Design and Fabrication of a Kirigami-Inspired Electrothermal MEMS Scanner with Large Displacement

micromachines

Article Design and Fabrication of a Kirigami-Inspired Electrothermal MEMS Scanner with Large Displacement

Masaaki Hashimoto 1,2 and Yoshihiro Taguchi 3,*

1 School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Yokohama, Japan; [email protected] 2 Research Fellow of Japan Society for the Promotion of Science, 5-3-1 Kojimachi, Tokyo, Japan 3 Department of System Design Engineering, Keio University, 3-14-1 Hiyoshi, Yokohama, Japan * Correspondence: [email protected]; Tel.: +81-45-566-1809

Received: 12 March 2020; Accepted: 28 March 2020; Published: 30 March 2020

Abstract: Large-displacement microelectromechanical system (MEMS) scanners are in high demand for a wide variety of optical applications. Kirigami, a traditional Japanese art of paper cutting and folding, is a promising engineering method for creating out-of-plane structures. This paper explores the feasibility and potential of a kirigami-inspired electrothermal MEMS scanner, which achieves large vertical displacement by out-of-plane film actuation. The proposed scanner is composed of film materials suitable for electrothermal self-reconfigurable folding and unfolding, and microscale film cuttings are strategically placed to generate large displacement. The freestanding electrothermal kirigami film with a 2 mm diameter and high fill factor is completely fabricated by careful stress control in the MEMS process. A 200 µm vertical displacement with 131 mW and a 20 Hz responsive frequency is experimentally demonstrated as a unique function of electrothermal kirigami film. The proposed design, fabrication process, and experimental test validate the proposed scanner’s feasibility and potential for large-displacement scanning with a high fill factor.

Keywords: electrothermal scanner; kirigami ﬁlm; large displacement; microelectromechanical system (MEMS)

1. Introduction Microelectromechanical system (MEMS) scanners with large vertical actuation of both the micromirror and microlens have a wide range of applications, including optical pickup [1], multiphoton microscopy [2], Fourier transform spectrometry [3,4], confocal microscopy [5], optical coherence tomography [6], and micro optical diﬀusion sensing [7–9]. MEMS scanners based on electrostatic [2,7,10,11], piezoelectrical [12], and electromagnetic [13] actuation mechanisms can achieve high-speed scanning. For example, Oda et al. [11] reported an electrostatic comb-drive MEMS mirror with a sensing function, which achieved a vertical displacement of 3 µm with approximately 40 V. Chen et al. [12] demonstrated 145 µm out-of-plane actuation with a 2 kHz resonant frequency using symmetrical eight piezoelectric unimorph driving. Compared with electrostatic, piezoelectrical, and electromagnetic actuations, an electrothermal MEMS scanner can achieve large displacement (several hundred micrometers) without resonant operation. To date, various novel designs for electrothermal MEMS scanners with vertical out-of-plane actuation have been proposed [3–6,8,14–16]. For example, Zhang et al. [14] presented a lateral shift-free actuator design using three bimorph hinges and two multimorph segments to compensate for the lateral shift. Their actuator achieved a vertical displacement of 320 µm. Zhou et al. [15] recently reported an electrothermal MEMS mirror with a

Micromachines 2020, 11, 362; doi:10.3390/mi11040362 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 362 2 of 12 high reliability and 114 µm scanning using an inverted-series-connected structure, which survived significant long-term operation with little characteristic change. Kirigami, a variation of origami, is a promising design method for building out-of-plane structures by paper cutting and folding. The design concepts of kirigami and origami have been introduced in the engineering of a large variety of nano-, micro-, and macroscale functional films, such as mechanical materials [17–23], photonic materials [24–27], biomedical devices [28–30], biomimetic robotics [31], and electronic devices [32,33]. Mechanically-actuated devices particularly require the ability to control the transition between folded and unfolded states. A planer stretchable film is kinematically manipulated with external stretching tethers [20,24]. The responsive film materials provide self-reconfigurable folding and unfolding when exposed to a change in environmental temperature [34–36], the addition of a solvent [37,38], or irradiation by lasers [39]. For example, Tolley et al. [35] demonstrated self-folding origami shapes composed of shape memory polymer, which is activated by uniform heating in an oven for less than 4 min. Jamal et al. [37] reported that a differential photo-crosslinked epoxy polymer, SU-8, was reversibly folded and unfolded by de-solvation and re-solvation to develop microfluidic devices that flatten out and curl up. However, these folding and unfolding mechanisms are not applicable to the fast-scanning MEMS actuator that is necessary for electrical control on a microscale. Moreover, electrically responsive film materials compatible with the fabrication of microscale architecture are required. In this study, we explore the feasibility and potential of a kirigami-inspired electrothermal MEMS scanner that enables large vertical actuation with a high fill factor. Based on the concept of a thermal bimorph being folded and unfolded by the thermal expansion difference induced by Joule heating and natural cooling, the freestanding kirigami film on which bimorphs are placed is electrothermally folded into an out-of-plane structure. In this design, the film material combinations suitable for electrothermal self-reconfigurable folding and unfolding are determined, and the kirigami cuttings and thermal bimorphs are aligned to generate vertical displacement with a high area efficiency. To fabricate the freestanding electrothermal kirigami film with a 2 mm diameter and high fill factor, spontaneous film folding due to residual stress, which determines the initial position, is controlled. Finally, the potential of fast, large-displacement scanning with a high fill factor is experimentally examined.

