Research On Laser-Cleaning Micron And Submicron Contaminate Particles From Super-Smooth Optical Glass Substrate

Particle contamination adversely affects performance of optical substrate and optical devices. Especially for the high performance and quality of super-smooth optical glass substrate that includes mechanically polished optical substrate and chemical-mechanically polished optical substrate, particulate contamination becomes the main problem of total yield loss. Therefore, cleaning of micron and submicron contaminate particles has become an important procedure in the process of glass substrate with super-smooth optical surface, and also become one of the key solving technology to ensure the quality of substrate. Laser cleaning is a newly-developed cleaning method, which is characterized by powerful removing force, non-contact cleaning, high flexibility and environment-friendly method. Therefore, it is tar-reaching to introduce laser-cleaning technology to clean the super-smooth surface of optical glass substrate.This paper first analyzes the particle removing mechanisms by means of dry laser cleaning and stream laser cleaning respectively in detail. Based on those, the effective ways to cleaning classic super-smooth optical wafers are put forward and optimized cleaning parameters are gained through a serial of experiments. The main contents are as follows:1. Using Scan Electron Microscope(SEM) and Electron Probe Analysis(EPA) to analyze the post-polishing surface dealt with traditional cleaning method, we discover that the main residual contaminate is micron and submicron polishing particles adhered to the super-smooth surface. According to the research on the adhesion between particle and surface, the adhesion force between optical surface and micron particle is discussed. Van der Waals attraction, electrostatic force and capillary adhesion force are the three main forces for micron particle adhered to surface, andfurther research demonstrates the van der Waals attraction is the most important factor to our research objects. At last the quantified adhesion force is given by a simplified formula.2. Temperature field is the key factor affecting the process of laser material processing. Therefore, the temperature field is simulated by building laser-heating models, describing them by differential equations and solving them by finite difference method. Then, the removing force acting on the particles is presented in laser dry cleaning and the advantages and disadvantages of it are discussed. For the laser stream cleaning, the removing force is qualified by setting up a model, in which laser heats the medium film and produces bubbles in a film of medium near substrate surface, then the growing bubble impels the medium around it, at last the particles on the surface are removed by the rapidly moving medium. According to analysis on the particle removing mechanism, the optimized cleaning procedure is achieved for super-smooth optical surface by laser stream cleaning.3. Under the guidance of the mechanism analysis, a serial of laser dry cleaning and stream cleaning experiments are carried out on classic super-smooth optical glass substrates. By analyzing and synthesizing the experimental results many optimized experimental parameters are achieved.4. In order to value the cleanliness of super-smooth surface before and after laser cleaning rapidly and precisely, a software is developed to automatically value it by processing the surface images, which can present key parameters about the surface cleanliness, such as maximal contaminant particle size, the total particle number per unit, the contaminant area per unit, etc.