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Shear mode coupling and properties dispersion in 8 GHz range AlN thin film bulk acoustic wave (BAW) resonator
Thin Solid Films 514 (2006) 341 – 343 www.elsevier.com/locate/tsf
Shear mode coupling and properties dispersion in 8GHz range AlN thin film bulk acoustic wave (BAW) resonator F. Martin a,⁎, M.-E. Jan a , B. Belgacem a , M.-A. Dubois b , P. Muralt a a
Ceramics Laboratory, Department of Materials, Swiss Federal Institute of Technology of Lausanne, 1015 Lausanne, Switzerland b RF Microelectronics, CSEM, 2007 Neuchâtel, Switzerland Received 20 April 2005; received in revised form 11 October 2005; accepted 6 March 2006 Available online 5 April 2006
Abstract 8 GHz range solidly mounted resonator (SMR) based on high quality direct current pulsed sputtered aluminum nitride (AlN) thin films have been fabricated on 100mm diameter silicon wafer. Dispersion in AlN film thickness has been measured and on-wafer distribution of operating frequency, mechanical coupling and quality factor of the SMR has been investigated. Data is presented showing efficient coupling of a shear mode resonating in the 4 GHz range. This coupling was found to increase with wafer radius and related to the increasing tilt of crystalline c-planes of AlN thin film. © 2006 Elsevier B.V. All rights reserved. Keywords: Aluminum nitride; Physical vapour deposition (PVD); Piezoelectric effect
1. Introduction For the past decade, AlN thin films have attracted much attention for high frequency acoustic wave devices, especially thin film bulk acoustic resonator and solidly mounted resonator [1]. Both have their advantages and drawbacks and the choice mainly depends on product specifications and design options. Although many techniques, such as chemical vapor deposition [2], pulsed laser ablation [3] and molecular beam epitaxy [4] have been used to fabricate AlN thin films on various substrates, the most successful technique for the growth of AlN has been by reactive sputtering [5,6]. The deposition of AlN thin film with high uniformity and excellent crystal orientation is the key to bulk acoustic wave (BAW) technology. In order to study feasibility of BAW filters for a wide frequency range above 2 GHz and to achieve productivity specifications in expanding the system use to the manufacturing floor, the requirements of the resonator specifications accuracy must be typically below 0.5%, which imposes roughly the same range of thickness
⁎ Corresponding author. Tel.: +41 216932944; fax: +41 216935810. E-mail address: [email protected] (F. Martin). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.03.005
uniformity of the piezoelectric layer. Typical requirement for BAW devices using longitudinal acoustic mode is high electromechanical coupling, which is characterized by large values of piezoelectric coefficient d33,f and can be related to the tilt of the crystalline c-planes in AlN thin films [6]. However, the coupling of unwanted acoustic mode tends to degrade the performances of BAW resonators, e.g. for filtering applications, by increasing insertion loss and distorting the response in the bandwidth. In this paper, we report on the electrical properties dispersion of 8 GHz range BAW devices fabricated on 100 mm diameter wafers and on the thickness shear mode excited in such devices. 2. Fabrication and testing The devices were fabricated on 100 mm diameter, 525 μm thick Si(100) wafers. The Bragg reflector consisted of five pairs of sputter-deposited SiO2/AlN quarter wavelength thick layers (160/330 nm), using radiofrequency magnetron sputtering and pulsed direct current (DC) reactive magnetron sputtering, respectively. To promote an excellent c-texture of AlN, the substrate was subsequently prepared with a Pt/Ti (100/10nm) bottom electrode and patterned by dry etching. The 300nm
