Acknowledgement

[0001 ] The research project leading to the present invention has been supported by the National Research Fund, Luxembourg (Ref. FNR/P12/4853155/Kreisel).

Description

Technical field

[0002] The invention is directed to the manufacturing by nanoimprint lithography and in particular to a mould for nanoimprint lithography. The invention concerns many applications, namely the manufacturing of components in the fields of magnetic storage, optical storage (EBR), opto-electronics, optical coatings, displays (OLEDs), polymer electronics, biological applications, molecular electronics, semi-conductors, etc.

Background art

[0003] The paper "Toward residual-layer-free nanoimprint lithography in large- area fabrication" by Yoon et al., Korea-Australia Rheology Journal, February 2014, lists the state of the art of nanoimprint lithography techniques and exposes the problem of residual layer. This layer of polymer remains on the substrate after the imprint step and this residual layer needs to be removed through a further step in the process, for instance a reactive-ion-etch step. This paper identifies a couple of solutions to ensure that no residual layer remains. In particular, the known solutions are the use of flexible mould, the selective filling, the reverse imprint or the dewetting-induced removal of the residual layer. Each of these solutions has some drawbacks: flexible moulds generate a high dispersion in the dimensions that are manufactured and the flexible moulds are prone to premature wear; selective filling and reverse imprint are complex processes which involve supplementary method steps and material; and dewetting-induced removal generates some problem of bubbles and is less flexible for what concerns the nature of the substrate that can be used.

Summary of invention

Technical Problem

[0004] The invention has for technical problem to provide a nanoimprint lithography mould and a process of manufacturing by nanoimprint lithography that overcome the above-mentioned drawbacks and in particular a mould and a process which prevent the formation of a residual layer and which are simple, cost-efficient and present a great flexibility with respect to the technical applications in which they can be used. Technical solution

[0005] The invention is directed to a nanoimprint lithography mould comprising: a body and a plurality of protrusions extending from the body, each protrusion having a proximal area at its proximal end and a distal area at its distal end, away from the body, wherein the distal area is smaller than or equal to a quarter of the proximal area.

[0006] The protrusions extend from the body in a general direction. This general direction is common to all protrusions of the mould and is substantially perpendicular to the surface of the body which receives the protrusions.

[0007] The proximal area can be defined as the area of the section of the protrusion orthogonally to the general direction at the base of the protrusion, i.e. where the protrusion "contacts" the body ("contacts" is in quotations because the protrusions and the body are integrally part of the mould and are not made of separate parts).

[0008] The distal area can be defined as the area of the section of the protrusion orthogonally to the general direction at the distal end of the protrusion, i.e. the end of the protrusion furthest away from the body.

[0009] Through various experiments it has been shown that by providing a difference between the two areas, no residual layer remains on the substrate. This is in particular due to the substantively slanted peripheral surface of the protrusions which "pushes" away the polymer during the downwards movement of the mould.

[0010] Preferably, the ratio of the distal area over the proximal area should be bigger than 10%. With regular shape factor (height of the protrusion over its diameter), a ratio below 10% will make the protrusions be too thin at their proximal end. This could lead to breakage of the protrusions under the pressure of the mould or during removal of the mould from the polymer.

[001 1 ] According to a preferred embodiment, each protrusion has a frustoconical shape at least in part of its length. In this embodiment, the section of the protrusion decreases linearly from the proximal end to the distal end.

[0012] According to a preferred embodiment, each protrusion has a concave peripheral surface. Thus, the section of the protrusion does not decrease linearly. The curve can be a polynomial or any other basic or more complex mathematical function. The concave surface accentuates the effect of the mould on the repulsion of the polymer away from the protrusions. The peripheral surface can be partly frustoconical for instance in a portion of the length of the protrusion close to its proximal end, and partly concave, for instance in a portion of the protrusion close to its distal end.

[0013] According to a preferred embodiment, the section of the protrusion is a disc or a polygonal form, such as a square, a hexagon or an octagon. Indeed, depending on the application and the purpose of the product that is realized, various shapes of section can be used.

