Photolithography Part1

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ENSC-E131 NanoFabrication and NanoAnalysis

Lithography

Jiangdong (JD) Deng, Ph.D



9/26/2011 Jiangdong Deng 2

Photolithography (Talk 1) -

Fundamentals, Processes

• Overview of Photolithography

– Micro/Nano fabrication processes

– Basic steps of photolithography

• Fundamentals of photolithography – Photomask

– UV exposure (light source, exposure optics, equipment) – Photoresist and related processes

• Resolution Limits and Profile Control




Photolithography (Talk 2)

Advanced Lithography for Nanofabrication – E-beam lithography

– Imprint Lithography

– X-ray Lithography

– Focus Ion-beam (FIB) Lithography

– SPM lithography





5

Trend of Nanofabrication



Metrology

6

Thin Films

(Photo)-lithography

Cleaning

Front-End

Processes

EtchIon

Implantation

Planarization

Test & Back End

DesignWafer

Preparation

Design

Wafer Preparation

- Material Growth, cutting, polishing Front-end Processes

- LPCVD, MOCVD, MBE, ALD, wafer-bonding

Lithography (photo-, E-Beam, FIB…)

Etch- RIE, wet etching,

Cleaning- Wet, plasma, O-zone

Thin Films

- PVD, PECVD, ALD, LPCVD Ion Implantation

Planarization- Spin coating, CVD,

Test and Back-end- Dicing/cleaver, Wire bonding, assembly

Metrology

Micro/Nano Fabrication Processes




• Photo-litho-graphy: light-stone-

writing

• Photolithography is a technique that is

used to define the shape of nano/micro-

machined structures on a wafer

– Purpose: Transfer same circuit patterns

onto a large number of wafers.

• Key role in Nanofabrication

– Critical dimension generations

• Major Concerns in Photolithography

– Resolution

– Profile control

– Overlay accuracy

– Process latitude

Photolithography

•• 3 key parts/components in3 key parts/components inphotolithographyphotolithography

•• MaskMask

•• PhotoresistPhotoresist

•• UV exposure systemUV exposure system



Photolithography used for Pattern Formation

Beginning of Integrated Circuits in 1959

Kilby (TI) and Noyce (Fairchild Semiconductors)

Substrate

Film deposition Photoresist application

Deposited Film

Photoresist

Exposure

Development Etching Resist removal

Mask

Etch mask

Light

• Basic lithography process

which is central to today’s

chip fabrication.

• Sensitive to light

• Durable in etching




8 Steps in Photolithography

8) Developinspect

5) Post-exposurebake (option)

6) Develop 7) Hard bake(option)

UV Light

Mask

4) Alignment

and Exposure

Resist

2) Spin coat 3) Soft bake1) Surface preparation

HMDS

•Photoresist and Process

•PhotoMask

•UV Exposure

Ph li h h Ph i l



9/26/2011 Jiangdong Deng

• Resolution

• Contrast (profile control)

• Sensitivity

• Viscosity

• Adhesion• Etch resistance

• Surface tension

• Surface roughness• Storage and handling

• Contaminants and particles

•Photoresist and Process

•PhotoMask

•UV Exposure

10

Photolithography Physical

Characteristics




8) Developinspect



UV Light

Mask

4) Alignment

and Exposure

Resist


HMDS

Photolithography Processes

Ph li h h (T lk 1)





Fundamentals, Basic Process


– Micro/Nano fabrication processes – Basic steps of photolithography

• Fundamentals of photolithography

- Photomask – UV exposure (light source, exposure optics, equipment)

– Photoresist and related processes

• Photolithography limits

• Pattern profile control in Photolithography




Photomasks

• Master patterns that will be transferred to wafers

• Transparent substrate coated with patterned, UV-opaque material

• Types: – Laser-plotting on plastic film (cheapest)

– Cr on soda lime glass

– Cr on quartz glass (most expensive, needed for deep UV litho)

• Polarity: – “light-field” = mostly clear, drawn feature = opaque

– “dark-field” = mostly opaque, drawn feature = clear




Photomasks

Mask MaterialsWriting

Method

Smallest

Feature

Pattern

QualityPrice Vendors Others

Hard' MaskCr on glass

plate (Soda-

lime or Quartz )

