Abstract: The present invention is directed to a method for forming a thin silicon dioxide deposited layer on a semiconductor structure. The silicon dioxide layer is formed in a hot wall furnace typically used for diffusions. A convenient temperature is selected from those which are suitable for decomposing the TEOS source of the silicon dioxide into its constituent parts out of which the silicon dioxide layer is formed. A vacuum is used to pull the TEOS gas through the diffusion tube. Care is exercised to assure that the flow of the TEOS material through the diffusion tube is kept at a constant rate. Prior to the first use of a new TEOS source bottle, the bottle is placed in a vacuum at least low enough to complete the boiling of impurities from the source material. This predeposition vacuum step is calculated to remove all the impurities contained in the source bottle and caused by the in situ decomposition of the source material during transit and storage.