Low-voltage electron-optical system for the high-speed inspection of integrated circuits

Abstract

Low-voltage scanning electron microscopes enjoy many advantages over light microscopes for both dimensional measurements and the analysis of small features. Their combination of high-resolution and large depth-of-focus has made them a standard instrument in the semiconductor industry. However, until now, they have been many orders of magnitude too slow to be used for automated wafer inspection. In this article, the authors will determine the optical requirements of high-speed inspection at low voltage and show how this leads to a novel optical system design. This optical system shares similarities with a 20 keV column designed for an x-ray mask inspection system [W. D. Meisburger, A. Desai, and A. D. Brodie, J. Vac. Sci. Technol. B 9, 3010 (1991)]. However, the objective lens and electron detection system are unique. The objective lens combines superimposed electric and magnetic fields to decelerate the primary beam from 20 keV to ∼800 eV, and to effectively collimate and reaccelerate the resulting secondary electrons. Auxiliary electrodes are used to control the electric field on the semiconductor’s surface. A Wien filter is used to separate the secondary electrons from the primary beam. A second Wien filter is used to cancel the transverse chromatic aberration of the first filter. This optical system is capable of producing current densities of 1000 A/cm2 at 800 eV into spots as small as 50 nm. Images are acquired at a rate of 100 million pixels/second. Both theoretical and experimental performance results for this system will be presented.