U.S. patent application number 11/124779 was filed with the patent office on 2005-09-22 for photocathode.
This patent application is currently assigned to Burle Technologies, Inc.. Invention is credited to Bryan, John G., Habib, Youssef M., Steinbeck, John W., Stoll, Charles W..
Application Number | 20050206314 11/124779 |
Document ID | / |
Family ID | 23380386 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050206314 |
Kind Code |
A1 |
Habib, Youssef M. ; et
al. |
September 22, 2005 |
Photocathode
Abstract
An electron-emitting photocathode includes a base and a large
number of projecting elements such as microscopic wires projecting
from a surface of the base. The photocathode has high quantum
efficiency, and hence can be used as the emitting element in a
sensitive phototube.
Inventors: |
Habib, Youssef M.;
(Lancaster, PA) ; Bryan, John G.; (Middletown,
PA) ; Stoll, Charles W.; (Mountville, PA) ;
Steinbeck, John W.; (Fitzwilliam, NH) |
Correspondence
Address: |
LERNER, DAVID, LITTENBERG,
KRUMHOLZ & MENTLIK
600 SOUTH AVENUE WEST
WESTFIELD
NJ
07090
US
|
Assignee: |
Burle Technologies, Inc.
Lancaster
PA
|
Family ID: |
23380386 |
Appl. No.: |
11/124779 |
Filed: |
May 9, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11124779 |
May 9, 2005 |
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10293014 |
Nov 13, 2002 |
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6908355 |
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60351308 |
Nov 13, 2001 |
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Current U.S.
Class: |
313/542 |
Current CPC
Class: |
H01J 40/16 20130101;
B82Y 10/00 20130101; H01J 1/34 20130101; H01J 9/12 20130101; B82Y
20/00 20130101 |
Class at
Publication: |
313/542 |
International
Class: |
H01J 040/06 |
Claims
1. A photocathode comprising: (a) a base having horizontal
directions; (b) a set of elements projecting from the base in a
vertical direction transverse to said horizontal directions, said
elements being adapted to emit electrons upon absorption of light
by said elements.
2. A photocathode as claimed in claim 1 wherein said elements are
substantially parallel to one another.
3. A photocathode as claimed in claim 1 wherein said projections
have substantially equal height in said vertical direction.
4. A photocathode as claimed in claim 3 wherein said height is
between 0.1 and 201 .mu.m.
5. A photocathode as claimed in claim 1 wherein said elements are
substantially evenly distributed in at least one of said horizontal
directions.
6. A photocathode as claimed in claim 5 wherein said elements are
substantially evenly distributed in at least two of said horizontal
directions.
7. A photocathode as claimed in claim 1 wherein said elements are
elongated structures.
8. A photocathode as claimed in claim 7 wherein said elements are
wires.
9. A photocathode as claimed in claim 1 wherein said set of
elements has an aggregate surface area, and said base has a surface
area, said aggregate surface area of said set of elements being
substantially greater than said surface area of said base.
10. A photocathode as claimed in claim 1 wherein said set of
elements is provided at a density of greater than 10.sup.9 elements
per square centimeter over at least a part of said base.
11. A photocathode as claimed in claim 1 wherein said set of
elements includes an ordered array of said elements in a hexagonal
arrangement.
12. A photocathode as claimed in claim 1 wherein said elements have
surfaces formed from a low-work function material.
13. A photocathode as claimed in claim 1 wherein said
low-work-function material is selected from the group consisting of
alkali metals, alkali metal antimony compounds, cesium, cesium
oxide, diamond, silicon, carbon nanotubes, III-V compound
semiconductors and combinations thereof.
14. A photocathode as claimed in claim 1 wherein said base and said
elements are formed integrally with one another.
15. A method of converting light to an electrical signal
comprising: (a) maintaining a photocathode as claimed in claim 3 at
a negative potential relative to an anode structure in a vacuum,
and (b) applying the light to the photocathode, whereby electrons
are emitted from said element to said anode structure.
16. A method as claimed in claim 15 wherein the height of said
projections is substantially equal to: n.lambda./4 where: n is an
integer; and .lambda. is the wavelength of the light.
