eTraining Introduction

Specimen Preparation

Leica Ultracut UCT

Histology

JEOL JSM-6400 SEM

Hitachi S-4700 FE-SEM

Hitachi FB-2000A FIB

Microanalysis

Veeco Dim 3000 AFM

Fluorescence Microscopes

Support

Charging

Types

Charging occurs when a specimen material cannot conduct the charge of the beam. There are two types of charging.

Negative ChargingNegative

Negative charging is a result of electrons becoming trapped within the sample, causing the charge to build and the sample to "glow". The glowing is merely an artifact of an increase in signal production.

Positive ChargingPositive

Positive charging is a result of emission of more electrons from the sample than are provided by the beam. A dark spot appears on the image.

Positive and Negative ChargingPositive and Negative

Determination

How do you know which type of charging is present?

  1. Focus on an area at high magnification. (Use the magnification at which you will be imaging, as charging is related to magnification strength.)
  2. Let the beam sit on the sample for a few seconds.
  3. Reduce the magnification and observe the sample.
  4. If there is a bright spot, the sample is experiencing negative charging; lower the voltage.
  5. If a dark square appears and then quickly disappears, the sample is experiencing positive charging; raise the voltage. If the dark square remains, it is probably a result of contamination on the sample.

Charge Balance

Charging Graph

Material E2 (keV)
PVC 1.65
Teflon 1.82
Quartz 3.0
Alumina 4.2

There are two voltages at which the specimen achieves charge balance: E1 and E2. These values are constants for each material. Zones of positive and negative charging occur outside of the optimum voltage, E2.

Left: E2 values for common, uncoated materials. (Echlin, Patrick, and Dale E. Newbury. Scanning Electron Microscopy and X-Ray Microanalysis. 2nd ed. New York, NY: Springer, 1992. 111.)

 

 

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