Laser Writing, or

E-beam

Lithography

(EBL)

1um (EBL

can reach

sub-um)

Smooth

edge

expensive

($80/hour

in CNS)

CNS,

Advance

Reproductions

…

good for

alignment

Transparent

FilmInk on plastic

film

High-precision

laser printer (5k -

20k dpi)

>10umRough

edge

cheap

(<$35)

Outputcity,Advance

Reproductions

…

not good for

alignment

Two Types of Photomasks in CNS

I t f M k O l




Importance of Mask Overlay

Accuracy

PMOSFET NMOSFET

Cross section of CMOS inverter

Top view of CMOS inverter

The masking layers

determine the accuracy by

which subsequent

processes can be

performed.

The photoresist mask

pattern prepares individual

layers for proper

placement, orientation, and

size of structures to be

etched or implanted.

Small sizes and low

tolerances do not providemuch room for error. Figure 13.4




Alignment Marks

2nd Mask

1st Mask

2nd mask layer

1st mask layer

RA, Reticle alignment marks, L/R

GA, Wafer global alignment marks,

L/R

FA, Wafer fine alignment marks, L/R

+ +

++

RAL

RAR

+ GA

+ FAL

+ FAR

+ GAR

+ GAL

Notch, coarsealignment

FAL

FAR

FAL/R +

+FAL/R

+

For 2ndmask

+From

1stmask

{

Ph t lith h (T lk 1)







– Micro/Nano fabrication processes

– Basic steps of photolithography


- Photomask – UV exposure (light source, exposure optics, equipment) – Photoresist and related processes

• Resolution limits


Wavelengths of Exposure




UV

Wavelength

(nm)

Wavelength

NameUV Emission Source Energy (eV)

436 g-line Mercury arc lamp 2.84

405 h-line Mercury arc lamp 3.1

365 i-line Mercury arc lamp 3.4

248Deep UV

(DUV)

Mercury arc lamp or

Krypton Fluoride (KrF)

excimer laser4.96

193Deep UV

(DUV)

Argon Fluoride (ArF) excimer

laser

6.42

157Vacuum UV

(VUV)Fluorine (F2) excimer laser

7.9

13.4EUV (Extreme

UV, soft-X-ray)

Plasma from Xe gas excited by

high power laser

92.6

0.5 X-Ray X-ray 2480

0.062 Electron 20 keV

0.012 Ion 100 keV

Decreasing feature sizes requires shorter λ.

Wavelengths of Exposure

Sources

Light source:



Spectrum image from http://zeiss-campus.magnet.fsu.edu/

I H G

Light source:

Mercury Arc Lamp

Th B i E M h d




2b

Three Basic Exposure Methods

1:1 Exposure 1:1 Exposure ~5:1 Exposure

K 1,2

~0.3-0.9

Resolution (b):

d, the thickness of photoresist, s, the gap of mask-resit, k~3

Light Profile Comparisons of




Light Profile Comparisons of

three exposure modes

b

Contact/Proximity exposure: Fresnel diffraction (near-field)

Projection exposure: Frounhoffer diffraction (far-field)

Comparisons of three exposure




Comparisons of three exposure

modes

Exposure System Advantage Disadvantage Application

High resolution (Near-filed)Mask contamination anddamage

R&D

Low cost (<200K) Sensitive to wafer bowing

High throughput Defects impact

Proximity Low mask/samplecontamination

Poor resolution R&D

Hight resolution

Low mask contamination Expensive (>$5M)

High throughput (50 wafer/h)

Contact

ProjectionDominates inproduction




Exposure Equipments

• Contact/Proximity Aligner

• Direct write lithography (Heidelburg, DWL66)

• Scanning Projection Aligner (scanner)

• Step-and-Repeat Aligner (stepper, production tool)• Step-and Scan System

Contact/Proximity Aligner Systems




Contact/Proximity Aligner Systems

in CNS

Suss-MJB4 Suss-MA6

Suss-MJB3

AB-M

Illuminator

Alignmentscope (split

vision)

Mask

Wafer

Vacuumchuck

Mask stage

(X, Y , Z ,θ

Wafer stage

(X, Y, Z,θ

Mercuryarc lamp



Heidelburg, DWL66Heidelburg, DWL66

Wafer stepper / wafer scanner Wafer stepper / wafer scanner

Exposure Procedure




p

(Contact Mask Aligner)

Mask loading

Wafer loading

WEC contacting

Separation

Alignment

Exposure Contacting

Alignment/contact

Checking

Exposure

NO

Yes

Unload wafer/mask

• WEC- Wedge Error Compensation,

-to make the sample surfaceconformably contact the mask without

‘wedge’.

