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1
Photolithography
References:
Introduction to Microlithography
Thompson, Willson & Bowder, 1994
Microlithography, Science and Technology
Sheats & Smith, 1998
Any other Microlithography or
Photolithography book
Contributors: Dr Nuri Amir, Dr Nava Ariel, Inbar Lifshitz and Oded Cohen
2
Contents
• Chapter 1: Introduction to Photolithography
Chapter 2: Basic Photolithography Optics
• Chapter 3: Resist Bulk Effects
• Chapter 4: Characterization of Process Window
4
Isolation
N Well P Well P N N P
Metal
Integrated Circuits:
5
N Well P Well P N N P
Metal
Isolation
6
Isolation
Photo - Resist
Light
Mask
N Well P Well P N N P
Metal
7
Isolation
Photo - Resist
Developer
N Well P Well P N N P
Metal
8
Isolation
Photo - Resist
After rinse:
N Well P Well P N N P
Metal
9
Isolation
Photo - Resist
Etch:
N Well P Well P N N P
Metal
10
Isolation
Photo - Resist
Strip: Remove of the Photo resist
N Well P Well P N N P
Metal
11
Isolation
N Well P Well P N N P
Metal
12
Photolithography Introduction
Definition
Photolithography: The process of duplicating two-
dimensional master pattern with the use of light
Basic requirements:
Mask with the desired pattern (Reticle)
Illumination system
Flat surface, covered with Photosensitive material
(Photo-resist)
Carefully controlled environment: vibrations,
pressure, humidity, temperature, and light.
Introduction
13
2005 requirements in Semiconductors industry (ITRS road map):
Print 2D layout with lines as
narrow as ~60nm with variation
less than 6nm
Align plates with maximum error
of 20-30nm depending on layer
Introduction Current requirements
Photolithography: ~1/3 of one chip manufacture total cost
* Source: International Technology Roadmap for Semiconductors website:
http://www.itrs.net/Links/2005ITRS/Litho2005.pdf
Table LITH2 Lithography Technology Requirements
Year of Production 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026
DRAM
DRAM ½ pitch (nm) 36 32 28 25 23 20 18 16 14 13 11 10.0 8.9 8.0 7.1 6.3
CD control (3 sigma) (nm) [B] 3.7 3.3 2.9 2.6 2.3 2.1 1.9 1.7 1.5 1.3 1.2 1.0 0.9 0.8 0.7 0.7
Contact after etch (nm) 36 32 28 25 23 20 18 16 14 13 11 10 8.9 8.0 7.1 6.3
Overlay [A] (3 sigma) (nm) 7.1 6.4 5.7 5.1 4.5 4.0 3.6 3.2 2.8 2.5 2.3 2.0 1.8 1.6 1.4 1.3
k1 (13.5nm) EUVL 0.66 0.59 0.52 0.62 0.55 0.49 0.44 0.51 0.45 0.40 0.47 0.42 0.37 0.33 0.29 0.26
Flash
Flash ½ pitch (nm) (un-contacted poly) 22 20 18 17 15 14.2 13.0 11.9 10.9 10.0 8.9 8.0 8.0 8.0 8.0 8.0
CD control (3 sigma) (nm) [B] 2.3 2.1 1.9 1.8 1.6 1.5 1.4 1.2 1.1 1.0 0.9 0.8 0.8 0.8 0.8 0.8
Bit line Contact Pitch (nm) [D] 131 120 110 101 93 113 104 95 87 80 71 64 64 64 64 64
Contact after etch (nm) 36 32 28 25 23 20 18 16 14 13 11 10 8.9 8.0 7.1 6.3
Overlay [A] (3 sigma) (nm) 7.2 6.6 6.1 5.6 5.1 4.7 4.3 3.9 3.6 3.3 2.9 2.6 2.6 2.6 2.6 2.6
k1 (13.5nm) EUVL 0.40 0.37 0.34 0.41 0.38 0.35 0.32 0.38 0.35 0.32 0.37 0.33 0.33 0.33 0.33 0.33
MPU / Logic
MPU/ASIC Metal 1 (M1) ½ pitch (nm) 38 32 27 24 21 19 17 15 13 12 11 9.5 8.4 7.5 6.7 6.0
MPU gate in resist (nm) 35 31 28 25 22 20 18 16 14 12 11 9.9 8.8 7.9 6.8 5.9
MPU physical gate length (nm) * 24 22 20 18 17 15 14 13 12 11 9.7 8.9 8.1 7.4 6.6 5.9
Gate CD control (3 sigma) (nm) [B] ** 2.5 2.3 2.1 1.9 1.7 1.6 1.5 1.3 1.2 1.1 1.0 0.9 0.8 0.8 0.7 0.6
Contact after etch (nm) 43 36 30 27 24 21 19 17 15 13 12 11 9.5 8.4 7.5 6.7
