Cavities Etched by Pulsed and Continuous Flow of XeF2 for Multispectral Infrared Sensors

The voltage response of thermoelectrically coupled nanoantennas (TECNA) to infrared light (IR) is greatly enhanced by suspending them above quasi-spherical cavities. The cavities behave as optical elements that focus the IR radiation onto the nanoantennas. The wavelength response of a cavity depends on its geometrical profile, for which the resonant cavity mode must match the resonant wavelength of the antenna. In this work, TECNAs were designed to detect four different wavelengths in the mid- and long-wave IR and fabricated in a single chip, requiring different cavity profiles. Cavities were fabricated by isotopically etching Si using xenon difluoride (XeF2). XeF2 gas etches the Si substrate through circular windows patterned in resist. The antenna is patterned before the cavity is formed to avoid difficulties in filling and planarizing the supporting layer for antenna patterning and is left suspended over the cavity after etching. The different cavity profiles were formed in a single etch, since subsequent resist patterning and etching of unfilled cavities would destroy previous antennas. This work compares cavity profiles etched using both pulsed and continuous-flow XeF2 etch methods using different etch window sizes to produce variously sized and shaped cavities in a single etch. The cavity profiles were characterized using a laser-scanning confocal microscope. Piecewise equations made up of a torus, a cone, and a cylinder were fitted to the profiles and used in COMSOL simulations of electromagnetic standing-wave patterns within the cavity. Antennas were then chosen to have peak responses at wavelengths that match a simulated cavity's peak response.