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A study on mobility degradation in nMOSFETs with HfO2 based gate oxide
Materials Science and Engineering B 165 (2009) 129–131
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Review
A study on mobility degradation in nMOSFETs with HfO2 based gate oxide G. Hyvert ∗ , T. Nguyen, L. Militaru, A. Poncet, C. Plossu Université de Lyon, Institut des Nanotechnologies de Lyon INL-UMR CNRS 5270, INSA de Lyon, Villeurbanne, F-69621, France
a r t i c l e
i n f o
Article history: Received 20 June 2008 Received in revised form 16 February 2009 Accepted 18 February 2009 Keywords: Electron mobility Hafnium oxide Low field transport Phonon–electron interaction Remote coulomb scattering interaction
a b s t r a c t In this paper, we investigate the causes for electron mobility reduction inside the conduction channel of nMOSFETs with TiN/HfO2 /SiO2 gate stack. The use of such a high-k gate dielectric stack induces new interactions compared to conventional SiO2 gate oxide, modifying the electrons momentum during their transport along the channel. Experimental results, obtained by split-CV at different temperatures and charge pumping techniques, allow us to separate the contribution of each known interaction in the mobility degradation. Remote interactions are found to be the main phenomena at stake, specifically remote coulomb scattering, which modifies the screened potential seen by electrons in the channel. We finally discuss about the nature and the localization of such an interaction within the gate stack. © 2009 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
129 129 130 131 131 131
1. Introduction
2. Experimental
Further increase of conventional CMOS performances implies a reduction of transistors dimensions. However, keeping a high drain current level with a reduced gate length can only be achieved by reducing the gate equivalent oxide thickness (EOT). The introduction of alternative high-k dielectrics, such as hafnium based oxides (HfO2 , HfSiOx, . . .), is currently the easiest industrial solution. The elaboration of hafnium based gate stacks raises two important problems. On the one hand, the main technological problem encountered is the formation of an interfacial SiO2 layer, which leads to an increase in the EOT. On the other hand, and despite this high-k dielectric has been already introduced for the 45 nm node, it is well established that hafnium based oxides drastically reduce electron mobility. The origin of this phenomenon is still not clearly identified. In this paper, we performed an experimental study on electron mobility in HfO2 /TiN nMOSFETs. Data are discussed regarding the different interactions that can lower the mobility.
Devices are nMOS transistors with TiN metal gate deposited by CVD (chemical vapour deposition). The gate dielectric is 2 nm thick SiO2 for reference transistors and 2, 2.5, 3 and 4.5 nm HfO2 , with a 0.8-nm underlying interfacial SiO2 layer, for high-k devices (respectively, EOT: 1.15; 1.24; 1.33; 1.6 nm). The HfO2 layer was deposited by ALD (atomic layer deposition) and has undergone a post-deposition annealing in N2 ambient at 600 ◦ C. The size of our devices is large (W × L = 10 m × 10 m, 100 m × 100 m and 250 m × 160 m) to avoid measurement resolution problems that could affect the extracted electron mobility. Indeed, near the resolution limit, the measured capacitance can be overestimated due to a non-negligible contribution of parasitic capacitance, which leads to an apparent higher inversion charge and, then, to an apparent reduced mobility. The main physical and electrical parameters were extracted from quasi-static drain current (ID ) measurements, as explained in Ref. [1]. The effective electron mobility was evaluated using the split-CV method [2]. The inversion and the accumulation capacitances are separately measured and are integrated to obtain the inversion charge (Qi ) and the accumulation charge (Qd ). Finally the effective mobility (eff ) and the effective electric field (Eeff ) are
∗ Corresponding author. Tel.: +33 472437036; fax: +33 472436080. E-mail address: [email protected] (G. Hyvert). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.02.016
130
G. Hyvert et al. / Materials Science and Engineering B 165 (2009) 129–131
calculated according to the following relations [2]: eff (VG ) =
L ID (VG ) W Qi (VG )VD
(1)
