Patent Publication Number: US-2023149965-A1

Title: Method and an apparatus for applying thin film material onto a substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of and priority to EP Patent Application Serial No. 21209014.6, filed Nov. 18, 2021, the entire contents of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present inventive concept relates to a method and an apparatus for applying thin film material onto a substrate, whereby a thin film may be formed. 
     BACKGROUND 
     Thin films are being increasingly used in various application fields. In particular, there is an interest in ultra-thin films having a thickness in a nanometer scale, such as films being thinner than 100 nm. However, for thinner films, it is increasingly difficult to produce films having a homogeneous thickness. 
     Material for forming thin films may be deposited on a surface using deposition of small droplets in which the material to be deposited on the surface is dissolved. A solvent is allowed to evaporate to leave the deposited material on the surface. However, rate of evaporation may be higher at edges of a droplet such that a hydrodynamic fluid flow may be initiated within the droplet. Thus, a solute in the droplets are carried by the fluid flow such that irregular ring-like structures know as coffee rings are formed in the deposited thin film. 
     Thus, there is a need for an improved technique for applying thin film material onto substrates for enabling thin films with homogeneous thickness to be formed. 
     SUMMARY 
     An objective of the present inventive concept is to enable forming of thin films having homogeneous thickness. 
     This and other objectives of the present inventive concept are at least partly met by the invention as defined in the independent claims. Preferred embodiments are set out in the dependent claims. 
     According to a first aspect, there is provided a method for applying thin film material onto a substrate, said method comprising: forming microdroplets of a solvent and a solute material forming the thin film material; depositing the microdroplets on an upper surface of a micro-structured mesh, wherein the microdroplets are deposited to allow coalescing of microdroplets on the micro-structured mesh into droplets extending at least into the micro-structured mesh and possibly protruding from a bottom surface of the micro-structured mesh; and arranging a surface of the substrate in close relation to the bottom surface of the micro-structured mesh such that a capillary force draws liquid of the droplets extending at least into the micro-structured mesh and possibly protruding from the bottom surface of the micro-structured mesh onto the surface of the substrate, whereby forced dynamic wetting of the surface of the substrate is provided to form a liquid film on the surface of the substrate. 
     Thanks to the present inventive concept, droplets are deposited on a mesh and then allowed to coalesce to form larger droplets on the mesh. Then, the mesh is used for transferring the liquid to the surface of a substrate on which a thin film is to be formed. The solute material is arranged in the liquid film such that the thin film material is applied onto the surface of the substrate when the liquid film is formed. Thanks to using the mesh for transferring the liquid to the surface, a forced dynamic wetting of the surface can take place instead of separate droplets being formed on the surface. This implies that a uniform layer (having uniform thickness) of the liquid from the droplets can be formed on the surface. Having a uniform layer of the liquid on the surface, evaporation of the solvent may occur with a constant evaporative flux profile over the surface, such that a uniform thin film of the solute material can be formed on the surface of the substrate. 
     Thanks to the present inventive concept, the liquid film is formed on the surface of the substrate by being transferred by capillary force from the mesh to the surface. Microdroplets are not formed on the surface of the substrate, but rather a uniform thin film can be directly formed. It is realized that once a microdroplet is deposited on a substrate, it is very difficult to manipulate wetting behavior of the microdroplet. Hence, thanks to avoiding the liquid film to be formed through deposition of microdroplets on the surface, the wetting behavior of microdroplets on the surface is avoided. 
     There is no restriction on an area being used for depositing the microdroplets on the micro-structured mesh or for forced dynamic wetting of the surface of the substrate. Thus, the method can be used for applying a liquid film such that forming a uniform thin layer of the solute material may be achieved over a large area of the surface. 
     The applying of the thin film material onto the substrate does not involve vacuum deposition (as is often used in alternative methods for fabrication of thin films). Thus, the method provides a cost-efficient approach for applying thin film material onto the substrate. Further, the method consumes little power, which may be beneficial in ensuring that fabrication of thin films is carbon neutral. 
     The applying of the thin film material onto the substrate does not require any pre-treatment or post-treatment of the substrate. This implies that temperature-sensitive substrates, i.e. substrates formed by temperature-sensitive materials or carrying structures that are temperature-sensitive, may be used. Thus, the thin film material is applied without requiring the substrate to be exposed to an elevated temperature that could harm a temperature-sensitive substrate. 
     It should be realized that the micro-structured mesh is not used for transferring a pattern defined by the mesh onto the surface of the substrate. Rather, the mesh is used for capturing droplets, which may then be transferred onto the surface of the substrate for forming a uniform liquid film. The micro-structured mesh allows small droplets to be captured and formed on the mesh and also allows transfer of liquid onto the surface of the substrate throughout a large area of the mesh. 
     The microdroplets refer to droplets that are deposited on a micro-structured mesh, i.e. before the droplets reach the mesh. The microdroplets may have a size that is adapted to dimensions of features of the micro-structured mesh in order for a suitable formation of droplets on the micro-structured mesh. It should be realized that microdroplets have a size in microscopic scale, i.e. smaller than what can be seen by a naked eye. Thus, the microdroplets may have a size, i.e. diameter, in a range of a few μm to hundreds of μm, such as microdroplets being smaller than 500 μm, such as smaller than 200 μm. It should further be realized that the forming of microdroplets does not necessarily generate microdroplets that are all equally sized. Rather, there may be a distribution of sizes in the formed microdroplets. When discussing sizes of microdroplets below, an average size of the microdroplets is referred unless stated otherwise. 
     The micro-structured mesh refers to a mesh having dimensions in microscopic scale, i.e. smaller than what can be seen by a naked eye. The mesh may comprise threads arranged in a pattern for forming spacings between adjacent threads. Thus, a size of individual spacings in the mesh and a diameter of individual threads are features of the micro-structured mesh that have dimensions in the microscopic scale. 
     The micro-structured mesh may have dimensions that are adapted for capturing microdroplets impinging on the micro-structured mesh and also adapted for transferring the liquid of the droplets to the surface of the substrate for forming the uniform liquid film. Thus, size of the microdroplets and the dimensions of the micro-structured mesh may need to be related to each other. 
     Typically, the spacings of the micro-structured mesh should not be much larger than the size of the microdroplets in order to avoid that a large proportion of the microdroplets pass through the mesh unaffected. However, it should be realized that once microdroplets are deposited on the upper surface of the micro-structured mesh, the microdroplets formed on the micro-structured mesh will aid in capturing subsequent microdroplets impinging on the upper surface of the micro-structured mesh so as to prevent the microdroplets from passing through the spacings of the mesh. Thus, the spacings of the micro-structured mesh need not necessarily be smaller than the size of the microdroplets. 
     The threads of the micro-structured mesh may be arranged in two sets of parallel lines, wherein the sets are perpendicular to each other, such that square or rectangular spacings may be formed. A size of a spacing may be defined by a largest side of the spacing. The spacings may have a size in a range of a few μm to hundreds of μm or even as large as a few millimeters, such as spacings being smaller than 500 μm, such as smaller than 200 μm. 
     The threads may have a similar size (diameter or other cross-sectional size for non-circular cross-sections) to the spacings, such that a relatively large proportion of an area of the micro-structured mesh is defined by the threads. Thus, the threads may have a size in a range of a few μm to hundreds of μm, such as threads being smaller than 500 μm, such as smaller than 200 μm. 
     The depositing of the microdroplets on the upper surface of the micro-structured mesh may involve a flux of microdroplets being incident on the micro-structured mesh. For instance, microdroplets may be formed and deposited on the micro-structured mesh through spraying of microdroplets on the micro-structured mesh. Once microdroplets are deposited, the microdroplets may coalesce with further microdroplets being deposited on the micro-structured mesh so as to form larger droplets on the micro-structured mesh than are initially deposited on the micro-structured mesh. The microdroplets may form milli-droplets, i.e. droplets in a millimeter scale, on the micro-structured mesh. For instance, the droplets may coalesce towards droplets having a size (diameter and thickness) corresponding to a capillary length of the liquid of the droplets. These droplets may extend over a plurality of threads of the micro-structured mesh and may form a droop extending into the spacings allowing for the liquid to be further transferred to the surface of the substrate when the surface of the substrate is close to the bottom surface of the micro-structured mesh. 
     Formation and profile of the droplets on the upper surface of the micro-structured mesh may depend on spreading coefficient and a geometry of texture of the micro-structured mesh. Partial wetting of the upper surface of the micro-structured mesh may occur, which may imply that droplets are formed on the upper surface of the micro-structured mesh with a droop extending into spacings of the micro-structured mesh from the upper surface of the micro-structured mesh but not necessarily protruding from the micro-structured mesh (i.e. an extension of the droplet into the spacing is smaller than a diameter of the thread that may define a distance between the upper surface of the micro-structured mesh and the bottom surface of the micro-structured mesh). However, in partial wetting of the micro-structured mesh, the droplets may extend into the spacing to protrude from the bottom surface of the micro-structured mesh. If full wetting of the micro-structured mesh occurs, the droplets may coalesce to form droplets having a top half extending on an upper side of the micro-structured mesh and a bottom half extending on a lower side of the micro-structured mesh such that the droplets protrude from the bottom surface of the micro-structured mesh. 
     The method comprises arranging a surface of the substrate so close to the bottom surface of the micro-structured mesh such that liquid of the droplets is affected by a capillary force for wetting of the surface of the substrate. This may imply that the surface of the substrate is arranged closer to the droplets than a capillary length of the droplets. At such distance, a capillary force may be sufficient to draw the liquid of the droplets from the micro-structured mesh onto the surface of the substrate. However, it should be realized that a sufficient capillary force may still be present, even if the surface of the substrate is arranged farther away from the bottom surface of the micro-structured mesh (and from the droplets) than the capillary length. Thus, the arranging of the surface of the substrate in close relation to the bottom surface of the micro-structured mesh may imply that the surface of the substrate is arranged within a few mm, such as closer than 10 mm from the bottom surface of the micro-structured mesh. However, according to embodiments, the surface of the substrate is arranged closer than 1 mm or closer than 200 μm from the bottom surface of the micro-structured mesh. In fact, the surface of the substrate may even be arranged in contact with the bottom surface of the micro-structured mesh. However, if the surface of the substrate is arranged in contact with the bottom surface of the micro-structured mesh, the micro-structured mesh may need to be very clean so as to avoid that any particles are transferred from the mesh to the surface of the substrate. 
     For instance, if droplets protrude from the bottom surface of the micro-structured mesh, the surface of the substrate may even be arranged to touch the droplets. The capillary force may then further act to spread the liquid onto the surface of the substrate. However, it is not necessary for the substrate to be arranged so close to the micro-structured mesh that the surface of the substrate initially touches the droplets. Rather, the capillary force may act to draw the liquid from the droplets towards the surface of the substrate and to further spread the liquid onto the surface of the substrate. 
