Controlled experiments and trials have led to improved throughput and cost reduction in filter glass polishing. This presentation explores how to achieve a deterministic process to reduce variability in optical polishing. Learn more: http://nanophase.com/markets/optical-surface-polishing/ "Deterministic Polishing from Theory to Practice" was presented by Abigail Hooper at Optifab 2015 in Rochester, NY.

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Deterministic Polishing
from Theory to Practice
Abigail Hooper, Nathan Hoffmann, and Harry
Sarkas (Nanophase)
John Escolas and Zachary Hobbs (Sydor Optics)

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Outline
• Deterministic polishing and how to achieve it
• DOE to explore impact of slurry stability in reducing cycle
time and rework
• Verification on fabrication line
• Key findings and conclusions
• Questions
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Deterministic Polishing
• Deterministic: A process which
produces a predictable output from
a given starting condition
• Polishing notoriously contains high
levels of variability
– Multiple variables and interactions
• Variability often leads to material
rework
– Yield is often calculated at end of
processing
– Reworking parts just once adds time to
the polishing process
– Deterministic polishing improves single
pass yield
Process
variability
Rework
Increased
Costs
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The missing piece
• Slurry Type
– Powder or Dispersion
– Particle Size and
Distribution
– Oxide density and purity
– Chemical Stability
• Ion sensitivity
• pH range
– Additives
• Dispersing Agents
• Lubricants
• Rheology modifiers
4
Slurry Type
Flow Rate
Downforce
RPM
Operator
Glass Type
Slurry Concentration
Pad Type
Slurry Type

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Slurry “Break-In” Period
• Powder in water or unstabilized slurries require a break in period
before they reach their ability to deliver optimal surface quality
• Oxides of lower density and purity are highly susceptible to particle
fracturing
• Polishing process results in a mechanical break down of the particle
size distribution
• The break-in behavior leads to process variability and restarts each
time the slurry take is topped off
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Slurry “Break-In” Period
6
0
5
10
15
0.0 0.1 1.0 10.0 100.0
Frequency(%)
Particle Size (Microns, Dist. Base Volume)
0 mins 0.5 mins 2 mins 7 mins 17 mins 37 mins
0
5
10
15
0.0 0.1 1.0 10.0 100.0
Frequency(%)
Effect of Sonciation Time on Particle Size Distribution Using Horiba LA-910 Laser Light Scattering (RRI 1.50 - 0.10i)
NanoArc CE-6752
Commercial Cerium Oxide Slurry
Starting
End Point

