ISSN 10637850, Technical Physics Letters, 2015, Vol. 41, No. 7, pp. 658–660. © Pleiades Publishing, Ltd., 2015. Original Russian Text © D.A. Usanov, A.V. Skripal, R.K. Yafarov, 2015, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2015, Vol. 41, No. 13, pp. 95–101.
Fabrication and Diagnostics of Planar Cellular Carbon Structures D. A. Usanov*, A. V. Skripal, and R. K. Yafarov Saratov State University, Saratov, 410012 Russia *email:
[email protected] Received December 5, 2014
Abstract—New planar carbon structure with carbon atoms in both sp2 and sp3 states has been fabricated with the use of strongly ionized lowpressure nonequilibrium microwave plasma. It was demonstrated by using nearfield microwave and atomicforce microscopy that the carbon structure is a macroscopic formation with a honeycomb structure, hexagonal macrocell size of 7.5–9.0 µm, and wall thickness at base of 3.0–4.0 µm. It was found that the presence of breaks in walls of the hexagonal macrocells is a distinctive feature of a struc ture of this kind. DOI: 10.1134/S1063785015070159
Scientific and technological problems in the devel opment and characterization of nanostructures to be used in nanoelectronics are associated with the struc turing of lowdimension materials on the scale of sev eral lattice constants. Development of nanostructures of this kind requires that new procedures and tech niques for their fabrication and diagnostics should be created. Another approach in creating structures of this kind involves a search for and development of technological processes the run of which is due to nat ural specific features of the materials themselves, manifested in that prescribed architectural forms are produced on the basis of molecular selfassembly and/or selforganization phenomena [1]. To raise the output of suitable microstructures and devices, it is highly important to diagnose the presence of buried (subsurface) defects in the materials used to create devices and localize defects with as high a precision as possible and in a short time. These opportunities are opened up, in particular, by the use of nearfield microwave microscopy [2, 3]. The application area of the nearfield diagnostics covers a wide variety of prac tical problems of modern electronics, materials sci ence, defect detection, and medicine. The goal of the study was to fabricate planar carbon structures by plasmachemical deposition from highly ionized nonequilibrium microwave plasma and to study their characteristics. The carbon coatings were deposited in a vacuum installation with an ionplasma microwave source operating at a frequency of 2.45 GHz [4]. The micro wave radiation power and the magnetic induction were 250 W and 875 G (0.0875 T), respectively. Deposition was performed onto glass substrates with ethanol vapor at a pressure of 0.05 Pa as the working substance. This satisfied the existence conditions of the electron
cyclotron resonance. The degree of ionization of the microwave plasma was determined from the current characteristics with a doubleprobe method [5] to be about 5%, which exceeds by more than two orders of magnitude the degree of plasma ionization in the dc or highfrequencydischarge mode. The substrates were heated in our experiments to a temperature of 300 ± 10°C. The carbon film coatings were studied by probe methods of atomicforce, electron, and nearfield microwave microscopy and by Raman spec troscopy. Raman spectra were obtained with laser light at a wavelength of 473 nm and positioning time of 35 ms. Figures 1 and 2 show images of selfstructured car bon film coatings deposited in a microwave ethanol vapor plasma obtained by scanning probe nearfield microwave microscopy and atomicforce microscopy (AFM), respectively. Analysis of the nearfield micro wave microscopic images demonstrated that the car bon coatings have the form of planar macrocells of hexagonal configuration, which resemble in shape cells in graphite monolayers and form, in aggregate, a honeycomb structure with “windows” transparent in the visible spectral range. AFM images of the same planar film structures, obtained with an Agilent Tech nologies AFM5600 scanning probe atomicforce microscope, demonstrated that the size of a separate hexagonal macrocell of the honeycomb structure is 7.5–9.0 μm, which is several thousands of times the size of a similarly shaped hexagonal cell in the graphite monolayer (a ≈ 0.142 nm, d ≈ 0.246 nm). The walls separating the macrocells have in crosssection the form of a triangle with curvature radius of ~100– 400 nm, height of about 0.6 μm, and thickness at base of 3.0–4.0 μm (Fig. 3). A distinctive feature of the honeycomb planar structures of this kind is that the
658
FABRICATION AND DIAGNOSTICS OF PLANAR CELLULAR CARBON STRUCTURES
TECHNICAL PHYSICS LETTERS
Vol. 41
No. 7
Phase, deg 0.86 0 5 15
0 10 20
25 35 µm 45
30 40 50 µm
55
60
65
70
75
Fig. 1. Phasecontrast visualization of a carbon coating by scanning probe nearfield microwave microscopy.
