Low Stress under Bump Metallizations for Direct Chip Attach
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27 Mat. Res. Soc. Symp. Proc. Vol. 555 01999 Materials Research Society
EXPERIMENT Solder/Ni-Containing Alloys Reaction Cold rolled thick film (-20ptm, referred to as "thick film sample" later) of five different Cu/Ni alloys, pure Cu, Cu90%/NilO%, Cu80%/Ni2O, Cu70%/Ni3O%, and Cu55%/Ni45 are used in this study. Eutectic Pb/Sn solder is melted in a ceramic crucible on a hot plate with a temperature of 213±2°C. The Cu/Ni alloy films are dipped in the flux first, then placed in the molten solder for various lengths of time: 1, 2, 5, 10, 15, 20 and 30 minutes. Once taken out of the molten solder, these films are quenched in water to acquire a fast cooling rate. Reacted thick films are then cold-mounted in the epoxy, and then cross-section is ground, polished and etched to expose the intermetallic layer. The thickness of the intermetallic compounds is obtained by integrating the area of intermetallic compound with an image analyzing program. Residual Stress Measurement Several different UBM's are prepared with e-beam evaporatin (referred to as "thin film sample" later). As shown in Fig. 1,the UBM's consist of an adhesion Ti layer, an oxidizationpreventing Au layer, and the main wetting layer in between. The Cu/Ni concentration of the wetting layers tested are Cu 100%, Cu60%/Ni4O%, Cu30%/Ni7O%, and Cul0/Ni90%. The wafer curvature measurement method is used to determine the stress before and after the evaporation, and a "net" curvature is calculated. Au
1000A
WVettingLayer IOOOOA ( various Cu/Ni ratios )
ri
IOOOA
SiO2
2000A
Si
Fig. I Stnicture of UBM's
Intermetallic Compound in the Metallization Film After the stress measurements, the UBM's are patterned into circular 25 mil pads with lithography and ion milling, flux is applied, and 16 rnil eutectic Pb/Sn solder balls are reflowed on these pads with the heating profile shown in Fig. 2. The formed solder bumps are then coldmounted in the epoxy and the cross-section is ground, polished, and etched to expose the intermetallic compound layer for SEM observation.
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250 ,
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Eo 01 0
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time (minute)
Fig. 2 Heating profile of the reflow process RESULTS AND DISCUSSION Effects of Ni on the Intermetallic Morphology In both the Cu/Sn and Ni/Sn systems, two types of intermetallic phases are present at the solder/substrate interface[2]: a Sn-rich phase (Cu 6Sn5 , Ni3 Sn4 ), and a Sn-poor phase (Cu 3Sn, Ni3 Sn). At short reaction times, the Sn-poor phases usually do not form or the amount is too small to be detected. In this study, no obvious Sn-poor intermetallic phases were observed. For the Cu/Sn system, the Sn-rich intermetallic phase usually has a rough interface at the solder/intermetallic side and a smooth interface at the intermetallic/substrate side. The rough interface is believed [2,3,5] to be formed by fast diffusion of Cu through the channels between the intermetallic grains. This fast diffusion path is also suggested to be one of the reasons that the growth rate of the intermetallic compound does not obey the parabolic growth la
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