WO2014076613A1

WO2014076613A1 - Method for patterning of graphene and graphene like materials

Info

Publication number: WO2014076613A1
Application number: PCT/IB2013/059969
Authority: WO
Inventors: Kamal Asadi; Cornelis Eustatius Timmering; Dagobert Michel De Leeuw; Johannes Franciscus Maria Cillessen; Marius Gabriel Ivan
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-11-14
Filing date: 2013-11-07
Publication date: 2014-05-22

Abstract

The present invention relates to a method of patterning a two-dimensional (two-dimensional) crystal layer such as e.g. a graphene layer, a structure comprising the patterned two-dimensional crystal layer, and a lighting device comprising the structure. The two-dimensional crystal layer is patterned by forming a metal layer (110) on the two- dimensional crystal layer (120), forming a masking layer (130) on the metal layer, forming a pattern (140) in the masking layer, transferring the pattern formed in the masking layer to the metal layer, and transferring the pattern formed in the metal layer to the two-dimensional crystal layer.

Description

Method for patterning of graphene and graphene like materials

FIELD OF THE INVENTION

The present invention relates to the field of patterning two-dimensional (2D) crystal layers, such as graphene, and in particular to a method for achieving such structures.

BACKGROUND OF THE INVENTION

2D crystals such as e.g. graphene, boron nitride, and molybdenum disulfide are of interest for various industrial applications thanks to their electrical and mechanical properties such as high electron mobility, high thermal conductivity, and high physical strength, which are examples of valued properties in e.g. the electronics industry.

For industrial applications, patterning of structures on a micro- or nanometre scale in graphene is required. There is therefore a general need for methods of patterning graphene layers.

In for example WO2012094045, a method based on selective removal of one or several monolayers from a graphene flake is described. In this method, a layer of zinc is sputtered on the graphene surface and then dissolved. At removal of the zinc, the zinc causes a simultaneous removal of one or several graphene layers beneath the metal. The number of graphene layers removed from the graphene material can be controlled by the sputtering power. By applying the metals to various areas of the surface of the graphene material, for example by e-beam lithography prior to sputter-coating, a patterned zinc layer is formed. After dissolving the zinc layer, and thereby removing the underlying areas of the graphene material, a patterned graphene material is obtained which corresponds to the areas that were not covered by the zinc.

Although such a method may provide a patterned graphene layer, there is still a need for more reliable methods and also methods enabling large-scale production.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of patterning 2D crystals that is more reliable and/or more suitable for large-scale production. The object is achieved by the invention as defined in the independent claims. The dependent claims provide advantageous embodiments.

According to the invention there is provided, a method of patterning a 2D crystal layer wherein a metal layer is formed on a 2D crystal layer. A masking layer is formed on the metal layer, in which masking layer a pattern is formed and transferred to the metal layer. The pattern formed in the metal layer is then transferred to the 2D crystal layer.

According to the invention there is further provided a structure comprising a patterned 2D crystal layer on a surface, where the pattern is such that the surface is covered by the 2D crystal region in one part and exposed (not covered) by the 2D crystal layer in another part of the surface.

According to the invention there is further provided a device including the structure. The device may comprise a structure according to the second aspect of the present invention. The structure and device may be formed in accordance with the method as defined in the first aspect of the invention.

The term two-dimensional crystal layer in the context of the invention has to be interpreted as defining a layer of material having physical properties that differ from those of a layer of the same material but with bulk crystal structure. For example, the physical properties of graphene (an example of a 2D crystal layer of carbon atoms) differ from the properties of graphite (bulk structure of same material). Thus, the term can mean that the 2D crystal layer is a layer of material comprising at least one mono layer of atoms having an arrangement with a periodicity in at least two dimensions where the layer of material has a thickness that is less than or equal to 10 nm. Preferably the thickness is less than 5 nm or even less than 2 or 1 nm. The atoms of one monolayer can form a monomolecular layer through bonding. The two-dimensional crystal layer can comprise a plurality of stacked monolayers. Preferably the number of stacked monolayers is equal to or less than 10, or 5. Preferably the number of monolayers is 2 or 1.

