U.S. patent application number 09/406647 was filed with the patent office on 2001-08-16 for reflective liquid crystal display device having an array of display pixels.
This patent application is currently assigned to U.S. PHILIPS CORPORATION. Invention is credited to YOUNG, NIGEL D..
Application Number | 20010013913 09/406647 |
Document ID | / |
Family ID | 10839757 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013913 |
Kind Code |
A1 |
YOUNG, NIGEL D. |
August 16, 2001 |
REFLECTIVE LIQUID CRYSTAL DISPLAY DEVICE HAVING AN ARRAY OF DISPLAY
PIXELS
Abstract
In a reflective liquid crystal display device comprising on a
substrate (12) an array of reflective pixel electrodes (45) which
are each connected to the output of a respective switching device
(18), e.g. a TFT, carried on the substrate and which are provided
on an insulating layer (40) that extends over the switching device,
each pixel electrode (45) is connected to the output (31) of its
associated switching device through a plurality of tapered contact
openings (47) in the insulating layer (40) which form depressions
(50) in the pixel electrode surface for enhancing the pixel's light
reflection characteristics. The number, shape, size and relative
disposition of such openings can be varied to tailor these
characteristics. Preferably, a conductive layer (35) extends from
the switching device output (31) beneath the area of the pixel
electrode (45) for contacting the electrode at each opening and may
have a rough surface resulting in asperities at the pixel electrode
surface which further enhance its reflection properties.
Inventors: |
YOUNG, NIGEL D.; (REDHILL,
GB) |
Correspondence
Address: |
CORPORATE PATENT COUNSEL
U S PHILIPS CORPORATION
580 WHITE PLAINS ROAD
TARRYTOWN
NY
10591
|
Assignee: |
U.S. PHILIPS CORPORATION
|
Family ID: |
10839757 |
Appl. No.: |
09/406647 |
Filed: |
September 27, 1999 |
Current U.S.
Class: |
349/113 |
Current CPC
Class: |
G02F 1/133553 20130101;
G02F 1/1368 20130101; G02F 1/136227 20130101; G02F 1/133345
20130101; G02F 1/133504 20130101 |
Class at
Publication: |
349/113 |
International
Class: |
G02F 001/133 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 1998 |
GB |
9821311.9 |
Claims
1. A reflective liquid crystal display device comprising first and
second substrates between which liquid crystal material is disposed
and electrodes provided on the substrates defining an array of
display pixels, the first substrate having an array of optically
reflective pixel electrodes each of which is connected to the
output of a respective switching device carried on the first
substrate and is provided on the surface of an insulating layer
extending over the first substrate and covering the switching
devices, characterised in that the pixel electrode is connected to
the output of the switching device via a plurality of contact
openings in the insulating layer at spaced locations over the area
of the pixel electrode and having sloping sidewalls over which the
pixel electrode extends.
2. A reflective liquid crystal display device according to claim 1,
characterised in that the contact openings are arranged regularly
over a substantial part of the pixel electrode area.
3. A reflective liquid crystal display device according to claim 2,
characterised in that the contact openings are arranged in rows and
columns.
4. A reflective liquid crystal display device according to claim 2
or 3, characterised in that the contact openings of each pixel are
of substantially identical size and shape.
5. A reflective liquid crystal display device according to any one
of the preceding claims, characterised in that the pixel electrode
contacts via the plurality of contact openings an electrically
conductive layer extending beneath the insulating layer over the
area occupied by the plurality of contact openings and connected to
the switching device output.
6. A reflective liquid crystal display device according to claim 5,
characterised in that the surface of the electrically conductive
layer is rough so as to form surface asperities in the pixel
electrodes via the insulating layer extending thereover.
7. A reflective liquid crystal display device according to claim 5
or claim 6, characterised in that the switching device comprises a
thin film transistor and in that the electrically conductive layer
comprises an integrally formed extension of the drain electrode of
the thin film transistor.
Description
[0001] The present invention relates to a reflective liquid crystal
display device comprising first and second substrates between which
liquid crystal material is disposed and electrodes provided on the
substrates defining an array of display pixels, the first substrate
having an array of optically reflective pixel electrodes each of
which is connected to the output of a respective switching device
carried on the first substrate and is provided on the surface of an
insulating layer extending over the first substrate and covering
the switching devices.
