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Interfacial reactions of rf-sputtered TiNi thin films on (100) silicon with a SiN
diffusion barrier
S. K. Wu a; J. J. Su a; J. Y. Wang b
a Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan b
Materials and Optical-Electronics Division, Chung-Shang Institute of Science and Technology, Lung-Tan, Tao-Yuan 325, Taiwan
Online Publication Date: 21 April 2004
To cite this Article Wu, S. K., Su, J. J. and Wang, J. Y.(2004)'Interfacial reactions of rf-sputtered TiNi thin films on (100) silicon with a SiN diffusion barrier',Philosophical Magazine,84:12,1209 — 1218
To link to this Article: DOI: 10.1080/14786430310001646745
URL: http://dx.doi.org/10.1080/14786430310001646745
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Philosophical Magazine, 21 April 2004 Vol. 84, No. 12, 1209–1218
Interfacial reactions of rf-sputtered TiNi thin films on
(100) silicon with a SiN diffusion barrier
S. K. Wuy, J. J. Su
Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan
and J. Y. Wang
Materials and Optical-Electronics Division, Chung-Shang Institute of Science and Technology, Lung-Tan, Tao-Yuan 325, Taiwan
[Received 11 August 2003 and accepted in revised form 28 October 2003]
Abstract
Silicon nitride (SiN) with a 50 nm thickness on Si(100) as a thermal barrier was obtained by plasma-enhanced chemical vapour deposition (PECVD). TiNi thin films were rf sputtered on a SiN/Si substrate and then annealed at 400–700C for 30 min. Their interfacial reactions were studied using transmission electron microscopy, X-ray diffraction and Auger electron spectroscopy analyses. Experimental results show that the thickness of reaction layer in TiNi/SiN/Si specimens is clearly reduced, compared with that in TiNi/Si specimens under the same annealing conditions. The significant effect of the SiN layer as a diffusion barrier in TiNi/SiN/Si can be recognized. N and Si atoms diffuse from the SiN layer to react with TiNi films at 500C and 600C respectively. The TiN1 xphase is formed in specimens annealed at 500C, and mixed Ti2Ni3Si and Ti4Ni2O compounds are found at 600C. In the specimen annealed at 700C, the reaction layer has sublayers in the sequence TiNi/Ti4Ni2O/Ti2Ni3Si/TiN1 x/ SiN/Si. The SiN thermal barrier obtained by PECVD caused quite different diffusion species to cross the interfaces between TiNi/SiN/Si and TiNi/Si specimens during the annealing.
} 1. Introduction
TiNi shape memory thin films are materials with a high potential for fabricating the microactuators and micropumps used in microelectromechanical systems because of the advantages of their large shape recovery force and strain. Hence, efforts have been made with TiNi thin films using rf sputtering techniques (Wolf and Heuer 1995, Bendahan et al. 1996, Stemmer et al. 1997, Miyazaki and Ishida 1999, Chen and Wu 1999, Matsunaga et al. 2000, Wu et al. 2001). Because the TiNi thin films are amorphous when initially deposited on to low-temperature substrates, thermal annealing is necessary for the crystallization of these films (Stemmer et al. 1997, Chen and Wu 1999, Wu et al. 2001). However, the film–substrate interface will interact during thermal annealing. These interfacial products will influence the
Philosophical MagazineISSN 1478–6435 print/ISSN 1478–6443 online # 2004 Taylor & Francis Ltd http://www.tandf.co.uk/journals
DOI: 10.1080/14786430310001646745 yAuthor for correspondence. Email: [email protected].
