Formation of a Hermetic Copper-Glass Joint By Copper Plating for Direct Metallization of Glass
Authors: Cordonier Christopher Ernest John , Okabe Kyohei , Noh Joohyong , Honma Hideo 
Published: February 2020

As a silicon substitute for interposer substrates, RF modules and 2.5/3.0D packaging, much attention has turned to glass. Glass presents a number of advantages over traditionally used silicon such as low cost, small footprint, transparency, electrical insulation and potential for roll to roll or large area processing. Solutions to the disadvantages of glass such as low thermal conductivity and difficulty in layer to layer interconnect via formation have progressed. Since the economic advantage is a major incentive for developing glass packaging technology, high cost fabrication techniques used for silicon, like sputtering, hinder the potential for application. Recently, a number of methods for copper plating directly on glass substrate employing a metal oxide adhesion layer have been reported. Use of a copper catalyst inclusive amorphous titanium oxide adhesion layer produced copper films on glass with up to 0.5 kN/m vertical peel adhesion strength and was independent on the type of glass. Similar results have also been attained using a solution deposited crystalline titanium oxide adhesion layer that was plated using a conventional process. In each account, the titanium oxide phase varied and an EDTA electroless copper bath was used with the copper catalyst on amorphous titanium oxide and a Rochelle salt bath with the palladium catalyst on crystalline titanium oxide. Although the result is similar the nature of the glass-copper bond was quite different. In this presentation, the amorphous titanium oxide adhesion system was investigated.
First, a photodefinable titanium copper complex solution was formulated for deposition of the adhesion layer on glass. Plating on glass substrates was performed according to the procedure outlined below. A dip coating method was employed such that substrates with through substrate interconnect holes could be accommodated. With this method, vertical peel adhesion strengths of over 1 kN/m were attained. Furthermore, there was no notable variation in the adhesion strength on soda-lime, borosilicate, aluminoborosilicate and fused quartz. Interdiffusion at the titanium oxide/copper-glass interface could not be observed by TEM and EDS analysis of the interface cross-section, only a smooth (Ra less than 0.5 nm) well defined joint line. Investigation of the adhesion mechanism revealed the following. First a nano-porous titanium oxide layer structure 40-60 nm thick fused to the substrate was formed by pyrolysis of TiCu complex films followed by a reduction of the copper in aqueous borohydride solution. Next, residual copper was displaced with palladium then the porous titanium oxide structure was filled with copper during deposition of the seed layer in an electroless copper bath. After thickening the copper deposit to 0.02 mm by electrolytic deposition, re-annealing the deposits resulted in reduction of Ti4+ oxide to Ti3+ oxide and oxidation of Cu0 to Cu1+ oxide.
Hermetic copper-glass bonding has been historically applied to form vacuum seals like those used in light-bulbs and vacuum tube transistors. When copper with a layer of divalent copper oxide is heated while in contact with a glass surface, a redox reaction occurs to form a monovalent copper oxide layer that has high affinity for glass resulting in a strong joint. In the case a catalytic copper-titanium oxide anchor layer was used to plate on glass, a type of Ti3+ and Cu1+ oxide Hermetic seal was formed resulting in abnormal adhesion to all types of silicate glass. This technique was applied to formation of copper micro patterns and copper plated TGV substrates.

Figure 1