17. A phototube comprising: (b) an anode structure; (a) a
photocathode for receiving light, said photocathode having a base
and a plurality of elements that project from a surface of said
base toward said anode structure, said photocathode being capable
of emitting electrons upon receipt of said light; (c) an enclosure
maintaining said photocathode and said anode in a vacuum.
18. A system comprising a phototube as claimed in claim 17 and a
potential source electrically coupled to said anode and said
photocathode, said source maintaining said photocathode at a
negative potential relative to said anode.
19. A system as claimed in claim 18 further comprising a light
source for providing said light.
20. A phototube as claimed in claim 17 wherein said anode structure
includes an electron multiplying device for receiving said emitted
electrons.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/293,014, filed Nov. 13, 2002, which application claims
benefit of the filing date of U.S. Provisional Patent Application
No. 60/351,308, filed on Nov. 13, 2001, the disclosures of which
are hereby incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to photocathodes, to
devices incorporating photocathodes, and to methods of making
photocathodes.
[0003] A photocathode is an opto-electronic device that emits
electrons when it is struck by photons of light. A photocathode
typically is used in a vacuum tube with an anode structure. In the
simplest case, the anode structure can be a simple plate, where
electrons emitted from the photocathode pass directly to the anode
structure. In this simple structure, the current passing through
the device is essentially equal to the number of electrons emitted
by the photocathode. In more complex structures, e.g.,
photomultiplier tubes, the anode structure includes known
electron-multiplying devices such as microchannel plates and
dynodes. These devices act to emit large numbers of electrons in
response to a few electrons emitted by the photocathode. In such a
tube, the current passing through the device is many times greater
than the emission current from the photocathode, i.e., the number
of electrons emitted from the photocathode. In all of these
structures, however, the current passing through the device is
directly related to the emission current from the photocathode.
Thus, to make a device that is highly sensitive, it is desirable to
use a photocathode with a high quantum efficiency, i.e., a
photocathode which emits a relatively large number of electrons for
a given amount of light impinging on the photocathode. This is
especially desirable where the device is used to detect extremely
dim light as, for example, in so-called "single photon detectors"
used in certain scientific applications. A single photon detector
is intended to provide a measurable electrical current in response
to a single photon impinging on the photocathode.
[0004] Conventional photocathodes are formed by depositing a layer
of polycrystalline material on a planar substrate. The layer of
polycrystalline material forms the photoemissive surface for
absorbing the light and releasing the electrons. The photoemissive
surface of a conventional photocathode is relatively smooth and of
the same size as the underlying substrate. The planar substrate is
formed of an electrically conductive material and is electrically
coupled to the polycrystalline layer.
[0005] The quantum efficiency of a photocathode is the ratio of the
number of released electrons over the number of incident photons of
a given wavelength. The maximum efficiency is 100%. Conventional
photocathodes have a peak quantum efficiency of 25% at a materials
dependent wavelength between the range of 200 nm-900 nm. This means
that approximately 75% of the incident photons do not cause
emission of electrons.
[0006] There exists a need for photocathodes that have high quantum
efficiency and for devices incorporating such photocathodes. There
also exists a need for a method to manufacture these high quantum
efficiency photocathodes.
SUMMARY OF THE INVENTION
[0007] One aspect of the present invention provides a photocathode
that includes a base and a plurality of elements projecting
outwardly from the base. For example, the projecting elements
desirably may be in the form of elongated microscopic wires
referred to herein as "nanowires," and desirably project
substantially parallel to one another in a direction normal to the
base. The base may be a platelike or sheetlike element formed
integrally with the projecting elements. For convenience, the base
is referred to herein as extending in "horizontal" directions, and
the elements are referred to herein as projecting in a "vertical"
direction. Merely by way of example, the projecting elements may
each have an equal height in the vertical direction, and such
height may be, for example, between 0.1 .mu.m and 200 .mu.m. The
projecting elements also may be, for example, between 4 nm and 200
nm in diameter. The projecting elements may be disposed as an array
of elements that are evenly distributed over at least part of the
base. Such an array may incorporate a vast number of individual
elements as, for example, 10.sup.9 to 10.sup.13 elements per square
centimeter of base area covered by the array.
