiO
2
sintered material, the porosity is relatively large. When sputtering is performed using a target sintered material having a large porosity, air contained in the sintered material is released, and to maintain the atmosphere in the sputtering apparatus at a high vacuum level during sputtering is difficult. When a composite material is deposited without maintaining the atmosphere in a high vacuum, the composition of the resulting thin film may become different from that of the sintered material. Further, when the atmosphere is not maintained in a high vacuum, the sputtering rate is lowered, which lowers productivity.
Although the thin film of a ZnS element has a relatively high refractive index due to addition of SiO
2
thereto, the refract
Noguchi Yukio
Ueno Takashi
Bell Mark L.
Hailey Patricia L.
Kyocera Corporation
Synnestvedt & Lechner LLP
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