Thick optical quality synthetic polycrystalline diamond material with low bulk absorption and low microfeature density

Abstract

A poly crystalline chemical vapour deposited (CVD) diamond wafer comprising: —a diameter >40 mm; —a thickness >1.0 mm; —an absorption coefficient ≤0.1 cm.sup.−1 at 10.6 μm; and ⋅a micro feature density, especially in the form of “black spots”, meeting the following specification: —in a central area of the polycrystalline CVD diamond wafer from 0 to 20 mm radius there are no more than 100 micro features of a size between 0.002 and 0.008 mm.sup.2, no more than 50 micro features of a size between 0.008 and 0.018 mm.sup.2, no more than 25 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2, and ⋅in an outer region of the polycrystalline CVD diamond wafer from 20 to 40 mm radius there are no more than 200 microfeatures 2 of a size between 0.002 and 0.008 mm.sup.2, no more than 150 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 100 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2.

Claims

1. A polycrystalline chemical vapour deposited (CVD) diamond wafer comprising: a diameter ≥40 mm; a thickness ≥1.0 mm; an absorption coefficient ≤0.1 cm.sup.−1 at 10.6 μm; and a plurality of microfeatures, wherein the diamond wafer has a microfeature density meeting the following specification: in a central area of the polycrystalline CVD diamond wafer from 0 to 20 mm radius there are between 5 and 100 microfeatures of a size between 0.002 and 0.008 mm.sup.2, between 1 and 50 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 25 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2, and in an outer region of the polycrystalline CVD diamond wafer from 20 to 40 mm radius there are between 10 and 200 microfeatures of a size between 0.002 and 0.008 mm.sup.2, between 2 and 150 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 100 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2.

2. A polycrystalline CVD diamond wafer according to claim 1, wherein the microfeature density meets the following specification: in the central area of the polycrystalline CVD diamond wafer from 0 to 20 mm radius there are between 5 and 50 microfeatures of a size between 0.002 and 0.008 mm.sup.2, between 1 and 10 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 5 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2, and in the outer region of the polycrystalline CVD diamond wafer from 20 to 40 mm radius there are between 10 and 100 microfeatures of a size between 0.002 and 0.008 mm.sup.2, between 2 and 20 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 10 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2.

3. A polycrystalline CVD diamond wafer according to claim 1, wherein the absorption coefficient is ≤0.09 cm.sup.−1, ≤0.08 cm.sup.−1, ≤0.07 cm.sup.−1, or ≤0.06 cm.sup.−1 at 10.6 μm.

4. A polycrystalline CVD diamond wafer according to claim 1, wherein the diameter of the polycrystalline CVD diamond wafer is equal to or greater than 50 mm, 60 mm, 70 mm, 80 mm, 90 mm, or 100 mm.

5. A polycrystalline CVD diamond wafer according to claim 1, wherein the thickness of the polycrystalline CVD diamond wafer is equal to or greater than 1.1 mm, 1.2 mm, 1.3 mm, 1.4 mm, or 1.5 mm.

6. A polycrystalline CVD diamond wafer according to claim 1, wherein the polycrystalline CVD diamond wafer has a thermal conductivity of at least 1900 Wm.sup.−1K.sup.−1.

7. A polycrystalline CVD diamond wafer according to claim 1, wherein the polycrystalline CVD diamond wafer has a fracture strength of at least 400 MPa with a growth face of the polycrystalline CVD diamond wafer in tension and a fracture strength of at least 800 MPa with a nucleation face of the polycrystalline CVD diamond wafer in tension.

8. A polycrystalline CVD diamond wafer according to claim 1, wherein the polycrystalline CVD diamond wafer has a nitrogen concentration in a range 500 to 600 ppb as measured by electron paramagnetic resonance spectroscopy (EPR).

9. A polycrystalline CVD diamond wafer according to claim 1, wherein a total integrated scatter in a forward hemisphere no more than 1%, 0.5%, or 0.1% at 10.6 μm for a sample thickness of 1 mm with front and rear surfaces polished to a root mean squared roughness of less than 15 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic plan view of the growth face of a thick (>1 mm) polycrystalline CVD diamond wafer comprising a plurality of microfeatures.

