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Inductively coupled plasma and laser-induced chemical vapour deposition of thin carbon nitride films
Surface and Coatings Technology 116–119 (1999) 261–268 www.elsevier.nl/locate/surfcoat
Inductively coupled plasma and laser-induced chemical vapour deposition of thin carbon nitride films C. Popov a,b, *, J. Bulir c,d, B. Ivanov e, M.-P. Delplancke-Ogletree c, W. Kulisch a a Institute of Technical Physics, University of Kassel, Heinrich-Plett-Straße 40, 34109 Kassel, Germany b Central Laboratory of Photoprocesses ‘Academician Jordan Malinowski’, Bulgarian Academy of Sciences, Acad. Georgi Bontchev St., Bl. 109, 1113 Sofia, Bulgaria c Universite Libre de Bruxelles, Service Metallurgie-Electrochimie, 50 Avenue F.D. Roosevelt, 1050 Brussels, Belgium d Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8, Czech Republic e University of Chemical Technology and Metallurgy, Dept. of Semiconductors, 8 Kliment Ohridski St., 1756 Sofia, Bulgaria
Abstract Thin CN films have been deposited on silicon substrates by inductively coupled plasma chemical vapour deposition (ICPx CVD) and by laser-induced chemical vapour deposition (LCVD) utilizing the focused beam of a copper bromide vapour laser. Different gas mixtures were used: CH /N and CCl /N /H for ICP-CVD, and CCl /NH for LCVD. Plasma properties (electron 4 2 4 2 2 4 3 temperature, electron and ion densities, plasma potential ) were studied by Langmuir probe and optical emission spectroscopy. The growth rates ranged up to 80 nm min−1 for ICP-CVD and 700 mm min−1 for LCVD. The surface morphology was studied using atomic force microscopy for ICP-CVD, and scanning electron microscopy for LCVD. Beside carbon and nitrogen, silicon and oxygen for LCVD and chlorine for ICP-CVD (in the case of the CCl /N /H system) was detected by Auger electron 4 2 2 spectroscopy. Fourier transform infrared (FTIR) spectroscopy was used to determine the chemical bonding structure. Bands assigned to CNC and CNN (graphite-like domains) and to CMH bonds were detected in the films deposited by both processes. In addition, different bands attributed to other types of carbon nitride bonds or to due to the presence of impurity atoms were observed in the FTIR spectra of the CN films deposited by the ICP-CVD and LCVD. The results of both deposition techniques x were compared and discussed on the base of the processes peculiarities. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbon; Laser CVD; Nitrides; PACVD
1. Introduction Various methods have been used for the deposition of carbon nitride (CN ) films since the hypothetical, x superhard b-C N was predicted [1]. Physical methods 3 4 such as r.f. diode sputtering [2], magnetron sputtering [3,4], pulsed laser deposition [5,6 ], and ion-assisted deposition methods [7–9] use energetic particle bombardment of the growing film in order to affect its structural properties and composition. In contrast, chemical methods, represented by various modifications of chemical vapour deposition (CVD) — CVD [10], plasma-assisted CVD (PACVD) [11,12], laser CVD (LCVD) [13,14] — are based on chemical reaction kinetics. In this case, enhanced substrate temperatures or energy-enhanced activation of the precursors are the * Corresponding author. Fax: +49-561-804-4136. E-mail address: [email protected] (C. Popov)
usual mechanisms driving the chemical reactions. Up to now in almost all cases the carbon nitride obtained films are quite different from the desired b-C N : they are 3 4 nitrogen deficient, amorphous (in some case with embedded nanocrystals) and involve various carbon–nitrogen bonds. In most of the published papers the results for the parameter dependence study of CN films prepared x by only one deposition method are presented. The comparison of techniques using the same method of excitation and different chemical systems or vice versa can give useful information about the process mechanism and the way for optimization of film properties. In this paper we report on the deposition of thin CN films by two types of chemical vapour deposition, x namely ICP-CVD which combines chemical reaction kinetics with physical treatment of the growing films by intensive ion bombardment, and LCVD, based on the chemical processes induced by the laser irradiation. The chemical system CCl /NH was chosen as a suitable 4 3
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candidate for CN deposition by activated CVD prox cesses. It can be supposed that, as a result of the chemical interaction, carbon nitride film will be deposited and the greater part of the chlorine and hydrogen atoms will be released in the form of HCl. This chemical system was used in the LCVD experiments. As NH 3 molecules decompose to nitrogen and hydrogen molecules, atoms and ions in the plasma, N /H mixtures 2 2 instead of NH were used for ICP-CVD which allowed 3 varying of the nitrogen to hydrogen ratio. In addition ICP-CVD experiments were performed using the CH /N system for comparison. Unfortunately the 4 2 methane system was not appropriate for LCVD and no films were deposited as shown by initial experiments probably because of the stronger CMH compared with CMCl bonds or intensive etching of the deposited films by reactive hydrogen within the laser spot. The aim of the study was to compare these two types of CVD methods with respect to the deposition rate, surface morphology, elemental composition and bonding structure of the obtained CN films and to draw x some conclusions on the influence of the method of activation on the film properties.
2. Experimental
ter was used to analyse the visible spectrum emitted by the plasma in the region from 250 to 750 nm with resolution of 0.65 nm, and in spectral region from 410 to 470 nm with resolution of 0.065 nm. 2.2. LCVD The LCVD set-up consists of a stainless steel chamber, the laser and its optics, an X–Y computer-controlled stepping stage, gas supply and vacuum systems. The silicon wafers used as substrates are placed on a holder inside the reactor resistively heated to 350°C. A focused beam (20 mm in diameter) of a copper bromide vapour laser is introduced perpendicular to the substrate through a quartz window in the upper part of the chamber. The CuBr vapour laser has wavelengths of 510 and 578 nm, a repetition rate of 20 kHz and pulse duration of 60 ns. The incident laser power was varied in the range from 1 to 2 W during the experiments. The laser beam scanned the substrate surface and two types of configurations, single stripes and meanders for larger deposition areas required for some analyses, were directly written with scanning speeds from 80 to 400 mm s−1. Ammonia (NH ) and carbon tetrachloride 3 (CCl ) were used as starting materials; they were intro4 duced into the closed chamber at pressures of 1 and 100 kPa, respectively.
