Signal Modulation of Super Read Only Memory with ...

Japanese Journal of Applied Physics Vol. 47, No. 7, 2008, pp. 5845–5847 #2008 The Japan Society of Applied Physics

Signal Modulation of Super Read Only Memory with Thermally Activated Aperture Model June Seo K IM, Keumcheol KWAK1 , and Chun-Yeol YOU
Department of Physics, Inha University, Incheon 402-751, Korea 1 LG Electronics Institute of Technology, 16 Woomyeon-dong, Seocho-gu, Seoul 137-724, Korea (Received November 22, 2007; accepted March 3, 2008; published online July 18, 2008)

We describe the signal modulation of super read only memory (ROM) with thermally activated aperture model using a threedimensional finite-difference time-domain method. The thermally activated aperture is modeled using a spatially varied refractive indices of the GeSbTe layer. No meaningful signal modulation is observed without thermally activated aperture below the resolution limit of 120 nm. When we open the thermally activated aperture by considering the temperature dependence of the refractive indices in the GeSbTe layer, the 2.8 and 1.7% signal modulations are observed for 120 and 80 nm pits, respectively. The experimentally observed signal modulation under the resolution limit can be explained using the thermally activated aperture model. [DOI: 10.1143/JJAP.47.5845] KEYWORDS: super-RENS/ROM, finite-difference time domain, thermally activated aperture

1.

Introduction

A super-resolution near-field structure (super-RENS) or super read only memory (super-ROM) is a prospective technology to overcome the resolution limit and achieve the capacity of optical storage over 100 GB.1–3) According to the diffraction theory, the reading signal from a mark vanishes when the size of the mark is below the resolution limit. However, on observable reading signal is obtained in many experiments below the resolution limit in super-RENS/ ROM structures.3) Even though the reading signal is observed in various experiments, the detailed physics of the reading mechanisms in super-RENS/ROM is not yet clear. To reveal the underlying physics of super-RENS/ ROM, the finite-difference time-domain (FDTD) simulation will be an appropriate tool. FDTD is one of the successful computational methods for solving the Maxwell equations for various electromagnetic wave problems.4) In this study, we were able to include the effect of temperature-dependent refractive indices for a thermally activated aperture by employing FDTD simulation. We simulated the signal modulations for various pit and aperture sizes, and it was found that the experimental observations can be explained using the thermally activated aperture model. 2.

Optical Disk Simulator

We have developed an optical disk simulator in order to study the super-RENS/ROM by a three-dimensional FDTD method. Figure 1 shows the schematic of the three steps of the simulations. In the first step, we utilize the vector diffraction theory to calculate the input beam profile on the disk surface.5) The plane wave or Gaussian laser beam is focused on the objective lens, so that the beam profile on the surface of the disk depends on the numerical aperture (NA) and focal length of the lens, and the distance between the lens and the disk. In the second step, we use the calculated beam profile as an input for the main FDTD simulation part on the surface of a multilayer disk structure. Because typical disk structures are multilayer stacks, we have to perform the FDTD calculation more than 10 periods of the input beam period in order to incorporate the multiple reflections from


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Fig. 1. Schematic of simulation procedures. The grid represents the FDTD calculation space.

the interfaces. After the FDTD simulations have been performed, the reflected beam profile at the surface of the disk is obtained. A near-to far-field transformation from the surface of the disk to the detector plane is carried out for the reflected signal in the third stage. Finally, we obtain the beam profile at the detector plane. We study the reading signal from various disk structures as a function of the laser beam position. In this paper, we calculate the reflectivity of a multilayer structure that consists of a polycarbonate substrate/AgPdCu (78 nm)/ZnS–SiO2 (10 nm)/GeSbTe (20 nm)/ZnS–SiO2 (30 nm)/cover layer as shown in Fig. 2. The resolution limit is approximately 120 nm for the wavelength of 405 nm and NA ¼ 0:85. Figure 2 shows a typical multilayer structure that we study. We tabulate the refractive indices of each layer in Table I. The experimental refractive index of the amorphous GeSbTe (GST) layer at 405 nm has not been reported yet, to the best of our knowledge. Therefore we predict the refractive index by using the reported data at the wavelength of 780 nm.1) According to the previous study, the real and imaginary parts of the refractive index of the GST are abruptly reduced to 37 and 40% at 700  C compared with the room-temperature values, respectively. We simply assume that the variation rate of the refractive indices does not depend on the wavelength and apply the variation rate to the refractive indices at the wavelength of 405 nm. Therefore, the values in Table I not measured ones, but we consider that the detailed

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Cover layer Lower Dielectric layer, 30 nm GeSbTe, 20 nm Upper Dielectric layer, 10 nm Reflective layer, 78 nm Substrate

Fig. 2. Schematic layer structure of typical multilayer system. Fig. 3. Reflectivity of super-ROM structure with 240 nm pit size and 60 nm pit depth without thermally activated apertures. The inset shows the disk surface with a pit. The arrows in the inset represent the beam positions.

Table I. Refractive indices used in simulation. Material ZnS–SiO2

Refractive index (n þ ki) 2.34

GeSbTe (crystal)

0:79 þ 2:76i

GeSbTe (molten) AgPdCu

0:5 þ 1:57i 0:01 þ 1:92i

Polycarbonate substrate

1:66 þ 0:01i

values of the refractive indices do not change the main results of our study. 3.

