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Formation of carbon nitride films by high-energy nitrogen ion implantation into glassy carbon
Surface and Coatings Technology, 68/69 (1994) 616—620
616
Formation of carbon nitride films by high-energy nitrogen ion implantation into glassy carbon A. Hoffmana,b H. Ge11er~,I. Gouzmanab, C. Cytermannb, R. Brenerb, M. Kenny~’ a Chemistry Department, bsoljd State Institute, cMaterials Engineering Department, Technion, Ha(fa 32000, Israel dCSIRO Division ofApplied Physics, POB 218, Lindfield, NSW2O7O, Australia
Abstract Carbon nitride fl-C
3N4 thin films are presently attracting increasing interest both scientifically and technologically. This is due to their expected mechanical and tribological properties, superior to those of diamond. In the present work the nitriding process of the near-surface region of glassy carbon by high-energy nitrogen ion implantation has been investigated with particular emphasis on the implantation temperature and post-annealing processes. The 2. The implantations distribution were of the performed implanted at room nitrogen temperature and its bonding and at 400 states °Cusing have 25 and been studied 50 keY bynitrogen a number ions of complementary up to doses of 1 techniques: x 1018 cm Auger electron spectroscopy, secondary ion mass spectrometry and X-ray photoelectron spectroscopy (XPS). The possibility of carbon nitride phase formation and the effect of implantation on the glassy carbon microstructure was assessed by Raman measurements. Volume effects were studied by measurements of the step height between the implanted and unimplanted regions. The maximum amount ofnitrogen in the implanted layer obtained in the present study reaches 25—30 at.%. Annealing the RT implanted layer up to 500°Cdoes not result in a measurable diffusion of the implanted nitrogen. However, during annealing to 1000 °Ca diffusion of the implanted nitrogen occurs. Hot implantation at 400°Cresults in a broad and nearly homogeneous distribution of the implanted nitrogen with an average concentration of 18 at.%. XPS measurements indicate that hot implantation results in a preferred population of a rather covalent nitrogen bonding state in the implanted layer compared with that obtained after RT implantation and annealing.
1. Introduction The existence of a /~-C3N4stable phase, similar in structure to /3-Si3N4, with mechanical properties superior to those of diamond has been predicted by theoretical calculations [1,2]. Over the last few years a great deal of effort has been directed towards the formation of carbon nitride thin by a number of J3-C deposition processes [3—6]. Thefilms possibility of forming 3N4 by ion beam implantation can be encouraged by the successful formation of nearly stoichiometric Si3N4 by highenergy and high-dose nitrogen implantation into silicon [7—9]. A number of implantation parameters, such as implantation energy, ion dose, substrate temperature and subsequent thermal annealing, were shown to play a vital role in the proper compound phase formation. Recently, the possibility of carbon nitride formation by 500 eV N~nitrogen ion irradiation of graphite was investigated in our laboratory by in-situ X-ray photoelectron spectroscopy (XPS) with particular attention paid to the effect of implantation temperature and postannealing processes [10—12]. Analysis of the N(ls) core level line indicated the existence of three different carbon—nitrogen bonding states inhot thenitrogen room temperature (RT) implanted layer, whereas implantation at 500 °C resulted in a predominant population of a
0257—8972/94/$7.00 SSDI 0257-8972(94)08002-G
covalent bonding state. Such a nitrogen bonding state is expected to exist in the elusive /3-C 3N4 compound [1,2]. These results suggest that a covalently bonded carbon nitride compound may be formed by ion-beamassisted methods. In the present work our investigations of low-energy nitrogen ion implantation into graphite are high-energy extended to 2) and high-dose (up to 1 x 1018 ions cm (25 and 50 keY) nitrogen implantation into glassy carbon (GC). To the best of our knowledge this is the first study of the possibility of carbon nitride formation by high-energy ion implantation into a carbon substrate. The properties of the implanted layer are studied, with particular attention given to the effect of the implantation dose, temperature and post-annealing processes.
2. Experimental Substrates of GC (V25) were prepared from plates 2 mm thick supplied by Atomergic Chemetals. The manufacturer’s specifications quote a heat treatment temperature 2500°Cand a densitythebetween 1.5 and 3. of Prior to implantation GC substrates 1.55 gpolished cm were with 1 p.m diamond paste, ultrasonically cleaned in acetone and rinsed in deionized water.
