Laser lift-off of polyimide thin-film from glass carrier using DPSS laser pulses of top-hat square profiles

Laser lift-off (LLO) process using a diode pumped solid state (DPSS) laser with wavelength of 355 nm and pulse width of 37 ns was investigated experimentally and theoretically for non-contact delamination of the polyimide (PI) substrate from the glass carrier. Each pulse emitted from the DPSS laser was shaped into a top-hat square beam by an optical system including a diffractive optical element lens and a spatial filter. The typical FWHM size of the top-hat beam was 315μm×328μm. The train of pulses was irradiated on the specimen in the form of a quasi-line beam through a scanner system for two representative pulse repetition rates (PRR) of 1 and 150 kHz. Experimental observations and theoretical simulations using the photothermal ablation model clarified that the intensive laser pulse induced extremely fast ablation inside the PI substrate, which caused the PI to delaminate internally along the PI/glass interface, leaving a residual PI layer of about 100 nm in thickness on the glass carrier. The successful non-overlap LLO by each pulse was confirmed optically by the Fizeau fringes at the threshold fluence of 240 mJ/cm2. The overlapping irradiation of laser pulses, which minimized overlay or stitch errors, was able to lower the threshold fluence of each pulse by about 50% compared to the non-overlapping irradiation by a single pulse. With increasing PRR for the same number of irradiation overlaps, the threshold fluence was reduced considerably due to the thermal accumulation effect, but the thickness of the residual PI layer remaining on the glass carrier increased slightly.