Oxide/LPCVD nitride stacks on silicon: the effects of high temperature treatments on bulk lifetime and on surface passivation

Abstract

Silicon dioxide / silicon nitride stacks are potentially useful for solar cell applications due to many favourable properties of the silicon nitride. If the thickness of the oxide and nitride is chosen correctly, an oxide / nitride stack behaves as a near ideal antireflection coating. Nitride layers allow significantly increased process flexibility and hence the realisation of novel cell structures. We used LPCVD deposition and in this paper, show that the effective lifetime of an oxide / LPCVD nitride wafer drops after a high temperature anneal in an inert gas ambient. This loss is due almost entirely to a loss of hydrogen from the silicon / oxide interface and a subsequent loss of surface passivation. Damage of the wafer, due to thermal expansion mismatch between nitride and silicon was minimal for all the stack parameters we used, the most extreme being a 25nm oxide under a 94nm nitride. The loss of surface passivation has been characterised as a function of oxide thickness, nitride thickness, anneal temperature and anneal time. Hydrogen can be reintroduced to the interface in a number of ways. We have shown complete repassivation of the interface using a high temperature forming gas anneal.