WO2004090618A1

WO2004090618A1 - Transflective active matrix liquid crystal display device and method of manufacture

Info

Publication number: WO2004090618A1
Application number: PCT/IB2004/001072
Authority: WO
Inventors: Nigel D. Young
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-04-12
Filing date: 2004-03-30
Publication date: 2004-10-21
Also published as: GB0308484D0

Abstract

A transflective active matrix liquid crystal display device comprises a row and column array of transmissive electrodes (32) which at least partly overlies a corresponding array of reflective pixel electrodes (30) within respective pixel regions on a first substrate (10). A transparent common electrode (55) is supported by a second substrate (50), spaced from the first substrate. By forming the common electrode and the transmissive pixel electrodes from the same material, DC voltage offsets across the LC cell are minimised thus reducing undesirable display artefacts associated therewith.

Description

DESCRIPTION

TRANSFLECTIVE ACTIVE MATRDS LIQUID CRYSTAL DISPLAY DEVICE AND METHOD OF ftflANUFACTURE

The invention relates to transflective active matrix liquid crystal display (AMLCD) devices and particularly to transflective AMLCD devices having an array of reflective pixel electrodes and an array of transmissive pixel electrodes. The invention further relates to a method of manufacturing such a display device.

AMLCDs typically comprise a row and column array of thin film transistors (TFTs) supported on a first substrate and addressed by crossing sets of address conductors which are connected to row and column drive circuits. Video data signals from the column drive circuits are supplied to pixel electrodes, also carried on the first substrate, via respective TFTs one row at a time. A second substrate, spaced from the first substrate, carries a transparent common electrode. Voltages applied across individual LC pixel cells, i.e. between the respective pixel electrodes and the common electrode, determine the orientation of the LC molecules and therefore the transmissivity. In this way, each LC pixel cell acts as an optical shutter to modulate light passing through the cell.

The pixel electrodes each cover respective pixel regions which define the area of the pixel. In a transmissive type AMLCD the pixel electrodes are formed of a transparent conductive material such as Indium Tin Oxide (ITO). Light is usually provided by a backlight and passes through both substrates. In a reflective type AMLCD the pixel electrodes are formed of a reflective (and conductive) material such as Aluminium-Titanium alloy (AITi). The display is viewed using ambient light which enters the LC cell via the second substrate, passes through the cell and reflected by the respective pixel electrodes.

There is great interest in providing low power display devices which consume little energy and can therefore operate for longer periods when powered by a battery. This is particularly beneficial for small displays employed in mobile phones and PDAs for example where the reduced need to periodically charge a device is an attractive property for potential customers. The backlight in transmissive type AMLCDs requires a significant amount of power. For this reason reflective type displays are favoured over transmissive type displays for portable, battery-powered applications. However, reflective AMLCDs require ambient lighting in order to be viewed which makes viewing in low light level environments difficult.

Transflective AMLCDs typically comprise pixel regions which have a transparent portion and a reflective portion. The light passing through each portion is modulated by respective transmissive and reflective pixel electrodes. This allows the display to operate in one of two modes depending on the surroundings: a transmissive mode for viewing in a low light level environment; and a reflective mode for viewing in sunlit conditions for example. Such a display maintains the advantage of relatively low power consumption whilst enabling viewing in the dark.

US-6,501 ,519, describes a transflective AMLCD and a method of manufacturing the same. US-6,501 ,519 describes a reflective pixel electrode overlying a transmissive pixel electrode, each serving to modulate light passing through respective areas of the pixel region.

It is an object of the present invention to provide an improved transflective AMLCD device.

According to a first aspect of the invention there is provided a transflective active matrix liquid crystal display device comprising a row and column array of reflective pixel electrodes carried on a first substrate, each reflective pixel electrode covering a portion of a respective pixel region and connected to an associated thin film transistor for applying data voltages thereto, and a row and column array of transmissive pixel electrodes each overlying at least a portion of a respective one of said reflective pixel 2004/090618

electrodes and covering an area of the corresponding pixel region not covered by that one reflective pixel electrode.

