SrTiO3 bilayer dielectrics for metal–insulator–metal capacitors

Thin Solid Films 519 (2011) 5734–5739

Contents lists available at ScienceDirect

Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Enhanced leakage current behavior of Sr2Ta2O7−x/SrTiO3 bilayer dielectrics for metal–insulator–metal capacitors C. Baristiran Kaynak a,⁎, M. Lukosius a, I. Costina a, B.Tillack a,b, Ch. Wenger a, G. Ruhl c, T. Blomberg d a

IHP Im Technologiepark 25, 15236 Frankfurt Oder, Germany Technische Universität Berlin, HFT4, Einsteinufer 25, 10587, Berlin, Germany Infineon Technologies AG, Wernerwerkstr. 2, 93049 Regensburg, Germany d ASM Microchemistry Ltd., Väinö Auerin katu 12 A, 00560 Helsinki, Finland b c

a r t i c l e

i n f o

Available online 12 January 2011 Keywords: MIM capacitor High-k dielectric Leakage current density

a b s t r a c t Metal–Insulator–Metal (MIM) capacitors are one of the most essential components of radio frequency devices and analog/mixed-signal integrated circuits. In order to obtain high capacitance densities in MIM devices, high-k materials have been considered to be promising candidates to replace the traditional insulators. The challenging point is that the dielectric material must demonstrate high capacitance density values with low leakage current densities. In this work, SrTiO3 based MIM capacitors have been investigated and the electrical performance of the devices have been optimized by using bilayered systems of Sr2Ta2O7−x/SrTiO3 with different thicknesses of Sr2Ta2O7−x. Sputtering X-Ray photoelectron spectroscopy (XPS) measurements have been applied to investigate the interfaces between the thin film constituents of the MIM stacks. The optimized bilayered system provides a leakage current density of 8 10− 8 A/cm2 at 2 V (bottom electrode injection) and a high capacitance density of 13 fF/μm2. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Besides the focus of integrated circuit (IC) miniaturization, there is an increasing need for the “More than Moore” technology, which is mainly based on silicon technology. Typical examples are Radio Frequency (RF) circuits, passives, sensors, Micro-Electro-Mechanical Systems (MEMS), bio-chips or solid-state lighting. As one of the key passive components in RF circuits, MIM capacitors have attracted much attention. Passive circuit elements like MIM capacitors have already been successfully integrated on-chip. MIM capacitors are valuable in many applications, such as RF circuits in mixed-signal ICs and for the decoupling in Microprocessor Units (MPU). Whereby, the key performance attributes of MIM capacitors for wireless applications (RF and mixed-signal) include high voltage linearity of capacitance, low series resistances, high capacitance densities and low parasitic capacitances. In order to reduce the parasitic capacitances, MIM capacitors are integrated in the back-end of IC processes. The integration of high-k dielectrics reduces the footprint area of MIM devices and therefore the contributions to the parasitic capacitance are reduced. However, with the integration of high-k dielectrics the

⁎ Corresponding author. Tel.: + 49 335 5625 580; fax: + 49 335 5625 327. E-mail address: [email protected] (C.B. Kaynak). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.001

reduction of leakage current densities is becoming a critical issue, especially for applications such as decoupling capacitors. Among various candidates, strontium tantalates have been investigated for MIM applications due to their high dielectric constants in the crystalline state (up to 100), low leakage currents and chemical stability [1]. Several phases in the strontium tantalate system have been reported and could be written under SrxTa2O5 + x formula [2], where x could be 1, 2, 4 and 6. The Sr5Ta4O15 phase has also been observed [3]. Thin films of strontium tantalates have been prepared by metalorganic vapor deposition (MOCVD) [4,5], atomic layer deposition (ALD) [6–8], sol–gel [9], or chemical solution deposition [10] techniques. However, dielectric constants in the range of 18–25 are typical for amorphous strontium tantalate films [11,12]. The leakage current densities at 2 V are in the range of 10− 8 to 10− 9 A/cm2 [11,13]. As a function of stoichiometry, the band gap of strontium tantalate is in the range of 4.5 to 4.8 eV [14]. Due to the high-k value of about 150 in the crystalline phase, strontium titanate (SrTiO3) films are considered to be the most promising materials for dielectric of dynamic random access memory (DRAM) capacitors [15]. However, they suffer from high leakage currents which may be related to a small oxide band gap of 3.5 eV [16]. The most used methods for the preparation of SrTiO3 films are the sol–gel technique [17,18], chemical vapor deposition (CVD) [19,20], hydrothermal synthesis method [21], and MOCVD [22,23]. The approach of this work is to combine the excellent leakage behavior of amorphous Sr2Ta2O7−x films with the high-k values of