film–substrate adhesion and other interfacial properties, thus affecting the perfor-mance of TiNi thin films as microactuators and micropumps (Wolf and Heuer 1995). In our previous study, interfacial microstructures of TiNi thin films rf sputtered on to Si(100) and post-annealed at 400–700C for 30 min were investigated (Wu et al. 2001). For annealing temperatures below 600C, Ni atoms are the primary diffusion species and NiSi2compound forms towards the Si substrate at 400C. TiNi thin films
initially crystallize at 500C. Si and Ti atoms begin to migrate in specimens annealed at 600C. At this temperature, a near-Ti4Ni4Si7compound on the NiSi2side and a
near-TiNiSi compound on the TiNi film side are simultaneously produced at the interface. For specimens annealed at 700C for 30 min, Ti4Ni4Si7forms a layer with a
thickness greater than 0.3 mm, while TiNiSi forms a layer with a thickness greater than 0.5 mm. Clearly, the amorphous rf-sputtered TiNi films crystallize at 500C, but Ni atoms in TiNi films have diffused into the Si substrate at 400C. In other words, the interfacial NiSi2compound forms at the TiNi–Si interface during the TiNi films
crystallization if there is no diffusion barrier existing in between the TiNi thin film and the Si substrate.
Silicon nitride obtained by plasma-enhanced chemical vapour deposition (PECVD) is widely used in the microelectronics industry as a diffusion barrier coating (Smith 1995). The silicon nitride obtained by PECVD (abbreviated here as SiN) is highly non-stoichiometric and has bonded H (Mar and Samuelson 1980, Chang et al. 1988). In this study, a layer of SiN was deposited on to Si(100); then TiNi thin films were rf sputtered on to it; finally they were annealed at temperatures between 400 and 700C for 30 min. The reaction phases and microstructures at the TiNi–SiN–Si interfaces resulting from the post-deposition thermal annealing are investigated. The difference between the interfacial reactions at the TiNi–Si and TiNi–SiN–Si interfaces are also discussed.
} 2. Experimental procedure
A SiN layer 50 nm thick was deposited by PECVD on to the (100) surface of cleaned and oxide-etched Si wafers of 3 in diameter at the National Nano-Device Laboratory, National Science Council, Hsinchu, Taiwan, using an STS multiplex cluster system with frequency 380 kHz. In this system, silane (SiH4) and NH3were
used as reaction gases, and N2as the carrying gas. Then near-equiatomic TiNi thin
films were sputtered on to SiN/Si at room temperature using a magnetron gun in a high-vacuum (1 107Torr) base pressure chamber. The sputtering conditions were as follows: sputtering pressure, 10 mTorr, target–substrate distance, 50 mm; dc power, 200 W; deposition rate, about 5 A˚ sec1. The targets used in this study were a Ti49Ni51
disc of 2 in diameter. The sputter-deposited TiNi specimens with thicknesses of 0.4 and 1.5 mm were scaled in evacuated quartz tubes, thermally annealed at 400, 500, 600 and 700C for 30 min and then furnace cooled to room temperature.
The annealed TiNi films of 0.4 mm thickness were used to analyse the composi-tion depth profile of TiNi–SiN–Si interfaces by Auger electron spectroscopy (AES), using a Perkins–Elmer 600 model with Ta2O5as the reference for depth estimation.
The ion-bombardment rate of AES is about 25 A˚ min1. The elemental profiles of O, N, Ti, Ni and Si across the TiNi–SiN–Si interfaces were obtained. The annealed TiNi films of 1.5 mm thickness were used to detect the phases and to observe the cross-sectional microstructures of interfaces. Cross-sectional transmission electron microscopy (TEM) specimens were fabricated by grinding, dimpling and ion milling at 5 kV to perforation. The sector speed control whisperlock of the Gatan Duomill at
liquid-N2temperature was used to minimize the effects of different ion-milling rates
of TiNi, SiN and Si in cross-sectional TEM specimens. Microstructural observation was performed using a 4000 FX (JEOL) transmission electron microscopy operated at 400 kV with 2.6 A˚ point-to-point resolution. The camera constant was 41 A˚ mm. X-ray diffraction (XRD) tests were carried out on a Philips PW1710 X-ray diffracto-meter using Cu Ka radiation. The power was 40 kV 30 mA and the 2 scanning rate was 0.008s1. The range of scanning angles was from 20oto 90o. All XRD tests were taken from the top of sputtered specimens and normal to the TiNi–SiN–Si interfaces.
