Patent Publication Number: US-2021183907-A1

Title: Semiconductor devices

Description:
CLAIM OF PRIORITY 
     This application claims priority to Great Britain Patent Application No. 1918626.1, dated Dec. 17, 2019. 
     FIELD OF THE INVENTION 
     Semiconductor devices may comprise a stack of layers defining at least two conductor patterns at two levels of the stack, and one or more semiconductor channels each in electrical series between parts of one of the conductor patterns, and each capacitively coupled to a conductor of the other conductor pattern. 
     Conventionally, an insulator layer or stack of insulator layers both provides the coupling dielectric and protects the lower conductor pattern during the process of creating the upper conductor pattern. 
     The inventor for the present application has invented a technique aimed at facilitating an improvement in the performance and/or stability of such semiconductor devices. 
     There is hereby provided a device comprising a stack of layers defining one or more electronic elements, wherein the stack comprises at least: one or more semiconductor channels; a dielectric; a first conductor pattern defining one or more coupling conductors, wherein the one or more coupling conductors are capacitively coupled to the one or one or more semiconductor channels via the dielectric; a planarisation layer; a second conductor pattern defining one or more routing conductors, wherein the second conductor pattern is in contact with the first conductor pattern via through holes in at least the planarisation layer, and wherein the semiconductor channel regions are at least partly outside the through hole regions. 
     According to one embodiment, the through holes are wholly outside the semiconductor channel regions. 
     According to one embodiment, the stack comprises a third conductor pattern including conductor elements connected in electrical series via the semiconductor channels; and wherein the through holes are in one or more regions unoccupied by any conductor element of the third conductor pattern. 
     According to one embodiment, the second conductor pattern comprises a metal pattern, and the first conductor pattern comprises a conductive metal oxide pattern. 
     According to one embodiment, the stack comprises a third conductor pattern below the dielectric and a patterned semiconductor layer defining the one or more semiconductor channels; wherein the patterned semiconductor layer and the dielectric overlap parts of the third conductor pattern; and wherein the first and third conductor patterns are configured such that the first and third conductor patterns do not overlap. 
     According to one embodiment, the third conductor pattern is formed over parts of a fourth conductor pattern, wherein parts of the fourth conductor pattern uncovered by the third conductor pattern define electrodes connected by the one or more semiconductor channels. 
     There is also hereby provided a method, comprising: forming at least a semiconductor, a dielectric, and a first conductor pattern on the support substrate, wherein the semiconductor defines one or more semiconductor channels capacitively coupled via the dielectric to one or more coupling conductors of the first conductor pattern; forming a planarisation layer on the support substrate; forming through holes in at least the planarisation layer, wherein the semiconductor channels are in regions at least partly outside the through hole regions; and forming a routing conductor layer on the support substrate, and etching the routing conductor layer to define a second, routing conductor pattern in contact with the first conductor pattern via the one or more through holes. 
     According to one embodiment, the method further comprises: forming a third conductor pattern at least before forming the dielectric, wherein the third conductor pattern comprises conductor elements in electrical series via the semiconductor channels; and wherein the etching comprises removing portions of the routing conductor layer in one or more regions occupied by the third conductor pattern. 
     According to one embodiment, the through hole regions are wholly outside the semiconductor channel regions. 
     According to one embodiment, the semiconductor and dielectric comprise semiconductor and dielectric patterns both substantially matching and coincident with the second conductor pattern, and the one or more through holes are in one or more regions unoccupied by any conductor element of the third conductor pattern. 
     According to one embodiment, forming the semiconductor, dielectric and first conductor pattern comprises forming semiconductor, dielectric and coupling conductor layers on the support substrate; patterning the coupling conductor layer to form the coupling conductor pattern, and patterning the semiconductor and dielectric layers using the coupling conductor pattern and/or a mask used to form the first, coupling conductor pattern. 
     According to one embodiment, the second, routing conductor pattern comprises a metal pattern, and the first, coupling conductor pattern comprises a conductive metal oxide pattern. 