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thick piezoelectric functional AlN thin film was then deposited by pulsed DC reactive magnetron sputtering. The target was a 200 mm diameter 99.999% pure Al disk. The system was pumped down to a base pressure of less than 2 · 10− 7 mbar. Then, the substrate was elevated to 300 °C in an atmosphere containing N2 of 4 · 10− 3 mbar with a power of 1500 W applied to Al target. Finally, the 100 nm Al0.99Si0.01 top electrode was DC magnetron sputtered and patterned by dry etching. AlN thin film uniformity was measured and mapped by means of noncontact spectroscopic reflectometry (nanospec 6100) and tested for the clamped longitudinal piezoelectric coefficient d33,f by using a double beam Mach-Zehnder interferometer. The SMR devices, of piezoelectric-area 50 × 50 μm2, were characterized by using a Hewlett-Packard 8722D network analyzer. The electrical admittance was calculated from the S11 scattering parameter. To take into account parasitic elements of the circuitry, a resonator model was used and the mechanical 2 coupling coefficient, keff and series quality factor, Qs, were extracted [7]. 3. Results The properties of the functional AlN thin film can be found in a previous work from the authors [6]. The mapping of the functional AlN thin film is showed in Fig. 1A, with an edge exclusion of 5 mm. From Fig. 1A, the thickness dispersion is clearly symmetrical from the wafer centre. There's an excellent uniformity (less than 0.3%) in the [− 24.5/ + 24.5mm] zone, representing half of the total wafer surface. This uniformity is degraded when 2/3 or 9/10 of the total wafer surface is considered, with a uniformity of 1.5% and 3.7%, respectively. This dispersion can be explained by the inhomogeneous target erosion of the magnetron source, where maximum erosion was observed at 75 mm from the target centre. Hence, the operating frequency of the SMR is expected to exhibit a substantial dispersion from the centre to the edge of the wafer. Fig. 1B shows the on-wafer frequency range of the SMR devices. For simplicity, only the wafer radius has been considered. As expected by the AlN film uniformity (Fig. 1A), the anti-resonance frequency increases quadratically with wafer radius and exhibits a maximum deviation of ∼10%,
Fig. 1. Radial distribution of the functional AlN film thickness from the wafer centre to the edge [0/± 45mm] (A) and measured anti-resonance frequency of corresponding SMR devices along the wafer radius (B). Lines are guides to the eye.
Fig. 2. Mechanical coupling (k2eff, open diamonds) and quality factor (Qs, full circles) versus anti-resonance frequency of SMR devices across the wafer— inset: figure-of-merit defined as the product of k2eff times Qs (stars) versus antiresonance frequency of SMR devices across the wafer. Lines are guides to the eye.
which is three times the film thickness maximum dispersion. From scanning electron microscope (SEM) observations, a range of 10% was also found on the electrodes thickness, which must be taken in the dispersion of SMR devices frequency [7]. A resonator circuit model was used to extract the coupling 2 and series quality factors, keff and Qs, respectively. In Fig. 2, 2 keff and Qs are plotted against anti-resonance frequency from 2 Fig. 1B. From Fig. 2, both keff and Qs are shown to strongly depend on the position of the SMR devices on the wafer, where 2 keff was found to decrease with higher frequencies, from 5.05% to 4.35% at 7.65GHz and 8.35 GHz, respectively. Unlike the coupling factor, the quality factor first decreases with frequency and then has its maximum at 8 GHz (Qs = 140). Indeed, the Bragg mirror was optimized for operating at 8 GHz. Hence, a maximum in the SMR output should be found at that frequency, as confirmed by the figure of merit, defined as the product of 2 keff by Q s (insert of Fig. 2). The authors previously demonstrated the dependence of the clamped film piezoelectric coefficient d33,f on the film thickness [6]. However, from Fig. 2, the change in coupling factor with film thickness should only account for less than 0.1%. The clamped film piezoelectric coefficient, d33,f, was measured as a function of the wafer radius (Fig. 3). It is clearly seen from Fig. 3 that d33,f decreases with wafer radius, in other words that the coupling of the longitudinal mode coupling is expected to decrease in the same manner. Note
Fig. 3. Clamped longitudinal piezoelectric coefficient d33,f, as a function of distance from wafer centre.
