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Annealing effects on zirconium nitride films
Applied Surface Science 230 (2004) 88–93
Annealing effects on zirconium nitride films H.B. Bhuvaneswarib, V. Rajagopal Reddyb, R. Chandramanic, G. Mohan Raoa,* a
Department of Instrumentation, Indian Institute of Science, Bangalore 560012, India b Mysore University, PG Centre, Hassan, India c Bangalore University, Bangalore, India Accepted 3 February 2004 Available online 22 April 2004
Abstract ZrN films were deposited by dc reactive magnetron sputtering on silicon substrates under optimized nitrogen partial pressure of 6 105 mbar. Structural, electrical and optical properties were systematically investigated. Films deposited at room temperature exhibited Schottky structure without any silicide interfacial layer. These films have electrical resistivity of 4:23 103 O cm, which were crystalline in nature, with cubic (1 1 1) orientation. Refractive index and extinction coefficient were found to be 1.95 and 0.43, respectively at a wavelength of 350 nm. Samples were annealed for 1 h in air at two temperatures, 350 and 550 8C. Scanning electron microscopy (SEM) and energy dispersive analysis of X-rays (EDAX) showed alloy penetration pits. Extent of penetration was greater in the films, which were annealed at higher temperature (550 8C). Variation in refractive index was observed in the range of 1.95–1.80 at 350 nm, for the annealed films, with increase in grain size from 7.25 to 11.10 nm. Poly-crystalline nature has been observed with (1 1 1) and (2 0 1) orientations. Resistivity is found to increase from 4:23 103 to 6:21 103 O cm. # 2004 Elsevier B.V. All rights reserved. Keywords: Sputtering; Pits formation; Nano-particles; Schottky devices; Resistivity; Annealing
1. Introduction The present day development in microelectronics is towards newer type of devices like DRAM, switching devices, sensors, etc. Metal–semiconductor contacts have been a subject of interest in the field of electronic circuits-advanced VLSI/ULSI devices. Recently, many scientists [1–3] have reported their efforts on the preparation and physical properties of metallic nano-particles. Zirconium nitrides reveal interesting *
Corresponding author. Tel.: þ91-80-309-2349; fax: þ91-80-347-3758. E-mail address: [email protected] (G. Mohan Rao).
optical, electrical, structural properties, which highly depend on nitrogen stoichiometry. Indeed, this material exhibits a transition from stable metallic ZrN to meta-stable semi-transparent insulating Zr3N4. Transition metal nitrides find applications in many fields [4]. Metallization in semiconductor devices is not only for carrying current, but also plays an active role in determining device parameter, as in case of gate electrode/Schottky barrier diode. Metallization also plays two important roles, viz., it controls the speed of the circuit by virtue of the resistance of the interconnection runners and it controls the so-called flat band voltage. Some of the desired properties of
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.055
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
metallization for integrated circuits in VLSI/ULSI are low resistivity, mechanical stability, good adherence, low stress, low contact resistance, minimum junction penetration and low electromigration. Apart from these, it should also enable easy pattern generation by etching techniques and high stability. Most of the metals normally used with silicon form Schottky barrier diodes. Many react with substrate during the process temperature excursions, to form metal silicide. Silicide formation is advantageous for reducing the contamination, since the critical metal–semiconductor rectifying interface is within the silicon substrate, where it has not been exposed to the environment. Studies on Schottky barrier is interesting as electrical characteristics are extremely sensitive to conditions of the interface between metal and semiconductor, contamination, oxide layers or
Fig. 1. X-ray diffraction patterns for ZrN films: (a) as deposited (room temperature), (b) annealed at 350 8C and (c) 550 8C.
89
metallurgical reactions which can cause major variations in diode behavior. Current materials of interest as alternatives to the conventional metal contacts are the nitrides of titanium, tantalum and zirconium. Titanium nitride as a barrier layer for microelectronic applications is widely studied [5–8]. In the present study, we have deposited zirconium nitride on silicon substrate and studied the Schottky barrier thus formed by annealing at different temperatures.
2. Experimental ˚ ) were deposiThin films of ZrN (thickness 1800 A ted on silicon substrates by dc reactive magnetron sputtering from pure zirconium disc of 100 mm diameter under optimized N2 partial pressure of
Fig. 2. Optical reflectance of the annealed films.
