硫族合金薄膜之電遷移行為及其熱性質對相變化記憶體操作性質之研究

Abstract

鍺銻鍗（Ge2Sb2Te5，GST）是相變化記憶體（Phase-change Memory，PCM）最常見之編程層（Programming Layer）材料。由於PCM之信號記錄係由電致加熱所致，GST之電遷移（Electromigration，EM）與熱性質遂成PCM元件可靠度的關鍵。本論文探討GST及摻雜氮氮（Nitrogen，N）與鈰（Cerium，Ce）元素的GST薄膜在直流以及脈衝電流下之EM行為。在直流電場的作用下，平均故障時間（Mean-time-to-failure）與Black理論之分析顯示摻雜降低了GST的EM活化能，此乃因摻雜所致的晶粒細化作用同時增加了晶界數量，而晶界為熟知之快速擴散（Short-circuit Diffusion）路徑，反而加速了EM的毀壞，對應之微結構與組成分析亦顯示了摻雜並不能大幅抑制GST元素之電致偏移。Blech結構分析顯示EM驅動力與試片長度有關：短尺度試片之EM由靜電場力（Electrostatic Force）主導，而長尺度試片之EM則由電子風力（Electron-wind Force）主導。在脈衝電場的作用下，當電脈衝頻率大於10 MHz時，EM破壞行為可利用『平均電流模型』描述之，當試片為奈米尺度，表面擴散可能參與試片之EM破壞。 在GST的熱性質對PCM操作性質的研究中，首先以3ω法（3-omega Method）量測GST及組成PCM的各種薄膜的熱傳導係數（Thermal Conductivity）與界面熱阻（Thermal Boundary Resistances，TBR），將量測結果代入有限元素模擬分析，以三度空間全電熱耦合模型求解PCM元件之操作性質，研究結果顯示在GST與氮化鈦（Titanium Nitride）接觸層（Contact Layer）界面之TBR阻礙了熱傳導且抑制了元件編程電流，透過編程層之寬高比設計及GST之摻雜可完成PCM元件編程效率之最佳化。

Ge2Sb2Te5 (GST) is the most common chalcogenide materials serving as the programming layer of phase-change memory (PCM). Since the signal recording of PCM is induced by the electrical heating, the electromigration (EM) behaviors and thermal properties of GST are hence the key issues affecting the device reliability. This dissertation studies the EM and thermal characteristics of pristine GST, nitrogen- and cerium-doped GSTs and their influence on the operational properties of PCM devices. The EM behaviors of pristine and doped GST thin-films under direct-current bias and pulse bias were investigated. The mean-time-to-failure (MTTF) analysis in conjunction with Black’s theory indicated that the decrease of activation energy in doped GST samples. This is ascribed to the increase number of grain boundaries due to the grain refinement in doped samples, which amplifies the short-circuit diffusion and accelerates the EM failure. Microstructure and composition analysis indicated doping alleviates the mass segregation in GST only in a moderate manner. The Blech-structure analysis illustrated that sample length affected the mechanism of EM failure: the electrostatic force dominates in short-strip samples while the electron-wind force dominates in long-strip samples. On the other hand, pulse bias analysis revealed that the average current model could be applied to delineate their EM failure at frequencies greater than 10 MHz and the surface diffusion might involve in EM failure mechanism of strip in nano-scale dimension. In the study regarding of the thermal properties of GSTs and the operational properties of PCM, the 3-omega method was adopted to measure the thermal conductivity and thermal boundary resistances (TBR) of GSTs and thin-films involved in PCM. The measured results were then implanted in the finite-element simulation utilizing the three-dimensional fully coupled electric and thermal model for analyzing the operational properties of PCM devices. The simulation results indicated that the TBR at the interface of GST and titanium nitride contact layer impedes the heat propagation and suppresses the programming current and the programming efficiency in PCM could be optimized by modulating the aspect ratio and doping in GST programming layer.