[0002] An example of such a display device is described in
EP-A-0617310. In this device, a row and column matrix array of
display pixels is provided, each of which is driven via an
associated switch device in the form of a TFT (thin film
transistor). The TFTs are carried on the surface of a first
substrate together with sets of row, selection, conductors and
column, data, conductors through which the TFTs are addressed for
driving the display pixels. As in conventional active matrix LCDs
using TFTs, each TFT is disposed adjacent the intersection between
respective ones of the row and column conductors. The gates of all
the TFTs associated with a row of display pixels are connected to a
respective row conductor and the sources of all the TFTs associated
with a column of pixels are connected to a respective column
conductor. Unlike conventional active matrix LCDs, however, in
which the individual pixel electrodes are arranged substantially
co-planar with, and laterally of, the TFTs, the reflective pixel
electrodes in this device are carried on an insulating film which
extends over the first substrate and covers the TFTs and the sets
of address conductors so that the pixel electrodes are positioned
generally above the level of the TFTs and the address conductors.
Each individual pixel electrode is connected to the drain electrode
of its associated TFT through a respective opening formed in the
insulating film directly over the drain-electrode. An advantage of
this type of construction, in which the array of pixel electrodes
and the array of TFTs are provided at different levels above the
substrate surface, is that the pixel electrodes can be enlarged
such that at two opposing sides they extend slightly over adjacent
row conductors and at their two other opposing sides they extend
slightly over adjacent column conductors rather than being sized
smaller than the spacing between adjacent row conductors and
adjacent column conductors with small gaps provided between each
edge of the pixel electrode and the adjacent conductor, as in
conventional display device arrangements. In this way, therefore,
the pixel aperture is increased and in operation more light which
passes through the liquid crystal layer and reaches the pixel
electrode is reflected back to produce a brighter display output.
Moreover, parts of a deposited metal layer which is patterned to
form the reflective pixel electrodes can be left immediately
overlying the TFTs during the patterning process so as to act as
light shields for the TFTs to reduce photoelectric effects in the
TFTs due to light incident thereon, thereby avoiding the need to
provide black matrix material on the other substrate for this
purpose as is usual. This other, transparent, substrate carries a
continuous transparent electrode common to all pixels in the array
and, in the case of a colour display, an array of colour filter
elements corresponding to the array of pixels with each filter
element overlying a respective pixel electrode.
[0003] In order to improve the reflection characteristics of the
pixels, particularly the resulting intensity of light scattering in
the direction perpendicular to the display panel with respect to
light incident on the pixel electrode at any angle, the pixel
electrodes in the display device of EP-A-0617310 are made
undulating by forming the region of the insulating film underlying
the reflective pixel electrode with a plurality of randomly
arranged bumps so that the pixel electrode deposited thereon, and
comprising a metal layer of substantially constant thickness,
similarly has surface bumps. These bumps on the pixel electrode
serve to scatter light so that a greater proportion of light
incident on the electrode from any angle is reflected in a
direction normal to the panel to increase pixel luminance. The
bumps in the insulating film are themselves formed by patterning a
deposited photoresist layer with the aid of a mask, light exposure
and development to leave discrete dots of photoresist whose area
and position are determined by the mask, and then depositing a
further organic insulating layer over these dots. Thereafter a
contact opening is formed at each pixel in the insulating film
overlying the drain electrode of the TFT and a reflective metal
layer is deposited which extends through these openings to contact
the underlying drain electrodes and which is patterned to define
the individual pixel electrodes.
[0004] The formation of the undulating pixel electrodes is thus
complicated, involving the deposition and processing of a number of
separate layers including photoresist and organic insulating films
which add significantly to the complexity of manufacture.
Importantly, it is necessary for the dots of photoresist material
to be shaped appropriately to avoid sharp edges and the like so
that suitably shaped bumps result at the surface of the pixel
electrodes and also for the region overlying the drain electrodes
to be kept free of bumps.
[0005] It is an object of the present invention to provide an
improved reflective LCD of the kind described in the opening
paragraph.
[0006] It is another object of the present invention to provide a
reflective LCD of the aforementioned kind which is relatively
simple to manufacture.
[0007] According to the present invention, there is provided a
reflective LCD of the kind described in the opening paragraph which
is characterised in that the pixel electrode is connected to the
output of the switching device via a plurality of contact openings
in the insulating layer at spaced locations over the area of the
pixel electrode and having sloping sidewalls over which the pixel
electrode extends. The plurality of contact openings serve to
enhance the reflection characteristics of the pixel electrode.