According to a second aspect of the present invention there is provided a method of manufacturing a transflective active matrix liquid crystal display device comprising the steps of forming a row and column array of spaced reflective pixel electrodes over a substrate, each covering a portion of a respective pixel region and each reflective pixel electrode being connected to a respective switching element for applying data voltages thereto, and, forming a corresponding row and column array of transmissive pixel electrodes each overlying at least a portion of one of said reflective pixel electrodes and covering an area of the corresponding pixel region which is not covered by the associated reflective pixel electrode.

In a preferred embodiment according to the invention, the forming of the transmissive pixel electrodes over the reflective pixel electrodes enables the use of the same material for the two opposing electrodes across each LC pixel cell. A transparent conductive material, typically ITO, is employed for the transmissive pixel electrodes, carried on the first substrate, and the common electrode which is carried on a second substrate. A major proportion of each pixel region is covered by the transmissive electrodes. Advantageously, DC offset across the LC pixel cells is then reduced. This has been recognised to be a consequence of dissimilar materials being used to form the two opposing electrodes adjacent the LC material in the cells. DC offset across the LC pixel cells is known to cause unwanted display artefacts such as image flicker. Such adverse effects can be avoided by employing the structure of the invention. Therefore, in a preferred embodiment of a device according to the invention, the device further comprises a second substrate spaced from the first substrate and which supports a common electrode, the common electrode comprising the same material as the transmissive pixel electrodes, thus minimising the effects of DC offset caused by opposing electrodes of dissimilar materials. Each reflective pixel electrode contacts the associated transistor through a contact hole in an insulating layer which separates the reflective pixel electrodes from the associated transistors. The insulating layer provides a planar surface to support the pixel electrode array. Also, this enables the pixel electrodes to overlap underlying row and column address conductors to maximise the pixel aperture. The insulating layer is preferably formed by photodefining a polymer material. This increases the flexibility of the device and requires no deposition or etching steps thus reducing the process cost. An example of such a polymer is JSR PC405G which is commercially available from Japan Steelworks (JSW).

The transflective AMLCD device may further comprise a plurality of bumps disposed under the reflective pixel electrodes and between the pixel regions, and insulating spacer members for maintaining the spacing of the first and second substrates, wherein each spacer member overlies at least a portion of one of the bumps between the pixel regions. It is known to form bumps underneath the reflective pixel electrodes to create an undulating, scattering, surface. By also forming bumps between the pixel regions in accordance with the invention, the thickness of any layer required to form the spacer members is reduced. Therefore, the bump which forms part of the spacer member also serves as an elevated support for the remaining portion of the spacer. This allows more accurate control of the cell gap spacing when forming the spacers as it is easier to control the thickness of a thinner layer. The reflective pixel electrodes and the transmissive pixel electrodes are preferably defined by etching in a single step. By defining both sets of pixel electrodes together using a single mask, the pixel aperture, i.e. the area of the pixel regions, is maximised. In this way, an alignment overlap at the edges of the first formed electrode, otherwise necessary when patterning separately is not required. Therefore, the gap between adjacent pixel regions can be minimised.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic plan view of part of a transflective-type AMLCD device in accordance an embodiment of the invention; F gure 2 is a cross-sectional view along the line X-X of the device of

Figure 1

F gure 3 is a cross-sectional view along the line Y-Y of the device of

Figure 1

Fi igure 4 is a flowchart showing a method of manufacturing a transflective AMLCD device according to the invention; and,

Figure 5 is a diagrammatic view of an example circuit arrangement of an array of TFTs comprised in a transflective AMLCD.

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. For example, the lower substrate layers have been exaggerated. The same reference numerals are used throughout the figures to indicate the same or similar parts.

Figures 1 , 2 and 3 show various views of part of a transflective type AMLCD device in accordance with the invention. The plan view of Figure 1 shows the upper layers supported by a first substrate 10. These layers overlie a row and column array of thin film transistors 8 which is also supported by the first substrate 10. A corresponding array of pixel regions is separated from the TFT array by an insulating layer 20. For simplicity, only four complete pixel regions are shown in Figure 1. A second substrate 50 which carries a common electrode 55 on its inner surface is separated from the first substrate by insulating spacer members 35 disposed between the pixel regions. The device comprises a plurality of bumps 25 disposed over the insulating layer 20 both within the pixel regions and therebetween. Spaced reflective pixel electrodes 30 are disposed over the bumps 25 at the pixel regions and each of which is connected to a respective transistor through the insulating layer 20 via contact holes 28. Transmissive pixel electrodes 32 are disposed over the insulating layer 20, each of which overlies, and is connected to, an associated reflective pixel electrode 30 and covers an area of the respective pixel region not covered by that reflective pixel electrode 30. Advantageously, the opposing electrodes across each LC pixel cell, i.e. the common electrode 55 and transmissive pixel electrodes 32 are both formed of ITO. Therefore, the DC voltage offset across the cells is minimised, preventing adverse display effects associated therewith. Such effects include image flicker and ion accumulation on the LC alignment layers which can lead to image sticking.

