Chemical vapor deposition (CVD) is a process wherein a solid material, such as nanotubes, nanowires, particle, thin film, and much more, is deposited onto a substrate by forming reactive species in the gaseous phase. The reactive species are generated when precursor gases come in contact with or pass over the heated substrate. There are several CVD processes available, including low pressure chemical vapor deposition (LPCVD), metal-organic chemical vapor deposition (MOCVD), and atmospheric pressure chemical vapor deposition (APCVD). In the MOCVD process, thin films of metals and metallic compounds such as metal nitrides and metal oxides are formed by using metal-organic species as precursors.
Figure 1. Typical CVD Reactor.
Atomic layer deposition (ALD) is a special form of CVD process that enables atomic scale deposition control. In this process, the reaction chamber is alternatingly fed with different precursors one at a time. The precursors then undergo self-limiting surface reactions in order to deposit the same amount of material in each reaction cycle, resulting in the formation of very smooth, homogenously thick, alternating layers of high dense materials with fewer defects.
Applications of CVD/ALD Processes
CVD/ALD processes gain attraction due to their ability to grow thin films of high homogeneity and conformality with a precisely controlled thickness. The CVD process has found use in the following applications:
- Production of semiconductor devices
- Synthesis of exotic new powdered/fibrous materials
- Fabrication of optical storage media
- Production of protective coatings, including high temperature-, corrosion-, and wear-resistance coatings
- Formation of ceramic composites, optical fibers and dense structural components.
Especially, the ALD process is more popular due to its ability to provide even greater control over the formation of thin films. The unique process has found use in the following applications:
- Production of new high-k gate dielectrics that hold potential to serve as an alternative to silica in next generation of metal oxide semiconductor field effect transistors
- Development of electroluminescent device technology
- Fabrication of microelectronic devices, including thin-film capacitors, radiation detectors, switches, ferroelectric memories, integrated circuits, and microelectromechanical structures (MEMS).
Key Characteristics of Precursors Used in CVD/ALD Processes
Precursors must to be volatile in nature. On the other hand, they must exhibit thermal stability in order to eliminate decomposition during vaporization. Their preferred form at room temperature is liquid and they are soluble in an inert solvent. In addition, they must exhibit preferential reactivity with the substrate and the growing film. Particularly, ALD precursors need to show self-limiting reactivity with the film surface and the substrate.
Figure 2. Precursor selection.
As process conditions greatly affect the properties of the materials formed by the CVD/ALD processes, it is essential to select proper precursors in order to produce the desired material. Initially, metal halides and hydrides were utilized as CVD precursors. Currently, many different metal organic compounds are used as precursors, which include metal carbonyls, metal amidinates, metal diketonates, metal alkoxides, metal alkyls, and much more.
Proper precursors show promise to develop customized systems for lower-temperature deposition processes, thus preventing the complications related to higher temperatures, including reduced adhesion of mismatching overlayers, interlayer atomic diffusion, and changes in the morphology and crystallinity. Furthermore, they can get rid of halogens, which can be corrosive in the deposition process and even after the formation of the film.
Figure 3. Metal alkoxide precursors.
Majority of the precursors can contribute only one element to the film being deposited and all other molecules are vaporized as part of the process. Nevertheless, certain compounds are capable of contributing over one element, thus reducing the number of reactants required for a particular process. Moreover, in certain cases, some metal organic precursors may unintentionally incorporate oxygen and carbon to the deposited films, an issue needs to be considered. Furthermore, it is necessary to evaluate the possibility for the undesired pre-reaction of precursors in the vapor phase.
Metal Alkoxide Precursors
In the fabrication of metal oxide thin films utilizing CVD/ALD, a metal organic compound often reacts with H2O2, O3, O2 plasma, or water as the oxygen source. Nonetheless, this approach often involves the formation of interfacial oxide layers between the substrate and the deposited film. On the other hand, in the approach that involves the reaction between metal alkoxides and other metal complexes such as metal chlorides or metal alkyls, the metal alkoxides can serve as the oxygen source as well as the metal. This alternative approach reduces the formation of an undesired interfacial layer as the oxygen is bound to the metal.
Applications of Metal Alkoxide Precursors
Metal alkoxides of copper, tantalum, vanadium, zirconium, titanium, hafnium, and aluminum have been explored as precursors to form oxide thin films using CVD/ALD. Using proper precursors, the CVD/ALD process is able to deposit ferroelectric materials such as PbTiO3. The thermal decomposition of niobium(V) ethoxide facilitates the production of Niobium(V) oxide layers.
Hafnium oxide films hold potential to serve as a silica replacement for gate dielectrics in next generation of semiconductor field effect transistors. Vanadium oxide films have found potential use in electrochemical battery applications.
Metal Alkoxide Precursors from Strem Chemicals
Strem Chemicals, a provider of high-purity specialty chemicals, offers a wide variety of metal organic precursors, such as metal alkyl, halide, cycopentadienyl, carbonyl, beta-diketonate, amidinate, alkoxide, alkyl amide, and other derivatives of roughly 60 metals. The following are the selected examples of metal alkoxide precursors offered by Strem Chemicals:
- Titanium(IV) i-propoxide
- Dimethylaluminum i-propoxide
- Zirconium(IV) t-butoxide
- Germanium(IV) ethoxide
- Antimony(III) ethoxide
- Titanium(IV) n-butoxide
- Niobium(V) ethoxide
- Tantalum(V) ethoxide.
Strem Chemicals also supplies additional alkoxide compounds of zirconium, vanadium, titanium, tin, thallium, silicon, lanthanum, hafnium, and aluminum.
This information has been sourced, reviewed and adapted from materials provided by Strem Chemicals.
For more information on this source, please visit Strem Chemicals.