The present invention generally relates to process architecture for color filter array (COA) in active matrix liquid crystal display (AMLCD). More specifically, the invention relates to reducing thickness of color filter array for enhanced performance of the AMLCD.
Liquid crystal displays (LCDs) generally display images by transmitting or blocking light through the action of liquid crystals. LCDs have been used in a variety of computing displays and devices, including notebook computers, desktop computers, tablet computing devices, mobile phones (including smart phones) automobile in-cabin displays, on appliances, as televisions, and so on. LCDs often use an active matrix to drive liquid crystals in a pixel region. In some LCDs, a thin-film transistor (TFT) is used as a switching element in the active matrix.
For fabrication of the AMLCD, the conventional manufacturing process selectively uses a relatively thick color filter, because its dielectric constant is normally about twice of the dielectric constant of a photoactive compound (PAC), which has a relatively low dielectric constant. Generally, the color filter array is about 3 μm to 4 μm thick. Such a large thickness of the color filter array (COA) reduces the coupling between common electrode and data line (CD). This CD coupling depends upon the capacitance between common electrode and the data line, and is proportional to the dielectric constant of the color filter, and is inversely proportional to the thickness of the color filter.
There are several issues with thicker COA for fringe field switching (FFS) mode. First, it is difficult to coat a thick COA. Second, the large thickness of the COA may require alignment of the color filter array with TFTs, which is an additional step in the process. Third, the large thickness of the color filter array also makes difficult to fill via holes such that the via holes need to be larger to enable completely filling the via holes. Fourth, larger via hole sizes may be required as a result of thicker COA. The larger via holes may reduce a ratio of aperture area to via hole area, which reduces the optical transmission or brightness of the AMLCD. Additionally, the deep via holes may cause non-uniformity in a planarization layer that covers the via holes and the color filter array. Liquid crystal molecules are arranged on the top of the planarization layer.
Therefore, it is desirable to develop techniques to reduce the thickness of the COA and to improve optical transmission and performance of the AMLCD.
Embodiments described herein may provide a design architecture for the color filter array (COA) in active matrix liquid crystal display (AMLCD). Embodiments also provide a process architecture for fabrication of such AMLCD.
In one embodiment, an active matrix liquid crystal display having an array of pixels is provided. The display includes a thin film transistor (TFT) for each pixel. The TFT has a gate electrode, a source electrode overlapping with a first area of the gate electrode, and a drain electrode overlapping with a second area with the gate electrode. The display also includes a color filter layer disposed over the TFT. The color filter layer has a first via hole to expose a portion of the drain electrode. The display further includes a metal layer disposed over the color filter layer and covering the gate electrode. The metal layer is configured to connect to the drain electrode through the first via hole. The display also includes an organic insulator layer disposed over the metal layer. The organic insulator layer has a second via hole to expose a first portion of the metal layer and a third via hole to expose a second portion of the metal layer.
In another embodiment, an active matrix liquid crystal display having an array of pixels is provided. The display includes a TFT for each pixel. The TFT has a gate electrode, a source electrode overlapping a first area with the gate electrode, and a drain electrode overlapping with a second area with the gate electrode. The display also includes an organic insulator layer disposed over the TFT. The organic insulator layer has a first via hole to expose a portion of the drain electrode. The display further includes a metal layer disposed over the organic insulator layer and covering the gate electrode. The metal layer is configured to connect to the drain electrode through the first via hole. The display also includes a color filter layer disposed over the metal layer. The color filter layer has a second via hole to expose a portion of the metal layer and a third via hole to expose a second portion of the metal layer.
