Link between hot electrons and interface damage in n-MOSFETs: A Monte Carlo analysis

Abstract

Typical measurements on n-channel MOSFETs show that interface damage, causing threshold and transconductance shifts, tends to peak when V GS≈0.5 V DS. In such conditions, more impact ionization (substrate current) is measured. At higher gate voltage, one expects that channel hot electrons would impact the interface at increased energy and numbers. Therefore, it has normally been assumed that hot carriers in the channel cannot directly contribute to damage, and impact-generated holes must be injected into the oxide for damage to take place. However, there is increasing experimental evidence that hot electrons should be able to remove hydrogen atoms which passivate interface dangling bonds, even through a multiple collision process, at energies below oxide-injection threshold (typically 3.0 eV). A detailed full-band Monte Carlo analysis shows that in a typical 0.25 μm gate device there are, indeed, more energetic electrons hitting the surface when V GS= V DS than at the condition of maximum impact damage V GS≈0.5 V DS. However, the electrons impact the surface at much shallower angles, with lower chance of perpendicular momentum transfer to the interface. This analysis reconciles damage measurements with the physical picture suggesting direct damage generation by energetic electrons.