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Formation of niobium oxynitrides by rapid thermal processing (RTP)
Applied Surface Science 252 (2005) 205–210 www.elsevier.com/locate/apsusc
Formation of niobium oxynitrides by rapid thermal processing (RTP) V.A. Matylitskaya a, W. Bock b, K. Thoma c, B.O. Kolbesen a,* a
Institut fu¨r Anorganische und Analytische Chemie, Johann Wolfgang Goethe-Universita¨t Frankfurt/M, Marie-Curie-Street 11, D-60439 Frankfurt/M, Germany b Institut fu¨r Oberfla¨chen-und Schichtenanalytik (IFOS), Universita¨t Kaiserslautern, Germany c Experimentalphysik, Universita¨t Kassel, Germany Available online 26 February 2005
Abstract The potential of rapid thermal processing (RTP) for the preparation of thin films of niobium oxynitrides was investigated. The 200 and 500 nm niobium films were deposited via sputtering on oxidized silicon(1 0 0)- and on sapphire(1 1 0 2)-substrates. At first, oxidation of niobium films in molecular oxygen and then nitridation in ammonia using an RTP-system was performed. The films were characterized before and after the oxidation and nitridation processes by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS). The influence of the two different substrates, amorphous SiO2 and single crystalline sapphire on the reactivity of the niobium films was studied in dependence of temperature, time of reaction and film thickness. The existence of niobium oxynitride formation was verified for some of the films. In some of the experiments, crack formation in the films or even delamination of the Nb-films from the substrates was observed. # 2005 Elsevier B.V. All rights reserved. Keywords: Niobium oxynitride; Rapid thermal processing (RTP); Thin films; X-ray diffraction (XRD); SIMS depth profile
1. Introduction Transition metal nitrides and oxynitrides have received increasing attention in recent years, because of their interesting and attractive chemical and * Corresponding author. Tel.: +49 69 798 29154; fax: +49 69 798 29235. E-mail address: [email protected] (B.O. Kolbesen).
physical properties [1]. As thin films, transition metal nitrides are used in a number of advanced tools and technologies [2]. Rapid thermal processing (RTP) is a key technology in microelectronics manufacturing for thin oxide or silicide film preparation [3]; so, this technology might also be established for further applications, for example solid-state reactions or formations of nitride films [4]. In this work, the possibility of niobium oxynitride formation by RTP was studied.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.119
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2. Experimental
3. Results and discussion
The substrates used in this study were 150 mm in diameter silicon wafers with a thermally grown 100 nm top oxide SiO2 (to prevent metal–silicon interactions and silicide formation) and sapphire (1 1 0 2). The 200 and 500 nm niobium layers were deposited by magnetron sputtering using Leybold Heraeus Z 700 sputtering device, equipped with Leybold sputtering cathodes PK 75 and niobium pellets of 99.9% purity by Superconductive Components Inc. The deposition rate was 27.72 nm min1 at a partial pressure of argon, pA = 102 mbar and plasma power of 100 W. Rapid thermal processing experiments were performed under atmospheric pressure in oxygen and ammonia gas flow (1.5 l min1, 99.999% gas purity), using a SHS 100 RTP system by Steag-a.s.t. with a wafer tray inside a quartz chamber. Substrate pieces with niobium films (1 cm 0.5 cm) were heated by top and bottom tungsten lamps with heating rates between 50 and 100 K s1. Temperature control during the experiments was accomplished by pyrometry [4]. At first, niobium films were oxidized by RTP in molecular oxygen at temperatures ranging from 350 to 500 8C and for times of 1, 2 and 5 min. After oxidation, the samples were nitridated in the RTP system by 1 min reaction with ammonia at 1000 8C. For analysis of the products, different methods were applied. The crystalline phases were characterized by X-ray diffraction (XRD) using a STOE u/udiffractometer with a multilayer mirror, using Cu Ka radiation. Elemental analysis of the films was carried out with an Amray SEM/Oxford Instruments EDX system using excitation energies between 5 and 20 kV. Surface morphology of the niobium films before and after oxidation and nitridation was characterized by atomic force microscopy (AFM) with a Nanoscope III A SPM (Digital Instruments) operated in tapping mode. The nominal cantilever spring constant was 42 N m1. Depth profile analysis was performed in a Cameca IMS 4 f system. Sputtering was carried out using Cs+ions of 5.5 keV energy, scanned on an area of 125 mm 125 mm, at a current density of 0.19 mA cm2.
