WO1999057544A1

WO1999057544A1 - Examining diamonds

Info

Publication number: WO1999057544A1
Application number: PCT/GB1999/001358
Authority: WO
Inventors: Paul Martyn Spear; Philip Maurice Martineau
Original assignee: Gersan Establishment
Priority date: 1998-04-30
Filing date: 1999-04-30
Publication date: 1999-11-11
Also published as: AU3721799A

Abstract

A method of examining a diamond (3), for example, to distinguish natural from synthetic diamonds or to detect CVD/natural diamond doublets, comprising the steps of irradiating the diamond (3) with ultraviolet radiation (1) having a wavelength of about 193 nm, substantially at room temperature, and observing or measuring the resulting free exciton emission spectrum using high efficiency collection optics with a numerical aperture of about f2.

Description

Examining Diamonds Background to the Invention

The present invention relates to distinguishing natural diamond from synthetic diamond. The present invention can be particularly useful for detecting high pressure/high temperature (HPHT) synthetic diamond and CVD diamond material, and for detecting doublets (natural diamonds on which synthetic diamond layers have been deposited).

The value of a diamond is in part dependent upon its weight. Accordingly, synthetic diamond material may be deposited onto natural gem diamonds, before or after cutting of the diamond, to increase the weight of the finished product. Synthetic diamond material may be deposited on an uncut or part- worked natural diamond which is then worked, for example, into a round brilliant cut. Alternatively, a synthetic diamond coating may be deposited onto a fully fashioned brilliant stone after working of the stone. The thickness of the synthetic diamond material layer may be very thin - perhaps in the range 1 μm to 0.1 mm.

However, the value of a diamond also resides in its qualities of authenticity and uniqueness and in the fact that it is an entirely natural product. Thus, a diamond that has not been enlarged by deposition of synthetic diamond material has a value over a diamond that has.

Over the years, a number of methods of synthesising diamond material have been developed. One of these methods is the chemical vapour deposition (CVD) technique, which is a low pressure technique involving deposition of synthetic diamond (referred to as CVD diamond material in this specification) onto a substrate from a gas. CVD is the method which is most likely to be used to deposit synthetic diamond material onto a diamond.

CVD diamond material may be deposited on a diamond substrate. A diamond artificially enlarged by deposition of CVD diamond material is referred to in this 2 specification as a "CVD/natural diamond doublet". The CVD diamond material can replicate the structure of the diamond substrate (referred to as "homoepitaxial growth"). The CVD/natural diamond doublet produced can be identical in appearance, density and other common physical properties to an entirely natural stone and there may be a problem in identifying such a CVD/natural diamond doublet.

Methods of identifying CVD/natural diamond doublets have been proposed in the past. However, under certain conditions, such doublets may be passed as entirely natural diamonds. Thus, one object of the present invention is to provide a method and apparatus for accurately detecting CVD/natural doublets.

It is also an object of the present invention to provide an effective and accurate method and apparatus for quickly and simply distinguishing natural diamond from synthetic diamond, such that it may be put into operation by a person with relatively little training, and which is reliable and relatively inexpensive. Another object of the present invention is to provide a method and apparatus for accurately distinguishing between natural diamond and synthetic diamond, particularly pure and high purity CVD diamond and near colourless HPHT synthetic diamond. It is further a general object of the invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

Pure CVD diamond exhibits phosphorescence but no 575 nm luminescence when irradiated. High purity CVD diamond exhibits little or no phosphorescence and also no 575 nm luminescence when irradiated.

WO86/07457 discloses a method for distinguishing diamond, by visually detecting the Raman signal emitted from a specimen which is irradiated with suitable exciting radiation.

Diamond simulant comprises dense non-diamond material (e.g. metal oxides, particularly zirconium dioxide) which has similar refractive properties to diamond. Synthetic diamond comprises diamond material (i.e. crystalline carbon) produced by an industrial process. The technique disclosed by WO 86/07457 is only suitable for distinguishing diamond from diamond-like simulant. All diamonds, natural or synthetic, show the Raman emission when irradiated with suitable exciting radiation, and cannot be distinguished by this technique. 3

J. Appl. Phys. 77 (4), 15 February 1995, pp. 1729 - 1734 "Cathodoluminescence from high-pressure synthetic and chemical-vapor-deposited diamond" (Lawson et al.) describes a method of performing spectroscopic studies of CVD diamonds by observing the exciton emission in a diamond using electron beam excitation or cathodoluminescence (CL).

