Cesium ion sputtering with oxygen flooding: Experimental SIMS study of work function change

Negative ion yield and sputter yield variations for Cs+ bombardment of Si with O2 gas flooding

“…Table 2 summarizes the useful ion yields U Y (M − ) for comparison of (14.5 keV, ∼25°) 133 Cs + bombardment of silicon at base pressure and with O 2 gas flooding (3.5 × 10 −6 torr, O/Si ≈ 1.8). Under our experimental conditions, useful ion yields of electronegative elements U Y (M − ) (M = P, As, Sb, range of initial kinetic emission energy 65–185 eV (‘H’)) were lowered by oxygen flooding, rather than enhanced as had been reported previously in Refs 12,18,19, respectively. Kudriatsev et al 12 and Berghmans and Vandervorst13 had previously also observed that U Y (Si − ) is lowered by O 2 gas flooding of a Cs + sputtered silicon surface.…”

“…Under oxygen flood, U Y (Cs − ) increases to ≈ 1.0 × 10 −5 , a factor of about 34 increase at O/Si ≈ 1.8, whereas U Y (B − ) is enhanced by a factor of 3.0, U Y (Si − ) is lowered by a factor of 3.2 and U Y (O − ) remains almost unchanged. These elemental differences with respect to the response of U Y (M − ) to oxidation of a partially cesiated silicon surface cannot be explained simply by a change of the (average) sample work function,12 but need to be interpreted within a local picture of negative secondary ion formation.…”

“…3 and 4 in Ref. 12). Similarly, Bergmans and Vandervorst13 also observed a decrease of the useful ion yield of Si − with O 2 flooding, except at the highest flood pressure.…”

“…One easy way to vary the sputter yield is to flood the surface with gas‐phase oxygen: the sputter yield of silicon, for example, drops by a factor of up to ∼4 with heavy oxygen coverage. Although the effects of oxygen on negative ion yields on cesium‐sputtered surfaces are not readily predicted11–13 it seemed worthwhile to investigate the effect of oxygen on cesium concentration and negative ion yields. (a) Three decades ago Chelgren et al 4 had suggested that additional gas flooding during Cs + bombardment should reduce the sputter yield, and thus might increase the surface cesium concentration.…”

“…To our knowledge, the present work is the first study in which, within the limitations of the above caveat, the oxygen vs cesium vs silicon content at a Cs + sputtered and O 2 flooded Si surface has been quantified together with the resultant negative ion yields. Kudriatsev et al 12 recently studied negative ion yield variations of Si − , B − , P − , As − , Sb − , F − for O 2 flooding of Cs + sputtered Si; they found most ion yields to increase (by up to a factor of ∼10) with oxygen flooding, except for the ion yield of Si − that was lowered by a factor of about 2 (Figs. 3 and 4 in Ref.…”