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Structural and surface properties of NiCr thin films prepared by DC magnetron sputtering under variation of annealing conditions
Microelectronic Engineering 82 (2005) 314–320 www.elsevier.com/locate/mee
Structural and surface properties of NiCr thin films prepared by DC magnetron sputtering under variation of annealing conditions Yong Kwon a, Nam-Hoon Kim b, Gwang-Pyo Choi b, Woo-Sun Lee c, Yong-Jin Seo d, Jinseong Park a,* a
Department of Advanced Materials Engineering, Chosun University, 375 Seosuk-Dong, Dong-Gu, Gwangju 501-759, Korea b Research Institute of Energy Resources Technology,Chosun University, Gwangju 501-759, Korea c Department of Electrical Engineering, Chosun University, Gwangju 501-759, Korea d Department of Electrical Engineering, Daebul University, Chonnam 526-702, Korea Available online 18 August 2005
Abstract NiCr thin films are widely used in several applications in microelectronics such as thin film resistors, filaments, and humidity sensors because of their relatively large resistivity, more resistant to oxidation and a low temperature coefficient of resistance (TCR). These interesting properties of NiCr thin films are dependent upon the preparation conditions including the deposition environment and subsequent annealing treatments. NiCr thin films were deposited by DC magnetron sputtering on Al2O3/Si substrate using 2-inch Ni/Cr (80/20) alloy. Annealing treatments were performed at 400, 500, and 600 C for 6 h in air and H2 ambient, respectively. The clear crystal boundaries without crystal growth and the densification were accomplished when the pores disappeared in air ambient. Most of surfaces are oxidic including NiO, Ni2O3 and CrxOy (x = 1,2, y = 2,3) after annealing in air ambient. The crystal growth in H2 ambient was formed and stabilized by combination with each other due to the suppression of oxidized substance on the film surface. Most Nioxides were reduced when the Cr-oxides were present due to their stability in high-temperature H2 ambient. 2005 Elsevier B.V. All rights reserved. Keywords: NiCr thin film; DC magnetron sputtering; Annealing; X-ray photoelectron spectroscopy (XPS)
1. Introduction
*
Corresponding author. Tel.: +82 62 230 7193; fax: +82 62 233 0302. E-mail address: [email protected] (J. Park).
With the miniaturization and lightness of electronic parts, the researches on preparing thin films have been actively conducted to use oxidic ceramic and alloy as electronic or semiconductor materials.
0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.07.040
Y. Kwon et al. / Microelectronic Engineering 82 (2005) 314–320
NiCr alloy, which is one kind of electrical heating materials, has been widely used as resistance or heating material due to its good oxidization resistance and corrosion property. Furthermore, its in-depth study has been recently done using chip resistor or micro-heater [1,2]. Since NiCr thin film has low temperature coefficient of resistance (TCR) and large resistivity, it became increased in importance and technical application as a resistor in the field of electronic industry [3–5]. Because the properties of NiCr thin film depend on manufacturing processes, deposition conditions, and subsequent annealing treatments [6], it is needed to correctly understand the chemical formation of NiCr thin film surface. Thereby the effects of oxidation and annealing on NiCr thin film have been researched [7,8]. However, it still leaves much to be desired to research on the changes in film properties according to annealing treatments in the manufacturing process [9]. To analyze the properties including the formation of oxidation layer and the changes in micro structure in NiCr thin films, analytical techniques like, auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were generally used [6]. In this study, the NiCr thin films using a NiCr target were prepared by DC magnetron sputtering. And the characteristics of these films such as crystal structure, changes of micro structure, changes of formation, and surface properties were analyzed as functions of annealing temperature and ambient conditions.