2. Kirigami-Inspired Electrothermal MEMS Scanner

2.1. Design Figure1 shows a 3D paper model of the proposed scanner. Inspired by the kirigami concept in which a plane paper is transformed into out-of-plane architecture by cutting and folding, the freestanding film was electrothermally folded into an out-of-plane structure. When switching the voltage on or off, the platform for the microlens and micromirror was vertically lifted or lowered. Figure2 illustrates the design schematic for the electrothermal kirigami MEMS scanner. As Figure2a shows, the freestanding SiN film on which the spiral-curved cuttings were strategically placed was formed on a Si substrate. The SiN film (1.0 µm thickness) was 2 mm in diameter, and the platform for the micromirror and microlens was 1.3 mm in diameter. The fill factor (i.e., the ratio of the area of the platform to the area of the freestanding kirigami film) was 42%. The extra Si substrate could be removed by the fabrication process to make a small circular chip. As Figure1b shows, NiCr patterns (0.5 µm thickness) and W patterns (0.2 µm) were deposited on the backside of the SiN film. Figure2c shows the details of the thermal bimorph beam. The platform was connected to the bimorph with a serpentine-shaped mechanical spring. To suppress heat leakage from the bimorph, the NiCr guard heater was introduced to the bottom area. To suppress the temperature increase at the spring, W patterns, which have a higher electrical conductivity, were deposited on the spring. When voltage was applied to the one-stroke electrical circuit composed of NiCr and W patterns, the all-spiral curved NiCr/SiN bimorph area bent and folded in the vertical direction by Joule heating. Micromachines 2020, 11, 362 3 of 12

The material properties of the kirigami film are important for reconfigurable electrothermal actuation with large displacement. Film materials with large coefficient of thermal expansion (CTE) differences must be folded so as not to exceed the metal yield strength or fracture strength. Table1 MicromachinesMicromachines 2020 2020, ,10 10, ,x x 3 3of of 12 12 compares the properties of common materials used in MEMS actuators. SiN was selected as a rigid, freestanding film material with a higher Young’s modulus and yield stress than SiO or Poly-Si. NiCr freestandingfreestanding film film materialmaterial withwith aa higherhigher Young's modulusmodulus andand yieldyield stress stress than than SiO SiO2 2or2 or Poly-Si. Poly-Si. NiCr NiCr andandand SiN SiN SiN were were were chosen chosen chosen as asas the thethe thermal thermalthermal bimorphs.bimorphs. Accordin Accordinggg toto TableTable Table 1, 11,, the the yield yield stress stress of of of NiCr NiCr NiCr is is isseveral several several timestimestimes higher higher higher than than than that thatthat of ofof Cu CuCu and andand Al, Al,Al, whilewhile thethe CTEsCTEs of of AlAl andand CuCu Cu areare are slightly slightly slightly higher higher higher than than than the the the CTE CTE CTE of of of NiCr.NiCr.NiCr. The The The thicknesses thicknessesthicknesses of ofof SiN SiNSiN and and NiCrNiCr areare 1 and 0.5 μμµm,m, respectively,respectively, andand and were were were determined determined determined by by bythe the the calculationcalculationcalculation of of of cantilever cantilevercantilever displacement. displacement.displacement.

((aa)) ((bb))

FigureFigureFigure 1. 1. 1.Paper Paper Paper model modelmodel of ofof the the proposedproposed kirigamikirigami scanner: (( (aaa)) ) unfoldedunfolded unfolded state state state and and and ( b(b ()b )folded )folded folded state. state. state.

