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Abstract
Longwave infrared spectral imaging (LWIR-SI) has potential in many important civilian and military fields. However, conventional LWIR-SI systems based on traditional dispersion elements always suffer the problems of high cost, large volume and complicated system structure. Micro-electro-mechanical systems Fabry-Perot filtering chips (MEMS-FPFC) give a feasible way for realizing miniaturized, low cost and customizable LWIR-SI systems. The LWIR MEMS-FPFC ever reported can’t meet the demands of the next-generation LWIR-SI systems, due to the limitation of small aperture size and nonlinear actuation. In this work, we propose a large-aperture, widely and linearly tunable electromagnetically actuated MEMS-FPFC for LWIR-SI. A multi-field coupling simulation model is built and the wafer-scale bulk-micromachining process is applied to realize the design and fabrication of the proposed MEMS-FPFC. Finally, with the rational structural design and fabrication process, the filtering chip after packaging has an aperture size of 10 mm, which is the largest aperture size of LWIR MEMS-FPFC ever reported. The fabricated electromagnetically actuated MEMS-FPFC can be tuned continuously across the entire LWIR range of 8.39-12.95 µm under ±100 mA driving current with a pretty good linear response of better than 98%. The developed electromagnetically actuated MEMS-FPFC can be directly used for constructing miniaturized LWIR-SI systems, aiming for such applications as military surveillance, gas sensing, and industry monitoring.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Owing to the rich two-dimensional (2D) morphology and one-dimensional (1D) thermal radiation spectrum information, longwave infrared spectral imaging (LWIR-SI) has been widely used in many scientific fields in the past decades, such as liquid and gas diagnostics [1,2], geologic mapping [3,4], environmental protection [5,6], military targets detection and early warning [7–10]. Dispersion element used for spectral division is one of key components for spectral imagers, which determines the working mode, spectral performance and structural configuration [11,12]. According to the working principles, dispersion elements can be classified into three types: dispersive elements (e.g. grating, prism), filtering elements (e.g. bandpass filter, acousto-optical tunable filter and liquid crystal tunable filter), and Fourier-transform interferometers (e.g. Michelson interferometer, Mach-Zender interferometer). Up to now, LWIR-SI systems based on above-mentioned dispersion elements have been adopted in practical applications [13–15]. However, because of the disadvantages of conventional dispersion elements, those LWIR-SI systems always suffer the problems of high cost, large volume and complicated system structure, which limit the widespread utilization in many desirable application scenarios. Especially, with the popularization of miniaturized, unmanned and smart equipments, like smart-phones and unmanned aerial vehicles (UAVs), the miniaturization of LWIR-SI system has become an inevitable trend.
Micro-electro-mechanical systems (MEMS) based filtering devices, including micro-grating [16,17], photonic crystal filters [18,19] and mosaic multi-band filters [20,21], have brought the revolutionary technical change to the miniaturized spectral imagers. In recent years, MEMS-based Fabry-Perot filtering chips (MEMS-FPFC) have attracted great interest around the world, due to their inherent merits of simple structure, customizable performance, good system compatibility and capability of high-yield production. Ever since Mallinson [22] reported the first electrostatically actuated MEMS-FPFC for wavelength division multiplexing, many research institutions have invested lots of resources to the development of MEMS-FPFC for the spectral imaging. After thirty-year development, MEMS-FPFC based on different actuating strategies and working in different wavebands have been reported, and some mature devices have also been commercially available [23,24]. In visible and near-infrared wavelength, VTT technical research center of Finland has realized several types of miniaturized spectral imaging payloads for some ultra-compact platforms, such as UAVs, nanosatellites, and smart phones, employing the piezo-actuated MEMS-FPFC [25–27] and surface micromachining electrostatically actuated MEMS-FPFC [28,29], respectively. NASA developed a silicon-based mid-infrared MEMS-FPFC for wide-field spectral imaging in space flight, and a bulk micromachining fabrication process was adopted to meet the demand of large aperture (11 mm) [30]. As for the applications in LWIR, although the LWIR MEMS-FPFC have already been reported by InfraTec GmbH [31–33], VTT [34,35] and other groups [36–38], the optical performance or the aperture size of those devices could hardly meet the demands of LWIR-SI.
