Field emission tips, cleaning

FIG. 30. SEM micrographs taken with a microscope with a field emission tip (A) clean Au on Si substrate (B) CdTe deposit formed using 200 cycles on a substrate equivalent to that shown in A. 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

Directly across the deposition chamber from the FEM tip was a fluorescent screen and viewport with which the field emission pattern from the tip could be observed during cluster deposition. The tip was first cleaned by joule heating. The two fold symmetry characteristic of a clean W(110) field emitter was observed and the voltage necessary to observe this pattern was recorded. The voltage was then reduced to about 1/2 of the original value and the tip was exposed to clusters that had been slowed to thermal speeds in the gas cell. Clusters landing near the apex of the tip appear as bright dots on the fluorescent screen. This procedure was repeated until an individual cluster positioned near the apex of the tip was obtained. 

Figure 1 (a) Field emission image of a (1120) - oriented rhenium tip cleaned by thermal... 

This is a specialised technique which has been applied in field emission and field ion microscopy (see Section 2.1.5c). It is achieved by giving the tip a positive potential. Tungsten can then be removed at liquid helium temperatures with an applied field of 5.7 x 10 V.cm Perfectly regular surface structures are exposed containing many different lattice planes. Clean surfaces have been produced on tungsten, nickel, iron, platinum, copper, silicon and germanium. It is potentially applicable to a wide range of materials, but the area of clean surface exposed is only about 10 ° cm . 

Fig. 40. Effect of contamination on field emission pattern (61). (a) Clean Ni surface, showing smooth variation of emission intensity with orientation, (b) [100] oriented Ni tip, after exposure to oxygen (p 5 X 10 mm) and heat treating at T 900°K.

Fig. 13- Field emission micrographs from a llO -oriented W tip, all taken at an accelerating potential of 4.7 kV and photographed under identical conditions, (a) Clean pattern after flashing to 2500 K. (b) After deposition of 1 dose of W, from direction indicated- (c) After deposition of 7 doses of W. (d) Annealed tip exposed to N2 for 2 X 10 s Torr s. (e) Tip shown in (c) after exposure to N2 for 2 X 10 s Torr s, N2 pumped away, and then shadowed with 7 doses of W. (g) Up shown in (f) after annealing in vacuum at 700 K for 5 s. (h) Tip shown in (g) after annealing in vacuum at 800 K for 5 s. (i) Tip shown in (e) after annealing in vacuum at 800 K for 5 s. (Taken from ref. 245.)...

Reimann (57), who mounted two U-shaped hairpin filaments of different radii, one inside the other, with their mid-points about 1 mm. apart (see Fig. 18b). Under these conditions, the emission produced by a small accelerating field was restricted to the tip of the filament, thus eliminating the need for the internal shielding used by Langmuir and Kingdon (93). In this apparatus the S.P. for the adsorption of O2 on W was —1.70 v., in agreement with the value recalculated from Kingdon s thermionic data (57). Later Reimann (94) obtained a C.P.D. of d-1.7 v. between clean and thoriated W filaments. 

Fig. 59, Interaction of N2 with W at T = 300°K, as observed in the ion microscope, (a) Clean surface, formed by field evaporation at 20°K. Radius 400A. (b) Tip after first room-temperature addition of nitrogen. Arrow points to one of the new emission centers formed by interaction with nitrogen, (c) Same surface after heavy dosing. All photographs at 20<>K.

An FEEM image of each tip was taken without breaking vacuum to record the field-electron emission pattern of the clean tip surface (Figure 5A and 5D). 

In practice, only two materials are in common use. A Pt-Ir (typically 80 20) alloy wire tends to be used for STM scanning in air, while W wire is used as tip material for UHV operation. Electrochemical polishing is the preferred method in both cases for producing good tips with small radii of curvature (=10 nm) and high aspect ratios (better than 3 1). There are recipes to be found in the literature [26,27]. When W tips are used, it is essential that the tips be cleaned in UHV by field desorption/emission in order to remove the oxide layer and any other contamination. 

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