Overlay [A] (3 sigma) (nm) 7.6 6.4 5.4 4.8 4.2 3.8 3.4 3.0 2.7 2.4 2.1 1.9 1.7 1.5 1.3 1.2
k1 (13.5nm) EUVL 0.70 0.59 0.50 0.58 0.52 0.46 0.41 0.48 0.43 0.38 0.44 0.39 0.35 0.31 0.28 0.25
Chip size (mm2)
Maximum exposure field height (mm) 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26
Maximum exposure field length (mm) 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33 33
Maximum field area printed by exposure tool (mm2) 858 858 858 858 858 858 858 858 858 858 858 858 858 858 858 858
Wafer site flatness at exposure step (nm) [C] 38 34 30 27 24 21 19 17 15 13 12 11 10.0 9.0 8.0 7.0
Number of mask Counts MPU [E] 50 54 44 50
Number of mask Counts DRAM [E] 41 33 38
Number of mask Counts Flash [E] 43 31
Wafer size (diameter, mm) 300 300 300 300 300 450 450 450 450 450 450 450 450 450 450 450
NA required for logic (single exposure) 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35
NA required for double exposure (Flash) 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35
NA required for double exposure (logic) 1.12 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35 1.35
EUV (13.5nm) NA 0.25 0.25 0.25 0.33 0.33 0.33 0.33 0.43 0.43 0.43 0.56 0.56 0.56 0.56 0.56 0.56
Manufacturable
solutions exist,
and are being
optimized
Manufacturable
solutions are
known
Interim
solutions are
known
Manufacturable
solutions are
NOT known
14
2014 requirements in Semiconductors industry (ITRS road map): Print 2D layout with lines as
narrow as ~60nm with variation
less than 6nm
Align plates with maximum error
of 20-30nm depending on layer
Introduction Current requirements
Photolithography: ~1/2of one chip manufacture total cost * Source: International Technology Roadmap for Semiconductors website: http://www.itrs.net/Links/2005ITRS/Litho2013.pdf
15
(1) Coat (Spin)
(2) Expose
(3) Develop
The Photolithography Process:
Introduction The process
Printing process consists of 3 steps: Coat, Expose, & Develop
Performed by two machines linked together: Stepper & Track
1. Track: Coats the Si wafer
with Photosensitive
resist material
2. Stepper: Exposes the resist
by the Mask pattern
3. Track: Develops the exposed resist
the Mask pattern is left on the wafer
16
Track (Coater/Developer)
Stepper & Track link:
Chill plate
Introduction The process
17
The Mask (Reticle)
One mask per wafer layer
Made of Quartz (transparent at UV) & Chrome
Must be perfect (+/-2 nm divided by 4)
Cost: $1K - $500K, depend on it’s complexity
Side view Top view
Introduction The Mask The process
18
Performance Trend Motivation for scaling (reduction of transistor size):
Functionality, Speed, Power
Economics - more chips per wafer higher yield
Necessary progress - photolithography printing machine
Introduction photolithography printing machine
From: Intel technology Journal Q3’98
http://www.intel.com/technology/mooreslaw/index.htm
19
Technology Evolution
Wave-length Light Source Size
Ratio
Light Projection
Method
Year
463nm (G-Line) Hg Lamp 1:1 Contact 1970
463nm (G-Line) Hg Lamp 1:1 Proximity 1980
463nm (G-Line) Hg Lamp 1:5 Step & Repeat 1985
365nm (I-Line) Hg Lamp 1:5 Step & Repeat 1991
256nm (DUV) Hg Lamp 1:4 Step & Scan 1994
248nm Excimer Laser 1:4 Step & Scan 1998
193nm Excimer Laser 1:4 Step & Scan 2001
193nm Wet+Excimer Laser 1:4 Step & Scan 2009
13.6nm EUV 1:4 Step & Scan 201?
