A metal oxide adhesion layer prepared with water based coating solution for wet Cu metallization of glass interposer

Abstract

Inorganic interposers made of glass are attractive for advanced high frequency applications and ultra- fine line patterning technology. Because glass combines a couple of benefits like large form factor, good coefficient of thermal expansion (CTE) matching to silicon, smooth surface and a low dielectric constant and loss tangent. Recently much progress has been made with respect to glass electrical and physical properties. This allows for handling of thin glass sheets down to 100 μm in a typical PCB panel format. Also advances have been made in the area of laser drilling allowing aspect ratio up to 1:10 for 25 μm diameter of through glass via (TGV). Another major challenge is the cost competitive and reliable metallization of smooth glass, a critical prerequisite for the use of glass substrates in the electronic packaging market. Plated copper does not adhere directly to glass. Sputtering technology typically also requires a 50 nm thick adhesion promoting metal layer (like Ti) before copper can be seeded. This metal layer could not be etched together with the copper and needs to be removed between traces by etching in an additional step. A volatile flammable solvent based metal oxide precursor coating solution has been used to make an adhesive metal oxide layer by a modified sol-gel process. To prevent potential safety issue for mass production water based metal oxide precursor coating solution so called VitroCoat GI W has been developed. The VitroCoat GI W solution can be dip-coated on flat glass surface and TGVs followed by sintering to form an ultrathin metal oxide adhesion layer (about 10nm). The thin adhesive layer enables electroless and electrolytic copper plating directly onto glass substrates without changing any of the glass properties or impacting high frequency performance. The thin metal oxide adhesive layer is non-conductive and can be easily removed from the area between circuit traces. This paper will focus on the coating uniformity and capability of VitroCoat GI W on flat glass surface and TGVs and the adhesion of wet chemical metallization on glass interposer. This adhesion layer can be used for copper fine line patterning on glass and radio frequency (RF) device fabrication.