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Improved EOT and leakage current for metal–insulator–metal capacitor stacks with rutile TiO2
Microelectronic Engineering 88 (2011) 1517–1520
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Improved EOT and leakage current for metal–insulator–metal capacitor stacks with rutile TiO2 Mihaela Popovici ⇑, Min-Soo Kim, Kazuyuki Tomida, Johan Swerts, Hilde Tielens, Alain Moussa, Olivier Richard, Hugo Bender, Alexis Franquet, Thierry Conard, Laith Altimime, Sven Van Elshocht, Jorge A. Kittl Imec, 75 Kapeldreef, 3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Available online 6 April 2011 Keywords: Atomic layer deposition High-k dielectrics Rutile TiO2
a b s t r a c t Downscaling of the metal–insulator–metal capacitor (MIMCAP) for Dynamic Random Access Memory (DRAM) requires the introduction of high permittivity dielectrics. MIMCAP structures formed with RuO2/Ru as bottom electrode, rutile TiO2 as dielectric and TiN as top electrode are described. Ozone (O3) is needed as oxidant in the TiO2 atomic layer deposition (ALD) process in order to obtain the rutile phase (permittivity > 80), while anatase TiO2 (permittivity 40) is obtained with H2O. As O3 etches the ruthenium substrate, an ultra-thin interlayer of TiO2 was first grown with H2O, followed by the thick TiO2 layer deposited with O3. In order to minimize the content of anatase in the TiO2 layer, responsible for a reduced dielectric constant, we investigate the effect of scaling down the thickness of the protective H2O based inter-layer on the equivalent oxide thickness (EOT) and leakage current density (Jg). The four times reduction in thickness without affecting the integrity of the ruthenium substrate resulted in a significant decrease of both EOT and Jg. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Fabrication of DRAM capacitors with an equivalent oxide thickness (EOT) lower than 0.5 nm requires oxide materials with a dielectric constant (k) of at least 50 [1]. One of the oxides that fulfill this condition is TiO2, but only in the rutile phase which has a high dielectric constant (k) reported to be 90 or as high as 170, depending on lattice orientation [2]. Moreover, to avoid the reduction of EOT via formation of a ‘‘dead’’ layer at the interface electrode/dielectric, an electrode that remains electrically conductive even when oxidized is required. Such an electrode is Ru with RuO2 as its conductive oxide [1]. Given these two conditions, the use of RuO2 as bottom electrode becomes a very interesting option as it also acts as a template to enable the growth of rutile TiO2 at low temperatures via atomic layer deposition [3–8]. The films are crystalline as deposited [4–6] or after post-deposition anneal at temperatures up to 500 °C [3,4,7]. Crystallization at such low temperatures is favored by the small lattice mismatch between tetragonal RuO2 (lattice parameters: a = b = 0.450 nm, c = 0.310 nm) and tetragonal rutile TiO2 (lattice parameters: a = b = 0.459 nm, c = 0.296 nm). Following a similar approach as reported in literature [3–5,7], we deposited rutile TiO2 using O3 based atomic layer deposition (ALD) at 250 °C. However, we found that direct ⇑ Corresponding author. Tel.: +32 16287759. E-mail address: [email protected] (M. Popovici). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.063
exposure of RuO2/Ru to O3 will partially etch the Ru substrate resulting in the lost of template near the gas inlet. We demonstrated that a thin H2O-based TiO2 inter-layer deposited prior to the O3-based TiO2 top layer can prevent the observed etching of the Ru substrate [8]. However, the extracted k value of the films was only 55, i.e. lower than the reported values for the rutile phase, which we believe is mostly due to the presence of a small amount of anatase (k 40). In this work we show how the process can be further improved to obtain higher dielectric constant TiO2 films with superior EOT and Jg values in the TiN (top electrode)/ TiO2/RuO2/Ru/TiN/Si capacitor stacks. 2. Experimental TiO2 films were deposited in a cross-flow ASM Pulsar 3000 reactor on 5 nm PEALD Ru/10 nm PVD TiN/300 mm Si(1 0 0) substrates. Prior to TiO2 growth, the Ru substrates were oxidized in an O2 flow (O2/N2 = 0.2) at low pressure (2 Torr) and 250 °C that yielded a 1.4 nm RuO2 at the surface. ALD of TiO2 was done at the same temperature with Ti(OCH3)4 and H2O or O3 as oxidant. Similar growth rate of 0.04 nm/cycle are observed for both H2O and O3 based processes. A H2O-based thin TiO2 film was deposited prior to the thick O3-based TiO2 layer (Fig. 1). The characterization of TiO2 layers was done by spectroscopic ellipsometry (SE), X-ray reflectometry (XRR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
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M. Popovici et al. / Microelectronic Engineering 88 (2011) 1517–1520
TiN TiO (O ) 2
3
TiO2(H 2O)
Ru
RuO2
Fig. 1. Schematic representation of the studied capacitor stack (TiN (top electrode)/ TiO2/RuO2/Ru/TiN).
