Field of the Invention

The present invention relates to a method of forming a structure in a material and relates particularly, though not exclusively, to a method that may be used for

formation of nano-scale structures.

Background of the Invention

Diamond, in particular single-crystalline diamond, has a number of extreme material properties and unigue

electrical and photonic properties that are of interest for device applications. The fabrication of structures in diamond or diamond like carbon materials typically involves forming a mask on a surface of the material and subseguent etching. Surface masking typically involves the deposition of a sacrificial layer followed by lithographic patterning and etching steps to form the structure.

A Focused Ion Beam Hard Mask (FIBHM) was recently

developed and provides a direct write masking process. In the FIBHM process a surface of the diamond is patterned by exposure to a low dose of a focused gallium ion beam flux. Subseguent etching (oxygen or argon) forms the structure without the need for a sacrificial layer and conseguently has a significant practical advantage. Additional observed advantages are a relatively short exposure time, an improved resolution and a reduced sidewall roughness. Summary of the Invention

The present invention provides in a first aspect a method of forming a structure in a material, the method

comprising the steps of:

providing first and second materials, the first material having a surface and having atoms or ions of the second material in a portion of the surface;

providing a third material in the proximity of the second material on the portion of the surface of the first material; and simultaneously or thereafter

exposing the first material to a selectively abrasive surface treatment that removes a portion of the first material;

wherein the second and third materials are selected such that the third material adsorbs in the proximity of the second material on the portion of the surface of the first material whereby a masking layer is formed such that the selectively abrasive surface treatment preferentially removes the first material at a surface region outside that portion of the surface of the first material.

Throughout this specification the term "selectively abrasive surface treatment" is used for a surface

treatment that has properties that depend on a material composition of the surface.

As the formation of the masking layer involves the third material that adsorbs on the first material, the above defined method may be used for forming a structure in a variety of first materials, such as carbon-based materials (for example diamond or diamond-like carbon material), but also including other materials such as semiconductor materials (for example silicon)

The method typically comprises the step of providing a source of at least a component of the third material. The step of providing the source of at least a component of the third material may comprise providing at least the component of the third material in solid state form and subseguently using a process that removes atoms or ions of at least the component of the third material from the source and moves the removed atoms or ions such that at least some of the removed atoms or ions adsorb on the portion of the surface of the first material. The step of exposing the surface of the first material to a

selectively abrasive surface treatment may be conducted such that the selectively abrasive surface treatment removes atoms or ions of at least the component of the third material from the source and moves the removed atoms or ions such that at least some of the removed atoms or ions adsorb on the portion of the surface of the first material .

The method may comprise providing the first and second material separate from each other. The second material may for example be gallium, such as gallium provided by a focused ion beam. It will be appreciated that

alternatively also other materials may be used.

Alternatively, the first material may be provided with atoms or ions of the second materials positioned within the first material. For example, the atoms or ions of the second material may be provided in the form of impurities within the first material.

In one specific example the third material comprises atoms or ions of a metal such as iron. It will be appreciated that alternatively other materials, such as chromium, may be used that preferentially adsorbs in the proximity of the atoms or ions of the second material in the surface of the first material.

For example the method may comprise forming the structure in a vacuum chamber that comprises a metallic material, such as iron, in a chamber wall or includes a component that comprises the metallic material. The step of exposing the surface of the first material to a selectively abrasive surface treatment may then comprise removing iron atoms or ions and forming a compound including iron on the surface of the first material. The third material typically also comprises oxygen in addition to the metallic material. In this case the third material typically is an oxide. The oxygen of the oxide may be provided by etching (in case of an oxygen etch) or in the form of residual oxygen present in the vacuum chamber in which the structure is formed or in the form of surface oxides present on the surface of the first material .

The method may comprise providing atoms or ions of the second material and selecting the portion of the surface. In one example the second material is provided by exposing the portion of the surface of the first material to a flux of energetic ions of the second material (such as gallium) such that at least a portion of the energetic ions is incorporated into the selected portion of the surface of the first material.

