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Material properties of interfacial silicate layer and its influence on the electrical characteristics of MOS devices using hafnia as the gate dielectric
Thin Solid Films 504 (2006) 192 – 196 www.elsevier.com/locate/tsf
Material properties of interfacial silicate layer and its influence on the electrical characteristics of MOS devices using hafnia as the gate dielectric Hei Wong a,*, B. Sen a, V. Filip a, M.C. Poon b b
a Department of Electronic Engineering, City University, Tat Chee Avenue, Kowloon, Hong Kong, China Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
Available online 24 October 2005
Abstract This work deals with some fundamental material properties of the hafnia or hafnium oxide and the silicate layer at the Si/HfO2 interface. It is realized that one of the fundamental features of the HfO2 film is that its lowest conduction band states are composed mainly by d states instead of s and p states as is in the case for SiO2. There is only a very small covalent component contributed from Hf d and O 2p states and the Hf – O bond is predominantly ionic. This is the reason for the small band gap and small conduction band offset energy. This feature also gives rise to thermal instabilities and the formation of an interfacial silicate layer. Although the formation of the silicate structure will help to build a more stable interface, long metallic silicide bonds will result as a by-product when the metal oxide reacts with the Si substrate to form this silicate layer. The existence of metallic silicide bonds is one of the major reasons for the observed high-interface trap density of the HfO2/Si structure. Because of the large difference in dielectric constant between the bulk hafnia and interfacial silicate, the electric field will be distributed mainly across the low-j interface layer, which in effect reduces the interface barrier height for electron tunneling injection and the effective breakdown voltage of the lowj layer. D 2005 Elsevier B.V. All rights reserved. Keywords: Dielectric film; Hafnium oxide; Interface properties; Reliability
1. Introduction According to the ITRS prediction, the feature size or physical channel length of MOS devices will soon enter the sub-10-nm range and the gate oxide thickness will be approaching the physical limit (0.7 nm) of silicon dioxide [1– 3]. Tremendous efforts would be needed to achieve this goal as we are pushing both the technological capability and the material properties to their ultimate limits [4 – 7]. To maintain a proper control of the channel potential, the gate oxide thickness needs to be reduced almost with the same factor of the channel length. However, one cannot make oxide layers thinner than 0.7 nm for the future MOS applications. Also, regarding the leakage current, it is still too large even for 1-nm-thick gate oxide [4,5]. Hence, if MOS structure is still to be used for future technology nodes, then obtaining a physically thicker high-j dielectric is a necessary problem. * Corresponding author. Fax: +852 2788 7791. E-mail address: [email protected] (H. Wong). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.123
All the transition metal (TM) oxides have values of dielectric constants higher than SiO2. Unfortunately, most of the high-j materials are ionic metal oxides. This fundamental fact results in several undesirable instability issues when interfacing TM oxides with silicon and with the CMOS processes. To be good substitutes for SiO2, the high-j materials must have several additional features besides the high-j value. Hafnium oxide (hafnia) has shown much promise in overall material properties [8,9]. Although a lot of investigations on the material properties and applications of HfO2 have been conducted recently, the interface physics and properties of HfO2/Si and their effect on the electrical properties are still not very clear [10 – 12]. This work aims to study the interfacial properties and their effects on the electrical properties. 2. Material properties of HfO2/Si interface The large leakage current in most of the high-j metal oxide/ Si structure can be partially attributed to the small conduction band offset energy with respect to the substrate. The small
H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
conduction band offset does not only result in large gate direct tunneling or Fowler – Nordheim (F-N) current but also gives rise to large hot-carrier emission into the gate insulator. There are some controversial experimental results on the band gap of high-j materials. Here we present a different approach in investigating the band gap of HfO2 from a systematic way. Theoretically, the top of the valence band of hafnium oxide or other TM metal oxides and silicon oxide are determined mainly by the unoccupied O 2pk states. The d valence electrons, loosely bounded, determine the lowest conduction band states of HfO2 [13]. This is the reason for the small band gap value as well as for the high-j value of the TM oxides. The different band gaps amongst different metal oxides can be attributed to the electronegativity differences (Pauling or its modified version, Phillips scale [14]) or the ionicity of the oxide bonds. It should be noted that the thermal stability is also governed by the ionicity of the chemical bonding. The dielectric constant of an insulator is governed by its energy gap and the ionicity of the chemical bonds. According to Phillips, the static dielectric constant of a heteropolar dielectric is [14]: i 2 h 2 j ¼ 1 þ ¯hxp = Eg0 þ C2 A ð1Þ where ¯hxp is the plasma energy; A is the number of order unity; E g0 is the symmetric part and C is the anti-symmetric part of the potential energy characterizing the bond covalency and the electronegativity difference of the bond, respectively. Based on this theory, a clear separation of the homopolar (covalence) and the heteropolar dielectrics in the j – E g relationship can be found which is shown in Fig. 1. Data of other materials are taken from [8]. A very good linear plot is found for heteropolar dielectric (metal oxides). This indicates that the covalency component of the metal oxide bonds (then E g0) is so small that the 1/(j 1) values are almost linearly scaled by E g2 (i.e., CåE g). For homopolar dielectrics, it is anticipated that the energy gap and the dielectric constant are mainly governed by the covalence fraction of the oxide bonds
Fig. 1. Plot of the dielectric constant as a function of the band gap using Phillips’ theory [14]. The linear relationship of the metal oxides suggests that their dielectric constants and the band gaps are predominantly governed by the ionic component of the metal – oxygen bond.
