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Characteristics essential for the readout durability of a superresolution near-field structure (super-RENS) disk are studied experimentally by using a home-built optical measuring setup and atomic force microscope, based on a simplified PtOx super-RENS disk. The experimental results show that for a super-RENS disk with constant structure and materials, readout signals including transmittance and reflectance vary with changes in bubble shape and size, indicating that the readout durability of the disk has a strong dependence on bubble stability, which is closely related to the thickness of the cover layer, the recording power and readout power, and the mechanical properties of the dielectric layer. Based on our experimental results, the main direction for improving readout durability is also proposed.
©2008 Optical Society of America
1. Introduction
Readout durability is one of the important factors for an optical disk. It relies not only on the material and structural system of a disk, but also is related to recording and reading power. Excellent readout durability is necessary for a commercial disk. A few years ago, a super-resolution near-field structure (super-RENS) disk, which can overcome the optical diffraction limit, was successfully developed [1]. Its practicality and rapid progress have attracted much attention in the field of high-density storage [2–12], and its commercialization is being carried out. A short time ago, TDK declared a PtOx super-RENS disk to have a carrier-to-noise-ratio (CNR) suitable for practical use, namely, more than 40 dB for a 60 nm mark train [13]. However, the readout durability must still be improved for commercialization. Recently, the readout durability was improved by use of SiO2-PtOx instead of PtOx, because SiO2-PtOx has better antistress deformation and better thermal diffusion [14]. The importance of thermal diffusion was also identified by superresolution recording with Si as an underlayer [15]. In order to improve the readout durability, clarifying essential characteristics is very important.
A schematic for a recorded PtOx super-RENS disk is shown in Fig. 1(a). In the disk recording process, a focused laser beam results in a permanent bubble deformation with Pt nanoparticles and O due to decomposition of PtOx in the PtOx layer. The bubble structure, which consists of a PtOx layer and ZnS-SiO2 dielectric layer, looks like the tip used in a scanning near-field optical microscope; its top surface is just the aperture of the tip. The light passing the aperture reaches the Ag60.0In4.5Sb60.8Te28.7 (AIST) layer and causes an AIST phase transition from amorphous to crystal. Generally speaking, any light-induced mark in a recording process will modify readout properties, but the readout durability in a reading process depends mainly on the stability of the recorded bubble rather than the phase transition, because a reading power much smaller than recording power cannot produce an inverse phase transition. However, the readout durability has little been studied so far, since it is quite difficult in an experiment to extract the signal of just the bubble from the total optical readout response.
In this study, the relationship between the readout durability and recorded-bubble transformation induced by thermal accumulation in the readout process is investigated by means of atomic force microscope (AFM) image analysis and measurement in transmittance (T) and reflectance (R) for the bubble, based on the simplified super-RENS disk shown in Fig. 1(b). In the simplified super-RENS disk, only the AIST layer and the protection layer shown in Fig. 1(a) are omitted, so that the influence of the two layers can be removed from the total optical response and the transformation of the bubble surface can be easily observed microscopically. Based on our experimental results, some directions for improving readout durability will be proposed.
2. Experiment
The experimental setup is shown in Fig. 2. A continue wave (CW) diode laser (405 nm) with a pulse signal generator (PG-230), which can provide both CW and pulsed laser emission, was used as a light source for reading and recording. A lens L with a 0.6 numerical aperture focused the incident beam on the sample; an objective lens OL with 10× magnification and 0.21 numerical aperture was used to collect light transmitted through the sample. Reflected and transmitted light were detected by using two Hamamatsu S2281-01 silicon photodiodes with a C2719 photosensor amplifier. The signals detected by the sensor system were integrated into a gated integrator (Stanford Research System SR 250). The simplified super-RENS samples with three different cover-layer thicknesses shown in Fig. 1(b) were fabricated by depositing a (ZnS)85(SiO2)15[130 nm]/PtOx[4 nm]/(ZnS)85(SiO2)15[40/80/140 nm] multilayer on a polycarbonate (PC) disk substrate, by means of radio-frequency sputtering method. The (ZnS)85(SiO2)15 and PtOx were sputter deposited, separately, by a composite target in an Ar atmosphere (5N) and by a Pt target in gas mixture of Ar (5N) and O2 (5N) with a gas-mass-flow ratio of (O2/O2+Ar)=0.2.
