Analysis of Phase Transformation Kinetics by Intrinsic Stress Evolutions During the Isothermal Aging of Amorphous Ni(P)
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T.Y. Lee Department of Materials Engineering, Hanbat National University, San 16-1 Dukmyung-dong, Yuseong-gu, Daejeon 305-719, Korea (Received 5 October 2003; accepted 26 January 2004)
The kinetics for the crystallization of amorphous Ni(P) films and the formation of intermetallic compounds in Sn/Ni(P) films during isothermal aging treatment were studied with in situ intrinsic stress measurements. The intrinsic stress changes from crystallization were about 200 and 150 MPa for Ni(14P) and Ni(11.7P) films, respectively, and according to Johnson–Mehl–Avrami analysis, the Avrami exponents were about 3.6 ± 0.46 and 4.2 ± 0.39, and the activation energies were 242 and 240 kJ/mol, respectively, for the crystallization of Ni(14P) and Ni(11.7P) films. The stress due to the formation of intermetallic compounds such as Ni3Sn4 and Ni3P in Sn/Ni(11.7P) films was about 320 MPa. Application of in situ stress measurements to the empirical growth model during isothermal phase transformation of Sn/Ni(P) showed that the intermetallic compounds growth was interface reaction-controlled (n ⳱ 0.91 ± 0.08) in the early stage and then became diffusion-controlled (n ⳱ 0.38 ± 0.01), and the activation energy was about 35.9 kJ/mol. I. INTRODUCTION
Electroless plated (EL) amorphous Ni(P) films have been used as under bump metallization (UBM) for lowcost flip-chip technology because they are not only good diffusion barriers but also can be patterned without the lithography process. Beginning with the work by Brenner and Riddell,1 amorphous2–8 and nanocrystalline9,10 films of Ni(P) have been studied extensively for the last several decades. Amorphous Ni(P) phase crystallizes by thermal treatment with increasing the density.2 Lu et al.3 found three distinct stages during crystallization of amorphous Ni80P20 and measured the Avrami exponents and the activation energy with magnetothermal analysis and differential scanning calorimetry. The microstructure changes and phase transitions of Ni(P) were investigated with transmission electron microscopy (TEM),4–6 which showed the crystallization of amorphous Ni(P) phase into crystalline Ni and Ni3P phases through transition phase of NixPy.5 In situ stress measurement has been used to study the phase transformation for Pt2Si from Pt/Si films,11 Mo/Si a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0163 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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multilayers,12 TiSi2 from Ti/Si multilayers,13 Nb/Al multilayers,14 NiCu films,15 Co films of martensitic transformation,16 and amorphous Ni(P) films of the crystallization during thermal cycling.17 The intrinsic stress resulted from the density changes during the phase transformation or crystallization.18 The stress change can be used as a scale for phase transformation. The intrinsic stress changes were compressive11 or tensile.12–17 Electroless Ni(P) UBM is usually used with Sn-based solders in microelectronics packaging. The reactions between UBM and solder r
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