# An Interlaboratory Comparison of On-Wafer S-Parameter Measurements up to 1.1 THz

Faisal Mubarak, Gia Ngoc Phung, Uwe Arz, Kamel Haddadi, Isabelle Roch-Jeune, Guillaume Ducournau, Thomas Flisgen, Ralf Doerner, Djamel Allal, Divya Jayasankar, et al.

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Faisal Mubarak, Gia Ngoc Phung, Uwe Arz, Kamel Haddadi, Isabelle Roch-Jeune, et al. An Inter-laboratory Comparison of On-Wafer S-Parameter Measurements up to 1.1 THz. IEEE Transactions on Terahertz Science and Technology, 2025, pp.1-16. 10.1109/TTHZ.2025.3537461. hal-04926307

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# An Interlaboratory Comparison of On-Wafer S-Parameter Measurements up to 1.1 THz

Faisal Ali Mubarak<sup>D</sup>, Member, IEEE, Gia Ngoc Phung<sup>D</sup>, Member, IEEE, Uwe Arz<sup>D</sup>, Senior Member, IEEE, Kamel Haddadi<sup>D</sup>, Member, IEEE, Isabelle Roch-Jeune, Guillaume Ducournau<sup>D</sup>, Member, IEEE, Thomas Flisgen<sup>D</sup>, Ralf Doerner<sup>D</sup>, Member, IEEE, Djamel Allal<sup>D</sup>, Divya Jayasankar<sup>D</sup>, Graduate Student Member, IEEE, Jan Stake<sup>D</sup>, Senior Member, IEEE, Robin Schmidt<sup>D</sup>, Member, IEEE, Gavin Fisher, Nick M. Ridler<sup>D</sup>, Fellow, IEEE, and Xiaobang Shang<sup>D</sup>, Senior Member, IEEE

**Abstract**—This article reports on an interlaboratory measurement comparison involving on-wafer S-parameter measurements from 10 GHz to 1.1 THz. Seven laboratories are involved, and each participant has measured an individual reference substrate fabricated from a high-resistivity silicon wafer in the same batch. One- and two-port co-planar waveguide (CPW) structures are designed, simulated, and fabricated. The measurements from 10 GHz to 1.1 THz, extending across six frequency bands, are conducted using

Received 13 September 2024; revised 2 January 2025; accepted 21 January 2025. Date of publication 30 January 2025; date of current version 28 April 2025. This work was supported in part by the EMPIR project under Grant 18SIB09 TEMMT, in part by the EPM project under Grant 23IND10 OnMicro, in part by the Dutch Ministry of Economic Affairs and Climate, in part by the Frech Renatech network, in part by the Contrat de Plan Etat Region (CPER) WAVETECH @HdF project and Haut de-France Regional council, IEMN UHD flagship project, in part by the IEMN CHOP platform, in part by the France 2030-PEPR FUNTERA and SYSTERA projects, funded by ANR (Agence Nationale de la Recherche) under Grant ANR-22-PEEL-0006 and Grant 22-PEFT-0006, in part by the French RENATECH network, and in part by the Equipex + Nanofutur under Grant IA-21-ESRE-0012. (Corresponding author: Faisal Ali Mubarak.)

Faisal Ali Mubarak is with the Electricity and Time Department of VSL, the National Metrology Institute of The Netherlands, 2629 JA Delft, The Netherlands (e-mail: fmubarak@vsl.nl).

Gia Ngoc Phung and Uwe Arz are with the Physikalisch-Technische Bundesanstalt (PTB), 38116 Braunschweig, Germany.

Kamel Haddadi is with the Université de Lille, CNRS, Centrale Lille, Université Polytechnic Hauts-De-France, UMR 8520 - IEMN - Institut d'Electronique de Microélectronique et de Nanotechnologie, F-59000 Lille, France.

Isabelle Roch-Jeune and Guillaume Ducournau are with the CNRS, UMR, Univ. Lille, 8520 Lille, France, and also with the IEMN - Institut d'Electronique de Microélectronique et de Nanotechnologie, 59652 Lille, France.

Thomas Flisgen is with the Leibniz-Institut für Höchstfrequenztechnik, Ferdinand-Braun-Institut gGmbH, 12489 Berlin, Germany, and also with the Brandenburgische Technische Universität Cottbus- Senftenberg, 03046 Cottbus, Germany.

Ralf Doerner is with the Leibniz-Institut für Höchstfrequenztechnik, Ferdinand-Braun-Institut gGmbH, 12489 Berlin, Germany.

Djamel Allal is with the Laboratoire national de métrologie et d'essais (LNE), 78197 Trappes Cedex, France.

Divya Jayasankar is with the Terahertz and Millimetre Wave Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden, and also with the Research Institutes of Sweden, SE-50462 Borås, Sweden.

Jan Stake is with the Terahertz and Millimetre Wave Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Gothenburg, Sweden.

Robin Schmidt is with Keysight Labs, 9051 Sint-Denijs-Westrem, Belgium.

Gavin Fisher is with Formfactor GmbH, 01561 Thiendorf, Germany.

Nick M. Ridler and Xiaobang Shang are with National Physical Laboratory, TW11 0LW Teddington, U.K.

Color versions of one or more figures in this article are available at
https://doi.org/10.1109/TTHZ.2025.3537461

Digital Object Identifier 10.1109/TTHZ.2025.3537461

different equipment in terms of vendors and specifications (e.g., probe pitch size). Despite such differences, this interlaboratory study has shown a generally good agreement between results from different participants when uncertainties are considered. The comparison with simulated reference values demonstrates agreement within 0.08 for $|S_{11}|$ and 2 dB for $|S_{21}|$ measurements of matched devices up to 1.1 THz. The measurement comparison demonstrates the need for a standardized measurement approach and, with that, a potential to achieve accurate on-wafer CPW measurements up to THz frequencies, underpinning the development of integrated circuits for such high frequencies.

**Index Terms**—Calibration, comparison, coplanar waveguides (CPWs), on-wafer, S-parameter measurements, terahertz metrology.

## I. INTRODUCTION

THE ability to sense terahertz waves in a chip-scale technology has the potential for transformative applications in sensing [1], imaging [2], and security [3]. Operational frequencies between 100 and 300 GHz are expected to meet the large bandwidth requirements necessary for supporting individual user data rates of up to 100 Gbit/s envisioned for 6th generation (6G) networks [4]. All these applications require devices and integrated circuits operating at these high frequencies, with semiconductor foundries demonstrating active devices with operating frequencies exceeding 1 THz [5], [6].

The measurement infrastructure for on-wafer device characterization is critical for developing such novel devices and systems operating beyond 1 THz. Commercially available measurement equipment for Terahertz characterization is increasing steadily, with frequency extender heads for vector network analyzers (VNAs) [7], on-wafer calibration kits [8], and ground-signal-ground (GSG) probes [9] as essential components. The complexity and diversity in developing such Terahertz testbeds make it increasingly difficult to establish accurate on-wafer measurements up to the WR 1.0 band. This is also evident from the lack of traceability for on-wafer coplanar waveguide (CPW) measurements above 118 GHz, with PTB as the only National Metrology Institutes (NMIs) worldwide having CMC's for on-wafer measurements [10].

The literature review reveals increasing efforts to develop measurement techniques for on-wafer S-parameter characterization in the millimetre-wave and sub-millimetre-wave range.
Williams demonstrated sub-millimetre wave transistor characterization by developing a dedicated calibration kit for the desired technology [11], an approach widely employed in on-wafer device characterization. Recent works have explored novel techniques and devices for WR 1.0 band characterization using planar goubau lines [12], [13]. A multiline-thru-reflect-line (mTRL) calibration up to 1 THz was demonstrated with inverted grounded-CPW devices in an InP HBT Process [14]. Likely, traceable CPW measurements on fused silica substrates [15] and membrane technology [16] have been demonstrated for frequencies up to 110 GHz, including a comprehensive uncertainty budget for mTRL on-wafer measurements, dominated by uncertainties arising from the reference calibration standards [15]. While [7], [17] demonstrated waveguide measurement up to WR 1.0 band and investigated the impact of repeatability and cable movements on the combined measurement uncertainty. Furthermore, other studies [18], [19] have focused on investigating the impact of drift error due to the frequency extender heads for the WR 1.0 band. Consequently, [20] recently demonstrated an on-wafer capacitor characterization up to 1.0 THz supported with uncertainty estimates.

Measurement comparisons are fundamental for the validation of such uncertainty budgets and provide increased confidence in corresponding measurements. Interlaboratory comparisons in on-wafer measurements have highlighted several considerations necessary for comparable on-wafer RF device characterization [21], [22], [23], [24], [25] such as strong dependency on the probe topology, chuck material, environmental conditions, measurement hardware, and operator skills [21].