© 1994
—
Elsevier Science S.A. All rights reserved
A. Hoffman et a!.
/
Formation of carbon nitride films
The nitrogen implantations were performed using a Freeman ion implanter scanning the sample through the ion beam. N~ ions accelerated to 50 or 25 keY were used. The ion beam current was 10 p.A with a diameter of 1 mm. The samples were implanted at RT using doses between 1 x 1017 and 1 x 1018 ions cm2. In order to obtain a homogeneously distributed nitrogen-implanted near-surface region, a high-dose double implantation using 25 and 50 keY nitrogen ions was performed. Annealings up to 1000 °Cwere performed in a vacuum furnace with a base pressure of 1 x iO~Torr. Annealing up to 500°C was performed in situ in the ultrahigh vacuum Auger electron spectroscopy (AES)/XPS analysis chamber with a base pressure of 1 x 10-8 Torr. Hot ion implantation was performed at 400 °C using ion energies of 50 keY and a dose of 6.2 x iO~N~cm2. In Table 1 all implantations into GC and thermal treatments conditions are summarized. The secondary ion mass spectrometry (SIMS) depth profiles of the as-implanted and annealed samples were measured in a Cameca IMS4f ion microscope. A 10 keY Cs + ion beam was used to simultaneously monitor the CsC + and CsN + ions. The concentration of nitrogen was calculated after normalization of the CsN~intensities to the CsC + signal and using a relative sensitivity factor derived from the 1 x 1017 N~cm2 RT implanted GC sample. Raman measurements were carried out using the 514.5 nm Ar line, and the scattered light was analysed with a SPEX 1403 double spectrometer. The Raman spectrum was measured in the 1000—2400 cm1 range using a 200 p.m2 laser beam spot and 200 mW power. From Raman measurements the possibility of carbon nitride phase formation and changes in the GC microstructure were assessed. The AES experiments were performed in a scanning Auger spectrometer (Perkin-Elmer, PHI model 590A) operated in the first derivative mode with a 6 V peakto-peak modulation voltage using a 3 keY, 1.0 p.A rast-
617
ered primary beam. Depth profiles were obtained by monitoring the C(KLL) and N(KLL) peak-to-peak signals during 3 keY Ar~ion-sputter etching. The sputter rate was determined by profilometer measurements. To calculate the nitrogen concentration as a function of depth the C(KLL) and N(KLL) Auger transitions sensitivity factors were calculated based on a cross-calibration with the SIMS measurements performed for the 1 x 1017N + cm 2 implantation dose. The chemical state of the implanted nitrogen was analysed by XPS using a Perkin-Elmer PHI model 555 system with a nonmonochromatized Al K~x(1486.5 eY) X-ray radiation and a double-pass cylindrical mirror analyser operating at a pass energy of 25 eY. Changes in the volume induced by the implantation were studied by measuring the step height between the implanted and unimplanted regions using a profilometer with a depth resolution of about 50 A. 3. Results and discussion Following implantation the entire surface of the GC samples appeared shiny and the implanted zone was easily distinguished. For doses lower than 1 x 1017 N~cm2 this effect was found to be due to an increase of the refractive index of the implanted GC and was associated with a densification of the affected volume by the implantation process [13, 14]. Measurements of the step height between the implanted and unimplanted GC are shown in Fig. 1. As observed from this figure for the 1 x 1017 and 5 x 1017 N~cm2 samples a clear compaction of the implanted region occurs. However, for the higher-dose implanted samples a reverse effect was observed. These results suggest that for the higher doses a chemical effect occurs in the implanted layer. In Fig. 2 Auger depth profiles are shown. For samples implanted with doses of 1 x 1017 and 5 x 10’~N~cm2 the nitrogen concentration profile is symmetric and reaches its maximum value at 1100 ±100 A with a full
Table 1 Summary of different implanted samples and heat treatments Sample
Implantation dose (N~cm2)
Implantation temperature (°C)
Implantation energy (keY)
1 2 3
lxlO’7 5x10’7 0.29 x 1018 1.05 x 1018 0.29 x 1018 1.05 x l0’~ 0.29 x 1018 1.05 x 1018 6.2 x 10~
RT RT RT
50 50 25
RT
50
RT RT RT RT 400°C
25 50 25 50 50
4 5 6
Heat treatment
500°C,3 h 1000°C,3.5 h —
618
A. Hoffman et al.
UNIMPLANTED
0.)
IMPLANTED
DOSE
I
(N~CM
0
i~~
/
Formation of carbon nitridefilms
concentration obtained in the implanted samples is 25—30 For at.%. the sample with a double-energy (high dose)
2)
implantation (Fig. 2) a broad distribution of the implanted nitrogen with an average concentration value of 25 at.% is observed, with two very clear maxima located at 1300 ±100 A and 750 ±100 A respectively. These values correspond to nitrogen ion ranges of 50 and 25 keY as calculated from TRIM89 using a carbon density of 1.8 g cm3. However, as the implanted dose is very high and the nitrogen and carbon concentrations are comparable, the use of TRIM89 to calculate the ion range is not justified. After annealing the RT double-energy-implanted sample to 500°Cfor 3 h a similar Auger depth profile (not shown) was obtained to that of the as-implanted
~
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7
~‘°
°°~
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~ oo1
~
-oi
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______
_______
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__
-. -
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5OKeV-62I07 IMPL.TE~1P4O0~C 0
250
500
~
750
LATERAL nSPLACEMENT
(i.Lm)
Fig. 1. Profilometer measurements across the interface between implanted and virgin glassy carbon.
