The role of oxygen excitation and loss in plasma-enhanced deposition of silicon dioxide from tetraethylorthosilicate

Abstract

The deposition rate of silicon dioxide from tetraethylorthosilicate/O2 capacitively coupled plasmas increases with increasing applied rf power, increasing total pressure and decreasing wafer temperature. These measured deposition rate dependences can be explained by a simple plasma deposition model in which deposition occurs through both an ion-assisted and an oxygen atom initiated pathway. The relative contributions of these pathways were roughly isolated using limiting step coverage measurements on low aspect ratio trenches. Limiting step coverages decreased, and hence directionality increased, with increasing rf power density, decreasing total pressure, and increasing wafer temperature. A simple bulk plasma chemistry model combined with an analytical sheath model was developed to qualitatively explain our experimental findings. The model suggests that the ion-enhanced deposition rate is directly proportional to oxygen ion flux, with a reactive sticking coefficient approaching unity. Using literature values for reaction rate parameters and rate forms for oxygen plasma reactions, the rate of the neutral-induced deposition reaction was found to be nearly independent of temperature and tetraethylorthosilicate concentration and directly proportional to oxygen atom concentration. The model reveals that the influence of an activated atomic oxygen surface recombination reaction on oxygen atom concentration is responsible for the apparent negative activation energy for deposition of −2 kcal/mol. The model further shows that under the conditions of the experiments, oxygen atom loss was controlled by surface reaction processes and convective flow; volume recombination reactions were found to be relatively unimportant.