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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
