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Optimizing the processing of sapphire with ultrashort laser pulses
Geoff Lott1, Nicolas Falletto1, Pierre-Jean Devilder2, and Rainer Kling3
1Electro Scientific Industries, 2Eolite Systems, 3Alphanov
October 20, 2015 – ICALEO
Motivation
Biocompatible
Scratch resistant Optical
transparency
Chemically inert
Hard
ne
ss
Intrinsic properties
of sapphire
Laser processing of sapphire
QCW lasers for cutting and dicing
Mendes, M. et al. (2015), Fiber laser micromachining in high-
volume manufacturing, www.industrial-lasers.com.
Internal features with helical cutting
Zibner, F. et al. (2014), Ultra-high precision helical
laser cutting of sapphire, ICALEO, San Diego, USA,
M301.
Motivation
Broader utilization of sapphire for many applications has been slowed by the difficulty of laser
machining fine features onto it with industrially acceptable throughput and quality
What is the optimized industrially-viable process for micromachining of sapphire with current state-of-
the-art laser systems and standard beam delivery components?
What are the limitations of this process, and expectations going forward?
Laser and experimental apparatus
• Wavelength: 1030nm
• Pulse duration: 0.8ps
• M2 <1.2
• Max pulse energy: 25uJ spec.
• Max average power : 40W spec.
• Repetition rate: up to 3MHz
λ/4
0
10
20
30
40
50
60
70
0 1000 2000 3000
Ave
rag
e P
ow
er
(w)
Pu
lse
En
erg
y (
µJ)
Seeder Repetition Rate (kHz)
4x beam
expander
Scanlabs
hurrySCAN 20
galvo
100mm f(θ)
Aerotech
ALS130H-150 (Z)
ABL15020 (X-Y)
Sapphire wafers:
c-plane sapphire, double side polish
Thickness: 430µm (effective sample thickness of 245µm)
TTV: ≤10µm
Micro-roughness: ≤0.3nm
Chinook IR
Data analysis with
Keyence laser
profilometer
Scanlabs hurrySCAN 20 galvo
100mm f-theta lens
Bottom-up ablation process
Beam waist starts below bottom surface;
translated upwards at constant speed
while pattern is repeated continuously
400µm diameter pattern consisting of:
Inward spiral
+ Outward spiral
+ Outer circle
Methodology and process parameters
Generate holes with 400µm diameter (aspect ratio ~1)
• Learn general rules that can be modified to suit smaller or larger features
Consider realistic throughput goals: are taper (if any) and throughput related?
Pulse energy: 26.4µJ on sample
Waist diameter: 18µm
Polarization: circular
Spiral pitch: 9µm
Repetition rate varied: 21kHz, 104kHz, 260kHz, 521kHz 1042kHz
9 21kHz not shown – poor hole quality
Overlap varied: 70%, 80%, 90%, 95%, 98%
Dynamic z-speed: varied from 10µm/s to >100µm/s
9 lower speed determined by customer cycle time requirements (equivalent to 25s/hole)
9 cycle time inversely proportional to z-speed
for complete bottom-up ablation process
Process Parameters
• Good process low
taper, no cracks or
chips
• Bad process
Significant taper,
cracking, damage
rings
• Low taper, not zero
taper due to molten
sapphire redeposition
Bottom-up hole quality comparison for sapphire
High quality Low quality
Top surface
Bottom surface
x20
x10x20
x20
• Available overlap conditions
limited by max galvo speed
• In general, taper is smaller
at lower z-speed values
• Taper decreases at higher
repetition rates for identical
overlap
• Not a cold ablation process
– thermal accumulation
plays a critical role
Taper vs. z-speed for varied overlap and repetition rate
260kHz
1042kHz
104kHz
521kHz
Ta
pe
r (d
eg
ree
s)
Ta
pe
r (d
eg
ree
s)
Z-axis Translation Speed (µm/s) Z-axis Translation Speed (µm/s)4020 60 10080 120 200140 160 180 4020 60 10080 120 200140 160 180
4020 60 10080 120 200140 160 180 4020 60 10080 120 200140 160 180
10
8
6
4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
25s12s
8s6s
98%98%
95%
98%
95%
90%
95%
90%
80%
70%
Accumulation and how it affects the bottom-up ablation process
Process
window below
top surface
threshold
Threshold position for
top surface machining
Full bottom-up ablation – large accumulation effects
Transition from bottom-up to top-down (hybrid) – lower accumulation
sapphire top
sapphire bottom
Process
window
overlaps top
surface
threshold
260kHz, 90% overlap
40µm/s
260kHz, 90% overlap
45µm/s
260kHz, 90% overlap
150µm/s
Bottom-up
Top-down
Hybrid
Observation of switch from bottom-up to
top-down process was easily observed by
eye, but curvature of wall taper can also be
used to identify process type(s).
