Nuclear Stopping Power

# Nuclear vs. Electronic Stopping Power in Ion Implantation

An implanted ion doesn't travel in a straight line to a fixed depth and stop. It loses energy continuously along a jagged path, through two completely different physical mechanisms that trade off with its own speed — and which one dominates at any given moment decides whether the ion's final resting depth is sharp and predictable or smeared into a long, damage-heavy tail. That tradeoff is stopping power: how fast the ion sheds energy per unit path length, split into a nuclear term and an electronic term that don't peak at the same energy.

Nuclear stopping is a billiard-ball collision; electronic stopping is friction. Nuclear stopping happens when the incoming ion's positively charged nucleus gets close enough to a target atom's nucleus to feel its screened Coulomb repulsion, and the two scatter off each other elastically — each collision can transfer a large chunk of energy in one event, and it kicks the target atom clean out of its lattice site, which is the mechanism that actually creates crystal damage. Electronic stopping is the ion dragging through the target's sea of valence and conduction electrons, continuously losing small amounts of energy to electronic excitation the way a fast object loses energy to viscous drag — no single electronic-stopping event displaces an atom, it just slows the ion down.

Two Ways an Implanted Ion Loses Energy same ion, same path — nuclear collisions displace atoms, electronic drag just slows it down NUCLEAR STOPPING — collision knocked-out atom (vacancy left behind) large-angle scatter, big energy transfer creates the damage that annealing has to repair ELECTRONIC STOPPING — drag electron sea continuous, no single large deflection slows the ion down, lattice stays intact

The balance flips as the ion itself slows down. Nuclear stopping needs the ion to linger near a target nucleus long enough for the Coulomb interaction to act, which favors slow ions; electronic stopping scales with how fast the ion moves through the electron sea, which favors fast ions over most of the practical implant-energy range. A decelerating ion enters mostly electron-drag-dominated near the surface and crosses over into nuclear-collision-dominated as it slows near the end of its range — so most of an implant's lattice damage concentrates near the end of the ion's path, not spread evenly along it.

Stopping Power vs. Ion Energy — Where the Crossover Happens electronic drag dominates while the ion is fast; nuclear collisions take over as it slows near end-of-range ion energy (keV) — high at left, low at right as it decelerates → stopping power electronic stopping (drag) nuclear stopping (collisions) crossover — damage ramps up here end-of-range damage pile-up Light, fast ions (e.g. boron) stay electronic-dominated longer; heavy, slow ions (e.g. arsenic) cross over earlier and damage more per unit depth.

This isn't textbook physics left on the shelf — it's what every implant-profile simulator actually computes. James Ziegler, Jens Biersack, and Mark Littmark's stopping-power tables (the "ZBL" formalism) underlie SRIM/TRIM, still the reference Monte Carlo codes process integration teams run to predict projected range and straggling before committing a recipe to a production implanter. Species and energy selection trade directly on the nuclear/electronic balance: light ions like boron stay electronic-dominated over more of their path, giving deeper range and less damage per unit depth, while heavy ions like arsenic or antimony cross into nuclear-dominated stopping earlier, giving shallower range with damage concentrated near the surface — a lever foundries pull deliberately when a target junction depth and a damage budget conflict. Preamorphization implants, typically germanium run on the same Applied Materials or Axcelis platforms used for the dopant species, exploit nuclear stopping's damage mechanism on purpose, disordering the surface so a later dopant implant's channeling tail gets suppressed.

Stopping power is the physics a dose-and-energy recipe is quietly built on top of: it decides how deep an ion goes and how much lattice damage it leaves on the way, before the dose-rate effect ever gets a chance to decide whether the wafer survives delivering it.

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