SbTe結晶促進層對GeSbTe碟片結晶行為與性質的影響

Abstract

傳統相變化光碟都是經由濺鍍製程來製作，其缺點是在光碟可以做為讀寫之用前需做初始化過程使得光碟中之記錄層由非晶相變成結晶相。為了要發展免初始化的製程，SbTe合金材料用來添加在記錄層的上方或下方當做添加層以研究對記錄層結晶行為之影響。 本研究是利用六靶磁控濺鍍機以Sb40.6Te59.4 和Ge21.9Sb23.1Te55.0為靶材通以氬氣為輸送氣體分別濺鍍添加層及記錄層。這種層狀結構在不同基板(Si,PC,銅網)上鍍製以觀測其反射率(n & k分析儀)，晶體結構(XRD，ED)，非晶/結晶轉換溫度(DSC)，表面形貌(SEM)和微觀結構(TEM)。也製作完整光碟片針對不同的結晶促進層厚度對反射率，抖動值(jitter) ，調變率(modulation) ，信號雜訊比(CNR)與J-M-A速率方程式之“q”參數的影響。 實驗結果顯示上結晶促進層對記錄層的結晶行為沒有顯著影響，SbTe愈厚會造成反射率愈高。結晶促進層之作用基本上是誘發記錄層由SbTe之(100)面開始結晶。除了兩種晶格之非契合性很小之外，這是由於SbTe的結晶溫度比記錄層低 85℃。上述結果跟J-M-A結晶動力學分析碟片由非晶轉變成結晶的主要速率控制方式在SbTe厚度超過15 nm以上時是由成核所主導(q = 2.53 ~ 2.79 > 2.5)。 有關SbTe膜層對碟片性質的影響，其結果顯示碟片中之下SbTe層厚度20 nm 和 13 nm分別是反射率和調變率有最大值。換言之，在GeSbTe厚度是 10 nm下，SbTe輔助碟片之下結晶促進層厚度介於13 nm到 20 nm間顯示有最佳反射率和調變率的組合。結果亦指出SbTe輔助光碟片之結晶時間在70 ns以內，比傳統商用碟片之結晶時間100 ns~120 ns短很多，顯示具備有較高的記錄速度的可能性。這製程最重要的優點是SbTe輔助光碟片不需要初始化的過程，因為其初鍍完成的光碟片可以直接做寫入和擦拭的動作。

The conventional phase-change DVD-RAM is generally fabricated by the sputtering process, which has a drawback of requiring an initialization process to change the as-deposited recording layer in the disk from amorphous to crystalline phases, before the disk can be used for reading or writing. In order to develop an initialization-free process, the SbTe alloy was used as an additional layer below or above the recording GeSbTe layer to study its effect on crystallization behaviors of the recording layer. The additional and recording layers were deposited by a six-gun sputter with Sb40.6Te59.4 and Ge21.9Sb23.1Te55.0 as targets and Ar as carrier gas. The layer structures were deposited on substrates of Si wafer, PC board, Cu-mesh to examine their reflectivity (n & k analyzer), crystal structure (XRD, ED), amorphous-to-crystallization transformation (DSC) and microstructure (TEM). The complete disk specimens were also fabricated to examine the effects of thickness of SbTe crystallization-induced layer on reflectivity, jitter, modulation, carrier signal to noise ratio (CNR) and parameter “q” in J-M-A rate equation. The results show that upper SbTe layer has no significant effect on crystallization behavior of the recording layer. The greater thickness of the SbTe layer shows a greater reflectivity. Effect of SbTe layer is essentially to induce crystallization of GeSbTe recording layer from (110) plane of SbTe crystals. This is due to the fact that the crystallization temperature of SbTe crystal is 85℃ below that of GeSbTe crystal, in addition to a lower lattice mismatch between two crystals. This is in agreement with the J-M-A kinetic analyses that the rate controlling step for amorphous-crystal transformation in disk specimens with SbTe layer over 15 nm thickness is mainly governed by nucleation with q = 2.53 ~ 2.79 > 2.5 in J-M-A equation. Regarding the effects of SbTe layer on disk properties, the results show that the reflectivity and modulation of the disks depict a maximum value at the lower SbTe layer thicknesses 20 nm and 13 nm, respectively. In other words, under the 10 nm GeSbTe layer thickness, the SbTe-assisted disks with lower SbTe layer thickness between 13 and 20 nm show the best combination of reflectivity and modulation. The results also indicate that the crystallization time of the SbTe-assisted disks is below 70 ns, which is much lower than 100 ~ 120 ns for commercial disks, implying a higher writing speed is possible. The most important advantage of this process is that the SbTe-assisted disks are required no initialization process, because the as-deposited disks can be directly written on and erased off.