CN114981986A

CN114981986A - Layer deposition method

Info

Publication number: CN114981986A
Application number: CN202080093147.3A
Authority: CN
Inventors: 乔瓦尼·弗朗西斯科·科泰拉; 吕泉; 骆欣涛
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2020-01-15
Filing date: 2020-01-15
Publication date: 2022-08-30
Also published as: WO2021144021A1; KR20220128399A

Abstract

A method for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate, the method employing a transfer tool having a patterned relief, the transfer tool defining a patterned transfer surface of the transfer tool, the method comprising: forming a layer of a coating material on a donor substrate; providing the transfer surface to the donor substrate in a collecting step to cause the coating material on the donor substrate to adhere to the transfer surface; in a separating step, moving the transfer surface and the donor substrate away from each other, wherein the coating material adheres to the transfer surface; providing the transfer surface to a target substrate in a deposition step to cause the coating material on the transfer surface to adhere to the target substrate; and in a return step, moving the transfer surface and the target substrate away from each other, wherein the coating material adheres to the target substrate; wherein the material of the transfer tool defining the transfer surface is harder than the donor substrate.

Description

Layer deposition method

Technical Field

The present invention relates to methods for depositing layers onto substrates for forming electronic devices and coating patterns. In particular, the method involves depositing a patterned surface coating of a coating material onto a target substrate using transfer printing.

Background

Transfer printing is a method of depositing a substance onto other final substrates by transferring the substance from at least one intermediate substrate. A simple and well-known example of this general concept is transfer printing on textiles such as T-shirts. The ink material forming the design is first deposited on the patterned stamp. The patterned surface of the stamp facilitates the application of the ink to the textile and can be repeated more quickly than drawing the design directly on the textile. Then, the application of the ink to the textile in the shape of the pattern is easily performed by simply pressing the inked stamp onto the textile. In the last decade this basic printing concept has evolved into a highly technical printed circuit approach and creates active components of the device on a microscopic level. One example is the transfer printing of quantum dots into specific shapes to produce various products such as components of televisions and other display screens.

Transfer Printing (TP) is a method that can be used to manufacture display technology. For example, a pattern of Quantum Dots (QDs) is transferred from a source substrate to a target substrate.

Quantum dots are semiconductor nanocrystals having specific optical and electrical properties due to quantum confinement of electrons in the nanocrystal. Quantum Dots (QDs) can emit light of a certain wavelength by exciting nanoparticles with incident light. The wavelength of the emitted light of the quantum dots can be tuned by acting on both the QD chemical composition (including crystal conformation and dot structure, i.e., single core, core shell, or multiple shell) and size. The size of the quantum dots may range from a few nanometers in diameter (e.g., about 2nm in diameter) to hundreds of nanometers in diameter (e.g., about 200nm in diameter). The larger the size of the QD, the more shifted the light emitted by the QD toward longer wavelengths (and the redder the color).

The standard TP method consists of three steps. The first step includes fabricating QDs on a donor substrate. The second step includes inking the micropatterned elastomeric stamp with QDs via an abrupt pickup step. The third step comprises transferring the quantum dots to a target substrate via a mild release step. This process can be solvent-free and is suitable for large-scale production.

An improvement over the standard TP method, known as the Intaglio TP (ITP) method, was achieved. This is achieved because in the ITP method, the micro-scale pattern of the QDs is defined via a high yield release step from PDMS (PDMS) to a gravure, rather than a low yield pickup step by the TP method. QD patterns with well-defined shapes, e.g. low to medium scale, can be transferred.

The standard transfer printing process has some weaknesses. These include low yield and low uniformity on the microscopic scale. This is mainly due to the high die peel speed of the pick-up step. The fabrication of the source substrate (or donor substrate) requires the formation of a self-assembled monolayer of organic molecules on silicon or glass, and thus the fabrication of the source substrate (or donor substrate) can also be a slow and somewhat unreliable process.

The gravure transfer printing process also has some weaknesses. These include: a large portion of the coating material is wasted (the coating material can be expensive); an additional step in the process compared to standard TP; using a flat stamp that is not always suitable for a target substrate having an uneven surface or for multiple printing steps on the same target area; and the use of expensive materials such as patterned silicon.

It is desirable to develop a transfer printing process that is efficient (e.g., by using materials that are easily and quickly prepared), suitable for use in mass production, and can provide high quality end results.

Disclosure of Invention

According to one aspect, there is provided a method for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate, the method employing a transfer tool having a patterned relief, the transfer tool defining a patterned transfer surface of the transfer tool, the method comprising: forming a layer of a coating material on a donor substrate; providing the transfer surface to a donor substrate in a collecting step to cause the coating material on the donor substrate to adhere to the transfer surface; in a separating step, moving the transfer surface and the donor substrate away from each other, wherein the coating material adheres to the transfer surface; providing the transfer surface to a target substrate in a deposition step to cause the coating material on the transfer surface to adhere to the target substrate; and in a return step, moving the transfer surface and the target substrate away from each other, wherein the coating material adheres to the target substrate; wherein the material of the transfer tool defining the transfer surface is harder than the donor substrate. The proposed method uses materials with a hardness gradient between them to provide a high yield and high quality transfer printing method with improved efficiency and reliability.

