© Japanese Society of Electron Microscopy
Journal of Electron Microscopy 48(3): 183-190 (1999)
Full-length paper
High-voltage, high-resolution electron microscopy study on thin sections of phase change optical disk prepared by an ultramicrotome Gyeong-Su Park1*, Hyeon-Chang Hong2, Jun-Mo Yang3 and Daisuke Shindo 3 'Samsung Advanced Institute of Technology, San 14-1, Nong-Seo Ri, Ki-Hung Ueb, Yong-In Gun, Kyung-Ki Do, 449-900, Korea, 2DVD R&D Center, Samsung Electronics Co., Ltd, 416, Mae-Tan 3 Dong, Pal-Dai Gu, Su-Won City, Kyung-Ki Do, 441-742, Korea and institute for Advanced Materials Processing, Tohoku University, Katahira 2-1-1, Aobaku, Sendai, 980-8577, Japan *To whom correspondence should be addressed. E-mail:
[email protected]
Abstract
Keywords Received
Thin cross-sections of a phase change optical disk were prepared by ultramicrotomy without embedding of the resin. Transmission electron microscope images of the thin sections directly show the constitution of the phase change optical disk, that is, the coating layer (UV resin: 2.05 |xm), the reflective layer (Al: 90 nm), the upper dielectric layer (ZnSu.SiC>2: 37 nm), the recording layer (Ge2Sb2Te5: 20 nm) and the bottom dielectric layer (ZnSSiO2: 160 nm) on the pre-grooved polycarbonate substrate. Its structure on an atomic scale is analysed by high-voltage, high-resolution electron microscopy. Moreover, it is found that the characteristic of the phase change of the optical disk is attributed to the GeTe intermetallic compound of the recording layer formed during the initializing process. phase change optical disk, ultramicrotomy, high-resolution electron microscopy, recording layer, Ge2Sb2Te5, GeTe intermetallic compound 30 April 1998, accepted 12 November 1998
Introduction Due to a growing interest in digital versatile disk (DVD) technology, the magneto-optical and the phase change optical disks have been developed to enhance the recording density. In addition to the higher recording density, phase change optical disks have the advantage of large capacity, one pass optical overwrite performance and high reliability compared with conventional optical disks [1,2]. The phase change optical recording makes use of the amorphous and the crystalline structural phases of a recording material to encode data. The read out of the disk applies different optical properties (reflectivity) between the two phases. Amorphization (writing process) can be easily accomplished by means of short laser pulses to induce melting and rapid solidification whereas crystallization (erasing process) usually requires longer laser pulses to induce both crystal nucleation and growth [3,4]. In general, a phase change optical disk is manufactured by depositing a coating layer, a reflective layer, an upper
dielectric layer, a recording layer and a bottom dielectric layer on the pre-grooved polycarbonate substrate [5]. The initial crystalline state of the recording layer can be obtained by irradiating laser beams onto the as-deposited recording layer on the rotating disk. The microstructure of the laser-induced crystalline state of the recording layer strongly depends on the power of laser source and the linear velocity of the disk. It has been reported that the microstructure of the laser-induced crystalline state of the recording layer affects the important dynamic characteristics such as the carrier-to-noise (C/N) ratio and the initial jitter [6,7]. Some results on the microstructure of phase change materials have been reported mainly by observing planview transmission electron microscopy (TEM) samples [8-10]. To investigate the structure of the phase change optical disk by TEM in detail, the preparation of thin cross-sections of constituent layers is the key. This is because thin cross-sections of the constituent layers are very difficult to obtain by the conventional ion milling technique due to the different ion milling rates among
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Fig. 1 Cross-sectional SEM image of a phase change optical disk revealing the shapes of the land and the groove. L: land, G: groove, T: track pitch
Fig. 2 Low magnification TEM image of the thin section of the phase change optical disk prepared with an ultramicrotome revealing the shapes of the land and the groove. L: land, G: groove, T: track pitch.
the layers. Thus, ultramicrotomy instead of the ion milling technique is applied to prepare thin cross-sections of the phase change optical disk in this study. Moreover, a highvoltage electron microscope can be utilized to clarify the structure of constituent layers on an atomic scale owing to their higher resolution limit and higher transmission capacity. Previously, we reported the results on the
internal structure of pseudocubic [11,12] and peanuttype [13,14] fine hematite particles investigated by highvoltage, high-resolution electron microscopy (HREM) coupled with ultramicrotomy. This paper presents the detailed structure of a phase change optical disk investigated using high-voltage, HREM coupled with ultramicrotomy.
