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Piezoelectric AlN Films for FPW Sensors with Improved Device Performance
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 1040 – 1043
30th Eurosensors Conference, EUROSENSORS 2016
Piezoelectric AlN films for FPW sensors with improved device performance Markus Reuscha,b, *, Katarzyna Holcb, Lutz Kirsteb, Philip Katusc, Leonhard Reindlc, Oliver Ambachera,b, and Vadim Lebedevb a
Laboratory for Compound Semiconductor Microsystems, IMTEK, University of Freiburg, Freiburg, Germany b Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany c Laboratory for Electrical Instrumentation, IMTEK, University of Freiburg, Freiburg, Germany
Abstract Reactively sputtered piezoelectric aluminum nitride (AlN) films for flexural plate wave (FPW) electroacoustic sensors equipped with buried interdigital transducers (IDTs) for sensing in liquids were fabricated and characterized. In order to assess the crystallographic and elastic film properties, residual film stress, and piezoelectric response, comprehensive analyses were performed on AlN layers and bimorph AlN structures, including X-ray diffraction, atomic force microscopy, and nanoindentation, in combination with analysis of the piezoelectric charge coefficient d33,f. To demonstrate the applicability of the optimized AlN films to electro-acoustic devices, IDT equipped devices were fabricated and characterized by means of laser Doppler vibrometry. © Published by Elsevier Ltd. This © 2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Aluminum nitride; reactive sputtering; FPW sensor; laser Doppler vibrometry
1. Motivation Acoustic-wave based devices are being used in a wide range of sensing applications including physical, chemical and bio-sensing. The antisymmetric A0 Lamb wave mode (also called flexural plate wave, FPW) of a thin membrane
* Corresponding author. Tel.: +49-761-5159359; fax: +49-761-515971359. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.335
Markus Reusch et al. / Procedia Engineering 168 (2016) 1040 – 1043
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is well-suited for bio/chemical sensors operating in liquid media [1]. State-of-the-art electro-acoustic devices are designed as a unimorph structure, having the interdigital transducer (IDT) for the excitation and detection of the acoustic wave on top of the piezoelectric film [2]. One of the major drawbacks of this design is the lack of an electrical shielding of the IDT against the surrounding liquid. Fig. 1 presents our improved design of the FPW sensor. Here the IDT is designed as a buried electrode which is embedded between two piezoelectric aluminum nitride (AlN) films. Due to this bimorph design the IDT is electrically shielded by the top and bottom metallization. Moreover, both sides of the membrane can be used for sensing, and the electromechanical coupling coefficient can be enhanced by the bilayer design [3]. Consequently the sensitivity of the sensor is improved.
Fig. 1. Schematic representation (exploded view) of the FPW sensor. The metal IDT (Mo) is embedded between the two piezoelectric AlN layers.
For this bimorph structure, the growth of the second AlN layer on top of the first AlN surface, which is patterned by the IDT, is required. The AlN material properties should be carefully tailored and should be comparable for both AlN films as the device performance strongly depends on the film stress, piezoelectric response, as well as the Young’s modulus of the AlN membrane. In previous work [4] aluminum and chromium were evaluated as IDT metallization. Due to differences in film stress and piezoelectric response overall device performance was lowered. In this work we show that AlN material properties can be improved by using molybdenum as buried IDT metallization. Furthermore the RF sputter process was enhanced by tuning the nitrogen concentration in the Ar/N2 reactive atmosphere during film growth, resulting in AlN films with improved structural properties. IDT equipped devices were fabricated in order to demonstrate the applicability of the optimized AlN films to electro-acoustic devices. 