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Love-wave devices with continuous and discrete nanocrystalline diamond coating for biosensing applications
Sensors and Actuators A 298 (2019) 111584
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Love-wave devices with continuous and discrete nanocrystalline diamond coating for biosensing applications L. Drbohlavová a,b,∗ , L. Fekete a , V. Bovtun a , M. Kempa a , A. Taylor a , Y. Liu c , O. Bou Matar c , A. Talbi c , V. Mortet a,b a
FZU - Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Czech Technical University, Faculty of Biomedical Engineering, Kladno, Czech Republic c Univ. Lille, Centrale Lille, UVHC, ISEN, LIA LICS/LEMAC, IEMN UMR CNRS 8520, F-59000, France b
a r t i c l e
i n f o
Article history: Received 3 May 2019 Received in revised form 10 August 2019 Accepted 2 September 2019 Available online 4 September 2019 Keywords: Love-waves Acoustic-wave devices Nanocrystalline-diamond Phase velocity COMSOL simulations ST-cut quartz
a b s t r a c t The integration of diamond layers brings biocompatibility and enhanced stability of biomolecules to Love wave devices for promising biosensor applications. Love-Wave Surface Acoustic Wave (LW-SAW) devices consisting of an ST-cut quartz substrate with a silicon oxide layer coated by a thin diamond layer have been fabricated. The effect of nucleation and growth of diamond on the properties of LW-SAW devices was studied. The Love-wave phase velocity was shown to be affected by the diamond coating and dependent on its thickness. Experimental results are in a good agreement with simulations carried out with COMSOL Multiphysics software. Deposition of isolated diamond grains results in a reduction in velocity in the Love waves, which is attributed to a mass loading effect. Conversely, coalesced diamond layers increase the stiffness of the surface, which results in faster propagation of the Love waves. Improved entrapment of the Love waves in the guiding layer was observed in the case of isolated diamond grains, which should ensure higher sensitivity for this type of coating. Diamond coated LW-SAW devices can be used for biosensing applications with appropriated functionalization of the diamond surface. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Due to their high frequency selectivity, Surface Acoustic Wave (SAW) devices have been widely used in signal processing in the telecommunication industry, such as in TV sets, mobile phones and base stations [1–3]. Apart from these applications, SAW devices are gaining importance for physical, chemical and biological detection [4] thanks to their high sensitivity, compact size, low fabrication cost as well as wireless capabilities [1,3]. With surface functionalization, SAW devices can be used for rapid, label-free pathogen detection and point-of-care testing devices [1,5,6]. As bacterial or pathogen detection is performed in aqueous or buffered solutions, acoustic waves have to be polarized, in order to reduce the waves attenuation caused by liquid loading. Rayleigh surface acoustic wave devices (RSAW) are commonly used for gas sensing. They suffer from high attenuation due to particle displacement perpendicular to the surface causing this acoustic energy to be radiated into the liquid. However, devices using pure shear hor-
∗ Corresponding author at: FZU, Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic. E-mail address: [email protected] (L. Drbohlavová). https://doi.org/10.1016/j.sna.2019.111584 0924-4247/© 2019 Elsevier B.V. All rights reserved.
izontal (SH) waves with a displacement parallel to the surface do not have radiation losses [5,7,8]. Love-wave surface acoustic wave sensors (LW-SAW) are electromechanical sensors based on a piezoelectric delay line with metallic interdigital transducers (IDTs) with a thin wave-guiding top layer. IDTs generate and receive pure SH waves confined in a thin guiding layer with a lower acoustic wave velocity than the piezoelectric substrate’s bulk [9–11]. The velocity and amplitude of Love waves is influenced by the variation of the mechanical properties of the media at the sensor’s surface [12]. To date, the most common piezoelectric substrates for the generation of SH waves are ST-cut quartz (acoustic shear wave velocity is 5060 m/s), 64◦ YX lithium niobate (∼4600 m/s) or 36◦ YX lithium tantalate (4212 m/s) [4,13]. Commonly used materials for fabrication of the guiding layer are amorphous SiO2 (2850 m/s), polymethylmethacrylate (1200 m/s) or ZnO (2747 m/s) [10,14]. LW-SAW sensors fabricated by combinations of the mentioned materials were successfully used to detect different biological analytes, such as uric acid [4], bacteriophages [15,16], Influenza A virus [17], E. coli l-asparaginase [18], or bacterial nucleic acids [19,20]. Diamond is an attractive material for bio-sensing applications because of its chemical inertness, bio-compatibility and prolonged stability of covalently attached biomolecules [21,22]. The novelty of this research is the integration of a thin diamond layer onto Love-
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wave surface acoustic wave devices. However, integration of the diamond layers is not straightforward because of the high transversal acoustic wave velocity (12 820 m s−1 ) of diamond [21] and its high deposition temperature (>700 ◦ C) by conventional plasma enhanced chemical vapor deposition [23], which is incompatible with the low Curie temperature of quartz (537 ◦ C) [24]. The studied LW-SAW devices consist of a ST-cut quartz substrate with a silicon oxide (SiO2 ) layer coated by a thin diamond layer. In this work, the effect of nucleation and growth of low temperature grown diamond on the properties of LW-SAW sensors are studied experimentally and theoretically. 2. Materials and methods 2.1. COMSOL simulations Diamond/SiO2 /ST-cut quartz structures have been simulated using COMSOL Multiphysics software. The substrate consists of an ST-cut quartz crystal with Euler angles (90◦ , 132.75◦ , 0◦ ) for shear wave propagation along the x-axis and particle displacement along the y-axis. Floquet periodic conditions were applied along the yaxis and x-axis. The thickness of the silicone oxide hSiO2 and the Love wave’s wavelength were arbitrary fixed to 2 m and 20 m respectively, which corresponds to a normalized SiO2 thickness hSiO2 / of 0.1. This hSiO2 / was chosen due to the high electromechanical coupling coefficient K2 (0.26%) of LW-SAW sensors. The theoretical study of the coalescence effect of diamond on the propagation of SH waves was carried out in three steps: 1/ deposition of an increasing number (4, 9, 16, 36, 64, 144) of diamond grains (modelled as cubes) with a fixed width (274 nm) and increasing thickness over the range of 45 nm–200 nm (see Fig. 7a–b), 2/ followed by the connection of diamond grains resulting in a decreasing number (64, 36, 16, 9, 4, 1) of grains but keeping the whole surface covered (see Fig. 7c–d). The thickness of the diamond cubes increased over the range from 210–420 nm. And finally, 3/ the growth of a fully coalesced diamond layer. The results of this study were compared to the growth of a diamond layer and the effect of a rapid coalescence. The shear horizontal ratio (RSH ) was used to distinguish waves with shear horizontal polarization among the appearing SAW:
RSH =
(ux u∗x V
v v∗ dxdydz V SH SH + vy v∗y + wz wz∗ )dxdydz
Fig. 1. Micrographs of fabricated aluminium IDTs a) without any coating, b) with amorphous SiO2 coating, c) with amorphous SiO2 and NCD layers.