[0008] The projecting elements are adapted to emit electrons upon
absorption of light, and hence form a part or all of the
photoemissive surface of the photocathode. Thus, the projecting
elements may incorporate a low-work function material at their
respective surfaces. As further discussed herein, the "work
function" of a material is a measure of the energy required to
eject an electron from the material into vacuum. Preferably, the
low work function material has a work function less than about 2
electron volts (ev). Merely by way of example, materials such as
alkali metals, alkali metal antimony compounds, cesium, cesium
oxide, diamond, silicon, carbon nanotubes, III-V compound
semiconductors and combinations thereof can be used.
[0009] The projecting elements provide the photocathode with a
large photoemissive surface area to absorb photons from a light
source. Thus, the surface area of a cathode in accordance with this
aspect of the invention is substantially greater than the surface
area of the base. Although the present invention is not limited by
any theory of operation, it is believed that a relatively large
photoemissive surface area contributes to an improved quantum
efficiency of the photocathode. It is also believed that the small
size of the projecting elements further facilitate electron
emissions from the projecting elements. Moreover, although here
again the present invention is not limited by any theory of
operation, it is believed that the elements serve to intensify the
electrical field in locations such as the tips of the elements,
which in turn facilitates electron emissions from the photocathode.
This increases the likelihood that light absorbed by the
photocathode will result in a released electron, and thus increases
the quantum efficiency of the photocathode.
[0010] As discussed in more detail hereinafter, the size of the
elements can be designed to improve the absorption of light having
a particular wavelength. This can include light in either the
visible spectrum or the invisible spectrum. For example, the length
or height of each of the elements can be designed to resonate with
the wavelength of light received by the photocathode, which
increases the likelihood that the light will be absorbed and not
reflected by the elements.
[0011] A further aspect of the invention provides phototubes that
incorporate an anode structure, a photocathode as discussed above,
and an enclosure maintaining the photocathode and the anode
structure in a vacuum. The projecting elements desirably extend
from the base in a direction towards the anode structure. Such a
phototube can be used in conjunction with a potential source
connected to the anode structure and to the photocathode. In
operation, the anode structure is maintained at a positive
electrical potential relative to the photocathode. As discussed
above, the geometrical attributes of the elements and the
positioning of the photocathode results in enhanced electric field
strength particularly near the tips of each of the elements, which
facilitate the ejection of electrons from the photocathode. The
anode structure may be a simple anode or may include an
electron-multiplying device.
[0012] In accordance to yet another embodiment of the invention,
there is described a method of making a photocathode comprising the
steps of providing a template having a plurality of pores with each
of the pores having an open end, forming elements by depositing a
material in the pores of the template, providing a base that is
electrically connected to the elements, and removing a least a
portion of the template so as to expose the elements. The exposed
portion of the elements can further be coated with a low work
function material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A is a diagrammatic elevational view of a phototube
according to one embodiment of the invention.
[0014] FIG. 1B is a fragmentary, diagrammatic view on an enlarged
scale taken along line 1B-1B in FIG. 1A.
[0015] FIG. 2 is a diagrammatic perspective view of a photocathode
according to a further embodiment of the invention.
[0016] FIG. 3 is a top plan view of a photocathode according to yet
another embodiment of the invention.
[0017] FIGS. 4A-4D are side view representations showing the
formation of a photocathode in successive steps of a manufacturing
method according to a further embodiment of the invention.
[0018] FIGS. 5A-5D are views similar to FIGS. 4A-4D but depicting a
method according to another embodiment of the invention.
