U.S. patent number 3,921,022 [Application Number 05/502,669] was granted by the patent office on 1975-11-18 for field emitting device and method of making same.
This patent grant is currently assigned to RCA Corporation. Invention is credited to Jules David Levine.
United States Patent |
3,921,022 |
Levine |
November 18, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Field emitting device and method of making same
Abstract
A non-thermionic field emitting device includes at least one
pyramidal shaped field emitting element on one surface of an
electrically conductive substrate. At least one needle-like
projection is located on the tip of the pyramidal shaped field
emitting element. An electron extracting electrode is mounted in
parallel spaced relation to and is electrically insulated from the
surface of the substrate having the field emitting element thereon.
The electron extracting electrode has at least one aperture
therein, the aperture being positioned substantially coaxially with
a corresponding pyramidal shaped field emitting element.
Inventors: |
Levine; Jules David (East
Brunswick, NJ) |
Assignee: |
RCA Corporation (New York,
NY)
|
Family
ID: |
23998842 |
Appl.
No.: |
05/502,669 |
Filed: |
September 3, 1974 |
Current U.S.
Class: |
313/309;
250/423F; 313/336; 313/351; 445/50 |
Current CPC
Class: |
H01J
29/52 (20130101); H01J 63/00 (20130101); H01J
9/025 (20130101); H01J 1/3042 (20130101); H01J
3/022 (20130101); H01J 2893/0031 (20130101) |
Current International
Class: |
H01J
3/02 (20060101); H01J 3/00 (20060101); H01J
29/52 (20060101); H01J 63/00 (20060101); H01J
1/30 (20060101); H01J 9/02 (20060101); H01J
1/304 (20060101); H01J 001/02 () |
Field of
Search: |
;313/309,336,351,95 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Chatmon, Jr.; Saxfield
Attorney, Agent or Firm: Bruestle; Glenn H. Murray; William
H.
Claims
I claim:
1. A non-thermionic field emitting device comprising:
a. an electrically conductive substrate having opposed
surfaces;
b. at least one field emitting element on said one surface of said
substrate, said field emitting element having a pointed tip which
projects away from said substrate; and
c. at least one needle-like projection disposed only on said
pointed tip of said field emitting element whereby electron
emission is enhanced by convergent lines of force of an electric
field.
2. A non-thermionic field emitting device in accordance with claim
1 in which said field emitting element has a substantially
pyramidal shape.
3. A non-thermionic field emitting device in accordance with claim
2 having an electron extracting electrode insulatingly spaced from
and substantially parallel to said one surface of said substrate,
said electron extracting electrode comprising a layer of
electrically conductive material having at least one aperture
therein which is positioned substantially coaxially with a
corresponding field emitting element.
4. A non-thermionic field emitting device in accordance with claim
3 having a plurality of field emitting elements in a matrix array
on said one surface of said substrate.
5. A device for modulating the flow of electrons comprising the
following in a vacuum tight relationship:
a. an electron source comprising at least one field emitting
element having a pointed tip with at least one needle-like
projection disposed only on said pointed tip;
b. an electron target;
c. a modulating element positioned beetween said electron source
and electron target to control the quantity of electrons impinging
upon said electron target, said modulating element having a known
capacitance with respect to said electron source;
d. means whereby said known capacitance can be charged to a
predetermined voltage level;
e. an electrode for extracting electrons from said field emitting
element, said electrode in insulatingly spaced-relation between
said field emitting element and said modulating element and
structured such that the electron extracting electrode will not
impede the passage of the electrons from said field emitting
element to said modulating element.
6. A device in accordance with claim 5 in which said electron
extracting electrode comprises a layer of electrically conductive
material having at least one aperture therein in coaxially spaced
relation with a corresponding field emitting element.
7. A device in accordance with claim 6 in which the modulating
element is a screen electrode parallel to and insulatingly spaced
from said electron extracting electrode, said screen electrode
comprising a sheet of an electrically conductive material having a
mesh-like structure formed by a multiplicity of finely spaced
apertures.
8. A device in accordance with claim 7 in which the electron target
comprises a phosphor coated display screen.
