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An investigation of structural phase transformation and electrical resistivity in Ta films
Applied Surface Science 257 (2010) 1211–1215
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An investigation of structural phase transformation and electrical resistivity in Ta films A. Javed a,∗ , Ji-Bing Sun b a b
Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e
i n f o
Article history: Received 20 June 2010 Received in revised form 17 July 2010 Accepted 4 August 2010 Available online 11 August 2010 Keywords: Metallic films Growth parameters Phase transformation Surface roughness Electrical resistivity
a b s t r a c t In this work, we report the effect of substrate, film thickness and sputter pressure on the phase transformation and electrical resistivity in tantalum (Ta) films. The films were grown on Si(1 0 0) substrates with native oxides in place and glass substrates by varying the film thickness (t) and pressure of the working gas (pAr ). X-ray diffraction (XRD) analysis showed that the formation of ␣ and  phases in Ta films strongly depend on the choice of substrate, film thickness t and sputter pressure pAr . A stable ␣-phase was observed on Si(1 0 0) substrates for t ≤ 200 nm. Both ␣ and  phases were found to grow on glass substrates at all thicknesses except t = 100 nm. All the films grown on Si(1 0 0) substrates for pAr ≤ 6.5 mTorr had ␣-phase with strong (1 1 0) texture normal to the film plane. The glass substrates promoted the formation of -phase in all pAr except pAr = 5.5 mTorr. The resistivity was observed to decrease with t, whereas was increased with pAr on Si(1 0 0) substrates. In all films, the measured resistivity was greater than the bulk resistivity. The resistivity was influenced by the effects of surface roughness and grain size. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Tantalum (Ta) thin films are of interest for technological applications in microelectronics industry such as diffusion barriers, decorative and protective coatings due to their excellent corrosion, wear resistance, high melting point (≈3000 ◦ C) and high hardness [1,2]. Ta thin films generally grow in two crystal structures; a body-centered-cubic (bcc) structure known as ␣-phase and a metastable tetragonal structure known as -phase [3]. Each phase has different microstructural, electrical and mechanical properties [4]. For example, stable bcc ␣-phase Ta films which have low resistivity (15–70 cm) are commonly used in thin film capacitors. A metastable tetragonal -phase which has high resistivity (140–210 cm) is used in thin film resistors [5] and heaters [1]. The coexistence of ␣ and  phases in thin films limited their use in microelectronics industry. Thin Ta films are also used (by the magnetic community) as seed layer (or as protective layer) during the growth of magnetic thin films and multilayer systems to improve the magnetic properties [6,7]. The presence of two phases may not be useful to improve the properties of magnetic thin films. Due to high melting point of Ta, the Ta films are usually grown using sputtering technique rather than evaporation. The formation of these
∗ Corresponding author. E-mail address: a.javed@sheffield.ac.uk (A. Javed).
phases strongly depend on the deposition conditions; for example, sputter pressure, substrate temperature and film thickness [8]. For the growth of single phase (␣ or ) Ta films, several approaches have been adopted by various groups [9–13]. For example, Zhang et al. [3] fabricated low resistivity ␣-Ta films on Si(1 0 0) substrates by varying the Cr under layer thickness. They found that ␣-Ta films with low resistivity (∼20 cm) can be obtained with ˚ Liu et al. [8,10] studied the annealing Cr under layer thickness 20 A. effect on the structure of 550 nm thick Ta films grown on Si(1 0 0) and SiO2 /Si substrates. They observed phase transformation from  to ␣ at annealing temperature of 500 ◦ C in the films grown on Si substrates. They concluded that there was no phase transformation at annealing temperature of 500 ◦ C in the films grown on SiO2 substrates. In the films grown on Si(1 0 0) substrates, they observed 80% -Ta phase transformation into ␣-Ta at 600 ◦ C, whereas 20% of Ta transformation into ␣-Ta occurs at this temperature in the films grown on SiO2 substrates. They suggested that the oxygen diffusion into Ta/SiO2 interface could be the possible reason to limit the phase transformation from  to ␣. Ren and Sosnowski [9] fabricated Ta films on Si(1 0 0) and Al substrates using RF magnetron sputtering. They studied phase transformation in Ta films by varying the substrate bias voltage (0–300 V) and found that both ␣ and  phases co-existed on both substrates for substrate bias voltage of 0 V and phase disappeared with increase in substrate bias. They found that the grain size decreased linearly with the increase in bias voltage. Recently, Zhou et al. [4] investigated the effect of deposition param-
0169-4332/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.023
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eters (sputter power and substrate temperature) on the structure and transport properties of Ta films. They fabricated 540 nm thick Ta films on the glass substrates by varying the sputter power from 25 to 100 W. They observed a metastable tetragonal -phase in all films grown on glass substrates. They investigated the effect of substrate temperature (300–650 ◦ C) on the film sputtered at 100 W and observed that a (2 0 0) reflection corresponding to ␣-phase appeared at substrate temperature of 650 ◦ C. They found that the resistivity significantly reduced (320–50 cm) with increase in substrate temperature (300–650 ◦ C). Recently, Grosser and Schmid [2] reported the effect of sputter conditions on microstructure and resistivity of Ta films fabricated on glass substrates. They observed -phase in all films of thickness up to 940 nm. They found that a stable ␣-phase occurred in the film of thickness 1300 nm. From SEM studies they observed that the microstructure change from dense to voided structure with increase in back pressure. They also found that the resistivity of the films increased with the increase in back pressure. One drawback in their work was that the influence of the variation in film thickness was not systematically studied, see Ref. [2]. The phase transformation and the resistivity in Ta films of thickness down to few nanometer needs to be studied for applications, where as-deposited films are required without complex processing [14,15] of the films to achieve single phase (␣ or ). The purpose of this work was to study the effects of substrate, film thickness and sputter pressure on the structural phase transformation and electrical resistivity in Ta films by extending the work reported by Grosser and Schmid [2].