The returning step is a returning step because it returns the transfer surface to a state where the transfer surface is separated from the target substrate.

The surface energy of the donor substrate immediately prior to the forming step is less than the surface energy of the transfer surface immediately prior to the collecting step. The method may apply a difference in surface energy to provide a gradient to facilitate release of the coating material onto the transfer surface.

According to another aspect, there is provided a method for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate, the method employing a transfer tool having a patterned relief, the transfer tool defining a patterned transfer surface of the transfer tool, the method comprising: forming a layer of a coating material on a donor substrate; providing the transfer surface to the donor substrate in a collecting step to cause the coating material on the donor substrate to adhere to the transfer surface; in a separating step, moving the transfer surface and the donor substrate away from each other, wherein the coating material adheres to the transfer surface; providing the transfer surface to a target substrate in a deposition step to cause the coating material on the transfer surface to adhere to the target substrate; and in a return step, moving the transfer surface and the target substrate away from each other, wherein the coating material adheres to the target substrate; wherein the surface energy of the donor substrate immediately before the forming step is less than the surface energy of the transfer surface immediately before the collecting step. The proposed method uses materials with a surface energy gradient between them to provide a high yield and high quality transfer printing method with improved efficiency and reliability.

The method may include the step of treating the donor substrate to reduce the surface energy of the donor substrate. Thus, the method may enable control of the surface energy of the donor substrate such that a greater surface energy gradient may be applied, thereby facilitating collection of the coating material.

The method may comprise treating one or both of the transfer surface and the target surface to increase its/their surface energy. Thus, the method may enable control of the surface energy of one or both of the transfer surface and the target surface such that a greater surface energy gradient may be applied, thereby facilitating collection or deposition of the coating material.

The separating step may comprise taking off the transfer surface from the donor substrate over a span of the transfer surface, and wherein the linear speed of the taking off may be 1mm/s or less than 1mm/s for at least 80% of the span. Thus, the method can enable release of the coating material from the donor substrate in a release process prior to removal of the donor substrate, thereby facilitating collection of the coating from the donor substrate.

The coating material may comprise one of: luminescent polymers, semiconducting polymers, conducting nanoparticles, luminescent nanoparticles, metals, and oxide materials. Thus, the method can be used with a variety of coating materials.

The forming step may include forming the layer as a set of dots. Thus, the method can be used for transfer printing of discontinuous coating materials.

The average diameter of the dots may be less than 200 nm. Thus, the method can be used to transfer brush coat materials on a nanometer scale.

The collecting step may include compressively deforming the donor substrate. Thus, the method may account for transfer printing onto a surface having a relief pattern.

The surface energy of the transfer surface immediately prior to the collecting step may be less than the surface energy of the target surface immediately prior to the depositing step. Thus, the method may apply a difference in surface energy to provide a gradient to facilitate release of the coating material onto the target surface.

The target substrate may be translucent. Thus, the method may be used to deposit a coating material on a translucent material.

The target substrate may comprise silicon, silica glass or indium tin oxide. Thus, the method may be used with target substrates comprising multiple materials.

The transfer surface may be defined by an organic material (e.g., including an organometallic complex). Thus, the method may be used with transfer surfaces comprising a variety of organic materials, such as small molecules, oligomers, and polymers.

The donor substrate can comprise the same polymer as the defined transfer surface. Thus, the method may use the same type of material for multiple components of the method, thereby increasing the overall efficiency of the method.

The method can include forming an optoelectronic device, wherein the coating material adhered to the target substrate constitutes optically active quantum dots of the device. Thus, the method can be used to produce high quality devices by printing optically active quantum dots with high yield and high level of precision. The method also reduces the number of wasted QDs and reduces the amount of silicon-based substrate preparation.

Drawings

The invention will now be described by way of example with reference to the accompanying drawings. In the drawings:

fig. 1a shows a schematic diagram illustrating an example of three steps in a standard method of transfer printing;

figure 1b shows a schematic diagram illustrating an example of four steps in an ITP method of transfer printing;

FIG. 2 shows an image containing six RGB quantum dot pixels that have been applied to a target substrate using standard transfer printing methods;

fig. 3 shows a schematic diagram illustrating the steps of the proposed method;

FIG. 4 is a flow chart of steps of a proposed method for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate;

FIG. 5 is a flow chart including additional or alternative collection steps of the proposed method for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate;

FIG. 6 shows the schematic of FIG. 3 and a macro-scale photograph taken at each step of the proposed transfer printing process;

fig. 7 shows a series of three micro-scale photographs of a substrate and a transfer surface during the proposed transfer printing method;

fig. 8 shows six micro-scale photographs of the substrate and the transfer surface during the proposed transfer printing method.

Detailed Description

As described above, Transfer Printing (TP) is a method that can be used in manufacturing display technologies to transfer a pattern of Quantum Dots (QDs) from a source substrate to a target substrate. This process can be solvent-free and is suitable for large-scale production. The standard TP method consists of three steps.

