WO2023177289A1

WO2023177289A1 - Method and apparatus for producing high aspect ratio surface structures

Info

Publication number: WO2023177289A1
Application number: PCT/NL2023/050128
Authority: WO
Inventors: Rebecca SAIVE; Mathis Alexandre Nathan VAN DE VOORDE; Janis Aivars ANDERSONS
Original assignee: Universiteit Twente
Priority date: 2022-03-14
Filing date: 2023-03-14
Publication date: 2023-09-21

Abstract

The invention relates to an apparatus and a method for producing a high aspect ratio surface structures on a surface of a substrate. The method comprises the steps of: providing a transfer member, wherein the transfer member extends in a first direction and has a width substantially perpendicular to the first direction, providing a layer of a substance onto the transfer member, wherein the substance and/or the transfer member is/are configured so that there is an adhesion between them, at least partially transferring the substance from the transfer member onto the surface of the substrate by: moving the transfer member and/or the substrate so that the transfer member and the substrate move towards each other until at least the substance contacts the surface of the substrate, and moving the transfer member and/or the substrate so that the transfer member and the substrate move away from each other.

Description

Method and apparatus for producing high aspect ratio surface structures

BACKGROUND

The invention relates to a method and apparatus for producing high aspect ratio surface structures . An example of such surface structures is a front contact, in particular for solar cells . Front contacts are an important topic for the photovoltaic industry as their ability to conduct generated charge carriers while simultaneously accepting photons has a maj or influence on the performance of solar cells . Furthermore, metallization is a signi ficant consumer of raw material ( i . e . silver) . Currently, the most prevalent front contact fabrication technique is screen printing .

Alternative surface structures encompass , for example, surface structures made from electrically conductive materials for use as sensors , antennas , thermoelectric structures , etc . , surface structures made from semiconducting material , surface structures made from electrically non-conductive materials for use as photonic structures , etc . , or a combination thereof .

SUMMARY OF THE INVENTION

A disadvantage of the use of screen printing for front contact fabrication is that screen printing, while being highly scalable, produces thick and low aspect ratio ( i . e . height to width ratio ) contacts . In particular for use as front contacts for solar cells , the thick and low aspect ratio contacts result in a decreased cell performance due to optical shading . In addition, screen printing for front contact fabrication focuses on high throughput, but not necessarily on the best performances , nor in the reduction of raw material consumption .

It is an obj ect of the present invention to provide a new or optimized surface structure manufacturing method and apparatus which improves at least partially at least one of performance , throughput and reduces materials consumption, at least when compared with screen printing .

According to a first aspect , the present invention provides a method for producing high aspect ratio surface structures on a surface of a substrate, wherein the method comprises the steps of : providing a transfer member, wherein the transfer member extends in a first direction and has a width in a second direction, wherein the second direction is substantially perpendicular to the first direction, and wherein said width is in a range from 5 - 500 micrometers , providing a layer of a liquid substance or paste onto the transfer member, wherein the liquid substance or paste and/or the transfer member is/are configured to comprise a tendency so that the liquid substance or paste clings to the trans fer member, at least partially transferring the liquid substance or paste from the transfer member onto the surface of the substrate by : moving the trans fer member and/or the substrate so that the trans fer member and the surface of the substrate move towards each other at least until the liquid substance or paste contacts the surface of the substrate , and moving the trans fer member and/or the substrate so that the trans fer member and the surface of the substrate move away from each other .

The method of the invention thus deposits an elongated structure or track of the liquid substance or paste onto the surface of the substrate , wherein the elongated structure or track extends over the surface of the substrate in a direction in which the first direction is oriented when the liquid substance or paste contacts the surface of the substrate . The elongated structure or track has a width substantially equal to or larger than the width of the trans fer member .

As the liquid substance or paste is configured to comprise a tendency to cling to the trans fer member, the moving apart of the trans fer member and the surface of the substrate pulls an amount of the fluid substance or paste away from the surface of the substrate to provide a narrow bridge of the fluid substance or paste between the surface of the substrate and the trans fer member . As the trans fer member and the surface of the substate move further apart from each other, this bridge becomes narrower and narrower, until it breaks and the trans fer member is free from the substrate . This mechanism provides an elongated structure or track with an upward tapered cross-section and a relatively high aspect ratio ( i . e . height to width ratio ) .

Due to the upward tapered cross-section of the elongated structures or tracks as produced by the method of the invention, the optical shading is reduced which yields an increase performance and throughput , in particular when using electrically conductive tracks of the invention as front contacts for solar cells .

When using screen printing, a mesh is used to trans fer the liquid substance or paste onto the substrate , except in areas of the mesh made impermeable to the liquid substance or paste , for example using a blocking stencil . Accordingly, in screen printing the liquid substance or paste is also provided in areas of the screen where no structures or tracks need to be trans ferred onto the substrate . In the method of the invention the liquid substance or paste is only provided onto the trans fer member, and thereby only to the parts of the substrate where a structure or track needs to be provided . Accordingly, the amount of liquid substance or paste needed for the fabrication of structures or tracks on a substrate , can be reduced considerably .