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(i.e., E g0åE g). Hence, from that plot, we can confirm that for intrinsic bulk hafnia, the band gap and dielectric constant should be around 5.7 eV and 25, respectively; and the Hf – O bonding is predominantly ionic. However, the actual value of the band gap may be affected by the chemical composition and bonding. It was found in some ZrO2 samples that the gap value is 4.7 eV, which is much smaller than other reported figures [15]. Since the Hf – O bond is predominantly ionic as evident from the plot in Fig. 1, the value of the conduction band offset and valence band offset are more or less closer to each other. In the ionic oxides, the valence band moves to the lower energy end. Hence, in the metal oxide/Si system, the conduction band offset is much smaller than the valence band offset. This effect together with the small band gap make the conduction band offset very small. TiO2 is more ionic (with more negative dstate energy) and the conduction band offset is only 0.1 eV. The electronic barrier for ZrO2/Si is also low, just about 0.8– 1.4 eV. Although hafnium oxide may have the same average electronic structure because they belong to the same group, it has a relative large conduction band offset energy of about 1.3– 1.5 eV and a valence band offset of about 3.3 eV because HfO2 is less ionic (with less negative atomic d-state energy) when compared to TiO2 and ZrO2. Robertson estimated theoretically the conduction band offset at the Si/HfO2 interface as 1.5 eV [16]. This value is less than half of the Si/SiO2 value. The band offset energies can be influenced by the interface bonding structure and experimental measurement techniques. Most high-j materials (transition metal oxides) are not stable when interfacing with Si. Silicide or silicates can be formed [17]. The HfO2/Si interface is stable with respect to the formation of silicide and the HfO2/SiO2 interface is marginally unstable with respect to the formation of silicate, which is more stable [17,18]. As the metal oxides are more ionic and without fixed coordination, the interface bonding turns out to be more unstable when compared to that of silicon oxide. The bonding structure at the metal oxide/Si interface is much more complicated than the SiO2/Si structure because of the presence of one more element. This element can form bonds with both silicon and oxygen. The bonding of these Si surface-dangling bonds with the metal oxide will affect the amount of interface states. It was found with XPS studies that the HfO2 film prepared by using direct sputtering is thermally unstable at post-metallization annealing temperature (> 500 -C) [9]. The HfO2 film is decomposed and some oxygen atoms are released upon the rapid thermal annealing in nitrogen. The interface structure of HfO2/Si was investigated using X-ray photoelectron spectroscopy (XPS) [10]. Fig. 2 shows the XPS Hf 4f features at the HfO2/Si interface with and without thermal annealing. The samples were prepared by direct sputtering of pure Hf in oxygen ambient. Details of the sample prepared can be found in Ref. [10] and the XPS measurements were conducted with a Physical Electronics PHI 5600 with a monochromatic Al K a X-ray source which was used and the excitation energy was 1486.6 eV. The silicide bonding (Hf –Si) with an Hf 4f doublet in the range of 14.3– 16.2 eV is obvious for all traces. It was proposed that the interface silicate layer could be a random mixture of Hf –Si (silicide), HfO4
H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
Bulk HfO2 Hf-O, Si-O
Transition Silicate Layer
Si-O, Hf-Si
As-deposited
Hf-Si
Si substrate
Annealed 12
14
16
18
20
22
24
Binding Energy (eV) Fig. 2. Hf 4f core level X-ray photoemission spectra of the interface silicate layer close to the Si substrate. The sample was annealed at 700 -C in oxygen ambient for 10 min. The spectrum of as-deposited interface silicates layer is also shown for comparison. The Hf – Si peak (14 – 16 eV) is found for all samples. The inset shows the proposed structure of the transition silicate layer.
(hafnium oxide) and SiO4 (silicon oxide) tetrahedral, and excess Hf and Si atoms [10]. Particularly for samples annealed at high temperature (> 900 -C), phase separation effects will lead to this random mixture interface. It was found by XPS that at the bottom of the silicate/Si interface, the Si 2p spectra are a mixture of the separated Si and HfSi2 phases. As depicted in the inset of Fig. 2, in the third layer (just above the silicon substrate), both Hf and O content reduce to minimum and the hafnium atoms mainly exist in the form of silicide (Hf – Si) bonds. The effect of post-deposition annealing in the oxygen ambient was also studied. Oxygen can diffuse very fast through the HfO2 film to react with Si and Hf – Si at the interface. However, at the location very close to the Si substrate, the silicide bond is still notable due to two reasons. The oxidation of Hf – Si at the interface involves structure reconfiguration and requires a large energy. In addition, the substrate has a large amount of Si to react with the incoming oxygen atoms. Theoretically, the dangling bonds (DBs) on Si surface can be terminated with either excess oxygen or excess metal atoms (Fig. 3). In the excess oxygen case, the interfacial Si DBs are empty and are in the form of Si+ whereas the metal-terminated Si DBs are filled and are in the form of Si [16]. The oxygenterminated interface poses fourfold coordinated oxygen atoms and would contribute to the insulating property of the oxide films. When the Si DBs are metal terminated, silicide bonds are formed. In this case, the transition layer must be thicker. For the oxide to appear as a bulk insulating layer (with band gap as wide as the bulk material), at least two rows of oxygen atoms are required. The existence of Hf – Si can also be used explain
3. Electrical properties related to the interface silicate layer It was often reported that the leakage current which is the prime issue in ultra-thin oxide may be reduced with physically thicker high-j dielectric from the EOT point of view [8]. However, the current transport characteristics in high-j oxides are much poor than SiO2 when the comparison is based on the same physical thickness. A typical current– voltage (I –V) plot (dots) for the Al/HfO2/Si capacitors is plotted in Fig. 4. It was found in the low-voltage range that the I – V dependence is almost linear. This part of the experimental diagram can be