2. Fabrication AlN films with thickness ranging from 950 to 1000 nm were sputter deposited on (001)-oriented boron doped silicon wafers with diameter of 100 mm and resistivity of 10-20 PȍFPusing a magnetron sputtering system von Ardenne CS730S. Further details on the sputter equipment, experimental procedure and film characterization are described elsewhere [5]. In order to compensate the characteristic stress gradient in thick AlN films the nitrogen concentration in the Ar/N2 reactive atmosphere was gradually increased, resulting in enhanced energy of the plasma species striking the film surface. From 15 % N2 in the beginning of film growth the N2 ratio was increased to 67 % N2 until 750 nm film thickness was reached and was kept constant until the final film thickness was grown. Mo films with thickness of 100 nm were DC sputtered in pure Ar plasma on top of the prepared AlN films. To fabricate the IDT structure the Mo film was patterned by UV-photolithography and dry etching. Subsequently the second AlN layer was grown on the patterned surface. For this process the nitrogen concentration was increased from 18 % N2 in the beginning to 70 % for 700 nm film thickness. 3. Characterization Fig. 2 (a) shows AFM micrographs of AlN/Si, Mo/AlN/Si, AlN/AlN/Si and AlN/Mo/AlN/Si layer stacks. All AlN films exhibited the typical pebble-like surface morphology indicating c-axis orientation of the AlN film. The root-mean-square surface roughness was in the range of 1.8 to 2.8 nm. Fig. 2 (b) shows the representative ;5'șș
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Markus Reusch et al. / Procedia Engineering 168 (2016) 1040 – 1043
scan of the AlN/Mo/AlN/Si layer stack. No other AlN orientations than the preferred [00.l] orientation were PHDVXUHG 7KH IXOO ZLGWK DW KDOI PD[LPXP RI WKH Ȧ-scan measured around the 00.2 reflection of AlN was 1.8° indicating improved crystal quality of the film. The piezoelectric response was measured by using a commercial Berlincourt piezometer system (PM300 from Piezotest). An effective piezoelectric charge coefficient d33,f of -7.2 pC/N was obtained for the AlN single layers and -6.6 pC/N for the AlN/Mo/AlN layer stacks (no correction for clamping effects).
Fig. 2. (a) AFM surface scans (2x2 μm2) of (I) AlN, (II) Mo/AlN, (III) AlN/AlN, and (IV) AlN/Mo/AlN layer stacks. All AlN films exhibit the typical, pebble-like surface morphology. (b) ;5'șșVFDQRIthe AlN/Mo/AlN layer stack.
Scanning confocal Raman measurements have been employed to analyze the local film stress in the AlN layer stacks. The film stress was calculated from the peak shift of the AlN E2 (high) phonon. Fig. 3 shows the AlN film stress across a fabricated structure before releasing the AlN film from the Si-substrate. The film stress was homogeneous across the scan and in the range of 100-200 MPa for AlN/AlN/Si and 50-100 MPa for AlN/Mo/AlN/Si stacks.
Fig. 3. (a) AlN film stress calculated form the measured Raman shift of the AlN E2 (high) phonon across an AlN/AlN and AlN/Mo/AlN layer stack. (b) Scan direction of the Raman measurement across the sample.
Nanoindentation tests have been performed by means of cycles load control method [5]. The nanoindentation measurements of the 1 μm thick AlN films revealed an indentation modulus of 365 GPa, demonstrating the improved elastic film properties. Before growing the second AlN film and release of the membrane by backside etching of the silicon substrate the transducer response was evaluated by means of laser Doppler vibrometry measurements (UHF120 from Polytec). Fig. 4 (a) shows the deflection of the transducer surface in resonance. Three different devices have been evaluated
Markus Reusch et al. / Procedia Engineering 168 (2016) 1040 – 1043
with different ZDYHOHQJWKȜGHILQHGE\WKH,'7RIȜ PȜ μm, and Ȝ μm. Fig. 4 (b) presents the vibrational spectra of the different devices. From the resonance frequencies the SAW velocity was calculated. The SAW velocity was in the range of 5105 m/s to 5141 PV IRU Ȝ IURP to 42.2 μm. These values are comparable to the data found in the literature [7] and therefore prove that the optimized AlN films are well suited for fabrication of acoustic-wave based FPW sensors. It should be noted that the measured velocity was also affected by the Si substrate, caused by the thin piezoelectric film compared to the wavelength.
Fig. 4. (a) Optical microscope image of the transducer surface and overlay of the surface deflection, measured by laser Doppler vibrometry Ȝ 100.7 μm). (b) Vibrational spectra of devices with three different wavelength Ȝ ranging from 100.7 to 26.4 μm.
