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Fabrication of Ultrathin Large-area Dielectric Membrane Stacks for use as Interference Filters
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 1342 – 1345
30th Eurosensors Conference, EUROSENSORS 2016
Fabrication of ultrathin large-area dielectric membrane stacks for use as interference filters M. Ghaderia,∗, G. de Graafa , R. F. Wolffenbuttela a Electronic
Instrumentation Laboratory, Microelectronics Department, Faculty of EEMCS, Delft University of Technology, Delft, the Netherlands
Abstract The design and CMOS-compatible fabrication of stacked dielectric membranes are presented. The structures are intended for use as airgap-based interference filters operating in the UV-visible spectrum. In optical filters, maximizing the fill-factor, i.e. the ratio of the active area to the total area of the filter, is essential, which calls for minimum dimensions of the anchoring pillars and the openings that are required for the fabrication of the structures. This requirement necessitates the fabrication of large area freestanding membranes. Maintaining flatness over such a large area (> 1000 μm2 ) membrane, as required by the optical application (deformation< λ/10), is challenging. While the thickness of the free-standing membrane is defined by the optical specification, the residual stress is the main force acting on the structure. Although an overall tensile residual stress can effectively stretch the membrane, the presence of a residual stress gradient causes the membrane to deform. Furthermore, a high residual stress results in the rupture of the membrane. These challenges in the fabrication of airgap-dielectric Bragg gratings are discussed. Higher-order optical designs were investigated to simultaneously satisfy both the mechanical and optical requirements. Bragg reflectors with one and two periods have been fabricated and characterized. The preliminary results are presented. © Published by Elsevier Ltd. This c 2016 by Elsevier Ltd. is an open access article under the CC BY-NC-ND license 2016The TheAuthors. Authors. Published (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: Optical MEMS; Membranes; Optical filter; Interference; Residual stress.
1. Main Text As opposed to the all-dielectric optical filters, airgap-based designs have several promising advantages for use in the UV-visible spectrum. Firstly, the refractive index contrast (≈ Δn/n) of most dielectric pairs in the UV-visible range is limited to about 0.3 [1]. Using air (n = 1) as the low-index layer results in an improved refractive index contrast and a more pronounced optical response. Secondly, the absorption peaks of many dielectrics are located in the UV spectrum which limit the applicability of the many dielectrics for the optical application, whereas air (k ≈ 0) does not significantly absorb light over the spectrum from the UV to the infrared. Thirdly, the process compatibility of the materials, both in terms of the material properties and the deposition temperature, is highly relevant in MEMS fabrication. This especially limits the choice of the suitable optical material for use in the UV-visible spectrum. ∗
Corresponding author. Tel.: +31-15-2789177. 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.2016.11.369
M. Ghaderi et al. / Procedia Engineering 168 (2016) 1342 – 1345
Airgap-based filters operating at UV-visible spectrum have been previously introduced [2–4]. The focus in literature is mainly on III-V semiconductor based LEDs, which do not require a large active area [3]. However, application such as optical MEMS sensors require a high fill-factor, hence a large optically active area. While the MEMS technology provides the essential tools for the fabrication of the airgap filters, the fabrication tolerances are more restrictive in optical applications. For the proper operation, an optical filter would require an RMS roughness/flatness of less than λ/10. Therefore, the main challenge in the fabrication of large-area membranes is maintaining the optical flatness over the entire area of the filter. The design and fabrication of large-area silicon-based optical filters have been investigated by our group [4]. This paper investigates the fabrication of optically flat airgap optical filters using higher-order optical designs.
2. The Design and Fabrication The fabrication process of the airgap optical filters is schematically shown in Figure 1. The fabrication is based on an alternating deposition of LPCVD polysilicon and PECVD silicon-oxide layers, as the sacrificial and the structural/optical layers, respectively. The selective removal of the sacrificial polysilicon layers using TMAH-based etching results in free-standing membranes [5]. In the released membranes, the residual stress gradient is the main force acting on the structure [6]. Although silicon-oxide layers with a tensile stress can be deposited using PECVD, the stress gradient (0.2 − 0.7 MPa/nm) over the thickness of the membranes results in a significant deformation of the released membranes [6]. Stress engineering methods, such as external straining using backside layers and internal straining using compound membrane structures, have been introduced and studied [5]. While the results showed a qualitative improvement in the membrane flatness when using straining methods, the optical flatness was only achieved in a compound membrane structure.