103
1
10
n-Si ( ρs = 20 Ω-cm) / HfO2 / Al Potential energy (eV)
Intensity (Arb. Unit)
Annealed
the Fermi-level pinning effects in poly-Si gate MOS transistors [19]. The formation of the silicate structure will help in reducing the amount of interface states [10]. In the silicate structure, the large intrinsic lattice mismatch produces a reduction of the effective coordination of both oxygen and the metal atoms and hence has fewer interface states (as compared to the metal oxide/Si interface). Unfortunately, silicide bonds are byproducts in the reaction of the metal oxide with the Si substrate to form a silicate layer. The existence of silicide bonds should be a major reason for the observed high interface trap density of the HfO2/Si structures [10,20]. The Hf –Si bonds are polar electron pair bonds where the Si atoms are negatively charged and the bond length is much larger. As the top of valence band of silicate is still determined by the d state electron of Hf, the band gap and the band offset should not be changed by the formation of the interface silicate layers; only the density of state increases. However, the effective barrier will be reduced in the hafnia/silicate structure because of the electric field distribution. Since silicate has much lower j value (¨10) [18] than the bulk hafnium oxide, the electric field will be largely distributed in the low-j region according to the Gauss’s law (i.e., E low-j /E high-j = j high-j / j low-j ) and this results in the interface barrier lowering. This effect explains the different barrier heights obtained from electrical measurements.
Current density (μA/cm2)
194
4
n-Silicon
10
HfO2
Fermi level
Vg = 6.0 V
-4
ng eli n nn uctio u T nd co
-6
Vg = 1.0 V
-5
0 5 10 15 Vg = 4.0 V Position (nm)
tion: condu6c Ohmic 0 Ω-cm 1 x .5 4 ρ = -3
Al gate
0 -2
-10
-1
SiO2
2
Total oxide thickness = 10 nm SiO2 layer thickness = 3.44 nm f 1 = 0.490; KHfO = 25
ox
10
2
SiO2 layer barrier = 3.12 eV HfO2 barrier = 1.7 eV
10-5 0
1
2
3
4
5
6
7
Gate voltage (V) Fig. 3. Schematic diagram showing the O-terminated and Hf-terminated (100) Si/HfO2 interfaces.
H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
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Fig. 4. Typical current – voltage characteristics of a Al/HfO2/Si capacitor showing the effect of interface layer.
very well fitted by an ohmic curve corresponding to a rather high resistivity q ox (see the dashed line on the figure). The physical origin of such a behavior should be the trap-assisted leakage current through the hafnium oxide films. It is known that the as-deposited hafnium oxide films are non-stoichiometric. Therefore, they should contain high amounts of oxygen vacancies, which could trap electrons and assist in the current conduction [21]. Grain boundary conduction due to the nanocrystallites in the dielectric film could be another mechanism [22]. This behavior shifts quite abruptly to an almost exponential dependence in the higher voltage range, suggesting the fact that a tunneling mechanism is suddenly turned on. This I –V diagram can be explained with a two-layer dielectric model. As seen in the band diagrams computed in this model and presented in the inset of Fig. 4, for low gate voltages, the electrons in the interface layer see a rather thick composite barrier. Consequently, the tunneling current is almost entirely negligible. The very good fitting in the highvoltage regime (see the continuous curve in Fig. 4) confirms the double-layer structure of the as-deposited HfO2 on a silicon substrate [10,15,16]. Other conduction mechanisms may still exist in hafnium oxide. Since the leakage current and the
Breakdown Field (MV/cm)
16 14
Heteropolar Dielectrics Homopolar Dielectrics
SiO2
12 Si3N4
10 8 6
HfO2
Al2O3
SrTiO3
4
ZrO2
2
Ta2O5
TiO2
0
2
4
6
8
10
emission rate are strongly governed by the temperature and by the electric filed [9], it is reasonable to anticipate that phononassisted tunneling will participate in the current conduction. The channel carriers can be polarized and the optical phonons can be produced [23]. The phonons interact with the electrons injected into the localized states of the dielectric and assist the electrons in the tunneling process. High-j materials are often found to have low breakdown fields when compared to silicon oxide [24,25]. In high-j metal oxides, the local electric field is substantially larger than the applied electric field because of the polarization effect. This polarization effect is directly proportional to the dielectric constant. The large local field distorts the molecular bonds and makes them more susceptible to breakage. Again, the reported breakdown voltages for HfO2 were quite different [25]. They may be affected by several different factors. We found that the breakdown voltage also correlates very well with the band-gap energy if we separate the homopolar and heteropolar materials (see Fig. 5). From this plot, it can be inferred that the intrinsic breakdown voltage of HfO2 is about 4.3 MV/cm, which is more reliable because we also take the breakdown voltages of other metal oxides into account. The polarization effect in the silicate layer is governed by the bonding structure. If the silicate is a random bonding of the Hf, Si and O atoms, then the polarization effect should be smaller than the bulk HfO2 and the silicate material should have a larger breakdown voltage. However, the actual breakdown mechanism is more complicated since it involves two layers with different j values. For high-j/low-j stacked dielectric film structure, the applied electric field will be largely distributed in the low-j region. In addition, the critical defect density for causing the low-j layer to break down is much smaller as it is much thinner than the bulk high-j layer. As a result, the first low-voltage breakdown (soft breakdown) happens in the low-j layer and the hard breakdown voltage of the whole dielectric [25] is lowered. 4. Conclusions
Bandgap (eV) Fig. 5. The intrinsic breakdown voltage is governed by the band gap energy. The sources of experimental data are from McPherson [24] and references therein.
