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Formation of Si3N4 by nitrogen implantation into SiC
Surfaceand Coatings Technology83 (1996) 128-133
Formation of S&N4 by nitrogen implantation into Sic S. Miyagawa a, K. Baba b, M. Ilceyama a, K. Saitoh a, S. Nakao ‘, Y. Miyagawa a a National Industrial Research Institute of Nagoya, 1-l Himte-cho, Kita-ku, Nagoya 462, Jqm b Technology Centre qf Nagasaki, 2, Ikeda, Ohmurn, Nagasaki 8.56, Japm
Abstract Polycrystalline Sic and a bilayer of thin Zr depositedon Sic (Zr-Sic) were implantedwith 50keV 15Nions at fluencesin the range (0.25-1.5) x 1OL8ionscrne2 at elevatedtemperatureup to 1100“C. After implantation, the depth profiles of the elements were measuredusing Rutherford backscattering spectroscopy(RBS), nuclear reaction analysis (NRA) and Auger electron spectroscopy(AES). The structure and chemicalbonding state of the implanted layer were investigatedby glancingangleX-ray diffraction (G-XRD) and X-ray photoelectron spectroscopy(XPS). It wasfound that the maximum concentrationand halfwidth of the nitrogen profile implantedat 1100“C werestrongly decreased in comparisonwith thoseat room temperature,and nitrogen implantation into Sic at 1100“C! resultedin a compositelayer of /?-S&N, and Sic. In Zr-Sic, the interfacial reaction of Zr and Sic was observedat high temperature,and a compositelayer of ZrC, /?-S&N, and Sic wasformed by nitrogen implantation. Keywords:Sic; Si3N4;Nitrogen implantation; Zr-SiC
1. Introduction Ion implantation into ceramics and into bilayer systems consisting of thin metal films on ceramics is a powerful tool for the synthesis of new phases and the modification of the near-surface properties of ceramics. Silicon carbide is a prospective material due to its potential application in diverse technological fields, e.g. as a high-temperature, wide-band-gap, semiconducting material or in high-temperature structural applications. Ion implantation into Sic has been widely investigated [l-6]; it is well known that Sic is amorphized at relatively
low
deposited
energy
during
room-
temperature implantation and a reduction in hardness results_ frsrn~ _the amorphization of the Sic surface. Recently, it has been shown using X-ray photoelectron spectroscopy (XPS) that Sic implanted by nitrogen ions at room temperature is composed of an intermediate state associated with SiC,N,, ruling out the coexistence
of two distinct phases Sic and S&N, [3,5,6]. However, &systematic studies have been performed on nitrogen implantation into Sic at high temperature [4], where a more marked replacement of carbon by nitrogen atoms may be expected as such a change is thermodynamically
favoured. In this paper, in order to study the formation
process
of composites of Sic, Si,N, and ZrC on Sic, nitrogen
implantation
at temperatures
up to 1100 “C was per-
0257-8972/96/S15.00 0 1996 El sevier Science S.A. All rights reserved ccnr 11117 Qn???IL)~\r)lQ11 1~
formed for Sic and a Zr-SiC bilayer system. The depth profiles of the elements were measured using Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA) and Auger electron spectroscopy (AES). The chemical state and structure of the implanted surface were evaluated by XPS and glancing angle X-ray diffraction (G-XRD).
2. Experimental
details
Samples were polycrystalline P-Sic and a bilayer system consisting of thin Zr films on Sic (&-Sic). Polycrystalline P-Sic, provided by Toshiba Ceramic Co., Ltd., had a thickness of about 100 pm deposited on a carbon substrate by the CVD method. Sic samples had near-theoretical density (3.2 g cmm3) and the size of the specimens was 15 mm x 15 mm x 0.5 mm. Zr-SiC samples were prepared by depositing a Zr (99.9%) layer of 200 or 400 A thickness onto an Sic substrate by electron beam deposition at a pressure of 7 x 10m7 Torr. Sic samples were optically polished and rinsed in acetone and alcohol with an ultrasonic cleaner before nitrogen implantation and Zr deposition. Nitrogen implantation was performed with a massanalysed 15N2+ beam (energy, 100 keV). During implantation, samples on an MO plate were heated to 1100 “C by electron beam bombardment on the back side of the
S. Miyagawa et al./Surface and Coatings Technology 83 (1996) 128-133
sample holder using an electron beam gun (4 kV, 200 mA). The substrate temperature during implantation was monitored by an infrared thermometer, which was calibrated by a W-Re thermocouple embedded in an Sic plate. The beam current was measured using a Faraday cup in front of the sample holder at intervals of a fixed period. The implanted area was 7 mm in diameter with a raster scanned beam. The beam current density of 15Nzf ions was maintained in the range 4-6 uA cm-‘, and the total implantation fluences were in the range (0.25-1.5 x 1018 ions crnw2 for 15N ions with an energy of 50 keV. The implantation chamber was evacuated to 8 x lo-‘Torr by cryopumping and the residual gas pressure was below 5 x 10m7 Torr during implantation at elevated temperature. After nitrogen implantation, samples were examined by RBS, NRA and AES. RBS analyses were conducted with 1.8 MeV He ions and a scattering angle of 170”. AES, combined with 3 keV Ar ion sputtering, was also used to obtain the atomic concentration in Sic. NRA measurements were performed using a proton beam with a fixed energy of 450 keV from a tandem pelletron accelerator. Details of the NRA measurements have been given elsewhere [7]. Briefly, the proton energy incident on the sample surface was regulated in the range 420-480 keV by directly supplying a retardation or acceleration potential between -30 and +30 kV to a sample holder which was insulated from the ground. The potential supplied to the sample holder was computer controlled by signals from the current monitor. yRays (4.43 MeV) emitted by the nuclear reaction 15N(p,ay)12C at 429 keV were detected with an NaI(T1) scintillation detector. The energy resolution of the measurements is mainly determined by the resonance width and energy stability of the proton beam. Consequently, I 5000
I
3
the overall energy resolution was estimated to be about 370 eV and thi! value corresponds to a depth resolution of about 40 A at the surface. X-Ray photoelectron spectra were recorded to identify and quantify the elements and chemical bonding states in the nitrogenimplanted layer and depth analyses were obtained by sputtering the surface intermittently with an Ar ion beam of 2 keV. G-XRD equipment (Cu Ka; 40 kV; 25 mA; incident angle of 2”) was used to examine the crystal structure of the implanted layer. The surface topography was observed by scanning electron microscopy (SEM).
