Polishing of diamond thick films by Ce at lower temperatures

Diamond & Related Materials 15 (2006) 1412 – 1417 www.elsevier.com/locate/diamond

Polishing of diamond thick films by Ce at lower temperatures Yujing Sun *, Shubin Wang, Shi Tian, Yihong Wang School of Material Science and Engineering, P. O. Box 101, Beijing University of Aeronautics and Astronautics (BUAA), Beijing 100083, PR China Received 5 May 2005; received in revised form 8 October 2005; accepted 25 October 2005 Available online 15 December 2005

Abstract An efficient coarsely polishing and thinning method for CVD diamond films is achieved using solid or molten rare earth—Ce at the lower temperatures such as 700 -C for 2 h and 820 -C for 0.5 h. The factors affecting the surface roughness (Ra) and the rate of removal of diamond films are discussed thoroughly in this paper. In addition, the polishing mechanism is investigated primarily. The results show the content of diamond on the surface of the polished films has increased to a certain extent due to the etching out of impurities mostly at the grain boundaries and the FMHW of diamond Raman peak for polished diamond films has a visible increase. A large number of diamond films may be polished simultaneously at the temperatures lower than reported previously without noticeable contaminants on the polished surfaces of diamond films. D 2005 Elsevier B.V. All rights reserved. Keywords: Diamond films; Polishing; Ce; Surface roughness

1. Introduction Diamond is a unique engineering material with known highest hardness and thermal conductivity, excellent wear resistance, chemical stability and optical transparency in the infrared-visible-ultraviolet regions [1,2]. The combination of these properties enables its extensive use in modern industry, including cutting tools, heat sinks, semiconductors, optical windows, etc. [3– 5]. However, most CVD diamond films are always very rough and usually cannot be used without polishing. An evident example is surface reflection resulted from rough surface as used for optical windows. To date, many methods such as mechanical lapping, hot iron metal polishing, laser polishing, etc. [6 –12] have already been used to reduce the surface roughness to meet the need of practical applications. However, no single method can receive completely satisfactory results by far. Therefore, to obtain a fine finish of the polished surface both efficiently and economically, the combination of polishing diamond film coarsely with fine polishing should be adopted. Molten rare earth etching method is considered to be of great potential for polishing diamond films coarsely. For example, the perpendicular (through the thickness) thermal conductivity K – of diamond films polished by cerium at 920 -C utilizing high * Corresponding author. Tel.: +86 10 82317101; fax: +86 10 82316976. E-mail address: [email protected] (Y. Sun). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.10.057

diffusivity of carbon atoms in liquid rare metals is remarkably improved from ¨14 – 15 to 20 –22 W/cm -C [13], a value approaching that of high quality Type IIa single crystal diamond. M. McCormack et al. achieved thinned CVD diamond films using Ce – 22%Ni eutectic alloy at 690 -C for 36 h [14], which is so time-consuming that it almost has no significance of practical use. The roughness of diamond film polished by rare earth La at 940 -C decreased from its original value of 12 to 1.6 Am [15]. To our knowledge, the experimental temperature employed in almost all the reports related to the polishing of rare earths are above their molten point, and neither on polishing parameters nor on changes in microstructure induced by polishing are detailed described, Ce

Ce

Al2O3 sheet

diamond film

crucible diamond

a

b

Fig. 1. Schematic illustration of Ce polishing method (a) T < 800 -C; (b) T > 800 -C.