In yet another embodiment, a method for fabricating an active matrix liquid crystal display is provided. The method includes forming a thin film transistor (TFT) over a substrate. The TFT has a gate electrode, a source electrode overlapping with a first area of the gate electrode, and a drain electrode overlapping with a second area with the gate electrode. The method also includes depositing a color filter layer over the TFT. The color filter has at least a first via hole to expose a portion of the drain electrode. The method further includes forming a metal layer over the color filter. The metal layer fills the first via hole to connect to the drain electrode. The method also includes depositing an organic insulator layer over the metal layer. The organic insulator layer has a second via hole to expose a first portion of the metal layer and a third via hole to expose a second portion of the metal layer. The method further includes forming a common electrode over the organic insulator. The common electrode is connected to the metal layer through the third via hole. The method also includes depositing a first passivation layer over the common electrode. The second via hole is through the first passivation layer. The method further includes forming a pixel electrode over the first passivation layer. The pixel electrode is connected to the metal layer through the second via hole.
In still yet another embodiment, a method for fabricating an active matrix liquid crystal display is provided. The method includes forming a thin film transistor (TFT) over a substrate. The TFT has a gate electrode, a source electrode overlapping with a first area of the gate electrode, and a drain electrode overlapping with a second area with the gate electrode. The method also includes depositing an organic insulator over the TFT. The organic insulator has a first via hole to expose a portion of the drain electrode. The method further includes forming a metal layer over the organic insulator. The metal layer fills the first via hole. The method also includes depositing a color filter layer over the metal layer. The color filter has a second via hole to expose a first portion of the metal layer and a third via hole to expose a second portion of the metal layer. The method further includes forming a common electrode over the organic insulator. The common electrode is connected to the metal layer through the third via hole. The method also includes depositing a first passivation layer over the common electrode. The second via hole is through the first passivation layer. The method further includes forming a pixel electrode over the first passivation layer. The pixel electrode is connected to the metal layer through the second via hole.
Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the invention. A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.
The present disclosure may be understood by reference to the following detailed description, taken in conjunction with the drawings as described below. It is noted that, for purposes of illustrative clarity, certain elements in various drawings may not be drawn to scale.
The present disclosure provides a design architecture for the AMLCD, which includes a third metal layer and also an organic insulator layer in the AMLCD. The design architecture allows the thickness of the color filter layer to be reduced. The present disclosure also provides a sample process architecture for fabricating various embodiments.
The TFT/CF/LC stack 200 further includes a CF layer 214 and a black matrix 228 disposed over the first passivation layer 210. The CF layer 214 includes a number of sub-color filters (CF), such as red, green and blue colors. The black matrix 228 may be arranged to divide the CF layer 214 into the sub-color filters. Some embodiments may replace one or more of the red, green and blue filters with a yellow filter, a cyan filter, a clear filter, or another color filter. Further, the number of color filters and their arrangement may vary in certain embodiments.
The common electrode 208 is disposed over the substrate 202 and is arranged under the color filter 214. The TFT/CF/LC stack 200 further includes a polyimide layer disposed over the pixel electrode to provide a flat surface of deposition of liquid crystal molecules, and a liquid crystal (LC) layer 242 on top of the polyimide layer 240.
The third metal layer 336 covers at least a portion of TFT 338, such that there is no photo-leakage when the TFT is in an off-current state. The TFT 338 is arranged to be outside the pixel region or aperture area rather than within the aperture area, which further helps improve optical transmission of the AMLCD.