Table 1 Amorphous SiO2 and NCD layers deposition conditions. NCD layer
SiO2 layer
Gas composition
92% H2 , 5% CH4 , 3% CO2
MW power Process pressure Substrate temperature Growth time/rate
2 × 2.75 kW 0.25 mbar 320 < T < 500 ◦ C 20 nm/h
150 sccm SiH4 , 700 sccm N2 O 20 W 1.3 mbar 300 ◦ C 67.5 nm/min
(1)
where ux , vy and wz are displacement components in x, y and z direction respectively, vSH is SH component of the displacement, V denotes the whole volume of unit cell. Complex conjugates are marked with the star symbol (*) [25].
Fig. 2. Structure of diamond coated LW-SAW sensor: a) cross-section, b) top view and c) image of fabricated LW-SAW sensor.
2.2. Love-wave device fabrication LW-SAW sensors were fabricated on ST-cut quartz substrates with a 1.6 m thick amorphous SiO2 layer and aluminium IDTs with a spatial period of 16 m resulting in a normalized SiO2 thickness of 0.1. 200 nm thick IDTs were patterned by RF sputtering, photolithography and wet etching in HNO3 :H3 PO4 :H2 O (1:4:4). Each consists of 110 finger pairs with an acoustic aperture of 840 m and propagation length of 320 m between input and output IDTs electrode. Fabricated electrodes are shown on Fig. 1. An amorphous SiO2 layer was deposited by low temperature plasma enhanced chemical vapor deposition using a Plasmalab 80 plus from Oxford Instruments. IDTs pads were mechanically protected during SiO2 layer deposition. Consecutive depositions of nano-crystalline diamond (NCD) layers were carried out at low temperature by microwave plasma enhanced chemical vapour deposition with linear antennas (MWLA-PECVD) [26] in order to determine the effect of diamond grain
size, coalescence and diamond thickness. Prior to NCD growth, LW-SAW samples were seeded with nanodiamond particle water based colloids (NanoAmando® B from NanoCarbon Research Institute Ltd.) by spin coating, to produce high and low particle densities, which act as nucleation sites for subsequent NCD growth, to enable growth thin coalesced NCD layers and non-coalesced NCD layers. The IDT contact pads were mechanically protected by clean lab tape as solid mask (F04xx tape from Semiconductor Equipment Corp.) during spin coating to ensure they were not subsequently coated with insulating diamond. All depositions (aluminum, SiO2 and NCD) were carried out at temperatures below 500 ◦ C to preserve the piezoelectric properties of the quartz substrates. Deposition conditions of SiO2 and NCD layers are reported in Table 1 and the structure of the LW-SAW sensor is shown in Fig. 2.
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Fig. 3. AFM images of deposited NCD layers with different nucleation density: 1/ deposition of NCD grains (upper part, white bar indicates distance of 270 nm) and 2/ deposition of closed NCD layers (lower part, white bar indicates distance of 1 m).
3. Results and discussion 3.1. Nanocrystalline-diamond layer characterization NCD coatings were characterized by Atomic Force Microscopy (AFM) using a Dimension Icon ambient AFM in Peak Force Tapping mode with Tab150AL-g tips and a Renishaw InVia Raman microscope with a 488 nm excitation laser at a power of 25 mW. Raman spectra were normalized to the diamond peak. Fig. 3 shows AFM pictures of deposited NCD coatings using two different seeding colloids. Use of the lower density colloid resulted in the growth of discrete NCD grains after the 1st deposition. A coalesced NCD layer was formed only after the 6th deposition, which corresponded to a thickness of 236 nm. Use of the higher density colloid resulted in a coalesced NCD layer after the 1st deposition (thickness of 30.8 nm). Raman spectra of coalesced NCD layers as well as discrete NCD grains are shown in Fig. 4. The diamond zone-center phonon peak, located at 1332 cm−1 , can be seen clearly for all NCD thicknesses, layers and grains while no significant sp2 carbon fraction can be observed. The peak at 1100 cm−1 for NCD layer thicknesses of 30 nm is attributed to the substrate, while peaks at 1480 and 1490 cm−1 are related to acetylene C H chains [27,28].
