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Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering
Vacuum 83 (2009) 286–291
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering Y.M. Zhou a, Z. Xie a, *, H.N. Xiao b, P.F. Hu b, J. He c a
School of Physics and Microelectronics Science, Hunan University, Hunan 410082, China College of Materials Science and Engineering, Hunan University, Hunan 410082, China c The 44th Research Institute of China Electronics Technology Group Corporation, 14 Huayuan Road, Nanping, Chongqing 400060, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2008 Received in revised form 30 June 2008 Accepted 6 July 2008
Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering were studied. The results indicated that the electrical properties were relative to the oxygen and other impurities rather than to growth orientation. As the sputtering power increases from 25 to 100 W, the preferred-growth orientation of Ta films changes from (200) to (202) and the oxygen and impurities content in the films decrease. The temperature coefficient of resistance also reduces from 289.79 to 116.65 ppm/ C. The O/Ta ratio decrease and grain size reduction related to a change of electrical resistivity were observed at substrate temperatures in the range 300–500 C. At 650 C, partial stable aTa associated with a sharp decrease of the electrical resistivity was also found. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
Keywords: Tantalum Deposition by sputtering X-ray diffraction Crystal structure
1. Introduction Tantalum films offer a number of attractive properties for mechanical and microelectronic industry applications, including a high melting temperature, high ductility, and good wear and corrosion-resistance [1–5]. However, vapor-deposited tantalum films are often found to contain one or both of two distinct crystalline phases. There are two main phases in the sputtered tantalum film: one is a-phase tantalum (body centered cubic) with low resistivity of 15–60 mU-cm and the other is b-phase tantalum (tetragonal) with high resistivity of 170–210 mU-cm. The low resistivity a-phase tantalum is preferred to reduce interconnection resistance and is desirable for thin film interconnection [6,7]. The high resistivity b-phase tantalum has good stability and is a good candidate for thin film resistor [8–10]. However, the phase of Ta films is usually effected by substrate temperature. The structure of tantalum films deposited by sputtering at different substrate temperatures can be only the bcc a-phase, the metastable b-phase or a mixture of the two phases. Previous studies have pointed out that a a-phase tantalum film with giant-grained microstructure was easily formed at a high temperature environment, while a bphase was easily formed at a room-temperature process [11]. On the other hand, the phase of Ta films is also influenced by different substrates. As has been reported earlier, the choice of the substrate or under-layer materials is often decisive for the structure of the
* Corresponding author. Tel.: þ86 731 8822892; fax: þ86 731 8822858. E-mail address: [email protected] (Z. Xie).
sputtered tantalum layer. Tantalum films deposited on thermally oxidized silicon wafers [4,12,13], silicon [14,15] and Cu [16,17] usually leads to the formation of the tetragonal b-phase tantalum, while bcc a-Ta is formed on Al [18]. Covering the substrate with a thin under-layer of material can also promote the growth of bcc aphase of Ta [19]. The microstructure, component, and defect density play very important roles on the electrical properties of tantalum films. Crystallization orientation and grain size are amongst the important microstructure characteristics that affect significantly the electrical properties of the films. In this paper we present the results of investigation on the effects of sputtering power (25–100 W) and substrate temperature (300–650 C) on the electrical properties, the phase composition and the crystallization orientation of the tantalum deposited by direct current magnetron sputtering. The relationships between the temperature coefficient of resistance (TCR) and the orientation of the films as well as oxygen and other impurities content in the films are also reported.
