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Physical Review Applied

Low-Energy Ions from Laser-Cooled Atoms

Gholamreza Shayeganrad, A. Fioretti, Irene Guerri, Francesco Tantussi, Donatella Ciampini, M. Allegrini, Matthieu Viteau, F. Fuso

Published May 27, 2016• 7 citations
Publisher

Abstract

Focused ion beam technology is key to both academic and industrial sample processing, for material deposition or ablation, but its progress toward nanometer resolution is hampered by chromatic aberration of the beam. The authors use laser cooling, a magneto-optical trap, and photoionization to deliver a well-controlled, low-energy, low-current ion beam with a narrow energy spread. Their ion source is a promising basis to advance the state of the art.

DOI
10.1103/physrevapplied.5.054020

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Focused ion beam technology is key to both academic and industrial sample processing, for material deposition or ablation, but its progress toward nanometer resolution is hampered by chromatic aberration of the beam. The authors use laser cooling, a magneto-optical trap, and photoionization to deliver a well-controlled, low-energy, low-current ion beam with a narrow energy spread. Their ion source is a promising basis to advance the state of the art.
The authors describe a new method for creating a high quality focused ion beam using laser-cooled neutral atoms in a magneto-optical trap as an ion source. They show that this new technique can provide spot resolutions and brightness values that are better than the state of the art in focused ion beams. The source can be used with a range of different ionic species and can be combined with laser cooling techniques to exert unprecedented control over the ion emission, for example, producing single ions “on demand.” The beam quality is a result of a high brightness and a narrow energy distribution, both of which stem from the cold temperature (≈100μK) of the atoms. The ions are produced by subjecting the cold neutral atoms to a photoionization laser, after which they become a compact source of nearly monoenergetic ions. With the application of a potential gradient, the ions form a beam that can be focused via standard ion optical techniques. They discuss estimations based on the initial size of the ion cloud and the energy distribution and show that the resulting beam has a low emittance.
The evolving field of nanofabrication demands that more precise fabrication and evaluation tools be developed. We describe a method for creating a high quality focused ion beam with enhanced capabilities using an ion source based on laser-cooled neutral atoms in a magneto-optical trap. This technique will improve resolution and brightness beyond the current state of the art, can be used with the full range of atomic species that can be laser cooled and trapped, and will allow unprecedented control over the ion emission, allowing, for example, the production of single ions ``on demand.'' We give estimates for the emittance and present a realistic ray tracing analysis of a basic focusing system demonstrating the feasibility of focusing the beam to a spot size of less than 10nm10\phantom{\rule{0.3em}{0ex}}\mathrm{nm}10nm.
High-brightness ion sources are important for several applications such as focussed ion beam (FIB) systems, in which an ion beam is focussed on a sample to a spot size of a few nanometer. These systems have become an important tool in a wide array of material science and technological applications because they offer both high-resolution imaging as well as micromachining capabilities. The resolution in the most common FIB system, the Gallium-Liquid Metal Ion Source (Ga-LMIS) based FIB, is limited by chromatic aberrations in the lens column due to the energy spread of o2 eV. To reach a higher resolution, a high-brightness ion source is needed with a lower energy spread. In this thesis such a new source is presented, the ultra-cold ion source (UCIS) based on the photo-ionization of a laser-cooled atomic gas confined in a magneto-optical trap. The low temperature (_ 150 µK) of the source makes it possible to reach high-brightness with an extended source size. The charge density at the source is thereby reduced compared to sources with a sharp tip, such as the LMIS, and thus also the Coulombic interactions that deteriorate the beam and increase the longitudinal energy spread. Additionally many different atomic species can be laser-cooled, making the UCIS a versatile source for both heavy and light ion species. Light ion species such as Li are very suitable for imaging purposes without directly damaging the sample. Heavy ion species on the other hand, can be used for manipulation of a sample by sputtering away material at a small scale. A analytical model has been developed to estimate the initial source performance. The extractable current from the ionization volume is estimated based on the influx of atoms, which depends on temperature and atomic density. According to the model, the energy spread oU is proportional to the finite length of the ionization volume, limited by the size of the laser focus waist, and the acceleration field E0. To study the influence of the ionic interactions in the beam in more detail, particle tracking simulations have been performed which take all mutual Coulombic interaction into account under realistic experimental conditions. From these simulations we learned that when the current is increased, the brightness is lowered due to increased stochastic heating. Beam currents between 1 and 100 pA are feasible, with an energy