3.2. Love-wave device characterization Fig. 5 shows spectra of the transmission coefficient S21 measured with an Agilent E8364B vector network analyser and a Summit 9000 Analytical Probe Station with Infinity probes at room temperature. Responses of delay lines coated by coalesced NCD layers and NCD grains with different thicknesses are shown in Fig. 5. We can notice, first, several oscillations with presence of dips in the filter band-pass instead of perfect sin-lobe response of a periodic transducer. This behaviour can be attributed to destructive interferences that occur in the presence of defects like short-circuit between IDTs. For coalesced NCD layers, an increase in the operating frequency fr
Fig. 4. Raman spectra of a) coalesced NCD layers and b) NCD grains with different thicknesses.
can be clearly seen with increasing NCD layer thickness. It can be seen, that the insertion loss decreased after the deposition of NCD layer. Conversely, the delay lines coated by discrete NCD grains (see Fig. 5b) show a decrease in operating frequency fr , and the insertion loss slightly increases. Once a coalesced NCD layer is formed (thickness 236 nm), fr increases and we can observe a 2 dB increase in insertion loss. 3.3. Phase velocity dispersion Fig. 6 shows the effect of NCD thickness on the experimentally recorded normalized phase velocity that is compared with simulated results. Phase velocity was normalized to 4373 m/s (minimum measured velocity) and 5060 m/s (propagation velocity value for acoustic waves in ST-cut quartz substrates). For coalesced NCD lay-
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Fig. 5. Spectra of transmission coefficient S21 for LW-SAW sensors coated by a) coalesced NCD layer and b) NCD grains with different thicknesses. From a thickness of 236 nm, the NCD layer became coalesced.
ers the phase velocity steeply increases for initial values of NCD thickness and becomes almost constant as it approaches 5000 m/s. This result indicates that the Love waves are no longer trapped within the SiO2 guiding layer, but that they propagate only within the quartz substrate (see Fig. 7e). However, after deposition of discrete NCD grains, the phase velocity increases slightly for the first two depositions, this is attributed to annealing of the quartz substrate and amorphous SiO2 layer during diamond deposition, then the phase velocity starts to decrease until the NCD layer coalesces. The additional mass on the surface of the sensor created by isolated grains slows down the acoustic waves, which results in a decrease
Fig. 6. Normalized phase velocity of Love waves as a function of NCD layer thickness from simulation (solid lines) and experimental measurements (dashed lines) for coalesced NCD layers (circles) and discrete NCD grains (squares) on SiO2 /ST-cut quartz structure.
in operating frequency. It can also be observed, that the phase velocity is lower than for coalesced NCD coatings, which indicates improved confinement of Love waves in the guiding layer, which can be clearly observed in the Fig. 7 and therefore ensure a higher sensitivity for the discrete NCD grain coated LW-SAW sensors than for the sensors with coalesced NCD layers.
Fig. 7. Love wave mode shapes for SiO2 /ST-cut quartz structures with different numbers of NCD grains: a) 4 grains, hNCD = 45 nm, b) 64 grains, hNCD = 120 nm, c) 64 grains covering whole surface, hNCD = 210 nm, d) 4 grains, hNCD = 300 nm and e) coalesced NCD layer with the same thicknesses.
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Czech Ministry of Education, Youth and Sports (Project No. SOLID21 - CZ.02.1.01/0.0/0.0/16 019/0000760). Appendix A See Table A1. Table A1 Constants of materials used for COMSOL Multiphysics simulations.
Fig. 8. Band structure of unit cell: geometrical parameters case (Fig. 5c) and cell parameter fixed to 10 m. a) Coalesced NCD layer and b) discrete NCD grains.
Density (g·cm−3 ) Rel. Permitivity εr (−) Young’s modulus (GPa) Elastic constants (GPa)
3.4. Band structure of SH modes To explain results in depth, we calculated the band structure of SH modes (SH ratio > 0.5) for each of the cases shown in Fig. 7c. The result is shown in Fig. 8, X points are the irreducible Brillouin zone limit of the unit cell in the x direction (cell parameter is fixed to 10 m). The black continuous and broken lines correspond to the shear bulk mode in quartz and SiO2 substrates respectively. In the case of coalesced NCD layers, the Love mode is located above the limit of the substrate shear mode. This means that the mode radiates into the bulk of the substrate, which is confirmed by displacement field distribution (see Fig. 7e). For discrete NCD grain coating, the Love mode is below the limit of the substrate shear mode, whilst approaching the limit of the Brillouin zone which implies a good confinement of the mode in the SiO2 layer. 4. Conclusions We investigated experimentally and by finite element simulations the effect of nucleation and the growth of diamond, in the form of nano-crystalline diamond, on the properties of SiO2 /STcut quartz LW-SAW sensors. We successfully deposited isolated diamond grains as well as coalesced diamond layers. The phase velocity is increasing due to the increasing rigidity of the surface of the sensor as well as deeper propagation of the acoustic waves in devices with coalesced diamond layers. The phase velocity is decreasing because of the prevailing effect of mass loading in devices with the growth of discrete diamond grains. As the diamond layer becomes coalesced, the phase velocity steeply increases. An increase in phase velocity above 5060 m/s was not observed, confirming that Love waves are not propagating within the deposited diamond layer, but that it propagates within the quartz substrate. These results confirm the possible use of thin diamond coating of SiO2 /ST-cut quartz LW-SAW sensors with two different types of diamond layers. These structures can be used for biosensing applications following appropriate diamond surface functionalization. Acknowledgements This work was supported by the Technology Agency of the Czech Republic project TH2030874, the Ministry of Industry and Trade of the Czech Republic project FV10312, the LO1409 project supported by the Ministry of Education, Youth and Sport of the Czech Republic, Infrastructure SAFMAT LM2015088 project supported by the Ministry of Education, Youth and Sport of the Czech Republic and the J.E. Purkynˇe fellowship awarded to V. Mortet by Academy of Sciences of the Czech Republic. This work has also been supported by Operational Program Research, Development and Education financed by European Structural and Investment Funds and the