9. A display device comprising the following in a vacuum tight
relationship:
a. a plurality of field emitting electron sources, each field
emitting electron source having a pointed tip and at least one
needle-like projection disposed only on said pointed tip;
b. an electron target;
c. a modulating element disposed between each of said electron
sources and said electron target to control uniformity of the
quantity of electrons impinging upon said electron target from each
source, each modulating element having a known capacitance with
respect to its associated electron source; and
d. means whereby each known capacitance can be charged to a
predetermined voltage level.
10. A display device in accordance with claim 9 in which said
plurality of electron sources comprises a matrix array of electron
emitting sites, each site comprising:
a. a field emitting element disposed on an electrically conductive
substrate, said field emitting element having a pointed tip which
projects away from said substrate and at least one needle-like
projection disposed only on said tip; and
b. an electrode for extracting electrons from said field emitting
element, said electrode comprising a layer of an electrically
conductive material disposed in insulatingly spaced relation
between said field emitting element and said modulating element,
said layer having an aperture therein in coaxially spaced relation
with said field emitting element.
11. A display device in accordance with claim 10 in which said
modulating element is a screen electrode disposed parallel to and
insulatingly spaced from said electron extracting electrode, said
screen electrode comprising a layer of an electrically conductive
material having a mesh-like structure formed by a multiplcity of
finely spaced apertures.
12. A display device in accordance with claim 11 in which the
electron target comprises a phosphor coated display screen.
Description
BACKGROUND OF THE INVENTION
The present invention relates to field emitting devices and more
particularly to non-thermionic field emitting devices and methods
of making the same.
Non-thermionic field emitting devices are well known in the art. It
is known that electron emission can be stimulated by an electric
potential applied near a pointed cathode. The prior art sharply
pointed field emitters can be broadly categorized by the type of
material used for fabrication. One such category includes the use
of semiconductor material such as silicon or germanium to construct
arrays of sharply pointed field emitters. Although these devices
appear to be particularly well suited for use as photodetectors,
their use as large area cold cathode sources is severely limited.
For example, the physical properties and cost of the single crystal
semiconductor materials used limit the size of the arrays. In their
paper entitled "Fabrication and Some Applications of Large-Area
Silicon Field Emission Arrays", Solid State Electronics, 1974, Vol.
17, pages 155-163, Thomas et al. consider large-area arrays to be
on the order of 10 cm.sup.2. In addition to being size limited, the
current densities obtainable from semiconductor field emitters are
less than those obtainable from metals.
Another prior art category of field emitting devices encompasses
the use of sharply pointed metallic field emmitters. Typical of
these devices is that disclosed in U.S. Pat. No. 3,755,704 issued
to Spindt et al. These devices utilize individual needle-like
protuberances deposited on an electrode, said protuberances being
of a higher resistivity material than the electrodes. These devices
suffer from two major disadvantages. First, the use of deposition
techniques to form the protuberances limits the area over which
uniform arrays can be formed. This technique includes projecting a
source of emitter material onto a surface essentially normal to the
surface while at the same time directing a source of masking
material at the same surface at a shallow grazing angle from all
sides. This is a critical operation which does not lend itself to
use in forming very large quantities of emitter elements over very
large surfaces. Second, the fabrication of these prior art devices
also entails the use of thin film techniques which produce
relatively delicate structures that are sensitive to strong
electrical forces characteristics of field emission. In addition,
the relative thinness of the insulators used in the prior art
devices, typically on the order of 1 micron, can cause
manufacturing problems in that a single pin hole in the insulation
can ruin an entire field emitter array.
SUMMARY OF THE INVENTION
A non-thermionic field emitting device comprises an electrically
conductive substrate having opposed surfaces with at least one
field emitting element on one of the opposed surfaces. The field
emitting element has a pointed tip, with at least one needle-like
projection thereon, which projects away from the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of one form of a field emitting device
of the present invention.
FIG. 2 is a sectional view taken along line 2--2 of FIG. 1.
FIGS. 3, 4, 5 and 6 are sectional views showing steps of making the
field emitting device of the present invention.
FIG. 7 is a portion of an enlarged plan view of a sheet of
electrically conductive material having a hole pattern therein.
FIGS. 8, 9, 10. 11 and 12 are sectional views showing further steps
of making the field emitting device of the present invention.
FIG. 13 is a cross section of one embodiment of the device of the
present invention.
FIG. 14 is an electrical schematic diagram of the device depicted
in FIG. 13.
