GB2361671A - Diamond marking - Google Patents

Abstract

An information mark invisible to the naked eye is applied to the polished facet of a diamond gemstone by irradiating the diamond gemstone surface, and cleaning the diamond surface with an oxidizing agent (e.g molten potassium nitrate) to reveal a mark having an appropriate depth, which does not detrimentally affect the clarity or colour grade of the diamond.

Description

2361671 Diamond Marking

Background to the Invention

The present invention relates to a method of marking a surface of a diamond or gemstone. The mark may be any mark, but the invention is particularly though not exclusively directed to applying an information mark to the diamond or gemstone. The diamond may be for instance an industrial diamond such as a wire-drawing die or diamond optical component, 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 x 10 loupe, when the mark can be applied to a polished facet of the gemstone without detracting from its clarity or colour grade. When a loupe is used, the visibility is assessed under the internationally accepted conditions for clarity grading, i.e. using a 10x magnifying achromatic, aplanatic loupe under normal light, this being a white diffuse light, not a spot light. 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 97/03 846, in which the marks are applied by irradiating a diamond gemstone with ultraviolet laser radiation using a projection mask. US 4 425 769 describes providing an identifying mark on a diamond or other gemstone by applying a photoresist to the surface, forming a contact mask by a photographic method, and etching the gemstone through the mask by cathode bombardment with an ionised gas to provide sputter etching. Sputter etching gives poor control of the depth of the mark and low resolution.

In accordance with the present invention, there is provided a method of marking the surface of a diamond or gemstone, comprising the steps of irradiating at least a portion 2 of said diamond or gemstone to form a crystal lattice disordered layer (a damaged layer) thereon, and removing said disordered layer using an oxidizing agent.

Acid-cleaning is another way of removing the disordered layer. An advantage of the 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. Also, the use of such oxidizing agents to remove a disordered layer allows a mark of a desired depth to be produced using a relatively low dose of ions, lower than using acid cleaning 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 oxidizing agent is preferably molten potassium nitrate. The diamond or gemstone is preferably covered with potassium nitrate and heated to a temperature of around 380550 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 XY,, where the group X may be Li+, Na+, K+, Rb+, Cs+, or other cation, and the group Y may be OH-, N03_ 02 2-, 02-, C03 2-, or other anion, the integers n and in being used to maintain charge balance. Mixtures of compounds may be used. Air or other oxygen-containing compounds may also be present.

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 is preferably about 1014/CM2 to 1015/CM2. However, good marks can be applied with a fairly modest dose, the preferred maximum dose being about 1016/CM2 or even up to about 1017/CM2. 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.

f 3 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 electricallyconducting 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 dose, 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 3 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 a charge neutralising device, such as an electron flood gun, such as that described in US 4 639 3 0 1, to prevent charging of the diamond surface.

In a preferred embodiment, the diamond or gemstone is irradiated with an ion beam as in GB 2 339 727, ftom which the present application is divided, and most preferably with a Gallium ion beam. The preferred embodiment of the present invention resulted in a remarkably efficient process, with each incident Gallium ion ultimately resulting in the removal of approximately 2,700 carbon atoms. In most materials other than diamond, this figure would be around 1 - 10.

It is this property of diamond that allows the relatively large structures such as alphanumeric characters covering an area of 0.43 mm by 0. 16 mm to be machined in a reasonably economic time of about 10 seconds.

The method of the present invention may also be used to mark the surface of a synthetic gemstone, such as the silicon carbide gemstones described in WO 97109470.

0 4 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 10'5 to 1016/CM2, the ion source being Gallium, the beam current I nA and the beam energy 30 to 50 keV. The sample is removed from the vacuum chamber. In the procedure carried out, the sample was acid cleaned to remove the disordered layer and the gold layer and there was a shallow mark typically about 30 nm deep, with no evidence of blackening. By replacing the acid cleaning with covering the gemstone with potassium nitrate and heating to a temperature of around 380' to 550'C for a period of approximately one hour, the diamond can be cleaned by removing the disordered layer.

