1. 1
INTRODUCTION
Although material ablation by pulsed light sources has been studied since the invention of
laser, reports in 1982 of polymers etched by uv excimer lasers stimulated widespread
investigations aimed at using the process for micromachining. In the intervening years
scientific and industrial research in this field has proliferated to a staggering extent –
probably spurred on by the remarkably small features that can be etched with little
apparent damage to surrounding unirradiated regions of material. Manufacturing industry
now uses laser beam micromachining in many high-tech application areas for which micro
fabrication is an enabling technology.
It is now known that clean ablative etching can also be achieved using pulsed laser sources
at wavelengths other than ultraviolet .Provided photons are absorbed strongly in submicron
depths in timescales less than the time it takes for heat to diffuse away from the irradiated
region, then pulsed lasers like copper vapor (CVL), CO2 and Nd and its harmonics can be
effective for ablative micromachining. For a particular micromachining application the
choice of laser is now judged as much by criteria such as process speed, part throughput,
reliability, service intervals, capital and operating costs of the overall machine tool rather
than solely by the quality of the processed part.
As compared to conventional techniques such as LIGA, and dry as well as wet etching, laser
based techniques offer many advantages for micromachining purposes. They can be used
for a wide variety of processes including cutting, wafer dicing, via drilling, small feature
patterning, surface modification and thin film large area material removal. A wide range of
materials can be machined with high resolution and flexibility at high yield without the
costly lithographic steps. Absence of the need for chemical waste management as well as
minimal mechanical and thermal loading on the workpiece and non-contact machining are
added benefits.

2. 2
LASER: Light Amplification by
Stimulated Emission of
Radiation

3. 3
LASER:
A device that converts some form of energy (electrical, optical, chemical, etc.) into a narrow
beam of light which is monochromatic (single pure color) and coherent (all waves in step
with one another).
Monochromatic means that it consist of one single color or wavelength. Even though some
lasers can generator more than one wavelength, the light is extreme "pure" and consists of a
very narrow spectral range.
Directional means that the beam is very well collimated and travels over long distances
with very little spread in diameter.
Coherent means that all individual waves of light are moving precisely together through
time and space, or are in phase. The effect of one wave enhances the strength of every other
wave, so that the overall effect of coherent light is much greater than if the waves were not
in phase.
Because of these properties laser light can be focused to an extremely small spot, which
results in a very large power density which produces a very high temperature.
So briefly we can say that:
 A laser is a device which generates or amplifies light
 LASER is the acronym for Light Amplified Stimulated Emission of Radiation
 LASER Medium (gas, liquid, solid)
 Pumping Process - must achieve a population inversion
 Optical Feedback Elements - single or multiple pass

4. 4
 Properties of LASER beams: Monochromatic - single wavelength
Directional - low divergence, beam spreads very little
Intense - high density of usable photons
Coherent - same phase relationship
The first successful laser was constructed by T.H. Maiman in 1960. He used a small Ruby
Rode (Al2O3 doped with .05% Cr oxide) known as Ruby Laser. May others have also been
constructed e.g. Helium-Neon Laser, Carbon dioxide laser etc.
LASERS FOR MATERIALS PROCESSING:
Lasers can replace mechanical material removal methods in several engineering
applications because of their following salient features:
1. Non-contact
· No tool wear as with traditional milling machines or EDM
· Reduces chance of damage to process material due to shock or handling
2. No solvent chemicals
· Reduced waste handling costs - environmentally clean
· Lasers provide one-step alternative to chemical etching process
3. Selective material removal
· Proper selection of laser power on-target allows removal of one type of material
Without damage to the under layers.
· Examples: skiving, wire stripping
4. Flexibility
· Laser materials processing systems incorporate advanced computer control with
programming interfaces that permit “soft retooling”
· Good for prototype work where high tool-up costs must be avoided

5. 5
BRIEF REVIEW OF LASER PHYSICS:
Quantum theory of Light: The Quantum theory of light was
developed by Plank & Einstein in the early 1900s. The theory states:
 Light is quantized in discrete bundles of energy called “photons”.
 Photons are emitted when atoms or molecules drop from an excited energy state to
a lower state.
 Each light photon has an associated energy that depends on its frequency
E photon = h γ = E2 – E1
where E2 and E1 are upper and lower energy levels of the atom, h is Plank’s constant (h =
6.626 x 10-34
J-s ) , and γ is the frequency of oscillation (s-1
).
 Although light is packaged in discrete photons (particle theory of light), light also is
characterized by frequency and wavelength(wave theory of light)/
 Consequently photon energy is proportional to frequency but inversely
proportional to wavelength.
Physical description of wavelength

6. 6
Coherence and divergence of beam: A laser beam is highly
coherent and has small divergence. Coherence is where the phase relationship between any
two points in the beam remains exactly the same.
Any light that exits a confined space will undergo divergence. In laser physics divergence is
the degree of spreading a laser beam exhibits after it exits the front aperture. In machining
applications, divergence is undesirable because it leads to reduced energy and distorted
images at the target surface.
Divergence of laser beam.
Photon interactions with matter:Possible photon interactions
with matter include the following:
 Photon absorption – the photon is absorbed by the atom or the molecule.
 Photon scattering - the photon is scattered either elastically or inelastically.

7. 7
 Spontaneous emission – Atoms at higher energy levels tend to, or spontaneously
jump to lower energy levels, in the same time they give out the electromagnetic
radiation having the energy equal to the difference between the two energy levels.
This is called spontaneous decay or spontaneous emission.
 Stimulated emission – the photon with the corresponding frequency stimulates the
atom which has the same possibility to jump to the corresponding lower levels,
emitting electromagnetic waves or photons with the same frequency, direction and
phase with the incident waves. This process is called Stimulated Emission.
Stimulated emission reduces the upper level population, increases the lower level
population.
Reaction cross section is a measure of probability that a reaction will take place, assuming
the basic constituents needed for reaction are present. Therefore stimulated emission cross-
section is the probability that stimulated emission will occur between an excited atom or
molecule and an incident photon.

8. 8
Essential elements of a laser oscillator: A laser requires a
lasing medium, a pump process and a resonator cavity to sustain oscillation. The lasing
medium can be a gas, solid, liquid or in the case of semiconductor lasers, electrons. The
pump process excites the atoms or molecules of the lasing medium to their upper energy
states by electronic means or kinetic energy transfer.
Laser transmission is initiated by spontaneous emission and amplified by stimulated
emission along the axis of the resonator cavity. The cavity mirrors reflect a portion of the
photons back and forth through the laser medium for increased amplification.
Elements of a laser
Characteristics of the laser cavity are:
 Rear resonator mirror is fully reflective.
 Front optic part is partly reflective and partly transmissive – for a helium-neon
laser, the front optic is 98.5% reflective; for an excimer laser; the front optic is
nearly 10% reflective because of the high gain in the laser medium.
 Resonator optics can be concave, as in a helium-neon laser, or flat, as in an excimer
laser.
 Lasing medium must have high stimulated emission cross section so more photons
are produced than absorbed.
 Methods of laser pumping: gas discharge, optical (flashlamp), chemical pump, laser
pump, electron collision excitation.
Energy is introduced into the laser through the pumping process, but only a fraction of the
wall plug energy is present in the laser beam as it exits the front aperture. A typical laser
might be less than 10% efficient. Most of the energy is lost in the form of heat.

9. 9
FORCE EXCERTED MY LASER BEAM:
Each second, the light source emits photons, each one carries an energy
, for a total power of . This gives:
In one second then, photons hit the mirror and bounce back, which gives:
APPLICATIONS OF LASER:
Lasers have proved a boon to the growth of modern scientific world. It is used in many
modes.
1. In communication : There are used to send signals over long distances through optical
fibres. They allows to send thousands of signals at the time due to narrow band width.
2. In measuring long distances : Being highly coherent these can travel long distances
without any loss their intensity e.g. To measure distance of moon.
3. In surgery : These are used to perform practically blood less surgery. As the laser beam
cuts through tissue, it heat seals blood vessels and this technique is used in removing
tumour from the body.
4. In Industry : Laser is used to drill holes and cuts of sheets of metals. They do this job
accurately and quickly.
5. Nuclear Power Production : A day is not far when loses beams will be used in
carrying out nuclear fusion reactions with much ease for power production.
6. For national defence : Three are used for controlling and guiding rockets, missiles,
satellites and locating planes.
7. In Weather forecasting : Laser beams are used to obtain clear picture of clouds and wind
movements for forecasting of weather.

10. 10
CATEGORIZATION OF DIFFERENT
TYPE OF LASERS

11. 11
Lasers come in many different types, each with a different power level and wavelength
(colour). Some are so weak that you cannot feel the beam on your hand (i.e., supermarket
scanners), while others might have an invisible beam that can burn a hole through a steel
plate (large CO2 laser).
Laser categorization according to wavelength in the electromagnetic spectrum
Lasers are categorized according to lasing medium. The basic types of lasers are: gas, solid
state, chemical, semiconductor and liquid dye.
GAS LASERS:
A gas laser is a laser in which an electric current is discharged through a gas to produce
light. The first gas laser, the Helium-neon, was co-invented by American physicist William
R Bennett, Jr. and Iranian physicist Ali Javan in 1960.The different types of gas laser are:
helium-neon laser, nitrogen laser, carbon dioxide laser, ion laser, gas dynamic laser.
SOLID STATE LASERS:
Solid state laser materials are commonly made by doping a crystalline solid host with ions
that provide the required energy states. For example, the first working laser was a ruby
laser, made from ruby (chromium doped corundum).

