ELSEVIER Materials Science and Engineering A209 (1996) 414-419

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Enhancement of diamond thin film quality by a cyclic depositionmethod under MPECVD

Y.S. Parka, S.H. Kima, S.K. Junga

, M.N. Shinna, J.-W. Leea, S.K. Hongb, J.Y. LeeC

aNew Materials Laboratory, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South KoreabAnalytical Engineering Laboratory, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea

CDepartment of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Kusong Dong 373-1, Yusung Gu,Taejon, South Korea

Abstract

Diamond thin films were deposited by a cyclic deposition method, which can be carried out periodically on and off the methanegas flow in the CHc Hz system, using a microwave plasma enhanced chemical vapour deposition system. The property ofdiamond thin films deposited using the cyclic process was investigated and compared with that of the normal process. The qualityof the diamond films deposited by the cyclic process is better than that by the normal process under the same conditions, althoughthe growth rate of diamond film was lower in the cyclic process. However, introducing the interlayer on silicon substrate prior tothe cyclic process leads to the increase in the growth rate which is comparable with that in the normal process while maintainingthe same high quality of the films.

Keywords: Diamond thin films; Cyclic deposition; Microwave plasma CVD

1. Introduction

Diamond has many outstanding physical and chemi­cal properties, which makes it desirable for mechanical,thermal, optical and electronic applications [1]. For thethermal applications, such as heat spreader of highpower laser diodes and substrate for multi-chip mod­ules, diamond has been regarded as an ideal materialbecause of its high thermal conductivity and excellentinsulating properties. Since these applications requirediamond films of more than several hundred microme­ters thickness, the increase in the deposition rate overthe large surface area has been a major goal in diamondchemical vapour deposition.

Recently a cyclic deposition, periodically interruptingthe growth and exposing the grown film to hydrogen,has been reported to increase the deposition rate andimprove the film quality [2-6]. This technique uses avery aggressive etching period with atomic hydrogen,gasifying spz phase much faster than diamond.

According to the surface reaction mechanism of thecyclic process [4], atomic hydrogen removes the spzcarbon on Sp3 diamond surface, thereby increasing the

number of available sites for further growth of dia­mond. Because the growth rate of spz carbon is lowerthan that of diamond [7], the net growth rate of filmdeposition increases by removing spz carbon. Althoughthis mechanism explains the role of atomic hydrogen toobtain the high quality diamond films and the highergrowth rate, it is only concentrated on the growth step.Many researchers have reported the detrimental effectsof atomic hydrogen on diamond deposition, especiallyat the nucleation step. The atomic hydrogen removesthe sub-critical size of diamond nuclei as well as thenucleation site on silicon surface [8]. This leads to alonger incubation time for the nucleation, and as aresult, the growth rate of diamond films decreases. Inorder to utilize the advantage in the cyclic process, therole of atomic hydrogen has to be fully understood.Many attempts have been made in the cyclic process toobserve the effect of atomic hydrogen on the growth ofdiamond films. Nevertheless, there have been few de­tailed studies on the nucleation step.

In this paper, we investigate the effect of atomichydrogen on the nucleation and the growth in the cyclicprocess. The substrate effect on the diamond growthduring the cyclic process was also studied.

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Y.S. Park et al. IHatcrials Sciellcc alld Enliincerinli 04209 (/996) 4/4 4/9 415

The gas flow was controlled by a mass flow controllerand the methane gas inlet was electronically controlledto allow periodically interrupting methane gas flow,whereas hydrogen flowed continuously into the reactionchamber. As shown in Fig. I, two solenoid valves wereused to keep methane gas at a constant flow rate. Thetwo computer controlled solenoid valves were on andoff alternately, allowing methane gas to introduce intothe reaction chamber or vent the methane to the vac­uum chamber. Thus, chamber pressure was not fluctu­ated at the beginning of introducing the methane gasinto the chamber.

The typical growth conditions for cyclic process wereas follows: total flow rate, 500 sccm; microwave power,1100 W; pressure, 40 torr; temperature, 770°C; anddeposition time,S h. In this work, total cyclic time was20 s, and the ratio of the growth time to the total cyclewas 0.5 throughout. The normal deposition was alsocarried out to compare with the cyclic process, and thegrowth conditions are the same as in the cyclic process.The films were characterized by field emission scanningelectron microscopy (Hitachi model S-4500II) and mi­cro-Raman spectroscopy (Renishaw Ltd.) employing anAr ion laser.