FIGS. 15 and 16 are potential energy diagrams depicting the
relative potential energies of electrons at different locations
within the device depicted in FIG. 13.
FIG. 17 is another electrical schematic diagram of the device
depicted in FIG. 13.
FIG. 18 is a potential energy diagram depicting the relative
potential energies of electrons at different locations within the
device shown in FIG. 13.
DETAILED DESCRIPTION
Referring initially to FIGS. 1 and 2, one form of a nonthermionic
field emitting device of the present invention is generally
designated as 10. The field emitting device 10 comprises a
substrate 12 of an electrically conductive material such as copper,
having a matrix array of field emitting elements, generally
referred to as 14, on one surface thereof. Each field emitting
element 14 comprises a conically or a pyramidally shaped field
emitter 16 with at least one needle-like projection hereinafter
referred to as tiplets 18, located on the tip thereof. The field
emitter 16 and the tiplets 18 are composed of a material having
good field emission characteristics, such as copper. A layer of
insulating material 20, such as glass, is on and bonded to the
surface of the substrate having the field emitting elements 14
thereon. The insulating layer 20 has an array of apertures 21
therethrough which are positioned such that the insulating layer
covers the surface of the substrate while leaving the field
emitting elements 14 exposed through the apertures 21.
An electron extracting electrode 22, of an electrically conductive
material such as a beryllium copper alloy is on and bonded to the
insulating layer 20. The electron extracting electrode 22 has a
plurality of apertures 24 therein, the number of apertures
corresponding to the number of field emitting elements 14 in the
array. The apertures are positioned such that each aperture is
aligned substantially coaxially with a corresponding field emitting
element.
To obtain the desired emission of electrons from the field emitting
elements 14, the positive terminal of a voltage source 26 is
connected to the electron extracting electrode 22 and the negative
terminal is connected to the array of field emitting elements 14
through the substrate 12. Electrons are emitted from the tiplets 18
under the influence of the applied voltage. The emitted electrons
pass through the apertures 24 in the electron extracting electrode
22 toward a suitable anode electrode such as a phosphor coated
screen, not shown. The electron emitting device 10 of the present
invention can consist of a single field emitting element 14 having
an electron extracting electrode 22 with a single aperture therein
in order to generate a single stream of electrons or it can consist
of a large array of field emitting elements and electron extracting
electrodes generating a large number of individually addressable
electron streams.
To make the field emitting device 10, one starts with a substrate
12 of an electrically conductive material such as copper. One
surface of the copper substrate is prepared in order to insure that
the surface is substantially clean, flat and free from blemish
prior to the application of a layer 28, see FIG. 3, of
photosensitive, etch-resistant material or what is commonly known
as a photoresist material. The photoresist material is then applied
to the prepared surface and exposed to a light source through a
transparency having an array of black dots. As shown in FIg. 3, the
unexposed area of the photoresist layer 28 is washed away leaving
an array of holes 30. It is to be noted here that, although the
previous steps have been described using a negative photoresist
material and transparency, these steps can be equally well carried
out using a positive photoresist material and transparency and such
embodiment is to be considered within the scope and contemplation
of this invention. The surface of the copper substrate is then
under etched through the holes 30, leaving an array of
interconnected hemishperical valleys 32 as shown in FIG. 4.
Next, referring to FIG. 5, the photoresist layer 28 is stripped off
leaving an array of mesa-like structures 34. The copper substrate
is then oxidized, by any well-known oxidizing method such as
heating in air, forming a layer of copper oxide 36 having a
thickness of approximately 2 mils. As shown in FIG. 6, the valleys
32 are then partially filled with a layer of an electrically
insulating material 38 such as glass. One method of filling the
valleys with glass is to place a sheet of glass approximately 3
mils thick across the tops of the mesa-like structures 34 and place
in a vacuum oven. A vacuum is drawn and the glass is heated until
it becomes semi-molten and settles down into the valleys 32 leaving
only a thin layer of glass 40 covering the tops of the mesa-like
structures 34. The vacuum is used to substantially eliminate any
air which may be trapped between the glass layer and the valleys
32.
