Synthetic CVD diamond

Abstract

The present disclosure relates to methods for synthesizing synthetic CVD diamond material and high quality synthetic CVD diamond materials.

Claims

1. Synthetic CVD diamond material comprising single substitutional nitrogen (N.sub.s.sup.0) at a concentration of greater than about 0.5 ppm and having a total integrated absorption in the visible range from 350 nm to 750 nm the synthetic CVD diamond material having a low concentration of defects other than N.sub.s.sup.0 that absorb in the visible range from 350 nm to 750 nm, wherein at least about 85% of the integrated absorption in the visible range from 350 nm to 750 nm is attributable to N.sub.s.sup.0.

2. Synthetic CVD diamond material according to claim 1, having a hue angle greater than about 80 for a transmission pathlength of 1 mm.

3. Synthetic CVD diamond material according to claim 1, wherein the material has a photoluminescence spectrum at 77 K using the 488 nm excitation of an argon-ion laser which shows a peak at from about 543.0 to about 543.2 nm, with an intensity ratio of this peak normalized to the 1st order diamond Raman of greater than about 0.005.

4. Synthetic CVD diamond material according to claim 1, wherein the concentration of N.sub.s.sup.0 is greater than about 2.5 ppm, as measured using the 270 nm peak using UV-visible absorption spectroscopy.

5. Synthetic CVD diamond material according to claim 1, wherein the elemental concentration of individual chemical impurities other than nitrogen and hydrogen is less than 0.1 ppm.

6. Synthetic CVD diamond material according to claim 1, wherein at least about 50% of the volume of the synthetic CVD diamond material is formed from a single growth sector.

7. Synthetic CVD diamond material according to claim 1, having the colour parameters a* between 20 and 1; b* between 5 and 20; C* between 0 and 30 and L* between 40 and 100.

8. Synthetic CVD diamond material according to claim 1, wherein the synthetic CVD diamond material is in the form of a freestanding entity having a thickness of greater than about 0.2 mm.

9. Synthetic CVD diamond material according to claim 1, wherein the synthetic CVD diamond material is in the form of a layer having a thickness of about 0.5 mm or less.

10. Synthetic CVD diamond material according to claim 1, wherein the synthetic CVD diamond material is in the form of a doublet.

11. A gemstone comprising synthetic CVD diamond material according to claim 1.

12. An electronic device comprising synthetic CVD diamond material according to claim 1.

Description

(1) The present invention is now described, by way of illustration only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a ternary diagram for the CHO space;

(3) FIG. 2 shows a comparison of a UV/visible optical absorption spectrum obtained from Sample 1, Layer 2 of Example 1 with a spectrum obtained from an HPHT Type Ib synthetic single crystal diamond;

(4) FIG. 3 shows a UV/visible optical absorption spectrum obtained from Sample 1, Layer 2 of Example 1 deconvolved into its components;

(5) FIG. 4 shows UV/visible optical absorption spectra obtained from Sample 2, Layer 1 and Sample 2, Layer 2 of Example 2;

(6) FIG. 5 shows the UV/visible optical absorption spectrum Sample 2, Layer 2 of Example 2 deconvolved into its components;

(7) FIG. 6 shows a photoluminescence (PL) spectrum obtained at 77 K from Sample 2, Layer 1 and Sample 2, Layer 2 of Example 2 by exciting with radiation having a wavelength of 488.2 nm using a 50 mW Ar-ion laser;

(8) FIG. 7 shows a photoluminescence (PL) spectrum obtained at 77 K from Sample 2, 30 Layer 1 and Sample 2, Layer 2 of Example 2 by exciting with radiation having a wavelength of 514.5 nm using a 50 mW Ar-ion laser;

(9) FIG. 8 shows a PL image obtained from Sample 3 of Example 3 showing the clear distinction between Sample 3, Layer 1 and Sample 3, Layer 2, overlaid with the spatially resolved 737 nm PL trace from the sample;

(10) FIG. 9 shows UV/visible optical absorption spectra obtained from Sample 4, Layer 2 of Example 4;

(11) FIG. 10 shows a PL spectra obtained from Sample 5, Layer 2 and Sample 6, Layer 2 of Example 5 obtained by exciting with radiation having a wavelength of 458 nm; and

(12) FIG. 11 shows a projection X-ray topograph obtained from Sample 6 of Example 5.

EXAMPLES

(13) The foregoing examples are intended to describe the current invention without limiting the invention to the content of the examples.