DETAILED DESCRIPTION

(3) Details of suitable microwave plasma reactor technology are provided in WO2012/084661, WO2012/084657, WO2012/084658, WO2012/084659, WO2012/084655, WO2012/084661, and WO2012/084656. Furthermore, WO2013/087702 and WO2013/087797 describe improvements to this microwave plasma reactor technology and its use in fabricating thick, large area polycrystalline CVD diamond wafers.

(4) Microfeatures in the form of black spots were mentioned in both WO2013/087702 and WO2013/087797. It was indicated that a polycrystalline CVD diamond wafer preferably has an average black spot density no greater than 1 mm.sup.−2, 0.5 mm.sup.−2, or 0.1 mm.sup.−2 and/or a black spot distribution such that there are no more than 4, 3, 2, or 1 black spots within any 3 mm.sup.2 area. However, it has been found to be difficult to provide a polycrystalline CVD diamond wafer which combines a relatively large diameter and thickness with a low bulk absorption coefficient and a low density of microfeatures. As such, research work following WO2013/087702 and WO2013/087797 has focussed on modifying the polycrystalline CVD diamond growth conditions to produce thick polycrystalline CVD diamond wafers which have low bulk absorption coefficient and a low density of microfeatures. In addition, a greater appreciation of how the size and location of microfeatures affects functional performance has led to a modification in the product specification away from a basic microfeature density and distribution criteria for the polycrystalline CVD diamond wafer to a specification as set out in the summary of invention section which takes into account the size and location of the microfeatures across the polycrystalline CVD diamond wafer.

(5) It has been found that typically nitrogen addition into the CVD growth chamber can aid in reducing the density of micro features in thick polycrystalline CVD diamond material. However, nitrogen itself contributes significantly to the optical absorption of the polycrystalline CVD diamond material. As such, it has been found that a balance must be struck between adding sufficient nitrogen to reduce the density of microfeatures while not adding too much nitrogen that optical absorption is unduly increased. That is, a gas phase nitrogen concentration window can be identified for the synthesis of thick optical grade polycrystalline CVD diamond material which results in the best compromise between absorption from nitrogen and absorption from microfeatures in the thick polycrystalline CVD diamond product material. This nitrogen window has been found to be in the range 400 to 550 parts per billion (ppb), more preferably 425 to 460 ppb, nitrogen in the CVD synthesis atmosphere as calculated from optical emission spectroscopy (OES) measurements and/or calculated based on input gas flows.

(6) It is also important to balance methane and nitrogen concentrations in the gas phase during synthesis. A relatively low methane concentration is utilized as otherwise the nitrogen added to the synthesis process would result in a higher growth rate, lower quality polycrystalline CVD diamond product wafer. For example, during the main growth stage the CVD synthesis atmosphere may be controlled to have a methane concentration in a range 0.8 to 1.5%. One option is to utilize a higher methane concentration during the nucleation stage of diamond growth and then drop the methane concentration for the main growth stage. This ensures good nucleation while also ensuring that the methane concentration is not too high during the main growth stage. The methane concentration can be further reduced in a stepwise or continuous fashion during the main growth stage to maintain the quality of the polycrystalline CVD diamond material as is grows thicker.

(7) In addition, or as an alternative, to adding a higher concentration of methane during nucleation and then reducing the methane during the main CVD diamond growth stage, it has also been found to be advantageous to add a higher concentration of nitrogen during nucleation and then reduce the nitrogen concentration during the main diamond growth stage. For example, 2 to 8 ppm nitrogen may be provided in the CVD synthesis atmosphere during diamond nucleation and then the nitrogen level dropped to a level in the range 400 to 550 ppb for the main diamond growth stage.

(8) As an alternative, or in addition, to using a higher nitrogen and/or methane concentrations during the diamond nucleation phase and then dropping the concentrations during the main diamond growth phase, it is advantageous to provide one or more of: a stepped or continuous reduction in substrate temperature (by adjusting the temperature control system for the substrate); a stepped or continuous reduction in microwave power input to the growth chamber (by adjusting the microwave power source); and a stepped or continuous increase in pressure within the growth chamber (by regulating gas flow).

(9) Again, the adjustment of these parameters during the growth run aids in maintaining the quality of the polycrystalline CVD diamond material as is grows thicker. The example described later in this specification uses a combination of a stepwise decrease in substrate temperature, a stepwise decrease in power, and a stepwise increase in pressure during growth of the polycrystalline CVD diamond wafer.