2. Experimental detail Four sets of Ta films were grown using a DC magnetron sputtering technique. Two sets were grown on Si(1 0 0) substrates with native oxides in place and the other two sets were grown on glass substrates. High purity Ta (99.99%) target of thickness 2 mm and diameter 50.8 mm was DC sputtered with sputter power P = 100 W for the growth of all four film sets. The deposition conditions for all four film sets were as follows: set-1 (on Si substrates) was grown at fixed P = 100 W and sputter pressure pAr = 5.5 mTorr, but varying the film thickness t from 20 to 500 nm. Set-2 was grown on glass substrates under similar conditions as set-1. Set-3 of film thickness 50 ± 2 nm were grown (on Si substrates) at fixed P = 100 W but varying pAr from 5.5 to 7.0 mTorr. Set-4 was grown on glass substrates under similar conditions as set-3. The substrate-target distance d = 7.5 cm was kept constant during the growth of all film sets. Before growing Ta films, the chamber was first calibrated for thickness so that the film thickness could be controlled by controlling the deposition time. All film sets were grown at room temperature. A piece (1 cm × 1 cm) of Si substrate was cut from a 3 in. diameter Si(1 0 0) wafer and a 1 cm × 1 cm glass pieces were cut from 2 cm × 5 cm glass slide. Prior to film deposition, the substrates were cleaned in ultrasonic bath with acetone and industrial methylated spirits (IMS) to ensure that the substrate is free from contamination. After cleaning, the substrate was immediately placed in the deposition chamber. Before deposition, the base pressure inside the chamber was achieved better than 4.0 × 10−6 Torr. The phase transformation and crystal structure of all the film sets were studied using X-ray diffraction (XRD) technique. A Siemens D5000 X-ray diffractometer (in standard –2 geome˚ try) operating at 30 kV and 40 mA with Co radiation ( = 1.78896 A) was used. Surface morphology, grain size and surface roughness of all the films were studied using an atomic force microscope (AFM; IIIa Veeco Digital Nanoscope). Scanning electron microscope (SEM) was used to study the preferred grain growth in the films. The square resistance (R) of all film sets was measured using a
Fig. 1. XRD spectra of Ta films grown on (a) Si(1 0 0) substrates and (b) glass substrates. (i) t = 50 nm, (ii) t = 100 nm, (iii) t = 200 nm, (iv) t = 300 nm, (v) t = 400 nm and (vi) t = 500 nm.
standard four-point-probe method. Finally, the electrical resistivity, was measured using the relation, = R(w × t)/l, where w and l represents the width and the length of a thin film sample, t is film thickness and R is resistance of a square (l = w = 1 cm) sample. 3. Results and discussion Fig. 1(a) shows the XRD spectra of Ta films of different thicknesses grown on Si(1 0 0) substrates. The XRD spectra showed that the films with t ≤ 200 nm grew with body-centred-cubic (bcc) structure (␣-phase). For t ≤ 200 nm, all films had (1 1 0) texture normal to the film plane and no peak other than the (1 1 0) reflection was observed. For t ≥ 300 nm, the phase transformation (␣ to ) occurs. A strong peaks corresponding to -phase were observed in 400 nm thick film. Reflections corresponding to both ␣ and  phase were found to be co-existed in 300 and 500 nm thick films [Fig. 1(a)]. Fig. 1(b) shows XRD spectra of Ta films of different thicknesses grown on glass substrates. On glass substrates (Fig. 1(b)], it was observed 50 nm thick film crystallized into ␣-phase as a dominant phase, but a small trace of -phase at 2 ≈ 51.4◦ was also observed at this film thickness [Fig. 1(b)]. A pure ␣-phase was observed in the film of t = 100 nm. With the increase in film thickness, the phase transformations (␣ to ) were found in the film of t = 200 nm. At this film thickness, it was found that the -phase dominated with a small trace of ␣-phase (2 2 0) at 2 ≈ 99.9◦ . For 200 nm ≤ t ≤ 400 nm, it was found that Ta films grew with a dominant -phase. Both ␣
A. Javed, J.-B. Sun / Applied Surface Science 257 (2010) 1211–1215
Fig. 2. XRD spectra of Ta films grown at different pAr on (a) Si(1 0 0) substrates and (b) glass substrates.
