Mechanism of Modification of Fluorocarbon Polymer by Ultraviolet Irradiation in Oxygen Atmosphere

A fluorocarbon polymer generated by plasma polymerization of CF4/CH2F2 used as a model polymer for sidewall residues was subjected to ultraviolet (UV) irradiation (λ = 254 nm). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated that the as-deposited fluorocarbon polymer mainly contains CF, CF2, and a small concentration of unsaturated, fluorinated C=C bonds, and carbonyl functionalities. A partial removal of the polymer occurred during UV irradiation in oxygen. Experimental results showed that UV irradiation resulted in a slight decrease in fluorine content concomitant with the formation of carbonyl groups. The presence of reactive species such as oxygen during UV treatment (by production of singlet oxygen and radicals) was necessary to allow bond cleavage and an increase of hydrophilicity of the polymer fragments thus making the removal of the polymer possible in a subsequent wet clean. In terms of polymer bonding structure, the presence of C=C bonds and oxygen in the polymer backbone, in particular of C-O bonds, was crucial for light absorption at the wavelength used and played a key role in the bond scissioning process. Quantum chemical calculation was also performed to support the experimental results.