U.S. patent number 3,863,332 [Application Number 05/374,444] was granted by the patent office on 1975-02-04 for method of fabricating back panel for liquid crystal display.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Hans G. Dill, Alex M. Leupp.
United States Patent |
3,863,332 |
Leupp , et al. |
February 4, 1975 |
METHOD OF FABRICATING BACK PANEL FOR LIQUID CRYSTAL DISPLAY
Abstract
In forming the back panel of a liquid crystal display, layers of
an insulating material and aluminum are successively deposited on
the surface of a semiconducting substrate having an array of
electrodes thereon. Openings are then formed through the insulating
material and the aluminum layer to expose the electrodes, thereby
defining a spacer lattice which is integral with the substrate and
whose walls are of a uniform height which corresponds to the
desired spacing between the front and back panels of the liquid
crystal display.
Inventors: |
Leupp; Alex M. (Newport Beach,
CA), Dill; Hans G. (Costa Mesa, CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
23476846 |
Appl.
No.: |
05/374,444 |
Filed: |
June 28, 1973 |
Current U.S.
Class: |
438/27; 29/592.1;
205/157; 445/24; 438/30; 438/587; 349/156; 349/143; 349/187;
216/51; 216/23 |
Current CPC
Class: |
G02F
1/13394 (20130101); G02F 1/13439 (20130101); G02F
1/13392 (20130101); Y10T 29/49002 (20150115) |
Current International
Class: |
G02F
1/13 (20060101); G02F 1/1343 (20060101); G02F
1/1339 (20060101); B01j 017/00 () |
Field of
Search: |
;29/578,580,590,591,625,592 ;350/16LC ;204/33,38A,58 ;156/17 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Tupman; W. C.
Attorney, Agent or Firm: MacAllister; W. H. Szabo; J. E.
Claims
We claim:
1. A method of fabricating a backplate with electrodes and
integrated spacers for a liquid crystal display comprising the
steps of:
a. forming an array of reflective electrodes in spaced apart
columns and rows on a surface of a substrate,
b. depositing an oxide layer on said surface and over said
electrodes,
c. depositing an aluminum layer over said oxide layer, and
d. forming a two-layered lattice extending from the spaces between
said columns and rows of electrodes by removing those portions of
said layers which are over the central portions of said electrodes
and anodizing the aluminum portions of said two-layered lattice
prior to removing the portions of the oxide layer.
2. The method of claim 1 characterized further in that said step of
forming a lattice includes the steps of:
a. forming a grid-shaped mask upon said aluminum layer, said mask
covering the aluminum layer above the spaces between said columns
and rows of electrodes and exposing the aluminum layer above at
least the central portion of said electrodes,
b. etching away the exposed portions of said aluminum layer with an
etchant which does not attack said oxide layer, thereby exposing
those portions of said oxide layer which lie above at least the
central portion of said electrodes,
c. anodizing the remaining portions of said aluminum layer, and
d. using said anodized aluminum portions as a mask, etching away
the exposed portions of said oxide layer with an etchant which does
not attack anodized aluminum.
3. A method of fabricating a backplate with electrodes and
integrated spacers for a liquid crystal display comprising the
steps of:
a. forming a plurality of electrodes on a surface of a substrate by
depositing aluminum upon said substrate and etching away portions
thereof,
b. forming a multi-layer on said substrate and over said electrodes
by successively forming an oxide layer and an aluminum layer on
said substrate, and
c. converting said multi-layer into a plurality of spaced apart
walls of equal height by successively etching through said aluminum
layer and said oxide layer so as to expose said central portions of
said plurality of electrodes, said aluminum layer being anodized
after it has been etched but prior to the etching of said oxide
layer, so as to make said aluminum layer resistant to attack by
nematic liquid crystal material.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to liquid crystal displays
and more particularly to a method for fabricating the backplate for
such displays having thereon spacers to maintain the thickness of
the liquid crystal display uniform throughout.
