U.S. patent application number 10/135182 was filed with the patent office on 2003-01-30 for method of making light emitting diode displays.
This patent application is currently assigned to Kopin Corporation. Invention is credited to Dingle, Brenda, Fan, John C. C., McClelland, Robert W., Shastry, Shambhu, Spitzer, Mark B..
Application Number | 20030020084 10/135182 |
Document ID | / |
Family ID | 24581294 |
Filed Date | 2003-01-30 |
United States Patent
Application |
20030020084 |
Kind Code |
A1 |
Fan, John C. C. ; et
al. |
January 30, 2003 |
Method of making light emitting diode displays
Abstract
Light emitting diodes (LEDs) and LED bars and LED arrays formed
of semiconductive material such as III-V and particularly
AIGaAs/GaAs material are formed in very thin structures using
organometallic vapor deposition (OMCVD). Semiconductor p-n
junctions are formed as deposited using carbon as the p-type
impurity dopant. Various lift-off methods are described which
permit back side processing when the growth substrate is removed
and also enabled device registration for LED bars and arrays to be
maintained.
Inventors: |
Fan, John C. C.; (Chestnut
Hill, MA) ; Dingle, Brenda; (Mansfield, MA) ;
Shastry, Shambhu; (Franklin, MA) ; Spitzer, Mark
B.; (Sharon, MA) ; McClelland, Robert W.;
(Norwell, MA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
Kopin Corporation
Taunton
MA
|
Family ID: |
24581294 |
Appl. No.: |
10/135182 |
Filed: |
April 29, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10135182 |
Apr 29, 2002 |
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08363150 |
Dec 23, 1994 |
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6403985 |
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08363150 |
Dec 23, 1994 |
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08165025 |
Dec 9, 1993 |
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5453405 |
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08165025 |
Dec 9, 1993 |
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07643552 |
Jan 18, 1991 |
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5300788 |
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Current U.S.
Class: |
257/92 ;
257/E21.614; 257/E25.021; 257/E27.026; 257/E27.111; 257/E33.068;
348/E5.141; 348/E5.143; 348/E5.145 |
Current CPC
Class: |
G02F 1/13336 20130101;
H01L 2221/68363 20130101; H01L 2924/01039 20130101; H01L 2924/00
20130101; H01L 2924/00 20130101; G02F 2202/105 20130101; H01L
2221/68359 20130101; H01L 2924/01006 20130101; H01L 2924/01077
20130101; H01L 2924/10349 20130101; H01L 2924/01005 20130101; H01L
2924/12041 20130101; H01L 27/1266 20130101; H01L 2924/01082
20130101; H01L 2924/00 20130101; G02F 1/13454 20130101; H01L
2924/01032 20130101; G09G 3/36 20130101; H01L 2924/01049 20130101;
Y10S 438/928 20130101; H01L 33/0062 20130101; H01L 2924/351
20130101; H01L 2924/01027 20130101; G02B 2027/0187 20130101; G09G
3/3607 20130101; H01L 2924/01015 20130101; G02B 5/30 20130101; G09G
2370/042 20130101; G02B 27/0172 20130101; H01L 2924/01074 20130101;
G09G 3/32 20130101; H01L 33/10 20130101; H01L 2924/01013 20130101;
H01L 2924/01078 20130101; H01L 2924/09701 20130101; H01L 2924/12042
20130101; G09G 2300/023 20130101; H01L 24/96 20130101; H01L 33/465
20130101; H01S 5/0217 20130101; Y10S 438/977 20130101; H01L 33/405
20130101; G02B 2027/0198 20130101; H01L 2924/01051 20130101; H01L
27/0688 20130101; H01L 27/156 20130101; H05B 33/12 20130101; H01L
2924/01014 20130101; H01L 21/8221 20130101; H01S 5/423 20130101;
H01L 25/50 20130101; H01L 2924/01012 20130101; H01L 2924/01019
20130101; H01L 2924/01322 20130101; H01L 2924/01023 20130101; G02B
2027/0132 20130101; G02B 27/017 20130101; H01L 2924/0103 20130101;
H01L 2924/10329 20130101; H04N 5/7491 20130101; H01L 2924/12041
20130101; H01L 2924/01075 20130101; H01L 2924/01072 20130101; H01L
2924/12042 20130101; G09G 3/3648 20130101; H01L 25/0756 20130101;
H01L 2924/01033 20130101; H01L 2924/10336 20130101; G02B 27/0093
20130101; H01L 2924/14 20130101; H04N 5/7441 20130101; H01L 27/1214
20130101; G02B 2027/0138 20130101; H01L 2924/351 20130101; H01L
21/84 20130101; H04N 9/3141 20130101 |
Class at
Publication: |
257/92 |
International
Class: |
H01L 033/00 |
Claims
What is claimed is:
1. A light emitting array alphanumeric display device comprising: a
plurality of light emitting pixel diode elements formed with a
III-V semiconductor material, the pixel elements positioned to form
an array of columns and rows of an alphanumeric display device; a
column driver circuit formed with a silicon material and connected
to the columns of pixel elements; a row driver circuit formed with
a silicon material and connected to the rows of pixel elements; and
a display control circuit connected to the column driver circuit
and the row driver circuit, the display control circuit selectively
actuating the pixel elements to display alphanumeric
information.
2. The light emitting alphanumeric display device of claim 1
wherein the array is positioned over a silicon layer with which the
column driver circuit and row driver circuit are formed, the array
being monolithically interconnected to the column driver circuit
and the row driver circuit.
3. The light emitting alphanumeric display device of claim 1
wherein the display device is mounted to a leadless chip
carrier.
4. The light emitting alphanumeric display device of claim 1
further comprising a video input circuit.
5. The light emitting alphanumeric display device of claim 1
wherein the III-V material comprises gallium arsenide.
6. The light emitting alphanumeric display device of claim 1
wherein the array is bonded to the silicon material with an
adhesive layer.
7. The light emitting alphanumeric display device of claim 1
wherein the thickness of the light emitting diodes is less than 5
microns.
8. The light emitting alphanumeric display device of claim 1
further comprising a lens positioned to magnify an image generated
on the display.
9. The light emitting alphanumeric display device of claim 1
further comprising a support that mounts the display on a user's
head.
10. The light emitting alphanumeric display device of claim 1
wherein there are more than about 1000 pixel elements.
11. The light emitting alphanumeric display device of claim 1
wherein the column driver circuit and the row driver circuit are
formed from a common layer of silicon material.
12. The light emitting alphanumeric display device of claim 1
wherein the common substrate is a silicon substrate.
13. A method of forming a light emitting matrix display device
comprising: forming a plurality of light emitting pixel elements
with a light emitting material, the pixel elements positioned to
form an array of columns and rows of a matrix display device;
forming a column driver circuit with a silicon material; forming a
row driver circuit formed with a silicon material; connecting the
column driver circuit to the columns of pixel elements and
connecting the row driver circuit to the rows of pixel elements;
and connecting the column driver circuit and the row driver circuit
to a display control circuit, the display control circuit
selectively actuating the pixel elements to display alphanumeric
information.
14. The method of claim 13 further comprising forming the light
emitting pixel elements in a gallium arsenide material.
15. The method of claim 13 further comprising transferring the
light emitting array from a first substrate to mount the array over
a region of the silicon material.
16. The method of claim 15 further comprising bonding the array to
the silicon material with an adhesive.
17. The method of claim 13 further comprising mounting the display
to a support that mounting on a user's head.
18. A light emitting display device comprising: a plurality of
light emitting pixel elements formed from a light emitting
material, the pixel elements positioned to form an array of columns
and rows of a display panel; a column driver circuit formed from a
layer of semiconductor material and connected to the columns of
pixel elements; a row driver circuit formed from the layer of
semiconductor material and connected to the rows of pixel elements;
an optically transparent substrate to which the array of pixel
elements, the column driver circuit and the row driver circuit are
mounted; and a display control circuit connected to the column
driver circuit and the row driver circuit, the display control
circuit selectively actuating the pixel elements to display an
image.
19. The device of claim 18 wherein the substrate comprises
glass.
20. The device of claim 18 wherein the device comprises a personal
communications device.
21. The device of claim 18 wherein the device comprises a
telephone.
22. The device of claim 18 wherein the pixel elements comprise an
array of light emitting diodes in a thin film of gallium arsenide
material.
23. The device of claim 18 wherein the column driver and the row
driver are connected to a microprocessor circuit.
24. The device of claim 18 further comprising a display input
circuit formed with the layer of semiconductor material.
25. The device of claim 18 further comprising a lens to magnify an
image generated on the display.
26. The device of claim 18 further comprising an adhesive to bond
the display panel to the substrate.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 08/363,150, filed Dec. 23, 1994, which is a continuation of
U.S. application Ser. No. 08/165,025, filed Dec. 9, 1993, now U.S.
Pat. No. 5,453,405, which is a divisional of U.S. application Ser.
No. 07/643,552 filed Jan. 18, 1991, now U.S. Pat. No. 5,300,788.
The entire teachings of the above applications are incorporated
herein by reference.
TECHNICAL FIELD
[0002] This invention is in the field of light emitting diodes
(LEDs).
BACKGROUND OF THE INVENTION
[0003] The background to the invention may be conveniently
summarized in connection with four main subject matters: LEDs, LED
bars, LED arrays and Lift-off methods, as follows:
[0004] Leds
[0005] LEDs are rectifying semiconductor devices which convert
electric energy into non-coherent electromagnetic radiation. The
wavelength of the radiation currently extends from the visible to
the near infrared, depending upon the bandgap of the semiconductor
material used.