2.1. ICP-CVD 2.3. Characterization The reaction chamber is made of a cylindrical fused silica tube; a shallow slope substrate holder is introduced in it from the pumping side. Its steel head, which is electrically isolated from ground, includes a silicon carbide heating element, a thermocouple to measure the temperature, and a connection to measure the self-bias of the head. An inductive plasma is generated inside the tube by the use of a coil, which is connected to an r.f. generator (13.56 MHz) and its matching unit. The r.f. power was varied in the range 25–400 W. The gases are distributed separately, controlled using mass flow controllers, and mixed at the reactor entrance. In the case of the N /CH system the gas ratio was 33. For the 2 4 CCl /N /H system a vaporiser was used for carbon 4 2 2 tetrachloride with pure nitrogen as carrier gas. The vapour pressure of CCl is determined by the vaporiser 4 temperature which was maintained by a thermostat at 25°C. The resulting CCl /N ratio was 0.07 (25 sccm 4 2 flow). Hydrogen was introduced separately into the reactor and the H /CCl ratio was varied in the range 2 4 1–6. The total pressure in the reactor of 25 Pa (CH /N ) and 50 Pa (CCl /N /H ) was set using a 4 2 4 2 2 throttle valve in the vacuum line between the reactor and the two-stage rotary pump. A single Langmuir probe was used for studying the main plasma properties. Pure nitrogen was used for this investigation. An OMA III optical emission spectrome-
The thickness of the deposited CN films was meax sured by a profilometer. The surface morphology was investigated by atomic force microscopy (AFM ) working in constant force contact mode for ICP-CVD films, and by scanning electron microscopy (SEM ) for LCVD films. The composition of the films was studied using a PHI 590 Auger electron spectrometer using 5 keV primary electrons. Depth profiling was carried out with 2 keV Ar+ ions. N/C ratios were calculated from the ‘peak-to-peak’ amplitudes using relative sensitivities of the elements C:N of 0.15:0.21. Fourier transform infrared spectroscopy ( FTIR) was used for studying the chemical bonding structure. The absorption of the bare silicon substrate was subtracted from the spectra.
3. Results 3.1. ICP-CVD Fig. 1 illustrates the Langmuir probe results and gives the dependence of the plasma properties on r.f. power in the range 25–300 W. The electron temperature varies in the narrow range from 1.6 to 1.9×105 K. The electron density increases by almost three orders of magnitude from 3×1014 to 1×1016 m−3 with increasing r.f. power.
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Fig. 1. Results of Langmuir probe measurement. Spectra were taken in the ICP-CVD reactor in N plasma at a pressure of 50 Pa for various 2 r.f. powers.
The plasma potential shows a strongly increasing dependence on the r.f. power, reaching −53 V at 300 W. This is important for the deposition process since a higher plasma potential induces a more energetic bombardment of the growing coating. Optical emission spectra were taken in the spectral region extending from 250 to 750 nm. Most of the peaks are related to N . CN-related peaks were found at 633 2 and 709 nm, and a peak at 672 nm was assigned to C 2 (most probably due to atomic carbon recombination in the plasma). In the region 410–470 nm (taken with better resolution) additional peaks of CN, N , N+ and 2 2 NH+ species can be distinguished. Major N peaks were 2 observed at 414, 420, 427, 434, 441.5, 449, 457.5 and 466 nm. A group of CN peaks at 415.2, 415.8, 416.4, 416.7 and 421.6 nm was observed. The intensity of these peaks is reduced by the addition of H into the reactive 2 gas mixture. Peaks at 423.5, 427, and 451.8 nm are associated with N+ species. The intensity of the peaks 2 related to N+is significantly lowered by the addition of 2 CCl in the reactive gas mixture, however, NH+ and 4 N+ lines (at 431 nm) appear with increasing injected r.f. 2 power. At 50 W, we detected only peaks of N . This 2 confirms that r.f. power is a key parameter for the chemical reaction leading to a film. The deposition rate of the CN films prepared by x ICP-CVD depends strongly on the process parameters such as substrate temperature, r.f. power, precursor composition and gas pressure, and the maximum values reached are 80 nm min−1 for CH /N precursors, and 4 2 15 nm min−1 for CCl /H /N precursors. The thickness 4 2 2 of the films is between 220 and 600 nm in the range of parameters varied. A typical example of the r.f. power influence on the deposition rate is given in Fig. 2. Three regions are observed as shown in the figure. In the range from 50 to 150 W, the deposition rate increases with increasing the r.f. power. Films in this range are typically homogeneous and smooth as shown in Fig. 3a. In the
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Fig. 2. Deposition rate versus r.f. power of films created in ICP-CVD. The films were grown in N /CH plasma (N /CH ratio was 33) at a 2 4 2 4 pressure of 25 Pa and different r.f. powers. Substrate temperature was influenced by self-heating from 50°C for 50 W to 160°C for 350 W.
Fig. 3. Atomic force microscopy of films deposited by ICP-CVD under conditions described in the caption of Fig. 2: (a) r.f. power of 50 W; (b) r.f. power of 270 W.