12. 12
Neodymium is a common dopant in various solid state crystal lasers, including yttrium
orthovanadate (YVO4), yttrium lithium fluoride (YLF) and yttrium aluminium garnet
(YAG).All these lasers can produce high powers in the infrared spectrum at 1064nm. They
are used for cutting, welding and marking of metals and other materials, and also in
spectroscopy and for pumping dye lasers. These lasers are also commonly frequency
doubled, tripled or quadrupled to produce 532 nm (green, visible), 355nm (UV) and
266nm (UV) light when those wavelengths are needed.
CHEMICAL LASER:
Chemical lasers are powered by chemical reaction, and can achieve high powers in
continuous operation. For example, in the Hydrogen fluoride laser (2700-2900nm) and the
Deuterium fluoride laser (3800nm) the reaction is the combination of hydrogen or
deuterium gas with combustion products of ethylene in nitrogen trifluoride. They were
invented by George C. Pimentel.
SEMICONDUCTOR LASER:
Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm, and wavelengths
of over 3 µm have been demonstrated. Low power laser diodes are used in laser printers and
CD/DVD players. More powerful laser diodes are frequently used to optically pump other
lasers with high efficiency. The highest power industrial laser diodes, with power up to 10
kW, are used in industry for cutting and welding. External-cavity semiconductor lasers
have a semiconductor active medium in a larger cavity. These devices can generate high
power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or
ultrashort laser pulses.
DYE LASER:
A dye laser is a laser which uses an organic dye as the lasing medium, usually as a liquid
solution. Compared to gases and most solid state lasing media, a dye can usually be used for
a much wider range of wavelengths. The wide bandwidth makes them particularly suitable
for tunable lasers and pulsed lasers. Moreover, the dye can be replaced by another type in
order to generate different wavelengths with the same laser, although this usually requires
replacing other optical components in the laser as well.

13. 13
CONTINUOUS WAVE AND PULSED
OPERATION OF LASERS

14. 14
A laser may either be built to emit a continuous beam or a train of short pulses. This makes
fundamental differences in construction, usable laser media, and applications.
CONTINUOUS WAVE OPERATION:
In the continuous wave mode of operation the output of a laser is relatively consistent with
respect to time. The population inversion required for lasing is continually maintained by a
steady pump source.
PULSED OPERATION:
In the pulsed mode of operation, the output of a laser varies with respect to time, typically
taking the form of alternating 'on' and 'off' periods. In many applications one aims to deposit
as much energy as possible at a given place in as short time as possible. Due to the short
pulse durations and the potential for strong focusing, optical pulses can be used for
generating extremely high optical intensities even with moderate pulse energies. For
example, a 10-fs pulse with only 10 mJ energy has a peak power of the order of 1 TW =
1000 GW, corresponding to the combined power of roughly 1000 large nuclear power
stations. This power may be focused to spots with only a few micrometers diameter.
Therefore, amplified ultrashort pulses are very important for high intensity physics,
studying phenomena like multi-photon ionization, high harmonic generation, or the
generation of even shorter pulses with attosecond durations.
Depending on the required pulse duration, pulse energy, and pulse repetition rate, different
methods of pulse generation and pulse characterization are used, in total covering
extremely wide parameter regimes. Ultrashort pulses (with picosecond or femtosecond
durations) are usually generated with mode-locked lasers, while Q-switched lasers
typically allow for nanosecond pulse durations. The methods for obtaining pulsed lasers are
discussed below:
 Q-switching: In a Q-switched laser, the population inversion (usually
produced in the same way as CW operation) is allowed to build up by making the
cavity conditions (the 'Q') unfavorable for lasing. Then, when the pump energy
stored in the laser medium is at the desired level, the 'Q' is adjusted (electro- or
acousto-optically) to favorable conditions, releasing the pulse. This results in high
peak powers as the average power of the laser (were it running in CW mode) is
packed into a shorter time frame.

15. 15
 Modelocking: A modelocked laser emits extremely short pulses on the
order of tens of picoseconds down to less than 10 femtoseconds. These pulses are
typically separated by the time that a pulse takes to complete one round trip in the
resonator cavity. Due to the Fourier limit (also known as energy-time uncertainty), a
pulse of such short temporal length has a spectrum which contains a wide range of
wavelengths. Because of this, the laser medium must have a broad enough gain
profile to amplify them all. An example of a suitable material is titanium-doped,
artificially grown sapphire (Ti: sapphire).
PULSED PUMPING:
Another method of achieving pulsed laser operation is to pump the laser material with a
source that is itself pulsed, either through electronic charging in the case of flashlamps, or
another laser which is already pulsed. Pulsed pumping was historically used with dye lasers
where the inverted population lifetime of a dye molecule was so short that a high energy,
fast pump was needed. The way to overcome this problem was to charge up large capacitors
which are then switched to discharge through flashlamps, producing a broad spectrum
pump flash. Pulsed pumping is also required for lasers which disrupt the gain medium so
much during the laser process that lasing has to cease for a short period. These lasers, such
as the excimer laser and the copper vapour laser, can never be operated in CW mode

16. 16
DESCRIPTION OF SOME LASERS
HAVING INDUSTRIAL
APPLICATIONS

17. 17
Of the five basic types of lasers discussed earlier, only gas and liquid lasers are practical for
most industrial machining applications. Mainly CO2 , YAG and Excimer lasers are employed
in micromachining operations.
CO2 LASERS:
The characteristics of CO2 lasers are as follows:
 Most common laser used in industry.
 Inexpensive.
 Wide range of power output capabilities.
 High efficiency.
 Long penetration depth.
 Machining is a thermal process - the beam performs its cutting and drilling
functions by overloading the target surface thermally.
 Usually used in focal point machining mode except for CO2 TEA lasers.
CO2 LASER OPERATIONAL THEORY: A carbon dioxide laser uses a gas
mixture of CO2: N2: He. The CO2 molecules constitute the active lasing medium, the N2 gas
serves as the energy transfer mechanism and the He atoms enhance the population
inversion by depopulating the lower energy states. The population inversion and lasing
transition in a CO2 laser is established between vibrational and rotational energy states.
Most CO2 lasers are pumped by gas discharge.

18. 18
TYPES OF CO2 Lasers:
Type Beam Delivery Method Applications
CW Lower order mode
Gaussian
Focal spot High speed profile
cutting; seam welding;
cladding; engraving
Gate pulsed Lower order mode
Gaussian
Focal spot Cutting and drilling in
metals; spot welding
Enhanced pulsed Lower order mode
Gaussian
Focal spot Cutting and drilling in
IR reflective metals
TEA High order multi-
mode
Near-field imaging Marking in thermally
insensitive materials;
wire stripping; flex
circuits
Increased laser output power can be achieved either by increasing the volume of the cavity
or gas pressure. Unfortunately these measures also increase heat generated inside the cavity
until lasing can no longer be sustained. The Lumonics IMPACT and Lasrtechnics Blazer
6000 both employ a slow gas purge and water cooling system to alleviate this problem. The
IMPACT and Blazer 6000 are used for machining Resonetics or are installed in materials
processing systems for customers. These lasers are called TEA (transverse excitation
atmospheric) lasers because the electrodes are constructed along the axis of the resonator
cavity.
IMPORTANT CO2 MACHINING CHARACTERISTICS:
 Material interaction via thermal overload and vaporization.
 Penetration depth is 5 to 10 m.
 Ultimate feature resolution ~10 m.
 Practical feature resolution ~50 m.
 Inert gas used to limit oxidation in process area.
SOLID STATE Nd3+
LASERS:
Most solid state lasers are constructed by doping a rare earth element or metallic element
into a variety of host materials. The most common host materials are Y3Al5O12 (YAG), LiYF4

19. 19
(YLF) and amorphous glass. The Nd: YAG and Nd: YLF lasers are discussed because they are
the most common solid state lasers in industry.
Nd: YAG (neodymium-doped yttrium aluminium garnet; Nd: Y3Al5O12) is a crystal
that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium,
typically replaces yttrium in the crystal structure, since they are of similar size. Generally
the crystalline host is doped with around 1% neodymium by weight.
Nd: YLF (Neodymium-doped yttrium lithium fluoride) is a lasing medium for arclamp-
pumped and diode-pumped solid-state lasers. The YLF crystal (LiYF4) is naturally birefringent, and
commonly-used laser transitions occur at 1047 nm and 1053 nm.
THE CHARACTERISTICS OF Nd LASERS:
 Typical solid state lasers are pumped optically by arc lamps or flashlamps. Arc
lamps typically are used for continuous wave (cw) pumping; flashlamps are used
with pulsed lasers.
 Solid state lasers are electronically excited. The atoms of the active medium
become excited when an electron jumps to a different orbit around the nucleus. In
Nd: YAG and Nd: YLF lasers, the neodymium ions (3+
) constitute the active
medium.
 Nd lasers are easy to pump. All Nd lasers (Nd: YAG, Nd: YLF, Nd: glass) are four
level laser systems with numerous absorption bands above the upper lasing
energy state 4
F3/2. Atoms at these states readily decay to 4
F3/2 making it easy to
establish the required population inversion.
 Emission wavelength of Nd doped lasers varies somewhat with different host
materials. Some host materials have a less defined lattice structure than the others.
The energy linewidths in these materials are “broadened” such that the transition
wavelengths are different.
 Nd lasers output can be frequency doubled, tripled or quadrupled through
harmonic generation.
 Nd lasers respond well to Q-switching.

