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Carbon films produced by fast atom bombardment
LHE8-6223i88 $3.00 + .oO Copyright Q 1988 Per@mon Press plc
Carbon Vol. 26. No. 4, pp. 547-552, 1988 Printed in Great Britam.
CARBON FILMS PRODUCED BY FAST ATOM BOMBARDMENT A. G. FITZGERALD Carnegie Laboratory of Physics, University of Dundee, Dundee DDl 4HN, Scotland Department
M. SIMPSON and G. A. DEDERSKI of Physics, University of Leeds. Leeds LS2 9JT, England
B. E. STOREY and P. A. MOIR Carnegie Laboratory of Physics, University of Dundee, Dundee DDl 4HN, Scotland
and Department
D. TITHER of Physics, University of Leeds, Leeds LS2 9JT, England
(Received
10 L)ecember
t987; accepted
19 Junuary
1988)
Abstract-A fast atom bombardment (FAB) source has been used to produce thin carbon films using butane (calor gas) and butane-argon mixtures. The films were produced on CR39 polycarbonate, glass, aluminium foil, and single-crystal sodium chloride substrates. The films were studied by X-ray photoelectron spectroscopy, Auger electron spectroscopy, transmission electron microscopy, and transmission electron diffraction. The carbon films were largely amorphous with small crystallites scattered across the film surface. The majority of these crystallites have been identified as either the alphacarbine or the Ries crater forms of carbon, Some of the crystallites were found to decompose in the electron beam during transmission electron microscope studies. It is suggested that hydrogen, argon, or a hydrocarbon gas is trapped between the layers during the growth of these hexagonal layer structured forms of carbon. Prolonged electron irradiation is likely to lead to the evolution of this intercalated material. Auger spectra are presented that suggest that argon ion bombardment also produces evolution of intercalated material and hence significantly modifies the surface of the films. Key Words-Carbon
films, surface analysis, electron microscopy, electron diffraction.
2. EXPERIMENTAL
1. INTRODUCTION
Recently a great deal of interest has been shown in carbon films, more particularly diamond, diamondlike carbon (DLC), amorphous hydrogenated carbon (a-C:H) and ion beam deposited carbon (icarbon); the latter being grown under conditions of ion bombardment. As part of a long-term program of work we have investigated carbon films using a wide array of surface analysis techniques and compared our results with those appearing in the literature[l-111. This article shows that carbon films produced by fast atom bombardment (FAB) have a generally amorphous structure[ll] with some complex crystallites that show behavior which could be due to the inclusion of hydrogen, argon, or hydrocarbon gas, possibly by intercalation. The work we have carried out under a variety of conditions and with butane and other feed gases and mixtures with argon suggest, in fact, that more complex forms of carbon can be produced by this technique. Films of a similar nature can also be produced by sputtering a graphite target using argon gas in a triode system[lO]. No evidence has been obtained so far of the decomposition of crystallites in films prepared using the triode system. CAR26:1-l
DETAILS
2.1 Materials A 4.5kg butane gas cylinder (calor gas) was used for preparing all the films, and zero-grade argon gas was also used. Films were prepared on ordinary catering foil and on glass microscope slides conforming to B.S. 3836-1974. FOT transmission electron microscopy the carbon films were deposited on 99.99% pure single-crystal sodium chloride cleaved to give a flat surface. The sodium chloride was obtained as off-cuts from Hilger Analytical Limited, Margate, Kent, England. After depositing the carbon on the sodium chloride substrates, the sodium chloride was dissolved away in distilled water and the films were mounted onto copper electron microscope grids. CR39 lens material was also coated with a carbon film, but these samples were difficult to analyze because of degassing of the substrate in the U.H.V. analysis system. 2.2 Sample preparation Substrates of glass, CR39, aluminium foil, and sodium chforide were attached to a copper substrate holder using clamps. This assembly was then mounted onto a water-cooled plate over the FAB source (Ion Tech Ltd., type: FAB 110-2). The sub547