QUARTZPLATE/

GRAPHITE

Mo SAMPL

HOLDE'l

Microwave

THERMOCOUPL

PUMP.-

+PUMP

Fig. 1. Schematic diagram of a microwave plasma CYO system.

2. Experimental

All diamond films were grown on the n-type (100) Siwafers (2 cm x 2 cm) using a microwave plasma en­hanced chemical vapor deposition (ASTeX Co. 1.5 kW)system. The substrates were pretreated with 30 pmdiamond powder-acetone solution in an ultrasoniccleaner for 5 h, and heated by induction heating of thegraphite susceptor. The temperature, measured by anoptical pyrometer, was maintained at about 770°C.

3. Results and discussion

Fig. 2 shows the surface morphologies of diamondfilms deposited by the cyclic process with different CH4

concentrations. The surface is composed of incom­pletely covered and well faceted diamond particles at

IoIlJS

1 : m ..""I

·~-".b

Fig. 2. SEM images of diamond films deposited on Si substrates at the total cyclic time or 20 s with different methane concentrations: (a) 0.5'1.,.(b) 1%. (c) 1.5%. and (d) 2%.

416 Y.S. Park et al. / Materials Science and Engineering A209 (1996) 414-419

Fig. 3. SEM images of diamond films deposited on Si substrates by a normal process with different methane concentrations: (a) 0.5'/'), (b) 1%, (c)1.5'/'0, and (d) 2%.

below 1% CH4 • The continuous diamond film is ob­tained at over 1.5% CH4 . The grain size of diamond filmbecomes larger with increasing methane concentration.

Diamond films were also deposited by a normalprocess in order to compare with the cyclic process, andtheir SEM images are presented in Fig. 3. The deposi­tion was performed under the same conditions as inFig. 2. Diamond films are grown as a continuous filmeven at 0.5% CH4. With increasing CH4 concentration,small crystallites start to protrude on the surface anddevelop into featureless cauliflower-like structures. Thisimplies that higher CH4 concentration leads to thesecondary nucleation and the deterioration of quality ofdiamond films.

Diamond films in Figs. 2 and 3 were analysed with amicro-Raman system, and the results are shown in Fig.4. The relative intensity ratio of diamond (~1332

cm - 1) to non-diamond components (~1500 cm - I)(/d/Ig) decreases with increasing methane concentrationfor both the normal and cyclic process. However, ahigher intensity ratio of Id/Ig can be obtained using thecyclic process at all methane concentrations in thisexperiment, compared with the normal deposition pro­cess. These results suggest that the relative fraction ofdiamond present in the films increases in the cyclicprocess. The full width at half-maximum (FWHM)obtained at ~ 1332 cm - I also indicates the enhance­ment of the homogeneity in diamond films for thecyclic process (see Fig. 4 (a)).

From the SEM images and Raman analysis, it isevident that the cyclic process produces more well­faceted morphologies and the high quality diamond

film. This can be rationalized in terms of growth kinet­ics of the cyclic process. By periodically interrupting thedeposition after the growth of a thin layer, atomichydrogen removes the surface-covered Sp2 carbons, andthe growth of the well-faceted surface is maintainedduring the cyclic process [4,5]. This surface reactionmechanism explains the growth of a high quality ofdiamond films in the cyclic process, in connection withthe promoting effect of atomic hydrogen on the growthof diamond film. However, in our work, the cyclicprocess is believed to prohibit the nucleation of dia­mond particles (see Figs. 2 (a) and (b)). When the cyclicprocess is performed at lower methane concentration(at higher atomic concentration), the surface is com­posed only of small diamond particles. This implies thatthe atomic hydrogen in cyclic process have a detrimen­tal effect on the nucleation of diamond.

Evidently, the atomic hydrogen in the cyclic processhas a promoting effect on the growth of diamond films,while it has a detrimental effect on the nucleation. Thisleads to a longer incubation time for the nucleation,and as a result, the growth rate of diamond film willdecrease. Therefore, in order to take advantage of thecyclic process, the effect of atomic hydrogen on nucle­ation must be considered.