For the MEMS-FPFC applied in LWIR-SI, several critical demands need to be took into account, including device design and system integration. Firstly, the aperture size of MEMS-FPFC needs to be as large as possible to allow high information and energy throughput, because the infrared radiation energy in LWIR is much weaker than the radiation or reflected energy in other wavebands [10]. Compared with surface micromachining, thanks to the relatively low-stress structural design and fabrication process, bulk micromachining always matches the fabrication of large-aperture MEMS-FPFC [23]. And secondly, to accommodate the diverse and complex application scenarios, a wide-range tunability covering the whole LWIR waveband is important to LWIR-SI. However, as the mainstream actuation strategy, due to the limitations of nonlinear response and pull-in phenomenon, electrostatic actuation usually needs significantly high driving voltage to realize large-range FP cavity modulation, which intrinsically has a limited working range of about one-third the initial FP cavity length [39]. The linearly electromagnetically actuated MEMS-FPFC was firstly reported by Lee, which utilized the Lorentz force as the driving force to change the FP cavity length, but the aperture size was just 50 µm [40]. To the best of our knowledge, the MEMS-FPFC for LWIR-SI with a large aperture size more than 2 mm, and a wide and linear tunability covering the whole LWIR waveband, has not been reported before.
In this work, a large-aperture, wide tuning range, electromagnetically actuated MEMS-FPFC for LWIR-SI is proposed, which can be tuned linearly and continuously in the entire 8-12 µm LWIR waveband under low driving current. The multi-field coupling simulation model is built to assist the customized, efficient design procedure, and a wafer-scale bulk micromachining process is developed to guarantee the high-quality and high-efficiency fabrication for this large-aperture electromagnetically actuated MEMS-FPFC. As a dispersion element, the developed MEMS-FPFC could be utilized to the construction of miniaturized LWIR-SI system, which breaks the shackles of conventional LWIR-SI systems and has great potential applications in the civilian fields and military detection.
2. Principle and design
2.1. Working principle
The fundamental structure and working principle of electromagnetically actuated MEMS-FPFC are illustrated in Fig. 1. The same as the conventional bulk micromachined MEMS-FPFC, two parallel micromechanical plates are necessary, i.e. the fixed plate and the movable plate with a spring structure. High-reflectance mirrors are deposited on the inner side of the two plates, and separated by a certain distance to form the FP cavity. The transmitted central wavelength λ obeys the following equation [41]:
where, n, d, θ, and m are the refractive index of the FP cavity material and its length, incident angle and interference order of the light source and transmitted light, respectively. A permanent magnet is attached on the backside of the movable plate, as one of the electromagnetically actuating parts. The filtering chip is packaged on a PCB with the designed circuitry and coil. When the driving current signal is input into the coil, a magnetic field is induced, which interacts with the permanent magnet to generate the needed electromagnetic force, further driving the suspension structures to change the FP cavity length d. As a result, according to Eq. (1), the transmitted light with different central wavelength λ can be achieved. In particular, by varying the direction of driving current, the direction of electromagnetic force can also be oppositely changed. In other words, the electromagnetically actuated FP cavity can be bidirectionally tuned, ensuring a wider operational waveband range of this new MEMS-FPFC.
Fig. 1. The fundamental structure and working principle of electromagnetically actuated MEMS-FPFC. (a) gives the fundamental structure, by applying actuating current to actuating coil, the movable plate can be driven by the electromagnetic force to move up or down, which leads to the FP cavity length variation, and (b) the transmitted wavelength is changed correspondingly.