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~°
~
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~ .......—~7
2 N /CM
,
• 30
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/ • / \ 0 / \
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~
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os
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io 820
25 KV-0 291018 N~CM2 50 ?IV-105’ 08 N/CM2
0
ANNEALING TEMP.
_____________________________ 20I0
~
5OKV-2-l0’7 N~CM2 A~ IMPLANTATION ~°\.~TEMP. -4OO~C
/ I
0
00
~
200
300
400
DEPTH (NANOMETER)
Fig. 2. Auger depth profile measurements of the implanted samples and after annealing the double-energy implant to 1000°Cfor 3.5 h.
width at half-maximum of 700 ±100 A. These values are in agreement with the calculated range of the implanted nitrogen into GC using a density of 2.1 g cm3 (TRIM89 [15]). Such a high matrix density value is justified on the basis of physical densification of the GC matrix [13, 14]. As observed from Fig. 2 the maximum nitrogen
sample. However, after 1000 °C,3.5 h annealing a redistribution of the implanted nitrogen occurs (Fig. 2). These results clearly show that during annealing to 1000°C diffusion of the implanted nitrogen from the implanted layer into the bulk GC and towards the sample surface occurs. For this sample the nitrogen concentration became homogeneous and reached a value of 7 at.%. These results are supported by the SIMS measurements described below. The Auger depth profile of the hot-implanted sample (Fig. 2) shows a very broad nitrogen distribution, which extends to almost 400 nm. This value is much larger than expected from TRIM89 calculation. Considering that swelling of the implanted region occurs during implantation (Fig. 1) this broad distribution is explained as due to a build-up of the modified layer during the implantation process. For this sample the nitrogen concentration obtained an average value of 18 at.%. In Fig. 3 SIMS depth profiles of the RT implanted .
samples are shown. Basically, the SIMS results are in good agreement with the Auger depth profiles of the different samples. XPS measurements were performed for the doubleenergy-implanted and hot-implanted samples. For the double-energy-implanted sample the measurements were carried out without Ar ion sputter-cleaning of the sample as its nitrogen concentration was finite at the surface as ascertained from the Auger profile and SIMS analysis (Figs. 2 and 3). In the case of the hot-implanted sample the nitrogen concentration at the surface was below the XPS detection limit. Therefore, XPS measurements of this sample were performed after Ar ion sputtering to a depth of 100 A. The XPS N( is) lines measured for these two samples are shown in Fig. 4 in the 408—395 eY binding energy range. Deconvolution of these lines was performed using a standard curve synthesis procedure [16]. The results of the deconvolution show the presence of three different possible nitrogen bonding states celltered at approximately 405—403, 401—400, and approxi-
A. Hoffman et a!. 23_
/ Formation
of carbon nitride films
-
i0
50KeVIIO~~N8/CM2
020
619
/‘\~
~ o~~50KeV
N
Anneal. Temp.
(Is)
-
a)
R.T.
§-I0~N~/CM2
-~~-ii~~ ~
~
~I 2I 0
I0~___ 25KeV-O29d°N~,tM2
500°C
.
0
____
I
00200300400
—
-,
400°C
Fig. 3. SIMS depth profile measurements of the different implanted samples.
peaks appear clearly in the non-deconvoluted spectrum. A similar effect was observed in the case of low-energy implantation experiments [10—12]. As observed from the deconvoluted spectra, the effect of annealing an RT implanted layer is a relative increase in the lower binding energy peak integral intensity with respect to the higherbinding-energy nitrogen states. These results show that annealing to 500°C of the double-implanted sample resulted in the population of a less covalent local C—N bonding. A similar conclusion was obtained also in our in-situ low-energy studies [12]. In Fig. 4(c) the nitrogen spectrumimplantation measured for the sputtercleaned-hot-implanted sample is shown (a similar spectrum was measured after heating this sample to 400°Cfor 3 h). From this spectrum, the N( is) peak at 400 eY binding energy is preferentially populated relative to the 398 and 404 eY states. This result demonstrates that a rather covalent nitrogen bonding state can be preferentially populated in the implanted layer produced by high-energy hot implantation. These results
b)
Implant.Temp.
DEPTH (NANOMETER)
mately 398 eY Peaks at similar binding energies were obtained in our in-situ low nitrogen energy implantations into graphite [10—12]. Whereas the 405—403 eV binding energy state may be associated with molecular nitrogen present in the implanted layer [17], the lower binding energy states are related to nitrogen—carbon chemical bonds. In Fig. 4(a),(b) the XP spectra are shown for the double-energy as-implanted sample and after annealing to 500°Cfor 3.0 h in the 408—395 eY binding energy range. After annealing, the N( is) line splits and two
\
/1
~~V-L05-IO’8N~CM2
j
~
-408
-406
-404
-402
i’\-_~ -
I
I
-400
-398
-396
-394
Binding-Energy Levi Fig. 4. XPS N( is) core level lines for: (a) the double-energy implanted sample; (b) the double-energy implanted layer after annealing to 500°C for 3 h; (c) after 100 A Ar ion sputtering the hot-implanted sample.
