Importance of Precursor Selection in CVD/ALD Processes

The process of deposition of a solid material such as nanowires or nanotubes, particle or thin film on a substrate by the generation of reactive species in the gaseous phase is referred to as chemical vapor deposition. When precursor gases come in contact with the heated substrate a reactive species is generated. There are a number of CVD forms that include atmospheric pressure chemical vapor deposition, metal organic chemical vapor deposition and low pressure chemical vapor deposition. Metal-organic species are used as precursors to form thin films of metal oxides, metal nitrides and other kinds of metallic compounds.

Atomic layer deposition (ALD) is a special kind of CVD in which it is possible to have atomic scale deposition. A number of cursors are fed alternatingly into the reaction chamber one at a time and undergo surface reactions that are self-limiting such that during each reaction cycle the same amount of material is deposited. By this, alternating layers of different materials that are very smooth, of uniform thickness, highly dense and have minimal defects are formed.

Figure 1. General MOCVD Mechanism.

CVD/ALD processes are highly attractive since by them it is possible to have the growth of thin films that conform to specifications and are uniform with a precise thickness control.

Basic applications of CVD include producing wear-, corrosion- and high temperature-resistant protective coatings and the formation of optical fibers, dense structural parts, ceramic composites, and interesting new powdered/fibrous materials. CVD is suitable for fabricating optical storage media and for fabricating semiconductor devices.

However, with more precise control of film formation, ALD is becoming highly attractive for depositing thin films with microelectronic device applications such as ferroelectric memories, integrated circuits, radiation detectors, switches, MEMS, thin film capacitors and also as new high-k gate dielectrics for replacement of silica in future generations of metal oxide semiconductor field effect transistors. They also are essential for advancing electroluminescent device technology.

Significance of Precursor Selection in CVD/ALD

Process conditions have a considerable impact on material properties that are produced with the help of ALD/CVD, the right selection of precursors is very important in order to obtain the desired material. Initially CVD precursors include metal hydrides and halides but today a large array of metal organic compounds are used that include metal alkoxides, metal alkyls, metal diketonites, metal amidinates, metal carbonyls and others.

It is essential that precursors are volatile but thermally stable so that they do not decompose during vaporization, and are preferably soluble in an inert solvent or liquid at room temperature. Furthermore, they must have preferential reactivity towards the substrate and the growing film. It is also important that ALD precursors have self-limiting reactivity with the substrate and the film surface.

Only one element is contributed to the deposited film by most precursors, with the rest of the molecules vaporized during the process. Certain compounds can contribute more than one element and bring down the number of reactants needed for a specific process.

Figure 2. ALD precursors.

Certain metal organic precursors can contribute to incorporating oxygen and carbon unintentionally into thin films and this must also be considered. Furthermore, the potential for the undesired pre-reaction of precursors in the vapor phase must also be studied.

CVD/ALD Precursors from Strem

This information has been sourced, reviewed and adapted from materials provided by Strem Chemicals.

For more information on this source, please visit Strem Chemicals.