Note: Descriptions are shown in the official language in which they were submitted.
CA 02316957 2003-08-22
WO 99I336~1 PCTlG898/03889
DIAMOND OR GEMSTONE MARKING BY PLURALrIY OF GROOVES
Background to the Invention
The present invention relates to a method and apparatus for marking a surface
of a
diamond or other gemstone. The mark may be any mark, but the invention is
particularly but not exclusively directed to applying an information mark to
the
diamond and the indicia can be alphanumeric characters or the like. 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, and
especially
for applying a mark which is invisible to the naked eye or invisible to the
eye using
a x10 loupe (which is the loupe used by jewellers), 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, but it 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
visibility when
viewed using appropriate magnification and illumination conditions, the marks
being
such that they do not detract from the value and appearance of the diamond or
other
gemstone.
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The Invention
In accordance with a first aspect of the present invention, there is provided
a method of
forming a mark on a surface of a diamond or gemstone, which mark comprises one
or
more unitary indicia, the method comprising the step of forming a plurality of
elongate,
parallel, equally-spaced grooves on said surface of the diamond or gemstone,
which
grooves define the indicium or indicia, the mark being such that it cannot be
distinguished by the naked eye, the grooves producing a visible diffraction
effect under
certain lighting (or illumination) and magnification conditions.
Also in accordance with the first aspect of the present invention, there is
provided an
apparatus for performing the above-mentioned method. The present invention
further
extends to a diamond or gemstone which has been marked by the above-mentioned
method.
In accordance with the second aspect of the present invention, there is
provided a
method of forming a mark on the surface of a diamond or gemstone, comprising
the step
of forming a plurality of elongate, parallel, equally-spaced grooves on the
surface of the
diamond or gemstone, said grooves producing a visible diffraction effect under
certain
lighting (or illumination) and magnification conditions, without detrimentally
affecting
the clarity grade of the diamond or gemstone.
Also in accordance with the second aspect of the present invention, there is
provided an
apparatus for performing the above-mentioned method. The present invention
further
extends to a diamond or gemstone which has been marked by the above-mentioned
method.
The mark cannot in general be distinguished or read by the naked eye. However,
the
grooves provide a visible diffraction effect under certain lighting
conditions. The
greater the depth of the grooves, the more visible the mark will be when
viewed. The
grooves should be of a suitable depth so that the mark is highly visible under
appropriate viewing conditions, but not so deep that the clarity grade of the
diamond or
other gemstone is detrimentally affected. In one preferred embodiment, each
groove is
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WO 99/33671 PCT/GB98/03889
3
not less than about 10 nm deep and/or not more than about 50 nm deep with no
evidence of blackening. A specific example would be around 30 nm.
The gooves may be in the form of parallel lines, or even a plurality of
intersecting
grooves forming cross-hatched pattern, depending on the effect desired.
Although the marking can be carried out using any suitable means, e.g. etching
with an
excimer laser or plasma etching, marking is preferably carried out using an
ion beam,
and most preferably by direct writing on the diamond surface with a focused
ion beam.
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. Typically Gallium ions are used, but a beam of other suitable
ions
may alternatively be used.
It is thought that each incident ion displaces a number of carbon atoms from
their sites
to create interstitials and vacancies in the diamond crystal. As the amount of
damage
(crystal lattice disorder) increases there is a tendency for the diamond spa
bonds to be
replaced by the graphite like sp2 bonds. These bonds can be attacked by a
chemical
etch to remove the disordered layer. By limiting the dose, and providing there
is
sufficient dose, the incident ions cause disordering that converts the diamond
to a
graphite-like or other non-diamond structure that can be cleaned using, for
example, a
powerful oxidizing agent, such as molten potassium nitrate, at a temperature
of
approximately 380-550 Centigrade for a period of between a few minutes and
several
hours.
The use of potassium nitrate has been found to be more effective in removing
disordered diamond than other known processes, thus allowing a mark of a given
depth
to be produced with a relatively low dose of ions.
