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 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 52μm, a current of (170±4) 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 1×107 A/(m2sreV)

Design and experimental validation of a compact collimated Knudsen source

In this paper we discuss the design and performance of a collimated Knudsen source which has the benefit of a simple design over recirculating sources. Measurements of the flux, transverse velocity distribution and brightness at different temperatures were conducted to evaluate the performance. The scaling of the flux and brightness with the source temperature follow the theoretical predictions. The transverse velocity distribution in the transparent operation regime also agrees with the simulated data. The source was found able to produce a flux of $10^{14}$ s$^{-1}$ at a temperature of 433 K. Furthermore the transverse reduced brightness of an ion beam with equal properties as the atomic beam reads $1.7 \times 10^2$ A/(m${}^2$ sr eV) which is sufficient for our goal: the creation of an ultra-cold ion beam by ionization of a laser-cooled and compressed atomic rubidium beam

Ultracold ion beams using laser cooling

Focused ion beam instruments a re indispensable tools for the semiconductor industry due to their ability to image and modify structures on the nanometer length scale. For milling and deposition, the industry standard is the gallium liquid-metal ion source which enables a resorution of 5-10 nm at a current of a few pA. with the quest towards smaller features on integrated circuits, there is a need for novel ion sources that allow for better resolution. Several research groups are working towards applying laser-intensified alkali-metal ion beams for this purpose [1]. s-uch ultra-low temperature (1 mK) ion beams can be created by lasercooling and photo-ionization of a thermal atomic beam or vapor. The Rb ion source under development in Eindhoven in collaboration with FEI

Granularity effects in high-brightness electron beams

Electron sources based on laser-cooling and trapping techniques are a relatively new reality in the field of charge particle accelerators. The dynamics of these sources are governed by stochastic effects, and not by the usually dominant space-charge forces. As the high-brightness field moves towards increasingly higher brightness, these stochastic effects will play an increasingly important role. In this presentation I will discuss the physics of these granularity effects and show their effect using molecular dynamics simulations with the GPT code where we track each and every particle in realistic fields and including all pair-wise interactions

Two step photo-ionization of a laser cooled and compressed thermal atomic beam for use in a focused ion beam

Photo-ionization is applied to a laser cooled and compressed atomic rubidium\u3cbr/\u3ebeam in order to generate a high brightness ion beam. When focused, this ion beam can be used to image and edit integrated circuits at the nano-scale which is important for the ongoing reduction of feature sizes in the semiconductor industry. Experiments have shown that an atomic beam brightness in excess of 106 A/(m2 sr eV) can be achieved with a flux equivalent to 500 pA in a compact magneto-optical compressor which should be sufficient to generate ion spots of 1 nm. Currently, photo-ionization experiments are being carried out that aim at ionizing the majority of the atoms within a small longitudinal\u3cbr/\u3erange in order to minimize the longitudinal energy spread. The two step ionization setup uses a tightly focused excitation laser beam and a powerful blue laser coupled to a build-up-cavity

Direct magneto-optical compression of an effusive atomic beam for application in a high-resolution focused ion beam

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 properties similar to 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 (1.0+0.8−0.4)×106  A/(m2 sr eV) is achieved with a beam flux equivalent to (0.6+0.3−0.2)  nA. The temperature of the beam is further reduced with an optical molasses after the 2D MOT. This optical molasses increases the equivalent brightness to (6+5−2)×106  A/(m2 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 6× improvement over the liquid-metal ion source and could improve the resolution in focused ion-beam nanofabrication.\u3cbr/\u3e\u3cbr/\u3

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 magneto-optical compressor, 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 magneto-optical compression step an equivalent transverse reduced brightness of $(1.0\substack{+0.8\\-0.4})$ $\times 10^6$ A/(m$^2$ sr eV) was achieved with a beam flux equivalent to $(0.6\substack{+0.3\\-0.2})$ nA. The temperature of the beam is further reduced by applying sub-Doppler cooling behind the magneto-optical compressor. This increased the equivalent brightness to $(6\substack{+5\\-2})$ $\times 10^6$ A/(m$^2$ sr eV). When fully ionized this will be a six times improvement over the liquid metal ion source, which would improve the resolution in focused ion beam nanofabrication