A. G. FITZGERALDetal.
548
strate was oscillated through the beam of material from the FAB source so that equal exposure was received by all the substrates. The substrates were cleaned by fast atom bombardment, using argon gas only, before the carbon film was deposited using butane or butane-argon gas mixtures. Table 1 details the parameters used on the FAB source for cleaning substrates and depositing the carbon films. The vacuum chamber used for this work is essentially a sputtering system fitted with a fast atom bombardment source for cleaning substrates. Modification of the gas handing system allowed a variety of gases to be admitted into the FAB source including gas mixtures using a crude needle valve and flow meter. The vacuum chamber was evacuated to background pressures of -1tY6 Torr using a cryopump and rotary pump. A mass spectrometer was routinely used to monitor background gases in the vacuum system. The major contaminant was water vapor with a partial pressure of approximately 5 x lo-’ Torr, hydrocarbon cont~ination normally being an order of magnitude lower. During sample cleaning and preparation, pressures were between 1O-2 and BY3 Torr depending on conditions and gas flows, see Table 1. Deposition rates were found to be extremely low, estimated to be between 5 and 10 8, min-r depending on conditions. Film thickness measurements were taken using a Talystep instrument on steps produced using a dry photoresist material (Letraset brand) as a subfilm mask that could be easily removed in acetone after depositing the carbon films, The Talystep instrument was used to measure the step height from the A/10 optical flat and the deposited film. However, the time to grow a sufficiently thick film for
accurate measurement by this method was prohibitively long and only estimated values were obtained. A quartz crystal thin film thickness monitor was also used, but results were rather poor, presumably because of the low deposition rate, the nature of the film being produced, and difficulty in obtaining good correction factors (tooling factors). The FAB source also showed signs of carbon building up on the electrodes and walls inside the source. This carbon was extremely fine powder but was easily removed by a very fine soft brush and a vacuum cleaner. If the carbon was allowed to accumulate, the FAB source showed erratic discharge behavior and would be switched off by the current overload protection on the power supply. Analysis of the carbon from inside the FAB source is being undertaken in a separate study to be presented later. 2.3 Analysis of films The carbon films were examined in a V.G. HBlOO ultra-high-vacuum scanning electron microscope (UHV-SEM) and JEOL 1OOCscanning transmission electron microscope (STEM). The HBlOO SEM was fitted with a CLAM 100 electron spectrometer for surface analysis by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). For surface analysis, the carbon films were deposited on glass slides and aluminium foil. Portions of these substrates were cut to a diameter of 1 cm and mounted on a stainless steel stub, Aluminium foil was used as a substrate to reduce charging in the electron beam during Auger eiectron spectroscopy measurements. Surface charging effects were also observed during X-ray photoelectron spectroscopy and resulted in the displacement of X-ray photo-
Table 1. Conditions used for producing carbon films with the FAB 110-2 fast atom bombardment source N(E)
(a) FAB cleaning of substrates Pressure: 2.1 X 10-j mhar @ - 35 seem argon* Power: Time: Oscillation: Distance:
Oscillation: Distance:
Power: Time: Oscillation: Distance:
only
8.0 x lo-) mbar @ -50 seem* 33.8 W = 750 V, 45 mA 15-100 min, (- 150-1000 A) (i.e., - 10 A min-‘) 2 20” (22.5 in.) @.-- 1%Hz frequency 100 mm from top of FAB source
Cc) Deposition by FAB-butane Pressure:
1s
49.5 W = 1.1 kV, 45 mA 3 min t 20” (52.5 in.) @ - % Hz frequency 100 mm from top of FAB source
(b) Deposition by FA B-butane
Pressure: Power: Time:
c
f
b
and argon - 50:50 mixture
5.9 x 10mJ mbar t@ - 45 seem total flow* 31.2 W = 650 V, 48 mA 15-100 min (- 75-500 A) (i.e., - 5 A min-I) + 20” (~2.5 in.) @ - 1%Hz frequency 100 mm from top of FAB source
*NJ. For these results, a crude flow controller with a single-flow meter has been used to measure flow rates. Gas flow ratios were set using precision needle valves hence, the ratios and total flows are approximate. A more sophisticated flow controller is to be installed shortly.
t 0
I
200
1
,
400
600
a00
Binding
Energy
I
1
1000
1
1200
faV)
Fig. 1. X-ray photoelectron spectrum from a carbon fiim {a) prepared using calor gas in a FAB source, (b) this film after a 30s argon ion etch.
549
Carbon films N(E)
a
b I
I
288
292
I
1 280
284 Binding
Energy
(eV)
Fig. 2. The carbon 1s X-ray photoelectron peak from a film prepared with a FAB source using (a) calor gas, (b)
a mixture of calor gas and argon. electron peaks from their true position. To determine the exact position of these peaks the films were given a flash of gold and the Au 4f,,, peak was recorded as a reference to establish the magnitude of this peak displacement due to charging. Throughout these measurements the Au 4f,,Z peak was situated at 83.9 eV for gold films deposited directly onto a stainless steel stub. The carbon films examined in the STEM were deposited on rock salt. The film was floated off this substrate in distilled water and scooped onto copper electron microscope grids.
(a)
Fig. 3. Electron diffraction pattern area of a film prepared by deposition with calor gas.