Although the transfer member may also comprises a thin blade, for example a thin knife blade, it is preferred that the trans fer member, in an embodiment, comprises a thread, wherein the first direction is along a longitudinal direction of the thread and the second direction is perpendicular to the longitudinal direction of the thread . A thread is a convenient way to provide a very thin template for manufacturing narrow, elongated, substantially straight structures or tracks , which for example can be used as front contacts for solar cells .

It is noted that, where the transfer member comprises a thin blade , the edge of the blade facing the substrate holder has a width in the range from 5 - 500 micrometers .

Although the thread may be made by any suitable material , for example using glass fibers or polymer fibers as threads , preferably a metal thread is used in the method of the present invention, because thin metal threads are readily available and they provide a robust template . Preferably, the thread comprises a monofilament comprising Carbon, a Tungsten alloy or a Titanium alloy .

In an embodiment , the thread has a diameter in a range from 25 - 100 micrometers . In the alternative, where the transfer member comprises a thin blade, the edge of the blade facing the substrate holder has a width in the range from 25 - 100 micrometers . Thinner threads or blades are preferred, because the experiments of the inventors suggest that thinner threads allow to produce structures or tracks on the substrate with a higher aspect ratio . However, the threads or blades are preferably also robust enough to allow multiple depositions of liquid substance or paste onto a substrate, and preferably onto multiple substrates . In an embodiment, before moving the transfer member towards the surface of the substrate , the method further comprises the step of arranging the tread and/or the surface of the substrate with respect to each other so that the longitudinal direction of the thread is substantially parallel to the surface of the substrate . By arranging the longitudinal direction of the thread parallel to the surface of the substrate, a distance between the thread and the surface of the substrate is substantially constant along the length of the thread . This allows to produce an elongated structure or track with a more constant shape and/or conductivity along the length of the elongated structure or track .

In an embodiment, before moving the transfer member towards the surface of the substrate , the method further comprises the step of providing an amount of the liquid substance or past onto the thread and subsequently pulling the thread through an orifice , wherein a part of the thread that has been pulled through the orifice is used for the at least partially trans ferring the liquid substance or paste onto the surface of the substrate . The orifice or opening comprises a smallest diameter which is a predetermined amount larger than the diameter of the thread . When the thread with an amount of the liquid substance or paste is pulled through the orifice , an excess of the liquid substance or paste is removed from the thread, leaving a coating with a cross-section substantially equal to the cross-section of the orifice or opening at its smallest diameter . This step allows to repeatedly load the thread with a well-defined coating of the liquid substance or paste for producing homogeneous and even surface structures , preferably time and time again .

In an embodiment, the paste comprises a suspension of granular material in a background fluid, in particular a liquid or background liquid . After the paste has been trans ferred onto the surface of the substrate , the background fluid evaporates and the granular material substantially retains the structure with the high aspect ratio .

In an embodiment, the granular material comprises a granular electrically conductive material , which allows to produce electrically conductive elongated structures or tracks , for example for front contacts for solar cells , but also for sensors or antennas .

In an embodiment, the granular electrically conductive material comprises silver, which provides a high electrical conductivity .

In an embodiment , the paste comprises a solid content of 50 weight % of the granular material or more, preferably a solid content of 60 weight % of the granular material or more . Experiments of the inventors suggest that a paste with a higher solid content allows to produce structures or tracks on the substrate with a higher aspect ratio .

It is noted that a paste also has a relative high viscosity . Experiments of the inventor suggest that a more homogeneously fabricated structure or track, as well as higher aspect ratio can be obtained when using a more viscous paste .

In an embodiment, the method further comprises the step of solidifying the liquid substance or paste after the liquid or paste has been at least partially transferred from the trans fer member onto the surface of the substrate .

In an embodiment , the step of solidi fying comprises a heating of the substrate for sintering the liquid substance or paste , or for evaporating a background fluid of the paste .

In an embodiment, wherein the liquid substance or paste comprises photopolymer or light-activatable resin, wherein the step of solidifying comprises an illumination of the structures or tracks by light, preferably ultraviolet or visible light . This illumination initiates a hardening of the photopolymer or light-activated resin, and thereby the solidi fication of the structures or tracks on the substrate .

In an embodiment, wherein the liquid substance or paste comprises an electrically conductive or magnetic substance, the method further comprises the step of applying a voltage difference between the trans fer member and the substrate or between the transfer member and the substrate holder . The voltage dif ference provides an electric field between the trans fer member and the substrate or the substrate holder which can be used to assist the formation of a desired shape of the structure and/or for initiating the solidi fication of the structures .

According to a second aspect, the invention provides an apparatus for producing a high aspect ratio surface structures on a surface of a substrate, wherein the apparatus comprises : a substrate holder for holding the substrate, a transfer member, wherein the transfer member extends in a first direction and has a width in a second direction, wherein the second direction is substantially perpendicular to the first direction, and wherein said width is in a range from 5 - 500 micrometers , and a transfer member holder for holding the transfer member, wherein the substrate holder and/or the trans fer member holder are configured for moving the trans fer member and/or the substrate with respect to each other so that the transfer member and the surface of the substrate are moveable towards each other and away from each other .