range 200–300 W the deposited films are very inhomogeneous in thickness and rough (see Fig. 3(b)) thus the thickness measurement is difficult and not precise. Above 300 W, the deposition process ceases and no film is obtained. The average roughness of the first group of
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˚ while that of the second samples does not exceed 20 A ˚ as determined from group of samples is around 400 A the areas displayed in Fig. 3. The film composition was studied using Auger electron spectrometry (AES ). In the case of CH /N 4 2 precursors, the AES spectrum consists of the carbon peak at 270 eV and the nitrogen peak at 380 eV. For very thin and/or porous films silicon and oxygen peaks are also visible at 80 and 510 eV, respectively. The nonuniform covering combined with small thickness resulting in uncovered areas is responsible of the detection of oxidized silicon substrate. The N/C ratios in these experiments vary between 0.40 and 1.10 for ‘as-deposited’ films and 0.10–0.40 after Ar+ ion sputtering. Both slightly increase with increasing r.f. power ( Fig. 4). In the case of the CCl /H /N system, an additional chlo4 2 2 rine peak is visible at 181 eV. Typical AES spectra are shown in Fig. 5. The chlorine and nitrogen peaks diminish after Ar ion sputtering indicating that chlorine as well as nitrogen are weakly bonded in the structure. For
this system, the N/C ratios vary in the range 0.40–0.80 for ‘as-deposited’ films and 0.05–0.20 after Ar+ ion sputtering. The Cl/C ratio in the film varies between 0.40 and 0.45 for films deposited at 50 W; however, it drops to 0.10 for an r.f. power of 200 W. Fig. 6 illustrates the comparison between the FTIR spectra of CH /N -deposited samples and two samples 4 2 deposited from CCl /H /N mixture. The bands around 4 2 2 1600 and 3350 cm−1 are dominating in all spectra. The broad absorption around 1600 cm−1 is composed of the several overlapping vibrational bands including CNC and CNN stretching modes and NMH bending mode. The shape of this band in the three spectra is very variable, indicating that the intensities of the components are different. The broad band around 3350 cm−1 is assigned to the NMH stretching mode. The FTIR spectrum of films prepared from CH /N precursors 4 2 contains a clear peak around 2180 cm−1 which can be associated with CONM triple bonding and/or MNNCNNM bonding characterizing isonitril and carbodiimide group, respectively. This band is barely detectable in the spectra of films deposited from CCl /H /N 4 2 2 mixtures. The presence of CMH bonds in the CH /N 4 2 deposited films is confirmed by CMH stretching mode around 2950 cm−1 and CMH bending mode around 1450 cm−1. In the case of CCl /H /N precursors, the 4 2 2 CMH stretching mode is shifted by 100 cm−1 to higher wavenumbers as a result of the presence of heteroatoms close to the CMH bonds. The significant band around 800 cm−1 in the bottom spectrum shows a presence of CMCl bonds in films deposited from CCl /H /N mix4 2 2 tures at an r.f. power of 50 W. 3.2. LCVD
Fig. 4. N/C ratio in the films as deposited and after Ar sputtering versus r.f. power. The films were deposited by ICP-CVD in N /CH 2 4 plasma.
Fig. 5. Example of typical AES spectra of films deposited by ICP-CVD in CCl /H /N plasma. The lower spectrum was taken after in situ 4 2 2 sputtering with Ar ions at a bias of 2 kV for 1 min.
The thickness of the CN films prepared by LCVD x is between 0.9 and 8 mm and the maximum deposition rate in the centre of the laser spot is 700 mm min−1. The profile of the deposited stripes follows the Gaussian energy intensity distribution in the laser beam. The
Fig. 6. Example of typical FTIR spectra of ICP-CVD deposited films in CCl /H /N and N /CH plasmas. 4 2 2 2 4
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increase of the laser power initially causes a rise in the deposition rate for kinetic reasons, i.e. due to the higher induced temperature ( Fig. 7). However, a further increase of the laser power changes this trend. Probably part of the deposited material is decomposed owing to the high surface temperature. The surface of the CN films is very rough which x does not allow AFM investigations. Thus the morphology was studied by SEM, and typical micrographs are presented in Fig. 8. The observed morphology is probably a result of the pulsed time structure of the laser irradiation. The surface temperature increases markedly during each pulse but then it drops until the next pulse. Higher laser power increases the roughness of the films (Fig. 8(b)) and in this case secondary decomposition of already deposited film in the centre of the laser spot as well as melting of the silicon substrate cannot be excluded. Typical Auger spectra of the deposited films are shown in Fig. 9. The peaks of carbon, nitrogen, oxygen and silicon are detected in the spectra at 274, 385, 512 and 83–93 eV, respectively. The presence of silicon in the films reveals that substrate material has penetrated into the deposit as a result of melting caused by the high surface temperature in the laser spot. The detected oxygen is due to residual molecules in the gas phase during the deposition as well as to the film oxidation after exposure to the air. No chlorine peak is observed in the AES spectra. The N/C ratio in the films varies between 0.05 and 0.14 for ‘as-deposited’ films and 0.26– 0.40 after Ar+ sputtering depending on the deposition conditions, and it increases slightly with the increase in the laser power ( Fig. 10). The FTIR spectra of the films show well-defined bands (Fig. 11). The absorption bands around 1730 and 1550 cm−1 could be attributed to CNC and CNN stretching modes. The band around 1250 cm−1 can be assigned to CMN bonds [15]. The presence of hydrogen in the films is indicated by the broad band in the region
2850–2970 cm−1 and the band around 1460 cm−1 due to CMH stretching and bending modes, respectively. The absorption bands around 960 and 840 cm−1 which appear in the spectra of the films deposited at higher laser power can be associated with bonds of silicon with nitrogen and carbon formed as a result of the interaction of the molten substrate with the deposit in the laser spot.
Fig. 7. Deposition rate versus laser power for films prepared by LCVD at room temperature and 120 mm s−1 scanning speed.
Fig. 9. Typical AES spectra of films deposited by LCVD from an NH /CCl mixture. The lower spectrum was taken after in situ sputter3 4 ing with Ar ions at a bias of 2 kV for 1 min.
Fig. 8. SEM micrographs of films deposited by LCVD at room temperature and 120 mm s−1 scanning speed: (a) laser power of 1 W; (b) laser power of 2 W.
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Fig. 10. N/C ratio in the films as deposited and after Ar sputtering versus laser power. The films are deposited by LCVD at room temperature and 120 mm s−1 scanning speed.
Fig. 11. FTIR spectra of films deposited by LCVD from CCl /NH at 4 3 350°C substrate temperature and 400 mm s−1 scanning speed.
4. Discussion 4.1. Deposition rate and morphology The deposition rate of CN films depends mainly on x the type of process activation (by plasma or by laser beam). Hence, it is difficult to use its absolute value for a comparison of the deposition techniques. However, the dependence of the rate on the deposition parameters can be used for studying and modelling the deposition mechanisms which can be different and unique for each method. The deposition rates for LCVD (of the order of some hundred mm min−1) are much higher than those for ICP-CVD (of the order of some ten nm min−1). The main reasons for this difference are the higher pressure of reagents during the LCVD experiments, on the one hand, and the intensive transport of gaseous reagents and products to and from the microreaction zone on the substrate surface, on the other hand. As a result, the rates of the chemical reaction and of the deposition, respectively, increase. In addition, the higher deposition rate for LCVD is also enhanced by the high surface temperature in the laser spot.