3. Results and discussion 3.1. Sic samples
Fig. 1 shows the Rutherford backscattering spectra of 1.8 MeV He ions incident on an SIC sample before and after nitrogen implantation at room temperature and 1100 “C with a fluence of 7.5 x 101’ ions cmH2. It is obvious that the maximum atomic concentration and width of the depth profile of nitrogen implanted at 1100 “C are reduced significantly in comparison with the profiles implanted at room temperature. For the room-temperature implantation, the maximum nitrogen atomic concentration exceeds by 50% the implantation fluence of 5 x 101’ ions cmv2 and dome-shaped blisters with a diameter of 2-5 urn are observed by SEM. The broadening of the nitrogen profile of the roomtemperature implantation is correlated with the formation of nitrogen gas bubbles and blisters in the implanted layer [S]. However, it should be noted that nitrogen I
,
I
/
I
50 keV 15N --) Sic
-
7.5 x1017 ions/cm” 4000
129
i
unimplanted RT llooOc
3000
E s o 2000
040
60
80
100 CHANNEL
120
140
160
180
NUMBER
Fig. 1. He (1.8 MeV) Rutherford backscattering spectra of Sic before and after 50 keV 15N implantation with a fluence of 7.5 x 1Ol7 ions cmm3. Implantation at room temperature and 1100 “C.
130
S. Miyagawa ef aLlSurface axd Coatings Technology 53 (1996) 128-133
implanted at 1100“C does not diffuse into the deeper layer of Sic. In order to evaluate the depth profiles of nitrogen implanted into Sic at high temperature in detail, NRA excitation curves were obtained for different fluences up to 1100 “C. The excitation curves were converted to the nitrogen depth profiles using iterative simulation analysis programs [7], and the results at 1100“C are shown in Fig. 2. In the figure, the horizontal axis shows the depth measured by atom number per unit area (cm2), and, for instance, 1000x 1Ol5atoms crnd2 corresponds to 1040A in stoichiometric Sic. The nitrogen depth profiles show a nearly gaussiandistribution for a fluence of 0.25 x 1Ol8 ions cmm2.The maximum nitrogen concentration saturates at a level of around 40% above the fluence of 0.5 x 1018ions crnv2 at a depth near the mean projected range of 50 keV 15N in Sic, and the profile shifts towards the surface with increasing implantation fluence. Using the SASAMAL simulation code, which takes into account the diffusion of the delocalizedatoms in the implanted layer and sputtering, the projected range of 50 keV 15N ions in Sic is 930 A for a low fluence [9], and this value agreeswell with the experimental value. The shift of the nitrogen profile towards the surface during implantation at high fluence is mainly ascribed to surface recession due to sputtering. The broadening of the nitrogen profile of the roomtemperature implantation was confirmed by NRA measurement (not shown in the figure). To determine the depth profiles of the atomic concentrations of C, Si, 0 and N atoms, AES measurements on Sic implanted at 1100“C with a fluence of 1.5 x 10” ions cm-’ were conducted (Fig. 3). The carbon concentration is significantly increased at the surface. Carbon depletion occurs in a subsurfaceregion extending to the I
/
I
50keV 75N+
1 Y. 1018 .-
SIC
80 T @
*’
50 keV N +
SIC (11OO'C)
0 0
0
IO
20 SPUTERING
30
40
TIME (Mln)
Fig. 3. Depth profiles of Auger signals from Sic implanted with 50 keV “N at a fluence of 1.5 x 10” ions cmq2 at 1100 “C.