and  phases were observed to be existed in 500 nm thick Ta film on glass substrates with a strong ␣-(1 1 0) peak [Fig. 1(b)]. Our results in Fig. 1(b) differ with the work of Grosser and Schmid [2]. Grosser and Schmid observed a pure -phase in the films of 210 nm ≤ t ≤ 940 nm on glass substrates. Their work showed that the phase transformation was independent of t in this thickness regime. The minimum thickness of the film studied by Grosser and Schmid [2] was 210 nm. They reported that the ␣-phase appeared for t ≥ 1300 nm; whereas our results indicated that the film thickness had a significant effect on the phase transformation. It can be seen that a pure ␣-phase was formed in the film of thickness t = 100 nm, while mixed phases were observed for all other thicknesses studied [see Fig. 1(b)]. This could be due to the difference in (i) substrate–target distance (ii) size of the target used or (iii) deposition conditions. It should be noted that the phase formation in Ta films is also sensitive to growth rate [16]. It is known that the higher growth rate promoted the formation of ␣-phase [2,16]. According to Ref. [2], the growth rate associated with the ␣-phase was 0.2 nm s−1 , which was less than that we achieved (∼0.5 nm s−1 ) according to our sputter conditions. Fig. 2 shows the XRD spectra of 50 ± 2 nm thick Ta films grown on Si and glass substrates by varying pAr . In all the films grown on Si(1 0 0) substrates, ␣-phase was observed for pAr ≤ 6.5 mTorr. A small satellite peak (3 1 1) at 2 ≈ 37.8◦ corresponding to phase was found to be appeared at pAr = 7.0 mTorr. On glass substrates, a completely different phase formation was observed. For pAr ≥ 6.0 mTorr, at best only a low intensity (330) reflection cor-
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responding to -phase was observed above the noise [Fig. 2(b)]. Other peaks may be present but it was difficult to observe their presence due to background noise signal in the XRD spectra. At pAr = 5.5 mTorr, both ␣ and  phases were found to be co-existed. From Fig. 1(a), it can be seen that the (1 1 0) peak shifted towards higher 2 value with increase in t from t = 20 nm up to t = 100 nm. For t = 200 nm film, the peak shifted towards lower 2 value. For the films which were grown on Si(1 0 0) substrates by varying pAr (set-3), the (1 1 0) peak shifted towards lower 2 value [Fig. 2(a)] with the increase in pAr . The shift in (1 1 0) peak position attributed the presence of homogeneous strain in these films. For the films, which grew with pure bcc ␣-phase (set-1 with t ≤ 200 nm and set3), the lattice constant, ‘a’ normal to the film plane was determined from the (110) peak position. It was found that the thin film lattice constant (a) decreased from 3.388 ± 0.006 A˚ to 3.355 ± 0.001 A˚ with an increase in t from 20 to 200 nm. Further, it was found that the thin film lattice constant, a was greater than the bulk lattice ˚ The homogeneous strain ε = (a − ao )/ao was constant, ao = 3.306 A. determined from the lattice constant data and it was found that the films (t ≤ 200 nm) were in the state of tensile strain. The lattice constant was also determined for set-3 films (see Table 1), and it was found that the lattice constant increased from 3.365 ± 0.003 A˚ to 3.443 ± 0.005 A˚ linearly with the increase in pAr from 5.5 to 7.5 mTorr. The estimated homogeneous strain was positive for all the films and increased with pAr which meant that the tensile strain increased with an increase in pAr (Table 1). It is known that the stress (i.e. strain) in sputtered metallic thin films changes from compressive to tensile depending on the product of the pressure pAr and the substrate-target distance d [17]. In our deposition setup, for d = 7.5 cm, the chosen range of pAr gave tensile stress in our films over the range of pAr studied. For set-3 films (␣-phase), the stress can be estimated using, = (ε/2)Y, where Y is the Young’s modulus and is the Poisson ratio of Ta. For stress evaluation, the constants Y = 1.962 × 1011 Pa and = 0.35 were taken from Ref. [9]. The estimated stress in set-3 films was increased from 5 to 11.5 GPa with the increase in strain from 1.8 to 4.1%, respectively. The (1 1 0) peak broadened due to both grain size and inhomogeneous strain effects. The grain size was also calculated using Scherrer equation [18], which was less than the grain size calculated from AFM measurements. The extra breadth of the (1 1 0) peak was due to the inhomogeneous strain. This point is discussed later in this paper after presenting the AFM data. Fig. 3 shows the cross-sectional SEM images of 400 nm and the 500 nm thick films grown on Si substrates. A clear columnar grain growth was observed in these films. For t ≤ 200 nm, the columnar grain growth was not clearly resolved using SEM. Fig. 4 shows the AFM 3D topographic images of Ta films of different t, grown on Si substrates. From the analysis of the AFM images, both the average grain size (D) and the root-mean-square (RMS) surface roughness were found to increase with t. For the films grown on Si substrates, the RMS surface roughness was found to be increased from 1.1 ± 0.3 to 4.0 ± 0.8 nm with the increase in t from 20 to 500 nm. On glass substrates, the surface roughness was found to be varied from 1.0 ± 0.1 to 2.5 ± 1.2 nm over the thickness range studied. On Si(1 0 0) substrates, the grain size increased from 25 ± 3 nm to 60 ± 5 nm with the increase in t. On glass substrates, the grain size, D varied over a narrow range (66 ± 2 to 70 ± 4 nm) with the increase in t, and was effectively constant. Using the AFM grain size data (for films set-1 and set-3), the FWHM was estimated using Scherrer equation and was compared with the FWHM determined from the XRD (1 1 0) peak. It was found that the FWHM estimated from the AFM grain size data was less than the FWHM measured from XRD (1 1 0) peak (Table 1). As mentioned above, in thin films, the peak broadened due to the effect of both grain size and the inhomogeneous strain. The difference in FWHM showed that the extra breadth of the (1 1 0) peak was a sig-
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Table 1 Summary of some structural parameters for set-3 films derived from XRD and AFM data. pAr (mTorr)
Phase
˚ Lattice constant, a (A)
5.5 6.0 6.5 7.0
␣ ␣ ␣ ␣
3.365 3.412 3.409 3.443
± ± ± ±
0.003 0.004 0.006 0.005
Strain (%)
FWHMXRD (◦ )
1.8 3.2 3.1 4.1
5.8 6.3 5.7 6.0
Fig. 3. Cross-sectional SEM images of Ta films grown on Si(1 0 0) substrates (a) t = 400 nm and (b) t = 500 nm.
