When silicon dioxide is forming on the single-crystal silicon surface, there is an abrupt change at the silicon-silicon dioxide interface. There are always some dangling bonds at the silicon-silicon dioxide interface because of the crystal structure mismatch.
The dangling bonds induce so-called interface state charge, which is a positive charge that strongly affects IC chip performance and reliability.
During IC chip manufacture hydrogen or some other atoms can diffuse to the silicon-silicon dioxide interface and attach to the dangling bonds resulting in changes in the interface state charge and this in turn changes the threshold voltage (VT) of the MOS transistors and finally the performance of the IC device.
Introducing HCl in the oxidation helps to minimize the number of dangling bonds as some of the chlorine atoms can be integrated into the silicon dioxide film and bind with silicon at the Si-SiO2 interface.
Fig. 5 Dangling bonds induce interface state charge.