(10) Nucleation temperature, in addition to nucleation chemistry, is another factor which has been found to affect microfeature formation in thick polycrystalline CVD diamond wafers. This is surprising and somewhat counterintuitive as microfeatures only tend to form after 800 μm to 1 mm of polycrystalline CVD diamond growth. That being the case, one would not expect that this phenomenon would be particularly sensitive to conditions during the nucleation stage at the start of diamond growth. However, the present inventors have found that microfeature formation later in the growth run is affected by the conditions during nucleation at the start of the growth run and that it is advantageous to initiate nucleation at a lower temperature prior to ramping up to full growth conditions. For example, power and pressure can initially be ramped up to an intermediate stage below the main growth conditions, methane introduced to initiate nucleation, and then the power and pressure can be further ramped up to full growth conditions. For example, methane may be added at a substrate temperature in a range 550° C. to 650° C., nucleation may occur at a central substrate temperature in a range 800° C. to 840° C., before ramping up to a starting growth temperature in a range 875° C. to 920° C. Substrate temperature can be measured using calibrated optical pyrometry operating at 2.2 micrometres and assuming an emissivity of 0.9. Variations are possible in terms of the exact temperatures at which methane addition, nucleation, and the main growth stage occur. The important feature is that methane is added and nucleation occurs at a significantly lower temperature than the main diamond growth temperature for the thick polycrystalline CVD diamond wafer.

(11) Furthermore, during growth the edge temperature of the polycrystalline CVD diamond wafer is lower than the central temperature with a temperature differential within 60° C. Temperature is controlled as described in WO2013/087702 and WO2013/087797 utilizing a gas gap and adjustable gas flow under the substrate in combination with the provision of a metallic temperature modifying ring disposed around the substrate. The stepped reduction in substrate temperature as previously described is relatively small compared with the absolute starting temperature during growth. For example, the substrate temperature may be reduced in three 20° C. steps during polycrystalline CVD diamond growth.

(12) High power, pressure, and gas flow rate are utilized during polycrystalline CVD diamond growth. For example, total gas flow may be in a range 2800 standard cubic centimetres per minute (sccm) to 3700 sccm. This may comprise a hydrogen flow rate in a range 2500 to 3500 sccm, a methane flow rate in a range 25 to 50 sccm, optionally 20 to 50 sccm of argon, and nitrogen added to a concentration of 400 to 550 ppb. Microwave power may be in a range 25 to 28 kW, and pressure may be in a range 180 to 220 Torr. Again, stepped changes in these parameters during growth are relatively small compared with absolute starting values. For example, pressure may be raised by 5 to 10 Torr during growth whereas power may be reduced by 1 to 2 kW during growth in order to maintain the quality of the polycrystalline CVD diamond material as it grows up to and beyond 1 mm in thickness.

(13) Synthesis parameters as described above are selected in order to control the growth texture of the polycrystalline CVD diamond material while maintaining intrinsic material quality for optical applications. It is important to understand that the texture of the material in the early stages of polycrystalline CVD diamond growth has a significant impact on the material quality in the later stages of polycrystalline CVD diamond growth as discussed below.

(14) As nominally transparent polycrystalline CVD diamond is grown to greater thickness (typically >1 mm thick), phenomena of defect uptake and intergranular competition and its resultant stresses begin to impact the aesthetic appearance and potential optical performance of the material. A major contributor to this is the formation of microfeatures in later growth. For the purposes of this specification, microfeatures are dark spots detected and counted using the microfeature density measurement method as described later in this specification. In terms of a physical explanation as to what microfeatures actually are, one definition is that a microfeature is a cluster of microfractures. The microfractures can cause reflections resulting in high angle scatter and, when they contain non-diamond carbon, increase optical absorption. That is, microfeatures may be formed of a discrete network of microscopic fractures within the continuous diamond structure, the internal faces of which are decorated with non-diamond carbon. These features appear opaque under the conditions used to image and analyse them as documented herein.