Fig. 1. Schematics of the fabrication process. a. Deposition of sacrificial and optical layer stack, b. Patterning and etching of anchoring pins, c. Deposition of anchoring layer and d. patterning and etching of the excess area, e. Patterning and etching of windows for (d.) sacrificial etching and release (f.).
In a free-standing membrane, the stiffness increases with the third power of the thickness (t3 ) of the membrane [7]. In the optical filters, however, the optical design dictates the thickness of each layer. For instance in the case of an airgap-based Bragg reflector, the structure is composed of quarter-wave optical-thick (QWOT) layers of dielectrics (H) separated by QWOT airgap (L) layers. While higher odd-multiples of QWOT layers still satisfy the interference condition at the center wavelength, the operating bandwidth of the filter decreases. Therefore, the trade-off between the optical performance and mechanical requirements has to be considered. In this paper, a DBR design based on 3-QWOT membranes and 1-QWOT airgaps was designed. Table 1 lists the values of the layer thickness for the designs of 2H (L 3H)N with one (N = 1) and two (N = 2) periods designed at λ = 400 nm center wavelength. Due to the three-fold increase in the thickness of the membranes, the stiffness of the membrane increases by a factor of 27, which results in a highly reduced deformation of the membrane, while the optical response to a great extent remains unaffected.
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M. Ghaderi et al. / Procedia Engineering 168 (2016) 1342 – 1345
Table 1. Typical airgap-based designs for Bragg gratings centered at 400 nm for two designs; one with one period (N = 1) and another with two periods (N = 2) are listed.
1 2 3 4 5
Layers
N = 1, Thickness [nm]
N = 2, Thickness [nm]
Si wafer SiO2 Airgap SiO2 Airgap SiO2
140 100 210
140 100 210 100 210
Fig. 2. a. Microscope image of a N = 1 Bragg grating structure. The scale bar in the image is 25 μm long. b. 45-degree tilted SEM image of the same structure.
Fig. 3. a. Microscope; and b. tilted SEM images of a N = 2 Bragg grating structure. The scale bar in the microscope image is 25 μm long.
3. Preliminary optical characterization Several samples were fabricated according to the designs listed in Table 1. The sacrificial etching was terminated before the complete removal of the sacrificial material in order to determine the maximum achievable length considering the process yield. Figure 2 and Figure 3 show the microscope and SEM images of the released Bragg reflector with N = 1 and N = 2 respectively. Various areas can be identified in the microscope images. These areas correspond to the released membranes and unreleased areas. The results qualitatively indicate the optical flatness over the released area. The optical characterization was performed using a reflection probe (FCR-7UVIR200, Avantes) coupled to a wideband light source (Deuterium-Halogen light source, DH-2000, OceanOptics) and a spectrometer module (Flame, OceanOptics). The probe tip was placed in a close vicinity of the sample area. Due to the comparatively large core diameter of the fiber (200 μm) and its numerical aperture (N.A. = 0.22), the probe measures the spectral reflectance over an area of several mm2 . Therefore, the measured spectral reflectance is a weighted average of the released and unreleased areas, together with the anchor pillars and the sacrificial openings. Figures 4a. and b. show the normalized spectral reflectance measured for the samples with a fill-factor varied from 10% to about 60% (or 80% for N = 2). An increase of the released fill factor, hence a larger released area, results in the intended peak at 400 nm to increase,
M. Ghaderi et al. / Procedia Engineering 168 (2016) 1342 – 1345
Fig. 4. The normalized reflectance of the structure with a. N = 1 and b. N = 2. The 10% and 60% (or 80%) lines indicate the fill-factor percentage, i.e. the under-etched area to the total area.
