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How is ALD atomic layer deposition technology applied in the field of metal powders?

How is ALD atomic layer deposition technology applied in the field of metal powders?

  • Categories:Industry News
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  • Time of issue:2021-11-24 15:40
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(Summary description)Atomic layer deposition (ALD) technology is a chemical vapor deposition (CVD) technology. It was originally proposed by Finnish scientists for the development of polycrystalline fluorescent materials

How is ALD atomic layer deposition technology applied in the field of metal powders?

(Summary description)Atomic layer deposition (ALD) technology is a chemical vapor deposition (CVD) technology. It was originally proposed by Finnish scientists for the development of polycrystalline fluorescent materials

  • Categories:Industry News
  • Author:
  • Origin:
  • Time of issue:2021-11-24 15:40
  • Views:

Atomic layer deposition (ALD) technology is a chemical vapor deposition (CVD) technology. It was originally proposed by Finnish scientists for the development of polycrystalline fluorescent materials ZnS, Mn and amorphous Al2O3 insulating films. These materials are used in flat panel displays. In the mid-1990s, the development of silicon semiconductors enabled the advantages of atomic deposition to be truly manifested, and set off an upsurge in ALD research. After nearly 30 years of development, ALD technology has played a very important role in the fields of catalysis, semiconductors, and optics, and has become a key technology for the preparation of functional films.

1. ALD technology principle

ALD is a method in which substances are deposited layer by layer on the surface of a substrate in the form of a monoatomic film. In the atomic layer deposition process, the chemical reaction of the new layer of atomic film is directly related to the previous layer. In this way, only one layer of atoms is deposited per reaction.

Each cycle of ALD consists of two half reactions. The chemical adsorption and surface chemical reaction in each step are obviously self-limiting and complementary. This self-limiting characteristic is the basis of atomic layer deposition technology. This self-limiting reaction is repeated continuously to form the desired film. An atomic layer deposition cycle can be divided into four steps: 1) pass the first precursor gas into the substrate, and the substrate will adsorb or react with the substrate surface; 2) flush the remaining gas with inert gas; 3) pass the second The precursor gas chemically reacts with the first precursor gas adsorbed on the surface of the substrate to form a coating, or the product reacted with the first precursor continues to react with the substrate to form a coating; 4) Use the flushing gas again to flush out the excess gas . By controlling the deposition cycle, precise control of the film thickness can be achieved.

2. ALD technology to prepare metal powder

The typical ALD reaction process is approximately a displacement reaction. The most commonly used method is to react metal precursors with their corresponding hydrides (H2O, H2S, NH3), and the metal precursors exchange their ligands with these reaction assistants to obtain the corresponding compounds. For the deposition of pure metals, what is needed is to reduce the metal state and remove the ligands attached to the metal atoms. When ALD deposits metals, there is a nucleation latency. It is usually difficult to remove metal organic ligands, and metal atoms are easy to diffuse and aggregate. In the initial stage of deposition, metals usually form dispersed crystal grains, and then the crystal grains gradually increase and become dense, and connect in parallel to form a continuous thin film.

◆ Precious metals

In ALD growth, noble metals are usually generated by the reaction of noble metal organic compounds with oxygen. This is because precious metals represented by platinum are easier to produce stable metal elements than compounds. Oxygen as one of the reactants will enhance this tendency. The organic ligands of the metal precursors are oxidized, and the combustion products CO2 and H2O are released during the reaction, causing ALD to grow precious metals to react, just like oxygen burning the hydrocarbon groups of the metal, the so-called combustion reaction.

Application of Atomic Layer Deposition Technology in the Field of Metal Powder

Taking the metal organic precursor methylcyclopentadiene trimethylplatinum to prepare platinum as an example, the platinum precursor ligand is substituted and adsorbed on the surface, and part of the distribution body reacts with the oxygen adsorbed on the surface; the oxygen pulse burns off the remaining ligand, The platinum surface leaves oxygen-containing groups, including catalytic and surface chemical functions, thereby forming a cyclic reaction.

◆ Transition metal

Different from oxidation-resistant precious metals, ALD deposition of other metals requires the selection of suitable reducing agents, commonly used reducing agents such as hydrogen, ammonia and their plasma. At present, the reaction mechanism of transition metal ALD growth is mainly divided into three categories: hydrogen reduction reaction, oxide reduction reaction and fluorosilane elimination reaction.

