Methods for Direct Deposition of Copper on Glass Substrates
Authors: Cordonier Christopher Ernest John , Okabe Kyohei , Watanabe Mitsuhiro , Saruwatari Tetsuya , Honma Hideo 
Published: February 2020

The properties of glass make it an attractive substrate material in a number of applications such as interposers, RF modules and opto-electronics. Smooth surfaces and low dielectric constant have been shown to reduce transmission loss in high frequency applications and improve the Q factor when used as an inductor substrate. Transparency, electrical insulation, low cost, freedom of shape, and potential for roll to roll or large area panel processing also can be beneficial. Shortcomings such as low thermal conductivity and difficulty in via formation are continually improving, therefore development of the metallization technology is also important. As economy is a major incentive for developing glass packaging technology, high cost fabrication techniques hinder the potential for application.
To contribute to glass processing development, recent advances in research of methods for copper deposition directly on glass substrate are discussed where copper seed layers are deposited followed by electrolytic copper plating, then a thermal treatment. Two methods of copper seed layer deposition were investigated. One is by an electroless plating technique developed for deposition on a nanometer scale thick solution processed TiO2 layer formed on the glass surface. The electroless seed layer deposition method utilizes a Pd amino acid complex bearing a cationic moiety that is selectively adsorbed into TiO2 layer material for catalyst deposition. The other seed layer deposition method investigated was by a high power, low vacuum sputtering technique in combination with optimized plasma treatment of the glass surfaces. Because high vacuum is not necessary, expensive vacuum equipment is not required and chamber evacuation is rapid. After plating, thermal treatments caused a reaction at the glass-copper interface which chemically bonded the glass and copper resulting in high adhesion while maintaining smooth interfaces. The methods are illustrated in the attached scheme. In the electroless plating method, TiO2 + Cu reacted to form Ti2O3/Cu2O at the interface after thermal treatment and form SiO2 + Cu, SiO/Cu2O in the sputtering case. The formation of oxygen deficient species in both cases was found to manifest with the adhesion of Cu plating to the glass substrate. However, in the sputtering method with higher thermal treatment temperatures, Cu diffusion into the glass surface was observed with a further increase in adhesion. When the sputtered samples were heated above 500°C, the mode of delamination in peel strength tests changed from interface delamination to destruction of the glass surface layer where glass was observed on the delaminated copper layer. In the electroless plating case, even when adhesion strength was close to 1.5 kN/m, delamination always occurred at the TiO2-Cu interface. Both seed layer deposition methods present pros, cons, and unique characteristics that will be discussed in detail. The TiO2 and electroless seed layer can be deposited by economic solution processing and be applied to large area. Alternatively, the low vacuum sputter method consumes very little time compared to electroless plating methods and at less energy and economic cost compared to conventional sputtering. After the thermal treatment, both methods resulted in 20 mm thick copper deposits with 90° peel strength of over 1 kN/m for aluminoborosilicate glass. Using the electroless plating technique ca. 1 kN/m for quartz glass and ca. 0.5 kN/m for alumina and alumina nitride ceramics was attained. Compatibility with subtractive and semi-additive patterning methods were illustrated, and environmental endurance evaluations performed. The electroless plating process is currently in the evaluation stage for application in industrial scale manufacture.

Figure 1