Ion Implantation for Ultra-Shallow Junctions in FinFETs

Introduction#

Ion implantation remains the backbone of doping in modern semiconductor manufacturing. In advanced FinFET devices, precise control of junction depth and lateral abruptness is critical to suppress short-channel effects and maintain electrostatic integrity. This guide explains how implant energy, dose, and tilt angle affect junction formation, and how alternative doping and annealing techniques such as plasma doping (PLAD) and laser spike annealing (LSA) meet the ultra-shallow junction (USJ) challenge.

The vertical junction depth and lateral diffusion are not independent; they are governed by the same ion-solid interaction and subsequent thermal budget. Therefore, process integration engineers must co-optimize the implant recipe with the annealing strategy to achieve both low resistance and minimal dopant movement.

Impact of Ion Implantation Energy#

Implant energy controls the projected range of ions in the substrate. Higher energy places dopants deeper, resulting in a deeper junction after annealing. For ultra-shallow n-type and p-type regions, energies in the hundreds of electron volts or a few kiloelectron volts are required. For example, boron implants for extension regions often use energies below 0.5 keV to keep the as-implanted profile confined to the near-surface region.

However, lowering energy also reduces beam current efficiency and increases sensitivity to wafer charging and surface contamination. The implanted profile must be tailored with an energy that places the peak concentration just below the intended final position, because subsequent annealing will redistribute dopants further.

Role of Dose and Amorphization#

Dose determines the peak dopant concentration and the degree of lattice damage. A high dose can amorphize the silicon, which suppresses channeling during implantation but introduces end-of-range defects that promote transient enhanced diffusion (TED) during annealing. This is especially problematic for boron, which diffuses via an interstitial mechanism and can diffuse several tens of nanometers even during rapid thermal processing.

To achieve a high activated concentration for low sheet resistance, the dose must exceed the solid solubility limit while the anneal must be short enough to prevent excessive diffusion. For FinFETs, the dose also determines conformality into the fin sidewalls and under the gate spacer. Tilt and rotation angles are adjusted to implant the fin sidewalls and top surfaces as uniformly as possible.

Tilt Angle and Lateral Diffusion in FinFETs#

Tilt angle is used to suppress ion channeling along crystal axes. A conventional 7-degree tilt prevents axial channeling in planar substrates. In FinFETs, however, a tilt creates a shadowing effect: the fin that faces the beam receives a higher dose, while the opposite sidewall may receive a reduced dose or be completely shadowed. A quad implant with multiple rotations is often needed to dope all sides, yet this may still leave asymmetric lateral diffusion and junction depth variations across the fin height.

Lateral diffusion, or the sideways movement of dopants under the gate spacer, determines the effective channel length. Since lateral diffusion scales with the vertical diffusion length, any process that reduces the thermal budget will also reduce the encroachment of source and drain dopants into the channel. In a three-dimensional fin, lateral diffusion occurs both horizontally and vertically, so the as-implanted profile must be highly uniform to avoid localized doping spikes that degrade device matching.

Plasma Doping for Conformal Ultra-Shallow Profiles#

Plasma doping (PLAD) is an alternative to beamline ion implantation for ultra-shallow junctions. In PLAD, the wafer is immersed in a plasma containing the dopant species, and a negative bias is applied to the wafer to accelerate ions from the plasma sheath onto the surface. The implant energy is defined by the bias voltage, which can be reduced to very low values for junction depths below 10 nanometers.

The key advantage of PLAD in FinFET structures is its non-line-of-sight nature. Although ions are accelerated perpendicular to the wafer surface, the plasma sheath conforms to the fin topography to some degree, allowing doping of the top and sidewalls with a single process. However, maintaining uniform sheath thickness along the fin height is challenging. The bias power and pressure must be tuned to minimize sidewall shadowing and to prevent excessive sputtering. After PLAD, a millisecond anneal such as laser spike annealing is typically required to activate the dopants without redistributing them.

Laser Annealing for High Activation with Minimal Diffusion#

Laser spike annealing (LSA) uses a scanned laser beam to heat only the near-surface region to a high temperature (usually above 1100 degrees Celsius) for a period of hundreds of microseconds to a few milliseconds. Because the thermal budget is so short, dopants do not have enough time to diffuse significantly, even though the temperature is high enough to exceed the solid solubility limit. This enables a high activation percentage and a low sheet resistance for ultra-shallow source and drain extensions.

For FinFET integration, LSA is particularly valuable because the heat is absorbed near the surface where the fins are located, while the substrate remains comparatively cooler. This reduces the risk of dopant deactivation caused by thermal gradient issues. However, the patterned structure can cause reflectivity variations, so capping layers are often used to equalize optical absorption and to avoid melting. When LSA is combined with PLAD or with a low-energy beamline implant that creates a pre-amorphized layer, the junction depth can be kept below 10 nanometers with an abruptness of just a few nanometers per decade.

Process Integration Recommendations#

Selection of the proper implant and anneal sequence depends on the device architecture. In a FinFET, a common flow is: pre-amorphization implant with germanium, followed by a low-energy boron or phosphorus implant, then a laser spike anneal. If a plasma doping tool is used,