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Si surface
Applied Surface Science 235 (2004) 21–25
X-ray photoelectron spectroscopy characterisation of high-k dielectric Al2O3 and HfO2 layers deposited on SiO2/Si surface R.G. Vitcheva, J.J. Pireauxa,*, T. Conardb, H. Benderb, J. Wolstenholmec, Chr. Defranouxd a Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), Faculte´s Universitaires Notre-Dame de la Paix (FUNDP), Rue de Bruxelles 61, B-5000 Namur, Belgium b IMEC, Kapeldreef 75, B-3001 Leuven, Belgium c Thermo Electron, Imberhorne Lane, RH19 1UB East Grinstead, West Sussex, UK d SOPRA, 26 rue Pierre-Joigneaux, F-92270 Bois-Colombes, France
Available online 6 July 2004
Abstract Ultra thin Al2O3 and HfO2 films (up to 6 nm) were deposited on SiO2/Si wafers by atomic layer chemical vapour deposition (ALCVDTM,1) and studied by exsitu X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). The thickness of these high-k layers (including the thickness of the interfacial SiO2 film) were determined by XPS and compared to the ellipsometry values. No silicate was detected at the interface. The XPS results for the bandgap (6.7 eV for Al2O3 and 5.25 eV for HfO2) are in good agreement with the SE measured values (6.26 eV for Al2O3 and 5.78 eV for HfO2). The growth rate of the high-k oxide layers and the effective attenuation length (EAL) of Si2p electrons in them were estimated; the measured EAL (2.75 nm in Al2O3 and 2.3 nm in HfO2) are in fair agreement with the NIST database values. # 2004 Elsevier B.V. All rights reserved. PACS: 77.55.þf; 79.60.j Keywords: High-k; XPS
1. Introduction Scaling down of CMOS devices results in a decrease of the gate dielectric thickness. This presents a considerable challenge to the microelectronics as the SiO2 gate oxide thickness reaches its physical limits leading to high leakage currents and reduced relia-
* Corresponding author. Tel.: þ32 81 72 4606; fax: þ32 81 72 4595. E-mail address: [email protected] (J.J. Pireaux). 1 Trademark of ASM International NV.
bility. Using materials with dielectric constant higher than that of SiO2 will allow to keep the gate capacitance with a physically thicker gate dielectric layer, thus avoiding the problems mentioned above. Al2O3 and HfO2 are some of the main candidates to replace SiO2 in the future CMOS devices. Characterisation of such thin dielectric layers is quite a difficult task. Xray photoelectron spectroscopy (XPS) is a convenient non-destructive surface sensitive technique for such thin films. It allows to determine the composition and thickness of the thin films, the bandgap of the material and the chemical state of constituent elements. The aim of this work was to characterise Al2O3 and HfO2
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.135
22
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25
layers with different thickness up to 6 nm, deposited on SiO2/Si wafers by using XPS. The results were compared to those obtained by ellipsometry.
2. Experimental The 1 nm thin SiO2 layer was grown in situ by rapid thermal oxidation (RTO) on silicon wafers subjected to a standard cleaning procedure. Al2O3 layers were deposited by atomic layer chemical vapour deposition (ALCVDTM,1) on top of this silicon oxide film using alternating surface saturation reactions of Al(CH3)3 (TMA) and H2O at 300 8C. HfO2 layers were deposited by ALCVD with alternating pulses of HfCl4 and H2O. Measurements were performed on a HP5950 ESCA spectrometer (FUNDP) (Al Ka monochromator, electron exit angle y ¼ 51:5 3 ) and on an SSX 100 ESCA equipment (FUNDP and IMEC) (Al Ka monochromator, emission angles 08 and 558, acceptance angle 308). The experimental work at Thermo VG Scientific was performed using Theta Probe, a small spot XPS instrument with a microfocusing Al Ka monochromator and the ability to collect angle resolved XPS data in parallel without the need to tilt the sample. Thus, the analysis area and analysis position remain constant during the measurement. Data are collected over the angular range 238–838, divided typically into 16 angular channels during the spectrum acquisition process, each channel covering an angular range of 3.758. Ellipsometry measurements were performed with a variable angle ES4G ellipsometer in the 250– 850 nm range as well as with a SOPRA purged UV spectroscopic ellipsometer (SE) in the 137– 620 nm range.
