Advanced resists for e-beam lithography: processing ... · Advanced resists for e-beam lithography: processing, exposure and characterization (Part I) Dra. Mariana Pojar de Melo Prof

Advanced resists for e-beam lithography: processing, exposure

and characterization(Part I)

Dra. Mariana Pojar de MeloProf. Dr. Antonio Carlos SeabraDep. Eng. de Sistemas Eletrônicos

Escola Politécnica da [email protected], [email protected]

M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 2

Goals

Familiarize potential users with processing steps involved in electron beam lithography for fabrication of high resolution structures;

Highlight some processes associated with these technologies;

Use of advanced resists seeking nanolithography.


Processing Steps for Lithography

HardBake

DehydrationBake

AdhesionPromoter

ResistSpinning Pre-bake

ExposurePost-exposureBakeDevelopmentPlasma

Flash

Etching

Deposition(Lift-off)

Substrate cleaning


Dose (C/cm2)

Fin

al T

hic

knes

s

Dexp

Dose (C/cm2)

Fin

al T

hic

knes

s

Dexp

Resists Positive Resist

• Solubility (in the developer) of the exposed regions is much higher than the regions not exposed

Negative Resist• Solubility (in the developer) of the exposed regions is much smallerthan the regions not exposed

Resists are generally organic materials with properties tailored for specific applications

Chemical composition is given basically by two components: array of resin (which servers as a binder

for the material) sensitizer (which provides adequate

sensitivity to exposure)

Positive Resist

Negative Resist

e-beam


Resists Characteristics of good resists

• Contrast ()• Sensibility (S)

• Etching resistance for transfer in substrates

• Resolution (R)

• Easy to manage: exposure, development, removing…

• Good adhesion properties

• Safer solvent

It is important to emphasize that these features are strongly influenced byprocess conditions (baking conditions, thickness, substrate, etc.). So, youhave to determine these characteristics for your sample and application.

A good exposure tool is essential for nanolithography but pay attention toother aspects, mainly the resist and its processing conditions. Please besure to characterize your processing to get the most from the tool!

Four main properties


Resists (contrast and sensitivity)

Log dose Log dose

PositiveNegative

pD

10

log

n

n

D

D

nD0nD

1

0log

p

p

D

D

pD0pD

Sensitivity (S)

Contrast (curve slope)

High contrast:vertical wallshigher process latitudehigher resolutionless sensible to proximity effects

Low contrast:"3D" lithography – grey level lithography - e.g. fresnel microlens fabrication

S S



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


Substrate Cleaning Wafer

• brand new: IPA 5min@ 80C

• If not, optical inspection in a microscope

Native oxide removal (p.ex. dip in BOE 5s para Si)

Cleaning with solvents: • Acetone or NMP PA 5min@ 80C +

IPA 5min@ 80C

If it has been used for other litho steps• Resist removal (Apprpriate resist remover / O2 Plasma)

• Acetone or NMP 5min@ 80C +

IPA 5min@ 80C

Always make a carefull microscope optical inspection!



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


Dehydration Bake

substrateSubstrate adsorbs water

easilyH2O

substrate

O O OH

H

(Hot Plate)

O

O HH

O

10min@ 200C 30min@ 200C

(Oven)

O

O HH

O



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


Adhesion Promoter

HMDS is widely used for Si

It can be applied:• In liquid form, diluted in solvent (20% HMDS em PGMEA), on a

dehydrated surface, (10~20s @1500rpm), immediately before applying the resist.

• In vapor form, in an oven(~ 35min@ 150C), in pure form. This form is much more efficient.

Caution: depending on the surface, the treatment may vary


Adhesion PromoterO

substrateSi substrate oxidizes

naturally

O

H2O

substrate

O O OH

H

substrateSi water absorption

O O OH

H

+H2O

OH

H

OH HO

H

H

HMDS (oven or spray/spinning)(Hexamethildisilazane)

CH3

CH3

CH3

Si N

H

CH3

CH3

Si CH3

H2O

N

H

CH3

CH3

Si CH3

H

CH3

CH3

CH3

Si O H +

substrate

N

H

H HCH3 CH3

CH3

Si

O



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


Resist spinning

Follow manufacturer recommendations (typically 30sec spinning)

The basic spinning characteristics depends on solvent evaporation rate.


Resist spinning

Samples with sizes less than 10mm don´t allow a uniform coverage!