Other suitable oxidising agents may be molten compounds such as alkali metal
salts;
compounds in the form XnYm where the group X may be Li+, Na+, K+, Rb+, Cs+ or
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4
other ration, and the group Y may be OH', N03 ; Ozz-, Oz-, C03z- or other
anion, the
integers n and m being used to maintain charge balance. Mixtures of such
compounds
may be used. Air or other oxygcn-containing gases may also be present.
As an alternative, the disordered layer of the diamond can be removed using an
acid or
potassium nitrate dissolved in acid. However, the use of, for example, molten
potassium nitrate eliminates acid fumes. Furthermore, the need to dispose of
spent acid
is eliminated, thereby offering safety, environmental and economic benefits.
It is required to minimise the depth of disordering inflicted by the ion beam
on the
surface of the diamond. The depth of disordering is determined by the range of
ions.
For 50 keV Gallium, this range is about 30 nm. The minimum dose may be around
10'3 / cmz and is preferably about 10'4lcmz to 101slcmz; but good marks can be
applied
with a fairly modest dose, the preferred maximum dose being about 10'6/cm z or
even
up to about 10"lcmz. 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 0.5
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 an ion beam, to prevent
charging. The
thickness of the gold, or other coating alters the variation of depth of the
mark with
beam energy and may thus be chosen to optimise the mark produced. However, it
is
preferred to irradiate the region to be marked with a low energy source of
electrons
CA 02316957 2003-06-17
(e.g. around 1-100e~ from, for example, an electron flood gun, during the
marking
process to prevent charging.
If a focused ion beam is used to form the plurality of grooves, the accuracy
of the
method is such that no masking is required: the ion beam is applied directly
to the
surface of the diamond at the positions where the grooves are required to be
formed.
However, if other, less accurate methods of forming the grooves are to be
used, then it
may be necessary to mask the areas between the grooved areas to avoid marking
them.
In accordance with the third aspect of the present invention, there is
provided apparatus
for viewing a mark on the surface of a diamond or gemstone, said mark
comprising a
plurality of elongate, parallel, equally spaced grooves which produce a
visible
diffractive effect when lit and magnified, the apparatus comprising two
illumination
means for illuminating the mark with light rays each at an angle which
corresponds to
the diffraction angle of light of a predetermined wavelength or band of
wavelengths, the
illumination means being such that the light rays are at substantially the
same angle to
the normal to said surface, but, as seen looking normal to said surface, are
in
substantially opposite directions, and viewing and magnification means for
viewing said
mark and for magnifying the viewed image of said mark. The invention also
extends to
a method of viewing the mark corresponding to the apparatus defined above.
The mark is preferably viewed against a dark background, ie. it is preferred
that the
illuminating light is substantially prevented from reflecting through the
stone and
appearing directly behind or close to the mark. It will be apparent to a
person skilled in
the art that, in order to achieve this, the angle and direction from which the
illuminating
light is supplied (and hence the orientation and spacing of the lines) must be
chosen so
as to ensure that no light can follow the undesired path.
The typical range ofmagnification required to view the mark is x10 to x50.
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d.sin6 = tn.~,
where d is the distance between each groove, 8 is the angle of the incident
light, ~, is the
wavelength of the diffracted light and n is an integer. Preferably n=1.
Thus, when a mark has been formed on a diamond, d and n are fixed, and the
wavelength of diffracted light, i.e. the colour which the mark will appear
when viewed,
can be varied by varying the angle of incident light. Thus, if it is desired
that the mark
appears blue when viewed, then the angle of the incident light, i.e. 8, is set
so that 7~ is
around 450nm; using the above equation. Similarly, if the mark is to appear
red, then 8
is set so that 7~ is around 620nm.
