Shadow images for in-line holography in a STEM instrument

In a dedicated STEM instrument equipped with a field emission gun, shadow images are easily obtained and have many uses. They are very sensitive to misalignment of the instrument and astigmatism, and therefore can be used for rapid and accurate alignment of the microscope. For crystalline materials, the shadow image contains both the bright-field and dark-field images. It is a summation of the transmitted and diffracted beams, and is basically a kind of Gabor's in-line hologram. Under small or medium defocus, shadow images of a thin, well-orientated crystalline specimen take the characteristic form of Ronchigrams, which offer a unique means to calibrate the microscope operation parameters, such as the spherical aberration coefficient Cs and defocus settings of the objective lens, with high accuracy. With the calibrated values of Cs and delta, a transfer function of the objective lens may be generated. In the stage of numerical reconstruction, by adapting this transfer function to the experimentally recorded hologram the lens aberration introduced in forming the hologram may be corrected and an improved resolution may be achieved for electron microscope images.