(14) Several of the examples were made using processes in which the synthesis environment was changed partway through the CVD diamond synthesis so that a CO.sub.2/CH.sub.4/H.sub.2 process could be compared directly with a CH.sub.4/Ar/H.sub.2.

Example 1

(15) Example 1 describes the preparation of single crystal diamond substrates suitable for the deposition of the diamond material of the invention, the deposition of a layer of diamond material using a CH.sub.4/H.sub.2 synthesis process, and the subsequent deposition of a layer of material made by the method of the invention.

(16) 1) The substrate was prepared using the following steps: a) A single crystal diamond was selected from a stock of material (type Ia natural stones and type Ib HPHT stones) on the basis of microscopic investigation and birefringence imaging to identify a stone which was substantially free of strain and imperfections. b) A parallel-surfaced plate with lateral dimensions of approximately 4 mm4 mm and approximately 500 m thick and surface R.sub.a of less than 1 nm, with all faces being within 5 of {100} surfaces, was prepared from the selected diamond using processes including laser sawing, and mechanical lapping and polishing. The processes used had previously been optimised to minimise subsurface defects using a method of a revealing plasma etch to determine the defect levels being introduced by the processing.

(17) A substrate produced by above steps typically has a density of defects measurable after a revealing etch that is dependent primarily on the material quality and is below about 510.sup.3 defects/mm.sup.2, and generally below about 10.sup.2 defects/mm.sup.2.

(18) 2) The diamond substrate was mounted on a tungsten carrier using a AuTa high temperature diamond braze. This was introduced into an 896 MHz microwave plasma CVD diamond reactor.

(19) 3) The reactor was started and the substrate was subjected to a two stage pre-growth etching sequence consisting of: (a) an in situ oxygen plasma etch performed using gas flows of 40/20/3000 sccm of O.sub.2/Ar/H.sub.2 at a pressure of about 23610.sup.2 Pa (about 180 Torr) and a substrate temperature of about 716 C. for a duration of about 30 minutes; (b) followed without interruption by a hydrogen etch with the removal of the O.sub.2 from the gas flow for a duration of about 30 minutes.

(20) 4) A first layer of CVD diamond (Sample 1, Layer 1) was deposited on the etched substrate by the introduction of CH.sub.4 into the gas flow, giving a gas flow comprising 140/20/3000 sccm of CH.sub.4/Ar/H.sub.2 at a pressure of about 23610.sup.2 Pa (about 180 Torr). The source gas additionally comprised an atomic fraction of nitrogen of 1.4 ppm. The substrate temperature was 840 C. Sample 1, Layer 1 has been produced by a process of a type well known in the art and thoroughly characterised, and therefore provides an in situ standard against which Sample 1, Layer 2 can be compared. The inventors have found that the optical properties of this diamond material are highly consistent and repeatable between synthesis runs.

(21) 5) A second layer, Sample 1, Layer 2, was prepared using the following conditions: gas flows of 290/250/230 sccm for CO.sub.2/CH.sub.4/H.sub.2, a C.sub.f:O.sub.f:H.sub.f ratio of 0.21:0.22:0.57, a C.sub.f:O.sub.f ratio of 0.95:1, a pressure of 18410.sup.2 Pa (138 Torr), nitrogen present at 18 ppm of atomic nitrogen equivalent, and a substrate temperature of 830 C. Of the atomic fraction of hydrogen in the source gases, 0.18 is added as H.sub.2 and 0.39 is added from sources other than H.sub.2. The H.sub.f value of 0.57 gives a P.sub.arc value of P.sub.arc=170(H.sub.f+0.25)=139 Torr.

(22) The surfaces of the sample were processed sufficiently to facilitate optical characterisation of the diamond material.

(23) UV/visible absorption spectra were recorded by illuminating one of the polished side surfaces of the sample such that the light path was entirely in either Layer 1 or Layer 2. The pathlength used for obtaining the absorption spectra was roughly the lateral dimension of the layer.

(24) Prior to the optical characterisation, the sample was exposed to a deuterium lamp (15 Watt electrical power consumption) for 10 minutes with the sample approximately 80 mm from the filament of the bulb.

(25) The experimental absorption spectrum was obtained as described in the body of the text FIG. 2 shows a comparison of the UV/visible absorption spectrum for Sample 1, Layer 2 with a spectrum from an HPHT type Ib synthetic diamond.