depth enriched with nitrogen, whereas changes in the silicon concentration caused by nitrogen implantation are small. These features have also been reported in a previous study on amorphous Sic films implanted with nitrogen at room temperature [6,7]. The Si-C bonds are broken by nitrogen bombardment; carbon depletion is caused by the preferential migration of free carbon atoms produced by nitrogen ion bombardment due to the formation of strong Si-N bonds, and thus carbon is gradually substituted with nitrogen. The crystal structure of the nitrogen-implanted layer was measured by G-XRD as a function of the implantation temperature and nitrogen fluence. Fig. 4 shows the XRD patterns of Sic implanted at 1100“C with fluences of 0.5 x 10” and 1.5 x 1018ions cms2. In Fig. 4, XRD peaksfrom P-S&N4are obvious. The peak intensity from P-S&N, increaseswith increasing nitrogen fluence and above 0.75 x 1018ions crnm2the peak height of P-S&N4 is saturated. No peaks from Si3N4 implanted in Sic below 930 “C are observed. These results indicate that a high temperature process during implantation is i 3 > 7
500 DEPTH
1500 1000 ( 1015 atoms.cm.2)
2000
Fig. 2. Nitrogen depth profiles in Sic obtained by NRA. The implantation temperature was 1100 “C and the implantation fluence was varied: 0.25 x lOIs, 0.5 x 101*, 0.75 x 10” and 1.0 x 10” ions cm-‘.
-
Fig. 4. XRD patterns of Sic implanted with lSN ions at fluences of 0.5 x lo’* and 1.5 x 10” ions crnm2, Implantation temperature, 1100 “C.
S! Miyagawa 6000
et al./Surface
and Coatings
I
I
50 keV ‘5N ++
Technology
60
80
120
Fig. 5. He (1.8 MeV) Rutherford backscattering spectra of Zr(400 &-Sic 1.5 x 1O1* ions cm-‘. Implantation temperature, 1100 “C.
necessary to promote the formation of the j-S&N4 phase in Sic, which is proportional to the amount of retained nitrogen. For high-temperature implantation, it may be considered that the remaining carbon in the layer of carbon depletion exists mainly as Sic and, as a result, a composite of /-S&N, and Sic can be formed on the Sic surface by 1100 “C implantation. The reduction in the nitrogen concentration in the profile implanted at 1100 “C compared with that at room temperature can be interpreted in terms of the re-emission of excess nitrogen through the grain boundary during the formation of /?-S&N,, in Sic. 3.2. Zr-Sic
.---..-. _--
unimplanted ,.. x ,o’8
-
1.5x
140
CHANNEL
160
180
,
lo’*
200
220
240
NUMBER
before and after 50 keV “N implantation
60-
1
1
Zr / SIC (1100°C)
-2
at fluences of 1.0 x 10” and
1
/
-
50
60
Unimplanted
@
bilayer system
Fig. 5 shows the Rutherford backscattering spectra of Zr-SiC samples implanted at 1100 “C with fluences of 1.0 x 1~018and 1.5 x 1O1’ ions cmm2, and of an unimplanted area on the same sample. The Zr layer (400 A) is sputtered by nitrogen implantation and the amount of Zr on Sic decreases with increasing nitrogen fluence. Since the sputtering yield of Zr by 50 keV 15N ions is estimated to be 0.24 atoms ion-r [lo], the thickness of 400 A is removed with a fluence of 7 x 1Ol7 ions cmm2. However, a considerable amount of Zr still remains on the SIC surface after higher fluence implantation. The remaining Zr on Sic is due to the interdiffusion of Zr with the Sic substrate at high temperature and the formation of a ZrC layer rather than ballistic mixing, as shown later. The depth proties of the atomic concentrations of N, C, Si, Zr and 0 atoms, measured by AES, are shown in Fig. 6. Figs. 6(a) and 6(b) show the depth profiles of an unimplanted area and Zr(200 A)-SiC implanted at
131 I
Zr(400&SiC
100
128-133
I
1100 oc
40
83 (1996)
0
0
10
20
30
SPUTTERING
40
TIME (Min)
80 T @
50keVN+Zr/SiC(ll0O”C)
0
10
20
SPUTTERING
30
40
TIME (Min)
Fig. 6. Depth profiles of Auger signals from Zr-SiC before (a) and after (b) implantation with 50 keV 15N at a fluence of 1.5 x 1Ol8 ions crnm2 at 1100 “C.