microscopy (TEM), and time of flight secondary ion mass spectrometry (TOFSIMS). A 40 nm thick PVD TiN top electrode was deposited. The MIM capacitors obtained after patterning with Deep-Ultraviolet (DUV) lithography and reactive ion etching (RIE) have a rectangular structure with sizes ranging between 1 and 1000 lm2 (such as 1, 2, 5, 10, 20, 30, etc.). The C–V, J–V measurements were done on 59 sites per wafer for capacitor sizes of 50, 70, 100, and 200 lm2.
was evaluated to be 75(±2)% (‘‘12 cy’’ films) or 84(±2)% (‘‘6 cy’’ or ‘‘3 cy’’ films) of rutile bulk density (4.27 g/cm3), confirming the incomplete densification or crystallization of the as-grown films. The difference between the density values of the ‘‘6 cy’’ and ‘‘12 cy’’ films could be related to the relative ratio between rutile and anatase phase. By X-ray diffraction we could identify the presence of rutile TiO2 as indicated by the (1 1 0) diffraction line (Fig. 3). The low intensity of the peak suggests poorly crystallized films or a very small grain size. TEM cross section analysis (Fig. 4) and SEM top view images (Fig. 5) allowed estimation of the grain size, i.e. 10–12 nm. The Fast Fourier Transform (FFT) patterns obtained from different areas of the TEM image (‘‘6 cy’’ film) along the O3 gas flow direction indicate the presence of rutile TiO2 (e.g. inset Fig. 4). TiO2 rutile has the {1 1 0}rutile crystallographic planes nearly parallel to the interfaces that confirms a preferential growth along the a axis [3]. Next to the rutile phase also the anatase was found in the region outside the gas flow direction, being probably present in a small quantity. This indicates that an increase of the O3 dose could have a further beneficial role in maximizing the amount of rutile phase across the wafer. The RuO2 interfacial layer could not be observed by TEM due to its low thickness and/or roughness of the different interfaces.
To test the scalability of the TiO2 under-layer, its thickness was varied (3, 6 and 12 cycles of the Ti(OCH3)4–H2O reaction). The film stacks were simply abbreviated throughout the text as ‘‘3 cy’’, ‘‘6 cy’’ and ‘‘12 cy’’, respectively. XRR data of the layer stacks obtained after TiO2 deposition were fitted using a TiO2/RuO2/Ru /TiN/Si stack layer model. The thickness of RuO2 layer was evaluated to be 1.4 nm. The thickness of the Ru substrate (4 nm after oxidation) determined from 49 points spectroscopic ellipsometry remained constant (showing the protective effect of the ultrathin H2O based TiO2 inter-layer. Refractive index (RI) of the TiO2 layers recorded at 633 nm ranged between 2.0 and 2.2, indicating that the films are either not fully crystallized (as anatase and/or rutile) or not completely densified, as the refractive indices of anatase (2.4) and rutile (2.7) are higher. We had previously shown that a H2O based process led to anatase formation while the use of O3 resulted in crystallization of rutile phase [8]. In spite of the very low amount of H2O used to grow the interfacial TiO2 layer, the refractive index of the TiO2 films is sensitive to the number of cycles of the TiO2 interlayer, as shown in Fig. 2, likely due to variations in crystalline phase. Thinner H2O based TiO2 inter-layer, the higher refractive index and the lower the within wafer non-uniformity WIWNU (1r, %) of RI. The density of the films (as extracted from XRR data fits)
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 20
RuO2 tetragonal
(110)
TiO2 rutile TiO2 anatase " 6 cy " film
25
30
35
40
45
50
2 θ [degree] Fig. 3. GIXRD of the ‘‘6cy’’ TiO2 film (11 nm TiO2 layer grown at 250 °C on RuO2 using a 6 cycles thick TiO2, H2O-based inter-layer. The (1 1 0) diffraction line of rutile is well visible. Diffraction lines of tetragonal RuO2, rutile TiO2 and anatase TiO2 are indicated in the figure.