These contact openings result in depressions in the pixel electrode
surface which, in comparison with the structure of EP-A-0617310,
are effectively negative, or inverse, bumps but which behave in
similar, light scattering, fashion for reflecting incident light.
Moreover, the plurality of contact openings serves also to provide
a degree of redundancy in the electrical connection between the
pixel electrode and the switching device output.
[0008] The plurality of contact openings at each pixel location can
be provided in simple manner, for example by patterning the
insulating layer photolithographically using a mask to define the
contact openings and their relative disposition. The layer may be
etched or, in the case of the insulating layer comprising a
photo-resist material, photodeveloped. It is to be noted that in
the device of EP-A-617310, a single contact opening is provided in
the insulating layer by a photolithographic process before the
pixel electrode is deposited so as to enable electrical connection
between the pixel electrode and the underlying drain electrode of
the TFT to be established. The provision of a plurality of contact
openings in the device of the present invention does not add
significantly to the complexity of the processing in this
manufacturing stage and can be accomplished without any additional
processing operations being required.
[0009] Preferably, the plurality of contact openings are arranged
regularly over a substantial part of the pixel electrode area, for
example in a generally uniform row and column array occupying 50%
or more of the overall area of the pixel electrode. The number,
size, shape and relative disposition of the openings can be varied.
For example, the openings may be generally circular or square. As a
consequence of the openings being formed by a photolithographic
method such as etching the sidewalls of the openings in the
insulating layer will be sloping to some extent so that the shape
of the resulting depressions in the subsequently deposited pixel
electrodes will be tapering, e.g. generally conical in the case of
circular openings. The exact shape of the depressions will depend
though on the relative thicknesses of the insulating layer, and
thus the depth of the openings, and the material of the pixel
electrode layer as well as the width of the openings. If the pixel
electrode layer is relatively thick then the resulting depressions
formed therein will tend to smoothed out to some extent, for
example in the nature of inverted domes. The openings may be spaced
apart from one another so that substantially flat areas of the
pixel electrode layer exist between adjacent openings or arranged
close together so that the extent of the pixel electrode material
between the depressions is reduced or minimised.
[0010] Depending on the number and position of the contact openings
with respect to the switching device, connection between the pixel
electrode and the output of the switching device may be achieved
via respective, individual, electrically conductive tracks
underlying the insulating layer and extending from the switching
device. Preferably, however, an electrically conductive layer
connected to the output of the switching device is provided beneath
the insulating layer extending over a substantial part of the area
of the pixel electrode, corresponding to the region in which the
contact openings are formed. In the case of the switching device
comprising a TFT, this conductive layer may be formed integrally
with the drain electrode of the TFT, as an extension, from a single
deposited layer. Again, comparing this with the display device of
EP-A-0617310, it will be appreciated that the provision of this
underlying electrically conducting layer requires no significant
additional processing operations. Thus, the reflection property
enhancing depressions in the pixel electrode of the device of the
present invention can be provided in simple manner merely by
modifying certain existing fabrication operations.
[0011] Preferably, the surface of this electrically conductive
layer is rough so that after depositing the insulating layer and
pixel electrode thereon the surface of the pixel electrode
possesses a degree of roughness, providing surface asperities. Such
roughness in the surface of the pixel electrodes in the regions
around the contact openings assists in achieving desirable
scattering reflection characteristics. The roughness of the
conductive layer may be introduced deliberately or achieved as a
natural consequence of fabrication processing by appropriately
selecting the deposition conditions. In the case, for example, of
the switching device comprises a polysilicon TFT having source and
drain contacts of laser crystallised, n type, polysilicon, the n
type polysilicon material inherently has a degree of surface
roughness which may be adequate for this purpose. If a metal is
used for this layer then depositing the metal in a substantially
pure form will tend to create bigger grains, and hence roughness.
Also, the deposited material of this underlying electrically
conducting layer may be deliberately roughened by further
processing in a known manner to provide this effect.
[0012] It will be appreciated that switching devices other than
TFTs may be employed, for example two-terminal non linear switching
devices such as MIMs or TFDs (thin film diodes). When using such
devices it is necessary to provide only one set of address
conductors, e.g. the row, selection, address conductors on the same
substrate as the switching devices and reflective pixel electrodes,
the other set of address conductor, e.g. the column, data, address
conductors, being provided on the other substrate.
