Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Low workfunction layer for electron emission
Reexamination Certificate
2000-03-02
2002-11-05
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Low workfunction layer for electron emission
C257S077000, C438S020000, C313S311000
Reexamination Certificate
active
06476408
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a field-emission display device. It is applicable to display screens of the flat-screen type and in particular high-resolution (100 &mgr;m pixel spacing), high-luminance (up to 500 cd/M
2
) and low-consumption screens. It is also applicable to the production of a planar microgun electron source applicable especially in microlithography.
2. Discussion of the Background
A field-emission display (FED) screen is schematically composed of a cathode, an anode and an interelectrode space under vacuum. The cathode is a matrix of electron emitters which illuminate the anode where various phosphors, that is to say receptors, are placed. Since corresponding to each emitter there is a receptor, the resolution of a direct-viewing screen is defined by the interpixel spacing with which it is manufactured.
For small (less than 14 inch diagonal) high-resolution screens, this spacing is about 100 to 300 &mgr;m by 100 to 300 &mgr;m. Direct-viewing screens having the highest resolution are without doubt avionic screens which have to be manufactured with a pixel pitch of about 100 &mgr;m by 100 &mgr;m. In colour displays, the dot pitch is greater since a dot is composed of three-red, green and blue-pixels.
SUMMARY OF THE INVENTION
In order to avoid the phenomenon of colour crosstalk, 99% of the electrons emitted by an emitter must strike the receptor which corresponds to it. The size (f
T
by f
T
) of the beam, emitted by an emitter of size f
E
by f
E
, at the anode is equal to: f
T
(&mgr;m)=f
E
+2X, 2X being the broadening of the beam with respect to its initial size. For example, for a 40 by 40 &mgr;m emitter size, X must be less than or equal to 30 &mgr;m.
If each element emits a beam of electrons having an initial velocity v
i
in a cone of half-angle q, the anode-cathode distance d
ca
may be written in the form of the following formula:
d
ca
=
qE
2
m
·
t
2
+
v
0
t
with E: cathode-anode field (v/m)
m: electron mass: 9.1×10
−31
kg
q: electron charge: 1.6×10
−19
C
t: cathode-anode transit time (s)
v
0
: orthogonal component of v
i
(m/s).
Since ½mv
i
2
=qE
i
and v
0
=v
i
cos &thgr;,
where qE
i
is the initial energy of the electrons (eV), then:
t
2
+
8
mE
i
cos
2
θ
qE
2
t
-
2
m
qE
d
ca
=
0
The solution of this equation is:
t
=
2
mE
i
cos
2
θ
qE
2
+
2
md
ca
qE
-
2
mE
i
cos
2
θ
qE
2
Since
X
=
v
p
t
=
2
qE
i
m
sin
θ
·
t
,
where v
p
is the parallel component of v
i
(m/s), then:
X
=
2
sin
θ
(
E
i
2
cos
2
θ
E
2
+
d
ca
E
i
E
-
E
i
2
cos
2
θ
E
2
)
X
=
2
E
i
E
sin
θ
(
d
ca
+
E
i
E
cos
2
θ
-
E
i
E
cos
2
θ
)
In general (see examples described below), in order to avoid cathode-anode breakdown phenomena, d
ca
is chosen to be equal to d
ca
(mm)=½ Va(kV), which corresponds to a field E=2×10
6
V/m.
It should be noted that for low-energy (≈1 eV) electrons, the term (E
i
/E)cos
2
&thgr; becomes negligible. This is because (E
i
/E)cos
2
&thgr;≦E
i
/E≦5×10
7
m<<d
ca
.
The constraint on the luminance (500 cd/M
2
) corresponds to a luminosity of 1600 Lm/m
2
and therefore to 1.6×10
−5
Lm per pixel (100 by 100 &mgr;m pixel). Taking a phosphor efficiency of 5 Lm/W (for electrons having an energy of 5 keV), we obtain 3.2 &mgr;W per pixel, which corresponds to an average current of 0.64 nA. Since each pixel emits during the time that the corresponding line is being addressed, the emission current per pixel must be 0.64 &mgr;A (for a screen with 1000 lines). This pixel current corresponds to current densities of 10 mA/cm
2
, 18 mA/cm
2
and 40 mA/cm
2
for 80 by 80 &mgr;m, 60 by 60 &mgr;m and 40 by 40 &mgr;m emissive sources, respectively.
