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

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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

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(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

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Track (Coater/Developer)

Stepper & Track link:

Chill plate

Introduction The process

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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Chapter 2

Basic Photolithography Optics

Light

Geometrical Optics

Wave optics

Projection optics

Resolution limit

Coherence of light

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The duality of light

Is it a ray of particles?

Is it a wave?

The Electro-Magnetic Wave Optics Basics

De Broglie’s equation

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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

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The Electro-Magnetic Spectrum

The Electro-Magnetic Wave Optics Basics

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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

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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

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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

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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

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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

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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

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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!

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Wave Optics: Diffraction

Geometrical optics:

An opaque object

makes a sharp

shadow.

Reality:

The light bends

around the edge.

Wave Optics Optics Basics Diffraction

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Wave Optics: Diffraction

Geometrical optics:

An opaque object

makes a sharp

shadow.

Reality:

The light bends

around the edge.

Wave Optics Optics Basics Diffraction

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.