22 The Electrochemical Society Interface • Spring 2003

hemical vapor deposited (CVD)diamond materials, grown fromlow pressure gases activated byplasmas, hot filaments, andcombustion flames, were a nov-

elty and scientific curiosity severaldecades ago. Now, CVD diamond is amaturing material with a wide variety oftechnological impacts, and with evenmore exciting applications to come inthe future.

The driving forces for this rapidexpansion in the development andapplication of CVD diamond materialsinclude the exceptional material proper-ties of diamond; growth methods thatcan be used to engineer desired materialsproperties via control of doping, crys-tallinity (size and texture), and morphol-ogy; the relative intellectual and compu-tational simplicity of a single elementmaterial (carbon) with a low number ofelectrons; and the extensive knowledgeavailable of organic and combustionchemistries.

The defining characteristic of CVDdiamond materials, as compared to othercarbon materials, is the domination ofthe crystalline cubic phase of carbon indomains of variable size. CVD diamondmaterials span a continuum of proper-ties. Purity ranges from the intrinsic

material with impurity concentrationsless than ppm, to materials (un)inten-tionally doped with boron, nitrogen,phosphorous, sulfur, silicon, hydrogen,etc. The crystallinity of these materialsvaries from ultrananocrystalline,through nanocrystalline, and polycrys-talline to single crystal materials. In CVDdiamond materials, the crystallinedomain size and extended defect struc-tures (dislocations, twins, incoherentgrain boundaries) all exist in a mannerthat can be engineered to varyingdegrees. This variability enables thedesign and fabrication of structures thatexploit many of the desirable propertiesof diamond in a controllable manner.

Single Crystal CVD Diamond

Beginning on one end of the crys-talline scale is single crystal diamond,most typically encountered with the nat-ural diamond used in jewelry, or syn-thetic diamond grown by high pressureat high temperatures (HPHT), where dia-mond is the stable crystalline form ofcarbon. Single crystal CVD diamondshave been produced recently by numer-ous groups. Figure 1a presents a photo ofa polished (faceted) CVD single crystaldiamond. This diamond contains the

nitrogen–vacancy (N-V) and substitu-tional nitrogen, Ns, defects (which con-tribute to the color observed). For a num-ber of reasons, diamond is a candidatehost material for quantum computing.Optically detected magnetic resonanceof the N-V defect has been used recentlyto demonstrate single-qubit operations,an element of quantum computing,exploits the very long spin-spin coher-ence lifetimes (ca. 50 microseconds atroom temperature) observed in this dia-mond.1 In related work, the N-V defecthas also been exploited recently for sin-gle photon quantum cryptography.2

Recent measurements on extremelypure CVD single crystal diamonds indi-cate a quality much better than that ofthe best diamonds found in nature orgrown by HPHT processes. For extremelypure device grade CVD single crystals,with dimensions of the order of 3-5 mm,electron and hole mobilities (4400 and3800 cm2/sec, respectively) exceedingtwice the previously measured best val-ues were reported recently.3

In Fig. 1b, a ca. 4 x 4 mm single crys-tal CVD plate (white/clear) is shown inrelation to the HPHT seed diamond onwhich it was grown. This homoepitaxialcrystal was separated from the seed crys-tal using a patented liftoff technique

Chemical Vapor Deposited DiamondMaturity and Diversity

James E. Butler

C

FIG. 1. Photographs of singlecrystal diamond grown bychemical vapor deposition

(CVD) (Courtesy of ApolloDiamond): (a) a faceted

stone ca. 3 mm diameter, inwhich single qubit operationsfor quantum computing have

been demonstrated at roomtemperature using the N-V

defect; and (b) a homoepitax-ial layer ca. 4 x 4 x 1 mm

(white/clear) after separationfrom the adjacent to the orig-

inal HPHT seed crystal(yellow).

(a) (b)

The Electrochemical Society Interface • Spring 2003 23

based on ion implantation, followed bygrowth, and then an electrochemicaletch,4 showing how high quality seedcrystal can be reused multiple times.Various groups have also demonstratedthat CVD on dense ‘tiling’ of similaroriented seed crystals can be used togrow larger area quasi-single crystalplates.

