diamond nucleation suppression in chemical vapor deposition process

Diamond and Related Materials 8 (1999) 2110–2117www.elsevier.com/locate/diamond

Diamond nucleation suppression in chemicalvapor deposition process

C.L. Fritzen, N.M. Balzaretti, R.P. Livi, J.P. de Souza, J.A.H. da Jornada *Instituto de Fısica da UFRGS, C.P. 15051, 91501-970, Porto Alegre, RS, Brazil

Received 3 May 1999; accepted 20 July 1999

Abstract

A systematic study of the effect of different pre-treatments of the Si substrate surface in suppressing diamond nucleation wasperformed to investigate the nature of the nucleation centers in chemical vapor deposition (CVD) of diamond. The Si substrateswere initially scratched with diamond powder and then submitted to one of the following pre-treatments: thermal annealing inhigh vacuum and in air, deposition of an amorphous silicon film, and 84Kr+ ion implantation. The pre-treated substrates wereused in a hot filament CVD diamond process, and the diamond films obtained were analyzed by different techniques. The resultssuggest that the observed nucleation reduction under certain pre-treatment conditions is related to modifications induced on theoriginal topographical features of the scratched substrate surface, which would be responsible for the CVD diamond nucleation.The dimensions of these surface features are estimated to be of the order of 5 nm. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Chemical vapor deposition; Nucleation suppression; Silicon

1. Introduction In the literature, there is some controversy about thenature of the physical process responsible for this nucle-ation enhancement: some authors [1–4] claim it isDiamond is a very important material for technologi-related to the strain generated by the scratching, whilecal applications, presenting a variety of unique andothers [5–7] propose that it is related to the seeding ofinteresting properties. Chemical vapor deposition (CVD)diamond nanocrystals released by the abrasion.is a relatively new method of producing polycrystallineMoreover, the nucleation can be enhanced by scratchingdiamond films, which has motivated enormous work inwith non-diamond materials [8,9], and also by appro-the last few years. Despite the large amount of scientificpriate surface pre-treatment that changes the substratework, several aspects of the deposition process, especi-topography without scratching [10–13]. Jiang et al. [3,4]ally the diamond nucleation over the substrate, are stillproposed that the abrasion modifies the surface topogra-a matter of controversy.phy by introducing valleys and grooves that can act asSilicon, the basic material for microelectronics, isnucleation sites. In the case of substrate abrasion, thewidely used as a substrate material for diamond filmdiamond nucleation is always randomly oriented withdeposition, despite the high lattice mismatch and highrespect to the substrate, and its density is aboutsurface energy difference between diamond and silicon.108 cm−2. A very high nucleation density of aboutHowever, a clear understanding of the microscopic1010 cm−2 can be obtained by seeding with ultrafinemechanisms that control the nucleation is still lacking.diamond powders [14].

Therefore, the possibility to control and enhance the Another method of enhancing diamond nucleationdiamond nucleation density by different Si substrate consists in apply a suitable substrate bias voltage ofpre-treatments is very important from both technologi- about 200–300 V prior the diamond deposition [15–22].cal and scientific points of view. This in situ pre-treatment is as good as or better than

The most usual method to enhance diamond nucle- any other to enhance diamond nucleation, attaining upation is to scratch the substrates with diamond powder. to 1011 nuclei cm−2 [15], without any scratches on the

substrate. The advantage of this pre-treatment is thatthe diamond film growth is nearly oriented over the Si* Corresponding author. Fax: +55 51319 1762.

E-mail address: [email protected] (J.A.H. da Jornada) substrate.

0925-9635/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved.PII: S0925-9635 ( 99 ) 00173-9

2111C.L. Fritzen et al. / Diamond and Related Materials 8 (1999) 2110–2117

Ion implantation of 28Si+ or 75As+ on pristine Si perature by a Pt–PtRh thermocouple embedded into thesubstrate holder. The substrate was positively biased insubstrates has also been demonstrated to enhance the

diamond nucleation density [1,23,24]. The induced sur- relation to the filament (about 60 V ), with a constantcurrent of 5 mA flowing between them. After this warm-face strain/stress is assumed to be the main cause for

the observed enhancement. However, 40Ar+ ion ing time of about 20 min, 1% methane (99.9% purity)was introduced in the chamber, diluted in the hydrogenimplantation on scratched Si substrates decreases the

diamond nucleation density, which is probably related at a constant total flow rate of 200 sccm (standard cubiccentimeters per minute), starting the diamond depositionto modifications of the substrate crystallinity or mor-

phology induced by ion implantation, or it could also that lasted for 3 h.After the CVD process, the samples were analyzedbe related to modifications in the diamond seeds crystall-

