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Lithography
• Lithography in the MEMS context is typically the
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transfer of a pattern to a photosensitive material
by selective exposure to a radiation source such
as light.
A photosensitive material is a material that
experiences a change in its physical properties
when exposed to a radiation source. If we
selectively expose a photosensitive material to
radiation (e.g. by masking some of the
radiation) the pattern of the radiation on the
material is transferred to the material exposed,
as the properties of the exposed and unexposed
regions differs
STEPS IN LITHOGRAPHY
• COATING THE SUBSTRATE WITH PHOTO
SENSITIVE MATERIAL (PHOTO RESIST)
• FIXING THE MASK WITH THE FEATURES
ON THE COAT
• EXPOSURE TO RADIATION
• SPRAY OF DEVELOPER TO OBTAIN
EITHER ‘POSITIVE’ OR ‘NEGATIVE
• ETCH OR DEPOSIT
• STRIP THE PHOTO RESIST
MASK, EXPOSURE & DEVELOP
ETCH(SUBTRACT) or
DEPOSIT(ADD)
CARE TO BE TAKEN
• Alignment
• Exposure
Alignment
• In order to make useful devices the patterns for different
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lithography steps that belong to a single structure must be aligned
to one another.
The first pattern transferred to a wafer usually includes a set of
alignment marks, which are high precision features that are used as
the reference when positioning subsequent patterns, to the first
pattern
Often alignment marks are included in other patterns, as the original
alignment marks may be obliterated as processing progresses.
It is important for each alignment mark on the wafer to be labeled
so it may be identified, and for each pattern to specify the
alignment mark (and the location thereof) to which it should be
aligned.
By providing the location of the alignment mark it is easy for the
operator to locate the correct feature in a short time. Each pattern
layer should have an alignment feature so that it may be registered
to the rest of the layers
Exposure
• The exposure parameters required in order to
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achieve accurate pattern transfer from the mask
to the photosensitive layer depend primarily on
the wavelength of the radiation source and the
dose required to achieve the desired properties
change of the photoresist.
Different photoresists exhibit different
sensitivities to different wavelengths.
The dose required per unit volume of photoresist
for good pattern transfer is somewhat constant
EFEECTS OF OVER EXPOSURE
• if an image is overexposed, the dose received by
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photoresist at the edge that shouldn't be exposed may
become significant.
If we are using positive photoresist, this will result in the
photoresist image being eroded along the edges,
resulting in a decrease in feature size and a loss of
sharpness or corners
If we are using a negative resist, the photoresist image
is dilated, causing the features to be larger than desired,
again accompanied by a loss of sharpness of corners.
If an image is severely underexposed, the pattern may
not be transferred at all, and in less sever cases the
results will be similar to those for overexposure with the
results reversed for the different polarities of resist
Industrial Process steps
• Dehydration bake - dehydrate the wafer to aid
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resist adhesion.
prime - coating of wafer surface with adhesion
promoter. Not necessary for all surfaces.
Resist spin/spray - coating of the wafer with
resist either by spinning or spraying. Typically
desire a uniform coat.
Soft bake - drive off some of the solvent in the
resist, may result in a significant loss of mass of
resist (and thickness). Makes resist more viscous.
Alignment - align pattern on mask to features on
wafers.
• Exposure - projection of mask image on resist to
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cause selective chemical property change.
Post exposure bake - baking of resist to drive off
further solvent content. Makes resist more
resistant to etchants (other than developer).
Develop - selective removal of resist after
exposure (exposed resist if resist is positive,
unexposed resist if resist is positive). Usually a
wet process (although dry processes exist).
Hard bake - drive off most of the remaining
solvent from the resist.
Descum - removal of thin layer of resist scum that
may occlude open regions in pattern, helps to
open up corners.
Resolution
• Resolution or the critical dimension is the
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minimum feature size that could be printed
The ability to project a clear image of a small
feature onto the wafer is limited by the
wavelength of the light that is used.
The minimum feature size that a projection
system can print is given approximately by
• λ = wave length,
NA = numerical aperture, K = constant (0.4)
NUMERICAL APERTURE
• In most areas of optics, and
especially in microscopy, the
numerical aperture of an optical
system such as an objective
lens is defined by
• NA = n Sinθ
• where n is the index of
refraction of the medium in
which the lens is working (1.0
for air, 1.33 for pure water, and
up to 1.56 for oils), and θ is the
half-angle of the maximum
cone of light that can enter or
exit the lens
• Photolithography has used ultraviolet light
from gas-discharge lamps using mercury,
sometimes in combination with noble gases
such as xenon. These lamps produce light
across a broad spectrum with several
strong peaks in the ultraviolet range. This
spectrum is filtered to select a single
spectral line, usually the "g-line" (436 nm)
or "i-line" (365 nm).
