奈米雙晶結構銅膜與奈米線之製備與性質研究

Fabrication and Characterization of Nanotwinned Copper Films and Nanowires

詹宗晟(Tsung-Cheng Chan)

指導教授 ： 廖建能 ； 闕郁倫

國立清華大學/工學院/材料科學工程學系/博士(2013年)

摘要

應用在下世代積體元件中的氣隙型內連接導線結構或3D封裝技術之一的矽穿孔皆需要擁有高機械強度，低電阻率的導線材料來支撐整體結構與維持低時間延遲(RC delay)。 最近研究指出奈米雙晶銅不僅擁有高機械強度與低電阻率，同時也具有抗電遷移的特性，因此被認為是下一世代積體元件中內連接導線的候選材料之一。 高密度雙晶結構藉由脈衝電鍍與低溫下高能量氬離子轟擊引入銅膜與銅奈米線中，其硬度與雙晶晶界間距存在著類似於Hall-Petch關係式一樣的關係，此外，藉由調配硫酸銅電鍍液中之氯離子濃度，本研究可以控制銅膜之晶體優選方向，隨著電鍍液中的氯離子濃度增加，在直流電鍍銅膜中，{110}之結晶方向呈現穩定增強，而在脈衝電鍍銅膜中，則在氯離子10−4 – 10−5體積莫耳濃度間呈現出{111}與{110}優選結晶方向之轉變。 基於不同晶面擁有不同交換電流密度與脈衝電鍍製程中的工作周期效應之物理機制來解釋為何奈米雙晶電鍍銅膜之優選結晶方向會受到電鍍液中氯離子濃度改變而轉變。在另外一方面，藉由陽極氧化鋁為模板，竹節狀奈米雙晶銅奈米線可以利用低溫下脈衝電鍍法製備。在脈衝電流密度為0.4 A/cm2下，雙晶晶界間距的平均值僅為14.6 奈米。高密度雙晶之形成機制可歸於為了釋放在電鍍過程中所引發之應力與在二維成長模型中，堆疊出錯所造成的。而此高密度雙晶銅奈米線可承受高達2.4 × 108 A/cm2 的電流密度。電遷移所造成的銅原子移動於雙晶晶界會被延遲，因此延長孔洞佔據整個奈米線所需要的時間進而提升其電流承受度。
對於利用離子轟擊所製備的奈米雙晶銅而言，雙晶晶界-差排與差排-差排間的交互作用都會對於其高硬度有不可忽視之貢獻。此外，離子轟擊所引發之機械性質強化區域可以延伸到轟擊面以下數百奈米。一個基於在離子轟擊過程中熱震(thermal spike)所引發之應力來解釋離子能量與轟擊溫度對於雙晶晶界形成的影響。由於擁有高的機械強度和良好的電流承受密度，奈米雙晶銅成為應用在先進奈微米元件中互連接導線的淺力材料之一。

關鍵字

銅製程 ； 雙晶 ； 奈米線 ； 機械性質 ； 電遷移 ； 離子轟擊

並列摘要

An air-gap structure or through silicon via (TSV) employed in interconnect technology of integrated-circuits requires interconnecting materials of high mechanical strength and low electrical resistivity. Recently, Cu with nano-scaled twins has been intensively researched due to its high yield strength, good ductility and reasonably low electrical resistivity. In addition, electromigration-induced atomic diffusion would be slowed down at the twin-modified grain boundary in Cu line, which may improve electromigration resistance of Cu interconnects. In this study, dense nanoscale twins were introduced in copper films and nanowires through pulse electrodeposition and bombardment of high-energy Ar+ ions at low temperatures. In the electrodeposited nanotwinned Cu films, the nanoindentation hardness increases inversely with the square root of twin lamella width, and follows the Hall-Petch like relationship. In addition, crystallographic texture of nanotwinned Cu films was achieved through adjustment of chloride concentration in copper sulfate electrolyte using direct-current and pulsed-current deposition methods. With increasing chloride concentration in the electrolyte the DC-deposited Cu film showed a monotonically strengthening {110} crystallographic texture, while the PC-deposited one revealed a {111} to {110} transition at the chloride concentration of 10−4 – 10−5 M. We found that change of Cu film texture with varying chloride concentration is attributed to the distinct exchange current density of different Cu crystallographic planes and duty cycle of pulse current. On the other hand, bamboo-like nanotwinned Cu nanowires with 55 nm in diameter were fabricated by pulse electrodeposition at low temperature with anodic aluminum oxide as template. At pulse current density of 0.4 A/cm2, the mean value of twin-lamella width is only 14.6 nm. The formation of high density of twin boundaries (TBs) is attributed to relaxation of coalescence induced stress and twin fault stacking when Cu NWs grow under two-dimensional kinetics. The endurance of electrical current density before breakdown of nanotwinned Cu NWs reaches 2.4 × 108 A/cm2, which is comparable with carbon nanotubes or graphene nanoribbons. The suppression of electromigration induced void growth at triple junction where twin boundaries meet surface or grain boundaries is responsible for the raise of failure current density.
Besides, both TB-dislocation and dislocation-dislocation interactions contribute to the strengthening of ion-irradiated Cu films. The strengthened region can be extended to several hundreds of nanometers below the irradiated surface of embedded nanowires and patterned lines. A mechanism based on thermal-spike induced stress is proposed to explain the influences of ion energy and bombardment temperature on the formation of nanotwins. With the advantages of high mechanical strength and good electric endurance, nanotwinned Cu becomes a good candidate of interconnect material for advanced micro- and nano-electronic devices.

並列關鍵字

Cu metallization ； Twin ； Nanowires ； Mechanical property ； Electromigration ； Ion bombardment

參考文獻

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