Etching SiO2 films in aqueous 0.49% HF

In both buffered (40% NH4F) and unbuffered 0.49% HF the thickness removed of undensified tetraethylorothosilicate (TEOS) is extremely linear vs. time with a slope (etch rate) near 140 Angstrom/min. A 97 Angstrom y-intercept (corresponding to etch during the transfer and rinse) is found with buffering; this zero-time intercept is only 13 Angstrom without the NH4F buffer. Thus it is impossible to etch less than 97 Angstrom of TEOS in 40 weight percent (w/o) buffered hydrofluoric acid; for borophosphosilicate glass the minimum exceeds 140 Angstrom. Buffering increases the etch rate of thermal oxide from 20 to 65 Angstrom/min, but the intercept is invariant. The etch rate of BPSG in 0.49% HF is dependent of buffering until the NH4F level exceeds 15 w/o, while TEOS and thermal oxide etch rates increase by a factor of five over the same range. The selectivity of TEOS to thermal oxide is strongly time dependent in both buffered and unbuffered HF. The rinse rate for TEOS in BHF does not depend on the flow velocity of the rinse water; stagnant water shows identical results with high flow rates. The large zero intercept with buffering is the result of the fact that etch rates increase significantly as water is added to the carry-over etchant during the rinse. It is only after dilution has approached 80% that the etch rates finally maximize and begin to drop toward zero.