     According to one embodiment, the method further comprises forming a third conductor pattern before forming the semiconductor; wherein the semiconductor and the dielectric overlap parts of the third conductor pattern; and wherein the first and third conductor patterns are configured such that the first and third conductor patterns do not overlap. 
     According to one embodiment, the third conductor pattern is formed over at least some regions of a fourth conductor pattern, wherein parts of the fourth conductor pattern uncovered by the third conductor pattern define electrodes connected by the one or more semiconductor channels. 
     There is also hereby provided a device comprising a stack of layers defining one or more electronic elements, wherein the stack comprises at least: a patterned semiconductor layer defining one or more semiconductor channels; a patterned dielectric; a first conductor pattern defining one or more coupling conductors, wherein the one or more coupling conductors are capacitively coupled to the one or one or more semiconductor channels via the patterned dielectric; a planarisation layer; a second conductor pattern defining one or more routing conductors, wherein the second conductor pattern is in contact with the first conductor pattern via through holes in at least the planarisation layer; and wherein the first conductor pattern is configured not to overlap with a third conductor pattern below the patterned semiconductor layer in the stack, wherein the patterned semiconductor layer and patterned dielectric overlap with parts of the third conductor pattern. 
     According to one embodiment, the third conductor pattern is formed over parts of a fourth conductor pattern, wherein parts of the fourth conductor pattern uncovered by the third conductor pattern define electrodes connected by the one or more semiconductor channels. 
     There is also hereby provided a method, comprising: forming at least a semiconductor pattern, a dielectric pattern, and a first conductor pattern on the support substrate, wherein the semiconductor pattern defines one or more semiconductor channels capacitively coupled via the dielectric to one or more coupling conductors of the first conductor pattern; forming a planarisation layer on the support substrate; forming through holes in at least the planarisation layer; and forming a routing conductor layer on the support substrate, and patterning the routing conductor layer to define a second, routing conductor pattern in contact with the first conductor pattern via the one or more through holes; wherein the first conductor pattern is configured not to overlap with a third conductor pattern formed before the semiconductor pattern; wherein the semiconductor pattern and the dielectric pattern overlap with the third conductor pattern. 
     According to one embodiment, the third conductor pattern is formed over parts of a fourth conductor pattern, wherein parts of the fourth conductor pattern uncovered by the third conductor pattern define electrodes connected by the one or more semiconductor channels. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the invention are described in detail hereunder, by way of example only, with reference to the accompanying drawings, in which: 
         FIGS. 1-6  are schematic cross-sectional illustrations of an example of a technique according to an embodiment of the invention; 
         FIGS. 7-10  are schematic cross-sectional illustrations of another example of a technique according to another embodiment of the invention; and 
         FIG. 11  is a schematic plan illustration showing positional relationships between elements shown in  FIGS. 1-6  and  FIGS. 7-10 ; and 
         FIGS. 12-14  illustrate one example variation of the techniques of  FIGS. 1-6  and  FIGS. 7-10 . 
     
    
    
     In one example embodiment, the technique is used for the production of an organic liquid crystal display (OLCD) device, which comprises an organic transistor device (such as an organic thin film transistor (OTFT) device) for the control component. OTFTs comprise an organic semiconductor (such as e.g., an organic polymer or small-molecule semiconductor) for the semiconductor channels. 
     The terms “row” and “column” are used below to indicate a substantially orthogonal pair of directions, without indicating any absolute direction. 
     An embodiment of a technique according to an embodiment of the present invention is described in detail below for the example of a thin-film-transistor (TFT) array for the control component of a fringe field switching (FFS) liquid crystal device, but the technique is also applicable, for example, to TFT arrays for any kind of semiconductor device, including e.g., the control component for other types of liquid crystal display devices (LCDs); the control component for other kinds of display devices such as electrophoretic display devices (EPDs) and organic light-emitting diode (OLED) display devices; electrical circuitry for sensor devices; and electrical circuitry for logic devices. 
