high aspect ratio micro/nanomachining with proton beam...

High aspect ratio micro/nano machining with proton beam writing

M. Chatzichristidi, E. Valamontes, N. Tsikrikas, P. Argitis, I. RaptisInstitute of Microelectronics, NCSR ‘Demokritos’ Athens, 15310 Greece

J.A.van KanEngineering Science Programme and Centre for Ion Beam Applications (CIBA), National University of Singapore, Singapore

F. Zhang, F. WattCentre for Ion Beam Applications (CIBA), Physics Dept. National University of Singapore, Singapore

[email protected], [email protected]

Proposed approach

Develop a resist formulation suitable for

� Formation of thick films

� High resolution structures

� High Aspect ratio

� High sensitivity

� Processing compatible with Standard Silicon processes

� Stripping with conventional stripping schemes

Develop a patterning technology suitable for

� Fast prototyping

� Thick polymer film structuring

� High resolution patterning

MNE 07 Copenhagen, Denmark25 September 2007

Presentation Outline

• Conventional HAR Patterning TechnologiesX-Ray lithography (XR-LIGA)I-line lithograpy (UV-LIGA)

• Proton Beam Writing (PBW)Principle of operationAdvantages & disadvantagesTypical results

• Typical HAR ResistsPMMASU-8

• TADEP resistFormulationPhysicochemical propertiesFormulation & processing optimizationLithographic resultsElectroplating

• PBW simulationSimulation of Proton beam – Matter interactionSimulation of the thick polymeric films patterning

• Conclusions








• Conclusions

UV-LIGA (typical literature results)


SEM photos of SU-8 microstructure with thickness of 400 µm. Linewidth: 10 µm

X. Tian, G. Liu, Y. Tian, P. Zhang, X. Zhang Micros. Techn. 11 265(2005)

SU-8 patterns with thickness 210 µm, width 10 µm Aspect ratio is 21.

J. Liu, B. Cai, J. Zhu, G. Ding, X. Zhao, C. Yang, D. ChenMicros. Tech. 10 265(2004)

X-Ray LIGA (typical literature results)


Three-level SU-8 structure with step heights of 300, 600 and 900 µm fabricated by X-ray lithography.

1500 µm tall SU-8 gears.

J. Hormes, J. Göttert, K. Lian, Y. Desta, L. Jian Nucl. Instr. Meth. B 199 332(2003)

Proton Beam Writing (PBW) (I)

Comparison between p-beam writing, FIB, and (c) e-beam writing. The p-beam and e-beam images were simulated using SRIM and CASINO software packages, respectively.

Schematic of the p-beam writing facility at CIBA. MeV protons are produced in a proton accelerator, and a demagnified image of the beam transmitted through an object aperture is focused onto the substrate material (resist) by means of a series of strong focusing magnetic quadrupole lenses. Beam scanning takes place using magnetic or electrostatic deflection before the focusing lenses.

F.Watt, M.B.H.Breese, A.A.Bettiol, J.A.van KanMaterials Today 10(6) 20(2007)


Proton Beam Writing (PBW) (II)

Typical Application areas� Photonics (waveguides, lens arrays, gratings, …)� Microfluidic devices, biostructures, and biochips (imprinting, …)� High resolution patterning (X-ray masks, …)� Porous Si (patterning, Distributed Bragg reflectors, …)

Advantages� Maskless process (ideal for prototyping)� Vertical side walls� Ultra high resolution� Ultra high aspect ratio pattering (provided the resist properties)

Disadvantages� Serial patterning process (moderate speed)� Limited penetration depth (50µm for 2MeV proton beam)


Proton Beam Writing (PBW) (III)

High aspect ratio test structures in SU-8 (60 nm wide, 10µm deep structures).

Parthenon’s copy with a reduction of 1 million times


J. A. van Kan, P. G. Shao, K. Ansari, A. A. Bettiol, T. Osipowicz, F. Watt,

Microsyst Technol 13 431(2007)

Second exposureFirst exposure

I.Rajta, M.Chatzichristidi, E.Baradács, I.Raptis, Nucl. Instrum. Meth. B 260 414(2007)

Side view of the “lambda” structures.

