Abstract
In this thesis, we study the positive secondary ion yield enhancement by addition electron beam irradiation in regular secondary ion mass spectrometry (SIMS) measurements.Secondary ion emission was enhanced by electron beam in a typical SIMS operation, which was demonstrated using the shallow p/n junction implanted with BF2+ ions of a dosage of 2.3E15 cm-2 at 20 keV. In the SIMS mass spectra measurements, the 19F+ signal is enhanced by 39 times at an electron beam current of 100 μA. In contrast, the 11B+ signal is slightly enhanced by a factor of ~50%. In order to characterize the spatial distribution of secondary ions along the surface normal direction, the energy spectra of 19F+, 11B+ and 30Si+ ions were measured by altering the sample potential by ±50 V (relative the original sample bias 2 kV). The energy spectra reveal that most 19F+ ion signal is generated by electron beam stimulated desorption on the sample surface and some by the electron induced post-ionization in the gas phase above the sample surface. In contrast, only small enhancements of 11B+ and 30Si+ occur on the sample surface. No 11B+ and 30Si+ ions are generated in the gas phase above the sample surface. The enhancement of 11B+ increases linearly with electron beam current based on the depth profile data of 11B+. This supports the occurrence of Penning ionization for 11B+ on the sample surface. The ionization enhancement of 11B+ is dominated by oxygen radicals O2*.The ionization enhancement strongly depended on the electron impact energy. The enhancement factors of the 11B+ and 30Si+ signals increase linearly with electron beam current. At an electron beam current of 50 nA, the 11B+ enhancement factor reaches ~21, ~29, ~40 and ~60% at electron beam energies of 3, 6, 9 and 12keV, respectively. Similarly, the 30Si+ enhancement factor reaches ~25, ~31, ~42 and ~66% at 3, 6, 9 and 12 keV electron beam energies, respectively.The enhancement of secondary ion signals has been also investigated for the Si(100) wafers implanted with different chemical elements. 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 □A. 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+ and Ar+ in SIMS operations.