2. Experiments To prepare NiCr thin films, a 2-inch NiCr alloy target (Ni:Cr = 80:20) was used. The substrate used for depositing NiCr thin films was the (1 0 0) oriented p-type silicon single crystal. Al2O3 thin film of about 120 nm thickness was deposited between substrate and NiCr thin film as an insulator using sintered alumina (99.8%, Aldrich Co.) by e-beam evaporator under the conditions of 3 sccm oxygen flux, and 7.23 kW of e-beam power. NiCr thin films of 250 nm were deposited on the Al2O3/Si substrate by DC magnetron sputtering at room temperature for 45 min under the condi-
315
tions of 50 sccm of Ar gas flux, 8.0 · 106 Torr vacuum, 450 V of DC voltage and 50 mA of current. To examine the surface oxidation properties and the changes in micro structure of deposited NiCr thin films with respect to annealing conditions, the annealing treatments were performed at 400, 500, and 600 C for 6 h in air and H2 ambient, respectively. The changes in micro structures were investigated through field emission scanning electron microscope (FESEM; Hitachi S-4700). AES (Perkin–Elmer PHI-660) was used to observe the changes in depth profile of NiCr thin film and thermal behavior at the boundary between NiCr thin film and Al2O3 insulating film due to annealing. AES analysis was carried out at 5-volt beam voltage, 30-degree tilt, and 12-second interval sputter time. XPS (VG-Scientific ESCALAB 250) was used to analyze the chemical properties of the surface. Al Ka (1486.6 eV) radiation was used as the excitation source. All binding energy values were compensated to C 1s (284.5 eV). The scan intervals used were 1 eV for wide scan spectrum and 0.05 eV for narrow scan spectrum, respectively.
3. Results and discussion FESEM surface images of the deposited NiCr thin films are shown in Figs. 1 and 2. Fig. 1 shows the FESEM images of specimens annealed in air ambient. It can be observed from Fig. 1(b) that small crystal grains were formed at 400 C. And the pores along with fine crystal growth were seen at 500 C as depicted in Fig. 1(c). After annealing at 600 C, the grain boundaries became clear without any crystal growth and the densification as well as the disappearance of pores was accomplished as shown in Fig. 1(d). It is generally known that densification and grain growth occurs at the same time in annealing process; pores grow when grains grow but pores are contracted when grains are densified [10]. In several cases, the combination of densification and grain growth made pores grow or be contracted. In particular, in case of forming the similar pore size to the specimens annealed at 500 C, densification would be accomplished more quickly [10]. In other words, the large quantities of small-sized grains inhibited the grain growth in
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Fig. 1. FESEM surface images of NiCr thin films annealed in air at various temperatures for 6 h: (a) as-deposited; (b) 400 C; (c) 500 C; and (d) 600 C.
case of annealing in air. And there also formed a small and uniform pore structure, eventually resulted in quick densification. On the one hand, Fig. 2 shows the FESEM surface images of the specimens annealed in H2 ambient. The crystals grown above 100–300 nm appeared at 400 C as shown in Fig. 2(b). When annealing temperature increased to 500 C, the formed crystals were combined each other and became larger as shown in Fig. 2(c). At 600 C, a huge crystals growth of about 400–600 nm were observed in Fig. 2(d). The formation of oxide was inhibited in the surface, in case of specimens annealed in H2 ambient, resulting in producing relatively fine crystal grains. Whereas, Ni and Cr crystals formed the oxides such as CrxOy or NijOk in the surface, in case of specimens annealed in air, giving the small and
uniform grain distribution and uniform pore structure in stable condition. Then formed crystal grains were combined and accomplished crystal growth to form large crystal grains. The specimens annealed at 500 C in H2 ambient were analyzed by AES depth profile as shown in Fig. 3, in order to examine the composition of crystal grains formed in H2 ambient as well as the diffusion behavior at the boundary between NiCr thin film and Al2O3 insulating film. Fig. 3(a) shows the AES spectrum of the asdeposited specimens and Fig. 3(b) shows the annealed spectrum at 500 C for 6 h in H2 ambient. It is observed that both Ni and Cr elements were uniformly distributed from surface to inside of the film in specimens without annealing. It is thought that the target used for a long time made
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Fig. 2. FESEM surface images of NiCr thin films annealed in H2 at various temperatures for 6 h: (a) as-deposited; (b) 400 C; (c) 500 C; and (d) 600 C.
