As scaling and 3D structuring advance, film uniformity, thickness accuracy, and material properties have come to determine performance, reliability, and yield.

Particularly at the 3nm and 2nm generations, the difficulty of deposition has risen sharply due to factors such as:

● Increasingly complex channel structures (GAA / nanosheets)

● More fine protrusions and steps on sidewalls

● More wiring layers and increasingly fragile low-k films

CVD is the most widely used deposition method in semiconductor manufacturing.

Strengths of CVD

・High productivity (fast deposition rate)

・Uniform films can be obtained relatively easily

・The equipment is mature and cost-efficient

Limits of CVD

・Film thickness uniformity in high-aspect-ratio structures is a challenge (films do not deposit uniformly all the way to the bottom of deep trenches)

・Insufficient thickness control accuracy in fine regions

・Depending on the material, reaction temperatures are high and cause thermal damage

Derivative Technologies

・LPCVD (low-pressure CVD)

・PECVD (plasma-enhanced CVD)

・HDPCVD (high-density plasma CVD)

Even with these improved technologies, from 2nm onward there are a growing number of steps where CVD alone cannot meet the requirements.

ALD is now becoming the leading technology in cutting-edge processes.

Features of ALD

・Forms a film at the atomic-layer level in each cycle

・Because the chemical reaction is self-limiting, thickness accuracy is extremely high

・Can deposit films uniformly even on the sidewalls of high-aspect-ratio structures

Typical Areas Where ALD Is Required

・High-k gate dielectrics (such as HfO₂)

・Metal gates (TiN, W)

・Barrier metals (TaN)

・Filling trenches and deep-hole structures

With the transition from FinFET to GAA, perfect control of sidewalls has become essential, making ALD indispensable.

A method that compensates for the drawbacks of ALD and is further adapted to scaling.

Advantages of PEALD

・Enables high-quality films to be formed at low temperatures

・Promotes reactions and improves deposition rate

・Enables denser metal films

・Helps reduce the device’s thermal budget

PEALD is used extensively, particularly for GAA gate stacks, insulating films in wiring layers, and barrier film formation.

In 3D NAND, vertical structures exceeding 300 layers mean that hole depths reach the 10 µm class.

Within these holes:

● Top: thick

● Middle: thin

● Bottom: no film deposited

— an “elephant foot” profile like this tends to occur.

ALD is what can overcome this. With CVD, uniform coverage is fundamentally difficult.

High-k Films

Too thin increases leakage; too thick degrades performance. An area where ALD is essential.

Metal Gates

Work-function tuning is difficult, and material purity is important.

Low-k Insulating Films

Vulnerable to plasma; optimizing PECVD conditions determines yield.

Cu Barrier Metals

Must achieve both thinner films and diffusion prevention.

Selective Deposition

Deposits film only where it is needed, reducing processing steps.

Spatial ALD (High-Speed ALD)

Moves the wafer at high speed to make ALD ultra-fast.

Hybrid ALD (Thermal × Plasma)

Pursues the optimal balance between film quality and throughput.

Expansion of Material-Based ALD (Ru, Co, Ultra-Thin Barriers)

Aims to reduce wiring resistance.

In-Situ Monitoring (Atomic-Layer Level)

Moving toward an era of depositing films while evaluating film quality in real time.

● Scaling and 3D structuring have placed deposition technology at the core of the process

● CVD is strong in volume production but has limits in fine structures

● ALD enables precise control at the atomic-layer level and is key to advanced nodes

● PEALD contributes to lower temperatures, better film quality, and higher speed

● Deposition technology determines competitiveness in terms of high aspect ratios, material-dependent challenges, and yield

1.Why is ALD indispensable for GAA transistors?

2.What is the biggest difference between CVD and ALD?

3.Give two reasons why PEALD is adopted.

 

Column Supervisor: Koji Kakumoto (President, OTIS Co., Ltd.)

After studying abroad and working in planning and development at a trading company, he joined OTIS 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 President, 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.