Replacing silicon dioxide as a gate dielectric material in present day complementary metal-oxide-semiconductor technology is an active area of research. This is because fundamental concerns have emerged regarding the increased leakage current and reduced dielectric reliability as a result of rapidly shrinking $SiO_2$-based gate dielectric dimensions. This situation has led to the search for an alternative material with a higher dielectric constant than $SiO_2$, but with a suitably large band gap like $SiO_2$, to keep the gate leakage current within reasonable limits. Amorphous $Al_2O_3$, which has a dielectric constant of about 8-10 and a band gap of about 7-9 eV, is an attractive alternative for a gate oxide material.
To determine the usefulness of $Al_2O_3$ film in future CMOS devices, it is important that the film be thermally stable on the Si surface. However, it is known that depositing an $Al_2O_3$ film on the Si surface often results in the formation of an interfacial aluminum silicate and/or a $SiO_2$ layer. The $Al_2O_3$-Si interface is expected to be stable against the formation of silicate and/or $SiO_2$, as such interfacial reactions are not predicted from bulk equilibrium thermodynamics of the Si and $Al_2O_3$. Bulk thermodynamics is not always a good predictor of interfacial reactions on an atomic scale. Therefore, to obtain a stable $Al_2O_3$-Si interface, it is important to understand the kinetics of deposition and interface formation.
We grew thin $Al_2O_3$ films on Si (100) surface using atomic layer deposition (ALD) and plasma enhanced ALD (PEALD), and investigated the $Al_2O_3$-Si interface structure and stability for each case. In ALD process, we used tri-methyl-aluminum (TMA) or methyl-pyrrolidine-alane (MPA) as Al sources and used iso-propyl-alcohol (IPA) or $H_2O$ vapor as oxygen sources. In PEALD process, $Al_2O_3$ films were grown by using MPA and $O_2$ plasma. The difference in saturated deposition thickness/cycle of each ALD process was explained by the adsorption of Al sources and secondary adsorption of oxygen sources. Because the effective area of one adsorbed molecule is smaller in the case of MPA than of TMA, the amount of adsorbed MPA in unit area would be more than that of adsorbed TMA. Therefore the saturated deposition thickness/cycle would be increased when MPA was used as Al precursor. When oxygen source was supplied to the reactor, the IPA or $H_2O$ would react with Al precursor previously adsorbed on the surface and form $Al_2O_3$. The oxygen sources are then likely adsorbed on both of the outermost surfaces of the newly formed $Al_2O_3$ and the newly exposed surface, which had been screened with adsorbed Al precursor before. This is referred to as the secondary adsorption. These secondary adsorbed oxygen sources would also react with Al precursor during the following Al precursor supply. $H_2O$ is more effective in the secondary adsorption, the saturated deposition thickness/cycle would be increased when $H_2O$ is used as oxygen source.
The stoichiometry and the carbon contamination of as-deposited and annealed $Al_2O_3$ films were characterized by X-ray photoelectron spectroscopy (XPS) and Auger electron spectrometry (AES). The interface stability of the $Al_2O_3$-Si systems was characterized by cross-sectional transmission electron microscopy (XTEM) and XPS. The formation of interfacial layer in as-deposited films depended upon the oxygen source used in process. An interfacial layer was not detectable on the as-deposited film grown by TMA-IPA or MPA-IPA ALD process. On the other hand, an interfacial layer with a thickness of ~ 12Å was generated on the as-deposited film that was grown by ALD using $H_2O$ instead of IPA. Similarly, an interfacial layer was formed on the as-deposited film that was grown by PEALD using $O_2$ plasma as oxygen source. $H_2O$ vapor or $O_2$ plasma oxidized Si surface at a deposition temperature until the surface was completely covered with deposited $Al_2O_3$ layer. After annealing at 800 ℃ for 5 min., an interface layer was newly formed or grown even under the neutral ambient of Ar, and it grew thicker under the oxidizing ambient of $O_2$. Oxygen, which is needed for the formation of the interface layer during the annealing process, was supplied from both of the ambient oxygen and the excess oxygen in the films.
We proposed the AlN interlayer to protect Si surface from oxygen source during an early stage of $Al_2O_3$ ALD/PEALD and excess oxygen in the films during post-deposition annealing. The AlN films were deposited by PEALD using MPA and $NH_3$ plasma. XTEM results showed the as-deposited and annealed $Al_2O_3$-Si interface was abrupt with the introduction of ultra-thin AlN interlayer ~ 10 Å before $Al_2O_3$ PEALD. The effective dielectric constant was improved from 8 of the film without AlN interlayer to 9.6 of the film with AlN interlayer