the high oxygen concentration in the as-deposited sample. However, it was a very low concentration if the unit would be changed to wt% commonly used in alloys. After annealing, small Cr elements existed around the surface and large Cr elements were existed in Al2O3 insulating film, whereas large Ni elements distributed around the surface, gradually decreasing inside. It is also found that oxygen decreased in quantity in the surface, but increased inside. The results of AES analysis demonstrated that the annealing in H2 ambient inhibited the formation of oxide in the surface, forming Ni-metal, and small Ni-oxide and Cr-oxide. Also, both oxygen and Cr gradually and simultaneously increased in quantity inside, suggesting that Croxide was formed inside of the film. In other words, it was identified that Ni-metal, and small
Ni-oxide and Cr-oxide formed in the films surface. The combined area of Ni and Cr elements existed inside. And Cr-metal and Cr-oxide existed around the Al2O3 insulating film. In the process of annealing treatment, the inter-diffusion of Cr elements and Al2O3 insulating film happened in a boundary between NiCr deposited film and Al2O3 insulating film. It is due to the reason that Cr2O3 and Al2O3 are isomorphic composites in which inter-diffusion occurs in the process of annealing treatment [11]. It means that Cr2O3 exists around the Al2O3 insulating film. The XPS analysis was performed to examine the surface properties of NiCr thin films according to annealing temperature and ambient as shown in Fig. 4. Fig. 4(a) shows the narrow scan spectra of Ni 2p for the specimen before annealing treatment.
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Fig. 3. Auger depth profiles of NiCr thin films: (a) asdeposited; (b) after annealing at 500 C for 6 h in H2.
Ni-metal and Ni(OH)2 were detected at binding energies of 852.3 and 853.6 eV, respectively. Ni(OH)2 and NiO were detected at 531.2 and
529.85 eV respectively, as shown in Fig. 4(b) of the narrow scan spectra of O 1s. Considering the detection of large quantity of Ni(OH)2 in both spectrums, most oxygen exists in the form of OH in the surface of specimens before annealing treatment. Such OH combination may be caused by exposure to atmospheric moisture while specimens move or native oxide inside the target [12]. In Fig. 4(c) of the narrow scan spectra of Cr 2p, a variety of Cr-oxides (CrxOy: x = 1,2, y = 2,3) were overlapped between 575 and 580 eV. In the Fig. 4(b) of the narrow scan spectra of O 1s, a variety of Cr-oxides (CrxOy: x = 1,2, y = 2,3) were overlapped between 530 and 531 eV. In other words, it was found that the most Ni elements existed in Ni-metal, Ni(OH)2, and the relatively small quantity of NiO oxide before annealing. Similarly, Cr elements existed in small quantity of Cr-oxide in the surfaces of specimens before annealing. Then, for the specimens annealed in air ambient, Ni-metal and Ni(OH)2 disappeared, but NiO (853.9 and 854.95 eV) and Ni2O3 (855.8 eV) increased as seen in Fig. 4(c) Ni 2p spectrum. It was confirmed from Fig. 4(b) of O 1s spectrum that Ni2O3 (531.7 eV) was clearly detected. In the Fig. 4(c) of Cr 2p spectrum, Cr-oxide decreased meaning that Cr elements were relatively smaller than Ni elements in the surface, which is consistent with AES results that Cr elements moved into the inside of film in the process of annealing treatment and remained in a small quantity in surfaces [13]. On the other hand, for the
Fig. 4. XPS narrow scan spectra obtained on NiCr thin films: (a) Ni 2p; (b) O 1s; and (c) Cr 2p.
Y. Kwon et al. / Microelectronic Engineering 82 (2005) 314–320
specimens annealed in H2 ambient, in the Fig. 4(a) of Ni 2p spectrum, NiO (853.9 and 854.95 eV) and Ni2O3 (855.6 eV) appearing after annealing in air disappeared, but Ni-metal (852.73 eV) and Ni(OH)2 (855.3 eV) were detected. Also, extremely small NiO (854.3 eV) was overlapped in detection. In the Fig. 4(b) of O 1s spectrum, Ni(OH)2 (531.2 eV) and small NiO (529.7 eV) were overlapped in detection. In the Fig. 4(c) of Cr 2p spectrum, the detection of Cr-oxide (578.3 and 576.6 eV) was identified. In the Fig. 4(b) of O 1s spectrum, Cr-oxides (CrxOy: x = 1,2, y = 2,3) were also identified around 530 eV. These results suggest that Ni-group oxides were reduced into Ni-metal in H2 ambient. Interestingly, although annealing treatments were performed in H2 ambient, Cr-oxide was detected. It is the reason that Cr2O3 can maintain the stable condition although it is annealed in H2ambient at a high temperature [14]. XPS analysis results suggest that while Nimetal, Ni(OH)2, and small quantity of Cr-oxide existed in specimen surfaces before annealing, NiO and Ni2O3 were formed after annealing in air ambient, and Ni-oxide was reduced and Croxide remained after annealing in H2 ambient. The reason that Ni(OH)2 appeared in H2 ambient while Cr-OH composites did not be detected and which may be explained as the relatively unstable Cr-group hydroxides were removed quickly when hydrogen atomic fluid produces moisture and removes it in the surface layer [15]. For large grains formed after annealing treatment in H2 ambient, Ni elements existed in a large quantity in surfaces as observed in AES analysis, suggesting Ni-group composites. Then XPS analysis results showed that Ni(OH)2 (855.3 eV) in the Ni 2p spectrum and Ni(OH)2 (531.2 eV) in the O 1s spectrum of specimen annealed in H2 ambient were exactly identical, confirming that the large grains were Ni(OH)2.
