Etching_Surface_Roughness_Reduction

1.

1
Etching and
Surface Roughness
Reduction
Internal-wall smoothing
sequence
as-etched
cleaned
annealed
Focused scientific summary for wet-etched glass
channels
with emphasis on SG 3.3 vs SG 3.4 process strategy.
Key question
Which post-etch route reduces wall
roughness
without adding geometry drift or
residue risk?
ETCHING • SURFACE ROUGHNESS
HF etch
→ HCl residue removal
→ mild polish
→ cautious anneal
Scientific summary deck

2.

2
Material choice sets the roughness-control
problem
SG 3.3 is the safer wet-etch baseline; SG 3.4 is flatter and stronger but more chemistry-sensitive.
SG 3.3
SG 3.4
Corning PYREX
borosilicate
Si-matched carrier glass
• Better documented HF / buffered-
• As-supplied surface roughness < 1.0
fluoride behavior
More predictable wet-etch response
• Lower published HF durability loss:
0.89 mg/cm² (10% HF)
Lower annealing point: 546 °C
nm
• Higher stiffness / hardness /
thermal robustness
• No current published HF durability
table
• Proxy data suggest much faster HF
attack and higher residue sensitivity
HF durability signal
(lower is easier to
manage)
SG 3.3
0.89
SGW3
proxy
5.18
Interpretatio
n
SG 3.4 likely demands
tighter control of chemistry,
residue removal, and coupon
screening before transfer to
real parts.
Critical nuance: the SG 3.4 HF number
is not a direct published value; it is an
SGW3-family proxy.
Source summary in speaker notes
ETCHING • SURFACE ROUGHNESS
Scientific summary deck

3.

3
Why post-etch roughness rises
The rough surface is not only an etch-rate issue; it is also a residue, leaching, and cleanup issue.
1
1 Fluoride
attack
Glass dissolves, but
local chemistry
depends on
composition and acid
balance.
2
2 Insoluble
products
Reaction byproducts
and sludge can linger
on the wall and seed
microroughness.
3
3 Local
topography
change
Preferential leaching
and chemistrysensitive cleanup
amplify nanoscale
height variation.
4
4 Optical /
flow penalty
Scattering, matte
walls, and higher
drag become visible
even when total
material removal is
similar.
Process implication for SG 3.4: prioritize residue removal and chemistry-screening before
assuming that a faster etch will polish the surface.
Source summary in speaker notes
ETCHING • SURFACE ROUGHNESS
Scientific summary deck

4.

4
Chemical routes with the strongest support
Best-supported improvement: combine HF with HCl cleanup/polish instead of relying on HF
alone.
Direct evidence on
borosilicate
1 Residue removal
Reported surface
roughness
HF only
34 nm
2 Mild polishing step
HF/HCl 10:1
Takeaway
~5 nm
HCl is useful because it helps clear
insoluble fluoride products that
otherwise leave the wall roughened
during longer HF exposure.
3 Secondary options
After HF etch, use HCl cleanup first to
remove fluoride residue/sludge. This is the
strongest low-risk action for SG 3.4-like
behavior.
If roughness is still too high, test a very mild
HF/HCl polish on coupons before moving to
device parts or buried channels.
Buffered NH₄F/HF can improve uniformity;
KOH/ethanol is promising on borosilicate
coverslips but remains narrow evidence for
SG 3.4.
Source summary in speaker notes
ETCHING • SURFACE ROUGHNESS
Scientific summary deck

5.

5
Recommended screening sequence for SG
3.4
channels
Keep the
sequence short, evidence-based, and coupon-driven before transfer to internal
features.
1
Baseline
HF etch
Establish
starting
roughness and
visible residue
state.
Stop criteria for chemistry
loop
2
HCl
cleanup
Remove fluoride
products before
judging wall
quality.
Low residue, lower scattering, no obvious
channel widening, repeatable coupon result.
3
Mild HF/HCl
polish
Apply only on
coupons; tune
time
conservatively.
4
Optional
furnace
anneal
Use when
internal walls
remain rough
after chemistry
control.
Transfe
r
5
Move only
the winning
condition to
real parts.
What not to do
Do not transfer a Pyrex recipe directly to SG 3.4. The
uploaded summary explicitly warns that the best
recipe is glass-specific.
Source summary in speaker notes
ETCHING • SURFACE ROUGHNESS
Scientific summary deck

6.

6
Thermal annealing targets high-frequency
roughness
Useful for buried or internal channels because the whole part can be smoothed without line-ofsight access.
Reported effect in FLICE
literature
Practical operating
window for SG 3.4
Average
roughness
Asetched
49
nm
Anneale
d
19
nm
722 °C anneal
screen
here
669 °C strain
Benefit
Best path for internal
High-frequency asperities are reduced
more than low-frequency form.
walls when chemistry
alone cannot smooth
them.
Possible
feature rounding,
Risk
slight widening, and
bow/warp drift if the
thermal cycle is too
aggressive.
Source summary in speaker notes
ETCHING • SURFACE ROUGHNESS
Scientific summary deck

7.

7
Decision summary
Use the minimum sequence that removes residue first, then adds polishing or annealing only
when needed.
Recommended default
path
For SG 3.4 internal
HF
etch → HCl residue
channels
removal → coupon-only mild
HF/HCl polish → cautious
anneal only if chemistry
leaves internal-wall scatter.
SG
3.3 is
the safer glass when
Bottom
line
smooth, predictable wet etching is
the priority. SG 3.4 can still work,
but the process window is
narrower.
What each route is best
for
HCl cleanup
Highest confidence first
step
Strongest literature support
Low geometry risk
HF/HCl polish for smoother borosilicate
Must be screened
on coupons
Buffered
NH₄F/HF
Uniformity tuning
Supportive, not
primary evidence
for SG 3.4
Furnace
anneal
Buried/internal walls after
chemistry is optimized
Watch form drift
and widening
surfaces
Source summary in speaker notes
ETCHING • SURFACE ROUGHNESS
Scientific summary deck