Method of forming gated photocathode for controlled single...

Details Method of forming gated photocathode for controlled single... Method of forming gated photocathode for controlled single...

1999-11-24

2001-04-24

Ramsey, Kenneth J. (Department: 2879)

Electric lamp or space discharge component or device manufacturi

Process

With assembly or disassembly

C445S050000

Reexamination Certificate

active

06220914

ABSTRACT:

BACKGROUND
1. Field of the Invention
This invention relates to electron beam sources and, more particularly, to photocathodes for the generation of single or multiple electron beams.
2. Prior Art
High resolution electron beam sources are used in systems such as scanning electron microscopes, defect detection instruments, VLSI testing equipment, and electron beam (e-beam) lithography. In general, e-beam systems include an electron beam source and electron optics. The electrons are accelerated from the source and focused to define an image at a target. These systems typically utilize a physically small electron source having a high brightness.
Improvements in optical lithography techniques in recent years have enabled a considerable decrease in the linewidths of circuit elements in integrated circuits. Optical methods, however, will soon reach their resolution limits. Production of smaller line-width circuit elements (i.e., those less than about 0.1 &mgr;m) will require new techniques such as X-ray or e-beam lithography.
In e-beam lithography, a controllable source of electrons is desired. A photocathode used to produce an array of patterned e-beams is shown in FIG.
1
. U.S. Pat. No. 5,684,360 to Baum et al., “Electron Sources Utilizing Negative Electron Affinity Photocathodes with Ultra-Small Emission Areas,” herein incorporated by reference in its entirety, describes a patterned photocathode system of this type.
FIG. 1
shows a photocathode array
100
with three photocathodes
110
comprising a transparent substrate
101
and a photoemission layer
102
. The photocathode is back-illuminated with light beams
103
which are focused on photoemission layer
102
at irradiation region
105
. As a result of the back-illumination onto photoemission layer
102
, electron beams
104
are generated at an emission region
108
opposite each irradiation region
105
. Other systems have been designed where the photoemitter is front-illuminated, i.e. the light beam is incident on the same side of the photoemitter from which the electron beam is emitted.
Often, light beams
103
or electron beams
104
are masked. In
FIG. 1
, light beams
103
are masked using mask
106
which allows light onto irradiation spots
108
but prevents light from being incident on other areas of photoemission layer
102
.
FIG. 1
also shows mask
107
which allows electrons to exit photoemission layer
102
only at certain surface spots corresponding to emission regions
105
. A photocathode may also have a mask between transparent substrate
101
and photoemission layer
102
to block light beam
103
so that it is only incident at irradiation spots
105
. In general, photocathode
110
may include no masking layers or may have one or more masking layers.
Each irradiation region
105
may be a single circular spot representing a pixel of a larger shape, the larger shape being formed by the conglomerate of a large number of photocathodes
110
in photocathode array
100
. In that case, irradiation region
105
may be as small as is possible given the wavelength of the light beam incident on photocathode
100
. Typically, a grouping of pixel irradiation regions has dimensions of 100-200 &mgr;m. Each pixel can have dimensions (i.e. diameter) as low as 0.1 &mgr;m. Alternatively, irradiation spot
105
and emission region
108
can be a larger shape. In either case, the image formed by emission region
108
will be transferred to e-beam
104
so long as the entirety of irradiation region
105
is illuminated by light beam
103
.
Photoemission layer
102
is made from any material that emits electrons when irradiated with light. These materials include metallic films (gold, aluminum, etc.) and, in the case of negative affinity (NEA) photocathodes, semiconductor materials (especially III-V compounds such as gallium arsenide). Photoemission layers in negative electron affinity photocathodes are discussed in Baum (U.S. Pat. No. 5,684,360).
When irradiated with photons having energy greater than the work function of the material, photoemission layer
102
emits electrons. Typically, photoemission layer
102
is grounded so that electrons are replenished. Photoemission layer
102
may also be shaped at emission region
108
in order to provide better irradiation control of the beam of electrons emitted from emission region
108
. Further control of the e-beam is provided in an evacuated column as shown in FIG.
2
.
Light beams
103
usually originate at a laser but may also originate at a lamp such as a UV lamp. The laser or lamp output is typically split into several beams in order to illuminate each of focal points
105
. A set of parallel light beams
103
can be created using a single laser and a beam splitter. The parallel light beams may also originate at a single UV source. Alternatively, the entire photoemission array
100
may be illuminated if the light source has sufficient intensity.
Photons in light beam
103
have an energy of at least the work function of photoemission layer
102
. The intensity of light beam
103
relates to the number of electrons generated at focal point
105
and is therefore related to the number of electrons emitted from emission region
108
. Photoemission layer
102
is thin enough and the energy of the photons in light beam
103
is great enough that a significant number of electrons generated at irradiation region
103
migrate and are ultimately emitted from emission layer
108
.
Transparent substrate
101
is transparent to the light beam and structurally sound enough to support the photocathode device within an electron beam column which may be a conventional column or a microcolumn. Transparent substrate
101
may also be shaped at the surface where light beams
103
are incident in order to provide focusing lenses for light beams
103
. Typically, transparent substrate
101
is a glass although other substrate materials such as sapphire or fused silica are also used.
If mask
106
is present either on the surface of transparent substrate
101
or deposited between transparent substrate
101
and photoemission layer
102
, it is opaque to light beam
103
. If mask
107
is present, it absorbs electrons thereby preventing their release from emission region
108
. Mask
107
may further provide an electrical ground for photoemission layer
102
provided that mask
107
is conducting.
Photocathode
100
may be incorporated within a conventional electron beam column or a microcolumn. Information relating to the workings of a microcolumn, in general, is given in the following articles and patents: “Experimental Evaluation of a 20×20 mm Footprint Microcolumn,” by E. Kratschmer et al., Journal of Vacuum Science Technology Bulletin
14(6
), pp. 3792-96, November/December 1996; “Electron Beam Technology—SEM to Microcolumn”, by T. H. P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; “Electron Beam Microcolumn Technology And Applications”, by T. H. P. Chang et al.,
Electron
-
Beam Sources and Charged
-
Particle Optics
, SPIE Vol. 2522, pp. 4-12, 1995; “Lens and Deflector Design for Microcolumns”, by M. G. R. Thomson and T. H. P. Chang, Journal of Vacuum Science Technology Bulletin
13(6
), pp. 2445-49, November/December 1995; “Miniature Schottky Electron Source”, by H. S. Kim et al., Journal of Vacuum Science Technology Bulletin
13(6
), pp. 2468-72, November/December 1995; U.S. Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Chang et al., all of which are incorporated herein by reference.
FIG. 2
shows a typical electron beam column
200
using photocathode array
100
as an electron source. Column
200
is enclosed within an evacuated column chamber (not shown). Photocathode array
100
may be completely closed within the evacuated column chamber or transparent substrate
101
may form a window to the vacuum chamber through which light beams
103
gain access from outside the vacuum chamber. Electron beams
104
are emitted from emission region
108
into the evacuated column chamber and carry an image of emission region
108
. Electron

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