Each spacer member 35 comprises a respective one of the bumps 25 which provides an elevated support for the remaining portion of the spacer member 35'. The height contribution made by the underlying bump advantageously reduces the thickness required by the remainder of the spacer member.

Now, with reference especially to Figures 2 and 3, a method of manufacturing this transflective AMLCD device will be described. The first substrate 10 is formed of a polymer material. The advantages of polymer substrates over traditional glass substrates are well known and include reduced weight and increased flexibility. However, other substrates, including glass, may be employed for the purposes of the invention. Example polymers which are commercially available include polyarylate (FPE), polynorbonene (PNB), polyimide (PI), polyethersulphone (PES), PET and PEN. The polymer substrate is pre-shrunk in a vacuum at the maximum processable temperature, cleaned and dried at 150^SC before the formation of the TFT array commences.

Various layers are formed on the first substrate 10 by deposition, patterning and etching for example using known methods such as sputter deposition, PECVD and photodefineing. It will be appreciated that various combinations of such methods can be employed. A layer of SiO₂ 12 is formed by sputter deposition onto the surface of the first substrate 10 to a uniform thickness of around 100nm. PECVD is then used to deposit onto this a layer of SiN_x 14 to a thickness of about 200nm, a layer of SiO₂ 16 to a thickness of about 200nm, and then a layer of amorphous silicon to a thickness of about 40nm. Typical temperatures for the deposition of the layers in this stack are 180 ^BC for PES substrates and 200 ^aC for PI and FPE substrates. The purpose of the dielectrics in this stack are (i) to obtain good adhesion to the polymer substrate, (ii) to insulate the substrate from heating during subsequent laser processing stages, and (iii) to act as barriers to the diffusion of water and oxygen.

A row and column array of TFTs is then formed on the first substrate 10. Ion doping is used to form respective pairs of lightly-doped drain (LDD) regions 22 and source and drain regions (n⁺ and p⁺) 23, 24 in the deposited amorphous silicon layer for each TFT. The amorphous silicon layer is then dehydrogenated and laser crystallised using two passes of a laser. Firstly, a scan at a laser energy density of 230mJ/cm²/pulse (with 95% pulse overlap) is made to dehydrogenate the material. The beam profile used has a flat top, with a ramped leading edge, so that the silicon is first exposed to some lower energy pulses. Secondly, a scan at a laser energy density of 320mJ/cm²/pulse (again with 95% pulse overlap) is made to melt and crystallise the amorphous silicon. The latter stage is identical to that used on glass substrates, using a beam with a ramped trailing edge. The laser crystallisation process also serves to activate the dopants and remove any damage caused by the ion doping. The polysilicon layer is then dry etched to form a respective individual island, comprising the regions 22, 23 and 24, for each TFT in the row and column array. A gate insulating layer 26 is then formed by depositing a layer of SiN_x to a thickness of about 150nm over the polysilicon islands and intervening regions of the exposed surface of the substrate structure using PECVD. Row address conductors (not shown) with integrated gate electrodes 27 are then formed by depositing and patterning a later of chromium over the gate insulating layer 26. Each gate electrode corresponds to a TFT and overlies a channel region of the island between the source and drain electrodes 23, 24 so as to form a TFT device.

An insulating layer 20 is then formed to cover the row and column array of TFTs. This consists of a photodefinable polymer at a thickness of about 200nm An example of a suitable polymer is JSR PC405G which is commercially available from Japan Steelworks (JSW). This insulating layer serves to insulate the row address conductors from the overlying column address conductors. Also, it provides a substantially planar surface for the formation of both the pixel electrodes and a scattering structure thereon. By using a photodefinable polymer, the flexibility of the overall device is increased and fewer process steps are required to form the layer compared to conventional photolithography, thereby reducing manufacturing costs.