Especially for such layers the method is advantageous as it provides good stack layer thickness control in contrast to the prior art method.

The two-dimensional crystal layer can preferably comprise or consist of graphene. Alternatively, the layer can comprise or consist of molybdenum disulfide, tungsten disulfide and/or boron nitride. Graphene is a substance forming monomolecular layers made of carbon atoms arranged in a regular hexagonal pattern, like in graphite, but in a one-atom thick sheet. It is a single planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb two-dimensional crystal lattice. In the method of the present invention, the metal layer is used as a (patterned) protective mask for realization of the pattern in the 2D crystal layer. At the same time, the method does not rely on the inherently unreliable lift of process of the prior art. Instead, in the method according to the invention, the patterned metal layer provides for a selective removal of the exposed 2D crystal material, wherein regions of the 2D crystal layer not covered by the metal are removed. Patterning a masking layer formed on the metal layer which can then be used for protection of the 2D crystal material during the patterning of the 2D crystal layer provides a more reliable patterning process of the 2D crystal material.

In the prior art technology referred to above, a patterned zinc layer is provided on a graphene surface. As the zinc is removed, parts of the underlying graphene material are simultaneously removed. However, the amount of graphene removed simultaneously as the zinc is difficult to control, depending among others on the sputtering power and not only the etching step for removal of the zinc, thereby resulting in a method limited in terms of reliability and reproducibility. Further, any residual graphene left in the patterned areas after removal of the zinc has to be removed by forming a new zinc layer and by repeating the same patterning and transferring process to remove further material, which is time-consuming. Repetition of almost the whole procedure is rather inconvenient.

The present invention is advantageous in that evaluation of the patterning result is possible without removal of the masking metal layer. The transferring of the pattern to the 2D crystal layer may be resumed without forming and patterning a new metal layer. Any undesirable residual 2D crystal material may be removed using the same patterned metal layer as protective mask, e.g. by dry etching. A more reliable patterning process is provided and the need for reworking may be reduced.

The masking layer may comprise any suitable resist that can be patterned. These may be imprint resists that are patterned using an imprinting or embossing technique known in the art.

According to an embodiment, the masking layer may be a photoresist layer. 2D crystals, such as e.g. graphene, may exhibit a large contact angle and thus inferior wetting properties. A poor wetting may render deposition of photoresist more difficult, and may reduce the quality of the photoresist layer due to difficulties in controlling the deposition process and the thickness of the layer. In the present embodiment, the photoresist is deposited on a metal layer, which facilitates the deposition process and also increases the quality of the masking layer. With the present invention, the patterning of the masking layer on the metal layer may be obtained by photolithography, using e.g. UV-light. The inventors have realized that 2D crystals, such as e.g. graphene, may be sensitive to UV-exposure since UV-exposure may damage the C=C bonds in graphene and cause the formation of C-0 or C=0 bonds, thereby leading to a graphene layer with deteriorated properties. An advantage with using a protective metal layer between the masking layer and the 2D crystal layer is that the underlying UV-sensitive 2D crystal layer may be protected from UV-exposure during the formation of the pattern in the masking layer. The present embodiment is therefore advantageous in that it provides a method of patterning a 2D crystal layer with e.g. UV-light, with a reduced risk of causing UV-induced damages to the 2D crystal layer.

Further, it has been observed by the inventors that graphene, which is an example of a 2D-Crystal, supplied by commercial suppliers cannot withstand the shear forces that occur during spin coating of a resist layer on top of the graphene layer it. The resist layer was to be used for a patterning procedure based on a lift-off process. The masking layer according to the invention provides protection of the graphene layer and prevents damage due to mechanical forces such as e.g. the shear forces occurring during spin coating.