In this effort, this work gathers five NMIs, two European universities, one research institute, and one leading manufacturer of millimetre-wave and THz test and measurement instrumentation to provide a comprehensive on-wafer ultra-broadband 10 GHz-1.1 THz CPW measurement comparison. To the best of the authors' knowledge, this is the first interlaboratory comparison conducted over a frequency range spanning up to 1.1 THz. Where initial results were made available in [26], this work offers valuable and detailed insights into the expected accuracy of on-wafer measurements and EM simulation tools at high frequencies. The manuscript is organized as follows. Section II presents the design and fabrication process of the reference substrates. A wide range of calibration and validation standards are designed and fabricated to support increased calibration and verification accuracy. In addition, a parametrization study is presented to determine electrical reference values and related uncertainties. Then, Section III proceeds with describing the simulation-based approach for reference values and uncertainties evaluation of the devices used during the interlaboratory comparison. Subsequently, measurement details crucial for organizing a comparison are provided in Section IV. Finally, Section V presents the measurement campaign results and related discussions.

## II. REFERENCE SUBSTRATE FABRICATION

### A. Device Selection and Design

A set of calibration and validation CPW structures has been designed on high-resistivity silicon (HR Si) wafer to establish

<figure><img src="image_2.png" /><figcaption>Fig. 1. A 2-D mapping of the silicon oxide thickness with an average of 53 and 51 nm for both wafers, respectively.</figcaption></figure>

metrology-grade accuracy in planar S-parameter measurements. The selection of the utilized technology is based on its availability. The reference substrate consists of a low-frequency set (kit 1) including 35 devices operating from 10 to 330 GHz and a high-frequency set (kit 2) with also 35 devices operating from 330 GHz to 1.1 THz, as listed in Table I. Two separate sets are designed to ensure there is one well-defined fundamental propagation mode to ensure accurate implementation of mTRL calibration. To ensure single mode of propagation, the following condition is ensured in the design of CPW devices:

$$f_{\max} < \frac{c_0}{10d\sqrt{\epsilon_{r,eff}}}. \qquad (1)$$

Here, $f_{\max}$ is 330 GHz for kit-1 devices, and 1100 GHz for kit-2 devices. Furthermore, $d$ is the ground-to-ground distance of the CPW devices in kit-1 and kit-2, and $c_0$ is the speed of light. Finally, $\epsilon_{r,eff}$ is the effective dielectric permittivity of the substrate.

Each set includes eight transmission lines with varying lengths ranging between 610 and 4280 µm for kit 1 and between 120 and 1320 µm for kit 2. Besides transmission lines, both kits include multiple offset shorts, opens, matched loads, and fixed-distanced two-port structures embedding two one-port devices. This variety of devices allows several calibration techniques to be implemented, i.e., TRL, mTRL, unknown thru (SOLR), and 16-term methods. Furthermore, identical access structures are included in the design of every device to allow for an accurate definition of the measurement reference plane and to realize consistent and accurate measurement results.

### B. Fabrication Process

Two wafers have been manufactured, each with eight identical dies, including kit 1 and kit 2 structures. The microfabrication process on a 3-inch (76.2 ± 0.3 mm) high resistivity (>5000 Ω/cm) silicon (Si) wafer from Siltronix, with 275 µm (±15 µm) thickness, started with thermal oxidation of the wafer in a furnace to produce a 50 nm layer of stable silicon oxide shown in Fig. 1. After deoxidation with Hydrogen Fluoride (HF), a 50 nm dry thermal oxide (TOX) was grown by APCVD (atmospheric pressure chemical vapour deposition) at 1100 °C. Then, a resist bilayer was spin-coated and patterned using e-beam lithography with a Nano Maker EBPG-5000 Plus (Vistec)
<table><caption>TABLE I
OVERVIEW OF CPW DEVICES IN KIT 1 AND KIT 2</caption><thead><tr><th>Structure</th><th>Physical length (μm)</th><th>Ground type</th><th>Structures in each kit</th><th>Purpose</th></tr></thead><tbody><tr><td>Thru</td><td>kit 1: 500<br>kit 2: 120</td><td>NA</td><td>3</td><td>system calibration</td></tr><tr><td>Lines</td><td>kit 1: 610 - 4280<br>kit 2: 155 - 1320</td><td>NA</td><td>8</td><td>system calibration</td></tr><tr><td>Short-Short</td><td>kit 1: 944<br>kit 2: 340</td><td>Ground Connected & Ground Open</td><td>1 & 1</td><td>system calibration</td></tr><tr><td>Mismatched line</td><td>kit 1: 1200, 2940<br>kit 2: 342, 894</td><td>NA</td><td>2</td><td>Interlaboratory comparison</td></tr><tr><td>Attenuator</td><td>kit 1: 944<br>kit 2: 340</td><td>NA</td><td>1</td><td>Interlaboratory comparison</td></tr><tr><td>Filter</td><td>kit 1: 944<br>kit 2: 340</td><td>NA</td><td>1</td><td>Interlaboratory comparison</td></tr><tr><td>Short-Short</td><td>kit 1: 0, 1044, 1264, 1444<br>kit 2: 0, 380, 450, 520</td><td>GC & GO</td><td>4 & 4</td><td>Interlaboratory comparison</td></tr><tr><td>Open-Open</td><td>kit 1: 944<br>kit 2: 340</td><td>Ground Connected & Ground Open</td><td>1 & 1</td><td>Probe crosstalk</td></tr><tr><td>Load-Load</td><td>kit 1: 944<br>kit 2: 340</td><td>Ground Connected & Ground Open</td><td>1 & 1</td><td>Probe crosstalk</td></tr><tr><td>Load-Open</td><td>kit 1: 944<br>kit 2: 340</td><td>Ground Connected & Ground Open</td><td>1 & 1</td><td>Probe crosstalk</td></tr><tr><td>Open-Short</td><td>kit 1: 944<br>kit 2: 340</td><td>Ground Connected & Ground Open</td><td>1 & 1</td><td>Probe crosstalk</td></tr><tr><td>Load-Short</td><td>kit 1: 944<br>kit 2: 340</td><td>Ground Connected & Ground Open</td><td>1 & 1</td><td>Probe crosstalk</td></tr></tbody></table>

for the definition of thin-layer resistors. Following exposure, the resist was developed to remove the exposed areas and create the desired pattern. Subsequently, a 23 nm Titanium (Ti) layer (measured mean value) was evaporated, and the lift-off process was performed to form the set of resistors. Afterward, spin-coating, alignment, and e-beam lithography of a new resist bilayer were conducted to define the CPW structures. The resist was again developed to ensure clean patterning before metal deposition. Finally, a 25 nm adhesion Ti layer and a Gold (Au) layer were deposited and lifted off (resulting in a Ti/Au average thickness of 485 nm). Eight identical reference dies were fabricated on the same wafer. In Fig. 2, Scanning Electron Microscope (SEM) images of the reference substrate are shown.

### C. Parameterization and Characterization

Relevant parameters affecting the electrical properties of the reference structures are identified and measured to assess high-frequency behavior through 3-D electromagnetic (EM) simulations, as outlined Section III. First, before the fabrication of the reference substrate, the sheet resistance was calibrated on a 0.25-in. gallium arsenide (GaAs) wafer to achieve an experimental value of 50.5 Ω. Then, after the fabrication of the reference kits, a mechanical profilometer was used to measure Ti and Ti/Au thicknesses corresponding to the resistive and conductive layers, respectively. The sheet resistance was measured using a noncontact resistivity measurement technique. A Tescan Mira XMU SEM was used to measure the critical dimensional parameters of CPW structures. For each of the eight dies, 15 parameters of CPW devices from kit 1 and kit 2 have been characterized. The images were processed offline to limit electron charging by the SEM, as shown in Fig. 2. The uncertainty for each parameter

<table><caption>TABLE II
NOMINAL VALUES AND UNCERTAINTIES OF THRU DEVICE PARAMETERS FROM KIT 1 AND KIT 2</caption><thead><tr><th>Parameter</th><th></th><th>kit 1 value</th><th>kit 2 value</th><th>Uncertainty</th></tr></thead><tbody><tr><td rowspan="6">Width (μm)</td><td>A, A'</td><td>100</td><td>29.2</td><td>2-4 %</td></tr><tr><td>D, D'</td><td>54</td><td>17.4</td><td>2-4 %</td></tr><tr><td>G, G'</td><td>100</td><td>29.2</td><td>2-4 %</td></tr><tr><td>C</td><td>25</td><td>9.0</td><td>2-4 %</td></tr><tr><td>F</td><td>11</td><td>3.5</td><td>3-4 %</td></tr><tr><td>I</td><td>25</td><td>9.0</td><td>2-4 %</td></tr><tr><td>Length (μm)</td><td>L</td><td>500</td><td>120</td><td>2 %</td></tr><tr><td rowspan="3">Spacing (μm)</td><td>B, B'</td><td>15.96</td><td>6.3</td><td>3-5 %</td></tr><tr><td>E, E'</td><td>8</td><td>2.6</td><td>3-5 %</td></tr><tr><td>H, H'</td><td>15.96</td><td>6.3</td><td>3-5 %</td></tr><tr><td rowspan="2">Substrate</td><td>ε<sub>r</sub></td><td>11.80</td><td>11.80</td><td>3 %</td></tr><tr><td>T<sub>Si</sub> (μm)</td><td>275</td><td>275</td><td>5.5 %</td></tr><tr><td>Thickness (μm)</td><td>T<sub>gold</sub></td><td>0.525</td><td>0.525</td><td>4 %</td></tr></tbody></table>

across eight dies is determined, as shown in Table II for the thru structure. The values obtained for the CPW dimensions were taken into account in the uncertainty estimation.