Eeff (VG ) =
Qi (VG ) + Qd (VG ) εSi
(2)
with VG the gate voltage, VD the drain voltage, εSi the silicon dielectric constant and the inversion charge reduction factor (1/2 for electrons, 1/3 for holes). Measurements were performed at VD = 50 mV. For devices with very low EOT, the measured drain current can be affected by the gate leakage current. In this case, the measured drain current ID was corrected following the method described in Ref. [3]. 3. Results and discussion The main electrical parameters are summarized in Table 1. They provide useful information about the quality of our devices. The source and drain access resistances are low, though slightly higher than the ITRS recommendations [4] (for a 10-m wide MOSFET, Racc = 480 m compared to 180 m in the ITRS). As expected, the low field mobility (0 ), calculated in the linear regime, is strongly reduced for high-k devices, compared to SiO2 reference one. This low field mobility (0 ) is equivalent to the effective mobility near the threshold voltage Vth . Negara et al. [5] and Simoen et al. [6] have published the extracted parameters they found for similar devices. There is a reasonable agreement between our results and the literature. Indeed, when comparing devices with similar EOT, the electrical parameters turn out to be of the same magnitude. The effective mobility values, extracted from split-CV measurements, are compared in Fig. 1 for HfO2 and SiO2 reference devices. We can see that 0 for SiO2 (199 cm2 /V s) corresponds to eff obtained for an effective field of 0.33 MV/cm. Similarly, the low field mobility for HfO2 (96 cm2 /V s) equals the effective mobility for a comparable effective field (0.34 MV/cm). For both 0 and eff , HfO2 devices exhibit an effective mobility reduced by a factor of two compared to SiO2 one. This reduction can only be explained by some specific interactions within the HfO2 devices. In order to compare the impact of each possible interaction on the electron mobility, we first investigated the magnitude of trapping phenomena in the gate dielectric, which are known to induce Vth instabilities in HfO2 devices [7]. The origin of such a phenomenon is still unclear. The main explanation would be that oxygen vacancies within the HfO2 layer create a defect energy band beneath the HfO2 conduction band [8]. Therefore transient carrier exchanges may occur between the channel and the dielectric layer during quasi-static measurements. This is revealed by the shift observed between the ID –VG characteristics recorded in both pulsed and quasi-static measurement conditions [7,9]. Charge trapping occurring into the gate dielectric during a quasi-static voltage ramp induces a lowering of the drain current and, therefore,
Fig. 1. Effective electron mobility eff as a function of effective electric field Eeff , extracted by split-CV for reference SiO2 and HfO2 transistors. The mobility is greatly reduced for high-k devices. The devices surface is W × L = 250 m × 160 m.
an apparent reduced mobility. However, by plotting mobility data obtained from both dynamic and static measurements, it can be observed in Fig. 2 that the gain in mobility extracted from pulsed measurements remains very low and is not sufficient to explain the difference between HfO2 and SiO2 devices. So, other mechanisms have to be considered. It is well known that the presence of interface states at the Si/SiO2 interface can reduce the effective channel electron mobility. Interface state densities (Dit ) were measured by means of the two levels charge pumping technique [10]. Results are given in Table 2. Similar densities are achieved for both HfO2 and SiO2 devices (≈1011 cm−2 eV−1 ) so it can be concluded that interfacial defects are not relevant to explain mobility reduction in HfO2 devices. Surface roughness may also be an active factor for the reduction of mobility. However, several works [11,12] show that this effect is only relevant for high effective fields (above 1 MV/cm). One can investigate this effect further by raising the applied field when studying devices with dramatically different surfaces. We only made measurements at low fields, in order to preserve the devices. Moreover, we were lacking devices with such different geometries. However, as the mobility reduction due to the highk is already visible at low fields, it is highly improbable that surface roughness alone might by its cause.
