News from McPherson
Plasma Cleaning Vacuum Ultraviolet Optics to Obtain Good %T and %R Measurements
Contaminants adsorbed on substrate or film surfaces markedly affect investigative processes in the vacuum ultraviolet (VUV). Contamination is of particular concern because many materials from polishing compounds to cleaning solvents absorb strongly in the ultraviolet. We found plasma cleaning provides an easy method to consistently control the state of samples prior to transmittance and reflectance measurements in the VUVAS vacuum spectrophotometer. First tests using the plasma cleaning method with VUV Calcium Fluoride optics show 15% improvement in transmission at 140 nanometers. Plasma cleaning is an environmentally safe method of organic removal and possibly modification of surface character.
The cleaning process consists of placing the test object or sample in a glass reaction chamber that is evacuated by oil free pump. For our first tests, pure nitrogen gas was metered into the reaction chamber and pressure maintained at 200 to 500 millitorr. The low pressure gas was ionized to form plasma by applying a 2.45GHz microwave frequency. The plasma exposure required about 27 seconds process time and the improvement in vacuum ultraviolet transmission is significant.
Preliminary data sheet for plasma cleaner available here | Summary paper describing the plasma cleaning process
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