As an outstanding engineering plastic, polyimide (PI) features exceptional thermal stability, favourable mechanical properties and low dielectric constant. It has become a core material for flexible‑electronic devices and achieved large‑scale deployment in flexible circuitry. Benefiting from its structural stability, conformal circuits fabricated on rigid PI substrates hold great value for extreme‑condition aerospace applications characterised by high temperature and heavy overload. Nevertheless, the inherently smooth and chemically inert rigid‑PI surface yields insufficient bonding strength against deposited copper layers. This bottleneck restricts its widespread adoption in high‑reliability electronic packaging and microelectronic fabrication.

Conventional PI‑surface modification techniques mostly strengthen physical interlocking by boosting surface roughness, yet standalone modification approaches carry obvious drawbacks. Mechanical roughening methods such as sanding and sand‑blasting are low‑cost and high‑efficiency, but they tend to inflict mechanical damage on the bulk substrate. Compared with traditional sanding and chemical etching procedures, plasma‑activation treatment possesses prominent strengths. It is a dry‑type process free from contaminated chemical waste and environmentally benign. The plasma only acts upon the outermost material layer and preserves the bulk mechanical performance and structural integrity of PI substrates.

Surface modification on rigid PI by plasma activation proceeds as follows: external excitation sources such as high‑intensity electric fields ionise working gases (hydrogen, oxygen, nitrogen, argon) into plasma. High‑energy plasma particles bombard the rigid‑PI surface and trigger complicated physic‑chemical surface reactions.
From the physical perspective, bombardment by energetic plasma species produces micro‑etching and roughening, generating uneven micro‑topography for enlarged contact area and mechanical interlocking.
Chemically, plasma‑generated reactive species graft polar functional groups including hydroxyl and carboxyl groups onto the PI surface. This raises the surface energy and wettability of the substrate and enables chemical‑bond formation with copper atoms, therefore reinforcing interfacial adhesion.

Variation in PI Surface Hydrophilicity Before and After Plasma Activation

Water contact angle serves as an intuitive indicator for surface hydrophilicity; a smaller contact angle corresponds to superior hydrophilicity and wettability. As shown in Figure 1, the pristine rigid‑PI surface presents strong hydrophobicity. After plasma‑activation treatment, the average contact angle drops sharply to 8.02°, proving that plasma activation can greatly improve PI surface hydrophilicity.

Figure 1. Water contact angle variation of PI before and after plasma activation treatment

Mechanism for Enhanced PI‑Cu Interfacial Bonding Strength Through Plasma‑Activation Treatment

Altered surface chemical composition constitutes a primary factor behind strengthened PI‑copper adhesion. X‑ray photoelectron spectroscopy (XPS) characterisation was conducted on PI surfaces before and after plasma treatment (see Figure 2). High‑resolution C1s‑spectrum comparison reveals an obvious rise in the proportion of O‑C=O groups after plasma processing. It demonstrates that oxygen‑plasma treatment introduces abundant oxygen‑containing polar groups such as carboxyl groups.
Such surface modification improves PI surface polarity, wettability and chemical activity. Favourable interfacial chemical conditions are prepared for subsequent activation and electroless copper plating, facilitating the nucleation and continuous growth of copper crystals and ultimately enhancing the bonding strength of the PI‑copper interface.

Figure 2. Highresolution C1s XPS spectra of the PI surface: (a) before plasma activation treatment; (b) after plasma activation treatment