Fig. 1. Schematic of the super-RENS disk: (a) recorded super-RENS disk, (b) simplified super-RENS disk.
Fig. 2. Schematic configuration for experimental setup of transmittance T and reflectance R measurement. L, focused lens with 0.6 numerical aperture; BSP, beam splitter prism; OL, objective lens lens with a 10×magnification and a 0.21 numerical aperture; PD, photodetector; M, mirror; PS, pulse signal simulator; Sample, the simplified super-RENS disk on a nanoscale moving stage.
In the experiment, we used a focused laser pulse (λ=405 nm) to generate a bubble on the simplified super-RENS sample, then took down the mirror M and irradiated the bubble by using a CW laser beam (405 nm); meanwhile R and T, varying with irradiating time, were measured by two detectors, PD–R and PD-T, and calculated automatically by a computer. This is like an accelerated experiment for a readout durability test of the super-RENS disk. The incident power on the sample surface was obtained by using an optical powermeter (Advantest TQ 8210). Here the decomposition of PtOx was monitored by using a pulse signal simulator (PS) combining with a digital phosphor oscilloscope (Textronix TDS 3052). A transmitted rectangular signal (yellow) that went through the sample in Fig. 3(a) showed no PtOx decomposition in the sample, as indicated by a wave shape that is the same as the incident laser pulse (blue). When a change in the transmitted pulse signal shown in Fig. 3(b) appeared, a bubble deformation induced by PtOx decomposition had been generated, which was confirmed by micro-observation. For an incident power of 9.2 mW, the pulse energy threshold for the PtOx decomposition in the three samples with cover-layer thicknesses of 140, 80 and 40 nm was seen to be 13.8 µJ (pulse width Pw=1.5 µs), 18.4 µJ (Pw=2.0 µs) and 22.08 µJ (Pw=2.4 µs), respectively. The energy threshold increases with a decrease in cover-layer thickness; this is because the thicker cover layer has a higher thermal-accumulation effect.
3. Experimental Results and Analyses
R and T are plotted against irradiating time for different cover-layer thicknesses, 140, 80, and 40 nm, in Figs. 4(a), 4(b), and 4(c), respectively. Here the bubbles were formed by a laser pulse of 27.6 µJ (Pw=3 µs), and then a CW laser with a power of 1 mW was used to irradiate the bubbles. For the different thicknesses, all R and T curves had a short plateau, which should obviously correspond to the stable state of the bubble; then there was a dramatically change, which should indicate that the bubble, including the distribution of Pt nanoparticles within the bubble, had changed in size and shape. The bubble stability times for the samples were t 140=1.0 s, t 80=0.5 s, and t 40=0.3 s, respectively, as shown in Figs. 4(a), 4(b), and 4(c). This indicates that the bubble stability decreased with a decrease in cover-layer thickness.
Fig. 4 Curves for R and T versus irradiating time for different cover-layer thicknesses: (a) 140 nm, (b) 80 nm, (c) 40 nm.
To clarify the relationship between the bubble’s change in size and the R and T measurement, the radius (r) and height (h) of the bubble corresponding to 0, 10, and 20 s of CW laser radiation with a power of 1 mW were measured by use of AFM images, respectively, for a 140, 80, and 40 nm thick cover layer. Measurement results were that (h, r)140=(71.36 nm, 445.31nm), (76.24 nm, 453.10 nm), (170.00 nm, 644.00 nm) and (220.00 nm, 859.00 nm); (h, r)80=(94.26 nm, 445.30 nm), (98.21 nm, 451.10 nm), (240.02 nm, 860.05 nm) and (258.88 nm, 890.20 nm); (h, r)40=(121.01 nm, 457.03 nm), (127.00 nm, 462.12 nm), (250.30 nm, 896.50 nm), and (258.02 nm, 910.04 nm). Figure 5 shows the AFM images and section analyses for a 140 nm thick cover-layer sample. It is easy to see that the bubble in Figs. 5(a) and 5(b) has little change, corresponding well to the stable R and T scope in Fig. 4(a), and that in Figs. 5(b)–5(d) there is a rapid increase in size, corresponding to the change in R and T in Fig. 4(a). Almost the same relationship also existed between Fig. 4(b) and (h, r)80 as well as between Fig 4(c) and (h, r)40 in the 80 and 40 nm cover-layer samples. These relationships indicate that an optical response change in R and T depends on the bubble’s change in size.
