Review of MEMS Based Fourier Transform Spectrometers

micromachines

Review Review of MEMS Based Fourier Transform Spectrometers

Junyu Chai 1,2,3, Kun Zhang 4, Yuan Xue 5, Wenguang Liu 1,2,3,*, Tian Chen 1,2,3, Yao Lu 1,2,3 and Guomin Zhao 1,2,3

1 College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China; [email protected] (J.C.); [email protected] (T.C.); [email protected] (Y.L.); [email protected] (G.Z.) 2 State Key Laboratory of Pulsed Power Laser Technology, Changsha 410073, China 3 Hunan Provincial Key Laboratory of High Energy Laser Technology, Changsha 410073, China 4 College of Computer Science, National University of Defense Technology, Changsha 410073, China; [email protected] 5 Wuxi WiO Technologies Co., Ltd., Wuxi 214000, China; [email protected] * Correspondence: [email protected]; Tel.: +86-1318-706-5510

Received: 31 January 2020; Accepted: 18 February 2020; Published: 20 February 2020

Abstract: Fourier transform spectrometers (FTS), mostly working in infrared (IR) or near infrared (NIR) range, provide a variety of chemical or material analysis with high sensitivity and accuracy and are widely used in public safety, environmental monitoring and national border security, such as explosive detection. However, because of being bulky and expensive, they are usually used in test centers and research laboratories. Miniaturized FTS have been developed rapidly in recent years, due to the increasing demands. Using micro-electromechanical system (MEMS) micromirrors to replace the movable mirror in a conventional FTS system becomes a new realm. This paper ﬁrst introduces the principles and common applications of conventional FTS, and then reviews various MEMS based FTS devices.

Keywords: Fourier transform spectrometers; Fourier transform infrared spectrometers (FTIR); micro-electromechanical system; micromirror; microactuator

1. Introduction Fourier transform infrared spectrometers (FTIR, or more generally FTS) are a type of powerful instruments for chemical and biological sensing. An FTS obtains spectrograms through the Fourier transform of the interferograms generated by an interferometer (Michelson interferometer and lamellar grating interferometer as the two main types) [1]. The spectral ranges of FTS systems can extend from visible to NIR or even MIR. Compared to other spectrometers, such as those based on Fabry-Perot interferometers or gratings, FTS have unparalleled advantages: (1) Multiplexing, so called Fellgett advantage [2], i.e., using only one single photodetector can acquire the full spectral range spectra without the need of any dispersive components and photodetector arrays, greatly reducing the product’s cost and form factor; (2) high throughput, namely Jacquinot advantage [3], resulting from the fact that no slits in the system lead to a higher signal-to-noise ratio (SNR); and (3) linearity, namely Connes advantage [4], so that stable gas lasers, such as a He-Ne laser can be introduced as a reference light source to tackle the interference distortion. Currently, FTS are no longer restricted to laboratory applications. Microelectromechanical systems (MEMS) technology has been successfully employed to make FTS portable, inexpensive and miniaturized [5,6], so these portable FTS devices can be used for real-time, on-site measurement and

Micromachines 2020, 11, 214; doi:10.3390/mi11020214 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 214 2 of 28 analysis. The key to realizing miniature FTS is to replace the bulky scanning mirror module in a conventional FTS with a MEMS mirror. Various MEMS based FTS devices have been developed using electrostatic, electromagnetic and electrothermal microactuators [7–9]. This paper is organized as follows. The working principles and applications of conventional FTS are reviewed in Section2. Several miniaturized FTS utilizing various MEMS based actuation mechanisms are introduced in Section3.

2. Theoretical Background of Fourier Transform Spectrometers

2.1. Fourier Transform Spectroscopy and Its Applications Fourier transform infrared (IR) spectroscopy reflects the transitions of electrons among the vibrational energy states upon absorbing light [4]. The absorption occurs when an incoming photon excites a molecule to a higher energy state. The transitions between the vibrational states of the molecule occur in the IR range. The wavelength of each IR absorbance peak is dependent on the specific physical and chemical properties of the corresponding molecule, which is a fingerprint of that functional group [10]. Fourier transform spectroscopy analysis has benefits of rapid analysis, minimum sample preparation, noninvasive and nondestructive operation, no reagents needed, simultaneous multicomponent analysis, and compatibility with fiber optics. FTS can also be used to test and characterize optical components, such as light sources, filters, detectors, and fiber-optic components [11]. FTS has been widely used in quality and process control, color measurement, and chemical analysis [4,12]. Compact FTS systems are needed in various fields, such as environmental monitoring, food industry, medical diagnostics, life science, and telecommunication [13–15].

2.2. Working Principles Micromachines 2020, 11, x 3 of 26 2.2.1. Michelson Interferometer-Based FTS As the core of commonly-used FTS systems, Michelson1 interferometers (see Figure1a) are typically 𝛽 (4) employed to obtain Fourier Transform spectra [1620∙𝐷∙𝜈]. As shown in Figure1a, the sample light is split by a beam splitter into two light beams: One is directed into a ﬁxed mirror, while the other is incident to a where D is the diameter of the beam and νmax is the wavenumber of the shortest wavelength movable mirror. The two reﬂected light beams recombine at the beam splitter and generate interference component of the light source. For example, if D is 0.1 cm and νmax is 15,800 cm−1, a tilting angle must signalsbe less thatthan are0.002°. detected Hence, by athe photodetector. extension of Thethe tr interferogramavel range and signal the pickedcompensation up by the of photodetectorthe tilting are variestwo key with factors the pistonthat can movement improve ofthe the spectral movable resolution. mirror.

(a) (b)

Figure 1.1. Schematic of a Michelson interferometer: ( (aa)) An ideal Michelson interferometer; (b) aa Michelson interferometer withwith thethe movablemovable mirrormirror tilting.tilting.

2.2.2.When Lamellar the sampleGrating absorbs Interferometer-Based a speciﬁc wavelength, FTS the interference signal I(δ) can be expressed as,

A lamellar grating interferometer, whichZ + is basically a binary grating with a variable depth that ∞ operates in the zeroth order of the diffractionI(δ) = patteB(υrn,) cos is (also2πδυ applied)dυ as a FTS [18]. Compared with(1) a Michelson interferometer, which splits the−∞ wave amplitude via a beam splitter, a lamellar grating interferometer divides the wave front. The lamellar grating (see Figure 2) is made of two sets of facets: One fixed, and the other actuated to move up and down, producing an OPD between two coherent light beams which are reflected off the top surfaces of the two sets of facets. The intensity I of the diffraction pattern is given by Reference [19] 𝑠𝑖𝑛 𝐾 𝑠𝑖𝑛 2𝑛𝐾 𝜑 𝐼∝ 𝑐𝑜𝑠 (5) 𝐾 𝑠𝑖𝑛 2𝐾 2 where K = (πa) sinα/(2λ), λ is the incident wavelength, a is the grating period, and n is the number of illuminated periods. The diffraction angle α and the phase difference φ are described as follows:

𝛼 = 𝑎𝑟𝑐𝑠𝑖𝑛 , (6) 𝜑= 𝛿, where m is the diffraction order, and δ is the OPD, given by δ = d (1 + cosα + asinα/(2d)), which is the sum of distances AB, BC, and CD in Figure 2 [19]. When m = 0 and α = 0, the intensity I(δ) of the zeroth diffraction order at different OPDs is picked to retrieve the spectrum using Equation (2).

Figure 2. Schematic of a lamellar grating interferometer. (Reprinted with permission from Reference [19] The Optical Society).

Micromachines 2020, 11, 214 3 of 28 where δ is the optical path diﬀerence (OPD), υ is the wave number, and B(υ) is the spectral power density. B(υ) can be obtained by performing Fourier transform on I(δ), as shown in Equation (2):

Z + ∞ i2πδ B(υ) = I(δ)e− dδ (2) −∞ For the FTS system, the theoretical spectral resolution, ∆σ, is given by Equation (3):

1 1 ∆σ = = (3) 2 ∆Zmax δmax × where ∆Zmax is the maximum displacement of the movable mirror. As shown in Equation (3), the spectral resolution of the FTS is determined by the displacement of the movable micromirror. The resolution increases as the displacement extends. On the other hand, the movable mirror requires to remain in good alignment during the motion. However, in practice, it is diﬃcult to achieve because tilting always exists when the mirror is moving. The tilting of the mirror signiﬁcantly deteriorates the interferogram (see Figure1b), thus, causing a reduction of the usable displacement range and degradation of spectral resolution [4]. Therefore, the maximum tilt angle, βmax, without degrading the resolution needs to satisfy the following relationship [17]:

1 βmax < (4) 20 D νmax · · where D is the diameter of the beam and νmax is the wavenumber of the shortest wavelength component 1 of the light source. For example, if D is 0.1 cm and νmax is 15,800 cm− , a tilting angle must be less than 0.002◦. Hence, the extension of the travel range and the compensation of the tilting are two key factors that can improve the spectral resolution.

sinK 2sin2nK 2 ϕ I cos2 (5) ∝ K sin2K 2 where K = (πa) sinα/(2λ), λ is the incident wavelength, a is the grating period, and n is the number of illuminated periods. The diﬀraction angle α and the phase diﬀerence ϕ are described as follows:

mλ α = arcsin , a (6) 2π ϕ = δ, λ where m is the diffraction order, and δ is the OPD, given by δ = d (1 + cosα + asinα/(2d)), which is the sum of distances AB, BC, and CD in Figure2[ 19]. When m = 0 and α = 0, the intensity I(δ) of the zeroth diffraction order at different OPDs is picked to retrieve the spectrum using Equation (2). Micromachines 2020, 11, x 3 of 26

1 𝛽 (4) 20∙𝐷∙𝜈 where D is the diameter of the beam and νmax is the wavenumber of the shortest wavelength component of the light source. For example, if D is 0.1 cm and νmax is 15,800 cm−1, a tilting angle must be less than 0.002°. Hence, the extension of the travel range and the compensation of the tilting are two key factors that can improve the spectral resolution.

(a) (b)

Figure 1. Schematic of a Michelson interferometer: (a) An ideal Michelson interferometer; (b) a Michelson interferometer with the movable mirror tilting.

2.2.2. Lamellar Grating Interferometer-Based FTS A lamellar grating interferometer, which is basically a binary grating with a variable depth that operates in the zeroth order of the diffraction pattern, is also applied as a FTS [18]. Compared with a Michelson interferometer, which splits the wave amplitude via a beam splitter, a lamellar grating interferometer divides the wave front. The lamellar grating (see Figure 2) is made of two sets of facets: One fixed, and the other actuated to move up and down, producing an OPD between two coherent light beams which are reflected off the top surfaces of the two sets of facets. The intensity I of the diffraction pattern is given by Reference [19] 𝑠𝑖𝑛 𝐾 𝑠𝑖𝑛 2𝑛𝐾 𝜑 𝐼∝ 𝑐𝑜𝑠 (5) 𝐾 𝑠𝑖𝑛 2𝐾 2 where K = (πa) sinα/(2λ), λ is the incident wavelength, a is the grating period, and n is the number of illuminated periods. The diffraction angle α and the phase difference φ are described as follows:

𝛼 = 𝑎𝑟𝑐𝑠𝑖𝑛 , (6) 𝜑= 𝛿, where m is the diffraction order, and δ is the OPD, given by δ = d (1 + cosα + asinα/(2d)), which is the Micromachinessum of distances2020, 11 AB,, 214 BC, and CD in Figure 2 [19]. When m = 0 and α = 0, the intensity I(δ) of the zeroth4 of 28 diffraction order at different OPDs is picked to retrieve the spectrum using Equation (2).

Figure 2. Schematic of a lamellar grating interferometer. (Reprinted with permission from Reference [19] Figure 2. Schematic of a lamellar grating interferometer. (Reprinted with permission from Reference The Optical Society). [19] The Optical Society). 3. Fourier Transform Spectrometers Based on MEMS Mirrors Using MEMS micromirrors to replace the movable mirror modules in conventional Michelson interferometer-based FTS systems is a common solution for miniaturizing FTS. MEMS micromirrors are driven by microactuators. Currently, there are mainly three types of MEMS micromirrors, namely, electrostatic, electromagnetic and electrothermal, that have been used for FTS miniaturization. Another type of micromirrors driven by piezoelectric actuation has also been developed [20,21], but not applied for FTS miniaturization so far, due to the fact that piezoelectric MEMS mirrors tend to have limited piston motion [22,23]. According to the requirements of chemical analysis applications, the resolutions of MEMS based 1 FTS must be 100 cm− or better [24]. Equation (3) yields that the needed OPD has to be over 100 µm, thus, corresponding the movable mirror needs to travel more than 50 µm, which is a big challenge for MEMS microactuators. On the other hand, the form factor of a MEMS based FTS is also crucial, which directly aﬀects the application range. Therefore, the miniaturization of MEMS based FTS is currently focused on increasing the travel range of the micromirror and reducing its overall size.

