implant depth / junction depth

Implant depth (projected range, Rp) and junction depth (Xj) define how deep implanted ions penetrate into silicon and where the dopant concentration equals the background doping—critical parameters determining transistor channel length, junction capacitance, and leakage current. Projected range (Rp) is the average depth of the implanted ion distribution, determined by implant energy, ion mass, and target material. Higher energy = deeper Rp; heavier ions = shallower Rp at same energy. For example, boron at 10 keV has Rp ≈ 35nm, while arsenic at 10 keV has Rp ≈ 7nm. The implanted profile approximates a Gaussian distribution centered at Rp with standard deviation ΔRp (straggle). Junction depth (Xj) is where the implanted dopant concentration equals the substrate background concentration—this is the metallurgical junction that defines the p-n junction location. Xj is always deeper than Rp because the Gaussian tail extends beyond the peak. Xj increases significantly during post-implant annealing as dopants diffuse thermally. For advanced CMOS nodes: source/drain extension Xj targets are 5-15nm (sub-7nm nodes), requiring ultra-low energy implants (0.2-2 keV), heavy ions (BF₂⁺, As⁺), and millisecond annealing to activate dopants with minimal diffusion. Measurement techniques include SIMS (secondary ion mass spectrometry) for dopant concentration profiles, spreading resistance profiling (SRP) for carrier concentration, and four-point probe for sheet resistance (Rs), which relates to Xj through Rs = 1/(q × μ × N × Xj) for uniform profiles.