Lithography. Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material by selective exposure to a radiation source such as light.
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Lithography in the MEMS context is typically the 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
In order to make useful devices the patterns for different 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
The exposure parameters required in order to 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 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 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 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 or the critical dimension is the 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)
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 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
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
Focused ion beam
Multiphoton lithography (also known as 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 three-dimensional periodic or non-periodic pattern.
This method could also be used for rapid prototyping of structures with fine features
In laser physics the numerical aperture is 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 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 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
Because of the sputtering capability, the FIB is used as a micro-machining tool, to modify or machine materials at the micro- and nanoscale.
nano machining with FIB is a field that still needs developing.
The common smallest beam size is
ion beam induced deposition
FIB-assisted chemical vapor deposition occurs when a 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