Towards programmable multi-element Boersch phase shifters for electron wavefront modulation

With an aim to contributing to the ongoing research of solving a well-known ‘phase problem’ in transmission electron microscopy, we fabricate a programmable device that will synthesize the phase of the incident electron wave at will. For this purpose, we target to realize a Boersch phase plate with multiple phase shifter elements that can be programmed by applying small voltages to each element. A three-layer Boersch phase shifter device is fabricated in metal-insulator-metal configuration with state-of-the-art nanofabrication methods such as electron beam lithography and reactive ion etching. The phase-shifting properties of the device are tested in a high-energy (200 keV) transmission electron microscope. The three-layer device comprised of three phase shifter elements could successfully alter the phase of the electron beam with a phase shift efficiency of ∼ 0.9π/V observed as three-beam electron interference on the detector. The phase shifter elements are micro-/nano elements capable of selectively altering the phase of the propagating electron beam by electrical biasing to the element. However, the key disadvantage of the three layer device is the parasitic deflection of the electron beam caused by the metal contact wires. The second-generation device in five-layer, metal-insulator-metal insulator-metal configuration is fabricated and tested within similar experimental conditions. The device’s phase shifting performance is improved by significant suppression of the beam deflection caused by contact wires and exhibits a phase shift efficiency of ∼ 0.55π/V. The deflection of the electron beam due to contact wire is 100 times lower for the new device. The three- and five-layer Boersch devices can operate on voltages below 5 V for a maximum of 2π phase shift. The low-voltage operation allows the integration with active electronics to program a device with an array of such phase shifter elements. The suppression of beam deflection is also very important as the complexity of the deflection increases with an increasing number of elements. Such a device would also eliminate the need to align the phase plate relative to the electron beam as is the case for thin film-based phase plates such as Zernike phase plate. A device with an array of Boersch phase shifter elements has great potential in wavefront modulation for applications ranging from real-time phase retrieval of unknown molecular structures to designing a programmable aberration corrector for electron microscopes. The electrostatic phase shifters would open another possibility to visualize transparent objects with increased resolution and also study structures of biological specimens like proteins, viruses, cells, etc.