Y. Sun et al. / Diamond & Related Materials 15 (2006) 1412 – 1417

TG

100

0.0

98

-0.4

800

DSC

96

719

-0.8

94

en do

Mass Loss / %

Heat Flow / (mW / mg)

0.4

1413

energy resolution of 0.1 eV, the primary electron energy and current used in which were 3.0 keV and 0.5 nA, respectively; the value of average surface roughness Ra of diamond film was determined by Surface Topography Meter (TaylorSurf 5P-120); phase structure of these species and products were investigated by X-ray Diffractometer D/max 2200PC (XRD). 3. Discussion and results

-1.2

92

3.1. DSC and TG results of rare earth Ce -1.6 200

400

600

800

Temperature/ºC Fig. 2. DSC and TG curves of Ce.

especially for rare earth solid polishing method. However, a good understanding of it will be of great value whether in science or for special applications. In this work, we not only polished the CVD diamond films at lower temperatures by solid cerium (Ce), but also studied effects of polishing conditions on the surface state, microstructure and removal rate of diamond using solid cerium polishing in detail, which is of great originality. 2. Experimental procedure As the experimental temperature was below 800 -C, freestanding CVD diamond films (10  10  0.3 mm; China Research institute of Non-metal Minerals) were sandwiched between in two sheets of Ce with purity of 99.5% polished well in advance [Fig. 1(a)]; whereas the experimental temperature was above 800 -C, the diamond film was placed at the bottom of a crucible covered with a sheet of porous Al2O3 disc, on which Ce block was laid [Fig. 1(b)]. All the experiments were carried in the heat-treated furnace at 600 – 850 -C for some time in a flowing high-purity argon gas. Both the unreacted and reacted rare earth metal Ce were removed from diamond film surfaces by immerged into excessive aqua regia (HNO3 : HCl = 1 : 3) for enough time. The Differential Scanning Calorimeter (DSC) and Thermogravimetric (TG) tests of Ce were carried out on DSC analysis instrument (NETZSCH STA 449C) with the rate of 15 K/min; the microstructure of the films was observed using Scanning Electronic Microscopy (SEM, JSM 5800); Raman spectra were measured with a Microscopic confocal Raman Spectrometer RM2000 (Renishaw) with 514.5 nm argon-ion laser emitting wavelength; Auger electron spectra (AES) were measured by PHI 700 scanning Auger nanoprobe with an

According to reference [16], g-Ce (s) is converted into h-Ce (s) at 726 -C and then changed into Ce (l) at 798 -C. DSC and TG curves of Ce used in this work are shown in Fig. 2. It is found that the solid phase transformation temperature of g-Ce (s) to h-Ce (s) is 719.4 -C and its melting point is 800.4 -C. In principle, the experiments below 800 -C are considered as solid etching and the experiments above that are considered as liquid etching. 3.2. The influences of polishing conditions on microstructure and polishing efficiency of diamond films A various samples polished in different conditions and their polished results are all listed in Table 1. Fig. 3(a) shows the SEM micrographs of the top view of original polycrystalline CVD diamond film and its Ra value of is 5.9762 Am. As is shown in Table 1, the Ra value of diamond films after polished by Ce at 680 -C for 2 h (sample 1) is 4.3155 Am, which indicates Ce can polish diamond film as low as 680 -C. It is evident shown in Table 1 that a substantial removal of the diamond after only 2 h at 700 -C, 50 Am reduction in thickness, has been accomplished. Moreover, its Ra can be reduced from its original value of 5.9762 to 2.0247 Am. With temperature increasing, the rate of diamond removal increases from 25 Am/h (sample 3) to 35 Am/h (sample 5) and Ra changes from 2.0247 to 2.1523 Am. The elevating of temperature enhances the thermodynamic driving force to reduce the surface energy, hence the reaction between Ce and C becomes more quickly, thus the rate of diamond removal has been improved. When the protrudent grains have been totally flattened, the subsequent reaction will be faster at grain boundaries because impurities distribute in the grain boundaries mostly. So surface status of diamond film will be instead more inferior with the further increase of temperature at T < 800 -C. It is distinct that the grain boundary of diamond films polished at 750 -C in Fig. 3(c) is deeper than those polished at 700 -C in (b). In the similar way, longer time can

Table 1 Ra and rate of removal of diamond films after processed in a various conditions Species number

Original

1

2

3

4

5

6

7

Temperature /-C Processing time/h Pressure /kPa Ra/Am Rate of diamond removal/Am/h

