Al2O3 based MIM capacitors: Impact of the bottom electrode material

Microelectronic Engineering 88 (2011) 1521–1524

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Single SrTiO3 and Al2O3/SrTiO3/Al2O3 based MIM capacitors: Impact of the bottom electrode material C. Baristiran Kaynak a,⇑, M. Lukosius a, B. Tillack a,b, Ch. Wenger a, T. Blomberg c, G. Ruhl d a

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

a r t i c l e

i n f o

Article history: Available online 3 April 2011 Keywords: MIM High-k Multilayer dielectric

a b s t r a c t Metal–Insulator–Metal (MIM) capacitors with atomic layer deposited (ALD) single SrTiO3 dielectric and Al2O3/SrTiO3/Al2O3 multilayer dielectric have been deposited on TaN and TiN bottom electrodes. The MIM stacks have been analyzed and compared in terms of electrical and structural properties. The results indicate that MIMs with multilayer dielectrics provide better leakage current performance than the ones with single dielectrics while capacitance density is decreased. Additional Al2O3 layers prevented the crystallization of SrTiO3 in the multilayer dielectric stack. The decreased capacitance density in MIMs with multilayer dielectric is attributed to the amorphous structure of SrTiO3 and the series capacitance of top and bottom Al2O3 layers. Furthermore, MIM capacitors with single SrTiO3 dielectric layer on TiN electrodes indicated better capacitance density compared to the one with TaN electrodes. The lower capacitance density of the single SrTiO3 dielectric on TaN electrodes is correlated to the interfacial layer formation between SrTiO3 and TaN electrodes. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction According to the international technology roadmap for semiconductors (ITRS), the main design requirements for Metal–Insulator–Metal (MIM) capacitors are a higher capacitance density accompanied with lower leakage current [1]. Several high k dielectric materials have been investigated and analyzed in terms of the required parameters for MIM applications, but to date no single dielectric material has been identified with the required properties from the practical use point of view [2]. As an alternative approach, stacked dielectrics are receiving an increasing attention for MIM applications [3–5]. Among several dielectric materials, SrTiO3 is one of the most promising insulators mainly due to its high dielectric constant of 150. However, the high k value of SrTiO3 is achieved, when the SrTiO3 layer is crystalline [6]. It is well known that grain boundaries in dielectric layer create channels for electrons and MIMs with crystalline dielectric suffer from high leakage current densities [7]. The challenge of practical use of SrTiO3 in MIM capacitor application could be solved by the use of stacked dielectric structures. As Al2O3 has an amorphous matrix at the crystallization temperature of SrTiO3 (550 °C) and exhibits a rather large band gap

⇑ Corresponding author. Tel.: +49 335 5625 580; fax: +49 335 5625 327. E-mail address: [email protected] (C.B. Kaynak). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.022

value of 8.7 eV [8], the stack dielectric constituted of SrTiO3 and Al2O3 is a good candidate for MIM applications. In this work, SrTiO3 dielectric layers with thickness of 50 nm were deposited between thin amorphous Al2O3 layers in order to enhance the leakage current performance of MIM capacitor. Single SrTiO3 dielectric MIM capacitors have also been prepared and compared with stack dielectric MIM in terms of electrical performances and structural properties. Furthermore, the impact of the bottom electrode materials TiN and TaN have been investigated in single and stacked dielectrics MIM capacitors.

2. Experimental TaN and TiN have been deposited on 8 in. silicon wafers by physical vapor deposition (PVD) and chemical vapor deposition (CVD) technique, respectively. SrTiO3 and Al2O3 layers were deposited by atomic layer deposition (ALD) technique using ALD PulsarÒ 2000 type R&D reactor. Sr(tBu3Cp)2, Ti(OMe)4 and O3 were used for the deposition of SrTiO3 layers with thickness of 50 nm. The pulsing ratio of 2 was used for Sr/Ti, which produced Sr-rich films with 60 at.% Sr/(Sr + Ti), determined by Rutherford backscattering spectroscopy (RBS). Al(CH3)3 and O3 precursors were used for the Al2O3 deposition process. The targeted thickness of the Al2O3 was 4 nm. The reactor temperature was kept at 250 °C during all ALD depositions. The single SrTiO3 capacitors as well as the Al2O3/

C.B. Kaynak et al. / Microelectronic Engineering 88 (2011) 1521–1524

Au

Au

SrTiO3

SrTiO3

TiN

TaN

(I)

(II)

Au

Au

Al2O3

Al2O3

SrTiO3

SrTiO3

Al2O3

Al2O3

TiN

TaN

(III)

(IV)

40 20

-2

10

-3

10

-4

10

-5

10

-6

10

-7

10

-8

10

-9

O1s_III Al2p_III N1s_III

80

Au Al2O3 SrTiO3 Al2O3

60 40

TiN (III)

50

55

60

65

70

75

80

85

90

95

100

Sputter Time (min) Fig. 4. XPS depth profiling of the interface region between SrTiO3/Al2O3/TaN or TiN for MIM stacks No. III and IV (just O1s, Al2p and N1s profiles).

