G.J.F. Legge

The Melbourne proton microprobe

A scanning proton microprobe is described which operates in ultra‐high vacuum with a resolution of 10 μm. The operating principles and main features of the design are discussed and the ability of such an instrument to detect trace elements down to a few parts per million by mass is illustrated.

Aberrations of single magnetic quadrupole lenses

Abstract A number of analytical instruments make use of strong focusing magnetic quadrupole lenses. Such lenses suffer from aberrations which may degrade the performance of the instruments. This paper presents a modification of the grid shadow method which may be used to measure the aberrations of magnetic quadrupole lenses. The method is applied to a system which consists of a single quadrupole and a single octupole and the third order effect of the octupole is shown to be in good agreement with theory. The method is also applied to measure the second and third order parasitic aberrations of the single quadrupole lens components of the Melbourne Proton Microprobe.

Scanning transmission microscopy with a 2 MeV alpha particle microbeam

Abstract A scanning 2 MeV He + microbeam has been used to form images of targets by scanning transmission ion microscopy. Silicon surface barrier detectors at 60 ° and 0 ° detected Rutherford scattered and transmitted α particles. Collimation of the 0 ° detector allowed bright field and dark field imaging. Image contrast was due to the target thickness dependences of multiple scattering and energy loss. Cu grids, diatoms and 0.22 μm diameter latex calibration spheres have been imaged rapidly and efficiently. Spatial resolution of 0.3 μm fwhm is demonstrated and thickness sensitivity is discussed. Comparisons with other imaging methods are made and applications are discussed.

Microbeam imaging at micron and submicron resolution

Abstract The accurate location, identification and irradiation of a specimen in a scanning proton microprobe or similar instrument and the correct interpretation of the collected data often depends critically on the spatial resolution and information content of any images that can be formed. The accommodation of all imaging requirements in turn places severe demands on the instrumentation. The imaging techniques discussed include light optical, secondary electron, PIXE, backward and forward scattering, channeling contrast and transmission imaging with energy loss and scattering contrast. The images may be bright field or dark field and live or processed and they make available a wide range of information concerning the specimen. Some images, such as the PIXE maps, provide information at 1 μm resolution. Others, such as scanning transmission maps, are an order of magnitude better. The imaging requirements for several fields of application are discussed, with particular reference to the fields of semiconductors, biology and medicine.

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.

Channeling scanning transmission ion microscopy

Scanning transmission ion microscopy (STIM) has been used, in conjunction with channeling, to explore transmission channeling in 50‐μm‐thick epitaxially grown n‐type silicon with 3.9 MeV H+ beam currents of 0.1 fA focused to spot sizes of less than 200 nm. The technique is extremely efficient, causes negligible damage, and is capable of very high resolution. High‐resolution images of crystal damage were obtained with this first demonstration of channeling contrast in STIM.

The measurement and correction of spherical aberration in a magnetic quadrupole quadruplet lens system

Abstract The aberrations of a magnetic quadrupole quadruplet lens used as a probe forming lens in a microprobe system have been studied with the grid shadow method. From a comparison of theoretical and experimental grid shadow patterns the system is shown to suffer from parasitic aberration (possibly from mechanical misalignments) as well as spherical aberration. Three magnetic octupole lenses were designed and built with the aim of correcting the spherical aberration. It was found that the parasitic aberration prevented complete correction of all spherical aberration by the three octupoles but the largest part of the spherical aberration, the cross terms, could be corrected with a single octupole.

Multipole lenses and their application in nuclear microprobe lens systems

Abstract In principle, aberrations of nuclear microprobe lens systems can be corrected with multipole lenses. This paper discusses multipole lenses and how they may be used to correct divergence dependent aberrations in a quadrupole probe forming lens system. Also discussed is the grid shadow method which provides a practical means for experimental study of the properties of the multipole lenses as well as a potential means for adjustment of the multipoles to achieve correction. Results are presented showing the effect of sextupole, octupole and duodecapole lenses in simplified lens systems that consisted of a single magnetic quadrupole and a single magnetic multipole. Further results are presented that show the grid shadow method applied to the magnetic quadrupole Russian quadruplet of the Melbourne Proton Microprobe. In this case the system was found initially to suffer from large parasitic aberration, probably octupole, which was later significantly reduced by replacement of some damaged pole tips.

The use of a scanning proton microprobe to observe anti-HIV drugs within cells

A series of inorganic polyanions (viz. heteropolytungstates) has been shown to have antiviral activity but there was no evidence to indicate that the drugs reached their site of antiviral (HIV) activity intact. We have shown that with a scanning proton microprobe it is possible to analyse the metal content of individual cells (PBLs) treated with such a polyoxometalate drug and to determine the atomic ratio of the metals within the cell. This was found to be near that in the drug. The distribution of the metals (tungsten and cobalt) within the cell was measured and it was shown that both metals were located in the same region within the cell. These findings would suggest that the drug had entered the cells intact.

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.