G.-S.Park et al. High voltage, HREM on phase change optical disk
1
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" *• Coating layer (UV resin)
— -'-Reflective layer (Al) Upper dielectric layer (ZnS-SiQO Recording Layer (Ge2Sb2Tes) 1 Bottom dielectric layer (ZnS-Sia)
• Polycarbonate substrates
Fig. 3 Schematic diagram of 5-layered disk structure on the substrate.
I 0.5
Fig. 4 Low magnification TEM image of the thin section of a phase change optical disk revealing the uniformly sliced coating layer.
Experimental procedure Thin sections of a phase change optical disk were prepared by an ultramicrotome without embedding of the resin. Trimming of a trapezoidal shape was carried out by a glass knife. The slicing speed and thickness were set at 1 mm/s and 50-70 nm, respectively. The sections were put on a microgrid covered with a carbon thin film, and then immersed in chloroform to remove the poly-
carbonate which might cause sample drift by strong electron irradiation during TEM observation. Highvoltage, HREM study was carried out using a JEMARM 12 50 electron microscope with an accelerating voltage of 1250 kV. Electron microscope observation at relatively low magnification was also carried out by a Hitachi S-4500 scanning electron microscope (SEM) and a H-9000NA TEM with accelerating voltages of 20 and 300 kV, respectively.
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150 nm Fig. 5 TEM image of the cross-section of a phase change optical disk showing four layers (A: reflective layer; B: upper dielectric layer; C: recording layer; D: bottom dielectric layer) on the pre-grooved polycarbonate substrate
strate. Figure 4 shows a low magnification TEM image of Figure 1 shows a cross-sectional SEM image of a phase the thin cross-section of a phase change optical disk change optical disk revealing the shapes of the land and revealing the uniformly sliced coating layer. It is seen that the groove. The length of the land, the groove arid the the thickness of the coating layer is about 2.05 u.m. Figure track pitch is found to be about 0.55 um, 0.40 u.m and 5 indicates a TEM image of the cross-section of the phase 1.10 nm, respectively. However, it is rather difficult to change optical disk showing four layers (reflective layer, clearly differentiate constituent thin layers of the disk upper dielectric layer, recording layer, bottom dielectric from SEM images only. Figure 2 shows a low magnification layer) on the pre-grooved polycarbonate substrate. The TEM image of the thin cross-section of the phase change arrows indicate the crack in the section caused by the optical disk prepared with an ultramicrotome. The uni- slicing process. It is shown from the image that the formly sliced thin section maintains the shapes of the thickness of the reflective layer, the upper dielectric layer, land and the groove. It is confirmed from this image that the recording layer and the bottom dielectric layer is the length of the land, the groove and the track pitch is 90 nm, 37 nm, 20 nm and 160 nm, respectively. Besides, 0.55 u.m, 0.40 nm and 1.10 u.m, respectively. These results the cross-section of the reflective layer shows that the layer coincide with the SEM data as illustrated in Fig. 1. consists of polycrystallites with the columnar structure. To Figure 3 indicates a schematic diagram of a 5-layered disk gain insight into more detailed atomic arrangements and structure on the substrate. It depicts that the 5-layered crystallographic structures of the main layers of the disk, disk is composed of the coating layer (UV resin), the HREM study was carried out with a high-voltage electron reflective layer (Al), the upper dielectric layer (ZnS-SiO2), microscope carefully operated for little damage of specithe recording layer (Ge2Sb2Te5) and the bottom dielectric mens by electron beam irradiation. Figure 6 shows a highlayer (ZnS-SiO2) on the pre-grooved polycarbonate sub- voltage, HREM image of the thin cross-section of the
Results and discussion
ff a]. H:.qh vrfimjr. HRFM cw r>basr -fiarj.Hf rpiira! *''k
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'Reflective laver (AI)
Upper dielectnc layer (ZnS-SiO2)
Recording layer (Ge2Sb2Te5)
Fig. 6 High-voltage, HREM image of the thin seaion of a phase change optical disk revealing the structures of the refleaive layer, the dielearic layer and the recording layer on an atomic scale. The digital diffraaogram obtained from the HREM image of the recording layer is also shown in the inset.