20. 20
Nd: YLF vs. Nd: YAG
 The Nd:YLF and Nd:YAG are not identical in laser characteristics:
 Nd: YAG wavelength is 1.064 m, Nd: YLF wavelength is 1.047 m.
 Nd: YLF pulse energy is greater than that for a similarly constructed Nd: YAG
laser at low pulse rates as more energy can be stored per Q-switched pulse
because the Nd: YLF upper state energy level lifetime.
 The YAG host material has better thermal conductivity and more stable
refractive index than YLF. The resulting parabolic temperature profile within the
YLF laser rod produces thermal lensing phenomenon that focuses the beam just
outside the laser rod.
 YLF host lasers are relatively new technology and are still undergoing research.
EXCIMER LASERS:
An excimer laser (sometimes, and more correctly, called an exciplex laser) is a form of
ultraviolet chemical laser which is commonly used in eye surgery and semiconductor
manufacturing. The term excimer is short for 'excited dimer', while exciplex is short for
'excited complex'. An excimer laser typically uses a combination of an inert gas (Argon,
krypton, or xenon) and a reactive gas (fluorine or chlorine). Under the appropriate
conditions of electrical stimulation, a pseudo-molecule called a dimer is created, which can
only exist in an energized state and can give rise to laser light in the ultraviolet range.
The UV light from an excimer laser is well absorbed by biological matter and organic
compounds. Rather than burning or cutting material, the excimer laser adds enough energy
to disrupt the molecular bonds of the surface tissue, which effectively disintegrates into the
air in a tightly controlled manner through ablation rather than burning. Thus excimer
lasers have the useful property that they can remove exceptionally fine layers of surface
material with almost no heating or change to the remainder of the material which is left
intact. These properties make excimer lasers well suited to precision micromachining
organic material (including certain polymers and plastics), or delicate surgeries such as eye
surgery (LASIK).
The first excimer laser was invented in 1970 by Nikolai Basov, V. A. Danilychev and Yu. M.
Popov, at the Lebedev Physical Institute in Moscow, using a xenon dimer (Xe2) excited by an
electron beam to give stimulated emission at 172 nm wavelength. A later improvement,
developed by many groups in 1975 was the use of noble gas halides (originally XeBr). These

21. 21
groups include the United States Government's Naval Research Laboratory, the Northrop
Research and Technology Center, the Avco Everett Research Laboratory, and Sandia
Laboratories.
Laser action in an excimer molecule occurs because it has a bound (associative) excited
state, but a repulsive (disassociative) ground state. This is because noble gases such as xenon
and krypton are highly inert and do not usually form chemical compounds. However, when
in an excited state (induced by an electrical discharge or high-energy electron beams,
which produce high energy pulses), they can form temporarily-bound molecules with
themselves (dimers) or with halides (complexes) such as fluorine and chlorine. The excited
compound can give up its excess energy by undergoing spontaneous or stimulated emission,
resulting in a strongly-repulsive ground state molecule which very quickly (on the order of
a picoseconds) disassociates back into two unbound atoms. This forms a population
inversion between the two states.
Most "excimer" lasers are of the noble gas halide type, for which the term excimer is strictly
speaking a misnomer (since a dimer refers to a molecule of two identical or similar parts):
The correct but less commonly used name for such is exciplex laser.
IMPORTANT PROPERTIES OF KrF AND OTHER EXCIMER
LASERS:
 Since the atoms undergoing decay to the ground state are repelled, photon
absorption to the upper state is non-existent. This condition and the high stimulated
emission cross section for KrF make a population inversion easy to establish and the
laser medium gain very high.
 The frequency of the excimer uv emission is sufficiently energetic to break the
chemical bonds of most materials. Machining is accomplished through ablation
instead of thermal overload.
 The pumping mechanism for excimer lasers is a gas discharge with 45 kv peak
excitation voltage. High voltages and currents of this magnitude test the limits of
electronic technology. High voltage power supply failures in excimer lasers are an
issue in proper design.
 The partial reflectance of the front resonator mirror is only 10% because of the high
gain.

22. 22
TYPES OF EXCIMER LASERS: Some well-known manufacturers of industrial
excimer lasers are Lambda Physik (GmbH, Goettingen, Germany), Lumonics, and MPB
Technologies, Inc (Dorval, Quebec). Performance characteristics for several excimer lasers
manufactured by these companies are provided below.
Parameter(KrF) EMG103
MSC
Lambda
1000
LPX
220i
Lumonics
PM-848
Lumonics
Index
886
MPBTech
PSX-100
Max pulse
energy
300mJ 300mJ 450mJ 450mJ 600mJ 3mJ
Max average
power output
55W 60W 80W 80W 30W 0.3W
Beam size 8 x 22
mm
8 x 24
mm
8 x 23
mm
10 x 25
mm
14 x 30
mm
2 x 2
mm
Beam
divergence(mrad)
4.5 x 1.5 4.5 x
1.5
1 x 3 1 x 3 1 x 3 2 x 3.6
Pulse repetition
rate(pulses/sec)
0-200 0-200 0-200 0-200 0-50 0-100
Laser parameters for several industrial lasers.

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TYPES OF MICROMACHINING

24. 24
PHOTOLITHOGRAPHY:
Photolithography (or "optical lithography") is a process used in microfabrication to
selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a
geometric pattern from a photo mask to a light-sensitive chemical photo resist, or simply
"resist," on the substrate. A series of chemical treatments then engraves the exposure pattern
into the material underneath the photo resist. In complex integrated circuits, for example a
modern CMOS, a wafer will go through the photolithographic cycle up to 50 times.
Photolithography shares some fundamental principles with photography in that the pattern
in the etching resist is created by exposing it to light, either using a projected image or an
optical mask. This procedure is comparable to a high precision version of the method used
to make printed circuit boards. Subsequent stages in the process have more in common with
etching than to lithographic printing. It is used because it affords exact control over the
shape and size of the objects it creates, and because it can create patterns over an entire
surface simultaneously. Its main disadvantages are that it requires a flat substrate to start
with, it is not very effective at creating shapes that are not flat, and it can require extremely
clean operating conditions.
The wafers are chemically cleaned to remove particulate matter, organic ionic and organic,
ionic, metallic impurities .High-speed centrifugal whirling of silicon wafers known as "Spin
Coating" produces a thin uniform layer of photoresist (a light sensitive polymer) on the
wafers. Photoresist is exposed to a set of lights through a mask often made of quartz.
Wavelength of light ranges from 300-500 nm(uv). Wavelength of light ranges from 300-
nm (UV) and X-rays (wavelengths 4-50 Angstroms).

25. 25
Two types of photoresist are used:
o Positive: whatever shows, goes
o Negative: whatever shows, stays
ETCHING:
Etching is the process of using strong acid or mordant to cut into the unprotected parts of a
metal surface to create a design in intaglio in the metal (the original process—in modern
manufacturing other chemicals may be used on other types of material
DRY ETCHING: Dry etching refers to the removal of material, typically a masked
pattern of semiconductor material, by exposing the material to a bombardment of ions
(usually a plasma of reactive gases such as fluorocarbons, oxygen, chlorine, boron
trichloride; sometimes with addition of nitrogen, argon, helium and other gases) that
dislodge portions of the material from the exposed surface. Unlike with many (but not all,
see isotropic etching) of the wet chemical etchants used in wet etching, the dry etching
process typically etches directionally or anisotropically. Dry etching is used in conjunction
with photolithographic techniques to attack certain areas of a semiconductor surface in
order to form recesses in material, such as contact holes (which are contacts to the
underlying semiconductor substrate) or via holes (which are holes that are formed to
provide an interconnect path between conductive layers in the layered semiconductor
device) or to otherwise remove portions of semiconductor layers where predominantly
vertical sides are desired. In conjunction with semiconductor manufacturing,
micromachining and display production the removal of organic residues by oxygen
plasmas is sometimes correctly described as a dry etch process. However, also the term
plasma ashing may be used.Dry etching is particularly useful for materials and
semiconductors which are chemically resistant and could not be wet etched, such as silicon
carbide or gallium nitride.
WET ETCHING:
1. Residual stresses in the part are removed prior to the machining process to prevent
warping after the chemical milling process.
2. To ensure good adhesion between the masking and part, the part undergoes a
thorough degreasing and cleaning process. There needs to be good adhesion

26. 26
between the masking and the part to ensure uniform material removal during the
milling process.
3. The masking material is applied to the surfaces of the part to prevent etching from
taking place on these surfaces. Some masking types include tapes or paints which
are referred to as maskants. Other maskants which are commonly used as well are
elastomers (rubber and neoprene) and plastics (polyvinyl chloride, polyethylene, and
polystyrene).
4. Masking which covers areas of desired etching can be removed by utilizing the
scribe-and-peel technique.
5. The exposed surfaces of the part are etched by the reagent. The machining process is
affected by the temperature and the agitation of the reagent in the tank. A good
removal rate requires careful consideration of these two parameters.
6. After machining, the masking is removed and the parts are washed thoroughly to
prevent further etching by residual reagent.
7. The etched part can be further machined by other finishing operations.
The wet etching process involves:
o Transport of reactants to the surface
o Surface reaction
o Transport of products from surfaces
The key ingredients are:
o Oxidizer (e.g. H2O2 , HNO3)
o Acid or base to dissolve the oxidized surface (e.g. H2SO4, NH4OH)
o Dilute media to transport the products through (e.g. H2O)
PLASMA ETCHING: Plasma etching is a form of plasma processing used to
fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of
an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as
etch species, can be either charged (ions) or neutral (atoms and radicals). During the
process, the plasma will generate volatile etch products at room temperature from the
chemical reactions between the elements of the material etched and the reactive species
generated by the plasma. Eventually the atoms of the shot element embed themselves at or
just below the surface of the target, thus modifying the physical properties of the target.

