GB2325392A

GB2325392A - Diamond marking

Info

Publication number: GB2325392A
Application number: GB9727365A
Authority: GB
Inventors: James Gordon Charters Smith; Andrew David Garry Stewart
Original assignee: Gersan Ets
Current assignee: Gersan Ets
Priority date: 1997-05-23
Filing date: 1997-12-24
Publication date: 1998-11-25
Also published as: GB9710738D0; GB9727365D0; ZA200201200B; ZA984377B; KR100509546B1; KR20010012916A

Abstract

An information mark invisible to the naked eye is applied to the polished facet of a diamond gemstone by irradiation with a low energy ion beam to prevent charging, forming the mark with a focussed ion beam and cleaning the diamond with a powerful oxidising agent. The surface may be coated with gold to which an electrical connection is made to prevent charging. Gallium ions may be used with a molten salt eg KNO 3 as oxidising agent.

Description

Diamond Marking Background to the Invention The present invention relates to a method of marking a surface of a diamond. The mark may be any mark, but the invention is particularly though not exclusively directed to applying an information mark to the diamond. The diamond may be for instance an industrial diamond such as a wire-drawing die, though the invention is of particular interest in marking gemstone diamonds, for instance for applying a mark which is invisible to the naked eye or invisible to the eye using a xlO loupe, when the mark can be applied to a polished facet of the gemstone without detracting from its clarity grade.

The marks can be used to uniquely identify the gemstone by a serial number or as a brand or quality mark. In general, the mark should be capable of being viewed under suitable magnification and viewing conditions, and, if applied to a gemstone, should not detract from the value or appearance of the stone and should preferably not exhibit blackening.

There is a detailed description of the nature of the marks that can be applied in WO 97/03846, in which the marks are applied by irradiating a diamond gemstone with ultraviolet laser radiation using a projection mask.

It is generally desirable to produce marks of improved resolution and to reduce the time required to apply the marks so that for instance serial numbers can be applied.

The Invention According to a first aspect of the present invention, the surface of a diamond is marked with an ion beam. The invention extends to a diamond which has been marked by the method of the invention, and to apparatus for carrying out the method.

The marking can be carried out by direct writing on the diamond surface with a focused ion beam. Typically Gallium ions are used, but a beam of other suitable ions may alternatively be used. By limiting the dose, sputtering of carbon atoms can be avoided, sputtering causing direct material removal; this enables a mark to be applied with a controlled depth and good resolution. By limiting the dose, and providing there is sufficient dose, the incident ions cause disordering of the crystal lattice that converts the diamond to a graphite-like or other non-diamond structure that can then be cleaned, e.g.

using an acid or potassium nitrate dissolved in acid, to leave a shallow mark say not less than 20 nm deep and/or not more than about 50 nm deep, typically about 30 nm deep, with no evidence of blackening. Plasma etching may be used as an alternative to acid cleaning.

However, in a preferred embodiment, the disordered layer produced on the diamond by the ion beam is removed by means of a powerful oxidizing agent, such as molten potassium nitrate. This method allows a mark to be produced at a lower dose and therefore in less time at a given beam current. Alternatively, a lower beam current, giving a smaller spot size may be used to produce marks with higher resolution features, such as diffraction gratings.

The depth of the lattice disordering is determined by the range of the ions. For 50 keV Gallium, this range is about 30 nm. The minimum dose may be as low as 1013/cm2, but 14 2 15 2 is preferably about 10'4/cm2 to 10'51cm2 However, good marks can be applied with a fairly modest dose, the preferred maximum dose being about 10 16/cm2 or even up to 1? 2 about 10 /cm However, the dose depends upon the ions being used and their energy (as measured in keV). The ion beam dose is a total number of incident ions per unit area at the sample surface, during the marking. The beam current may be about 1 nA, and the beam energy not less than about 10 keV or about 30 keV and/or not greater than about 100 keV or about 50 keV.

It has been found that if depth of mark is plotted against ion beam dose for a series of different beam energies, there is an increase of depth of mark with increasing beam energy. Characteristics of the mark may be optimised by selecting from the dose/energy combinations which will result in the desired depth of mark.

The region to be marked and/or the surrounding area may be coated with an electrically-conducting layer, for instance gold, prior to forming the mark, so that an electrical connection can be provided before marking with the ion beam, to prevent charging. The thickness of the gold, or other, coating alters the variation of depth of mark with beam energy and may thus be chosen to optimise the mark produced.

Other suitable methods to reduce charging may be used. One method is to irradiate the region to be marked with a low energy ion beam, e.g. about 5 to about 10 keV, prior to forming the mark, to modify the diamond surface to cause it to become electrically conductive, the electrical connection being made to that region. In a preferred embodiment, the ion beam used for marking may be used in conjunction with an electron flood gun, such as that described in US patent specification number US4639301, to prevent charging of the diamond surface.

In accordance with a second aspect of the present invention, there is provided a method of marking the surface of a diamond, comprising the steps of irradiating at least a portion of said gemstone to form a damaged or crystal lattice disordered layer thereon, and removing said disordered layer using an oxidizing agent.

A further advantage of the second aspect of the present invention over acid-cleaning is that no acid fumes are produced and also that spent acid does not have to be disposed of, thereby improving the safety of the process as well as offering environmental and economic benefits.

The oxidizing agent is preferably molten potassium nitrate. The diamond is preferably covered with potassium nitrate and heated to a temperature of around 380-550 Centigrade for a period of between a few minutes and several hours, preferably approximately one hour.

However, other suitable powerful oxidizing agents include molten compounds such as alkali metal salts. Suitable compounds may be in the form XnYm where the group X may be Li, Na, K, Rb, Cs, or other cation, and the group Y may be OK, NO3, 022 O o 2., 32- or other anion; the integers n and m being used to maintain charge balance.

