Simulation of Simulation of Proton Beam Proton Beam HHigh-Aspect-Ratio Micro / igh-Aspect-Ratio Micro / Nano MachiningNano Machining

E. ValamontesE. Valamontes1, 21, 2, M. Chatzichristidi, M. Chatzichristidi33, C. Potiriadis, C. Potiriadis44, D. Kotsiampasis, D. Kotsiampasis33, D. Niakoula, D. Niakoula33,,A. KarydasA. Karydas55, S. Harissopoulos, S. Harissopoulos55, D. Goustouridis, D. Goustouridis33, I. Raptis, I. Raptis33

11 Department of Electronics, TEI of Athens, 12210 Aegaleo, Greece Department of Electronics, TEI of Athens, 12210 Aegaleo, Greece 22 Department of Telecommunications, University of Peloponnese, GR-22100 Tripoli, GreeceDepartment of Telecommunications, University of Peloponnese, GR-22100 Tripoli, Greece

33 Institute of Microelectronics, NCSR “Demokritos” 15310 Athens, Greece Institute of Microelectronics, NCSR “Demokritos” 15310 Athens, Greece44 Greek Atomic Energy Commission, Agia Paraskevi, Attiki 15310, Greece Greek Atomic Energy Commission, Agia Paraskevi, Attiki 15310, Greece55 Institute of Nuclear Physics, NCSR “Demokritos” 15310 Athens, Greece Institute of Nuclear Physics, NCSR “Demokritos” 15310 Athens, Greece

Among the patterning technologies proposed and applied for the Among the patterning technologies proposed and applied for the realization of high aspect ratio structures in the micro and nano scale, realization of high aspect ratio structures in the micro and nano scale, the Proton Beam Writing (PBW) is considered as a valuable tool for the Proton Beam Writing (PBW) is considered as a valuable tool for maskless patterning of such structures due to the unique ability of maskless patterning of such structures due to the unique ability of protons to maintain a straight path over long distances [1]. In the protons to maintain a straight path over long distances [1]. In the present work, the PBW capabilities are shown through simulation results present work, the PBW capabilities are shown through simulation results of fine structures in resist films. These results prove the capability of of fine structures in resist films. These results prove the capability of PBW to produce very tall structures with almost vertical sidewall, with PBW to produce very tall structures with almost vertical sidewall, with the aspect ratio limited practically only by the resist performance and the aspect ratio limited practically only by the resist performance and the beam diameter provided. The performance of PBW is explored and the beam diameter provided. The performance of PBW is explored and proved through the patterning of an aqueous base developable negative proved through the patterning of an aqueous base developable negative chemically amplified resist (TADEP). By employing PBW on 2.0 μm thick chemically amplified resist (TADEP). By employing PBW on 2.0 μm thick TADEP, patterns with 110nm linewidth and aspect ratio of 18 were TADEP, patterns with 110nm linewidth and aspect ratio of 18 were resolved.resolved.

ReferencesReferences[1][1] F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. van Kan Materials Today 10(6) (2007) F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. van Kan Materials Today 10(6) (2007) 20.20.[2] J. Biersack, L. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257.[2] J. Biersack, L. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257.[3] J. F. Ziegler, J. P. Biersack, U. Littmark, The Stopping and Range of Ions in [3] J. F. Ziegler, J. P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, The Solids, The Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon Press Inc., Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon Press Inc., 1985.1985.[4] Stopping Powers and Ranges for Protons and Alpha Particles, International [4] Stopping Powers and Ranges for Protons and Alpha Particles, International Commission on Radiation Units and Measurements, Report 49, 1993.Commission on Radiation Units and Measurements, Report 49, 1993.[5] [5] M. Chatzichristidi, I. Rajta, Th. M. Chatzichristidi, I. Rajta, Th. Speliotis, E. Valamontes, D. Goustouridis, P. Speliotis, E. Valamontes, D. Goustouridis, P. Argitis, I. Raptis Microsyst. Argitis, I. Raptis Microsyst. Technol. (in press 2008)Technol. (in press 2008)

AcknowledgementsAcknowledgementsThis paper is part of the This paper is part of the 05-NONEU-46705-NONEU-467 research project research project (PB.NANOCOMP) (PB.NANOCOMP), co-, co-funded by E.U.-European Social Fund (75%) and the Greek Ministry of Development-funded by E.U.-European Social Fund (75%) and the Greek Ministry of Development-GSRT (25%).GSRT (25%).

ResultsResults

Figure 2Figure 2: T: The energy deposition vs. lateral he energy deposition vs. lateral dimension for various TADEPdimension for various TADEP resist films resist films (350nm, 2(350nm, 2μμm and m and 1818μμm) at the resist/Si m) at the resist/Si interface. The beam broadening is very small interface. The beam broadening is very small regardless the very high film thicknesses.regardless the very high film thicknesses.

Simulation modules and materialsSimulation modules and materials

AbstractAbstract

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Figure 3Figure 3: : SEM images from the PBW on 2SEM images from the PBW on 2m thick TADEP resist on Si. a) Low magnification top-m thick TADEP resist on Si. a) Low magnification top-view of the irradiated area. b) High magnification side view of the fine patterns. Each line is a two view of the irradiated area. b) High magnification side view of the fine patterns. Each line is a two pixels pass line with beam step size of 10nm. The pitch values were 1pixels pass line with beam step size of 10nm. The pitch values were 1m and 4m and 4m and the m and the calculated aspect ratio 18.calculated aspect ratio 18.