27. 27
BULK MICROMACHINING:
Bulk micromachining is a process used to produce micromachinery or microelectromechanical
systems (MEMS).Usually, silicon wafers are used as substrates for bulk micromachining, as they can
be anisotropically wet etched, forming highly regular structures. Wet etching typically uses alkaline
liquid solvents, such as potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH) to
dissolve silicon which has been left exposed by the photolithography masking step. These alkali
solvents dissolve the silicon in a highly anisotropic way, with some crystallographic orientations
dissolving up to 1000 times faster than others. Such an approach is often used with very specific
crystallographic orientations in the raw silicon to produce V-shaped grooves. The surface of these
grooves can be atomically smooth if the etch is carried out correctly, and the dimensions and angles
can be precisely defined.
Bulk micromachining starts with a silicon wafer or other substrates which is selectively
etched, using photolithography to transfer a pattern from a mask to the surface. Like
surface micromachining, bulk micromachining can be performed with wet or dry etches,
although the most common etch in silicon is the anisotropic wet etch. This etch takes
advantage of the fact that silicon has a crystal structure, which means its atoms are all
arranged periodically in lines and planes. Certain planes have weaker bonds and are more
susceptible to etching. The etch results in pits that have angled walls, with the angle being a
function of the crystal orientation of the substrate. This type of etching is inexpensive and is
generally used in early, low-budget research.

28. 28
SURFACE MICROMACHINING:
Surface micromachining is a process used to produce micromachinery or
Microelectromechanical systems. Unlike Bulk micromachining, where a silicon substrate
(wafer) is selectively etched to produce structures, surface micromachining builds
microstructures by deposition and etching of different structural layers on top of the
substrate. Generally polysilicon is commonly used as one of the layers and silicon dioxide is
used as a sacrificial layer which is removed or etched out to create the necessary void in the
thickness direction. Added layers are generally very thin with their size varying from 2-5
Micro metres. The main advantage of this machining process is the possibility of realizing
monolithic microsystems in which the electronic and the mechanical components
(functions) are built in on the same substrate. The surface micromachined components are
relatively smaller compared to their counterparts, the bulk micromachined ones.
As the structures are built on top of the substrate and not inside it, the substrate's properties
are not as important as in bulk micromachining, and the expensive silicon wafers can be
replaced by cheaper substrates, such as glass or plastic. The size of the substrates can also be
much larger than a silicon wafer, and surface micromachining is used to produce TFTs on
large area glass substrates for flat panel displays. This technology can also be used for the
manufacture of thin film solar cells, which can be deposited on glass, but also on PET
substrates or other non-rigid materials.

29. 29
LIGA:
LIGA is a German acronym for Lithographie, Galvanoformung, Abformung (Lithography,
Electroplating, and Molding) that describes a fabrication technology used to create high-
aspect-ratio microstructures.
There are two main LIGA-fabrication technologies, X-Ray LIGA, which uses X-rays
produced by a synchrotron to create high-aspect ratio structures, and UV LIGA, a more
accessible method which uses ultraviolet light to create structures with relatively low aspect
ratios.
The notable characteristics of X-ray LIGA-fabricated structures include:
 high aspect ratios on the order of 100:1
 parallel side walls with a flank angle on the order of 89.95°
 smooth side walls with Ra = 10 nm, suitable for optical mirrors
 structural heights from tens of microns to several millimetres
 structural details on the order of micrometers over distances of centimetres
LIGA Process Description:
 Deep X-ray lithography and mask technology
o Deep X-ray (0.01 – 1nm wavelength) lithography can produce high aspect
ratios (1 mm high and a lateral resolution of 0.2 µm)
o X-rays break chemical bonds in the resist; exposed resist is dissolved using
wet-etching process
 Electroforming
o The spaces generated by the removal of the irradiated plastic material are
filled with metal (e.g. Ni) using electro-deposition process
o diamond slurry-based metal plate used Precision grinding with to remove
substrate layer/metal layer.
 Resist Removal
o PMMA resist exposed X-ray and removed by to X exposure to oxygen plasma
or through wet-etching.
 Plastic Molding
o Metal mold from LIGA used for injection molding of MEMS structures.

30. 30
LASER ABLATION:
Laser ablation is the process of removing material from a solid (or occasionally liquid)
surface by irradiating it with a laser beam. At low laser flux, the material is heated by the
absorbed laser energy and evaporates or sublimates. At high laser flux, the material is
typically converted to a plasma. Usually, laser ablation refers to removing material with a
pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the
laser intensity is high enough.
The depth over which the laser energy is absorbed, and thus the amount of material
removed by a single laser pulse, depends on the material's optical properties and the laser
wavelength.
Laser pulses can vary over a very wide range of duration (milliseconds to femtoseconds)
and fluxes, and can be precisely controlled. This makes laser ablation very valuable for both
research and industrial applications.
Laser Ablation Process Description:
 High-High power laser pulses are used to evaporate matter from a target surface.
 A supersonic jet of particles (plume) is ejected normal to the target surface which
condenses on substrate opposite to target.
 The ablation process takes place in a vacuum chamber - either in vacuum or in
the presence of some background gas.

31. 31
Laser Beam Micromachining

32. 32
DEFINITION:
Laser Beam micromachining is defined as laser cutting, drilling, etching, stripping, skiving
materials such as plastics, glass, ceramic and thin metals with dimensions from 1 micron
(0.00004”) to 1mm (0.040"). Recommended maximum machining thickness is 1mm.
Typically, laser drilling holes are tapered where the entrance diameter is larger than the
exit diameter. The typical half angle side wall taper is 3 to 5 degrees, which is material
dependent.
METHODS OF LASER BEAM MICROMACHINING:
Machining dimensions are proportional to approximately twice the laser wavelength. The
shorter the laser wavelength, the smaller the feature size is realizable. By using short
wavelength lasers, with short pulse duration, machining dimension as small as 1 micron is
realizable. Laser micromachining is performed by two different methods:
 Direct Write
 Mask Projection.
DIRECT WRITE: Two direct write methods are commonly used. The fixed beam
approach involves moving the part on an X-Y motion stage; alternatively the laser beam
can be moved very quickly by a pair of galvonometers, programmable spinning mirrors to
direct the beam in the X and Y axis within a defined field.

33. 33
i) Mask less: There is no mask pattern. Just point and shoot the laser beam.
ii) Ease of programming: The drawing is converted by a CAD/CAM program to the
machine code, used to drive the motion controller of the laser system. The laser beam is
focused to a small spot size (typically 10-25 microns) and the beam traces the pattern to
be cut. If the laser is drilling a hole larger than the spot size, a method called trepanning
is used where the beam is directed in a series of concentric circles to "fill" the larger
hole. Often, to accelerate processing times, the beam is directed by galvanometer or
scanning mirrors to direct the beam to a specific location.
MASK PROJECTION: Mask Projection has a number of advantages:
i) Complex pattern: Any complex pattern such as an "8" or "S" can be produced
flawlessly without any stitching or scalloping issues (caused by overlapping a circular
laser spot) because the entire pattern is machined at one time.
ii) Edge quality: When laser micromachining blind channels, the excimer laser can
image a long thin rectangular image with perfectly straight line edges. When drilling
holes, perfectly circular holes with "ink jet-type" precision is achieved.
iii) Throughput: A large process area can be laser micromachined at one time,
maximizing process throughput and lowering costs.
iv) The business case for using excimer lasers in a mask projection method usually
comes down to process throughput. Having UV laser sources up to 100W in average
power, as opposed to 2.5W to 10W with DPSS, allows excimer lasers to be a cost-
effective manufacturing solution.
v) This is especially the case at 193nm laser wavelength that is not currently attainable
by DPSS lasers at reasonable average powers (ie; 30W). The 193nm laser wavelength is
used for laser micromachining specific materials such as pebax, nylon, glass and
bioabsorbable materials. To automate the mask projection technique, programmable
mask changers are deployed where multiple mask pattern are mounted on a high speed
linear mask stage, permitting different mask patterns to be shuttled in on-the-fly.

