DTIC ADP011840: Multi-Level Recording in Erasable Phase-Change Media by Light Intensity Modulation

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Defense Technical Information Center
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ADPO 11840
TITLE: Multi-Level Recording in Erasable Phase-Change Media by Light
Intensity Modulation
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TITLE: Optical Storage and Optical Information Held in Taipei, Taiwan on
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UNCLASSIFIED
Multi-level Recording in Erasable Phase-Change Media
by Light Intensity Modulation
Yen-Lin Chen and Han-Ping D. Shieh
Institute of Electro-Optical Engineering, National Chiao Tung University
Hsin-Chu, Taiwan, 30010, Rep. of China
Tel: [phone] Fax: [phone] Email: [email protected]
ABSTRACT
Multi-level recording has advantage of increasing the recording density without extensively altering the current optic
configuration. In this paper, a new four-level recording phase-change disk using three-recording layers was demonstrated.
By modulating the intensity of the laser pulse, marks were chosen to record in any of recording layers, to form four
recording regions. Utilizing the property of the phase change media whose amorphous and crystalline states differ largely in
refractive indices, each recording region corresponds to different reflectance. For the four-level recording disk, each
recording region represents two-bit data, then the recording density of the four-level disk is a factor of two higher than that
of a conventional disk recorded by adopting the pulse position modulation (PPM).
Keywords: light intensity modulation, multi-level recording optical disk, pulse position modulation, multi-layer
1. INTRODUCTION
Recording density of optical disks is limited by the optical diffraction limit. For a conventional disk whose marks are
recorded by the pulse position modulation (PPM) method, each mark length corresponds to one bit. With the multi-level
recording method, each recording region of the four-level disk corresponds to two-bit data. Therefore, the effective mark
length for one bit is half of the actual mark length, and the recording density can be doubled without using the short
wavelength diode laser or high numerical aperture (N.A) lens. Several multi-level recording methods have been reported.
For example, the magnetic multi-valued (MMV) recording was realized in MO media, applying an exchange coupled bi-
layerl'1.Using partial crystallization properties of phase-change optical disks to achieve multi-level recording was also
reportedI21. In this paper, a new four-level recording technique on the three-recording layers disk was demonstrated.
2. PRINCIPLE AND DISK DESIGN
The four-level optical disk structure (Substrate /ZnS-SiO /1st recording layer /ZnS-SiO /2nd recording layer /ZnS-
2 2
SiO /3rd recording layer /ZnS-SiO /Al) is composed of three recording layers. All of the recording layers are composed of
2 2
phase-change material GeTeSb of the same composition, as shown in Fig. 1. Marks can be recorded in any of recording
layers. The dielectric layers of ZnS-SiO are used to control the reflectance and the thermal diffusion of the four-level disk.
2
The Al reflective layer increases the cooling rate of the recording layers, and is as a reflector to optimize write/read
properties.
The amorphous and crystalline states of the recording layer correspond to different refractive indices. The total
reflectance of the disk is different when the recording layers are in the different phase. Four different reflectance, Rccc,
Racc, Raac, Raaa of the four-level disk are produced, as shown in Fig. 2. For example, Racc denotes the reflectance when
the upper recording layer is in the crystalline state, and the middle recording layer and lower recording layer are in the
amorphous state, respectively.
Four recording regions, ccc, acc, aac, aaa are formed by using the different laser power to write marks in different
recording layers. Each recording region represents two bits, as shown in Fig. 3(a). All recording layers should be initialized
60 In Optical Storage and Optical Information Processing, Han-Ping D. Shieh, Tom D. Milster,
Editors, Proceedings of SPIE Vol. 4081 (2000) * 0277-786X/00/$15.00
to the crystalline state, as phase change disks, before to record amorphous marks. In the recording process, the laser beam is
incident from the substrate. Most energy is absorbed by the 1st recording layer so that the temperature of the irradiated
region of the 1st recording layer is higher than that of the 2nd and 3rd recording layers. The temperature of the 2nd
recording layer is also higher than that of the 3rd recording layer. The amorphous state can be formed when the irradiated
regions of the recording layer are heated over the melting point, then cool down rapidly. By controlling the laser power, the
highest temperature of each recording layer is different, and the marks can be chosen to record in the different recording
layers, as shown in Fig. 3(b). For example, in order to record marks only in the 1st recording layer, the irradiated region of
the 1st recording layer is over melting point; meanwhile, the temperature of other two recording layers are lower than
melting point. Thus, the amorphous marks are only recorded in the 1st recording layer.
The temperature difference between any two recording layers, which is controlled by the disk structure, should be
high enough to avoid recording marks on the unexpected recording layers. By increasing the thickness of the dielectric
layers between any two recording layers, the power margin of recording marks in different recording layers will be
increased because of higher temperature difference. However, the mark size difference between any two recording layers
will also be enlarged due to the effect of extensive heat diffusion. It will affect the quality of the readout signal. Therefore,
there is a tradeoff between the power margin and the mark size.