In copending application Ser. No. 352,397 filed by Hans G. Dill, et
al, on Apr. 18, 1973, and entitled Liquid Crystal Display System
with Integrated Signal Storage Circuitry, there is described a
liquid crystal display panel having a plurality of spacers between
the back and front panels of the liquid crystal display. The
purpose of these spacers is to maintain a uniform spacing between
the front and back panels of the liquid crystal display. One of the
features of the invention described in the referenced application
is the provision of addressing circuitry which is fabricated in the
back panel of the display, which for this purpose is a
semiconducting wafer.
It is a principal object of the present invention to provide a
method for the fabrication of spacers of the type disclosed in the
referenced patent application which method is compatible with the
steps required to fabricate liquid display panels of the type
therein described.
It is a related object of the invention to provide a method for the
fabrication of back panels for liquid crystal displays with spacers
which are accurately located with respect to an array of
electrodes, also on the back panel.
A further object of the present invention is to provide a method
for fabricating liquid crystal display back panels with integral
spacers thereon wherein the spacers are resistant to attack by the
liquid crystal material, and may serve to shield electrically
conductors that run to the electrodes.
In accordance with the invention the above and other objects are
accomplished by first forming an array of relective electrodes in
spaced apart columns and rows on a surface of a substrate panel
which is preferably a semiconducting wafer. This step may be
preceded by several steps directed to the formation of a plurality
of switching devices in the surface of the semiconducting substrate
as described in the referenced patent application. Following the
formation of the reflective electrodes there is deposited a layer
of insulating material, preferably oxide, on top of the substrate
surface and over the electrodes, after which an additional layer,
preferably of aluminum, is deposited over the layer of insulating
material. A two-layered spacer lattice is formed from the
successively deposited layers by removing those portions of the
layers which are over the central portions of the electrodes.
Preferably, so much of the layers is removed as to expose all but
the extreme perimeters of the array of electrodes, thereby
providing a slight overlap of the two-layered lattice over the
electrodes. The resulting structure, comprising the back panel
having a plurality of electrodes on its surface and a spacer
lattice extending integrally from that surface, may then be used to
complete the fabrication of a liquid crystal display by adding a
front transparent panel having a transparent electrode thereon and
placing a nematic liquid crystal material between the front and
back panels. Further objects and features of the invention will
become apparent from the following description and drawings in
which:
FIG. 1 is a perspective view of a liquid crystal display having a
less desirable peripheral spacer between the front and back panels
thereon;
FIG. 2 is a cross section through the display illustrated in FIG.
1;
FIG. 3 is a diagrammatic perspective view of a liquid crystal
display incorporating the spacer lattice configuration produced in
accordance with the present invention;
FIG. 4 is a cross section through the display illustrated in FIG.
3; and
FIGS. 5-13 are a series of plan views and cross sections
therethrough illustrating a back panel fabricated in accordance
with the present invention at successive stages of such
fabrication.
Referring now to the figures, a liquid crystal display of
conventional construction is illustrated in FIGS. 1 and 2. A
nematic liquid crystal material 11 is confined between back and
front plates 13 and 15 by a peripherally extending spacer 17. An
array of electrodes 19 is disposed on the surface of the backplate
13 and a transparent common electrode (not shown) is disposed on
the inside surface of the transparent front plate 15. Desired
images may be displayed by the selective actuation of desired ones
of the array of electrodes 19 so as to establish an electric field
between them and the front electrode across the liquid crystal
material 11 lying between them. The particular theory of operation
of liquid crystal displays is not of concern in this application
but may be gleaned from the above referenced application which is
hereby incorporated by reference.
It is an inherent disadvantage of the peripheral spacer 17 that it
permits the bowing of the front electrode carrying plate 15,
thereby causing uneven electric fields to be applied across the
width and length of the display. As a result, different field
strengths will exist across the liquid crystal material when
various ones of the back electrodes 19 are energized, causing
uneven changes in the appearance of the liquid crystal material
across the display.
