Lithography is a process that uses light to transfer geometric patterns from a photomask to a light-sensitive chemical "photoresist" on a semiconductor substrate. The key steps in the lithography process include cleaning and preparing the wafer surface, depositing and spinning photoresist, soft baking to evaporate solvents, aligning the mask and exposing the photoresist to light, developing to remove exposed or unexposed areas of photoresist, hard baking to harden the photoresist, plasma etching or depositing additional layers, cleaning, and inspecting the final patterned wafer. Lithography is critical for manufacturing integrated circuits and is capable of printing ever smaller semiconductor features.

1. Lithography Fabrication
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Jadhav Avinash J
 2K13E11
 Savitri Bai Phule Pune University

2. What is Lithography?
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 Lithography(Greek word) means printing is done
on stone.
 Photo-litho-graphy: light-silicon wafer-printing.
 Components in photolithography:
(1)Mask (2)Photoresist (3)UV exposure system

3. Overview of the Photolithography Process
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4. Steps Used in Photolithography
 Surface cleaning
 Spin coating with photoresist
 Soft baking
 Mask alignment
 Exposure
 Development
 Hard baking
 Plasma Etch-Or Add Layer
 Post process cleaning
 Final Inspection
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5. Surface Cleaning
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 Typical contaminants that must be removed prior to
photoresist coating: dust from scribing or cleaving
(minimized by laser scribing)
 photoresist residue from previous photolithography
(minimized by performing oxygen plasma ashing)
 atmospheric dust (minimized by good clean room
practice)
 bacteria (minimized by good DI water system)

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 films from other sources:
-solvent residue
-H2 O residue
-photoresist or developer residue
-silicone
 For particularly troublesome grease, oil, or wax
stains: Start with 2-5 min. soak in 1,1,1-
trichloroethane (TCA) or trichloroethylene (TCE)
with ultrasonic agitation prior to acetone

7. Spin coating with photoresist
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 Wafer is held on a spinner chuck by vacuum and
resist is coated to uniform thickness by spin coating.
 Typically 3000 - 6000 rpm for 15-30 seconds.
 Resist thickness is set by: primarily resist viscosity
secondarily spinner rotational speed
 Most resist thicknesses are 1-2 μm for commercial Si
processes.

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 Resist thickness is given by
t =square of( kp)/root of(w1),
where,
k = spinner constant, typically 80-100
p = resist solids content in percent
w = spinner rotational speed in rpm/1000

9. Stages of Resist Coating
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10. Soft baking
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 Used to evaporate the coating solvent.
 Typical thermal cycles:90-100°C for 20 min. in a
convection oven 75-85°C for 45 sec. on a hot plate
 Microwave heating or IR lamps are also used
 Optimizes light absorbance characteristics of
photoresist

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12. Mask alignment & Exposure
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Transfers the mask image to
the resist-coated wafer
Activates photo-sensitive
components of photoresist
Three types of masking
(1) Contact printing
(2) Proximity printing
(3) Projection printing

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 Contact printing capable of high resolution but has
unacceptable defect densities. May be used in
Development but not manufacturing.
 Proximity printing cannot easily print features below
a few mm in line width. Used in nano-technolgy.
 Projection printing provides high resolution and low
defect densities and dominates today.They print » 50
wafers/hour.

15. Positive resist & Negative resist
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16. Development
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 Soluble areas of photoresist are dissolved by
developer chemical
 Visible patterns appear on wafer
 Quality measures:
 –line resolution
 –uniformity
 –particles & defects

17. Hard baking
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 Used to stabilize and harden the developed
photoresist prior to processing steps that the resist
will mask.
 Postbake removes any remaining traces of the
coating solvent or developer.
 Higher temperature than soft bake (120-150 degree)

18. Plasma Etch-Or Add Layer
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19. Post process cleaning
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 Plasma etching with O2 (ashing)
 Simple solvents are generally sufficient for non-postbaked
photoresists:
 Positive photoresists:
aceton
trichloroethylene (TCE)
 Negative photoresists:
methyl ethyl ketone
methyl isobutyl ketone


20. Final Inspection
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 Photoresist has been completely removed
 Pattern on wafer matches mask pattern (positive
resist)
 Quality issues:
–defects
–particles
–step height
–critical dimensions

21. Any Questions???
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