無電鍍之低溫低壓的銅-銅接合技術開發

Abstract

近年來，電子產品往輕、薄、微型化已是未來的趨勢。為了要因應此趨勢, 利用晶片垂直堆疊整合三維積體電路的矽穿孔技術 (3D IC Integration with Through silicon vias) 被認為是目前最有可能突破現今傳統封裝的限制方案之一。隨著接點逐漸地縮小, 高溫高壓的熱壓接合技術將是目前在2.5D與3D IC的主流接合方式。高溫高壓的組裝方式將會產生熱與機械應力, 使得晶片在組裝的過程中容易造成毀損。本實驗將利用無電鍍鎳的特性開發出新穎的低溫且無壓力的接合方式。 本實驗將分為兩個部分，第一個部分為無電鍍鎳應用在銅銅接合的可行性評估，在經過不同條件的測試下，使用外加壓力在微流道內之控制流場的方式, 可以把圓弧形的銅接點完全的連接起來，接點內部不會以任何的孔洞或縫隙產生。第二個部分為把接點的高度大幅縮小至35 μm，然後利用無電鍍鎳在微流道內進行接合。此部分將會對於無電鍍鎳在銅接點的接合性進行深入的討論, 包含整體均勻性的評估、銅柱表面粗糙度的評估、無電鍍鎳接點之熱處理的影響、無電鍍鎳接點之機械性質的量測、以及無電鍍鎳接點之電性的量測。無電鍍鎳的液體將使用間接性的流體方式進行傳輸至微流道內，在每個脈衝的流體之間將會有5秒的停滯時間, 實驗結果顯示脈衝性的流體將有效的移除無電鍍鎳過程中在鍍層表面吸附的氣泡，以防止接點之間產生孔洞。另外，無電鍍鎳鍍層會發現有明顯的lamellae的結構, 利用脈衝式的流體傳輸會使此結構變的更加的明顯。無電鍍鎳的接合機制將可以利用條狀的lamellae結構完整分析出來，兩邊的Ni(P)層最後碰觸的時候會先形成點對點的接觸, 進行兩邊的merging process, 最後再把剩下的空間給填滿。使用微流道的無電鍍方式能在小尺度的接點也有著很高的均勻度, 接點能完整的連接在一起, 不會有孔洞以及接縫的生成。此外, 不論是在錯位的接點或著是有著很高粗糙度的銅表面, 無電鍍鎳都有很良好的接合性, 不會生成內部的孔洞。利用EPMA mapping與line scan的分析可以得到磷的含量在銅住的表面與中間的接合處有很高的含量。表面高磷含量主要是因為銅表面的鈀的活化粒子能提高次磷酸鈉的反應速率, 因此加速磷的沈積反應速率。而在接合處也有著高磷含量主要是因為狹小的空間限制使得液體傳輸受到限制, 造成內部的氫離子的濃度大幅上身, 進而提高磷的沈積反應速率。此外, Ni(P)接點在經過不同熱處理條件 (150 °C, 1000 h, 300 °C, 1 h, 400 °C, 1 h) 都可以保持很好的接點完整性, 沒有任何的內部孔洞或裂痕出, 此接點在長時間工作溫度下與經歷多次的回焊製程還是能夠維持很好的完整性。在經過熱處理400°C, 1h下, Ni(P)的接點會從非晶轉變成晶體的結構, 使得內部磷的含量能夠進行擴散, 得到一個比較均勻的分佈。Ni(P)接點不論在電性與機械性質也都有很好的表現。無電鍍鎳在經過熱處理後還是能有好的強度, 在結晶之後會有著較低電阻性質。最後, 本研究提出了一個利用添加穩定劑(Pb)來達成選擇性無電鍍Ni(P)的方式來避免銅柱的側壁上鍍, 可以進而應用於更小的接點尺度。

Increasing demands for high-performance miniaturized electronic devices have driven the semiconductor industry toward finer pitch and higher interconnect density. 3D IC integration has considered to be one of the most potential solutions to increase vertical interconnect density. The industry standard bonding technology for high-density interconnect 2.5D or 3D IC is thermo-compression bonding. However, the high heat and high force applied to the components during the bonding process often induce high thermal-mechanical stress that causes severe damage to the devices and low-k dielectric layers. Because the mechanical properties of porous, low-k materials decreases with lower dielectric constants, this issue will become more severe in the future when lower dielectric constant materials are used. Therefore, it is imperative to develop a low-temperature, low-stress bonding process. The study is divided into two parts. The first part of the study mainly focused on the preliminary study on the feasibility of using electroless plating for an alternative low-temperature and low-stress bonding technology. It is concluded that void-free, seamless Cu-to-Cu interconnections can be achieved by electroless plating with a controlled pressure-driven flow. The key to achieving void-free Cu interconnects is to utilize the curved surface of the dome pillar. The second part of the study takes this concept to the next level by extending to smaller stand-off height joints (35 μm), and a more in-depth analysis of various aspects of microfluidic electroless plating interconnection will be present, including uniformity assessment, the influence of surface roughness, effect of heat treatment, mechanical property measurement, and joint electrical characterization. The microfluidic electroless plating interconnection process was operated in a pulse-beat process, with a delay time of 5 s between pulses. The intermittent flow can reduce the risk of hydrogen bubble entrapment during bonding process. The results showed that a striking lamellar structure would formed in the deposits when electroless Ni solution was fed into a microfluidic platform intermittently, and through this visible structure, the bonding mechanism of the process can be characterized fully. Preliminary results showed that a high level of plating uniformity across the die could be obtained using the microfluidic interconnection process, and that the copper pillars can be joined completely by electroless Ni plating without voids or seams. Further, the process is able to compensate not only for non-uniform copper surfaces, but also for the misalignment of copper pillars, which provides a competitive edge over other bonding methods. Inhomogeneous phosphorus distribution was observed across the bonding joint with phosphorus-rich regions located at the bonding interface and on copper pillar’s surface. Moreover, the direct shear test results showed that the bond strength of the electroless Ni bonds was quite strong. Thermal aging treatments have no substantial effects on the structural integrity and mechanical properties of the joints. The distribution of phosphorus became more homogenous after crystallization process at a temperature of 400 °C. The electrical resistance of the Ni(P) joint has improved after aging process. Last but not least, selective Ni deposition can be achieved by poising the sidewall of copper pillar bump with addition of stabilizer (Pb), which presents a viable solution to enable electroless interconnection with very fine pitch without the risk of bridging.