Study of oxide/silicon charge trapping using second harmonic generation

The single most important interface in semiconductor technology is that between silicon and its thermally grown oxide. This interface plays a crucial role in the performance of the silicon metal oxide semiconductor field effect transistor(MOSFET). Issues related to this interface include trap states in the bulk oxide and near the interface. Although much understanding has been gained in recent years, there still remains controversy and debate over the nature of trap state generation and the related physical phenomena. This is due to the limitations of the existing tools used to characterize and monitor the degradation. Therefore, new techniques have to be developed to allow a thorough and direct study of this complex problem. Second harmonic generation(SHG) is developing into a surface probe. In nature, it is highly surface-specific and has a submonolayer sensitivity and is also capable of in-situ probing of the surface. It is applicable to all interfaces accessible by light. This dissertation focuses on the development of this new technique into a diagnostic tool for semiconductor device interface research and uses it to study trap generation caused by high electric fields and reactive ion etching(RIE).; We built a sensitive, high repetition rate, femtosecond laser system, which also contains data acquisition hardware and software. SHG is able to detect electric-field-induced nonlinearity at the surface and interface and is therefore sensitive to electric field variations across the Si/SiO 2 interface. Reduction of the interface SHG intensity from Fowler-Nordheim(F-N) stressed MOS diodes directly reveals the weakened control of gate voltage over the channel surface for the first time. Trapped electrons in the oxide near the oxide/silicon interface produce an electric field along the silicon crystalline [100] direction. The electric field is strong enough to cause band bending in the silicon substrate. With the highly sensitive SHG system, for the first time the effects of electric-field induced and RIE processing-induced interface and oxide bulk trapped charge from the silicon-oxide interface can be detected and studied.; The laser pulses simultaneously excite electrons and provide for the detection of newly trapped and accumulated charge. To make fall use of this unique property, time-dependent second harmonic (TDSH) signal measurements are used to study neutral traps in the oxide from a F-N stressed oxide/silicon structure. A hypothetical model of laser-induced electron captured by oxide traps is established. The model is used to explain the time-dependent interface electric field variation induced by trapped electrons that tunnel through the interface via multiphoton absorption. New information about high electric field-induced neutral traps is also obtained.; The dependence of oxide trap generation on the oxide thickness and plasma conditions is studied. The temperature dependence of the capture and re-emission of electrons by the trap energy states is also investigated. The decay constant obtained from TDSHG reveals information on the oxide trap density and the spatial distribution of the traps. These studies demonstrate that SHG can become a non-invasive, sensitive monitoring of surface charge that accumulates on the oxide during plasma processing. (Abstract shortened by UMI.)...