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This paper reports the development of an electrothermal microelectromechanical systems (MEMS) mirror with serpentine shape actuators. A micro Fresnel mirror with fringe-spacing tunability is required to realize a compact and high-speed diffusion sensor for biological samples whose diffusion coefficient changes significantly because of a conformational change. In this case, the measurement time-constant is dependent on the fringe-spacing and diffusion coefficient of the sample. In this study, a fringe-tunable MEMS mirror with an actuation voltage less than 10 V was developed. The characteristics of the fabricated mirror were investigated experimentally. A high-visibility optical interference fringe was successfully demonstrated using both an ultranarrow-linewidth solid-state laser and a low-cost compact laser diode. The experimental results demonstrated a distinct possibility of developing a measurement device using only simple and low-voltage optical components.
© 2017 Optical Society of America
1. Introduction
The measurement of the diffusion coefficient represents a new approach in fields such as medical diagnosis and biochemistry. Terazima et al. used the diffusion coefficient of DNA to investigate the DNA repair process [1] and conformational change in DNA [2]. The difference between the processes of water-molecule diffusion in healthy subjects and patients with Parkinson’s disease can be used as a diagnostic tool [3,4]. Diffusion-coefficient measurement can be applied to the analysis of biological samples, clinical diagnosis via point-of-care testing (POCT), and multianalyte sequential measurement, as described above. Several methods have been developed to measure diffusion coefficients, e.g., dynamic light scattering [5], the transient grating method [1,2], Soret forced Rayleigh scattering [6], fluorescence correlation spectroscopy [7], phase-shifting interferometry [8], and nuclear magnetic resonance spectroscopy [9]. However, no diffusion sensors suitable for use in POCT have yet been reported because of the difficulty in applying the sensing principle to a sufficiently compact device. In this study, a miniaturized sensor that allows high-speed measurement of the diffusion coefficient with a low actuation voltage is proposed. The proposed sensor is applicable to POCT or massively parallel measurement. Oka et al. were the first to develop a micro Fresnel mirror. They used it to detect a high-speed diffusion process in a striped microscale concentration distribution of the samples formed by dielectrophoretic (DEP) force [10]. Matoba et al. used a comb-driven micro Fresnel mirror (CD-MFM) operated by electrostatic force to measure a range of samples [11]. The CD-MFM was able to tune the fringe spacing to provide the appropriate measurement conditions; however, it required a driving voltage of 150 V to the mirror. The tunable piezoelectric microelectromechanical systems (MEMS) mirror proposed by Wallrabe et al. for a nanomachining process required a large driving voltage of 350 V [12]. Of the various drive systems that have been proposed [13–17], the electrothermal drive system requires a low driving voltage, which is desirable. Samuelson et al. demonstrated a compact, high-fill-factor electrothermal MEMS mirror with a scanning range of ± 23° using Al/SiO2/Pt actuators and applied it to optical coherence tomography at a driving voltage of only 4.6 V [18]. Liu et al. proposed an electrothermal MEMS mirror with a scanning range of ± 11° at 0.6 V that achieved a high fill factor using curved Al/SiO2/W actuators [19]. More recently, Duan et al. reported a 45°-tilted electrothermal MEMS mirror with a scanning range of ± 20° at 5.5 V that comprised Al/SiO2/Pt actuators for use in side-view imaging [20]. In this study, we developed a novel angular-adjustable electrothermal micro Fresnel mirror using polymer actuators. The proposed mirror requires a driving voltage of less than 10 V, which makes it promising for applications such as POCT or massively parallel measurement. This article reports the characteristics of the fabricated mirror as well as its successful application to the generation of a clear fringe pattern using a low-cost laser. The experimental results confirm the possibility of developing a compact sensor using simple low-voltage optical components.
2. Design of the electrothermal MEMS mirror
Herein, an interference fringe was formed by splitting an incident laser and creating interference between the beams using a V-shaped MEMS mirror. This enabled manipulation of the sample via an optical DEP force and generation of a microscale concentration fringe in the microchannel (a detailed description of the measurement principles can be found in [10]). Because fringe spacing depends on both the wavelength of the laser and angle of the mirrors, it could be varied by altering the mirror angle. The proposed device comprised two movable mirrors supported by two torsion springs and a beam. As shown in Fig. 1, the two beams are overlapped each other immediately after splitting by V-shaped MEMS mirror. In this case, the high contrast interference is realized because the optical path difference is ideally reduced to less than the coherent length of the laser source. At the center of the mirror, torsion springs 5 μm high and 5 μm wide were used as hinges supporting the mirrors. This design minimizes the spring constant under the limitation of the minimum linewidth of the fabrication process of the photolithography. The serpentine shape of the electrothermal actuator was proposed because it enlarges the displacement and is flexible under twisting. Four serpentine shape electrothermal actuators were connected to the beam beneath the mirror edge. The material used for each actuator was selected to allow precise angle control with a low-voltage drive (rather than an extremely low-voltage drive). In this study, NiCr and SU-8 were selected as the active layers based on finite element analysis (FEA) with temperature-dependent electrical and thermophysical properties in Table 1. SU-8 has a high coefficient of thermal expansion and a low Young’s modulus; this results in a large displacement in a small temperature rise. When the temperature of the actuator exceeds the glass transition temperature (Tg = 473 K) of SU-8, the drive repeatability of the actuator is lost. High electric resistance was required not to exceed the Tg under the operating voltage, therefore, 0.2-μm-thick NiCr was selected. Under this condition, in order to achieve the angular change of 3°, the 30-μm-wide serpentine shape 700 μm long and 320 μm wide with 30 μm gap and 10-μm-thick SU-8 was utilized. The deformation of the electrothermal actuator resulted from the temperature change due to the Joule heating of NiCr; this changed the MEMS mirror from its initial flat state to a V-shaped configuration. Figure 2(a) shows the analytical result of the electrothermal MEMS mirror under a driving voltage of 7 V. A V-shaped mirror was formed with no undesirable curvatures of the mirrors or actuators. The mirror angle increased up to 3.7° with the applied voltage as shown in Fig. 2(b).