3. Results and discussion 3.1. XPS layer thickness determination 3.1.1. Theoretical model For a sample consisting of a thin homogeneous layer of material A with thickness d on a substrate of material S, we can write the following expressions for the XPS signal intensities assuming exponential
attenuation in the overlayer [1]: IA IA1 ð1 expð d=ðlA;A cos yÞÞÞ ¼ expðd=ðlS;A cos yÞÞ IS IS1
(1)
where IA1 and IS1 are the signal intensities from thick layers of material A and S, lA;A and lS;A are the effective attenuation lengths in the overlayer of photoelectrons emitted from the overlayer and the substrate, respectively. y is the emission angle of photoelectrons measured towards the surface normal. Due to elastic scattering, the effective attenuation length l is known to depend not only on the material and photoelectron kinetic energy but also on the experimental geometry [2–4]. It was shown that for emission angles less than about 608 and for film thickness of practical relevance (such that the substrate signal is attenuated down to 10% of its initial value), l does not vary significantly with thickness and emission angle [2–4]. In that case the exponential attenuation approximation is valid and we can use an average practical effective attenuation length l. For the case of a SiO2 layer on top of a Si substrate where the kinetic energy of Si2p photoelectrons from oxidized and elemental Si differ by less than 4 eV (0.3%) lA;A ¼ lS;A ¼ l and ðIA =IA1 Þ IA d ¼ l cos y ln 1 þ ¼ l cos y ln 1 þ K ðIS =IS1 Þ IS (2) where A and S stand for the Si2p signal of SiO2 and Si, respectively, and l is the effective attenuation length of Si2p electrons in SiO2. Eq. (2) can be used to determine d from a single variable angle XPS measurement. It can also be implemented for buried SiO2 films since the overlayers attenuate the Si2p signal of Si and SiO2 equally. A linear fit through the origin of ln (1 þ K (IA/IS)) versus sec y was plotted to extract the d/l values from the angle resolved measurements . The data used for the fit was restricted to angles up to 608 (see above). The parameters K and l could be determined either theoretically or experimentally (K being the ratio of the intensities of the appropriate peaks from thick samples) [1,5]. In this work, K and l were chosen to be 1.82 nm and 3.44 nm, respectively. These values are close to the theoretically predicted ones [4–6].
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25
while the ratio of the signals from two adjacent layers i and i þ 1 is Ii Iiþ1
Ii1 ð1 expð di =lii cos yÞÞ 1 ð1 expð diþ1 =liþ1;iþ1 cos yÞÞ Iiþ1 " !# j¼i j¼i1 X X dj 1 dj exp cos y j¼1 liþ1;j l j¼1 ij
¼
(4)
Here lij denotes the effective attenuation length of electrons originating from layer i in layer j. Using a method of least squares fitting to these equations, the thickness of each of the layers can be determined. A software package developed by Thermo VG Scientific was used to calculate the thickness of the high-k layers and SiO2 interfacial layers. The input parameters are the materials density, bandgap, stoichiometry, kinetic energy and emission angles of photoelectrons. The l values are calculated in a way similar to that used in the NIST database [6]. 3.1.2. Results The XPS and SE measured thickness of Al2O3 layers deposited by ALCVD are plotted in Fig. 1 versus the deposition cycles. The precision of the SE thickness values is better than 0.1 nm . As far as the XPS measurements are concerned, their accuracy and precision are 5% and 0.5%, respectively. It is seen that the XPS thickness is all the time smaller than the one measured by SE. This could be due to the different properties of the thin films compared to the bulk material (e.g. different density) and to the presence of an adsorbed contamination layer. Further investigation is necessary to clarify that point. The XPS measured thickness indicates some incubation for small number of cycles as the measured thickness is
ALCVD Al2O3/SiO2/Si
6
Al2O3 SE Al2O3 ARXPS SiO2 ARXPS SiO2 XPS SiO2 XPS 2 angle
Thickness [nm]
5 4 3 2 1 0 0
10
20
30
40
50
No. of deposition cycles
Fig. 1. Thickness of the ALCVD Al2O3 layers deposited on 1 nm RTO SiO2 and of the SiO2 interfacial layer determined by XPS and spectroscopic ellipsometry. For data accuracy, see the text.
