Copyright 1999 Society of Photo-Optical Instrumentation Engineers.
This paper was published on the website accompanying Philips Research Password, and is made available as an electronic reprint with permission of SPIE.
Single print or electronic copies for personal use only are allowed. Systematic or multiple reproduction, or distribution to multiple locations through an
electronic listserver or other electronic means, or duplication of any material in this paper for a fee or for commercial purposes is prohibited. By choosing to
view or print this document, you agree to all the provisions of the copyright lawprotecting it.
AgInSbTe materials for high-speed phase change recording
H.J. Borga,*, P.W.M. Bloma, B.A.J. Jacobsa, B. Tiekea, A.E. Wilsonb, I.P.D. Ubbensb, and G.F. Zhoua
a
Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands
Philips Optical Disc Technology Centre, Glaslaan 1, 5616 LD Eindhoven, The Netherlands
*
corresponding author: phone: +31-40-2744287, fax: +31-40-2744282, e-mail: borg@natlab.research.philips.com
b
1
Introduction
Currently applied phase change rewritable optical discs are mainly based on two families of phase change materials, i.e.
pseudo-binary alloys on the GeTe-Sb2Te3 tie-line (here referred to as GeSbTe) or quaternary AgInSbTe alloys. The choice for
one of these materials is mostly governed by the specific requirements of the application. GeSbTe-materials are preferred for
land-groove recording formats, have fast crystallization properties (crystallization times down to 20 ns resulting in data rates
up to 35 Mbit/s) and generally exhibit an excellent direct overwrite (DOW) cyclability of more than 10 5 times [1]. AgInSbTealloys are typically used for groove-only formats, where the lower radial density must be compensated by a higher linear
density to achieve the same overall data capacity on the disc. The overwrite cyclability of these materials is typically one to
two orders of magnitude lower than that of GeSbTe, but still sufficient for video recording applications. Based on CD-RW
experience, application of AgInSbTe for high-speed recording has been doubted in the past. However, a data rate over 22
Mbit/s has recently been reported for DVD 4.7 GB recording conditions [2]. In this work we evaluate the prospects of
AgInSbTe materials for high-speed phase change recording applications, at recording wavelengths λ around 650 nm and 400
nm in combination with numerical apertures NA of 0.60 and 0.85.
Jitter after 10 DOW [% of Tch]
2
AgInSbTe versus GeSbTe materials: recording density
The AgInSbTe materials that are being used have
compositions close to the eutectic Sb69Te31, in which some
14
of the Te is replaced by Ag and In. During re-crystallization
a number of crystalline phases are formed [3], complicating
AgInSbTe
detailed studies on the kinetics of the re-crystallization
GeSbTe
process. Interestingly, transmission electron microscopy
12
studies and static tester experiments indicate that the erase
process of amorphous marks proceeds via growth of the
surrounding crystalline edge to the center of the mark, rather
than by nucleation and subsequent growth (as in the
10
stoichiometric GeSbTe compositions). Consequently, the
marks written in AgInSbTe-based media have a welldefined shape with sharp edges, leading to intrinsically
lower jitter values than observed for GeSbTe-based media.
8
As jitter is the limiting factor in increasing the linear density,
the AgInSbTe-materials allow a higher linear density than
GeSbTe. Figure 1 shows the average multi-track jitter values
as a function of bit length, under DVD recording conditions,
6
for typical AgInSbTe and Ge2Sb2Te5-media. Considering a
bottom jitter value of 9% as limit, the maximum linear
0.24
0.26
0.28
0.30
0.32
0.34
0.36
density of GeSbTe amounts 0.30 μm/bit, whereas AgInSbTe
Bit length [m]
allows a data bit length of 0.267 μm, equal to that in DVD
read-only. Combining the data bit length of 0.267 μm with
Figure 1. Average jitter after 10 DOW cycles as
the DVD read-only track pitch of 0.74 μm, a user capacity
a function of linear density, under DVD recording
of 4.7 GB has been achieved in a DVD-compatible grooveconditions (λ=658 nm, NA=0.60, EFMplus code),
only format (DVD+RW 4.7 GB).
for GeSbTe and AgInSbTe-based media.
