MEMS: Fabrication
Lecture 12:
Micromachining with
Laser
Prasanna S. Gandhi
Assistant Professor,
Department of Mechanical Engineering,
Indian Institute of Technology, Bombay,
Recap
ƒ Laser fundamentals
ƒ Stimulated emission
ƒ Laser beam characteristics
ƒ Laser demo: Excimer laser
Today
ƒ Practical laser design: Excimer laser
example
ƒ Laser micromachining: Excimer laser
example
Population Inversion
dN12 = BuN1dt
(absorption)
dN 21, spon = AN 2 dt
(spontaneous emission)
dN 21, stim = BuN 2 dt
(Stimulated emission)
N1 = no of atoms in E1 state
dN = no of transitions in time dt
A, B = Eienstein Coefficients
A 8πhν
=
B
C03
3
N2
=e
N1
−
hν
κT
ƒ This probability increase in spontaneous emission to
power 3 of inverse wavelength makes it difficult to
design lasers with short wavelength
Principle Components
of Laser System
ƒ Active medium
ƒ Source of pumping energy
ƒ Discharge excitation (electrical current thro medium)
ƒ Electrical or ion beam excitation (pulses of e/ions
deposited in medium from accelerator)
ƒ Microwave excitation
ƒ Chemical excitation (exothermic reaction)
ƒ Nuclear excitation (nuclear reaction)
ƒ Resonating cavity
L
plane-parallel resonator
(marginal stability)
Light oscillator
(laser cavity, laser resonator)
r1 = L
r1 hemiconfocal resonator
(stable)
Laser
medium
mirror
(R=100%)
r 1 = r2 = L
Output mirror
(R<100%)
r1 confocal resonator r2
r1 = r2 =
Principle set-up of various mirror
configurations for a laser resonator
L
2
r1 concentric resonator r2
r1 + r2
= L
2
r1
confocal unstable r2
resonator
Source of Pumping
ƒ Electron or ion beam excitation (pulses of e/ions
deposited in medium from accelerator)
ƒ High pulse energy, Low pulse frequency
ƒ Short tube life and higher cost
ƒ Microwave excitation
ƒ Low pulse energy (100µJ),Good pulse freq (upto 8kHz)
ƒ Chemical excitation (exothermic reaction)
ƒ Ok but not feasible
ƒ Discharge excitation:
ƒ After 10-30ns discharge in excimerÆ spark discharge:
pulse limit
Excimer Laser
• Created by IBM, Excimer lasers (the name is derived from the terms
excited and dimers) use reactive gases, such as chlorine and fluorine,
mixed with inert gases such as argon, krypton or xenon. When
electrically stimulated, a diatomic pseudo molecule (dimer) usually of an
inert gas atom and a halide atom is produced that in the excited state.
These diatomic molecules have very short lifetimes and dissociate
releasing the excitation energy through UV photons.
Consider collision between hydrogen and
chlorine atoms. At very small energies, the
two atoms come close to each other and the
repulsive forces are converted to attraction
forces primarily due to interaction of outer
shells. This is called ‘molecular bonding’. Not
all elements can form such bonds.
In Xe and Ar atoms, the forces remain
repulsive. Under normal circumstances no
bonding is possible. But in the excited stage
chemical reaction takes place and a bond is
formed between them.
Excimer Laser: KrF
Pumping
e + Kr Î Kr+ + e + e
- positive inert gas ion formation
e + Kr Î Kr* + e
- inert gas in metastable condition
e + F2 Î F- + F+
- negative halogen ion formation
Kr* + F- + M Î KrF* + M
-KrF production
Kr* + F2 Î KrF* + F - KrF
production
Stimulated emission
KrF* + hv Î Kr + F + 2hv (248 nm)
- laser emission.
Excited dimer
Excited Complex : Exciplex
Excimer Laser
Excitation: the method of excitation can be either by electron beams
(highest pulse energy, high cost), micorwaves (with two electrodes
outside the a laser gas filled capillary tube) low pulse energy) or gas
discharge.