(15) It is unknown whether micro features initiate as the material is growing, or whether they are a result of the cooling process from growth conditions to atmospheric temperatures and pressures, but they are present in all nominally transparent CVD diamond films of sufficient thickness, and it is known that for a given material type, the number density and size distribution of microfeatures increases with both material thickness and growth rate. With increasing growth rate and thickness, the number of microfeatures and distribution of microfeature sizes increases, eventually to the point where, for fast-growing, thick CVD diamond (e.g. >3.0 mm thick), discrete clusters of microscopic fractures coalesce into a visually continuous network of microscopic fractures. Historically, for a given diamond film thickness, the initiation of such features has be delayed by slowing the growth rate, reducing the aggressiveness of intergranular competition.

(16) CVD diamond films nucleate and early growth initiates with thousands of grains per mm.sup.2, however different relative lateral and vertical growth rates between grains and their neighbours result in local granular competition for space and an evolution of the crystallographic properties of the diamond film, eventually towards a single, dominant crystallographic texture. The extinction of particular growth directions and overgrowth of slower-growing neighbouring grains is a major source of stress relief for the growing film, but in later growth, as the distribution of growth directions narrows, that stress relief mechanism is decreasingly available. The single dominant texture and the rate at which that texture evolves from a near-random distribution at nucleation to become pure, depends on the relative growth rates of different crystallographic directions, defined by the synthesis parameters used for growth, i.e. [N]/[C]/substrate temperature/power density/chamber pressure.

(17) However, it has been observed that as the overall texture of the growing top face tends towards 100% <110> orientation (perpendicular to the substrate surface), a second distribution of isolated, large microfeatures starts to emerge. This type of feature is aesthetically unattractive but is also more prone to cracking and chipping during polishing than the usual smaller micro features and will contribute to reduced optical performance. The <110> growth texture tends to dominate more quickly in slower growing wafers, which would be assumed to be of higher optical quality.

(18) A parameter window exists between these two situations where diamond growth is slow enough to maintain the optical absorption and sufficiently low numbers of smaller microfeatures initiate in thicker material (˜1.5 mm thick), but the resultant texture evolution rate is limited, so that the proportion of grains with a <110> growth direction does not reach the required level to initiate the formation of the isolated, large micro features. Diamond growth on either side of this parameter window would result in material unsuitable for use in demanding optical applications such as for extreme ultraviolet lithography (EUVL). That is, using a growth process optimized to achieve polycrystalline CVD diamond material having a low optical absorption and a low concentration of small microfeatures results in the formation of large microfeatures when grown to high thicknesses. Conversely, using a growth process optimized to achieve polycrystalline CVD diamond material having a low concentration of large microfeatures when grown to high thicknesses results in a material which has higher optical absorption and a higher concentration of small microfeatures. It is important to note that there are two distinct types of microfeatures (small and large) which are affected in different ways by changes to the growth conditions. The growth parameters as described herein represent a window in parameter space to achieve polycrystalline CVD diamond material having a low optical absorption, a low concentration of small microfeatures, and a low concentration of large microfeatures.

(19) Using the aforementioned methodology, it is possible to fabricate a polycrystalline CVD diamond wafer comprising: a diameter ≥40 mm, ≥50 mm, ≥60 mm, ≥70 mm, ≥80 mm, ≥90 mm, or ≥100 mm (optionally less than 150 mm, 130 mm, or 110 mm); a thickness ≥1.0 mm, ≥1.1 mm, ≥1.2 mm, ≥1.3 mm, ≥1.4 mm, or ≥1.5 mm (optionally less than 2 mm); an absorption coefficient ≤0.1 cm.sup.−1, more preferably ≤0.09 cm.sup.−1, ≤0.08 cm.sup.−1, ≤0.07 cm.sup.−1, or ≤0.06 cm.sup.−1, at 10.6 μm (optionally no less than 0.03 cm.sup.−1); and a microfeature density meeting the following specification: in a central area of the polycrystalline CVD diamond wafer from 0 to 20 mm radius there are no more than 100 microfeatures of a size between 0.002 and 0.008 mm.sup.2, no more than 50 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 25 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2, and in an outer region of the polycrystalline CVD diamond wafer from 20 to 40 mm radius there are no more than 200 microfeatures of a size between 0.002 and 0.008 mm.sup.2, no more than 150 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 100 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2.

