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MEMS-based filter integrating tunable Fabry–Perot cavity and grating
Optics Communications 402 (2017) 472–477
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
MEMS-based filter integrating tunable Fabry–Perot cavity and grating Zhendong Shi a, *, Liang Fang b , Libo Zhong b a b
Southwest Institute of Technical Physics, Chengdu 610041, China Institute of Optics and Electronics, Chinese Academy of Science, Chengdu 610209, China
a r t i c l e
i n f o
Keywords: Optical microelectromechanical devices Tunable optical filters Fabry–Perot cavities Gratings
a b s t r a c t We present a novel Micro-Electro-Mechanical-Systems (MEMS) filter integrating a tunable Fabry–Perot cavity and a grating (FP-G). It fuses the functions of the Fabry–Perot (FP) cavity and grating into a single device so as to realize both narrow spectral linewidth given by FP cavity and wide spectral range given by grating. A MEMS FP-G filter with 200 μm cavity length was designed and fabricated. It is based on a W-shaped-section microbridge structure by which the electrostatic actuator is independent on the FP cavity and a long cavity length can be configured. A grating etched in the back of a cavity mirror serves to spatially separate the transmitted spectral lines corresponding to different interference orders of FP cavity. Experimental results indicated that the MEMS FP-G filter obtained a broader operation spectral range compared with the traditional MEMS FP filter, and achieved 1.2 nm spectral linewidth, which could be narrower by improving the effective finesse of the FP cavity in the fabrication process. Compared with the simulation results, there was considerable room for improvement in device performance by the optimal technique. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Tunable Fabry–Perot (FP) filter has been pursued for various applications including spectrographs [1,2], optical communications [3,4], sensors [5–9] and lasers [10]. The MEMS based tunable FP filters are desirable due to their miniaturization, high tuning speed, wide tuning range and potentially low cost at large volumes [11,12]. For conventional MEMS-based FP filter, the actuated plates of the electrostatic actuator usually locate on two micro-mirrors of FP optical cavity to benefit the manufacture process [13,14], so the FP cavity length is limited to a few micrometers to keep the driving voltage lower. Moreover, short enough cavity length can acquire adequately wide free spectral range (FSR) to guarantee only single transmission peak wavelength in spectral range. Though optical FP cavity with long cavity length separated from the electrostatic actuator using SOI wafer is developed [15], transmission peaks in the different interference orders are overlapped spatially, and the transmission spectrum contains more than one peak wavelength, which results in an inaccuracy of transmission peak wavelength measurement. In general, a spectrometer is required to analyze multiple transmission peak wavelengths, while FP cavity itself usually is unable to complete it. Furthermore, approach to using long cavity length is capable of obtaining narrow spectral linewidth but the operation spectral range is mainly limited by the FSR [16]. In * Corresponding author.
E-mail address: [email protected] (Z. Shi). http://dx.doi.org/10.1016/j.optcom.2017.06.043 Received 20 March 2017; Received in revised form 23 May 2017; Accepted 12 June 2017 0030-4018/© 2017 Elsevier B.V. All rights reserved.
general, there is a contradiction to meet the demands between narrow spectral linewidth and wide FSR. The combined use of optical devices is a method to overcome this contradiction in conventional instruments. The combination of the FP etalon and the grating has been carried out for the tunable external-cavity laser diode to achieve the narrowband spectral output [17–19] and for the versatile instrument to realize astronomical narrowband imaging [20]. Additionally, the array of filters with different narrow-bands is an effective way to cover a wide range of operation wavelengths [21–23]. Meanwhile, the optical filter employs the tapered cavity to achieve the change of cavity length, and the different peak wavelengths are transmitted at different positions of the filter, so that the operation bandwidth is effectively widened [6,24,25]. Based on the method of the integration of a tunable FP cavity and a grating, we have previously reported a dispersive element in a miniature spectrometer by a piezo-electrical transducer [26], and analyzed the influences of divergence angle on the device performance [27]. In order to miniaturize the device and take advantages of MEMS devices, the structure of MEMS device has been preliminarily designed [28]. Here, we improved the design in favor of the actual processing, fabricated optical MEMS device and experimentally performed function tests. This paper extends the method of the integration of a tunable FP cavity and a grating to MEMS devices. A MEMS filter based on W-shaped micro bridge is proposed, which integrates the tunable MEMS FP cavity and
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Optics Communications 402 (2017) 472–477
the grating. The proposed W-shaped micro bridge structure based on low-cost bulk micromachining processes was favorable to obtain a long FP cavity length depending on the silicon wafer thickness itself as well as a low driving voltage by the separation of the electrodes and the mirrors simultaneously in the design. For optical characterizations, peak wavelengths in multiple interference orders were produced through the MEMS FP cavity, then peak wavelengths could be spatially separated with the post-dispersion by the grating integrated with the FP cavity. Thus the MEMS FP-G filter could achieve the frequency filtering and spatial dispersing simultaneously, to overcome the limitation of the conventional MEMS FP filter that the operation wavelength range depending on the FSR became short at the long cavity length. Moreover, by changing the cavity length with the electrostatic force, the corresponding peak wavelengths would vary and cover a wide spectral range. The experimental results indicated the MEMS device achieved a broader spectral range in comparison with the traditional MEMS FP filter and obtained 1.2 nm spectral linewidth. It could be narrower by increasing the effective finesse of the FP cavity through optimizing the fabrication process.
