Evidence for strongly enhanced yields of negative molecular secondary ions due to bombardment with SF n cluster ions

Abstract

We have performed the first comparative study of the effect of molecular ion bombardment on the yield of positive and negative molecular ions sputtered from adsorbed layers. Mass spectra of differently treated silicon samples were measured by TOF-SIMS using 27–33 keV Xe + and SF − 6 primary ions at impact angles between 56° and 66°. The spectra of positively charged secondary ions are dominated by characteristic peaks due to C n H + m , ( m, n = 0, l, 2, …) ions and, prior to etching, by the well-known plasticizer peak at mass number 149 u. The secondary ion yields Y + are higher for SF − 6 than for Xe + impact, the yield ratio R + = Y + (SF − 6)/ Y + (Xe +) amounting to a factor of about 2 at low masses ( M < 35 u) and a factor of 4 to 6 at higher masses. This moderate yield enhancement may tentatively be attributed to a corresponding difference in the cross sections for ion induced desorption. The spectra of negatively charged secondary ions, observed under SF − 6 bombardment, exhibit high yields of H −, O −, OH −, C n H − m ( n = 0, 1, 2, …; m = 0 and 1), (SiO 2) − n and (SiO 2) n X −, where X can be H, O and OH. For M < 35 u the yield ratio R − (≈ 3) is similar to R +, but with increasing molecular ion mass R − increases rapidly to become as large as ≈ 100 for M > 80. Comparative studies using primary ion beams of Xe +, SF + 5 and SF − 5 showed that the strong yield enhancement observed in the negative SIMS mode is due to the molecular character of the projectile and only marginally (if any) related to the projectile charge. In an application of this cluster bombardment technique we show that changes in composition of the topmost layers, brought about by etching of the sample, can be studied in detail by comparing the yields of bulk-type high-mass molecular ions like (SiO 2) n X −( X = H or OH).