Introduction photolithography printing machine
(l)
20
Printing methods: Contact Printing
Properties
Mask is in physical contact with the wafer
Mask covers the entire wafer
Limitations
Mask gets dirty and damaged
Wafer non-flat surface affects printing quality
Contact Printing Printing Methods Introduction
21
Printing methods: Proximity
Properties
Mask covers the entire wafer
Small gap between mask and wafer
Limitations
Resolution limit: minimum feature size ~
(for l=365nm minimum feature size for
d~24m)
dl3
m3
d
Proximity Printing Printing Methods Introduction
22
Printing methods: Step & repeat
Projection printing
Expose one or more dies at a time (one field)
Use reduction lens (1:5)
Focus correction at each step
Limited ~25X25mm field size
Today steppers can
print 350nm
with l = 248nm
Stepping Printing Methods Introduction
Mask
wafer
23
Printing methods: Step & Scan
Step (between fields) and Scan (within field)
Scan: Both reticle and wafer move during exposure
Requires stage and reticle excellent sync
Reduced lens active area 25X8mm
– higher quality (uniformity)
Focus while scanning
Resolution: 65nm
Stepping & Scanning Printing Methods Introduction
Mask
wafer
24
Light Sources: Hg Lamp
Light Sources Introduction
Hg lamp (365nm: I-Line, 256nm:
DUV):
Simple for use / easy to replace
Low power at small wavelength –
a resolution limiter
Wide band width:
needs filtering at illuminator
– sensitive to chromatic (wavelength
driven)
aberrations
25
Light Sources: Excimer Laser
Advantages
High Power at UV
Bandwidth narrowing (highly coherent source)
Problems
Pulsed (~30nsec, ~100Hz) –
can damage the optics
Cost, space, facilities
Light Sources Introduction
26
Chapter 2
Basic Photolithography Optics
Light
Geometrical Optics
Wave optics
Projection optics
Resolution limit
Coherence of light
27
The duality of light
Is it a ray of particles?
Is it a wave?
The Electro-Magnetic Wave Optics Basics
De Broglie’s equation
28
The Electro-Magnetic Wave
• Light is composed of
perpendicular
components of
electrical and magnetic
fields
• Both components are
perpendicular to the
wave propagation
direction
•
The Electro-Magnetic Wave Optics Basics
C=lsn=const.
E=hsn h - plank’s constant
29
The Electro-Magnetic Spectrum
The Electro-Magnetic Wave Optics Basics
30
Geometrical Optics
The wave nature of light is neglected
D (feature size) >> l (wavelength)
Basic Principles:
Propagates in straight line within the
same medium
Incident angle i = reflectance angle r
Snell law - light changes direction by the
difference in refractive indices
n1
n2
i r
Geometrical Optics
n1*sin(α) = n2*sin(β)
Optics Basics
31
Interaction of Light and Matter
When an EM wave strikes
an atom, it make its
electrons cloud oscillate.
That oscillation produce
a time delayed EM wave.
[light in matter will travel at a different speed v.
