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Description  |
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FIELD OF THE INVENTION
The present invention pertains to multicolor organic LED arrays and more
specifically to a new method of fabricating multicolor organic LED arrays.
BACKGROUND OF THE INVENTION
Organic light emitting diodes (LEDs) are becoming very popular because of
the simplicity of fabricating the LEDs and the relatively low cost of
materials. While multi-color organics, i.e., organics that emit different
colors when properly activated, are known, generally all prior art
disclosures of organic LEDs discuss a single color LED. To form
multi-color displays and the like using the teachings of the prior art
requires extensive fabrication techniques with each LED, or each color of
LED, being formed separately on a substrate.
Accordingly, it would be highly advantageous to develop a method of
fabrication, and fabricated color LED array, which is simple and
inexpensive.
It is a purpose of the present invention to provide a new and improved
method of fabricating an integrated multicolor organic LED array and the
integrated multicolor organic LED array fabricated.
It is a further purpose of the present invention to provide a new and
improved method of fabricating an integrated multicolor organic LED array
which is relatively simple and inexpensive.
It is a still further purpose of the present invention to provide a new and
improved method of fabricating an integrated multicolor organic LED array
which is easily integrated into drive circuits using well known
technology.
It is another purpose of the present invention to provide a new and
improved method of fabricating an integrated multicolor organic LED array
which requires fewer steps and, therefore, is less labor intensive.
SUMMARY OF THE INVENTION
The above problems and others are at least partially solved and the above
purposes and others are realized in a method of fabricating an integrated
multicolor organic LED array including the steps of providing a negative
electrical layer having a surface and patterning a plurality of different
color LED organic layers, one at a time, on the surface of the negative
electrical layer to form a plurality of different color LEDs in a
plurality of areas of a selected array, a first color LED organic layer of
the plurality of organic layers being patterned on the surface of the
negative electrical layer in first areas and to define additional areas in
the selected array for additional LEDs laterally separated from the first
color LEDs in the first areas and a final color LED organic layer of the
plurality of organic layers being deposited on the surface in defined
final areas of the additional areas and on previously patterned color
organic layers to form a plurality of final color LEDs in the defined
final areas of the selected array. First transparent positive electrical
contacts are then formed on the final color LED layer in the first areas
so as to form positive electrical contacts to the plurality of first color
LEDs and final transparent positive electrical contacts are formed on the
final color LED layer in the final areas so as to form positive electrical
contacts to the plurality of final color LEDs.
The above problems and others are at least partially solved and the above
purposes and others are further realized in an integrated multicolor
organic LED array including a negative electrical layer having a surface
and a plurality of different color LED organic layers positioned on the
surface of the negative electrical layer to form a plurality of different
color LEDs in a plurality of areas of a selected array. A first color LED
organic layer of the plurality of organic layers is patterned on the
surface of the negative electrical layer in a first area and so as to
define additional areas in the selected array for additional LEDs
laterally separated from the first color LEDs in the first areas. A final
color LED organic layer of the plurality of organic layers is deposited on
the surface in defined final areas of the additional areas and on
previously patterned color organic layers to form a plurality of final
color LEDs in the defined final areas of the selected array. First
transparent positive electrical contacts are positioned on the final color
LED layer in the first areas so as to form positive electrical contacts to
the plurality of first color LEDs and final transparent positive
electrical contacts are positioned on the final color LED layer in the
final areas so as to form positive electrical contacts to the plurality of
final color LEDs.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings wherein like characters indicate like parts
throughout the various figures:
FIG. 1 is an enlarged, simplified sectional view of an intermediate
structure in the fabrication of an integrated multicolor organic LED array
in accordance with the present invention, portions thereof broken away;
FIG. 2 is a view in top plan of the structure of FIG. 1;
FIG. 3 is an enlarged, simplified sectional view similar to FIG. 1,
illustrating a next step in the fabrication process;
FIG. 4 is a view in top plan of the structure of FIG. 3;
FIG. 5 is an enlarged, simplified sectional view similar to FIG. 3,
illustrating a next step in the fabrication process;
FIG. 6 is an enlarged, simplified sectional view similar to FIG. 5,
illustrating a next step in the fabrication process
FIG. 7 is a view in top plan of the structure of FIG. 6;
FIGS. 8 and 9 are greatly enlarged top plan and sectional views,
respectively, of a portion of an edge of an integrated multicolor organic
LED array illustrated similar to that illustrated in FIGS. 6 and 7; and
FIG. 10 is a simplified view in top plan of the structure of FIG. 6
integrated into a display.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring specifically to FIG. 1, an enlarged simplified sectional view is
illustrated of an intermediate structure in the fabrication of an
integrated multicolor organic LED array 10 in accordance with the present
invention. The structure illustrated in FIG. 1 includes a supporting
member or layer of material, which for convenience is referred to herein
as a substrate 12. In a preferred embodiment, as will be explained more
fully later, substrate 12 includes a semiconductor material such as
silicon, or gallium arsenide.