Piezoelectric constants (C·m−2 )
C11 C12 C13 C14 C33 C55 C66 e11 e12 e14 e25 e26
Diamond
Quartz
SiO2
3.515 5.1 1050 – – – – – – – – – – – –
2.651 4.4 – 86.7 6.99 11.9 17.9 107.2 57.9 39.9 −0.17 0.17 −0.04 0.04 0.17
2.2 4.2 70 – – – – – – – – – – – –
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Biographies Lucie Drbohlavova obtained a Master’s degree in Biomedical, Clinical Technology from Czech Technical University, Prague, Czech Republic in 2016. She is pursuing a PhD, degree in Biomedical, Clinical Technology at Czech Technical University in Prague, at Institute of Physics, Czech Academy of Sciences in the field of biosensors development by using Love wave surface acoustic wave sensors. She spent 6, months at IEMN laboratory working on SAW devices fabrication, characterization, modelling. Currently, she is on 6, months internship at the National Centre of Biotechnology, Madrid, Spain. Ladislav Fekete was born 1980 in Kosice Slovak Republic, obtained Mgr (Master of Science in Physics) [2004] in and PhD (Doctor of Philosophy in Physics - Optics and Optoelectronics) [2009] in Charles University in Prague. Since 2003 is employed in the Czech Academy of Sciences and since 2009 is employed as a head of the AFM laboratory. Viktor Bovtun was born in Kiev, Ukraine, in 1954. He received the D.E., Ph.D., and habilitation degrees from the National Technical University of Ukraine (Kiev Polytechnic Institute) in 1977, 1986, and 1993, respectively. Since 1977 he worked in the Microelectronics Department of the Technical University of Ukraine as a researcher, senior researcher, and associate professor. Since 1998 he is employed in the Department of Dielectrics, Institute of Physics, Czech Academy of Sciences, Prague, as a senior researcher. His research deals mainly with broadband dielectric spectroscopy of the ferroelectric, high-permittivity dielectric, multiferroic and inhomogeneously conducting materials, including microwave range. Martin Kempa was born in 1979 in Uherské Hradiˇstˇe, Czech Republic. He obtained his Master degree in 2003 at the Charles University, Prague, Czech Republic. In 2008 he got his PhD at the Charles University in the field of condensed matter physics. Since 2003 he is employed at Institute of Physics of the AS CR, Prague. He is specialized in microwave dielectric spectroscopy and inelastic neutron scattering. He
spent two years in total at the Institute Laue-Langevin, Grenoble, France as a PhD student and research scientist, where he participated on commissioning and tests of the FlatCone and Lagrange multianalyzers. Andrew Taylor has over 28 years’ experience working in the field of diamond research, 18 years of which were at De Beers Technologies, UK. Whilst at De Beers he was a key part of the team investigating the properties of single crystal CVD grown diamond. Since February 2009 he has helped to establish a new laboratory within the Materials for Nanosystems and Biointerfaces group at FZU, Prague, Czech Republic for the growth of diamond layers using MW PECVD technologies. He has authored or co-authored 46 cited publications as well as 2 related patents and has an H-index of 10. Yuxin LIU was born in 1990 at Chengdu, China. She obtained a Master’s degree in Electronic from Beijing Beihang University, China and General Engineering degree from Ecole Centrale, France in 2016 (double degree). She is pursuing a PhD degree in microelectronics and nanotechnology at IEMN laboratory and works in the field of Love waves coupled phononic crystals and metamaterials for sensing applications. Olivier Bou Matar is currently Professor at Centrale Lille and head of the Acoustics Department of IEMN (UMR 8520). He received his PhD in Acoustics at the University of Tours in 1997. After his PhD, he became an Associate Professor at the IUT of Blois in 1998 and Full Professor at Ecole Centrale de Lille in 2005. His fields of research cover Nonlinear magneto-elasticity, Phononic crystal devices for sensing and RF applications, Nonlinear acoustic imaging and NDT, and Droplet manipulation by SAW. He co-authored 175 scientific publications including 65 papers in peer-reviewed journals, 2 books chapters and 4 patents. Abdelkrim Talbi is Full Professor at “Centrale Lille” and researcher at IMEN UMR CNRS 8520 Institute. He is a member of the International Laboratory on Critical and Supercritical phenomena in functional electronics, acoustics and fluidics (formerly LIA LICS- LEMAC). He received the MS degree in plasma, optics, electronics, and microsystems from the Universities of Metz, Nancy I, and Sup- Elec Metz, in 2000, and the PhD degree in 2003 (LPMIA) from the university of Nancy I. He has authored or coauthored of over 90-refereed publications and he is listed as co-inventor of 5 patents. Vincent Mortet holds a Ph.D. in Physics (2001) from the University of Valenciennes and Hainaut–Cambrésis. He had been a post-doctoral fellow at the Institute for Materials Research (Belgium) and Laboratory for Analysis and Architecture of Systems (France) between 2001 and 2013. He joined the Institute of Physics of the Czech Academy of Sciences as a senior researcher in December 2013. He is currently leading the Nanosystems and Biointerfaces Group from the Department of Functional Materials. His main research interests are the CVD diamond synthesis and their properties for their potential applications in electronics.