(a) (a)

(c) (b) (c) (b) FigureFigure 2. 2.Design Design of of the the electrothermal electrothermal kirigamikirigami scanner: ( (aa) )frontside frontside perspective perspective view view of offreestanding freestanding SiNFigureSiN ﬁlm film 2. (1.0 Design(1.0µ mμm thickness,of thickness, the electrothe 2 2 mm mmrmal inin diameter) diameter)kirigami scanner: on which (a )th the frontsidee spiral-curved spiral-curved perspective cuttings cuttings view are areof strategically freestanding strategically placed;SiNplaced; film (b )( b(1.0 backside) backside μm thickness, view view of of SiN 2SiN mm ﬁlm film in wherewherediameter) NiCr on patterns which (0.5th (0.5e μspiral-curvedµmm thickness) thickness) andcuttings and W W patterns are patterns strategically (0.2 (0.2 μm)µ m) areplaced;are deposited; deposited; (b) backside (c )(c backside) backside view of detailed detailed SiN film view view where ofof theNiCrthe thermalthermal patterns bimorph (0.5 μm beam. beam.thickness) and W patterns (0.2 μm) are deposited; (c) backside detailed view of the thermal bimorph beam.

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Table 1. Material selection of the electrothermal kirigami scanner [40–45]. Table 1. Material selection of the electrothermal kirigami scanner [40–45]. Coefficient of Young's Modulus Yield Strength Material Thermal Expansion Coeﬃcient of Thermal [GPa] /FractureYield Strength Strength [GPa]/Fracture Material [10−6/K] 6 Young’s Modulus [GPa] Expansion [10− /K] Strength [GPa] SiN 1.6 252 5.8 SiN 1.6 252 5.8 Poly-Si 3.0 179 1.1 Poly-Si 3.0 179 1.1 SiO2 0.4 70 0.8 SiO2 0.4 70 0.8 AlAl 23.6 23.6 70 70 0.2 0.2 CuCu 16.9 16.9 120 120 0.3 0.3 NiCrNiCr 14.2 14.2 220 220 2.2 2.2

2.2. Simulation Simulation Analysis To verifyverify thethe out-of-planeout-of-plane actuation,actuation, 3D modelsmodels werewere built,built, andand electro-thermo-mechanicalelectro-thermo-mechanical analyses were performed using the CoventorWare © finitefinite element element modeling modeling (FEM) (FEM) tool tool (Coventor, (Coventor, Inc., Fremont, CA, USA). The thermal conductivity of of the freestanding SiN film film was particularly considered. In general, general, the thermal conductivity of the nanoscale thin film film was lower than that of the bulk state due to phonon phonon scattering at the interfac interfacee grain. Figure Figure 33 shows the temperature distributiondistribution and vertical actuation using the film film value [[46].46]. A selective selective temperature rise in the bimorph area was observed, andand NiCrNiCr thermal thermal guard guard heaters heaters suppressed suppressed the temperaturethe temperature decrease decrease in the bimorphin the bimorph bottom bottomarea. As area. seen As in Figureseen in4 b,Figure the proposed 4b, the proposed electrothermal electrothermal actuator actuator could achieve could approximatelyachieve approximately 0.2 mm 0.2displacement mm displacement in the vertical in the direction.vertical direction. The resonant The resonant vibration vibration modes were modes simulated, were simulated, and Figure and4 Figureshows the4 shows results. the The results. first mode The wasfirst piston, mode andwas the piston, second and mode the was second tilting. mode The frequencieswas tilting. wereThe 1.4frequencies and 1.8 kHz, were respectively. 1.4 and 1.8 kHz, respectively.

(a) (b)

Figure 3. Finite element modeling (FEM) simulation of the microelectromechanical system (MEMS) scanner: ( (aa)) temperature temperature distribution distribution and and ( (bb)) out-of-plane actuation actuation wi withth approximately 0.2 mm vertical displacement.