2.15 1.0 2.10 0.5 2.05
RI WIWNU [%]
1.5
2.20
Refractive index at 633 nm
Intensity [a.u.]
3. Results and discussion
Refractive index (RI) RI WIWNU [%] 2.00
2
4
6
8
10
12
0.0
Cycles TiO2(H2O) Fig. 2. Mean refractive index value at 630 nm as function of the number of cycles of the H2O based TiO2 inter-layer and the RI WIWNU (1r, %) calculated from 49 points measurements.
Fig. 4. Cross section TEM of the ‘‘6 cy’’ TiO2 based stack. Inset: FFT of the selected area diffraction pattern shows the presence rutile oriented in the h0 0 1i crystallographic direction.
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M. Popovici et al. / Microelectronic Engineering 88 (2011) 1517–1520
It is likely that TiO2 and RuO2 intermix at the interface as the two oxides can form a solid solution due to their similar lattice parameters and ionic radii for sixfold coordination of Ru4+ (0.062 nm) and Ti4+ (0.0605 nm) [9]. The resulting TixRu1xO2 mixed oxide with a rutile structure is believed to act as an effective barrier against the etch effect that otherwise occurs during the first O3 pulses. This can explain the protective effect of the ultrathin H2O based TiO2 inter-layer in spite of not being a closed ALD layer as we have previously shown using TOFSIMS measurements [8]. The average EOT value extracted from capacitance data and within wafer non-uniformity (WIWNU) [%] on 59 sites measurements of the capacitor stacks as function of TiO2 inter-layer thickness are shown in Fig. 6 for a reduced number of Ti(OCH3)4–H2O cycles from 12 over 6 to 3, respectively. Reduction of both EOT and EOT WIWNU with the reduced number of cycles of H2O based TiO2 inter-layer is likely due to the decrease of the amount of anatase TiO2 present in the film driven by the H2O based process [8]. In Fig. 7 we display the Jg–V characteristics of the ‘‘3 cy’’, ‘‘6 cy’’ and ‘‘12 cy’’ films that indicate a reduction of the leakage current density with decreasing in thickness of the H2O based inter-layer. The results are in agreement with Grahn et al. [10] and confirm the lower values for the leakage current in the case of the rutile phase as compared to anatase. The effective k value was also estimated using thickness extracted from XRR data fit (Fig. 8). A k value of 79 was obtained for the ‘‘3 cy’’ film that is similar to those reported by Kim et al. [3] and Choi et al. [7]. Even though the k value
1.00
16 14
EOT WIWNU EOT
0.90
12
EOT [nm]
0.85 0.80
10
0.75
8
0.70
6
0.65
4
0.60
2
0.55 0.50
WIWNU EOT [%]
0.95
0
5
10
0 15
Cycles TiO2(H2O) Fig. 6. EOT and EOT WIWNU (1r, %) as function of the number of cycles of the H2Obased TiO2 inter-layer.
Fig. 7. Jg–V plot for the deposited TiO2 films with 3, 6, and 12 cycles of H2O-based TiO2 inter-layer.
85
Dielectric constant, k [a.u.]
Fig. 5. SEM plan view image of the ‘‘6cy’’ TiO2 film indicating a uniform grain growth with a diameter of 10–12 nm.
80 75 70 65 60 55 50 45
0
5
10
15
Cycles TiO2(H2O) Fig. 8. Dielectric constant (k) estimated for the deposited TiO2 films with 3, 6, and 12 cycles of H2O-based TiO2 inter-layer.
is lower compared to other reports [5,6], in our case the leakage current density is lower at the same EOT and applied voltage.
