(51) Int Cl. 7 : B44B 7/00, B28D 5/00. (86) International application number: PCT/GB98/ (56) References cited:

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1 (19) Europäisches Patentamt European Patent Office Office européen des brevets *EP B1* (11) EP B1 (12) EUROPEAN PATENT SPECIFICATION (4) Date of publication and mention of the grant of the patent: Bulletin 02/16 (21) Application number: (22) Date of filing: (1) Int Cl. 7 : B44B 7/00, B28D /00 (86) International application number: PCT/GB98/01493 (87) International publication number: WO 98/2773 ( Gazette 1998/47) (4) MARKING DIAMOND DIAMANTMARKIERVERFAHREN PROCEDE DE GRAVURE SUR DIAMANT (84) Designated Contracting States: AT BE CH CY DE DK ES FI FR GR IE IT LI LU MC NL PT SE () Priority: GB (43) Date of publication of application: Bulletin 00/ (73) Proprietor: GERSAN ESTABLISHMENT 9490 Vaduz (LI) (72) Inventors: SMITH, James, Gordon, Charters High Wycombe, Buckinghamshire HP13 QL (GB) GUY, Keith, Barry Twickenham, Middlesex TW2 6BY (GB) POWELL, Graham, Ralph Iver, Buckinghamshire SL0 9QB (GB) GAUKROGER, Michael, Peter Fleet, Hampshire GU13 8EZ (GB) (74) Representative: Lyndon-Stanford, Edward Willoughby Brooke MARKS & CLERK, 7/60 Lincoln s Inn Fields London WC2A 3LS (GB) (6) References cited: EP-A WO-A-97/03846 US-A US-A US-A EP B1 Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). Printed by Jouve, 7001 PARIS (FR)

2 1 EP B1 2 Description [0001] The present invention relates to a method of marking a surface of a diamond to produce a mark which is invisible to the naked eye. 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 x 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 x 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. [0002] US discloses marking the surface of a gemstone by depositing a photoresist resin on the surface of the gemstone, applying a photographic film to the photoresist layer, exposing the photoresist through the photographic film, developing the photoresist by etching, and then etching the surface of the gemstone by cathode bombardment with an ionised gas in a vacuum chamber. The marks applied are generally of rather poor resolution and the application of the marks takes a significant amount of time. [0003] There is a detailed description of the nature of the marks that can be applied in WO97/03846, in which the marks are applied by irradiating a diamond gemstone with ultraviolet laser radiation using a projection mask. [0004] 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 using an assembly or sequence of masks. The Invention [000] According to the invention, a layer of resist is applied to the surface of the diamond, a selected zone of the resist layer is ablated to form a mask on the diamond surface, and the diamond surface is etched through the mask, wherein an electrically-conducting layer is applied to the resist layer, and an electrical connection is provided to the electrically-conducting layer to prevent charging during etching. The invention extends to a diamond whose surface has been marked by the method, and to apparatus for carrying out the method [0006] The preferred form of etching is plasma etching. For plasma etching, it is especially advantageous to have an electrically-conducting layer, for example metal, and provide an electrical connection to the layer, to prevent charging of the diamond, the resist can then be non-electrically-conducting. The layer of metal can for instance be a layer of gold, for instance about 0.1 microns thick. It need not be applied to the whole of the resist layer, only to a region sufficiently large to prevent charging during plasma etching. The bilayer mask so formed may require different ablation conditions to a single layer, but generally both layers are ablated substantially simultaneously. It is found that the electrically-conducting layer effectively remains on the resist around the ablated zone, and thus prevents charging during plasma etching, whilst leaving the ablated zone clear of metal. The metal should have an ablation threshold no higher than that of the resist. A metal such as gold cannot be used on its own as a resist because it does not give high enough resolution, ablating too readily and leaving poorly defined edges. Furthermore, if a thicker layer of metal such as gold is used, there is a risk of the metal sputtering and redepositing in the ablated zone. [0007] A completely dry technique can be used (with no chemical etching or stripping steps); although wet cleaning may be required after plasma etching in order to remove the mask, this is not a critical step requiring controlled conditions. The bilayer mask can provide greatly improved resolution (particularly in relation to the laser etching technique disclosed in WO97/03846), and, in comparison with WO97/03846, requires a reduced pulse count if laser ablation is employed, for instance using about pulses or fewer, say pulses, rather than 00 pulses, making it practical to produce serial numbers using a sequence of masks, one for each number, for the resist ablation step. The ablation could be performed using a mask projection technique, but can be performed by direct beam writing. [0008] The resist can be any suitable resist, for instance a plastics (polymer) resist The thickness of the resist layer may for instance be not less than about 0. micron and/or not more than about 1 micron. [0009] In general, it is preferred that the plasma etching should be to a depth of not less than about nm and/or not greater than about 70 nm, more preferably not less than about nm and/or not greater than about 0 nm, a suitable value being about nm. [00] As an alternative to plasma etching, the diamond exposed by the mask can be etched using a broad ion beam to convert it to graphite or other non-diamond carbon which may then be removed by, for example, acid cleaning. [0011] The invention is particularly useful in association with etching methods which produce charging. Example [0012] A diamond gemstone is mounted on a holder 2

