Subsections

• 2.5.1 Different Nitridation Methods
• 2.5.2 Diffusion-Barrier Properties of Nitrided Layers
• 2.5.3 Nitrogen Incorporation by NO
• 2.5.4 Nitrogen Incorporation and Removal by NO${_2}$
• 2.5.5 Nitridation in N${_2}$ and NH${_3}$

2.5 Nitrided Oxide Films

While SiO$_2$ was the main material for gate dielectrics for more than three decades, the use of traditional SiO$_2$ gate dielectrics becomes questionable for sub-0.25 $\mu$m ULSI devices. Increasing problems with dopant penetration through ultrathin SiO$_2$ layers (<2nm) and direct tunneling for ultrathin oxide films dictate the search for new materials for future gate dielectrics with better diffusion barrier properties and higher dielectric constants [40]. At this time, ultrathin silicon oxynitrides (SiO$_x$N$_y$) are the leading candidates to replace pure SiO$_2$ [41].

Nitrogen suppresses boron penetration from the poly-Si gate and reduces hot-electron-induced degradation. The dielectric constant of the oxynitride increases linearly with the percentage of nitrogen from $\varepsilon_{SiO_2} =3.8$ to $\varepsilon_{Si_3N_4} =7.8$. Because most SiO$_x$N$_y$ films are currently grown by thermal methods, they are only lightly doped with N (< 10 at.%). Therefore, these silicon oxynitrides have a dielectric constant only slightly higher than that of pure SiO$_2$.

2.5.1 Different Nitridation Methods

The performance of MOS-based devices depends on both the concentration and distribution of the nitrogen atoms incorporated into the gate dielectric. The optimal nitrogen profile is determined by its application. One possibility is a SiO$_x$N$_y$ film with two nitrogen-enhanced layers: at first, nitrogen is placed at or near the Si/SiO$_2$ interface to improve hot-electron immunity, and second, an even higher nitrogen concentration is put at the SiO$_2$/polysilicon interface where it is best used to minimize the penetration of boron from the heavily doped gate electrode [42]. Typical amounts of nitrogen at each interface are in the order of $(0.5-1)\times10^{15}$ cm$^{-2}$.

Nitrogen may be incorporated into SiO$_2$ using either thermal oxidation/annealing or chemical and physical deposition methods. Thermal nitridation of SiO$_2$ in NO or N$_2$O generally results in a relatively low concentration of nitrogen in the films in the order of 10$^{15}$ N/cm$^{2}$ [42]. Since the nitrogen content increases with temperature, thermal oxynitridation is typically performed at high temperatures (T > 800$^{\circ }$C).

For more heavily N-doped SiO$_x$N$_y$ films, other deposition methods, such as chemical vapor deposition in different variants, or nitridation by energetic nitrogen particles (e.g. N atoms or ions), can be used. These nitridation methods can be performed at lower temperatures ($\sim$300-400$^{\circ }$C). Unfortunately, low temperature deposition methods result in non-equilibrium films, and subsequent thermal processing steps are often required to improve film quality and minimize defects and induced damage [43].

2.5.2 Diffusion-Barrier Properties of Nitrided Layers

An important property of nitrogen in nitrided oxides is that it forms a barrier against the diffusion of boron. Concurrent with this, it also lowers the diffusion rates for oxygen and other dopants, slowing down the growth rate of any further oxidation or nitridation [44]. For example, for a 2 nm oxynitride with one monolayer of nitrogen $6.8\times10^{14}$ N/cm$^{2}$ located near the interface, the oxidation rate decreases by at least a factor 4 relative to the pure oxide. The decrease in film growth rate results from a decreased rate of diffusion due to nitrogen.

One explanation for the lower diffusivity of NO, O$_2$, N$_2$ or other molecular species is the higher density of nitrides and oxynitrides compared with pure oxide. Furthermore, the lattice involves N bonds and therefore becomes more rigid. The three bonds connected to each nitrogen as in Si$_3$N$_4$ are more constrained than the two bonds of each O atom in SiO$_2$, where the Si-O-Si bond angles can go from 120$^{\circ }$ to 180$^{\circ }$ with little change in energy. These more constrained bonds are another important reason for decreasing the ability of nitrided lattices to permit the diffusion of atoms and small molecules.

2.5.3 Nitrogen Incorporation by NO

Oxidation of silicon and annealing of SiO$_2$ in nitric (NO) or nitrous (NO$_2$) oxide are the leading procedures for making nitrided oxides by conventional thermal processing methods. NO is the main species responsible for nitrogen incorporation into the film [45]. Oxynitridation in pure NO should be considered for ultrathin dielectrics, especially in processes where thermal budget and film thickness issues are crucial. When the temperature increases, the total amounts of both nitrogen and oxygen increase as well as the ratio of nitrogen to oxygen so that the film becomes more nitride-like at higher temperatures. For example the ratio increases by 40% if the temperature changes from 700 to 1000$^{\circ }$C [46]. With rising temperature the depth of the nitrogen profiles and so the width of the containing nitrogen region increase too.

The thicknesses of the films on clean silicon surfaces measured at 700-1000$^{\circ }$C after one hour were only $\sim$1.5-2.5 nm [46]. From the practical point of view, the slower growth of oxynitride compared with pure oxide facilitates good thickness control in the ultrathin regime during high-temperature processing. To make a thicker film, a thin preoxide (SiO$_2$) of desired thickness can first be formed and then annealed by NO. However, the nitrogen distribution in NO-annealed films is different compared to the one in NO-grown filmss (see Fig. 2.15).