F. Martin et al. / Thin Solid Films 514 (2006) 341–343
Fig. 4. Admittance curves (conductance, solid line–susceptance, dotted line) of 7.85GHz longitudinal mode (A) and 4.05GHz shear mode (B) of a single SMR device located at the edge of the 100mm wafer.
that, from 30 mm, disturbance to the measured d33,f was observed. This phenomenon can be related to the arising of parasitic acoustic mode coupling. Interestingly, an additional resonance could be observed at lower frequency, from 3.82GHz at the centre of the wafer to 4.30 GHz at the edge. Fig. 4A and B show the admittance curves of the longitudinal and shear modes, respectively, extracted from a single SMR device located at 33 mm from the wafer centre. Unambiguously, this additional resonance was identified as a shear mode, as previously described by other authors in similar conditions [8]. From Fig. 5, the shear mode coupling increases with the distance from the centre of the wafer (from ∼ 0.15% at the centre to ∼0.45% at the edge) and, conversely, the longitudinal mode coupling starts to decrease at a distance of 35 mm from the edge (from 5.00% at the centre to 4.35% at the edge). Indeed, the increase in shear mode coupling could be charged on the tilting of the crystalline c-axis of AlN crystals due to uneven distribution of the deposition flux over the whole wafer [8]. This postulation is confirmed by SEM observations from which a maximum of ∼ 10° in grain tilting of the AlN film was found at the edge of the 100mm wafer. The theoretical shear coupling in tilted AlN can be calculated from materials tensors of AlN [9] when the shear stress tensor normalized to the applied thickness electric field is optimized. Results of this calculation are depicted in Fig. 6. Thus, a range of shear coupling from 0.15% to 0.45% (Fig. 5) should correspond to a range of c-planes tilt of ∼ 4.5–8.0°. It must be noticed that
Fig. 5. Mechanical coupling (k2eff) of longitudinal mode (open circles) and shear mode (full circles) of SMR devices along the wafer radius. Lines are guides to the eye.
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Fig. 6. Computed shear electromechanical coupling versus AlN c-planes tilt— inset: zoom on the 0–10° range of c-planes tilt.
recent work from the authors demonstrated that the observed tilt in AlN grains does not correspond to the actual tilt of the cplanes [10], as previously observed by Löbl et al. [11]. 4. Conclusion 8GHz SMRs with good electromechanical properties were fabricated and analyzed with respect to functional film uniformity across the 100 mm wafer. In deposition conditions close to those of production, device resonant frequency shows a strong dependence on AlN film and metal electrode film thickness uniformity. A shear mode was excited in the 4 GHz range, which coupling increases with the tilt of the crystalline caxis, at the expense of the longitudinal mode. Computed electromechanical coupling for shear mode and SEM observations supported the assumption of graded tilting of crystalline cplanes across the wafer and competitive coupling between longitudinal and shear modes. Acknowledgments This work was supported by the Swiss Office for Education and Research in the frame of the E.U. project MARTINA. References [1] K.M. Lakin, J. Belsick, J.F. McDonald, K.T. McCarron, IEEE Ultrason. Symp., Atlanta, Oct. 7–10, 2001, IEEE Ultrason. Symp. Proc., 2001, p. 3E-5. [2] G.Z. Meng, S. Xie, D.K. Peng, Thin Solid Films 334 (1–2) (1998) 145. [3] R.D. Vispute, H. Wu, J. Narayan, Appl. Phys. Lett. 67 (11) (1995) 1549. [4] S. Yoshida, S. Misawa, Y. Fujii, S. Takada, H. Hayakawa, S. Gonda, A. Itoh, J. Vac. Sci. Technol. 16 (1979) 990. [5] M.-A. Dubois, P. Muralt, Appl. Phys. Lett. 74 (20) (1999) 3032. [6] F. Martin, P. Muralt, M.-A. Dubois, A. Pezous, J. Vac. Sci. Technol., A 22 (2) (2004) 361. [7] R. Lanz, P. Muralt, IEEE Ultrason. Ferroelectr. Freq. Contr., Honolulu, HI, Oct. 05–08, 2003, IEEE Ultrason. Symp. Proc., vol. 1–2, 2003, p. 178. [8] J. Bjurstrom, D. Rosen, I. Katardjiev, V.M. Yanchev, I. Petrov, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 51 (10) (2004) 1347. [9] K. Tsuboushi, N. Mikoshiba, IEEE Trans. Sonics Ultrason. (1985) 634. [10] F. Martin, M.-E. Jan, S. Rey-Mermet, B. Belgacem, M. Cantoni, P. Muralt, IEEE Ultrasonics, Ferroelectrics, and Frequency Control, unpublished, Editor: Jiashi Yang. [11] P. Löbl, M. Klee, R. Milsom, R. Dekker, C. Metzmacher, W. Brand, P. Lok, J. Eur. Ceram. Soc. 21 (2001) 2633.
