Effect of phosphorous content on phase transformation induced stress in Sn/Ni(P) thin films
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Jin Yu Center for Electronic Packaging Materials, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon 305-701, South Korea (Received 2 March 2006; accepted 28 April 2006)
The film stress evolutions induced by the phase transformation of Sn/Ni(P) films during thermal treatment were investigated using an in situ measurement of wafer curvature by laser scanning. Apparently, tensile stress developed due to the layer-by-layer formation of Ni3Sn4 and Ni3P phases for Sn/Ni(11.7P) films, and a compressive stress evolved for Sn/Ni(3P) films, despite the same phase transformation. The molar volume mismatch and x-ray diffraction analyses before and after the reaction between Sn and Ni(P) films suggested that a compressive stress existed in the Ni3Sn4 layer while the Ni3P layer was under a tensile stress state. The apparent stress states (tensile or compressive) for overall thickness of the films formed by the layer-by-layer transformation in Sn/Ni(P) were determined by the competition between compressive stress related to Ni3Sn4 formation and tensile stress caused by Ni3P formation. The stress states were dependent upon the relative thickness of the product layers with varying P content.
I. INTRODUCTION
In microelectronics packaging, Sn-based solders with underbump metallurgy (UBM) such as Cu, Ni, Ni(V), and Ni(P) has been widely used as interconnect materials.1 Of these materials, electroless plated Ni film, Ni(P), has been used for low-cost flip chip technology because of its merits, including selective deposition, good solderability, and effective diffusion barrier.1–4 Ni(P) film has nanocrystalline structure at low P content but amorphous structure at high P content.5 The Ni(P) film used as UBM in microelectronics packaging usually has an amorphous phase, which transforms to Ni3P nanocrystals during the solder reflow process. When there is Sn, the reaction between Sn and Ni(P) leads to the formation of Ni3Sn4, which subsequently enhances the crystallization of Ni(P) by making the Ni(P) layer Ni depleted or P rich. It is known as a reaction-assisted crystallization.2 Most of the previous work on the Ni(P) or Ni UBM has been focused on the kinetics of intermetallic compound (IMC) formation with Sn, such as Ni3Sn4, Ni3Sn2,
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0273 J. Mater. Res., Vol. 21, No. 9, Sep 2006
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Ni3Sn and Ni3P, and the effects on the solder joint reliability.6–11 Generally, the intermetallic compounds are brittle and frequently become sites of brittle failure,12–14 where residual stress plays an important role. Stresses in the solder joints mainly originate from two sources; one is the thermal stress caused by the difference in coefficients of thermal expansion (CTE) between dissimilar layers (e.g., polymer substrate and Si chip) during thermal cycling,15,16 and the other is the transformationinduced stress associated with the cry
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