[0013] The display device may be a monochrome display device or a
colour display device in which colour filter elements are provided
on the other substrate, for example as described in
EP-A-0617310.
[0014] Embodiments of reflective liquid crystal display devices in
accordance with the present invention will now be described, by way
of example, with reference to the accompanying drawings, in
which:
[0015] FIG. 1 is a schematic, cross-sectional view through part of
an embodiment of a reflective LCD in accordance with the
invention;
[0016] FIG. 2 is a schematic, plan view of the part of the LCD of
FIG. 1;
[0017] FIGS. 3A to 3G illustrate examples of possible variations in
the nature of the pixel electrodes; and
[0018] FIG. 4 is a schematic cross-sectional view through part of a
second embodiment of a display device according to the
invention.
[0019] It will be appreciated that the figures are merely schematic
and are not drawn to scale. In particular certain dimensions such
as the thickness of layers or regions may have been exaggerated
whilst other dimensions may have been reduced. The same reference
numerals are used throughout the figures to indicate the same or
similar parts.
[0020] The reflective LCD of FIGS. 1 and 2 comprises a matrix array
of display pixels arranged in rows and columns and is of the
so-called field shielded pixel type. FIGS. 1 and 2 are,
respectively, cross-sectional and plan views through a typical part
of the device showing just one display pixel, 10, for simplicity,
although it will be appreciated that the device typically may
comprise many thousands of display pixels. Referring to these
figures, the device comprises a pair of insulating substrates 11
and 12 which are spaced apart and sealed together around their
periphery so as to contain a layer of liquid crystal material 15
therebetween. Both substrates are of glass, although only the
substrate 11 needs to be optically transparent to transmit light in
operation.
[0021] The substrate 11 carries on its inner surface adjacent to
the liquid crystal layer 15 a transparent electrically conducting
layer 16, for example of ITO, which extends continuously over the
display area of the device, corresponding to the area of the pixel
array, and serves as a common electrode for the display pixels in
the array. Over this common electrode an LC orientating film 17 of
conventional form is provided.
[0022] The other substrate 12 carries on its surface active matrix
addressing circuitry and reflective pixel electrodes which define
respective individual display pixels. In this embodiment, TFTs 18
are used as the switching devices associated with the display
pixels in the active matrix addressing circuitry. The operation of
this circuitry and the manner in which the display pixels are
driven follows conventional practice, as is described for example
in U.S. Pat. No. 5,130,829 to which reference is invited for
further information in these respects and whose contents are
incorporated herein. The rows of pixels in the array are addressed
one at a time in sequence by means of a gating (selection) signal
applied to each row in turn which turns on the TFTs associated with
the pixels of the row allowing each pixel in the row to be loaded
with a respective data signal that determines its display output.
Following addressing of all the rows of pixels in this manner in
one field period, the rows are addressed again in similar manner in
successive fields.
[0023] The pixels are connected to sets of row (selection) and
column (data) address conductors 20 and 21 carried on the substrate
12 with the gates of all the TFTs 18 in a row of pixels being
connected to a respective row conductor 20 and the source
electrodes of all the TFTs in a column being connected to a
respective column conductor 21. The drain of each TFT is connected
to a pixel electrode 22 of its associated display pixel. In this
particular embodiment, the TFTs 18 comprise amorphous silicon,
bottom gate TFTs. However, top gate amorphous silicon TFTs, or
polysilicon TFTs, can be used instead.
[0024] The gates and source electrodes of the TFTs comprise
integrally formed extensions 25 and 30 of the row and column
conductors. The active matrix circuitry comprising the TFTs and the
set of rows and column address conductors is formed in conventional
manner using standard thin film processing techniques involving the
deposition and photolithographic patterning of various layers. The
row conductors 20 and gates 25 are provided by depositing and
patterning a conductive material such as aluminium or aluminium and
chromium. Following this, an insulating layer 26 of silicon nitride
or oxide constituting the gate insulator of the TFTs, and serving
also to separate the row and column conductors at their cross-over
points, is deposited over the entire surface of the substrate 12.
