High-Current Ion Implanter
Ion implanters are classified by beam current and energy — high-current, medium-current, high-energy, medium-energy, and ultra-low-energy — because each combination solves a different doping problem, and no single implanter design covers the full range well. High-current machines push dose into the wafer fast; high-energy machines push ions deep; ultra-low-energy machines do the opposite, barely pushing ions past the surface at all, which turns out to be its own serious engineering challenge.
Ultra-low-energy implantation is harder than it sounds, because the same positive charges that need to go into the wafer repel each other strongly enough to blow the beam apart. At very low energy, ions in the beam move slowly and spend more time near each other, so their mutual electrostatic repulsion — space-charge blow-up — spreads the beam out before it ever reaches the wafer, degrading both dose uniformity and the sharpness of the resulting junction. Ultra-low-energy implanters are specifically engineered with shortened beamlines and specialized beam transport to minimize how long ions spend fighting that repulsion, which is why they're a distinct equipment category rather than just a dial-down setting on a standard implanter.
High-energy implantation earns its own equipment class because reaching deep implant depths needs acceleration stages a standard medium-current machine simply doesn't have. Implants for retrograde wells or buried isolation layers need ions to penetrate microns into the silicon, well beyond what a standard medium-energy beamline can reach — high-energy implanters add additional RF or electrostatic acceleration stages specifically to push ion energy into that deep-implant range, at the cost of a physically larger, more complex machine than a shallow-implant tool needs.
The five implanter classes aren't marketing tiers so much as five distinct beamline engineering problems — handling high dose fast, handling deep penetration, or handling a beam that wants to blow itself apart at low energy — and a fab typically runs several different implanter classes side by side because no one design does all five jobs well.