Initiation of ablation
• Two regions:
• High speed = top-down
• Low speed = bottom-up
• Inflection between regions
signifies transition from
bottom-up to hybrid process
• At this point, cycle time is no
longer inversely proportional
to z-axis translation speed
Taper vs. z-speed for varied overlap and repetition rate
260kHz
1042kHz
104kHz
521kHz
Ta
pe
r (d
eg
ree
s)
Ta
pe
r (d
eg
ree
s)
Z-axis Translation Speed (µm/s) Z-axis Translation Speed (µm/s)4020 60 10080 120 200140 160 180 4020 60 10080 120 200140 160 180
4020 60 10080 120 200140 160 180 4020 60 10080 120 200140 160 180
10
8
6
4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
25s12s
8s6s
98%98%
95%
98%
95%
90%
95%
90%
80%
70%
Damage rings observed for top-down process
Origin previously observed experimentally
and modeled by Wolfgang Schulz et al.Sun, M. et al. (2013), Numerical analysis of laser ablation and damage in glass with multiple
picosecond laser pulses. Optics Express 21(7), 7858-7867.
Damage rings on back-side of sample
onset of damage ring
Bottom-up/top-
down hybrid onsetTop-down process
Entrance edge
acts as focusing
lens
10µm/s: 25s/hole 30µm/s: 8s/hole 50µm/s: 5s/holes
104kHz, 90% overlap
These processes
are too cold(low thermal
accumulation, transition
to top-down process more
likely)
1042kHz, 98% overlap
These processes
are too hot(melt on surface, HAZ,
filamentation)
260kHz, 90% overlap
521kHz, 95% overlap
These processes
are just right
415um max
380um min 2° taper
415um max
350um min 4.3° taper
• One data point for each individual set of examined process parameters (not a yield measurement)
• Low taper high chance of excellent hole quality
• Small, tightly spaced arrays of holes demonstrate repeatable, robust process
• High confidence that many sets of process parameters result in very high yield
Cracking/damage vs. average taper
0
1
0 2 4 6 8 10
Average Taper (degrees)
< 5° taper:
No cracks observed for
86% of holes > 5° taper:
No cracks observed for
24% of holes
No cracks/damage observed
Cracks/damage observed
Best process windows
260kHz
1042kHz
104kHz
521kHz
Ta
pe
r (d
eg
ree
s)
Ta
pe
r (d
eg
ree
s)
Z-axis Translation Speed (µm/s) Z-axis Translation Speed (µm/s)4020 60 10080 120 200140 160 180 4020 60 10080 120 200140 160 180
4020 60 10080 120 200140 160 180 4020 60 10080 120 200140 160 180
10
8
6
4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
98%98%
95%
98%
95%
90%
95%
90%
80%
70%
We are able to drill
holes with less than 2
degrees taper in
under 12 seconds, or
less than 5 degrees
taper in under 5
seconds
• We have demonstrated the ability to drill small holes in sapphire wafers
with throughput that meets known customer demands
• 400µm diameter holes with <5° taper in less than 5 seconds
• 400µm diameter holes with <2° taper in less than 12 seconds
• Process speed and quality both benefit from a bottom-up process
• Avoid damage rings and cracks, minimize taper
• Aggregates of melted sapphire particulates re-adhere to the hole
sidewalls, leading to non-zero taper for all examined parameter
combinations
• Post-processing did not eliminate aggregates
Conclusions
Thank you for your attention!
Can a post-process KOH bath decrease taper?
• One hour KOH etching bath, agitated with stir bar
• Aggregated material decreases, but is not eliminated
Before KOH:
After KOH:
At the start of processing, the trench is
very clean, and mostly free of debris.
This trench is <40 µm in depth (the
‘Step’ value on the left)
Note that for diagnostic purposes, the
outer diameter of these features is
large (~1.35 mm).
Bottom-up Debris Accumulation
As the trench is made deeper,
debris starts to accumulate along
the side walls. A maximum depth
of ~120 µm is reached before
debris accumulation clogs the
trench.
Bottom-up Debris Accumulation
Bottom-up Debris Accumulation
With further movement along the
z-axis, the trench has become
completely clogged with
sapphire particulates.