3.1. Electrostatic MEMS Based FTS Electrostatic actuation, widely used in the MEMS world for its high speed, low power consumption and easy fabrication, is realized by applying a varying potential difference in the device structure to produce a changing electrostatic force. For example, the digital micromirror device (DMD) (Texas Instruments) that have been successfully commercialized in projection displays is based on electrostatic actuation [25,26]. The essence of an electrostatic actuator is a capacitor, which is broadly defined as two conductors that can hold the same or opposite charges. When the distance and the relative position between two conductors change via stimulus, the capacitance value will change accordingly. When a voltage is applied across two conductors, an electrostatic force will be generated between them [27]. Parallel-plate and interdigitated fingers (comb drives) are two types of electrostatic actuators available to generate in-plane motions and out-of-plane motions. Two electrodes are involved in each type of electrostatic actuators: One is fixed on the substrate, and the other is suspended and free to move in the desired direction. The parallel-plate structure (see Figure3a) has planar electrodes facing each other, and the two parallel plates can move with respect to each other in two ways: Normal displacement or parallel sliding displacement [27]. Parallel-plate actuators were applied to form MEMS FTS by VTT [28], where multiple reflections between the two parallel mirror plates were used to increase the OPD (13 reflections at 45◦ angle, producing a 275 µm OPD). However, the mirror surface had a curvature of up to 200 nm, leading to serious wavefront distortion as the wavefront distortion was amplified upon every reflection. Moreover, the reflection loss would be quite high, due to the multiple reflections. Hence, comb-drive electrostatic actuators are more commonly applied in FTS. Micromachines 2020, 11, x 4 of 26

3. Fourier Transform Spectrometers Based on MEMS Mirrors Using MEMS micromirrors to replace the movable mirror modules in conventional Michelson interferometer-based FTS systems is a common solution for miniaturizing FTS. MEMS micromirrors are driven by microactuators. Currently, there are mainly three types of MEMS micromirrors, namely, electrostatic, electromagnetic and electrothermal, that have been used for FTS miniaturization. Another type of micromirrors driven by piezoelectric actuation has also been developed [20,21], but not applied for FTS miniaturization so far, due to the fact that piezoelectric MEMS mirrors tend to have limited piston motion [22,23]. According to the requirements of chemical analysis applications, the resolutions of MEMS based FTS must be 100 cm−1 or better [24]. Equation (3) yields that the needed OPD has to be over 100 μm, thus, corresponding the movable mirror needs to travel more than 50 μm, which is a big challenge for MEMS microactuators. On the other hand, the form factor of a MEMS based FTS is also crucial, which directly affects the application range. Therefore, the miniaturization of MEMS based FTS is currently focused on increasing the travel range of the micromirror and reducing its overall size.

3.1. Electrostatic MEMS Based FTS Electrostatic actuation, widely used in the MEMS world for its high speed, low power consumption and easy fabrication, is realized by applying a varying potential difference in the device structure to produce a changing electrostatic force. For example, the digital micromirror device (DMD) (Texas Instruments) that have been successfully commercialized in projection displays is based on electrostatic actuation [25,26]. The essence of an electrostatic actuator is a capacitor, which is broadly defined as two conductors that can hold the same or opposite charges. When the distance and the relative position between two conductors change via stimulus, the capacitance value will change accordingly. When a voltage is applied across two conductors, an electrostatic force will be generated between them [27]. Parallel-plate and interdigitated fingers (comb drives) are two types of electrostatic actuators available to generate in-plane motions and out-of-plane motions. Two electrodes are involved in each type of electrostatic actuators: One is fixed on the substrate, and the other is suspended and free to move in the desired direction. The parallel-plate structure (see Figure 3a) has planar electrodes facing each other, and the two parallel plates can move with respect to each other in two ways: Normal displacement or parallel sliding displacement [27]. Parallel-plate actuators were applied to form MEMS FTS by VTT [28], where multiple reflections between the two parallel mirror plates were used to increase the OPD (13 reflections at 45° angle, producing a 275 μm OPD). However, the mirror surface had a curvature of up to 200 nm, leading to serious wavefront distortion as the wavefront distortion was amplified upon every reflection. Moreover, the reflection loss would be quite high, Micromachinesdue to the multiple2020, 11, 214 reflections. Hence, comb-drive electrostatic actuators are more commonly applied5 of 28 in FTS.

(a) (b) (c) (d)

FigureFigure 3.3. VariousVarious conﬁgurationsconfigurations of electrostatic actuation: ( (aa)) Parallel-plate Parallel-plate structure structure (out-of-plane (out-of-plane motionmotion andand in-planein-plane motion);motion); (b) transverse motion motion comb comb structure; structure; ( (cc) )longitudinal longitudinal motion motion comb comb structure;structure; ((dd)) out-of-planeout-of-plane motionmotion combcomb structure.

A comb drive consists of two sets of electrodes placed in the same plane parallel to the substrate, which is usually actuated via three operation modes: Transverse, longitudinal and out-of-plane (see Figure3b–d. (1) Transverse comb drives: The set of free fingers moves in a perpendicular direction to the longitudinal axis of comb fingers; (2) longitudinal comb drives: The direction of relative movement is along the longitudinal axis of the fingers, allowable by the suspension; and (3) Comb drives designed to produce out-of-plane displacement: They are initially located at the same plane and actuated by the fringe capacitance fields. The comb drives, which are commonly used in MEMS FTS, have large length-to-width aspect ratio and rectangular shape viewed from the top and side. The amplitude of the displacement of a comb drive under DC or quasi-static biasing is rather limited. Large piston displacement can be generated from resonant actuation. Several in-plane and out-of-plane electrostatic comb drive MEMS micromirrors and FTS are introduced below.

3.1.1. In-Plane Electrostatic MEMS Micromirrors and FTS A MEMS micromirror driven by an in-plane electrostatic comb was successfully verified in FTS in 1999 by Manzardo et al. [7]. They developed a voltage-movable MEMS mirror to replace the moving mirror in a Michelson interferometer (see Figure4a). In a conventional comb actuator, the voltage-induced force relied on the square of the control voltage. To overcome this, two identical comb-drive A and B were placed opposite to each other, which could generate a linear movement with variable voltage. The mirror surface was formed by silicon sidewall via deep reactive ion etching (DRIE) (see Figure4b). When applying a 10 V-amplitude control voltage, the mirror could generate a maximum vertical placement of 77 µm, leading to an OPD of 154 µm. The estimated spectral resolution for He-Ne laser was 5.2 nm. Repeatability of 25 nm for the MEMS mirror position from the OPD was ± measured. However, the mirror size was only about 75 µm 500 µm, which was much smaller than × the size of the entire MEMS device (5 mm 4 mm). Since the device was based on silicon-on-insulator × (SOI) wafer processing, the sidewall mirror surface height was limited by the device layer thickness of the SOI wafer, and thus, greatly limited the FTS system’s luminous flux. An in-plane electrostatically-actuated Lamellar grating interferometer was also proposed in 2004 by Manzardo et al. (see Figure5)[ 19]. The actuator was fabricated using DRIE on an SOI wafer. The height of the mirrors was 75 µm, and the number of the illustrated periods of the grating was 12, where the grating period was 90 µm. The whole silicon chip measures at 5 mm 5 mm. In the × experiment, a maximum displacement of 145 µm was achieved with 65 V control voltage, corresponding 1 to a theoretical resolution of 70 cm− . The wavelength range extending from 380 nm to 1100 nm, due to no reflection coating on the silicon mirrors. Micromachines 2020, 11, x 5 of 26

A comb drive consists of two sets of electrodes placed in the same plane parallel to the substrate, which is usually actuated via three operation modes: Transverse, longitudinal and out-of-plane (see Figure 3b–d. (1) Transverse comb drives: The set of free fingers moves in a perpendicular direction to the longitudinal axis of comb fingers; (2) longitudinal comb drives: The direction of relative movement is along the longitudinal axis of the fingers, allowable by the suspension; and (3) Comb drives designed to produce out-of-plane displacement: They are initially located at the same plane and actuated by the fringe capacitance fields. The comb drives, which are commonly used in MEMS FTS, have large length-to-width aspect ratio and rectangular shape viewed from the top and side. The amplitude of the displacement of a comb drive under DC or quasi-static biasing is rather limited. Large piston displacement can be generated from resonant actuation. Several in-plane and out-of- plane electrostatic comb drive MEMS micromirrors and FTS are introduced below.

3.1.1. In-Plane Electrostatic MEMS Micromirrors and FTS A MEMS micromirror driven by an in-plane electrostatic comb was successfully verified in FTS in 1999 by Manzardo et al. [7]. They developed a voltage-movable MEMS mirror to replace the moving mirror in a Michelson interferometer (see Figure 4a). In a conventional comb actuator, the voltage-induced force relied on the square of the control voltage. To overcome this, two identical comb-drive A and B were placed opposite to each other, which could generate a linear movement with variable voltage. The mirror surface was formed by silicon sidewall via deep reactive ion etching (DRIE) (see Figure 4b). When applying a 10 V-amplitude control voltage, the mirror could generate a maximum vertical placement of 77 μm, leading to an OPD of 154 μm. The estimated spectral resolution for He-Ne laser was 5.2 nm. Repeatability of ± 25 nm for the MEMS mirror position from the OPD was measured. However, the mirror size was only about 75 μm × 500 μm, which was much smaller than the size of the entire MEMS device (5 mm × 4 mm). Since the device was based on silicon- on-insulatorMicromachines 2020 (SOI), 11 ,wafer 214 processing, the sidewall mirror surface height was limited by the device6 of 28 layer thickness of the SOI wafer, and thus, greatly limited the FTS system’s luminous flux.

(a) (b)

FigureFigure 4. AnAn in-plane in-plane electrostatic electrostatic comb comb micro-electromechanical micro-electromechanical system system (MEMS) (MEMS) based based Fourier Fourier transformtransform spectrometers spectrometers (FTS): (FTS): ( (aa)) Schematic Schematic of of FTS; FTS; ( (bb)) scanning scanning electron electron microscope microscope (SEM) (SEM) of of the the MEMSMEMS mirror. mirror. (Reprinted (Reprinted with with permission permission from from Reference Reference [7] [7] The The Optical Optical Society). Society). Micromachines 2020, 11, x 6 of 26 Micromachines 2020, 11, x 6 of 26 An in-plane electrostatically-actuated Lamellar grating interferometer was also proposed in 2004 by Manzardo et al. (see Figure 5) [19]. The actuator was fabricated using DRIE on an SOI wafer. The height of the mirrors was 75 μm, and the number of the illustrated periods of the grating was 12, where the grating period was 90 μm. The whole silicon chip measures at 5 mm × 5 mm. In the experiment, a maximum displacement of 145 μm was achieved with 65 V control voltage, corresponding to a theoretical resolution of 70 cm−1. The wavelength range extending from 380 nm to 1100 nm, due to no reflection coating on the silicon mirrors.

FigureFigure 5. SEMSEM of of an an in-plane electrostatic actuated La Lamellarmellar grating interferometer. (Reprinted (Reprinted with with Figure 5. SEM of an in-plane electrostatic actuated Lamellar grating interferometer. (Reprinted with permissionpermission from Reference [19] [19] The Optical Society). permission from Reference [19] The Optical Society). BasedBased on on this this actuator, actuator, a aminiature miniature FTS FTS was was designed designed by byBriand Briand et al. et al.in 2007 in 2007[29]. The [29]. Based on this actuator, a miniature FTS was designed by Briand et al. in 2007 [29]. The interferometricThe interferometric module module involved involved MEMS, MEMS, actuation actuation electronics electronics to drive the to driveMEMS the and MEMS give a feedback and give interferometric module involved MEMS, actuation electronics to drive the MEMS and give a feedback ofa feedbackthe motion, of theoptical motion, component optical component to bring the to ligh bringt onto the lightthe surface onto the of surfacethe MEMS of the mirrors, MEMS and mirrors, two of the motion, optical component to bring the light onto the surface of the MEMS mirrors, and two fibersand two for ﬁbers in-coupling for in-coupling and out-coupling and out-coupling the light the (see light Figure (see Figure6). The6). instrument The instrument was wascompact compact and fibers for in-coupling and out-coupling the light (see Figure 6). The instrumentµ was compactµ and portable,and portable, and andin principle, in principle, easily easily upgradeable upgradeable to any to any wavelength wavelength above above 2.6 2.6 μmm up up to to 5 5 μm,m, which which portable, and in principle, easily upgradeable to any wavelength above 2.6 μm up to 5 μm, which dependeddepended on on the photodetector. In In their their experime experiment,nt, a a single single InGaAs InGaAs photodiode photodiode was was applied applied to to depended on the photodetector. In their experiment, a single InGaAs photodiode was applied to recordrecord the the spectra. This This FTS FTS was was able able to to measure measure 400 400 spectra spectra per per second. record the spectra. This FTS was able to measure 400 spectra per second.

