WO2009074765A1

WO2009074765A1 - Patterning method

Info

Publication number: WO2009074765A1
Application number: PCT/GB2008/003827
Authority: WO
Inventors: Christopher Rider; Christopher Bower
Original assignee: Eastman Kodak Company
Priority date: 2007-12-13
Filing date: 2008-11-13
Publication date: 2009-06-18
Also published as: GB0724385D0

Abstract

A method of patterning a flowable material on a surface, the method comprising providing the surface with at least one channel and at least one through- hole with at least two openings, wherein at least one of the openings is located in the surface adjacent to the at least one channel, such that when flowable material is deposited adjacent to another of the at least two openings, the material is directed into the at least one through-hole by the action of capillary forces and emerges at the opening adjacent to the at least one channel whereupon it is further directed along said channel.

Description

PATTERNING METHOD

FIELD OF THE INVENTION The invention relates to a patterning method and in particular to a method of patterning a surface in the manufacture of devices and structures for electronic, optical and optoelectronic, sensing and security applications. The invention also relates to devices and structures manufactured by the method and to patterning surfaces and substrates on which devices and structures can be manufactured.

BACKGROUND OF THE INVENTION

The development of silicon-based thin-film transistor (TFT) technology has been an essential enabler for the development of large flat panel displays. Despite the huge cost of factories to manufacture TFTs on glass and the complexity of the TFT manufacturing process, the technology is now well-established for active matrix liquid crystal displays and is based largely on photolithographic techniques for depositing patterns of the various materials into multilayer structures.

In recent years great progress has been made on TFT technologies based on other semiconductors including polymers, metal oxides and semiconducting nanowires and nanotubes. Many of these approaches benefit from simpler processes that promise greatly reduced commercialisation investment compared to the current silicon-based factories. Another recent development has seen liquid processed semiconductors deposited onto flexible substrates using additive approaches such as inkjet and conventional printing and this promises further process simplification and cost-reduction as more processes can be performed in roll-to-roll configurations.

These new approaches to the production of TFTs can also be applied to the production of sensors, such as chemical sensor arrays and other optoelectronic devices such as photovoltaics, photodetector arrays for scanning applications or image capture and organic light emitting diode arrays for electronic displays or image sensors.

A common need for all thin film optoelectronic devices, especially arrayed structures such as displays or scanners, is the provision of conductive bus lines and electrodes. In a display, for example, data bus lines connect pixel electrodes together in rows and columns. Other bus lines provide power to the pixels. It is important that the bus lines should be highly conductive so that resistive losses are minimised and device non-uniformities are avoided. Usually, the requirements for the conductivity of the pixel TFT electrodes are not so severe as for the bus lines, but the need for high resolution electrode patterning is more important. It is preferred that TFT gate electrodes should not overlap substantially with the source and drain electrodes to minimise parasitic capacitance. Gate electrode widths below 10 microns are not uncommon and with the drive to increase switching speeds and reduce TFT footprint within the pixel area, there is a need to further reduce all the dimensions of the TFT.

When fabricating a multi-layer TFT device, accurate registration between the features in all the layers is very important if optimum device performance is to be achieved. Certain features, however, require greater alignment accuracy than others. Alignment of the gate electrode with the channel region of a TFT formed between the source and drain electrodes is very important and overlap between the gap and source and drain electrodes is to be avoided. On the other hand, the semiconductor and dielectric layers may significantly overlap the device electrodes without detriment to performance, provided that adequate isolation is achieved between neighbouring TFTs and between neighbouring pixels so that leakage currents do not cause inter-pixel cross-talk. Thus, some features of a TFT require accurate and high-resolution pattern registration, while others do not.

During a TFT manufacturing process many separate steps are typically required. Between each step and even during a step, environmental conditions such as ambient temperature and humidity may change, causing changes in dimensions of the carrier substrate for the TFTs and of masks or positioning equipment for deposition or patterning tools. Thus registration errors may accumulate between TFT layers. The temperature of the substrate and deposition or patterning tools may also change during an individual process step as sources of heat are frequently necessary for deposition, patterning or post deposition treatments. These temperature changes also cause dimensional changes within a single patterned layer and again registration may be affected. There is therefore a need to accurately control registration and alignment as well as the dimensions of individual patterns throughout the manufacturing process of thin film optoelectronic devices in general. Another important issue to be considered is the efficient use of materials.

In the well-known photolithographic process used for semiconductor device manufacture, materials are deposited over the whole device substrate and removed pattern-wise. This subtractive approach, while highly reliable and accurate, is wasteful of materials. It is preferable where possible to use additive processes in which materials are only deposited where they are required. In recent years, ink- jet deposition has been widely used to place small droplets of material onto surfaces. This technique enables significant reduction in material wastage because it is an additive process.

A disadvantage with additive processes is that defects can easily occur through spurious deposition of materials outside the desired deposition areas. For example, with inkjet deposition, it is known that satellite droplets can be formed. Also, it is known that splashing can occur, when a droplet impacts the deposition surface. Through these mechanisms, material can be deposited outside the desired areas of the surface to be patterned. It can occur that these spurious depositions cause short-circuits or deleterious performance to the devices being fabricated.

Another possible cause of spurious depositions in the case of printing from plates is plate wear which can give rise to overprinting in non-image areas.

For large feature sizes, additive processes offer great promise as patterned deposition technologies. For small feature sizes, however, additive processes, such as conventional printing and inkjet have more limited applicability. InkJet droplet sizes are typically of the order of a few picolitres. A 1 pi droplet has an in-flight diameter of 12 microns. When it lands it spreads and depending on the surface energy of the substrate and the surface tension of the liquid, the diameter of the circle that is now covered with liquid could be much larger than the diameter of the original droplet. This significantly limits the resolution of patterning that can be achieved. Furthermore, there is a limit to the accuracy with which inkjet droplets can be placed at a precise location on a surface, and the thickness of liquid that can be deposited.