4. Conclusion NiCr thin films were deposited on Al2O3/Si substrate using a NiCr alloy target (Ni:Cr = 80:20) by DC magnetron sputtering. To examine the surface oxidation properties and the changes in micro
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structure of deposited NiCr thin films according to annealing condition, the annealing treatments were performed at 400, 500, and 600 C in air or H2 ambient, respectively. Annealing treatments in air promoted easy formation of oxide and formed a large quantity of small oxide grains and inhibited the grains growth. At 500 C in air ambient, a small and uniform-sized pore structure was formed. Furthermore, it allowed densifying more quickly, resulting in uniform distribution of small grains and minute micro structure. On the other hand, annealing treatment in H2 ambient led to large crystal growth by inhibiting the formation of oxide, combining formed small grains each other, and stabilizing through crystal growth. In other words, annealing treatment in air allowed obtaining NiCr thin films with closer structure of smaller and more uniform grains, compared to annealing in H2 ambient. In the process of annealing treatment, Cr elements formed oxides in various combination forms of a small quantity in the surface, moved inside of film, and remained as Cr-metal. Furthermore, it formed Cr2O3around the Al2O3 insulating film. In particular, Cr2O3– Al2O3 was an isomorphic composite and diffused mutually in a boundary between NiCr thin film layer and Al2O3 insulating layer. Ni elements were diffused from inside to surface of film, oxidizing and forming Ni oxidation layer. XPS analysis results demonstrated that Ni-metal, Ni(OH)2, and small quantities of NiO and Cr-oxide were detected in the as-deposited specimen surfaces, and that oxygen in surfaces existed in the form of OH combination due to exposure to moisture in air. Then while NiO, Ni2O3, and Cr-oxides (CrxOy; x = 1,2, y = 2,3) remained in the surface after annealing in air, Ni-oxide was reduced and Croxide (Cr2O3) remained stable after annealing in H2 ambient. And it is identified that the formed large crystal grain was Ni(OH)2. This study examined the micro structure and surface properties related to annealing temperature and ambient conditions, inter-diffusion in a boundary between NiCr thin film and Al2O3 insulating layer, and oxidation properties according to annealing conditions. These results will contribute to the understanding of the properties of thermal behavior and stability of NiCr thin films, in the
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manufacturing processes of NiCr thin films with high stability and uniform and minute micro structure. Acknowledgement This work was supported by Korea Research Foundation Grant (KRF-2004-005-D00008).
References [1] A. Banovec, A. Zalar, Thin Solid Films 164 (1998) 129– 133. [2] A. Peled, J. Farhadyan, Y. Zloof, V. Baranauskas, Vacuum 45 (1) (1994) 5–10. [3] C.L. Au, M.A. Jackson, W.A. Anderson, J. Electron. Mater. 16 (4) (1987) 301–306. [4] G. Nocerino, K.E. Singer, J. Vac. Sci. Technol. B 16 (2) (1979) 147–150.
