Electric lamp and discharge devices – Cathode ray tube – Image pickup tube
Reexamination Certificate
1999-07-29
2004-07-06
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Image pickup tube
C313S542000, C313S523000, C445S047000, C445S024000
Reexamination Certificate
active
06759800
ABSTRACT:
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.
BACKGROUND
Electron beam sources are used in several fields of endeavor, including 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 use 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 will soon reach their resolution limits. Production of integrated circuit elements with smaller line widths (i.e., those with line widths less than about 0.1 &mgr;m) will require new techniques such as X-ray or e-beam lithography, which can provide accompanying resolutions well below 1 micrometer because of the shorter wavelengths associated with X-rays or electrons.
In e-beam lithography, a controllable source of electrons is required. A photocathode used to produce an array of patterned e-beams is shown in FIG.
1
. U.S. Pat. No. 5,684,360, 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.
Since modem lithographic systems must achieve fast writing times (high throughput rates) in addition to high resolution, their electron beams must also have a high brightness, which in the case of electron beams requires a high current density. This property is especially important for so-called direct write applications in which the electron beam is rapidly steered and modulated so as to effect a projection of the highly complex circuit directly onto a semiconducting chip substrate.
The primary motivation for using multiple beams in an electron-beam lithography system is to increase the total current that can be delivered while minimizing space-charge effects in each beam. In multiple e-beam lithography, multiple electron beams are created by focusing an array of light beams, where each light beam's intensity can be independently regulated, onto a photocathode in transmission mode (wherein the photocathode is back-illuminated with the light beams which are focused on a photoemission layer). The resulting electron beams from the photoemission layer are then accelerated, focused, and scanned across the wafer or mask using a conventional electron-optical column.
FIG. 1
shows in a side cross-sectional view a photocathode
100
having a transparent substrate
101
and a photoemission layer
102
. The photocathode array
100
is back-illuminated by light beams
103
(having an envelope defined by the lines adjacent thereto) which are focused on photoemision 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 are known where the photoemitter is front-illuminated, i.e., the light beams are incident on the same side of the photoemitter from which the electron beam is emitted.
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 compounds of Group III and Group V elements 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. The resulting electron beam is shown below region
108
and has a lateral extent shown by the lines crossing at region
108
. 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
.
Photons in light beam
103
have an energy of at least the work function of photoemission layer
102
. The number of emitted electrons is directly proportional to the intensity of the light beam. 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 its 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.
FIG. 2
shows in a side view 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
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 beam
104
may be further shaped by other components of column
200
.
Electron beams
104
are accelerated between photocathode array
100
and anode
201
by a voltage supplied between anode
201
and photoemission layer
102
. The voltage between photocathode array
100
and anode
201
, created by power supply
208
(housed outside of the vacuum chamber), is typically a few kilovolts to a few tens of kilovolts. The electron beam then passes through electron lens
204
that focuses the electron beam onto limiting aperture
202
. Limiting aperture
202
blocks those components of the electron beams that have a larger emission solid angle than desired. Electron lens
205
refocuses the electron beam. Electron lenses
204
and
205
focus and demagnify the image carried by the electron beam onto target
207
. Deflectors
203
cause the electron beam to laterally shift, allowing control over the location of the image carried by the electron beam on a target
207
.
One of the critical challenges in developing a photocathode as the electron beam source in multiple electron beam lithography featuring high current is the ability to conduct heat away from the focused regions of illumination on the photocathode. The laser power needed to produce a certain beam current depends on the conversion efficiency of the photocathode material. A considerable amount of energy per unit area is dissipated in these regions due to the relatively low conversion efficiency of the photoemission process. For example, if a gold film approximately 15 nm in thickness is used as the photoemission layer
102
, the efficiency is about 5×10
−5
, which implies that 5 mW of laser beam power is needed to produce a 100 nA beam.
When this amount of power is focused into a small spot (approximately 1 &mgr;m diameter) on a thin film, the heat flow is
Chen Xiaolan
Coyle Steven T.
Fernandez Andres
Mankos Marian
Thomas Timothy
Applied Materials Inc.
Kuo Jung-hua
Patel Vip
Williams Joseph
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