Fundamentally, the working principle of the electromagnetically actuated MEMS-FPFC is the interaction of physical fields and conversion of physical quantities, including the actuating current (I), electromagnetic force (F), mechanical strain (ɛ) and transmitted wavelength (λ). According to the electromagnetism, solid mechanics and FP interference principles, the mutual relationships among these physical quantities are all linear, and can be simply expressed as:
Here, the maximum ɛ is the variation (Δd) of FP cavity length. Thus,
and finally, the linear relation between I and λ can be derived:For m = 1 and θ=0, combining Eqs. (1) and (5), the value b is:
While a is the complex function of electromagnetic characteristics, material mechanical parameters and optical properties. If a is known, then λ can be accurately predicted by Eq. (5).2.2. Multi-field coupling simulation design
2.2.1. Design objectives
A multi-field coupling simulation model is built to facilitate the convenient design of the electromagnetically actuated MEMS-FPFC. The basic design objectives in this work are shown in Table 1, and the aperture size is expected to 10 mm to obtain more LWIR radiation, which exceeds the aperture size of LWIR MEMS-FPFCs ever reported. The initial central wavelength (λ0) is 10 µm, and the electromagnetically actuated MEMS-FPFC can be tuned continuously at two directions covering the entire 8-12 µm LWIR waveband under the driving current of ±100 mA. As the key parameters of optical performance, the spectral transmittance (T) should be larger than 40% and the full width at half maximum (FWHM) is expected to be smaller than 500 nm.
Table 1. The basic design objectives of the electromagnetically actuated MEMS-FPFC
2.2.2. Optical and mechanical structure considerations
Foundationally, the selection of movable and fixed plate substrates is important to the design and fabrication process, and the material of substrate needs to fit the bulk micromachining process. Si wafer is the most common substrate in semiconductors and has formed a complete micromachining industry foundation which is often used for fabricating MEMS devices with movable structures. The transmittance of Si wafer in LWIR largely depends on the resistance (i.e. doping content), as shown in Fig. 2, giving the actually testing results, the higher the resistance, the higher the transmittance (48%−54% for the most), because high-content doping in low-resistance Si wafer can significantly reflect or absorb LWIR wavelength leading to the decline of transmittance. In addition, according to the different production processes, Si wafer can be classified into czochralski silicon (CZ-Si) wafer and float-zone silicon (FZ-Si) wafer. High-resistance CZ-Si wafer has an absorption peak at 9 µm due to the high oxygen content, which is unseen in FZ-Si wafer benefiting from the low-oxygen production process. Thus, the high-resistance FZ-Si wafers (>10 kΩ/cm) are selected as the basic substrates of movable and fixed plates in this work.
To eliminate the interface reflection of air-silicon interface, the antireflection (AR) films are fabricated on the outer surfaces of the movable and fixed plates. For a Si wafer with a single-side AR film, the transmittance can be obviously promoted by 15.8%∼18.8% in 8-12 µm waveband, as the simulated result shown in Fig. 3.
Mirrors are the key optical structures to form the FP interference, of which the reflectance (R) determines the T and FWHM of filtering chip by Eqs. (7) and (8) [41]:
in which φ is the phase difference. Most of metallic films have high absorptance in LWIR that can’t be used as the mirrors of LWIR MEMS-FPFC. The distributed Bragg reflector (DBR) consists of high- and low-refractive multilayer film stacks, as shown in Fig. 4(a), has unique advantages of low absorption and customizable reflectance, according to the Eq. (9) [41]:Fig. 4. The simulated reflective/transmissive Ge/ZnS DBR: (a) the basic structure, and (b) the optical performance with different N.
Moreover, a permanent magnet and a copper actuating coil are set in the simulation model as the electromagnetically actuating parts, of which parameters are chosen according to the measured values. The residual flux density in axial direction of permanent magnet is measured to be 67 mT; the turns of copper actuating coil is 200, and the copper wire diameter is 0.25 mm.
2.2.3. Design results
Overall consideration of the above-mentioned conditions, the simulation model coupling the electromagnetic field, solid mechanics and ray optics is built. In this model, parameters with different combinations are swept to achieve the optimal solution of the electromagnetically actuated MEMS-FPFC. The desirable structural design results are shown in Fig. 5, showing the two plates of the same size of 25 mm × 25 mm × 0.4 mm, with 10-mm-diameter DBR and AR films at inner and outer sides. The central area of the movable plate is supported by three double-folded beams and etched to form a groove to integrate the permanent magnet. Under this configuration, the working frequency should be within the 1st-order resonant frequency (185 Hz) to ensure the vertical movement of the movable mirror, instead of the torsional motion within the 2nd- or higher-order resonant frequency.