Fig. 1. (a) 3D schematic of the proposed MEMS FP-G filter, optical process and operational principle of the device in the inset.
of spectral resolution. In Fig. 2(c) and (d), for the peak wavelength of 801.6 nm, the half angular width of 0.025◦ corresponds to spectral range of 0.625 nm through the grating equation, but the spectral range in the diffraction peak is measured by the FWHM of spectral line of 0.115 nm from FP cavity. Hence, spatial resolution of gratings does not affect spectral resolution of MEMS FP-G optical filter under the conditions, Δ𝜃 ≥ 𝛿𝜃. When the variation of cavity length is the step size (28 nm) obtained by the FWHM of spectral line, the change of peak wavelength is the FWHM of spectral line, and diffraction beam also shifts and double peaks of diffraction beam can be distinguished each other due to differences of transmission time, thereby the spectral information with the higher resolution at the corresponding position can be obtained. In addition, intensity distribution of single diffraction peak overlays spatial distributions of all spectral components in corresponding spectral line. Thus the diffraction peak intensity increases with increase of wavelengths due to the broadening of spectral linewidth as shown in the inset in Fig. 2(b), while the spatial half angular width of the peak wavelength (801.6 nm) is the same as the one of the diffraction peak without the spatial broadening as shown in Fig. 2(d). Moreover, due to intensity distribution through the grating similar to sinc function, a sub-peak exists between two main peaks as shown in Fig. 2(b). However, when the cavity length is determined, the peak wavelength and its diffraction angle is also determined, so the detection in specific spatial regions can reduce the influence of a sub-peak. In short, due to the miniaturization of MEMS devices, the number of grating period is reduced resulting in the increase of the half angular width and decrease of the spectral resolution. The spectral resolution is improved by MEMS FP-G filter, compared with the grating.
2. Device design 2.1. Optical considerations Fig. 1(a) presents 3D structure of a designed MEMS FP-G filter that consists of a W-shaped-section silicon top plate and a quartz bottom plate with a diffraction grating fabricated on its lower surface. The movable mirror of FP cavity on the top of the mesa is suspended by the cantilever and the fixed bottom mirror is provided on the quartz substrate. The operation principle of the device integrating the Fabry– Perot (FP) cavity and grating is illustrated in the inset. A collimated beam is normal incident on the FP cavity where the multi-orders resonant wavelengths are produced, and then the resonant wavelengths achieve the spatial separation by the grating. According to Rayleigh’s criterion, in order to distinguish the peak–peak wavelengths of adjacent interference orders of the FP cavity spatially, the difference of diffraction angle Δ𝜃 of the adjacent peak–peak wavelengths is required to be greater than the half angular width 𝛿𝜃 of single peak wavelength 𝜆k passing through the grating, Δ𝜃 ≥ 𝛿𝜃 [16,28]. Consequently, the restrictive relation between the parameters of the grating and FP cavity is obtained by 𝜆𝑘 1 ≥ . (1) 2𝑛𝐿 𝑁 where 𝜆k , 𝑛, 𝐿 and 𝑁 represent the peak wavelength corresponding to the interference order 𝑘, the refractive index inside the cavity, the cavity length and the number of grating period, respectively. The initial cavity length (𝐿) determined by the silicon wafer thickness itself is approximately 200 μm. This device is mainly used in the nearinfrared wavelength range (0.8–1.0 μm), According to Eq. (1), the period number of grating with 600 lines/mm should be greater than 500. To ensure the beam size, the mirror size of FP cavity is 2 × 2 mm2 . The aluminum mirror with the easy processing can maintain the consistency of reflectivity in a relatively broad spectrum range, and the mirror reflective of 80% is selected taking the absorption into account. Based on the device parameters above, optical process of the MEMS FP-G optical filter was simulated. The parallel light with the continuous spectrum normally transmits the tunable FP cavity to form a series of discrete spectral lines as shown in Fig. 2(a). Then, discrete spectral lines in spectral domain are separated by the grating in spatial domain, and there exists a corresponding relationship between the spectral lines and diffraction peaks as shown by red double arrows in Fig. 2(b). Thus, transmission spectra spread out in the space, to detect the wider spectral range just by MEMS FP-G filter. On the other hand, the spectral components of single peak in the diffraction beams rely solely on the spectral linewidth of the FP cavity and can be measured by the full width at half maximum (FWHM) of spectral line which is used as the standard
2.2. Mechanical design The MEMS FP-G filter is based on the W-shaped-section microbridge structure, as is denoted with red dashed lines in Fig. 3(a), making the cavity mirror independent on the electrostatic actuator to ensure the long cavity length and the short distance of electrodes simultaneously. The two plates are bonded with the adhesive wrapped in the diversion trench. The fundamental configuration of electrostatic actuators is similar to a parallel-plate capacitor. For the design of an electrostatic actuator, the pull-in displacement 𝑥PI , which limits the operation range of cavity length to approximately one third of original air gap (ℎ) between the top and bottom electrode [29], should be larger than the minimum variation of cavity length determined by the maximum wavelength 𝜆max , Δ𝐿min = 𝜆max ∕2 [26]. In order to satisfy the condition of 𝑥PI > Δ𝐿min , the original air gap (ℎ) should be larger than 1.5 μm, and we select the original air gap (ℎ) of 3 μm. To investigate electromechanical performance of the MEMS device, we employ the IntelliSuite software to analyze the relation between 473
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Optics Communications 402 (2017) 472–477
Fig. 2. Simulated transmission curve of the FP cavity (a). Intensity distribution about diffraction angle of MEMS FP-G filter (b). Discrete spectral lines in spectral domain (c) and intensity distribution in spatial domain (d) shifting with the variation of the FP cavity length (𝐿). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Schematic of the electrostatic actuator based on the W-shaped-section micro-bridge structure (a). Simulated results of the change of displacement of the movable mirror versus driving voltage (𝑉d ) at the different cantilever lengths 𝐶T (b), and air gaps of electrodes ℎ (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the displacement of the movable mirror and driving voltage (𝑉d ) under different parameters of structure including the thickness (𝐶T ) in Fig. 3(b) of the cantilever and the air gap (ℎ) of electrodes in Fig. 3(c) (Si: Young’s modulus of 170 Gpa, Poisson’s ratio of 0.26). When the thickness (𝐶T ) and air gap (ℎ) increases, the driving voltage (𝑉d ) increases rapidly. Nevertheless, there is an equal pull-in displacement 𝑥PI about 1 μm at the same original air gap (ℎ), so the larger driving voltage at the displacement of the Δ𝐿min is, more gently the change of driving voltage is. In addition, compared with the designed length of the cantilever (𝐶L = 1000 μm), the length error of the cantilever from the wet etching has a little effect on the driving voltage.
