Abstract
Electrostatic beam blankers are an alternative to photo-emission sources for generating pulsed electron beams for Time-resolved Cathodoluminescence and Ultrafast Electron Microscopy. While the properties of beam blankers have been extensively investigated in the past for applications in lithography, characteristics such as the influence of blanking on imaging resolution have not been fully addressed. We derive general analytical expressions for the spot displacement and loss in resolution induced by deflecting the electron beam in a blanker. In particular, we analyze the sensitivity of both measures to how precise the conjugate focus is aligned in between the deflector plates. We then work out the specific case of a beam blanker driven by a linear voltage ramp as was used in recent studies by others and by us. The result shows that the spot displacement and focus blur can be reduced to the same order as the electron beam probe size, even when using a beam blanker of millimeter or larger scale dimensions. An interesting result is that, by the right choice of the focus position in the deflector, either the spot displacement from the stationary position can be minimized, or the blur can be made zero but not both at the same time. Our results can be used both to characterize existing beam blanker setups and to design novel blankers. This can further develop the field of time-resolved electron microscopy by making it easier to generate pulses with a typical duration of tens of picoseconds in a regular scanning electron microscope at high spatial resolution.
Highlights
Electrostatic deflectors are commonly used in electron beam lithography and microscopy to avoid exposure of specific areas of a sample by deflecting, or blanking, the beam over an aperture [1,2,3,4]
Novel concepts based on microfabricated electrostatic deflectors [11,12] or deflecting by resonant magnetic field modes in a microwave cavity [13,14,15] have been proposed as a means to generate sub-picosecond pulses using the blanker concept, rivaling the pulse durations achieved with photoemission sources [16]
We illustrate the implications of the above derived equations using the parameters for a commercial beam blanker (FEI, Thermo Fisher) that has been used in recent works [1,2,20,21]
Summary
Electrostatic deflectors are commonly used in electron beam lithography and microscopy to avoid exposure of specific areas of a sample by deflecting, or blanking, the beam over an aperture [1,2,3,4]. Trajectory equations were derived in closed form for electrons in time-dependent electric fields by Gesley [10] and he analyzed the resulting beam motion This framework, cannot be readily adopted to a new design or a single pair of deflector plates. We present an analytical evaluation of SD and focus blur induced by a single pair of mm-sized deflector plates We analyze how both factors depend on the electron energy at the FBB position, the electron entry time with respect to the deflector voltage transient, and the mismatch between the focal plane and the center of the deflector plates. Our results provide insight into the intrinsic resolution limitations imposed by current FBBs and allow to evaluate how SD and resolution loss can be minimized or avoided with current or potentially new designs
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