     Also, even if the droplets do not initially protrude from the bottom surface of the micro-structured mesh, a force may be applied on the micro-structured mesh so as to deform the micro-structured mesh at a location in which droplets have been formed, which may cause the spacings of the micro-structured mesh to increase in size such that the droplets may then protrude from the bottom surface of the micro-structured mesh. This may be used when the micro-structured mesh is flexible so as to allow deformation of the micro-structured mesh. Thus, the surface of the substrate may be arranged to touch the droplets, thanks to deformation of the micro-structured mesh allowing the droplets to protrude further from the bottom surface of the micro-structured mesh. However, it is still not necessary for the substrate to be arranged so close to the micro-structured mesh that the surface of the substrate initially touches the droplets, even if the micro-structured mesh is deformed to allow the droplets to further protrude from the bottom surface of the mesh. Rather, the capillary force may act to draw the liquid from the droplets towards the surface of the substrate and to further spread the liquid onto the surface of the substrate. 
     Arranging the surface of the substrate in close relation to the bottom surface of the substrate may involve acting on the substrate to move the surface of the substrate towards the micro-structured mesh, acting on the micro-structured mesh to move the micro-structured mesh towards the surface of the substrate, or both. This may imply moving the entire substrate and/or the entire mesh or moving a part of the substrate and/or the mesh. For instance, the substrate may be formed by a roll, which may be transported past a wetting position at which the surface is wetted. At the wetting position, the roll may be manipulated to follow a path bringing the roll close to the micro-structured mesh so as to arrange the surface of the substrate in close relation to the bottom surface of the micro-structured mesh. 
     The surface of the substrate may be arranged in close relation to the bottom surface of the substrate after the microdroplets have been deposited and allowed to coalesce to form larger droplets. This implies that wetting of the surface of the substrate will only take place after a plurality of droplets have been formed on the micro-structured mesh so that wetting may occur simultaneously from a plurality of microdroplets, which may facilitate forming of a uniform liquid film on the surface of the substrate. 
     Forced dynamic wetting implies that an external force is provided to influence flow of liquid so as to wet the surface of the substrate. This is in contrast to spontaneous wetting, where only capillary force drives the flow of liquid. The forced dynamic wetting may be provided thanks to the presence of the micro-structured mesh close to the surface of the substrate. 
     When the surface of the substrate is arranged close to the bottom surface of the micro-structured mesh, a plurality of individual droplets will simultaneously try to wet the surface. The plurality of droplets could form a liquid film with an abrupt increase in thickness when the droplets interfere constructively on the surface. However, thanks to the presence of the mesh formation of abrupt increases in thickness may be prevented, due to an applied force by the micro-structured mesh over the liquid film resembling a force exerted by nano-hairs of water striders on a liquid. The applied force may cause hydrostatic pressure inside the liquid to increase, which may then force liquid to flow in lateral dimensions so as to spread over the surface of the substrate. 
     According to an embodiment, the method further comprises, after the liquid film has been formed on the surface of the substrate, separating the micro-structured mesh from the substrate. 
     Thus, the substrate and the micro-structured mesh may be moved in relation to each other after the liquid film has been formed leaving the liquid film on the surface of the substrate without the liquid film being in contact with the micro-structured mesh. 
     Separating the micro-structured mesh from the surface of the substrate may involve acting on the substrate to move the surface of the substrate away from the micro-structured mesh, acting on the micro-structured mesh to move the micro-structured mesh away from the surface of the substrate, or both. This may imply moving the entire substrate and/or the entire mesh or moving a part of the substrate and/or the mesh. For instance, the substrate may be formed by a roll, which may be transported past a wetting position at which the surface is wetted. After passing the wetting position, the roll may be manipulated to follow a path separating the roll from the micro-structured mesh. 
     According to an embodiment, the method further comprises, after the liquid film has been formed on the surface of the substrate, allowing the solvent to evaporate to leave the solute material on the surface of the substrate for forming a thin film on the substrate. 
     A time required for evaporating the solvent is dependent on an evaporation rate of the solvent. The solvent may evaporate spontaneously, by e.g. being exposed to room temperature. Thus, the solvent may be allowed to evaporate spontaneously based on an ambient temperature during producing of the thin film on the substrate. 
     However, it should be realized that solvent may be allowed to evaporate based on an active step of causing evaporation. For instance, the surface of the substrate may be exposed to an increased temperature for causing the solvent to evaporate, such as using a mild increase in temperature. The temperature used may depend on the solvent. As an example, the temperature may be below 60° C. when water is used as the solvent. Even if the solvent has a high evaporation rate and may be relatively quickly evaporated through spontaneous evaporation, it may still be advantageous to apply an increase temperature for controlling evaporation and for reducing time for formation of the thin film on the substrate. 
     A time required for forming of droplets on the micro-structured mesh and transferring the droplets to the surface of the substrate may be substantially shorter than a time required for evaporation of the solvent, such that the solvent will not be evaporated before the liquid film has been formed on the surface of the substrate. 
     The solute material may be homogeneously distributed in the solvent. Thus, when the solvent is evaporated, the solute material is left on the surface of the substrate to form the thin film on the substrate. Evaporation of the solvent may occur with a constant evaporative flux profile over the surface, such that a uniform thin film of the solute material can be formed on the surface of the substrate. 
     Evaporation of solvent may occur such that the solvent leaves the substrate through the micro-structured mesh. This may further ensure that a constant evaporative flux profile is provided over the surface. 
     According to an embodiment, the thin film being formed on the substrate has a thickness smaller than 50 nm, such as smaller than 15 nm. 
     Thus, a very thin film with a uniform thickness may be formed on a large area of the surface of the substrate. It should be realized that the desired thickness of the thin film may depend on an application in which the thin film is to be formed. The method may very well be used for forming films of the solute material with a greater thickness than 50 nm, such as thin films having a thickness up to 100 nm or a thickness up to 1 μm. However, forming of large-size, uniform thin films of a very small thickness using technology that may be used in mass production is particularly difficult and for such reason it is particularly interesting to use the method for forming a thin film having a thickness smaller than 50 nm, such as smaller than 15 nm. 
     According to an embodiment, the method further comprises controlling a concentration of the solute material in the microdroplets being formed such that a thickness of the thin film being formed on the substrate is controlled. 
     The thickness of the thin film being formed may at least be dependent on the thickness of the liquid film being formed on the surface of the substrate and on the concentration of the solute material. Any of these parameters may be controlled for controlling the thickness of the thin film. The thickness of the liquid film may e.g. depend on an amount of microdroplets being deposited on the micro-structured mesh (which may in turn depend on flow rate of the deposited microdroplets and deposition time), and parameters relating to transfer of droplets from the micro-structured mesh to the surface of the substrate. 
     The controlling of the concentration of the solute material has a direct relation to the thickness of the thin film being formed on the substrate. Thus, the thickness of the thin film may be simply controlled by controlling the concentration of the solute material. 
     According to an embodiment, the microdroplets are deposited to allow coalescing of microdroplets on the micro-structured mesh into droplets protruding from the bottom surface of the micro-structured mesh and wherein arranging the surface of the substrate in close relation to the bottom surface of the micro-structured mesh comprises bringing the substrate in contact with the droplets protruding from the bottom surface of the micro-structured mesh. 
     By ensuring that the droplets are formed so as to protrude from the bottom surface of the micro-structured mesh, the surface of the substrate can easily be brought in close relation to the droplets so that the capillary force draws liquid of the droplets onto the surface of the substrate. Thus, there is no need to arrange the surface of the substrate in contact with the micro-structured mesh for drawing the liquid onto the surface of the substrate and thereby need of having a very clean mesh is avoided. Further, the surface of the substrate may be brought into contact with the droplets such that a strong capillary force may act on the liquid of the droplets for ensuring proper wetting of the surface of the substrate. 
     The microdroplets may be allowed to coalesce to droplets protruding from the bottom surface of the micro-structured mesh before the surface of the substrate is brought into contact with the droplets. Thus, wetting of the surface of the substrate is not initiated in some places (due to the droplets touching the surface) while droplets are still coalescing in other places. Such wetting could otherwise lead to non-uniform deposition of the surface of the substrate. 
     The droplets formed by coalescing of microdroplets may have different shapes depending e.g. on surface tension of the solvent and surface energy of the micro-structured mesh. If partial wetting of the micro-structured mesh occurs, droplets may be formed according to Cassie-Baxter model forming an air pocket between the mesh and the droplet at a center of the droplet. At edges of the droplet, the droplet may extend into the micro-structured mesh, and protrusion of the droplet into the micro-structured mesh may depend on thickness of threads of the micro-structured mesh and on a distance between adjacent threads. For such droplets, the droplets will typically not protrude from the bottom surface of the micro-structured mesh. If complete wetting of the micro-structured mesh occurs, droplets may be formed according to Wenzel model penetrating the micro-structured mesh to extend on both sides of the micro-structured mesh. 
     Dimensions and material used for the micro-structured mesh as well as material used for the solvent may be selected for controlling a type of droplets being formed when microdroplets coalesce. This may be used for allowing coalescing of microdroplets on the micro-structured mesh into droplets protruding from the bottom surface of the micro-structured mesh. 
     According to an embodiment, the micro-structured mesh is formed by interlaced threads defining spacings between the threads. 
     This is a suitable manner of forming the micro-structured mesh providing a structure of the mesh that is suitable for depositing of microdroplets that coalesce into droplets. 
     According to an embodiment, a distance between adjacent threads in the micro-structured mesh is smaller than five times a diameter of the formed microdroplets. 
     Thus, the micro-structured mesh used may be dimensioned in relation to the size of the microdroplets that will be formed. Similarly, the forming of the microdroplets may be controlled such that the size of the formed microdroplets is adapted to the dimensions of the micro-structured mesh. In this regard, it should be understood that there may be a distribution in the size of the formed droplets and that the distance between adjacent threads is related to an average diameter of the formed droplets. 
     Having a micro-structured mesh with spacings (distance between adjacent threads) being smaller than five times the diameter of the droplets may ensure that the microdroplets are deposited on the micro-structured mesh. Even if some microdroplets may initially pass through the spacings, droplets will quickly form on the micro-structured mesh to fill the spacings and thereby start to prevent microdroplets from passing through the micro-structured mesh. Hence, the spacings of the micro-structured mesh may be larger than the microdroplets without a large portion of the microdroplets passing through the micro-structured mesh. Having relatively large spacings may also be advantageous for allowing the liquid of the droplets to be drawn onto the surface of the substrate. 
