Experimental and theoretical studies of surface and volume changes in dielectrics induced by long-pulse RF CO₂ laser irradiation

Abstract

This thesis describes research into infrared (IR) laser irradiation and damage of four commercially significant polymers: polyimide (PI), polyether ether ketone (PEEK), polyethylene terephthalate (PET) and polypropylene (PP). Many research groups have studied the laser ablation and irradiation of polymers, but they have focussed mainly on ultraviolet and pulsed infrared sources. There appears to be little published data for laser irradiation of polymers with IR lasers operating with pulse durations in the range 50µs to 1ms. Laser coupling to polymers is strongly dependent on the absorption coefficient at the emission wavelength. These properties are widely known and used to inform experimental practice but the absorption coefficient used in the literature is usually that measured at room temperature and low power. In this way it does not truly represent typical experimental conditions. It is also commonly assumed that the laser wavelength is constant. In this work the laser wavelength has been determined as a function of time during a typical pulse for a radio frequency (RF) excited CO2 laser. It was found that the emission wavelength could move from as short as 10.53µm to as long as 10.63µm during a 200µs duration pulse. This alone was seen to affect the absorption cofficient of the polymers studied. The absorption coefficient as a function of polymer temperature was measured over all wavelengths. This allowed any changes in the optical coupling during laser heating to be inferred. The change in absorption coefficient as a function of temperature was determined as being -0.40cm-1K-1, 0.86cm-1K-1, 0.48cm-1K-1and 0.04cm-1K-1 for PI, PEEK, PET and PP respectively at a wavelength of 10.59µm. The threshold fluence for damage was determined as a function of the laser pulse duration. Damage included any permanent change to the polymer surface and in this way took into account decomposition and melting, as well as ablation. Together with the absorption coefficient data, this allowed the energy densities to be calculated. For PI and PEEK these were found to be 2.4kJ/cm3 and 1.9kJ/cm3 respectively and agreed with existing data. The threshold energy density was 0.1kJ/cm3 for PET and 0.2kJ/cm3 for PP. These results were smaller than those expected from the literature due to melting rather than ablation taking place. The threshold fluence for each polymer was found to be mostly independent of laser pulse duration over the range investigated. The small thermal diffusivity of the materials was thought to be the reason for this. Calculations using solutions to the heat diffusion equation and a rate limited thermal decomposition model were found to be consistent with the experimental results. Some initial calculations of the effect of including the temperature dependent absorption coefficient indicated that this does indeed affect the temperature profile during and after the laser pulse. It has been shown that the RF CO2 laser is suitable for polymer processing, particularly for applications where spot size and high resolution etching are not an issue. Laser marking, cutting and hole-drilling would be acceptable applications for this laser which offers more choice in terms of duty-cycle and pulse duration than the pulsed TEA CO2 alternatives. Quantification of the thermal and optical properties and the interaction between these two parameters could be extended to other polymers and it is expected that similar behaviours would be observed.EThOS - Electronic Theses Online ServiceGBUnited Kingdo