Electric lamp and discharge devices – Photosensitive – Photocathode
Reexamination Certificate
2000-08-10
2003-07-22
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Photocathode
C313S544000
Reexamination Certificate
active
06597112
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the manufacturing of image intensifier devices, and more particularly, to a process of manufacturing a photocathode.
BACKGROUND OF THE INVENTION
Image intensifiers, also known as a night vision systems, multiply the amount of incident light received by the image intensifier to provide a visible image. These devices typically require some low-level residual light, such as moon or star light, in which to operate. However, the present generation of image intensifiers can also make visible the light from the near-infrared (invisible) portion of the light spectrum. As used herein, the term “light” means electromagnetic radiation, regardless of whether or not this light is visible to the human eye. The image intensification process involves conversion of the received ambient light into electron patterns and projection of the electron patterns onto a phosphor screen for conversion of the electron patterns into light visible to the observer. This visible light can then be viewed directly by the operator or through a lens provided in the eyepiece of the system.
Image intensifiers are constructed for a variety of applications and therefore vary in both shape and size, with proximity focused image intensifiers comprising a particular type of image intensifier having the smallest size and weight of all categories of image intensifiers. These devices are particularly useful for both industrial and military applications. For example, image intensifiers are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are also employed in security cameras, photographing astronomical bodies and in medical instruments to help alleviate conditions such as retinitis pigmentosis, more commonly known as night blindness. Such an image intensifier device is exemplified by U.S. Pat. No. 5,084,780, entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING by Earl N. Phillips issued on Jan. 28, 1992, and assigned to ITT Corporation the assignee herein.
Image intensifiers are currently manufactured in two types, commonly referred to as Generation II (GEN
2
) and Generation III (GEN
3
) type image intensifier tubes. The primary difference between these two types of image intensifier tubes is in the type of photocathode employed in each. Image intensifier tubes of the GEN
2
type have a multi-alkali photocathode with a spectral sensitivity in the range of 400-900 nanometers (nm). This spectral range can be extended to the blue or red by modification of the multi-alkali composition and/or thickness. GEN
3
image intensifier tubes have a p-doped gallium arsenide (GaAs) photocathode that has been activated to negative electron affinity by the adsorption of cesium and oxygen on the surface. This material has approximately twice the quantum efficiency of the GEN
2
photocathode. An extension of the spectral response to the near infrared can be accomplished by alloying indium with gallium arsenide. A third type of image intensifier is current being introduced and will be know as Generation IV (GEN
4
) image intensifier. A GEN
4
image intensifier is similar to the GEN
3
except improvements have been made to the microchannel plate.
A GEN
3
image intensifier tube according to the prior art is illustrated in FIG.
1
. The image intensifier tube
10
comprises an evacuated envelope or vacuum housing
22
having a photocathode
12
disposed at one end of the housing
22
and a phosphor-coated anode screen
30
disposed at the other end of the housing
22
. A microchannel plate
24
is positioned within the vacuum housing
22
between the photocathode
12
and the phosphor screen
30
. The photocathode
12
comprises a glass faceplate
14
coated on one side with an antireflection layer
16
; a aluminum gallium arsenide (Al
x
Ga
1−x
As) window layer
17
; a gallium arsenide active layer
18
; and a negative electron affinity coating
20
.
The microchannel plate
24
is located within the vacuum housing
22
and is separated from the photocathode
12
by a gap
34
. The microchannel plate
24
is generally made from a thin wafer of glass having an array of microscopic channel electron multipliers extending between input surfaces
26
and output surfaces
28
. The wall of each channel is formed of a secondary emitting material. The phosphor screen
30
is located on a fiber optic element
31
and is separated from the output surface
28
of the microchannel plate
24
by a gap
36
. The phosphor screen
30
generally includes an aluminum overcoat
32
to stop light reflecting from the phosphor screen
30
from reentering the device through the negative electron affinity coating
20
.
In operation, photons from an external source impinge upon the photocathode
12
and are absorbed in the GaAs active layer
18
, resulting in the generation of electron/hole pairs. The electrons generated by the photo cathode
12
are subsequently emitted into the gap
34
of the vacuum housing
22
from the negative electron affinity coating
20
on the GaAs active layer
18
. The electrons emitted by the photo cathode
12
are accelerated toward the input surface
26
of the microchannel plate
24
by applying a potential applied across the input surface
26
of the microchannel plate
24
and the photo cathode
12
of approximately 800 volts.
When an electron enters one of the channels of the microchannel plate
24
at the input surface
26
, a cascade of secondary electrons is produced from the channel wall by secondary emission. The cascade of secondary electrons are emitted from the channel at the output surface
28
of the microchannel plate
24
and are accelerated across gap
36
toward the phosphor screen
30
to produce an intensified image. Each microscopic channel functions as a secondary emission electron multiplier having an electron gain of approximately several hundred. The electron gain is primarily controlled by applying a potential difference across the input and output surfaces of the microchannel plate
24
of about 900 volts.
Electrons exiting the microchannel plate
24
are accelerated across the gap
36
toward the phosphor screen
30
by the potential difference applied between the output surface
28
of the microchannel plate
24
and the phosphor screen
30
. This potential difference is approximately 6000 volts. As the exiting electrons impinge upon the phosphor screen
30
, many photons are produced per electron. The photons create an intensified output image on the output surface of the optical inverter or fiber optics element
31
.
A method of manufacturing a GEN
3
image intensifier photocathode according to the prior art is illustrated in
FIGS. 2
a
-
2
e
. In a series of steps, a buffer layer or layers
42
are grown on a single crystal substrate
40
and an etching stop layer
44
is provided over the buffer layer
42
, as illustrated in
FIG. 2
a
. These three layers
40
,
42
,
44
will be subsequently removed during the formation of the faceplate/photocathode structure, and the stop layer
44
facilitates the removal of the substrate
40
during a later processing step. The substrate
40
is typically formed from GaAs, and the stop layer
44
is typically formed from a different composition than the substrate
40
, such as Al
x
Ga
1−x
As.
As illustrated in
FIG. 2
b
, a p-type conductivity GaAs active layer
18
is deposited on the Al
x
Ga
1−x
As stop layer
44
. This is typically accomplished using organometallic chemical vapor deposition. After deposition of the active layer
18
, an Al
x
Ga
1−x
As window layer
17
is deposited over the active layer
18
, as illustrated in
FIG. 2
c
. Additionally, an antireflective coating
16
can be formed over the window layer
17
. This complete structure forms a photocathode wafer
12
a.
As illustrated in
FIG. 2
d
, the photocathode wafer
12
a
is bonded to a faceplate
14
. This is typically accomplished by heating the photocathode wafer
12
a
and faceplate
14
to a temperature close to the softeni
ITT Manufacturing Enterprises Inc.
Patel Ashok
Zimmerman Glenn
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