Proceedings of the 7th World Congress on Mechanical, Chemical, and Material Engineering (MCM'21)
Prague, Czech Republic Virtual Conference – August, 2021
Paper No. MMME 120
DOI: 10.11159/mmme21.120

Atomic Migration of Cu in Ti/Ni/Cu/Ag Backside Metallization on Si
Substrate
Pei-Ing Lee1, Po-Ching Wu1, Tung-Han Chuang1,2*
1

Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, R.O.C.
leepeiing@gmail.com; brian19960510@gmail.com; tunghan@ntu.edu.tw[JR1];
2
Ag Materials Technology Co., LTD, Hsinchu Science Park, Hsinchu 30078, Taiwan, R.O.C.
*Corresponding author: tunghan@ntu.edu.tw

Extended Abstract
To improve the adhesion between the Ni and Ag films in the traditional Ti/Ni/Ag backside metallized Si wafer,an additional layer of Cu was added between the Ni/Ag interfaces. The thicknesses of the Ti/Ni/Cu/Ag layers inthis study were 1-3-1-9 kÅ. It was observed that, during heating at 200 °C for durations of up to 8 hours, the Cu
layer remained largely unchanged. However, when the high temperature storage was extended to 9 hours, slightfluctuation was observed in the Cu layer, which formed a wavy layer. As no Cu was found on the surface of theAg layer, it was speculated that the thermal energy at 200 °C was insufficient to overcome the energy barrier forthe transition, and hence the Cu atoms could not cross over the Ag layer. In this case, EDS mapping showed thatthe Ag/Cu interface was still intact and no diffusion reaction had occurred. When the temperature was increased
to 300 °C, the cross-sections of the Ti/Ni/Cu/Ag metallization showed significant growth of the Ag grains, while
the Cu layer showed no signs of fluctuation. As the heating time increased, the Ag/Cu became uneven, and someCu atoms diffused to the surface of the Ag layer while the Ag grains continued to grow. At 4 hrs 30 min[JR2], mostof the Cu atoms had migrated to the surface and the intermediate Cu layer had decreased in thickness. The EDS
mapping revealed that the Cu layer had split into two distinct layers after heating for 4 hrs. When the heating time
was increased to 4 hrs 30 min, the Cu layer eventually migrated to the surface of the Ag layer. The Cu layer became
wavy and discontinuous after holding at 400 °C for 2 hrs and for 2 hrs 30 min. After holding at 400 °C for 2 hrs
30 min, partial migration of Cu atoms to the surface had occurred. The same phenomenon was observed withincreases in heating time. However, the Cu layer had completely migrated to the surface after holding at 400 °Cfor 3 hrs. At 500 °C, however, the Cu layer quickly became discontinuous clusters. The high temperature destroyed
the metal stack structure, where voids started to appear within the layers. It is preliminarily speculated that the film
adhesion was degraded by the high temperature, and the weakened interface caused subsequent signs of peelingwithin the metal films, causing the diffusion of Cu to become slow and inconspicuous. Summarizing theobservations in this study, it can be speculated that, as heat was applied to the metal stack, the aggressive Ag grain
growth initiated void formation near the grain boundaries. The Cu atoms were energized by the heat energy andstarted to diffuse through the Ag grain boundaries and voids to the surface, where they eventually spread over theAg layer to form a uniform layer. The Cu atomic migration phenomenon can be inhibited through the depositionof a Sn layer on the surface of Ti/Ni/Cu/Ag metallization and the bonding strength at the Ni/Ag interface can beeffectively increased.
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[2] Roy, R. and Sen, S.K., 1993. “The study of diffusion of copper in thin films of silver and Ag-Al alloys as afunction of increasing aluminium concentration.” Thin solid films, 223(1), pp.189-195.
[3] Chamberlain, M.B., 1982. “Diffusion of copper in thin TiN films.” Thin Solid Films, 91(2), pp.155-162.
[4] Kehoe, R.B., Newman, R.C. and Pashley, D.W., 1956. “The migration of copper atom on the surface ofsilver.” Philosophical Magazine, 1(8), pp.783-785.
MMME 120-1