1. Lithography
Instructor
Abu Syed Md. Jannatul Islam
Lecturer, Dept. of EEE, KUET, BD
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Department of Electrical and Electronic Engineering
Khulna University of Engineering & Technology
Khulna-9203
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Electrical and mechanical properties of the wafer depend on the orientation of the crystalline
structure, the impurity concentrations, and the type of impurities present.
The surface of the wafer is then polished to a mirror finish using chemical and mechanical
polishing (CMP) techniques.
Crystal is then sawed (like a loaf of bread) to produce circular wafers that are 400μm to 600μm
thick
Solid cylinder 10 cm to 30 cm in diameter and can be 1 to 2 m in length.
Very-high-purity, single-crystal silicon ingot
Basic Things
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When designating the relative doping concentrations in semiconductor material, it is common
to use the + and – symbols.
If a large number of impurity atoms is added, the silicon will be heavily doped (e.g.,
concentration > ∼10^18 atoms/cm−3).
Depending on the types of impurity, either holes (in p-type silicon) or electrons (in n-type
silicon) can be responsible for electrical conduction.
A specific amount of impurities known as doping allows the alteration of the electrical
properties of the silicon, in particular its resistivity.
These variables are strictly controlled during crystal growth
Basic Things
4. 4
The ability to control the type of impurities and the doping concentration in the silicon permits
the formation of diodes, transistors, and resistors in integrated circuits.
Similarly, p+ and p− designations refer to the heavily doped and lightly doped p-type regions,
respectively.
A heavily doped (low-resistivity) n-type silicon wafer is referred to as n+ material, while a lightly
doped material (e.g., concentration < ∼1016 atoms/cm−3) is referred to as n−.
Basic Things
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Simple layers of thin films do not make a Device.
Why Lithography?
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It was invented in 1796 by German author and
actor Alois Senefelder as a cheap method of
publishing theatrical works.
History of Lithography
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What is Lithography ?
Lithography comes from two Greek words,
“lithos” which means stone and “graphein” which
means write.
“ writing a pattern on stone”
Lithography is the transfer of geometric shapes on a mask
to a smooth surface
It uses light or other forms of radiant energy to change the
chemical properties of thin layers of films that have been
coated on a substrate.
Typically 8-25 lithography steps and several hundred
processing steps between exposure are required to
fabricate a packed IC.
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Lithography is one of the 4 major processes in the top-down
model
Lithography
Etching
Deposition
Doping
What is lithography ?
In order to perform the other 3 processes,
we must precisely define where to do them
Lithography Does This!
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Photolithography is the process
of transferring patterns of geometric shapes
on a mask
to a thin layer of photosensitive material (called photoresist)
covering the surface of a semiconductor wafer.
Photolithography
A light sensitive photoresist is spun onto the wafer forming a thin layer
on the surface.
The resist is then selectively exposed by shining light through a mask
which contains the pattern information for the particular being
fabricated.
The resist is then developed which completes the pattern transfer from
the mask to the wafer.
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Photolithography is an optical means for transferring patterns
onto a substrate
Overview of the Photolithography Process
1. Surface Preparation (Get rid of H2O, RCA clean, apply adhesion
promoter
2. Deposit (Photoresist Coating by Spin Casting)
3. Soft Bake (90 – 120°C for 60 –120 sec to remove solvent from liquid
photoresist
4. Photo Mask Alignment
5. Exposure (Pattern transfer)
6. Development (Remove soluble photoresist)
7. Hard Bake (100 – 180°C) to increase adhesion
8. Etching (Remove oxide)
9. Stripping (Photoresist removal)
10. Post Processing/Cleaning (Ashing)
Photolithography
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The surface patterns of the various integrated-circuit components
can be defined repeatedly using photolithography
Here, a photographic plate with drawn patterns will be used to
selectively expose the photoresist under a deep ultraviolet
illumination (UV)
The exposed areas will become softened (for positive photoresist)
The exposed layer can then be removed using a chemical developer,
causing the mask pattern to be duplicated on the wafer
Silicon dioxide, silicon nitride, polysilicon, and metal layers can be
selectively removed using the appropriate etching methods
After the etching step(s), the photoresist is stripped away, leaving
behind a permanent pattern of the photomask on the wafer surface
Photolithography
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Photoresist Composition
The most commonly used positive resist consist of diazonaphtoquinone (DQ), which is the
photoactive compound (PAC), and novolac (N), a matrix material called resin. Upon absorption of
UV light, the PAC undergoes a structural transformation which is followed by reaction with water to
form a base soluble carboxylic acid, which is readily soluble in basic developer (KOH, NAOH, TMAH
etc.)