Fig. 1 Schematic of the proposed electrothermal MEMS mirror. The thickness of the mirror plate was 5 µm. The thicknesses of NiCr and SU-8 were 0.2 µm and 10 µm, respectively.
Table 1. Material properties of NiCr and SU-8 used in finite element analysis.
Fig. 2 (a) Analytical results for the displacement of the proposed mirror (the image is enlarged three times along the z-axis) and (b) Analytical angular shift of the proposed MEMS mirror.
3. Fabrication of the electrothermal MEMS mirror
The MEMS mirror was fabricated on a 385-μm-thick silicon substrate via chemical vapor deposition (CVD) that was well known process to form a high purity and dense film for the thin film devices and applications such as optics [25] and solar cells [26,27], followed by standard photolithography, plasma etching, and chemical vapor etching. Initially, a 1-μm-thick CVD SiO2 layer in the shape of the mirror and the alignment mark was formed on the silicon substrate as shown in Fig. 3(a). A silicon substrate, which later formed the mirror plate, was then bonded to the SiO2 layer via direct wafer bonding and polishing to thickness of 5 µm [Fig. 3(b)]. Next, after partially etching the bonded 5-μm-thick silicon layer on the alignment mark by deep-reactive ion etching (DRIE), a 0.2-μm-thick NiCr alloy (80%Ni–20%Cr) layer was deposited by a RF magnetron sputtering and patterned using a lift-off process to act as both the actuator and heater [Fig. 3(c)], and a 10-μm-thick SU-8 layer was spin-coated and patterned using photolithography [Fig. 3(d)]. For the backside processing, a metal mask (Cr) was patterned on a 385-μm-thick silicon substrate using bottom-side alignment for the backside trenches to release the mirrors. The Si layers were etched from the thick silicon-substrate side and through the patterned SiO2 layer using DRIE. This formed the mirror plate as shown in Fig. 3(e). Finally, the SiO2 layer was removed via vapor hydrofluoric acid release at a device temperature of 313 K to avoid sticking of the mirror plate [Fig. 3(f)].
Figure 4 shows a scanning electron microscope (SEM) image of the fabricated MEMS mirror. No damage to the mirrors or actuators was detected, and torsion springs 5 μm in width and thickness were fabricated without breakage or residue.
Fig. 4 SEM image of the fabricated electrothermal MEMS mirror. (a) View of the entire mirror and (b) enlarged view of the actuator and torsion spring.
4. Device characterization
4.1 Static response
The temperature distribution was characterized using infrared thermography (TVS-8500, Nippon Avionics, the measurement accuracy of ± 2°C at T≦373 K, and ± 2% at T>373 K). Figures 5(a) and 5(b) show the temperature distribution of the mirror under direct-current voltage operation. The results confirmed that the temperature of the actuators increased with the voltage applied to the NiCr electrodes. No temperature rise was observed on the substrate connected to the NiCr electrodes. The measured temperature never exceeded the Tg of SU-8. In Fig. 5(c), the experimental findings were in reasonable agreement with the analytical results, with the deviation between the two never exceeding 2%. This small deviation was caused by nonuniform heat convection in the vicinity of the actuators.
Fig. 5 (a) Measured temperature distribution at the initial state, (b) measured temperature distribution at 3 V, and (c) analytical and measured temperatures of the proposed MEMS mirror.
4.2 Dynamic response
The response time of the mirror was measured by monitoring the position of a laser beam reflected from the mirror to a position sensitive detector (PSD). A square-wave voltage was applied to the four actuators. Figure 6 shows the results obtained under an output voltage from the PSD and an input voltage from the current source. The measured rise time from 10% to 90% was 60.0 ms, and the fall time from 90% to 10% was 60.1 ms. The targeted time resolution of the mass diffusion measurement is 1 s, which is significantly faster than the sensing period of the conventional techniques. In this timing period, the proposed MEMS mirror can sufficiently follow the change of the sample structure.
