高<111>優選方向之奈米雙晶銅膜拉伸及疲勞測試之研究

Abstract

電鍍沉積銅已廣泛地應用在晶圓級封裝（WLP, Wafer Level Packaging）的銅導線製備上，然而銅導線在製程中若遇熱，周圍材料之熱膨脹係數差異容易造成線路的損壞，造成產品壽命減少，例如在RDL（Redistribution Layer）周圍，Molding Compound及Si的熱膨脹係數差異容易在加熱時產生應力造成RDL的斷裂，所以提升銅導線的機械強度有助於增加產品的可靠度。 奈米雙晶銅具有優秀的機械強度、延性，同時也具有低電阻率的特性，使奈米雙晶銅成為有潛力運用在銅導線上的材料，本實驗利用直流電鍍方式製造奈米雙晶銅薄膜，以拉伸試驗機測試其機械性質表現，同時也進行耐折疲勞試驗觀察其運用在軟性電路板（FPC, Flexible Printed Circuits）上之潛力。 本實驗結果發現，具高<111>優選方向之奈米雙晶銅薄膜進行拉伸試驗，其最大抗拉強度（Ultimate Tensile Stress）可達近500 MPa、伸長率百分比（Percent of Elongation）達11.1%、韌性（Toughness）達45.4 MJ/m3，其最大抗拉強度較不具奈米雙晶結構之銅薄膜高出128 MPa、伸長率百分比高出5.7 %、韌性也高出了28 MJ/m3。本實驗亦比較退火後之拉伸強度，不論退火200℃ 4小時抑或是退火400℃ 1小時，具高<111>優選方向之奈米雙晶銅薄膜之拉伸強度、延展性及韌性均較不具奈米雙晶結構之銅薄膜高，從這些性質來看，<111>奈米雙晶銅非常有機會能應用於下一世代之RDL。 而本實驗之疲勞試驗中，退火前具高<111>優選方向之奈米雙晶銅薄膜之失效循環次數Nf（Fatigue Life）為172，而不具奈米雙晶結構之銅薄膜之失效循環次數Nf為140，高<111>優選方向之奈米雙晶銅薄膜在運用於軟性電路板上之潛力亦高於不具奈米雙晶結構之銅薄膜。 本論文亦研究微結構及晶界角度（Misorientation）對應變硬化指數n、破裂模式及疲勞壽命之影響，其影響方式將於內文中詳盡探討。

Electroplated Cu has been extensively used in wafer level packaging (WLP) interconnect material such as RDL (Redistribution Layer). However, it is easier to lead to failure because of the mismatch of the thermal expansion coefficient between Si and molding compounds. Thus, the strength of Cu becomes a critical issue. Previous paper has reported that nanotwinned Cu (nt-Cu) has excellent mechanical properties. In addition, highly <111>-oriented nt-Cu films have been fabricated by direct-current (DC) electrodeposition in 2012. In this research, we used tensile tester to investigate the mechanical properties of nt-Cu films. Control Cu specimens without nanotwinned structure are also fabricated for comparison. The result shows that nt-Cu has 128 MPa higher tensile strength, 5.7% larger elongation and 28 MJ/m3 higher toughness than Cu films without nanotwinned structure. After annealing at 200℃ for 4 hr and 400℃ for 1hr, the mechanical properties of nt-Cu films are also better than Cu films without nanotwinned structure. Thus, nt-Cu films might become a promising application in WLP interconnects. In addition, folding fatigue test is also done to know the potential of nt-Cu films applying to Flexible Printed Circuits (FPC). Before annealing, the fatigue life of nt-Cu film is 172 cycles, and the fatigue life of Cu film without nanotwinned structure is 140 cycles. The nt-Cu films might be better materials than normal Cu films to apply to FPC. In this study, we also discussed the influences on strain-hardening exponent, fracture mode and fatigue life in detail by microstructures and grain boundary misorientation.