while the peak at longer wavelengths, which is attributed to the unreleased area, decreases. These results are in good agreement with the expected spectral response of the filters. 4. Conclusion The design and fabrication of 3-QWOT membranes that are separated by 1-QWOT DBRs have been presented. Obtaining a sufficient flatness over the membrane area is essential to the proper operation of the filter. The fabrication of airgap filters for the UV and visible spectrum is especially challenging due to stricter optical requirements and higher sensitivity to the mechanical characteristics. The implementation of thicker membranes results in a higher stiffness in the membrane and thus in a reduced deformation after sacrificial release of the membranes. Therefore, the optical layer design has to be such that both optical and mechanical requirements are satisfied. Accordingly, the 3-QWOT membrane structures result in a sufficient optical flatness, while the appropriate optical response required for the application is obtained. Future work is aimed at an improved process with reduced variation to obtain a higher yield in the fabrication of large-area and fully released optical filters. Acknowledgements This work has been supported by the Dutch Technology Foundation STW under Grant DEL.11476. The process was carried out in the Else Kooi Laboratory (formerly known as DIMES Technology Center) and Kavli nanolab Delft both at the Delft University of Technology. References [1] M. Ghaderi, N. P. Ayerden, G. de Graaf, R. F. Wolffenbuttel, Optical characterization of mems-based multiple air-dielectric blue-spectrum distributed bragg reflectors, Proc. SPIE 9517 (2015) 95171M–95171M–6. [2] D. Chen, J. Han, High reflectance membrane-based distributed Bragg reflectors for GaN photonics, Applied Physics Letters 101 (2012). [3] R. Sharma, Y.-S. Choi, C.-F. Wang, A. David, C. Weisbuch, S. Nakamura, E. L. Hu, Gallium-nitride-based microcavity light-emitting diodes with air-gap distributed Bragg reflectors, Applied Physics Letters 91 (2007). [4] M. Ghaderi, N. P. Ayerden, G. de Graaf, R. F. Wolffenbuttel, Surface-micromachined Bragg reflectors based on multiple airgap/SiO2 layers for CMOS-compatible Fabry-Perot filters in the UV-visible spectral range, Procedia Engineering 87 (2014) 1533–1536. [5] M. Ghaderi, G. de Graaf, R. F. Wolffenbuttel, Design and fabrication of ripple-free CMOS-compatible stacked membranes for airgap optical filters for uv-visible spectrum, Proc. SPIE 9888 (2016) 98880R–98880R–8. [6] M. Ghaderi, E. Karimi Shahmarvandi, G. de Graaf, R. F. Wolffenbuttel, Analysis of the effect of stress-induced waviness in airgap-based optical filters, Proc. SPIE 9889 (2016) 98890A–98890A–9. [7] M. Strassner, J. C. Esnault, L. Leroy, J. L. Leclercq, M. Garrigues, I. Sagnes, Fabrication of ultrathin and highly flexible InP-based membranes for microoptoelectromechanical systems at 1.55 m, IEEE Photonics Technology Letters 17 (2005) 804–806.
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ScienceDirect Procedia Engineering 168 (2016) 1342 – 1345
30th Eurosensors Conference, EUROSENSORS 2016
Fabrication of ultrathin large-area dielectric membrane stacks for use as interference filters M. Ghaderia,∗, G. de Graafa , R. F. Wolffenbuttela a Electronic
Instrumentation Laboratory, Microelectronics Department, Faculty of EEMCS, Delft University of Technology, Delft, the Netherlands
Abstract The design and CMOS-compatible fabrication of stacked dielectric membranes are presented. The structures are intended for use as airgap-based interference filters operating in the UV-visible spectrum. In optical filters, maximizing the fill-factor, i.e. the ratio of the active area to the total area of the filter, is essential, which calls for minimum dimensions of the anchoring pillars and the openings that are required for the fabrication of the structures. This requirement necessitates the fabrication of large area freestanding membranes. Maintaining flatness over such a large area (> 1000 μm2 ) membrane, as required by the optical application (deformation< λ/10), is challenging. While the thickness of the free-standing membrane is defined by the optical specification, the residual stress is the main force acting on the structure. Although an overall tensile residual stress can effectively stretch the membrane, the presence of a residual stress gradient causes the membrane to deform. Furthermore, a high residual stress results in the rupture of the membrane. These challenges in the fabrication of airgap-dielectric Bragg gratings are discussed. Higher-order optical designs were investigated to simultaneously satisfy both the mechanical and optical requirements. Bragg reflectors with one and two periods have been fabricated and characterized. The preliminary results are presented. © Published by Elsevier Ltd. This c 2016 by Elsevier Ltd. is an open access article under the CC BY-NC-ND license 2016The TheAuthors. Authors. Published (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: Optical MEMS; Membranes; Optical filter; Interference; Residual stress.
