WO2014090938A1

WO2014090938A1 - Conductive atomic force microscope tips coated with graphene

Info

Publication number: WO2014090938A1
Application number: PCT/EP2013/076362
Authority: WO
Inventors: Huiling DUAN; Marc PORTI PUJAL; Mario LANZA MARTINEZ; Albin BAYERL; Montserrat NAFRÍA MAQUEDA
Original assignee: Universitat Autonoma De Barcelona; Peking University
Priority date: 2012-12-14
Filing date: 2013-12-12
Publication date: 2014-06-19

Abstract

The present invention relates to coating with a graphene single layer film the conductive surface of an atomic force microscope tip. The process of coating the conductive surface with graphene consists in three steps: first, immobilization of the tip on a Silicon block as a base using a thin film of Poly-methyl methacrylate (PMMA) in between. Then, the solid PMMA/AFM tip/PMMA/Silicon block was used as target substrate on which it is transferred the graphene sheet. Finally, it is removed the different PMMA layers using Acetone. Once PMMA is removed, the graphene is completely attached. The resulting tip is perfectly coated with graphene, and therefore much more resistant to both high currents and frictions than commercially available metal-varnished CAFM tips, and also to much larger lifetimes and more reliable imaging due to a lower tip-sample interaction.

Description

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CONDUCTIVE ATOMIC FORCE MICROSCOPE TIPS COATED WITH GRAPHENE

DESCRIPTION OBJECT OF THE INVENTION

The present patent application is related to conductive atomic force microscope tips, and has remarkable advantages. More specifically, the present invention relates to coating with a graphene single layer film the conductive surface of an atomic force microscope tip, leading to a much more resistant to both high currents and frictions than commercially available metal-varnished CAFM tips, and also to much larger lifetimes and more reliable imaging due to a lower tip-sample interaction.

BACKGROUND OF THE INVENTION

It is known from the Prior Art that electrical characterization at the nanoscale is an essential procedure for analyzing the performance of many materials used at both industry and academia. In this field, one of the most powerful tools is the Conductive Atomic Force Microscope (CAFM), which can characterize the electrical properties of both conductive and thin insulating materials at areas as small as 3 nm². The main challenge associated with this technique is the poor reliability of the conductive tips. Traditionally, metal-varnished silicon tips are commonly utilized in these kinds of applications. However, due to the low stability of the metallic varnish, these tips can wear out very fast when measuring high currents and/or because of intense tip-sample frictions. The low reliability of the tips results in false imaging and unnecessarily high costs of measurements. A good approach to preserve CAFM tip properties is to varnish them with a very stable material, such as doped diamond. However, this approach not only reduces the lateral resolution of measurement (due to a larger tip radius), but also increases the price of the tips. Moreover, the same happens when using solid metallic AFM tips. Therefore, finding a new method to avoid fast tip wearing is essential for cheap and reliable nanoscale electrical characterization.

On the other hand, graphene (a two-dimensional sheet of carbon atoms arranged in a dense honeycomb lattice) is a material that has excellent mechanical, physical and electronic properties. In the beginning, the only known method to obtain graphene was from mechanical exfoliation (repeated peeling) of small mesas of highly oriented pyrolytic graphite, and it was mainly used as a conductive channel in Field Effect Transistors. The new method to produce graphene by Chemical Vapor Deposition (CVD) is a revolutionary approach for graphene growth and manipulation, since, by CVD, graphene can not only be grown on different substrates but is also transferrable to arbitrary substrates. This method extends the use of graphene to other applications, for example commercially available CAFM tip being coated with a sheet of CVD-grown graphene.

It is also known multilayer graphene sheets directly grown on Au-varnished CAFM tips by a CVD process. In our case, graphene single layer (GSL) sheets were grown on copper foils by CVD (an already known process) and directly transferred onto different Pt-lr varnished CAFM tips. The resulting probe has been used to perform nanoscale electrical characterization of both conductive and thin insulating materials. Our graphene-coated tips are shown to be extremely stable and resistant even under high currents and frictions, leading to longer device lifetime, and the tips can also protect the material under test from interaction with the tip varnish, which could lead to false imagining.