     According to another embodiment, the distance between adjacent threads in the micro-structured mesh is smaller than three times the diameter of the formed microdroplets, such as equal to the diameter of the formed microdroplets. This may ensure that the very few microdroplets pass through the micro-structured mesh during depositing of the microdroplets on the micro-structured mesh. 
     The distance between adjacent threads may thus advantageously be related to the size of the formed microdroplets. According to an embodiment, the distance between adjacent threads may be smaller than 200 μm, such as smaller than 100 μm. 
     A size of a cross-section of the threads, such as a diameter of the threads for circular cross-sections may be similar to the distance between adjacent threads. According to an embodiment, the size of the cross-section of the threads may be smaller than 200 μm, such as smaller than 100 μm. The size of the threads also functions to facilitate forced dynamic wetting using threads that are sufficiently small so as not to break a surface of the liquid when a liquid film is formed on the surface of the substrate. Thus, in order to facilitate forced dynamic wetting, the size of the threads should not be too large. On the other hand, in order to ensure that the microdroplets are properly deposited on the upper surface of the micro-structured mesh, the size of the threads should not be too small. 
     According to an embodiment, the micro-structured mesh comprises a regular mesh pattern. 
     This may ensure that droplets having a spherical symmetry may be formed, which may ensure a good control of wetting of the surface of the substrate. Further, this may ensure that the surface of the substrate is wetted in a similar manner over an entire area of the surface. 
     According to an embodiment, a diameter (an average diameter) of the microdroplets is at least a factor 10 times smaller than a capillary length of the solvent. 
     Droplet size may affect wetting of the micro-structured mesh. Using small microdroplets allow the microdroplets to be deposited on the micro-structured mesh and also allows the microdroplets to coalesce into larger droplets that are appropriate for drawing liquid form the droplets to the surface of the substrate. 
     The diameter of the microdroplets may further be related to the dimensions of the micro-structured mesh, as discussed above. Thus, the diameter of the microdroplets may be larger than a fifth of the distance between adjacent threads of the micro-structured mesh. 
     According to an embodiment, the diameter of the microdroplets may be smaller than 200 μm, such as smaller than 100 μm. 
     According to an embodiment, arranging the surface of the substrate in close relation to the bottom surface of the micro-structured mesh comprises exerting a force on the micro-structured mesh for bringing the micro-structured mesh in close relation to the surface of the substrate. 
     This implies that the micro-structured mesh may be lowered towards the surface of the substrate when the liquid of the droplets of the micro-structured mesh is to be drawn onto the surface of the substrate. This may be a simple manner of bringing the micro-structured mesh in close relation to the surface of the substrate. For instance, the micro-structured mesh may be arranged in a frame, such that the entire frame may be lowered in order to bring the micro-structured mesh in close relation to the surface of the substrate. 
     According to an embodiment, the micro-structured mesh is flexible for bringing a portion of the micro-structured mesh in close relation to the surface of the substrate. 
     This implies that the micro-structured mesh may be deformed by exerting a force on the micro-structured mesh. A force may be exerted on the upper surface of the micro-structured mesh for pushing the portion of the micro-structured mesh towards a surface of the substrate, such that the portion of the micro-structured mesh is brought in close relation to the surface of the substrate. 
     When deforming the micro-structured mesh, adjacent threads in the portion of the micro-structured mesh may be pulled apart in order to allow the micro-structured mesh to deform. This implies that spacings in the micro-structured mesh may be enlarged so as to allow droplets to extend further into the micro-structured mesh from the upper surface or protrude further from the bottom surface of the micro-structured mesh. Hence, while the micro-structured mesh is brought in close relation to the surface of the substrate, the droplets are also pushed towards the surface of the substrate so as to promote liquid of the droplets being drawn by the capillary force to wet the surface of the substrate. 
     According to an embodiment, forming and depositing of microdroplets is made using ultrasonic spray coating. 
     Ultrasonic spray coating provides a technique for forming and depositing microdroplets with high accuracy, good resolution, and very high transfer rate. Thus, ultrasonic spray coating is suitable for forming and depositing of microdroplets. 
     Having a high transfer rate implies that material waste is very low, such that the use of ultrasonic spray coating may be environment-friendly. 
     According to a second aspect, there is provided an apparatus for applying thin film material onto a substrate, said apparatus comprising: a microdroplet generator for generating microdroplets of a solvent and a solute material forming the thin film material; a micro-structured mesh being arranged in relation to the microdroplet generator so as to receive microdroplets on an upper surface of the micro-structured mesh, wherein the micro-structured mesh is configured to promote microdroplets on the upper surface of the micro-structured mesh to coalesce into droplets extending at least into the micro-structured mesh and possibly protruding from a bottom surface of the micro-structured mesh; and a controller for controlling a relation of the substrate to the bottom surface of the micro-structured mesh such that a capillary force is formed to draw liquid of the droplets extending at least into the micro-structured mesh and possibly protruding from the bottom surface of the micro-structured mesh onto the surface of the substrate, whereby the controller is configured to control the relation of the substrate to the bottom surface of the micro-structured mesh such that forced dynamic wetting of the surface of the substrate is provided to form a liquid film on the surface of the substrate. 
     Effects and features of this second aspect are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect. 
     The apparatus enables applying thin film material onto a substrate in a well-controlled manner such that forming of a thin film having a uniform thickness on the surface of the substrate is facilitated. 
     As used herein, the terms “upper surface” and “bottom surface” of the micro-structured mesh should be construed as defining two opposite surfaces of the mesh. In use of the apparatus, the micro-structured mesh may be arranged in the apparatus such that the micro-structured mesh is arranged in relation to a gravity field with the gravity field acting from the upper surface towards the bottom surface. 
     The controller may be configured to control actuator(s) to act on the substrate to move the surface of the substrate towards the micro-structured mesh, act on the micro-structured mesh to move the micro-structured mesh towards the surface of the substrate, or both. 
     The apparatus may further comprise a heater, configured to provide thermal energy to the liquid film for promoting evaporation of the solvent after the liquid film has been formed on the surface of the substrate. 
     The heater may provide control of evaporation and may also reduce time for formation of the thin film on the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features, and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise. 
         FIG.  1    is a schematic view illustrating forming of microdroplets and depositing the microdroplets on a micro-structured mesh according to an embodiment. 
         FIGS.  2   a - b    are schematic views of a micro-structured mesh according to an embodiment. 
         FIGS.  3   a - c    are schematic views illustrating partial wetting of the micro-structured mesh. 
         FIGS.  4   a - c    are schematic views illustrating complete wetting of the micro-structured mesh. 
         FIG.  5    is a schematic view illustrating arranging a surface of a substrate in close relation to the micro-structured mesh according to an embodiment. 
         FIG.  6    is a schematic view illustrating liquid of a droplet being drawn by capillary force from the micro-structured mesh onto the surface of the substrate. 
         FIG.  7    is a schematic view illustrating evaporation of solvent from a liquid film formed on the surface of the substrate. 
         FIG.  8    is a flow chart of a method according to an embodiment. 
         FIG.  9    is a schematic view of an apparatus according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS.  1 - 7   , a method for applying thin film material onto a substrate  100  for forming a thin film on the substrate  100  will be described. 
       FIG.  1    illustrates forming of microdroplets  102  of a solvent and a solute material and depositing the microdroplets  102  on a micro-structured mesh  110 . The deposited microdroplets  102  coalesce on the micro-structured mesh  110  into larger droplets  104 . 
     The microdroplets  102  may be deposited by spray coating using a microdroplet generator  120 . For instance, the microdroplets may be deposited using ultrasonic spray coating (USSC), which may involve utilizing vibrations in ultrasonic range, such as 20-180 kHz, inside a nozzle to atomize small droplets that are output from the nozzle. A flow of a carrier gas, such as nitrogen, may be provided for forcing the microdroplets to impinge on an upper surface  112  of the micro-structured mesh  110  and be deposited thereon. 
     The ultrasonic vibrations may be produced by mechanical expansion and contraction of piezoelectric transducer(s) inside the nozzle. A frequency of the ultrasonic vibrations may control a size of the microdroplets  102 , such that higher frequencies may be used for generating smaller microdroplets  102 . 
     It should be realized that USSC is not necessarily used for forming and depositing of the microdroplets  102  on the micro-structured mesh  110 . According to alternatives, other spray coating techniques or other techniques of generating microdroplets and forcing the microdroplets onto the micro-structured mesh  110  may be used. For instance, pressurized spray coating may be used. 
     USSC is advantageous in that USSC provides a capability of forming homogeneous microdroplets  102  having a size smaller than 100 μm. Further, velocity of the microdroplets  102  towards the micro-structured mesh  110  may be easily controlled by controlling a pressure of the carrier gas. This may reduce singularity which may exist for other spray coating technologies, i.e. velocity and/or temperature-induced droplet interaction with the micro-structured mesh  110 . 
     The solvent may be any liquid that is suitable for carrying the solute material and for wetting of the micro-structured mesh  110  as well as a surface of the substrate  100 . The solvent to be used may be selected based on the solute material to be used, material used in the micro-structured mesh  110  and on the surface of the substrate  100  as well as on a surface tension of the liquid. For instance, water or isopropanol alcohol (IPA) may be used as a solvent, but it should be realized that other liquids may be used as solvent instead. 
     The solute material is to be used for forming a thin film on the surface of the substrate. Thus, the solute material may be selected based on the material desired for forming the thin film. The solute material may thus be dependent on an application of the substrate with the thin film arranged thereon. The thin film may be used to provide a functionality to a device in many different fields of application, such as for organic photo-voltaic devices, organic light-emitting diodes, micro-batteries, wear-resistant coatings, bio-medical devices, micro-electro-mechanical systems, piezoelectric films, etc. Hence, the solute material may be selected to provide the desired functionality in the fields of application in which it is to be used. 
     The solute material may typically be a polymer which is suitable for forming a thin film. For instance, the solute material may be poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), which may be used with water as solvent and may for instance be used in organic electronics providing low sheet resistance and optical transparency, or 80% ethoxylated polyethyleneimine (PEIE), which may be used with IPA as a solvent and may for instance be deposited on an indium tin oxide (ITO) electrode for providing low dark current and high detectivity of a photodetector. 
     The diameter of the microdroplets  102  formed by the microdroplet generator  120  may be smaller than a capillary length of the solvent, such as at least a factor 10 times smaller than the capillary length of the solvent. The size and shape of the microdroplets  102  when impinging on the micro-structured mesh  110  affect wetting of the micro-structured mesh  110 . For a small microdroplet  102 , where the size of the microdroplet  102  is smaller than the capillary length, an effect of surface tension is dominant over gravity such that spherical microdroplets  102  are formed. This may be particularly suitable for providing a well-controlled wetting of the micro-structured mesh  110 . 
     The capillary length l c  is given by: 
     