### III. REFERENCE SUBSTRATE EVALUATION

This section outlines the method for evaluating the reference values and uncertainties corresponding to the devices used for the interlaboratory comparison. First, EM simulations in CST Studio Suite are employed to evaluate the S-parameters corresponding to each CPW device from kit 1 and kit 2, as listed in Table I. However, the fabrication process tolerances, as shown in Table II, introduce variations in the S-parameters of the CPW devices. The dimensional and material parameter variations resulting from the fabrication process tolerances are propagated via EM simulations using the method proposed in [27] to quantify the S-parameter uncertainty.
<figure><img src="image_3.png" /><figcaption>Fig. 2. SEM image of (a) reference substrate showing a section including four CPW structures from kit 1 and (b) the thru standard from kit 2 including various device parameters measured for uncertainty evaluation.</figcaption></figure>

The second step involves evaluating the reference value and combined uncertainty for each device resulting after the calibration. The fabrication process uncertainty of every calibration device contributes to the combined uncertainty corresponding to the devices used in the comparison. For this, Monte-Carlo simulations are employed to evaluate the combined uncertainty of each comparison device.

## A. Simulation Approach

To calibrate each CPW device and determine the combined uncertainty, EM simulations are employed to acquire the necessary data for all CPW devices embedded in kit-1 and kit-2. First, the so-called uncorrected (RAW) data is acquired with EM simulation of all structures, including advanced probe models designed for kit-1 and kit-2 operation. Dedicated probe models are designed for kit-1 and kit-2 structure simulations. In Fig. 3(a), for brevity, only the probe model for kit-1 is shown. The probe is positioned to emulate realistic measurement conditions to the device, as it would during an actual on-wafer measurement.

<figure><img src="image_4.png" /><figcaption>Fig. 3. (a) Acquisition of EM simulation-based raw data for the calibration standards. (b) Acquisition of EM simulation-based reference data for the calibration standards at the reference plane position.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. S-parameter variations for kit-1 short structure estimated with EM simulations. The S-parameter results acquired for the various parameter settings are normalized to the S-parameter results acquired for nominal parameter values.</figcaption></figure>

Different combinations of parameter variations, resulting from the fabrication process tolerances shown in Table II, are then propagated through these EM simulations to determine a configuration that produces the maximum deviation from nominal S-parameter results. The S-parameter deviation from the nominal values of a kit-1 offset-short structure is shown in Fig. 4 for assorted combinations of parameter variations. The configuration with the most significant offset in S-parameters, denoted with the combination (max), is subsequently used to acquire the uncorrected S-parameter and uncertainty of all devices in kit-1 and kit-2. In Table III, a summary of CST Studio Suite simulation settings is shown. It was not possible to measure all
<table><caption>TABLE III
SUMMARY OF CST STUDIO SUITE SIMULATION SETTINGS</caption><thead><tr><td>Parameter</td><td>Value</td></tr></thead><tbody><tr><td>Background</td><td>Normal (vacuum)</td></tr><tr><td>Boundaries</td><td>Open boundary</td></tr><tr><td>Solver</td><td>Frequency domain</td></tr><tr><td>Solver order</td><td>3rd order</td></tr><tr><td>Mesh type</td><td>Hexahedral mesh</td></tr><tr><td>Mesh resolution</td><td>&gt; 1·10<sup>6</sup> cells</td></tr><tr><td>Accuracy</td><td>1·10<sup>-4</sup></td></tr><tr><td>Excitation</td><td>Waveguide ports</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Fig. 5. Monte-Carlo simulation setup used to propagate wafer fabrication tolerances for mTRL calibration.</figcaption></figure>

dimensions of every structure throughout the eight calibration dies manufactured for the comparison participants, and only 15 selected dimensions on each die were measured. Hence, due to the lack of information, the configuration with the most significant offset is used to estimate device uncertainty instead of the most probable combination.

## B. Reference S-Parameter & Uncertainty

Using the uncorrected S-parameter results acquired in the previous section, the corrected S-parameter values for all devices are determined using the mTRL calibration method. For this, first, S-parameters of only the TRL calibration devices are acquired with EM simulation at the pre-defined reference plan position, as shown in Fig. 3(b). Once the reference data of the calibration devices is acquired, the uncorrected data of all devices is corrected using the TRL calibration method. Simultaneously, the combined uncertainty of each device is estimated using the Monte-Carlo approach, as shown in Fig. 5. In total, 1000 samples were used for each frequency point during the Monte-Carlo simulations, which were found sufficient during the Monte-Carlo verification experiment.

The corrected simulation results, referred to as reference values, for all comparison devices, are shown in Fig. 6. The blue lines depict the reference value for each device, and the corresponding uncertainty is identified with the blue-colored region. The reference value uncertainty accounts for the fabrication process uncertainty of every calibration device used for correcting the comparison device S-parameters and the probe crosstalk uncertainty detailed in Section V-A. The uncertainty of

the EM simulation itself is not accounted for in this uncertainty analysis, which can typically range up to 5% or even higher for 3-D structures, as it depends on several factors, i.e., solver type, mesh quality, material properties, and frequency considerations [28]. The simulation accuracy most likely degrades with increasing frequencies due to the lack of material accuracy at higher frequencies.

## IV. MEASUREMENT DETAILS

This section outlines the interlaboratory measurement comparison details for on-wafer S-parameter measurements up to 1.1 THz. The comparison covers an extensive frequency range, including six frequency bands collectively supported by all participants. The measurement comparison participants and details of the various measurement systems are provided in Section IV-A. Detailed information on the comparison devices and the corresponding measurement parameters are provided in Section IV-B. The most critical part of a measurement comparison is the reference value corresponding to each parameter being compared. Description of the reference value and uncertainty corresponding to the comparison devices are provided in Section IV-C. The reference substrate is designed to support mTRL calibration to support the extensive frequency range of the comparison, with more information provided in Section IV-D. Finally, Section IV-E summarizes uncertainty sources involved in on-wafer measurements and details of the combined measurement uncertainty supported by PTB measurements.

### A. Participants

Each participant is provided with an individual reference substrate due to the destructive nature of on-wafer measurements. Each reference substrate includes two kits, with kit-1 supporting measurements in the coaxial frequency range up to 110 GHz, the WR6, WR5, and WR3 waveguide bands. Whereas kit-2 supports measurements in the WR2, WR1.5, and WR1 waveguide bands. Section II provides detailed information on the reference substrate design and fabrication. The measurements for kit 1 range from 10 to 330 GHz and are conducted by the National Physical Laboratory (NPL), Physikalisch-Technische Bundesanstalt (PTB), Ferdinand-Braun-Institut (FBH), and Keysight Technologies (Keysight). Whereas measurements for kit 2 range from 330 GHz to 1.1 THz and are conducted by the University of Lille (Lille), Chalmers University of Technology (Chalmers), and Form Factor. Table IV shows an overview of the measurement systems used by the participants.