2. Experimental details Tantalum films in this study were deposited on glass substrates by direct current magnetron sputtering from a pure tantalum target. The target was a diameter of 2 inch and thickness of 5 mm tantalum plate with 99.97% purity. The substrates were cleaned in the ultrasonic baths of acetone, ethanol and de-ionized water for 10 min, separately, and then dried with N2 gas before inserting into the vacuum chamber. The distance between the target and the substrate holder was 100 mm. The base pressure of the sputtering
0042-207X/$ – see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.07.002
Y.M. Zhou et al. / Vacuum 83 (2009) 286–291
chamber was kept at better than 5 104 Pa, while the depositing pressure was kept at 0.7 Pa. The flow of argon (purity of 99.9995%) was regulated by mass-flow controller to the chamber. The Ta target was cleaned by pre-sputtering for 10 min in an argon gas flow of 20 sccm before deposition. Then mass-flow controllers regulated an argon gas flow of 25 sccm to the vacuum chamber during the deposition. Two sets of tantalum samples were prepared. For the first set of tantalum films, the substrate temperature was fixed at 300 C and sputtering power is varied from 25 to 100 W. Sputtering time between 20 and 60 min was used to produce films approximately 540 nm thick. For the second set of tantalum films, the sputtering power was fixed at 100 W and the substrate temperature was varied from 300 to 650 C. For the first set of samples, a pure tantalum film dot was initially deposited on both the left and right border area of the glass substrate to act as electrode. Electrical leads were then bonded to this electrode. The tantalum film was then deposited on the glass substrate, overlapping the pre-deposited tantalum electrode. The electrical leads were connected with an Agilent 34401A digital multimeter outside the chamber by two feedthrough terminals to achieve the measurement of the electrical properties of samples without being exposed the sample to atmosphere after deposition. After deposition, films were cooled spontaneously to 100 C in the vacuum better than 5 104 Pa and then temperature dependence of resistance of the deposited tantalum films was measured in range of 100 C to room temperature. For the second sets of samples, the electrical resistivity was determined from the sheet resistance measured by four-point probe at room temperature. The phase and crystal structure of the deposited tantalum films was characterized by X-ray diffraction (XRD) with a RIGAKU D/ MAX2550VBþ, operating at 40 kV and 30 mA with Cu Ka radiation. The q–2q scans ranged between 10 and 90 with a step size of 0.02 and a fixed dwelling time of 0.2 s. The morphology of the surface of the films was studied by both scanning electron microscopy and atomic force microscopy. A field-emission-gun scanning electron microscope (JSM-6700F) working at 5.0 kV and an atomic force microscope (SolverP47-Pro) operating in noncontact mode were carried out. The composition and impurities of the tantalum films were detected by energy dispersive spectrometer (OXFORD INCA) and secondary ion mass spectroscopy (CAMECA IMS-4F). The thickness of the samples was measured by auger electron spectroscopy (ULVAC, PHI-700). 3. Results and discussions Fig. 1 shows the temperature dependence of resistance of the deposited tantalum films measured under vacuum better than 5 104 Pa in range of 100 C to room temperature and the temperature coefficient of resistance (TCR) as a function of sputtering power ranging between 25 and 100 W. It is clear from Fig. 1 that the temperature coefficient (TCR) of resistance is more negative for lower sputtering power and becomes less negative for higher sputtering power. The tantalum film deposited with sputtering power of 100 W yields a temperature coefficient of resistance (TCR) of 116.65 ppm/ C. This temperature coefficient of resistance (TCR) is in good agreement with those of previous reports [15,20]. The X-ray diffraction spectra of tantalum films grown at different sputtering powers for the deposition temperature of 300 C are shown in Fig. 2. For the sample deposited with sputtering power of 25 W, only two distinct peaks, with 2q positions at 33.5 and 38.2 , are identified as (200) and (202) peaks for the b phase of tantalum, respectively. As shown in Fig. 2, besides the two strongest reflections with 2q positions at approximately 33.34 and 37.34 , several weaker peaks of b-Ta of (333), (225), (400), and (404) appeared deposited with sputtering power of 60 W. The diffraction peak at approximately 37.34 may be the sum of the two peaks indexed as
287
Fig. 1. Effect of sputtering power on the temperature coefficient of resistance (TCR) of the films.
(330) and (202) of b-tantalum and thus showed a scalar form and broadened appearance rather than a single sharp peak. As the sputtering power was increased to 100 W, the intensities of b-Ta of (333), (225), (400), and (404) increased significantly. However, there were significant changes in the absolute intensity of the two strongest reflections of b-Ta of (200) and (202) for the three samples. At low sputtering power of 25 W, the strongest reflection was that from (200) planes of the tetragonal b-Ta phase. At high sputtering power of 100 W, however, the (202) reflection was the strongest. The ratio of the peak intensity of b-Ta (202) to b-Ta (200) increased rapidly from 0.63 to 1.44 as the sputtering power was increased from 25 to 100 W. Previous studies [12] observed that the preferred orientation of the b-Ta films changes from (200) to (202) as the oxygen content in the films increase. And the increase of the ratio of the intensities I(202)/I(200) was accompanied by a continuous temperature coefficient of resistance (TCR) reduced to less negative value. In fact, our results were entirely different. The relative intensities of these two orientations, i.e. I(202)/I(200), increased from about 0.63 in the case of the film sputtered with 25 W to 1.05 in the film sputtered with 60 W and then further increased to 1.44 in the film sputtered with 100 W, corresponding to a continuous temperature coefficient of resistance (TCR) reduced from 289.79 ppm/ C to 116.65 ppm/ C. In our case, no
Fig. 2. XRD spectra obtained on tantalum films deposited on glass substrate at 300 C at the increased sputtering powers of 25, 60, and 100 W.