3. RESULTS
AND
from an amorphous from a FAB source
DISCUSSION
3.1 X-ray photoelectron spectroscopy A large number of X-ray photoelectron spectra were recorded from the different film preparations. Figure 1 is an X-ray photoelectron spectrum obtained over the range of binding energies between zero and approximately 1100 eV from a film deposited using calor gas in a FAB source. Similar spectra were obtained from films deposited on aluminium foil and from films prepared with a mixture of calor gas and argon in the FAB source. These spectra confirmed the presence of carbon and oxygen at the film surface. The carbon 1s peaks from films prepared by deposition from calor gas and from the calor gas-argon mixture in the FAB source have been examined at higher resolution and are shown in more detail in Fig. 2. The C 1s peak from films prepared from calor gas in the FAB source is situated at 284.6 eV. This
(b)
Fig. 4. (a) Electron micrograph from crystallite in a carbon film prepared by deposition from a FAB source with calor gas. (b) Electron diffraction pattern from this crystallite. The pattern can be attributed to either the alpha-carbine or the Ries Crater forms of carbon. The zone axis of the pattern for both is (001).
A. G.
550
FITZGERALD
et al.
Table 2. Interplanar spacings from a polycrystalline region of the film-comparison with interplanar spacings in carbon crystal structures o-carbine Observed d(A) 4.38 2.52 2.22 1.66 1.47
Fig. 5. Polycrystalline electron diffraction pattern from an area of film prepared by deposition from a FAB source with calor gas.
peak was found at 284.5 eV in films prepared with a calor gas-argon mixture in the FAB source. These binding energies for the C 1s peak should be compared with binding energies of 284.5 eV obtained for C 1s a-C:H films, 284.3 eV for graphite, and 284.4 eV for diamond. To investigate the nature of the oxygen peak on the surface each carbon film was given a light argon ion etch (Fig. 1). The spectrum obtained after this etch from the film deposited from the FAB source using calor gas is shown in Fig. lb. The oxygen peak was virtually removed by this light etch. The oxygen is therefore present only at the surface of the carbon film. Similar results were obtained with the films prepared with a mixture of calor gas and argon in the FAB source. 3.2 Transmission electron microscopy and diffraction Both film preparations the scanning transmission
had a similar structure in electron microscope, bas-
(a)
d(A) (4.465 { (4.256 2.542 2.232 1.488
hkl
Ries Crater d(A)
110 (4.465 103 1 (4.256 301 2.536 220 2.232 1.688 1.495 330
graphite
hkl
d(A)
hkl
110 111 301 220 140 227
1.68 -
004 -
ically an amorphous film (Fig. 3) with small platelike lamellar crystallites distributed over the surface[9]. One of these crystallites is shown in Fig. 4a. Electron diffraction patterns from this type of crystallite have been identified as originating from the alpha-carbine form of carbon (Fig. 4b). Polycrystalline electron diffraction patterns have also been obtained from areas of the film (Fig. 5). The spacings of the interatomic planes obtained from measurement of these polycrystalline ring diameters are listed in Table 2 with interatomic plane spacings from graphite, alpha carbine, and the Ries Crater form of carbon[6,9]. These polycrystalline regions also appear to be either alpha-carbine or the Ries Crater form of carbon. Prolonged observation of carbine crystals has shown that some of these crystals are sensitive to the electron beam in the transmission electron microscope. Figure 6 shows electron micrographs obtained from a carbine crystal with a space of a few minutes between exposures. Areas of the crystal show enhanced electron transmission and it appears that some of the crystal material has evaporated. Similar effects, with the formation of voidlike regions, have been observed during the decomposition of cadmium hydroxide crystals in the electron microscope to form cadmium oxide[ 121.
0))
Fig. 6. Electron micrograph from a crystallite that has been identified as either alpha-carbine or the Ries Crater form (a) taken immediately after exposure to the electron beam in the STEM, (b) after a period of 2 min electron irradiation, and (c) part of the area in this crystallite at higher magnification after 2 min electron irradiation.
