Electrically active defects generated by MERIE and RIE-mode plasmas in thin SiO 2-Si structures

Abstract

The effect of both RIE and high-density nonuniform magnetically enhanced reactive ion etching (MERIE)-type plasmas on the properties of thin oxide (11–13 nm) MOS capacitors as well as FETs without gate has been investigated. The results reveal the vulnerability of the oxide and its interface with Si to the plasma process — the interface is much more sensitive. The creation of defects in the form of fixed oxide charge, bulk traps, slow states and interface states is found. The damage level is a function of both the discharge conditions (including plasma exposure time) and the initial Si-SiO 2 structure parameters, the plasma conditions having a priority impact. The damage process is very rapid particularly in the first seconds (up to 30 s) of plasma exposure. The effects become highly process dependent as the plasma time increases. The plasma induced defects degrade the inversion carrier mobility and change the dominant scattering mechanism in the inversion channel. The damage leads to an excess leakage current and decreases the breakdown fields. A strong linear correlation between plasma induced leakage current and plasma created positive charge is detected. It is established that the build-up damage depends on plasma nonuniformity, but the non uniformity is neither the only nor the dominating factor. The nature of process induced defects and the influence of plasma components are discussed. It is proposed that generated interface states are mainly attributed to VUV and ion bombardment, whereas the high values of positive oxide charge are due to the charging effect. The type of plasma induced defects (oxide traps or interface states) and the energy distribution of interface states strongly depend on the relative contribution (or domination) of the different plasma components. A room temperature annealing of MERIE-type plasma induced interface states is established. The reduction depends only on the starting postplasma treatment level of interface states and the effects responsible for this reduction take place very close to the Si-SiO 2 interface. (The fixed oxide charge is stable and it does not change at all.) The process seems to be controlled by moisture transport to the Si-SiO 2 interface. By means of X-ray photo electron spectroscopy it is found that 5 min exposure of thin thermal SiO 2 to N 2-RIE mode plasma causes structural modifications, which manifest only as a deterioration of oxide quality without actual nitration of the oxide. The presence of a small constant amount of SiO species through the oxide and a broadening of Si-SiO 2 interface region are detected. The nature of the electrically active plasma induced defects by both plasma processes — RIE and MERIE is equal — the bond defects in the oxide and at the interface: the oxide charge is associated with E′ centers and the interface states with P b centers.