Atomic Layer DepositionAtomic Layer Deposition (ALD) belongs to the family of chemical vapor deposition methods (CVD).

It is a deposition process at a nano-scale level within a vacuum chamber.

The deposition process forms ultra-thin films (atomic layers) with extremely reliable film thickness control.

This provides for highly conformal and dense films at extremely thin layers (1-100nm).

What is actually deposited in ALD?

ALD principally deposits metal oxide ceramic films.

These films range in composition from the most basic and widely used aluminum oxide (Al2O3), titanium oxide (TiO2) all the way to mixed metal oxide multilayered or doped systems.

How does ALD actually work?

The ALD deposition technique is based upon the sequential use of a gas phase chemical process. Gases are used to grow the films onto the substrate within a vacuum chamber.

The majority of ALD reactions use two chemicals called precursors. These precursors react with the surface of a material one at a time in a sequential, self-limiting, manner.

Through the repeated exposure to alternating gases there is a buildup of a thin film through deposition.

What family is ALD from?

ALD belongs to the family of chemical vapor deposition methods (CVD).

It was initially developed for manufacturing nano-laminate insulators and zinc sulfide phosphor films for thin film electroluminescent displays.

The unique properties of the coatings, together with the high repeatability, were the main factors leading to successful industrial production.

ALD is a mature technology and widely used in an array of commercial applications.

Where is ALD used?

ALD is used in many different areas including:

  • Micro-electronics
  • Semiconductors
  • Photovoltaics
  • Biotechnology
  • biomedical
  • LEDs
  • Optics
  • Fuel cell systems

The ultra-thin films can be grown onto virtually any substrate. They have been demonstrated on highly patterned wafers, polymer films, and fine powders of most compositions.

The films deposit as well on single composition materials as they do on multi material composition objects or materials.

What is the future of ALD?

A major driving force for the recent interest is the possibility of using ALD in scaling down microelectronic devices according to Moore’s law.

It is an active field of research, with hundreds of different processes being examined right now.

Advantages and Disadvantages of ALD

Advantages

  • Self-Limiting. The ALD process limits the film thickness. Many other processes like Parylene are dependent upon amount of dimer and will continue to deposit successive polymer layers until it is completely used up.
  • Conformal films. ALD film thickness can be uniform from end to end throughout the chamber. Other coatings like Parylene can have a varied coating thickness across the chamber and the devices being coated.
  • Pinhole free. ALD films can be pinhole-free at a sub-nanometer thickness. Parylene and some other materials are only pinhole-free at micron levels.
  • ALD allows layers or laminates. Most other films including Parylene are single component layers.

Disadvantages

  • High purity substrate. This is very important to the quality of the finish similar to many other vapour deposition processes.
  • ALD System can range anywhere from $200,000 to $800,000 based on the quality and efficiency of the instrument. This tends to be 3-4 times the prices of a Parylene system.
  • Reaction time. Traditionally, the process of ALD is very slow and this is known to be its major limitation.
  • The ALD masking process must be perfect. Any pin hole in the masking process will allow deposition beyond the masking barrier.

What are some of the ALD coatings that can be deposited?

A wide variety of chemistries are possible with Atomic Layer Deposition. They include:

  • Oxides
  • Nitrides
  • Metals
  • Carbides
  • Sulfides

Article by Dr Lee Hitchens

Want to find out more?

Contact us at Nexus and discuss ALD now.