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intentional change in sputtering conditions except for sputtering power was made for the samples. An available explanation to the increased temperature coefficient of resistance (TCR) may be attributed to the sputtering power. A low sputtering power was known to produce a low deposition rate. The tantalum films deposited at low oxygen or no oxygen content with a low deposition grown rate would cause the background oxygen in the vacuum chamber have time to react with tantalum atoms to form tantalum suboxides. The impurities also had effect on the grain size, which would affect the electrical properties of the tantalum films. To clarify results mentioned above, the surface morphology of samples deposited at 300 C with sputtering power of 25, 60 and 100 W was examined by scanning electron microscopy (SEM) and the micrographs were shown in Fig. 3a–c. As shown in Fig. 3a, the tantalum film sputtered at 25 W was smooth in nature and was consisted of many particles with the diameter ranging from 10 to 100 nm. As the sputtering power was increased to 60 W, the film appeared to be dense, with a granular structure and a smooth surface, with the exception of some grains that were varied more in size, complex shape and sharper edges, seen in Fig. 3b. X-ray diffraction measurement indicated that the film deposited at 60 W had (333) texture, as in Fig. 2. In addition, the surface consisted of a few elongated grains of 100–150 nm long and typically more than 30 nm wide. These elongated grains may indicate that the aforementioned diffraction peak at approximately 37.34 contained the peak indexed as (330) of b-Ta. With increasing sputtering power up to 100 W, the elongated grains disappeared and the size of
particulates with sharp edges became larger, while the amount of them decreased, seen in Fig. 3c. The tantalum film deposited at 25 W showed a more negative temperature coefficient of resistance (TCR) than that of dense film deposited at 60 W or 100 W, which was probably related to its less dense structure than that of film deposited at 60 W or 100 W. In fact, for its less dense microstructure, the residual oxygen or impurities in the vacuum chamber could easily fill in the voids and boundaries between grains in the tantalum film deposited at 25 W. Hence, although possessed a lower ratio of the intensities I(202)/I(200), the sample deposited at 25 W showed a more negative temperature coefficient of resistance (TCR) than the another two samples in our case. The tantalum film deposited at 100 W had the same dense structure as the sample at 60 W but the bigger grain size was observed, seen in Fig. 3b and c. The tantalum grain with smaller size at lower sputtering power would dissolve more easily residual oxygen or other impurities in vacuum chamber than that with bigger grain size at higher sputtering power. Thus, the bigger of the grains in tantalum films deposited at 100 W should bring the less amount oxygen or impurities pickup than that of film at 60 W. Fig. 4 shows the evolution of the Auger spectra of tantalum films depending on the sputtering power. The surface of the samples was sputtered off with argon ions for 3 min and then elements of the films were analyzed. For the two samples the O(KLL) position in the spectra does not change in energy and appears at 515 eV. Carbon peak at 269 eV is also observed, indicating the presence of carbon contamination. In our process, the carbon contamination of the
Fig. 3. SEM images of surface of tantalum films deposited on glass substrate at 300 C: (a) sputtered with 25 W; (b) sputtered with 60 W; (c) sputtered with 100 W.
Y.M. Zhou et al. / Vacuum 83 (2009) 286–291
289
Fig. 4. Auger spectra of tantalum films deposited on glass substrate at 300 C: (a) sputtered with 25 W; (b) sputtered with 60 W.
samples was probably resulted from base vacuum or glass substrate which was heated at 300 C. The films deposited at 60 W shows the tantalum peak at 178 eV which can be assigned to Ta(NVV). The film deposited at 25 W gives a spectrum of Ta(NVV) similar to that of 60 W. A slight shift of the peak towards lower energies was observed with decreasing sputtering power, indicating a higher lever of oxidation which probably induces the more negative TCR value. Besides peaks shown in Fig. 4, four obvious peaks around 1260, 1470, 1670 and 1730 eV have been also identified in these two samples obtained under condition used. They have been assigned to the MNN transition of Ta. Depth profile of the film deposited at 60 W was measured by Auger depth analysis, as shown in Fig. 5. Fig. 5 shows that the O/Ta ratio is about 0.33. The high ratio of O to Ta on the top surface is probably the presence of adsorbed oxygen. Both Auger surface spectra and depth analysis indicate the formation of partial tantalum suboxide in the films. The chemical composition of the two samples deposited at 60 and 100 W, measured by energy dispersive spectrometer (EDS), were given in Fig. 6. Compared with the inset tables in Fig. 6a and b, carbon impurity is also observed in the films. The result is content with the Auger analysis. Increasing the sputtering power resulted in a decrease in the oxygen and impurities content of the films. The increase to more negative temperature coefficient for the two samples was consistent with an increase of oxygen dissolution and impurities in the films. On the other hand, the secondary ion mass spectroscopy (SIMS) surface analyses of the two samples deposited
Fig. 5. Auger depth profile of tantalum deposited with 60 W at 300 C.
Fig. 6. Energy dispersive spectrometer (EDS) spectra of tantalum films deposited on glass substrate at 300 C: (a) sputtered with 60 W; (b) sputtered with 100 W.
at 60 and 100 W were also studied. The SIMS results (not shown here) showed that there were impurity positive ions of Hþ, OHþ, Cþ and Oþ in these two samples. However, for the sample deposited at 100 W, the absolute intensity of these impurity positive ions is
Fig. 7. XRD spectra obtained on tantalum films deposited on glass substrate with sputtering power of 100 W at the increased substrate temperature of 300, 400, 500, and 650 C.