± ± ± ±
0.2 0.2 0.5 0.4
Grain size, DAFM (nm) 32.3 35.5 33.2 36.0
± ± ± ±
1.2 1.5 1.7 1.2
Surface roughness (nm) 1.2 2.3 3.6 3.8
± ± ± ±
1 1 1 1
nature of the dominant contribution into the peak broadening from the inhomogeneous strain. Comparing the XRD spectra in Fig. 1(a) and (b), a relatively sharp (1 1 0) peak can be seen in the films grown on glass substrates. The electrical resistivity of all the film sets is presented in Fig. 5. Fig. 5(a) shows the electrical resistivity as a function of film thickness t. It can be seen from Fig. 5(a) that the decreases with the increase in t for the films grown on glass substrates. On Si(1 0 0) substrates, the was found to be decreased with t for t ≤ 100 nm. The maximum was observed in 200 nm thick film. Beyond t = 200 nm, the decreased with t [Fig. 5(a)]. Comparing two data sets in Fig. 5(a), it can be seen that the films grown on glass substrates had higher as compared to the films grown on Si(1 0 0) substrates for t ≤ 100 nm. For t ≥ 100 nm, the was almost similar (except, t = 200 nm) in all the films grown on Si and glass substrates. However, the measured in all films of different t was larger than the bulk resistivity, bulk = 13.5 cm. The dependence of on t was found to be decreased (approaching to bulk ) with the increase in t. The increase in grain size with film thickness causes a decrease in resistivity in the films grown on Si(1 0 0) and glass substrates. Fig. 5(b) shows the as a function of pAr for the film grown on Si and glass substrates. It can be seen that the dependence of on pAr is significantly different on Si(1 0 0) and glass substrates, particularly for low and high value of pAr . On Si substrates, the was found to be increased with the increase in pAr . With an increase in pAr , the mean free path l (l ∝ (1/pAr )) decreased due to increase in collisions between sputter particles and working gas, which results in different surface morphology.
Fig. 4. AFM 3D topographic images of Ta films grown on Si(1 0 0) substrates (a) t = 20 nm, (b) t = 100 nm, (c) t = 200 nm and (d) t = 400 nm.
A. Javed, J.-B. Sun / Applied Surface Science 257 (2010) 1211–1215
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4. Summary In summary, we have studied the effect of substrate, film thickness and sputter pressure on the microstructure and electrical resistivity in Ta films. It was observed that the growth of Ta films on Si(1 0 0) substrates promoted ␣-phase to grow for t ≤ 200 nm. Above 200 nm, ␣-phase was found to be co-existed with metastable tetragonal -phase. On glass substrates, both ␣ and  phases were observed at all film thicknesses except for t = 100 nm. On Si(1 0 0) substrates, a strong (1 1 0) peak corresponding to ␣-phase was observed in all the films grown by varying pAr except pAr = 7.0 mTorr. A columnar grain growth was observed for t ≥ 300 nm. The resistivity decreased with an increase in film thickness on both the substrates, whereas increased with the increase in pAr . It was found that the grain size and the surface roughness influence the resistivity in the films. In conclusion, formation ␣ and  phase depend on the growth conditions and a single phase (␣ or  phase) films can be grown with the careful choice of the substrate, film thickness and sputter pressure. Acknowledgements We acknowledge the support from Professor M.R.J. Gibbs during this work. We are also thankful to Paul Hawksworth for his technical assistance during the fabrication of films. References [1] [2] [3] [4] [5] [6] Fig. 5. Electrical resistivity in Ta films grown on Si(1 0 0) and glass substrates as a function of (a) film thickness, t and (b) Ar-pressure, pAr .
