Method for forming apparatus comprising two dimensional material

A method and apparatus, the method comprising: forming a layer of two dimensional material (23), in particular graphene, on a first release layer; forming, possibly a (gate) insulating layer (35), and at least two, preferably three, electrodes (25); forming a second release layer overlaying at least a portion of the layer of two dimensional material; providing a mouldable polymer (24, 26, 28) overlaying the at least two electrodes and the second release layer; and removing the first and second release layers to provide a cavity (29) between the mouldable polymer (26) and the layer of two dimensional material (23).

RELATED APPLICATION

This application was originally filed as PCT Application No. PCT/FI2016/050550 filed Aug. 5, 2016 which claims priority benefit from EP Patent Application No. 15182976.9 filed Aug. 28, 2015.

TECHNOLOGICAL FIELD

Examples of the disclosure relate to a method for forming apparatus comprising two dimensional material. In particular, they relate to a method for forming electronic apparatus comprising a two dimensional material such as graphene.

BACKGROUND

Apparatus comprising two dimensional materials such as graphene are well known. For instance graphene can be provided in devices such as resistive sensors or field effect transistors to enable parameters such as chemicals or light to be detected. In other devices graphene field effect transistors can be used as logic elements or other electronic components.

It is useful to provide improved methods of forming such devices.

BRIEF SUMMARY

According to various, but not necessarily all, examples of the disclosure there may be provided a method comprising: forming a layer of two dimensional material on a first release layer; forming at least two electrodes; forming a second release layer overlaying at least a portion of the layer of two dimensional material; providing mouldable polymer overlaying the at least two electrodes and the second release layer; and removing the first and second release layers to provide a discontinuity between the mouldable polymer and the layer of two dimensional material.

In some examples the discontinuity may set the layer of two dimensional material and the mouldable polymer apart from each other.

In some examples the method may comprise providing a dielectric overlaying at least a portion of the layer of two dimensional material.

In some examples the second release layer may be formed so that at least part of the second release layer contacts the first release layer.

In some examples the method may comprise providing an inert material in the discontinuity.

In some examples the method may comprise providing nitrogen in the discontinuity.

In some examples the layer of two dimensional material and the at least two electrodes may form at least part of a field effect transistor. The field effect transistor may comprise a gate electrode and the gate electrode may be provided adjacent to the layer of two dimensional material. The field effect transistor may comprise a gate electrode and the gate electrode may be provided overlapping at least part of the layer of two dimensional material.

In some examples the method may comprise providing a rigid portion aligned with the discontinuity.

In some examples the two dimensional material may comprise graphene.

In some examples the method may comprise activating the layer of two dimensional material.

In some examples the method may comprise activating the layer of two dimensional material with quantum dots.

According to various, but not necessarily all, examples of the disclosure there may be provided an apparatus formed by any of the methods described above.

According to various, but not necessarily all, examples of the disclosure there may be provided an apparatus comprising: a layer of two dimensional material and at least two electrodes wherein the layer of two dimensional material was formed on a first release layer;mouldable polymer overlaying the at least two electrodes; and a discontinuity between the mouldable polymer and the layer of two dimensional material wherein the discontinuity is formed by a second release layer which has been removed.

In some examples the discontinuity may set the layer of two dimensional material and the mouldable polymer apart from each other.

In some examples the apparatus may comprise a dielectric overlaying at least a portion of the layer of two dimensional material.

In some examples the second release layer may be formed so that at least part of the second release layer contacts the first release layer.

In some examples the apparatus may comprise an inert material in the discontinuity.

In some examples the apparatus may comprise nitrogen in the discontinuity.

In some examples wherein the layer of two dimensional material and the at least two electrodes may form at least part of a field effect transistor. The field effect transistor may comprise a gate electrode and the gate electrode may be provided adjacent to the layer of two dimensional material. The field effect transistor may comprise a gate electrode and the gate electrode may be provided overlapping at least part of the layer of two dimensional material.

In some examples the apparatus may comprise a rigid portion aligned with the discontinuity.

In some examples the two dimensional material may comprise graphene.