Contact person: Herman Borg, borg@natlab.research.philips.com
Philips
Research
Complete erasure time [ns]
3
AgInSbTe versus GeSbTe materials: crystallization rate
The maximum data rate that can be achieved in phase
100
change recording depends strongly on the crystallization rate
of the amorphous phase change material. Valuable
AgInSbTe
information on the possible recording speed of a material
GeSbTe
can be obtained by studying the re-crystallization (erase)
80
process of amorphous marks in a static tester setup. During
these experiments, amorphous marks are written by using
high-power laser pulses, and these marks are subsequently
60
erased (re-crystallized) by applying erase pulses of varying
power and pulse width. The minimum time required to
completely re-crystallize an amorphous mark is the so-called
40
complete erasure time (CET). Figure 2 shows static tester
results for typical AgInSbTe- and GeSbTe-based media. In
this experiment the size of the amorphous marks was
20
changed by varying the write power P write in fixed steps
relative to the threshold power P melt-threshold for melting the
phase change material, and the complete erasure times were
measured as described above. For GeSbTe-media, where re0
crystallization proceeds via nucleation and growth, the CET
1.0
1.1
1.2
1.3
1.4
1.5
1.6
appears to depend only marginally on the size of the
Amorphous mark size (Pwrite/Pmelt-threshold)
amorphous mark. Here, the re-crystallization rate is mainly
determined by the nucleation probability per unit area. In the
case of AgInSbTe, however, the CET increases rapidly with
Figure 2. The effect of the amorphous mark size
increasing mark size. A simple explanation for this behavior
on its complete erasure time, for Ge2Sb2Te5 and
is that, in the absence of nucleation within the mark, the
AgInSbTe materials. Experiments were performed
crystalline edge has to grow over a distance equal to the
at 658 nm wavelength and a numerical aperture
radius of the mark to completely erase it.
of 0.60.
4
Recording speed of AgInSbTe materials at different laser spot size
The observed dependence of the re-crystallization time on the size of the written amorphous mark supports the hypothesis
that the maximum data rate that can be obtained with a particular AgInSbTe material depends strongly on the spot size of the
focussed laser beam. To verify this, we performed a series of recording experiments at different laser wavelengths and
numerical apertures. For this purpose, 4-layer recording stacks of the rapid-cooling structure type
(ZnS:SiO2/AgInSbTe/ZnS:SiO2/Al-alloy) were sputter-deposited onto 1.2 mm and 0.6 mm thick polycarbonate (PC)
substrates. For recording experiments at NA=0.85, the recording stack was deposited onto a 1.1 mm PC substrate in reverse
order, and a 0.1 mm thick cover layer was spin-coated onto the disc [4]. The structure (materials and thickness of the layers)
of the various stacks was kept the same, for a fair comparison of the recording performance at varying spot size. Also, the
recording conditions were kept as comparable as possible. The linear density was scaled by the laser spot diameter d
(d=1.22*λ/NA), using the density of the Digital Video Recording (DVR) system as reference [5]. The recording strategy
consists of 0.5T write pulses, with a first pulse length of 1T and a cooling pulse length of 0.5T: this simple strategy gives
good results under all spot size conditions. The most important recording parameters are summarized in Table 1.
Table 1. Recording conditions and results
CD-RW light path
DVD light path
DVR light path
Wavelength [nm]
780
658
658
Numerical aperture
0.50
0.60
0.85
Substrate thickness [mm]
1.2
0.6
0.1
Data bit length [μm]
0.431
0.301
0.21
Max. linear velocity [m/s]
9.6
9.8
9.2
Max. user data rate [Mbit/s]2
18
26
35
1
scaled by spot size from DVR conditions
2
here reported data rates are for a (1,7) RLL code with rate 2/3 and a format efficiency of 80%
Contact person: Herman Borg, borg@natlab.research.philips.com
Philips
Research
The maximum data rate at a certain spot size was
determined as the rate at which the jitter after 10 DOW
cycles was equal to 10%. The exact value of this jitter level
has no significant effect on the result, because the jitter
50
steeply increases at higher recording speeds. The reported
user data rate values are based on a (1,7) RLL code with rate
40
2/3, taking into account a format efficiency of 80%.