Fig. 1a) shows a schematic diagram illustrating an example of three steps in a standard method of transfer printing, here exemplified using quantum dots. From left to right, the method 100 includes a first step of QD donor substrate fabrication. This includes forming a layer 102 of coating material, in this case a QD layer, on a donor substrate 104 (e.g. silicon). In a second step, the micropatterned elastomeric stamp 105 is inked with QDs via an abrupt pick-up technique. The snap-off involves pressing the micropatterned elastomeric stamp onto the inked donor substrate for a certain amount of time, then snapping off to leave a subset 102a of coating material QDs remaining on the stamp 105. The relief portion of the patterned stamp 105, which is the portion closest to the donor substrate 104, is the only part of the stamp that picks up the QDs, and thus defines the shapes of the QDs on the stamp surface. The third step includes transferring the QDs to the target substrate 106. When the patterned stamp supporting the subset 102a is provided to the target substrate and then removed, the subset 102a of QDs is deposited onto the target substrate 106, resulting in a transfer printing of the pattern of the stamp 105. When the picking up step is described herein as fast or abrupt, this is intended to describe a picking up step in which the stamp peeling speed is higher than 1 mm/s.

Standard transfer printing processes have some weaknesses, particularly in the printing of quantum dots.

Standard quantum dot transfer printing methods have low yield and low uniformity when the pattern to be printed is in the microscopic range. This is a significant problem for display applications where the pixel size is <100 micrometers (μm) and the emissive layer (coating material) QD must be as uniform as possible.

The poor yield of TP is mainly due to the fast pick-up step used when inking the stamp. This has a significantly lower yield when the pattern is in the microscopic range, since the pattern edges typically suffer from a lower pick-up yield. On this scale, the edge regions that are not picked up may represent a significant portion of the designed micro-pattern. Therefore, the pickup failure of the boundary area will correspond to a proportionally significant change in the transfer pattern. In contrast, the release step is a mild and high yield step. The mild release step is performed when the coating material (e.g., quantum dots) is transferred from a typical elastomer transfer surface to the target substrate. The yield of this type of release step can be as high as almost 100%.

The fabrication of a source or donor substrate in a TP requires the formation of a self-assembled monolayer of organic molecules on silicon or glass. This is a slow process and typically not a highly reliable process.

The development of the TP method, known as the "gravure transfer printing" method (ITP), was described in 2015. Fig. 1b) shows a schematic diagram illustrating an example of four steps in an ITP method 150 of transfer printing, here exemplified using quantum dots as coating material.

The ITP method is almost the same as the standard TP method. However, at the second step, QD picking using a patterned stamp by the flat elastomer 152 instead of the TP method is included. The QDs are picked up using a flat, i.e., unpatterned, layer 152 of Polydimethylsiloxane (PDMS).

Additional steps of the TP method are then performed at step three. This step involves a so-called intaglio plate 154 in which a pattern is engraved. QDs that are not needed to form the desired final pattern on the target substrate 106 are released onto the additional intermediate engraved silicon (Si) substrate or intaglio 154. The engraved pattern on the intaglio plate 154 is complementary to the desired final pattern. Thus, only a subset 102a of QDs in the desired pattern shape remains on the flat elastomer 152.

In the fourth step, the desired pattern in QDs 102a is transferred from the planar elastomer 152 to the target substrate 106 with a yield of almost 100%. That is, the QDs are transferred from the planar elastomer 152 to the target substrate 106 with almost 100% transfer efficiency. Therefore, by using the intaglio 154, patterns in the microscopic range can be accurately transferred to the target substrate with high efficiency.

An improvement from the standard TP method to the Intaglio TP (ITP) method is achievable because in the ITP method, the micron-scale patterns of QDs are defined in the high-yield release step from the flat elastomer to the Intaglio, rather than by the low-yield pick-up step of the TP method.

However, ITP also has disadvantages associated with its method. Some of these disadvantages are explained below.

Due to the materials required for intaglio and the difficulties associated with the reuse and cleaning of these materials, a significant portion of the QDs are wasted on the "intaglio" substrate. The QDs themselves cannot be easily released from the intaglio plate and are therefore lost during cleaning of the intaglio plate. QDs are very expensive active materials and this waste is an expensive loss.

The ITP method also adds a step to the standard TP processing. The time taken for this additional step reduces the overall process efficiency and therefore makes it more expensive.

The ITP process requires at least one additional silicon substrate that is also engraved to form a gravure. The intaglio plate also needs to be cleaned between transfers. The fabrication and cleaning of multiple engraved silicon substrates (now gravure and donor substrates) reduces the efficiency of the overall process and increases the overall cost.

The use of a flat stamp for the target substrate for the final transfer step may not always be compatible with the target substrate. This is because target substrates, particularly for manufacturing electronic devices, often have some other components already attached to them. These additional components, such as electrical contacts and electrical connections, are often applied to a flat base substrate to create a target substrate. Such a difference in height may produce a relief pattern on the target substrate, where different contacts even have different respective heights. Thus, using a flat PDMS transfer surface that is then pressed onto an uneven target substrate may result in a portion of the pattern not contacting the target substrate at all. This may be due to a portion of the planarization layer being lifted up by the relief pattern of the features on the target substrate, such that a portion of the patterned coating on the planarization layer is also lifted up and prevented from contacting the target substrate.