v ≤ C ALWAYS
The index of refraction: n=C/v
Optics Basics Interaction of Light an Matter
n2
n1
32
Lens
From practical considerations spherical lens are
used:
Spherical lens – approximation for “near main
axis” illumination
Focal length f:
Image Formation: object
image
f
Lens Geometrical Optics
)11
)(1(1
21 RRn
fi
R1,2 - lens rad
D1,2 - object, image
F - focal distance
fdd
111
21
Optics Basics
33
Lens Aberration
Lens imperfections (aberrations) are inevitable
As long as the total aberration induced error << l (1/10)
the effect on patterning/resolution will be acceptable –
diffraction limited optics
Aberration types:
Chromatic
Spherical
Coma
Astigmatism
Lens aberrations Geometrical Optics Optics Basics
Hubble Space Telescope
34
Dispersion: n=n(l)
Optics Basics Interaction of Light an Matter
Typical lens material have High n change at UV
l
n
35
Spherical
The focal plane
depends on the ray’s
distance from the
focal axis (h)
Optimal focal plane
changes with h(max)
Can be minimized
with a proper choice
of lens
Spherical Lens aberrations Geometrical Optics Optics Basics
h
Optimal
focal plane
36
Chromatic
n=n(l)
Optimal focal
plain depends on
the band width
Two wavelengths
can be
achromatized by
a proper lenses
combination
Chromatic
Photolithography: Using a very narrow bandwidth or
monochromatic light , for example laser: l=248nm , 0.08pm
bandwidth
Lens aberrations Geometrical Optics Optics Basics
37
Stepper optics
Stepper simplified optical scheme:
Designed objective lens involves complex set of ~30
individual lenses:
Lens Stepper optics Geometrical Optics Optics Basics
Light Source
Condenser
lens
Objective
lens
Wafer Mask
•Disadvantage – power loss!
38
Wave Optics: Diffraction
Geometrical optics:
An opaque object
makes a sharp
shadow.
Reality:
The light bends
around the edge.
Wave Optics Optics Basics Diffraction
39
Wave Optics: Diffraction
Geometrical optics:
An opaque object
makes a sharp
shadow.
Reality:
The light bends
around the edge.
Wave Optics Optics Basics Diffraction
40
Fresnel (near-field) diffraction:
The screen is close to the source
The image of the aperture is projected
onto the screen.
Fraunhofer (far-field) diffraction:
The screen in very far:
R – the distance between aperture and screen.
d – the aperture’s greatest width.
l - the wavelength of the light
The diffraction pattern will be the
Fourier Transform of the aperture.
Fresnel and Fraunhofer diffraction
Wave Optics Optics Basics Diffraction
l2dR
41
Diffracted Light
Light passing trough a grid is diffracted
Diffracted rays are on the order of m = 0, (+/-)1 , (+/-)2
... 0 1
2
-1
-2
•The diffracted pattern originates from interference of
different waves and the phase relations between them
Single slit (w wide) diffraction: Sin m lw
42
Collecting the diffracted rays
The order of the diffracted rays
follows the Fourier series terms.
For perfect image transfer all orders
should pass through the lens
In order to form an image at least
two orders must pass through the
lens
Projection Optics Optics Basics
43
Projection Optics
Mask to lens:
FT of image -carries
information about
image’s spatial
frequencies
Lens to Wafer:
Lens transforms the FT
back into image at
focal plane
Projection Optics
Objective lens Wafer
Mask
FT Inv(FT)
Optics Basics
44
m – diffraction order
l wavelength of light
d – The narrowest line-period
at the reticle (Line + space)
Critical Dimension: CD= ~d/2 (AKA – pitch)
d * sin f = m*l , m= 1,2,… )(
0 1
f -2 2
-1
d
CD
Optical Limits of resolution
f – Angle of diffraction order
NA=sin
NA(min)= l/d=sin f
45
Optical Limits of resolution
CD effect:
As CDs (d) get smaller – angle
between the orders of diffraction
increases:
min CD (d/2) ~ 0.5 l / NA
Resolution Limit Optics Basics
d * sin f = m*l , m= 1,2,… a NA(min)= l/d )(
l1
l2 l1>l2
l effect: The smaller the
wavelength: more orders of
interference, hence better
image quality
46
Optical Limits of resolution
Depth of focus: Wafer printed
out of optimal focus
signal is smeared
Loss of CD control
High NA reduces depth of focus
but improves resolution:
Depth of Focus = k2 l / NA2
Resolution = k1 l / NA
(Rayleigh’s formula)
Low NA