The atomic hydrogen suppresses the diamond nucle­ation in two ways: re-evaporating sub-critical size ag­gregates and etching the nucleation sites on thesubstrate. The sub-critical size aggregates are trans­formed to stable nuclei through the atom adsorption.However, if an atom has a chance of being taken awayfrom the sub-critical size aggregate, it dissociates again.

Y.S. Park et a/. Ma[erials Science and EngineerinJ; A209 (/996) 414 419 417

Fig. 4. Raman characteristics of diamond films shown in Figs. 2 and3.

a'-" ...

<i. \.

r-"-' ..~ .. ,

. ~ ~~.. _"*d'..... .~

·iI ' .•~

. .- . ,.: J ,..,......,_."'~i

Fig. 5. Surface morphologies of diamond films deposited by (a)normal, and (b) two-step process.

time for 2 h. This is an early stage of the diamond filmgrowth shown in Fig. 3 (b). The morphology is con­sisted of diamond particles with small protuberances,indicating the secondary nucleation. Fig. 5 (b) is thesurface morphology of the diamond film obtained by asubsequent growth with the cyclic deposition after thenormal growth obtained in Fig. 5 (a). Growth condi­tions for the cyclic process were the same as in Fig. 2(a): tetclllllg:tgrowth = 10 s:1O s, 0.5% CH4 , and growthtime for 5 h. It is clear that the surface morphologyfrom small grains with some protuberances developsinto the well-defined facet structure with large grainsize. The surface structure exhibits no secondary nucle­ation, implying a high quality of diamond film.

Figs. 6 (a) and (b) show the cross sectional SEMimages of diamond films in Figs. 5 (a) and (b), respec­tively. From Fig. 6 (b), it is apparent that the additionalgrowth in the cyclic process proceeds with growing acontinuous layer after the normal process. When thetwo-step process is used, the nucleation of diamonddoes not seem to be a problem, whieh is found only inthe cyclic process (Fig. 2 (a)).

The growth rate of diamond films deposited by thenormaL cyclic, and two-step process, as describedabove, is presented as a function of methane concentra­tions in Fig. 7. To obtain growth rates, the films werefractured and the thicknesses were measured fromcross-sectional SEM images. Since the deposits weregrown as isolated particles in the typical cyclic processat below 1% CH4 the growth rates under that conditioncan not be determined. In the two-step process, aninterlayer was deposited under the same conditions as

2.5

2.50.5 1 1.5 2Methane Concentration (%)

0.5 1 1.5 2Methane Concentration (%)

,-----normal•cyclic

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40

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10

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Generally, in order to nucleate diamond on the foreignsubstrate, nucleation sites have to be supplied byscratching the substrate. Park and Lee [8], however,reported that the nucleation density of diamond onsilicon wafer decreases with increasing hydrogen expo­sure time due to the etching of nucleation sites. Theabove two detrimental effects of atomic hydrogen areresponsible for the difficulty of growing the deposits ascontinuous films when the cyclic process performs di­rectly on silicon surface.

The atomic hydrogen effects on the nucleation areapplicable to any substrate. However, each substratehas its own particular atomic arrangement that deter­mines the bond strength between the nuclei and thesubstrate. The ability of atomic hydrogen to re-evapo­rate nuclei depends on the bond strength of nuclei tosubstrate. This means that the nucleation behavior canbe controlled through the modification of substratestate. This is evident from Fig. 5, which exhibits thesurface morphologies of diamond films grown throughthe normal and subsequent cyclic growth, namely, two­step process. Fig. 5 (a) shows the surface morphologyof diamond film grown by the normal process under thetypical growth conditions of 1'1.) CH4, and the growth

418 Y.S. Park 1'1 al. / Materials Science and Engineerinf? A209 (/996) 4/4 4/9

1.9 12

1.811.5

1.7 ....- 11 ""-1.6 8

eo C)- '-'

~1.5 10.5 ~-1.4

10 ~

1.3

9.51.2

1.1 9a 0.5 1 1.5 2 2.5

Methane Concentration (%)

a

b

'i:e;'e'fm'-15.0k~.

UD?

UD?

... _.,.~

Fig. 8. Raman characteristics of diamond films deposited by two-step

process.

0.05

000576 15,0kV X20, 'j:strate conditions although the same cyclic process isperformed under the same condition.

