Lecture # 08

Lecture # 08
o PLA (Irradiation in Non-reactive Atmosphere)
- Fluence, Ablation rates, Penetration depth
o Effect of Wavelength, Fluence and no. of pulses on ablation
rates
o Effect of ambient on ablation rates
Ref.: Springer Series in Surface Sciences Volume 53
“Pulsed Laser Ablation of Solids-Basics, Theory and Applications”
by Mihai Stafe, Aurelian Marcu, Niculae N. Puscas (Springer)
PHL702_L08
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PLA (Irradiation in Non-reactive Atmosphere)
 Pulsed laser ablation (PLA) represents the process of material
removal under the action of short laser pulses involving heating,
melting, vaporization/ionization
 PLA becomes effective when the laser fluence >a threshold value Fth
which results in removal of at least a mono-atomic layer
 Fth depends on the material optical properties and laser wavelength
 The efficiency of material removal upon irradiation with short and
intense laser pulses in different ambient conditions is described by
the ablation rate, which gives the maximum thickness of the layer
removed during irradiation with a laser pulse
 PLA produces micro & nano craters and grooves on the irradiated
surfaces, which modify the surface properties (hardness,
hydrophobic properties, optical absorptivity, etc.). This is used to
produce micro-components for mechanical/optical devices (microlenses for optoelectronic circuits, cooling holes for aircrafts engines
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 Thermal and optical properties of the target material are very
important parameters to be accounted for in laser processing
(They determine the magnitude of the thermal and optical
penetration depths)
 Micro-machining is essential in creating intricate and useful
microstructures in a variety of configurations on different materials
 Three different important classes of materials from the electric
perspective, i.e. dielectric (borosilicate glass), semiconductor (single
crystal silicon) and metal (aluminium alloy) should be considered
Deep & Regular Ablation
Periodic pattern (∼3 microns gap) on Al (a), Si (b) and BSi (c) obtained with a
ArF laser (20 ns, 2.5 J/cm2, 193 nm, 250 pulses)
Al: 650C
(mp)
Si: 1400C
(mp)
BSi: 700C (softening temp)
238 W/mK () PHL702_L08 157W/mK ()
1.1 W/mK ()
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 Different pulse fluence leads to diff. ablated groove cross-sections
Grooves Ablated on Glass
Low F ∼0.1 J/cm2
V shaped
groove
F ∼1.4 J/cm2
High F ∼2 J/cm2
U shaped groove
 Low F (<1J/cm2): Photothermal ablation is more dominant and the thermal
energy always gradually transfers to the neighborhood between pulses.
The central region has a higher temperature than that of the adjacent
regions. This makes the central region accumulating enough energy sooner
and be ablated sooner which, is more favorable to produce a V-shape
profile, i.e., the central region is always ablated first
 High F (>2J/cm2): Photochemical effects become noticeable and the
energy is more uniformly distributed into the target area, so that this
situation is more favorable to ablation of U-shape profiles
 Effect of wavelength: Ablation rate decreases with wavelength due to
reduced optical absorptivity and high reflectivity of the target surfaces at
large wavelengths [see Fig (a), next page]
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 Ablation rate depends upon Wavelength, Fluence & Beam
diameter
LiNbO3
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 Effect of no. of pulses: At higher laser pulses, since the depth of the
crater increase, the ablation plasma is trapped inside of the crater,
leading to rapid increase of the plasma density and absorption
coefficient, and hence to a weak direct coupling of the laser energy
to the sample. The decrease of the effective laser irradiance on the
crater walls leads to the decay of the ablation rate with pulse no.
Ablation rate drops as the light is scattered and trapped within
the structures, eventually reaching an effective fluence that is
unable to cause appreciable material removal
 Effect of ambient on ablation rates: The ablation rate at high
fluences is higher in vacuum than air (see Fig., next page)
Cause: 1. Under high laser fluence, air breakdown occur
2. Greater chance for the ablated atoms and ions to escape from the
irradiated surface into vacuum
 At low fluences, the ablation rates are slightly higher in air than in
vacuum (due to formation of micro-holes in presence of gas)
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100 pulses
500 pulses
1000 pulses
Vacuum
(10 mtorr)
Air
Craters (∼150 microns diameter) drilled in Al with Ti-sapphire laser (150 fs; 10 J/cm2)
At high fluence, ablation rates are high in vacuum: The formation of a
main central channel in the vacuum is evident. In air, Al2O3 also forms
on walls of the crater
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Vacuum (10 mtorr)
Air
1000 pulses
Craters drilled at 1 J/cm2 in Al with Ti-sapphire laser
(150 fs, 810 nm) with 1,000 pulses in
vacuum (10 mtorr) (a) and air (b)
At low fluence, ablation rates are high in air: Formation of micro-holes
which progress at faster rate & presence of gas favor their formation
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Pulsed Laser Deposition
 The ablated material from the target i.e., atoms, clusters and even
droplets usually have an initial speed (perpendicular to the target
surface) that could reach values of tens of km/s, decreasing gradually
while interacting with ambient atmosphere
 By placing an object surface in front
of the ablated particles plume, part
of the particles will hit the surface
and some of them will remain on it,
gradually forming a thin film. Such a
deposition technique is called laser
deposition
 Because the material ablates as macroparticles rather than
vaporizing as atoms or molecules, PLD received much less attention
as technique for thin films
 Globally, the recognition to PLD came originated by very good “highTc” superconducting (HTSC) ceramic thin films in O2 ambient (1988)
A typical HTSC material is “Y-123” or “YBCO”. It is a mixed ternary
oxide of approximate composition (Y2O3)0.5(BaO)2(CuO)3 or
Y1Ba2Cu3O7- (Y-123). Obtaining a higher Tc, requires close
composition control. The key advantage with PLD is that it permits
congruent evaporation of these materials as such!
(In sputtering the YBCO target, the O- ions emitted from target
bombard and affect the oxygen stoichiometry in Y-123 film with a
result in the form of lower Tc. Higher oxygen pressure then helps
 Note: The sputtered films, on other hand, are free from
macroparticles (advantage). They grow atom-by-atom.
 PLD requires deposition of an intense energy pulse in a shallow
depth in the target/source material and a consequent explosive
evaporation of a thin layer before it has a time to disproportionate
 The depth involved is optical absorption depth= 1/T (T =optical
absorption coefficient or thermal diffusion depth , whichever is
larger
e.g., taking a typical value of T =105 /cm, 1/T =100 nm for YBCO for
KrF excimer laser  of 248 nm
 Thermal diffusion depth  can be defined analogous to mass
diffusion length using,
  4 t
where, =thermal diffusivity.
 For BaO (at room temp),
  s  (4W / cm / K )(6.5 g / cm3 )(0.31J / g / K )  2cm 2 / s
 For typical pulse length of 20 ns,  = 4000 nm >> 1/T of YBCO
Oxygen inlet

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