Adhesive enabling technology for directly plating copper onto glass

An economical and reliable copper metallization of dimensionally stable, non-conductive substrates, such as glass and ceramic, is becoming a major focus in electronic packaging technologies. Conventionally, adhesion on these substrates is achieved either by sputtering a thin metallic adhesive (Ti, Cr) followed by a copper seed layer or by high temperature lamination. This study has demonstrated that a metal oxide adhesion promoter (MOAP) layer functions as a strong adhesion layer between Cu and glass/ceramic, even when Cu films are formed near room temperature. In this new approach a 10-200 nm thick adhesive metal oxide layer is deposited by a modified sol gel process followed by sintering, thus enabling electroless, and galvanic metal plating directly on glass/ceramic. With the new approach, Cu can be plated on different glass types or glass with different roughness. The new approach also can be used on ceramics such as Al2O₃ and BaTiO₃. Cu film thickness of over 50 μm can be applied without delamination. Adhesion at 15 μm Cu thickness as measured by 90° peel strength tests can achieve 5 N/cm or even higher values. The plated layer stands up well to reflow shock (260C) and HAST without significant loss of adhesion. Good coverage of MOAP and adhesion have been also demonstrated inside the via holes of patterned substrates without indication of blockages by the process. Reliable adhesion of copper to glass is a major hurdle for the entry of glass substrates into the electronic packaging market. Otherwise, glass is a strong competitor to organic substrates due to its superior flatness, dimensional stability, thermal and dielectric properties. These are essential requirements for high density interconnects, high speed signal transfer and IC substrate packaging. Typically, adhesion on non-conductive substrates is often achieved by sputtering a thin metallic adhesive (Ti, Cr) and copper seed layer or by mechanical anchoring. However, sputtering requires high vacuum technology which has a relatively low throughput and high capital investment cost. Besides, the thickness of the galvanic copper layer constitutes a major challenge leading to its facile delamination from the glass. Mechanical anchoring, on the other hand, requires strong roughening of the substrate surface which negatively impacts the functionality of the metallized surface and the roughening process is difficult to control due to the variable glass compositions. Moreover, the improvements in adhesion that come from such treatments on glass substrates have not always been significant. The use of sol-gel deposition to produce a thin film layer on substrates is widely known in the semiconductor industry. A metal oxide based sol-gel process, which retains the original flatness of the substrate, is discussed in this paper.