Submicron Oxidation Thickening

The thinning of oxides that are grown in narrow windows (<0.5 μm) or over edges and the crotch of a deep trench can be a scaling limitation and result in device problems. In this paper, we present data on the fluorine-assisted oxidation of isolation structures contrasted with the present state of art oxidation process. Dry oxidations were carried out using the addition of several hundred parts per million of NF 3 added to oxygen at temperatures between 950 and 1100°C for times of 30 min to 8 h. The data from this study show that oxides, when normalized to that grown in very large windows can be very uniform vs. window size in contrast to the normal case where thinning is experienced. Samples of both poly-buffered LOCOS and shallow trench structures were studied. Using specially arranged testers, the role of masking stack size (i.e., nitride block width) vs. window opening was assessed. These testers point to the role that stress plays especially in narrow windows. These studies suggest that the selection of proper conditions can result in uniform oxides or for the fluorinated oxides thickening in the very smallest windows.