Atomic Layer Deposition - An Introduction
Atomic Layer Deposition (ALD) is a vacuum deposition technique that provides digital control over thickness on high aspect ratio substrates. ALD relies on self-limiting surface reactions between vapor phase reactants. The reactants are separated in time providing sequential exposures of the surface. Sequential exposures provide unparalleled control over the thickness and film composition. Initial development around 1965 leading up to increasing maturity today.
At A Glance
The non-line of sight process provides advantages over other deposition techniques at small scales such as;
- High aspect ratio substrates up to 4000:1 (HiPIMS is 30:1)
- 3D structures of large physical size (compatible with process chamber)
- Small feature sizes such as below 10 nm
- Tuning of metal-semiconductor interfaces
- Tuning fermi-states of semiconductor-semiconductor interfaces
- Production of amorphous films
Image credit: Erwin Kessels, ALD turns 45: the history of ALD in a timeline and an animated version of the ALD periodic table, Atomic Limits blog (2019)
Atomic Layer Deposition
The first references to what would become known as ALD appeared in 1965 and was identified as “Molecular Layering”. After years of research by multiple laboratories, the first production plant using ALD was made by Electroluminescent Displays. ALD is now widely used across many industries including semiconductors, batteries, microelectronics, and photovoltaics to name a few. There are over a thousand well characterized ALD chemistries, even more substrates, and uncountable applications. For the purposes of this discussion, we will focus on thermal ALD carried out in the typical temporal manner.
Trench & Via
Excellent conformal deposition of high aspect ratio’s.
The conformality offered by thermal ALD is unparalleled because the process relies on gas diffusion and surface reactions instead of flux-controlled precursor delivery. Any surface that is reachable by the gas can be reliably conformally coated. A clean, reactive surface provides a platform for a pinhole free film deposition. A necessary characteristic of high-𝓀 dielectrics, diffusion barriers, anti-corrosion films, etc.
Conformal coating of cavities
Resonance cavities can be deposited with tuned layers
Uniformites <1% on low aspect ratio Wafers
Left & Right images: K. Arts, M.A. Verheijen, W.M.M. Kessels and H.C.M Knoops (CC BY 4.0 license), image library at www.AtomicLimits.com, 2021. Corresponding paper DOI: 10.1021/acs.chemmater.1c00781
Conformality is what sets the upper limit for the highest aspect ratio that a vacuum process can be used for coating. ALD has been shown to coat aspect ratios (L/d) of 5000:1 with high conformality. (Elam, 2003) There are a few terms that are used to describe ALD films. They are conformality, uniformity, cycles, thickness, and growth rate. Uniformity refers to the variation in film thickness over the entire coated surface. Many studies have shown uniformities below 4% are achievable for high aspect ratio substrates. (Elers, 2006) When well characterized thermal chemistries are used with low aspect ratio substrates, like wafers, uniformities of 1% are common. For best results a reactor and process should be tuned to the substrate and chemistry of interest. This has the added benefit of optimizing cycle times as well. There are many types of reactors for different applications.
There are a few different ways that the term “cycles” is used when discussing ALD. In regular thermal ALD a cycle consists of four basic stages: Precursor A Dose, Precursor A Purge, Precursor B Dose, Precursor B Purge. At any given time only, a single precursor is present in the deposition chamber. The purge stages consist of only an inert process gas flowing to remove excess precursor and reaction products. The purge step ensures that material isn’t deposited in a chemical vapor deposition (CVD) manner. Some materials require more than two precursors to be deposited. In these instances, the cycle simply takes on another “unit” as described in the figure below. When depositing more than one material each material can be described by its relevant precursor cycle. These “super cycles” are used to deposit nanolaminate materials, graded layers, alloys, etc.
A nanolaminate is a repeated unit of multiple layers, two to three. Repeating structures can regulate the crystal size in crystalline or polycrystalline film. They also allow for unique applications such as free-standing x-ray mirrors on polymer substrates. (Fabreguette, 2007)
Harm Knoops, Status and prospects of plasma-assisted atomic layer deposition, JVSTA, 37, 030902 (2019), DOI: 10.1116/1.5088582. CC BY
Harm Knoops, AtomicLimits image library (2020). A similar figure to this figure was published in: Harm Knoops, Atomic Layer Deposition in Handbook of Crystal Growth, DOI: 10.1016/B978-0-444-63304-0.00027-5
Harm Knoops, AtomicLimits image library (2020). A similar figure to this figure was published in: Harm Knoops, Atomic Layer Deposition in Handbook of Crystal Growth, DOI: 10.1016/B978-0-444-63304-0.00027-5
The self-limiting nature of the precursors results in sub-monolayer to monolayer surface coverage. The degree of surface saturation has a correlation with the roughness and thickness of material deposited per cycle known as the growth per cycle. Under ideal conditions the surface is saturated to nearly 100% coverage of reactive sites. However, the precursors being used, and reactivity of the surface may lead to much less than 100% surface coverage. When a system is tuned properly the surface exposures, called doses, will reach saturation for that combination of precursor and surface. A reliable growth rate is the result. The growth rate for ALD chemistries is characterized in Å/cycle. Growth rates from 0.2 to 1.3 Å/cycle are common. These slow growth rates are often cited as a negative when it comes to the use of ALD in industrial processes. This is a misconception. Slow growth rates are not necessarily negative because the pinhole free nature of ALD films results in thinner films that meet the same performance criteria as films from other methods. When creating water/oxygen diffusion barriers ALD films can show superior performance by 15 nm in thickness. (Higgs et al., 2014)
Karsten Arts, adapted from “Film Conformality and Extracted Recombination Probabilities of O Atoms during Plasma-Assisted Atomic Layer Deposition of SiO2, TiO2, Al2O3, and HfO2”, J. Phys. Chem. C 123, 27030–27035 (2019). DOI: 10.1021/acs.jpcc.9b08176. CC-BY-NC-ND
GP Plasma together with Maxima Sciences, builds lab scale ALD equipment to allow new market entrants to de-risk the technology exploration, critically providing equipment that can deposit on complex 3D geometries. For high throughput and batch applications with hybrid deposition capability to mix ALD with PVD and PECVD, GP Plasma also represents Swiss Cluster for North America.
R-ALD Machine
Maxima Sciences RALD machine built by GP plasma fills the gap for cost effective R&D equipment for Atomic Layer Deposition systems to coat 3D geometries.
References
Elam, J. W. (2003). Conformal Coating on Ultrahigh-Aspect-Ratio Nanopores of Anodic Alumina by Atomic Layer Deposition. Chem. Mater., 15(18), 3507–3517. https://pubs.acs.org/doi/abs/10.1021/cm0303080
Elers, K. E. (2006). Film Uniformity in Atomic Layer Deposition. Chemical Vapor Deposition, 12(1), 13-24. https://onlinelibrary.wiley.com/doi/abs/10.1002/cvde.200500024
Fabreguette, F. H. (2007). X-ray mirrors on flexible polymer substrates fabricated by atomic layer deposition. Thin Solid Films, 515(18), 7177-7180. https://www.sciencedirect.com/science/article/abs/pii/S0040609007003586
Higgs, D. J., Bertrand, J. A., Young, M. J., & George, S. M. (2014, August 14). Oxidation Kinetics of Calcium Films by Water Vapor and Their Effect on Water Vapor Transmission Rate Measurements. J. Phys. Chem. C, 118(50), 29322–29332. https://pubs.acs.org/doi/abs/10.1021/jp505508c
Kessels, E. AtomicLimits ImageBase – Atomic Limits. Atomic Limits. Retrieved September 6, 2022, from https://www.atomiclimits.com/imagebase/