EMPIR-14INDD02: Microwave measurements for planar circuits and components (PlanarCal)

• Forschungsstelle: METAS
• Projektnummer: F-5217.30060
• Projekttitel Englisch: EMPIR-14INDD02: Microwave measurements for planar circuits and components (PlanarCal)

Schlüsselwörter (Englisch)

On-wafer microwave measurements; vector network analysis; planar circuits and components; microwave, millimeter-wave and sub-millimeter-wave frequencies; calibration substrates; parasitic modes; interconnect characterization; microwave probe crosstalk compensation; surface roughness; RF nanotechnology and nanodevices

Kurzbeschreibung (Englisch)

Although on-wafer HF measurements already have an economic impact on chip fabrication costs, industrial assurance and traceability have not yet been established. Boundary conditions of the measurement system setup and parasitic modes are often not sufficiently considered, leading to inconsistent results. This project will use the most advanced vector network analysers (VNAs) that are currently available together with state-of-the-art numerical simulation techniques to fully capture all relevant effects. At the end of this project, industry will be provided with methods to perform reliable on-wafer scattering-parameter measurements down to sub-mm wavelengths (i.e. 325 GHz).

This is a joint research project carried out in the framework of the European Metrology Programme for Innovation and Research (EMPIR) (see:http://www.euramet.org/research-innovation/empir/). The EMPIR initiative is co-funded by the European Unions's Horizon 2020 research and innovation programme and the participating states. METAS is one of the project partners in the project.

Projektziele (Englisch)

The specific technical objectives of the project are:

1. To establish traceability of planar scattering parameter measurements on reference calibration substrates. These reference calibrations will provide the lowest possible uncertainties for scattering parameter measurements of devices embedded on the same wafer. One possible candidate for such calibrations are airline-like interconnects in membrane technology. In addition, substrates such as GaAs will be investigated, because they allow for comparisons to electro-optic waveform metrology.
2. To transfer uncertainties to calibration standards in conventional technology to be used in industry. This will address the difficulty of moving between different substrate materials and different planar waveguide types. Residual errors of the calibration will be quantified with regard to the selected calibration algorithms and the influences of probe geometry and technology.
3. To improve planar transmission lines models accounting for surface roughness and radiation losses. This will not only help develop reliable uncertainty budgets for planar S-parameter measurements, but will be of fundamental importance to the entire microwave design and circuit community.
4. To develop calibration substrates and algorithms for planar scattering parameter measurements up to at least 325 GHz. This will be achieved by fully characterising calibration standards built in selected substrate materials through dimensional measurements, wideband substrate permittivity extraction and numerical simulation. Guidelines will be developed to suppress excitation of unwanted parasitic modes.
5. To develop suitable calibration standards and methods for measurements of RF nano-devices. Calibrated S-parameter measurements of nano-devices require solutions for the impedance mismatch problem and the challenge of probing at nanoscale dimensions.
6. To engage with manufacturers of planar microwave circuits and components to facilitate the take-up of the technology and measurement infrastructure developed by the project.

Abstract (Englisch)

The scope of the project was RF&MW measurements at small scales. METAS had the lead in WP4, which addressed measurement uncertainties and calibration algorithms in on-wafer S-parameter measurements, and also contributed to WP5, which was related to even smaller dimensions using scanning probe microscopy. In WP4 the main task of METAS was to extend the VNA metrology software VNA Tools with on-wafer capabilities. In WP5 METAS was using the Scanning Microwave Microscope (SMM) to measure nano and submicrometer structures.

On-wafer part:
Characterizations of measurement errors typical of on-wafer measurements were carried out. Subsequently VNA Tools was extended with on-wafer capabilities, i.e. measurement models and on-wafer standards, and on-wafer specific calibration algorithms. For this purpose METAS developed an algorithm that determines suitable starting points for the overdetermined line reflect calibration, which was implemented in VNA Tools. Systematic investigations were carried out to compare the performance of different on-wafer specific calibration algorithms. 

SMM part:
IEMN (Institute for Electronics, Microelectronics und Nanotechnology of the University of Lille) produced on-wafer structures for test measurements in the project. METAS was involved in the design of these structures.

METAS positioned subsequently Silicon nanowires on these structures. The positioning of the nanowires was performed at the Poggio lab at the University of Basel. The home-built METAS SMM was upgraded with new positioners to master the subsequent measurement tasks

In the final phase of the project, the SMM was used to perform a variety of measurement tasks on different combined arrangements of on-wafer structures and nanowires. Whereas structural integrity of on-wafer structures could be tested with the SMM, it was much harder to perform the measurements of nanowires. The results were inconclusive and it turned out that further effort would be needed to increase the adhesive force of the nanowires on the substrate. Resources and time were lacking to address these issues in the framework of this project. The topic could therefore be addressed again in a follow-up project.

Umsetzung und Anwendungen (Englisch)

Miniaturization and integration are a common trend in electronics and will also increasingly impact metrology. Acquisition of competences in this fields will prove valuable in the future.

The enhancement of VNA Tools with on-wafer capabilities will expand its user base and make it interesting for other industries. In case METAS should decide to build up its own on-wafer measurement service, an integral part would already be available. Many things have been learned in the course of the project, which would be useful when evaluating the equipment for the own measurement setup.

The SMM technology could be further tested in a different field and the results and the experience gained will prove helpful to develop the technology towards further applications and measurement services. The suitability of the SMM measurement technique could not be shown for the detection of nanowires. Further studies will be needed to clarify this point before any conclusions in terms of utilization for this particular task can be drawn. Furthermore, measuring nanowires is very specific and does not allow conclusions about the usability of the SMM for measuring other nano-devices.

Publikationen / Ergebnisse (Englisch)

M. Wollensack, J. Hoffmann, D. Stalder, J. Ruefenacht, M. Zeier,VNA Tools II: Calibrations Involving Eigenvalue Problems, 89th ARFTG Microwave Measurement Symposium, Conference Digest, 2017.