For the deposition of thin functional films tailoring the molecule structure or solution formulation requires many factors to be considered, and a compromise is often needed to engineer the best process system.
In the case of Chemical Vapour Deposition (CVD), critical parameters include precursor volatility, reactivity and thermal stability. For a stable vapour delivery the compound must possess a suitable vapour pressure to supply the requisite concentration of gas to the substrate, and not degrade over time. On reaching the deposition area the product must decompose at the surface cleanly to the target material with all by-products readily removed to exhaust.
In binary/ternary/quaternary and beyond, systems interaction of the different precursors in the gas phase prior to deposition must be limited, whilst at the growth surface they must react completely. Different equipment set ups and films targeted necessitate precursor matching for process optimisation, but obtaining the same properties for wildly different chemistry is not straightforward. This makes the the role of the R&D chemist is key.
Typically to enhance volatility a small ligand is employed to minimise molecular weight, however this makes the metal centre very reactive which can lead to polymerisation which lowers volatility. Conversely, use of a multifunctional bulky ligand can reduce metal-metal interactions leading to volatile molecular precursors. Attempts to tailor parameters quickly shows the interdependence of properties and the sometimes unpredictable results that can be achieved.
Combinations of different ligands has also been explored to perfect the properties of specific molecules. In one example, the use of substituted cyclopentadienyl, alkyl and alkoxide groups in the same compound was found to provide the optimum balance of reactivity, stability and volatility demonstrating the degree to which material design can be tailored.
Versatile chemistry has been instrumental in enabling different thin film technologies that yield better performing end devices such as computers, displays, solar cells, smart windows, longer life architecture, detectors, medical imagers and many more.
Exchange of ligand on a metal centre can alter many physical properties of the resulting molecules, and by choosing the correct combination of compounds, significant improvement in precursor matching can be achieved.
The experts at EpiValence have a wealth of experience in designing new precursor technologies and are available for free consultation to ensure the best chemicals are provided for customer processes. New products are continuously being added to our range of precursors based on collaborative research outputs and for the most up to date list visit our website www.epivalence.com.