It is also known what the CN102353817 patent describes, which is the manufacturing and use of conductive AFM probes made of Ni and graphene layers. The nickel layer can be made by thermal evaporation or sputtering and the graphene layer is made by Chemical Vapor Deposition (CVD). The graphene layer can be a multi or single layer. There is no disclosure about a particular process involving polymeric intermediate layers dissolved or etched by acetone, in liquid or in vapor state.

It is also known from the state of the art deposition technologies for coating SiNx cantilevers, or Si tips. However no document of the state of the art discloses entirely the graphene layer deposition process of the invention.

It could be considered also known by the person skilled in the art the existence of acetone as a solving agent of the Poly-methyl methacrylate (PMMA), and also the technique of transferring the PMMA/graphene layer on a AFM tip. However it could not considered as obvious the immobilization process steps of the AFM tips for a proper graphene layer deposition, and also the effective and simultaneous dissolving operation of the intermediate polymeric layers so that a graphene coated tip is safely obtained, ready for its use. Also the result of this manufacturing process, a conductive AFM probe consisting of a Pt-lr coated silicon tip coated with a graphene single layer, would not be disclosed, neither would be obvious in the light of the state of the art. This conductive Pt-lr alloy external layer would offer better properties versus the state of the art (gold-coated silicon AFM-tip coated with a graphene multilayer by CVD, or totally metallic tip) in terms of economic cost, and in terms of dimensions variation respect the original shape of the commercial tip. DESCRIPTION OF THE INVENTION

The present invention has been developed with the aim of improving the performance of the conductive atomic force microscope tips, expecting that such invention will be a novelty in the field of the application of such technology, resolving the current disadvantages mentioned above, and presenting further additional advantages that will be evident from the following description.

It is therefore an object of the present invention to develop a coating process over the conductive surface of an atomic force microscope tip with a graphene single layer film. The resulting tip will be totally coated with graphene, and therefore much more resistant to both high currents and frictions than commercially available metal-varnished CAFM tips, and also to much larger lifetimes and more reliable imaging due to a lower tip-sample interaction.

More specifically, the invention is related to a graphene coating process of a probe for an atomic force microscope comprising a step of transference of a graphene layer from a metal substrate to a polymeric substrate, further comprising the steps of:

- coating a base by a first polymeric layer

- probe attachment over said first polymeric layer

- coating the probe by a second polymeric layer

- deposition of said polymeric substrate with a graphene layer over said second polymeric layer

- dissolving all the polymeric layers by a solvent agent such that the graphene layer is attached to the surface of the probe. In this way all the polymeric layers that take part on the process (the fixing layer of the probe to the base, the intermediate layer between the probe and the graphene layer, and the substrate of the graphene layer) becomes simultaneously dissolved and by the same solvent agent, leaving the probe ready for its use. On the other hand the polymeric layers work as holding means during the process, immobilizing the probe for the purpose of a proper deposition of the graphene layer on the probe's external surface. Preferably the graphene layer is a single atomic sheet. In this way the additional layer minimizes the dimensional variation of the probe tip. According to another aspect of the invention, the coating of the base by a first polymeric layer is carried out by a spin coating technique, which has the advantage of producing a uniform thin film (thicknesses -200 nm) in flat substrates. This is achieved rotating at high speed the substrate in order to spread the fluid place on it by centrifugal force. With this technique it is possible to dimension very precisely the thicknesses of this first polymeric layer, which is useful in terms to define the time necessary to be solved, in relation to what it is needed with the other polymeric layers of the sample structure. More specifically, the probe comprises a support, a cantilever, and a tip, and it is the bottom surface of said support the part attached to the first polymeric layer. The right moment to do so is before it completely dries, such that the probe stays along the whole process in a substantially horizontal position. In this way the risk of a probe fall and breakage during the process is minimized. At the end the probe will rest over the base till it is picked up. Keeping the horizontal position facilitates also the union between the graphene layer and the upper surface of the probe.