may be of importance, as a rather covalent bonding configuration is expected to exist in the elusive /3-C 3N4 phase [5]. However, we do not have any evidence that the carbon nitride formed in our implantation experiments consists of or partially contains a crystalline phase. Only electron or X-ray diffraction studies of the implanted layer may determine the presence of such a phase. Such studies are under way in our laboratories. In Fig. 5 Raman spectra for the different samples in the 1000—1800 cm~range are shown. As observed from this figure for the unimplanted sample two broad peaks characteristic of GC in the spectrum 1350 and 1. After all appear RT implantations theseat peaks are 1580 cm completely washed out and instead a broad peak at 1550 cm characteristic of highly disordered carbon, dominates the spectrum. After annealing the characteristic Raman peaks of GC appear in the spectrum. This is an evidence that some micro-recrystallization processes in the damaged GC near surface layer occurred. No new peaks which may be suggestive of the formation of a new crystalline carbon nitride phase became apparent in the Raman spectrum up to 2400 cm’ [3]. — ~,
620
A. Hoffman et al. 1111111111
/
Formation of carbon nitridefilms
II I~II~I
pTIIIPIIIFII
Swelling of the implanted region occurs, suggesting that large physicochemical changes of the modified layer has occurred. (4) XPS measurements show that by high-energy hot implantation a rather covalent nitrogen bonding state can be preferentially populated in the implanted layer. A less covalent bonding state was prevalent in the RT implanted layer and after annealing this layer up to 500°Cfor 3 h. (5) The implantation processes at RT results in an amorphization of the near-surface region of GC. Annealing results in some re-microcrystallization of this disordered layer.
UThIUIIIIHIIIILIILHIIIHIII
1000
1200
1400
1600
1800
References
RAMAN SHIFT [cm~]
1 range of: (a) polished Fig. 5. Raman spectra in the 1000—1800cm GC; (b) after implantation to 5 x 1017 N~cm2 (c) double-energy implanted sample after annealing to 500°Cfor 3 h; (d) double-energy implanted sample after annealing to 1000 °Cfor 3.5 h; (e) after implantation to 62 x 1017 N~cm2 at 400°C.
[1] A. Y. Liu and M. L. Cohen, Science, 245 (1989) 841. [2] A. Y. Liu and M. L. Cohen, Phys. Rev. B, 41(1990)10727. [3] M. Y. Chen, D. L. X. Lin, V. P. Dravid, Y. W. Chung, M. S. Wong and W. D. Sproul, J. Vac. Sci. Technol., All (1993) 521. [4] J. F. D. Chubaci, T. Sakai, T. Yamamoto, K. Ogata, A. Ebe and F. Fujimoto, Nuci. Instrum. Methods Phys. Res., B80/81 (1993) 463.
4. Summary
[5] [6] [7] [8]
(1) The near-surface region of GC can be nitrided by high-energy high-dose nitrogen ion implantation. The maximum amount of nitrogen in the implanted layer obtained in the present study reaches 25—30 at.%. (2) The nitrogen distribution in the RT double-ionenergy high-dose implanted sample obtained an average value of 25 at.%. Swelling of the implanted region of this sample was observed. Annealing to 500°Cfor 3 h of this sample does not result in a measurable diffusion of the implanted nitrogen. However, nitrogen diffusion occurs at 1000 °C.After annealing at this temperature for 3.5 h the total amount of nitrogen present in the implanted layer decreases by 70%. (3) A 50 keY N~implantation into GC at a temperature of 400°Cwith a dose of 6.2 x io’~ cm2 results in a broad and nearly homogeneous distribution of the implanted nitrogen up to a depth of 250 nm with a nearly constant nitrogen concentration of 18 at.%.
[9] [10] [11] [12] [13] [14] [15] [16] [17]
C. Niu, Y. Z. Lu and C. M. Lieber, Science, 261 (1993) 334. S. Kumar and T. L. Tansley, Solid State Commun., 88(1993)803. 5. Hasegawa and P. C. Zalm, J. App!. Phys., 58 (1985) 2539. J. Petruzzello, T. F. McGee, M. H. Frommer, V. Rumennik, P. A. Walters and C. J. Chou, J. AppI. Phys., 58 (1985) 4605. P. H. Oosting, J. Petruzzello and T. F. McGee, J. Appi. Phys., 62 (1987) 4118. A. Hoffman, I. Gouzman and R. Brener, App!. Phys. Lett., 64 (1994) 845. I. Gouzman, R. Brener, C. Cytermann and A. Hoffman, Surf. Interface Anal. (1994) in press. I. Gouzman, R. Brener and A. Hoffman, Thin Solid Films, 253 (1994) in press. A. Hoffman, P. J. Evans, D. D. Cohen and P. J. K. Paterson, J. App?. Phys., 72 (1992) 5687. D. McCulloch, A. Hoffman and S. Prawer, J. AppI. Phys., 74 (1993) 135. J. F. Ziegler, J. P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. P. M. A. Sherwood, in D. Briggs and M. D. Seah (eds.) Practical Surface Analysis, Wiley, Chichester, 1983, p. 445. A. Nilson, 0. Bjorneholm, H. Tillborg, B. Hernnas, R. J. Guest, A. Sandell, R. F. Palmer and N. Maternsson, Surf. Sd., 287 (1993) 758.