In some examples the layer of two dimensional material may be activated. The layer of two dimensional material may be activated with quantum dots.

According to various, but not necessarily all, examples of the disclosure there is provided examples as claimed in the appended claims.

DETAILED DESCRIPTION

The Figures illustrate example methods and apparatus21. The methods may be used to form apparatus21comprising two dimensional material. The apparatus21may form electronic components within electronic devices or parts of electronic components within electronic devices. In some examples the apparatus21which are formed may be for sensing. The apparatus21may be for sensing environmental parameters such as light, temperature, chemicals or other parameters.

FIG. 1illustrates a method according to examples of the disclosure. The method may be used to form apparatus21comprising one or more electronic components. The electronic components may comprise a two dimensional material such as graphene.

The method comprises, at block11, forming a layer of two dimensional material23on a first release layer33. The method also comprises, at block13, forming at least two electrodes25and, at block15, forming a second release layer37overlaying at least a portion of the layer of two dimensional material23. At block17the method comprises providing mouldable polymer27overlaying the at least two electrodes25and the second release layer37. At block19the method comprises removing the first and second release layers33,37to provide a discontinuity29between the mouldable polymer27and the layer of two dimensional material23.

It is to be appreciated that the electrodes25and the layer of two dimensional material23may have any configuration which enables an electronic component to be formed. An example method for forming field effect transistor (FET) devices is illustrated in more detail inFIGS. 3A to 3I. Other methods for forming FETs and other types of devices may be used in other examples of the disclosure.

FIG. 2illustrates a cross section of an example apparatus21which may be formed using methods such as the method ofFIG. 1. The apparatus21comprises a layer of two dimensional material23and at least two electrodes25. The layer of two dimensional material23was formed on a first release layer33. The apparatus21also comprises mouldable polymer27overlaying the at least two electrodes25and a discontinuity29between the mouldable polymer27and the layer of two dimensional material23. The discontinuity29is formed by a second release layer37which has been removed.

The layer of two dimensional material23may comprise a very thin layer of material. In some examples the layer of two dimensional material23could be an atomic monolayer. In some examples the layer of two dimensional material23could comprise several atomic monolayers. The layer of two dimensional material23could comprise graphene, molybdenum disulphide, boron nitride or any other suitable material.

The layer of two dimensional material23may have been formed on the surface of a first release layer33. This may enable the other components of the apparatus21to be formed around the layer of two dimensional material23.

In some examples the surface of the first release layer33may be smooth so that once the first release layer33has been removed this leaves a smooth layer of two dimensional material23. As the first release layer33provides a smooth flat surface for forming the layer of two dimensional material23this reduces the amount of discontinuities and/or impurities in the layer of two dimensional material23and may provide for improved charge transfer characteristics of the two dimensional material23.

In the example apparatus21ofFIG. 2the layer of two dimensional material23is provided between the at least two electrodes25. The at least two electrodes25and the layer of two dimensional material23are arranged to form an electronic device or at least part of an electronic device. The example apparatus21ofFIG. 2could form a resistive sensor or other electronic device. It is to be appreciated that other electrodes or components could be comprised within the apparatus21to enable other electronic devices to be formed. For instance, in some examples the apparatus21could comprise source drain and gate electrodes which could be arranged around the layer of two dimensional material23to form a channel within an FET.

The electrodes25may comprise any suitable conductive material. The electrodes25may be electrically connected to the layer of two dimensional material23. The electrodes25may be electrically connected to the layer of two dimensional material23to enable direct current to flow through the electrodes25and the layer of two dimensional material23.

The electrodes25may be formed on the same first release layer33as the layer of two dimensional material23. This may enable the electrodes25and the layer of two dimensional material23to be provided in the same plane. This reduces the number discontinuities such as step edges in the apparatus21which may provide for improved charge transfer characteristics of the two dimensional material23.

The apparatus21also comprises a mouldable polymer27. The mouldable polymer27may form a thin substrate. The thin substrate may support at least some of the components of the apparatus21.