inity [25]. by scanning electron microscopy (SEM), X-ray diffrac-tion and micro-Raman spectroscopy. The SEM micro-As can be seen, there are several studies on diamond

nucleation based on alternative approaches to generate graphs of the pre-treated and control samples weredigitized and the nucleation density for each case wasnucleation sites. In this work the nature of the nucleation

sites was approached from a different point of view, manually analyzed. The ratio n/n0 of the nucleationdensities of the pre-treated sample (n) and the controlnamely, scratching the substrate surface with diamond

paste to produce nucleation sites, and then investigating sample (n0) was then calculated.X-ray diffractometry and Raman spectroscopy resultsdifferent methods to suppress these sites. The study of

the site annihilation should contain complementary confirmed that the chemically vapor-deposited crystalsobserved by SEM were diamond.information on the physical nature of the nucleation

process. Some preliminary results have already beenpresented elsewhere [26,27].

3. Experimental results and discussion

In this section, the results obtained for the behavior2. Experimental setupof the nucleation density for each of the pre-treatmentswill be presented and discussed.All the substrates used were mirror-polished p-type

Si(100) wafers, scratched with diamond paste (4 mmparticle size), cleaned with acetone and deionized water, 3.1. Heat treatment in vacuumand then cleaved in squared pieces of about 1 cm2. Inorder to suppress the nucleation density, the following In this case, the scratched Si substrates were annealed

under vacuum of 10−6 Torr up to a temperature ofsubstrate pre-treatments were investigated: (1) heattreatment up to 1100°C under high vacuum 1100°C, before the CVD process. During this pre-

treatment, the stress field associated with the scratches(~10−6 Torr) for 1 h; (2) heat treatment in air atdifferent temperatures in the range 20–900°C for 1 h; should decrease considerably. After the diamond film

deposition, however, there was almost no difference(3) deposition of an amorphous silicon film over thesubstrate with film thicknesses of 2, 5 and 10 nm; (4) between the nucleation densities of the vacuum-annealed

substrate and of the control substrate. This behavior84Kr+ ion implantation at room temperature (RT) usingdifferent ion fluences and different ion energies. indicates that the nucleation is probably independent of

the stress induced by the scratches.Before the amorphous Si film deposition or the ionimplantation pre-treatments, half of the scratched Si It is known that strain and stress shift the Si Raman

line [28]. In order to investigate the decrease of thesubstrate surface was masked providing a control regionto analyze the effect of the pre-treatment on the nucle- stress field with the increase of the annealing temper-

ature, the Raman spectrum of the region containing aation density. For the other cases, the control wasprovided by a scratched Si substrate from the same scratch over the substrate was measured as a function

of the annealing temperature. The laser spot size usedwafer placed simultaneously with the pre-treated sampleinside the hot filament CVD reactor. in the micro Raman analysis was less than 5 mm and

covered more than the lateral extension of the scratch.The diamond film was deposited in a typical hotfilament CVD reactor. The substrate was placed 5 mm The heat treatment under vacuum lasted 1 h for each

temperature. Additionally, a pristine Si substrate wasaway from the filament and a initial vacuum of about10−2 Torr was established. A hydrogen (99.99% purity) indented with a Vickers diamond indenter with loads of

50, 100 and 200 gf and the substrate was submitted toflow was started and a chamber pressure of 50 Torr wasmaintained. After that, the coiled tungsten filament was the same vacuum annealing treatment. For loads of 50

and 100 gf, there were no cracks at the ends of theheated to 1900°C and the substrate temperature wasincreased to 750°C. The filament temperature was mea- indentations, but for 200 gf load some cracks were

observed. The Raman spectrum of the center and of thesured by a two-color pyrometer and the substrate tem-

2112 C.L. Fritzen et al. / Diamond and Related Materials 8 (1999) 2110–2117

corner of the indentation was recorded for each annea- deposition. Fig. 2 shows SEM pictures of the diamondfilms deposited over these substrates. For substrate pre-ling temperature.

Fig. 1 shows the behavior of the Raman shift as a treatments at temperatures higher than 700°C in air, adrastic reduction in the diamond nucleation density wasfunction of temperature for the following samples: pris-

tine Si; a scratch over the Si substrate; and indentations observed.The ratio n/n0 as a function of the temperature isover the Si substrate generated by 50, 100 and 200 gf.