Fig. 8 exhibits the characteristics of Raman spectrafor the diamond films deposited by the two-step pro­cess. The FWHM obtained at ~ 1332 cm - I is almostidentical to that for the diamond films obtained by thetypical cyclic process (see Fig. 4). However, the relativeintensity ratio of diamond to non-diamond components(Id/lg) is much lower than that in Fig. 4. This is owingto the film thickness and the different sensitivity ofRaman to diamond and non-diamond components. Inorder to get a Raman spectrum only for the diamondfilm deposited by the cyclic process, film thickness mustbe greater than the focusing depth of the laser beam. Inour Raman system, the depth resolution of the laserbeam is about 3 ,um. However, the maximum thicknessof diamond films in our measurement was 2.3 ,um at2'% CH4 . Thus, the Raman spectra in Fig. 8 indicate themixed characteristics of two layers. The Raman spectraof the interlayer (deposited by the normal process at 1'%CH4 ) shows that the FWHM is not much different fromthat of diamond films in the cyclic process (see Fig. 4).However, the relative intensity ratio of diamond tonon-diamond for the interlayer is much lower than thatin the cyclic process. Thus, the interlayer might affectthe ld/lg more significantly than FWHM. This seems tobe the reason why the ld/lg is much lower in thetwo-step process than in the typical cyclic process.

From the growth rate (Fig. 7) and the Raman char­acteristics (Fig. 8) of diamond films in the two-stepprocess, we can confirm that introducing the interlayeron silicon substrate leads to the increase in growth ratewhich is comparable with that in the normal process,while maintaining the same high quality of the films asin the typical cyclic process.

oL----I--------"-----'-----L--~

o 0.5 1 1.5 2 2.5Methane Concentration (%)

{ 0.25

'-' 0.2~

~ 0.15

1'wi 0.1

0.3

Fig. 6. Cross-sectional SEM images for diamond films shown in Fig.

5.

0.35~-------------~

in Fig. 5 (a) and the growth rate for the cyclic processwas obtained by subtracting the thickness of the inter­layer from the total thickness. The growth rate in thenormal process (Fig. 7 (a)) is higher than that in thecyclic process (Fig. 7 (c)) for all conditions of methaneconcentrations. If using the two-step process (Fig. 7(b»), the growth rate can be very close to that in thenormal process. It is evident from these results that thegrowth rate is quite different, depending on the sub-

Fig. 7. Growth rates of diamond films deposited by (a) normalprocess, (b) two-step process, (c) cyclic process at the total cyclic time

of 20 s.

Y.S. Park et al. Materials Scicnce and Enr;ineering AlO'.l (1996) 414 419 419

4. Conclusions

Diamond films were deposited by the normal andcyclic process on the silicon wafer at various methaneconcentrations. The normal deposition method give riseto different surface morphologies with increasingmethane concentration, well faceted at 0.5';-;, CH4 andrough morphology with some cauliflower-like featuresat 2'Yo CH4 . However, the cyclic process leads to wellfaceted morphology even at 2% CH4 . Although a highquality of diamond film is obtained by the cyclic pro­cess, the growth rate in this process is lower than thatin the normal process. The cyclic process on the siliconsubstrate suppresses the nucleation of diamond due tothe aggressive etching of atomic hydrogen. This resultsin a longer incubation period for the nucleation and thelower growth rate. By introducing diamond interlayerprior to the cyclic process, nucleation problems in the

cyclic process can be solved. Thus, even in the cyclicprocess, a higher quality of diamond films can beobtained at a comparable growth rate with the normaldeposition under the same condition.

References

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[2] W.N. Howard. K.E. Spear and M. Frenklach. Appl. PhI's. Lell ..63 (1993) 2641.

[3] M.A. Kelly. D.S. Olson. S. Kapoor and S.B. Hagstorm. Appl.PhI'S. Lell .. nO (1992) 2502.

[4] D.S. Olson. M.A. Kelly. S. Kapoor and S.B. Hagstorm. 1. Maler.

Res .. 9 (1994) 1546.[5] H. Wang and M. Frenk1ach. 1. Appl. Phl'l .. 70 (1991) 7132.[6] 1. Wei and Y. Tzeng. 1. Cn·sl. GrOlllh .. IlR (1993) 413.[7] M. Frcnklach and H. Wang. PhI's. R('l'. B. 43 (1991) 1520.[8] S.S. Park and JY. Lee. J. Maler. Sci .. lR (1993) 1799.