90
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
6 105 mbar. Optimization, fabrication and characterization of ZrN Schottky devices have been reported earlier [4,9]. ZrN/Si structure was fabricated at room temperature on polished p-type silicon wafer (r ¼ 10 O cm). The deposited films were characterized for crystallographic orientation and surface morphology at room temperature. The films were uniform, adhesive, and smooth with crystalline nature and have a resistivity of 4:23 103 O cm. Later, the samples were annealed at 350 and 550 8C for 1 h in air. The crystalline structure of all these annealed films was analyzed with X-ray diffractometer (Siefert XRD ˚ ). PW 3710) using Cu Ka radiation (l ¼ 1:5418 A Electrical resistivity of the films was measured using four-point probe technique. Reflectance of the films was measured with Hitachi (330A) double-beam spectro-photometer. Variations in refractive index (n) and extinction co-efficient (k) were calculated from the optical reflectance data [10]. Surface morphological changes, alloy penetration pits/cracks formations due to heat treatment of ZrN films were examined with scanning electron microscope (JSM 5600LV). In order to study the effect of heating on the diffusion characteristics of ZrN into silicon and vice versa, energy dispersive analysis of X-rays was carried out on the
cross-section of the sample at the substrate as well as on the surface.
3. Results and discussion 3.1. X-ray diffraction studies Fig. 1 shows X-ray diffraction pattern of ZrN films deposited at room temperature and annealed films at 350 and 550 8C for 1 h in air. X-ray diffraction peaks indicate that as-deposited ZrN films were crystalline with cubic phase having (1 1 1) orientation and the annealed films showed both cubic (1 1 1) and hexagonal (2 0 1) phases [11], corresponding to ZrN. It also shows peaks corresponding to those of ZrO2 and unreacted Zr. During reactive sputtering, some atoms of zirconium do not react with nitrogen and this results in the presence of unreacted zirconium. Intensity of peaks corresponding to (1 1 1) and (2 0 1) peaks have increased with the increasing annealing temperature and this phenomenon is predominant in both the annealed samples. At 350 8C, intensity of ZrO2 also increases because at higher temperatures the possibility of unreacted Zr undergoing oxidation is more.
Fig. 3. Optical constants of ZrN films at a wavelength of 350 nm.
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
This is similar to the earlier results obtained using RF reactive sputtering [12]. The grain size (D) of the cubic (1 1 1) ZrN phase has been calculated using Scherrer formula [13]. D¼
0:9l B cos y
˚, B where l is the wavelength of X-ray ¼ 1:5418 A 2 2 2 is calculated from B ¼ ðD2yÞ b . Here, D2y is full-width at half-maximum (in radians) and b the instrumental broadening taking the peak at 2y ¼ 33:968. The grain size for the films annealed at 350 8C was 7.2 nm and it increased to 11.1 nm in case of films annealed at 550 8C. The increase in grain size can also indicate a relaxation stress by the formation of dislocation/small crystallites with increasing temperatures. Scanning electron microscopy has confirmed the increase in pits formation/dislocation. Thus, film becomes more compact. Grain size variation in the films is due to annealing/electro-migration. Larger grain minimizes grain boundary area and the diffusion and electro-migration is expected to be reduced further. Thermally, larger grains provide more stability.
91
3.3. Optical studies The optical reflectance of the as-deposited and annealed films is shown in Fig. 2. Optical constants were determined for ZrN films by using reflectance spectrum in the wavelength range of 300–800 nm. It is seen that as annealing temperature is increased, difference in Rmax Rmin is increased. Also, we can observe that the interference pattern shifts towards lower wavelength side indicating deficiency in nitrogen and formation of oxide as explained earlier.
3.2. Electrical resistivity Electrical resistivity of ZrN films (films deposited at room temperature and annealed films at 350 and 550 8C) was measured by using four-point probe technique. The annealed films showed an increase in resistivity from 4:23 103 to 6:21 103 O cm. As the films were subjected to annealing, stoichiometric deviations lead to defects such as vacancies or interstitials that behave like impurities, which cause an increase in resistivity. Annealing in ambient air influences the oxidation rate, by increasing formation of voids, formation of ZrO2 and helps the diffusion process. Thus, solubility increases because of vacancies, which is initially due to oxidation [17]. The presence of oxide in the films can be seen in the reflectance spectrum as interference pattern shifts towards lower wavelengths. This argument is in accordance with the published data [14,17]. Apart from oxidation, the increase in resistivity for the annealed sample could also be due to dislocation, voids/cracks, grain boundaries and increase in number of pits formation at higher annealing temperature, which is confirmed from SEM and EDAX.