[0014] According to a preferred embodiment, the protrusions are arranged in a grid-like network wherein the pitch of the network is comprised between 10 nm and 10 μητι, preferably between 800 and 850 nm, more preferably 840 nm. The pitch is defined as being the distance between the central vertical axis of two neighbouring protrusions. The pitch may vary across the mould to obtain irregular pattern. The network can be made of honey-comb-like pattern or rectangular. The pitch can also be identical in all directions and therefore a diamond-shape pattern or square pattern is defined. Alternatively, a one-dimensional pattern, such as a line, a circle or a spiral, or any combination of these, can also be used.

[0015] According to a preferred embodiment, the distal area is comprised between 1 nm ² and 70 000 nm ² , and preferably between 8 000 and 18 000 nm ² . For instance, when the section of the protrusion is a disc, the diameter of the disc at the distal end can be comprised between 100 nm and 150 nm, preferably 120 nm. When the section is not a disc, an equivalent range of dimension can be calculated.

[0016] According to a preferred embodiment, the proximal area is comprised between 100 nm ² and 300 μιτι ² , and preferably between 70 000 and 200 000 nm ² . For instance, when the section of the protrusion is a disc, the diameter of the disc at the proximal end can be comprised between 300 nm and 500 nm, preferably 400 nm. When the section is not a disc, an equivalent range of dimension can be calculated.

[0017] The smallest ratio between the diameters of the proximal end and the distal end is therefore 300/150 = 2, which in terms of area is equivalent to a quarter of the surface area.

[0018] According to a preferred embodiment, the length of the protrusion is comprised between 10 nm and 20 μιτι, preferably from 1 .8 and 2.2 μιτι, and is more preferably about 2 μιτι. The ratio between the height of the protrusion and the diameter of the protrusion (if the section of the protrusion is a disc) can be from around 3 (= 1 .8 μιτι / 500 nm) to 7 (= 2.2 μιτι / 300 nm) at the proximal end and from 12 (= 1 .8 μιτι / 150 nm) to 22 (= 2.2 μιτι / 100 nm) at the distal end. These ratios make it possible to further optimise the imprint process in particular by not putting the peripheral surface of the protrusions under too much pressure of the polymer.

[0019] The body of the mould of the present invention can have overall dimensions of from 2x2 μιτι ² to a few dozen of square meters, and preferably from 5 mm x 5 mm to a few dozen centimetres square. The protrusions can be distributed on all the surface of the body or can be only in part of it. Several patterns of protrusions can be used on a single mould, with different pitches, different forms or surface area. To this end, the skilled person would know how to combine the various teachings of the various embodiments of the present invention and would know how to adapt a mould to his needs.

[0020] The invention also relates to a process of manufacturing by nanoimprint lithography comprising the steps of: depositing a polymer resist on a substrate; heating above the glass transition temperature of the polymer; pressing down a mould on the polymer; cooling and releasing the pressure of the mould; wherein the mould is according to any of the embodiments disclosed above. [0021 ] The temperature of heating is such that the polymer becomes fluid enough to be deformed by the mould. For example, the polymer employed can be a PMMA with a temperature of glass transition of 105°C. The substrate and the mould can be heated at 155°C for 60 seconds prior to the operation of imprint. The duration and temperature can be selected differently, as long as the polymer is soft enough to be plastically deformed.

[0022] The mould can be pressed down with a pressure of 40 bars applied on the substrate for about 300 seconds.

[0023] The step of cooling can be done at 70°C while gradually releasing the pressure applied on the mould.

[0024] The mould can be made of or comprise hard silicon. It can be silanized with an anti-sticking layer, as for example Trichlorosilane. Alternatively, it can be made of any rigid material such as resin or quartz.

[0025] The mould is made by ebeam lithography (period 840nm) and Displacement Talbot lithography (period 780nm). After the lithography step the structures are etched into Si using RIE. Ebeam is a standard method and the Displacement Talbot Lithography is well known as described in the paper of H. Solak et AI., Opt. Express 19, 10686 (201 1 ).

