1. ALD coating of porous materials and powders
Erxiong Ding

2. Introduction of ALD and the coating of porous materials
Principle of ALD: Al2O3 coating using
TMA (trimethylaluminium) and H2O
Pore size:
Mesoporous: 2~50nm
Microporous: <2nm
Ref.: C. Detavernier, et al. Chem. Soc. Rev., 2011, 40, 5242–5253.
Figure 1. Principle of Al2O3 coating

3. Problem Advantages of ALD: Excellent conformality
Atomic scale thickness control
Low growth temperature
Definition of the problem:
For some certain films (e.g. on polymer or plastic substrates, coating of polymers or biomaterials), it requires much lower temperature (<150℃).

4. Problem solution 1 Using plasma
Plasma is a kind of physical state comprising of free electrons and ions. It has high energy radicals which can activate the reactant molecules, inducing decomposition of the molecules.
Ref.: A. Niskanen, et al. J. Electrochem. Soc., 2005, 152, F90- F93.
Figure 2. Growth rate and film density dependence on the deposition temperature. (Al2O3 coating by plasma enhanced ALD)

5. Problem solution 2 Using catalysis
J. W. Klaus et al employed pyridine as a catalyst to achieve room temperature ALD of SiO2 using SiCl4 and water.
Ref.: J. W. Klaus, et al. Science, 1997, 278,
Figure 3. Thickness of SiO2 film as a function of coating temperature. (pyridine as a catalyst )

6. Conclusions
Although ALD has some certain advantages compared with other coating methods, some details still should be considered for better quality film.
Plasma and catalysis are helpful for deceasing coating temperature using ALD method.

7. Information Slide
Atomic layer deposition (ALD) consists of four essential steps: 1) precursor exposure, 2) evacuation or purging of the precursors and any byproducts from the chamber, 3) exposure of the reactant species, typically oxidants or reagents, and 4) evacuation or purging of the reactants and byproduct molecules from the chamber [1].
Advantages:
1. Excellent conformality and step coverage on 3D surface structures, ability to deposit within high aspect ratio features;
2. Control of the layer thickness at the Angstrom level—the limited deposition rate renders ALD especially suitable for the deposition of ultrathin films ( e.g. <10 nm) [2];
3. Since the adsorbed precursor molecules are subject to complete reaction with reactants, the incorporated impurity level is expected to be low compared to CVD even at low growth temperatures. However, thermal activation for precursor molecule adsorption and surface reaction with reactant is necessary for ALD; thus, substrate heating is usually required [1].

8. Information Slide-continued
For most polymer materials (polymer or plastic-based devices and for coating heat-sensitive materials such as polymers or biomaterials), the ALD process temperature should be limited to lower than 150 °C.
A more general way of lowering the growth temperature is the use of plasma to activate the reactants.
A. Niskanen et al. (2005) reported that aluminum oxide was deposited by radical enhanced atomic layer deposition using trimethylaluminum (TMA) and oxygen radicals in the temperature range °C. The radicals were produced by dissociating oxygen gas in a remote microwave plasma discharge. Oxygen was mixed with argon which was also used as the carrier and purge gas. Films were grown on silicon, glass, and indium tin oxide coated glass substrates. Additional growth experiments were conducted on heat-sensitive materials: polyethene, polypropene, and wool. The films were amorphous according to X-ray diffraction. The films were also very smooth; the surface root-mean-square roughness was less than 0.8 nm for 180 nm thick films [3].

9. Information Slide-continued
Another interesting possibility to achieve very low temperature ALD is the use of catalysis.
J. W. Klaus et al. reported that films of silicon dioxide (SiO2) were deposited at room temperature by means of catalyzed binary reaction sequence chemistry. The binary reaction SiCl4 + 2H2O SiO2 + 4HCl was separated into SiCl4and H2O half-reactions, and the half-reactions were then performed in an ABAB...sequence and catalyzed with pyridine. The pyridine catalyst lowered the deposition temperature from > 600 to 300 kelvin and reduced the reactant flux required for complete reactions from ~109 to ~104 Langmuirs. Growth rates of ~2.1 angstroms per AB reaction cycle were obtained at room temperature for reactant pressures of 15 millitorr and 60-second exposure times with 200 millitorr of pyridine [4].

10. References
Hyungjun Kim, et al. Applications of atomic layer deposition to nanofabrication and emerging nanodevices. Thin Solid Films, 2009, 517, 2563–2580.
Christophe Detavernier, et al. Tailoring nanoporous materials by atomic layer deposition. Chemical Society Review, 2011, 40, 5242–5253.
Antti Niskanen, et al. Low-temperature deposition of aluminum oxide by radical enhanced atomic layer deposition. Journal of The Electrochemical Society, 2005, 152, F90-F93.
Jason W. Klaus, et al. Growth of SiO2 at room temperature with the use of catalyzed sequential half-reactions. Science, 1997, 278,

11. Thanks for your attention!