Polycrystalline CVD Diamond

Polycrystalline diamond films andplates are grown on non-diamondmaterials, usually refractory metals orceramics, e.g. W, Ta, Mo, WC, Si, SiO2,and others, by seeding the surface withparticles of diamond, or by specifictreatments that evolve into small dia-mond particles, e.g. biased enhancednucleation5,6 followed by homoepitax-ial growth on the surface of these parti-cles until they coalesce into a continu-ous coating. As a result of this growthprocess, the grain size is smallest on thenucleation surface (substrate side) andcoarsens into larger grains as the filmthickens due to the overgrowth of slow-er growing surfaces (grains) by morerapidly growing surfaces (grains).Additionally, impurity incorporationand intentional doping (with B, N, P, Si,etc.) depend on the specific growthmechanisms on each local facet (typi-cally a (111) or (100) surface), resultingin different incorporation rates on eachsurface (or growth sector under eachsurface).7 Examples are shown in Fig. 2for a high quality, boron doped, poly-crystalline diamond film of thickness,ca. 450 µm and diameter of 50 mm,grown by microwave plasma enhancedCVD. Figure 2a displays a scanningelectron micrograph (SEM) of thegrowth surface of the film with charac-teristic microfacets of predominately(111) and (100) crystal orientation.Most of the crystallites have a <110>axis nearly parallel to the growth direc-tion. Figure 2b is a SEM of the cross-sec-tion through a film (laser cut andmechanically polished) showing thegrain coarsening as the film grows.Figure 2c is an optical picture of thewhite light transmission through amechanically polished diamond platesuch that the lower part of the picturewas the material near the substrate(nucleation side) with small grain sizeand the top was near the growth sidewith larger grain size. The degree ofblue color of a grain relates to theamount of boron incorporated in thegrowth sector and can vary by as muchas a factor of ten.

FIG. 2. Polycrystalline CVD diamond (boron doped): (a) SEM image of the faceted growth surface; (b) SEMimage of the edge of the film after mechanically polishing; and (c) a transmitted light microscope imagethrough section of the film thinned by polishing both sides such that the top portion of the image is close tothe growth surface and the bottom is closer to the nucleation surface.

In polycrystalline CVD diamond,many of the apparently random grains(see Fig. 2a, for example) are really crys-tallographically related, generally

through twinning (sharing a common(111) plane, or <110> axis), and formgrain clusters which probably originatefrom a single nuclei.8 Internal bound-

(a)

(b)

(c)

24 The Electrochemical Society Interface • Spring 2003

aries within the grain cluster are crystal-lographically related, while incoherentgrain boundaries are formed betweenclusters evolving from different (random-ly) oriented nuclei.9 Figure 3 displaysexamples of this, again for a mechanical-ly polished, boron doped polycrystallinediamond film grown by microwave plas-ma CVD. The contrast shown in Figs. 3a-d results from properties most sensitiveto individual growth sectors or grains,e.g. wear resistance in polishing, surfacework function, and impurity concentra-tions. On the other hand, the contrast inFig. 3e results from electron backscatterintensities (a measure of electron chan-neling), which are more sensitive to thecrystallographic orientation (alignment)of the growth sectors (grains). As seen inthe figure, growth sectors (grains) thatshare a common <110> axis have similarbackscatter intensities and thus formcrystallographically related grain clusters.

Freestanding plates of polycrystallinediamond have been made industriallywith dimensions up to 30 cm diameterand several mm thickness.10 The color ofsuch materials can vary from transparentfor optical applications, to blue for elec-trically conductive applications, togray/black found generally in cutting toolor thermal management applications.

Nanocrystalline CVD Diamond

Nanocrystalline CVD diamond is aform of polycrystalline diamond inwhich the initial seeds are of nanometerdimensions resulting in extremely highnucleation densities, typically between1010 nuclei/cm2 to over 1012 nuclei/cm2.As with polycrystalline CVD diamond,the grain size coarsens with growth,reaching 100s of nm with films of severalmicrons thickness. The purity of individ-ual growth sectors in nanocrystalline dia-mond can approach that of the singlecrystal CVD diamond resulting in optical-ly transparent, or colored doped films.But the existence of incoherent clusterboundaries and higher order twin bound-aries often causes localized stress and sp2

bonded carbon regions, which in turn,results in higher dislocation densities.

A related and unique material, ultra-nanocrystalline diamond, can be grownunder conditions where the renucle-ation, or formation of new diamondnuclei, predominates over the homoepi-taxial growth on diamond seeds (nuclei)and results in a composite material com-prised of multi-nanometer sized crys-talline diamond domains bounded bysp2 bonded surface or intergranularmaterial. This novel material is the focus

of a separate article in this issue (Ed note:See the article by Carlisle and Auciello, p.28).

