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44228-G5
Studies of Interface Structure and Properties of Thin Films
The objective of our work is to study the nucleation of metal oxide thin films on different starting surfaces using Atomic Layer Deposition (ALD). In ALD, film formation is achieved by alternating exposure of a surface to the vapor of two chemical reagents that react in a complimentary, self-limiting manner. Complimentary means that each of the two reagents must prepare the surface for reaction with the other chemical so the process is cyclical.
We have used a hot wall ALD flow reactor to compare two different HfO2 ALD chemistries: tetrakis (ethylmethylamino) hafnium (TEMAH) and H2O, and tetrakis (dimethylamino) hafnium (TDMAH) with H2O on H-terminated Si (Si-H) and SC1 chemical oxide starting surfaces. Based on the prevailing models for ALD, Si-H surfaces are inert and should impose a barrier for the efficient nucleation of metal oxide films whereas oxide surfaces are suitable for efficient film nucleation. Spectroscopic ellipsometry (SE) confirms linear growth of the films with a growth rate of ~1.2 Å/cycle at 250°C for the TEMAH process, and ~1.0 Å/cycle at 275°C for the TDMAH process. Rutherford backscattering spectrometry indicates an initial 4-7 cycle nucleation barrier for the Si-H surface with both metal-organic precursors, before the growth rates on that surface match those on SC1. Steady-state coverage is attained after a ‘conditioning' period of about 45 ALD cycles for TEMAH (Figure 1), as opposed to only 20 cycles with TDMAH (Figure 2). However, TEMAH results in a higher steady-state coverage rate of 3.1x1014 Hf/cm2 on both starting surfaces, compared to 2.4x1014 Hf/cm2 and 2.7x1014 Hf/cm2 for the Si-H and SC1 surfaces with TDMAH. A comparison of the growth rate measurements with RBS and SE indicates that the HfO2 films grown from TEMAH appear to reach about 85% of their bulk density on both starting surfaces while TDMAH results in 75% bulk density for the Si-H surface and 85% density on the SC1 surface. Angle resolved X-ray photoelectron Spectroscopy is used to examine the film composition and the Si/HfO2 interfacial region for films grown on the Si-H starting surface. The films deposited from TEMAH are close to stoichiometric with a Hf/O ratio of 2.1 while the films deposited from TDMAH are O-rich (Hf/O=2.4). For the TEMAH films, we detect a ~7 Å SiOx/silicate interfacial layer which remains practically unchanged for 50 ALD cycles. In the TDMAH/H2O process, the interfacial layer is thicker (8-10 Å) but still remains practically unchanged up to 25 cycles (Figure 3). These results are corroborated by HRTEM data presented in Figure 4. The thicker interfacial layer detected in the TDMAH films may be due to the excess O present in the films.
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