甲基磺酸鍍液系統之銅及銅銀的電鍍與微結構研究以及在導線之應用

Electroplating and microstructure investigations of copper and copper-silver deposits from methanesulfonic acid and their application to electronic circuits

姜智豪(Chi-Haw Chiang)

指導教授 ： 胡啟章

國立清華大學/工學院/化學工程學系/博士(2021年)

摘要

電鍍銅沉積廣泛應用於微電子線路製作，隨著電子元件持續地進行微小化的趨勢，銅互連導線因尺寸微縮將可能造成電遷移效應產生，此種現象是一種牽涉擴散、熱與電等影響，造成金屬原子移動引發短路與斷路等可靠度疑慮。電鍍銅沉積薄膜的結晶特徵及微結構與微電子導線的特性及可靠度有密切相關。此外，隨著線路微縮的技術不斷前進，適形化亦是電子元件發展的另一個趨勢，此已正逐漸應用於通訊電子、車用電子、醫療電子等領域。
本項研究以甲基磺酸鍍液系統進行銅與銅銀電鍍技術開發，並以硫脲(TU)或烯丙基硫脲(ATU)作為添加劑，藉由一系列調整添加劑與其濃度、電流密度等因素，呈現鍍層結晶結構的轉變，透過電化學分析與鍍層的結晶優選方向、表面粗糙度與形貌等微結構觀察作影響探討。此外，電鍍層厚度的均勻度將影響線路定義加工與其品質，本項研究利用數值模擬對於非平面基材表面的電鍍，進行鍍銅厚度分佈計算與影響因子探討。
本論文的第一部分主要研究添加劑硫脲(TU)和烯丙基硫脲(ATU)對於甲磺酸(MSA)鍍液系統的銅電鍍沉積行為和微觀結構影響。藉由控制添加劑類型、添加劑濃度和電流密度，瞭解銅鍍層的晶體結構變化。從電化學分析來看，TU和ATU均顯示出對電鍍銅有抑制能力，且在鍍液中TU與銅離子有著較強的交互作用。隨著在鍍液中引入 TU 和 ATU 添加劑，可使得鍍銅(111)結晶方向優取的電流密度操作範圍變得更寬廣。 此外，儘管TU和ATU這兩種添加劑具有相似的分子結構，並顯示出對銅沉積的抑制能力，但對於銅電鍍過程中的晶體成核和生長的影響有明顯差異，導致銅鍍層的表面形貌和粗糙度顯著不同。
本論文的第二部分更進一步應用甲基磺酸系統進行銅銀雙成分電鍍共沉積與其微結構特性探討，藉由引入TU作為錯合劑，可以改善與消除酸性鍍銅銀系統中的氯化銀沉澱現象。透過調整電鍍液成分與電流密度，電沉積層的銀含量可控制於0.7至43 wt. %的範圍。此外，對於鍍層的相關特徵，例如銀含量、結晶晶粒尺寸、結晶方向與粗糙鍍等均有完整探討，以期本項開發在未來可應用於微電子導線。
本論文的第三部分探討在非平面基材表面的電鍍銅製程設計，藉由數值計算的方式進行電鍍銅厚度分佈的模擬預估與改善，對於均勻鍍銅厚度分佈提供最適化的製程條件與反應器設計，此項將有助於非平面線路製作與其他相關的高精密度電子線路製作。

關鍵字

電化學沉積 ； 電鍍銅 ； 電鍍銅-銀 ； 硫脲 ； 微結構 ； 導線 ； 數值計算

並列摘要

Copper electrodeposition is extensively applied in the field of the microelectronic interconnects. With the trend of the miniaturization of the electronics, nowadays, the continuous pursuit of downscaling in copper interconnects probably causes reliability concern resulting from the electromigration (EM) effect. It is a kind of mass transfer phenomenon related to diffusion, heat, and electrical effects, which causes the metal movement and short-circuit issue. The crystalline characteristics and microstructures of copper electrodeposition are highly related to the properties and reliability of interconnects. Besides, with the continuous advancement of the miniaturization technology, conformalization of the electronics elements has gradually been applying to communication electronics, automotive electronics, medical electronics and other fields.
This study aims to develop the copper and bimetallic copper-silver plating from the methanesulfonic acid (MSA) bath and discuss the effects of the additives, thiourea (TU) and allyl thiourea (ATU). The deposition can be manipulated by adjusting the type and concentration of the additives and the current density, which has been investigatived through the electrochemical analysis and micro-structure observation, including preferential crystallographic orientation, surface roughness and morphology. Besides, the uniformity of the deposition thickness may affect the pattern define and quality of the circuit. This study utilizes the numerical to discuss the thickness distribution of the copper plating on the non-planar substrate and its affecting factors.
The first part of this thesis mainly focuses on the effects of two additives, thiourea (TU) and allyl thioura (ATU), on the electrodeposition behavior and microstructure development of copper deposits plated from the methane-sulfonic acid (MSA) bath. Three variables, including additive types, additive concentration, and current density, have been investigated in order to observe the variation in the crystallographic texture of Cu deposits. From the electrochemical analysis, TU and ATU show the suppression ability of Cu deposition and the interaction strength between Cu2+ and ATU is weaker than that between Cu2+ and TU. The operating current density range of the preferential (111) Cu deposition becomes wider with the introduction of TU and ATU additives in the plating bath. The influences of TU and ATU on the nucleation and growth of Cu grains are significantly different, leading to the very different surface morphologies and surface roughness of resultant Cu films although both additives are of the similar molecular structures and show a suppression ability on Cu deposition.
The second part of this thesis further investigates the bimetallic copper-silver deposits with variable compositions, which were prepared by electrodeposition from the methanesulfonate bath containing chloride ions. The precipitation of AgCl in this acid copper-silver bath can be avoided by the introduction of thiourea (TU) as the complexing agent. The composition and microstructure of such deposits are significantly affected by the introduction of TU, silver ion concentration, and deposition potential. The silver content varying from 0.7 to 43 w/o (weight percent) in the deposits is effectively adjusted by the systematic variation in the electrolyte composition and current density. The microstructures, such as the silver content, grain size, crystalline preferential orientation, and roughness of this series of Cu-Ag deposits are systematically compared for future electronic applications.
The third part of this thesis utilized the numerical calculation to improve the thickness distribution of the copper deposits prepared by electroplating. The results also gave the feedback to the reactor design and process factor optimization. This technique will be beneficial to the development of the nonplanar circuit and other related high-precision conformal electronics.

並列關鍵字

electrochemical deposition ； copper plating ； copper-silver plating ； thiourea ； microstructure ； interconnect ； numerical calculation

參考文獻

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