Characteristics of DC magnetron sputtered ternary cobalt–nickel silicide thin films for ultra shallow junction devices

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Microelectronic Engineering 85 (2008) 559–565 www.elsevier.com/locate/mee

Characteristics of DC magnetron sputtered ternary cobalt–nickel silicide thin films for ultra shallow junction devices D. Panda, A. Dhar, S.K. Ray

*

Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India Received 18 June 2007; received in revised form 6 October 2007; accepted 10 October 2007 Available online 30 October 2007

Abstract Ternary cobalt–nickel silicide thin films were synthesized by DC magnetron sputtering from an equiatomic cobalt–nickel alloy target. Grazing incidence XRD, Rutherford back scattering, high-resolution cross-sectional TEM analysis and electrical study were carried out to investigate the formation of silicide, stoichiometry, film thickness, depth profile and sheet resistance of as-deposited and post-deposition annealed films. The ternary silicide layer thickness was calculated from RBS simulated data, which was found to vary 20–43 nm for as-deposited and different vacuum annealed films. A minimum value of sheet resistance 2.73 X/sq corresponding to a resistivity of 8.4 lX-cm was obtained for optimized deposition and annealing conditions. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ternary cobalt–nickel silicides; Sputtering; Annealing; Surface diffusion; Rutherford back scattering; Resistivity

1. Introduction Transition metal silicides (TMS) play an important role in silicon-based device technology as gate electrodes, Schottky barriers, interconnects and contacts for their self-passivating properties in oxygen-rich environment [1–3]. Suitable metal silicides with low silicidation temperature and Si consumption ratio are indispensable for processing ultra shallow junctions in scaled-Si CMOS technology. Although, CoSi2 and NiSi have been extensively investigated for the above applications, there are some limitations for both the silicides in aggressively scaled ultra large scale integrated (ULSI) devices. CoSi2 not only exhibits low resistivity (15 lX-cm), but also possesses a cubic structure with close lattice matching to Si, high thermal stability, low formation temperature and complete silicidation on narrow Si lines etc. [4–6]. However, formation of CoSi2 is sensitive to the presence of native oxide and results in rough interfaces with agglomeration due to non-uniform reaction between Co and Si [7,8]. NiSi is attractive for the fabrication of *

Corresponding author. Tel.: +91 3222 283838; fax: +91 3222 255303. E-mail address: [email protected] (S.K. Ray).

0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.10.001

advanced sub-quarter, micron devices because of its lower growth temperature and less silicon consumption ratio as compared to CoSi2 [9–12], both of which are prerequisites for low thermal budget ultra shallow junction technology. However, its thermal stability is limited to 750 °C, above which the NiSi phase transforms into high resistive NiSi2 [6,13,14]. Although NiSi2 is highly resistive, it has best lattice match with Si among all silicide phases. Recently, there have been growing interests in ternary cobalt–nickel silicide (Co1 xNixSi) films [15–20], to exploit the beneficial properties possessed by both CoSi2 and NiSi. Highly conducting ternary cobalt–nickel silicide with a resistivity of 16– 20 lX-cm has been reported at a growth temperature lower than that of CoSi2 and stability up to higher temperature as compared to NiSi [21]. However, only few results have been reported on the synthesis with structural and electrical properties of ternary Co1 xNixSi films. In this paper, we report the formation of ternary cobalt– nickel silicide thin films on Si (1 0 0) using DC magnetron sputtering from a single Co–Ni alloy target. The effect of post-deposition annealing on the phase formation, structural properties and resistivity of the resultant films are reported.