Shear mode coupling and properties dispersion in 8GHz range AlN thin film bulk acoustic wave (BAW) resonator F. Martin a,⁎, M.-E. Jan a , B. Belgacem a , M.-A. Dubois b , P. Muralt a a
Ceramics Laboratory, Department of Materials, Swiss Federal Institute of Technology of Lausanne, 1015 Lausanne, Switzerland b RF Microelectronics, CSEM, 2007 Neuchâtel, Switzerland Received 20 April 2005; received in revised form 11 October 2005; accepted 6 March 2006 Available online 5 April 2006
Abstract 8 GHz range solidly mounted resonator (SMR) based on high quality direct current pulsed sputtered aluminum nitride (AlN) thin films have been fabricated on 100mm diameter silicon wafer. Dispersion in AlN film thickness has been measured and on-wafer distribution of operating frequency, mechanical coupling and quality factor of the SMR has been investigated. Data is presented showing efficient coupling of a shear mode resonating in the 4 GHz range. This coupling was found to increase with wafer radius and related to the increasing tilt of crystalline c-planes of AlN thin film. © 2006 Elsevier B.V. All rights reserved. Keywords: Aluminum nitride; Physical vapour deposition (PVD); Piezoelectric effect
1. Introduction For the past decade, AlN thin films have attracted much attention for high frequency acoustic wave devices, especially thin film bulk acoustic resonator and solidly mounted resonator [1]. Both have their advantages and drawbacks and the choice mainly depends on product specifications and design options. Although many techniques, such as chemical vapor deposition [2], pulsed laser ablation [3] and molecular beam epitaxy [4] have been used to fabricate AlN thin films on various substrates, the most successful technique for the growth of AlN has been by reactive sputtering [5,6]. The deposition of AlN thin film with high uniformity and excellent crystal orientation is the key to bulk acoustic wave (BAW) technology. In order to study feasibility of BAW filters for a wide frequency range above 2 GHz and to achieve productivity specifications in expanding the system use to the manufacturing floor, the requirements of the resonator specifications accuracy must be typically below 0.5%, which imposes roughly the same range of thickness
⁎ Corresponding author. Tel.: +41 216932944; fax: +41 216935810. E-mail address: [email protected] (F. Martin). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.03.005
uniformity of the piezoelectric layer. Typical requirement for BAW devices using longitudinal acoustic mode is high electromechanical coupling, which is characterized by large values of piezoelectric coefficient d33,f and can be related to the tilt of the crystalline c-planes in AlN thin films [6]. However, the coupling of unwanted acoustic mode tends to degrade the performances of BAW resonators, e.g. for filtering applications, by increasing insertion loss and distorting the response in the bandwidth. In this paper, we report on the electrical properties dispersion of 8 GHz range BAW devices fabricated on 100 mm diameter wafers and on the thickness shear mode excited in such devices. 2. Fabrication and testing The devices were fabricated on 100 mm diameter, 525 μm thick Si(100) wafers. The Bragg reflector consisted of five pairs of sputter-deposited SiO2/AlN quarter wavelength thick layers (160/330 nm), using radiofrequency magnetron sputtering and pulsed direct current (DC) reactive magnetron sputtering, respectively. To promote an excellent c-texture of AlN, the substrate was subsequently prepared with a Pt/Ti (100/10nm) bottom electrode and patterned by dry etching. The 300nm
342
F. Martin et al. / Thin Solid Films 514 (2006) 341–343
