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Atomic-hydrogen mapping in hot-filamentchemical-vapor deposition

Jussi Larjo, Heidi Koivikko, Daming Li, and Rolf Hernberg

Two-dimensional maps of the atomic-hydrogen concentration distribution were acquired with two-photonlaser-induced fluorescence. The environment was a hot-filament chemical-vapor-deposition reactorused for polycrystalline diamond-film deposition. The maps were measured in situ under diamond-deposition conditions with variation of the growth parameters. The parameters investigated werefilament temperature, input methane concentration, and total pressure. © 2001 Optical Society ofAmerica

OCIS codes: 300.6910, 310.1860.

1. Introduction

Atomic H has a key role in crystalline diamond-filmproduction by chemical-vapor deposition1 ~CVD!. It

articipates both in the gas phase, generating hydro-arbon radicals that act as growth precursors, and athe surface, generating dangling bonds for the radi-als that attach to the diamond lattice and etching-offhe generated nondiamond carbon deposit. Because

atoms are consumed by all these reactions, thectivation mechanism of diamond CVD must producehe required atomic H for the growth process. Un-er equilibrium conditions the H concentration is tooow for practical film growth at typical substrate tem-eratures of 800 °C–1000 °C. Therefore CVD dia-ond deposition typically relies on chemical

onequilibrium at the substrate surface with convec-ive or diffusive mass-transfer mechanisms providinghe necessary number of H atoms. The relevantass-transfer mechanism is determined by the gas-

hase conditions in the reactor. A boundary layerith significant concentration and temperature gra-ients will be generated adjacent to the substrate.One of the most popular methods of producing

tomic H is hot-filament activation2 in which an elec-trically heated wire is located in close proximity tothe substrate. The filament material is typically W

The authors are with the Optics Laboratory, Tampere Univer-sity of Technology, P.O. Box 692, FIN-33101 Tampere, Finland.J. Larjo’s e-mail address is larjo@cc.tut.fi.

Received 22 May 2000; revised manuscript received 28 August2000.

0003-6935y01y060765-05$15.00y0© 2001 Optical Society of America

or Ta that is converted to carbide during the process.The reactor atmosphere is H, with CH4 the mostcommonly used carbon carrier. In a H atmospherethe filament surface dissociates H2 effectively, pro-ducing typically a few percent dissociation when thefilament’s surface temperature is higher than2000 °C. The CH4 content is less than 1% in a typ-ical hot-filament reactor, but it can be as high as 12%if the filament temperature is sufficiently high.3,4 Itis generally believed that diffusion is the dominantmass-transfer mechanism in this kind of reactor5

when there is no forced-gas convection. Naturalconvection might occur, depending on the reactor ge-ometry, but its effects are not typically accounted forin reactor models.

The optimal atomic-H mass transfer to the sub-strate is crucial to the deposition process. One of thepractical goals of diamond-film processing is homoge-neous large-area growth. Requirements for uniformgrowth over a large substrate are a uniform substratetemperature and homogeneous mass transfer overthe boundary layer. These characteristics are noteasily attained with techniques that employ convec-tive mass transfer, like flame6 or thermal-plasma7

deposition. In other techniques convection is hardto eliminate entirely with the strong temperaturegradients present. It is therefore necessary to ana-lyze the particular reactor setup with flow modelsand experimental techniques to optimize growth con-ditions.

In situ atomic-H detection and concentration-distribution measurements are not trivial in temper-atures that are too low for emission spectroscopy.The most frequently used technique for this taskis two-photon laser-induced fluorescence ~LIF!

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spectroscopy.8–10 We demonstrated a two-dimensional H-concentration-distribution measure-ment in the reactive layer of diamond CVD in athermal inductively coupled plasma environment.11

In this paper, we present the same measurement in ahot-filament CVD reactor.

2. Experiment

A special hot-filament reactor was constructed foroptical diagnostics of the deposition process ~Fig. 1!.

he deposition system consists of an electricallyeated TaC filament and a water-cooled Si substrate,imilar to the system presented in an earlier paper.3

The configuration was inverted, i.e., the filament wasplaced below the substrate at a distance of 10 mm.The filament was arranged as a grid of five parallelwires with a 5-mm separation. The gas atmospherewas a mixture of H2 and CH4 with the total pressurein the range of 20–40 mbars. The substrate was a28-mm-square Si wafer. The gas-feed rate, pres-sure, and filament temperature are monitored duringthe deposition process. Optical access to the reactoris provided through two pairs of windows at the sub-strate level.