The TFT/CF stack 300 further includes a black matrix layer 334 disposed over the third metal layer 336. The black matrix layer 334 is photo-insensitive and has a low overall dimension. For example, it may be very thin and small in area. The black matrix layer 334 may cover at least a portion of the third metal layer 336 and may block light reflected from the third metal layer. In one embodiment, the black matrix may be made of a polymer, such as polyimide. The black matrix layer 334 may reduce light reflected from the LCD panel by absorbing the light and may also reduce optical transmission of the backlight. Therefore, the black matrix layer 334 may be configured to help block light reflection from the third metal layer while retaining optical transmission of a backlight. Note that this black matrix layer 334 is different from the black matrix 228 which is arranged between two sub-color filters 214A and 214B, as shown in
The TFT/CF stack 300 further includes an organic insulator layer 332 disposed over the black matrix layer 334. By including the organic insulator 332, the color filter layer 314 of the present TFT/CF stack 300 becomes thinner than that of conventional TFT/CF stack 200, because a combination of the organic insulator and a thinner color filter layer provides equivalent CD coupling to that of a thicker color filter layer of a conventional TFT/CF stack. Additionally, the organic insulator has a lower dielectric constant than the color filter, which also helps make the color filter thinner for the TFT/CF stack 300. Thickness estimation of the color filter will be described later in details. The organic insulator layer 332 may be formed of a photoactive compound (PAC), among other suitable material to provide a flat surface for forming more layers, such as a common electrode 308 and a pixel electrode 318 among others.
The common electrode 308 is disposed over the organic insulator layer 332 for all pixels. By connecting the third metal layer 336 to the drain electrode 324 and covering the gate 306, common electrode Vcom 308 is arranged above the color filter 314. The common electrode 308 includes both an ITO layer and the third metal layer, and has a lower resistivity than the common electrode of the conventional TFT/CF stack 200.
The pixel electrode 318 is disposed over a second passivation layer 316 that is disposed over the common electrode 308 and organic insulator layer 332. The pixel electrode 318 connects to the third metal 336 above the color filter 314 through via hole CNT2 330B and thus connects to the drain electrode 324B or source electrode 324A (not shown) through via hole CNT1 330A. The source and drain electrodes may be interchangeable.
Via holes formed in the present TFT/CF stack 300 are less deep than that of a conventional TFT/CF stack due to the presence of the third metal layer. The common electrode 308 is now arranged above the color filter layer 314, as shown in
The pixel region includes an active pixel area 350 (also referred to herein as an “aperture area”) above a dashed line and a TFT area below the dashed line. The pixel region edges are defined by first and second data lines 360A-B and first and second gate lines 370A-B. The pixel region may be generally rectangular in shape or square in shape. Still other embodiments may have differently-shaped pixel regions.
Each gate line 370A or 370B may be connected to multiple TFTs. The first data line 360A may be connected to drain electrode 324B. The second data line 360B may be connected to a neighboring drain electrode. The first gate line 370A may be connected to gate electrode 306 for the pixel regions shown in
The TFT 338 switches a respective pixel for each active pixel area 350 on and off. Each active pixel area includes a liquid crystal layer. A voltage between the pixel electrode and the common electrode may be applied to the liquid crystal layer for each active pixel area. The voltage may control the alignment of liquid crystal molecules in the liquid crystal layer and to control light for each pixel of the LCD.
The data lines 360A-B and the gate lines 370A-B may be controlled by a controller for a LCD (not shown) to change the “on” and “off” states of the TFT. The pixel electrode 318 may be connected to the drain electrode 324B and may receive a signal from the TFT, such that a voltage between the pixel electrode 318 and the common electrode 310 may be applied to the respective pixel.
The third metal layer 336 covers the L-shaped TFT 338, via holes CNT2 330B and CNT1 330A. The TFT 338 may overlap with via hole CNT2 in a particular embodiment, although other embodiments of arrangements of CNT1 and CNT2 are provided and described below, for example, in
The via holes CNT1 330A and CNT2 330B are arranged to be in the TFT area, but beyond the aperture area or the active pixel area. The via holes CNT2 may overlap with a portion of the TFT area. The third metal layer 336 covers the TFT 338 which is illustrated as “L” shaped, via holes CNT1 and CNT2.
The first passivation layer 310 in TFT/CF stack 300 is optional, and may be removed.
Additionally, the position of color filter layer and the PAC may be exchanged.
Additionally, the black matrix layer 334 may be removed when the third metal layer 336 is configured to remove reflection of light from the LCD panel.
Various arrangements of the via holes CNT1 and CNT2 are provided.
