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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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 3
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 4
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 5
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 6
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 7
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 8
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 9
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!
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 10
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 11
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 12
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 13
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 14
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 15
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 16
Resist spinning
Follow manufacturer recommendations (typically 30sec spinning)
The basic spinning characteristics depends on solvent evaporation rate.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 17
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 18
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 19
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 20
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 21
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 22
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 23
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)
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 24
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 25
(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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 26
Processing Steps for Lithography
HardBake
DehydrationBake
AdhesionPromoter
ResistSpinning Pre-bake
ExposurePost-exposureBakeDevelopmentPlasma
Flash
Etching
Deposition(Lift-off)
Substrate cleaning
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 27
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 28
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!)
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 29
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)
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 30
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
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 31
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).
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 32
Resists for Nanofabrication...ZEP520A
ZEP 60nm @ 75kV – 30pA: positive tone
sensitivity: ………. 256µC/cm2
resolution: ………. 40nm
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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 33
Resists for Nanofabrication...calixarene Calixarene 30nm @ 50kV :
negative tone
sensitivity: ………. 64µC/cm2
resolution: ………. 15nm
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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 34
Resists for Nanofabrication... HSQ HSQ 100nm @ 50kV – 1,1nA:
negative tone
inorganic resist
sensitivity: ………. 450µC/cm2
resolution: ………. 60nm
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).
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 35
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 -
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 36
Development Processes – PMMARESIST 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
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).
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 37
Development Processes – ZEP 520
hexyl acetate
RESIST THICKNESS
(nm)
DEVELOPER STOPPER RINSE CONTRAST SENSITIVITY
(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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 38
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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 39
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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 40
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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 41
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.
M. Pojar & A.C.Seabra Advanced resists for e-beam lithography 03/2015 42
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.).