Atomic Layer Deposition is the conformal (fig. 1), homogeneous and surface-controlled thin-film deposition technique. ALD involves two self-limiting and complementary reactions sequentially and alternatingly for the depositing of a perfectly homogeneous and ultra-pure very thin-film. In fact, the ALD incorporates angstrom (A°) or monolayer film-deposition control possiblity.
Historically, ALD was introduced in year 1970. It used to be termed as Atomic Layer Epitaxy (ALE). Inital research on ALE was carried out by the Research Group from University of Helsinky, Finland led by Prof. Dr. Tuomo Suntola. In year 1977, Pro. Suntola filed a patent for the production of compound thin-films, which was the first patent in the development of ALD process . The first pioneering research work on ALE from Prof. Suntola was published in Materials Science Reports in 1989 . The inital phase of ALE was entirely dedicated towards the growth of single crystal of III-V and II-VI compounds and ordered heterostructures like superlattices and superalloys. Those days ALE was studied to meet the requirement for the improvement of ZnS thin-films and dielectric thin-films for electrolumniscent thin-film displays  .
Fig. (1): Figure 3. Cross-sectional SEM image of an conforma Al2O3
ALD ﬁlm with a thickness of 300 nm on a Si-wafer with a trench structure. (Source: From ref 42. Copyright 1999 John Wiley & Sons.)
With huge interest from all around the world and due to the vast applications of ALD process in micro-nano electronic fabrication compared with other thin-film deposition process, profound research in ALD process and novel devices has been already carried out -. As a result, the scope of ALD in not only limited to the growth of single crystal compounds, but also it has lots of application in deposition of conformal layer in the microelectronics industry. Conformal deposition of seed layer for back-end-process Cu-interconnection metallization, thin-film bareer for organic light emitting diodes (OLEDs) and organic photo-voltaics (OPV), bareer layer against metal (e.g. Cu) diffusion, metal electrode for gate stack capacitance for DRAM (fig. 2), dielectric for magnetic read-write head, ultra-pure magneto resistive material for magneto-resistive memory (MRM) unit, high dielectric constant insulating material on semiconductor are some notable applications of ALD in future microelectronics industry.
Fig. 2: (a) TEM image depicting conformal coating on Germanium(Ge) Nanowire (NW) by ALD Ta2O3 and (b) potential application of ALD nanowire as 1D-nanoOFET .
1. TFT Display
– For large area thin-film electroluminescent displays (TFEL): ALD Al2O3 is applied for Ion Barrier Difussion layer for dielectric e.g. and transparent oxide layers , similar to as shown in fig. 1 and 2.
2. Transistors and Semiconductor Optical Devices
– ALD of GaAs to grow AlGaAs or InGaAs , and also Phosphides .
3. High dielectric oxides and Nitrides for Microelectronics
– ALD of High-k dielectrics as well as diffusion barriers in ULSI copper metallization  (seed layer deposition), DRAM unit stacked-capacitor , gate electrodes .
4. Organic Electronics
– ALD of capping layer to protect OTFTs or OFETS , and OLEDs  against diffusion of gas or moisture.
5. Organic Photovoltaics
– Deposition of Passivation layer  and semiconducting material  for organic solar cells.
More systematic study of chemistry behind ALD process of AL2O3 on Silicon substrate (which is termed as an ideal ALD process) has been extensively presented by Prof. Steven M. George, Department of Chemistry and Biochemistry and Department of Chemical and Biological Engineering, University of Colorado . Later, Prof. Francisco Ziera published an article elaborating the ovolution of thin-film on the substrate surface during ALD process .
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