       
         
           
             
               
                 l 
                 c 
               
               = 
               
                 
                   
                     ρ 
                     ⁢ 
                     g 
                   
                   γ 
                 
               
             
             , 
           
         
       
     
     where ρ is density and γ is surface tension of the liquid solvent and g is gravitational acceleration. For instance, the capillary length of water is approximately 2.7 mm. 
     The diameter of the microdroplets  102  formed by the microdroplet generator  120  may for instance be smaller than 200 μm or smaller than 100 μm. 
       FIGS.  2   a - b    illustrate details of the micro-structured mesh  110  according to an embodiment.  FIG.  2   a    illustrates a pattern of the micro-structured mesh  110  and  FIG.  2   b    is an enlargement of details of the micro-structured mesh  110 . 
     The micro-structured mesh  110  may be formed by interlaced threads  114 . The threads  114  may be arranged in two sets of parallel lines, wherein the sets are perpendicular to each other. A distance between adjacent threads  114  may be equal between the two sets, such that square spacings  116  may be formed between the threads  114 . The micro-structured mesh  110  may form a regular mesh pattern, which is identical throughout an area of the micro-structured mesh  110 . 
     The threads  114  may be formed from a material that is lyophobic to the solvent, i.e. having little or no affinity to the liquid of the solvent. If water is used as the solvent, the threads  114  may thus be formed by a hydrophobic material. The use of a lyophobic or hydrophobic material of the threads  114  is advantageous in inducing forced dynamic wetting of the surface of the substrate  100  as will be described later. The use of a lyophobic or hydrophobic material of the threads  114  further controls wetting of the micro-structured mesh  110  and forming of coalesced droplets on the micro-structured mesh  110 . 
     According to an embodiment, the micro-structured mesh  110  may be formed by polyester threads  114 . 
     The mesh  110  is referred to as a micro-structured mesh  110  because the mesh  110  has features in micrometer scale. Cross-sectional dimension of the threads  114  and a distance between adjacent threads for defining spacings  116  may thus be in micrometer scale. 
     The spacings  116  may be dimensioned in relation to the diameter of the formed microdroplets  102  such that the microdroplets  102  will mainly be deposited on the micro-structured mesh  110  and will not pass through the spacings  116 . However, the spacings  116  may further be dimensioned to allow droplets that are formed on the upper surface  112  of the micro-structured mesh  110  to extend into the micro-structured mesh  110  and possibly protrude from a bottom surface  118  of the micro-structured mesh  110 . The spacings  116  should thus be sufficiently large to allow liquid of the droplets to be drawn through the micro-structured mesh  110  by capillary force, as will be explained in further detail below. 
     According to an embodiment, the distance between adjacent threads  114  may be smaller than five times, such as smaller than three times, a diameter of the formed microdroplets  102 . This may imply that initially some microdroplets  102  may pass through the micro-structured mesh  110 , but as droplets are starting to be deposited on the micro-structured mesh  110 , microdroplets  102  will be captured by the droplets  104  formed on the micro-structured mesh  110  such that microdroplets  102  will no longer be able to pass through the micro-structured mesh  110 . 
     However, it should be realized that the size of the spacings  116  may alternatively correspond to the diameter of the microdroplets  102  such that the microdroplets  102  cannot pass unaffected through the micro-structured mesh  110 . 
     The size of the cross-section of the threads  114  may be similar to the distance between adjacent threads  114 . This implies that a relatively large proportion of the upper surface  112  of the micro-structured mesh  110  is defined by the threads  114 , such that a likelihood that a microdroplet  102  will impinge on a thread  114  is large. This implies that there is a relatively low risk of microdroplets  102  passing through the micro-structured mesh  110  even if the microdroplets  102  have a smaller diameter than the distance between adjacent threads  114 . 
     For instance, the threads  114  may have a circular cross-section and a diameter of the threads  114  may be smaller than five times, such as smaller than three times, a diameter of the formed microdroplets  102 . 
     According to an embodiment, the distance between the threads  114  is smaller than 200 μm or smaller than 100 μm. Further, the diameter of the threads  114  may be smaller than 200 μm or smaller than 100 μm. 
     In a particular embodiment illustrated in  FIG.  2   b   , the diameter t of the threads may be 40 μm and the distance W between adjacent threads  114  may be 50 μm. 
     Referring now to  FIGS.  3   a - c  and  4   a - c   , coalescing of microdroplets  102  into larger droplets  104  on the micro-structured mesh  110  will be described in further detail. 
     Thousands of microdroplets  102  may be generated by the microdroplet generator  120  and may hit the micro-structured mesh  110  within the same time-frame. These microdroplets  102  will coalesce and form larger droplets  104 , which may be called milli-droplets below. The formation and profile of the milli-droplets  104  upon coalescence of microdroplets  102  depends on a spreading coefficient S and a geometry of texture of the upper surface  112  of the micro-structured mesh  110 . The spreading coefficient S is usually calculated to distinguish between different wetting states. In general, the spreading coefficient S represents surface energy difference between partial and complete wetting stages, i.e. at partial wetting, an equilibrium contact angle θ between the droplet  104  and the upper surface  114  would be between 90° and 180° and at complete wetting, the equilibrium contact angle θ would be 180°. 
     The spreading coefficient S may be expressed as: 
         S=γ   sv −(γ sl +γ iv ),
 
