Field Emission Electron Probe (FEP) Elemental Analysis Tests at Low Voltage

Introduction

Field Emission Electron Probe (FEP) is a technique used for elemental analysis at low voltages.

FEP tips are typically prepared using Electrochemical etching methods, often from Tungsten wire due to its favorable properties.

The JEOL JXA-8500F and JXA-8530F are examples of field emission electron microprobes used for high-resolution elemental analysis.

FEP allows for better light element analyses and faster mapping of low abundance elements.

The process involves generating electrons at relatively low voltages, which is beneficial for analyzing delicate samples.

Preparation of FEP Tips

Material: Tungsten wire is commonly used due to its low work function and high mechanical strength.

Method: Electrochemical etching is the preferred method for preparing FEP tips.

Process: The tungsten wire is dipped in a KOH solution and connected to a DC power supply.

Formation: A sharp tip is formed through a 'drop-off' method where the lower part of the wire falls off.

Cleaning: Tips are rinsed with distilled water and cleaned with HF acid.

Figure 1: Comparison of the JEOL in-lens thermal FEG with a conventional thermal gun and lens. The in-lens FEG produces significantly higher beam currents.

Advantages of FEP

Resolution: Provides very small probe sizes, enhancing resolution.

Low Voltage: Capable of generating electrons at lower voltages, reducing sample damage.

Light Element Analysis: Improved analysis of light elements.

Speed: Faster mapping of low abundance elements.

Contamination Reduction: Features like a Cold finger reduce contamination during analysis.

Figure 2: Basic structure of FEG.

Applications of FEP

Material Analysis: Used for analyzing rocks, crystals, glasses, alloys, and thin films.

Tephrochronology: Extensively used for tephra identification and dating.

Consulting: Utilized by Consulting companies and Government agencies for material analysis.

Academic Research: Supports research in various Academic institutions.

Elemental Mapping: Provides detailed elemental (X-ray) maps and Backscatter electron images.

Experimental Setup

Tip Holder: The FEP tip is mounted into a holder with a teflon plate and steel foil.

Vacuum Chamber: The assembly is kept in a chamber evacuated to a pressure of 4.6 x 10-6 mbar.

Voltage Application: High voltage is applied to the metal foil, with the collector plate and FEP at ground potential.

Initial Emission: High voltage is needed initially due to surface contamination.

Repeated Operation: Voltage required decreases with repeated operation due to self-sputtering.

Figure 3: Calculating the difference of ΔG binding between Ligand A and Ligand B to a target using alchemical changes.