In one specific embodiment exposing the portion of the surface of the first material to the flux of energetic ions comprises exposing the portion of the surface to a focused ion beam that scans across the at least one portion of the surface. It has been observed that a spatial resolution with which the structure is formed from the first material largely depends on a probe diameter of the focused ion beam. For example, the spatial resolution may be of the order of lOOnm, 50nm, 20nm or even lOnm or smaller .

In one example the portion of the surface of the first material is selected and the selectively abrasive surface treatment performed such that a photonic waveguide is formed. The photonic waveguide may also comprise a source of photons in the form of a colour centre . In an

alternative example the surface portion of the first material is selected and the selectively abrasive surface treatment is performed so that an imprint die, such as a die for nano-imprint lithography, is formed. In a further example the surface portion of the first material is selected and the selectively abrasive surface treatment is performed such that a field emitter cathode is formed.

In an alternative embodiment the method comprises

providing the first material with the second material being incorporated into an atomic matrix of the first material. For example, the second material may be provided in the form of impurity ions or atoms that are

incorporated into the atomic matrix of the first material.

The method m accordance with the alternative embodiment of the present invention may for example comprise forming projections such as high aspect ratio pillars or "nano- whiskers" from the first material and at locations of the impurities. For example, the method may comprise forming field emitter cathode or a high surface area nano-whisker structure suitable for electrochemical sensors .

The step of exposing the surface of the first material to a selectively abrasive surface treatment may for example comprise etching, such as oxygen or argon plasma etching, or physical sputtering and typically comprises continuous formation of the masking layer at the at least one portion of surface.

As mentioned above, the structure that is formed by the defined method may for example be a photonic waveguide, a die for imprint lithography, a field emitter cathode, an electrochemical sensor, a micro-electromechanical system device, substrate for high density patterned media, a high surface area structure for thermal management, an

electronic device, an optoelectronic device, a photonic device, a micro-fluidic structure, a shadow mask for photon exposure based lithography, optical elements including diffractive optics and antireflective surfaces, an antibacterial surface or a super hydrophobic or self cleaning surface.

The present invention provides in a second aspect an optical element formed by the method in accordance with the first aspect of the present invention. For example, the optical element may be a photonic waveguide, a diffractive element and an element comprising an anti- reflective coating.

The present invention provides in a third aspect a die for imprint lithography formed by the method in accordance with the first aspect of the present invention.

The present invention provides in a fourth aspect a field emitter cathode formed by the method in accordance with the first aspect of the present invention .

The present invention provides in a fifth aspect an electrochemical sensor formed by the method in accordance with the first aspect of the present invention.

The invention will be more fully understood from the following description of specific embodiments of the invention. The description is provided with reference to the accompanying drawings .

Brief Description of the Drawings

Figure 1 illustrates processing steps of a method of forming a structure in a material in accordance with a specific embodiment of the present invention;

Figures 2 a) to 2 d) illustrate structures formed by a method according to a specific embodiment of the present invention ; Figure 3 shows micrographs of structures formed using a method according to an embodiment of the present

invention ;

Figure 4 shows an energy dispersive X-ra (EDX) depth profile for a structure formed using the method in accordance with the specific embodiment of the present invention ;

Figures 5 a) and 5 b) show energy dispersive X-ray spectra for a structure formed using the method in accordance with the specific embodiment of the present invention;

Figures 6 shows an X-ray photoelectron spectroscopy measurement for a structure formed using the method in accordance with the specific embodiment of the present invention; and

Figures 7 and 8 illustrate structures that were formed using the method in accordance with the specific

embodiment of the invention.

Detailed Description of Specific Embodiments Referring initially to Figure 1, a method 10 of forming a structure in a material is now described. The method 10 includes the initial step 12 of providing a first

material, such as a diamond or diamond-like carbon material. It will be appreciated that in variations of the described embodiment the material may not necessarily be diamond or a diamond-carbon like material, but may for example be silicon or another suitable material. The first material has atoms or ions of a second material in the surface of a portion of surface. Step 12 may comprise implanting the atoms or ions into the first material.