4. Conclusion We report a potential industry friendly process for 300 mm wafer with excellent results in terms of EOT/Jg for TiO2 based capacitors (Jg 6 108 A/cm2 and EOT = 0.68 nm at +1 V) using a three cycles H2O based TiO2 inter-layer. The small WIWNU of EOT value (2.1% 1r) indicates better crystalline phase homogeneity across the 300 mm wafer. While the H2O-based TiO2 inter-layer is needed to prevent Ru from being etched by O3, better results in respect of EOT and uniformity are obtained when the inter-layer thickness is scaled down. Also a substantial reduction of leakage current density with the decrease of H2O-based inter-layer thickness is observed, which is attributed to the reduction of the anatase fraction present in the films. The influence of other parameters such as ruthenium oxidation condition and O3 dose to further reduce the EOT–Jg characteristics of the films are currently being investigated. Acknowledgements P. Fazan, S. Vidya and C. Caillat from Micron Technology at imec’s Industrial Affiliation program on Memory Devices are acknowledged.
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References [1] S.K. Kim, S.W. Lee, J.H. Han, B. Lee, S. Han, C.S. Hwang, Adv. Funct. Mater. 20 (2010) 2989–3003. [2] U. Diebold, Surf. Sci. Rep. 48 (2003) 53–229. [3] S.K. Kim, W.-D. Kim, K.-M. Kim, C.S. Hwang, J. Jeong, Appl. Phys. Lett. 85 (2004) 4112–4114. [4] S.K. Kim, K.-M. Kim, O.S. Kwon, S.W. Lee, C.B. Jeon, W.Y. Park, C.S. Hwang, J. Jeong, Electrochem. Solid State Lett. 8 (2005) F59–F62. [5] S.K. Kim, G.W. Hwang, W.-D. Kim, C.S. Hwang, Electrochem. Solid State Lett. 9 (2006) F5–F7.
[6] K. Fröhlich, M. Tˇapajna, A. Rosová, E. Dobrocˇka, K. Hušeková, J. Aarik, A. Aidla, Electrochem. Solid State Lett. 11 (2008) G19–G21. [7] G.-J. Choi, S.K. Kim, S.Y. Lee, W.Y. Park, M. Seo, B.J. Choi, C.S. Hwang, J. Electrochem. Soc. 156 (2009) G71–G77. [8] M. Popovici, J. Swerts, K. Tomida, D. Radisic, M.-S. Kim, B. Kaczer, O. Richard, H. Bender, A. Delabie, A. Moussa, C. Vrancken, K. Opsomer, A. Franquet, M.A. Pawlak, M. Schaekers, L. Altimime, S. Van Elshocht, J.A. Kittl, Phys. Status Solidi RRL 5 (2011) 19–21. [9] K.T. Jacob, R. Subramanian, J. Phase Equilib. Diff. 29 (2008) 136–140. [10] J.V. Grahn, M. Linder, E. Fredriksson, J. Vac. Sci. Technol. A 16 (1998) 2495– 2500.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Improved EOT and leakage current for metal–insulator–metal capacitor stacks with rutile TiO2 Mihaela Popovici ⇑, Min-Soo Kim, Kazuyuki Tomida, Johan Swerts, Hilde Tielens, Alain Moussa, Olivier Richard, Hugo Bender, Alexis Franquet, Thierry Conard, Laith Altimime, Sven Van Elshocht, Jorge A. Kittl Imec, 75 Kapeldreef, 3001 Leuven, Belgium
a r t i c l e
i n f o
Article history: Available online 6 April 2011 Keywords: Atomic layer deposition High-k dielectrics Rutile TiO2
a b s t r a c t Downscaling of the metal–insulator–metal capacitor (MIMCAP) for Dynamic Random Access Memory (DRAM) requires the introduction of high permittivity dielectrics. MIMCAP structures formed with RuO2/Ru as bottom electrode, rutile TiO2 as dielectric and TiN as top electrode are described. Ozone (O3) is needed as oxidant in the TiO2 atomic layer deposition (ALD) process in order to obtain the rutile phase (permittivity > 80), while anatase TiO2 (permittivity 40) is obtained with H2O. As O3 etches the ruthenium substrate, an ultra-thin interlayer of TiO2 was first grown with H2O, followed by the thick TiO2 layer deposited with O3. In order to minimize the content of anatase in the TiO2 layer, responsible for a reduced dielectric constant, we investigate the effect of scaling down the thickness of the protective H2O based inter-layer on the equivalent oxide thickness (EOT) and leakage current density (Jg). The four times reduction in thickness without affecting the integrity of the ruthenium substrate resulted in a significant decrease of both EOT and Jg. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Fabrication of DRAM capacitors with an equivalent oxide thickness (EOT) lower than 0.5 nm requires oxide materials with a dielectric constant (k) of at least 50 [1]. One of the oxides that fulfill this condition is TiO2, but only in the rutile phase which has a high dielectric constant (k) reported to be 90 or as high as 170, depending on lattice orientation [2]. Moreover, to avoid the reduction of EOT via formation of a ‘‘dead’’ layer at the interface electrode/dielectric, an electrode that remains electrically conductive even when oxidized is required. Such an electrode is Ru with RuO2 as its conductive oxide [1]. Given these two conditions, the use of RuO2 as bottom electrode becomes a very interesting option as it also acts as a template to enable the growth of rutile TiO2 at low temperatures via atomic layer deposition [3–8]. The films are crystalline as deposited [4–6] or after post-deposition anneal at temperatures up to 500 °C [3,4,7]. Crystallization at such low temperatures is favored by the small lattice mismatch between tetragonal RuO2 (lattice parameters: a = b = 0.450 nm, c = 0.310 nm) and tetragonal rutile TiO2 (lattice parameters: a = b = 0.459 nm, c = 0.296 nm). Following a similar approach as reported in literature [3–5,7], we deposited rutile TiO2 using O3 based atomic layer deposition (ALD) at 250 °C. However, we found that direct ⇑ Corresponding author. Tel.: +32 16287759. E-mail address: [email protected] (M. Popovici). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.03.063
exposure of RuO2/Ru to O3 will partially etch the Ru substrate resulting in the lost of template near the gas inlet. We demonstrated that a thin H2O-based TiO2 inter-layer deposited prior to the O3-based TiO2 top layer can prevent the observed etching of the Ru substrate [8]. However, the extracted k value of the films was only 55, i.e. lower than the reported values for the rutile phase, which we believe is mostly due to the presence of a small amount of anatase (k 40). In this work we show how the process can be further improved to obtain higher dielectric constant TiO2 films with superior EOT and Jg values in the TiN (top electrode)/ TiO2/RuO2/Ru/TiN/Si capacitor stacks. 2. Experimental TiO2 films were deposited in a cross-flow ASM Pulsar 3000 reactor on 5 nm PEALD Ru/10 nm PVD TiN/300 mm Si(1 0 0) substrates. Prior to TiO2 growth, the Ru substrates were oxidized in an O2 flow (O2/N2 = 0.2) at low pressure (2 Torr) and 250 °C that yielded a 1.4 nm RuO2 at the surface. ALD of TiO2 was done at the same temperature with Ti(OCH3)4 and H2O or O3 as oxidant. Similar growth rate of 0.04 nm/cycle are observed for both H2O and O3 based processes. A H2O-based thin TiO2 film was deposited prior to the thick O3-based TiO2 layer (Fig. 1). The characterization of TiO2 layers was done by spectroscopic ellipsometry (SE), X-ray reflectometry (XRR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
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M. Popovici et al. / Microelectronic Engineering 88 (2011) 1517–1520
TiN TiO (O ) 2
3
TiO2(H 2O)
Ru
RuO2
Fig. 1. Schematic representation of the studied capacitor stack (TiN (top electrode)/ TiO2/RuO2/Ru/TiN).