[5] S. Hofmann, A. Zalar, Thin Solid Films 39 (1976) 219– 225. [6] G.B. Hoflund, W.S. Epling, Thin Solid Films 307 (1/2) (1997) 126–132. [7] J. Steffen, S. Hofmann, Surf. Sci. 202 (3) (1988) L607– L611. [8] S.P. Jeng, P.H. Holloway, D.A. Asbury, G.B. Hoflund, Surf. Sci. 235 (2/3) (1990) 175–185. [9] W. Bru¨cker, S. Baunack, J. Appl. Phys. 85 (2) (1999) 935– 940. [10] R.M. German, Sintering Theory and Practice, John Wiley and Sons, New York, 1996, p. 73. [11] R.M. German, Sintering Theory and Practice, John Wiley and Sons, New York, 1996, p. 198. [12] J.H. Ryu, N.H. Kim, H.S. Kim, G.Y. Yeom, E.G. Chang, C.I. Kim, J. Vac. Sci. Technol. A 18 (4) (2000) 1377–1380. [13] Y. Kun, Y. Park, S. Choi, J. Jung, G. Choi, H. Ryu, J. Park, J. Korean Ceram. Soc. (in Korean) 41 (6) (2004) 450– 455. [14] B.H. Kim, Physics and Chemistry Dictionary (in Korean), Korea Dictionary Research Co., Seoul, 1995, p. 573. [15] W.S. Epling, C.K. Mount, G.B. Hoflund, Thin Solid Films 304 (1/2) (1997) 273–277.
Structural and surface properties of NiCr thin films prepared by DC magnetron sputtering under variation of annealing conditions Yong Kwon a, Nam-Hoon Kim b, Gwang-Pyo Choi b, Woo-Sun Lee c, Yong-Jin Seo d, Jinseong Park a,* a
Department of Advanced Materials Engineering, Chosun University, 375 Seosuk-Dong, Dong-Gu, Gwangju 501-759, Korea b Research Institute of Energy Resources Technology,Chosun University, Gwangju 501-759, Korea c Department of Electrical Engineering, Chosun University, Gwangju 501-759, Korea d Department of Electrical Engineering, Daebul University, Chonnam 526-702, Korea Available online 18 August 2005
Abstract NiCr thin films are widely used in several applications in microelectronics such as thin film resistors, filaments, and humidity sensors because of their relatively large resistivity, more resistant to oxidation and a low temperature coefficient of resistance (TCR). These interesting properties of NiCr thin films are dependent upon the preparation conditions including the deposition environment and subsequent annealing treatments. NiCr thin films were deposited by DC magnetron sputtering on Al2O3/Si substrate using 2-inch Ni/Cr (80/20) alloy. Annealing treatments were performed at 400, 500, and 600 C for 6 h in air and H2 ambient, respectively. The clear crystal boundaries without crystal growth and the densification were accomplished when the pores disappeared in air ambient. Most of surfaces are oxidic including NiO, Ni2O3 and CrxOy (x = 1,2, y = 2,3) after annealing in air ambient. The crystal growth in H2 ambient was formed and stabilized by combination with each other due to the suppression of oxidized substance on the film surface. Most Nioxides were reduced when the Cr-oxides were present due to their stability in high-temperature H2 ambient. 2005 Elsevier B.V. All rights reserved. Keywords: NiCr thin film; DC magnetron sputtering; Annealing; X-ray photoelectron spectroscopy (XPS)
1. Introduction
*
Corresponding author. Tel.: +82 62 230 7193; fax: +82 62 233 0302. E-mail address: [email protected] (J. Park).
With the miniaturization and lightness of electronic parts, the researches on preparing thin films have been actively conducted to use oxidic ceramic and alloy as electronic or semiconductor materials.
0167-9317/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2005.07.040
Y. Kwon et al. / Microelectronic Engineering 82 (2005) 314–320
NiCr alloy, which is one kind of electrical heating materials, has been widely used as resistance or heating material due to its good oxidization resistance and corrosion property. Furthermore, its in-depth study has been recently done using chip resistor or micro-heater [1,2]. Since NiCr thin film has low temperature coefficient of resistance (TCR) and large resistivity, it became increased in importance and technical application as a resistor in the field of electronic industry [3–5]. Because the properties of NiCr thin film depend on manufacturing processes, deposition conditions, and subsequent annealing treatments [6], it is needed to correctly understand the chemical formation of NiCr thin film surface. Thereby the effects of oxidation and annealing on NiCr thin film have been researched [7,8]. However, it still leaves much to be desired to research on the changes in film properties according to annealing treatments in the manufacturing process [9]. To analyze the properties including the formation of oxidation layer and the changes in micro structure in NiCr thin films, analytical techniques like, auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were generally used [6]. In this study, the NiCr thin films using a NiCr target were prepared by DC magnetron sputtering. And the characteristics of these films such as crystal structure, changes of micro structure, changes of formation, and surface properties were analyzed as functions of annealing temperature and ambient conditions.