Table 1 Extracted physical and conduction parameters for SiO2 and HfO2 devices. Cox is the oxide capacitance, Vth the threshold voltage, L the difference between designed and effective channel lengths, the mobility degradation factor, 0 the low field electron mobility, Racc the drain and source access resistance, Vfb the flat band voltage. Values for SiO2 and two thicknesses of HfO2 are given here as examples. Devices
Ref. SiO2 2 nm
HfO2 2 nm
HfO2 2.5 nm
Cox (F/m2 ) Vth (V) L (m) (V−1 ) 0 (cm2 /V s) Racc () Vfb (V)
1.73 × 10−2 0.76 0.1–0.105 0.17–0.2 192–218 48 −0.5
2.99 × 10−2 0.8 0.1 0.04 138 32 −0.552
2.78 × 10−2 0.78 0.1 0.017 93–109 33 −0.535
Fig. 2. Comparison of effective electron mobility extracted from both static and pulsed ID –VG , for the 3 and 4 nm HfO2 devices. The static mobility is lower that the pulsed one. The device surface is W × L = 10 m × 10 m.
G. Hyvert et al. / Materials Science and Engineering B 165 (2009) 129–131
131
Table 2 Interface states densities Dit extracted by two levels charge pumping measurements. Devices −1
Dit (eV
−2
cm
)
Ref. SiO2 2 nm
HfO2 2 nm
1.2–1.5 × 10
3.2–5.8 × 10
11
11
Fig. 3. Effective mobility eff versus inversion charge Qi for temperatures between 90 and 300 K and comparison of reference 2 nm SiO2 and HfO2 (EOT 1.33 nm). The devices surface is 100 m × 100 m. The mobility increases with decreased temperature but remains lower for HfO2 devices compared to SiO2 ones.
Two other interactions may reduce the mobility: high-k remote phonons scattering (RPS) and remote coulomb scattering (RCS) [13,14]. The first assumes that electrons in the inversion channel can interact with high-k phonons, created by the vibrations of the soft polarisable Hf–O bonds. The later is a remote electrostatic interaction caused by charged species into the gate dielectric, which modifies the electrostatic potential seen by electrons during their transport along the conduction channel. To investigate phonon scattering impact, we performed splitCV measurements for temperatures varying from 90 to 300 K. Effective mobility values are plotted in Fig. 3. We can see, on these curves, two distinct domains of mobility which can be explained by considering the mobility dependency on coulomb and phonon channel interactions [11]. Following the Mathiessen’s rule, the electron mobility can be decomposed between the influence of coulomb interactions inside the channel (cb ), different from the RCS which is located inside the gate oxide, and phonons interactions (ph ). According to literature, Mathiessen’s rule is valid as long as the different individual scatterings are not correlated, even in microstructures [15,16]. The former is proportional to the temperature (T), the later to T−1.5 . In the low inversion charge (<3 × 10−7 C/cm2 ) domain, channel coulomb interactions are dominant since eff increases with temperature for both HfO2 and SiO2 devices. Nevertheless it can be seen in Fig. 3 that the coulomb interaction domain at low inversion charges is larger for HfO2 devices than for SiO2 ones. In the high inversion charge domain, eff increases greatly while decreasing the temperature showing that remote phonon scattering play an important role for both HfO2 and SiO2 devices. However, the mobility at low temperature in HfO2 transistors remains inferior to SiO2 ones. Therefore, RPS can only be described as a secondary phenomenon for the mobility reduction and RCS, which is independent of temperature, appears to be the main mechanism that can impact mobility. Finally, considering that RCS is the main cause of mobility degradation, we need to know where the involved charged species are
HfO2 2.5 nm
HfO2 3 nm
HfO2 4.5 nm
2.4–4.8 × 10
1.8 × 10
4.2 × 1011
11
11