34. 34
MATERIAL PROPERTIES AND CUTTING PRINCIPLE:
Most materials absorb light more strongly as the optical wavelength decreases. For example,
at the ArF laser wavelength deep in the ultraviolet, oxygen absorbs the laser photons and
even air becomes weakly attenuating. Many polymers, crystals and metals that are either
highly transmissive or highly reflective at infrared and visible wavelengths – for example,
borosilicate glass, polyurethane, or silver – absorb strongly in the ultraviolet. This allows
ultraviolet lasers to be used in cutting applications that are unsuitable for longer
wavelength sources. Some materials show dramatic changes in absorption even within the
ultraviolet. Figure below shows the absorption depth of PMMA at three excimer laser
wavelengths. Between the ArF and XeF wavelengths the absorption depth of PMMA changes
by a factor of 100. At the longer 351 nm wavelength this material requires high laser
power for cutting, and the quality of the cut is poor. At 193 nm the same material cuts
cleanly with significantly reduced power requirements.
Absorption depth of PMMA at three excimer laser wavelengths.
Ultraviolet lasers also benefit from the fact that a high-quality laser beam can be focused
with high-quality optics into a spot of dimensions comparable to its wavelength. A suitable
ultraviolet source can be used to machine features that are more than an order of
magnitude smaller than those available with similar CO2 laser equipment. Since many laser
micromachining applications require production of feature dimensions in the micron
range, short laser wavelength can be essential to the feasibility of a process.

35. 35
When polymers absorb ultraviolet light of sufficiently short wavelength, molecular bonds
are broken and the material is converted to small particles and gaseous fragments. In UV
laser cutting of plastics, this "cold cutting" effect minimizes damage to surrounding
material. All ultraviolet lasers used for cutting produce pulsed light with individual pulse
duration in the tens of nanosecond range. The short pulse duration of UV laser sources also
limits the time available for heat to spread from the cutting zone to adjacent material. The
net result of short wavelength and short pulse duration is a higher quality cut in most
materials.
The figure below shows the essential processes involved in machining with pulsed UV light
and the effect produced on a worksurface by a single laser pulse. When the worksurface
strongly absorbs the light, the penetration depth often is less than one micron and there is
little opportunity for lateral or axial diffusion of the absorbed energy. The excited volume is
very well defined, and material within it is ejected in a vapour plume. The shallow pit
produced by the single pulse has a predictable depth and a lateral profile that mimics the
optical energy distribution in the focused spot. If the laser light is uniformly distributed in
the focal spot, the ablated area will have a "top hat" profile, a Gaussian beam will produce a
Gaussian profile, and more complex distributions will tend to produce more complex
profiles.
(a) During the ablation process, a pulse of UV laser light is focused onto a highly-absorbing
substrate. Vaporization of material in the illuminated region produces a plume of ejected molecules
and particles. (b) After the optical pulse, a thin layer of material has been removed with little
damage to unilluminated adjacent areas.

36. 36
The depth of material removed by a single pulse strongly depends on the composition of the
worksurface, the wavelength of the laser, and the energy per unit area in the focal spot. The
table and the figure below shows the depth of material removed from a polyimide surface
by single pulses of 248 nm krypton fluoride laser light at various energy densities.
Vaporization does not occur when the focused energy density, or fluence, is less than
approximately 10 millijoules/cm2. As the fluence increases above this threshold level, the
ablation depth rapidly increases and then begins to plateau. This type of threshold
behaviour at low fluence and plateau behaviour at high fluence is typical of most materials.
Highest cutting efficiency is attained when the fluence is near the onset of the plateau
region
.
Ablation depth and associated fluence range for polymers, metals, and ceramics
Ablation (etch) depth as a function of energy density (fluence) for ablation of Kapton (polyimide) by
a 248 nm KrF excimer laser.

37. 37
DEPTH CONTROL AND 3-D MACHINING:
Since each pulse removes only a very thin layer of material from the worksurface, good
depth control is usually achievable with pulsed UV laser micromachining. With careful
choice of materials, three-dimensional structures with very smooth surfaces also can be
fabricated. An approach that is quite effective involves scanning the laser beam over the
substrate surface to remove a thin layer of material in selected areas, as shown in the fig.
below. By repetitively scanning a selected area and removing sequential layers a three
dimensional structure can be produced. Best results are obtained when the material is
strongly absorbing at the laser wavelength and vaporizes without going through a melt
phase.
Production of three-dimensional shapes on a substrate surface by repetitive scan patterning with a
UV laser.

38. 38
LASER BEAM MICROMACHINING OF
DIFFERENT MATERIALS

39. 39
LASER BEAM MICROMACHINING OF SILICON:
The key technologies in silicon micromachining are Bulk micromachining and surface
micromachining. Bulk micromachining features a sculpturing capability in three
dimensions, but at the expense of tapered sidewalls and limited aspect ratio of structures.
Surface micromachining enables the fabrication of microstructures of small feature size,
but with a structural thickness limited to that of the deposited film. Many devices exhibit
sensitivity depending on the mass of a suspended microstructure (accelerometer, gyroscope,
etc.) and large but closely separated opposing areas are required for effective capacitive
sensing or electrostatic actuating. Neither conventional bulk micromachining nor surface
micromachining is suitable for the fabrication of such structures and high-aspect-ratio
deep-plasma-etching techniques offer a huge potential in this respect. Recently very thick
(several microns) polysilicon layers have been obtained using epitaxial growth of
polysilicon in an epitaxial reactor . The epitaxial polysilicon layer can be formed on a
sacrificial oxide layer. After deep directional plasma etching to obtain high-aspect-ratio
structures, conventional wet sacrificial etching processes can be applied to release the
structure. Therefore the problem of limited structural thickness in conventional surface
micromachining is solved. However, many applications require single-crystalline silicon
microstructures due to their superior mechanical properties to those of polysilicon.
Basically, it is desirable to micromachine bulk silicon with directional plasma etching so as
to achieve small lateral dimensions, large structural layer thickness and single-crystalline
silicon microstructures. Micromachining techniques based on SOl (silicon on insulator) or
porous silicon formation are very interesting and promising developments . The former uses
an epitaxial process to form a thick single-crystal silicon layer on a SOI substrate. The
epitaxial layer serves as the structural layer, while the insulating oxide layer is the
sacrificial layer. The latter uses a single-crystal silicon substrate and a porous silicon layer is
formed underneath the micromechanical structures. The porous layer serves as the
sacrificial layer. Therefore the thickness of the structural layer in these techniques is much
less limited compared with the conventional surface micromachining. Anisotropic plasma
etching can be applied to pattern the structure to achieve fine feature size. However, since
the releasing process in both techniques is based on wet chemical etching of a sacrificial
layer, stiction similar to that in conventional surface micromachining is often a problem.

40. 40
Therefore, it would be beneficial to use also plasma etching instead of wet etching in the
releasing process.
A novel bulk-micromachining technique called SIMPLE ( silicon micromachining by
plasma etching ) is described. This technique uses a CI2/BCI3 plasma chemistry which etches
p- or lightly n-doped silicon anisotropically but heavily n-doped silicon laterally. As shown
in Fig. 1 below, an n + buried layer is formed on the substrate before the growth of a lightly
n-doped epitaxial layer. The Cl2/BCl3 plasma etches the epitaxial layer anisotropically but
lateral etching occurs when the n + buffed layer is exposed to the plasma. Thus the silicon
beam with vertical sidewalls can be patterned and released from the substrate in a single
plasma etch due to the lateral etching of the n + layer. As will be shown, the technique
features simplicity and fabrication compatibility with microelectronic circuits.
Experimental details: The SIMPLE technique has been experimentally
optimized to maximize the lateral etching of the n + buried layer, while keeping the etching
of the epitaxial silicon layer anisotropic. In the experiment b-doped (100) wafers with 1-5
fl cm resistivity were used as the starting material. The n + buried layer was formed by ion
implantation of n-type dopants, using either phosphorus, arsenic or antimony, followed by
a thermal anneal to remove the crystal damage and to activate the implanted dopant.
Subsequently, a 4 p,m silicon epitaxial layer doped with I × 1016 cm- 3 arsenic was formed.
After deposition and plasma patterning of a 2 p,m thick PECVD oxide mask, the wafers were
etched in the Alcatel GIR300 Reactive Ion Etching (RIE) machine [ 11 ] using a two-step
process. The first step of the plasma etching, which used 15 sccm BCI3 with 0.34 W cm- 2
r.f. ( 13.56 MHz) power density and 22.5 mtorr pressure, was necessary to remove any
residual oxide in the windows to eliminate non-uniform silicon etching in the second step.

