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1. Polylithic Integration for RF/MM-Wave Chiplets using Stitch-Chips: Modeling, Fabrication, and Characterization

   https://doi.org/10.1109/IMS30576.2020.9223887  

   Zheng, T.; Jo, P.; Rajan, S.; Bakir, M. (January 2020, IEEE MTT-S Int. Microw. Symp. (IMS))

   A polylithic integration technology is demonstrated for seamless stitching of RF and digital chiplets. In this technology, stitch-chips with compressible microinterconnects (CMIs) are used for low-loss and dense interconnection between chiplets. A testbed using fused-silica stitch-chips with integrated CMIs is demonstrated including modeling, fabrication, assembly, and characterization. A 500 µm-long stitch-chip signal link is measured to have less than 0.4 dB insertion loss up to 30 GHz. A simulated eye diagram for 1000 µm-long stitch-chip signal link has a clear opening at 50 Gbps data rate. Moreover, the S-parameters of the CMIs are extracted from this testbed and show less than 0.17 dB insertion loss up to 30 GHz. Benchmarking to silicon interposer based interconnection is also reported. 

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2. Low-Cost Sub-THz Linear to Circular Polarizer

   https://doi.org/10.1007/s10762-026-01157-w  

   Mutai, Kevin K; Stults, Chloe; Lei, Jeffrey; Fang, Zhaoji; Mittleman, Daniel M (July 2026, Journal of Infrared, Millimeter, and Terahertz Waves)

   We introduce an approach to produce inexpensive linear to circular polarizers for the sub-THz spectral regime with either left or right handedness. Our design consists of a cross-dipole array of slot antennas, fabricated on paper using a fast and inexpensive process. We demonstrate a prototype designed for a frequency of 200 GHz, showing a low axial ratio and acceptable insertion loss. 

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3. Sub-terahertz devices and test metrology

   Li, Lei; Asadi, Mohammad J; Li, Tianze; Wang, Xiaopeng; Hwang, James CM (September 2023, SRC)

   As 6G wireless communications push the operation frequency above 110 GHz, it is critical to have low-loss interconnects that can be accurately tested. To this end, D-band (110 GHz to 170 GHz) substrate-integrated waveguides (SIWs) are designed on a 100-μm-thick SiC substrate. The fabricated SIWs are probed on-wafer in a single sweep from 70 kHz to 220 GHz with their input/output transitioned to grounded coplanar waveguides (GCPWs). From CPW-probed scattering parameters, two-tier calibration is used to de-embed the SIW-GCPW transitions and to extract the intrinsic SIW characteristics. In general, the record low loss measured agrees with that obtained from finite-element full-wave electromagnetic simulation. For example, across the D band, the average insertion loss is approximately 0.2 dB/mm, which is several times better than that of coplanar or microstrip transmission lines fabricated on the same substrate. A 3-pole filter exhibits a 1-dB insertion loss at 135 GHz with 20-dB selectivity and 11% bandwidth, which is order-of-magnitude better than typical on-chip filters. These results underscore the potential of using SIWs to interconnect transistors, filters, antennas, and other circuit elements on the same monolithically integrated chip. 

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4. A Radio Frequency Comb Filter for Sparse Fourier Transform-Based Spectrum Sensing

   https://doi.org/10.1109/ULTSYM.2018.8579645  

   Lu, Ruochen; Manzaneque, Tomas; Yang, Yansong; Zhou, Jin; Hassanieh, Haitham; Gong, Songbin (October 2018, 2018 IEEE International Ultrasonics Symposium (IUS))

   This work demonstrates a passive low-insertion-loss (IL)RF filter with periodic passbands and capable of sparsifying the spectrum from 238 to 526 MHz for sparse Fourier transform (SFT)based spectrum sensing. The demonstrated periodic filter employs LiNbO 3 lateral overtone bulk acoustic resonators (LOBARs)with high-quality factors (Qs), large electromechanical coupling (k t 2 ), and multiple equally spaced resonances in a ladder topology. The fabricated LOBARs show k t 2 larger than 1.5% and figure of merits (k 2 Q) more than 30 for over 10 tones simultaneously and are both among the highest demonstrated in overmoded resonators. The multi-band filter centered at 370 MHz have then been obtained with a passband span of 291 MHz, a spectral spacing of 22 MHz, an IL of 2 dB, FBWs around 0.6%, and a sparsification ratio between 7 and 15. An out-of-band rejection around 25 dB has also been achieved for more than 14 bands. The great performance demonstrated by the RF filter with 14 useable periodic passbands will serve to enable future sparse Fourier transform-based spectrum sensing. 

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5. A Compact F-band Filter Based on SiC Substrate-integrated Waveguides

   https://doi.org/10.1109/APMC57107.2023.10439892  

   Wang, Xiaopeng; Asadi, Mohammad Javad; Li, Lei; Li, Tianze; Hwang, James_C M; Thome, Fabian; Brückner, Peter; Schwantuschke, Dirk (December 2023, IEEE)

   F-band substrate-integrated waveguides (SIWs) are designed, fabricated, and characterized on a SiC wafer, along with SIW-based filters, impedance standards, and transitions to grounded coplanar waveguides (GCPWs). The GCPW-SIW transitions not only facilitate wafer probing, but also double as resonators to form a 3-pole band-pass filter together with an SIW resonator. The resulted filter exhibits a 1.5-dB insertion loss at 115 GHz with a 34-dB return loss and a 19-GHz (16%) 3-dB bandwidth. The size of the filter is only 63% of previous filters comprising three SIW resonators. These results show the feasibility for monolithic integration of highquality filters with high-efficiency antennas and amplifiers in a single-chip RF frontend above 110 GHz, which is particularly advantageous for 6G wireless communications and nextgeneration automobile radars. 

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