### B. Comparison Devices

The selection of the comparison devices is such to provide coverage across the match and highly mismatched loading conditions for the reflection coefficient ($S_{11}$) parameter. Therefore, two one-port devices are selected to compare the $S_{11}$ parameter: a matched load and a highly mismatched open device. For the transmission ($S_{21}$) parameter, the selection of the devices enables comparison across a broad transmission parameter dynamic range. Hence, a mismatched transmission line and an
<figure><img src="image_7.png" /><figcaption>Fig. 6. Comparison device reference values determined by the median of the measurement results for kit 1 and kit 2. Data depicted with round markers correspond to kit 1 devices, and data using square markers belong to kit 2 devices.</figcaption></figure>

<table><caption>TABLE IV
DETAILS OF THE MEASUREMENT SYSTEMS USED BY THE PARTICIPANTS</caption><thead><tr><th>Frequency band</th><th>Laboratory</th><th>VNA</th><th>Probe (pitch)</th><th>Frequency extender</th><th>Chuck</th><th>Probing system</th></tr></thead><tbody><tr><td>coaxial</td><td>Keysight FBH</td><td>Keysight PNA-X 5247B<br>Anritsu MS4647B</td><td>Infinity (100 µm)<br>T220A (50 µm)</td><td>N5295A (M4)<br>Anritsu MA25400A</td><td>Ceramic<br>Ceramic</td><td>Form Factor CMX300<br>Cascade PA200</td></tr><tr><td>WR6<br>WR5</td><td>Keysight FBH<br>NPL<br>PTB</td><td>Keysight PNA-X N5292A<br>Anritsu MS4647B<br>Keysight PNA-X N5247B<br>Anritsu VectorStar</td><td>DMPI (50 µm)<br>T220A (50 µm)<br>GGB (75 µm)<br>GGB (50 µm)</td><td>VDI Mini<br>Anritsu MA25400A<br>VDI<br>VDI</td><td>Ceramic<br>Ceramic<br>Ceramic<br>Ceramic</td><td>Form Factor CMX300<br>Cascade PA200<br>MPI TS-150<br>MPI TS-150</td></tr><tr><td>WR3</td><td>Keysight PTB</td><td>Keysight PNA-X N5292A<br>Anritsu VectorStar</td><td>DMPI (50 µm)<br>GGB (50 µm)</td><td>VDI Mini<br>VDI</td><td>Ceramic<br>Ceramic</td><td>Form Factor CMX300<br>MPI TS-150</td></tr><tr><td>WR2</td><td>LILLE<br>FormFactor</td><td>Rohde & Schwarz ZVA24<br>Keysight PNA</td><td>T-Wave (25 µm)<br>T-Wave (25 µm)</td><td>Rohde & Schwarz Z500<br>VDI</td><td>Absorber<br>Ceramic</td><td>FormFactor EPS200<br>FormFactor Summit 200</td></tr><tr><td>WR1.5</td><td>LILLE</td><td>Rohde & Schwarz ZVA24</td><td>T-Wave (25 µm)</td><td>VDI VNAX</td><td>Absorber</td><td>FormFactor EPS200</td></tr><tr><td>WR1</td><td>LILLE<br>Chalmers<br>FormFactor</td><td>Rohde & Schwarz ZVA24<br>Keysight PNA-X N5242A<br>Keysight PNA</td><td>T-Wave (25 µm)<br>T-Wave (25 µm)<br>T-Wave (25 µm)</td><td>Rohde & Schwarz ZC1100<br>VDI<br>VDI</td><td>Absorber<br>Absorber<br>Ceramic</td><td>FormFactor EPS200<br>MPI TS-150<br>FormFactor Summit 200</td></tr></tbody></table>

attenuator device are selected. Both the magnitude and phase parameters are considered in the comparison. All comparison devices have different electrical characteristics compared to calibration standards used for the mTRL calibration employed to correct the data and, therefore, are appropriate for verification and comparison. In Table V, an overview of the comparison devices and the corresponding parameter is provided.

<table><caption>TABLE V
AN OVERVIEW OF THE COMPARISON DEVICES AND PARAMETERS</caption><thead><tr><th>Device</th><th>Parameter</th><th>Unit</th></tr></thead><tbody><tr><td>Matched load (1-port)</td><td>S<sub>11</sub></td><td>Linear magnitude</td></tr><tr><td>Open (1-port)</td><td>S<sub>11</sub></td><td>Linear magnitude</td></tr><tr><td>Attenuator (2-port)</td><td>S<sub>21</sub></td><td>Log magnitude (dB)</td></tr><tr><td>Mismatched line (2-port)</td><td>S<sub>21</sub></td><td>Log magnitude (dB)</td></tr><tr><td>Mismatched line (2-port)</td><td>S<sub>21</sub></td><td>Phase (degrees)</td></tr></tbody></table>
### C. Comparison Reference Value

The interpretation of the comparison reference value is preferably based on traceable measurement results acquired during the comparison using methods as outlined in [29]. However, as only PTB provided the uncertainty estimates corresponding to their measurements, it is not possible to determine the reference values for the comparison devices using participant measurement results. Hence, the reference value and the corresponding uncertainty for each comparison device shown in Table V are determined with EM simulations using methods outlined in Section III.

### D. Calibration Method

The mTRL method is widely regarded as one of the most accurate on-wafer calibration techniques [30]. For instance, in the interlaboratory study [31] performed by three laboratories in the 140–220 GHz frequency band with different calibration approaches (Short-Open-Load-Thru (SOLT), on-wafer mTRL and off-wafer TRL) reliable results could only be obtained by on-wafer mTRL calibration. On-wafer mTRL currently represents the best choice for calibrations of millimetre-wave and THz VNA measurement systems. Hence, the mTRL method is employed in the measurement comparison campaign. Participants were allowed to use a different combination of eight transmission lines for the mTRL calibration of their system, as shown in Table I.

It is common practice to verify the quality of the calibration using the measured complex propagation constant or the measured complex effective relative dielectric constant of the transmission lines. Higher-order modes of propagation and probe cross-talk may be observable, allowing us to relate these irregularities to the discrepancies observed in calibration results. Unfortunately, we did not collect this data during the interlaboratory comparison. In addition, the participants calibrated their measurements based on different implementations of the mTRL algorithm [32], [33], [34]. A more particular case is Keysight, whose results were calibrated based on an optimal mTRL with additional probe misplacement compensation as realized in [35].

All measurement results presented in this interlaboratory comparison correspond to a reference impedance defined by the characteristic impedance of the transmission line standards. Therefore, the characteristic impedance $Z_0$ was not computed, and no renormalization to 50 Ω was applied to the corrected results. Consequently, any discrepancies related to the reference impedance will most likely arise from the variations observed between different calibration substrates.

### E. Measurement Uncertainty

Several types of uncertainty sources generally stem from external and internal disturbances in on-wafer measurements. Internal effects originate from the transmission lines, i.e., radiation and dispersion effects. To incorporate the internal disturbances from radiation or dispersion effects, the uncertainty estimation has been revised to implement the new improved CPW model [36]. External effects, on the other hand, stem from disturbances such as e.g., the influence of microwave probes, the impact

<table><caption>TABLE VI
OVERVIEW OF UNCERTAINTY SOURCES ACCOUNTED FOR BY THE PTB MEASUREMENTS AND SIMULATED REFERENCE VALUES</caption><thead><tr><td>Uncertainty source</td><td>PTB measurements</td><td>Simulated values</td></tr></thead><tbody><tr><td>calibration standards</td><td>yes</td><td>yes</td></tr><tr><td>system noise</td><td>yes</td><td>no</td></tr><tr><td>system linearity</td><td>yes</td><td>no</td></tr><tr><td>system drift</td><td>yes</td><td>no</td></tr><tr><td>cable stability</td><td>yes</td><td>no</td></tr><tr><td>cross-talk</td><td>yes</td><td>yes</td></tr><tr><td>contact repeatability</td><td>yes</td><td>no</td></tr></tbody></table>

of the neighbourhood, propagation of parasitic modes (e.g., surface waves and parallel plate modes) due to the measurement boundary condition and radiation effects from the probes. Due to the complexity of the nature of these effects, a series of studies has been performed to allow for a better understanding of these effects [37], [38], [39].

With the benefit of hindsight, the layout of both kits has been optimized to mitigate the influence of these uncertainty sources as much as possible. Thus, the remaining uncertainty sources stem from the VNA, cables, and frequency extenders which can still be estimated according to the guide for VNA-based measurement methods [40].

The uncertainties corresponding to the PTB measurements are calculated according to the comprehensive measurement model and the procedure described in [15], making use of the improved CPW model of [36]. The PTB uncertainty budget takes into account instrumentation errors such as VNA noise, linearity, drift, cable stability, contact repeatability, uncertainties from crosstalk between the measurement ports, and CPW calibration standard uncertainties. Due to differences in the measurement equipment used by the participants, these uncertainties are valid for the PTB measurements only, which were performed in D-band and J-band, respectively. Table VI shows an overview of the uncertainty sources supported by the PTB and simulation uncertainty analysis.

Furthermore, participants did not provide details corresponding to the exact combination of transmission lines used for the implementation of their mTRL calibration. Hence, no fixed and optimum combination of transmission lines was set, and additional measurement errors may have been introduced if delays between each pair of transmission lines had not been well chosen by the participants. For example, PTB used all eight transmission lines to implement the mTRL calibration.