(20) Preferably the polycrystalline CVD diamond wafer meets the more restrictive criteria for microfeature density as follows: in the central area of the polycrystalline CVD diamond wafer from 0 to 20 mm radius there are no more than 50 microfeatures of a size between 0.002 and 0.008 mm.sup.2, no more than 10 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 5 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2, and in the outer region of the polycrystalline CVD diamond wafer from 20 to 40 mm radius there are no more than 100 microfeatures of a size between 0.002 and 0.008 mm.sup.2, no more than 20 microfeatures of a size between 0.008 and 0.018 mm.sup.2, no more than 10 microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2.

(21) While it is advantageous to have zero microfeatures of any size at any position on the polycrystalline CVD diamond wafer, many wafers will still have a small number of microfeatures but well within the tolerable specification. For example, lower limits for microfeature density may be defined as follows: in the central area of the polycrystalline CVD diamond wafer from 0 to 20 mm radius there are no less than 5 microfeatures of a size between 0.002 and 0.008 mm.sup.2, no less than 1 microfeatures of a size between 0.008 and 0.018 mm.sup.2, zero microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2, and in the outer region of the polycrystalline CVD diamond wafer from 20 to 40 mm radius there are no less than 10 microfeatures of a size between 0.002 and 0.008 mm.sup.2, no less than 2 microfeatures of a size between 0.008 and 0.018 mm.sup.2, zero microfeatures of a size between 0.018 and 0.05 mm.sup.2, and zero microfeatures of a size between 0.05 and 0.1 mm.sup.2.

(22) Of course, there should also be zero microfeatures anywhere in the region from 0 to 40 mm radius which are larger than 0.1 mm.sup.2. Outside of the 40 mm radius region, more micro features can be tolerated as in use only the central region of the wafer is disposed in the light path.

(23) The polycrystalline CVD diamond wafer may also have one or more of the following characteristics: a thermal conductivity of at least 1900 Wm.sup.−1K.sup.−1, preferably at least 2000 Wm.sup.−1K.sup.−1 (optionally no more than 2200 Wm.sup.−1K.sup.−1); a fracture strength of at least 400 MPa (optionally no more than 1000 MPa) with a growth face of the polycrystalline CVD diamond wafer in tension and a fracture strength of at least 800 MPa (optionally no more than 1400 MPa) with a nucleation face of the polycrystalline CVD diamond wafer in tension; and a total integrated scatter in a forward hemisphere no more than 1%, 0.5%, or 0.1% at 10.6 μm for a sample thickness between 1 mm and 1.5 mm with front and rear surfaces polished to a root mean squared roughness of less than 15 nm.

(24) The polycrystalline CVD diamond wafer may also have a nitrogen concentration (N.sub.s.sup.0) in a range 500 to 600 ppb as measured by EPR.

(25) In addition to the improved optical performance of thick wafers of polycrystalline CVD diamond as described herein, the reduction in micro features also allows such wafers to be more readily surface processed to high flatness and low roughness without microfeatures causing large pits to be formed in the wafer during processing. For example, the polycrystalline CVD diamond wafer may be processed to have a surface roughness no more than 40 nm, 20 nm, or 10 nm without microfeatures causing large pits to be formed in the processed surface. Furthermore, an antireflective or diffractive structure may be readily formed in or on a surface of the polycrystalline CVD diamond wafer.

(26) Measurement Techniques

(27) Absorption Coefficient

(28) Laser calorimetry is the method of choice for measuring the absorptance of low loss materials and optical components. Details of this measurement can be found in Test method for absorptance of optical laser components ISO/FDIS 11551, International Organisation for Standardisation, Geneva (1995) and G. Turri et al, Optical absorption, depolarization, and scatter of epitaxial single-crystal chemical-vapor-deposited diamond at 1.064 μm, Optical Engineering 46(6), 064002 (2007). Laser calorimetry involves measuring the time-dependent rise and fall in temperature of a test specimen exposed to a laser of known power for a fixed time period. From an analysis of the temperature response of the specimen, the absorbance A can be determined, where A is defined as the fraction of incident laser power absorbed by the specimen. When the product of the absorption coefficient α with the sample thickness d is much less than unity, then α≈A/d. This approximation is reasonable for optical quality polycrystalline diamond. In addition, optical quality polycrystalline diamond is commonly used for CO.sub.2 laser optics operating at 10.6 μm. Performing the laser calorimetry measurement using the 10.6 μm line of a CO.sub.2 laser is therefore particularly appropriate for the present invention. Test specimens for calorimetry are prepared as follows. Firstly the growth and nucleation faces of the wafer are lapped and polished to a uniform, desired thickness. A minimum of 20 μm is polished off the nucleation face in order to remove any contamination which may have incorporated during the nucleation stage of synthesis. Secondly, a series of calorimetry test specimens are laser machined from the polished wafer. These specimens are further polished on both sides to an rms roughness of <15 nm.