[0006] Homojunction LEDs operate as follows: For a zero-biased p-n
junction in thermal equilibrium, a built-in potential at the
junction prevents the majority charge carriers (electrons on the n
side and holes on the p side) from diffusing into the opposite side
of the junction. Under forward bias, the magnitude of the potential
barrier is reduced. As a result, some of the free electrons on the
n-side and some of the free holes on the p-side are allowed to
diffuse across the junction. Once across, they significantly
increase the minority carrier concentrations. The excess carriers
then recombine with the majority carrier concentrations. This
action tends to return the minority carrier concentrations to their
equilibrium values. As a consequence of the recombination of
electrons and holes, photons are emitted from within the
semiconductor. The energy of the released photons is close in value
to that of the energy gap of the semiconductor of which the p-n
junction is made. For conversion between photon energy (E) and
wavelength (.lambda.), the following equation applies: 1 E ( eV ) =
1.2398 ( m )
[0007] The optical radiation generated by the above process is
called electroluminescence. The quantum efficiency .eta. for a LED
is generally defined as the ratio of the number of photons produced
to the number of electrons passing through the diode. The internal
quantum efficiency .eta..sub.i is evaluated at the p-n junction,
whereas the external quantum efficiency .eta..sub.e is evaluated at
the exterior of the diode. The external quantum efficiency is
always less than the internal quantum efficiency due to optical
losses that occur before the photons escape from the emitting
surface. Some major causes for the optical losses include internal
re-absorption and absorption at the surface. The internal
efficiency can exceed 50% and, sometimes, can be close to 100% for
devices made of a very high-quality epitaxial material. The
external quantum efficiency for a conventional LED is much lower
than the internal quantum efficiency, even under optimum
conditions.
[0008] Most commercial LEDs, both visible and infrared, are
fabricated from group III-V compounds. These compounds contain
elements such as gallium, indium and aluminum of group III and
antimony, arsenic and phosphorus of group V of the periodic table.
With the addition of the proper impurities, by diffusion, or
grown-in; III-V compounds can be made p- or n-type, to form p-n
junctions. They also possess the proper range of band gaps to
produce radiation of the required wavelength and efficiency in the
conversion of electric energy to radiation. The fabrication of LEDs
begins with the preparation of single-crystal substrates usually
made of gallium arsenide, about 250-350 .mu.m thick. Both p- and
n-type layers are formed over this substrate by depositing layers
of semiconductor material from a vapor or from a melt.
[0009] The most commonly used LED is the red light-emitting diode,
made of gallium arsenide-phosphide on gallium arsenide substrates.
An n-type layer is grown over the substrate by vapor-phase
deposition followed by a diffusion step to form the p-n junction.
Ohmic contacts are made by evaporating metallic layers to both--and
p-type materials. The light resulting from optical recombination of
electrons and holes is generated near the p-n junction. This light
is characterized by a uniform angular distribution; some of this
light propagates toward the front surface of the semiconductor
diode. Only a small fraction of the light striking the top surface
of the diode is at the proper angle of incidence with respect to
the surface for transmission beyond the surface due to the large
difference in the refractive indices between semiconductor and air.
Most of the light is internally reflected and absorbed by the
substrate. Hence a typical red LED has only a few percent external
quantum efficiency, that is, only a few percent of the electric
energy results in external light emission. More efficient and
therefore brighter LEDs can be fabricated on a gallium phosphide
substrate, which is transparent to the electroluminescent radiation
and permits the light to escape upon reflection from the back
contact. For brighter LEDs, AlGaAs, with the Al percentage equate
to 0-38%, grown on GaAs substrates is used. The AlGAs LEDs are
usually about 50 .mu.m thick and are grown on GaAs by liquid-phase
epitaxy (LPE). The p-n junction are diffused. For even brighter
LEDs, the AlGAs layers are grown even thicker (.about.150 .mu.m),
and the GaAs substrates are etched off. The thick AlGAs layer
becomes the mechanical support. With no substrate and a reflector
at the back side one can double the external efficiency.
[0010] Visible LEDs are used as solid-state indicator lights and as
light sources for numeric and alphanumeric displays. Infrared LEDs
are used in optoisolators, remote controls and in optical fiber
transmission in order to obtain the highest possible
efficiency.
[0011] The advantages of LEDs as light sources are their small
size, ruggedness, low operating temperature, long life, and
compatibility with silicon integrated circuits. They are widely
used as status indicators in instruments, cameras, appliances,
dashboards, computer terminals, and so forth, and as nighttime
illuminators for instrument panels and telephone dials. Visible
LEDs are made from III-V compounds. Red, orange, yellow and green
LEDs are commercially available. Blue LEDs may be formed of II-VI
materials such as ZnSe, or ZnSSe, or from SiC.
[0012] LEDs can also be employed to light up a segment of a large
numeric display, used for example, on alarm clocks. A small numeric
display with seven LEDs can be formed on a single substrate, as
commonly used on watches and hand-held calculators. One of the
major challenge for LEDs is to make very efficient LEDs, with high
external efficiency.
[0013] Led Bars
[0014] A linear, one-dimensional array of LEDs can be formed from a
linear series of sub-arrays, wherein the sub-arrays comprise a
semiconductor die with several hundred microscopic LEDs. Each LED
is separately addressable and has its own bond pad. Such a die is
referred to as an LED bar and the individual LEDs in the array are
referred to as "dots" or "pixels".
[0015] LED bars are envisioned as a replacement for lasers in
laser-printer applications. In a laser printer, the laser is
scanned across a rotating drum in order to sensitize the drum to
the desired pattern, which is then transferred to paper. The use of
electronically scanned LED bars for this purpose can result in
replacement of the scanning laser with a linear stationary array of
microscopic LEDs that are triggered so as to provide the same
optical information to the drum, but with fewer moving parts and
possibly less expensive electro-optics.
[0016] Currently, commercial LED bars are of two types: GaAsP on
GaAs substrates and GaAlAs on GaAs. The GaAsP/GaAs bars are grown
by Vapor Phase Epitaxy (VPE). Because of the lattice mismatch
between GaAsP and GaAs, thick GaAsp layers must be grown of about
50 microns or more thickness and growth time per deposition run is
long (5-6 hours). LED bars produced in this fashion are not very
efficient and consume much power, and have relatively slow response
times.
[0017] The second type of LED bar, i.e., GaAlAs/GaAs is grown by
Liquid Phase Epitaxy (LPE). LPE growth is cumbersome and does not
lead to smooth growth, or thin uniform layers, and is not well
suited to the growth of complex structures requiring layers of
different III-V compositions.
[0018] One of the most important performance requirements for LED
bars is dot-to-dot uniformity of the optical output or
electroluminescence (or .eta..epsilon.). Uniformity of 10 to 15% is
currently typical but the marketplace desires .+-.2% or better.
Another major requirement is output stability over the lifetime of
the LED bar. Currently stability is poor. Another important feature
is high brightness, which is presently not very good. Elimination
of wire bonding which is currently not available is also highly
desirable. Thermal sinking is also important, particularly in the
case of inefficient GaAsP bars, in which the brightness is
dependent upon operating temperature.
[0019] Led Arrays
[0020] Currently, arrays of LEDs, addressable in two directions
(i.e., and X-Y array or X-Y matrix), have been formed of discrete
LED chips mounted on printed circuit boards. The resolution of such
arrays is limited by the pixel size which is one the order of 200
microns square.
[0021] An alternate approach has been to use LED bars to project
the light on scanning mirrors. The inclusion of moving parts causes
life and speed limitations.
[0022] A need exists, therefore, for a monolithic X-Y addressable
array with high resolution properties.
[0023] Lift-Off Methods
[0024] In the fabrication of LEDs, LED bars and LED arrays, it is
desirable for a number of reasons, chiefly relating to quantum
output efficiency, to utilize thin film epitaxial semiconductor
layers for device fabrication. Furthermore, as stated in U.S. Pat.
No. 4,883,561 issued Nov. 28, 1989 to Gnitter et al.:
[0025] "In thin film technology there has always been a tradeoff
between the material quality of the film and the ease of depositing
that thin film. Epitaxial films represent the highest level of
quality, but they must be grown on and area accompanied by
cumbersome, expensive, bulk single crystal wafer substrates. For
some time, research has focused on the possibility of creating
epitaxial quality thin films on arbitrary substrates while
maintaining the ultimate in crystalline perfection.
[0026] The main approach has been to attempt to rescue the
substrate wafer by separating it from the epitaxially grown film;
however, to undercut a very thin film over its entire are without
adversely affecting the film or the underlying substrate, the
selectivity must be extremely high. This is very difficult to
achieve. For example, J. C. fan has described in Journale de
Physique, re, Cl. 327 (1982) a process in which an epitaxial film
is cleaved away from the substrate on which it is grown. Such
cleavage, at best, is difficult to achieve without damage to the
film and/or substrate, r without removal of part of the substrate.
Also, in some instances, the cleavage plane (<110>) and the
growth plane (<110>) of the film may be mutually
exclusive.
[0027] In a paper by Konagai et al. appearing in J. or Crystal
Growth 45, 277-280 (1978) it was shown that a Zn doped
p-Ga.sub.1-xAl.sub.xAs layer can be selectively etched from GaAs
with HF. This observation was employed in the production of thin
film solar cells by the following techniques. In one technique,
zinc doped p-Ga.sub.1-xAl.sub.xAs was grown by liquid phase epitaxy
(LPE) on a n-GaAs grown layer on a GaAs single crystal substrate.
During this LPE growth of the Zn doped Ga.sub.1-xAl.sub.xAs, Zn
diffuses into the surface of the underlying GaAs to form a p-type
GaAs layer and hence p-n GaAs junction. The surface
p-Ga.sub.1-xAl.sub.xAs is then selectively etched away leaving the
p-n junction GaAs layers on the GaAs substrate.