     For simplicity of explanation,  FIGS. 1-10  show only two semiconductor channels and  FIG. 11  shows only four semiconductor channels, but a semiconductor device may comprise a very large number of semiconductor channels. 
     With reference to  FIG. 1 , the description of a technique according to a first example embodiment of the present invention begins at the stage of a workpiece comprising a planarised plastics film component  2  supporting: a lower metal pattern  4   a ,  4   b ,  4   c ; a transparent conductor pattern  6 ; a continuous semiconductor layer  8 ; a continuous first gate dielectric layer  10  interfacing the semiconductor layer  8 ; a continuous second gate dielectric layer  12 ; and a continuous conductive metal oxide layer  50 . 
     In this example, the plastics film component  2  comprises an ultra-thin plastics support film (such as a 40-micron or 60-micron thickness cellulose triacetate (TAC) film, and may also be supporting one or more functional elements such as a patterned metal layer configured to shield the semiconductor channels against light incident on the rear surface of the plastics film component  2 . 
     In this example, the lower metal pattern defines at least (i) an array of source conductors each providing the source electrodes  4   a  for a respective column of TFTs, and each comprising a conductor line  4   c  extending outside the array of TFTs; and (ii) an array of drain conductors  4   b  each associated with a respective pixel electrode (discussed below). Here, the term “source conductor” refers to a conductor in electrical series between the semiconductor channel and driver chip/circuitry (not shown), and the term “drain conductor” refers to a conductor in electrical series with the driver chip/circuitry via the semiconductor channel. The lower metal pattern may also define gate terminals and routing conductors via which the gate lines at a higher level are connected to the gate terminals. 
     In this example, the lower metal pattern  4   a ,  4   b ,  4   c  comprises silver (Ag). The lower metal pattern may comprise e.g., a non-noble metal even less resistant to oxidation and corrosion than silver, or may comprise a noble metal more resistant to oxidation and corrosion than silver. 
     The transparent conductor pattern  6  is more transparent than the lower metal pattern  4   a ,  4   b ,  4   c  at least in the visible part of the electromagnetic spectrum. In this example, the transparent conductor pattern  6  comprises conductive indium-tin-oxide (ITO). The ITO pattern  6  defines pixel electrodes each contacting a respective drain conductor  4   b . The ITO pattern  6  also comprises ITO on at least the whole area of the conductor lines  4   c  defined by the lower metal pattern (but not on the parts of the source and drain conductors in closest proximity to each other). In this example, retaining ITO on the silver conductor lines  4   c  serves to protect the silver conductor lines  4   c  during subsequent processing steps (e.g., the ITO functions as a good etch-stop during the below-mentioned dry-etching of organic layers in regions over the conductor lines  4   c ). For other examples of devices (such as e.g., EPD, OLED display, sensor and logic devices, and some other types of LCD devices) in which a highly transmissive (highly transparent) pixel electrode is not required at the same level as the source and drain conductors: then (a) the ITO pattern  6  may be limited to regions in which the ITO may be required or desirable as a good etch-stop during subsequent patterning steps, or (b) the ITO pattern may be omitted completely, if the lower metal pattern  4   a ,  4   b ,  4   c  itself exhibits the required etch-stop functionality. A gold (Au) metal pattern is an example of a lower metal pattern that is resistant to dry etching processes that can be used to pattern overlying organic layers. 
     In this example, the semiconductor layer  8  comprises an organic conjugated polymer semiconductor, and the semiconductor layer  8  is formed by e.g., spin-coating a solution of the semiconductor polymer material. The semiconductor layer  8  defines semiconductor channels in electrical series between each drain conductor  4   b  and the part of the source conductor  4   a  in closest proximity to that drain conductor  4   b . The regions of the semiconductor  8  in which the source and drain conductors are in closest proximity are hereafter referred to as the channel regions  9 . One or more layers (such as a self-assembled monolayer of an organic material) may be formed selectively on exposed surfaces of the lower metal pattern  4   a ,  4   b  to facilitate the transfer of charge carriers between the source-drain conductors  4   a ,  4   b  and the semiconductor  8 . 