The deposition rate for both processes initially increases with an increase in the activation power (r.f. or laser) indicating chemical reaction kinetics as a ratelimiting step (power regions I in Figs. 2 and 7). The higher r.f. power leads to higher electron density (see Fig. 1) resulting in higher dissociation rates and thus in higher concentrations of reactive species, respectively. In case of LCVD the higher laser power corresponds to higher surface temperatures. Both trends result in an increase of the chemical reaction rate and consequently of the deposition rate. The films deposited in these power regions are rather smooth (Figs. 3(a) and 8(a)). It should be noted that the surface roughness of the LCVD films is much higher than that of ICP-CVD films. Probably this is a result of the pulsed time structure of the laser irradiation leading to peak changes in the surface temperature after each pulse. A further increase of the introduced energy results in saturation (LCVD) or even decrease (ICP-CVD) of the deposition rate (power regions II in Figs. 2 and 7). The most probable reason for the observed dependence is the onset of film destruction induced by the higher energy impact to the deposited material: chemical etching by reactive species (e.g. H or Cl atoms) or sputtering of the deposited film (as the plasma potential and the electron density are increased, Fig. 1), both resulting in the formation of volatile products in the case of ICPCVD, and decomposition of the CN films due to the x higher surface temperature and chemical etching in the case of LCVD. In addition, processes affecting the reagents concentration — homogeneous reaction (ICPCVD) and enhanced desorption (LCVD) — may also contribute to the observed deposition rate dependence at higher powers. All these processes are also responsible for the increased surface roughness of the films deposited at higher r.f. or laser power ( Figs. 3(b) and 8(b)). 4.2. Composition and N/C ratio The elemental analysis shows that the films are composed of carbon and nitrogen. In addition, chlorine (in the case of ICP-CVD from the system CCl /H /N ), 4 2 2 and silicon and oxygen (in the case of LCVD) are detected in the AES spectra of the films ( Figs. 5 and 9). The high surface temperature induced by the laser beam suppresses the formation of chlorine-containing solid by-products of the chemical interaction (e.g. NH Cl ) in 4 the microreaction zone on the one hand, but also causes melting of the substrate and penetration of silicon in the deposited layer. The chlorine contamination is highly reduced in the ICP-CVD films prepared at higher r.f. power. The study of the N/C ratio in the films showed that it exhibits a rather similar dependence on the power to that of the deposition rate. Initially the nitrogen content slightly increases with the increase of the power in the
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‘kinetic region’ discussed above ( Figs. 4 and 10). The higher power results in higher efficiency of breaking of the chemical bonds in the reaction gases, and of the product formation. A change in the composition of the ICP-CVD films prepared at high r.f. powers is observed — the N/C ratio decreases similarly to the deposition rate. This shows that part of the nitrogen in the films is only loosely bonded and can be released easily on the impact of energetic particles during the process which has been established for ion-assisted deposition methods [16 ]. The N/C ratio in the ICPCVD films as deposited is much higher than that in LCVD films. Most probably the surface composition of the latter is affected by contamination (mainly with carbon and oxygen) adsorbed from the gas phase in the chamber and after exposure to the air. Ar+ sputtering has different effects on the N/C ratio in the films. It decreases for ICP-CVD films owing to the ion bombardment as a result of the preferential nitrogen sputtering [17]. In contrast, the N/C ratio in LCVD films increases after sputtering owing to the cleaning of the surface by the Ar+ ions. It could be also supposed that only nitrogen steadily bonded to carbon and silicon remain in the films while the weakly bonded nitrogen is released during the deposition process owing to the high surface temperature.
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laser power. Probably the high surface temperature enhances the formations of such bonds. It should also be mentioned that bands due to the impurities detected by AES — Cl in ICP-CVD films form CCl /H /N and 4 2 2 Si in LCVD films — can also be detected in the spectra.
5. Conclusions The similarities found between ICP-CVD and LCVD with respect to thin CN films preparation, e.g. in the x dependence on the deposition rate and N/C ratio on the activation power (r.f. or laser), are due to the common process on which they are based, namely chemical vapour deposition. However, the different methods of the process activation and of transport of reactants to the substrate, as well as the different impacts on the growing films — ion bombardment (ICP-CVD) and high surface temperature (LCVD) — result in much higher deposition rates for LCVD and higher nitrogen contents in ICP-CVD films. In addition, different contaminants, chlorine in the case of ICP-CVD and silicon for LCVD, and some differences in the bonding structure such as hydrogen bonded to carbon and nitrogen in ICP-CVD layers but only to carbon in LCVD ones, are found depending on the deposition technique.
4.3. Chemical bonding structure The data obtained by FTIR supported the conclusions drawn from the elemental composition analysis. Bands which can be assigned to graphite-like CNC and CNN bonds, and to CMH stretching and bending modes are observed in the spectra of the films prepared by both techniques ( Figs. 6 and 11). Additional bands associated with NMH stretching and bending modes are present in the spectra of all ICP-CVD films. In such a way the presence of hydrogen in the deposited CN x films is clearly shown by FTIR although AES is not capable of detecting it. The main difference is that the hydrogen is bonded mainly to carbon atoms in LCVD films and to carbon and nitrogen in ICP-CVD films which is probably due to different reaction mechanisms enhanced by the plasma and the laser beam. Another difference that should be addressed is the presence of a band associated with CONM and/or MNNCNNM groups in the FTIR spectra of ICP-CVD films deposited form the CH /N system. This band is barely detectable 4 2 for films deposited by both techniques using CCl as 4 carbon precursor. These observations are in agreement with the compositional analysis discussed above. As has been shown previously, this band although ‘not desired’ in view of b-C N , is mainly present in films with high 3 4 nitrogen content [18]. In the case of LCVD films, a band which can be attributed to CMN single bonds is observed in the spectra of the films deposited at higher
Acknowledgement The authors gratefully acknowledge the financial support of the European Community (INCO/ COPERNICUS Project IC 15CT 960757).