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(a) (b)

Figure 4. ModalModal simulation simulation of of the the MEMS MEMS scanner: scanner: (a) ( afirst) first resonant resonant mode, mode, vertical vertical piston, piston, 1.4 1.4kHz; kHz; (b) (secondb) second resonant resonant mode, mode, lateral lateral tilt, tilt, 1.8 1.8kHz. kHz. 3. Fabrication 3. Fabrication Figure5 represents the fabrication process flow of the kirigami-film actuator. To fabricate the Figure 5 represents the fabrication process flow of the kirigami-film actuator. To fabricate the freestanding SiN film on which NiCr patterns were deposited with a high area efficiency, it was freestanding SiN film on which NiCr patterns were deposited with a high area efficiency, it was necessary to control the residual stress of both SiN and NiCr. First, an SiO2 film with low residual stress necessary to control the residual stress of both SiN and NiCr. First, an SiO2 film with low residual was deposited on a single-side polished Si wafer with a 300 µm thickness and 100 mm diameter, as stress was deposited on a single-side polished Si wafer with a 300 μm thickness and 100 mm diameter, shown in Figure5a. To prevent the SiN film from shrinking after removal of the SiO 2 film underneath, as shown in Figure 5a. To prevent the SiN film from shrinking after removal of the SiO2 film the SiO2 film with low residual stress, estimated to have a 30 MPa compressive strength, was grown underneath, the SiO2 film with low residual stress, estimated to have a 30 MPa compressive strength, by plasma-enhanced chemical vapor deposition (PECVD) using TEOS. This functioned as an etch was grown by plasma-enhanced chemical vapor deposition (PECVD) using TEOS. This functioned stop layer during two processes: (1) SiN film reactive-ion etching to align the kirigami cuttings on as an etch stop layer during two processes: (1) SiN film reactive-ion etching to align the kirigami the SiN film and (2) Si deep reactive-ion etching (DRIE) to form the freestanding SiN/SiO2 film. A cuttings on the SiN film and (2) Si deep reactive-ion etching (DRIE) to form the freestanding SiN/SiO2 low-residual SiN (1.0 µm) film was then deposited by PECVD. To produce a low-stress SiN film, which film. A low-residual SiN (1.0 μm) film was then deposited by PECVD. To produce a low-stress SiN is composed of alternating tensile and compressive layers, low and high radio frequency (RF) mix film, which is composed of alternating tensile and compressive layers, low and high radio frequency fabrication was used [47]. The residual stress of the SiN film was adjusted to a 25 MPa compressive (RF) mix fabrication was used [47]. The residual stress of the SiN film was adjusted to a 25 MPa strength, approximately equal to the residual stress of the SiO2 film, to further prevent film shrinkage compressive strength, approximately equal to the residual stress of the SiO2 film, to further prevent by different residual stresses on the SiO2 film. Next, the W pattern was deposited (0.2 µm) by RF film shrinkage by different residual stresses on the SiO2 film. Next, the W pattern was deposited (0.2 magnetron sputtering and a lift-off process, as shown in Figure5b. NiCr alloy (80% Ni–20% Cr) μm) by RF magnetron sputtering and a lift-off process, as shown in Figure 5b. NiCr alloy (80% Ni– patterns (0.5 µm) were deposited by RF sputtering and wet etching, as shown in Figure5c. The residual 20% Cr) patterns (0.5 μm) were deposited by RF sputtering and wet etching, as shown in Figure 5c. stress of the NiCr patterns caused the initial curling of the bimorphs, resulting in the initial elevation of The residual stress of the NiCr patterns caused the initial curling of the bimorphs, resulting in the the platform. Moreover, the compressive/tensile state determined the direction of the initial elevation. initial elevation of the platform. Moreover, the compressive/tensile state determined the direction of The residual stress of NiCr patterns was controlled by adjusting the sputtering gas pressure. Figure6 the initial elevation. The residual stress of NiCr patterns was controlled by adjusting the sputtering shows the residual stresses of NiCr films (0.5 µm) versus process pressure using fixed sputtering gas pressure. Figure 6 shows the residual stresses of NiCr films (0.5 μm) versus process pressure power. The residual stresses were estimated from the curvature of the film-coated substrates using using fixed sputtering power. The residual stresses were estimated from the curvature of the film- Stoney’s formula [48]. As the sputtering Ar pressure was increased, the sputtered film transitioned coated substrates using Stoney’s formula [48]. As the sputtering Ar pressure was increased, the from a compressive state to tensile state. After reaching a maximum tensile strength, the stress was sputtered film transitioned from a compressive state to tensile state. After reaching a maximum decreased with a further increase in the pressure. This tendency, which has also been reported in W tensile strength, the stress was decreased with a further increase in the pressure. This tendency, which films [49] and Ta films [50], can be attributed to the change of the film qualities caused by mean free has also been reported in W films [49] and Ta films [50], can be attributed to the change of the film paths of Ar and NiCr atoms. The NiCr film with 180 MPa in a tensile state was deposited because the qualities caused by mean free paths of Ar and NiCr atoms. The NiCr film with 180 MPa in a tensile tensile stress caused the opposite direction to actuate, which did not result in displacement reduction. state was deposited because the tensile stress caused the opposite direction to actuate, which did not After photoresist (PR) masking pattern-inverse kirigami-cutting geometry, the SiN film was etched by result in displacement reduction. After photoresist (PR) masking pattern-inverse kirigami-cutting reactive-ion etching (RIE) and stopped at the SiO2 layer, as shown in Figure5d. After backside Cr geometry, the SiN film was etched by reactive-ion etching (RIE) and stopped at the SiO2 layer, as mask patterning by sputtering and wet etching followed by frontside PR removal, the Si substrate was shown in Figure 5d. After backside Cr mask patterning by sputtering and wet etching followed by etched by the backside DRIE to form the SiO2/SiN film, as shown in Figure5e. To prevent erosion of frontside PR removal, the Si substrate was etched by the backside DRIE to form the SiO2/SiN film, as the frontside metal pattern by backside Cr etchant, the PR was removed by acetone immersion and O shown in Figure 5e. To prevent erosion of the frontside metal pattern by backside Cr etchant, the PR2 ashing after Cr pattering. Finally, the SiO2 film was removed by vapor hydrofluoric acid (HF) release, was removed by acetone immersion and O2 ashing after Cr pattering. Finally, the SiO2 film was removed by vapor hydrofluoric acid (HF) release, and the freestanding kirigami film on which NiCr and W were patterned was formed, as shown in Figure 5f.

