Engineering of chemical and physical properties of low-k materials by different wavelength of UV light

SiCOH low-k films were deposited by PECVD with variable flow rates of porogen and matrix precursors and variable substrate temperature and RF power. Two precursor ratios were chosen leading to 1) material with higher target porosity around 33% and target k-value 2.3 and 2) material with lower porosity around 25% and target k-value 2.5. After deposition, the samples were UV-cured on temperature above 400 C in nitrogen ambient. Two types of curing lamps were used for the experiments: 1) lamps A emitting photons with energies higher than 6.5 eV (190 nm) and 2) lamps B with photon energies below 6.2 eV (200 nm). For all properties evaluated, irradiation at wavelengths below 190 nm resulted in more pronounced changes than at longer wavelengths. Lamps A provide fast decrease of porogen content, conversion of the Si-O-Si bonds from cage to network. However, degradation of Si-CH3 bonds is significant, as well as formation of H-SiO (Si-H) bonds and amorphous carbon like porogen residue. Lamps B provide more slow porogen removal, do not decrease Si-CH3 concentration and leave less porogen residues. Application of UV ellipsometry (138 nm -165 nm) for the low-k films, pure matrix material and pure porogen allowed obtaining additional results important for discussions. These findings are well confirmed by quantum-chemical calculations.