The document discusses photolithography, which is a key process in microfabrication. It describes the basic photolithography process which involves coating a wafer with photoresist, exposing it to UV light through a mask, developing it to remove either exposed or unexposed areas of resist, and then using the patterned resist for etching or deposition. The document provides details on the components used, including masks, photoresist materials, and exposure systems. It also explains the various steps such as surface cleaning, soft/hard baking, development, and inspection.

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Photo-lithography
Manufacturing
Book: Micro Electro Mechanical System Design by James J Allen

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What is Lithography?
• Lithography(Greek word) means printing on stone.
• Photo-lithography: light-silicon wafer-printing.
• Components in photolithography:
(1)Mask (2) Photoresist (3)UV exposure system
Lithography is the most widely used method to pattern
layers in microelectronic and MEMS processing. 2

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Overview of the Photolithography
Process
(1)Mask
(2) Photoresist
(3)UV exposure system

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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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Surface Cleaning
• 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 Deionized water system)

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• films from other sources:
-solvent residue
-H2O 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
Surface
Cleaning

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Spin coating with photoresist
• 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( k p )/root of(w),
where,
k = spinner constant
p = resist solids content in percent
w = spinner rotational speed in rpm/1000

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Stages of Resist Coating

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Stages of Resist Coating
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Soft baking
• 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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Soft baking

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Mask alignment & Exposure
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.
• Projection printing provides high resolution and low
defect densities and dominates today. The number of
print 50 wafers/hour.

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Positive resist & Negative
resist
16

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Development
• Soluble areas of photoresist are dissolved by
developer chemical
• Visible patterns appear on wafer
• Quality measures:
• –line resolution
• –uniformity
• –particles & defects

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Hard baking
• 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)

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Plasma Etch-Or Add Layer
19

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Post process cleaning
• 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

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

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Photolithography
Requirements• High Resolution
• High PR Sensitivity
• Precision Alignment
• Precise Process Parameters Control
• Low Defect Density
• Photo sensitive material
• Temporarily coated on wafer surface
• Transfer design image on it through exposure
• Very similar to the photo sensitive coating on the film for
camera
Photoresist

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Negative Photoresist
• Becomes insoluble after
exposure
• When developed, the
unexposed parts dissolved.
• Cheaper
Positive Photoresist
• Becomes soluble after exposure
• When developed, the exposed
parts dissolved
• Better resolution
Photoresist

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• Polymer
• Solvents
• Sensitizers
• Additives
POLYMER
• Solid organic material
• Transfers designed pattern to wafer surface
• Changes solubility due to photochemical reaction when exposed
to UV light.
• Positive PR: from insoluble to soluble
• Negative PR: from soluble to insoluble
Photoresist Composition

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SOLVENTS
• Dissolves polymers into liquid
• Allow application of thin PR layers by spinning.
SENSITIZERS
• Controls and/or modifies photochemical reaction of resist
during exposure.
• Determines exposure time and intensity.
ADDITIVES
• Various added chemical to achieve desired process results, such
as dyes to reduce reflection..
Photoresist Composition

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• Most negative PR are polyisoprene type
• Exposed PR becomes cross-linked polymer
• Cross-linked polymer has higher chemical etch resistance.
• Unexposed part will be dissolved in development solution.
Negative Resist
Disadvantages
• Polymer absorbs the development
solvent
• Poor resolution due to PR swelling
• Environmental and safety issues
due to the
main solvents xylene.

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• Exposed part dissolve in developer solution
• Image the same that on the mask
• Higher resolution
• Commonly used in IC fabs
Positive Resist

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Comparison of Photoresists

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• High resolution
– Thinner PR film has higher the resolution
– Thinner PR film, the lower the etching and ion
implantation resistance
• High etch resistance
• Good adhesion
• Wider process latitude
– Higher tolerance to process condition change
Requirement of Photoresist

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• Photoresist must be able to withstand process conditions
• Coating, spinning, baking, developing.
• Etch resistance
• Ion implantation blocking
Photoresist Physical Properties
Photoresist Performance Factor
• Resolution • Adhesion
• Expose rate, Sensitivity and Exposure Source
• Process latitude/ liberty • Pinholes
• Particle and Contamination Levels
• Step Coverage • Thermal Flow

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• The smallest opening or space that can produced in a photoresist layer.
• Related to particular processes including expose source and developing
process.
• Thinner layer has better resolution.
• Etch and implantation barrier and pinhole-free require thicker layer
• Positive resist has better resolution due to the smaller size of polymer.
Resolution Capability