Maya ALAWAR's thesis: "Micro-machining and packaging of active probes for on-slice mmW measurements" 18/12 at 10.30 a.m.

Jury

• Eric RIUS (Professor, University of Brest) - Rapporteur
• Florence GARRELIE (Professor, Jean Monnet University - Saint-Etienne) - Rapporteur
• Marina DENG (Lecturer, University of Bordeaux - IMS) - Examiner
• Emmanuel DUBOIS (CNRS Research Director, IEMN) - Thesis supervisor
• Guillaume DUCOURNAU (Professor, University of Lille-IEMN) - Thesis co-director
• Daniel GLORIA (R&D engineer, STMicroelectronics) - Co-supervisor
• Jean-François ROBILLARD (Lecturer and researcher, Junia/IEMN) - Guest
• Victor FIORESE (R&D Engineer, STMicroelectronics) - Guest

 Summary:

Advanced silicon technologies, such as STMicroelectronics' BiCMOS B55X, targeting fT/fmax cut-off frequencies in excess of 400 GHz, enable the development of circuits in the 140-220 GHz (G-band) range. To validate these technologies, microwave characterisation methods for on-wafer measurements are essential to extract figures of merit from transistors, passive circuits and associated parasitics. However, broadband circuits such as noise sources (NS), noise receivers and impedance tuners at these frequencies are not widely available on the market. Previous research has shown that it is possible to directly integrate on-wafer measurement functions into BiCMOS B55 technology, but this in-situ or built-in self test (BIST) approach has limitations, particularly in terms of silicon area and because the integrated BIST instrumentation cannot be used for any other technology.

This thesis extends the applicability of measurement instrumentation beyond the B55X process and aims to reduce test costs by evolving from BIST to smart probes. This new approach focuses on the integration of measurement functions into a compact system placed as close as possible to the measurement probes for ex-situ measurements.

Building on previous thesis results, the first packaged noise source (NS), based on 55-nm SiGe BiCMOS technology, was developed and characterised in two distinct configurations. In the first approach, on-wafer noise measurements were used to extract an excess noise ratio (ENRav) level of 37 dB in the 140-170 GHz range. In an alternative approach, the NS was encapsulated in a split-block package with a WR5.1 flange termination for connection to commercial passive probes, achieving an ENRav level of up to 25 dB in the 140-220 GHz range, corresponding to a 12 dB reduction in ENR compared with on-wafer measurements.

To enhance this work, one of the key results of this thesis is the development of Ground-Signal-Ground (GSG) probes for on-wafer measurements, fabricated by femtosecond laser micromachining with a resolution of between 5 and 10 µm. These probes, made from a 100µm-thick Schott AF32 glass substrate coated with a 10µm nickel foil, offer improved mechanical durability and electrical performance. While probes made from a glass substrate without a nickel layer reached mechanical failure at a contact force of 196 mN, landing tests revealed that the nickel-glass probes withstood up to 667 mN. In addition, these probes demonstrated a very low DC electrical contact resistance of up to 0.05 Ω for a contact force in excess of 6 mN.

In addition, this research introduces a new substrate technology that embeds a chip integrating an amplified noise source in B55X technology on a glass interposer to reduce dielectric and propagation mode transition losses. Thanks to femtosecond laser micromachining, the interconnections are precisely structured, enabling the NS chip to be integrated on the same substrate as that used to manufacture the coplanar probe tips, with the advantage of simplifying the signal propagation path. This system has achieved an ENRav level adjustable down to 29 dB in the 140-170 GHz range, with output impedance matching better than -12 dB over the whole frequency band.

This research opens up new prospects for cost-effective and scalable active millimetre probes for on-wafer measurements.

Abstract:

Advanced silicon technologies, such as BiCMOS B55X from STMicroelectronics, which target fT/fmax cutoff frequencies above 400 GHz, are enabling the development of silicon circuits in the 140-220 GHz range (G-band). To validate these technologies, microwave characterization methods for on-wafer measurements are essential to extract the figures of merit of transistors, passive circuitry and associated parasitics. However, broadband circuits like noise sources (NS), noise receivers and impedance tuners at these frequencies are very incompletely covered by the market offer. Previous research demonstrated that embedding measurement functions directly onto silicon in BiCMOS B55 technology is possible, but this in-situ approach or built-in self-test (BIST) has certain limitations, particularly in terms of the silicon surface allocated to the test circuits alone and also because embedded BIST instrumentation cannot be used for another technology.

This thesis broadens the applicability of measurement instrumentation beyond the B55X process and aims to reduce testing costs by transitioning from BIST to smart probes. This new approach focuses on integrating measurement functions into compact systems placed as close as possible to the measurement probes for ex-situ measurements.

Building on earlier research achievement as part of a previous thesis, the first-ever packaged NS based on SiGe BiCMOS 55-nm technology was developed and characterized in two distinct configurations. In a first flavour, on-wafer noise measurements yielded an extracted excess noise ratio (ENRav) level of 37 dB in the 140-170 GHz. In an alternative approach, the NS was packaged in a split-block with a WR5.1 flange termination for connection to commercial passive probes, achieving an ENRav level of up to 25 dB in 140-220 GHz corresponding to a 12 dB ENR reduction when compared to the on-wafer measurements.

To improve on this work, a key achievement of the present thesis is the development of Ground-Signal-Ground (GSG) probes for on-wafer measurements fabricated using femtosecond laser micromachining with a resolution between 5-10 µm. These probes made from 100 µm thick Schott AF32 glass substrate bonded to a 10 µm thick nickel sheet, demonstrate improved mechanical durability and electrical performance. Nickel was chosen for the tip contacts due to its mechanical hardness and superior electrical properties, which minimize contact resistance and extend probe lifespan. Mechanical testing revealed that while glass-only probes failed at a contact force of 196 mN, the nickel-glass probes withstood forces up to 667 mN. Additionally, these probes achieved low-resistance electrical contacts (0.05 Ω above 6 mN), as verified through four-wire measurements on a single contact point.

Furthermore, this research introduces a novel substrate technology that integrates an amplified NS B55X chip onto a glass interposer to reduce dielectric and transition losses. Using femtosecond laser micromachining, the interconnects are precisely structured, allowing the integration of the NS chip on the same substrate used to manufacture the coplanar probing tips, with the advantage of simplifying the signal propagation path. This system achieved a tunable ENRav level of up to 29 dB in the 140-170 GHz range, with constant output impedance matching better than -12 dB across the entire frequency band. This innovation allows for the integration of the GSG probes with the NS to perform on-wafer noise measurements.

This research opens new possibilities for cost-effective, scalable millimeter-wave active probes for on-wafer measurements. Their adaptable design makes them suitable for diverse applications, advancing circuit characterization and high-frequency semiconductor testing.