Many approaches have been suggested to reduce the pattern resolution limitations arising from inkjet droplet sizes. US 2005/0170550 describes the use of banks of appropriate wettability formed on a surface to contain liquid droplets that are incident on the surface between a pair of banks, The process for forming the banks and for profiling the wettability of the sides requires several steps. US 7115507 describes a method of restricting the lateral spreading of a liquid droplet on a surface by the use of indent regions.

When making electronic devices such as TFTs using largely solution-based deposition processes, there have been many approaches to overcome the problem of aligning the gate electrode with the semiconductor channel region. WO 03/034130 describes a method of using the topology of a liquid film while it is still wet to align a second liquid, immiscible with the first, deposited on top of it. US 2005/0071969 describes a method of embossing a groove and building an electronic device in the groove. WO 2004/055920 describes a method for making electronic devices in which a surface topology is defined in a lower layer, preferably by embossing, and a non-planarising upper layer is deposited such that liquid applied to the upper layer conforms to the topology defined originally in the lower layer. Various methods of constructing TFTs are proposed but these require many extra process steps to manipulate the wettability of surfaces arid to achieve alignment of the gate and the TFT semiconductor channel. A further difficulty is the use of raised topologies in some embodiments which suffer greatly reduced capillary flow speeds due to the convex profile of the surface of the liquid as it flows along the channel.

Managing the flow of liquid on the surface of the substrate on which the electronic devices are fabricated is a key issue, especially given the need to reduce processing time to a minimum so that fabrication costs are low. Care must be taken to avoid droplets of functional material both bridging between two wettable features closely spaced on a surface and creating voids within a feature which become defects. US 2006/0091547 describes a method for providing a linear region and a wider region both enclosed by banks, such that the thickness of the dried film formed after jetting droplets into the region between the banks is substantially uniform. US 2005/0005799 describes a method for jetting a series of spaced droplets into a long narrow lyophilic channel so that the droplets do not wet the top surfaces of the channel but flow off them into the channel. The spacing between neighbouring droplets is such that they fill the channel and merge with one another to form a continuous strip of liquid. No liquid remains on top of the channel walls, even if the original incidence of the droplet was partly on the top as well as the walls of the channel. In this method, adjacent channels must have a lyophobic surface between them that is wider than the droplet width on impact to ensure that no droplet ever bridges the gap between two adjacent channels. This limits the closest approach between neighbouring channels. Furthermore, it is generally much harder to get good adhesion between a lyophobic surface and a layer that is deposited on top of it. This can lead to mechanical weakness in multilayer devices of the kind addressed by the present invention.

The use of via-holes or through-holes as a means of soldering components onto a printed circuit board is well documented in the prior art. For example

US6402531 describes the use of capillary action to wick molten solder into the gap between a hole in a circuit board and the pin of a component which is to be connected electrically to the board. It is also known in the field of microfluidics that through-holes in a substrate can be used as reservoirs in fluidic devices. US6086825 describes a fluid analysis system that uses a series of through holes in a thick substrate to act as reservoirs for the test liquid. Closed channels are provided within the substrate of the fluid analysis device which guide liquid away from the reservoir through a mechanical filter in one embodiment. One of the surfaces around the opening of the through hole is hydrophobic to prevent the aqueous liquid in the through-hole wetting the surface of the substrate and contaminating other through holes.

A further limitation not addressed in the prior art is that of the restrictions placed upon the functional materials that can be reliably applied by a printing process such as, for example, inkjet printing, flexographic printing or gravure printing. The material to be patterned generally has to be formulated with a specific rheology, viscosity and surface tension to be suitable for a specific deposition process. This can be restrictive, for example when patterning a conductive ink that must have a very high solids content, and is therefore very viscous, in order to achieve good conductivity. These kind of conflicting "requirements may necessitate compromises either in the quality of the patterned deposition or in the functional performance of the deposited material.

Another issue with the use of a substrate that has a series of narrow channels to direct flowable material, is the requirement for a deposition region, where the material is first deposited before being drawn into the capillary channels. The width of the deposition region is necessarily greater than the width of the channel to ensure that capillary forces will drive the flowable liquid into the channel. Each channel that is to be filled must be connected to a deposition region. In some circumstances the deposition region forms part of the finished device, for example it may become a conductive pad, but otherwise the deposition region or regions, reduce the amount of space on the substrate available for patterning.

PROBLEM TO BE SOLVED BY THE INVENTION

The present invention aims to solve the problem of how to pattern a wide range of materials with improved resolution, high feature density and high quality using a wide range of deposition techniques. It further aims to solve the problem of how to make interconnections between various points in the pattern.

SUMMARY OF THE INVENTION According to the present invention there is provided a method of patterning a flowable material on a surface, the method comprising providing the surface with at least one channel and at least one through-hole with at least two openings, wherein at least one of the openings is located in the surface adjacent to the at least one channel, such that when flowable material is deposited adjacent to another of the at least two openings, the material is directed into the at least one through-hole by the action of capillary forces and emerges at the opening adjacent to the at least one channel whereupon it is further directed along said channel.

The invention further provides an element having a patterning surface for patterning flowable material, the surface being provided with at least one channel and at least one through-hole with at least two openings wherein one of the at least two openings is located in the surface adjacent to the at least one channel such that the relative positioning of said opening and said channel would result in flowable material emerging from said opening being directed along said channel and wherein the through-hole and openings are of such a size that flowable material deposited adjacent to another of the at least two openings would be directed into said opening and through the through-hole by the action of capillary forces.

