Electric lamp and discharge devices – With positive or negative ion acceleration – Means for deflecting or focusing
Reexamination Certificate
1999-03-05
2002-05-21
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With positive or negative ion acceleration
Means for deflecting or focusing
C313S363100, C313S153000, C313S336000, C313S155000, C313S158000
Reexamination Certificate
active
06392333
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electron guns (sources) for use for instance in electron beam lithography, and to the cathodes (electron emitters) of such guns.
BACKGROUND
Electron beam columns are well known for use, for instance, in electron beam lithography for imaging a pattern onto a substrate typically coated with a resist sensitive to electron beams. Subsequent development of the exposed resist defines a pattern in the resist which later can be used as a pattern for etching or other processes. Electron beam columns are also used in electron microscopy for imaging surfaces and thin samples. Conventional electron beam columns for electron microscopy and lithography are well known and typically include an electron gun, including an electron emitter, that produces an electron beam. The beam from the gun may be used to produce a scanning probe, or may be used to illuminate a sample or an aperture using a series of electron beam lenses, which are magnetic or electrostatic lenses.
A well-known variant is called a microcolumn which is a very short and small diameter electron beam column typically used in an array of such columns; See “Electron beam technology—SEM to microcolumn” by T. H. P. Chang et al.,
Microelectronics Engineering
, 32 (1996) p. 113-130. See also U.S. Pat. No. 5,122,663 to T. H. P. Chang et al. issued Jun. 16, 1992, also describing microcolumns. These documents are incorporated herein by reference.
Both conventional electron beam columns and microcolumns include a source of electrons. In one version this source is a conventional Schottky emission gun or a field emission gun (generally referred to as electron guns) which typically includes an emitter (cathode) and the triode region surrounding the emitter (see
FIG. 1
) downstream of which, with respect to the direction of the electron beam, is an electrostatic pre-accelerator lens that focuses and accelerates the electron beam to its final energy. As described above, this gun optics is followed by a series of lenses which refocuses and images the source aperture or sample onto the target.
It has generally been difficult in the prior art to obtain very high beam currents using high brightness electron sources. Although the brightness of the cathode (the electron emitter) is high in such sources, the angular intensity of the electron beam emerging from the emitter region is limited by the properties of the cathode itself. This means that a rather high aperture angle must be used in the electron gun optics. (Optics here refers to structures for handling and manipulation of electron beams and not to conventional light optics.) This makes spherical and chromatic aberration in the gun lens a major factor limiting the beam current that may be obtained in a small spot (spot here refers to the diameter of a cross-section of the beam).
Optimization of such electron guns is described in “Some general considerations concerning the optics of the field emission illumination system,” L. H. Veneklasen,
Optik
, 36 (1972)p. 410-433. This document shows that sources with not only high brightness but also high angular intensity are needed for optimal high current performance.
Modern electron beam lithography systems for use, for instance, in the semiconductor field (mask making or direct writing of integrated circuit patterns on a semiconductor wafer) favor a shaped beam (other than Gaussian in cross-section) so that more than one pixel is exposed simultaneously. Pixel refers to a picture element in the exposed image. Optimum formation of very small high current density shaped beams uses shadow projection optics as disclosed in, for instance, U.S. patent application Ser. No. 09/058,258 filed Apr. 10, 1998 entitled “Shaped Shadow Projection for Electron Beam Column”, Lee H. Veneklasen et al., incorporated herein by reference in its entirety. In such optics, the beam shape is a projection shadow of one or more shaping apertures. It has been shown that the distortion of such shadow projection shapes depends upon the spherical aberration of the electron gun and objective lens. This kind of optics requires a high brightness cathode for good feature edge resolution in the projected image and also requires a high angular intensity cathode to minimize gun aberrations. Thus high current shadow projection optics as well as Gaussian optics (referring to a rounded somewhat diffused spot) benefit from a high angular intensity source.
FIG. 1
shows in a side cross sectional view a prior art high brightness electron source and triode region
10
which is typically part of an electron beam column and also referred to generally as an electron gun. The remainder of the electron beam column is not shown. This is exemplary of a field emission or Schottky emission gun. Details of such a device are shown in L. Swanson and G. Schwind, “A Review of the ZRO/W Schottky cathode”,
Handbook of Charged Particle Optics
editor Jon Orloff, CRC Press LLC, New York, (1997) incorporated herein by reference. The depicted rays
26
in
FIG. 1
show the limits
29
(envelope) of the useful high brightness electron beam and those electrons
30
passing through the extractor aperture
28
. The angular intensity is the total current in beam
30
divided by the solid angle into which the cathode
14
is emitting. The remainder of the “shank” emission
20
is thermionic and contributes to the total emission current but not to the useful beam
30
current and hence is effectively wasted, since there it impinges on the outside of the extractor electrode structure
24
and is not part of the final beam
30
. The cathode
14
is typically an oriented single crystal tungsten structure with a sharp point (approximately 1 micrometer radius) and mounted on a hair pin filament (not shown).
This assembly is surrounded by the negatively biased suppressor electrode
16
which is typically a conductive structure that prevents thermionically emitted electrons from leaving the cathode
14
anywhere but near its tip. The pointed tip of cathode
14
protrudes slightly from the suppressor electrode
16
and faces the extractor electrode (anode)
24
Which defines small diameter hole
29
. The extractor electrode
24
is biased positively with respect to the cathode
14
and defines an aperture
28
below the upper hole
29
to shape the final beam
30
entering the downstream gun lens (not shown). It is the combination of the electric field and temperature that causes emission from low work function facets of the cathode
14
.
There have been prior attempts to improve the performance of such Schottky and field emission guns. One attempt is to reduce the aberrations of the electrostatic gun lens. This is difficult using standard electrostatic lenses whose size and focal length are limited by the need for high voltage stand-off distances (the lens voltages are typically extremely high in the thousands or tens of thousands of volts).
It is also possible to use a magnetic lens near the cathode. For an example see “A New Design of a Field Emission Electron Gun with a Magnetic Lens” Delong et al., Optik, 81:3 (1989), pp. 103-108. This discloses that the cathode is deeply immersed in the magnetic field of a miniature magnetic lens of low chromatic and spherical aberration. This leads to an appreciable increase in the operating solid angle of emission compared with other designs. The magnetic pole pieces are built into the gun to provide a preaccelerator lens with very low aberrations. This preaccelerator lens uses coils to provide the magnetic lens, and uses two iron pole pieces to form the focusing field. However, this approach disadvantageously requires major modifications of the gun design and geometry and so is not suitable for installation in a conventional electrostatic electron gun as in FIG.
1
.
Another approach is to change the electron gun operating conditions so as to increase the angular intensity of the electrons leaving the cathode surface, thus allowing more electrons into the usual acceptance area of the gun lens. This requires increasing the current density
DeVore William J.
Garcia Rudy F.
Veneklasen Lee H.
Applied Materials Inc.
Janah & Associates
Patel Nimeshkumar D.
Santiago Mariceli
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