THE ELEMENT SIX CVD DIAMOND HANDBOOK
THE ELEMENT SIXCVD DIAMONDHANDBOOKContact Element Six Technologies at technologies@e6.com
CONTENTSIN T R O D U C IN G D I A M O NDP H Y S I C A L P R O P E R T IE SD I A M O ND C L A S S IF I C AT I O ND I A M O ND S Y N T HE S I ST Y P E S O F C V D D I A M O NDC R Y S TA L L O G R A P H YME C H A NI C A L S T R E N G T HP O L I S HIN G O F D I A M O NDD I A M O ND S UR FA C E S34567891011PROPERTIESO P T I C A L P R O P E R T IE SO P T I C A L C O N S TA N T SR A M A N S C AT T E R IN GS IN G L E C R Y S TA L O P T I C SP O LY C R Y S TA L L INE O P T I C SE MI S S I V I T Y A ND R F W IND O W SP R E C I S I O N C O MP O NE N T ST HE R M A L P R O P E R T IE SE L E C T R O NI C P R O P E R T IE SE L E C T R O C HE MI C A L P R O P E R T IE S12131415161718192021D ATA S H E E T SE L E C T R O NI C G R A D E SO P T I C A L A ND R F G R A D E SME C H A NI C A L G R A D E ST HE R M A L G R A D E SE L E C T R O C HE MI C A L G R A D E2223242526FURTHER READING27Advances in the synthesis and processingtechnology for CVD diamond has resultedin materials with exceptional diamondproperties in practical components.Engineered single crystal CVD diamond,with ultra low absorption and birefringencecombined with long optical path lengths,has made Monolithic Diamond RamanLasers a practical reality.The Element Six CVD Diamond Handbook2
INTRODUCING DIAMONDDiamond is characterised by itsexceptional hardness, robustness and itsoptical and thermal properties;pre-eminent as a gemstone and anindustrial tool.Natural diamond has an inherentvariability and scarcity that limits its use inengineering applications. Developmentsin synthesis processes have enabled theproduction of consistently engineeredsynthetic diamond; firstly in the 1950susing high pressure and high temperatureand later using chemical vapourdeposition in the 1980s to produce theexceptional covalent crystal diamond.I T IS ALL IN T HE S T RUC T UREDiamond’s properties derive from itsstructure; tetrahedral covalent bondsbetween its four nearest neighbours,linked in a cubic lattice. This stronglybonded, tightly packed, dense and rigidstructure gives rise to its outstandingproperties. Manipulating the impactof defects and the synthesis conditionsmeans that material scientists have beenable to optimise and tailor the remarkableproperties of diamond for a wide range ofapplications.The modern industrial world consumesapproximately 800 tonnes of syntheticdiamond, around 150 times the amount ofnatural diamond mined as 007aluminiumsiliconphosphorus1314AL Si26.9811528.085P30.973High pressure high temperature synthetic diamond isusually nitrogen doped, giving it a distinctiveyellow hue.Carbon’s position as a group IV element above siliconin the periodic table.The Element Six CVD Diamond HandbookFurther Reading2. Science’s Gem3
PHYSICAL PROPERTIESP R OP E R T YVA L U ENUMBER DENSITY3 . 5 1 5 2 4 x 1 0 3 k g m -31 . 7 7 x 1 0 23 p e r c m 3L AT T I C E T Y P EC u b i c F d 3 m - O h7L AT T I C E S PA C I N GLattice constant between0.356683 /-0.000001 and0.356725 /-0.000003 nm at 298 KDENSITYF R A C T U R E T O U G H N E S S K 1C5 M P a m 0.5 S i n g l e C r y s t a l8 . 5 M P a m 0.5 P o l y c r y s t a l l i n eP O I S S O N ’ S R AT I O0.1YOUNG’S MODULUS1050 GPaFRACTURE STRENGTH2.5 to 3 GPa Single Crystalsurface finish dependent200 to 1100 MPa Polycrystallinegrain size and grade dependentFAT I G U E L I F E 9 5% s t r engt h af t er 10 7 c y cl es t o 7 0 % of F SFRACTURE PLANE{111} and occasionally {110}2.55 Single CrystalWEIBULL MODULUSHARDNESSFRICTION COEFFICIENT (µ)1020Growth SurfaceNucleation surfacePolycrystallinePolycrystalline70 to 120 GPa Single crystal(Plane and direction of indent dependent) 81 18 GPa Polycr ystalline(Grain orientation dependent)0.05 to 0.15 (orientation dependent)In air (requires surface termination)The Element Six CVD Diamond HandbookFurther Reading3. Mechanical Properties of Diamond4