Fig. 4. Scanning electron micrographs of ZrN films: (a) as deposited (room temperature), (b) annealed at 350 8C and (c) 550 8C.
92
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
The refractive index (n) and extinction coefficient (k) of ZrN films (at a wavelength 350 nm) as a function of different temperatures are shown in Fig. 3. The as-deposited films show a high refractive index of 1.95, which decreases to 1.80 with increasing in annealing temperature. A similar variation has been observed for ZrN films prepared by double ion beam sputtering [15,16]. This can be explained to be due to structural changes and changes in density of the films. The decrease in refractive index could be due to densification of film and crystallization of ZrN when annealed up to 550 8C. Pichon et al. [15] reported variation in refractive index between 1.6 and 2.3 at 350 nm and k was found to be between 0.4 and 1.2 at 350 nm. The optical index variation could have been simulated by free electrons which is characteristic of a good conducting sample. These variations can also be due to changes in stoichiometry of films at different annealing temperature, change in packing density due to oxygen incorporation in films, change in crystal structure and due to cluster/pits formation as observed in the diffusion studies and explained later. 3.4. Diffusion studies ZrN/Si films deposited at room temperature on ˚ thick without silicon were adherent, dense, 1800 A any voids formation as shown in Fig. 4. Metallization
ZrN FILM
si
can be completely destroyed by reactions induced by thermally activated processes with the substrates or layers on top. Diodes fabricated with ZrN metallization in contact with silicon, when annealed at 350 8C and 550 8C for 1 h undergo a process called alloy penetration [18]. This process reduces native oxides if any, in the contacts. This process depends on the annealing temperature and duration of annealing. As annealing temperature increases dissolution of silicon by diffusion into the metal leads to pits formation. This is similar to annealing effects of aluminium on Si [17]. As annealing temperature increases, silicon from the substrate diffuses into ZrN through grain-boundary paths to satisfy solubility. Concomitantly, ZrN can also diffuse into the substrate. This process is shown in Fig. 5. Thus, as the pit size varies, Si-to-ZrN contact area increases, allowing pits with different sizes to form. The dissolution process during annealing is highly non-uniform due to the presence of non-uniform interfacial layer which leads to isolated crystallographic etch pits. Pit growth is non-uniform and pit size is directly related to square root of annealing time [17]. Thus, the contact between ZrN–Si structures fails due to precipitates of dissolved silicon (from ZrN) on cooling and it is expected to cause an undesirable increase in resistance [18]. Extent of penetration is greater at higher temperature, thus leading to a higher
si
OXIDE SILICON
ZrN FILM OXIDE SILICON
Fig. 5. Diffusion process in ZrN/Si structure. (Formation of alloy penetration pits.)
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
93
Table 1 Composition changes at the substrate due to annealing
References
Elements in %
[1] W.P. Halperin, Rev. Mod. Phys. 58 (1986) 533. [2] J. Musil, J. Vicek, Surf. Coat. Technol. 142 (2001) 557. [3] C.J. Tavarer, L. Rebouta, B. Almeida, J. Bessa e sousa, M.F. da Silva, J.C. Soares, Thin Solid Films 317 (1998) 124. [4] H.B. Bhuvaneswari, I. Nithiya Priya, R. Chandramani, V. Rajagopal Reddy, G. Mohan Rao, J. Cryst. Res. Technol. 38 (2003) 1047. [5] C.-H. Ma, J.-H. Huang, H. Chen, Surf. Coat. Technol. 133 (2000) 289. [6] S. Kanamori, Thin Solid Films 136 (1986) 195. [7] M. Ostring, S. Nygren, C.S. Petersson, et al., Thin Solid Films 145 (1986) 81. [8] K. Hinode, Y. Homma, M. Horiuchi, T. Takahashi, J. Vac. Sci. Technol. A 15 (4) (1997) 2017. [9] H.B. Bhuvaneswari, V. Rajagopal Reddy, R. Chandramani, G. Mohan Rao, in: the Conference on Recent Developments and Challenges in Physics, University of Hyderabad, December 2002. [10] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [11] JCPDS Data—International Center for Diffraction Data, v. 1.30 (40-1275, 35-0753), 1997. [12] M. Nose, M. Zhou, T. Nagae, et al., Surf. Coat. Technol. 132 (2000) 163. [13] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Workingham, UK, 1967. [14] T.K. Subramanyam, B. Srinivasulu Naidu, S. Uthanna, Appl. Surf. Sci. 169 (2001) 529. [15] L. Pichon, T. Girardeau, A. Straboni, et al., Appl. Surf. Sci. 150 (1999) 115. [16] L. Pichon, T. Girardeau, A. Straboni, Surf. Coat. Technol. 125 (2000) 100. [17] S.M. Sze, VLSI Technology, second ed., McGraw-Hill, Bell Laboratories, Murray Hill, NJ, 1988. [18] J.M. Poate (Ed.), Thin Film Inter-Diffusion and Reaction, Bell Laboratories, Murray Hill, NJ, 1978.
