Pure, dense, and stoichiometric MgO thin films have been deposited at temperatures as low as 225 °C by chemical vapor deposition using a recently reported magnesium precursor, magnesium N,N-dimethylaminodiboranate, which has the highest room-temperature vapor pressure among known Mg-containing compounds, with water as a co-reactant. The films are characterized by x-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and spectroscopic ellipsometry. Conformal coating on a trench with 35:1 aspect ratio is achieved at a film growth rate of 2 nm/min. The growth rate can be tuned between 2–20 nm/min according to the requirement of the structure to be coated.

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 °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.

TiO2 films are synthesized by chemical vapor deposition using the recently synthesized precursor Ti(H3BNMe2BH3)2 with H2O as the co-reactant. Films grown between 350 and 450 °C are crystalline and consist of a mixture of rutile and anatase phases; the fraction of rutile/anatase is larger at 450 °C. The films are continuous, dense, and pure, with the sum of B, C, and N impurities <1 at. %. The growth rate is ∼1.2 nm/min, limited by the precursor feed rate and therefore independent of temperature. The growth rate decreases monotonically with increasing H2O pressure due to the competition between precursor and co-reactant molecules for adsorption sites on the surface. The advantages of this system compared with other available Ti-bearing precursors are the absence of halogen and the synthesis of mixed-phase material at modest temperatures.

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.

Complete filling of a deep recessed structure with a second material is a challenge in many areas of nanotechnology fabrication. A newly discovered superconformal coating method, applicable in chemical vapor deposition systems that utilize a precursor in combination with a co-reactant, can solve this problem. However, filling is a dynamic process in which the trench progressively narrows and the aspect ratio (AR) increases. This reduces species diffusion within the trench and may drive the component partial pressures out of the regime for superconformal coating. We therefore derive two theoretical models that can predict the possibility for filling. First, we recast the diffusion-reaction equation for the case of a sidewall with variable taper angle. This affords a definition of effective AR, which is larger than the nominal AR due to the reduced species transport. We then derive the coating profile, both for superconformal and for conformal coating. The critical (most difficult) step in the filling process occurs when the sidewalls merge at the bottom of the trench to form the V shape. Experimentally, for the Mg(DMADB)2/H2O system and a starting AR = 9, this model predicts that complete filling will not be possible, whereas experimentally we do obtain complete filling. We then hypothesize that glancing-angle, long-range transport of species may be responsible for the better than predicted filling. To account for the variable range of species transport, we construct a ballistic transport model. This incorporates the incident flux from outside the structure, cosine law re-emission from surfaces, and line-of-sight transport between internal surfaces. We cast the transport probability between all positions within the trench into a matrix that represents the redistribution of flux after one cycle of collisions. Matrix manipulation then affords a computationally efficient means to determine the steady-state flux distribution and growth rate for a given taper angle. The ballistic transport model predicts a deeper position for the peak of the super-conformal growth rate than the diffusion-reaction model, and successfully explains the observation of complete filling. These models can be used to predict the behavior of any system given a small set of kinetic coefficients to describe the growth rate.