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Fig. 8. The resistivity and O/Ta ratios of tantalum films as a function of substrate temperature.
lower than that deposited at 60 W. Hence, both EDS results and SIMS analysis implied that the diffusion of the residual oxygen and impurities in vacuum chamber into the grains of tantalum film rather than the preferred-grown orientation changes from (200) to (202) seemed to be a main influence on the change of TCR in our process. According to the results that the tantalum film deposited at power of 100 W dissolved small oxygen and impurities and had a less negative temperature coefficient of resistance (TCR) of 116.65 ppm/ C, we choose sputtering power 100 W as the base deposition conditions for different substrate temperatures in range from 300 to 650 C. The sputtering time of 20 min was applied to this set of samples. Fig. 7 showed the evolution of the X-ray diffraction patterns of these tantalum films with substrate temperature from 300 to 650 C. As shown in Fig. 7, except for the sample deposited at 650 C, all of the other three samples showed clear XRD peaks attributed to the b phase of tantalum and no clear change in texture was observed with substrate temperature variation. With the increase in the substrate temperature to 650 C, two
Fig. 9. Surface profiles measured by AFM, for the films deposited at (a) 300 C, (b) 400 C, and (c) 500 C.
Y.M. Zhou et al. / Vacuum 83 (2009) 286–291
peaks of a-Ta (200), and (211) was observed, indicating that the metastable b-Ta phase was partially transformed into the stable aTa phase. Depth profiles of tantalum films for different substrate temperatures were also measured by Auger electron spectroscopy working at 10 keV and 50 mA. Fig. 8 showed the resistivity of the tantalum films at room temperature and the O/Ta ratios with substrate temperature. As shown in Fig. 8, the O/Ta ratios of the films decreased slightly from 0.26 to 0.23 as the substrate temperature was increased from 300 to 500 C, and then the O/Ta ratio decreased to 0.18 for the sample obtained at 650 C. The ratio decrease was accompanied by a resistivity decrease. The resistivity decreased from 312 to 243 mU-cm as the substrate temperature was increased from 300 to 500 C, and then the resistivity decreased sharply to 71 mU-cm for the sample deposited at 650 C. The sharp decrease of resistivity for the sample deposited at 650 C was relative to the formation of partially a-Ta phase and the lower O/Ta ratio. These results might indicate that the O/Ta ratio tends to decrease with the substrate temperature. The probable explanation was that the tendency towards dissociation of the oxide compound increased with the substrate temperature which might result in lower O/Ta ratios and smaller grains. Hence, average grain size and morphology of the films deposited at 300, 400, and 500 C were examined by an atomic force microscope (AFM) operating in noncontact mode. The average grain size based on AFM images was calculated. The average grain size and morphology observation were shown in Fig. 9. As was clear from Fig. 9, large grains were distinctly visible in the case of sample deposited at 300 C. The morphology observation as shown in Fig. 3c also supported this result. The average grain size decreased from 24.601 to 10.672 nm as the substrate temperature was increased from 300 to 400 C, and then the average grain size decreased to 6.214 nm for the sample deposited at 500 C. Thus, although no obvious change in texture was shown for the three samples deposited at 300, 400, and 500 C (seen in Fig. 7). The decrease of the resistivity with the increase of substrate temperatures was probably relative to the O/Ta ratios decrease and the grain size reduction in the films. 4. Conclusion The effects of the processing parameters on the properties of tantalum films deposited by direct current reactive magnetron sputtering of a tantalum target have been investigated. Electrical properties and microstructure of the deposited tantalum films were found to be dependent on the sputtering power and substrate temperature in range 300–650 C. The increase of the sputtering power, from 25 to 100 W, gave rise to an increase of the temperature coefficient of resistance (TCR) of the deposited tantalum films,
291
from 289.79 to 116.65 ppm/ C. This variation was associated with an increase of the ratio of the intensities I(202)/I(200) from 0.63 to 1.44 in the films, which did not agree well with previously reports in literature. From the analysis of the deposition process, preferred-growth orientation changes and chemical composition, the major role on the electrical properties of the deposited tantalum films has been discussed. With the decrease of sputtering power, the energy position showed a slight shift towards lower energies. This evolution of microstructure resulted in an oxygen and other impurities dissolution degrease in the films, which was assumed to be an important effect on the electrical properties of tantalum films. No obvious variation in texture except for the reduction electrical resistivity of the samples was observed when the substrate temperature rises from 300 to 500 C. This variation might be related to the decrease of O/Ta ratio and grain size in the films as the increase of substrate temperature. Partial stable a-Ta phase in the film was observed when the film deposited at 650 C. Both the smaller O/T ratio and the partial transformation from b-Ta phase to stable a-Ta phase in the film were related to a drastic decrease of the electrical resistivity.