     In this example, the gate dielectric comprises a stack of two dielectric layers  10 ,  12 , but the gate dielectric may alternatively comprise only one dielectric layer, or a stack of more than two dielectric layers. 
     In this example, the interface gate dielectric layer  10  and uppermost gate dielectric layer  12  also comprise respective polymers, and are each formed by e.g., spin-coating a solution of the respective dielectric polymer. The uppermost gate dielectric polymer  12  material has a higher dielectric constant than the interface gate dielectric polymer  10 , but the interface gate dielectric polymer  10  is better suited than the uppermost gate dielectric polymer  12  to forming a good interface with the semiconductor  8 . In this example, the uppermost gate dielectric polymer  12  is formed from a cross-linkable dielectric polymer available from the Merck group under the product name Lisicon® AP048, but the uppermost gate dielectric polymer  12  may also be formed from a non-cross-linkable polymer. 
     In this example, the continuous conductive metal oxide layer  50  over the uppermost gate dielectric layer  12  comprises indium-tin-oxide (ITO). Other examples of conductive metal oxides may also be used, such as e.g., TiOx. A resistivity of less than about 104 Ohm/square is preferred for the conductive metal oxide layer. In this example, ITO and other conductive metal oxides can provide good etch-stop functionality in the dry-etching patterning process (discussed below) to produce through holes in overlying organic layer(s). ITO is a conductive material exhibiting a high white-light transmittance, but less transmissive (more opaque) conductive materials may also be used. Gold (Au) metal is another example of a conductor material that has good electrical conductivity and provides good etch-stop functionality in dry etching patterning processes for patterning organic layers. If the conductive layer  50  need not also provide good etch-stop functionality in the patterning process (discussed below) to produce through holes in overlying layer(s), another example option is to instead use an organic material which has the necessary level of conductivity and does not negatively affect the semiconductor channels. 
     This stack of layers  8 ,  10 ,  12 ,  50  may comprise additional layers, such as e.g., a layer of light-absorbing material (e.g., dispersion of carbon black particles in an insulating organic polymer) between the upper gate dielectric layer  12  and the ITO layer  50  to better protect the semiconductor channels in the finished device against damage by white light. 
     With reference to  FIG. 2 , the workpiece is further processed to achieve patterning of the sub-stack comprising the semiconductor layer  8 , gate dielectric layers  10 ,  12  and metal oxide layer  50 . The patterning defines isolated islands  100 , each associated with a respective TFT. In this example, this patterning is done by a photolithographic technique involving: forming a photoresist layer over the metal oxide layer  50 : projecting an image (positive or negative depending on the type of photoresist used) of the pattern desired for the sub-stack, using radiation that induces a change in the solubility of the photoresist, to thereby create a latent solubility pattern in the photoresist layer; developing the latent solubility pattern; using the resulting physical photoresist pattern as an etching mask, wet etching the ITO layer  50  (using an etchant (e.g., an etchant comprising oxalic acid) to which at least the uppermost gate dielectric layer  12  is substantially attack-resistant and impermeable); and dry-etching the semiconductor and gate dielectric layers  8 ,  10 ,  12  using at least the ITO pattern as an etching mask. The dry etching is compatible with the choice of metal for the lower metal pattern  4   a ,  4   b ,  4   c . Plasmas created from gas mixtures excluding chemically reactive gases such as oxygen etc. are compatible with a wide range of metals for the lower metal pattern  4   a ,  4   b ,  4   c . More chemically reactive plasmas may only be compatible with the use of noble metals such as e.g., gold or platinum for the lower metal pattern  4   a ,  4   b ,  4   c.    
     In the example shown in  FIGS. 1-6 , none of the photoresist mask is retained in the product device. In one example variation, the photoresist etching mask comprises a material that fulfils a function in the product device and is retained. For example: the photoresist mask may comprise a mixture of a photoresist material and a light-absorbing material such as carbon black particles; and the photoresist mask is retained in the product device to provide extra white light-shielding for the semiconductor channels. In order to retain a sufficient thickness of photoresist mask in the product device, the starting thickness of the photoresist mask is calculated taking into account the amount by which the dry etching process reduces the thickness of the photoresist mask. 
