Electron-beam-assisted scanning tunneling microscopy of insulating surfaces.

摘要

Insulating materials are widely used in electronic devices. Bulk insulators and insulating films pose unique challenges for high resolution study since most commonly used charged particle surface analysis techniques are incompatible with insulating surfaces and materials. A, method of performing scanning tunneling microscopy (STM) on insulating surfaces has been investigated. The method is referred to as electron-beam assisted scanning tunneling microscopy (e-BASTM).;It is proposed that by coupling the STM and the scanning electron microscopy (SEM) as one integrated device, that insulating materials may be studied, obtaining both high spatial resolution, and topographic and electronic resolution. The premise of the technique is based on two physical consequences of the interaction of an energetic electron beam (PE) with a material. First, when an electron beam is incident upon a material, low level material electrons are excited into conduction band states. For insulators, with very high secondary electron yields, the population of conduction band states could be quite significant. Second, for specific incident primary beam energies, the resulting electron yield will be equal to the incoming beam intensity. These are referred to as the cross over energies (E1 and E2). For a stationary primary beam at E2 the current entering the sample and the current leaving sample are equal so that a state of dynamic equilibrium is quickly reached whereby the charge density distribution local to primary beam, both at the surface and within the material, is fixed. Thus, if the surface of an insulator is illuminated with an energetic electron beam at E2, the surface will be locked to some potential and there will be filled conduction band states. Under these conditions, it may be possible to make STM measurements of material even though it is insulating. That is, from an STM point of view, it may be possible to make an insulator 'act' like a conductor.;In order to test the principle of e-BASTM, metals, thin insulating films, and bulk insulators have been examined. For metals, as expected, we observe no alteration of the tunneling signal due to the PE beam. However, with SiO 2, there is a significant increase in the tunneling current which can be directly attributed to the PE beam. For Al2O3 and CaF2 it is found that the surfaces are damaged too quickly by the PE beam for this technique to be applied suggesting that e-BASTM may only be suitable for a small class of materials.;The STM (not e-BASTM) has been used to electrically stress thin films of SiO2 (native oxide thickness). The stressing is observed to create trapping states which have been connected to stress induced leakage currents (SILC) in metal/SiO2/Si devices. The effect of the stress is observed to depend on the polarization of the applied bias (positive or negative). The trapping site density is observed to reach levels on the order of 1013--1014 traps/cm2 which is about a factor of 10--100 higher than what has been previously been reported.