According to Mattiessen’s formula [19], the resistivity in metals and metallic films [20] can be explained on the basis of various electron scattering mechanisms. The scattering processes involved are collision of electrons with impurities, defects, grain boundaries and surface scattering in ultrathin films. The analysis of the AFM images of set-3 films showed that the surface roughness increased from 1.2 ± 0.8 to 3.8 ± 1.2 nm with the increase in pAr . It is known that the carrier mobility is inversely proportional to the film’s surface roughness [20]. Thus the increase in surface roughness causes an increase in resistivity in these films. The surface roughness of the films grown on glass substrates by varying pAr was almost constant ∼1.8 ± 0.6 nm over the whole range of pAr , thus the resistivity in these films did not vary over a wide range.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An investigation of structural phase transformation and electrical resistivity in Ta films A. Javed a,∗ , Ji-Bing Sun b a b
Department of Engineering Materials, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China
a r t i c l e
i n f o
Article history: Received 20 June 2010 Received in revised form 17 July 2010 Accepted 4 August 2010 Available online 11 August 2010 Keywords: Metallic films Growth parameters Phase transformation Surface roughness Electrical resistivity
a b s t r a c t In this work, we report the effect of substrate, film thickness and sputter pressure on the phase transformation and electrical resistivity in tantalum (Ta) films. The films were grown on Si(1 0 0) substrates with native oxides in place and glass substrates by varying the film thickness (t) and pressure of the working gas (pAr ). X-ray diffraction (XRD) analysis showed that the formation of ␣ and  phases in Ta films strongly depend on the choice of substrate, film thickness t and sputter pressure pAr . A stable ␣-phase was observed on Si(1 0 0) substrates for t ≤ 200 nm. Both ␣ and  phases were found to grow on glass substrates at all thicknesses except t = 100 nm. All the films grown on Si(1 0 0) substrates for pAr ≤ 6.5 mTorr had ␣-phase with strong (1 1 0) texture normal to the film plane. The glass substrates promoted the formation of -phase in all pAr except pAr = 5.5 mTorr. The resistivity was observed to decrease with t, whereas was increased with pAr on Si(1 0 0) substrates. In all films, the measured resistivity was greater than the bulk resistivity. The resistivity was influenced by the effects of surface roughness and grain size. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Tantalum (Ta) thin films are of interest for technological applications in microelectronics industry such as diffusion barriers, decorative and protective coatings due to their excellent corrosion, wear resistance, high melting point (≈3000 ◦ C) and high hardness [1,2]. Ta thin films generally grow in two crystal structures; a body-centered-cubic (bcc) structure known as ␣-phase and a metastable tetragonal structure known as -phase [3]. Each phase has different microstructural, electrical and mechanical properties [4]. For example, stable bcc ␣-phase Ta films which have low resistivity (15–70 cm) are commonly used in thin film capacitors. A metastable tetragonal -phase which has high resistivity (140–210 cm) is used in thin film resistors [5] and heaters [1]. The coexistence of ␣ and  phases in thin films limited their use in microelectronics industry. Thin Ta films are also used (by the magnetic community) as seed layer (or as protective layer) during the growth of magnetic thin films and multilayer systems to improve the magnetic properties [6,7]. The presence of two phases may not be useful to improve the properties of magnetic thin films. Due to high melting point of Ta, the Ta films are usually grown using sputtering technique rather than evaporation. The formation of these
∗ Corresponding author. E-mail address: a.javed@sheffield.ac.uk (A. Javed).
phases strongly depend on the deposition conditions; for example, sputter pressure, substrate temperature and film thickness [8]. For the growth of single phase (␣ or ) Ta films, several approaches have been adopted by various groups [9–13]. For example, Zhang et al. [3] fabricated low resistivity ␣-Ta films on Si(1 0 0) substrates by varying the Cr under layer thickness. They found that ␣-Ta films with low resistivity (∼20 cm) can be obtained with ˚ Liu et al. [8,10] studied the annealing Cr under layer thickness 20 A. effect on the structure of 550 nm thick Ta films grown on Si(1 0 0) and SiO2 /Si substrates. They observed phase transformation from  to ␣ at annealing temperature of 500 ◦ C in the films grown on Si substrates. They concluded that there was no phase transformation at annealing temperature of 500 ◦ C in the films grown on SiO2 substrates. In the films grown on Si(1 0 0) substrates, they observed 80% -Ta phase transformation into ␣-Ta at 600 ◦ C, whereas 20% of Ta transformation into ␣-Ta occurs at this temperature in the films grown on SiO2 substrates. They suggested that the oxygen diffusion into Ta/SiO2 interface could be the possible reason to limit the phase transformation from  to ␣. Ren and Sosnowski [9] fabricated Ta films on Si(1 0 0) and Al substrates using RF magnetron sputtering. They studied phase transformation in Ta films by varying the substrate bias voltage (0–300 V) and found that both ␣ and  phases co-existed on both substrates for substrate bias voltage of 0 V and phase disappeared with increase in substrate bias. They found that the grain size decreased linearly with the increase in bias voltage. Recently, Zhou et al. [4] investigated the effect of deposition param-
0169-4332/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.08.023
1212
A. Javed, J.-B. Sun / Applied Surface Science 257 (2010) 1211–1215
eters (sputter power and substrate temperature) on the structure and transport properties of Ta films. They fabricated 540 nm thick Ta films on the glass substrates by varying the sputter power from 25 to 100 W. They observed a metastable tetragonal -phase in all films grown on glass substrates. They investigated the effect of substrate temperature (300–650 ◦ C) on the film sputtered at 100 W and observed that a (2 0 0) reflection corresponding to ␣-phase appeared at substrate temperature of 650 ◦ C. They found that the resistivity significantly reduced (320–50 cm) with increase in substrate temperature (300–650 ◦ C). Recently, Grosser and Schmid [2] reported the effect of sputter conditions on microstructure and resistivity of Ta films fabricated on glass substrates. They observed -phase in all films of thickness up to 940 nm. They found that a stable ␣-phase occurred in the film of thickness 1300 nm. From SEM studies they observed that the microstructure change from dense to voided structure with increase in back pressure. They also found that the resistivity of the films increased with the increase in back pressure. One drawback in their work was that the influence of the variation in film thickness was not systematically studied, see Ref. [2]. The phase transformation and the resistivity in Ta films of thickness down to few nanometer needs to be studied for applications, where as-deposited films are required without complex processing [14,15] of the films to achieve single phase (␣ or ). The purpose of this work was to study the effects of substrate, film thickness and sputter pressure on the structural phase transformation and electrical resistivity in Ta films by extending the work reported by Grosser and Schmid [2].