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Improving productivity
• Objective: Develop a deterministic polishing processes that reduces
cycle time and improves efficiencies on a commercial high volume
filter glass polishing line when using a stabilized polishing slurry.
– Keys to success:
– Stabilized polishing slurry
– Establish optimized polishing parameters
– Model in the lab and verify at commercial scale
– Reduce commercial scale cycle time from 2-3 hours to a predictable 90 minutes
or less
– Maintain or exceed current optic quality
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Design of Experiments (DOE)
• Factors
– Ceria Slurry Solids
• 1.25 wt% - 10 wt%
– Flow Rate
• 50 mL/min – 400 mL/min
– RPM
• 26 RPM – 60 RPM
• Responses
– Removal Rate (Å/min)
– Surface Roughness (RMS, nm)
– Transmitted Wavefront (PV, λ at 632.8 nm)
– Scratch-Dig
• Central Composite Design: Box-Wilson
• Inscribed within specified parameter
minimum and maximums
• Rotatable around the center point
• Randomized run schedule
• 20 total experiments including 6 center
point conditions
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Lab Experiment Setup
• PR Hoffman PR-1 66T Double-Sided Polisher
• Fixed downforce at 2.0 PSI
• Suba X Embossed Pad, no change throughout experiment
• 5 kilograms of fresh NanoArc® CE-6752 used per run
• Schott B-270 Discs
– Cutting and grinding completed by Sydor
– 3 discs per run, 1-disc per 66T carrier
– Discs are 6.5 cm in diameter and 0.5 cm thick
• Run time dictated by total glass removal per disc
– Pre-experiments conducted to estimate removal rate
– All discs had 30-40 microns of glass removal
– Discs were flipped in carriers at estimated halfway point
• Controls staggered throughout the experiment to help distribute any experimental noise
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Measurement of Responses
• Gravimetrically determined removal rate (Å/min)
• Zygo NewView 8000 for surface roughness (RMS, nm)
– 20X Mirau objective, 2X zoom
– Zernike piston, tilt, power and sphere removal
• Zygo Mark GPI Interferometer for Transmitted Wave-front (PV, ʎ at 632.8 nm)
– 6” beam expander
– 90% aperture, no trim
• Scratch-Dig
– MIL-PRF-13830B, 45 W Fluorescent bulb, Edmunds Paddle
– 90% Aperture
– Scored: 1= Exceed (<60/40), 3=Meets Spec. (=60/40), 6=Fails Spec. (>60/40)
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Statistical Analysis
• Data analysis completed with JMP® (SAS) statistical software
• Examined effect of each factor and interaction between factors
• F ratio is used to test whether the hypothesis is real and statistically significant
when compared against experimental noise
– F ratio > 2 indicates a significant effect
– F ratio > 4 indicates a highly significant effect
• “Prediction Profiler” used to visualize predicted responses and identify a
preferred set of run conditions based on the objective
– Error bars denote 95% confidence limits
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Statistical Responses
12
• DOE identified strong responses:
– Removal Rate
• ↑RPM = ↑ Removal Rate (F Ratio = 797)
• ↑wt% Solids = ↑ Removal Rate (F Ratio = 41)
– Surface Roughness
• ↑RPM = ↓Surface Roughness (F Ratio = 13)
• ↑wt% Solids = ↑ Surface Roughness (F Ratio = 14)
– Transmitted Wavefront
• ↑wt% Solids = ↓Transmitted Wavefront (F Ratio = 20)
– Scratch-Dig
• Scratch-Dig = No strong response
RPM 43
Flow Rate 225
wt% Ceria 5.625
Hold Values
Flow Rate*RPM
60504030
400
300
200
1 00
wt% Ceria*RPM
60504030
1 0
8
6
4
2
wt% Ceria*Flow Rate
4003002001 00
1 0
8
6
4
2
>
–
–
–
–
< 1 .0
1 .0 1 .2
1 .2 1 .4
1 .4 1 .6
1 .6 1 .8
1 .8
RMS
Roughness,
Surface
Contour Plots of Surface Roughness, RMS
RPM 43
Flow Rate 225
wt% Ceria 5.625
Hold Values
Flow Rate*RPM
60504030
400
300
200
1 00
wt% Ceria*RPM
60504030
1 0
8
6
4
2
wt% Ceria*Flow Rate
4003002001 00
1 0
8
6
4
2
>
–
–
–
–
–
< 0.1 5
0.1 5 0.20
0.20 0.25
0.25 0.30
0.30 0.35
0.35 0.40
0.40
PV
Wavefont,
Transmitted
Contour Plots of Transmitted Wavefont, PV

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Optimum Condition Selection
• Used the “Prediction Profiler” in the JMP®
software to determine optimum condition that
met all material specifications:
– Surface Roughness of 1.0 – 1.5 nm, RMS
– Transmitted Wavefront Error < 0.25 ʎ, PV
– Scratch-Dig ≤ 60/40
– Maximize Removal Rate
• Optimal condition outputs on PR-1:
– Ceria solids: 7.5 wt% (Baume: 10°)
– Machine RPM: 53 RPM
– Flow rate: 125 mL/min
• Model validated through confirmation run-
confirms deterministic polishing can be achieved
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Examination of Removal Rate and Surface Roughness on
Schott B-270 Using Optimal Run Parameters
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13,950
15,967 16,066
14,989
17,480 17,782
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
8 18 28
SurfaceRoughness(RMS,nm)
RemovalRate(Å/min)
Microns of B-270 glass Removal
53 RPM, 7.5 wt% ceria, 2.0 PSI downforce on a Suba X Embossed pad; PR-1 double-sided polisher
CE-6752 (MRR) Competitor Powder (MRR) CE-6752 (RMS) Competitor Powder (RMS)