High NA
Resolution Limit Optics Basics
47
Optical Limits of resolution
Resolution Limit Optics Basics
Min CDs : ~ l / NA
The smaller the better
Depth of Focus: ~ l / NA2
The larger the better
Need to find the balance
48
Chapter 3
Photo-resist Properties
Resist Properties
Resist Reaction
Resist Adhesion
Resist thickness control
Standing waves effect
Proximity effect
49
Resist properties
Basic resist requirements:
Sensitivity to Radiation at desired wavelength –
solubility in developer
Good adhesion
Flat and homogeneous coating of the wafer
Controlled resist thickness
Long shelf life
Photo- Resist Properties
50
Resist properties
Resist main components:
Polymers: A long chain of molecules (phenolic
resins) combined with a radiation sensitive
compound (carbon rings with changing cross
linking induced by light)
Solvent
Some resists need to be heated pre exposure
in order to alter solubility
Photo- Resist Properties
51
The Photo-Resist
Negative and Positive Resist:
Expose:
Negative Positive
Develop:
Etch:
Strip:
Resist
Base
Introduction The Resist The process
Light
52
Chemically Amplified Resist – positive resist reaction:
Photo- Resist Properties Resist reaction
•Phenolic resins are hydrophilic and soluble in solvents
•DQ is hydrophobic and causes the phenolic resins to be
hydrophobic as well
•Light transforms DQ into an acid - ICA (Indene Carboxylic Acid)
which is hydrophilic, developer can dissolve the exposed areas
53
Chemically Amplified Resist:
After Exposure: Acid formation at exposed areas
Unexposed resist film
Exposure
H+ H+ H+
H+ H+ H+
H+ H+
H+ H+ H+
H+ H+ H+
H+ H+
PEB: Post Expose Bake: • Acid makes Polymer soluble and hydrophilic
• More acid is evolved (Chain reaction)
• Acid diffuses into unexposed area – not
desired
• End of PEB will stop the chain reaction.
H+
H+ H+
H+
H+
H+ H+
H+ H+
H+ H+
H+
H+
H+ H+
H+
PEB
Photo- Resist Properties Resist reaction
Develop Develop: • Developer spreads on wafer.
• Developer and developed resist are rinsed
Line width at wafer: Smaller than optical printed line
54
1D Proximity Effects Features on reticle will come out with different size
on the wafer depending on their proximity
(interference)
1D Effects Proximity Effects Resist Properties
Wafer CD's as function of
reticle pitch
0 Space between features
Featu
res w
idth
(C
D)
Isolated features
Resolution limit
Dense features
(Pitch =Chrome line + space )
55
2D Proximity Effect
Lines shorter
Rounded corners
2D Effects Proximity Effects Resist Properties
Pattern on Mask: Pattern on wafer:
Destructive intereference in
corners – resist was not
exposed, corner is rounded
56
2D Proximity Effect
Can be fixed by OPC:
Optical Proximity Correction
2D Effects Proximity Effects Resist Properties
OPC require smaller CDs
57
Chapter 4
Characterization of Working Window
Focus Exposure Matrix
DOF
58
Working window D
OF
Positive focus Focus above the resist
Negative focus Focus below the resist
Resist line profile:
Wafer
Wafer
Optimal focus
Wafer
•Changing focus is done by controlling the
distance between the wafer and the lens
59
Positive focus Focus below the resist
Working window D
OF
Negative focus Focus above the resist
60
Working window
Resist profile:
Positive focus Focus below the resist
Negative focus Focus above the resist
Exposure dose
Fo
cu
s p
osit
ion
Working window Characterization Of Working Window
61
Focus Exposure Matrix
Selecting optimal focus point: Wafer stage in optimal location – when CDs don’t vary with focus
Selecting optimal energy dose: print bias: mask size to wafer size delta – depending on desired
CDs
Focus Exposure Matrix Characterization Of Working Window
Exposure
energy
Focus
Proximity effect – focus by isolated
62
Resist profile – Anti Reflective Coating
ARC is used to reduce standing waves which damage
resist profile
63
Back up files
64
Smaller chips Higher yield
65
Smaller chips Higher yield
Edge
Defects
66
EBR – Edge Bid removal
The resist at the edge of the wafer needs to be
removed since it creates particles and makes
the wafer edge sticky.
There are two ways to remove it:
Optical exposure (if the resist is positive)
Chemical removal.