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Lithography

Prof. Yosi Shacham-Diamand

Fall 2004

33rdrd lecture: introductionlecture: introduction

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List of contentFundamental principlesCharacteristics parametersExposure systems

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Fundamental principles

Aerial ImageExposure DevelopmentCritical dimension (CD) control

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Lithography characterization

Post exposure characterizationLine-width control, photo-resist profile, resolution, process window

RegistrationProcess compatibility

Etch resistance, thermal stability, adhesion, chemical compatibility, resist removal

ManufacturabilityCost, safety, defects, stability, shelf life

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Lithography: information transfer process

Mask

Optical image

Latent image in the photo-lithographic material

Image development

Transfer onto the wafer

Design

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Image transfer from the mask to the wafer

Single field exposure, includes: focus, align, expose, step, and repeat process

UV light source

Reticle (may contain one or more die in the reticle field)

Shutter

Wafer stage controls position of wafer in X, Y, Z, θ

Projection lens (reduces the size of reticle field for presentation to the wafer surface)

Shutter is closed during focus and alignment and removed during wafer exposure

Alignment laser

Figure 14.1

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Typical CMOS masks

4) Poly gate etch1) STI etch 2) P-well implant 3) N-well implant

8) Metal etch5) N+ S/D implant 6) P+ S/D implant 7) Oxide contact etch

Top view

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23

45

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8 Cross section

Resulting layers

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Exposure systemsMask illumination from the backsideThe light interacts with the maskThe electromagnetic wave reaches the lensThe lens collects part of the light and forms and image on the waferThere is an information loss due to the finite size of the lens

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Light – an electromagnetic wave phenomena

λ

λ =vf

Laser

v = velocity of light, 3 x108 m/secf = frequency in Hertz (cycles per second)λ = wavelength, the physical length of one

cycle of a frequency, expressed in meters

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Diffraction

A

B

A+B

Waves in phase Waves out of phase

Constructive

Destructive

DestructiveDestructive

Waves out of phase

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Optical filtering

Secondary reflections (interference)

Coating 1 (non-reflecting)

Coating 3

Glass

Coating 2

Reflected wavelengths

Transmitted wavelength

Broadband light

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Spectrum UVλ (nm)

7004 550 600 65050045040035030025020015010050

Ultraviolet spectrum Visible spectrum

Mercury lampExcimer laser

Photolithography light sources

ghi365 40524819313 436157126

Violet RedBlue Green YellowOrangeMid-UVEUV DUVVUV

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High pressure Hg lamp120

100

80

60

40

20

0

200 300 400 500 600

Wavelength (nm)

Rel

ativ

e In

tens

ity (%

)

h-line405 nm

g-line436 nm

i-line365 nm

DUV248 nm

Emission spectrum of high-intensity mercury lamp

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UV Light Wavelength (nm) Descriptor CD Resolution (µm)

436 g-line 0.5405 h-line 0.4365 i-line 0.35248 Deep UV

(DUV)0.25

High pressure Hg lamp

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100

80

60

40

20

0

Rel

ativ

e In

tens

ity (%

)

KrF laser

280210 240 260220

Wavelength (nm)

Hg lamp

Hg lamp vs. Excimer laser

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Resist UV absorptionPhotoresist (after develop)

Substrate

Sloping profile

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Eximer lasers for lithography

Material Wavelength (nm)

Max.Output

(mJ/pulse)

Frequency(pulses/sec)

PulseLength (ns)

CD Resolution(µm)

KrF 248 300 – 1500 500 25 ≤ 0.25

ArF 193 175 – 300 400 15 ≤ 0.18

F2 157 6 10 20 ≤ 0.15

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Spatial CoherenceIncoherent light sourceof a single wavelength

Slit

Coherent cylindrical wave front

Two coherent cylindrical wave fronts

Interference patterns

Two slitsclosely spaced

Black box illuminator

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Optical lithography

•Optics

Reflection

Refraction

Lens

Diffraction

Typical parameters

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Reflection laws

θi θrIncident light Reflected light

Law of Reflection: θi= θr

The angle of incidence of a light wavefront with a plane mirror is equal to the angle of reflection.

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Reflection lithography

Mask

Flat mirror

Ellipsoidal mirror

Flat mirror

Illuminator for a simple aligner

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RefractionSnell’s Law: sin θi = n sin θr

Index of refraction, n = sin θi / sin θr

θθ

air (n ≅ 1.0)

glass (n = 1.5)

fast medium

slow medium

θθ

air (n ≅ 1.0)

glass (n = 1.5)

fast medium

slow medium

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Refraction index

Material Index of Refraction (n)

Air 1.000293

Water 1.33

Fused Silica (AmorphousQuartz)

1.458

Diamond 2.419

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Lens based lithography

Mercury lamp

Lamp position knob

Lamp monitor

Ellipsoidal mirror

Shutter

Fly’s eye lens

Flat mirror

Masking unitMirror

MirrorCollimator lens

Condenser lensCondenser lens

Optical filter

Fiber optics

Reticle

Reticle stage (X, Y, θ)

