Stress effects in silicon oxidation—simulation and experiments

Abstract

Oxidation is a key step in the manufacture of silicon integrated circuits. Thermally grown oxides constitute the gates of MOS transistors, the dielectric in trench capacitors, and the isolation between devices. The growth of oxide on nonplanar surfaces generates significant stress, which retards the growth rate and reduces the thickness grown at silicon corners or nitride mask edges. This dissertation describes a computer program developed to predict oxide shapes on nonplanar surfaces, and new experiments carried out to characterize the stress-dependent models. The computer program, incorporated in the process simulator SUPREM-IV, uses finite elements to analyze the diffusion of oxidant and the flow of oxide. Grids are generated automatically as the oxide grows and changes shape. The stress in the oxide is calculated and fed back into the growth and flow coefficients, and the feedback loop is solved self-consistently. This allows the calculation of horn and cusp development at silicon corners, as well as conventional isolation features such as the bird's beak. The stress-dependent models used in SUPREM-IV are based on planar oxidation experiments. Annealing experiments on low temperature oxides showed a logarithmic decay of density with time, consistent with a viscosity model of the Eyring type. Two-step oxidation experiments, where a pre-stressed layer is reoxidized, were carried out for thin and thick films. History effects were found in both cases, showing stress dependences in both the linear and parabolic growth coefficients. Numerical values of the stress-dependent coefficients were derived by fitting the cylinder oxidation data of Kao et al.