Efficient Picosecond-Laser Lift-Off of Copper Oxide from Copper: Modelling and Experiment

Laser-induced lift-off of functional surface layers is a key process in micro- and nano-fabrication; however, optimization criteria for maximizing the lifted-off area remain insufficiently defined. In analogy to the well-established theory of efficient laser ablation, where the maximum ablated volume per pulse is achieved at a peak fluence of F_0^{\mathrm{opt}} = e^{2} F_{\mathrm{th}}, we develop a theoretical framework for efficient laser lift-off driven by Gaussian beams. By analytically describing the lift-off area as a function of peak fluence, beam radius, and focus position, we demonstrate that the maximum lifted-off area is achieved at a substantially lower optimal fluence, namely F_0^{\mathrm{opt}} = e^{1} F_{\mathrm{th}}. Closed-form expressions for the optimal beam radius, maximal lift-off area, and optimal focus position are derived and validated by numerical modeling. The theory is applied to picosecond laser lift-off of copper oxide from copper, showing excellent agreement between experimental observations and model predictions. The results reveal fundamental differences between ablation- and lift-off-dominated material removal and provide practical guidelines for maximizing process efficiency in laser-assisted delamination, selective coating removal, and surface functionalization.

💡 Research Summary

This paper establishes a comprehensive theoretical framework for optimizing the laser-induced lift-off (LLO) process and validates it through both numerical modeling and experiment. Laser lift-off, a critical technique for selectively removing functional surface layers in micro- and nano-fabrication, has historically lacked a fundamental theory for maximizing its efficiency, often relying on empirical optimization. Inspired by the well-established theory of efficient laser ablation—where maximum ablated volume per pulse occurs at a peak fluence of F0_opt = e^2 * F_th—the authors develop an analogous theory for efficient lift-off driven by Gaussian beams.

The core of the theoretical model derives an analytical expression for the area delaminated per pulse (A) as a function of key processing parameters: peak fluence (F0), beam radius (w), pulse energy (Ep), and the lift-off threshold fluence (F_th). By analyzing this function, the authors derive closed-form solutions for the optimal processing conditions that maximize the lift-off area. The central finding is that the optimal peak fluence for maximum lift-off area is F0_opt = e^1 * F_th, which is substantially lower than the optimal condition for efficient ablation. This highlights a fundamental physical difference: lift-off is governed by interfacial failure due to thermomechanical stress, requiring sufficient energy to overcome adhesion but not so much as to cause volumetric ablation of the substrate. The model also provides formulas for the optimal beam radius (w_opt) and the optimal sample position relative to the focal plane (z_opt) to achieve this maximum area, offering a complete set of guidelines for process optimization.

Numerical simulations visually map the lift-off area across a wide parameter space of pulse energy, beam radius, sample position, and peak fluence, clearly demonstrating the existence of pronounced maxima at the predicted optimal conditions (Figure 1). For experimental validation, the theory was applied to picosecond laser lift-off of a native copper oxide (Cu2O) layer from a copper substrate. The Gaussian beam propagation characteristics were meticulously characterized. Single-pulse experiments were then conducted across a range of pulse energies and defocus distances (z-positions). The resulting lift-off areas showed excellent agreement with the model’s predictions. The experimental data confirmed that the lift-off area peaks at a specific defocus position for a given energy, and that the general scaling behavior aligns with the theoretical curves.

In conclusion, this work successfully bridges a gap in laser-material interaction theory by introducing the concept of “efficient lift-off.” It provides a powerful and practical analytical tool for optimizing laser-assisted delamination, selective coating removal, and surface functionalization processes. The demonstrated agreement between theory and experiment underscores the model’s validity and its potential to move LLO process development from trial-and-error towards scientifically grounded, efficiency-driven design.