A negative contact layer 13 is positioned on the surface of substrate 12
and includes either a layer of metal or heavily doped areas adjacent the
surface of substrate 12. In this specific embodiment, an optional electron
transport layer 14 is positioned on the surface of layer 13. Layers 12 and
13 are referred to herein jointly as a negative electrical layer or
contact and serve as the negative terminal of diode array 10. It should be
understood that electron transport layer 14 is generally very thin (in the
range of approximately 200-700 angsttoms) so that electrons are
predominantly transported vertically (across layer 14) therethrough with
little or no lateral or horizontal movement in layer 14. Thus, to form row
connections negative contact layer 13 is formed into rows, as can be seen
most clearly in FIG. 2, and electron transport layer 14 is formed in a
blanket deposition to provide a generally planar upper surface 16 for
convenient deposition of additional layers.
An organic layer or layers 15 includes one or more layers of polymers or
low molecular weight organic compounds. Hereinafter, for simplification of
this disclosure, the term organic/polymer will be shortened to "organic"
but it should be understood that this term is intend to encompass all
polymers or low molecular weight organic compounds. The organic materials
that form layers 15 are chosen for their combination of electrical,
luminescent and color properties, and various combinations of hole
transporting, electron transporting, and luminescent materials can be
used. In this embodiment, for example, layer 15 is a luminescent hole
transport layer and layer 14 is an electron transport layer which
cooperates with layer 15 to provide the desired light emission. Also, in
this specific embodiment layer 15 is chosen from a material structure with
an electroluminescent emission spectrum having a dominant wavelength in
the red, so as to emit red light when properly energized and will be
referred to hereinafter as a red organic layer.
Referring specifically to FIG. 2, it can be seen that, in this specific
embodiment, organic layer or layers 15 are patterned in columns or strips
which are spaced horizontally apart with areas of surface 16 of electron
transport layer 14 exposed therebetween.
Referring specifically to FIGS. 3 and 4, a second organic layer or layers
20 including one or more layers of polymers or low molecular weight
organic compounds is patterned onto the exposed surface 16 of electron
transport layer 14. Layer 20 is again patterned in columns or strips, as
can be seen more clearly in FIG. 4, which are spaced horizontally apart
with areas of surface 16 of electron transport layer 14 exposed
therebetween. It should be noted that layer 20 is also formed over the
surface of layer 15. Further, in this specific embodiment layer 15 is
chosen so as to emit green light when properly energized and will be
referred to hereafter as a green organic layer.
Referring specifically to FIG. 5, a third, or final, organic layer or
layers 25 including one or more layers of polymers or low molecular weight
organic compounds is deposited onto the exposed surface 16 of electron
transport layer 14 and on the surface of layer 20. Because layer 20 was
patterned in columns or strips so as to leave strips of surface 16 of
electron transport layer 14 exposed, layer 25 is also deposited on surface
16 of electron transport layer 14 in strips. Also, in this specific
embodiment layer 25 is chosen so as to emit blue light when properly
energized and will be referred to hereafter as a blue organic layer.
In general, in organic electroluminescent or LED devices it should be
understood that organic layers 15, 20 and 25 do not conduct electrons well
and the electron resistivities (e.g., approximately 10e.sup.-7) are much
higher than the hole resistivities (e.g., approximately 10e.sup.-3) in the
same material. Also, electron transport layer 14 conducts electrons
relatively well but does not conduct holes well and can thus be thought of
as a hole blocking layer. Further, it should be understood that generally
light, or photons, is generated when electrons and holes combine. Thus,
because holes are transported readily through organic layers 15, 20 and 25
and because electrons are transported readily through electron transport
layer 14, substantially all recombination of holes and electrons occurs at
or near surface 16 of layer 14. Further, the color of the light emitted is
dependent upon the electroluminescent spectrum of the organic layer in
which the recombination takes place. Thus, recombinations in layer 25 near
surface 16 results in blue light being emitted. Similarly, recombinations
in layer 20 near surface 16 results in green light being emitted and
recombinations in layer 15 near surface 16 results in red light being
emitted. In this specific embodiment, layer 15 for the red LED is
positioned at the bottom with layer 25 for the blue LED deposited last
because any effect (absorption, etc.) the extra layers might have on light
passing therethrough will be minimized by the longer wavelengths of light
passing through the shorter wavelength materials. More specifically, the
absorption spectrum of the shorter wavelength emitting materials (i.e.
green, blue) is poor in the longer wavelengths which the materials below
it are emitting. This is further assisted by the fact that each of these
layers (15, 20 and 25) is thin, generally in the range of approximately
200-1000 angstroms.
