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.

铬（Chromium）凭借其优异的综合性能，成为掩膜介质（mask medium）的首选，广泛应用于光刻（lithography）、半导体（semiconductor）及光子器件制造（photonic device fabrication）领域。然而，当前仍存在一项核心挑战：如何在制备工序完成后快速且彻底地去除铬掩膜，同时避免对衬底造成任何损伤。本文采用基于飞秒激光（femtosecond laser）烧蚀阈值（ablation thresholds）差异的选择性刻蚀方案，以期实现铬掩膜的剥离。有趣的是，我们在实验中观察到，在多次扫描过程中，表面烧蚀阈值超过了所施加的激光通量（laser fluence）。该现象主要由冲击波诱导应力（shockwave-induced stresses）引发的表面致密化（surface densification）所驱动，会显著阻碍多次扫描后铬掩膜的完全去除。此外，进一步提升激光通量可能会损害衬底的结构完整性。为规避这一技术局限，我们引入了一项策略性改进方案：在铬掩膜表面覆盖一层水层，以增强冲击波诱导应力，从而实现铬掩膜的纯机械剥离，且不会对衬底的表面粗糙度、损伤阈值或微观结构造成任何破坏。有限元模拟（finite-element simulations）验证：相较于空气环境，水约束可将冲击波诱导应力指数级放大近两个数量级，该应力超过铬掩膜的断裂强度（fracture strength），引发其断裂并实现后续剥离。这种新型方法具有对衬底无侵入性、无污染物残留且操作简便等显著优势，为其应用于更复杂的掩膜结构铺平了道路。