Fig. 6 Step response of the MEMS mirror. (a) Entire wave, (b) enlarged view of the rising edge, and (c) enlarged view of the falling edge.
The frequency response was measured by applying a sinusoidal wave to the four actuators, as shown in Fig. 7. The mirror angle was calculated from the relationship between the scan length on the PSD and the distance from the mirror to the PSD. The first-mode mechanical resonant frequency was estimated to be in the kHz range, and the thermoelectric response was relatively slow. No resonant peak of the fabricated mirror was observed in the experiment, and the cutoff frequency was estimated to be 8 Hz.
The repeatability of the angular shift was characterized by applying a 1-Hz sinusoidal wave at 8 Vpp with an offset voltage of 4 V. Figure 8 shows the deviation between the optical angle at each measurement point and the optical angle at 0 h. The deviations never exceeded 7% at any point in the 24-h testing period. Because of the annealing effect, the mirror exhibited tilting of less than 1% after 13 h into the experiment until the measurements ended. This stability of angular shifting was attributed to two factors. First, the mirror was designed such that the stress applied to the torsion spring was three times less than the tensile strength of silicon. Second, the temperature of the serpentine shape actuators was designed to remain below the Tg of SU-8. After 24 h of operation, which comprised 86400 cycles, no breakage of the mirrors or actuators was observed.
5. Characterization of the interference fringe
The interference fringe generated by the fabricated mirror and the laser at a wavelength of 532 nm was observed using a charge-coupled device (CCD) camera. In the measurement system, the laser beam was split by the V-shaped mirror, and the two beams were collimated and superposed by two lenses. Figures 9(a) and 9(b) show the image acquired by the beam profiler. Voltages of 0 V and 7 V were applied to the mirror. A clear fringe pattern was formed, and the fringe spacing became narrower with increasing applied voltage. Figure 9(c) and 9(d) shows the optical-intensity distribution at 0 V and 7 V. The proposed mirror at 0 V and 7 V formed a high-contrast fringe pattern with calculated visibilities of 0.85 and 0.72, respectively.
Fig. 9 (a) Image acquired by the beam profiler at 0 V, (b) image acquired by the beam profiler at 7 V, (c) optical-intensity distribution at 0 V, and (d) optical-intensity distribution at 7 V.
The proposed MEMS mirror generated a sharp interference fringe even when a low-coherency laser source was used. This was because of the small gap (~5 µm) between mirrors resulting in the extremely small difference in the optical path length. Figures 10(a) and 10(b) compares the optical-intensity distribution generated by the ultranarrow-linewidth solid-state laser and a low-cost compact laser diode, whose coherent lengths were 2.00 × 102 m and 2.42 × 10−3 m (at a center wavelength of 532 nm), at a driving voltage of 7 V. The same fringe spacing was formed by each source with visibilities of 0.72 and 0.71, respectively, at 7 V. As the voltage was increased, the fringe spacing reduced to 50% of its initial value for both sources, as shown in Fig. 10(c). This result indicated that the mirror angle was increased to twice of its initial value resulting from the mirror actuation. The experimental value of the fringe spacing of 17.8 µm at a driving voltage of 7 V corresponded to a mirror angle of 0.43°. The difference between the analytical and experimental results was attributed to the residual stress of bi-material due to the thermal stress on the actuators generated during the fabrication process of NiCr layer. These results confirm that the proposed MEMS mirror could generate an interference fringe with a width and contrast same as those generated by an ultranarrow-linewidth solid-state laser and a low-cost laser.
Fig. 10 (a) Optical-intensity distribution of the ultranarrow-linewidth solid-state laser at 7 V, (b) optical-intensity distribution of the low-cost compact-laser-diode coherency laser at 7 V, and (c) fringe spacing obtained using each laser under different voltages.
6. Conclusions
We developed an electrothermal MEMS mirror with serpentine shape actuators for use in a compact and high-speed device to measure the diffusion coefficient. The fabricated mirror was designed to generate low-Joule heat at a desired angular shift. Experimental tests on the fabricated mirror showed the same temperature dependence as that obtained analytically. Thus, we confirmed the fringe-tunability of the proposed electrothermal MEMS mirror experimentally and analytically. Fringe spacing at a driving voltage of 7 V and an operating temperature of 394 K decreased to half of its initial value resulting from the appropriate electrothermal actuation of the mirrors. We confirmed that the same interference fringe was generated when an ultranarrow-linewidth solid-state laser and a low-cost compact laser diode were used, because the optical path difference of two beams was negligibly small. The experimental results demonstrated the possibility of developing measurement devices using only simple and low-voltage optical components.
Funding
JSPS KAKENHI Grant Numbers JP24226006 and JP15K13890, Kawasaki city subsidy for promoting R&D of nano–micro technology by SMEs based on academia-industry cooperation.
Acknowledgments
Authors would like to thank “Global nano micro technology business incubation center (NANOBIC), Kawasaki city, Japan” and the academic consortium for nano and micro fabrication of four universities (Keio University, Waseda University, Tokyo Institute of Technology, and the University of Tokyo).
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