Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- _ R~A 68,018
O
1 This invention relates to field emitting
devices, particularly non-thermionic field emitting devices,
and methods of making the same.
Non-thermionic field emitting devices, wherein
electron emission is stimulated by an electric potential
applied near a pointed cathode, are well known. Prior
art sharply pointed field emitting devices can be broadly
categorized by the type of material used in fabrication.
One such category is based upon the use o semiconductor
material, e.g., silicon or germanium, particularly for
photodetectors; however, such use as large area cold
cathode sources is severly limited. The physical properties
and costs alone of the single crystal semiconductor
materials limit the size of the arrays. Thus, in
"Fabrication and Some Applications of Large-Area Silicon
Field Fmission Arrays", Solid State Electronics, 1974,
Vol. 17, pages 155-163, Thomas et al. consider large-area
arrays to be on the order of only 10 cm2. In addition to
; being size limited, the current densities obtainable from
semiconductor field emitters are less than those obtainable
from metals.
Another category of devices encompasses the use
of sharply pointed metallic field emitters, such as that
disclosed in United States Patent No. 3,755,704 issued
28 August 1973 to Spindt et al. These devices, which
utilize individual needle-like protuberances deposited on
an electrode, suffer from two major disadvantages. First,
the deposition procedure used to form the protuberances
limits the area over which uniform arrays can be formed.
z
~ /\ 6~,()18
1(~41'~
1 This procedure includes projecting ~ source of emitter
material essentially normal to a given surface while
directin~ a source of masking material at the same surface,
but at a shallow grazing angle--a critical operation which
does not lend itself to forming very large quantities of
emitter elements over very large surfaces. Second, the
fabrication process entails the use of thin film techniques
which produce relati~ely delicate structures that are
sensitive to strong electrical forces characteristic of
~ield emission. Also, the relative thinness of the
insulators used in the prior art devices, ~ypically on the
order of l micron, causes manufacturing Prohlems since a
single pin hole in the insulation can ruin an entire field
emitter array.
In accordance with the invention, a non-thermionic
field emitting device comprises an electrically conductive
substrate having at least one field emitting element on
a surface thereof. The field emitting element has a
; pointed tip, with at least one needle-like projection
disposed thereon, which projects away from the substrate.
In the drawings:
FIGURE l is an isometric view of one emhodiment
of the novel field emitting device;
FIGURE 2 is a sectional view taken along line
2-2 of FIGIlRE l;
FIGURES 3, 4, 5 and 6 are sectional views showing
steps of a novel method of making the field emitting
device of FIGURE l;
FIGURE 7 is a portion of an enlarged plan view
of a sheet employed in the novel method;
--~ R~A 68,018
1~41'~0
1 FIGURES 8~ 9, 10, 11 and 12 are sectional views
showing further steps of the novel method;
FIGURE 13 is a cross section of a novel display
device incorporating the device of FIGURE l;
FIGURE 14 is an electrical schematic diagram of
the device depicted in FIGURE 13;
FIGURES 15 and 16 are potential energy diagrams
depicting the relative potential energies of electrons at
d~fferent locations within the device depicted in FIGURE 13;
FI~ E 17 is another electrical schematic
diagram of the device depicted in FIGURE 13; and
FI~URE 18 is another potential energy diagram
depicting the relative potential energies of electrons at
diferent locations within the device shown in FIGURE 13.
Referrring initially to FIGURES 1 and 2, an
embodiment of a non-thermionic field emitting device
according to the present invention is generally designatèd
as 10. The field emitting device 10 comprises a substrate
.: .
12 o an electrically conductive material, such as copper,
2!C having a matrix array of field emitting elements 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 or "tiplet" 18 located
on the tip thereof. The field emitter 16 and the tiplets
18 are composed of a material having good field emission
characteristics 9 such as copper. A layer of insulating
material 20, such as glass, is bonded to the surface of
the substrate having the field emitting elements 14 thereon.
The insulating layer 20 has an array of apertures 21
3 therethrou~h which are positioned such that the layer covers
-4-
R~A 68,018
J4~20
l the surface of the suhstrate while leaving the field
emitting elements 14 exposed through the apertures 21.
An electron extracting elec~rode 22, of an
electrically conductive material such as a beryllium
copper alloy, is bonded to the insula~ing layer 2n. The
electron extracting electrode 22 has a plurality o
apertures 24 therein, the number of aPertures corresponding
to the number of field emitting elements 14 in the matrix
array. The apertures 24 are positioned such that each
aperture is aligned substantially coaxially with a
correspondin~ field emitting element 14.