The lack of difference in height of the coated and uncoated portions of the flat PDMS substrate layer also means that it is not easy to apply additional pressure to ensure contact between the coated areas and the target substrate. Such pressure would have to be applied in the correct place to prevent damage to the already sufficiently coated areas of the target substrate, with the additional risk of damaging the PDMS layer itself by applying such pressure.

ITP processes still rely on the fabrication of silicon or glass based donor substrates. As described above with reference to the cliche layer, the use of these materials can be expensive and time consuming, thus resulting in less efficient and more expensive overall processing.

Fig. 2 shows an image containing six RGB quantum dot pixels 202a to 202f that have been applied to a target substrate using standard transfer printing methods. The dimensions of each of the six pixels were 200 microns high by 400 microns wide. Each pixel comprises three

rectangular areas

204a, 204b, 204c, each having red, green or blue QDs deposited thereon. It can be seen how the rectangular region 204 has different levels of QD coverage. For example, consider the top left pixel arrangement 202 a. The leftmost rectangle 204a of the pixel 202a has very little, if any, discernable QD coverage. However, the right rectangle 204b has some coverage with distinct lines where the coverage is broken or lost. The third rectangle 204c has a more solid coverage area in the center, but it can be seen that the significant part is hardly or not covered by QDs, especially the lower half. It would be extremely difficult to obtain functional RGB pixels from transfer printing of this quality.

Many coating materials can be transferred using transfer printing methods. QDs with well-defined shapes are just one example of a coating material that can be successfully transfer printed onto a substrate.

It would be desirable to create a transfer printing method that retains some of the benefits of the TP and ITP methods, but also eliminates some of their drawbacks as described above.

The presently proposed method is used to eliminate these drawbacks in an improved transfer printing process. In the proposed method, a transfer tool with a patterned relief defines a patterned transfer surface of the tool. The material of the transfer tool, and hence the transfer surface, is selected to be harder than the material making up the donor substrate. For example, the donor substrate material may be an elastomeric sheet material that is softer than the selected elastomeric material of the different formulation used to make the transfer tool and its surface. Alternatively, the donor substrate may be specifically selected to be softer than a harder transfer surface material of a different formulation. The important aspect is that the gradient from softest material to hardest material is in the direction of the donor substrate to the transfer surface. This gradient promotes the desired transfer of the coating material from the donor substrate to the transfer surface.

In the proposed method, the material of the transfer tool, and hence of the transfer surface, may alternatively or additionally be selected to be a material having a larger surface energy than the material constituting the donor substrate. For example, the donor material may be an elastomeric sheet material having a surface energy less than the surface energy of the selected elastomeric material of a different formulation used to make the transfer surface. Alternatively, the donor substrate may be specifically selected to have a surface energy that is less than the surface energy of a transfer surface material of a different formulation. An important aspect is that the surface energy gradient of the material increases in the direction from the donor substrate to the transfer surface. This gradient promotes the desired transfer of the coating material from the donor substrate to the transfer surface. Thus, in practice, the surface energy of the donor substrate immediately prior to the formation step of forming the layer of coating material on the donor substrate is less than the surface energy of the transfer surface immediately prior to the collection step of providing the transfer surface to the donor substrate to cause the coating material on the donor substrate to adhere to the transfer surface.

The term "provide," as used herein, has a definition as used in a general engineering sense. That is, the act of initiating the approach or contact of one item or mechanical part to another item or mechanical part is provided. In some cases, one may continue to assemble the items or parts together. It is therefore intended herein to position the donor substrate and the transfer surface close to each other so that the coating material can be transferred from one to the other. They may not be in direct contact with each other. However, a certain degree of direct contact between the donor substrate and the transfer surface is not always avoided, nor does it have to have a negative effect on the proposed method if direct contact does occur. The same definition of the term "provided" in relation to the transfer surface and the target substrate can also be inferred.

The presently proposed method may be performed using any suitable coating material that can be transfer printed onto a target substrate using the described method. The coating material may comprise any of a luminescent polymer, a semiconducting polymer, a conducting nanoparticle, a luminescent nanoparticle, a metal, an oxide material and one or more optically active quantum dots. The layer of coating material may be formed by a set of dots. For example, the forming step may include forming the layer as a set of dots. The average diameter of the dots may be less than 200 nm. For ease of understanding, the method has been described below with reference to an example coating material comprising optically active quantum dots. It should be understood that this is not the only coating material that can be used with the presently proposed transfer printing method.

