The reference window for reduced perturbation of the reference wave in electrical biasing off-axis electron holography.

The perturbation of the reference wave due to electric stray fields represents a major challenge in quantitative electron holographic investigations. By introducing a focused-ion-beam-milled rectangular hole, the reference window, in an area of nearly constant electrostatic potential of the sample, this perturbation can be significantly reduced. The edge of the window forms a closed conducting loop, acting similarly to a Faraday cage, shielding the influence of the stray field on the reference wave to some extent. In this work, the shielding effect of the reference window is systematically investigated by comparing electron holograms of an electrically biased coplanar capacitor, as a well-known reference sample, with finite element simulations. It is shown that the introduction of the reference window into electrical biasing samples both suppresses unknown lateral phase distortions substantially and in addition improves the agreement of the experimentally observed phase slope with that expected by simulation significantly, particularly for small object-reference wave distances. Consequently, a slight adjustment of the sample geometry results in an improved reproducibility of electron holographic electrical biasing experiments, which is a significant step towards quantitative evaluation.