41. 41
The second step, for the real bulk silicon etching, used a gas mixture of 20 sccm C12 + 5
sccm BOa with 0.57 W cm- 2 power density and 15 mtorr pressure. After the etching the
wafer was cleaved and the etch rate and sidewall profile were examined using a Philips
SEM525E scanning electron microscope.
Lateral etching of the n + buried layer formed
by different n-type dopants: In principle undoped or lightly doped
silicon is not etched by Cl atoms or O2 molecules unless energetic ion bombardment to the
substrate in a Cl-containing plasma is present . The ion bombardment causes damage in the
silicon crystal and therefore induces chemical reactions between the silicon atoms and Cl
atoms and/or O2 molecules. In other words, the etching takes place only along the direction
of ion bombardment, which is perpendicular to the substrate. This is why the lightly n-
doped epitaxial layer is etched anisotropically in the experiments. Using the Cl2/BCl3
chemistry, the vertical etch rate of lightly doped silicon is = 2000 A min- '. By contrast,
heavily n-doped silicon reacts rapidly and spontaneously with Cl atoms, thus etching takes
place also on the surface of the buffed layer that does not receive energetic ion
bombardment, leading to the lateral etching observed. This spontaneous reaction is related
to the concentration of active n-type carriers, rather than the chemical identity of the
dopant. Therefore, no lateral etching was found in the unannealed buried layers.
Some mechanisms have been proposed to explain the spontaneous etching of n + silicon in
Cl plasmas . It is generally agreed that n-type doping raises the Fermi level and thereby
reduces the energy barrier for charge transfer to chemisorbed chlorine. As shown in Fig. 3
below, Cl atoms are covalently bound to specific sites in the case of an undoped silicon
surface. Steric hindrance impedes impinging Cl atoms from penetrating the surface to reach
sub-surface Si--Si bonds. However, a more ionic Si-Cl surface bond is formed on a + silicon
surface due to the n-type doping and enhanced electron transfer, providing additional
chemisorption sites and facilitating Cl penetration into the substrate lattice. This makes it
possible for impinging Cl atoms to chemisorb, penetrate the lattice and react more readily.
Therefore, on the surface of n + silicon Cl atoms can react with silicon spontaneonsly,
resulting in lateral etching of the n + buried layer.

42. 42
Effects of the n + doping level on the lateral
etch rate: Fig. 4 shows the lateral etch rate of n + buried layer formed by arsenic
implantation, after annealing. The etch rate increases linearly with the logarithm of the
implantation dose. With doses of less than 1X 10-15
cm -2
no significant lateral etching can
be observed. Experiments with an annealed phosphorus-implanted buried layer result in
similar etching.
The results indicate that the lateral etching takes place only when the carrier concentration
is above a threshold. Note that the maximum solid solubility of antimony in silicon is 7 × 10
-9
cm- 3
and its electrically active surface concentration is limited to2-5 × 1019
cm -3
. This
carrier concentration is below the threshold concentration indicated, thus preventing the
etching.

43. 43
LASER BEAM MICROMACHINING OF SYNTHETIC DIAMOND:
The laser ablation technique has been extensively recognized to be a unique method for the
micromachining and designing of micro components. Chemical vapor deposited (CVD)
diamond films or synthetic diamond have been processed by various types of pulsed lasers
for this purpose. Nanosecond pulsed excimer lasers, and recently femtosecond pulsed lasers
have been used for the micromachining of diamond films. CVD diamond films were subject
to various processing environments, such as atmospheric condition, vacuum condition and
gas stream condition. Observed damage depends on behavior of the plasma in the different
processing environments. In the gas stream condition, it has been clearly shown that proper
dissipation of the high-temperature plasma leads to precisely irradiated surfaces, which are
almost free from thermal damage.
The laser ablation technique has been extensively recognized to be a unique method for
micromachining and designing of micro components. CVD diamond films have been
processed by various types of pulsed lasers for this purpose. Nanosecond pulsed, excimer
lasers have been applied for surface modifications of CVD diamond films. In these
investigations, the excimer lasers have shown their capa-bilities in polishing, etching, and
patterning of the diamond films. Reduced surface roughness, improved optical
transmittance, and patterned structures were reported from the applications. However, the
processes have also exhibited some damage, such as transformed surface layers, and spoiled
spot edges. These drawbacks have been obstacles to the application of the nanosecond
pulsed excimer lasers to high resolution micromachining of CVD diamond. Recently,
femtosecond pulsed UV lasers have been introduced to overcome the drawbacks of
nanosecond pulsed lasers.
A CVD diamond film was subjected to this preliminary investigation. The polycrystalline
diamond film has an average grain size and thickness of 2 mm and 7 mm, respectively. The
diamond film was subjected to three different processing environments, such as
atmospheric condition, vacuum condition and gas-stream condition. The interaction,
between the CVD diamonds and excimer laser pulses, showed drastic changes among the
different processing environments.

44. 44
Experimental setup for the excimer laser irradiation in atmospheric condition
Atmospheric condition: SEM micrographs in Fig. below are taken from
the irradiation under stationary air in ambient atmosphere. In Fig.a, the surrounding of the
irradiated spot shows a distinctive plasma affected zone (PAZ), which has somewhat
symmetrical, ring-shaped patterns with spikes. The areas of PAZ indicated by 1 and 2 in
Fig.b are magnified in Fig.b1 and b2, respectively.
(a) the irradiated spot and surroundings;(b) the irradiated spot; (b1) magnified view of area 1 from
(b); and (b2) magnified view of area 2 from (b).
Vacuum condition: A vacuum chamber was constructed to examine the
interaction in an airless condition. Using a rotary-vane vacuum pump, a low-vacuum

45. 45
condition (10 KPa) was gained in the chamber. In Fig. 2a below, the PAZ, around the
circular spot at the center, was considerably reduced as compared to that in Fig. a, in
atmospheric environment. However, in Fig. 2b below, the area surrounding the irradiated
spot still shows an oval-patterned PAZ.
Gas-stream condition: In the gas-stream condition, a nozzle was
installed to blow a 99.9% argon gas over the surface of the diamond thin film, upon the
laser pulse irradiation. The nozzle was positioned approximately 3 cm from the irradiated
spot and 30° to the film surface. Diameter of the nozzle was 2mm and the argon gas flow
rate was 4.72×10−4
m3
/s. In Fig.a below, no PAZ was observed around circular spot at the
center, in contrast to atmospheric and vacuum conditions. In Fig. b below, areas near the
irradiated spot show no visible damage.
In the gas stream condition, it has been clearly shown that proper dissipation of the high-
temperature plasma leads to a precisely irradiated surface, which is almost free from
plasma damage.

46. 46
Experimental setup and SEM micrographs for the excimer laser irradiation in gas-stream condition:
(a) the irradiated spot and surroundings;
(b) the irradiated spot
LASER BEAM MICROMACHINING OF GLASS:
UV nanosecond pulsed laser machining can be used to machine glass. The production of 3D
structures in glass is required for applications such as a microchemical chips and optical
waveguides. Methods using a laser with a short wave length or a short pulse, glass with
high absorptivity, and the addition of absorbents all have been proposed for the laser
machining of glass. Methods using a femtosecond laser and an absorbent have been
reported, in particular, for machining the interior of the glass material.
Machining system: The light source, wavelength and the pulse width are a
third harmonic of YAG laser, 355 nm(UV nanosecond laser), and 40 ns, respectively. A lens
with a focal length of 42.27 mm was used to concentrate the beam. Minimum spot radius of
the laser was 9.17 mm. A stage with the resolution of 1.0 mm was adopted for the X and Y
motions. The workpiece was a silica glass with the size of 20mmX10mmX5 mm. The
bottom and the top of the workpiece contact with the absorbent slurry and the air,

47. 47
respectively. The target was machined while the absorbent slurry was continuously stirred
with a magnetic stirrer, to prevent the deposition of the slurry and to remove any generated
bubbles. CeO2 (average powder diameter: 14 nm), TiO2(36 nm) and ZnO(34 nm),which are
generally used as UV blockers in products such as cosmetics, were used as the absorbent
after mixing with water.
Groove machining on the surface: The machining
characteristics were investigated through groove machining experiments on the surface of
the workpiece. A groove with length of 500 mm was machined by the linear motion of the
laser. The relationship between the groove depth and the machining conditions was studied.
The groove shape was measured using the laser microscope after washing with an aqueous
solution of sodium hydroxide (1 mol/l).

48. 48
Relationship between the pulse energy and the
machining depth: The machining depth increased as the pulse energy
increased as shown in Fig. 7. The relationship between the machining depth and the pulse
energy is linear when the focus is located in the absorbent as shown in Fig. 7(a). Crack
generation increased the groove depth at 20 mJ/pulse for ZnO. The data for more than 40
mJ/ pulse are lacking because of crack generation. The machining depth is saturated, even
when the pulse energy increased for CeO2 and TiO2, when the focus is located in the
workpiece glass, as shown in Fig. 7(b). The result is the same as the simulation described in
the previous section. Therefore, the machining phenomenon is considered as thermal
melting caused by laser heating. The tendency for ZnO is different from other absorbent
materials. This is because the machining energy threshold is large and it is expected that the
depth will be saturated at a larger pulse energy. However, cracks are generated by the
energy absorption of the glass itself. The machining rate is high when the focus is located in
the absorbent. Furthermore, cracks occur often. The reason is that bubbles are generated
when the laser energy is absorbed in the narrow region. Therefore, the removal rate in the
melting area is large, due to a cavitation effect.

49. 49
Relationship between the defocus and the
machining depth: The machining amount increased as the defocus became
small, as shown in Fig. 8. Cracks were generated around the focus area, similar to the
investigation for the pulse energy. The groove depth is large when the defocus is negative.
At the focus area, on the contrary, it is large when the defocus is positive. However, the
beam radius does not vary so much when the defocus is less than 150 mm, because the
Rayleigh range is approximately 150 mm.
Relationship between the absorbent
concentration and the machining depth: The groove
depth increased as the absorbent concentration increased for CeO2 and ZnO, as shown in
Fig. 9. The groove depth was almost the same for TiO2 when the concentration was more
than 0.5 wt%. The reason is presumed to be that the concentration of absorbent becomes
constant around the machining area, in particular, for more than 0.5 wt% because TiO2
easily adheres to the glass wall. Absorbent state, such as adhesion and doping, around the
machining area is crucial for removal efficiency.

