Silicon-Integrated Arrays for Passive Terahertz Cameras

Abstract

The short wavelengths and wide available bandwidth in the terahertz (THz) regime make it an attractive frequency range for commercial, passive imaging applications. Fully exploiting these benefits requires low-cost THz imaging solutions with a high density of beams and excellent temperature sensitivity. Thanks to advancements in the temperature sensitivity of direct detectors integrated in processes such as CMOS and SiGe, commercially available silicon platforms have become the most promising candidates for a low-cost THz camera. Integrating these detectors in dense, large focal plane arrays (FPAs) operating over ultra-wide bandwidths would yield a camera capable of passive imaging with a diffraction-limited resolution. This thesis describes the design and characterization of arrays integrated in both CMOS and SiGe BiCMOS towards realizing such a passive THz camera.

To explore the realization of such a camera, we first extensively characterized a previously proposed chessboard FPA that was designed to achieve a near diffractionlimited resolution. This chessboard FPA was integrated with direct detectors based on Schottky diodes in 22-nm CMOS, and operates from 200 GHz to 600 GHz. Characterization was performed using an over-the-air measurement setup, enabling 2-D, multi-beam pattern characterization with a high dynamic range. Measurements of both the aperture efficiency and beam overlap between adjacent pixels demonstrated that the fabricated chessboard FPA improves gain at the edge-of-coverage by 1.2dB compared to an ideal hexagonal FPA of uniform feeds. The chessboard configuration therefore combines state-of-the-art focal plane sampling with minimal penalty in antenna efficiency.

Building on these results, we designed a quasi-optical system to validate the imaging performance of the CMOS-integrated FPA in a practical scenario. Since the Schottky diodes in the FPA did not enable passive imaging, active illumination of the sample was required to achieve sufficient imaging dynamic range. The quasi-optical system was optimized to ensure that the imaging resolution was dictated solely by the focal plane sampling in the chessboard FPA, while maintaining sufficient coupling between all pixels and the active source. Using this system, we performed an imaging demonstration of an ivy leaf hidden in an envelope, further emphasizing the state-of-the-art spatial resolution of the chessboard FPA.

While the CMOS-integrated chessboard FPA demonstrated excellent spatial resolution, its temperature sensitivity was insufficient for passive imaging. To overcome this limitation, we designed the chessboard FPA in a 130-nm SiGe BiCMOS process, utilizing heterojunction bipolar transistors (HBTs) to implement the direct detectors. The implemented chessboard FPA operates between 250 GHz – 600 GHz, and features detectors employing deeply saturated HBTs in a common-base configuration. The prototype was characterized using the quasi-optical system designed for the imaging demonstration. Measurements of the radiation patterns and responsivity showed good agreement with simulations, although the noise-equivalent power was higher than expected. Nevertheless, it is still competitive with the state-of-the-art and should enable passive THz imaging. The noise-equivalent temperature difference was estimated to be 1.6K for a 1 s integration time.

A critical challenge for passive THz cameras is low-frequency noise injected by the direct detectors. To address this, we proposed a solid-state chopper based on a reconfigurable periodic surface. The chopping operation is realized by electronically controlling the transmission through this surface, which is both more compact and faster than a mechanical chopping wheel, while preserving the spatial sampling of the chessboard FPA. The chopper was implemented in 130-nm SiGe BiCMOS and consists of sub-wavelength metal patches in a chessboard geometry loaded with varactor-connected MOSFETs. For characterization, a quasi-optical system was developed, consisting of two elliptical silicon lenses with the chopper located at the shared focus. Measurements showed that the chopper transmission was significantly lower than simulated. When using impedance measurements of a single MOSFET device to re-simulate the chopper, a considerably better match with simulations was obtained. To achieve the desired chopper performance, an alternative implementation was designed based on the measured impedance of an HBT-based load. Although this design has not yet been fabricated, simulations indicate it should enable passive imaging performance when combined with a state-of-the-art antenna-coupled direct detector. Since this design is based on device measurements, there is a high confidence that this proposed chopper design contributes towards the realization of a compact, passive THz camera with near diffraction-limited resolution.