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Examination of Removal Rate and Surface Roughness on
Schott B-270 Using Optimal Run Parameters
15
13,950
15,967 16,066
14,989
17,480 17,782
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
8 18 28
SurfaceRoughness(RMS,nm)
RemovalRate(Å/min)
Microns of B-270 glass Removal
53 RPM, 7.5 wt% ceria, 2.0 PSI downforce on a Suba X Embossed pad; PR-1 double-sided polisher
CE-6752 (MRR) Competitor Powder (MRR) CE-6752 (RMS) Competitor Powder (RMS)
Average RMS: 1.38 nm
78% of discs in spec
Average RMS: 2.36 nm
6% of discs in spec
Average RMS: 1.21 nm
100% of discs in spec
Average RMS: 2.39 nm
0% of discs in spec

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Results Summary
CE-6752
Commercial
Powder
Average Removal Rate
(Å/min)
15,328 16,989
Surface Roughness
(RMS, nm)
1.21 2.26
Transmitted Wavefront
(PV, ʎ @ 632.8nm)
0.139 0.136
Scratch-Dig <60-40 <60-40
Time
(Minutes)
18 16
Glass Removed
(Microns)
27.6 27.2

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NewView
NanoArc® CE-6752 Commercial Ceria Powder
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What about removal rate?
• Commercial competitor powder is 11% faster in MRR than CE-6752
• Based on disc roughness, estimate another 10 microns of glass needs to be
removed (38 microns total) on commercial competitor powder polished discs to
get RMS in spec (2.3 nm to <1.5 nm)
• CE-6752 removed 28 microns in 18 minutes
• Commercial competitor powder removed 28 microns in 16 minutes
– 1.6989 microns/min
– Additional 10 microns would require 6 additional minutes of polishing
– Net 4 minutes additional polishing over CE-6752 polished discs
• Result is a 25% longer polishing cycle with commercial competitor powder
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Setup: Fabrication Line Testing
• Test conducted by Sydor Optics on an active polishing fabrication line currently
using the previously tested commercial competitor 1-2 micron ceria powder to
polish Schott B-270 glass
• Current fabrication line experiences processing times of 2-3 hours to reach target
specifications
– Goal time is 90 minutes or less
• Target Specifications:
– Surface Roughness < 1.5 nm, RMS
– Meet or exceed 60/40 scratch-dig
– Transmitted Wavefront < 1 ʎ
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Fabrication Line Testing Results
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*Parameters resulting in quality and time changes between test 1 – 3 are proprietary to Sydor Optics

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Further Testing
• Additional testing to validate the deterministic polishing program past the
testing phase was completed using NanoArc CE-6755, a stabilized and
rheologically modified high solids slurry
• The stabilized slurry allowed for deterministic polishing over several consecutive
days with no slurry top-offs on various glass types including Fused Silica, BK7,
B-270 and Borofloat
– All quality and MRR metrics were achieved
– Machine cleanability was improved over the tested commercial competitive ceria
21
Removal Rates in µm/hr
Glass Type CE-6755
Commercial
1 -2 µm Ceria
Fused Silica 10.4 10.0
BK7 14.0 14.2
Borofloat 16.3 17.6
B-270 25.0 24.0

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Conclusions
• Deterministic polishing can be achieved when using a robust,
stabilized polishing slurry
• Optimal conditions for deterministic polishing can be determined
using a statistical approach (DOE)
• Robust stabilized polishing slurry together with optimal processing
conditions determined in the lab can be successfully transferred to
a commercial polishing line
• Sydor Optics was able to achieve improved part quality while
drastically reducing cycle time from 2-3 hours to 45 minutes
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Acknowledgments
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