Fig. 5. AFM images and section analyses of the bubble for a 140 nm thick cover layer: (a) 0 s, (b) 1 s, (c) 10 s, (d) 20 s.
Fig. 6. Bubble stability for a 140 nm thick cover layer. For a recording pulse energy of 27.6 µJ, stability changes with readout powers of (a) 0.5 mW, (b) 1.0 mW, and (c) 1.5 mW. For a fixed readout laser power (1 mW) stability variation with recording pulse energies of (d) 18.4 µJ, (e) 27.6 µJ, (f) 36.8 µJ.
The bubble stability was also investigated for different reading powers of the CW laser and different recording pulse energies. Figures 6(a)–6(c) show that for the 140 nm cover layer, the stability time of the bubble generated by 27.6 µJ (Pw=3 µs) was 1.6, 0.98, and 0.6 s for CW irradiations of 0.5, 1.0 and 1.5 mW, respectively; Figs. 6(d)–6(f) indicate that the stability time for a 140 nm cover layer at a readout power of 1 mW was 1.3, 1.0 and 0.6 s for bubbles generated by 18.4 µJ (Pw=2 µs), 27.6 µJ (Pw=3 µs) and 36.8 µJ (Pw=4 µs), respectively. This demonstrates that the bubble stability decreases with an increase in either recording pulse energy or readout CW irradiating power.
To sum up, the bubble stability, based on the simplified PtOx super-RENS disk, has been identified as correlated with such parameters as recording power, cover layer thickness, and readout power. The experimental results have made it clear that the bubble’s change in size strongly corresponds to the readout signal change. It is indicated that readout durability depends strongly on the bubble stability. Therefore, the readout durability for the simplified super-RENS disk increases with an increase of cover-layer thickness and decreases with an increase of either recording power or readout power. At the same time, for the actual super-RENS disk in Fig. 1(a), the bubble’s increase in size can also press the relatively soft AIST layer and lead to a bigger change in T and R with respect to the simplified super-RENS disk because of the high absorption coefficient of the AIST layer. In addition, the strong absorption of AIST can increase the thermal-accumulation effect, so that bubble stability will decrease. Therefore, bubble stability definitely has a direct effect on readout durability for an actual PtOx super-RENS disk and is essential for readout durability.
4. Conclusion
In an actual PtOx super-RENS disk, readout durability strongly depends on the bubble stability, which is influenced by recording and reading power, AIST layer thickness, and ZnS-SiO2 as the dielectric layer. Therefore, improvement of readout durability is complicated systemic work. Although there are a lot of apparent phenomena affecting the readout durability, no doubt the bubble change induced by thermal-accumulation in the readout process is crucial for readout durability. Therefore, increasing the antistress deformation of the bubble is a main direction for improvement. For this goal, a dielectric layer material with excellent performance is key. In order to improve antistress, better thermal diffusion and antistress dielectric materials should be developed by means of dopants, adopting new materials, and so on. Also, a reading power as low as possible and an appropriate AIST thickness are helpful for increasing readout durability. In addition, research and development of new recording materials is also necessary for improving the readout durability of the super-RENS disk.
Acknowledgement
This work is partly supported by NSFC (90606025, 90406024), KIP of CAS (KJCX2-YW-M06, Kjcx-h12-sw-02), 863 Program (2006AA03Z353), NBRPC (No. 2006CB705600), and BNSF (4072027).
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