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering Y.M. Zhou a, Z. Xie a, *, H.N. Xiao b, P.F. Hu b, J. He c a
School of Physics and Microelectronics Science, Hunan University, Hunan 410082, China College of Materials Science and Engineering, Hunan University, Hunan 410082, China c The 44th Research Institute of China Electronics Technology Group Corporation, 14 Huayuan Road, Nanping, Chongqing 400060, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 May 2008 Received in revised form 30 June 2008 Accepted 6 July 2008
Effects of deposition parameters on tantalum films deposited by direct current magnetron sputtering were studied. The results indicated that the electrical properties were relative to the oxygen and other impurities rather than to growth orientation. As the sputtering power increases from 25 to 100 W, the preferred-growth orientation of Ta films changes from (200) to (202) and the oxygen and impurities content in the films decrease. The temperature coefficient of resistance also reduces from 289.79 to 116.65 ppm/ C. The O/Ta ratio decrease and grain size reduction related to a change of electrical resistivity were observed at substrate temperatures in the range 300–500 C. At 650 C, partial stable aTa associated with a sharp decrease of the electrical resistivity was also found. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
Keywords: Tantalum Deposition by sputtering X-ray diffraction Crystal structure
1. Introduction Tantalum films offer a number of attractive properties for mechanical and microelectronic industry applications, including a high melting temperature, high ductility, and good wear and corrosion-resistance [1–5]. However, vapor-deposited tantalum films are often found to contain one or both of two distinct crystalline phases. There are two main phases in the sputtered tantalum film: one is a-phase tantalum (body centered cubic) with low resistivity of 15–60 mU-cm and the other is b-phase tantalum (tetragonal) with high resistivity of 170–210 mU-cm. The low resistivity a-phase tantalum is preferred to reduce interconnection resistance and is desirable for thin film interconnection [6,7]. The high resistivity b-phase tantalum has good stability and is a good candidate for thin film resistor [8–10]. However, the phase of Ta films is usually effected by substrate temperature. The structure of tantalum films deposited by sputtering at different substrate temperatures can be only the bcc a-phase, the metastable b-phase or a mixture of the two phases. Previous studies have pointed out that a a-phase tantalum film with giant-grained microstructure was easily formed at a high temperature environment, while a bphase was easily formed at a room-temperature process [11]. On the other hand, the phase of Ta films is also influenced by different substrates. As has been reported earlier, the choice of the substrate or under-layer materials is often decisive for the structure of the
* Corresponding author. Tel.: þ86 731 8822892; fax: þ86 731 8822858. E-mail address: [email protected] (Z. Xie).
sputtered tantalum layer. Tantalum films deposited on thermally oxidized silicon wafers [4,12,13], silicon [14,15] and Cu [16,17] usually leads to the formation of the tetragonal b-phase tantalum, while bcc a-Ta is formed on Al [18]. Covering the substrate with a thin under-layer of material can also promote the growth of bcc aphase of Ta [19]. The microstructure, component, and defect density play very important roles on the electrical properties of tantalum films. Crystallization orientation and grain size are amongst the important microstructure characteristics that affect significantly the electrical properties of the films. In this paper we present the results of investigation on the effects of sputtering power (25–100 W) and substrate temperature (300–650 C) on the electrical properties, the phase composition and the crystallization orientation of the tantalum deposited by direct current magnetron sputtering. The relationships between the temperature coefficient of resistance (TCR) and the orientation of the films as well as oxygen and other impurities content in the films are also reported.
