Compact Plasma and Focused Ion Beams

342 Compact Plasma and Focused Ion Beams

divergence (obtained from

γ

) is largest, while the beams spot size (obtained

from

β

) is minimum. The heavier ions focus at nearly the same point along the

beam axis (~1.7 cm), while the focal point of H

2

is closer to the last electrode.

It is evident that beam-limiting apertures with aperture sizes <100 μm will

only be effective in reducing the beam spot size further, which is a mechani-

cally challenging job. By varying the potential applied to the beam-limiting

aperture, the beam spot size can be controlled. This is investigated next.

*E –2 m

(a)

(b)

2D plot

0 –15

PE E

1

E

2

EL

1

EL

2

f

E

3

L

1

L

2

L

3

0 –15

–20 –30 0 –30

(k