Thereafter, a layer 27 of the amorphous silicon is deposited and
patterned to leave portions at the locations of the TFTs
constituting the channel regions of the TFTs. Doped (n type)
amorphous silicon source and drain contact electrodes (not shown)
may then be formed over the intrinsic amorphous silicon layer 27 at
opposite sides of the gate if desired. The column conductors 21 and
TFT source and drain electrodes are then defined by depositing a
layer of, for example, a metal, such as Al, a metal alloy, or ITO,
and patterning this layer to leave the column conductors with
integral extensions 30, for the source electrodes, and the drain
electrodes 31. Although a particular structure of TFT of simple
form is shown here, it will be appreciated that other kinds of TFT
structures which are known in the art can be used instead, with
either a top or bottom gate, and that the order in which the layers
are provided, the nature of these layers, and the materials
employed can be varied accordingly.
[0025] The drain electrode 31 of each of the TFTs is provided with
an integrally formed extension 35, defined simultaneously with
patterning of the drain electrode layer, which extends laterally of
the TFT over the substrate surface, and above the insulator layer
26, so as to occupy a substantial proportion of the eventual pixel
area. As can be seen particularly from FIG. 2, the electrically
conductive extension layer 35 is generally rectangular with its
edges extending alongside, and slightly inwards of, adjacent pairs
of row and column conductors which border the pixel. This drain
extension 35 may be formed separately from a deposited conductive
material different to that of the drain electrode 31 and with the
extension overlapping, and in electrical contact with, the drain
electrode at the region of the TFT, rather than being integrally
formed from the same deposited layer.
[0026] Over this structure on the substrate 12 a comparatively
thick film 40 of insulating material such as silicon nitride or
oxide, or an organic insulating material such as a polyimide or a
photo-resist, is disposed. The film 40 extends continuously over
the entire surface of the substrate and completely covers the TFTs
18, the drain extension layers 35 and the sets of row and column
conductors to provide a surface parallel to the substrate surface
and spaced from those components upon which the pixel reflective
electrodes 45 are formed. Prior to forming these electrodes,
however, a series of tapering contact holes, or vias, 47 are
provided by a photolithographic process over the area of the
extension layer 35 at each pixel which extend through this
insulating layer. When using silicon nitride or oxide or a
polyimide material, a standard photolithographic process using a
resist, exposure through a mask and etching may be employed to form
the contact holes. When using a photo-resist polymer, the contact
holes can be formed simply by photodeveloping.
[0027] The pixel electrodes are then formed by depositing a layer
of electrically conductive, light--reflective, material such as
aluminium, an aluminium alloy, or silver over the substrate which
covers the surface of the layer 40 and the sloping sidewalls of the
tapering contact openings 47 to contact electrically with the
underlying drain extension layer 35 at each contact opening
location. This reflective layer is then photolithographically
patterned to leave the array of discrete, mutually-spaced, pixel
electrodes 45, each of which is electrically connected with its
respective underlying layer 35 via the plurality of contact
openings. Each pixel electrode 45 in this embodiment is generally
rectangular, for example, approximately 100 .mu.m square, and so as
to provide a high aperture ratio extends completely over the area
between the sections of the two row conductors 20 and the two
column conductors 21 bordering the pixel 10 and partly over these
conductor sections as well. Each pixel electrode is separated from
its adjacent pixel electrodes, which also overlap these conductors,
by small gaps so as to maintain electrical isolation between the
electrodes. The comparatively thick insulating layer 40 ensures
that there are no significant capacitive coupling effects between a
pixel electrode 45 and the address conductors at the regions of
overlap. The array of pixel electrodes and intervening surface
areas of the layer 40 are covered by a continuous LC orientation
layer 52 of conventional kind.
[0028] The liquid crystal material of the layer 15 comprises a
guest-host LC material, for example of the kind described in
EP-A-0617310. However, other suitable known types of LC materials
could be used. Although desirably the material should be of a type
which does not require the provision of a polarising film which
would reduce the amount of light utilised for display purposes, a
twisted--nematic liquid crystal material could be employed together
with a polarising film provided on the outer surface of the
substrate 11.
[0029] In operation, each row of pixels is addressed in sequence by
means of a gating signal applied by a conventional peripheral row
driver during a respective row address period which turns on the
TFTs associated with the row of pixels so as to transfer image data
voltage signals present on the column conductors 21, supplied by a
peripheral column driver circuit, to the respective pixels
electrodes 45 via the drain extension layers 35 to cause the
required display effect from the row of pixels. Light entering the
display device through the substrate 11 is modulated by the LC
material and reflected by the pixel electrodes back through this
substrate, depending on the display state of the individual pixels,
to produce a display image.
