Plasma Doping

Plasma doping (PLAD) and plasma immersion ion implantation (PIII) both dope a wafer by immersing it directly in a plasma and pulling dopant ions out of that plasma with a bias voltage, rather than accelerating ions through a beamline the way conventional implanters do. That difference lets both techniques dope every exposed surface of a complex 3D structure simultaneously — something a directional beamline implanter, which only implants along its beam's line of sight, can't do at all.

No single beam angle can dope a trench sidewall the way a 3D-structure process often needs, and that geometric limitation is exactly what plasma immersion solves. A conventional beamline implanter delivers ions along one fixed direction; a FinFET fin's vertical sidewalls or a deep trench's walls sit at angles that a single beam simply can't reach efficiently, leaving those surfaces under-doped relative to horizontal surfaces facing the beam directly. Immersing the wafer in plasma surrounds every exposed surface with ions simultaneously, so sidewalls, trench bottoms, and top surfaces all receive dose from the same process step — a geometric capability a line-of-sight beam fundamentally cannot match.

Pulsed bias voltage, rather than continuous bias, is what keeps plasma doping from simply arcing or overheating the wafer surface. Applying a continuous high voltage to pull ions from the plasma sheath would sustain a continuous, hard-to-control discharge at the wafer surface; pulsing the bias instead delivers ion acceleration in short, controlled bursts with recovery time between pulses, giving process engineers a way to tune total dose and surface damage independently of each other — a level of control a continuous-bias approach would make much harder to achieve.

Plasma doping and PIII exist specifically because 3D device structures created a doping problem beamline implanters were never built to solve — reaching every exposed surface of a structure, not just the one facing the beam — and plasma immersion's non-directional geometry is the direct answer to that specific limitation.