Plasma-assisted thermal-cyclic atomic-layer etching of tungsten and control of its selectivity to titanium nitride

Plasma-assisted thermal-cyclic atomic-layer etching (ALE) of tungsten (W), the selectivity of which with respect to TiN can be tuned between highly selective and nonselective by changing infrared-heating time, has been demonstrated. It is a two-step process consisting of exposure to radicals generated in a CHF3/O2 or CH2F2/O2/Ar plasma followed by infrared heating. Analysis by in situ x-ray photoelectron spectroscopy confirmed that a WFx-based modified surface formed on the surface of a W film exposed to radicals at −22 °C. The modified surface on the W film is volatile at a heating temperature lower than that required for removing the modified surface on the TiN film. Cyclic etching of both W and TiN was performed by using the CH2F2/O2/Ar plasma and infrared heating in a 300-mm ALE apparatus. When the infrared-heating time was not long enough to remove the modified surface on TiN, the cyclic repetition etched only the W film. On the other hand, when the infrared-heating time was long enough to remove modified surfaces on both W and TiN, the cyclic repetition etched both W and TiN films. Therefore, both highly selective and nonselective ALEs for W and TiN were obtained by choosing an optimum infrared-heating time. This shows that material selectivity in plasma-assisted thermal-cyclic ALE can be controlled by changing the infrared-heating time.