Using standard photo etching techniques, a hole pattern 42, as
partially shown in FIG. 7, is etched into a sheet of conductive
material 44 such as a beryllium copper alloy. The hole pattern 42
is laid out such that when the copper alloy sheet 44 is placed on
the etched surface of the copper substrate 12, the holes 42 will be
aligned with and surround the mesa-like structures 34. After the
glass has settled into the valleys 32 it is slowly cooled to
substantially room temperature. The copper alloy sheet 44 is then
placed on the etched surface of the copper substrate 12 and
positioned such that the mesa-like structures 34 protrude through
the holes 42 while the remainder of the copper alloy sheet 44 is
mounted on the glass insulating material 38 (see FIG. 8). The
resulting structure is heated in order to bond the copper alloy to
the glass insulating material 38. The structure is then removed
from the oven and allowed to cool to substantially room
temperature. Next, the thin layer of glass 40 which covers the
mesa-like structures 34 is removed to expose the copper oxide 36.
The copper oxide is then etched away to form pyramidally shaped
field emitters 16 as shown in FIG. 9. These pyramidially shaped
field emitters 16 typically have a tip diameter on the order of
approximately 5 microns.
After the pyramidally shaped field emitters 16 are formed, a porous
layer 46 of a material which does not form an appreciable oxide
layer such as chromium, gold or rhodium, is deposited on the tips
of said field emitters as shown in FIG. 10. The porous layer 46 in
the preferred embodiment comprises chromium which has been
electroplated on the tips of the field emitters using known porous
plating techniques for example as described in the textbook
Chromium by A. H. Sully, Butterworth, London (1954), Chapter 5. The
chromium layer has pores or cracks 47 therethrough which are
typically 1 micron apart. The field emitters 16 with the porous
layer 46 deposited on the tips thereof, are then heated in air to
oxidize those surfaces of the field emitters which are exposed
beneath the pores or cracks 47. This oxidation process forms sharp
points of copper covered by copper oxide beneath the solid portions
of the porous layer 46 as indicated by the dotted line 48 in FIg.
11. After cooling to approximately room temperature, the oxide is
chemically stripped from the tips of the field emitters 16 causing
the porous layer 46 to fall off leaving an array of tiplets 18
exposed as shown in FIG. 12.
The operation of the device of the present invention is described
making reference to FIG. 13 through 18. FIG. 13 shows a cross
section of a single electron beam display device generally
designated as 49 which includes a non-thermionic field emitting
device 10 having as an electron souorce a single field emitting
element 14 and an electron extracting electrode 22 having a single
aperture 24 therein. Also included in the display device 49 is an
electron target in the form of a phosphor coated display screen 50
and a screen electrode 52 having a mesh-like structure formed by a
multiplicity of finely spaced apertures 54. The components of the
display device 49 are joined together to form an airtight cavity 55
which is evacuated to provide a vacuum environment between the
field emitting element 14 and the display screen 50. FIG. 14 is a
schematic representation of the single emitter display device 49
depicted in FIG. 13 which has been connected to the external
components required for basic device operation. The reference
numeral used to identify the elements of the schematic diagram in
FIG. 14 correspond to the reference numerals used to identify the
parts of the embodiment depicted in FIg. 13.
As shown in FIG. 14, the positive terminal of a voltage source 26
is connected to the electron extracting electrode 22 and the
negative terminal is connected to ground. The voltage applied to
the electron extracting electrode 22 is on the order of 100 volts.
The positive terminal of a first bias voltage source 56 is
connected to the switch 58 and the negative terminal is connected
to ground. The negative terminal of a second bias voltage aource 60
is connected to the switch 58 and the positive terminal is
connected to ground. The voltage of the first 56 and second 60 bias
voltage sources is typically 5 volts each. The positive terminal of
a high voltage source 62 is connected to the anode electrode or
phosphor coated screen 50 in this embodiment and the negative
terminal is connected to ground. The voltage applied to the
phosphor coated screen 50 is on the order of 20,000 volts.
