A Non-galvanic Chip-to-Waveguide Transition for Mm-Wave Characterisation Probes

Abstract

To solve extant complications with standard wafer-probing techniques, such as probe-to-probe coupling and probe-tip deterioration, a novel probe tip device has been designed and verified by means of 3D EM-simulation for the 220-325 GHz frequency band. The new probing technique uses an on-wafer cavity-backed slot to couple the signal to a tapered structure in an open-ended waveguide which is held above the cavity, and acts as a fully shielded transition from the transverse electromagnetic mode of the planar stripline to the fundamental transverse electric mode of the waveguide. The on-wafer structure used for the transition is limited to the back-end-of-line of the process, and is shielded from the ill-characterised substrate. The transition achieves an insertion loss of <1.8 dB across the entire 220-325 GHz band and does not require galvanic contact with the die. The layout for the on-wafer structure has been designed for a 0.25 µm SiGe process and a simplified version has been used for a simulated sample measurement using the thru-reflect-line calibration algorithm. Due to coupling to nearby structures, the novel probing technique show significant improvement over the standard coplanar probes only when a chip area greater than the cross-section of the open-ended waveguide is reserved. Simulated measurement of an independent passive structure show a reduction of the average worst case bound from 0.041 to 0.017 in the 220-325 GHz band.