The theory of exciton emission in diamond goes back to the early 1960s, and will now be briefly described. Free exciton (FE) luminescence is intrinsic to all diamond. It consists of a series of resolvable emission lines in the deep ultra-violet at a wavelength of around 235 nm. In order to generate FE luminescence, electric charge carriers (i.e. electrons) within the diamond must be excited across the indirect band gap, from the valence band to the conduction band. The indirect band gap is approximately 5.5 eV. Photons of this energy have wavelength of 225 nm.

When an electron reaches the lowest point of the conduction band, there is a probability of this electron existing together with the hole it left behind in the valence band for a time of the order of 10 ns, during which time the electron exists in a level just under the conduction band. This is known as a free (or unbound) exciton. When the exciton recombines, i.e. the electron recombines with the hole, a photon is emitted. In order for the electron to return to the valence band, across the indirect band gap, a momentum-conserving phonon will also be emitted. There are three principal phonons. Therefore, the principal emission will be three lines around 234.5 nm, with the dominant emission being from a line at 234.5 nm.

Exciton emission known as bound exciton emission can also be detected from natural or synthetic semiconducting type lib diamond; the intensity of the emission can be used as a measure of the concentration of the uncompensated boron impurities present in these types of diamond.

Conventionally, as disclosed in the above-mentioned reference, exciton emission in diamond has been observed using electron beam excitation, or cathodoluminescence (CL) using custom made electron guns or Scanning Electron Microscopes (SEM). An HPHT diamond sample was irradiated with an electron beam having a diameter of approximately 2 μm, with a typical beam current of around 10^" A. An accelerating voltage of 15 kV was found to produce an efficient CL at around 2 μm below the 4 surface of the sample. The CL was then focused onto the entrance slit of a monochromator via an ellipsoidal mirror and the exciton emission spectra were observed using conventional spectroscopy techniques.

However, there are a number of disadvantages associated with the method described in the above-mentioned reference. Firstly, CL requires a vacuum. In the case of instruments which operate under a partial vacuum, there are critical issues of deposits building up on the diamond from the influence of the electron beam on the hydrocarbons from the vacuum pump, and the possibility of ion beam etching of the diamond, a symptom of the partial vacuum conditions. Therefore, high vacuum conditions must be maintained for the best results. Also, this reference does not describe a method and apparatus suitable for distinguishing between natural and synthetic diamond. Furthermore, FE emission under electron excitation must be measured with the sample at liquid nitrogen temperatures, principally to ensure that the intensity and resolution of the emission lines are sufficient. In the above-mentioned reference, the sample is held at a temperature of around 110 K by a liquid-nitrogen- cooled cold stage. This is because, at room temperature, the intensity of the FE emission is at least 100 times weaker than at liquid nitrogen temperatures, and is therefore extremely difficult to detect.

A paper by Takiyama et al, in Solid State communications, Vol. 99, No. 11, 1996, pages 793 to 797, describes experimental work on the FE emission from HPHT synthetic diamonds and natural diamonds, notes the lack of FE emission from the natural diamonds and the FE emission from the synthetic diamonds, and notes that increase in temperature causes a remarkable reduction in the FE emission.

The Invention

In accordance with the present invention, there is provided a method of distinguishing between natural and synthetic diamond, comprising the steps of: irradiating the diamond with ultraviolet radiation while the diamond is at room temperature; and observing or measuring the intensity of the resulting free exciton emission. 5

In accordance with the present invention, there is also provided apparatus for performing the above method.

It has surprisingly been found that natural and synthetic diamond can be distinguished using free exciton emission at room temperature, which can give enormous advantages in commercial practice in that there is no requirement for the employment of liquid nitrogen. The term "room temperature" should be interpreted broadly and covers any reasonable ambient temperature. Using the invention, it is found that diamonds can be examined sufficiently swiftly for commercial use, say with a maximum dwell of about 10 seconds per diamond, provided the power density of the ultraviolet radiation on the surface of the diamond is sufficiently high and provided there is sufficient efficiency of collection of the FE emission.