• A uniform coverage has typically less than 10nm variation all over the useful area

• In GaAs thats important

• Try to avoid small samples



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


Prebake (a.k.a. softbake, PB or SB)

To evaporate remaining solvent from resist layer.

Follow manufacture´s recommendations, but typically 1min@ 90C in a hot plate or 30min@ 90C in an oven.

Thick resists (> 5m) requires that the wafer cools slowly to room temperature to avoid cracks



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


Exposure Process

STEP GRIDY

STEP GRID X

E-BEAMCURRENT DENSITY

IDEAL STEP GRID

< ½ E-BEAMDIAMETER

FWHM

(BEAMDIAMETER)

STEP GRID

E-BEAM FWHM DIAMETER

STEP GRID X

STEP GRID

Y

SCAN

SUBSTRATERESIST

FIELD

STRUCTURE DESIGN

FIELD

SUBFIELD

E-BEAM


Exposure Process – Current Influence

ADVANTAGES: Weak Coulomb interactions; High depth of focus;

DISADVANTAGES: Small exposure areas; Increases time of exposure; Difficulty of focusing (and correction of astigmatism); Increase noise/signal ratio.

ADVANTAGES: Short time of exposure; Large exposure areas;

DISADVANTAGES: Only when high resolution is not required.

Pitch ~ ½ electron beam diameter; Homogenous exposure; Limited by the digital-to-analog (DAC) board;

)2

(cm

(s)A)(μ)

2C/cm(μ

S

.tIDose


Resist contrast and sensitivity revisited

Log dose

Contrast and Sensitivity are process dependant!!!

Be sure to put in your design some process checking structures (they act like a fingerprint)

0

1Negative

10

log

n

n

D

D

nD0nD

PEB Time

Development time

SNR 200 (Chemical Amplifier Resist)



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


(Post-exposure bake, PEB)

For optical resists, it reduces standing waves phenomena

For chemically amplified resists it defines the crosslinking/scision rates

Conventional resists like PMMA don´t need this step



HardBake

DehydrationBake

AdhesionPromoter



Flash

Etching


Substrate cleaning


Development Always buy together the resist and its ancillaries

Alkaline solutions may be used but contains mobile ions that severaly degrades some electronic devices performance.

Try to use mobile ion free develorpers (MIF developers) like MF312 or 300-MIF, which are TMAH based and contain surfactants

PMMA developers in general don´t have this problem

Dilution is specific to each resist and even process. You may have to try a little bit.

Take care of overused developers!

Some developers may etch your sample! That´s common with aluminium


Resists (resolution) The ultimate resolution on a resist is set by:

• Electron scattering (both in resist and substrate);

• Chemical resist properties: molecular size, weight, chemical structureand reactivity to electrons. ADVANCED RESISTS

Thinner resist layer (as thin as possible for your application!)


Main Advanced Resists for Nanofabrication

Resist Tone Composition Molecularweight Thinner

PMMA positive Polymethylmethacrilate 495k-950k Chlorobenzene or anisole

ZEP 520 positive - 55000 o-dichlorobenzene

Calixarene negative

MC6AOAc(Hexaacetate p-

methylcalix[6]arene 972

o-dichlorobenzene or

monochlorobenzene

HSQ negativeHydrogen

Silsesquioxane 11000Methylisobutylketone

(MiBK)


Main Advanced Resists for Nanofabrication

Resist Tone Thickness (nm) Dose Resolution

(nm) Contrast

PMMA positive 30 100μC/cm2 at 25kV 10 >4

ZEP 520 positive 120 30μC/cm2 at 20kV 15 >4

Calixarene negative 30 7mC/cm2 at 50kV 7 2,5

HSQ negative 100 300µC/cm2 at 70kV 7 >5


Resists for Nanofabrication ... PMMA PMMA 150nm @ 20kV – 13pA:

positive tone

the most commonly positive resist used (very good shelf life, may be used several times)

sensitivity: ………. 200µC/cm2

resolution: ………. 20nm

minimum period: 80nm

lift-off

Drawbacks: Poor dry etching resistance (~30nm/min in CF4)

Advantages: Resolution – best resolution achieved 6nm

versatility - wide process latitude

“low” proximity effects

easy to remove

grating period 80nm Si etching depth: 150nmmask PMMA

Macanter et al.; Sub-35nm gratings fabricated using PMMA with high contrast developers, Microelectronic Engineering, 41/42(1998).