132
S. Miyagawa et al./Sqface and Coatings Technology 83 (1996) 128-133
1100“C with a fluence of 1.5 x 101*ions cmm2respectively. The horizontal axis indicates the sputtering time with Ar ions; the Ar ion current density is different for Figs. 6(a) and 6(b). In unimplanted Zr-Sic, the’carbon concentration is significantly increasedin the subsurface -layer, and Zr atoms diffuse into the Sic substrate.The interfacial reaction between Zr and Sic releasescarbon and silicon which interdiffuse with the Zr layer and form ZrC. Such an interfacial reaction has also been observed in the Ti-SiC bilayer system [ll]. A slight increase in nitrogen concentration in the subsurfaceregion is mainly causedby reaction with the residual nitrogen gas during implantation, In implanted Zr-Sic, most of the Zr is sputtered and the carbon concentration increasesat the surface and decreasesin the subsurface region with increasing nitrogen fluence. The depth profiles of N, C, Si and 0 atoms are similar to those of the Sic samples. Carbon loss from the subsurfaceregion can be explained by diffusion to the surface during nitrogen implantation as observed for Sic. The crystal structure of the nitrogen-implanted layer was measured by G-XRD. Fig. 7 shows the XRD patterns of Zr-SiC implanted at 1100“C with fluences of 0.5 x 1Or8 and 1.5 x 101* ions cmm2. In Fig. 7, XRD peaks from /?-S&N, are observed in addition to the ZrC( 111) peak. Sic decreaseswith an increase in the nitrogen fluence. Diffraction lines of zirconium silicides are not observed in these XRD scans. To verify the chemical bonding state of Zr(200 A)-Sic implanted at 1100“C with a fluence of 1.5 x 1O1* ions crnm2, XPS analyseswere performed. Figs. 8(a) and 8(b) show the peak shifts of the Si 2p and C 1s spectra respectively as a function of the Ar ion sputtering time. The Si 2p peak at the surface layer shifts to higher binding e.nergy,’indicating that the surface is partly oxidized. The Si 2p spectrum in the subsurface layer is composedof Si-N’( 102.0eV) and Si-C (100.2 eV) peaks. Si-C becomes the main component with increasing Ar ion sputtering time.
36
38 110
106
102 98 BINDING ENERGY (ev)
94
294
290
286
282
BINDING ENERGY (ev)
Fig. 8. Peak shifts of Si 2p (a) and C Is (b) spectra in nitrogenimplanted Zr-Sic.
The C 1s spectrum in the surface layer is composed of C-C (285 eV) bonds which correspond to carbon in the graphite phase.The C 1s spectrum in the subsurface layer is much broader than the spectrum for the deeper layer, A shift to lower binding energy of the C is peak near the surface layer is ascribed to the C-Zr (281.1 eV) and C-Si (283.5 eV) bonds. The C-Si peak becomesthe main component with increasing Ar ion sputtering time. Peak deconvolution of the C 1s spectrum with curve fitting analysis indicates that the C 1s spectrum is composed of C-C and C-Si in the intermediate region. Considering the appearance of C-C and C-Si bonding, the implanted layer may be composed of a composite of Sic and free carbon. The N 1s spectrum displays a peak located at 397.8eV, which is typical of the binding energy of N in Si,N4, and no peak corresponding to N-C bonds is observed. 4. Conclusions
Fig. 7. XRD .patterns of Zr-SiC implanted with “N ions at fluences of;0.5 x 10” and 115x lOI4 ions’cm-‘. j Implantation temperature, 1100 “C.
Polycrystalline /?-Sic and a bilayer consisting of thin Zr on Sic (Zr-Sic) were implanted with 50 keV 15N ions to a total fluence of 1.5 x lo’* ions cmM2 at an elevated temperature up to 1100“C. Nitrogen depth profiles and surface structures were measured using NRA, RBS, AES, G-XRD and XPS. The following results were obtained. 1. The maximum concentration and halfwidth of the nitrogen profile implanted at 1100“C were significantly decreasedin comparison with those at room temperature, and the formation of ,L?-S&N4 crystallites on the Sic surface implanted at 1100“C was confirmed by G-XRD analysis. 2. In the caseof Zr-Sic, the interfacial reaction of Zr
S. Miyagawa
et al./Surfaee
and Coatings
and Sic was observed at 1100 “C!, and a composite layer composed of ZrC, ,!?-S&N4 and Sic was formed by hightemperature implantation.
References [l]
J.M. Williams, C.J. McHargue and B.R. Appleton, Nucl. Instruct. 2091210 (1983) 317. [2] Y. Miyagawa and S. Miyagawa, Nucl. Inst~um. Methods B, 28 (1987) 27. [ 31 A. Nakao, M. Iwaki, H. Sakairi and K. Terasima, Nucl. Instrum. Methods,
Methods
B, 65 (1992)
352.
Technology
83 (1996)
128-133
133
[4] C.J. McHargue and J.M. Williams, Nucl. Instrum Methods B, SO/81 (1993) 889. [S] R. Kelly, I. Bertoti and A. Miotello, Nucl. Instrum. Methods B, so/s1 (1993) 1154. [6] N. Laidani, M. Bone& A. Miotello, L. Guzman, L. Calliari, M. Elena, R. Bertoncello, A. Glisenti, R. Capelletti and P. Ossi, J. Appl. Phys., 74 (1993) 2013. [7] Y. Miyagawa, K. Saitoh and S. Miyagawa, Nucl. Instr. Merhods, in press. [S] S. Miyagawa, S. Nakao, K. Saitoh, M. Ikeyama, Y. Miyagawa and K. Baba, J. A&. Phys., 78 (1995) 7018. [9] Y. Miyagawa, Y. Sakai, M. Ikeyama, K. Saitoh, S. Nakao and S. Miyagawa, Radiat. Eff., 1301131 (1994) 471. [lo] Y. Miyagawa, M. Ikeyama, K. Saitoh, S. Nakao and S. Miyagawa, Surf. Coat. Technol., in press. [ 111 M.B. Chamberlain, Thin Solid Films, 72 (1980) 305.
