Sub-surface damage is a serious issue in the manufacturing of precision optical elements. For very lightweight mirrors,
changes in surface stresses through various process steps that sequentially relieve stored up strain energy lead to poor
convergence to eventually desired figures. For high fluence laser applications damage sites can prove to be deleterious
to the functioning of the optic. For precision refractive optics, birefringence resulting from damage and stress can be an
issue as well.
Conventional methods of optical finishing rely mostly on mechanical abrasion, requiring an iterative process of subsurface
damage mitigation from earlier process steps while minimizing damage from the current process step. This
manufacturing paradigm leads to very long lead times and costs in producing high precision optics.
Reactive Atom Plasma (RAP) based figuring is introduced as a technique to simultaneously remove damage from prior
steps while imparting no further damage and figuring the surface of the optic. RAP based figuring demonstrates a new
approach to the figuring of precision optics using a non-contact sub-aperture atmospheric plasma footprint to shape the
surface. RAP figuring has been illustrated to remove Twyman stresses caused by conventional optical processing
technologies. Twyman stresses on coupons of various glass materials and ceramics have been characterized and RAP
removals of the damage layer have led to removal of the strains and thence the associated stress. The process is
deterministic, enabling the figuring of high-precision surfaces with little to no sub-surface damage.
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 Ra <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.