The donor substrate can also be chosen to be flat. For example, QDs can be transferred from a flat softer elastomer sheet of a first formulation to a patterned harder elastomer transfer tool or stamp having a second, different formulation. In the described example, the elastomeric material may be Polydimethylsiloxane (PDMS) with a formulation ratio of polymer to curing agent ranging between 25:1 and 2:1 (by weight). QDs can be disposed on a flat donor elastomer via solution deposition or via standard large area pickup steps. In other cases, the material to be transferred may be vacuum deposited on the donor elastomer. QDs can be released from a flat elastomer onto a patterned harder elastomer with high yield. The elastomer of the donor substrate and the transfer surface may be PDMS. The coating of the donor substrate using a large area pick-up step does not adversely affect the proposed transfer printing method, since the coating material can be released from the central area of the donor substrate to the patterned transfer substrate, i.e. edge areas that may be more severely affected by a low yield pick-up step are avoided.

The preparatory step of disposing the QDs onto the donor substrate may require that a specific film coating be disposed on the donor substrate before the QDs are disposed on the donor substrate. Such a coating may enhance the wettability of the elastomer to the solvent in which the material to be printed is dissolved. In other cases, they can protect the elastomeric material from alteration or damage during the inking process due to interaction with solvents or the operating conditions of vacuum deposition. Examples of such coating materials may be polymeric materials such as Poly (p-xylene), Polyvinyl Alcohol (PVA), polymethyl Methacrylate (PMMA) and fluoropolymers or one of their derivatives.

Fig. 3 shows a schematic diagram illustrating the steps of the proposed method 300. Step one is shown in the first part of the schematic and involves forming a layer of coating material on the donor substrate 302. This formation may be performed using a variety of methods known to be used to fill such donor substrates 302 with films of various coating materials 102. In the example of fig. 3, the coating material is a Quantum Dot (QD) layer. As mentioned above, the proposed method may comprise a step of treating the donor substrate before inking with the coating material. Such treatment may be used to reduce the surface energy of the donor substrate. For example, as mentioned above, a particular film coating may be disposed on the donor substrate before the QDs are disposed on the donor substrate.

The pick-up step, which typically has a low yield as explained above, is not used in the proposed method to define the pattern to be transferred to the target substrate. This lack of pattern definition pick-up step can be considered similar to the intaglio ITP method, in which QDs are acquired from silicon donors by a flat elastomer without patterning in the pick-up step. However, in the proposed process, the only potentially used pick-up step would be directly to the donor substrate and therefore not form part of the transfer printing method. The donor substrate of the proposed method is for example an elastomeric substrate, instead of a hard silicon based substrate. In the proposed method, the pick-up step is not necessary, since the coating material can also be applied to the donor substrate using other methods, for example by solution. Thus, although a pick-up step may be used to fill the donor substrate (which is still a relatively low yield step), it has little impact on the yield of the overall transfer printing process and is not used during the pattern definition step. The pattern, which may be a micropattern, is instead defined only by the release step. This enables the desired pattern to be transferred to the target substrate in an efficient manner. Thus, a transfer printing process is defined that does not require the use of a sudden and low-yield pick-up step. The entire transfer printing process is based only on a high yield release step. The entire process can also be done with only one transfer step, e.g. from a flat elastomer to a patterned elastomer, before printing onto the target substrate.

The second part of the schematic illustrates the collecting and separating steps of the proposed method and shows that the patterned transfer surface 304 has coating material 102a already released onto it, the coating material 102a being in the shape of the desired pattern as engraved or molded into the transfer surface 304. The patterned transfer surface 304 can be made of a harder elastomer than the donor substrate. The coating material in this depiction includes QDs. In the collecting step, the transfer surface is provided to a donor substrate to cause the coating material on the donor substrate to adhere to the transfer surface. The collecting step may include compressively deforming the donor substrate. In the separation step, the transfer surface and donor substrate are moved away from each other, with the coating material 102a adhering to the transfer surface 304. The separating step may also include peeling the transfer surface 304 from the donor substrate 302 over the span of the transfer surface. The linear speed of stripping can be 1mm/s or less than 1mm/s for at least 80% of the span. The transfer surface may be defined by a polymer. The donor substrate can comprise the same polymer as the defined transfer surface.

The proposed method may additionally comprise the step of treating one or both of the transfer surface and the target surface to increase their surface energy. In this way, it is possible to ensure that the surface energy gradient from the donor substrate to the transfer surface, or the surface energy gradient from the transfer surface to the target substrate, or both the surface energy gradient from the donor substrate to the transfer surface and the surface energy gradient from the transfer surface to the target substrate, go from low to high. That is, the surface energy of the transfer surface immediately prior to the collecting step may be less than the surface energy of the target surface immediately prior to the depositing step.

The QD pattern can have well-defined edges that produce a desired pattern that is distinctly delineated on the transfer surface 304. This is because the QDs are released from the donor substrate to the transfer surface and not picked up as in the standard TP method.

The results described above can be based onDifferent compositions of the surface, such as different viscoelastic (mechanical) properties and surface energies of different compositions between different elastomer formulations. The different surface energies may be achieved by using specific coatings or treatments (e.g., UV-O) on the donor substrate surface and the transfer surface as desired ₃ 、O ₂ Plasma, etc.) to enhance. In certain example cases, this may be achieved using PDMS with different compositions. For example, under the right conditions, QDs can be transferred from soft PDMS to hard PDMS at high yield via a mild release step.