(29) Tensile Strength

(30) Strength testing of materials can be performed using different techniques, all of which have their advantages and disadvantages. These are well-known to persons skilled in the art. One method of testing for strength is the so-called 3-point bend test. The application of this technique to polycrystalline diamond specimens is detailed in Philosophical Magazine Vol. 83, No. 36, 4059-4070 (2003), Strength of free-standing chemically vapour-deposited diamond measured by a range of techniques, A. R. Davies, J. E. Field, C. S. J. Pickles. An as-grown CVD wafer is prepared for 3-point bend strength testing as follows. First, the growth face of the wafer is lapped and polished to a uniform, desired thickness. Secondly, a series of 40 rectangular strength test specimens of lateral dimension 18 mm×2 mm are laser machined from the polished wafer. These specimens are extracted from across the entire wafer in order to provide an even sampling. The 3-point bend test is performed on a first 20 samples in which the nucleation face is in tension and a second 20 samples in which the growth face is in tension. The mean strength of the nucleation and growth faces is determined by calculating the arithmetic mean of each sample set.

(31) Microfeature Density

(32) A partially automated method for analysis of micro feature density and size distribution within specific regions of processed polycrystalline CVD diamond wafers has been devised. Cleanliness during preparation and image capture is of utmost importance as contamination, dust and fibres can skew measurements.

(33) Sample Preparation: The polycrystalline diamond wafer must be planarised and polished on both faces. If any surface texture remaining from the grinding stage is included in the measurement, the procedure will give an inaccurate representation of the microfeature density. Following polishing, the wafer should be thoroughly cleaned, so that the surface is free from particles, residual glue or other contaminants.

(34) Image Capture: A clean, dust-free wafer is located centrally on the clean platen of high-resolution imaging system (ideally ≥4800×4800 pixels per inch or 189×189 pixels per mm) with bright field, transmission-mode illumination. After previewing and cropping the image to the desired region, an image is captured using the following parameters: 8-bit grayscale (256 shades) High resolution (4800×4800 pixels per inch) Transmission-mode The captured image is saved as a .tiff file

(35) Image Analysis: The full resolution 8-bit .tiff file is uploaded into an image analysis software suite, e.g. ImageJ. The image scaling (pixels/inch or pixels/mm) is applied, according to the procedure used in capturing the image. The original image can be cropped to isolate specific regions of the wafer. These can then be analysed separately if required. For each cropped image, grayscale thresholding is applied to isolate microfeatures (darker, non-transparent regions) from the transparent background. Each thresholded pixel or cluster thereof (microfeature) is treated as a ‘particle’ and the number of particles along with their areas are calculated according to the number of pixels. Output data for each cropped image comprises a list of particles, each with their associated area measurement (mm.sup.2)

(36) Statistical Analysis: For each cropped image, the list of particle areas is copied into a data handling software package such as an MS Excel spreadsheet template. The entire list of measured particle areas is compiled into a histogram with appropriately spaced bins, according to the data spread, or any pre-defined specifications.

(37) Thermal Conductivity

(38) Thermal conductivity is measured in thick diamond wafers using the proven relationship between thermal conductivity and the CH.sub.x component of the FTIR absorption spectrum. This relationship is described in “Thermal conductivity measurements on CVD diamond”, by Twitchen et al, Diamond and related materials, 10 (2001) 731-735. The integrated area of the CH.sub.x components in the region 2760 cm.sup.−1 to 3030 cm.sup.−1 of the IR spectrum of the diamond window, once corrected with a linear baseline, has been shown to be quantitatively related to the thermal conductivity of diamond.