the lower oxide layer was patterned, and the wafer was dipped into a 25% TMAH (Tetramethylammonium hydroxide) solution at 80 ◦ C for anisotropic wet etching about 8 min, in order to etch a 3 μm silicon layer where the air gap between the top and bottom electrode and the diversion trench were formed. Thirdly, a 600 nm silicon nitride layer was deposited by low pressure chemical vapor deposition (LPCVD) to act as a hard mask in the following deep wet etching. Fourthly, a square opening window on the lower silicon nitride layer was opened by the reactive ion etching (RIE), and a 20 μm silicon layer was etched in the TMAH solution to control the thickness of the cantilever in advance. Subsequently, a square annular window on the upper silicon nitride layer was accomplished by the RIE. Then, the deep volume cavity and the cantilever were formed during double-side anisotropic TMAH etching, and the film covering the aperture was removed to form an optical aperture in the center, because the deformation of the large area film occurred and the quality of the mirror was declined, when the cantilever moved down. Next, the Aluminum (Al) layer with the thickness of 30 nm was evaporated on the lower surface of the silicon wafer, which was used for the top electrode. Finally, the Pyrex glass with the 30 nm thick Al mirror was bond with the top plate, and the fabricated top plate is show in the inset.
3. Device fabrication Fig. 4 shows the schematic process flow of the MEMS FP-G filter. In the top plate of MEMS FP-G filter, the W-shaped micro-bridge structure was fabricated by the wet etching technology as shown in Fig. 4(a). Firstly, a 60 nm thermal oxide layer for masking layer in wet etching process was grown on a double side polished (100) silicon wafer with the thickness of 200 μm. Secondly, an opening window on 474
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Optics Communications 402 (2017) 472–477
Fig. 4. Schematic fabrication sequence of the MEMS FP-G filter, (a) Top plate, a photo of the fabricated top plate in the inset (b) Bottom plate, AFM image of the grating and a photo of the fabricated bottom plate in the inset, (c) Photograph of the fabricated MEMS FP-G filter. 𝑆: width of ten grating period (𝛬 = 1.7 μm), ℎ: the original air gap between the top and bottom electrode or of the diversion trench.
Fig. 4(b) illustrates the processing flow of the bottom plate. Firstly, a Cr grating was fabricated on the lower surface of a quartz plate by laser interference lithography (𝜆 = 363 nm, Spectra-Physics 2065). Surface morphology of the grating (𝛬 = 𝑆∕10 = 1.7 μm) was observed by Atomic Force Microscopy (AFM, Veeco DI). The Al grating with a duty cycle of 0.5, theoretical value of diffraction efficiency was 10.13% (1st order) and the experimental value was about 9.0%. Subsequently, a 30 nm Al layer [30] was deposited and patterned on its upper surface to form the square annular electrode and bottom mirror. At step 3, a shield block was placed on the pad and a 500 nm SiO2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD), which acted as an insulating layer of the top and bottom electrode. Finally, the shield block was removed to uncover the pad for the following wirebonding. The fabricated bottom plate is show in the inset, where the size of the Al mirror was 2 × 2 mm2 , and its reflective was about 78%, which approximately agreed with the designed reflectivity of 80%. After the fabrication of the top and bottom plate, the two plates of tunable FP cavity were bonded with the adhesive which was wrapped in the diversion trench. The fabricated MEMS FP-G filter is shown in Fig. 4(c).
Fig. 5. Simulated (solid line, ℎ = 3 μm, 𝐶T = 30 μm, 𝐶L = 1000 μm) and experimental displacement of the moveable mirror as a function of the driving voltage.
and on account of the decrease of the wetting speed in a long etching time, the thickness (𝐶T ) of the cantilever was larger than the expected value. These factors could lead to the rapid increase of the driving voltage at the same displacement as shown in Fig. 3(b) and (c), while the effect of these factors could be reduced by improving the existing technology and optimizing process parameters further.
4. Experimental results and discussion 4.1. Micromachine properties Fig. 5 shows micromachine performance test of the MEMS FP-G filter about the static characteristic between the driving voltage and the displacement of the moveable mirror. The displacements under various driving voltages were measured by employing a white light interferometer (Zygo Newview 7300). The electrostatic actuator was driven by a DC-biased voltage source (Pragmatic 2416A). With the increase of the driving voltage, the movable mirror moved down to decrease the cavity length. The required minimum variation of cavity length Δ𝐿min of 0.5 μm was obtained at about the driving voltage of 234 V, and with the change of driving voltages, the displacements changed slowly and gently. The discrepancy of the driving voltage between the simulation and the experiment could result from the difference of the material parameters and the structure dimensions between the model parameters and the actual values of the device. During the specific processing, the thickness error of the glue in the assembly process of the top and bottom plate caused a wider gap (ℎ) between electrodes than the design value,
4.2. Optical performance of MEMS FP-G filter Fig. 6(a) illustrates the schematic diagram of experimental setup to test the optical performance of the fabricated MEMS FP-G filter. The light source with the continuous spectrum was employed, and its spectrum was measured by a commercial optical spectrum analyzer (OSA) (Ocean Optics HR4000). The light source was attached to an optical fiber with core diameter of 50 μm. Subsequently, the beam from the optical fiber was collimated by a collimating lens with 100 mm effective focal length. Then the collimated beam was modulated by the MEMS FP-G filter. The zero order emergent beam was coupled into the fiber and measured by the OSA with a resolution of 0.75 nm. The only optical performance of FP cavity in MEMS FP-G filter was analyzed without dispersion effect of the grating on the transmission beam. The 0th order 475
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Optics Communications 402 (2017) 472–477
Fig. 6. (a) Schematic of the experiment setup, spectrum of light source in the inset, (b) Zero order transmission spectrum of the fabricated MEMS FP-G filter from optical spectrum analyzer.