10
0
20
30
µm 40
50
60
70 0.8
B
10
0.7 20
0.6
40
0.5 µm
µm
30
0.4
A
50
0.3
60
0.2 0.1
70
0 Fig. 2. AFM image of a carbon coating.
Profile, deg
walls of the hexagonal cells have breaks (there are no closed hexagonal cells). The breaks in the cell walls may have widths of several nanometers to a few micrometers. A study of the samples by electron microscopy with a MIRAII scanning electron microscope demon strated that they exhibit a highintensity cathodolumi nescence in the visible spectral range. It is known that this indicates that carbon atoms in the structure are in the sp3 hybridization state, as in diamond or methane [6, 7]. These results are correlated with the Raman data. The carbon coatings characteristically had a doublepeak spectrum with a rather strong and narrow line at around 1330 cm–1, which confirms the pres ence of carbon atoms in the sp3 hybridization state (diamond structure), and a second, weaker peak at around 1580 cm–1, which is due to the presence of car bon atoms with sp2 hybridization of valence bonds in the form of graphite, both crystalline and amorphous [8, 9]. Because ethanol is the working substance in plasmachemical deposition from highly ionized low pressure nonequilibrium microwave plasma, the decomposition of ethanol in the plasma of the micro wave gas discharge yields hydrogen, which is chemi sorbed on the surface of a growing film in the form of C–H groups. These films are amorphous hydroge nated films (aC:H) and contain, in addition to car bon atoms in the sp3 and sp2 hybridization valence states, also bound hydrogen in C–H groups. According to cluster model of the structure of amorphous carbon elaborated in [10, 11], graphite atomic clusters with an sp2 valence state are distributed within the sp3 stressbound rigid network, a matrix in which mixed bonds are predominant and which deter mines the tunneling barrier between them. The size of the hydrocarbon clusters with the sp2 valence state may vary from 4 to 100 nm, depending on the deposition conditions and thickness of films and on the hydrocar bon used for their deposition. A decrease in the con tent of weakly bound hydrogen in aC:H films in the form of CH groups favors formation of C=C double bonds and leads to an increase in the size of πbound clusters characteristic of graphite structures. By con trast, the presence of CH groups in the sp3 state in the structure of the films favors a decrease in the size of π bound clusters and localization of their π electrons as a result of the increase in the tunneling barrier between these. With decreasing content of CH groups in the sp3 state, the packing density of the structure of aC:H films grows. According to these concepts and to the Raman spectra of the carbon films with honeycomb macro cells, the fact that the Raman spectra of aC:H films deposited from the microwave plasma with ethanol vapor have no 1250 cm–1 band associated with vibra tions of C–C bonds at branch points of the structure makes the peak at around 1330 cm–1 narrower and stronger than the peak at around 1580 cm–1, which
659
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
10
20
30 40 A−B, µm
50
60
Fig. 3. AFM profile of the carbon coating along the A–B straight line in Fig. 2.
indicates that films have carbon atoms in the sp2 state. This transformation of the peaks evidences that the unit cell of the sp3bound matrix and the length of sp2 clusters distributed within this matrix become larger
2015
660
USANOV et al.