3 3 EP B1 4 (or a plurality of diamond gemstones can be so mounted). A layer of non-conducting polymer plasma etch resist is applied to the exposed surface of the diamond, for instance by spin coating using e.g. a Novalac photoresist or by evaporation. The resist layer is 0. to 1 microns thick. [0013] A layer of gold about 0.1 microns thick is deposited on the resist layer on at least part of the facet to be marked. [0014] The resist and gold layers are patterned by laser ablation with about pulses to leave a clean diamond surface. The laser wavelength is selected to give the best results with the chosen resist, shorter wavelengths permitting greater resolution than longer ones. 248 nm or other wavelengths may be used, but the preferred wavelength is 193 nm. [001] Using the holder, an electrical connection is made to the metal layer and the diamond is plasma etched in a standard manner, preferably under a partial pressure of oxygen. Zones of the facet not protected by the resist are etched to a depth of about nm, providing a clean etch with no evidence of blackening. The electrical connection to the metal layer prevents charging. [0016] The stone or stones is/are removed from the holder. The mask is removed by wet cleaning. [0017] The apparatus used for the laser ablation can be similar to that shown in Figure 2 of WO 97/ The method of any of Claims 1 to 4, wherein the thickness of the electrically-conducting layer is about 0.1 microns. 6. The method of any of Claims 1 to, wherein the selected zone of the layer is ablated using laser ablation. 7. The method of Claim 6, wherein about pulses or fewer are used for the laser ablation. 8. The method of any of the preceding Claims, wherein the diamond surface is etched to a depth of about 1 to about 70 nm. 9. The method of any of the preceding Claims, wherein the diamond surface is etched to a depth of about to about 0 nm.. The method of any of the preceding Claims, wherein the diamond surface is etched by plasma etching. 11. The method of any of Claims 1 to 9, wherein the diamond surface is etched using a broad ion beam. 12. The method of any of the preceding Claims, wherein an information mark is applied to the diamond. Claims 13. The method of Claim 1, wherein the mark applied is invisible to the eye using a x loupe. 1. A method of marking a surface of a diamond to produce a mark thereon which is invisible to the naked eye, the method comprising: applying to said surface a layer of resist; ablating a selected zone of the resist layer to form a mask on the diamond surface; and etching the diamond surface through the mask in order to mark the diamond surface; characterised in that: before ablating the selected zone of the resist layer, an electrically-conducting layer is applied to said resist layer, and an electrical connection is provided to the electrically-conducting layer during etching, to prevent charging during etching. 2. The method of Claim 1, wherein the thickness of the resist layer is about 0. to 1 microns The method of any of the preceding Claims, wherein the diamond is a gemstone. 1. The method of Claim 14, wherein the mark is applied to a polished facet of the gemstone. Patentansprüche 1. Verfahren zur Markierung einer Oberfläche eines Diamanten, um darauf eine Kennung zu erzeugen, die für das bloße Auge unsichtbar ist, wobei das Verfahren folgendes umfaßt: Auftragen einer Resistschicht auf die Oberfläche, Abschmelzen einer ausgewählten Zone der Resistschicht zur Bildung einer Maske auf der Diamantenoberfläche und Ätzen der Diamantenoberfläche durch die Maske, um die Diamantenoberfläche zu markieren, 3. The method of Claim 1 or Claim 2, wherein the electrically-conducting layer is metal. 4. The method of any of Claims 1 to 3, wherein the resist layer is non-electrically-conducting. dadurch gekennzeichnet, daß vor dem Abschmelzen der ausgewählten Zone der Resistschicht eine elektrisch leitende Schicht auf die Resistschicht aufgebracht wird und während des Ätzens eine elektrische Verbindung 3

