Principles of Optical Lithography, 2002 November 20. Principles of Optical Lithography Layout:...

Principles of Optical Lithography, 2002 November 20.

Principles of Optical Lithography

Layout:• Motivation• IC manufacturing• Traditional method and trends• Resolution enhancement techniques

University ofSzeged


„From Lithography to Optical Lithography”

lithography: Graphic Arts. any of several printing processes,such as offset lithography, in which the image areas of a plateare treated (photographically or directly by hand, as with alithographic crayon) to accept greasy inks and repel water, whilethe nonimage area accept water and repel ink.

/Dictionary of Science and Technology/

“Object” “Image”printing process


„From Lithography to Optical Lithography”

Optical lithography is the process used to transfer a Optical lithography is the process used to transfer a pattern to a layer of an integrated circuitpattern to a layer of an integrated circuit..

application: cameras, microscopes, manufacturing of integrated circuits (IC)

“Object” “Image”optical imaging process


Manufacturing of Integrated Circuits

Chip fabrication occurs as a cycle of steps carried out as many as 20 times!


Silicon crystal (ingot)

1

polished wafer

2

SiO2

Thermal oxidation3

Si - substrate

4

Si substrate

Light

PhotoresistSiO2



Photolithography - Equipment

To HouseVacuum

Hollow Shaft

VacuumChuck

Resist Dispenser

Spin Coater:

Used for the application of photoresist, primer, and developer.

Photoresist Thickness Control:•Photoresist Viscosity•Spin Speed•Temperature•Humidity


Spin Speed vs. Thickness:

Photolithography - Equipment


Positive vs. Negative Resist

• Positive Resist: Photo Mask

Resist

Silicon Substrate

Oxide

Resist Resist

Oxide

• Light areas on the mask => photoresist is removed during development.• Long-chained molecules in resist are broken down to smaller chains by the

UV light, which are easily dissolved by the developer solution.• Capable of smaller features, better resolution, but has poor adhesion and

costs much more.


Positive vs. Negative Resist

• Negative Resist:

Resist

Photo Mask

Silicon Substrate

Oxide

Resist

Oxide

• Dark areas on the mask => photoresist is removed during development.• Short-chain molecules in the resist are “cross-linked” by the UV light. Resultant longer chains are resistant to developer solution.• Better adhesion, but incapable of producing submicron features.• Only used (anymore) for certain specialty applications.


Threshold of Photoresists


4

Si substrate

Light

PhotoresistSiO2



„From Optical Lithography to Projection Optical Lithography”

Contact Printing

Proximity Printing

Projection Printing

•simple design (M=1x, no optics)•easy to operate•limited to ~1 m features (diffraction)•some resolution loss due to gap

•simplest design (M=1x, no optics)•easy to operate but requires mask cleaning•limited to ~1m features

•complex design (M=1x, no optics)•limited by field size, optical aberrations etc.


Clean Room Environment

“first line of defence”“2. line of defence”


Transistor miniaturization

The first transistor1948

Salt-size transistors1964

Early integrated circuit1973

DRAM chip1997


Transistor miniaturization

• Moore’s Law: the number of transistors quadruples every four years

• Rock’s Law: the cost of a fab doubles every four years

• Result 1: fewer advanced fabs and fewer fab operators, fabless/foundry partnerships

• Result 2: Extend the lifetime of every generation, by means of super resolution techniques.


Rayleigh resolution limit


Rayleigh resolution limit

Lateral ResolutionCritical Feature SizeCritical Dimension

Axial ResolutionDepth of FieldDepth of Focus

NAkCD

1

22:NA

kDOF

How can we enhance the resolution?

• Reducing the wavelength ()

• Increasing the numerical aperture (NA)

• Manipulating the k1 factor


Lithography Roadmap (reduction of )


Lithography Roadmap - Why Excimers

• It is difficult to design and manufacture anachromatic projection lens for excimerlasers in deep UV due to the small differencein chromatic dispersion between fused quartzand fluorite, in addition to other materialrelated problems, such as UV-induced defectformation.

< 0.6 pm

• Beam shaping


Development of Projection Optics (increasing of NA)

Lithography represents the greatestcapital expense in an advanced fab

Strategies to stretch machinecapabilities and extend the lifetimeof every generation


Development of Projection Optics (evolution of the stepper)


Stepper


Importance of DOF

DOF is limited by

• thickness of the resist• surface smoothness (although the wafer is polished, aftert several fabrication cycles the surface has a complex topology)


Chemical-Mechanical Polishing (CMP)


Importance of DOF

The required minimum feature size reduces

from 0.24 micron to 0.04 micron

The required DOF is 0.5 micron in 2010,

/limited by the thickness of the resist/


Mask writing process(e-beam or laser)

Stepper(NA, M, , , etc.)

