Lithography 1

MP4004 Advanced Manufacturing and

Nanotechnology

A/P Yeo Swee Hock

Content

1. Lithography; Etching; Additive TechniquesS.

H.

r Techniques2. Micromachining3. Advanced Metrology

4. Nanotechnology

Yeo

S24

hr

ture

r

5. Nanometrology

Co-le

c15

hr

2The link: EdveNTUre for all course content, announcement and CA

Reference Books Madou M.J., Fundamentals of microfabrication:

the science of miniaturization, CRC Press, 2002M G h J Mi hi i f E i i McGeough J., Micromachining of Engineering Materials, Marcel Dekker, 2002

Dotson C.L., Fundamentals of Dimensional Metrology, Thomson Delmar Learning, 2006

Poole C. P. and Owens F. J., Introduction to N t h l WILEY I t i 2003Nanotechnology, WILEY-Interscience, 2003.

Wolf E. L., Nanophysics and Nanotechnology, WILEY-VCH Verlag GmbH, 2004.

3

MP4004 LT 2A (Full-time)WEEK MON 1.30 MON 2.30 WED 12.30

1 (8/8/11) Yeo SH Yeo SH Yeo SH2 Yeo SH Yeo SH Yeo SH3 Yeo SH Yeo SH Yeo SH4 Yeo SH Yeo SH Yeo SH4 Yeo SH Yeo SH Yeo SH5 Yeo SH Yeo SH Yeo SH6 Yeo SH Yeo SH Yeo SH7 Yeo SH Yeo SH Yeo SH8 R e c e s s9 YeoSH YeoSH YeoSH10 Liu EJ Liu EJ Liu EJ11 Liu EJ Liu EJ Liu EJ

4

12 Liu EJ Liu EJ Liu EJ13 Liu EJ Liu EJ Liu EJ14 Liu EJ Liu EJ Liu EJ

Lithography 2

Continuous Assessment 1 Using on-line access (thru Edventure) Scope: Front end silicon processing (litho,

& )etch & additive)

When? Week 5: Wed 23 Sep 11 (TBC) Time? 12.30pm (TBC) Location? (TBC)

5

In Module 1:

LithographyEtching: subtractiveEtching: subtractive

techniquesAdditive techniques

6

Wafer Fabrication High capital investment 300 mm wafers; 90 nm

technologytechnologyWafer cycle time: 30 – 80

days A few hundreds of

processes; process sequencessequences

Full material automation Cleanroom: Class 10 or

1007

Front End Silicon Processing

A multi-trillion US$ electronics industry with wide ranging applications (aerospace, automotive, consumer electronics, etc)

Basic knowledge of semiconductor materials, devices, and processes is essential

Here, we will deal with the basic processes involved in IC fabrication.

B k E d

8

Lithography 3

“Chef at work”

Processing Concept: involve many stepsDeposition – Oxidation,

PVD, CVDEtchEtch

LithographyChemicals

Lithography

Doping

CMP9

Transistor in 3D……..0.13m

0.15 to 10m

+ + + +

1.2 V

+ + + + 0 V1.2 V

STI

P well

Source Drain

P- well

MOS (metal-oxide semiconductor) transistor 10

Outline: LithographyUnit 1: Photolithography ProcessUnit 2: ResistsUnit 2: ResistsUnit 3: Photolithography Systems

Reading: Chapter 1 of Madou 11

What is Photolithography? litho-graphy: latin word for “stone-

writing” A key semiconductor manufacturing

light source A key semiconductor manufacturing

function condenser lens

Reticle / mask

Projection lens

entrance pupil

A process to transfer the circuitry pattern from a mask (a quartz plate, master copy) to a coated wafer.

Exit pupil

wafer

Mask patterns of windows that are first transferred to a light-sensitive material called photoresist

[Wafer is a silicon material on which chips are made.] 12

Lithography 4

Photolithography Overview1. Masks2. Wafer Priming3. Spinning Resist and Soft Baking4. Exposure and Postexposure Treatment5. Development 6. Postbaking7. Resist Stripping7. Resist Stripping 8. Wafer Cleaning and Contaminants: The

Clean Room9. Resists

13

Photolithography Process Ultilized mask, resist and UV light exposure Basic process steps from A to F Note: not all below is related to photolithography Note: not all below is related to photolithography

process

A Oxidation

B Coat with Photoresist

SiO2 (~ 1μm)n-type Silicon

Negative photoresist coat (± 1μm)

SiO2Si

Unexposed PhotoresistRemoved by Developer

D

UV light

C Exposure by shining light through mask

Glass PlateOpaque PatternHardened Resist

B Coat with Photoresist

SiO2 Etched with NH4F + HF

E

Exposed Photoresistremoved with H2SO4

F14

Masks A stencil used to

repeatedly form a pattern on resist-coated

UV lightGlass MaskOpaque PatternHardened Resist

pattern on resist coated surface – called mask

Quartz – transparent to UV radiation

Chromium – opaque to UV radiation

C Exposure

UV radiation Complete IC needs

more than 20 masks, cost about US$800,000

15

Wafer Priming

Low adhesion for high humidityWafer may be vapor primed with reactiveWafer may be vapor primed with reactive

silicone primers before resist coating Adhesion promoter: hexamethyldisilazane

(HMDS), for oxide and resist interface HMDS: dipping or exposing the wafer to

HMDS vapor

16

Lithography 5

Spin Coating (of resist) Resist is dispensed from a

viscous polymer solution to the center of wafer.

