THE ELEMENT SIX CVD DIAMOND HANDBOOK Contact Element Six Technologies at [email protected] CONTENTS INTRODUCING DIAMOND 3 PHYSICAL PROPERTIES 4 DIAMOND CLASSIFICATION 5 DIAMOND SYNTHESIS 6 TYPES OF CVD DIAMOND 7 CRYSTALLOGRAPHY 8 MECHANICAL STRENGTH 9 POLISHING OF DIAMOND 10 DIAMOND SURFACES 11 PROPERTIES OPTICAL PROPERTIES 12 OPTICAL CONSTANTS 13 RAMAN SCATTERING 14 SINGLE CRYSTAL OPTICS 15 POLYCRYSTALLINE OPTICS 16 EMISSIVITY AND RF WINDOWS 17 PRECISION COMPONENTS 18 Advances in the synthesis and processing THERMAL PROPERTIES 19 technology for CVD diamond has resulted ELECTRONIC PROPERTIES 20 in materials with exceptional diamond properties in practical components. ELECTROCHEMICAL PROPERTIES 21 Engineered single crystal CVD diamond, with ultra low absorption and birefringence DATASHEETS combined with long optical path lengths, ELECTRONIC GRADES 22 has made Monolithic Diamond Raman OPTICAL AND RF GRADES 23 Lasers a practical reality. MECHANICAL GRADES 24 THERMAL GRADES 25 ELECTROCHEMICAL GRADE 26 FURTHER READING 27 The Element Six CVD Diamond Handbook 2 INTRODUCING DIAMOND Diamond is characterised by its exceptional hardness, robustness and its optical and thermal properties; pre-eminent as a gemstone and an industrial tool. Natural diamond has an inherent variability and scarcity that limits its use in engineering applications. Developments in synthesis processes have enabled the production of consistently engineered synthetic diamond; firstly in the 1950s using high pressure and high temperature and later using chemical vapour deposition in the 1980s to produce the IT IS ALL IN THE STRUCTURE exceptional covalent crystal diamond. Diamond’s properties derive from its The modern industrial world consumes structure; tetrahedral covalent bonds approximately 800 tonnes of synthetic between its four nearest neighbours, diamond, around 150 times the amount of linked in a cubic lattice. This strongly natural diamond mined as gemstones. bonded, tightly packed, dense and rigid structure gives rise to its outstanding boron carbon nitrogen properties. Manipulating the impact of defects and the synthesis conditions 5 6 7 means that material scientists have been able to optimise and tailor the remarkable properties of diamond for a wide range of B C N applications. 10.811 12.011 14.007 aluminium silicon phosphorus 13 14 15 AL Si P 26.981 28.085 30.973 High pressure high temperature synthetic diamond is Carbon’s position as a group IV element above silicon usually nitrogen doped, giving it a distinctive in the periodic table. yellow hue. The Element Six CVD Diamond Handbook Further Reading 2. Science’s Gem 3 PHYSICAL PROPERTIES PROPERTY VALUE DENSITY 3 . 51524 x 103 kgm-3 NUMBER DENSITY 1 . 77 x 1023 per cm3 7 LATTICE TYPE Cubic Fd3m-Oh Lattice constant between LATTICE SPACING 0.356683 +/-0.000001 and 0.356725 +/-0.000003 nm at 298 K 5 MPa m0.5 Single Crystal FRACTURE TOUGHNESS K 1C 8.5 MPa m0.5 Polycrystalline POISSON’S RATIO 0.1 YOUNG’S MODULUS 1050 GPa 2.5 to 3 GPa Single Crystal surface finish dependent FRACTURE STRENGTH 200 to 1100 MPa Polycrystalline grain size and grade dependent FATIGUE LIFE >95% strength after >>107 cycles to 70% of FS FRACTURE PLANE {111} and occasionally {110} 2.55 Single Crystal WEIBULL MODULUS 10 Growth Surface Polycrystalline 20 Nucleation surface Polycrystalline 70 to 120 GPa Single crystal (Plane and direction of indent dependent) HARDNESS ~81 ±18 GPa Polycrystalline (Grain orientation dependent) 0.05 to 0.15 (orientation dependent) FRICTION COEFFICIENT (µ) In air (requires surface termination) The Element Six CVD Diamond Handbook Further Reading 3. Mechanical Properties of Diamond 4 DIAMOND CLASSIFICATION N > 5PPM N < 5PPM Type I - Contains nitrogen Type II - Very low nitrogen Typically 100 3000 ppm concentration > 98% OF MOST CVD VERY RARE IN NATURAL DIAMOND NATURE DIAMOND (UNLESS DOPED) TYPE Ia TYPE Ib TYPE IIa TYPE IIb Contains aggregated Contains Nitrogen as Boron as nitrogen single the major the major substitional impurity impurity p-type nitrogen Electrically semiconductor insulating Boron doped CVD TYPE IaA TYPE IaB Contains Contains nitrogen nitrogen in the A in the B aggregate aggregate form form Natural diamonds were first classified classification scheme remains relevant to according to their optical properties. synthetic diamond, in that most HPHT The majority, being Type I, have an grown diamond falls into the type Ib absorption edge of around 330 nm and classification and most CVD into type a small group, being Type II, have an IIa, due to their different nitrogen levels. absorption edge of about 220 nm. Over However, within this there are now the years this natural classification multiple polycrystalline and single crystal scheme has been extended and further grades, developed