4. Summary Piezoelectric AlN films have been fabricated by reactive magnetron sputtering. In order to compensate the stress gradient in thick AlN films the nitrogen concentration in the reactive atmosphere was tuned during the growth process. This enabled the growth of 1 μm thick AlN films with low intrinsic stress of 50-200 MPa on silicon substrates as well as on AlN/Mo patterned surfaces. Due to the high degree of c-axis orientation the AlN films exhibited a high piezoelectric response of up to d33,f -7.2 pC/N. N-face polarity was measured for all AlN-based layer stacks. This can be attributed to the low power density sputter process [5]. Moreover the measured indentation modulus of 365 GPa and the analyses of the SAW velocity, which was in the range of 5105 m/s to 5141 m/s, both demonstrated that the stress optimized AlN films are well suited for FPW device fabrication. Acknowledgements The authors would like to thank N. Lehmann, S. Bühler, and K. Schäuble for technical assistance. M. Prescher is acknowledged for carrying out the XRD analysis. This work was funded by the Deutsche Forschungsgemeinschaft (DFG) within the project AM 105/24-1. References [1] S.W. Wenzel and R. M. White. A Multisensor Employing an Ultrasonic Lamb-Wave Oscillator, IEEE Trans. on Electronic Devices, Vol. 35 (1988), 735-743. [2] I.-Y. Huang et al., Development of a flexural plate-wave (FPW) immunoglobulin-E (IgE) allergy bio-sensing microsystem, Sens. Actuators, B, Vol. 162 (2012) 184– 193. [3] M. Reusch et al., Aluminium nitride membranes with embedded buried IDT electrodes for novel flexural plate wave devices, 18th IEEE Transducers 2015, pp. 1291–1294. [4] P. Katus, Entwurf und Modellierung eines mikroakustischen Sensors zur Analytik in Flüssigkeit, PhD thesis, University of Freiburg (2015). [5] M. Reusch et al., Analysis and optimization of sputter deposited AlN-layers for flexural plate wave devices, J. Vac. Sci. Technol. B. 34 (2016), in press. [6] M. Datcheva et al., Determination of Anodized Aluminum Material Characteristics by Means of Nano-Indentation Measurements Mater. Sci. Appl. 02 (2011) 1452–64. [7] Si-Hong Hoang and Gwiy-Sang Chung, Surface acoustic wave characteristics of AlN thin films grown on a polycrystalline 3C-SiC buffer layer, Microelectronic Engineering 86.11 (2009) 2149-2152.
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ScienceDirect Procedia Engineering 168 (2016) 1040 – 1043
30th Eurosensors Conference, EUROSENSORS 2016
Piezoelectric AlN films for FPW sensors with improved device performance Markus Reuscha,b, *, Katarzyna Holcb, Lutz Kirsteb, Philip Katusc, Leonhard Reindlc, Oliver Ambachera,b, and Vadim Lebedevb a
Laboratory for Compound Semiconductor Microsystems, IMTEK, University of Freiburg, Freiburg, Germany b Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany c Laboratory for Electrical Instrumentation, IMTEK, University of Freiburg, Freiburg, Germany
Abstract Reactively sputtered piezoelectric aluminum nitride (AlN) films for flexural plate wave (FPW) electroacoustic sensors equipped with buried interdigital transducers (IDTs) for sensing in liquids were fabricated and characterized. In order to assess the crystallographic and elastic film properties, residual film stress, and piezoelectric response, comprehensive analyses were performed on AlN layers and bimorph AlN structures, including X-ray diffraction, atomic force microscopy, and nanoindentation, in combination with analysis of the piezoelectric charge coefficient d33,f. To demonstrate the applicability of the optimized AlN films to electro-acoustic devices, IDT equipped devices were fabricated and characterized by means of laser Doppler vibrometry. © Published by Elsevier Ltd. This © 2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Aluminum nitride; reactive sputtering; FPW sensor; laser Doppler vibrometry
1. Motivation Acoustic-wave based devices are being used in a wide range of sensing applications including physical, chemical and bio-sensing. The antisymmetric A0 Lamb wave mode (also called flexural plate wave, FPW) of a thin membrane
* Corresponding author. Tel.: +49-761-5159359; fax: +49-761-515971359. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.335
Markus Reusch et al. / Procedia Engineering 168 (2016) 1040 – 1043
1041
is well-suited for bio/chemical sensors operating in liquid media [1]. State-of-the-art electro-acoustic devices are designed as a unimorph structure, having the interdigital transducer (IDT) for the excitation and detection of the acoustic wave on top of the piezoelectric film [2]. One of the major drawbacks of this design is the lack of an electrical shielding of the IDT against the surrounding liquid. Fig. 1 presents our improved design of the FPW sensor. Here the IDT is designed as a buried electrode which is embedded between two piezoelectric aluminum nitride (AlN) films. Due to this bimorph design the IDT is electrically shielded by the top and bottom metallization. Moreover, both sides of the membrane can be used for sensing, and the electromechanical coupling coefficient can be enhanced by the bilayer design [3]. Consequently the sensitivity of the sensor is improved.