In the various embodiments as shown in
The semiconductor layer 320 may be amorphous silicon. Alternatively, the TFT may be an oxide TFT, such as an indium-gallium-zinc oxide (IGZO) TFT.
By including the organic insulator 332, the color filter layer 314 of the present TFT/CF stack 300 becomes thinner than that of a conventional TFT/CF stack 200. The organic insulator and the thinner color filter layer together provide equivalent CD coupling to the capacitance of the thicker color filter layer of the conventional TFT/CF stack. The thickness of the organic insulator 332 (e.g. PAC) may be estimated as follows. As shown in
1/CPAC=1/CCF+1/COC Equation (1)
where CPAC is the capacitance between common electrode and data line, CCF is the capacitance for the color filter layer, and COC is the capacitance for organic insulator. Assume that the dielectric constant of the PAC is ∈, and the dielectric constant of the color filter is 2∈, based upon Equation (1), the thickness for each of the PAC layer, OC layer, and CF layer can be represented in Equation (2) as follows:
dPAC/∈=dCF/2∈+dOC/∈ Equation (2)
In a particular example, assuming that dPAC is about 2.0 μm, dCF is about 2.2 μm, then dOC is estimated to be about 0.9 μm based upon Equation (2).
Although conventional color filter material can still be used, a lower dielectric constant for the color filter helps further reduce the CD coupling. The color filter may use transparent color polymer materials, such as polycarbonate or polyester among others.
It will be appreciated by those skilled in the art that the source and drain electrodes 324A and 324B may be interchangeable.
In a particular embodiment, the semiconductor layer may be an indium-gallium-zinc-oxide (IGZO). The IGZO may be replaced by other semiconductors. It will be appreciated by those skilled in the art that the semiconductor layer may include other materials, for example, zinc oxide (ZnO), indium oxide (InO), gallium oxide (GaO), tin oxide (SnO2), indium gallium oxide (IGO), indium zinc oxide (IZO), zinc tin oxide (ZTO), or indium zinc tin oxide (IZTO) among others.
The organic insulator layer 332 may be formed of an organic material, such as a photoactive compound (PAC), an acrylate, or an organic-inorganic hybrid like siloxane to provide a flat surface for forming more layers, including the common electrode 308 and the pixel electrode 318. Furthermore, the photoactive compound (PAC) could be positive tone or negative tone material. The polymer bases may be acrylate, cyclic olefin polymer, or siloxane among others. The PAC has a relatively low dielectric constant, considerably lower than the first and second passivation layers 310 and 316. The first passivation layer 310 and the second passivation layer 316 may be formed of a dielectric material, such as silicon nitride (g-SiNx) which has a relatively high dielectric constant.
The substrate 302 may be transparent, such as a glass substrate. The gate electrode 306, source electrode 324A, the drain electrode 324B, and the third metal layer 336 may be formed of a conductive material having low electrical resistance, such as copper or aluminum and the like. The common electrode 308 and the pixel electrode 318 may be formed of a transparent conductor, such as indium-tin oxide (ITO), indium zinc oxide (IZO) among others.
The gate insulator 304 may be formed of an inorganic insulation film including silicon oxide (SiO2), silicon nitride (SiNx), a dielectric oxide film such as aluminum oxide (Al2O3), or an organic material, and the like. The gate insulator 304 may be formed by a chemical vapor deposition (CVD) method using a plasma enhanced chemical vapor deposition system or formed by a physical vapor method using a sputtering system. Other deposition processes may also or alternatively be used. The gate insulator 304 may also include multiple layers of the above materials. For example, the gate insulator 304 may include one or more passivation layers. In a particular embodiment, the gate insulator 304 may have a two-layer structure. A silicon nitride layer may be formed as a first insulating layer and a silicon oxide layer may be formed as a second insulating layer. This gate insulator 304 may prevent an impurity such as moisture or alkali metal or copper contamination from diffusing into a TFT element and a display device and may also improve reliability of a semiconductor element formed in an element formation layer, or the like.