The temperature profile of the recording layers need to be carefully controlled. By adopting single pulse writing, the
highest temperature of the rear part of the recorded mark is higher than that of the front part of the same recorded mark due
to the heat diffusion along the direction of the laser spot scanning. It may cause those marks be recorded in the unexpected
recording layers. To decrease the effect of the thermal diffusion, a multi-pulse recording method3t1 was utilized, as shown in
Fig. 4.
When the focussed laser spot scans along the track, different recording regions produce different reflectance levels, as
shown in Fig. 5. The C/N of the four-level disk is defined at the nearest interval of reflectance levels. The contrast of any
two reflectance levels of the readout signal is determined by the reflectance of the disk and the recorded mark size. When
the recording density increases, the reduced recorded mark size decreases, so does the readout signal. In order to achieve
high signal quality, the reflectance contrast between each level needs as high as possible. With the optical simulation using
the Jones matrix methodl41, the reflectance was defined by controlling the thickness of each layer of the disk.
According to the optical and thermal simulations, the disk structure of (Substrate / 150nm ZnS-SiO / 8rim 1st
2
recording layer / 40nm ZnS-SiO / 8nm 2nd recording layer / l2nm ZnS-SiO / 7nm 3rd recording layer / 50nm ZnS-SiO, /
2 2
40nm Al) is designed for the experiments. The simulated reflectances of the four-level disk, Rccc, Racc, Raac, Raaa are
0.24, 0.03, 0.09, 0.18, respectively.
3. EXPERIMENT AND DISCUSSION
The optical and thermal designed four-level disk was measured by a dynamic tester. The wavelength is 660nm and
N.A is 0.6. The linear velocity of the disk is about 4.8mi/s.
When the recorded mark size of the four-level disk was about 0.8um, four recording regions were produced by
different multi-pulse waveform, as shown in Fig. 6. The readout signal with the DC-read detection method was observed, as
shown in Fig. 7. When the recording pattern was arranged as Fig. 7(b), the readout signal of four reflectance levels was no
longer detectable.
The pulse-read detection method15 l to distinguish the four reflectance levels was adopted. The readout signal of the
same recording pattern as shown in Fig. 7(b) was tested by the pulse-read method, as shown in Fig. 8. The four-level
readout signal can be clearly differential. When the recorded marks became smaller than 0.6um, the readout signal was
distorted and the reflectance level corresponding to each region no longer kept the original level, as shown in Fig. 9. Thus,
the smallest recording region which was detectable in this four-level disk was about 0.6um using the above mentioned
optical system, laser, and light intensity modulation writing method.
The C/N of the four-level disk is defined at the nearest interval of reflectance levels which is between the acc and aac
regions in the experiment, as shown in Fig. 10. The C/N of the four-level disk for different effective mark size, half of the
actual mark length, is compared to a conventional disk, as shown in Fig. 11. The C/N of the four-level disk for the actual
mark size of 0.6um which corresponds to the effective mark size of 0.3um was about 36dB. It shows that the C/N of the
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four-level disk for the small effective mark size was higher than that of the conventional disks. The C/N was not high
enough for the large mark size because the contrast between the reflectance levels of acc and aac regions was small, and
adjacent marks interfered with each other.
4. CONCLSION
Multi-level recording using the light intensity modulation method on the four-level disk of three erasable phase
change recording layers was studied. Multi-level recording has advantage of increasing the recording density without
altering the current optic configuration. At the laser wavelength of 660nm and N.A of 0.6, the C/N for the effective mark
size of 0.3um on the four-level disk is above 36dB by light intensity modulation recording. The C/N can be increased
further by optimizing the structure of the disk and the writing schemes.
5. ACKNOWLEDGEMENT
This work was supported by National Science Council of the Republic of China under contract no. NSC 88-2622-L-
009-002.
6. REFERENCES
[1] K.SHIMAZAKI. M. YOSHIHIRO. 0. ISHIZAKI. S. OHNUKI and N. OHTA: Proc. MORIS '94 (1995). pp.429-430.
[2] L.P.SHI, T.C. CHONG, P.K. TAN, X.S. MIAO, Y.M. HUANG and R. ZHAO: Jpn. J. Appl. Phys. Vol. 38 (1999)
pp. 1645-1648.
[3] T. Ishida, M. Shoji, Y. Miyabata, Y. Shibata, E. Ohno, and S. Ohara, Proc. SPIE 2338, 121 (1994)
[4] Hecht, Optics, 2nd Edition, Addison-Wesley Publisher, U.S.A., 373 (1987).