The third part of the schematic illustrates the deposition and return steps of the proposed method and shows that the patterned elastomeric transfer surface 304 has released the subset of QDs 102a to the target substrate 106. In the deposition step, the transfer surface 304 is provided to the target substrate 106, for example, to adhere the coating material (e.g., QDs) on the transfer surface 304 to the target substrate 106. In the return step, the transfer surface 304 and the target substrate 106 are moved away from each other, wherein the coating material 102a adheres to the target substrate 106. The target substrate may be translucent. The target substrate may comprise silicon, quartz glass or indium tin oxide.

The proposed method may comprise forming an optoelectronic device, wherein the coating material adhered to the target substrate constitutes the optically active quantum dots of the device.

The sequence of release-only steps of the method can be used to create a series of gradients from soft to hard, or low surface energy to high surface energy, or soft to hard and low surface energy to high surface energy. For example, the material of the transfer tool defining the transfer surface is harder than the donor substrate, and additionally or alternatively, the surface energy of the donor substrate immediately prior to the forming step is less than the surface energy of the transfer surface immediately prior to the collecting step.

The method can be implemented in various ways. Some specific examples of the embodiments are described below.

In example embodiments, an unpatterned elastomeric substrate may be inked with quantum dots or nanoparticles to produce a donor substrate. The unpatterned elastomeric substrate may beMade of a multilayer structure and may additionally or alternatively be treated to achieve desired mechanical properties and surface energy. The treatment used may be, for example, UV-O ₃ 、O ₂ Plasma and heat treatment.

In an example embodiment, the elastomeric material involved in the process may have mechanical viscoelastic properties. Viscoelasticity is a property of some materials that includes both viscous and elastic properties of the material when deformed.

In an example embodiment, the elastomeric material used in the process may have a weight ratio range of polymer to curing agent of 25:1 to 2: 1.

In example embodiments, the inking process may be achieved via one or more solution processing methods or via one or more transfer printing processing methods or via one or more other compatible methods or via a combination of any such suitable methods.

In an example embodiment of the proposed method, a patterned elastomeric substrate may be brought into contact with an inked unpatterned elastomer.

In an example embodiment, sufficient pressure may be applied between the different elastomers to achieve conformal contact.

In example embodiments, the elastomeric substrates can be pulled apart from one another at a peel speed of one millimeter per second or less than one millimeter per second (≦ 1 mm/s).

In example embodiments, the QD-containing coating material may be transferred from a flat elastomeric substrate to a patterned elastomeric substrate.

In example embodiments, the inked patterned elastomeric transfer surface may be brought into conformal contact with a target substrate.

In example embodiments, the transfer surface of the tool or stamp may be pulled away from the target substrate at a peel speed of one millimeter per second or less (<1 mm/s).

In example embodiments, the coating material comprising the QDs may be transferred to a target substrate.

Fig. 4 shows a flow chart of steps of a proposed method 400 for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate, employing a transfer tool having a patterned relief defining a patterned transfer surface of the transfer tool.

Step 402 includes forming a layer of coating material on a donor substrate.

Step 404 is a collection step, which includes: the transfer surface is provided to a donor substrate such that the coating material on the donor substrate adheres to the transfer surface. The material of the transfer tool defining the transfer surface may be harder than the donor substrate.

Step 406 is a separation step comprising: the transfer surface and donor substrate are moved away from each other with the coating material adhering to the transfer surface.

Step 408 is a deposition step comprising: the transfer surface is provided to a target substrate such that the coating material on the transfer surface adheres to the target substrate.

Step 410 is a return step, which includes: the transfer surface and the target substrate are moved away from each other, wherein the coating material adheres to the target substrate.

Fig. 5 shows an additional or alternative collection step 502 in the proposed method 400. Step 502 is a collection step, which includes: the transfer surface is provided to a donor substrate such that the coating material on the donor substrate adheres to the transfer surface. The surface energy of the donor substrate immediately prior to the forming step may be less than the surface energy of the transfer surface immediately prior to the collecting step. Step 502 may be additional in that the material of the transfer tool defining the transfer surface may be harder than the donor substrate and the surface energy of the donor substrate immediately prior to the forming step may be less than the surface energy of the transfer surface immediately prior to the collecting step.

The proposed method not only eliminates any fast pick-up step during definition of the desired pattern to be transfer printed, but also provides a solution to many of the above-described problems specifically associated with the ITP method.

In the currently proposed method, there is no gravure substrate. Thus, after use in such a step, the coating material wasted during cleaning of the intaglio plate (or other intermediate substrate made of a similar material) is significantly reduced. In the described embodiments, the number of wasted QDs is therefore reduced.

Additional steps involving an intaglio plate and thus the extra costs and time of manufacturing and cleaning the intaglio plate are not necessary. Thus, the proposed transfer printing method is more efficient than the previous standard TP and ITP methods.