The present invention has been described above purely by way of example.

Claims (44)

CLAIMS:

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

2. The method of Claim 1, wherein the gemstone is a silicon carbide gemstone.

3. The method of Claim 1, wherein the gemstone is a diamond.

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

5. The method of any of the preceding Claims, wherein the oxidizing agent is at least one compound in the form X,,Y. where the group X is a cation and the group Y is an anion.

6. The method of Claim 5, wherein the group X is Li+, Na+, K+, Rb+ or Cs+.

7. The method of Claim 5 or 6, wherein the group Y is OH-, N03-5 02 2'02- or C203

8. The method of Claim 5, wherein the oxidizing agent is potassium nitrate.

9. The method of Claim 8, comprising removing said disordered layer by substantially covering the disordered layer with molten potassium nitrate.

10. The method of Claim 9, wherein the temperature of the gemstone or diamond and molten potassium nitrate is maintained for approximately one hour.

11. The method of any of Claims I to 8, wherein said disordered layer is removed using an oxidizing agent dissolved in acid.

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12. The method of Claim 11, wherein said disordered layer is removed using potassium nitrate dissolved in acid.

13. The method of any of the preceding Claims, wherein the depth of the mark is not less than about 10 rim.

14. The method of Claim 13, wherein the depth of the mark is not less than about 20 rim.

15. The method of any of the preceding Claims, wherein the depth of the mark is not more than about 100 nm.

16. The method of Claim 15, wherein the depth of the mark is not more than about 70 nm.

17. The method of Claim 15, wherein the depth of the mark is not more than about 50 rim.

18. The method of Claim 15, wherein the depth of the mark is not more than about 30 rim.

19. The method of any of the preceding Claims, wherein the mark comprises characters whose height is about 50 microns.

20. The method of any of the preceding Claims, wherein the mark comprises lines of a width of not less than about 2 microns.

21. The method of any of the preceding Claims, wherein the mark comprises lines of a width of not more than about 3 microns.

22. The method of any of the preceding Claims, wherein the mark comprises lines the ratio of the width to depth of which is greater than about 20: 1.

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

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

25. The method of any of Claims 1 to 23, wherein the mark is invisible to the naked eye using a x 10 loupe.

26. The method of any of the preceding Claims, wherein the mark is applied to a polished facet of the gemstone or diamond.

27. The method of any of the preceding Claims, wherein said portion is irradiated using an ion beam.

28. The method of Claim 27, including coating said surface with an electricallyconductive layer prior to forming the mark.

29. The method of Claim 28, wherein the layer is gold.

30. The method of any of Claims 27 to 29, 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.

31. The method of Claim 30, wherein the energy of said low energy ion beam is about 3 to about 10 keV.

32. The method of Claim 27, wherein the region to be marked is simultaneously irradiated using a charge neutralising device.

33. The method of any of Claims 27 to 32, wherein the mark is formed at a dose of not more than about 10 1 1/CM2.

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34. The method of Claim 33, wherein the mark is formed at a dose of not more than about 1016/CM2.

35. The method of Claim 33, wherein the mark is formed at a dose of not more than about 1015/rM2.

36. The method of any of Claims 27 to 35, wherein the mark is formed at a dose of not less than about 1013/CM2.

37. The method of Claim 36, wherein the mark is formed at a dose of not less than about 10 14/CM2.

38. The method of any of Claims 27 to 37, wherein the beam energy is greater than about 10 keV.

39. The method of Claim 38, wherein the beam energy is greater than about 30 keV.

40. The method of any of Claims 27 to 39, wherein the beam energy is less than about 100 keV.

41. The method of Claim 40, wherein the beam energy is less than about 50 keV.

42. The method of any of Claims 27 to 41, wherein the ion beam is a gallium ion beam.

43. A gemstone which has been marked by the method of any of the preceding Claims.

44. A diamond which has been marked by the method of any of Claims 3 to 42.