[0028] In another solar cell fabrication process Konagai et al
describe a "peeled film technology," which will be referred to here
as lift-off technology. A 5 micron thick Ga.sub.0.3Al.sub.0.7As
film is epitaxially grown on a GaAs<111>substrate by LPE. A
30 micron thick Sn doped n-GaAs layer is then grown over the
Ga.sub.0.3Al.sub.0.7As layer and a p-n junction is formed by
diffusing Zn into the specimen utilizing ZnAs.sub.2 as the source
of Zn. Appropriate electrical contacts are then formed on the films
using known photoresist, etch and plating techniques. The surface
layer is then covered with a black wax film support layer an the
wafer is soaked in an aqueous HF ethcant solution. The etchant
selectively dissolves the Ga.sub.0.3Al.sub.0.7As layer which lies
between the thin solar cell p-n junction device layers and the
underlying substrate, allowing the solar cell attached tot he wax
to be lifted off the GaAs substrate for placement on an aluminum
substrate. The wax provides support for the lifted off film.
[0029] While the technique described above has been described in
the literature for over ten years, it was not adopted by the
industry. One reason for this was a difficulty encountered in
completely undercutting the Ga.sub.0.3Al.sub.0.7As "release" layer
in a reasonable time, especially when the area of the film to be
lifted-off was large. This difficulty arose due to the formation
and entrapment of gas formed as a reaction product of the etching
process, within the etched channel. The gas created a bubble in the
channel preventing or diminishing further etching and causing
cracking in the epitaxial film. The problem could only be partially
obviated by using very slow reaction rates (very dilute HF
solutions). Since both the time required for lift-off and the risk
of damage to the overlying film are important, the process was
virtually abandoned."
[0030] In the Gmitter et al. patent, a lift-off approach was used
which comprised selectively etching away a thin release layer
positioned between an epitaxial film and the substrate upon which
it grows, while causing edges of the epitaxial film to curl upward
as the release layer is etched away, thereby providing means for
the escape and outdiffusion of the reaction products of the etching
process from the area between the film and substrate.
[0031] The Gmitter et al. process uses Apiezon (black) wax applied
to the front side layer to be separated. The stress in the wax
imparts a curvature to the layer being separate or lifted, thereby
allowing etching fluid access to the etching front. This process is
inherently limited to relatively small areas. The etching front
must commence from the outer edge of the total area being lifted
off. This results in long lift-off times, for example, up to 24
hours for a 2 cm.sup.2 area.
[0032] In addition, the curvature necessary for lift-off is caused
by a low temperature wax so that no high temperature processing can
be done on the backside of the lifted area. This results from the
fragile nature of the thin film which must be supported at all
times. The film, when supported by the wax on the front side, is
curved and cannon be further processed int hat shape, without a
great deal of difficult. If the wax is dissolved to allow the film
to lay flat, the film must first be transferred to a support by
applying the backside surface to a support, in which case, access
to the backside is no longer feasible without a further transfer.
Presently, samples are cleaved to size, which precludes substrate
reuse in full wafer form. Thus, this process is useful only for
individual small areas that do not require backside processing.
More importantly, there is no known method of registration from one
lifted-off area to another. Thus, large scale processing for LED
bars and LED arrays using this technique is not presently
practical.
SUMMARY OF THE INVENTION
[0033] The invention is directed to novel LEDs and LED bars and
arrays, per se. The present invention is also directed to a new and
improved lift-off method and to LEDs, LED bars and LED arrays made
by such method.
[0034] Lift-Off Methods
[0035] In one embodiment of the novel lift-off method, a thin
release layer is positioned between an epitaxial film and the
substrate upon which it is grown. A coating of materials having
different coefficients of expansion is applied on the epitaxial
film layers. The top structure comprising the coating and the
epitaxial layers is then patterned as desired to increase the
amount of etchant front by cutting channels to completely laterally
separate individual lift-off areas or by cutting slits part way
into the epitaxial film.
[0036] The entire structure is then brought to a suitable
temperature which causes thermal stress between the coating
compositions while the structure is subjected to a release etchant
resulting in lift-off individual thin film areas supported by the
coating.
[0037] Where registration between film areas is desired, such as in
the fabrication of LED bars or LED arrays, a coating of material,
such as uncured UV epoxy, which is capable of being transformed
from a more readily soluble state to a less soluble state by UV
radiation is applied over a thin film epilayer formed on a release
layer over a substrate. A UV light transparent grid with a
plurality of openings is affixed over the transformable
coating.
[0038] A photomask, with an opaque patter to cover the opening in
the grid, is affixed over the grid. The transformable coating is
cured everywhere except beneath the covered openings by exposing
the layer to UV light through the photomask.
[0039] The mask is then removed. The uncured portions, i.e., in the
openings of the grid are then removed by a solvent down to the
epitaxial surface leaving a cured grid layer of epoxy over the thin
film surface.
[0040] Next, the epitaxial layer is etched away down to the release
layer using the openings in the grid to create access for th
etchant at the many points across the structure.
[0041] The thin film layer may then be lifted off while attached to
the support grid of remaining cured transformable material. The
backside may then be processed on the wafer (substrate) scale with
the wafer registration still retained.
[0042] In one of several alternative lift-off embodiments, release
and registration is accomplished by forming channels between device
areas directly on the thin film and thereby exposing areas of the
release layer between lift-off areas. The exposed areas are then
filled with etchant material. While the exposed areas are so
filled, a lift-off support structure, such as UV curable epoxy
tape, or other fairly rigid material, is affixed to the frontside
of the wafer trapping the etchant in the channels. Eventually, the
trapped etchant consumes enough release layer material to enable
the lift-off support, together with the underlying lift-off areas,
to be removed from the underlying substrate with registration
intact.
[0043] Led and Led Bars
[0044] In accordance with the present invention, thin film
epitaxial GaAs/AlGaAs LEDs and LED bars are formed by an
Organo-Metallic-Chemical Vapor Deposition (OMCVD) lattice matched
process. The p-n junctions are grown during OMCVD of an active GaAs
layer which is sandwiched between AlGaAs cladding layers formed on
a GaAs or Ge substrate. Preferably, carbon is used for the p-type
dopant.
[0045] The cladding layers confine injected minority carriers to
regions near the p-n junction.
[0046] A thin top surface of GaAs (light emitting surface) layer of
about 1000 A, or less, is formed to assist in current spreading at
the pixel region. Current spreading is desired at the pixel region
to provide uniform current through the p-n junction, but is
undesirable beyond the pixel region where it would tend to cause a
non-uniform pixel boundary, and for ease in contacting the device.
The thin top surface layer also prevents oxidation of the AlGaAs
cladding layer.
[0047] Various methods are employed to isolate the LED bar dots
from each other and to preclude current spreading beyond the
desired pixel boundary. One such method is ion or proton
bombardment to destroy the crystal quality between dot regions and
another is etch isolation through the p-n junction between pixels.
A simple but elegant alternate solution to the problem is to
initially grow very thin cladding layers which serve the minority
carrier confinement function near the p-n junction region, but are
poor lateral conductors due to their thinness and thereby serve to
prevent current spreading laterally.
[0048] By way of contrast, the currently known art uses a thick
cladding layer to spread the current. Also, most LED bars use
patterned Zn-diffused junctions to define the pixels. In that case,
a thick top layer is used because the Zn diffuses quite deeply.
This deep diffusion is useful for current spreading, but may not be
easily controllable. In the present invention, which discloses
grown epitaxial junctions, ion implantation, etching, or
anodization may be used, as aforesaid, to create the high
resistance region between pixels. A fourth approach, outlined
above, uses a thin highly conductive patterned GaAs layer for
current spreading to the pixel boundaries, and a thin and much less
laterally conductive AlGaAs cladding layer. The thin GaAs layer
(between 500 A and 1000 A, and preferably less than 1000 A)
transmits a large fraction of the light and conducts current to the
edges of the pixel, provided the pixel size is not too much larger
than 30 .mu.m square. Thus, by patterning the GaAs, the current
spreading is limited to the edge of the GaAs contact layer, and the
cladding layer does not have to be patterned, leading to better
planarity of the surface, and also avoiding the formation of
exposed junction edges and associated deleterious perimeter leakage
currents.
[0049] Optionally, the lift-off methods previously discussed may be
employed to separate a front processed LED bar from its substrate,
or the back surface fo the substrate may simply be metallized to
form a back contact for current flow.
[0050] Led Arrays
[0051] In accordance with the invention, LED arrays are formed on a
suitable substrate comprising a III-V epitaxial heterojunction,
preferably comprising AlGaAs cladding layers with a GaAs carbon
doped p-n junction formed between the AlGaAs layers using the OMCVD
process described above. Optionally, an etch stop or release layer
is formed between the substrate and the epi-layers when it is
desired to separate the substrate after front side processing.
[0052] A pattern of contact pads and bus bars is then formed on the
top (or light emitting) surface. Next, each LED dot, or pixel, is
isolated by etching part way through the epi-layers forming
isolated dot mesas.
[0053] A planar support structure (preferably of light transparent
material, such as glass) is then bonded to the to of the mesas by a
suitable adhesive, such as light transparent epoxy.
[0054] After the support is attached, the substrate is etched off,
or cleaved off, leaving the Led film patterned on one side (front
side) with contact pads and bus bars attached to the support
structure. The remaining side (called the backside) is exposed when
the substrate is removed. The backside contacts (running orthogonal
to the top side contacts) and bus bars are then formed by
photolithography followed by electroplating or evaporation of the
metal for the contact to form and Led array of LED pixels
addressable in two orthogonal directions.