The LIF detection system is essentially similar tothose used in the earlier experiments.11 The laser isa frequency-tripled pulsed dye laser that is pumpedwith a Q-switched Nd:YAG laser. The excitation

avelength that we used was 205 nm with a maximumulse energy of 1000 mJ and a 10-ns pulse duration.he laser beam was focused with a lens system with a500-mm focal length, which produced a uniform focal-rea length of approximately 100 mm with the beamenerated by the laser system. The two-photon exci-ation was n 5 1 3 n 5 3; the fluorescence was gen-

erated by the spontaneous Hb transition at 656 nm anddetected with a gated, intensified CCD camera. Thedetection system was improved by use of on-line laserpulse-energy monitoring of the measured signal to cor-rect the signal for laser intensity fluctuations. Thesingle-shot fluorescence distribution along the beamwas captured by the camera. The vertical displace-ment of the beam was controlled manually by adielectric-mirror translation stage. The two-dimensional signal distribution was composed of sev-eral one-dimensional lateral distributions that weremeasured from different positions under the same dep-

Fig. 1. Two-dimensional measurement setup.

66 APPLIED OPTICS y Vol. 40, No. 6 y 20 February 2001

osition conditions but with background subtraction.Capturing one full signal map took approximately 10min when using the signal averaging of 100 pulses forone lateral distribution.

To determine the relation between the atomic-Hnumber density and the LIF signal intensity requiresthat a number of factors be determined. These canbe enumerated briefly, as follows:

1. The processes competing with the monitoredH spontaneous emission must be accounted for.These processes include stimulated emission andphotoionization. Their roles depend strongly on thepopulation of the excited state. Therefore they canusually be rendered insignificant by proper selectionof the laser pulse energy. Because the two-photonexcitation rate has a quadratic dependence on thelaser intensity, the detected LIF signal should beproportional to the square of the pulse energy. Thecompeting processes will cause the signal to deviatefrom this behavior so that it will be lower than ex-pected. Thus one can determine their presence bythe signal behavior described in items 2 and 3.

2. Collisional de-excitation ~quenching! of the ex-cited state of the atom must be determined. This pro-cess is different from the other competing processesbecause it depends on only the local gas-phase condi-tions ~number density, temperature, and chemicalcomposition!. Quenching will always be an issue in acollision-dominated environment and must be ac-counted for separately in a quantitative analysis.

3. The photodissociation of H2 compounds bythe excitation laser can produce nonambient atomicH, which will, in turn, generate spurious fluorescencesignals.12 This situation is particularly troublesomewhen connected with the competing deexcitationprocesses because their effects can cancel each other,leading to an apparently quadratic power-to-signaldependence. Probably the best way to eliminate theeffect of photodissociation is to monitor the signalthat is detected from a location far away from thefilament, where the presence of atomic H is not ex-pected. In our experience it is possible to optimizethe laser energy to a level at which no LIF signal isdetected in the surrounding gas and a strong signal isstill emitted from the reactive layer. Together withthe quadratic-power-dependence check, which is re-peated at different locations in the measurement re-gion, this finding was held to be sufficient evidencefor an insignificant amount of photodissociation inthe process.

4. The excitation rate of the H atoms dependson the overlap of the respective spectral-line profilesof the laser and the selected atomic transition. Ifthe dominant line broadening in the gas is Dopplerbroadening the line overlap will vary according to thelocal kinetic temperature. This variation can be ac-counted for if the temperature is known.

The absence of ionization, stimulated emission,and photodissociation effects was verified by the com-parison of the quadratic signal versus the pulse-

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Table 1. Experimental Parameters

energy check. This was also the model for the pulse-energy correction factor. The optimal laser pulseenergy was found to be between 300 and 400 mJ,depending on the CH4 input to the reactor. Detailedcalculations of the line-broadening and the quench-ing effects for the hot-filament environment showthat, with our laser linewidth of 0.14 cm21, the com-bined variation of quenching and line-overlap factorsis less than 3% in the 1000–3000-K temperaturerange. This result is in accord with the results pre-sented in Ref. 13. Therefore the LIF signal was ap-proximately proportional to the H number densitywithout further adjustment. The statistical errorcaused by shot noise and laser beam-quality fluctua-tions was less than 10% at the profile maximum withsignal averaging of 100 pulsesypoint.

Several maps were measured under different dep-osition parameters ~see Table 1!. The imaging wasdone by use of an fy1.2 standard camera lens. Thespatial resolution of the measurement was 0.18 mmypixel, as determined by use of a resolution target.

