Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2002-04-11
2004-01-27
Ometz, David L. (Department: 2653)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S284000, C369S013380
Reexamination Certificate
active
06683845
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium, a method of producing the same, and an optical recording and reproduction device, more particularly an optical recording medium used in the near field and preventing a decrease of intensity of reproduced signals accompanying fluctuation of the distance between the optical recording medium and optical system and preventing damage due to collision with an optical system and a method of producing the same and an optical recording and reproduction device including the optical recording medium.
2. Description of the Related Art
Up to now, a hard disk or other magnetic recording medium has been used in a state with a head for recording and reproduction brought into extremely close proximity to the disk or other medium for the purpose of obtaining good signal characteristics. As opposed to this, a phase change type optical disk, magneto-optical disk, or other optical recording medium has been used in a state with the optical system or head for recording and reproduction separated from the recording medium by a predetermined distance.
However, in recent years, in the devices used for optical recording media, the system of bringing the optical system or head into close proximity, for example, 200 nm with the disk (near field) has begun to be employed for the purpose of increasing a numerical aperture (NA) of the optical system and thereby increasing a recording density of the disk.
As an optical recording medium device used at the near field, for example, there are an optical hard disk structured with a lens mounted on a slider, an optical disk device with a lens made movable by an electromagnetic actuator, etc. In these devices, light for recording and reproduction is focused on the recording medium by an optical system comprised of a plurality of lenses including at least an objective lens and a solid immersion lens (SIL). Due to this, an NA of over
1
has been obtained.
FIG. 1
is a schematic view of a hard disk. The disk
1
is structured with a recording layer
3
and a lubrication film
4
stacked on a substrate
2
. A recording and reproducing head
5
for changing the magnetization of the recording layer
3
is mounted on a slider
6
and movable in the direction of the disk plane. The lubrication film
4
is provided for preventing abrasion of the head
5
and the disk
1
. The lubrication film
4
can be formed, for example, by coating a fluorine compound. In the case of an optical disk, consideration of optical conditions is required for a layer formed on a recording layer, but the lubrication film
4
of a hard disk does not require consideration of optical conditions as required for the optical disk. Therefore, it can be relatively easily formed.
FIG. 2
is a schematic view of a conventional optical disk with a large distance between an optical system or head and the disk, for example, a phase change type optical disk or a magneto-optical disk. The optical disk of
FIG. 2
is structured with a dielectric protective layer
12
, a recording layer
13
, a dielectric protective layer
14
, a reflective film
15
, and a resinous protective layer
16
sequentially stacked on a substrate
11
. In the case of a phase change type optical disk, a material changing in phase by focusing of light is used for the recording layer
13
. In the case of a magneto-optical disk, a material changing in magnetization state using focusing of light is used for the recording layer
13
.
In the optical disk of
FIG. 2
, both surfaces of the recording layer
13
are protected by the dielectric protective layers
12
,
14
. These surfaces are further protected by the substrate
11
or the resinous protective layer
16
. A distance between a lens
17
and the disk is much larger than that of a hard disk. A film for dealing with friction or collision between the lens
17
or head and the disk is usually preferable, but not necessary.
FIG. 3
is a cross-sectional view of an optical disk used in the near field. It is structured by a reflective layer
22
, a second dielectric layer
23
, a recording layer
24
, and a first dielectric layer
25
sequentially stacked on a substrate
21
. In the case of the optical disk shown in
FIG. 2
, light is focused from the side at which the transparent substrate
11
is formed. On the other hand, in the case of the optical disk for near-field use shown in
FIG. 3
, light is focused from the side at which the first dielectric layer
25
is formed. Due to this, the increase in the coma along with an increase in the NA is moderated.
In the optical disk of
FIG. 3
, the four layers of the first dielectric layer
25
, the recording layer
24
, the second dielectric layer
23
, and the reflective layer
22
are optimized in design for obtaining good signal characteristics for light striking the disk surface perpendicularly.
On the other hand, in the case of a near-field optical disk device having a short distance between the head and the disk as described above, the risk of collision of a head or a lens and the rest of the optical system with the disk becomes extremely high. However, it is very difficult to uniformly coat a lubricating substance such as used for the lubrication film
4
of the hard disk on the surface of an optical disk to form a thin film satisfying the optical conditions. Also, in the case of a near-field configuration, the fluorine material used for the lubrication film
4
of a hard disk cannot be used because the refractive index is too low. There are few other suitable materials.
When an antireflection coating (AR coating) is provided on the surface of the lens, once the AR coating at the lens side is damaged due to collision, the recording and reproduction are influenced by the damage at all times. That is, a change of the optical characteristics of the entire device is caused. However, it is difficult to find a suitable coating material resistant to damage by collision as the material for the AR coating.
According to a film configuration of the above conventional near-field optical disk, there is a problem that light of a component of a high NA, that is, the light having a large incident angle with respect to the recording layer
24
is hard to reach the recording layer
24
. This is caused by a large reflectance at the surface of the first dielectric layer
25
with respect to the light of the component of the high NA. Also, in a state where a space (below, referenced as t) between the lens and the surface of the disk is extremely small, when t fluctuates slightly, along with this, the reflectance of the surface of the first dielectric layer
25
greatly changes.
In a phase change type optical disk of the conventional configuration shown in
FIG. 3
, the reflectance of the surface of the first dielectric layer was calculated with an air layer between the lens and the surface of the disk regarded as a thin layer having a refractive index n of 1 and a thickness of t when light from an optical part (lens) of n=1.8 strikes the disk with various incident angles (an increase of the incident angle corresponds to an increase of NA). The calculation was performed when the incident angle is 0°, 100°, 20°, 30°, 40° or 50°. The results of calculation are shown in FIG.
4
.
As shown in
FIG. 4
, along with an Increase of the incident angle, the reflectance of the first dielectric layer becomes higher. Also, when the thickness t of the air layer changes to about 0 to 100 nm, the reflectance of the first dielectric layer drastically rises. In the actual near-field optical disk device, the space t between the lens and the surface of the disk is usually about 50 to 200 nm. It overlaps the region where the reflectance of the first dielectric layer greatly fluctuates. Therefore, when t changes slightly due to rotation of the disk etc., a distribution of energy of light in a disk plane easily fluctuates and it becomes hard to stabilize the reproduced signal level.
Further, in the above conventional near-field optical disk, when the space t betw
Ichimura Isao
Kishima Koichiro
Kuroda Yuji
Osato Kiyoshi
Saito Kimihiro
Ometz David L.
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
LandOfFree
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