Chessboard FPA in 130-nm SiGe BiCMOS for High-Resolution Passive Terahertz Imaging

This work presents a chessboard focal plane array (FPA) camera with state-of-the-art thermal and spatial resolution in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$200$</tex-math></inline-formula>–<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$600 \,\mathrm{G}\mathrm{Hz}$</tex-math></inline-formula> frequency range. The FPA is implemented in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$130$</tex-math></inline-formula>-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{n}\mathrm{m}$</tex-math></inline-formula> SiGe BiCMOS technology, where each antenna element is loaded with a direct detector based on heterojunction bipolar transistors (HBTs). The antenna and detector architecture, including the vias and biasing network, were optimized to achieve a noise-equivalent power (NEP) suitable for passive <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{T}\mathrm{Hz}$</tex-math></inline-formula> imaging. Overall, the estimated loss of the FPA is better than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4 \,\mathrm{d}\mathrm{B}$</tex-math></inline-formula> between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$350$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$600 \,\mathrm{G}\mathrm{Hz}$</tex-math></inline-formula>, of which <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.5 \,\mathrm{d}\mathrm{B}$</tex-math></inline-formula> is due to ohmic losses in the FPA, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \,\mathrm{d}\mathrm{B}$</tex-math></inline-formula> to mutual coupling between detectors, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0.7 \,\mathrm{d}\mathrm{B}$</tex-math></inline-formula> to the impedance mismatch between the detector and antenna. A prototype of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$24$</tex-math></inline-formula> pixels was manufactured and mounted on the base of a silicon hyperhemispherical lens with an anti-reflection coating. Excellent spatial resolution is achieved through a tight element spacing in the fabricated FPA, which is only half the wavelength in silicon at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$350 \,\mathrm{G}\mathrm{Hz}$</tex-math></inline-formula> and therefore consistent with the state-of-the-art. Its responsivity, noise, and radiation patterns were characterized using a quasi-optical measurement setup. The measured radiation patterns are within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \,\mathrm{d}\mathrm{B}$</tex-math></inline-formula> of simulations, demonstrating that the integrated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{T}\mathrm{Hz}$</tex-math></inline-formula> camera achieves excellent spatial resolution. Between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$330 \,\mathrm{G}\mathrm{Hz}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$500 \,\mathrm{G}\mathrm{Hz}$</tex-math></inline-formula>, the NEP was measured to be on the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10 \,\mathrm{p}\mathrm{W}\mathrm{/}\sqrt\text{Hz}$</tex-math></inline-formula>. When considering the entire operational band, this NEP results in a noise-equivalent temperature difference (NETD) of the camera is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1.6 \,\mathrm{K}$</tex-math></inline-formula> for an integration time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$1 \,\mathrm{s}$</tex-math></inline-formula> per pixel, which is comparable to the state-of-the-art. While <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{T}\mathrm{Hz}$</tex-math></inline-formula> detectors with state-of-the-art sensitivity are limited to single-pixel designs, the presented work combines a multi-pixel implementation with competitive sensitivity.