smaller than the expected one according to a linear growth model. The thickness of the interfacial SiO2 layers determined by XPS is also shown. It is seen that the results obtained by the three different equipment are centered around 1.2 nm and are with good consistency. This mean thickness value is somewhat higher than the nominal value (1.0 nm). The thickness of ALCVD HfO2 films deposited on 1 nm RTO SiO2 was also measured (Fig. 2). It is seen that the SE thickness values are always higher than those obtained by XPS. The reasons for that are similar to those for the Al2O3 case. The mean thickness of the interfacial SiO2 layers is about 1.23 nm HfO2 SE HfO2 ARXPS HfO2 XPS SiO2 ARXPS SiO2 XPS SiO2 XPS 2 angle
3.5 3.0 Thickness [nm]
If the attenuation length of the electrons from the substrate differs significantly from that of the electrons from the overlayer, the data should be fitted to Eq. (1). The model used can be generalized for n layers deposited on a substrate S. In that case the ratio of the signal from layer i versus the substrate signal can be written as follows: Ii Ii1 di ¼ 1 1 exp Is Is lii cos y " !# j¼n j¼i1 X X dj 1 dj (3)
exp cos y j¼1 lsj l j¼1 ij
23
2.5 2.0
ALCVD HfO2/SiO2/Si
1.5 1.0 0.5 0.0 0
10
20
30
40
50
No. of deposition cycles
Fig. 2. The same as in Fig. 1, but for ALCVD HfO2 layers.
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25
except for the case of 50 deposition cycles where it is slightly higher, 1.46 nm. The consistency of the SiO2 thickness values obtained by different XPS equipment is again very good. The composition of the interfacial layer is important for the application of the high-k films. For our studies, a SiO2 film was grown intentionally. A silicate may be formed depending on the initial SiO2 interfacial layer thickness, deposition conditions and subsequent heat treatment of the wafers. The chemical state of Si in the interfacial layer can be determined by the position of its XPS Si2p peak. The Si2p peak of Si in SiO2 is shifted by about 4 eV towards lower binding energy relative to the Si2p peak of elemental Si [1,7]. This shift is smaller for the silicates of interest in this work [1,7]. The shift of the Si2p peak obtained for both ALCVD Al2O3 and HfO2 cases was in the range 3.8– 4.0 eV. Therefore, we can conclude that no silicate was formed on the interface. 3.1.3. Effective attenuation length and growth rate If we have a set of layers with known different thickness deposited on a substrate, we can estimate the l by measuring the attenuation of the substrate signal passing through the layers. The main assumptions for this are exponential attenuation of the substrate signal IS passing through a homogeneous layer A with a thickness d and similar superficial carbon contamination for all samples: d IS ¼ IS1 exp ðl cos yÞ An example is presented in Fig. 3, in which the Al Ka excited Si2p signal from the Si substrate is plotted versus the thickness of the ALCVD deposited HfO2 films measured by ellipsometry. l for Si2p in the HfO2 films is extracted by fitting the Si2p signal with an exponential function. The obtained l is 2.3 nm, a value somewhat higher than that given by the NIST database (1.8 nm) [6]. One of the possible reasons for this could be some errors in the SE thickness determination and in XPS measurements due to the presence of adsorbed layers (hydrocarbons, water, etc.). Other reasons could include the lower density of HfO2 in the thin films compared to the bulk value due to island growth and/or rough layer and/or porous material. We can estimate the growth rate of ALCVD grown films from the same data under the same assumptions.
ALCVD HfO 2/SiO 2/Si 1000 Si2p intensity [a.u.]
24
100 Aexp(-d/ λ cos θ)
10
0
1
2 3 4 5 SE thickness HfO 2 [nm]
6
Fig. 3. Estimation of the effective attenuation length of Si2p photoelectrons in ALCVD HfO2 layers. For data accuracy, see the text.
