隨著電子產品的發展趨勢─輕、薄、短小，電子構裝技術成為學界與業界研究的重要領域。目前電子構裝產業中的兩大研究主題，一為現今封裝基礎技術─軟焊；另一為下一世代的電子構裝技術─三維度積體電路構裝，本研究中將分成此兩大部分作探討。
在過去幾十年中電子構裝技術之發展，軟焊技術扮演著關鍵腳色，最常見的軟焊銲料為Sn-Pb合金，並廣泛用於之前封裝技術，然而在歐盟管制電子產品中有害物後，無鉛銲料開始被廣泛運用。在低溫無鉛銲料的使用，大多為富Sn合金，如Sn-Ag-Cu、Sn-Cu、Sn-Bi等。除此之外，因為Cu具有良好的接合特性，是常見金屬墊片材料。Sn基合金與Cu基板之界面反應常生成易碎並且產生孔洞之介金屬化合物，伴隨接點可靠度降低，使焊點壽命減少。為了避免此現象的發生，人們會微量添加金屬元素在Sn基銲料中，並在本實驗室先前的研究，Ga之添加將大幅影響Sn-Bi/Cu液固界面反應之生成相。在本研究中，為了模擬在裝置、元件的運作過程之焊點變化，將於固固界面反應之溫度進行退火處理。Sn-Ag-Cu銲料擁有良好的接合可靠度與發展性，並且Ag於界面反應中不參與反應，為了簡化實驗變數，本研究選用Sn-0.7Cu作為基礎銲料，並在其中添加不同比例的Ga，以此銲料與Cu基板在200 oC進行界面反應研究，同時以CALPHAD方式建構出Cu-Ga-Sn 200 oC三元外推等溫橫截面圖與進行Cu-Ga-Sn相平衡實驗，以探討界面反應中的相變化過程以及機制，並比較Ga添加於銲料中，影響界面生成相之情況。實驗結果發現，當Ga添加於Sn基銲料之比例在3 wt. % 以上時，生成物為具極低Sn溶解度之γ-Cu9Ga4相，當γ-Cu9Ga4生成相均勻覆蓋於Cu基板上，Sn將難以穿過Cu-Ga介金屬化合物與基板進行反應，可成為穩定的界面結構。
除此之外，在三維度積體電路構裝，關鍵技術為直通矽穿孔與Cu對Cu連結。本研究中引用新穎接合方式之Cu對Cu連結製程，並利用銲料接合法，在Cu接點間引進聲化學法合成(Cu,Ga)奈米粒子，取代先前本實驗室研究之液態Ga，利用奈米粒子接觸面積大、熔點降低的特性進行Cu對Cu接合反應，同時對合成之(Cu,Ga)奈米粒子進行分析研究。以純Cu作為基板之反應偶中，無法產生穩定Cu對Cu之FCC-(Cu,Ga)固溶體，而應用Ni之凸塊下金屬化處理於Cu基板上進行界面反應則能夠產生穩定之FCC-(Cu,Ni)固溶體，此接點將可證實應用(Cu,Ga)奈米粒子對於三維度積體電路構裝中Cu對Cu接合之可能性。

SUMMARY
In recent years, soldering and three dimensional integrated-circuit (3D IC) take important parts of electronic packaging technology. Sn-based Pb-free solders have been widely used for low-temperature soldering. However, voids and brittle intermetallic compounds form at interfaces through Sn-based solder/Cu interfacial reactions. Doping elements into Sn-based solders is a common approach for improving joint reliability. In our previous work, minor addition of Ga has been found to effectively mitigate the soldering reactions between Sn-58Bi solders and Cu substrates. In this study, the reactions between Sn-0.7Cu-xGa (x = 1~3) solders and Cu substrates at 200 oC for various lengths of time up were investigated using electron probe micro analysis (EPMA) and CALPHAD thermodynamic modeling. The effect of Ga addition in Sn-0.7Cu solder is reported and the phase transformation in the Sn-0.7Cu-xGa/Cu couples is elaborated based on the Cu-Ga-Sn phase equilibria in the study. It is found that the reaction phase formation is strongly influenced by Ga concentrations. Besides, TSV process and Cu-to-Cu bonding are the crucial processes of 3D IC technology. The reactions of Cu/(Cu,Ga)NPs/Cu, Cu/Ni/(Cu,Ga)NPs/Ni/Cu and synthesized nanoparticles were investigated in this study. We proposed a bonding process for Cu-to-Cu in 3D IC with nanoparticles (NPs) synthesized by sonochemistry. After bonding process, a solid solution phase formed in the interface of Cu/Ni/(Cu,Ga)NPs/Ni/Cu and this result shows (Cu,Ga) nanoparticles have the great potential to develop in Cu-to-Cu process of 3D IC technology.