2. Experimental details Tantalum films in this study were deposited on glass substrates by direct current magnetron sputtering from a pure tantalum target. The target was a diameter of 2 inch and thickness of 5 mm tantalum plate with 99.97% purity. The substrates were cleaned in the ultrasonic baths of acetone, ethanol and de-ionized water for 10 min, separately, and then dried with N2 gas before inserting into the vacuum chamber. The distance between the target and the substrate holder was 100 mm. The base pressure of the sputtering
0042-207X/$ – see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.07.002
Y.M. Zhou et al. / Vacuum 83 (2009) 286–291
chamber was kept at better than 5 104 Pa, while the depositing pressure was kept at 0.7 Pa. The flow of argon (purity of 99.9995%) was regulated by mass-flow controller to the chamber. The Ta target was cleaned by pre-sputtering for 10 min in an argon gas flow of 20 sccm before deposition. Then mass-flow controllers regulated an argon gas flow of 25 sccm to the vacuum chamber during the deposition. Two sets of tantalum samples were prepared. For the first set of tantalum films, the substrate temperature was fixed at 300 C and sputtering power is varied from 25 to 100 W. Sputtering time between 20 and 60 min was used to produce films approximately 540 nm thick. For the second set of tantalum films, the sputtering power was fixed at 100 W and the substrate temperature was varied from 300 to 650 C. For the first set of samples, a pure tantalum film dot was initially deposited on both the left and right border area of the glass substrate to act as electrode. Electrical leads were then bonded to this electrode. The tantalum film was then deposited on the glass substrate, overlapping the pre-deposited tantalum electrode. The electrical leads were connected with an Agilent 34401A digital multimeter outside the chamber by two feedthrough terminals to achieve the measurement of the electrical properties of samples without being exposed the sample to atmosphere after deposition. After deposition, films were cooled spontaneously to 100 C in the vacuum better than 5 104 Pa and then temperature dependence of resistance of the deposited tantalum films was measured in range of 100 C to room temperature. For the second sets of samples, the electrical resistivity was determined from the sheet resistance measured by four-point probe at room temperature. The phase and crystal structure of the deposited tantalum films was characterized by X-ray diffraction (XRD) with a RIGAKU D/ MAX2550VBþ, operating at 40 kV and 30 mA with Cu Ka radiation. The q–2q scans ranged between 10 and 90 with a step size of 0.02 and a fixed dwelling time of 0.2 s. The morphology of the surface of the films was studied by both scanning electron microscopy and atomic force microscopy. A field-emission-gun scanning electron microscope (JSM-6700F) working at 5.0 kV and an atomic force microscope (SolverP47-Pro) operating in noncontact mode were carried out. The composition and impurities of the tantalum films were detected by energy dispersive spectrometer (OXFORD INCA) and secondary ion mass spectroscopy (CAMECA IMS-4F). The thickness of the samples was measured by auger electron spectroscopy (ULVAC, PHI-700). 3. Results and discussions Fig. 1 shows the temperature dependence of resistance of the deposited tantalum films measured under vacuum better than 5 104 Pa in range of 100 C to room temperature and the temperature coefficient of resistance (TCR) as a function of sputtering power ranging between 25 and 100 W. It is clear from Fig. 1 that the temperature coefficient (TCR) of resistance is more negative for lower sputtering power and becomes less negative for higher sputtering power. The tantalum film deposited with sputtering power of 100 W yields a temperature coefficient of resistance (TCR) of 116.65 ppm/ C. This temperature coefficient of resistance (TCR) is in good agreement with those of previous reports [15,20]. The X-ray diffraction spectra of tantalum films grown at different sputtering powers for the deposition temperature of 300 C are shown in Fig. 2. For the sample deposited with sputtering power of 25 W, only two distinct peaks, with 2q positions at 33.5 and 38.2 , are identified as (200) and (202) peaks for the b phase of tantalum, respectively. As shown in Fig. 2, besides the two strongest reflections with 2q positions at approximately 33.34 and 37.34 , several weaker peaks of b-Ta of (333), (225), (400), and (404) appeared deposited with sputtering power of 60 W. The diffraction peak at approximately 37.34 may be the sum of the two peaks indexed as
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Fig. 1. Effect of sputtering power on the temperature coefficient of resistance (TCR) of the films.
(330) and (202) of b-tantalum and thus showed a scalar form and broadened appearance rather than a single sharp peak. As the sputtering power was increased to 100 W, the intensities of b-Ta of (333), (225), (400), and (404) increased significantly. However, there were significant changes in the absolute intensity of the two strongest reflections of b-Ta of (200) and (202) for the three samples. At low sputtering power of 25 W, the strongest reflection was that from (200) planes of the tetragonal b-Ta phase. At high sputtering power of 100 W, however, the (202) reflection was the strongest. The ratio of the peak intensity of b-Ta (202) to b-Ta (200) increased rapidly from 0.63 to 1.44 as the sputtering power was increased from 25 to 100 W. Previous studies [12] observed that the preferred orientation of the b-Ta films changes from (200) to (202) as the oxygen content in the films increase. And the increase of the ratio of the intensities I(202)/I(200) was accompanied by a continuous temperature coefficient of resistance (TCR) reduced to less negative value. In fact, our results were entirely different. The relative intensities of these two orientations, i.e. I(202)/I(200), increased from about 0.63 in the case of the film sputtered with 25 W to 1.05 in the film sputtered with 60 W and then further increased to 1.44 in the film sputtered with 100 W, corresponding to a continuous temperature coefficient of resistance (TCR) reduced from 289.79 ppm/ C to 116.65 ppm/ C. In our case, no
Fig. 2. XRD spectra obtained on tantalum films deposited on glass substrate at 300 C at the increased sputtering powers of 25, 60, and 100 W.