Alternatively, the second material may for example be provided in the form of impurities in the first material.

The method 10 also includes step 14 of providing a third material in the proximity of the second material on the surface portion of the first material. For example, the third material may include ions or atoms of a selected metallic material such as iron. In addition, the method 10 includes the step of exposing the first material to a selectively abrasive surface treatment that removes a portion of the first material. The second and third materials are selected such that the third material (such as an oxide) preferentially adsorbs in the proximity of the second material in the surface of the first material and a masking layer is formed thereon such that the selectively abrasive surface treatment preferentially removes the first material at a surface region outside that portion of the surface of the first material.

Referring now to Figures 2 to 3, the method 10 is

described in further detail. Figure 2 a) illustrates a diamond substrate 102 (the "first material") positioned in a vacuum chamber (not shown) and a selected surface area 104 is exposed to a low dose of gallium ions (the "second material") 106. In this embodiment the low dose of the gallium ions is provided in the form of a focused gallium ion beam comprising gallium ions having a kinetic energy of 30keV and a dose of approximately 10 ¹⁵ ions per cm ² .

Figure 2 b) illustrates a portion of the diamond substrate 102 in more detail. Figure 2 b) shows a portion of the selected surface area 104 into which the gallium ions are implanted and/or onto which the gallium ions are deposited by the gallium ion beam. After the exposure of the diamond substrate 102 to the flux of gallium ions 106, the diamond substrate 102 is then exposed to selective plasma etching 108, which is illustrated in Figure 2 c) . In this example the plasma etching 108 is oxygen plasma etching. However, it will be appreciated that alternatively other types of selectively abrasive surface treatments may be provided, such as argon plasma etching or physical sputtering, or halogen based reactive ion etching with, for example, SF ₆ . Figure 2 d) illustrates effects of the plasma etch 108 in more detail. The oxygen etching 108 predominately removes diamond material outside the selected area 104. However, as illustrated in Figure 2 b), the diamond material 102 includes impurities 109, such as metallic impurities (the second material) . High aspect ratio pillars 110 ("nano- whiskers") are formed at positions of the impurities 109, which is illustrated in Figure 2 c) and 2 d) .

In this embodiment the vacuum chamber in which the structure is formed is a stainless steel chamber and conseguently comprises iron (included in the "third material") . The etching will remove iron ions or atoms into the plasma (in case of plasma etching) by sputtering, which results in a deposition of the iron-containing species for example on surfaces of the diamond substrate 102. It is concluded that the gallium together with oxygen on the surface of the selected region 104 and also the impurities 109 in the environment of surface oxygen provide a base, with a sufficiently high affinity for iron and oxygen, on which the sputtered iron together with oxygen preferentially adsorbs. This results in an iron rich masking layer that grows during the plasma etching procedure and provides a hard mask for the plasma etching. Consequently, the diamond material of the diamond

substrate 102 is predominately etched at areas that are outside the selected surface area 104 and outside surface areas that include impurities 109.

Figure 2 d) shows the formed iron containing masking layer 112 which is positioned over the selected surface area 104 of the diamond substrate 102 and over the impurities 109. The masking layer 112 also contains some oxygen and it consequently is likely that some of the iron is oxidised. A likely source of the oxygen of the layer 112 is the oxygen etching 108. However, even if the oxygen etching 108 is replaced by argon etching, an oxygen concentration was observed in the layer 112. Consequently, it is likely that a further source for the oxygen is residual oxygen present in the vacuum chamber, and/or surface oxygen present in interior surfaces of the vacuum chamber and on the diamond substrate 102 and/or on the selected surface area 104.

The formed hard mask also includes carbon in the case of a carbon-containing substrate, but it is to be appreciated that the hard mask may also be formed on other materials that do not include carbon, such as silicon.