C.B. Kaynak et al. / Thin Solid Films 519 (2011) 5734–5739

crystalline SrTiO3 layers. The electrical performance of bilayered Sr2Ta2O7−x/SrTiO3 MIM capacitors is optimized by varying the thickness of Sr2Ta2O7−x films. 2. Experimental 200 mm Si(100) wafers with 100 nm TiN/40 nm TaN layers, grown by physical vapor deposition (PVD), were used as substrates (Si/TiN/ TaN). Sr2Ta2O7−x films were deposited on a TaN layer using an atomic vapor deposition (AVD) in Tricent reactor. The principle of AVD is based on repetitive injections of micro doses of solution of a volatile metal-organic precursor, flash evaporation at 200 °C, vapor transport by a carrier gas (Ar), and oxidant-assisted (O2) decomposition on a hot substrate (400 °C). An Sr[Ta(OEt)5(me)]2 precursor, dissolved in toluene, was used for the deposition of films. The composition of the amorphous film was proved as Sr2Ta2O7−x by XPS measurement in which the standard single element sensitivity factors from the PHI-MULTIPAK software are utilized (Sr3d = 1.992, Ta4f = 3.384, O1s = 0.733). SrTiO3 films were grown on Sr2Ta2O7−x layer by ALD in Pulsar 2000 reactor at 250 °C. (1,2,4-tertiarybutyl-Cp)2Sr (from SAFC) and Ti(OMe)4 were used as the metal precursors and H2O was used as the oxidizing agent. Composition of the film was controlled by changing the cycle ratios of the SrO and TiO2 deposition's steps:


z⋅ x⋅ ð1; 2; 4−tertiarybutyl−CpÞ2 Sr + H2 O + y⋅ TiðOMeÞ4 + H2 O x = y = Sr = Ti cycle ratio z = number of deposition cycles; adjusted to get the desired thickness of the films: Fig. 1 presents the composition of the films measured by Rutherford Backscattering Spectrometry (RBS) as a function of the Sr/Ti cycle ratio. 2:1 Sr/Ti cycle ratio (corresponding to ≈1.3 Sr/Ti atomic ratio in the films) was used throughout the depositions. For the optimization of Sr2Ta2O7−x/SrTiO3 bilayered MIM capacitor system, 3 nm, 6 nm, and 9 nm thickness of Sr2Ta2O7−x have been combined with 50 nm SrTiO3 as a dielectric stack. Single layer dielectric SrTiO3 (50 nm) system has been utilized as a reference. After the deposition of the SrTiO3 layer, the complete stacks of all the samples are annealed at 550 °C in nitrogen ambient for 10 min. For the investigation of electrical properties in the MIM capacitors,