[0030] The plurality of tapering contact openings 47 distributed
over the pixel area result in the reflective pixel electrode 45
having reflective depressions 50 at these opening locations which
serve to enhance the light scattering characteristics of the pixel
electrode, for example, to ensure that light is reflected towards a
viewer in a direction generally normal to the plane of the
substrate 11 so as to produce a display output of adequately high
luminance, bearing in mind that light can be incident on the
reflective pixel electrodes from various different angles. The
intensity of light reflected perpendicular and near perpendicular
to the substrate 11 is thus increased, resulting in a bright
display output and improved display quality. The improved
scattering characteristics can also be beneficial to increasing the
viewing angle.
[0031] In the example illustrated in FIG. 2, the contact openings
47, and hence the depressions 50, are organised in a regular row
and column array, of five rows with each row, apart from the first,
having six openings, and with adjacent depressions 50 being
separated in both the row and column directions by a similar,
predetermined, distance. In this example, the size of the
depressions at their opening will typically be around 5 .mu.m in
width or greater. The size, shape and relative dispositions of the
openings, and hence depressions, may be varied according to, for
example, the overall size of the pixel electrode and the particular
reflection characteristics desired. Typically, there will be a few
tens of opening per pixel distributed over a substantial
proportion, greater than around 50%, of the overall pixel area. The
openings may be circular in cross-section or polygonal, for example
square, and arranged closer together or further apart, the shape
and mutual position being determined by the mask used in the
photolithographical patterning of the layer 40. The
openings/depressions need not be in a regular, row and column,
array but could instead be arranged in a quasi-random fashion. For
polygonally-shaped contact openings, the orientation of the
individual openings in the array may be varied. A mixture of
differently sized and/or differently shaped openings may be
provided over the area of the pixel electrode. A smaller number of
relatively large openings may be used. Examples of some possible
variations are illustrated in FIGS. 3A to 3G which show the shape
of the depressions 50 towards their open end in portions of typical
pixel electrodes. In FIGS. 3A, 3B, 3D, and 3E the openings are of
rectangular, circular, square and hexagonal shape respectively and
of substantially similar dimensions. In FIG. 3G, the orientations
of the openings, here square shape, are randomly varied.
[0032] The depth of the depressions can also be varied to some
extent, although this is dependent on the thickness of the layer
40, the degree of tapering of the contact openings 47, and also the
thickness of the layer used to form the pixel electrodes. With
relatively small openings 47, and/or a relatively thick layer of
reflective material for example, the depressions 50 resulting in
the pixel electrode may be more generally rounded, and in the form
of inverted domes.
[0033] These possible variations can be utilised so as to tailor
the scattering effects produced and optimise the scattering
reflection characteristics of the pixel electrodes.
[0034] Preferably, the nature of the depressions 50, i.e. the
number, size, shape, positioning, etc is the same for each pixel
electrode 45 in the array so that similar, uniform, reflection
characteristics are obtained from all the pixels in the array.
[0035] In order to enhance still further the scattering effects of
the pixel electrodes, the drain extension layer 35 at each pixel is
provided with a rough upper surface, as shown at 36 in FIG. 1,
which, as a result of the layers 40 and 45 being deposited in
sequence thereon, translates through these layers and produces a
degree of roughness at the surface of the pixel electrodes 45 in
the regions between the depressions. The amount of roughness
produced at the surface of the pixel electrodes depends on various
parameters, particularly the thickness of the layer 40, but it can
be expected that some of the roughness of the layer 35 will be
translated to the pixel electrode surface to form a microscopic
unevenness at the surface of the pixel electrode which will enhance
the light scattering capability of those regions. Such unevenness
may be in the form of protrusions, undulations, or other kinds of
asperities typically having on average a pitch of around 1 to 3
.mu.m and a height of around 0.5 to 1 .mu.m.