In the proposed method, the final transfer surface 304 used as stamp is the surface that is patterned. Since the surface to which the coating material is ultimately transferred to the target substrate is an engraved or molded surface, the transfer surface can be better adapted to follow the geometry of the target substrate. For example, where the target substrate is a display backplane.

Since the coating material is initially deposited directly on the softer donor substrate (e.g., an elastomer such as PDMS), there is no need to prepare a donor substrate that is typically formed of a glass-based or silicon-based material. Typically, on the surface of such substrates, thin layers of covalently bonded organic molecules (e.g., self-assembled monolayers) are prepared to reduce the overall surface energy of the substrate. These typical glass-based or silicon-based materials and their surface modification processes are often time consuming, unreliable, and expensive to manufacture. Cleaning them between each print also requires time and expense. Thus, the need for donor substrates typically made of these materials is eliminated, which makes the overall transfer printing process faster and more reliable.

Additionally, lower pressures are necessary during printing compared to standard TPs. Thus, less stress is applied to the transfer tool or stamp including the transfer surface. This in turn means that the transfer tool is more likely to be able to be reused for more printing cycles. Similarly, a lower pressure may be applied on the layer of coating material, which may reduce the risk of damaging the coating and reducing its performance. This is particularly important when the coating material is an active material comprising quantum dots. Excessive pressure may damage the QDs to the point of compromising their performance, thereby degrading the quality of the results even when the QDs are successfully deposited onto the target substrate.

In summary, the proposed method achieves the same or better performance than the ITP method for transferring high yield coating materials for micro-patterns; but avoids many of the disadvantages of ITP, such as waste of coating material (e.g., expensive QDs), a shorter and more efficient process.

Fig. 6 shows the schematic of fig. 3 and a macro-scale photograph taken at each step of the proposed transfer printing process. The photographs show the different substrates and their respective coating material coverage after each step. In this example, QDs are used as the coating material, where two PDMS elastomer substrates form the soft donor substrate and the hard transfer surface, and the target surface is made of glass.

At the first step, a flat and soft PDMS donor substrate is covered with a layer of QDs. Photograph 602 shows a donor substrate with a solid area covered by coating material in the center of the substrate.

At a second step, the hard patterned PDMS transfer surface has QDs released from the softer PDMS donor substrate onto the hard patterned PDMS transfer surface according to a relief pattern. In photograph 604, the donor substrate is shown after the pattern of QDs has been released. The pattern in this example is a simple rectangular shape, such as the one that may be used to make a portion of a pixel in a display and that was previously shown in fig. 2. Photograph 604 shows a clear rectangular gap in the center of the previously solid area covered by QDs. On the right, another photograph 606 shows the complementary side of the second step-the patterned hard PDMS transfer surface to which the QDs have been released in a pattern (rectangle) molded or engraved onto the patterned hard PDMS transfer surface.

At the third step, the hard PDMS transfer surface releases the QDs to the target substrate. In this example, the target substrate is a glass target. In photograph 608, the hard patterned PDMS is shown after releasing its QD coating material to a glass target substrate. It can be seen that none of the QDs remained on the transfer surface of the hard PDMS. This indicates that a high yield transfer process has occurred. On the right, photograph 610 shows a glass target substrate. The QD coating material has been successfully transferred to the glass substrate and it can be seen that the shape shown on the transfer surface in photograph 606 is retained. It can also be seen from the series of photographs that the quality of the coverage is maintained, wherein no significant holes or jagged edges appear due to the transfer process itself.

Fig. 7 shows a series of three micro-scale transfer print photographs. The desired rectangular pattern in this example measures 100 micrometers (μm) in its shortest dimension.

The first photograph 702 shows a portion of the donor substrate made of PDMS inked with QDs after it has released the QDs to the transfer surface. The center of the image has a rectangular void in which no quantum dots are present. The voids correspond to a rectangular pattern molded or engraved into the transfer surface. The voids were free of remaining QD plaques and showed that the QD coating material had successfully transferred to the transfer surface with high yield.

The second photograph 704 shows a portion of the final deposition on the target substrate. In this example, the target substrate is made of glass. The QD coating material has been deposited on the target substrate in a rectangular pattern of transfer surfaces. The coating on the target substrate is of high quality with very few holes and jagged edges. The difference between the jaggies of the edges of the printed rectangle in the photograph in fig. 6, as compared to the transfer rectangle in the photograph in fig. 7, can be attributed to the scale of the image. For example, the dimensions of the rectangular pattern in fig. 7 are about one-thousandth of the dimensions of the rectangular pattern in fig. 6, and thus the 5-10 micron variation in the straightness of the rectangular edge in fig. 7 is more pronounced.

The third photograph 706 shows a portion of the elastomeric transfer surface after the QD coating material has been released onto the glass substrate. The raised rectangular pattern of the transfer surface can be distinguished from the rest of the transfer tool. It can be seen that some QD coating material is still attached to the edges of the rectangular pattern. The jagged edges of the QDs remaining on the transfer surface match the mirror image of the jagged edges of the coating material deposited on the glass target substrate shown in the second photograph 704. It is therefore evident that the transfer from the donor substrate to the transfer surface defining the pattern to be transfer printed has a very high yield. The use of a release mechanism rather than a quick pick-up to perform this defining step is a distinction of the proposed method from the standard TP method described above and is a reason for enabling an increase in the overall coating material yield that is ultimately deposited onto the target substrate.

