MEEN 489-500 Nanoscale
Issues in Manufacturing
Lithography
Lecture 1: The Lithographic
Process
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Discuss Reading Assignment
a 1 Introducing Nano
a 2 Size Matters
a 3 Interlude One-The Fundamental Science
Behind Nanotechnology
`Electrons, Atoms, Ions, Molecules
`Metals, Ceramics, Polymers
`Biosystems
`Electrical Conduction and Ohm’s Law
`Quantum Effects
`Optics
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Nanofabrication - 2 principal
approaches
a Top-Down
a Bottom-Up
` Miniaturizing existing
processes at the
Macro/Microscale
` Traditional approach in
industrial applications
` E.g. Lithography, backbone of
computing systems
` Assembling structures from
the atomic/molecular level
` Novel approach, conceptually
imitating nature
` E.g. chemical self-assembly
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Lithography
a Lithography in Art
a How lithography works
a Materials used for lithography drawing
a Photolithography
a Photolithographic process
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Lithography in Art
a Invented by Alois Senefelder in 1798
a Used for book illustrations, artist's prints,
packaging, posters etc.
a In 1825, Goya produced a series of lithographs.
a In the 20th and 21st century, become an
important technique with unique expressive
capabilities in the Art field
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How Lithography started
a Lithography (Greek for "stone
drawing") relies on the fact that
water and grease repel
a Draw a pattern onto a flat stone
surface with a greasy substance
a Paint the printing ink onto the
stone
a While the stone background
absorbs water, the greasy
substance retains wet ink on top
a Press paper against the stone to
transfer the pattern
a Positive! Repeatable!
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Materials used for
lithography drawing
a Litho crayons and
pencils (containing
wax, pigment, soap
and shellac), conte
crayons, pens and
graphite pencils, etc.
“Bulls of Bordeaux” by Goya
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Lithography, to date
a Miniaturized computing circuits require mass
manufacturing of small features ⇒ push
lithographic approach to new limits
a Some lithography approaches for manufacturing
`Optical lithography (including ultraviolet)
`X-Ray lithography
`Electron Beam lithography
`Ion Beam lithography
`“Dip-Pen” lithography
`…
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Optical/UV Lithography
a Workhorse of current chip manufacturing
processes
a Limited by wave length of light employed
a Smaller features ⇒ reduce wave length ⇒ UV
light
a Here is how it works
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Photolithographic process
a Wafer cleaning
a Barrier layer formation
a Photoresist application
a Soft baking
a Mask alignment
a Exposure and development
a Hard-baking
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Wafer cleaning
a Cleans wafer chemically to remove particulate
matter on the surface
(traces of organic, ionic, and metallic impurities)
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Barrier layer formation
a After cleaning, silicon dioxide (a barrier
layer) is deposited on the surface of
the wafer
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Photoresist application
a After the formation of the SiO2 layer, photoresist
is applied to the surface of the wafer
a Spin coating produces a uniform thin film
a There are two types of photoresist: positive and
negative
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(Photolitho_main_process.jpg)
http://britneyspears.ac/physics/fabrication/photolithography.htm
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Soft baking
a During this stage, most of the solvents are
removed from the photoresist coating
a The photoresist coatings become
photosensitive (imageable) after softbaking
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Mask alignment
a Use a photomask - square glass plate with a
patterned emulsion of metal film on one side
a The mask is aligned with the wafer to
transfer the pattern onto the wafer surface.
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Exposure
a Photoresist - exposed through pattern on the
mask with a high intensity ultraviolet light.
a Three primary exposure methods:
- contact
- proximity
- projection
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(plithp2.gif)
http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html
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Development
a At low-exposure energies, the negative
resist remains completely soluble in the
developer solution…a solvent mixture
delivered the resist to the surface and the
‘masked’ area did not chemically change.
a For positive resists, the resist solubility in its
developer is finite even at zero-exposure
energy…the masked area did not chemically
change, the exposed area must become
MORE soluble.
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(plithp3.gif)
http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html
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Hard-baking
a The final step in photolithographic process
a Harden the photoresist and improve
adhesion of the photoresist to the wafer
surface
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References:
a http://www2.mmlc.nwu.edu/c303/levavy/lith1.html
a http://www.ece.gatech.edu/research/labs/vc/theory/pho
tolith.html
a http://www.ece.gatech.edu/research/labs/vc/theory/pho
tolith.html
a http://en.wikipedia.org/wiki/Photolithography
a http://britneyspears.ac/physics/fabrication/photolithogra
phy.htm
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Assignment
aReview this PowerPoint set.
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