Transient Thermoelastic Response of Nanofilms Under Radiation Heating From Pulsed Laser-Induced Plasma

Measuring transient surface temperatures of substrates excited by nanosecond impulsive-type thermal sources is a nontrivial problem due to limited response times of many current sensors and the thinness of thermal skin at the nanosecond-time scale. An indirect transient surface temperature measurement technique for nanofilms subjected to a nanosecond dynamic loading is presented and demonstrated. The intensity profile calibrated based on the plasma radiation energy measurements is used as a boundary condition for finite-element analysis to estimate the transient surface temperature and the stress tensor induced in a 100-nm chromium film bonded to a quartz substrate due to the thermal radiation heating of the laser-induced plasma. The current approach is useful for predicting the damage threshold of nanofilms in laser-induced plasma (LIP) particle cleaning, as the direct and indirect transient temperature measurements currently available are unreliable for nanosecond impulsive thermal excitations. Particle cleaning techniques based on LIP have been under development for damage-free removal of sub-100-nm particles. The plasma core formed in this cleaning approach is a source of nanosecond-range impulsive radiation and subsequent thermomechanical excitation of the substrate and, consequently, possible substrate damage. The transient temperature measurements are used to estimate the peak surface temperature and the thermomechanical stresses induced in the substrate.