DIAMOND CL ASSIFICATIONN 5PPMN 5PPMType I - Contains nitrogenTypically 100 3000 ppmType II - Very low nitrogenconcentration 98% OFNATURALDIAMONDMOST CVDDIAMOND(UNLESS DOPED)VERY RARE INNATURET Y PE IaT Y PE IbT Y PE IIaT Y PE IIbContains enNitrogen asthe majorimpurityElectricallyinsulatingBoron asthe majorimpurity p-typesemiconductorBoron dopedCVDT Y PE Ia AT Y PE IaBContainsnitrogenin the AaggregateformContainsnitrogenin the Baggregateformclassification scheme remains relevant tosynthetic diamond, in that most HPHTgrown diamond falls into the type Ibclassification and most CVD into typeIIa, due to their different nitrogen levels.However, within this there are nowmultiple polycrystalline and single crystalgrades, developed with specific tailoredproperties that this simple classificationsystem does not distinguish between.Natural diamonds were first classifiedaccording to their optical properties.The majority, being Type I, have anabsorption edge of around 330 nm anda small group, being Type II, have anabsorption edge of about 220 nm. Overthe years this natural classificationscheme has been extended and furtherlinked to different predominantdefects, such as nitrogen and nitrogenclusters. In its broadest sense, thisThe Element Six CVD Diamond HandbookFurther Reading4. The “Type” Classification5
DIAMOND SYNTHESISHPH TThe vast majority of synthetic diamondThe plasma can be heated by microwaves,is made by high pressure and highradio frequency, lasers, direct current,temperature diamond (HPHT) methods.hot filament and chemical reactions. TheHPHT aims to mimic the thermodynamicnucleation and growth of continuousconditions in nature that diamond formsdiamond requires a substrate within, but with the addition of a molten metal refractory characteristics, stable carbidesolvent / catalyst to reduce the largeformation and a low thermal expansionkinetic barrier and act as transport mediacoefficient.for dissolved carbon. Material grownthis way typically has a yellow hue, as aconsequence of nitrogen incorporationWaveguideinto the diamond lattice from theMicrowavesatmosphere and growth materials.AntennaCHEMICAL VAPOUR DEPOSI T ION (C V D)ChamberCVD diamond exploits the relativelysmall difference in stability betweenthe two allotropes (sp2 & sp3) of carbon.Create the right surface conditions, thepresence of atomic hydrogen and surfacetemperatures above 600 C and theformation of diamond depends on havinga faster nucleation and growth rate thangraphite.PlasmaGas inletSubstratePressurecontrolThe growth conditions are created bythermal disassociation of hydrogen, anda gaseous source of carbon in plasma,with a gas temperature above 2000 C.VacuumAfter nearly 4 decades of stop-startresearch into CVD diamond growth,microwave plasma enhanced CVDdiamond synthesis emerged as acommercial synthesis method in the1990s. The growth rates and controlover purity of this method lends itself tomanufacturing high quality, free-standing,polycrystalline and single crystal CVDdiamond.The Element Six CVD Diamond HandbookFurther Reading5. CVD: The early years6. Synthesis MethodsWindow6
T YPES OF CVD DIAMONDT ECHNICAL CER AMICSingle crystaldiamond growthCVD diamond is sometimes classified byits grain size: ultra nano crystalline ( 10nm), nano crystalline ( 50 nm) microcrystalline ( 500 µm) and single crystaldiamond. The grain size depends on thesynthesis conditions, substrate and layerthickness. Layers above 50 µm can beremoved from the carrier substrates,leaving free standing bulk CVD diamondmaterials.Single crystal substrateSubstrate removalPoly diamondgrowthPOLYCRYS TALLINE C V D DIAMONDNon-diamond substrateBy controlling the impurities andthe grain boundaries, free standingpolycrystalline diamond wafers can befabricated 120 mm in diameter, withthermal and infrared optical propertiesthat approach highest quality perfectdiamond. Free-standing polycrystallinediamond wafers 140 mm in diameterthat are more durable, with finer grains,with thermal conductivities still morethan 2.5 times that of copper are alsomanufactured.Substrate