2. Experiments To prepare NiCr thin films, a 2-inch NiCr alloy target (Ni:Cr = 80:20) was used. The substrate used for depositing NiCr thin films was the (1 0 0) oriented p-type silicon single crystal. Al2O3 thin film of about 120 nm thickness was deposited between substrate and NiCr thin film as an insulator using sintered alumina (99.8%, Aldrich Co.) by e-beam evaporator under the conditions of 3 sccm oxygen flux, and 7.23 kW of e-beam power. NiCr thin films of 250 nm were deposited on the Al2O3/Si substrate by DC magnetron sputtering at room temperature for 45 min under the condi-
315
tions of 50 sccm of Ar gas flux, 8.0 · 106 Torr vacuum, 450 V of DC voltage and 50 mA of current. To examine the surface oxidation properties and the changes in micro structure of deposited NiCr thin films with respect to annealing conditions, the annealing treatments were performed at 400, 500, and 600 C for 6 h in air and H2 ambient, respectively. The changes in micro structures were investigated through field emission scanning electron microscope (FESEM; Hitachi S-4700). AES (Perkin–Elmer PHI-660) was used to observe the changes in depth profile of NiCr thin film and thermal behavior at the boundary between NiCr thin film and Al2O3 insulating film due to annealing. AES analysis was carried out at 5-volt beam voltage, 30-degree tilt, and 12-second interval sputter time. XPS (VG-Scientific ESCALAB 250) was used to analyze the chemical properties of the surface. Al Ka (1486.6 eV) radiation was used as the excitation source. All binding energy values were compensated to C 1s (284.5 eV). The scan intervals used were 1 eV for wide scan spectrum and 0.05 eV for narrow scan spectrum, respectively.
3. Results and discussion FESEM surface images of the deposited NiCr thin films are shown in Figs. 1 and 2. Fig. 1 shows the FESEM images of specimens annealed in air ambient. It can be observed from Fig. 1(b) that small crystal grains were formed at 400 C. And the pores along with fine crystal growth were seen at 500 C as depicted in Fig. 1(c). After annealing at 600 C, the grain boundaries became clear without any crystal growth and the densification as well as the disappearance of pores was accomplished as shown in Fig. 1(d). It is generally known that densification and grain growth occurs at the same time in annealing process; pores grow when grains grow but pores are contracted when grains are densified [10]. In several cases, the combination of densification and grain growth made pores grow or be contracted. In particular, in case of forming the similar pore size to the specimens annealed at 500 C, densification would be accomplished more quickly [10]. In other words, the large quantities of small-sized grains inhibited the grain growth in
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Fig. 1. FESEM surface images of NiCr thin films annealed in air at various temperatures for 6 h: (a) as-deposited; (b) 400 C; (c) 500 C; and (d) 600 C.
case of annealing in air. And there also formed a small and uniform pore structure, eventually resulted in quick densification. On the one hand, Fig. 2 shows the FESEM surface images of the specimens annealed in H2 ambient. The crystals grown above 100–300 nm appeared at 400 C as shown in Fig. 2(b). When annealing temperature increased to 500 C, the formed crystals were combined each other and became larger as shown in Fig. 2(c). At 600 C, a huge crystals growth of about 400–600 nm were observed in Fig. 2(d). The formation of oxide was inhibited in the surface, in case of specimens annealed in H2 ambient, resulting in producing relatively fine crystal grains. Whereas, Ni and Cr crystals formed the oxides such as CrxOy or NijOk in the surface, in case of specimens annealed in air, giving the small and
uniform grain distribution and uniform pore structure in stable condition. Then formed crystal grains were combined and accomplished crystal growth to form large crystal grains. The specimens annealed at 500 C in H2 ambient were analyzed by AES depth profile as shown in Fig. 3, in order to examine the composition of crystal grains formed in H2 ambient as well as the diffusion behavior at the boundary between NiCr thin film and Al2O3 insulating film. Fig. 3(a) shows the AES spectrum of the asdeposited specimens and Fig. 3(b) shows the annealed spectrum at 500 C for 6 h in H2 ambient. It is observed that both Ni and Cr elements were uniformly distributed from surface to inside of the film in specimens without annealing. It is thought that the target used for a long time made
Y. Kwon et al. / Microelectronic Engineering 82 (2005) 314–320
317
Fig. 2. FESEM surface images of NiCr thin films annealed in H2 at various temperatures for 6 h: (a) as-deposited; (b) 400 C; (c) 500 C; and (d) 600 C.