2.5.4 Nitrogen Incorporation and Removal by NO $\boldsymbol{_2}$

Under equivalent conditions, oxynitridation in NO$_2$ results in less nitrogen incorporation than in NO. However, NO$_2$ is particularly attractive, because
1) it allows to incorporate an appropriate amount of nitrogen near the SiO$_x$N$_y$/Si interface (typically $\sim 5\times10^{14}$ atoms/cm$^{2}$),
2) its processing with O$_2$ gas permits NO$_2$ to replace oxygen in the oxidation reactors/furnaces.
Among other factors, oxynitridation in NO$_2$ is complicated by the fast gas-phase decomposition of the molecule into N$_2$, O$_2$, NO, and O at typical oxidation temperatures 800-1000$^{\circ }$C [47], in contrast to NO, which is a relatively stable molecule.

The fundamental difference between oxynitridation in NO$_2$ and NO is that, while both incorporate nitrogen by NO reactions near the interface, in the NO$_2$ case the nitrogen incorporation occurs simultaneously with nitrogen removal from the upper layers of the film (see Fig. 2.15). In experiments it was observed that NO does not effectively remove nitrogen from the oxynitride [48]. So it can be concluded that other products of the NO$_2$ gas-phase decomposition, like O, are responsible for the nitrogen removal. The final nitrogen concentration and distribution is influenced by a competition between N incorporation and removal.

NO$_2$ rapidly decomposes in the gas phase to N$_2$ and O, and then the O initiates a further series of reaction to form NO, the key oxynitriding agent, and other species. NO, from gas or decomposition, is similar to O$_2$ when it interacts with silicon or SiO$_2$, in that the dominant oxynitride growth mechanism involves NO diffusion through a SiO$_x$N$_y$ overlayer, followed by a reaction with silicon at and near the SiO$_x$N$_y$/Si interface [48].

Figure 2.15: Several nitridation processes and resultant nitrogen profiles.

2.5.5 Nitridation in N $\boldsymbol{_2}$ and NH $\boldsymbol{_3}$

Direct nitridation via reaction of silicon with N$_2$ requires very high temperatures (T $\geq$ 1200$^{\circ }$C) and, therefore, a too high thermal budget. To reduce the thermal budget, oxynitrides were grown in pure N$_2$ by rapid thermal processing (RTP). Although the input N$_2$ gas stream is purified at the point of use and therefore extremely free of contaminants such as N$_2$O, O$_2$, CO$_2$, and CO (less than 1 ppb each of them), it was found in experiments, that in a cold wall RTP module, the growth chamber contributes impurities to the ambient through outgasing from the walls [49].

Therefore, although the Si/N$_2$ system may be inert for T $\leq$ 1200$^{\circ }$C, the de facto oxidation ambient is not so. Thus, it was observed that N$_2$ reacts with silicon at moderate temperatures (760-1050$^{\circ }$C) in an RTP module [49], due to the presence of gas impurities, to form ultrathin (less than 1.2 nm) SiO$_x$N$_y$ films.

Nitridation in ammonia (NH$_3$) was one of the first methods used to incorporate relatively high concentrations of nitrogen ( $\sim 10-15$ at.%) into SiO$_2$ films. The nitridation atmosphere of NH$_3$ introduces high concentrations of hydrogen into SiO$_2$ films, which then can act as traps. One of the advantages of the thermal nitridation of SiO$_2$ in NH$_3$ is the simultaneous nitridation of the interface and the SiO$_2$ surface, while one disadvantage is the introduction of hydrogen in the oxynitride film. This disadvantage can be overcome by performing a thermal reoxidation of the oxynitride film in dry O$_2$, which completely removes the hydrogen from the film and serves also to decrease the concentration of nitrogen at the SiO$_2$/Si interface, improving the electrical characteristics of this interface [50].

The role of hydrogen is crucial, because, if hydrogen is not contained in the nitriding molecule as in the case of thermal treatments of SiO$_2$ films in N$_2$ or N$_2$/H$_2$ mixtures, incorporation of nitrogen in the films does not occur. Hydrogen participates in the transport of the nitriding species from the film surface towards the SiO$_2$/Si interface. Ammonia reacts in the surface region of silica at temperatures above 650$^{\circ }$C as

$$\mathrm{Si-O-Si + NH_3 \to SiNH_2 + SiOH}.$$ (2.4)

At the SiO$_2$/Si interface, because of the existence of free silicon atoms and by considering the change of free energy of the chemical reactions, the following reactions can take place

$$\mathrm{SiO_2 + 3 Si + 4 NH_3 \to 2 Si_2N_2O + 6 H_2},$$ (2.5)

$$\mathrm{2 SiO_2 + Si + 4 NH_3 \to Si_3N_4 + 4 H_2O + 6 H_2}.$$ (2.6)

In the bulk of the silicon oxide, on the other side, the nitriding species will react mostly with silicon oxide

$$\mathrm{2 SiO_2 + Si + 4 NH_3 \to Si_3N_4 + 4 H_2O + 6 H_2}.$$ (2.7)

The incorporation of a nitrogen atom will often be accompanied by the intake of a hydrogen atom which removes an oxygen atom (in form of water or OH), the nitridation of the SiO$_2$ films proceeds essentially by an exchange of N for O atoms [50].