## V. MEASUREMENT RESULTS AND DISCUSSION

This section presents the interlaboratory comparison measurement results for kit 1 and 2 devices for frequencies up to 1.1 THz. The measurement results submitted by all participants for the comparison devices are shown in Fig. 6. Here, measurement results for kit 1 and kit 2 devices are denoted with round and square markers, respectively. The results are collected across three frequency bands with kit 1 devices measured from 10 up to 330 GHz. While measurement results for kit 2 devices range from 330 GHz to 1.1 THz and are accumulated across three waveguide bands. In Table IV, participant details are provided for each frequency band.
<figure><img src="image_8.png" /><figcaption>Fig. 7. Probe crosstalk measured with two-port devices comprising of two one-port structures as shown in Table I, i.e. Load-Open (LO), Open-Short (OS), and Load-Short (LS). Data depicted with markers are estimated using the set of structures without a common ground plane, and data shown with dotted lines are for structures with a common ground plane.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Comparison results for reflection coefficient measurements of one-port load and open from kit 1 and kit 2.</figcaption></figure>

Comparison results are analyzed by calculating the difference between the measurement, and the simulation-based reference value for each participant, as shown in Figs. 8 and 9. First, the probe crosstalk measurements are evaluated, and then the measurement results for each frequency band are separately discussed.

## A. Results for Probe Crosstalk

The probe crosstalk is estimated by taking the maximum value acquired for the $|S_{21}|$ and $|S_{12}|$ measurement results acquired for devices comprising of two one-port equidistant structures

as listed in Table I. This includes all devices identified with “probe crosstalk,” i.e., Load-Open (LO), Open-Short (OS), and Load-Short (LS). The devices are designed to maintain a constant distance between the probes; for kit-1, the two one-port devices are distanced by 944 µm, and for kit-2, the distance is 340 µm. Furthermore, each set of devices is manufactured twofold: a set with both devices sharing a common ground and a set with both devices having an isolated ground. The selection of devices is such to offer a wide range of loading conditions to the probes and allows the compilation of crosstalk performance for a variety of different probe types, as shown in Table IV.
<figure><img src="image_10.png" /><figcaption>Fig. 9. Comparison results for transmission coefficient measurements of two-port attenuator and mismatched line from kit 1 and kit 2.</figcaption></figure>

We estimate probe crosstalk for the set of devices with a common ground plane and the set of devices without a common ground plane separately. This approach includes sensitivity to the load impedance, as eight devices are used for probe crosstalk analysis using the following:

$$ \text{probe crosstalk} = \max_{(x \neq y \in 1,2)} (|S_{xy}(\text{SS})|, |S_{xy}(\text{OO})|, \\ |S_{xy}(\text{LL})|, |S_{xy}(\text{LO})|, |S_{xy}(\text{OS})|, |S_{xy}(\text{LS})|) . \quad (2) $$

Here, subscript denotes the combination of devices fabricated, i.e., SS denotes Short-Short, and LO denotes Load-Open pair.

The probe crosstalk results are shown in Fig. 7. First, the probe crosstalk values for the coaxial frequency range are evaluated. Here, only Keysight could provide measurement data, including crosstalk information. Here, superior crosstalk performance is visible when comparing results acquired at 10 GHz with those collected at 110 GHz, an intuitive attribute. The more interesting attribute is the difference between the crosstalk performance for structures with an isolated ground shown in markers and those devices with a common ground depicted with dotted lines in Fig. 7. Up to 110 GHz, devices with isolated ground provide better crosstalk performance.

In the WR5 frequency range, three participants submitted crosstalk data. Here, two participants (PTB and NPL) used GGB (by GGB Industries Inc.) probes, and Keysight used DMPI (by

Dominion MicroProbes Inc.) probes. It is evident from Fig. 7 that the DMPI probes show better crosstalk performance as compared with GGB probes. In the WR5 frequency band, the GGB probes exhibit crosstalk degradation proportional to the frequency, a finding confirmed by data provided by NPL and PTB. The WR3 frequency range shows comparable crosstalk performance for the DMPI and GGB probes, comparable to the values noted in the WR2 band. Also, no significant difference in crosstalk performance is evident for the devices with and without a shared ground.

The crosstalk data for the WR2, WR1.5, and WR1 bands is acquired using T-Wave probes (by FormFactor Inc.), collectively submitted by three participants (Lille, Chalmers, and FormFactor). The WR2 band shows slightly larger crosstalk values as compared to those acquired for the WR1.5 band. It is interesting to see notable 5-10 dB lower crosstalk values in the WR1.5 band when using an absorber, and there is no clear reason for this, as similar probes are also used in the other frequency bands. Also, devices with common ground show a better crosstalk performance in the WR2 band, while no notable difference is evident for the WR1.5 and WR1 bands.

The crosstalk measurement results acquired up to 1.1 THz, covering six frequency bands, do not show abrupt shifts when transitioning between the different frequency bands. It is evident from Fig. 7 that crosstalk performance does not show noteworthy
degradation proportional to the frequency, a key attribute of this study. Also, no significant difference in crosstalk performance is noted for devices with and without a common ground. Finally, the cross-talk values shown in Fig. 7 do not indicate the cross-talk performance seen in typical commercial nanoscale technologies, i.e., CMOS and SiGe BiCMOS. The limitations in the available area in such technologies will most likely not allow usage of probe distances as in this interlaboratory comparison.

## B. Results for the Coaxial Frequency Range

Fig. 6(a), (c), (e), and (g) shows the measurement and reference values for the kit-1 comparison devices. Here, two participants, Keysight and FBH, provided their measurement results acquired with test beds with frequency extenders as the most notable difference.

When evaluating the reflection coefficient parameters for the matched load and open device shown in Fig. 8, a good agreement is observed as the differences between measurement and reference values are smaller than the reference value uncertainty up to 110 GHz.

However, the transmission parameter magnitude results shown in Fig. 9 illustrate a considerable systematic difference between the measurement and reference values, which is larger than the reference value uncertainty. When evaluating the transmission parameter values for mismatched transmission line shown in Fig. 6(g), the measurements results already show around 1 dB loss at 10 GHz, whereas the simulation results at 10 GHz are closer to 0.2 dB which seems more intuitive. The magnitude of the transmission parameters in the coaxial band seems to be influenced by the aging of the structures on the substrate, which might have caused considerable differences. Furthermore, measurement results for the transmission parameter phase are found to be in good agreement with the reference values.

Finally, no notable difference is observed between the measurement results submitted by the two participants resulting from the differences in the measurement systems listed in Table IV.

## C. Results for the WR6 and WR5 Frequency Range

For the WR6 and WR5 frequency bands, four participants provided their measurement results. Here, FBH and Keysight provided data for the entire frequency range, while PTB submitted data from 110 to 170 GHz, and NPL provided data ranging from 130 to 220 GHz. Furthermore, PTB is the only participant who submitted measurement results supported with uncertainty evaluation. All four participants collected the measurement data using a ceramic chuck, while different combinations of probes and frequency extender units were used. Analyzing comparison results from Figs. 8 and 9, no notable differences are observed as a result of different hardware usage.

When evaluating the reflection coefficient parameter values for the matched load and open device shown in Fig. 8, like for the coaxial band, also here a good agreement is observed as the differences between measurement and reference values. Furthermore, PTB measurement uncertainties also show a strong agreement with the simulation values.

However, like for the coaxial band, the transmission parameter magnitude results shown in Fig. 9 illustrate large differences between the measurement and reference values, larger than the reference value uncertainty for most of the WR5 frequency band. Also here, the transmission parameters in the WR6 and WR5 bands seem to be influenced by the aging of the structures on the substrate. Measurement uncertainties of PTB for the transmission parameter magnitude also seem similar to those evaluated for the reference values. However, analyzing the differences observed between the four participants, it seems that substrate aging may have a considerable impact on the transmission parameter magnitude in the coaxial and WR5 bands. Meanwhile, measurement results for the transmission parameter phase agree with the reference values.

## D. Results for the WR3 Frequency Range

Two participants, Keysight and PTB, contributed with measurement data in the WR3 band. Here, the most notable difference in the employed measurement systems is the used probe, where Keysight used DMPI probes, and PTB used the GGB probes.

First, the reflection coefficient results are analyzed, as shown in Fig. 8(a)–(c). The Keysight results for the matched load are in better agreement with the reference values. It is worth mentioning the difference between all measurement and reference values is still smaller than the reference value uncertainties. Here, PTB measurements are supported by a detailed uncertainty evaluation and are of similar order to reference value uncertainties.

Unlike results for the coaxial WR6, and WR5 bands, the transmission parameter magnitude results for the WR3 band shown in Fig. 9(a)–(c) are in good agreement with the reference values, as the differences for all measurement results are smaller than the reference value uncertainties. It seems the transmission parameters of the structures on the substrate are less sensitive for frequencies exceeding 220 GHz. Finally, measurement results for the transmission parameter phase are also in strong agreement with the reference values.

## E. Results for the WR2 Frequency Range

Fig. 6(b), (d), 6(f), and 6(g) shows the measurement and reference values for the kit-2 comparison devices. Two participants, LILLE and FormFactor, contributed with measurement data in the WR2 band. Here, the most notable difference in the operated measurement systems is the chuck, where LILLE used an absorber, and FormFactor used a ceramic chuck.

