High aspect ratio micro/nano machining with proton beam writing

High aspect ratio micro/nano machining with proton beam writing M. hatzichristidi, E. Valamontes, N. Tsikrikas, P. Argitis, I. Raptis Institute of Microelectronics, NSR Demokritos Athens, 15310 Greece J.A.van Kan Engineering Science Programme and entre for Ion Beam Applications (IBA), National University of Singapore, Singapore F. Zhang, F. Watt entre for Ion Beam Applications (IBA), 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

Presentation utline onventional 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 onclusions

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. ai, J. Zhu, G. Ding, X. Zhao,. Yang, D. hen Micros. Tech. 10 265(2004) SU-8 patterns with thickness 210 µm, width 10 µm Aspect ratio is 21.

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) omparison between p-beam writing, FIB, and (c) e-beam writing. The p-beam and e-beam images were simulated using SRIM and ASIN 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 IBA. 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.

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) J. A. van Kan, P. G. Shao, K. Ansari, A. A. Bettiol, T. sipowicz, F. Watt, Microsyst Technol 13 431(2007) High aspect ratio test structures in SU-8 (60 nm wide, 10µm deep structures). Parthenon s copy with a reduction of 1 million times Side view of the lambda structures. First exposure Second exposure I.Rajta, M.hatzichristidi, E.Baradács, I.Raptis, Nucl. Instrum. Meth. B 260 414(2007)

Typical resists for HAR structures Positive resists PMMA Novolak-diazonaphtoquinone resist platform Negative resists epoxy based, chemically amplified SU-8

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)

PMMA (II) H 2 H 3 n H 3 H 2 H 3 H 2 H 3 H 3 H 3 n H 3 leavage leavage of methyl of ester group hv H 2 H 3 H 3 H 2 H 3 +. n H 3. H 3 evolution of gaseous, volatile products:, 2,. H 3, H 3.. Evolution of gaseous, Volatile products:, 2, H 3,H 3. PMMA H 2 hain hain scission H 2 +. further reactions H 3 Further reactions hain scission mechanism upon exposure PMMA thickness: 350 nm. Proton beam: 2MeV Structure s width: 50nm F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. van Kan Materials Today 10(6) 20(2007) a) SEM image of 5-µm thick PMMA by PBW at 1.7 MeV (fluence: 100 n/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)

SU-8 resist (I) Absorption 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

SU-8 resist (II) hemical formulation of SU-8 resist H 2 H H 2 H 2 H 2 H H 2 H 2 H 2 H H 2 H 2 H 2 H 2 H H 3 H 3 H 3 H 3 H 3 H 3 H 3 H 3 S + S H 2 H 2 H H 2 H H 2 H 2 H 2 H H 2 H H 2 F F F Sb F F F rosslinking mechanism + PEN F EPXY RINGS AND BNDING

SU-8 resist (characteristic PBW results) 1 mm 2 area of single pixel irradiation on SU-8 (top view). Side view of irradiation on SU-8. 130 nm wide lines, 15 aspect ratio I.Rajta, M.hatzichristidi, E.Baradács, I.Raptis, Nucl. Instrum. Meth. B 260 414(2007) J.A. van Kan, P.G. Shao, K. Ansari, A.A. Bettiol, T. sipowicz F. Watt, Microsyst. Technol. 13 431(2007)

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

TADEP formulation [ H H 2 ] [ H n H2 ] m [ H H Partially Hydrogenated PHS from Maruzen o. Epoxy novolac Fractionation of EPITE 164 from Shell + S H 3 H S + F F F Sb F F F Triphenyl Sulfonium Hexafluoro Antimonate (TPS-SbF 6 ) F S F F 1-(4-hydroxy-3-methylphenyl) tetrahydrothiophenium triflate (o-s-triflate)

TADEP rosslinking mechanism H H H H H H H H * * n H H H H H H H H * * n H H H H H H H H H H H H H H H H * H H H H H H H H n* H +, heating * n* H H H H H H H H * * n * * n H H H H H H H H H H H H H H H H Unexposed Resist Areas: Soluble in aqueous base developers H + SbF 6 R 1 H 2 H H rosslinking R 1 H H 2 + R 2 H - R 1 H H + H 2 R 1 H H H 2 R 2 R 2 + H Exposed Resist Areas: Insoluble in aqueous base developers R 1 H H + H + SbF 6 - H 2 Lewis acid regeneration R 1 H H H 2 + R 2 H

Thickness- Absorption data 0.8 PAG s UV spectra TADEP film thickness (after PAB) vs. spinning speed 60 Absorbance (A) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 o-s Triflate TPS Antimonate 365 nm Film Thickness (µm) 50 40 30 20 10 45% TADEP 30% TADEP 0.0 300 320 340 360 380 400 Wavelenght (nm) 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Spinning speed (rpm) It is shown that at 365nm, where the film is exposed, the TPSantimonate does not absorb whereas o-s triflate absorbs slightly. 55µm thick film can be achieved with one spinning