[5] Gung-Chian YIN, Bing-Mau CHEN, Chia-Ping TAI and Han-Ping D. SHIEH: Jpn. J. Appl. Phys. Vol. 39 (2000)
pp.830-3. Japan
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Substrate
dielectric layer (ZnS-SiO,)
1st recording layer (Ge Te- Sb )
21 3 26
dielectric layer (ZnS-SiO )
2
2nd recording layer (Ge Te Sb )
21 53 26
dielectric layer (ZnS-SiO )
2
3rd recording layer (Ge Te Sb )
21 53 26
dielectric layer (ZnS-SiO,)
reflective layer (Al-Cr)
Fig. 1 The structure of the four-level recording using erasable phase change media
Rccc Race Raac Raaa
(cid:127)<:;:=(cid:127)[=-(cid:127) = ~~~~ (cid:127) (cid:127) ==:st recording layer
Dielectric layer i===:=' 2nd recording layer
L J==z-
3rd recording layer
nca
(njc) Crystalline state
f
Amorphous state C a
S(na)
Fig. 2 Four different reflectances of the disk when both recording layers are in the different phase states.
63
Actual mark C Effective mark length
2
length for one bit
.................................................. ............ ............................................................. ... ....... . Crystalline
1 st recording
-*at
layer
2nd recording
layer 4-
3rd recording
layer 4
01 10
Four Amorphous mark
acc aac aaa ccc
recording
re g io n s..................... ....................................................... .......... ..................... ........................................
(a)
S...... .................. ............................ ................................... ...... .......... ......... ...........
S1 1st recording layer
... -. -.--.-.-.. . .. . -x - -. . . .. / . .. ..Tmm -- 2ndrecordinglayer
/............. 3rd recording layer
Time
acc aac aaa
Laser Power
(b)
Fig. 3 (a) Four different recording regions, acc, aac, aaa, ccc. (b) Temperature profiles on the recording layers at different recording power
to produce different recording regions. Tx and Tm are the melting and crystallize temperature, respectively.
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Pw_______
Pw =writing power
Pe = erase power
Pe Pr =read power
Pr
Fig. 4 Multi-pulse recording waveformn.
1s t record'mg*- 4
layer
2nd recordmn4-4
layer
3rd recording
layer 9
reflectance
... ...
levels(3
Fig. 5 The readout signal of the disk of four reflectance levels.
Pw3 Pwl 9mW
Pwl w Pw2= llmW
i ,... ... ..- Pe Pw3= 14.5mW
Pe= 4mW
I1st recording layer 4 -. INNOW!
2nd recordin2 laver 4I I II
3rd recording layer 4 BPI II
IO.8um
Fig. 6 When the recorded mark size is about O.8um, the different recording regions are produced by pulse readout method.
65
1
st recording layer
4 I
2nd 4 -'
I___
rd.
. .........
__________. .......... .. ....-..-.-------.
.1. .. ... .... . .. .... .... ... .. ... .....
I I I I I
.6 O~WL -62.6P.
12 V DC
22 V DC SOO MSZ.'
So nýV 50D E,-±IS DC 0.80 V SOO
56 V 500 [1 STOPPED
PI I 1, I ID-
21.18:41I
I ___ ___ I I IT
....... .. .
1
................
S 0 V
......... . .
~
1 2 V D C '
2V DC 2S6 MSZ.'
06m SoDo E~t3D DC 0-8D V 6son
.6 -V Co0 1. TOPPED
Fig. 7 (a) The readout signal of the repetitive recording regions with the DC-read detection when the mark size is about O.8um. (b) The
readout signal of the different recording pattern.
66
~: ~:(a)
(b)
21:.16: 26
i.
~ Z ..........
S~~rn')
\77V
1 2 V DC
2 2 V DC lee Ms/.
so .V saDc] E~t10 DC 9.80 V 500SOPE
Fig. 8 (c)Th e readout signalof the repetitive recording regions with (b)th e pulse-read detection (a)w hen the mark size isa bout O.8um.
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7I
i
1%J1
JK
-23 -- J~
-,q vi
~ ~
- _ T+__
.2 ps BWL 1.708776 ms
1 2 V DC
2~ 501 V DC 2510 MS/s
81 5111 Ext DC 246r,¶ 5012
.5 V 5811 LI STOPPED
Fig. 9 The readout signal with the PULSE-read detection when the mark size isa bout O.6um.
67
14-M8-98 Fdir9, Flop' M.4 i,
21:43:6?
SI II. . . [... . .i....
IIi. ... i I i .. Is . ......... i ,. ....... ...! ......... ..
1A.1. ......... ...
I I S........... i... .. .(cid:127) _._.1.. . .._.. :. ._ ... _.. ....... ...............
I I I I I H I .
'. ........ .-.. . .
~s
.5 Bit 1.99820 I
12S 22:5 VVV S0Vi0CM -- ExtlG DC 8.8 V Sao ie8 Is/s
1 i . STOPPED
Fig. 10 The C/N of the four-level disk is measured with this repetitive recording pattern.
50
45
40~
35
30 DVD RAM
-0 4-level
25~
20 I
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
effective mark size (urn)
Fig. I1]C /N as a function of the effective mark length in a four-level disk and a conventional disk.
68