Fig. 8 shows a total of six photographs, a series of four micro-scale photographs 802 to 808 of the proposed method of transfer printing, and two

further photographs

810 and 812 of the donor and target substrates taken under uv light. The desired pattern in this example measures about 50 micrometers (μm) in its shortest dimension.

The first photograph 802 shows the donor substrate (made of PDMS in this example) after the coating material (QDs in this example) has been transferred to the patterned transfer surface. There is a distinct region in the center of the donor substrate where no QDs are present. The QDs have been released to the transfer surface, leaving voids in the coating material remaining on the donor substrate.

The image above the first photograph is another photograph 810 of the same area of the donor substrate as in photograph 802, but this time taken under uv light. The areas of QDs remaining on the donor substrate can be clearly distinguished from the PDMS donor substrate that has been exposed after the coating material is released to the transfer surface.

The second photograph 804 shows the QDs released onto the patterned transfer surface. In this example, QDs have clearly defined edges, in contrast to the upper 100 micrometer (μm) scale pattern of fig. 7. This is a direct result of this being a gentle release step rather than a pick-up step.

The third photograph 806 shows the QDs having been transferred onto a target substrate, which in this example is a glass substrate. The transferred QDs retain their well-defined edges and the uniformity of coverage of the transfer surface. Again, this is due to the fact that this step is also defined as a gentle release step rather than an abrupt pick-up step.

The image above the third photograph is another photograph 812 of the same area of the same target substrate as in photograph 806, but this time taken under ultraviolet light. The regions of QDs that have been released to the target substrate can be clearly distinguished from the regions of the glass target substrate without the coating material.

The fourth photograph 808 shows the transfer surface with the relief pattern after the coating material has been released to the target substrate. The edges 814 of the relief pattern can be seen, but very few QDs remain on the raised central portion of the transfer surface.

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A method for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate, the method employing a transfer tool having a patterned relief, the transfer tool defining a patterned transfer surface of the transfer tool, the method comprising:

forming a layer of the coating material on a donor substrate;

providing the transfer surface to the donor substrate in a collecting step to adhere the coating material on the donor substrate to the transfer surface;

in a separating step, moving the transfer surface and the donor substrate away from each other, wherein the coating material adheres to the transfer surface;

providing the transfer surface to the target substrate in a deposition step to cause the coating material on the transfer surface to adhere to the target substrate; and

in a return step, moving the transfer surface and the target substrate away from each other, wherein a coating material adheres to the target substrate;

wherein the material of the transfer tool defining the transfer surface is harder than the donor substrate.

2. The method of claim 1 wherein the surface energy of the donor substrate immediately prior to the forming step is less than the surface energy of the transfer surface immediately prior to the collecting step.

3. A method for forming an electronic device by depositing a patterned surface coating of a coating material on a target substrate, the method employing a transfer tool having a patterned relief, the transfer tool defining a patterned transfer surface of the transfer tool, the method comprising:

forming a layer of the coating material on a donor substrate;

moving the transfer surface and the donor substrate away from each other in a separation step, wherein a coating material adheres to the transfer surface;

wherein a surface energy of the donor substrate immediately prior to the forming step is less than a surface energy of the transfer surface immediately prior to the collecting step.

4. A method according to claim 2 or claim 3, including the step of treating the donor substrate to reduce the surface energy of the donor substrate.

5. A method according to any one of claims 2 to 4, comprising treating one or both of the transfer surface and the target surface to increase its/their surface energy.

6. A method according to any preceding claim, wherein the separating step comprises taking off the transfer surface from the donor substrate over a span of the transfer surface, and wherein the linear speed of the taking off is equal to or less than 1mm/s for at least 80% of the span.

7. The method of any preceding claim, wherein the coating material comprises one of: luminescent polymers, semiconducting polymers, conducting nanoparticles, luminescent nanoparticles, metals, and oxide materials.

8. A method according to any preceding claim, wherein the forming step comprises forming the layer as a set of dots.

9. The method of claim 8, wherein the dots have an average diameter of less than 200 nm.

10. A method according to any preceding claim, wherein the collecting step comprises compressively deforming the donor substrate.

11. A method according to any preceding claim, wherein the surface energy of the transfer surface immediately prior to the collecting step is less than the surface energy of the target surface immediately prior to the depositing step.

12. A method according to any preceding claim, wherein the target substrate is translucent.

13. A method according to any preceding claim, wherein the target substrate comprises silicon, silica glass or indium tin oxide.

14. A method according to any preceding claim, wherein the transfer surface is defined by a polymer.

15. A method according to any preceding claim, wherein the donor substrate comprises the same polymer as that defining the transfer surface.

16. A method according to any preceding claim, comprising forming an optoelectronic device, wherein the coating material adhered to the target substrate constitutes optically active quantum dots of the device.