Zr Si
As deposited
– 100
Film, annealed at 350 8C
550 8C
19.53 80.47
18.87 81.13
diffusion rates and solubility. SEM clearly indicates alloy penetration pits and this has been confirmed from EDAX data taken at the substrate as shown in Table 1.
4. Conclusions ZrN films were prepared onto silicon substrates by dc reactive magnetron sputtering technique. Structural studies revealed that the films were crystalline in nature at room temperature and showed cubic phase, which changed to cubic and hexagonal phase after annealing. Presence of zirconium oxide was indicated in the annealed films and these changes showed an effect on the properties of the films. The electrical resistivity for annealed samples has increased from 4:23 103 to 6:21 103 O cm. The variation in refractive index and extinction co-efficient was found to be from 1.95 to 1.80 and 0.44 to 0.15 for as-deposited and annealed films, respectively. Alloy penetration pits have been observed in annealed films. Further studies are in progress to study zirconium nitride coating as a conducting layer in MOS structures.
Annealing effects on zirconium nitride films H.B. Bhuvaneswarib, V. Rajagopal Reddyb, R. Chandramanic, G. Mohan Raoa,* a
Department of Instrumentation, Indian Institute of Science, Bangalore 560012, India b Mysore University, PG Centre, Hassan, India c Bangalore University, Bangalore, India Accepted 3 February 2004 Available online 22 April 2004
Abstract ZrN films were deposited by dc reactive magnetron sputtering on silicon substrates under optimized nitrogen partial pressure of 6 105 mbar. Structural, electrical and optical properties were systematically investigated. Films deposited at room temperature exhibited Schottky structure without any silicide interfacial layer. These films have electrical resistivity of 4:23 103 O cm, which were crystalline in nature, with cubic (1 1 1) orientation. Refractive index and extinction coefficient were found to be 1.95 and 0.43, respectively at a wavelength of 350 nm. Samples were annealed for 1 h in air at two temperatures, 350 and 550 8C. Scanning electron microscopy (SEM) and energy dispersive analysis of X-rays (EDAX) showed alloy penetration pits. Extent of penetration was greater in the films, which were annealed at higher temperature (550 8C). Variation in refractive index was observed in the range of 1.95–1.80 at 350 nm, for the annealed films, with increase in grain size from 7.25 to 11.10 nm. Poly-crystalline nature has been observed with (1 1 1) and (2 0 1) orientations. Resistivity is found to increase from 4:23 103 to 6:21 103 O cm. # 2004 Elsevier B.V. All rights reserved. Keywords: Sputtering; Pits formation; Nano-particles; Schottky devices; Resistivity; Annealing
1. Introduction The present day development in microelectronics is towards newer type of devices like DRAM, switching devices, sensors, etc. Metal–semiconductor contacts have been a subject of interest in the field of electronic circuits-advanced VLSI/ULSI devices. Recently, many scientists [1–3] have reported their efforts on the preparation and physical properties of metallic nano-particles. Zirconium nitrides reveal interesting *
Corresponding author. Tel.: þ91-80-309-2349; fax: þ91-80-347-3758. E-mail address: [email protected] (G. Mohan Rao).
