ALD(Atomic Layer Deposition) technology and application

Atomic Layer Deposition (ALD) was originally called Atomic Layer Epitaxy (ALE). ALD is also a chemical vapor deposition technology (CVD). The difference from traditional CVD is: like CVD the reaction process is divided into two half-reactions (Half-reactions), each half-reaction only occurs on the surface of the substrate (Surface reaction only), and has the characteristics of self-limiting; alternately completing the two half-reactions.With ALD, a single layer of film (1~2 Å) is deposited, so the growth of ALD film is controlled within the thickness of a single atomic layer.

Among them, self-limitation means: one ALD cycle, only a single atomic layer of film is grown on the surface of the substrate (Substrate); the precursor (Precursor) only produces a single molecular layer of adsorption on the substrate, and the adsorption capacity is mainly related to the surface active site (Site) is related to the saturation state, and the precursors will not react with each other, so it can also be called a self-terminating gas-solid reaction.

Epitaxy means: growing a single crystal thin film on a single crystal substrate. However, due to the self-limiting characteristics of ALD, the substrate does not necessarily need to be a single crystal; and the grown film is not necessarily a single crystal film, but can also be a polycrystalline or amorphous structure. Therefore, after 2000, the term Atomic Layer Epitaxy (ALE) gradually changed to Atomic Layer Deposition (ALD).

Development History

• In 1965, Professor V.B. Aleskovski of the Soviet Union (USSR) first proposed the theory of molecular layer surface modification.
• In 1974, Finland T. Suntola et al. pioneered the ALE coating process and grew zinc sulfide (ZnS) thin films. Since then, ALD technology has gained the attention of scientists.
• In 1977, T. Suntola and others applied for the first ALE patent (U.S. Patent #4,058,430).
• In 1980, T. Suntola and others published the first ALE journal paper (Thin Solid Films 1980, 65, 301)

Deposition mechanism and process

ALD is mainly deposited by two basic mechanisms: one is the chemical adsorption saturation process of precursors, and the other is the Sequential surface chemical reaction process. The ALD deposition process can be divided into four stages:

1. The first precursor feed (Pulse) is introduced into the chamber cavity, and the precursor produces a single atomic layer of chemical adsorption on the surface of the substrate, so that functional groups are generated on the surface of the substrate.
2. Inert gas is introduced to purge excess precursors.
3. The second precursor feed (Pulse) is introduced into the chamber cavity and reacts with the surface functional groups of the substrate to form a single atomic layer.
4. Inert gas is introduced to purge excess precursors or by-products. 

ALD film growth example

• The growth process of traditional CVD：

Using Trimethylaluminum (Al(CH3)3, TMA) and water (H2O) to grow Al2O3 film, the traditional CVD growth process will simultaneously pass the two precursors of TMA and H2O into the chamber cavity for chemical reaction. Chemical reactions (Overall reactions) are expressed as follows: 2Al(CH3)3(g) + 3H2O(g) →Al2O3(s)+ 6CH4(g) where (s) represents solid and (g) represents gas. 

•  ALD's growth process：

1. Using ALD technology to grow Al2O3 film, the following discusses the growth of Al2O3 film on the surface of hydrogen passivated silicon substrate (SiH), and describes the reaction of each step: First, pass the first precursor H2O into the chamber cavity, and the water will be in the SiH The chemical adsorption of a single atomic layer on the surface of the substrate produces OH functional groups on the surface of the substrate to form Si-OH(s), which is a half-reaction; SiH + H2O → Si-OH(s) + H2(g).
2. Pass in inert gas (Ar or N2) to blow off excess water molecules.
3. Pass the second precursor TMA into the chamber cavity to make the surface adsorb to produce Al(CH3)2 functional groups to form Si-O-Al(CH3)2(s), which is another half reaction; Al(CH3)3 + Si-OH(s) → Si-O-Al(CH3)2(s) + CH4(g).
4. Pass in inert gas (Ar or N2) to blow off excess TMA molecules or by-products.
5. Pass the first precursor H2O into the chamber cavity to make the surface adsorb to produce OH functional groups to form Si-O-Al(OH)2(s); Si-O-Al(CH3)2(g) + 2H2O → Si -O-Al(OH)2(s) + 2CH4(g).
6. Pass in inert gas (Ar or N2) to blow off excess water molecules.
   
7. Pass the second precursor TMA into the chamber cavity to make the surface of the substrate adsorb to produce Al(CH3)2 functional groups, expressed as Si-O-Al-O-Al(CH3)2(s); Al(CH3)3 + Si-O-Al(OH)2(s) → Si-O-Al-O-Al(CH3)2(s) + CH4(g).
   
8. Blow in inert gas (Ar or N2) to blow off excess TMA molecules or by-products; the above steps (1) ~ (4) or steps (5) ~ (8), that is, each is an ALD cycle; step (1) It mainly involves the activation process of the substrate surface, and the subsequent ALD process mainly follows the sequence of steps (5) to (8) for growth.

ALD's  Growth time

The feed time of the precursor is usually very short, about 0.1 seconds, but the actual feed time depends on factors such as gas flow rate, reactor volume, vapor pressure of the precursor, chemical kinetics of the precursor and functional groups on the substrate surface. In contrast, the blowing step takes a long time, about several seconds to several tens of seconds, but it depends on factors such as the size and shape of the chamber cavity, the desorption rate of byproducts, and the removal rate of residues; If it is a process at low temperature, the blowing time may even take 3 minutes. Regardless of the blowing time, the growth rate of ALD is about 6 nm/min. Therefore, blowing off is the rate determining step.

Precursor requirements

1. Volatility: In order to effectively transport the precursor to the chamber cavity, the precursor must have a sufficiently high vapor pressure (more than 0.1 Torr). The precursor is preferably liquid or gas to facilitate delivery. The liquid precursor will not decompose at the volatilization temperature.
2. Reactivity: It can react quickly with the functional groups on the surface of the substrate instead of chemically reacting with the substrate itself; and it cannot produce self-decomposition reaction or gas phase reaction, otherwise it will destroy the self-limited growth mechanism.
3. The precursors and by-products cannot react with the as-deposited film to be etched or dissolved.
   

Advantages of ALD

• Films with large area, uniformity, and chemical dose ratio can be grown, and the structure with high aspect ratios still has excellent step coverage.
• The film thickness can be accurately controlled with atomic level accuracy.
• Can do low temperature process.
• Can deposit dense and pinhole-free thin films.

Disadvantages of ALD

• Slow deposition rate: The growth rate of each layer of ALD is about 0.1 to 1 second, the thickness is about 1 to 2 Å, and each cycle duration (Cycle duration) is about several seconds to 1 minute. Due to the self-limiting characteristics of ALD, its growth rate has a linear relationship with the ALD cycle.
• Precursors are usually expensive and highly toxic, and must be monitored and carefully controlled to avoid potential hazards.

Application of ALD

ALD technology can be applied to high dielectric materials (Al2O3, HfO2, ZrO2, Ta2O5), catalyst catalytic materials (Pt, Ir, Co, TiO2), biomedical coatings (TiN, ZrN, CrN), electroluminescence (SrS) :Cu, ZnS:Mn, ZnS:Tb), gas barrier film (Al2O3), transparent conductive film (ZnO:Al, ITO)...etc.

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Remarks: Quote from Dah Young Vacuum - R&D Department