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 800nm 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 μm and heights of 75 nm to 2 μm. 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-μm 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.

Surface adhesion of back-illuminated ultrafast laser-treated polymers

“…Despite the losses in the substrate, the remaining energy delivered to the polymer film at the interface leads to a blister formation. Increasing the energy, especially for a low NA lens, will lead to continuum generation and breakdown in the substrate, in addition to blister formation [13]. Here, we accounted for the substrate-mediated absorption in glass using the lawn-mower model published in our earlier work [13,14].…”

“…Increasing the energy, especially for a low NA lens, will lead to continuum generation and breakdown in the substrate, in addition to blister formation [13]. Here, we accounted for the substrate-mediated absorption in glass using the lawn-mower model published in our earlier work [13,14]. In this section, we report pulse energies delivered through the substrate accounting for the nonlinear absorption in the substrate.…”

“…In addition, a void is present beneath the This observation is consistent with what we expect. When an ultrafast laser pulse is focused into the material interface, it forms a plasma [13]. At the temperature that we achieve, the material vaporizes and undergoes a chemical change creating the embedded layer and the solidified material on glass, as seen in Fig.…”

“…If laser-induced surface texture is added to an initially flat surface in back illumination, it changes only the adhesive properties (preserving the surface chemistry) by minimizing the contact area fraction via pointlike blister formation [13]. When large surface areas are patterned using an ultrafast laser, the laser-induced surface texture contains complex rough patterns (in 3D) causing a locally increased hydrophobicity due to an increase in the density of air pockets.…”

“…In contaminant-free LIFT, the nonlinear interaction of an ultrafast pulse with the polymer film (often called a dynamic release layer) between a glass substrate and transfer material ensures the confinement of the energy deposition to the glass-polymer interface, leaving the transfer material chemically intact [12]. When an ultrafast femtosecond pulse interacts with a polymer film at the interface, it creates a localized hot plasma [13,14]. The hot plasma expands, leading to formation of a blister.…”