[0055] A multicolor array can be formed by two or more such arrays.
In the multicolor embodiment, each array is formed with a different
bandgap material to create light emissions of different wavelength
and, hence, different colors. The larger bandgap material is formed
closer to the top or light emitting surface. The material with the
larger bandgap will be transparent to radiation from the smaller
bandgap material.
[0056] A "smart" switch can be formed using an x-y LED array, as
described above. The LED array is mounted inside alight transparent
pushbutton. The LED X-Y contacts are addressed by a microprocessor,
so that a message can be displayed on the face of the button
indicated, for example, the button function.
[0057] A digital multiplexed infrared (IR) and visible image
converter/enhancement system can be formed using the previously
described lift-off processes and backside processes to form X-Y
arrays of photodetectors and X-Y LED arrays of very thin epi-layers
with registered dots and metallization on both sides.
[0058] An image, focused on the X-Y detector array, is converted to
an electrically signal by sequentially detecting the charge or
current in each IR detector element. An X-Y photo-detector array,
formed as above, is coupled to a microprocessor controlled digital
multiplexor comprising an array of transistor gates.
[0059] The detected signal is amplified and drives a corresponding
visible light emitting dot or pixel in an LED array, resulting in
conversion of the IR image to a visible light image. The pixel size
can be as small as 25 microns or even less, depending on the
wavelength of the light and up to the layer thickness, i.e.,
approximately 1 micron, resulting in very high resolution and
fairly low cost.
[0060] The above summary will now be supplemented by a more
complete description of the invention in the various embodiments
described in connection with the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] FIGS. 1A-1D are a series of schematic cross-sectional flow
diagrams showing various steps in a first embodiment of a lift-off
process of the invention.
[0062] FIG. 1E is an enlarged view of a portion of the LED layer 16
of FIGS. 1A-1D.
[0063] FIG. 1F is a cross-sectional view of a single LED formed
according to the process of FIGS. 1A-E.
[0064] FIG. 2A is a partial perspective view of a portion of a LED
wafer during lift-off processing, according to a second embodiment
of a lift-off process.
[0065] FIG. 2B is a sectional view taken along lines II-II of FIG.
2a of the lift-off structure after one step in the process.
[0066] FIG. 3a is a partial perspective view of a portion of an LED
wafer during lift-off processing in an alternate embodiment wherein
registration is maintained.
[0067] FIGS. 3b and 3c show cross-sections of the structure of 3a
after additional steps in the process.
[0068] FIGS. 4a-4e are schematic drawings of a wafer during various
steps in the processing flow involved in a further lift-off
embodiment of the invention.
[0069] FIGS. 5a-5b are schematic cross-section drawings of a wafer
subjected to another lift-off procedure in accordance with the
invention.
[0070] FIGS. 6a-6d are schematic drawings of a wafer in yet another
Led lift-off process in accordance with the invention.
[0071] FIGS. 7a-7c are schematic sectional drawings of a last
embodiment of the lift-off method of the invention.
[0072] FIGS. 8aa-8af and 8bg-8bk are a process flow diagram of the
main steps in fabricating an LED bar in accordance with a mesa etch
isolation process with a corresponding schematic sectional view of
a wafer structure so processed shown beneath each step.
[0073] FIG. 9 is a cross-sectional side view of a wafer during step
k of FIG. 8b.
[0074] FIGS. 10a-10h are a process flow diagram of the main steps
in fabricating an LED bar in accordance with an alternate process
with a corresponding schematic sectional view of a wafer structure
so processed shown beneath each step.
[0075] FIGS. 11aa-11af and 11bg-11bj are a process flow diagram of
the main steps in fabricating an LED bar in accordance with yet
another alternate process with a corresponding schematic sectional
view of a wafer structure so processed shown beneath each step.
[0076] FIG. 12a is a plan view of a wafer before being diced into
separate Led bars.
[0077] FIG. 12b is a plan view of an LED bar 200 made in accordance
with any of the processes described in connection with FIGS. 8, 10
or 11.
[0078] FIG. 13 is a perspective view of a LED pixel from an X-Y
addressable LED array embodiment of the invention.
[0079] FIGS. 14a-b are schematic sectional views of a wafer being
processed to form an X-Y addressable Led array.
[0080] FIGS. 14c-e are schematic partial perspectives showing a
wafer during successive additional process steps.
[0081] FIG. 15 is a plan view of an X-Y addressable LED array
mounted on a silicon substrate with associated silicon electronic
circuitry.
[0082] FIG. 16 is a side view of a pixel of a tri-color X-Y
addressable LED array.
[0083] FIG. 17 is a plan view of the array of FIG. 16.
[0084] FIG. 18 is a schematic side view of an IR to visible light
converter embodiment of the invention.
[0085] FIG. 19 is a schematic diagram of the converter of FIG.
18.
[0086] FIG. 20 is a side view of an alternate embodiment in
accordance with the invention.
[0087] FIG. 21 is a top view of a "Smart" button embodiment in
accordance with the invention.
[0088] FIG. 22 is a schematic side view of the button of FIG.
21.
[0089] FIG. 23 is a plan view of the top of an LED bar in
accordance with the invention.
[0090] FIG. 24 is a schematic side view of FIG. 23 taken along
lines XXII-XXII.
[0091] FIG. 25 is a plan view of a silicon wafer adapted to mate
with the LED of FIGS. 23 & 24.
[0092] FIG. 26 is a schematic side view of a silicon structure
taken along lines XXV-XXV of FIG. 25 adapted to mate with the LED
bar of FIGS. 23 & 24 to form a hybrid circuit.
DETAILED DESCRIPTION OF THE INVENTION
[0093] I. Lift-Off Methods
[0094] A first embodiment of the invention will now be described in
connection with item 10 of the cross-sectional drawings of FIGS.
1a-1f. A substrate 12, which may comprise any suitable substrate
material upon which to grow epitaxial hetero-layers, is provided. A
release layer 14 is grown, preferably by OMCVD, on substrate 14.
Layer 14 is preferably formed of AlAs for an AlGaAs/GaAs device.
For an InP device, an InGaAs release layer is preferred. AlAs is
preferentially etched by HF acid, while InGaAs is preferentially
etched by sulfurix/hydrogen peroxide and water solution.
[0095] There are a number of ways to achieve liftoff depending on
whether large continuous sheets of material, discrete areas, or
registered discrete areas are desired. The first liftoff embodiment
is intended to improve upon the prior art for lifting off large
continuous areas of material. In this case, curvature is needed to
speed up the process. A coating 18 & 19 is formed on the front
side processed LED structure 16 formed on a release layer 14 grown
on substrate 12 (FIG. 1A). The coating may consist of a single film
or combination of thick or thin film materials with various thermal
coefficients of expansion. For example, coating 18 & 19 may
comprise a nitride, metal, bi-metal or coated thin glass. The
stress in the coating can be tailored for the exact application to
impart a controlled curvature to the thin film of material as it is
released, thereby assisting the flow of etchant to the liftoff
front.
[0096] The curvature can be tailored for use at room temperature,
in which case it would need to be removed afterward for further
processing without curvature, or it could be tailored to lie flat
at room temperature, and to impart the compressive stress to the
liftoff material at elevated or reduced temperature. In one
example, a thin glass sample 18 can be coated with a layer of
purposely stressed nitride 19, in order to put the glass under
compression. Stress is induced by varying the rate of deposition
and by depositing at a temperature which is different than the
temperature at which the structure will be used. For example, the
film materials may be sputter at low deposition temperatures. The
glass can be coated after it is affixed to the front of the liftoff
material (i.e. LED structure 16) or before. The coating is then
patterned and the structure is then exposed to a release etchant
such as, HCL, HF or H.sub.2SO.sub.4/H.sub.2O.sub.2/H.sub.2O using
the coating layers 18/19 as a mask. Due to the stress, the glass 18
and thin film sandwich 16 will curve up as the material is released
from it's rigid substrate, allowing the etchant access to the
front, FIG. 1C. After complete liftoff, the nitride coating can be
removed, allowing the sample to lay flat again, while still being
supported by the glass for backside processing.
[0097] This process could also be applied for lifting off small
area devices because although curvature is not necessary, it can
still be beneficial in speeding up the release process. In this
case, the coating layer 18 plus the LED structure 16 would simply
be patterned in the desired configuration, FIG. 1B, using for
instance well known photolithography techniques followed by wet
etching, and subsequently removed down to the release layer 14 as
seen in FIG. 1B.
[0098] An alternate liftoff process for lifting discrete devices
will now be described in connection with FIGS. 2A and B, wherein
corresponding items in FIG. 1 retain the same reference numeral in
FIG. 2. By dividing the total wafer area into small areas, or
equivalently, bringing the undercutting etchant in contact with
many points across the wafer, the requirement for curvature is
avoided. Areas as large as 0.5 cm wide have been shown to lift
readily without curvature. As shown in the partial perspective
cross-section of FIG. 2A, a substrate 12 has formed thereon a
release layer 14, followed by an LED structure 16, as described in
connection with FIG. 1. All front side processing, such as bonding
pads and metal contacts (not shown) to the LED structure 16 are
completed.
[0099] A state-transformable material, that is, a material which
can be selectively transformed from one state to another state is
formed on, or applied to, the front side processed LED structure
16. In the original state the transformable material may be
substantially insoluble and when transformed may become soluble.
The material is selectively transformed where desired and the
material which is left in its original state is then removed. For
example, a UV curable epoxy 30 may be spread over the structure 16.