• CD is 200 to 150nm
• Current state-of-the-art photolithography
tools use deep ultraviolet (DUV) light with
wavelengths of 248 and 193 nm
• which allow minimum feature sizes down
to 100 nm
Immersion lithography
• Immersion lithography is
a photolithography
resolution enhancement
technique that replaces the
usual air gap between the
final lens and the wafer
surface with a liquid
medium that has a refractive
index greater than one. The
resolution is increased by a
factor equal to the refractive
index of the liquid. (CD =
60nm)
Other issues in photo lithography
• Low depth of field and depth of focus
• Depth of field is a measurement of depth
of acceptable sharpness in the object
space, or subject space.
• Depth of focus is a measurement of how
much the film / substrate can be displaced
while an object remains in acceptably
sharp focus
Depth of field diagram
Depth of field and depth of focus
Depth of focus
Depth of Focus
• t is the depth of focus,
•N is the f-number of the optical system
•C is the circle of confusion
•v is the distance of the object from lens
•f is the focal length
maskless lithography
• In maskless lithography, the radiation
that is used to expose a photosensitive
emulsion (or photoresist) is not projected
from, or transmitted through, a
photomask. Instead, most commonly, the
radiation is focused to a narrow beam.
The beam is then used to directly write
the image into the photoresist, one or
more pixels at a time
FORMS OF MASKLESS LITHOGRAPHY
• Laser (Optical)
• Focused ion beam
• Electron beam
Multiphoton lithography
• Multiphoton lithography (also known as
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direct laser writing) is a technique for
creating small features in a photosensitive
material, without the use of complex optical
systems or photomasks.
By scanning and properly modulating the laser,
a chemical change (usually polymerization)
occurs at the focal spot of the laser and can be
controlled to create an arbitrary two or threedimensional periodic or non-periodic pattern.
This method could also be used for rapid
prototyping of structures with fine features
Multiphoton lithography
• In laser physics the numerical aperture is
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defined slightly differently
The NA of a Gaussian laser beam is related to its
minimum spot size by
• D = beam dia
Focused ion beam
• Focused ion beam (FIB) systems operate
in a similar fashion to a scanning electron
microscope (SEM) except, rather than a
beam of electrons and as the name
implies, FIB systems use a finely focused
beam of ions (usually gallium) that can be
operated at low beam currents for imaging
or high beam currents for site specific
sputtering or milling
Why Ions ?
• ions are larger than electrons
• they cannot easily penetrate within individual
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atoms of the sample. Interaction mainly involves
outer shell interaction resulting in atomic
ionization and breaking of chemical bonds of the
substrate atoms.
The penetration depth of the ions is much lower
than the penetration of electrons of the same
energy.
Why Ions ?
• ions are heavier than electrons
• ions can gain a high momentum. For the same
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energy, the momentum of the ion is about 370
times larger.
For the same energy ions move a lot slower than
electrons. However, they are still fast compared
to the image collection mode and in practice this
has no real consequences.
The magnetic lenses are less effective on ions
than they would be on electrons with the same
energy. As a consequence the focused ion beam
system is equipped with electro-static lenses and
not with magnetic lenses
USE of FIB
• Unlike an electron microscope, FIB is
inherently destructive to the specimen.
When the high-energy gallium ions strike
the sample, they will sputter atoms from
the surface.
• Gallium atoms will also be implanted into
the top few nanometers of the surface
• FIB assisted deposition
• the surface will be made amorphous
FIB
• Because of the sputtering capability, the
FIB is used as a micro-machining tool, to
modify or machine materials at the microand nanoscale.
• nano machining with FIB is a field that still
needs developing.
• The common smallest beam size is
2.5-6 nm
ion beam induced deposition
• FIB-assisted chemical vapor deposition occurs when a
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gas, such as tungsten hexacarbonyl (W(CO)6) is
introduced to the vacuum chamber and allowed to
chemisorb onto the sample.
By scanning an area with the beam, the precursor gas
will be decomposed into volatile and non-volatile
components; the non-volatile component, such as
tungsten, remains on the surface as a deposition.
From nanometers to hundred of micrometers in length,
tungsten metal deposition allows to put metal lines right
where needed.
Other materials such as platinum, cobalt, carbon, gold,
etc., can also be locally deposited