Figure 3 a) shows a scanning electron microscopy image of a structure 200 formed using the method 10. The structure 200 is surrounded by high aspect ratio pillars 202, which are a conseguence of impurities 109 in the diamond as described above. Figure 3 b) shows a transmission electron microscopy image showing a cross-sectional representation of a structure 204 that was formed using the method 10. A layer 206 is clearly visible. The layer 206 comprises amorphous carbon and gallium. The structure 204 is surrounded by high aspect ratio pillars (not visible in Figure 3b) . The layer 206 comprises an outer layer portion 209 which corresponds to layer 112 illustrated in Figure 2.

Figure 4 shows an energy dispersive X-ray spectroscopy depth profile taken along line 210 shown in Figure 3b) (the profile was taken from the outer layer 209 towards the diamond material) . The depth profile identifies an outer layer surface region that is an iron rich region 300 followed by an oxygen rich region 302, a transition region 304 containing amorphous carbon, gallium, oxygen and crystalline carbon, and a diamond region 306.

Consequently, the region 300 and 302 correspond to the layer 209, the region 304 corresponds to the layer 206 and the depth profile region 306 represents the underlying diamond material .

Figure 5 a) shows an energy dispersive X-ray spectrum of a region of the layer 209. Carbon, oxygen, iron and gallium are clearly detectable. Figure 5 b) shows energy

dispersive X-ray spectra that were taken for different times of exposure of the material to the plasma etch and consequently Figure 5b) illustrates growth of the layer 209. Figure 6 shows an X-ray photoelectron spectroscopy (XPS) measurement of a structure that was formed using the above-described method. The surface of a diamond material was exposed to a focused gallium ion beam to define a pattern and then exposed to oxygen plasma etching in a stainless steel vacuum chamber. Again, a hard mask was formed including the above described elements, and the XPS measurement also identified the presence of Cr in the hard mask. It is concluded that the Cr together with the Fe originate from the vacuum chamber.

The method 10 has been described with reference to a specific example. However, it will be apparent that the method 10 may be performed in many different ways. For example, the method 10 is not limited to the use of gallium ion beam. For example, other suitable focussed ion beam systems include sources of Co, Ni, Ge, In, Si, Au, Mn and Pb . Exposures to non-focussed ions from most elements would also be feasible and may be applied in select areas using a shadow mask. Further, the method 10 may not necessarily comprise exposing a surface with energetic ions. In this case the method 10 may for example be used to exclusively form high aspect ratio pillars. In

addition, and as discussed above, the structure may not necessarily be formed in a carbon-containing material, but may alternatively be formed for example in silicon or another suitable material (such as another suitable semiconductor material) .

It has been observed that the resolution with each the structure can be formed using a focused ion beams largely depends on the diameter of a probe of the ion beam. Consequently, if the probe of the focused ion beam is sufficiently narrow, structures with a resolution of lOOnm, 50nm or even lOnm or less can be formed. It will be apparent that the method 10 can be used for a range of applications. Walls of structures that are formed with the method 10 are typically very smooth, which is particularly advantageous for photonic applications. The method 10 may for example be used to fabricate nano- photonics waveguides that may also include sources of the photons (such as a nano-photonic waveguide formed from diamond and having colour centres that can be excited by an external source) . Further, dies for imprint lithography (for example nano-imprint lithography), and

electrochemical sensors may be formed. Further, optical elements such as diffractive optical elements or anti- reflective coatings may be formed.

Figures 7 and 8 illustrate examples of structures that may be formed using the method 10. Figure 7 shows formed structures 400 that are composed of diamond and a pattern for formation of the structures 400 was formed using a scanning focus gallium ion beam as illustrated in Figure 2 a) . The structures 400 are composed of polycrystalline diamond and are positioned on a silicon surface 402.

Figure 8 shows a cross-sectional representation of a structure 400 along section 404 as illustrated in

Figure 7. The electron micrograph shown in Figure 8 was taken using a scanning transmission electron microscope. The uniform cross-sectional shape of the structure 400 is apparent. Such structures are ideally suited for photonic waveguide applications . In the claims which follow and in the preceding

description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.