and the content of CH groups in the sp3 state decreases [8]. This, in turn, must lead to an increase in the size of πbound hydrocarbon clusters and to localization of their conjugation system. According to the experimental data obtained in this study, at certain relative sizes of carbon atom clusters in the sp3 and sp2 hybridization states in aC:H films controlled by the amount of bound carbon in a grow ing film, and at certain thicknesses of these films, the observed honeycomb macrocellular structure can be formed from various branching chains, which consti tute, together with other similarly branching chains, a quasiclosed honeycomb hexagonal carbon macro structure. The linear sizes of chains of this kind may be as large as 100 μm. The driving force of this process is minimization of the free energy of the system via attainment of equilibrium between the internal com pressing stresses caused by the presence of carbon atom groups in the sp3 hybridization valence state and the formation of strong C=C double bonds accompa nied by an increase in the size of πbound clusters characteristic of graphite structures. These processes enable implementation of a new lowtemperature (200–300°C) method of plasmachemical deposition of carbon coatings having the form of a planar honey comb structure with hexagonal macrocells on glass and silicon substrates. The synthesis temperature used in this process is two to three times lower than that in the chemical vapor deposition (CVD) method and is compatible with processes employed in semiconduc tor electronics. Another advantage of the method is the possibility of obtaining coatings directly on dielec tric substrates. This makes it possible to deposit coat ings onto such substrates as glass, plastics, etc. Thus, a new planar carbon structure was obtained for the first time by the plasmachemical deposition method with highly ionized lowpressure nonequilib rium microwave plasma and studied by scanning probe nearfield microwave microscopy and atomicforce microscopy. In this structure, carbon atoms are in both sp2 and sp3 hybridization states. It was shown that the carbon structure is a macroscopic formation of honey
comb architecture with micrometersize hexagonal macrocells. The study demonstrated the high sensitivity of the nearfield microwave probing to changes in morpho logical parameters of the planar structures under study, which is comparable with that of probe atomicforce microscopy. Acknowledgments. This study was financially sup ported by the Ministry of Education and Science of the Russian Federation, State assignments nos. 1376 and 1575. REFERENCES 1. Nanotechnologies in Electronics, Ed. by Yu. A. Chaply gina (Tekhnosfera, Moscow, 2005) [in Russian]. 2. D. A. Usanov, NearField Scanning Microwave Micros copy and Its Application Areas (Izd. Saratov. Univ., Sara tov, 2010) [in Russian]. 3. D. A. Usanov, S. A. Nikitov, A. V. Skripal, A. V. Abra mov, A. S. Bogolubov, B. N. Korotin, V. B. Feklistov, D. V. Ponomarev, and A. P. Frolov, in Proceedings of the 41st European Microwave Conference, October 9–14, 2011, Manchester, pp. 210–213. 4. R. K. Yafarov, Physics of Microwave VaccumPlasma Nanotechnologies (Fizmatlit, Moscow, 2009) [in Rus sian]. 5. F. F. Chen, in Plasma Diagnostic Techniques Ed. by R. H. Huddlestone and S. L. Leonard (Mir, Moscow, 1967) [in Russian], pp. 94–164. 6. Diamonds in Electronics, Ed. by V. B. Kvaskova (Ener goatomizdat, Moscow, 1990) [in Russian]. 7. A. N. Obraztsov, I. Yu. Pavlovskii, and A. P. Volkov, Tech. Phys. 46, 1437 (2001). 8. E. A. Konshina, Fiz. Tekh. Poluprovodn. (S.Peter burg) 33, 469 (1999). 9. Ch. P. Pul and F. J. Owens, Nanotechnologies, Ed. by Yu. I. Golovina (Tekhnosfera, Moscow, 2005) [in Rus sian]. 10. J. Robertson, Adv. Phys. 35, 317 (1986). 11. J. Robertson, Surf. Coat. Technol 50, 185 (1992).
Translated by M. Tagirdzhanov
TECHNICAL PHYSICS LETTERS
Vol. 41
No. 7
2015