(26) The experimental absorption spectrum was then de-convolved, as described elsewhere in the specification, to determine the concentration of N.sub.s.sup.0. FIG. 3 shows the measured spectrum and the Type Ib component for Sample 1, Layer 2.

(27) The measured optical absorption spectrum was integrated over the visible wavelength range (i.e. 380 nm, equivalent to 3.2168 eV, to 750 nm, equivalent to 1.6527 eV) giving a value (C) with units of eV.Math.cm.sup.1. The absorption attributable to N.sub.s.sup.0 was similarly determined in eV.Math.cm.sup.1 over the same wavelength range giving a value (B). The ratio B/C and the difference CB are then calculated and are used for characterising the material.

(28) TABLE-US-00003 Integrated Integrated absorption absorption in in visible Difference Sample [N.sub.s.sup.0] visible (C), attributable to N.sub.s.sup.0 (C B), Ratio 1 (ppm) (eV .Math. cm.sup.1) (B), (eV .Math. cm.sup.1) (eV .Math. cm.sup.1) (B/C) Layer 2 1.95 0.5989 0.5202 0.0787 0.87

(29) The results of the optical analysis show that for Sample 1, Layer 2, the proportion of the optical absorption over the range 350 nm to 750 nm that is due to N.sub.s.sup.0 is greater than 0.35 or 35%.

(30) In addition, the absorption coefficients for the 360 nm and 510 nm bands were measured from the deconvolved spectrum at the peaks of the respective bands.

(31) TABLE-US-00004 Absorption coefficient (cm.sup.1) Sample 1 360 nm band (cm.sup.1) 510 nm band (cm.sup.1) Layer 2 0.1 0.2

(32) The CIE L*a*b* coordinates for an optical pathlength of 1 mm were derived from the absorption spectrum, in the way described in the detailed description of the invention, and are tabulated below.

(33) TABLE-US-00005 CIE L* a* b* Coordinates Hue angle at 1 mm at 1 mm thickness optical path Sample 1 a* b* C* L* length (degrees) Layer 2 0.1 2.1 2.1 85.3 93.6

(34) Sample 1, Layer 2, made by the method of the invention, has a hue angle for an optical pathlength of 1 mm of greater than 80 in addition to at least 35% of the optical absorption in the visible spectrum being due to N.sub.s.sup.0.

(35) Photoluminescence (PL) spectra from Sample 1, Layer 1 and Sample 1, Layer 2 were recorded at 77 K excited using 514.5 nm light from a 50 mW Ar-ion laser. The ratio of the intensities of the peaks at 637 nm and 575 nm were 0.8 and 1.4 for layers 1 and 2 respectively.

(36) The inventors have found that the higher the value of the ratio of the 637 nm PL line to the 575 nm PL line, the closer the optical absorption spectrum is to that of a pure Type Ib component. In a diamond containing only N and NV centres, the ratio of NV.sup.:NV.sup.0 (i.e. the ratio of the intensities of the 637 nm and 575 nm lines, 637 nm:575 nm) is thought to be largely governed through the following equation:
N+NV(575 nm).fwdarw.N.sup.++NV.sup.(637 nm)

(37) However in diamond material which is found to contain significant contributions from absorptions at around 360 nm and 510 nm, another trap(s) competes for this electron transfer. Using X to denote this trap(s), it is found that:
N+X.fwdarw.N.sup.++X.sup.
where N.sup.+ can be characterized through an absorption band with a peak at the one-phonon energy of 1332 cm.sup.1.

(38) This competing electron trap mechanism results in the 637 nm:575 nm intensity ratios reducing.

Example 2

(39) The method of Example 1 was followed for steps 1) to 4)

(40) A second layer of CVD diamond (Sample 2, Layer 2) was deposited on the first layer by gradually changing the input gas mixture to 375/430/290 sccm of CO.sub.2/CH.sub.4/H.sub.2 at a pressure of about 19010.sup.2 Pa (about 142 Torr) over a period of about 10 minutes. The source gas additionally comprised 20 ppm of atomic N. The substrate temperature was 840 C. The synthesis conditions for Sample 2, Layer 2 have C.sub.f:O.sub.f:H.sub.f atomic fractions of 0.21:0.19:0.60 and a C.sub.f:O.sub.f ratio of 1.1:1. Of the atomic fraction of hydrogen in the source gases, 0.15 is added as H.sub.2 and 0.45 is added from sources other than H.sub.2. The H.sub.f value of 0.60 gives a P.sub.arc value of 144.5 Torr, so the operating pressure is approximately 2.5 Torr below P.sub.arc.

