A Phenomenological Model of the Screen-Printed, Silver Paste Contact to Si Substrate

Abstract

A phenomenological model of screen-printed silver contact to an n-doped, p-type multi-crystalline Si wafer, based on extensive electrical, morphological, and compositional evaluations, has been developed. Rapid and quasi steady state heating configurations over broad (150–925°C) temperature ranges were investigated. Conventional rapid thermal annealing (RTA) with a conveyor belt was used for a rapid and custom-designed three-zone quartz tube furnace (QTF) for slow temperature variations. Lowest contact resistivity at 0.15 mΩ cm2 was observed in RTA horizontal configuration which was 25 times smaller than the same in QTF. RTA contact resistivity measurements revealed a minimum at 870°C while linear reduction in contact resistance was observed for the QTF configuration. The silver/silicon contact was based on three physical mechanisms: (1) migration of Si into glass and silver regions of the paste, (2) intermixing of silver and silicon (nano- and micrometer scale), and (3) epitaxial growth of silver/silicon crystallites. Experimental evidence of silicon migration was supported through extensive phosphorous concentration measurements from silicon and silver/silicon regions. The glass film with a colloidal distribution of randomly-distributed silver/silicon crystallites leads to lower contact resistance. Rapid temperature fluctuations facilitate development of Ag/Si crystallites. The higher contact resistance in quasi steady state thermal configuration was attributed to glass films with reduced density of Ag/Si crystallites. This disadvantage may be eliminated through post-contact, forming gas annealing at lower temperatures.