Electron Holography at Low Energy

Abstract

Shadow images, magnified by projection from a small electron source, were amongst the first images to be obtained by electron microscopes.24 In the same year (1939) pointprojection shadow images of carbon grids were also reported at a few hundred volts using a field-emission electron tip.320 The authors comment on the absence of aberrations in their lensless system, the removal of which has proved a major obstacle for conventional electron holography at higher voltages. Fig. 14.1 (a) shows the optical arrangement for point-projection microscopy (PPM). A tungsten field emitter S is placed at a distance z1 (a few hundred nanometers) from a thin sample T. and an image recorded on a channel plate D about z2 = 10 cm away. The magnification is approximately M = z2/z2 ≈ 106 and the resolution in the image is about equal to the source size. Fig. 14.1(b) emphasises that the resulting image is equivalent to a TEM image with defocus z1 (under certain conditions). The conducting sample, grounded, acts as the anode. Since field emitters of atomic dimensions can be obtained,107 atomic resolution should be possible. However, at the very low voltages (about 100 V) needed to sustain field emission with z1 = 100 nm, electron penetration in carbon is only about 0.8 nm, and most samples are completely opaque. Then the main features seen in the images are Fresnel fringes around edges, and occasionally interference fringes between waves passing through different pinholes. This arrangement constitutes exactly Gabor's original proposal for electron holography, and also corresponds to the in-line STEM Fresnel holography case described in Chapter 2, Section 3 (with sub-nanometer aberration coefficients appropriate to the PPM virtual field-emission source). For STEM, the probe-forming lens shown in Fig. 2.6(a) is used. A low voltage PPM instrument was built at NIST in 1968302 and several more have been built more recently23,108,318,418 using STM-based construction methods.