The apparatus of the invention allows to utilize the method of the invention for producing high aspect ratio surface structures or tracks on a surface of a substrate, with an upward tapered cross-section of the elongated structures or tracks as described above . Accordingly, the optical shading can be reduced, which yields an increase performance and throughput , in particular when using electrically conductive tracks of the invention as front contacts for solar cells . In addition, the amount of liquid substance or paste needed for the fabrication of the surface structures or tracks on a substrate , can be reduced considerably, at least when compared with screen printing .

In an embodiment , the transfer member holder and the substrate holder are configured to arrange the transfer member and/or the surface of the substrate with respect to each other so that the first direction, which is a longitudinal direction of the transfer member, is substantially parallel to the surface of the substrate that in use is facing the trans fer member . By arranging the longitudinal direction of the transfer member parallel to the surface of the substrate, a distance between the transfer member and the surface of the substrate is substantially constant along the length of the trans fer member . This allows to produce a surface structure or track with a more constant shape and/or conductivity along the length of the surface structure or track .

In an embodiment, the apparatus further comprises an application device, wherein the application device is configured for providing a layer of a liquid substance or paste onto the trans fer member, in particular on a surface of the transfer member which, in use, is facing the substrate . In an embodiment , the application device and/or the transfer member holder are configured for moving the transfer member and/or the application device with respect to each other so that the application device and the transfer member are moveable towards each other and away from each other . Such an application device allows to provide a desired distribution of liquid substance or paste along the longitudinal direction of the transfer member .

When the previous embodiment is used for producing high aspect ratio front contacts , the liquid substance or paste comprises a electrically conductive liquid substance or paste . In positions where an electrical contact pad is desired, the amount of electrically conductive liquid substance or paste on the transfer member may be increased to obtain a locally broadening of the front contact , which can make it easier to connect electrical wiring to this broadened part of the front contact .

In an embodiment, the application device is configured for providing a substantially even layer of the paste onto at least a part of the surface of the transfer member which in use is facing the substrate, preferably wherein the paste comprises a suspension of granular material in a background fluid or liquid, in particular with substantially the same amount of the paste substantially everywhere on the surface of the trans fer member which in use is facing the substrate .

In an embodiment, the granular material comprises an electrically conductive granular material , preferably the electrically conductive granular material comprises silver, which provides a high electrical conductivity . This material is highly suitable for the production of high aspect ratio front contacts , in particular for solar cells .

In an embodiment, the application device is configured for providing an even layer of a paste which comprises a solid content of 50 weight % of the granular material or more , preferably a solid content of 60 weight % of the granular material or more .

Although the trans fer member may also comprise a thin blade, for example a thin knife blade, it is preferred that the trans fer member, in an embodiment, comprises a thread, preferably a metal thread, wherein the first direction is along a longitudinal direction of the thread and the second direction is perpendicular to the longitudinal direction of the thread .

In an embodiment, the thread has a diameter in a range from 25 - 100 micrometers . In the alternative, where the transfer member comprises a thin blade, the edge of the blade facing the substrate holder has a width in the range from 25 - 100 micrometers . Thinner threads or blades are preferred, because the experiments of the inventor suggests that thinner threads allow to produce structures or tracks on the substrate with a higher aspect ratio .

In an embodiment , the apparatus comprises : a transfer position where , in use, at least part of a liquid substance or past is transferred onto the substrate, an ori fice, wherein the orifice is arranged between the application device and the transfer position, wherein the thread is configured to pass through the orifice, and a thread conveyor device configured for moving the thread from the application device, through the ori fice, to the transfer position . The orifice or opening comprises a smallest diameter which is a predetermined amount larger than the diameter of the thread . When the thread with an amount of the liquid substance or paste is pulled through the ori fice, an excess of the liquid substance or paste is removed from the thread, leaving a coating with a cross-section substantially equal to the cross-section of the orifice or opening at its smallest diameter . The use of an orifice, for example in the form of a through opening in a die, for pulling the thread therethrough allows to repeatedly load the thread with a well-defined coating of the liquid substance or paste for producing homogeneous and even surface structures , preferably time and time again .

In an embodiment, the apparatus further comprises a tensioning member configured for applying a tension to the thread, preferably wherein the tensioning member is configured for setting the tension in the thread to a desired level . The inventors have found that a low tension in the thread might cause an uneven withdrawal of the thread and thus uneven surface structures . To solve this problem, the apparatus preferably comprises a tensioning member for setting the tension in the thread to a desired level . Preferably, the tension in the thread is set as high as possible . However, care should be taken to not over- tension the thread which may cause necking or fracture . Preferably, the tension in the thread is under the yield strength of the thread material .

