6.1.2 Oxide Growth with Native Oxide Present

When some native oxide is present, both interfaces, Si-SiO$_2$ and SiO$_2$-ambient already exist and the oxide does not need to grow from scratch. How much additional oxide will grow depends on the oxidation time, temperature, ambient, pressure, crystal orientation of the silicon, as well as the thickness of native oxide already present on the surface. The implementation of such velocity fields within the LS is described in Section 3.2.1, while the Massoud model is used to determine the oxidation rate.

When 10nm of native oxide is present, the oxidation process begins with $x_o=10$nm, as shown in Figure 6.2a, where the surface representing the Si-SiO$_2$ is the bottom (blue) surface, the SiO$_2$-ambient interface is the top (red) surface and the volumes depicted are of the silicon substrate (yellow-bottom) and the native oxide (blue-top). As the process begins, the Si-SiO$_2$ interface moves deeper into the Si wafer, while the SiO$_2$-ambient interface grows towards the ambient.

Figure 6.2: Results of the oxidation of (100) oriented silicon in a dry ambient at 1atm pressure and 1000 $^{\textrm {o}}$C temperature for 100 minutes with 10nm of native oxide present. The top surface (red) depicts the SiO$_2$-ambient interface, while the lower surface (blue) depicts the location of the Si-SiO$_2$ interface. The volumes shown are the original location of the silicon substrate and the native oxide.

(a) Initial state of the processing environment before oxidation.

(b) State of the processing environment after oxidation.

Using the Massoud model, after 100 minutes of oxidation of a (100) oriented silicon wafer in a dry ambient at 1atm pressure and 1000 $^{\textrm {o}}$C temperature, a total oxide thickness of 73.8nm is found, meaning that 63.8nm of new oxide has been grown, depicted in Figure 6.2b. Even though the oxidation conditions were identical to the process shown in Figure 6.1, less oxide is grown, when there is already some initial oxide present. This is expected since oxidation is fastest during the initial stages of oxidation.