This epoxy has the property that exposure to intense UV light
causes it to cure and become a solid. Uncured epoxy is soluble in
solvents such as acetone and trichloroethylene while the cured
epoxy is not. The epoxy is irradiated with UV light in the desired
pattern using for instance a standard chrome photolithography mask
34 to block the light where curing is not desired. After exposure,
the mask 34 is removed and the uncured epoxy is removed with a
solvent such as trichloroethylene.
[0100] Next the LED structure 16 is removed down to the release
layer 14. The cured epoxy 30 is left on the Led structure to serve
as a support for the thin film LED structure 16 after separation
from the substrate. In this manner, the etching front is increased
by dividing up the total wafer area of structure 16 into smaller
areas by cutting channels 40 down to the release layer 14. In this
way, a whole wafer's worth of devices can be lifted in much shorter
times than it would take to lift a continuous sheet of LED material
from the same size wafer, and it is achieved without the need for
curvature.
[0101] Where registration between LED devices separated from the
same wafer is required, as in LED bars and arrays, the liftoff
method of the alternate embodiment of FIGS. 3a-3d offers many
advantages. Like numerals are used in FIGS. 3a-3d and subsequent
figures for corresponding items in the previous figures.
[0102] This alternate process of FIG. 3 solves the difficult
problem of trying to register small device or pixel areas of
material with respect to each other, while at the same time,
allowing the etching medium access to the exposed release layer.
The ability to do this allows for easy retrieval from the solution,
wafer scale processing on the backside, and short liftoff times due
to the smaller areas and maximum etching front. This approach also
enables registration of devices throughout the entire wafer area
while still providing the etching solution access to many points
across the wafer.
[0103] Turning now to FIG. 3(a), there is shown a rectangular
partial section of a conventional III-V planar circular 3 inch
wafer. The wafer is formed of a semiconductor substrate 12 upon
which a release layer 14 is deposited by OMCVD followed by a front
processed LED structure 16, all as previously described above.
[0104] Transformable material, such as uncured liquid UV epoxy 50
is spread onto the top or front surface of structure 16. The point
of departure with the previous embodiment occurs in the next step,
when a perforated planar grid 52, made of material transparent to
UV light such as glass, is aligned on top of the epoxy 50. The
perforations 56 extend orthogonal to, and through, the plane of
grid 52.
[0105] A photomask 58 with opaque areas designed to cover the
perforations 56 is then positioned over the grid 52 (FIG. 3a). (An
optional UV transparent mask release layer can be placed between
the mask 58 and the grid 52 if intimate contact is desired to
facilitate mask removal). UV light is focused onto the mask, curing
the underlying epoxy 50a everywhere except beneath the opaque areas
58, as shown in FIG. 3b. Wherein the cured sections of epoxy 50 are
shown in shaded section and the uncured sections are in blank. The
mask 58 is removed. The uncured epoxy 50 is removed from the
openings 56 by a suitable solvent and the LED structure 16 etched
away through the openings down to the release layers 14. The
release layer is then etched away using openings 56, as provided
above. Access for the etchant is thus achieved at many points
across the wafer, resulting in a large area LED structure attached
to grid 52 by cured epoxy 50a (FIG. 3c).
[0106] Another approach to registration is to form channels 60
directly in the LED material by etching down to the release layer
14, thereby forming channels in the LED material alone (FIG. 4a).
Any other method that forms channels 60 or access streets between
the areas 70 to be separated, as shown in the plan view of FIG. 4c,
could also be used including the previous UV cured epoxy
technique.
[0107] A support 80 can then be attached to the material 70 over
the channels 60 and then the etchant can be allowed to run along
the channels, thereby giving the etchant access to the center of
the wafers (FIG. 4d/4e). The support materials must be rigid enough
that they do no fill in the channels that have been formed and
thereby force the etchant out. As shown in FIGS. 7a-7c, rigid
alumina support 110, combined with double-sided UV release tape
120, may be used. One side of the tape 120 is adhered to the
alumina, and the other side of the tape is adhered to the front of
the structure 16 after the channels have been formed.
[0108] These UV release tapes are well known in the industry for
dicing and shipping small devices and have proven to be an
excellent support/adhesive choice in the present application for
several reasons. These tapes have the property that when they are
exposed to intense UV radiation, they lose most of their adhesion,
so they can be easily removed. In addition, moisture does not seem
to effect the adhesive, and they can be applied with great success,
even if submerged in liquid.
[0109] The second, rigid support 110 should be formed from material
which is transparent to UV radiation if it will be desired to
release the devices later. It should also not be attached by the
specific etchant used. After the support 110, 120 is applied, the
etchant is then allowed access tot he sample and will run up the
channels and undercut the devices. The devices are then released
attached by UV releasable tape 120 to the alumina 110. Taller
channels may assist in speeding up the capillary action t achieve
faster release times. Other methods may also be employed to speed
along the movement of the etchant up the channels 60, including
vacuum assistance, ultrasonic assistance, etc.
[0110] Another way to speed up the action of the etchant is to
avoid the problem altogether by placing the etchant on the surface
of the layer to be lifted after the channels are made (FIG. 6a) but
prior to the affixing of the support (FIG. 6c). This can be done by
pouring the liquid onto the surface (FIG. 6b), or submerging the
sample during application of the support material 80.
[0111] The tape's adhesion can then be released by UV irradiation
through the alumina (FIG. 7b) and the tape can be taken off the
alumina carrier with the devices still attached. Further UV
exposure will decrease the adhesion of the devices to the tape,
allowing them to be removed by vacuum wand or to be transferred
directly from the tape to any other tape, epoxy, or adhesive medium
(See FIGS. 7b or 7c). FIG. 7b shows release directly to another
tape 100a whereas FIG. 7c illustrates release by affixing with
epoxy 15 to a rigid substrate 25. Discrete areas as large as 0.5 cm
in width have been lifted, released and transferred by this
non-curvature method. The total wafer area which can be lifted
simultaneously in a registered fashion by way of this channel
method appears to be limited only by the wafer size.
[0112] Along the same lines, channels 60, as shown in FIG. 5a can
be made in the LED material 16 to expose the release layer 14
below. A porous material 90 is then spun on, or otherwise formed or
attached to the front surface (FIG. 5a). This material is rigid or
semi-rigid when cured by light, heat, or solvent release, and
therefore will be able to support the lifted film (FIG. 5b) after
separation from the substrate 12. The material is sufficiently
porous to pass the etchant fluid without being altered by the
etchant. In this way, the etchant passes through the porous
material and is given access to the release layer at its exposed
points.
[0113] The channel method and porous material method for liftoff
allow for a solid support medium to be employed without requiring
perforation for etchant access. The liftoff time is very short
because the etchant has access to the release layer from many
points on the wafer surface. Also, this method results in devices
which are registered with respect to each other and are supported
by the alumina or other rigid material for backside processing.
[0114] II. Leds and Led Bars
[0115] Formation of LED's or LED Bars having a unique thin film
structure and other features which provide a highly efficient light
emitting structure will now be described in connection with FIG. 1
in which an LED semiconductor-layer structure 16 is formed on
release layer 14, preferably by OMCVD. Note that while FIG. 1 has
previously been used to describe lift-off procedures, the lift-off
method is considered to be optional, but preferred in this
discussion. Structure 16 is formed of materials having appropriate
bandgap for the desired emission wavelength, such as III-V
materials. Structure 16 is preferably comprised of an n-doped upper
cladding layer 16a, an active p-doped layer 16b and a lower p-doped
cladding layer 16c (See FIG. 1e). Preferably, the cladding layers
16a and c are formed of III-IV materials, such as
Al.sub.xGa.sub.yAs, (y=x-1) and the active layer 16b of
Al.sub.zGa.sub.1-zAs. A contact layer 16d, preferably of FaAs, is
also formed on the top (light emitting surface), of upper cladding
layer 16a and an optional bottom contact layer 16e is provided
beneath layer 16c.
[0116] Note: As a matter of convention, the light emitting side of
the LED will generally be referred to as the "front side" herein.
Also, the conductivity of the layers may be reversed, such that,
the upper layers 16d and a are p-type and the lower layers 16b and
c are n-type.
[0117] By using OMCVD and by using the above-mentioned isolation
systems, the LED structure 16 can be made very thin, i.e., less
than about 5 microns and, preferably, less than 3 microns, with the
contact layer 16d being less than 0.1 micron thick, the cladding
layers 16a and c about 2 microns thick and the active layer 16b
less than about 1 micron.
[0118] A section of an LED pixel formed as above is shown in FIG. 1
with the contact structure added. For more efficient operation, the
cladding layer 16a on the light side of the pixel is roughened; as
shown in FIG. 1F to increase the probability of multiple bounces of
light rays within the LED and thereby increase the probability of
achieving a good exit angle. Note: the roughened surface can be on
the bottom side instead of the front side. Also the LED structure
16a, b, c is a thin transparent double heterostructure and the
substrate is removed and replace by alight reflector layer 13 of
material such as al or a white diffused surface such as MgO on
white ceramic, for improve quantum efficiency. Finally, unlike the
conventional cubic geometry of discrete LEDs, the LED of FIG. 1(F)
has a rectangular geometry with a high ratio of width to thickness.
For example, the width W preferably ranges from about 50 microns to
about 150 microns while the thickness T is about 5 microns or less
for a 10 or 20 to 1 ratio of W/T. The pixel length L is generally
about 200 to 400 microns.
[0119] Contacts 33 and 31 are formed on the front contact layer 16d
and the back contact layer 16e respectively. Preferably the back
layer contact is extended dup to the front surface to form a planar
structure. A passivating, isolating spacer 37 of Si.sub.3N.sub.4 or
SiO.sub.2 is formed between the LED structure and contact 31.
