Etching is the process of selectively removing unwanted portions of a film in order to form a circuit pattern.

 

Using the pattern drawn in the photoresist during lithography as a mask, etching removes the underlying film to shape fine structures.

 

In semiconductor manufacturing, lithography and etching work together as a combined technology that transforms a two-dimensional design into a three-dimensional structure.

Etching falls broadly into two categories:

(1) Wet etching

Dissolves a film using a liquid chemical.

 

(2) Dry etching

Uses plasma to remove a film through a gas-based reaction.

 

Today, as devices have continued to shrink, dry etching has become the dominant method used across nearly all applications.

Wet etching chemically dissolves a film using a liquid, such as HF or H₃PO₄.

Characteristics

・Relatively simple equipment

・Low cost

・Isotropic etching (dissolves at the same rate both vertically and horizontally)

・Poorly suited to fine-scale processing

・Also used for removing photoresist (resist stripping)

 

Representative examples

・SiO₂ → etched using HF

・Al → etched using a phosphoric-acid-based etchant

 

In leading-edge manufacturing, wet etching is now mainly used for CMP cleaning and in back-end processing.

Dry etching can be divided into three main categories:

(1) Plasma etching

Converts a gas into plasma and removes the film through a chemical reaction.

・SiO₂ → etched using CF₄ or CHF₃

・Si → etched using Cl₂ or HBr

Characteristics

・Etching occurs via a chemical reaction

・High selectivity, removing the target film while protecting the resist and underlying layers

 

(2) Reactive Ion Etching (RIE)

A hybrid method combining plasma with ion bombardment.

・Provides directionality (anisotropy), allowing vertical sidewalls to be formed

・Essential for miniaturization

・The dominant method used today

 

(3) Deep etching (DRIE: Deep RIE)

A method for etching deep holes, used in applications such as MEMS.

Representative technology: the Bosch process

Characteristics

・Capable of etching vertically to depths of several hundred micrometers

・Essential for devices such as smartphone accelerometers

Understanding these two concepts is essential to understanding etching profiles.

Isotropic etching

・Etches equally in the horizontal direction as well.

・Can undercut beneath the mask.

 

Anisotropic etching

・Etches preferentially in the vertical direction.

・Capable of precisely reproducing fine patterns.

 

As a result, anisotropic etching (RIE) is essential for wiring and gate formation in leading-edge logic devices.

One of the most important metrics in etching is selectivity — the ratio between how quickly the target film is removed and how slowly an unwanted film is removed.

Example

When etching SiO₂, it’s essential to avoid etching away the underlying silicon or the resist as much as possible.

 

Low selectivity can lead to:

・Pattern collapse

・Thinning resist, leading to pattern failure

・Device damage caused by over-etching

High selectivity is a hallmark of advanced etching technology.

Etching becomes increasingly difficult as devices continue to shrink:

・Pattern collapse

・Line edge/width roughness (LER/LWR)

・The “high aspect ratio problem” — uniformly etching deep into narrow 3D trenches

・Thinner resist layers lacking sufficient durability

・Dimensional variation caused by over-etching

 

In particular, as devices transition from FinFET to GAA (nanosheet) structures, 3D etching technology capable of precisely etching sidewalls and top/bottom surfaces simultaneously has become essential.

Etching is used throughout virtually every part of a semiconductor device:

・Transistor gate formation

・STI (Shallow Trench Isolation)

・Multilayer wiring (vias and trenches)

・Forming the deep holes used in 3D NAND

・Forming MEMS structures

 

In particular, 3D NAND requires etching deep vertical holes exceeding 100μm in height, demanding some of the highest aspect-ratio etching capability in the world.

・Low-damage etching techniques that reduce plasma damage

・Improved selectivity techniques suited to nanosheet and GAA structures

・Development of new reactive gases compatible with EUV resists

・Improved uniformity control for high-aspect-ratio structures

・AI-driven process stabilization for etching

Going forward, etching is expected to become an increasingly integrated discipline, combining materials science, plasma physics, and AI.

・Etching is the process of removing unwanted material to give circuitry its three-dimensional shape.

・Dry etching (RIE) has become the dominant method over wet etching.

・Anisotropic etching is essential to achieving further miniaturization.

・Selectivity determines processing precision.

・The difficulty of etching continues to increase year by year as structures become more three-dimensional.

・DRIE is an important technology for deep etching in MEMS and 3D NAND applications.

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

2.Why has dry etching become the dominant method over wet etching in modern processes?

3.What is selectivity?

 

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.