ALD is actually a sub-set of CVD. CVD encompasses all deposition techniques in which the deposition depends on some sort of chemical reaction (e.g. SiH4 + 2*N2O ->2* N2 + 2*H2 + SiO2) In ALD, the growth progresses layer by layer by alternatively pulsing the source gases. This enables ultra fine thickness control. In most other CVD techniques, all source gases flow simultaneously and some energy source is provided to aid the reaction (high-temperature or plasma).

ALD is actually a sub-set of CVD. CVD encompasses all deposition techniques in which the deposition depends on some sort of chemical reaction (e.g. SiH4 + 2*N2O ->2* N2 + 2*H2 + SiO2) In ALD, the growth progresses layer by layer by alternatively pulsing the source gases. This enables ultra fine thickness control. In most other CVD techniques, all source gases flow simultaneously and some energy source is provided to aid the reaction (high-temperature or plasma).

Agree with Derek. CVD is a more general name for the process of depositing some materials onto some other (not necessarily other) materials from the gas phase, utilizing a chemical reaction. ALD differs from other CVD processes by the fact that a surface catalytic reaction is employed in ALD. This means, that a gas phase reactant, referred to as a precursor, on impact with a solid-state reactant - substrate (might as well be hot, or cold, but again, not necessarily) - as a result of a surface catalytic reaction, produces a deposit on the substrate. The surface reaction normally requires clean substrate surface, since it proceeds through the catalytic sites on the surface. Once all the surface of the substrate is covered with the deposit material, the reaction stops, since no more active catalytic sites are subjected to the gas-phase precursor. The reaction is therefore called self-limited, or self-terminated. However, the deposit might provide the active sites for a catalytic reaction with other precursors, and so on. This effect is put in the basis of thin films engineering with atomic layer precision (hence the name). One of the good examples is growth of graphene on the surface of catalytically active metals using a hydrocarbon precursor:

C2H4 + Me(t = 500-1000C) ->graphene/Me + 2*H2

Finally, I looked it up on Wikipedia and found the pages on CVD/ALD there quite worthy, so if you are interested in an overview of CVD techniques, you can refer to Wikipedia.

Regards

I visited a laboratory a few weeks ago practising ALD and they emphasized that the distinctive feature of ALD is that the layer deposition process automativally stops when the surface functional groups are exhausted, therefore it is easier to prodduce unofmrly thin layers depositen on the top of each other.

it already the deposition rate is not the same, and the quality of the deposit, ALD remains slower and give better structural quality

ALD proceeds via 2 half-reactions, done one after the other, while CVD is a continuous process where all reactants are supplied at the same time to grow the film. As an example, one can make aluminum oxide via CVD by admitting both trimethylaluminum and oxygen to the reactor, reacting on a substrate at fairly high temperature. By ALD, the temperature would not need to be as high, and one would first admit only trimethylaluminum but not oxygen, and the TMA would react with hydroxyl groups on the substrate until they are all covered, then stop. Then the chamber is purged, then water vapor can be admitted to react with methyl groups on the surface organo-aluminum groups, leaving hydroxyl groups and evolving methane. The reactor is purged again, and a second cycle of reaction with trimethyl aluminum reaction is carried out... etc. Using ALD you will make extremely even, thin films with controlled thickness, which can be useful for (as an example) a gate oxide, where CVD would give higher growth rates but not the same level of uniformity.

I guess this question is thoroughly answered now. I'd like to take a slight aside .....

Can someone comment on the stoichiometry control of ALD SiO2 (or any material) vs. other techniques (including reactive sputtering, evaporation, etc). Which would give the best proportion and least impurities? Which would be worst?

Impurities in both sputtering and evaporation dependends upon the purity of the original cathode material/evaporant. ALD should be better in that respect ; but partial decomposition of the precursors may retain some unwanted components in the film. For example carbon content can appear in alumina deposition using trimethyl aluminum and water. Stoichiometry in the case of sputtering depends upon the starting material. In the case of evapoation we will have to use reactive conditions in the chamber and right substrate temperature to get good stoichiometry. As far as ALD is oncerned one should obtain good stoichiometry..well..at least in principle; but my experience shows that it depends on several factors like substrate temperature, precursor flow (should saturate the chamber), pulse time etc.

@Rajeev Thanks for your answer. Apologies for my delay in responding. Just noticed your answer now when I was showing a colleague this discussion thread. :))

Now a days ALD (fast ALD from veeco, and spatial ALD) can achieve high growth rates and better in quality as compared to conventional ALD tools..in ALD the chemistry is totally different from the CVD method. However, ALD is subset of CVD. In ALD low temp growths can be achieved in combination with FLASH, PLASMA, LT-remote plasma techniques.

Questions:

Vacuum Systems

Research a type of pump that we did NOT discuss: the ion pump

- What are the principles of operation?

- What is its ultimate pressure?

- What limits its performance practically?

- What type of system is it generally used on?

Thin Films

Research and describe a layering technology we did NOT discuss: Atomic Layer Deposition (ALD)

• What type of deposition is this and how does it differ from other techniques of the same type?

• What are the key components/reactions?

• Describe the process

• Give at least one example of its use in current (>2000) literature (describe why it was important in that case)

Can you please help to find answer for these  Qs?

Atomic Layer Deposition (ALD)

• What type of deposition is this and how does it differ from other techniques of the same type?

• What are the key components/reactions?

• Describe the process

• Give at least one example of its use in current (>2000) literature (describe why it was important in that case)

-ALD is in the sense CVD, the only difference is reaction in ALD case is reaction doesn't takes place in gaseous phase. Both the precursors (A & B) are not present in the chamber at same time. They are introduced into the reactor sequentially with purging by N2 or Ar gas (G) in between. for eg, (A + G + B + G) + (A + G + B + G) + ... (one cycle)

- ALD is surface saturation process where surface chemistry plays vital role on the film growth and properties. Film growth takes place till self termination is achieved  or surface is saturated.

Due to its unique property to provide high uniform and conformality it is possible to deposit thin films on high aspect ratio substrate.

- Well, very slow technique compare to conventional CVD.

I will suggest to go through: http://www.vtt.fi/inf/julkaisut/muut/2010/Puurunen.pdf

Its a nice review paper.

ı wat to grwoth HfAlO layer in ALD system. how can ı dizayn recipe. sent firt H2O then Hf source or Al source. Or Hf and Al sent smultaneusly.