optical, electrical, structural properties, which highly depend on nitrogen stoichiometry. Indeed, this material exhibits a transition from stable metallic ZrN to meta-stable semi-transparent insulating Zr3N4. Transition metal nitrides find applications in many fields [4]. Metallization in semiconductor devices is not only for carrying current, but also plays an active role in determining device parameter, as in case of gate electrode/Schottky barrier diode. Metallization also plays two important roles, viz., it controls the speed of the circuit by virtue of the resistance of the interconnection runners and it controls the so-called flat band voltage. Some of the desired properties of
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.02.055
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
metallization for integrated circuits in VLSI/ULSI are low resistivity, mechanical stability, good adherence, low stress, low contact resistance, minimum junction penetration and low electromigration. Apart from these, it should also enable easy pattern generation by etching techniques and high stability. Most of the metals normally used with silicon form Schottky barrier diodes. Many react with substrate during the process temperature excursions, to form metal silicide. Silicide formation is advantageous for reducing the contamination, since the critical metal–semiconductor rectifying interface is within the silicon substrate, where it has not been exposed to the environment. Studies on Schottky barrier is interesting as electrical characteristics are extremely sensitive to conditions of the interface between metal and semiconductor, contamination, oxide layers or
Fig. 1. X-ray diffraction patterns for ZrN films: (a) as deposited (room temperature), (b) annealed at 350 8C and (c) 550 8C.
89
metallurgical reactions which can cause major variations in diode behavior. Current materials of interest as alternatives to the conventional metal contacts are the nitrides of titanium, tantalum and zirconium. Titanium nitride as a barrier layer for microelectronic applications is widely studied [5–8]. In the present study, we have deposited zirconium nitride on silicon substrate and studied the Schottky barrier thus formed by annealing at different temperatures.
2. Experimental ˚ ) were deposiThin films of ZrN (thickness 1800 A ted on silicon substrates by dc reactive magnetron sputtering from pure zirconium disc of 100 mm diameter under optimized N2 partial pressure of
Fig. 2. Optical reflectance of the annealed films.
90
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
6 105 mbar. Optimization, fabrication and characterization of ZrN Schottky devices have been reported earlier [4,9]. ZrN/Si structure was fabricated at room temperature on polished p-type silicon wafer (r ¼ 10 O cm). The deposited films were characterized for crystallographic orientation and surface morphology at room temperature. The films were uniform, adhesive, and smooth with crystalline nature and have a resistivity of 4:23 103 O cm. Later, the samples were annealed at 350 and 550 8C for 1 h in air. The crystalline structure of all these annealed films was analyzed with X-ray diffractometer (Siefert XRD ˚ ). PW 3710) using Cu Ka radiation (l ¼ 1:5418 A Electrical resistivity of the films was measured using four-point probe technique. Reflectance of the films was measured with Hitachi (330A) double-beam spectro-photometer. Variations in refractive index (n) and extinction co-efficient (k) were calculated from the optical reflectance data [10]. Surface morphological changes, alloy penetration pits/cracks formations due to heat treatment of ZrN films were examined with scanning electron microscope (JSM 5600LV). In order to study the effect of heating on the diffusion characteristics of ZrN into silicon and vice versa, energy dispersive analysis of X-rays was carried out on the
cross-section of the sample at the substrate as well as on the surface.
3. Results and discussion 3.1. X-ray diffraction studies Fig. 1 shows X-ray diffraction pattern of ZrN films deposited at room temperature and annealed films at 350 and 550 8C for 1 h in air. X-ray diffraction peaks indicate that as-deposited ZrN films were crystalline with cubic phase having (1 1 1) orientation and the annealed films showed both cubic (1 1 1) and hexagonal (2 0 1) phases [11], corresponding to ZrN. It also shows peaks corresponding to those of ZrO2 and unreacted Zr. During reactive sputtering, some atoms of zirconium do not react with nitrogen and this results in the presence of unreacted zirconium. Intensity of peaks corresponding to (1 1 1) and (2 0 1) peaks have increased with the increasing annealing temperature and this phenomenon is predominant in both the annealed samples. At 350 8C, intensity of ZrO2 also increases because at higher temperatures the possibility of unreacted Zr undergoing oxidation is more.
