Abstract: Scanning microspot Auger spectroscopy and microscopy as a device diagnostic technique

Abstract

Auger spectroscopy with a focused electron beam in the scanning electron microscope (SEM) has been applied as a diagnostic technique for solid state devices. The focused electron beam in a SEM allows one to determine the surface topography (SEM mode) of the specimen and then determine the Auger spectra of selected areas. In addition, a scanning Auger image may be obtained by using the amplitudes of the Auger peaks for intensity or amplitude modulation of the cathode ray tube. In the present system, a SEM designed with a dry-pump UHV system and equipped with an Auger electron detector and sputter etching capability has been applied to device failure diagnostics. Vacuum levels of 3×10−9 Torr (4×10−7 Pa) at the specimen stage have been obtained with an electro-ion-titanium sublimation pumping system. A secondary electron image resolution of less than 2000 Å with Auger spectra of features less than 1 μm have been attained. Gray-scale images from Auger electrons have been obtained at beam currents of 10−6 to 10−8 A. The Auger electron detector used has an energy resolution of 0.5% and transmission of 10%. A sputter-etching capability for sputter cleaning of specimen surfaces prior to analysis is provided. The energy source of the SEM is a triode electron gun with a high-emission filament (3.5×105 A cm−2 sr−1 at 20 kV). The relationship between probe current and diameter for a fixed working distance was measured in order to determine the optimum spatial resolution for AES. For optimum conditions and high beam currents (10−6 A) spatial resolution is essentially determined by the probe diameter. For low beam currents, it is shown that parameters such as escape depth, atomic number, surface area, and beam energy affect spatial resolution. AES recording time and its variation with signal-to-noise ratio has also been determined. The normal modes for microspot AES investigations in the SEM include spot analysis, line scan, and Auger electron imaging. For Auger imaging the vacuum levels and probe currents are shown to be the critical parameters since the time to record such an image is usually greater than five minutes.