In terms of optical properties, the λ0 of the designed filtering chip is 10 µm where the FWHM is 171 nm (Fig. 6(a)), and the filtering chip works at the first interference order (m = 1, d0 = 5 µm) to ensure a wide tuning range. As predicted, the movable plate and the transmitted central wavelength can be bidirectionally, linearly and continuously tuned in 8-12 µm waveband under the ±100 mA driving current (Fig. 6(b)), and the FWHM in the whole waveband is smaller than 500 nm (Fig. 6 (c)). Figure 6(d) shows the simulated Δd-I and λ-I linear plots, which are the key advantages of the proposed electromagnetically actuated MEMS-FPFC as compared with the previously reported counterparts. Furthermore, the linearity is evaluated by the residual sum of squares (RSS) and coefficient of determination (R2), and the (RSS, R2) of the Δd-I and λ-I plots are (0.01099, 0.99971) and (0.06859, 0.99889), respectively. In this condition, the linear response of the electromagnetically actuated MEMS-FPFC is nearly 100%, and the a is computed as 0.021, so the Eq. (5) can be decided as:
Fig. 6. The simulated optical performance and tunability of the electromagnetically actuated MEMS-FPFC: (a) the initial central wavelength λ0 = 10 µm; (b) the designed tunable performance of the electromagnetically actuated MEMS-FPFC in LWIR; (c) the FWHM at different central wavelengths in LWIR; (d) the linear fitting results of Δd-I and λ-I plots.
The simulated results give a dependable guidance to the actual fabrication of electromagnetically actuated MEMS-FPFC.
3. Fabrication and characterizations
As mentioned in the above context, a wafer-scale bulk micromachining process flow is developed to achieve high-quality and high-efficiency fabrication of the designed electromagnetically actuated MEMS-FPFC, combining the lithography, film deposition, deep reactive ion etching (DRIE) and wafer bonding, as shown in Fig. 7. Initially, two pieces of 4-inch FZ-Si wafers are prepared as the substrates of the movable and fixed plates, respectively, which are cleaned in hydrofluoric acid solution, deionized water and isopropanol to remove the native oxide layer and possible contaminants (Figs. 7(a) and 7(f)). And then, the Ge/ZnS DBRs and AR films are formed in succession by thermal evaporation deposition and lift-off process accordingly on front surface (Figs. 7(b) and 7(g)) and back surface (Figs. 7(c) and 7(h)) of the two substrates. For the next, the permanent magnet groove is etched on the backside of movable plate (Fig. 7(d)), followed by another DRIE process for the double-folded beams (Fig. 7(e)). Finally, an epoxy-based polymer bonding layer is patterned on the front side of fixed plate, which acts as the spacing layer to interconnect the two plates, as illustrated in Figs. 7(i) and 7(j).
During the micromachining process, the performances of the fabricated optical structures are characterized. The reflectance of three five-layer Ge/ZnS DBRs tested by FTIR (Bruker, ALPHA II) is in the range of 80% to 95% over the 8-12 µm waveband (Fig. 8(a)), and the average thickness of the DBRs measured by stylus profile (Bruker, Dektak XT) is about 4.29 µm (Fig. 8(b)), which keeps in good agreement with the designed results. In addition, as shown in Fig. 8(c), the roughness profiles of the DBRs are measured by stylus profiler, and in the 5 mm testing length, the roughness profiles of DBRs are within 60 nm, and the root-mean-square (RMS) of roughness is smaller than 6 nm, which mean that the fabricated DBRs have very good surface quality. Compared with the naked FZ-Si substrate, the transmittance with the deposited single-side AR film can be promoted by 11.6%−15.8% in 8-12 µm waveband and has a maximum value at λ0 = 10 µm, as demonstrated in Fig. 9.
Fig. 8. The performance of five-layer Ge/ZnS DBRs: (a) the reflectance achieved by FTIR, (b) the average thickness measured by stylus profile, and (c) the roughness profiles of the DBRs.
