Surface analysis by secondary-ion mass spectroscopy during etching with gas-cluster ion beam

Abstract

Primary ions from a gas-cluster ion beam (GCIB) are used to investigate metal and silicon surfaces by mass spectroscopy of the secondary ions, as well as to study the nature of cluster-ion interactions with surfaces. The GCIB consists of condensed nanodroplets of either argon, oxygen, or nitrogen gas from which the singly charged cluster ions are generated. The effects of beam acceleration (∼10–25 kV) and flux (ion current density ∼50–500 nA/cm2) are reported. Secondary-ion analysis is done with a quadrupole mass spectrometer in an ultrahigh vacuum chamber. Argon GCIB incident upon metal surfaces of Al, Au, Cu, Ta, and NiFe all result in strong emission of small metal-cluster ions, metal-argon excimers and various other compound ions. Argon GCIB incident upon Al films and Si wafer surfaces generate strong fluxes of small cluster ions (i.e., Aln+ and Sin+ for n=2–10) which decay in emission intensity (with increasing n) approximately by a power law with exponent ∼2.8 and ∼1.0, respectively. Oxygen GCIB upon Al and Si generate strong fluxes of AlnOm+ and SinOm+ compound ions, respectively, with n=1–5 and m⩾n. Nitrogen GCIB upon Al and Si generate fluxes of nitride compounds. The beam conditions utilized are similar to those known to significantly reduce fine-scale surface roughness (i.e., ion smoothen) and concurrently etch at a rate of ∼1 nm/min. Native oxide films on Al, Cu, Ta, and NiFe metals are depth profiled under various GCIB conditions, thus characterizing mechanisms of cluster–ion interaction with metals. The native surface of a Ta film is investigated in some detail. The TaO+ and C+ ion emissions show a two-part decay and growth, respectively, with accumulating argon-GCIB fluence. These are well fit with exponential functions and the characteristic rates are found to depend linearly on GCIB flux and acceleration. The oxidized surfaces exhibit a minimum threshold in acceleration of ∼7 kV which corresponds to only ∼3 eV per incident argon atom. After the surface oxide is removed by the GCIB, no threshold is observed. The native oxides and Fe/Ni, ratio are depth profiled for a permalloy (NiFe) film showing resolution of the thin oxide layer differentiated from the somewhat thicker metal layer that is enriched with Fe.