Ion Implantation Effects on Junction Depth in FinFETs

Introduction to Ion Implantation and Junction Depth#

Ion implantation is the primary doping method in advanced logic because it offers precise control over dopant species, dose, energy, and angle. In a FinFET process, the key metrics are junction depth, lateral diffusion, dopant activation, and profile abruptness. Junction depth is set mainly by the implant energy, which determines the projected range and straggle of the dopant distribution. A lower energy places the peak closer to the surface and enables an ultra-shallow junction, while a higher energy drives the peak deeper and broadens the profile.

Dose controls the peak concentration and the total number of dopant atoms, but it also influences damage accumulation and amorphous layer thickness. Tilt angle is used to suppress channeling, to place dopants under the gate or spacer, and to manage shadowing in three-dimensional fins. The final junction depth, however, is not only an implant result. It is the combined outcome of implantation, damage, and subsequent thermal anneal steps that cause diffusion and activation.

How Energy, Dose, and Tilt Angle Shape the Profile#

Energy is the strongest lever on junction depth. In a simplified Gaussian profile, the peak concentration occurs near the projected range, and the depth where concentration falls to background defines the electrical junction. Lowering energy reduces the projected range and shortens the tail, which is essential for ultra-shallow junctions in FinFET source/drain extensions. However, very low energy can reduce beam current, increase surface contamination risk, and create a shallower but more defective layer.

Dose primarily changes concentration and damage. A higher dose can amorphize the silicon, which later helps solid-phase epitaxial regrowth and high activation during laser anneal. Excess dose can also broaden the profile through concentration-dependent diffusion during any high-thermal-budget step. Tilt angle affects channeling and geometry. A small tilt, typically a few degrees, reduces axial channeling that would otherwise create deep tails. A larger tilt can place dopants laterally under the spacer or gate, but it also increases shadowing in dense fin arrays and can create asymmetric doping on fin sidewalls.

For process integration, the practical sequence is to fix the required junction depth, then select the lowest energy that provides acceptable throughput and damage. Next, set the dose to meet resistance and activation targets without exceeding the solid solubility limit. Finally, tune tilt and twist to control lateral placement, channeling, and shadowing across the fin pitch. This order prevents rework because energy and dose changes shift the whole profile, while angle changes fine-tune the lateral and vertical coupling.

Lateral Diffusion and the FinFET Geometry Challenge#

Lateral diffusion is driven by thermal budget and by concentration gradients. In a planar transistor, lateral diffusion mainly affects overlap capacitance and short-channel effects. In a FinFET, the same diffusion occurs in a three-dimensional structure with a narrow fin, multiple crystal orientations, and thin sidewalls. Dopants can diffuse from the source/drain into the channel under the spacer, increasing overlap capacitance and degrading electrostatics. They can also diffuse along the fin sidewall, creating non-uniform junctions and higher parasitic resistance.

The fin geometry amplifies the impact of tilt angle and anneal. Because the fin is tall and narrow, an angled implant can shadow one side of the fin and dope the other side more heavily. The fin sidewalls may have different channeling behavior than the top surface. The effective junction depth on the sidewall is therefore not the same as the depth measured from the top. This is why lateral straggle, dopant diffusivity, and surface orientation must be co-optimized with the fin pitch and spacer width.

To control lateral diffusion, the process must minimize the thermal budget after the extension implant. Any high-temperature step, such as a spike anneal or an extended furnace anneal, broadens the profile. The goal is to activate dopants while keeping the dopant atoms close to their as-implanted positions. This requirement pushes advanced nodes toward millisecond and sub-millisecond anneals, and toward plasma-based doping methods that can place dopants very near the surface with low energy.

Plasma Doping and Laser Annealing for Ultra-Shallow Junctions#

Plasma doping, often called plasma-immersion ion implantation or plasma doping, offers a low-energy,