Lihan Chen

A Micromachined Terahertz Waveguide 90

Waveguide twists are often necessary to provide polarization rotation between waveguide-based components. At terahertz frequencies, it is desirable to use a twist design that is compact in order to reduce loss; however, these designs are difficult if not impossible to realize using standard machining. This paper presents a micromachined compact waveguide twist for terahertz frequencies. The Rud-Kirilenko twist geometry is ideally suited to the micromachining processes developed at the University of Virginia. Measurements of a WR-1.5 micromachined twist exhibit a return loss near 20 dB and a median insertion loss of 0.5 dB from 600 to 750 GHz.

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An improved micromachined on-wafer probe covering frequencies 500-750 GHz is demonstrated in this paper to address sub-millimeter-wave integrated-circuit testing. Measurements of a prototype WR-1.5 micromachined on-wafer probe exhibit a return loss better than 12 dB and a mean insertion loss of 6.5 dB from 500 to 750 GHz. The repeatability of on-wafer measurements with the micromachined probe is investigated. Monte Carlo simulations are used to identify the dominant error source of on-wafer measurement and to estimate the measurement accuracy. The dominant error source is positioning error, which results in phase uncertainty. Reliability tests show the probe is robust and can sustain over 20 000 contacts.

Twist

Although progress has been made in the development of submillimeter-wave monolithic integrated circuits, the evaluation of these circuits still relies on test fixtures, which makes testing expensive and time consuming. Based on a W-band prototype, a micromachined on-wafer probe covering frequencies 500–750 GHz is built to simplify submillimeter-wave integrated circuits testing. This paper demonstrates the repeatability and the robustness of this terahertz micromachined on-wafer probe.

Terahertz Micromachined On-Wafer Probes: Repeatability and Reliability

An experimentally based technique for characterizing calibration uncertainty is presented. The approach described calculates uncertainty metrics at the output of the calibration processing chain as opposed to the input. In doing so, this method replaces the complexities of error propagation with the computational effort associated with performing numerous calibrations. Practical applications are demonstrated for a variety of scenarios focused on one-port calibration, illustrating the versatility of the technique. An open-source implementation has been made publicly available as part of the Python module scikit-rf.

Terahertz micromachined on-wafer probes: Repeatability and robustness

The accuracy of an on-wafer probe system operating at 625 GHz is analyzed. A weighted least squares analysis is applied to the calibration of a one-port measurement system to propagate the non-systematic errors introduced by probe contact and probe placement variation. The worst-case errors of the 625 GHz on-wafer probe system are found and the combined effects of the VNA extenders power drop-out at the low end of the band with the poor matching of the probe at the high-end results in the 625 GHz probe system being most accurate in the center of the WR-1.5 waveguide band. Between 560 and 625 GHz the worst case error is 0.26 in linear magnitude and 15° in phase for a 0 dB reflection.

An Experimental Technique for Calibration Uncertainty Analysis

This paper describes a new process for using room temperature liquid metals as the interconnect material for flip chip bonding. The proposed liquid metal interconnects are not susceptible to damage caused by thermo-mechanical stress and are therefore an attractive alternative to solid phase solder bumps. A new interconnect structure is presented involving a patterned underfill dielectric material to contain the liquid metal on the bottom carrier, and electroplated pins on the top chip to mate with the socket and make contact with the liquid metal. RF and DC measurements were performed on the liquid metal transition showing an insertion loss of ≪0.5dB up to 20 GHz.

Calibration accuracy of a 625 GHz on-wafer probe

A micromachined on-wafer probe has been designed to facilitate the development of integrated circuits in the 600-900 GHz frequency range. The probe tip is fabricated on a 5-micrometer thick high-resistivity silicon substrate using a silicon-on- insulator fabrication process. This letter updates previous work on WR-1.2 wafer probes and presents for the first time the full RF characterization of the probe. These are the first reported on-wafer measurements above 750 GHz.

Liquid metal vertical interconnects for RF flip-chip assembly

This paper presents the design, simulation and measurement of a vertical interconnect with mechanical fit for three-dimensional heterogeneous integration. The mechanical fit is a strategy employing interlocking SU-8 structures to transition between flip-chip style stacked chips through vertical CPW transmission lines. The mechanical fit is introduced in this paper to reduce flip-chip alignment difficulty and increase the reliability of the interconnects. This paper also describes a process for using pre-fabricated active ICs in a mechanical fit vertical configuration. Experimental results show excellent RF performance up to 50 GHz, with extremely low insertion loss (better than 0.25 dB at 40 GHz per transition). The transitions have been fabricated and tested for 380 ¿m-thick silicon substrates with passive components and experiments are being conducted on active components.

Characterization of Micromachined On-Wafer Probes for the 600–900 GHz Waveguide Band

This paper presents the design, simulation, and measurement of a vertical RF interconnect with mechanical fit for 3-D heterogeneous integration. The mechanical fit is a flip-chip strategy employing interlocking SU-8, an ultra-thick photoresist, structures to prevent misalignment during assembly and increase the reliability of the interconnects. To determine the electromagnetic characteristics, such as insertion loss and the coupling between face-to-face chips, different test structures were fabricated and measured. Experimental results show excellent RF performance up to 110 GHz with low insertion loss (better than 0.1 dB per transition at 40 GHz).