W f t hi h d

photoresist dispenser

Wafer spun at high speed, between 1500 and 8000 rpm to achieve uniform film thickness.

Film thickness,

T = 1 to 2 m

α

γβ

ωηKCT

vacuum chuckspindle

to vacuum pump T 1 to 2 m

Uniformity of film thickness, 5 nm to ensure reproducible line widths and development times in subsequent steps

(rpm)min per rotations y viscositintrinsic

solution mL g/100in ion concentratpolymer Cconstantn calibratio overall

K

pump

17

Soft Baking

To remove solvents and stress built-up after spin coating

Soft bake promote adhesion of resist to the substrate; uniformity; etch resistance; linewidth control; light adsorbent characteristicscharacteristics

Typical 75 to 100C for 10 min

18

Exposure

Alignment of features on the mask

Exposure using near ultra UV light Exposure using near ultra-

violet (UV) or deep ultra-violet (DUV) radiation

Near UV – 350 to 500 nm wavelength; e.g. “g-line” or 436 nm, and “i-line” or 365 nm

Opaque PatternHardened Resist

C Exposure

Glass Mask

Printing of the masknm DUV – 150 to 300 nm

wavelength

Printing of the mask pattern onto the resist to form the latent image

19

Development Transform latent resist image into relief image;

Soluble areas of photoresist are dissolved by developerdeveloper

Wet & Spray DevelopmentPositive resists use aqueous alkaline

solutionsNegative resists use organic solutions

D d l t Dry developmentPlasma-based: Oxygen- Reactive Ion Etching (O2-RIE) with differential etch rate rather than differential solubility to a solvent (i.e. wet)

20

Lithography 6

Postbaking

Also called hard baking Postbaking – Before etching the Postbaking Before etching the

substrate or adding a material, this is to remove residual solvents, promote interfacial adhesion of the resist, increase hardness of the film; 120C for 20 min is t picalfor 20 min is typical

21

At which step does Postbaking being deployed?

Deployed after

DA Oxidation

SiO2 (~ 1μm)n-type Silicon

step D

UV lightGlass PlateOpaque PatternHardened Resist

A Oxidation

B Coat with Photoresist

Negative photoresist coat (± 1μm)

SiO2Si

Unexposed Photoresist Removed by Developer

D

SiO2 Etched with NH4F + HF

E

C Exposure by shining light through maskExposed Photoresist removed with H2SO4

F

22

Resist Stripping To remove the resist after etching, without

damaging the device under construction. Wet Stripping (Removal by causing resist to Wet Stripping (Removal by causing resist to

swell and lose adhesion to substrate)Strong acid e.g. H2SO4 or acid-oxidant

combination e.g. H2SO4-Cr2O3Commercial strippers are RCA Clean

(Radio Corp of America) and Piranha with(Radio Corp of America) and Piranha with mixture of H2SO4:H2O2 = 5:1

Dry stripping – oxidizing (burning) it an oxygen plasma system, a.k.a. resist ashing

23

Contaminants include Solvent stains (methyl alcohol acetone etc)

Wafer Cleaning

Solvent stains (methyl alcohol, acetone, etc)Dust from operators, equipment, etc

Significance of particle sizeFeature size of a 4 MB DRAM is 0.5 mCan only tolerate 0.05 m particle sizeCan only tolerate 0.05 m particle size

Methods: wet and dry

26

Lithography 7

Wafer CleaningWet CleaningE.g. RCA Clean: The most common clean

solution for ICsolution for ICSC-1 (Standard Clean): NH4OH : H2O2 : H2O

= 1:1:5 to 1:2:7 at 70-80Co To remove organic dirt

QDR (quick dry rinse) in DI water SC-2: HCL : H2O2 : H2O = 1:1:6 to 1:2:8 at

C70-80Co To remove metal ions

1x or 2x QDR Spin or IPA Dry (IPA = isopropyl-alcohol)

27

Clean Room Enclosed area:

airborne particulates, temperature,

Class 1 10 100 1000 10000No of particles 1 10 100 1000 10000

Fed. Std. airborne particle cleanliness classes (particles/ft3)

pressure, humidity, vibration and lighting

Class 10: Less than 10 particles with size of 0.5 m and larger, per cubic foot

p0.5 m

No. of particles 0.1 m

35 350 3500 35000 350000

p ISO standard cleanliness class N with

particle concentration, Cn (particles/m3) is calculated as:

Cn = 10N x (0.1 m / D)2.08

Where D is particle size in m 30

Three ways in which dust particles interfere with mask patterns

Particle 1 – cause formation offormation of pinhole

Particle 2 – cause constriction of current flow

Features on mask

Dust particles

1 2

3

Particle 3 – cause short circuit

31

Is this Class 100 or Class 10,000 ?

