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Fabrication of force sensors based on two-dimensional photonic crystal technology
Microelectronic Engineering 84 (2007) 1450–1453 www.elsevier.com/locate/mee
Fabrication of force sensors based on two-dimensional photonic crystal technology T. Stomeo a, M. Grande a, A. Qualtieri a, A. Passaseo a, A. Salhi a, M. De Vittorio a,*, D. Biallo b, A. D’orazio b, M. De Sario b, V. Marrocco b, V. Petruzzelli b, F. Prudenzano a
c
National Nanotechnology Laboratory (NNL) – CNR-INFM, Universita` di Lecce, Via Arnesano, 73100 Lecce, Italy b Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, V. Re David 200, 70125 Bari, Italy c DIASS, Politecnico di Bari, Viale del Turismo 8, 74100 Taranto, Italy Available online 9 February 2007
Abstract We propose the simulation and the fabrication of a photonic crystal (PhC) strain-sensitive structure, showing that the optical properties of photonic crystals can be used to realize sensing devices characterized by a high degree of compactness and good resolution. The force/pressure optical sensor has been realized by designing a bulk GaAs/AlGaAs photonic crystal microcavity operating in the wavelength range 1300–1400 nm. The simulations show that the resonant wavelength of the mode localized in the microcavity shifts its spectral position following a linear behaviour when a pressure ranging between 0.25 Gpa and 5 GPa is applied, thus allowing the possibility to achieve pressure resolution of 5.82 nm/GPa. High-resolution electron beam lithography technique followed by inductively coupled plasma process were used to transfer the designed geometry on the sample. 2007 Elsevier B.V. All rights reserved. Keywords: Photonic crystal; Sensors; Microcavity; FDTD method
1. Introduction In the last years the interest for microsensors has drastically grown thanks to their low cost, small dimensions, easy integration. At the same time, the development of nanotechnologies allowed the possibility to fabricate photonic crystal (PhC) structures [1,2], periodic patterns that can control and guide the photons by tailoring the transmission and reflection properties in a specific direction and frequency range. Photonic crystals have been used in a wide range of applications such as 90 bend waveguides, filters, lasers, amplifiers, resonators, non-linear devices. Their application as sensors is a recent research field which seems to be very promising due to their extreme miniaturization, high spectral sensitivity and MEMS integration. So far, a few architectures based on PhCs technology have
*
Corresponding author. Tel.: +39 832298200. E-mail address: [email protected] (M.D. Vittorio).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.227
been proposed in the literature such as bio-sensors [3] and displacement sensors [4]. In this paper we report on the design and fabrication of a PhC strain-sensitive device consisting of a PhC Waveguide (PhCW) coupled to a PhC microcavity obtained by creating a point defect in the regular 2D periodic pattern. The waveguide is designed in order to allow the propagation of an infrared signal centred around k = 1330 nm. The coupling with the point defect causes a dip in the transmission spectrum corresponding to the microcavity resonating wavelength. When a force is applied to the sample, a variation in the refractive index of the structure occurs due to the photoelastic effect in GaAs. This modifies the optical properties of the cavity, thus shifting the resonance wavelength in the transmission spectrum. The amount of such spectral displacement can be therefore exploited to measure the applied strain. In particular, the numerical results, obtained by a home-made FDTD based code, reveal that the spectral position of the resonating dip detected at the output of the PhCW shifts towards higher
T. Stomeo et al. / Microelectronic Engineering 84 (2007) 1450–1453
wavelengths as the pressure value is increased. The shift linearly depends on the applied pressure, being displaced of about 5 nm per GPa. A detailed description of the nanotechnology process necessary for the fabrication of the bulk force/pressure optical sensor is reported. It includes the definition of 2D-PhC pattern by means of electron beam lithography (EBL) and an inductively coupled plasma (ICP) etching process which employed a SiCl4/He gas mixture. 2. Simulation results The sensor is modelled by a microcavity coupled to a PhC single line defect waveguide (W1) with a triangular lattice of air holes, having period a = 360 nm and radius r = 0.35a realized on a GaAs/AlGaAs slab waveguide. The microcavity is designed by removing one hole and by modifying the radii of the surrounding six holes. This variation provides at the same time a better coupling between the W1 waveguide and the defect and improves the Q factor of the microcavity because of the envelope function of the in-plane mode profile varies more gently than the single defect case (Fig. 1). A preliminary analysis of the unperturbed PhC has been performed by using the RSoft (FullWave) FDTD software. The equivalent 2D structure was obtained by the refractive effective index method. The analysis performed in this work has been concentrated on the TM polarization (mag-