When the wafer-scale bulk micromachining process is finished, the bonded wafer is diced into single MEMS-FPFCs, whose geometrical parameters are 25.5 mm × 25.5 mm × 0.8 mm, as shown in Fig. 10(a). After the permanent magnet is inserted into the magnet groove, the electromagnetically actuated MEMS-FPFC is packaged on the PCB with the designed driving coil and protected by a 3D-printed shell to further facilitate the construction of LWIR-SI system, as presented in Fig. 10(b). The actuating PCB uses the digital output of a microprocessor to realize the pulse-wide modulation (PWM) of the driving current. The aperture size of the packaged electromagnetically actuated MEMS-FPFC is 10 mm, to the best of our knowledge, which is the largest among the LWIR MEMS-FPFCs ever reported.
The realized optical performance of the packaged electromagnetically actuated MEMS-FPFC is experimentally characterized and given in Fig. 11, with the tested central wavelength λ0 of 10.57 µm and its FWHM of 254 nm (Fig. 11(a)). As shown in Fig. 11(b), the driving current is applied in the range of ±100 mA at a step of 5 mA, and the transmitted spectral responses exhibit a bidirectional and continuously tuning capability covering an actual waveband of 8.39-12.95 µm with the transmittance between 45% to 70%, revealing the key functionality of this filtering chip. Correspondingly, the FWHMs at different central wavelengths are mainly smaller than 350 nm in the entire tuning range (Fig. 11(c)). Compared with the design, the practical results have certain deviation in the aspects of λ0, FWHM and T, which may be due to the comprehensive influence of the intrinsic factors including surface roughness of wafers and films, fabrication errors and micromechanical stresses. However, the developed electromagnetically actuated MEMS-FPFC still exhibits a very good linear tunability, as shown in Fig. 11(d), and ignoring the phase-shifting region of the Ge/ZnS DBR, the (RSS and R2) of the Δd-I and λ-I plots are (0.00623, 0.99994) and (0.02492, 0.99994), respectively. According to the fitting results, a linear response better than 98% is achieved, and the Eq. (5) in our practical situation can be decided as:
Fig. 11. The experimentally testing results of the fabricated electromagnetically actuated MEMS-FPFC: (a) the initial central wavelength λ0 = 10.57 µm; (b) the bidirectional tuning capability of the filtering wavelength, covering the entire LWIR; (c) the corresponding FWHMs at different central wavelengths; (d) the linear fitting results of Δd-I and λ-I plots.
Therefore, the central wavelength λ of the developed LWIR electromagnetically actuated MEMS-FPFC at different driving currents can be accurately calculated and controlled according to Eq. (11) in the practical applications.
4. Conclusions
In this work, a large-aperture, widely and linearly tunable, electromagnetically actuated MEMS-FPFC for LWIR-SI is demonstrated. By building an efficient multi-field coupling simulation model, the feasibility verification and optimization design of the electromagnetically actuated MEMS-FPFC are realized. Moreover, a wafer-scale bulk micromachining process is proved to be effective to meet the high-quality fabrication of the proposed large-aperture electromagnetically actuated MEMS-FPFC. Benefiting from the rational actuating strategy, materials selection, structural design and fabrication process, the successfully fabricated MEMS-FPFC prototype has the largest aperture size among LWIR MEMS-FPFCs ever reported, and behaves very well in LWIR waveband of 8.39-12.95 µm under ±100 mA driving current, showing a very good linear response of better than 98%, which is also rarely reported. The proposed electromagnetically actuated MEMS-FPFC will be used for the construction of miniaturized LWIR-SI system which can serve the various civilian and military fields.
Funding
Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20180508151936092, YFJGJS1.0); Science and Development Program of Local Lead by Central Government, Shenzhen Science and Technology Innovation Committee under Grant (2021Szvup112); National Natural Science Foundation of China (51975483); Natural Science Foundation of Ningbo (202003N4033); Key Research and Development Projects of Shaanxi Province (2020ZDLGY01-03).
Acknowledgments
We sincerely acknowledge the useful discussions and assistance from Dr. Xingchen Xiao and Dr. Jiancun Zhao.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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