Research on Wideband Terahertz On-Chip Antenna


Student thesis: Doctoral Thesis

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Award date18 Sept 2023


This thesis presents a series of wideband terahertz (THz) on-chip antennas and a one-port de-embedding method for extracting the dielectric constant of SiO2 in standard complementary metal-oxide-semiconductor (CMOS). On-chip antennas using CMOS technologies have triggered significant interest at high frequencies due to the advantages of full integration, overall system size reduction, lower cost, and high yield for massive production. It is regarded as a promising candidate in future fully integrated THz wireless communication systems such as imaging, defect detection, and short-range wireless communications. This thesis proposes different techniques to improve the antenna performance, including gain enhancement, bandwidth expansion, filtering response, and size miniaturization.

Firstly, a one-port de-embedding method is studied for extracting the dielectric constant of SiO2 in standard CMOS at terahertz frequencies. Instead of extracting dielectric constant using absolute parameters from one cavity resonator, the presented method extracts key information from the relative differences of three designated cavity resonators. By doing so, this method gains a unique de-embedding technique that can remove the effect of the feeding network and other repeatable errors, simplifying the calibration and increasing measurement accuracy. Furthermore, compared with conventional resonant methods requiring widely swept data or complex data processing, the proposed method only requires fL and QL, which can be directly measured even from a narrow frequency band. Therefore, it reduces the measurement requirement and lowers the detuned error.

Secondly, a wide impedance-bandwidth and gain-bandwidth terahertz on-chip antenna are demonstrated. The antenna employs the inherent silicon substrate in CMOS technologies as a rectangular dielectric resonator (DR). This inherent DR can be seen as a standard dielectric resonator antenna (DRA) or a magnetic-wall cavity with in-phase reflected waves from the ground. A versatile comb-shaped dipole antenna is designed above the inherent DR, functioning both as a feeder for the DRA and an independent radiator. With the proper DR and dipole design, hybrid-DR-cavity modes are simultaneously excited with multiple higher-order DR modes and the cavity mode. The hybrid modes greatly expand the bandwidth and improve the gain. Simulations show that the −10 dB impedance bandwidth and 3-dB gain band widths are 50% and 34.5%, while a peak gain of 6.5 dBi is achieved at 305 GHz. The fabricated sample shows a peak gain of 8.6 dBi and a maximum radiation efficiency of 44% at 295 GHz.

Thirdly, two 450-GHz on-chip dual-patch antennas with expanded bandwidth are investigated: one optimized for higher gain and the other aiming for filtering response. By using the dual-patch structure, both antennas achieve an impedance bandwidth of wider than 15 % under an extremely low profile of 0.013 λ0. Other techniques, such as I-shaped or C-shaped slots and open stubs, are also used for different purposes, including size miniaturization, improved design freedom, and filtering enhancement. Most importantly, using the topmost metal layer (TM1) as the patch and the bottommost metal layer (M1) as the ground, two proposed antennas are entirely separated from the active region with an integrated on-chip ground. The gain-optimized antenna, Ant#1, demonstrates a measured impedance bandwidth of approximately 15.9 %, from 428.3 to 500 GHz. Two distinct resonances are observed at 445 and 498 GHz, corresponding to TM10 and TM01 modes, respectively. The measured peak gain of Ant#1 is 3.1 dBi, occurring at 450 GHz. On the other hand, Ant#2, designed with an emphasis on filtering response, presents a measured impedance bandwidth of 15.3% and a peak gain of 1.6 dBi at 499 GHz, albeit with a slight decrement compared to Ant#1. Notably, Ant#2 offers an improved filtering response, with good out-of-band suppression and distinct radiation nulls.

Lastly, an innovative 425-GHz wideband on-chip sequential-phase-fed CP antenna is presented. Unlike traditional sequential-phase feed relying on the physical distance of the delay line to achieve phase progression, the presented unique sequential-phase feed uses highly compact reactance-loaded delay lines. This design exploits the multiple metal layers and their close distances in the standard CMOS BEOL process, strategically creating reactive components to shift the phase and reduce the length of the delay line. This method significantly miniaturizes the sequential phase feed network while achieving the desired sequential phases. The overall dimensions of the antenna are approximately 390 μm × 390 μm, equivalent to 0.55 λ0 × 0.55 λ0 at 425 GHz. This antenna has an extremely low profile of 8.8 µm, corresponding to 0.012 λ0 at 425 GHz. Measurements show an overlapped impedance-AR bandwidth of 19% and a peak gain of 1.2 dBic. Symmetrical and stable broadside LHCP radiation with low RHCP levels are seen at multiple frequencies from the measurement.

All on-chip antennas presented in this thesis achieve wide bandwidths and good radiation performances without any off-chip components. Therefore, these antennas are promising candidates for future fully integrated THz wireless communication systems.