Formation of S&N4 by nitrogen implantation into Sic S. Miyagawa a, K. Baba b, M. Ilceyama a, K. Saitoh a, S. Nakao ‘, Y. Miyagawa a a National Industrial Research Institute of Nagoya, 1-l Himte-cho, Kita-ku, Nagoya 462, Jqm b Technology Centre qf Nagasaki, 2, Ikeda, Ohmurn, Nagasaki 8.56, Japm
Abstract Polycrystalline Sic and a bilayer of thin Zr depositedon Sic (Zr-Sic) were implantedwith 50keV 15Nions at fluencesin the range (0.25-1.5) x 1OL8ionscrne2 at elevatedtemperatureup to 1100“C. After implantation, the depth profiles of the elements were measuredusing Rutherford backscattering spectroscopy(RBS), nuclear reaction analysis (NRA) and Auger electron spectroscopy(AES). The structure and chemicalbonding state of the implanted layer were investigatedby glancingangleX-ray diffraction (G-XRD) and X-ray photoelectron spectroscopy(XPS). It wasfound that the maximum concentrationand halfwidth of the nitrogen profile implantedat 1100“C werestrongly decreased in comparisonwith thoseat room temperature,and nitrogen implantation into Sic at 1100“C! resultedin a compositelayer of /?-S&N, and Sic. In Zr-Sic, the interfacial reaction of Zr and Sic was observedat high temperature,and a compositelayer of ZrC, /?-S&N, and Sic wasformed by nitrogen implantation. Keywords:Sic; Si3N4;Nitrogen implantation; Zr-SiC
1. Introduction Ion implantation into ceramics and into bilayer systems consisting of thin metal films on ceramics is a powerful tool for the synthesis of new phases and the modification of the near-surface properties of ceramics. Silicon carbide is a prospective material due to its potential application in diverse technological fields, e.g. as a high-temperature, wide-band-gap, semiconducting material or in high-temperature structural applications. Ion implantation into Sic has been widely investigated [l-6]; it is well known that Sic is amorphized at relatively
low
deposited
energy
during
room-
temperature implantation and a reduction in hardness results_ frsrn~ _the amorphization of the Sic surface. Recently, it has been shown using X-ray photoelectron spectroscopy (XPS) that Sic implanted by nitrogen ions at room temperature is composed of an intermediate state associated with SiC,N,, ruling out the coexistence
of two distinct phases Sic and S&N, [3,5,6]. However, &systematic studies have been performed on nitrogen implantation into Sic at high temperature [4], where a more marked replacement of carbon by nitrogen atoms may be expected as such a change is thermodynamically
favoured. In this paper, in order to study the formation
process
of composites of Sic, Si,N, and ZrC on Sic, nitrogen
implantation
at temperatures
up to 1100 “C was per-
0257-8972/96/S15.00 0 1996 El sevier Science S.A. All rights reserved ccnr 11117 Qn???IL)~\r)lQ11 1~
formed for Sic and a Zr-SiC bilayer system. The depth profiles of the elements were measured using Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA) and Auger electron spectroscopy (AES). The chemical state and structure of the implanted surface were evaluated by XPS and glancing angle X-ray diffraction (G-XRD).
2. Experimental
details
Samples were polycrystalline P-Sic and a bilayer system consisting of thin Zr films on Sic (&-Sic). Polycrystalline P-Sic, provided by Toshiba Ceramic Co., Ltd., had a thickness of about 100 pm deposited on a carbon substrate by the CVD method. Sic samples had near-theoretical density (3.2 g cmm3) and the size of the specimens was 15 mm x 15 mm x 0.5 mm. Zr-SiC samples were prepared by depositing a Zr (99.9%) layer of 200 or 400 A thickness onto an Sic substrate by electron beam deposition at a pressure of 7 x 10m7 Torr. Sic samples were optically polished and rinsed in acetone and alcohol with an ultrasonic cleaner before nitrogen implantation and Zr deposition. Nitrogen implantation was performed with a massanalysed 15N2+ beam (energy, 100 keV). During implantation, samples on an MO plate were heated to 1100 “C by electron beam bombardment on the back side of the
S. Miyagawa et al./Surface and Coatings Technology 83 (1996) 128-133
sample holder using an electron beam gun (4 kV, 200 mA). The substrate temperature during implantation was monitored by an infrared thermometer, which was calibrated by a W-Re thermocouple embedded in an Sic plate. The beam current was measured using a Faraday cup in front of the sample holder at intervals of a fixed period. The implanted area was 7 mm in diameter with a raster scanned beam. The beam current density of 15Nzf ions was maintained in the range 4-6 uA cm-‘, and the total implantation fluences were in the range (0.25-1.5 x 1018 ions crnw2 for 15N ions with an energy of 50 keV. The implantation chamber was evacuated to 8 x lo-‘Torr by cryopumping and the residual gas pressure was below 5 x 10m7 Torr during implantation at elevated temperature. After nitrogen implantation, samples were examined by RBS, NRA and AES. RBS analyses were conducted with 1.8 MeV He ions and a scattering angle of 170”. AES, combined with 3 keV Ar ion sputtering, was also used to obtain the atomic concentration in Sic. NRA measurements were performed using a proton beam with a fixed energy of 450 keV from a tandem pelletron accelerator. Details of the NRA measurements have been given elsewhere [7]. Briefly, the proton energy incident on the sample surface was regulated in the range 420-480 keV by directly supplying a retardation or acceleration potential between -30 and +30 kV to a sample holder which was insulated from the ground. The potential supplied to the sample holder was computer controlled by signals from the current monitor. yRays (4.43 MeV) emitted by the nuclear reaction 15N(p,ay)12C at 429 keV were detected with an NaI(T1) scintillation detector. The energy resolution of the measurements is mainly determined by the resonance width and energy stability of the proton beam. Consequently, I 5000
I
3
the overall energy resolution was estimated to be about 370 eV and thi! value corresponds to a depth resolution of about 40 A at the surface. X-Ray photoelectron spectra were recorded to identify and quantify the elements and chemical bonding states in the nitrogenimplanted layer and depth analyses were obtained by sputtering the surface intermittently with an Ar ion beam of 2 keV. G-XRD equipment (Cu Ka; 40 kV; 25 mA; incident angle of 2”) was used to examine the crystal structure of the implanted layer. The surface topography was observed by scanning electron microscopy (SEM).