Fig. 1. Microcavity coupled to the photonic crystal. The red holes have r = 0.35a, the blue holes have r = 0.38a and the cyan holes have r = 0.34a. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
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netic field component parallel to air hole axis), for which the PhC structure exhibits a large band gap between 1150 nm and 1800 nm; the device also shows a smaller stop band for TE polarization between 1650 nm and 1850 nm. A proprietary parallel 3D-FDTD computer code, running on a Linux Cluster CLX CINECA, having 1024 CPUs, has been used to analyze the nature of the resonant modes. For all simulations, a grid size has been chosen equal to a/20, whereas the temporal step size is 3.178 · 1017 s, corresponding to 90% of the Courant limit, and uniaxial perfectly matched layers (UPML) are used to limit the computational domain. The structure has been excited by an input Gaussian pulse characterized by a wide spectrum centred at k = 1310 nm. Fig. 1a shows the simulation of light propagation through the force-sensitive structure. The height of the air holes has been fixed equal to 800 nm, by 3D simulations, value that guarantees a good resolution of the band gap and smaller losses. In absence of applied pressure, the cavity shows a resonant mode at the wavelength k = 1332 nm (Fig. 2b).
Fig. 2. (a) Simulation of light propagation through the force-sensitive structure; (b) output transmission spectrum in absence of applied pressure. The resonant wavelength is equal to 1332 nm.
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T. Stomeo et al. / Microelectronic Engineering 84 (2007) 1450–1453
3. Strain effect analysis We investigate the modification of band gap in a photonic bandgap structure undergoing force application. The operation principle of the PhC pressure sensor is based on the fact that the force application provokes, due to the photoelastic, piezoelectric and electrooptic properties of the materials constituting the structure, a change of the refractive index which modifies the transmission spectrum of the regular photonic crystal or that of the localized state when a defected photonic crystal is considered. To model the pressure action, the change of the dielectric impermeability tensor b of the GaAs and AlGaAs, accounting for the strain and stress effects acting on the lattice of the PhC, has been evaluated. The final expression for the dielectric impermeability tensor is obtained by combining all the effects mentioned above: Db ¼ ðpE : s rT gT Þ : T
ð1Þ
where p is the photoelastic fourth-order tensor at constant electric field; s is the fourth-rank elastic compliance sensor,
rT = rS + p: dS is the electrooptic third-rank tensor at constant stress in which rS is the third-order electrooptic tensor at constant strain u and dS is the third-rank piezoelectric tensor at constant strain; gT is the third-rank piezoelectric tensor at constant stress, and T is the stress tensor. Simulation results show that the refractive index change is mainly due to the photoelastic effect when a force is applied normally to the plane of photonic crystal. We calculate the transmission evaluated at the W1 output for different values of the applied pressure. The drop peak corresponds to the resonant wavelength of the mode localized in the microcavity, and it shifts its spectral position, following a linear law when a pressure ranging between 0.25 Gpa and 5 GPa is applied (Fig. 3a). By the simulation data shown in Fig. 3a it is possible to estimate the sensor sensitivity that is equal to 5.82 nm/GPa. Therefore, if the pressure is applied on an area equal to 1 mm2, the numerical simulations show that the detectable minimum force is about 0.3 mN. Fig. 3b shows that there is a linear relationship between the output drop wavelength and the applied pressure. 4. Experimental process
Fig. 3. (a) Transmission spectra for different applied pressure. The force range is 0–5 GPa. (b) Calibration curve: the relation between the pressure and the output drop wavelength is a linear dependence.