The mouldable polymer27is provided overlaying at least the two electrodes25. The mouldable polymer27may be deposited overlaying the electrodes25while the electrodes25and layer of two dimensional material23are still positioned on the first release layer33. Before the mouldable polymer27is deposited a second release layer37may be positioned overlaying at least part of the layer of two dimensional material23. The second release layer37may be arranged to prevent the mouldable polymer27from contacting the layer of two dimensional material23.

The mouldable polymer27may comprise any polymer material which is fluid enough to embed the electrodes25and layer of two dimensional material23. Once the mouldable polymer27is provided around the electrodes25and the second release layer37the mouldable polymer27may be cured or otherwise hardened. Once the mouldable polymer27has hardened it may form a substrate for the at least two electrodes25. Once the mouldable polymer27has hardened the second release layer37may be removed.

The mouldable polymer27has different thicknesses across the cross section of the apparatus21. In the example ofFIG. 2the mouldable polymer27comprises a first portion24, a second portion26and a third portion28. The first portion24supports a first electrode25and the second portion supports a second electrode25. The first and second portions24,26of the mouldable polymer27may contact the electrodes25. The first and second portions24,26of the mouldable polymer27may have the same thickness.

The third portion28of the mouldable polymer27extends between the first and second portions24,26. The third portion28has a different thickness to the first and second portions24,26. The third portion28is thinner than the first and second portions24,26so that the third portion28does not contact the electrodes25or the layer of two dimensional material23. This creates a discontinuity29between the mouldable polymer27and the layer of two dimensional material23. The discontinuity29sets the layer of two dimensional material23and the mouldable polymer27apart from each other.

The discontinuity29may provide a cavity within the substrate formed by the mouldable polymer27. In some examples a material may be comprised within the cavity. In some examples the material within the cavity may comprise a fluid such as a gas. The fluid or gas may prevent the layer of two dimensional material23from contacting other layers of the apparatus21.

In some examples a material within the cavity may be arranged to prevent contamination of the layer of two dimensional material23. In some examples the material may comprise nitrogen or an inert material which may be arranged to prevent the layer of two dimensional material23from reacting with contaminants.

In some examples a vacuum could be provided within the discontinuity29. The vacuum could be arranged to protect the layer of two dimensional materials from contaminants.

It is to be appreciated that the apparatus21could comprise other components that are not illustrated inFIG. 2. For instance the apparatus21could comprise layers of dielectric or additional electrodes25or contacts.

FIGS. 3A to 3Iillustrate example methods which may be used to form example apparatus21.

InFIG. 3Aa first release layer33is provided on a carrier substrate31. In the example ofFIG. 3Athe carrier substrate31may provide a rigid or substantially rigid substrate which may provide support while the layer of two dimensional material23and/or other components of the apparatus21are being fabricated. The carrier substrate31may comprise a silicon wafer or any other suitable material. The carrier substrate31may be flat or substantially flat.

The first release layer33is provided overlaying the carrier substrate31. The first release layer33may comprise a sacrificial layer which may enable the components of the apparatus21such the layer of two dimensional material23to be removed from the carrier substrate31. The material that is used for the first release layer33may depend on the components that are being fabricated and the materials that are being used for those components. In some examples the first release layer33may comprise copper, nickel or any other suitable material. Using a material such as copper or nickel for the first release layer33may enable the layer of two dimensional material23to be grown on the first release layer33.

InFIG. 3Ba layer of two dimensional material23is deposited onto the first release layer33. In the example ofFIGS. 3A to 3Gthe two dimensional material23comprises graphene. Other two dimensional materials may be used in other examples of the disclosure.

In some examples the graphene may be grown directly on the first release layer33. This may avoid having to transfer the graphene between different substrates. This may reduce stresses and/or other defects within the graphene and provide improved charge carrier characteristics.

In other examples the graphene may be deposited on the first release layer33using chemical vapour deposition, a wet transfer process, a dry transfer process or any other suitable process.

The graphene may be patterned on the first release layer33in order to provide the correct channel dimensions for the apparatus21.

InFIG. 3Ca dielectric35is deposited over the graphene. The dielectric35may be provided in a thin layer. The dielectric35may be deposited so that it completely covers the graphene. In the example ofFIG. 3C, where the graphene is still attached to the first release layer33, the first release layer33and the dielectric35completely envelop the graphene.

