DYNAMICS OF ION-ASSISTED ETCHING IN THE SI(100) XEF2/AR+ SYSTEM ON A TIME-SCALE 100-MU-S-1000-S/

To investigate the reaction layer dynamics of ion-assisted etching we have measured the time response of product formation in an ultrahigh v acuum beam-surface experiment on a time scale ranging from 100 mus to 1000 s. Both the step function response and the delta function respons e are investigated. For the latter the pseudorandom cross correlation method is used. The system investigated is the classic Si(100)/XeF2/Ar + example, at low flux conditions of 0.6 ML/s XeF2 and 0.04 ML/s Ar+ i ons at 1 keV energy. We observe a consistent picture of the fourfold a ction of the bombarding ions. First, on a 1 ms time scale and shorter, the release of tightly bound intermediate radical species such as SiF and SiF2 by physical sputtering, i.e., by momentum of the impinging i ons, is the main effect. Second, on a time scale of 40 ms, we observe ion-enhanced formation of SiF4, most likely by the influence of ion bo mbardment on a rate limiting step in the reaction chain such as the fo rmation of SiF4 from SiF3. Third, on a time scale of 4 s, there is a r edistribution of intermediate SiF(x) products in the reaction chain, b oth in depth profile and in absolute density. Finally, on an even long er time scale, detectable after some 10 s at an ion bombardment of 0.0 4 ML/s, the production of vacancies and/or broken bonds as reactive si tes deep in the substrate becomes important, as observed by a long-ter m enhanced etch rate after switching off the ion beam. The results are consistent with a model that for low temperatures (T<600 K) the dispr oportionation reaction, 2 SiF3-->SiF4+SiF2, is the rate limiting step, while at high temperatures (T>700 K) the reaction step leading from S iF2 to SiF3 plays this role.