合金電鍍製作錫銀焊料凸塊於覆晶接合技術的應用

Abstract

隨著電子元件高速與輕薄短小的需求，高密度的導體連線技術，如覆晶構裝，因應而生。覆晶接合是利用許多小晶圓凸塊，固定積體電路晶片至基板的一種方法。在電子構裝中，焊料凸塊扮演著電流傳導、機械支撐以及散熱的功用。 在2016 年以後，國際上強制要求必須全面使用無鉛焊料凸塊來取代傳統的錫鉛焊料凸塊。 錫銀合金因為有良好的接合可靠度，被大家認定為很好的凸塊材料。 低成本高效率的電鍍，被視為一種很有用的技術來大量製作晶圓凸塊，目前也被應用於量產上。然而錫銀合金電鍍是一個非常複雜的製程，因為它同時包含了上千個凸塊的沉積以及複雜的金屬與添加劑的反應。在金屬合金電鍍製程中，局部的電流密度以及每一個凸塊的品質，不但會被質傳條件影響，也會被金屬與電鍍液界面間的電荷反應所影響。 本論文利用掃描電子顯微鏡、能量分散X 光線儀、四點探針與X 光繞射儀研究電鍍中的轉速、電鍍液溫度與離子濃度對錫銀鍍膜表面型態、電鍍速率、化學組成以及結構的影響。目的是想得知如何有效控制錫銀凸塊中銀的含量以得到接近共晶相的錫銀凸塊。 本論文研究當電鍍行為在質量傳送主導區, 增加離子濃度的補充速率會降低極化現象, 而導致所電鍍之錫銀晶粒變大, 金屬膜表面變粗糙。 但當電鍍行為在純粹質量傳送區, 增加離子濃度的補充速率會降低離子缺乏及表面群聚現象, 而使所電鍍之錫銀晶粒變小, 降低金屬膜表面粗糙度。增強質量傳送效應, 或是提高電鍍液銀離子濃度, 會增加錫銀電鍍合金中的銀含量, 而提高電鍍電壓則反而會降低錫銀電鍍合金中的銀含量。 本論文也提出一個錫銀合金電鍍的等效電路以供進一步探討。

The trend of electronic products toward high performance, compact size, and lightweight requires an interconnection technology of high density such as a flip chip package. Flip-chip bonding is a method for mounting integrated circuit chips onto a substrate using very small protruding electrodes called bumps. In electronic packaging, solder bumps serve for signal propagation, electrical conduction, mechanical support and heat dissipation. After 2016, the alternative Pb-free solders must be adopted to replace traditional Pb-Sn solders without major changes in manufacturing process or significant investments. Sn-Ag alloy is thought to be a promising material for fabricating bumps with better connection reliability. Eelectroplating is a superior batch formation technique of bumps and is widely used in practical applications due to low cost and high throughput. However, Sn-Ag alloy electroplating is a complicated process as the process involves the simultaneous deposition of thousands of solder bumps and complicated metal and additive reactions. In metal alloy electroplating process, the local current density and each bump quality may be influenced not only by mass-transport conditions but also by the charge-transfer kinetics at the metal-electrolyte interface. In this thesis, the effects of rotation speed, bath temperature and ionic concentration on the surface morphology, deposition rate, chemical composition, and texture of Sn-Ag alloys were investigated by scanning electron microscope, energy dispersive X-ray, four-point-probe and X-ray diffraction. The purpose is to explore how to well control Ag content in the Sn-Ag deposits and obtain near-eutectic Sn-Ag bumps. In mass transfer dominant region, increasing ion replenishment will decrease polarization and then increase Sn-Ag grain size and surface roughness. However, in pure mass transfer region, increasing ion replenishment will decrease ion depletion and surface aggregation and then decrease Sn-Ag grain size and surface roughness. Increasing mass transfer effect or Ag ion concentration will increase Ag% of Sn-Ag deposits whereas increasing applied voltage will decrease it. And a proposed equivalent circuit was setup in this study.