Nickel silicon thin film as barrier in under-bump-metallization by magnetronsputtering deposition for Pb-free chip packa
- PDF / 1,591,807 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 108 Downloads / 190 Views
T. Sun and E. Ko Siliconware Precision Industries Co., Tantzu, Taichung, Taiwan, Republic of China (Received 12 April 2005; accepted 28 June 2005)
In the search for a Pb-free compatible under-bump-metallization (UBM) barrier material, magnetron sputtering of a nonmagnetic nickel silicon alloy (NiSi) target was used to deposit a low-stress polycrystalline NiSi thin film. In the reaction with SnAg3.0Cu0.5 solder, NiSi was found to have greatly reduced reaction and intermetallics (IMC) formation rates compared with sputtered NiV. A continuous layer of IMCs in NiSi UBM-solder joint remains stable after 10 reflows and 1000 h thermal aging tests, resulting in preferred bulk solder failure mode in ball shearing/pull testing. Our results demonstrate that NiSi is a highly promising thin film barrier material suitable for Pb-free solder bumping.
I. INTRODUCTION
In recent years, the transition from PbSn solder to environment-friendly Pb-free solder in flip-chip packaging has been intensively studied and gradually adopted by the semiconductor chip industry. One research focus is the search for a suitable underbump metallization (UBM) stack for Pb-free solders. Because of the distinctively different reaction phenomena between Pb-free and conventional eutectic PbSn solders with UBM, some solder joint reliability issues have been raised.1,2 Among various metals and alloys studied as the UBM barrier material, nickel stands out due to its much slower reaction rate with Pb-free solders at solder reflow temperatures higher than what PbSn solders can sustain. A Ni-based barrier layer can be deposited by either electroless plating or magnetron sputtering. Electroless plating can deposit an amorphous 10–20-m-thick Ni(P) film (∼15 at.% P) at low cost. However, serious concern has been raised in solder reaction assisted selfcrystallization of Ni(P) into Ni3P during reflows, which results in voids between Ni3P and (Cu, Ni)6Sn5 IMCs.2,3 Magnetron sputtering, on the other hand, is able to deposit high-quality, dense, pure Ni or nonmagnetic NiV (∼7 wt% V) thin films for high-end semiconductor chip packaging. NiV/Cu UBM, though a good choice for
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0346 2622
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 10, Oct 2005 Downloaded: 14 Mar 2015
PbSn solder1 is not compatible with Pb-free solder, as electromigration failure has been reported due to current crowding and void formation when (Cu, Ni)-Sn intermetallic compounds (IMCs) dissolve in Pb-free solders and the NiV layer is in direct contact with Sn.4,5 Furthermore, the fact that only a thin pure Ni sputtering target can be used to permit sufficient pass-through magnetic flux increases costly maintenance down time in production by at least 4-fold. Therefore, to date, a new thin film UBM compatible with Pb-free solder bumping is still under investigation. In this paper, we report our work on using a nickel silicon alloy film deposited by magnetron sputtering as a new UBM barrier. We
Data Loading...