interval (Δ𝜆𝑝 ) of 0.093 nm. Thus the peak–peak wavelength interval was about 2.3 nm corresponding to the 25 pixels, which was accord with the value from the OSA approximately. The FWHM of single diffraction peak covering about 15 pixels corresponded to 1.4 nm by the grating, while spectral resolution of the MEMS FP-G filter with 1.2 nm spectral linewidth depending on the FP cavity was slightly better than the one of the grating. However, due to the aberration of the focus lens and divergence of the incident beam, the size of light spot changed from 6.5 pixels under ideal conditions in the simulation to 15 pixels in the experiment, but as long as Rayleigh’s criterion (Δ𝜃 ≥ 𝛿𝜃) was met, the decrease of the spatial resolution of the grating had no effect on the spectral resolution of MEMS FP-G filter. By reducing the defects of the FP cavity, the spectral linewidth of FP cavity could be further reduced to improve the spectral resolution of the MEMS FP-G filter. Due to the limited spectral linewidth of FP cavity, the change of cavity length was required only once. Spatial shift of the focus spots corresponding to the different interference orders occurred simultaneously by the variation of the FP cavity length under the driving voltages of 0 and 187.5 V in favor of quickly realizing the wavelength scanning in the operation spectral range. After completing the wavelength scanning, transmission spectrum at different cavity length was superposed to broaden operation spectral range of the MEMS FP-G filter, compared with that of the traditional MEMS FP filter about 2.03 nm, which was limited by its own FSR, at the same conditions.
Fig. 7. Intensity distributions of the first order transmission beam of the fabricated MEMS FP-G filter at driving voltages of 0 and 187.5 V. Intensity curves coming from the cross section rows of light spots.
transmission spectrum is shown in Fig. 6(b). The FSR of the peak wavelength (𝜆peak ) of 972 nm was about 2.2 nm, and the FWHM of spectral line 𝛿𝜆 was about 1.2 nm. The effective Finesse 𝐹𝐸 of the FP cavity is about 1.83 calculated from equation, 𝐹𝐸 = FSR/FWHM, which is far less than the reflective Finesse of 12.6, because of the defects of the FP cavity, including surface irregularities, spherical defects and especially parallelism defects [31]. In addition, according to the formula [26], 𝐿 = 𝜆2 ∕(2Δ𝜆), the calculated initial cavity length of 214.7 μm was slightly greater than the silicon wafer thickness (200 μm), possibly due to the thickness error of the adhesive. Finally, the transmission of FP cavity about 10% was measured by OSA. Furthermore, we will need to optimize bonding technique or try other bonding methods (i.e, Si–SiO2 bonding) in order to decrease the parallelism deviation, and to adopt multilayer dielectric film with the purpose of decreasing the absorptivity of metal mirrors and increasing the transmission and reflectivity. The first order emergent beam of the MEMS FP-G filter passed through the focusing lens with the focal length 𝑓 of 100 mm (Thorlabs AC254-100-B), and the intensity distribution of the beam was recorded by a charge-coupled device (Coherent Inc Beam View Analyzer). Fig. 7 shows the intensity distribution of light spots at the focal plane which could be distinguished spatially, and the intensity curve was extracted from the cross section rows of light spots. The differential of grating equation is, Δ𝜆𝑝 = 𝛬 cos 𝜃 arctan(𝑥𝑝𝑖𝑥𝑒𝑙 ∕𝑓 )
5. Conclusions A tunable MEMS FP cavity based on the W-shaped-section microbridge structure is proposed and combines the grating to make up MEMS FP-G filter. The W-shaped-based FP cavity was designed and fabricated to realize long cavity length. The single MEMS FP-G filter with dual functions of the frequency filtering and spatial dispersing could realize the capabilities of both traditional MEMS FP filters and spectrometers to broaden the operation spectral range and ensure narrow spectral linewidth. Compared with the simulation results, the fabricated device performances such as the maximum driving voltage and effective finesse of FP cavity were restricted by the fabrication and bonding technique, meanwhile there was considerable room for improvement in device performance by the optimal technique. The tunable MEMS FP-G filter with multiple transmission peak wavelengths could been pursued for various applications, e.g., spectrographs where the spectral resolution could be improved in wide operation spectral range, sensors where spectrum characteristic of various materials could been analyzed simultaneously, and so on. Acknowledgments
(2)
This work was supported by the National Natural Science Foundation of China (No. 61308064); the National Basic Research Program of
where 𝛬 is the grating period; 𝜃 is the diffraction angle. The size of single pixel, 𝑥pixel = 6.7 μm, in the CCD corresponded to the wavelength 476
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China (973 Program) (No. 2014CB744204); Open Research Fund of Key Laboratory of Optical Fiber Communications ( Ministry of education of China). We thank Professor Miao Lv (Xiamen University) for experimental assistance and discussions.