     based on thermodynamic equilibrium between three surface tensions: solid/vapor (γ sv ), solid/liquid (γ sl ), and liquid/vapor (γ iv ). 
     If the spreading coefficient is equal to or more than zero, then the droplet  104  completely wets the surface and if it is less than zero then the droplet  104  partially wets the surface. Based on the surface tension of the liquid and the surface energy of the material of the threads  114  of the micro-structured mesh  110 , S could be determined. 
     Below, an embodiment is discussed, wherein a micro-structured mesh  110  formed by threads  114  of low surface energy polyester (43 mN/m) is used. Further, two different liquids at opposite ends of surface tension spectrum may be used with the micro-structured mesh  110 . It should be realized that other materials with other properties may be used in the micro-structured mesh  110  and that other liquids may be used. 
     For water as a solvent (surface tension γ=72 mN/m), being a high surface tension liquid, partial wetting occurs. Therefore, there could be two situations, a) the deposited microdroplets  102  would coalesce into a sizable hemispherical milli-droplet  104 , b) the microdroplets  102  would be pushed towards the bottom surface  118  of the micro-structured mesh  110  by a force of the carrier gas carrying the microdroplets  102 . The latter would happen only if the diameter of the microdroplet  102  is smaller than the spacings  116  of the micro-structured mesh  110 . 
     In  FIG.  3   a   , a Cassie-Baxter model of partial wetting of a textured surface is illustrated. As shown, the droplet may be formed on top of the textured surface with air trapped underneath the droplet. Growth and profile of the droplet are dominated by the roughness of the surface texture. 
     In  FIG.  3   b   , a cross-section through a droplet  104  formed on the micro-structured mesh  110  is illustrated. When the droplets  104  are formed on the micro-structured mesh  110 , the Cassie-Baxter state of wetting may exhibit an air pocket at a center of the droplet  104 . At edges of the droplet  104 , the droplet may extend into the micro-structured mesh  110 . 
     In the case of  FIG.  3   b   , a relation of an apparent contact angle θ* is given by the Cassie-Baxter model, cos(θ*)=rφ s  cos(θ E )+φ s −1, where r is a roughness parameter of the micro-structured mesh, θ E  is an equilibrium contact angle between the liquid droplet  104  and a smooth flat substrate of the material of the threads  114 , 1−φ s  is a fraction of liquid/solid interface where the droplet  104  is in contact with the surface  112 . Since φ s ≤0, the apparent contact angle θ* is larger than or equal to 180° even for a surface whose θ E &lt;90°. 
     As further illustrated in  FIG.  3   c   , which is an enlargement of an edge of the droplet  104  in  FIG.  3   b   , the droplet  104  may protrude into the micro-structured mesh  110 . Protrusion of the droplet into the micro-structured mesh may depend on thickness of threads  114  of the micro-structured mesh  110  and on a distance between adjacent threads  114 . The droplet may form a meniscus around the thread  114 , having a droop from a center of the micro-structured mesh  110  between the upper surface  112  and the bottom surface  118 . At equilibrium, a maximum droop would be at a center of the spacing  116  of the mesh  110 , with a droop length of δ=(√{square root over (2)}P−D) 2 /8r d , where r d  is radius of the droplet  102 , D is a thickness of the threads  114 , and P is a pitch of the micro-structured mesh  110 . 
     Referring now to  FIGS.  4   a - c   , complete wetting will be discussed. For IPA as a solvent (surface tension γ=22 mN/m), being a low surface tension, wetting would be complete, i.e. the liquid will completely wet the threads  114  of the micro-structured mesh  110 . Any further deposition would result in increasing the thickness of the deposited droplet  104  on the thread  114  of the mesh  110 . The droplet  104  may then grow until the droplets  104  from two neighboring threads  114  reach a critical thickness (W/2, W being the distance between the neighboring threads). Then, coalescence would result in a continuous liquid film being formed between the neighboring threads  114  extending across a spacing  116 . Upon further deposition of the microdroplets  102 , the continuous thin film between the threads  114  would keep spreading over further neighboring threads  114  and simultaneously grows both in width and thickness. The milli-droplet  104  would acquire a hemispherical shape, with a top half being above the mesh  110  and a bottom half being below the mesh  110 . 
     In  FIG.  4   a   , a Wenzel model of complete wetting of a textured surface is illustrated. As shown, the droplet may be formed to extend into the textured surface. In  FIG.  4   b   , a cross-section through a droplet  104  formed on the micro-structured mesh  110  is illustrated. When the droplets  104  are formed on the micro-structured mesh  110 , the Wenzel state of wetting may exhibit the droplet  104  being partly formed above the micro-structured mesh  110  and partly below the micro-structured mesh  110 . 
     As further illustrated in  FIG.  4   c   , the droplet  104  may protrude from the bottom surface  118  of the micro-structured mesh  110 . A curvature of the droplet  104  is governed by the Laplace equation and the curvature is the same on top and bottom of the mesh  110 . The Laplace equation relates the pressure inside the droplet  104  to its curvature. Further increase in deposition of microdroplets  102  increases the width and thickness of the coalesced droplet  104  until a size of the droplet  104  reaches the capillary length. For the complete wetting, e.g. for the case of IPA wetting on the micro-structured mesh, the relationship between the apparent or macroscopic contact angle (θ*) to the equilibrium contact angle (θ E ) between the liquid droplet  104  and a smooth flat substrate formed by the material of the threads  114  is given by the Wenzel equation, cos(θ*)=rcos(θ E ), where r is the roughness parameter that describes the ratio between the actual area wetted by the liquid to the area projected onto a flat substrate. 
     As illustrated in the discussion above in relation to  FIGS.  3   a - 3   c  and  4   a - 4   c   , spreadability of the deposited liquid over the micro-structured mesh  110  depends i.a. on the surface tension of the solvent being used. 
     Referring now to  FIG.  5   , the surface  106  of the substrate  100  on which the thin film material is to be applied is arranged in close relation to the bottom surface  118  of the micro-structured mesh  110  on which the droplets  104  have been formed. 
     The arranging of the surface  106  of the substrate  100  in close relation to the bottom surface  118  of the micro-structured mesh  110  may thus take place after the microdroplets  102  have been deposited and coalesced to milli-droplets  104  on the upper surface  112  of the micro-structured mesh  110 . The arranging of the surface  106  of the substrate  100  in close relation to the bottom surface  118  of the micro-structured mesh  110  may involve acting on the micro-structured mesh  110  for moving the micro-structured mesh  110  or acting on the substrate  100  for moving the substrate  100  or both. Movement of the micro-structured mesh  110  and/or the substrate  100  may be performed in a direction perpendicular to the bottom surface  118  of the micro-structured mesh  110  and/or to the surface  106  of the substrate  100 . However, it should be realized that movement may alternatively be performed in a lateral direction parallel to the bottom surface  118  of the micro-structured mesh  110  and/or to the surface  106  of the substrate  100 . In such case, the micro-structured mesh  110  and/or the substrate  100  may be moved in a plane for aligning the bottom surface  118  of the micro-structured mesh  110  above the surface  106  of the substrate  100 . It should be further realized that a combination of a perpendicular and parallel movement may be performed. 
     Hence, according to an embodiment, a force may be exerted on the substrate  100  for bringing the surface  106  of the substrate  100  in close relation to the bottom surface  118  of the micro-structured mesh  110 . The force may be exerted on the substrate  100  for moving the entire substrate  100 , such as moving a carrier holding the substrate  100  for moving the substrate  100  with the carrier. However, in particular if the substrate  100  is flexible material, such as being defined by a foil or film that may form a web, the force may be exerted on the substrate  100  for bringing a portion of the substrate  100  in close relation to the bottom surface  118  of the micro-structured mesh  110 . Thus, the portion of the substrate  100  may form the surface  106  of the substrate  100  on which the thin film material is to be applied. If the substrate  100  is formed by a web, the web may be continuously, or intermittently, moved past a position at which thin film material is applied onto the surface  106 . Thus, as the web is moved, an entire surface  106  of the web may eventually be provided with a thin film. 