Most IC fab areas: Class 100 (i.e. dust count is 4 orders of magnitude lower than that of ordinary room air)

Class 10: lithography area

Gown with “bunny suits”

32

Lithography 8

Characteristics of Clean Rooms1. Air is recirculated through

HEPA filters with about 20% make up. Vapors are entrained, so

t i ti t ti l icontamination potential is very high

2. Temperature is controlled to 20 - 22 °C

3. Humidity is controlled to 40 - 46 % RH.

4. Room is held at positive pressurepressure Positive pressure constantly blows dust OUT Doors open inward, so room pressure (creates a force) closes them shut

33

Practice Problems: Identify the major steps in the figure

n - Si(1)

SiO2

ResistSiO2n - Si

1.Surface Preparation2.Oxidation

n - Si2

(2)

n - Si

Resist

(3)

SiO2

(6)

ResistSiO

3.Photoresist Coating4.Soft Bake5.Align & Expose6.Develop7 H d B k

ResistSiO2

ResistSiO2n - Si

(7)

(5)

ResistSiO2

Maskhv

n - Si

SiO2n - Si(8)

SiO2n - Si(9)

7.Hard Bake 8.Etch9.Resist Strip

n - Si(4)

2

34

Practice Problems1. A certain silicon-gate NMOS transistor occupies an area

of 25 2, where is the minimum lithographic feature size. How many MOS transistors can fit on a 5x5 mm die if = 1 µm? Ans: 1 milliondie, if = 1 µm? Ans: 1 million

2. A 200-mm wafer is exposed for 1 minute to an air stream under a laminar-flow condition at 30 m/min. Estimate the amount of dust particles that will land on the wafer in a Class 10 clean room. Note that for Class 10 has 350 particles per m3. Ans: 330

3. Is RCA Clean the only method used in wafer cleaning?4. What is the main aim of wafer cleaning? Suggest a

method which can enhance the cleaning baths for removal of particles (in wafer cleaning).

5. Oxygen plasma can be used for stripping of resist. True or false

35

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36

Lithography 9

LithographyUnit 1: Photolithography Processg p yUnit 2:ResistsUnit 3: Photolithography Systems

Resists 2 main roles of resist in lithography

processRespond to exposing radiation in such a

way that mask pattern can be replicated in the resist.

Remaining resist areas must protect the underlying substrate during subsequent

42

y g g qprocess, etch or ion implantation.

Processing after lithography puts varying demands on resists

Wet etching

Resist pattern

Plasma etching

etching

43

Ion implantation

Sputter Metal in Lift-off

Electro-plating

Resists Principal components in conventional

photoresists1 Resin (base material) – polymer as binder1. Resin (base material) polymer, as binder,

establishes mechanical properties of the films e.g. adhesion, etch resistance, thickness

2. Active ingredient (sensitizer) – photoactive compound (PAC), undergoes a chemical reaction in response to radiation

44

p3. Solvent – keep resist in liquid until it is

applied; ie. allows spin application and formation of thin layers on the wafer surfaces

Lithography 10

Positive Vs Negative ResistLight

Mask

Exposed PhotoresistSili Di id

Iluminated A

Positive resistdeveloped in the exposed region

Negative resistremain in the exposed

Silicon DioxideSilicon Substrate

Areas

Negative Resist :Rendered Insoluble

Positive Resist :Rendered Soluble

45

remain in the exposed region

Etched Film Patterns

Positive Resist

During exposure, photochemical

Positive Resist :Rendered Soluble

reaction weakens the polymer chains. Exposed resists becomes more soluble

in developing solutions. Development rate for the exposed resist

is faster than the unexposed resist.

46

is faster than the unexposed resist. Example of PAC: diazonaphthoquinone

(DNQ)

Negative Resist During exposures, polymer is

strengthen by random cross-linkage of main chains or

Negative Resist :Rendered Insoluble

linkage of main chains or pendant side chains.