Simulation Results and Discussion

The typical calculated reflectivities are shown in Fig. 3. The reflectivity of the super-ROM structure is obtained with various laser beam center positions. As shown in Fig. 3, the reflectivity is varied as a function of the laser beam position. Position 4 is the center of the two pits, and the reflectivities must show mirror symmetry with respect to position 4, and this has been confirmed (not shown). With the position dependent reflectivities, we calculate the signal modulation defined as jðRmax  Rmin Þ=ðRmax þ Rmin Þj; it has a close relationship with the experimentally measured carrier-to-noise (CNR) values. In Fig. 3, the signal modulation is about 1.51%, where the pit size is 240 nm and the pit depth is 60 nm. Figure 4(a) shows a typical disk structure with array of pits. In our calculation, the track pitch (TP) is fixed at 320 nm and the pit pitch (PP) is set to have the same value as the diameter of the pits. In the Fig. 4(b), the signal modulations for various pit sizes are depicted. The pit sizes and depths varies from 120 to 240 nm and from 1/8 to 2=3
=n, respectively. Larger signal modulations are obtained for the larger pit sizes, as we expected. It is also confirmed that the signal modulation depends on the pit depth, which has been experimentally reported.3) Let us focus on the signal modulation of the 120 nm pit case. At this stage, we do not consider the temperaturedependent refractive indices, which indicates that there is no thermally activated aperture. Therefore, it is physically reasonable that there is no meaningful signal modulation under the resolution limit. However, experimentally the meaningful signals are reported under the resolution limit.2)

Fig. 4. (a) Disk surface structure for modeling of super-ROM disk (PS: pit size, TP: track pitch, PP: pit pitch). (b) Signal modulations from ODS as a function of the pit depth for various pit sizes without thermally activated apertures.

This indicates that some underlying physics exist in the reading mechanism in the super-ROM. In real experiments, the GST layer is heated by laser beam irradiation, and the temperature increase of the GST layer causes the change in the refractive index. At high temperature, the complex refractive index of the GST decreases, and it becomes more transparent. Now, we incorporate such effect by employing temperature-dependent refractive indices at each unit cell in the FDTD calculation. We assume that the high-temperature area has a different refractive index and we call that this area is the thermally activated aperture. Figure 5 shows the details of the simulation. We assume that only the inside of the circular aperture has a different refractive index for simplicity. Because the shape of the beam is approximately Gaussian, the temperature of the layer has a circular symmetry. The actual size and shape of the temperature profile depends on the power of the laser irradiation, velocity of the disk, and layer structure, but we ignore such effect in this study. Because the thermally activated aperture size is mainly determined by the laser power, we perform the simulation with various aperture sizes in order to investigate the effect of the laser power on the CNR.8,9) In Fig. 6, from (a) to (e) indicate the positions of the beam and thermally activated apertures with pits. The beam is moving from left to right, when the disk is rotated. Signal

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Fig. 5. Schematic of thermally activated aperture and pits. The molten area of the GeSbTe layer acts as a thermally activated aperture.

Fig. 7. Signal modulations as a function of size of thermally activated aperture for 80 and 120 nm pits. The pit depth is fixed at 60 nm.

can conclude that the local changes in refractive indices at the GST layer activate a dynamic aperture, and it can be a possible reason for the experimentally observed signal under the diffraction limit. 4.

Fig. 6. (a)–(e) Schematic figures of moving thermally activated apertures. The center of the aperture is the same as the center of the laser beam. (f) Reflectivity of 80 nm pit and aperture size of 120 nm as a function of laser beam/thermally activated aperture positions.

modulations are calculated from the reflectivity of each point for various sizes of the thermally activated apertures. Figure 6(f) shows the variation of the reflectivity as a function of the laser beam or the aperture positions. Basically, this is the same calculation with the Fig. 3, except for the consideration of the temperature-dependent refractive indices. In this case, the calculated signal modulation is about 1.7 (2.8)% for the 80 (120) nm pit sizes with 60 nm pit depth. For the fixed 80 (120) nm pit size and 60 nm pit depth, we depict the signal modulation as a function of the thermally activated aperture sizes in Fig. 7. Meaningful signal modulation is obtained even for the pit size below the resolution limit. We also find that the signal modulation depends on the aperture size as we expect. In the experiments, the CNR depends on the laser power. The CNR increases with laser power, and then it decreases.3,6) Because the aperture size increases with the laser power, our calculation results can explain the experimental observations. In the x-axis in Fig. 7, the crystal (amorphous) indicates that the entire GST layer is crystal (molten); therefore, no thermally activated aperture is formed. In these cases, the signal modulations are approximately zero. We

Conclusions

We investigated the signal modulations in super-ROM using a home made three-dimensional FDTD simulator. We consider the thermally activated aperture model in order to explain the experimentally observed CNR below the resolution limit. When we do not consider the temperaturedependent refractive index of the GST layer, no meaningful signal modulation is obtained under the diffraction limit. However, with the thermally activated aperture concept, we can obtain clear signal modulations even below the diffraction limit. The thermally activated aperture model can be a possible explanation for the experimentally observed signal modulations. Acknowledgement This work was supported by Korea Research Foundation Grant funded by the Korean Government (Grant No. KRF2006-312-C00529).

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