According to another aspect of the invention the coating of the probe by a second polymeric layer may be carried out specifically by a drop casting technique, which has the advantage the polymer fills the gap under the cantilever and the tip.

Advantageously, the deposition of said polymeric substrate with a graphene layer over said second polymeric layer is carried out typically inside water, or another similar inert liquid. More specifically the graphene layer (still attached its substrate polymeric layer) rests over the water surface in a recipient. The probe, already covered with the intermediate polymeric layer, will be manipulated to come up from inside of the liquid and to pick up the graphene layer with its substrate polymeric layer attached.

More specifically, the polymer of the polymeric layers is Poly-methyl methacrylate (PMMA) due to its suitable properties for this process. The solvent agent is acetone also due to its suitable properties for this process.

According to another aspect of the invention the dissolving step is carried out inside a camera with acetone steam, and more particularly over a boiling acetone container. Being exposed the structure Graphene/PMMA/AFM tip/PMMA/Base just to vaporized acetone, all the PMMA layers will be dissolved gradually and simultaneously without losing a substantive horizontal position, and therefore avoiding its slippage into the boiling acetone. Once finalized this process step, the graphene coated probe will rest on the base being ready to be easily picked up. Just to mention additionally that the camera serves as vapor enclosure to increase etching yield and constant etching on the entire sample. Preferably the camera remains open on the top to avoid that drops of condensed acetone precipitate on the sample. The dissolving step is carried out over a period of approximately 30 minutes to assure the smoothness of the etching process, and to avoid falls and breakages of the probe.

According to another aspect of the invention, the graphene coated probe itself, as result of the described process above, comprises a support, a cantilever, and a tip, including, at least the tip, a silicon substrate varnished with a conductive layer of a Pt-lr alloy. Said probe, Pt-lr alloy further coated with a graphene layer, is cheaper than probes with other materials (as, for example, doped diamond) as conductive layer, and also cheaper than fully metallic AFM tips. More specifically, the graphene layer is a single atomic sheet, not having the probe tip any dimensional variation after being graphene coated.

Other characteristics and advantages of the graphene coating process of conductive Atomic Force Microscope tips, object of the present invention, will be evident from the preferred embodiment description, which it is shown in the enclosed drawings and described below.

DESCRIPTION OF THE DRAWINGS

With the aim of complementing the description that follows, and of facilitating understanding of the characteristics of the invention, and according to an example of a practical preferred embodiment thereof, this description is accompanied by a set of drawings which include, but are not limited to, the following features:

FIG. 1 is a schematic view for illustrating an as-received commercial Pt-lr varnished probe; FIG. 2 is a schematic view for illustrating an as-grown GSL on both sides of a Cu foil;

FIG. 3 is a schematic view for illustrating a sample of FIG. 2 covered on one side with spin coated PMMA;

FIG. 4 is a schematic view for illustrating a sample of FIG. 3 wherein the bottom GSL and the Cu foil are etched;

FIG. 5 is a schematic view for illustrating a base, typically a piece of Silicon, covered with spin coated PMMA;

FIG. 6 is a schematic view for illustrating a sample of FIG. 5 wherein the tip is attached on a heated first layer of PMMA in order to fix the structure;

FIG. 7 is a schematic view for illustrating a sample of FIG. 6 wherein the AFM tip / PMMA / Si structure is again coated with PMMA;

FIG. 8 is a schematic view for illustrating the Graphene/PMMA stack of FIG. 4 being transferred onto the sample of FIG. 7;

FIG. 9 is a schematic view for illustrating a sample of FIG. 8 with the PMMA removed using Acetone and maintaining the tip completely horizontal.