The invention also provides a patterning substrate for patterning flowable material having two surfaces and at least one through-hole connecting the two > surfaces, one surface being provided with at least one channel wherein an opening of the through-hole is located in the surface adjacent to said channel such that the relative positioning of said opening and said channel would result in flowable material emerging from said opening being directed along said channel and wherein the through-hole and openings are of such a size that flowable material deposited adjacent to an opening on the other surface would be directed into said opening and through the through-hole by the action of capillary forces. ADVANTAGEOUS EFFECT OF THE INVENTION

The method of the invention is self-aligning. It is insensitive to substrate distortion and very accurately aligns, for example, the gate electrode of a TFT to the semiconductor channel region between the source and drain electrodes with no overlap. This reduces parasitic capacitance and improves device speed.

The invention allows the distance between the source and drain electrodes (i.e. the semiconductor channel length) of a TFT to be reduced substantially below printing resolutions. It enables gaps to be brought much closer with respect to the width of an inkjet droplet. Bringing the lines closer together reduces TFT conduction path length thus increasing switching speeds and reducing the device footprint.

The method of the invention also increases the speed of forming the patterns. It is possible to deposit a functional fluid onto a single location on the patterning substrate and have the patterns form by virtue of capillary flow whilst the deposition head is being moved to the next location. It is not necessary to actually deposit fluid at all locations in the pattern to be created.

The invention provides better adhesion of overlayers to the substrate since it does not rely on having a lyophobic characteristic on the areas where the flowable material should not flow. It is possible to avoid the channel overflow and create short circuits. The closest approach of two channels can be much closer because there is no need to have a large lyophobic land between the channels.

The invention also allows the pre-patterning step to be simplified. There is no requirement for the formation of banks or areas of surface energy contrast. In the preferred embodiment channels are defined as recessed regions in the surface and may be formed for example by photolithography, embossing, laser ablation, cutting and moulding. In a further preferred embodiment, embossing alone is enough to prepare the substrate for patterning. Embossing is a low cost technique and may be done roll-to-roll so that incremental costs in preparing the substrate for deposition are minimised versus some of the other routes. The method of the invention may be readily used to pattern functional materials to achieve feature sizes of lOOnm. With great care and careful selection of materials it is possible to make patterns by the method with feature sizes of a few tens of nanometres. The method enables very high resolution features to be made and is therefore applicable to the manufacture of frequency selective surfaces, metamaterials, as well as all kinds of electronic devices and optoelectronic devices, switchable Bragg gratings, hybrid fluidic-optoelectronic or electro wetting devices. The method of the invention may also be used to pattern biological liquids and to make sensor arrays. The method allows the usable area for the channels to be maximised, since the deposition can be performed on the opposite side of the substrate from the high resolution pattern. The technique used to deposit the material to be patterned can be a low resolution method since it is no longer critical that the material be accurately deposited. This considerably broadens the range of deposition technologies that can be used in combination with the capillary patterning method. For instance, in one implementation, a continuous liquid film coating method such as blade coating, rod coating, slide-hopper coating or the like can be used to deposit material onto one side of a substrate that has through holes which connect to a series of patterned channels on the opposite side of the substrate. The method allows for a pattern to be created by any suitable printing technique on one side of the substrate, and for this pattern to be connected to another pattern by wicking some of the flowable material through holes in the substrate into channels on an opposing side of the substrate.

The method allows for a pattern to be created by depositing a liquid over a surface energy pattern using a suitable continuous-liquid coating technique on one side of the substrate, and for this pattern to be connected to another pattern by wicking some of the flowable material through holes in the substrate into channels on an opposing side of the substrate. This creates patterns on at least two sides of a substrate, one directed by surface energy, the other directed by the channels. The method allows for interconnections to be made between devices patterned on one side of the substrate with those on the opposite side of the substrate. A device fabricated using a series of pre-formed channels on one side of a substrate can be connected to a device on an opposing side of the substrate formed by an additive patterning technique such as inkjet printing, flexographic printing, gravure printing, screen printing or similar by using a through hole to wick a flowable conductive material. Alternatively a device fabricated using a series of pre-formed channels on one side of a substrate, can be connected to a device on an opposing side of the substrate formed by over-coating a surface energy contrast pattern with a flowable liquid using a continuous liquid film coating method such as blade coating, rod coating, slide-hopper coating or the like by using a through hole to wick a flowable conductive material.

The method is insensitive to drop volume variations and thickness variations in the deposition technique, because the technique provides a "self- metering" effect to fill channels but not to overfill.

The method is largely insensitive to splatter because the high resolution pattern is located on the opposite side of the substrate from the deposition. Spurious deposition may give rise to harmless islands of material on the deposition side, whereas had they occurred on the patterned side, short-circuits or other defects might have arisen in the fabricated device.

The method is further insensitive to gravity, which enables printing to be done on the upper surface of a substrate and the pattern to be formed on the lower surface, since capillary forces associated with through-holes and channels are much stronger than gravitational forces. The method is also applicable to the patterning of surfaces on three- dimensional objects.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference to the following drawings in which: Figures Ia to Ij illustrate various channel geometries defined by a surface topology;

Figure 2 illustrates the definition of channel angle and a condition for capillary flow; Figure 3 illustrates an embodiment in which the surface of a three dimensional object is provided with a channel in two different regions of the surface, both of which are connected by a through-hole to the deposition region on the other side of the object;

Figure 4 shows five examples of through-holes intersecting with channels on the patterning surface;

Figure 5 illustrates an embodiment in which a pattern on one side of the substrate is created by an additive printing process, and connects using through- holes to a series of channels on the opposite side of the substrate to allow two-sided patterning of the substrate, with both patterns being connected by the through- holes;

Figure 6 shows an example of a patterning substrate with a series of V groove stripes, each of which is provided with a through-hole;

Figure 7 shows the patterning substrate of Figure 6 in which a pattern is made on the deposition side by additive printing of stripes and on the other side by capillary wicking along channels;

Figures 8 illustrates an embodiment in which a surface energy contrast mask on one side of the substrate is used to direct a continuous coated liquid film into a pattern which connects using through-holes to a series of channels on the opposite side of the substrate to allow two-sided patterning of the substrate, with both patterns being connected by the through-holes;