In one embodiment of the apparatus according to the third aspect of the
present
invention, the illuminating means may comprise a light source and an opaque
screen
located in the incident light path, the screen having two apertures formed
therein, the
apertures being formed on either side of a generally central position such
that two
angular directional light sources are produced. The distance between the
apertures
determines the angle of the incident light sources. However, it is envisaged
that the
directional light sources) may be provided by any convenient means, e.g. two
separate
light sources. A conventional microscope may include illumination means
comprising
a circular ring-shaped source comprising optical fibres illuminated by a
remote tungsten
light bulb. The illumination means of the present invention can be obtained by
masking
off all but two diametrically opposite sections of the illuminator.
Embodiments of the present invention will now be described by way of examples
only
and with reference to the accompanying drawings, in which:
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Figure 1 is a magnified schematic diagram of the type of mark which is formed
using
the method and apparatus of the first and second aspects of the present
invention;
Figure 2 is a further magnified cross-sectional view along line A - A of
Figure 1; and
Figure 3 is a schematic view of an embodiment of apparatus according to the
third
aspect of the present invention.
Referring to Figure 1 of the drawings, a mark in the form of an alphanumeric
character
may be formed by means of a plurality of equally spaced, parallel elongate
grooves 10
each separated by a distance d. Each groove 10 may have a generally square or
rectangular cross-section, as shown in Figure 2. Alternatively, a sinusoidal
profile may
be preferred to reduce unwanted higher order diffraction.
A specific method of forming each groove will now be described.
A diamond gemstone is mounted in a suitable holder and placed in a vacuum
chamber
equipped with a focused ion beam source such as, supplied by FEI or Micrion.
During
exposure, the region to be marked may be irradiated using an electron flood
gun
supplied by Micrion, providing a low energy, e.g. 1-100 eV, source of
electrons, to
prevent the diamond from becoming charged.
Using a focused ion beam with a raster scan or similar to scan the beam with,
for
instance, electrostatic deflection (as an alternative, the diamond may be
moved but this
is less practical), and optionally any suitable software for controlling the
ion beam, a
series of closely spaced parallel lines are 'written' on the diamond facet.
The sample is removed from the vacuum chamber, placed in a stainless steel
crucible,
and covered with a powerful oxidising agent, such as molten potassium nitrate,
for a
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period of around one to two hours. The sample is subsequently cooled and
removed
from the potassium nitrate before being cleaned using water and ethanol,
thereby
removing the portions of the diamond surface which have been disordered by the
ion
beam, and leaving a series of closely spaced grooves each around 30 - 35 nm
deep, with
no evidence of blackening.
Upon examination before cleaning, the exposed region is identifiable by its
graphite-
like appearance when examined, for example, in a reflected light microscope.
Such a
mark would not be acceptable to a diamond grader, in that it would
substantially reduce
the clarity grade of the diamond. However, after cleaning using the powerful
oxidising
agent, the mark is not easily visible in a microscope, with no contrast
between the mark
and surrounding areas. The mark only becomes visible when illuminated by
preferably
two directional light sources at an angle which corresponds to the angle of
diffracted
light of a particular wavelength, for example blue light, at which time the
mark appears
blue. Such a mark is acceptable to a diamond grader in that it does not
detrimentally
affect the clarity grade of the diamond.
The closely spaced grooves are preferably formed within an 'invisible outline'
of an
alphanumeric character or the like, as shown in Figure 1 of the drawings.
Referring now to Figure 3 of the drawings, a method and apparatus for viewing
the
mark produced by the process described above will now be described by way of
example only.
The marked diamond 104 is placed on the viewing surface 100 of a conventional
microscope 102. The diamond 104 is illuminated by two directional light
sources 106
having an angle 8 relative to the vertical axis Y. As described above, 8 is
chosen so
that the mark appears to be, for example, blue or red, as desired. Thus, if
the mark is to
appear blue, and d is approximately 1200nm, then 8 is chosen to satisfy:
d.sin9 = (approximately) 450nm
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where n=1 and 450nm is the approximate wavelength of blue light, which is the
wavelength of the diffracted light at X in Figure 3. In this case, 8 =
22°.
The directional light sources may be provided by a generally ring-shaped
illuminator,
all but two diametrically opposite portions thereof being masked off. However,
any
suitable light source may be used to produce the same effect.
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.
