Secondary ion yield changes in Si and GaAs due to topography changes during O+2 or Cs+ ion bombardment

Abstract

Changes in secondary ion yields of matrix and dopant species have been correlated with changes in surface topography during O+2 bombardment of Si and GaAs. In Si, profiles were measured in (100) wafers at 6- and 8-keV impact energy. At 6 keV, a yield increase of about 70% occurred for Si+ over a depth range of 2.5 to 3.5 μm, with changes in other species ranging from a decrease of ∼20% for Si+3 to an increase of more than 25% for O+. The development of a rippled surface topography was observed in scanning electron micrographs over the same depth range. Similar effects occurred over a 3–5 μm depth range for 8-keV ions, and in (111) silicon at a depth of 3 to 4 μm for 6-keV ions. No differences were noted between p- and n-type silicon, or implanted and unimplanted silicon. In GaAs, profiles were measured in (100) wafers at 2.5-, 5.5-, and 8-keV impact energies. At 8 keV, a yield increase of about 70% was found for GaO+ in the range 0.6–1.0 μm, with smaller changes for other matrix species. At 5.5 keV, similar effects were observed, but over a depth interval of 0.3 to 0.7 μm. No yield changes were detected at 2.5-keV impact energy. The yield changes at the higher energies were again correlated with the onset of changes in topography. No change in ion yield or surface topography was noted for Cs+ bombardment of Si or GaAs. The topography and ion yield changes are affected by the angle of incidence and, for Si, the oxygen coverage. The results show that the practice of normalizing secondary ion mass spectrometry dopant profiles to a matrix signal must be modified for situations where matrix yield changes occur.