Key words: Sn-0.7Cu solders, minor Ga addition, Cu-Ga-Sn phase equilibria, Sonochemistry, Nanoparticles, Cu-to-Cu bonding

INTRODUCTION

In recent years, soldering and three dimensional integrated-circuit (3D IC) take important parts of electronic packaging technology.
Soldering has been changed to the key assembly and interconnection technology for electronic products including flip chip, ball grid array (BGA) process and 3D IC packaging. Sn-based Pb-free solders have been widely used for low-temperature soldering. However, some voids and brittle intermetallic compounds usually form at interfaces through Sn-based solder/Cu interfacial reactions. These brittle layer are called Kirkendall voids and these layer reduce the reliability of Cu-Sn joint. Doping elements into Sn-based solders is a common approach for improving joint reliability. Besides, in our previous research, adding the minor Ga into the Sn-58Bi alloy make a large difference of Sn-58Bi-xGa/Cu reaction. In this study, the reactions between Sn-0.7Cu-xGa (x=1~3) solders and Cu substrates at 200 oC were investigated while the reaction phase formation was also identified. The effect of Ga addition in Sn-0.7Cu solder is reported and the phase transformation in the Sn-0.7Cu-xGa/Cu couples is elaborated based on the Cu-Ga-Sn phase equilibria and CALPHAD thermodynamic modeling in the study.
3D IC packaging is the most important interconnection technology in the next generation electronic packaging industry including two important processes namely trough-silicon-via (TSV) process and Cu-to-Cu bonding. In our previous research, the Cu/Ga/Cu sandwich interfacial reactions were examined and the reaction phase formation was also identified. However, a brittle interface was found between Cu substrate and Ga solder. In this study, we proposed a bonding process for Cu-to-Cu in 3D IC with nanoparticles (NPs) synthesized by sonochemistry. The Cu/(Cu,Ga)NPs/Cu and Cu/Ni/(Cu,Ga)NPs/Ni/Cu reaction couples were bonded at 300 oC to set all condition the same as the real Cu-to-Cu interconnection in 3D IC technology. Furthermore, sonochemically synthesized nanoparticles was investigated and discussed in the paper.

MATERIALS AND METHODS

In the Sn-0.7Cu-xGa/Cu (x=1~3) interfacial reactions, the minor Ga is added into Sn-0.7Cu solders which were prepared by mixing proper amounts of pure Sn shot, pure Cu foil and pure Ga. The Cu foils were cut into pieces, and then metallographically grinded and polished with Al2O3 powders down to 1 μm.
The samples were annealed at 200 oC under a 10-5 bar vacuum for predetermined lengths of times. Besides, the Cu-Ga-Sn ternary phase diagram was constructed based on calculation of phase diagram (CALPHAD) method. Furthermore, series of ternary Cu-Ga-Sn alloys were designed to do the phase equilibrium experiments at 200 °C to verify the phase relation of calculation isothermal section. This phase equilibria of the Cu-Ga-Sn ternary system were applied to investigate the mechanism of phase transformation and microstructural evolution in the interfacial reaction.
In addition, the Cu/(Cu,Ga)NPs/Cu and Cu/Ni/(Cu,Ga)NPs/Ni/Cu sandwich-type reaction couples were prepared. The (Cu,Ga) nanoparticles were sonochemically synthesized by reducing copper sulfate with excessed Ga. The Ni under-bump metallization (UBM) layer was electroplated on each Cu substrate. Then the reaction couples were annealed at 300 oC for 6 hours.
Finally, the compositions of IMCs were evaluated and determined by using EPMA, and the crystallography of compounds were detected by XRD to realize the mechanism of phase transformation and microstructural evolution.

