AVS2015 Session HI-WeA: GFIS Based Nanostructuring

We report on the status and application of the gas field ion source (GFIS) focused ion beam (FIB) nanofabrication system [1] located at the Japan Advanced Institute of Science and Technology (JAIST). The atomic emission tip is biased against the extractor, generating high electric field strengths at the tip apex leading to ionization of the source gas. The ions are then accelerated to a typical energy of 25 Kv, focused, and scanned over the device under test (DUT). Such beams can be used to mill the DUT or generate images by simultaneous detection of the secondary electrons by the Everhart-Thornley detector. The GFIS-FIB employs a newly designed emission tip technology to use nitrogen (N₂ → N₂⁺) as a high sputter-yield source gas. Other ion species, such as helium and hydrogen, can be generated with the tip technology as well. The latter has been demonstrated to suppress the sub-surface damage formation [2], but is not currently in use at JAIST. Carbon is available for deposition, while iodine, xenon difluoride (XeF₂) and H₂O can be used for gas assisted milling.

Our current efforts involve carving graphene into sub-10-nm wide ribbons and electrically separating gold electrodes on top of graphene with the goal of realizing graphene tunnel field effect transistors. We also work on fabrication of graphene resonators based on suspended graphene that are narrowed down after the resist-based fabrication and hydrofluoric acid release. The second main area is the preparation of nanoimprint lithography (NIL) masters. Line-and-space pattern with a half pitch of 15 nm were milled into quartz substrates by the ion beam and will be applied to the fabrication of cutting-edge semiconductor devices. Lastly, we use the nitrogen ion beam for quantum point contact fabrication (QPC) in high-in-content InGaAs [3]. The size of the QPCs is ~30 nm × 150 nm, and the QPCs show step-like structures of 2e²/h without magnetic field and of e²/h and 3e²/h with magnetic fields. Additionally, we will show the unique material contrast observed in nitrogen ion excited secondary electron images of graphene-based structures, and talk about low-energy operation, where 15 nm resolution has been achieved with a 5 Kv nitrogen beam. Such low energy beams will be used for defect generation and doping of graphene and other materials.

This work was partially supported by Kakenhi No. 25220904 from JSPS and the Center of Innovation Program from Japan Science and Technology Agency.

[1] F. Aramaki et al., in Proc. SPIE 8441, 84410D, Yokohama, Japan, 2012.

[2] F. Aramaki et al., in Proc. SPIE 7969, 79691C, San Jose, USA, 2011.

[3] M. Akabori et al., Jpn. J. Appl. Phys., 53, 118002, 2014.

Plasmonic nanoantennas are versatile tools for coherently controlling and directing light on the nanoscale. For these antennas, current fabrication techniques such as electron beam lithography (EBL) or focused ion beam (FIB) milling with Ga⁺-ions routinely achieve feature sizes in the 10 nm range. However, they suffer increasingly from inherent limitations when a precision of single nanometers down to atomic length scales is required, where exciting quantum mechanical effects are expected to affect the nanoantenna optics. Here, we demonstrate that a combined approach of Ga⁺-FIB and milling-based He⁺-ion lithography (HIL) for the fabrication of nanoantennas offers to readily overcome some of these limitations (1). Gold bowtie antennas with 6 nm gap size were fabricated with single-nanometer accuracy and high reproducibility. Using third harmonic (TH) spectroscopy, we find a substantial enhancement of the nonlinear emission intensity of single HIL-antennas compared to those produced by state-of-the-art gallium-based milling. Moreover, HIL-antennas show a vastly improved polarization contrast. This superior nonlinear performance of HIL-derived plasmonic structures is an excellent testimonial to the application of He⁺-ion beam milling for ultrahigh precision nanofabrication, which in turn can be viewed as a stepping stone to mastering quantum optical investigations in the near-field.

(1) H. Kollmann, X. Piao, M. Esmann, S.F. Becker, D. Hou, C. Huynh, L.-O. Kautschor, G. Boesker, H. Vieker, A. Beyer, A. Gölzhäuser, N. Park, R. Vogelgesang, M. Silies, C. Lienau: Towards Plasmonics with Nanometer Precision: Nonlinear Optics of Helium-Ion Milled Gold Nanoantennas, Nano Letters 14, 4778 (2014)

Scaling the dimensions of materials to the nanoscale creates new opportunities and many applications. Nanomaterials display unique properties relative to their bulk counterparts due to surface/interface effects, quantum confinement/coherence, which have been investigated to understand new materials physics. Applications driven by nanostructures can benefit from their enhanced functionalities by modifying material properties and geometries via ion irradiation. Specifically in nanostructures, ion irradiation with a focused ion beam can play a critical role in modifying properties and geometries locally in those nanostructures. This local modification had already been proven to have applications in quantum optics and circuits by creating localized material modification at the nanoscale in bulk materials via focused helium ion beam (FHIB) irradiation However, local modification may occur in different ways in nanostructures. Therefore, one needs to investigate the effect of FHIB irradiation on nanostructures in order to control local modification in a desirable way and understand new physics.