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Love-wave devices with continuous and discrete nanocrystalline diamond coating for biosensing applications L. Drbohlavová a,b,∗ , L. Fekete a , V. Bovtun a , M. Kempa a , A. Taylor a , Y. Liu c , O. Bou Matar c , A. Talbi c , V. Mortet a,b a
FZU - Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic Czech Technical University, Faculty of Biomedical Engineering, Kladno, Czech Republic c Univ. Lille, Centrale Lille, UVHC, ISEN, LIA LICS/LEMAC, IEMN UMR CNRS 8520, F-59000, France b
a r t i c l e
i n f o
Article history: Received 3 May 2019 Received in revised form 10 August 2019 Accepted 2 September 2019 Available online 4 September 2019 Keywords: Love-waves Acoustic-wave devices Nanocrystalline-diamond Phase velocity COMSOL simulations ST-cut quartz
a b s t r a c t The integration of diamond layers brings biocompatibility and enhanced stability of biomolecules to Love wave devices for promising biosensor applications. Love-Wave Surface Acoustic Wave (LW-SAW) devices consisting of an ST-cut quartz substrate with a silicon oxide layer coated by a thin diamond layer have been fabricated. The effect of nucleation and growth of diamond on the properties of LW-SAW devices was studied. The Love-wave phase velocity was shown to be affected by the diamond coating and dependent on its thickness. Experimental results are in a good agreement with simulations carried out with COMSOL Multiphysics software. Deposition of isolated diamond grains results in a reduction in velocity in the Love waves, which is attributed to a mass loading effect. Conversely, coalesced diamond layers increase the stiffness of the surface, which results in faster propagation of the Love waves. Improved entrapment of the Love waves in the guiding layer was observed in the case of isolated diamond grains, which should ensure higher sensitivity for this type of coating. Diamond coated LW-SAW devices can be used for biosensing applications with appropriated functionalization of the diamond surface. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Due to their high frequency selectivity, Surface Acoustic Wave (SAW) devices have been widely used in signal processing in the telecommunication industry, such as in TV sets, mobile phones and base stations [1–3]. Apart from these applications, SAW devices are gaining importance for physical, chemical and biological detection [4] thanks to their high sensitivity, compact size, low fabrication cost as well as wireless capabilities [1,3]. With surface functionalization, SAW devices can be used for rapid, label-free pathogen detection and point-of-care testing devices [1,5,6]. As bacterial or pathogen detection is performed in aqueous or buffered solutions, acoustic waves have to be polarized, in order to reduce the waves attenuation caused by liquid loading. Rayleigh surface acoustic wave devices (RSAW) are commonly used for gas sensing. They suffer from high attenuation due to particle displacement perpendicular to the surface causing this acoustic energy to be radiated into the liquid. However, devices using pure shear hor-
∗ Corresponding author at: FZU, Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic. E-mail address: [email protected] (L. Drbohlavová). https://doi.org/10.1016/j.sna.2019.111584 0924-4247/© 2019 Elsevier B.V. All rights reserved.
izontal (SH) waves with a displacement parallel to the surface do not have radiation losses [5,7,8]. Love-wave surface acoustic wave sensors (LW-SAW) are electromechanical sensors based on a piezoelectric delay line with metallic interdigital transducers (IDTs) with a thin wave-guiding top layer. IDTs generate and receive pure SH waves confined in a thin guiding layer with a lower acoustic wave velocity than the piezoelectric substrate’s bulk [9–11]. The velocity and amplitude of Love waves is influenced by the variation of the mechanical properties of the media at the sensor’s surface [12]. To date, the most common piezoelectric substrates for the generation of SH waves are ST-cut quartz (acoustic shear wave velocity is 5060 m/s), 64◦ YX lithium niobate (∼4600 m/s) or 36◦ YX lithium tantalate (4212 m/s) [4,13]. Commonly used materials for fabrication of the guiding layer are amorphous SiO2 (2850 m/s), polymethylmethacrylate (1200 m/s) or ZnO (2747 m/s) [10,14]. LW-SAW sensors fabricated by combinations of the mentioned materials were successfully used to detect different biological analytes, such as uric acid [4], bacteriophages [15,16], Influenza A virus [17], E. coli l-asparaginase [18], or bacterial nucleic acids [19,20]. Diamond is an attractive material for bio-sensing applications because of its chemical inertness, bio-compatibility and prolonged stability of covalently attached biomolecules [21,22]. The novelty of this research is the integration of a thin diamond layer onto Love-
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wave surface acoustic wave devices. However, integration of the diamond layers is not straightforward because of the high transversal acoustic wave velocity (12 820 m s−1 ) of diamond [21] and its high deposition temperature (>700 ◦ C) by conventional plasma enhanced chemical vapor deposition [23], which is incompatible with the low Curie temperature of quartz (537 ◦ C) [24]. The studied LW-SAW devices consist of a ST-cut quartz substrate with a silicon oxide (SiO2 ) layer coated by a thin diamond layer. In this work, the effect of nucleation and growth of low temperature grown diamond on the properties of LW-SAW sensors are studied experimentally and theoretically. 2. Materials and methods 2.1. COMSOL simulations Diamond/SiO2 /ST-cut quartz structures have been simulated using COMSOL Multiphysics software. The substrate consists of an ST-cut quartz crystal with Euler angles (90◦ , 132.75◦ , 0◦ ) for shear wave propagation along the x-axis and particle displacement along the y-axis. Floquet periodic conditions were applied along the yaxis and x-axis. The thickness of the silicone oxide hSiO2 and the Love wave’s wavelength were arbitrary fixed to 2 m and 20 m respectively, which corresponds to a normalized SiO2 thickness hSiO2 / of 0.1. This hSiO2 / was chosen due to the high electromechanical coupling coefficient K2 (0.26%) of LW-SAW sensors. The theoretical study of the coalescence effect of diamond on the propagation of SH waves was carried out in three steps: 1/ deposition of an increasing number (4, 9, 16, 36, 64, 144) of diamond grains (modelled as cubes) with a fixed width (274 nm) and increasing thickness over the range of 45 nm–200 nm (see Fig. 7a–b), 2/ followed by the connection of diamond grains resulting in a decreasing number (64, 36, 16, 9, 4, 1) of grains but keeping the whole surface covered (see Fig. 7c–d). The thickness of the diamond cubes increased over the range from 210–420 nm. And finally, 3/ the growth of a fully coalesced diamond layer. The results of this study were compared to the growth of a diamond layer and the effect of a rapid coalescence. The shear horizontal ratio (RSH ) was used to distinguish waves with shear horizontal polarization among the appearing SAW:
RSH =
(ux u∗x V
v v∗ dxdydz V SH SH + vy v∗y + wz wz∗ )dxdydz
Fig. 1. Micrographs of fabricated aluminium IDTs a) without any coating, b) with amorphous SiO2 coating, c) with amorphous SiO2 and NCD layers.