The dielectric35may be arranged to provide an electrically insulating layer between the graphene and one or more of the electrodes25or other components of the apparatus21. The dielectric35may comprise any suitable electrically insulating material. In some examples the dielectric35may comprise aluminum oxide which could be deposited using atomic layer deposition or any other suitable process. In some examples the dielectric35may comprise a polymer or any other suitable material.

In some examples the dielectric35may also be arranged as a barrier to protect the graphene from contamination. For instance the dielectric35could comprise a material which is impermeable, or partially impermeable to contaminants such as water or oxygen or anything else which could contaminate or affect the charge carrier characteristics of the graphene.

In some examples the graphene may be activated before the dielectric35is deposited. The activation of the graphene may counteract the low surface energy of the graphene and may enable uniform deposition of the dielectric35over the graphene. For instance, a seed layer may be evaporated onto the graphene to enable the atomic layer deposition of the dielectric35.

InFIG. 3Da plurality of electrodes25are deposited onto the first release layer33. In the example ofFIGS. 3A to 3Gthe electrodes25comprise source, drain and gate electrodes. The electrodes25are deposited so that at least part of the electrodes25extend over the dielectric35. The source and drain electrodes25are deposited so that at least part of the source and drain electrodes25are in direct contact with the surface of the first release layer33. This may enable contacts to be provided between the electrodes25and the graphene once the first release layer33has been removed.

In the example ofFIG. 3Dthe electrodes25are formed so that at least the source and drain electrodes25are in the same plane as the graphene. This reduces the number of step edges in the apparatus21which may provide for improved charge mobility.

The electrodes25may be formed using any suitable technique. For instance, in some examples the electrodes25may be formed by photolithography followed by evaporation of the electrode material or by evaporating through a shadow mask or any other suitable technique.

InFIG. 3Ea second release layer37is formed. The second release layer37is formed overlaying at least a part of layer of two dimensional material23. In the example ofFIG. 3Ethe second release layer37is formed overlaying both the dielectric35and the graphene. In such examples the dielectric35may act as a protective barrier for the graphene. In the example ofFIG. 3Ethe second release layer37also overlays at least some of the electrodes25.

The second release layer37may be formed so that the second release layer37can be removed with the first release layer33. The second release layer37may be formed so that least part of the second release layer37contacts the first release layer33.

The second release layer37may be formed using any suitable technique. For instance, in some examples the second release layer37may be formed by photolithography followed by evaporation of the release layer material37, by evaporating through a shadow mask, by using a liftoff process using a photo resist mask or any other suitable technique.

The second release layer37may be formed from any suitable material. The second release layer37may be formed form a material which will not react or otherwise contaminate the graphene. The second release layer37may comprise a material which can be removed by chemical etching or any other suitable technique. In some examples the second release layer37may comprise the same material as the first release layer33.

InFIG. 3Fmouldable polymer27is provided overlaying the layer of graphene, the electrodes25, the dielectric35and the second release layer37. The mouldable polymer27is deposited on the first release layer33overlaying the layer of graphene, the electrodes25, the dielectric35and the second release layer37. The mouldable polymer27comprises any polymer which will embed the components of the apparatus21. The mouldable polymer27may form a planar surface against the surface of the first release layer33.

The second release layer37may be positioned so as to prevent the mouldable polymer27from contacting the layer of graphene. In the example ofFIGS. 3A to 3Ithe second release layer37prevents the mouldable polymer27from coming into contact with the dielectric35. The second release layer37may provide a barrier between the mouldable polymer27and the layer of graphene and/or the dielectric35. The second release layer37may be positioned so that the mouldable polymer27forms different portions24,26,28of different thicknesses. The different portions24,26,28of different thicknesses may be as described above.

In some examples the mouldable polymer27may comprise a liquid polymer which may be deposited onto the first release layer33via spin coating, spray coating or any other suitable process. In other examples the mouldable polymer27may comprise a polymer foil which may be deposited by hot embossing or any other suitable process.

