ALD grown NbTaOx based MIM capacitors

Microelectronic Engineering 88 (2011) 2447–2451

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ALD grown NbTaOx based MIM capacitors T. Blomberg a,⇑, Ch. Wenger b, C. Baristiran Kaynak b, G. Ruhl c, P. Baumann d a

ASM Microchemistry Ltd., Väinö Auerin katu 12A, Helsinki 00560, Finland IHP, Im Technologiepark 25, Frankfurt 15236, Germany c Infineon Technologies AG, Wernerwerkstr. 2, Regensburg 93049, Germany d AIXTRON AG, Kaiserstr. 98, Herzogenrath 52134, Germany b

a r t i c l e

i n f o

Article history: Available online 22 January 2011 Keywords: MIM capacitor High-k dielectric NbTaOx

a b s t r a c t NbTaOx mixed oxides were deposited by ALD in ASM PulsarÒ 2000 R&D reactor using TaF5 and NbF5 as metal precursors. Two deposition processes were evaluated, one with only H2O as the oxidizer, and the other with a combination of H2O and O3. The depositions where done at 225 °C on Si (200 mm, p-type (1 0 0) with native oxide), Ru(AVD), TiN(ALD) and TaN(PVD) bottom electrodes. After the dielectric depositions, N2 PDA’s were applied on the samples and finally Au dot (e-beam evaporation with shadow mask) top electrodes were deposited for electrical characterization. Best electrical results (bottom electrode injection) achieved for the different stacks with 45 nm thick NbTaOx films were: Ru(AVD) k = 17, 9  10 11 A/cm2 @ 3 V, TiN(ALD) k = 39, 2  10 2 A/cm2 @ 3 V, TaN(PVD) k = 44, 7  10 4 A/cm2 @ 3 V. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Further shrinkage of microelectronic devices calls for novel high-k and metallic materials to meet the future device requirements. The deeper 3D features with high aspect ratios in the chips also put great demands on the materials depositions techniques and thus require conformal deposition processes, such as ALD. Today Al2O3, ZrO2 and HfO2 based high-k materials have received the main attention. Nb2O5 and Ta2O5 can be considered as alternatives due to their possibly higher k-values in the crystalline phase. As binary oxides they are already used as dielectric materials in RF and decoupling capacitors. Typically, these materials are deposited with wet chemical, CVD or PVD techniques. In this paper we will present the results from ALD of Nb2O5, Ta2O5 and their mixtures and will also show how the electrical properties of NbTaOx vary with the Nb/Ta ratio and type of bottom electrode used. 2. Experimental The NbTaOx films were deposited at 225 °C by the atomic layer deposition (ALD) method in an ASM PulsarÒ 2000 R&D reactor at ASM Microchemistry Ltd. TaF5 and NbF5 were used as metal precursors and H2O or O3 as the oxidizers. Two hundred and twenty-five degree Celsius was chosen as the deposition temperature, because at higher temperatures NbF5 started to etch the growing film leading to no growth or nonuniform thickness or composition of the films. Self etching has also been reported earlier in ALD processes ⇑ Corresponding author. E-mail address: [email protected] (T. Blomberg). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.050

using niobium and tantalum halides [1,2]. Two deposition processes were evaluated, one with only H2O as the oxidizer (Process 1), and the other with a combination of H2O and O3 (Process 2). Film compositions were controlled by changing the pulsing ratio of the Nb2O5 and Ta2O5 subcycles in the pulsing sequence. Film thicknesses were controlled by changing the number of supercycles in the deposition process. Compositions of the mixed oxides studied ranged from 6:1 to 1:3 Nb2O5/Ta2O5 pulsing ratios. The substrates used for the depositions were blanket Si wafers (200 mm, p-type (1 0 0) with native oxide) for the process behaviour evaluation and blanket Si wafers (200 mm) with atomic vapor deposited Ru(AVD), atomic layer deposited TiN(ALD) or physical vapor deposited TaN(PVD) for analytical and electrical evaluation. Additionally, conformality of the depositions process was evaluated with 150 mm patterned wafer (placed on a 200 mm pocket wafer during the deposition) with 5/1 lm (depth/diameter) pore structures. After the dielectric depositions the films were annealed in nitrogen at 600 °C for 5 min in order to crystallize the layers. The films were analyzed by spectroscopic ellipsometry, SEM, RBS, TOFERDA and XRD. For the electrical evaluation, Au dots were deposited on the films with e-beam evaporation and shadow mask. Then the electrical evaluation of the formed capacitors was done by measuring the C–V at 100 kHz and I–V curves. 3. Results 3.1. Process behaviour Process 2 was tested with pure Nb2O5, pure Ta2O5 and with 1:3, 1:1 and 6:1 Nb2O5/Ta2O5 pulsing ratio mixed oxides. In all cases

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the thicknesses and refractive index values determined with spectroscopic ellipsometry were identical with Process 1, suggesting that H2O functioned as the ligand removal agent and the effect of O3 on the process behaviour was not detectable. Therefore, only Process 1 behaviour was studied in more detail. Fig. 1 shows the growth rate curves for the NbTaOx films grown as a function of subcycles. All the compositions studied showed linear relations with the deposition subcycles. Furthermore, the growth rates of all the compositions could be quite accurately estimated as the arithmetic weighted average of the ALD growth rates of the binary oxides, 1.33 Å/cycle for Nb2O5 and 0.96 Å/cycle for Ta2O5 as shown in Fig. 2.