The geometry of the bulk 2D-PhC force-sensitive device has been obtained by patterning a GaAs/ Al0.7Ga0.3As heterostructure by means of EBL and ICP. The sample has been grown by molecular beam epitaxy (MBE): it consists of a 300 nm thick GaAs slab waveguide on a 2000 nm thick cladding of Al0.7Ga0.3As. The sample has been subsequently covered with 300 nm spincoated positive e-beam resist. We choose ZEP-520A resist from Zeon Chemicals L.P. due to high selectivity in SiCl4 ICP processes. In order to minimize the proximity effect due to the scattering of electrons in the 300 nm thick resist layer, we have performed preliminary e-beam exposure resolution tests at three different values of e-beam energy carried out by a Raith150 lithography system. From the analysis of the cross-sectional profile of the 300 nm thick resist layer after the e-beam exposures at 10 keV, 15 keV and 20 keV, we found that a vertical profile is achieved with an e-beam energy of 20 keV, which also guarantees the vertical transfer down to the GaAs slab of the photonic crystal pattern during the dry-etching process. The 2D-PhC patterns of air holes, arranged in a triangular lattice, has been fabricated by means of high-resolution EBL process performed at 20 keV, 15.95 pA and an area dose of 40 lC/cm2. Subsequently the pattern has been transferred down to the GaAs guiding layer by an ICP process which employed a SiCl4/He gas mixture. The rf coil power and the rf platen power have been set to 300 W and 60 W, respectively. The selectivity of our ZEP-520A mask to GaAs is of about 1:7 whilst the height of the air holes is equal to about 800 nm. Fig. 4 shows the top-view scanning electron micrograph (SEM) of the bulk force-sensitive device after EBL and ICP processes. Thus
T. Stomeo et al. / Microelectronic Engineering 84 (2007) 1450–1453
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5. Conclusion We have proposed to apply the optical properties of photonic crystals in order to realize sensing devices characterized by a high degree of compactness and a good resolution. A force/pressure optical sensor has been realized by designing and fabricating a GaAs/AlGaAs photonic crystal microcavity for the wavelength range 1300–1400 nm, with a sensitivity in the mN range. The fabricated structure is currently under optical characterization in order to experimentally evaluate its strain sensitivity. This sensitivity could be further improved by either maximizing the quality factor of the resonance state or by fabricating the same 2DPhC structure on a membrane configuration; the integration of these two devices, on the same substrate, would enable the realization of strain sensors covering a broad strain range. It is noteworthy that the same architecture and technology can be directly applied for the realization of photonic crystal actuators, where the applied strain can be used to tune the emission properties of active photonic crystal devices. Acknowledgements
Fig. 4. (a) SEM top-view of the bulk force-sensitive device after EBL and ICP processes; (b) SEM detail of the microcavity coupled to the waveguide.
highly ordered and periodical geometry of the pattern of air holes indicates the good quality of the bulk 2D-PhC force/pressure optical sensor. The PhCW is connected to 10 lm wide guides by two modal adapter for optical characterization.
The authors gratefully thank the expert technical help of Gianmichele Epifani. This work was supported by MIURPRIN2005, in the framework of the national research project ‘‘Photonic Bandgap Nanosensor’’. References [1] E. Yablonovitch, JOSA B 19 (1997) 283–295. [2] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Princeton University Press, 1995. [3] E. Chow, A. Grot, L.W. Mirkarimi, M. Sigalas, G. Girolami, Opt. Lett. 29 (2004) 1093–1095. [4] W. Suh, M.F. Yanik, O. Solgaard, S. Fan, Appl. Phys. Lett. 82 (2003) 1999–2001.