Becoming less soluble, slower dissolving in organic and water-based developersE l f ti ti

47

Example of negative-acting, two-component resists: bis(aryl)azide rubber resists

Positive resist vs Negative resistCharacteristic Positive Negative

Adhesion to Si Fair ExcellentAvailable compositions

Many VastcompositionsContrast, Higher, e.g., 2.2 Lower, e.g., 1.5Cost More expensive Less expensiveDeveloper Aqueous based

(ecologically sound)Organic solvent

Influence of oxygen No YesLift off Yes Yes

48

Lift-off Yes Yes

Minimum feature 0.5 m and below 2 mPhotospeed Slower Faster

Lithography 11

Characteristic Positive NegativePinhole count Higher LowerPinholes in mask Prints mask pinholes Not so sensitive to

mask pinholesR id ft d l t M tl t 1 d Oft bl

… cont’d

Residue after development Mostly at < 1m and high aspect ratio

Often a problem

Sensitizer quantum yield, 0.2 to 0.3 0.5 to 1Step coverage Better LowerStrippers of resist over Oxide stepsMetal steps

AcidSimple solvent

AcidChlorinated solvent

d

49

compoundsSwelling in developer No YesThermal stability Good FairWet chemical resistance Fair Excellent

Most useful metrics1. Sensitivity Amount of dose or light energy

(mJ/cm2) to create the chemical change2. Resolution How fine a line the resist can reproduce

from an aerial image

50

Resolution is determined by contrast, thickness, swelling and contraction after development

Resist Sensitivity Input energy (mJ/cm2) required to cause

chemical response in resist, which results (after development) in the desired resist (a te de e op e t) t e des ed es stpattern

Higher sensitivity allows shorter exposure time but too high resist undergo thermal reaction at room temperature

Positive system = 0 2 to 0 3

51

Positive system = 0.2 to 0.3 Negative system = 0.5 to 1, making it

more sensitive than positive resists

= Quantum yield

Resist Profiles Photons strike resist at different angle Scattering at the reflective interface results in

overexposed of resist Negative resist: broadening of the remaining

resist features Positive resist: quenching of top resist leading to

better resolution

A. Negative

DEVELOPMENT STEP Exterior Scatter Zone

52

B. Positive resist image

A. Negative resist image

Exterior Scatter Zone

Lithography 12

Resist Profiles• Undercut difficult to form in positive resist, easy to

UsesProfile Dose Developer Influence R/ Ro γ

Ion Implant, lift-off. Not good for plasma etching. Often only obtained through image reversal

High (often with back scatter radiation)

Low >10 >6

A: Positive resists Undercut

95-110°

a)

form in negative resist• R = Develop’t rate of exposed region • Ro = Develop’t rate of unexposed

image reversal

Lift-off, Reactive ion etch wet etch Ion Beam etch Perfect fidelity

Typically for positive resists, wet etch,

Normal Dose

Low Dominant

Moderate <4

<3

5-10

<5

Vertical

Normal or overcut

75-95°

b)

°

c)

53

rate of unexposed region• = Resist contrast

metallization <20% resist loss

Permanent resists, larger devices, MEMS

Dominant Little Influence

<3<0.1

B: Negative resists Undercut

45-75°

Pattern Transfer (by additive metal lift off)

Liftoff process sequence

Identify the resist

Starting Si substrate

Apply Resist Identify the resist type used – positive

A discontinuity or gap in the metal deposit is necessary for solvent to get at

Apply Resist

Mask alignment and Exposure

After Chlorobenzene Soak and Development

54

gthe uncoated resist wall; Identify the resist profile type –undercut

Sputter Metal

Metallized patternLiftoff to remove resist

Resist Contrast, Critical in determining the smallest line width to

be patterned (ie. resolution) and profile A measure of the resist’s ability to distinguish A measure of the resist s ability to distinguish

light from dark areas in the aerial image the exposure system produces

Exposure Response Curve: Normalized resist thickness Vs Dose

is determined by the slope of the curve; it is

55

y pnot constant for a particular resist composition; it depends on bake times, temperatures before and after exposure, wavelength of light, etc.

Contrast curves

st re

mai

ningD0

rem

aini

ng

0.8

1.0 1.0

0.8

D100

Frac

tion

of re

si

(A)Fr

actio

n of

resi

st

0

0.2

0.4

0.6

10 100 1000Exposure dose (mJ/cm2)

D100

0.6

0.4

0.2

010 100 1000

Exposure dose (mJ/cm2) (B)

D0

Approx slope portion of curve by a straight line Line extends from lower energy dose for which all of the

resist is removed, i.e. at energy density called D100

Lowest energy to drive the photochemistry, is D0

(A)Exposure dose (mJ/cm ) Exposure dose (mJ/cm ) (B)

Lithography 13

Contrast, Higher gives sharper line edges

)/(log1

010010 DD

g g p g Typical is 2 to 4 How many times is D100 larger that D0, if is 2?