DETAILED DESCRIPTION
[0019] Referring now to the drawings wherein like reference
numerals represent like elements, there is shown in FIG. 1 a
phototube 101 having a photocathode 100 according to one embodiment
of the invention. The photocathode 100 has a generally planar base
102 with a plurality of elements or projections 104 that extend
from a surface of the base 102 in a direction perpendicular to such
surface. The directions H.sub.1 and H.sub.2 (FIG. 1B) along the
surface of the base are referred to herein as the "horizontal"
directions, whereas the direction V (FIG. 1A) towards and away from
the surface of the base is referred to herein as the "vertical"
direction. The extent of projections 104 in the vertical direction
from the surface of base 102 is indicated by I.sub.1 in FIG.
1A.
[0020] The base 102 may be formed from essentially any electrically
conductive material as, for example, a metal or semiconductor. The
elements or projections 104 may be formed entirely of a
electron-emitting material having a low work function, desirably
less than about 2 ev and more desirably less than about 1 ev.
Suitable electron-emitting materials are well known in the art.
Examples include alkali metals, alkali metal antimony compounds,
cesium, cesium oxide, diamond, silicon, carbon nanotubes, III-V
compound semiconductors and combinations thereof. In the
alternative, the elements 104 can include an electrically
conductive structural material such as a metal or semiconductor
coated with a low work function electron-emitting material. For
example, metals such as nickel, gold, silver and combinations
thereof can be employed as structural materials. Also, the
electron-emitting material may be formed by reaction of another
material with the structural material at the surfaces of the
elements 104.
[0021] The elements 104 are in electrical contact with the base
102. As further discussed below, the elements 104 optionally may be
formed integrally with the base 102. The elements 104 are sized and
shaped to increase the quantum efficiency of the photocathode 100.
Each of the projecting elements 104 preferably is a narrow,
elongated structure with a base 106 and a tip 108. Preferably the
elements 104 each have a diameter that is between 4 nm-200 nm and a
length that is between 0.1 .mu.m-200 .mu.m. The elements 104 are
uniformly distributed along the surface of the base 102 at a
density between 10.sup.9 to 10.sup.13 elements per square
centimeter. As best seen in FIG. 1B, the elements 104 are disposed
in a hexagonal array. The elements 104 provide the photocathode 100
with an active surface area that is substantially greater than a
smooth surface of a comparable size. For example, an array of
elements 104 having a density of 10.sup.10 elements per square
centimeter with each element having a diameter of 75 nm and a
length of 2 .mu.m has a surface area that is about 100 times
greater than a smooth surface of a comparable size. This increase
in surface area increases the likelihood that energy from an
incident photon will be absorbed by the photocathode 100.
[0022] The phototube 101 depicted in FIG. 1A includes an anode
structure 110 and an enclosure 103 encompassing the photocathode
100 and the anode structure 110. The enclosure 103 maintains the
photocathode 100 and anode structure 110 in a vacuum, typically at
an absolute pressure of less than about 10.sup.-7 Pa. The enclosure
103 is formed, at least in part, from a material transparent to the
light to be detected by the phototube 101. The anode structure 110
is depicted in FIG. 1A as a simple, platelike electrically
conductive element. In practice, the anode structure 110 may
include additional elements such as an electron-multiplying
structure as, for example, a dynode chain or microchannel plate.
Such electron-multiplying structures are known in the art. Merely
by way of example, the electron-multiplying structures described in
U.S. Pat. No. 6,384,519, the disclosure of which is hereby
incorporated by reference herein, may be employed as part of the
anode structure 110. The photocathode 100 is positioned in the
enclosure 103 so that the elements 104 project in a direction
towards the anode structure 110. The phototube 101 also includes
appropriate electrical connections, schematically indicated at 105
and 107, for connecting the photocathode 100 and the anode
structure 110 to externally-applied electrical potentials.
[0023] A system incorporating the phototube 101 includes a circuit
having a source of electrical potential such as a battery 112
coupled to the anode structure 110 and to the electrically
conductive base 102 of the photocathode 100. The potential source
is arranged to maintain the photocathode 100 at a negative
potential relative to the anode structure 110. Where the anode
structure 110 includes an electron multiplier, the circuit also
incorporates appropriate devices (not shown) for providing the
requisite power to the electron multiplier. The circuit further
includes a detector 111 adapted to detect current flowing from the
anode structure 110.