Fabrication of force sensors based on two-dimensional photonic crystal technology T. Stomeo a, M. Grande a, A. Qualtieri a, A. Passaseo a, A. Salhi a, M. De Vittorio a,*, D. Biallo b, A. D’orazio b, M. De Sario b, V. Marrocco b, V. Petruzzelli b, F. Prudenzano a
c
National Nanotechnology Laboratory (NNL) – CNR-INFM, Universita` di Lecce, Via Arnesano, 73100 Lecce, Italy b Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, V. Re David 200, 70125 Bari, Italy c DIASS, Politecnico di Bari, Viale del Turismo 8, 74100 Taranto, Italy Available online 9 February 2007
Abstract We propose the simulation and the fabrication of a photonic crystal (PhC) strain-sensitive structure, showing that the optical properties of photonic crystals can be used to realize sensing devices characterized by a high degree of compactness and good resolution. The force/pressure optical sensor has been realized by designing a bulk GaAs/AlGaAs photonic crystal microcavity operating in the wavelength range 1300–1400 nm. The simulations show that the resonant wavelength of the mode localized in the microcavity shifts its spectral position following a linear behaviour when a pressure ranging between 0.25 Gpa and 5 GPa is applied, thus allowing the possibility to achieve pressure resolution of 5.82 nm/GPa. High-resolution electron beam lithography technique followed by inductively coupled plasma process were used to transfer the designed geometry on the sample. 2007 Elsevier B.V. All rights reserved. Keywords: Photonic crystal; Sensors; Microcavity; FDTD method
1. Introduction In the last years the interest for microsensors has drastically grown thanks to their low cost, small dimensions, easy integration. At the same time, the development of nanotechnologies allowed the possibility to fabricate photonic crystal (PhC) structures [1,2], periodic patterns that can control and guide the photons by tailoring the transmission and reflection properties in a specific direction and frequency range. Photonic crystals have been used in a wide range of applications such as 90 bend waveguides, filters, lasers, amplifiers, resonators, non-linear devices. Their application as sensors is a recent research field which seems to be very promising due to their extreme miniaturization, high spectral sensitivity and MEMS integration. So far, a few architectures based on PhCs technology have
*
Corresponding author. Tel.: +39 832298200. E-mail address: [email protected] (M.D. Vittorio).
0167-9317/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.227
been proposed in the literature such as bio-sensors [3] and displacement sensors [4]. In this paper we report on the design and fabrication of a PhC strain-sensitive device consisting of a PhC Waveguide (PhCW) coupled to a PhC microcavity obtained by creating a point defect in the regular 2D periodic pattern. The waveguide is designed in order to allow the propagation of an infrared signal centred around k = 1330 nm. The coupling with the point defect causes a dip in the transmission spectrum corresponding to the microcavity resonating wavelength. When a force is applied to the sample, a variation in the refractive index of the structure occurs due to the photoelastic effect in GaAs. This modifies the optical properties of the cavity, thus shifting the resonance wavelength in the transmission spectrum. The amount of such spectral displacement can be therefore exploited to measure the applied strain. In particular, the numerical results, obtained by a home-made FDTD based code, reveal that the spectral position of the resonating dip detected at the output of the PhCW shifts towards higher
T. Stomeo et al. / Microelectronic Engineering 84 (2007) 1450–1453
wavelengths as the pressure value is increased. The shift linearly depends on the applied pressure, being displaced of about 5 nm per GPa. A detailed description of the nanotechnology process necessary for the fabrication of the bulk force/pressure optical sensor is reported. It includes the definition of 2D-PhC pattern by means of electron beam lithography (EBL) and an inductively coupled plasma (ICP) etching process which employed a SiCl4/He gas mixture. 2. Simulation results The sensor is modelled by a microcavity coupled to a PhC single line defect waveguide (W1) with a triangular lattice of air holes, having period a = 360 nm and radius r = 0.35a realized on a GaAs/AlGaAs slab waveguide. The microcavity is designed by removing one hole and by modifying the radii of the surrounding six holes. This variation provides at the same time a better coupling between the W1 waveguide and the defect and improves the Q factor of the microcavity because of the envelope function of the in-plane mode profile varies more gently than the single defect case (Fig. 1). A preliminary analysis of the unperturbed PhC has been performed by using the RSoft (FullWave) FDTD software. The equivalent 2D structure was obtained by the refractive effective index method. The analysis performed in this work has been concentrated on the TM polarization (mag-
