Key Findings
Copper films deposited on polyimide exhibit wildly different adhesion strengths even when surface energies are matched within a narrow band of 46.43 to 47.22 mJ/m². The strongest adhesion was achieved via carbon and nitrogen co-implantation, yielding a peel strength of 1.63 N/mm. This is approximately 11.6 times the adhesion of carbon-implanted surfaces (0.14 N/mm) and 8.2 times that of nitrogen-implanted surfaces (0.20 N/mm). Untreated polyimide showed the lowest adhesion at 0.06 N/mm.
Technical Details
The study compared four conditions using 25-micrometer polyimide films: untreated, carbon-ion implanted (20 keV), nitrogen-ion implanted (25 keV), and co-implanted with carbon (15 keV) and nitrogen (8 keV), all at fluences of 5 × 10^15 ions/cm². Copper films of 2 micrometers were deposited via DC magnetron sputtering and electroplated to 18 micrometers for testing per IPC-TM-650 2.4.9 E.
- Electrical Performance: Sheet resistance varied slightly, with nitrogen-implanted films showing the lowest resistance (14 mΩ/sq) compared to co-implanted (17 mΩ/sq) and carbon-implanted (20 mΩ/sq) films.
- Morphology: AFM revealed root-mean-square roughness rose from 1.90 nm (untreated) to 5.34 nm (co-implanted). However, carbon-implanted surfaces (4.61 nm) remained weak despite high roughness.
- Chemistry: XPS showed nitrogen implantation caused substantial degradation of imide structures and a rise in surface carbon (94.09 atomic %). Co-implantation retained elemental concentrations closer to untreated polyimide, including 5.52 atomic % oxygen.
Background & Context
Reliable metallization of polymer substrates is critical for flexible electronics and high-frequency systems. Previous plasma processing studies often altered roughness, surface energy, and chemical functionality simultaneously. By tuning ion energy and fluence to match surface energies, researchers neutralized wettability as a variable to isolate the factors controlling adhesion.
Significance & Outlook
The results indicate that interfacial adhesion and electrical continuity are associated with different reconstructed surface states and should be treated as decoupled optimization targets. The findings suggest that nanoscale roughness is a contributing but non-sufficient condition for strong bonding.
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