InFIG. 3Gthe carrier substrate31and the first release layer33and second release layer37are removed. The mouldable polymer27may be hardened or cured before the carrier substrate31and release layers33,37are removed so that the mouldable polymer27provides a substrate for the apparatus21.

The first release layers33and the second release layers37may be removed simultaneously. The release layers33,37may be removed using any suitable technique such as chemical etching.

InFIG. 3Hcontacts39are provided between the source and drain electrodes25and the graphene. The contacts39may provide a direct current path between the source and drain electrodes25and the graphene. The contacts39may comprise any conductive material, such as a metal, which may be deposited between the electrodes25and the graphene. The contacts39may enable the graphene and electrodes25to form an FET.

The contacts39may be deposited using photolithography, metal evaporation or any other suitable process. In some examples the apparatus21may be treated with argon plasma before the contacts39are deposited to reduce the resistance of the contacts39.

InFIG. 3Ithe graphene is activated. The activation of the graphene may enable the FET to be used as a sensor. In the example ofFIG. 3Ithe graphene is activated with quantum dots41. The material that is used to activate the graphene may depend on the parameters that the FET is intended to detect. In some examples the graphene might not be activated.

In the examples ofFIGS. 3A to 3Ithe apparatus21comprises a bottom gate FET. It is to be appreciated that the methods could be modified to provide an apparatus21comprising a top gate FET. For instance the gate electrode of the top gate FET could be deposited on the surface of the moldable polymer27after the first release layer33has been removed. Other apparatus21could be formed using other similar methods.

FIG. 4is a plan view of another example apparatus21. The example apparatus ofFIG. 4may be formed using the methods described above. The apparatus21comprises a layer of two dimensional material23, a plurality of electrodes25moldable polymer27and a discontinuity29which may be as described above. Corresponding reference numerals are used for corresponding features. The plurality of electrodes25and the layer of two dimensional material23are arranged to form an FET.

The layer of two dimensional material23is provided spanning the discontinuity29. A first end51of the layer of two dimensional material23is supported by the moldable polymer27on a first side of the discontinuity29. A second end53of the layer of two dimensional material23is supported by the moldable polymer27on a second side of the discontinuity29. The first side may be opposite to the second side. A middle portion55is provided between the two ends51,53of the layer of two dimensional material23. The middle portion55of the layer of two dimensional material extends over the discontinuity29so that the middle portion of the layer of two dimensional material23is set a distance apart from the moldable polymer27.

In the example ofFIG. 4a dielectric35layer is provided underneath the layer of two dimensional material23. In some examples the dielectric35may be arranged to support the layer of two dimensional material23across the discontinuity29.

InFIG. 4the dielectric35is provided spanning the discontinuity29. A first end52of the layer of dielectric35is supported by the moldable polymer27on the first side of the discontinuity29. A second end54of the dielectric35is supported by the moldable polymer27on the second side of the discontinuity29. The first side may be opposite to the second side. A middle portion56is provided between the two ends52,54of the dielectric35. The middle portion56of the dielectric35extends over the discontinuity29so that the middle portion of the dielectric35is set a distance apart from the moldable polymer27.

The apparatus21also comprises a source electrode45and a drain electrode47. The source electrode45and drain electrode47may be supported by the mouldable polymer27so that the mouldable polymer27bears the weight of both the source electrode45and the drain electrode47.

The source electrode45is provided on the first side of the discontinuity29and may be arranged to be connected to the first end51of the layer of two dimensional material23. The drain electrode47is provided on the second side of the discontinuity29and is arranged to be connected to the second end53of the layer of two dimensional material23.

The apparatus21also comprises a gate electrode43. In the example ofFIG. 4the gate electrode overlaps a portion of the layer of two dimensional material23. In the example ofFIG. 4a portion of the gate electrode43extends across the discontinuity29underneath the middle portion56of the dielectric35and the middle portion55of the layer of two dimensional material23. The portion of the gate electrode43which extends underneath the layer of two dimensional material23is indicated by the dashed line57inFIG. 4. The dielectric35may insulate the layer of the two dimensional material23and the gate electrode43.