the high oxygen concentration in the as-deposited sample. However, it was a very low concentration if the unit would be changed to wt% commonly used in alloys. After annealing, small Cr elements existed around the surface and large Cr elements were existed in Al2O3 insulating film, whereas large Ni elements distributed around the surface, gradually decreasing inside. It is also found that oxygen decreased in quantity in the surface, but increased inside. The results of AES analysis demonstrated that the annealing in H2 ambient inhibited the formation of oxide in the surface, forming Ni-metal, and small Ni-oxide and Cr-oxide. Also, both oxygen and Cr gradually and simultaneously increased in quantity inside, suggesting that Croxide was formed inside of the film. In other words, it was identified that Ni-metal, and small
Ni-oxide and Cr-oxide formed in the films surface. The combined area of Ni and Cr elements existed inside. And Cr-metal and Cr-oxide existed around the Al2O3 insulating film. In the process of annealing treatment, the inter-diffusion of Cr elements and Al2O3 insulating film happened in a boundary between NiCr deposited film and Al2O3 insulating film. It is due to the reason that Cr2O3 and Al2O3 are isomorphic composites in which inter-diffusion occurs in the process of annealing treatment [11]. It means that Cr2O3 exists around the Al2O3 insulating film. The XPS analysis was performed to examine the surface properties of NiCr thin films according to annealing temperature and ambient as shown in Fig. 4. Fig. 4(a) shows the narrow scan spectra of Ni 2p for the specimen before annealing treatment.
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Fig. 3. Auger depth profiles of NiCr thin films: (a) asdeposited; (b) after annealing at 500 C for 6 h in H2.
Ni-metal and Ni(OH)2 were detected at binding energies of 852.3 and 853.6 eV, respectively. Ni(OH)2 and NiO were detected at 531.2 and
529.85 eV respectively, as shown in Fig. 4(b) of the narrow scan spectra of O 1s. Considering the detection of large quantity of Ni(OH)2 in both spectrums, most oxygen exists in the form of OH in the surface of specimens before annealing treatment. Such OH combination may be caused by exposure to atmospheric moisture while specimens move or native oxide inside the target [12]. In Fig. 4(c) of the narrow scan spectra of Cr 2p, a variety of Cr-oxides (CrxOy: x = 1,2, y = 2,3) were overlapped between 575 and 580 eV. In the Fig. 4(b) of the narrow scan spectra of O 1s, a variety of Cr-oxides (CrxOy: x = 1,2, y = 2,3) were overlapped between 530 and 531 eV. In other words, it was found that the most Ni elements existed in Ni-metal, Ni(OH)2, and the relatively small quantity of NiO oxide before annealing. Similarly, Cr elements existed in small quantity of Cr-oxide in the surfaces of specimens before annealing. Then, for the specimens annealed in air ambient, Ni-metal and Ni(OH)2 disappeared, but NiO (853.9 and 854.95 eV) and Ni2O3 (855.8 eV) increased as seen in Fig. 4(c) Ni 2p spectrum. It was confirmed from Fig. 4(b) of O 1s spectrum that Ni2O3 (531.7 eV) was clearly detected. In the Fig. 4(c) of Cr 2p spectrum, Cr-oxide decreased meaning that Cr elements were relatively smaller than Ni elements in the surface, which is consistent with AES results that Cr elements moved into the inside of film in the process of annealing treatment and remained in a small quantity in surfaces [13]. On the other hand, for the
Fig. 4. XPS narrow scan spectra obtained on NiCr thin films: (a) Ni 2p; (b) O 1s; and (c) Cr 2p.
