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Abstract
This paper proposes an electrothermally-actuated circular pyramidal kirigami microscanner with a millimeter-range low-power lens drive for endoscopic biomedical applications. A variation of Japanese origami art, kirigami involves creation of out-of-plane structures by paper cutting and folding. The proposed microscanner is composed of freestanding kirigami film on which the spiral-curved thermal bimorphs are strategically placed. The kirigami microscanner is electrothermally transformed into an out-of-plane circular multistep pyramid by Joule heating. The circular pyramidal kirigami microscanner on a small footprint of 4.5 mm × 4.5 mm was fabricated by microelectromechanical system processes. A large four-step pyramidal actuation was successfully demonstrated, and a large 1.1-mm lens travel range at only 128 mW was achieved.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Various optical biomedical sensing techniques enable patient diagnosis without requiring tissue sample removal. These techniques have been studied using confocal microscopy [1–2], optical coherence tomography [3–6], multiphoton microscopy [7–12], and Raman spectroscopy [13–14]. Miniaturization of their bulky optomechanical components into optical microelectromechanical systems (MEMS) scanners is expected to foster their use in endoscopic applications involving in situ observation of internal organs. In our previous study, we developed a motion-robust focal-tracking sensing system for endoscopic laser Doppler blood flow measurement [15] and autofluorescence lifetime measurement [16]. In our method, focal tracking of the moving object enables sensitive signal detection, which eliminates motion artifacts induced by inevitable organ motilities. These motilities arise from gastric digestion, beating of the heart, and respiration/expiration. Although the motilities are slow (less than 1.5 Hz), displacement in the optical axis typically occurs in the millimeter range [17–19]. Consequently, a MEMS microlens scanner for motion tracking—a key component of in vivo endoscopic measurement—is required to fulfill the long millimeter range displacement. To implement the scanner in a miniature probe, the long-range scanner must be small and operate at low power.
MEMS microlens scanners have been proposed for different applications based on electrostatic [20–25], piezoelectrical [26–28], electromagnetic [29], and electrothermal [30–33] actuation mechanisms. Li et al., [23] reported an electrostatic comb-shaped tracking and focusing actuator, which can drive in both a 24.6-µm horizontal direction and a 5.7-µm vertical direction. Chen et al., [28] demonstrated a piezoelectric 228-µm out-of-plane resonant actuator, which was enhanced by the residual stress control of a piezoelectric unimorph. Moreover, electrothermal actuation can be achieved in a millimeter range without a resonant operation. Chen et al., [31] presented the design and fabrication of a single-layer step-bridge actuator. It provided upward polymer lens positioning with an amplitude near 13 µm when driven at 54 mW. Wu et al., [32] proposed a scanner that achieved large-vertical 880-µm actuation with a small lateral shift and tilt. Zhou et al., [33] reported a large vertical displacement over 100 µm with a high piston resonant frequency. Despite the above important advancements and other studies on various electrothermal micromirrors [34–42], few studies have been conducted on an out-of-plane lens microscanner in the optical axis direction. Development of a millimeter-range microlens electrothermal microscanner with low driving power (one hundredth of a milliwatt) remains a significant challenge on account of the difficulty of achieving Joule heating thermal control.
In this paper, we propose a millimeter-range low-power electrothermal lens microscanner with a small device footprint for endoscopic applications. Its unique specification is enabled by an electrothermally-actuated circular pyramidal kirigami. A variation of Japanese origami art, kirigami is a design method for building out-of-plane structures by paper cutting and folding. In our previous study, the feasibility of electrothermally-actuated kirigami film was validated using a simple kirigami structure [43]. An advantage of the kirigami concept is its design flexibility. Various shapes of an out-of-plane film structure can be created by adjusting the kirigami cutting patterns. Kirigami cuttings are aligned with high area efficiency to generate large-displacement circular pyramidal actuation. The proposed electrothermal pyramidal microscanner has a small footprint and was fabricated by MEMS processes. The potential of millimeter-range scanning with low-power driving was experimentally characterized.
2. Design of electrothermally-actuated pyramidal kirigami microscanner
The concept of electrothermally-actuated circular pyramidal kirigami for millimeter-range microlens scanning is illustrated in Fig. 1. The circular pyramidal kirigami is mainly composed of a thin substrate film and spiral-curved thermal bimorphs. When electrical power is applied to the bimorphs, the kirigami film is electrothermally transformed into an out-of-plane circular multistep pyramid by Joule heating. The large displacement of the top lens stage is obtained by amplifying the external multistep actuation. To build the circular multistep pyramid with a minimally wasted area, the spiral-curved bimorphs are placed with high area efficiency. In electrothermal actuation, the thermal coupling between the thin bimorphs and the bulk components increases the power consumption and the response time on account of the larger thermal capacitance. The freestanding kirigami film has no bulk support components in the practical actuation area. The thermal resistivity between the actuation area and the device handling frame is enhanced by the long thermal conductive path of the pyramidal kirigami structure, enabling suppression of the considerable heat leakage.