As can be seen, in the case of the scratch and indent- shown in Fig. 3. As can be seen, for temperatures above700°C and up to 900°C, n/n0 decreases by about threeations, there is a shift to higher wave numbers for low

annealing temperatures. A broadening of the Raman orders of magnitude. A possible explanation for thedifferent behavior of the nucleation density for sub-peak was also observed in these cases. As the annealing

temperature increases, the Raman peak position of all strates heated in vacuum and in air is the existence of athin oxide layer over the samples heated in air, whichsamples tend to the value corresponding to pristine Si,

indicating a stress relief at temperatures above 700°C. could be responsible for this drastic reduction in thediamond nucleation density at high temperatures.The peak FWHM follows a similar trend to that of the

peak position. The aligned Rutherford backscattering (RBS) tech-nique was used to measure the thickness of the oxideIn the present work, since the substrate temperature

during the CVD process was 750°C, the stress field layer formed in a silicon substrate heated up to 750°Cin air. The result obtained was 12 nm. However, duringinduced by the scratches should be relieved, according

to the results described above. Moreover, no evidence the CVD process the highly reactive atomic hydrogenshould attack the oxide layer by etching or even reduc-was found for diamond nucleation enhancement near

or at the several Vickers indentations produced by tion to Si. This effect was investigated using a thickoxide layer (110 nm) over the substrate, submitted to adifferent loads on the pristine Si substrates. It is interes-

ting to note that Hirakuri et al. [29,30] found a diamond hydrogen atmosphere under the same conditions as usedfor diamond deposition, except for the absence of meth-nucleation enhancement on pristine Si substrates

mechanically stressed during the diamond deposition ane. Using the RBS technique it was possible to observethat the 110 nm original oxide layer was completelyprocess. However, in that case the stress was continu-

ously applied by an external force, offsetting the annea- removed after 1 h. This result indicates that the nucle-ation reduction for the samples heated in air is probablyling effect. This continuous stress would possibly lead

to the formation of etch pits by the active hydrogen not related to the oxide layer itself, but to the effect thatthe oxidation process has on the topography of theatmosphere.substrate surface by rounding the sharp corners andleveling the Si surface.3.2. Heat treatment in air

In order to determine what was the role of the oxidelayer, scratched Si substrates heated in air at a temper-In this case, the scratched Si substrates were heated

in air for 1 h at different temperatures, prior the diamond ature of 800°C were etched with hydrofluoric acid toremove completely the oxide layer prior to the CVDprocess. The result was an even lower diamond nucle-ation density, about one-half of that obtained withoutremoving the oxide layer. This result shows that theremoval of the oxide layer does not enhance the CVDdiamond nucleation. In summary, the observed signifi-cant decrease in the nucleation density for substratesheated in air was probably due to the reduction of theoriginal roughness of the scratched Si surface during theoxidation process.

3.3. Deposition of an amorphous silicon layer

The effect of the deposition of an amorphous Si filmover the scratched Si substrate, prior to the CVDprocess, was investigated. The film was deposited usingelectron gun thermal evaporation, and the film thicknesswas determined using a quartz crystal oscillator. In

Fig. 1. Crystalline Si Raman peak position as a function of the annea-Fig. 4, the n/n0 ratio is shown as a function of the filmling temperature for various Vickers indented substrates at variousthickness and, as can be seen, there is a film thicknessloads, as well as for a scratch and for pristine Si. The dotted lines are

for guidance only. between 2 and 5 nm where a significant reduction in


(a) (b)

(c) (d)

Fig. 2. Typical micrographs showing diamond deposition on scratched Si substrates heat treated in air at various temperatures for 1 h: (a) RT; (b)750°C; (c) 800°C; (d) 900°C.

Fig. 4. Relative diamond nucleation density (n/n0) on scratched Si sub-strates coated with amorphous silicon films of various thicknesses,

Fig. 3. Relative diamond nucleation density (n/n0) as a function of the subjected to CVD diamond deposition, as a function of the siliconfilm thickness.previous heat treatment temperature of the scratched Si substrates.


n/n0 takes place. In principle, the deposited silicon film In the second approach, 84Kr+ ions were implantedon the scratched substrates with a fixed ion fluence ofwould lie over the substrate surface topography, repro-

ducing its features. However, the coated substrate 4×1014 cm2, at RT, using different ion energies. Theresults obtained are also shown in Fig. 5. As can bereaches 750°C during the warming up time in the CVD

process, where the recrystallization of the amorphous seen, the ratio n/no increases as the ion energy increasesfor a given fluence. A possible explanation for thissilicon layer takes place. In fact, Witvrouv and Spaepen

