PE-CVD of fluorocarbon/SiO composite thin films using C4F8 and HMDSO

Plasma copolymerization of hexamethyldisiloxane (HMDSO, (CH3)3-Si-O-Si-(CH3)3) and C4F8 was performed using an RF plasma enhanced chemical vapor deposition method for application to low dielectric constant intermetal dielectrics. Structure of the films was investigated by X-ray photoelectron spectroscopy and Fourier transform infrared (FT-IR) spectroscopy. The film composition was controlled gradually from fluorinated carbon to organic siloxane by changing the mixing ratio of HMDSO/Ar. Dielectric constant of the films ranged from 2-3.3. Thermal stability of the films, which was characterized by intensity loss of IR absorbance peak around 1000-1500 cm-1 corresponding to C-Fn Si-O-Si and Si-(CH2)n-Si bonds, was inferior to that from C2F4/HMDSO/ Ar. In situ gasphase FT-IR spectroscopy revealed that there was a marked difference between the gas phase of C4F8/HMDSO/Ar and that of C2F4HMDSO/Ar discharges. The IR spectrum of the former combination plasma contained a peak at 1250 cm-1 with full width at half maximum as large as 150 cm-1, which suggests that fluorocarbon particles and/or dusts are formed in the plasma. This suggests also that deposition precursors are not only CFn(n = 1, 2, and 3) but also larger precursors such as CxFx (x>1, y4F8/HMDSO/Ar discharges, which is presumably the cause of difference in thermal stability of the films prepared from C4F8/HMDSO/Ar and C2F4/HMDSO/Ar mixtures.