616
Formation of carbon nitride films by high-energy nitrogen ion implantation into glassy carbon A. Hoffmana,b H. Ge11er~,I. Gouzmanab, C. Cytermannb, R. Brenerb, M. Kenny~’ a Chemistry Department, bsoljd State Institute, cMaterials Engineering Department, Technion, Ha(fa 32000, Israel dCSIRO Division ofApplied Physics, POB 218, Lindfield, NSW2O7O, Australia
Abstract Carbon nitride fl-C
3N4 thin films are presently attracting increasing interest both scientifically and technologically. This is due to their expected mechanical and tribological properties, superior to those of diamond. In the present work the nitriding process of the near-surface region of glassy carbon by high-energy nitrogen ion implantation has been investigated with particular emphasis on the implantation temperature and post-annealing processes. The 2. The implantations distribution were of the performed implanted at room nitrogen temperature and its bonding and at 400 states °Cusing have 25 and been studied 50 keY bynitrogen a number ions of complementary up to doses of 1 techniques: x 1018 cm Auger electron spectroscopy, secondary ion mass spectrometry and X-ray photoelectron spectroscopy (XPS). The possibility of carbon nitride phase formation and the effect of implantation on the glassy carbon microstructure was assessed by Raman measurements. Volume effects were studied by measurements of the step height between the implanted and unimplanted regions. The maximum amount ofnitrogen in the implanted layer obtained in the present study reaches 25—30 at.%. Annealing the RT implanted layer up to 500°Cdoes not result in a measurable diffusion of the implanted nitrogen. However, during annealing to 1000 °Ca diffusion of the implanted nitrogen occurs. Hot implantation at 400°Cresults in a broad and nearly homogeneous distribution of the implanted nitrogen with an average concentration of 18 at.%. XPS measurements indicate that hot implantation results in a preferred population of a rather covalent nitrogen bonding state in the implanted layer compared with that obtained after RT implantation and annealing.
1. Introduction The existence of a /~-C3N4stable phase, similar in structure to /3-Si3N4, with mechanical properties superior to those of diamond has been predicted by theoretical calculations [1,2]. Over the last few years a great deal of effort has been directed towards the formation of carbon nitride thin by a number of J3-C deposition processes [3—6]. Thefilms possibility of forming 3N4 by ion beam implantation can be encouraged by the successful formation of nearly stoichiometric Si3N4 by highenergy and high-dose nitrogen implantation into silicon [7—9]. A number of implantation parameters, such as implantation energy, ion dose, substrate temperature and subsequent thermal annealing, were shown to play a vital role in the proper compound phase formation. Recently, the possibility of carbon nitride formation by 500 eV N~nitrogen ion irradiation of graphite was investigated in our laboratory by in-situ X-ray photoelectron spectroscopy (XPS) with particular attention paid to the effect of implantation temperature and postannealing processes [10—12]. Analysis of the N(ls) core level line indicated the existence of three different carbon—nitrogen bonding states inhot thenitrogen room temperature (RT) implanted layer, whereas implantation at 500 °C resulted in a predominant population of a
0257—8972/94/$7.00 SSDI 0257-8972(94)08002-G
covalent bonding state. Such a nitrogen bonding state is expected to exist in the elusive /3-C 3N4 compound [1,2]. These results suggest that a covalently bonded carbon nitride compound may be formed by ion-beamassisted methods. In the present work our investigations of low-energy nitrogen ion implantation into graphite are high-energy extended to 2) and high-dose (up to 1 x 1018 ions cm (25 and 50 keY) nitrogen implantation into glassy carbon (GC). To the best of our knowledge this is the first study of the possibility of carbon nitride formation by high-energy ion implantation into a carbon substrate. The properties of the implanted layer are studied, with particular attention given to the effect of the implantation dose, temperature and post-annealing processes.
2. Experimental Substrates of GC (V25) were prepared from plates 2 mm thick supplied by Atomergic Chemetals. The manufacturer’s specifications quote a heat treatment temperature 2500°Cand a densitythebetween 1.5 and 3. of Prior to implantation GC substrates 1.55 gpolished cm were with 1 p.m diamond paste, ultrasonically cleaned in acetone and rinsed in deionized water.
