LaserMaterial Interaction

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Laser-Material Interaction

• B.K. Paul

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Course Schedule
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Laser-Material Interaction

• Material Absorption
• Material Removal Mechanisms
• Plasma/Plume Effects
• Laser Selection
• Ultra-Fast Lasers

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Power Density
For photothermal processes
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Beam Attentuation in Matter
Infinitely small laser beam with uniform
intensity, Isurf
Beer-Lambert Law where a absorption
coeff. (m-1) 1/a penetration depth (m)
distance to 1/e (37) of beam intensity
remaining Isurf surface intensity
Beam Intensity, I
Isurf
Depth into material, z
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Laser/Material Interaction
Electron Energy
Allowed
C.B.
Band Gap (Eg) lt Eg 0.2 3 eV
(semiconductors) Eg gt 3 eV (insulators)
V.B.
Allowed
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Absorbed Intensity, Iabs(W/cm2)
Infinitely small laser beam with uniform
intensity, Isurf
Absorbed Intensity, Iabs
Depth into material, z
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Material Exposure per Pulse, E i.e. Energy
Density (J/cm2)
Infinitely small laser beam with uniform
intensity, Isurf
Exposure, E

• where
• tp FWHM pulse duration (s)
• Assumes
• pulsed laser

Depth into material, z
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Absorbed Dosage per Pulse, D (J/cm3)
Infinitely small laser beam with uniform
intensity, Isurf
Dmach
Dosage, D
zmach
Depth into material, z
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Simplified Energy Deposition FunctionOne Single
Pulse
Gaussian beam
Beam radius, r
Locus of points where the material dosage is
equivalent to the dosage needed for machining.
For photothermal machining, should Dmach be the
enthalpy for melting or evaporation?
Depth into material, z
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Enthalpy to Remove Materials
Process energy can be reduced if a material is
removed as liquid
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Laser-Material Interaction

• Material Absorption
• Material Removal Mechanisms
• Plasma/Plume Effects
• Laser Selection
• Ultra-Fast Lasers

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Photo-Chemical vs. Photo-Thermal
Harmonics of YAG
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Photo-Thermal Machining

• Recoil force from exiting vapor drives liquid
  splashing.
• Shorter pulse lasers generate higher recoil force
• Higher recoil force drives liquid farther from
  the site
• Better coupling of energy faster processing
  worse quality

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Blast Wave by a CO2 Laser
Blast Wave of PMMA by a 9.17 µm CO2 Laser pulse.
(a) 60 ns (b) 350 ns (c) 700 ns (d) 1.1 µs
(e) 3 µs (f) 5 µs (g) 10 µs (h) ?
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Photo-Chemical Ablation

• UV ablation of polymeric materials

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Blast Wave by a UV Laser
0.5 mm
Blast wave of PMMA by a UV(248 nm) Laser pulse.
(a) 750 ns (b) 1.0 µs (c) 3.0 µs (d) 6.1 µs
(e) 9.7 µs (f) 15.0 µs (g) 20.7 µs (h) ?
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Material Removal Mechanism

• Photo-Thermal
• Materials/processing conditions
• metals
• IR processing of polymers
• Effects
• Photon absorbed - heat - melting - evaporation.
• Large heat affected zone.
• Poorer qualityDebris, melt splashes
• Photo-Chemical
• UV processing of polymers
• Excellent definition

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Laser-Material Interaction

• Material Absorption
• Material Removal Mechanisms
• Plasma/Plume Effects
• Laser Selection
• Ultra-Fast Lasers

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Why does curve flatten out?
On resin, with shaped 355 nm UV
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Laser-Induced Plasma/Plume
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Plasma Reduces Reflection
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Expansion Speed of the Plume
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Effect of Gas Blow
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Laser-Material Interaction

• Material Absorption
• Material Removal Mechanisms
• Plasma/Plume Effects
• Laser Selection
• Ultra-Fast Lasers

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Choosing a Laser Wavelength
Laser Type
5th H. 4th H. 3rd H. Ar-Ion
2nd H.
NdYAG
CO2 NdYAG NdYAG NdYAG
NdYAG NdYLF
1000 nm
10,000 nm
100 nm
212 nm
266 nm
355 nm
488 nm
532 nm
1064 nm
1321 nm
510 nm, Cu/Br
308 nm, XeCl
VISIBLE
INFRARED
400 nm
750 nm
Wavelength
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Choosing a Laser Wavelength

• Type of cut
• Through cut
• Interactions between laser beam and target
  material(s)
• Blind cut
• Interactions between laser beam and laser beam
  induced plasma/plume
• Beam spot size/focusing depth
• Reliable/economical laser source

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Type of Cut
Through-cut (Single pass)
Blind-cut (Multi-pass)
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Via Drilling Lasers

• CO2 Laser
• Reliable TEA laser, high power (gt100 W), low
  rep. rate (200 Hz)
• Q-switched, new, lower power (20 W), high rep.
  rate (50 KHz)
• Solid state laser and its harmonic generation
• Reliable diode pumping, 30-50 W IR, 15 W 532 nm
  green,
• 10 W at 355 nm UV, high rep. rate (80 KHz),
• Cu/Br Laser
• Gas discharge laser, 510/580 nm, 20 W, 10 KHz

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Via Formation - CO2 Laser

• Through-cut
• wavelength at 10.6 / 9.3 µm, reasonable
  absorption by many dielectric materials
• mature technology, high output power, single pass
• rougher/darker surface helps surface absorption
• Critical for single pulse process
• Difficult to make high aspect ratio via
• larger spot
• higher wavelength, poor coupling to metals
• bad for Cu PCBs
• pure photo-thermal process, poor quality

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Via Formation - UV Laser

• Through-cut
• not as much power
• multi-pass more likely
• Better for high aspect ratio via processing
• small beam spot size - satisfactory focusing
  depth, working distance
• better coupling to dielectrics and metals
• direct drill through copper layer
• less interaction between laser beam and
  plasma/plume
• shorter than 100 ns pulse width, plume clears out
• UV not as absorptive in plume as IR
• Blind-cut
• beam shaping possible

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Beam Imaging Shaping
Scan Lens
Imaging
Upcollimator
UV Laser
Sample or Work Piece
Beam Shaper
Galvos
Before
After
Patent exclusively licensed by ESI from Sandia
National Lab.
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Shaped Beam Profile
Uniform Energy Distribution
Precise Roundness
Shaped Beam Profile (before aperture)
Uniformly-illuminated 200um Aperture
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SEM of 45um Imaged Via

• 70um thick resin-coated foil
• Via roundness gt 90
• Clean copper pad
• Excellent quality

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Laser-Material Interaction

• Material Absorption
• Material Removal Mechanisms
• Plasma/Plume Effects
• Laser Selection
• Ultra-Fast Lasers

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Machining with Ultra-Fast Lasers

• Pulse width shorter than 10 ps is commonly
  considered as ultra-fast
• Non-thermal process
• not enough time for the excited free electron to
  pass its energy to the lattice
• laser pulse is completed before plasma is formed
• Typical laser parameters
• 150 fs, 1 W _at_ 1 KHz, _at_ 800 nm wavelength
  (TiSapphire)
• However, at lower power density, metal
  melting/splashing is observed

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Via Drilling with Ultra-Fast Laser (400 nm)
 Copper drilled with multiple fs laser pulses
Copper drilled with four fs laser
pulses (0.8 µJ/pulse)
(0.9
µJ/pulse)