Plasma process is extremely exciting and any in-vacuum optics is easily contaminated by unexpected coatings on mirrors or lenses which are essential to keep the best optical performance for metrology instruments. A special designed in-vacuum optics protection device is able to prevent most of the strong contaminations from the plasma processes, such as sputtering, plasma-CVD or etching. With applying this device, the monitoring stability of plasma process control is assured and preventive maintenance is extended tremendously as well.~~~ Supported by Dr. Thomas Schütte of PLASUS ~~~