2. Experimental detail Four sets of Ta films were grown using a DC magnetron sputtering technique. Two sets were grown on Si(1 0 0) substrates with native oxides in place and the other two sets were grown on glass substrates. High purity Ta (99.99%) target of thickness 2 mm and diameter 50.8 mm was DC sputtered with sputter power P = 100 W for the growth of all four film sets. The deposition conditions for all four film sets were as follows: set-1 (on Si substrates) was grown at fixed P = 100 W and sputter pressure pAr = 5.5 mTorr, but varying the film thickness t from 20 to 500 nm. Set-2 was grown on glass substrates under similar conditions as set-1. Set-3 of film thickness 50 ± 2 nm were grown (on Si substrates) at fixed P = 100 W but varying pAr from 5.5 to 7.0 mTorr. Set-4 was grown on glass substrates under similar conditions as set-3. The substrate-target distance d = 7.5 cm was kept constant during the growth of all film sets. Before growing Ta films, the chamber was first calibrated for thickness so that the film thickness could be controlled by controlling the deposition time. All film sets were grown at room temperature. A piece (1 cm × 1 cm) of Si substrate was cut from a 3 in. diameter Si(1 0 0) wafer and a 1 cm × 1 cm glass pieces were cut from 2 cm × 5 cm glass slide. Prior to film deposition, the substrates were cleaned in ultrasonic bath with acetone and industrial methylated spirits (IMS) to ensure that the substrate is free from contamination. After cleaning, the substrate was immediately placed in the deposition chamber. Before deposition, the base pressure inside the chamber was achieved better than 4.0 × 10−6 Torr. The phase transformation and crystal structure of all the film sets were studied using X-ray diffraction (XRD) technique. A Siemens D5000 X-ray diffractometer (in standard –2 geome˚ try) operating at 30 kV and 40 mA with Co radiation ( = 1.78896 A) was used. Surface morphology, grain size and surface roughness of all the films were studied using an atomic force microscope (AFM; IIIa Veeco Digital Nanoscope). Scanning electron microscope (SEM) was used to study the preferred grain growth in the films. The square resistance (R) of all film sets was measured using a
Fig. 1. XRD spectra of Ta films grown on (a) Si(1 0 0) substrates and (b) glass substrates. (i) t = 50 nm, (ii) t = 100 nm, (iii) t = 200 nm, (iv) t = 300 nm, (v) t = 400 nm and (vi) t = 500 nm.
standard four-point-probe method. Finally, the electrical resistivity, was measured using the relation, = R(w × t)/l, where w and l represents the width and the length of a thin film sample, t is film thickness and R is resistance of a square (l = w = 1 cm) sample. 3. Results and discussion Fig. 1(a) shows the XRD spectra of Ta films of different thicknesses grown on Si(1 0 0) substrates. The XRD spectra showed that the films with t ≤ 200 nm grew with body-centred-cubic (bcc) structure (␣-phase). For t ≤ 200 nm, all films had (1 1 0) texture normal to the film plane and no peak other than the (1 1 0) reflection was observed. For t ≥ 300 nm, the phase transformation (␣ to ) occurs. A strong peaks corresponding to -phase were observed in 400 nm thick film. Reflections corresponding to both ␣ and  phase were found to be co-existed in 300 and 500 nm thick films [Fig. 1(a)]. Fig. 1(b) shows XRD spectra of Ta films of different thicknesses grown on glass substrates. On glass substrates (Fig. 1(b)], it was observed 50 nm thick film crystallized into ␣-phase as a dominant phase, but a small trace of -phase at 2 ≈ 51.4◦ was also observed at this film thickness [Fig. 1(b)]. A pure ␣-phase was observed in the film of t = 100 nm. With the increase in film thickness, the phase transformations (␣ to ) were found in the film of t = 200 nm. At this film thickness, it was found that the -phase dominated with a small trace of ␣-phase (2 2 0) at 2 ≈ 99.9◦ . For 200 nm ≤ t ≤ 400 nm, it was found that Ta films grew with a dominant -phase. Both ␣
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Fig. 2. XRD spectra of Ta films grown at different pAr on (a) Si(1 0 0) substrates and (b) glass substrates.
