Si Waveguide Technology for High Performance Millimeter-Wave/Terahertz Integrated Systems
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The terahertz (THZ) spectrum (0.3 – 3 THz) offers new opportunities to a wide range of emerging applications which demand high-quality THz sources, detectors, amplifiers, and integrated circuits. On-chip integration of planar transmission line passive components degrades their performance due to the conduction loss. Therefore, a hybrid integrated technology in which all of the high-quality passive components are implemented using a suitable off-chip planar integrated technology and the active devices are placed on-chip, has become the most promising approach. In this thesis, a low-cost and low-loss silicon-on-glass (SOG) integrated circuit technology is proposed for THz/millimeter-wave (mmW) applications. Highly-resistive intrinsic silicon (Si) is selected as the main guiding region due to its high transparency at mmW/THz frequency ranges and the maturity of Si-devices fabrication. In the proposed technology, all of the passive components and waveguide connections are made of highly-resistive Si on a glass substrate. The proposed technique leads to a high-precision and low-cost fabrication process, wherein the alignment between the sub-structures is automatically achieved during the fabrication process. This is performed by photolithography and dry etching of the entire integrated passive circuit layout through the Si layer of the SOG wafer. The SOG dielectric ridge waveguide, as the basic component of the SOG integrated circuit, is theoretically and experimentally investigated. A test setup is designed to measure propagation characteristics of the proposed SOG waveguide. Measured dispersion diagrams of the SOG dielectric waveguides show average attenuation constants of 0.63 dB/cm, 0.28 dB/cm, and 0.53 dB/cm over the frequency ranges of 55 – 65 GHz, 90 – 110 GHz, and 140 – 170 GHz, respectively. Extending the SOG platform toward the THz range is achieved by new SOG waveguide structures wherein the glass substrates below the Si channels are etched to reduce the effect of greater glass material loss at higher frequencies (i.e., > 200 GHz). To fabricate these structures, the glass substrate is etched in hydrophilic acid before bonding to the Si. Four new SOG configurations, called the suspended SOG, corrugated SOG, rib SOG, and U-SOG waveguides are proposed with their respective fabrication techniques for the THz range of frequencies. In the suspended SOG waveguide, a periodic configuration of Si beams supports the Si guiding channel over the etched grove on the glass substrate. Measurements of two suspended SOG waveguides show low attenuation constants of 0.031 dB/λ0 and 0.042 dB/λ0 (on average) over the frequency ranges of 350 - 500 GHz and 400 - 500 GHz, respectively. It is theoretically demonstrated that the rib SOG and U-SOG waveguides are promising candidates for THz high-density and low-loss integrated circuits. Rib SOG waveguide and U-SOG waveguide test devices are designed over the frequency bands of 0.8 – 0.9 THz and 0.9 – 1.1 THz. The proposed SOG waveguide technology can easily be extended to several THz with no limitations. A new mmW low-loss dielectric phase shifter integrated in the corrugated SOG platform is designed, fabricated, and measured. Phase shifts of 111 ° and 129 ° at frequencies of 85 GHz and 100 GHz, with maximum insertion losses of 0.65 dB and 2.5 dB, are achieved during measurements of the proposed phase shifter. Millimeter-wave integrated SOG tapered antennas are developed and implemented. The idea of a suspended SOG tapered antenna is demonstrated to enhance the radiation efficiency and the gain of the SOG tapered antenna over 110 – 130 GHz. The suspended SOG tapered antenna, which can function under two orthogonal mode excitations, shows measured efficiencies of higher than 90 % for the two vertical polarizations.