## 4 Discussion  
It was shown in Section 3 that the particular choice of cleaning process has a strong influence on the density of the detectable particles remaining after cleaning and the LIDT of subsequently AR-coated samples. Though a low particle density does not guarantee higher LIDT values:  
(1) It is well known that non-appropriate ultrasonic frequency and power during precision cleaning can negatively influence the substrate surface's quality. The worst case is a degradation of the surface quality. Hence, the laser damage resistance also decreases significantly. Figure 7 shows microscopic pictures of typical damage morphologies for the samples cleaned with different ultrasonic frequencies and power. In the sample cleaned prior to coating with A-CP3 (Figure 7a), we assume a slight but hardly detectable change in the surface quality (sub-surface damaged layer), leading to the 50% decreased LIDT compared to the standard cleaning of A-CP1 with adjusted ultrasonic power. This is confirmed by comparing the damage morphologies for samples cleaned with adjusted ultrasonic power (Figure 7b,c). Figure 5: Laser damage testing results of the sample cleaned using process A-CP1: (**a**) damage probability curve and (**b**) characteristic damage curve for the 0% and 50% laser-induced damage threshold (LIDT) as a function of the number of pulses. Figure 6: The 0% and 50% LIDT for (**a**) cleaning process set A and (**b**) cleaning process set B. The estimated fluences are normalized to the values of the standard cleaning A-CP1. (2) With the help of alcoholic solutions applied prior to the precision cleaning step, many organic compounds can be removed from the substrate surface. Figure 8 shows the comparison of the damage morphology between samples that were pre-cleaned with an alcoholic solution (Figure 8a) and samples that were not pre-cleaned (Figure 8b). The difference in the damage morphology and the appearance clearly shows the influence of the organic residuals. These organic surface residuals bring a decrease in the LIDT of about 35%. (3) Using the inspection microscope Leica type INM200, only particles with a size above 83 nm were detected. Potential smaller particles were not detected. In the case of B-CP3 and B-CP4, the total number of particles was 14 and 12, respectively. The particle size distribution also shows no significant difference. However, for B-CP3, the LIDT is roughly 30% lower compared to B-CP4. A cross-comparison can lead to the assumption that particles smaller than 83 nm remain undetected on the cleaned surface prior to coating. It is known that the adhesion force of particles increases with decreasing particle size [17]. Furthermore, increasing ultrasonic frequencies up to megasonics increased the efficiency of removing particles with decreasing size down to nanoparticles. Hence, we assume that these particles' cleaning efficiency is higher for B-CP4 and the applied 500 kHz during fine rinsing compared to B-CP3 without ultrasonic assistance during fine rinsing. This is confirmed by comparing the LIDT results of B-CP4 with A-CP1, which was equipped with a quick dump rinse (QDR). The QDR technology is well known to reduce back contamination with chemicals and particles from the cleaning solution due to an efficient and fast removal step (few seconds) in which the water is in a quasi \"free fall\" [18]. Under consideration of surface effects (polishing quality, organic residuals), Figure 9 summarizes the normalized LIDT (see Figure 6) as a function of the total particle count after cleaning, excluding results from A-CP2 and A-CP3. Considering that the LIDT in case of B-CP3 (indicated by the red circle in Figure 9) is influenced by particles smaller than 83 nm, a correlation between LIDT and particle density exists. The same tendency was observed for each single particle class (not shown in detail). The highest detected particle density leads to the lowest LIDT values. With decreasing particle density, the LIDT values increase by about 20%.