Multilayered Thin-Film Materials for Phase-Change Erasable Storage
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es. Currently the subject of number of reviews,3"9 the field of phase-change materials promises to broaden and intensify in the 1990s. The active layer, where the storage occurs, is typically a tellurium-based alloy with a variety of soluté species. Early work studied the recôrding properties of single-layered films, but it has been clearly shown that multilayered films, where the active layer is sandwiched between two or more dieléctric layers, hâve superior recôrding properties and résistance to irréversible damage caused by laser heating. The
dieléctric layers (typically Si0 2 , Si3N4, or ZnS) provide barriers to active-layer oxidation and contamination, help prevent the hole formation associated with ablative write-once storage methods, and act as crucibles and heat sinks which contain the molten spot and influence its cooling properties, respectively. A typical multilayer structure is shown in the cross-sectional transmission électron micrograph of Figure 1. The processes of writing, reading, and erasing a bit of information are illustrated schematically in Figure 2. The différent thin-film layers vary in thickness between roughly 200 Â and 2,500 À. The active layer is typically less than 800 Â and is amorphous in the vapor-deposited state. It is crystallized in a process known as initialization by low-power CW laser exposure. A bit of information is then written by applying a 50-100 ns focused laser puise ranging in incident power from roughly 15-50 mW depending on the optical and thermal properties of the film. This writing puise raises the local température of the active layer above its melting température, Tm. When the puise ends, the molten spot cools at rates of order 109-1010 K/s and amorphizes. The bit can la ter be identified by using the same focused laser at much lowef incident powers simply as a light source to measure the reflection or transmission of the spot relative to its surroundings. Erasure is achieved by exposing the film to intermediate laser power which raises the local température of the active layer above its crystallization température, Tx, but below Tm. The amorphous région thus crystallizes while the surrounding matrix remains
Figure 1. Cross-sectional transmission électron micrograph of a tri-layer phase-change recôrding film (2,500 A ZnS/750 A (TeGe)85Sn1512,500 A ZnS). The grooves give rise to a signal used to servo the read/write laser head along the data track,1 and the data bits are written on the track between the grooves.
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MRS BULLETIN/APRIL1990
Multilayered Thin-Film Materials for Phase-Change Erasable Storage
Initialized Active Layer
Written Bit
the small amounts of molten material involved in the process. Similarly, for growth-controlled solidification, the critical cooling rate is defined by:
Erased Bit
wmmwÊm
L=
f71 VJTdT
(2)
•Toys
A
Write
Read
Erase
- •
Time
figure 2. Thermal and microstructural évolution of a phase-change bit during the write, read, and erase processes. The active storage layer is sandwiched between two refractory dielectric loye
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