Fig. 1. Microcavity coupled to the photonic crystal. The red holes have r = 0.35a, the blue holes have r = 0.38a and the cyan holes have r = 0.34a. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
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netic field component parallel to air hole axis), for which the PhC structure exhibits a large band gap between 1150 nm and 1800 nm; the device also shows a smaller stop band for TE polarization between 1650 nm and 1850 nm. A proprietary parallel 3D-FDTD computer code, running on a Linux Cluster CLX CINECA, having 1024 CPUs, has been used to analyze the nature of the resonant modes. For all simulations, a grid size has been chosen equal to a/20, whereas the temporal step size is 3.178 · 1017 s, corresponding to 90% of the Courant limit, and uniaxial perfectly matched layers (UPML) are used to limit the computational domain. The structure has been excited by an input Gaussian pulse characterized by a wide spectrum centred at k = 1310 nm. Fig. 1a shows the simulation of light propagation through the force-sensitive structure. The height of the air holes has been fixed equal to 800 nm, by 3D simulations, value that guarantees a good resolution of the band gap and smaller losses. In absence of applied pressure, the cavity shows a resonant mode at the wavelength k = 1332 nm (Fig. 2b).
Fig. 2. (a) Simulation of light propagation through the force-sensitive structure; (b) output transmission spectrum in absence of applied pressure. The resonant wavelength is equal to 1332 nm.
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T. Stomeo et al. / Microelectronic Engineering 84 (2007) 1450–1453
3. Strain effect analysis We investigate the modification of band gap in a photonic bandgap structure undergoing force application. The operation principle of the PhC pressure sensor is based on the fact that the force application provokes, due to the photoelastic, piezoelectric and electrooptic properties of the materials constituting the structure, a change of the refractive index which modifies the transmission spectrum of the regular photonic crystal or that of the localized state when a defected photonic crystal is considered. To model the pressure action, the change of the dielectric impermeability tensor b of the GaAs and AlGaAs, accounting for the strain and stress effects acting on the lattice of the PhC, has been evaluated. The final expression for the dielectric impermeability tensor is obtained by combining all the effects mentioned above: Db ¼ ðpE : s rT gT Þ : T
ð1Þ
where p is the photoelastic fourth-order tensor at constant electric field; s is the fourth-rank elastic compliance sensor,
rT = rS + p: dS is the electrooptic third-rank tensor at constant stress in which rS is the third-order electrooptic tensor at constant strain u and dS is the third-rank piezoelectric tensor at constant strain; gT is the third-rank piezoelectric tensor at constant stress, and T is the stress tensor. Simulation results show that the refractive index change is mainly due to the photoelastic effect when a force is applied normally to the plane of photonic crystal. We calculate the transmission evaluated at the W1 output for different values of the applied pressure. The drop peak corresponds to the resonant wavelength of the mode localized in the microcavity, and it shifts its spectral position, following a linear law when a pressure ranging between 0.25 Gpa and 5 GPa is applied (Fig. 3a). By the simulation data shown in Fig. 3a it is possible to estimate the sensor sensitivity that is equal to 5.82 nm/GPa. Therefore, if the pressure is applied on an area equal to 1 mm2, the numerical simulations show that the detectable minimum force is about 0.3 mN. Fig. 3b shows that there is a linear relationship between the output drop wavelength and the applied pressure. 4. Experimental process
Fig. 3. (a) Transmission spectra for different applied pressure. The force range is 0–5 GPa. (b) Calibration curve: the relation between the pressure and the output drop wavelength is a linear dependence.
