## 1 Introduction  
Polishing optical substrates produces numerous different surface contaminants that remain on the surface after process completion [1; 2; 3; 4]. The contaminants include particles originating from the polishing abrasive and removed substrate material and organic films from optical paint and cement. The remaining residuals negatively affect the properties and performance of the optical component and can even cause total failure. For example, particles and organic films lead to increased scattering and absorption and can reduce the thin film layer stack's adhesion. Especially in the case of laser components, surfaces with extremely low particle densities must be achieved. In particular,the initiation of laser-induced damages can be addressed to absorbing nanometer-sized particles referred to as \"nano precursors\" [5]. In consequence, particles down to the nanometer-size must be reduced to a minimum. Any remaining organic films negatively influence the laser damage threshold [6; 7]. Therefore, appropriate cleaning of the optical substrates is a must for further processing, e.g., coating, assembling [8; 9; 10; 11; 12; 13; 14]. However, increasing demands and tight specifications for optical coatings require well understood, clearly defined, and stable cleaning procedures. For research purposes and in the scientific environment, time-consuming and expensive techniques such as short-time etching in hydrofluoric acid (referred to as HF dip) [15] or spin cleaning can be used. It is worth mentioning that for an HF dip, a rinsing and drying step is necessary, which is critical regarding back-contamination with particles. In case of expensive components, single-part processes like spin cleaning and manual cleaning are also applied in industrial environments. However, usually automatic batch processes for cost reduction and process optimization are preferred. Such processes are ideally well defined and controlled compared to \"daily-performance depending\" manual processes. Therefore, the most valuable cleaning method is by means of a fully automatic ultrasonic cleaning bench. Surprisingly, detailed investigations on the influence of different process parameters during the ultrasonic cleaning on the cleaning results (e.g., residual particles) are rare. This is partially due to confidential, proprietary issues. Especially for anti-reflective (AR) coatings for laser optics in the nanosecond pulse regime, the laser-induced damage threshold depends strongly on the substrate surface quality prior to coating [8; 10]. To achieve optimal cleaning results, one has to take the substrate material, the polishing quality, and residuals from the polishing process itself (e.g., polishing material, cement, protective layer) into account when choosing a suitable cleaning procedure. Regarding the substrate material, essential properties are climatic resistance, stain resistance, acid resistance as well as alkaline and phosphate resistance, Knoop hardness, and the thermal expansion coefficient. It also has to be mentioned that permeate and deionized water act as acids due to their chemical behavior. Inadequate cleaning can subsequently attack the substrate surface. Consequently, the roughness can increase dramatically, and the chemical composition of the surface can change. Standard automatic cleaning processes consist of the following elements:  
1. Various cleaning steps make use of acids or alkaline solutions, depending on the glass material used, to remove polishing residuals or fingerprints from handling. 2. By increasing the ultrasonic frequency in the subsequent steps, particles with ever-decreasing dimensions can be removed. 3. Between successive cleaning steps, the chemical solution has to be removed quickly from the substrate surface using permeate as well as deionized water. 4. Drying of the substrates is crucial and can be done by infrared radiation, hot airflow, or an alcoholic vapor.