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Inductively coupled plasma and laser-induced chemical vapour deposition of thin carbon nitride films C. Popov a,b, *, J. Bulir c,d, B. Ivanov e, M.-P. Delplancke-Ogletree c, W. Kulisch a a Institute of Technical Physics, University of Kassel, Heinrich-Plett-Straße 40, 34109 Kassel, Germany b Central Laboratory of Photoprocesses ‘Academician Jordan Malinowski’, Bulgarian Academy of Sciences, Acad. Georgi Bontchev St., Bl. 109, 1113 Sofia, Bulgaria c Universite Libre de Bruxelles, Service Metallurgie-Electrochimie, 50 Avenue F.D. Roosevelt, 1050 Brussels, Belgium d Institute of Physics, Czech Academy of Sciences, Na Slovance 2, 182 21 Prague 8, Czech Republic e University of Chemical Technology and Metallurgy, Dept. of Semiconductors, 8 Kliment Ohridski St., 1756 Sofia, Bulgaria
Abstract Thin CN films have been deposited on silicon substrates by inductively coupled plasma chemical vapour deposition (ICPx CVD) and by laser-induced chemical vapour deposition (LCVD) utilizing the focused beam of a copper bromide vapour laser. Different gas mixtures were used: CH /N and CCl /N /H for ICP-CVD, and CCl /NH for LCVD. Plasma properties (electron 4 2 4 2 2 4 3 temperature, electron and ion densities, plasma potential ) were studied by Langmuir probe and optical emission spectroscopy. The growth rates ranged up to 80 nm min−1 for ICP-CVD and 700 mm min−1 for LCVD. The surface morphology was studied using atomic force microscopy for ICP-CVD, and scanning electron microscopy for LCVD. Beside carbon and nitrogen, silicon and oxygen for LCVD and chlorine for ICP-CVD (in the case of the CCl /N /H system) was detected by Auger electron 4 2 2 spectroscopy. Fourier transform infrared (FTIR) spectroscopy was used to determine the chemical bonding structure. Bands assigned to CNC and CNN (graphite-like domains) and to CMH bonds were detected in the films deposited by both processes. In addition, different bands attributed to other types of carbon nitride bonds or to due to the presence of impurity atoms were observed in the FTIR spectra of the CN films deposited by the ICP-CVD and LCVD. The results of both deposition techniques x were compared and discussed on the base of the processes peculiarities. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Carbon; Laser CVD; Nitrides; PACVD
1. Introduction Various methods have been used for the deposition of carbon nitride (CN ) films since the hypothetical, x superhard b-C N was predicted [1]. Physical methods 3 4 such as r.f. diode sputtering [2], magnetron sputtering [3,4], pulsed laser deposition [5,6 ], and ion-assisted deposition methods [7–9] use energetic particle bombardment of the growing film in order to affect its structural properties and composition. In contrast, chemical methods, represented by various modifications of chemical vapour deposition (CVD) — CVD [10], plasma-assisted CVD (PACVD) [11,12], laser CVD (LCVD) [13,14] — are based on chemical reaction kinetics. In this case, enhanced substrate temperatures or energy-enhanced activation of the precursors are the * Corresponding author. Fax: +49-561-804-4136. E-mail address: [email protected] (C. Popov)
usual mechanisms driving the chemical reactions. Up to now in almost all cases the carbon nitride obtained films are quite different from the desired b-C N : they are 3 4 nitrogen deficient, amorphous (in some case with embedded nanocrystals) and involve various carbon–nitrogen bonds. In most of the published papers the results for the parameter dependence study of CN films prepared x by only one deposition method are presented. The comparison of techniques using the same method of excitation and different chemical systems or vice versa can give useful information about the process mechanism and the way for optimization of film properties. In this paper we report on the deposition of thin CN films by two types of chemical vapour deposition, x namely ICP-CVD which combines chemical reaction kinetics with physical treatment of the growing films by intensive ion bombardment, and LCVD, based on the chemical processes induced by the laser irradiation. The chemical system CCl /NH was chosen as a suitable 4 3
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candidate for CN deposition by activated CVD prox cesses. It can be supposed that, as a result of the chemical interaction, carbon nitride film will be deposited and the greater part of the chlorine and hydrogen atoms will be released in the form of HCl. This chemical system was used in the LCVD experiments. As NH 3 molecules decompose to nitrogen and hydrogen molecules, atoms and ions in the plasma, N /H mixtures 2 2 instead of NH were used for ICP-CVD which allowed 3 varying of the nitrogen to hydrogen ratio. In addition ICP-CVD experiments were performed using the CH /N system for comparison. Unfortunately the 4 2 methane system was not appropriate for LCVD and no films were deposited as shown by initial experiments probably because of the stronger CMH compared with CMCl bonds or intensive etching of the deposited films by reactive hydrogen within the laser spot. The aim of the study was to compare these two types of CVD methods with respect to the deposition rate, surface morphology, elemental composition and bonding structure of the obtained CN films and to draw x some conclusions on the influence of the method of activation on the film properties.
2. Experimental
ter was used to analyse the visible spectrum emitted by the plasma in the region from 250 to 750 nm with resolution of 0.65 nm, and in spectral region from 410 to 470 nm with resolution of 0.065 nm. 2.2. LCVD The LCVD set-up consists of a stainless steel chamber, the laser and its optics, an X–Y computer-controlled stepping stage, gas supply and vacuum systems. The silicon wafers used as substrates are placed on a holder inside the reactor resistively heated to 350°C. A focused beam (20 mm in diameter) of a copper bromide vapour laser is introduced perpendicular to the substrate through a quartz window in the upper part of the chamber. The CuBr vapour laser has wavelengths of 510 and 578 nm, a repetition rate of 20 kHz and pulse duration of 60 ns. The incident laser power was varied in the range from 1 to 2 W during the experiments. The laser beam scanned the substrate surface and two types of configurations, single stripes and meanders for larger deposition areas required for some analyses, were directly written with scanning speeds from 80 to 400 mm s−1. Ammonia (NH ) and carbon tetrachloride 3 (CCl ) were used as starting materials; they were intro4 duced into the closed chamber at pressures of 1 and 100 kPa, respectively.
