Reading Assignments: Plummer, Chap 5.1~5.4,...

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Lithography

Reading Assignments:

Plummer, Chap 5.1~5.4, 5.6

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Lithography Is the Designer’s “Brush”

Lithography is indispensible for defining locations and configurations of circuit elements/functions.

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ITRS 2007

The major challenge in litho: CD, CD control, Overlay, Defect Control, Cost.

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FSG

Metal 4 Copper

Passivation 1, USGPassivation 2, nitride

Lead-tin alloy bump

FSG

CopperMetal 2

FSG

FSG

CopperMetal 3

FSG

P-epiP-wafer

N-wellP-welln+STI p+ p+USGn+

PSG TungstenFSG

Cu Cu

Tantalum barrier layer

Nitride etch stop layer

Nitride seal layer

M 1

Tungsten local Interconnection

Tungsten plug

PMD nitride barrier layer

T/TiN barrier & adhesion layer

Tantalum barrier layer

Critical Layers

At least the following 4

• Active

• Gate

• Contact

• M1

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Photo Lithography

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Photo Lithography

Photolithography is the patterning process that transfers the designed patterns from the mask to the resist on the wafer surface. It is the core of the manufacturing process flowProcess Sequence: photoresist coating, alignment and exposure, photoresist developingRequirements• High resolution• High sensitive• Precise alignment• Low defect density

High yield and good imaging

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Photolithography Process

Substrate

Photoresist

Light Source

Mask Positive Resist

Alignment & ExposureResist Coating Developing

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“Positive” and “Negative”Resist Processes

Mask/reticle

Exposure

UV light

Substrate

NegativePhotoresist

Substrate

PositivePhotoresist

Substrate

Photoresist

SubstratePhotoresist

After Development

Dr. Ko, Institute of Nanotechnology, NCTU

Positive PR: from insoluble to soluble Negative PR: from soluble to insoluble

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PhotoresistPhotoresists are the photosensitive material coated temporarily on the wafer surface and used to transfer the image of designed circuit on the mask to the wafer surface The photosensitive chemical reaction (break bonds) mainly sensitive to the UV light and not sensitive to the visible light.Since resist is not sensitive to yellow light, all the fabs use yellow to illuminate lithography areas

Ingredients: polymer, solvent, sensitizer, additives

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Photoresist Material ParameterPrimary two functions of resist• Precise pattern formation• Protection of the substrate during etch

Parameters can be categorized as follows:• Optical properties: resolution, photosensitivity, and

index of refraction• Mechanical/chemical properties: solid content,

viscosity, adhesion, etch resistance, thermal stability, and sensitivity to ambient

• Processing and safety related properties: particle count, metals content, process latitude, shelf life, flash point, and threshold limit value

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Ingredients in I-line Resist• Polymer: Novolak

(Etch mask)

• Photoactive compound (PAC, or called sensitizer): diazonaphthoquinone (DNQ)

(Control photochemical reaction during exposure)

• Additive: phenolic materials

(Modify photochemical reaction during exposure)

• Solvent: PGMEA, EL

(Liquid suspension)H3C

O

CH2 C O

H

CH3

C

O

CH3

H3C C

H

OH

C

O

O C2H5Source:Prof. Ko in NCTU

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Chemical Reaction in I-line Resist

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Photo-ResistPositive PR• Becomes soluble after exposure• When developed, the exposed

parts dissolved• Better resolution

Negative PR• Exposed PR becomes cross-

linked polymer• Cross-linked polymer has higher

chemical etch resistance.• Becomes insoluble after

exposure• When developed, the unexposed

parts dissolved.• Cheaper material• Swelling lead to poor resolution

1

1

ln : ratioContrast −

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛≡

EETγ

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Positive and Negative Resists

Azide/isoprene negative resist

Novolak resin resist

Swells during develop No swelling during develop

Marginal step coverage Good step coverage

Organic solvent developer Aqueous developer

Toxic strippers Environmentally benign resist stripper

Sensitive to ambient oxygen Operate well in air

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DUV Resists• g-line and i-line resists have maximum quantum efficiencies < 1 and are

typically ≈ 0.3. • Chemical amplification can improve this substantially.• DUV resists all use this principle. A catalyst is used. • Photo-acid generator (PAG) is converted to an acid by photon exposure. Later,

in a post exposure bake, the acid molecule reacts with a “blocking” molecule on a polymer chain, making it soluble in developer AND REGENERATING THE ACID MOLECULE.