57

Critical Modulation Transfer Function CMTF is the min optical modulation

function necessary to obtain a pattern D fi iti CMTF Definition – CMTFresist

= If the MTF of an aerial image is less than

the CMTF, the image will not be resolved

110110

/1

/1

0100

0100

DDDD

the CMTF, the image will not be resolved 1.0 -

0.75 -

0.5 -

0.25 -

0

Comparison of Aerial Images

D100

Do

Position

Rel

ativ

e Ex

posu

re D

ose

Quality of aerial image and resist contrast combine to produce resist edge profile

Practice Problems1. Suggest another method of Pattern Transfer in

lieu of the additive lift off process.2. If you want to create metal features on wafers y

via lithography, only negative photoresist is used to pattern metal by additive lift-off. True or false.

3. A 0.6-m thick layer of a particular photoresisthas D0 = 40 mJ/cm2 and D100 = 85 mJ/cm2. (a) Calculate the resist contrast, . (b) Calculate

59

Calculate the resist contrast, . (b) Calculate the CMTF. Ans: 3.05 & 0.36

4. Show that: 110110

/1

/1

0100

0100

DDDDCMTFresist

Blank page

65

Lithography 14

Lithography

Unit 1: Photolithography ProcessUnit 2: ResistsUnit 3: Photolithography SystemsUnit 3 Photolithography Systems

(5)

ResistSiO2

Maskhv

n - Si

Photolithography Systems Corner stone of

wafer processingE t Exposure systems –3 types

Performance parameters –projection systemprojection system

Technology Scaling 0.18-micron & 0.13-micron technology are used

to make IC; latest is the 90 nanometer technology – what does it mean? gy

Technology node to describe the minimum feature size

Critical Dimension (CD) – Minimum feature size in IC or miniature device and it is a measure of the resolution of a lithographic process (e.g. line width spacing contact dimension)line width, spacing, contact dimension)

Overall Resolution of a process – Ability to print a minimum size image, a critical dimension, under conditions of reasonable manufacturing variation

Energy Sources – waves or particles Energy sources are required to modify the resist The energy source is aerial imaged on the resist Bright sources are usually required for high Bright sources are usually required for high

throughputWavelength Energy

Light UV 400 nm 3.1 eV

Deep UV 250 nm 4 96 eVDeep UV 250 nm 4.96 eVX-Ray 0.5 nm 2480 eV

Particles Electrons 0.62 Å 20 keVIons 0.12 Å 100 keV

Lithography 15

Photolithography – printing methods Optical-based - three exposure systems

1. Contact Printing2 Proximity Printing2. Proximity Printing3. Projection Printing

Comparison of three exposure systemsLight intensity profile forthree exposure systems

OpticalSystem

LightSource

(1) Contact (2) Proximity (3) Projection

1.2 II

Mask

SiliconWafer

Photoresist

Mask

System

Gap0

0.2 I

0.4 I

0.6 I

0.8 I

I (1) Contact(2) Proximity

(3) Projection

1. Contact Printing Photo-resist covered

wafer, put in intimate contact with photomask, exposed with

Ilumination lightexposed with broadband-visible and near UV light

Defects on mask, due to repeated mask-to-wafer contacting steps

Mask image: Resist

Condensor lensCondensor lens

Mask patternPhotoresistWafer

Contact

Mask image: Resist image is 1:1, not limited by diffraction

Not applicable to VLSI and ULSI

2. Proximity Printing

Mask is placed in closed proximity, gap Ilumination light

Gap

about 2-20 µm to wafer Avoid mask damage Limited by near field

(Fresnel) diffraction of transmitted light;

Condensor lens

Mask pattern

PhotoresistWafer transmitted light;

reduces resolutionMinimum feature size

about 3 µm

Wafer

Resolution: min resolvable feature size

Generally, resolution in shadow printing

3 zsbR (1)

Incident UV light on mask

Mask plate

bmin = half the grating periods = gap between mask & resist = incident wavelengthz = resist thickness

Contact Printing

22min sbR

3

(1)

b b bs

p

Mask pattern

Resist

Wafer

Ideal transferA t l

Proximity Printing, s >> z22

3min

zbR

sbR 23

min (3)

(2)

0 1 2 3

Actual transfer

Sb 32 min

POSITION ON WAFER

4

b b bMinimum period transferable

Lithography 16

Example on Proximity PrintingGap,s = 10 m; = 400 nm From Eq.(3) R = bmin = 3 m sbR

23

min

Limitation: Practical UV proximity printing is about 2 to 3 m

In contact printing: If thickness resist, z = 1 m, from Eq.(2) R = bmin = 0.67 m(for extremely flat wafers) 3 z(for extremely flat wafers)