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2. Experimental Ternary cobalt–nickel silicide thin films were grown by DC magnetron sputtering using a cobalt–nickel circular shaped alloy target having an equal atomic ratio of Co and Ni. The base pressure of the chamber was 0.1 Pa. Metal layers were deposited on bulk p-type boron doped Si (1 0 0) substrates with resistivity 7–14 X-cm at different substrate temperatures. Silicon wafers were degreased by ultrasonic agitation in acetone followed by rinsing in DIwater. They were treated in a mixture of H2SO4 and H2O2 solution (1:1) for 20 min followed by rinsing thoroughly in DI-water. Finally, the samples were dipped in 1% HF solution and immediately loaded into the deposition chamber for metal (Co–Ni) deposition after washing with prompt DI-water. The deposition pressure, discharge current and deposition time were maintained constant during each set of deposition. Post-deposition annealing was carried out in nitrogen and vacuum ambient at temperatures varying from 300 °C to 750 °C, to study the formation of ternary Co1 xNixSi silicides. Unreacted metal was removed from the annealed samples before characterisations. Typical deposition conditions and post-deposition annealing of samples carried out in the present study are summarized in Table 1. Grazing angle X-ray diffraction (Philips X’Pert-Pro MRD) and Rutherford backscattering spectroscopy (RBS) were carried out on different samples to study the phase formation, film thickness and composition of the silicide films. The grazing angle (1.0°) XRD data were collected with a step size of 0.01° in the point focus mode of the X-ray beam. RBS measurement using 2 MeV He2+ ion beam delivered from a tandem pelletron accelerator was carried out with a silicon surface barrier detector fixed at a backscattering angle of 165° and the spectrum was analyzed using simulation software SIMNRA 6.03 [22,23]. High-resolution cross-sectional transmission electron microscopy (HRXTEM) observation is performed using a JEOL JEM–2100, microscope operating at 200 kV. Sheet resistance of the series of silicide samples was measured using four-probe technique (VEECO model FPP-5000). 3. Results and discussion Fig. 1a shows the grazing incidence XRD pattern of the samples deposited for 6 min at room temperature (Set 1) and films annealed in nitrogen at different temperatures. The thickness of the deposited metal (Co–Ni) layer is 20– 24 nm, measured from the surface profile-meter. In 400 °C annealed film, a single Ni3Si (2 2 0) [ICDD data card no: 03-1048] binary phase is present; other peaks are attributed to ternary silicides. Silicide phases are absent in the asdeposited sample because of the amorphous nature of the film. In general, both Ni and Co have relatively high diffusion coefficient (D) in Si, so Ni and Co could easily migrate into Si to form the silicide. The diffusion coefficient of Ni (DNi) in Si is, however, slightly higher than that of Co

Table 1 Different deposition conditions and post-deposition treatments of cobalt– nickel silicide thin film processing Sample name

Annealing temperature (°C)

Substrate temperature (°C)

Annealing ambient and time

Set 1

CS1-00 CS1-40 CS1-45 CS1-50 CS1-55 CS1-60 CS1-65

As-deposited 400 450 500 550 600 650

Room temperature

Nitrogen, 1-h

Set 2

CS2-00 CS2-30 CS2-40 CS2-45

As-deposited 350 400 450

200

Nitrogen, 1-h

Set 3

CS3-00 CS3-30 CS3-35 CS3-45 CS3-45 CS3-50

As-deposited 300 350 400 450 500

200

Vacuum, 20-min

Set 4

CS4-00 CS4-33 CS4-36 CS4-39

300

200

As-deposited Nitrogen, 30-min Nitrogen, 60-min Nitrogen, 90-min

Set 5

CS5-00 CS5-21 CS5-22 CS5-23 CS5-24

200

200

As-deposited Vacuum, 10-min Vacuum, 20-min Vacuum, 30-min Vacuum, 40-min

(DCo) [24]. Therefore, Ni3Si (2 2 0) is preferentially formed at a lower annealing (400 °C) temperature. Diffusion to grow Co and Ni silicide was studied in 1980s [25]. In many cases, the growth of a silicide layer at a metal–Si interface was shown often to take place mainly by the diffusion of silicon through silicide. The dominance of this process is understandable when silicide is polycrystalline and transport occurs via grain boundaries. On a typical single-crystal Si substrate, the process is most likely to proceed by diffusion of metal like Co and Ni into silicon, which is applicable in the present study. For all the annealed samples a number of additional peaks appear due to the formation of silicides. Two peaks at 43.56° and 62.86°, which do not match either with the deposited metal or binary silicides, are attributed to the ternary cobalt–nickel silicide phases. The peak at 2h of 36.36° between the planes of Ni3Si (2 0 0) and CoSi (2 0 0) is may be due to the formation of CoxNi1 xSi (2 0 0) phase, in agreement with reported results [21]. The normalized peak intensity of three different peaks from the XRD data is plotted as a function of annealing temperature and is shown in Fig. 1b. As seen from this figure, the intensity of the peak at angles 36.36° increases, when the sample is annealed above 500 °C. The intensity of the peak at 43.56° decreases, while that of 62.86° varies randomly as a function of annealing temperature. Fig. 2a shows the grazing incidence XRD pattern of

