Laser cooling of a rubidium atomic beam:towards a high-brightness ion beam for nm-resolution focused ion beam milling

Steinar H. W. Wouters, G. ten Haaf, Jasper F. M. van Rens +4 more

Steinar H. W. Wouters, G. ten Haaf, T. C. H. de Raadt +2 more

Shw Steinar Wouters, S.B. van der Geer, G. ten Haaf +2 more

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.

G. Offermans

Brenda R. J. Jansen

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.

Steinar H. W. Wouters, G. ten Haaf, Peter H. A. Mutsaers +2 more

Ultra-low temperature (1 mK) ion beams can be created by photo-ionization of a laser cooled and compressed thermal atomic beam. Full device simulations show that for rubidium ions, spot sizes down to 1 nm can be achieved at a current of 1 pA.