(39) Optical Scatter

(40) Total integrated scatter in the forward direction is measured using a so-called Coblentz sphere capable of collecting forward scattered light at an angle ≥2.5° with respect to the incident light beam. The technique is described in J. C. Stover, Optical Scattering: Measurement and Analysis, SPIE Press Monograph (1995). The 10.6 μm line of a CO.sub.2 laser is used for these measurements. Test specimens for scatter measurements are prepared as follows. Firstly the growth and nucleation faces of the wafer are lapped and polished to a uniform, desired thickness. A minimum of 20 μm is polished off the nucleation face in order to remove any contamination which may have incorporated during the nucleation stage of synthesis. Secondly, a series of test specimens are laser machined from the polished wafer. These specimens are further polished on both sides to an rms roughness of <15 nm.

EXAMPLE

(41) The example given here is for a 100 mm diameter wafer grown to a thickness >1.5 mm. This example targets low microfeature densities, particularly in thick products while maintaining a low absorption coefficient at 10.6 μm. The process detailed below is optimised for achieving high yields of 60-85 mm diameter, 1.3 mm thick optical grade windows from such as-grown wafer. It should be noted that the thickness of the as-grown wafer must be significantly higher than that required for the product as material is removed during lapping and polishing to achieved the desired flatness and roughness criteria for extreme optical applications.

(42) Synthesis conditions are as follows:

(43) TABLE-US-00001 Stage Nucleation Growth Hydrogen flow 2950 ± 50 sccm 2950 ± 50 sccm Methane flow 35 ± 1 sccm 35 ± 1 sccm Power 26.5 ± 0.5 kW 26.5 ± 0.5 kW Pressure 206 Torr 206 Torr Centre 820° C. 900° C. Temperature Edge ~780-810° C. ~840-870° C. Temperature Gas phase 425 ± 25 ppb nitrogen concentration Start-up Flow process H.sub.2 and Ar. Microwaves on, stabilise at 100 Torr (10 kW)  0:00:00 Ramp to 195 Torr (23 kW) ~0:01:30 Start CH.sub.4/N.sub.2 ~0:03:30 Complete ramp to 206 Torr (26 kW) ~0:05:30 Control substrate temperature at 820° C. ~4:00:00 Increase substrate temperature to 900° C.

(44) After growth conditions are established a stepped synthesis profile is utilized with respect to substrate temperature, power, and pressure to manage growth rate, control uniformity, and maintain material quality as the wafer grows to high thickness. Substrate temperature is lowered 60° C. during growth in three 20° C. steps; power is reduced by 1.5 kW in three 0.5 kW steps; and pressure is increased by 6 Torr in three 2 Torr steps. Each stepped change is performed slowly over a 1 to 2 hour time interval.

(45) Material properties of the product are as follows:

(46) TABLE-US-00002 Lower Upper Method of specification specification Attribute measurement Typical value limit limit Thermal Laser Flash >2000 Wm.sup.−1K.sup.−1 1900 Wm.sup.−1K.sup.−1 N/A Conductivity Absorption 10.6 μm Laser 0.060 cm.sup.−1 N/A 0.070 cm.sup.−1 Coefficient @ Calorimetry 10.6 μm Fracture 3-point 450 MPa (G) 400 MPa on N/A Strength bend test 1050 MPa (N) growth face 800 MPa on nucleation face Minimum Micrometry >1500 μm 1500 μm N/A Wafer (41 pt) Thickness Microfeature High Definition See Below N/A See Below Density Scan and Image Analysis

(47) Microfeature Density

(48) Wafers are processed to give a ˜1400 μm thick polished plate. Plates are acid cleaned prior to removing all possible dust from the surface. The plate is scanned and the image analysed according to the procedure outlined previously in this specification. Results are indicated in the below table:

(49) TABLE-US-00003 0-20 mm 20-40 mm Key Data Typical Upper Spec. Typical Upper Spec. 0.002-0.008 mm.sup.2 20-50 100 40-100 200 0.008-0.018 mm.sup.2  2-10 50 5-20 150 0.018-0.05 mm.sup.2 0-5 25 0-10 100 0.05-0.1 mm.sup.2 0 0 0 0

(50) While this invention has been particularly shown and described with reference to embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention which is defined by the appending claims.