Contact holes 28 are formed in the insulating layer 20 to allow contact of column address conductors and pixel electrodes contacts with the source and drain regions 23, 24, respectively of the associated underlying TFT. The column address conductors, referenced at 29, and the pixel electrode contacts, referenced at 33, are then formed directly over the insulating layer 20 by depositing and patterning a layer of AITi. The column address conductors are each aligned with the spacing between adjacent pixel regions.

A plurality of bumps 25 are then formed over the insulating layer 20. These bumps are photodefined from a polymer photoresist material, such as JSR PC412G which is commercially available. These are dome shaped typically with circular cross-section, and formed in the pixel regions and may vary in both height and width across the layer. Bumps formed in the "dead" pixel aperture, i.e. the region separating adjacent pixels, are substantially rectangular and aligned with underlying column address conductors. These bumps serve as a base for, and part of, the spacer members which separate the first and second substrates 10, 50. It can be seen from Figure 1 that the bumps which form these bases overlap partly with the adjacent pixel regions, but this need not be the case. It will be appreciated that the shape of the bumps 25 may be varied.

A photopolymer layer 39 is spun onto the substrate so that it directly overlies the bumps and spaces therebetween. This layer 39 may be formed of JSR PC412G for example and have a thickness of about 1500nm. The bumps 25 and overlying photopolymer layer 39 provide a scattering structure which establishes an uneven, undulating, reflective surface on the subsequently formed reflective pixel electrode, for example in order to avoid unwanted display effects caused by direct reflections, thereby achieving a more diffuse image. The layer 39 also serves to insulate the pixel electrodes 30 from the underlying column address conductors 29. Contact holes are formed in this photopolymer layer over the pixel electrode contacts 33 and aligned with the contact holes 28

An electrically conductive layer of AITi, which is subsequently used to form the reflective pixel electrodes 30, is then deposited over the scattering structure 25, 39 to a thickness of about 500nm. A portion of each pixel region is then etched to remove parts of this layer of AITi and the underlying scattering structure 25, 39. These portions will form the transmissive windows 38 of the pixel regions. A transparent conductive layer of ITO is then deposited over the substrate 10 to cover the AITi layer and the etched areas. In a single dry etching step, the conductive layers of AITi and ITO are patterned to define reflective pixel electrodes 30 and the transmissive pixel electrodes 32 in conformity with the array of pixel regions. By defining both sets of pixel electrodes together using a single mask, the pixel aperture, i.e. the area of the pixel regions, is maximised. In this way, an alignment overlap at the edges of the first formed electrode, otherwise necessary when patterning separately is not required. Therefore, the gap between adjacent pixel regions can be minimised.

The cell spacer members 35 are then completed by forming the upper component 35' of each member 35 on each respective underlying bump 25 together with the immediately overlying portion of the layer 39 and comprising a photopolymer material. Each upper component 35' is formed in a similar manner to the bumps 25 and at the gap between adjacent pixel regions. This is by depositing a photopolymer material over the pixel electrodes and the spaces therebetween, exposing the desired regions with incident light radiation, and developing the layer to remove the unwanted material. The bump provides a support for the formation of this upper component of the cell spacers 35' in addition to contributing to the overall height of the spacer member 35. By forming the complete spacer members at the location of these bumps 25, the thickness/height required from this photopolymer layer employed to form the upper component 35' is reduced. As forming thick photodefined layers is very time consuming then this is of considerable advantage. The height of each spacer member lies in the range of 1000nm to 5000nm and which will determine the cell gap. This completes the fabrication of the active plate for the AMLCD device. The thicknesses and/or heights of the photopolymer layers and bumps which form the complete spacer members 35 are precisely controlled so as to ensure that the overall height of each spacer member is uniform in order to provide a well defined cell gap.

The fabrication of the display device is then completed by fixing a passive plate to the active plate with LC material sandwiched therebetween. The passive plate comprises the second substrate 50 with a common electrode 55 supported on its inner surface. The common electrode is formed of a transparent conductive material such as ITO. Conventional techniques are used for completing the assembling of the transflective type AMLCD device.

