High aspect ratio micro/nano machining with proton beam writing

High aspect ratio micro/nano machining 

with proton beam writing 

M. Chatzichristidi, E. Valamontes, N. Tsikrikas, P. Argitis, 

I. Raptis 

Institute of Microelectronics, NCSR ‘Demokritos’ Athens, 15310 

Greece 

J.A.van Kan 

Engineering Science Programme and Centre for Ion Beam Applications 

(CIBA), National University of Singapore, Singapore 

F. Zhang, F. Watt 

Centre for Ion Beam Applications (CIBA), Physics Dept. National 

University of Singapore, Singapore 

mchatz@imel.demokritos.gr, raptis@imel.demokritos.gr

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, Denmark 

25 September 2007

Presentation Outline 

• Conventional HAR Patterning Technologies 

X-Ray lithography (XR-LIGA) 

I-line lithograpy (UV-LIGA) 

• Proton Beam Writing (PBW) 

Principle of operation 

Advantages & disadvantages 

Typical results 

• Typical HAR Resists 

PMMA 

SU-8 

• TADEP resist 

Formulation 

Physicochemical properties 

Formulation & processing optimization 

Lithographic results 

Electroplating 

• PBW simulation 

Simulation of Proton beam – Matter interaction 

Simulation of the thick polymeric films patterning 

• Conclusions 


25 September 2007










PMMA 

SU-8 


Formulation 








• 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) 

J. Liu, B. Cai, J. Zhu, G. Ding, X. Zhao, C. Yang, D. Chen 

Micros. Tech. 10 265(2004) 

SU-8 patterns with 

thickness 210 µm, width 10 µm 

Aspect ratio is 21. 


25 September 2007

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. 

J. Hormes, J. Göttert, K. Lian, Y. Desta, L. 

Jian Nucl. Instr. Meth. B 199 332(2003) 

1500 µm tall SU-8 gears. 


25 September 2007

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. 

F.Watt, M.B.H.Breese, A.A.Bettiol, J.A.van Kan 

Materials Today 10(6) 20(2007) 

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. 


25 September 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) 


25 September 2007

Proton Beam Writing (PBW) (III) 

High aspect ratio test 

structures in SU-8 (60 nm 

wide, 10µm deep structures). 

First exposure 

Parthenon’s copy with a 

reduction of 1 million times 

Second exposure 

I.Rajta, M.Chatzichristidi, E.Baradács, I.Raptis, 

Nucl. Instrum. Meth. B 260 414(2007) 

J. A. van Kan, P. G. Shao, 

K. Ansari, A. A. Bettiol, 

T. Osipowicz, F. Watt, 

Microsyst Technol 13 431(2007) 

Side view of the 

“lambda” structures. 


25 September 2007

Typical resists for HAR structures 

Positive resists 

PMMA 

Novolak-diazonaphtoquinone resist platform 

Negative resists 

epoxy based, chemically amplified SU-8 


25 September 2007

PMMA (I) 

Advantages 

High resolution 

Stripping with conventional stripping schemes ( e.g. acetone) 

Disadvantages 

Limitation in the film thickness obtained by spin coating 

Low sensitivity 

Development in organic solvents (MIBK or MIBK/IPA) 


25 September 2007

CH3 H2 C C 

n 

C O 

O 

CH 3 

PMMA 

PMMA thickness: 350 nm. 

Proton beam: 2MeV 

Structure’s width: 50nm 

PMMA (II) 

CH 3 

H 2 

C C 

C 

O 

OCH 3 

F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. 

van Kan Materials Today 10(6) 20(2007) 

a) 

H 2 

C 

CH 3 

C 

C O 

OCH 3 

C 

H 2 

n 

CH 3 

C 

Chain Chain scission 

Cleavage Cleavage of methyl of methyl ester group 

CH 3 

hv H2 C C 

CH 3 

. 

Chain scission mechanism upon exposure 

H 2 

C 

CH 3 

C 

C O 

OCH 3 

CH2 + . C further reactions 

C O 

OCH 3 

n 

+ 

Further reactions 

. 

C O 

OCH 3 

evolution of gaseous, 

volatile products: 

CO, CO2, . CH3, CH3O . Evolution of gaseous, 

Volatile products: 

CO, CO . 

2 , CH3 ,CH3O . 

SEM image of 5-μm thick PMMA by PBW 

at 1.7 MeV (fluence: 100 nC/mm 2 ) 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) 


25 September 2007

Advantages 

High sensitivity 

Thick films by spin coating 

Disadvantages 

Development in organic solvent 

SU-8 resist (I) 

Very difficult stripping by conventional 

stripping schemes (needs piranha etch, 

plasma ash e.t.c.) 