Compositions of the films could also be estimated with the same methodology. This suggests that ALD Nb2O5 grows on Ta2O5 in the same way as on Nb2O5 and vice versa. Conformality of the deposition process was found to be 100% in 5/1 lm (depth/diameter) pore structures as shown in Fig. 3. 3.2. Analytical characterization of the films All the studied films, thicknesses 45 nm, compositions ranging from 10% to 70% Ta/(Ta + Nb), were amorphous on Si and crystalline on Ru(AVD), TiN(ALD) and TaN(PVD). Fig. 4 presents the XRD patterns of some of the films. On Ru(AVD) TiN(ALD) and TaN(PVD)

45 1:3 Nb/Ta 1:1 Nb/Ta 3:1 Nb/Ta 6:1 Nb/Ta Linear (1:3 Nb/Ta) Linear (1:1 Nb/Ta) Linear (3:1 Nb/Ta) Linear (6:1 Nb/Ta)

35

Thickness, nm

y = 0.129x + 0.0333

30 25 20

40

y = 0.1215x + 0.2067 y = 0.106x + 0.5833 y = 0.0962x - 0.0867

15

Measured Th, nm

40

35

y = 0.9994x - 0.4834 2 R = 0.9899

30 25 20 15 10

10

5 5

0

0

0 0

100

200

300

400

Subcycles Fig. 1. Growth rate curves for NbTaOx films with varying cycle ratios.

10

20

30

40

50

Calculated Th, nm Fig. 2. Correlation of the measured thicknesses (ellipsometry) of the NbTaOx films (six different compositions) with the calculated ones.

Fig. 3. SEM images of the NbTaOx film (1:1 Nb/Ta pulsing ratio) deposited in 5/1 lm pore structure.

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Fig. 4. XRD patterns of the NbTaOx films on different bottom electrodes after 5 min, 600 °C anneals. Standard = only H2O as oxidizer, Ozone = H2O + O3 combination process. (a) On Si (with native oxide), Nb2O5/Ta2O5 pulsing ratio = 1:1. (b) On TiN(ALD), Nb2O5/Ta2O5 pulsing ratio = 1:1. (c) On TaN(PVD), Nb2O5/Ta2O5 pulsing ratios = 1:1 and 1:3, the TiN peak is from the buffer layer under the TaN(PVD) layer. (d) On Ru(AVD), Nb2O5/Ta2O5 pulsing ratio = 1:1.

Fig. 5. Leakage current density at 1 V (TaN electrode injection) of NbTaOx films on TaN(PVD), annealed at 600 °C in N2 as function of Ta content. NbTaOx films processed with O3 and H2O (closed circles) provides smaller leakage values then pure H2O processed NbTaOx films (open circles).

the films crystallized in the orthorhombic (Nb,Ta)2O5 phase. On Ru(AVD), (0 0 1) peak was pronounced and on TiN(ALD), (2 0 0) peak was the predominant one indicating differences in either orientations of the films, or lattice defects in the dielectric [3]. The contamination (F, C, H) levels of the films (three samples on bare Si wafer with the H2O process, compositions 10%, 40% and 70% Ta/(Ta + Nb)) were low, F <0.3 at.%, C <0.1 at.% and H <0.3 at.%, all below the detection limit of TOF-ERDA method used for the analysis. For Process 2 we did not measure directly, but contamination wise it should result in similar levels or even purer films. However,

Fig. 6. Dielectric constants of NbTaOx films on TaN(PVD), annealed at 600 °C in N2 as function of Ta content. NbTaOx films processed with O3 and H2O (closed circles) provides slightly higher k-values than pure H2O processed NbTaOx films (open circles).

because already Process 1 led to contamination levels lower than the detection limit, it would have been impossible to see any difference between the processes in the physical characterization. 3.3. Electrical characterization of the films k-Values and leakage current densities of the Au/NbTaOx/ TaN(PVD) capacitors are shown in Figs. 5 and 6. Increasing Nb content of the films increased the leakage current densities of the capacitors. The trends are somewhat different than reported earlier for NbTaOx based capacitor with different precursors or electrode

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Fig. 7. IV and CV curves at 100 kHz of the Au/NbTaOx/BE capacitors with 45 nm thick ALD NbTaOx films, 1:1 Nb2O5/Ta2O5 pulsing ratio, grown with Process 1 (H2O as the oxidizer).