located within the TiN/HfO2 /SiO2 /Si gate stack. As the flat-band voltage is not affected, these charges could be either located near the metal gate interface or could be associated to neutral electrostatic dipoles [13]. If RCS was situated near the gate, decreasing the HfO2 layer thickness should make the charged species closer to the channel, increasing the strength of the coulomb interactions, and reducing even more the effective mobility. However, as can be observed in Fig. 1, the HfO2 thickness seems to have no influence on the effective electron mobility and no coherent pattern, linking the effective electron mobility and the HfO2 thickness, can be established. So, RCS can hardly be associated to fixed charges near the gate. The presence of dipoles, near the HfO2 /SiO2 interfacial layer, might be the source of RCS; these dipoles would induce potential fluctuations at the Si/SiO2 interface along the channel and consequently degrade the electron mobility, as it is drawn in other works [17]. Demonstrating this hypothesis will need further works based on channel transport simulations. Other theoretical works have demonstrated that a numerical model can be developed using Monte Carlo simulations [18,19] or by using a Poisson–Schrödinger solver [20]. We are rather studying the use of an analytical model to emulate the existence of remote coulomb scattering. 4. Conclusion We have investigated the different causes for electron mobility degradation in competitive high-k NMOS transistors with different HfO2 thicknesses. It has been shown that the reduction of mobility is mainly due to a remote coulomb scattering mechanism which might be associated to the presence of neutral electrostatic dipoles located near the HfO2 /SiO2 interface. Further work will model the impact of remote coulomb scattering induced by the presence of dipoles at the HfO2 /SiO2 by means of channel transport simulations. Acknowledgment This work was supported by the European Commission as part of project PULLNANO. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
G. Ghibaudo, Electron. Lett. 24 (1998) 543–545. C. Sodini, T. Eksted, J. Moll, Solid-State Electron. 25 (1982) 833–841. F. Lime, C. Guiducci, R. Clerc, Solid-State Electron. 47 (2003) 1147–1153. International Technology Roadmap for Semiconductors, http://www.itrs.net/. A. Negara, K. Cherkaoui, P. Majhi, Microelectron. Eng. 84 (2007) 1874–1877. E. Simoen, A. Mercha, C. Clayes, Appl. Phys. Lett. 85 (2004) 1057–1059. A. Kerber, E. Cartier, L. Pantisano, IEEE Electron Device Lett. 24 (2003) 87–89. E. Simoen, A. Mercha, L. Pantisano, IEEE Trans. Electron Devices 51 (2004) 780. C. Leroux, J. Mitard, G. Ghibaudo, IEDM Technical Digest (2004) 737–740. G. Groeseneken, H.E. Maes, N. Beltran, IEEE Trans. Electron Devices 31 (1984) 42–53. D. Jeon, D. Burk, IEEE Trans. Electron Devices 36 (1989) 1456. W. Zhu, J. Han, T.P. Ma, IEEE Trans. Electron Devices 51 (2004) 98–105. O. Weber, M. Cassé, L. Thevenod, Solid-State Electron. 50 (2006) 626–631. M.V. Fischetti, D.A. Neumayer, E.A. Cartier, J. Appl. Phys. 90 (2001) 4587. S. Saito, K. Torii, Y. Shimamoto, J. Appl. Phys. 98 (2005) 113706. T. Ando, A.B. Fowler, F. Stern, Rev. Mod. Phys. 54 (1982) 437. M. Cassé, L. Thevenod, B. Guillaumot, IEEE Trans. Electron Devices 53 (2006) 759–768. N. Rodriguez, J.B. Roldan, F. Gamiz, Semicond. Sci. Technol. 22 (2007) 348–353. G. Ferrari, J.R. Watling, S. Roy, J. Non-Cryst. Solids 353 (2007) 630–634. G.S. Lujan, W. Magnus, L.-A. Ragnarsson, Microelectron. Reliab. 45 (2005) 794–797.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Review
A study on mobility degradation in nMOSFETs with HfO2 based gate oxide G. Hyvert ∗ , T. Nguyen, L. Militaru, A. Poncet, C. Plossu Université de Lyon, Institut des Nanotechnologies de Lyon INL-UMR CNRS 5270, INSA de Lyon, Villeurbanne, F-69621, France
a r t i c l e
i n f o
Article history: Received 20 June 2008 Received in revised form 16 February 2009 Accepted 18 February 2009 Keywords: Electron mobility Hafnium oxide Low field transport Phonon–electron interaction Remote coulomb scattering interaction
a b s t r a c t In this paper, we investigate the causes for electron mobility reduction inside the conduction channel of nMOSFETs with TiN/HfO2 /SiO2 gate stack. The use of such a high-k gate dielectric stack induces new interactions compared to conventional SiO2 gate oxide, modifying the electrons momentum during their transport along the channel. Experimental results, obtained by split-CV at different temperatures and charge pumping techniques, allow us to separate the contribution of each known interaction in the mobility degradation. Remote interactions are found to be the main phenomena at stake, specifically remote coulomb scattering, which modifies the screened potential seen by electrons in the channel. We finally discuss about the nature and the localization of such an interaction within the gate stack. © 2009 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
