transient enhanced diffusion
**Transient Enhanced Diffusion (TED)** is the **anomalously rapid diffusion of dopants driven by an excess population of silicon interstitials released from ion implantation damage** — it causes boron junction profiles to spread far beyond equilibrium predictions during annealing, degrading short-channel control and historically limiting transistor miniaturization.
**What Is Transient Enhanced Diffusion?**
- **Definition**: A non-equilibrium diffusion phenomenon in which the diffusivity of boron (and other interstitial-diffusing species) is enhanced by orders of magnitude above its equilibrium value for a brief transient period following ion implantation annealing.
- **Interstitialcy Mechanism**: Boron diffuses primarily through a kick-out or interstitialcy mechanism — a mobile silicon interstitial displaces a substitutional boron atom, which then migrates as a boron-interstitial pair until it is re-incorporated at a new substitutional site.
- **Damage Release**: Ion implantation creates a supersaturation of silicon self-interstitials concentrated near the end-of-range. During annealing, these interstitials are released from {311} defect reservoirs and dislocation loops, flooding the region with mobile interstitials that dramatically accelerate boron diffusion.
- **Transient Duration**: TED persists until the excess interstitials recombine at surfaces, sinks, or with vacancies — typically a few milliseconds to seconds at temperatures above 900°C — after which diffusion returns to the equilibrium rate.
**Why Transient Enhanced Diffusion Matters**
- **Junction Blooming**: TED causes boron p+/n source and drain junctions to deepen and spread laterally by 10-50nm beyond what equilibrium diffusivity would predict, directly worsening drain-induced barrier lowering and short-channel threshold voltage roll-off.
- **Scaling Limiter**: TED was one of the primary physical barriers to transistor miniaturization below the 130nm node — conventional furnace anneals produced too much boron diffusion through TED, forcing the industry to adopt rapid thermal processing and eventually millisecond laser annealing.
- **Millisecond Anneal Solution**: Laser spike annealing heats the surface to 1300°C for only microseconds — too short for significant interstitial-driven diffusion to occur — enabling high activation with sub-nanometer junction movement, effectively suppressing TED.
- **Carbon Suppression**: Carbon co-implanted before boron traps excess interstitials through carbon-interstitial binding, reducing the interstitial supersaturation that drives TED and limiting boron profile spreading during anneal.
- **TCAD Modeling**: Accurate simulation of boron diffusion in implanted silicon requires coupled point-defect diffusion and reaction models (the two-state model) that track interstitial and vacancy concentrations self-consistently with dopant profiles.
**How TED Is Managed in Practice**
- **Pre-Amorphization Implant (PAI)**: Creating an amorphous layer with Ge or Si self-implantation before boron implantation localizes damage and separates the EOR defect band from the boron profile, reducing interstitial injection into the boron-containing region.
- **Low-Energy Implantation**: Using lower implant energies reduces the range of implant damage, keeping EOR defects shallower and further from the junction and reducing the interstitial flux driving TED.
- **Rapid Thermal Anneal Optimization**: Spike anneal profiles with very fast ramp rates and minimal time at peak temperature minimize TED by limiting the total time available for interstitial-boosted diffusion.
Transient Enhanced Diffusion is **the implant-damage penalty that forced the entire semiconductor industry to abandon furnace annealing** — understanding its physics drove the development of rapid thermal processing, laser annealing, and pre-amorphization that define modern source/drain engineering at advanced nodes.