Fig. 8. IV and CV curves at 100 kHz of the Au/NbTaOx/BE capacitors with 45 nm thick ALD NbTaOx films, 1:1 Nb2O5/Ta2O5 pulsing ratio, grown with Process 2 (H2O + O3 as the oxidizers).

materials [4,5]. H2O + O3 combination process led to lower leakage current densities compared to the H2O process only, presumably because of lower oxygen vacancy levels in the dielectric films. The CV and I–V curves of the 1:1 pulsing ratio films on different bottom electrodes are presented in Figs. 7 and 8. Calculated k-val-

ues of the stacks as well as leakage current densities show high variability depending on the bottom electrode used. The physical explanation to the differences in the electrical behaviour may lie in the different orientations these films adapted on the different bottom electrodes as shown in Fig. 4. Table 1 summarizes the com-

T. Blomberg et al. / Microelectronic Engineering 88 (2011) 2447–2451 Table 1 Leakage current densities and k-values of the Au/NbTaOx/BE with 45 nm thick ALD NbTaOx films, 1:1 Nb2O5/Ta2O5 pulsing ratio, grown with Process 1 (H2O as the oxidizer) or Process 2 (H2O + O3 as the oxidizers). Negative voltage = Au injection, positive voltage = bottom electrode injection. Bottom electrode

NbTaOx process

Ru(AVD) Ru(AVD) TiN(ALD)

k-Value

Process 1 15 Process 2 17 Process 1 Too high leakage TiN(ALD) Process 2 39 TaN(PVD) Process 1 40 TaN(PVD) Process 2 44

j @ 1V (A/cm2) 8  10 6  10 1  10

7

6  10 1  10 1  10

6

11 1

6 6

j @ +1 V (A/cm2) 1  10 4  10 1  10

7

9  10 5  10 1  10

5

11 1

6 6

j @ 3 V j @ +3 V (A/cm2) (A/cm2) 3  10 1  10 2  10

5

2  10 1  10 1  10

5

9 1

5 5

6  10 9  10 3  10

5

2  10 1  10 7  10

2

11 1

2 4

2451

[6]. Furthermore, inside a given crystallographic phase a highly anisotropic electrical behaviour is possible, which makes the NbTaOx a challenging material to be integrated in microelectronics. On TaN(PVD), 2:1 and 1:1 Ta/Nb cycle ratio films (60 at.% and 45 at.% Ta/(Ta + Nb)) showed the most promising combinations of k-values and leakage current densities. On TiN(ALD) bottom electrodes, the k-values of the films were comparable to the k-values measured on TaN(PVD) bottom electrodes, but the leakage current densities were higher. In summary, low enough leakage (<10 7 A/ cm2) for chip capacitor applications could only be achieved with Ru bottom electrodes, but these films showed also low k-values. None of the films gave a combination of high k-value and low leakage current density. Acknowledgments

parison of the effect of the different bottom electrodes on the kvalues and leakage current densities. Ru bottom electrodes resulted in lowest leakages and lowest k-values, TaN bottom electrodes resulted is highest k-values and second best leakage performance, whereas TiN bottom electrodes lead to k-values close to TaN, but the worst leakage behaviour. Using H2O + O3 combination process lowered the leakage current densities of the films on all bottom electrodes. Especially on Ru the improvement was substantial. For the k-values, the addition of O3 to the process seemed to have only a minor effect. 4. Conclusions Electrical characteristics of the ALD grown NbTaOx films were highly variable. k-Values ranged from 15 to 45 and leakage current densities from 10 11 to 10 2 A/cm2 at 3 V. The high variation can be explained by the large number of crystalline phases and orientations these films can adapt on different bottom electrodes. These phases are known to have highly varying electrical characteristics

This work has been supported by the MaxCaps Project within the European MEDEA+ Research Program. Dr. Wim Besling from NXP is thanked for providing the 3D structured wafers. Dr. Timo Sajavaara from University of Jyväskylä is thanked for the TOF-ERDA and RBS analyses. Andy Zauner from Air Liquide and Simon Rushworth from SAFC are gratefully thanked for providing the precursor chemicals. References [1] J. Aarik, K. Kukli, A. Aidla, L. Pung, Appl. Surf. Sci. 103 (1996) 331–341. [2] K. Knapas, A. Rahtu, M. Ritala, Chem. Vap. Deposition 15 (2009) 269–273. [3] A. Ramadan, A. Abd El-Mongy, A. El-Shabiny, A. Mater, S. Mostafa, E. ElSheheedy, H. Hashem, Cryst. Res. Technol. 44 (2009) 111–116. [4] K. Kukli, M. Ritala, M. Leskelä, J. Appl. Phys. 86 (1999) 5656–5662. [5] M. Strømme1, G.A. Niklasson, M. Ritala, M. Leskelä, K. Kukli, J. Appl. Phys. 90 (2001) 4532–4542. [6] S. Clima, G. Pourtois, A. Hardy, S. Van Elshocht, M.K. Van Bael, S. De Gendt, D.J. Wouters, J.A. Kittl, J. Electrochem. Soc. 157 (2010) G20–G25.