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specimens annealed in H2 ambient, in the Fig. 4(a) of Ni 2p spectrum, NiO (853.9 and 854.95 eV) and Ni2O3 (855.6 eV) appearing after annealing in air disappeared, but Ni-metal (852.73 eV) and Ni(OH)2 (855.3 eV) were detected. Also, extremely small NiO (854.3 eV) was overlapped in detection. In the Fig. 4(b) of O 1s spectrum, Ni(OH)2 (531.2 eV) and small NiO (529.7 eV) were overlapped in detection. In the Fig. 4(c) of Cr 2p spectrum, the detection of Cr-oxide (578.3 and 576.6 eV) was identified. In the Fig. 4(b) of O 1s spectrum, Cr-oxides (CrxOy: x = 1,2, y = 2,3) were also identified around 530 eV. These results suggest that Ni-group oxides were reduced into Ni-metal in H2 ambient. Interestingly, although annealing treatments were performed in H2 ambient, Cr-oxide was detected. It is the reason that Cr2O3 can maintain the stable condition although it is annealed in H2ambient at a high temperature [14]. XPS analysis results suggest that while Nimetal, Ni(OH)2, and small quantity of Cr-oxide existed in specimen surfaces before annealing, NiO and Ni2O3 were formed after annealing in air ambient, and Ni-oxide was reduced and Croxide remained after annealing in H2 ambient. The reason that Ni(OH)2 appeared in H2 ambient while Cr-OH composites did not be detected and which may be explained as the relatively unstable Cr-group hydroxides were removed quickly when hydrogen atomic fluid produces moisture and removes it in the surface layer [15]. For large grains formed after annealing treatment in H2 ambient, Ni elements existed in a large quantity in surfaces as observed in AES analysis, suggesting Ni-group composites. Then XPS analysis results showed that Ni(OH)2 (855.3 eV) in the Ni 2p spectrum and Ni(OH)2 (531.2 eV) in the O 1s spectrum of specimen annealed in H2 ambient were exactly identical, confirming that the large grains were Ni(OH)2.
4. Conclusion NiCr thin films were deposited on Al2O3/Si substrate using a NiCr alloy target (Ni:Cr = 80:20) by DC magnetron sputtering. To examine the surface oxidation properties and the changes in micro
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structure of deposited NiCr thin films according to annealing condition, the annealing treatments were performed at 400, 500, and 600 C in air or H2 ambient, respectively. Annealing treatments in air promoted easy formation of oxide and formed a large quantity of small oxide grains and inhibited the grains growth. At 500 C in air ambient, a small and uniform-sized pore structure was formed. Furthermore, it allowed densifying more quickly, resulting in uniform distribution of small grains and minute micro structure. On the other hand, annealing treatment in H2 ambient led to large crystal growth by inhibiting the formation of oxide, combining formed small grains each other, and stabilizing through crystal growth. In other words, annealing treatment in air allowed obtaining NiCr thin films with closer structure of smaller and more uniform grains, compared to annealing in H2 ambient. In the process of annealing treatment, Cr elements formed oxides in various combination forms of a small quantity in the surface, moved inside of film, and remained as Cr-metal. Furthermore, it formed Cr2O3around the Al2O3 insulating film. In particular, Cr2O3– Al2O3 was an isomorphic composite and diffused mutually in a boundary between NiCr thin film layer and Al2O3 insulating layer. Ni elements were diffused from inside to surface of film, oxidizing and forming Ni oxidation layer. XPS analysis results demonstrated that Ni-metal, Ni(OH)2, and small quantities of NiO and Cr-oxide were detected in the as-deposited specimen surfaces, and that oxygen in surfaces existed in the form of OH combination due to exposure to moisture in air. Then while NiO, Ni2O3, and Cr-oxides (CrxOy; x = 1,2, y = 2,3) remained in the surface after annealing in air, Ni-oxide was reduced and Croxide (Cr2O3) remained stable after annealing in H2 ambient. And it is identified that the formed large crystal grain was Ni(OH)2. This study examined the micro structure and surface properties related to annealing temperature and ambient conditions, inter-diffusion in a boundary between NiCr thin film and Al2O3 insulating layer, and oxidation properties according to annealing conditions. These results will contribute to the understanding of the properties of thermal behavior and stability of NiCr thin films, in the
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manufacturing processes of NiCr thin films with high stability and uniform and minute micro structure. Acknowledgement This work was supported by Korea Research Foundation Grant (KRF-2004-005-D00008).
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