Ammonium fluoride deposition during plasma etching of silicon nitride

Abstract

During oxygen and carbon tetrafluoride plasma etching of silicon nitride, a light blue and water soluble film is deposited upon the surface of the silicon nitride layer. Depending upon the thickness, this film retards or completely inhibits the desired etching process. This film has been identified by infrared spectrophotometry as ammonium fluoride. The thickness of the ammonium fluoride layer has been determined to be a function of the ammonia to dichlorosilane mole ratio of the gas flow into the LPCVD reactor during deposition. For mole ratios greater then 4.0, the ammonium fluoride film thickness was observed to decrease to approximately one-third of the observed value at a mole ratio of 3.0. A possible mechanism for the formation of the ammonium fluoride layer is the reaction of ammonia molecules and hydrogen atoms (which have been occluded in the silicon nitride layer during deposition) with fluorine atoms from the plasma etching process. Increasing the mole ratio during deposition improves the stoichiometry of the silicon nitride layer and reduces the number of hydrogen atoms available for subsequent formation of ammonium fluoride during etching. The etching variations caused by ammonium fluoride deposition during plasma etching of silicon nitride can be minimized by optimizing the ammonia to dichlorosilane mole ratio during the deposition of the silicon nitride layer.