DETERMINATION OF THE ETCHING KINETICS FOR THE HYDROFLUORIC-ACID SILICON DIOXIDE SYSTEM

Etching kinetics for several silicon dioxide films in various hydrofluoric acid solutions have been studied. The low temperature silicon dioxide films were deposited at 450-degrees-C and 300 mTorr in SiH4, O2, and PH, and annealed at 950-degrees-C for 1 h. The thermal oxides were grown at 1100-degrees-C in H2O and 02. Four hydrofluoric acid solutions were used: dilutions of 49 weight percent HF with deionized water, buffered hydrofluoric acid, surfactant-buffered hydrofluoric acid, and hydrofluoric acid/hydrochloric acid mixtures. Expressions for etch rate as a function of hydrofluoric acid concentration are presented. Notably, the etching of the low temperature oxides was of the order 1.6-2.0 in HF solutions and of the order 0.5-1.0 in buffered HF solutions. The etching of the thermal oxide was of the order 1.37-1.5 in HF and of the order 0.75-1.07 in buffered HF solutions. The results for the HF solutions provide evidence of a two-part etching mechanism: first, the acidity causes the formation of the silanol bonds on the silicon dioxide surface and, then, the fluorine species react with the silicon to form SiF4(g) which is soluble in water and forms H2SiF6(aq). Significant enhancement (as much as 600%) in the etch rate was observed when hydrochloric acid was added to the HF solutions.