} 3. Results and discussion
3.1. As-deposited specimens and specimens annealed at 400C for 30 min
Figure 1 (a) shows the cross-sectional TEM image of an as-deposited TiNi/ SiN/Si specimen, and figure 1 (b) is the XRD result of the specimen annealed at 400C for 30 min. Figures 1 (a) and (b) indicate that the as-deposited TiNi films are amorphous, and that these films are stable even after annealing at 400C for 30 min. Figure 1 (c) shows the AES results for the as-deposited specimen. From figures 1 (a) and (c), it can be seen that the interface between the Si substrate and the SiN layer is quite sharp, but the interface between the SiN layer and the TiNi film is not clear and has a relatively higher O concentration. The reason for the latter may be that the specimens have a long holding time in the air in between different deposition procedures. In figure 1 (a), one can observe a layer of about 10 nm thickness with varying contrast in the Si substrate neighbouring the SiN–Si interface. This layer can still be observed even after annealing at 700C for 30 min, and it is speculated that high-density defects induced by high-energy particle bombardment during the SiN PECVD occur. In figure 1, there is no Ni atom diffusion crossing the TiNi–SiN–Si interfaces, although that has been observed in the TiNi–Si interface annealed at 400C for 30 min (Wu et al. 2001).
3.2. Specimens annealed at 500C for 30 min
Figures 2 (a), (b) and (c) show the cross-sectional TEM image, the XRD results and the AES results respectively of a TiNi/SiN/Si specimen annealed at 500C for 30 min. From figure 2 (b), it can be seen that the TiNi film now has clearly crystal-lized to be a mixture of the B2 parent phase and B190martensite. Its crystallization behaviour is similar to that of previous reports (Stemmer et al. 1997, Chen and Wu 1999, Wu et al. 2001). From figures 2 (a) and (c), the interface between the SiN layer and the Si substrate is still sharp, but that between the SiN layer and the TiNi film now forms a reaction layer with a thickness of about 10 nm on the TiNi side. This extra layer contains many N atoms, compared with figure 1 (c), indicating that N atoms have diffused from the SiN layer to react with the TiNi film at 500C. From figure 2 (b), this reaction layer is expected to be a TiN1 xphase because the
impor-tant XRD peaks of the TiN1 xphase can be identified (Joint Committee for Powder
Diffraction Standards 1995b). The sharp SiN–Si interface means that there is no clear atom diffusion crossing over this interface, even after annealing at 500C for 30 min.
3.3. Specimens annealed at 600C for 30 min
Figures 3 (a), (b) and (c) show the cross-sectional TEM image, the XRD results and the AES results respectively of a TiNi/SiN/Si specimen annealed at 600C for 30 min. Figure 3 (d) is the selected-area diffraction pattern of the area covering Interfacial reactions of TiNi/SiN/Si 1211
the Si–SiN–TiNi interfaces in figure 3 (a). There are spot and ring patterns super-imposed in figure 3 (d). The spot pattern is identified as the [011]Si zone axis, but
the ring pattern has ring radii of about 4.15–4.63 nm1. The ring of about 4.15 nm1 radius is proposed to come from the {111} plane of the TiN1 x phase with
d ¼0.2447 nm (Villars and Calvert 1985) and/or the (002)–(020) plane of Ti2Ni3Si
with d ¼ 0.24 nm (Bardos et al. 1961). The ring of about 4.63 nm1radius consists of the spots that come from the {011}B2plane of the TiNi parent phase with the lattice
20 30 40 50 60 70 80 90 0 1000 2000 3000 4000 amorphous TiNi Si(400) Intensity (counts) 2θ(degree) (b) (c)
Figure 1. (a) The cross-sectional TEM image of the as-deposited TiNi/SiN/Si specimen, (b) the XRD results of the TiNi/SiN/Si specimen annealed at 400C for 30 min and (c) the AES results of the as-deposited TiNi/SiN/Si specimen.