Micromachines 2020, 10, x 6 of 12

MicromachinesFigure 2020 7 shows, 10, x the scanning electron microscope (SEM) images of an electrothermal kirigami6 of 12 MEMS scanner. The freestanding SiN film on which the spiral-curved cuttings were strategically Figure 7 shows the scanning electron microscope (SEM) images of an electrothermal kirigami placed was formed on the Si substrate, as shown in Figure 7a. No destruction was observed in the MEMS scanner. The freestanding SiN film on which the spiral-curved cuttings were strategically bimorph area, including the guard heater and serpentine spring (Figures 7b,d). Initial displacement placed was formed on the Si substrate, as shown in Figure 7a. No destruction was observed in the of the platform was 20 μm above the substrate level. Platform tilting was estimated to be Micromachinesbimorph area,2020 including, 11, 362 the guard heater and serpentine spring (Figures 7b,d). Initial displacement6 of 12 approximately 0.6° by microscope focusing. The one-stroke electrical circuit composed of NiCr and of the platform was 20 μm above the substrate level. Platform tilting was estimated to be W patterns was successfully deposited on the SiN film (Figure 7c). The measured resistance of the approximately 0.6° by microscope focusing. The one-stroke electrical circuit composed of NiCr and andscanners the freestanding was 4.6 kΩ at kirigami room temperature. ﬁlm on which NiCr and W were patterned was formed, as shown in W patterns was successfully deposited on the SiN film (Figure 7c). The measured resistance of the Figure5f. scanners was 4.6 kΩ at room temperature.

Figure 5. Fabrication flow of the electrothermal kirigami MEMS scanner: (a) SiO 2/SiN film with an Figureapproximately 5. Fabrication 30 MPa flow compressive of the electrothermal strength was kirigami deposited MEMS by plasma-enhanced scanner: (a) SiO 2chemical/SiN film vapor with Figure 5. Fabrication flow of the electrothermal kirigami MEMS scanner: (a) SiO2/SiN film with an andeposition approximately (PECVD). 30 MPa (b) W compressive patterns were strength deposited was by deposited lift-off processes. by plasma-enhanced (c) NiCr films chemical with 180 vapor MPa approximately 30 MPa compressive strength was deposited by plasma-enhanced chemical vapor depositiontensile residual (PECVD). stress (b )were W patterns deposited were by deposited sputtering by lift-oand ffpatternedprocesses. by (c )wet NiCr etching. films with (d) 180After MPa PR deposition (PECVD). (b) W patterns were deposited by lift-off processes. (c) NiCr films with 180 MPa tensilepatterning, residual microscale stress werekirigami deposited cuttings by were sputtering placed andon SiN patterned film by by reactive-ion wet etching. etching (d) After(RIE). PR(e) tensile residual stress were deposited by sputtering and patterned by wet etching. (d) After PR patterning,After Cr mask microscale patterning kirigami and cuttingsPR removal, were the placed Si substrate on SiN film was by etched reactive-ion by the backside etching (RIE). DRIE (e to) After form patterning, microscale kirigami cuttings were placed on SiN film by reactive-ion etching (RIE). (e) Crthe mask SiO2/SiN patterning film. (f and) Freestanding PR removal, SiN the kirigami Si substrate film on was wh etchedich NiCr by and the backsideW patterns DRIE were to deposited form the After Cr mask patterning and PR removal, the Si substrate was etched by the backside DRIE to form SiOwas2/ formedSiN film. by ( fSiO) Freestanding2 removal with SiN vapor kirigami hydrofluoric film on which acid NiCr(HF) andetching. W patterns were deposited was the SiO2/SiN film. (f) Freestanding SiN kirigami film on which NiCr and W patterns were deposited formed by SiO2 removal with vapor hydrofluoric acid (HF) etching. was formed by SiO2 removal with vapor hydrofluoric acid (HF) etching.

