Pulsed Electron Beam Deposition of Nanocrystalline Diamond

Laurentian University

We wish to thank the Canada Foundation for Innovation (CFI) and the Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support.

1. Robertson, J. Diamond-like amorphous carbon, Materials Science and Engineering: R: Reports, vol. 37, no. 4-6, pp. 129-281, 2002. 2. Ferrari, A.C., and Robertson, J. Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond, J. The Royal Society, vol. 362, pp. 2477-2512, 2004. 3. Sharda, T., Rahaman, M.M., Nukaya, Y., Soga, et al. Structural and optical properties of diamond and nano-diamond films grown by microwave plasma CVD, J. Diamond Relat. Mater., vol. 10, pp. 561, 2001. 4. Strikovski, M., and Harshavardhan, K.S. Parameters that control PED ablation of materials and film deposition processes, J. Appl. Phys. Lett., vol. 82, pp. 853, 2003. 5. Strikovski, M., Kim, J. & Kolagani, S. "Plasma Energetics in Pulsed Laser and Pulsed Electron Deposition" in Springer Handbook of Crystal Growth, eds. G. Dhanaraj, K. Byrappa, V. Prasad & M. and Dudley, Springer-Verlag, Berlin Heidelberg, pp. 1193-1211, 2010. 6. Krueger, A. The structure and reactivity of nanoscale diamond, Journal of Materials Chemistry, vol. 18, pp. 1485-1492, 2008. 7. Yadav, V.S., Sahu, D.K., Singh, M., and Kumar, K. Study of Raman spectra of NC DLC films composition (sp2:sp3) with substrate temperature, World Congress on Engr. Comp. Sci., vol. 1, pp. 1-4, 2009.

! Nanocomposites of NCD consist of nanometer sized diamond grains (crystallites) surrounded by an amorphous carbon matrix (Figure 1). Diamond grains contain only sp3 hybridized carbon atoms, while the amorphous carbon phase consists of both sp3 and sp2 hybridized C atoms (and possibly hydrogen). At the interface of these two phases are grain boundaries (Figure 2) (1-3). The attractive properties of NCD such as chemical inertness, electronic and mechanical properties, and other characteristics make NCD an ideal material for many applications.

! Numerous attempts have been made to deposit NCD. These include plasma enhanced chemical vapor deposition, ion beam deposition, cathodic arc deposition, magnetron sputtering of carbon targets, and pulsed laser deposition (PLD). It has proven to be quite challenging to prepare NCD films and optimize their properties to meet certain requirements of a given application. Therefore, more advanced/novel deposition techniques need to be assessed for their potential to prepare NCD thin films of various properties. This study is a contribution towards this concerted research effort.

! Channel-spark pulsed electron beam ablation (PEBA)* is based on the non-equilibrium evaporation (4, 5) of a target [highly order pyrolytic graphite (HOPG), in this case] through the application of a self-pinched pulsed electron beam (Figure 3). PEBA has an electric-to-electric energy conversion efficiency of > 30% (5), making it an effective technique for the preparation of thin films. Many parameters that control the quality of deposited thin films via PED must be judiciously selected. These include energy (power) density, electron energy, pulse rate, background gas and other geometrical features of the chamber.

Figure 1: SEM image of nanocrystalline diamond (6)

Figure 2: Structure model for NCD (6)

Figure 3: Simplified schematics of a PED system

OBJECTIVES

EXPERIMENTAL DETAILS

BACKGROUND

RESULTS & DISCUSSION

CONCLUSIONS

Thin films of nanocrystalline diamond have been prepared from a HOPG target (HOPG ZYB, Optigraph GmbH, Germany) (Figure 4) under a background of argon gas. A typical deposition run is shown in Figure 5. The thickness of the resulting thin films is in the range 50 nm – 120 nm (see inset, Figure 9). NCD Films have been deposited on various substrates of silicon (Figure 6), namely, Si(100) and Si(111), at two different substrate temperatures, namely, room temperature and 150°C (run # 2), and at ablating pulses of 3,000 (runs # 1-3) and 5,000 (run # 4). The films have been characterized using AFM (Bruker), visible Raman spectroscopy (HORIBA Jobin Yvon XploRA), and visible reflectance spectroscopy (Semiconfsoft), and SEM (FEI Quanta 650F, 20 kV, spot size = 3.5).