As used herein, the term "ultraviolet radiation" means electromagnetic radiation comprising UVA, UVB and UVC radiation as defined by the International Commission on Illumination (CIE) in 1970 as follows:-

UVA 315-400 nm

UVB 280-315 nm

UVC 100-280 nm

The ultraviolet radiation utilised by the present invention is preferably in the UVC range, specifically preferably less than about 225 nm but preferably at least about 180 nm as ultraviolet radiation of wavelengths less than about 180 nm is absorbed by air. This allows the resulting free exciton emission spectrum to be satisfactorily observed substantially at room temperature. The ultraviolet radiation is preferably of wavelength less than about 215 nm, and most preferably less than about 200 nm. In a preferred embodiment, the ultraviolet radiation has a wavelength of around 193 nm, and is supplied from a UV laser, although it is envisaged that the present invention could be operated using any convenient coherent or non-coherent ultraviolet source. It has been discovered that if ultraviolet radiation preferably of less than about 225 nm is used to irradiate the diamond, then the FE emission from pure and high purity CVD diamond material (which does not exhibit other competing luminescence bands such as the nitrogen impurity related orange 575 nm emission) and near colourless HPHT synthetic diamond, is very intense at liquid nitrogen temperatures, and therefore, surprisingly, 6 even sufficiently intense to be measured if the diamond is irradiated at room temperature using, for example, a spectrograph and a liquid nitrogen cooled CCD camera for the measurement.

In contrast, the FE emission from natural type I diamond is very weak or undetectable even at liquid nitrogen temperatures. Type I diamonds contain significant quantities of nitrogen and constitute the great majority of natural diamonds. Even in the case of natural type II diamond, the FE emission is weak compared to the FE emission from CVD or near-colourless HPHT synthetic diamond. Thus, by observing or measuring the intensity of the FE emission, CVD diamond material or a near-colourless HPHT synthetic diamond can in general be distinguished from a natural diamond. However, the FE emission from nitrogen-containing CVD diamond material or nitrogen-containing HPHT synthetic diamond is also weak so that it may be impossible to detect such materials.

FE emission is quenched by the presence of impurities such as nitrogen and by the presence of lattice defects. However, pure and high purity CVD diamond material contains relatively few lattice defects. As pure and high purity CVD diamond material contains nitrogen impurities at concentrations that are ineffective at quenching the exciton emission, the intensity of FE emission can also be used as a figure of merit for sample purity.

It is a particular advantage of the invention that pure and high purity CVD diamond material can be detected as this enables diamond doublets to be detected.

In carrying out the method of the invention, the resulting FE spectrum can be observed, but there is no need to isolate the separate FE lines; the intensity of the spectral range of all these FE lines can be observed or measured or just the intensity of one of the lines, particularly of the strongest line, that at 234.5 nm. As explained later, in another mode, the fits of the spectra can be compared.

The preferred embodiment of the method of the present invention is simple and convenient to carry out, because it does not require the diamond to be held at liquid nitrogen temperatures, although it is envisaged that the method of the present invention could be based on observing the free exciton emission spectrum at liquid nitrogen 7 temperatures or at any other suitable temperature if a higher intensity spectrum is required, or if the emission lines are to be highly resolved.

The upper limit of the excitation density at the surface of the diamond is determined by having to avoid marking the diamond in any way, but it is say less than about 1 or 0.5 J/cm /pulse at the surface of the diamond. However, the excitation density at the surface of the diamond is preferably not less than about 0.05 or 0.1

9 9

J/cm /pulse, a preferred value being 0.2 J/cm /pulse.

In order to maximise the effect, the ultraviolet radiation is preferably focused to a spot on the diamond, which is preferably less than about 5 mm diameter or 5 mm x 5 mm, but is preferably greater than about 0.5 mm diameter or 0.5 mm x 0.5 mm, to avoid damage to the diamond, a preferred spot being about 3 mm diameter or 3 mm x 3 mm.