The XRD diagrams of the 200 nm niobium films on SiO2/Si after oxidation in molecular oxygen at 350 8C have a slight shift of the Nb reflection to lower values of 2u. This shift enlarges with increasing oxidation time. The slight lattice expansion might be caused by the incorporation of oxygen into the metal, which is equivalent to the formation of the Nb(O) phase (solid solution). Formation of the substoichiometric phase NbO0.7 occurs by oxidation of Nb-films at 400 8C. The presence of two oxide phases can be observed after 1 min of RTP at 450 8C with O2: NbO0.7 and NbO. Parallel with these two phases, formation of Nb2O5 starts after 2 min reaction with O2 at 450 8C. The SIMS depth profiles of this sample (Fig. 1) show that concentration of oxygen decreases to the Nb/SiO2 interface. The depth distribution of the phases in this film in agreement with XRD (not shown) and SIMS data are as follows: close to the surface, Nb2O5; middle region, NbO and NbO0.7; close to the interface, solid solution Nb(O). Complete conversion of the Nbfilm to Nb2O5 occurs after 5 min RTP in oxygen at 450 8C. This temperature and time of the reaction are critical for the oxidation of niobium films, because beginning delamination of the oxidized niobium film from substrate becomes visible. The presence of four different phases (NbO2, NbN0.95, Nb3.49N4.56O0.44 and Nb4N5) can be observed after 1 min nitridation in ammonia at 1000 8C for all oxidized 200 nm Nb-films (except for the film oxidized during 5 min at 450 8C). As typical example, the XRD of a 200 nm Nb-film after oxidation at 450 8C for 2 min and nitridation in NH3 at 1000 8C for 1 min is shown in Fig. 2. The phase NbO2 dominates in these films. Content of oxynitrides in Nbfilms after nitridation enlarges with increasing oxidation temperature. Comparing XRD (Fig. 2) with SIMS depth profiles (Fig. 3), the following distribution of phases can be derived: close to the surface, NbN0.95 and Nb4N5; middle region, Nb3,49N4.56O0.44; close to the interface, NbO2. The nitrogen and oxygen concentration profiles in Fig. 3 indicate a snowplough effect: the in-diffusing nitrogen pushes the oxygen deeper into the bulk of the film resulting in a pile-up of oxygen towards the substrate interface. A similar effect has been observed in the nitridation of Ti-films [5]. The formation of the two phases, niobium
V.A. Matylitskaya et al. / Applied Surface Science 252 (2005) 205–210
207
Fig. 1. SIMS depth profile of 200 nm Nb-film on SiO2/Si-substrate after RTP in O2 for 2 min at 450 8C.
oxynitride and niobium nitride (Nb4N5), occurs after nitridation of Nb-films, oxidized during 5 min at 450 8C in oxygen. According to the XRD-data these two phases or one of them (Nb3,49N4.56O0.44 or Nb4N5) could be present in this film. Nitridation of this film
leads to further proceeding delamination of Nb-films from substrate in comparison with oxidized film. Oxidation of 500 nm Nb-films on SiO2/Si-substrate in oxygen at 350 and 400 8C, and during 1 min at 450 8C leads to the formation of solid solution of niobium due to
Fig. 2. XRD 200 nm Nb-film on SiO2/Si-substrate after RTP in O2 at 450 8C for 2 min and in NH3 at 1000 8C for 1 min.