Normalized peak intensity

CoxNi1-xSi CoxNi1-xSi

CoxNi1-xSi

Intensity (a.u.)

o

650 C o

600 C o

550 C o

500 C o

*

o

400 C as-deposited

40

60

(b)

(a)

450 C

20

561

0.48

* Ni3Si (-311)

CoxNi1-xSi

CoNi3Si (200)

D. Panda et al. / Microelectronic Engineering 85 (2008) 559–565

80

0.42

0.36 o

2θ = 36.36 o 2θ = 43.56 o 2θ = 62.86

0.30

0.24

0.18

100

400

450

500

550

600

650

Annealing temperature (oC)

2θ (Degree)

Normalized peak intensity

Cox Ni1-x Si

Cox Ni1-x Si

Cox Ni1-x Si

Cox Ni1-x Si

o

450 C o

400 C as-deposited

40

0.8

(a) Cox Ni1-x Si

Cox Ni1-x Si

CoxNi1-xSi (200)

Intensity (a.u.)

Fig. 1. (a) XRD spectra of N2 annealed cobalt–nickel silicide thin films (Set 1) and (b) plot of normalized peak intensity for three dominant peaks as a function of annealing temperature [intensity normalized using I1 normalize = I1/(I1 + I2 + I3)].

60

80

100

2θ (Degree)

(b)

0.6 o

2θ = 43.56 o 2θ = 51.44 o 2θ = 75.97

0.4

0.2

300

350

400

450

Annealing temperature (oC)

Fig. 2. (a) XRD pattern of N2 annealed film at substrate temperature 200 °C (Set 2) and (b) plot of normalized intensity of three selected peak as a function of annealing temperature.

the sample deposited at 200 °C and annealed in nitrogen at different temperatures (Set 2). Because of the elevated substrate temperature, even the as-deposited sample exhibits a signature of silicide formation. In addition, Fig. 2a does not show any binary silicide phases; only ternary silicide phases are found on annealing. Normalized peak intensities plotted in Fig. 2b show a trend similar to that shown in Fig. 2a. The XRD spectra of vacuum annealed films (Set 3) are shown in Fig. 3. Some additional peaks of ternary silicide phases are observed, few of them are identical with the ternary cobalt–nickel silicide phases reported by Guo and Tsai [20] at angle 47.24°. In general at high temperature nickel monosilicide phase is formed. But we observed one N2Si phase formed at 350 °C and still this phase present at 500 °C although its intensity decreasing. That’s indicates this binary silicide phase is not stable at high temperature. So we conclude that those phases are may be due to the ternary silicides which agrees with the

reported results [21]. On the other hand, it was found from the XRD pattern that an annealing conditions, N2 annealed (Fig. 1a) and vacuum annealed (Fig. 3), influences on the development of the solid-state reactions as well as crystallinity in the CoNi/Si(1 0 0) thin film system. The vacuum annealed samples shows more solid-state reactions in compare to N2 annealed one due to the presence of some residual oxygen on a atmosphere of the furnace, which are retarded solid-state reactions on the formation of silicide phases [26]. RBS random spectra of different vacuum annealed cobalt–nickel silicide samples are shown in Fig. 4a, along with those simulated by SIMNRA 6.03 program. The peaks corresponding to individual Co and Ni elements are not completely resolved because of the nearly same atomic masses (Co = 58.9332 a.m.u. and Ni = 58.6934 a.m.u.) [16,21]. But the formation of silicides due to diffusion of metals in silicon is clearly detected. The valley on silicon

Ni2Si CoxNi1-xSi CoxNi1-xSi

* NiSi (200)

CoxNi1-xSi

Intensity (a.u.)

