E Beam Lithography.pdf - 123SeminarsOnly

PHOTOLITHOGRAPHY

Photolithography…
• Process of transferring geometrical
patterns from mask on to layer of
radiation sensitive material covering
the surface of wafer.
• Steps involved:
• Coat wafer with resist.
• Exposing to radiation.
• Developing.
• Etching.

MASK: Chrome coated quartz plate

Photo-resist
• Is a radiation sensitive material which changes
chemically 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

Positive Lithography

Negative Lithography

Mask:
• Same size as final chip or an integral factor of
final 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.

Aligning the masks
• Each successive layer has to be
aligned with the previous layer.
• Each mask layer consists of
alignment marks which help in aligning
the layers on top of each other.

Features:
• Resolution
– Minimum feature size that can be
transferred precisely.
• Throughput
– Number of wafers processed per hour.
• Depth of focus
– Masks must be aligned.

Lithography optical
X-ray
E-beam
Contact printing
Proximity printing
Projection printing

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

Optical lithography
• Proximity printing:
• Mask close to wafer.
• No contact (10 to 25 μm)
• Lesser resolution
• Higher mask life.
• Projection printing:
• Mask kept at higher distance.
• Highly focused image.
• Higher mask life.
• Compromise on cost.

E-beam lithography
• Optical lithography: ~0.5 microns
• For sub-micron fabrication: e-beam
lithography.
• Size < 1 μm possible.
• Direct writing.
• Easy automation.
• Greater DOF.

HOW…
• E-beam diameter: 0.01 to 0.5
microns.
• Focused on substrate.
• Scanned over required area.
• Substrate placed on movable table.
– After one scan field, need to move
wafer for next imprinting.

E-beam lithography
• For higher resolution– smaller
diameter.
• Smaller diameter low throughput.
• Minimum feature size ~ 4 x diameter.
• Scan field ~ 2000 x diameter.
• Hence higher time.

E-beam resist
• Basically organic polymers.
• 2 types : +ve and –ve
• Positive resist
• Softens with e-beam.
• Chemical bond breaking.
• Better resolution.
• Poor sensitivity.

• Negative resist:
• Hardens when exposed to radiations.
• Cross-linking.
• Increase in molecular weight.
• Poor resolution due to swelling.

Issues regarding e-beam
lithography
• Slow process.
• Proximity effect.
• Due to electron scattering.
• Inter proximity effect.
• Intra proximity effect.
• USED MAINLY FOR MASKS

X-ray lithography
• An extension of optical lithography.
• No direct writing.
• Proximity printing.
• Distance – 40 to 50 μm

Characteristics..
• Faster throughput.
• Good resolution.
• No proximity effect.
• No effect of dirt.
• Straight walls due to low absorption.

HOW…
• Electron beam focused to water cooled
Palladium target.
• Generates x-rays ( λ=4.37 angstrom)
• Passed through beryllium window and sent
to chamber.
• Chamber filled with He.
• He doesn’t absorb x-rays.
• Mask and substrate kept at close
proximity.
• Substrate is developed by x-rays.

• Smaller λ higher resolution.
• X-ray mask: thin membrane coated
with gold.
• All e-beam resist can be used as xray
resist.
• Mainly PMMA

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