(41) On completion of the growth period, the substrate was removed from the reactor and the CVD diamond layer removed from the substrate, the top and bottom surfaces and two opposed side surfaces of the CVD diamond layer were polished sufficiently for the layer to be optically characterised. The final product was a layer of CVD diamond with a total thickness of 2.4 mm (approximately equally split between Layer 1 and Layer 2), with lateral dimensions of approximately 3.8 mm3.8 mm.

(42) Prior to the optical characterisation, the sample was exposed to a deuterium lamp (15 Watt electrical power consumption) for 10 minutes with the sample approximately 80 mm from the filament of the bulb.

(43) UV/visible absorption spectra were recorded by illuminating one of the polished side surfaces of the sample such that the light path was entirely in either Layer 1 or Layer 2. Thus the pathlength used for obtaining the absorption spectra was roughly the lateral dimension of the layer.

(44) The optical absorption spectra for Sample 2, Layer 1 and Sample 2, Layer 2 are shown in FIG. 4. The spectrum for Sample 2, Layer 2 was analysed as follows:

(45) The Type Ib component was de-convolved from the measured spectrum (FIG. 5).

(46) The measured optical absorption spectrum was integrated over the visible wavelength range (i.e. 380 nm, equivalent to 3.2168 eV, to 750 nm, equivalent to 1.6527 eV) giving a value with units of eV.Math.cm.sup.1 (C). The absorption attributable to N.sub.s.sup.0 was similarly determined in eV.Math.cm.sup.1 over the same wavelength range (B). The ratio B/C and the difference CB are then calculated and are used for characterising the material. The values for Sample 2, Layer 2 are given below.

(47) TABLE-US-00006 Integrated Integrated absorption absorption in in visible Difference Sample [N.sub.s.sup.0] visible (C), attributable to N.sub.s.sup.0 (C B), Ratio 2 (ppm) (eV .Math. cm.sup.1) (B), (eV .Math. cm.sup.1) (eV .Math. cm.sup.1) (B/C) Layer 2 1.25 0.737 0.3334 0.4037 0.45

(48) For Sample 2, Layer 2, the proportion of the absorption over the range 380 nm to 750 nm due to N.sub.s.sup.0 is 0.45 or 45%.

(49) In addition, the absorption coefficients for the 360 nm and 510 nm bands were determined by further deconvolution of the spectrum as described elsewhere in the specification.

(50) TABLE-US-00007 Absorption coefficient (cm.sup.1) Sample 2 360 nm band (cm.sup.1) 510 nm band (cm.sup.1) Layer 2 0.3 0.3

(51) The CIE L*a*b* coordinates were derived from the absorption spectrum, in the way described in the detailed description of the invention. The values tabulated below are those calculated from these values for an optical pathlength of 1.0 mm.

(52) TABLE-US-00008 CIE L* a* b* Coordinates Hue angle at 1.0 mm Sample 1 a* b* C* L* optical pathlength, degrees Layer 2 0.4 2.1 2.0 86.2 81.2

(53) PL spectra from Sample 2, Layer 1 and Sample 2, Layer 2, recorded at 77 K using 488.2 nm light from a 50 mW Ar-ion laser, are shown in FIG. 6. The spectra have been normalised by ratioing the integrated area beneath the first-order Raman line at 521.9 nm in FIG. 6. The PL spectrum for Sample 1, Layer 2 shows a peak at 543.1 nm that is absent in Sample 1, Layer 1, and is believed to be related to the use of large fractions of oxygen in the synthesis process.

(54) PL spectra recorded at 77 K using 514.5 nm light from a 50 mW Ar-ion laser from Sample 2, Layer 1 and Sample 2, Layer 2, are shown in FIG. 7. The ratio of the peaks at 637 nm and 575 nm are 0.7 for Sample 2, Layer 1, and 1.1 for Sample 2, Layer 2. The higher value of this ratio for Sample 1, Layer 2 compared with Sample 2, Layer 1 indicates that Sample 2, Layer 2 has a higher concentration of N.sub.s.sup.0 in the material.