FIG. 10 is a schematic view for illustrating a home-made engine to remove all the PMMA layers while maintaining the sample completely horizontal.

PREFERRED EMBODIMENT OF THE INVENTION

As shown in the referred drawings, and according to the included numeric references, it can be seen a preferred embodiment of the invention, which comprises the parts, elements and steps that are described in detail below.

One preferred implementation of the graphene coating process of a probe (1 ) for an atomic force microscope comprises a step of transference of a graphene layer (4) from a metal substrate (9) to a polymeric substrate (33), further comprising the steps of:

- coating a base (2) by a first polymeric layer (31 )

- probe (1 ) attachment over said first polymeric layer (31 )

- coating the probe (1 ) by a second polymeric layer (32)

- deposition of said polymeric substrate (33) with a graphene layer (4) over said second polymeric layer (32)

- dissolving all the polymeric layers (31 , 32, 33) by a solvent agent (5) such that the graphene layer (4) is attached to the surface of the probe (1 ), being preferably the graphene layer (4) a single atomic sheet.

More particularly, the coating of the base (2) by a first polymeric layer (31 ) is carried out by a spin coating technique. The probe (1 ) comprises a support (13), a cantilever (12), and a tip (1 1 ), and in that the bottom surface of said support (13) is attached to the first polymeric layer (31 ) before it completely dries, such that the probe (1 ) stays in a substantially horizontal position. More specifically, the conditions of this part of the process are:

The solid PMMA/AFM tip/PMMA/Silicon block was created by:

i) the Silicon substrate was spin-covered with 200 nm of PMMA, (spinner working at 1000 rpm during 1 minute);

ii) before the PMMA dries, the AFM tip is attached and cured the whole structure for 10 minutes at 130 °C on a hot plate to immobilize it.

Then, the AFM tip/PMMA/Silicon is again spin coated with 200 nm PMMA using the spinner and a hot plate. Graphene over Cu is commercially available, but one possible growth process consists of: i) Load the cut Cu foil (25 μπΊ, Alfa Aesar) into the 25 mm inner tube, flush the chamber with 400 seem Ar and 50 seem H2 for 5min, close the gas flows and then pump the chamber down to 1 mTorr;

ii) Introduce 10 seem H₂ into the system, heat the chamber to the growth temperature (1000°C) and anneal the Cu foil for 30 min at this temperature to enlarge the Cu grains, remove residual copper oxide and impurities;

iii) Introduce 20 seem methane gas into the chamber, perform the synthesis for 15 min and then close the methane flow;

iv) Cool the samples down to room temperature with protection of 10 seem H₂ flow.

The transfer process of the Graphene on the solid PMMA/AFM tip/PMMA/Silicon block was developed by

i) covering the GSL/Cu stack with 200 nm PMMA media using the spinner;

ii) etching the Copper using FeCI₃;

iii) washing the PMMA GSL in HCI;

iv) washing the PMMA GSL in pure water;

and v) picking up the PMMA/GSL stack with the target substrate, which in our case was the PMMA/AFM tip/PMMA/Silicon structure Alternatively, the coating of the probe (1 ) by a second polymeric layer (32) is carried out by a drop casting technique.

As another aspect of the invention the deposition of said polymeric substrate (33) with a graphene layer (4) over said second polymeric layer (32) is carried out inside water.

More particularly, the polymer of the polymeric layers (31 , 32, 33) is PMMA, and the solvent agent (5) is acetone, being the dissolving step carried out inside a camera (6) with acetone steam (52), and more specifically over a boiling acetone container (7), lasting this dissolving step over a period of 30 minutes.

