Atomic Layer Deposition (ALD) belongs to the family of chemical vapor deposition methods (CVD). It is a nano-scale deposition process 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).
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 material 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) and titanium oxide (TiO2) all the way to mixed metal oxide multilayered or doped systems.
Why was ALD originally developed?
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.
What areas are ALD used?
ALD is used in many different sectors including:
- Microelectronics
- 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.
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
A high purity substrate is required. This is very important to the quality of the finish similar to many other vapour deposition processes.
Price. ALD Systems 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.
Masking challenges. The ALD masking process must be perfect. Any pin hole in the masking process will allow deposition beyond the masking barrier.
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