Example 3

(55) Sample 3 was prepared using the same sequence of steps described in Example 1 except that the following conditions were used for step 5 to form Sample 3, Layer 2: gas flows of 501/604/500 sccm for CO.sub.2/CH.sub.4/H.sub.2, a C.sub.f:O.sub.f:H.sub.f ratio of 0.20:0.18:0.62, a C.sub.f:O.sub.f ratio of 1.11:1, a pressure of 19010.sup.2 Pa (about 143 Torr), nitrogen present as 15 ppm of atomic nitrogen equivalent, and a substrate temperature of 860 C. The of the hydrogen atomic fraction of 0.62, 0.18 was added as H.sub.2 and 0.44 was added in other forms (in this case as CH.sub.4). The H.sub.f value of 0.62 gives a value for P.sub.arc of 148 Torr, so the operating pressure is 5 Torr less than P.sub.arc.

(56) For this sample, the amount of silicon incorporated in each of the layers was characterised using the intensity of the 737 nm PL line that is believed to be associated with the silicon-vacancy defect. FIG. 8 shows a PL image (excited with 633 nm radiation from a HeNe laser) of the cross-section (substrate to the left-hand-side of the image) overlaid with the intensity plot for the 737 nm line. This example demonstrates that the method of the invention suppresses the uptake of Si into the material compared with H.sub.2/CH.sub.4 chemistries. The peak intensity of the 737 nm line is near the interface between substrate and the first diamond layer and is typically associated with higher levels of contaminants during the first stage of diamond growth, for example due to exposure of sources of silicon within the growth environment.

(57) Analysis of PL spectra obtained at 77 K using 514.5 nm light from a 50 mW Ar-ion laser for Sample 3, Layer 1 and Sample 3, Layer 2 gave ratios of the intensities of the 637 nm and 575 nm peaks of 0.5 and 1.1 respectively.

Example 4

(58) This comparative example demonstrates the effect that not providing some of the hydrogen atoms in the form of H.sub.2 molecules has on the synthesis process.

(59) The procedure set out in Example 1 was repeated with the following variation in conditions for step 5 to form Sample 4, Layer 2: synthesis conditions of CO.sub.2/CH.sub.4 based plasma growth process without interruption by a ramped change in gas composition and process (pressure and power) window. The final pressure (limited by control issues due to no H.sub.2 addition) was fixed at 13010.sup.2 Pa (about 97 Torr), with a C.sub.f:O.sub.f ratio of 1.07:1, and C.sub.f:O.sub.f:H.sub.f ratios of 0.246:0.229:0.525. The gas flows were 375/430 sccm of CO.sub.2/CH.sub.4. The source gas additionally comprised 20 ppm of atomic N. Substrate temperature was 810 C. The proportion of H.sub.f that was added in the form of H.sub.2 molecules was zero. For the H.sub.f fraction in the source gas for the synthesis of Sample 4, Layer 2, the value of P.sub.arc would be expected to be 132 Torr; this is substantially higher than the operating pressure of about 97 Torr and is believed by the inventors to be due to the absence of H added as H.sub.2 in the source gas mixture.

(60) The UV/visible optical absorption spectrum for Sample 4, Layer 2 is shown in FIG. 9. In contrast to the previous examples grown at the higher pressures made possible by the addition of hydrogen as H.sub.2 to the source gas mixture, the spectrum from Sample 4, Layer 2 shows considerable absorption in addition to the type Ib component.

(61) The optical properties of Sample 4, Layer 2 were deconvolved, as previously described, to determine the concentration of N.sub.s.sup.0. The hue angle was measured and converted to the hue angle for an optical pathlength of 1 mm.

(62) TABLE-US-00009 Hue angle for 1 mm CIE L* a* b* Coordinates at 1 mm optical path length Sample 4 a* b* C* L* (degrees) Layer 2 1.7 4.8 5.0 81.3 70.5 Integrated absorption Integrated in visible absorption in attributable to Difference [N.sub.s.sup.0] visible (C) N.sub.s.sup.0 (B), (C B) Ratio Sample 4 (ppm) (eV .Math. cm.sup.1) (eV .Math. cm.sup.1) (eV .Math. cm.sup.1) (B/C) Layer 2 2.8 5.314 0.75 4.564 0.14

(63) The ratio B/C of 0.14 means that the proportion of the optical absorption in the visible region that is due to N.sub.s.sup.0 is only 0.14 or 14%, very much less than for those samples prepared by the method of the invention, demonstrating the importance of H.sub.2 and the desirability of operating at high pressures.