Assuming d ¼ GN, where N is the number of deposition cycles, we can plot the Si2p signal intensity versus N, fit with an exponential function and calculate G. The l value of Si2p in the corresponding film is obtained from the NIST database. The growth rate of HfO2 on SiO2/Si was estimated to be 0.04 nm/cycle assuming the l value of 1.8 nm (NIST) and 0.05 nm/ cycle for l ¼ 2.3 nm. The effective attenuation length for Si2p photoelectrons in ALCVD deposited Al2O3 layers was determined by the same method to be 2.75 nm, a value slightly lower than that given by the NIST database (2.88 nm). The growth rate of ALCVD Al2O3 on SiO2/ Si was estimated to be 0.10 nm/cycle and 0.09 nm/ cycle assuming l to be 2.88 nm and 2.75 nm, respectively. 3.1.4. Bandgap The bandgap values of the thin high-k dielectric layers can be obtained from the XPS data by using the electron energy loss signal for the O1s peak [8]. The bandgap is determined as the energy separation between the peak energy and the threshold of the inelastic losses corresponding to band-to-band excitations. The onset of the energy loss spectrum is defined by linearly extrapolating the segment of maximum negative slope to the background level. This method was implemented successfully in this work for the case of SiO2, HfO2 and Al2O3 layers. The results are shown in Fig. 4. It is seen that for very thin
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25 9,0
Al2O3 Al2O3 HfO2 HfO2
8.9 eV (SiO2) [9]
8,5
Bandgap [eV]
8,0 7,5 7,0
6.7 eV (Al2O3) [10]
6,5 6,0
5.7 eV (HfO2) [9]
5,5 5,0 4,5 0
20
40
60
80
25
spectroscopic ellipsometry results revealed a systematic deviation probably due to film density being different from that of the bulk materials. The interfacial SiO2 layer thickness was shown to be somewhat larger than the nominal value. No silicate was detected at the interface. The effective attenuation lengths of Si2p photoelectrons in ALCVD HfO2 and Al2O3 layers were obtained. The results are in reasonable agreement with the NIST database predicted values. The bandgap of the thin films was measured in good agreement with the results cited in the literature.
100
No. of deposition cycles
Fig. 4. Bandgap values of the ALCVD Al2O3 and HfO2 layers measured by XPS. Open and solid symbols show the results obtained by the SSX and HP XPS equipment. Horizontal lines present the corresponding literature bandgap values. For data accuracy, see the text.
ALCVD HfO2 films, the bandgap values are close to that of SiO2 (8.9 eV) [9]. The onset of the losses due to band-to-band transitions in HfO2 is detectable for the samples with 10 and more deposition cycles. The HfO2 bandgap values are lower than the bulk value of 5.7 eV [9] with the mean of all measurements being 5.25 eV 0.2 eV. The mean value of the bandgap of ALCVD Al2O3 films is 6.7 eV 0.1 eV, smaller than that cited in Ref. [9] (8.7 eV), but identical to that determined in Ref. [10]. The good consistency of the data obtained by using different XPS equipment is also evident. The bandgap can also be obtained from the onset of the rise of the absorption coefficient in ellipsometry measurements. This requires the use of wavelengths in the vacuum UV range and relatively thick high-k layers. The method is described in detail elsewhere [11]. The corresponding values obtained for HfO2 (5.78 eV) and for Al2O3 (6.26 eV) are in good agreement with the XPS results.
4. Conclusions The thickness of ALCVD HfO2 and Al2O3 thin films was determined by XPS. Comparison with the
Acknowledgements The work was performed with the financial support of the EU-CUHKO project.
References [1] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis: vol. 1—Auger and X-ray Photoelectron Spectroscopy, Wiley, Chichester, 1990. [2] P.J. Cumpson, M.P. Seah, Surf. Interface Anal. 25 (1997) 430. [3] A. Jablonski, C.J. Powell, Surf. Sci. Rep. 47 (2002) 33. [4] C.J. Powell, A. Jablonski, J. Vac. Sci. Technol. A 19 (2001) 2604. [5] M.P. Seah, S.J. Spencer, Surf. Interface Anal. 33 (2002) 640. [6] NIST Electron Effective Attenuation Length Database, Version 1, NIST, Gaithersburg, MD, 2001. [7] J. Chastain (Ed.), Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Corporation, Eden Prairie, Minnesota, 1992. [8] S. Miyazaki, Appl. Surf. Sci. 190 (2002) 66. [9] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243. [10] H. Nohira, W. Tsai, W. Besling, E. Young, J. Petry, T. Conard, W. Vandervorst, S. De Gendt, M. Heyns, J. Maes, M. Tuominen, J. Non-cryst. Solids 303 (2002) 83. [11] P. Boher, C. Defranoux, S. Boultauld, J.P. Piel, Analytical Techniques for Semiconducting Materials and Process and The 202nd Meeting of The Electrochemical Society, in: B.O. Kolbesen, C. Claeys, P. Stallhofer, F. Tardiff, D.K. Schroder, T.J. Shaffner, M. Tajima, P. Rai-Choudhury (Eds.), Joint Proceedings of ALTECH 2003, Diagnostic Techniques for Semiconductor Materials and Devices VI, Salt lake City, Utah, ECS Proc. vol. 2003-03, p. 305.
