Determination of critical temperature regimes for laser carbonization and ablation of polyimide films of various thicknesses using spectral pyrometry method

The development of polymer laser carbonization methods opens up broad opportunities for creating flexible electronics components. However, a key problem remains the lack of precise data on the technological regimes of laser pyrolysis; in particular, the optimal temperature has not been established, making it impossible to ensure high quality of the synthesized materials. This paper investigates the temperature dependence of 100and 200μ m-thick polyimide films within the laser irradiation zone (wavelength 1064 nm) on laser parameters (power and pulse repetition rate) and scanning modes (beam velocity and hatch spacing) in continuous and pulsed regimes. The temperature was determined by spectral pyrometry: thermal radiation spectra were recorded using a compact HR2000+ (Ocean Optics, USA) visible-range spectrometer (350–760 nm) and transmitted to a computer for processing via specialized “Spectral Pyrometry” software.

The transmittances of the pristine films and those processed by laser scanning were determined using a spectrometric method and by measuring the transmitted power. It was found that the transmittance of the pristine 100and 200-μm-thick polyimide films at the operating laser wavelength is 87.0 % and 85.5 %, respectively. It is shown that increasing the scan line density (hatch spacing) increases the temperature in the processing zone and decreases the transmittance of the treated films. During laser processing of 100and 200μ m-thick films in the pulsed scanning mode with a hatch spacing of 20 lines/mm, their transmittance decreases to 7.5 % and 3.0 %, respectively. It was established that a heat-affected zone is formed along the laser scanning line, in which changes in the surface microstructure and optical properties of the polyimide are observed. The width of this zone depends on the laser scanning modes. The results of this work will be useful for selecting optimal laser exposure regimes in tasks involving the controlled synthesis of carbon structures, such as laser-induced graphene. 

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