UV laser polishing of thick diamond films for IR windows

Abstract

Diamond has very suitable properties for infrared (IR) window applications. The rugged surface of a free-standing 320 μm thick diamond film grown by chemical vapour deposition (CVD) is polished with the UV light of an ArF excimer laser ( λ=193 nm). The angle of incidence is 85° and irradiation is subsequently performed at three planar directions around the perpendicular axis. Scanning electron microscopy (SEM) is used to visualise the surface before and after the polishing procedure. With a profilometer (tip radius 5 μm) the improvement of the average roughness of the surface is measured. Polishing is analysed in dependence on the size of surface irregularities by means of fast Fourier transformation (FFT) of the signal of the profilometer. Surface roughness measurements are also performed on the polished surface with an atomic force microscope (AFM). With IR transmittance measurements the optical quality is analysed over a wavelength range of 2.5 to 25 μm before and after polishing. The oxidative removal of the laser induced graphitic layer is studied with optical spectroscopy techniques.