2.1. ICP-CVD 2.3. Characterization The reaction chamber is made of a cylindrical fused silica tube; a shallow slope substrate holder is introduced in it from the pumping side. Its steel head, which is electrically isolated from ground, includes a silicon carbide heating element, a thermocouple to measure the temperature, and a connection to measure the self-bias of the head. An inductive plasma is generated inside the tube by the use of a coil, which is connected to an r.f. generator (13.56 MHz) and its matching unit. The r.f. power was varied in the range 25–400 W. The gases are distributed separately, controlled using mass flow controllers, and mixed at the reactor entrance. In the case of the N /CH system the gas ratio was 33. For the 2 4 CCl /N /H system a vaporiser was used for carbon 4 2 2 tetrachloride with pure nitrogen as carrier gas. The vapour pressure of CCl is determined by the vaporiser 4 temperature which was maintained by a thermostat at 25°C. The resulting CCl /N ratio was 0.07 (25 sccm 4 2 flow). Hydrogen was introduced separately into the reactor and the H /CCl ratio was varied in the range 2 4 1–6. The total pressure in the reactor of 25 Pa (CH /N ) and 50 Pa (CCl /N /H ) was set using a 4 2 4 2 2 throttle valve in the vacuum line between the reactor and the two-stage rotary pump. A single Langmuir probe was used for studying the main plasma properties. Pure nitrogen was used for this investigation. An OMA III optical emission spectrome-
The thickness of the deposited CN films was meax sured by a profilometer. The surface morphology was investigated by atomic force microscopy (AFM ) working in constant force contact mode for ICP-CVD films, and by scanning electron microscopy (SEM ) for LCVD films. The composition of the films was studied using a PHI 590 Auger electron spectrometer using 5 keV primary electrons. Depth profiling was carried out with 2 keV Ar+ ions. N/C ratios were calculated from the ‘peak-to-peak’ amplitudes using relative sensitivities of the elements C:N of 0.15:0.21. Fourier transform infrared spectroscopy ( FTIR) was used for studying the chemical bonding structure. The absorption of the bare silicon substrate was subtracted from the spectra.
3. Results 3.1. ICP-CVD Fig. 1 illustrates the Langmuir probe results and gives the dependence of the plasma properties on r.f. power in the range 25–300 W. The electron temperature varies in the narrow range from 1.6 to 1.9×105 K. The electron density increases by almost three orders of magnitude from 3×1014 to 1×1016 m−3 with increasing r.f. power.
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Fig. 1. Results of Langmuir probe measurement. Spectra were taken in the ICP-CVD reactor in N plasma at a pressure of 50 Pa for various 2 r.f. powers.
The plasma potential shows a strongly increasing dependence on the r.f. power, reaching −53 V at 300 W. This is important for the deposition process since a higher plasma potential induces a more energetic bombardment of the growing coating. Optical emission spectra were taken in the spectral region extending from 250 to 750 nm. Most of the peaks are related to N . CN-related peaks were found at 633 2 and 709 nm, and a peak at 672 nm was assigned to C 2 (most probably due to atomic carbon recombination in the plasma). In the region 410–470 nm (taken with better resolution) additional peaks of CN, N , N+ and 2 2 NH+ species can be distinguished. Major N peaks were 2 observed at 414, 420, 427, 434, 441.5, 449, 457.5 and 466 nm. A group of CN peaks at 415.2, 415.8, 416.4, 416.7 and 421.6 nm was observed. The intensity of these peaks is reduced by the addition of H into the reactive 2 gas mixture. Peaks at 423.5, 427, and 451.8 nm are associated with N+ species. The intensity of the peaks 2 related to N+is significantly lowered by the addition of 2 CCl in the reactive gas mixture, however, NH+ and 4 N+ lines (at 431 nm) appear with increasing injected r.f. 2 power. At 50 W, we detected only peaks of N . This 2 confirms that r.f. power is a key parameter for the chemical reaction leading to a film. The deposition rate of the CN films prepared by x ICP-CVD depends strongly on the process parameters such as substrate temperature, r.f. power, precursor composition and gas pressure, and the maximum values reached are 80 nm min−1 for CH /N precursors, and 4 2 15 nm min−1 for CCl /H /N precursors. The thickness 4 2 2 of the films is between 220 and 600 nm in the range of parameters varied. A typical example of the r.f. power influence on the deposition rate is given in Fig. 2. Three regions are observed as shown in the figure. In the range from 50 to 150 W, the deposition rate increases with increasing the r.f. power. Films in this range are typically homogeneous and smooth as shown in Fig. 3a. In the
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Fig. 2. Deposition rate versus r.f. power of films created in ICP-CVD. The films were grown in N /CH plasma (N /CH ratio was 33) at a 2 4 2 4 pressure of 25 Pa and different r.f. powers. Substrate temperature was influenced by self-heating from 50°C for 50 W to 160°C for 350 W.
Fig. 3. Atomic force microscopy of films deposited by ICP-CVD under conditions described in the caption of Fig. 2: (a) r.f. power of 50 W; (b) r.f. power of 270 W.
