Super-polished aluminum mirrors through the application of chemical mechanical polishing techniques

Chemical Mechanical Polishing, also referred to Chemical Mechanical Planarization (CMP), is one of the enabling technologies which allows the fabrication of high performance multi-level metal structures in IC fabrication. In this paper we will discuss the specific application of CMP techniques to aluminum mirror polishing and the resultant super polished finish obtained. Current aluminum mirror processing methods use combinations of machining, lapping and diamond turning operations to achieve required surface accuracy and quality. Optimum results from diamond turning yields surface figure with an error of no less than half a wave and surface roughness no less than 50 angstrom aluminum substrates. In addition, diamond turning puts "grooves" onto the surface that act as a diffractive element resulting in specular beam power loss and ghost images. Often these diffractive and scatter effects, inherent to grooved surfaces, are too severe to provide adequate performance in the UV and visible range. Further, the low signal to noise ratio of the optical system reduces resolution and the overall efficiency of the optical system. A new procedure for polishing bare 6061-T6 Aluminum monolithic mirrors using Chemical Mechanical Planarization (CMP) slurry and techniques yields extremely high quality, low scatter mirrors. Planar aluminum mirrors with flatness equivalent to lambda/10 and R_a <2 nm have been polished and measured on a Veeco NT3300 white light Interferometer (at 20X). Comparison of the power spectral density curves of mirrors produced via CMP with those presently produced with diamond turning shows reduction across the range of spatial frequencies (1-10³ mm^-1) and elimination of the grooving frequency. Both white light interferometer and AFM images show the polished surfaces to be smooth, pit free with no pull outs.