3. Results The leakage current densities of the MIM devices as well as the capacitance density measurements with the extracted k values of the corresponding dielectrics are represented in Fig. 2a and b, respectively. It was observed that MIM structures with stacked dielectrics (III and IV) exhibit lower leakage current densities than the ones with single layer dielectric (I and II). However the evaluated k-values of the samples (III and IV) are also reduced as seen in 20

I II

kI=60

18

2

Capacitance Density (fF/um )

2

Leakage Current (A/cm )

10

TaN (IV)

0 100

20

SrTiO3/Al2O3 multi stack structures were deposited in a single process step. After the deposition process, the structures were annealed at 600 °C in N2 ambient for 5 min. For electrical characterization, Au dots with the area of 1.2  10 3 cm2 were deposited as top electrodes by thermal evaporation. The cross-sectional schematic diagrams of the realized MIM capacitors are shown in Fig. 1. Capacitance–voltage (C–V) measurements were performed in serial mode using two top Au dot electrodes at 100 kHz. X-ray diffraction (XRD) measurements were performed to characterize the morphology of the layers. X-ray photoelectron spectroscopy (XPS) measurements were done by using a monochromatic Al Ka radiation at the angle of 45°. Ar+ with ion energy of 2 keV was used for the XPS depth profiling. The cross section transmission electron microscopy (TEM) technique was used to analyze the interfaces of MIM layers. -1

Au Al2O3 SrTiO3 Al2O3

60

0

TiN / TaN

Al2O3

O1s_IV Al2p_IV N1s_IV

80

Fig. 1. The cross-section schematic diagrams of the MIM configurations.

10

SrTiO3

100

Atomic Concentration (%)

1522

III IV

-10

10

bottom electrode injection

-11

10

0

1

(a)

16 14

kII=42

12 kIII=11

2

II IV

-3

3

(b)

III

0

2

kIV=10

I

1

-2

-1

Voltage (V)

0

1

2

3

Voltage (V)

Fig. 2. (a) Leakage current densities versus voltage, (b) capacitance densities versus voltage and extracted k values of the corresponding dielectrics.

(b)

TaN

on stack III

on stack I

TaN

Intensity (a. u.)

Intensity (a. u.)

(a)

on stack IV on stack II

powder SrTiO3 data 20

25

30

35

40

45

2 theta (degree)

50

55

60

powder SrTiO3 data 20

25

30

35

40

45

50

55

60

2 theta (degree)

Fig. 3. XRD patterns of MIM stack structures (a) No. I and III, (b) No. II and IV. The simulated spectra of powder SrTiO3 are indicated on the bottom of the spectra.

C.B. Kaynak et al. / Microelectronic Engineering 88 (2011) 1521–1524

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Fig. 5. Cross section TEM images on MIM stack No. III (a) and IV (b).

Ta4f interfacial layer

bottom electrode

90. min

Intensity (a.u)

80. min 70. min 60. min 50. min 40. min 30. min 20. min 10. min 0. min

40

36

32

28

24

20

16

Binding Energy (eV) Fig. 6. XPS spectra in Ta4f transition region of MIM stack No. II.

Fig. 2b. The lower k-values can be attributed to the series capacitances of additional Al2O3 layers. The additional layers on the top and the bottom of SrTiO3 films increase the effective dielectric thickness and cause the lower effective k-value due to the low dielectric constant (8) of Al2O3. In addition the crystallization of SrTiO3 at 600 °C is suppressed when SrTiO3 is deposited between Al2O3 layers as indicated by XRD measurements, shown in Fig. 3a and b. Single SrTiO3 layers on TiN and TaN are crystallized independent from the bottom electrode material (see peak at 46.5°), while the SrTiO3 layers in between thin Al2O3 films remain amorphous. The amorphous structure of SrTiO3 reduces the effective k-values of the stacked structures as well as the leakage current densities. It has been reported that additional Al2O3 decreases the leakage due to its amorphous matrix and its high band gap value of 8.7 eV [8,9]. The large conduction band offset of Al2O3 is attributed for the suppression of leakage currents [9]. Moreover, the better leakage performance in the case of structure No. IV compared to No. III is associated with the presence of the Al2O3 layer between SrTiO3 and TaN bottom electrode. Fig. 4 illustrates the XPS depth profiles of the structures No. III and IV. For simplicity, just the O1s, Al2p and N1s profiles are considered. The XPS depth profiles correspond exactly to the interface regions of samples No. III and IV between SrTiO3/Al2O3/TaN or TiN. The Al2p profile of structure No. IV shows the distinctive Al2O3 layer between SrTiO3 and TaN layers. The Al2p profile of sample No. III

Fig. 7. Cross section TEM images on MIM stack No. I (a) and II (b).