Thus, by selecting organic materials in which the hole conductivity is
sufficiently high (which includes substantially all of the known organic
luminescent materials), and by forming layers 15, 20 and 25 relatively
thin, the presence of layers 20 and 25 over layer 15 in the formation of
red LEDs and layer 25 over layer 20 in the formation of green LEDs has
very little effect. This feature allows a substantial simplification of
the fabrication of multicolor organic layers for an LED array because it
is not necessary to protect the earlier deposited organic layers from
subsequently deposited layers.
The presence of additional organic layers in the structure, not near the
interface or surface 16, only effects the resulting structure to the
extent of any voltage drop across the additional layers. Thus, in the
event that it is desired to utilize thicker organic layers or if it is
necessary in specific applications to reduce the voltage drop across the
additional layers, a partial etch can be utilized to thin the organic
layers overlying a previously patterned organic layer. As an example and
referring to FIG. 3, the portion of layer 20 overlying layer 15 could be
thinned before applying layer 25. Further, the portions of layer 25
overlying layer 20 could be thinned if desired.
It should also be noted that lateral spreading of current in organic layers
15, 20 and 25 is small enough that it is generally unnecessary to etch or
separate the organic layers 15, 20 and 25 into individual LEDs. Thus, for
example, layer 15 is patterned into columns but there is no necessity to
further separate the column into individual LEDs. This feature allows a
further and substantial simplification of the fabrication of multicolor
organic layers for LED array 10.
Referring specifically to FIG. 6, positive terminals for the blue LEDs are
formed by depositing columns 30 of conductive material, such as metal, on
the surface of organic layer 25 overlying the areas in which organic layer
25 is in contact with surface 16 of electron transport layer 14.
Similarly, positive electrical contacts for the green and red LEDs are
formed by depositing columns 35 and 40 of conductive material on the
surface of organic layer 25 overlying the areas in which organic layers 20
and 15, respectively, are in contact with surface 16 of electron transport
layer 14. Thus, because layers 14, 15, 20 and 25 are formed relatively
thin and because there is very little lateral current flow in any of these
layers, current flows between the rows formed in layer 13 and columns 30,
35 and 40 only at the crossover points. Therefore, a multicolor organic
LED array is described wherein three different colored LEDs are
addressably formed into pixels in rows and columns.
It should be understood that while three color LEDs are illustrated in a
specific embodiment to illustrate full color pixels and, hence, a full
color LED array, other LED arrays can be fabricated including simplified
versions in which only two colors are utilized. Further, it should also be
understood that various individual colored LEDs may be included as pixels
in an array for some specific applications.
Generally, either the anode (positive electrical contacts) or the cathode
(negative electrical contacts) of an LED must be optically transparent to
allow the emission of light therethrough. In this embodiment conductive
layers 30, 35 and 40 are formed of indium-tin oxide (ITO) which is
optically transparent. In some applications a very thin metal film may be
used as a transparent conductor instead of the ITO. Also, to reduce the
potential required in embodiments not incorporating electron transport
layer 14, the cathode is generally formed of a low work function
metal/conductors or combination of metals/conductors, at least one of
which has a low work function. In this embodiment the cathode is formed of
low work function material, such as heavily doped diamond, or the cathode
may be a conductive metal incorporating cesium, calcium or the like.
A list of some possible examples of materials for the organic layer or
layers 15, 20 and 25 of the above described organic LEDs follows. As a
single layer of organic, some examples are: poly (.rho.-phenylenevinylene)
(PPV); poly (.rho.-phenylene) (PPP); and poly [2-methoxy,
5-(2'-ethylhexoxy) 1,4-phenylenevinylene] (MEH-PPV). As an electron
transporting electroluminescent layer between a hole transporting layer or
one of the single layer organics listed above and a low work function
metal cathode, an example is: 8-hydroxquinoline aluminum (ALQ). As an
electron transporting material, an example is:
2-(4-tert-butylphenyl)-5-(.rho.-biphenylyl)-1,3,4-oxadiazole (butyl-PBD).
As a hole transport material, some examples are:
4,4'-bis[N-phenyl-N-(3-methylphenyl)amino]biphenyl (TPD); and 1,1-bis
(4-di-p-tolyaminophenyl)cyclohexane. As an example of a fluorescent that
may be used as a single layer or as a dopant to an organic charge
transporting layer is coumarin 540, and a wide variety of fluorescent
dyes. Examples of low work function metals include: Mg:In, Ca, and Mg:Ag.