Typical resists for HAR structures

Positive resistsPositive resists

PMMANovolak-diazonaphtoquinone resist platform

Negative resistsNegative resists

epoxy based, chemically amplified SU-8


PMMA (I)

Advantages

High resolutionStripping with conventional stripping schemes ( e.g. acetone)

Disadvantages

Limitation in the film thickness obtained by spin coatingLow sensitivity

Development in organic solvents (MIBK or MIBK/IPA)


PMMA (II)

PMMA


H2C C

CH3

C O

O

CH3

n

Chain scission mechanism upon exposure

H2C C

CH3

C

C

O

OCH3

CH3

C O

OCH3

H2C

n

hv H2C C

CH3

C

CH3

C O

OCH3

H2C

n. + C O

OCH3

CH2

C

CH3

CH2 + C

CH3

C O

OCH3

. further reactions

.

Cleavage of methyl ester group

Chain scission

evolution of gaseous, volatile products: CO, CO2,

.CH3, CH3O..

Chain scission

Further reactions

Evolution of gaseous,Volatile products:CO, CO2, CH3,CH3O

.

Cleavage of methyl ester group

PMMA thickness: 350 nm.Proton beam: 2MeVStructure’s width: 50nm

F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. van Kan Materials Today 10(6) 20(2007)

SEM image of 5-µm thick PMMA by PBW at 1.7 MeV (fluence: 100 nC/mm2) with writing patterns of dots (1.25µm diameter).

N. Uchiya, T. Harada, M. Murai,H. Nishikawa, J. Haga, T. Sato, Y. Ishii, T. Kamiya, Nucl. Instr. and Meth. in Phys. Res. B 260405(2007)

a)

SU-8 resist (I)

Advantages

� High sensitivity

� Thick films by spin coating

Disadvantages

� Development in organic solvent

� Very difficult stripping by conventional

stripping schemes (needs piranha etch,

plasma ash e.t.c.)

www.microchem.com MNE 07 Copenhagen, Denmark25 September 2007

Absorption

SU-8 resist (II)

CH2CH

OH2CO

C CH3H3C

OCH2C

H

OCH2

CH2CH

OH2CO

C CH3H3C

OCH2C

H

OCH2

H2C

H2C

CH2CH

OH2CO

C CH3H3C

OCH2C

H

OCH2

H2C

CH2CH

OH2CO

C CH3H3C

OCH2C

H

OCH2

S+

S

Sb

F

F

F

FF

F

Chemical formulation of SU-8 resist

Crosslinking mechanism

+

OPEN OF EPOXY RINGS AND BONDING

SU-8 resist (characteristic PBW results)

1 mm2 area of single pixel irradiation on SU-8 (top view).


I.Rajta, M.Chatzichristidi, E.Baradács, I.Raptis, Nucl. Instrum. Meth. B 260

414(2007)

Side view of irradiation on SU-8.130 nm wide lines, 15 aspect ratio

J.A. van Kan, P.G. Shao, K. Ansari, A.A. Bettiol, T. Osipowicz F. Watt, Microsyst. Technol. 13 431(2007)







• Conclusions

TADEP formulation

Goal:

Develop a resist formulation suitable for•Formation of thick films•High resolution structures•High Aspect ratio•High sensitivity•Processing compatible with Standard Silicon process•Stripping with conventional stripping schemes


Thick Aqueous Developable EPoxy resist

TADEP formulation

[ CH

OH

CH2[ ]CH

OH

n CH2 ]m

Partially Hydrogenated PHSfrom Maruzen Co.

[

O

O

Epoxy novolacFractionation of EPICOTE 164 from Shell

S+

Sb

F

F

F

FF

F

Triphenyl Sulfonium HexafluoroAntimonate (TPS-SbF6)

S

CH3

OH+

S O

O

O

FF

F

1-(4-hydroxy-3-methylphenyl) tetrahydrothiophenium triflate(o-CS-triflate)


TADEP Crosslinking mechanism

Exposed Resist Areas: Insoluble in aqueous base developers

Unexposed Resist Areas: Soluble in aqueous base developers

H +, heating

OHOH OH OH OH OH OH OH

OHOHOHOHOHOH

* *

OH OH

n

O

O

*O

*

O

n


OHOHOHOHOHOH

* *

OH OH

n

OHOH OH OH OH OH OH

OHOHOHOHOHOH

* *

OH OH

n

O

O

*O

*

O

n

OH OHO


OHOHOHOH

* *

OH OH

n

H


CH

O

R1

H

CH2

O+ H

R2

CH

O

R1

H

CH2

O+ H

R2

CH

O

R1

H

CH2

O

R2

+

CH2CH

OR1

H+SbF6-

CH2CH

O+

R1

H

R2 OH

CH2CH

O+

R1

H

H+SbF6- Lewis acid

regeneration

Crosslinking

Thickness- Absorption data

It is shown that at 365nm, where the film is exposed, the TPS-antimonate does not absorb whereas o-Cs triflate absorbs

slightly.