• Exposure Contact Control

•Proximity•Soft-contact

•Hard Contact

•Vacuum Contact

• Alignment/Contact Check•Shadow effect

Two Keys for a goodexposure:

1, Right exposure dosage

2. Good contact!!!

SU-8 Photolithography




g p y

Process Conditions

Jiangdong Deng, “Soft-Lithography-summary-2-1-06-user-meeting-02”, SLF at Harvard CNS





Photolithography (Talk 1)





– Photomask – UV exposure (light source, exposure optics, equipment)

– **Photoresist and related processes

• Lithography limits and Profile Control

Photoresist Basic




-Positive tone:

•Exposure increases solubi lity

•Low molecular-weight (<10,000) polymer•Pre-cross linked before exposure. exposure

weakens polymer by rupture or scission of polymer

chains

•Mask image is same as wafer image

•Typical Resist, Shipley 1800, AZ series

-Negative tone

•Exposure decreases solubil ity

•High molecular-weight (~65,000) polymer

•Exposure causes cross-linking of polymer chains

•Wafer image is opposite of mask image

•Typical Resist, SU-8,

(Positive and Negative resist)

Components of Conventional




Additives:•chemicals that control specific

aspects of resist material

Solvent:•gives resist its flowcharacteristics

•PGMEA (Propylene Glycol MethylEther Acetate-C6H12O3))

Sensitizers: (inhibitor, PAC)•photosensitive component of

the resist material

Resin:

•mix of polymers used as binder;

•gives resist mechanical andchemical properties

p

Photoresist

Positive Photoresist




-(DNQ-novolak resist)

Resin in photoresist (positive)

W.S. Deforest, ‘Photoressit’, McGraw Hill (1975)

Photo Chemical Reaction in P-PR




(DNQ-novolak resist)

Photo Reaction in P-PR




(DNQ-novolak resist)

Photo Reaction in Positive resist

DiazoNaphtoQuinone

(DNQ)- sulfonate• The position of SO3-R

determine the

photoreaction wavelength

(I,g,h line)

• Lower solubility

Indene Carboxylic Acid

• Higher solubility (1000times higher) in alkaline

than DNQsulfonate

Release N2,

absorpt H2O,

Photo Recation in Negative PR




(KTFR resist)

Sensitizer,-bis arylazide…

Sensitizer reacts with light.

-Exposure

-releasing N2 and

-very active

Polymer(resin)-sensitizer-

cross link

-Post exposure baking (PEB)

- Higher insolubility

KTFR – Kodak Thin Film Resist,

-invented in 1957, dominated in the early years of semiconductor

Cyclized poly-isoprene

Photoresist Processes




8) Developinspect5) Post-exposurebake (option) 6) Develop 7) Hard bake(option)

UV Light

Mask

4) Alignmentand Exposure

Resist


HMDS


Surface Preparation




Surface Preparation

• Cleaning:

- Remove any contaminants on the wafers prior to photoresist coating

• Dusts/particles, oil, residue, grease, wax…

• Cleaning approaches: solvent cleaning, TCA (1,1,1-trichloroethane), TCE

(trichloroethylene), Piranha cleaning (H2SO4 & H2O2), RCA (I, II), acid-

based Nanostriper, O2 plasma…

• Dehydration:

– remove water prior to priming and coating

• 200 C hotplate >5 min, or 15 min 80C oven

• Priming:

– form a polar (electrostatic) surface to isolate moisture adhesion on

wafer surface and

– Increase the adhesion between wafers and photoresist coating layer

Surface Preparation




• For Si, SiO2, and other dielectric material: – primers form bonds with surface and produce a polar