FIGS. 15 and 16 are potential energy diagrams depicting the
relative potential energies of electrons at different locations
within the device of the present invention when the voltage applied
to the electron extracting electrode 22 is 100 volts and the
voltage on the phosphor coated screen 50 is 20,000 volts. Point 64
represents the potential energy of electrons at the tiplets 18,
point 66 is the potential energy of electrons at the electron
extracting electrode 22, point 68 is the potential energy of
electrons at the screen electrode 52 and point 70 is the potential
energy of electrons at the phosphor coated screen 50. FIG. 15 shows
the electron potential energy diagram for the device when the
switch 58 is in position A. In this position, the voltage on the
field emitting element 14 is +5 volts and the voltage on the screen
electrode 52 is -5 volts. Since the potential energy of electrons
at the screen electrode 52 is higher than the potential energy of
electrons at the field emitting element 14, as represented by
points 68 and 64 respectively in FIG. 15, electrons which have been
extracted from the field emitting element will not be able to
transit the screen electrode toward the phosphor coated screen 50
because the region of higher potential energy acts as a barrier to
the passage of the electrons. When the switch 58 is placed in
position B, the voltages on the field emitting element 14 and the
screen electrode 52 are reversed resulting in an electron potential
energy relationship as shown in FIG. 16. Since the potential energy
of electrons at the field emitting element 14 is now higher than
the potential energy of electrons at the screen electrode 52, as
represented by points 64 and 68 respectively in FIG. 16, the
potential energy barrier has been eliminated and electrons which
have been extracted from the field emitting element will pass
through the screen electrode and strike the phosphor coated screen
50 thereby producing light.
Referring to FIG. 17, represents the capacitance between the screen
electrode 52 and the field emitting element 14. A signal having a
positive voltage with respect to the field emitting element is
momentarily applied to the screen electrode causing the capacitance
C to charge up to a voltage corresponding to the value of the
applied signal voltage. After the capacitance C has been charged,
the potential energy of electrons at the screen electrode, as
represented by point 68 (a) in FIG. 18, is less than that at the
field emitting element 14 as indicated by point 64 and reference
line 74. With an appropriate voltage, for example 100 volts,
applied to the electron extracting electrode 22, some of the
electrons emanating from the field emitting element 14 will transit
the screen electrode 52 and strike the phosphor coated screen 50
while other will strike the screen electrode causing the voltage
stored on the capacitance C to be reduced. As more electrons strike
the screen electrode, the voltage on the capacitance C will be
further reduced until the potential energy of electrons at the
screen electrode, as represented by point 68(b), is substantially
equal to that at the field emitting element 14 at which time
further passage of electrons through the screen electrode will
cease. Consequently, the quantity of electrons striking the
phosphor coated screen 50 can be regulated by varying the voltage
of the applied signal.
One of the major advantages of the device of the present invention
over prior art devices are the improved structural stength and heat
transfer characteristics afforded by the combination of a slender
tiplet on a relatively large pyramidal or conical base. Another
advantage lies in the enhanced reliability afforded by the presence
of a plurality of tiplets at each emission site as opposed to a
single point which is characteristic of the prior art. The failure
of a single tiplet will not appreciably degrade the performance of
a particular site as would the failure of a pointed tip in a prior
art device. Still another advantage accrues from the method
disclosed herein which permits the fabrication of large (on the
order of 10.sup.6) quantities of uniform emission sites over a
large area (on the order of 10ft..sup.2). Additional uniformity of
emission from site to site is obtained by the use of a screen
electrode as disclosed herein.
The device of the present invention, when embodied as an array of
field emitting elements, can be used in those applications
requiring a large area cathode having a plurality of electron
sources such as the Electron Deflection System for Image
Reproduction disclosed in U.S. Pat. No. 3,176,184, the Electron
Beam Scanning Device disclosed in U.S. Pat. No. 3,539,719 or the
Plural Beam Electron Beam Scanner Utilizing a Modulation Grid
disclosed in U.S. Pat. No. 3,708,713 for example. In addition, the
display device disclosed herein, having an electron source
comprising an array of field emitting elements, can be used as a
programmable display device by selectively generating one or more
streams of electrons and controlling the quantity of electrons
which impinge upon the phosphor coated screen. The selective
generation of electron streams may be accomplished, for example,
utilizing strips of field emitting elements disposed in a matrix
relationship with screen electrode strips. Generation of an
electron stream can be effected from any desired field emitting
element by causing the proper differential voltage to be applied
between said field emitting element and the intersecting screen
electrode strip. Modulation of the quantity of electrons striking
the phosphor coated screen can be accomplished by, for example,
selectively applying a signal voltage to the intersecting screen
electrode strip in order to charge the associated screen electrode
emitting element capacitance for the purposes of controlling the
passage electrons as described above.
* * * * *