Non-Invasive Mask Removal by Femtosecond Laser: Pure Mechanical Stripping Achieved Via Shockwave-Induced Stresses

Chromium, favored for its superior performance, is the mask medium of choice in lithography, semiconductor, and photonic device fabrication. However, a significant challenge persists in the rapid and complete removal of Cr masks post-fabrication without inflicting substrate damage. In this paper, selective etching based on the difference in ablation thresholds with a femtosecond laser was attempted to achieve the stripping of the Cr mask. Intriguingly, we observed that during multiple scanning passed the surface ablation threshold exceeded the applied laser fluence. This phenomenon was primarily driven by surface densification caused by shockwave-induced stresses, significantly hinders the complete removal of the Cr mask after multiple scans. Moreover, further increasing the fluence may risk compromising the substrate integrity. To circumvent this limitation, a strategic modification was introduced: a water layer was covered on the Cr mask to enhance shockwave-induced stresses, enabled purely mechanical stripping of Cr masks without damaging the surface roughness, damage threshold, or structure of substrates. Finite-element simulations validated that water confinement exponentially amplified shockwave-induced stresses by nearly two orders of magnitude compared to air, surpassing the fracture strength of the Cr mask and initiating its fracture and subsequent stripping. This novel methodology, characterized by its non-invasiveness to substrates, absence of contamination, and operational simplicity, paves the way for application to more intricate mask structures.