In order to maximise the efficiency of collection of the FE's, the collection angle or numerical aperture of the collection optics should be as wide as possible, i.e. the f number should be as small as possible. However, if the numerical aperture is too large, magnification problems cause imperfect imaging on the observing or measuring means, e.g. the slit of a grating spectrometer. Preferably, the collection optics have a maximum numerical aperture equivalent to about f 1. Preferably, the collection optics have a minimum numerical aperture equivalent to about f 4. A preferred numerical aperture is equivalent to about f 2.

The collection optics should be suitably matched to the observing or measuring means to avoid excessive loss of energy. In general, the collection optics should image the spot of ultraviolet radiation on the diamond.

A preferred collection time is less than about two seconds, and in one procedure, about one second or about Vi second collection times are used, though it would be preferred to aim for a collection time of about 0.1 second.

In order to improve efficiency and speed up collection times, the background emission should be kept as low as possible, such background emission in the FE emission range being due mainly to background emission from the source of ultraviolet, i.e. the tail of the e.g. laser excitation line, and emission from materials within the instrument, particularly the materials supporting the diamond. Thus it is preferable to operate the method in such a way that the ultraviolet radiation impinges only on the 8 diamond, specular and refractive emissions from the diamond are suppressed by being absorbed on suitable black bodies, and the diamond holder does not emit radiation in the FE emission range under irradiation by ultraviolet.

In the preferred embodiment, an ultraviolet laser is used. In an alternative embodiment, the ultraviolet laser could be replaced by an array of xenon flash lamps, for example. In this case, means are preferably provided to remove any extraneous light at the FE wavelength, e.g. a filter means. The apparatus preferably comprises an e.g. static low bandpass filter between the ultraviolet source and the diamond, so as to allow only the laser wavelength and not the laser discharge to impinge on the diamond, for substantially eliminating spurious or extraneous radiation at the wavelength of the free exciton. The apparatus preferably comprises a bandpass filter between the diamond and the observing or measuring means, for substantially eliminating radiation at the wavelength of the irradiating ultraviolet radiation.

The method and apparatus of the present invention provides a very powerful way of discriminating high purity CVD diamond material and near colourless HPHT synthetics from natural diamond. Moreover, it allows the detection of the pure or high purity CVD diamond material component in a loose (i.e. not set) diamond doublet. Some conventional detecting techniques may pass a CVD doublet as natural if the natural diamond component of the doublet exhibits absorption due to the N3 impurity centre and the CVD diamond material component does not exhibit either 575 nm luminescence or phosphorescence. Thus, the present invention provides a method and apparatus for detecting doublets, prior to screening using other instruments for classification or further distinguishing between natural and synthetic diamonds, which even identifies a doublet having, for example, a natural diamond component exhibiting N3 absorption and a CVD diamond material component which does not exhibit orange 575 nm luminescence or phosphorescence, such doublets being passed as natural by conventional detection instruments.

The FE emission may be detected using a spectrograph comprising a rotating grating and synchronised photomultiplier (PMT). Such a system could comprise a Monolight Optical Spectrum Analyser Scanning Monochromator type 6121 fitted with a 9 grating optimised for the FE region (supplied by Macam Photometries Ltd) and fitted with a solar blind PMT type R166 (supplied by Hamamatsu Photonics).

Alternatively the FE emission may be detected using a static narrowband filter, e.g. of bandwidth 10 nm at half maximum, the signal through which is compared to the background signal through a similar narrowband filter centred away from the FE emission at, for example, 200 nm. Both signals are preferably each detected using a PMT. The difference in the signals through the two filters is a measure of the intensity of the FE emission.

As another alternative, the FE emission may be detected using a rotating narrowband filter covering the dominant FE wavelength (234.5 nm). The thus modulated signal is then preferably detected using a PMT.

In a different embodiment, the sample may be placed at the centre of an integrating sphere, effective in the ultraviolet, and the emission at the FE wavelength detected at a port on the integrating sphere by means of a static narrowband filter and PMT. The background signal could be detected at a second port on the integrating sphere by means of a static narrowband filter, at for example 200 nm, and a PMT.

Preferred Embodiment

An embodiment of the present invention will now be described by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic block diagram of an apparatus for performing the method of the present invention;

Figure 2 shows the UVFE spectrum from high purity CVD diamond under 193 nm laser at room temperature.

