Subsections

• 2.4.1 Oxidant Species
  • 2.4.1.1 Dry Oxidation
  • 2.4.1.2 Wet Oxidation
  • 2.4.1.3 Mixed Flows of O${_2}$ with H${_2}$O, HCL, and Cl${_2}$
• 2.4.2 Influence of Temperature
• 2.4.3 Influence of Pressure
• 2.4.4 Influence of Crystal Orientation

2.4 Oxidation Parameters

The desired characteristics and requirements of the fabricated oxide can be mainly influenced by the used oxidant species. For a chosen oxidant species the oxide growth rate usually is controlled by the temperature. Additionally, it is possible to vary the hydrostatic pressure in the reaction chamber, if the oxidation system offers such possibilities. Furthermore, the oxidation rate is also influenced by the crystal orientation of the used silicon substrate.

2.4.1 Oxidant Species

The most important characteristic of oxidant molecules is that they contain oxygen atoms, which are needed for the transformation from silicon to SiO$_2$. The classical oxidant species are pure oxygen, which is also declared as dry oxidation, and water vapour, which is also declared as wet oxidation. In the middle of the 70's people started to mix pure oxygen mostly with Chlorine or Hydrocloric Acid to improve oxide quality and speed up growth rate. The state of the art are nitrided oxides for MOS-gates, which are in principle also produced by dry oxidation. Because of their extension and importance this species is described separately in Section 2.5

2.4.1.1 Dry Oxidation

During dry oxidation the silicon wafer is settled to a pure oxygen gas atmosphere (O$_2$). The oxidation rate is low (< 100 nm/hr) and so the final oxide thickness can be controlled accurately. Compared with other oxides the dry oxide has the best material characteristics and quality. The chemical reaction between silicon (solid) and oxygen (gas) is

$$\mathrm{Si + O_2 \to SiO_2}.$$ (2.1)

With dry oxidation normally high quality thin oxide films up to 100nm thickness are produced. Dry oxides are especially used as gate oxides in MOS technology. The actually fabricated gate oxide thickness is in the magnitude of about only 2nm in the currently used 90nm process technology, whereas the exact thickness depends on the respective manufacturing setup. Unfortunately, at such thicknesses SiO$_2$ generated from pure oxygen does not fulfill all demands for a good gate oxide.

2.4.1.2 Wet Oxidation

During wet oxidation the silicon wafer is settled to a water vapour atmosphere (H$_2$O). Wet oxides grow really fast compared to dry oxidation, which is the biggest advantage. The reason for the much higher growth rate is the oxidant solubility limit in SiO$_2$, which is much higher for wet (H$_2$O) than for dry oxidation (O$_2$). For 1000$^{\circ }$C the typical solubility limit value is 5.2$\times$10$^{16}$ cm$^{-3}$ for dry oxidation compared to 3$\times$10$^{19}$ cm$^{-3}$ for wet oxidation, which is nearly 600 times higher.

Therefore, wet oxidation is applied for thick oxides in insulation and passivation layers, where thick oxide buffers are needed to suppress electric currents or to ensure high threshold voltage of parasitic transistors. The chemical reaction is

$$\mathrm{Si + 2 H_2O \to SiO_2 + 2 H_2}.$$ (2.2)

Because of its water content, wet oxide films exhibit a lower dielectric strength and more porosity to impurity penetration than dry oxides. Therefore, wet oxidation is used when the electrical and chemical properties of the film are not critical.

2.4.1.3 Mixed Flows of O $\boldsymbol{_2}$ with H $\boldsymbol{_2}$O, HCL, and Cl $\boldsymbol{_2}$

The gas flow of O$_2$ can be mixed in the furnace with H$_2$O, HCL, and Cl$_2$ to get acceptable oxide quality at a higher growth rate. Besides a higher growth rate, Hydrocloric Acid (HCL) or Chlorine (Cl$_2$) is often used in oxidation in order to prevent metallic contamination and to help avoiding defects in the oxidation layer [31]. HCL and Cl$_2$ have a cleaning effect of the furnace as well as an improvement of the oxide reliability. This means that HCL and Cl$_2$ additions provide benefits to the resulting device structures such as better ion passivation, higher and more uniform oxide dielectric strength, and improved junction properties due to lower current leakage.

The mixed flows were investigated among others by Deal and Hess in the late 70's, especially for the influence on the growth rate. The addition of H$_2$O as well as Cl is investigated in [32], and of HCL in [33]. In order to see the effect of the different mixed flows on the growth rate in a clear manner, the oxide thickness over time for a (100) oriented Silicon at 1000$^{\circ }$C is plotted in Figs. 2.7-2.9. It is notable that a double logarithmic scale of the plots leads to nearly linear curves also for the mixtures.

Figure 2.7: Oxide thickness versus oxidation time for (100) oriented silicon in various H$_2$O/O$_2$ mixtures at 1000 $^{\circ }$C.

Figure 2.8: Oxide thickness versus oxidation time for (100) oriented silicon in various Cl$_2$/O$_2$ mixtures at 1000 $^{\circ }$C.

Figure 2.9: Oxide thickness versus oxidation time for (100) oriented silicon in various HCL/O$_2$ mixtures at 1000 $^{\circ }$C.