223

minzbR

3. Projection PrintingUse of imaging (projection)

optics in between the mask and the wafer

Ilumination light

Condensor lens

k1 = empirical constant NA

k 1min

bR

Limited by far field (Fraunhofer) diffraction Modified Rayleigh criterion

Condensor lens

Mask pattern

Diffracted lightProjection lens

(resists, process and mask aligner optics)

NA = numerical aperture of projection lens system

PhotoresistWafer

Projection printers: Performance Parameters

1. Resolution Determined by optical system, resist, etch

process2. Alignment accuracy Alignment of pattern to previous layer is to

ensure overlay is within the specification Determined by the optical system and Determined by the optical system and

aligner3. Throughput How many wafers/hour Determined by optical system, resist

Rayleigh Resolution (Fraunhofer diffraction pattern) - 1 Rayleigh criterion defines that two images are

just resolved when the maximum intensity of the first Airy disk falls on the first zero intensity offirst Airy disk falls on the first zero intensity of the second disk

Where n = refraction index

sin 0.61

) sin 2( f 1.22

Df 22.1

nfnR

Where n refraction index of material (usually air with n = 1 for the system); = max half-angle of diffracted light = wavelength

Lithography 17

Numerical Aperture NA characterizes the ability

of lens to transmit light, θsin nNA

Mask

2D

2D θ and

air)(for 1 Since

2D

fNA

ff

n

f

D

Projector

Wafer 2 fEffective F-number = Focal Length (f)

Clear Aperture (D)= 1/ (2 NA)

Wafer

NA DNA = 0, Lens collects no lightNA = 1, Lens collects all light

Rayleigh Resolution (Fraunhofer diffraction pattern) - 2

610

:gives

0.61 into sin n ngSubstituti

sinnRNA

0.61 factor is based on Fraunhofer diffraction pattern for an Airy disk using point

NANAR k 61.0

1

sources k1 is used in actual system; 0.6 to 0.8

Resolution – three ways to improve

NAR k ,Resolution 1

Lower k1

Smaller Light source

Ability of resist to distinguish between small changes in intensity; Improved optical

Higher NA

intensity; Improved optical schemesLens design improvements;Limited by lens aberrations

Problems Calculate the resolution by a 0.65 NA lens when

i-line wavelength is used. Assume k1 = 0.6.R = k / NAR = k1 / NAAns: R = 0.34 µm

Repeat the calculations for 0.75 NA lens when (c) DUV 248 nm (d) DUV 193 nm wavelength is used. Assume k1 = 0.4.

ANS: (c) 0.13 µm (d) 0.10 µm

Lithography 18

Depth of Focus To obtain good line width control, the image must

remain in focus through the depth of the resist layer If DOF, is the on-axis path length difference at the limit

of focus, then the path length difference for a ray from , p g ythe edge of the entrance aperture (mask window) is simply cos.

Rayleigh criteria for DOF: difference of these two lengths is not more than /4

(MASK) Object plane Lens

lm

Best focusing plane

(Wafer) Image plane

Lens axisDOF,

Simple image system

D

f

Depth of Focus (DOF)22

0

2

2 - 1 - 1 cos -

4

DOF becomes an issue

2

5.0 ,DOF f 2

D sin ,SinceNA

NA

22 DOFNA

k

Decreases as Decreases as k2 Decreases as NA

Example: Estimate the resolution and depth of focus of an excimer stepper using KrF light source (=248 nm) with a NA=0.6. Assume k1=0.75 and k2=0.5.

μm 0.34

0.60.248 0.5 k

μm 0.31 0.6

0.248 0.75 k

222

1

NADOF

NAR

What is maximum possible resist layer thickness?What is maximum possible resist layer thickness? Note that very flat topography is requiredMax resist layer thickness isSuggest other methods to improve R other than

improving the light source. Ans: optical methods

0.34 m

Modulation Transfer Function (MTF) Diffraction effects, interference, dark regions in

the image never reach complete darkness, i.e. intensity = 0intensity = 0

Quality of image, modulation index, M is defined as

Use to characterize the capability of projection system to reproduce mask feature on wafer

minmax

minmax

IIIIMTFim

surface Ideal case, MTFim=1 Practical case, MTFim<1

Lithography 19

Modulation Transfer Function (MTF)Incident UV light on mask

b b b

Mask plate

Mask patternb b b

Resist

Wafer

Ideal transfer, Mmask= 1

LensIncident UV light on Resist

Res

ist

0 1 2 3

Actual transfer, Mim

POSITION ON WAFER (Arbitrary Units)

4

b b bminmax

minmaxim

IIIIMTF

Inci

dent

Inte

nsity

on

R

Imax

Imin

MTF What is MTF? A measure of the contrast in

the aerial image produced by the exposure systemsystem

Generally MTF 0.5 for resist to resolve features

Value is dependent on (a) feature size in the image (b) spatial coherence of the light source

5

Spatial coherence – a measure of degree to which light emitted from the light source stays in phase at all points along the emitted wave fronts.