© 1994
—
Elsevier Science S.A. All rights reserved
A. Hoffman et a!.
/
Formation of carbon nitride films
The nitrogen implantations were performed using a Freeman ion implanter scanning the sample through the ion beam. N~ ions accelerated to 50 or 25 keY were used. The ion beam current was 10 p.A with a diameter of 1 mm. The samples were implanted at RT using doses between 1 x 1017 and 1 x 1018 ions cm2. In order to obtain a homogeneously distributed nitrogen-implanted near-surface region, a high-dose double implantation using 25 and 50 keY nitrogen ions was performed. Annealings up to 1000 °Cwere performed in a vacuum furnace with a base pressure of 1 x iO~Torr. Annealing up to 500°C was performed in situ in the ultrahigh vacuum Auger electron spectroscopy (AES)/XPS analysis chamber with a base pressure of 1 x 10-8 Torr. Hot ion implantation was performed at 400 °C using ion energies of 50 keY and a dose of 6.2 x iO~N~cm2. In Table 1 all implantations into GC and thermal treatments conditions are summarized. The secondary ion mass spectrometry (SIMS) depth profiles of the as-implanted and annealed samples were measured in a Cameca IMS4f ion microscope. A 10 keY Cs + ion beam was used to simultaneously monitor the CsC + and CsN + ions. The concentration of nitrogen was calculated after normalization of the CsN~intensities to the CsC + signal and using a relative sensitivity factor derived from the 1 x 1017 N~cm2 RT implanted GC sample. Raman measurements were carried out using the 514.5 nm Ar line, and the scattered light was analysed with a SPEX 1403 double spectrometer. The Raman spectrum was measured in the 1000—2400 cm1 range using a 200 p.m2 laser beam spot and 200 mW power. From Raman measurements the possibility of carbon nitride phase formation and changes in the GC microstructure were assessed. The AES experiments were performed in a scanning Auger spectrometer (Perkin-Elmer, PHI model 590A) operated in the first derivative mode with a 6 V peakto-peak modulation voltage using a 3 keY, 1.0 p.A rast-
617
ered primary beam. Depth profiles were obtained by monitoring the C(KLL) and N(KLL) peak-to-peak signals during 3 keY Ar~ion-sputter etching. The sputter rate was determined by profilometer measurements. To calculate the nitrogen concentration as a function of depth the C(KLL) and N(KLL) Auger transitions sensitivity factors were calculated based on a cross-calibration with the SIMS measurements performed for the 1 x 1017N + cm 2 implantation dose. The chemical state of the implanted nitrogen was analysed by XPS using a Perkin-Elmer PHI model 555 system with a nonmonochromatized Al K~x(1486.5 eY) X-ray radiation and a double-pass cylindrical mirror analyser operating at a pass energy of 25 eY. Changes in the volume induced by the implantation were studied by measuring the step height between the implanted and unimplanted regions using a profilometer with a depth resolution of about 50 A. 3. Results and discussion Following implantation the entire surface of the GC samples appeared shiny and the implanted zone was easily distinguished. For doses lower than 1 x 1017 N~cm2 this effect was found to be due to an increase of the refractive index of the implanted GC and was associated with a densification of the affected volume by the implantation process [13, 14]. Measurements of the step height between the implanted and unimplanted GC are shown in Fig. 1. As observed from this figure for the 1 x 1017 and 5 x 1017 N~cm2 samples a clear compaction of the implanted region occurs. However, for the higher-dose implanted samples a reverse effect was observed. These results suggest that for the higher doses a chemical effect occurs in the implanted layer. In Fig. 2 Auger depth profiles are shown. For samples implanted with doses of 1 x 1017 and 5 x 10’~N~cm2 the nitrogen concentration profile is symmetric and reaches its maximum value at 1100 ±100 A with a full
Table 1 Summary of different implanted samples and heat treatments Sample
Implantation dose (N~cm2)
Implantation temperature (°C)
Implantation energy (keY)
1 2 3
lxlO’7 5x10’7 0.29 x 1018 1.05 x 1018 0.29 x 1018 1.05 x l0’~ 0.29 x 1018 1.05 x 1018 6.2 x 10~
RT RT RT
50 50 25
RT
50
RT RT RT RT 400°C
25 50 25 50 50
4 5 6
Heat treatment
500°C,3 h 1000°C,3.5 h —
618
A. Hoffman et al.
UNIMPLANTED
0.)
IMPLANTED
DOSE
I
(N~CM
0
i~~
/
Formation of carbon nitridefilms
concentration obtained in the implanted samples is 25—30 For at.%. the sample with a double-energy (high dose)
2)
implantation (Fig. 2) a broad distribution of the implanted nitrogen with an average concentration value of 25 at.% is observed, with two very clear maxima located at 1300 ±100 A and 750 ±100 A respectively. These values correspond to nitrogen ion ranges of 50 and 25 keY as calculated from TRIM89 using a carbon density of 1.8 g cm3. However, as the implanted dose is very high and the nitrogen and carbon concentrations are comparable, the use of TRIM89 to calculate the ion range is not justified. After annealing the RT double-energy-implanted sample to 500°Cfor 3 h a similar Auger depth profile (not shown) was obtained to that of the as-implanted
~
0. 0
7
~‘°
°°~
02
~ oo1
~
-oi
8
______
_______
02
__
-. -
-0,1
5OKeV-62I07 IMPL.TE~1P4O0~C 0
250
500
~
750
LATERAL nSPLACEMENT
(i.Lm)
Fig. 1. Profilometer measurements across the interface between implanted and virgin glassy carbon.
