| AVS 64th International Symposium & Exhibition | |
| Thin Films Division | Thursday Sessions |
| Session TF+SE-ThM |
| Session: | Control, Characterization, and Modeling of Thin Films I |
| Presenter: | Henrik Pedersen, Linköping University, Sweden |
| Authors: | H. Pedersen, Linköping University, Sweden L. Souqui, Linköping University, Sweden M. Imam, Linköping University, Sweden R. Tonner, Philipps Universität Marburg H. Högberg, Linköping University, Sweden |
| Correspondent: | Click to Email |
Organoborons with short alkyl groups; trimethylboron (TMB), B(CH3)3, triethylboron (TEB), B(C2H5)3, and tributylboron (TBB), B(C4H9)3, were suggested as alternative, highly reactive, less-poisonous, non-explosive B-precursors in the mid 1990’s1.TEB was found to exhibit the best properties for CVD of boron-carbon films, making it a popular CVD precursor2. TMB and TBB were deemed not suitable as CVD precursors as no boron was found in the films deposited from these molecules. Consequently, these molecules are less investigated in CVD.
We study the gas phase chemistry of TMB in a thermal CVD process, using a combination of B-C film deposition experiments at several temperatures in both hydrogen and argon atmosphere and quantum chemical calculations for a wide range of possible gas phase reactions. We suggest that without assistance from the carrier gas, i.e. in argon ambient, TMB most likely decomposes by α-H elimination of CH4 to form H2CBCH3. Methane is not highly reactive in CVD at deposition temperatures below 1000 °C, meaning that the H2CBCH3 species is the major film forming species. This correlates well with the B/C ratio of about 0.5 observed for films deposited in Ar at 700-900 °C. At higher temperatures, the B/C ratio of films increases as attributed to further decomposition to H2BCH3.
With assistance from the hydrogen carrier gas, TMB can also decompose to HB(CH3)2 that can further decompose to H2BCH3 and finally to BH3, all with negative Gibbs free energies, albeit with some high energy barriers. This in combination with the unimolecular α-H elimination with a somewhat lower energy barrier, can explain the higher B/C ratios of films deposited in H2.
Furthermore, we note that the onset of film deposition from TMB is 700 °C and at then at a very low deposition rate. Interestingly the film thickness does not increase with longer deposition time at 700 °C, indicating a surface poisoning effect. As this is seen both in Ar and H2, we speculate that this is caused by CH4 or H2CBCH3, which is currently the subject of our further investigations.
1J. S. Lewis et al. Chemical vapor deposition of boron-carbon films using organometallic reagents. Mater. Lett. 1996, 27, 327.
2M. Imam et al. Gas phase chemical vapor deposition chemistry of triethylboron probed by boron–carbon thin film deposition and quantum chemical calculations. J. Mater. Chem. C 2015, 3, 10898.