Chemical and mechanical adhesion mechanisms of sputter-deposited metal on epoxy dielectric for high density interconnect printed circuit boards

Strong chemical reactions between metal and polymer substrates significantl y enhance adhesion of the metal to the polymer. This study investigated the adhesion of three types of thin film metals, including Cu, NiCr, and Cr, t o a fully epoxy-based polymer. Before depositing these thin film metals, th e epoxy surface was treated with either an Ar or O-2 plasma etch. It was fo und that NiCr and Cr produced higher peel strengths than Cu, but NiCr and C r did not produce different peel strengths than each other. It was also fou nd that O-2 plasma etch produced significantly higher peel strengths than A r plasma etch for Cu and Cr, but not for NiCr. An XPS (X-ray photoelectron spectroscopy) study was performed to investigate the reactivities and possi ble chemical adhesion mechanisms of the metal thin films with the epoxy. It was determined that Cr reacted more strongly than Ni in forming metal oxid e at the metal-epoxy interface. Cu was not seen to react strongly in formin g oxide with the epoxy. Thermodynamic information supported the relative am ounts of oxides found by XPS. Thermodynamic information also suggested that O-2 plasma etch did not produce significantly higher adhesion than Ar plas ma etch on the NiCr samples due to the large Ni component of the NiCr thin film. An AFM (atomic force microscopy) study was performed to investigate p ossible mechanical adhesion mechanisms. Implications of the AFM results wer e that the main adhesion mechanism for all samples was chemical and that th e Cu oxide that was available on the Cu samples was beyond the detection li mits of the XPS equipment.