phase change optical disk revealing the structures of the reflective layer, the upper dielectric layer and the recording layer on an atomic scale. The digital diffractog-
ram obtained from the HREM image of the recording layer is shown in the inset. The lattice fringe of the refleaive layer corresponds to the (111) plane (0.234 nm)
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Fig. 7 Typical plan-view TEM image of the recording layer showing the amorphous recording marks and its electron diffraction pattern in the inset.
of the aluminum. The high-voltage, HREM image of the dielectric layer in Fig. 6 indicates that ZnS crystals (about 3-4 nm in size) are evenly dispersed in the amorphous SiO2 matrix. Although part of the recording layer outlined by dotted lines is moved to the dielectric layer due to the strain field introduced by slicing, no interdiffusion between two layers is observed across the boundary. Moreover, from the HREM image of the recording layer and its digital diffractogram, it is found that two types of sharp lattice fringes of the recording layer correspond to the {202} and {224} planes of the GeTe compound with rhombohedral structure where the indices of planes are based on the hexagonal system. Thus, it is concluded that the crystalline phase of the recording layer (Ge2Sb2Te5) is mainly composed of the GeTe intermetallic compound. Figure 7 shows a typical plan-view TEM image of the recorded recording layer with a corresponding electron diffraction pattern in the inset. Cracks induced during the sampling process are observed. The plan-view TEM image
and its electron diffraction pattern demonstrate that most of the recording layer is composed of crystalline grains except bright regions which correspond to the recording marks obtained after laser beam irradiation. Diameters of the recording marks are about 0.5 nm. In order to evaluate the structure change of the recording layer before and after laser beam irradiation on the disk on an atomic scale, HREM study was carried out on three types of the recording layer (as-deposited, initialized and recorded). Figures 8a, 8b and 8c show HREM images of as-deposited, initialized and recorded recording layers, respectively. Figure 8a shows that the as-deposited recording layer is composed of an amorphous phase with partially coexisting nanocrystals. The size of the nanocrystals is about 0.9-1.2 nm as indicated by arrows in Fig. 8a. Figure 8b represents two types of sharp lattice fringes of the initialized recording layer corresponding to the {202} and {021} planes of the GeTe compound. It should be noted that the as-deposited recording layer is crystallized to the
G.-S.Park et al. High voltage, HREM on phase change optical disk
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Fig. 8 (a) HREM image of the as-deposited recording layer showing the amorphous phase with partly coexisting nanocrystals. (b) HREM image of the initialized recording layer showing sharp lattice fringes of the GeTe mtermetallic compound, (c) HREM image of the recorded recording layer turned into amorphous phase by irradiating the laser beams in a pulse mode for recording. The nanocrystals are also shown enlarged in size compared with (a).
GeTe intermetallic compound by the initializing process. The initializing process was carried out at a laser power of 400 mW and a constant linear velocity of 7 m/s. The result also indicates that the recording layer represented in Fig. 6 has been initialized. Figure 8c shows that the recorded recording layer is composed of the amorphous phase changed from the crystalline phase of the initialized recording layer. The recorded recording layer can be obtained by irradiating the laser beams in a pulse mode for recording. In this case, the recording power and the linear velocity were 15 mW and 11 m/s, respectively. The nanocrystals are also shown enlarged in size compared with Fig. 8a. The result seems to indicate that the nanocrystals in the as-deposited recording layer have grown during the initializing or recording process. The grown nanocrystals in the recorded recording layer lessen the reflectivity change and cause the formation of a heterogen-
eous crystalline phase during the erasing cycle. Thus, the nanocrystals in the amorphous phase of the recorded recording layer may be an important factor to worsen the dynamic characteristics such as the C/N ratio and the initial jitter of a phase change optical disk.
Concluding remarks Based on the present results, it is clear that ultramicrotomy is useful for preparing the thin cross-sections of a phase change optical disk layered with metal, ceramic and polymeric materials. The thin cross-sections of a phase change optical disk clearly reveal the thickness of constituent layers and the structure of the disk. Highvoltage, HREM images demonstrate that the ZnS-SiO2 dielectric layer and the initialized Ge2Sb2Te5 recording layer are composed of ZnS nanocrystals (about 3-4 nm
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in size) in the amorphous SiO2 matrix and the crystalline GeTe compound, respectively. Moreover, from the highvoltage, HREM images of the recording layer before and after laser beam irradiation on the disk, it is concluded that the phase change of the recording layer is accomplished by the GeTe intermetallic compound formed during the initializing process.
Acknowledgements The authors thank Dr J-Y. Kim and Ms S-Y. Lee for their useful discussions.
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