thick piezoelectric functional AlN thin film was then deposited by pulsed DC reactive magnetron sputtering. The target was a 200 mm diameter 99.999% pure Al disk. The system was pumped down to a base pressure of less than 2 · 10− 7 mbar. Then, the substrate was elevated to 300 °C in an atmosphere containing N2 of 4 · 10− 3 mbar with a power of 1500 W applied to Al target. Finally, the 100 nm Al0.99Si0.01 top electrode was DC magnetron sputtered and patterned by dry etching. AlN thin film uniformity was measured and mapped by means of noncontact spectroscopic reflectometry (nanospec 6100) and tested for the clamped longitudinal piezoelectric coefficient d33,f by using a double beam Mach-Zehnder interferometer. The SMR devices, of piezoelectric-area 50 × 50 μm2, were characterized by using a Hewlett-Packard 8722D network analyzer. The electrical admittance was calculated from the S11 scattering parameter. To take into account parasitic elements of the circuitry, a resonator model was used and the mechanical 2 coupling coefficient, keff and series quality factor, Qs, were extracted [7]. 3. Results The properties of the functional AlN thin film can be found in a previous work from the authors [6]. The mapping of the functional AlN thin film is showed in Fig. 1A, with an edge exclusion of 5 mm. From Fig. 1A, the thickness dispersion is clearly symmetrical from the wafer centre. There's an excellent uniformity (less than 0.3%) in the [− 24.5/ + 24.5mm] zone, representing half of the total wafer surface. This uniformity is degraded when 2/3 or 9/10 of the total wafer surface is considered, with a uniformity of 1.5% and 3.7%, respectively. This dispersion can be explained by the inhomogeneous target erosion of the magnetron source, where maximum erosion was observed at 75 mm from the target centre. Hence, the operating frequency of the SMR is expected to exhibit a substantial dispersion from the centre to the edge of the wafer. Fig. 1B shows the on-wafer frequency range of the SMR devices. For simplicity, only the wafer radius has been considered. As expected by the AlN film uniformity (Fig. 1A), the anti-resonance frequency increases quadratically with wafer radius and exhibits a maximum deviation of ∼10%,
Fig. 1. Radial distribution of the functional AlN film thickness from the wafer centre to the edge [0/± 45mm] (A) and measured anti-resonance frequency of corresponding SMR devices along the wafer radius (B). Lines are guides to the eye.
Fig. 2. Mechanical coupling (k2eff, open diamonds) and quality factor (Qs, full circles) versus anti-resonance frequency of SMR devices across the wafer— inset: figure-of-merit defined as the product of k2eff times Qs (stars) versus antiresonance frequency of SMR devices across the wafer. Lines are guides to the eye.
which is three times the film thickness maximum dispersion. From scanning electron microscope (SEM) observations, a range of 10% was also found on the electrodes thickness, which must be taken in the dispersion of SMR devices frequency [7]. A resonator circuit model was used to extract the coupling 2 and series quality factors, keff and Qs, respectively. In Fig. 2, 2 keff and Qs are plotted against anti-resonance frequency from 2 Fig. 1B. From Fig. 2, both keff and Qs are shown to strongly depend on the position of the SMR devices on the wafer, where 2 keff was found to decrease with higher frequencies, from 5.05% to 4.35% at 7.65GHz and 8.35 GHz, respectively. Unlike the coupling factor, the quality factor first decreases with frequency and then has its maximum at 8 GHz (Qs = 140). Indeed, the Bragg mirror was optimized for operating at 8 GHz. Hence, a maximum in the SMR output should be found at that frequency, as confirmed by the figure of merit, defined as the product of 2 keff by Q s (insert of Fig. 2). The authors previously demonstrated the dependence of the clamped film piezoelectric coefficient d33,f on the film thickness [6]. However, from Fig. 2, the change in coupling factor with film thickness should only account for less than 0.1%. The clamped film piezoelectric coefficient, d33,f, was measured as a function of the wafer radius (Fig. 3). It is clearly seen from Fig. 3 that d33,f decreases with wafer radius, in other words that the coupling of the longitudinal mode coupling is expected to decrease in the same manner. Note
Fig. 3. Clamped longitudinal piezoelectric coefficient d33,f, as a function of distance from wafer centre.
F. Martin et al. / Thin Solid Films 514 (2006) 341–343
Fig. 4. Admittance curves (conductance, solid line–susceptance, dotted line) of 7.85GHz longitudinal mode (A) and 4.05GHz shear mode (B) of a single SMR device located at the edge of the 100mm wafer.
