l4 Lithography

8/3/2019 l4 Lithography

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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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