     The web may follow a path which may be defined e.g. by rollers. Thus, as the web is moved along its path, forces may act on the web so as to bring the web in close relation to the bottom surface  118  of the micro-structured mesh  110  at a position along the path of the web in which liquid of the droplets  104  is to be transferred from the micro-structured mesh  110  to the surface  106 . 
     According to another embodiment, as illustrated in  FIG.  5   , a force, as illustrated by arrow FA, may be exerted on the micro-structured mesh  110  for bringing the surface  106  of the substrate  100  in close relation to the bottom surface  118  of the micro-structured mesh  110 . The force may be exerted on the micro-structured mesh  110  for moving the entire micro-structured mesh  100 , such as moving a frame in which the micro-structured mesh  110  is arranged for moving the micro-structured mesh  110  with the frame. 
     However, the micro-structured mesh  110  may be flexible so as to allow deformation of the micro-structured mesh  110 , as illustrated in  FIG.  5   . In such case, the force may be exerted on the micro-structured mesh  110  for bringing a portion of the bottom surface  118  of the micro-structured mesh  110  in close relation to the surface  106  of the substrate  100 . Thus, the microdroplets  102  may be deposited on the portion of the bottom surface  118  of the micro-structured mesh  110  and then a force may be applied on the portion for bringing the portion of the micro-structured mesh  110  in close relation to the surface  106  of the substrate  100  for allowing the liquid of the droplets  104  to be transferred onto the surface  106  of the substrate  100 . 
     For instance, a substrate  100 , e.g. a substrate  100  provided in the form of a web, may be arranged to extend parallel to the micro-structured mesh  110  when the micro-structured mesh  110  is undeformed. Then, droplets  104  may be formed along a first strip, forming a first portion of the micro-structured mesh  110 , extending between two edges of the micro-structured mesh  110 . Once the droplets  104  are ready to be transferred onto the surface  106  of the substrate  100 , an actuator may exert a force on the first strip to bring the first strip towards the surface  106  of the substrate  100 . While the liquid in the droplets  104  on the first strip of the micro-structured mesh  110  is transferred onto the surface  106  of the substrate  100 , microdroplets  102  may be deposited at a second strip, forming a second portion of the micro-structured mesh  110 , adjacent to the first portion, so that the microdroplets  102  are coalesced into droplets  104  on the second portion. Then, when the liquid of the droplets  104  has been transferred to the surface  106  of the substrate  100 , the actuator may be moved in a direction, illustrated by arrow A in  FIG.  5   , or the micro-structured mesh  110  may be moved along the edges of the micro-structured mesh  110  so as to act on the second portion of the micro-structured mesh  110  and transfer liquid from the droplets  104  onto another portion of the surface  106  of the substrate  100 . 
     This may allow transferring of liquid onto a web that may be moved past a position in which a liquid is applied onto the web for eventually forming a thin film on the web. Thus, the process of applying a thin film onto a substrate may be made roll-to-roll compatible, which implies that a very high throughput of thin film formation may be provided. 
     The surface  106  of the substrate  100  may be brought in so close relation to the micro-structured mesh  110  that a capillary force may draw liquid from the droplets  104  onto the surface  106  of the substrate  100 . As described above, the microdroplets  102  may be deposited to allow coalescing of microdroplets  102  into droplets  104  protruding from the bottom surface  118  of the micro-structured mesh  110 . Thus, the surface  106  of the substrate  100  would come in contact with the droplets  104  before being in contact with the bottom surface  118  of the micro-structured mesh  110 . In such case, the surface  106  of the substrate  100  may be arranged in contact with the droplets  104  for allowing liquid of the droplets  104  to be drawn by a capillary force onto the surface  106  of the substrate  100 . 
     It should be realized that, when a force acts on a flexible micro-structured mesh  110 , the micro-structured mesh  110  may be deformed such that the size of spacings  116  may be increased. This may imply that a relation of the droplets  104  and the micro-structured mesh  110  may change so that the droplets  104  extend further into the micro-structured mesh  110  or protrude (or protrude further) from the bottom surface  118  of the micro-structured mesh  110 . This may facilitate the liquid of the droplets  104  being drawn by capillary force onto the surface  106  of the substrate  100 . 
     It should be realized that the surface  106  of the substrate  100  need not be in contact with the droplets  104  in order to allow a capillary force to draw liquid of the droplets  104  onto the surface  106  of the substrate  100 . However, the substrate  100  should be brought relatively close to the droplets  104  in order for the capillary force to be sufficiently strong to draw the liquid of the droplets  104  onto the surface  106  of the substrate  100 . 
     In an embodiment, particularly if the droplets  104  do not protrude from the bottom surface  118  of the micro-structured mesh  110 , the surface  106  of the substrate  100  may be brought in contact with the micro-structured mesh  100 . However, this may require the micro-structured mesh  110  to be very clean to avoid contamination in the thin film material being applied onto the surface  106  of the substrate  100 . Therefore, it may be beneficial to arrange the surface  106  of the substrate  100  so as not to be in contact with the micro-structured mesh  110 . Also, the capillary force that acts to draw the liquid of the droplets  104  onto the surface  106  of the substrate  100  may also pull the micro-structured mesh  110  towards the surface  106  of the substrate  100 . Thus, it may be beneficial to have a distance between the surface  106  of the substrate  100  and the bottom surface  118  of the micro-structured mesh  110  so that contact is not made during transfer of liquid from the droplets  104  to the surface  106  of the substrate  100 . 
     According to an embodiment, the surface  106  of the substrate  100  is arranged at a distance of at least 100 μm from the bottom surface  118  of the micro-structured mesh  110 . This may be used regardless whether the droplets  104  protrude from the bottom surface  118  of the micro-structured mesh  110  or not. The surface  106  of the substrate  100  may be arranged with a distance between the surface  106  of the substrate  100  and the bottom surface  118  of the micro-structured mesh  110  being smaller than the capillary length of the solvent. This may imply that the capillary force may be strong enough to draw liquid of the droplets  104  onto the surface  106  of the substrate  100 , even if the surface  106  of the substrate  100  is not initially in contact with the droplets  104 . 
     Regardless of the surface tension of the solvent being used, once the droplet  104  touches the surface  106  of the substrate  100 , the liquid will be drawn from the droplets  104  onto the surface  106  of the substrate  100  in a similar manner. 
     The capillary force arising between the substrate  100 , the liquid on the micro-structured mesh  110  and an air interface would result in the spreading of the liquid onto the surface  106  of the substrate  100  in order to reach a global minimum of the surface energy of the substrate  100 . This initiates a hydrodynamic flow of the droplet  104 . Once the liquid deposited on the micro-structured mesh  110  touches the surface  106  of the substrate  100 , the liquid will be drawn from the droplets  104  and would form a meniscus between the micro-structured mesh  110  and the substrate  100 , as illustrated in  FIG.  6   . 
     An attractive force is generated by the formed meniscus. The attractive force is mainly due to the existence of the surface tension of the liquid around the periphery of the meniscus and due to the reduced pressure inside the meniscus as compared to the capillary pressure. This attractive force then pulls the micro-structured mesh  110  towards the substrate  100  and drains the liquid on the upper surface  112  of the micro-structured mesh  110 . By assuming the height of the meniscus m h  to be smaller than the capillary length l c , the capillary force F is given by: 
     
