Cleaning multilayer dielectric pulse compressor gratings with top layer of HfO2 by Piranha solution

In order to improve the resistance of multilayer dielectric pulse compressor gratings (PCGs) with top layer of HfO2 to laser damage, the residues including carbon fluorinated compounds and metal compounds containing fluorine, which are left on the surface of PCG during ion reactive etching of HfO2 with working gas of CHF3, have been cleaned by Piranha solution(the mixture of 98% H2SO4 and 30% H2O2). The parameters of Piranha solution cleaning, such as cleaning temperature, composition, cleaning time, and times of cleaning, were studied systematically. Scanning electron microscope(SEM) was presented to observe the grooves of PCG and X-ray photoelectron spectroscopy(XPS) was employed to evaluate different elements on the surface of PCG before and after cleaning. The removal mechanism of residual contaminants was analyzed. The experimental results show that higher removal efficiency can be achieved with more times of cleaning and higher cleaning temperature. In the cleaning active time of 60 minutes at the temperature of 90°C, the atomic fraction of residual fluorine on the surface of PCG which were cleaned for three times or more was close to 0.1% (the resolution limitation of XPS), when the volume ratio of H2SO4 to H2O2 was 2:1.