Although HfO2 was considered as the most promising highj material for replacing the conventional SiO2 gate dielectric, it still gives rise to several reliability problems which greatly
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H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
deteriorates the performances of the MOS devices. These disadvantages are not easy to solve because they are governed by fundamental chemical and material properties of transition metal oxides. The low-energy d-states limit the band gap of the HfO2. Combining from the d-state electrons, the bond will be more or less ionic and requires less energy for oxidation. The ionicity of the HfO2 is confirmed with a plot for the j-band gap relationship for several different high-j materials. As Hf atoms can also react with the substrate Si atoms at low energy, they produce silicate and silicide bonds. The interfacial metallic silicide bonds, working as interface traps, can also lower the conduction band offset energy. The ionic or polarized metaloxygen bonds is also the reason for the high-j value and the existence of soft optical phonons, which further induce a large leakage current and channel mobility degradation. These properties constitute the fundamental limitation for the application of hafnia as the ultimate MOS gate dielectric material. According to the fundamental principles, forming the interface silicate layer should not change the interface barrier height but may increase the intrinsic breakdown voltage. However, the significant difference in j values between the bulk hafnium oxide and the silicate layer would in effect reduce the interface barrier height and the breakdown voltage. Acknowledgment The work described in this paper was support by a UGC Competitive Earmarked Research Grant from the Research Grants Council of Hong Kong SAR [Project No. CityU 1167/ 03E]. References [1] International Technology Roadmap for Semiconductors, 2003 Edition, Semiconductor Industry Association (SIA), Austin, Texas: SEMATECH, USA, 2706 Montopolis Drive, Austin, TX 78741;http://www.itrs.net/ntrs/ publntrs.nsf.
[2] H. Wong, Proc. 2004 Int’l Conf. on Communications, Devices, and Intelligent Systems, 2004 (Jan.), pp. 393 – 396 (Kolkata). [3] H. Wong, H. Iwai, Physics World, 18 (9) (2005) 40. [4] D. Misra, H. Iwai, H. Wong, Electrochem. Soc. Interface 14 (2) (2005) 30. [5] H. Wong, V.A. Gritsenko, Microelectron. Reliab. 42 (2002) 597. [6] H. Wong, M.C. Poon, Y. Gao, T.C.W. Kok, J. Electrochem. Soc. 148 (2001) G275. [7] H. Wong, M.C. Poon, C.W. Kok, P.J. Chan, V.A. Gritsenko, IEEE Trans. Electron Devices 50 (2003) 1941. [8] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [9] N. Zhan, M.C. Poon, C.W. Kok, K.L. Ng, H. Wong, J. Electrochem. Soc. 150 (2003) F200. [10] H. Wong, K.L. Ng, N. Zhan, M.C. Poon, C.W. Kok, J. Vac. Sci. Technol., B 22 (2004) 1094. [11] H. Wong, Proc. Int’l Conf. Integrated Circuits and Solid-State Technol., Beijing, IEEE, Piscataway, 2004, p. 378. [12] H. Wong, N. Zhan, K.L. Ng, M.C. Poon, C.W. Kok, Thin Solid Films 462 (2004) 96. [13] N. Ikarashi, K. Manabe, J. Appl. Phys. 94 (2003) 480. [14] J.C. Phillips, Phys. Rev. Lett. 20 (1968) 550. [15] V. Gritsenko, D. Gritsenko, S. Shaimeev, V. Aliev, K. Nasyrov, S. Erenburg, V. Tapilin, H. Wong, M.C. Poon, J.H. Lee, J.-W. Lee, C.W. Kim, Microelectron. Eng., 81 (2005) 524. [16] J. Robertson, P.W. Peacock, Phys. Status Solidi, B Basic Res. 241 (2004) 2236. [17] M. Gutowski, J.E. Jaffe, C.-L. Liu, M. Stoker, R.I. Hegde, R.S. Rai, P.J. Tobin, Appl. Phys. Lett. 80 (2002) 1897. [18] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 87 (2000) 484. [19] C. Hobbs, L. Fonseca, V. Dhandapani, S. Samavedam, B. Taylor, J. Grant, L. Dip, D. Triyoso, R. Hegde, D. Gilmer, R. Garcla, D. Roan, L. Lovejoy, R. Rai, L. Hebert, H. Tseng, B. White, P. Tobin, Symp. VLSI Technol. Sympo. Dig. Technical Papers, 2003, p. 9. [20] A. Kawamoto, K. Cho, P. Griffin, R. Dutton, J. Appl. Phys. 90 (2001) 1333. [21] K.L. Ng, N. Zhan, C.W. Kok, M.C. Poon, H. Wong, Microelectron. Reliab. 43 (2003) 1289. [22] H. Wong, P.G. Han, M.C. Poon, J. Korean Phys. Soc. 35 (1999) S196. [23] M. Fischetti, J. Appl. Phys. 89 (2001) 1232. [24] J. McPherson, J. Kim, A. Shanware, H. Mogul, J. Rodriguez, Tech. Digest-IEEE IEDM, p. 633. [25] N. Zhan, M.C. Poon, H. Wong, K.L. Ng, C.W. Kok, Microelectron. J. 36 (2005) 29.