CoxNi1-xSi

D. Panda et al. / Microelectronic Engineering 85 (2008) 559–565

NiSi CoxNi1-xSi CoxNi1-xSi (200)

562

o

500 C

*

o

450 C o

400 C o

350 C o

300 C

as-deposited 20

40

60

80

2θ θ (Degree) (

100

ing increase of annealing temperature, which also attributed to increase of film roughness with the increase of higher annealing temperature [22]. The experimental data have been simulated using a structure consisting of ternary silicide layers of different thicknesses and compositions. Typical depth profiles of Co, Ni and Si in the silicided region for the samples vacuum annealed at 500 °C are shown in Fig. 4b. The schematic diagram of RBS experiment is shown in inset of Fig. 4b. RBS depth profile clearly indicates the formation of monosilicide phases. When the samples are annealed at higher temperature, total thickness of the ternary silicide layers increases. Fig. 5 shows the extracted thickness of ternary silicides from the best fitted RBS data as a function of annealing temperature. It may be noted that, thickness of the silicide layer increases with

Fig. 3. XRD spectra of vacuum annealed film deposited at a substrate temperature 200 °C (Set 3) cobalt–nickel silicide films.

45

40

Thickness (nm)

edge, as shown in Fig. 4a, is due to the formation of silicide layers. In the RBS spectra we could not find any signature of oxygen and nitrogen contaminations following the annealing steps and it attributes to the formation of selfpassivating silicide layers. A visible plateau in Si edge is observed in 350 °C vacuum annealed samples, but not such plateau noticed at higher annealing temperature. This plateau affects to the RBS slopes and hence to film roughness [22]. Since our scattering angle is 165° and non-grazing incidence and emergence angles, so here we neglected correlation effects, such as incidence through a hump and emergence through a valley or multiple surface scattering. Those effects are strongly effected at 180° back scattering angles. Roughness of the films is affected at low energy edges and gets broader, but higher energy edge of the films is not affected. As seen from Fig. 4a, Si edges broaden dur-

35

30

25

350

as-deposited

400

450

500

o

Annealing temperature ( C) Fig. 5. Variation of ternary silicide thickness with annealing temperature.

Energy (keV) 1000

800

1200

1400

Silicide

(b) Element concentration

350 C o 450 C o 500 C simulated

Normalised yield

1.0

Ni & Co

o

(a)

1600

2 MeV He2+

0.8

Si

15º Detector

0.4

0.2

Co & Ni

Plateau

Si substrate 0

Si

0.6

0.0

300

375

450

Channel

525

600

675

0

100

300

200

400

15

Layer thickness (10 atoms/cm2)

Fig. 4. (a) RBS spectrum of vacuum annealed cobalt–nickel silicide films (Set 3) and (b) typical depth profile of 500 °C vacuum annealed film extracted from the SIMNRA 6.03 simulated data.

D. Panda et al. / Microelectronic Engineering 85 (2008) 559–565

increase in annealing temperature due to higher silicon consumption. The extracted thickness is very close to that observed in high-resolution cross-sectional TEM micrograph of 500 °C annealed cobalt–nickel silicide thin films, shown in Fig. 6. The HRXTEM micrograph exhibits a fairly uniform diffusion of metals into Si (1 0 0) substrate with the formation of fully silicided film having a thickness of 40–42 nm formed from 20 to 24 nm metal layer which attributes to the formation of monosilicide phase. Which is agrees with the XRD and RBS data. Fig. 7 shows the comparison of sheet resistance of different samples deposited at various conditions (Table 1) as a function of annealing temperature and time. The deposition temperature and post-deposition annealing conditions have a noticeable effect on the measured sheet resistance,

Fig. 6. Cross-sectional TEM micrographs of ternary cobalt–nickel silicide film formed by annealing at vacuum in 500 °C.