1. Main Text As opposed to the all-dielectric optical filters, airgap-based designs have several promising advantages for use in the UV-visible spectrum. Firstly, the refractive index contrast (≈ Δn/n) of most dielectric pairs in the UV-visible range is limited to about 0.3 [1]. Using air (n = 1) as the low-index layer results in an improved refractive index contrast and a more pronounced optical response. Secondly, the absorption peaks of many dielectrics are located in the UV spectrum which limit the applicability of the many dielectrics for the optical application, whereas air (k ≈ 0) does not significantly absorb light over the spectrum from the UV to the infrared. Thirdly, the process compatibility of the materials, both in terms of the material properties and the deposition temperature, is highly relevant in MEMS fabrication. This especially limits the choice of the suitable optical material for use in the UV-visible spectrum. ∗
Corresponding author. Tel.: +31-15-2789177. 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.2016.11.369
M. Ghaderi et al. / Procedia Engineering 168 (2016) 1342 – 1345
Airgap-based filters operating at UV-visible spectrum have been previously introduced [2–4]. The focus in literature is mainly on III-V semiconductor based LEDs, which do not require a large active area [3]. However, application such as optical MEMS sensors require a high fill-factor, hence a large optically active area. While the MEMS technology provides the essential tools for the fabrication of the airgap filters, the fabrication tolerances are more restrictive in optical applications. For the proper operation, an optical filter would require an RMS roughness/flatness of less than λ/10. Therefore, the main challenge in the fabrication of large-area membranes is maintaining the optical flatness over the entire area of the filter. The design and fabrication of large-area silicon-based optical filters have been investigated by our group [4]. This paper investigates the fabrication of optically flat airgap optical filters using higher-order optical designs.
2. The Design and Fabrication The fabrication process of the airgap optical filters is schematically shown in Figure 1. The fabrication is based on an alternating deposition of LPCVD polysilicon and PECVD silicon-oxide layers, as the sacrificial and the structural/optical layers, respectively. The selective removal of the sacrificial polysilicon layers using TMAH-based etching results in free-standing membranes [5]. In the released membranes, the residual stress gradient is the main force acting on the structure [6]. Although silicon-oxide layers with a tensile stress can be deposited using PECVD, the stress gradient (0.2 − 0.7 MPa/nm) over the thickness of the membranes results in a significant deformation of the released membranes [6]. Stress engineering methods, such as external straining using backside layers and internal straining using compound membrane structures, have been introduced and studied [5]. While the results showed a qualitative improvement in the membrane flatness when using straining methods, the optical flatness was only achieved in a compound membrane structure.
Fig. 1. Schematics of the fabrication process. a. Deposition of sacrificial and optical layer stack, b. Patterning and etching of anchoring pins, c. Deposition of anchoring layer and d. patterning and etching of the excess area, e. Patterning and etching of windows for (d.) sacrificial etching and release (f.).
In a free-standing membrane, the stiffness increases with the third power of the thickness (t3 ) of the membrane [7]. In the optical filters, however, the optical design dictates the thickness of each layer. For instance in the case of an airgap-based Bragg reflector, the structure is composed of quarter-wave optical-thick (QWOT) layers of dielectrics (H) separated by QWOT airgap (L) layers. While higher odd-multiples of QWOT layers still satisfy the interference condition at the center wavelength, the operating bandwidth of the filter decreases. Therefore, the trade-off between the optical performance and mechanical requirements has to be considered. In this paper, a DBR design based on 3-QWOT membranes and 1-QWOT airgaps was designed. Table 1 lists the values of the layer thickness for the designs of 2H (L 3H)N with one (N = 1) and two (N = 2) periods designed at λ = 400 nm center wavelength. Due to the three-fold increase in the thickness of the membranes, the stiffness of the membrane increases by a factor of 27, which results in a highly reduced deformation of the membrane, while the optical response to a great extent remains unaffected.
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M. Ghaderi et al. / Procedia Engineering 168 (2016) 1342 – 1345
Table 1. Typical airgap-based designs for Bragg gratings centered at 400 nm for two designs; one with one period (N = 1) and another with two periods (N = 2) are listed.
1 2 3 4 5
Layers
N = 1, Thickness [nm]
N = 2, Thickness [nm]
Si wafer SiO2 Airgap SiO2 Airgap SiO2
140 100 210
140 100 210 100 210
Fig. 2. a. Microscope image of a N = 1 Bragg grating structure. The scale bar in the image is 25 μm long. b. 45-degree tilted SEM image of the same structure.
Fig. 3. a. Microscope; and b. tilted SEM images of a N = 2 Bragg grating structure. The scale bar in the microscope image is 25 μm long.