       
         
           
             
               F 
               = 
               
                 2 
                 ⁢ 
                 π 
                 ⁢ 
                 γ 
                 ⁢ 
                 
                   R 
                   ⁡ 
                   ( 
                   
                     
                       2 
                       ⁢ 
                       c 
                     
                     - 
                     
                       d 
                       
                         m 
                         r 
                       
                     
                   
                   ) 
                 
               
             
             , 
           
         
       
     
     where γ is the surface tension of the liquid, R is a droop radius of the droplet  104 , β is a filling angle, d is a distance between the drooped liquid and substrate  100 , and m r  is a radius of the meniscus, with constant c being given by: 
     
       
         
           
             
               c 
               = 
               
                 
                   
                     cos 
                     ⁡ 
                     ( 
                     
                       β 
                       + 
                       
                         Φ 
                         1 
                       
                     
                     ) 
                   
                   + 
                   
                     cos 
                     ⁡ 
                     ( 
                     
                       Φ 
                       2 
                     
                     ) 
                   
                 
                 2 
               
             
             , 
           
         
       
     
     where Φ 1  and Φ 2  are the contact angle of the liquid meniscus with the drooped liquid and with the substrate  100 , respectively. 
     This force created would be then applied by the micro-structured mesh  110  on the liquid which influences the wetting behavior and hence forced dynamic wetting will happen. Also, as there would be many milli-droplets  104  formed, these individual milli-droplets  104  would simultaneously try to wet the surface  106  of the substrate  100 . Finally, without presence of the micro-structured mesh  110 , these multiple droplets  104  would form a liquid film on the surface  106  of the substrate  100  with an abrupt increase in thickness of the liquid film when the droplets  104  deposited on the micro-structured mesh  110  interfere constructively on the surface  106  of the substrate  100 . Fortunately, the presence of the micro-structured mesh  110  prevents the formation of abrupt thin liquid coatings, due to the applied force by the micro-structured mesh  110  over the liquid film, which resembles the force exerted by nano-hairs of water striders on a liquid. The hydrostatic pressure inside the liquid increases, which forces the liquid to flow in lateral dimensions. This induces dynamic wetting, which follows the hydrodynamic flow induced by the capillary force. The combination of capillary force and the external force applied by the micro-structured mesh  110  would assure that the liquid wets the surface  106  irrespective of the solvent and type of substrate  100  being used. 
     Referring now to  FIG.  7   , the liquid film  108  is shown on the surface  106  of the substrate  100 . 
     After the liquid film with uniform thickness has been formed on the surface  106  of the substrate  100 , the bottom surface  118  of the micro-structured mesh  110  may be separated from the surface  106  of the substrate  100 . Similar to the description above for bringing the surface  106  of the substrate  100  in close relation to the bottom surface  118  of the micro-structured mesh  110 , either the micro-structured mesh  110  or the substrate  100  or both may be moved for bringing the surface  106  of the substrate  100  away from the bottom surface  118  of the micro-structured mesh  110 . Again, this may imply that the entire micro-structured mesh  110  and/or the entire substrate  100  is moved or that a portion of the micro-structured mesh  110  and/or a portion of the surface  106  of the substrate  100  is moved. 
     It should be realized that separating the micro-structured mesh  110  from the substrate  100  does not necessarily imply that the micro-structured mesh  110  and the substrate  100  have been in contact with each other, but rather that a distance between the micro-structured mesh  110  and the substrate  100  is increased. 
     When the micro-structured mesh  110  has been separated from the substrate  100 , a liquid film  108  is left on the surface  106  of the substrate  100 . The liquid film  108  may have a uniform or at least almost uniform thickness and may extend over a large area or an entire area of the surface  106  of the substrate  100 . 
     The solute material is arranged within the solvent which forms the liquid film  108 . After the liquid film  108  has been applied to the surface  106  of the substrate  100 , the solvent may be allowed to evaporate leaving the solute material on the surface  106  of the substrate  100  so as to form a very thin film of the solute material on the substrate  100 . 
     Since the liquid film  108  has a uniform or almost uniform thickness, evaporation of the solvent may occur with a constant or almost constant evaporative flux profile (illustrated by arrows B in  FIG.  7   ) over the area of the surface  106 . Thanks to the constant evaporative flux profile, there will not be any substantial flow laterally within the liquid film  108  during evaporation, such that the solute material may form a thin film having a uniform or almost uniform thickness and formation of coffee rings in the thin film is avoided. 
     The solvent of the liquid film  108  may be allowed to evaporate spontaneously based on an ambient temperature in a manufacturing environment during producing of the thin film on the substrate  100 . However, in order to increase control of evaporation, the substrate  100  may be exposed to an increased temperature, which may promote evaporation of the solvent of the liquid film  108 . 
     The substrate  100  may be generally exposed to an increased temperature, e.g. by bringing the substrate  100  into a space (oven) with an increased temperature (above room temperature). However, the substrate  100  may alternatively be exposed to a local increase in temperature by providing local heating to the liquid film  108  on the surface  106  of the substrate  100 . 
     Evaporation of the solvent may also or alternatively be promoted by controlling humidity of ambience and/or controlling an air flow above the liquid film. 
     The evaporation may be controlled for reducing a time required for fully evaporating the solvent. However, it may be desired that the solvent of the liquid film  108  changes from liquid form to gaseous form through evaporation, without any boiling occurring. Boiling could cause movements within the liquid that may affect the thin film of the solute material that is eventually formed. Hence, the control of evaporation may involve exposing the substrate  100  to an increased temperature, while ensuring that the temperature of the liquid is not close to a boiling point. 
     Control of evaporation may be different, e.g. using different parameters, dependent on the solvent used. For instance, the ambient temperature may be different depending on the solvent used to promote evaporation while ensuring that the temperature of the solvent does not reach the boiling point. 
     As mentioned, after evaporation of the solvent, the solute material may form a thin film having a uniform or almost uniform thickness. A very thin film of the solute material may be formed, and thickness of the thin film may be dependent on requirements of the field of application in which the thin film on the substrate  100  is to be used. Thus, in embodiments, the thickness of the thin film may be smaller than 100 nm, such as smaller than 50 nm, such as smaller than 15 nm. It is noted that the method allows applying of thin film material to the surface  106  of the substrate  100  for forming exceptionally thin films with uniform thickness, such as thin films having a thickness smaller than 15 nm. 
     The thickness of the thin film may be controlled by controlling different parameters for applying the thin film material onto the surface  106  of the substrate  100 . However, when control of the thickness of the thin film is desired, controlling concentration of the solute material in the microdroplets  102  offers a suitable and easy manner of controlling the thickness of the thin film. 
     The controlling of the concentration of the solute material has a direct relation to the thickness of the thin film being formed on the substrate  100 . Thus, the thickness of the thin film may be simply controlled by controlling the concentration of the solute material in the microdroplets formed by the microdroplet generator  120 . 
     Referring now to  FIG.  8   , the method for applying thin film material onto a substrate and eventually forming a thin film on the substrate according to an embodiment is summarized. 
     The method comprises forming  202  microdroplets  102  of a solvent and a solute material, wherein the solute material is the thin film material. 
     The method further comprises depositing  204  the microdroplets  102  on the upper surface  112  of the micro-structured mesh  110 . The microdroplets  102  are then allowed to coalesce on the micro-structured mesh  110  into droplets  104  extending at least into the micro-structured mesh  110  and possibly protruding from the bottom surface  118  of the micro-structured mesh  110 . 
     The method further comprises arranging  206  the surface  106  of the substrate  100  in close relation to the bottom surface  118  of the micro-structured mesh  110 . This implies that a capillary force will draw liquid of the droplets  104  from the micro-structured mesh  110  onto the surface  106  of the substrate  100 . Further, the micro-structured mesh  110  provides a force on the liquid and forced dynamic wetting of the surface  106  of the substrate  100  is provided to form a liquid film  108  on the surface  106  of the substrate  100 . 
     The liquid film  108  may be formed with a uniform thickness on the surface  106  of the substrate  100 . The method may further comprise controlling  208  evaporation of the liquid of the solvent in the liquid film  108  such that the thin film material is left on the surface  106  of the substrate  100  to form a thin film of uniform thickness on the substrate  100 . 
     Referring now to  FIG.  9   , an apparatus  300  for applying thin film material onto the substrate  100  according to an embodiment will be described. 
     The apparatus  300  comprises a microdroplet generator  120  for generating microdroplets  102  of a solvent and a solute material forming the thin film material. The microdroplet generator  120  may be configured to generate and output a directed spray of microdroplets  102 , for instance using USSC, but it should be realized that the microdroplet generator  120  may generate microdroplets  102  in different manners. 
     The apparatus  300  further comprises the micro-structured mesh  110 . The micro-structured mesh  110  is arranged in the apparatus  300  in relation to the microdroplet generator  120  so as to receive microdroplets  102  on the upper surface  112  of the micro-structured mesh  110 . Thus, the microdroplet generator  120  and the micro-structured mesh  110  may be arranged such that the directed spray output by the microdroplet generator  120  impinges on the micro-structured mesh  110 . 
     The micro-structured mesh  110  may thus be arranged in the apparatus  300  such that the microdroplets  102  generated by the microdroplet generator  120  are deposited on the upper surface  112  of the micro-structured mesh  110 . 
     The micro-structured mesh  110  is configured as explained above such that the microdroplets  102  deposited on the upper surface  112  of the micro-structured mesh  110  coalesce into larger droplets  104 . The droplets  104  extend at least into the micro-structured mesh  110  and possibly the droplets  104  protrude from the bottom surface  118  of the micro-structured mesh  110 . 
     The apparatus  300  further comprises a controller  304  for controlling a relation of the substrate  100  to the bottom surface  118  of the micro-structured mesh  110 . The control of the relation of the substrate  100  to the bottom surface  118  of the micro-structured mesh  110  may be provided in many different manners, as discussed above, by controlling the substrate  100  and/or the micro-structured mesh  110 , or by controlling a portion of the substrate  100  and/or a portion of the micro-structured mesh  110 . 
     In the embodiment illustrated in  FIG.  9   , the substrate  100  is provided in form of a web that may be continuously transported past a position for applying thin film material onto the upper surface  106  of the substrate  100 . A direction of movement of the web of the substrate  100  is indicated by arrow C in  FIG.  9   . Further, the micro-structured mesh  110  is also provided in form of a web that may be transported continuously along a path above the substrate  100  in the position for applying thin film material onto the upper surface  106  of the substrate  100 . A direction of movement of the web of the micro-structured mesh  110  is indicated by arrow D in  FIG.  9   . 
     The apparatus  300  comprises a roller  304  in the path of the micro-structured mesh  110 , such that the micro-structured mesh  110  when passing the roller  304  is brought towards the substrate  100 . The microdroplet generator  120  is arranged in relation to the web of the micro-structured mesh  110  such that the microdroplets  102  are deposited on the micro-structured mesh  110  before it reaches the roller  304 . The microdroplets  102  are also allowed to coalesce into larger droplets  104  before the micro-structured mesh  110  reaches the roller  304 . 
     When the micro-structured mesh  110  reaches the roller  304 , the micro-structured mesh  110  is brought in close relation to the surface  106  of the substrate  100  such that a capillary force is formed to draw liquid of the droplets  104  from the micro-structured mesh  110  onto the surface  106  of the substrate  100  and the micro-structured mesh  110  is controlled in relation to the substrate  100  such that forced dynamic wetting of the surface  106  of the substrate  100  is provided to form a liquid film  108  on the surface  106  of the substrate  100 . 
     The micro-structured mesh  110  is configured to follow a path after passing the roller  304  such that the micro-structured mesh  110  is brought away from the substrate  100 , leaving the liquid film  108  on the substrate  100 . 
     As illustrated in  FIG.  9   , the substrate  100  is configured to pass the position for applying thin film material onto the upper surface  106  of the substrate  100  and after passing the position, a continuous liquid film  108  is provided on the upper surface  106  of the substrate  100 . 
     The substrate  100  may further be configured to be transported in the apparatus  300  into a position for promoting evaporation of the solvent of the liquid film  108 . The apparatus  300  may thus comprise a space which is controlled to provide an elevated ambient temperature. The substrate  100  may be transported through this space so as to allow controlled evaporation of the solvent leaving the solute material on the surface  106  of the substrate  100  forming a thin film thereon. 
     To corroborate flexibility of the method and apparatus described above in formation of ultra-thin coatings, PEDOT:PSS solved in water; PEIE solved in IPA, and a polymer-metal oxide composite of Zinc oxide (ZnO) nanoparticles with a diameter of 10-15 nm mixed with PEIE in IPA as a solvent were deposited on an ITO substrate with a size of 2.5×2.5 cm 2 . The surface energy of the ITO is calculated by using Fowke&#39;s theory, and it was found to be 117.37 mN/m. 
     The deposition condition for all three cases are given in table 1. The distance between the substrate  100  and a nozzle for USSC is kept constant for all set of experiments at 6 cm and the room temperature is kept constant for all set of experiments at 22° C. A nitrogen pressure for carrying the microdroplets  102  towards the micro-structured mesh  110  was kept constant for all set of experiments at 1.5 kPa. 
     A micro-structured mesh  110  based on polyester threads  114  was used, with a dimension of thickness of the threads (t)=50 μm and the distance between neighboring threads (w)=50 μm. The micro-structured mesh  110  possessed an equilibrium water contact angle of 104.6 degrees. 
     Upon the deposition of microdroplets  102  on the micro-structured mesh  110 , the micro-structured mesh  100  was made to touch the substrate  100  so that capillary force would draw the liquid from the upper surface  112  of the micro-structured mesh  110  and would induce forced dynamic wetting. It should be noted that the micro-structured mesh  110 , which is held by capillary force, should not be in contact with the substrate  100  for a long time, such as a minute. Otherwise the deposited material could be deposited only at positions of intersections of threads  114  in the micro-structured mesh  110 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Deposition parameters for various materials and solvents used 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Flow 
                 Path 
               
