[Paper Review] Few pulses femtosecond laser exposure for high efficiency 3D glass micromachining

Enrico Casamenti, Sacha Pollonghini+1 more|arXiv (Cornell University)|Jul 23, 2021

Laser Material Processing Techniques48 references50 citations

TL;DR

This study demonstrates that few-pulse femtosecond laser exposure at ultra-low energy doses (~1.5 J/mm²) enables high-efficiency 3D glass micromachining using sodium hydroxide (NaOH) etching, achieving unprecedented etching rates of 300 µm/h and aspect ratios up to 1:400. The method relies on laser-induced defects—particularly non-bridging oxygen hole centers (NBOHC) and oxygen deficiency centers (ODC)—to drive selective etching, replacing hazardous hydrofluoric acid (HF) and improving process speed by over tenfold.

ABSTRACT

Advanced three-dimensional manufacturing techniques are triggering new paradigms in the way we design and produce sophisticated parts on demand. Yet, to fully unravel its potential, a few limitations have to be overcome, one of them being the realization of high-aspect-ratio structures of arbitrary shapes at sufficiently high resolution and scalability. Among the most promising advanced manufacturing methods that emerged recently is the use of optical non-linear absorption effects, and in particular, its implementation in 3D printing of glass based on femtosecond laser exposure combined with chemical etching. Here, we optimize both laser and chemical processes to achieve unprecedented aspect ratio levels. We further show how the formation of pre-cursor laser-induced defects in the glass matrix plays a key role in etching selectivity. In particular, we demonstrate that there is an optimal energy dose, an order of magnitude smaller than the currently used ones, yielding to higher process efficiency and lower processing time. This research, in addition to a conspicuous technological advancement, unravels key mechanisms in laser-matter interactions essential in chemically-based glass manufacturing and offers an environmentally-friendly pathway through the use of less-dangerous etchants, replacing the commonly used hydrofluoric acid.

Motivation & Objective

• To overcome limitations in 3D glass micromachining, particularly low aspect ratios and slow processing speeds, by optimizing laser and chemical etching parameters.
• To identify a safer, more efficient alternative to hydrofluoric acid (HF) and potassium hydroxide (KOH) etchants by evaluating sodium hydroxide (NaOH) as a substitute.
• To investigate the role of laser-induced defects—such as NBOHC and ODC—in enhancing etching selectivity and enabling high-aspect-ratio structures.
• To determine the optimal laser exposure dose and pulse energy regime that maximize etching efficiency while minimizing material damage and processing time.
• To clarify the dominant etching mechanism in femtosecond laser-processed glass, distinguishing between defect-driven and pore/nanograting-driven etching pathways.

Proposed method

• Fused silica substrates (Corning 7980) were exposed to a 270 fs, 1030 nm ytterbium-fiber femtosecond laser at 333 kHz repetition rate, with pulse energies from 160 to 260 nJ.
• Laser patterns were written using a 0.4 NA objective, focusing the beam to a 1.94 µm waist, with scanning speeds varied from 0.5 to 85 mm/s to achieve exposure doses from ~0.5 to ~100 J/mm².
• The exposure dose was calculated using the formula: Φ = πEp f / (4wv), where Ep is pulse energy, f is repetition rate, w is beam waist, and v is scanning speed.
• Three etchants were compared: 2.5 v% HF at room temperature, 45 wt% KOH at 90 °C, and 5 wt% NaOH at 90 °C.
• Etching efficiency was quantified by measuring etched pattern length after 4 hours using a digital microscope on samples cut perpendicular to laser lines to expose modified zones.
• Annealing experiments at 300 °C and 500 °C were performed to assess the thermal stability of laser-induced defects and their role in etching enhancement.

Experimental results

Research questions

• RQ1 What is the optimal laser exposure dose that maximizes etching rate and selectivity in 3D glass micromachining?
• RQ2 How does sodium hydroxide (NaOH) compare to hydrofluoric acid (HF) and potassium hydroxide (KOH) in terms of etching rate, selectivity, and environmental safety?
• RQ3 What is the dominant etching mechanism—defects (NBOHC, ODC) or nanogratings/pores—under low-dose femtosecond laser exposure?
• RQ4 How does laser polarization and pulse energy influence the anisotropy and efficiency of the etching process?
• RQ5 To what extent do thermal annealing treatments affect the etching behavior of laser-modified glass, and what does this reveal about the nature of the laser-induced defects?

Key findings

• Sodium hydroxide (NaOH) achieved an etching rate of 300 µm/h, which is twice that of KOH and nearly four times higher than HF, demonstrating superior etching efficiency.
• The highest etching rate for NaOH, KOH, and HF occurred at a low exposure dose of ~1.5 J/mm², corresponding to only about ten overlapping pulses, indicating an optimal dose regime.
• NaOH exhibited anisotropic etching with a ratio of ~1200:1 µm in etched depth between parallel and perpendicular polarization states, indicating strong defect alignment effects.
• Annealing experiments showed that etching enhancement by NaOH and KOH is primarily driven by defects (NBOHC, ODC), as their etching rate decreased significantly after annealing, unlike HF.
• The etching rate dip at intermediate exposure doses (region II) is attributed to the transition from defect-dominated to pore-dominated modification, though the exact cause remains unknown.
• The study demonstrates that nanogratings are not necessary for high etching selectivity, as the highest efficiency occurs before their formation, confirming that defect-driven etching is the dominant mechanism at low doses.