Fig. 3. Optical constants of ZrN films at a wavelength of 350 nm.
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
This is similar to the earlier results obtained using RF reactive sputtering [12]. The grain size (D) of the cubic (1 1 1) ZrN phase has been calculated using Scherrer formula [13]. D¼
0:9l B cos y
˚, B where l is the wavelength of X-ray ¼ 1:5418 A 2 2 2 is calculated from B ¼ ðD2yÞ b . Here, D2y is full-width at half-maximum (in radians) and b the instrumental broadening taking the peak at 2y ¼ 33:968. The grain size for the films annealed at 350 8C was 7.2 nm and it increased to 11.1 nm in case of films annealed at 550 8C. The increase in grain size can also indicate a relaxation stress by the formation of dislocation/small crystallites with increasing temperatures. Scanning electron microscopy has confirmed the increase in pits formation/dislocation. Thus, film becomes more compact. Grain size variation in the films is due to annealing/electro-migration. Larger grain minimizes grain boundary area and the diffusion and electro-migration is expected to be reduced further. Thermally, larger grains provide more stability.
91
3.3. Optical studies The optical reflectance of the as-deposited and annealed films is shown in Fig. 2. Optical constants were determined for ZrN films by using reflectance spectrum in the wavelength range of 300–800 nm. It is seen that as annealing temperature is increased, difference in Rmax Rmin is increased. Also, we can observe that the interference pattern shifts towards lower wavelength side indicating deficiency in nitrogen and formation of oxide as explained earlier.
3.2. Electrical resistivity Electrical resistivity of ZrN films (films deposited at room temperature and annealed films at 350 and 550 8C) was measured by using four-point probe technique. The annealed films showed an increase in resistivity from 4:23 103 to 6:21 103 O cm. As the films were subjected to annealing, stoichiometric deviations lead to defects such as vacancies or interstitials that behave like impurities, which cause an increase in resistivity. Annealing in ambient air influences the oxidation rate, by increasing formation of voids, formation of ZrO2 and helps the diffusion process. Thus, solubility increases because of vacancies, which is initially due to oxidation [17]. The presence of oxide in the films can be seen in the reflectance spectrum as interference pattern shifts towards lower wavelengths. This argument is in accordance with the published data [14,17]. Apart from oxidation, the increase in resistivity for the annealed sample could also be due to dislocation, voids/cracks, grain boundaries and increase in number of pits formation at higher annealing temperature, which is confirmed from SEM and EDAX.
Fig. 4. Scanning electron micrographs of ZrN films: (a) as deposited (room temperature), (b) annealed at 350 8C and (c) 550 8C.
92
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
The refractive index (n) and extinction coefficient (k) of ZrN films (at a wavelength 350 nm) as a function of different temperatures are shown in Fig. 3. The as-deposited films show a high refractive index of 1.95, which decreases to 1.80 with increasing in annealing temperature. A similar variation has been observed for ZrN films prepared by double ion beam sputtering [15,16]. This can be explained to be due to structural changes and changes in density of the films. The decrease in refractive index could be due to densification of film and crystallization of ZrN when annealed up to 550 8C. Pichon et al. [15] reported variation in refractive index between 1.6 and 2.3 at 350 nm and k was found to be between 0.4 and 1.2 at 350 nm. The optical index variation could have been simulated by free electrons which is characteristic of a good conducting sample. These variations can also be due to changes in stoichiometry of films at different annealing temperature, change in packing density due to oxygen incorporation in films, change in crystal structure and due to cluster/pits formation as observed in the diffusion studies and explained later. 3.4. Diffusion studies ZrN/Si films deposited at room temperature on ˚ thick without silicon were adherent, dense, 1800 A any voids formation as shown in Fig. 4. Metallization
ZrN FILM
si
can be completely destroyed by reactions induced by thermally activated processes with the substrates or layers on top. Diodes fabricated with ZrN metallization in contact with silicon, when annealed at 350 8C and 550 8C for 1 h undergo a process called alloy penetration [18]. This process reduces native oxides if any, in the contacts. This process depends on the annealing temperature and duration of annealing. As annealing temperature increases dissolution of silicon by diffusion into the metal leads to pits formation. This is similar to annealing effects of aluminium on Si [17]. As annealing temperature increases, silicon from the substrate diffuses into ZrN through grain-boundary paths to satisfy solubility. Concomitantly, ZrN can also diffuse into the substrate. This process is shown in Fig. 5. Thus, as the pit size varies, Si-to-ZrN contact area increases, allowing pits with different sizes to form. The dissolution process during annealing is highly non-uniform due to the presence of non-uniform interfacial layer which leads to isolated crystallographic etch pits. Pit growth is non-uniform and pit size is directly related to square root of annealing time [17]. Thus, the contact between ZrN–Si structures fails due to precipitates of dissolved silicon (from ZrN) on cooling and it is expected to cause an undesirable increase in resistance [18]. Extent of penetration is greater at higher temperature, thus leading to a higher
si
OXIDE SILICON
ZrN FILM OXIDE SILICON
Fig. 5. Diffusion process in ZrN/Si structure. (Formation of alloy penetration pits.)