Referring to Figure 1 of the drawing, the apparatus comprises a 193 nm ultraviolet laser radiation source 1 such as a LAMBDAPHYSIK Minex excimer laser (supplied by Coherent (UK) Ltd), which gives a fundamental frequency at 193 nm, has a pulse frequency of 10 to 50 Hz and is preferably operated at 25 Hz, and has a power of 0.2 to 1.0 watts at its output. There is preferably a static low bandpass filter 2 (substantially transmitting only radiation of wavelength 200 nm or below) between a 10 diamond sample 3 and the ultraviolet laser 1. The apparatus further comprises a static long bandpass filter 4 (such as a 3 mm thick BG24A filter), a suitable lens assembly 5 positioned anywhere that allows collection of the FE emission from the surface of the diamond sample 3, and a quartz fibre 6 matched to the entrance slit of a spectrograph 7 fitted with a grating optimised for the FE region and a CCD detector 8. By way of example, the spectrograph 7 could be a Spex Industries 270M and the detector 8 a Spectrum One cooled silicon array CCD detector supplied by Instruments S.A. It is not essential to use a liquid-nitrogen-cooled CCD detector as thermoelectrically-cooled CCD detectors are available.

The diamond sample 3 is irradiated with radiation of wavelength of around 193 nm by the source 1, focused to form a spot on the surface of the diamond sample 3. The static low bandpass filter 2 is arranged to allow only the wavelength of the radiation source to impinge on the diamond sample 3, thereby eliminating spurious radiation at the wavelength of the free exciton. The lens assembly 5 collects the FE emission which is emitted substantially at the surface of the diamond. The lens 5 has a numerical aperture equivalent to about f 2 and focuses the spot on the entrance to the fibre 6 so that the spot on the diamond is imaged on the slit of the spectrograph 7. The filter 4 substantially eliminates radiation or scatter at the laser wavelength, preventing the 193 nm radiation reaching the spectrograph 7. The quartz fibre 6 is flexible and merely helps set up the instrument, and has a numerical aperture of 0.22. This is matched to the lens assembly 5, which, if it has as preferred a diameter of 2.5 cm will be 5.7 cm from the entrance to the fibre 6 with magnification of FE emission. If the fibre 6 has a typical diameter of 1 mm, it is overfilled with the 3 mm x 3 mm image of the spot on the diamond 3, providing reasonably efficient matching. The spectrograph 7 produces a complete FE spectrum (see Figure 2) on the CCD detector 8, which is then read and analysed. In Figure 2, it should be noted that the third FE emission line, at about 247.5 nm, is very slight.

A holder 9 for the diamond 3 is indicated schematically as being a pair of metal prongs on a metal goniometer, designed to keep background emission as low as possible and avoid specular reflections, but for instance one can use a putty-like material such as "Blutack" (trademark), which gives low emission under irradiation with ultraviolet. 11

The ultraviolet source 1 is focused to provide a 0.3 mm x 0.3 mm spot on the surface of the diamond sample 3. The source 1 is operated to provide an excitation density at the surface of the diamond of about 0.2 J/cm /pulse. Software 10 associated with the spectrograph 7 operates to look at all wavelengths in the FE emission range, as illustrated in Figure 2, and to up-date every half second. The software 10 causes the apparatus to operate in two modes. Intensity can be measured across the FE emission range, say from 227 nm to 257 nm. Alternatively, ratioing can be used to discriminate against the background emission and to determining the intensity of the 234.5 nm line alone, say ratioing from trough to trough on either side of this line, say from 225 nm to 237 nm. The intensity value is thresholded, any diamond giving a reading above the threshold being considered as being pure or high purity CVD diamond material or HPHT diamond, or possibly a natural type Ila diamond. Ninety eight per cent of all natural diamonds are type I, though this depends on the mine, and thus natural type Ila diamonds are rare. However, the FE emission from natural type Ila diamonds is significantly less intense than for pure or high purity CVD diamond material of HPHT diamonds, and thus a threshold could be set above the natural type Ila diamond level. Another way of operating is to keep a low threshold and to treat all rejected diamonds as merely suspect, for further sorting.

The other mode of operation is to precalibrate the FE emission curve (generally as in Figure 2), which will have fundamentally the same profile for both pure or high purity CVD diamond material and HPHT diamond material (though the background emission will vary), and to fit the emission profile to the precalibrated profile. If there is a fit, the diamond sample 3 is probably synthetic.

