**Figure 5.**

*The ion source magnet with optimized poles and yoke for a more uniform source magnetic field, as used with a 150 mm broad expanding beam in the implanter of Figure 4.*

SHC-80 [12] design shown in **Figure 3** used a uniform field 70-degree magnet, which provided the correct amount of horizontal focusing to render the strongly diverging beam from the resolving aperture parallel, but also provided some vertical focusing, thereby delivering a substantially parallel beam in two dimensions to the endstation of the implanter.

The control of uniformity was a more difficult problem. The first approach was crude and simple: to trim those regions of the ribbon beam where the current was too high with mechanically adjustable trimmers, nicknamed 'magic fingers'. To minimize sputter contamination of the silicon, these fingers were located inside the first magnet, at the local maximum in the beam width. Unfortunately, it was found that these only rendered the beam less uniform, for as each finger neared the beam, a plasma sheath formed beside it and the electric field in the sheath deflected the beam ions that came close to it sufficiently to generate peaks and valleys on the current profile across the final beam.

However, this provided a hint for the successful development and use of linear multipole lenses to correct the uniformity [16]. To raise the linear current density (measured in mA/cm along the beam breadth dimension) it would be necessary to slightly deflect ions from neighboring regions toward the low-density zone. To lower

**Figure 6.** *The multipole for uniformity control from the SHC-80 implanter.*

it, the opposite would be true. We therefore used linear multipole lenses, being a horizontal array of pairs of magnetic poles above and below the ribbon beam, to successfully achieve this end (**Figure 6**).

An alternative mechanical multipole style consisted of a movable set of fingers made of magnetic steel at the edge of one of the magnet poles, thereby allowing the local field to be slightly raised or lowered. This can be seen in **Figure 3**.

Subsequently, White developed a multipole variation of the 'Piccioni' quadrupole lens [17], to simplify the manufacture and implementation of these multipoles, comprising two parallel steel bars, each wrapped with an array of coils. Energizing one pair of opposite coils on these bars creates a local quadrupole field component, which raises or lowers the linear current density of the region of the ribbon beam passing between that pair of coils. This arrangement produces a smoother control of the uniformity from a simpler structure, though the underlying theory is the same. Passing a similar current through all the coils in this multipole generates a uniform broad quadrupole field, which can be used for correcting the beam parallelism. A pair of such quadrupoles can be used to simultaneously fine tune the breadth, the parallelism and the uniformity of a ribbon beam (**Figure 7**).

A further variation of this was developed by White [15], in which one multipole has the two steel bars linked by a piece of magnetic steel at one end, creating a

**Figure 7.** *A multipole lens based on the Piccioni quadrupole lens concept.*

U-shaped magnetic steel yoke. This version can develop an overall dipole field superimposed on quadrupole and multipole field components, and it can be useful for introducing an overall deflection of 5–15 degrees into the beam. This can be seen in **Figure 4**, as used in Mitsui Zosen flat-panel display implanters.
