Abstract

Putting germanium nitride thin-films between Sb-Te and ZnS-SiO₂ was effective to improve super-resolution readout durability of a super-RENS disc using PtO_x-SiO₂ recording layer. 260,000 times readout was possible with CNR over 40 dB.

1. Introduction A remaining issue of the super-resolution near-field structure (super-RENS) disc [1, 2] for the practical use is readout durability, and we have demonstrated that it can be partly improved by adding SiO₂ to platinum oxide (PtO_x) recording layer [3]. However, it was still 7.5 x 10⁴ times for 100-nm monotone marks (using HD DVD based optics) until carrier-to-noise ratio (CNR) decreases 3 dB from the original value over 40 dB (at the readout laser power, P_r= ~3 mW). Previous studies have shown better durability results that can be achieved by using ZnO as a material for the super-resolution readout [4] and by lowering P_r condition of a super-RENS disc (P_r= 1.5 mW) [5]. In this study, we did not replace Sb-Te (Sb₇₅Te₂₅) to other readout materials since it shows better super-resolution performance, and we intended to find a way of improving the durability that is also applicable to high P_r condition (~3 mW). We will demonstrate that putting germanium nitride (GeN_y) layers at both side of the Sb-Te layer is promising, and it suppressed some of the variation that becomes evident when repeating the readout [6]. We show that reflected light intensity from the disc was almost stable after the readout of 100- and 350-nm patterned marks to at least 5 x 10⁴ times. We also show that the readout of 2.6 x 10⁵ times was possible until the CNR (43 dB) decreases 3 dB for 100-nm (monotone) marks, and this is more than 3 times improvement by the GeN_y layer addition.

2. Experimental condition All the disc samples were fabricated by conventional RF magnetron sputtering method at room temperature. The details concerning PtO_x-SiO₂ recording layer can be found in ref. 3. GeN_y layer was prepared by sputtering pure Ge target in Ar and N₂ gas atmosphere. N₂ gas ratio was controlled to obtain nearly a transparent film (n= 2.49 and k= 0.16 at the wavelength of 400 nm). Figure 1 shows the disc structure prepared in this study (hereafter, with GeN_y). Fig. 1 Disc structure with GeN_y. For a comparison, we have also prepared a disc with no GeN_y and film thickness of the neighboring ZnS-SiO₂ layer is increased by 5 nm each (hereafter, without GeN_y) [3]. The disc properties were evaluated using an optical disc drive tester (DDU-1000, Pulstec Industrial Co.) with a laser wavelength (λ) and numerical aperture (NA) of 405 nm and 0.65, respectively. The resolution limit of the optics is thus 156 nm (= λ/4NA). The disc was rotated at a constant linear velocity of 4.4 m/s.

3. Results and discussion Figure 2 shows the CNR properties of 100-nm marks (below the resolution limit) as a function of P_r for both with GeN_y (recorded at 9.5 mW) and without GeN_y (recorded at 10.0 mW). A high CNR of 43 dB was obtained at P_r= 3.0 mW with GeN_y, and no large CNR property differences were recognized by the interface layer addition, probably since the layer thickness was thin enough. To evaluate more realistic condition than just the CNR of monotone marks, we started to study on the waveform of patterned marks combining the mark lengths of 100 nm (2T) and 350 nm (7T). The pattern Fig. 2 P_r vs. CNR of 100-nm marks. was 7T_s-7T_m-7T_s-7T_m-7T_s-{(2T_m-2T_s) x 19}-2T_m, where the subscript s and m letters describe space and mark, respectively. Writing (recording) strategy of 7T mark is slightly adjusted to make the waveform symmetric and to prevent a gradual waveform change at the boundary between 2T and 7T marks. It should be noted that the writing strategy control examined here is used for the GeN_y study, and more precise control is really necessary for the practical use as is discussed in ref. 7. Figures 3a) and 3b) are for without GeN_y (P_r= 2.8 mW), and Figs. 3c) and 3d) are for with GeN_y (P_r= 3.0 mW). Figures 3a) and 3c) are the waveforms just after starting the (super-resolution) readout, and Figs. 3b) and 3d) are the ones Fig. 3 Waveforms of 100-nm (2T) and 350-nm (7T) patterned marks (GND: ground). a) without GeN_y, readout just started, b) without GeN_y, after about 5 x 10⁴ times readout, c) with GeN_y, readout just started, d) with GeN_y, after about 5 x 10⁴ times readout. after about 5 x 10⁴ times. The results in Fig. 3 demonstrated that the waveform variation and reflected light intensity change from the disc were small with the GeN_y interface layers. This is probably explained by that Sb₇₅Te₂₅ layer was preserved during the super-resolution readout, and GeN_y layer stopped various reactions with the neighboring ZnS-SiO ² presumably similar to as it was effective in increasing overwrite cycles of a rewritable media [8]. We have also evaluated the readout durability by CNR for 100-nm (monotone) marks, and the results are shown in Fig. 4. In the case without GeN_y (P_r= 2.8 mW), CNR dropped 3 dB after 7.5 x 10⁴ times readout, but it improved with GeN_y (P_r= 3.0 mW) to be 2.6 x 10⁵ times. Both results in Figs. 3 and 4 indicate that interface-layer addition is a promising way of durability improvement for the super-RENS media. g. 4 Readout durability of 100-nm marks.

4. Summary We have studied on the effect of adding GeN_y interface layer (5 nm thickness) between Sb₇₅Te₂₅ and ZnS-SiO₂ layers of a super-RENS disc. The interface layer did not deteriorate high CNR property (>40 dB at 100-nm monotone marks) of the original disc. Although fairly high laser power condition (P_r= 3.0 mW) was used for the super-resolution readout, the layer kept the waveform of 100-nm and 350-nm patterned marks nearly unchanged after at least 5 x 10⁴ times readout and the CNR of 100-nm (monotone) marks stable to at the order of 10⁵ times readout.

© 2007 Optical Society of America