RESULTS AND DISCUSSIONS

In the Sn-0.7Cu-1Ga/Cu annealed at 200 oC for 120 h couple, shown in Figure 1, according to EPMA analysis and Sn-Cu binary phase diagram, the compositions of the thick light gray and the scallop-type dark gray interfacial phases are determined to be η-Cu6Sn5 and ε-Cu3Sn phase with 2.34 at. % Ga and 1.97 at. % Ga, respectively. The phase formation of Sn-0.7Cu-2Ga/Cu annealed at 200 oC for 120 h couple, shown in Figure 2, were presumed to be the γ-Cu9Ga4 phase with 2.24 at. % Sn, η-Cu6Sn5 phase with 1.69 at. % Ga and the ε-Cu3Sn with 0.87 at. % Ga. Besides, the interfacial reaction of Sn-0.7Cu-3Ga/Cu annealed at 200 oC for 120 h couple is shown in Figure 3. With a slightly higher doping level of Ga, only one integral IMC layer on the interface was observed. This IMC phase was presumed to be γ-Cu9Ga4 phase with 2.53at. % Sn according to the analysis of EPMA and the Cu-Ga binary phase diagram. The difference between these three reaction couples can be due to the formation of the γ-Cu9Ga4 phase. The solid solubility of Sn in γ-Cu9Ga4 layer is around 2 at. %. After γ-Cu9Ga4 phase formed in the interface, Sn hardly diffused from solder to Cu substrates. The γ-Cu9Ga4 phase act as a native diffusion barrier of Sn to suppress the growth of Cu-Sn compound.

Figure 1: Sn-0.7Cu-1Ga/Cu couples reacted at 200 °C for 120 hours

Figure 2: Sn-0.7Cu-2Ga/Cu couples reacted at 200 °C for 120 hours

Figure 3: Sn-0.7Cu-3Ga/Cu couples reacted at 200 °C for 120 hours

Because the Kidkendall voids formation will cause serious reliability concern of joints, the major objective of this research is to avoid the Kirkendall voids formation. In the literature, several voids are formed due to the rapid diffusion of Cu in the ε-Cu3Sn phase. We compared Ga addition effect to the IMCs thickness, shown in Figure 4. It is indicated that the Ga addition increases as the thickness of total IMC decreases, and the thickness of ε-Cu3Sn decreases as well. It is suggested the Ga addition effectively reduces the thickness of ε-Cu3Sn growth, and even the interfacial reaction only forms the Cu-Ga compounds in the 3 wt. % Ga addition couple. In this situation, the Kirkendall voids can be avoid.

Figure 4: IMCs thickness in different Ga addition at 200 °C for 120 hours

Figure 5 is XRD analysis of nanoparticles synthesized by reducing copper sulfate with excessed Ga. The results show Cu peak slightly shift toward left indicating lattice expansion. Cu peak shifting is due to a part of Ga dissolves into FCC-(Cu) solid solution and the remaining Ga still exists in the nanoparticles after the synthesis of nanoparticles. After bonding process, γ2-Cu9Ga4 is formed at the interface which had a brittle structure in Cu/(Cu,Ga)NPs/Cu couple. However, a uniform joint of FCC-(Cu,Ga,Ni) solid solution phase formed at the interface of Cu/Ni/(Cu,Ga) NPs/Ni/Cu, shown in Figure 6. It is indicated that (Cu,Ga) nanoparticles dissolved into FCC-(Cu,Ni) because Ga has high solubility of FCC-(Cu) and FCC-(Ni).

Figure 5: XRD plots of (Cu,Ga) prepared using copper sulphate solution (0.175 M, 20 mL) and Ga metal (0.5 g) after 3 min sonication

Figure 5: Cu/Ni/(Cu,Ga)NPs /Ni/Cu couples annealed at 300 °C for 6 hours

CONCLUSIONS

In this research, the phase transformation in the Sn-0.7Cu-xGa/Cu (x=1~3) interfacial reactions was presented. The solubility of Sn is very low in γ-Cu9Ga4 layer, so γ-Cu9Ga4 layer can act as a native diffusion barrier of Sn to suppress the growth of Cu-Sn compound. The Ga addition strongly reduces the thickness of ε-Cu3Sn growth, and even the interfacial reaction only forms the Cu-Ga compounds in the 3 wt. % Ga addition couple. In this situation, the Kirkendall voids can be avoid. Furthermore, a part of Ga dissolves into FCC-(Cu) solid solution and the remaining Ga still exists in the nanoparticles after the synthesis of nanoparticles. In addition, the Cu/Ni/(Cu,Ga) NPs/Ni/Cu couple had a uniform joint of FCC-(Cu,Ga,Ni). The joints proved this process has great potential to be applied and developed in Cu-to-Cu process of 3D IC technology.

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