When solid materials such as particles, thin films, nanotubes or nanowires are deposited on a substrate by the generation of reactive species in the gaseous phase, the process is referred to as chemical vapor deposition. When precursor gases come in contact or pass over the heated substrate, reactive species is generated. There are a number of forms of CVD that include the following:
Figure 1. Typical CVD reactor.
In the case of MOCVD, metal organic species are used as precursors to form thin films of metal oxides, metals, metal nitrides and other types of metallic compounds. Atomic layer deposition or ALD is a special form of CVD in which it is possible to achieve atomic scale deposition control. One at a time, different precursors are fed alternatingly and undergo self-limiting surface reactions such that the same amount of material is deposited during each reaction cycle. Highly smooth, uniformly thick, alternating layers of different materials that have few defects and are highly dense.
Figure 2. General MOCVD mechanism.
Since CVD/ALD processes allow the growth of thin films of high uniformity and conformity with a precise thickness control. Basic CVD applications include the manufacture of high temperature resistant, corrosion-resistant and wear-resistant protective coatings and formation of optical fibers, ceramic composites and interesting new fibrous or powdered materials.
CVD is suitable for fabricating optical storage media and is used in manufacturing semiconductor devices. The precise control of film formation has rendered ALD highly attractive for thin film applications in microelectronic devices such as ferroelectric memories, radiation detectors, switches, integrated circuits, MEMS and as new high-k gate dielectrics to replace silica in future generations of metal oxide semiconductor field effect transistors. They are also essential for improving electroluminescent device technology.
The key for obtaining the desired material is to select precursors appropriately even though process conditions also play a significant role in deciding the material properties produced. In early times, typical CVD precursors include metal hydrides and halides however today a large number of metal organic compounds are used that include metal alkoxides, metal alkyls, metal amidinates, metal diketonates, metal carbonyls and others.
Figure 3. Precursor selection.
They also help develop customized systems for a deposition process at low temperature and avoids the complexities related with higher temperatures. These complexities may include interlayer atomic diffusion, reduced adhesion of mismatching overlayers, changes in the crystallinity and morphology. Furthermore, elimination of halogens that can be corrosive during deposition and also in the formed film, if included is advantageous.
Precursors need to be volatile as well as thermally stable as they should not decompose during vaporization and are mostly soluble in an inert solvent. Furthermore, they must have preferential reactivity towards the substrate and the growing film. It is also required that ALD precursors have self-limiting reactivity with the film surface and the substrate.
Only one element is contributed by most precursors to the deposited film with the remaining vaporized during the process. Certain compounds may contribute more than one element and bring down the number of reactants given for a specific process.
Figure 4. ALD precursors.
Certain metal inorganic precursors can contribute to incorporating oxygen and carbon into thin films and this must also be considered. Furthermore the potential for undesired pre-reaction of precursors in the vapor phase must also be studied.
Cyclopentadienyl (Cp) complexes have become attractive precursors for CVD/ALD as they are normally volatile and reactive towards water at considerable temperatures and often form films with minimal impurity levels.
Cp complexes of Mg, Sc, Y, Hf, and Zr have been used to form oxide films, typically with water as the oxygen source. Pure metal films have been produced from metallocenes of Va, Cr, Mn, Ir, Co, Ru, Pt, Pd, and Ni at temperatures typically above 500ºC.
For the removal of carbon, hydrogen gas may be required. The carbon contamination issue is addressed by using Cp carbonyl derivatives.
Cyclopentadientyl precursors have been used for the deposition via ALD of electroluminescent (EL) SrS and BaS thin films doped with Cu, Ce, Pb, Mn, or Eu.
The study results show that the use of the cyclopentadienyl-based dopants may lead to improvements in the performance of EL devices. Depositing noble metal films using ALD to be used in magnetic recording media and integrated circuits has been achieved using Cp complexes, such as Ru(EtCp)2, as precursors. Separately, ZrO2 thin films have been grown at 350ºC on silicon (100) substrates via ALD using (CpMe)2ZrMe2 and (CpMe)2Zr(OMe)Me with ozone as the oxygen source, causing deposition of highly conforming films onto high aspect ratio trenches.
HfO2 thin films with good dielectric properties have also been deposited onto p-Si(100) substrates through ALD using Cp2Hf(CH3)2 and water. Ultra-thin films of hafnium and zirconium oxides are considered to have the greatest potential to replace SiO2 as high-k gate oxides as semiconductor technology moves to 45 nm technology.
Selected examples of metal cyclopentadienyl precursors from Strem include:
Additional cyclopentadienyl compounds of barium, bismuth, cerium, chromium, cobalt, copper, dysprosium, erbium, gadolinium, hafnium, indium, iron, lanthanum, magnesium, manganese, molybdenum, neodymium, nickel, niobium, osmium, platinum, praseodymium, rhenium, rhodium, ruthenium, samarium, thallium, thulium, titanium, tungsten, vanadium, ytterbium, yttrium, and zirconium are also offered.
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