In gas discharge a high pressure gas vessel is used.
0.1 to 0.5 % - Halogen component
5 to 10 % of inert gas component
Buffer gas
•The preionization pins produce
a spark discharge which
produce UV radiation that is
sufficient to preionize the laser
gas between the electrodes.
•Discharge circuits::
•10 ns Limit on pulse duration
because of arcing
Excimer Laser
The excimer lasers operating at about 2% conversion efficiency
between electrical power input and optical power output, the
excess energy is to be removed as heat.
A heat exchanger with water as a cooling medium is used.
Optics for Laser
Micromachining
ƒ Beam conditioning optics
ƒ Homogenizer (array of cylindrical lenses)
ƒ Beam collimator
ƒ Optics for reduction and focussing
Laser Micro-machining - Definitions
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Ablation: The use of a laser to remove any material by
vaporization.
Absorption: The loss of light as it passes through a material,
generally due to its conversion to other energy forms (typically
heat).
Avalanche Ionization: Free electrons colliding with a surrounding
atoms, and breaking off more free electrons, create additional
free electrons at an exponential rate.
Conductivity: A material property that is the inverse to its
resistance to the flow of electricity.
Features: While we have yet to create features this small to date
in materials, the principle has been demonstrated.
Free Electrons: Electrons in the outer orbit around the nucleous
of an atom, they can be moved out of orbit comparatively easily.
Laser Micro-machining - Definitions
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Heat-Affected Zone (HAZ): The process has a threshold. Below
that threshold energy from the laser pulse may be absorbed into
the material and converted to heat that will dissipate into the
surrounding material. Since the beam profile typically does not
have sharp edges, some energy in the beam may be below the
threshold for ablation. How much gets into the surrounding
material depends on the exact beam shape, its relation to the
threshold for ablation and the repetition rate of the laser.
Typically, however, some set of conditions can be chosen to
minimize these effects. The price for this minimization may be
thruput; i.e. how fast material can be removed from the target. In
some cases this may be unacceptably slow.
Heat-diffusion time: All tool bits deposit mechanical energy into
the material that is being machined, a portion of which is
converted to heat energy.
Laser Micro-machining - Definitions
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Ultrafast lasers can produce this state of matter because they
pack so many particles of light called photons into so small a
time interval that when they interact with the atoms in the
surface of the material, they strip as many as 15 electrons off
the atom. Physicists call this process multiphoton ionization.
Peak Power: The maximum power supplied by a laser pulse.
Picosecond: A fraction of a second (10-¹²). Abbrieviated as p.
Power Density: In laser beam welding or heat treating, the
instantaneous laser beam power per unit area. This parameter
is key in determining the fusion zone profile (area of base metal
melted) on a workpiece.
Recast Layer: Molten metal which forms a layer of debris on the
surface of the material during picosecond machining.
Slag: The unwanted material that is removed from metal when it
is heated to a liquid state.
Terawatt: A unit or power equal to one trillion watts.
Laser Micro-machining - Definitions
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Ultrafast: As it relates to micromachining, a laser capable of
generating light pulses that last only a few femtoseconds. This
can be achieved by nonlinear filtering to increase bandwidth and
compress the pulse or by passive modelocking or synchronous
pumping in conjunction with pulse-shaping techniques.
Laser Micro-machining
The laser beam is characterized by the half divergence angle (θ =
D/2f) and radius of beam waist, w
For an ideal beam θw = λ/π (an invariant over the whole beam
trajectory). A beam quality parameter is M2 number = [(θw) / (λ/π)]
The minimum spot diameter of a beam δ is given by –
δ = (4/π) M2 λ (f/D)
For micromachining smallest spot size is obtained when M2 ~1;
with a short wavelength and short focal length lens.
Laser Micro-machining
Laser Micro-machining
The primary requirement for laser micro-machining is availability of
laser energies in a very small pulse durations.
Ultrafast pulses of light interact with materials in micromachining
process is one on a different time scale.
Ultrafast pulses of light interact with materials therefore
micromachining process is one on a different time scale.