Nanocrystalline CVD diamond differsfrom polycrystalline CVD diamond inthat the critical dimension of the growthsector or grain cluster (nanometers tohundreds of nanometers) is less than typ-ical characteristic dimensions for varioustransport properties, e.g. phonon scatter-ing, electrical carrier scattering, crackpropagation length. Hence, there areimportant differences in some of theproperties of these two classes of materi-als. Among the improved properties ofnanocrystalline CVD diamond films arehigher toughness and lower light scatter-ing, while other properties such as carriermobility and thermal diffusivity aredecreased. Surprisingly, in fully densenanocrystalline CVD diamond films, theYoung’s Modulus is nearly that of singlecrystal diamond.11 In addition, many ofthe other properties of diamond areretained, such as corrosion resistance,surface termination and chemistry, den-sity, optical transparency, and others.

Recently, nanocrystalline CVD dia-mond materials have been exploited forapplications in micro-electro-mechanicalsystems (MEMS) and the nano variant,NEMS, where the mechanical, electrical,and corrosion properties of these fullydense films extend the range and appli-cation of these novel devices.12-14 Forexample, the high stiffness (Young’sModulus) of diamond can push resonantstructures of a given dimension to muchhigher frequencies than achievable inmore traditional MEMS materials (Si,SiO2, Si3N4, etc.), or relax the lithograph-ic constraints required to achieve a par-ticular frequency. DiamondMEMS/NEMS may also prove to be moredurable, capable of surviving harshchemical, radiation, or mechanical shockenvironments. The optical transparencywill permit novel coupling of waveguidesusing MEMS structures and novel sensingconfigurations. Examples of the structur-al diversity that can be fabricated usingnanocrystalline CVD diamond and rela-tively standard Si micro/nano litho-graphic/processing techniques are shownin Fig. 4. Figure 4a is a SEM image of anano-tennis-racquet resonator fabricatedfrom a ca. 80 nm thick diamond filmdeposited onto a sacrificial silicon oxidelayer on a silicon wafer. The ghostlinessof the structure is due to the partial elec-tron transparency of the very thin dia-mond. In Fig. 4b, a SEM image of a later-al resonant filter fabricated from electri-cally conductive boron doped diamondis shown where the outer combs are fixed

FIG. 3. Comparison of microscopy techniques forstudying a bulk polished sample of boron dopedCVD diamond. The images are of the same area ofa <110> growth: (a) Nomarski DIC optical micro-graph; (b) low voltage (1 keV) secondary electronimage; (c) room temperature cathodoluminescencetopograph; (d) low temperature cathodolumines-cence image, passing the free and boron-boundexcitons; (e) high voltage (15 keV) SEM image, thecontrast produced is due to differences in the elec-tron backscatter intensities.9

(a)

(b)

(c)

(d)

(e)

The Electrochemical Society Interface • Spring 2003 25

FIG. 5. A method for the covalent attachment of DNAto nanocrystalline diamond and the detection fluo-rescently tagged complementary DNA with excellentselectivity.15

to the substrate and the inner comb shut-tle is clamped at two pads and nanocrys-talline diamond bars form the spring. Thefilter operated at 27.4 KHz with a Q of36,460. Figure 4c shows a SEM image of anano-xylophone where mechanical reso-nances have been detected from less than10 MHz to over 640 MHz.

The diversity of applications for dia-mond materials continues to expand. Inparticular, one can exploit the surfacechemistry specific to diamond to sense ordetect specific molecules or classes ofmolecules. A recent step in this directionis the exploitation of the biological com-patibility of diamond surfaces. The sur-face chemistry of nanocrystalline andultrananocrystalline CVD diamond filmshas been used for covalent attachment ofspecific DNA oligonucleotides in pat-

terned areas in a prototype sensor forcomplementary DNA oligonucleotides.Complementary DNA (tagged with a flu-orescent chromophore) is bound withvery high specificity (sensitive to oneacid/base pair mismatch) and the detec-tor surface may be reused many timeswith no measurable degradation aftermany cycles of denaturing and rehy-bridizing.15 Figure 5 shows a schematic ofthe surface modification process using uvlithography and uv fluorescence detec-tion of the fluorescently labeled oligonu-cleotides. �

Summary

This brief article attempts to highlightsome of the great progress that has beenmade in the quality and diversity of CVD

diamond materials. Great flexibility forengineering the diamond materials tomatch the desired properties of an appli-cation has been demonstrated. Theserecent results indicate a renaissance inthe interest and application of CVD dia-mond growth and materials.

