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

1. 110nm Simulation of Proton Beam High-Aspect-Ratio Micro / Nano Machining E. Valamontes1, 2, M. Chatzichristidi3, C. Potiriadis4, D. Kotsiampasis3, D. Niakoula3, A. Karydas5, S. Harissopoulos5, D. Goustouridis3, I. Raptis3 1 Department of Electronics, TEI of Athens, 12210 Aegaleo, Greece 2Department of Telecommunications, University of Peloponnese, GR-22100 Tripoli, Greece 3 Institute of Microelectronics, NCSR “Demokritos” 15310 Athens, Greece 4 Greek Atomic Energy Commission, Agia Paraskevi, Attiki 15310, Greece 5 Institute of Nuclear Physics, NCSR “Demokritos” 15310 Athens, Greece Abstract Results Among the patterning technologies proposed and applied for the realization of high aspect ratio structures in the micro and nano scale, the Proton Beam Writing (PBW) is considered as a valuable tool for maskless patterning of such structures due to the unique ability of protons to maintain a straight path over long distances [1]. In the present work, the PBW capabilities are shown through simulation results of fine structures in resist films. These results prove the capability of PBW to produce very tall structures with almost vertical sidewall, with the aspect ratio limited practically only by the resist performance and the beam diameter provided. The performance of PBW is explored and proved through the patterning of an aqueous base developable negative chemically amplified resist (TADEP). By employing PBW on 2.0 μm thick TADEP, patterns with 110nm linewidth and aspect ratio of 18 were resolved. Figure 1: 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, Simulation Dz=50nm). Figure 2: The energy deposition vs. lateral dimension for various TADEPresist films (350nm, 2μm and 18μm) at the resist/Si interface. The beam broadening is very small regardless the very high film thicknesses. Simulation modules and materials • Simulation Strategy • For the PBW simulation several modules (exposure, thermal processing, development) should be coupled in a software tool. The first simulation module calculates the energy loss distribution in the resist film and the substrate due to a point proton beam irradiation, the second performs the convolution of the energy deposition with 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 amplified resists a simulation of the PEB follows. The last simulation module is the dissolution which in the present work is a simple absorbed energy thresholding. In order to simplify the simulation study of the PBW, PMMA films are considered in the present study. • Proton Beam – Matter Interaction • The formalism adopted for simulating protons propagation is that of TRIM / SRIM [2]. At high energies, we have decided, for the sake of the computer efficiency, to base the calculations on the Coulomb potential [3]. • 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 [4]. • TADEP resist • In the present work, TADEP resist (Thick Aqueous Developable EPoxy resist, TADEP) is patterned with PBW. TADEP consists of partially hydrogenated poly(hydroxy styrene) (PHPHS), epoxy novolac (EP) and a sulfonium salt as the photoacid generator (PAG) and is capable to provide film thickness up to 55m with one spin coating step. The processing steps are: a) spin coating from the suitable solution, b) Post apply Bake (PAB) on a leveled hot plate at 950C for a time depending on the film thickness, c) Proton Beam exposure, d) Post Exposure Bake (PEB) at 1100C for a time depending on the film thickness, exposure, e) development in TMAH 0.26N (AZ-726MIF from AZ-EM) for the dissolution of the uncrosslinked areas and f) rinsing in deionized H2O. The stripping is performed in acetone in ultrasonic bath. Detailed information on the resist’s chemistry and of resist’s processing can be found in [5]. Figure 3: SEM images from the PBW on 2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 pixels pass line with beam step size of 10nm. The pitch values were 1m and 4m and the calculated aspect ratio 18. Figure 4: Side view SEM images from the PBW on 12m thick TADEP resist on Si. Each line is a two pixels pass line with beam step size of 10nm. The pitch values were 1m and 4m and the calculated aspect ratio 42. References [1]F. Watt, M.B.H. Breese, A.A. Bettiol, J.A. van Kan Materials Today 10(6) (2007)20. [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 Solids, The Stopping and Ranges of Ions in Matter, Vol. 1, Pergamon Press Inc., 1985. [4] Stopping Powers and Ranges for Protons and Alpha Particles, International Commission on Radiation Units and Measurements, Report 49, 1993. [5] M. Chatzichristidi, I. Rajta, Th. Speliotis, E. Valamontes, D. Goustouridis, P. Argitis, I. RaptisMicrosyst. Technol. (in press 2008) Acknowledgements This paper is part of the 05-NONEU-467 research project (PB.NANOCOMP), co-funded by E.U.-European Social Fund (75%) and the Greek Ministry of Development-GSRT (25%).