In this work, we primarily study the interaction of a FHIB and nanostructures with two materials, single crystal silicon and diamond. Our experimental approach using thin vertical membranes fabricated by focused gallium ion beam enabled us to observe ion-nanostructure interaction in 3-dimension by preserving defects by ion irradiation. We have investigated new physical phenomena; (1) strain-induced volume expansion, (2) long-range ion propagation, and (3) material behavior’s transition from a bulk to a small-scale where the size-dependent characteristic exists. We have explained these mechanisms between nanostructure (material, crystal orientation, and geometry) and helium ion (energy and dose). We have also extended our study to focused neon ion beam (FNIB) irradiation. We have investigated and compared the difference of ion-nanostructure interaction between neon and helium.

Furthermore, we have expanded our study to new nanofabrication method by embedding a 3D geometry on nanostructures with the consideration of a geometrical constraint, decided by crystal-to-amorphous boundary.

Abstract

Rapid advanced in nanoscience rely on continuous improvements of manipulating matter at near atomic scales. Currently, well understood, robust resist-based lithography, carries the brunt of nanofabrication, however local electron, ion and physical probe methods are improving as well, driven largely in part of their ability to fabricate without multi-step preparation processes that can result in sample contamination from the resists and solvents. Furthermore probe based methods extend beyond nanofabrication to nanomanipulation and imaging, vital ingredients to rapid transition to testing and manufacturing of layered 2D heterostructured devices.

In this work we demonstrate that helium ion interaction in a Helium Ion Microscope (HIM) with the surface of bulk copper indium thiophosphate (CITP) CuM_IIIP₂X₆ (M = Cr, In; X= S, Se) result in the controlled loss of ferrielectric domains, and growth of cylindrical nanostructures with enhanced conductivity, with material volumes scaling to the dosage of the beam. The nanostructures are oxygen rich, sulfur poor, with the copper concentration virtually unchanged as confirmed by Energy Dispersive X-ray (EDX). Scanning Electron Microscopy (SEM) image contrast as well as Scanning Microwave Microscopy (SMM) measurements suggest enhanced conductivity of the formed particle, whereas Atomic Force Microscopy (AFM) based measurements indicate that the resulting structures have lower dissipation and a lower young’s modulus.

Acknowledgements

Research was supported (A. B., V. I., A.T., D. J., S. V. K., S. J., A. J. R. O. S. O) and partially conducted (AFM, HIM) at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. This work was also supported (M. S., M. M.) and partially conducted (material growth) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.

material [2].

Here, we present a novel route utilizing a Helium Ion Microscope (HIM) to form nano-sized magnets of arbitrary shape using very low fluences (6x10¹⁴ cm^-2) of 20 keV-25 keV Neon ions. The fine Neon beam available in the HIM is used to locally switch 40 nm thin Fe₆₀Al₄₀ films from the well ordered paramagnetic B2 structure into the ferromagnetic A2 structure [3,4]. Planar structures potentially useful for applications such as spin valves or other spin--transport devices have been formed this way. Kerr Microscopy and off-axis TEM holography has been used to analyse the resulting magnetic nano-structures. Results on the energy depended depth of magnetization as well as on the lateral definition of the magnetic structures due to scattering are presented.

FR and HL gratefully acknowledge funding from the European Union Seventh Framework Programme under Grant Agreement 312483 - ESTEEM2 (Integrated Infrastructure Initiative - I3).

1. Hlawacek, G., Veligura, V., van Gastel, R. & Poelsema, B. Helium ion microscopy. J. Vac. Sci. Technol. B Microelectron. Nanom. Struct. 32, 020801 (2014).

2. Veligura, V. et al. Digging gold: keV He+ ion interaction with Au. Beilstein J. Nanotechnol. 4, 453–460 (2013).

3. Bali, R. et al. Printing nearly-discrete magnetic patterns using chemical disorder induced ferromagnetism. Nano Lett. 14, 435–41 (2014).