X-ray photoelectron spectroscopy characterisation of high-k dielectric Al2O3 and HfO2 layers deposited on SiO2/Si surface R.G. Vitcheva, J.J. Pireauxa,*, T. Conardb, H. Benderb, J. Wolstenholmec, Chr. Defranouxd a Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), Faculte´s Universitaires Notre-Dame de la Paix (FUNDP), Rue de Bruxelles 61, B-5000 Namur, Belgium b IMEC, Kapeldreef 75, B-3001 Leuven, Belgium c Thermo Electron, Imberhorne Lane, RH19 1UB East Grinstead, West Sussex, UK d SOPRA, 26 rue Pierre-Joigneaux, F-92270 Bois-Colombes, France
Available online 6 July 2004
Abstract Ultra thin Al2O3 and HfO2 films (up to 6 nm) were deposited on SiO2/Si wafers by atomic layer chemical vapour deposition (ALCVDTM,1) and studied by exsitu X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). The thickness of these high-k layers (including the thickness of the interfacial SiO2 film) were determined by XPS and compared to the ellipsometry values. No silicate was detected at the interface. The XPS results for the bandgap (6.7 eV for Al2O3 and 5.25 eV for HfO2) are in good agreement with the SE measured values (6.26 eV for Al2O3 and 5.78 eV for HfO2). The growth rate of the high-k oxide layers and the effective attenuation length (EAL) of Si2p electrons in them were estimated; the measured EAL (2.75 nm in Al2O3 and 2.3 nm in HfO2) are in fair agreement with the NIST database values. # 2004 Elsevier B.V. All rights reserved. PACS: 77.55.þf; 79.60.j Keywords: High-k; XPS
1. Introduction Scaling down of CMOS devices results in a decrease of the gate dielectric thickness. This presents a considerable challenge to the microelectronics as the SiO2 gate oxide thickness reaches its physical limits leading to high leakage currents and reduced relia-
* Corresponding author. Tel.: þ32 81 72 4606; fax: þ32 81 72 4595. E-mail address: [email protected] (J.J. Pireaux). 1 Trademark of ASM International NV.
bility. Using materials with dielectric constant higher than that of SiO2 will allow to keep the gate capacitance with a physically thicker gate dielectric layer, thus avoiding the problems mentioned above. Al2O3 and HfO2 are some of the main candidates to replace SiO2 in the future CMOS devices. Characterisation of such thin dielectric layers is quite a difficult task. Xray photoelectron spectroscopy (XPS) is a convenient non-destructive surface sensitive technique for such thin films. It allows to determine the composition and thickness of the thin films, the bandgap of the material and the chemical state of constituent elements. The aim of this work was to characterise Al2O3 and HfO2
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.135
22
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25
layers with different thickness up to 6 nm, deposited on SiO2/Si wafers by using XPS. The results were compared to those obtained by ellipsometry.