50. 50
Relationship between the pulse frequency and
the machining depth: A linear relationship was found between the pulse
frequency and the groove depth, as shown in Fig. 10. The figure shows that the machining
amount per pulse is almost constant. The total number of irradiation pulses is determined
by the pulse frequency.
Relationship between the scan speed and the
machining depth: The groove depth increased in inverse proportion to the
laser scan speed, as shown in Fig. 11. This is because the pulse number is distributed along a
unit length that is proportional to the laser scan speed

51. 51
Relationship between the scan number and the
machining depth: The groove depth increased proportional to the scan
number. Therefore, it is expected that the machining efficiency does not vary even if the
surface has some roughness.

52. 52
SPECIFIC FIELDS OF APPLICATION
OF LASERS IN MICROMACHINING

53. 53
Selection of the optimum laser wave length, laser pulse parameters, beam delivery optics,
assist gases has a critical effect on the quality for each application for each type of
material. A sequence of controlled laser pulses (pulse duration and frequency) –
maximizing peak power density and optimizing pulse parameters) delivers a precise
amount of energy to the material.
Laser processing; drilling, cutting and scribing of various materials; metals, plastics,
ceramics, silicon, composites are widely used laser processes for numerous industries and
applications, e.g., electronic - semiconductor, medical, sheet metal, solar cell, micro cooling,
machining and fabrication.
MICRO-HOLE DRILLING:
The ability to drill ever smaller holes down to ~1μm diameter is an underpinning
technology in many industries that manufacture high-tech products. By providing solutions
to critical problems in manufacturing integrated circuits, hard disks, displays,
interconnects, computer peripherals and telecommunication devices, laser micromachining
is a key enabling technology allowing the current revolution in information technology to
continue. The requirement for material processing with micron or submicron resolution at
high-speed and low-unit cost is an underpinning technology in nearly all industries that
manufacture high-tech products. The combination for high-resolution, accuracy, speed and
flexibility is allowing laser micromachining to gain acceptance in many industries.

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Figure 9: 100 micro m holes drilled in 75m high-density polyethylene with (a) a twist drill bit (b) a
KrF laser
As shown in Fig. 9 above, a practical illustration of laser micromachining can be seen when
comparing the quality of holes drilled by an excimer laser and a mechanical twist drill.
While the mechanically drilled hole is close to the minimum size that can be drilled, the
improved quality of this and much smaller holes drilled by lasers is obvious.
Microvia hole drilling in circuit
interconnection packages: Almost as important as the rapid
improvements in speed and memory of integrated circuits (IC's) are the parallel
developments in interconnection packaging made during the last 20 years. So speed, power
and area (real estate) are not compromised, packages on which chips are mounted for
connection to other devices have had to keep pace with the rapid advances made in IC's.
Thus there is a demand for an ever-increasing packing density of interconnections - for
example mountings in current mobile phones and camcorders have around 1200
interconnections/cm2
. There are now more than a dozen generic types of chip
interconnection packages which include multichip-modules (MCM's), chip-scale-packages
(CSP's) like ball-grid-arrays (BGA's), chip-on-boards (COB's), tape-automated-bonds
(TAB's). Generally these consist of multilayer sandwiches of conductor-insulator-conductor
with electrical connection between layers made by drilling small vias through the dielectric
and metal plating metal down the hole. Such blind via holes provide high-speed
connections between surface-mounted components on the board and underlying power
and signal planes while minimizing valuable real estate occupation. For example, due to
difficulties in soldering IC's with greater than ~200 pins, peripheral lead mounting
packages like TAB's must be made larger than the chip. By placing microvia connections in
the package at the base of the chip instead of around its periphery, a BGA is no larger than

55. 55
20% the size of the chip. Typically then the requirement is to drill 100μm diameter
microvias on ~500μm centers. The cost for drilling these vias on such high-density
packages can represent 30% of the overall cost of the board.
In the electronics industry, a “via” is a hole through a material which, when filled with a
conductive material such as copper or silver-conductive ink, creates an electrical
connection between at least two conductive layers. The simplest form involves two
conductive layers separated by an insulating substrate, commonly either polyimide (PI),
polyester (PET), or epoxy resin. Vias can be “through” or “blind” type, as shown in the
illustration.
Cross sectional diagrams of laser drilled vias.
Through vias are drilled through all layers of the laminant structure while blind vias are
initiated at one surface and stop at the layer interface within the laminant.
Drilling microvias by ablation was first investigated in the early 1980's using pulsed Nd:
YAG and CO2 lasers. Excimer lasers led the way in applying it to volume production when
the Nixdorf computer plant introduced polyimide ablative drilling of 80μm diameter vias in
MCM's - as used to connect silicon chips together in high-speed computers. Other
mainframe computer manufacturers such as IBM rapidly followed suite and installed their
own production lines for this application .With fewer process steps than other methods,
laser-drilling is regarded as the most versatile, robust, reliable and high-yield technology

56. 56
for creating microvias in thin film packages. Trillions of vias have now been drilled with
excimer lasers at yields >99.99% whose mean time between failure (MTBF) has been logged
at >1,000 hours.
Interconnection densities on rigid and flexible printed circuit boards (PCB's and FPC's) are
also increasing, driving the requirement for drilling ever-smaller vias in these packages. In
such lower cost packages the current common practice is to mechanically drill the vias. As
diameters decrease to <100μm it is generally recognized lasers will displace mechanical
drills, although for these packages excimer lasers are too slow and expensive. Because
~100μm diameter tungsten-carbide drills are expensive, frequently break and rapidly
wear, drilling costs skyrocket to several $ per 1,000holes. Using TEA, rf-excited slab CO2 or
Q-switched Nd: YAG lasers, drilling speeds for precisely positioned vias can be as high as
200holes/sec at costs as low as 0.6¢ per 1,000holes. Trepanning the hole with a small focal
spot under galvo-mirror scanner control allows hole positions and sizes to be programmed
from CNC drill files containing the circuit layout. Since copper is highly reflective at 10μm,
Q-switched Nd: YAG lasers (fundamental or 3rd-harmonic) are used to drill the metal,
while either CO2 (rf-excited or TEA) or 3rd-harmonic YAG lasers drill the dielectric
material. When drilling blind microvias, CO2 lasers have the advantage that drilling
naturally self-terminates at the copper level below without damaging it. Holes defined
previously in the top copper (either by a YAG laser or chemically etched using
photolithography) can be used as a conformal mask for cleanly drilling the dielectric
material – as shown in Fig. 11(b) below.
Fig11. 100μm diameter blind microvia drilled in a PCB. (a) Step 1. Nd laser trepanned hole in top
copper conductive layer. (b) Step 2. CO2 laser drilling of fiber reinforced composite FR4 layer to
copper below.

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There is a large potential market for laser microvia drilling tools. Many companies now use
pulsed CO2 lasers for drilling 80-100μm blind vias through dielectric layers on MCM's,
CSP's, and TAB's which then get incorporated into flat panel displays, hard-disk drives,
printers, cameras, cellular phones, photocopiers, fax machines, notebook and palmtop PC's.
With laser-drilling now producing twice as many microvias than any other method, the
annual market for laser-drilling tools in Japan alone is estimated to be ~600 units.
Ink jet printer nozzle drilling: Inkjet printers comprise a row of
small tapered holes through which ink droplets are squirted onto paper. Adjacent to each
nozzle, a tiny resistor rapidly heats and boils ink forcing it through the orifice. Increased
printer quality is achieved by simultaneously reducing the nozzle diameter, decreasing the
hole pitch and lengthening the head. Modern printers like HP's Desk Jet 800C and 1600C
have 300x 28μm input diameter nozzles giving a resolution of 600 dots-per-inch (dpi).
Earlier 300dpi printers consisted of a 100nozzle row of 50μm diameter holes made by
electroforming thin nickel foil. Trying to fabricate more holes with smaller diameters
reduced even further the already low 70-85% production yield. Laser drilling of nozzle
arrays allowed manufacturers to produce higher performance printer heads at greater
yields. At average yields of >99%, excimer laser mask projection is now routinely used for
drilling arrays of nozzles each having identical size and wall angle. Most of the ink jet
printer heads sold currently are excimer laser drilled on production lines in the US and
Asia. Figure 12(a) shows some excimer laser drilled nozzles in a modern printhead.
Fig12. (a) Array of 30μm diameter ink jet printer nozzles drilled in polyimide. (b) Array of nonlinear
tapered nozzles aiding laminar fluid flow

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Figure 12(b) shows nozzles with nonlinear tapers to aid the laminar flow of the droplet
through the orifice. More advanced printers sometimes use piezo-actuators. Rather than
being constrained to give shapes characteristic of the process, excimer laser
micromachining tools with appropriate CNC programming can readily engineer custom-
designed 2½D and 3D structures. Fig. 13(a) shows an example of a rifled tapered hole that
spins the droplet to aid its accuracy of trajectory, while Figure 13(b) shows an array with
ink reservoirs machined behind each nozzle.
Fig13. (a) Tapered nozzle with rifling. (b) Nozzle array with machined reservoirs
MICRO-MILLING:
Laser micro-milling is emerging as an important technology for rapid prototyping, and
serial production of microdevices using batch fabrication methods. The attraction to pulsed
lasers in particular, available with high repetition rates up to 500 kHz and short ns- or ps-
pulse durations, stems from their ability for very finely controlled material removal at
reasonable processing speeds. Ultra fast (pico and femto second) lasers can be optimised to
process more than one class of material. Competing micro-milling techniques such as
micro electrodischarge machining (microEDM), ion milling or lithography are generally
considered material dependent or slow. Although laser ablation suffers from the usual
compromise between high ablation rates and good surface quality, we show that careful
laser choice and process optimization can result at a satisfactory compromise for both.

