MICRO AND NANO STRUCTURING OF A DIAMOND SUBSTRATE

Abstract

A process of structuring a diamond substrate, comprising the steps of (a) depositing an adhesion layer on a face of the diamond substrate; (b) coating a resist layer on the adhesion layer; (c) removing parts of the resist layer so as to expose parts of the adhesion layer and form a corresponding structuring mask; (d) etching the adhesion layer and the diamond substrate (2) through the structuring mask so as to structure the diamond substrate; wherein the adhesion layer is a non-metallic compound comprising oxides.

Claims

1.-16. (canceled)

17. A process of structuring a diamond substrate, comprising the steps of: (a) depositing an adhesion layer on a face of the diamond substrate; (b) coating a resist layer on the adhesion layer; (c) removing parts of the resist layer so as to expose parts of the adhesion layer and form a corresponding structuring mask; and (d) etching the adhesion layer and the diamond substrate through the structuring mask so as to structure said diamond substrate, wherein at least one of the structures comprise one or more colour centres; and wherein the adhesion layer comprises substantially a non-metallic compound.

18. The process according to claim 17, wherein the non-metallic compound comprises at least one of the following list: silicon, silicon oxide, germanium and germanium oxide, and any combination thereof.

19. The process according to claim 17, wherein at step (a) the adhesion layer is deposited by electron beam physical vapour deposition, plasma activated chemical vapour deposition, thermal deposition or atomic layer deposition.

20. The process according to claim 17, wherein the adhesion layer extends homogeneously over the whole face of the diamond substrate.

21. The process according to claim 17, wherein the adhesion layer has a thickness of at least 1 nm and of not more than 200 nm.

22. The process according to claim 17, wherein step (d) comprises the following sub-steps: (d1) etching the adhesion layer through the structuring mask so as to expose parts of the diamond substrate; and (d2) etching the diamond substrate through the structuring mask so as to structure said diamond substrate.

23. The process according to claim 22, wherein sub-steps (d1) and (d2) are achieved by plasma with reactive gases.

24. The process according to claim 23, wherein the reactive gases in step (d1) is fluorine-based and the reactive gas in step (d2) is oxygen based.

25. The process according to claim 17, wherein the resist layer is an electron beam resist layer or a photo resist layer and step (c) is achieved by electron beam lithography or optical lithography, and subsequent development of the resist.

26. The process according to claim 17, wherein the resist layer comprises a silsesquioxane.

27. The process according to claim 26, wherein the silsesquioxane comprises Hydrogen silsesquioxane HSQ.

28. The process according to claim 17, comprising the further step of: (e1) removing the structuring mask and the adhesion layer that remains further to step (d).

29. The process according to claim 28, wherein step (e2) is achieved by wet etching.

30. The process according to claim 17, comprises the further steps of: (e2) removing the structuring mask so that the adhesion layer remains on the structures; (f) coating a further resist layer on the adhesion layer remaining on the structures; (g) removing parts of the further resist layer so as to expose parts of the adhesion layer on the structure and form a further structuring mask; (h) etching the adhesion layer and the diamond substrate through the further structuring mask so as to further structure the structures of said diamond substrate.

31. The process according to claim 30, wherein step (h) comprises the following sub-steps: (h1) etching the adhesion layer through the further structuring mask so as to expose parts of the structures of the diamond substrate; and (h2) etching the diamond substrate through the further structuring mask so as to further structure said diamond substrate.

32. The process according to claim 30, wherein step (e2) is achieved by wet etching.

33. The process according to claim 17, wherein the structures comprise structures with a height of at least 1 nm and of not more than 10 000 nm.

34. The process according to claim 17, wherein at least one of the structures comprise nitrogen-vacancy centres.

35. The process according to claim 17, wherein the adhesion layer is amorphous.

Description

DRAWINGS

[0037] FIG. 1 exemplarily illustrates a step of depositing an adhesion layer on a diamond substrate by electron beam physical vapour deposition, in accordance with various embodiments of the invention.