Material properties of interfacial silicate layer and its influence on the electrical characteristics of MOS devices using hafnia as the gate dielectric Hei Wong a,*, B. Sen a, V. Filip a, M.C. Poon b b
a Department of Electronic Engineering, City University, Tat Chee Avenue, Kowloon, Hong Kong, China Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
Available online 24 October 2005
Abstract This work deals with some fundamental material properties of the hafnia or hafnium oxide and the silicate layer at the Si/HfO2 interface. It is realized that one of the fundamental features of the HfO2 film is that its lowest conduction band states are composed mainly by d states instead of s and p states as is in the case for SiO2. There is only a very small covalent component contributed from Hf d and O 2p states and the Hf – O bond is predominantly ionic. This is the reason for the small band gap and small conduction band offset energy. This feature also gives rise to thermal instabilities and the formation of an interfacial silicate layer. Although the formation of the silicate structure will help to build a more stable interface, long metallic silicide bonds will result as a by-product when the metal oxide reacts with the Si substrate to form this silicate layer. The existence of metallic silicide bonds is one of the major reasons for the observed high-interface trap density of the HfO2/Si structure. Because of the large difference in dielectric constant between the bulk hafnia and interfacial silicate, the electric field will be distributed mainly across the low-j interface layer, which in effect reduces the interface barrier height for electron tunneling injection and the effective breakdown voltage of the lowj layer. D 2005 Elsevier B.V. All rights reserved. Keywords: Dielectric film; Hafnium oxide; Interface properties; Reliability
1. Introduction According to the ITRS prediction, the feature size or physical channel length of MOS devices will soon enter the sub-10-nm range and the gate oxide thickness will be approaching the physical limit (0.7 nm) of silicon dioxide [1– 3]. Tremendous efforts would be needed to achieve this goal as we are pushing both the technological capability and the material properties to their ultimate limits [4 – 7]. To maintain a proper control of the channel potential, the gate oxide thickness needs to be reduced almost with the same factor of the channel length. However, one cannot make oxide layers thinner than 0.7 nm for the future MOS applications. Also, regarding the leakage current, it is still too large even for 1-nm-thick gate oxide [4,5]. Hence, if MOS structure is still to be used for future technology nodes, then obtaining a physically thicker high-j dielectric is a necessary problem. * Corresponding author. Fax: +852 2788 7791. E-mail address: [email protected] (H. Wong). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.123
All the transition metal (TM) oxides have values of dielectric constants higher than SiO2. Unfortunately, most of the high-j materials are ionic metal oxides. This fundamental fact results in several undesirable instability issues when interfacing TM oxides with silicon and with the CMOS processes. To be good substitutes for SiO2, the high-j materials must have several additional features besides the high-j value. Hafnium oxide (hafnia) has shown much promise in overall material properties [8,9]. Although a lot of investigations on the material properties and applications of HfO2 have been conducted recently, the interface physics and properties of HfO2/Si and their effect on the electrical properties are still not very clear [10 – 12]. This work aims to study the interfacial properties and their effects on the electrical properties. 2. Material properties of HfO2/Si interface The large leakage current in most of the high-j metal oxide/ Si structure can be partially attributed to the small conduction band offset energy with respect to the substrate. The small
H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
conduction band offset does not only result in large gate direct tunneling or Fowler – Nordheim (F-N) current but also gives rise to large hot-carrier emission into the gate insulator. There are some controversial experimental results on the band gap of high-j materials. Here we present a different approach in investigating the band gap of HfO2 from a systematic way. Theoretically, the top of the valence band of hafnium oxide or other TM metal oxides and silicon oxide are determined mainly by the unoccupied O 2pk states. The d valence electrons, loosely bounded, determine the lowest conduction band states of HfO2 [13]. This is the reason for the small band gap value as well as for the high-j value of the TM oxides. The different band gaps amongst different metal oxides can be attributed to the electronegativity differences (Pauling or its modified version, Phillips scale [14]) or the ionicity of the oxide bonds. It should be noted that the thermal stability is also governed by the ionicity of the chemical bonding. The dielectric constant of an insulator is governed by its energy gap and the ionicity of the chemical bonds. According to Phillips, the static dielectric constant of a heteropolar dielectric is [14]: i 2 h 2 j ¼ 1 þ ¯hxp = Eg0 þ C2 A ð1Þ where ¯hxp is the plasma energy; A is the number of order unity; E g0 is the symmetric part and C is the anti-symmetric part of the potential energy characterizing the bond covalency and the electronegativity difference of the bond, respectively. Based on this theory, a clear separation of the homopolar (covalence) and the heteropolar dielectrics in the j – E g relationship can be found which is shown in Fig. 1. Data of other materials are taken from [8]. A very good linear plot is found for heteropolar dielectric (metal oxides). This indicates that the covalency component of the metal oxide bonds (then E g0) is so small that the 1/(j 1) values are almost linearly scaled by E g2 (i.e., CåE g). For homopolar dielectrics, it is anticipated that the energy gap and the dielectric constant are mainly governed by the covalence fraction of the oxide bonds
Fig. 1. Plot of the dielectric constant as a function of the band gap using Phillips’ theory [14]. The linear relationship of the metal oxides suggests that their dielectric constants and the band gaps are predominantly governed by the ionic component of the metal – oxygen bond.