2. Experimental The 1 nm thin SiO2 layer was grown in situ by rapid thermal oxidation (RTO) on silicon wafers subjected to a standard cleaning procedure. Al2O3 layers were deposited by atomic layer chemical vapour deposition (ALCVDTM,1) on top of this silicon oxide film using alternating surface saturation reactions of Al(CH3)3 (TMA) and H2O at 300 8C. HfO2 layers were deposited by ALCVD with alternating pulses of HfCl4 and H2O. Measurements were performed on a HP5950 ESCA spectrometer (FUNDP) (Al Ka monochromator, electron exit angle y ¼ 51:5 3 ) and on an SSX 100 ESCA equipment (FUNDP and IMEC) (Al Ka monochromator, emission angles 08 and 558, acceptance angle 308). The experimental work at Thermo VG Scientific was performed using Theta Probe, a small spot XPS instrument with a microfocusing Al Ka monochromator and the ability to collect angle resolved XPS data in parallel without the need to tilt the sample. Thus, the analysis area and analysis position remain constant during the measurement. Data are collected over the angular range 238–838, divided typically into 16 angular channels during the spectrum acquisition process, each channel covering an angular range of 3.758. Ellipsometry measurements were performed with a variable angle ES4G ellipsometer in the 250– 850 nm range as well as with a SOPRA purged UV spectroscopic ellipsometer (SE) in the 137– 620 nm range.
3. Results and discussion 3.1. XPS layer thickness determination 3.1.1. Theoretical model For a sample consisting of a thin homogeneous layer of material A with thickness d on a substrate of material S, we can write the following expressions for the XPS signal intensities assuming exponential
attenuation in the overlayer [1]: IA IA1 ð1 expð d=ðlA;A cos yÞÞÞ ¼ expðd=ðlS;A cos yÞÞ IS IS1
(1)
where IA1 and IS1 are the signal intensities from thick layers of material A and S, lA;A and lS;A are the effective attenuation lengths in the overlayer of photoelectrons emitted from the overlayer and the substrate, respectively. y is the emission angle of photoelectrons measured towards the surface normal. Due to elastic scattering, the effective attenuation length l is known to depend not only on the material and photoelectron kinetic energy but also on the experimental geometry [2–4]. It was shown that for emission angles less than about 608 and for film thickness of practical relevance (such that the substrate signal is attenuated down to 10% of its initial value), l does not vary significantly with thickness and emission angle [2–4]. In that case the exponential attenuation approximation is valid and we can use an average practical effective attenuation length l. For the case of a SiO2 layer on top of a Si substrate where the kinetic energy of Si2p photoelectrons from oxidized and elemental Si differ by less than 4 eV (0.3%) lA;A ¼ lS;A ¼ l and ðIA =IA1 Þ IA d ¼ l cos y ln 1 þ ¼ l cos y ln 1 þ K ðIS =IS1 Þ IS (2) where A and S stand for the Si2p signal of SiO2 and Si, respectively, and l is the effective attenuation length of Si2p electrons in SiO2. Eq. (2) can be used to determine d from a single variable angle XPS measurement. It can also be implemented for buried SiO2 films since the overlayers attenuate the Si2p signal of Si and SiO2 equally. A linear fit through the origin of ln (1 þ K (IA/IS)) versus sec y was plotted to extract the d/l values from the angle resolved measurements . The data used for the fit was restricted to angles up to 608 (see above). The parameters K and l could be determined either theoretically or experimentally (K being the ratio of the intensities of the appropriate peaks from thick samples) [1,5]. In this work, K and l were chosen to be 1.82 nm and 3.44 nm, respectively. These values are close to the theoretically predicted ones [4–6].
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25
while the ratio of the signals from two adjacent layers i and i þ 1 is Ii Iiþ1
Ii1 ð1 expð di =lii cos yÞÞ 1 ð1 expð diþ1 =liþ1;iþ1 cos yÞÞ Iiþ1 " !# j¼i j¼i1 X X dj 1 dj exp cos y j¼1 liþ1;j l j¼1 ij
¼
(4)
Here lij denotes the effective attenuation length of electrons originating from layer i in layer j. Using a method of least squares fitting to these equations, the thickness of each of the layers can be determined. A software package developed by Thermo VG Scientific was used to calculate the thickness of the high-k layers and SiO2 interfacial layers. The input parameters are the materials density, bandgap, stoichiometry, kinetic energy and emission angles of photoelectrons. The l values are calculated in a way similar to that used in the NIST database [6]. 3.1.2. Results The XPS and SE measured thickness of Al2O3 layers deposited by ALCVD are plotted in Fig. 1 versus the deposition cycles. The precision of the SE thickness values is better than 0.1 nm . As far as the XPS measurements are concerned, their accuracy and precision are 5% and 0.5%, respectively. It is seen that the XPS thickness is all the time smaller than the one measured by SE. This could be due to the different properties of the thin films compared to the bulk material (e.g. different density) and to the presence of an adsorbed contamination layer. Further investigation is necessary to clarify that point. The XPS measured thickness indicates some incubation for small number of cycles as the measured thickness is
ALCVD Al2O3/SiO2/Si
6
Al2O3 SE Al2O3 ARXPS SiO2 ARXPS SiO2 XPS SiO2 XPS 2 angle
Thickness [nm]
5 4 3 2 1 0 0
10
20
30
40
50
No. of deposition cycles
Fig. 1. Thickness of the ALCVD Al2O3 layers deposited on 1 nm RTO SiO2 and of the SiO2 interfacial layer determined by XPS and spectroscopic ellipsometry. For data accuracy, see the text.