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intentional change in sputtering conditions except for sputtering power was made for the samples. An available explanation to the increased temperature coefficient of resistance (TCR) may be attributed to the sputtering power. A low sputtering power was known to produce a low deposition rate. The tantalum films deposited at low oxygen or no oxygen content with a low deposition grown rate would cause the background oxygen in the vacuum chamber have time to react with tantalum atoms to form tantalum suboxides. The impurities also had effect on the grain size, which would affect the electrical properties of the tantalum films. To clarify results mentioned above, the surface morphology of samples deposited at 300 C with sputtering power of 25, 60 and 100 W was examined by scanning electron microscopy (SEM) and the micrographs were shown in Fig. 3a–c. As shown in Fig. 3a, the tantalum film sputtered at 25 W was smooth in nature and was consisted of many particles with the diameter ranging from 10 to 100 nm. As the sputtering power was increased to 60 W, the film appeared to be dense, with a granular structure and a smooth surface, with the exception of some grains that were varied more in size, complex shape and sharper edges, seen in Fig. 3b. X-ray diffraction measurement indicated that the film deposited at 60 W had (333) texture, as in Fig. 2. In addition, the surface consisted of a few elongated grains of 100–150 nm long and typically more than 30 nm wide. These elongated grains may indicate that the aforementioned diffraction peak at approximately 37.34 contained the peak indexed as (330) of b-Ta. With increasing sputtering power up to 100 W, the elongated grains disappeared and the size of
particulates with sharp edges became larger, while the amount of them decreased, seen in Fig. 3c. The tantalum film deposited at 25 W showed a more negative temperature coefficient of resistance (TCR) than that of dense film deposited at 60 W or 100 W, which was probably related to its less dense structure than that of film deposited at 60 W or 100 W. In fact, for its less dense microstructure, the residual oxygen or impurities in the vacuum chamber could easily fill in the voids and boundaries between grains in the tantalum film deposited at 25 W. Hence, although possessed a lower ratio of the intensities I(202)/I(200), the sample deposited at 25 W showed a more negative temperature coefficient of resistance (TCR) than the another two samples in our case. The tantalum film deposited at 100 W had the same dense structure as the sample at 60 W but the bigger grain size was observed, seen in Fig. 3b and c. The tantalum grain with smaller size at lower sputtering power would dissolve more easily residual oxygen or other impurities in vacuum chamber than that with bigger grain size at higher sputtering power. Thus, the bigger of the grains in tantalum films deposited at 100 W should bring the less amount oxygen or impurities pickup than that of film at 60 W. Fig. 4 shows the evolution of the Auger spectra of tantalum films depending on the sputtering power. The surface of the samples was sputtered off with argon ions for 3 min and then elements of the films were analyzed. For the two samples the O(KLL) position in the spectra does not change in energy and appears at 515 eV. Carbon peak at 269 eV is also observed, indicating the presence of carbon contamination. In our process, the carbon contamination of the
Fig. 3. SEM images of surface of tantalum films deposited on glass substrate at 300 C: (a) sputtered with 25 W; (b) sputtered with 60 W; (c) sputtered with 100 W.
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Fig. 4. Auger spectra of tantalum films deposited on glass substrate at 300 C: (a) sputtered with 25 W; (b) sputtered with 60 W.
samples was probably resulted from base vacuum or glass substrate which was heated at 300 C. The films deposited at 60 W shows the tantalum peak at 178 eV which can be assigned to Ta(NVV). The film deposited at 25 W gives a spectrum of Ta(NVV) similar to that of 60 W. A slight shift of the peak towards lower energies was observed with decreasing sputtering power, indicating a higher lever of oxidation which probably induces the more negative TCR value. Besides peaks shown in Fig. 4, four obvious peaks around 1260, 1470, 1670 and 1730 eV have been also identified in these two samples obtained under condition used. They have been assigned to the MNN transition of Ta. Depth profile of the film deposited at 60 W was measured by Auger depth analysis, as shown in Fig. 5. Fig. 5 shows that the O/Ta ratio is about 0.33. The high ratio of O to Ta on the top surface is probably the presence of adsorbed oxygen. Both Auger surface spectra and depth analysis indicate the formation of partial tantalum suboxide in the films. The chemical composition of the two samples deposited at 60 and 100 W, measured by energy dispersive spectrometer (EDS), were given in Fig. 6. Compared with the inset tables in Fig. 6a and b, carbon impurity is also observed in the films. The result is content with the Auger analysis. Increasing the sputtering power resulted in a decrease in the oxygen and impurities content of the films. The increase to more negative temperature coefficient for the two samples was consistent with an increase of oxygen dissolution and impurities in the films. On the other hand, the secondary ion mass spectroscopy (SIMS) surface analyses of the two samples deposited
Fig. 5. Auger depth profile of tantalum deposited with 60 W at 300 C.
Fig. 6. Energy dispersive spectrometer (EDS) spectra of tantalum films deposited on glass substrate at 300 C: (a) sputtered with 60 W; (b) sputtered with 100 W.
at 60 and 100 W were also studied. The SIMS results (not shown here) showed that there were impurity positive ions of Hþ, OHþ, Cþ and Oþ in these two samples. However, for the sample deposited at 100 W, the absolute intensity of these impurity positive ions is
Fig. 7. XRD spectra obtained on tantalum films deposited on glass substrate with sputtering power of 100 W at the increased substrate temperature of 300, 400, 500, and 650 C.