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(i.e., E g0åE g). Hence, from that plot, we can confirm that for intrinsic bulk hafnia, the band gap and dielectric constant should be around 5.7 eV and 25, respectively; and the Hf – O bonding is predominantly ionic. However, the actual value of the band gap may be affected by the chemical composition and bonding. It was found in some ZrO2 samples that the gap value is 4.7 eV, which is much smaller than other reported figures [15]. Since the Hf – O bond is predominantly ionic as evident from the plot in Fig. 1, the value of the conduction band offset and valence band offset are more or less closer to each other. In the ionic oxides, the valence band moves to the lower energy end. Hence, in the metal oxide/Si system, the conduction band offset is much smaller than the valence band offset. This effect together with the small band gap make the conduction band offset very small. TiO2 is more ionic (with more negative dstate energy) and the conduction band offset is only 0.1 eV. The electronic barrier for ZrO2/Si is also low, just about 0.8– 1.4 eV. Although hafnium oxide may have the same average electronic structure because they belong to the same group, it has a relative large conduction band offset energy of about 1.3– 1.5 eV and a valence band offset of about 3.3 eV because HfO2 is less ionic (with less negative atomic d-state energy) when compared to TiO2 and ZrO2. Robertson estimated theoretically the conduction band offset at the Si/HfO2 interface as 1.5 eV [16]. This value is less than half of the Si/SiO2 value. The band offset energies can be influenced by the interface bonding structure and experimental measurement techniques. Most high-j materials (transition metal oxides) are not stable when interfacing with Si. Silicide or silicates can be formed [17]. The HfO2/Si interface is stable with respect to the formation of silicide and the HfO2/SiO2 interface is marginally unstable with respect to the formation of silicate, which is more stable [17,18]. As the metal oxides are more ionic and without fixed coordination, the interface bonding turns out to be more unstable when compared to that of silicon oxide. The bonding structure at the metal oxide/Si interface is much more complicated than the SiO2/Si structure because of the presence of one more element. This element can form bonds with both silicon and oxygen. The bonding of these Si surface-dangling bonds with the metal oxide will affect the amount of interface states. It was found with XPS studies that the HfO2 film prepared by using direct sputtering is thermally unstable at post-metallization annealing temperature (> 500 -C) [9]. The HfO2 film is decomposed and some oxygen atoms are released upon the rapid thermal annealing in nitrogen. The interface structure of HfO2/Si was investigated using X-ray photoelectron spectroscopy (XPS) [10]. Fig. 2 shows the XPS Hf 4f features at the HfO2/Si interface with and without thermal annealing. The samples were prepared by direct sputtering of pure Hf in oxygen ambient. Details of the sample prepared can be found in Ref. [10] and the XPS measurements were conducted with a Physical Electronics PHI 5600 with a monochromatic Al K a X-ray source which was used and the excitation energy was 1486.6 eV. The silicide bonding (Hf –Si) with an Hf 4f doublet in the range of 14.3– 16.2 eV is obvious for all traces. It was proposed that the interface silicate layer could be a random mixture of Hf –Si (silicide), HfO4
H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
Bulk HfO2 Hf-O, Si-O
Transition Silicate Layer
Si-O, Hf-Si
As-deposited
Hf-Si
Si substrate
Annealed 12
14
16
18
20
22
24
Binding Energy (eV) Fig. 2. Hf 4f core level X-ray photoemission spectra of the interface silicate layer close to the Si substrate. The sample was annealed at 700 -C in oxygen ambient for 10 min. The spectrum of as-deposited interface silicates layer is also shown for comparison. The Hf – Si peak (14 – 16 eV) is found for all samples. The inset shows the proposed structure of the transition silicate layer.
(hafnium oxide) and SiO4 (silicon oxide) tetrahedral, and excess Hf and Si atoms [10]. Particularly for samples annealed at high temperature (> 900 -C), phase separation effects will lead to this random mixture interface. It was found by XPS that at the bottom of the silicate/Si interface, the Si 2p spectra are a mixture of the separated Si and HfSi2 phases. As depicted in the inset of Fig. 2, in the third layer (just above the silicon substrate), both Hf and O content reduce to minimum and the hafnium atoms mainly exist in the form of silicide (Hf – Si) bonds. The effect of post-deposition annealing in the oxygen ambient was also studied. Oxygen can diffuse very fast through the HfO2 film to react with Si and Hf – Si at the interface. However, at the location very close to the Si substrate, the silicide bond is still notable due to two reasons. The oxidation of Hf – Si at the interface involves structure reconfiguration and requires a large energy. In addition, the substrate has a large amount of Si to react with the incoming oxygen atoms. Theoretically, the dangling bonds (DBs) on Si surface can be terminated with either excess oxygen or excess metal atoms (Fig. 3). In the excess oxygen case, the interfacial Si DBs are empty and are in the form of Si+ whereas the metal-terminated Si DBs are filled and are in the form of Si [16]. The oxygenterminated interface poses fourfold coordinated oxygen atoms and would contribute to the insulating property of the oxide films. When the Si DBs are metal terminated, silicide bonds are formed. In this case, the transition layer must be thicker. For the oxide to appear as a bulk insulating layer (with band gap as wide as the bulk material), at least two rows of oxygen atoms are required. The existence of Hf – Si can also be used explain
3. Electrical properties related to the interface silicate layer It was often reported that the leakage current which is the prime issue in ultra-thin oxide may be reduced with physically thicker high-j dielectric from the EOT point of view [8]. However, the current transport characteristics in high-j oxides are much poor than SiO2 when the comparison is based on the same physical thickness. A typical current– voltage (I –V) plot (dots) for the Al/HfO2/Si capacitors is plotted in Fig. 4. It was found in the low-voltage range that the I – V dependence is almost linear. This part of the experimental diagram can be
103
1
10
n-Si ( ρs = 20 Ω-cm) / HfO2 / Al Potential energy (eV)
Intensity (Arb. Unit)
Annealed
the Fermi-level pinning effects in poly-Si gate MOS transistors [19]. The formation of the silicate structure will help in reducing the amount of interface states [10]. In the silicate structure, the large intrinsic lattice mismatch produces a reduction of the effective coordination of both oxygen and the metal atoms and hence has fewer interface states (as compared to the metal oxide/Si interface). Unfortunately, silicide bonds are byproducts in the reaction of the metal oxide with the Si substrate to form a silicate layer. The existence of silicide bonds should be a major reason for the observed high interface trap density of the HfO2/Si structures [10,20]. The Hf –Si bonds are polar electron pair bonds where the Si atoms are negatively charged and the bond length is much larger. As the top of valence band of silicate is still determined by the d state electron of Hf, the band gap and the band offset should not be changed by the formation of the interface silicate layers; only the density of state increases. However, the effective barrier will be reduced in the hafnia/silicate structure because of the electric field distribution. Since silicate has much lower j value (¨10) [18] than the bulk hafnium oxide, the electric field will be largely distributed in the low-j region according to the Gauss’s law (i.e., E low-j /E high-j = j high-j / j low-j ) and this results in the interface barrier lowering. This effect explains the different barrier heights obtained from electrical measurements.