55µm thick film can be achieved with one spinning


300 320 340 360 380 4000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Abs

orba

nce

(A)

Wavelenght (nm)

o-Cs Triflate TPS Antimonate

365 nm

PAG’s UV spectraTADEP film thickness (after PAB) vs. spinning speed

500 1000 1500 2000 2500 3000 3500 4000 4500 50000

10

20

30

40

50

60

45% TADEP 30% TADEP

Film

Th

ickn

ess

( µµ µµm

)

Spinning speed (rpm)

Glass transition temperature studies


23.5°C (H )0.2129J/g /°C

108.6°C (I)

88 .74°C (I)

95.75°C (I)

38.51°C (I)

134.74°C (I)

-0 .4

-0 .3

-0 .2

-0 .1

0 .0

0 .1

0 .2

0 .3

0 .4

Rev

Hea

t Flo

w (

W/g

)

0 50 100 150 200 250 300

Temperature (°C )Exo U p U niversa l V3 .9A TA Ins trum ents

Epoxy

PHS

PHS-EP

PHS-EP TPS Ant. unexposed

PHS-EP TPS Ant. exposed

PHS-EP o-Cs trifl. unexposed PHS-EP o-Cs trifl.

exposed

SU-8

The resist components are fully miscibleThe exposed areas are crosslinked and show no Tg

Dissolution study (PAG molecule)

In all cases dissolution proceeds linearly with time. NO swelling effect is observed except in the UVI case which is very

hydrophobic. In the SU-8 resist case, the development is abrupt (impossible to monitor).

Film thickness: ~2µmDeveloper: AZ-726 (AZ-EM) 0.26N TMAH


0 60 120 180 240 300 3600

300

600

900

1200

1500

1800

2100 no PAGo-Cs Triflate 3.4%TPS Antimonate 5%TPS Triflate 4%UVI 6%TPS Ant. 2%TPS Ant. 3,2% o-Cs1.1%

Thi

ckne

ss (

nm)

Development time (sec)

Concept: Controlled development process

Tool:White Light Reflectance Spectroscopy DRM

P-RES-4 “Processing effects on the dissolution properties of thin chemically amplified photoresist films”

PAB study

Solvent evaporation during the PAB step. Most of the solvent evaporates during the first 10min (heat up from RT to 95oC). During

the rest period (4h) ~0.8µm of resist thinning is observed.


50 100 150 200

0.76

0.78

0.80

0.82

0.84

0.86

0.88

0.90

30

40

50

60

70

80

90

100

Interference signal

Inte

refe

renc

e S

igna

l (a.

u.)

Time (min)

Temperature

Tem

pera

ture

(o C

)

Detector

θ2

n1θ1

n2 z1resist

n3substrate

Laser

Operating principle

Single Wavelength Interferometer Set-up

PEB study


a

The increased PEB temperature helps significantly the crosslinking reaction

b

100°C for 8 minexposure dose (238 nC/mm2)

110°C for 8 minexposure dose (116 nC/mm2)

Ι. Rajta, E. Baradacs, M. Chatzichristidi, E.S. Valamontes, I. Raptis, Nucl. Inst. Meth. B, 231 423 (2005)

Preliminary Lithographic evaluation (I)

Thickness = 35 µm Layout = 5 µm L/SLinewidth = 5 µm


Thickness = 11 µm Aspect Ratio = 7

Substrate: Silicon wafer with plating base (Au surface)

UV-LIGA

I-line lithography (MJB3 Karl-Suss)

Preliminary Lithographic evaluation (II)SINGLE PIXEL AND SINGLE LINE EXPOSURES

The achieved smallest feature size was the same as the measured beam spot size (limited by the proton beam dimensions). The highest aspect ratio for this type of structures was 7 (for the selected film thickness).

Side view of single pixel irradiation on TADEP.Resolution 2.8µmAspect ratio 7

Top view of single line irradiation on TADEP.