The disclosure also provides methods for fabricating the color filter array (COA) with back channel etching (BCE).
For forming the TFT, it may require up to five masks. For example, in the case of forming the TFT according to the first embodiment, at most four masks are used for the depositions of the gate metal, the gate insulator, the semiconductor layer and the source/drain metal as well as the first passivation layer with respective photo patterning.
In the case of forming the TFT according to the second embodiment without the first passivation layer, at most three masks may be used for the depositions of the gate metal, the gate insulator, the semiconductor layer or active layer, the source/drain metal and the respective photo patterning as well as active back channel etching. The number of masks is reduced because the first passivation layer is not present in this embodiment.
In the case of forming the TFT according to the third embodiment where the color filter and the organic insulator are exchanged, at most four masks may be used for depositions of the gate metal, the gate insulator, the active layer, and the source/drain metal with respective photo patterning, for active back channel photo etching and deposition of the first passivation layer and photo patterning.
In the case of forming the oxide TFT with an etch-stop layer according to the eighth embodiment, at most five masks may be used for the depositions of the gate metal, gate insulator, active layer, etch-stop layer, source/drain/metal layer, and the first passivation layer with their respective photo patterning. An additional mask is needed for the deposition of the etch-stop layer.
The method 1200 continues with depositing color filter such as red-green-blue (RGB) color filters and patterning at operation 1204, followed by depositing the third metal layer for common electrode and photo patterning at operation 1206. The method 1200 also includes depositing a black matrix layer and photo patterning, which may be an optional operation (as shown in dash line) 1208. The method 1200 further includes depositing organic insulator layer and photo patterning at operation 1210, depositing common ITO and photo patterning at operation 1212, depositing a second passivation layer and photo patterning at 1214, and depositing pixel ITO and photo patterning at operation 1216.
For photo patterning or lithography, a photoresist is first deposited on a surface, and then light is selectively passed through a patterned mask that may block light in certain areas. The exposed photoresist film is developed through the patterned mask to form the photoresist patterns as shown. The exposed photoresist film protects the layers underneath during an etching process, such that the portion exposed by the photoresist may be completely removed by the etching process, such as a wet etching. Portions of underlying layers that are protected by photoresist generally are not removed or otherwise etched. After etching to form a pattern of a deposited layer by using photoresist, the insoluble photoresist is removed prior to the next deposition operation. Different masks may be provided to form various films with different patterns. In alternative embodiments, different photoresist may be used.
The photoresist film may be made of a photosensitive material; exposure to light (or particular wavelengths of light) to develop the photoresist. The developed photoresist may be insoluble or soluble to a developer. There may be two types of photoresist, a positive photoresist and a negative photoresist. The positive photoresist is soluble to the photoresist developer. The portion of the positive photoresist that is unexposed remains insoluble to the photoresist developer. The negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.
The process architecture of the present disclosure provides several benefits over of the conventional COA technology. The benefits include (A) increasing optical transmittance by reducing via hole sizes, and re-arranging the TFT beyond the pixel region compared to the conventional design; (B) simplifying the process by eliminating the operation of the alignment between the COA and TFT due to thinner COA; (C) minimizing color mixing from inter-pixel optical interference for high resolution display by using black matrix on the bottom glass substrate and also low misalignment between pixel and the color filter; (D) reducing CD coupling between data line or data signal and common electrode/pixel electrode by adding the organic passivation layer. Although conventional color filter material with high dielectric constant may still be used, other color filter materials with lower dielectric constant are desirable to further help reduce the CD coupling.
Having described several embodiments, it will be recognized by those skilled in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, a number of well-known processes and elements have not been described in order to avoid unnecessarily obscuring the present invention. Accordingly, the above description should not be taken as limiting the scope of the invention.
Those skilled in the art will appreciate that the presently disclosed embodiments teach by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween.
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