3. Preliminary optical characterization Several samples were fabricated according to the designs listed in Table 1. The sacrificial etching was terminated before the complete removal of the sacrificial material in order to determine the maximum achievable length considering the process yield. Figure 2 and Figure 3 show the microscope and SEM images of the released Bragg reflector with N = 1 and N = 2 respectively. Various areas can be identified in the microscope images. These areas correspond to the released membranes and unreleased areas. The results qualitatively indicate the optical flatness over the released area. The optical characterization was performed using a reflection probe (FCR-7UVIR200, Avantes) coupled to a wideband light source (Deuterium-Halogen light source, DH-2000, OceanOptics) and a spectrometer module (Flame, OceanOptics). The probe tip was placed in a close vicinity of the sample area. Due to the comparatively large core diameter of the fiber (200 μm) and its numerical aperture (N.A. = 0.22), the probe measures the spectral reflectance over an area of several mm2 . Therefore, the measured spectral reflectance is a weighted average of the released and unreleased areas, together with the anchor pillars and the sacrificial openings. Figures 4a. and b. show the normalized spectral reflectance measured for the samples with a fill-factor varied from 10% to about 60% (or 80% for N = 2). An increase of the released fill factor, hence a larger released area, results in the intended peak at 400 nm to increase,
M. Ghaderi et al. / Procedia Engineering 168 (2016) 1342 – 1345
Fig. 4. The normalized reflectance of the structure with a. N = 1 and b. N = 2. The 10% and 60% (or 80%) lines indicate the fill-factor percentage, i.e. the under-etched area to the total area.
while the peak at longer wavelengths, which is attributed to the unreleased area, decreases. These results are in good agreement with the expected spectral response of the filters. 4. Conclusion The design and fabrication of 3-QWOT membranes that are separated by 1-QWOT DBRs have been presented. Obtaining a sufficient flatness over the membrane area is essential to the proper operation of the filter. The fabrication of airgap filters for the UV and visible spectrum is especially challenging due to stricter optical requirements and higher sensitivity to the mechanical characteristics. The implementation of thicker membranes results in a higher stiffness in the membrane and thus in a reduced deformation after sacrificial release of the membranes. Therefore, the optical layer design has to be such that both optical and mechanical requirements are satisfied. Accordingly, the 3-QWOT membrane structures result in a sufficient optical flatness, while the appropriate optical response required for the application is obtained. Future work is aimed at an improved process with reduced variation to obtain a higher yield in the fabrication of large-area and fully released optical filters. Acknowledgements This work has been supported by the Dutch Technology Foundation STW under Grant DEL.11476. The process was carried out in the Else Kooi Laboratory (formerly known as DIMES Technology Center) and Kavli nanolab Delft both at the Delft University of Technology. References [1] M. Ghaderi, N. P. Ayerden, G. de Graaf, R. F. Wolffenbuttel, Optical characterization of mems-based multiple air-dielectric blue-spectrum distributed bragg reflectors, Proc. SPIE 9517 (2015) 95171M–95171M–6. [2] D. Chen, J. Han, High reflectance membrane-based distributed Bragg reflectors for GaN photonics, Applied Physics Letters 101 (2012). [3] R. Sharma, Y.-S. Choi, C.-F. Wang, A. David, C. Weisbuch, S. Nakamura, E. L. Hu, Gallium-nitride-based microcavity light-emitting diodes with air-gap distributed Bragg reflectors, Applied Physics Letters 91 (2007). [4] M. Ghaderi, N. P. Ayerden, G. de Graaf, R. F. Wolffenbuttel, Surface-micromachined Bragg reflectors based on multiple airgap/SiO2 layers for CMOS-compatible Fabry-Perot filters in the UV-visible spectral range, Procedia Engineering 87 (2014) 1533–1536. [5] M. Ghaderi, G. de Graaf, R. F. Wolffenbuttel, Design and fabrication of ripple-free CMOS-compatible stacked membranes for airgap optical filters for uv-visible spectrum, Proc. SPIE 9888 (2016) 98880R–98880R–8. [6] M. Ghaderi, E. Karimi Shahmarvandi, G. de Graaf, R. F. Wolffenbuttel, Analysis of the effect of stress-induced waviness in airgap-based optical filters, Proc. SPIE 9889 (2016) 98890A–98890A–9. [7] M. Strassner, J. C. Esnault, L. Leroy, J. L. Leclercq, M. Garrigues, I. Sagnes, Fabrication of ultrathin and highly flexible InP-based membranes for microoptoelectromechanical systems at 1.55 m, IEEE Photonics Technology Letters 17 (2005) 804–806.
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