removalS IN GL E C RY S TA L C V D D I A M OND GR A DE SSINGLEC R Y S TA LEngineered replacement fornatural type IIa diamondOPTICALGRADELow absorption andbirefringence diamondDETECTORGRADEUltrahigh purity for quantumoptics and electronicsP O LY C RY S TA L L INE C V D D I A M ONDOPTICALGRADESINGLE CRYS TAL C V D DIAMONDEngineered for far infraredlaser optical applicationsE L E C T R O N I C Ultrahigh purity material forlarge area passive electronicsGRADESingle crystal diamond materials, withuniformly exceptional Type IIa optical,thermal and mechanical properties, areroutinely available up to 8 8 2 mm.THERMALGRADESHigh thermal conductivitydiamond heat spreadingM E C H A N I C A L High strength diamond forprecision machiningGRADESELECTROCHEMISTRYGRADESBoron doped diamond forelectrochemical applicationsThe Element Six CVD Diamond HandbookFurther Reading7. Technology and Applications7
CRYSTALLOGRAPHYDIAMOND S T RUC T URE{100}Each carbon atom in diamond is bondedto its four nearest neighbours in a regulartetrahedron. These are arranged ina variation of the face centred cubicstructure Fd3m-0h74pt (point)The three primary planes to considerfor the octahedral diamond crystal arethe (111), (100) and (110) planes. Thenomenclature, more commonly usedwithin the diamond industry, describethese as 3 point, 4 point and 2 pointrespectively.The majority of CVD single crystaldiamond is available with (100) orientatedsurfaces, with (110) edges, while the (111)is the cleavage plane. All of these surfacescan be presented in CVD diamondengineered products.{110}2pt (point){111}3pt (point)FA C EP L A NE4PT( 100)2PT( 110)3PT(111)4PT (100)00900450900540 44’2PT (110)45090000600900350 16’9003PT (111)540 44’350 16’90000720 32’POLYCRYS TALLINE DIAMONDThe grain structure of polycrystallineCVD diamond has a non-uniformcomposition, due to the growth process.Small randomly orientated grains formduring the nucleation process, thengrains with facets favoured by the growthconditions rapidly form large grains,that are 10% of the film thickness indiameter, elongated in the direction ofgrowth. When processed it still presentsan inhomogeneous surface of manyorientations.SEM image of the polished surface of boron dopeddiamond, highlighting the different orientations.A polished cross section, illustrating the elongatedstructure originating from the nucleation surfacealong the direction of growth.The Element Six CVD Diamond HandbookFurther Reading8. Structure of Diamond8
MECHANICAL STRENGTHWith grain size increasing with layerthickness, strength is also dependent onthickness. Grain size is also dependent onthe growth conditions and the orientationof the layer; with the nucleation surfacein tension, the fracture stress is higherand the differences between grades lesspronounced. In use, consideration shouldbe given to which side is under tension.S T RENG T H OF DIAMONDThe covalent carbon to carbon bondsmake diamond a very high strengthmaterial. The extreme rigidity of thediamond lattice also makes it a very stiffmaterial, thus diamond is the ultimatehigh strength ceramic. The stress tofracture diamond is related to the size ofthe critical flaws in the material.After 107 cycles to 70% of fracturestress, there has been found to beno degradation expected in fracturestrength.S T RENG T H OF SINGLE CRYS TALGROWTH SIDE FRACTURE STRESS (MPa)Tensile fracture testing of single crystaldiamond finds the critical flaw size tobe 100 nm. In mechanical testing thefracture strength is dependent on thesurface finish and volume under test.With careful surface preparation,3 5 0.2 mm samples achieve fracturestresses in the region of 2.5 - 3.0 GPa.S T RENG T H OF POLYCRYS TALLINEDIAMONDBulk polycrystalline diamond is in therange 200 to 1100 MPa. It is significantlyweaker due to the critical flaw found tobe proportional to the grain size. Sincethe critical flaw / grain size is large at 50 to 300 µm, polycrystalline diamondstrength is relatively independent ofsurface finish, with a Weibull modulus of 20 when grain size is above 100 cal GradesOptical and Thermal Grades0.40.81.01.2The strength of polycrystalline diamond is dependenton the grain size. Finer grained mechanical recipesare stronger, whilst material from all recipes exhibita dependency with thickness. This effect is mostpronounced on the growth surface.The Element Six CVD Diamond HandbookFurther Reading3. Mechanical Properties of Diamond0.6THICKNESS (mm)9