– – – 5.9762 –

680 2 10 4.3155 5

700 2 5 2.2047 –

700 2 10 2.0247 25

700 4 10 2.3843 –

750 2 10 2.1523 35

820 0.5 – 1.5721 200

>850 0.1 – – >400

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Y. Sun et al. / Diamond & Related Materials 15 (2006) 1412 – 1417

also induce a slight increase in the value of Ra. Furthermore, the application of proper pressure should be available to smoother surface. At T > 800 -C, a rate of diamond removal as high as hundreds of micrometers, which is about 5 to 10 times higher than that at T < 800 -C, is achieved because of its higher diffusivity in the liquid metal than in solid one. From cross-sectional view of original and sample 7 in Fig. 3(g) and (h), the thickness reduction of 100 Am after only 0.5 h at 820 -C can be observed, and Ra of diamond films can down to 1.5721 Am accordingly. As the treating temperature is above

850 -C, a smoother surface can be obtained within several minutes. The striking difference between the morphology of polished (III) and unpolished regions (I) can be seen from Fig. 3(e). Fig. 3(f) provides a magnified SEM micrograph of the polished regions (part III) in Fig. 3(e). It can be seen the facets have been essentially removed in the polished region. Compared with the diamond films polished by solid Ce shown in Fig. 3(b) and (c), the grains in the liquid-polished films shown in Fig. 3(d), and (f) become more clear and the surface is flatter in the whole. With the progressing of the

Fig. 3. SEM photographs of diamond films. Top view of (a) before polished (b) sample 3 (c) sample 5 (d) sample 6 (e) sample 7: I—unpolished region; II— transitional region; III—polished region; (f) enlarged part III of Fig. 3 (e); cross sectional view of (g) original and (h) sample 7 in Table 1.

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polishing, big grains on the top disappear and small grains in the bottom emerge gradually.

The polished diamond films washed by excessive aqua regia were subsequently examined by energy-dispersive X-ray analysis (EDXA)in a scanning electron microscope for the traces of Ce; none was found within the detection limits. The SEM image of sample polished by the same condition before washed is shown in Fig. 4(a). The elements included in the white squares at the grain boundaries in the micrograph examined by EDXA are C and Ce, and it is presumed as Ce3C2 according to Ce – C binary phase diagram. In addition, the XRD result of powders on the surface of Ce bulks reacted with diamond indicated that a mixture of CeO2 and Ce3C2 are produced in the process of polishing. Apparently, Ce is easily oxided in the atmosphere, so the existence of CeO2 on the surface of Ce is inevitable, and Ce3C2 is certainly the reaction product of Ce and C. Therefore, the conclusion that there is a diffusion and reaction between Ce and C at the interface at the

Intensity / (a.u.)

(a) 3.3. Investigation of polishing mechanism

1000

(b)

(c)

1200

1400

1600

Raman shift

1800

2000

/cm-1

Fig. 5. Raman spectra of diamond films (a) original; (b) sample 3; (c) sample 7.

given conditions and Ce3C2 is their main reaction product can be obtained. Fig. 4(b), (c), and (e) show SEM micrographs of diamond films polished at 680 -C for different time, respectively, which involves the whole course of polishing. As shown in Fig. 4(b),

Fig. 4. SEM micrographs of CVD diamond films polished by Ce (a) Unwashed polished diamond film at 820 -C; (b) polished at 680 -C—initial stage; (c) polished at 680 -C—middle stage; (d) magnified details of (c); (e) polished at 680 -C—final stage.