constant 0.3015 nm and d011¼0.213 nm (Philip and Beck 1957). From figure 3, the
sharp interface between the SiN layer and the Si substrate is still observable, but the reaction layer between the SiN layer and the TiNi film is thicker (about 60 nm) than that observed in specimen annealed at 500C for 30 min. The results in figures 3 (b), (c) and (d) indicate that the reaction layer contains a sublayer of TiN1 x on the
near-SiN side and a sublayer of the mixture of Ti2Ni3Si and Ti4Ni2O on the
near-TiNi film. It is well known that the Ti2Ni phase can absorb much more O than TiNi
can to form Ti4Ni2O (Honma 1987), and the cubic structures of Ti2Ni and Ti4Ni2O
are similar, having almost the same lattice constant (Joint Committee for Powder Diffraction Standards 1995a, c). As seen from figure 2 (c), the high concentration Interfacial reactions of TiNi/SiN/Si 1213
(c) 20 30 40 50 60 70 80 90 0 1000 2000 3000 4000 (222) TiNi B2(hkl) TiNi B19'(hkl) TiN(hkl) Si(hkl) (211) (002) (-111) (200), (220) (400) (111) Intensity (counts) 2θ (degree) (b) (110), (200)
Figure 2. (a) The cross-sectional TEM image, (b) the XRD results and (c) the AES results of the TiNi/SiN/Si specimen annealed at 500C for 30 min.
20 30 40 50 60 70 80 90 0 1000 2000 3000 4000 (211) (222) (400) (440) (111), (201)/(102) (103), (511), (-111) (422) (111), (422) (002) (200), (220) (110), (200) TiNi B2(hkl)TiNi B19'(hkl) TiN(hkl) Ti2Ni3Si(hkl) Si(hkl) Ti4Ni2O(hkl) Intensity (counts) 2θ (degree) (b) (c)
Figure 3. (a) The cross-sectional TEM image, (b) the XRD results, (c) the AES results and (d) the selected-area diffraction pattern of (a) of the TiNi/SiN/Si specimen annealed at 600C for 30 min.
of O in this sublayer suggests the possibility of formation of Ti4Ni2O. It is also
suggested that Ti2Ni3Si forms in the reaction sublayer of figure 3 (a) because
Ti2Ni3Si has been observed at the TiNi–Si interface of the specimen annealed at
about 525C for 30 min, when Si atoms have diffused across the interface (Stemmer et al. 1997). At the same time, the formation of TiN1 x and Ti4Ni2O will cause
deficit Ti atoms in TiNi film. This feature will favour the formation of the Ti2Ni3Si
phase. Figure 3 also clearly indicates that Si atoms have migrated from the SiN layer to react with TiNi film at 600C on the TiNi/SiN/Si specimen.
3.4. Specimens annealed at 700C for 30 min
Figures 4 (a), (b) and (c) show the cross-sectional TEM image, the XRD results and the AES results respectively of a TiNi/SiN/Si specimen annealed at 700C Interfacial reactions of TiNi/SiN/Si 1215
Figure 4. (a) The cross-sectional TEM image, (b) the XRD results and (c) the AES results of the TiNi/SiN/Si specimen annealed at 700C for 30 min.
for 30 min. In figures 4 (a) and (c), the SiN–Si interface is still sharp, the SiN–TiNi interface has a reaction layer with a thickness of about 150 nm. In figures 4 (b) and (c), the reaction layer of SiN–TiNi contains the sublayers of TiN1 x, Ti2Ni3Si and
Ti4Ni2O, which are the same as those observed in figure 3 except that now Ti2Ni3Si
and Ti4Ni2O have separated into two sublayers. Figure 4 (c) suggests that these
sublayers have the sequence TiNi/Ti4Ni2O/Ti2Ni3Si/TiN1 x/SiN. However, figure
4 (a) shows that there are no clear interfaces between these sublayers. This feature indicates that these reaction compounds are mixed at the interfaces of the sublayers. In figure 4 (b), no sharp (400)Si peak can be observed. This may be due to the
formation of a thick reaction layer at the TiNi–SiN interface after annealing at 700C for 30 min. From figure 4 (b), some minor XRD peaks still cannot be identified, a feature that merits further study.
