Figure 1.  SRM 484f sample and its micrograph.

A scanning electron microscope (SEM) with ultra-high vacuum (UHV) environment has a cold field emitter as its beam source. Less than 10 nm beam resolution can be achieved by this microscope.

The SEM can operate as an ordinary SEM and is also equipped with a commercial He­Ne laser interferometer on the outside of its chamber. A single-axis piezo flexure stage (100 micrometers maximum scan) with all the necessary optics resides inside of the vacuum chamber. The distance of two features of interest can be measured by scanning the stage with the sample across the stationary electron beam.

Since 1981, the instrument has been used to calibrate SRM 484, an artifact to calibrate the magnifications of SEM's. A photograph of an SRM­484f sample and its micrograph are pictured at left.  In the micrograph, the line spacings range from 0.5 micrometers to 5 micrometers. A part of the indentation mark, which determines the calibrated area, is shown on the left side of the sample (see Figure 1).

A SEM micrograph of 3-micrometer latex spheres and an intensity-distance plot of a single row of spheres taken with the metrology electron microscope are pictured below (see Figures 2a and 2b).