Superconformal chemical vapor deposition of thin films in deep features

The authors report a new and potentially widely applicable method for the chemical vapor deposition (CVD) of films with a superconformal thickness profile in recessed features, i.e., the rate of growth increases with depth away from the opening. Provided that the aspect ratio of the feature is not too large, deposition initially affords a V shaped profile; continued deposition eventually fills the feature without leaving a void or seam of low-density material along the centerline. Superconformal deposition occurs under the following set of conditions: (1) growth involves two coreactants; (2) the deposition rate depends directly on the surface concentrations of both coreactants; (3) the molecular diffusivities of the coreactants are different; and (4) the partial pressures of the coreactants are chosen such that the surface coverage of the more rapidly diffusing coreactant is relatively small, and therefore rate-limiting, near the opening. The latter condition can be fulfilled if the more slowly diffusing coreactant is employed in excess or has an intrinsically higher sticking coefficient. Under these circumstances, the deposition rate will increase deeper in the feature for the following reason: the pressure of the slowly diffusing coreactant necessarily drops more quickly with depth than that of the rapidly diffusing coreactant, which increases the fractional surface coverage of the fast-diffusing coreactant and with it the growth rate. At sufficiently large depths, eventually the surface concentration of the more slowly diffusing coreactant will become rate limiting and the growth rate will begin to fall; to obtain superconformal growth, therefore, conditions must be chosen so that the growth rate does not surpass its peak value. As a specific example of how this new approach can be implemented, MgO is deposited at 220 degrees C using the aminodiboranate precursor Mg(DMADB)(2) and H2O. Under properly chosen conditions, the growth rate increases from 1.0 nm/min at the trench opening to 1.8 nm/min at a depth/width ratio of 18. The authors propose a kinetic model that quantitatively explains these observations and, more generally, predicts the film profile as a function of the partial pressures of the coreactants in the gas feed, the molecular diffusivities, and the aspect ratio of the feature. An additional benefit of the model is that it can be used to predict conditions under which perfectly conformal CVD depositions will result. The present method should enable the fabrication of nanoscale devices in which high aspect ratio recessed features need to be completely filled. The method is intrinsic in nature and does not require special surface preparation, the use of a catalyst, or cycles of deposition and etching. (C) 2014 American Vacuum Society.