Fig. 5 Cross section elemental x-ray analysis of Ag/Cu contacts
Challenges in the printing process manifests themselves in two distinct
forms; lift-off of Ag/Ni stack from the front surface and delamination
of Ag/Cu contacts between the Ag and Cu. As illustrated by SEM images,
these issues not only compromised the mechanical stability of the
contacts but also influenced their electrical performance. Figure 6a
provides a cross-sectional SEM image of an Ag/Cu contact after
undergoing a contact co-firing process at 780°C. The clear separation
between the Ag and Cu layers might be attributable to the differential
thermal expansion of these metals during the firing process. However,
this delamination issue is not apparent in contacts co-fired at peak
temperatures below 780°C, underlining the significance of optimal firing
conditions for the stability of these bilayer contacts. The SEM image in
Fig. 6b exhibits a lift-off scenario with the Ag/Ni stack, where the
contact stack detaches from the Si substrate in the middle, while
remaining intact at the edges. This occurrence could stem from multiple
factors, including differential thermal expansion, interfacial
reactions, and inadequate surface preparation, among others. These
observations align with previous study [25], which reported similar
lift-off and delamination issues in aerosol printed bi and tri stack
layers of Ag frit/Ni and Ag frit/Ni/Cu when fired above 790°C peak
firing temperature.