(electrostatic) surface

– most are based upon siloxane linkages (Si-O-Si)• 1,1,1,3,3,3-hexamethyldisilazane (HMDS), (CH3)3SiNHSi(CH3)3

• trichlorophenylsilane (TCPS), C6H5SiCl3

• bistrimethylsilylacetamide (BSA), (CH3)3SiNCH3COSi(CH3)3

• For GaAs and other III-V materials: – GaAs already has a polar surface

• monazoline C

• trichlorobenzene

• xylene

• Priming approach:

1) Vapor coating (Industry)

2) Spin-coating (R&D, 2000rpm for 20s)

-Surface Priming (1)

Surface Preparation

S f (2)




HMDS adhesion promotion on SiO2

-Surface Priming (2)

Surface tension (dyne/cm) H2O on surface (molecule/um^2)

Pre-HMDS 78 >35

Post-HMDS 21 <1






UV Light

Mask


Resist


HMDS


PR Spin Coat



9/26/2011 Jiangdong Deng

Process Summary

40

:• Wafer is held onto vacuum chuck

• Typical coating steps: Dispensing (low speed ~500rpm for

4-10s) Level out (high speed-1000-to

5000rpm, 30-60s)

• Quality measures:

– thickness – uniformity – particles and defects

Vacuum chuck

Spindle

connected to

spin motor

To vacuumpump

Photoresistdispenser

Time

Speed

dispensing

Spin

down

Level out

p

SU-8 2000 Photoresist Spin Curve




SU 8 2000 Photoresist Spin Curve

0

50

100

150

200

250

300

350

500 1000 1500 2000 2500 3000 3500

T h i c k n e s s ( u m )

Coating Speed (RPM)

Su-8 Spin Curve (MicroChem and CNS)

2100-CNS

2100-MicroChem

2050-CNS2050-MicroChem

2025-CNS

2025-MicroChem

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

500 1000 1500 2000 2500 3000 3500

T h i c k n e s s ( u m )

Coating Speed (RPM)

SU-8 Spin Curve (MicroChem and CNS-LISE)

2015-CNS

2015-MicroChem2010-CNS

2010-MicroChem

2005-CNS

2005-MicroChem

-Resist coating performance depends on:

a) Solvent Concentration, b) Speed control, c) Coating environment

(air flow, T, Huminity), d) surface conditions (adhesion, energy…)






UV Light

Mask


Resist


HMDS


Soft baking




• Drives Off Most of Solvent in Photoresist

– For positive resist, pre-crosslink happens between resin and sensitizer

• Impacts to post fabrication processes

– Photoresist-to-Wafer Adhesion

– Thermal stress within PR-film

– Resist Uniformity on Wafer

– Etching rate and Linewidth Control During Development

• Typical Bake Temperatures are 90 to 115°C

– Shipley 1800, 115°C; SU-8, 65/95 °C

– Baking time (>2min) varies with the thickness

– On a Hot Plate (better uniformity than oven baking)






UV Light

Mask


Resist


HMDS

Post-Exposure Bake (PEB)




p ( )

• Required for negative PR (like SU-8), to ensure

cross-link in the resist• Typical Temperatures 90 to 110°C on a hot

plate

– SU-8, 65/95 °C






UV Light

Mask


Resist


HMDS

Photoresist Development




Process Summary:

• Soluble areas of photoresistare dissolved by developerchemical

• Developer:

• For Positive Resist(Shipley 1800) KOH (CD-351),

NaOH (CD-30), or

(CH3)4NOH (TMAH, CD-26,

MF319) Time: 30-90s

• For negative resist (SU-8) PGMEA (SU-8 developer,

or BTS-220) Time: 1-20min

Vacuum chuck

Spindle

connected to

spin motor

To vacuumpump

Developdispenser

•Spinning option ( bath option in CNS)

p






UV Light

Mask


Resist


HMDS

Hard Bake




• An Optional Post-Development Thermal Bake to

– Evaporate Remaining Solvent

– Stabilize and harden the developed photoresist

• Impacts to the post processes

– Improve Resist-to-Wafer Adhesion

– Improve the resistance for dry or wet etching – Improve the edge roughness due to plastic flowing