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Laser micro-milling of metals: Laser milling of metals finds
specific industrial applications but is often considered a complex operation. In many laser
milling and engraving systems, long pulses (microsecond duration) are used. The long pulse
duration produces relatively high ablation rates but the extensive thermal penetration
depth, laser melting and recast dominate the process resulting in poor feature quality. More
recently, progress in laser milling of metals has been achieved using shorter laser pulses at
higher repetition rates. Tungsten has a high melting and boiling temperature therefore
requires very high peak power for efficient laser machining. Figure 16 shows a SEM image
of a CVL machined feature used in biomedical tissue repair. The inverse pyramidal shape
was machined in layers of 20μm thickness. Figure 17 shows removal rate versus laser
fluence and shows that relatively high ~106 μm3/sec rates could be achieved. A
combination of high quality and attractive laser milling speed was achieved by careful
optimization.
Fig 16.Laser micro-milled inverse pyramidal feature in tungsten
Fig 17.Volume removal rate (x106
μm3
/pulse) versus laser fluence for tungsten micro-milling

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Initial tests have also been conducted using the 1064nm picosecond laser. Figure 18 shows
a high magnification view of the bottom of picosecond laser milled feature in stainless steel.
The surface roughness is Ra < 0.3 μm which is substantially better than that achieved with
longer laser pulses. Figures 18, 19 and 20 show examples of laser milled bottom surfaces in
stainless steel using pico, nano and micro-second laser pulses respectively. It is clear that
the surface roughness improves with decreasing pulse duration. Note that the nanosecond
example is shown at a higher SEM magnification than the other examples. However, the
picosecond milled features show higher taper than the nanosecond milled examples and so
further process optimization is required.
Fig 18.Surface roughness in stainless steel achieved with picosecond laser (preliminary results). SEM
image at X1000 magnification
Fig 19.Surface roughness in stainless steel achieved with nanosecond laser. SEM magnification
X3500
Fig 20.Surface roughness in stainless steel achieved with microsecond laser. SEM magnification
X1000

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Laser micro-milling of ceramics: The interaction of most hard
ceramics with high intensity laser pulses is quite different to that with metals. The threshold
for ablation is higher (factor of 2 – 10) and more obviously defined. In many ceramics such
as alumina and silicon nitride there is no evidence of melting. One challenge for laser
micro-milling of ceramics is the large scattering exhibited at common laser wavelengths,
which restrict energy absorption. A combination of short pulse and short wavelength
usually shows best results. Figure 21 illustrates high quality ps-laser milling of alumina
using the modelocked laser at 1064 nm. The 1mm square was milled using rastering steps
of 6μm, scanning speed of 300 mm/s and repetition rate of 50 kHz with a laser spot size
~17.5μm. The milled area appears flat with no evidence of laser-induced cracking or other
collateral damage and given the already granular texture of alumina, the milled surface can
be considered smooth.
Fig 21.Laser milled pocket in alumina using 1064nm picosecond laser
The floor surface roughness measured as Ra~0.9 μm, compares extremely well with the
unirradiated parent surface (Ra~0.73μm) and is smaller than the irradiating wavelength.
This combined with the high volume removal rates of ~107
μm3
/sec and above, makes the
laser very attractive for micro-milling as compared to UV excimers with lower removal
rates reported earlier (~104-105 μm3/sec). The surface roughness increases gradually
from Ra~1μm to 1.45μm with increasing rastering step from 4 μm to 10 μm. Similarly
surface roughness increases with increasing volume removal rate as seen in Figure 22 in

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the next page. For the highest recorded Ra value of 1.7μm, the rastering step used during
milling was 10 μm.
Fig 22.Surface roughness versus material removal rate in μm3
/sec for alumina using a 1064nm
picosecond laser
The direct-write method of many laser machining processes can make it a very flexible tool
for micromachining. Figure 23 that shows an example of a component fabricated using
drilling, cutting and milling in a single operation using the nanosecond CVL. The
component was fabricated from a 0.3mm thick sheet of alumina. The 45o
chamfers along
the edge of the bar were created using controlled depth ablation. The depth being varied by
locally adjusting the total number of pulses used.
Fig 23. Example of drilling, cutting and milling in alumina ceramic

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Laser micro-milling of dielectrics: Laser machining of
dielectrics is usually limited by their optical properties. Many dielectrics, such as fused
silica, are transparent to most common laser wavelengths and controlled laser ablation can
only take place at VUV wavelengths or by using ultrashort laser pulses. Other materials
such as synthetic CVD diamond are not totally transparent owing imperfections in the
material and trace elements. As such it is possible to process CVD diamond with longer
wavelength nanosecond lasers. Figure 24 illustrates an example of a laser milled sensor
mold on synthetic diamond using high peak power from the CVL. The device consists of a
meander-line ridge with 0.12mm height. The meander lines have a period of 170μm, wall
thickness of 30μm and inner and outer radius of curvature of 25μm. A total area of 11.2
mm2
was milled in 14 min. A layer of approximately 3-4 μm in depth was milled out for
each laser pass of the rastered area. The overall volume removal rate in this case was 0.09
mm3
/min. The shot overlap used was maintained 50% and 55% in x- and y-axis
respectively.
Fig 24.CVL milled sensor mold in CVD diamond
Fused silica is a wide band gap material, transparent at 511nm and so it was not possible to
successfully micro-mill using the CVL. But a tightly focused 1064 nm ps-laser beam of
average power 0.5W was capable of 3D micro-milling. The laser was rastered on the
surface to create 1mm square pockets using a rastering step of 1-2μm. The scanning speed
ranged between 50-200 mm/s and the repetition rate was fixed at 50 kHz. By varying the
number of repeat passes over the exposed area the milled depth ranged between 27-210
μm. Although ps laser ablation of fused silica has been shown before, it is interesting that
high repetition rate laser ablation enables such high quality micro-milling of a brittle

64. 64
material without evidence of microcracking or other collateral damage, see figure 25. Some
edge chipping is present but generally the surface roughness on the floor of the milled area
was kept low with measured values of Ra~0.9μm albeit higher than the unirradiated
surface (Ra ~20nm).
Fig 25.Picosecond laser (1064nm) milling of fused silica.
3D laser micro-milling of fused silica was also attempted with the 355nm frequency
unconverted mode locked laser. It was noted that the average floor surface roughness was
reduced with increasing milled depth as shown in figure 26 and that for depths greater
than 60 μm, the surface roughness was better than that achieved with 1064nm.
Fig 26.Surface roughness Ra, versus laser milled depth in fused silica using 355nm picosecond laser

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The lowest Ra value measured was Ra~ 0.434 μm. Generally an uneven surface
morphology on the milled floor is commonly encountered in ultrafast laser machining of
glasses and is attributed to the resolidification of the molten layers generated during the
laser pulse overlap. Pressure gradients can be generated by the plasma formed above the
molten surface, particularly large near the edges of the laser spot. During the melt lifetime,
the thin layer of molten material will flow from the centre of the laser spot outwards. It is
proposed that an averaging effect from a repetitive action of this process with increasing
milled depth might produce the improving Ra observed.

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CONCLUSION
In this term paper, a brief overview have been given about the idea of laser, micro
machining, advantages of micro machining over conventional machining, the advantages of
using laser ablation compared to other material removal processes are discussed, a short
review of the laser physics and laser operations is given. Different types of lasers, which are
widely used in industry, are discussed in short and principal laser beam micromachining
operations and their applications are highlighted.
The future areas of development in laser beam micro-machining include the use of shorter
excimer laser wavelengths (157nm) which can access more materials with better precision
(e.g. PTFE) and the use of ultra-short pulses to machine the widest choice of materials with
very high quality. Also, many areas of laser beam micro-machining applications are
increasingly demanding the ability to be able to machine non-planar samples and this
challenge is already being taken up by both excimer laser mask projection systems and
direct write tools. The problems with this approach are similar, irrespective of the technique
which is considered - how to handle a sample with an arbitrary shape, how to maintain
proper imaging or focusing on the sample surface and how to program the system to
micro-machine the desired pattern with sufficient accuracy.
Laser beam micromachining has been already recognized by government-supported
initiatives in Japan and the European Union, and this technology will be a key
manufacturing tool in emerging nanotechnologies. The economic advantages of mass
production at low unit cost are of the highest importance and will open up many new
industrial application areas.