In order to determine a quality criterion for a screen with respect to the power dissipated for its operation, it is possible to define a parameter characteristic of the power needed to go from a black pixel to a white pixel, namely:
P
=
1
2
C
p
V
scan
2
t
c
where C
p
is the capacitance of a pixel, V
scan
is the difference between the addressing voltage for a white pixel and for a black pixel and t
c
is the charging time of the pixel, which is of the order of 10 &mgr;s. Consequently:
P
(&mgr;
W)=
0.05 ×C
p
(p
F
)×
V
scan
2
.
It should be noted that in the case of a liquid-crystal screen (C
p
≈0.6 pF and V
scan
=10 V), this parameter P is equal to 3 &mgr;W.
Within the technology of field-effect screens, the screen manufactured by the company Pixtech [1] is known. This screen uses a cathode with field-emission tips. Each emitter is composed of about 30 tips or more. According to S. T. Purcell et al. [2], the beam emitted by this type of cathode is composed of primary electrons having an initial energy of about 10 eV less than the gate voltage and of secondary electrons having an average energy of 7 eV. Assuming electrons with an initial energy of 90 eV (gate voltage=100 V) emitted in a cone of about 30° half-angle and striking an anode biased at 400 V, a distance d
ca
equal to 0.2 mm and X=69 &mgr;m are obtained. Since the emitting surface seems to be about 40 &mgr;m along the axis for which the pixel pitch is 100 &mgr;m, a beam size of the order of 180 &mgr;m is obtained. According to Futaba [1], &phgr;
T
is equal to 230 &mgr;m for 95% of the electrons emitted by an emitter. In order to obtain a beam size of less than 100 &mgr;m, Futaba and Pixtech use the switched-anode technique: dual anode [1] and triple anode [3]. In these configurations, a switched anode is flanked by non-selected and therefore non-biased, anodes. As a result, the electrons are focused onto the selected anode. The size of the beam at the anode is then less than 100 &mgr;m. However, since the distance between anodes is of the order of 30 &mgr;m, it would seem to be impossible to use a high anode voltage (greater than 1 kV). Since low-voltage phosphors have a low efficiency, the present results are not very satisfactory since the luminance of the screen obtained is low: 80 cd/m
2
instead of 500 cd/m
2
for an avionics screen.
Since the capacitance of a pixel is given by:
C
p
=&egr;
0
.&egr;
r
S
.1/e=0.009 p
F
where e is the thickness of silica between the gate and the base of the tip: 1 &mgr;m
&egr;
r
(silica): 4
S is the coverage area per pixel: 50 by 50 &mgr;m.
The value of P(&mgr;W) obtained is 0.05×C
p
(pF)×V
2
scan
=4 &mgr;W with V
scan
=30 V i.e. a value equivalent to that obtained for a liquid-crystal screen.
In order to obtain a high-resolution luminous screen, it is necessary to have a screen operating with an anode voltage ranging from 4 kV to 6 kV, for which the parameter X is small (≈30 &mgr;m). To do this, the beam emitted by the cathode must have a low divergence and a low energy.
Materials with a low electron affinity are known, such as carbon with a diamond structure. This is a low-field emissive material, for example for a field of between 1 and 50 V/&mgr;m, the emissivity of which is commonly ascribed to the low electron affinity of the material but which may be due to other phenomena. In the rest of the description, this material will be called “material with a low electron affinity” as is done in the art. These materials have the great advantage of emitting electrons for low extraction fields (of the order of 10 V/&mgr;m). Since it is easy to obtain such fields over a plane thin layer, it is no longer necessary to produce tips, thereby facilitating the fabrication process. For example, in a cathode with tips, it is absolutely essential to control
Legagneux Pierre
Pribat Didier
"Thomson-CSF"
Jackson, Jr. Jerome
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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