The reflection coefficient measurement results for the matched load and open device shown in Fig. 8(b)–(d) reveal a good agreement, as the differences between measurement and reference values are smaller than the reference value uncertainty throughout the WR2 band. However, for LILLE at some frequencies, the matched load results are outside the reference value uncertainty.

The transmission parameter magnitude results for the attenuator device are shown in Fig. 9(b) and are in good agreement with the reference values, as the differences for all measurement results are smaller than the reference value uncertainties.
However, measurement results for the mismatched line shown in Fig. 9(d) depict a considerable difference between the measurement and reference values, larger than the reference value uncertainty. When evaluating the results of both participants for the mismatched line shown in Fig. 6(h), a significant discrepancy is noted throughout the WR2 band, which is not observed for the attenuator device. A possible cause for this might be the design of the mismatched line itself, as measurement results for other verification devices seem to be correct. Another possible cause for this might have been the selection of the transmission lines for implementing the mTRL calibration, and errors might result if the phase delay between each pair of lines is not well chosen by the participants.

### F. Results for the WR1.5 Frequency Range

In the WR1.5 band, only LILLE submitted measurement data. The reflection coefficient measurement results for the matched load and open device shown in Fig. 8(b)–(d) reveal a satisfactory agreement, as the differences between measurement and reference values are smaller than the reference value uncertainty throughout the WR2 band, excluding few outliers.

Unlike for the WR2 band, here, the transmission parameter magnitude results for the attenuator and mismatched line shown in Fig. 9(b)–(d) are in adequate agreement with the reference values, as the differences for all measurement results are smaller than the reference value uncertainties.

### G. Results for the WR1 Frequency Range

Three participants, LILLE, Chalmers, and FormFactor, contributed with measurement data in the WR1 band. Here, the most notable difference in the operated measurement systems is the chuck, where LILLE and Chalmers used an absorber, and FormFactor used a ceramic chuck.

The reflection coefficient measurement results for the matched load and open device shown in Fig. 8(b)–(d) reveal a reasonable agreement, as the differences between measurement and reference values are smaller than the reference value uncertainty for a majority of the measurement values throughout the WR1 band. However, measurement values shown in Fig. 6(b)–(d) indicate large frequency-dependent variation observed for all participant data. However, reference value uncertainties appearing proportional to frequency demonstrate a strong increment in the WR1 band and support the increased variance observable in the measurement results.

The transmission parameter magnitude results for the attenuator device are shown in Fig. 9(b) and are in good agreement with the reference values for Chalmers and FormFactor data, as the differences for the majority of the measurement results are smaller than the reference value uncertainties. However, measurement results for Lille show larger differences, especially at the start of the WR1 band. The transmission parameter results by Lille seem to show a strong variation around 950 GHz and might be caused by a poor probe contact during the through or line measurements for implementing the mTRL calibration in WR1 band. The mismatched line results shown

in Fig. 9(d) depict a considerable difference between the measurement and reference values for Lille as well. However, reasonable agreement is observable for Chalmers and FormFactor data. Measurement results beyond 950 GHz seem to have a good agreement between the participants and depict less loss compared with simulation results. It seems likely the simulated reference values for the mismatched line exceeding 950 GHz are accounting for too much material loss than reality. Furthermore, the reference value uncertainties decline after 1 THz and cause the measurement agreement to exceed the uncertainty region. Overall, the WR1 band shows a reasonable agreement up to 1.1 THz.

### H. General Remarks

The comparison results for the reflection coefficient magnitude of one-port devices are shown in Figs. 6 and 8. For kit 1 and kit 2, two one-port standards are considered, including a 50 Ω match and an open device. None of these devices is part of the calibration devices.

Analyzing the differences between the measurement results and reference values acquired for the coaxial and WR5 bands, it seems that substrate aging may have a considerable impact on the transmission parameter magnitude. As the substrate conductivity properties seem susceptible to the ageing effect, they appear inversely proportional to frequency. This is evident from the improving agreement for the transmission parameter magnitude results observed for the upper half of the WR5 band and, subsequently, higher frequency bands. Several participating labs (among them PTB and FBH) observed these issues with the measurement repeatability and stability over longer periods of time, especially when analyzing the real part of the propagation constant extracted from the test structures. In an attempt to quantify the uncertainties associated with the effects observed, PTB determined the measurement repeatability from four repeat measurements of the entire kit 1 set of calibration standards and verification devices on two consecutive days. Consequently, for most measurands, repeatability formed the most significant component in the PTB uncertainty budget. More details on the composition of the PTB budget for various measurands are given in Section IV-E.

No noteworthy impact, i.e., results do not show discrepancies at transitions between frequency bands marked up to 330 GHz, is notable due to the differences in the used equipment in the used measurement systems, including different VNAs, probes, frequency extenders, and chucks as listed in Table IV.

As none of the other laboratories estimated the uncertainty contributions corresponding to their measurements, the combined uncertainty budget provided by PTB for kit 1 was analyzed further. As mentioned, the measurement repeatability was impaired by a lack of long-term stability in the test structures. It represents one of the most significant contributors to the PTB budget for most measurands. This is especially true for the magnitude of the reflection coefficient of the devices shown in Figs. 8 and 9. For the magnitude of the transmission coefficient of the 2-port devices shown in Fig. 9, repeatability and crosstalk between the ports (defined as “DUT uncertainty” in [15]) are the major
contributors to an almost equal level, followed by the uncertainty contributions from the reference substrate calibration standards. This can explain the lack of agreement in the coaxial and WR5 band for the transmission parameter, as the reference value uncertainties do not take into account the repeatability uncertainty contributor. Only for the phase of the transmission coefficient of the 2-port devices of Fig. 9, the uncertainty contributions from the calibration standards are typically the dominant component. Hence, it explains the good agreement for the transmission parameter phase for the kit-1 mismatched line.

Comparison of the uncertainty analysis corresponding to the PTB measurements and the simulation results highlights noteworthy differences, as evident from Fig. 9(a) and (c). Here, PTB uncertainty analysis accounts for more error sources as compared with the simulation results, which accounts only for the calibration standards and cross-talk uncertainty sources, as shown in Table VI. Furthermore, PTB and simulation uncertainty evaluations are performed entirely independently, i.e., calibration standard uncertainty corresponding to the simulations applies the maximal offset approach due to lack of information, which most likely results in overestimation of the uncertainty. This might be the reason for the larger uncertainties corresponding to the simulation results for the mismatched transmission line, as shown in Fig. 9(c).

While no shifts are observed at the transition between WR2 and WR1.5 bands, a substantial shift is evident at the transition between WR1.5 and WR1 bands. Furthermore, WR1 results also show degradation in calibration accuracy, as evident from the Short and Open values exceeding 1 in linear magnitude. Here, the increased noisy behavior observed throughout the measurement results might suggest a degraded dynamic range or generation of higher-order modes, both error sources are not accounted for by the uncertainty analysis corresponding to the simulated reference values.

## VI. CONCLUSION

This article presents an on-wafer CPW measurement campaign ranging from 10 GHz to 1.1 THz. The measurement comparison is conducted using a dedicated reference substrate comprising two kits. The first kit (kit 1) is used for measurements from 10 to 330 GHz, whereas the second kit (kit 2), focused on the 330 GHz to 1.1 THz range.

Measurement setups used by the participants include different vendor VNAs, frequency extender units, and probes, as shown in Table IV. Furthermore, probes also varied in pitch size between the laboratories.

Four laboratories participated in the measurement campaign up to 330 GHz for kit 1 and included measurements across the coaxial, WR5, and WR3 waveguide bands. The measurement results demonstrated excellent agreement despite the differences as mentioned earlier in the setups, and no abrupt shifts in results are evident when transitioning between frequency bands, see Fig. 6. PTB estimated the combined uncertainty contribution corresponding to their measurements. The reference value and uncertainty estimates for the comparison devices primarily focused on accounting for calibration standards and probe crosstalk uncertainties. Both uncertainty estimates

demonstrate similarity up to 330 GHz. The comparison results for kit 1 are in good agreement with the reference values and uncertainties.

Three laboratories participated in the measurements from 330 GHz up to 1.1 THz for kit 2, including measurements across WR2, WR1.5, and WR1 waveguide bands. The comparison results for kit 2 are in agreement with the estimated uncertainty as described earlier. However, the measurement results depict abrupt shifts when transitioning between the WR1.5 and WR1 frequency bands shown in Fig. 6. The calibration accuracy seems to degrade between 750 GHz and 1.1 THz range, as the reflection coefficient magnitude result for high reflect open device exceeds 1. Many possible reasons can cause this discrepancy, e.g., calibration standards and probe errors.

The agreement in reflection coefficient measurements of a one-port matched device is better than 0.08 in linear magnitude up to 330 GHz for kit 1 and 0.08 up to 1.1 THz for kit 2. The differences are well within the estimated uncertainties.

