We have used Medium Energy Ion Scattering (MEIS) and other techniques to in
vestigate the structure and formation mechanisms of ultrathin (less than 10
nm) layers of Ta2O5 on Si. We find that a compositionally graded oxide wit
h < 2 nm effective thickness can be formed. The him degenerates at high ann
ealing temperatures both by roughening at the outer surface. and reacting a
t the interface, hut a buffer layer of Si3N4 can prevent the latter effect
to a certain extent. Introducing a TiN/Ti layer between Ta2O5 and Si (which
may be desirable for DRAM applications) has an adverse effect on the therm
al stability of the Ta2O5 overlayer due to migration and subsequent reactio
n of oxygen with titanium.