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
MEMS-based filter integrating tunable Fabry–Perot cavity and grating Zhendong Shi a, *, Liang Fang b , Libo Zhong b a b
Southwest Institute of Technical Physics, Chengdu 610041, China Institute of Optics and Electronics, Chinese Academy of Science, Chengdu 610209, China
a r t i c l e
i n f o
Keywords: Optical microelectromechanical devices Tunable optical filters Fabry–Perot cavities Gratings
a b s t r a c t We present a novel Micro-Electro-Mechanical-Systems (MEMS) filter integrating a tunable Fabry–Perot cavity and a grating (FP-G). It fuses the functions of the Fabry–Perot (FP) cavity and grating into a single device so as to realize both narrow spectral linewidth given by FP cavity and wide spectral range given by grating. A MEMS FP-G filter with 200 μm cavity length was designed and fabricated. It is based on a W-shaped-section microbridge structure by which the electrostatic actuator is independent on the FP cavity and a long cavity length can be configured. A grating etched in the back of a cavity mirror serves to spatially separate the transmitted spectral lines corresponding to different interference orders of FP cavity. Experimental results indicated that the MEMS FP-G filter obtained a broader operation spectral range compared with the traditional MEMS FP filter, and achieved 1.2 nm spectral linewidth, which could be narrower by improving the effective finesse of the FP cavity in the fabrication process. Compared with the simulation results, there was considerable room for improvement in device performance by the optimal technique. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Tunable Fabry–Perot (FP) filter has been pursued for various applications including spectrographs [1,2], optical communications [3,4], sensors [5–9] and lasers [10]. The MEMS based tunable FP filters are desirable due to their miniaturization, high tuning speed, wide tuning range and potentially low cost at large volumes [11,12]. For conventional MEMS-based FP filter, the actuated plates of the electrostatic actuator usually locate on two micro-mirrors of FP optical cavity to benefit the manufacture process [13,14], so the FP cavity length is limited to a few micrometers to keep the driving voltage lower. Moreover, short enough cavity length can acquire adequately wide free spectral range (FSR) to guarantee only single transmission peak wavelength in spectral range. Though optical FP cavity with long cavity length separated from the electrostatic actuator using SOI wafer is developed [15], transmission peaks in the different interference orders are overlapped spatially, and the transmission spectrum contains more than one peak wavelength, which results in an inaccuracy of transmission peak wavelength measurement. In general, a spectrometer is required to analyze multiple transmission peak wavelengths, while FP cavity itself usually is unable to complete it. Furthermore, approach to using long cavity length is capable of obtaining narrow spectral linewidth but the operation spectral range is mainly limited by the FSR [16]. In * Corresponding author.
E-mail address: [email protected] (Z. Shi). http://dx.doi.org/10.1016/j.optcom.2017.06.043 Received 20 March 2017; Received in revised form 23 May 2017; Accepted 12 June 2017 0030-4018/© 2017 Elsevier B.V. All rights reserved.
general, there is a contradiction to meet the demands between narrow spectral linewidth and wide FSR. The combined use of optical devices is a method to overcome this contradiction in conventional instruments. The combination of the FP etalon and the grating has been carried out for the tunable external-cavity laser diode to achieve the narrowband spectral output [17–19] and for the versatile instrument to realize astronomical narrowband imaging [20]. Additionally, the array of filters with different narrow-bands is an effective way to cover a wide range of operation wavelengths [21–23]. Meanwhile, the optical filter employs the tapered cavity to achieve the change of cavity length, and the different peak wavelengths are transmitted at different positions of the filter, so that the operation bandwidth is effectively widened [6,24,25]. Based on the method of the integration of a tunable FP cavity and a grating, we have previously reported a dispersive element in a miniature spectrometer by a piezo-electrical transducer [26], and analyzed the influences of divergence angle on the device performance [27]. In order to miniaturize the device and take advantages of MEMS devices, the structure of MEMS device has been preliminarily designed [28]. Here, we improved the design in favor of the actual processing, fabricated optical MEMS device and experimentally performed function tests. This paper extends the method of the integration of a tunable FP cavity and a grating to MEMS devices. A MEMS filter based on W-shaped micro bridge is proposed, which integrates the tunable MEMS FP cavity and
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the grating. The proposed W-shaped micro bridge structure based on low-cost bulk micromachining processes was favorable to obtain a long FP cavity length depending on the silicon wafer thickness itself as well as a low driving voltage by the separation of the electrodes and the mirrors simultaneously in the design. For optical characterizations, peak wavelengths in multiple interference orders were produced through the MEMS FP cavity, then peak wavelengths could be spatially separated with the post-dispersion by the grating integrated with the FP cavity. Thus the MEMS FP-G filter could achieve the frequency filtering and spatial dispersing simultaneously, to overcome the limitation of the conventional MEMS FP filter that the operation wavelength range depending on the FSR became short at the long cavity length. Moreover, by changing the cavity length with the electrostatic force, the corresponding peak wavelengths would vary and cover a wide spectral range. The experimental results indicated the MEMS device achieved a broader spectral range in comparison with the traditional MEMS FP filter and obtained 1.2 nm spectral linewidth. It could be narrower by increasing the effective finesse of the FP cavity through optimizing the fabrication process.