Absorption 

www.microchem.com MNE 07 Copenhagen, Denmark 

25 September 2007

H 3C 

H 2C 

O 

O 

C 

H 

C CH 3 

O 

C 

H 2 

CH 2 

O 

C 

H 

H 2 

C 

CH 2 

H 3C 

H 2C 

O 

O 

C 

H 

C CH 3 

O 

CH 2 

O 

C C 

H2 H 

H 2 

C 

CH 2 

SU-8 resist (II) 

Chemical formulation of SU-8 resist 

H 3C 

H 2C 

O 

O 

C 

H 

C CH 3 

O 

C 

H 2 

CH 2 

O 

C 

H 

H 2 

C 

CH 2 

H 3C 

H 2C 

O 

O 

C 

H 

C CH 3 

O 

O 

C C 

H2 H 

CH 2 

CH 2 

Crosslinking mechanism 

+ 

S + 

S 

F 

F 

F Sb F 

F F 

OPEN OF EPOXY RINGS 

AND BONDING

SU-8 resist (characteristic PBW results) 

1 mm 2 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) 


25 September 2007










PMMA 

SU-8 


Formulation 








• Conclusions

Thick Aqueous Developable EPoxy resist 

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 


25 September 2007

[ CH CH2 ] 

OH 

TADEP formulation 

[ CH 

n CH2 ] m 

OH 

Partially Hydrogenated PHS 

from Maruzen Co. 

S + 

F 

F 

F Sb 

F F 

F 

Triphenyl Sulfonium Hexafluoro 

Antimonate (TPS-SbF 6 ) 

[ 

O 

Epoxy novolac 

Fractionation of EPICOTE 164 from Shell 

O 

F 

+ 

S 

O 

S O 

F 

F 

CH 3 

OH 

1-(4-hydroxy-3-methylphenyl) 

tetrahydrothiophenium triflate 

(o-CS-triflate) 


25 September 2007 

O

TADEP Crosslinking mechanism 

OH OH OH OH OH OH OH OH 

* * 

n 

OH OH 


* * 

n 

OH OH 

OH 

OH 

* 

O 

O 

OH 

OH 

O 

OH 

O 

Unexposed Resist Areas: Soluble in 

aqueous base developers 

R 1 

O 

C 

H 

Crosslinking 

CH 2 

OH 

R 1 

n* 

CH 

OH 

OH 

H + SbF 6 - 

H 

O 

CH 2 

O + H 

R 2 

OH 

OH 

OH 

OH 

R 1 

H 

O 

CH2 + 

C 

H 

H + , heating 

R 1 

CH 

H 

O 

CH 2 

O 

R 2 


* * 

n 

R 2 

OH OH 

OH OH 

HO 

O 

OH OH OH OH OH 

* * 

n 

+ 

OH 

OH OH 

OH OH O OH 

* 

OH 

O 

OH 

O 

n* 

Exposed Resist Areas: Insoluble in 

aqueous base developers 

R 1 

C 

H 

H 

H + SbF 6 - 

O 

CH2 + 

Lewis acid 

regeneration 

OH 

R 1 

OH 

OH 

CH 

H 

O 

CH 2 

OH 

O + H 

R 2 

OH 

OH 

OH 


25 September 2007

Absorbance (A) 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

PAG’s UV spectra 

Thickness- Absorption data 

0.0 

300 320 340 360 380 400 

Wavelenght (nm) 

o-Cs Triflate 

TPS Antimonate 

365 nm 

It is shown that at 365nm, where 

the film is exposed, the TPSantimonate 

does not absorb 

whereas o-Cs triflate absorbs 

slightly. 

Film Thickness (μm) 

TADEP film thickness (after PAB) 

vs. spinning speed 

60 

50 

40 

30 

20 

10 

0 

45% TADEP 

30% TADEP 

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 

Spinning speed (rpm) 

55µm thick film can be achieved 

with one spinning 


25 September 2007

Rev Heat Flow (W/g) 

0 .4 

0 .3 

0 .2 

0 .1 

0 .0 

-0 .1 

-0 .2 

-0 .3 

-0 .4 

Glass transition temperature studies 

23.5°C (H ) 

0.2129J/g/°C 

38.51°C (I) 

108.6°C (I) 

95.75°C (I) 

88.74°C (I) 

Epoxy 

134.74°C (I) 

0 50 1 00 1 50 2 0 0 2 50 30 0 

Exo U p Tem perature (°C ) 

U niversal V3.9A TA Instrum ents 

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 miscible 

The exposed areas are crosslinked and show no Tg 


25 September 2007

Dissolution study (PAG molecule) 

Concept: Controlled development 

process 

Tool: White Light Reflectance 

Spectroscopy DRM 

Thickness (nm) 

0 

0 60 120 180 240 300 360 

Film thickness: ~2μm 

Developer: AZ-726 (AZ-EM) 0.26N TMAH 

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). 

2100 

1800 

1500 

1200 

900 

600 

300 

Development time (sec) 

P-RES-4 “Processing effects on the dissolution properties of 

thin chemically amplified photoresist films” 

no PAG 

o-Cs Triflate 3.4% 

TPS Antimonate 5% 

TPS Triflate 4% 

UVI 6% 

TPS Ant. 2% 

TPS Ant. 3,2% o-Cs1.1% 


25 September 2007

Laser 

n 1 

θ 1 

Detector 

n 2 z1 

resist 

n 3 

substrate 

θ 2 

Operating principle 

Single Wavelength 

Interferometer Set-up 

PAB study 

Intereference Signal (a.u.) 

