Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Isolation by pn junction only
Reexamination Certificate
1998-07-30
2002-08-20
Patel, Nimeshkumar D. (Department: 2879)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Isolation by pn junction only
C438S020000, C438S139000, C313S495000, C313S309000
Reexamination Certificate
active
06436788
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to visual displays for electronic devices and more particularly to an improved emitter substructure for active matrix field emission displays.
BACKGROUND OF THE INVENTION
FIG. 1
is a simplified side cross-sectional view of a portion of a display
10
including a faceplate
20
and a baseplate
21
in accordance with the prior art.
FIG. 1
is not drawn to scale. The faceplate
20
includes a transparent viewing screen
22
, a transparent conductive layer
24
and a cathodoluminescent layer
26
. The transparent viewing screen
22
supports the layers
24
and
26
, acts as a viewing surface and as a wall for a hermetically sealed package formed between the viewing screen
22
and the baseplate
21
. The viewing screen
22
may be formed from glass. The transparent conductive layer
24
may be formed from indium tin oxide. The cathodoluminescent layer
26
may be segmented into pixels yielding different colors for color displays. Materials useful as cathodoluminescent materials in the cathodoluminescent layer
26
include Y
2
O
3
:Eu (red, phosphor P-
56
), Y
3
(Al, Ga)
5
O
12
:Tb (green, phosphor P-
53
) and Y
2
(SiO
5
):Ce (blue, phosphor P-
47
) available from Osram Sylvania of Towanda Pa. or from Nichia of Japan.
The baseplate
21
includes emitters
30
formed on a planar surface of a semiconductor substrate
32
. The substrate
32
is coated with a dielectric layer
34
. In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer
34
is formed to have a thickness, measured in a direction perpendicular to a surface of the substrate
32
as indicated by direction arrow
36
, that is approximately equal to or just less than a height of the emitters
30
. This thickness is on the order of 0.4 microns, although greater or lesser thicknesses may be employed. An extraction grid
38
comprising a conductive material is formed on the dielectric layer
34
. The extraction grid
38
may be realized, for example, as a thin layer of polysilicon. The radius of an opening
40
created in the extraction grid
38
, which is also approximately the separation of the extraction grid
38
from the tip of the emitter
30
, is about 0.4 microns, although larger or smaller openings
40
may also be employed. This separation is defined herein to mean being “in close proximity.”
Another dielectric layer
42
is formed on the extraction grid
38
. A chemical isolation layer
44
, such as titanium, is formed on the dielectric layer
42
. A soft X-ray blocking layer
46
, such as tungsten, is formed on the chemical isolation layer
44
for reasons that will be explained below.
The baseplate
21
also includes a field effect transistor (“FET”)
50
formed in the surface of the substrate
32
for controlling the supply of electrons to the emitter
30
. The FET
50
includes an n-tank
52
formed in the surface of the substrate
32
beneath the emitter
30
. The n-tank
52
serves as a drain for the FET
50
, and may be formed via conventional masking and ion implantation processes. The FET
50
also includes a source
54
and a gate electrode
56
. The gate electrode
56
is separated from the substrate
32
by a gate oxide layer
57
and a field oxide layer
58
.
The substrate
32
may be formed from p-type silicon material having an acceptor concentration N
A
ca. 1-5×10
15
/cm
3
, while the n-tank
52
may have a surface donor concentration N
D
ca. 1-2×10
16
/cm
3
. A depletion region
60
is formed at a p-n junction between the n-tank
52
and the p-type substrate
32
. The depletion region
60
provides electrical isolation from other circuitry contained on or integrated in the substrate
32
. These values for the acceptor and donor concentrations allow the FET
50
to operate at the voltages required for displays
10
and provides a higher avalanche breakdown voltage than would be provided by, e.g., transistors used in conventional CMOS logic circuitry. The capacitance of the depletion region
60
is reduced compared to that of conventional logic circuitry because the doping levels are less and the operating voltages are higher, resulting in a larger depletion region
60
than would exist for transistors used in conventional logic circuitry. This provides increased electrical isolation of the FET
50
from other circuitry integrated into the substrate
32
, compared to transistors used in conventional logic circuitry.
In operation, the extraction grid
38
is biased to a voltage on the order of 40-80 volts, although higher or lower voltages may be used, while the substrate
32
is maintained at a voltage of about zero volts. Signals coupled to the gate
56
of the FET
50
turn the FET
50
on, allowing electrons to flow from the source
54
to the n-tank
52
and thus to the emitter
30
. Intense electrical fields between the emitter
30
and the extraction grid
38
then cause field emission of electrons from the emitter
30
. A larger positive voltage, ranging up to as much as 5,000 volts or more but often 2,500 volts or less, is applied to the faceplate
20
via the transparent conductive layer
24
. The electrons emitted from the emitter
30
are accelerated to the faceplate
20
by this voltage and strike the cathodoluminescent layer
26
. This causes light emission in selected areas, i.e., those areas adjacent to where the FETs
50
are conducting, and forms luminous images such as text, pictures and the like. Integrating the FETs
50
in the substrate
32
to provide an active display
10
yields advantages in size, simplicity and ease of interconnection of the display
10
to other electronic componentry.
Visible photons from the cathodoluminescent layer
26
and photons that travel through the faceplate
20
can also travel back through the openings
40
. When photons travel through portions of the extraction grid
38
that are exposed by the openings
40
and impinge on the depletion region
60
, electron-hole pairs are generated. When electron-hole pairs are produced within the depletion region
60
associated with the p-n junction between the n-tank
52
and the p-type substrate
32
, the electrons and holes are efficiently separated by the electrical fields associated with the depletion region
60
. The electrons are swept into the n-tank
52
and the holes are swept into the p-type substrate
32
surrounding the n-tank
52
. The electrons provide an undesirable component to electrons emitted by the emitter
30
. This results in distortion in the images produced by the display
10
.
For example, a blue pixel emitting blue light could provide a photon that reaches semiconductor material underlying the emitter
30
associated with an adjacent red pixel, which is not intended to be emitting light. This may cause an emitter current component resulting in an anode current in the red pixel, thus providing unwanted red light and thereby distorting the color intended to be displayed.
Alternatively, an area intended to be a dark area in the display
10
may emit light when that area is exposed to high ambient light conditions. These effects are undesirable and tend to reduce display dynamic range in addition to distorting the intended image.
There is therefore a need for a way to render p-n junctions associated with monolithic emitters less sensitive to incident photons for use in field emission displays.
SUMMARY OF THE INVENTION
Various aspects of the present invention include an emitter substrate and methods for manufacturing the substrate as well as displays incorporating the substrate and a computer using the substrate. The inventive substrate includes a semiconductor material of one type in which a tank of the opposite type semiconductor material is formed. An emitter is formed on and electrically coupled to the tank. An insulating region is formed at a lower boundary of the tank. The insulating region electrically isolates the emitter and the tank along at least a portion of the lower boundary. As a result, a depletion region associated with a boundary betwee
Lee John K.
Moradi Behnam
Dorsey & Whitney LLP
Micro)n Technology, Inc.
Patel Nimeshkumar D.
Quarterman Kevin
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