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Lithography

• Optical lithography– Contact/Proximity lithography– Projection lithography (Stepper)

• Electron-Beam lithography• X-ray lithography

lithos, 'stone' + γράφω - graphο

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Optical Lithography

• Exposure tools– mask aligners– illumination sources

• Mercury arc lamp• Excimer lasers

• Photomasks, phase shift masks• Photoresists

– Spin curves– Exposure curves– Resist profile

• Pattern transfer

ContactPrinting

ProximityPrinting

ProjectionPrinting(stepper)

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Mask aligner

Contact Mask Aligner

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Resolution limitk~1, g: gap– Best flatness of glass: 2 μm– Best flatness of wafer: 2 μm– Dust particle: ~ 1 μm– Resist thickness varietion: 100 nm

gkW λ=min

Source Gap Limitg-line (436 nm) 20 μm 3.0 μm

i-line (365 nm) 20 μm 2.7 μm

i-line (365 nm) 10 μm 1.9 μm

Conventional

Diffraction reducing

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Projection printing (stepper)

Projection printing / Stepper

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45nm immersion lithography

Eximer lasers (deep UV lithography)

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Photomasks

• Glass plate with a patterned Cr or FeO2 layer.

• It is designed with a CAD software and converted to the glass plate with EB or optical pattern generator.

• It could be a dark-field or light-field.

• Cost of mask depends on the smallest feature size.

1/20/2010 Nanofabrication 18

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Mask making: E-beam writer

Ebeam raster scanMinimum feature size: 10nmNeeds vacuum

Mask making: Laser writer

Laser raster scanMinimum feature size: 800nmNo need for vacuum

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Photoresist

• Positive vs. negative resist• Spin curves• Contrast curves• Resist profile

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Substrate Substrate

Positive resist

Negativeresist

Positive resist

Negativeresist

Substrate Substrate

Mask Mask

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Positive resist

Negativeresist

Substrate Substrate

Exposed resistis removed

Exposed resistremains

Positive NegativeAdhesion to Si fair excellent

Relative cost more less

Developer base aqueous organic

Minimum feature size

0.5 μm 2 μm

Step coverage better lower

Wet chemical resistance

fair excellent

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• When irradiated, the diazonaphthaquinoneundergoes the photochemical Wolf rearrangement.

• Produced carboxylic acid acts as dissolution enhancer that increases the rate of resin dissolution by orders of magnitude.

• The unirradiated regions are effectively insoluble, leading to a positive tone photoresist.

• Example: Two component system– Photoresist contains a polymer (resin) and a

photoactive component (compound with azide group).

– Upon exposure to light, nitrene radicals are produced

– The radicals promote the crosslinking of the polymer matrix which makes it less soluble in solvent.

23 : NNRNR +−→−hν

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AZ P4000 series resist

•Resist thickness varies with the spin speed.•Resist can pile up at the wafer edge.

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⎟⎟⎠

⎞⎜⎜⎝

⎛=

0

10010log

1

DD

γ

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• Cross-sectional view of resist as it covers vertical step.

Step

Resist

• Alignment marks– Optical vs. E-beam

• Lithography qualification– Resolution patterns– Vernier

• Automatic alignment– Moire pattern

Alignment marks

Features on wafer

Features on mask

Mask over wafer

Aligned

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• Photoresist is sculpted by a spatially variable exposure. “analog process”

• The grayscale is created on a binary photomask by usingasubresolution array openings.

• The thickness remaining after development depends on the local dose.

• Local dose takes into account the nonlinear photoresponseof the resist and the etch contrast ratio (10:1 – 1:1).

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Lift-Off Etching

Substrate Substrate

Layer to be etched

Resist afterdeveloping

Lift-Off Etching

Substrate Substrate

DepositedMaterial

Etchedregion

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Lift-Off Etching

Substrate Substrate

Layer patternedby lift-off

Layer patternedby etching

1 μm

PR

200 nm Aluminum