In an embodiment, the apparatus comprises a tension sensing member configured for sensing the tension in the thread and for providing an indication of a magnitude of the tension in the thread and/or a signal corresponding to the magnitude of the tension in the thread . The tension sensing member allows to measure the tension in the thread and, in combination with the tensioning member, to set the tension accurately to a predetermined or desired level , for producing homogeneous and even surface structures , preferably time and time again .

In an embodiment, the apparatus further comprises a heating element which is configured for heating the structures or tracks on the surface of the substrate . By heating the structures or tracks , the liquid substance of paste is sintered or the background fluid of the paste is evaporated, in order to solidi fy the structures or track on the surface of the substrate . In an embodiment , the heating element is arranged to be part of the substrate holder, and is configured for heating the substrate .

In an embodiment, the apparatus further comprises a light source which is configured for illuminating the structures or tracks on the surface of the substrate . This embodiment is particularly advantageous in combination with a liquid substance or paste that comprises photopolymer or light-activatable resin . An illumination of structures or tracks on the substrate initiates a hardening of the photopolymer or light-activated resin, and thereby provides the solidi fication of the structures or tracks on the substrate . In an embodiment the light source is configured for emitting ultraviolet or visible light .

In an embodiment, the apparatus further comprises a voltage supply which is configured for applying a voltage dif ference between the trans fer member and the substrate or between the transfer member and the substrate holder . The voltage di fference provides an electric field between the transfer member and the substrate or the substrate holder which can be used to assist the formation of a desired shape of the structure and/or for initiating the solidi fication of the structures , in particular when the liquid substance or paste comprises an electrically conductive or magnetic substance .

According to a third aspect , the present invention provides a computer-readable medium, preferably a non-transitory computer-readable medium, having computerexecutable instructions adapted to cause an apparatus , or an embodiment thereof , as described above under the second aspect of the invention to perform a method, or an embodiment thereof , for producing a high aspect ratio front contacts on a surface of a substrate as described above under the first aspect of the invention .

The various aspects and features described and shown in the specification can be applied, individually, wherever possible . These individual aspects , in particular the aspects and features described in the attached dependent claims , can be made subj ect of divisional patent applications .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be elucidated on the basis of an exemplary embodiment shown in the attached drawings , in which :

Figure 1 schematically shows a first example of an apparatus of the invention;

Figures 2A - 2F schematically show several steps of a method of the invention;

Figure 3 shows a schematic cross-section of a high aspect ratio front contact made by using a platinum thread with a diameter of 50 micrometer and 60 weight % Silver paste ;

Figure 4 schematically shows a second example of an apparatus of the invention using a thin blade as transfer member;

Figure 5 schematically shows a third example of an apparatus of the invention comprising additional features such as a heating element, light source and/or voltage supply; and

Figure 6 schematically shows a fourth example of an apparatus according to the invention comprising a string printing device .

DETAILED DESCRIPTION OF THE INVENTION

The below described examples are predominantly directed to the production of high aspect ratio surface structures for use as front contact for solar cells . However, it will be apparent for a person skilled in the art that the same method and apparatus can also be used for the production of high aspect ratio surface structures for other applications . Examples are, surface structures made from electrically conductive materials for use as sensors , antennas , thermoelectric structures , etc . , surface structures from semiconducting materials , or surface structures made from electrically non-conductive or dielectric materials for use as photonic structures , etc . , or a combination thereof .

Recent work by one of the inventors , R . Saive, on effectively transparent contacts (ETCs ) have shown excellent performance metallization with transparencies up to 99% due to a triangular geometry and high aspect ratio of the contacts . However, the fabrication of these contacts is more complicated than traditional screen-printing, making an entry into the solar cell mass production market dif ficult .

The present invention provides a fabrication process to tackle performance , throughput and materials consumption simultaneously using a novel method called string-printing . In this process , silver paste is coated on ultra-thin ( 25-100 pm) metal threads . The paste is then transferred onto a silicon substrate or solar cell by approach and withdrawal of the coated thread to and from the substrate, and the released silver on the surface of the silicon substrate or solar cell then forms the contact .

The method of the invention promises to be able to compete with the throughput of screen-printing, while also providing performances similar to ETCs , by the fabrication or high aspect ratio triangular contacts on Si substrates using string-printing .

For the deposition of silver paste on the silicon substrates , the present invention provides an apparatus as schematically shown in figure 1 . The apparatus 1 comprises a substrate holder 2 for holding a substrate 3 , a transfer member 4 , and a transfer member holder 5 for holding the transfer member 4 .

As schematically shown in figure 1 , the trans fer member 4 comprises a thread 41 which extends in a first direction X and has a width in a second direction Y, wherein the second direction Y is perpendicular to the first direction X, and wherein said width or diameter of the thread 41 is in a range from 10 - 500 micrometers , preferably in a range from 25 - 100 micrometers . The thread 41 is mounted such that an amount of tension is provided on the thread 41 in order to ensure that the thread 41 is straight . In the example of figure 1 the tension is provided by connecting a first end 42 of the thread 41 to an actuator 51 which is configured for moving the first end 41 of the thread 41 along the first direction X, whereas a second end 42 of the thread 41 is connected to a spring 52 .