An indicator shown schematically at 11 can give a signal whether the diamond 3 is wholly or partly synthetic.

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. 12 Unless the context clearly requires otherwise, throughout the description and the claims, the words 'comprise', 'comprising', and the like are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Claims

13 CLAIMS:

1. A method of distinguishing natural from synthetic diamond, comprising the steps of irradiating the diamond with ultraviolet radiation while the diamond is at room temperature, and observing or measuring the intensity of the resulting free exciton emission.

2. A method according to claim 1, and used for distinguishing natural diamond from pure or high purity CVD diamond material.

3. A method according to claim 1 or 2, wherein the free exciton emission spectrum is observed.

4. A method according to any of the preceding claims, wherein said ultraviolet radiation has a wavelength less than about 225 nm.

5. A method according to any of the preceding claims, wherein the fluence of the ultraviolet radiation at the surface of the diamond is not less than about 0.05

J/cm /pulse.

6. A method according to any of the preceding claims, wherein the ultraviolet radiation is focused to a spot on the diamond having a diameter of less than about 5 mm or a size of less than about 5 mm x 5 mm.

7. A method according to any of the preceding claims, wherein the ultraviolet radiation is focused to a spot on the diamond having a diameter of greater than about 0.5 mm or a size of greater than about 0.5 mm x 0.5 mm.

8. A method according to any of claims 1 to 5, wherein the ultraviolet radiation is focused to a spot on the diamond that has a diameter of about 3 mm or a size of about 3 mm x 3 mm. 14

9. A method according to any of the preceding claims, wherein collection optics are used to collect the free exciton emission, which collection optics have a minimum numerical aperture equivalent to about f 4.

10. A method according to any of the preceding claims, wherein collection optics are used to collect the free exciton emission, which collection optics have a maximum numerical aperture equivalent to about f 1.

11. A method according to any of claims 1 to 8, wherein collection optics are used to collect the free exciton emission, which collection optics have a numerical aperture equivalent to about f 2.

12. A method according to any of the preceding claims, wherein the ultraviolet radiation is focused to a spot on the diamond, and the spot on the diamond is imaged by collection optics onto means for observing or measuring the resulting free exciton emission.

13. A method according to any of the preceding claims, wherein the free exciton emission is collected for a time of less than about 2 seconds.

14. A method according to any of the preceding claims, wherein the ultraviolet radiation is substantially confined to the diamond, and wherein the diamond is held in a holder which does not emit any substantial radiation in the free exciton emission range under irradiation by ultraviolet radiation.

15. A method according to any of the preceding claims, wherein said ultraviolet radiation is supplied by an ultraviolet (UV) laser.

16. A method according to any of claims 1 to 14, wherein said ultraviolet radiation is supplied by an array of xenon flash lamps. 15

17. A method according to any of the preceding claims, comprising providing a bandpass filter between a source of the ultraviolet radiation and the diamond, for substantially eliminating radiation at the wavelength of the free exciton emission.

18. A method according to any of the preceding claims, comprising providing a bandpass filter between the diamond and observing or measuring means, for substantially eliminating radiation at the wavelength of the irradiating ultraviolet radiation.

19. A method according to any of the preceding claims, wherein said free exciton emission is observed using a spectrograph.

20. A method according to any of claims 1 to 18, wherein said free exciton emission is observed using a rotating filter covering the dominant free exciton wavelength of substantially 234.5 nm.

21. A method according to any of claims 1 to 18, wherein said free exciton emission is observed using a static filter.

22. A method according to any of claims 1 to 18, wherein said free exciton emission is observed using an integrating sphere.

23. A method of distinguishing between natural and synthetic diamonds, substantially as herein described with reference to the accompanying drawings.

24. An apparatus for distinguishing natural from synthetic diamond, comprising irradiating means for irradiating the diamond with ultraviolet radiation while the diamond is at room temperature, and means for observing or measuring the intensity of the resulting free exciton emission spectrum. 16

25. An apparatus according to claim 24, and arranged to carry out the method of any of claims 2 to 22.

26. An apparatus for distinguishing between natural and synthetic diamonds, substantially as herein described with reference to the accompanying drawings.