Modelling and observations of electron beam charging of an insulator/metal bilayer and its impact on secondary electron images in defect inspection equipment

Abstract

A self-consistent simulation of secondary electron (SE) emission and charging of a SiO(2) layer with the thickness of several tens of nanometres on Si is incorporated into a trajectory simulation of emitted SEs above the surface, the centre area of which is charged by electron beams (EBs) at the energy range from 300 to 2000 eV. In order to study the influence of the charging of an insulating layer on defect inspection, a pseudo-image is reconstructed from net SE yields calculated at each point of the SiO(2) surface locally applied the positive voltage. The image contrast between charged and uncharged areas is compared with the observation of thermally oxidized layer with the thickness of 24-106 nm on a Si wafer. The image contrast is very sensitive to the thickness of the SiO(2) layer, which is verified by both observed and calculated images. The calculated changes of the images with the layer thickness and the primary electron energy reproduce the experimental observations fairly well. This confirms a highly sensitive detection mechanism for tiny defects in insulating patterns on a metal hard mask for an EB defect inspection equipment.

Modeling of Electron Beam Charging of an Insulating Layer on a Silicon Substrate

“…This model has been applied to a very thin SiO 2 layer on a Si substrate. Calculated changes in pseudo-SE images with PE energy reproduced experimental observations of thermally oxidized layers with thicknesses of 24-106 nm on a Si wafer [2]. Thick SiO 2 layers on Si become positively charged during irradiation by 300-1600-eV EBs, and attract emitted SEs back to the surface.…”

“…Therefore, charging can- not be reduced by the Si substrate. Figure 4 reveals that the surface voltage increases with increasing number of PEs until it saturates above ∼5,000 PEs at a steady-state value of ∼16 V. The steady-state voltage depends on both the incident energy and the SiO 2 step height (if it is of the order of tens of nanometers) [2]. Figures 5 and 6 show the trajectories of SEs emitted into the vacuum for 1 keV PEs incident on different positions on the SiO 2 step and the Si substrate together with the corresponding charge distributions that accumulate in the SiO 2 .…”

Photoresist cross-sectional shape change caused by scanning electron microscope-induced shrinkage

“…This means that the SE yield of the SOG is larger for a 500-V EB irradiation than for a 100 V one. Although the SE yield of SiO 2 is difficult to determine by experiment due to charging, a simulation study which eliminates charging effect reported that the yield is larger for 500 V than for 100 V. 24 Detailed investigations on the SE yield of SiO 2 are needed for a comprehensive interpretation of the observed accelerating-voltage dependency.…”