Wafer reflectionThin film effects

PhotochemistryDiffusion

Development

Pattern transfer

Images in lithography

Designed Pattern

Latent Image

Device Layer

Mask

Real Image

Resist Image

Aerial Image


Aerial versus Resist image simulated by Solid-C


Image formation based on Fourier Optics

)},()},({{),( 1yx ffPyxmyxE FF

m(x,y): electric field transmittance of the mask

P(fx,fy): transfer function of the optical system (different interpretations are possible)


Image formation Manhattan Geometry


Image formation

Image quality

Number of diffracted orders(Spatial Fourier components)

Conventional imaging requiresminimum 3 diffraction orders


Fourier series of a grating

f x( )1

2

2

0

k

i

1k sin 2 i 1( ) x( )

2 i 1

=

k=0

k=1

k=2

k=40

f(x)=1/2

0--2-3 2 30

1

0

1

0

1

0

1/2

mask

threshold of the photoresist


Resolution Enhancement Techniques

Light Source

Filter

Condenser Lens

Mask

Projection Lens System

Wafer withPhotoresist

Mirror

Pupil-PlaneFilter

* Bandwidth

* Filter (annular, quadrupole, etc.)

* Spatial coherence ()

* Binary or PS mask (Manhattan geometry)* Magnification (M)* Numerical Aperture (NA)

* Defocus * Pupil-plane filter/Aberrations

* Image model

* Wavelength ()

* Image flare

* Resist system* Thickness* Absorp. Param. A, B* Rate Constant C* Refractive Index* Development Model* etc.

Input Parameters

* Exposure


Resolution Enhancement Techniques - OPC

The light intensity profile is not significantly different for the isolated1.0 micron line as for the 1.0 micronline/1 micron spacing grating.

Significantly different light intensityprofiles depending upon whether thelines are isolated or part of a gratingstructure

Isolated and grouped lines are expectedto print approximately the same with.

Linewidth variation

The size of a line is dependent uponits proximity to other geometries



Original Layout0.18 m

Conventional (no OPC)

OPC Layout

Silicon Image w/o OPC

Silicon Image with OPC



Solution:

OPC (Optical proximity correction):is the technique of predistorting mask patterns such that the printed patterns are as close to the desiredshape as possible.

Problems:

•Proximity effects•Nonlinearity•Line shortening•Corner rounding



OPC processes1. Catastrophic OPC: The goal is to guarantee presence of a pattern without feature-size control worries.Precise dimension control is not critical. An example is to ensure a wire is continuous and that no open circuit will result.

2. One dimensional OPC (Linewidth variation minimization):Vary the size of each feature on the photomask depending on its nominal dimension and environment /For example, dense and sparse lines are made smaller on the mask whereas lines of intermediateperiodicities are made larger/

3. Line shortening method:

4. Corner rounding:

biasing if there is room



Limited by the pixel size of the mask writing process!!!

Uncorrected Test PatternVertex Count=94

Super Aggressive CorrectionVertex Count=1106


Resolution Enhancement Techniques/Off-Axis Illumination/

0. +1. +2.-1.-2.

Photo-Mask

Projection Lens

Photo-Resist

Intensity in the Pupil-plane

+1.

-1.

0.

ConventionalIllumination

Off-AxisIllumination

TWO BEAM IMAGING!

NApitchCD

2

1

2

1

pitchNA

maxsin

%1.974

421

20

10

aa

aaC

NACD

4

1

pitchNA

2

%6.902

21

20

10

aa

aaC


Resolution Enhancement TechniquesPhase Shifting Techniques



Phase-shifting masks are classified as “strong” and “weak”, according to theirability to suppress the zero order diffraction component.



Phase Shifting LayerChrome

Alt

erna

ting

/Lev

enso

n P

SMA

tten

uate

d P

SM

Phase Shifting Layer (T0.04-0.15)

Phase Shifting Layer Dark Field Grating(PS Layer)

Chr

omel

ess

PSM

Phase Shifting LayerChrome

Out

rigg

er P

SMR

im P

SM

Phase Shifting Layer

Chrome

Inte

rfer

omet

ric

PSM

?

Phase Shifting Method without Phase Shifting Layer.

a.

b.

c.

d.

e.

f.



tE0tE1

rE1rE0

BackIllumination

FrontIllumination

2·

Fused Silica

Substrate

Reflective ChromeLayer

x

z

Four-beam imaging



CCD

L3 L1L2

Mask

Attenuator

PiezoTranslator

M2

Optical Axis

M1

Beam Splitter

Ar+-ion laser(=457.9) nm

Image Plane

1°0.7°



1

00 3.5

Inte

nsi

ty

X(m)

1

0

Inte

nsi

ty

0 3.5X(m)1

0

Inte

nsi

ty

1

0In

ten

sity

0 3.5X(m)0 3.5X(m)1

0

Inte

nsi

ty

1

0

Inte

nsi

ty

0 3.5X(m)0 3.5X(m)1

0

Inte

nsi

ty

1

0

Inte

nsi

ty

0 3.5X(m) 0 3.5X(m)

Calculated Measured CCD Image

a

b

c

d

e

f

g

h

R im

age

T im

age

Cor

rect

Ph

ase

Wro

ng

Ph

ase


What’s next for Optical Lithography


Lithography Roadmap

Development of Projection Optics (increasing of NA)

Catadioptric and immersion objectives

NANO Fórum 2006 December 13.

Principles of Optical Lithography, 2002 November 20. Principles of Optical Lithography Layout:...

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