Fig. 1. Schematic representation (exploded view) of the FPW sensor. The metal IDT (Mo) is embedded between the two piezoelectric AlN layers.
For this bimorph structure, the growth of the second AlN layer on top of the first AlN surface, which is patterned by the IDT, is required. The AlN material properties should be carefully tailored and should be comparable for both AlN films as the device performance strongly depends on the film stress, piezoelectric response, as well as the Young’s modulus of the AlN membrane. In previous work [4] aluminum and chromium were evaluated as IDT metallization. Due to differences in film stress and piezoelectric response overall device performance was lowered. In this work we show that AlN material properties can be improved by using molybdenum as buried IDT metallization. Furthermore the RF sputter process was enhanced by tuning the nitrogen concentration in the Ar/N2 reactive atmosphere during film growth, resulting in AlN films with improved structural properties. IDT equipped devices were fabricated in order to demonstrate the applicability of the optimized AlN films to electro-acoustic devices. 2. Fabrication AlN films with thickness ranging from 950 to 1000 nm were sputter deposited on (001)-oriented boron doped silicon wafers with diameter of 100 mm and resistivity of 10-20 PȍFPusing a magnetron sputtering system von Ardenne CS730S. Further details on the sputter equipment, experimental procedure and film characterization are described elsewhere [5]. In order to compensate the characteristic stress gradient in thick AlN films the nitrogen concentration in the Ar/N2 reactive atmosphere was gradually increased, resulting in enhanced energy of the plasma species striking the film surface. From 15 % N2 in the beginning of film growth the N2 ratio was increased to 67 % N2 until 750 nm film thickness was reached and was kept constant until the final film thickness was grown. Mo films with thickness of 100 nm were DC sputtered in pure Ar plasma on top of the prepared AlN films. To fabricate the IDT structure the Mo film was patterned by UV-photolithography and dry etching. Subsequently the second AlN layer was grown on the patterned surface. For this process the nitrogen concentration was increased from 18 % N2 in the beginning to 70 % for 700 nm film thickness. 3. Characterization Fig. 2 (a) shows AFM micrographs of AlN/Si, Mo/AlN/Si, AlN/AlN/Si and AlN/Mo/AlN/Si layer stacks. All AlN films exhibited the typical pebble-like surface morphology indicating c-axis orientation of the AlN film. The root-mean-square surface roughness was in the range of 1.8 to 2.8 nm. Fig. 2 (b) shows the representative ;5'șș
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Markus Reusch et al. / Procedia Engineering 168 (2016) 1040 – 1043
scan of the AlN/Mo/AlN/Si layer stack. No other AlN orientations than the preferred [00.l] orientation were PHDVXUHG 7KH IXOO ZLGWK DW KDOI PD[LPXP RI WKH Ȧ-scan measured around the 00.2 reflection of AlN was 1.8° indicating improved crystal quality of the film. The piezoelectric response was measured by using a commercial Berlincourt piezometer system (PM300 from Piezotest). An effective piezoelectric charge coefficient d33,f of -7.2 pC/N was obtained for the AlN single layers and -6.6 pC/N for the AlN/Mo/AlN layer stacks (no correction for clamping effects).
Fig. 2. (a) AFM surface scans (2x2 μm2) of (I) AlN, (II) Mo/AlN, (III) AlN/AlN, and (IV) AlN/Mo/AlN layer stacks. All AlN films exhibit the typical, pebble-like surface morphology. (b) ;5'șșVFDQRIthe AlN/Mo/AlN layer stack.
Scanning confocal Raman measurements have been employed to analyze the local film stress in the AlN layer stacks. The film stress was calculated from the peak shift of the AlN E2 (high) phonon. Fig. 3 shows the AlN film stress across a fabricated structure before releasing the AlN film from the Si-substrate. The film stress was homogeneous across the scan and in the range of 100-200 MPa for AlN/AlN/Si and 50-100 MPa for AlN/Mo/AlN/Si stacks.