Miliseconds
1x10-3 second
Microseconds
1x10-6 second
Nanoseconds
1x10-9 second
Picoseconds
1x10-12 second
Femtoseconds 1x10-15 second
In other words, a femtosecond is a million times shorter than a
nanosecond. Femtosecond pulses are the fastest man-made
“objects”. Nothing, absolutely nothing, is faster!
Mechanics of
Micromachining
Simple model
I(X) = (1-R)I0 e(-αx)
α: absorption coefficient
R: reflectance
⎛ε ⎞
−1
h = α In ⎜⎜ ⎟⎟ ε 0 : threshold fluence
⎝ ε0 ⎠
Mechanics of
Micromachining
Simple model
ƒ Temperature variation
⎛ x ⎞
⎟⎟
T(x,t) = T0 erf ⎜⎜
⎝ 2kt ⎠
T0: surface temperature
κ: coefficient of temperature
diffusion
erf: error function
http://mathworld.wolfram.com/
Mechanics of
Micromachining
Two important parameters:
ƒ Optical penetration depth
ƒ Thermal penetration depth
Mechanics of
Micromachining
To get proper machining with excimer:
ƒ Absorption
ƒ Optical length vs thermal diffusion length
Table
Material
Aluminum
Micromachining
α-1[µm]
L30=2
(τ=30 ns)
0.007
3.3
Silicon nitride 0.06
Alumina
0.08
PI
0.07
PET
0.1
PC
0.2
1.0
0.8
0.16
0.14
0.13
PE
PMMA
0.16
0.11
6
16
Quartz glass >108
0.32
Ratio
kE=α-1/L30
0.002
0.06
0.1
0.4
0.7
1.5
38
145
>108
Material Characteristics
Metallic behavior
kE<0.01
Strong absorber
0.01<kE < 10
Weak absorber
10<kE < 1000
Transparent region
1000 < kE
Thank You
Summary:
Laser Micro-machining – By long pulses
Absorption: depends upon the w/p material, power
density and wavelength. CO2 lasers – 20% absorption
whereas Nd: YAG and exciemer 40-80% is absorbed.
The optical penetration depth is a depth for which power
density is reduced to 1/e of the initial density. With CO2
lasers this depth is 15 nm and with Nd: YAG it is 5 nm.
The heat flow in the bulk of the material is a approx. one
dimensional phenomenon and the temperature at the
surface is given by –
T = (Ia/λ) (4at/π)1/2
Where, Ia – power density (W/cm2) ; a - thermal
diffusivity = λ/ρc; t - time.
For Ia = 109 W/cm2 on steel, the melting point is reached
in 300 ns. If power density is increased 10 times, time to
melt is reduced to 3 ns.
Laser Micro-machining – By long pulses
The high vaporization rate causes a shock wave and a high vapor
pressure at the liquid surface considerably increases the boiling
temperature. Finally, the material is removed by the expulsion of melt
and explosive like boiling of the superheated liquid at the end of a laser
pulse. Machining of metals generate a rim of resolidified material.
In plastics, the material is removed by breaking of chemical bonds
of macro-molecules, and is dispersed as gas or small particles so
no melting is found.
Laser Micro-machining – By long pulses
Machining with long pulse lasers – Long pulse machining
Example of a 25 micron (1 mil) channel machined in 1 mm (40 mils) thick
INVAR with a nanosecond laser. INVAR, an alloy formed of Nickel and Iron,
has an extremely small coefficient of thermal expansion at room
temperature. INVAR is often called for in the design of machinery that must
be extremely stable. This sample was machined using a “long” pulse laser.
The laser pulse parameters are: pulse duration 8 ns, energy 0.5 mJ. The
machining was not assisted by an air jet.
Laser Micro-machining – By long pulses
Plume or cloud development in laser micromachining takes a time. If a
detectable plume is assumed to form at a time tp when the surface
reaches a temperature of Tv (vaporization temperature) then
tp = π/4 (E/Ia2) where E- erosion resistance of the material given by
λρcTv2.
