Improvement of Adhesion between High-Frequency-Compatible Substrates with Low Dielectric Constants and Electroless Plated Copper Films Using a Vacuum Ultraviolet Treatment

With the development of sophisticated and faster semiconductors, research is focusing on semiconductors that can work within high-frequency bands ranging from millimeter waves to sub-terahertz waves. To develop these semiconductors, the circuit substrates used for communication terminals and base stations should be capable of reducing signal attenuation or transmission loss, which increases at higher frequencies. Transmission loss is defined as the sum of dielectric loss and conductor loss. Dielectric losses can be reduced using a substrate material with a low dielectric constant (Dk) and a dielectric loss tangent (Df). Conductor losses are primarily due to the surface roughness and skin effect losses of its material at high frequencies. To reduce conductor losses, good adhesion should be achieved between the substrate and the conductor film without roughening the surface of the packaged substrate, i.e., the roughness at the interface between the wiring and the substrate should be low (Fig. 1). Cyclo olefin polymer (COP) is a suitable material for the next generation flexible substrates that allow high-speed transmission because of its dielectric properties. Vacuum ultraviolet (VUV, λ = 172 nm) excimer radiation with a photon energy large enough to break down organic matter can be used for the hydrophilization of polymer surfaces. Furthermore, under air atmosphere, the O 2 can be photolyzed by absorbing a VUV light to generate reactive oxygen species such as O(1D), O(3P), and O 3 . The reactive oxygen species that are produced can then initiate chemical reactions on the surface of materials being processed. (Fig. 2). This allows the formation of stronger interactions, which improve the adhesion between the surface and coating without the need for a conventional anchoring effect. [1], [2] In our previous study, [3], [4] we reported the surface modification of COP films by a VUV light, which resulted in an improvement of the adhesion strength between an electroless deposited copper and the COP substrate without roughening the surface. In this study, we analyzed the role of interface composition for achieving a good adhesion between the COP substrates and the deposited metal using a VUV-light irradiation without roughening their surfaces. Moreover, the mechanisms for achieving a strong adhesion between the substrates and electroless deposited copper by VUV-light irradiation were investigated.