The transfer member holder 5 is arranged on a first linear actuator 6 which is configured for moving the thread 41 up and down along the z-direction . The substrate holder 2 is mounted on top of a second linear actuator 7 which is configured for moving the substate holder 2 with the substrate 3 along the y-direction . Accordingly, the apparatus 1 is configured for moving the transfer member 4 and/or the substrate 3 with respect to each other so that the transfer member 4 and the surface of the substrate 3 are moveable towards each other and away from each other at various positions along the y-directions on the surface of the substrate 3 .

It is noted that the first example shown in figure 1 is a prototype . Any modifications for making an apparatus for producing high aspect ratio front contacts on full si ze solar cells , such as a larger substrate holder or a modified trans fer member holder for holding a longer transfer member, are apparent for a person skilled in the art .

Figures 2A - 2F schematically show several steps of a method of the invention . It is noted that figures 2A - 2 F schematically show a cross-section in the plane YZ along the line I I- I I of figure 1 .

First, as shown in figure 2A, the thread 41 is arranged above the surface 31 of the substrate 3 , preferably above the position on the surface 31 of the substrate 3 where the high aspect ratio front contact needs to be provided .

Subsequently, as shown in figure 2B, a layer of an electrically conductive substance 8 is provided onto the surface of the thread 41 which is facing the surface 31 of the substrate 3 . Preferably, the layer of substance 8 is provided by means of an application device (not shown) comprising a noz zle or a syringe needle . During the application of the substance 8 , a droplet of said substance 8 is formed at the end of the nozzle or the syringe needle, and said droplet is carefully run along the side of the thread 41 facing the surface 31 of the substrate 3 . The material of the thread 41 and/or the material of the electrically conductive substance 8 are selected such that there is an adhesion between them . Accordingly, the electrically conductive substance 8 is configured to comprise a tendency to cling to the thread 41 .

Once the electrically conductive substance 8 is applied, the thread is lowered along the z-direction as schematically, shown in figure 2C, at least until the electrically conductive substance 8 contacts the surface 31 of the substrate 3 , as shown in figure 2D .

Subsequently, as shown in figure 2E, the thread 41 is moved away from the surface 31 of the substrate 3 . Due to the adhesion between the thread 41 and the electrically conductive substance 8 , the thread 41 pulls an amount of the electrically conductive substance 8 away from the surface 31 of the substrate 3 to provide a narrow bridge of electrically conductive substance 8 between the surface 31 of the substrate 3 and the thread 41 . As the thread 41 and the surface 31 of the substate 3 move further apart from each other, this bridge becomes narrower and narrower, until it breaks and the thread 41 is free from the substrate 3 , as shown in figure 2 F . accordingly, at least a part of the electrically conductive substance 8 is trans ferred from the thread 41 onto the surface 31 of the substrate 3 .

Once the process is completed, the substrate 3 is placed on a hot plate at 120 ° C to evaporate the remaining solvent of the electrically conductive substance 8 on the substrate 3 .

This mechanism provides an electrically conductive track 81 with an upward tapered cross-section and a relatively high aspect ratio ; i . e . height h to width w ratio , as schematically shown in figure 3 .

In first experiments , two commercially available low viscosity silver inks ( 10 cP and 100 cP at 1000 s- 1 ) from Novacentrix® with dif ferent solid contents ( 40 weighti and 60 weighti , respectively) as electrically conductive substances or pastes , have been tested with the previously explained procedure . The printing was performed with the apparatus shown in Figure 1 . Dif ferent diameters of threads have also been used to print the contacts . The dif ferent threads used had diameters of 50 micrometers and 60 micrometers . Both inks have an aqueous-based vehicle , so the di fference in evaporation rates is mainly caused by the dif ference in solid content .

For the low solid content paste using a thread of 60 micrometers , a very thin film is printed . The relative large amount of fluid in this paste , seems to prevent the formation of a structure in height, leading to a flattened structure .

When using the higher solid content paste, on the other hand, a small bump is formed . The lesser amount of fluid in the paste and a quicker evaporation thereof , seems to lead to the formation of a structured contact during the withdrawal of the thread .

Subsequently, the experiments were continued with a platinum thread with a thread diameter of 50 pm and using the higher solid content paste . This setup allowed to produce front contacts with an aspect ratio h/w approximately equal to 1 .

With a more viscous paste and/or smaller diameter threads , it may be expected that a more homogeneously printed line, as well as higher aspect ratio structures can be produces .

Figure 4 schematically shows a second example of an apparatus 1 ' for producing high aspect ratio surface structures on a surface of a substrate . Instead of using a thread 41 as shown in figure 1 , this alternative apparatus 1 ' comprises a thin blade 4 ' as transfer member . The thin blade 4 ' is connected to the trans fer member holder 5 via a set of springs 51 ' , 52 ' . The edge of the blade 4 ' facing the substrate holder 2 has a width in the range from 25 - 100 micrometers . This alternative apparatus 1 ' provides substantially the same results as the apparatus 1 as shown in figure 1 and as described in more detail above .

