High Brightness Electron Gun Using a Field Emission Cathode

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100101219 ◽

1968 ◽

Vol 26 ◽

pp. 294-295

Author(s):

A. N. Broers

Keyword(s):

Field Emission ◽

Electron Gun ◽

Field Emission Cathode ◽

High Brightness ◽

Probe Size ◽

Test Grid ◽

Two Stages ◽

Emission Cathode ◽

Scanning Electron ◽

Better Than

A field emission cathode electron gun with two stages of acceleration has been built in order to measure the electron beam brightness that can be produced in practice from a tungsten field emission cathode. The gun is similar to that reported by A. Crewe except that the accelerating electrodes are plane, rather than shaped, apertures, and the cathode is located by a gimbal mechanism which allows the cathode to be tilted over an arc of 70° in any direction and positioned laterally. The gun electrodes have been precisely machined with the apertures round within 0.25 micron and aligned with respect to each other to better than 10 micron. The second accelerating electrode is followed by scan plates, a test grid, and an electron detector which together allow the probe size to be measured in the usual scanning electron microscope mode.

Download Full-text

Optical characteristics of the hitachi HF-2000 cold field-emission TEM

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100151210 ◽

1993 ◽

Vol 51 ◽

pp. 1076-1077

Author(s):

L. F. Allard ◽

E. Völkl ◽

T. A. Nolan

Keyword(s):

Field Emission ◽

Recent Literature ◽

Electron Gun ◽

High Brightness ◽

Condenser Lens ◽

Illumination System ◽

High Resolution Images ◽

Imaging Mode ◽

Better Than ◽

Illuminating System

The illumination system of the cold field emission (CFE) Hitachi HF-2000 TEM operates with a single condenser lens in normal imaging mode, and with a second condenser lens excited to give the ultra-fine 1 nm probe for microanalysis. The electron gun provides a guaranteed high brightness of better than 7×l08 A/cm2/sr, more than twice the guaranteed brightness of Schottky emission guns. There have been several articles in the recent literature (e.g. refs.) which claim that the geometry of this illumination system yields a total current which is so low that when the beam is spread at low magnifications (say 10 kX), the operator must “keep his eyes glued to the binoculars” in order to see the image. It is also claimed that this illuminating system produces an isoplanatic patch (the area over which image character does not vary significantly) at high magnification which is so small that the instrument is ineffective for recording high resolution images.

Download Full-text

Interference Experiments with a Field Emission Source

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100069661 ◽

1970 ◽

Vol 28 ◽

pp. 534-535

Author(s):

A. V. Crewe ◽

J. Saxon

Keyword(s):

Field Emission ◽

Spherical Aberration ◽

Electron Gun ◽

Partial Coherence ◽

Vacuum System ◽

High Brightness ◽

Small Spot ◽

Tungsten Tip ◽

Effective Source ◽

Better Than

Field emission from a tungsten tip provides a source with very high brightness and high partial coherence. An electron gun of low spherical aberration is used to focus the electrons from the tip to a small spot about 100 Å in diameter. Since the voltages applied to the tip and gun are stable to better than 5 ppm, the temporal coherence is limited by the energy spread of the source, about 200 mv.Using the focused spot a few centimeters below the gun as an effective source, a metalized quartz fiber about 2 μ in diameter is positioned a few centimeters below the source, as shown in Fig. 1. Two cylindrica11y symmetric magnetic lenses are used to magnify the resulting Fresnel diffraction pattern. The image is produced on a fluorescent coating deposited on the vacuum side of a fiber optic window. The image is recorded directly on film placed against the window outside the vacuum system.

Download Full-text

Noise and energy broadening in field emission systems

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100107423 ◽

1978 ◽

Vol 36 (1) ◽

pp. 56-57 ◽

Cited By ~ 1

Author(s):

K.C.A. Smith ◽

A.D.G. Cumming

Keyword(s):

Field Emission ◽

Axial Magnetic Field ◽

Chromatic Aberration ◽

Electron Gun ◽

High Current ◽

Field Emission Cathode ◽

Transmission Microscopy ◽

Control Electrode ◽

Tip Radius ◽

Emission Cathode

To improve the performance of the field emission electron gun requires that greater total currents are drawn from the field emission cathode. This involves various practical difficulties associated with the vacuum environment, and in addition, the Boersch effect energy broadening may become important at high currents and for certain applications, for example in pulsed transmission microscopy (Smith and Cleaver, 1976). As part of a study of field emission guns under high current conditions we have measured the energy spreads in a pulsed triode gun system. Positive going pulses on the control electrode, or Wehnelt, were used to draw peak currents of up to 2 mA for 10 μS durations, the pulses being of 4 kV amplitude. A (111) orientated single crystal tungsten cathode was used, of 150 mm tip radius, and the energy of the emitted electrons measured in an analyser due to Brack (1962). This consists of a three electrode einzel lens with a superimposed axial magnetic field, and is designed to have a high coefficient of chromatic aberration.

Download Full-text

A review of Schottky and cold field-emission cathode characteristics

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100172425 ◽

1994 ◽

Vol 52 ◽

pp. 938-939

Author(s):

L. W. Swanson

Keyword(s):

Field Strength ◽

Field Emission ◽

Full Width ◽

Half Maximum ◽

Continuous Change ◽

Field Emission Cathode ◽

High Brightness ◽

Surface Electric Field ◽

Electron Focusing ◽

Emission Cathode

Two high brightness, point cathodes currently being used in TEM's and nanometer electron focusing systems are the cold field emission (CFE) and Schottky emission (SE) cathodes. In terms of emission mechanisms, CFE and SE represent two extremes of a continuous change in surface electric field strength F and temperature T of a pointed cathode of work function ϕ. The conditions for CFE and SE emission modes can be stated as follows:CFE: 4π(2mϕ(1/2 kT/heF < 0.5SE: he1/4F3/42<2m1/2 < 0.5When the above conditions are met most electrons are tunnelling from at or near the Fermi level inthe case of CFE and thermally excited over the vacuum barrier in the case of SE. The expressions for the current density J are:CFE: JCFE = a(F2/ϕ)exp(10.4/ϕ1/2)exp(-bϕ3/2/F)(A/cm2SE: JSE = 1202exp(-(ϕ - 3.8F1/2)/)kT)(A/cm2)where a = 1.54×10-6, b = 0.644 and F and ϕ are in units of V/Å and eV respectively.One important aspect for source optics applications is the energy spread of the emitted electrons for the two emission regimes. Fig. 1 gives the theoretical values of the full width at half maximum (FWHM) of the energy distribution for the case of CFE with ϕ = 4.0 eV, T = 300 K, J = 1×105 A/cm2 and SE with ϕ = 3.0 eV, T = 1800, J = 1×103 A/cm2.

Download Full-text