Carbon films
3.3 Auger electron spectroscopy Auger spectra obtained from both carbon film preparations confirmed the XPS observations that oxygen was present on the film surface and could be removed with a light argon ion etch (Fig. 7). Figure 8 shows the detailed structure of the carbon KW Auger spectra from the two film preparations. The main difference between Auger electron spectra from the surfaces of graphite, diamond, and hydrogenated carbon films occurs in the Auger line shapes for the three forms. This variation in line shape reflects the different band structure of the three forms. The HBlOO UHV-SEM can be used to obtain Auger electron spectra from areas with diameters of the order of 10 nm. This ability to analyze small areas is important where there is a possibility that variations in composition or microstructure occur across the film surface. Electron microscopy and diffraction studies in this investigation have shown that crystall&es are scattered across these film surfaces. It was important therefore to investigate the form of the Auger line shape obtained from small areas of the surface, of the order of 2 km, that appeared as small particles and protuberances on the surface in the secondary electron image. Figure 8 shows Auger spectra taken from both film preparations from general areas of 100 p,rn in diameter and from particles of the order to 2 pm in diameter lying on the surface. The Auger electron spectra from the particles in each case show a shoulder that is similar to that obtained by Moravec and Orent[8] from ion beam deposited
551
carbon films. This shoulder was not present in spectra obtained from general areas of the order of 100 pm in diameter. When each film was argon ion etched to remove oxygen from the surface, this shoulder also appeared in spectra taken from 100 Frn diameter areas.
4.
CONCLUSIONS
X-ray photoelectron spectroscopy and Auger electron spectroscopy show that oxygen is present on the surface of carbon films prepared with the FAB
source under the conditions discussed in this study. The binding energy of C 1s photoelectron peak in these films is located at a position characteristic of hydrogenated carbon films. Transmission electron microscopy has shown that these carbon films are largely amorphous with small crystallites scattered across the film surface. The majority of these crystallites have been identified as either the alpha-carbine or the Ries Crater forms of carbon. An interesting effect observed during transmission electron microscope studies is the gradual decomposition of some of these crystallites in the electron beam. Alpha-carbine and Ries Crater car-
a b
b
I-C KLL
0
d
e
KLL
a
I
225
I
250 Electron
1 100
I
I
300
Electron
I 700
500
Energy
(eV)
Fig. 7. Auger electron spectrum from a carbon film (a) prepared using a mixture of calor gas and argon, (b) this film after a 60s argon ion etch.
I
1
275 Energy
300 (eV)
Fig. 8. Structure of the KVV carbon Auger peak obtained from a carbon film prepared with a FAB source from (a) a 5+m diameter crystallite (calor gas), (b) a lOO-pm area (calor gas), (c) a 2-pm diameter area (calor gas plus argon), (d) a 200~pm diameter area (calor gas plus argon), and (e) a lOO-p.m diameter area calor gas plus argon) after a 60s argon ion etch, An arrow indicates the position of the shoulder that develops on Auger peaks after intense electron irradiation or an argon ion etch.
552
A. G. FITZGERALD et al.
bon have complex hexagonal crystal layer structures. It is possible that during the growth of these crystallites, hydrogen, argon, or a hydrocarbon gas is trapped between the layers. This intercalation is commonly observed in graphite and many other layer structures such as molybdenum disulphide[l3]. Prolonged electron irradiation is likely to lead to the evolution of this intercalated material. A shoulder has been observed in KW Auger spectra from particles on the surface. This shoulder is characteristic of the whole film surface after argon ion etching. In the case of ion and small area electron bombardment, an intense beam of particles is directed at the surface. During the acquisition of Auger spectra from particles on the surface the electron beam is scanned over a 2-km diameter area giving rise to intense electron irradiation over this small area. During argon ion irradiation the complete specimen surface is irradiated. Transmission electron microscopy observations suggest that at high irradiation intensities there is evolution of gas from the crystallite surface. It is probable that the appearance of this shoulder in the Auger spectra is therefore also associated with the decomposition of the hydrocarbon material present at the surface of the film and with the release of intercalated material from alpha-carbine crystallites after high intensity particle irradiation. Acknowledgments-Mr.
P. A. Moir wishes to thank SERC for a research studentship. Mr. G. A. Dederski is grateful
for funding from the University of Leeds, Department of Physics during his training period on secondment from Sheffield Citv Polvtechnic. Deoartment of Chemistrv. Dr. M. Simpson is grateful to ‘SERC for supporting him.
REFERENCES
1. A. R. Nyaiesh, R. E. Kirby, F. K. King, and E. L. Garwin, J. Vat. Sci. Tech. A 3(3), 610, May/June (1985). 2. R. G. Cavell, S. P. Kowalezyk, L. Ley, R. A. Pollak, B. Mills, D. A. Shirley, and W. Perry, Phys. Rev. B7, 5315 (1973). 3. V. I. Kasatochkin, V. V. Korshak, Yu. P. Kudryavtsev, A. M. Sladkov, and I. E. Sterenberg, Carbon 11, 70 (1973). 4. V. I. Kasatochkin, I. E. Sterenberg, M. E. Kasakov, V. N. Slezarev, and L. V. Belousova. Dokladv Akademii Nauk. UiSR 209, 388 (1973).
