Wet-cleaning of contaminants on the surface of multilayer dielectric pulse compressor gratings by the Piranha solution

Abstract

Contaminants are deemed to be the initial source of optical-damage mechanisms as well as the main reason to decrease the Laser Induced Damage Threshold (LIDT) of multilayer dielectric pulse compressor gratings (PCG). In this paper, the Piranha solution was used effectively and nondestructively to remove the residual contaminants on the surface of PCG. The surface cleanliness was evaluated by Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS).The higher removal efficiency was achieved with more times of multiple-cleaning by fresh Piranha solution which has an effectiveness for a given period of time.

Improving the performance of high-laser-damage-threshold, multilayer dielectric pulse-compression gratings through low-temperature chemical cleaning

“…Carbon is attributed to organic photoresist/BARC layers, etch residues, and environmental contamination. Fluorine contamination most likely occurred from the production of fluorinated by-products during reactive-ion beam etching of the grating's groove structure, as has been reported by others [11,13,14]. The detection of molybdenum motivated the inclusion of a hydrochloric-acid-based ionic cleaning step to specifically target molybdenum and other trace metal contaminants.…”

“…Standard operating procedure for acid piranha varies, but typical acid/peroxide ratios are in the range of 2∶1-7∶1 (two to seven parts 99% sulfuric acid to one part 30% hydrogen peroxide) and typical processing temperatures are in the range of 90°C-140°C [32,33]. Optimized piranha-cleaning processes for MLD gratings documented in the open literature have been consistent with these ranges [12][13][14]. Ashe et al [12] found that laser-damage resistance was maximized when high cleaning temperatures were used and when the proportion of H 2 O 2 in the piranha solution was high.…”

“…Acid piranha, a mixture of sulfuric acid (H 2 SO 4 ) and hydrogen peroxide (H 2 O 2 ), was identified as the most promising chemistry for MLD grating cleaning based on postcleaning DE and LIDT results. Other groups [13][14][15][16] have also reported on the successful use of acid piranha to clean MLD gratings. Britten and Nguyen [15] developed a cleaning method for diffraction gratings that involved stripping bulk photoresist with an aqueous base and employing an oxidizing acid solution to remove residues, with oxygen plasma used as an intermediate step to remove fluorinated hydrocarbon residues.…”

“…Since many grating manufacturers do not use BARC, little information is available on stripping it from MLD gratings during final cleaning. Finally, wet cleaning of MLD gratings has typically been performed at high temperatures (60°C-110°C), especially when acid piranha is used to strip photoresist [11][12][13][14]. Such elevated processing temperatures have recently raised concerns about thermal-stress-induced defects, such as blistering and localized coating delamination, that can occur during cleaning.…”

“…Surface contamination can dramatically reduce a grating's resistance to laser-induced damage [5,[8][9][10][11][12][13][14][15]. OMEGA EP pulse compressor gratings are fabricated by etching a periodic groove structure (1740 lines∕mm) into the top layer of a hafnia/silica multilayer mirror using interference lithography.…”