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Positive Photoresist
Most commonly used in
the IC industry.
Become soluble after
exposure
Better resolution
Cheaper
Negative Photoresist
Becomes insoluble after
exposure
When developed, the
unexposed parts
dissolved
Cheaper
Types of Photoresist
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Soft Bake
Used to evaporate the coating solvent and to densify the
resist after spin coating.
Typical thermal cycles: 90-100°C for 20 min. in a
convection oven, 75-85°C for 45 sec. on a hot plate
Commercially, microwave heating or IR lamps are also
used in production lines.
Improves adhesion
Improves uniformity
Improves etch resistance
Improves line width control
Optimizes light absorbance characteristics of photoresist
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Photomasks are high precision plates containing microscopic images
of electronic circuits.
There are four types of material used to make photomasks;
quartz (the most commonly used and most expensive), LE, soda lime,
and white crown.
What Is a Photomask?
Material Used to make Photomasks:
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Contact Printing
The mask is directly in contact with
the wafer
Advantages
Simple
Low Cost
Disadvantages
Poor for small features
Mask damage may occur from
contact
Defects from contaminants on
mask or wafer due to contacting
surfaces
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Proximity Printing
The mask is above the wafer surface
Advantages
Mask damage is minimal
Good registration possible
Disadvantages
Poorer resolution due to distance
from the surface
Diffraction errors
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Projection Printing
An optical system focuses the
light source and reduces the
mask image for exposure on
the surface
Advantages
Higher resolution
Lens system reduces
diffraction error
Disadvantages
Errors due to focus of lens
system may occur
Limiting factor in resolution
can be due to optical
system
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Hard Bake
Evaporate remaining photoresist
Improve adhesion
Used to stabilize and harden the
developed photoresist prior to
processing steps
Eliminates the solvent burst effects
in vacuum processing
Introduces some stress into the
photoresist.
Needed for acid etching, e.g. BOE.
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Etching
Etch oxide with hydrofluoric acid (HF)
Only attacks oxide where resist has been exposed
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Want to remove the photoresist and any of its residues.
– Positive photoresists:
• acetone
• trichloroethylene (TCE)
• phenol-based strippers
– Negative photoresists:
• methyl ethyl ketone (MEK), CH3COC2H5
• methyl isobutyl ketone (MIBK), CH3COC4H9
Photoresist Removal (Stripping)
Plasma etching with O2 (Ashing) is also effective for
removing organic polymer debris.
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Resolution: minimum feature dimension that can be transferred with
high fidelity to a resist film.
Registration: how accurately patterns on successive masks can be
aligned (or overlaid) with respect to previously defined patterns.
Throughput: number of wafers that can be exposed/unit time for a
given mask level.
Performance Metrics
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Limitations of Optical Lithography
Resolution becoming a challenge for deep-submicron IC process
requirements
Complexity of mask production and mask inspection
High cost of masks
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Electron Beam Lithography
• Involves direct exposure of the resist by a focused electron
beam without a mask
• Resolution as low as 10 – 25 nm
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Electron gun
generates beam of
electrons
Condenser lenses
focus the e-beam
Beam-blanking
plates turn beam
on and off
Electron Beam Lithography
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Electron Beam Lithography
Advantages
Generation of submicron resist geometries
Highly automated and precisely controlled operation
Greater depth of focus than that available from optical lithography
Direct patterning on wafer without using a mask
Disadvantages
Low throughput
Expensive resists
Proximity effect: backscattering of electrons irradiates adjacent regions
and limits minimum spacing between features
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Extreme ultraviolet : EUV
Vacuum operation
Laser plasma source
Very expensive system
Uses very short 13.4 nm light
Step and scan printing
All reflective optics (at this wavelength all materials absorb!)
Uses reduction optics (4X)
Optical tricks seen before all apply: off axis illumination
(OAI), phase shift masks and OPC
Next Generation Lithography
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Extreme ultraviolet : EUV
Challenges:
• EUV is strongly absorbed in all materials.
• Lithography process must be performed in vacuum
• Mask blank must also be multilayer coated to minimize its reflection.
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1nmAdvantages:
Low diffraction
Shorter exposure time
Scattering is minimum
X –rays pass through spots
Problems:
Masks are the most
Difficult and critical
Element of an XRL system
lacking of photoresist
1:1 printing
High energy x-ray destroy
conventional optics
X-ray Lithography