Figure 5 schematically shows a third example of an apparatus 1" for producing high aspect ratio surface structures on a surface of a substrate . In particular figure 5 shows some additional features such as a heating element, a light source and/or a voltage supply, which can be provided to the apparatus 1" individually or in combination .

In particular, the apparatus 1" may further comprise a heating element 21 which is configured for heating the structures or tracks on the surface of the substrate 3 . The heating element 21 is arranged to be part of the substrate holder 2 , and is configured for heating the substrate . The heating element 21 is connected to a heating controller 22 , which is configured to activate the heating element 21 ones the desired surface structures or tracks have been provided onto the surface of the substrate 3 . By heating the substrate, and thereby the structures or tracks , the liquid substance of paste is sintered or the background fluid of the paste is evaporated, in order to solidi fy the structures or tracks on the surface of the substrate .

In addition or alternatively, the apparatus 1" may further comprise a light source 23 which is configured for illuminating the structures or tracks on the surface of the substrate 3 . The light source 23 is connected to a illumination controller 24 , which is configured to activate the light source 23 ones the desired surface structures or tracks have been provided onto the surface of the substrate 3 . This exemplary embodiment is particularly advantageous in combination with a liquid substance or paste that comprises photopolymer or light-activated resin . An illumination of structures or tracks on the substrate 3 by means of the light source 23 initiates a hardening of the photopolymer or light-activated resin, and thereby provides the solidi fication of the structures or tracks on the substrate 3 . Preferably the light source 23 is configured for emitting ultraviolet and/or visible light .

In addition or alternatively, the apparatus 1" may further comprises a voltage supply 25 which is configured for applying a voltage di fference between the transfer member 4 and the substrate 3 or between the transfer member 4 and the substrate holder 2 . The voltage dif ference provides an electric field between the transfer member 4 and the substrate 3 or the substrate holder 2 which can be used to assist the formation of a desired shape of the structure and/or for initiating the solidi fication of the structures , in particular when the liquid substance or paste comprises an electrically conductive or magnetic substance .

Figure 6 shows a schematic cross-section view of a fourth example of an apparatus 81 according to the invention comprising a string printing device . The apparatus 81 comprises a substrate holder 82 for holding the substrate 83 at a transfer position 94 where , in use , at least part of a liquid substance or past is trans ferred from the thread 84 onto the substrate 83 . The substrate holder 82 is configured for moving the substrate 83 towards or away from the thread 84 in a first direction, for depositing at least part of the liquid substance or paste from the thread 84 onto the substrate 83 .

It is noted that the substrate holder 82 may also be provided with a further translation device (not shown) for moving the substrate 83 in a second direction perpendicular to the first direction and perpendicular to the length direction of the thread 84 , in order to print several surface structures next to each other on the substrate 83 .

In addition or alternatively, the substrate holder 82 may be provided with an angle adj ustment member (not shown) for moving the substrate holder to ensure that the surface of the substrate 83 is parallel to the thread 84 .

As schematically shown in figure 6, the apparatus 81 comprises a thread conveyor device comprising a first drum 88 and a second drum 91 . The thread 84 extends from the first drum 88 to a first wheel 89, through an orifice 86, a cleaning member 87 , a second wheel 90 to the second drum 91 . The first drum 88 and second drum 91 are rotationally fixed coupled, for example using a gear train, and can be driven to rotate synchronously and unidirectionally . Accordingly, there is a fixed length of thread 84 between the first drum 88 and the second drum 91 in the apparatus 81 .

In this example the second wheel 90 is provided with a tensioning member 92 for applying a tension to the thread 84 , for example by pulling the second wheel 90 with a predetermined force in the direction of the arrow .

Furthermore , the first wheel 89 is provided with a tension sensing member, comprising a load cell 93 for measuring the force applied to the first wheel 89 .

In a further elaboration, the load cell 93 is configured to send a signal to a controller, which signal comprises information about the measured tension in the thread 84 , and wherein the controller is coupled to the tensioning member 92 for controlling the tensioning member 92 to provide a desired level of tension in the thread 84 . This allows to provide a control loop in order to establish and keep a desired level of tension in the thread 84 .

The apparatus 81 further comprises an application device 85, which is configured for providing a layer of a liquid substance or paste onto the thread 84 . Downstream of the application device 85, an orifice 86 is arranged . The ori fice 86 or opening comprises a smallest diameter which is a predetermined amount larger than the diameter of the thread 84 . When the thread 84 with an amount of the liquid substance or paste from the application device 85 is pulled through the ori fice 86 by driving the first and second drums , an excess of the liquid substance or paste is removed from the thread 84 , leaving a coating with a crosssection substantially equal to the cross-section 86' of the orifice 86 or opening at its smallest diameter . Thus the part of the thread 84 that has been pulled through the ori fice 86 is provided with a well-defined coating of the liquid substance or paste .

