Table 10.1: Substrate properties before submersion superpolishing
Table 10.2: Submersion polishing proved to reduce RMS surface roughness
from >7Å to <1Å. More details may be found in our SPIE conference
proceedings2,4
www.edmundoptics.co.uk/LO 31
Increasing spatial frequency range typically comes at a tradeoff of a decreasing
field of view. AFM can directly measure sub-angstrom surfaces,
but a small field of view and high sensitivity make AFM more ideal for
laboratory use rather than production environments. Data correlation
between WLI and AFM, in tandem with steps to maximize the performance
of the WLI, have allowed Edmund Optics® to confirm WLI as an
effective tool for measuring sub-angstrom RMS surface roughness in a
production setting.2
Section 10.2: Manufacturing
Superpolished Optics
Conventional optical polishing is a subtractive and iterative processwhere
increasingly finer grits of abrasives are used to remove damage
from an optical surface caused by earlier grinding and polishing stages.
No matter how fine a grit is used, loose abrasive polishing will result in
some level of subsurface damage. This damage at and below the optical
surface increases surface roughness, energy absorption, and scatter,
which generates heat and decreases system efficiency. Optical scatter is
proportional to the surface roughness squared.
However, the superpolishing process developed by Edmund Optics®
eliminates subsurface damage by changing focus from a mechanical
loose abrasive polishing process to the chemical reactions between the
glass, slurry, and polishing lap. Mechanical force is only used to remove
particles from the surface as reactions occur in the Beilby layer. Silica
glass is insoluble in water, but during polishing a silica layer modified by
the diffusion of hydroxyl ions is formed called the Beilby layer. Once this
is formed it protects the optic from further change.3
Submersion polishing is used to create superpolished sub-angstrom
roughness optics. A hydrated lap with slurry is kept at the same temperature
as the substrate while temperature and pH level are highly controlled.
This facilitates a chemical reaction while surface tension creates
a protective barrier against contaminants.4
Section 10.3: Superpolished Optics
from Edmund Optics®
Edmund Optics demonstrated that sub-angstrom optics could be repeatedly
manufactured on planar and spherical fused silica substrates
(Figure 10.2). No observable surface structure or subsurface damage
were left behind on the optics from polishing (Tables 10.1 and 10.2).
References
1. Leslie L. Deck, Chris Evans, "High performance Fizeau and scanning
whitelight interferometers for mid-spatial frequency optical testing of
free-form optics," Proc. SPIE 5921, Advances in Metrology for X-Ray and
EUV Optics, 59210A (31 August 2005); doi: 10.1117/12.616874
2. Shawn Iles, Jayson Nelson, "Sub-angstrom surface roughness metrology
with the white light interferometer," Proc. SPIE 11175, Optifab 2019,
1117519 (15 November 2019); https://doi.org/10.1117/12.2536683
3. Finch, G. Ingle. “The Beilby Layer on Non-Metals.” Nature, vol. 138, no.
3502, 1936, pp. 1010–1010., doi:10.1038/1381010a0.
4. Jayson Nelson, Shawn Iles, "Creating sub angstrom surfaces on planar and
spherical substrates," Proc. SPIE 11175, Optifab 2019, 1117505
(15 November 2019); https://doi.org/10.1117/12.2536689
5. Peter D. Groot, “The Meaning and Measure of Vertical Resolution in
Optical Surface Topography Measurement.” Applied Sciences, 7(1), 54
(5 January 2017) doi:10.3390/app7010054
Fused Silica Optics Before Superpolishing
P-V (Å) RMS (Å) Ra (Å)
Average 183.42 7.42 5.70
Range 2089.92 18.24 11.19
Standard Deviation 186.88 2.91 1.82
Fused Silica Optics After Superpolishing
P-V (Å) RMS (Å) Ra (Å)
Average 14.24 0.91 0.77
Range 2.26 0.03 0.21
Standard Deviation 1.14 0.02 0.06
/LO