INVESTIGATION OF THE CHEMICAL MECHANISMS OF TA(110)/TA(110)-SUBOXIDE ETCH SELECTIVITY USING CL-2 MOLECULAR-BEAMS

XPS and LEED techniques were used to investigate the interaction of a Cl-2 molecular beam with a Ta(110) single crystal surface as a function of temperature. Due to a sample heating limitation of 1500 degrees C, the starting surface contained an ordered oxide layer that originated from a bulk contamination. The surface oxygen coverage was similar to 1/3 monolayer. The oxide was ordered along the [110] azimuth but slightly disordered along the root 3/4 close-packed direction rotated 55 degrees from the [110] direction. An unequal population of Ta1+ and Ta2+ oxidation states were identified and found to exhibit an inequivalent reactivity toward chlorine. Three temperature regimes were distinguished. At room temperature chlorine adatoms initially bond to both Ta and TaO sites causing further oxidization of Ta. Heating to 300 degrees C induced a state selective etching reaction involving Cl and O. The reaction preferentially depleted oxygen bonded at Ta2+ sites which were formed by Cl adsorption at Ta1+ sites in disordered domains. Between 300 degrees C and 650 degrees C chlorine adsorption passivates the surface in ordered domains of p(4 x 1) orientation exhibiting some disorder along the [110] site stacking direction. Above 650 degrees C chlorine is thermally depleted and surface segregation of bulk oxygen occurs. Oxygen segregation occurs at similar to 400 degrees C in the absence of adsorbed chlorine. The propensity for Cl to react at Ta-O sites has implications in understanding the overwhelming etch selectivity of Ta metal to Ta-oxide in Cl-rich plasma etching applications for microelectronic device fabrication which is discussed.