微機電LIGA製程之銅合金電鑄技術開發

Abstract

摘 要 由於半導體銅鑲嵌製程的發展，且銅因具有高導電、高導熱等優點，故本研究以低成本之直流電鍍技術來沉積銅合金，並應用於微機電LIGA製程中微結構之製作。 本研究探討了銅鉬、銅鈷及銅鎳三種合金。實驗首先藉由哈爾氏槽試驗來測試鍍液與添加劑對鍍層的影響，再挑選較合適的鍍液組成進行不同電流密度之銅合金電鍍。研究中採用不同的錯合劑、電流密度、鍍液成分、鍍液濃度及沉積時間來進行銅合金電鍍，再經由SEM、EDS、Alpha-Step及ESCA等設備觀察量測鍍層之表面形貌、金屬沉積比率、粗糙度及厚度等特性。最後，從三種銅合金中選定較佳的鍍層與電鍍參數，經微影及電鑄等步驟完成以LIGA製程製作微結構之應用。 由實驗結果可知：(1)以焦磷酸銅鍍液進行銅鉬合金電鍍時，在電流密度2~5 ASD下具有銅鉬合金沉積，但鉬的含量僅2~3 at%。此外，因合金中有大量的氧原子沉積，造成鍍層出現嚴重的裂痕。(2)銅鈷合金可藉由添加檸檬酸鈉於硫酸銅鍍液中被沉積出，鍍層中鈷離子沉積量會隨著電流密度、鍍液中鈷離子濃度及檸檬酸鈉濃度的增加而增加，但會隨著沉積厚度的增加而逐漸減少，造成此現象的原因可能是鍍層中鈷原子易遭銅離子置換所造成。(3)銅鈷合金中，鍍層在電流密度4~5 ASD時，鈷離子含量可達50~60 at%。若鍍浴中加入銅光澤劑時，在電流密度6 ASD以上可沉積出具金屬光澤之銅鈷鍍層，但因過多的氫氣泡阻礙鍍層的沉積，導致坑洞的產生及電沉積效率的降低。(4)硫酸銅鍍液中添加檸檬酸鈉可沉積出銅鎳合金，但所沉積出之鍍層色澤偏暗且具粉末狀顆粒。光澤劑的添加，於電流密度2~3 ASD時，可得金屬光澤之銅鎳合金。(5)經微影及電鑄過程後，5 m厚之銅鎳合金微結構可被製作，但所沉積的結構有應力、粗糙度及厚度等問題，仍有待進一步探討。Abstract Because of the development of copper damascene process and the good physical properties of copper, DC electroplating was used to deposit the copper alloys which applied to fabricate microstructures in LIGA process. Three copper alloys, including Cu-Mo, Cu-Co and Cu-Ni, were discussed in the thesis. In the investigation, Hull cell was used to observe the effect of electroplating bath and additives. Better electroplating parameters were chosen to deposit copper alloys under different current density. Various deposited films were obtained by changing different kinds of complexings, current density, deposition time, constituent, and the concentration of bath. The surface morphology of film was observed by SEM. The atomic percent of alloy in film was determined by EDS. The roughness and the thickness were calculated by Alpha-Step 500. The depth profiling was determined by ESCA. Finally, better electroplating parameters were chosen for LIGA process application. From the experimental results: (1) Cu-Mo alloy could be deposited from the copper pyrophosphate bath, and the atomic percent of Mo in film is 2~3 % between current density 2 to 5 ASD. Serious cracks appeared in the films due to lots of deposited oxygen atoms. Therefore, Cu-Mo alloy was unsuitable to be deposited by electroplating. (2) Cu-Co alloy could be deposited from the acid copper bath by adding sodium citric. The atomic percent of Co in film increases with the increase of the current density, sodium citric and the concentration of Co in bath, but decreases with the increase of thickness. This is because Co in film was easily replaced by Cu in the duration of deposited process. (3) The atomic percent of Co in film was 50~60 % between current density 4 to 5 ASD. Metal luster of Cu-Co region was obtained above current density 6 ASD by adding brightener into the bath, but too many hydrogen bubbles appeared, which resulted in low current efficient and rough surface. (4) Cu-Ni alloy could be deposited form acid copper bath by adding sodium citric, but the deposited film was dull and granular. Bright film of Cu-Ni alloy was obtained between current density 2 to 3 ASD by adding brightener into the bath. (5) The microstructures of Cu-Ni alloy, which was 5 m thick, were fabricated after lithography and electroforming, but the roughness, stress and thickness of film have to be improved in further investigations.