[0026] The substrate can be a rigid silicon or a flexible material. It can be made of other materials such as glass or steel.

[0027] The invention also relates to a process of manufacturing by roll-to-roll imprint lithography comprising the steps of: depositing a polymer resist on a substrate; rolling on the polymer a mould wrapped around a roller; wherein the mould is according to any of the previously detailed embodiments. This technique is particularly useful for large-area fabrication, in particular for manufacturing solar cells, optical films or OLEDs.

[0028] The invention also relates to a product manufactured at least in part through one of the processes of manufacturing disclosed above. It can be observed on the final product, potentially through SEM observations of a cut-through product, that the layer of polymer presents holes that are not cylindrical and are the complementary shape of the protrusions of the mould, i.e. conical, convex, octagonal, ...

[0029] Sometimes, a product can be further processed by filling the holes left in the layer of polymer with a filler prior to destructing/dissolving the layer. In such a case, the filler will have the exact shape of the protrusions. It is therefore possible to identify that a product has been manufactured with the mould of the present invention, even when the layer of polymer has disappeared.

Advantages of the invention

[0030] The invention is particularly interesting in that the formation of a residual layer is prevented and the mould or process used are not complex. Brief description of the drawings

[0031 ] Figure 1 shows a SEM picture of a mould according to the invention;

[0032] Figure 2 represents a process according to the invention;

[0033] Figure 3 illustrates a cross-section view of the polymer layer after imprint.

Description of an embodiment

[0034] A mould 1 according to the invention is represented on figure 1A in a picture shot by scanning electron microscopy (SEM). The mould, also sometimes called stamp, has a body 2 from which extends a plurality of protrusions 3.

[0035] As can be seen on figure 1 B, the protrusions 3 have a proximal end 3.1 which is the base of the protrusion where it extends from the body 2. The protrusion 3 then extends until its distal end 3.2 which is the furthest point of the protrusion away from the body 2.

[0036] The protrusion extends along a general direction indicated as axis A. The protrusion can be axisymmetric around axis A, or can be of any odd asymmetric shape. At the proximal end 3.1 , the protrusion 3 has a section orthogonal to the direction A which has an area that is the proximal area. At the distal end 3.2, the protrusion 3 has a section orthogonal to the direction A which has an area that is the distal area. According to the invention, the proximal area is at least four times the distal area. Figure 1 B illustrates a particular embodiment of the invention, where the sections have a disc shape of a diameter of 120 nm at the distal end and 400 nm at the proximal end. In this specific case, the proximal area is 1 1 times the distal area. The length of the protrusion can be of about 2 μιτι. In this case, the diameters are between 5 and 16 times smaller than the length of the protrusion.

[0037] Figure 2 shows the process of nanoimprint lithography according to the invention. A layer of polymer 4 is deposited on a substrate 5. The mould 1 of the invention and the layer are heated for instance at 155°C during 60 seconds.

[0038] Figure 2B represents the mould in its descent phase. As illustrated with the arrows, the specific shape features of the protrusion push away the polymer layer 4 to ensure that no residual layer remains on the substrate.

[0039] Figure 2C represents the mould in its extreme downwards position. A pressure of about 40 bars is applied on the mould during 300 seconds.

[0040] Figure 2D represents the substrate with the layer after the process of imprint lithography. We can see that there is no residual layer remaining at the bottom of the cavities formed by the protrusions. The shape of the cavity is complementary to the shape of the protrusions. There is therefore a trace of the use of the mould of the invention on the layer of polymer.

[0041 ] Figure 3 is a SEM cross-section view of a layer of polymer 4 on a substrate 5 after an imprint process of the invention. We see clearly the absence of any residual layer at the bottom of the cavity. EDS/TEM analysis at the bottom of the cavity confirms this. Should a further step of the manufacturing of the component include the filling of the cavities with a filler and the destruction of the layer, the filler will take the form complementary to the cavities, i.e. the same form as the mould. There would therefore also be traces of the process of manufacturing visible on the component.