Fig. 1. (a) 3D schematic of the proposed MEMS FP-G filter, optical process and operational principle of the device in the inset.
of spectral resolution. In Fig. 2(c) and (d), for the peak wavelength of 801.6 nm, the half angular width of 0.025◦ corresponds to spectral range of 0.625 nm through the grating equation, but the spectral range in the diffraction peak is measured by the FWHM of spectral line of 0.115 nm from FP cavity. Hence, spatial resolution of gratings does not affect spectral resolution of MEMS FP-G optical filter under the conditions, Δ𝜃 ≥ 𝛿𝜃. When the variation of cavity length is the step size (28 nm) obtained by the FWHM of spectral line, the change of peak wavelength is the FWHM of spectral line, and diffraction beam also shifts and double peaks of diffraction beam can be distinguished each other due to differences of transmission time, thereby the spectral information with the higher resolution at the corresponding position can be obtained. In addition, intensity distribution of single diffraction peak overlays spatial distributions of all spectral components in corresponding spectral line. Thus the diffraction peak intensity increases with increase of wavelengths due to the broadening of spectral linewidth as shown in the inset in Fig. 2(b), while the spatial half angular width of the peak wavelength (801.6 nm) is the same as the one of the diffraction peak without the spatial broadening as shown in Fig. 2(d). Moreover, due to intensity distribution through the grating similar to sinc function, a sub-peak exists between two main peaks as shown in Fig. 2(b). However, when the cavity length is determined, the peak wavelength and its diffraction angle is also determined, so the detection in specific spatial regions can reduce the influence of a sub-peak. In short, due to the miniaturization of MEMS devices, the number of grating period is reduced resulting in the increase of the half angular width and decrease of the spectral resolution. The spectral resolution is improved by MEMS FP-G filter, compared with the grating.
2. Device design 2.1. Optical considerations Fig. 1(a) presents 3D structure of a designed MEMS FP-G filter that consists of a W-shaped-section silicon top plate and a quartz bottom plate with a diffraction grating fabricated on its lower surface. The movable mirror of FP cavity on the top of the mesa is suspended by the cantilever and the fixed bottom mirror is provided on the quartz substrate. The operation principle of the device integrating the Fabry– Perot (FP) cavity and grating is illustrated in the inset. A collimated beam is normal incident on the FP cavity where the multi-orders resonant wavelengths are produced, and then the resonant wavelengths achieve the spatial separation by the grating. According to Rayleigh’s criterion, in order to distinguish the peak–peak wavelengths of adjacent interference orders of the FP cavity spatially, the difference of diffraction angle Δ𝜃 of the adjacent peak–peak wavelengths is required to be greater than the half angular width 𝛿𝜃 of single peak wavelength 𝜆k passing through the grating, Δ𝜃 ≥ 𝛿𝜃 [16,28]. Consequently, the restrictive relation between the parameters of the grating and FP cavity is obtained by 𝜆𝑘 1 ≥ . (1) 2𝑛𝐿 𝑁 where 𝜆k , 𝑛, 𝐿 and 𝑁 represent the peak wavelength corresponding to the interference order 𝑘, the refractive index inside the cavity, the cavity length and the number of grating period, respectively. The initial cavity length (𝐿) determined by the silicon wafer thickness itself is approximately 200 μm. This device is mainly used in the nearinfrared wavelength range (0.8–1.0 μm), According to Eq. (1), the period number of grating with 600 lines/mm should be greater than 500. To ensure the beam size, the mirror size of FP cavity is 2 × 2 mm2 . The aluminum mirror with the easy processing can maintain the consistency of reflectivity in a relatively broad spectrum range, and the mirror reflective of 80% is selected taking the absorption into account. Based on the device parameters above, optical process of the MEMS FP-G optical filter was simulated. The parallel light with the continuous spectrum normally transmits the tunable FP cavity to form a series of discrete spectral lines as shown in Fig. 2(a). Then, discrete spectral lines in spectral domain are separated by the grating in spatial domain, and there exists a corresponding relationship between the spectral lines and diffraction peaks as shown by red double arrows in Fig. 2(b). Thus, transmission spectra spread out in the space, to detect the wider spectral range just by MEMS FP-G filter. On the other hand, the spectral components of single peak in the diffraction beams rely solely on the spectral linewidth of the FP cavity and can be measured by the full width at half maximum (FWHM) of spectral line which is used as the standard
2.2. Mechanical design The MEMS FP-G filter is based on the W-shaped-section microbridge structure, as is denoted with red dashed lines in Fig. 3(a), making the cavity mirror independent on the electrostatic actuator to ensure the long cavity length and the short distance of electrodes simultaneously. The two plates are bonded with the adhesive wrapped in the diversion trench. The fundamental configuration of electrostatic actuators is similar to a parallel-plate capacitor. For the design of an electrostatic actuator, the pull-in displacement 𝑥PI , which limits the operation range of cavity length to approximately one third of original air gap (ℎ) between the top and bottom electrode [29], should be larger than the minimum variation of cavity length determined by the maximum wavelength 𝜆max , Δ𝐿min = 𝜆max ∕2 [26]. In order to satisfy the condition of 𝑥PI > Δ𝐿min , the original air gap (ℎ) should be larger than 1.5 μm, and we select the original air gap (ℎ) of 3 μm. To investigate electromechanical performance of the MEMS device, we employ the IntelliSuite software to analyze the relation between 473
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Fig. 2. Simulated transmission curve of the FP cavity (a). Intensity distribution about diffraction angle of MEMS FP-G filter (b). Discrete spectral lines in spectral domain (c) and intensity distribution in spatial domain (d) shifting with the variation of the FP cavity length (𝐿). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Schematic of the electrostatic actuator based on the W-shaped-section micro-bridge structure (a). Simulated results of the change of displacement of the movable mirror versus driving voltage (𝑉d ) at the different cantilever lengths 𝐶T (b), and air gaps of electrodes ℎ (c). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
the displacement of the movable mirror and driving voltage (𝑉d ) under different parameters of structure including the thickness (𝐶T ) in Fig. 3(b) of the cantilever and the air gap (ℎ) of electrodes in Fig. 3(c) (Si: Young’s modulus of 170 Gpa, Poisson’s ratio of 0.26). When the thickness (𝐶T ) and air gap (ℎ) increases, the driving voltage (𝑉d ) increases rapidly. Nevertheless, there is an equal pull-in displacement 𝑥PI about 1 μm at the same original air gap (ℎ), so the larger driving voltage at the displacement of the Δ𝐿min is, more gently the change of driving voltage is. In addition, compared with the designed length of the cantilever (𝐶L = 1000 μm), the length error of the cantilever from the wet etching has a little effect on the driving voltage.
