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.

Fluidics

Fluidics (also known as Fluidic logic) is the use of a fluid or compressible medium to perform analog or digital operations similar to those performed with electronics. 

The physical basis of fluidics is pneumatics and hydraulics, based on the theoretical foundation of fluid dynamics. The term Fluidics is normally used when the devices have no moving parts, so ordinary hydraulic components such as hydraulic cylinders and spool valves are not referred to as fluidic devices. The 1960s saw the application of fluidics to sophisticated control systems, with the introduction of the fluidic amplifier. 

A jet of fluid can be deflected by a weaker jet striking it at the side. This provides non-linear amplification, similar to the transistor used in electronic digital logic. It is used mostly in environments where electronic digital logic would be unreliable (e.g., systems exposed to high levels of electromagnetic interference or ionizing radiation).

Nanotechnology considers fluidics as one of its instruments. In this domain, effects such as fluid-solid and fluid-fluid interface forces are often highly significant. Fluidics have also been used for military applications.

Amplifiers :

The basic concept of the fluidic amplifier is shown here. A fluid supply, which may be air, water, or hydraulic fluid, enters at the bottom. Pressure applied to the control ports C₁ or C₂ deflects the stream, so that it exits via either port O₁ or O₂. The stream entering the control ports may be much weaker than the stream being deflected, so the device has gain.

Given this basic device, flip flops and other fluidic logic elements can be constructed. Simple systems of digital logic can thus be built.

Fluidic amplifiers typically have bandwidths in the low kilohertz range, so systems built from them are quite slow compared to electronic devices.

Systems :

Fluidic components appear in some hydraulic and pneumatic systems, including some automotive automatic transmissions. As digital logic has become more accepted in industrial control, the role of fluidics in industrial control has declined.Fluidic injection is being researched for thrust vectoring in aircraft jet engine nozzles, and for ships. Such systems divert thrust via fluid effects . Tests show that air forced into a jet engine exhaust stream can deflect thrust up to 15 degrees. Such nozzles are desirable for their lower: mass, cost (up to 50% less), inertia (for faster, stronger control response), complexity (mechanically simpler, no moving parts or surfaces), and radar cross section for Stealth. This will likely be used in many unmanned aircraft and 6th generation fighter aircraft.

Professor Nikolai Priezjev of the Department of Mechanical Engineering writes about fluidics and micro - fluidics  here .The transport and manipulation of small amounts of fluids are crucial for emerging technologies. A brisk demand for micro-devices, which are  used for the transportation of nanovolume  liquid samples, gave rise to a new exciting field called microfluidics. This field combines various  disciplines including engineering, chemistry, physics, and biology. The long range goal in modern technology is a reduction in size and the further development of microfluidic devices, which could be used for diagnoses of diseases, for the autonomous or remote detecting of biological and chemical  agents, and for gene and drug delivery.  

A very good introduction to microfluidics can be found at the wiki page. Microfluidics news is another kind of a aggregator for papers published on this subject . 

Food for thought :

  Is it possible to use constructal theory as a basis to design applications using microfluidics  ?? What would be the implications of doing so ?

Sources of information :

Wikipedia and Google .

A not-so-ordinary insight into everything around us ... Constructal theory .

This is one of the most fascinating theories developed in recent times which actually gives a very different insight into nature and its structure . For example in the field in thermodynamics it replaces the black boxes , we as mechanical engineers assume while designing any system by actually delving into the flow configuration.

The core of the theory is the constructal law :

“For a finite-size flow system to persist in time (to survive) its configuration must evolve in such a way that it provides an easier access to the currents that flow through it”

This principle predicts natural configuration across the board: river basins, turbulence, animal design (allometry, vascularization, locomotion), cracks in solids, dendritic solidification, Earth climate, droplet impact configuration, etc. The same principle yields new designs for electronics, fuel cells, and tree networks for transport of people, goods, and information.

An amazing way to describe phenomena around us . To what extent is it adopted by the industry is something we have to wait n watch ....

Cya ... Have a gr8 day ahead ....

Sources :

Constructal theory of generation of configuration in nature and engineering - Adrian Bejan¹ and Sylvie Lorente²

Constructal.org - A site dedicated to it .