smaller than the expected one according to a linear growth model. The thickness of the interfacial SiO2 layers determined by XPS is also shown. It is seen that the results obtained by the three different equipment are centered around 1.2 nm and are with good consistency. This mean thickness value is somewhat higher than the nominal value (1.0 nm). The thickness of ALCVD HfO2 films deposited on 1 nm RTO SiO2 was also measured (Fig. 2). It is seen that the SE thickness values are always higher than those obtained by XPS. The reasons for that are similar to those for the Al2O3 case. The mean thickness of the interfacial SiO2 layers is about 1.23 nm HfO2 SE HfO2 ARXPS HfO2 XPS SiO2 ARXPS SiO2 XPS SiO2 XPS 2 angle
3.5 3.0 Thickness [nm]
If the attenuation length of the electrons from the substrate differs significantly from that of the electrons from the overlayer, the data should be fitted to Eq. (1). The model used can be generalized for n layers deposited on a substrate S. In that case the ratio of the signal from layer i versus the substrate signal can be written as follows: Ii Ii1 di ¼ 1 1 exp Is Is lii cos y " !# j¼n j¼i1 X X dj 1 dj (3)
exp cos y j¼1 lsj l j¼1 ij
23
2.5 2.0
ALCVD HfO2/SiO2/Si
1.5 1.0 0.5 0.0 0
10
20
30
40
50
No. of deposition cycles
Fig. 2. The same as in Fig. 1, but for ALCVD HfO2 layers.
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25
except for the case of 50 deposition cycles where it is slightly higher, 1.46 nm. The consistency of the SiO2 thickness values obtained by different XPS equipment is again very good. The composition of the interfacial layer is important for the application of the high-k films. For our studies, a SiO2 film was grown intentionally. A silicate may be formed depending on the initial SiO2 interfacial layer thickness, deposition conditions and subsequent heat treatment of the wafers. The chemical state of Si in the interfacial layer can be determined by the position of its XPS Si2p peak. The Si2p peak of Si in SiO2 is shifted by about 4 eV towards lower binding energy relative to the Si2p peak of elemental Si [1,7]. This shift is smaller for the silicates of interest in this work [1,7]. The shift of the Si2p peak obtained for both ALCVD Al2O3 and HfO2 cases was in the range 3.8– 4.0 eV. Therefore, we can conclude that no silicate was formed on the interface. 3.1.3. Effective attenuation length and growth rate If we have a set of layers with known different thickness deposited on a substrate, we can estimate the l by measuring the attenuation of the substrate signal passing through the layers. The main assumptions for this are exponential attenuation of the substrate signal IS passing through a homogeneous layer A with a thickness d and similar superficial carbon contamination for all samples: d IS ¼ IS1 exp ðl cos yÞ An example is presented in Fig. 3, in which the Al Ka excited Si2p signal from the Si substrate is plotted versus the thickness of the ALCVD deposited HfO2 films measured by ellipsometry. l for Si2p in the HfO2 films is extracted by fitting the Si2p signal with an exponential function. The obtained l is 2.3 nm, a value somewhat higher than that given by the NIST database (1.8 nm) [6]. One of the possible reasons for this could be some errors in the SE thickness determination and in XPS measurements due to the presence of adsorbed layers (hydrocarbons, water, etc.). Other reasons could include the lower density of HfO2 in the thin films compared to the bulk value due to island growth and/or rough layer and/or porous material. We can estimate the growth rate of ALCVD grown films from the same data under the same assumptions.
