Contribution of dynamic charging effects into dopant contrast mechanisms in silicon
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In this work, SEM capability for imaging of both p- and n-doped regions in Si was demonstrated. The best dopant contrast was found when the primary electron range (R) is comparable or larger than the maximum escape depth of secondary electrons (∼5λ) (λ stands for mean free path). Beyond this scale (R < 5λ, R > > 5λ) the contrast between p-, n-doped and intrinsic regions gradually disappears. The dopant profiles obtained by SEM were judged using scanning capacitance microscopy (SCM), dopant selective etch (DSE) and secondary ion mass spectrometry (SIMS) measurements, and excellent matching was demonstrated. A novel dopant contrast mechanism incorporating dynamic charging effects that take place during e-beam/specimen interaction is suggested. Under threshold steady-state imaging conditions, an Ebi field in Si near the surface region is formed. This field, governed by secondary electron (SE) emission and trapping of some incident and generated SE, accelerates electrons towards the surface in p-type regions and decelerates them in n-type regions, compared with the intrinsic material. This results in the observed dopant contrast: C(n) < C(i) < C(p). Use of the SEM for 2D-dopant imaging provides many advantages; giving fast results, covering a wide range of dopant concentrations, applicable to real devices, and does not require sample preparation needed by SCM and DSE. In addition, SEM-dopant contrast data quantification is possible using SIMS standards which needs to be defined with more details.
KeywordsSecondary Electron Primary Electron Interaction Volume Dopant Profile Secondary Electron Emission
The authors wish to thank E. Kaganer for the SIMS measurements and I. Vidoshinsky for his help with the dopant selective etching recipe development.
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