     With reference to  FIG. 3 , the workpiece is further processed by the formation in situ on the upper surface of the workpiece of a continuous planarisation layer  14  of electrically insulating material over at least the whole area of the TFT array (including the whole area of the above-mentioned islands  100  and the whole area between the islands  100 ). In this example, the planarisation layer  14  is formed by a solution processing technique such as e.g., spin-coating. 
     With reference to  FIG. 4 , the workpiece is further processed by patterning the planarisation layer  14  to define through holes  54  (interconnect vias ICVs), each through hole  54  exposing the ITO layer (of a respective island  100 ). In this example, each through hole  54  is formed in a region that is wholly outside the respective channel region  9  to best protect the semiconductor  8   a  in the channel region  9  from the process of creating the through holes  54 . However, in one example variation (particularly suited to high-resolution display devices), the regions in which the through holes  54  are formed may occupy a part of the semiconductor channel regions. In both cases, for each island  100  there is at least part of the semiconductor channel region that is occupied by the patterned ITO layer  50   a  but not the through hole  54 ; and good operation of the TFT across the whole channel region thus relies on the ITO layer  50   a.    
     The regions in which the through holes  54  are formed are also in regions in which the lower metal pattern does not define any conductor element. This feature (shown in  FIG. 7 ) minimises the risk of shorts between the gate metal pattern (described below) and the lower metal pattern. In this example, the planarisation layer  14  comprises a cross-linkable polymer (such as an epoxy-based cross-linkable polymer, e.g., SU-8 negative photoresist available from MicroChem Corp.); and the patterning comprises: projecting a negative image of the desired through hole pattern (i.e., positive image of the pattern desired for the planarisation layer  14 ) onto the cross-linkable planarisation layer  14 , using radiation that induces cross-linking and thereby decreases solubility of the insulating material of the planarisation layer  14 , to create a latent solubility image in the planarisation layer  14 ; and developing the latent solubility image. 
     The patterning of the planarisation layer  14  may also comprise forming other through holes (not shown), such as through holes in regions outside the TFT array (active area) for the gate conductors  56   a  (mentioned below) to contact routing conductors defined by the lower metal pattern. 
     In addition to functioning as the gate electrode, the ITO layer  50   a  at the top of each island  100  can function as a dry-etch stop, which facilitates the alternative use of dry etching to create the through holes  54  in the planarisation layer  14 . For the alternative example of using dry etching to form the through holes  54 , a photoresist, etching mask is formed over the planarisation layer  14 , and is removed after it has been used to pattern the planarisation layer  14 . More particularly, the ITO at the top of each island  100  facilitates dry etching the full thickness of planarisation layer  14  without exposing the uppermost gate dielectric  12  to the dry-etching conditions. The use of a dry-etching technique for creating the through holes  54  in the planarisation layer  14  can provide an upper workpiece surface with a more gradual step in the border regions between inside and outside the regions of the through holes  54 , and thereby better facilitate the formation of gate conductor lines  56   a  (discussed below) that extend without breaks over the upper surface of the workpiece. 
     For the alternative example of using dry etching, the planarisation layer  14  comprises the same negative photoresist material (e.g., SU-8) as in the wet-etching example, and the photoresist layer used to pattern the planarisation layer  14  comprises a positive photoresist material. 
     With reference to  FIG. 5 , the workpiece is further processed by the formation of at least one continuous metal layer  56  in situ on the upper surface of the workpiece defined together by the uppermost gate dielectric layer  12  and patterned ITO layer  50   a  (in the regions of the through holes  54 ). The at least one metal layer  56  is formed continuously over the whole area of the array of TFTs and beyond. In this example, a sub-stack of metal layers is used, comprising an aluminium (Al) layer sandwiched between two molybdenum (Mo) layers. In this example, each metal layer of this sub-stack is formed in situ on the workpiece by a vapour deposition technique such as e.g., sputtering. 
