Write-once optical recording using WO 2 films prepared by pulsed laser deposition

Abstract

WO 2 films were deposited onto the glass substrates (Corning #7059 with an area of 26 × 38 mm) by the pulsed laser deposition method using an ArF excimer laser. It was found that after annealing at 500 °C for 10 min, the film thickness became 1.9 times compared with that (approximately 40 nm) in the as-deposited state. At that time, the difference in the transmittance, ΔT, between the annealed state and the as-deposited state was about 70% at the wavelength of 405 nm. From the XRD and XPS spectra, it was considered that oxygen was absorbed into the tungsten oxide films through the annealing process, accelerating the formation of WO 2 or WO 3 structure which caused the incremental effect in the transmittance and the expansion effect in the film thickness. For the revolution-test of the sample without the protection layer in which the WO 2 films were deposited upon the digital versatile disk (DVD-R) substrate, a write peak power dependence of CNR (at λ = 405 nm, NA = 0.65) of 3T signal (58.5 MHz) was measured at a linear velocity of 5 m/s and a read power of 0.6 mW. The values of CNR of 35 ∼ 40 dB were obtained at the peak power of 5 ∼ 7 mW, and increased largely up to 60 dB at 8 mW, and then reached a maximum of 62 dB at 8.5 mW. After that, the values of the CNR suffered some deterioration at more than 9 mW because of too much expansion of the written dot by which the portion between the dot and neighbor dots became to be invaded. For the dots formed in the writing performance on the DVD-R disk, the expansion effect in direction along the groove or over the neighboring lands was recognized by SEM images. It was found that bits of 0.18 ∼ 0.30 μm size, corresponding to the maximal storage capacity of 25 GB in the ‘Blu-ray disk’ specification, were made.