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Fig. 3. SIMS depth profile of 200 nm Nb-film on SiO2/Si-substrate after RTP in O2 for 2 min at 450 8C and 1 min in NH3 at 1000 8C.
the incorporation of oxygen into the metal lattice. Formation of two phases, NbO0.7 and Nb2O5, takes place after 2 min of RTP in oxygen at 450 8C. Increasing of the oxidation time up to 5 min leads to the formation of Nb2O5 only, accompanied by delamination of Nbfilms from the SiO2/Si-substrate. After 1 min nitridation in ammonia at 1000 8C of oxidized Nb-films, the presence of six different phases in accordance with XRD-measurements can be observed: NbN0.95, Nb2N, NbO, NbO2, Nb3.49N4.56O0.44 and Nb4N5 (except for the film oxidized during 5 min at 450 8C). According to the intensities of the reflections, NbN0.95 and Nb2N are the main products in the films, oxidized at 350, 400 and 450 8C for 1 min. NbO dominates in the film oxidized during 2 min at 450 8C. Nitridation of the Nb-films, oxidized during 5 min at 450 8C and 1 min at 500 8C in oxygen, leads to the formation of two phases, Nb3,49N4.56O0.44 and Nb4N5 (Fig. 4). AFM measurements show that surface roughness of this film significantly increases compared with the as deposited 500 nm niobium film on SiO2/Si-substrate (as deposited film ! Rms = 6.786 nm, after reaction 1 min O2 500 8C + 1 min NH31000 8C ! Rms = 52.376 nm). At the surface, AFM and SEM investigations reveal cracks of this sample. This seems to be the pre-stage of delamination. Oxidation of 200 nm Nb-films on sapphire at 350 8C, and during 1 and 2 min at 400 8C leads to the
formation of solid solution of niobium Nb(O) and niobium oxide NbxO y. This oxide cannot be clearly identified according to the available XRD data base. Formation of two phases, NbxO y and Nb6O, occurs after oxidation during 5 min at 400 8C and 1 and 2 min at 450 8C. Parallel with these two phases, formation of Nb2O5 starts after 5 min reaction with O2 at 450 8C. Complete conversion of the films to niobium pentoxide after 1 min reaction with O2 at 500 8C leads to visible delamination of Nb-films from substrate. The presence of five different phases: NbO2, NbN0.95, Nb2N, Nb3.49N4.56O0.44 and Nb4N5 can be observed after 1 min nitridation in ammonia at 1000 8C of all oxidized 200 nm Nb-films on sapphire (except for the film oxidized during 1 min at 500 8C). NbO2 dominates in these films. Formation of niobium oxynitride (Nb3.49N4.56O0.44) and niobium nitride (Nb4N5) occurs by nitridation of the film, oxidized during 1 min at 500 8C. SEM measurements show holes on the surface of Nb-films after nitridation, which very likely is the result of hydrogen diffusion to and agglomeration at/below the surface. The formation of hydrogen occurs due to decomposition of ammonia. XRD diagrams of 500 nm niobium films on sapphire after oxidation at 350 and 400 8C show formation of solid solution of niobium Nb(O) and niobium oxide NbxO y (Table 1). The formation of two
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Fig. 4. XRD 500 nm Nb-film on SiO2/Si-substrate after RTP in O2 at 500 8C for 1 min and in NH3 at 1000 8C for 1 min.
phases NbxO y and Nb6O occurs after 1 min of RTP in oxygen at 450 8C. Parallel with these two phases, a formation of Nb2O5 takes place after 2 and 5 min reaction with O2 at 450 8C. Complete conversion of the films to niobium pentoxide occurs after 1 min reaction with O2 at 500 8C. This temperature and time of reaction are critical for the oxidation of 500 nm Nb-films on sapphire, because beginning delamination of niobium films from substrate becomes visible. Nitridation of these samples (except for the films oxidized during 1 min at 500 8C and 5 min at 450 8C), according to XRDdata, leads to the formation of six phases: NbN,
NbN0.95, Nb2N, NbO, Nb3.49N4.56O0.44 and Nb4N5. According to the intensities of the reflections, niobium nitrides are the main products in the films, oxidized at 350 and 400 8C, and during 1 and 2 min at 450 8C. Parallel with these six phases formation of NbO2 takes place after nitridation of the films, oxidized during 5 min at 450 8C. The main products in these films are Nb3.49N4.56O0.44 and Nb4N5. Nitridation of the films oxidized during 1 min at 500 8C in O2 leads to the formation of two phases Nb3.49N4.56O0.44 and Nb4N5. According to XRDdata, increasing oxidation temperature leads to increasing oxynitride content in the film by
Table 1 The reaction products of 500 nm niobium films on sapphire with oxygen and ammonia by RTP
Temperature (8C)
Time (min)
Products of the reaction
Products of the reaction with NH3, 1 min 1000 8C
350 400
1, 2, 5 1, 2, 5 1 2
Solid solution Nb(O), NbxOy NbxOy, Nb6O
NbN, NbN0.95, Nb2N, NbO, Nb4N5, Nb3.49N4.56O0.44
Reaction with O2
5
NbxOy, Nb6O, Nb2O5 Nb2O5
Nb4N5, Nb3.49N4.56O0.44, NbO2, NbO, NbN, NbN0.95, Nb2N
1
Nb2O5
Nb4N5, Nb3.49N4.56O0.44
450
500
210
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nitridation. SEM measurements show that increasing oxidation temperature enlarges the roughness of the films.