Subsequently, the coated thread 84 is moved to the trans fer position 94 adj acent to the substrate 83 by driving the first and second drums . When the coated thread 84 substantially extends over the transfer position 94 , the driving of the first and second drums is stopped and at least a part of the liquid substance or paste is transferred from the thread 84 onto the surface of the substrate 83 to provide an elongated surface structure by : moving the substrate 83 towards the thread 84 until at least the liquid substance or paste contacts the surface of the substrate 83 , and moving the substrate 83 away from the thread 84 . After the deposition of the elongated surface structure on the surface of the substrate 83 , the first and second drums are driven to move the part of the thread 84 in the transfer position 94 towards the second drum 91 in order to provide a freshly coated thread part in the transfer position 94 for providing a subsequent elongated surface structure onto the surface of the substrate 83 . Preferably, the substrate 83 has been moved in the second direction such that the subsequent elongated surface structure is arranged ad acent and parallel to the previous provided elongated surface structure .

The part of the thread 84 which is moved out of the transfer position 94 is moved through the cleaning member 87 to further remove liquid substance or paste from the thread 84 , and ultimately, the cleaned thread 94 is would up at the second drum 91 . Once all of the thread 84 has been trans ferred from the first drum 88 to the second drum 91 as a result of driving the first and second drum in rotation, the thread conveyor device can run in reverse to return the thread 84 back to the first drum 88 .

The above described apparatus 81 is a convenient device to provide multiple narrow, elongated, substantially straight structures or tracks on a substrate 83 , in particular for the manufacturing of front contacts for solar cells .

It is noted that in stead of using a first drum 88 for storing a long section of thread 84 and moving the long section of thread 84 stepwise to the second drum 91 , as described above , the string conveyor may also be provided with a closed loop of thread, where the same length of thread is constantly being repurposed as it moved through the loop .

It is further noted that the above example is described with having a thread 84 . However the principle of the thread conveyor and ori fice can be utilized with multiple threads and orifices arranged adj acent to each other in the second direction to provide a plane parallel threads in the transfer position over the surface of the substrate , in order to trans fer multiple narrow, elongated, substantially straight structures or tracks in one transferring step onto the surface of the substrate .

Accordingly, the apparatus and method for producing high aspect ratio surface structures on a surface of a substrate according to the present invention is scalable to be competitive with existing metalli zation processes , at least in view of the throughput .

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention . From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the scope of the present invention .

In summary, the invention relates to an apparatus and a method for producing a high aspect ratio surface structure on a surface of a substrate . The method comprises the steps of : providing a transfer member, wherein the transfer member extends in a first direction and has a width substantially perpendicular to the first direction, providing a layer of a substance onto the transfer member, wherein the substance and/or the transfer member is/are configured so that there is an adhesion between them, at least partially transferring the substance from the transfer member onto the surface of the substrate by : moving the trans fer member and/or the substrate so that the transfer member and the substrate move towards each other until at least the substance contacts the surface of the substrate, and moving the transfer member and/or the substrate so that the transfer member and the substrate move away from each other .

Claims

C L A I M S

1 . A method for producing a high aspect ratio surface structures on a surface of a substrate , wherein the method comprises the steps of : providing a trans fer member, wherein the transfer member extends in a first direction and has a width in a second direction, wherein the second direction is substantially perpendicular to the first direction, and wherein said width is in a range from 5 - 500 micrometers , providing a layer of a liquid substance or paste onto the transfer member, wherein the liquid substance or paste and/or the transfer member is/are configured to comprise a tendency so that the liquid substance or paste clings to the transfer member, at least partially transferring the liquid substance or paste from the transfer member onto the surface of the substrate by : moving the transfer member and/or the substrate so that the transfer member and the surface of the substrate move towards each other until at least the liquid substance or paste contacts the surface of the substrate , and moving the transfer member and/or the substrate so that the trans fer member and the surface of the substrate move away from each other .

2 . The method according to claim 1 , wherein the transfer member comprises a thread, preferably a metal thread, wherein the first direction is along a longitudinal direction of the thread and the second direction is perpendicular to the longitudinal direction of the thread .

3 . The method according to claim 2 , wherein the thread has a diameter in a range from 25 - 100 micrometers .

4 . The method according to claim 2 or 3 , wherein, before moving the transfer member towards the surface of the substrate , the method further comprises the step of arranging the thread and/or the surface of the substrate with respect to each other so that the longitudinal direction of the thread is substantially parallel to the surface of the substrate .

5 . The method according to claim 2 , 3 or 4 , wherein, before moving the transfer member towards the surface of the substrate, the method further comprises the step of providing an amount of the liquid substance or past onto the thread and subsequently pulling the thread through an ori fice, wherein a part of the thread that has been pulled through the orifice is used for the at least partially transferring the liquid substance or paste onto the surface of the substrate .

6. The method according to any one of the claims 1 - 5, wherein the liquid substance or paste comprises a suspension of granular material in a background fluid, preferably wherein the granular material comprise a granular electrically conductive material , preferably wherein the granular electrically conductive material comprises silver .