Mixtures of compounds may be used. Air or other oxygen-containing compounds may also be present.

The use of such oxidizing agents to remove a disordered diamond layer allows a mark of a desired depth to be produced using a relatively low dose of ions.

In a preferred embodiment, the gemstone is irradiated with an ion beam as in the first aspect of the present invention, and most preferably a Gallium ion beam. The preferred embodiment of the method of the second aspect resulting in a remarkably efficient process, with each incident Gallium ion ultimately resulting in the removal of approximately 2,700 carbon atoms. In materials other than diamond, this figure would be around 1-10.

It is this unique property of diamond that allows the relatively large structures such as alphanumeric characters covering an area of 0.43 mm by 0.16mum to be machined in a reasonably economic time of about 10 seconds.

Example A diamond gemstone is mounted in a suitable holder and a facet is coated with a layer of gold. The sample is placed in a vacuum chamber equipped with a focused ion beam source such as supplied by FEI or Micrion, the holder making an electrical connection to the gold layer to prevent the diamond becoming charged. Using a focused beam with a raster scan or similar to scan the beam for instance with electrostatic deflection (as an alternative, the diamond may be moved, but this is less practical), a mark is written on the diamond facet with ions to a dose of 1015 to 10 16/cm2, the ion source being Gallium, the beam current 1 nA and the beam energy 30 to 50 keV. The sample is removed from the vacuum chamber and acid cleaned to remove the disordered layer and the gold layer. There is a shallow mark typically about 30 nm deep, with no evidence of blackening.

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention, which extends to the equivalents of the features described. The invention also consists in any individual features described or implicit herein or shown or implicit in the drawings or any combination of any such features or any generalisation of any such features or combination.

Claims

CLAIMS:

1. A method of marking the surface of a diamond, comprising forming the mark with an ion beam.

2. A method of marking the surface of a diamond, comprising the steps of irradiating at least a portion of said diamond to form a disordered layer thereon, and removing said disordered layer using an oxidizing agent.

3. The method of Claim 2, wherein the diamond is irradiated using an ion beam.

4. The method of Claim 1, wherein the surface of the diamond is irradiated by means of said ion beam to form a disordered layer thereon, and said disordered layer is removed using an acid.

5. The method of Claim 4, wherein said disordered layer is removed using an oxidising agent dissolved in acid.

6. The method of Claim 5, wherein said disordered layer is removed using potassium nitrate dissolved in acid.

7. The method of Claim 1 or Claim 3, wherein the mark is formed at a dose of not more than about 10 17/cm2

8. The method of Claim 1 or Claim 3, wherein the mark is formed at a dose of not more than about 10 '6/cam2

9. The method of any of Claims 1, 3, 7 or 8, wherein the mark is formed at a dose of not more than about

10 15/cm2 10. The method of any of Claims 1, 3 or 7 to 9, wherein the mark is formed at a dose of not less than about 10'4/cm2

I 1. The method of any one of Claims 1, 3 or 7 to 9, wherein the mark is formed at a dose of not less than about 10 13/cm2

12. The method of any of Claims 1, 3 or 7 to 11, wherein the beam current is about 1 nA.

13. The method of any of Claims 1, 3 or 7 to 11, wherein the beam current is about 0.5 nA.

14. The method of any one of Claims 1, 3 or 7 to 13, wherein the beam energy is about 10 to about 100 keV.

15. The method of Claim 14, wherein the beam energy is about 50 keV.

16. The method of any one of Claims 1, 3 or 7 to 15, wherein the ion beam is a Gallium ion beam.

17. The method of any of the preceding Claims, wherein the depth of the mark is about 20 to about 50 nm.

18. The method of any of the preceding Claims, wherein the depth of the mark is about 30 to about 35 nm.

19. The method of any one of Claims 1,3 or 7 to 18, including coating said surface with an electrically-conductive layer prior to forming the mark.

20. The method of Claim 19, wherein the layer is gold.

21. The method of any of Claims 1, 3 or 7 to 18, wherein the region to be marked is irradiated with a low energy ion beam prior to forming the mark, to modify the diamond surface to cause it to become electrically conductive.

22. The method of Claim 21, wherein the energy of said low energy ion beam is about 5 to 10 keV.

22. The method of any of the preceding Claims, wherein the mark is an information mark.

23. The method of any of the preceding Claims, wherein the mark is invisible to the naked eye.

24. The method of Claim 23, wherein the mark is invisible to the eye using a x10 loupe.

25. The method of any of the preceding Claims, wherein the diamond is a diamond gemstone.

26. The method of Claim 25, wherein the mark is applied to a polished facet of the gemstone.

27. The method of Claim 4, wherein the disordered layer is removed using an oxidizing agent.

28. The method of Claim 2 or 27, wherein the oxidizing agent is potassium nitrate.

29. The method of Claim 2, 5 or 28, wherein the oxidizing agent is at least one compound in the form XnYm where the group X is Lit, Nat, K+, Rbt, Cyst, or other cation, and the group Y is OK, NO3, o22 02., CO32 or other anion; the integers n and m being used to maintain charge balance.

30. The method according to any preceding claim comprising the steps of irradiating at least a portion of a diamond with an ion beam to form a damaged layer 'thereon and removing said damaged layer by substantially covering the damaged layer with molten potassium nitrate.

31. The method of Claim 30, wherein the temperature of said diamond and molten potassium nitrate is maintained for approximately one hour.

32. A method of marking the surface of a diamond, substantially as herein described in the foregoing Examples.

33. A diamond which has been marked by the method of any of the preceding Claims.