Projection optics

Optical focus sensor

Interferometer mirrorX-drive motor

Y-drive motor

θ-Z drive stage

Vacuum chuck

Wafer stage assembly

Light sensorIll

umin

ator a

ssem

bly

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Converging lens

OF F´ S´S

f

2f f = focal lengthF = focal pointS = 2fO = origin, center of lens

Real image

Object

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Diverging lens

OF F´ S´S

Virtual image

Object

f = focal lengthF = focal pointS = 2fO = origin, center of lens

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Laser-Induced Lens Compaction

Compacted area of lens

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Interference Pattern from Light Diffraction at Small Opening

Light travels in straight lines.Diffraction occurs when light hits edges of objects.Diffraction bands, or interference patterns, occur when light waves pass through narrow slits.

Diffraction bands

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Diffraction in a Reticle Pattern

Slit

Diffracted light rays

Plane light wave

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Bragg’s condition for constructive diffraction

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Lens Capturing Diffracted Light

UV

0

12

3

4

12

3

4

Lens

Quartz

Chrome Diffraction patterns

Mask

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Effect of Numerical Aperture on Imaging

Exposure light

Lens NA

Pinhole masks

Image results

Diffracted light

Good

Bad

Poor

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Image formation - example

θn

λn=)sin(θp n

Bragg’s law

Quartz plate with chromium pattern with a periodicity (pitch) of p.

Coherent illumination

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The lens

Focal plane

Lens with diameter D & distance f from the focal plane

Mask

fDnNA

2)sin( ≈= α

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Typical NA Values for Photolithography Tools

Type of Equipment NA Value

Scanning Projection Aligner with mirrors(1970s technology) 0.25

Step-and-Repeat 0.60 – 0.68

Step-and-Scan 0.60 – 0.68

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What determines the resolution ?

The resolution is proportional to the wavelength, λThe resolution is proportional to the numerical aperture, NA; larger lens collects more information.

NAksolution λ

1Re =

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Definition of line-width (L) and line-spacing (S)

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Image contrast

0

1Intensity, IImax

Imin

minmax

minmaxIII-Icontrast Image

+=

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The limitations of the optical contrast as a figure of merit

It represents only an image of a simple pattern with lines and spacesNot practical for large or complex featuresToo sensitive to the minimum intensity than the real imageLow correlation to the lithography quality.

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Photoresist Reflective Notching Due to Light Reflections

Exposed photoresist

Surfacereflection

Polysilicon

SubstrateSTISTI

UV exposure light

Mask

Unexposed photoresist

Notched photoresist

Edgediffraction

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Incident and Reflected Light Wave Interference in Photoresist

Standing waves cause non-uniform exposure along the

thickness of the photoresist film.

Incident waveReflected wave

PhotoresistFilm

Substrate

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Standing waves in the resist

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Solution – Anti Reflective Coating (ARC)

The use of antireflective coatings, dyes, and filters

can help prevent interference.

Incident wave Antireflective coating

Photoresist Film

Substrate

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BARC - Bottom Antireflective Coating

BARCPolysilicon

Substrate

STISTI

UV exposure light

Mask

Exposed photoresist

Unexposed photoresist

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BARC Phase-Shift Cancellation of Light

(A) Incident light

Photoresist

BARC (TiN)Aluminum

C and D cancel due to phase difference

(B) Top surface reflection

(C)(D)

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Top Antireflective Coating

Incident light

Photoresist

Resist-substrate reflections

Substrate

Incident light

Photoresist

Substrate reflection

Substrate

Top antireflective coating absorbs substrate reflections.

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Optical Lithography

ResolutionCalculating ResolutionDepth of FocusResolution Versus Depth of Focus

Surface Planarity

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Resolution of Features

2.0

1.0

0.5

0.10.25

The dimensions of linewidths and spaces must be equal. As feature sizes decrease, it is more difficult to separate features from each other.

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Calculating Resolution for a given λ, NA and k

Lens, NA

Wafer

Mask

Illuminator, λ

R

k = 0.6

λ ΝΑ R365 nm 0.45 486 nm365 nm 0.60 365 nm193 nm 0.45 257 nm193 nm 0.60 193 nm

i-line

DUV

k λNAR =

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Depth of Focus (DOF)

+

-

Photoresist

Film

Depth of focusCenter of focusCenter of focus

Lens

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Example to focal plane problems due to a non-planar surface

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Resolution Versus Depth ofFocus for Varying NA

λ 2(NA)2DOF =

Photoresist

Film

Depth of focusDepth of focusCenter of focus

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--Lens, NA

Wafer

Mask

Illuminator, λ

DOF

λ ΝΑ R DOF365 nm 0.45 486 nm 901 nm365 nm 0.60 365 nm 507 nm193 nm 0.45 257 nm 476 nm193 nm 0.60 193 nm 268 nm

i-line

DUV

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New approaches – super resolution imaging