Patterning a 2D crystal layer using photolithography is also advantageous in that large scale production of patterned 2D crystal structures may be obtained since photolithography has the advantage of being a high speed process, at least as compared to other patterning techniques such as e.g. e-beam lithography, drop-on-demand jetting, or printing.

According to an embodiment, the metal layer may comprise a metal that is less prone to react with the underlying 2D crystal layer, thereby enabling the metal to be stripped without damaging the underlying layer. The metal may also be chosen such that it allows for a selective removal of material during transfer of the pattern to the 2D crystal layer, i.e. that any technique used for the transfer may (selectively) etch the 2D crystal layer (material) and not, or at least much less, the metal layer acting as a protective mask during transfer of the pattern. Similarly, the metal layer is preferably possible to pattern using a masking layer of e.g. photoresist. Examples of metals include gold, copper, nickel, titanium, chromium, aluminium, Silver, platinum, palladium and molybdenum, which may be deposited by e.g. sputtering, evaporation, plating, or other suitable techniques.

The transferring of the pattern formed in the masking layer to the protective metal layer may be performed by etching of the metal with a liquid etchant (i.e. by wet etching), such as e.g. FeCl3. As mentioned above, the masking layer such as a photoresist is preferably not, or at least much less, sensitive to such liquid etchant. Further, FeCl₃ is advantageous in that it may etch e.g. copper with a relatively high selectively in relation to e.g. graphene. As a result, the pattern may be transferred to the metal layer, down to the 2D crystal layer, without removing the underlying 2D crystal layer.

The patterned metal layer may act as a mask protecting the unexposed area of the underlying 2D crystal layer from being removed as the pattern is transferred to the 2D crystal layer. According to an embodiment, the 2D crystal layer may be etched by reactive ion etching (RIE), using e.g. 0₂ plasma. This embodiment is advantageous in that it allows for a selective etch of the 2D crystal layer, thereby providing a transfer of the pattern formed in the metal layer to the 2D crystal layer. The metal pattern may also protect the unexposed area of the 2D crystal layer from other potential damage, such as defects induced by UV-light that may be present during etching.

After the pattern has been transferred to the 2D crystal layer, another pattern (also referred to as a second pattern) may be formed in the metal layer to define e.g. a contact layer which may be used for electrically contacting the 2D crystal structures. According to an embodiment, the second pattern may be formed in the masking layer and then transferred to the metal layer by e.g. a selective etching process. According to another embodiment, the second pattern may be formed in another masking layer that has been formed on the metal layer after the patterning of the 2D crystal layer.

One advantage of forming a second pattern in the metal layer is that a single metal layer may be used both as a mask during the patterning of the 2D crystal layer, and as an electrical contact layer comprising structures such as e.g. contact pads, leads, and heat sinks. Forming e.g. contact pads in the metal layer provides a potential reduction of the number of required processing steps.

It will be appreciated that other embodiments than those described above are also possible, in which further metal layers and masking layers may be formed and patterned after the patterning of the 2D crystal layer.

It will also be appreciated that any of the features in the embodiments described above for the method of patterning a 2D crystal layer according to the first aspect of the present invention may be combined with the structure and device according to the second and third aspects of the present invention, respectively.

The structure obtainable with the method can be obtained with improved reliability. E.g. less defective two-dimensional crystal layers may be formed in a structure. The structure can be part of any electronic device (chip or integrated circuit) or lighting device (illuminating device or display device) or energy scavenging device (e.g. solar cell device).

Further objectives of, features of, and advantages with the present invention will become apparent when studying the following detailed disclosure, the drawings, and the appended claims. Those skilled in the art will realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, in which:

Figs, la-f show a general outline of a method of patterning a 2D crystal layer according to an embodiment of the present invention;

Figs. 2a-g show an outline of a method according to another embodiment;

Fig. 3a-i show an outline of a method according to yet another embodiment of the present invention;

Fig. 4 schematically shows a top view of a structure according to an embodiment of the present invention; and

Figs. 5a-k show an outline of a method according to another embodiment of the present invention.

the Figs, are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With reference to Figs, la-f, there is shown a general outline of a method of patterning a 2D crystal layer in accordance with an embodiment of the present invention.

