Cu diffusion is one major problem that inhibits low-k dielectric to be integrated with existing fabrication technology effectively. This paper demonstrates the effects of surface modification towards polymethylsiloxane low-k dielectric (LKD 5109) from JSR Micro using gas mixture of H-2 +N-2 plasma in order to improve Cu diffusion barrier. C-V plots indirectly indicated that plasma treatment reduces Cu+ ions penetration during Cu deposition using magnetron sputtering. XPS confirmed that short duration (10 to 30 s) of H-2+N-2 plasma treatment could cause surface densification of LKD 5109 low-k thin film through formation of N-C bonds. However, the negative effect of plasma treatment is the increment of dielectric constant (k) due to possible surface densification. (c) 2005 Published by Elsevier B.V.