5735

formed by evaporation of Au top electrodes on top of the SrTiO3 layer after the annealing process, Capacitance–Voltage (C–V) at 100 kHz and Current–Voltage (I–V) measurements were done. The crystallinity of the deposited films was studied by X-Ray diffraction (XRD) using CuKα radiation. XPS with 2 keV Ar+ ion energy and 1 min sputter interval time was used for the investigation of stoichiometry of dielectric films and dielectric/metal interfaces of MIM capacitors by recording the spectrum after each step. Thicknesses of the films were determined by ellipsometry, by cross sectional Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) techniques. 3. Results and discussion The XRD pattern, recorded after the annealing step at 550 °C of the pure SrTiO3 structure (reference) as well as the bilayered Sr2Ta2O7−x (9 nm)/SrTiO3 film on the bottom electrode is shown in Fig. 2. The SrTiO3 films are crystallized in the cubic perovskite phase, while the Sr2Ta2O7−x film remains XRD amorphous. Typical crystallization temperature of strontium tantalate is 700-800 °C, as shown by Regnery et al. [4]. Cross sectional TEM images of the bilayered dielectrics with 3 nm, 6 nm and 9 nm Sr2Ta2O7−x layers are shown in Fig. 3. The amorphous character of the Sr2Ta2O7−x interlayer after the annealing step is visible for each stack. Although the TiN/TaN bottom electrode stack has a high roughness, even the 3 nm thick Sr2Ta2O7−x layer covers the electrode surface uniformly due to the advantage of AVD technique. In addition, there is not any visible interfacial layer between Sr2Ta2O7−x and TaN layer. Electrical measurements were performed on the TaN/Sr2Ta2O7−x/ SrTiO3/Au MIM capacitors. Capacitance variation, measured at 100 kHz can be approximated by the polynomial law:   2 CðVÞ = C0 αV + βV + 1

where α and β are the quadratic and linear coefficients, respectively. C0 represents capacitance at zero bias voltage. The extracted C0 values as a function of thickness are shown in Fig. 4. The single layer SrTiO3 MIM capacitor provides a high capacitance density of 17 fF/μm2. With increasing thickness of the underneath Sr2Ta2O7−x, the capacitance density is reduced to 9 fF/μm2. The dielectric constant of the pure

1,25

104

b)

1,00

SrTiO3 211

Intensity (cps)

1,50

SrTiO3 200

SrTiO3 110

105

SrTiO3 111

1,75

TiN 111

Si 200 TaN 111

106

2,00

Sr/Ti Atomic Ratio

ð1Þ

103

a)

0,75

102

0,50 1

2

3

Sr/Ti Cycle Ratio Fig. 1. Composition of the films (measured by RBS) as a function of SrO/TiO2 cycle ratio. (Film thicknesses of ~ 30 nm deposited on 200 mm Si wafers with native oxide.).

20

30

40

50

60

Two Theta (degree) Fig. 2. XRD patterns of samples after annealing process: a) reference sample (Si/TiN/ TaN/SrTiO3), b) 9 nm Sr2Ta2O7−x deposited sample (Si/TiN/TaN/Sr2Ta2O7−x/SrTiO3).

5736

C.B. Kaynak et al. / Thin Solid Films 519 (2011) 5734–5739

Fig. 3. Cross section TEM images of samples after annealing process: a) reference sample: (TiN/TaN/SrTiO3), b) 3 nm Sr2Ta2O7−x, c) 6 nm Sr2Ta2O7−x, d) 9 nm Sr2Ta2O7−x deposited sample: (TiN/TaN/Sr2Ta2O7−x/SrTiO3).

SrTiO3 film is 95, as shown in Fig. 4. Due to the low k-value of the amorphous Sr2Ta2O7−x film of 20, the effective dielectric constant of the insulating bilayer is reduced to 62 for the 9 nm Sr2Ta2O7−x/50 nm SrTiO3 stack. The total capacitance Ctotal, and consequently the effective k value are defined by the lowest k value of Sr2Ta2O7−x according to the equation for capacitors connected in series: Ctotal =

CSrTiO3 ⋅ CSr2 Ta2 O7−x CSrTiO3 + CSr2 Ta2 O7−x

ð2Þ

where, CSrTiO3 and CSr2Ta2O7 − x are the contributions of SrTiO3 and Sr2Ta2O7−x, respectively. The capacitance variation of Eq. (1) can also 2

Capacitance Density (fF/μm ) Effective dielectric constant (k)

17

100 95

16

90

15

85

14

80

13

75

12

70

11

65

10

60

9

55

Effective Dielectric Constant (k)

Capacitance Density (fF/μm2)

18

50

8 0

3

6

9

Sr2Ta2O7-x Layer Thickness (nm) Fig. 4. Capacitance density (●) and effective dielectric constant (○) vs. thickness of Sr2Ta2O7−x layer in the bilayered Sr2Ta2O7−x/SrTiO3 dielectric MIM capacitors.

