透過氧化鈦緩衝層增強插層石墨烯互連的界面黏附力

Abstract

隨著半導體技術的發展進步，電晶體尺寸遵循著摩爾定律持續微縮。然而，在後端製程上，傳統的銅互連將因導線尺寸微縮而面臨諸多問題，例如電子散射的機率增加將導致電阻率大幅上升，以及一定厚度的阻擋層導致銅在整體導線的比例逐漸下降，還有電致遷移效應的顯著增加將大幅縮短導線壽命等。為了解決銅互連面臨的瓶頸，本研究使用感應式耦合型電漿輔助化學氣相沉積系統生長多層石墨烯作為互連導線材料，藉由石墨烯材料的特性以及封裝效果的改善，提升小尺寸互連的性能及可靠度。 在本研究中，首先引入鈦薄膜並使其自然氧化成為石墨烯互連和封裝層之間的緩衝層材料，在不影響互連電阻率的情況下，使互連的崩潰電流密度提升29.4 %，並達到50.3 MA/cm²，這不僅明顯優於銅金屬互連的崩潰電流密度，也顯示鈦緩衝層能夠有效改善石墨烯互連和封裝層之間的界面黏附力，使封裝層能夠實際發揮隔絕大氣的效果，從而增加石墨烯互連的可靠度。接著，為了進一步使石墨烯互連的電阻率降低，本研究透過引入插層技術，成功使用氯化鐵分子對石墨烯互連進行插層，不僅讓互連的電阻值大幅下降95 %，使電阻率達到16.7 μΩ∙cm，也能藉由氯化鐵分子的保護作用，對互連表面進行電漿清潔處理以去除製程中的殘留物質。藉由穩定性量測發現，不管有無進行插層或覆蓋緩衝層，石墨烯互連皆可以在10 MA/cm²的電流密度以及100 ℃的溫度下穩定維持超過36 hr，顯示互連具有高度穩定性應且不受到電致遷移效應的影響。最後，透過減少石墨烯的生長時間將互連厚度降低，證實石墨烯互連的性能及可靠度皆沒有因為厚度減小而產生負面的影響。 總結來說，本研究製備的石墨烯互連不僅與相同尺寸下的銅互連有相近的電阻率，其崩潰電流密度和穩定性皆明顯高於銅互連。未來隨著互連尺寸微縮，石墨烯將有極高的潛力取代銅金屬，成為下一代互連的首選材料。

With the advancement of semiconductor technology, transistor sizes have continued to shrink according to Moore's Law. However, in the back-end process, traditional copper interconnects face numerous challenges due to the downscaling of wire dimensions. For example, a significant increase in resistivity caused by the higher probability of electron scattering, a decrease in the proportion of copper in the overall wire due to the fixed thickness of the barrier layer, and a marked increase in electromigration effects, which significantly shorten the lifetime of the wires. In order to solve the bottlenecks faced by copper interconnects, this study utilizes an inductively coupled plasma enhanced chemical vapor deposition system to grow multilayer graphene as an interconnect material. By improving the properties of graphene and the encapsulation effect, the performance and reliability of small-sized interconnects are enhanced. In this study, a titanium film was first introduced and naturally oxidized to become a buffer layer material between the graphene interconnection and encapsulation layers. This buffer layer increased the breakdown current density of the interconnect by 29.4%, reaching 50.3 MA/cm², without affecting the wire’s resistivity. This result not only significantly outperforms the breakdown current density of copper interconnects but also demonstrates that the titanium buffer layer effectively improves the interfacial adhesion between the graphene interconnect and the encapsulation layer, allowing the encapsulation layer to effectively isolate the interconnect from atmospheric exposure, thereby enhancing the reliability of the graphene interconnect. Next, to further reduce the resistivity of the graphene interconnect, this study successfully employed intercalation technology using ferric chloride molecules. This process not only significantly reduced the resistance value of the interconnections by 95%, but also achieving a resistivity of 16.7 μΩ∙cm. Additionally, the protective effect of the ferric chloride molecules allowed for plasma cleaning of the interconnect surface to remove residual substances from the manufacturing process. Stability measurements revealed that, regardless of whether intercalation or a buffer layer was used, the graphene interconnect could stably maintain its performance for over 36 hours at a current density of 10 MA/cm² and a temperature of 100°C, indicating that the interconnect has high stability and is not affected by electromigration effects. Finally, the interconnection thickness was reduced by reducing the graphene growth time, confirming that the performance and reliability of the graphene interconnect were not negatively impacted by the reduction in thickness. In summary, the graphene interconnects fabricated in this study not only have a resistivity comparable to that of copper interconnects of the same dimensions, but they also exhibit significantly higher breakdown current density and stability than copper interconnects. As interconnect dimensions continue to shrink in the future, graphene has the potential to replace copper as the preferred material for the next generation of interconnects.