ammonia. Increasing oxidation temperature leads to the enlargement of the film roughness. The number of phases coexisting in 500 nm films is higher than in 200 nm films; 200 nm niobium films show a better reactivity than 500 nm films.
4. Summary and conclusions The 200 and 500 nm niobium films have been deposited via sputtering on oxidized silicon(1 0 0) wafers and on sapphire(1 1 0 2). Oxidation of niobium films in molecular oxygen followed by nitridation in ammonia using RTP has been investigated. Formation of niobium oxynitrides occurs by nitridation in ammonia of niobium films on SiO2/ silicon and on sapphire oxidized before in oxygen. Content of oxynitrides in film after nitridation enlarges with increasing oxidation temperature. Beginning delamination of the niobium films starts by oxidation at 450 8C according to SEM data. Surface roughness of 200 and 500 nm niobium films increases during the reactions with oxygen and
References [1] S.T. Oyama (Ed.), The Chemistry of Transition Metal Carbides .and Nitrides, Blackie Academic and Professional, London, 1996 [2] A. Kaloyeros, X. Chen, T. Stark, K. Kumar, S.-C. Seo, G. Peterson, H. Frish, B. Arkles, J. Sullivan, J. Electrochem. Soc. 146 (1999) 170. [3] R.B. Fair (Ed.), Rapid Thermal Processing Science and Technology, Academic Press, London, 1993. [4] A. Berendes, O. Brunkahl, C. Angelkort, W. Bock, F. Hofer, P. Warbichler, B.O. Kolbesen, Anal. Bioanal. Chem. 379 (2004) 554. [5] G.J.P. Krooshof, F.H.P.M. Habracken, W.F. van der Weg, L. van den Hove, K. Maex, R.F. De Keersmaecker, J. Appl. Phys. 63 (10) (1988) 5110.
Formation of niobium oxynitrides by rapid thermal processing (RTP) V.A. Matylitskaya a, W. Bock b, K. Thoma c, B.O. Kolbesen a,* a
Institut fu¨r Anorganische und Analytische Chemie, Johann Wolfgang Goethe-Universita¨t Frankfurt/M, Marie-Curie-Street 11, D-60439 Frankfurt/M, Germany b Institut fu¨r Oberfla¨chen-und Schichtenanalytik (IFOS), Universita¨t Kaiserslautern, Germany c Experimentalphysik, Universita¨t Kassel, Germany Available online 26 February 2005
Abstract The potential of rapid thermal processing (RTP) for the preparation of thin films of niobium oxynitrides was investigated. The 200 and 500 nm niobium films were deposited via sputtering on oxidized silicon(1 0 0)- and on sapphire(1 1 0 2)-substrates. At first, oxidation of niobium films in molecular oxygen and then nitridation in ammonia using an RTP-system was performed. The films were characterized before and after the oxidation and nitridation processes by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS). The influence of the two different substrates, amorphous SiO2 and single crystalline sapphire on the reactivity of the niobium films was studied in dependence of temperature, time of reaction and film thickness. The existence of niobium oxynitride formation was verified for some of the films. In some of the experiments, crack formation in the films or even delamination of the Nb-films from the substrates was observed. # 2005 Elsevier B.V. All rights reserved. Keywords: Niobium oxynitride; Rapid thermal processing (RTP); Thin films; X-ray diffraction (XRD); SIMS depth profile
1. Introduction Transition metal nitrides and oxynitrides have received increasing attention in recent years, because of their interesting and attractive chemical and * Corresponding author. Tel.: +49 69 798 29154; fax: +49 69 798 29235. E-mail address: [email protected] (B.O. Kolbesen).
