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arXiv:1710.11477

Measurements of the energy distribution of an ultracold rubidium ion beam

G. ten Haaf, Steinar H. W. Wouters, D. F. J. Nijhof, Peter H. A. Mutsaers, Edgar J. D. Vredenbregt

Published Oct 31, 2017• 0 citations
arXivHTMLPDF

Abstract

The energy distribution of an ultracold rubidium ion beam, which is intended to be used as the source for a focused ion beam instrument, is measured with a retarding field analyzer. The ions are created from a laser-cooled and compressed atomic beam by two-step photoionization in which the ionization laser power is enhanced in a build-up cavity. Particle tracing simulations are performed to ensure the analyzer is able to resolve the distribution. The lowest achieved full width 50% energy spread is $\left(0.205\pm0.006\right)$ eV. The energy spread originates from the variation in the ionization position of the ions which are created inside an extraction electric field. This extraction field is essential to limit disorder-induced heating which can decrease the ion beam brightness. The ionization position distribution is limited by a tightly focused excitation laser beam. Energy distributions are measured for various ionization and excitation laser intensities and compared with calculations based on numerical solutions of the optical Bloch equations including ionization. A good agreement is found between measurements and calculations.

DOI
10.48550/arxiv.1710.11477

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The energy distribution of an ultracold rubidium ion beam, which is intended to be used as the source for a focused ion beam instrument, is measured with a retarding field analyzer. The ions are created from a laser-cooled and compressed atomic beam by two-step photoionization in which the ionization laser power is enhanced in a build-up cavity. Particle tracing simulations are performed to ensure the analyzer is able to resolve the distribution. The lowest achieved full width 50% energy spread is (0.205±0.006)\left(0.205\pm0.006\right)(0.205±0.006) eV. The energy spread originates from the variation in the ionization position of the ions which are created inside an extraction electric field. This extraction field is essential to limit disorder-induced heating which can decrease the ion beam brightness. The ionization position distribution is limited by a tightly focused excitation laser beam. Energy distributions are measured for various ionization and excitation laser intensities and compared with calculations based on numerical solutions of the optical Bloch equations including ionization. A good agreement is found between measurements and calculations.
A two-step photoionization strategy of an ultracold rubidium beam for application in a focused ion beam instrument is analyzed and implemented. In this strategy the atomic beam is partly selected with an aperture after which the transmitted atoms are ionized in the overlap of a tightly cylindrically focused excitation laser beam and an ionization laser beam whose power is enhanced in a build-up cavity. The advantage of this strategy, as compared to without the use of a build-up cavity, is that higher ionization degrees can be reached at higher currents. Optical Bloch equations including the photoionization process are used to calculate what ionization degree and ionization position distribution can be reached. Furthermore, the ionization strategy is tested on an ultracold beam of 85Rb^{85}\mathrm{Rb}85Rb atoms. The beam current is measured as a function of the excitation and ionization laser beam intensity and the selection aperture size. Although details are different, the global trends of the measurements agree well with the calculation. With a selection aperture diameter of $52\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$, a current of $\left(170\ifmmode\pm\else\textpm\fi{}4\right)$ pA is measured, which according to calculations is 63% of the current equivalent of the transmitted atomic flux. Taking into account the ionization degree the ion beam peak reduced brightness is estimated at $1\ifmmode\times\else\texttimes\fi{}{10}^{7}$ A/(m2sreV{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}\mathrm{sr}\phantom{\rule{0.16em}{0ex}}\mathrm{eV}m2sreV).
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.
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.
Photoionization of a laser-cooled and compressed atomic beam from a high-flux thermal source can be used to create a high-brightness ion beam for use in focused-ion-beam instruments. Here we show using calculations and Doppler cooling simulations that an atomic rubidium beam with an equivalent brightness of $2.1\ifmmode\times\else\texttimes\fi{}{10}^{7} \mathrm{A}/(\mathrm{m}{}^{2} \mathrm{sr} \mathrm{eV})$ can be created using a compact 5 cm long two-dimensional magneto-optical compressor. If this can be conserved during the photoionization process, this leads to an ion beam brightness an order of magnitude higher than produced by a liquid metal ion source. The source is also capable of producing a flux of $6.2\ifmmode\times\else\texttimes\fi{}{10}^{9} \mathrm{s}{}^{\ensuremath{-}1}$ that results in a substantial beam current of 1 nA once fully ionized.
In the ultracold FIB project a high brightness ion beam is developed for usage in a focused ion beam (FIB) source. This Focused Ion Beam can be used in applications where milling, imaging and deposition at the nanoscale needs to be done with increasing demands on control and resolution. The generation of the ion beam done in three stage principle. The first stage is the generation of the atomic beam and this is the subject of this work. The following stages first transversely cool and compress the atomic beam by using laser cooling techniques and then ionize and accelerate the atoms to form the ion beam. The generation of the atomic beam is done using a Knudsen cell; an effusive oven containing a vapor of rubidium atoms. The pressure difference between the vapor pressure in the oven and the vacuum outside the oven generates a flow of atoms from the oven to the vacuum. Only atoms that have a low transverse velocity can be captured by the laser cooling stage and will attribute to the ion beam. Therefore it is important to minimize the divergence in the beam and to this extent a collimating tube is used to connect the effusive oven to the vacuum system. The collimating tube minimizes beam divergence and compared to other collimating mechanisms it reduces the amount of rubidium loss, therefore greatly increasing the lifetime of the system.In order to measure the transverse velocity distribution, the transverse beam profile and the total flux, Laser-Induced Fluorescence measurements have been performed. These measurements show that the source generates an atomic beam with a particle flux ranging from 107 s-1 at a source temperature of 60 ºC to 1010 s-1 at a source temperature of 160 ºC. The flux is lower than predicted by theory and the transverse velocity distribution is broader than predicted by theory. However, the particle flux is stable within 1% and the installation of an active temperature controller promises to improve this stability even further. In the ultracold FIB project a high brightness ion beam is developed for usage in a focused ion beam (FIB) source. This Focused Ion Beam can be used in applications where milling, imaging and deposition at the nanoscale needs to be done with increasing demands on control and resolution. The generation of the ion beam done in three stage principle. The first stage is the generation of the atomic beam and this is the subject of this work. The following stages first transversely cool and compress the atomic beam by using laser cooling techniques and then ionize and accelerate the atoms to form the ion beam. The generation of the atomic beam is done using a Knudsen cell; an effusive oven containing a vapor of rubidium atoms. The pressure difference between the vapor pressure in the oven and the vacuum outside the oven generates a flow of atoms from the oven to the vacuum. Only atoms that have a low transverse velocity can be captured by the laser cooling stage and will attribute to the ion beam. Therefore it is important to minimize the divergence in the beam and to this extent a collimating tube is used to connect the effusive oven to the vacuum system. The collimating tube minimizes beam divergence and compared to other collimating mechanisms it reduces the amount of rubidium loss, therefore greatly increasing the lifetime of the system.In order to measure the transverse velocity distribution, the transverse beam profile and the total flux, Laser-Induced Fluorescence measurements have been performed. These measurements show that the source generates an atomic beam with a particle flux ranging from 107 s-1 at a source temperature of 60 ºC to 1010 s-1 at a source temperature of 160 ºC. The flux is lower than predicted by theory and the transverse velocity distribution is broader than predicted by theory. However, the particle flux is stable within 1% and the installation of an active temperature controller promises to improve this stability even further.
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.