Infrared reflection spectroscopy of adsorbed intermediates in real time during chemical vapor deposition of oxides

In this paper, we develop multilayer optical theory to model the real-time (in operando) optical response of a growing dielectric film being deposited by chemical vapor deposition (CVD), with a particular emphasis on understanding the deposition mechanism through direct detection of the adsorbates responsible for film growth by infrared reflection-absorption spectroscopy (IRRAS). The model involves a four-layer stack consisting of a vacuum over a monolayer or submonolayer of molecules adsorbed on the surface of a dielectric thin film, which, in turn, is growing on a metal substrate. It is well known that, in IRRAS, the sensitivity of p-polarized light to absorption by the molecular adsorbates is a function of the incident angle of the IR beam: at high angles, the sensitivity is highest. We show that, for incident beam angles above 70° (which are typically used in IRRAS experiments), the sensitivity also depends on the thickness and refractive index of the insulating thin film; as a result, the sensitivity changes dynamically during the growth of the dielectric layer. Our analysis shows that, at incident beam angles of ∼60°–70°, the sensitivity to molecular adsorbates is somewhat lower, but is almost independent of the oxide thickness from 0 to 100 nm and also independent of the oxide refractive index from 1.0 to 2.5. Despite the loss of sensitivity relative to that achievable at higher incident beam angles, 1000 scans at an incident angle of 60° are sufficient to obtain IR spectra of the adsorbed molecules with reasonable signal-to-noise ratios even at submonolayer coverages. Because the sensitivity at this incident beam angle is not thickness dependent, it is not an issue (as it is at higher beam angles) that additional oxide grows during the time required to acquire 1000 scans. Experiments can be performed using a conventional vacuum deposition system, in which the internal beam path is tens of cm. We demonstrate the use of these smaller incident beam angles to study the mechanism of a CVD process in real time by polarization-modulation IRRAS, obtained by subtracting the s-polarized from the p-polarized infrared spectra in order to eliminate the unpolarized component due to molecules in the beam path and on windows. We explore the surface coverage of various adsorbed intermediates during CVD of HfO2 from tetrakis-(dimethylamido)-hafnium (TDMAH) and water in the presence of the consumable inhibitor magnesium N,N-dimethylamino-diboranate [Mg(DMADB)2]. We find that the addition of the Mg(DMADB)2 inhibitor causes a decrease in the IR absorption from the adsorbed TDMAH precursor that correlates with the observed decrease in the HfO2 growth rate; this result indicates that the mechanism of inhibition involves Mg(DMADB)2 acting as a dynamic site-blocker that lowers the surface coverage of TDMAH.