(64) The ratio of the peaks at 637 nm and 575 nm is 0.7 for Sample 4, Layer 1, and 1.0 for Sample 4, Layer 2.

(65) In addition, the absorption coefficients for the 360 nm and 510 nm bands were determined by further deconvolution of the measured spectrum as described elsewhere in this specification.

(66) TABLE-US-00010 Absorption coefficient (cm.sup.1) Sample 4 360 nm band (cm.sup.1) 510 nm band (cm.sup.1) Layer 2 3.2 1.5

Example 5

(67) This example demonstrates that provided the atomic fractions of O, C and H in the source gas are the same, the optical properties of the diamond material that is produced will be substantially similar.

(68) The methodology used in steps 1) to 3) of Example 1 was repeated to produce two further samples, referred to as Sample 5, Layer 2 and Sample 6, Layer 2. The starting gas compositions for Sample 5, Layer 2 and Sample 6, Layer 2 are summarized in the table below (gas flows in sccm). The table also shows the fractions of C, H and O in the gas phase.

(69) TABLE-US-00011 Proportion, Flow, sccm atomic fraction Sample CO.sub.2 CO H.sub.2 CH.sub.4 Total C H O Sample 5, 500 0 500 590 1590 0.20 0.62 0.18 Layer 2 Sample 6, 0 614 902 75 1591 0.20 0.62 0.18 Layer 2

(70) The starting gases were chosen to result in the same C, H and O atomic fractions in the plasma. The C.sub.f:H.sub.f ratio is 1.11:1. For both samples, thick single crystal diamond bodies were produced (3.7 mm for Sample 5 and 3.6 mm for Sample 6), grown at a gas pressure of 17010.sup.2 Pa (approximately 127 Torr), with the source gas also comprising 14 ppm of equivalent atomic nitrogen. In this case P.sub.arc for H.sub.f=0.62 is 148 Torr.

(71) After completion of the synthesis process, Sample 5 and Sample 6 were processed such that they could be optically characterised. UV-visible absorption spectra were obtained. The optical absorption spectrum was deconvolved, as described elsewhere in this specification, to determine the concentration of N.sub.s.sup.0. The absorption coefficients for the 360 nm and 510 nm bands were measured at the peaks of the respective bands. The key parameters are given in the table below.

(72) TABLE-US-00012 N.sub.s.sup.0 360 nm band 510 nm band Sample (ppm) (cm.sup.1) (cm.sup.1) Sample 5 1.5 0.5 0.45 Sample 6 1.2 0.4 0.35

(73) The deconvolution and absorption coefficients of the optical absorption spectrum show that different gas mixtures give essentially the same result in terms of the absorption due to each of the components.

(74) PL spectra were obtained from Sample 5 and Sample 6 using 514.5 nm excitation from an Ar-ion laser. The ratio of the peaks at 637 nm and 575 nm are 0.94 for Sample 5 and 0.84 for Sample 6.

(75) PL spectra obtained from Sample 5, Layer 2 and Sample 6, Layer 2 using 458 nm excitation are shown in FIG. 10. Both the characteristics and the relative intensities of the spectral features are very similar; similar results were found at other PL excitation wavelengths studied (325 nm, 488 nm, 515 nm and 660 nm).

(76) A projection X-ray topograph of Sample 6 is shown in FIG. 11. There is little X-ray contrast indicating high crystalline quality and low dislocation density. This material property makes the material suitable for some optical and mechanical applications.

Example 6

(77) In this example, the variation of the CIELAB parameters as a function of optical path length is shown. These results are derived from the model described and referred to in the specification.

(78) In this case, the sample was produced according to Example 1, Layer 2 and the absorption spectrum required for calculating the CIE L*a*b* coordinates was obtained from a 1.37 mm thick sample.

(79) TABLE-US-00013 Thickness (mm) 0.5 1 1.5 2 3 4 5 6 10 L* 86.4 85.3 84.3 83.4 81.4 79.5 77.6 75.8 68.8 a* 0.1 0.1 0.2 0.2 0.3 0.3 0.2 0.2 0.4 b* 1.1 2.1 3.2 4.2 6.1 7.9 9.6 11.2 16.6 c* 1.1 2.1 3.2 4.2 6.1 7.9 9.6 11.2 16.6 Hue 93.9 93.6 93.4 93.1 92.5 91.9 91.4 90.8 88.8 angle,

(80) It can be seen that, in particular, the hue angle is greater than 80 for all the thicknesses where it has been calculated.