[0024] In the particular embodiment illustrated, the system further
includes a light source 116 adapted to provide light to be detected
at a preselected wavelength. For example, in a communication system
such as a fiber optic communication system, source 116 may be
disposed remote from the phototube 101 and connected to the
phototube 101 by fiber optic (not shown). Alternatively, the source
116 can include a bandpass filter for selecting light at a
predetermined wavelength from an outside source.
[0025] In operation, the battery 112 maintains an electrical field
between the photocathode 100 and the anode structure 110. The
average value of the electrical field, as measured over the
distance between the photocathode 100 and the anode structure 110,
typically is about 1 to about 1000 V/mm. Electrons are released in
a vacuum towards the anode structure 110 in response to the light
being absorbed by the elements 104. The resulting current is
detected by the detector 111. This current is directly related to
the intensity of light impinging on the photocathode 100. For
example, in a fiber optic communications system, the intensity of
light provided by source 116 varies with the signal being
transmitted, and hence the current detected by detector 111 also
varies with the signal.
[0026] The elements 104 can also be designed to improve the
absorption of light of a certain desired wavelength. The length or
height (l) of each of the elements 104, as measured from its base
106 to its tip 108, corresponds to the resonant wavelength of light
to which the photocathode 100 is designed to absorb. Preferably,
the height (l) of each element 104 is substantially equal to:
n.lambda./4
[0027] where n is an integer and .lambda. is the wavelength of the
light to be detected. Preferably, n is 1, 2 or 4, and hence the
height or length (l) of each projecting element 104 is 1/2, 1/4, or
the same as the wavelength of the light to be detected. Preferably,
the center-to-center spacing (d) between adjacent elements 104 is
less than .lambda.. These conditions result in a photocathode 100
that is more likely to absorb light of the desired wavelength.
[0028] The response of the photocathode 100 varies with the
polarization of the incident light. The photocathode 100 is more
sensitive to light having an electric field vector parallel to the
vertical direction V of the photocathode 100, and hence parallel to
the length dimension (l) of the elements 104, than to other light.
Thus, the light source 116 desirably is arranged to provide
polarized light 114 having an electric field vector e with a
component c in the vertical direction V.
[0029] The shape of the elements 104 also creates a field focusing
effect that increases the likelihood that electrons will be
released from the photocathode 100. The electric field
intensification factor describes the effect that the geometry has
on the field strength compared to a planar geometry.
[0030] In the illustrated embodiment, the elements 104 are
relatively long structures having a small diameter. The electric
field strength is concentrated at the tips 108 of the elements 104,
which increases the likelihood that an electron will be released in
response to the absorption of light. Furthermore, the diameters of
each of the elements 104 are relatively small, which further
improves the emissions of electrons by the photocathode 100.
Although the present invention is not limited by any theory of
operation, it is believed that a smaller diameter provides electric
field enhancements by reducing the distance by which the charge
must flow to reach a surface through which an electron is released.
In a structure having a small diameter, the electron must travel a
distance of half the diameter to reach an emission surface.
[0031] In FIG. 1, the photocathode 100 is shown to have elements
104 that are wire-shaped (e.g., nanowires), that are roughly the
same height, and that are uniformly distributed across the entire
surface of the flat base 102. However, the invention is not so
limited. The elements 104 can have a different shape, can have a
wide range of heights, and can be unevenly distributed along a base
102 that is not flat e.g., curved, wavy or pointed.
[0032] The photocathode 100 is also shown to be formed of a single
structure. In the alternative, the photocathode 100 can be formed
of at least two structures. For example, the base 102 can be one
structure of one material and the elements 104 can be of another
material attached to the base 102.
[0033] In an alternative embodiment illustrated in FIG. 2, a
photocathode 120 has a relatively long and narrow base 122 and
elements 124 that extend out from the base 122. The elements 124
are in the shape of fins having a cross-section that is elongated.
The platelike elements are disposed in a row and are spaced apart
from one another by a distance d.