’
5. A. El Goresy and G. Donnay; Science 161,363 (1968). 6. M. B. Guseva. N. F. Savchenko. and V. G. Babaev. Sov. Phys. Dokl. 30(8), 686-687’(1985). 7. A. K. Green and V. Rehn, J. Vat. Sci. Tech. Al(4) 1877, Oct.-Dec. (1983). 8. T. J. Moravec and T. W. Orent, J. Vat. Sci. Tech. 18(2), 226 March (1981). 9. V. M. Melnitchenchenko, B. N. Smirnov, V. P. Varlakov, Yu. N. Nikulin, and A. M. Sladkov. Carbon 21, (2), 131-133
(1983).
10. A. G. Fitzgerald, M. Simpson, G. A. Dederski, P. A. Moir, A. Matthews, and D. Tither, Carbon in press. 11. J. Robertson, Advances in Physics 35(4), 317-374 (1986).
12. A. G. Fitzgerald, V. Ripper, and A. D. Yoffe, Phys. Stat. Sol. 3, K445 (1964). 13. R. R. Chianelli, J. Cryst. Growth 34, 239 (1976).
Carbon Vol. 26. No. 4, pp. 547-552, 1988 Printed in Great Britam.
CARBON FILMS PRODUCED BY FAST ATOM BOMBARDMENT A. G. FITZGERALD Carnegie Laboratory of Physics, University of Dundee, Dundee DDl 4HN, Scotland Department
M. SIMPSON and G. A. DEDERSKI of Physics, University of Leeds. Leeds LS2 9JT, England
B. E. STOREY and P. A. MOIR Carnegie Laboratory of Physics, University of Dundee, Dundee DDl 4HN, Scotland
and Department
D. TITHER of Physics, University of Leeds, Leeds LS2 9JT, England
(Received
10 L)ecember
t987; accepted
19 Junuary
1988)
Abstract-A fast atom bombardment (FAB) source has been used to produce thin carbon films using butane (calor gas) and butane-argon mixtures. The films were produced on CR39 polycarbonate, glass, aluminium foil, and single-crystal sodium chloride substrates. The films were studied by X-ray photoelectron spectroscopy, Auger electron spectroscopy, transmission electron microscopy, and transmission electron diffraction. The carbon films were largely amorphous with small crystallites scattered across the film surface. The majority of these crystallites have been identified as either the alphacarbine or the Ries crater forms of carbon, Some of the crystallites were found to decompose in the electron beam during transmission electron microscope studies. It is suggested that hydrogen, argon, or a hydrocarbon gas is trapped between the layers during the growth of these hexagonal layer structured forms of carbon. Prolonged electron irradiation is likely to lead to the evolution of this intercalated material. Auger spectra are presented that suggest that argon ion bombardment also produces evolution of intercalated material and hence significantly modifies the surface of the films. Key Words-Carbon
films, surface analysis, electron microscopy, electron diffraction.
2. EXPERIMENTAL
1. INTRODUCTION
Recently a great deal of interest has been shown in carbon films, more particularly diamond, diamondlike carbon (DLC), amorphous hydrogenated carbon (a-C:H) and ion beam deposited carbon (icarbon); the latter being grown under conditions of ion bombardment. As part of a long-term program of work we have investigated carbon films using a wide array of surface analysis techniques and compared our results with those appearing in the literature[l-111. This article shows that carbon films produced by fast atom bombardment (FAB) have a generally amorphous structure[ll] with some complex crystallites that show behavior which could be due to the inclusion of hydrogen, argon, or hydrocarbon gas, possibly by intercalation. The work we have carried out under a variety of conditions and with butane and other feed gases and mixtures with argon suggest, in fact, that more complex forms of carbon can be produced by this technique. Films of a similar nature can also be produced by sputtering a graphite target using argon gas in a triode system[lO]. No evidence has been obtained so far of the decomposition of crystallites in films prepared using the triode system. CAR26:1-l
DETAILS
2.1 Materials A 4.5kg butane gas cylinder (calor gas) was used for preparing all the films, and zero-grade argon gas was also used. Films were prepared on ordinary catering foil and on glass microscope slides conforming to B.S. 3836-1974. FOT transmission electron microscopy the carbon films were deposited on 99.99% pure single-crystal sodium chloride cleaved to give a flat surface. The sodium chloride was obtained as off-cuts from Hilger Analytical Limited, Margate, Kent, England. After depositing the carbon on the sodium chloride substrates, the sodium chloride was dissolved away in distilled water and the films were mounted onto copper electron microscope grids. CR39 lens material was also coated with a carbon film, but these samples were difficult to analyze because of degassing of the substrate in the U.H.V. analysis system. 2.2 Sample preparation Substrates of glass, CR39, aluminium foil, and sodium chforide were attached to a copper substrate holder using clamps. This assembly was then mounted onto a water-cooled plate over the FAB source (Ion Tech Ltd., type: FAB 110-2). The sub547