7 . The method according to claim 6 , wherein the paste comprises a solid content of 50 weight % of the granular material or more , preferably a solid content of 60 weight % of the granular material or more .

8 . The method according to any one of the claims 1 - 7 , wherein the method further comprises the step of solidi fying the liquid substance or paste after the liquid or paste has been at least partially trans ferred from the transfer member onto the surface of the substrate .

9. The method according to claim 8 , wherein the step of solidifying comprises a heating of the substrate for sintering the liquid substance or paste, or for evaporating a background fluid of the paste .

10 . The method according to claim 8 , wherein the liquid substance or paste comprises photopolymer or light- activated resin, wherein the step of solidi fying comprises an illumination of the structures or tracks by light, preferably ultraviolet or visible light .

11 . The method according to any one of the claims 1 - 12 , wherein the liquid substance or paste comprises an electrically conductive or magnetic substance, wherein the method further comprises the step of applying a voltage dif ference between the trans fer member and the substrate or between the trans fer member and the substrate holder .

12 . An apparatus for producing a high aspect ratio surface structures on a surface of a substrate , wherein the apparatus comprises : a substrate holder for holding the substrate, a transfer member, wherein the transfer member extends in a first direction and has a width in a second direction, wherein the second direction is substantially perpendicular to the first direction, and wherein said width is in a range from 5 - 500 micrometers , and a transfer member holder for holding the transfer member, wherein the substrate holder and/or the trans fer member holder are configured for moving the trans fer member and/or the substrate with respect to each other so that the transfer member and the surface of the substrate are moveable towards each other and away from each other .

13 . The apparatus according to claim 12 , wherein the trans fer member holder and the substrate holder are configured to arrange the trans fer member and/or the surface of the substrate with respect to each other so that the first direction, which is a longitudinal direction of the trans fer member, is substantially parallel to the surface of the substrate that in use is facing the transfer member .

14 . The apparatus according to claim 12 or 13 , wherein the apparatus further comprises an application device , wherein the application device is configured for providing a layer of a liquid substance or paste onto the transfer member, in particular on a surface of the trans fer member which in use is facing the substrate .

15. The apparatus according to claim 14 , wherein the application device and/or the transfer member holder are configured for moving the transfer member and/or the application device with respect to each other so that the application device and the transfer member are moveable towards each other and away from each other .

16. The apparatus according to claim 14 or 15 , wherein the application device is configured for providing an even layer of the paste onto at least a part of the surface of the transfer member which in use is facing the substrate, preferably wherein the paste comprises a suspension of granular material in a background fluid, preferably wherein the granular material comprises an electrically conductive granular material , preferably wherein the electrically conductive granular material comprises silver .

17 . The apparatus according to claim 16 , wherein the application device is configured for providing an even layer of a paste which comprises a solid content of 50 weight % of the granular material or more, preferably a solid content of 60 weight % of the granular material or more .

18 . The apparatus according to any one of the claims 12 - 17 , wherein the trans fer member comprises a thread, preferably a metal thread, wherein the first direction is along a longitudinal direction of the thread and the second direction is perpendicular to the longitudinal direction of the thread .

19. The apparatus according to claim 18 , wherein the thread has a diameter in a range from 25 - 100 micrometers .

20 . The apparatus according to claim 18 or 19 , when dependent on claim 14 , wherein the apparatus comprises : a transfer position where , in use, at least part of a liquid substance or past is trans ferred onto the substrate , an ori fice, wherein the orifice is arranged between the application device and the transfer position, wherein the thread is configured to pass through the ori fice, and a thread conveyor device configured for moving the thread from the application device, through the ori fice, to the transfer position .

21 . The apparatus according to claim 18 , 19 or 20 , wherein the apparatus further comprises a tensioning member configured for applying a tension to the thread, preferably wherein the tensioning member is configured for setting the tension in the thread to a desired level .

22 . The apparatus according to claim 21 , wherein the apparatus comprises a tension sensing member configured for sensing the tension in the thread and for providing an indication of a magnitude of the tension in the thread and/or a signal corresponding to the magnitude of the tension in the thread .

23 . The apparatus according to any one of the claims 12 - 22 , wherein the apparatus further comprises a heating element which is configured for heating the structures or tracks on the surface of the substrate, preferably the heating element is arranged to be part of the substrate holder, and is configured for heating the substrate .

24 . The apparatus according to any one of the claims 12 - 23 , wherein the apparatus further comprises a light source which is configured for illuminating the structures or tracks on the surface of the substrate, preferably the light source is configured for emitting ultraviolet or visible light .

25. The apparatus according to any one of the claims 12 - 24 , wherein the apparatus further comprises a voltage supply which is configured for applying a voltage dif ference between the trans fer member and the substrate or between the transfer member and the substrate holder .

26. Computer-readable medium having computerexecutable instructions adapted to cause an apparatus according to any one of the claims 12 - 25, to perform a method for producing a high aspect ratio front contacts on a surface of a substrate according to any one of the claims 1 - 11 .