– Not easy to be removed, thus not good for lift-off process

– May cause extra thermal stress• Higher Temperature than Soft Bake, for >5min

– Shipley 1800, 140-160°C

– SU-8, 150°C





8) Developinspect



UV Light

Mask


Resist


HMDS

Inspection




Resist liftoff

• Inspect to Verify a Quality Pattern

– Identify Quality Problems (Defects)

– Characterize the Performance of the Photolithography Process

– Prevents Passing Defects to Other Areas

• Etch

• Implant

– Rework Mis-processed or Defective Resist-coated Wafers


Fundamentals Basic Process







- Photoresist and related processes

- Photomask

– UV exposure (light source, exposure optics, equipment)

• Resolution limits



Resolution Limit and Profile Control




Linewidth Space

Thickness

Substrate

Photoresist

Three Dimensional Pattern in Photoresist

Criterias for ‘good photolithography’

- Line/space size -Profile control - Etching resistance

- Adhesion -Sidewall roughness - Surface tension- Uniformity -Defects

Exposure Systems




2b


K 1,2

~0.2-0.8Resolution (2b):


Photolithography-Diffraction



g p y

• At smaller dimensions,

diffraction effectsdominate

• If the aperture is on the

order of λ, the lightspreads out after passing

through the aperture. (The

smaller the aperture, the

more it spreads out.)

Photolithography-NA

j ti t



-projection system

NA-Numerical Aperture, represents

the light collected by the condenser

or objective lens

•If we want to image the

aperture on an image

plane (resist), we cancollect the light using a

lens and focus it on the

image plane.•But the finite diameter

of the lens means some

information is lost(higher spatial

frequency components).


projection system



• Image formed by a small

circular aperture (Airy disk)

as an example

• Image by a point source

forms a circle with diameter1.22λf/d surrounded by

diffraction rings (airy

pattern)

-projection system


projection system



-projection system

• Rayleigh suggested that areasonable criterion forresolution (b = distance betweenA and B) is that the centralmaximum of one point sourcelies at the first minimum of theAiry pattern of the other point (b

= diameter of circle)• The numerical aperture (NA) of

a lens represents the ability ofthe lens to collect diffracted lightand is given by NA = n sin α inthis expression n is the index ofrefraction of the mediumsurrounding the lens and α is theacceptance angle of the lens ( n= 1 for air)

NA f n

f

d

f b

λ

α

λ λ 61.0

)sin2(

22.122.1===

Reyleigh Resolution:

Practical resolution:

NAk b λ

•= 1 (0.25< k1 <0.8)

Photolithography Resolution

projection system



-projection system

• Three ways to improveresolution bmin

– Reduce wavelength

– Increase NA

– Reduce k 1

NAk b λ

•= 1min

Electromagnetic Spectrum




Visible

Radio wavesMicro-wavesInfraredGamma rays UVX-rays

f (Hz) 10 10 10101010101010 10 4681012141622 1820

(m) 420-2-4-6-8-14 -10-1210 10 10101010101010 10

365 436405248193157

g hi DUVDUVVUV (nm)

Common UV wavelengths used in optical lithography.


Wavelengths of ExposureSources




UVWavelength

(nm)

WavelengthName

UV Emission Source Energy (eV)

436 g-line Mercury arc lamp 2.84

405 h-line Mercury arc lamp 3.1

365 i-line Mercury arc lamp 3.4

248Deep UV

(DUV)

Mercury arc lamp or

Krypton Fluoride (KrF)

excimer laser4.96

193Deep UV

(DUV)

Argon Fluoride (ArF) excimer

laser

6.42

157Vacuum UV

(VUV)Fluorine (F2) excimer laser

7.9

13.4EUV (Extreme

UV, soft-X-ray)

Plasma from Xe gas excited by

high power laser

92.6

0.5 X-Ray X-ray 2480

0.062 Electron 20 keV

0.012 Ion 100 keV


Photolithography-λ



Photolithography λ

Photolithography-NA



• Increasing NA

– Improvements in lens

design.• In the mid eighties,

NA ~0.4,

• In 2000, for 248nmexposure systems, NA> 0.8.

• Now, for 193nm (ArFsystem), 0.93 is possible in the air

– Immerse into the high

index medium (n>1,water n=1.47@193nm)

Photolithography-Immersion Litho



52nm in the air!52nm in the air!

