Collection of low-energy secondary electrons and imaging in a low-voltage SEM

Diana Nyyssonen

doi:10.1117/12.240145

21 May 1996 Collection of low-energy secondary electrons and imaging in a low-voltage SEM

Diana Nyyssonen

Proceedings Volume 2725, Metrology, Inspection, and Process Control for Microlithography X; (1996) https://doi.org/10.1117/12.240145
Event: SPIE's 1996 International Symposium on Microlithography, 1996, Santa Clara, CA, United States

Abstract

One of the factors that influences the nature of secondary electron imaging and the subsequent metrology of submicron features in a SEM is the system's ability to collect low energy secondary electrons (LES). In a SEM, a bias voltage is applied to the detector grid to enhance collection and pull electrons from the wafer. However, at low beam voltages the bias voltage that can be applied is limited in order not to deflect or interfere with the incident beam. For even moderate aspect ratios of submicron features (3:1), there is a reduction in the collection efficiency that may affect imaging. In this paper, we use a simple model of LES trajectories to simulate both symmetric and asymmetric collection systems. Formulas for the collection angles are given as a function of the aspect ratio of the feature and parameters of the collection system. These formulas can be incorporated into Monte Carlo, diffusion or other phenomenological modeling to predict the effect of collection system design on SEM imaging. An example is given using a previously published model [Nyyssonen, Proc. SPIE Vol. 921, pp. 48 - 56, 1988]. This model computes the surface integral of a probability density function to predict SEM images. It is shown that reduced LES collection efficiency may introduce artifacts in the SEM image of both lines and trenches. These artifacts can be used as a method of characterizing the efficiency of the collection system when well- characterized samples are used.

Citation Download Citation

Diana Nyyssonen "Collection of low-energy secondary electrons and imaging in a low-voltage SEM", Proc. SPIE 2725, Metrology, Inspection, and Process Control for Microlithography X, (21 May 1996); https://doi.org/10.1117/12.240145

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