The design of the proposed electrothermal pyramidal kirigami microscanner is shown in Fig. 2. The four-step circular pyramid actuation was designed for a large-displacement demonstration. The maximum travel range and power consumption was determined by the bimorph length of the kirigami structure, number of steps, mechanical, electrical and thermal properties of the bimorph materials. The travel range and the response frequency are in a trade-off relationship. NiCr (thickness: 0.5 µm) and W metal patterns (0.2 µm) were deposited on the backside of a free-standing SiN film (1.0 µm), on which the spiral-curved cuttings were strategically placed (Fig. 2(a)). As shown in Fig. 2(a), the diameters of the freestanding SiN film and the lens top stage are 4 and 1.3 mm, respectively. The extra edge of the Si frame can be removed in the same fabrication process to create a smaller circular device chip, which is favorable for integration in the miniature endoscopic cylindrical probe. Figure 2(b) shows the backside of the scanner. The spiral-curved NiCr/SiN bimorphs with a large coefficient of thermal expansion (CTE) difference are aligned with high area efficiency. The NiCr and SiN were selected as a bimorph material because of their high Young’s modulus and high yield/fracture strength for reliable long-range actuation without any breakage. On the other hand, W has similar CTE to SiN, therefore, W electrode pattern does not affect the bimorph actuation. The ratio of the bimorph-occupied area to the actuation area is about 30%. All NiCr patterns are electrically connected by W patterns to form a one-stroke electrical heater. The bimorphs are mechanically and electrically connected to the upper level step with a serpentine-shaped spring. This spring, which has a small thermal capacitance and large heat resistivity, also acts as a guard heater to suppress heat leakage to the lower step. In addition, NiCr guard heaters are placed at the bimorph bottom to prevent heat leakage to the step layer area. Figure 2(c) shows the out-of-plane circular pyramidal actuation built by a finite element modeling tool. When applying power to the one-stroke electrical circuit, all spiral-curved NiCr/SiN bimorph beams bend in the vertical direction, and the top stage is vertically lifted.
Fig. 2. Design of the circular pyramidal kirigami microscanner. (a) Composition of the scanner. (b) Backside of the scanner. (c) FEM actuation model.
3. Device fabrication
The proposed microscanner was fabricated by four-photomask MEMS processes. To fabricate the 4-mm-diameter freestanding SiN kirigami film, on which NiCr patterns occupied approximately 30% of the area, fabrication processes with film residual stress control were utilized. By changing the deposition pressure of the thin films, the residual stress causing the deformation of the device was relieved. These processes were developed in a previous study [43]. Figure 3 depicts a simplified fabrication flow of the scanner. (a) Initially, a SiO2/SiN (0.2 µm/1.0 µm) film with 30 MPa compressive strength was deposited by plasma-enhanced chemical vapor deposition. This procedure was followed by W patterning (0.2 µm) by RF magnetron sputtering and lift-off processes. NiCr alloy (80% Ni–20% Cr) patterns (0.5 µm) with 180 MPa tensile residual stress were then deposited by RF magnetron sputtering and wet etching. (b) After mask patterning by photolithography, the SiN layer was partially etched by reactive-ion etching. (c) After backside mask patterning, the free-standing SiN/SiO2 film was formed using backside Si deep-reactive-ion etching. Finally, the SiO2 layer was removed by vapor hydrofluoric acid release. A microscopic image of the fabricated electrothermal scanner is shown in Fig. 4(a). The microscale NiCr and W patterns are successfully deposited on the backside of the transparent freestanding SiN kirigami film with a 4-mm diameter. Figure 4(b) shows a detailed view of the bimorphs, including the serpentine spring and NiCr guard heater. The measured electrical resistance of the one-stroke electrical circuit is 18 kΩ at room temperature.
Fig. 4. Microscopic images of the fabricated scanner. (a) Freestanding kirigami film. (b) Detailed view of the bimorph area.
To verify the mechanical circular pyramidal out-of-plane actuation of the fabricated scanner, the chip was placed on a hot plate with different temperatures for the preliminary experiment. The freestanding kirigami film was heated by thermal conduction through the Si frame. The heat loss in the device handling frame was negligible in this experiment. Figure 5 depicts the circular pyramidal actuation of the fabricated device with a 4.5 mm × 4.5 mm footprint. The multistep circular-pyramidal actuation is clearly observed by a charge-coupled device camera. By increasing the heater temperature, the displacement of the top stage is increased. From the side view, an approximately 2-mm displacement is obtained at the surface heater temperature of 200°C. There is no break in the freestanding kirigami after 2-mm-range actuation. This circular pyramidal kirigami structure has the mechanical potential of 2-mm actuation.
Fig. 5. Demonstration of circular pyramidal out-of-plane actuation stimulated by external heating with different temperatures.