3. Results and Discussion

The measured signal maps covered the 0–4-mm dis-tance from the substrate surface. Below this pointthe LIF signal was too weak to be detected by thecamera. This falloff is not predicted by earlier in-vestigations8,14 in which the signal increased towardthe filament. One possible explanation for the dif-ferent behavior seen in this study is the experimentalarrangements: In the study of Ref. 8 the substratewas absent, and in the study of Ref. 14 the normalfilament-above-substrate configuration was used.As the distance of 6–10 mm could not be coveredbecause of the filament’s background luminescence,which saturated the detector, it could not be deter-mined how far the decrease might continue in thiscase.

A typical structure of the boundary-layer signal dis-tribution is presented in Fig. 2. In this and subse-quent figures x denotes the distance from the centerxis and z the distance from the substrate surface. It

can be seen that, far from the substrate, the H densityincreases with the distance from the filament. Thistrend reflects that there is little homogeneous H re-combination in the gas phase and that the numberdensity follows the inverse of the gas-temperature pro-file between the filament and the substrate. This de-

FigureNumber

FilamentTemperature Tf

~°C!

ReactorPressure p

~mbars! CH4 Input

2 2400 40 1%3 2600 40 8%4 2600 40 2%5 Reference figure 40 1%6 2750 Reference figure 8%7 2600 40 Reference figure

velopment continues to the surface level outside thesubstrate area, i.e., when uxu . 20 mm.

The role of the substrate surface in diamondgrowth can be seen as that of a catalyzer to the het-erogeneous recombination of H, producing activesites for diamond-film growth and the etching off ofthe nondiamond carbon. The effect can be seen at acloser than 2-mm distance from the surface as analmost linear decline of the number density in boththe vertical and the lateral directions. The bound-ary layer exhibits features that are very similar tothose seen from the maps that we measured in theinductively coupled plasma reactor, even though themass transport was strongly convective in the latterenvironment.

Figure 3 shows a typical lateral LIF profile at anapproximately 0.2-mm distance from the substrate.The profile is nearly symmetrical and uniform overthe substrate region. Sharp dips in the signal areseen close to the substrate edges. This feature ispossibly related to the inverted filament–substrateconfiguration, as the gas layer cooled by the sidewallof the substrate holder flows down because of buoy-

Fig. 2. Typical two-dimensional signal distribution.

Fig. 3. Samples of lateral signal profiles that are close to thesubstrate.

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ancy forces. In light of simple diamond-growth mod-els,15 the drop in the H concentration along the edgess seen as a narrow strip of poor-quality film in theubstrate edge. This effect was observed in relatedrowth experiments ~Fig. 4!.Comparative measurements were performed

between conditions with differing deposition para-meters. Vertical one-dimensional profiles were ex-tracted from the central axis of the substrate. Theexperimental variables were the filament tempera-ture Tf ~Fig. 5!, the reactor pressure p ~Fig. 6!, and the

H4:H2 input ratio ~Fig. 7!. All the profiles haveimilar qualitative behaviors, peaking at a 2–3-mmistance and falling rapidly when approaching theubstrate. The decline toward the filament is slowernd depends on the filament temperature.The effect of the filament temperature on the overalldensity is obvious. It can be seen from Fig. 5 thatlarge increment is observed between 2400 °C and

600 °C, whereas the difference is less notable in the

68 APPLIED OPTICS y Vol. 40, No. 6 y 20 February 2001

000 °C–2400 °C interval. We reported diamondrowth with filament temperatures approaching000 °C in a different reactor3,4; a further increment in

the H density is expected under these conditions.Pressure variation has little effect on the signal

intensity, as can be seen from Fig. 6. This is ex-pected because, as the gas density decreases, quench-ing will decrease by the same amount. So the molefraction of atomic H will remain nearly invariantwith respect to pressure, whereas the absolute den-sity halves. Generally, pressure variation in thisrange does not affect growth significantly.

Increasing the CH4 input will decrease the H den-sity significantly ~Fig. 7!. We attribute this decreaseo the participation of atomic H in the reaction

CH4 1 Hº CH3 1 H2, (1)

which, according to the models, is a rapid enoughreaction to be in partial equilibrium in a hot-filamentenvironment. The added CH4 will consume H atoms

Fig. 4. Results of diamond-film growth on ~a! the substrate center and ~b! the substrate edge. The resolution scale is 10 mm.

Fig. 5. Vertical LIF profiles obtained under different filamenttemperatures.

Fig. 6. Vertical LIF profiles obtained under different gas pres-sures.

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to produce additional CH3 in the gas phase. Fila-ment poisoning was not observed.

