Electromigration studies of Sn(Cu) and Sn(Ni) alloy stripes
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C.Y. Liua) Department of Chemical Engineering and Materials Engineering, and Institute of Materials Science and Engineering, National Central University, Chung-Li, Taiwan 320, Republic of China (Received 16 December 2004; accepted 14 April 2005)
Using Blech’s structure, we studied the electromigration (EM) behaviors of four different Sn(Cu) and Sn(Ni) alloys, which are Sn, Sn0.7Cu, Sn3.0Cu, and Sn1Ni. The order of the EM rates was determined to be Sn0.7Cu > Sn > Sn3Cu > Sn1Ni. We believe that the lower yielding strength and higher grain boundary density of Sn0.7Cu were the main reasons that Sn0.7Cu had the fastest EM rate. Moreover, we found that the addition at Ni could effectively retard EM. The critical products of Sn, Sn0.7Cu, and Sn3.0Cu, which were determined to be 1500, 500, and 1580 A/cm, respectively. The diffuse-effective charge (DZ*) values of the solder stripes from this present work have same order magnitude of the reported values, which were measured by using actual solder bumps.
I. INTRODUCTION
The on-chip metal interconnection line is considered to be the most likely electromigration (EM) failure site in a high performance central processing units (CPUs), since it usually carries large current density. However, as the size of controlled collapsed chip connections (C4) solder bumps continues to shrink, the current density in the C4 solder bumps has increased dramatically. People are starting to realize that C4 solder bumps have the potential to become the most serious EM failure sites. According to the semiconductor technology roadmap, by 2007, the pitch of these C4 interconnections will be as small as 75 m.1 As a result, the current density in the C4 solder bumps could reach the order of 104 A/cm2. Many EM studies of solder bumps under such a high current density have demonstrated that the EM damage could be fatal for the C4 solder bumps.2–5 Currently, in the electronic packaging industries, Pbfree solders are replacing eutectic SnPb.6,7 Hence, it is of great important to study the EM behaviors of Pb-free solders. Currently, the major metal element in Pb-free solders is Sn, and Cu is one of the most common elements to be alloyed with Sn. It is well known that a small amount of Cu added to Al interconnection lines greatly enhances their EM lifetime.8–10 Thus, how the Cu additive will affect the EM behavior should be understood in
a)
Address all correspondence to this author. e-mail: [email protected]. DOI: 10.1557/JMR.2005.0257
great detail. In this study, we chose a Sn(Cu) binary alloy system to study the effect at Cu alloying on the EM of Sn(Cu) alloys. Also, by using the famous Blech structure, the length effect, and the important EM parameters, the critical products obtained can be symmetrically studied. II. EXPERIMENTAL PROCEDURE
The EM test setup in this study used the well-known Blech structure, as depicted in Fig. 1(a).11,12 The preparation of the EM test samples is described in the following. A 1-m Si3N4 layer was deposited on an n-type silicon wafer by plasma enhanced chemical