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Fig. 8. The resistivity and O/Ta ratios of tantalum films as a function of substrate temperature.
lower than that deposited at 60 W. Hence, both EDS results and SIMS analysis implied that the diffusion of the residual oxygen and impurities in vacuum chamber into the grains of tantalum film rather than the preferred-grown orientation changes from (200) to (202) seemed to be a main influence on the change of TCR in our process. According to the results that the tantalum film deposited at power of 100 W dissolved small oxygen and impurities and had a less negative temperature coefficient of resistance (TCR) of 116.65 ppm/ C, we choose sputtering power 100 W as the base deposition conditions for different substrate temperatures in range from 300 to 650 C. The sputtering time of 20 min was applied to this set of samples. Fig. 7 showed the evolution of the X-ray diffraction patterns of these tantalum films with substrate temperature from 300 to 650 C. As shown in Fig. 7, except for the sample deposited at 650 C, all of the other three samples showed clear XRD peaks attributed to the b phase of tantalum and no clear change in texture was observed with substrate temperature variation. With the increase in the substrate temperature to 650 C, two
Fig. 9. Surface profiles measured by AFM, for the films deposited at (a) 300 C, (b) 400 C, and (c) 500 C.
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peaks of a-Ta (200), and (211) was observed, indicating that the metastable b-Ta phase was partially transformed into the stable aTa phase. Depth profiles of tantalum films for different substrate temperatures were also measured by Auger electron spectroscopy working at 10 keV and 50 mA. Fig. 8 showed the resistivity of the tantalum films at room temperature and the O/Ta ratios with substrate temperature. As shown in Fig. 8, the O/Ta ratios of the films decreased slightly from 0.26 to 0.23 as the substrate temperature was increased from 300 to 500 C, and then the O/Ta ratio decreased to 0.18 for the sample obtained at 650 C. The ratio decrease was accompanied by a resistivity decrease. The resistivity decreased from 312 to 243 mU-cm as the substrate temperature was increased from 300 to 500 C, and then the resistivity decreased sharply to 71 mU-cm for the sample deposited at 650 C. The sharp decrease of resistivity for the sample deposited at 650 C was relative to the formation of partially a-Ta phase and the lower O/Ta ratio. These results might indicate that the O/Ta ratio tends to decrease with the substrate temperature. The probable explanation was that the tendency towards dissociation of the oxide compound increased with the substrate temperature which might result in lower O/Ta ratios and smaller grains. Hence, average grain size and morphology of the films deposited at 300, 400, and 500 C were examined by an atomic force microscope (AFM) operating in noncontact mode. The average grain size based on AFM images was calculated. The average grain size and morphology observation were shown in Fig. 9. As was clear from Fig. 9, large grains were distinctly visible in the case of sample deposited at 300 C. The morphology observation as shown in Fig. 3c also supported this result. The average grain size decreased from 24.601 to 10.672 nm as the substrate temperature was increased from 300 to 400 C, and then the average grain size decreased to 6.214 nm for the sample deposited at 500 C. Thus, although no obvious change in texture was shown for the three samples deposited at 300, 400, and 500 C (seen in Fig. 7). The decrease of the resistivity with the increase of substrate temperatures was probably relative to the O/Ta ratios decrease and the grain size reduction in the films. 4. Conclusion The effects of the processing parameters on the properties of tantalum films deposited by direct current reactive magnetron sputtering of a tantalum target have been investigated. Electrical properties and microstructure of the deposited tantalum films were found to be dependent on the sputtering power and substrate temperature in range 300–650 C. The increase of the sputtering power, from 25 to 100 W, gave rise to an increase of the temperature coefficient of resistance (TCR) of the deposited tantalum films,
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from 289.79 to 116.65 ppm/ C. This variation was associated with an increase of the ratio of the intensities I(202)/I(200) from 0.63 to 1.44 in the films, which did not agree well with previously reports in literature. From the analysis of the deposition process, preferred-growth orientation changes and chemical composition, the major role on the electrical properties of the deposited tantalum films has been discussed. With the decrease of sputtering power, the energy position showed a slight shift towards lower energies. This evolution of microstructure resulted in an oxygen and other impurities dissolution degrease in the films, which was assumed to be an important effect on the electrical properties of tantalum films. No obvious variation in texture except for the reduction electrical resistivity of the samples was observed when the substrate temperature rises from 300 to 500 C. This variation might be related to the decrease of O/Ta ratio and grain size in the films as the increase of substrate temperature. Partial stable a-Ta phase in the film was observed when the film deposited at 650 C. Both the smaller O/T ratio and the partial transformation from b-Ta phase to stable a-Ta phase in the film were related to a drastic decrease of the electrical resistivity.
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