The method comprises forming a metal layer 110 of e.g. copper on a 2D crystal layer 120 arranged on a substrate 100. The 2D crystal layer 120 may e.g. be a monolayer or a few layers of graphene 120 that is produced by chemical vapor deposition (CVD) and deposited on a 4-inch (4") glass or silicon (Si) wafer 100. The copper layer 110 may be deposited by any suitable deposition method, such as e.g. sputtering or evaporation. The resulting structure is shown in fig. la.

In the next step (Fig. lb), a masking layer 130 is formed on the copper layer 110. The masking layer 130 may e.g. be a spin coated photoresist 130 having a thickness of e.g. 0.01-10 μιη. The photoresist 130 may then be pre-exposure baked at e.g. 50-200°C for 10-120 seconds.

A pattern 140 is formed in the photoresist 130 (fig. lc) by e.g. UV-exposure, post exposure bake at 50-200°C for 10-120 seconds, and development for 10 seconds to a few minutes.

After the pattern 140 is formed in the photoresist 130, the pattern 140 is transferred (fig. Id) to the copper layer 110 by selectively removing the exposed parts 140 of the copper layer 110. Copper may be removed e.g. by etching with FeCl₃. The etching may be continued until the underlying graphene layer 120 is exposed. When the pattern has been transferred to the copper layer 110, the photoresist mask 130 may be removed (not shown in Figs. la-f). This can be done by e.g. stripping with acetone.

In the next step (fig. le), the pattern 140 of the copper mask 110 is transferred to the graphene layer 120 by e.g. etching with 0₂ plasma at 600 W for 2 minutes in order to remove the exposed graphene areas. This step may be repeated to reduce the amount of possible residues of graphene or other undesired substances, such as e.g.

photoresist residues.

The etching of the graphene layer 120 may be followed by a removal of the masking copper layer 110, e.g. by another etch with FeCl₃, such that the remaining product is a patterned graphene layer 120 arranged on e.g. a glass substrate 100, as shown in fig. If. Figs. 2a-g show an outline of an alternative method of patterning a 2D crystal 220 layer in accordance with another embodiment of the invention. The method comprises the same first three steps as those described with reference to Figs, la-c, wherein a copper layer 210 is formed on a graphene layer 220 (fig. 2a), a photoresist layer 230 is formed on the copper layer 210 (fig. 2b), and a pattern 240 is formed by lithography in the photoresist layer 230 (fig. 2c).

After the etching of the copper layer 210 (fig. 2d), wherein the pattern 240 formed in the photoresist mask 230 is transferred to the copper layer 210, there is however no stripping of the photoresist 230. The photoresist mask 230 is instead kept for further processing. The etching of the copper layer 210 is followed by an etching step of the exposed graphene layer 220, in accordance with what has been described above with reference to Figs. ld-e.

After the patterning of the graphene layer 220 (Figs. 2d-e), a second pattern 250, differing from the first pattern 240, may be formed in the photoresist mask 230 by e.g. UV-lithography. In the Figs., the patterns 240 and 250 are represented by open areas

(however seen as a cross-section) in the photoresist layer 230. After another development of the photoresist 230, the exposed copper area 250 is removed (fig. 2f) in a second etching step (or process) using e.g. FeCl₃. The remaining photoresist 230 may be removed by stripping with acetone, thereby resulting in the structure shown in fig. 2g.

The later part of the process according to this embodiment may e.g. be used for defining electrical contacts and leads for contacting the structures fabricated in the graphene layer 220.

