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Ion beams from laser-cooled gases

MP Merijn Reijnders

Published Jan 1, 2010• 2 citations
Publisher

Abstract

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.

DOI
10.6100/ir684840

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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.
UltraCold Ion Beam Source. Focused ion beam (FIB) machines are largely used in the semiconductor industry mainly for imaging, milling and deposition of material, at the nanometer scale. To keep up with the trend of making smaller and smaller components, FIBs need to be continuously improved. A FIB is mainly made of an ion source and a focusing column. In this thesis, two concepts of ion sources are presented, modeled and discussed. The ultra-cold ion source (UCIS) is based on creating very cold ion beams (T <1 mK) by near-threshold photo-ionization of a laser-cooled and trapped atomic gas. In our experiment, 85Rb atoms are confined in a magneto optical trap built directly inside an accelerator structure. A spherical portion of the atom cloud is ionized with a 2-step ionization process and the bunched ions are accelerated by either a DC or a pulsed electric field. The liquid-metal ion source (LMIS) is the current state-of-art for focused ion beam technology, having a reduced brightness of 106 A/m2 sr eV. The reduced brightness characterizes the source and is proportional to the ion current and inversely proportional to the beam emittance (closely related with the source temperature). A high brightness is of importance as much as a low energy spread of the ion beam, since the latter is related with the focus ability of the beam. We have already shown that the UCIS can provide much lower energy spread (20 meV) than LMIS (˜ 4.5 eV), see Reijnders et al. (PRL, 2009), but not yet in combination with a high brightness as well. The energy spread is the limiting factor for the final achievable spot size in a FIB based on a LMIS. Chapter 3 and 4 present measurements of the source temperature and of the extracted current to complete the characterization of the UCIS. The ultra low temperature of the source permits collimated bunches to be created at low kinetic energy (down to few eV), which allows using time-dependent fields for accelerating and focusing. With this lens, a transversal ion temperature of (3±2) mK has been measured, as shown in chapter 3. This temperature is an upper limit since it is ten times higher than the expected value. The novel technique of focusing with time-dependent electric field is successfully applied to an ultra cold ion beam. In chapter 4, a dynamic model of the source describing its properties under pulsed operation has been developed and experiments validated it. A maximum average ion current of (13 ± 1) pA has been extracted from the UCIS and this is enough for imaging (1-100 pA are used). Those results correspond to a source reduced brightness of (8 ± 1) × 104 A/m2 sr eV with 0.9 eV energy spread. This result is 12.5 times lower than the brightness of a LMIS. An interesting "pushing effect", due to the laser used to excite the atoms in the cloud, is described as well. Particle tracking simulations show that disorder induced heating has an important effect on the brightness, lowering its value by 1-2 orders of magnitude depending on the extracted current. Calculations and simulations of a different ion source, the atomic beam laser-cooled ion source (ABLIS), are presented in chapter 6. The follow-up project is based on a similar idea as an UCIS but aims for production of ion beams from a much higher initial atomic flux. The atoms originate from a Knudsen cell and, after being cooled and compressed transversally with laser cooling and trapping techniques, are photo-ionized and accelerated. The source size is reduced as well with respect to the UCIS. Realistic particle tracking simulations with 87Rb show that disorder induced heating is still very important, but in certain conditions, the so-called "pencil-beam" regime is reached and the disorder-induced heating does not deteriorate the brightness anymore. Depending on the extracted current, it is possible to reach a reduced brightness of 2 × 107 A/m2 sr eV with an ion current of 22 pA (corresponding to a final aperture diameter df = 1.6 µm) and an energy spread of 0.7 eV can be achieved. This source is likely capable of provide a high brightness and a low energy spread at the same time. The calculations are done assuming a purified 87Rb source. Eventually, it is possible to reach very high current (1 nA), making the ABLIS ideal also for milling applications with FIBs.
A new type of high-brightness ion source is under development which employs transverse laser cooling and compression of a thermal atomic rubidium beam, followed by in-field photo-ionization. When attached to a focusing column, this Focused Ion Beam (FIB) has the advantage of supplying a higher current in a smaller spot compared to conventional LMIS-based FIBs, thus increasing both the resolution and the speed of the FIB. Furthermore, different types of ion species can be used, broadening the range of applications of the FIB. Simulations using a 10 cm long laser cooling and compression stage and a realistic ionization and acceleration structure, predict an achievable brightness for 87 Rb + of order 10 7 A/m 2 sr eV at an energy spread of less than 1 eV and a current of tens of pA. This would lead to a spot size below 5 nm. Simulations and modeling on the ionization process have led to a better understanding of stochastic heating. 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 and compressor. Progress on simulations and experimental results will be reported.
At present, the smallest spot size which can be achieved with state-of-the-art focused ion beam (FIB) technology is mainly limited by the chromatic aberrations associated with the 4.5 eV energy spread of the liquid-metal ion source. Here we numerically investigate the performance of an ultracold ion source which has the potential for generating ion beams which combine high brightness with small energy spread. The source is based on creating very cold ion beams by near-threshold photoionization of a laser-cooled and trapped atomic gas. We present ab initio numerical calculations of the generation of ultracold beams in a realistic acceleration field and including all Coulomb interactions, i.e., both space charge effects and statistical Coulomb effects. These simulations demonstrate that with existing technology reduced brightness values exceeding 105 A m−2 sr−1 V−1 are feasible at an energy spread as low as 0.1 eV. The estimated spot size of the ultracold ion source in a FIB instrument ranges from 10 nm at a current of 100 pA to 0.8 nm at 1 pA.
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.
The ultra-cold ion (UCI) source is based on creating very cold ion beams by nearthreshold\nphoto-ionization of a laser-cooled and trapped atomic gas. The UCI source has\nthe potential of producing ion beams with a brightness and current comparable to the\nliquid-metal ion source (LMIS) [1], which is the current state-of-art for focused ion beam\n(FIB) technology. We have already shown [2] that the UCI source can provide much\nlower energy spread than LMIS, and may therefore offer a route toward 1-nm ion beam\nmilling [3].
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.