Femtosecond-Laser-Induced Nanoscale Blisters in Polyimide Thin Films through Nonlinear Absorption

Abstract

Nonlinear absorption of femtosecond laser pulses provides a unique opportunity to confine energy deposition in any medium to a region that is below the focal diameter of a pulse. Illumination of a polymer film through a transparent high-band-gap material such as glass, followed by nonlinear absorption of 800-nm light in polymers, allows us to further restrict absorption to a very thin layer along the propagation direction. We demonstrate this confinement by simulating femtosecond-laser-induced polymer modification by linear, two-photon, and three-photon absorption, and discuss the control over energy absorption in polymers that multiphoton processes offer. Energy deposited in a thin polymer film induces a protruding blister. We present experimental results for blister diameter and height scaling with variation of pulse energy. Using pulse energies of 20--200 nJ and 0.4-NA focusing, we fabricate blisters with diameters of $1--5.5\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and heights of $75\phantom{\rule{0.2em}{0ex}}\mathrm{nm}$ to $2\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. Using 0.95-NA focusing, we obtain laser-induced blisters with diameters as small as 700 nm, suggesting blister-based laser-induced forward transfer is possible on and below the 1-$\ensuremath{\mu}\mathrm{m}$ scale. Submicrometer blister formation with use of femtosecond lasers also offers a method of direct, precise laser writing of microstructures on films with single laser pulses. This method is a possible alternative to lithography, laser milling, and laser-based additive machining.