Resolution Performance of Programmable Proximity Aperture MeV Ion Beam Lithography System
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1020-GG03-04
Resolution Performance of Programmable Proximity Aperture MeV Ion Beam Lithography System Sergey Gorelick1, Timo Sajavaara1, Mikko Laitinen1, Nitipon Puttaraksa2, and Harry J. Whitlow1 1 Dept. of Physics, University of Jyväskylä, Jyväskylä, 40014, Finland 2 Dept. of Physics, Chiang Mai University, Chiang Mai, 50200 (FNRF), Thailand
ABSTRACT An ion beam lithography system for light and heavy ions has been developed at the University of Jyväskylä’s Accelerator Laboratory. The system employs a programmable proximity aperture to define the beam. The proximity aperture is made up of four Ta blades with precise straight edges that cut the beam in the horizontal and vertical directions. The blade positions and dimensions are controlled by a pair of high-precision linear-motion positioners. The sample is mounted on a X-Y-Z stage capable of moving with 100 nm precision steps under computer control. The resolution performance of the system is primarily governed by the proximity aperture. Pattern edge sharpness is set by the beam divergence, aperture blade straightness, and secondary and scattered particles from the aperture blade edges. Ray tracing simulations using object oriented toolkit GEANT4 were performed to investigate the beam spatial resolution on the sample defined by the proximity aperture. The results indicate that the edge-scattering does not significantly affect the pattern edge sharpness. INTRODUCTION MeV proton beam writing (PBW) is a rapidly evolving lithography technique capable of patterning high 3D nanopatterns with very high line-width to resist-thickness aspect ratio (more than 100), and with a high resolution of better than 20 nm [1-3]. The PBW technique is analog to electron beam lithography (EBL). A beam of protons from an electrostatic accelerator is magnetically focused and scanned over the resist surface. However, the MeV protons, as opposed to keV electrons, can penetrate deep into the resist along a straight path with minimal scattering. Proton beams from cyclotrons generally have higher energies (tens of MeV), which enables pattern writing in thicker resists (up to 400 µm for 6 MeV protons in PMMA [4]). However, even if large beam currents are available (up to 100’s of µA), the rather large divergence (about 1 mrad) and poor energy resolution imply that it is not straightforward to use a focusing in order to reach µm beam spot sizes. An alternative approach is to raster the target relative to a small beam spot defined by an aperture [5,6]. In MeV ion programmable proximity aperture lithography (PPAL), which is used in our system, a rectangular beam spot is defined by a “shadow” of a computer-controlled variable aperture in close proximity to the sample (Fig. 1). The aperture is made up of two L-shaped Ta blades with straight edges (each blade is made up of two 100 µm thick Ta sheets glued together). Precise movement of each L-shaped blade in the X’ and Y’ directions defines the size of the beam spot (Fig. 1(a)). By combining X’ and Y’ movement of the defining aperture with x and y
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