Figure 9 illustrates the results from Example 1 which is also shown schematically in Figure 7; and

Figure 10 illustrates the results from Example 2 where a surface energy pattern on the deposition side of a substrate connects via a through-hole to a series of V groove channels on the other side of the substrate. DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved method for manufacturing electronic and optoelectronic devices. The present invention enables the patterning of flo wable materials on a surface which is provided both with at least one through-hole and at least one channel. Typically the surface would be provided with many through-holes and channels so that complex multilayer structures can be fabricated. The surface to be patterned may be situated on a solid object or it may be one surface of a planar substrate. The present invention is particularly suited to roll-to-roll processes where multilayer devices or structures are built up in subsequent deposition and patterning steps on a flexible web. In the case where it is desired to pattern a thin planar substrate, the flowable material is usually supplied to one surface of the substrate (the deposition surface) by any convenient method such as by ink-jet printing. The invention is not however limited to this additive method. It will be understood by those skilled in the art that any suitable printing method may be used such as by conventional printing, e.g. lithographic printing, flexographic printing, screen printing, pad printing, gravure printing, intaglio printing and also by digital printing techniques such as electrophotographic printing and thermal transfer. In another embodiment of the invention, flowable material is supplied by continuous liquid film coating techniques such as blade coating, rod coating, slide-hopper coating or the like. The invention is neither limited to deposition being only on one surface of the substrate. It is also possible to deposit some parts of the pattern directly onto the surface to be patterned and have the flowable material pass through the substrate to channels on the deposition surface. It is normally the case however, that devices are built on one side of the substrate and connections made on the other as will be described in further embodiments.

Flowable material is deposited adjacent to a through-hole such that it is in immediate contact with, or spreads to make contact with the through-hole whereupon it is drawn into the hole by capillary forces, passes through it and on emerging at the other side, it is further directed into and along the channel by capillary forces. The liquid will continue to flow into the channel, until either it reaches the end of the channel or until the volume of liquid in the deposition region is reduced to the point that the pressures of the liquid on the opposite side of the substrate and the channel region are equal. The liquid adjacent to a hole may be constrained by boundaries, such as banks, surface energy contrast, or a recessed region or it may be entirely or partially unconfined on the surface of the substrate. Furthermore, when material is deposited in overlapping regions a continuous pattern may be formed. By manipulating the wetting characteristics of the substrate around the through-hole it is possible to control how the liquid is distributed both around the hole and within the channel on the opposite side of the substrate. For instance, liquid may be prevented from being completely drawn into the through-hole by creating a zero receding angle surface around the through-hole. "Channel" 1, as illustrated in Figure Ia includes any open linearly extended topology on a substrate surface 2 that confines liquid. Here, the term "open" is used to mean that the channel is open to the atmosphere at all points along its length. It includes, but is not limited to, recessed areas, raised "lands" bound by sharp-edged descending walls and steps where the flowable material is confined to and directed along the base of the step. Channel topology may be characterized either as convex or concave by which is meant that a channel can be said to be concave if the liquid is confined within the channel structure. A convex channel is one in which the liquid is confined outside the channel structure. A flat surface, which is by definition neither convex nor concave, cannot confine liquid by surface topology and liquid placed on a flat surface spreads freely until the contact angle at the wetting line is less than the advancing contact angle. Figure 2 defines the channel angle β for a V groove channel. Note that the channel angle is defined by the intercept of the projection of the side walls at the wetting lines, rather than the actual shape of the base of the channel. Figure Ib illustrates a V groove channel with a channel angle of just less than 90° showing liquid confined within the channel. This channel geometry is concave. Figure Ic illustrates a channel with a channel angle of 90° configured as a step. Liquid is confined at the base of the step. Capillary forces are much stronger than gravitational forces over distances less than the capillary length (typically a few millimetres) and so the orientation of the channel in space is irrelevant. Liquid would be confined in the V groove of Figure Ib or the step of Figure Ic whether or not the substrate was orientated upwards or downwards. Therefore the structures illustrated in Figures Ib and Ic are almost identical in terms of their confinement efficiency. A difference would arise if the channel in Figure Ib were filled to the top, at which point the liquid would pin on the topological discontinuities on either side of the channel. In the case of Figure Ic, there is a discontinuity to the left hand side of the channel that would act as a pinning point, but such a discontinuity ₍ is not shown on the right hand side and so liquid would not be confined and would be free to flow until the advancing contact angle is reached.

Figure Id is an example of a convex channel with a channel angle of 180°. Liquid is contained on the outside of the channel structure. In fact, the flat land on the top of the protrusion does not contain the liquid in the same way as the channels of Figures Ib and Ic where capillary interactions with the walls and the nature of the wall geometry contain the liquid. In Figure Id, it is only the discontinuities at either side of the flat land that pin the wetting line and confine the liquid. If a small droplet of liquid were placed on the flat land and allowed to spread, it would do so freely as on a flat surface, without the enhancement that comes from interaction between the wetting line at its leading edge and the surface profile at the base of a V groove channel.

Figure Ie shows a rectangular channel, also an example of a concave channel, with a channel angle of 0° since the planes of the channel walls where the wetting lines on either side of the channel are situated do not intersect. Figure If illustrates another much wider rectangular channel in which the volume of liquid contained is not enough to wet the whole of the bottom of the channel. The channel therefore behaves as two step channels as shown in Figure Ic that do not communicate with each other. Figure If is therefore illustrating two concave channels with channel angles of 90°. It is noted that within a rectangular channel such as Ie and If, it is possible to have liquid at the leading edge of the capillary flow filling the channel as illustrated in figure If whereas further back from the leading edge, where the channel has a higher volume of liquid contained within it, it may fill the channel as illustrated in Figure Ie.