Figure 6. A miniature FTS, including driving electronics, MEMS and fibers. (Reprinted with FigureFigure 6.6. A miniature FTS, including driving electronics, MEMS and fibers. (Reprinted with permissionA from miniature Reference FTS, including[29] IEEE). driving electronics, MEMS and ﬁbers. (Reprinted with permission frompermission Reference from [29 Reference] IEEE). [29] IEEE). Further, FTS systems for near-infrared (ARCspectro ANIR) and mid-infrared (ARCspectro Further,Further, FTS FTS systems systems for for near-infrared near-infrared (ARCspectro (ARCspectro ANIR) ANIR) and mid-infrared and mid-infrared (ARCspectro (ARCspectro AMIR) AMIR) were developed based on this type of MEMS by Merenda et al. in 2010 [30]. The new design wereAMIR) developed were developed based on based this typeon this of MEMStype of byMEMS Merenda by Merenda et al. in et 2010 al. in [30 2010]. The [30]. new The design new design had a had a pitch of 100 μm and an 8 mm2 active area. This FTS consisted of an interferometer, a VCSEL pitchhad a of pitch 100 µofm 100 and μ anm 8and mm an2 active 8 mm area.2 active This area. FTS This consisted FTS consiste of an interferometer,d of an interferometer, a VCSEL a diode VCSEL to diode to control the grating movement, and USB communication on a single PCB (see Figure 7). The diode to control the grating movement, and USB communication on a single PCB (see Figure 7). The overall size was 10 cm × 15 cm × 7 cm and the weight was 850 g. It could generate a maximal OPD of overall size was 10 cm × 15 cm × 7 cm and the weight was 850 g. It could generate a maximal OPD of more than 1 mm, corresponding to a spectral resolution of fewer than 8 cm−1. The wavelength stability more than 1 mm, corresponding to a spectral resolution of fewer than 8 cm−1. The wavelength stability was <0.5 nm at 1500 nm at stable conditions. was <0.5 nm at 1500 nm at stable conditions.

Figure 7. Portable FTS for near-infrared/mid-infrared (ARCspectro ANIR/AMIR) utilizing an in-plane Figure 7. Portable FTS for near-infrared/mid-infrared (ARCspectro ANIR/AMIR) utilizing an in-plane electrostatic MEMS lamellar grating. (Reprinted with permission from Reference [30] SPIE). electrostatic MEMS lamellar grating. (Reprinted with permission from Reference [30] SPIE).

Micromachines 2020, 11, x 6 of 26

Figure 5. SEM of an in-plane electrostatic actuated Lamellar grating interferometer. (Reprinted with permission from Reference [19] The Optical Society).

Based on this actuator, a miniature FTS was designed by Briand et al. in 2007 [29]. The interferometric module involved MEMS, actuation electronics to drive the MEMS and give a feedback of the motion, optical component to bring the light onto the surface of the MEMS mirrors, and two fibers for in-coupling and out-coupling the light (see Figure 6). The instrument was compact and portable, and in principle, easily upgradeable to any wavelength above 2.6 μm up to 5 μm, which depended on the photodetector. In their experiment, a single InGaAs photodiode was applied to record the spectra. This FTS was able to measure 400 spectra per second.

Figure 6. A miniature FTS, including driving electronics, MEMS and fibers. (Reprinted with permission from Reference [29] IEEE).

MicromachinesFurther,2020 FTS, 11 , 214systems for near-infrared (ARCspectro ANIR) and mid-infrared (ARCspectro7 of 28 AMIR) were developed based on this type of MEMS by Merenda et al. in 2010 [30]. The new design hadcontrol a pitch the gratingof 100 μ movement,m and an 8 and mm USB2 active communication area. This FTS on consiste a singled PCBof an (see interferometer, Figure7). The a overallVCSEL diodesize was to control 10 cm the15 grating cm 7 cmmovement, and the weightand USB was communication 850 g. It could on generate a single a PCB maximal (see Figure OPD of 7). more The overall size was× 10 cm × ×15 cm × 7 cm and the weight was 850 g. It could1 generate a maximal OPD of than 1 mm, corresponding to a spectral resolution of fewer than 8 cm− . The wavelength stability was

Figure 7. 7. PortablePortable FTS FTS for for near-infrared/mid-infrared near-infrared/mid-infrared (ARCspectro (ARCspectro ANIR/AMIR) ANIR/AMIR) utilizing an in-plane in-plane electrostatic MEMS lamellar grating. (Reprinted (Reprinted with permission from Reference [[30]30] SPIE).

An in-plane integrated FTS microsystem was implemented based on another sidewall electrostatically driven MEMS micromirror by Yu et al. in 2006 [31]. The system integrated a MEMS movable mirror, a fixed mirror, a beam splitter, and fiber U-grooves into a single SOI wafer (see Figure8a). The U-grooves were used for passive alignment of optical fibers. All the components were made of the (110) silicon device layer of SOI wafers. The fabrication process combined deep reactive ion etching (DRIE) with wet potassium hydroxide KOH etching, realizing a good surface quality of the sidewall. The crystallographic (110) silicon device layer only allows two possible directions, which makes the mirrors oriented at 70.53◦ not typical 45◦. Figure8b,c show the details of the fixed mirror and the movable mirror, respectively. The fixed mirror had a large surface roughness as it could only be fabricated by DRIE. The device realized in-plane integration of the FTS system, and the overall size was only 4 mm 8 mm 0.6 mm, but the area of the comb drive structure occupied about × × 4 mm 4 mm. The maximum displacement of the movable mirror was about 25 µm under 150 V × at a frequency of 5 Hz. The corresponding maximum OPD was 50 µm, and the spectral resolution measured at a wavelength of 1500 nm was about 45 nm. Khalil et al. successively proposed two fiber-coupled in-plane integration FTS systems based on the classical Michelson interferometer and Mach-Zehnder interferometer in 2009 and 2011, respectively [32,33]. All of the optical components were located on a single SOI chip. A platform, which was called silicon-integrated micro-optical system technology (SiMOSTTM, Si-Ware Systems, Cairo, Egypt) [34], was developed and utilized in the manufacturing process. The Michelson interferometer system (see Figure9a) was realized by applying a special single-medium interface (silicon-air) beam and a vertical comb-based translational mirror. The reflective surfaces of both the moving mirror and fixed mirror could either be metal-coated or an air-silicon Bragg mirror using DIRE. The comb-drive actuator had 126 fingers which could achieve a vertical displacement of 48 µm at resonance. This MEMS chip was mounted on a printed circuit board, and mixed laser sources of 1310 nm and 1550 nm were inserted to the FTS using an optical fiber. Micromachines 2020, 11, x 7 of 26

Micromachinesfrequency2020 of, 115, 214Hz. The corresponding maximum OPD was 50 μm, and the spectral resolution8 of 28 measured at a wavelength of 1500 nm was about 45 nm.

(b)

(a)

(c)

MicromachinesFigureFigure 8.2020 An8., 11An in-plane, x in-plane electrostatic electrostatic comb-based comb-based FTS: FTS: (a) ( SEMa) SEM of theof the micromachined micromachined FTS; FTS; (b) ( SEMb) SEM of8 of 26 thethe movable movable micromirror micromirror with with the the sidewall sidewall obtained obtained by potassiumby potassium hydroxide hydroxide (KOH) (KOH) etching; etching; (c) SEM(c) SEM DIRE.of The theof the ﬁxedcomb-drive fixed mirror mirror withactuator with the the sidewall had sidewall 126 deﬁnedfingers defined bywhich by deep d eepcould reactive reactive achieve ion ion etching a etching vertical (DRIE). (DRIE). displacement (Reprinted (Reprinted withof 48with μ m at resonance.permissionpermission This from fromMEMS Reference Refere chipnce [31 was ][31] Elsevier). mountedElsevier). on a printed circuit board, and mixed laser sources of 1310 nm and 1550 nm were inserted to the FTS using an optical fiber. Khalil et al. successively proposed two fiber-coupled in-plane integration FTS systems based on the classical Michelson interferometer and Mach-Zehnder interferometer in 2009 and 2011, respectively [32,33]. All of the optical components were located on a single SOI chip. A platform, which was called silicon-integrated micro-optical system technology (SiMOSTTM, Si-Ware Systems, Cairo, Egypt) [34], was developed and utilized in the manufacturing process. The Michelson interferometer system (see Figure 9a) was realized by applying a special single-medium interface (silicon-air) beam and a vertical comb-based translational mirror. The reflective surfaces of both the moving mirror and fixed mirror could either be metal-coated or an air-silicon Bragg mirror using

(a) (b)

Figure 9. InIn plane-integrated plane-integrated FTS: FTS: ( (aa)) Classical Classical Michelson Michelson interferometer interferometer configuration; conﬁguration; ( (bb)) Mach- Mach- Zehnder configuration. conﬁguration. (Reprinted with permission from Reference [[33]33] SPIE).

The Mach-Zehnder interferometer (see Figure9 9b)b) reportedreported by byKhalil Khalilet etal. al. hadhad twotwo transmissiontransmission outputs that were utilizedutilized toto cancelcancel thethe source source ﬂuctuation fluctuation noise noise and and increase increase the the SNR. SNR. Two Two silicon-air silicon- airbeam beam splitters splitters and and two two metallic metallic mirrors mirrors were were used used in the in system.the system. The The overall overall size ofsize the of chip the chip was onlywas 1only mm 1 mm2 mm. × 2 mm. When When applying applying a voltage a voltage of 70 of V, 70 the V, micromirror the micromirror could could achieve achieve a maximum a maximum OPD × OPDdisplacement displacement of 135 ofµ m135 at μ resonance,m at resonance, and the and measured the measured spectral spec resolutiontral resolution of 1550 of nm 1550 was nm about was about25 nm. 25 nm. Mortada et al. presented another on-chip Michelson interferometer based FTS in 2014 [35]. This MEMS actuator (see Figure 10a) was working in a push-pull condition to produce a large displacement. A double folded flexure structure was applied to reduce the parasitic torque usually happened in fabrication tolerance. The whole FTS device comprised a half-plane beam splitter, fixed mirror, moving mirror attached to the MEMS actuator, and fiber grooves employed for optical fiber insertion in an alignment-free manner (see Figure 10b). Etching depth larger than 300 μm was achieved with vertical surface angle and scalloping depth smaller than 0.1° and 60 nm, respectively. To demonstrate the improvement with larger depth, multi-mode optical fibers with core diameters of 62.5 μm and 200 μm were applied to transmit the white light to SOI chips with 90 μm and 200 μm device layer heights. Experimental results showed there was a 12 dB increase in the interference signal, which was a significant boost for the SNR.

(a) (b)

Figure 10. On-chip in-plane electrostatic comb-based FTS: (a) Comb-drive actuator; (b) optical components. (Reprinted with permission from Reference [35] IEEE).

Eltagoury et al. developed a cascaded low-finesse Fabry-Perot interferometers based FTS (see Figure 11) in 2016 [36]. Two Fabry-Perot interferometers were applied to replace a traditional Michelson interferometer in this system. A fixed Fabry-Perot interferometer was a silicon block that could generate spectral modulation, and thus, creating a shifted version of the interferogram away

Micromachines 2020, 11, x 8 of 26

DIRE. The comb-drive actuator had 126 fingers which could achieve a vertical displacement of 48 μm at resonance. This MEMS chip was mounted on a printed circuit board, and mixed laser sources of 1310 nm and 1550 nm were inserted to the FTS using an optical fiber.

(a) (b)

Figure 9. In plane-integrated FTS: (a) Classical Michelson interferometer configuration; (b) Mach- Zehnder configuration. (Reprinted with permission from Reference [33] SPIE).