Yet another alternative method according to an embodiment of the present invention is outlined in Figs. 3a-i. The steps shown in Figs. 3a-e are similar to the steps described above with reference to Figs. la-e. After the reactive ion etch of the graphene layer 320, however, there is no stripping of the copper layer 310. Instead another (or second) photoresist layer 360 may be formed (fig. 3f) on the copper masking layer 310 and the exposed graphene area. The photoresist layer 360 may then be patterned (fig. 3g) with another (or second) pattern 350, as previously described. The second pattern 350 is then etched (fig. 3h) into the copper layer 310 with e.g. FeCl₃, and the remaining photoresist 360 is stripped with e.g. acetone, thereby resulting in the structure shown in fig. 3i.

Fig. 4 illustrates a top view of a structure comprising a patterned graphene layer 420 in accordance with an embodiment of the present invention. A monolayer of graphene 420 is arranged on a side (surface) of a substrate, which may be a 4" glass wafer. The graphene layer 420 is also provided with a pattern 440 defined by an area in which the glass substrate is exposed, and a metal layer comprising e.g. contact pads 410 for electrically connecting the graphene layer 420.

The graphene layer 420 may be patterned by ion etching through a masking metal layer of e.g. copper (not shown). The exposed areas 440 of the graphene layer 420, i.e. the areas not protected by the copper layer, may be subject to the etchant plasma until all, or most, of the carbon in the graphene layer 420 has been consumed. The pattern in the copper layer may be defined in a photolithography process, in which process a photoresist layer (not shown) is formed on the copper layer, pre-exposure baked, UV-exposed, post-exposure baked, and eventually developed. The exposed pattern of the masking layer is then transferred to the copper layer by a wet etching process using e.g. FeCl₃.

After transferring the pattern to the copper layer, the resist may be stripped and the pattern may be further transferred to the graphene layer 420. The remaining contact pads 410 may be defined by the formation of another photoresist layer (not shown) which is patterned with another pattern that may define the contact pads 410 and other structures that can be used for e.g. electrically connecting the graphene structures 420. After a second etching process, in which the exposed metal areas are removed, the remaining photoresist is stripped and a structure such as the one illustrated in fig. 4 is obtained.

Figs. 5a-k outline an alternative method according to another embodiment of the present invention. The steps shown in Figs. 5a-d are similar to the steps described above with reference to Figs, la-d, wherein a copper layer 510 is formed on a graphene layer 520 (fig. 5a), a photoresist layer 530 is formed on the copper layer 510 (fig. 5b), and a pattern 540 is formed by lithography in the photoresist layer 530 (fig. 5c) and transferred to the copper layer 510, thereby resulting in the structure depicted in fig. 5d.

After the etching of the copper layer 510 (fig. 5d), a second metal layer 515 may be formed (fig. 5e) on the photoresist layer 530 and the exposed areas of the graphene layer 520. The second metal may for example comprise gold. The parts of the second metal layer 515 that are formed on the photoresist layer 530 may then be removed (fig. 5f) by stripping the photoresist layer 530 with e.g. acetone. The copper layer 510 is then exposed, while the areas of the graphene layer 520 that correspond to the pattern 540 of the copper layer 510 remain covered by gold (from the second metal layer 515), in accordance with what is shown in fig. 5f.

With reference to fig. 5g, a second photoresist layer 535 may be formed on the copper layer 510 and the gold layer 515. A second pattern 545 may be formed (fig. 5h) in the photoresist layer 535 by e.g. UV-exposure followed by e.g. post-exposure bake and development. The second pattern 545 is then transferred (fig. 5i) to the copper layer by e.g. a selective FeCl₃ etching process that exposes the underlying graphene layer 520. The second pattern 545 may be transferred (fig. 5j) to the graphene layer 520 by a reactive ion etch, using the copper layer 510 as an etch mask.