MTF MTF

L S ll0

1

1

Light Source (point source)

CondensorLens

Light Source (area)

CondensorLens

sMTF versus feature size in the image.

LargerFeatures

SmallerFeatures SizeFeature

Waves are in phase

Examples of spatially coherent (left) and partially coherent (right) light sources.

Lens

Mask

Lens

Mask

D

Manufacturing Methods and Equipment: Projection Methods

1. Scannero 1:1 scanning projection aligners first use in

1970s; capability of 1-m features and1970s; capability of 1 m features and throughput of 110 wafers/hr (WPH)

2. Step-and-repeat (stepper) Projection ratio of 4:1 reduction lens; capable

of 0.13 m production and 30 WPH throughput; alignment of each chip individually;

St d ( i t h b id)3. Step-and-scan (scanning stepper or hybrid) Reduction lenses makes mask-making much

easier and gives tighter line-width tolerance on wafer

Lithography 20

Types of Projection1. Scan System (Scanner) Only narrow slit of entire wafer is illuminated

until the whole field is scanned; Both reticle & wafer are moving;wafer are moving;

Larger field; 1X masks that contain the pattern info for all the chips cost effective and high throughput

What happens when geometries shrink or wafer size increases?

Mask pattern

Projection lens

PhotoresistWafer

Types of Projection2. Step and Repeat System (Stepper) Reduction lenses by 4X or more Exposed one part of the wafer follow by Exposed one part of the wafer, follow by

new position Reticle remains stationary while the wafer

is movingMask pattern

Projection lens

PhotoresistWafer

3. Step and Scan SystemHybrid type which

Types of Projection

Exposure Field

Scan

y ypuses both stepping and scanning

Takes the advantages of scanners (exposure field) and steppersfield) and steppers (4X- reduction masks)

3. Step and Scan System (con’td) Both reticle and wafer moved in opposite

direction at the same time, with very precise alignment p g

4X reduction: reticle moves 4X faster than wafer

Reticle and wafer in synchronized movement

Two dimensional translations

M υR

Two-dimensional translations of the wafer with speed, v and one dimensional translation of the mask with a speed, Mtimes that of the wafer speed.

υ

R

Lithography 21

Step and Scan System: ASML PAS 5500/950B

Reticle (Mask)

193 nm Excimer Laser Source Computer

Console

Exposure Column(Lens)

Wafer

Alignment in Photolithography

RECAP

Alignment and Overlay Overlay – general pattern placement; each level

must be aligned to the previous levels; Alignment – specific spots on the wafer; it is g p p ;

limited to specific structures (chip edge) and it not a full guarantee of overlay elsewhere

Overlay is the outcome of alignment Overlay tolerance requirement is 1/3 of the min

feature size E g overlay tolerance to print x m min linewidth E.g. overlay tolerance to print x m min linewidth

structure is capable of x/3 m registration between levels

Global Alignment & Local Alignment

o

a

A 5X projection stepper with 0.5 m min lines is capable of 0.16 m registration between levels

Lithography 22

Summary: Resolution Limit

Maskbbs

2z

23

min bR1) Contact

2) Proximity sbR 23

min

ResistFilm

bbsz

2) Proximity

3) Projection2min

NAbR 1k

min 22

)(kNA

DOF

Photolithograph Resolution Enhancement Technology (RET)

Present: Current lithography techniques g p y quse deep ultraviolet range 248 nm wavelengths to print 150 to 120 nm size features

Future: New techniques to create smaller featuresfeatures

Cost of lithography in IC production: 35% of total chip cost

Moore’s Law in IC Fabrication Gordon Moore, former chairman of Intel

states that the number of transistors per square cm doubles every two years without q y yincrease in cost.

http://news.bbc.co.uk/2/hi/technology/7080646.stm

Resolution Enhancement Technique (RET) Improve the resolution by Mask

engineering1 Optical Proximity Correction (OPC)1. Optical Proximity Correction (OPC)2. Phase-Shifting Mask (PSM)3. Off-Axis Illumination (OAI) 4. Immersion

Lithography 23

Improvement of R (Extension of Photolithography)

Wavelength 365nm 248nm 193nm 157nm Deep UV Excimer Laser Sources

gi-line KrF ArF F2

NA Method

k1 0.65 0.70 0.80 0.70 0.85 0.70 0.85

Conventional 0.6 335nm 213 185nm

Off axis ill i i

0.5 280nm 177 155nm 138 115nm 110 90nm illumination

Strong OAI + OPC and/or PSM

0.4 142 125nm 110 90nm 90 75nm

1. Optical Proximity Correction (OPC)

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

- Example: corner rounding, line shortening

OPC is used to pre-compensate the reticle/mask pattern to account for expectedreticle/mask pattern to account for expected pattern distortion due to diffraction effect