The transmissive pixel electrodes 32 overlie the reflective pixel electrodes 30 and therefore directly oppose the transparent common electrode 55. These are typically formed from the same material (e.g. ITO) as the common electrode 55 which advantageously eliminates any DC offset which may exist across the LC cells. Such DC offset is known to create unwanted display artefacts such as flicker. These artefacts can therefore be avoided or even completely removed.

Various adaptations to the devices and methods of fabrication thereof will be apparent to those skilled in the art. The thickness of the layers deposited on the first substrate may vary depending on the desired properties and application of the device. The materials used for each component of the devices can deviate from those mentioned above provided they maintain the desired effect. In particular, the invention is not limited to the use of polymer substrates and glass or metal may be used instead for example. Specific layouts of the pixel electrodes, address conductors, scattering structures and spacer members can differ from those shown in the accompanying drawings. It should also be appreciated that the substrates will normally carry other layers common to AMLCD devices such as LC alignment layers, polarisers and colour filters which are not shown in the drawings.

Figure 4 illustrates manufacturing steps 410, 420 in conformity with the above description. With reference to Figure 5, the first substrate 10 carries a row and column array of TFTs 8. The TFTs are addressed in a conventional manner as described in US-5, 130,829 for example to which reference is invited. In summary, the TFTs 8 are addressed via row and column address conductors 74, 29. Each row of TFTs is selected one at a time by selection signals in the form of voltage pulses supplied by a row driver 76. As each TFT 8 is switched on (during selection) video data voltages from the pixel by column driver 78 are supplied by the column address conductors via the respective TFTs 8. A reference voltage is applied to the common electrode 55 carried on the second opposing substrate 50. Therefore, voltages are periodically applied across the LC cells to modify their transmissivity thereby creating an output image.

In summary therefore, a transflective active matrix liquid crystal display device is provided which comprises a row and column array of transmissive electrodes 32 which at least partly overlies a corresponding array of reflective pixel electrodes 30 within respective pixel regions on a first substrate 10. A transparent common electrode 55' is supported by a second substrate 50, spaced from the first substrate. By forming the common electrode and the transmissive pixel electrodes from the same material, DC voltage offsets across the LC cell are minimised thus reducing undesirable display artefacts associated therewith.

Claims

1. A transflective active matrix liquid crystal display device comprising a row and column array of reflective pixel electrodes (30) carried on a first substrate (10), each reflective pixel electrode covering a portion of a respective pixel region and connected to an associated switching element (8) for applying data voltages thereto, and a row and column array of transmissive pixel electrodes (32) each overlying at least a portion of a respective one of said reflective pixel electrodes and covering an area (38) of the corresponding pixel region not covered by that one reflective pixel electrode.

2. A transflective active matrix liquid crystal display device according to Claim 1 , wherein the device further comprises a second substrate (50) spaced from the first substrate and which supports a common electrode (55), the common electrode comprising the same material as the transmissive pixel electrodes.

3. A transflective active matrix liquid crystal display device according to Claim 1 or Claim 2, wherein each reflective pixel electrode contacts the associated switching element through a contact hole (28) in an insulating layer (20) which separates the reflective pixel electrodes from their associated transistors.

4. A transflective active matrix liquid crystal display device according to Claim 2 or Claim 3, further comprising a plurality of bumps (25) disposed under the reflective pixel electrodes and between the pixel regions, and insulating spacer members (35) for maintaining the spacing of the first and second substrates, wherein each spacer member overlies at least a portion of one of the bumps between the pixel regions.

5. A method of manufacturing a transflective active matrix liquid crystal display device comprising the steps of:

- forming a row and column array of spaced reflective pixel electrodes (30) over a substrate (10), each covering a portion of a respective pixel region and each pixel electrode being connected to a respective switching element (8) for applying data voltages thereto; and,

- forming a corresponding row and column array of transmissive pixel electrodes (32) each overlying at least a portion of one of said reflective pixel electrodes and covering an area (38) of the corresponding pixel region which is not covered by the associated reflective pixel electrode.

6. A method according to Claim 5, wherein the reflective pixel electrodes and the transmissive pixel electrodes are defined by etching in a single step.

7. A method according to Claim 5 or 6, further comprising the step of:

- forming an insulating layer (20) over the transistors by photodefining a polymer material before the reflective pixel electrodes are formed.