the lower oxide layer was patterned, and the wafer was dipped into a 25% TMAH (Tetramethylammonium hydroxide) solution at 80 ◦ C for anisotropic wet etching about 8 min, in order to etch a 3 μm silicon layer where the air gap between the top and bottom electrode and the diversion trench were formed. Thirdly, a 600 nm silicon nitride layer was deposited by low pressure chemical vapor deposition (LPCVD) to act as a hard mask in the following deep wet etching. Fourthly, a square opening window on the lower silicon nitride layer was opened by the reactive ion etching (RIE), and a 20 μm silicon layer was etched in the TMAH solution to control the thickness of the cantilever in advance. Subsequently, a square annular window on the upper silicon nitride layer was accomplished by the RIE. Then, the deep volume cavity and the cantilever were formed during double-side anisotropic TMAH etching, and the film covering the aperture was removed to form an optical aperture in the center, because the deformation of the large area film occurred and the quality of the mirror was declined, when the cantilever moved down. Next, the Aluminum (Al) layer with the thickness of 30 nm was evaporated on the lower surface of the silicon wafer, which was used for the top electrode. Finally, the Pyrex glass with the 30 nm thick Al mirror was bond with the top plate, and the fabricated top plate is show in the inset.
3. Device fabrication Fig. 4 shows the schematic process flow of the MEMS FP-G filter. In the top plate of MEMS FP-G filter, the W-shaped micro-bridge structure was fabricated by the wet etching technology as shown in Fig. 4(a). Firstly, a 60 nm thermal oxide layer for masking layer in wet etching process was grown on a double side polished (100) silicon wafer with the thickness of 200 μm. Secondly, an opening window on 474
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Fig. 4. Schematic fabrication sequence of the MEMS FP-G filter, (a) Top plate, a photo of the fabricated top plate in the inset (b) Bottom plate, AFM image of the grating and a photo of the fabricated bottom plate in the inset, (c) Photograph of the fabricated MEMS FP-G filter. 𝑆: width of ten grating period (𝛬 = 1.7 μm), ℎ: the original air gap between the top and bottom electrode or of the diversion trench.
Fig. 4(b) illustrates the processing flow of the bottom plate. Firstly, a Cr grating was fabricated on the lower surface of a quartz plate by laser interference lithography (𝜆 = 363 nm, Spectra-Physics 2065). Surface morphology of the grating (𝛬 = 𝑆∕10 = 1.7 μm) was observed by Atomic Force Microscopy (AFM, Veeco DI). The Al grating with a duty cycle of 0.5, theoretical value of diffraction efficiency was 10.13% (1st order) and the experimental value was about 9.0%. Subsequently, a 30 nm Al layer [30] was deposited and patterned on its upper surface to form the square annular electrode and bottom mirror. At step 3, a shield block was placed on the pad and a 500 nm SiO2 layer was deposited by plasma enhanced chemical vapor deposition (PECVD), which acted as an insulating layer of the top and bottom electrode. Finally, the shield block was removed to uncover the pad for the following wirebonding. The fabricated bottom plate is show in the inset, where the size of the Al mirror was 2 × 2 mm2 , and its reflective was about 78%, which approximately agreed with the designed reflectivity of 80%. After the fabrication of the top and bottom plate, the two plates of tunable FP cavity were bonded with the adhesive which was wrapped in the diversion trench. The fabricated MEMS FP-G filter is shown in Fig. 4(c).
Fig. 5. Simulated (solid line, ℎ = 3 μm, 𝐶T = 30 μm, 𝐶L = 1000 μm) and experimental displacement of the moveable mirror as a function of the driving voltage.
and on account of the decrease of the wetting speed in a long etching time, the thickness (𝐶T ) of the cantilever was larger than the expected value. These factors could lead to the rapid increase of the driving voltage at the same displacement as shown in Fig. 3(b) and (c), while the effect of these factors could be reduced by improving the existing technology and optimizing process parameters further.
4. Experimental results and discussion 4.1. Micromachine properties Fig. 5 shows micromachine performance test of the MEMS FP-G filter about the static characteristic between the driving voltage and the displacement of the moveable mirror. The displacements under various driving voltages were measured by employing a white light interferometer (Zygo Newview 7300). The electrostatic actuator was driven by a DC-biased voltage source (Pragmatic 2416A). With the increase of the driving voltage, the movable mirror moved down to decrease the cavity length. The required minimum variation of cavity length Δ𝐿min of 0.5 μm was obtained at about the driving voltage of 234 V, and with the change of driving voltages, the displacements changed slowly and gently. The discrepancy of the driving voltage between the simulation and the experiment could result from the difference of the material parameters and the structure dimensions between the model parameters and the actual values of the device. During the specific processing, the thickness error of the glue in the assembly process of the top and bottom plate caused a wider gap (ℎ) between electrodes than the design value,
4.2. Optical performance of MEMS FP-G filter Fig. 6(a) illustrates the schematic diagram of experimental setup to test the optical performance of the fabricated MEMS FP-G filter. The light source with the continuous spectrum was employed, and its spectrum was measured by a commercial optical spectrum analyzer (OSA) (Ocean Optics HR4000). The light source was attached to an optical fiber with core diameter of 50 μm. Subsequently, the beam from the optical fiber was collimated by a collimating lens with 100 mm effective focal length. Then the collimated beam was modulated by the MEMS FP-G filter. The zero order emergent beam was coupled into the fiber and measured by the OSA with a resolution of 0.75 nm. The only optical performance of FP cavity in MEMS FP-G filter was analyzed without dispersion effect of the grating on the transmission beam. The 0th order 475
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Fig. 6. (a) Schematic of the experiment setup, spectrum of light source in the inset, (b) Zero order transmission spectrum of the fabricated MEMS FP-G filter from optical spectrum analyzer.