1000

1000 750

750 500

500 250 Tensile state

250 Tensile state Residual stress [MPa] stress Residual 0

Residual stress [MPa] stress Residual 0 Compressive state −250 Compressive state −250 00.511.52 Sputtering pressure [Pa] 00.511.52 FigureFigure 6.6. ResidualResidual stressstress ofof thethe NiCrNiCr filmfilm versusversusSputterin ArArg sputteringspu pressurettering [Pa] pressure.pressure. InIn this this study,study, thethe ArAr pressurepressure waswas setset toto 0.230.23 PaPa forfor deposition deposition of of the the NiCr NiCr film film with with 180 180 MPa MPa tensile tensile stress. stress. Figure 6. Residual stress of the NiCr film versus Ar sputtering pressure. In this study, the Ar pressure Figurewas set7 to shows 0.23 Pa the for scanningdeposition electronof the NiCr microscope film with 180 (SEM) MPa imagestensile stress. of an electrothermal kirigami MEMS scanner. The freestanding SiN film on which the spiral-curved cuttings were strategically placed was formed on the Si substrate, as shown in Figure7a. No destruction was observed in the bimorph area, including the guard heater and serpentine spring (Figure7b,d). Initial displacement of the platform was 20 µm above the substrate level. Platform tilting was estimated to be approximately Micromachines 2020, 11, 362 7 of 12

0.6◦ by microscope focusing. The one-stroke electrical circuit composed of NiCr and W patterns was successfully deposited on the SiN ﬁlm (Figure7c). The measured resistance of the scanners was 4.6 k Ω atMicromachines room temperature. 2020, 10, x 7 of 12

(a) (b)

FigureFigure 7.7. SEM images of the the fabricated fabricated el electrothermalectrothermal kiri kirigamigami scanner: scanner: (a ()a frontside) frontside perspective perspective view view ofof freestandingfreestanding SiNSiN ﬁlmfilm (2 mm in diameter) on on which which the the spiral-curved spiral-curved cuttings cuttings were were strategically strategically placed;placed; ((bb)) detaileddetailed viewview of thermal bimorph bimorph area, area, includ includinging the the serpentine serpentine spring spring and and guard guard heater; heater; (c(c)) backside backside viewview ofof thethe SiNSiN ﬁlmfilm where NiCr patterns patterns and and W W patterns patterns were were deposited; deposited; (d ()d detailed) detailed viewview of of thethe serpentineserpentine mechanicalmechanical spring.

4.4. ExperimentalExperimental TestsTests 4.1. Static Response 4.1. Static Response TheThe directdirect currentcurrent (DC)(DC) responses of the the propos proposeded scanners scanners were were characterized. characterized. Through Through contact contact ofof the the electricalelectrical probeprobe andand the W electrode patterned patterned on on the the Si Si frame, frame, the the voltage voltage was was applied applied to tothe the one-strokeone-stroke circuitcircuit composedcomposed of W and NiCr patterns patterns that that was was deposited deposited on on the the SiN SiN film. ﬁlm. The The power power suppliedsupplied to to thethe scannerscanner waswas calculated from from the the inpu inputt voltage voltage and and the the measured measured current. current. The The vertical vertical displacementdisplacement waswas preciselyprecisely measured by by microscope microscope focusing focusing of of a aselected selected point point on on the the platform. platform. Figure 8 shows the vertical displacements of the platform versus the applied voltage and power. A vertical displacement of 200 μm was achieved at only 131 mW, as shown in Figure 8b. The temperature rise at the NiCr/SiN bimorph was measured using infrared thermography (TVS-8500, Nippon Avionics, Tokyo, Japan, measurement accuracy of ± 2 °C at T ≤ 373 K). Figure 9 shows the