Adding serifs enhance the amount of light transmitted through the corner

Cont’d OPC

Mask with OPC

PatternPattern (without OPC) Pattern

2. Phase Shifting Mask (PSM) Improved mask technology: most dramatic

improvement in resolution that can be contributed by reticlecontributed by reticle

Without PSM: NA=0.6, =0.248 nm, CD=0.3 µmWith PSM: CD can be improved to 0.18 µm

Concept of improving resolution by modifying the optical phase of the mask transmission was first suggested in 1982 by Levensonfirst suggested in 1982 by Levenson

Lithography 24

Alternating Phase Shifting Mask (AltPSM)

Adding an extra layer of transmissive material to the optical path

Basically involves the Amplitude at Mask

180° Phase Shift

PSMChrome

ydestructive interference of light occurring along the transition edge between the 0 and 180 phase-shifted areas.

Better resolution and

at Mask

Amplitude at Wafer

contrast can be achieved due to destructive interference between the two adjacent diffracted wavefronts. (After Levenson)

Intensity at Wafer

Conventional Phase-shift mask

3. Off-Axis Illumination (OAI)

Incorporated into steppers in 1992 OAI = Illumination light strike the projectionOAI Illumination light strike the projection

lens at the edge of entrance pupil Conventional illumination, light strike the

center of lens Imaging ≤ 0th order + one additional

diffracted order

Cont’d - OAI R is improved by letting light impinging at an

angle to the mask to capture higher-order diffracted light

ApertureProjection

Lens ApertureProjection

Lens

“Capture” Diffracted

Light

“Lost” Diffracted

Light Wafer“Capture” Diffracted

Light

“Lost” Diffracted

Light Wafer

On-axis illumination (left) and off-axis illumination (right) (Plummer p.237)

Limits of Optical Lithography CD of optical lithography With RETs, CD 30% smaller than exposure wavelength, i.e.

0.18 µm for 248 nm wavelength Rate of shrinkage of CD faster than reduction of exposure

NAb 1

mink

g p Current DUV generation: 193 nm with combinations of PSM

and OAI can be extended to beyond 100 nm

0.8

0.9

0.7

G-linel-line

248nm193nm

1.0

0.8

0 6Ape

rture

Physical limit for air

l-line248nm

0.6

0.5

0.4

0.3

k1

1982 1985 1990 1995 2000

193nm

Year of system introduction

0.6

0.4

0.2

0.0

Num

eric

al A

1980 1985 1990 1995 2000

G-linel line

Lithography 25

4. Immersion Lithography (IL)

Recap:DθsinnNA

Liquid recovery

Projection optics Liquid

supply 2

θsin f

nNA

Where n is the refractive index of the medium surrounding the lens and is the acceptance

l f th l

Wafer stage

(Scanning motion)Immersion liquid

Wafer

angle of the lens. Sine of any angle is always ≤ 1 and n =1 for air What if a medium with a higher index of

refraction is substituted with air?

IL – refractive index Refractive index, n > 1 Ultrapure water, n 1.47 (it has low optical

b ti tibl ith i t dabsorption, compatible with resist and non-contaminating

sin k k

11

min nNAb

Increase n improves resolutionIncrease n improves resolution

Issues – presence of bubbles; cause light scattering

Next Generation Lithography (NGL)

Energy Source1. Extreme

UltravioletUltraviolet (EUV); 20 nm

2. X-Ray Lithography; 1 nm

3. Charged-P ti lParticle Beam (E-beam and Ion beam); 0.1 nmOthers: Nanoimprint for lithography and Soft lithography

Summary of Lithography Components of

Lithography Energy – modify resist gy y

dissolution rate; UV, DUV, X-ray, E-beam

Mask – pattern energy to resist; transparent + opaque

Exposure systems/ aligner – contact, proximity, projection, optics; resolution

Resist

Lithography 26

Practice Problems1. It has been determined that there are no clear

choices for lithography systems beyond optical projection tools based on 193-nm ArF excimerlasers One possibility is an optical projectionlasers. One possibility is an optical projection system using 157-nm F2 excimer laser.

a. Assuming a numerical aperture of 0.8 and k1=0.75, what is the expected resolution of such a system using a first order estimate of resolution? Ans: 147 nm

b. Actual projections for such systems suggest that they might be capable of resolving features suitable for the 2009 70 nm generation Suggest three approaches to2009 70 nm generation. Suggest three approaches to actually achieving this resolution with these systems? Hint: refer to the resolution equation and section on RET

2. In a 4X reduction scanner lithography system, the wafer must move 4 times faster than the

ti l T f l ?reticle. True or false?3. Once the _______ image has been formed in

the polymer film after exposure, it must be developed to produced the final 3-D ______ image.