4. Device characterization
The thermal DC response of the kirigami film with Joule heating was characterized. Figure 6 shows the temperature DC response captured by high-speed infrared thermography (TVS-8500, Nippon Avionics). The measurement accuracy is ±2°C at T≦373 K, and the frame rate is 120 fps. Figure 6(a) shows the temperature distribution when 51 and 98 mW are respectively applied. A temperature increase occurs at the bimorph area at each step level. The thermal cross-talk coupling between the bimorphs at the same step level is not significant. This is because the thermal resistance between bimorphs of the same level is increased by the low thermal conductive SiN film. Hot spots, which act as a heating guard, are observed at the serpentine spring area.
Fig. 6. DC temperature response of the electrothermally-actuated kirigami microscanner. (a) Temperature distributions at 51 and 98 mW. (b) Temperature changes at bimorphs on different pyramidal steps.
Figure 6(b) shows the temperature change at the bimorph of each level with respect to the applied electrical power. White arrows, as shown in Fig. 6(a), indicate the measurement points. The maximum temperature rise of 103 K at bimorph 1 is obtained only at 128 mW. The temperatures at the inner bimorphs are higher than those of the outer bimorphs because of the high thermal resistance, which is enhanced by the multistep pyramidal structure. The effective thermal resistance of the bimorphs at each step level is roughly estimated from the slope average in Fig. 6(b) (bimorph 1: 0.81 K/mW; bimorph 2: 0.51 K/mW; bimorph 3: 0.31 K/mW; bimorph 4: 0.17 K/mW). The thermal resistance of bimorph 1 is approximately five times higher than that of bimorph 4.
The thermal step response at bimorph 1, which has the maximum high thermal resistance, was acquired via a high-speed infrared thermal camera. Figure 7 shows the temperature response when applying a square wave of 89 mW. By switching the power on and off, the temperature at bimorph 1 is immediately changed. The rise time (10% to 90%) and fall time (90% to 10%) are each 91 ms. A significant response difference between the respective Joule heating and cooling processes is not observed.
Fig. 7. Thermal step response of the electrothermal pyramidal kirigami scanner. (a) Entire view. (b) Enlarged view of the rising edge. (c) Enlarged view of the falling edge.
The mechanical DC response of the proposed lens microscanner was characterized. A ball lens, which can be manually fitted in the circular aperture of the lens holder by tweezers under the microscope, was utilized to evaluate the actuation in the lens assembled state. The specifications of the ball lens are summarized in Table 1. Figure 8 shows the vertical actuation of the microscanner, which was assembled with a polymer ball lens. The microlens stage is initially elevated under a substrate level by the downward bending of NiCr/SiN bimorphs due to the residual stress. The 0.8-mm diameter microlens (0.3 mg) is vertically lifted. Figure 9 shows the vertical displacement of both the unloaded and assembled lens scanner with different applied power levels. The vertical displacement was precisely measured by a microscope focused on a selected point of the top lens stage. A large vertical displacement of 1.1 mm in both unloaded and assembled states was achieved at only 128 mW. As shown in Fig. 9, the exponential actuation responsivity changes to a linear mode. This is because the downward bending state of bimorphs 4 was transited to upward bending state by the temperature rise. The transition point slightly shifts on account of the microlens load. This mixed-mode responsivity can be solved by the residual stress control of bimorphs in the fabrication process.
Fig. 8. Electrothermal circular pyramidal actuation captured by a CCD camera when the power is off and at 106 mW.
Fig. 9. Static mechanical response of the electrothermally-actuated pyramidal kirigami microscanner: Vertical displacement versus the total applied power.
Table 1. Parameters of assembled polymer ball lens
The mechanical frequency response of the lens-assembled microscanner was then characterized. Figure 10 shows the measured frequency response when applying a 58-mW sinusoidal wave to the scanner. The displacement of the beam spot reflected by the actuating stage was utilized to measure the frequency response. The displacement was detected by a position-sensitive detector. As depicted in Fig. 10, the 3-dB cutoff frequency is 8 Hz. The targeted motion of the organ motilities is less than 1.5 Hz. Therefore, the proposed MEMS scanner can sufficiently follow the targeted motion.
5. Conclusion
In this paper, we proposed an electrothermally actuated circular pyramidal kirigami microscanner with a millimeter lens travel range and low driving power for endoscopic biomedical applications. The kirigami-inspired film microscanner is electrothermally transformed into an out-of-plane circular multistep pyramid by Joule heating. The large displacement of the lens is obtained by amplifying the external multistep actuation. The four-step pyramidal microscanner on a small footprint of 4.5-mm squares was fabricated by MEMS processes. The thermal and mechanical responses of the microscanner were respectively characterized. A large 1.1-mm lens travel range at only 128 mW was achieved.
Funding
Japan Society for the Promotion of Science (JP18J20513); Keio University.
Acknowledgments
The fabrication was performed in a clean room at the Global Nano Micro Technology Business Incubation Center (NANOBIC), Kawasaki, Japan, with support from the Academic Consortium for Nano and Micro Fabrication of Four Universities (Keio University, Waseda University, Tokyo Institute of Technology, and The University of Tokyo).
Disclosures
The authors declare no conflicts of interest.
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