ultrashort laser pulses

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)


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)


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.



With further movement along the

z-axis, the trench has become

completely clogged with

sapphire particulates.

ultrashort laser pulses

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