Introduction to ALD and MLD

ALD and MLD refer to "Atomic Layer Deposition" and "molecular layer Deposition". They are used to produce both organic and inorganic polymers. Miniaturization to the nanometer scale has been one of the most important trends in science and technology over the last several years. The chemistry to fabricate nanolayers, the engineering for nanocomposite design and the physics of nanostructure properties have created many exciting opportunities for research. These new interdisciplinary areas in nanoscience and nanotechnology supersede the more traditional disciplines and demand new paradigms for collaboration.





Many of our surface chemistry and thin film growth investigations utilize atomic layer deposition (ALD) techniques. ALD is based on sequential, self-limiting surface reactions as illustrated in the accompanying figure. This unique growth technique can provide atomic layer control and allow conformal films to be deposited on very high aspect ratio structures. ALD methods and applications have developed rapidly over the last few years. In particular, ALD is on the semiconductor road map for high-k gate oxides and diffusion barriers for backend interconnects.


ALD is based on sequential, self-limiting surface chemical reactions. For example, for Al2O3 deposition, the binary reaction: 2Al(CH3)3 + 3H2O -> Al2O3 + 6CH4 can be split into the following two surface half-reactions


A) AlOH* + Al(CH3)3 -> AlOAl(CH3)2* + CH4
B) AlCH3* + H2O -> AlOH* + CH4


where the asterisks denote the surface species. In the (A) reaction, Al(CH3)3 reacts with the hydroxyl (OH*) species and deposits aluminum and methylates the surface. The (A) reactions stops after all the hydroxyl species have reacted with Al(CH3)3. In the (B) reaction, H2O reacts with the AlCH3* species and deposits oxygen and rehydroxlates the surface. The (B) reactions stops after all the methyl species have reacted with H2O. Because each reaction is self-limiting, the Al2O3 deposition occurs with atomic layer control. By applying these surface reactions repetitively in an ABAB... sequence, Al2O3 ALD is achieved with a growth rate of 1.1 Å per AB cycle. We have also extended the ALD method to deposit single-element metal films.





Similar self-limiting surface reactions can be employed for the growth of organic polymer films. This film growth is described as molecular layer deposition (MLD) because a molecular fragment is deposited during each reaction cycle.The precursors for MLD have typically been homobifunctional reactants. A cartoon illustrating the MLD process is shown in the nearby figure. MLD methods have been developed for the growth of organic polymers such as polyamides.The polyamides have been deposited using dicarboxylic acid and diamines as the reactants. New approaches to MLD involve heterobifunctional and ringopening precursors. In addition to organic polymers, the precursors for ALD and MLD can be combined to grow hybrid organic-inorganic polymers.


Source:
One of the research groups that pioneers in this study can be visited from here.