microscopy (TEM), and time of flight secondary ion mass spectrometry (TOFSIMS). A 40 nm thick PVD TiN top electrode was deposited. The MIM capacitors obtained after patterning with Deep-Ultraviolet (DUV) lithography and reactive ion etching (RIE) have a rectangular structure with sizes ranging between 1 and 1000 lm2 (such as 1, 2, 5, 10, 20, 30, etc.). The C–V, J–V measurements were done on 59 sites per wafer for capacitor sizes of 50, 70, 100, and 200 lm2.
was evaluated to be 75(±2)% (‘‘12 cy’’ films) or 84(±2)% (‘‘6 cy’’ or ‘‘3 cy’’ films) of rutile bulk density (4.27 g/cm3), confirming the incomplete densification or crystallization of the as-grown films. The difference between the density values of the ‘‘6 cy’’ and ‘‘12 cy’’ films could be related to the relative ratio between rutile and anatase phase. By X-ray diffraction we could identify the presence of rutile TiO2 as indicated by the (1 1 0) diffraction line (Fig. 3). The low intensity of the peak suggests poorly crystallized films or a very small grain size. TEM cross section analysis (Fig. 4) and SEM top view images (Fig. 5) allowed estimation of the grain size, i.e. 10–12 nm. The Fast Fourier Transform (FFT) patterns obtained from different areas of the TEM image (‘‘6 cy’’ film) along the O3 gas flow direction indicate the presence of rutile TiO2 (e.g. inset Fig. 4). TiO2 rutile has the {1 1 0}rutile crystallographic planes nearly parallel to the interfaces that confirms a preferential growth along the a axis [3]. Next to the rutile phase also the anatase was found in the region outside the gas flow direction, being probably present in a small quantity. This indicates that an increase of the O3 dose could have a further beneficial role in maximizing the amount of rutile phase across the wafer. The RuO2 interfacial layer could not be observed by TEM due to its low thickness and/or roughness of the different interfaces.
To test the scalability of the TiO2 under-layer, its thickness was varied (3, 6 and 12 cycles of the Ti(OCH3)4–H2O reaction). The film stacks were simply abbreviated throughout the text as ‘‘3 cy’’, ‘‘6 cy’’ and ‘‘12 cy’’, respectively. XRR data of the layer stacks obtained after TiO2 deposition were fitted using a TiO2/RuO2/Ru /TiN/Si stack layer model. The thickness of RuO2 layer was evaluated to be 1.4 nm. The thickness of the Ru substrate (4 nm after oxidation) determined from 49 points spectroscopic ellipsometry remained constant (showing the protective effect of the ultrathin H2O based TiO2 inter-layer. Refractive index (RI) of the TiO2 layers recorded at 633 nm ranged between 2.0 and 2.2, indicating that the films are either not fully crystallized (as anatase and/or rutile) or not completely densified, as the refractive indices of anatase (2.4) and rutile (2.7) are higher. We had previously shown that a H2O based process led to anatase formation while the use of O3 resulted in crystallization of rutile phase [8]. In spite of the very low amount of H2O used to grow the interfacial TiO2 layer, the refractive index of the TiO2 films is sensitive to the number of cycles of the TiO2 interlayer, as shown in Fig. 2, likely due to variations in crystalline phase. Thinner H2O based TiO2 inter-layer, the higher refractive index and the lower the within wafer non-uniformity WIWNU (1r, %) of RI. The density of the films (as extracted from XRR data fits)
1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 20
RuO2 tetragonal
(110)
TiO2 rutile TiO2 anatase " 6 cy " film
25
30
35
40
45
50
2 θ [degree] Fig. 3. GIXRD of the ‘‘6cy’’ TiO2 film (11 nm TiO2 layer grown at 250 °C on RuO2 using a 6 cycles thick TiO2, H2O-based inter-layer. The (1 1 0) diffraction line of rutile is well visible. Diffraction lines of tetragonal RuO2, rutile TiO2 and anatase TiO2 are indicated in the figure.
2.15 1.0 2.10 0.5 2.05
RI WIWNU [%]
1.5
2.20
Refractive index at 633 nm
Intensity [a.u.]
3. Results and discussion
Refractive index (RI) RI WIWNU [%] 2.00
2
4
6
8
10
12
0.0
Cycles TiO2(H2O) Fig. 2. Mean refractive index value at 630 nm as function of the number of cycles of the H2O based TiO2 inter-layer and the RI WIWNU (1r, %) calculated from 49 points measurements.
