Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
2003-09-26
2004-12-07
Wells, Nikita (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S310000
Reexamination Certificate
active
06828565
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electron beam source, a method of operating an electron beam source and an electron optical apparatus using such beam source.
An electron optical apparatus, such as an electron microscope and an electron lithography apparatus using electrons for imaging purposes, comprises at least one electron beam source for providing an electron beam which is used in the apparatus for electron optical imaging or other purposes.
2. Description of the Related Art
Typical demands which an electron beam source should fulfill are the provision of an electron beam having properties such as a high beam current, a high brightness, an intensity which is sufficiently constant over time, and a low width of a distribution of kinetic energies of the electrons in the beam. The width of such distribution is often referred to as FWHM (full width at half maximum). Often it is desirable to operate the electron beam source under conditions which do not permit obtaining a particularly high vacuum.
The conventional electron beam source comprises a cathode body having a source surface from which the electrons emanate, and an anode disposed at a distance from the source surface for providing an electrical extraction field for supporting the emission of electrons from the source surface. A heater may be provided for heating the source surface to further assist the emission process of the electrons from the surface.
Depending on a strength of the extraction field and the temperature of the source surface, plural physical processes may be identified which cause emission of the electrons from the source surface. These processes are illustrated in e.g. Reimer, Scanning Electron Microscopy:Physics of Image Formation and Microanalysis 2
nd
edition, Springer series in optical sciences, 1998). To leave the source surface the electron has to traverse a potential barrier at the metal-vacuum interface, which potential barrier is referred to as work function &phgr;
w
or as chemical potential &mgr;
e
.
In a thermionic emission process, the temperature of the source surface is high enough such that the electrons from the Fermi level E
F
of the cathode material can overcome the potential barrier by thermionic excitation. For example, thermionic emission is achieved at temperatures of the cathode material above 2500 K to 3000 K using a cathode made of tungsten.
At the low temperatures such that thermionic excitation does not substantially contribute to electron emission and at high electrical excitation fields, a field emission process is the dominating process in electron emission from the surface. Electron sources operating in such regime are referred to as field emission sources. Field emission from a tungsten tip having a radius of about 0.1 &mgr;m starts when the electrical field strength at the surface is 10
7
V/cm or higher. Such high fields decrease the width of the potential barrier in front of the source surface to a few nanometers so that electrons from the Fermi level E
F
can penetrate the potential barrier by a wave mechanical tunneling effect.
The conventional electron beam source further comprises a Schottky emission gun in which the potential barrier or the work function &phgr;
w
is decreased by the Schottky effect. The electrical extraction field in the Schottky emission source is about ten times lower, as compared to the field emission gun, such that a sufficient narrowing of the potential barrier allowing a substantial contribution of the wave mechanical tunneling effect to the total emission does not occur. The Schottky emission source is heated to a temperature which is substantially lower than the operating temperature of a corresponding thermionic emission source. However, the temperature is sufficiently high that the electrons may overcome the remaining potential barrier which is reduced by the Schottky effect.
In view of a low energy width (FWHM) of the electron source, the source surface should be at a low temperature to avoid a thermal broadening of the energy width. From this point of view the field emission source is preferred since this type of source may be operated at room temperature. As a drawback, the field emission source requires operation at ultra high vacuum conditions for preventing destruction of the source surface by ion bombardment. The field emission source is also insufficient with respect to a maximum beam current.
Schottky emission sources are often used as a compromise between low temperatures of the source surface in view of a low energy width, and avoiding making high demands in terms of vacuum conditions. A drawback of the Schottky emission source is a reduced stability of the beam current. Small changes in operating conditions, such as changes of temperature and surface contamination, already result in comparatively high changes of the beam current.
The conventional electron beam source further comprises a photo emission source as illustrated in e.g. U.S. Pat. Nos. 4,460,831 and 5,808,309. In the photo emission source, the source surface is illuminated with a photon beam for releasing electrons from the source surface by a photo effect. The photo emission source is used in applications where the electron beam has to be rapidly switched on and off. Rapidly switchable light sources are readily available, and the electron beam intensity immediately follows in time with the switched photon intensity. However, the photo effect requires using radiation of a particularly short wavelength in photo emission sources using source surfaces made of typical materials employed as electron sources. The energy of the photons incident on the source surface must be higher than the potential barrier or the work function &phgr;
w
. Light sources of sufficiently short wavelength are expensive and complicated to operate.
From U.S. Pat. No. 5,041,724 there is known a rapidly switchable photo emission source in which the photon energy necessary for generating photo emission is reduced by reducing the height of the potential barrier by applying an additional strong electrical extraction field, resulting in field assisted photo emission, or by heating the source surface, resulting in thermally assisted photo emission.
Further, U.S. Pat. No. 5,763,880 discloses reducing the potential barrier or work function &phgr;
W
of a cathode body by applying an oxide or nitride layer to the source surface.
SUMMARY OF THE INVENTION
As illustrated above, electron sources having a reduced operating temperature lack adjustability of the beam intensity due to an increased contribution of the wave mechanical tunneling effect.
Accordingly, it is an object of the present invention to provide an electron beam source operated at a reduced temperature of the source surface while allowing for an improved adjustability of a desired beam intensity.
Further, it is an object of the present invention to provide an electron optical apparatus, in particular an electron microscope, generating an electron beam having a reduced energy width and an improved adjustability of the beam current.
It is a further object of the present invention to provide a corresponding method of operating an electron source.
The invention provides an electron beam source for generating a beam of electrons wherein an intensity of a photon beam incident on a source surface for emission of electrons is adjusted dependant on an intensity of the generated electron beam, and wherein heating of the source surface by some process different from the illumination with the photon beam assists in releasing electrons from the source surface.
According to an embodiment, the electron beam source comprises a cathode body having a source surface for emitting electrons, and an anode disposed at a distance from the cathode for generating an electrical extraction field. The extraction field is provided to assist the electrons in overcoming the potential barrier, i.e. to decrease the potential barrier at least by some amount as illustrated above with respect to the Schottky emission source, an
Burns Doane , Swecker, Mathis LLP
LEO Elektronenmikroskopie GmbH
Leybourne James J.
Wells Nikita
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