Chromium precursors Cr(CO)6, Cr(C6H6)2, Cr(C6H5iPr)2, Cr(C5H5)2 - have been compared in terms of growth of chromium carbide coatings by HW LP CVD. The comparison of Cr-ligand bond energies, decomposition temperature, molar fraction fo precursor used for the growth, total flow rate and growth rate are presented in Table [948]
Cr(CO)6 is effectively decomposed at the lowest temperature because of the lowest chromium—ligand bond strength (25.6 kJ/mol). However, the disadvantage of Cr(CO)6 is the large amount of oxygen in the films originatinbg from carbonyl ligands; it incorporates as chromium oxides in the coatings. A deviating from thermodynamic equilibrium (pobably metastable) cubic CrC1-x phase with the NaCl structure was obtained because of low deposition temperature and the high total flow rate.
Cr(C6H5iPr)2 and Cr(C6H6)2 generally resulted in the Cr7C3 phase contaminated with free carbon. The films grown using Cr(C6H6)2 also contained small amounts of Cr3C2. The coatings grown with chromium bis-arenes were polycrystalline over 450°C and amorphous at lower temperatures. The total carbon content and free carbon ratio was independent of the deposition temperature and the nature of the ligand, despite varying C/Cr atomic ratios in the precursors (Table 5), f.e. bis(benzene)chromium and bis(cumene)chromium result in similar carbon incorporation. This leads to the conclusion that carbon incorporation is not coming from decomposition of alkyl groups bonded to the benzene ring, but rather from the benzene ring fragmentation. The heterogeneous decomposition of hydrogen-deficient cyclopentadienyl ligand on the growing chromium surface is signigicantly different from that of benzene or substituted benzene ligands, resulting in rougher morphology.
Chromium arene tricarbonyl complexes were applied as CVD precursors for Cr-based films. The early reports it was claimed that the selection of suitable arene ligands was able to control carbon content for a given deposition temperature. [917] However, according to literature data, film composition widely varied with deposition temperature and no correlation coukd be found between the arene structure and the carbon content of the layers. The reactivity of the π-arene ligands in terms of formation of carbide coatings seems to be greater than that of carbonyl, as was observed in thermal decomposition of (C7H8)Cr(CO)3 [911].
In order to avoid oxygen contamination which can reach 8 wt.% in the films obtained from mixed arene tricarbonyl complexes [917], precursors without carbonyl ligands must be be chosen [948]
The comparison of vapor pressure of various chromium CVD/ALD precursors is presented in Fig.