Journey of a molecule from the solid to the gas phase and vice versa: direct estimation of vapor pressure of alkaline-earth metalorganic precursors for atmospheric pressure vapor phase deposition of fluoride films

Abstract

Atmospheric pressure (AP) vapor phase processes such as spatial atomic layer deposition (S-ALD) and AP-metalorganic chemical vapor deposition (AP-MOCVD) are becoming increasingly appealing for their use in a variety of academic and industrial applications. Evaluation of precursor vapour pressures is crucial for their application in AP processes and to this aim the Langmuir equation has been applied as a simple and straightforward method for estimating the vapor pressure and vaporization enthalpy of various metalorganic precursors. Using benzoic acid as a calibration reference, the vapour pressure–temperature curves for several alkaline-earth β-diketonate fluorinated compounds, with molecular formula “M(hfa)₂·L” (with M = Mg, Ca, Sr, Ba; Hhfa = 1,1,1,5,5,5-hexafluoroacetylacetone and L = diglyme, triglyme, and tetraglyme) are derived from their termogravimetric curves. Thus, the enthalpy of vaporization of all complexes has been estimated using the Clausius–Clapeyron equation. As a proof of concept, preliminary results on the use of [Mg(hfa)₂·2H₂O]·2diglyme and [Ca(hfa)₂·diglyme·H₂O] or [Ca(hfa)₂·triglyme] as precursors for AP-MOCVD deposition of MgF₂ and CaF₂ in the form of thin films are presented. This approach may be used to easily determine vapor pressures of complexes and thus evaluate “a priori” the suitability of a compound as precursor for AP-MOCVD and/or spatial ALD.