The Mach-Zehnder interferometer (see Figure 9b) reported by Khalil et al. had two transmission outputs that were utilized to cancel the source fluctuation noise and increase the SNR. Two silicon- air beam splitters and two metallic mirrors were used in the system. The overall size of the chip was Micromachinesonly 1 mm 2020× 2 ,mm.11, 214 When applying a voltage of 70 V, the micromirror could achieve a maximum9 of 28 OPD displacement of 135 μm at resonance, and the measured spectral resolution of 1550 nm was about 25 nm. Mortada et et al. al. presented presented another another on-chip on-chip Michelson Michelson interferometer interferometer based based FTS FTSin 2014 in 2014[35]. This [35]. ThisMEMS MEMS actuator actuator (see (seeFigure Figure 10a) 10 wasa) was working working in ina apush-pull push-pull condition condition to to produce produce a a large displacement. A A double folded flexure flexure structure was applied to reduce the parasitic torque usually happened in in fabrication fabrication tolerance. tolerance. The The whole whole FTS FTS dev deviceice comprised comprised a half-plane a half-plane beam beamsplitter, splitter, fixed fixedmirror, mirror, moving moving mirror mirror attached attached to the to MEMS the MEMS actuat actuator,or, and fiber and fibergrooves grooves employed employed for optical for optical fiber µ fiberinsertion insertion in an in alignment-free an alignment-free manner manner (see (see Figure Figure 10b). 10b). Etching Etching depth depth larger larger than than 300 300 μmm was achieved withwith verticalvertical surfacesurface angle angle and and scalloping scalloping depth depth smaller smaller than than 0.1 0.1°◦ and and 60 60 nm, nm, respectively. respectively. To demonstrateTo demonstrate the improvementthe improvement with with larger larger depth, depth multi-mode, multi-mode optical optical fibers fibers with core with diameters core diameters of 62.5 µ µ µ µ ofm 62.5 and μ 200m andm 200 were μm applied were applied to transmit to transmit the white the light white to light SOI chipsto SOI with chips 90 withm and90 μ 200m andm 200 device μm layerdevice heights. layer heights. Experimental Experimental results showed results there showed was th a 12ere dB was increase a 12 dB in theincrease interference in the signal, interference which wassignal, a significant which was boost a significant for the SNR. boost for the SNR.

(a) (b)

Figure 10.10. On-chipOn-chip in-plane in-plane electrostatic electrostatic comb-based comb-based FTS: ( a) Comb-drive actuator; ( b) opticaloptical components. (Reprinted with permission from Reference [[35]35] IEEE).

Eltagoury et al. developed a cascaded low-finesselow-finesse Fabry-Perot interferometers based FTS (see Figure 1111)) in in 2016 2016 [36 ].[36]. Two Two Fabry-Perot Fabry-Perot interferometers interferometers were appliedwere applied to replace to a replace traditional a traditional Michelson interferometerMichelson interferometer in this system. in this A fixed system. Fabry-Perot A fixed interferometerFabry-Perot interferometer was a silicon block was a that silicon could block generate that Micromachinesspectralcould generate modulation, 2020 , spectral11, x and modulation thus, creating, and a thus, shifted creating version a shifted of the interferogram version of the awayinterferogram from the pointaway9 of 26 of zero spacing between the two mirrors. The interferogram was then produced by the scanning fromFabry-Perot the point interferometer of zero spacing attached between to a relativelythe two mirrors. large-stroke The electrostaticinterferogram comb-drive was then actuator.produced This by thecompact scanning device Fabry-Perot was fabricated interferom on aneter SOI attached substrate to a using relatively DRIE larg withe-stroke a simple electrostatic MEMS process comb-drive flow. actuator.The Fabry-Perot This compact interferometer device was configuration fabricated on simply an SOI arranged substrate the using mirrors DRIE in a with line, a making simple itMEMS much processtolerant flow. to the The misalignment Fabry-Perot errors. interferometer This cascaded configuration Fabry-Perot simply interferometer arranged the based mirrors FTS in has a beenline, makingdemonstrated it much reliably tolerant through to the misalignment a narrow laser errors. source This of cascaded 1550 nm Fabry-Perot and a wide bandinterferometer spectral sourcebased FTScomposed has been of a demonstrated SLED centered reliably around through 1300 nm. a narrow laser source of 1550 nm and a wide band spectral source composed of a SLED centered around 1300 nm.

Figure 11. Cascaded FP interferometers based FTS. (Repri (Reprintednted with permission from Reference [36] [36] SPIE).

3.1.2. Out-of-Plane Electrostatic MEMS Micromirrors and FTS The in-plane electrostatic MEMS micromirrors have good movement stability without tilting problems. However, the mirror quality is commonly poor and unable to coat gold, aluminum, silicon film which have high reflectance to near infrared light. The FTS system’s luminous flux is limited, due to the height of the mirrors determined by the device layer thickness of the SOI wafer, and the displacement is also restricted. Thus, out-of-plane electrostatic MEMS designs have been explored. Sandner et al. designed an out-of-plane moving electrostatic translational MEMS micromirror (type A design) in 2007, which was composed of the central mirror, long cantilever and comb-drive components (see Figure 12) [37]. The actual mirror was suspended on two long bending springs. The chip size was 1.8 mm × 9 mm, and the mirror size was 1.5 mm × 1.1 mm. The mirror was hermetically packaged in a 100 Pa vacuum chamber to reduce the impact of air damping, and thus, achieved a maximum vertical displacement of 200 μm with 40 V driving voltage at 5 kHz resonance. It could produce a spectral resolution of 25 cm−1 theoretically.

(a)

(b)

Figure 12. Layout (a) and SEM image (b) of a type A translational MEMS micromirror, using two long bending springs as mirror suspension. (Reprinted with permission from Reference [37] SPIE).

Type A translational MEMS micromirror was integrated into a small vacuum chamber with optical windows located parallel to the front and back side of this MEMS mirror. A compact FTS was fabricated using type A MEMS micromirror (see Figure 13) in 2007. This MEMS micromirror was

Micromachines 2020, 11, x 9 of 26 from the point of zero spacing between the two mirrors. The interferogram was then produced by the scanning Fabry-Perot interferometer attached to a relatively large-stroke electrostatic comb-drive actuator. This compact device was fabricated on an SOI substrate using DRIE with a simple MEMS process flow. The Fabry-Perot interferometer configuration simply arranged the mirrors in a line, making it much tolerant to the misalignment errors. This cascaded Fabry-Perot interferometer based FTS has been demonstrated reliably through a narrow laser source of 1550 nm and a wide band spectral source composed of a SLED centered around 1300 nm.

MicromachinesFigure 11.2020 Cascaded, 11, 214 FP interferometers based FTS. (Reprinted with permission from Reference [36]10 of 28 SPIE).

3.1.2.3.1.2. Out-of-Plane Out-of-Plane Electrosta Electrostatictic MEMS MEMS Micromirrors Micromirrors and and FTS FTS TheThe in-plane in-plane electrostatic electrostatic MEMS MEMS micromirrors micromirrors have have good good movement movement stability stability without without tilting tilting problems.problems. However, However, the the mirror mirror quality quality is is commonly commonly p pooroor and and unable unable to to coat coat gold, gold, aluminum, aluminum, silicon silicon filmfilm which which have have high high reflectance reflectance to to near near infrared infrared light. light. The The FTS FTS system’s system’s luminous luminous flux flux is is limited, limited, duedue to to the the height height of of the the mirrors mirrors determined determined by by the the device device layer layer thickness thickness of of the the SOI SOI wafer, wafer, and and the the displacementdisplacement is is also also restricted. restricted. Thus, Thus, out-of-plane out-of-plane electrostatic electrostatic MEMS MEMS designs designs have have been explored. SandnerSandner et et al. al. designed designed an an out-of-plane out-of-plane moving moving electrostatic electrostatic translational translational MEMS MEMS micromirror micromirror (type(type A A design) design) in in 2007, 2007, which which was was composed composed of of the the central central mirror, mirror, long long cantilever cantilever and and comb-drive comb-drive componentscomponents (see (see Figure Figure 12)12)[ [37].37]. The The actual actual mirror mirror was was suspended suspended on on two two long long bending bending springs. springs. The The chip size was 1.8 mm 9 mm, and the mirror size was 1.5 mm 1.1 mm. The mirror was hermetically chip size was 1.8 mm ×× 9 mm, and the mirror size was 1.5 mm ×× 1.1 mm. The mirror was hermetically packagedpackaged in in a a 100 100 Pa Pa vacuum chamber chamber to to reduce reduce the the impact impact of of air damping, and and thus, thus, achieved achieved a a maximummaximum vertical vertical displacement displacement of of 200 200 μµmm with with 40 40 V V driving voltage at 5 kHz resonance. It It could could 1 produceproduce a a spectral spectral resolution of 25 cm −1 theoretically.theoretically.

(a)

(b)

FigureFigure 12. 12. LayoutLayout (a (a)) and and SEM SEM image image (b (b) )of of a a type type A A translational translational MEMS MEMS micromirror, micromirror, using using two two long long bendingbending springs springs as as mirror mirror suspensi suspension.on. (Reprinted (Reprinted with with permission permission from from Reference Reference [37] [37] SPIE). SPIE).

TypeType A translational MEMS micromirror micromirror was was integrated integrated into into a a small vacuum vacuum chamber chamber with with opticaloptical windows windows located located parallel parallel to to the the front front and and back back side side of this of this MEMS MEMS mirror. mirror. A compact A compact FTS was FTS fabricatedwas fabricated using using type typeA MEMS A MEMS micromirror micromirror (see Figure (see Figure 13) in 13 2007.) in 2007. This MEMS This MEMS micromirror micromirror was was operated at 100 Pa in a vacuum chamber. A Peltier cooled IR detector was applied for recording interference signals. A temperature-controlled VCSEL diode monitored the position and velocity of the MEMS mirror in real time to increase the spectral accuracy. This MEMS FTS could produce a resolution 1 of 35 cm− for a wavelength range of 2.22 µm to 5.55 µm. The measured polystyrene spectrum was consistent with the standard data. Based on the design of type A translation MEMS micromirror, an improved design of type B, applying two pantograph-like spring mechanisms for mirror suspension was realized (see Figure 14) in 2007. In order to acquire a suﬃciently large aperture for optical throughput, circular shape of 3 mm diameter was chosen for this mirror plate. A reduced resonance frequency of 500 Hz was chosen to improve the SNR. This actuator was designed for a stroke of 500 µm, greatly increasing spectral 1 resolution to 10 cm− theoretically. However, only 280 µm displacement could be measured at a maximum under the inﬂuence of superimposed parasitic torsional oscillation modes. Micromachines 2020, 11, x 10 of 26

operated at 100 Pa in a vacuum chamber. A Peltier cooled IR detector was applied for recording interference signals. A temperature-controlled VCSEL diode monitored the position and velocity of the MEMS mirror in real time to increase the spectral accuracy. This MEMS FTS could produce a resolution of 35 cm-1 for a wavelength range of 2.22 μm to 5.55 μm. The measured polystyrene spectrum was consistent with the standard data.

Micromachines 2020, 11, x 10 of 26 operated at 100 Pa in a vacuum chamber. A Peltier cooled IR detector was applied for recording interference signals. A temperature-controlled VCSEL diode monitored the position and velocity of the MEMS mirror in real time to increase the spectral accuracy. This MEMS FTS could produce a resolutionMicromachines of2020 35, 11cm, 214-1 for a wavelength range of 2.22 μm to 5.55 μm. The measured polystyrene11 of 28 spectrum was consistent with the standard data.

Figure 13. A compact FTS using type A MEMS micromirror. (Reprinted with permission from Reference [37] SPIE).

Based on the design of type A translation MEMS micromirror, an improved design of type B, applying two pantograph-like spring mechanisms for mirror suspension was realized (see Figure 14) in 2007. In order to acquire a sufficiently large aperture for optical throughput, circular shape of 3 mm diameter was chosen for this mirror plate. A reduced resonance frequency of 500 Hz was chosen to improveFigure 13.the SNR.A compact This actuator FTS using was type designed A MEMS for micromirror. a stroke of (Reprinted500 μm, greatly with permission increasing from spectral Figure 13. A compact FTS using type A MEMS micromirror. (Reprinted with permission from resolutionReference to 10 [37 ]cm SPIE).-1 theoretically. However, only 280 μm displacement could be measured at a Reference [37] SPIE). maximum under the influence of superimposed parasitic torsional oscillation modes. Based on the design of type A translation MEMS micromirror, an improved design of type B, applying two pantograph-like spring mechanisms for mirror suspension was realized (see Figure 14) in 2007. In order to acquire a sufficiently large aperture for optical throughput, circular shape of 3 mm diameter was chosen for this mirror plate. A reduced resonance frequency of 500 Hz was chosen to improve the SNR. This actuator was designed for a stroke of 500 μm, greatly increasing spectral resolution to 10 cm-1 theoretically. However, only 280 μm displacement could be measured at a maximum under the influence of superimposed parasitic torsional oscillation modes.