It is not necessary to have sharp geometries to confine liquid as shown in Figures Ig and Ih, which are both examples of concave geometries with channel angles of approximately 10° and 60° respectively. The channel of Figure Ii, however, despite having the same topology as the channel of Figure Ih has a greater volume of liquid being confined such that the inner channel shape cannot contain all the liquid. The channel angle is now approximately 325°. Liquid is still confined, but the confinement of the wetting line is now on the outside of the structure and this is therefore an example of a convex channel. It will be recognised that the geometry of Figure Ii is highly unstable and there is a significant risk that the liquid will run down the walls of the channel at the slightest perturbation. Figures Ih and Ii illustrate that it is sometimes not possible to define a channel as concave or convex simply by its topology as the position of the wetting lines are needed to understand the nature of the confinement. For the sake of clarity, in Figure Ii, had only one side of the inner channel overflowed so that only one of the wetting lines had been situated on the outside of the structure, it would still be functionally a convex channel. It is necessary for both the wetting lines to be on the inside of the structure for it to be defined as concave. Figure Ij is another example of a concave structure but with a channel angle of -90°. Channels with a negative channel angle are narrower at their upper opening than at their base. Although this structure is very effective at wicking liquid and confining it, owing to difficulties in manufacture of such structures by low cost processes, it is not a preferred embodiment. Channels of the present invention are concave, by which is meant that they have a channel angle, β, greater than -180° and less than 180°. More preferably channel angles of the present invention are greater than -150° and less than 150°, such that the capillary enhancement to the flow of the functional material in the channel is more significant. Most preferably the channel angles are greater than 0° (for ease of manufacture of the channel by stamping or embossing) and less than 90° to further enhance capillary wicking and widen the range of functional materials that can be used. It is not necessary for channels of the present invention to have sharp-edged pinning points defining the boundaries of the channel and preventing overflow, but channels with preferred topologies have at least one sharp-edge pinning point and preferably two.

When considering wetting behavior of surfaces, conventionally an advancing contact angle is defined. If a sessile drop is formed and liquid added slowly, then the advancing angle (θ_a) is defined as the angle between the tangent of the liquid surface and the substrate measured at the three-phase line and through the liquid as the line just begins to advance. For more information on this topic see for example "Capillarity and Wetting Phenomena" by Gilles De-Gennes, Brochard- Wyart and Quere published by Springer 2003. Likewise, the receding contact angle (θ_r) can also be defined. If a sessile drop is formed and liquid removed slowly from the drop, then the receding contact angle is defined as the angle between the tangent of the liquid surface and the substrate measured at the three- phase line and through the liquid as the line just begins to recede. If the wetting line does not recede the contact angle is defined as zero degrees. A low receding contact angle can be created by roughening the substrate or by chemical treatment such as corona discharge treatment or a non-planarising layer with an intrinsically low receding contact angle. Wetting hysteresis is defined as the difference between the advancing and receding angles. A surface is termed reversible if it has zero wetting hysteresis. The rate of wicking along the channel may be controlled by the channel angle, β, defined in Figure 2.

The greater the channel angle, the slower the rate of wicking will be. When the advancing contact angle, θ_a, is less than (90° - β/2) wicking will occur as described in "Flow of simple liquids down narrow V grooves", Mann et al., Phys. Rev.E. 52, p3967, 2005. Preferably, θ_a should be set somewhat below this value to facilitate rapid wicking along the channel and thus minimise evaporation losses from the liquid which may change its flow characteristics as it wicks. The movement of flowable material into the channel stops when either the pressure in the flowable material body is everywhere the same, or when the flowable material solidifies. Thus provided the region adjacent to the through-hole has a non-zero receding contact angle and the channel volume is greater than the volume of the body of flowable material, then the equalisation of pressure implies that the flowable material will continue to move until it has been drawn completely into the channel. In situations where this is undesirable, it can be avoided by arranging the liquid flow to prevent this. This is achieved by ensuring a receding contact angle of less than 30°, preferably less than 15°, at least within the region on the surface where the liquid was originally deposited but possibly over the entire surface. Even more preferably the receding contact angle is less than 5°, or zero. Note that this constraint also ensures good adhesion for the added layer.

In the present invention, liquid flow may be arranged such that the channel cannot overflow and cause undesirable bridging between circuit elements. This may be achieved by setting a further condition on the receding contact angle, θ_r in the region where the liquid was originally deposited. The flowable material will continue to flow until the pressure in the liquid body is everywhere equal, or until the material solidifies. Given the small scale of the structures under consideration here (i.e. much smaller than a capillary length) and the low Reynolds number of the flow, the local pressure is entirely determined by the Laplace pressure, i.e. the ratio of surface tension and local surface curvature. The nature of capillary wicking, i.e. flow into the channels, is such that at the advancing front there has to be a concave surface with radius of curvature of order {cos(contact angle)/channel dimension}, whereas the deposition region has a curvature of order {cos(contact angle)/source dimension} . If the region adjacent to the through-hole has a non-zero receding contact angle and the channel has a volume greater than the flowable material body, then as the flowable material is drawn into the channel the volume of liquid in the deposition region drops and its radius of curvature drops. If either the channel volume is too small, or the viscous pressure drop along the channel is high, then as the radius of curvature falls, the pressure in the deposition region may be sufficient to cause a quantity of flowable material to be expelled from the channel. These conditions are specific, and can be avoided under all situations if the deposition region has a zero receding contact angle. For defined situations created by a particular embodiment, this condition can be relaxed to a requirement of a minimum level of contact angle hysteresis. If the diameter of the through-hole is less than the width of the adjacent channel or channels, then the preferred channel cross-section is concave or 'V shaped, since in this case the capillary forces are more likely to drive the flowable material along the channel as it emerges from the hole, by creating concave curvature at the advancing contact line. If the channel cross-section is not concave or 'V shaped then the flowable material emerging from the hole must have a sufficiently low advancing contact angle on the channel material to allow unconstrained spreading wetting with a convex curvature at the advancing contact line, to allow the flowable material to spread until it encounters the walls of the channel when a concave wetting line may then be formed and the flowable material driven along the channel by capillary forces.