Table 1 Amorphous SiO2 and NCD layers deposition conditions. NCD layer
SiO2 layer
Gas composition
92% H2 , 5% CH4 , 3% CO2
MW power Process pressure Substrate temperature Growth time/rate
2 × 2.75 kW 0.25 mbar 320 < T < 500 ◦ C 20 nm/h
150 sccm SiH4 , 700 sccm N2 O 20 W 1.3 mbar 300 ◦ C 67.5 nm/min
(1)
where ux , vy and wz are displacement components in x, y and z direction respectively, vSH is SH component of the displacement, V denotes the whole volume of unit cell. Complex conjugates are marked with the star symbol (*) [25].
Fig. 2. Structure of diamond coated LW-SAW sensor: a) cross-section, b) top view and c) image of fabricated LW-SAW sensor.
2.2. Love-wave device fabrication LW-SAW sensors were fabricated on ST-cut quartz substrates with a 1.6 m thick amorphous SiO2 layer and aluminium IDTs with a spatial period of 16 m resulting in a normalized SiO2 thickness of 0.1. 200 nm thick IDTs were patterned by RF sputtering, photolithography and wet etching in HNO3 :H3 PO4 :H2 O (1:4:4). Each consists of 110 finger pairs with an acoustic aperture of 840 m and propagation length of 320 m between input and output IDTs electrode. Fabricated electrodes are shown on Fig. 1. An amorphous SiO2 layer was deposited by low temperature plasma enhanced chemical vapor deposition using a Plasmalab 80 plus from Oxford Instruments. IDTs pads were mechanically protected during SiO2 layer deposition. Consecutive depositions of nano-crystalline diamond (NCD) layers were carried out at low temperature by microwave plasma enhanced chemical vapour deposition with linear antennas (MWLA-PECVD) [26] in order to determine the effect of diamond grain
size, coalescence and diamond thickness. Prior to NCD growth, LW-SAW samples were seeded with nanodiamond particle water based colloids (NanoAmando® B from NanoCarbon Research Institute Ltd.) by spin coating, to produce high and low particle densities, which act as nucleation sites for subsequent NCD growth, to enable growth thin coalesced NCD layers and non-coalesced NCD layers. The IDT contact pads were mechanically protected by clean lab tape as solid mask (F04xx tape from Semiconductor Equipment Corp.) during spin coating to ensure they were not subsequently coated with insulating diamond. All depositions (aluminum, SiO2 and NCD) were carried out at temperatures below 500 ◦ C to preserve the piezoelectric properties of the quartz substrates. Deposition conditions of SiO2 and NCD layers are reported in Table 1 and the structure of the LW-SAW sensor is shown in Fig. 2.
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Fig. 3. AFM images of deposited NCD layers with different nucleation density: 1/ deposition of NCD grains (upper part, white bar indicates distance of 270 nm) and 2/ deposition of closed NCD layers (lower part, white bar indicates distance of 1 m).
3. Results and discussion 3.1. Nanocrystalline-diamond layer characterization NCD coatings were characterized by Atomic Force Microscopy (AFM) using a Dimension Icon ambient AFM in Peak Force Tapping mode with Tab150AL-g tips and a Renishaw InVia Raman microscope with a 488 nm excitation laser at a power of 25 mW. Raman spectra were normalized to the diamond peak. Fig. 3 shows AFM pictures of deposited NCD coatings using two different seeding colloids. Use of the lower density colloid resulted in the growth of discrete NCD grains after the 1st deposition. A coalesced NCD layer was formed only after the 6th deposition, which corresponded to a thickness of 236 nm. Use of the higher density colloid resulted in a coalesced NCD layer after the 1st deposition (thickness of 30.8 nm). Raman spectra of coalesced NCD layers as well as discrete NCD grains are shown in Fig. 4. The diamond zone-center phonon peak, located at 1332 cm−1 , can be seen clearly for all NCD thicknesses, layers and grains while no significant sp2 carbon fraction can be observed. The peak at 1100 cm−1 for NCD layer thicknesses of 30 nm is attributed to the substrate, while peaks at 1480 and 1490 cm−1 are related to acetylene C H chains [27,28].