[0038] FIG. 2 exemplarily illustrates a step of spin coating a resist layer on the adhesion layer of the diamond substrate, deposited as in FIG. 1, in accordance with various embodiments of the invention.

[0039] FIG. 3 exemplarily illustrates a step of electron beam lithography on the resist layer coated as in FIG. 2, in accordance with various exemplary embodiments of the invention.

[0040] FIG. 4 exemplarily illustrates the diamond substrate after removal of areas of the resist layer that have not been subject to the electron beam lithography as in FIG. 2, so as to form a structuring mask, in accordance with various embodiments of the invention.

[0041] FIG. 5 exemplarily illustrates a step of structuring by plasma etching the adhesion layer and the diamond substrate, in accordance with various embodiments of the invention.

[0042] FIG. 6 exemplarily illustrates a step of removing the structuring mask and the adhesion layer after structuring in FIG. 5, in accordance with various embodiments of the invention.

[0043] FIG. 7 exemplarily illustrates an alternative step to removing the structuring mask and adhesion layer as in FIG. 6, consisting in removing the structuring mask only, in accordance with various embodiments of the invention.

[0044] FIG. 8 exemplarily illustrates a step of spin coating a further resist layer on the diamond substrate, in accordance with various embodiments of the invention.

[0045] FIG. 9 exemplarily illustrates a step of electron beam lithography on the further resist layer coated in FIG. 8, in accordance with various embodiments of the invention.

[0046] FIG. 10 exemplarily illustrates a step of removing areas of the further resist layer that have not been subject to the electron beam lithography as in FIG. 9, so as to form a further structuring mask, in accordance with various embodiments of the invention.

[0047] FIG. 11 exemplarily illustrates a step of further structuring by plasma etching the further adhesion layer and the diamond substrate, in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

[0048] FIGS. 1 to 5 illustrate successive steps of a process of structuring a diamond substrate according to exemplary embodiments of the invention. In the context of this invention, structuring means providing a structured surface on a face of the diamond substrate. The structured surface is obtained for instance by removing material of the diamond substrate so as to form various shapes like pillars, wires, cantilevers or disks. These shapes can be in the micro-scale and/or in the nano-scale.

[0049] The structuring process of the diamond substrate consists essentially in coating a resist layer on a face of the diamond substrate, removing parts of the resist layer for forming a structuring mask, and etching the diamond substrate through the structuring mask. To that end, the structuring mask made of the resist layer must properly adhere to the diamond structure. Etching of diamond requires however high-energy and/or oxygen plasmas which under certain circumstances require specific resits that might not properly adhere to the diamond substrate. The invention provides depositing an adhesion layer on the diamond substrate before coating the resist layer. The adhesion layer is made of a non-metallic compound and comprises oxides. Metallic compounds tend indeed to oxidize rapidly in contact with ambient air, showing thereby poor adhesion properties with the resist layer. Oxides of non-metallic compound are at the contrary formed during deposition and provide a good adhesion to the diamond substrate.

[0050] Before structuring the diamond substrate, the later can be cleaned, for instance mechanically and/or chemically. Mechanical cleaning can comprise rubbing with a wipe wetter with solvent, dipping in a solvent like acetone, ultrasound bath and drying. Chemical cleaning can comprise immersion in acid, in various instances a mixture of 3 acids HNO.sub.3, H.sub.2SO.sub.4 and HClO.sub.4. The acid mixture can be heated during immersion until it changes colour from transparent to yellow, corresponding to the production of chlorine dioxide radicals responsible for the cleaning. The ratio of these acids is advantageously 1.sub.v:1.sub.v:1.sub.v.

[0051] A stress relief etching can be performed after cleaning and before structuring. This etching can be made with a plasma, for instance an inductively coupled plasma, reactive ion etching ICP/RIE reactor, so as to remove a thin layer, e.g., between 3 and 5 μm, on the face that is to be structured.

[0052] The diamond substrate can be placed on a silicon chip and attached thereto by gluing. It can then be nitrogen ion implanted in an ion implanter and thereafter be annealed, e.g., at a temperatures of at least 400° C., in various instances 900° C., during at least 100 minutes.

