Abstract: Improved techniques for imaging high-aspect-ratio structures such as contact holes utilize two signal detection sub-systems, one optimized for imaging at the top and another optimized for imaging at the base of submicrometer structures. These detection systems produce signals that can be combined in real-time to produce an image which resembles the "extended focus" images obtained with confocal optical microscopes. Unlike the confocal image, however, the resulting image has the inherent linearity and resolution characteristics of electron-beam technology. Using the new approach, the signal, rather than exhibiting a near-zero minimum at the base of the structure as is typical of the prior art, exhibits its maximum at the base of the structure, allowing high-precision measurement with no need for extrapolation.