3.2. Love-wave device characterization Fig. 5 shows spectra of the transmission coefficient S21 measured with an Agilent E8364B vector network analyser and a Summit 9000 Analytical Probe Station with Infinity probes at room temperature. Responses of delay lines coated by coalesced NCD layers and NCD grains with different thicknesses are shown in Fig. 5. We can notice, first, several oscillations with presence of dips in the filter band-pass instead of perfect sin-lobe response of a periodic transducer. This behaviour can be attributed to destructive interferences that occur in the presence of defects like short-circuit between IDTs. For coalesced NCD layers, an increase in the operating frequency fr
Fig. 4. Raman spectra of a) coalesced NCD layers and b) NCD grains with different thicknesses.
can be clearly seen with increasing NCD layer thickness. It can be seen, that the insertion loss decreased after the deposition of NCD layer. Conversely, the delay lines coated by discrete NCD grains (see Fig. 5b) show a decrease in operating frequency fr , and the insertion loss slightly increases. Once a coalesced NCD layer is formed (thickness 236 nm), fr increases and we can observe a 2 dB increase in insertion loss. 3.3. Phase velocity dispersion Fig. 6 shows the effect of NCD thickness on the experimentally recorded normalized phase velocity that is compared with simulated results. Phase velocity was normalized to 4373 m/s (minimum measured velocity) and 5060 m/s (propagation velocity value for acoustic waves in ST-cut quartz substrates). For coalesced NCD lay-
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Fig. 5. Spectra of transmission coefficient S21 for LW-SAW sensors coated by a) coalesced NCD layer and b) NCD grains with different thicknesses. From a thickness of 236 nm, the NCD layer became coalesced.
ers the phase velocity steeply increases for initial values of NCD thickness and becomes almost constant as it approaches 5000 m/s. This result indicates that the Love waves are no longer trapped within the SiO2 guiding layer, but that they propagate only within the quartz substrate (see Fig. 7e). However, after deposition of discrete NCD grains, the phase velocity increases slightly for the first two depositions, this is attributed to annealing of the quartz substrate and amorphous SiO2 layer during diamond deposition, then the phase velocity starts to decrease until the NCD layer coalesces. The additional mass on the surface of the sensor created by isolated grains slows down the acoustic waves, which results in a decrease
Fig. 6. Normalized phase velocity of Love waves as a function of NCD layer thickness from simulation (solid lines) and experimental measurements (dashed lines) for coalesced NCD layers (circles) and discrete NCD grains (squares) on SiO2 /ST-cut quartz structure.
in operating frequency. It can also be observed, that the phase velocity is lower than for coalesced NCD coatings, which indicates improved confinement of Love waves in the guiding layer, which can be clearly observed in the Fig. 7 and therefore ensure a higher sensitivity for the discrete NCD grain coated LW-SAW sensors than for the sensors with coalesced NCD layers.
Fig. 7. Love wave mode shapes for SiO2 /ST-cut quartz structures with different numbers of NCD grains: a) 4 grains, hNCD = 45 nm, b) 64 grains, hNCD = 120 nm, c) 64 grains covering whole surface, hNCD = 210 nm, d) 4 grains, hNCD = 300 nm and e) coalesced NCD layer with the same thicknesses.
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Czech Ministry of Education, Youth and Sports (Project No. SOLID21 - CZ.02.1.01/0.0/0.0/16 019/0000760). Appendix A See Table A1. Table A1 Constants of materials used for COMSOL Multiphysics simulations.
Fig. 8. Band structure of unit cell: geometrical parameters case (Fig. 5c) and cell parameter fixed to 10 m. a) Coalesced NCD layer and b) discrete NCD grains.
Density (g·cm−3 ) Rel. Permitivity εr (−) Young’s modulus (GPa) Elastic constants (GPa)
3.4. Band structure of SH modes To explain results in depth, we calculated the band structure of SH modes (SH ratio > 0.5) for each of the cases shown in Fig. 7c. The result is shown in Fig. 8, X points are the irreducible Brillouin zone limit of the unit cell in the x direction (cell parameter is fixed to 10 m). The black continuous and broken lines correspond to the shear bulk mode in quartz and SiO2 substrates respectively. In the case of coalesced NCD layers, the Love mode is located above the limit of the substrate shear mode. This means that the mode radiates into the bulk of the substrate, which is confirmed by displacement field distribution (see Fig. 7e). For discrete NCD grain coating, the Love mode is below the limit of the substrate shear mode, whilst approaching the limit of the Brillouin zone which implies a good confinement of the mode in the SiO2 layer. 4. Conclusions We investigated experimentally and by finite element simulations the effect of nucleation and the growth of diamond, in the form of nano-crystalline diamond, on the properties of SiO2 /STcut quartz LW-SAW sensors. We successfully deposited isolated diamond grains as well as coalesced diamond layers. The phase velocity is increasing due to the increasing rigidity of the surface of the sensor as well as deeper propagation of the acoustic waves in devices with coalesced diamond layers. The phase velocity is decreasing because of the prevailing effect of mass loading in devices with the growth of discrete diamond grains. As the diamond layer becomes coalesced, the phase velocity steeply increases. An increase in phase velocity above 5060 m/s was not observed, confirming that Love waves are not propagating within the deposited diamond layer, but that it propagates within the quartz substrate. These results confirm the possible use of thin diamond coating of SiO2 /ST-cut quartz LW-SAW sensors with two different types of diamond layers. These structures can be used for biosensing applications following appropriate diamond surface functionalization. Acknowledgements This work was supported by the Technology Agency of the Czech Republic project TH2030874, the Ministry of Industry and Trade of the Czech Republic project FV10312, the LO1409 project supported by the Ministry of Education, Youth and Sport of the Czech Republic, Infrastructure SAFMAT LM2015088 project supported by the Ministry of Education, Youth and Sport of the Czech Republic and the J.E. Purkynˇe fellowship awarded to V. Mortet by Academy of Sciences of the Czech Republic. This work has also been supported by Operational Program Research, Development and Education financed by European Structural and Investment Funds and the