Figure 14. 14. SEMSEM and suspension and suspension layout layoutof type ofB transl typeational B translational MEMS micromirro MEMSr, micromirror, using pantograph- using pantograph-typetype suspensions. suspensions. (Reprinted with (Reprinted permission with from permission Reference from [37] Reference SPIE). [37] SPIE). An optimized translator MEMS with a new version of pantograph design was later developed An optimized translator MEMS with a new version of pantograph design was later developed by Sandner et al. in 2014 with a displacement of 1.2 mm at 30 Pa and 50 V environment [38]. It by Sandner et al. in 2014 with a displacement of 1.2 mm at 30 Pa and 50 V environment [38]. It was was designed to enable a spectral resolution of 8 cm 1 for a wavelength range 2.5 µm to 16 µm at designed to enable a spectral resolution of 8 cm−1 for a− wavelength range 2.5 μm to 16 μm at 500 Hz 500 Hz scan rate using a 5 mm diameter round micromirror. The modified translatory MEMS actuator scan rate using a 5 mm diameter round micromirror. The modified translatory MEMS actuator was was made up of four pantograph suspensions in the fourfold rotationally symmetric configuration made up of four pantograph suspensions in the fourfold rotationally symmetric configuration in in contrastFigure 14. to SEM two and pantographs suspension usedlayout forof type previous B transl typeational B MEMS design micromirro (see Figurer, using 15a). pantograph- It was driven contrast to two pantographs used for previous type B design (see Figure 15a). It was driven electrostaticallytype suspensions. at resonance (Reprinted by with in-plane permission vertical from comb Reference drives placed[37] SPIE). at each of the four pantographs to increase the driving efficiency (see Figure 15b). This MEMS device has been encapsulated in a hybrid opticalAn vacuumoptimized package translator (see MEMS Figure with15c) toa new reduce vers theion significantof pantograph viscous design gas was damping later developed in normal byambient. Sandner While et al. at in the 2014 normal with atmospherica displacement pressure, of 1.2 themm driving at 30 Pa voltage and 50 of V 40 environment V could only [38]. produce It was a −1 designeddisplacement to enable of about a spectral 160 µm resolution at resonance. of 8 The cm large for sizea wavelength of the electrostatic range 2.5 MEMS μm to micromirror 16 μm at 500 could Hz scanbe helpful rate using in improving a 5 mm diameter the displacement round micromirro output. However,r. The modified a large mirror translatory of 5 mm MEMS produced actuator 200 was nm madedistortion up of in four the drivingpantograph process, suspensions which aff inected the thefourfold divergence rotationally of the lightsymmetric beam. configuration In addition, thein contrastcomb structure to two usually pantographs occupied used a large for spaceprevious and seriouslytype B design affected (see the Figure fill-factor 15a). of theIt micromirror,was driven which also indicated that the displacement generated by the unit drive area was relatively low. Micromachines 2020, 11, x 11 of 26 electrostatically at resonance by in-plane vertical comb drives placed at each of the four pantographs to increase the driving efficiency (see Figure 15b). This MEMS device has been encapsulated in a hybrid optical vacuum package (see Figure 15c) to reduce the significant viscous gas damping in normal ambient. While at the normal atmospheric pressure, the driving voltage of 40 V could only produce a displacement of about 160 μm at resonance. The large size of the electrostatic MEMS micromirror could be helpful in improving the displacement output. However, a large mirror of 5 mm produced 200 nm distortion in the driving process, which affected the divergence of the light beam. In addition, the comb structure usually occupied a large space and seriously affected the fill- factorMicromachines of the2020 micromirror,, 11, 214 which also indicated that the displacement generated by the unit drive12 area of 28 was relatively low.

(a) (b) (c)

Figure 15.15.A A large large displacement displacement vertical vertical electrostatic electros comb-driventatic comb-driven MEMS micromirrors: MEMS micromirrors: (a) Photograph (a) Photographof new MEMS of devicenew MEMS at 400 deviceµm mechanical at 400 μm predeﬂected; mechanical (bpredeflected;) modiﬁed MEMS (b) modified design withMEMS D = design5 mm withand aD stroke = 5 mm of and 1 mm; a stroke (c) MEMS of 1 mm; vacuum (c) MEMS package vacuum with package ZnSe window. with ZnSe (Reprinted window. with(Reprinted permission with permissionfrom Reference from [ 38Reference] SPIE). [38] SPIE).

Ataman et al. reported reported An An out-of-plane out-of-plane vertical vertical comb-drive actuator for Lamellar grating based FTS in 2006 [39,40]. [39,40]. Two Two sets sets of of electrostatic electrostatic comb comb fingers fingers were were simultaneously applied as an actuator and a variable depth diffraction diffraction grating (see Figure 1616a–c).a–c). In In order to enhance the robustness, the comb fingers fingers were symmetrically placed on a rigidrigid backbone. The The length length and and width width of the comb fingersfingers forfor bothboth staticstatic and and movable movable were were 1.2 1.2 mm mm and and 70 µ70m, μ respectively,m, respectively, where where the gap the betweengap between them themwas 5 wasµm. 5 A μ highm. A width-to-gap high width-to-gap of the comb of the fingers comb realizedfingers realized a high fill a factorhigh fill and factor well opticaland well effi opticalciency, efficiency,but the grating but periodthe grating was alsoperiod increased, was also which incr ledeased, to smaller which separation led to smaller between separation diffraction between orders. diffractionThe movable orders. fingers The were movable carried fingers by 250 wereµm carried wide H-shaped by 250 μm backbone wide H-shaped connected backbone to the fixed connected frame tothrough the fixed four frame folded through flexure four beam. folded The foldedflexure flexurebeam. The structure folded (see flexure Figure structure 16d) had (see uniform Figure stress 16d) haddistribution uniform and stress could distribution provide low and sti ffcouldness within provide a compact low stiffness structure within by varying a compact the cross-sectional structure by varyingwidth. Only the cross-sectional the moving part width. of the MEMSOnly the device movi wasng thepart resonating of the MEMS structure. device Out-of-plane was the resonating resonant structure.mode operation Out-of-plane of the combresonant actuator mode couldoperation generate of the a comb peak-to-peak actuator could 106 µ mgenerate deflection a peak-to-peak in ambient pressure under 28 V square-wave excitation, and could provide a large clear aperture of 3 mm 3 mm 106 μm deflection in ambient pressure under 28 V square-wave excitation, and could provide× a large clearfor the aperture incident of beam.3 mm × The3 mm Lamellar for the incident grating beam based. The FTS Lamellar was constructed grating based by employing FTS was constructed the MEMS bydevice, employing a single the detector, MEMS device, and an a electronic single detector, circuit and for processing an electronic the circuit detector for output.processing The the theoretical detector output.spectral The resolution theoretical of this spectral FTS was resolution 0.4 nm inof the this visible FTS was range 0.4 andnm 3.2in the nm visible at telecom range wavelengths. and 3.2 nm at telecomSeren wavelengths. et al. implemented another out-of-plane resonant mode electrostatic MEMS for Lamellar grating based FTS in 2010 [41]. The device had a clear aperture of 10 mm 10 mm. The fabricated × MEMS consisted of fixed and movable parallel fingers, which operated as a variable depth grating (see Figure 17). This actuator transferred the motion to micromirror through four pantograph structures. Two comb structures were carried by either side of each pantograph to motivate the device. In addition, the width of fingers and gap between fingers for grating were kept constant. This device was actuated through both comb fingers and grating fingers which were placed at the pantographs to make the electrostatic force maximization. A maximum peak to peak deflection of 355 µm was achieved under 76 V and 971 Hz excitation environment.

Micromachines 2020, 11, x 12 of 26

Micromachines 2020, 11, x 12 of 26 Micromachines 2020, 11, 214 13 of 28

(a) (b) (c)

( d) (a) Figure 16. An electrostatic actuator for Lamellar grating(b) based FTS: (a) FTS structure schematic:(c) (1) Fixed fingers, (2) Movable fingers, (3) Rigid backbone, (4) Folded flexures; (b) fabricated MEMS device; (c) packaged device; (d) closed-up view of the serpentine flexure. (Reprinted with permission from References [39,40] IOP Publishing, Ltd.).

Seren et al. implemented another out-of-plane resonant mode electrostatic MEMS for Lamellar grating based FTS in 2010 [41]. The device had a clear aperture of 10 mm × 10 mm. The fabricated MEMS consisted of fixed and movable parallel fingers, which operated as a variable depth grating (see Figure 17). This actuator transferred the motion to micromirror(d) through four pantograph structures. Two comb structures were carried by either side of each pantograph to motivate the FigureFigure 16.16. An electrostatic actuator actuator for for Lamellar Lamellar grating grating based based FTS: FTS: (a ()a FTS) FTS structure structure schematic: schematic: (1) device. In addition, the width of fingers and gap between fingers for grating were kept constant. This (1)Fixed Fixed fingers, fingers, (2) (2) Movable Movable fingers, fingers, (3) (3) Rigid Rigid backbone, backbone, (4) Folded flexures;flexures; ((bb)) fabricatedfabricated MEMS MEMS devicedevice;device; was ( cactuated()c) packaged packaged through device; device; ( d(bothd)) closed-up closed-up comb view fingersview of of the theand serpentine serpentine grating flexure. flfingersexure. (Reprinted (Reprinted which were with with permission placedpermission at the pantographsfromfrom References References to make [ 39[39,40] the,40] electrostatic IOP IOP Publishing, Publishing, force Ltd.). Ltd.). maximi zation. A maximum peak to peak deflection of 355 μm was achieved under 76 V and 971 Hz excitation environment. Seren et al. implemented another out-of-plane resonant mode electrostatic MEMS for Lamellar grating based FTS in 2010 [41]. The device had a clear aperture of 10 mm × 10 mm. The fabricated MEMS consisted of fixed and movable parallel fingers, which operated as a variable depth grating (see Figure 17). This actuator transferred the motion to micromirror through four pantograph structures. Two comb structures were carried by either side of each pantograph to motivate the device. In addition, the width of fingers and gap between fingers for grating were kept constant. This device was actuated through both comb fingers and grating fingers which were placed at the pantographs to make the electrostatic force maximization. A maximum peak to peak deflection of 355 μm was achieved under 76 V and 971 Hz excitation environment.

Figure 17. SEM picture of the fabricated deﬂected device. (Reprinted with permission from Figure 17. SEM picture of the fabricated deflected device. (Reprinted with permission from Reference Reference [41] IEEE). [41] IEEE). 3.1.3. Summary 3.1.3. Summary Table1 summarizes several parameters of electrostatic MEMS micromirrors and FTS. The electrostaticTable 1 summarizes actuators haveseveral fast parameters response speed. of el However,ectrostatic high MEMS voltage micromirrors is required, and and hundredsFTS. The electrostaticof volts only actuators produce have several fasttens response of micrometers. speed. However, Moreover, high voltage high voltage is required, introduces and hundreds electronics of voltscomplexity only produce and material several compatibility tens of micrometers. issues. The Moreover, electrostatic high mechanisms voltage introduces commonly electronics operate in resonant frequency and vacuum encapsulation to increase the displacement. On the other hand, pull-in occurs in electrostatic actuators, which may lead to irreversible damages for short circuit, arcing and surfaceFigure bonding. 17. SEM The picture pull-in of the behavior fabricated prevents deflected the device displacement. (Reprinted with of an permission actuator from Reference reaching its full-gap[41] allowable IEEE). range, so that only 1/3 of the initial gap size could be used.

3.1.3. Summary Table 1 summarizes several parameters of electrostatic MEMS micromirrors and FTS. The electrostatic actuators have fast response speed. However, high voltage is required, and hundreds of volts only produce several tens of micrometers. Moreover, high voltage introduces electronics

Micromachines 2020, 11, 214 14 of 28

Table 1. Comparison of electrostatic micro-electromechanical system (MEMS) micromirrors and Fourier transform spectrometers (FTS).