Micromachines 2020, 11, 362 8 of 12

Figure8 shows the vertical displacements of the platform versus the applied voltage and power. A verticalMicromachines displacement 2020, 10, x of 200 µm was achieved at only 131 mW, as shown in Figure8b. The temperature8 of 12 rise at the NiCr/SiN bimorph was measured using infrared thermography (TVS-8500, Nippon Avionics, Micromachines 2020, 10, x 8 of 12 Tokyo,temperature Japan, change measurement with respect accuracy to the of applied2 ◦C at elec T trical373 K). power Figure of9 theshows single the bimorph. temperature The changeapplied ± ≤ withpowertemperature respect was tocalculated change the applied with by dividing electricalrespect to the power the total applied of powe the elec singler bytrical bimorph.the power number Theof ofthe applied bimorphs. single powerbimorph. The was measurement The calculated applied bypointpower dividing was was set thecalculated to totalthe center power by dividingpoint by the of thethe number thermaltotal ofpowe bimorphs.bimorph.r by the A Thenumbertemperature measurement of bimorphs. rise of point approximately The was measurement set to 90 the K centerwaspoint obtained pointwas set of atto the 200the thermal μcenterm displacement. point bimorph. of the A thermal temperature bimorph. rise A of temperature approximately rise 90of Kapproximately was obtained 90 at K 200 µm displacement. was obtained at 200 μm displacement. 200 200 200 200

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50 50 Vertical displacement [µm] displacement Vertical 50 [µm] displacement Vertical 50 Vertical displacement [µm] displacement Vertical Vertical displacement [µm] displacement Vertical 0 0 0 0 0 5 10 15 20 25 0 20 40 60 80 100 120 Total voltage [V] Total electrical power [mW] 0 5 10 15 20 25 0 20 40 60 80 100 120 Total(a) volta ge [V] Total electrical(b) power [mW]

Figure 8. Static response(a) of electrothermal kirigami scanner when the voltage(b) was applied to eight FigureNiCr/SiNFigure 8. 8.Static bimorphs.Static response response The of verticalof electrothermal electrothermal displacement kirigami kirigami was scannerprecisely scanner when measuredwhen the the voltage voltageby microscope was was applied applied focusing to to eight eight of a NiCrselectedNiCr/SiN/SiN point bimorphs. bimorphs. on the The platform:The vertical vertical (a displacement) displacementvertical displacement was was precisely precisely versus measured measured total applied by by microscope microscope voltage, and focusing focusing (b) vertical of of a a selecteddisplacementselected point point onversus on the the platform:total platform: electrical ( a(a)) vertical vertical power. displacement displacement versus versus total total applied applied voltage, voltage, and and ( b(b)) vertical vertical displacementdisplacement versus versus total total electrical electrical power. power.

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20 Temperaturechange [K] 20 Temperaturechange [K] 0 0 0 5 10 15 0Electrical 5 power 10 [mW] 15 Electrical power [mW] FigureFigure 9.9. MeasuredMeasured temperaturetemperature changechange versusversus appliedapplied electricalelectrical powerpower forfor each each bimorph. bimorph. TheThe temperature rise at the NiCr/SiN bimorph was measured using infrared thermography. The power temperatureFigure 9. Measured rise at the temperature NiCr/SiN bimorph change wasversus me applieasuredd using electrical infrared power thermography. for each bimorph. The power The applied to one bimorph was estimated from the total power. appliedtemperature to one rise bimorph at the wasNiCr/SiN estimated bimorph from was the totalmeasured power. using infrared thermography. The power applied to one bimorph was estimated from the total power. 4.2. Dynamic Response 4.2. DynamicThe dynamic Response responses in the low-frequency range of the MEMS scanner were characterized. FigureThe 10 dynamicshows the responses measured in frequency the low-frequency response whenrange applyingof the MEMS a sinusoidal scanner wavewere voltagecharacterized. to the singleFigure electrical10 shows circuit.the measured The frequency frequency response response of when the verticalapplying actuation a sinusoidal was wavemeasured voltage by to spot the displacementsingle electrical of the circuit. beam The reflected frequency by the responseplatform . ofThe the displacement vertical actuation of the beam was spotmeasured was measured by spot displacement of the beam reflected by the platform. The displacement of the beam spot was measured