563

shown in Fig. 7a. The sheet resistance of the as-deposited Set 1 sample is around 186 X/sq. The sheet resistance was increased abruptly to 802 X/sq on annealing at 450 °C. At higher anneal temperatures, the sheet resistances are found to decrease slightly. The high sheet resistance of the without substrate heated samples is may be due to the presence of oxides on the metal silicon interface. The interface oxide was removed at substrate temperature due to the heating of hydrogen passivation layers and as a result low resistive silicide phase formed. Another reason for it may be due to the agglomeration of the silicide films. Such type high sheet resistance of the silicide films reported by Zhu et al. [17], d’Heurle et al. [25]. This can be correlated with the normalized intensity of the XRD data, shown in Fig. 1b. The high sheet resistance is attributed to the highly resistive ternary phase at angle 62.86°, which also shows a random variation as a function of annealed temperature. Moreover we think that CoxNi1 xSi (2 0 0) is of low resistive phase; because the sheet resistivity does not increase much after the formation of this phase (2h = 36.36°), although high resistive phase (2h = 62.86°) is present. This result is in qualitative agreement with those reported by Shong et al. [19] Guo and Tsai [20] and Zhang et al. [27]. It is known that binary nickel silicide exhibits a sharp rise in sheet resistance above 700 °C due to the formation of NiSi2 [28]. The resistivity of the nitrogen annealed films (Set 2) remains almost constant over the studied temperature range. In this case both high and low resistive ternary silicide phases are absent, (as shown in Fig. 2a), for the sample annealed at 400 °C. Due to the absence of high resistive phase, the sheet resistivity is low. However, samples are annealed at higher temperature, the low resistive (CoxNi1 xSi (2 0 0)) phase are formed and sheet resistivity decreases although high resistive phase (2h = 62.86°) also evolved. The sheet resistivity depends upon the relative fraction of CoxNi1 xSi (2 0 0), 43.56° and 62.86° phases shown in Fig. 1b for Set 1 and 43.56°,

25

(b)

(a)

600 400 200 60 40

without substrate heating N2 annealed with substrate heating Vacuum annealed with substrate heating N2 annealed

20

Sheet resistance (Ω/Sq.)

Sheet resistance (Ω/Sq.)

800

o

N2 annealed (300 C) o

Vacuum annealed (200 C) 20

15

0 10

as-deposited 300

400

500 o

Annealing temperature ( C)

600

700

as-deposited 20

40

60

80

100

Annealing time (Minute)

Fig. 7. Variation of sheet resistance as a function of (a) annealing temperature and (b) annealing time for different cobalt–nickel silicide films.

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Resistivity (μΩ-cm)

80

60

40

20

as-deposited

350

400

450

500

Annealing temperature (oC) Fig. 8. Variation of resistivity as a function of annealing temperature of cobalt–nickel silicide films.

51.44° and 75.97° shown in Fig. 2b for Set 2. We achieved very low sheet resistance for the samples deposited with substrate heating. The minimum sheet resistance is found to be 2.73 X/sq, when the samples are vacuum annealed at 400 °C (Set 3). However, the sheet resistance is found to increase above 450 °C, due to the decomposition of low resistive phase at 2h 31.68°. This sheet resistance value obtained in our study is comparable with reported results [17,21]. Shong et al. [19] and Wang et al. [29] also reported similar sheet resistivity value, by using Ti and TiN as capping layers for their Co/Ni multilayer system, respectively. Fig. 7b shows the measure sheet resistance of the best condition deposited samples (Set 4 and Set 5) as a function of annealing time. In this case we observed that the 20 min vacuum annealing and 1-h nitrogen annealing shows minimum sheet resistivity. Resistivity of the vacuum annealed films as a function of annealing temperature has been calculated using extracted thickness from RBS data. Fig. 8 shows the variation of resistivity as a function of annealing temperature of ternary silicide films. Initially resistivity is found to decrease with the increase of annealing temperature achieving minimum value (8–10 lX-cm) in temperature ranges 400–450 °C. This resistivity value is lowest among the reported results on ternary silicides. However, at higher temperature (500 °C) the resistivity increases in consistent with the sheet resistivity results. Our study shows the cobalt–nickel silicide has a lower silicidation temperature as compared to CoSi2 and better thermal stability than NiSi, which is in agreement with previously reported results [25]. Therefore, ternary silicide with low resistivity is attractive for application as a contact material for future shallow junction devices. 4. Conclusions We have studied the synthesis of ternary cobalt–nickel silicide thin films by DC magnetron sputtering from an