∴ catalytic action ∴ sensitivity is enhanced.

PAG

INSOL INSOL

Exposure

INSOL INSOL

Acid

Post ExposureBake (PEB)

SOL INSOL

Acid

SOL SOL

AcidAcid

a) b)

c)d)

Polymer Chain Polymer Chain

Polymer Chain Polymer Chain

Acid

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MaskPhotoresist

Si Wafer

LightSource

OpticalSystem

Contact Printing Proximity Printing Projection Printing

Gap

1:1 Exposure Systems Usually 4X or 5XReduction

Wafer Exposure System

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Projection Printing

Project an image of the mask pattern onto a resist coated wafer several centimeters away from the mask.• 1:1 projection optical system is easier to

design.• M:1 projection mask is easier to fabricate

(5:1 most common or 10:1).

Projection method• Scan• Step-and-repeat• Step-and-scan

Light Source

Lens

Mask

Photoresist

Wafer

Synchronized mask and wafer movement

Slit

Lens

Wafer Stage

ProjectionLens

LightSource

Reticle

Wafer

ProjectionLens

Scan

Step and repeat

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Lens Field Size Overlay ThroughputNA Resolution Max X & Y 2 pt. Global

Alignment200 mm Wafers

70 Exp., 200 mJ/cm2

Variable0.48 to 0.60

< 280 nm 22 X 27.4 mm < 40 nm > 100 wph

I-Line Stepper

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Diffraction

Light in free space Light through a small aperture

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Fresnel Diffraction

WaferResist MaskAperture

Incident Plane Wave

Light Intensityat Resist Surface

g

W

λ < g < W2

λFresnel diffraction (near-field diffraction) applies when

Within this range, the minimum resolvable feature size is Wmin ≈ λg

Thus if g = 10 µm and an i-line (365nm) light source is used, Wmin ≈ ??

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Shadowing Printing

Contact printing• Resolution ~ 1um• Dust on mask will damage PR pattern.• Mask pattern may be contaminated.

Proximity printing• A small gap of 10-50 um.• Longer mask lifetime.• Poorer resolution.

gapR ×= λ

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Fraunhofer DiffractionImagePlane

PointSources d

AB

A'

B'

α

EntranceAperture

Fraunhofer diffraction (far-field diffraction) is dominate in the projection system. According to the Rayleigh Criterion:

R

1.22 1.22 0.61 (2 sin ) sin

f fRd n f nλ λ λ

α α= = =

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Resolution1

KRN Aλ

= λ: wavelength, K1 : system constant=0.61 (ideal)NA = n sinα= d/2f

NA (numerical aperture): the capability of the lens to collect the diffraction light. NA is proportional to the lens diameter (camera with large lens) and reversely proportional to the distance between the wafer and the lens.

Diffracted light collected by the lens

Strayed refracted light

Lens

Ideal light Intensity pattern

Less diffraction after focused by the lens

Mask

ro

D

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Resolution Comparison

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* Source: Renesas

Immersion Lithography

NA of 1.35 – 1.44

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22( )

KDOFNA

λ= NA : numerical aperture, λ : wavelength, K2 : system constant =0.5 (ideal)

DOF

Depth of Focus

Lens

Ex: Point and shot cameras use small lens without focus but the resolution won’t be great!

Very flat wafer surface is needed -> CMP

PhotoresistSubstrate

Depth of focusCenter of focus

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Modulation Transfer Function (MTF)

MTF = IMAX − IMIN

IMAX + IMIN

Intensityat Mask

Intensityon Wafer

0

1

0

1IMAX

IMIN

Aperture

LightSource

Condenser Lens

Objective orProjection Lens

Photoresist on Wafer

Mask

Position Position

The MTF is a measure of the contrast in the aerial image produced by the exposure system. Usually MTF > 0.5 is necessary fir the resist to properly resolve the feature.