3. Results and discussion 3.1. Sic samples
Fig. 1 shows the Rutherford backscattering spectra of 1.8 MeV He ions incident on an SIC sample before and after nitrogen implantation at room temperature and 1100 “C with a fluence of 7.5 x 101’ ions cmH2. It is obvious that the maximum atomic concentration and width of the depth profile of nitrogen implanted at 1100 “C are reduced significantly in comparison with the profiles implanted at room temperature. For the room-temperature implantation, the maximum nitrogen atomic concentration exceeds by 50% the implantation fluence of 5 x 101’ ions cmv2 and dome-shaped blisters with a diameter of 2-5 urn are observed by SEM. The broadening of the nitrogen profile of the roomtemperature implantation is correlated with the formation of nitrogen gas bubbles and blisters in the implanted layer [S]. However, it should be noted that nitrogen I
,
I
/
I
50 keV 15N --) Sic
-
7.5 x1017 ions/cm” 4000
129
i
unimplanted RT llooOc
3000
E s o 2000
040
60
80
100 CHANNEL
120
140
160
180
NUMBER
Fig. 1. He (1.8 MeV) Rutherford backscattering spectra of Sic before and after 50 keV 15N implantation with a fluence of 7.5 x 1Ol7 ions cmm3. Implantation at room temperature and 1100 “C.
130
S. Miyagawa ef aLlSurface axd Coatings Technology 53 (1996) 128-133
implanted at 1100“C does not diffuse into the deeper layer of Sic. In order to evaluate the depth profiles of nitrogen implanted into Sic at high temperature in detail, NRA excitation curves were obtained for different fluences up to 1100 “C. The excitation curves were converted to the nitrogen depth profiles using iterative simulation analysis programs [7], and the results at 1100“C are shown in Fig. 2. In the figure, the horizontal axis shows the depth measured by atom number per unit area (cm2), and, for instance, 1000x 1Ol5atoms crnd2 corresponds to 1040A in stoichiometric Sic. The nitrogen depth profiles show a nearly gaussiandistribution for a fluence of 0.25 x 1Ol8 ions cmm2.The maximum nitrogen concentration saturates at a level of around 40% above the fluence of 0.5 x 1018ions crnv2 at a depth near the mean projected range of 50 keV 15N in Sic, and the profile shifts towards the surface with increasing implantation fluence. Using the SASAMAL simulation code, which takes into account the diffusion of the delocalizedatoms in the implanted layer and sputtering, the projected range of 50 keV 15N ions in Sic is 930 A for a low fluence [9], and this value agreeswell with the experimental value. The shift of the nitrogen profile towards the surface during implantation at high fluence is mainly ascribed to surface recession due to sputtering. The broadening of the nitrogen profile of the roomtemperature implantation was confirmed by NRA measurement (not shown in the figure). To determine the depth profiles of the atomic concentrations of C, Si, 0 and N atoms, AES measurements on Sic implanted at 1100“C with a fluence of 1.5 x 10” ions cm-’ were conducted (Fig. 3). The carbon concentration is significantly increased at the surface. Carbon depletion occurs in a subsurfaceregion extending to the I
/
I
50keV 75N+
1 Y. 1018 .-
SIC
80 T @
*’
50 keV N +
SIC (11OO'C)
0 0
0
IO
20 SPUTERING
30
40
TIME (Mln)
Fig. 3. Depth profiles of Auger signals from Sic implanted with 50 keV “N at a fluence of 1.5 x 10” ions cmq2 at 1100 “C.