For highly reflective open devices, the difference in the reflection coefficient parameter is smaller than 0.1 for kit 1 and smaller than 0.18 for kit 2. Also, the differences between the mismatched devices are within the estimated uncertainties.

For two-port attenuator and mismatched transmission line devices, the transmission parameter $S_{21}$ is compared.

The differences in the attenuator $S_{21}$ magnitude with a nominal attenuation between 10 and 20 dB are smaller than 1.5 dB for kit 1 and smaller than 2 dB for kit 2. The results of the mismatched transmission line $S_{21}$ magnitude are within 2 dB for both kits. For both two-port devices, the results agree reasonably well with the reference value uncertainties.

The $S_{21}$ phase results for the mismatch transmission line show good agreement and are within reference value uncertainty up to 330 GHz. In 330 GHz–1.1 THz frequency range, it was not possible to compare the measurement values with reference values as the measurements were acquired at the different reference plane positions.

The design of the reference substrate on a semi-insulating high resistivity ($>5000 \ \Omega/cm$) silicon (Si) wafer has proven to be suitable for designing high-accuracy reference substrates up to 750 GHz. At frequencies higher than 750 GHz, the calibration accuracy shows degradation and needs further investigation.

Furthermore, the use of EM simulations to acquire reference values for the comparison devices has proven highly promising. Where state-of-the-art simulation tools, such as CST Studio Suite, can accurately model 3-D structures, they are susceptible if inaccurate material characteristics are used, as evident from transmission parameter magnitude comparison results. Here, the material characteristics, frequency response, and its accuracy are critical elements for EM simulation tools to realize accurate results for frequencies exceeding 1 THz.

Finally, the interlaboratory comparison aimed at demonstrating the measurement accuracy of S-parameter systems up to 1.1 THz by using several verification devices with different properties, such as an attenuator and a mismatched transmission line. This allowed a detailed investigation into the sensitivity of the various error sources and the calibrated S-parameters. However, application-based sensitivity analysis between systems
error sources and typical industrial measurement parameters of interest, i.e. transistor small signal equivalent circuit and model parameters such as $f_{max}$ and noise figure, is not performed and will be considered in future work.

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[25] T. Karpisz et al., “On-wafer calibration comparisons of multiline TRL with platinum and gold conductors,” in *Proc. IEEE/MTT-S Int. Microw. Symp.*, IEEE, Jun. 2024, pp. 1020–1023.

[26] X. Shang et al., “Interlaboratory investigation of on-wafer S-parameter measurements from 110 GHz to 1.1 THz,” in *Proc. 53rd Eur. Microw. Conf.*, IEEE, Sep. 2023, pp. 624–627.

[27] F. Mubarak et al., “Parameterization models for traceable characterization of planar CPW sol calibration standards,” in *Proc. Conf. Precis. Electromagn. Meas.*, IEEE, Jul. 2018, pp. 1–2.

[28] L. Sevgi, “Electromagnetic modeling and simulation: Challenges in validation, verification, and calibration,” *IEEE Trans. Electromagn. Compat.*, vol. 56, no. 4, pp. 750–758, Aug. 2014.

[29] R. Kacker, R. Datla, and A. Parr, “Statistical interpretation of key comparison reference value and degrees of equivalence,” *J. Res. Nat. Inst. Standards Technology.*, vol. 108, p. 439, Nov. 2003.

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[32] D. DeGroot, J. Jargon, and R. Marks, “Multiline TRL revealed,” in *Proc. 60th ARFTG Conf. Dig.*, Fall 2002, IEEE, pp. 131–155.

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[39] “Ind02 ‘planarcal’ best practice guide for planar S-parameter measurements using vector network analysers.” Sep. 2018. [Online]. Available: https://planarcal.ptb.de/14ind02home.html

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<figure><img src="image_11.png" /></figure>

Faisal Ali Mubarak (Member, IEEE) received the M.Sc. degree in electrical engineering, in 2009, and the Ph.D. degree in microwave measurements, in 2025, both from Delft University of Technology, Delft, The Netherlands.

In 2009, he joined the VSL, the national measurement institute of the Netherlands, where he is presently a Principal Scientist with RF & MW measurements. In 2017, he was one of the co-founders of Vertigo Technologies, Delft, a company developing innovative measurement techniques and instruments.

His research interests include developing RF measurement systems and techniques up to millimetre-wave frequencies.

Mr. Mubarak is a member of the International Consultative Committee for Electricity and Magnetism Working Group on Radiofrequency Quantities and the European Association of National Metrology Institutes technical sub-committee on Radio frequencies and Microwaves (WG-RF). He is also a serving member of the European Metrology Network on Quantum Technologies (EMNQ) and the IEEE MTT-11 Technical Committee.
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**Gia Ngoc Phung** (Member, IEEE) received the Dipl.-Ing. and Ph.D. degrees in electrical engineering from the Technical University of Berlin, Berlin, Germany, in 2012 and 2021, respectively. From 2015 to 2019, she received the Ph.D. degree in the field of modeling and characterisation of on-wafer measurements and microwave packaging at millimeter-wave frequencies from the Ferdinand-Braun-Institut (FBH), Berlin.

From 2013 to 2015, she worked with Biotronik SE & Co. KG, Berlin, where she worked on electromagnetic simulation of cardiac devices. Since October 2019, she has been working with the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. Her main research interests are on-wafer, dielectric material, and RF power metrology.

Dr. Phung was a recipient of the Young Scientist Award of the Kleinheubach Conference 2021 and was co-recipient of the European Microwave Prize 2011. She is currently leading the European Partnership on Metrology (EPM) project OnMicro 23IND10 as a project coordinator.

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**Uwe Arz** (Senior Member, IEEE) received the Dipl.-Ing. degree in electrical engineering and the Ph.D. degree (*summa cum laude*) in on-wafer microwave measurements from the University of Hannover, Hannover, Germany, in 1994 and 2001, respectively.

In 2001, he served as a Post-Doctoral Research Associate with the National Institute of Standards and Technology, Boulder, CO, USA. In 2002, he joined the Physikalisch-Technische Bundesanstalt (PTB), Brunswick, Germany, where he develops metrology for on-wafer measurements. From 2015 to 2018, he led the European EMPIR Project PlanarCal as a Coordinator. He is currently the Head of the Working Group On-Wafer Scattering Parameter Measurements at PTB. He has authored and coauthored more than 100 publications in the fields of on-wafer S-parameter measurements, broadband characterization of high-speed interconnects, high-impedance probing, network analyzer calibration, and dielectric material measurements.

Dr. Arz is the recipient of the first ARFTG Microwave Measurement Student Fellowship Award in 1999 and the 2003 AHMT Measurement Award presented by the Association of German University Professors for Measurement Science. He was also a recipient of three ARFTG Best Poster and two ARFTG Best Paper Awards, and was a co-recipient of the 2011 European Microwave Prize. In 2023, he was honored as an ARFTG Life Member.

<figure><img src="image_14.png" /></figure>

**Kamel Haddadi** (Member, IEEE) received the M.Sc. and Ph.D. degrees from the University of Lille, Villeneuve-d'Ascq, France, in 2003 and 2007, respectively.

He is currently a Full Professor with the Institute of Electronics, Microelectronics and Nanotechnology, joint research unit between the University of Lille and the CNRS. He is the Team Leader of RF-2S (radio frequency sensing and services) dedicated to the development of microwave/mm-wave instrumentation for metrology and sensing RF applications.

Dr. Haddadi is a Member of the International Committees: IEEE MTT-8 RF Nanotechnology and IEEE MTT-24 Microwave/mm-Wave Radar, Sensing and Array Systems, and a TPC member in various IEEE conferences. He is an Associate Editor for IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT and *IET Measurement, Science and Technology*.

**Isabelle Roch-Jeune**, photograph and biography not available at the time of publication.

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**Guillaume Ducournau** (Member, IEEE) received the Ph.D. degree from the University of Rouen, Rouen, France, in 2005.

He has been with the Institute of Electronics, Microelectronics and Nanotechnology (IEMN), University of Lille, Villeneuve d'Ascq, France, since 2007. He is the Leader of the THz wireless communications activity with IEMN using optoelectronic THz photomixers, electronic receivers, THz instrumentation, and millimeter-wave (mm-wave) characterization. He worked on several European projects: STREP Roothz 2010–2013, ThoR H2020, GRAPH-X, TIMES (6G SNS) as well as the Marie-Skłodowska Curie TERAOPTICS network. At national level, he was the Coordinator of the COM'TONIQ Project from 2014 to 2017 funded by ANR (INFRA 2013) dedicated to THz communications in the 300 GHz band, the ANR/DFG TERASONIC project for the use of THz photonics technologies and electrical solid-state technologies for THz communications and SPATIOTERA for spatially-distributed photomixers.

