Reactive ion beam etching: Dissociation of molecular ions upon impact

This paper reports on etch yield measurements on Au films by ion beams generated from Ar, Ne, O2, N2, CF4, and C2F6 in a Kaufman-type ion gun for ion energies between 0.2 and 1.4 keV. Ions and neutral species emanating from the gun are characterized by mass spectrometry. For CF4 and C2F6 the ion composition is a strong function of the magnetic field in the gun, CF+3 being the major ion only for CF4 at low magnetic field. The physical etch yield of CF+3 is found to be close to that of Ne+, not Ar+. The results on the dependence of the etch yields on ion mass, ion energy, and incident angle can be correlated quantitatively by assuming complete dissociation of molecular ions upon impact on the substrate. The same model is also shown to explain the data by Tachi et al. on etching Si by mass-selected ions of the form CF+x and BF+x. The implications of these results for plasma and reactive ion etching are discussed.