ABSTRACT
The a-Si/Cu bilayer was first proposed as a recording thin film for BD-Rs by Inoue et al. from TDK in 2003 [16] and is still being considered as the most promising candidate for high-speed (12X) BDs [18]. The phase formation and crystallization behaviors of a-Si/Cu bilayers with different stacks under thermal annealing and/or pulsed laser irradiation have been investigated by different research groups to reveal the operative mechanism. Inoue et al. [16] proposed a possible recording mechanism for a BD-R consisting of a reflective layer (Ag alloy), a protective layer (ZnS-SiO2), a recording stack (Cu alloy layer and Si layer), and a protective layer (ZnS-SiO2). At recording, when a laser pulse is
irradiated on to the recording stack, the irradiated region will be heated and the laser-induced mixing of the two different films of Cu alloy and Si will form a recorded mark with a reflection coefficient that differs significantly from the nonirradiated region. Her and Wu investigated the crystallization kinetics of an a-Si/Cu bilayer under thermal and pulsed laser annealing [9]. In their experiment, a recording stack with 20 nm a-Si/5 nm Cu was deposited on pregrooved PC substrates. For comparison, a recording stack with only 20 nm a-Si was also prepared. Pulsed laser annealing was carried out by a two-laser static tester from Tueoptics. The reflectivity variation of the a-Si/Cu bilayer recording film with time during the heating and cooling periods of the recording process was monitored. Laser 1 with a wavelength of 399 nm was used in pulsed mode for recording, while laser 2 with a wavelength of 422 nm was used in continuous-wave (cw) mode for monitoring reflectivity variations. The recording powers of laser 1 were chosen to be 6 mW, 8 mW, and 10 mW, and the pulse duration varied from 20 ns to 100 ns. Figure 7.1a shows the transmission electron microscopy (TEM) images of an a-Si/Cu bilayer and an a-Si single layer after irradiation by a 405 nm blue pulsed laser with a power of 6 mW for 50 ns, 100 ns, 150 ns, 200 ns, and 250 ns. It is seen that recording marks were formed in the a-Si/Cu bilayer recording film, whereas no structural change was observed in the a-Si single layer. Obviously, the recording power of the a-Si/Cu bilayer used for the BD was much lower than that of the a-Si single layer because of the lower crystallization temperature and activation energy with the aid of Cu-induced crystallization. As they closely examined the microstructure of the recording mark formed in the a-Si/Cu bilayer recording film, as shown in Fig. 7.1b, Cu3Si precipitates with sizes of tens of nanometers were found to be uniformly dispersed in the polycrystalline Si matrix, which was the same as the microstructure found in the a-Si/Cu bilayer recording film after thermal annealing at 500°C for three minutes. On the basis of the observed results, Her and Wu proposed a two-step recording mechanism that in the laser spot the Cu layer first reacts with Si to form crystalline Cu3Si nanoparticles, which subsequently serve as nucleation sites for crystallization of the remaining amorphous Si. They also pointed out that the rapid solid-phase crystallization of a-Si with a thin Cu layer under pulsed laser annealing is different from the melt crystallization of a-Si under high-power pulsed laser
irradiation because the laser energy density applied in their study is only high enough for the crystallization of the a-Si/Cu bilayer but not enough for the crystallization or melting of pure a-Si.
