Traditionally, aluminum optics have been produced via a combination of machining, lapping, and diamond turning
techniques. The surface roughness and diffraction grating effects resultant from diamond turning have largely limited
the use of these optics to IR applications. Work arounds for this problem have included nickel coatings which are
subsequently polished to a required finish for use in visible and/or ultraviolet spectra. Unfortunately, this introduces
additional costs as well as bimetallic effects that can limit the application of such components.
We have developed chemical mechanical polishing (CMP) techniques that allow high quality optical surfaces to be
produced on bare aluminum alloy such as 6061-T6. Alloy properties such as grain size, inclusions, and voids can
impact all types of finishing processes. The CMP method, however, has been very robust in polishing performance over
a range of alloy types and properties. Surface roughness <20 Å rms is readily attainable with this process, and values
below 10 Å have been produced with proper process conditions and alloy properties.
The monolithic mirrors produced via CMP techniques have been compared against other current alternatives such as
diamond turned aluminum, nickel coated aluminum, and aluminized glass. Data indicate the aluminum mirrors
produced via CMP can provide performance improvements versus the alternatives based on measurements comparing
parameters such as surface roughness, surface quality, reflectivity, and bidirectional reflectance distribution function.
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<sub>a</sub> <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 (1-10<sup>3</sup> mm<sup>-1</sup>) 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.