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seemed to be diffused into TiN electrode. In addition, TEM images obtained on sample No. III and IV, as shown in Fig. 5, confirm the bottom Al2O3 layer between SrTiO3 and bottom electrode of sample No. IV. Although the single layer SrTiO3 dielectrics deposited with the same stoichiometry on TaN and TiN electrode are both crystalline (as depicted by XRD), the effective k-value is slightly different (60 and 42). The relatively low k-values of the crystalline SrTiO3 dielectrics are caused by the chosen Sr/Ti ratio of 1.5 [10]. In order to find the origin of the reduced k-value of the single SrTiO3 dielectric on TaN bottom electrode (No. II) compared to sample No. I, the interface analysis was done by sputter-XPS. Fig. 6 shows XPS spectra of Ta4f transition (sample No. II) after periodical Ar+ sputtering processes. After 30 min. sputtering, the Ta4f splitting peaks starts to appear at 26.3–28.2 eV which corresponds to oxidized tantalum ions at the interface [11]. The peaks observed at 22.4–24 eV are corresponding to the tantalum nitride bottom electrode [11]. Cross section TEM images (shown in Fig. 7) obtained from SrTiO3/TaN and SrTiO3/TiN stacks confirm an interfacial layer with thickness of about 10 nm between SrTiO3 and TaN, while there is not interface formation between SrTiO3 and TiN. The interfacial layer of sample No. II is attributed to the origin of the reduced k-value [12]. 4. Conclusion The MIM capacitors with single layer SrTiO3 and stacked Al2O3/ SrTiO3/Al2O3 dielectrics have been investigated on different electrode materials (TiN and TaN). The better leakage performances of the samples with multilayer dielectrics are attributed to the additional Al2O3 layers which lead to an increase in barrier heights. However, SrTiO3 layers deposited between thin Al2O3 films are not

crystalline after annealing at 600 °C, resulting in low k-values of the MIMs with stacked dielectrics. Due to the different interfacial reactivity, the electrode material influences the electrical performances of MIM capacitors, too. Acknowledgements This work was supported by the grant from German BMBF (Grant No. 13N9926). References [1] RF and Analog/Mixed-Signal Technologies for Wireless Communications, International Roadmap for Semiconductors (Semiconductor Industry Association, Palo Alto, 2009 update). [2] S.Y. Lee, H. Kim, P.C. McIntyre, K.C. Saraswat, J.S. Byun, Appl. Phys. Lett. 82 (2003) 2874. [3] S.J. Ding, H. Hu, H.F. Lim, S.J. Kim, F. Yu, C. Zhu, B.J. Cho, D.S.H. Chan, S.C. Rustagi, M.B. Yu, A. Chin, D.L. Kwong, IEEE Electron. Dev. Lett. 24 (2003) 730. [4] T. Ishikawa, D. Kodama, Y. Matsui, M. Hiratani, T. Furusawa, D. Hisamoto, IEDM (2002) 940. [5] S.H. Lin, K.C. Chiang, Albert Chin, F.S. Yeh, IEEE Electron. Dev. Lett. 30 (2009) 715. [6] K.C. Chiang, C.C. Huang, G.L. Chen, W.J. Chen, H.L. Kao, Y.H. Wu, A. Chin, S.P. McAlister, IEEE Trans. Electron. Dev. 53 (2006) 2312. [7] S.K. Kim, S.W. Lee, J.H. Han, B. Lee, S. Han, C.S. Hwang, Adv. Funct. Mater. 20 (2010) 2989. [8] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [9] Y.H. Wu, C.K. Kao, B.Y. Chen, Y.S. Lin, M.Y. Li, H.C. Wu, Appl. Phys. Lett. 93 (2008) 033511. [10] M. Popovici, S. Van Elshocht, N. Menou, J. Swerts, D. Pierreux, A. Delabie, B. Brijs, T. Conard, K. Opsomer, J.W. Maes, D.J. Wouters, J.A. Kittl, J. Electrochem. Soc. 157 (2010) G1–G6. [11] K. Kato, H. Toyota, Y. Jin, T. Ono, Vacuum 83 (2009) 592. [12] Ch. Wenger, M. Lukosius, H.J. Müssig, G. Ruhl, S. Pasko, Ch. Lohe, J. Vac. Sci. Technol. B 27 (2009) 286.