The oxidation process is the step in which a layer of SiO₂ (an oxide film) is formed on the surface of silicon.

 

This oxide layer plays an extremely important role in semiconductor manufacturing, serving functions such as:

・The gate insulating layer of a transistor

・Insulation between wiring layers

・Passivation (surface protection)

・A masking layer (protecting against ion implantation or etching)

 

In particular, the gate insulating layer of a MOSFET is a core component that directly determines device performance. The precision of the oxidation process determines both electrical characteristics and reliability.

There are two main methods for forming an oxide layer:

(1) Thermal oxidation

The silicon wafer is placed in a high-temperature furnace (900–1,100°C) and reacted with oxygen or water vapor, allowing SiO₂ to grow naturally.

・Dry oxidation (using O₂)

・Wet oxidation (using H₂O / water vapor)

Dry oxidation produces high quality and is well suited to thin films, while wet oxidation grows faster and is well suited to thicker films.

 

(2) CVD oxide films (LPCVD / PECVD)

SiO₂ films are deposited using a chemical reaction.

・Can be performed at low temperatures

・Well suited to forming thicker films

・Commonly used as the insulating layer in BEOL processing

 

The key distinction to remember: thermal oxidation involves “growing” the film, while CVD oxidation involves “depositing” it.

Dry oxidation

・Produces the highest film quality.

・Used for high-quality thin films, such as gate oxide layers.

・Grows relatively slowly, allowing precise control at the few-nanometer scale.

 

Wet oxidation

・Uses water vapor to quickly form thicker films.

・Used for field oxide layers, such as in LOCOS processes.

 

Film quality is somewhat lower than that achieved with dry oxidation

SiO₂ was traditionally the material of choice, but as miniaturization pushed film thickness below 1nm, tunneling leakage current began to increase significantly.

 

As a result, the industry has shifted toward the following materials:

・HfO₂ (hafnium oxide)

・ZrO₂

・Al₂O₃

 

These are known as high-k dielectric materials. Using them as the gate insulator makes it possible to maintain an effectively larger thickness while still reducing leakage current.

Oxidation furnaces fall broadly into two types:

・Batch furnaces (processing many wafers at once)

・Single-wafer furnaces (processing one wafer at a time, with high uniformity)

 

Key points that must be controlled include:

・Film thickness uniformity

・Temperature uniformity

・Growth rate

・Film quality (density, defects, interface quality)

・Management of particle and metal contamination

 

Oxide film quality directly affects transistor lifespan and reliability.

A MOSFET’s electrical characteristics are extremely sensitive to the quality of the interface between silicon and SiO₂.

 

A poor interface can cause:

・Reduced carrier mobility

・Variation in threshold voltage

・Increased flicker noise

・Reduced reliability (TDDB degradation)

 

For this reason, an annealing step (using N₂ or H₂) is often performed after oxidation to repair the interface.

Oxide layers are used throughout virtually every part of a semiconductor device:

・Gate insulating layers

・STI (shallow trench isolation) insulation

・Interlayer insulation between wiring layers

・Passivation layers

・Ion implantation masks

・LOCOS processes

 

Given this range of uses, oxidation is one of the foundational technologies of semiconductor processing.

・The oxidation process is a critical step for forming SiO₂ films.

・The distinction between thermal oxidation (high quality, “growth”) and CVD oxidation (deposition) is important.

・As devices shrink, gate oxide layers are increasingly shifting to high-k materials.

・Interface quality has a major impact on device performance.

・Oxide layers are used throughout the manufacturing process, from device formation all the way through to wiring.

1.Explain the difference between dry oxidation and wet oxidation.

2.Why are high-k materials used?

3.What role does the oxide layer play in a MOSFET?

 

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.