Referring to FIG. 7, a view in top plan of LED array 10 is illustrated.
Each row of layer 13 has an external connection, represented herein as a
terminal 42, with alternate terminals being accessible at opposite ends to
allow the maximum amount of room for connection pads. Also, each column
30, 35 and 40 have an external connection, represented herein as a
terminal 43, with alternate terminals being accessible at opposite ends.
Array 45 includes a plurality of pixels 45 (outlined in heavy broken
lines) each including a red, green and blue LED. Thus, it should be
understood that by properly addressing each of the pixels and each of the
LEDs in each pixel, a complete color image can be generated by array 10.
Referring to FIGS. 8 and 9, one example of very simple integrated drive
circuitry is disclosed in simplified top plan and sectional views,
respectively. In this specific embodiment a single field effect transistor
(FET) 50 is integrated onto substrate 12 and utilized to provide drive
current to each row of negative contact layer 13. In this simplified
example, a source 51 of FET 50 is a doped area in substrate 12 which is
formed in contact with a specific row of layer 13. A drain 52 of FET 50 is
a doped area which is connected to a relatively negative potential, such
as ground. A gate 53 of FET 50 is connected to an external connection pad
54. In operation the rows of layer 13 are sequentially turned on by
supplying an appropriate potential to the external pad 54 to turn on FET
50 and connect the selected row of layer 13 to ground potential. It should
of course be understood that FET 50 is only for purposes of example to
illustrate the ease with which circuitry can be integrated with multicolor
organic LED arrays fabricated in accordance with the present invention.
Referring specifically to FIG. 10, a display source 60 is illustrated in
top plan. Display source 60 includes a multicolor organic LED array 62,
which is a plurality of pixels 45 arranged in rows and columns. Display
source 60 further includes a column decoder 66 electrically connected to
pixels 45 and having a plurality of signal input terminals 68 and a row
decoder 70 electrically connected to pixels 45 and having a plurality of
signal input terminals 72. In this specific embodiment, each pixel 45
includes three light emitting organic elements, or diodes, with driver
portions connected to be operated by column and row decoders 68 and 70. By
addressing specific pixels 45 by row and column in a well known manner,
the specific pixels 45 are energized to produce a real image. Further, by
addressing each color LED in each pixel 45, each pixel 45 will produce a
different desired color. Digital or analog data is received at an input
terminal and converted by data processing circuits, not shown, into
signals capable of energizing selected pixels 45 to generate the
predetermined real image. Thus, image data is supplied to input terminals
68 and 72 and multicolor organic LED array 62 produces a desired colored
image, which image may be continuous or changing in accordance with the
image data.
Display source 60 is illustrated simply as a display and decoders but it
will be understood that additional image generation apparatus, including
data processing circuits may be included on the same or additional chips.
Data processing circuits generally include logic and switching circuit
arrays, some of which are incorporated into decoders 66 and 70, for
controlling each pixel 45. Data processing circuits include, in addition
to or instead of the logic and switching arrays, a microprocessor or
similar circuitry for processing input signals to produce a desired real
colored image on multicolor organic LED array 62. It will be understood by
those skilled in the art that multicolor organic LED array 62 and decoders
66 and 70 are greatly enlarged in FIG. 10. The actual size of a
semiconductor chip containing the entire image generation apparatus is on
the order of a few milli-meters along each side with each LED in each
pixel 45 being on the order of as little as ten microns or less on a side.
Thus, a new and improved multicolor organic LED array is disclosed which is
relatively simple to fabricate, because many steps required in the
fabrication of prior art LEDs are eliminated. These advantages are
realized generally because of the low electron transport and high hole
transport of the organic materials, the hole blocking effect of the
electron transport material and the low lateral current conduction of the
organic materials. The lateral current conduction is further reduced by
the small thickness of the active layers. Generally, because of these
features the various organic layers can be deposited in overlying
relationship and there is no necessity to isolate individual LEDs.
However, in some specific examples, it may be desirable to isolate
individual LEDs and this can be accomplished with an oxygen etch. The
effects of depositing organic layers in overlying relationship are further
reduced by organizing the various colored LEDs in order of color or
wavelength, e.g. red at the bottom with blue on top, so that minimal
absorption of the colors occurs in the overlying layers.
While we have shown and described specific embodiments of the present
invention, further modifications and improvements will occur to those
skilled in the art. We desire it to be understood, therefore, that this
invention is not limited to the particular forms shown and we intend in
the append claims to cover all modifications that do not depart from the
spirit and scope of this invention.
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