ALCVD HfO 2/SiO 2/Si 1000 Si2p intensity [a.u.]
24
100 Aexp(-d/ λ cos θ)
10
0
1
2 3 4 5 SE thickness HfO 2 [nm]
6
Fig. 3. Estimation of the effective attenuation length of Si2p photoelectrons in ALCVD HfO2 layers. For data accuracy, see the text.
Assuming d ¼ GN, where N is the number of deposition cycles, we can plot the Si2p signal intensity versus N, fit with an exponential function and calculate G. The l value of Si2p in the corresponding film is obtained from the NIST database. The growth rate of HfO2 on SiO2/Si was estimated to be 0.04 nm/cycle assuming the l value of 1.8 nm (NIST) and 0.05 nm/ cycle for l ¼ 2.3 nm. The effective attenuation length for Si2p photoelectrons in ALCVD deposited Al2O3 layers was determined by the same method to be 2.75 nm, a value slightly lower than that given by the NIST database (2.88 nm). The growth rate of ALCVD Al2O3 on SiO2/ Si was estimated to be 0.10 nm/cycle and 0.09 nm/ cycle assuming l to be 2.88 nm and 2.75 nm, respectively. 3.1.4. Bandgap The bandgap values of the thin high-k dielectric layers can be obtained from the XPS data by using the electron energy loss signal for the O1s peak [8]. The bandgap is determined as the energy separation between the peak energy and the threshold of the inelastic losses corresponding to band-to-band excitations. The onset of the energy loss spectrum is defined by linearly extrapolating the segment of maximum negative slope to the background level. This method was implemented successfully in this work for the case of SiO2, HfO2 and Al2O3 layers. The results are shown in Fig. 4. It is seen that for very thin
R.G. Vitchev et al. / Applied Surface Science 235 (2004) 21–25 9,0
Al2O3 Al2O3 HfO2 HfO2
8.9 eV (SiO2) [9]
8,5
Bandgap [eV]
8,0 7,5 7,0
6.7 eV (Al2O3) [10]
6,5 6,0
5.7 eV (HfO2) [9]
5,5 5,0 4,5 0
20
40
60
80
25
spectroscopic ellipsometry results revealed a systematic deviation probably due to film density being different from that of the bulk materials. The interfacial SiO2 layer thickness was shown to be somewhat larger than the nominal value. No silicate was detected at the interface. The effective attenuation lengths of Si2p photoelectrons in ALCVD HfO2 and Al2O3 layers were obtained. The results are in reasonable agreement with the NIST database predicted values. The bandgap of the thin films was measured in good agreement with the results cited in the literature.
100
No. of deposition cycles
Fig. 4. Bandgap values of the ALCVD Al2O3 and HfO2 layers measured by XPS. Open and solid symbols show the results obtained by the SSX and HP XPS equipment. Horizontal lines present the corresponding literature bandgap values. For data accuracy, see the text.
ALCVD HfO2 films, the bandgap values are close to that of SiO2 (8.9 eV) [9]. The onset of the losses due to band-to-band transitions in HfO2 is detectable for the samples with 10 and more deposition cycles. The HfO2 bandgap values are lower than the bulk value of 5.7 eV [9] with the mean of all measurements being 5.25 eV 0.2 eV. The mean value of the bandgap of ALCVD Al2O3 films is 6.7 eV 0.1 eV, smaller than that cited in Ref. [9] (8.7 eV), but identical to that determined in Ref. [10]. The good consistency of the data obtained by using different XPS equipment is also evident. The bandgap can also be obtained from the onset of the rise of the absorption coefficient in ellipsometry measurements. This requires the use of wavelengths in the vacuum UV range and relatively thick high-k layers. The method is described in detail elsewhere [11]. The corresponding values obtained for HfO2 (5.78 eV) and for Al2O3 (6.26 eV) are in good agreement with the XPS results.
4. Conclusions The thickness of ALCVD HfO2 and Al2O3 thin films was determined by XPS. Comparison with the
Acknowledgements The work was performed with the financial support of the EU-CUHKO project.
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