The above shortcomings are minimized by the provision of a spacer
lattice in the liquid crystal display illustrated schematically in
FIGS. 3 and 4. The latter liquid crystal display is shown with the
same components as those illustrated in FIGS. 1 and 2 except that
in place of the peripherally extending spacer 17 there is provided
a lattice whose walls crisscross the surface of the back panel 13
between the individual electrodes 19. As best seen in FIG. 4, the
individual walls of the spacer lattice 21 include a base 23 which
rises above the surfaces of the electrodes 19 and a top portion 25
whose heights are uniform relative to the surface of the backplate
13. Preferably, the bases 23 are formed of an oxide layer which is
particularly convenient to produce on top of the backplate 13 when
that plate is made of silicon which is a preferable material for
the backplate of the type disclosed in the above-referenced patent
application. Because of the limitations which exist in the
thickness to which such an oxide layer can be grown, it is
preferable that the top portions of the walls of the lattice 21 be
formed of a different material and aluminum which is already used
in the process of fabricating the panel for forming the electrodes
19 is particularly suitable for this purpose.
Turning now to FIGS. 5-13, there will be next explained a method
for fabricating the spacer lattice of FIGS. 3 and 4 in accordance
with the present invention.
Prior to the fabrication of the spacer lattice, there is first
formed on a suitable backplate 13 an array of electrodes 19. As
described in the above-referenced patent application, the backplate
13 is preferably formed of a silicon substrate, typically in the
form of a wafer about two inches in diameter. Alternatively, of
course, the wafer may be square rather than circular in outline. In
addition to forming an array of electrodes 19, arranged in columns
and rows as shown in FIGS. 5 and 6, there are also formed on the
backplate 13 X and Y buslines or conductors 22 and 24. The purpose
of the respective X bulines 22 is to conditionally enable all of
the electrodes 19 in a row associated with a particular busline.
Similarly, it is the purpose of the respective Y buslines 24 to
conditionally enable all of the electrodes 19 in a particular
column associated with a particular y busline. Thus, when a
particular pair of X and Y buslines 22 and 24 is energized, this
will cause a unique one of the electrodes 19 to be actuated. This
is accomplished by providing a switching transistor 26 for each
electrode 19. Since each of the transistors 26 functions as an AND
gate, it is represented by the conventional symbol for such a gate
in FIG. 5. The switching transistors 26 may be formed in the manner
described in detail in the referenced patent application in the
body of the silicon backplate 13 and, since their fabrication is
not a part of the present invention, it will not be described in
detail herein. Briefly, however, the process described in the
referenced patent application produces field effect transistors in
the surface of the backplate 13, each of these transistors having a
source connected to its associated electrode 19, a drain connected
to one of the X and Y buslines 22 and 24, and a gate connected to
the other one of the buslines 22 and 24. Thus, by energizing a
particular pair of buslines 22 and 24, the necessary connections
are made to the gate and drain of a particular transistor 26 to
complete a circuit through that transistor to the electrode 19 with
which it is associated.
The transistors 26 are shown only schematically in FIG. 5 and are
not shown physically either in that Figure or in FIG. 6, which is a
cross section therethrough. It will be understood, however, that
the switching transistors 26 are physically located in the surface
of the backplate 13. Also disclosed in the referenced patent
application is a technique for providing cross-under connections in
the surface of the backplate 13 for either the X or the Y buslines
22 and 24. Thus, assuming that it is the X buslines 22 which are
provided with such cross-under connections, a doped region is
formed by conventional semiconductor doping techniques in the
surface of the substrate 13 under those regions of the Y buslines
24 where they intersect the X buslines 22. Each of the X buslines
22 makes contact with the doped cross-under connection on both
sides of the Y buslines to establish a continuous X bus conductor.
Since the deposition of the aluminum conductors and electrodes 22,
24 and 19 are preceded by the formation of an oxide layer (not
shown) in the process of forming the doped regions in the substrate
which comprise the cross-under connectors, as well as the field
effect transistors, the cross-under connections are prevented from
directly connecting the X conductors 22 to the Y conductors 24.
Contact between the X conductors 22 and their respective doped
cross-under connections is established by etching through the oxide
layer above these doped cross-under connections so that when the
metal layers, including the bus conductors 22 are formed, they
extend down to the cross-under connectors to establish contact with
them.
