Fabrication and characterization of an adaptive thin-film electrostatic phase plate

In this work, we establish a robust protocol for designing and fabricating adaptive electrostatic thin-film phase plates, as demonstrated by the first experimental realization of such a device and its characterization. A phase plate placed before the sample in a transmission electron microscope (TEM) can shape the beam to create user-configured irradiation with predesigned amplitude and phase distribution. It is, therefore, possible to use a phase plate to compensate for aberrations of the beam, which are a common problem in electron microscopy. The electron-optical index of refraction is proportional to the electric and magnetic potentials. Thus, the active part of a phase plate is designed to generate a local electric and/or magnetic field. By using a specific set of electrodes, the imprinted phase distribution of the electron beam can be adjusted externally by changing the potential applied to the electrodes. This is denoted as an adaptive electrostatic phase plate. Here, we present the design, fabrication, and characterization of an adaptive thin-film electrostatic phase plate, with gold electrodes supported on a thin silicon nitride membrane. Our design consists of six electrodes positioned as concentric and equidistant circles. We describe the fabrication of the phase plate, with particular focus on the less conventional steps involved. After careful process optimization, the fabricated phase plate features largely match the design, and approximately 95% of the 174 chips fabricated on each wafer show no visible defects in the active area. We characterize the functionality of the fabricated phase plate using both computational ( Multiphysics) and experimental (Fresnel imaging) methods. The phase profile reconstruction using the transport of intensity equation method generally follows the predicted phase profile. The desired quartic phase profile is partly achieved, although the presence of electrodes increase noise, and the electrode contacts introduce asymmetry. This opens up opportunities for further optimization before implementing the phase plates in a TEM for spherical aberration correction.