depth enriched with nitrogen, whereas changes in the silicon concentration caused by nitrogen implantation are small. These features have also been reported in a previous study on amorphous Sic films implanted with nitrogen at room temperature [6,7]. The Si-C bonds are broken by nitrogen bombardment; carbon depletion is caused by the preferential migration of free carbon atoms produced by nitrogen ion bombardment due to the formation of strong Si-N bonds, and thus carbon is gradually substituted with nitrogen. The crystal structure of the nitrogen-implanted layer was measured by G-XRD as a function of the implantation temperature and nitrogen fluence. Fig. 4 shows the XRD patterns of Sic implanted at 1100“C with fluences of 0.5 x 10” and 1.5 x 1018ions cms2. In Fig. 4, XRD peaksfrom P-S&N4are obvious. The peak intensity from P-S&N, increaseswith increasing nitrogen fluence and above 0.75 x 1018ions crnm2the peak height of P-S&N4 is saturated. No peaks from Si3N4 implanted in Sic below 930 “C are observed. These results indicate that a high temperature process during implantation is i 3 > 7
500 DEPTH
1500 1000 ( 1015 atoms.cm.2)
2000
Fig. 2. Nitrogen depth profiles in Sic obtained by NRA. The implantation temperature was 1100 “C and the implantation fluence was varied: 0.25 x lOIs, 0.5 x 101*, 0.75 x 10” and 1.0 x 10” ions cm-‘.
-
Fig. 4. XRD patterns of Sic implanted with lSN ions at fluences of 0.5 x lo’* and 1.5 x 10” ions crnm2, Implantation temperature, 1100 “C.
S! Miyagawa 6000
et al./Surface
and Coatings
I
I
50 keV ‘5N ++
Technology
60
80
120
Fig. 5. He (1.8 MeV) Rutherford backscattering spectra of Zr(400 &-Sic 1.5 x 1O1* ions cm-‘. Implantation temperature, 1100 “C.
necessary to promote the formation of the j-S&N4 phase in Sic, which is proportional to the amount of retained nitrogen. For high-temperature implantation, it may be considered that the remaining carbon in the layer of carbon depletion exists mainly as Sic and, as a result, a composite of /-S&N, and Sic can be formed on the Sic surface by 1100 “C implantation. The reduction in the nitrogen concentration in the profile implanted at 1100 “C compared with that at room temperature can be interpreted in terms of the re-emission of excess nitrogen through the grain boundary during the formation of /?-S&N,, in Sic. 3.2. Zr-Sic
.---..-. _--
unimplanted ,.. x ,o’8
-
1.5x
140
CHANNEL
160
180
,
lo’*
200
220
240
NUMBER
before and after 50 keV “N implantation
60-
1
1
Zr / SIC (1100°C)
-2
at fluences of 1.0 x 10” and
1
/
-
50
60
Unimplanted
@
bilayer system
Fig. 5 shows the Rutherford backscattering spectra of Zr-SiC samples implanted at 1100 “C with fluences of 1.0 x 1~018and 1.5 x 1O1’ ions cmm2, and of an unimplanted area on the same sample. The Zr layer (400 A) is sputtered by nitrogen implantation and the amount of Zr on Sic decreases with increasing nitrogen fluence. Since the sputtering yield of Zr by 50 keV 15N ions is estimated to be 0.24 atoms ion-r [lo], the thickness of 400 A is removed with a fluence of 7 x 1Ol7 ions cmm2. However, a considerable amount of Zr still remains on the SIC surface after higher fluence implantation. The remaining Zr on Sic is due to the interdiffusion of Zr with the Sic substrate at high temperature and the formation of a ZrC layer rather than ballistic mixing, as shown later. The depth proties of the atomic concentrations of N, C, Si, Zr and 0 atoms, measured by AES, are shown in Fig. 6. Figs. 6(a) and 6(b) show the depth profiles of an unimplanted area and Zr(200 A)-SiC implanted at
131 I
Zr(400&SiC
100
128-133
I
1100 oc
40
83 (1996)
0
0
10
20
30
SPUTTERING
40
TIME (Min)
80 T @
50keVN+Zr/SiC(ll0O”C)
0
10
20
SPUTTERING
30
40
TIME (Min)
Fig. 6. Depth profiles of Auger signals from Zr-SiC before (a) and after (b) implantation with 50 keV 15N at a fluence of 1.5 x 1Ol8 ions crnm2 at 1100 “C.