and  phases were observed to be existed in 500 nm thick Ta film on glass substrates with a strong ␣-(1 1 0) peak [Fig. 1(b)]. Our results in Fig. 1(b) differ with the work of Grosser and Schmid [2]. Grosser and Schmid observed a pure -phase in the films of 210 nm ≤ t ≤ 940 nm on glass substrates. Their work showed that the phase transformation was independent of t in this thickness regime. The minimum thickness of the film studied by Grosser and Schmid [2] was 210 nm. They reported that the ␣-phase appeared for t ≥ 1300 nm; whereas our results indicated that the film thickness had a significant effect on the phase transformation. It can be seen that a pure ␣-phase was formed in the film of thickness t = 100 nm, while mixed phases were observed for all other thicknesses studied [see Fig. 1(b)]. This could be due to the difference in (i) substrate–target distance (ii) size of the target used or (iii) deposition conditions. It should be noted that the phase formation in Ta films is also sensitive to growth rate [16]. It is known that the higher growth rate promoted the formation of ␣-phase [2,16]. According to Ref. [2], the growth rate associated with the ␣-phase was 0.2 nm s−1 , which was less than that we achieved (∼0.5 nm s−1 ) according to our sputter conditions. Fig. 2 shows the XRD spectra of 50 ± 2 nm thick Ta films grown on Si and glass substrates by varying pAr . In all the films grown on Si(1 0 0) substrates, ␣-phase was observed for pAr ≤ 6.5 mTorr. A small satellite peak (3 1 1) at 2 ≈ 37.8◦ corresponding to phase was found to be appeared at pAr = 7.0 mTorr. On glass substrates, a completely different phase formation was observed. For pAr ≥ 6.0 mTorr, at best only a low intensity (330) reflection cor-
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responding to -phase was observed above the noise [Fig. 2(b)]. Other peaks may be present but it was difficult to observe their presence due to background noise signal in the XRD spectra. At pAr = 5.5 mTorr, both ␣ and  phases were found to be co-existed. From Fig. 1(a), it can be seen that the (1 1 0) peak shifted towards higher 2 value with increase in t from t = 20 nm up to t = 100 nm. For t = 200 nm film, the peak shifted towards lower 2 value. For the films which were grown on Si(1 0 0) substrates by varying pAr (set-3), the (1 1 0) peak shifted towards lower 2 value [Fig. 2(a)] with the increase in pAr . The shift in (1 1 0) peak position attributed the presence of homogeneous strain in these films. For the films, which grew with pure bcc ␣-phase (set-1 with t ≤ 200 nm and set3), the lattice constant, ‘a’ normal to the film plane was determined from the (110) peak position. It was found that the thin film lattice constant (a) decreased from 3.388 ± 0.006 A˚ to 3.355 ± 0.001 A˚ with an increase in t from 20 to 200 nm. Further, it was found that the thin film lattice constant, a was greater than the bulk lattice ˚ The homogeneous strain ε = (a − ao )/ao was constant, ao = 3.306 A. determined from the lattice constant data and it was found that the films (t ≤ 200 nm) were in the state of tensile strain. The lattice constant was also determined for set-3 films (see Table 1), and it was found that the lattice constant increased from 3.365 ± 0.003 A˚ to 3.443 ± 0.005 A˚ linearly with the increase in pAr from 5.5 to 7.5 mTorr. The estimated homogeneous strain was positive for all the films and increased with pAr which meant that the tensile strain increased with an increase in pAr (Table 1). It is known that the stress (i.e. strain) in sputtered metallic thin films changes from compressive to tensile depending on the product of the pressure pAr and the substrate-target distance d [17]. In our deposition setup, for d = 7.5 cm, the chosen range of pAr gave tensile stress in our films over the range of pAr studied. For set-3 films (␣-phase), the stress can be estimated using, = (ε/2)Y, where Y is the Young’s modulus and is the Poisson ratio of Ta. For stress evaluation, the constants Y = 1.962 × 1011 Pa and = 0.35 were taken from Ref. [9]. The estimated stress in set-3 films was increased from 5 to 11.5 GPa with the increase in strain from 1.8 to 4.1%, respectively. The (1 1 0) peak broadened due to both grain size and inhomogeneous strain effects. The grain size was also calculated using Scherrer equation [18], which was less than the grain size calculated from AFM measurements. The extra breadth of the (1 1 0) peak was due to the inhomogeneous strain. This point is discussed later in this paper after presenting the AFM data. Fig. 3 shows the cross-sectional SEM images of 400 nm and the 500 nm thick films grown on Si substrates. A clear columnar grain growth was observed in these films. For t ≤ 200 nm, the columnar grain growth was not clearly resolved using SEM. Fig. 4 shows the AFM 3D topographic images of Ta films of different t, grown on Si substrates. From the analysis of the AFM images, both the average grain size (D) and the root-mean-square (RMS) surface roughness were found to increase with t. For the films grown on Si substrates, the RMS surface roughness was found to be increased from 1.1 ± 0.3 to 4.0 ± 0.8 nm with the increase in t from 20 to 500 nm. On glass substrates, the surface roughness was found to be varied from 1.0 ± 0.1 to 2.5 ± 1.2 nm over the thickness range studied. On Si(1 0 0) substrates, the grain size increased from 25 ± 3 nm to 60 ± 5 nm with the increase in t. On glass substrates, the grain size, D varied over a narrow range (66 ± 2 to 70 ± 4 nm) with the increase in t, and was effectively constant. Using the AFM grain size data (for films set-1 and set-3), the FWHM was estimated using Scherrer equation and was compared with the FWHM determined from the XRD (1 1 0) peak. It was found that the FWHM estimated from the AFM grain size data was less than the FWHM measured from XRD (1 1 0) peak (Table 1). As mentioned above, in thin films, the peak broadened due to the effect of both grain size and the inhomogeneous strain. The difference in FWHM showed that the extra breadth of the (1 1 0) peak was a sig-
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Table 1 Summary of some structural parameters for set-3 films derived from XRD and AFM data. pAr (mTorr)
Phase
˚ Lattice constant, a (A)
5.5 6.0 6.5 7.0
␣ ␣ ␣ ␣
3.365 3.412 3.409 3.443
± ± ± ±
0.003 0.004 0.006 0.005
Strain (%)
FWHMXRD (◦ )
1.8 3.2 3.1 4.1
5.8 6.3 5.7 6.0
Fig. 3. Cross-sectional SEM images of Ta films grown on Si(1 0 0) substrates (a) t = 400 nm and (b) t = 500 nm.