Beam size: ~ 3X3µm

I.Rajta, M.Chatzichristidi, E.Baradács, I.Raptis, Nucl. Instrum. Meth. B 260 414(2007)

MNE 07 Copenhagen, Denmark 25 Sept. 2007

Lithographic Evaluation using fine p-beam

Top view of PBW double line irradiation on TADEP resist. Line width ~110nm in X-direction.Resist thickness ~2.0 µm Aspect ratio: 18Beam size: ~100X200nmTwo pixels pass line

Pitch: 1µm, 4µm

110nm

Lithographic Evaluation (III)


Side view of dense lines by 2MeV PBW on TADEP resist.Resist thickness ~11.0 µm Beam size: ~100X200nmTwo pixels pass linePitch: 1µm, 4µm

Lithographic Evaluation (IV)

Side view of the pattern. Resist thickness 12µm, aspect ratio: 42.Beam size: 200nm in X-direction

Top view of PBW double line irradiation on TADEP resist. Line width 280nm in X-direction.


Electroplating results


Side view of Ni plated on gold features.Ni thickness 0.8µm

On the right side the resist pattern

167nm







• Conclusions

PBW simulation flow-chart


Point proton beam simulation

Convolution with Gaussian Profile

Realistic proton beam simulation

Thermal processing,PEB simulation (only for CARs*)

* P-RES5 “Stochastic simulation studies of molecular resists for the 32nm technology node”

Convolution with Layout

Layout

(top view)

0 500 1000 1500 2000107

108

109

1010

1011

1012

1013

1014

1015

Ene

rgy

(KeV

/cm

3 *ele

ctro

n)

Radial distance (nm)

Point Beam Energy Deposition

Point Beam exposure

Energy deposition on resist Energy deposition(cross section)

Development simulation

Resist Profile (development)

Proton Beam – matter interaction simulation

� The formalism adopted for simulating protons propagation is that of TRIM / SRIM [1]. At high energies, we have decided, for the sake of the computer efficiency, to base the calculations on the Coulomb potential [2].

� Stopping powers at high energies were calculated according to Bethe’s theory

� At low energies, electronic stopping powers were obtained from experimental data, closely related to the empirical fitting formulas developed by Andersen and Ziegler.

� The nuclear stopping power, which is important only at very low energies, was obtained by the method of Everhart et al [3].

[1] J. Biersack, L. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257.[2] J. F. Ziegler, J. P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, The Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon Press Inc., 1985.[3] Stopping Powers and Ranges for Protons and Alpha Particles, International Commission on Radiation Units and Measurements, Report 49, 1993.

PBW simulation results (energy deposition)


0 5 10 15 20 25 30 35 40 45 50 55 60 650

2

4

6

8

10

12

14

Ene

rgy

depo

sitio

n (e

V/Α

)

Depth (µm)

Bulk PMMA 10umPMMA/Si 20umPMMA/Si 30umPMMA/Si

0 5 10 15 20 25 30 35 40 45 50 55 60 65 700

2

4

6

8

10

12

Ene

rgy

depo

sitio

n (e

V/Α

)

Depth (µm)

Literature SRIM Bulk PMMA

Monte-Carlo (MC) simulation: Energy deposition vs. depth for various resist films.

Proton beam: 2MeV

Simulation Dz=50nm

Comparison of the MC simulation results in bulk with the literature and SRIM

Proton beam: 2MeV

Simulation Dz=50nm

Si substratePMMA

PBW simulation results (convolution)


Energy deposition due to point beam and gaussian proton beam at the resist (10µm)/substrate interface (Beam diameter 100nm)

0 500 1000 1500 2000107

108

109

1010

1011

1012

1013

1014

1015

Ene

rgy

(KeV

/cm

3 *ele

ctro

n)

Radial distance (nm)

Convoluted Energy Deposition Point Beam Energy Deposition

PBW simulation results (layout level)


Proton beam irradiated

Unexposed area

Top view (layout)

Si wafer Si wafer

Threshold: 0.01 x G Threshold: 0.1 x G

Energy deposition(cross section)

Cross-sectionafter development

(simulation)

200nm lines / 400nm spaces

Proton Beam Diameter 100nm

Film Thickness 10µm

Conclusions

Proton Beam Writing proved a powerful micro/nano machining tool, resolution depending on the beam size.

The strippable, aqueous base developable TADEP resist proved adequate for high aspect ratio nanomachining

Dense features with vertical & smooth sidewalls were revealed:Thickness 2µµµµm, Linewidth 110nm, Aspect ratio 18Thickness 12µµµµm, Linewidth 280nm, Aspect Ratio 42

Successful Ni electroplating is showed demonstrating the stripping capability of TADEP resist

Simulation software for proton beam writing is under development


Acknowledgments

The work was financially supported by the PB.NANOCOMP project (funded by the Greek Secretariat for Research & Technology contract number 05-NONEU-467).


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