physical properties [1]. As thin films, transition metal nitrides are used in a number of advanced tools and technologies [2]. Rapid thermal processing (RTP) is a key technology in microelectronics manufacturing for thin oxide or silicide film preparation [3]; so, this technology might also be established for further applications, for example solid-state reactions or formations of nitride films [4]. In this work, the possibility of niobium oxynitride formation by RTP was studied.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.119
206
V.A. Matylitskaya et al. / Applied Surface Science 252 (2005) 205–210
2. Experimental
3. Results and discussion
The substrates used in this study were 150 mm in diameter silicon wafers with a thermally grown 100 nm top oxide SiO2 (to prevent metal–silicon interactions and silicide formation) and sapphire (1 1 0 2). The 200 and 500 nm niobium layers were deposited by magnetron sputtering using Leybold Heraeus Z 700 sputtering device, equipped with Leybold sputtering cathodes PK 75 and niobium pellets of 99.9% purity by Superconductive Components Inc. The deposition rate was 27.72 nm min1 at a partial pressure of argon, pA = 102 mbar and plasma power of 100 W. Rapid thermal processing experiments were performed under atmospheric pressure in oxygen and ammonia gas flow (1.5 l min1, 99.999% gas purity), using a SHS 100 RTP system by Steag-a.s.t. with a wafer tray inside a quartz chamber. Substrate pieces with niobium films (1 cm 0.5 cm) were heated by top and bottom tungsten lamps with heating rates between 50 and 100 K s1. Temperature control during the experiments was accomplished by pyrometry [4]. At first, niobium films were oxidized by RTP in molecular oxygen at temperatures ranging from 350 to 500 8C and for times of 1, 2 and 5 min. After oxidation, the samples were nitridated in the RTP system by 1 min reaction with ammonia at 1000 8C. For analysis of the products, different methods were applied. The crystalline phases were characterized by X-ray diffraction (XRD) using a STOE u/udiffractometer with a multilayer mirror, using Cu Ka radiation. Elemental analysis of the films was carried out with an Amray SEM/Oxford Instruments EDX system using excitation energies between 5 and 20 kV. Surface morphology of the niobium films before and after oxidation and nitridation was characterized by atomic force microscopy (AFM) with a Nanoscope III A SPM (Digital Instruments) operated in tapping mode. The nominal cantilever spring constant was 42 N m1. Depth profile analysis was performed in a Cameca IMS 4 f system. Sputtering was carried out using Cs+ions of 5.5 keV energy, scanned on an area of 125 mm 125 mm, at a current density of 0.19 mA cm2.
The XRD diagrams of the 200 nm niobium films on SiO2/Si after oxidation in molecular oxygen at 350 8C have a slight shift of the Nb reflection to lower values of 2u. This shift enlarges with increasing oxidation time. The slight lattice expansion might be caused by the incorporation of oxygen into the metal, which is equivalent to the formation of the Nb(O) phase (solid solution). Formation of the substoichiometric phase NbO0.7 occurs by oxidation of Nb-films at 400 8C. The presence of two oxide phases can be observed after 1 min of RTP at 450 8C with O2: NbO0.7 and NbO. Parallel with these two phases, formation of Nb2O5 starts after 2 min reaction with O2 at 450 8C. The SIMS depth profiles of this sample (Fig. 1) show that concentration of oxygen decreases to the Nb/SiO2 interface. The depth distribution of the phases in this film in agreement with XRD (not shown) and SIMS data are as follows: close to the surface, Nb2O5; middle region, NbO and NbO0.7; close to the interface, solid solution Nb(O). Complete conversion of the Nbfilm to Nb2O5 occurs after 5 min RTP in oxygen at 450 8C. This temperature and time of the reaction are critical for the oxidation of niobium films, because beginning delamination of the oxidized niobium film from substrate becomes visible. The presence of four different phases (NbO2, NbN0.95, Nb3.49N4.56O0.44 and Nb4N5) can be observed after 1 min nitridation in ammonia at 1000 8C for all oxidized 200 nm Nb-films (except for the film oxidized during 5 min at 450 8C). As typical example, the XRD of a 200 nm Nb-film after oxidation at 450 8C for 2 min and nitridation in NH3 at 1000 8C for 1 min is shown in Fig. 2. The phase NbO2 dominates in these films. Content of oxynitrides in Nbfilms after nitridation enlarges with increasing oxidation temperature. Comparing XRD (Fig. 2) with SIMS depth profiles (Fig. 3), the following distribution of phases can be derived: close to the surface, NbN0.95 and Nb4N5; middle region, Nb3,49N4.56O0.44; close to the interface, NbO2. The nitrogen and oxygen concentration profiles in Fig. 3 indicate a snowplough effect: the in-diffusing nitrogen pushes the oxygen deeper into the bulk of the film resulting in a pile-up of oxygen towards the substrate interface. A similar effect has been observed in the nitridation of Ti-films [5]. The formation of the two phases, niobium
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207
Fig. 1. SIMS depth profile of 200 nm Nb-film on SiO2/Si-substrate after RTP in O2 for 2 min at 450 8C.
