Doping is the process of introducing a small amount of an impurity — a dopant — into a semiconductor (such as silicon) in order to control its electrical properties.

 

This makes it possible to create properties such as:

・N-type regions (rich in electrons)

・P-type regions (rich in holes)

・PN junctions (the foundation of the diode)

・The source and drain regions of a transistor

・Adjustment of threshold voltage (Vth)

 

Doping lies at the very center of why a semiconductor is able to function as a semiconductor at all.

Representative dopants include:

For N-type doping

・Phosphorus (P)

・Arsenic (As)

・Antimony (Sb)

 

For P-type doping

・Boron (B)

・Gallium (Ga)

・Aluminum (Al)

 

In modern logic devices, boron (for P-type) and arsenic/phosphorus (for N-type) are the dominant choices.

Doping falls broadly into two categories:

(1) Ion implantation

Ions are accelerated by an electric field and driven directly into the wafer.

Characteristics

・Allows extremely precise control over dose (concentration)

・Depth can be adjusted via implantation energy

・Occurs at low temperature, minimizing thermal effects

・The standard technique used across nearly all modern processes

 

(2) Thermal diffusion

Relies on the natural tendency of dopants to spread through silicon at high temperature.

Characteristics

・Produces uniform, deep diffusion

・Once the dominant method, now largely replaced by ion implantation

・Still used in certain sensors and power devices

Ion implantation is defined by two key parameters:

Dose The concentration of dopant introduced (per cm²).

・Low concentration: used for well formation and channel adjustment

・High concentration: used for source/drain formation (LDD/HDD)

 

Energy (acceleration energy) Determines how deep the ions are implanted (keV to MeV).

・Deeper implantation requires higher energy (used for wells and power devices)

・Shallower implantation uses lower energy (used in fine-pitch logic devices)

 

In particular, leading-edge logic devices require ultra-shallow junctions (USJ).

Immediately after ion implantation, the silicon’s crystal structure becomes disordered. Annealing — rapid heating — is used to repair this structure.

>Purposes

・Repairing damage to the semiconductor crystal

・Activating the dopant (enabling it to contribute to conductivity)

・Adjusting the depth of the PN junction

 

Representative technologies

・RTA (Rapid Thermal Anneal)

・SPA (Spike Anneal)

・Laser Anneal (localized heating)

 

As devices continue to shrink, there is a growing need for methods that activate dopants vertically without allowing them to diffuse laterally.

As devices shrink, the following challenges become increasingly apparent:

・Short-channel effects make voltage control increasingly difficult.

・Forming ultra-shallow junctions becomes more difficult.

・Damage caused by ion implantation becomes more severe.

・Dimensional variation in LDD/HDD regions increases.

・GAA (nanosheet) structures require uniform implantation across their sidewalls.

 

For these reasons, doping has become an increasingly demanding technology year after year.

The following trends are central to doping in today’s leading-edge logic devices:

Plasma doping (PLAD) Causes less damage to the resist mask.

Laser annealing Applies instantaneous, localized heating, suppressing lateral diffusion.

Monolayer doping (MLD) A research technology aiming for control at the level of a single atomic layer.

Wrap-around implantation for GAA structures A technique for achieving uniform implantation across the sidewalls of 3D structures.

In essence, the goal of miniaturization-era doping is to activate dopants vertically while preventing them from spreading laterally.

・Source/drain formation

・Well formation

・Threshold voltage (Vth) adjustment

・SRAM stabilization

・Voltage-tolerant structures in power devices

・Formation of resistive layers in sensors

 

Doping is an essential process for defining a semiconductor’s fundamental electrical character.

・Doping is the central process for giving a semiconductor its electrical properties.

・Ion implantation is the dominant method, with depth and concentration controlled via dose and energy.

・Annealing repairs the crystal structure and activates the dopant.

・Miniaturization has made forming ultra-shallow junctions increasingly difficult.

・3D structures such as GAA and FinFET require new doping techniques.

1.Explain the purpose of doping in a single sentence.

2.What do dose and energy each determine in ion implantation?

3.Why is annealing necessary?

 

As a bit of a side note — if I recall correctly, the topic of my own graduation thesis was the electric field distribution of delta doping in conductive superpolymers. I believe I wrote the program myself and plotted the results as graphs, but looking back on it now, I honestly find myself wondering how I ever managed to pull that off.

 

Column Supervisor: Koji Kakumoto (Otis Group Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined Otis Group Co., Ltd. in 2011. While primarily working in the Corporate Planning Department, he has also served concurrently in manufacturing and technical divisions, and since 2018 has served as Representative Director, working to drive business growth and strengthen the organization.

 

This article is a general technical explanation intended for educational purposes and does not refer to any specific company, product, or technology.