A. G. FITZGERALDetal.
548
strate was oscillated through the beam of material from the FAB source so that equal exposure was received by all the substrates. The substrates were cleaned by fast atom bombardment, using argon gas only, before the carbon film was deposited using butane or butane-argon gas mixtures. Table 1 details the parameters used on the FAB source for cleaning substrates and depositing the carbon films. The vacuum chamber used for this work is essentially a sputtering system fitted with a fast atom bombardment source for cleaning substrates. Modification of the gas handing system allowed a variety of gases to be admitted into the FAB source including gas mixtures using a crude needle valve and flow meter. The vacuum chamber was evacuated to background pressures of -1tY6 Torr using a cryopump and rotary pump. A mass spectrometer was routinely used to monitor background gases in the vacuum system. The major contaminant was water vapor with a partial pressure of approximately 5 x lo-’ Torr, hydrocarbon cont~ination normally being an order of magnitude lower. During sample cleaning and preparation, pressures were between 1O-2 and BY3 Torr depending on conditions and gas flows, see Table 1. Deposition rates were found to be extremely low, estimated to be between 5 and 10 8, min-r depending on conditions. Film thickness measurements were taken using a Talystep instrument on steps produced using a dry photoresist material (Letraset brand) as a subfilm mask that could be easily removed in acetone after depositing the carbon films, The Talystep instrument was used to measure the step height from the A/10 optical flat and the deposited film. However, the time to grow a sufficiently thick film for
accurate measurement by this method was prohibitively long and only estimated values were obtained. A quartz crystal thin film thickness monitor was also used, but results were rather poor, presumably because of the low deposition rate, the nature of the film being produced, and difficulty in obtaining good correction factors (tooling factors). The FAB source also showed signs of carbon building up on the electrodes and walls inside the source. This carbon was extremely fine powder but was easily removed by a very fine soft brush and a vacuum cleaner. If the carbon was allowed to accumulate, the FAB source showed erratic discharge behavior and would be switched off by the current overload protection on the power supply. Analysis of the carbon from inside the FAB source is being undertaken in a separate study to be presented later. 2.3 Analysis of films The carbon films were examined in a V.G. HBlOO ultra-high-vacuum scanning electron microscope (UHV-SEM) and JEOL 1OOCscanning transmission electron microscope (STEM). The HBlOO SEM was fitted with a CLAM 100 electron spectrometer for surface analysis by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). For surface analysis, the carbon films were deposited on glass slides and aluminium foil. Portions of these substrates were cut to a diameter of 1 cm and mounted on a stainless steel stub, Aluminium foil was used as a substrate to reduce charging in the electron beam during Auger eiectron spectroscopy measurements. Surface charging effects were also observed during X-ray photoelectron spectroscopy and resulted in the displacement of X-ray photo-
Table 1. Conditions used for producing carbon films with the FAB 110-2 fast atom bombardment source N(E)
(a) FAB cleaning of substrates Pressure: 2.1 X 10-j mhar @ - 35 seem argon* Power: Time: Oscillation: Distance:
Oscillation: Distance:
Power: Time: Oscillation: Distance:
only
8.0 x lo-) mbar @ -50 seem* 33.8 W = 750 V, 45 mA 15-100 min, (- 150-1000 A) (i.e., - 10 A min-‘) 2 20” (22.5 in.) @.-- 1%Hz frequency 100 mm from top of FAB source
Cc) Deposition by FAB-butane Pressure:
1s
49.5 W = 1.1 kV, 45 mA 3 min t 20” (52.5 in.) @ - % Hz frequency 100 mm from top of FAB source
(b) Deposition by FA B-butane
Pressure: Power: Time:
c
f
b
and argon - 50:50 mixture
5.9 x 10mJ mbar t@ - 45 seem total flow* 31.2 W = 650 V, 48 mA 15-100 min (- 75-500 A) (i.e., - 5 A min-I) + 20” (~2.5 in.) @ - 1%Hz frequency 100 mm from top of FAB source
*NJ. For these results, a crude flow controller with a single-flow meter has been used to measure flow rates. Gas flow ratios were set using precision needle valves hence, the ratios and total flows are approximate. A more sophisticated flow controller is to be installed shortly.
t 0
I
200
1
,
400
600
a00
Binding
Energy
I
1
1000
1
1200
faV)
Fig. 1. X-ray photoelectron spectrum from a carbon fiim {a) prepared using calor gas in a FAB source, (b) this film after a 30s argon ion etch.