Contact surface 39 may be formed of n.sup.+ GaAs if contact layer
16e is an n-type layer. Optionally layers 16a, b and c may be
stepped at the edges and a conformal coating of Si.sub.3N.sub.4 or
SiO.sub.2 deposited over the edges.
[0120] Although the above description was made using AlGaAs and
GaAs layers, the invention can be applied to other III-V
lattice-matched material system; for example, the active layer 16b
can be replaced with Ga.sub.xIn.sub.yP, which is lattice match with
GaAs and GaAlAs, and which has better light emitting properties.
For lattice-matching x should be roughly equal to y. The cladding
layers may also be replaced with Al.sub.xGa.sub.zIn.sub.yP. For
lattice matching to GaAs, x+z=y. Finally, the active layer GaInp
can be replaced with Al.sub.xGa.sub.zIn.sub.yP as long as it can be
lattice-matched to GaAs. Such LEDs, with AlGaInP compounds, can
range in emission wavelengths from about 0.55 .mu.m to 0.70
.mu.m.
[0121] Note that the active layer 16b is doped with a p-type
dopant, preferably carbon, during the OMCVD process. Zinc, the
conventional impurity used for p-type doping in LEDs, is typically
introduced by diffusion after the growth process and is highly
diffusive over the life of the LED. Consequently, the pixel or dot
location may vary due to such diffusion. Carbon is very
non-diffusive, leading to greater uniformity in dot location and
longer device life. Therefore, carbon-doping in the active layer of
LEDs is much preferred.
[0122] The A1 content "x" may be varied for different devices, but
the cladding layers of AlGaAs should be as heavily doped as
reasonable. The Ga content "y" should be less than "x". Also, the
A1 content, z, of the active layer should be less than x. Usually
both cladding layers have the same A2 content, but that is not
necessary.
[0123] The above-mentioned structures are excellent for LED
discrete devices as well as LED bars. Referring now to FIGS. 8-12,
further embodiments of the invention will be described in
connection therewith. FIGS. 8, 10 and 11 summarize the important
steps of three alternate processes for fabricating LED bars in
accordance with the invention. Beneath each step in the
corresponding wafer structure shown in side view.
[0124] Each LED bar 200, when processed, as shown in the plan view
of FIG. 12b, consists of a die 200 measuring about 0.5 mm by 20 mm
cut from a wafer 240, as shown in FIG. 12a, upon which several
hundred such dies are fabricated in accordance with FIGS. 8, 10 or
11. Each die or bar 200 is comprised of a row of many microscopic,
laterally isolated, LEDs 210, each LED forming a pixel of light or
a "dot". Each LED 210 in a row is addressable from its own bound
pad 220 connected by conductor 119 to a pixel contact area.
[0125] Note that while the invention is explained herein in
connection with LED Bars it is contemplated that the individual
LEDs 210 may be diced and separated along with each respective bond
pad, coupled by conductor 119, to form discrete LEDs of unique
configuration, in that, the bond pad is formed beside the LED and
is therefore highly accessible and does not need to be wire bonded
to the LED pixel.
[0126] The main steps in LED bar fabrication are:
[0127] a) epitaxial growth to form the required LED epitaxial
layers 16 and p-n junctions therein, as described in connection
with FIGS. 1-7;
[0128] b) dot definition to delineate the edge of the LED spot;
[0129] c) front side metallization for contacts;
[0130] d) optional lift-off procedure as previously described in
connection with FIGS. 1-7; and
[0131] e) backside metallization.
[0132] Only steps b, c and e will be described in detail herein, it
being assumed that any of the lift-off methods and growth methods
previously described can be used in connection with the steps
described herein to form registered or unregistered LEDs or LED
bars.
[0133] Referring now to FIG. 8, a mesa isolation method of dot
definition is shown therein. Note: for each process step block, the
corresponding structure is illustrated in section below. Step a)
comprises pre-epitaxial cleaning of wafer 12 using well known
techniques, such as soaking in H.sub.2SO.sub.4/H.sub.2O.sub.2 and
H.sub.2O, followed by OMCVD deposition of AlGaAs/GaAs epi-layers
16, in which a p-n junction is formed in the active GaAs layer
(Step b).
[0134] Next, using well known photolithography techniques,
individual dot junction areas 40 are defined over the surface of
epi-layers 16 beneath areas of photoresist 105 (Step c). Next, the
exposed epi-layers 16 are etched away down to just below the p/n
junction or alternatively all the way down to substrate 12 (Step
d). The resist 105 is removed and a protective coating 106 of
Si.sub.3N.sub.4 or oxy-nitride (SiON) is formed over the top
surface (Step e). Contact areas 171 are photolithographically
defined by resist 115 over the nitride 106 (Step f). The nitride
106 is etched away beneath the resist openings (Step g). The resist
is stripped away and a "lift-off" photo-resist layer 117 is formed
over the top surface, except where the metal contacts will reside
(Step h). Front metallization layer 119 is evaporated onto the
resist contacting the exposed epi-layer surface aligned in the LED
dot (Step i).
[0135] The resist 117 with metallization 119 is then removed using
well-known photoresist stripper liquids, leaving metal contacts
119' remaining and applied to each dot 16 (Step j). These contacts
extend over the nitride 116 to the edge of the chip (See FIGS. 9
and 12) where individual bond pads are formed to address each dot
16'. Contact metallization 121 is then applied to the back of the
substrate 12.
[0136] FIG. 10 illustrates an alternate dot definition method
utilizing ion beam implantation. Steps a and b are as set forth in
connection with FIG. 8. In step c, an implant mask of photoresist
105 is formed which defines regions 41 between LEDs which will be
ion bombarded to implant protons 111 (Step d) to laterally isolate
individual dots or pixels 16', separate by highly resistive
bombarded regions 41' (See FIG. 10 notes). Next (Step e), a
lift-off photoresist layer 115 is formed on the exposed top surface
of epi-layers 16 with openings left where contact metallization 119
will be evaporated (Step f). The metallization is removed
everywhere, except where desired, to form individual contacts 119'
for each dot 16'. Contact metallization 121 then applied to the
backside (Step h).
[0137] FIG. 11 depicts an alternate dot definition process that
does not require a separate deposit of a dielectric layer with
associated photolithography, as in FIG. 8. Steps a-b are as above.
In this alternate method, after defining the dot edges (Step c),
the cap or contact layer 16d [discussed in connection with FIG. 1
(E)] is etched away (Step d). The exposed epilayer surface 16 is
then anodized to form an insulating oxide 108, thus creating a
dielectric in the proper pattern. This method, as in the method of
FIG. 8, limits current spreading to the pixel area where it is
desirable for uniform current injection. But, by removing the cap
layer from regions between dots, illumination within the confines
of each dot is maintained. Current spreading is further eliminated
by growing an extremely thin upper cladding layer 16a, which will
have very high lateral resistivity. Conventional cladding layers
are 20 microns or higher. OMCVD enables fabrication of 0.5 micron,
or less, layers with 0.2 micron being a preferred thickness for
layer 16a.
[0138] The resist 105 is then removed (Step f) and a photoresist
layer 115 formed, except where contacts are desired. Metal 119 is
evaporated over and between the resist (Step h) and removed (Step
i) leaving contacts 119; to each dot 16'. The structure is then
ready for back metallization 121, as previously described in
connection with FIG. 8 (Step j).
[0139] In a variation of FIG. 11, the cap 16d and cladding layer
16a could both be anodized, eliminating the need for a cap etch
step.
[0140] The above processes offer many advantages over other known
systems of fabricating LEDs or LED bars. Some of these are the
following:
[0141] Lattice-Matched System. The epitaxy process is very nearly
perfectly lattice matched, since it is made in the GaAs/AlGaAs
system rather than the GaAs/GaAsP system. Thus, compositional
grading to achieve lattice matching is not required. The epi-layers
are thin (less than 3 microns) as opposed to 20 to 30 microns in
the GaAs/GaAsP system. Since the layers thinner and are made by
OMCVD, the layers yield much more uniform electroluminescence,
making the LED bar more uniform. Since the epitaxial layers are
lattice matched, it is also a simple matter to change the process
to grow LEDs of any wavelength in the range of about 650 nm to 870
nm. The above processes can also utilize GaInP for the active epi
layers and AlGaInP for the cladding layers. Another possible
lattice matched system is GaInAsP/InP.
[0142] Better Confinement of Injected Carriers. The beneficial
properties of AlGaAs layers can be used to enhance the optical
output of the LED devices, in a manner similar to heterojunction
lasers. The AlGaAs is used to reflect carriers so that they are
confined to the volume in which the optical radiation is to be
generated. This enables the generation of much higher efficiency
and optical output than is believed to be possible in the
GaAs/GaAsP system.
[0143] Epitaxially-grown P/N Junction. The junctions are grown
during the OMCVD process. In general, in GaAs/GaAsP technology, the
junction is diffused. The epitaxial junctions are of extremely high
quality and can be placed anywhere in the structure. Diffused-zinc
junctions used in GaAs/GaAsP have the following limitations: the
zinc causes p-type doping, so the structure must be p-on-n (whereas
epitaxial junctions can be p-on-n or n-on-p); the zinc
concentration must be highest at the surface and must have a
diffusion profile (whereas epitaxial doping can have any profile),
the diffused junctions are limited to zinc (whereas epitaxial
structures can be zinc, or carbon, or other dopant as desired).
[0144] Implant Isolation. In the FIG. 10 embodiment, the epitaxial
wafers are implanted with protons to destroy the crystal quality of
the regions between the dots. This isolation is used to prevent the
current from spreading beyond the desired dot perimeter. (The
GaAs/GaAsP technology uses patterned diffusion.) An additional
advantage of implant isolation is that the surface becomes
nonconducting so that the surface becomes nonconducting so that the
metallization can be placed directly on the semiconductor, without
dielectric insulators, and no short circuit will occur.
