122 research outputs found
Atomic Beam Laser-cooled Ion Source : towards sub-nm ion beam milling
This work discusses the predicted performance of the Atomic Beam Laser-cooled Ion Source (ABLIS) and the progress in its experimental realization. The ABLIS is a new source for focused ions beams (FIBs), which are tools that are used on a large scale in the semiconductor industry, to image and modify structures on the smallest possible length scale. On the contrary to other FIB sources such as the Liquid Metal Ion Source (LMIS), the ABLIS is based on the fact that the ions are created from atoms with a very small spread in velocity instead of a very small spread in position. The biggest application of an ABLIS-based FIB will be so called milling in which material is physically etched at the nanometer length scale.In the ABLIS setup a beam of atomic rubidium is created from a Knudsen cell. This beam is laser-cooled and -compressed after which it is photo-ionized by means of a very intense laser. The ions will be accelerated immediately to their demanded energy (mostly 30 keV) and finally focused to an as small as possible spot by a set of electrostatic lenses. Since the ionization will take place over a certain region, the energy spread of the ion beam, and therefore also the amount of chromatic aberration of the lens system, will be proportional to the electric field.The bottleneck of the setup will be disorder induced heating, which was investigated in this work. It is the effect that ions, which were created at random initial positions, will heat up due to relaxation of the potential energy associated with these random initial positions. Investigations in this work show that this effect can be counteracted by increasing the electric field at the position they are ionized. A relation was found between the electric field needed to suppress disorder induced heating and the beam current. This relation was used to calculate the amount of chromatic aberration of the lens system as a function of the current. Using this information, an analytical calculation was performed of the possible spot size of the ABLIS setup, including all individual contributions to the spot size, i.e., the brightness, spherical aberration and chromatic aberration. The result showed that a spot size of 0.2 nm is possible at a current of 1 pA, compared to the 5 nm spot size which is possible with the LMIS. The calculation was verified with particle tracking simulations, which were in good agreement.In order to perform laser cooling and compression, a laser is needed which is stable and can be precisely detuned from the cooling transition in rubidium. Furthermore a repump beam is needed which is tuned to a different transition in rubidium. The laser system which matches these requirements was finalized in the work discussed here. A double pass acousto-optic modulator (AOM) configuration was built to detune the laser frequency. Furthermore, an electro-optic modulator (EOM) was added to the setup, to create the repump beam.An experimental setup was built in which the efficiency of laser cooling can be tested with laser induced fluorescence. Simulations of this setup were performed, which showed the setup should be capable of measuring the effect of laser cooling on the atom beam. Furthermore, a simulation of the atoms in the collimating tube of the Knudsen cell is set up. Its results are in good agreement with earlier performed measurements and a theoretical model. This work discusses the predicted performance of the Atomic Beam Laser-cooled Ion Source (ABLIS) and the progress in its experimental realization. The ABLIS is a new source for focused ions beams (FIBs), which are tools that are used on a large scale in the semiconductor industry, to image and modify structures on the smallest possible length scale. On the contrary to other FIB sources such as the Liquid Metal Ion Source (LMIS), the ABLIS is based on the fact that the ions are created from atoms with a very small spread in velocity instead of a very small spread in position. The biggest application of an ABLIS-based FIB will be so called milling in which material is physically etched at the nanometer length scale.In the ABLIS setup a beam of atomic rubidium is created from a Knudsen cell. This beam is laser-cooled and -compressed after which it is photo-ionized by means of a very intense laser. The ions will be accelerated immediately to their demanded energy (mostly 30 keV) and finally focused to an as small as possible spot by a set of electrostatic lenses. Since the ionization will take place over a certain region, the energy spread of the ion beam, and therefore also the amount of chromatic aberration of the lens system, will be proportional to the electric field.The bottleneck of the setup will be disorder induced heating, which was investigated in this work. It is the effect that ions, which were created at random initial positions, will heat up due to relaxation of the potential energy associated with these random initial positions. Investigations in this work show that this effect can be counteracted by increasing the electric field at the position they are ionized. A relation was found between the electric field needed to suppress disorder induced heating and the beam current. This relation was used to calculate the amount of chromatic aberration of the lens system as a function of the current. Using this information, an analytical calculation was performed of the possible spot size of the ABLIS setup, including all individual contributions to the spot size, i.e., the brightness, spherical aberration and chromatic aberration. The result showed that a spot size of 0.2 nm is possible at a current of 1 pA, compared to the 5 nm spot size which is possible with the LMIS. The calculation was verified with particle tracking simulations, which were in good agreement.In order to perform laser cooling and compression, a laser is needed which is stable and can be precisely detuned from the cooling transition in rubidium. Furthermore a repump beam is needed which is tuned to a different transition in rubidium. The laser system which matches these requirements was finalized in the work discussed here. A double pass acousto-optic modulator (AOM) configuration was built to detune the laser frequency. Furthermore, an electro-optic modulator (EOM) was added to the setup, to create the repump beam.An experimental setup was built in which the efficiency of laser cooling can be tested with laser induced fluorescence. Simulations of this setup were performed, which showed the setup should be capable of measuring the effect of laser cooling on the atom beam. Furthermore, a simulation of the atoms in the collimating tube of the Knudsen cell is set up. Its results are in good agreement with earlier performed measurements and a theoretical model
Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams
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 Rb 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 m, a current of 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 A/(msreV).Comment: 13 pages, 9 figure
Performance predictions of a focused ion beam based on laser cooling
Focused ion beams are indispensable tools in the semiconductor industry because of their ability to image and modify structures at the nanometer length scale. Here we report on performance predictions of a new type of focused ion beam based on photo-ionization of a laser cooled and compressed atomic beam. Particle tracing simulations are performed to investigate the effects of disorder-induced heating after ionization in a large electric field. They lead to a constraint on this electric field strength which is used as input for an analytical model which predicts the minimum attainable spot size as a function of amongst others the flux density of the atomic beam, the temperature of this beam and the total current. At low currents (
Direct magneto-optical compression of an effusive atomic beam for high-resolution focused ion beam application
An atomic rubidium beam formed in a 70 mm long two-dimensional
magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen
source, is analyzed using laser-induced fluorescence. The longitudinal velocity
distribution, the transverse temperature and the flux of the atomic beam are
reported. The equivalent transverse reduced brightness of an ion beam with
similar properties as the atomic beam is calculated because the beam is
developed to be photoionized and applied in a focused ion beam. In a single
two-dimensional magneto-optical trapping step an equivalent transverse reduced
brightness of A/(m sr eV) was
achieved with a beam flux equivalent to nA. The
temperature of the beam is further reduced with an optical molasses after the
2D MOT. This increased the equivalent brightness to A/(m sr eV). For currents below 10 pA, for which disorder-induced
heating can be suppressed, this number is also a good estimate of the ion beam
brightness that can be expected. Such an ion beam brightness would be a six
times improvement over the liquid metal ion source and could improve the
resolution in focused ion beam nanofabrication.Comment: 10 pages, 8 figures, 1 tabl
Application of laser-cooling to achieve an ultra-cold ion beam for FIB
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
Optimization of Gutzwiller Wavefunctions in Quantum Monte Carlo
Gutzwiller functions are popular variational wavefunctions for correlated
electrons in Hubbard models. Following the variational principle, we are
interested in the Gutzwiller parameters that minimize e.g. the expectation
value of the energy. Rewriting the expectation value as a rational function in
the Gutzwiller parameters, we find a very efficient way for performing that
minimization. The method can be used to optimize general Gutzwiller-type
wavefunctions both, in variational and in fixed-node diffusion Monte Carlo.Comment: 9 pages RevTeX with 10 eps figure
Proof for an upper bound in fixed-node Monte Carlo for lattice fermions
We justify a recently proposed prescription for performing Green Function
Monte Carlo calculations on systems of lattice fermions, by which one is able
to avoid the sign problem. We generalize the prescription such that it can also
be used for problems with hopping terms of different signs. We prove that the
effective Hamiltonian, used in this method, leads to an upper bound for the
ground-state energy of the real Hamiltonian, and we illustrate the
effectiveness of the method on small systems.Comment: 14 pages in revtex v3.0, no figure
Issues and Observations on Applications of the Constrained-Path Monte Carlo Method to Many-Fermion Systems
We report several important observations that underscore the distinctions
between the constrained-path Monte Carlo method and the continuum and lattice
versions of the fixed-node method. The main distinctions stem from the
differences in the state space in which the random walk occurs and in the
manner in which the random walkers are constrained. One consequence is that in
the constrained-path method the so-called mixed estimator for the energy is not
an upper bound to the exact energy, as previously claimed. Several ways of
producing an energy upper bound are given, and relevant methodological aspects
are illustrated with simple examples.Comment: 28 pages, REVTEX, 5 ps figure
Performance predictions for a laser intensified thermal beam for use in high resolution Focused Ion Beam instruments
Photo-ionization 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 Focus Ion Beam (FIB) instruments. Here we show using calculations and
Doppler cooling simulations that an atomic rubidium beam with a brightness of
at a current of 1 nA can be created using a
compact 5 cm long 2D magneto-optical compressor which is more than an order of
magnitude better than the current state of the art Liquid Metal Ion Source.Comment: 8 pages, 7 figures submitted to: Phys. Rev.
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