HF‐Based Etching Processes for Improving Laser Damage Resistance of Fused Silica Optical Surfaces

Abstract

The effect of various HF‐based etching processes on the laser damage resistance of scratched fused silica surfaces has been investigated. Conventionally polished and subsequently scratched fused silica plates were treated by submerging in various HF‐based etchants (HF or NH4F:HF at various ratios and concentrations) under different process conditions (e.g., agitation frequencies, etch times, rinse conditions, and environmental cleanliness). Subsequently, the laser damage resistance (at 351 or 355 nm) of the treated surface was measured. The laser damage resistance was found to be strongly process dependent and scaled inversely with scratch width. The etching process was optimized to remove or prevent the presence of identified precursors (chemical impurities, fracture surfaces, and silica‐based redeposit) known to lead to laser damage initiation. The redeposit precursor was reduced (and hence the damage threshold was increased) by: (1) increasing the SiF62− solubility through reduction in the NH4F concentration and impurity cation impurities, and (2) improving the mass transport of reaction product (SiF62−) (using high‐frequency ultrasonic agitation and excessive spray rinsing) away from the etched surface. A 2D finite element crack‐etching and rinsing mass transport model (incorporating diffusion and advection) was used to predict reaction product concentration. The predictions are consistent with the experimentally observed process trends. The laser damage thresholds also increased with etched amount (up to ∼30 μm), which has been attributed to: (1) etching through lateral cracks where there is poor acid penetration, and (2) increasing the crack opening resulting in increased mass transport rates. With the optimized etch process, laser damage resistance increased dramatically; the average threshold fluence for damage initiation for 30 μm wide scratches increased from 7 to 41 J/cm2, and the statistical probability of damage initiation at 12 J/cm2 of an ensemble of scratches decreased from ∼100 mm−1 of scratch length to ∼0.001 mm−1.

Modeling max-of-n fluence distribution for optics lifetime

“…NIF uses laser-damage-resistant optics to withstand the high laser fluence 1 . NIF has developed optics lifetime models to predict when an optic needs to be refurnished or replaced as part of an integrated operational strategy.…”

Effect of thermal anneal on growth behavior of laser-induced damage sites on the exit surface of fused silica

“…All four edges were polished to allow imaging though the sides of the sample. Both samples were then exposed to a chemical etchant to render the surfaces with state-of-the-art resistance to laser-induced damage [ (Suratwala, Miller et al 2011)]. Damage sites were initiated on the bare surface using a 1-mJ, single laser pulse from a 355 nm, 3.5-ns Full Width at Half Maximum of intensity (FWHM) Gaussian pulsed laser operating at 10 Hz (EKSPLA).…”

“…While advances have been made in bulk purification and surface finishing methods for optical materials [ (Menapace, Penetrante et al 2002), (Suratwala, Miller et al 2011)], localized damage on optical components used in high power laser systems continues to limit their operational lifetime. The initial size of such damage sites is highly dependent on laser conditions [ (Carr, Trenholme et al 2007), (Carr, Cross et al 2011)] but range from a few microns to a few tens of microns for nanosecond pulses.…”

Correlation of UV damage threshold with post-annealing in CVD-grown SiO2 overlayers on etched fused silica substrates

“…HF-etching was used to remove ~27 m of surface material to eliminate any polishing contaminants, followed by a standard NaOH/detergent cleaning step, and deionized water rinse [11]. The samples were then piranha-etched (1:1, H 2 SO 4 :H 2 O 4 ) and coated with SiO 2 using PE CVD method in a SiH 4 /N 2 O precursor gas at 300 mTorr with a substrate temperature of 573 K. The thickness, d, of the resulting SiO 2 films was 1.5, 3.1 and 6.4 m.…”