be expressed as a function of the electrical field, indicating the following relation of Eq. (3) between α and film thickness d: −2

α∝d

ð3Þ

The inverse quadratic relation of α and d is reported for several dielectrics [24] and in agreement with the non-linear characteristics of pure Sr2Ta2O7−x and SrTiO3 films, as shown in Fig. 5. Due to the amorphous character of Sr2Ta2O7−x films, the C–V curves exhibit parabolic behavior with a positive sign. In contrast to Sr2Ta2O7−x, the polycrystalline SrTiO3 films show a negative parabolic behavior. The difference in sign is caused by the non-isotropic electrostrictive coefficients of polycrystalline SrTiO3 layers [24]. The combination of the dielectric Sr2Ta2O7−x (3, 6, 9 nm) and SrTiO3 (50 nm) leads to moderate α-values in the range of 250 to 300 ppm/V2, as illustrated in Fig. 6. The loss tangent at 100 kHz as a function of the applied voltage is shown in Fig. 7. Pure SrTiO3 based MIM capacitors exhibit a loss tangent of 0.024 at 0 V. By introducing thin amorphous Sr2Ta2O7−x films, tanδ is reduced by a factor of 2. The leakage currents for bottom electrode (TaN) injection are shown in Fig. 8. The leakage current density of the single layer SrTiO3 (50 nm) MIM capacitor is 4*10− 4 A/cm2 at 2 V. This leakage value can be drastically reduced to 8*10− 8 A/cm2 by introducing a 6 nm thick Sr2Ta2O7−x layer underneath the SrTiO3 films. However, the further increase of the Sr2Ta2O7−x thickness does not affect the leakage behavior of the bilayered MIM capacitors. Interfacial reactions which occur at metal/insulator interfaces can affect the electrical performance of MIM capacitors [25,26]. The reactive TaN electrodes can interact with the dielectric, creating defect states within the dielectric, and thus increase the leakage

C.B. Kaynak et al. / Thin Solid Films 519 (2011) 5734–5739

5737

0,030

50 nm SrTiO3 0,025

tanδ

50 nm SrTiO3 / 3 nm Sr2Ta2O7-x 0,020

50 nm SrTiO3 / 6 nm Sr2Ta2O7-x 0,015

50 nm SrTiO3 / 9 nm Sr2Ta2O7-x 0,010 -2

-1

0

1

2

Applied Voltage (V) Fig. 7. Loss tangent of bilayered Sr2Ta2O7−x/SrTiO3 dielectrics as a function of the applied voltage.

Fig. 5. Quadratic voltage linearity coefficient α as a function of pure Sr2Ta2O7−x (●) and SrTiO3 (○) layer thickness.

current density [27]. In order to study the interfacial characteristic of metal/dielectric (TaN/Sr2Ta2O7−x) interface, XPS depth profiles were obtained by performing a low energy Ar+ ion sputtering process. Fig. 9 illustrates the distribution of elements (only strontium and nitrogen are plotted for simplicity) across the particular layers of interest (TaN/Sr2Ta2O7−x/SrTiO3) for the investigation of dielectric/metal interface. The cross-over points of strontium and nitrogen profiles have been utilized as a representative point for the TaN/Sr2Ta2O7−x interface [28] and shown detailed in the inset figure of Fig. 9. It is visible that as the thickness of Sr2Ta2O7−x increases, the sputtering time to reach to the interface increases (141.min for 3 nm Sr4Ta2O9; 148.min for 6 nm Sr2Ta2O7−x; 152.min for 9 nm Sr2Ta2O7−x). The sputtering rate was determined

using SrTiO3 films of known thickness and found to be about 0.4 nm/ min. Fig. 10 demonstrates the photoelectron spectra (Ta4f, N1s, Sr3d, O1s) taken from each sample at the interface region between TaN/Sr2Ta2O7−x as it is indicated in the depth profile (Fig. 9) by the vertical lines. The spectra of each element obviously declare that there is no visible change in the binding energies at the interface between TaN/Sr2Ta2O7−x for different thicknesses of Sr2Ta2O7−x which indicates that dielectric/metal interfaces are identical.