Authors Institution Actuation Type Core of FTS Displacement Work Condition Device Size Reference MEMS chip: 5 4 mm2 Manzardo et al. UniNE In-plane MI 77 µm 10 V–amplitude × [7] (Mirror: 75 500 µm2) × Manzardo et al. UniNE In-plane LGI 145 µm 65 V MEMS chip: 5 5 mm2 [19] × ARCoptix and Merenda et al. In-plane LGI >500 µm Entire FTS: 10 15 7 cm3 [30] EPFL − × × SNU, Stanford, Yu et al In-plane MI 25 µm 150 V @ 5 Hz Entire FTS: 4 8 0.6 mm3 [31] and SNL × × Khalil et al. ASU and SWS In-plane MI 48 µm @ resonance - [32,33] Khalil et al. ASU and SWS In-plane MZI 62.5 µm 70 V @ resonance Entire FTS: 1 2 mm2 [33] × Mortada et al. SWS, EP, and ASU In-plane MI 62.5 µm or 200 µm [35] − − Eltagoury et al. ASU and SWS In-plane FPI [36] − − − 40 V @ 100 Pa MEMS chip: 1.8 9 mm2 Sandner et al. IPMS Out-of-plane 200 µm × [37] − vacuum, 5 kHz (Mirror: 1.5 1.1 mm2) × Sandner et al. IPMS Out-of-plane 500 µm 500 Hz Aperture: 3 mm in diameter [37] − 50 V @ 30 Pa Sandner et al. IPMS Out-of-plane 1.2 mm Aperture: 5 mm in diameter [38] − vacuum, 500 Hz Ataman et al. KU and IPMS Out-of-plane LGI 106 µm 28 V @ resonance Aperture: 3 3 mm2 [39,40] × Seren et al. KU and IPMS Out-of-plane LGI 355 µm 76 V @ 971 Hz Aperture: 10 10 mm2 [41] × (UniNE: University of Neuchâtel; EPFL: Ecole Polytechnique Federale de Lausanne; SNU: Seoul National University; SNL: Sandia National Lab; ASU: Ain-Shams University; SWS: Si-Ware Systems; EP: ESIEE Paris; IPMS: Fraunhofer Institute for Photonic Microsystems; KU: Koc University; MI: Michelson Interferometer; LGI: Lamellar Grating Interferometer; MZI: Mach-Zehnder Interferometer; FPI: Fabry-Perot Interferometer). Micromachines 2020, 11, 214 15 of 28

3.2. Electromagnetic MEMS Based FTS Electromagnetic based MEMS mirrors typically have such characteristics as relatively large displacement, low voltage, and high speed [27]. Magnetization is a phenomenon when a magnetic field causes internal magnetic polarization of a piece of magnetic material within the field. A piece of magnetic material comprises magnetic domains. Each magnetic domain involves a magnetic dipole. The intensity of the internal magnetization of the magnetic material relies on the ordering of these domains. A net internal magnetic field of the magnetic material is produced via these domains, if they are slightly aligned. The magnetic field may come from a permanent magnet, an integrated coil, an external solenoid, or their hybrid manner. Ferromagnetic materials (e.g., iron (Fe), nickel (Ni), or permalloy (Fe-Ni)) are often applied in MEMS actuators [27]. The force-generating structure of an electromagnetic microactuator is commonly placed on a chip, which can be a permanent magnet (hard ferromagnet), a soft ferromagnet, or an integrated coil. When an external magnetic field is present, an internal magnetization is generated in the ferromagnet. Hard ferromagnets can retain certain magnetic polarization when there is no external magnetic driving field, while soft ferromagnets can exhibit internal magnetization only when it is placed in a biasing, external magnetic field. Lorentz-type and magnetic pole type are two kinds of electromagnetic MEMS micromirrors for FTS, which are detailed below. Micromachines 2020, 11, x 14 of 26 3.2.1. Lorentz-Type Electromagnetic MEMS Micromirrors and FTS 3.2.1. Lorentz-Type Electromagnetic MEMS Micromirrors and FTS When a current is injected into a conductive coil which is perpendicular to an external magnetic field,When the coil a current will experience is injected a magneticinto a conductive force (Lorenz coil which force). is perpendicular The magnetic forceto an external can be applied magnetic to producefield, the either coil will in-plane experience or out-of-plane a magnetic linear force motion. (Lorenz The force). Lorenz The force magnetic varies force with can the inputbe applied current. to Coil-basedproduce either magnetic in-plane actuation or out-of-plane can generate linear either moti pistonon. The motion Lorenz or force rotation varies depending with the input on the current. spring designCoil-based and magnetic the external actuation magnetic can field generate direction either (see piston Figure motion 18). or rotation depending on the spring design and the external magnetic field direction (see Figure 18).

Figure 18. Coil-based magnetic actuators. Figure 18. Coil-based magnetic actuators. Wallrabe et al. reported an electromagnetic actuator driven large displacement movable Wallrabe et al. reported an electromagnetic actuator driven large displacement movable micromirror using a LIGA fabrication process in 2005 [42]. This electromagnetic micromirror, a micromirror using a LIGA fabrication process in 2005 [42]. This electromagnetic micromirror, a collimator, a photodetector and other optical elements were integrated together on a single substrate collimator, a photodetector and other optical elements were integrated together on a single substrate (see Figure 19)[8,43], forming a miniature FTS optical platform with a footprint of 11.5 mm 9.4 mm. (see Figure 19) [8,43], forming a miniature FTS optical platform with a footprint of 11.5 mm ×× 9.4 mm. The actuator had two assembled microcoils with soft magnetic Fe-Ni alloy cores. The movable plunger The actuator had two assembled microcoils with soft magnetic Fe-Ni alloy cores. The movable was suspended by a set of four folded cantilever beams, and the coils were employed at either end of plunger was suspended by a set of four folded cantilever beams, and the coils were employed at the plunger. The plunger could be pulled in both directions, thus, doubling the travel range. either end of the plunger. The plunger could be pulled in both directions, thus, doubling the travel The produced magnetic force was related to the number of windings in the coil. With 300 windings, range. the maximum displacement was about 110 µm at 12 mW. When the input power exceeds 12 mW, the actuator started to exhibit instability, due to the occurrence of pull-in behavior, which greatly limited the useful displacement of the movable mirror. The available displacement of the actuator was only 54 µm, and an 850 nm laser was applied to measure the mirror displacement. The spectral resolution of this FTS was 24.5 nm at 1540 nm. Although this electromagnetic actuator achieved a large scan range and large optical aperture, due to the presence of a pull-in behavior, only a fraction of the full scan range could be used. In addition, the LIGA process is very expensive and diﬃcult to access.

Figure 19. Top view of an electromagnetically actuated miniature FTS using the LIGA process. (Reprinted with permission from Reference [43] Elsevier).

The produced magnetic force was related to the number of windings in the coil. With 300 windings, the maximum displacement was about 110 μm at 12 mW. When the input power exceeds 12 mW, the actuator started to exhibit instability, due to the occurrence of pull-in behavior, which greatly limited the useful displacement of the movable mirror. The available displacement of the actuator was only 54 μm, and an 850 nm laser was applied to measure the mirror displacement. The spectral resolution of this FTS was 24.5 nm at 1540 nm. Although this electromagnetic actuator achieved a large scan range and large optical aperture, due to the presence of a pull-in behavior, only a fraction of the full scan range could be used. In addition, the LIGA process is very expensive and difficult to access. An electromagnetically driven Lamellar grating based FTS was developed by Yu et al. in 2008 [44]. The electromagnetic actuator was applied to drive the movable surfaces of the lamellar grating to move bi-directionally (see Figure 20a,b). The size of the permanent magnet was 700 μm × 700 μm × 500 μm, which was placed perpendicular to the central platform. The actuation of this FTS was achieved by a driving coil positioned under the device. When applying 129 mA-amplitude currents

Micromachines 2020, 11, x 14 of 26

3.2.1. Lorentz-Type Electromagnetic MEMS Micromirrors and FTS When a current is injected into a conductive coil which is perpendicular to an external magnetic field, the coil will experience a magnetic force (Lorenz force). The magnetic force can be applied to produce either in-plane or out-of-plane linear motion. The Lorenz force varies with the input current. Coil-based magnetic actuation can generate either piston motion or rotation depending on the spring design and the external magnetic field direction (see Figure 18).

Figure 18. Coil-based magnetic actuators.

Wallrabe et al. reported an electromagnetic actuator driven large displacement movable micromirror using a LIGA fabrication process in 2005 [42]. This electromagnetic micromirror, a collimator, a photodetector and other optical elements were integrated together on a single substrate (see Figure 19) [8,43], forming a miniature FTS optical platform with a footprint of 11.5 mm × 9.4 mm. The actuator had two assembled microcoils with soft magnetic Fe-Ni alloy cores. The movable plunger was suspended by a set of four folded cantilever beams, and the coils were employed at Micromachines 2020, 11, 214 16 of 28 either end of the plunger. The plunger could be pulled in both directions, thus, doubling the travel range.

Figure 19. TopTop view of an electromagnetically actuat actuateded miniature miniature FTS FTS using using the the LIGA LIGA process. process. (Reprinted with permission from Reference [[43]43] Elsevier).Elsevier).

TheAn electromagneticallyproduced magnetic driven force Lamellar was related grating to basedthe number FTS was of developed windings byin Yuthe et coil. al. in With 2008 [30044]. windings,The electromagnetic the maximum actuator displacement was applied was toabout drive 110 the μm movable at 12 mW. surfaces When of the the input lamellar power grating exceeds to move bi-directionally (see Figure 20a,b). The size of the permanent magnet was 700 µm 700 µm 12 mW, the actuator started to exhibit instability, due to the occurrence of pull-in behavior,× which 500 µm, which was placed perpendicular to the central platform. The actuation of this FTS was greatlyMicromachines× limited 2020 ,the 11, xuseful displacement of the movable mirror. The available displacement 15of ofthe 26 actuatorachieved was by a only driving 54 μ coilm, and positioned an 850 undernm laser the was device. applied When to applyingmeasure 129the mA-amplitudemirror displacement. currents The to the actuation coil, a large deflection of 125 µm was produced. The resultant force varied linearly with spectralto the actuation resolution coil, of a thislarge FTS deflection was 24.5 of 125nm μatm 1540 was nm.produced. Although The resultantthis electromagnetic force varied actuator linearly the injecting current, which greatly simplified control procedures. Moreover, this design avoids pull-in achievedwith the injecting a large scan current, range which and large greatly optical simplified aperture, control due toprocedures. the presence Moreover, of a pull-in this behavior, design avoids only instability so that larger deflections (hundreds of microns) can be generated only by increasing the apull-in fraction instability of the full so thatscan larger range deflections could be used. (hundreds In addition, of microns) the LIGA can be process generated is very only expensive by increasing and applied current. The measured spectral resolution was 3.8 nm at 632.8 nm and 3.44 nm at 532 nm. difficultthe applied to access. current. The measured spectral resolution was 3.8 nm at 632.8 nm and 3.44 nm at 532 nm. An electromagnetically driven Lamellar grating based FTS was developed by Yu et al. in 2008 [44]. The electromagnetic actuator was applied to drive the movable surfaces of the lamellar grating to move bi-directionally (see Figure 20a,b). The size of the permanent magnet was 700 μm × 700 μm × 500 μm, which was placed perpendicular to the central platform. The actuation of this FTS was achieved by a driving coil positioned under the device. When applying 129 mA-amplitude currents

(a) (b)

Figure 20. An electromagnetically driven Lamellar grating grating based based FTS: FTS: ( (aa)) SEM; SEM; ( (bb)) microscope microscope image. image. (Reprinted with permission fromfrom ReferenceReference [[44]44] IOPIOP Publishing,Publishing, Ltd.).Ltd.).

Baran et al. proposed proposed an an electromagnetically electromagnetically actuated actuated Flame-retardant-4 Flame-retardant-4 (FR4) scanner replacing the movable mirror in MichelsonMichelson interferometer in 2011 [[45].45]. FR4 was a composite material which has been used for PCB with well-engineered electrical,electrical, mechanical and thermal properties. The FR4 actuator (see Figure 2121a)a) could enhance the out-of-plane translationtranslation when applying two opposing magnets approach to suppress the torsion eeffects.ffects. The central movable part was anchored via four flexuresflexures to produce out-of-plane resonance frequency (see Figure 2121b).b). The serpentine shape flexures flexures were used to increase torsion mode stistiffness,ffness, andand decreasedecrease out-of-planeout-of-plane mode stistiffness.ffness. The mirror tilting was suppressed by applying a corner cube reflector.reflector. ThisThis actuatoractuator couldcould generategenerate ± 162.8 μµm ± out-of-plane translationtranslation at at 149 149 Hz Hz resonate resonate mode mode with with 120 mVpp120 mVpp applied applied voltage, voltage, leading leading to a spectral to a spectralresolution resolution<1 nm. The <1 nm. whole The device whole was device 7 cm was8 cm7 cm with × 8 a cm mirror with size a mirror of 1 cm size1 of cm 1 (seecm × Figure 1 cm 21(seec). × × FigureHowever, 21c). since However, the FR4 since material the wasFR4 extremelymaterial was soft, extremely the actuator soft, could the notactuator stop under could suchnot stop big current under suchandcould big current not come and back could to thenot originalcome back point, to whichthe original dramatically point, which decreases dramatically the accuracy. decreases the accuracy.

(a)

(b) (c)

Figure 21. Design of the FR4 Lorentz-type electromagnetic actuator: (a) Side view of the system; (b) top view of the system; (c) fully assembled device. (Reprinted with permission from Reference [45] AMER SOC MECHANICAL ENGINEERS).