Fig. 4. Cross section TEM of the ‘‘6 cy’’ TiO2 based stack. Inset: FFT of the selected area diffraction pattern shows the presence rutile oriented in the h0 0 1i crystallographic direction.
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It is likely that TiO2 and RuO2 intermix at the interface as the two oxides can form a solid solution due to their similar lattice parameters and ionic radii for sixfold coordination of Ru4+ (0.062 nm) and Ti4+ (0.0605 nm) [9]. The resulting TixRu1xO2 mixed oxide with a rutile structure is believed to act as an effective barrier against the etch effect that otherwise occurs during the first O3 pulses. This can explain the protective effect of the ultrathin H2O based TiO2 inter-layer in spite of not being a closed ALD layer as we have previously shown using TOFSIMS measurements [8]. The average EOT value extracted from capacitance data and within wafer non-uniformity (WIWNU) [%] on 59 sites measurements of the capacitor stacks as function of TiO2 inter-layer thickness are shown in Fig. 6 for a reduced number of Ti(OCH3)4–H2O cycles from 12 over 6 to 3, respectively. Reduction of both EOT and EOT WIWNU with the reduced number of cycles of H2O based TiO2 inter-layer is likely due to the decrease of the amount of anatase TiO2 present in the film driven by the H2O based process [8]. In Fig. 7 we display the Jg–V characteristics of the ‘‘3 cy’’, ‘‘6 cy’’ and ‘‘12 cy’’ films that indicate a reduction of the leakage current density with decreasing in thickness of the H2O based inter-layer. The results are in agreement with Grahn et al. [10] and confirm the lower values for the leakage current in the case of the rutile phase as compared to anatase. The effective k value was also estimated using thickness extracted from XRR data fit (Fig. 8). A k value of 79 was obtained for the ‘‘3 cy’’ film that is similar to those reported by Kim et al. [3] and Choi et al. [7]. Even though the k value
1.00
16 14
EOT WIWNU EOT
0.90
12
EOT [nm]
0.85 0.80
10
0.75
8
0.70
6
0.65
4
0.60
2
0.55 0.50
WIWNU EOT [%]
0.95
0
5
10
0 15
Cycles TiO2(H2O) Fig. 6. EOT and EOT WIWNU (1r, %) as function of the number of cycles of the H2Obased TiO2 inter-layer.
Fig. 7. Jg–V plot for the deposited TiO2 films with 3, 6, and 12 cycles of H2O-based TiO2 inter-layer.
85
Dielectric constant, k [a.u.]
Fig. 5. SEM plan view image of the ‘‘6cy’’ TiO2 film indicating a uniform grain growth with a diameter of 10–12 nm.
80 75 70 65 60 55 50 45
0
5
10
15
Cycles TiO2(H2O) Fig. 8. Dielectric constant (k) estimated for the deposited TiO2 films with 3, 6, and 12 cycles of H2O-based TiO2 inter-layer.
is lower compared to other reports [5,6], in our case the leakage current density is lower at the same EOT and applied voltage.
4. Conclusion We report a potential industry friendly process for 300 mm wafer with excellent results in terms of EOT/Jg for TiO2 based capacitors (Jg 6 108 A/cm2 and EOT = 0.68 nm at +1 V) using a three cycles H2O based TiO2 inter-layer. The small WIWNU of EOT value (2.1% 1r) indicates better crystalline phase homogeneity across the 300 mm wafer. While the H2O-based TiO2 inter-layer is needed to prevent Ru from being etched by O3, better results in respect of EOT and uniformity are obtained when the inter-layer thickness is scaled down. Also a substantial reduction of leakage current density with the decrease of H2O-based inter-layer thickness is observed, which is attributed to the reduction of the anatase fraction present in the films. The influence of other parameters such as ruthenium oxidation condition and O3 dose to further reduce the EOT–Jg characteristics of the films are currently being investigated. Acknowledgements P. Fazan, S. Vidya and C. Caillat from Micron Technology at imec’s Industrial Affiliation program on Memory Devices are acknowledged.
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