± ± ± ±
0.2 0.2 0.5 0.4
Grain size, DAFM (nm) 32.3 35.5 33.2 36.0
± ± ± ±
1.2 1.5 1.7 1.2
Surface roughness (nm) 1.2 2.3 3.6 3.8
± ± ± ±
1 1 1 1
nature of the dominant contribution into the peak broadening from the inhomogeneous strain. Comparing the XRD spectra in Fig. 1(a) and (b), a relatively sharp (1 1 0) peak can be seen in the films grown on glass substrates. The electrical resistivity of all the film sets is presented in Fig. 5. Fig. 5(a) shows the electrical resistivity as a function of film thickness t. It can be seen from Fig. 5(a) that the decreases with the increase in t for the films grown on glass substrates. On Si(1 0 0) substrates, the was found to be decreased with t for t ≤ 100 nm. The maximum was observed in 200 nm thick film. Beyond t = 200 nm, the decreased with t [Fig. 5(a)]. Comparing two data sets in Fig. 5(a), it can be seen that the films grown on glass substrates had higher as compared to the films grown on Si(1 0 0) substrates for t ≤ 100 nm. For t ≥ 100 nm, the was almost similar (except, t = 200 nm) in all the films grown on Si and glass substrates. However, the measured in all films of different t was larger than the bulk resistivity, bulk = 13.5 cm. The dependence of on t was found to be decreased (approaching to bulk ) with the increase in t. The increase in grain size with film thickness causes a decrease in resistivity in the films grown on Si(1 0 0) and glass substrates. Fig. 5(b) shows the as a function of pAr for the film grown on Si and glass substrates. It can be seen that the dependence of on pAr is significantly different on Si(1 0 0) and glass substrates, particularly for low and high value of pAr . On Si substrates, the was found to be increased with the increase in pAr . With an increase in pAr , the mean free path l (l ∝ (1/pAr )) decreased due to increase in collisions between sputter particles and working gas, which results in different surface morphology.
Fig. 4. AFM 3D topographic images of Ta films grown on Si(1 0 0) substrates (a) t = 20 nm, (b) t = 100 nm, (c) t = 200 nm and (d) t = 400 nm.
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4. Summary In summary, we have studied the effect of substrate, film thickness and sputter pressure on the microstructure and electrical resistivity in Ta films. It was observed that the growth of Ta films on Si(1 0 0) substrates promoted ␣-phase to grow for t ≤ 200 nm. Above 200 nm, ␣-phase was found to be co-existed with metastable tetragonal -phase. On glass substrates, both ␣ and  phases were observed at all film thicknesses except for t = 100 nm. On Si(1 0 0) substrates, a strong (1 1 0) peak corresponding to ␣-phase was observed in all the films grown by varying pAr except pAr = 7.0 mTorr. A columnar grain growth was observed for t ≥ 300 nm. The resistivity decreased with an increase in film thickness on both the substrates, whereas increased with the increase in pAr . It was found that the grain size and the surface roughness influence the resistivity in the films. In conclusion, formation ␣ and  phase depend on the growth conditions and a single phase (␣ or  phase) films can be grown with the careful choice of the substrate, film thickness and sputter pressure. Acknowledgements We acknowledge the support from Professor M.R.J. Gibbs during this work. We are also thankful to Paul Hawksworth for his technical assistance during the fabrication of films. References [1] [2] [3] [4] [5] [6] Fig. 5. Electrical resistivity in Ta films grown on Si(1 0 0) and glass substrates as a function of (a) film thickness, t and (b) Ar-pressure, pAr .
According to Mattiessen’s formula [19], the resistivity in metals and metallic films [20] can be explained on the basis of various electron scattering mechanisms. The scattering processes involved are collision of electrons with impurities, defects, grain boundaries and surface scattering in ultrathin films. The analysis of the AFM images of set-3 films showed that the surface roughness increased from 1.2 ± 0.8 to 3.8 ± 1.2 nm with the increase in pAr . It is known that the carrier mobility is inversely proportional to the film’s surface roughness [20]. Thus the increase in surface roughness causes an increase in resistivity in these films. The surface roughness of the films grown on glass substrates by varying pAr was almost constant ∼1.8 ± 0.6 nm over the whole range of pAr , thus the resistivity in these films did not vary over a wide range.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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