[0034] Referring now to FIG. 3, a photocathode 130 is shown. The
photocathode 130 has a disc shaped base 132 and an array of
elements 134 that extend from the base 132. In this embodiment, the
array of elements 134 extends over only the peripheral portion of
the base 132. This embodiment of the invention may be preferable in
applications where the incident light strikes only the outer
portion of a photocathode 130. This embodiment requires less
material to manufacture than a comparably-sized, circular
photocathode. Moreover, the elements 134 disposed adjacent the
periphery of the base 132 substantially block the light from
reaching the center of the base 132. Additional elements near the
center of the base 132 would not receive light, but could
contribute to the "dark current" or spontaneous electron emission
from the photocathode 130. Thus, by omitting the elements 134
adjacent the center of the base 132, the undesired dark current is
minimized without impairing the quantum efficiency of the
photocathode 130. In a further variant, the central portion of the
base 132 also may be omitted, so that the base 132 has a ring-like
shape.
[0035] Also, in the embodiments discussed above, the projecting
elements are of uniform height and are parallel to one another, but
this is not essential. For example, the projecting elements may be
of random height within a preselected range. Such random-height
elements can be formed by processes such as abrasion or dendritic
electroplating on the base.
[0036] The photocathode is preferably formed by a method that
utilizes the physical properties of the material. Two such methods
are described below.
[0037] In a first method, which is illustrated in FIGS. 4A-4D,
aluminum foil 140 (FIG. 4A) is anodized to form a porous template
142 (FIG. 4B). This technique is well established and is described
in detail in the following publications: J. Gruberger and E.
Gileadi, Electrochemical Acta, 31, 1531 (1986) and K. Shimuzu, K.
Kobayaski, G. E. Thompson, and G. C. Wood, Phiolosophical Mag. A,
66, 643 (1991), which are hereby incorporated by reference. Merely
by way of example, the anodizing bath may be an oxalic acid
solution. Alternatively, a bath such as H.sub.3PO.sub.4 and
H.sub.2SO.sub.4 can be used.
[0038] Referring now to FIG. 4B, the template 142 includes a
plurality of channels or pores 144, each having an opening 146 at
an outer surface 148. The channels 144 are surrounded by a layer of
alumina 150, which insulates the channels 144 from a layer of
aluminum 152 beneath the surface 148. The alumina layer 150 is
formed during the anodizing process.
[0039] In one exemplary process, the porous template 142 is formed
by anodizing aluminum 140 in a 0.45 weight percent oxalic acid
solution at 80 volts for 30 minutes. The anodization is performed
at 2.degree. C. to increase the ordering of channels 144 in the
aluminum foil 140 and prevent thermal run away. This treatment
yields channels 144 with a 40 nm diameter and 2 .mu.m depth. The
alumina layer 150 has a thickness of approximately 40 nm and is
formed around each of the channels 144 including at their bottoms
154. This layer 150 forms a barrier between the channels 144 and
the layer of unanodized aluminum 152. It is important to overcome
this insulating alumnia layer 150 to improve the effectiveness of a
subsequent electroplating step.
[0040] Referring now to FIG. 4C, the alumina layer 150 is overcome
by lowering the anodized voltage to 5 volts for 15 minutes. This
forms smaller sized channels 156 to bridge the gap between the
channels 144 and the aluminum layer 152. The template 142 is then
cleaned in water and etched in a 5 wt % phosphoric acid solution.
The phosphoric acid etch increases the diameter of the channels 144
to 80 nm and further etches away the alumina layer 150 at the
bottom of the channels 144. Accordingly, this step is sometimes
referred to as "pore widening."