range 200–300 W the deposited films are very inhomogeneous in thickness and rough (see Fig. 3(b)) thus the thickness measurement is difficult and not precise. Above 300 W, the deposition process ceases and no film is obtained. The average roughness of the first group of
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˚ while that of the second samples does not exceed 20 A ˚ as determined from group of samples is around 400 A the areas displayed in Fig. 3. The film composition was studied using Auger electron spectrometry (AES ). In the case of CH /N 4 2 precursors, the AES spectrum consists of the carbon peak at 270 eV and the nitrogen peak at 380 eV. For very thin and/or porous films silicon and oxygen peaks are also visible at 80 and 510 eV, respectively. The nonuniform covering combined with small thickness resulting in uncovered areas is responsible of the detection of oxidized silicon substrate. The N/C ratios in these experiments vary between 0.40 and 1.10 for ‘as-deposited’ films and 0.10–0.40 after Ar+ ion sputtering. Both slightly increase with increasing r.f. power ( Fig. 4). In the case of the CCl /H /N system, an additional chlo4 2 2 rine peak is visible at 181 eV. Typical AES spectra are shown in Fig. 5. The chlorine and nitrogen peaks diminish after Ar ion sputtering indicating that chlorine as well as nitrogen are weakly bonded in the structure. For
this system, the N/C ratios vary in the range 0.40–0.80 for ‘as-deposited’ films and 0.05–0.20 after Ar+ ion sputtering. The Cl/C ratio in the film varies between 0.40 and 0.45 for films deposited at 50 W; however, it drops to 0.10 for an r.f. power of 200 W. Fig. 6 illustrates the comparison between the FTIR spectra of CH /N -deposited samples and two samples 4 2 deposited from CCl /H /N mixture. The bands around 4 2 2 1600 and 3350 cm−1 are dominating in all spectra. The broad absorption around 1600 cm−1 is composed of the several overlapping vibrational bands including CNC and CNN stretching modes and NMH bending mode. The shape of this band in the three spectra is very variable, indicating that the intensities of the components are different. The broad band around 3350 cm−1 is assigned to the NMH stretching mode. The FTIR spectrum of films prepared from CH /N precursors 4 2 contains a clear peak around 2180 cm−1 which can be associated with CONM triple bonding and/or MNNCNNM bonding characterizing isonitril and carbodiimide group, respectively. This band is barely detectable in the spectra of films deposited from CCl /H /N 4 2 2 mixtures. The presence of CMH bonds in the CH /N 4 2 deposited films is confirmed by CMH stretching mode around 2950 cm−1 and CMH bending mode around 1450 cm−1. In the case of CCl /H /N precursors, the 4 2 2 CMH stretching mode is shifted by 100 cm−1 to higher wavenumbers as a result of the presence of heteroatoms close to the CMH bonds. The significant band around 800 cm−1 in the bottom spectrum shows a presence of CMCl bonds in films deposited from CCl /H /N mix4 2 2 tures at an r.f. power of 50 W. 3.2. LCVD
Fig. 4. N/C ratio in the films as deposited and after Ar sputtering versus r.f. power. The films were deposited by ICP-CVD in N /CH 2 4 plasma.
Fig. 5. Example of typical AES spectra of films deposited by ICP-CVD in CCl /H /N plasma. The lower spectrum was taken after in situ 4 2 2 sputtering with Ar ions at a bias of 2 kV for 1 min.
The thickness of the CN films prepared by LCVD x is between 0.9 and 8 mm and the maximum deposition rate in the centre of the laser spot is 700 mm min−1. The profile of the deposited stripes follows the Gaussian energy intensity distribution in the laser beam. The
Fig. 6. Example of typical FTIR spectra of ICP-CVD deposited films in CCl /H /N and N /CH plasmas. 4 2 2 2 4
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increase of the laser power initially causes a rise in the deposition rate for kinetic reasons, i.e. due to the higher induced temperature ( Fig. 7). However, a further increase of the laser power changes this trend. Probably part of the deposited material is decomposed owing to the high surface temperature. The surface of the CN films is very rough which x does not allow AFM investigations. Thus the morphology was studied by SEM, and typical micrographs are presented in Fig. 8. The observed morphology is probably a result of the pulsed time structure of the laser irradiation. The surface temperature increases markedly during each pulse but then it drops until the next pulse. Higher laser power increases the roughness of the films (Fig. 8(b)) and in this case secondary decomposition of already deposited film in the centre of the laser spot as well as melting of the silicon substrate cannot be excluded. Typical Auger spectra of the deposited films are shown in Fig. 9. The peaks of carbon, nitrogen, oxygen and silicon are detected in the spectra at 274, 385, 512 and 83–93 eV, respectively. The presence of silicon in the films reveals that substrate material has penetrated into the deposit as a result of melting caused by the high surface temperature in the laser spot. The detected oxygen is due to residual molecules in the gas phase during the deposition as well as to the film oxidation after exposure to the air. No chlorine peak is observed in the AES spectra. The N/C ratio in the films varies between 0.05 and 0.14 for ‘as-deposited’ films and 0.26– 0.40 after Ar+ sputtering depending on the deposition conditions, and it increases slightly with the increase in the laser power ( Fig. 10). The FTIR spectra of the films show well-defined bands (Fig. 11). The absorption bands around 1730 and 1550 cm−1 could be attributed to CNC and CNN stretching modes. The band around 1250 cm−1 can be assigned to CMN bonds [15]. The presence of hydrogen in the films is indicated by the broad band in the region
2850–2970 cm−1 and the band around 1460 cm−1 due to CMH stretching and bending modes, respectively. The absorption bands around 960 and 840 cm−1 which appear in the spectra of the films deposited at higher laser power can be associated with bonds of silicon with nitrogen and carbon formed as a result of the interaction of the molten substrate with the deposit in the laser spot.
Fig. 7. Deposition rate versus laser power for films prepared by LCVD at room temperature and 120 mm s−1 scanning speed.
Fig. 9. Typical AES spectra of films deposited by LCVD from an NH /CCl mixture. The lower spectrum was taken after in situ sputter3 4 ing with Ar ions at a bias of 2 kV for 1 min.
Fig. 8. SEM micrographs of films deposited by LCVD at room temperature and 120 mm s−1 scanning speed: (a) laser power of 1 W; (b) laser power of 2 W.
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Fig. 10. N/C ratio in the films as deposited and after Ar sputtering versus laser power. The films are deposited by LCVD at room temperature and 120 mm s−1 scanning speed.
Fig. 11. FTIR spectra of films deposited by LCVD from CCl /NH at 4 3 350°C substrate temperature and 400 mm s−1 scanning speed.
4. Discussion 4.1. Deposition rate and morphology The deposition rate of CN films depends mainly on x the type of process activation (by plasma or by laser beam). Hence, it is difficult to use its absolute value for a comparison of the deposition techniques. However, the dependence of the rate on the deposition parameters can be used for studying and modelling the deposition mechanisms which can be different and unique for each method. The deposition rates for LCVD (of the order of some hundred mm min−1) are much higher than those for ICP-CVD (of the order of some ten nm min−1). The main reasons for this difference are the higher pressure of reagents during the LCVD experiments, on the one hand, and the intensive transport of gaseous reagents and products to and from the microreaction zone on the substrate surface, on the other hand. As a result, the rates of the chemical reaction and of the deposition, respectively, increase. In addition, the higher deposition rate for LCVD is also enhanced by the high surface temperature in the laser spot.