Fig. 3. (a) AlN film stress calculated form the measured Raman shift of the AlN E2 (high) phonon across an AlN/AlN and AlN/Mo/AlN layer stack. (b) Scan direction of the Raman measurement across the sample.
Nanoindentation tests have been performed by means of cycles load control method [5]. The nanoindentation measurements of the 1 μm thick AlN films revealed an indentation modulus of 365 GPa, demonstrating the improved elastic film properties. Before growing the second AlN film and release of the membrane by backside etching of the silicon substrate the transducer response was evaluated by means of laser Doppler vibrometry measurements (UHF120 from Polytec). Fig. 4 (a) shows the deflection of the transducer surface in resonance. Three different devices have been evaluated
Markus Reusch et al. / Procedia Engineering 168 (2016) 1040 – 1043
with different ZDYHOHQJWKȜGHILQHGE\WKH,'7RIȜ PȜ μm, and Ȝ μm. Fig. 4 (b) presents the vibrational spectra of the different devices. From the resonance frequencies the SAW velocity was calculated. The SAW velocity was in the range of 5105 m/s to 5141 PV IRU Ȝ IURP to 42.2 μm. These values are comparable to the data found in the literature [7] and therefore prove that the optimized AlN films are well suited for fabrication of acoustic-wave based FPW sensors. It should be noted that the measured velocity was also affected by the Si substrate, caused by the thin piezoelectric film compared to the wavelength.
Fig. 4. (a) Optical microscope image of the transducer surface and overlay of the surface deflection, measured by laser Doppler vibrometry Ȝ 100.7 μm). (b) Vibrational spectra of devices with three different wavelength Ȝ ranging from 100.7 to 26.4 μm.
4. Summary Piezoelectric AlN films have been fabricated by reactive magnetron sputtering. In order to compensate the stress gradient in thick AlN films the nitrogen concentration in the reactive atmosphere was tuned during the growth process. This enabled the growth of 1 μm thick AlN films with low intrinsic stress of 50-200 MPa on silicon substrates as well as on AlN/Mo patterned surfaces. Due to the high degree of c-axis orientation the AlN films exhibited a high piezoelectric response of up to d33,f -7.2 pC/N. N-face polarity was measured for all AlN-based layer stacks. This can be attributed to the low power density sputter process [5]. Moreover the measured indentation modulus of 365 GPa and the analyses of the SAW velocity, which was in the range of 5105 m/s to 5141 m/s, both demonstrated that the stress optimized AlN films are well suited for FPW device fabrication. Acknowledgements The authors would like to thank N. Lehmann, S. Bühler, and K. Schäuble for technical assistance. M. Prescher is acknowledged for carrying out the XRD analysis. This work was funded by the Deutsche Forschungsgemeinschaft (DFG) within the project AM 105/24-1. References [1] S.W. Wenzel and R. M. White. A Multisensor Employing an Ultrasonic Lamb-Wave Oscillator, IEEE Trans. on Electronic Devices, Vol. 35 (1988), 735-743. [2] I.-Y. Huang et al., Development of a flexural plate-wave (FPW) immunoglobulin-E (IgE) allergy bio-sensing microsystem, Sens. Actuators, B, Vol. 162 (2012) 184– 193. [3] M. Reusch et al., Aluminium nitride membranes with embedded buried IDT electrodes for novel flexural plate wave devices, 18th IEEE Transducers 2015, pp. 1291–1294. [4] P. Katus, Entwurf und Modellierung eines mikroakustischen Sensors zur Analytik in Flüssigkeit, PhD thesis, University of Freiburg (2015). [5] M. Reusch et al., Analysis and optimization of sputter deposited AlN-layers for flexural plate wave devices, J. Vac. Sci. Technol. B. 34 (2016), in press. [6] M. Datcheva et al., Determination of Anodized Aluminum Material Characteristics by Means of Nano-Indentation Measurements Mater. Sci. Appl. 02 (2011) 1452–64. [7] Si-Hong Hoang and Gwiy-Sang Chung, Surface acoustic wave characteristics of AlN thin films grown on a polycrystalline 3C-SiC buffer layer, Microelectronic Engineering 86.11 (2009) 2149-2152.
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