132
S. Miyagawa et al./Sqface and Coatings Technology 83 (1996) 128-133
1100“C with a fluence of 1.5 x 101*ions cmm2respectively. The horizontal axis indicates the sputtering time with Ar ions; the Ar ion current density is different for Figs. 6(a) and 6(b). In unimplanted Zr-Sic, the’carbon concentration is significantly increasedin the subsurface -layer, and Zr atoms diffuse into the Sic substrate.The interfacial reaction between Zr and Sic releasescarbon and silicon which interdiffuse with the Zr layer and form ZrC. Such an interfacial reaction has also been observed in the Ti-SiC bilayer system [ll]. A slight increase in nitrogen concentration in the subsurfaceregion is mainly causedby reaction with the residual nitrogen gas during implantation, In implanted Zr-Sic, most of the Zr is sputtered and the carbon concentration increasesat the surface and decreasesin the subsurface region with increasing nitrogen fluence. The depth profiles of N, C, Si and 0 atoms are similar to those of the Sic samples. Carbon loss from the subsurfaceregion can be explained by diffusion to the surface during nitrogen implantation as observed for Sic. The crystal structure of the nitrogen-implanted layer was measured by G-XRD. Fig. 7 shows the XRD patterns of Zr-SiC implanted at 1100“C with fluences of 0.5 x 1Or8 and 1.5 x 101* ions cmm2. In Fig. 7, XRD peaks from /?-S&N, are observed in addition to the ZrC( 111) peak. Sic decreaseswith an increase in the nitrogen fluence. Diffraction lines of zirconium silicides are not observed in these XRD scans. To verify the chemical bonding state of Zr(200 A)-Sic implanted at 1100“C with a fluence of 1.5 x 1O1* ions crnm2, XPS analyseswere performed. Figs. 8(a) and 8(b) show the peak shifts of the Si 2p and C 1s spectra respectively as a function of the Ar ion sputtering time. The Si 2p peak at the surface layer shifts to higher binding e.nergy,’indicating that the surface is partly oxidized. The Si 2p spectrum in the subsurface layer is composedof Si-N’( 102.0eV) and Si-C (100.2 eV) peaks. Si-C becomes the main component with increasing Ar ion sputtering time.
36
38 110
106
102 98 BINDING ENERGY (ev)
94
294
290
286
282
BINDING ENERGY (ev)
Fig. 8. Peak shifts of Si 2p (a) and C Is (b) spectra in nitrogenimplanted Zr-Sic.
The C 1s spectrum in the surface layer is composed of C-C (285 eV) bonds which correspond to carbon in the graphite phase.The C 1s spectrum in the subsurface layer is much broader than the spectrum for the deeper layer, A shift to lower binding energy of the C is peak near the surface layer is ascribed to the C-Zr (281.1 eV) and C-Si (283.5 eV) bonds. The C-Si peak becomesthe main component with increasing Ar ion sputtering time. Peak deconvolution of the C 1s spectrum with curve fitting analysis indicates that the C 1s spectrum is composed of C-C and C-Si in the intermediate region. Considering the appearance of C-C and C-Si bonding, the implanted layer may be composed of a composite of Sic and free carbon. The N 1s spectrum displays a peak located at 397.8eV, which is typical of the binding energy of N in Si,N4, and no peak corresponding to N-C bonds is observed. 4. Conclusions
Fig. 7. XRD .patterns of Zr-SiC implanted with “N ions at fluences of;0.5 x 10” and 115x lOI4 ions’cm-‘. j Implantation temperature, 1100 “C.
Polycrystalline /?-Sic and a bilayer consisting of thin Zr on Sic (Zr-Sic) were implanted with 50 keV 15N ions to a total fluence of 1.5 x lo’* ions cmM2 at an elevated temperature up to 1100“C. Nitrogen depth profiles and surface structures were measured using NRA, RBS, AES, G-XRD and XPS. The following results were obtained. 1. The maximum concentration and halfwidth of the nitrogen profile implanted at 1100“C were significantly decreasedin comparison with those at room temperature, and the formation of ,L?-S&N4 crystallites on the Sic surface implanted at 1100“C was confirmed by G-XRD analysis. 2. In the caseof Zr-Sic, the interfacial reaction of Zr
S. Miyagawa
et al./Surfaee
and Coatings
and Sic was observed at 1100 “C!, and a composite layer composed of ZrC, ,!?-S&N4 and Sic was formed by hightemperature implantation.
References [l]
J.M. Williams, C.J. McHargue and B.R. Appleton, Nucl. Instruct. 2091210 (1983) 317. [2] Y. Miyagawa and S. Miyagawa, Nucl. Inst~um. Methods B, 28 (1987) 27. [ 31 A. Nakao, M. Iwaki, H. Sakairi and K. Terasima, Nucl. Instrum. Methods,
Methods
B, 65 (1992)
352.
Technology
83 (1996)
128-133
133
[4] C.J. McHargue and J.M. Williams, Nucl. Instrum Methods B, SO/81 (1993) 889. [S] R. Kelly, I. Bertoti and A. Miotello, Nucl. Instrum. Methods B, so/s1 (1993) 1154. [6] N. Laidani, M. Bone& A. Miotello, L. Guzman, L. Calliari, M. Elena, R. Bertoncello, A. Glisenti, R. Capelletti and P. Ossi, J. Appl. Phys., 74 (1993) 2013. [7] Y. Miyagawa, K. Saitoh and S. Miyagawa, Nucl. Instr. Merhods, in press. [S] S. Miyagawa, S. Nakao, K. Saitoh, M. Ikeyama, Y. Miyagawa and K. Baba, J. A&. Phys., 78 (1995) 7018. [9] Y. Miyagawa, Y. Sakai, M. Ikeyama, K. Saitoh, S. Nakao and S. Miyagawa, Radiat. Eff., 1301131 (1994) 471. [lo] Y. Miyagawa, M. Ikeyama, K. Saitoh, S. Nakao and S. Miyagawa, Surf. Coat. Technol., in press. [ 111 M.B. Chamberlain, Thin Solid Films, 72 (1980) 305.