Factors Affecting Thickness Variation of ${\rm SiO}_{2}$ Thin Films Grown by Wet Oxidation

Thin oxide films have been grown by wet oxidation on 200 mm silicon wafers at atmospheric pressure in three different furnaces. The effects of temperature tilt, purge flow rate to oxidation flow rate ratio, reactant mole fraction, and oxidation time on axial and radial thickness distributions have been studied. Two of the furnaces share a similar design while the third furnace has a different fluid flow configuration and torch design. The two similar furnaces show virtually identical axial profiles in average thickness after both the pre-oxidation steps and the combined oxidation-purge steps. Under conditions of a standardized run recipe, the wafer-averaged oxide thickness decreases monotonically by about 3 Å down the axis of the boat. Depletion of water due to oxide formation can account for only about 20% of this decrease in oxide growth. A reduction in the concentration of steam due to adsorption and condensation during the oxidation period, which is then reversed during the purge period, is proposed as a significant cause of those thinner oxide films. Thickness variations within single wafers are also significant, with thickness being highest at the side of a wafer corresponding to the furnace outlet and monotonically decreasing to the opposite side.