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4.2. Dynamic Response The dynamic responses in the low-frequency range of the MEMS scanner were characterized. Figure 10 shows the measured frequency response when applying a sinusoidal wave voltage to theMicromachines single electrical 2020, 10, x circuit. The frequency response of the vertical actuation was measured by9 spot of 12 displacement of the beam reﬂected by the platform. The displacement of the beam spot was measured using a position sensitive detector. The applied voltage was (Vo + Vosin(2πft), Vo = 7 V), corresponding using a position sensitive detector. The applied voltage was (V + V sin(2πft), V = 7 V), corresponding to the maximum 85 μm displacement of DC operation, becauseo ofo the spatial restrictiono of the optical to the maximum 85 µm displacement of DC operation, because of the spatial restriction of the optical path for the experiment on the frequency responsivity check. The low-frequency band was mainly path for the experiment on the frequency responsivity check. The low-frequency band was mainly determined by the thermal response of the scanner, and the 3 dB cutoff frequency was approximately determined by the thermal response of the scanner, and the 3 dB cutoﬀ frequency was approximately 20 Hz. The mechanical resonant frequency of the piston mode was estimated to be 1.4 kHz. Therefore, 20 Hz. The mechanical resonant frequency of the piston mode was estimated to be 1.4 kHz. Therefore, no resonant peak was observed in the low-frequency range. no resonant peak was observed in the low-frequency range.

−2

−4

−6

−8

Normarized vertical [dB] displacement −10 10−1 100 101 102 Frequency [Hz] FigureFigure 10.10. FrequencyFrequency responseresponse ofof thethe electrothermalelectrothermal kirigamikirigami scannerscanner whenwhen applyingapplying aa sinusoidalsinusoidal wavewave voltage. voltage. The The frequency frequency response response of of the the vertical vertical actuation actuation was was measured measured by by spot spot displacement displacement of theof the beam beam reﬂected reflected by theby the platform. platform.

5. Conclusions 5. Conclusions Kirigami, a traditional Japanese art of paper cutting and folding, is a promising engineering Kirigami, a traditional Japanese art of paper cutting and folding, is a promising engineering method for creating out-of-plane structures. This paper proposes a kirigami-inspired electrothermal method for creating out-of-plane structures. This paper proposes a kirigami-inspired electrothermal MEMS scanner that obtains large vertical displacement by out-of-plane film actuation. Film material MEMS scanner that obtains large vertical displacement by out-of-plane film actuation. Film material combinations suitable for electrothermal self-reconfigurable folding and unfolding were selected, combinations suitable for electrothermal self-reconfigurable folding and unfolding were selected, and microscale cuttings were strategically placed to generate a large displacement. The freestanding and microscale cuttings were strategically placed to generate a large displacement. The freestanding electrothermal kirigami film with a 2 mm diameter and high fill factor was completely fabricated by electrothermal kirigami film with a 2 mm diameter and high fill factor was completely fabricated by careful stress control in the microfabrication process. A 200 µm vertical displacement with 131 mW and careful stress control in the microfabrication process. A 200 μm vertical displacement with 131 mW a 20 Hz responsive frequency was experimentally demonstrated as a unique function of electrothermal and a 20 Hz responsive frequency was experimentally demonstrated as a unique function of kirigami film. The proposed design, fabrication process, and experimental tests validate the proposed electrothermal kirigami film. The proposed design, fabrication process, and experimental tests scanner’s feasibility and potential for large-displacement scanning with a high fill factor. validate the proposed scanner’s feasibility and potential for large-displacement scanning with a high Authorfill factor. Contributions: Conceptualization, M.H. and Y.T.; methodology, M.H. and Y.T.; software, M.H.; validation, M.H.; formal analysis, M.H.; investigation, M.H.; resources, M.H. and Y.T.; data curation, M.H. and Y.T.; Author Contributions: Conceptualization, M.H. and Y.T.; methodology, M.H. and Y.T.; software, M.H.; writing—original draft preparation, M.H.; writing—review and editing, M.H. and Y.T.; visualization, M.H.; supervision,validation, M.H.; Y.T. All formal authors analysis, have readM.H.; and investigation, agreed to the M. publishedH.; resources, version M.H. of theand manuscript.Y.T.; data curation, M.H. and Y.T.; writing—original draft preparation, M.H.; writing—review and editing, M.H. and Y.T.; visualization, M.H.; Funding: This research was funded by JSPS KAKENHI, Grant Number JP18J20513 and Keio University doctoral supervision, Y.T. All authors have read and agreed to the published version of the manuscript. student grant-in-aid program 2019. Funding: This research was funded by JSPS KAKENHI, Grant Number JP18J20513 and Keio University doctoral student grant-in-aid program 2019.

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).

Conflicts of Interest: The authors declare no conflicts of interest.

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