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PHOTOLITHOGRAPHY
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Photolithography
Process of transferring geometricalpatterns from mask on to layer ofradiation sensitive material coveringthe surface of wafer.
Steps involved: Coat wafer with resist. Exposing to radiation.
Developing. Etching.
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MASK: Chrome coated quartz plate
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Photo-resist
Is a radiation sensitive material which changeschemically on exposure to light. Usually a carbon based organic molecule. Two types of resist:
Positive Regions of resist exposed to light dissolve quickly in
developer Unexposed regions remain unchanged and are not removed
by developer Negative
Regions exposed to light are hard to remove by developer
Unexposed regions are easily removed by developer Positive resists result in better resolution than
negative resist
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Positive Lithography
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Negative Lithography
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Mask:
Same size as final chip or an integral factor offinal chip.
During exposure, the image size is reduced. Made of fused silica. Essential properties:
High degree of optical transparency. Small thermal expansion coefficient. Flat and polished surface.
Resistant to scratches. Opaque layer: Chromium.
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Aligning the masks
Each successive layer has to bealigned with the previous layer.
Each mask layer consists ofalignment marks which help in aligningthe layers on top of each other.
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Features:
Resolution Minimum feature size that can be
transferred precisely.
Throughput Number of wafers processed per hour.
Depth of focus
Masks must be aligned.
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Lithography optical
E-beam
X-ray
Contact printing
Proximity printing
Projection printing
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Optical lithography
Most popular and oldest. Follows four basic steps.
Contact printing:
Wafer in contact with mask. High resolution.
Life of mask is less.
contamination
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Optical lithography
Proximity printing: Mask close to wafer.
No contact (10 to 25 m)
Lesser resolution
Higher mask life. Projection printing:
Mask kept at higher distance.
Highly focused image.
Higher mask life. Compromise on cost.
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E-beam lithography
Optical lithography: ~0.5 microns
For sub-micron fabrication: e-beam
lithography. Size < 1 m possible.
Direct writing.
Easy automation.
Greater DOF.
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HOW
E-beam diameter: 0.01 to 0.5microns.
Focused on substrate. Scanned over required area. Substrate placed on movable table.
After one scan field, need to movewafer for next imprinting.
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E-beam lithography
For higher resolution smallerdiameter.
Smaller diameter low throughput. Minimum feature size ~ 4 x diameter.
Scan field ~ 2000 x diameter.
Hence higher time.
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Negative resist: Hardens when exposed to radiations.
Cross-linking. Increase in molecular weight.
Poor resolution due to swelling.
I di b
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Issues regarding e-beamlithography
Slow process.
Proximity effect.
Due to electron scattering. Inter proximity effect.
Intra proximity effect.
USED MAINLY FOR MASKS
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X-ray lithography
An extension of optical lithography.
No direct writing.
Proximity printing. Distance 40 to 50 m
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HOW
Electron beam focused to water cooledPalladium target. Generates x-rays ( =4.37 angstrom) Passed through beryllium window and sent
to chamber. Chamber filled with He. He doesnt absorb x-rays. Mask and substrate kept at close
proximity. Substrate is developed by x-rays.
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Smaller higher resolution.
X-ray mask: thin membrane coated
with gold. All e-beam resist can be used as x-
ray resist.
Mainly PMMA
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Limitations
Geometric effects. Mask is at a distance.
Instead of point, a blur region formed.
Depends on Diameter of x-ray
Distance between mask and substrate.
Distance between light source and mask
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