Chapter 6 Charge pumping techniques: Their use for diagnosis and interface states studies in MOS transistors

Abstract

The charge pumping (CP) technique is a powerful tool used to characterize the traps of the Si-SiO2 interface in submicrometer MOS devices. It is based on the exploitation of a repetitive process whereby majority carriers coming from the substrate recombine with minority carriers previously trapped in interface states, when the MOSFET is submitted to well-chosen biasing cycles. By taking into account the emission processes which control the exchange of charges at the interface, information concerning the capture cross-section and the energy distribution of the interface states can be obtained. In this chapter, we describe various charge pumping techniques and their applications. We first show that a “pumping current” is detected under certain circumstances in a MOS transistor and we present the fundamentals of the charge pumping technique. We then describe the early version of the two-level (2CP) technique. The first-order modeling is not sufficient to explain all the experimental results found. We thus introduce a more thorough analysis of the various physical phenomena which take place when a triangular, trapezoidal or sinusoidal voltage pulse is applied to the gate electrode. We then review the experimental parameters which affect the CP response of the MOS transistor (the frequency and the profile of the gate pulse, the reverse bias of the source and drain junctions, the device temperature) and we indicate the accuracy and the limitations of the classical 2CP method. We show next how the 2CP method enables us to calculate the average density of the interface states 〈Dit〉 and their average cross-section 〈σ〉. By adjusting some experimental parameters we also gain access to the energy distribution of the states Dit(E) and to their spatial distribution Dit(x). With the spectroscopic technique, we explain that it is possible to define an energy window by exploiting a differential signal and that Dit(E) and σ(E) are accessible by performing a temperature sweep. The introduction of a third voltage level in the gate pulse gives access to the same parameters as the 2CP but requires less simplifying assumptions. We show that the 3CP and its variants are powerful tools to determine the Dit(E) and σ(E) distributions. We end this chapter by describing some clever applications of the 2CP method: characterization of interfaces (degraded homogeneously or not) characterization of border traps (by using a frequency sweep) and characterization of grain boundary traps in polysilicon thin-film transistors. Given the shrinking dimensions of elementary transistors, the various CP techniques are to date the only techniques which make it possible to characterize the Si-SiO2 interface directly in the transistor itself. They thus have a promising future.