2D and 3D Interdigital Capacitors and Bias Tees Technologies on MnM Interposer for mmWave Applications

This paper presents two capacitors fabricated using the metallic nanowire membrane (MnM) interposer technology operating at mmWaves. Standard 2D interdigital capacitors (IDCs) are designed to operate up to 70 GHz, which presents a straightforward and non-complex fabrication. In comparison, this work also proposes an improved device that is more compact and exhibits large capacitance density, as high-performance vias enable the realization of high-depth capacitors. The fabrication process of 3D devices presents advanced maturity and innovation as it takes advantage of the porous nature of the interposer material to overcome the device complexity, and is also described in detail. Both capacitor types are modeled by a numerical lumped-element model that also considers parasitics. The 3D capacitors were successfully fabricated and characterized up to 70 GHz, displaying capacitance values between 30 fF and 160 fF and self-resonant frequencies in good agreement with mmWave applications. The quality factor of these devices, measured at 40 GHz, lies between 16 and 4, and the superficial capacitance density is between 4 pF/mm² and 8 pF/mm², showing that these devices are indeed promising for mmWave applications. These devices present considerably larger capacitance density compared to 2D traditional capacitors fabricated on the high-performance substrate, highlighting the advantage of 3D fabrication using nanowire growth. In addition, thin-film resistances are simulated and fabricated, projecting their functions as an RF-choke in a bias tee configuration using Ti thin film sputtering deposition step that is also part of the capacitors fabrication.