Superconformal coating and filling of deep trenches by chemical vapor deposition with forward-directed fluxes

Abstract

The authors report a superconformal chemical vapor deposition method that affords bottom-up filling of trenches with oxide: the film growth rate increases with depth such that the profile of material develops a “V” shape that fills in along the centerline without a seam of low density material. The method utilizes low pressures of a metal precursor plus a forward-directed flux of co-reactant (water) at a lower pressure than the precursor. Under these conditions, many of the co-reactant molecules travel ballistically to the trench bottom where a fraction of them reflect. This scattering, which creates a virtual source of co-reactant from the trench bottom, leads to a superconformal growth process whose rate is highest at the bottom and declines toward the opening. Simultaneous with this superconformal component is the typical subconformal growth process due to the portion of the co-reactant flux that enters the trench opening isotropically; with a sufficiently large forward-directed flux, however, the overall profile is superconformal. We demonstrate this approach for filling trenches with HfO2 using 0.09 mTorr tetrakis(dimethylamido)­hafnium (TDMA-Hf) precursor and 0.009 mTorr H2O co-reactant. Precursor-rich growth conditions at a substrate temperature of ≤270 °C are used to assure that the growth rate is kinetically limited (determined) by the H2O flux and is nearly independent of the TDMA-Hf flux. Under these conditions, the growth rate in a trench with an aspect ratio of 3.5 increases from 0.6 nm/min at the top to 1.0 nm/min at the bottom sidewalls (step coverage = 1.6). The authors simulate the precursor transport-reaction problem within the trench using a Markov chain model to account for both the forward-directed and isotropic reactant fluxes and for the multiple reemission events within the trench, as a function of the surface sticking probability β of the water flux. The model predicts the fraction of the total incident flux that must be forward-directed in order to afford seam-free filling as a function of the sticking probability and the starting aspect ratio. Experimentally, the authors find that the opening of the trench accumulates a slightly greater thickness (a “bread-loaf” profile) that tends to pinch off the trench just before complete filling. To eliminate this effect, a molecular inhibitor, H(hfac) or H(acac), is used to reduce the growth rate near to the opening. The result is seam-free filling of trenches with HfO2 up to an aspect ratio of 10.