129 129 130 131 131 131
1. Introduction
2. Experimental
Further increase of conventional CMOS performances implies a reduction of transistors dimensions. However, keeping a high drain current level with a reduced gate length can only be achieved by reducing the gate equivalent oxide thickness (EOT). The introduction of alternative high-k dielectrics, such as hafnium based oxides (HfO2 , HfSiOx, . . .), is currently the easiest industrial solution. The elaboration of hafnium based gate stacks raises two important problems. On the one hand, the main technological problem encountered is the formation of an interfacial SiO2 layer, which leads to an increase in the EOT. On the other hand, and despite this high-k dielectric has been already introduced for the 45 nm node, it is well established that hafnium based oxides drastically reduce electron mobility. The origin of this phenomenon is still not clearly identified. In this paper, we performed an experimental study on electron mobility in HfO2 /TiN nMOSFETs. Data are discussed regarding the different interactions that can lower the mobility.
Devices are nMOS transistors with TiN metal gate deposited by CVD (chemical vapour deposition). The gate dielectric is 2 nm thick SiO2 for reference transistors and 2, 2.5, 3 and 4.5 nm HfO2 , with a 0.8-nm underlying interfacial SiO2 layer, for high-k devices (respectively, EOT: 1.15; 1.24; 1.33; 1.6 nm). The HfO2 layer was deposited by ALD (atomic layer deposition) and has undergone a post-deposition annealing in N2 ambient at 600 ◦ C. The size of our devices is large (W × L = 10 m × 10 m, 100 m × 100 m and 250 m × 160 m) to avoid measurement resolution problems that could affect the extracted electron mobility. Indeed, near the resolution limit, the measured capacitance can be overestimated due to a non-negligible contribution of parasitic capacitance, which leads to an apparent higher inversion charge and, then, to an apparent reduced mobility. The main physical and electrical parameters were extracted from quasi-static drain current (ID ) measurements, as explained in Ref. [1]. The effective electron mobility was evaluated using the split-CV method [2]. The inversion and the accumulation capacitances are separately measured and are integrated to obtain the inversion charge (Qi ) and the accumulation charge (Qd ). Finally the effective mobility (eff ) and the effective electric field (Eeff ) are
∗ Corresponding author. Tel.: +33 472437036; fax: +33 472436080. E-mail address: [email protected] (G. Hyvert). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.02.016
130
G. Hyvert et al. / Materials Science and Engineering B 165 (2009) 129–131
calculated according to the following relations [2]: eff (VG ) =
L ID (VG ) W Qi (VG )VD
(1)
Eeff (VG ) =
Qi (VG ) + Qd (VG ) εSi
(2)
with VG the gate voltage, VD the drain voltage, εSi the silicon dielectric constant and the inversion charge reduction factor (1/2 for electrons, 1/3 for holes). Measurements were performed at VD = 50 mV. For devices with very low EOT, the measured drain current can be affected by the gate leakage current. In this case, the measured drain current ID was corrected following the method described in Ref. [3]. 3. Results and discussion The main electrical parameters are summarized in Table 1. They provide useful information about the quality of our devices. The source and drain access resistances are low, though slightly higher than the ITRS recommendations [4] (for a 10-m wide MOSFET, Racc = 480 m compared to 180 m in the ITRS). As expected, the low field mobility (0 ), calculated in the linear regime, is strongly reduced for high-k devices, compared to SiO2 reference one. This low field mobility (0 ) is equivalent to the effective mobility near the threshold voltage Vth . Negara et al. [5] and Simoen et al. [6] have published the extracted