Acknowledgments

The author gratefully acknowledgessupport from the Office of NavalResearch/Naval Research Laboratory,DARPA, and the assistance of many col-leagues including B. Feygelson, T.Feygelson, D. S. Y. Hsu, J. N. Russell, Jr., R.J. Colton, T. A. Kennedy, R. C. Linares, P.J. Doering, J. W. Steeds, S. Charles, A.Mora, R. Hamers, P. Hess, J. Phillip, S.Chattopadhyay, K. H. Chen, L. C. Chen,

FIG. 4. SEM images of MEMS and NEMS structures formed from nanocrystalline CVD diamond films: (a) a nano-tennis-racquet resonator structure;14 (b) a borondoped comb resonator electronic filter with very high Q;13 and (c) a nano-xylophone with mechanical resonances as high as 640 MHz.12

(a) (b) (c)

26 The Electrochemical Society Interface • Spring 2003

J. Wang, C. T.-C. Nguyen, L. Sekaric, H. G.Craighead, S. Rotter, M. W. Geis, and G.M. Swain.

References1. T. A. Kennedy, F. T. Charnock, J. S. Colton,

J. E. Butler, R. C. Linares, and P. J. Doering,Phys. Stat. Sol. (b), 223, 416 (2002).

2. A. Beveratos, R. Brouri, A. Villing, J.-P.Poizat, and P. Grangier, Phys. Rev. Lett., 89,187901 (2002).

3. J. Isberg, J. Hammersberg, E. Johansson, T.Wikstrom, D. J. Twitchen, A. J. Whitehead,S. E. Coe, and G. A. Scarsbrook, Science,297, 1670 (2002).

4. P. E. Pehrsson, M. Marchywka, U.S. Patent#5702586 (1997).

5. S.Yugo, T. Kanai, T. Kimura, and T. Muto,Appl. Phys. Lett., 58, 1036 (1991).

6. S. D. Wolter, B. R. Stoner, J. T. Glass, P. J.Ellis, D. S. Buhaenko, C. E. Jenkins, and P.Southworth. Appl. Phys. Lett., 62, 1215(1993).

7. M. Mermoux, B. Marcas, G. M. Swain, andJ. E. Butler, J. Phys. Chem., 106, 10816(2002).

8. A. E. Mora, J. W. Steeds, and J. E. Butler,Diam. and Rel. Mat’ls, 11, 697 (2002).

9. S. J. Charles, J. W. Steeds, D. J. F. Evans, andJ. E. Butler, Materials Letters, Submitted.

10. J. E. Butler and H. Windischmann, MRSBulletin, 23, 22 (1998).

11. J. Phillip, P. Hess, T. Feygelson, J. E. Butler,S. Chattopadhay, K. H. Chen, and L. C.Chen, J. Appl. Phys., In press.

12. J. Wang, J. E. Butler, D. S. Y. Hsu, C. T.-CNguyen, in The Fifteenth IEEE InternationalConference on Micro Electro MechanicalSystems 2002, 657, (2002).

13. J. Wang, J. E. Butler, D. S. Y. Hsu, and C. T.-C. Nguyen in Solid-State Sensor, Actuator,and Microsystems Workshop Proceedings,Hilton Head Island, South Carolina, 61,(2002).

14. L. Sekaric, J. M. Parpia, H. G. Craighead, T.Feygelson, B. H. Houston, and J. E. Butler,Appl. Phys. Lett., 81, 4455 (2002).

15. J. E. Butler, W. Cai, J. A. Carlisle, J. Gerbi, D.M. Gruen, T. Knickerbocker, T. L. Lasseter,J. N. Russell, Jr., L. M. Smith, and R. J.Hamers, Nature Materials, 1,253-257 (2002).

About the Author

JAMES E. BUTLER has activelyresearched many facets of diamondchemical vapor deposition includinggrowth mechanisms, in situ diagnos-tics, surface chemistry, defect charac-terization, materials properties, anddevice fabrication; and heads theGas/Surface Dynamics Section,Chemistry Division, Naval ResearchLaboratory. He may be reached by e-mail at butler@ccf.nrl.navy.mil.