Preparing a Celadonite Electron Source and Estimating Its Brightness

Summary

The article presents a protocol to prepare a celadonite source and estimate its brightness for use in a long-range imaging low-energy electron point-source projection microscope.

Abstract

The electron celadonite source described here performs well in a low-energy electron point-source projection microscope in long-range imaging. It presents major advantages compared to sharp metal tips. Its robustness affords a lifetime of months and it can be used under relatively high pressure. The celadonite crystal is deposited at the apex of a carbon fiber, maintained itself in a coaxial structure ensuring a spherical beam shape and easy mechanical positioning to align the source, the object and the electron-optical system axis. There is a single crystal deposition via generation of celadonite-containing water droplets with a micropipette. Scanning electron microscopy observation can be performed to verify the deposition. However, this adds steps and therefore increases the risk of damaging the source. Thus, after preparation, the source is usually inserted directly under vacuum in the projection microscope. A first high voltage supply provides the kick-off needed to start the electron emission. The field emission process involved is then measured: it has already been observed for dozens of electron sources prepared in this way. The brightness is under-estimated through an over-estimation of source size, intensity at one energy and cone angle measured in a projection system.

Introduction

Metal/insulator structures used for electron emission have been studied for almost 20 years due to their low macroscopic field¹. The electric field involved is only of the order of some V/µm^2,3,4, in contrast to the V/nm required for classic field emission with sharp metal tips^5,6,7. This probably explains the starting plasma discharges that are so useful in electron source technologies. Some years ago, we sought to explore this low field emission by depo....

Protocol

1. Preparation of the source

NOTE: In our microscope, the source-support is composed of a machinable glass ceramic plate from which emerges 1 cm of a stainless-steel tube of 90 µm internal diameter with an electrical connection on the plate.

1. Preparation of the fiber
   1. Fix the source support under an optical microscope.
   2. Insert the 10 µm carbon fiber into the stainless-steel tube. Glue the carbon fiber to the tube with silver lacquer.
   3. Cut the fiber with a cutting tweezers (under a binocular microscope) so that between 100 µm and 3 mm are left outside the stainless-steel tube.
      NOTE: The carbon ....

Results

Several scanning electron micrographies of carbon fibers prepared as detailed in protocol were obtained in an SEM at 15 kV. Sources exhibit one, sometimes two, crystals at their apex (Figure 1). However, the use of the SEM involves another support for the carbon fiber, which is hard to mount and demount without breaking. It is safer to attempt direct electron emission. Tested in a projection microscope (Figure 2), every source pr.......

Discussion

This protocol is not critical because the geometry of the source at a microscopic scale changes from one source to another one. The difficulty is that since a carbon fiber is brittle, its cutting can lead to an inappropriate length. An adequate length is about 500 µm; the microscopic shape of the cut is not crucial. The critical step is to have a very small number of crystals (ideally one) deposited on the apex of a conductive wire. Adapting the crystal concentration with the deposited volume is the most important p.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Carbon fiber filament	Goodfellow	C 005711
Carbon fiber filament	Mitsubishi Chemical	DIALEAD
Carbon fiber filament	Solvay	THORNEL P25
Carbon fiber filament	Zoltek	PX35 Continuous Tow
Celadonite			Verona Green earth / pigment
Dual-stage microchannel plate and fluorescent screen assembly	Hamamatsu	F2225-21S
Flow controller	Elveflow	OB1
Machinable glass ceramic	Macor
Micropipette Puller	Sutter Instruments	P2000
Piezo-electric actuators	Mechonics	MS30
Quartz capillary	Sutter Instrument	B100-75-15
Silver Lacquer	DODUCO GmbH	AUROMAL 38
Ultrasonic processor	Hielscher / sonotrode MS3	UP50H