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
elèctrode 22 and the negati~e terminal is connected to the
array of field emitting elements 14 through the substrate
12. Electrons, which are emitted from the tiplets 18 under
the influence of the applied voltage, pass through the
apertures 24 toward a suitable anode electrode, such as a
phosphor coated screen ~not shown).
In accordance with the invention, the device 10
c~ould comprise a single field emitting element 14 having
an electron extracting electrode 22 with a single aperture
therein, to generate a single stream of electrons, instead
of the shown large array of field emitting elements and
electron extracting elactrodes generating a large number
of individually addressable electron streams.
To make the field emitting device 10, one surface
of a substrate 12 of an electrically conductive material,
such as copper, is made substantially clean, flat and free
. ,:
RCA 68,018
0
l from blemish. Then, as shown in FIGUR~ 3, a layer 28
of photosensitive etch-resistant, i.e., ~hotoresist,
material is applied to the prepared surface and exposed to
a light source through a transparency having an array of
black dots. The unexposes area of the photoresist layer 28
is nex~ washed away, leaving an array of holes 30. The
surface of the substrate is then under etched through the
holes 30, leaving an array o interconnected hemispherical
valleys 32 shown in FIGURB 4. It is to be noted that,
l although the previous steps use a negative photoresist
material an~ transparency, they cou:ld be equally well
carried out using a positive photoresist material and
transparency.
Next, referring to FIGURE 5, the photoresist layer
28 is stripped off, leaving an array of mesa-like
~ structures 34. The copper substrate 12 is then oxidized,
;~ by any well-known method such as heating in airJ ~forming
a layer of copper oxide 36 havin~ a thickness of
~pproximately 2 mils. As shown in FIGURE 6, the valleys
32 àre then partially filled with a layer 38 of an
electrically insulating ma~erial, such as glass. Onemethod
of filling the valleys with glass is to place a sheet of
glass approximately 3 mils thick across the tops o~ the
mesa-like structures 34 in a ~acuum oven. A vacuum is
drawn, and the glass is heated until it hecomes 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 substantially eliminates any
air which may be trapped between the glass layer and the
valleys 32.
R~A 68,018
,.~
~ '2 ~
1 llsing stan~ard pho~o etching techniques, a
pattern of holes 42, as shown in FIGURE 7, is etche~ into
a sheet 44 of conductive material, such as a beryllium
copper alloy. The pattern is such that, when the copper
alloy sheet 44 is placed on the etched surface of the
copper substrate 12, the holes 42 will be ali~ned with and
surround the mesa-like structures 34. After the glass has
settled into the valleys 3Z, it is slowly cooled to
substan*ially room temperature. The copper alloy sheet 44
is then positioned on the etched sùrface o the copper
substrate 12, so that the mesa-li~e structures 34 protrude
through the holes 42 while the remainder o~ the co~ner alloy
sheet 44 is mounted on the glass insulating material 38
(see FI~URE 8). The resulting structure is heated to bond `~
~he copper alloy to the ~lass insulating material 38, and
; then removed from the oven and allowed to cool to
substantially room temperature. Next, the thin layer o
glass 40 which co~ers 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 FIGURE 9. These emitters 16 typically have
artip diameter on the order of approximately 5 mic~ons.
Next, a porous layer 46 o a material which does
not form an appreciable oxide layer, such as chromium, gold
or rhodium, is deposited on the~ tips of the field emitters
16, as shown in FIGURE 10. Preferably, layer 46 is deposited
by electroplatin~ chromium on the tips of the ~ield emitters
using known porous plating techniques, as described for
example by A.H. Sully in "Chromium", Butterworthsg London
(1954), Chapter 5. The chromium layer 46 has pores or
:
RG~ 68,018
~ ~ 1 2'~0
l 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. The oxidation forms sharp points
of copper oxide-covered copper beneath the solid ~ortions
of the porous layer 46, as indicated ~y the dotted line 48
in PIGURE 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 of and leave an array of exposed tiplets 18 as shown
in FIGURE 12.
To describe the operation of the novel device 10,
reference is made to PIGURES 13 through 18. FIGURE 13
shows a single electron beam display ~evice generally
deslgnated as 49, which includes a non-thermionic ield
emitting device 10 having a single field emitting element
14 and an electron extracting electrode 22 with a single
aperture 24 therein. Also included in the display device
49 is an electron target, here 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.