More specifically, the conditions of this part of the process are:

As bad etching would result in PMMA contamination that could reduce the performance of the tips (even if they would be effectively etched by annealing the sample at 200°C-400°C during 2 hours). Commercially available acetone is heated up to its boiling point of 68°C. In order to ensure a stable and continuous bubbling of the acetone a piece of clean silicon is dropped into the acetone container (7). To avoid tip contamination by residues left over in the acetone and carefully remove the PMMA, the sample is put on top of the inner glass container (7) and therefore exposed just to vaporized acetone. As the entire PMMA is etched away, the AFM tip / substrate system has to be kept horizontal otherwise it could easily fall into the acetone. The camera (6) serves as vapor enclosure to increase etching yield and constant etching on the entire sample. Camera (6) remains open on the top to avoid that drops of condensed acetone precipitate on the sample.

As result of the preferred implementation of the described process of graphene coating over conductive Atomic Force Microscope tips, it is obtained a probe (1 ) for an atomic force microscope comprising a support (13), a cantilever (12), and a tip (1 1 ), characterized in that at least the tip (1 1 ) includes a silicon substrate (81 ) varnished with a conductive layer (82) of a Pt-lr alloy, and further coated with a graphene layer (4), being preferably said graphene layer (4) a single atomic sheet. The commercial tips are made by silicon micromachining and are coated first with a 20 nm thick layer of Pt-lr, which is a 95% Platinum and 5% Iridium alloy (the Iridium is used to enhance the stability of the Platinum layer).

The details, shapes, dimensions and materials used in the coating process of a conductive Atomic Force Microscope tip, and in the product itself, described in the present patent application, may be conveniently changed by others that are technically equivalent and do not move aside of the essentiality of the invention neither of the scope defined by the claims included below.

Claims

1- Graphene coating process of a probe (1 ) for an atomic force microscope comprising a step of transference of a graphene layer (4) from a metal substrate (9) to a polymeric substrate (33), characterized in that it further comprises the steps of:

- coating a base (2) by a first polymeric layer (31 )

- probe (1 ) attachment over said first polymeric layer (31 )

- coating the probe (1 ) by a second polymeric layer (32)

- dissolving all the polymeric layers (31 , 32, 33) by a solvent agent (5) such that the graphene layer (4) is attached to the surface of the probe (1 ). 2- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 1 , characterized in that the coating of the base (2) by a first polymeric layer (31 ) is carried out by a spin coating technique.

3- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 1 , characterized in that the probe (1 ) comprises a support (13), a cantilever (12), and a tip (11 ), and in that the bottom surface of said support (13) is attached to the first polymeric layer (31 ) before it completely dries, such that the probe (1 ) stays in a substantially horizontal position. 4- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 1 , characterized in that the coating of the probe (1 ) by a second polymeric layer (32) is carried out by a drop casting technique.

5- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 1 , characterized in that the deposition of said polymeric substrate (33) with a graphene layer (4) over said second polymeric layer (32) is carried out inside water.

6- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 1 , characterized in that the graphene layer (4) is a single atomic sheet.

7- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 1 , characterized in that the polymer of the polymeric layers (31 , 32, 33) is PMMA. 8- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 1 , characterized in that the solvent agent (5) is acetone. 9- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 8, characterized in that the dissolving step is carried out inside a camera (6) with acetone steam (52).

10- Graphene coating process of a probe (1 ) for an atomic force microscope, according to claims 8 or 9, characterized in that the dissolving step is carried out over a boiling acetone container (7).

1 1 - Graphene coating process of a probe (1 ) for an atomic force microscope, according to claim 9, characterized in that the dissolving step is carried out over a period of 30 minutes.

12- Probe (1 ) for an atomic force microscope comprising a support (13), a cantilever (12), and a tip (1 1 ), characterized in that at least the tip (1 1 ) includes a silicon substrate (81 ) varnished with a conductive layer (82) of a Pt-lr alloy, and further coated with a graphene layer (4).

13- Probe (1 ) for an atomic force microscope, according to claim 12, characterized in that the graphene layer (4) is a single atomic sheet.