Resists for Nanofabrication...ZEP520A

ZEP 60nm @ 75kV – 30pA: positive tone



Drawbacks: Expensive

Advantages: Etch resistance: x2.5 PMMA

Track patterns with track pitch of 50nm, after RIE, fabricated on SiO2 substrate using ZEP 520.

Sbiaa et al.; Sub-50nm track pitch mold using electron beam lithography, J. Vac. Sci. Technol. B. 26(5), 2008.


Resists for Nanofabrication...calixarene Calixarene 30nm @ 50kV :

negative tone



Drawbacks: requires long exposure times

Extremely expensive

Advantages: high sensitivity resist

the small size of the molecule allows the

smoothness of sidewall patterning.

Grigorescu et al.; Review: Resist for sub-20nm electron beamlithography, with a focus on HSQ: state of the art, Nanotechnology20, 2009.

Dots having 15nm diameter and 35nm pitch were fabricated on Si substrate using calixarene.


Resists for Nanofabrication... HSQ HSQ 100nm @ 50kV – 1,1nA:

negative tone

inorganic resist



Drawbacks: expensive

Advantages: better dry etch resistance than PMMA

Schematic representation of the molecular structure ofHSQ: a-) cage structure for an eight-corner oligomer; b-)random structure of the resist solution.

Two 340nm optical wires separated by 60nm fabricatedon GaAs substrate with HSQ.

Lauvernier et al.; Optimization of HSQ resist e-beam processingtechnique on GaAs material, Microelectronic Engineering 75(2004).


Development Processes – different solvents

RESISTS THICKNESS

(nm)

DEVELOPER STOPPER RINSE CONTRAST SENSITIVITY

(C/cm2)

PMMA 70 MIBK:IPA (1:3)

for 30s

- IPA 4.3 100

70 IPA for 30s - - 6.1 250

ZEP-520 100 Xylene - 1:3

MIBK:IPA

4 -

100 hexyl acetate - IPA 5.3 -

HSQ 50 2,38% TMAH TMAH:H2O

(1:9)

H2O DI 5 -

200 2,5 % TMAH 3 123

200 25% TMAH 7 261

50 20% KOH 10 -


Development Processes – PMMARESIST THICKNESS

(nm)


(C/cm2)

PMMA 70 MIBK:IPA (1:3)

for 30s

- IPA 4.3 100

70 IPA for 30s - - 6.1 250

IPA: MIBK for 30 sec IPA for 5 sec

Jung et al., Enhanced Development Properties of IPA (isopropyl alcohol) on the PMMAelectron beam resist, Electronic Materials Letters, 3 (2007).


Development Processes – ZEP 520

hexyl acetate

RESIST THICKNESS

(nm)


(C/cm2)

ZEP-520 100 Xylene - 1:3

MIBK::IPA

4 -

100 hexyl acetate - IPA 5.3 -

xylene

Namatsu et al., Fabrication of sub-10nm silicon lines with minimum fluctuation, J. Vac. Sci. Technol.B13(4), 1995.


Developing at different temperatures – HSQ

30ºC 50ºC

TMAH developer

100nm equal lines/spaces

30nm equal lines/spaces

Chen et al., Effects of developing conditions on the contrast and sensitivity of hidrogen silsesquioxane,Microelectronic Engineering 83, 2006.


Development – Ultrasonic – HSQ

UAS – Ultrasonically assisted development

Without UAS With UASChen et al., Effects of developing conditions on the contrast and sensitivity of hidrogen silsesquioxane,Microelectronic Engineering 83, 2006.


Development Processes – ZEP 520UAS – Ultrasonically assisted development

Without UASWith UASDense arrays of resist nanostructures with 50 nm line/space in 145 nm thick ZEP-520positive resist

Lee et al., Ultrasonic and dip resist development processes for 50nm device fabrication, J. Vac. Sci. Technol. 1997.


Development Processes – ZEP 520UAS – Ultrasonically assisted developmenAlso helpful to remove a thin residue between lines

Without UAS With UASLee et al., Ultrasonic and dip resist development processes for 50nm device fabrication, J. Vac. Sci. Technol. 1997.


Trilayer Lift-off Process (PMMA/P(MMA)/ZEP)

Kim et al., Sub-100nm T-gate fabrication using a positive resist ZEP520/P(MMA-MAA)/PMMA trilayer by doubleexposure at 50kV e-beam lithography, Materials Science in Semiconductor Processing 7 (2004.).

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Advanced resists for e-beam lithography: processing ... · Advanced resists for e-beam lithography: processing, exposure and characterization (Part I) Dra. Mariana Pojar de Melo Prof