In the example ofFIG. 4at least part of the gate electrode43may be supported by the moldable polymer27. The discontinuity29may have a U-shape so that the portion of the gate electrode43which is not overlapping with the layer of two dimensional material23is provided on the moldable polymer27. The moldable polymer27may extend up to the edge of the layer of two dimensional material23to reduce bending forces on the gate electrode43.

FIG. 5is a plan view of another example apparatus21. The example apparatus ofFIG. 5is similar to the example apparatus21ofFIG. 4and may also be formed using the methods described above. The apparatus21ofFIG. 5comprises a layer of two dimensional material23, a plurality of electrodes25moldable polymer27and a discontinuity29which may be as described above. Corresponding reference numerals are used for corresponding features. The plurality of electrodes25and the layer of two dimensional material43are arranged to form an FET.

InFIG. 5the apparatus21comprises a layer of two dimensional material23which spans across a discontinuity29as inFIG. 4. The apparatus21also comprises a source electrode45and a gate electrode47which may also be as inFIG. 4.

However inFIG. 5the gate electrode43does not extend underneath the layer of two dimensional material23. InFIG. 5the gate electrode43is positioned adjacent to the middle portion55of the layer of two dimensional material23. The gate electrode43is positioned close to, but not touching, the middle portion55of the layer of two dimensional material23.

In the example ofFIG. 5the whole of the gate electrode43may be supported by the moldable polymer27. The discontinuity29may have a U-shape so that the gate electrode43can be provided on the moldable polymer27and positioned adjacent to the layer of two dimensional material23. The moldable polymer27may extend up to the edge of the layer of two dimensional material23to enable the gate electrode43to be provided close to but not touching the layer of two dimensional material23.

In the example apparatus21ofFIG. 5no dielectric35is provided supporting the layer of two dimensional material23. In the example ofFIG. 5air, nitrogen or other insulating fluid may be provided within the cavity formed by discontinuity29. The air or other fluid may act as a dielectric and provide electrical insulation between the gate electrode43and the layer of two dimensional material23.

It is to be appreciated that other components may be provided within the apparatus which are not illustrated inFIGS. 1 to 5. For instance, in some examples a rigid portion may be provided within the apparatus21. The rigid portion may be aligned with the discontinuity29and may be arranged to prevent the discontinuity29from bending or otherwise deforming. This may help to prevent damage being caused to the layer of two dimensional material23and other components within the apparatus21.

Examples of the disclosure provide methods of forming apparatus21comprising two or more electrodes25and a channel of two dimensional material23. Providing the discontinuity29between the moldable polymer27substrate and the layer of two dimensional material23prevents the moldable polymer27from deforming or otherwise contaminating the layer of two dimensional material23. This may reduce the number of defects within the layer of two dimensional material23which increases carrier mobility within the layer of two dimensional material23and provides for an improved apparatus21.

In some examples the discontinuity29may comprise a gas or other fluid which may be configured to prevent contamination of the layer of two dimensional material23. This may further reduce defects within the layer of two dimensional material23.

Examples of the disclosure may enable the apparatus21to be formed around the layer of two dimensional material23. This may avoid the need to transfer the layer of two dimensional material23which will reduce the chance of the layer of two dimensional material23being damaged or otherwise contaminated. As the layer of two dimensional material23does not need to be transferred between different substrates this may reduce stresses within the layer which could be formed as the layer is removed from and attached to different substrates. This may also reduce the number of defects within the layer of two dimensional material23

The methods of the disclosure may enable large numbers of apparatus21to be produced at low costs. The method may be fast as processes such as curing may only take several seconds to be completed. The method may avoid the use of high temperatures which could damage sensitive components. For instance the thermosetting resins may be set at temperatures of 200° C. which may be low enough to avoid damaging other components of the apparatus21.

In the above description the term “coupled” means operationally coupled. Any number of intervening components may be provided including no intervening components.

The term “comprise” is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use “comprise” with an exclusive meaning then it will be made clear in the context by referring to “comprising only one . . . ” or by using “consisting”.

Although examples of the disclosure have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Although features have been described with reference to certain examples, those features may also be present in other embodiments whether described or not.