[0053] FIG. 1 is a schematic illustration of a step of depositing an adhesion layer on the diamond substrate, for instance by electron beam assisted physical vapour deposition (PVD). To that end the diamond substrate 2 on the silicon chip 4 is placed in an electron beam evaporator machine 6. A target 8 on an anode 10 is bombarded with an electron beam given off by a charged tungsten filament 12 under high vacuum. The electron beam causes atoms from the target 8 to transform into the gaseous phase. These atoms then precipitate into solid form, coating the upper face of the diamond substrate 2 with a thin layer of the target material 8 on the anode 10, forming the adhesion layer 14. The adhesion layer 14 can show a thickness of at least 1 nm, in various instances 10 nm and/or not greater than 200 nm, for example 50 nm. The target 8 is made of non-metallic compound. That deposition step has been carried with a polycrystalline silicon target.

[0054] FIG. 2 is a schematic illustration of a coating step of a resist layer 16 on the adhesion layer 14 of the diamond substrate 4. That coating is for instance made by spin coating in a spin coating machine 18. The resist is for instance hydrogen silsesquioxane HSQ commercially available under the name FOX®-16 of the company Dow Corning®. The diamond substrate can be pre-heated up to 10 minutes at 120° to remove any possible moisture from the surface. The HSQ resist is poured on the centre of the diamond substrate while it is rotating, so as to form the resist layer 16. Optionally, the rotation speed can be varied during or after pouring the resist, e.g., from 1000 rpm to 3300 rpm. Also optionally, the diamond substrate 2 and the silicon chip 4 are post-heated, e.g., at 90° C. during 5 minutes.

[0055] FIGS. 3 and 4 are schematic illustrations of removing parts of the resist layer so as to form a structuring mask.

[0056] In FIG. 3, the diamond substrate 2 on the silicon chip 4 is placed in an electron beam lithography (EBL) machine 20. A focused beam of electrons 22 is scanned over the resist layer 16 to draw or write custom shapes on the resist layer. The electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a solvent (developing).

[0057] After the EBL writing of the resist layer, the diamond substrate is developed by immersion in a solvent. For instance, the diamond substrate (with the silicon chip) can be immersed in a solution comprising tetramethylammonium hydroxide (TMAH), e.g., at 25%, for a duration of at least 10 seconds. It can then be immersed into ultrapure water, and thereafter several times in acetone and afterwards in isopropyl alcohol until the non-exposed regions are totally removed.

[0058] In FIG. 4, shows the diamond substrate 2 with the remaining resist layer 16 further to EBL and development, forming then a structuring mask 24.

[0059] FIG. 5 illustrates schematically a step of etching the adhesion layer 14 and the diamond material through the structuring mask 24. This is done by reactive-ion etching (RIE), i.e., a chemically reactive plasma under low pressure (vacuum) by an electromagnetic field where high-energy ions from the plasma attack the substrate and react with it. For instance, the diamond substrate 2 and the silicon chip 4 are placed in a plasma machine or installation 26. The latter comprises a chamber supplied with gas and provided with two electrodes 26.1 and 26.2 supplied with an oscillating electrical power supply 26.3. The resulting oscillating electric field ionizes the gas molecules by stripping them of electrons, creating a plasma. The reactive-ion etching can be combined with an inductively coupled plasma (ICP). The ICP is employed as a high density source of ions which increases the etch rate, whereas a separate oscillating electric field bias is applied to the substrate to create directional electric fields near the substrate to achieve more anisotropic etch profiles.

[0060] For instance, the etching step is achieved in two sub-steps, namely a first sub-step of removal of the adhesion layer 14 by reactive ions of SF.sub.6 etching through the structuring mask 24, followed by a second sub-step of etching through the structuring mask 24 the material of the diamond substrate by reactive ions of O.sub.2.