parameters they found for similar devices. There is a reasonable agreement between our results and the literature. Indeed, when comparing devices with similar EOT, the electrical parameters turn out to be of the same magnitude. The effective mobility values, extracted from split-CV measurements, are compared in Fig. 1 for HfO2 and SiO2 reference devices. We can see that 0 for SiO2 (199 cm2 /V s) corresponds to eff obtained for an effective field of 0.33 MV/cm. Similarly, the low field mobility for HfO2 (96 cm2 /V s) equals the effective mobility for a comparable effective field (0.34 MV/cm). For both 0 and eff , HfO2 devices exhibit an effective mobility reduced by a factor of two compared to SiO2 one. This reduction can only be explained by some specific interactions within the HfO2 devices. In order to compare the impact of each possible interaction on the electron mobility, we first investigated the magnitude of trapping phenomena in the gate dielectric, which are known to induce Vth instabilities in HfO2 devices [7]. The origin of such a phenomenon is still unclear. The main explanation would be that oxygen vacancies within the HfO2 layer create a defect energy band beneath the HfO2 conduction band [8]. Therefore transient carrier exchanges may occur between the channel and the dielectric layer during quasi-static measurements. This is revealed by the shift observed between the ID –VG characteristics recorded in both pulsed and quasi-static measurement conditions [7,9]. Charge trapping occurring into the gate dielectric during a quasi-static voltage ramp induces a lowering of the drain current and, therefore,
Fig. 1. Effective electron mobility eff as a function of effective electric field Eeff , extracted by split-CV for reference SiO2 and HfO2 transistors. The mobility is greatly reduced for high-k devices. The devices surface is W × L = 250 m × 160 m.
an apparent reduced mobility. However, by plotting mobility data obtained from both dynamic and static measurements, it can be observed in Fig. 2 that the gain in mobility extracted from pulsed measurements remains very low and is not sufficient to explain the difference between HfO2 and SiO2 devices. So, other mechanisms have to be considered. It is well known that the presence of interface states at the Si/SiO2 interface can reduce the effective channel electron mobility. Interface state densities (Dit ) were measured by means of the two levels charge pumping technique [10]. Results are given in Table 2. Similar densities are achieved for both HfO2 and SiO2 devices (≈1011 cm−2 eV−1 ) so it can be concluded that interfacial defects are not relevant to explain mobility reduction in HfO2 devices. Surface roughness may also be an active factor for the reduction of mobility. However, several works [11,12] show that this effect is only relevant for high effective fields (above 1 MV/cm). One can investigate this effect further by raising the applied field when studying devices with dramatically different surfaces. We only made measurements at low fields, in order to preserve the devices. Moreover, we were lacking devices with such different geometries. However, as the mobility reduction due to the highk is already visible at low fields, it is highly improbable that surface roughness alone might by its cause.
Table 1 Extracted physical and conduction parameters for SiO2 and HfO2 devices. Cox is the oxide capacitance, Vth the threshold voltage, L the difference between designed and effective channel lengths, the mobility degradation factor, 0 the low field electron mobility, Racc the drain and source access resistance, Vfb the flat band voltage. Values for SiO2 and two thicknesses of HfO2 are given here as examples. Devices
Ref. SiO2 2 nm
HfO2 2 nm
HfO2 2.5 nm
Cox (F/m2 ) Vth (V) L (m) (V−1 ) 0 (cm2 /V s) Racc () Vfb (V)
1.73 × 10−2 0.76 0.1–0.105 0.17–0.2 192–218 48 −0.5
2.99 × 10−2 0.8 0.1 0.04 138 32 −0.552
2.78 × 10−2 0.78 0.1 0.017 93–109 33 −0.535
Fig. 2. Comparison of effective electron mobility extracted from both static and pulsed ID –VG , for the 3 and 4 nm HfO2 devices. The static mobility is lower that the pulsed one. The device surface is W × L = 10 m × 10 m.