549
Carbon films N(E)
a
b I
I
288
292
I
1 280
284 Binding
Energy
(eV)
Fig. 2. The carbon 1s X-ray photoelectron peak from a film prepared with a FAB source using (a) calor gas, (b)
a mixture of calor gas and argon. electron peaks from their true position. To determine the exact position of these peaks the films were given a flash of gold and the Au 4f,,, peak was recorded as a reference to establish the magnitude of this peak displacement due to charging. Throughout these measurements the Au 4f,,Z peak was situated at 83.9 eV for gold films deposited directly onto a stainless steel stub. The carbon films examined in the STEM were deposited on rock salt. The film was floated off this substrate in distilled water and scooped onto copper electron microscope grids.
(a)
Fig. 3. Electron diffraction pattern area of a film prepared by deposition with calor gas.
3. RESULTS
AND
from an amorphous from a FAB source
DISCUSSION
3.1 X-ray photoelectron spectroscopy A large number of X-ray photoelectron spectra were recorded from the different film preparations. Figure 1 is an X-ray photoelectron spectrum obtained over the range of binding energies between zero and approximately 1100 eV from a film deposited using calor gas in a FAB source. Similar spectra were obtained from films deposited on aluminium foil and from films prepared with a mixture of calor gas and argon in the FAB source. These spectra confirmed the presence of carbon and oxygen at the film surface. The carbon 1s peaks from films prepared by deposition from calor gas and from the calor gas-argon mixture in the FAB source have been examined at higher resolution and are shown in more detail in Fig. 2. The C 1s peak from films prepared from calor gas in the FAB source is situated at 284.6 eV. This
(b)
Fig. 4. (a) Electron micrograph from crystallite in a carbon film prepared by deposition from a FAB source with calor gas. (b) Electron diffraction pattern from this crystallite. The pattern can be attributed to either the alpha-carbine or the Ries Crater forms of carbon. The zone axis of the pattern for both is (001).
A. G.
550
FITZGERALD
et al.
Table 2. Interplanar spacings from a polycrystalline region of the film-comparison with interplanar spacings in carbon crystal structures o-carbine Observed d(A) 4.38 2.52 2.22 1.66 1.47
Fig. 5. Polycrystalline electron diffraction pattern from an area of film prepared by deposition from a FAB source with calor gas.
peak was found at 284.5 eV in films prepared with a calor gas-argon mixture in the FAB source. These binding energies for the C 1s peak should be compared with binding energies of 284.5 eV obtained for C 1s a-C:H films, 284.3 eV for graphite, and 284.4 eV for diamond. To investigate the nature of the oxygen peak on the surface each carbon film was given a light argon ion etch (Fig. 1). The spectrum obtained after this etch from the film deposited from the FAB source using calor gas is shown in Fig. lb. The oxygen peak was virtually removed by this light etch. The oxygen is therefore present only at the surface of the carbon film. Similar results were obtained with the films prepared with a mixture of calor gas and argon in the FAB source. 3.2 Transmission electron microscopy and diffraction Both film preparations the scanning transmission
had a similar structure in electron microscope, bas-
(a)
d(A) (4.465 { (4.256 2.542 2.232 1.488
hkl
Ries Crater d(A)
110 (4.465 103 1 (4.256 301 2.536 220 2.232 1.688 1.495 330
graphite
hkl
d(A)
hkl
110 111 301 220 140 227
1.68 -
004 -
ically an amorphous film (Fig. 3) with small platelike lamellar crystallites distributed over the surface[9]. One of these crystallites is shown in Fig. 4a. Electron diffraction patterns from this type of crystallite have been identified as originating from the alpha-carbine form of carbon (Fig. 4b). Polycrystalline electron diffraction patterns have also been obtained from areas of the film (Fig. 5). The spacings of the interatomic planes obtained from measurement of these polycrystalline ring diameters are listed in Table 2 with interatomic plane spacings from graphite, alpha carbine, and the Ries Crater form of carbon[6,9]. These polycrystalline regions also appear to be either alpha-carbine or the Ries Crater form of carbon. Prolonged observation of carbine crystals has shown that some of these crystals are sensitive to the electron beam in the transmission electron microscope. Figure 6 shows electron micrographs obtained from a carbine crystal with a space of a few minutes between exposures. Areas of the crystal show enhanced electron transmission and it appears that some of the crystal material has evaporated. Similar effects, with the formation of voidlike regions, have been observed during the decomposition of cadmium hydroxide crystals in the electron microscope to form cadmium oxide[ 121.