In the simplest embodiment, an object with a surface is provided with a through-hole, that emerges on another surface adjacent to a channel, such that when flowable material is deposited adjacent to the through-hole, it is drawn into and through the through-hole and along the channel by capillary forces. In another embodiment, the through-hole or holes may connect one or more channels on one or more surfaces as is shown in Figure 3. In this case, flowable material deposited adjacent to the through-hole 33, is directed into and through the hole whereupon it emerges at one or more different surfaces and is directed along the one or more channels 31, 32, by capillary forces. The opening of the through-hole must be located adjacent to the channel, meaning that the two must be sufficiently close that liquid emerging from the through-hole is able to flow along the channel. In the plan view of a surface to be patterned in Figure 4a, the through-hole 42 and the channel 41 are separated by a small distance comparable to the dimensions of the hole and the channel. Their cross-sections do not intersect at the surface. The topology of the circumference of the (in this case) circular hole at the surface is a circle in the same plane as the surface itself. Liquid emerging from the through-hole must be able to flow across the surface by unconstrained wetting until such time that the wetting line of the spreading liquid makes contact with the edge of the channel at which point, the topology of the channel modifies the shape of the leading edge of the wetting line to produce a concave shape which is advantageous in promoting rapid wicking of liquid into and along the channel. This configuration requires the through-hole and the channel to be close to each other and the surface between the through-hole and the channel to be very wettable and is not a preferred embodiment. Figure 4b shows a rectangular cross-sectioned channel 41 , i.e. one with a flat base and square edged walls as illustrated in Figure Ie with a hole 42 emerging from within the base of the channel. In this case, the channel and the hole do now intersect at the surface, but because at the point of emergence of the hole at the surface, the channel profile is flat, the topology of the circumference of the hole at the surface is again in a parallel plane to the substrate surface. Again, as in Figure 4a, the channel base must be very wettable in order for the liquid to emerge from the hole and wet the base of the channel by unconstrained wetting, but assuming this condition is met, the liquid will spread on the base of the channel until the wetting line touches one of the walls, whereupon, liquid will readily wick along the step channel formed by the base and the walls and so fill the channel. As with

Figure 4a, the wettability constraints placed on the channel surface do not make this a preferred embodiment, but it is more preferred than the example of Figure 4a.

Figure 4c shows in plan view a hole 42 emerging in the base of a V groove channel 43 where the dotted line 45 represents the deepest point of the channel. The topology of the top of the hole at the surface is no longer flat as in Figures 4a and 4b because the base of the channel is V shaped. The hole and the channel intersect at the surface to form a line which in this case is circular seen in plan view. If the intersection line is projected along the line C C and viewed from the side as shown by the arrow, the profile of the intersection as shown in 44 will be seen. The lowest point of the intersection will be at the base of the V groove. It is this concave profile which facilitates the flow of liquid from the hole 42 and into the channel 43 by capillary forces and is a preferred embodiment.

It is not necessary for the through-holes to be smaller than the channels. The only constraint on the hole diameter is that is should be small enough to promote flow of liquid through the hole by capillary forces. The capillary length is a well-known measure of the length scale below which capillary forces (rather than gravity) dominate flow and typically this will be of the order of several millimetres. It should also be noted that any hole cross section can used, not just circular, provided it is of such an extent that capillary forces dominate. Figure 4d shows in plan view a hole 48 of larger size than the channel emerging at the surface such that it partly intersects with a V channel 43 at one corner. If the line of intersection of the hole wall and channel at the surface is projected on to the line D D' and viewed from the side in the plane of the surface as in Figure 4c, the profile 46 will be seen. The profile of the line of intersection again shows a concave profile which shapes the surface of the liquid as it flows up the hole and thus facilitates the emergence of the liquid from the hole and into the channel. This is a preferred embodiment.

Figure 4e shows another preferred embodiment in which a hole 49 of larger extent than the channel 43 intersects with the surface of the channel to form a cross section 47. which also has a concave profile.

Figure 5 shows another embodiment in which a pattern is written on the deposition side of a substrate which connects two device structures on the other side via two through-holes . The substrate is provided with two 'V channels as shown in plan view in Figure 5b. A through-hole connects each channel with the deposition side of the substrate as shown in cross-sectional view in Figure 5a. The flowable material is deposited in a line on the deposition side of the substrate such that the line connects the openings of the two through-holes as shown in Figure 5c. As the flowable material comes into contact with the opening of a through-hole, it is directed into the hole by the action of capillary forces. The liquid flows through the hole and emerges at the other opening whereupon it is directed into and along the channel by the further action of capillary forces. In this way, connections may be made between two devices on the other side of the substrate as further shown in Figure 5c. The devices themselves are not shown in the diagram but the two channels direct material from the holes to the device to make the connections. A benefit of this embodiment of the present invention is that the positioning tolerances of the deposition technique may be much larger than the dimensions of the channel, and printing defects such as splatter, overprinting or satellite drops do not affect the final patterned coating within the channels. The deposition method may be any printing method, including but not limited to inkjet, electrophotographic methods, flexography, gravure, screen printing and offset lithography. In this way the interconnection between the two devices can be made using a lower resolution deposition process that can create much thicker, and so more conductive, layers.

