Toward Smart Probes for mmW On-Wafer Measurements: Advanced Packaging Using Laser Micromachining of Glass

This article introduces the development of ground-signal-ground (GSG) probes for on-wafer active measurements, fabricated using femtosecond laser micromachining with a resolution between 5 and 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m. The probes are made from a 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m-thick Schott AF32 glass substrate bonded to a 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m-thick nickel sheet, demonstrating enhanced mechanical durability and improved electrical performance. Nickel was selected for the tip contacts due to its superior mechanical hardness and electrical properties, which minimize contact resistance and extend probe lifespan. Mechanical testing showed that while glass-only probes failed at a contact force of 196 mN, nickel-glass probes withstood forces up to 667 mN. In addition, four-wire measurements confirmed low-resistance electrical contacts (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.05~\Omega $ </tex-math></inline-formula> above 6 mN) at a single contact point. Furthermore, this work introduces a glass interposer technology based on the same substrate material as the probe tips, enabling the integration of an amplified noise source (NS) based on 55 nm SiGe BiCMOS technology on a glass interposer. This approach mitigates dielectric and transition losses.Femtosecond laser micromachining was used to precisely structure the interconnects, allowing the NS to be integrated on the same substrate as the coplanar probing tips, thereby simplifying the signal propagation path. This system achieved a tunable available excess noise ratio (ENR<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$)_{\text {av}}$ </tex-math></inline-formula> 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. Using the same glass carrier substrate enables the possibility of integrating the GSG probe tips with the NS chip into a smart active probe for on-wafer noise measurements.