4. Conclusions Sr2Ta2O7−x/SrTiO3 based MIM capacitors have been investigated for the first time and the electrical performance of the devices have been optimized by varying the thickness of Sr2 Ta 2O 7−x. The optimized bilayered system constructed by 6 nm Sr2Ta2O7−x and 50 nm SrTiO3 provides a leakage current density of 8 10− 8 A/cm2 at 2 V (bottom electrode injection) and a high capacitance density of 13 fF/μm2 in combination with a small quadratic voltage linearity coefficient of 300 ppm/V2.

400

10-3

reference sample 3 nm Sr2Ta2O7-x

Leakage Current (A/cm2)

300

2

alpha (ppm/V )

10-4

200

6 nm Sr2Ta2O7-x 9 nm Sr2Ta2O7-x

10-5

10-6

10-7

10-8

100 3

6

9

Sr2Ta2O7-x thickness (nm) Fig. 6. Quadratic voltage linearity coefficient α of bilayered Sr2Ta2O7−x/SrTiO3 dielectrics as a function of the Sr2Ta2O7−x film thickness.

Bottom Injection

10-9 0

1

2

3

4

Electric Field (MV/cm) Fig. 8. Leakage current density vs. electric field of bilayered Sr2Ta2O7−x/SrTiO3 dielectric MIM capacitors with varied Sr2Ta2O7−x film thickness.

5738

C.B. Kaynak et al. / Thin Solid Films 519 (2011) 5734–5739

Sr2Ta2O7-x

SrTiO3 148.

141.

152.min

TaN 148. 152.min

Intensity (c/s)

Intensity (c/s)

141.

138

140

142

144

146

148

150

152

154

Sputter ring Time (min.)

Sr_3nm Sr2Ta2O7-x

Sr_6nm Sr2Ta2O7-x Sr_9nm Sr2Ta2O7-x N_3nm Sr2Ta2O7-x N_6nm Sr2Ta2O7-x N_9nm Sr2Ta2O7-x

80

100

120

140

160

Sputterring Time (min.) Fig. 9. XPS depth profiles for strontium and nitrogen (inset figure: zoom in version for the cross-over points of strontium and nitrogen profile).

Sr3d

N1s

Ta4p3/2

Ta4f

3nm_141. min 6nm_148. min 9nm_152. min

Intensity (arb. units)

O1s

130

132

134

136

138

526

528

530

532

534

536

394 396 398 400 402 404 406 408 16 18 20 22 24 26 28 30 32 34 36 38 40

Binding Energy (eV) Fig. 10. XPS spectra (Sr3d, Ta4f, O1s, N1s) for each sample obtained from their corresponding cross sections as representative of Sr2Ta2O7−x/TaN interface (141 min for 3 nm, 148 min for 6 nm, 152 min for 9 nm Sr2Ta2O7−x deposited bilayered Sr2Ta2O7−x/SrTiO3 dielectric MIM capacitors).

Acknowledgements This work was supported by the grant from German BMBF (grant No. 13N9926). References [1] W.-J. Lee, I.-K. You, S.-O. Ryu, B.-G. Yu, K.-I. Cho, S.-G. Yoon, C.-S. Lee, Jpn. J. Appl. Phys. 40 (2001) 6941–6944. [2] K. Yoshioka, V. Petrykin, M. Kakihana, H. Kato, A. Kudo, J. Catal. 232 (2005) 102–107. [3] S. Kamba, J. Petzelt, E. Buixaderas, D. Haubrich, P. Vanek, P. Kuzel, I.N. Jawahar, M.T. Sebastian, P. Mohanan, J. Appl. Phys. 89 (2001) 3900–3906. [4] S. Regnery, R. Thomas, P. Ehrhart, R. Waser, J. Appl. Phys. 97 (2005) 073521. [5] M. Silinskas, M. Lisker, B. Kalkofen, E.P. Burte, Mat. Sci. Semicond. Proc. 9 (2006) 1102–1107. [6] W.-C. Shin, S.-O. Ryu, I.-K. You, B.-G. Yu, W.-J. Lee, K.-J. Choi, S.-G. Yoon, J. Electrochem. Soc. 151 (2004) C292–C296.

