Four-Port Probe Stations and SOLR Calibration Standard Design up to 125 GHz on 28 nm CMOS

TL;DR

The work tackles the challenge of characterizing multiport RF front-ends at mmWave and sub-THz frequencies by presenting two commercial four-port probe stations (FFI and MPI) and a CMOS-based SOLR calibration standard capable of up to 125 GHz. It develops an SOLR-based calibration workflow, including a virtual mTRL process, to enable precise four-port S-parameter measurements and mitigate orthogonal-port calibration difficulties. The calibration die is taped out in UMC 28 nm CMOS with carefully engineered pads and interconnects to minimize parasitics, and simulations demonstrate robust performance and reciprocity across ports. Together, the four-port probing systems and the 125 GHz SOLR standard enable fast, accurate multiport RF characterization without repositioning probes, supporting next-generation multiport transceivers in mmWave/sub-THz regimes.

Abstract

This paper presents two innovative four-port probe stations developed by FormFactor Incorporated (FFI) and MPI Corporation (MPI), and a four-port calibration standard design up to 125 GHz for the probe stations. True four-port probing at mmWave and beyond does not yet exist, but is anticipated for future multi-band wireless devices using several antennas and RF chains. The four-port probe stations are housed in the THz measurement facility at NYU and allow simultaneous probing from East, West, North, and South orientations, which presents challenges for calibration. An on-chip Short-Open-Load-Reciprocal (SOLR) calibration (cal) standard is designed leveraging UMC's 28 nm CMOS process. S/O/L standard S-parameters are extracted using a virtual multiline Thru-Reflect-Line (mTRL) cal and used to validate SOLR cal performance via simulations up to 125 GHz. The novel probing solutions from MPI and FFI, along with the SOLR cal, open up considerable opportunities for precise RF characterization across wide frequency ranges.

Figures (5)

• Figure 1: Formfactor Inc. design of the four-port probing solution. Design includes three angled positioners and one flat North positioner.
• Figure 2: MPI Corporation design for the four-port probing platform. Zoomed-in views show direct mounting of waveguide probes onto frequency extenders.
• Figure 3: Simplified layout of the taped-out 4 mm $\times$ 4 mm SOLR cal chip. SOL standards line the die edges. Arced Thru (R) standard at corners. Diagonal and straight Thrus at chip center. Four SOLR cal set replicas included
• Figure 4: Simulated plots of s-parameters for the SOLR cal: (a) $S_{11}$ impedance of Open, Short, and Load; (b) $|S_{11}|$ of Load (left Y-axis); $|S_{11}|$ of Open and Short, $|S_{21}|$ of Orthogonal Thru (right Y-axis).
• Figure 5: Error correction results on diagonal Thru DUT: (a) $|S_{21}|$ ($<1.5$ dB) ; (b) $\angle S_{21}$. S,O,L standards evaluated w/ virtual mTRL (solid-blue line). Ideal standards used (dotted-black line). Raw uncorrected (dashed-red line)