G.R. Moloney

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.

Dead time corrected and charge normalised maps generated with the MicroDas fast data acquisition system

Abstract A new data acquisition system, called MicroDas, is capable of collecting data at rates of up to 100 k counts per second per station. To correct for the large dead times that are implicitly inherent with such high data rates, dead time information is also collected for each station. Since the count rate varies with different scan regions of a sample, dead time information is collected for each pixel of each station. To generate quantitative charge normalised maps of spectral features, charge information is also collected using an ultra sensitive charge-to-frequency converter. To evaluate this new system, examples are provided that demonstrate the improvement to maps when dead time and charge information are used to correct the original raw energy data. We conclude that the most quantitative accurate maps are generated when charge triggering with beam blanking is used.

High signal to noise level ion beam induced charge images

The use of MeV α particles to generate ion beam induced charge images with a signal to noise level approximately ten times larger than previously obtained using protons is described. The effect of α particle induced damage on the resultant image contrast is shown and a method of image formation in which the effects of ion induced damage are compensated for is described which enables the use of a higher ion dose.

Ion-optical review of the new Melbourne two-stage microprobe

Abstract A new two-stage microprobe is currently under construction at Melbourne. A magnetic quadrupole doublet in the first stage forms an intermediate beam focus midway along the beam line. The demagnified image is then demagnified again, by another doublet lens in the second stage. The calculated first-order properties, and aberration coefficients, of the system are presented. The relative advantages and disadvantages of the system are discussed. The two-stage microprobe is part of the ion-optical development program at Melbourne. Its current and envisaged role in this program is discussed.

A versatile system for the rapid collection, handling and graphics analysis of multidimensional data

Abstract The aim of this work was to provide a versatile system for handling multiparameter data that may arise from a variety of experiments — nuclear, AMS, microprobe elemental analysis, 3D microtomography etc. Some of the most demanding requirements arise in the application of microprobes to quantitative elemental mapping and to microtomography. A system to handle data from such experiments had been under continuous development and use at MARC for the past 15 years. It has now been made adaptable to the needs of multiparameter (or single parameter) experiments in general. The original system has been rewritten, greatly expanded and made much more powerful and faster, by use of modern computer technology — a VME bus computer with a real time operating system and a RISC workstation running Unix and the X Window system. This provides the necessary (i) power, speed and versatility, (ii) expansion and updating capabilities (iii) standardisation and adaptability, (iv) coherent modular programming structures, (v) ability to interface to other programs and (vi) transparent operation with several levels, involving the use of menus, programmed function keys and powerful macro programming facilities.

Measurement, analysis and modification of the fifth-order fringe field components of magnetic quadrupole lenses

Abstract Extensive, quantitative measurements of magnetic quadrupole lens fringe fields have been conducted. The fringe field region of magnetic quadrupole lenses has been shown to contain significant contamination by higher-order multipole fields. These multipole components will contribute to the aberration coefficients of the lens. One of the largest components is the duodecapole component, which contributes to the fifth-order geometric aberrations of the lens. The measured multipole profiles in the fringe field region of the Melbourne quadrupoles are presented. We also present the results of an investigation into the effect of modifying the quadrupole pole tip profile at the pole ends.

High velocity ion microprobes and their source requirements (invited) (abstract

For many years, beams of high velocity ions have been widely used for relatively nondestructive investigation of surfaces or thin films. Many properties may be investigated, associated with the large number of interactions, most of the information obtained being complementary to that from high velocity electron beams or low velocity ion beams. Recently there has been increasing interest in the application of microbeams of these high velocity ions and two streams of activity have emerged—one using traditional techniques and requiring beams of hundreds of picoamps to give spatial resolutions of the order of 1 μm, the other using many new techniques and requiring currents from femtoamps down to single ions to give resolutions of an order of magnitude better. In both cases, the resolution is limited by the low brightness of ion sources commonly used in high energy ion accelerators. These energies commonly are of the order of 3 MeV for very light ions, but there are several developing fields in which much high...

Design modifications to reduce duodecapole components in the fringe field region of magnetic quadrupole lenses

Abstract An investigation into minimisation of the duodecapole (fifth order) fringe field components of magnetic quadrupole lenses has been conducted at Melbourne. Detailed measurement and analysis of the fringe field region of the Melbourne magnetic quadrupole lenses has revealed the presence of significant fifth order multipole fields. A program of design modifications to the pole tip profiles has been conducted with the aim of reducing the duodecapole component of the fringe field. The design changes made and the results of detailed magnetic field mapping are presented.

Analysis of the fringe field region of magnetic quadrupole lenses: field measurements and ion optical calculations

An investigation into the minimisation of the duodecapole fringe field components of magnetic quadrupole lenses has been conducted at Melbourne. As part of this program, a detailed field mapping of the fringe field region of the Melbourne magnetic quadrupole lenses has been conducted (G.R. Moloney, D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 54 (1991) 24). The field mapping technique measures the radial magnetic field component, Br over the three cylindrical coordinates, r, α and z. We have also conducted a program of modifying the pole tip termination of the lenses to reduce the duodecapole field components. Complete magnetic field measurements for the modified lens pole tips have been presented previously (G.R. Moloney, D.N. Jamieson and G.J.F. Legge, Nucl. Instr. and Meth. B 77 (1993) 35–38). This paper presents an Ion Optical analysis of the effects of the fringe field components on the beam optics of the Melbourne Microprobe systems. The optical effects are calculated with a new computer code developed at Melbourne. Standard numerical ray tracing is performed utilising Runge-Kutta techniques.

Integrating microprobe laboratory automation with high speed data acquisition and analysis

We are developing a laboratory and beam line control system. The MpControl system utilises all the flexibility and power of a computer network. Target stages, power supplies, Faraday cups, beam monitors, event counters, etc. can all be controlled by any of a number of computers in the laboratory. Software on each of these computers allows control, monitoring and display of the state of the beam line, accelerator and target manipulation stage. The entire system may be simultaneously controlled from any computer terminal on the network. The system has the potential to allow a user to manipulate the target stage from the beam control room, or to adjust accelerator parameters from the target chamber at the end of the beam line. The system has been designed to be easily transportable across computer platforms, currently with support for UNIX, X-Windows and MS-DOS. We believe this is a critical factor in a world of rapidly advancing computer and instrumentation hardware systems. The system has been designed to integrate with the MpSys data acquisition system.