[7] M. Vehkamäki, M. Ritala, M. Leskelä, A.C. Jones, H.O. Davies, T. Sajavaara, E. Rauhala, J. Electrochem. Soc. 151 (2004) F69–F72. [8] W.-J. Lee, W.-C. Shin, B.-G. Chae, S.-O. Ryu, I.-K. You, S.-M. Cho, B.-G. Yu, B.-C. Shin, Integr. Ferroelectr. 46 (2002) 275–284. [9] E. Tokumitsu, G. Fujii, H. Ishiwara, Appl. Phys. Lett. 75 (1999) 575–577. [10] M.A. Rodriguez, T.J. Boyle, B.A. Hernandez, D.R. Taland, K. Vanheusden, J. Am. Ceram. Soc. 82 (1999) 2101–2105. [11] M. Lukosius, Ch. Wenger, S. Pasko, I. Costina, J. Dabrowski, R. Sorge, H.-J. Müssig, Ch. Lohe, IEEE Trans. Electron Devices 55 (2008) 2273–2276. [12] V.Y. Kunin, Y. Tarnopolskii, N.A. Shturbina, Russ. Phys. Journ. 32 (1989) 556–559. [13] L. Goux, H. Vander Meeren, D.J. Wouters, J. Electrochem. Soc. 153 (2006) F132–F136. [14] A. Hushur, G. Shabbir, J.-H. Ko, S. Kojima, J. Phys. D Appl. Phys. 37 (2004) 1127–1131. [15] K.C. Chiang, J.W. Lin, H.C. Pan, C.N. Hsiao, W.J. Chen, H.L. Kao, I.J. Hsieh, A. Chin, J. Electrochem. Soc. 154 (2007) H214–H216. [16] S. Piskunov, E. Heifetsb, R.I. Eglitisa, G. Borstela, Comput. Mater. Sci. 29 (2004) 165–178. [17] K.-Y. Chen, L.-L. Lee, D.-S. Tsai, J. Mater. Sci. Lett. 10 (1991) 1000–1002. [18] P.C. Joshi, S.B. Krupanidhi, Appl. Phys. Lett. 61 (3) (1992) 1525–1527.

C.B. Kaynak et al. / Thin Solid Films 519 (2011) 5734–5739 [19] W.A. Feil, B.W. Wessels, L.M. Tonge, T.J. Marks, J. Appl. Phys. 67 (8) (1990) 3858–3861. [20] H. Holzschuh, H. Suhr, Adv. Mater. 4 (5) (1992) 357–359. [21] M.E. Pilleux, C.R. Grahman, V.M. Fuenzalida, J. Am. Ceram. Soc. 77 (1994) 1601–1604. [22] J.V. Mantese, A.L. Micheli, A.H. Hamdi, R.W. West, MRS Bull. (1989) 48–538 October. [23] G. Braunstein, G.R. Paz-Pujalt, Thin Solid Films 216 (1992) 1–3. [24] Ch. Wenger, G. Lupina, M. Lukosius, O. Seifarth, H.-J. Müssig, S. Pasko, Ch. Lohe, J. Appl. Phys. 103 (2008) 104103.

5739

[25] N. Gaillard, L. Pinzelli, M. Gros-Jean, A. Bsiesy, Appl. Phys. Lett. 89 (2006) 133506. [26] J.P. Chang, R.L. Opila, G.B. Alers, M.L. Stelgerwald, H.C. Lu, E. Garfunkel, T. Gustafsson, Solid State Technol. 42 (1999) 43–48. [27] T. Remmel, R. Ramprasad, J. Walls, IEEE International, Dallas, Texas, March 30– April 4, 2003, pp. 277–281. [28] G. Sjöblom, J. Westlinder, J. Olsson, IEEE Trans. Electron Devices 52 (2005) 2349–2352.