     With reference to  FIG. 6 , the workpiece is further processed to pattern the metal sub-stack  56 , to define gate conductors  56   a  each providing the gate electrodes for a respective row of transistors, and each extending beyond the edge of the array of TFTs. In this example, this patterning is done by a photolithographic technique comprising: forming a photoresist layer over the metal sub-stack  56 ; projecting onto the photoresist an image (positive or negative depending on the type of photoresist used) of the pattern desired for the gate conductors  56   a , using radiation that induces a change in the solubility of the photoresist, to create a latent solubility image; developing the latent solubility image; and using the resulting physical pattern in the photoresist as an etching mask for patterning the metal sub-stack  56  using one or more patterning agents. In this example, the patterning agent is a wet metal etchant whose main component is phosphoric acid H3PO4. In this example, the planarisation layer  14  comprises a material that is less (compared to the uppermost gate dielectric material  12 ) susceptible to damage (surface roughening) by this wet metal etchant and/or less (compared to second gate dielectric material  12 ) permeable to this wet metal etchant. The regions in which the metal sub-stack  56  is removed include regions overlying conductor elements defined by the lower metal pattern (e.g., source line conductors  4   c ), and this relatively high attack-resistance and impermeability to the wet metal etchant better protects the underlying lower metal pattern against being etched by this wet metal etchant. 
     The upper surface of the planarisation layer  14  is higher than the upper surface of the uppermost gate dielectric layer  12 . This relatively large thickness for the planarisation layer  14  can be advantageous for improving dielectric breakdown properties; and the relatively small thickness for the gate dielectric layer  12  can be advantageous for improving capacitance between the ITO gate electrodes  50   aa  and the semiconductor  8   a  in the channel regions  9 . 
     The planarisation layer  14  and uppermost gate dielectric layer  12  may have different physical and/or chemical properties. For example, the two layers may differ in terms of one or more of the following properties: dielectric constant; layer thickness; etch resistance; adhesion to interfacing layers; density; dielectric breakdown strength; and purity. 
     In the example described above, both planarisation layer  14  and uppermost gate dielectric layer  12   a  are cross-linked layers in the final device; and the method by which cross-linking is achieved in the planarisation layer  14  may be different or the same as the method by which cross-linking is achieved in the uppermost gate dielectric layer  12   a.    
     Furthermore, the planarisation layer  14  comprises a material selected for its good (compared to the uppermost gate dielectric layer  12   a ) adhesion to at least one or more (and preferably all) of the gate metal pattern  56   a , the lower metal pattern  4   a ,  4   b ,  4   c  and the upper surface of the plastics film component  2  (e.g., an organic polymer planarisation layer at the upper surface of the plastics film component  2 ). For example, good adhesion to the gate metal pattern  56   a  better avoids the gate conductor lines  56   a  lifting off the underlying insulator surface. 
     Each pixel electrode is associated with a respective unique combination of source and gate conductors, whereby each pixel electrode can be independently addressed via the portions of gate and source conductors outside the array of TFTs. 
     Further processing (not shown in the drawings) comprises the formation on the upper surface of the workpiece (as defined by the gate conductors  56   a  and planarisation layer  14 ) of a continuous layer of transparent conductor material (e.g., ITO) in situ on the workpiece, followed by patterning of the continuous layer to define the patterned common electrodes for the FFS liquid crystal device. 
