Mass transfer by natural and forced convection in open cavities

Abstract

Time-dependent mass transfer by natural convection in two-dimensional open cavities is studied using the finite element method. Emphasis is placed on a system simulating selective chemical etching of thin solid films for microelectronic device fabrication. Time-dependent local and spatially-averaged Sherwood numbers are reported for a Schmidt number of 10 3, cavity aspect ratios (depth: width) of 1:4, 1:1, and 2:1, and for Rayleigh numbers of up to 10 5. The flow and concentration fields are symmetric at early times. However, symmetry breaking and oscillatory flows occur at later times. The formation of plumes result in effective communication between the external ‘fresh’ etching solution and the ‘contaminated’ solution within the cavity, especially for deep cavities which are otherwise difficult to access. Forced convection mass transfer is also studied for Peclet numbers of up to 10 4. When compared to forced convection, natural convection resulted in one order of magnitude better mass transfer in a 2:1 cavity. The results have important implications for deep anisotropic etching of thin solid films and other related processes.