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Exposure Wavelength

G-line(436)

H-line(405)

I-line(365)

300 400 500 600Wavelength (nm)

Inte

nsity

(a.u

)

Deep UV(<260)

• High pressure Hg or Hg/Se lamp

• Lamp intensity is too low at λ < 260 nm

R=k1λ/NA

• Rayleigh Equation

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Exposure Wavelength

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• Photon beam: 365nm (I-line/UV), 248nm (KrF/DUV), 193nm (ArF/DUV), 157nm (F2/VUV), 13.5nm (EUV)

Name Wavelength (nm) Application feature Size (nm)

G-line 436 500

Mercury Lamp H-line 405

I-line 365 350 to 250

XeF 351

XeCl 308

Excimer Laser KrF 248 250 to 110

ArF 193 110 to 65

Fluorine Laser F2 157 65 to 40

Photolithography Light Sources

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Pre-baking• 90-120°C for 60-120 sec.

Adhesion promoter vapor deposition or spin coatingPR spin coater• 3000-6000 RPM for 1um• with edge bead remove

Soft baking• 90-120°C for 60-120 sec.

ExposurePost exposure baking (PEB)• ~100°C for 10 min.

PR DevelopingHard baking• 100-180°C for 10-30 min.

Photolithography Process

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Wafer Clean

P-Well

USGSTIPolysilicon

Gate Oxide

Wafer Clean

• Removing all contaminants from the surface

• Standard cleaning process can remove particle, organic and metal

• The native oxide can be removed by dipping in diluted HF

• Improved adhesion


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• Promotes adhesion of PR to wafer surface• Widely used: Hexamethyldisilazane

(HMDS) • HMDS vapor coating prior to PR spin

coating• Usually performed in-situ with pre-bake• Chill plate to cool down wafer before PR

coating

Pre-bake and Primer Vapor

P-Well

USGSTIPolysilicon

Primer

Substrate Preparation

• Dehydration bake• Remove moisture from wafer

surface • Promote adhesion between PR

and surface• Usually around 100 ˚C• Integration with primer coating


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Primer Vapor CoatingDehydration Bake

Wafer

Prep Chamber Primer Layer

Wafer

Hot Plate Hot Plate

HMDS Vapor

Si Si晶圓表面

Si Si親水性表面

OH OH

H 3CH 3C–Si–N–Si–CH 3

H 3C CH 3

CH 3

H

Si Si疏水性表面

O OSiSi

H 3CH 3C CH 3 CH 3

CH 3H 3C+ H 2O + NH 3

Substrate Preparation


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Resist Coating

P-Well

USGSTIPolysilicon

Photoresist

Primer

• Wafer sits on a vacuum chuck• Liquid photoresist applies at center of wafer• Rotate at high speed• Photoresist spreads by centrifugal force• Evenly coat on wafer surface


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Resist Spin Coater

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•　Photoresist spread on spinning wafer surface

•　Wafer held on a vacuum chuck•　Slow spin ~ 500 rpm•　Ramp up to ~ 3000 - 7000 rpm

Dynamic Spin RateResist Thickness

Viscosity Effect

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Resist Spin Coater

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Soft Baking

• Hot plate heating is widely used• Evaporating most of solvents from PR film (5~20 % left)• Solvents help to make a thin PR but absorb radiation and affect

adhesion• Soft baking time and temperature are determined by the matrix

evaluations• Over bake: polymerized, less photo-sensitivity• Under bake: affect adhesion and exposure (solvent is not sensitive)


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Alignment & Exposure

P-Well

USGSTIPolysilicon

Photoresist

Gate Mask Gate Mask

P-Well

USGSTIPolysilicon

Photoresist

Alignment and exposure

• Alignment mark • Most critical process for IC fabrication• Most expensive tool (stepper or scanner) in an

IC fab• Most challenging technology• Determines the minimum feature size• Currently 65 nm and pushing to 45 nm


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Structure of Mask

quartz

Cr2O3

Cr

Cr2O3, 30nm

Cr, 70nm

quartz, 2.3mm

Electron beam lithography

Which illumination mode is correct ?