that, from 30 mm, disturbance to the measured d33,f was observed. This phenomenon can be related to the arising of parasitic acoustic mode coupling. Interestingly, an additional resonance could be observed at lower frequency, from 3.82GHz at the centre of the wafer to 4.30 GHz at the edge. Fig. 4A and B show the admittance curves of the longitudinal and shear modes, respectively, extracted from a single SMR device located at 33 mm from the wafer centre. Unambiguously, this additional resonance was identified as a shear mode, as previously described by other authors in similar conditions [8]. From Fig. 5, the shear mode coupling increases with the distance from the centre of the wafer (from ∼ 0.15% at the centre to ∼0.45% at the edge) and, conversely, the longitudinal mode coupling starts to decrease at a distance of 35 mm from the edge (from 5.00% at the centre to 4.35% at the edge). Indeed, the increase in shear mode coupling could be charged on the tilting of the crystalline c-axis of AlN crystals due to uneven distribution of the deposition flux over the whole wafer [8]. This postulation is confirmed by SEM observations from which a maximum of ∼ 10° in grain tilting of the AlN film was found at the edge of the 100mm wafer. The theoretical shear coupling in tilted AlN can be calculated from materials tensors of AlN [9] when the shear stress tensor normalized to the applied thickness electric field is optimized. Results of this calculation are depicted in Fig. 6. Thus, a range of shear coupling from 0.15% to 0.45% (Fig. 5) should correspond to a range of c-planes tilt of ∼ 4.5–8.0°. It must be noticed that
Fig. 5. Mechanical coupling (k2eff) of longitudinal mode (open circles) and shear mode (full circles) of SMR devices along the wafer radius. Lines are guides to the eye.
343
Fig. 6. Computed shear electromechanical coupling versus AlN c-planes tilt— inset: zoom on the 0–10° range of c-planes tilt.
recent work from the authors demonstrated that the observed tilt in AlN grains does not correspond to the actual tilt of the cplanes [10], as previously observed by Löbl et al. [11]. 4. Conclusion 8GHz SMRs with good electromechanical properties were fabricated and analyzed with respect to functional film uniformity across the 100 mm wafer. In deposition conditions close to those of production, device resonant frequency shows a strong dependence on AlN film and metal electrode film thickness uniformity. A shear mode was excited in the 4 GHz range, which coupling increases with the tilt of the crystalline caxis, at the expense of the longitudinal mode. Computed electromechanical coupling for shear mode and SEM observations supported the assumption of graded tilting of crystalline cplanes across the wafer and competitive coupling between longitudinal and shear modes. Acknowledgments This work was supported by the Swiss Office for Education and Research in the frame of the E.U. project MARTINA. References [1] K.M. Lakin, J. Belsick, J.F. McDonald, K.T. McCarron, IEEE Ultrason. Symp., Atlanta, Oct. 7–10, 2001, IEEE Ultrason. Symp. Proc., 2001, p. 3E-5. [2] G.Z. Meng, S. Xie, D.K. Peng, Thin Solid Films 334 (1–2) (1998) 145. [3] R.D. Vispute, H. Wu, J. Narayan, Appl. Phys. Lett. 67 (11) (1995) 1549. [4] S. Yoshida, S. Misawa, Y. Fujii, S. Takada, H. Hayakawa, S. Gonda, A. Itoh, J. Vac. Sci. Technol. 16 (1979) 990. [5] M.-A. Dubois, P. Muralt, Appl. Phys. Lett. 74 (20) (1999) 3032. [6] F. Martin, P. Muralt, M.-A. Dubois, A. Pezous, J. Vac. Sci. Technol., A 22 (2) (2004) 361. [7] R. Lanz, P. Muralt, IEEE Ultrason. Ferroelectr. Freq. Contr., Honolulu, HI, Oct. 05–08, 2003, IEEE Ultrason. Symp. Proc., vol. 1–2, 2003, p. 178. [8] J. Bjurstrom, D. Rosen, I. Katardjiev, V.M. Yanchev, I. Petrov, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 51 (10) (2004) 1347. [9] K. Tsuboushi, N. Mikoshiba, IEEE Trans. Sonics Ultrason. (1985) 634. [10] F. Martin, M.-E. Jan, S. Rey-Mermet, B. Belgacem, M. Cantoni, P. Muralt, IEEE Ultrasonics, Ferroelectrics, and Frequency Control, unpublished, Editor: Jiashi Yang. [11] P. Löbl, M. Klee, R. Milsom, R. Dekker, C. Metzmacher, W. Brand, P. Lok, J. Eur. Ceram. Soc. 21 (2001) 2633.