It is a significant process variable as it is a measure of absorbed power
density. It in collaboration with the focus distance, determines the
plume initiation time.
In the focal area the process is
mainly drilling, smooth
machining is possible at a
distance zopt.
Laser Micro-machining – By long pulses
Microscopic view of Ablation by (Evaporation) long pulses: It involves –
formation of a plasma and formation of a dielectric transparent layer on
the work surface and a melt front.
There can not be any heat absorption in
the super heated layer. Material above
the layer is removed by evaporation. At
side walls, the material is forced away
by plasma pressure. At the end of pulse,
pressure drops suddenly, the material is
removed by boiling of superheated
liquid and get redeposited around the
processing area.
Laser Micro-machining – By long pulses
In a numerical model of the process, a drop in absorptivity at the critical
temperature causes decrease in the specific heat of evaporation with
temperature.
Lv = Lv0 [1 - (T/Tc)2 ]1/2
At the critical temperature (about 1.4 times the boiling temperature) no
extra heat is required for evaporation. Thus, above a certain fluence
(energy per unit area), the critical temperature remains constant.
Ablation still occurs at the end of a pulse. For a given fluence, the
ablation depth is maximum at a given pulse length.
Effect of ratio of free to bound electrons is a parameter. If the laser field
is intense enough, a free electron colliding with a surrounding atom will
knock off an additional electron. They in turn can knock two more
electrons off atoms in the surrounding material and so on. This type of
multiplication effect is called an avalanche effect, and because it creates
electrons by ionizing atoms, it is called 'avalanche ionization.‘
More free electrons Less electrons Lesser free electrons Comparison
Laser Micro-machining – By Short pulses
The laser-material interaction consists of a set of physical steps
each characterized by its typical time constant.
The laser energy is transformed to the electrons first especially in
the case of metals. The electrons will transfer the energy to lattice
and finally within the lattice heat is distributed further by lattice
collisions.
The Absorption of a photon by an electron requires about 10-15 s
(1 femtosecond).
The relaxation time, the time required to transfer the energy to the
lattice is 10-12 s (1 picosecond).
The time to diffuse heat in the lattice by conduction over a
distance of optical penetration depth is 10-12 s (1 picosecond).
The time constants of these processes change the material
removal process laser micromachining.
Laser Micro-machining – By Short pulses
Femtosecond ablation: There is no transfer of energy to the lattice
during this process. All the energy is stored in a thin surface layer.
This energy will be more than the specific heat of evaporation and
there will be vigorous evaporation after the incidence of the pulse. The
ablation depth per pulse is given by –
Za ~ α-1 ln(Fa/Fth)
Where, Fa is the absorbed fluence and Fth is the threshold fluence =
energy required to evaporate the irradiated volume of material; α penetration depth (absorption). For α-1 = 10nm, Fth = 0.1 J/cm2.
For the material removal to occur, fluence should be about 3 times
that of the threshold fluence.
The ablation process is a direct solid-vapor transition.The energy is
transferred to the lattice from electrons after the pulse in a
picosecond.
The result is a precise and pure laser ablation of materials.
Laser Micro-machining – By Short pulses
Laser machining by ultra-short pulses – Femtosecond pulse
Picosecond Ablation: Here the pulse length is of the same order as
that of the transfer of energy from electrons to the lattice.
Due to the heat flow by the free electrons is also significantly
higher.
Ii results in the formation of a solid-vapor or solid-plasma transition
front but deeper in deeper in the material liquid phase is present.
Nanosecond Ablation: In this process, the heat of laser is used for
melting and evaporation of the surface. The main energy loss is by
conduction of heat into the work surface. Therefore, in the above
equation thermal diffusion depth (at)1/2 replaces α-1.
For 20ns the thermal diffusion depth is 0.5 µm, threshold fluence is
4J/cm2.
Laser Micro-machining – Organic
Polymers
In plastics, the mechanism of material removal is based on the
photochemical reactions with photons.
