Dual-Functional Ag-ZnO Nanocomposite Interlayers Enable Robust Copper Metallization on NiFe₂O₄ Substrates for High-Performance X-Band Ferrite Microstrip Isolators

The development of high-performance ferrite microstrip circulators/isolators for 5G/millimeter-wave systems demands cost-effective metallization techniques with robust copper-ferrite adhesion. While electroless copper plating (ECP) offers process advantages, its implementation on nickel ferrite (NiFe₂O₄) substrates is hindered by insufficient catalytic activity and weak metal-ceramic bonding. In this work, we demonstrate a breakthrough strategy using sol-gel synthesized silver-doped zinc oxide (Ag-ZnO) nanocomposite interlayers to enable adherent copper metallization. The ZnO matrix establishes chemical anchoring via the Zn-O-Fe bonding, while Ag nanoparticles act as catalytic seeds for autocatalytic deposition. The dual-functional design exhibits an interfacial adhesion strength of 9.392 N/mm². This material attains the highest 5B classification per the ASTM D3359 tape test standard. Conventional tin-palladium (Sn-Pd) systems show substantially inferior performance, achieving only the lowest (0B) classification. Acid-etchable Ag-ZnO further allows selective microstrip patterning without photolithography. Crucially, the deposited copper exhibits 2.20 μΩ·cm resistivity (1.31 times that of bulk copper), satisfying X-band device requirements. The optimized X-band microstrip isolator exhibits a relative bandwidth >20%, insertion loss <0.55 dB, port-to-port isolation >23 dB, and peak isolation >40 dB at 9.25 GHz, validating the efficacy of the Ag-ZnO-mediated metallization for millimeter-wave systems. This work resolves long-standing adhesion challenges in ferrite metallization and provides a scalable pathway for manufacturing millimeter-wave integrated systems.