Micromachines 2020, 11, x 15 of 26

to the actuation coil, a large deflection of 125 μm was produced. The resultant force varied linearly with the injecting current, which greatly simplified control procedures. Moreover, this design avoids pull-in instability so that larger deflections (hundreds of microns) can be generated only by increasing the applied current. The measured spectral resolution was 3.8 nm at 632.8 nm and 3.44 nm at 532 nm.

(a) (b)

Figure 20. An electromagnetically driven Lamellar grating based FTS: (a) SEM; (b) microscope image. (Reprinted with permission from Reference [44] IOP Publishing, Ltd.).

Baran et al. proposed an electromagnetically actuated Flame-retardant-4 (FR4) scanner replacing the movable mirror in Michelson interferometer in 2011 [45]. FR4 was a composite material which has been used for PCB with well-engineered electrical, mechanical and thermal properties. The FR4 actuator (see Figure 21a) could enhance the out-of-plane translation when applying two opposing magnets approach to suppress the torsion effects. The central movable part was anchored via four flexures to produce out-of-plane resonance frequency (see Figure 21b). The serpentine shape flexures were used to increase torsion mode stiffness, and decrease out-of-plane mode stiffness. The mirror tilting was suppressed by applying a corner cube reflector. This actuator could generate ± 162.8 μm out-of-plane translation at 149 Hz resonate mode with 120 mVpp applied voltage, leading to a spectral resolution <1 nm. The whole device was 7 cm × 8 cm with a mirror size of 1 cm × 1 cm (see Figure 21c). However, since the FR4 material was extremely soft, the actuator could not stop under Micromachinessuch big current2020, 11 ,and 214 could not come back to the original point, which dramatically decreases17 of the 28 accuracy.

(a)

(b) (c)

MicromachinesFigureFigure 2020 21. ,21. 11Design ,Design x of of the the FR4 FR4 Lorentz-type Lorentz-type electromagnetic electromagnetic actuator: actuator: (a) ( Sidea) Side view view of the of system;the system; (b) top (b) 16 of 26 viewtop view of the of system; the system; (c) fully (c) assembledfully assembled device. device. (Reprinted (Reprinted with permission with permission from Reference from Reference [45] AMER [45] 3.2.2. MagneticSOCAMER MECHANICAL Pole-TypeSOC MECHANICAL Electrom ENGINEERS). ENGINEERS).agnetic MEMS Micromirors and FTS Magnetic3.2.2. Magnetic pole-type Pole-Type electromagnetic Electromagnetic MEMS MEMS Micromirorsactuators (see and FTSFigure 22) with magnetic materials are depositedMagnetic on the pole-type movable electromagnetic parts. The MEMS magnetic actuators poles (see are Figure generated 22) with at magnetic the ends materials of the are magnetic materialdeposited coated on movable the movable part parts. when The an magnetic external poles magnet are generatedic field atis theapplied. ends of An the magneticinteractive material force, either attractivecoated or movable repulsive part force, when is an then external generated magnetic be fieldtween is applied. the movable An interactive part and force, the either magnetic attractive field. Such or repulsive force, is then generated between the movable part and the magnetic field. Such actuation actuation can be in-plane or out-of-plane regarding the external field direction. can be in-plane or out-of-plane regarding the external field direction.

Figure 22. Electromagnetic actuators with magnetic materials formed on the movable structure. Xue et al. introduced a large displacement with high surface quality in-plane electromagnetically Figure 22. Electromagnetic actuators with magnetic materials formed on the movable structure. actuated translation micromirror for FTS in 2016 [46]. The actuator comprised a nickel film (see Figure 23a) fabricated by MetalMUMPs [47], and a solenoid located underneath the film. The nickel Xue et al. introduced a large displacement with high surface quality in-plane electromagnetically film could curve up via the residual stress gradient and a curve-up mechanism, including four actuatedtrapezoidal translation plates micromirror and anchoring for springs. FTS in A 2016 size of[46]. 2 mm The actuator2 mm mirror comprised plate was a connectednickel film to (see the Figure × 23a) fabricatedcentral ring by of theMetalMUMPs nickel film (see [47], Figure and 23 b).a solenoid The translation located was underneath realized by a the solenoid film. attractingThe nickel film couldthe curve nickel up film via alongthe residual with the stress mirror gradient plate downwards. and a curve-up A quasi-static mechanism, displacement including of 123 fourµm trapezoidal was platesgenerated and anchoring at 400 mA. springs. The mirror A size achieved of 2 mm a high × 2 surface mm mirror quality plate with 15.6was nm connected curvature to radius the central and ring of the2 nickel nm surface film roughness.(see Figure23b). The translation was realized by a solenoid attracting the nickel film along with the mirror plate downwards. A quasi-static displacement of 123 μm was generated at 400 mA. The mirror achieved a high surface quality with 15.6 nm curvature radius and 2 nm surface roughness.

(a) (b)

Figure 23. Prototype pf the translation micromirror: (a) Nickel film; (b) nickel film bonded with the mirror plate. (Reprinted with permission from Reference [46] IOP Publishing, Ltd.).

A repulsive magnetic force driven out-of-plane micromirror with large displacement and high surface quality was later developed for FTS by Xue et al. in 2017 [48]. This design did not need the residual stress gradients to curl up the moving film. It applied a permanent magnet ring above and an electromagnet underneath the moving film (see Figure 24b) to generate repulsive magnetic force for moving the film to generate out-of-plane motion. The moving film (fabricated by MetalMUMPs [47]) was connected to a 2 mm × 2 mm mirror plate using non-touching bonding technology (see Figure 24a). The micromirror had a radius of curvature of 9.2 m and surface roughness of 12 nm. The maximum displacement of 144 μm was achieved when 140 mA was injected into the electromagnet. However, only 30 μm translation could be used in FTS, due to 0.3° tilting. The measurement accuracy of 2.19 % for a 532 nm laser beam was obtained.

Micromachines 2020, 11, x 16 of 26

3.2.2. Magnetic Pole-Type Electromagnetic MEMS Micromirors and FTS Magnetic pole-type electromagnetic MEMS actuators (see Figure 22) with magnetic materials are deposited on the movable parts. The magnetic poles are generated at the ends of the magnetic material coated movable part when an external magnetic field is applied. An interactive force, either attractive or repulsive force, is then generated between the movable part and the magnetic field. Such actuation can be in-plane or out-of-plane regarding the external field direction.

Figure 22. Electromagnetic actuators with magnetic materials formed on the movable structure.

Xue et al. introduced a large displacement with high surface quality in-plane electromagnetically actuated translation micromirror for FTS in 2016 [46]. The actuator comprised a nickel film (see Figure 23a) fabricated by MetalMUMPs [47], and a solenoid located underneath the film. The nickel film could curve up via the residual stress gradient and a curve-up mechanism, including four trapezoidal plates and anchoring springs. A size of 2 mm × 2 mm mirror plate was connected to the central ring of the nickel film (see Figure23b). The translation was realized by a solenoid attracting the nickel film along with the mirror plate downwards. A quasi-static displacement of 123 μm was generated at 400

mA. TheMicromachines mirror 2020achieved, 11, 214 a high surface quality with 15.6 nm curvature radius and 2 18nm of 28 surface roughness.

(a) (b)

FigureFigure 23. Prototype 23. Prototype pf the pf thetranslation translation micromirror: micromirror: ((aa)) NickelNickel ﬁlm; film; (b )( nickelb) nickel ﬁlm film bonded bonded with the with the mirror mirrorplate. plate.(Reprinted (Reprinted with with permission permission from from Reference Reference [ 46[46]] IOP IO Publishing,P Publishing, Ltd.). Ltd.).

A repulsive magnetic force driven out-of-plane micromirror with large displacement and high A repulsive magnetic force driven out-of-plane micromirror with large displacement and high surface quality was later developed for FTS by Xue et al. in 2017 [48]. This design did not need the surfaceresidual quality stress was gradients later developed to curl up thefor movingFTS by film. Xue It et applied al. in a 2017 permanent [48]. This magnet design ring above did not and anneed the residualelectromagnet stress gradients underneath to curl the up moving the moving film (see film. Figure It 24appliedb) to generate a permanent repulsive magnet magnetic ring force above for and an electromagnetmoving the film underneath to generate the out-of-plane moving motion.film (see The Figure moving 24b) film to (fabricated generate by repulsive MetalMUMPs magnetic [47]) force was connected to a 2 mm 2 mm mirror plate using non-touching bonding technology (see Figure 24a). for moving the film to generate× out-of-plane motion. The moving film (fabricated by MetalMUMPs [47]) wasThe connected micromirror to had a a2radius mm × of 2 curvature mm mirror of 9.2 plat m ande using surface non-touching roughness of 12 bonding nm. The technology maximum (see displacement of 144 µm was achieved when 140 mA was injected into the electromagnet. However, Figure 24a). The micromirror had a radius of curvature of 9.2 m and surface roughness of 12 nm. The only 30 µm translation could be used in FTS, due to 0.3◦ tilting. The measurement accuracy of 2.19 % maximum displacement of 144 μm was achieved when 140 mA was injected into the electromagnet. Micromachinesfor a 532 2020 nm, 11 laser, x beam was obtained. 17 of 26 However, only 30 μm translation could be used in FTS, due to 0.3° tilting. The measurement accuracy of 2.19 % for a 532 nm laser beam was obtained.

(a) (b)

FigureFigure 24. Concept 24. Concept of the of themicromirror: micromirror: (a ()a )Moving Moving filmfilm ofof the the micromirror; micromirror; (b) structure(b) structure of the of the micromirror.micromirror. (Reprinted (Reprinted with with permission permission fr fromom Reference Reference [ 48[48]] IOP IO Publishing,P Publishing, Ltd.). Ltd.). 3.2.3. Summary 3.2.3. Summary Table2 summarizes several parameters of electromagnetic MEMS micromirrors and FTS. TableElectromagnetic 2 summarizes actuators several designed parameters for FTS exhibit of el largerectromagnetic displacement MEMS and lower micromirrors response speed and FTS. Electromagneticcompared to actuators electrostatic designed actuators. for However, FTS exhibit electromagnetic larger displacement actuators typically and havelower larger response device speed comparedsize toto holdelectrostatic the external actuators. electromagnetic However, field, elec thus,tromagnetic reducing its compactness.actuators typically At the same have time, larger the device size tofabrication hold the external may cause electromagnetic asymmetry of the field, electromagnetic thus, reducing actuation its compactness. structure, and the At magnetic the same field time, the encapsulation is not precisely coaxial, leading to the tilting of the MEMS micromirror. fabrication may cause asymmetry of the electromagnetic actuation structure, and the magnetic field encapsulation is not precisely coaxial, leading to the tilting of the MEMS micromirror.

Table 2. Comparison of electromagnetic MEMS micromirrors and FTS.

Actuation Core of Work Authors Institution Displacement Device Size Reference Type FTS Condition Wallrabe Uni Freiburg Lorentz- Entire FTS: MI 110 μm 12 mW [8,43] et al. and FK type 11.5 × 9.4 mm2 NUS and Lorentz- 129 mA– Yu et al. LGI 125 μm − [44] DSI type amplitude MEMS chip: 7 Baran et Lorentz- 120 mVpp @ × 8 cm2 KU MI 325.6 μm [45] al. type 149 Hz (Mirror: 1 × 1 cm2) Magnetic Mirror: 2 × 2 Xue et al. RU − 123 μm 400 mA. [46] pole-type mm2 Magnetic Mirror: 2 × 2 Xue et al. RU − 144 μm 140 mA [48] pole-type mm2 (Uni Freiburg: University of Freiburg; FK: Forschungszentrum Karlsruhe; NUS: National University of Singapore; DSI: Data Storage Institute of Singapore; KU: Koc University; RU: Ryerson University).

3.3. Electrothermal Actuation MEMS Based FTS Electrothermal actuators are generally based on electrothermal bimorph actuation which is realized by temperature variable via Joule heating induced from injecting an electrical current to a resistive heater embedded in a bimorph beam. A bimorph structure (see Figure 25) comprises two layers of materials with different coefficients of thermal expansion (CTEs). When the temperature changes, thermal stress is generated, due to the difference of the CTEs of the two bimorph layers. The beam curls toward the layer with a lower CTE value when the temperature rises; then, a transverse beam bending is generated [9]. The electrothermal MEMS micromirrors designed for FTS can also produce in-plane and out-of-plane motions, which are described below.

Micromachines 2020, 11, 214 19 of 28

Table 2. Comparison of electromagnetic MEMS micromirrors and FTS.

