NF3 plasma treatment of polymeric dielectrics

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Abstract
Polyimide and polyphenylquinoxaline are important dielectric materials used in microelectronics fabrication. These polymers could be used even more extensively if they had greater moisture resistance. We demonstrate that plasma processing techniques can be used to improve the moisture resistance of polyimide and polyphenylquinoxaline films. Films are exposed to nitrogen trifluoride plasmas to introduce fluorine into the surface of the polymers. Fluorination is monitored with x-ray photoelectron spectroscopy and Fourier-transform infrared absorption spectroscopy. Water-contact angle measurements are used to assess the hydrophobicity of the treated surfaces. The polyimide and polyphenylquinoxaline film surfaces are quickly fluorinated during plasma exposure, and the moisture resistance improves consequently. The water-contact angle of our best film, 102°, compares favorably with polytetrafluoroethylene (109°). Plasma conditions can be controlled to minimize polymer degradation. The polyphenylquinoxaline films are etched by less than 0.01 μm/min in these experiments. Spectroscopic studies show that the aromatic carbons in the polymers are more readily fluorinated than carbonyl group carbons, and that the fluorination is restricted to the surface.

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