New Generation Nuclear Microprobe Systems

Over the past 20 years, the minimum probe size for nuclear microscopy has stayed around 1 micrometre. No breakthroughs in nuclear microprobe design have been forthcoming to produce dramatic improvements in spatial resolution. The difficulties of breaking the constraints that are preventing reduction of the probe size have been well recognised in the past. Over the past 5 years it has become clear that some of these constraints may not be as limiting as first thought. For example, chromatic aberration clearly is not as significant as implied from first order ion optics calculations. This paper reviews the constraints in view of the increased understanding of the past 5 years and looks at several new approaches, presently being evaluated in Melbourne and elsewhere, on how to make progress. These approaches include modified RF ion sources for improved beam brightness and exploitation of relaxed constraints on some lens aberrations allowing the use of high demagnification probe forming lens systems. INTRODUCTION AND MOTIVATION The quest for improved spatial resolution in nuclear microprobes has a long history and is the common aspiration of many workers in the field [1]. The first nuclear microprobe system was constructed at Harwell around 1970. It attained a spatial resolution of 2.2 microns with a beam current of 250 pA as recorded in a list of nuclear microprobes compiled by Cookson in 1987 [2]. The beam current is given because, traditionally, spatial resolution measurements are divided into two regimes. These are the high current regime, for a nominal 100 pA beam current, usually regarded as the minimum possible for measurements involving Rutherford Backscattering Spectrometry (RBS) or Particle Induced X-ray Emission (PIXE) and the low current regime for 1 fA or lower beam currents, usually regarded as being appropriate for the techniques of Scanning Transmission Ion Microscopy (STIM) or Ion Beam Induced Charge (IBIC). In the discussion to follow, it is the high beam current regime that applies unless otherwise stated. Paper MF-O1 Presented at the 7 International Conference on Nuclear Microprobe Technology and Applications, Cité Mondiale, Bordeaux, France, 10-15 September 2000