Efficient method for determination of laser conditions adopted in laser-induced micro-lithology based on laser polymerization size analysis

Since negative photoresist SU-8 has become a common material for multi-photon micro-lithology, it is necessary to study laser conditions adopted in lithology process. Optical transmittance of SU-8 was tested. According to Urbach optical-absorption theory and Gaussian laser lateral spatial intensity envelope, relationship between theory and actual polymerization size of SU-8 was shown. Experimentally, we investigated multi-photon polymerization threshold and laser-induced damage of SU-8 under femtosecond laser irradiation with the pulse width of 45 fs at 800 nm by 1-on-1 tests. The polymerization and damage threshold at 45 fs are 2.7 and 8.9 TW/cm2, respectively. Polymerization and damage morphologies are shown with high contrast and polymerization sizes are measured under SEM. Theoretical polymerization sizes versus laser fluence are calculated by laser-induce multi-photon polymerization size analysis (LMPSA), including Urbach optical-absorption theory and Gaussian laser lateral spatial intensity distribution. The calculated results show that diffusion exists in the femtosecond laser-induced polymerization.