Write-Erase Mechanism of Indium-Antimony Optical Disk Medium
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WRITE-ERASE MECHANISM OF INDIUM-ANTIMONY OPTICAL DISK MEDIUM
Y. GOTO, K. UTSUMI, A. USHIODA, I. TSUGAWA AND N. KOSHINO Fujitsu Laboratories Ltd., 10-1, Morinosato-Wakamiya, Atsugi, Japan.
243-01,
ABSTRACT Written and erased bits of the In-Sb phase change type optical disk medium were studied using transmission electron microscopy (TEN) and electron probe microanalysis (EPMA). Both the bits were separated into inner and outer areas and were composed of only rhombohedral Sb crystals and zinc blende In 5 oSb 5 0 crystals. The difference between the two bits were in crystal size and atomic distribution of the inner area. Models of the writing and erasing processes were derived from these observations and the In-Sb phase diagram. With these models, the thicknesses, grain sizes and optical contrasts of the both bits were consistently explained.
INTRODUCTION Thin film material in which reversible phase transitions can be induced by a laser beam show promise for use as an erasable optical disk medium. Many materials have been proposed for this medium. Almost all however, use the phase transition between amorphous and crystalline states [1,2,3]. These materials have some problems with areas such as the stability of the written and erased bits or repeated recording. In contrast, we found that the Se-In-Sb medium could stably repeat phase transition between crystals [4]. We also found that the In-Sb medium has the same characteristics and a simple composition [5]. These new media have the following good characteristics. (I) writing parameters of laser beam : 10 mW , 400 ns (2) erasing parameters of laser beam : 5 mW , 800 ns (3) reversible cycles : > 106 times (4) carrier to noise ratio : 45 dB (rotating at 600rpm and a 2 MHz data rate with a 30 kHz band width) Our investigation made it clear that the reversible change originates in the eutectic composition of the In-Sb alloy system. The present paper describes the method used to make samples for microanalysis and the results of TEM and EPMA observations. We also will explain the model developed from the results.
PREPARATION Figure 1 is the cross section of the sample disk. The sample was produced by the following procedure. A 1.2 mm thick glass substrate covered with 50 uim thick photo-polymer resin was put into a vacuum chamber. A 20 nm thick ZnS under layer was formed on the substrate by electron beam evaporation. Then, a 100 nm thick In-Sb layer was made on the ZnS under layer by co-evaporation technique. The composition of this layer was In4o5b60, Finally the medium was taken out of the vacuum chamber and coated with an organic protective layer 50 um thick. The written and erased bits were formed on the sample by 1 um diameter laser beams. Figure 2 is a photomicrograph of both the observed
Mat. Res. Soc. Syrup. Proc. Vol. 74. ' 1987 Materials Research Society
252
ln4oSb6 o Protective layer
Underlayer Substrate
/
5 pm
Laser'beam
Fig. 1. Cross section of a disk.
bits recorded on a stationary disk. Figure 3 shows the power and duration of the laser pulses. Th
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