Figure 4: HOPG Target Disk (25 mm diameter X 5 mm thickness)

Figure 5: Ablation and plume inside the deposition chamber

Figure 6: Silicon (100)

bare substrate

"  Reporting on the potential of PEBA toward the deposition of thin films of nanocrystalline diamond from a HOPG target.

"  Investigation of optimal deposition conditions. "  Characterization and analysis of the deposited films via numerous analytical methods.

•  Nanocomposites of NCD have been successfully obtained via PEBA under judiciously selected process conditions. •  NCD has been identified using visible Raman spectroscopy. •  AFM and SEM analyses of NCD films have shown cluster size to vary between 50 nm and up to the micron range. •  NCD clusters are better developed at room temperature in terms of their size and structure. •  The growth of larger NCD clusters also seems to be favoured by a large number of ablating pulses (thicker films).

! Visible Raman spectra (514 nm) of NCD coatings on Si (100) and Si (111) are shown in Figure 8. The spectra show five signals (obtained using a Gaussian deconvolution) at 1150, 1332, 1350, 1480 and 1550 cm!1. The recorded signals are indicative of the presence of NCD in the films (2). The position of the G peak has been used to determine sp3 contents in the deposited films using Yadav’s relation (7).

sp3 content = 0.24 - 48.9("G - 0.1580) Where "G (µm-1) represents the value of the G peak position. Table 1 shows the position of the G peak and sp3 content in films prepared from all runs. The sample prepared at 150˚C (run # 3) contains the lowest percentage of sp3. ! The morphology of the films as revealed by AFM analysis, see Figure 10, consists of clusters of crystallites in the range 50 nm to 1 µm. The clusters grow along a preferential direction. With respect to grain size criterion, films resulting from runs 1, 2, and 4, are consistent with the grain size expected for NCD. The cluster size does not appear to be uniformly distributed over the surface. AFM image of run # 3 (150˚C) does not show well developed clusters relatively to NCD films obtained at room temperature. This is consistent with the results obtained from Raman analysis. AFM images also show larger size NCD clusters as the number of electron beam pulses is increased from 3,000 pulses to 5,000 pulses. ! SEM images (Figure 11) of films reveal variations in the morphology of films deposited at RT (run # 1) and at 150°C (run # 3). The former seem to be more continuous than the latter, in good agreement with AFM data.

Run Sample G

position sp3%

1 Si(100) 1534 47.494 2 Si(111) 1536 46.516 3 Si(100) 1571 29.401 4 Si(100) 1537 46.027

Table 1: The position of G peak and sp3

content

Figure 8: Raman signal for film on Si in Run 1, 2, 3, and 4

Figure 9: Measured and calculated reflectance data

Figure 10: AFM of NCD film deposited on Si in Run 1, 2, 3, and 4

Figure 7: NCD coating on

Si (100)

Redhouane Henda and Omar Alshekhli Bharti School of Engineering/Laurentian University

Sudbury, Ontario P3E 2C6

Si(100) - Run1, Ra = 3.06 Si(111) – Run2, Ra = 22.2

Si(100) – Run3, Ra = 57.4 Si(100) – Run4, Ra = 71.7

* PEBA is a technology licensed from Forschungszentrum Karlsruhe

Run Substrate Thickness

(nm) 1 Si(100) 72.42 2 Si(111) 55.96 3 Si(100) 60.22 4 Si(100) 113.74

Figure 11: SEM images of films on Si at RT (left) and 150°C.

References