G. Hyvert et al. / Materials Science and Engineering B 165 (2009) 129–131
131
Table 2 Interface states densities Dit extracted by two levels charge pumping measurements. Devices −1
Dit (eV
−2
cm
)
Ref. SiO2 2 nm
HfO2 2 nm
1.2–1.5 × 10
3.2–5.8 × 10
11
11
Fig. 3. Effective mobility eff versus inversion charge Qi for temperatures between 90 and 300 K and comparison of reference 2 nm SiO2 and HfO2 (EOT 1.33 nm). The devices surface is 100 m × 100 m. The mobility increases with decreased temperature but remains lower for HfO2 devices compared to SiO2 ones.
Two other interactions may reduce the mobility: high-k remote phonons scattering (RPS) and remote coulomb scattering (RCS) [13,14]. The first assumes that electrons in the inversion channel can interact with high-k phonons, created by the vibrations of the soft polarisable Hf–O bonds. The later is a remote electrostatic interaction caused by charged species into the gate dielectric, which modifies the electrostatic potential seen by electrons during their transport along the conduction channel. To investigate phonon scattering impact, we performed splitCV measurements for temperatures varying from 90 to 300 K. Effective mobility values are plotted in Fig. 3. We can see, on these curves, two distinct domains of mobility which can be explained by considering the mobility dependency on coulomb and phonon channel interactions [11]. Following the Mathiessen’s rule, the electron mobility can be decomposed between the influence of coulomb interactions inside the channel (cb ), different from the RCS which is located inside the gate oxide, and phonons interactions (ph ). According to literature, Mathiessen’s rule is valid as long as the different individual scatterings are not correlated, even in microstructures [15,16]. The former is proportional to the temperature (T), the later to T−1.5 . In the low inversion charge (<3 × 10−7 C/cm2 ) domain, channel coulomb interactions are dominant since eff increases with temperature for both HfO2 and SiO2 devices. Nevertheless it can be seen in Fig. 3 that the coulomb interaction domain at low inversion charges is larger for HfO2 devices than for SiO2 ones. In the high inversion charge domain, eff increases greatly while decreasing the temperature showing that remote phonon scattering play an important role for both HfO2 and SiO2 devices. However, the mobility at low temperature in HfO2 transistors remains inferior to SiO2 ones. Therefore, RPS can only be described as a secondary phenomenon for the mobility reduction and RCS, which is independent of temperature, appears to be the main mechanism that can impact mobility. Finally, considering that RCS is the main cause of mobility degradation, we need to know where the involved charged species are
HfO2 2.5 nm
HfO2 3 nm
HfO2 4.5 nm
2.4–4.8 × 10
1.8 × 10
4.2 × 1011
11
11
located within the TiN/HfO2 /SiO2 /Si gate stack. As the flat-band voltage is not affected, these charges could be either located near the metal gate interface or could be associated to neutral electrostatic dipoles [13]. If RCS was situated near the gate, decreasing the HfO2 layer thickness should make the charged species closer to the channel, increasing the strength of the coulomb interactions, and reducing even more the effective mobility. However, as can be observed in Fig. 1, the HfO2 thickness seems to have no influence on the effective electron mobility and no coherent pattern, linking the effective electron mobility and the HfO2 thickness, can be established. So, RCS can hardly be associated to fixed charges near the gate. The presence of dipoles, near the HfO2 /SiO2 interfacial layer, might be the source of RCS; these dipoles would induce potential fluctuations at the Si/SiO2 interface along the channel and consequently degrade the electron mobility, as it is drawn in other works [17]. Demonstrating this hypothesis will need further works based on channel transport simulations. Other theoretical works have demonstrated that a numerical model can be developed using Monte Carlo simulations [18,19] or by using a Poisson–Schrödinger solver [20]. We are rather studying the use of an analytical model to emulate the existence of remote coulomb scattering. 4. Conclusion We have investigated the different causes for electron mobility degradation in competitive high-k NMOS transistors with different HfO2 thicknesses. It has been shown that the reduction of mobility is mainly due to a remote coulomb scattering mechanism which might be associated to the presence of neutral electrostatic dipoles located near the HfO2 /SiO2 interface. Further work will model the impact of remote coulomb scattering induced by the presence of dipoles at the HfO2 /SiO2 by means of channel transport simulations. Acknowledgment This work was supported by the European Commission as part of project PULLNANO. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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