G. Bench

High resolution STIM

Abstract Scanning transmission ion microscopy (STIM) is a technique whereby the imaging resolution of a scanning proton microprobe (SPMP) can be improved by one or two orders of magnitude. This has major applications in medicine and biology where it may be needed for identification, resolution of fine structure or imaging a specimen of intrinsically low contrast. Bright field energy loss contrast is the most efficient mode of STIM operation and provides a means of examining sensitive specimens with negligible damage.

Changes in organic materials with scanning particle microbeams

Abstract The effects of scanning a 2 MeV alpha beam and a 3 MeV proton beam on organic targets were investigated. As organic materials we chose ∼ 1 μm thick Elvamide nylon and ∼ 2.5 μm thick mylar foils. Elemental concentrations were measured using unscanned and continuously scanned microbeams for different areas (from ∼ 20 × 20 μm2 up to ∼ 100 × 100 μm2) with frequencies from ∼ 10 to ∼ 1000 Hz. Scanning transmission ion microscopy (STIM) measurements were made before and after scanning to observe changes in the areal density of the samples.

Submicron STIM tomography reconstruction techniques

Abstract One-dimensional projections of ion energy loss have been converted to areal density using tabulated stopping powers in a scanning transmission ion microscopy (STIM) reconstruction of a test object, displaying a spatial resolution of 0.37 μm. Location and registration of the centre of areal mass density in each projection provides a means of overcoming (a) the misalignment of the specimen rotation axis with the centre of the scan path and (b) deviations of the specimen from its precession around the axis of rotation. Multiplicative algebraic reconstruction technique (ART), filtered backprojection and maximum-entropy algorithms for the reconstruction of the object are considered. Maximum entropy is shown to produce an image with the least number of artifacts. However, when time considerations are taken into account, it is found that filtered backprojection is the preferred algorithm for a tomographic reconstruction.

STIM with energy loss contrast: an imaging modality unique to MeV ions

Abstract Scanning transmission ion microscopy (STIM) through measurement of energy loss of individual ions is a quantitative imaging technique with several unique capabilities. The uniqueness derives conjointly from the large penetration with small scattering of MeV ions in low-Z specimens, from the simple relationship between energy loss and projected or areal density, and from the almost 100% efficiency with which one obtains pixel data from individual ions. Since contrast is in energy loss and not in numbers of events, the statistics of energy loss straggling affects the image but the statistics of counting does not. Small scattering makes it possible to observe details within transparent specimens. High efficiency makes it possible to collect large data sets for computed tomography, stereo, or high-definition imaging with a small radiation dose. High efficiency allows one to minimize aberrations by use of small apertures, to achieve good precision in the determination of areal density, or even to image live biological specimens in air since only one or a few ions per pixel are required. This paper includes a bibliography on STIM with MeV ions, it discusses the accuracy that one can achieve in the areal density coloring of a pixel with data from one or a few ions, and it supplements that review with recent examples from the Melbourne and the Eugene microprobes.

The design of a versatile scanning proton microprobe of high resolution and efficiency

Abstract The designer of a scanning proton (or nuclear) microprobe must make many decisions, some of which may be compromises. There is a wide range of lens types and configurations. Microprobe performance will depend on performance of the accelerator and its ion source, on stability and control of the lens current supply, on the nature of the microprobe supports, on the vacuum system, on magnetic shielding and connection to the accelerator. There are many possible modes of observation and analysis to be considered when the specimen chamber is designed and a versatile chamber should make provision for most of them. They include optical microscopy of front and back surfaces of the specimen, secondary electron imaging, X-Ray imaging, channelling contrast microscopy, Rutherford backscattering and forward scattering, nuclear reaction analysis and scanning transmission ion microscopy in brightfield and darkfield modes. Microprobe performance will also inevitably depend on the ease of operation and the extent to which the operator has been considered in the overall design and layout of the microprobe. The equally important considerations involved in data collection and analysis are discussed in a second paper.

STIM tomography: a three-dimensional high resolution imaging tool

Abstract The ion microprobes now found in many accelerator laboratories were developed to perform quantitative elemental microanalysis with high sensitivity. The rapid evolution on this instrument of new computer based techniques has led to the development of high resolution quantitative 3D ion microtomography. This technique offers unique opportunities to examine internal structure of microscopic specimens.

Applications of energy loss contrast STIM

Abstract Scanning transmission ion microscopy (STIM) with energy loss contrast is a quantitative imaging technique. A focussed MeV ion microbeam is scanned over the sample and measured energy losses of residual ions at each beam location are used to provide the contrast in the image. The technique is highly efficient as almost every ion carries useful information from which quantitative data can be obtained. The high efficiency of data collection at present necessitates the use of small beam currents. Therefore small apertures can be used and fine spatial resolution can be achieved. High efficiency also makes it possible to collect large data sets for high definition imaging with a small radiation dose. Owing to the simple relationship between energy loss and areal density, STIM with energy loss contrast can provide a quantitative image that can be used to obtain areal density information on the sample. These areal density maps can be used not only to provide a high resolution image of the sample but also to normalise particle induced X-ray emission (PIXE) data. The small radiation dose required to form these areal density maps also allows one to use STIM with energy loss contrast to quantitatively monitor ion beam induced specimen changes caused by higher doses and dose rates used in other microanalytical techniques. STIM with energy loss contrast also provides the possibility of stereo imaging and ion microtomography. STIM has also been used in conjunction with channeling to explore transmission channeling in thin crystals. This paper will discuss these applications of STIM with energy loss contrast and look at further developments from them.

Application of channeling scanning transmission ion microscopy

Scanning transmission ion microscopy (STIM) has been used in conjunction with channeling, to explore transmission channeling in thin epitaxially grown n-type silicon crystal. 1 MeV H+ beams and 2.5 MeV α beams were used with beam currents of 0.1 fA focussed to spot sizes < 200 nm. Part of the sample was damaged in the channel and a random direction with the same beams focussed to a ~15 μm diameter spot size and current up to 2 nA. Theoretical calculations predict the general behaviour of collected energy spectra in both the random and the channel direction. n nThe channeling STIM (CSTIM) technique can be 100% efficient, the analysis is achieved in a short time with negligible damage compared to backscattering channeling contrast microscopy (CCM), and it is capable of very high resolution (50 nm). These features can be successfully applied to the investigation of crystal damage and small size imperfections in samples transparent to the beam.

The mapping of unresolved spatial structure in STIM images

Abstract With scanning transmission ion microscopy (STIM), images are formed by taking an “average” of the energies of a number of ions within a pixel. Moments can be calculated by summing various powers of energy difference from this average. This paper will explore the use of moments in STIM imaging. Moments from both median and mean averaging have been calculated and compared to theoretical calculations for a beam scanned across an edge. The use of moments to emphasize regions of unresolved structure and to display areas of complexity within an object has been examined with a 0.220 μm latex sphere as an example. In the presence of noise from slit scattered ions and incomplete charge collection within the surface barrier detector, the optimal moment has been found to be the second moment of ions whose energy is greater than the “average” value. Examples of moments emphasizing regions of unresolved structure in animal tissue are presented.

High resolution channeling STIM in a thin crystal

Abstract Scanning Transmission Ion Microscopy (STIM) has been combined with Channeling Contrast Microscopy (CCM) to examine damage in a thin crystal (4 μm thick Si) with high spatial resolution (200 nm). Such measurements require very low beam currents (