Penning ionization in the electron beam assisted secondary ion mass spectrometry-dependence of ionization potential

The enhancement of secondary ion signals has been investigated for the Si(1 0 0) wafers implanted with different chemical elements using the electron beam assisted secondary ion mass spectrometry (SIMS). The 7Li+, 40Ca+, 50Ti+, 24Mg+, 11B+ and 30Si+ secondary ion signals increase linearly with electron beam current at different rates, supported by the SIMS depth profiles. An enhancement factor (e-factor) is defined to characterize the increased percentage of a secondary ion signal. The 7Li+, 40Ca+, 50Ti+, 24Mg+, 11B+ and 30Si+ secondary ion signals exhibit e-factors of 0.1, 1.6, 6.1, 26.4, 59.2 and 71.4%, respectively at an electron beam current of 90 μA. The e-factor increases in an exponential function with ionization potential. A proposed Penning ionization mechanism on the sample surface can well explain the experimental results. The Penning ionization is dominated by the oxygen radicals since no secondary ion enhancements were observed when the O2+ primary ion was replaced by Cs+ in SIMS operations.