[0145] Uses of GaAs Cap. A very thin layer 16d, about 1000 A thick,
of GaAs is provided on the top surface for three reasons: ease of
contact, environmental stability, and improvement in current
spreading. The GaAs is kept thin to allow most of the generated
light to escape. If the cap is much thicker than 1000 A, it will
absorb a significant amount of light. Environmental stability is a
factor because AlGaAs can oxidize in air if left uncoated. The GaAs
cap 16d provides the required coating.
[0146] LED Bars fabricated as described above may be modified as
shown in FIGS. 23-26 to incorporate a cantilevered contact bar 240
which mates with a corresponding contact bar 242 on a processed
silicon wafer 260. In this way, a hybrid Si/LED structure can be
formed with a minimum of wire bonding and avoidance of alignment
problems.
[0147] Contact Bar 240 is bonded or otherwise affixed to the front
side of LED bar 270 and contact wires 210 extended on each pixel
261. Back contacts 280 are formed on the back of LED Bar 270.
[0148] A eutectic alloy 290, of, for example Au.sub.0.8Sn.sub.0.2
is formed on the side walls of the wafer 260 and/or the back of LED
Bar 270. The bar 270 and wafer are joined so that the contact bars
240 and 242 overlap. The joined structure is heated to the melting
point of the eutectic (i.e. about 252.degree. C.) and allowed to
cool to room temperature thereby bonding the contact bars and
structure together. The contact bars may then be laser trimmed or
etched or scribed to form bonding pads 250,252 (shown in dotted
lines) for interconnecting Si circuits 292 to specific pixels. Si
circuits 292 may comprise Si transistors connected to form driver
circuits for energizing the individual LED pixels 261.
[0149] III Led X-Y Arrays
[0150] Next, the fabrication of an X-Y multiplexed array, in
accordance with the invention, will be described. It begins with
the epitaxial growth of the required hetero-epi-layers of AlGaAs
and GaAs layers on a GaAs or Ge substrate. In the case of the GaAs
substrate 12, an optional layer 14 of AlAs is formed between the
active AlGaAs layers 16 and the substrate 12 to facilitate
substrate removal by the etch-off method. The AlAs forms an etch
stop layer. [Alternatively, the X-Y array can be removed from the
substrate by a CLEFT process (See U.S. Pat. No. 4,727,047 issued
Feb. 23, 1988 to Fan et al.) or chemical epitaxial lift-off]. In
the case of Ge substrates, a layer of AlAs can be used as an etch
stop, but AlAs is no really necessary, since the Ge substrate can
be dissolved in H.sub.2O.sub.2 without harm to the AlGaAs active
layers. FIG. 14a shows the epitaxial layer structure to comprise a
bottom cladding layer 16c of AlGaAs, an active GaAs (or AlGaAs)
layer 16b in which a p-n junction 17 is formed by carbon doping
during growth, a top cladding layer 16a of AlGaAs and thin GaAs
contact layer 16d, all, as previously described, formed by OMCVD. A
pattern of contact pads 119 and busbars (not shown) is formed by
photolithographic techniques, evaporation, and/or electroplating on
the front surface, as shown in FIG. 14b. This step is not
absolutely required at this point, however, it simplifies a later
etch step in the process.
[0151] The next stage of the process consists of bonding of the
wafer to a support 80, such as glass, ceramic, or thin stainless
steel. (If the support is transparent to infrared radiation,
downstream front-to-back alignments are facilitated, but the
alignments can also be carried out by careful registration to the
support edges.) The processed front side is bonded to the support
80 using a suitable adhesive (not shown) (FIG. 14c). After the
support 80 is attached, the wafer or substrate 12 is etched off (or
cleaved off) leaving the LED film 16 attached to the support 80, as
shown in FIG. 14d, in which the structure has been flipped over
onto the support to expose the backside B for processing.
[0152] Once the backside is exposed, any remaining non-essential
material is removed from the back by selective etching in HF to
expose a clean GaAs contact layer B. The backside (X-axis) contacts
121 and busbars 121x are now photolithographically patterned and
electroplated or evaporated onto the contact regions 16'.
[0153] Finally, the backside is exposed to the mesa etch to totally
separate the dots. At this point, all of the epi-material between
the pixels 16' is removed (FIG. 14e). Alternately, the isolation
may be completed by implant isolation, or by limiting the current
spreading, as described for LED bars in connection with FIGS. 8, 10
and 11. By not removing all of the interpixel material, a path for
lateral heat flow out of the pixel is preserved.
[0154] As shown in FIG. 15, the front and backside processed X-Y
array 300 may be mounted directly to silicon wafer 323 in a precise
location 310 with X and Y silicon driver circuits 320 and 322
formed in wafer 323 and coupled to the X and Y bonding pads 324 and
326, respectively. Bonding of array 300 to wafer 323 may also be
accomplished as described previously in connection with FIGS. 23-26
by having the contact pads 326 replaced by cantilevered bars which
extend over to pads on wafer 323 which can be trimmed to form
circuit bonding pads.
[0155] Suitable silicon logic circuits 330 and interface circuits
332 are formed on wafer 323 to control which pixel 16 is
illuminated in the X-Y matrix. Note that the driver circuits
activate individual pixels by applying a positive voltage to a
pixel in a top column, for example, pixel 1601 via bus bar 326a,
while a negative voltage is applied to the same pixel 1601 via
Y-driver 322 to bottom bus bar 324a, thus completing the current
circuit through the LED 1601.
[0156] It should be noted that the substrate removal methods for
fabrication of LED arrays include CLEFT, lift-off, and substrate
etch-off. CLEFT and lift-off are appropriate if the substrate is to
be reclaimed as a solid wafer. The etch-off process simply
comprises the chemical dissolution of the substrate. Note that the
substrate material may still be reclaimed in the etch-off process;
however, it must be precipitated from the etch solution. The
substrate can also be lapped off, as is conventionally done in the
industry.
[0157] Also note that in the first step of the backside process,
undesired epitaxial layers are removed; these layers are present to
initiate the epitaxy, or may be buffer layers that are not needed
in the final device. To make their removal simple, an AlAs etch
stop layer (not shown) may be provided in the epitaxy between these
layers and the epitaxial device structure. The layers can then be
removed in etches that stop at AlAs, such as the well known PA
etches. At a pH of about 8, these etches dissolve GaAs 1000 times
faster than AlGaAs. After the etch stops at the AlAs, the AlAs can
be removed in HF or HCl.
[0158] In the process described above, the backside of the
substrate is provided with multiplex-compatible metallization to
contact the back of each pixel. Note that this type of processing
requires front-to-back alignment. The pixels are then separated by
a mesa etch. Since the films are only about 5 microns thick, the
mesa etch is straightforward and quick. The etching may be
accomplished with either wet or dry processing. At this point, the
exposed semiconductor may be coated with a dielectric to prevent
oxidation.
[0159] Finally, the wafers are formed into individual dice. The
dice 300 (See FIG. 15) are mounted in a pin grid array (PGA) or
leadless chip carrier socket (neither shown). If the pixel count is
sufficiently high (>1000), the X-Y drivers 320, 322 and logic
multiplexing circuits 330 should be mounted within the chip
carrier. The reason for this is that the wire count becomes
excessive for high pixel numbers. The wire count is approximately
the square root of the pixel count. Preferably, the array is
mounted on the Si circuitry itself, and interconnected using thin
film techniques and photolithographic processing. The circuit and
array are then mounted in the leadless chip carrier or PGA.
[0160] As shown in FIG. 13, reflection from the back surface may be
sued to enhance emission. FIG. 13 is a perspective view of an LED
array pixel showing the upper an lower cladding layers 16a and 16c
with the active layer 16b between them. Thin contact layers 16d and
16e are formed on the front and back sides, respectively, and
conductors 119a and b run orthogonal to each other on the contact
layers. The back surface contact layer 16e of GaAs extends across
the total pixel surface and serves as a back surface reflector. The
back surface reflector reverses the light propagating toward the
back of the pixel, so that it is directed toward th front surface.
The back surface 16e may also serve to scatter light into the
escape cone; which is a range of angles that rays, propagating
within the LED crystal, must fall within for the ray to propagate
beyond the semiconductor/air interface.
[0161] Tuning of individual epi-layers may also be provided to
further improve LED efficiency. For example, assume a structure,
such as the LED shown in FIG. 13, in which the epi-layers have the
following properties:
1 Refractive Wavelength Composition Layer Index .lambda./n(A)
A1GaAs AIR 1 6500 N/A 16d 3.85 1688 0 16a 3.24 2006 80% 16b 3.60
1806 38% 16c 3.24 2006 80% 16e N/A N/A Metal
[0162] The active layer 16b, could be made "resonant by making the
active layer thickness a multiple of half the wavelength (i.e., a
multiple of 9103 A). For example, an active layer of thickness of
4510 A or 5418 a would be preferable to 5000 A. Such a resonant
structure could yield superluminescence or stimulated emission
which would enhance the optical output. A benefit of stimulated
emission in the resonant structure would be that all of the light
thus generated would be in the escape cone.
[0163] The front (top) cladding layer 16a is set for maximum
transmission (quarterwave or odd multiple). The quarterwave
thickness is 530 A, therefore the top layer should be 0.55 microns,
or if better current spreading is needed, 1.05 microns.
[0164] The back cladding layer can be tuned for maximum reflection
by using even multiples of 503 A, such as 10.times.503 or 5030
A.