0))
Fig. 6. Electron micrograph from a crystallite that has been identified as either alpha-carbine or the Ries Crater form (a) taken immediately after exposure to the electron beam in the STEM, (b) after a period of 2 min electron irradiation, and (c) part of the area in this crystallite at higher magnification after 2 min electron irradiation.
Carbon films
3.3 Auger electron spectroscopy Auger spectra obtained from both carbon film preparations confirmed the XPS observations that oxygen was present on the film surface and could be removed with a light argon ion etch (Fig. 7). Figure 8 shows the detailed structure of the carbon KW Auger spectra from the two film preparations. The main difference between Auger electron spectra from the surfaces of graphite, diamond, and hydrogenated carbon films occurs in the Auger line shapes for the three forms. This variation in line shape reflects the different band structure of the three forms. The HBlOO UHV-SEM can be used to obtain Auger electron spectra from areas with diameters of the order of 10 nm. This ability to analyze small areas is important where there is a possibility that variations in composition or microstructure occur across the film surface. Electron microscopy and diffraction studies in this investigation have shown that crystall&es are scattered across these film surfaces. It was important therefore to investigate the form of the Auger line shape obtained from small areas of the surface, of the order of 2 km, that appeared as small particles and protuberances on the surface in the secondary electron image. Figure 8 shows Auger spectra taken from both film preparations from general areas of 100 p,rn in diameter and from particles of the order to 2 pm in diameter lying on the surface. The Auger electron spectra from the particles in each case show a shoulder that is similar to that obtained by Moravec and Orent[8] from ion beam deposited
551
carbon films. This shoulder was not present in spectra obtained from general areas of the order of 100 pm in diameter. When each film was argon ion etched to remove oxygen from the surface, this shoulder also appeared in spectra taken from 100 Frn diameter areas.
4.
CONCLUSIONS
X-ray photoelectron spectroscopy and Auger electron spectroscopy show that oxygen is present on the surface of carbon films prepared with the FAB
source under the conditions discussed in this study. The binding energy of C 1s photoelectron peak in these films is located at a position characteristic of hydrogenated carbon films. Transmission electron microscopy has shown that these carbon films are largely amorphous with small crystallites scattered across the film surface. The majority of these crystallites have been identified as either the alpha-carbine or the Ries Crater forms of carbon. An interesting effect observed during transmission electron microscope studies is the gradual decomposition of some of these crystallites in the electron beam. Alpha-carbine and Ries Crater car-
a b
b
I-C KLL
0
d
e
KLL
a
I
225
I
250 Electron
1 100
I
I
300
Electron
I 700
500
Energy
(eV)
Fig. 7. Auger electron spectrum from a carbon film (a) prepared using a mixture of calor gas and argon, (b) this film after a 60s argon ion etch.
I
1
275 Energy
300 (eV)
Fig. 8. Structure of the KVV carbon Auger peak obtained from a carbon film prepared with a FAB source from (a) a 5+m diameter crystallite (calor gas), (b) a lOO-pm area (calor gas), (c) a 2-pm diameter area (calor gas plus argon), (d) a 200~pm diameter area (calor gas plus argon), and (e) a lOO-p.m diameter area calor gas plus argon) after a 60s argon ion etch, An arrow indicates the position of the shoulder that develops on Auger peaks after intense electron irradiation or an argon ion etch.
552
A. G. FITZGERALD et al.
bon have complex hexagonal crystal layer structures. It is possible that during the growth of these crystallites, hydrogen, argon, or a hydrocarbon gas is trapped between the layers. This intercalation is commonly observed in graphite and many other layer structures such as molybdenum disulphide[l3]. Prolonged electron irradiation is likely to lead to the evolution of this intercalated material. A shoulder has been observed in KW Auger spectra from particles on the surface. This shoulder is characteristic of the whole film surface after argon ion etching. In the case of ion and small area electron bombardment, an intense beam of particles is directed at the surface. During the acquisition of Auger spectra from particles on the surface the electron beam is scanned over a 2-km diameter area giving rise to intense electron irradiation over this small area. During argon ion irradiation the complete specimen surface is irradiated. Transmission electron microscopy observations suggest that at high irradiation intensities there is evolution of gas from the crystallite surface. It is probable that the appearance of this shoulder in the Auger spectra is therefore also associated with the decomposition of the hydrocarbon material present at the surface of the film and with the release of intercalated material from alpha-carbine crystallites after high intensity particle irradiation. Acknowledgments-Mr.
P. A. Moir wishes to thank SERC for a research studentship. Mr. G. A. Dederski is grateful
for funding from the University of Leeds, Department of Physics during his training period on secondment from Sheffield Citv Polvtechnic. Deoartment of Chemistrv. Dr. M. Simpson is grateful to ‘SERC for supporting him.
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