oxynitride and niobium nitride (Nb4N5), occurs after nitridation of Nb-films, oxidized during 5 min at 450 8C in oxygen. According to the XRD-data these two phases or one of them (Nb3,49N4.56O0.44 or Nb4N5) could be present in this film. Nitridation of this film
leads to further proceeding delamination of Nb-films from substrate in comparison with oxidized film. Oxidation of 500 nm Nb-films on SiO2/Si-substrate in oxygen at 350 and 400 8C, and during 1 min at 450 8C leads to the formation of solid solution of niobium due to
Fig. 2. XRD 200 nm Nb-film on SiO2/Si-substrate after RTP in O2 at 450 8C for 2 min and in NH3 at 1000 8C for 1 min.
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V.A. Matylitskaya et al. / Applied Surface Science 252 (2005) 205–210
Fig. 3. SIMS depth profile of 200 nm Nb-film on SiO2/Si-substrate after RTP in O2 for 2 min at 450 8C and 1 min in NH3 at 1000 8C.
the incorporation of oxygen into the metal lattice. Formation of two phases, NbO0.7 and Nb2O5, takes place after 2 min of RTP in oxygen at 450 8C. Increasing of the oxidation time up to 5 min leads to the formation of Nb2O5 only, accompanied by delamination of Nbfilms from the SiO2/Si-substrate. After 1 min nitridation in ammonia at 1000 8C of oxidized Nb-films, the presence of six different phases in accordance with XRD-measurements can be observed: NbN0.95, Nb2N, NbO, NbO2, Nb3.49N4.56O0.44 and Nb4N5 (except for the film oxidized during 5 min at 450 8C). According to the intensities of the reflections, NbN0.95 and Nb2N are the main products in the films, oxidized at 350, 400 and 450 8C for 1 min. NbO dominates in the film oxidized during 2 min at 450 8C. Nitridation of the Nb-films, oxidized during 5 min at 450 8C and 1 min at 500 8C in oxygen, leads to the formation of two phases, Nb3,49N4.56O0.44 and Nb4N5 (Fig. 4). AFM measurements show that surface roughness of this film significantly increases compared with the as deposited 500 nm niobium film on SiO2/Si-substrate (as deposited film ! Rms = 6.786 nm, after reaction 1 min O2 500 8C + 1 min NH31000 8C ! Rms = 52.376 nm). At the surface, AFM and SEM investigations reveal cracks of this sample. This seems to be the pre-stage of delamination. Oxidation of 200 nm Nb-films on sapphire at 350 8C, and during 1 and 2 min at 400 8C leads to the
formation of solid solution of niobium Nb(O) and niobium oxide NbxO y. This oxide cannot be clearly identified according to the available XRD data base. Formation of two phases, NbxO y and Nb6O, occurs after oxidation during 5 min at 400 8C and 1 and 2 min at 450 8C. Parallel with these two phases, formation of Nb2O5 starts after 5 min reaction with O2 at 450 8C. Complete conversion of the films to niobium pentoxide after 1 min reaction with O2 at 500 8C leads to visible delamination of Nb-films from substrate. The presence of five different phases: NbO2, NbN0.95, Nb2N, Nb3.49N4.56O0.44 and Nb4N5 can be observed after 1 min nitridation in ammonia at 1000 8C of all oxidized 200 nm Nb-films on sapphire (except for the film oxidized during 1 min at 500 8C). NbO2 dominates in these films. Formation of niobium oxynitride (Nb3.49N4.56O0.44) and niobium nitride (Nb4N5) occurs by nitridation of the film, oxidized during 1 min at 500 8C. SEM measurements show holes on the surface of Nb-films after nitridation, which very likely is the result of hydrogen diffusion to and agglomeration at/below the surface. The formation of hydrogen occurs due to decomposition of ammonia. XRD diagrams of 500 nm niobium films on sapphire after oxidation at 350 and 400 8C show formation of solid solution of niobium Nb(O) and niobium oxide NbxO y (Table 1). The formation of two
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Fig. 4. XRD 500 nm Nb-film on SiO2/Si-substrate after RTP in O2 at 500 8C for 1 min and in NH3 at 1000 8C for 1 min.