Figure 2.10: Oxidation rate of H$_2$O/O$_2$ mixture compared with HCL/O$_2$ mixture at 1000 $^{\circ }$C.

The mixture of H$_2$O/O$_2$ has the highest increase of the growth rate, because it is in principle a combination of wet and dry oxidation. We can see in Fig. 2.7 that the same percentage of H$_2$O leads to a much thicker oxide at any time than HCL or Cl$_2$. Another interesting aspect is that the admixture of the same percentage of HCL and Cl$_2$ always leads to the same oxide thickness (compare Fig. 2.8 with Fig. 2.9).

The chemical reaction of HCL with oxygen is

$$\mathrm{4 HCL + O_2 \to 2 H_2O + 2 Cl_2}.$$ (2.3)

Now it can be said that 2 moles of HCL produce 1 mol of H$_2$O and Cl$_2$. So the mixtures of HCL can be compared with H$_2$O. From the theoretical aspect the double percentage of HCL should lead to the same growth effect as the single percentage of H$_2$O. But in the practical experiment, as shown in Fig. 2.10, 5vol% H$_2$O results in a considerable thicker oxide than 10 vol% HCL. There are no more details known about this fact [32], only that the difference between the oxide thicknesses by H$_2$O and HCL becomes smaller with increasing temperature, so that the theory comes true for high temperatures (1100$^{\circ }$C).

In wet oxidation the addition of HCL does not increase the oxidation rate, rather the oxidation rate is decreased for the same percentage as the amount of HCL is added [34]. In H$_2$O-HCL ambients the thickness uniformity and appearance of these oxides were considerably better than in pure H$_2$O ambients. Also the defects in the oxide are considerably reduced.

2.4.2 Influence of Temperature

The oxidation rate increases significantly with the temperature in the furnace for dry as well as for wet oxidation. The temperature dependence of the oxidation rate is plotted in Fig. 2.11 for dry and Fig. 2.12 for wet oxidation. For wet oxidation in Fig. 2.12 it can be seen that 100$^{\circ }$C more temperature leads to approximately double the oxidation rate, if the temperature is increased from 900 to 1000$^{\circ }$C. The important temperature effect can also be observed for dry oxidation in Fig. 2.11, where the same temperature increase from 900 to 1000$^{\circ }$C leads to much more than double the oxidation rate.

Figure 2.11: Oxide thickness versus oxidation time for (100) oriented silicon by dry oxidation (O$_2$) for various temperatures.

Figure 2.12: Oxide thickness versus oxidation time for (100) oriented silicon by wet oxidation (H$_2$O) for various temperatures.

The main reason of this striking temperature influence on the oxidation rate is the temperature dependence of the diffusivity of oxygen (O$_2$) and water (H$_2$O) in fused silica. The diffusivity of the oxidants depends on the temperature $T$ in the way exp( $-\frac{c}{T}$). The oxidant diffusivity is exponentially increased with higher temperature and exponentially decreased with lower temperature. Higher diffusivity means that more oxidants can reach the Si/SiO$_2$ interface and react there with silicon to form SiO$_2$.

2.4.3 Influence of Pressure

The oxidation rate increases with the hydrostatic pressure in the furnace for dry and wet oxidation in nearly the same way. The principal advantages of higher pressure oxidation over conventional atmospheric oxidation are the faster oxidation rate (see Fig. 2.13) and the lower processing temperature generally employed [35,36]. Both lead to less impurity diffusion and minimum junction movement during the several oxidation steps which are necessary in the manufacturing of high-density multilayer IC devices. The quality and integrity of higher pressure oxides have been found to be comparable to atmospheric oxides. Oxidation-induced stacking faults are significantly reduced with higher pressure oxidation [37], which leads to improved device performance.

Figure 2.13: Oxide thickness versus oxidation time for (110) oriented silicon by dry oxidation at 1000 $^{\circ }$C for various pressures.

2.4.4 Influence of Crystal Orientation

The studies of oxidation have shown that the oxidation rate also depends on the crystal orientation of the silicon substrate. Experiments have demonstrated many times that the oxide growth is faster on (111) oriented surfaces than on (100) oriented at any temperature for dry as well as wet oxidation. Furthermore, as plotted in Fig. 2.14 for wet oxidation, it was found that the (111) and (100) orientation represent the upper and the lower bound for oxidation rates, respectively. Therefore, the growth rate for all other orientations lies between these two extremal values [38].

It is important to understand orientation effects on oxidation more generally because many structures actually use etched trenches and other shaped silicon regions as part of their structure. Ligenza [39] suggested that the crystal orientation effect might be caused by differences in the surface density of silicon atoms on the various crystal faces. He argued that since silicon atoms are required for the oxidation process, crystal planes that have higher densities of atoms should oxidize faster. Furthermore, he argued that not only the number of silicon atoms per cm$^2$ is important, but also the number of bonds matter, since it is necessary for Si-Si bonds to be broken for proceeding the oxidation. Ligenza calculated the ``available'' bonds per cm$^2$ on the various silicon surfaces and concluded that oxidation rates in H$_2$O ambients should be in the order (111)>(100), which was also observed experimentally.

Figure 2.14: Oxide thickness versus oxidation time for (100), (110), and (111) oriented silicon by wet oxidation (H$_2$O).