Piezoelectric constants (C·m−2 )
C11 C12 C13 C14 C33 C55 C66 e11 e12 e14 e25 e26
Diamond
Quartz
SiO2
3.515 5.1 1050 – – – – – – – – – – – –
2.651 4.4 – 86.7 6.99 11.9 17.9 107.2 57.9 39.9 −0.17 0.17 −0.04 0.04 0.17
2.2 4.2 70 – – – – – – – – – – – –
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Biographies Lucie Drbohlavova obtained a Master’s degree in Biomedical, Clinical Technology from Czech Technical University, Prague, Czech Republic in 2016. She is pursuing a PhD, degree in Biomedical, Clinical Technology at Czech Technical University in Prague, at Institute of Physics, Czech Academy of Sciences in the field of biosensors development by using Love wave surface acoustic wave sensors. She spent 6, months at IEMN laboratory working on SAW devices fabrication, characterization, modelling. Currently, she is on 6, months internship at the National Centre of Biotechnology, Madrid, Spain. Ladislav Fekete was born 1980 in Kosice Slovak Republic, obtained Mgr (Master of Science in Physics) [2004] in and PhD (Doctor of Philosophy in Physics - Optics and Optoelectronics) [2009] in Charles University in Prague. Since 2003 is employed in the Czech Academy of Sciences and since 2009 is employed as a head of the AFM laboratory. Viktor Bovtun was born in Kiev, Ukraine, in 1954. He received the D.E., Ph.D., and habilitation degrees from the National Technical University of Ukraine (Kiev Polytechnic Institute) in 1977, 1986, and 1993, respectively. Since 1977 he worked in the Microelectronics Department of the Technical University of Ukraine as a researcher, senior researcher, and associate professor. Since 1998 he is employed in the Department of Dielectrics, Institute of Physics, Czech Academy of Sciences, Prague, as a senior researcher. His research deals mainly with broadband dielectric spectroscopy of the ferroelectric, high-permittivity dielectric, multiferroic and inhomogeneously conducting materials, including microwave range. Martin Kempa was born in 1979 in Uherské Hradiˇstˇe, Czech Republic. He obtained his Master degree in 2003 at the Charles University, Prague, Czech Republic. In 2008 he got his PhD at the Charles University in the field of condensed matter physics. Since 2003 he is employed at Institute of Physics of the AS CR, Prague. He is specialized in microwave dielectric spectroscopy and inelastic neutron scattering. He
spent two years in total at the Institute Laue-Langevin, Grenoble, France as a PhD student and research scientist, where he participated on commissioning and tests of the FlatCone and Lagrange multianalyzers. Andrew Taylor has over 28 years’ experience working in the field of diamond research, 18 years of which were at De Beers Technologies, UK. Whilst at De Beers he was a key part of the team investigating the properties of single crystal CVD grown diamond. Since February 2009 he has helped to establish a new laboratory within the Materials for Nanosystems and Biointerfaces group at FZU, Prague, Czech Republic for the growth of diamond layers using MW PECVD technologies. He has authored or co-authored 46 cited publications as well as 2 related patents and has an H-index of 10. Yuxin LIU was born in 1990 at Chengdu, China. She obtained a Master’s degree in Electronic from Beijing Beihang University, China and General Engineering degree from Ecole Centrale, France in 2016 (double degree). She is pursuing a PhD degree in microelectronics and nanotechnology at IEMN laboratory and works in the field of Love waves coupled phononic crystals and metamaterials for sensing applications. Olivier Bou Matar is currently Professor at Centrale Lille and head of the Acoustics Department of IEMN (UMR 8520). He received his PhD in Acoustics at the University of Tours in 1997. After his PhD, he became an Associate Professor at the IUT of Blois in 1998 and Full Professor at Ecole Centrale de Lille in 2005. His fields of research cover Nonlinear magneto-elasticity, Phononic crystal devices for sensing and RF applications, Nonlinear acoustic imaging and NDT, and Droplet manipulation by SAW. He co-authored 175 scientific publications including 65 papers in peer-reviewed journals, 2 books chapters and 4 patents. Abdelkrim Talbi is Full Professor at “Centrale Lille” and researcher at IMEN UMR CNRS 8520 Institute. He is a member of the International Laboratory on Critical and Supercritical phenomena in functional electronics, acoustics and fluidics (formerly LIA LICS- LEMAC). He received the MS degree in plasma, optics, electronics, and microsystems from the Universities of Metz, Nancy I, and Sup- Elec Metz, in 2000, and the PhD degree in 2003 (LPMIA) from the university of Nancy I. He has authored or coauthored of over 90-refereed publications and he is listed as co-inventor of 5 patents. Vincent Mortet holds a Ph.D. in Physics (2001) from the University of Valenciennes and Hainaut–Cambrésis. He had been a post-doctoral fellow at the Institute for Materials Research (Belgium) and Laboratory for Analysis and Architecture of Systems (France) between 2001 and 2013. He joined the Institute of Physics of the Czech Academy of Sciences as a senior researcher in December 2013. He is currently leading the Nanosystems and Biointerfaces Group from the Department of Functional Materials. His main research interests are the CVD diamond synthesis and their properties for their potential applications in electronics.