[0061] FIG. 6 illustrates the diamond substrate 2 on the silicon chip 4 where the structuring mask 24, formed by the resist layer 16, and the adhesion layer 14 have been removed. The resist layer 16 can be removed by immersing the diamond substrate 2 in a chemically etching substance like a hydrogen fluoride (HF) based buffered oxide etch (BOE). The adhesion layer 14 can be removed afterwards by immersing the diamond substrate in a potassium hydroxide solution.

[0062] FIGS. 7 to 11 shows additional steps for further structuring the diamond substrate, following the step of etching the adhesion layer and the diamond material through the structuring mask as in FIG. 5.

[0063] FIG. 7 illustrates the diamond substrate 2 on the silicon chip 4 where the structuring mask 24, formed by the resist layer 16, has been removed whereas the remaining adhesion layer 14 is left. This can be achieved chemical etching, e.g., by immersing the diamond substrate 2 in a chemically etching substance like a hydrogen fluoride (HF) based buffered oxide etch (BOE).

[0064] FIG. 8 illustrates schematically a coating step of a further resist layer 30 on the remaining adhesion layer 14 and also directly on the diamond substrate 4. The absence of adhesion layer between the structures 28 is not an issue because the portion of the further resist layer 30 located between the structures 28 will be removed when forming a further structuring mask (see FIG. 10). That coating is for instance made by spin coating in a spin coating machine 18 similar or identical to the one in FIG. 2. The resist is for instance Hydrogen silsesquioxane HSQ available under the commercial name FOX®-16 of the company Dow Corning®. It is referred to the description of FIG. 2 for further details about this step.

[0065] FIGS. 9 and 10 are schematic illustrations of removing parts of the remaining resist layer 16 on the structures 28 so as to form a further structuring mask 32.

[0066] In FIG. 9, the further resist layer 30 coated in FIG. 8 is subject to lithography on the structures 28 for drawing or writing custom shapes on the structures 28. For instance, an electron beam 22 is applied in a controlled manner for locally changing the solubility of the further resist. This is done with the same or similar electron beam lithography machine 20 as in FIG. 3.

[0067] After the lithography writing of the further resist layer, the diamond substrate is developed by immersion in a solvent. The diamond substrate (with the silicon chip) can be immersed in a solution comprising tetramethylammonium hydroxide (TMAH), e.g., at 25%, for a duration of at least 10 seconds. It can then be immersed into ultrapure water, and thereafter several times in acetone and afterwards in isopropyl alcohol until the non-exposed regions are totally removed.

[0068] FIG. 10 shows the diamond substrate 2 with the remaining further resist layer 30 after lithography and development, forming then the further structuring mask 32.

[0069] FIG. 11 illustrates schematically a step of etching the adhesion layer 14 on the structures 28 through the further structuring mask 32 and the diamond material. This is done by reactive-ion etching (RIE) similarly or identically to the step illustrated in FIG. 5. The reactive-ion etching step comprises then a first sub-step of etching the remaining adhesion layer 14 on the structures 28, e.g., by SF.sub.6 ions, and a second sub-step of etching the material of the diamond substrate with O.sub.2 ions forming a further structuring 34 on the structures 28.

[0070] The further structuring process which has been described in relation with FIGS. 7 to 11 can be carried out several times, in particular when after etching the adhesion layer and the material of the diamond substrate through the last structuring mask, only the structuring mask is removed so as to leave the adhesion layer on the corresponding structures. That left adhesion layer can then be further used for coating a next further resist layer.

[0071] The reliability of this plasma transfer and further cleaning process and its influence on shallowly (10-15 nm) implanted colour centres in diamond was evaluated by means of optically detected magnetic resonance (ODMR). Negatively charged nitrogen vacancies, NV(−) possess electronic spin and exhibit a resonance at 2.87 GHz, which can be perceived as a fingerprint of this colour centre. The experimentally obtained ODMR spectrum and photoluminescence map based on nano-pillars of 200 nm diameter etched into the diamond by the method presented here show that the adhesion layer is totally removed and also that there is no bleaching after longer irradiation times, indicating no surface damage and therefore no alternation in the electronic structure of shallowly underlying nitrogen vacancy. This conclusion can be directly related to the reliability of the presented method and perfect adhesion of the tone resist.