Adhesive Enabling Technology for Directly Plating Copper onto Glass

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, thermal and dielectric properties. These are essential requirements for high density interconnects, high speed signal transfer and IC substrate packaging. Typically, adhesion on glass is achieved by sputtering a thin metallic adhesive (Ti) and copper seed layer followed by galvanic plating. This paper presents a promising wet-chemical alternative to sputtering. In this new approach a 50-200 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. Formerly the thickness of the galvanic copper layer constituted a major challenge leading to its facile delamination from the glass. With the new approach, Cu film thickness of over 50 μm can be applied without delamination. Adhesion at 15 μm Cu thickness as measured by 90o peel strength tests can achieve 5 N/cm or even higher values, while 2 N/cm appear to be sufficient to prevent delamination. In comparison, Ti/Cu sputtered glass substrates achieve at best 1.5 N/cm at the same copper thickness, while electroless Cu seeded glass substrates without the adhesive metal oxide layer show no adhesion. The effect of glass roughness on adhesion was also studied. It does appear to have only a marginal impact on adhesion. On the other hand, the glass type has bearing on the achievable adhesion values. The plated layer stands up well to reflow shock (260C) and HAST without significant loss of adhesion. Good adhesion has been also demonstrated inside the via holes of patterned substrates without indication of blockages by the process. The process is versatile in that it is also applicable to ceramic substrates such as aluminum oxide.