phases NbxO y and Nb6O occurs after 1 min of RTP in oxygen at 450 8C. Parallel with these two phases, a formation of Nb2O5 takes place after 2 and 5 min reaction with O2 at 450 8C. Complete conversion of the films to niobium pentoxide occurs after 1 min reaction with O2 at 500 8C. This temperature and time of reaction are critical for the oxidation of 500 nm Nb-films on sapphire, because beginning delamination of niobium films from substrate becomes visible. Nitridation of these samples (except for the films oxidized during 1 min at 500 8C and 5 min at 450 8C), according to XRDdata, leads to the formation of six phases: NbN,
NbN0.95, Nb2N, NbO, Nb3.49N4.56O0.44 and Nb4N5. According to the intensities of the reflections, niobium nitrides are the main products in the films, oxidized at 350 and 400 8C, and during 1 and 2 min at 450 8C. Parallel with these six phases formation of NbO2 takes place after nitridation of the films, oxidized during 5 min at 450 8C. The main products in these films are Nb3.49N4.56O0.44 and Nb4N5. Nitridation of the films oxidized during 1 min at 500 8C in O2 leads to the formation of two phases Nb3.49N4.56O0.44 and Nb4N5. According to XRDdata, increasing oxidation temperature leads to increasing oxynitride content in the film by
Table 1 The reaction products of 500 nm niobium films on sapphire with oxygen and ammonia by RTP
Temperature (8C)
Time (min)
Products of the reaction
Products of the reaction with NH3, 1 min 1000 8C
350 400
1, 2, 5 1, 2, 5 1 2
Solid solution Nb(O), NbxOy NbxOy, Nb6O
NbN, NbN0.95, Nb2N, NbO, Nb4N5, Nb3.49N4.56O0.44
Reaction with O2
5
NbxOy, Nb6O, Nb2O5 Nb2O5
Nb4N5, Nb3.49N4.56O0.44, NbO2, NbO, NbN, NbN0.95, Nb2N
1
Nb2O5
Nb4N5, Nb3.49N4.56O0.44
450
500
210
V.A. Matylitskaya et al. / Applied Surface Science 252 (2005) 205–210
nitridation. SEM measurements show that increasing oxidation temperature enlarges the roughness of the films.
ammonia. Increasing oxidation temperature leads to the enlargement of the film roughness. The number of phases coexisting in 500 nm films is higher than in 200 nm films; 200 nm niobium films show a better reactivity than 500 nm films.
4. Summary and conclusions The 200 and 500 nm niobium films have been deposited via sputtering on oxidized silicon(1 0 0) wafers and on sapphire(1 1 0 2). Oxidation of niobium films in molecular oxygen followed by nitridation in ammonia using RTP has been investigated. Formation of niobium oxynitrides occurs by nitridation in ammonia of niobium films on SiO2/ silicon and on sapphire oxidized before in oxygen. Content of oxynitrides in film after nitridation enlarges with increasing oxidation temperature. Beginning delamination of the niobium films starts by oxidation at 450 8C according to SEM data. Surface roughness of 200 and 500 nm niobium films increases during the reactions with oxygen and
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