               
                 Solute 
                 Substrate 
                 Solution 
                 Number 
                 rate 
                 speed 
               
               
                 material 
                 temp. ° C. 
                 concentration 
                 of passes 
                 ml/min 
                 mm/sec 
               
               
                   
               
               
                 PEDOT:PSS 
                 60 
                 1 ml 
                 1 
                 0.5 
                 10 
               
               
                   
                   
                 PEDOT:PSS 
               
               
                   
                   
                 in 15 ml water 
               
               
                 PEIE 
                 30 
                 0.01 ml in 30 
                 1 
                 0.8 
                 10 
               
               
                   
                   
                 ml IPA 
               
               
                 ZnO + PEIE 
                 30 
                 2 ml ZnO + 
                 1 
                 0.9 
                 10 
               
               
                   
                   
                 0.01 ml PEIE + 
               
               
                   
                   
                 50 ml IPA 
               
               
                   
               
            
           
         
       
     
     Flow rate relates to a flow of droplets being injected into the transducer of the USSC nozzle, where the droplets are atomized into microdroplets. 
     Path speed relates to speed of the USSC nozzle for moving the nozzle and scanning the nozzle in two directions so as to cover an entire surface of the substrate on which the thin film is to be formed. 
     Numbers of passes relates to a number of times the nozzle is passed over the surface. 
     Uniformity of the deposited ultra-thin films over the large area of the ITO substrate was investigated by optical microscopy. It was seen that irrespective of the materials and solvents being used, the deposited ultra-thin films show a uniform deposition without any coffee rings. 
     To determine the thickness of the different materials, tapping mode atomic force microscopy (AFM) was used. To measure the thickness, all materials were deposited on ITO-coated glass slides and the concentration of the solution was varied while keeping all other parameters constant. The desired ultra-low thickness of the material of interest was achieved by varying the solution concentration and the flow rate of the solution. PEDOT:PSS being diluted in water evaporated less during the flight from the nozzle to the micro-structured mesh  110  and hence the flow rate was less compared to IPA based materials (see Table 1). Also to evaporate the deposited liquid film  108  for PEDOT:PSS, the substrate temperature was kept at 60° C., which is also more than the substrate temperature used for IPA-based materials, which was 30° C. Excess of solution flow rate resulted in an edge effect, where more material was accumulated. Therefore, the flow rate was optimized according to the evaporative properties of the solvent being used. 
     As shown in Table 2, the minimum thickness achieved for PEDOT:PSS, PEIE and ZnO+PEIE is 14.51±0.627 nm, 5.95±0.199 nm, and 14.16±0.63 nm, respectively. The surface roughness (Ra) of the PEDOT:PSS, PEIE and ZnO+PEIE composite thin films deposited on ITO substrate is found to be around 857.2 pm, 245.3 pm, and 2.6 nm respectively. This clearly shows that the method and apparatus enable achieving ultra-low thickness of thin films with very low surface roughness of the deposited thin film. The method and apparatus allow forming of such thin films using an up-scalable fabrication regime. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Minimum thickness achieved for different 
               
               
                 materials over ITO substrate 
               
            
           
           
               
               
               
               
            
               
                   
                 PEDOT:PSS 
                 PEIE 
                 ZnO + PEIE 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Minimum 
                 14.51 ± 
                 5.95 ± 
                 14.16 ± 
               
               
                   
                 thickness 
                 0.627 nm 
                 0.199 nm 
                 0.63 nm 
               
               
                   
                 Surface 
                 857.2 pm 
                 245.3 pm 
                 2.6 nm 
               
               
                   
                 roughness (Ra) 
               
               
                   
                   
               
            
           
         
       
     
     To further investigate the effect of solution concentration on the thickness of the deposited thin films, a comprehensive investigation was done for all three materials with varying concentration. The results are shown in Table 3. The obtained results show that by varying the concentration of the solvent, the thickness of the deposited thin film could be varied. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Thickness variation for different material for varying concentration 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 0.01 ml 
                 0.025 ml 
                 0.05 ml 
                 0.07 ml 
                 0.1 ml 
               
               
                   
                 PEIE/35 
                 PEIE/35 
                 PEIE/35 
                 PEIE/35 
                 PEIE/35 
               
               
                 Material 
                 ml IPA 
                 ml IPA 
                 ml IPA 
                 ml IPA 
                 ml IPA 
               
               
                   
               
               
                 PEIE 
                 5.95 nm 
                 10.08 nm 
                 20.103 nm 
                 25.6 nm 
                 38.23 nm 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 1 ml 
                 1 ml 
                 1 ml 
                 1 ml 
                 1 ml 
               
               
                   
                 Material/ 
                 Material/ 
                 Material/ 
                 Material/ 
                 Material/ 
               
               
                   
                 30 ml 
                 200 ml 
                 15 ml 
                 10 ml 
                 5 ml 
               
               
                 Material 
                 water 
                 water 
                 water 
                 water 
                 water 
               
               
                   
               
               
                 PEDOT:PSS 
                 14.51 nm 
                 22.67 nm 
                 31.9 nm 
                 38.22 nm 
                 48.9 nm 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 2 ml 
                 2 ml 
                 2 ml 
                 2 ml 
                 2 ml 
               
               
                   
                 ZnO + 
                 ZnO + 
                 ZnO + 
                 ZnO + 
                 ZnO + 
               
               
                   
                 0.01 ml 
                 0.01 ml 
                 0.01 ml 
                 0.01 ml 
                 0.01 ml 
               
               
                   
                 PEIE/30 
                 PEIE/20 
                 PEIE/15 
                 PEIE/10 
                 PEIE/5 
               
               
                 Material 
                 ml IPA 
                 ml IPA 
                 ml IPA 
                 ml IPA 
                 ml IPA 
               
               
                   
               
               
                 ZnO + PEIE 
                 14.16 nm 
                 21.07 nm 
                 33.77 nm 
                 42.53 nm 
                 57.7 nm 
               
               
                   
               
            
           
         
       
     
     However, the previous results have been demonstrated on a substrate like ITO having high surface energy. The method and apparatus described above also provides versatility so that it may be used on substrate having low surface energy. Therefore, to investigate the flexibility of the method and apparatus, a low surface energy, temperature-sensitive super yellow light-emitting poly(1,4-phenylenevinylene) (PPV) copolymer is used as a substrate. The surface energy of this substrate is measured using Fowke&#39;s theory, and it was found to be around 56 mN/m. 
     The materials and the deposition condition used in the previous studies on ITO is being duplicated on the super yellow (SY) substrates. Atomic force measurements were performed to measure the surface morphology of the deposited PEDOT:PSS, PEIE and ZnO+PEIE composite. The surface roughness of the deposited films was 766.9 pm, 798.3 pm and 2.102 nm respectively for PEDOT:PSS, PEIE and ZnO-PEIE composite, which is almost similar to the results obtained for the films deposited on the ITO substrate. SEM image obtained on the same set of samples also confirmed that there are no presence of coffee rings and the film is uniform throughout, even though the SY substrate was of low surface energy and a soft substrate. These extraordinary results confirm independence of the technology of the method and the apparatus to the nature of the substrate. 
     In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.