Figure 4Figure 4: : Side view SEM images from the Side view SEM images from the PBW on 12PBW on 12m thick TADEP resist on Si. m thick TADEP resist on Si. Each line is a two pixels pass line with beam Each line is a two pixels pass line with beam step size of 10nm. The pitch values were 1step size of 10nm. The pitch values were 1m m and 4and 4m and the calculated aspect ratio 42.m and the calculated aspect ratio 42.

Simulation StrategySimulation Strategy

For the PBW simulation several modules (exposure, thermal For the PBW simulation several modules (exposure, thermal processing, development) should be coupled in a software tool. The processing, development) should be coupled in a software tool. The first simulation module calculates the energy loss distribution in the first simulation module calculates the energy loss distribution in the resist film and the substrate due to a point proton beam irradiation, the resist film and the substrate due to a point proton beam irradiation, the second performs the convolution of the energy deposition with the second performs the convolution of the energy deposition with the proton beam profile (Gaussian in the present case) and the third the proton beam profile (Gaussian in the present case) and the third the convolution with the layout of interest. Then in the case of chemically convolution with the layout of interest. Then in the case of chemically amplified resists a simulation of the PEB follows. The last simulation amplified resists a simulation of the PEB follows. The last simulation module is the dissolution which in the present work is a simple module is the dissolution which in the present work is a simple absorbed energy thresholding. In order to simplify the simulation study absorbed energy thresholding. In order to simplify the simulation study of the PBW, PMMA films are considered in the present study.of the PBW, PMMA films are considered in the present study.

Proton Beam – Matter InteractionProton Beam – Matter Interaction

• The formalism adopted for simulating protons propagation is that The formalism adopted for simulating protons propagation is that of TRIM / SRIM [2]. Aof TRIM / SRIM [2]. At high energiest high energies, we have decided, for the sake of , we have decided, for the sake of the computer efficiency, to base the calculations on the Coulomb the computer efficiency, to base the calculations on the Coulomb potential [3].potential [3].• Stopping powers at high energies were calculated according to Stopping powers at high energies were calculated according to Bethe’s theory.Bethe’s theory.• At low energies, electronic stopping powers were obtained from At low energies, electronic stopping powers were obtained from experimental data, closely related to the empirical fitting formulas experimental data, closely related to the empirical fitting formulas developed by Andersen and Ziegler.developed by Andersen and Ziegler.• The nuclear stopping power, which is important only at very low The nuclear stopping power, which is important only at very low energies, was obtained by the method of Everhart et al [4].energies, was obtained by the method of Everhart et al [4].

TADEP resistTADEP resist

In the present work, TADEP resist (Thick Aqueous Developable In the present work, TADEP resist (Thick Aqueous Developable EPoxy resist, TADEP) is patterned with PBW. TADEP consists of EPoxy resist, TADEP) is patterned with PBW. TADEP consists of partially hydrogenated poly(hydroxy styrene) (PHPHS), epoxy novolac partially hydrogenated poly(hydroxy styrene) (PHPHS), epoxy novolac (EP) and a sulfonium salt as the photoacid generator (PAG) and is (EP) and a sulfonium salt as the photoacid generator (PAG) and is capable to provide film thickness up to 55capable to provide film thickness up to 55m with one spin coating m with one spin coating step. The processing steps are: a) spin coating from the suitable step. The processing steps are: a) spin coating from the suitable solution, b) Post apply Bake (PAB) on a leveled hot plate at 95solution, b) Post apply Bake (PAB) on a leveled hot plate at 9500C for a C for a time depending on the film thickness, c) Proton Beam exposure, d) time depending on the film thickness, c) Proton Beam exposure, d) Post Exposure Bake (PEB) at 110Post Exposure Bake (PEB) at 11000C for a time depending on the film C for a time depending on the film thickness, exposure, e) development in TMAH 0.26N (AZ-726MIF from thickness, exposure, e) development in TMAH 0.26N (AZ-726MIF from AZ-EM) for the dissolution of the uncrosslinked areas and f) rinsing in AZ-EM) for the dissolution of the uncrosslinked areas and f) rinsing in deionized Hdeionized H22O. The stripping is performed in acetone in ultrasonic bath. O. The stripping is performed in acetone in ultrasonic bath. Detailed information on the resist’s chemistry and of resist’s Detailed information on the resist’s chemistry and of resist’s processing can be found in [5].processing can be found in [5].

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Figure 1Figure 1: : Monte-Carlo (MC) simulation. a) Energy deposition vs. depth for various resist films. b) Monte-Carlo (MC) simulation. a) Energy deposition vs. depth for various resist films. b) Comparison of the MC simulation results in bulk with the literature and SRIM. (Proton beam: 2MeV, Comparison of the MC simulation results in bulk with the literature and SRIM. (Proton beam: 2MeV, Simulation Dz=50nm).Simulation Dz=50nm).