4 EP B1 6 zu der elektrisch leitenden Schicht bereitgestellt wird, um ein Aufladen während des Ätzens zu verhindern. 2. Verfahren nach Anspruch 1, bei dem die Stärke der Resistschicht etwa 0, bis 1 um beträgt. 3. Verfahren nach Anspruch 1 oder 2, bei dem die elektrisch leitende Schicht aus Metall ist. 4. Verfahren nach einem der Ansprüche 1 bis 3, bei dem die Resistschicht elektrisch nichtleitend ist.. Verfahren nach einem der Ansprüche 1 bis 4, bei dem die Stärke der elektrisch leitenden Schicht etwa 0,1 µm beträgt. 6. Verfahren nach einem der Ansprüche 1 bis, bei dem die ausgewählte Zone der Schicht unter Anwendung des Laserabschmelzens abgeschmolzen wird. 7. Verfahren nach Anspruch 6, bei dem etwa oder weniger Impulse für das Laserabschmelzen angewendet werden. 8. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Diamantenoberfläche bis auf eine Tiefe von etwa 1 bis etwa 70 nm geätzt wird. 9. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Diamantenoberfläche bis auf eine Tiefe von etwa bis etwa 0 nm geätzt wird.. Verfahren nach einem der vorhergehenden Ansprüche, bei dem die Diamantenoberfläche durch Plasmaätzen geätzt wird. 11. Verfahren nach einem der Ansprüche 1 bis 9, bei dem die Diamantenoberfläche unter Anwendung eines breiten Ionenstrahls geätzt wird. 12. Verfahren nach einem der vorhergehenden Ansprüche, bei dem eine Informationskennung auf den Diamanten aufgebracht wird. 13. Verfahren nach Anspruch 1, bei dem die aufgebrachte Kennung unter Benutzung einer Lupe mit facher Vergrößerung für das Auge unsichtbar ist. 14. Verfahren nach einem der vorhergehenden Ansprüche, bei dem der Diamant ein Edelstein ist. 1. Verfahren nach Anspruch 14, bei dem die Kennung auf eine polierte Facette des Edelsteines aufgebracht wird Revendications 1. Procédé de marquage d'une surface d'un diamant pour y appliquer une marque invisible à l'oeil nu, le procédé comprenant les étapes ci-dessous: application d'une couche de réserve sur ladite surface; ablation d'une zone sélectionnée de la couche de réserve pour former un masque sur la surface du diamant; et gravure de la surface du diamant à travers le masque pour marquer la surface du diamant; caractérisé en ce que: l'ablation de la zone sélectionnée de la couche de réserve est précédée par l'application d'une couche conductrice d'électricité à ladite couche de réserve, une connexion électrique étant établie avec la couche conductrice d'électricité au cours de la gravure pour empêcher une charge au cours de la gravure. 2. Procédé selon la revendication 1, dans lequel l'épaisseur de la couche de réserve est comprise entre 0, et 1 micron. 3. Procédé selon les revendications 1 ou 2, dans lequel la couche conductrice d'électricité est composée de métal. 4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel la couche de réserve n'est pas conductrice d'électricité.. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel l'épaisseur de la couche conductrice d'électricité est de l'ordre de 0,1 micron. 6. Procédé selon l'une quelconque des revendications 1 à, dans lequel la zone sélectionnée de la couche et soumis à une ablation par laser. 7. Procédé selon la revendication 6, utilisant environ impulsions ou moins pour l'ablation par laser. 8. Procédé selon l'une quelconque des revendications précédentes, dans lequel la surface du diamant est gravée à une profondeur comprise entre environ 1 et environ 70 nm. 9. Procédé selon l'une quelconque des revendications précédentes, dans lequel la surface du diamant et gravée à une profondeur comprise entre environ et environ 0 nm.. Procédé selon l'une quelconque des revendications 4

5 7 EP B1 8 précédentes, dans lequel la surface du diamant est gravée par gravure au plasma. 11. Procédé selon l'une quelconque des revendications 1 à 9, dans lequel la surface du diamant est gravée par l'intermédiaire d'un large faisceau ionique. 12. Procédé selon l'une quelconque des revendications précédentes, dans lequel une marque d'information est appliquée sur le diamant. 13. Procédé selon la revendication 1, dans lequel la marque appliquée est invisible à l'oeil avec utilisation d'une loupe à grossissement x. 14. Procédé selon l'une quelconque des revendications précédentes, dans lequel le diamant est une pierre précieuse. 1. Procédé selon la revendication 14, dans lequel la marque est appliquée sur une facette polie de la pierre précieuse