     A second example technique according to another embodiment of the invention is shown in  FIGS. 7-10 . The second example is the same as the first example  FIGS. 1-6 ), except that an inorganic insulating passivation layer  60  (having a high white-light transmittance) is formed on the upper surface of the workpiece (over at least the whole active area) before formation of the organic planarisation layer  14 . The inorganic insulating layer  60  may, for example, comprise an inorganic nitride layer, such as e.g., an aluminium nitride (AlNx) layer, an aluminium oxide (AlOx) layer or a silicon nitride (SiNx) layer. The thickness of the inorganic insulating layer may, for example, between about 40 nm and 100 nm. The inorganic insulating layer  60  may, for example, be formed by a vapour deposition technique such as e.g., sputtering, plasma-enhanced chemical vapour deposition (PECVD) and atomic layer deposition (ALD). The inorganic insulating layer  60  is patterned ( FIG. 10 ) using the same photoresist etching mask as the planarisation layer, such that the through holes  54  extend down to the ITO layer  50   a . The patterning of the inorganic insulating layer  60  may be done by dry etching or wet etching (using an etchant compatible with retention of the underlying ITO layer  50   a ). The inorganic insulating layer  60  acts as an extra gate dielectric layer and can improve yield and reduce the risk of dielectric breakdowns. The inorganic insulating layer  60  also provides further protection for the lower metal pattern against the etchants used to pattern the upper metal sub-stack  56 . 
     The variations, additions and process details mentioned for the first example are also applicable to this second example. In particular, the gate dielectric may comprise a single dielectric layer or a stack of dielectric layers. 
     The above-described techniques facilitate the selection for the gate dielectric (e.g., upper most gate dielectric layer in the example of using a stack of gate dielectric layers for the gate dielectric) of a dielectric material optimised for good capacitance and TFT stability, by reducing the demands on the uppermost gate dielectric layer  12  for good attack-resistance and impermeability against the gate metal etchant, and good adhesion to interfacing layers. 
     According to one example variation of the techniques described above and illustrated in  FIGS. 1-10 , the ITO layer  50  is patterned separately to the underlying semiconductor and dielectric layers  8 ,  10  and  12 . Differently configured masks are used for the patterning of the ITO layer  50  and the patterning of the underlying semiconductor and dielectric layers  8 ,  10 ,  12 . As shown in schematic cross-sectional  FIG. 12  and schematic plan  FIG. 13 : the lower and upper ITO patterns  6 ,  150   a  are configured such that there is no overlap between the upper and lower ITO patterns  6 ,  150   a ; and the islands  100  of semiconductor and dielectric layers  8 ,  10 ,  12  are configured such that the islands  100  overlap with the lower ITO pattern  6 . As shown in schematic cross-sectional  FIG. 14 , this example variation involves forming an additional photoresist mask  80  for patterning the semiconductor and dielectric layers  8 ,  10 ,  12  after patterning the ITO layer  50 . This example variation can further improve yield by reducing the risks of shorts between the upper ITO pattern  150   a  and the lower ITO pattern  6 . 
     According to an example variation of all the techniques described above and illustrated in  FIGS. 1-14 , the upper conductor pattern  50   a ,  150   a  is instead formed from a conductive material having a lower white-light transmittance, such as a layer of a metal or metal alloy, such as e.g., an alloy comprising tantalum and molybdenum (MoTa alloy). MoTa alloy is an example of a material that is also resistant to the dry etching used to pattern the underlying semiconductor and dielectric layers  8 ,  10 ,  12  into islands  100  and/or the overlying planarisation layer  14  to create through holes  54  down to the conductor pattern  50   a ,  150   a . Depending on what technique is used to pattern the overlying planarisation layer  14 , resistance to dry etching may be less important in the above-described example variation illustrated in  FIGS. 12-14 , in which the conductive capping layer  150   a  has a different pattern to that of the underlying semiconductor and dielectric layers  8 ,  10 ,  12 , and is not used as a mask for patterning the underlying semiconductor and dielectric layers  8 ,  10 ,  12 . 
     As mentioned above, an example of a technique according to the present invention has been described in detail above with reference to specific process details, but the technique is more widely applicable within the general teaching of the present application. Additionally, and in accordance with the general teaching of the present invention, a technique according to the present invention may include additional process steps not described above, and/or omit some of the process steps described above. 
     In addition to any modifications explicitly mentioned above, it will be evident to a person skilled in the art that various other modifications of the described embodiment may be made within the scope of the invention. 
     The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features.