Mr. Ducournau was a recipient of the 2020 ISAP BEST PAPER award. He is involved in national France 2030 programs gathering several French laboratories under the “PEPR” programs funded by the ANR (Agence Nationale de la Recherche). In this framework, the FUNTERA project (6 partners) is investigating THz converters, while the SYSTERA project (12 partners) is dedicated to beyond 90 GHz systems for future networks. He also participates to the ST-IEMN common laboratory, and more specifically involved in the mm-wave technologies characterization part. He has authored or co-authored more than 180 publications in peer-reviewed international journals or peer-reviewed conferences proceedings and holds 1 patent.

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**Thomas Flisgen** received the Diploma degree in electrical engineering/telecommunications engineering from the Hochschule Niederrhein, Krefeld, Germany, in 2007, the master's and Ph.D. degrees in computational engineering from the University of Rostock, Rostock, Germany, in 2009 and 2015, respectively.

From 2017 to 2018, he was a Scientist for wakefields and impedances with Helmholtz-Zentrum Berlin. From 2018 to 2024, he was the head of the electromagnetic field simulation group with Ferdinand-Braun-Institut gGmbH, Leibniz-Institut für Höchstfrequenztechnik. Since autumn 2024, he has been a Full Professor with the Brandenburg University of Technology and holds the Chair of Electromagnetic Field Theory.

<figure><img src="image_17.png" /></figure>

**Ralf Doerner** (Member, IEEE) received the Dipl. Ing. degree in communications engineering from the Technische Universität Ilmenau, Ilmenau, Germany, in 1990.

Since 1989, he has been working on microwave measuring techniques. In 1992, he joined the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany. His current research is focused on calibration problems in on-wafer millimeter-wave and sub-millimeter-wave measurements of active and passive devices and circuits. Further research activities include nonlinear characterization of microwave power transistors and noise characterization and modeling of microwave devices.

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**Djamel Allal** received the Ing. degree in electronics from the University of Science and Technology Houari Boumediene, Algiers, Algeria, in 1991, and the Ph.D. degree in electronics from the University of Lille, Villeneuve d'Ascq, France, in 1997.

He joined the Laboratoire National de Métrologie et d'Essais (LNE), Trappes, France, in 2001, where he has been working in research activities in the field of RF and microwave metrology. His current activities include the development of measurement standards in power and in coaxial and on-wafer S parameters, the development of measurement methods for microwave and terahertz electromagnetic materials characterization, and the development of measurement capabilities of electromagnetic field and specific absorption rate.
<figure><img src="image_19.png" /></figure>

**Divya Jayasankar** (Graduate Student Member, IEEE) was born in India, in 1994. She received the bachelor's degree in electronics and communication engineering, in 2015, the master's degree in wireless, photonics and space engineering, in 2019, and the Ph.D. degree in terahertz electronics, in 2024, all from Chalmers University of Technology, Gothenburg, Sweden.

From 2015 to 2017, she worked as a Research Assistant with Raman Research Institute in India. Since 2025, she has been a Postdoctoral Researcher with the German Aerospace Centre (DLR). During her Ph.D., she worked as a Visiting Ph.D. Student with DLR in 2021, the University of Warwick in 2022, and Virginia Diodes Inc. (VDI) in 2023. Her research interests include the development of THz front-end receivers for space/airborne missions and THz metrology.

Dr. Jayasankar was a recipient of the Ericsson Research Foundation award, the European Microwave Association (EuMA) internship award, the IEEE-MTTs graduate fellowship, and the Roger Pollard fellowship for microwave measurements by ARFTG. In 2022, she also won second place in the best student paper competition at the ISSTT conference in Baeza, Spain.

<figure><img src="image_20.png" /></figure>

**Gavin Fisher** received the mechanical engineering degree in electronic systems from Brunel University, West London, U.K., in 1996.

He is applications specialist for the Centre of Expertise at FormFactor working on RF application layers for the systems group providing application support and technical services to FormFactor's customers as well as evaluating and developing new product. With two decades of experience working with Formfactor/Cascade probe systems and probes and a broad spectrum of applications such as high-frequency measurement, calibration and power device measurement, he educates and trains customers on best practices to achieve accurate measurement results. He has made several presentations at European Microwave Week, International Microwave Symposium, MOS-AK workshops, and Agilent/Keysight seminars. He holds an upper-second degree from Brunel University in Mechanical Engineering with electronic systems.

<figure><img src="image_21.png" /></figure>

**Jan Stake** (Senior Member, IEEE) was born in Uddevalla, Sweden, in 1971. He received the M.Sc. degree in electrical engineering and the Ph.D. degree in microwave electronics both from Chalmers University of Technology in Gothenburg, Sweden, in 1994 and 1999, respectively.

In 1997, he was a Research Assistant with the University of Virginia, Charlottesville, VA, USA. From 1999 to 2001, he was a Research Fellow with the Millimetre Wave Group at the Rutherford Appleton Laboratory, Didcot, UK. He then joined Saab Combitech Systems AB, Gothenburg, Sweden, as a Senior RF/microwave Engineer until 2003. From 2000 to 2006, he held different academic positions with the Chalmers University of Technology, and from 2003 to 2006, he was also the Head of the Nanofabrication Laboratory, Department of Microtechnology and Nanoscience (MC2). In 2006, he was appointed Professor and the Head of the Terahertz and Millimetre Wave Laboratory at the Chalmers University of Technology. He was a Visiting Professor with the Submillimeter Wave Advanced Technology (SWAT) Group at Caltech/JPL, Pasadena, CA, USA, in 2007 and at TU Delft, the Netherlands, in 2020. His research interests include high-frequency semiconductor devices, terahertz electronics, submillimeter wave measurement techniques, and terahertz systems.

Prof. Stake received an appointment as a visiting research fellow with the National Physical Laboratory, UK, in the year 2023. He is also the co-founder of Wasa Millimeter Wave AB, Gothenburg, Sweden. He served as the Editor-in-Chief for IEEE TRANSACTIONS ON TERAHERTZ SCIENCE AND TECHNOLOGY between 2016 and 2018 and as Topical Editor between 2012 and 2015. From 2019 to 2021, he was chairperson of the IEEE THz Science and Technology Best Paper Award committee. He is an elected member of the International Society of Infrared, Millimeter and Terahertz Waves (IRMMW-THz) board.

<figure><img src="image_22.png" /></figure>

**Nick M. Ridler** (Fellow, IEEE) received the B.S. degree in physics from King's College London, London, U.K., in 1981.

He is currently the Head of Science for Electromagnetic and Electrochemical Technologies with the UK's National Physical Laboratory (NPL), U.K. He is also an Honorary Professor with the Universities of Glasgow and Liverpool, U.K., and a Visiting Professor with the Universities of Kent, Leeds, and Strathclyde, U.K. He is Non-Executive Director of LA Techniques Ltd. and a member of the Board of

Directors of the European Microwave Association (EuMA). He has more than 40 years' experience working in industrial, government and academic research establishments. His current interests include making accurate electromagnetic measurements at high frequencies (i.e., from approximately 1 kHz to 1 THz).

Prof. Ridler is an NPL Fellow, and Fellow of the Institute of Electrical and Electronics Engineers (IEEE), the Institution of Engineering and Technology (IET) and the Institute of Physics (IOP). He was General Chair of the 2021 European Microwave Week.

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**Robin Schmidt** (Member, IEEE) received the graduate degree in electrical engineer from l'École Supérieur d'Électricité (Supélec), in 2018. He is currently working toward the Ph.D. degree in development of calibration for on-wafer S-parameters measurements in mm and sub-mm-wave frequency with University of Leuven, Leuven, Belgium.

He is currently a Researcher with Keysight Technology Belgium. His research interests include development of calibration for on-wafer S-parameters measurements in mm and sub-mm-wave frequency.

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**Xiaobang Shang** (Senior Member, IEEE) received the B.Eng. degree (First Class) in electronics and communication engineering, in 2008, and the Ph.D. degree in microwave engineering, in 2011, from the University of Birmingham, Birmingham, U.K.

He became a Senior Scientist with the National Physical Laboratory (NPL), Teddington, U.K., in 2017 and was promoted to Principal Scientist in 2023. He has authored or coauthored more than 120 scientific articles on microwave measurements and microwave circuits.

Dr. Shang is a member of IEEE MTT-S Technical Committee (TC)-3 and TC-21 and was an Associate Editor for the IEEE Microwave and Wireless Technology Letters (2020–2024). He was a recipient of several prestigious awards including the IEEE MTT-S Outstanding Young Engineer Award in 2025, the Roberto Sorrentino Prize in 2022, the IEEE Tatsuo Itoh Award in 2017, and the ARFTG Microwave Measurement Student Fellowship Award in 2009.