Compared to a light microscope, an electron microscope achieves a considerably higher lateral resolution since the wavelength of the electrons used for imaging (which have an energy of several 10 keV) is only one 100,000th of the light wavelength. For a long time, however, electron microscopes were clearly inferior to light microscopes in terms of depth resolution. This changed when it became possible to visualize on surfaces – with the aid of interfering electron waves – unevenness and structures whose height was less than 1 nanometre. Dr. Hannes Lichte achieved his breakthrough in this field with the construction of an interference microscope for electron waves. He was awarded the Helmholtz Prize in 1977 for his work “Ein Elektronen-Auflicht-lnterferenzmikroskop zur Präzisionsmessung von Unebenheiten und Potentialunterschieden auf Oberflächen” (An electron-reflected-light interference microscope for the precision measurement of unevenness and potential differences on surfaces).
Lichte’s electron interference microscope worked according to the principle of the light-optical Michelson interferometer. In the Michelson interferometer, a beam splitter divides the light waves emitted by a light source into two partial waves, each of which is reflected by a mirror. After that, these partial waves recombine and interfere. If one mirror is a smooth reference surface and the other one is uneven, the shape and the displacement of the occurring interference fringes can be used to determine the extent and height of the unevenness. In the electron interference microscope, the electron waves emitted by a source were split into two partial waves by a biprism.
The electron-optical biprism developed by Gottfried Möllenstedt and Heinrich Düker in 1954 consisted of a metallized quartz thread of 1 micrometre thickness which was suspended and had a negative electrostatic potential. A reversing solenoid deflected the two partial waves at right angles to an electron mirror, where they encountered two different areas which corresponded to the two interferometer mirrors. The negative potential of the electron mirror caused the electron waves to approach the mirror up to about 200 nm only before they returned. Then they passed the reversing solenoid again and encountered a second biprism which had a positive potential and recombined the two partial waves so that they interfered. It turned out that height differences of atomic order shift the phase of the electron waves by 2π. With this electron microscope, Lichte could visualize and measure surface unevenness with a height of less than 0.1 nm. Also, potential differences in a surface could be determined with an accuracy of less than 1 mV.
