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Scanning Electron Microscopy Charging Effect Simulation and Parameter Estimation Using a Resistor-Capacitor Circuit

IP.com Disclosure Number: IPCOM000124930D
Publication Date: 2005-May-13
Document File: 5 page(s) / 385K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method that simulates images that exhibit charging effects, and computes the parameters of the charging model for a given image. Benefits include significantly faster estimation of charging effects than is available using existing methods.

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Scanning Electron Microscopy Charging Effect Simulation and Parameter Estimation Using a Resistor-Capacitor Circuit

Disclosed is a method that simulates images that exhibit charging effects, and computes the parameters of the charging model for a given image. Benefits include significantly faster estimation of charging effects than is available using existing methods.

Background

Charging of specimens in electron microscopy is an important issue because it can lead to image coordinate distortions and anomalous image contrast. These effects can cause errors when one attempts to infer the size or shape of a specimen from an image. Also, when a scanning electron microscope (SEM) is used for a controlled exposure in electron-beam lithography, charging can distort the region of exposure, creating defects in the final product.

Given a specimen composed of an insulator or electrically isolated conductor, the difference between the number of incident electrons and the number of back-scattered electrons (including secondary electrons) results in either a positive or negative charge in the specimen. This fact allows the sign of the charge to be determined from models of secondary and back-scattered electron emissions. In the case of an insulator, a more complete picture is provided by Melchinger’s dynamic double layer model. In this model, the numerous secondary electrons which escape very close to the surface of the specimen result in a positively charged layer about 5 nm thick at the surface. Some of the primary electrons become trapped deep in the specimen, forming a negatively charged layer at a depth approximately equal to the maximum range of electrons in the material. In between these two layers, there is an electron beam-induced conduction (EBIC) or radiation induced conduction (RIC) layer that allows charges to conduct mainly in a vertical direction. The combination of secondary electrons escaping near the surface and a relaxation current that transports electrons from the lower layer up to the surface helps to explain time dependent charging behavior. Because the induced conduction lasts only for a short time after the beam is scanned to a new location, this only allows charges to migrate within a small volume at each beam position.

The total electron yield (i.e. the ratio between total emitted and incident electrons) is a function of the incident energy. Typically, there are two crossover points at energies E1 and E2, where the electron yield is 1 (see Figure 1).

The crossover points are significant because when the SEM accelerating voltage is set to the crossover energy, no charging occurs because the numbers of incident and emitted electrons are equal. Because of the dependence on topography and material, for many specimens it is impossible to find a single accelerating voltage that makes the electron yield 1 at all points. It is very difficult to measure the E1 crossover point, because it is unstable and simulations tend to be very inacc...