Figure 6 shows an embodiment where the flowable material may itself be a pattern of functional materials or a device created by printing, whilst serving as the source of flowable material to feed into the channels on the opposite side of the substrate. This embodiment utilises a substrate 60 having a series of channels 61 on one side 63 with through-holes 65 connecting the bottom of the channels to the opposite side of the substrate 62 which is planar as shown in the side view of Figure 6. Figure 7 illustrates Example 1 below in which a pattern of flowable material is deposited onto the planar side 72 of the substrate 70, and as the flowable material impinges upon a hole 75, it is drawn through the substrate and into the channel 71 on the opposite side. The resulting effect is a series of horizontal lines 76 printed on the planar side 72 of the substrate, and a series of vertical lines created in the channels on the opposite side of the substrate by capillary wicking. Figure 8 illustrates the embodiment of Example 2. In this case a surface energy contrast mask of lyophilic 83 and lyophobic 84 regions has been created on the substrate. Flowable material is deposited on the surface as a continuous coating. As the coating interacts with the surface energy pattern, it rearranges itself, preferentially settling on the lyophilic region 83 to form a pattern which furthermore serves as a source for the flowable material to feed the channels 81 on the opposite side of the substrate. The continuous liquid film coating can be deposited by techniques such as blade coating, rod coating, slide-hopper coating or the like, which allows much thicker layers of material to be deposited and have a wider formulation window. When the flowable material impinges upon a through- hole 82 it is drawn through the through-hole by capillary forces and into the adjacent channel on the opposite side of the substrate. In this manner low-cost, continuous liquid film coating techniques can be used to create patterned coatings on two different sides of a substrate, the pattern formation being directed first by surface energy contrast, and subsequently by capillary channels. Depending upon the wetting characteristics of the surface energy pattern, the flowable material may be distributed on both sides of the substrate and so create a connected double sided pattern, or it may completely withdraw from the surface energy patterned side and into the channels. It is understood that the viscosity of the deposited material may be reduced to make the material more flowable after deposition by a further treatment step, such as raising the temperature. In a preferred embodiment, the channel width is less than the resolution of the deposition method. All printing methods routinely achieve resolutions of 100 microns, but below this value, accurate definition of features becomes increasingly difficult. Screen printing for example, struggles to achieve gaps between features below 50 microns. Typical inkjet resolutions are limited by the width of the droplet after impact with the surface and typically achieve a minimum feature size of 40 microns. Offset lithography and gravure printing can achieve feature sizes of 30 microns when extremely well controlled. No conventional printing technique can achieve feature sizes and gaps between features of 10 microns that would be in the preferred dimension range for construction of TFT features.

The flowable material may include conductive, semi-conducting, light emissive, light reflecting or dielectric inks. The flowable material may comprise precursor materials, or catalysts that may enable functional material to be formed within the channels by a subsequent process. The flowable material may comprise a carrier solution with particles dispersed therein. The particles may be biological materials, high molecular weight molecules, viruses, cells, bacteria, nanoparticles, quantum dots and polymer particles. The particles may also be monodisperse spheres, or monodisperse core-shell spheres, or monodisperse metal coated spheres, such that photonic or plasmonic crystal structures may be created when a regular arrangement of spheres is formed within the channels. Flowable material also includes materials that are deposited in solid or highly viscous form, but which on heating become flowable and are able to move rapidly under the action of capillary forces. Solders and phase-change inks are examples of materials that become flowable when heated. Other flowable materials are polymers that can be cross- linked by the application of heat or high energy radiation. It will be understood that these are examples only and the invention is not limited to these examples.

The present invention is particularly suited to the forming of patterns for electronically active structures. Not only may the channel regions be used to form circuit elements such as TFTs, but the deposition regions may form circuit elements of larger dimension also. Examples of such circuit elements are conductive bus lines, capacitors, inductors and resistors.

In one embodiment of the present invention, the flowable material is a conductive ink and the channels direct the material to the electrodes of an electronic device such as a TFT. In this embodiment, it is preferable that flowable material is deposited on the opposite side of the substrate to the channels and devices, so as to maximize the amount of space available for devices. In this example, flowable material, a conductive ink, was deposited by inkjet, such that each droplet of ink is positioned to overlap with the previously deposited droplets. Through -holes connect to the channels provided in the surface as recesses such that when the deposited droplets of ink make contact with the through-holes ink is drawn into the hole by capillary forces and is subsequently directed along the channels by capillary forces. On the opposite side of the substrate, the conductive ink is unconstrained by physical or chemical patterning and allowed to form a continuously conductive structure which may be used as a highly conductive line, such as a bus line for power or data or data select functions for several devices. The smaller channels connect to devices on another part of the substrate. In this case it is essential to avoid open circuits in the connecting pattern between the two through holes. This can be achieved by manipulating the wetting characteristics of the deposition surface and the flowable material such that it has a low or zero receding contact angle to avoid the flowable material dewetting the surface.

In another embodiment material is deposited, but does not flow into the through-hole. Energy is applied to the material such that the material becomes flowable. Materials that display this property would include phase change inks, which can change viscosity by several orders of magnitude when the temperature is increased beyond a certain critical temperature. Another material that displays this property would be a material that melts above a certain temperature, such as, for example, solders and waxes. Once the viscosity has dropped, the material may flow and is able to be directed into the channel by capillary wicking from the deposition area. After the material has flowed down the channel the viscosity again returns to its normal ambient state as the material cools. The benefit of this approach is that very low-cost patterned deposition methods such as offset lithographic printing or flexographic printing may be used to achieve coarse positioning of a material on the surface of a patterned substrate. Fine positioning is achieved by capillary wicking after a change in material viscosity is achieved.

Example 1 (see Figures 6 and 9)

In this example, V shaped grooves with a pitch of approximately 300 micrometers in polycarbonate (3 M optical lighting film, SOLF ™) were used as the substrate material. The substrate was prepared as shown in Figure 6, having approximately 20 micrometer diameter holes laser cut into the bottom of the channels with a vertical pitch equal to the horizontal pitch of the grooves. The holes are circled in Figure 9. A Dimatix DMP 2800 printer was used to deposit horizontal lines of a conductive silver nanoparticle ink (Silverjet DGH 50LT, Advanced Nano Products, Korea ) with a vertical spacing of approximately 300 micrometers, onto the planar side of the substrate. In locations where the ink impinged upon the through-holes, it was wicked into the holes and subsequently driven along the channels by capillary forces.