FIGURE 14 is a schematic representation of the
single emitter display device 49 connected for basic
operation. The positive terminal of a voltage source 26 is
~ 2~ RCA 68,018
1 connected to the electron extracting electrode 22, and the
negative terminal thereof is connected to ground. Voltage
source 26 is on the order of 100 volts dc. The positive
terminal of a first bias voltage source 56 is connected
to a switch 58, and the negative terminal thereof is con-
nected to ground. The negative terminal of a second bias
voltage source 60 is connected to the switch 58, and the
positive terminal thereof is connected to ground. First
and second bias voltage sources 56 and 60, respectively,
are typically 5 volts dc each. The positive terminal of
a high voltage source 62 is connected to the phosphor coated
screen 50, and the negative terminal thereof is connected to
ground. High voltage source 62 is on the order of 20,000 ;~
volts dc.
15FIGU~ES 15 and 16 are potential energy diagrams
depicting the relative potential energies of electrons at
different locations within the device 49 when the voltages
applied to the electron extracting electrode 22 and the
phosphor coated screen 50 are 100 volts and 20,000 volts,
respectively. Point 64 represents the potential energy
` o~ 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. FIGURE 15
corresponds to position a of switch 58 in FIGURE 14, wherein
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 (point 68) is higher than the potential energy
of electrons at the field emitting element 14 (point 64),
RCA f)8,018
~ 4 ~
l electrons which have heen extracted from the field emitting
element will face a potential energy barrier and not be
able to transit the screen electrode toward the phosphor
coated screen 50.
When the switch 58 of FIGURE 14 is placed in
position b, the voltages on the field emittin~ element 14
and the screen electrode 52 are reversed, produclng the
electron potential energy diagram of FI~URE 16. Since
the potential energy of electrons at the field emitting
element 14 ~point 64) is now higher than the potential
energy of electrons at the screen electrode 52 (point 68)
the potential energy barrier is 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 to produce light.
;
In FIGUR~ 17, C represents the capacitance
between the screen electrode 52 and the field emitting
element 14. A pulse-type si~nal having a positive voltage
with respect to the field emitting element is applied to
the screen electrode, causing the capacitance C to cha~ge
up to an appropriate voltage. After the capacitance
chargin~, the potential energy of electrons at the scre~en
electrode, indicated by point 68 ~a) in FIGIJRF. 18, is less
than that at the field emitting element 14, indicated by
point 64 and reference line 74 in the same figure. When a
signal of, e.g.j 100 volts dc is 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 others will strike the screen electrode and
-- -10-
R~A 68,018
~ 2 ~
1 cause the voltage stored on the canacitance C to be
reduced. As more electrons strike the screen electrode,
the voltage on the capacitance C will be reduced further,
until the potential energy of electrons at the screen
electrode (point 68(b)) becomes substantially eaual to that
at the field emitting element 14 and further passa~e of
electrons through the screen electrode is prohibited.
Hence, the quantity of electrons striking the Phosphor
coated screen 50 can be regulated by varying the voltage
Of the applied signal.
Major advantages of the novel device of the
invention, over prior art devices, include 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
a plurality of tiplets,as opposed to the prior art single
point, at each emission site. The failure of one of a
plurality of tiplets will not appreciably degrade the
performance of a particular site as would the failure of
a single pointed tip in a prior art device. Still another
disadvantate accrues from the method which permits the
fabrication of large (on the order of 106) quantities
of uniform emission sites over a large area (on the order
2S of lOft.2). Additional uniformity of si~e-to-site
emission 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 e~ements, can be used in
applications requiring a large area cathode having a
. -~1~ -
.. . .
'
RCA 68,018
Canada
1~4:1Z~O
1 plurality of electron sources, such as disclosed in
United States Patents No. 3,176,184, issued 30 March 1965
to Mopkins; No. 3,539,719, issued 10 November 1970 to
Requa, et al.; and No. 3,708,713, issued 2 January 1973
to McCann. In addition, the display device disclosed
herein can be used as a programmable 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, by utilizing
strips of field emitting elements disposed in a matrix
relationship with screen electrode strips. Generation of
an electron stream can be effected by causing the proper
differential valtage to be applied between any desired
lS field emitting element and the intersecting screen electrode
strip. Modulation of the quantity of electrons striking
the phosphor coated screen can be accomplished, for
example, by selectively applying a signal voltage to the
intersecting screen electrode strip, to charge the
associated screen electrode emitting element capacitance to
control the passage of electrons.
.
'
:
-12-