Returning now to the description of the present invention, after
the formation of the display electrodes 19 and their associated
buslines 22 and 24, an insulating layer 25 is deposited over the
surface of the substrate 13 so as to cover both the surface and the
electrodes 19. Preferably the insulating layer is a silicon dioxide
film doped with phosphorus to effect a more uniform deposition. A
thickness of 1.5 microns can be readily achieved in a horizontal
resistance heated furnace at 450.degree.C, maintained for 30
minutes. The doped silicon dioxide layer is formed in the furnace
by the decomposition of the SiH4 and doping of the oxide can be
achieved by adding phosphine gas Other means of depositing the
insulating layer, such as sputtering and evaporating, may also be
employed.
There is next evaporated a layer of aluminum 27 on top of the doped
oxide layer 25. As will become apparent shortly, it is the total
thickness of the two layers 25 and 27 which ultimately determines
the total thickness of the liquid crystal display device in which
the fabricated assembly will be incorporated. The optimum cell
thickness and therefore the optimum spacing between the back
electrodes 19 and the electrode carried by the front plate 15
depends on the particular application for which the display device
is to be used. One of the advantages of the present invention is
that whatever is the desired thickness it can be readily achieved
by varying the thickness of the aluminum layer 27, since the
evaporation process can be controlled within the required
tolerances without difficulty. since front to back spacing of 6 to
10 microns is typical, the usual thickness of the aluminum layer
will be about four to eight microns, which with the 1.5 micron
oxide layer 25 results in a total spacer height of 5.5 to 9.5
microns. In comparison a typical spacing between adjacent walls of
the spacer will be about 10 mils, or about 25 times the height of
the spacer walls.
Windows 29 are next opened to the reflective back electrodes 19.
For this purpose, a photoresist layer is applied on top of the
aluminum layer 27 and a suitable pattern in the photoresist layer
is exposed after which the photoresist is developed, the exposed
portions are removed, thereby exposing through openings 31 in the
photoresist layer portions of the aluminum layer 27 corresponding
to the windows 29 which are to be formed therethrough. The masked
aluminum layer 27 is then exposed to an aluminum etchant which
removes the exposed portions of the aluminum layer down to the
bottom oxide layer 25.
Next the remainder of the photoresist mask is removed and the
remaining aluminum pattern 33 is anodized so as to minimize any
possible subsequent interaction between the aluminum pattern 33 and
the liquid crystal material with which it will interface.
Anodization is suitably performed electrolytically in a solution of
tartaric acid (concentration 3% Ph adjusted to 5.5). Application of
150 volts between the aluminum structure 33 and a negative
electrode, both submerged in the tartaric acid solution, will
result in an anodized layer of 2,100 angstroms.
Finally, using the anodized aluminum layer 35 as a mask, the
exposed portions of the underlying oxide layer 25 are etched away
in a solution of buffered hydrofluoric acid. This etchant will
expose the central portion of each of the electrodes 19. In other
words, the spacer lattice at its base overlaps the edges of the
electrodes 19.
This completes the fabrication of the backplate and its integrated
spacer lattice. Liquid crystal material may now be placed on the
backplate in the interstices of the spacer lattice, after which the
top plate 15 is secured in place.
What has been described in a preferred method for fabricating an
integrated spacer lattice for the backplate of a liquid crystal
cell. Modifications of the invention will readily occur to those
skilled in the art having the benefit of the above description. For
example, other materials may be used in place of those given above
for the two layers 25 and 27. Moreover, a single layer of an
insulating material which can be grown to satisfy the height
requirements for the spacer lattice can be used. Such a material
might, for example, be polycrystalline silicon. Moreover, whereas
there have been shown an array of electrodes arranged on the
backplate of a liquid crystal cell in columns and rows, it is
apparent that the method of the present invention for fabricating a
spacer lattice structure for such a backplate could be employed
with equal benefits with a different arrangement of such
electrodes. Generally, it will be true that such a spacer lattice
and method disclosed therefor will be found useful wherever there
are a plurality of electrodes spaced from one another on the
backplate of the liquid crystal cell regardless of the geometrical
configuration into which its distribution may fall.
* * * * *