interval (Δ𝜆𝑝 ) of 0.093 nm. Thus the peak–peak wavelength interval was about 2.3 nm corresponding to the 25 pixels, which was accord with the value from the OSA approximately. The FWHM of single diffraction peak covering about 15 pixels corresponded to 1.4 nm by the grating, while spectral resolution of the MEMS FP-G filter with 1.2 nm spectral linewidth depending on the FP cavity was slightly better than the one of the grating. However, due to the aberration of the focus lens and divergence of the incident beam, the size of light spot changed from 6.5 pixels under ideal conditions in the simulation to 15 pixels in the experiment, but as long as Rayleigh’s criterion (Δ𝜃 ≥ 𝛿𝜃) was met, the decrease of the spatial resolution of the grating had no effect on the spectral resolution of MEMS FP-G filter. By reducing the defects of the FP cavity, the spectral linewidth of FP cavity could be further reduced to improve the spectral resolution of the MEMS FP-G filter. Due to the limited spectral linewidth of FP cavity, the change of cavity length was required only once. Spatial shift of the focus spots corresponding to the different interference orders occurred simultaneously by the variation of the FP cavity length under the driving voltages of 0 and 187.5 V in favor of quickly realizing the wavelength scanning in the operation spectral range. After completing the wavelength scanning, transmission spectrum at different cavity length was superposed to broaden operation spectral range of the MEMS FP-G filter, compared with that of the traditional MEMS FP filter about 2.03 nm, which was limited by its own FSR, at the same conditions.
Fig. 7. Intensity distributions of the first order transmission beam of the fabricated MEMS FP-G filter at driving voltages of 0 and 187.5 V. Intensity curves coming from the cross section rows of light spots.
transmission spectrum is shown in Fig. 6(b). The FSR of the peak wavelength (𝜆peak ) of 972 nm was about 2.2 nm, and the FWHM of spectral line 𝛿𝜆 was about 1.2 nm. The effective Finesse 𝐹𝐸 of the FP cavity is about 1.83 calculated from equation, 𝐹𝐸 = FSR/FWHM, which is far less than the reflective Finesse of 12.6, because of the defects of the FP cavity, including surface irregularities, spherical defects and especially parallelism defects [31]. In addition, according to the formula [26], 𝐿 = 𝜆2 ∕(2Δ𝜆), the calculated initial cavity length of 214.7 μm was slightly greater than the silicon wafer thickness (200 μm), possibly due to the thickness error of the adhesive. Finally, the transmission of FP cavity about 10% was measured by OSA. Furthermore, we will need to optimize bonding technique or try other bonding methods (i.e, Si–SiO2 bonding) in order to decrease the parallelism deviation, and to adopt multilayer dielectric film with the purpose of decreasing the absorptivity of metal mirrors and increasing the transmission and reflectivity. The first order emergent beam of the MEMS FP-G filter passed through the focusing lens with the focal length 𝑓 of 100 mm (Thorlabs AC254-100-B), and the intensity distribution of the beam was recorded by a charge-coupled device (Coherent Inc Beam View Analyzer). Fig. 7 shows the intensity distribution of light spots at the focal plane which could be distinguished spatially, and the intensity curve was extracted from the cross section rows of light spots. The differential of grating equation is, Δ𝜆𝑝 = 𝛬 cos 𝜃 arctan(𝑥𝑝𝑖𝑥𝑒𝑙 ∕𝑓 )
5. Conclusions A tunable MEMS FP cavity based on the W-shaped-section microbridge structure is proposed and combines the grating to make up MEMS FP-G filter. The W-shaped-based FP cavity was designed and fabricated to realize long cavity length. The single MEMS FP-G filter with dual functions of the frequency filtering and spatial dispersing could realize the capabilities of both traditional MEMS FP filters and spectrometers to broaden the operation spectral range and ensure narrow spectral linewidth. Compared with the simulation results, the fabricated device performances such as the maximum driving voltage and effective finesse of FP cavity were restricted by the fabrication and bonding technique, meanwhile there was considerable room for improvement in device performance by the optimal technique. The tunable MEMS FP-G filter with multiple transmission peak wavelengths could been pursued for various applications, e.g., spectrographs where the spectral resolution could be improved in wide operation spectral range, sensors where spectrum characteristic of various materials could been analyzed simultaneously, and so on. Acknowledgments
(2)
This work was supported by the National Natural Science Foundation of China (No. 61308064); the National Basic Research Program of
where 𝛬 is the grating period; 𝜃 is the diffraction angle. The size of single pixel, 𝑥pixel = 6.7 μm, in the CCD corresponded to the wavelength 476
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China (973 Program) (No. 2014CB744204); Open Research Fund of Key Laboratory of Optical Fiber Communications ( Ministry of education of China). We thank Professor Miao Lv (Xiamen University) for experimental assistance and discussions.
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