Chemical structure characteristics of flexible low-k SiCOH thin films etched by inductively coupled plasma-reactive ion etching process using FTIR and XPS spectra analysis

Flexible low dielectric constant (low-k) SiCOH thin films were fabricated onto flexible indium tin oxide coated polyethylene naphthalate (ITO/PEN) substrates using plasma-enhanced chemical vapor deposition (PECVD) of a tetrakis(trimethylsilyloxy)silane (TTMSS) precursor. RF plasma powers of 20 and 60 W were utilized for the deposition. The k-values of the pristine SiCOH films deposited at 20 and 60 W were 2.46 and 2.00, respectively. Both films showed hydrophobic surfaces. An inductively coupled plasma-reactive ion etching (ICP-RIE) process was then performed on the flexible SiCOH thin films using CF₄, CF₄ + O₂, and CF₄ + Ar. The surface wettability of the films increased substantially following etching, with many of the etched films being considered hydrophilic. The Fourier transform infrared (FTIR) spectra of the pristine films identified four prominent absorption bands as CH_x stretching, Si-CH₃ bending, Si-O-Si stretching, and Si-(CH₃)_x stretching vibration modes. After the etching process, the peak area ratios of Si-O-Si stretching mode increased and those of Si-(CH₃)_x stretching mode decreased. The X-ray photoelectron spectroscopy (XPS) spectra analysis determined significant concentration of fluorine on the surface of the film following etching. From the high-resolution XPS scan, it was found that the peak intensity of the C1s and Si2p peaks decreased after etching process and the peak center of the F1s peak shifted depending on etching chemistry. The k-values of the films at 20 W were fairly consistent while those of the films at 60 W increased significantly following the etching process. The increase in k-value after etching for the films at 60 W correlated with surface hydrophilicity, increase in the refractive index, and change in the peak area ratios of Si-O-Si and Si-(CH₃)_x stretching modes in the FTIR spectra.