Secondary electron emission in scanning Ga ion, He ion and electron microscopes

Abstract

The secondary electron (SE) emission for Ga ion, He ion and electron impact have been calculated using Monte-Carlo simulations, in which the trajectories of all the collision partners (i.e., primary ions, recoiled target atoms, and excited electrons (electron cascade) along which SEs are excited) have been simulated. The SE yields for Ga ion impact show a gradual decrease with increasing Z 2 which is opposite to that found for electron impact. The main reasons for the decrease in SE yield for Ga are the decrease in number and energy of electrons excited by the primary ions and the decrease in contribution of the recoiled atoms to the SE yield with increasing Z 2. For electron impact, both primary electrons and backscattered electrons (BSEs) excite the SEs. The additional SE excitation created by the electron cascade by BSEs is enhanced for high- Z 2 metals especially at E > a few keV. For He ion impact, the Z 2-dependence is between that for the Ga ions and the electrons and is weak because the He ion is light but still much heavier than an electron. As to the lateral resolution, the electron excitations by trapped He ions dominate the SE yield, so that the SE excitation volume is narrower than for electron and Ga impacts. This small contribution of BS He ions to the SE yield does not increase the information depth determined by the trapped He ions, in contrast with the large contribution of BSEs to the SE yields for SEM imaging. The simulated incident-angle dependence in SE yields shows that the topography contrast for He-SIM imaging is clearer than that obtained by SEM and Ga-SIM imaging.