These findings are consistent with the simplegrowth models.15 Increased CH3 concentration inthe gas will improve the film-growth rate but will alsolead to poor film quality unless the atomic-H densityincreases as well. To maintain quality with the im-proved growth rate requires that a higher filamenttemperature be used to enhance the H dissociation.

4. Conclusions

We have presented preliminary results of two-dimensional LIF signal-distribution measurementsin a hot-filament CVD environment. It is plausiblethat the signal distribution describes the H-atomnumber density with good accuracy in this setup withcareful optimization of the laser pulse energy and thelinewidth. Variation of the growth parameters pro-duced the expected results in the measured distribu-tions, with the filament temperature and the CH4input having strong effects on the signal level.

With the results that we obtained it is possible toevaluate qualitatively the diamond-growth process interms of growth rate and uniformity and to test thevalidity of the gas-chemistry models with regard tovariation of the H-atom concentration. Further de-velopment of the diagnostic technique will involveabsolute calibration and implementation of the gastemperature measurement together with growth ex-periments.

Fig. 7. Vertical LIF profiles obtained under different CH4 inputamounts.

References1. M. Frenklach and K. E. Spear, “Growth mechanism of vapor-

deposited diamond,” J. Mater. Res. 3, 133–140 ~1988!.2. F. Jansen, M. A. Machonkin, and D. E. Kuhman, “The depo-

sition of diamond films by filament techniques,” J. Vac. Sci.Technol. A 8, 3785–3790 ~1990!.

3. D. M. Li, T. Mantyla, R. Hernberg, and J. Levoska, “Diamonddeposition by coiled and grid filaments using high meth-ane concentrations,” Diamond Rel. Mater. 5, 350–353~1996!.

4. D. M. Li, R. Hernberg, and T. Mantyla, “Diamond nucleationunder high CH4 concentration and high filament tempera-ture,” Diamond Rel. Mater. 7, 188–192 ~1998!.

5. D. G. Goodwin, “Scaling laws in diamond chemical vapor dep-osition. II. Atomic hydrogen transport,” J. Appl. Phys. 74,6895–6923 ~1993!.

6. J. Hwang, K. Zhang, B. S. Kwak, and A. Erbil, “Growth oftextured diamond films on Si~100! by C2H2yO2 flame method,”J. Mater. Res. 5, 2334–2336 ~1990!.

7. S. Matsumoto, M. Hino, and T. Kobayashi, “Synthesis of dia-mond films in a RF induction thermal plasma,” Appl. Phys.Lett. 51, 737–739 ~1987!.

8. L. Schafer, C.-P. Klages, U. Meier, and K. Kohse-Hoinghaus,“Atomic hydrogen concentration profiles at filaments used forchemical vapor deposition of diamond,” Appl. Phys. Lett. 58,571–573 ~1991!.

9. R. J. H. Klein-Douwel and J. ter Meulen, “Spatial distribu-tions of atomic hydrogen and C2 in an oxyacetylene flame inrelation to diamond growth,” J. Appl. Phys. 83, 4734–4745~1998!.

10. L. Cherigier, U. Czarnetski, D. Luggenholscher, V. Schultz-von der Gathen, and H. Dobele, “Absolute atomic hydrogendensities in a radio frequency discharge measured by two-photon laser induced fluorescence imaging,” J. Appl. Phys. 85,696–702 ~1999!.

11. J. Larjo, J. Walewski, and R. Hernberg, “Atomic hydrogenconcentration mapping in thermal induction plasma CVD,”Appl. Phys. B ~accepted for publication!.

12. K. Miyazaki, T. Kajiwara, K. Uchino, K. Muaroka, T. Okada,and M. Maeda, “Laser-induced dissociation of molecules dur-ing measurements of hydrogen atoms in processing plasmasusing two-photon laser-induced fluorescence,” J. Vac. Sci.Technol. A 14, 125–131 ~1996!.

13. U. Meier, K. Kohse-Hoinghaus, L. Schafer, and C.-P. Klages,“Two-photon excited LIF determination of H-atom concen-trations near a heated filament in a low-pressure H2 envi-ronment,” Appl. Opt. 29, 4993–4999 ~1990!.

4. L. L. Conell, J. W. Fling, H.-N. Chu, D. J. Vestyck, E. Jensen,and J. E. Butler, “Spatially resolved atomic hydrogen concen-trations and molecular hydrogen temperature profiles in thechemical-vapor deposition of diamond,” J. Appl. Phys. 78,3622–3634 ~1995!.

5. D. G. Goodwin, “Scaling laws for diamond chemical vapor dep-osition. I. Diamond surface chemistry,” J. Appl. Phys. 74,6888–6894 ~1993!.

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