Role of nitrogen in the downstream etching of silicon nitride

Chemical downstream etching of silicon nitride (Si3N4) requires the addition of nitrogen to the discharge for obtaining efficient etch rates. A 10% addition of N-2 to a CF4/O-2 discharge (CF4/O-2 = 1.2, 0.525 Torr) causes a factor of 6 increase in the Si3N4 etch rate and a 8% decrease in the silicon dioxide etch rate. The result is selectivities approaching 9:1. Importantly, the conversion of CF4 to F and F-containing reactive species by the discharge decreases or remains constant as nitrogen is added to the discharge mix, indicating that the etching reaction is not limited by delivery of these species to the substrate. By measuring the amount of NO and NO2 in the etch chamber, it is found that the NO concentration increases by a factor of 6 as N-2 is added, while the amount of NO2 remains small and constant. The NO signal is significantly reduced during nitride etching compared to the signal observed during a discharge with an empty etch chamber, implying that the increase in Si3N4 etch rate is related to the formation of NO in the discharge. This view is consistent with the observation that an NF3 plasma in a quartz discharge tube results in a nitride etch rate which is a factor of 2 higher than for the same discharge in a sapphire tube. The conclusion is that the oxygen liberated by erosion of the quartz tube allows the formation of NO. That NO is a key Si3N4 etch reactant was confirmed by performing a series of experiments in which N-2, NO, NO2, and N2O were injected into the discharge and then downstream in the reaction chamber during a CF4/O-2 discharge. Nitride etch rates increased significantly upon injection of NO into both discharge and etch chamber as compared to injection of the other NxOy species. (C) 1996 American Vacuum Society.