Figure 3 shows the measured maximum data rate of a
AgInSbTe-based recording stack as function of the inverse
30
spot diameter. The maximum data rate increases
658 nm
NA=0.85
significantly at decreasing spot size, in accordance with the
hypothesis. Interestingly, recording experiments with
20
658 nm
GeSbTe-based media at 658 nm wavelength do not exhibit
NA=0.6
an increase in data rate in going from NA=0.60 to NA=0.85,
780 nm 0
400 nm
also in line with the static tester results.
10
NA=0.5
NA=0.85
It is tempting to speculate about the data rates that can be
0
achieved with these materials at 400 nm wavelength and an
0
NA of 0.85. On the basis of our experience and the above
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
results, the speed that can be achieved with GeSbTe-1
-1
materials is mainly determined by the possibilities to further
(spot diameter) [m ]
increase the nucleation probability of the amorphous
Figure 3. The maximum user data rate of a
material (while at the same time the archival life stability of
AgInSbTe-based recording stack as function of
the amorphous state must be maintained at a sufficient level)
the inverse laser spot diameter d -1=NA/(1.22λ).
and the timing accuracy of the write electronics. No
Details on the recording conditions are listed in
significant correlation between the laser spot size and data
Table 1.
rate has been found for GeSbTe-media, so that similar data
rates are expected at red and blue wavelengths. For AgInSbTe, reduction of the spot size of the laser has an important effect
on the maximum data rate. Linear extrapolation of the curve in Figure 3 to a spot diameter corresponding with λ=400 nm and
NA=0.85 suggests that data rates above 50 Mbit/s can be achieved with conventional 4-layer AgInSbTe-media and a
straightforward recording strategy. Optimization of the recording material composition, the stack structure, and the write
strategy will further add to the data rate. The justification of these speculations is the subject of ongoing research.
Nevertheless, the use of AgInSbTe-materials or alternative materials with growth-determined re-crystallization kinetics will
become increasingly attractive at decreased laser spot size.
Maximum user data rate [Mbit/s]
60
5
Conclusions
AgInSbTe materials are currently the preferred materials for groove-only phase change recording formats, because they allow
higher linear densities than stoichiometric GeSbTe-compositions. The intrinsically low jitter of AgInSbTe is attributed to the
mechanism by which amorphous marks are erased, i.e. via growth of the crystalline edge towards the mark center, resulting
in marks with well-defined edges. The nature of this re-crystallization process also has an effect on the time required to erase
marks of different size: the re-crystallization time of amorphous marks decreases with decreasing mark size. As a result, the
maximum data rate of a particular AgInSbTe recording stack increases with decreasing laser spot size. On the basis of our
experiments, we expect that at 400 nm wavelength and an NA of 0.85 data rates over 50 Mbit/s will be realized with
conventional 4-layer AgInSbTe recording stacks and a simple recording strategy.
References
[1] G.F. Zhou and B.A.J. Jacobs, “High performance media for phase change optical recording”, Proc. ISOM’98, to be
published in Jpn. J. Appl. Phys. 38 (1999).
[2] K. Ito et al., “The capability of AgInSbTe phase change material for high speed rewritable 4.7 GB media”, Proc.
ISOM’98, to be published in Jpn. J. Appl. Phys. 38 (1999).
[3] H. Iwasaki et al., “Completely erasable phase change optical disc: application of AgInSbTe mixed-phase system for
rewritable CD”, Jpn. J. Appl. Phys. 32 (1993) 5241.
[4] K. Yamamoto et al., “0.8 Numerical aperture two-element objective lens for the optical disc”, Jpn. J. Appl. Phys. 36
(1997) 456.
[5] T. Narahara et al., “Optical disc system for Digital Video Recording”, Technical Digest of ISOM/ODS’99.
Contact person: Herman Borg, borg@natlab.research.philips.com
Philips
Research