Scanning Electron Microscopy and X-Ray Microanalysis

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31

SEM, to fully optimize the performance of the HIM it is nec-

essary to pay more attention to certain details. The ion source

offers just two adjustable controls—the “extractor” which

determines the field at the tip and so controls the emission of

the source, and the “accelerator” which determines the land-

ing energy of the beam on to the specimen surface. The

extractor module “floats” on the top of the potential deter-

mined by the accelerator setting, and both the brightness and

the stability of the ion source are affected by the extractor

module settings. As the extractor potential is increased, the

emission current rises before reaching a plateau at the so-

called “best imaging voltage” (BIV), which for helium ions

occurs at a field strength of about 44 V/nm depending on the

geometry of the tip itself. Exceeding the BIV will make the

beam less stable, and can result in so much damage to the

emitter that it may become necessary to reform the tip again

before reliable operation can be restored. The “accelerator”

control determines the actual landing energy of the ions on

the specimen. For a He+ beam this energy is usually in the

range 25–45 keV while for a heavier ion such as Ne+ the cor-

responding value is more typically in the 20–35-keV range.

In either case the yield of secondary electrons increases with

the accelerator setting, but continuous operation with the

system at energies of 45 keV or higher may cause problems

such as insulator breakdowns and discharges. It is therefore

good practice to record and save all the experimental param-

eters likely to be encountered.

31.3 Signal Generation in the HIM

Energetic electrons and ions travel through solid materials

while undergoing elastic and inelastic scattering events

until they either deposit all of their initial energy and come

to a halt, or are re-emitted from a sample surface and sub-

sequently escape while generating secondary electrons and

backscattered electrons or ions as they leave. Every such

beam particle trajectory is unique and so it is not possible

to predict in advance how deep, or how far, any particular

incident ion or electron might travel. Examples of Monte

Carlo simulations for ion beam trajectories are shown in

. Fig. 31.6.
The most probable depth reached by the beam, and the
horizontal spread of the beam, can both be estimated using
the formula developed by Kanaya and Okayama ( 1972 )
which assumes that the range R depends only on the incident
energy E of the incident particle and the density ρ of the
material through which the beam is traveling. The “K–O”
range is then given as

REKO− =κρp/ (31.2)

where RK-O is the beam range (in nm), ρ is the density of the

target material (in g/cm^3 ), k is a constant depending on the

choice of particle, i.e., electrons or ions, and P is a scaling

constant. For example, when using a helium ion beam then

H+

He+

Ne+

Ar+

Ga+

1

2

3

4

40keV beams in Mo (Blue=incident ion, red = recoiling target atom)

250 nm 80 nm 40 nm

250nm

250 nm 50 nm

50nm

. Fig. 31.6 Monte Carlo ion beam trajectory simulations for various ion species. E 0 = 40 keV

Chapter 31 · Ion Beam Microscopy