Onset Conditions for Gas Phase Reaction and Nucleation in the CVD of Transition Metal Oxides
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ONSET CONDITIONS FOR GAS PHASE REACTION AND NUCLEATION IN THE CVD OF TRANSITION METAL OXIDES J, Collinsa, D.E. Rosnera and J. Castillob aYale Univ,. Chem. Engrg. Dept., HTCRE Lab., New Haven CT 06520-2159, USA bU.N.E.D., Dept. Fisica Fundamental, Apdo 60141, Madrid 28080, Spain
ABSTRACT A combined experimental/theoretical study is presented of the onset conditions for gas phase reaction and particle nucleation in hot substrate/cold gas CVD of transition metal oxides. Homogeneous reaction onset conditions are predicted using a simple high activation energy reacting gas film theory. Experimental tests of the basic theory are underway using an axisymmetric impinging jet CVD reactor. No "vapor phase ignition" has yet been observed in the TiCI 4/0 2 system under accessible operating conditions (below substrate temperature Tw=1700 K) and further experiments are planned using more reactive feed materials. The goal of this research
is to provide CVD reactor design and operation guidelines for achieving acceptable deposit microstructures at the maximum deposition rate while simultaneously avoiding homogeneous reaction/nucleation and diffusional limitations. INTRODUCTION
The onset of gas phase reaction and particle nucleation is a common problem in the CVD of transition metal oxides, often resulting in decreased deposition rates and reduced film quality [1-31. This is particularly true in cold gas/hot substrate CVD systems in which homogeneous reactions in the thermal boundary layer adjacent to the hot substrate produce particles which are thermophoretically repelled from the surface and, for the most part, do not deposit [4]. We call the sudden onset of significant reagent consumption by homogeneous reactions which result in nondepositing products (e.g. TiO 2 particles) "vapor phase ignition" (VPI). Since the onset of homogeneous reactions can effectively starve the growing surface of reagent, it is often possible to detect vapor phase ignition by a sharp drop in deposition rates with increasing surface temperature. On the other hand, in hot gas/cool substrate systems the onset of homogeneous particle nucleation can lead to increased film growth rates due to interface roughening associated with thermophoretically driven particle/vapor co-deposition [5,6]. It must also be mentioned that high temperature deposition rate decreases may be due to other causes, such as reaction product thermodynamic instability [7]. Dramatic rate decreases attributed to VPI were first reported by Ghoshtagore [4] for titania deposition from TiCI4 in 02, and his early experimental studies may still be the most thorough. Unfortunately, it difficult to use his data to predict conditions under which vapor phase ignition will occur in other reactors (let alone other chemical systems) because the transport conditions in his experiments were not well characterized. We developed and are using an impinging-jet stagnation point reactor to experimentally study vapor phase ignition under well-defined transport conditions. In particular, we are inferring th
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