[0036] The surface of the layer 35 may be deliberately roughened to
this end, for example by etching or other known technique, after it
has been deposited or the material of the layer 35 may be deposited
in such a manner that the required roughness occurs naturally due
to the deposition conditions. A layer of polysilicon material
formed by a laser recrystallisation technique and having a rough
surface can be created without difficulty. In the case, therefore,
of the switching device comprising a polysilicon TFT instead of an
a-Si TFT then polysilicon material is conveniently available to be
used for this purpose. FIG. 4 is a cross-sectional view through
another embodiment of display device using a polysilicon TFT as the
switching device. In this device the polysilicon TFT is a top-gate
polysilicon TFT of relatively simple structure. The TFT comprises a
layer of polysilicon material 55 formed by laser crystallisation on
the surface of the substrate 12 over which a gate insulator layer
26, comprising silicon nitride or oxide, is disposed and with the
gate 25, for example of an Al alloy, being formed on the insulator
layer 26. Regions of the polysilicon layer to either side of the
gate are doped to form n+ polysilicon, for example by ion doping
using the gate as a mask, to provide source and drain contacts 30
and 31 with the intrinsic polysilicon material extending
therebetween constituting the channel. A region of the n+ doped
polysilicon layer is left after patterning to provide the drain
extension layer 35. Thus, the drain contact 31 and this layer 35
comprise different regions of the same deposited layer. At least
this region 35 of the polysilicon layer is formed deliberately in a
manner resulting in a rough upper surface 36 having asperities with
dimensions in the aforementioned range. The gate insulator layer 26
is also extended to cover the extension layer 35, and in this case
extends completely over the substrate surface, but may instead be
terminated adjacent the end of the drain contact 31 by appropriate
patterning. The layer 40 and the pixel electrodes 45 are then
formed over this structure as before with contact openings 47 being
provided through the layer 40, and the extension of the gate
insulator layer 26 if present, before deposition of the reflective
layer constituting the electrodes 45 so that the electrode material
penetrates the openings and contacts the underlying n+ polysilicon
layer 35, and with the roughness of the surface of the layer being
translated to the surface of the pixel electrodes to cause
microscopic surface irregularities and unevenness. Depending on the
particular kind of TFT structure used, a further insulating layer
may be deposited after forming the gates 25 and prior to depositing
the layer 40.
[0037] While the roughness of the layer 35 here is conveniently
obtained as a direct consequence of the manner of the layer
formation, other techniques may be used deliberately to introduce
such roughness, for example by an etching process. These techniques
are applicable also in the case where the layer 35 is formed
separately from, but in contact with, the drain electrode, for
example from a deposited metal layer.
[0038] The switching devices of the active matrix circuitry need
not comprise TFTs but may instead comprise two-terminal non-linear
switching devices such as MIMs or TFDs (thin film diodes). In a
display device using such switching devices, it is usual to provide
just the set of row, selection, address conductors on one substrate
and the set of column, data signal, address conductors on the other
substrate. The switching devices and the pixel electrodes can be
provided on either substrate but normally on the substrate carrying
the row conductors. The switching devices typically comprise a pair
of conductive contact layers between which a layer of insulating,
or semi-insulating, material is sandwiched. One contact is
connected to the row conductor, and may be an integrally formed
extension, and the other, output, contact is connected to the
associated pixel electrode. This output contact may be formed with
an integral extension to provide the contact layer 35 underlying
the pixel electrode 45 or the layer 35 may be formed from a
separately deposited conductive layer contacting the output contact
of the switching device.
[0039] For reflective mode operation, the substrate 12, unlike the
substrate 11, need not be transparent and may be of a semiconductor
material, such as a single crystal silicon wafer, rather than an
insulating material such as glass.
[0040] It will be appreciated that the display devices can be full
colour display devices as well as monochrome display devices. To
this end, an array of colour filter elements associated with the
pixel array may be carried in the substrate 11, for example, as
described in EP-A-0617310.
[0041] In summary, therefore, a reflective LCD has been disclosed
of the kind comprising on a substrate an array of reflective pixel
electrodes which are each connected to the output of a respective
switching device, e.g. a TFT, carried on the substrate and which
are provided on an insulating layer that extends over the switching
device, and in which each pixel electrode is connected to the
output of its associated switching device through a plurality of
tapered contact openings in the insulating layer which form
depressions in the pixel electrode surface for enhancing the
pixel's light reflection characteristics. The number, shape, size
and relative disposition of such openings can be varied to tailor
these characteristics. Preferably, a conductive layer extends from
the switching device output beneath the area of the pixel electrode
for contacting the electrode at each opening and may have a rough
surface resulting in asperities at the pixel electrode surface
which further enhance its reflection properties.
[0042] From reading the present disclosure, other modifications
will be apparent to persons skilled in the art. Such modifications
may involve other features which are already known in the field of
LCDs and component parts thereof and which may be used instead of
or in addition to features already described herein.
* * * * *