[0041] Following the pore widening step, the structural material
158 which will form the projecting elements of the photocathode is
deposited in the channels 144. Electrodeposition of Ni and/or Au
into the channels 144 can be performed using an electroplating
technique. For example, a nickel-plating bath is mixed with 160 g
of nickel sulfamate per liter of water. The solution is buffered
with 30 grams of boric acid per liter to keep the pH constant at a
value of approximately 4. An electrodeposition bath to produce gold
elements 158 or nanowires comprises a solution of potassium gold
cyanide and citric acid that is pH controlled using potassium
hydroxide and phosphoric acid. A pulse plating technique is used to
fill the channels 144 in the porous template 142 to form the
elements 158 A 1-5 volt pulse with a 50 .mu.s width for a 500 .mu.s
period provides satisfactory results. To promote penetration of the
electrodeposition solution into the channels or pores 144, the
template 142 should be kept wet as, for example, in deionized
water, from the time the channels are formed until the template is
immersed in the electrodeposition solution. Also, an ultrasonic
bath should be used to promote uniform penetration of the
electrodeposition solution into the channels 144. After
electrodeposition, the template 142, with the formed elements 158
disposed in the channels 144, is mechanically polished so as to
form a flat surface 159 remote from the base. Thus, each element
158 is formed to approximately the same length.
[0042] After the channels 144 are filled, the elements 158 are then
exposed by removing at least part of the template. This is
accomplished by etching the template 142 using phosphoric acid, so
as to bring the structure to the configuration shown in FIG. 4D. In
this configuration, the alumina layer 150 has been etched back to
produce a photocathode 160 having projecting elements 158 of length
(l). The aluminum layer 152 is electrically connected to the
elements 158 and together with the alumina layer 150 forms the base
of the photocathode 160. The amount of back etching determines the
length (l) of the elements 158. The exposed elements 158 can be
further treated by coating them with a layer of a low work function
material to improve the quantum efficiency.
[0043] A second method of manufacturing a photocathode is shown in
FIGS. 5A-5D. The steps of anodizing the aluminum foil 140 to form
the porous template 142, overcoming the insulating layer of alumina
150, and electrodepositing a structural material such as Ni and/or
Au to form elements 162 are identical to the first described
method. In the method of FIGS. 5A-5D, the electrodepositing step is
continued until the deposited material overflows above the outer
surface 148 of the template 142 and forms a continuous layer 164
that electrically connects the elements 162. The overflow layer
164, which is illustrated in FIG. 5C, forms a base that is
integrally formed with the elements 162. In an alternative method,
a separate, electrically conductive element (not shown) can be
placed on top of the template 142 after the electrodepositing step
to provide electrical conductivity with the elements 162. For
example, such a separately-formed layer could be solder-bonded,
diffusion-bonded or eutectic-bonded to the formed elements 162
immediately after a polishing step as discussed above with
reference to FIG. 4C.
[0044] Referring now to FIG. 5C, the elements 162 are then exposed
by removing the template 142. This is accomplished by chemically
etching off the template 142 with phosphoric acid. In this
embodiment, the length (l) of the finished elements 162 is
controlled by the depth of the formed channels 144 in the template
142. Optionally, the template 142 can be polished at its outer
surface 148 after anodization but before deposition of the
structural material so as to control the depth of the channels 144.
The exposed elements 162 can be further treated with a layer of a
low work function material to improve the quantum efficiency.
[0045] As mentioned above, the projecting elements of the
photocathode can be treated so as to provide a coating of a low
work function, photoemissive material on the surfaces of the
elements. This treatment may include processes such as vapor
deposition, chemical vapor deposition, sputtering, reactive
sputtering, electroplating, or electroless plating. Merely by way
of example, antimony can be applied on a metallic structural
material by evaporation from a platinum-antimony source. Cesium can
be applied by thermal evaporation from a commercially available
getter source, or by exposure to vapor from a thermally heated
elemental Cs source. In other embodiments, the entirety of the
projections can be formed from the photoemissive material as, for
example, by depositing such material in the template. However, the
photocathode may include elements that are formed from a metal not
normally regarded as a low work function, photoemissive material,
without adding any additional coating material. For example, a
photocathode having nanowire elements formed entirely from nickel
provides measurable photoemission. Although the present invention
is not limited by any theory of operation, it is believed that this
result is attributable to the emission-enhancing effects of the
physical configuration such as field intensification and resonant
absorption of light as discussed above.
[0046] Although the invention has been described in detail with
reference to a preferred embodiment, numerous variations and
modifications exist within the scope of the invention as defined by
the claims.
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