Photolithography-DOF

-projection system



projection system

DOFDOF--Depth of FocusDepth of Focus

The range over which there are clear optical images

+

-

Photoresist

Film

Depth of focusCenter of focus

Lens2)(5.0~ NA DOF

λ

λ ΝΑ R DOF365 nm 0.45 486 nm 901 nm

365 nm 0.60 365 nm 507 nm

193 nm 0.45 257 nm 476 nm

193 nm 0.60 193 nm 268 nm

Why need to meet DOF Requirement?

Photolithography-DOF



• The defocus tolerance (DOF)

• Much bigger issue in miniaturization

science than in ICs

A small aperture (NA) was used to ensure

the foreground stones were as sharp as the

ones in the distance.

What you need here is a use a telephoto

lens at its widest aperture.

How about contact/proximity system ?




2b


K 1,2

~0.2-0.8Resolution (2b):


Light Profile Comparisons ofthree exposure modes




b

Contact/Proximity exposure: Fresnel diffraction (near-field)

Projection exposure: Frounhofer diffraction (far-filed)

Light intensity profile

– contact/proximity system




Resolution Limit and Profile Control – Mask-resist Gap




Gap between mask and resist

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 T h e o r e t i c a l M a x . R

e s o l u t i o n ( u m )

Effective Mask-resist Gap (um)

Resoulution for Contact/ Proximity Exposure

I-line, 365nm

H-line, 405nm

G-line, 435nm

Effective mask-resist gap:

gd sb λ λ 5.1)5.0(5.1 =+=

d sg 5.0+=

Resolution for

Contact/proximity

exposure:

d, the thickness of photoresist,

s, the gap of mask-resit,

Resolution Limit and Profile Control- PR Contrast (1)



Mask

P-PR

- γ , Photoresist Contrast.

- E0- disolusion dose, Ef - Resist sensitivity, 100% solution dose

- Ideal: a) E0 ~Ef step function, γ~ ∞, b) Ef small

- Reality, γ ~ 2-8

Mask

N-PR

IntensityIntensity

Exposure Dose (mW/cm2)= Lamp Intensity (mJ/cm2) x exposure timeExposure Dose (mW/cm2)= Lamp Intensity (mJ/cm2) x exposure time (s)(s)





Ideal Exposure - Ideal Resist Real Exposure - Ideal Resist

maskmask





Real Exposure - Real Resist (positive)

P-PR

mask

γ↓ D↑

N-PR

Su-8-2025

Resolution Limit and Profile Control- Light Reflections Effects (1)




Polysilicon

Substrate

STISTI

UV exposure light

Mask

Exposedphotoresist

Unexposed

photoresist

Notched photoresist

Edgediffraction

Surfacereflection

Resolution Limit and Profile Control- Light Reflections Effects (2)




Planarization

Trick: Planarrization

Resolution Limit and Profile Control- Standwave Effects




Standing waves cause non-

uniform exposure along thethickness of the photoresist film.

Incident wave

Reflected wave

PhotoresistFilm

Substrate

Solution Trick: B-ARC (Bottom Anti-Reflection Coating)

polymer type, dielectric multilayer…

Resolution Limit and Profile ControlSummary




•Challenge Photolitho process:High resolution, with

High aspect ratio (>5:1), andDense patterns

•Keys to ‘good exposure’

1)Right photoresist (type, contrast, thickness)2)Right Dosage

Thickness, substrate, feature size and density

3)Good contact!!! (for contact/proximity system)Uniform PR film, particle/defect control, pre-patterns,

right exposure contact control…

Photolithography Track System




Automated Wafer Track forPhotolithography




Wafer Transfer System

Load station Transfer station

Vaporprime

Resistcoat

Develop andRinse

Edge-beadremoval

Softbake

Coolplate

Coolplate

Hardbake

Wafer stepper(Alignment/Exposure

system)

Illuminatoroptics

Excimer laser(193 nm ArF )

Step and Scan Exposure System




Beam line

Operatorconsole

4:1 Reduction

lensNA = 0.45 to 0.6

Wafer transportsystem

Reticle stage

uto-alignment system

Wafer stage

Reticle library(SMIF pod interface)

Documents

Photolithography Part1