equiatomic cobalt–nickel alloy target on p-Si (1 0 0) substrate. Grazing angle XRD shows the formation of ternary silicide phases. The layer thickness of ternary silicide films as extracted from RBS analysis varied from 20 nm to 43 nm in the mixed region of as-deposited and annealed samples. The simulated composition of ternary silicide phases agrees with XRD data. Cross-sectional TEM micrograph of the cobalt–nickel silicide films shows the formations of a smooth interface due to uniform interdiffusion. A minimum sheet resistance of 2.73 X/sq leading to resistivity 8.4 lX-cm has been achieve for samples are vacuum annealed at 400 °C. The ternary silicide shows a temperature window for low resistivity as compared to binary NiSi, a requirement of contact material in ULSI device applications. Acknowledgements The authors are grateful to Defense Research Development Organization (DRDO), New Delhi, India, for financial support of the work and Ion Beam Laboratory (IBL), Institute of Physics (IOP), Bhubaneswar, India for providing the RBS facility. References [1] S.-J. Wang, G.R. Misium, J.C. Camp, K.-L. Chen, H.L. Tigelaar, IEEE Electron. Dev. Lett. 13 (9) (1992) 471–472. [2] A. Lauwers, K.K. Larsen, M.V. Hove, R. Verbeeck, K. Maex, J. Appl. Phys. 77 (1995) 2525–2536. [3] G. Gewinner, C. Pirri, J.C. Peruchetti, D. Bolmont, J. Derrien, P. Thiry, Phys. Rev. B 38 (1988) 1879–1884. [4] B.-S. Chen, M.-C. Chen, J. Appl. Phys. 74 (2) (1993) 1035–1039. [5] A. Alberti, F. La Via, E. Rimini, Appl. Phys. Lett. 81 (2002) 55–57. [6] V. Teodorescu, L. Nistor, H. Bender, A. Steegen, A. Lauwers, K. Maex, J. Van Landuyt, J. Appl. Phys. 90 (1) (2001) 167–174. [7] A. Alberti, F. La Via, V. Raineri, E. Rimini, J. Appl. Phys. 86 (1999) 3089–3095. [8] S.P. Muraka, D.S. Williams, J. Vac. Sci. Technol. B 5 (1987) 1674– 1688. [9] A. Lauwers, A. Steegen, M. de Potter, R. Lindsay, A. Satta, H. Bender, K. Maex, J. Vac. Sci. Technol. B 19 (6) (2001) 2026–2037. [10] A. Lauwers, M. de Potter, O. Chamirian, R. Lindsay, C. Demeurisse, C. Vrancken, K. Maex, Microelectron. Eng. 64 (1–4) (2002) 131–142. [11] E.G. Colgan, J.P. Gambino, Q.Z. Hong, Mater. Sci. Eng. R 16 (2) (1996) 43–96. [12] E.C. Cahoon, C.M. Comrie, R. Pretorious, Appl. Phys. Lett. 44 (5) (1984) 511–513. [13] J.-S. Maa, Y. Ono, D.J. Tweet, F. Zhang, S.T. Hsu, J. Vac. Sci. Technol. A 19 (4) (2001) 1595–1599. [14] S.K. Ray, T.N. Adam, G.S. Kar, C.P. Swann, MRS Proc. 745 (2002) N6.6.1–N6.6.6. [15] M. Cannaerts, O. Chamirian, K. Maex, C. Van Haesendonck, Nanotechnology 13 (2002) 149–152. [16] A. Alberti, C. Bongiorno, F. La Via, C. Spinella, J. Appl. Phys. 94 (2003) 231–237. [17] S. Zhu, R.L. Van Meirhaeghe, S. Forment, G.-P. Ru, X.-P. Qu, B.Z. Li, Solid State Electron. 48 (2004) 1205–1209. [18] J.A. van Beek, P.J.T.L. Oberndorff, A.A. Kodentsov, F.J.J. Van Loo, J. Alloy Compd. 297 (2000) 137–143. [19] O. Shong, D. Kim, C.S. Yoon, C.K. Kim, Mater. Sci. Semicond. Process. 8 (5) (2005) 608–612. [20] S.S. Guo, C.J. Tsai, J. Vac. Sci. Technol. A 21 (3) (2003) 628–633.

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