Plasma ALD (Atomic Layer Deposition) is a thin-film deposition technique used in nanotechnology and semiconductor manufacturing. Plasma ALD is a variation of the standard ALD technique that involves the use of plasma to enhance the deposition process.
Plasma ALD allows for more freedom in the choice of materials and the use of plasma processes to pre-treat (or clean) the surface prior to deposition. See how plasma ALD can make it easier to achieve your goals.
How Does Plasma ALD Work?
In standard ALD, alternating pulses of precursor gases are introduced onto the substrate surface. Each pulse forms a monolayer of material by chemisorption. The process is repeated, allowing for precise control over layer thickness and composition. It’s important to note that in certain cases, using standard ALD may lead to slow deposition rates or challenges in achieving certain material properties.
When a plasma source is introduced into the ALD process, it helps activate precursor molecules and enhance surface reactions, leading to increased deposition rates and improved film properties.
Benefits of Plasma ALD
Here are a few key benefits of plasma-enhanced ALD (PE-ALD):
- Faster deposition rates: Plasma activation speeds up the surface reactions, allowing for quicker film growth compared to traditional ALD.
- Improved film properties: Plasma ALD can enhance film density, adhesion, and crystallinity, leading to better material properties.
- Compatibility with a broader range of precursors: Plasma activation can enable the deposition of materials that may not react efficiently through conventional ALD.
Plasma ALD finds applications in various industries, including semiconductor manufacturing, solar cells, display technology, microelectronics, and advanced coatings. It is particularly useful when high-quality thin films are needed, and when precise control over film properties and composition is essential.
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Frequently Asked Questions (FAQs)
Read these frequently asked questions regarding plasma-enhanced ALD.
What is Plasma ALD?
The use of plasma species as reactants allows for more freedom in processing conditions and for a wider range of material properties compared with the conventional thermally-driven ALD method
Plasma-assisted ALD is an energy-enhanced ALD method that is gaining popularity. In plasma ALD (also known as Plasma-assisted ALD or plasma enhanced ALD), the surface is exposed to the species generated by a plasma during the reactant step.
What Types of Plasmas Are Used?
Typical plasmas used during plasma-enhanced ALD (PE-ALD) are those generated in O2, N2 and H2 reactant gasses or some combination. Such plasmas can replace ligand-exchange reactions typical of H2O or NH3, and they can be employed to deposit metal oxides, metal nitrides and metal films.
Plasmas generated in gases or vapors such as NH3 and H2O have been reported, for which there can also be a combination of plasma and thermal ALD surface reactions taking place at the same time. Plasma-assisted ALD offers several merits for the deposition of ultra-thin films over thermal ALD and other vapor-phase deposition techniques.
The high reactivity of the plasma species on the deposition surface during the plasma- assisted ALD process allows for more freedom in processing conditions and for a wider range of material properties.
What are The Benefits of Plasma-ALD?
There are several benefits of Plasma-enhanced ALD.
Deposition at reduced substrate temperatures
As high reactivity is delivered to the deposition surface by the plasma species, less thermal energy is required at the substrate to drive the ALD surface chemistry
The reactivity delivered by the plasma species is not only provided by reactive plasma radicals but is also determined by the kinetic energy of the ions accelerated in the plasma sheath.
Improved material properties
Plasma-assisted ALD affords better material properties than thermal ALD in terms of film density, impurity content, and electronic properties. high reactivity provided by the plasma
Increased choice of precursors and materials
Reactive plasma radicals are delivered to the deposition surface, allowing for the use of precursors with relatively high thermal and chemical stabilities. The PE-ALD method can also yield solutions when depositing non-oxidic materials such as metal nitrides and metals.
In addition, plasma-assisted ALD processes also enable the deposition of more material systems, for example, of the elemental metals Ti and Ta plasma-assisted ALD allows for a wider choice of substrate materials to be used, particularly those which are temperature-sensitive.
Good control of stoichiometry and film Composition
Non-thermally-driven reactions can be induced at the deposition surface due to the nonequilibrium conditions in the plasma, which enables better control of the ALD surface chemistry and of the species incorporated into the film.
Therefore, the use of a plasma provides additional variables with which to tune the stoichiometry and composition of the films. This includes:
- plasma power
- operating pressure
- plasma exposure time
- admixing of additional gases into the plasma
- biasing voltage
As an example, it is relatively straightforward to incorporate N atoms into oxide thin films by the addition of N2 into a plasma generated with O2.
Such controlled doping of thin film materials is difficult to achieve with strictly thermally-driven ALD reactions.
Increased growth rate
Cases exist where plasma species create a higher density of reactive surface sites due to the high reactivity of the plasma. Consequently, this can lead to higher growth per cycle values.
Plasma can be switched off and on quickly. This enables the fast pulsing of the plasma reactant species and reduced purge times (depending on the gas residence time in the reactor).
Reduced purge times are especially important for the ALD of metal oxides at low temperatures (room temperature up to 150 C), where purging of H2O, in the case of thermal ALD, requires excessively long purge times and, therefore, long cycle times.
Shorter cycle times have a significant impact on the net throughput of an ALD reactor. The high plasma reactivity is also advantageous as nucleation delay is shorter for plasma-assisted ALD than for the equivalent thermal ALD process.
More processing versatility
The availability of a plasma source on an ALD reactor allows for several other in situ treatments of the deposition surface, deposited films, and reactor walls. Plasmas may be used for substrate pretreatment (as an example, oxidation by an O2 plasma and nitridation by NH3 or N2 plasmas), substrate cleaning, post-deposition treatments, and reactor wall conditioning and cleaning.
