Micro-machining of transparent materials with
nano, pico and femtosecond lasers
- a review
M.R.H. Knowles
Oxford Lasers Ltd., Unit 8, Moorbrook Park, Didcot, Oxon OX11 7HP.
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AILU 2007
1. Motivation – Applications & Markets
2. Laser processing of transparent media
3. Examples
4. Conclusions
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AILU 2007
“There is good reason to believe that the impact of
photonics in the 21st Century will be as significant as
electronics was in the 20th or steam in the 19th.”
Lord Sainsbury
QE2 Conference
13th July 2006
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AILU 2007
Photonic Applications
• Rely on transparent media
• Processing of transparent media on milli & micro & nano
scale is increasingly important.
• Transparent media encountered in Photonics include
glass, fused silica, sapphire, PMMA, polycarbonate
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AILU 2007
Photonic Applications
• Displays – TV, PC, mobile phones
• Sensors – healthcare,
healthcare security,
security industrial
• Lighting – LEDs, OLEDs
• Energy – solar cells
• Communications – high bandwidth fibre optics
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AILU 2007
Photonic Markets
• £150 billion for components & enabled products in 2004
• £300 - £600 billion estimate for 2015
If the 1970s – 1980s saw the birth of laser processing of
metals,
t l the
th 1980s
1980 – 1990s
1990 the
th breakthrough
b kth
h off laser
l
applications in electronics and semiconductors then it
seems 2000 – 2020 will be the era of growth into
processing of transparent photonic materials.
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AILU 2007
Laser Processing of Transparent Media
• Definition of transparent – lets visible light through
 laser processing with UV & IR would seem good choice
 also ultrafast (ps & fs) lasers
R t i t this
Restrict
thi talk
t lk to
t UV and
d ultrafast
lt f t lasers
l
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AILU 2007
Laser Processing of Transparent Media
The penetration to which a laser pulse interacts with material is
determined by optical and thermal penetration
L = L op + L th
In dielectrics optical penetration dominates over thermal and for long
pulses (>ns) strongly depends on wavelength.
 UV lasers <300nm





266nm Nd
255nm CVL
248nm KrF
93
ArF
193nm
157nm F2
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AILU 2007
Laser Processing of Transparent Media
Ult f t Lasers
Ultra-fast
L
High
g intensity
y “rips”
p electrons out of the lattice.
Resulting ions repel each other and cause a “Coulomb” explosion.
Coulomb explosion is a non-thermal ablation mechanism.
Wavelength of laser becomes less important as pulses become shorter
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AILU 2007
Examples
Fibre Bragg Gratings
- inscription of grating structure in fibres
- telecoms, sensors, fibre lasers
Fib structuring
Fibre
i
- milling
illi & d
drilling
illi off fib
fibres
- medical devices, sensors, telecoms
Micro-fluidics
- milling & drilling glass or polymers
- lab-on-chip
Display & Solar
- ablation of thin films
- structuring of “transparent” electrical circuits
Lighting
- ablation thin films & sapphire dicing
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AILU 2007
Fibre Bragg Gratings
• inscription of grating structure in fibres
• modification of the refractive index in a periodic manner
• induced refractive index modulation in the fibre core achieved by
exposing fibre to modulated UV beam
• satisfying Bragg condition means a single  can be selected,  ideal
as filters in optical
p
networks
• telecoms, sensors, fibre lasersUV
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AILU 2007
Fibre Bragg Gratings
• 2 methods used to produce FBGs
Interferometer
Phase mask
U V L aser
B eam
From
Light
Source
C y li n d r i c a l L e n s
P h ase
M a sk
Optical
Fiber

-1
Order
Cylindrical
Lens
x
z
c
UV L
Laser
Beam
O l
Overlap
Region
Phase
Mask
a
b
d
+1
Order
y
’
O p t ic a l F i b e r
O v e r la p
R e g io n
To
OSA
•easy to
t use
•harder
h d tto align
li
•fixed 
•flexible , omit zero order
•independent
independent of source coherence
requires good source coherence
•requires
high n using both methods
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12
AILU 2007
Fibre Bragg Gratings



Optical microscope image of grating written using 255nm
L
Lucent
t Ph
Photosil
t il graded
d d iindex
d B/G
B/Ge co-doped
d
d fibre
fib
Grating period of  1m
Outer Cladding
Inner Cladding
g
Core
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13
AILU 2007
10
10
0
0
-10
-10
-20
-20
-30
-30
-40
1539.5
1540.5
0
50 GHz
Transmiission (dB)
Reflec
ction (dB)
FBG filters for use in DWDM Systems
25 GHz
-40
1541.5
10
FibreCore 1250/1550
300mW at 255nm
uniform
if
1550
1550nm
gaussian, super gaussian
15-35mm
> -25dB
-10
0
-20
-10
-30
-20
-40
-30
-50
1556.0
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Transmission (dB)
Fiber Type Laser Power Ph
Phase
M
Maskk Apodised Scanning Reflectivity-
Reflection (dB)
Wavelength (nm)
-40
1557.0
1558.0
Wavelength (nm)
1559.0
14
AILU 2007
Fibre Bragg Gratings