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Table 2 Raman spectra characteristics of films Peak position/cm (a) (b) (c)

1

1333.1 1331.6 1330.1

Table 3 Auger C-KLL spectra characteristics of films

FWHM/cm

1

9.66 11.90 15.19

A D / A G (area)

Cd (wt.%)

0.791 1.707 1.579

79.81 98.84 98.74

etching happens at the grain boundary at the beginning, and then it goes deeply into the inside of grains until the entire grain tip is etched out along the grain boundary, just as shown in Fig. 4(c). The magnified photograph inside the etched grains is shown in Fig. 4(d), which reveals the tiny grains located few layers below the growth surface. As the polishing progressing, the protruding regions are gradually etched, finally all the regions around the diamond films are flattened and appear the same morphology as that in Fig. 4(e). Fig. 5 shows Raman spectra obtained from the surface of diamond films before polishing [Fig. 5(a)], after 2 h of polishing using solid reaction [Fig. 5(b)] and after 0.1 h of polishing using liquid reaction [Fig. 5(c)]. Table 2 lists the characteristics of Raman spectra in Fig. 5. The FWHM (full width at half maximum) of diamond Raman peak near 1332 cm 1 increases significantly after polished, especially for Ce’s liquid polishing (sample 7), and the diamond peak shifts towards lower wavenumber after polished. It is known that the FWHM of diamond films is relevant to many factors including crystallinity, grain size, defect density, impurities and content of non-diamond carbon, etc. Previous studies [17 – 20] indicate that the surface of diamond films at different depth from the substrate has its own Raman characteristic, especially FWHM of diamond peak by reasons of possessing different crystallinity, grain size, defect density and so on. In this work, the diamond removal of growth surface by polishing emerges the structure more close to substrate with polishing progressing, thus polished samples have a broader FWHM than unpolished ones, which is consistent with document [17] that FWHM

d[EN(E)] / dE (arbitary units)

(a)

200

(b)

(a) (b) (c)

Peak position/eV

G parameter

315 336 335

13.3 5.4 4.1

decreases along the growth direction and has a maximum value near the substrate. In the same way, the downshift of diamond Raman peak attributes to increase of tensile residual stress due to the emerging of deep-seated structure of films. Furthermore, according to phonon confinement model [18], the reducing of particle sizes (i.e. 60¨80 Am for original and 4~8 mm for ones polished by liquid cerium) shown in Fig. 4(a) and (d) will result in broaden of FWHM. In addition, the content of diamond in the films can be estimated by a relative Raman cross section of diamond to graphite of 1 / 50 [21] by the relation: Cd = 100A d / (A d + A g / 50). As is shown in Table 2, the content of diamond in the films Cd has a dramatic increase from 80% to 98.8% after polished due to the etching out of impurities mostly in the grain boundaries. As a result, the polishing of diamond films by Ce can reduce the impurities in diamond film. The same samples measured by Raman spectra are analyzed using AES and their C-KLL Auger spectra are shown in Fig. 6. It can be seen from Table 3, very severe positive shift for kinetic energy of diamond occurs for all the samples due to charging effect, in particular for polished ones. The charging effect is known to shift the Auger line to high kinetic energy in the spectrum. The differences of charging effect are generally considered to correspond to the conductivity of samples, so it is not surprising that it is regarded as the characterization of relative diamond content for diamond films contained different quantities of graphite because of electrically conducting of graphite and electrically insulating of diamond, consequently, polished diamond films showing more dramatic charging effect can have higher diamond content. Furthermore, a G-parameter [22] assessment of AES C-KLL spectra for each sample was performed and showed: G a = 13.3, G b = 5.4 and G c = 4.1. According to document [20], the G parameters of type IIA natural diamond and highly oriented pyrolytic graphite (HOPG) are 1.2 and 138, respectively, hence these semiquantitative results suggest that both unpolished and polished diamond surfaces have more diamond bonding character instead of graphite, additionally, polished diamond appear to be of higher content relative to the unpolished ones at this point, which is consistent with the results of Raman spectra. 4. Conclusions

(c)

Based on the experimental investigations and analytical presented in this paper, the following conclusions can be drawn: 240

280

320

360

400

Kinetic Energy (eV) Fig. 6. Auger C-KLL spectra for (a) original; (b) sample 3; (c) sample 7.

