H.N. Southworth

The preparation of field electron/field-ion emitters by ion etching

Abstract A new technique is described for the fabrication and resharpening of field emitters. This technique employs an ion gun and uses methods analogous to those used to produce thin foils for the transmission electron microscope. In this way it has proved possible to produce reliably sharp emitters of both metal and semiconducting materials and the technique is particularly useful when dealing with those materials normally difficult to electropolish or chemically etch. Applications lie not only in field electron emission and field-ion microscopy, but may also be relevant to the development of high current electron sources for use in electron microscopy.

Field-ion microscope observations of helium ion bombardment damage in tungsten

Abstract The field-ion microscope has been used in a pilot investigation of the damage caused to tungsten surfaces following bombardment with low-energy helium ions. Such effects could be important because the first wall of a fusion reactor will have to withstand high doses of these particles. The damage observed following bombardment with total doses in the range 10 13 to 5 × 10 18 ions cm −2 has been found to be dose dependent. These results have then been related to those derived from studies using more conventional techniques and this has led to the conclusion that the origin of the observed damage lies in the formation of near-surface gas bubbles and surface blisters.

The projection geometry of the field-ion image

Abstract An expression for the projection geometry of the field-ion microscope is derived and it is shown how a single parameter describing the projection relationship may be obtained from measurements on the micrograph. The relationship of experimentally obtained projections to the stereographic case is discussed, as is the influence of varying emitter radius.

The Moiré Pattern Produced by Overlapping Zone Plates

Two separate approaches have been put forward to account for the moire pattern produced by two overlapping zone plates. The analysis has then been extended to include the case of three overlapping zone plates. Several new relationships regarding the formation of the moire pattern have been deduced and these are expected to find a more general application, particularly in the interpretation of images formed in the field-ion microscope.

A combined fim, aes and LEED study of the structure and composition of ion bombarded tungsten surfaces

Abstract A number of experimental techniques have been used to gain information on the structure and chemical composition of tungsten surfaces following low energy ion bombardment using inert gases. Field-ion microscopy reveals the nature and depth distribution of the lattice defects that are formed, but is unable to determine whether any departure from true crystallinity has been induced; neither can it detect whether the inert gas species becomes trapped. On the other hand, low energy electron diffraction shows that the bombarded surface remains effectively crystalline, whilst Auger electron spectroscopy reveals the extent to which the different bombardment species become trapped, and the effect that such trapping has on the LEED patterns. In this way it has been possible to obtain a full description of the processes that occur, and to come to some conclusions regarding the relative effectiveness of the different ion species in sputter-cleaning metal surfaces.

Observations of sputtering damage using the field-ion microscope

Abstract The field-ion microscope has been used to study the effects of bombarding tungsten surfaces with low energy noble gas ions. This technique is particularly useful because the collision events which contribute to the process of sputtering occur essentially on the atomic scale. By bombarding with neon, argon and xenon ions in the energy range 100 eV to 1 keV it has been possible to obtain information on the type and distribution of the lattice damage produced, together with the effects of annealing. These results are relevant to those surface techniques such as LEED, AES and XPS which employ low energy ion bombardment as a means of removing surface contamination. Although surfaces treated in this way may be chemically clean their exact topography has previously remained undefined.

The influence of manganese on free surface segregation in a low alloy steel

Abstract Auger electron spectroscopy has been employed to study free surface segregation in a 3.5Ni-Cr-Mo-V steel of varying manganese content. The study was designed to complement an earlier investigation of grain boundary segregation in the same materials. In situ heating at 450 °C initially produced a substantial amount of sulphur segregation but only minimal phosphorus segregation to the free surface; in the grain boundary case, however, phosphorus was the principal segregant, and sulphur segregation was not detected. Examination of surfaces which had been heated-sputtered-heated revealed examples of site competition, particularly between sulphur and phosphorus. Alloying element interactions were also observed; for instance, materials with a high manganese content were found also to exhibit a higher surface nickel content, consistent with the previous grain boundary observations.

A moiré interpretation of field-ion microscopy

Abstract It is shown that the atomic arrangement on the surface of a field-ion emitter may be interpreted as a moire pattern. This method of analysis enables new kinds of information to be obtained from field-ion micrographs.

The optimum ion species for sputter-cleaning or ion profiling tungsten surfaces

Abstract Removal of atomic layers by means of low-energy ion bombardment (sputtering) has become an important technique in surface technology. It is used not only to prepare clean surfaces prior to in situ experiments using various surface analysis techniques, but also to obtain composition profiles. As well as these more scientific applications the technique is also utilised in more directly technological ways, such as in ion beam micromachining. However, this paper is more concerned with the former. The problem is that, although the technique is effective in producing surfaces which are chemically clean, the precise structural condition of the surface after such treatment is not well established. Moreover, little attention has been paid to effects arising from the inert gas atoms which may become implanted into the surface and sub-surface regions. The factors that need to be considered in formulating the optimum conditions for these ion erosion procedures are as follows: (a) the sputtering yield, (b) the extent of inert gas ion implantation, and (c) the depth of bombardment damage. Some field-ion microscope results on the last of these factors are combined with existing data on the remainder to make recommendations for the most effective gas species that may be used, consistent both with the amount of lattice disorder produced and the post-annealing treatments that are feasible.

The Moiré Pattern Formed on Superposing a Zone Plate with a Grating or Grid

A detailed analysis is made of the moire pattern formed on superposing a Fresnel zone plate with an equispaced grating or a rectangular grid. Information regarding the focal length, location and phase of the moire zone plates so produced is extracted. The relevance of this work to a number of physical situations is discussed.