The geometry of the bulk 2D-PhC force-sensitive device has been obtained by patterning a GaAs/ Al0.7Ga0.3As heterostructure by means of EBL and ICP. The sample has been grown by molecular beam epitaxy (MBE): it consists of a 300 nm thick GaAs slab waveguide on a 2000 nm thick cladding of Al0.7Ga0.3As. The sample has been subsequently covered with 300 nm spincoated positive e-beam resist. We choose ZEP-520A resist from Zeon Chemicals L.P. due to high selectivity in SiCl4 ICP processes. In order to minimize the proximity effect due to the scattering of electrons in the 300 nm thick resist layer, we have performed preliminary e-beam exposure resolution tests at three different values of e-beam energy carried out by a Raith150 lithography system. From the analysis of the cross-sectional profile of the 300 nm thick resist layer after the e-beam exposures at 10 keV, 15 keV and 20 keV, we found that a vertical profile is achieved with an e-beam energy of 20 keV, which also guarantees the vertical transfer down to the GaAs slab of the photonic crystal pattern during the dry-etching process. The 2D-PhC patterns of air holes, arranged in a triangular lattice, has been fabricated by means of high-resolution EBL process performed at 20 keV, 15.95 pA and an area dose of 40 lC/cm2. Subsequently the pattern has been transferred down to the GaAs guiding layer by an ICP process which employed a SiCl4/He gas mixture. The rf coil power and the rf platen power have been set to 300 W and 60 W, respectively. The selectivity of our ZEP-520A mask to GaAs is of about 1:7 whilst the height of the air holes is equal to about 800 nm. Fig. 4 shows the top-view scanning electron micrograph (SEM) of the bulk force-sensitive device after EBL and ICP processes. Thus
T. Stomeo et al. / Microelectronic Engineering 84 (2007) 1450–1453
1453
5. Conclusion We have proposed to apply the optical properties of photonic crystals in order to realize sensing devices characterized by a high degree of compactness and a good resolution. A force/pressure optical sensor has been realized by designing and fabricating a GaAs/AlGaAs photonic crystal microcavity for the wavelength range 1300–1400 nm, with a sensitivity in the mN range. The fabricated structure is currently under optical characterization in order to experimentally evaluate its strain sensitivity. This sensitivity could be further improved by either maximizing the quality factor of the resonance state or by fabricating the same 2DPhC structure on a membrane configuration; the integration of these two devices, on the same substrate, would enable the realization of strain sensors covering a broad strain range. It is noteworthy that the same architecture and technology can be directly applied for the realization of photonic crystal actuators, where the applied strain can be used to tune the emission properties of active photonic crystal devices. Acknowledgements
Fig. 4. (a) SEM top-view of the bulk force-sensitive device after EBL and ICP processes; (b) SEM detail of the microcavity coupled to the waveguide.
highly ordered and periodical geometry of the pattern of air holes indicates the good quality of the bulk 2D-PhC force/pressure optical sensor. The PhCW is connected to 10 lm wide guides by two modal adapter for optical characterization.
The authors gratefully thank the expert technical help of Gianmichele Epifani. This work was supported by MIURPRIN2005, in the framework of the national research project ‘‘Photonic Bandgap Nanosensor’’. References [1] E. Yablonovitch, JOSA B 19 (1997) 283–295. [2] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Princeton University Press, 1995. [3] E. Chow, A. Grot, L.W. Mirkarimi, M. Sigalas, G. Girolami, Opt. Lett. 29 (2004) 1093–1095. [4] W. Suh, M.F. Yanik, O. Solgaard, S. Fan, Appl. Phys. Lett. 82 (2003) 1999–2001.