Method 3 – Direct write inscription using femtosecond laser.
Optical
O
ti l microscope
i
image
i
off grating
ti written
itt using
i 1030nm
1030
ffs llaser
Aston University, Photonics Group
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15
AILU 2007
Fibre Structuring
Drilling or milling of features to create features that :
• interface/export the fibre photons to other devices (photodiodes)
• allow fibre photons to access a chemical for spectroscopic analysis
Bilumen catheter, hole drilled using
248nm KrF ns laser
Industrial Applications of Laser Micromaching
M. C. Gower (2000), Optics Express, 7(2) pp. 56-67
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AILU 2007
Fibre Structuring
Milling using 1030nm femtosecond laser
Laser and vision set-up
set up
View of fibre, showing fibre core
from vision system
Grooves milled into fibre
Aston Universtiy, Photonics Group
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AILU 2007
Micro-Fluidics
Microfluidics / BioMEMS becoming an important
tool for analytical chemistry
Gl
Glass,
P l
Polymers,Silicon
Sili
channels, mixers, reservoirs, diffusion chambers,
integrated electrodes, pumps, valves
chips 1
1-50
50 cm2,
channel width and depth 5-100 μm
fluid volumes handled 0.01 - 10 μL
Other etching techniques:
photolithography,
g p y DRIE
acid, p
Laser Micromachining: fast, simple, flexible, cheaper, ideal for rapid prototyping
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Optical Beam Delivery
Direct Writing
Mask Projection
Laser
Laser
Focal Plane
Image Plane
Mask Projection Technique
Direct Writing Method
(No
mask used)
Resulting Channel Profile
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UV Nanosecond Results (255nm) in Glass
Channel with cracking
Small process window with ns
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UV Nanosecond Results (193nm) in Glass
Bottom of channel
Channel with cracking
Excellent surface
Small process window with ns
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AILU 2007
UV Nanosecond Results (266nm) in PMMA
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AILU 2007
ps Laser Results - Fused Silica
Scan Velocity
Li pitch
Line
it h
Partiallyy optimized
p
results
Surface Roug
ghness, Ra [μm
m]
5 μm
= 100 mm/s
= 1 m

= 355nm
PRF
= 50kHz
Av. Pwr = 250mW
p Size = 15m

Spot
Fluence = 3 J/cm2
1.6
1.4
1.2
1
0.8
0.6
0.4
0
50
100
Laser Milled Depth [μm]
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150
Best Ra~0.434 μm
AILU 2007
ps Laser Results - Fused Silica
Surface Roughness vs Fluence
Surrf.Roughn ess, Ra [μ
μm]
3
Scan Velocity
Line pitch
2.5
2
= 1 mm/s
 = 5 m
= 3 m

= 355nm
PRF
= 10kHz
Spot Size = 2m
15
1.5
1
0.5
0
0
10
20
30
40
Ra 1
R
1.03
03 m att 80 J/
J/cm2
Ra 0.6 m at 40 J/cm2
Ave.Pow er [mW]
 Fluence plays important role in surface roughness
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AILU 2007
Femtosecond Results (100fs, 780nm) Glass
Dimitris Karnakis1, MRH Knowles1, KT Alty2, M.Schlaf2 & HV Snelling2
Comparison of Glass Processing using High Repetition Rate Femtosecond (800nm) and UV (255nm)
Nanosecond Pulsed Lasers
Photonics West 2005
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AILU 2007
Microfluidic Channels for Liquid Sample Manipulation
Flow restrictor
(8 parallel
8 µmchannels)
(8 parallel
8 µm wide
channels)
60µM
60
60 MMB
60µM
B
Bodipy
BodipyFL
di FL
FL
in 0.1M acetic acid/50%MeOH
To grounded plate
From CE channel
Electrokinetic
Electrokinetic flow channels
flow
channels
Hydrodynamic
flow channel
1µM 1µM
rhodamine
rhodamineB
B in 0.1M
in 0.1M
acetic
acetic
acid/20%
id/20%M
id/20%
MeOH
M OH
MeOH
OH
Device made in D263 glass (no dopants) with UV ns Copper laser (255nm)
Dimitris Karnakis1, MRH Knowles1, KT Alty2, M.Schlaf2 & HV Snelling2
Comparison of Glass Processing using High Repetition Rate Femtosecond (800nm) and UV (255nm)
Nanosecond Pulsed Lasers
Photonics West 2005
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AILU 2007
Glass & Sapphire Cutting
255nm ns laser (5eV)
•0.2mm
0.2mm thick borosilicate
•0.43mm
0.43mm thick sapphire
•bandgap ~4eV
•bandgap ~8eV
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AILU 2007
Sapphire substrate dicing for Blue LEDs
V-I curve shows no effect on component
Data courtesy of Institute of Photonics, University of Strathclyde, Glasgow, UK
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AILU 2007
Sapphire substrate dicing
266nm ns laser
Requires high power & high pulse freq for high throughput
Sapphire
S
pp
etch rate
2.5
etch rate
e ( m/pulse)
2
1.5
1
0.5
0
1
10
100
Fl
Fluence
(J/
(J/cm )
1000
2
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29
AILU 2007
Summary
•
Laser processing of transparent materials is an increasingly
important field, supporting several growing markets.
•
Ultrafast lasers often produce the best quality results but are still a
new technology.
•
UV ns lasers produce acceptable results in many transparent
materials
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AILU 2007