Current density (μA/cm2)
194
4
n-Silicon
10
HfO2
Fermi level
Vg = 6.0 V
-4
ng eli n nn uctio u T nd co
-6
Vg = 1.0 V
-5
0 5 10 15 Vg = 4.0 V Position (nm)
tion: condu6c Ohmic 0 Ω-cm 1 x .5 4 ρ = -3
Al gate
0 -2
-10
-1
SiO2
2
Total oxide thickness = 10 nm SiO2 layer thickness = 3.44 nm f 1 = 0.490; KHfO = 25
ox
10
2
SiO2 layer barrier = 3.12 eV HfO2 barrier = 1.7 eV
10-5 0
1
2
3
4
5
6
7
Gate voltage (V) Fig. 3. Schematic diagram showing the O-terminated and Hf-terminated (100) Si/HfO2 interfaces.
H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
195
Fig. 4. Typical current – voltage characteristics of a Al/HfO2/Si capacitor showing the effect of interface layer.
very well fitted by an ohmic curve corresponding to a rather high resistivity q ox (see the dashed line on the figure). The physical origin of such a behavior should be the trap-assisted leakage current through the hafnium oxide films. It is known that the as-deposited hafnium oxide films are non-stoichiometric. Therefore, they should contain high amounts of oxygen vacancies, which could trap electrons and assist in the current conduction [21]. Grain boundary conduction due to the nanocrystallites in the dielectric film could be another mechanism [22]. This behavior shifts quite abruptly to an almost exponential dependence in the higher voltage range, suggesting the fact that a tunneling mechanism is suddenly turned on. This I –V diagram can be explained with a two-layer dielectric model. As seen in the band diagrams computed in this model and presented in the inset of Fig. 4, for low gate voltages, the electrons in the interface layer see a rather thick composite barrier. Consequently, the tunneling current is almost entirely negligible. The very good fitting in the highvoltage regime (see the continuous curve in Fig. 4) confirms the double-layer structure of the as-deposited HfO2 on a silicon substrate [10,15,16]. Other conduction mechanisms may still exist in hafnium oxide. Since the leakage current and the
Breakdown Field (MV/cm)
16 14
Heteropolar Dielectrics Homopolar Dielectrics
SiO2
12 Si3N4
10 8 6
HfO2
Al2O3
SrTiO3
4
ZrO2
2
Ta2O5
TiO2
0
2
4
6
8
10
emission rate are strongly governed by the temperature and by the electric filed [9], it is reasonable to anticipate that phononassisted tunneling will participate in the current conduction. The channel carriers can be polarized and the optical phonons can be produced [23]. The phonons interact with the electrons injected into the localized states of the dielectric and assist the electrons in the tunneling process. High-j materials are often found to have low breakdown fields when compared to silicon oxide [24,25]. In high-j metal oxides, the local electric field is substantially larger than the applied electric field because of the polarization effect. This polarization effect is directly proportional to the dielectric constant. The large local field distorts the molecular bonds and makes them more susceptible to breakage. Again, the reported breakdown voltages for HfO2 were quite different [25]. They may be affected by several different factors. We found that the breakdown voltage also correlates very well with the band-gap energy if we separate the homopolar and heteropolar materials (see Fig. 5). From this plot, it can be inferred that the intrinsic breakdown voltage of HfO2 is about 4.3 MV/cm, which is more reliable because we also take the breakdown voltages of other metal oxides into account. The polarization effect in the silicate layer is governed by the bonding structure. If the silicate is a random bonding of the Hf, Si and O atoms, then the polarization effect should be smaller than the bulk HfO2 and the silicate material should have a larger breakdown voltage. However, the actual breakdown mechanism is more complicated since it involves two layers with different j values. For high-j/low-j stacked dielectric film structure, the applied electric field will be largely distributed in the low-j region. In addition, the critical defect density for causing the low-j layer to break down is much smaller as it is much thinner than the bulk high-j layer. As a result, the first low-voltage breakdown (soft breakdown) happens in the low-j layer and the hard breakdown voltage of the whole dielectric [25] is lowered. 4. Conclusions
Bandgap (eV) Fig. 5. The intrinsic breakdown voltage is governed by the band gap energy. The sources of experimental data are from McPherson [24] and references therein.
