Effects of electroplating additives on void formation at the Sn/electroplated Cu solder joints

在現今這高科技的時代中，我們的日常生活被電子產品所充斥著，幾乎人人身上都會配備著3C產品，為了提高這些電子產品的便攜性，其中的電子元件便朝著輕、薄、短、小發展，然而，電子元件的微型化會使得連接線路的銲點可靠度問題被放大，且近年來電鍍銅這項技術已被廣泛的應用於製作電子元件內的金屬導線，當銲料與金屬導線進行接合時，會在其界面生成層狀的介金屬化物(Intermetallic compounds, IMCs)並伴隨著孔洞的出現，過去文獻指出生成這些孔洞的主要原因，為兩種不同原子間的相互擴散速率不同所造成，當一方原子無法立即填補相互擴散所留下的空位時，空位在逐漸累積後便生成了孔洞，此現象稱為Kirkendall voiding。 近年來有文獻指出除了原子擴散速率不同會造成孔洞的生成之外，亦與電鍍銅中殘留有機添加劑有關，在電鍍銅的過程中，若電鍍添加劑被包覆於銅沉積層內，會造成電鍍銅內的雜質含量增加，而當此電鍍銅與銲料進行反應後，這些雜質會誘發孔洞的生成。常用的電鍍銅添加劑有聚乙二醇(Polyethylene glycol, PEG)與聚二硫二丙烷磺酸鈉(bis(3-sulfopropyl) disulfide, SPS)兩種，分別作為抑制劑與加速劑使用，從Liu等人的研究結果可以發現，PEG被包覆於電鍍銅層內為雜質的主要來源，而當SPS共存在時會與PEG競爭吸附於電鍍銅表面，可有效抑制PEG的摻入，Wafula等人則提出PEG的分子量會影響界面孔洞的生成，由於分子量越大的PEG具有越強的吸附能力，因此在電鍍的過程中越容易被包覆於電鍍銅層內。 本研究探討以不同分子量的PEG搭配不同濃度的SPS進行電鍍銅，並將電鍍銅與銲料錫進行不同溫度的固-固反應，觀察Cu/Sn界面中介金屬化合物與孔洞的生成行為，經由實驗結果可以發現，當PEG分子量越大時，所需SPS臨界濃度則越高，當SPS具有足夠濃度抑制PEG摻入電鍍銅層內，可使得Cu/Sn界面在經熱處理後呈現無孔洞的結構，經由二次離子質譜儀(Secondary Ion Mass Spectrometry, SIMS)分析電鍍銅層內殘留雜質含量，可以發現雜質含量隨SPS濃度增加而減少。而當Cu/Sn界面有孔洞生成時，可以發現經過不同熱處理溫度將得到不同的界面結構，在150 oC時，可以觀察到孔洞聚集在Cu3Sn/Cu界面，經由標記實驗可以發現，在熱處理72小時後孔洞的聚集阻擋Cu原子的擴散，使得IMCs成長速率變慢，為了觀察孔洞與雜質之間的關係，透過SIMS縱深分析可以發現，雜質訊號較強的位置接近於孔洞聚集的界面，說明雜質的摻入會誘發孔洞的生成。在200 oC時，可以觀察到孔洞分散於IMCs中，藉由標記實驗可以發現，在較高的熱處理溫度下，Sn原子具有較快的擴散速率，使得IMCs往Cu端的方向成長，當孔洞聚集在Cu3Sn/Cu界面的速率小於IMCs的成長速率時，可以觀察到在界面處生成的孔洞將被推往Sn端的方向，如此一來便形成多層結構的IMCs。In this generation of high technology, the electronic products are filled with our daily life. In order to elevate the portability of electronic products, the inner components are developed toward the miniature size. The reduced volume of solder joints make the reliability issue become more and more important. Nowadays, electroplating copper procedure is widely used to prepare the conductors in the microelectronics industry. The tin-based solders are commonly used to connect the metallic conductors. After reacting with the electroplated copper to form the solder joints, two general layered intermetallic compounds (IMCs) form at the interface between the electroplated copper and tin-based solder. The IMCs formation usually accompany with the voids appeared. Previous literatures indicated the void formation was caused by the different diffusion rate of Cu and Sn atoms. This phenomenon was called Kirkendall voiding. In the recent years, some literatures point out the voids formation is related to the electroplated copper. In the electroplating procedure, the additives could be incorporated into the deposit and make the impurity content increase which induce the voids formation. The most common additives were polyethylene glycol (PEG) and bis(3-sulfopropyl) disulfide (SPS) which were used as inhibitor and accelerator, respectively. Liu et al. observed the main source of impurities was PEG incorporated into the deposit during electroplating copper. However, SPS could compete with PEG to adsorb on the copper surface and inhibit effectively impurities incorporated. Wafula et al. indicated the molecular weight of PEG could affect the voids formed at the electroplated Cu/Sn interface. Because the adsorption intensity is proportional to the molecular weight of PEG, it is incorporated into deposit more easily during electroplating copper. In order to understand the synergism of PEG and SPS, we focus on the molecular weight of PEG and the concentration of SPS. From our results, we observe the higher critical concentration of SPS is need to inhibit the PEG which has higher molecular weight incorporated into the deposit. After electroplated Cu reacts with Sn at 150 oC and 200 oC, we could find the Cu/Sn interface was void-free. As the concentration of SPS increases, the impurity content decreases gradually by the secondary ion mass spectrometry (SIMS) analysis. Some of interesting phenomenon could be observed in our research. When the voids form at the Cu/Sn interface, the microstructure would be very different at various annealing temperature. At 150 oC, the voids would segregate at the Cu3Sn/Cu interface and decrease the growth rate of IMCs. By the marker experiment, we could find the voids block the diffusion of Cu atom after annealing for 72 hours. The SIMS result show the impurities are associated with the voids formed at the Cu3Sn/Cu interface. At 200 oC, we could find a completely different microstructure. Because of the higher temperature, Sn has the faster diffusion rate which make the IMCs grow toward Cu side. When the voids formed at the Cu3Sn/Cu interface could not segregate to inhibit the diffusion of atoms, new IMCs would push the voids toward Sn side. As a result, we observe massive voids distributed in the IMCs at higher annealing temperature and form a multilayer structure.誌謝 I 中文摘要 II Abstract III 目錄 V 表目錄 VI 圖目錄 VII 1. 前言 1 2. 文獻回顧 3 2.1 單一添加劑對於電鍍填孔之影響 3 2.2 電鍍添加劑間交互作用 7 2.3 電鍍添加劑對於雜質殘留之影響 11 2.4 有機雜質殘留對界面反應之影響 14 2.5 Cu-Sn系統的擴散機制 26 2.6 孔洞對於IMC生成的影響 31 3. 實驗方法 36 3.1 電鍍銅基材製備 36 3.2電鍍銅/錫固-固界面反應 37 3.3金相分析 37 3.4 場發射掃描式電子顯微鏡分析 38 3.4.1 截面結構觀察 38 3.4.2 表面形態觀察 38 3.5 聚焦離子束顯微鏡分析 39 3.6 二次離子質譜儀分析 39 4. 結果與討論 40 4.1 自熔銅片與純錫銲料之固-固界面反應 40 4.2 不同電鍍基材與純錫銲料之固-固界面反應 41 4.3 電鍍銅中雜質對於孔洞生成之影響 53 4.4 Cu/Sn界面孔洞對於IMCs生成之影響 59 5. 結論 65 6. 參考文獻 6