POLISHING OF DIAMONDE T CHING OF DIAMONDProcessing the hardest bulk material isnot trivial. To date most methods still relyon diamond to process diamond. The{111} plane of diamond is harder (up to45%) and more wear resistant than theothers.CVD diamond can be etched in high pHchemical mechanical polishing (CMP)slurries and in reactive ion etchingplasma based process.L APPINGRough processing of diamond is achievedusing lapidary processes with a diamondgrit slurry to remove material quickly.The physical wear mechanism is brittlefracture, leaving a rough surface withsignificant sub-surface damage.SCAIFE POLISHINGHigh quality, low damage surfaces preparedon single crystal diamond by traditional scaifepolishing. The shallow polishing grooves, 5 nmare 20 nm wide.For single crystal diamond, by confiningthe direction of the wear fracture plane,low damage and low surface roughnesscan be achieved. The diamond is pressedonto a high speed rotating cast iron plate,a scaife, which has diamond particlesembedded. It can achieve high surfacefinish Ra 1 nm with low damage.3.0124100RESIN BONDED W HEEL POLISHINGThis can be used to process both singlecrystal and polycrystalline diamond.Large areas and higher flatness canbe achieved, however the subsurfaceprocessing damage is more severe thanwith scaife 53.0Polycrystalline diamond, polished with a fixedgrit diamond wheel. The grain relief is due tothe different grain orientations, which polish atdifferent rates.The Element Six CVD Diamond HandbookFurther Reading9. Diamond Polishing10-100-117µmmm2.5
DIAMOND SURFACESSURFACE T ERMINAT IONCARBIDE FORMAT IONThe diamond lattice terminates withdangling carbon bonds and typically Oand OH groups covalently bond to them.The surface can also be terminatedwith hydrogen via a plasma treatmentprocess. Exposed diamond surfaces havean inherent affinity for oils and dustparticulates.Two distinct groups of metals react withdiamond. The first group form theirstable carbides, such as Si, W & Ti. Thesecond group, including Fe, Co, Cr & Niare solvents for carbon and exhibit poor/no adhesion via graphitised interfaces.ME TALISAT ION & OP T ICAL COAT ING SOhmic metal carbide contacts can beformed using thin film, multi-layer metalcoatings, with a carbide former as thebasis for adhesion, followed by a stableinert metal such as Au, Pt. Stable to 400 C, these coatings are used for theindirect attachment of contacts or heatsinks. Dialectic optical coatings can alsobe employed using extremely thin carbideadhesion layers.CLE ANING DIAMONDResidual surface contamination can beremoved from diamond using aggressivehigh temperature, 150 C oxidisingsolutions, such as a combination ofacid, e.g. H2SO4 and an oxidising agent,e.g. KNO3. These types of cleaningsteps are used in the manufacturingprocesses of bulk CVD diamond andleave the diamond surface with an oxygentermination. Cleaning diamond, and/orcare to avoid surface contamination, arerequired immediately prior to processessuch as bonding and metallisation.OX IDAT ION AND GR APHI T ISAT IONMounted ATR prisms for spectroscopy.The thermal oxidation in the air starts atabove 600 C. In a reducing atmosphere,e.g. H2, the onset of graphitisation isabove 1300 C. Surface graphitisationcan also occur under ion bombardment,such as with low pressure plasma cleaningprocesses.BR A ZINGCVD diamond can be attached usingactive brazes, formulated to form a stablecarbide interface, in high temperaturevacuum processes, at 800 C. Interfaceshear strengths 200 MPa can beachieved. The stresses generated by thethermal expansion mismatch betweendiamond and most mount materialsrequires careful consideration.The Element Six CVD Diamond HandbookFurther Reading10. Diamond surfaces:11. Brazing Parameters11