5
~°
~
I 0
~ .......—~7
2 N /CM
,
• 30
50KV
/ • / \ 0 / \
20
Ii ~
_—‘
5 I0~ N~/CM2
~
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25
820
KV-0.29lO
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os
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N/CM2
N~CM2
io 820
25 KV-0 291018 N~CM2 50 ?IV-105’ 08 N/CM2
0
ANNEALING TEMP.
_____________________________ 20I0
~
5OKV-2-l0’7 N~CM2 A~ IMPLANTATION ~°\.~TEMP. -4OO~C
/ I
0
00
~
200
300
400
DEPTH (NANOMETER)
Fig. 2. Auger depth profile measurements of the implanted samples and after annealing the double-energy implant to 1000°Cfor 3.5 h.
width at half-maximum of 700 ±100 A. These values are in agreement with the calculated range of the implanted nitrogen into GC using a density of 2.1 g cm3 (TRIM89 [15]). Such a high matrix density value is justified on the basis of physical densification of the GC matrix [13, 14]. As observed from Fig. 2 the maximum nitrogen
sample. However, after 1000 °C,3.5 h annealing a redistribution of the implanted nitrogen occurs (Fig. 2). These results clearly show that during annealing to 1000°C diffusion of the implanted nitrogen from the implanted layer into the bulk GC and towards the sample surface occurs. For this sample the nitrogen concentration became homogeneous and reached a value of 7 at.%. These results are supported by the SIMS measurements described below. The Auger depth profile of the hot-implanted sample (Fig. 2) shows a very broad nitrogen distribution, which extends to almost 400 nm. This value is much larger than expected from TRIM89 calculation. Considering that swelling of the implanted region occurs during implantation (Fig. 1) this broad distribution is explained as due to a build-up of the modified layer during the implantation process. For this sample the nitrogen concentration obtained an average value of 18 at.%. In Fig. 3 SIMS depth profiles of the RT implanted .
samples are shown. Basically, the SIMS results are in good agreement with the Auger depth profiles of the different samples. XPS measurements were performed for the doubleenergy-implanted and hot-implanted samples. For the double-energy-implanted sample the measurements were carried out without Ar ion sputter-cleaning of the sample as its nitrogen concentration was finite at the surface as ascertained from the Auger profile and SIMS analysis (Figs. 2 and 3). In the case of the hot-implanted sample the nitrogen concentration at the surface was below the XPS detection limit. Therefore, XPS measurements of this sample were performed after Ar ion sputtering to a depth of 100 A. The XPS N( is) lines measured for these two samples are shown in Fig. 4 in the 408—395 eY binding energy range. Deconvolution of these lines was performed using a standard curve synthesis procedure [16]. The results of the deconvolution show the presence of three different possible nitrogen bonding states celltered at approximately 405—403, 401—400, and approxi-
A. Hoffman et a!. 23_
/ Formation
of carbon nitride films
-
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Fig. 3. SIMS depth profile measurements of the different implanted samples.
peaks appear clearly in the non-deconvoluted spectrum. A similar effect was observed in the case of low-energy implantation experiments [10—12]. As observed from the deconvoluted spectra, the effect of annealing an RT implanted layer is a relative increase in the lower binding energy peak integral intensity with respect to the higherbinding-energy nitrogen states. These results show that annealing to 500°C of the double-implanted sample resulted in the population of a less covalent local C—N bonding. A similar conclusion was obtained also in our in-situ low-energy studies [12]. In Fig. 4(c) the nitrogen spectrumimplantation measured for the sputtercleaned-hot-implanted sample is shown (a similar spectrum was measured after heating this sample to 400°Cfor 3 h). From this spectrum, the N( is) peak at 400 eY binding energy is preferentially populated relative to the 398 and 404 eY states. This result demonstrates that a rather covalent nitrogen bonding state can be preferentially populated in the implanted layer produced by high-energy hot implantation. These results
b)
Implant.Temp.
DEPTH (NANOMETER)
mately 398 eY Peaks at similar binding energies were obtained in our in-situ low nitrogen energy implantations into graphite [10—12]. Whereas the 405—403 eV binding energy state may be associated with molecular nitrogen present in the implanted layer [17], the lower binding energy states are related to nitrogen—carbon chemical bonds. In Fig. 4(a),(b) the XP spectra are shown for the double-energy as-implanted sample and after annealing to 500°Cfor 3.0 h in the 408—395 eY binding energy range. After annealing, the N( is) line splits and two
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Binding-Energy Levi Fig. 4. XPS N( is) core level lines for: (a) the double-energy implanted sample; (b) the double-energy implanted layer after annealing to 500°C for 3 h; (c) after 100 A Ar ion sputtering the hot-implanted sample.
