Magneto-Optical-Trap-Based, High Brightness Ion Source for Use as a Nanoscale Probe

Abstract

We report on the demonstration of a low emittance, high brightness ion source based on magneto-optically trapped neutral atoms. Our source has ion optical properties comparable to or better than those of the commonly used liquid metal ion source. In addition, it has several advantages that offer new possibilities, including high resolution ion microscopy with ion species tailored for specific applications, contamination-free ion milling, and nanoscale implantation of a variety of elements, either in large quantities, or one at a time, deterministically. Using laser-cooled Cr atoms, we create an ion beam with a normalized rms (root-mean-square) emittance of 6.0 x 10 (-7) mm mrad M e V and approximately 0.25 pA of current, yielding a brightness as high as 2.25 A cm (-2) sr (-1) eV (-1). These values of emittance and brightness show that, with suitable ion optics, an ion beam with a useful amount of current can be produced and focused to spot sizes of less than 1 nm.

Low-Energy-Spread Ion Bunches from a Trapped Atomic Gas

“…This result will certainly hold for continuous operation of the source, because space charge effects will be much less important at demonstrated currents of 1.4 pA [9] and achievable currents of 100 pA [4]. The lowest energy spread observed here, U ¼ 0:02 eV, constitutes a 2 orders of magnitude improvement over the state-of-the-art liquid metal ion source.…”

“…After first preliminary experiments [8], the UCIS potential of delivering beams of LMIS brightness was recently demonstrated [9]. In this Letter we present detailed TOF energy spread measurements on rubidium ion bunches extracted from a UCIS.…”

“…Since energy spread can be conserved during further acceleration, chromatic aberration can be virtually eliminated at beam energies typical for FIB operation, i.e., 30 keV. In combination with the recently measured brightness of such beams [9], this implies that sub-nm focal spot sizes are within reach for Li þ , Cs þ and many other ionic species.…”

Time-dependent manipulation of ultracold ion bunches

“…This source, the ultracold ion source ͑UCIS͒, is based on the photoionization of laser-cooled atoms trapped in a magneto-optical trap ͑MOT͒. It can have a brightness that is comparable 3 to the current industry standard Ga-liquid metal ion source ͑LMIS͒ ͑Ref. 4͒ but with much lower energy spread, as we showed recently.…”

Ion microscopy based on laser-cooled cesium atoms

“…This means that ion or electron sources based on cold atom ionization would be able to create at low acceleration energy very small focal spots with relatively strong currents. Focused ion beams have been generated starting from both a magneto-optical trap (MOT) [10,11] and a slow and cold atomic beam [12]. Their properties as nanoprobes have been demonstrated first with chromium [13] and then with lithium [14] ions, the latter reaching a 27 nm resolution at 2 keV energy and 1 pA current.…”