The deposition rate for both processes initially increases with an increase in the activation power (r.f. or laser) indicating chemical reaction kinetics as a ratelimiting step (power regions I in Figs. 2 and 7). The higher r.f. power leads to higher electron density (see Fig. 1) resulting in higher dissociation rates and thus in higher concentrations of reactive species, respectively. In case of LCVD the higher laser power corresponds to higher surface temperatures. Both trends result in an increase of the chemical reaction rate and consequently of the deposition rate. The films deposited in these power regions are rather smooth (Figs. 3(a) and 8(a)). It should be noted that the surface roughness of the LCVD films is much higher than that of ICP-CVD films. Probably this is a result of the pulsed time structure of the laser irradiation leading to peak changes in the surface temperature after each pulse. A further increase of the introduced energy results in saturation (LCVD) or even decrease (ICP-CVD) of the deposition rate (power regions II in Figs. 2 and 7). The most probable reason for the observed dependence is the onset of film destruction induced by the higher energy impact to the deposited material: chemical etching by reactive species (e.g. H or Cl atoms) or sputtering of the deposited film (as the plasma potential and the electron density are increased, Fig. 1), both resulting in the formation of volatile products in the case of ICPCVD, and decomposition of the CN films due to the x higher surface temperature and chemical etching in the case of LCVD. In addition, processes affecting the reagents concentration — homogeneous reaction (ICPCVD) and enhanced desorption (LCVD) — may also contribute to the observed deposition rate dependence at higher powers. All these processes are also responsible for the increased surface roughness of the films deposited at higher r.f. or laser power ( Figs. 3(b) and 8(b)). 4.2. Composition and N/C ratio The elemental analysis shows that the films are composed of carbon and nitrogen. In addition, chlorine (in the case of ICP-CVD from the system CCl /H /N ), 4 2 2 and silicon and oxygen (in the case of LCVD) are detected in the AES spectra of the films ( Figs. 5 and 9). The high surface temperature induced by the laser beam suppresses the formation of chlorine-containing solid by-products of the chemical interaction (e.g. NH Cl ) in 4 the microreaction zone on the one hand, but also causes melting of the substrate and penetration of silicon in the deposited layer. The chlorine contamination is highly reduced in the ICP-CVD films prepared at higher r.f. power. The study of the N/C ratio in the films showed that it exhibits a rather similar dependence on the power to that of the deposition rate. Initially the nitrogen content slightly increases with the increase of the power in the
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‘kinetic region’ discussed above ( Figs. 4 and 10). The higher power results in higher efficiency of breaking of the chemical bonds in the reaction gases, and of the product formation. A change in the composition of the ICP-CVD films prepared at high r.f. powers is observed — the N/C ratio decreases similarly to the deposition rate. This shows that part of the nitrogen in the films is only loosely bonded and can be released easily on the impact of energetic particles during the process which has been established for ion-assisted deposition methods [16 ]. The N/C ratio in the ICPCVD films as deposited is much higher than that in LCVD films. Most probably the surface composition of the latter is affected by contamination (mainly with carbon and oxygen) adsorbed from the gas phase in the chamber and after exposure to the air. Ar+ sputtering has different effects on the N/C ratio in the films. It decreases for ICP-CVD films owing to the ion bombardment as a result of the preferential nitrogen sputtering [17]. In contrast, the N/C ratio in LCVD films increases after sputtering owing to the cleaning of the surface by the Ar+ ions. It could be also supposed that only nitrogen steadily bonded to carbon and silicon remain in the films while the weakly bonded nitrogen is released during the deposition process owing to the high surface temperature.
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laser power. Probably the high surface temperature enhances the formations of such bonds. It should also be mentioned that bands due to the impurities detected by AES — Cl in ICP-CVD films form CCl /H /N and 4 2 2 Si in LCVD films — can also be detected in the spectra.
5. Conclusions The similarities found between ICP-CVD and LCVD with respect to thin CN films preparation, e.g. in the x dependence on the deposition rate and N/C ratio on the activation power (r.f. or laser), are due to the common process on which they are based, namely chemical vapour deposition. However, the different methods of the process activation and of transport of reactants to the substrate, as well as the different impacts on the growing films — ion bombardment (ICP-CVD) and high surface temperature (LCVD) — result in much higher deposition rates for LCVD and higher nitrogen contents in ICP-CVD films. In addition, different contaminants, chlorine in the case of ICP-CVD and silicon for LCVD, and some differences in the bonding structure such as hydrogen bonded to carbon and nitrogen in ICP-CVD layers but only to carbon in LCVD ones, are found depending on the deposition technique.
4.3. Chemical bonding structure The data obtained by FTIR supported the conclusions drawn from the elemental composition analysis. Bands which can be assigned to graphite-like CNC and CNN bonds, and to CMH stretching and bending modes are observed in the spectra of the films prepared by both techniques ( Figs. 6 and 11). Additional bands associated with NMH stretching and bending modes are present in the spectra of all ICP-CVD films. In such a way the presence of hydrogen in the deposited CN x films is clearly shown by FTIR although AES is not capable of detecting it. The main difference is that the hydrogen is bonded mainly to carbon atoms in LCVD films and to carbon and nitrogen in ICP-CVD films which is probably due to different reaction mechanisms enhanced by the plasma and the laser beam. Another difference that should be addressed is the presence of a band associated with CONM and/or MNNCNNM groups in the FTIR spectra of ICP-CVD films deposited form the CH /N system. This band is barely detectable 4 2 for films deposited by both techniques using CCl as 4 carbon precursor. These observations are in agreement with the compositional analysis discussed above. As has been shown previously, this band although ‘not desired’ in view of b-C N , is mainly present in films with high 3 4 nitrogen content [18]. In the case of LCVD films, a band which can be attributed to CMN single bonds is observed in the spectra of the films deposited at higher
Acknowledgement The authors gratefully acknowledge the financial support of the European Community (INCO/ COPERNICUS Project IC 15CT 960757).
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