with specific tailored linked to different predominant properties that this simple classification defects, such as nitrogen and nitrogen system does not distinguish between. clusters. In its broadest sense, this The Element Six CVD Diamond Handbook Further Reading 4. The “Type” Classification 5 DIAMOND SYNTHESIS HPHT The vast majority of synthetic diamond The plasma can be heated by microwaves, is made by high pressure and high radio frequency, lasers, direct current, temperature diamond (HPHT) methods. hot filament and chemical reactions. The HPHT aims to mimic the thermodynamic nucleation and growth of continuous conditions in nature that diamond forms diamond requires a substrate with in, but with the addition of a molten metal refractory characteristics, stable carbide solvent / catalyst to reduce the large formation and a low thermal expansion kinetic barrier and act as transport media coefficient. for dissolved carbon. Material grown this way typically has a yellow hue, as a consequence of nitrogen incorporation Waveguide into the diamond lattice from the Microwaves atmosphere and growth materials. Antenna CHEMICAL VAPOUR DEPOSITION (CVD) Chamber Window CVD diamond exploits the relatively small difference in stability between Gas inlet Plasma 2 3 the two allotropes (sp & sp ) of carbon. Substrate Create the right surface conditions, the presence of atomic hydrogen and surface temperatures above ~ 600°C and the formation of diamond depends on having a faster nucleation and growth rate than Pressure control graphite. Vacuum The growth conditions are created by After nearly 4 decades of stop-start thermal disassociation of hydrogen, and research into CVD diamond growth, a gaseous source of carbon in plasma, microwave plasma enhanced CVD with a gas temperature above 2000°C. diamond synthesis emerged as a commercial synthesis method in the 1990s. The growth rates and control over purity of this method lends itself to manufacturing high quality, free-standing, polycrystalline and single crystal CVD diamond. The Element Six CVD Diamond Handbook Further Reading 5. CVD: The early years 6 6. Synthesis Methods TYPES OF CVD DIAMOND TECHNICAL CERAMIC Single crystal CVD diamond is sometimes classified by diamond growth its grain size: ultra nano crystalline (< 10 nm), nano crystalline (< 50 nm) micro crystalline (< 500 µm) and single crystal diamond. The grain size depends on the Single crystal substrate Substrate removal synthesis conditions, substrate and layer thickness. Layers above ~ 50 µm can be removed from the carrier substrates, Poly diamond leaving free standing bulk CVD diamond growth materials. POLYCRYSTALLINE CVD DIAMOND By controlling the impurities and Non-diamond substrate Substrate removal the grain boundaries, free standing polycrystalline diamond wafers can be SINGLE CRYSTAL CVD DIAMOND GRADES fabricated 120 mm in diameter, with SINGLE Engineered replacement for thermal and infrared optical properties CRYSTAL natural type IIa diamond that approach highest quality perfect diamond. Free-standing polycrystalline OPTICAL Low absorption and diamond wafers 140 mm in diameter GRADE birefringence diamond that are more durable, with finer grains, DETECTOR Ultrahigh purity for quantum with thermal conductivities still more GRADE optics and electronics than 2.5 times that of copper are also POLYCRYSTALLINE CVD DIAMOND manufactured. OPTICAL Engineered for far infrared laser optical applications SINGLE CRYSTAL CVD DIAMOND GRADE Ultrahigh purity material for Single crystal diamond materials, with ELECTRONIC GRADE large area passive electronics uniformly exceptional Type IIa optical, thermal and mechanical properties, are THERMAL High thermal conductivity routinely available up to 8 × 8 × 2 mm. GRADES diamond heat spreading MECHANICAL High strength diamond for GRADES precision machining ELECTRO- Boron doped diamond for CHEMISTRY electrochemical applications GRADES The Element Six CVD Diamond Handbook Further Reading 7. Technology and Applications 7 CRYSTALLOGRAPHY DIAMOND STRUCTURE {100} {110} {111} Each carbon atom in diamond is bonded to its four nearest neighbours in a regular tetrahedron. These are arranged in a variation of the face centred cubic 7 4pt (point) 2pt (point) 3pt (point) structure Fd3m-0h The three primary planes to consider for the octahedral diamond crystal are FACE 4PT 2PT 3PT the (111), (100) and (110) planes. The PLANE (100) (110) (111) nomenclature, more commonly used 0 0 450 4PT (100) 540 44’ within the diamond industry, describe 900 900 these as 3 point, 4 point and 2 point 0 0 respectively. 450 350 16’ 2PT (110) 600 900 900 The majority of CVD single crystal 900 diamond is available with (100) orientated 350 16’ 0 0 3PT (111) 540 44’ surfaces, with (110) edges, while the (111) 900 720