An apparatus (21) and method is disclosed for keeping interior walls (28) of a reaction vessel (22) free of undesirable deposits of solid materials in gas-to-solid reactions. The apparatus (21) includes a movable cleaning head (36) which is configured to be substantially complementary to the interior contour of the walls (28) of the reaction vessel (22) and which is disposed in close proximity to the walls (28). The head (36) ejects a stream of gas with a relatively high velocity into a narrow space between the head (36) and the walls (28), and in accordance with Bernoulli's principle the head (36) is biased towards the walls (28) by the pressure prevailing in the reaction vessel (22). The gas ejected from the head (36) is of such composition that it does not participate significantly in the chemical reaction conducted in the reaction vessel (22). The head (36) is moved substantially continuously to at least intermittently blow the stream of gas to substantially the entire surface of the walls (28) wherein undesirable solid deposition is likely to occur. The disclosed apparatus and process is particularly useful for keeping the walls (28) of a free-space silane-gas-to-solid-silicon reactor (22) free of undesirable silicon deposits.
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