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MasksBright field Dark field

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Post Exposure Baking

P-Well

USGSTIPolysilicon

Photoresist

• Photoresist has glass transition temperature (Tg)• Baking temperature higher than Tg

• Thermal movement of photoresist molecules• Rearrangement of the overexposed and underexposed

PR molecules• Average out standing wave effect• Smooth PR sidewall and improve resolution

Photoresist

λ/2nPR

Substrate

OverexposureUnderexposure

Photoresist

λ/2nPR

Substrate

OverexposureUnderexposure


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Development

P-Well

USGSTIPolysilicon

PR

• International standard of 2.38% TMAH (0.26 N) is used to reveal pattern

• Tetramethylammonium hydroxide, (CH3)4NOH• Developer solvent dissolves the softened part of photoresist• Transferring the pattern from mask or reticle to photoresist• Three basic steps:

- Development- Rinse- Dry Dr. Ko, Institute of Nanotechnology, NCTU

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Development

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Hard Baking

P-Well

USGSTIPolysilicon

PR

Evaporating all solvents in PRImproving etch and implantation resistanceImprove PR adhesion with surfacePolymerize and stabilize photoresistPR flow to fill pinholeUnder-bake • Photoresist is not fully

polymerized• High photoresist etch rate • Poor adhesion

Over-baking • PR flow and bad resolution


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Hard Baking

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P-Well

USGSTIPolysilicon

PR

Pattern Inspection

• Inspection: go next step or rework (stripping PR)– Photoresist pattern is temporary – Etch or ion implantation pattern is permanent

• Photolithography process can rework • Can’t rework after etch or implantation • Scanning electron microscope (SEM) • Optical microscope (OM)


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Pattern Inspection

Overlay or alignment• run-out, run-in, reticle rotation, wafer rotation,

misplacement in X-direction, and misplacement in Y-direction

Critical dimension (CD)Surface irregularities such as scratches, pinholes, stains, contamination, etc.

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Critical Dimension (CD) Measurement

Off-line SEM cross-sectional inspection

In-line SEM plane-view inspection

CD Atomic Force Microscope

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Critical Dimension (CD)

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Misalignment

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Photolithography Apparatus

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Process Flow Chart

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Optical Proximity Effect (OPE)What is Optical Proximity Effect ?

• Feature distortion (edges of printed or etched features do not conform to those of the designed patterns) incurred in the pattern transfer process.

• Manifest itself as line-width bias, line-end shortening, corner rounding, etc.• Mainly caused by loss of high spatial frequency components in imaging. Non-uniformity in the etch process also

contribute to OPE.

Line-endShortening

PitchEffect

CornerRounding

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Optical Proximity Correction (OPC)How to Minimize OPE ?

• Pre-distort the patterns on the mask to minimize OPE.

On wafer

On mask

Before OPC After OPCOptical effect

After OPCInclude PR process

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Example of OPC

Before OPC

After OPC

On Focus Defocus

SEM picturesOn mask

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Phase Shift Mask

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Advanced Lithography

E-beam lithography

X-ray lithography

Extreme UV (EUV) lithography

Nanoimprint

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E-Beam Lithography

W or LaB6 thermionic-emission tip

Focus beam diameter to 10-25 nm

Turn on/off beam

Computer controlled to direct the beam. Scan field ~ 1cm

Advantages : High resolution~30nm, Large DOF~20-30um, Maskless

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Evolution of e-Beam System

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Photolithography vs Electron Beam電子束照射

第一束電子

第二束電子

第n束電子

光束照射

光罩

一次曝光

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SCALPEL

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X-Ray Lithography (XRL)

Tool feature• Energy ~ 1-10 KeV• Wave length ~ 1nm• 1:1 proximity exposure

Advantages• High resolution• No reflection

Disadvantages• X-ray source (Synchrotron source)• Mask fabrication

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XRL MaskXRL mask

• No materials transmit X-rays easily.• The clear areas need to be made from thin layers of low mass materials such as Si,

Si3N4, SiC or BN of a few um thick.• The dark areas are made from heavy atoms such as W, Ta, or Au of about 0.5um

thick.• Mask absorbs 25%-35% of the incident flux and must be cooled.

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EUV System

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Light Source

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Nano Imprint Lithography

Reading Assignments: Plummer, Chap 5.1~5.4,...

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