Method to improve etch selectivity during silicon nitride spacer etch

A method of etching features on a substrate, the method comprising:

• positioning a substrate on a substrate holder in a plasma processing chamber, the substrate having a silicon nitride film conformally covering structures on the substrate, the structures including gate structures and fin structures of a FinFET structure;
• flowing a first process gas mixture into the plasma processing system, the first process gas mixture comprising a hydrofluorocarbon gas;
• flowing a second process gas mixture into the plasma processing system, the second process gas mixture comprising an oxygen-containing gas;
• forming plasma from the first process gas mixture and the second process gas mixture;
• repeatedly varying a flow rate of the first process gas mixture or repeatedly varying a flow rate of the second process gas mixture such that a gas flow ratio of the hydrofluorocarbon gas to the oxygen-containing gas repeatedly fluctuates between a first gas ratio defining an etch period and a second gas ratio defining an oxidation period and having a greater proportion of oxygen-containing gas as compared to the etch period such that a protection layer of SiOF forms on exposed silicon surfaces during the oxidation period and etching of silicon nitride occurs during the etch period, wherein the etch period has a duration that is at least three times greater than a duration of the oxidation period;
• and
• anisotropically directing products of the plasma toward the substrate at least until silicon nitride is removed from fin structures.

A method of etching features on a substrate, the method comprising:

• positioning a substrate in a plasma processing chamber, the substrate having a silicon nitride film conformally covering structures on the substrate, the structures including gate structures and fin structures of a FinFET structure;
• establishing a plasma in the plasma processing chamber from a gas mixture of a hydrofluorocarbon gas and an oxygen-containing gas;
• and
• performing a cyclic anisotropic etch using the plasma to achieve a target level of removal of the silicon nitride film from the fin structures and from a portion of the gate structures while leaving the silicon nitride film on at least a portion of the sidewalls of the gate structures to form sidewall spacers, wherein the cyclic anisotropic etch comprises repeatedly cycling between an etching step a) and an oxidation step b) until step a) achieves the target level of removal, wherein the etching step a) and the oxidation step b) comprise, respectively:
• a) for a first duration, flowing the hydrofluorocarbon gas and the oxygen-containing gas into the plasma processing system at respective first flow rates to provide a first gas flow ratio of the hydrofluorocarbon gas to the oxygen-containing gas that is greater than 1:1 and that is sufficient to etch the silicon nitride with greater etch selectivity to silicon nitride than to silicon;
• and
• b) for a second duration less than the first duration, flowing the hydrofluorocarbon gas and the oxygen-containing gas into the plasma processing system at respective second flow rates to provide a second gas flow ratio of the hydrofluorocarbon gas to the oxygen-containing gas that is less than the first gas flow ratio to form a protection layer of SiOF on exposed silicon surfaces at a thickness greater than a thickness of SiOF that forms on silicon nitride to protect the exposed silicon surfaces from etching during the next cycle of etching step a).