The typical bonding energy for many macromolucules is 3-15 eV. This
corresponds to the photon energy in the ultra violet range.
UV photons are absorbed in the top layer of thickness 0.2 microns.
Long chin of molecules is broken into parts. Molecules are removed
from the processing area in the form of vapor.
Laser Micro-machining - Applications
Laser micro-drilling: by direct focusing. The focal length should chosen
so that the focal diameter corresponds to the required hole diameter.
CO2 lasers for large number of holes in thin materials
Nd: YAG lasers for drilling of precision holes in harder materials.
Diamond is one of the difficult to machine materials, is optically
transparent over a wide range of wavelengths. Under high power
densities, the diamond is transformed into graphite which absorbs the
laser power and is removed by ablation.
Laser Micro-welding: Of wires of 0.1
mm diameter.
Laser micro-adjustment: generation
of thermal-mechanical stresses in
metal structures.
Laser cleaning: for particles below 1
µm size get strongly adherent to the
surfaces. They can be ablated by
124 nm eximer laser pulses.
Laser Micro-machining – Femtosecond
laser
All lasers produce light over some natural bandwidth or range of
frequencies which is determined primarily by the gain / laser medium
that the laser is constructed from.
For example, a typical helium-neon (HeNe) gas laser has a gain
bandwidth of approximately 1.5 GHz (around 0.002 nm wavelength
range), whereas a titanium-doped sapphire (Ti:Sapphire) solid-state
laser has a bandwidth of about 128 THz (a 300 nm wavelength range).
The second factor which determines a laser's emission frequencies is
the optical cavity or resonant cavity of the laser. For a simple planemirror cavity, the allowed modes are those for which the separation
distance of the mirrors L is an exact multiple of half the wavelength of
the light λ, such that L = q λ/2, when q is an integer known as the
mode order.
In practice, the separation distance of the mirrors L is usually much
greater than the wavelength of light λ, so the relevant values of q are
large (around 105 to 106).
Laser Micro-machining – Femtosecond
laser
Of more interest is the frequency separation between any two
adjacent modes q and q+1; this is given by ∆ν:
∆ν = c/2L c: speed of light
Using the above equation, a small laser with a mirror separation of 30
cm has a frequency separation between longitudinal modes of 0.5 GHz.
Thus for the two lasers referenced above, with a 30 cm cavity the 1.5
GHz bandwidth of the HeNe laser would support up to 3 longitudinal
modes, whereas the 128 THz bandwidth of the Ti:sapphire laser could
support approximately 250000 modes.
Modelocking theory:In a simple laser, each of these modes will oscillate
independently, with no fixed relationship between each other, in
essence like a set of independent lasers all emitting light at slightly
different frequencies. If instead of oscillating independently, each mode
operates with a fixed phase between it and the other modes, the laser
output behaves quite differently. Instead of a random or constant output
intensity, the modes of the laser will periodically all constructively
interfere with one another, producing an intense burst or pulse of light.
Laser Micro-machining – Femtosecond
laser
Such a laser is said to be mode-locked or phase-locked. These pulses
occur separated in time by τ = 2L/c, which is the time taken for the light to
make exactly one round trip of the laser cavity. This time corresponds to
a frequency exactly equal to the mode-spacing of the laser, ∆ν = 1/τ.
If there are N modes locked with a frequency separation ∆ν, the overall
modelocked bandwidth is N∆ν, and the wider this bandwidth, the shorter
the pulse duration from the laser.
For example, for a laser producing pulses with a Gaussian temporal
shape, the minimum possible pulse duration ∆t is given by:
∆t = 0.44/N∆ν
The value 0.44 is known as the time-bandwidth product of the pulse, and
varies depending on the pulse shape.
Using this equation, we can calculate the minimum pulse duration which
can be produced by a laser. For the HeNe laser with a 1.5 GHz bandwidth,
the shortest Gaussian pulse which can be produced would be around 300
picoseconds; for the 128 THz bandwidth Ti:sapphire laser, this duration
would be only 3.4 femtoseconds.