OPTICSOPTICAL PROPERTIEST R ANSMIS SION SPEC T RUMcut off at 225 nm (5.47 eV). It is thentransparent in the ultra violet, visible nearinfrared regions, far infrared and radiofrequencies, with only weak phonon bandabsorption, in the 2.5 to 7 µm regionpeaking at 14 cm-1 at 4.63 µm.The exceptional properties of syntheticCVD diamond place it at the pinnacle ofmodern optical materials, from ultravioletto radio frequency transmission. Theintrinsic optical properties are governedby its band gap in the deep UV, with aDIAMOND30A B S O R P T I O N C O E F F I C I E N T ( c m -1)SAPPHIREZINC SELENIDECALCIUM FLUORIDE202 and 3 phonon regionGORILLA GLASSAbsorption edge10visibleregion00.20.512510WAV E L E NG T H ( µ m)20RADIOFREQUENCIESAbsorption coefficient for ultra high purity CVD diamond from the UV cut-off 225 nm to far infrared region.Diamond has a relatively low dielectric constant making it highly suitable as a window for radio frequencyapplications.The Element Six CVD Diamond HandbookFurther Reading12. Single crystal optics12
REFR AC T I V E INDE XREFRACTIVE INDEX (η)0.5 0.3306λ4.3356λ n( λ ) 1 2 2 22 λ (175) λ (106) 2Wavelength226nm400nm500nm600nm700nm1064nm7. 0 µ m10.6 µm14.0 µm2 .7 5Diamond has a refractive index in therange of 2.7, at 220 nm, to 2.37 at 10.6 µm.The incident reflectivity is in the range21.3 % to 16.7%. The Sellmeier equationapproximates the dispersion curve for226 to 650 nm, where λ is in nm.2OPTICSOPTICAL CONSTANTS2 .7 2.4152.4062.3922.3762.3762.37510100WAV E L E NG T H ( µ m)In the range 2.5 to 25 µm, dispersion isbetter approximated by the Hertzbergerexpression.1.188971.0083 10 4 2.3676 10 5 λ 2 3.24263 10 8 λ 42λ 0.028 λ 0.028 1000 1000REFRACTIVE INDEX (η)n( λ ) 2.37837 The thermo-optic coefficient of refractiveindex (1/η ) (dη /dT) is in the range3.2 to 6.7 10 -6 K-1 IR region, 2 to 4.0 10 -6 K-1 in the UV to NIR 882.3872.386-50 -0 50 00 50 00 50 00 50 00 50 001 1 2 2 3 3 4 4 5T E M P E R A T U R E CThe Element Six CVD Diamond HandbookFurther Reading13. Refractive Index14. Thermo Optic Properties13
OPTICSRAMAN SCAT TERINGR AMAN SPEC T ROSCOPYR AMAN L ASERThe Raman frequency of diamond, at RT,is ω 1332.3 cm-1, line width 1.5 cm-1. Thepeak shape, position and luminescence inthe Raman spectroscopy can be used bymaterial scientists to assess diamond filmquality, including phase purity, crystallineperfection and stresses.CVD synthesis enables intra-cavity laseroptics, including Raman frequencyconvertors. In comparison with otherhigh gain Raman crystals, syntheticdiamond has a larger frequency shift atmuch higher brightness.T SOURCEDhcω 0k ln 1 C / ω 0 ω (T ) {(kgLdη / dT Δλ)G AT ERaman Figure of Merit In applications, the Raman shift canbe used as a sensor itself. Temperaturedependency is given by the semiempirical expression:DRAIN}where w 0 1333 0.6 cm-1, C 61.14 5and D 0.787 0.03. C & D are fittedconstants and w 0 is the Raman frequencyat T 0 K.DioamndThe approximate relationship of peakshift with hydrostatic pressure is given by:Δω (P) 3.2 cm 1 GPa 1The Raman peak shift of nanometre scale diamondcrystallites can be used as a micro temperature probemeasuring the gate junction temperature of electronicdevices such as transistor gate structures.SC CVD DIAMONDRAMAN GAINRAMAN SHIFT Δ λ cmK G W K G D ( W O 4) 2YVO4B A ( N O 3) 215451113329018921047C R Y S TA L L E N G T H L m m8252525T H E R M A L C
The Element Six CVD Diamond Handbook 3 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
CVD Diamond Pin Specimens The CVD diamond pin specimens were produced as follows: (1) a free-standing diamond film was pro-duced by the hot-filament CVD technique (ref. 5); (2) the film was brazed onto one end of a steel pin; and (3) the CVD diamond tip of the pin was then ground with a diamond wheel and polished with diamond powder.
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