Glass transition temperature studies 0.4 0.3 23.5 (H ) 0.2129J/g/ Epoxy 0.2 134.74 (I) 108.6 (I) PHS Rev Heat Flow (W/g) 0.1 0.0-0.1-0.2-0.3 38.51 (I) 95.75 (I) 88.74 (I) PHS-EP o-s trifl. unexposed SU-8 PHS-EP PHS-EP TPS Ant. unexposed PHS-EP TPS Ant. exposed PHS-EP o-s trifl. exposed -0.4 Exo U p 0 50 1 00 1 50 2 0 0 2 50 30 0 Tem perature ( ) U niversal V3.9A TA Instrum ents The resist components are fully miscible The exposed areas are crosslinked and show no Tg

Dissolution study (PAG molecule) oncept: ontrolled development process Tool: White Light Reflectance Spectroscopy DRM Thickness (nm) 2100 no PAG o-s Triflate 3.4% 1800 TPS Antimonate 5% TPS Triflate 4% UVI 6% 1500 TPS Ant. 2% TPS Ant. 3,2% o-s1.1% 1200 900 600 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. N 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). 300 Development time (sec) P-RES-4 Processing effects on the dissolution properties of thin chemically amplified photoresist films

PAB study Laser Detector 0.90 100 n 1 θ 1 n 2 z 1 resist n 3 substrate θ 2 perating principle Intereference Signal (a.u.) 0.88 0.86 0.84 0.82 0.80 0.78 0.76 Temperature Interference signal 50 100 150 200 90 80 70 60 50 40 30 Temperature ( o ) Single Wavelength Interferometer Set-up Time (min) Solvent evaporation during the PAB step. Most of the solvent evaporates during the first 10min (heat up from RT to 95 o ). During the rest period (4h) ~0.8µm of resist thinning is observed.

PEB study 100 for 8 min exposure dose (238 n/mm 2 ) 110 for 8 min exposure dose (116 n/mm 2 ) a b The increased PEB temperature helps significantly the crosslinking reaction Ι. Rajta, E. Baradacs, M. hatzichristidi, E.S. Valamontes, I. Raptis, Nucl. Inst. Meth. B, 231 423 (2005)

Preliminary Lithographic evaluation (I) UV-LIGA Thickness = 11 µm Aspect Ratio = 7 Substrate: Silicon wafer with plating base (Au surface) Thickness = 35 µm Layout = 5 µm L/S Linewidth = 5 µm I-line lithography (MJB3 Karl-Suss)

Preliminary Lithographic evaluation (II) SINGLE PIXEL AND SINGLE LINE EXPSURES 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.hatzichristidi, 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

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

Electroplating results 167nm Side view of Ni plated on gold features. Ni thickness 0.8µm n the right side the resist pattern

PBW simulation flow-chart Point proton beam simulation onvolution with Gaussian Profile Energy (KeV/cm 3 *electron) 10 15 10 14 10 13 10 12 10 11 10 10 10 9 10 8 Point Beam Energy Deposition Point Beam exposure Realistic proton beam simulation 10 7 0 500 1000 1500 2000 Radial distance (nm) Layout onvolution with Layout (top view) Energy deposition on resist Energy deposition (cross section) Thermal processing, PEB simulation (only for ARs*) Resist Profile (development) Development simulation * P-RES5 Stochastic simulation studies of molecular resists for the 32nm technology node

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 oulomb 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 ommission on Radiation Units and Measurements, Report 49, 1993.

PBW simulation results (energy deposition) 14 12 12 Energy deposition (ev/α) 10 8 6 4 2 Bulk PMMA 10umPMMA/Si 20umPMMA/Si 30umPMMA/Si Energy deposition (ev/α) 10 8 6 4 2 Literature SRIM Bulk PMMA 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Depth (µm) PMMA Si substrate 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Depth (µm) Monte-arlo (M) simulation: Energy deposition vs. depth for various resist films. Proton beam: 2MeV Simulation Dz=50nm omparison of the M simulation results in bulk with the literature and SRIM Proton beam: 2MeV Simulation Dz=50nm

PBW simulation results (convolution) 10 15 Energy (KeV/cm 3 *electron) 10 14 10 13 10 12 10 11 10 10 10 9 10 8 10 7 onvoluted 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)

PBW simulation results (layout level) Top view (layout) Energy deposition (cross section) Proton beam irradiated Unexposed area 200nm lines / 400nm spaces Proton Beam Diameter 100nm Film Thickness 10µm ross-section after development (simulation) Si wafer Threshold: 0.01 x G Si wafer Threshold: 0.1 x G

onclusions 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 2µm, Linewidth 110nm, Aspect ratio 18 Thickness 12µm, 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.NANMP project (funded by the Greek Secretariat for Research & Technology contract number 05-NNEU-467). THANK YU FR YUR ATTENTIN