H.B. Bhuvaneswari et al. / Applied Surface Science 230 (2004) 88–93
93
Table 1 Composition changes at the substrate due to annealing
References
Elements in %
[1] W.P. Halperin, Rev. Mod. Phys. 58 (1986) 533. [2] J. Musil, J. Vicek, Surf. Coat. Technol. 142 (2001) 557. [3] C.J. Tavarer, L. Rebouta, B. Almeida, J. Bessa e sousa, M.F. da Silva, J.C. Soares, Thin Solid Films 317 (1998) 124. [4] H.B. Bhuvaneswari, I. Nithiya Priya, R. Chandramani, V. Rajagopal Reddy, G. Mohan Rao, J. Cryst. Res. Technol. 38 (2003) 1047. [5] C.-H. Ma, J.-H. Huang, H. Chen, Surf. Coat. Technol. 133 (2000) 289. [6] S. Kanamori, Thin Solid Films 136 (1986) 195. [7] M. Ostring, S. Nygren, C.S. Petersson, et al., Thin Solid Films 145 (1986) 81. [8] K. Hinode, Y. Homma, M. Horiuchi, T. Takahashi, J. Vac. Sci. Technol. A 15 (4) (1997) 2017. [9] H.B. Bhuvaneswari, V. Rajagopal Reddy, R. Chandramani, G. Mohan Rao, in: the Conference on Recent Developments and Challenges in Physics, University of Hyderabad, December 2002. [10] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [11] JCPDS Data—International Center for Diffraction Data, v. 1.30 (40-1275, 35-0753), 1997. [12] M. Nose, M. Zhou, T. Nagae, et al., Surf. Coat. Technol. 132 (2000) 163. [13] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Workingham, UK, 1967. [14] T.K. Subramanyam, B. Srinivasulu Naidu, S. Uthanna, Appl. Surf. Sci. 169 (2001) 529. [15] L. Pichon, T. Girardeau, A. Straboni, et al., Appl. Surf. Sci. 150 (1999) 115. [16] L. Pichon, T. Girardeau, A. Straboni, Surf. Coat. Technol. 125 (2000) 100. [17] S.M. Sze, VLSI Technology, second ed., McGraw-Hill, Bell Laboratories, Murray Hill, NJ, 1988. [18] J.M. Poate (Ed.), Thin Film Inter-Diffusion and Reaction, Bell Laboratories, Murray Hill, NJ, 1978.
Zr Si
As deposited
– 100
Film, annealed at 350 8C
550 8C
19.53 80.47
18.87 81.13
diffusion rates and solubility. SEM clearly indicates alloy penetration pits and this has been confirmed from EDAX data taken at the substrate as shown in Table 1.
4. Conclusions ZrN films were prepared onto silicon substrates by dc reactive magnetron sputtering technique. Structural studies revealed that the films were crystalline in nature at room temperature and showed cubic phase, which changed to cubic and hexagonal phase after annealing. Presence of zirconium oxide was indicated in the annealed films and these changes showed an effect on the properties of the films. The electrical resistivity for annealed samples has increased from 4:23 103 to 6:21 103 O cm. The variation in refractive index and extinction co-efficient was found to be from 1.95 to 1.80 and 0.44 to 0.15 for as-deposited and annealed films, respectively. Alloy penetration pits have been observed in annealed films. Further studies are in progress to study zirconium nitride coating as a conducting layer in MOS structures.