In situ measurement of aspect ratio dependent etch rates of polysilicon in an inductively coupled fluorine plasma

Abstract

The etch rate of polysilicon in high aspect ratio structures has been studied in an inductively coupled fluorine based plasma (pure SF6). The change of the silicon etch rate with increasing aspect ratio [aspect ratio dependent etch rate or reactive ion etching (RIE) lag] has been measured in situ by interferometry. The experimental structures as well as the process conditions were chosen such that (i) the interaction of neutral fluorine atoms with the silicon surface determines the etch rate, (ii) the ion energy is minimized, (iii) the mass transport of neutrals is in a molecular flow (Knudsen) regime, and (iv) the interaction of the etch species with the sidewalls can be neglected. Under these conditions, the experimental findings indicate that the RIE lag effect is reduced for higher pressures and lower cathode temperatures, i.e., for higher fluorine atom coverages of the silicon surface. This is in agreement with the Knudsen transport model by Coburn and Winters [W. Coburn and H. F. Winters, Appl. Phys. Lett. 55, 2730 (1989)]. Probabilities for the reaction of fluorine atoms with the silicon surface between 0.03 and 0.11 can be derived when fitting the experimental data with this model. The findings reported in this work are of relevance for the formation of deep trench storage capacitors and polysilicon plugs. In particular, we show that we can control RIE lag and etch rate independently for the given process conditions.