Although HfO2 was considered as the most promising highj material for replacing the conventional SiO2 gate dielectric, it still gives rise to several reliability problems which greatly
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H. Wong et al. / Thin Solid Films 504 (2006) 192 – 196
deteriorates the performances of the MOS devices. These disadvantages are not easy to solve because they are governed by fundamental chemical and material properties of transition metal oxides. The low-energy d-states limit the band gap of the HfO2. Combining from the d-state electrons, the bond will be more or less ionic and requires less energy for oxidation. The ionicity of the HfO2 is confirmed with a plot for the j-band gap relationship for several different high-j materials. As Hf atoms can also react with the substrate Si atoms at low energy, they produce silicate and silicide bonds. The interfacial metallic silicide bonds, working as interface traps, can also lower the conduction band offset energy. The ionic or polarized metaloxygen bonds is also the reason for the high-j value and the existence of soft optical phonons, which further induce a large leakage current and channel mobility degradation. These properties constitute the fundamental limitation for the application of hafnia as the ultimate MOS gate dielectric material. According to the fundamental principles, forming the interface silicate layer should not change the interface barrier height but may increase the intrinsic breakdown voltage. However, the significant difference in j values between the bulk hafnium oxide and the silicate layer would in effect reduce the interface barrier height and the breakdown voltage. Acknowledgment The work described in this paper was support by a UGC Competitive Earmarked Research Grant from the Research Grants Council of Hong Kong SAR [Project No. CityU 1167/ 03E]. References [1] International Technology Roadmap for Semiconductors, 2003 Edition, Semiconductor Industry Association (SIA), Austin, Texas: SEMATECH, USA, 2706 Montopolis Drive, Austin, TX 78741;http://www.itrs.net/ntrs/ publntrs.nsf.
[2] H. Wong, Proc. 2004 Int’l Conf. on Communications, Devices, and Intelligent Systems, 2004 (Jan.), pp. 393 – 396 (Kolkata). [3] H. Wong, H. Iwai, Physics World, 18 (9) (2005) 40. [4] D. Misra, H. Iwai, H. Wong, Electrochem. Soc. Interface 14 (2) (2005) 30. [5] H. Wong, V.A. Gritsenko, Microelectron. Reliab. 42 (2002) 597. [6] H. Wong, M.C. Poon, Y. Gao, T.C.W. Kok, J. Electrochem. Soc. 148 (2001) G275. [7] H. Wong, M.C. Poon, C.W. Kok, P.J. Chan, V.A. Gritsenko, IEEE Trans. Electron Devices 50 (2003) 1941. [8] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [9] N. Zhan, M.C. Poon, C.W. Kok, K.L. Ng, H. Wong, J. Electrochem. Soc. 150 (2003) F200. [10] H. Wong, K.L. Ng, N. Zhan, M.C. Poon, C.W. Kok, J. Vac. Sci. Technol., B 22 (2004) 1094. [11] H. Wong, Proc. Int’l Conf. Integrated Circuits and Solid-State Technol., Beijing, IEEE, Piscataway, 2004, p. 378. [12] H. Wong, N. Zhan, K.L. Ng, M.C. Poon, C.W. Kok, Thin Solid Films 462 (2004) 96. [13] N. Ikarashi, K. Manabe, J. Appl. Phys. 94 (2003) 480. [14] J.C. Phillips, Phys. Rev. Lett. 20 (1968) 550. [15] V. Gritsenko, D. Gritsenko, S. Shaimeev, V. Aliev, K. Nasyrov, S. Erenburg, V. Tapilin, H. Wong, M.C. Poon, J.H. Lee, J.-W. Lee, C.W. Kim, Microelectron. Eng., 81 (2005) 524. [16] J. Robertson, P.W. Peacock, Phys. Status Solidi, B Basic Res. 241 (2004) 2236. [17] M. Gutowski, J.E. Jaffe, C.-L. Liu, M. Stoker, R.I. Hegde, R.S. Rai, P.J. Tobin, Appl. Phys. Lett. 80 (2002) 1897. [18] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 87 (2000) 484. [19] C. Hobbs, L. Fonseca, V. Dhandapani, S. Samavedam, B. Taylor, J. Grant, L. Dip, D. Triyoso, R. Hegde, D. Gilmer, R. Garcla, D. Roan, L. Lovejoy, R. Rai, L. Hebert, H. Tseng, B. White, P. Tobin, Symp. VLSI Technol. Sympo. Dig. Technical Papers, 2003, p. 9. [20] A. Kawamoto, K. Cho, P. Griffin, R. Dutton, J. Appl. Phys. 90 (2001) 1333. [21] K.L. Ng, N. Zhan, C.W. Kok, M.C. Poon, H. Wong, Microelectron. Reliab. 43 (2003) 1289. [22] H. Wong, P.G. Han, M.C. Poon, J. Korean Phys. Soc. 35 (1999) S196. [23] M. Fischetti, J. Appl. Phys. 89 (2001) 1232. [24] J. McPherson, J. Kim, A. Shanware, H. Mogul, J. Rodriguez, Tech. Digest-IEEE IEDM, p. 633. [25] N. Zhan, M.C. Poon, H. Wong, K.L. Ng, C.W. Kok, Microelectron. J. 36 (2005) 29.