[31] have shown that amorphous Si can flow viscously behavior is related to the fact that the depth where theamorphization layer induced by the implantation isat temperatures above 150°C. During the recrystalliza-

tion process, the Si atoms of the coating layer are very located, is directly proportional to the ion energy.Therefore, as the ion energy increases, the damagedmobile and, in order to minimize the surface energy,

they tend to eliminate small topographical irregularities region will be deeper inside the substrate, away fromthe surface, and the suppression of nucleation centersof the substrate surface. The observed reduction of the

nucleation density for films with thicknesses between 2 would be less effective.The RBS technique was used to analyze the crystall-and 5 nm would probably be related to the decrease in

the number of fine topographical features of the sub- inity of the scratched Si substrates as implanted with84Kr+ ions, 100 keV energy, at RT and at variousstrate surface with dimensions between 2 and 5 nm. For

thicker coating layers, the coating would cover up all fluences. Fig. 6 shows the results obtained for the caseswhen the incident beam was aligned with 100� channelsthe fine surface structures responsible for the CVD

diamond nucleation, which would be responsible for the and when it was randomly aligned. For fluences above2×1014 cm−2, the Si surface was completely amorphous,low level saturation behavior of n/n0 for thicknesses

greater than 5 nm. as shown by the high level of RBS counts close to thesubstrate surface. This amorphization was probablyresponsible for the significant decrease of n/no observed3.4. Ion implantationin Fig. 5 for fluences close to 2×1014 cm−2.

It is known that, for heavy mass ion implantation, aThe effect of ion implantation on the nucleationdensity was investigated by two different approaches. In minimum energy density of about 6×1021 keV cm−3

is necessary to amorphize the Si substrate surfacethe first case, 84Kr+ ions with a constant energy(100 keV ) were implanted on the scratched substrates completely [33]. In the present case, for 60 keV 84Kr+

ions, the corresponding energy density is 6.2×at RT, using different ion fluences. The obtained resultsfor n/no as a function of the fluence are shown in Fig. 5. 1021 keV cm−3 for a fluence of 4×1014 cm−2 and, there-

fore, the observed amorphization is in agreement withThe ratio decreases significantly as the fluence increasesfrom 2×1013 to 4×1014 cm−2. In a previous paper, this the previous results [33].

The amorphous layer of Si induced by ion implant-decrease was shown to depend also on the ion itself, onits energy and on the substrate temperature during ion ation would also recrystallize during the CVD process

owing to the high temperature of the substrate, similarlyimplantation [32]. The ion current density was less than200 nA cm−2 in all cases, and it was controlled by the to the behavior of the amorphous Si layer deposited

over the substrate (Section 3.3). The RBS techniquefluence and time of implantation.

Fig. 6. Aligned RBS results, corresponding to scratched Si substratesFig. 5. Relative diamond nucleation density (n/n0) as a function of ionfluence for various 84Kr+ ion implantation fluences and energies. as implanted with 84Kr+ ions, 100 keV, RT, at various fluences.


was used to detect this recrystallization after a heat in the present work. Therefore, the behavior of thenucleation density of diamond deposition over a Sitreatment of the as-implanted substrates under condi-

tions similar to those of the diamond deposition process substrate scratched with fine powders of different materi-als was investigated to examine whether the effect of thebut without methane. The heat treatment was carried

out in the CVD reaction chamber. After the warming diamond seeding is a necessary condition to the nucle-ation process. Table 1 contains the nucleation densitiesup time, the implanted sample was maintained for 2 min

at 750°C, and then the CVD reactor was switched off. measured for substrates scratched with four differentmaterials, including diamond, compared with the resultFig. 7 shows the aligned RBS results corresponding to

the (84Kr+, 100 keV, 4×1014 cm−2, RT) scratched Si for the pure Si without scratches [34]. As can be seen,the nucleation density is much higher when the diamondsubstrate as implanted and after the heat treatment. As

can be seen, the Si surface, amorphized during the ion paste is used to scratch the substrate. However, there isalso nucleation for scratching with materials other thanimplantation, recrystallizes completely after the heat

treatment in the CVD reactor. The aligned RBS studies diamond, and even without scratches. In addition, thebehavior of n/no was investigated for two pre-treatmentscorrelate well with the Raman spectroscopy results [32]

in determining the amorphization as well as the recrystal- of a Si substrate scratched with silicon powder: heattreatment in air at 900°C and ion implantation usinglization for similar ion-implanted Si samples.

