鍺-銻-鍗相變化薄膜電遷移行為之研究

Abstract

本論文研究應用於相變化記憶體（Phase-change Random Access Memory，PRAM）之鍺-銻-鍗（Ge2Sb2Te5，GST）合金薄膜及其摻有鈰（Cerium，Ce）與氮（Nitrogen，N）元素的GST試片之電遷移（Electromigration，EM）行為。掃描式電子顯微鏡（Scaning Electron Microscopy，SEM）之觀察發現，因電子流在負極端附近會逐漸累積達最大值，故所有GST試片皆在負極端發生斷裂；能量散佈光譜儀（Energy Dispersive Spectroscopy，EDS）分析發現所有試片皆有Te元素往正極端移動，而Sb與Ge往負極端移動的現象，摻雜（Doping）對阻擋EM元素偏析以N摻雜的效果較佳，但其效果並非顯著。利用Black方程式計算薄膜的EM活化能（Activation Energy of Electromigration，Ea）結果發現擁有細化晶粒結構的Ce與N摻雜GST薄膜的Ea值皆低於純GST薄膜（GST：1.07 eV；Ce摻雜GST：0.68 eV；N掺雜GST：0.56 eV），摻雜雖能細化晶粒結構而提升GST之熱穩定性與再結晶活化能，然其亦提高試片中之晶界數目，因晶界為物質快速傳輸之路徑，反而不利GST之EM性質。 因鈦（Titanium，Ti）電極會與GST中的Te反應，且EM實驗皆以高溫加速之方式進行，故亦以熱時效實驗釐清合金反應對EM之影響。X光繞射（X-ray Diffraction，XRD）分析發現GST與Ti的界面會形成Te2Ti介金屬化合物（Intermetallic Compound，IMC），但IMC之形成對GST之EM行為影響不大。熱時效實驗亦研究以TiN為Ti與GST間之阻障層，5 nm厚的TiN在250□C以下即有阻障效果，升溫至300□C則需使用15 nm以上厚度的TiN阻障層；實驗也發現摻雜Ce之GST在高達300□C的環境中亦不需要阻障層，推測其原因為入的Ce比Ti更容易與Te形成鍵結所致。

Electromigration (EM) of pristine Ge2Sb2Te5 (GST), cerium (Ce)-doped GST and nitrogen (N)-doped GST layers applied to phase-change random access memory (PRAM) were investigated. Scanning electron microscopy (SEM) observed, regardless of the sample type, the failure always occurs at the cathode side due the sufficienltly high current density. Energy dispersive speecctroscopy (EDS) revealed that Te migrates to the anode side while Ge and Sn move to the cathode side in all specimens. Doping might alleviate the element segregation during EM and N doping exhibited a better blocking effect. The activation energy (Ea) of EM deduced by using the Black equation found that and the Ea’s of doped samples are lower than that of pristine GST (1.07 eV for GST; 0.68 eV for Ce-doped GST; 0.56 eV for N-doped GST). The decrease of Ea’s in doped GST implies that the increase of grain boundaries due to the grain refinement in fact amplifies the short-circuit diffusion which might accelerate the EM failure. Since the EM experiment was performed at high temperatures, reaction of titanium (Ti) with GST was hence evaluated. Analytical results found that the formation of Te2Ti did not severely affect the EM failure of GST. Insertion of titanium nitride (TiN) buffer layer in between Ti and GST to inhibit the alloy reactions was also evlauated. A 5-nm-thick TiN was found to be an effective barrier at temperature below 250□C while the TiN layer thicker than 15 nm is required at temperature above 300□C. Probably due to the preference in forming the Ce-Te ionic bonds, Ce-doped GST required no TiN buffer layer for the EM study.