After the second pattern 545 is transferred to the graphene layer 520, the copper layer may be removed (fig. 5k) by a selective etching process that leaves a remaining gold layer 515 such that a structure as the one illustrated in fig. 5k is obtained. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. In particular, the invention is not limited to the specific values of the parameters provided in the description of the various processes of the manufacturing method. These parameter values are provided for illustrative purposes.

For example, although the invention described with reference to Figs. 1 to 5 comprises a 2D crystal layer of graphene, the present invention is also intended to be applied on 2D crystal layers comprising other materials such as e.g. boron nitride (BN), molybdenum disulfide (M0S₂), and Tungsten(IV) sulfide (WS₂). It will also be appreciated that the etching of the metal layer may not be restricted to wet etching processes, but rather include other processes such as e.g. reactive ion etching. This also applies for the etching of the graphene, with may be obtained by e.g. wet etching.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. It will be appreciated that, in the present invention, there may be provided a plurality of layers and patterns in addition to the metal layer, the masking layer, the 2D crystal layer and the pattern(s) described with reference to Figs. 1 to 5. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A method of patterning a two-dimensional crystal layer, comprising:

forming a metal layer (110) on the two-dimensional crystal layer (120);

forming a masking layer (130) on the metal layer;

forming a pattern (140) in the masking layer;

transferring the pattern formed in the masking layer to the metal layer; and

transferring the pattern formed in the metal layer to the two-dimensional crystal layer.

2. The method of claim 1 , further comprising removing at least a part of the metal layer and the masking layer on the metal layer after transferring the pattern formed in the metal layer to the two-dimensional crystal layer.

3. The method according to claim 1, or 2, wherein the two-dimensional crystal layer (120) comprises or consists of any one of graphene, molybdenum disulfide, boron nitride and tungsten disulfide.

4. The method according to any one of the previous claims, wherein the masking layer (130) is a resist layer.

5. The method according to claim 4, wherein the resist layer is a photoresist layer and the pattern in the masking layer is formed by photolithography.

6. The method according to any one of the preceding claims, wherein the metal layer comprises at least one of gold, copper, nickel, titanium, chromium, aluminium, platinum, palladium, and molybdenum.

7. The method according to any one of the preceding claims, wherein the transferring of the pattern formed in the masking layer to the metal layer comprises etching of the metal with a liquid etchant.

8. The method according to claim 7, wherein the wet etchant is an iron(III) chloride (FeCl₃) solution.

9. The method according to any one of the preceding claims, wherein the transferring of the pattern formed in the metal layer to the two-dimensional crystal layer comprises etching of the two-dimensional crystal layer by reactive ion etching.

10. The method according to any one of the preceding claims, comprising forming another pattern (250) in the masking layer after transferring the pattern formed in the metal layer to the two-dimensional crystal layer.

11. The method according to claim 10, further comprising transferring said another pattern formed in the masking layer to the metal layer.

12. The method according to any one of claims 1-9, comprising forming another masking layer (360) on the metal layer after transferring the pattern formed in the metal layer to the two-dimensional crystal layer.

13. The method according to claim 12, further comprising forming another pattern (350) in said another masking layer.

14. The method according to claim 13, comprising transferring said another pattern formed in said another masking layer to the metal layer.

15. A structure comprising a two-dimensional crystal layer having a pattern, the pattern being obtainable with the method as defined in any one of claims 1-14.

16 A structure comprising a two-dimensional crystal layer on a surface, the two dimensional crystal layer having a pattern such that the surface is covered by the two- dimensional crystal layer in a first region and the surface is not covered by the two dimensional crystal layer in a second region that is different from the first region and wherein the two-dimensional crystal layer has a thickness smaller than or equal to 10 nm.

17 A structure according to claim 15 or 16, wherein the two-dimensional crystal layer comprises any one of: at most five, at most 2 and 1 monolayers of atoms or molecules.

18. The structure of any one of claims 15 to 17, wherein the two-dimensional crystal is vulnerable to light and/or shear stress.

19. A device, comprising a structure according to any one of claims 15 to 18.