In summary, the Si amorphization would probably 84Kr+ ions of 100 keV, 4×1014 cm−2 at RT. Theobserved behavior of the relative nucleation density wasnot be responsible for the diamond nucleation suppres-

sion, because the Si recrystallizes at the temperature very similar to the case of scratching the substrate withdiamond powder, despite the lower level of the absoluteused during the CVD process. However, it can be

assumed that, during the recrystallization, the Si atoms’ nucleation densities compared with the diamondpowder case.motion would be able to eliminate some topographical

features of the substrate surface in order to minimize These results indicate that the seeding process is notthe main contributor to the nucleation process. Thethe surface energy and, consequently, the diamond

nucleation would be suppressed [32]. higher nucleation density in the case of substratesscratched with diamond paste could be related to theThe observed behavior of the nucleation density for

the different substrate pre-treatments, besides the elevated diamond hardness which, in turn, make it themost efficient material for abrasion. The sharp edges ofscratching with diamond paste, indicates that the stress

induced by the scratches is probably not the main the diamond grains would most effectively generate thefine topography to enhance the diamond film nucleationcontribution during the nucleation process, whereas the

topographical features of the substrate should play an during the CVD process.important role during this process. However, the ideaof seeding of diamond grains during the scratchingprocedure with diamond paste should also be important 4. Conclusionsand it would also be affected by the pre-treatments used

In this work, a systematic study of the suppressionof diamond nucleation by different pre-treatments onscratched Si substrates was carried out. The goal of thisstudy was to obtain relevant informations about thephysical nature of the nucleation sites.

The results obtained for all the different pre-treat-ments of the scratched Si substrate described in thiswork confirm the idea of the dependence of the diamondnucleation density on the surface topography on a

Table 1Average nucleation density for diamond deposition over Si substratesscratched with different materials, compared with the pure Si withno scratches

Material used to scratch the substrate Nucleation density (cm−2)diamond paste (4 mm) 1.1×107silicon powder 1.1×105

Fig. 7. Aligned RBS results with 100� alignment, corresponding to sintered alumina powder 5.4×104scratched Si substrates as implanted (84Kr+, 100 keV, 4×1014 cm−2, carbon powder 3.8×104RT ), and after subjected to CVD diamond deposition conditions (with- substrate with no scratch ~104out methane) for 2 min.


nanometric scale. Accordingly, the results can be sum- or even without any scratch, by an electrical bias betweenthe substrate and the filament [15–22]. Therefore, themarized as follows. (1) The substrate pre-treated in

vacuum heated to 1100°C did not show any change in seeding of diamond particles would enhance the dia-mond nucleation density but it is probably not a neces-the nucleation density, as can be expected if the surface

topography does not change during this pre-treatment. sary condition for the deposition of a diamond film overa Si substrate during a CVD process. In our case, theFurthermore, it was shown that the stress field associated

to the scratch is considerably reduced by the high highest nucleation density always occurred when thesubstrate was scratched only with diamond paste, withtemperature of the substrate during the CVD process.

(2) In contrast, for substrates heated in air before the no additional pre-treatment. The diamond paste wouldbe the most efficient material for abrasion owing to thediamond deposition, the nucleation density decreases,

and this decrease is probably related to the formation elevated diamond hardness, and the small edges of thediamond grains would generate the adequate topogra-of an oxide layer at the substrate surface for temper-

atures above 750°C in air, modifying the topographical phy to enhance the diamond film nucleation during theCVD process.features of the substrate. The critical oxide layer thick-

ness for the nucleation reduction is about 12 nm, repre-senting a layer of Si atoms of about 5 nm. (3) In thecase of a scratched substrate coated with an amorphous AcknowledgementsSi layer, recrystallization of the coating was observed asa result of the high temperature in the CVD process. The authors would like to thank Prof. B.V. Spitsyn

for helpful discussions and also to Dr. S.R. Teixeira forDuring the recrystallization, the Si atoms would elimi-nate some of the topographical features of the substrate the Si films deposition, to Mr. S. Menoschelli of

RIOCEL S.A. and to Mr. C.J. Mansan of the L.M.E.in order to reduce the surface energy. The reduction inthe diamond nucleation density occurs at a film thickness of the Museu de Ciencias Naturais — Fundacao

Zoobotanica — RS for the SEM observations. Thisbetween 2 and 5 nm, and this should be the size of thefine topographical features responsible for the diamond work was partially supported by CNPq, FINEP,

FAPERGS and Perto S.A.nucleation. This value is consistent with item (2) above.(4) In the case of the ion-implanted scratched substrate,the suppression of the diamond nucleation densityoccurs only when the fluence and ion energy are such Referencesthat the damaged region of the substrate is close toits surface. [1] S.J. Lin, S.L. Lee, J. Hwang, C.S. Chang, H.Y. Wen, Appl. Phys.

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diamond nucleation suppression in chemical vapor deposition process

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