3.5. Effect of SiN diffusion barrier on the interfacial reactions of TiNi/SiN/Si specimens
The thicknesses of SiN and reaction layers shown in figures 1–4 are measured. Figure 5 shows the curves of the thickness of SiN layer and that of reaction layer of SiN/TiNi versus the annealing temperature in TiNi/SiN/Si specimens annealed at 400–700C for 30 min. For comparison, the curve of the thickness of reaction layer versus the annealing temperature in TiNi/Si specimens annealed in the same condi-tions is also plotted in figure 5 (Wu et al. 2001). From figure 5, the significant effect of the SiN layer as a diffusion barrier in TiNi/SiN/Si can be recognized. In the case of TiNi/Si without the diffusion barrier, Ni atoms can diffuse to form NiSi2 after
annealing at 400C for 30 min, and they are the primary diffusion species for anneal-ing temperatures below 600C (Wu et al. 2001). However, in the case of TiNi/SiN/Si with SiN as a diffusion barrier, N and Si atoms diffuse from the SiN layer into the TiNi film at 500C and 600C respectively. Clearly, the Ni atoms are not the primary diffusion species in annealed TiNi/SiN/Si specimens owing to the existence of the SiN thermal barrier. This feature also causes the sharp SiN–Si interface in TiNi/SiN/ Si specimens, even after annealing at 700C for 30 min.
100 150 200 250 300 TiNi/Si TiNi/SiN/Si 20 30 40 50 60 700 °C 500 °C 600 °C 400 °C As-deposited Annealing Temperature (°C) ~800nm for 700°C 0 50 10 0 SiN Thickness(nm) Thickness of Reaction Layer (nm)
Figure 5. The curves of the thicknesses of the total reaction layer versus the annealing temperature for TiNi/SiN/Si and TiNi/Si specimens annealed at 400–700C for 30 min. The curve of the thickness of SiN layer versus the annealing temperature for TiNi/SiN/Si specimens is also plotted.
The interfacial microstructures of TiNi/SiN/Si specimens annealed at 400–700C for 30 min are also different from those of TiNi/Si specimens annealed under the same conditions. This characteristic can be understood from the fact that the diffusion species have changed at TiNi/SiN/Si and TiNi/Si interfaces. In this study, the speci-men annealed at 700C for 30 min has reaction sublayers of TiN1 x, Ti2Ni3Si and
Ti4Ni2O at the TiNi–SiN interface. Of these, TiN1 x, Ti2Ni3Si and Ti4Ni2O have
been found in specimen annealed at 600C for 30 min and TiN1 xis formed at much
lower annealing temperatures, for example in the specimen annealed at 500C for 30 min. This feature indicates that the formation of these phases is closely related to their free energies of formation, and is also dependent on the primary diffusion species crossing the interfaces. For example, according to the Ti–Si–N and Ni–Si–N ternary systems (Rogl and Schuster 1992), TiN1 xhas a lower free energy to form
compared with NiSi, NiSi2, Ti5Si3and Si3N4. At the same time, at 500C, N atoms
are the primary diffusion species at the TiNi–SiN interface; therefore TiN1 xcan be
formed at a lower temperature than other compounds in this study. } 4. Conclusion
SiN with a thickness of 50 nm on Si(100) as a thermal barrier was obtained by PECVD. TiNi films were rf sputtered on a SiN/Si substrate and then annealed at 400–700C for 30 min; and their interfaces are studied using TEM, AES and XRD analyses. Experimental results show that the thickness of reaction layer in TiNi/ SiN/Si specimens is clearly reduced, compared with that in TiNi/Si specimens under the same annealing conditions. The significant effect of the SiN layer as a diffusion barrier in TiNi/SiN/Si can be recognized. In this study, N and Si atoms diffuse from the SiN layer on to the TiNi film at 500C and 600C respectively. This feature allows the TiN1 xphase and mixed TiN1 xand Ti2Ni3Si–Ti4Ni2O phases to
be formed at 500C and 600C respectively. After annealing at 700C for 30 min, the reaction layer has sublayers in the sequence TiNi/Ti4Ni2O/Ti2Ni3Si/TiN1 x/SiN/Si.
The formation of TiN1 x, Ti2Ni3Si and Ti4Ni2O phases is suggested to be related to
their free energies of formation and to be dependent on the primary diffusion species crossing the interfaces. These results also demonstrate that the diffusion species observed in TiNi–SiN–Si specimens are quite different from those observed in TiNi/Si specimens.
Acknowledg em ent
The authors are grateful for the financial support of this research from the National Science Council, Taiwan, under grant NSC 88-2216-E002-013.
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