0.90 

0.88 

0.86 

0.84 

0.82 

0.80 

0.78 

0.76 

Temperature 

Interference signal 

50 100 150 200 

Time (min) 

Solvent evaporation during the PAB step. Most of the solvent 

evaporates during the first 10min (heat up from RT to 95 o C). During 

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

100 

90 

80 

70 

60 

50 

40 

30 

Temperature ( o C) 


25 September 2007

a 

100°C for 8 min 

exposure dose (238 nC/mm 2 ) 

PEB study 

The increased PEB temperature helps 

significantly the crosslinking reaction 

b 

110°C for 8 min 

exposure dose (116 nC/mm 2 ) 

Ι. Rajta, E. Baradacs, M. Chatzichristidi, E.S. Valamontes, I. Raptis, 

Nucl. Inst. Meth. B, 231 423 (2005) 


25 September 2007

Preliminary Lithographic evaluation (I) 

UV-LIGA 

Thickness = 11 µm 

Aspect Ratio = 7 

Substrate: Silicon wafer with plating base (Au surface) 

I-line lithography (MJB3 Karl-Suss) 

Thickness = 35 µm 

Layout = 5 µm L/S 

Linewidth = 5 µm 


25 September 2007

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 2.8µm 

Aspect 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) 


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: 18 

Beam size: ~100X200nm 

Two 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: ~100X200nm 

Two pixels pass line 

Pitch: 1µm, 4µm 


25 September 2007

Lithographic Evaluation (IV) 

Top view of PBW double line 

irradiation on TADEP resist. Line 

width 280nm in X-direction. 

Side view of the pattern. Resist 

thickness 12µm, aspect ratio: 42. 

Beam size: 200nm in X-direction 


25 September 2007

Electroplating results 

167nm 

Side view of Ni plated on gold features. 

Ni thickness 0.8µm 

On the right side the resist pattern 


25 September 2007










PMMA 

SU-8 


Formulation 








• Conclusions

PBW simulation flow-chart 

Point proton beam simulation 

Convolution with Gaussian Profile 

Realistic proton beam simulation 

Convolution with Layout 

Thermal processing, 

PEB simulation (only for CARs*) 

* P-RES5 “Stochastic simulation studies of molecular 

resists for the 32nm technology node” 

Energy (KeV/cm 3 *electron) 

10 15 

10 14 

10 13 

10 12 

10 11 

10 10 

10 9 

10 8 

10 7 

Point Beam Energy Deposition 

0 500 1000 1500 2000 

Radial distance (nm) 

Point Beam 

exposure 

Layout 

(top view) 

Energy deposition on resist Energy deposition 

(cross section) 

Development simulation 

Resist Profile 

(development) 


25 September 2007

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.

Energy deposition (eV/Α) 

PBW simulation results (energy deposition) 

14 

12 

10 

8 

6 

4 

2 

Bulk PMMA 

10umPMMA/Si 

20umPMMA/Si 

30umPMMA/Si 

0 

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

Depth (μm) 

Monte-Carlo (MC) simulation: 

Energy deposition vs. depth for 

various resist films. 


Simulation Dz=50nm 

PMMA 

Si substrate 

Energy deposition (eV/Α) 

12 

10 

8 

6 

4 

2 

Literature 

SRIM 

Bulk PMMA 

0 

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 

Depth (μm) 

Comparison of the MC simulation 

results in bulk with the literature 

and SRIM 


Simulation Dz=50nm 


25 September 2007

PBW simulation results (convolution) 

Energy (KeV/cm 3 *electron) 

10 15 

10 14 

10 13 

10 12 

10 11 

10 10 

10 9 

10 8 

10 7 

Convoluted Energy Deposition 

Point Beam Energy Deposition 

0 500 1000 1500 2000 

Radial distance (nm) 

Energy deposition due to point beam and gaussian proton beam at 

the resist (10μm)/substrate interface (Beam diameter 100nm) 


25 September 2007

PBW simulation results (layout level) 

Proton beam irradiated 

Unexposed area 

200nm lines / 400nm spaces 

Proton Beam Diameter 100nm 

Film Thickness 10µm 

Top view 

(layout) 

Si wafer Si wafer 

Threshold: 0.01 x G Threshold: 0.1 x G 

Energy deposition 

(cross section) 

Cross-section 

after development 

(simulation) 


25 September 2007

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 18 

Thickness 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 


25 September 2007

Acknowledgments 

The work was financially supported by the 

PB.NANOCOMP project (funded by the Greek 

Secretariat for Research & Technology contract 

number 05-NONEU-467). 

THANK YOU FOR YOUR ATTENTION 


25 September 2007

High aspect ratio micro/nano machining with proton beam writing

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