Take the reaction of the amide-based ligand of [Cu(*Bu amd)]2 with the silicon substrate as an example. After the introduction of the copper precursor, the ligand is heated to stimulate the hydrogenation reaction with the surface hydroxyl groups, and the bridging structure is replaced with a single ligand. Position the substrate Si-Cu-O bond structure. A hydrogen pulse is then introduced to reduce copper. At the same time, part of the silicon-oxygen bond is restored, which means that copper atoms can diffuse and aggregate into crystalline nanoparticles. As the bond between the copper and the silicon-oxygen substrate is broken, the reaction sites on the original surface are partially restored, allowing the ligand replacement reaction to continue.

◆ Lively metal

Positively charged metals include aluminum, titanium, iron, silver, and tantalum. Taking silver as an example, since its compounds are all +1 valence, it is difficult to adsorb metal ions that are only bound to one ligand. Therefore, some electrically neutral adduct ligands are needed. By displacement, the auxiliary metal cation is adsorbed onto the substrate. However, the binding of this ligand is usually weak. The silver precursor used in the report of successful ALD deposition of silver is (hfac)Ag(1,5-COD), where COD is the neutrality of the above-mentioned auxiliary silver ion adsorption. Ligand. When the COD is replaced, the silver adsorbed on the substrate has sufficient surface mobility and lifetime to diffuse and nucleate along the surface of the substrate in the subsequent high-purity nitrogen cleaning step. In the next propanol pulse, due to the alcohol's catalytic oxidation of hydrogen, the excess hfac ligand is removed and the deposited metallic silver is obtained.

◆ Metal oxide (carbide, nitride, sulfide)

Some transition metal oxides/hydroxides, such as RuO2, Fe3O4, MnO2, V2O5, Ni(OH)2, etc., have the ability of rapid redox reaction and have a higher theoretical specific capacity than electric double layer capacitor materials. As an ideal pseudo-capacitance supercapacitor material.

Li Ji uses nanoporous gold film as the substrate, Mn(thd)3 and O3 as precursors, high-purity nitrogen as carrier gas and scrubbing gas, and uses atomic layer deposition process to prepare NPG/MnO2 composite film. The obtained MnO2 film is uniform and dense, and is amorphous. When the current density is 100 μA/cm2, the maximum specific capacitance is 253F/g. The material has good cycle performance. After 4000 cycles of charging and discharging, the specific capacitance still maintains 98%.

3. ALD technology is used for surface coating modification of micro and nano particles

At present, the application of ALD in metal powders is mainly reflected in the surface coating modification of micro-nano particles. Coating, also known as coating, refers to the introduction of one or more layers of substances on the surface of powder particles to form a micro-nano-level coating, and finally form a composite material with a core-shell structure to achieve the purpose of surface modification. The coated particles can change the original surface chemical properties, surface charge properties and functional properties, while retaining their own optical, magnetic, electrical and catalytic properties, thereby improving the dispersion stability of the original particles, and even adding some novel and special physical properties. The chemical properties expand the application range of ultra-fine powders.

3.1 Micro-nano metal powder coating

Liu Yanfeng used the ALD method to coat and modify the micro-nano carbonyl iron powder (CIP). Using trimethylaluminum and water as precursors, a layer of nano-alumina is coated on the surface of carbonyl iron powder to form a core-shell structure composite material (CIP/Al2O3). Experiments show that: 1) The oxidation resistance of the carbonyl iron powder composite material has been greatly improved after coating. After 75 cycles, the oxidation onset temperature of the carbonyl iron powder reached 560°C, which was 360°C later than the original sample. 2) The prepared composite material reacts very slowly with hydrochloric acid, and the thin aluminum oxide layer can effectively protect the carbonyl iron powder from corrosion. 3) Due to the hydrophilicity of alumina material, the prepared carbonyl iron powder composite material also has better hydrophilicity and is easier to disperse in aqueous solution. 4) After coating aluminum oxide on the surface of carbonyl iron powder by atomic layer deposition, its electromagnetic parameters and absorption performance are significantly improved, and impedance matching is improved. Under the same conditions, the composite material after 75 ALD cycles has better performance than the raw material, the reflection loss of the sample is lower (-20.43dB), and the reduction rate is 40.9%.

Chen R et al. used ALD technology to deposit zirconia film on the surface of nano aluminum powder to achieve a complete coating of nano aluminum powder. The hydrothermal stability test results show that the zirconia nano-film has excellent hot water corrosion performance, which can effectively prevent the surface of the aluminum powder from reacting with hot water at 80°C. The ALD zirconia coating and alumina passivation layer on the surface of the aluminum powder can form a ZrAlxOy phase at the film interface layer, which has a good hydrophobic effect and can effectively prevent water molecules from penetrating into the surface of the aluminum powder.

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