may be of importance, as a rather covalent bonding configuration is expected to exist in the elusive /3-C 3N4 phase [5]. However, we do not have any evidence that the carbon nitride formed in our implantation experiments consists of or partially contains a crystalline phase. Only electron or X-ray diffraction studies of the implanted layer may determine the presence of such a phase. Such studies are under way in our laboratories. In Fig. 5 Raman spectra for the different samples in the 1000—1800 cm~range are shown. As observed from this figure for the unimplanted sample two broad peaks characteristic of GC in the spectrum 1350 and 1. After all appear RT implantations theseat peaks are 1580 cm completely washed out and instead a broad peak at 1550 cm characteristic of highly disordered carbon, dominates the spectrum. After annealing the characteristic Raman peaks of GC appear in the spectrum. This is an evidence that some micro-recrystallization processes in the damaged GC near surface layer occurred. No new peaks which may be suggestive of the formation of a new crystalline carbon nitride phase became apparent in the Raman spectrum up to 2400 cm’ [3]. — ~,
620
A. Hoffman et al. 1111111111
/
Formation of carbon nitridefilms
II I~II~I
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Swelling of the implanted region occurs, suggesting that large physicochemical changes of the modified layer has occurred. (4) XPS measurements show that by high-energy hot implantation a rather covalent nitrogen bonding state can be preferentially populated in the implanted layer. A less covalent bonding state was prevalent in the RT implanted layer and after annealing this layer up to 500°Cfor 3 h. (5) The implantation processes at RT results in an amorphization of the near-surface region of GC. Annealing results in some re-microcrystallization of this disordered layer.
UThIUIIIIHIIIILIILHIIIHIII
1000
1200
1400
1600
1800
References
RAMAN SHIFT [cm~]
1 range of: (a) polished Fig. 5. Raman spectra in the 1000—1800cm GC; (b) after implantation to 5 x 1017 N~cm2 (c) double-energy implanted sample after annealing to 500°Cfor 3 h; (d) double-energy implanted sample after annealing to 1000 °Cfor 3.5 h; (e) after implantation to 62 x 1017 N~cm2 at 400°C.
[1] A. Y. Liu and M. L. Cohen, Science, 245 (1989) 841. [2] A. Y. Liu and M. L. Cohen, Phys. Rev. B, 41(1990)10727. [3] M. Y. Chen, D. L. X. Lin, V. P. Dravid, Y. W. Chung, M. S. Wong and W. D. Sproul, J. Vac. Sci. Technol., All (1993) 521. [4] J. F. D. Chubaci, T. Sakai, T. Yamamoto, K. Ogata, A. Ebe and F. Fujimoto, Nuci. Instrum. Methods Phys. Res., B80/81 (1993) 463.
4. Summary
[5] [6] [7] [8]
(1) The near-surface region of GC can be nitrided by high-energy high-dose nitrogen ion implantation. The maximum amount of nitrogen in the implanted layer obtained in the present study reaches 25—30 at.%. (2) The nitrogen distribution in the RT double-ionenergy high-dose implanted sample obtained an average value of 25 at.%. Swelling of the implanted region of this sample was observed. Annealing to 500°Cfor 3 h of this sample does not result in a measurable diffusion of the implanted nitrogen. However, nitrogen diffusion occurs at 1000 °C.After annealing at this temperature for 3.5 h the total amount of nitrogen present in the implanted layer decreases by 70%. (3) A 50 keY N~implantation into GC at a temperature of 400°Cwith a dose of 6.2 x io’~ cm2 results in a broad and nearly homogeneous distribution of the implanted nitrogen up to a depth of 250 nm with a nearly constant nitrogen concentration of 18 at.%.
[9] [10] [11] [12] [13] [14] [15] [16] [17]
C. Niu, Y. Z. Lu and C. M. Lieber, Science, 261 (1993) 334. S. Kumar and T. L. Tansley, Solid State Commun., 88(1993)803. 5. Hasegawa and P. C. Zalm, J. App!. Phys., 58 (1985) 2539. J. Petruzzello, T. F. McGee, M. H. Frommer, V. Rumennik, P. A. Walters and C. J. Chou, J. AppI. Phys., 58 (1985) 4605. P. H. Oosting, J. Petruzzello and T. F. McGee, J. Appi. Phys., 62 (1987) 4118. A. Hoffman, I. Gouzman and R. Brener, App!. Phys. Lett., 64 (1994) 845. I. Gouzman, R. Brener, C. Cytermann and A. Hoffman, Surf. Interface Anal. (1994) in press. I. Gouzman, R. Brener and A. Hoffman, Thin Solid Films, 253 (1994) in press. A. Hoffman, P. J. Evans, D. D. Cohen and P. J. K. Paterson, J. App?. Phys., 72 (1992) 5687. D. McCulloch, A. Hoffman and S. Prawer, J. AppI. Phys., 74 (1993) 135. J. F. Ziegler, J. P. Biersack and U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. P. M. A. Sherwood, in D. Briggs and M. D. Seah (eds.) Practical Surface Analysis, Wiley, Chichester, 1983, p. 445. A. Nilson, 0. Bjorneholm, H. Tillborg, B. Hernnas, R. J. Guest, A. Sandell, R. F. Palmer and N. Maternsson, Surf. Sd., 287 (1993) 758.