spread much lower than that of the LMIS. At 1 pA the source has a peak brightness up to Br = 8 × 104 Am-2sr-1 with an energy spread _U = 0.08 eV. At 10 pA it has a brightness up to Br = 2 × 104 Am-2sr-1 with an energy spread oU = 0.14 eV. An experimental setup has been built, consisting of a rubidium magneto-optical trap inside a cylindrically symmetric accelerator structure. By using a two-step photo-ionization process, a small fraction of the cold atoms in the cross-section of two laser beams is ionized. A pulsed ionization laser was used to produce ion bunches. A short beam line with a multichannel plate detector was used to obtain both the temporal and the transverse spatial profileof the bunches. Time-of-flight measurements have been performed to characterize the longitudinal energy spread of the beam as function of the beam energy U. We have demonstrated, as predicted by the model, that the energy spread oU is indeed due to the length of the ionization volume in the acceleration field. By lowering the extraction field, well defined beams with an average energy of only 1 eV have been produced with an energy spread as low as 20 meV, more than two orders of magnitude lower than the LMIS. A transition from ballistic behavior at high beam energies, where oU / U, to a space-charge dominated regime at low beam energies, where oU / pU, has been observed. In the cross-over region an interesting new effect has been observed in which the space charge forces reduce the energy spread. The experimental results agree well with detailed particle tracking simulations. The combination of well-defined ion beams at low energy and time-dependent acceleration fields opens new possibilities. Precise manipulation of the bunch distribution in both longitudinal and transverse phase space has been demonstrated. Reduction of the longitudinal energy spread has been realized by switching the accelerator field off before the ions have had a chance to leave the accelerating structure. A reduction by a factor 3 is achieved, which is limited by the inhomogeneity of the field. Calculations show that much stronger reduction is achievable in a more homogenous acceleration field. This would make low-energy spread possible without the need to go to small _z0 and E0, enabling even lower energy spread. We also demonstrated that the accelerator structure, which normally acts as a fixed negative lens, can be transformed into a versatile lens by using time-dependent fields. Control over the focal length and sign, as well as the sign and strength of the spherical aberrations can be obtained by tailoring the applied time-dependent acceleration voltage. The possibility to create a negative spherical aberration may be used to correct for the spherical aberrations in the focal columns which are presently limiting the spatial resolution of FIBs. The dependence of the properties of this lens on the parameters of the applied tri-polar pulse has been studied in detail, both experimentally and with particle tracking simulations, which are in good agreement. Finally, based on lessons learned, the design of a next generation UCIS is briefly discussed, based on photo-ionization of a laser cooled and intensified atomic beam. The principle is similar as an UCIS based on a MOT, but with a higher current density. Calculations show that this source should be able to produce orders of magnitude brighter beams in a more compact configuration. With this new design it should be possible to beat the LMIS not only in energy spread but now also in brightness.
A new type of ultra-cold ion source is under development which employs transverse laser cooling and compression of a thermal atomic rubidium beam followed by photo-ionization. The resulting ultra-cold plasma is focused to a nanometer-sized spot using an existing Focused Ion Beam column and this spot can be used for the fabrication of nano-structures. Simulations of a 10 cm long laser-cooling stage and of disorder-induced heating of the resulting ion beam, predict an achievable brightness for87Rb+ of order 107 A/m2 sr eV at an longitudinal energy spread of less than 1 eV and a current of tens of pA, which is substantially better than conventional ion sources. Experimental realization of the compact ion source has recently started with the development of an efficient high-flux atom source and a 2D laser cooler. Progress on these items will be reported.
Focused ion beam instruments a"re indispensable tools for the semiconductor\nindustry due to their ability to image and modify structures on the nanometer\nlength scale. For milling and deposition, the industry standard is the gallium\nliquid-metal ion source which enables a resorution of 5-10 nm at a current\nof a few pA. with the quest towards smaller features on integrated circuits,\nthere is a need for novel ion sources that allow for better resolution. Several\nresearch groups are working towards applying laser-intensified alkali-metal ion\nbeams for this purpose [1].\ns-uch ultra-low temperature (1 mK) ion beams can be created by lasercooling\nand photo-ionization of a thermal atomic beam or vapor. The Rb ion\nsource under development in Eindhoven in collaboration with FEI c
Photoionizing a beam of ultracold atoms can provide a high-brightness ion beam, to improve resolution in focused-ion-beam nanofabrication. With this aim, the authors use laser cooling and compression to give a 85{}^{85}85Rb atomic beam an equivalent brightness better than the current standard for the semiconductor industry. This could represent an important step toward smaller feature sizes in integrated circuits.