Example 2 (see Figure 10)

In this example, polycarbonate film having V shaped grooves as was used in the previous example, was patterned with a lyophobic ink created using 25 % w/w Fluoropel PFC604 + 75% w/w Perfluorodecalin to create a surface energy contrast mask. The lyophilic regions 102 and lyophobic regions 103 were a series of lmm wide lines with lmm wide gaps horizontally across the substrate formed by printing onto the back of the polycarbonate using a Dimatix DMP 2800. 800um diameter holes 101 were drilled through the sample to connect the lyophilic areas on the back of the sample to the bottom of the V-grooves on the front of the substrate.

The surface energy mask on the back of the sample was then overcoated with a small volume of polythiophene (PEDOT-PSS ~0.25ml) consisting of Baytron P + % Ethane Diol + 0.1% Triton TXlOO, using a blade coating device (RK Print, Royston). After coating, the polythiophene layer was seen to retract from the horizontal lines of fluoropolymer to form horizontal lines on the lyophilic regions 102 of the substrate. In locations where the ink impinged upon the through- holes, it was wicked into the holes and subsequently driven along the channels 104 by capillary forces.

The present invention offers a method for making patterns of any flowable material, including liquids containing dispersions of solid particles. The liquid may contain at least one of a polymer, a monomer, a high molecular weight molecule, a latex, a solid particle, an emulsion, a reactive species, and/or a soluble metal complex. It is particularly suited to making patterns of conductive materials for connections between devices and for definition of electrode structures in optoelectronic thin film devices. The method can work on any surface on which is defined a channel. The method can be used to fabricate antennae, interconnections, bus lines, optoelectronic devices, and in particular, TFTs.

Devices fabricated as described above may be part of a more complex device or product in which one or more of the devices may be integrated with each other or with other devices. Examples include but are not limited to displays, RFIDs, touch screens and batteries.

The invention has been described in detail with reference to preferred embodiments thereof. It will be understood by those skilled in the art that variations and modifications can be effected within the scope of the invention.

Claims

CLAIMS:

1. A method of patterning a flowable material on a surface, the method comprising providing the surface with at least one channel and at least one through-hole with at least two openings, wherein at least one of the openings is located in the surface adjacent to the at least one channel, such that when flowable material is deposited adjacent to another of the at least two openings, the material is directed into the at least one through-hole by the action of capillary forces and emerges at the opening adjacent to the at least one channel whereupon it is further directed along said channel.

2. A method as in claim 1 wherein the at least one channel has a concave profile.

3. A method as in claim 2 wherein the at least one channel and the at least one through-hole are located on the surface such that the intersection of the perimeter of the at least one through-hole with the surface forms a line which has at least in part a concave profile.

4. A method as in claims 1, 2 or 3 wherein the surface is the first of two surfaces of a substrate and wherein the at least one through-hole connects the two surfaces such that when flowable material is deposited on the second surface of the substrate adjacent to an opening of the at least one through-hole, the material is directed into the through-hole by the action of capillary forces and emerges at an opening on the first surface adjacent to the at least one channel whereupon it is further directed along the at least one channel.

5. A method as in claims 1 to 4 wherein a further channel is provided adjacent to the opening of the through-hole into which the liquid is to be directed, such that when flowable material is deposited adjacent to the further channel, the material is directed into and along the further channel and is thus directed into the through-hole by the action of capillary forces.

6. A method as in claims 4 to 5 wherein the substrate is a flexible web.

7. A method as claimed in any preceding claim wherein the flowable material is deposited by a printing method.

8. A method as claimed in claims 1 to 6 wherein the flowable material is deposited by a coating method.

9. A method as claimed in any preceding claim wherein a surface energy contrast pattern is provided to both pattern flowable material on the surface where it is deposited and to direct flowable material to an opening of a through- hole.

10. A method as claimed in any preceding claim wherein the material as deposited has minimal flow and wherein energy is supplied to the material to make it more flowable.

11. A method as claimed in any preceding claim wherein channels are provided by embossing, stamping, moulding, spark erosion, etching or ablating.

12. A method as claimed in any preceding claim wherein through-holes are provided by laser drilling, stamping, etching, mechanical drilling, punching, spark erosion or water jets.

13. A method as in claim 4 in which there are at least two through-holes and wherein flowable material is deposited in a pattern on the second surface to materially connect the openings of the at least two through-holes.

14. An element having a patterning surface for patterning flowable material, the surface being provided with at least one channel and at least one through-hole with at least two openings .wherein one of the at least two openings is located in the surface adjacent to the at least one channel such that the relative positioning of said opening and said channel would result in flowable material emerging from said opening being directed along said channel and wherein the through-hole and openings are of such a size that flowable material deposited adjacent to another of the at least two openings would be directed into said opening and through the through-hole by the action of capillary forces.

15. An element as in claim 14 wherein the at least one channel has a concave profile.

16. An element as in claim 15 wherein the at least one channel and the at least one through-hole are located on the surface such that the intersection of the perimeter of the at least one through-hole with the surface forms a line which has at least in part a concave profile.

17. A patterning substrate for patterning flowable material having two surfaces and at least one through-hole connecting the two surfaces, one surface being provided with at least one channel wherein an opening of the through-hole is located in the surface adjacent to said channel such that the relative positioning of said opening and said channel would result in flowable material emerging from said opening being directed along said channel and wherein the through-hole and openings are of such a size that flowable material deposited adjacent to an opening on the other surface would be directed into said opening and through the through-hole by the action of capillary forces.

18. A patterning substrate as in claim 17 wherein the at least one channel is concave.

19. A patterning substrate as in claim 18 wherein the at least one channel and the at least one through-hole are located on the surface such that the intersection of the perimeter of the at least one through-hole with the surface forms a line which has at least in part a concave profile.

20. A device formed, at least in part, by the method according to claim 1