1. It is a high efficiency, low cost and coarse polishing method for the CVD diamond films to use Ce as polishing materials due to simple set-up and relative low temperature. The polishing of Ce in solid state proceeds relatively slowly

Y. Sun et al. / Diamond & Related Materials 15 (2006) 1412 – 1417

(commonly for 2 h), but temperature required is lower and controllable easily, whereas the polishing of molten Ce has a very high removal rate ten times as much as that of the former and a smoother surface can be achieved. 2. Utilizing solid Ce polishing method, increasing of temperature and time can improve the removal rate and surface roughness of diamond films, but very high temperature and too much time will make the surface worse due to preferred etching at the boundary, 3. Both diffusion of carbon to the cerium and the reaction between Ce and C play important roles in the polishing process. The relative content of diamond in the polished films has a notable increase, but diamond Raman FWHM in polished films has obviously broadened. However, since analysis of FWHM of Raman peak involves many aspects, the leading reason of changes in FWHM and what a role polishing plays in the process still need a further investigation. References [1] H.O. Pierson, Handbook of Carbon, Graphite, Diamond and Fullerene: Properties, Processing and Applications, Noyes, Park Ridge, NJ, 1993. [2] A.M. Zaitsev, Optical Properties of Diamond: A Data Handbook, Springer, 2002. [3] Cheng Guanghua, Zhang Yang, et al., Syntheses and Applications of Diamond Thin Film, Beijing, China 2004. [4] Gu Changzhi, Jin Zengsun, Funct. Mater. 28 (1997) 232.

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[5] Charles B. Willingham, Thomas M. Hartnett, Richard P. Miller, Proc. SPIE Int. Soc. Opt. Eng. 3060 (1997) 160. [6] A.P. Malshe, B.S. Park, W.D. Brown, H.A. Naseem, Diamond Relat. Mater. 8 (1999) 1198. [7] J.E. Graebner, S. Jin, M. McCormack, Patent US5328550. [8] A.M. Zaitsev, G. Kosaca, B. Richarz, V. Raiko, R. Job, T. Fries, W.R. Fahrner, Diamond Relat. Mater. 7 (1998) 1108. [9] C.J. Tang, A.J. Neves, A.J.S. Femandes, J. Ggracio, N. Ali, Diamond Relat. Mater. 12 (2003) 1141. [10] ABE Toshihiko, Hashimoto Hitoshi, Takeda Shuichi, Patent JP2003211361, 2003. [11] Daniel J. Gregoire, Wei Ronghua, Patent US6652763, 2003. [12] S. Jin, J.E. Graebner, M. McCormack, T.H. Tiefel, A. Katz, W.C. Dautremont-Smith, Nature 362 (1993) 822. [13] S. Jin, L.H. Chen, J.E. Graebner, M. McCormack, M.E. Reiss, Appl. Phys. Lett. 63 (1993) 622. [14] M. McCormack, S. Jin, J.E. Graebner, T.H. Tiefel, G.W. Kammlott, Diamond Relat. Mater. 3 (1994) 254. [15] Fu Yiliang, Lv Fanxiu, Wang Jianjun, Zhong Guogang, Wang Liang, Yang Rang, High Technol. Commun. 1 (1996) 1. [16] T.B. Massalski, Binary Alloy Phase Diagrams, 2nd edition, ASM International, Metals Park, Ohio, 1990, p. 834. [17] S.R. Sails, Appl. Phys. Lett. 65 (1994) 43. [18] I.H. Compbell, P.M. Fauchet, Solid State Commun. 58 (1986) 739. [19] Philip M. Fabis, Appl. Surf. Sci. 126 (1998) 309. [20] Tongchun Kuang, Zhengyi Liu, Physical Testing and Chemical Analysis Part A: Physical Testing, vol. 33, 1997, p. 21. [21] A.M. Zaitsev, Optical Properties of Diamond: A Data Book, Springer, 2001, p. 69. [22] Alon Hoffman, Steven Prawer, R. Kalish, Phys. Rev., B 45 (22) (1992) 12736.