Fabrication of low aspect ratio three-element Boersch phase shifters for voltage-controlled three electron beam interference

A Boersch phase plate can shift the phase of electrons proportionally to the applied electrical potential, thereby allowing for in situ control of the electron phase shift. A device comprising multiple Boersch phase shifter elements will be able to modulate the wavefront of a coherent electron beam and control electron interference. Recently, fabrication of single and 2x2 element Boersch phase shifter devices by focused ion beam milling has been reported. Realization of a large-scale Boersch phase shifter array would demand further developments in the device design and the fabrication strategy, e.g., using lithographic processes. In the present work, we develop a fabrication method utilizing the state-of-the-art electron beam lithography and reactive ion etching processes, a combination that is widely used for high-throughput and large-scale micro- and nanofabrication of electronic and photonic devices. Using the developed method, we fabricated a three-element phase shifter device with a metal-insulator-metal structure with 100-nm-thick ring electrodes and tested its electron transmission characteristics in a transmission electron microscope with a beam energy of 200keV. We observed voltage-controlled evolution of electron interference, demonstrating the voltage-controlled electron phase shift using the fabricated device with a phase shift of pi rad per 1V. We analyze the experimental results in comparison with a three-dimensional electrostatic simulation. Furthermore, we discuss the possible improvements in terms of beam deflection and crosstalk between phase shifter elements in a five-layer device structure.