[0165] Optional front and back Bragg reflector layers 16f and 16g,
respectively, may be incorporated into the device of FIG. 13 during
OMCVD growth, thereby converting the LED into a vertical cavity
laser. The laser cavity is bounded by the Bragg reflectors 16f and
16g and the emitted light will be phase coherent. The Bragg
reflectors are formed by alternating many Al.sub.xGaAs/Al.sub.zGaAs
layers. A sufficient number of layers will yield a high reflection
coefficient. The electrical cavity is formed by the AlGaAs cladding
layers. Thus, vertical cavity lasers can be in an X-Y array, or may
be formed in a laser bar. The feature that makes this possible is
the double-sided processing approach, which permits a wide range of
pixel structures, including LEDs, lasers and detectors.
[0166] A light detector array 450 can be formed in a similar
manner. To form a light detector array, the epitaxial films are
doped so as to form a p-i-n structure, rather than and LED. The
active layer comprises a semiconductor chosen for absorption over
the wavelength range of interest. For example, long wavelength
detection could utilize InAs grown on an InAs substrate.
Alternatively, InGaAs grown on InP or GaAs could be utilized for
mid-IR detection. Near IR is detected with GaAs or AlGaAs. The
fabrication of the detector must include edge passivation to
maintain minimal dark current, but is otherwise the same as the LED
array processing previously described.
[0167] The multiplexing electronic detector circuitry is somewhat
different than the LED driver circuit, sine it must sense the
current generated in each pixel in sequence, rather than supply
current. The electronics is nevertheless straightforward, and is
similar to charge coupled device (CCD) circuitry. In fact, the
device could be formed using a CCD array instead of a p-i-n
array.
[0168] An infrared-to-visible digital image converter can be formed
from a detector 450 and light emitting diode array 300 (as shown in
FIG. 19). The converter is useful for night vision devices, as well
as for digital processing of IR and visible video data.
[0169] Current image converters utilize a photocathode-based system
that converts IR radiation to visible. The conversion process is a
direct analog process. Owing to this design, the direct analog
process is not particularly amenable to digital image enhancement.
There are also various displays that could be superimposed over the
night vision display to provide the user with communication or
computer data. However, the photocathode display is not easily
adaptable to display applications.
[0170] A digital pixel-based system, in accordance with FIGS. 18
and 19, function both as an IR image converter, an image enhancing
device, and a display.
[0171] The converter invention consists of three main elements: The
IR detector array 450, the multiplexing electronics 470, and the
light emitting diode (LED) array 300. A diagram of the IR image
converter is shown in FIG. 19. An IR image is focused by lens 460
on a multiplexed X-Y array 450 of IR detectors. The pixel data from
the detectors is processed by the electronics 470, which then
drives a synchronous multiplexed LED array 300. Note that the
processor can accept external data via data port 472 to add to or
subtract from the image. In this way, image enhancement can be
accomplished, or communications or other data can be superimposed
on the display 300.
[0172] As noted above, the detector array 450 can comprise a Si
charge coupled device, or if longer wavelength detection is
required, can be made from p-i-n diodes formed from material in the
InGaAs system. The array 450 is fabricated using substrate etch-off
or lift-off processing, along with backside processing, to form
very thin structure with metallization on both sides, as more fully
described above in connection with the LED array 300.
[0173] The intensity of the image produced by array 300 may be
controlled by varying the duty cycle timing or modulating the drive
current of the LED pixels.
[0174] The electronics 470 consists of a multiplexing and
sequencing circuit that first detects the charge or current in each
IR detector, and then couples this input data to a current
amplifier that drives the corresponding LED pixel in the output
array 300. The electronic processor 470 also accepts signals from
an external source, such as a microprocessor that can be displayed
on the LED array. Moreover, the electronics can supply that video
data to the microprocessor for image enhancement and can accept a
return signal to be displayed on the LED array 300.
[0175] The LED array consists of multiplexed thin film LED pixels
formed from material in the AlGaInP family, and more particularly,
AlGaAs for bright red displays. The array is formed using the
previously described processing array steps. The pixel size can be
as small as 25 microns square and, consequently, the display can
offer extremely high resolution or alternatively, fairly low
cost.
[0176] As shown in FIG. 20, the detector 450 and LED array 300 can
be stacked into a hybrid assembly comprises of a top thin film IR
X-Y detector array 450 affixed by light transparent glue to lower
thin film LED array 300 mounted on glass substrate 620. A glass
lens 460 is affixed to the top surface of detector 450 and heat
transfer openings 460 provided as necessary for cooling purposes.
The entire structure can be quite thin (1 mil), with the
electronics 470 provided around the periphery. Ultimately, the
monolithic thin array can be mounted on ordinary glasses for image
enhancement of visible light, as well as for display of data
superimposed on video images.
[0177] The applications of the device of FIGS. 18-20 include
military night vision systems, range finders, advanced military
avionics, personal communications systems, and medical systems in
which real-time image enhancement is useful.
[0178] As shown schematically in FIGS. 16 and 17, X-Y arrays can
also be used to form a multicolor display. To make such a display,
individual X-Y arrays labeled LED1, LED2 and LED3, are formed from
two or more different epitaxial structures. The primary difference
in the structure is in the active layer material 161, 162 and 163,
which must have different band gaps to create different colors. For
example, red 163 can be created with AlGaAs, and green 162 can be
created with InAlGaP. The top device LED1 may be a blue LED formed
of II-VI material, such as ZnSe, ZnSSe or a group IV alloy such as
SiC.
[0179] The arrays must be stacked with the larger bandgap LED1
closer to the observer. The material with the larger bandgap will
be transparent to the radiation from the smaller bandgap. Thus, in
this way, the observer will be able to see both colors.
[0180] The creation of the stack of three LEDs 1020 is as follows:
First, the three separate LED arrays LED1, LED2 and LED3 are
formed, as previously described. Next, the are stacked together
with glass 600 between them.
[0181] Transparent glue or epoxy 400 is used to bond the stacks on
top of each other. The upper and lower bonding pads P1 and P2 on
each LED are laterally staggered with respect to other LEDs, so
that individual LED pixels may be addressed (See plan view FIG.
17).
[0182] An LED array, made in accordance with the above-described
methods, can also be provided behind a lens or pushbutton, so that
a message may be displayed. The message can be used to label the
function of the pushbutton. The label and function may be assigned
by a microprocessor.
[0183] Keyboards, pushbuttons and the like now dominate the
interface between advanced microprocessor-based electronic
instrumentation and the user. Commercial electronics and computers,
such as work stations, avionics, telecommunications centers, and
other advanced electronic systems, are limited by the functionality
of the keyboard and button based interface. Even consumer and
automotive electronics are beginning to reach the limits of the
user/microprocessor interface.
[0184] For example, typical PC keyboards now employ as many as 20
re-assignable function keys, the purpose of which changes with each
software package. It is not uncommon to see keyboards with numerous
assignment stickers, templates, or other means of tracking the
particular assignments of the keys. Even the alphabetical keys have
numerous assignments. Obviously, the functionality of the various
programs is becoming limited by the user's ability to handle
numerous registers of multifunction keys. In other electronic
instruments, a similar problem exists. Modern medical
instrumentation, for example, utilizes buttons that are positioned
next to a cathode ray tube (CRT). The CRT is used to label the
function of the buttons. The software determines the function of
the button of the label to be displayed on the CRT, hence, the term
"soft-key" has been used to describe this type of re-assignable
button.
[0185] In many applications, buttons cannot be placed next to a
CRT; the personal computer is a good example of such an application
(nevertheless, some software packages label the buttons this way,
but most users dislike the arrangement). In such a case, what is
needed is a button that has an internal display capable of
providing a label with a message provided by the resident active
software.
[0186] In accordance with the invention, an X-Y LED array 966 is
mounted within a pushbutton, as shown in FIG. 21. The X-Y array
matrix is formed as previously described above in connection with
FIGS. 14-17. The matrix is capable of providing two five letter
words 980, or more, such as "DELAY", to identify the reassignable
function of the pushbutton 960. The X-Y LED matrix 966 may be quite
small, in order to be manufactured at a low cost. In such a case,
the pushbutton also includes a small lens 964 that magnifies the
image. Ideally, the electronics 968 that drive the X-Y array are
located within the leadless chip carrier that houses the X-Y array
966. However, the electronics can also be located behind the array
within the switch housing 970.
[0187] The front lens 962 or rear-projection screen, which also
serves as the button, is mechanically attached by plunger 972 to a
mechanical switch (not shown), so that when the surface lens 962 is
depressed by the user, a signal may be sent to a microprocessor
(not shown).
[0188] Another embodiment which provides a wider viewing angle
comprises the projection of the X-Y image onto a rear-projection
screen; in such a case, the lens is positioned between the screen
and the matrix. The screen in made of a plastic that transmits red
light, so that the internal parts of the button are not visible,
but which nevertheless transmits the image from the LEDs.
[0189] The smart switch button or message center should have
numerous applications in advanced instrumentation and electronics.
One main application will be in workstations in which a large
number of reassignable function keys are needed. The second main
application is on instrumentation in which front panel space is
limited, such as in medical electronics and in avionics.
[0190] Equivalents
[0191] This completes the description of the preferred embodiments
of the invention. Those skilled in the art may recognize other
equivalent embodiments to those described herein; which equivalents
are intended to be encompassed by the claims attached hereto. For
example, while an OMCVD process is preferred for the reasons given
above, molecular beam epitaxy (MBE) and chemical vapor deposition
(CVD) and chemical beam epitax (CBE) based processes are also
envisioned. Likewise, other types of material removal processes, in
addition to chemical etching, such as reactive ion etching, are
contemplated. Also, while an GaAs active layer has been described
an AlGaAs layer can also be used wherein the aluminum percentage
may vary from 0-38%.
* * * * *