Actuation Work Device Authors Institution Core of FTS Displacement Reference Type Condition Size Entire FTS: Uni Freiburg Wallrabe et al. Lorentz-type MI 110 µm 12 mW 11.5 9.4 [8,43] and FK × mm2 129 Yu et al. NUS and DSI Lorentz-type LGI 125 µm [44] mA–amplitude − MEMS chip: 7 8 120 mVpp @ × Baran et al. KU Lorentz-type MI 325.6 µm cm2 [45] 149 Hz (Mirror: 1 1 cm2) × Magnetic Mirror: 2 Xue et al. RU 123 µm 400 mA. [46] pole-type − 2 mm2 × Magnetic Mirror: 2 Xue et al. RU 144 µm 140 mA [48] pole-type − 2 mm2 × (Uni Freiburg: University of Freiburg; FK: Forschungszentrum Karlsruhe; NUS: National University of Singapore; DSI: Data Storage Institute of Singapore; KU: Koc University; RU: Ryerson University).

3.3. Electrothermal Actuation MEMS Based FTS Electrothermal actuators are generally based on electrothermal bimorph actuation which is realized by temperature variable via Joule heating induced from injecting an electrical current to a resistive heater embedded in a bimorph beam. A bimorph structure (see Figure 25) comprises two layers of materials with different coefficients of thermal expansion (CTEs). When the temperature changes, thermal stress is generated, due to the difference of the CTEs of the two bimorph layers. The beam curls toward the layer with a lower CTE value when the temperature rises; then, a transverse beam bending is generated [9]. The electrothermal MEMS micromirrors designed for FTS can also produceMicromachines in-plane 2020, 11 and, x out-of-plane motions, which are described below. 18 of 26

FigureFigure 25.25. ElectrothermalElectrothermal bimorphbimorph structure.structure.

3.3.1.3.3.1. In-PlaneIn-Plane ElectrothermalElectrothermal MEMSMEMS MicromirrorMicromirror andand FTSFTS SinSin etet al.al. proposedproposed aa micromicro FTSFTS basedbased onon in-planein-plane electrothermalelectrothermal actuatoractuator inin 20062006 [[49].49]. AllAll thethe opticaloptical componentscomponents werewere fabricatedfabricated onon aa micro-opticalmicro-optical benchbench usingusing thethe DRIE process on an SOI wafer. V-beamV-beam shapeshape structuresstructures werewere applied as an electrothermal actuator. actuator. The The stroke stroke was was amplified amplified by by a alever lever mechanism mechanism connected connected to tothe the end end of ofthe the actuator actuator (see (see Figure Figure 26a). 26 a).A symmetric A symmetric structure structure of two of twoactuators actuators and and two two lever lever mechanisms mechanisms were were employed employed to to enhance enhance the the force force and and eliminate eliminate the stagestage rotation.rotation. TheThe sockets sockets were were female female mechanical mechanical flexure flexure structure structure that enables that enables a precise a mirror precise assembly. mirror Theassembly. mirror The could mirror be picked could up be bypicked a passive up by microgripper a passive microgripper by inserting by the insert grippering the tip intogripper the flexuretip into structure. The overall size of this interference platform was only 10 mm 10 mm (see Figure 26b), and the flexure structure. The overall size of this interference platform was× only 10 mm × 10 mm (see the area of the micromirror was about 0.5 mm 0.45 mm. A ball lens was used to collimate incoming Figure 26b), and the area of the micromirror ×was about 0.5 mm × 0.45 mm. A ball lens was used to lightcollimate when incoming the micro light FTS when was the combined micro FTS with was a fibercombined light sourcewith a fiber (see Figurelight source 26c). (see The Figure maximum 26c). µ displacementThe maximum of displacement the movable mirror, of the drivenmovable by mirror, a voltage driven of 22 Vby was a voltage about 30 of 22m, V corresponding was about 30 to μ 60m, corresponding to 60 μm OPD. The tilting angles of the mirror were from −2.5° to 0.8° during several experiments. The measured spectral resolution of this micro FTS was about 25 nm at 632 nm.

(a) (b) (c)

Figure 26. (a) An electrothermal scanning mechanism with a lever mechanism; (b) an assembled FTS; (c) an assembled FTS with a ball lens. (Reprinted with permission from Reference [49] SPIE).

Based on this micro FTS, another in-plane electrothermal actuator based FTS was developed by Das et al. in 2008 [50,51]. The whole device was still fabricated on a 10 mm × 10 mm die. The actuator involved 5 beams of 100 μm × 12 μm × 503 μm, generating a displacement of 45 μm under 45 V driving voltage and the vertical placed assembly gold-coated mirror was 1000 μm × 800 μm in size. The components of this FTS (see Figure 27) were different from the previous design, including two MEMS mirrors, two MEMS holders for ball lenses, two spherical glass lenses, one beam splitter, one single-mode optical fiber, and optionally, one SMT detector and one VCSEL laser source. The assembled FTS was carried out with precision robots using μ3 automated micro-assembly station (having 19 precision degrees of freedom). The new design was capable of two FTS instruments: One for the visible range (400–750 nm), the other for near infrared range (700–1800 nm). The calculated spectral resolution of two FTS devices was about 3.6 nm at 600 nm wavelength and 12 nm at 1100 nm wavelength, respectively.

Micromachines 2020, 11, x 18 of 26

Figure 25. Electrothermal bimorph structure.

3.3.1. In-Plane Electrothermal MEMS Micromirror and FTS Sin et al. proposed a micro FTS based on in-plane electrothermal actuator in 2006 [49]. All the optical components were fabricated on a micro-optical bench using the DRIE process on an SOI wafer. V-beam shape structures were applied as an electrothermal actuator. The stroke was amplified by a lever mechanism connected to the end of the actuator (see Figure 26a). A symmetric structure of two actuators and two lever mechanisms were employed to enhance the force and eliminate the stage rotation. The sockets were female mechanical flexure structure that enables a precise mirror assembly. The mirror could be picked up by a passive microgripper by inserting the gripper tip into the flexure structure. The overall size of this interference platform was only 10 mm × 10 mm (see MicromachinesFigure 26b),2020 and, 11 the, 214 area of the micromirror was about 0.5 mm × 0.45 mm. A ball lens was used20 of to 28 collimate incoming light when the micro FTS was combined with a fiber light source (see Figure 26c). The maximum displacement of the movable mirror, driven by a voltage of 22 V was about 30 μm, µm OPD. The tilting angles of the mirror were from 2.5 to 0.8 during several experiments. The corresponding to 60 μm OPD. The tilting angles of the− mirror◦ were◦ from −2.5° to 0.8° during several measured spectral resolution of this micro FTS was about 25 nm at 632 nm. experiments. The measured spectral resolution of this micro FTS was about 25 nm at 632 nm.

(a) (b) (c)

Figure 26. ((aa)) An An electrothermal electrothermal scanning scanning mech mechanismanism with a a lever lever mechanism; mechanism; ( (b)) an an assembled assembled FTS; FTS; (c) an assembled FTSFTS withwith aa ballball lens.lens. (Reprinted with permission from Reference [[49]49] SPIE).SPIE).

Based on this this micro micro FTS, FTS, anothe anotherr in-plane in-plane electrothermal electrothermal actuator actuator based based FTS FTS was was developed developed by byDas Das et al. et in al. 2008 in [50,51]. 2008 [50 The,51 whol]. Thee device whole was device still fabricated was still fabricated on a 10 mm on × 10 a 10mm mm die. The10 mmactuator die. × Theinvolved actuator 5 beams involved of 100 5beams μm × 12 of 100μm µ×m 503 12μm,µm generating503 µm, a generatingdisplacement a displacement of 45 μm under of 45 45µ mV × × underdriving 45 voltage V driving and the voltage vertical and placed the vertical assembly placed gold-coated assembly mirror gold-coated was 1000 mirror μm × was800 1000μm inµ size.m × 800The µcomponentsm in size. The of this components FTS (see ofFigure this FTS27) were (see Figure different 27) from were the diﬀ previouserent from design, the previous including design, two includingMEMS mirrors, two MEMS two MEMS mirrors, holders two MEMS for ball holders lenses, for two ball spherical lenses, two glass spherical lenses, one glass beam lenses, splitter, one beam one splitter,single-mode one single-modeoptical fiber, optical and ﬁber,optionally, and optionally, one SMT one detector SMT detectorand one and VCSEL one VCSEL laser lasersource. source. The Theassembled assembled FTS FTSwas was carried carried out out with with precision precision robots robots using using μµ3 3automatedautomated micro-assembly micro-assembly station (having 19 precision degreesdegrees ofof freedom).freedom). The new design was capable of two FTS instruments:instruments: One for the visible rangerange (400–750(400–750 nm),nm), thethe otherother forfor nearnear infraredinfrared rangerange (700–1800(700–1800 nm).nm). The calculatedcalculated spectral resolution of two FTS devices was about 3.6 nm at 600 nm wavelength and 12 nm at 1100 nm Micromachineswavelength, 2020 respectively., 11, x 19 of 26

Figure 27. Fully assembled electrothermal actuator-based FTS. (Reprinted with permission from Figure 27. Fully assembled electrothermal actuator-based FTS. (Reprinted with permission from Reference [50] IEEE). Reference [50] IEEE). Another miniature FTS was realized via in-plane thermal actuator by Reyes et al. (see Figure28a) Another miniature FTS was realized via in-plane thermal actuator by Reyes et al. (see Figure 28a) in 2008 [52,53]. A special monolithic silicon MEMS solution based on SUMMiT-V technology [54] was in 2008 [52,53]. A special monolithic silicon MEMS solution based on SUMMiT-V technology [54] was used to fabricate the key structures. Compared with the traditional SOI platform having only one used to fabricate the key structures. Compared with the traditional SOI platform having only one device layer, it could make complex multilayer structures of silicon oxide and polysilicon. After the device layer, it could make complex multilayer structures of silicon oxide and polysilicon. After the release was completed, the mirror plate was connected to the bottom movable base via a hinge (see release was completed, the mirror plate was connected to the bottom movable base via a hinge (see Figure 28b). The micromirror was erected by the external force ﬁeld, and the left and right structures of Figure 28b). The micromirror was erected by the external force field, and the left and right structures the mirror plate were raised to form a bracket, thus, locking and keeping the micromirror perpendicular of the mirror plate were raised to form a bracket, thus, locking and keeping the micromirror to the base. The movable base was connected to the micro-gear via a connecting rod (see Figure 28c). perpendicular to the base. The movable base was connected to the micro-gear via a connecting rod The transmission structure formed by the micro-gear and the crankshaft was connected to an in-plane (see Figure 28c). The transmission structure formed by the micro-gear and the crankshaft was thermal actuator via ratchet teeth. The actuator motion could drive the micro-gear to rotate in a speciﬁc connected to an in-plane thermal actuator via ratchet teeth. The actuator motion could drive the direction and then drive the connecting rod. A displacement of 600 µm was generated with an open micro-gear to rotate in a specific direction and then drive the connecting rod. A displacement of 600 μm was generated with an open loop stepwise control of the gear system, producing a spectral resolution of 8 cm-1. The complete mirror cycle was performed with 225 steps. However, the erection of the components was done manually rather than automatic self-assembly, causing high operating complexity. Moreover, the thin mirror caused mirror warping after metallization, and the curvature was measured at 600 nm. This actuator could only realize stepwise scanning, which limited its application in the short wavelength.

(a) (b) (c)

Figure 28. A three-dimensional miniature FTS based on thermal actuator: (a) After assembly completion; (b) before assembly; (c) actuator structure. (Reprinted with permission from References [52,53] SPIE).

3.3.2. Out-of-Plane Electrothermal MEMS Micromirror and FTS Since the in-plane electrothermal MEMS micromirrors sustain limited displacement, out-of- plane electrothermal MEMS configurations have been proposed successively. Wu et al. demonstrated a lateral-shift-free (LSF) large-vertical-displacement (LVD) actuator based electrothermal micromirror in 2008 [55] (see Figure 29), whose mirror plate size was 0.8 mm × 0.8 mm. The LSF-LVD actuator comprised two rigid frames and three aluminum (Al)/silicon dioxide (SiO2) bimorph beams, which could compensate the lateral shift. It could generate a maximum vertical displacement of 620 μm under 5.3 V driving voltage in an FTS. The scanning tilting of the micromirror was as high as 0.7°,

Micromachines 2020, 11, x 19 of 26

Figure 27. Fully assembled electrothermal actuator-based FTS. (Reprinted with permission from Reference [50] IEEE).

Another miniature FTS was realized via in-plane thermal actuator by Reyes et al. (see Figure 28a) in 2008 [52,53]. A special monolithic silicon MEMS solution based on SUMMiT-V technology [54] was used to fabricate the key structures. Compared with the traditional SOI platform having only one device layer, it could make complex multilayer structures of silicon oxide and polysilicon. After the release was completed, the mirror plate was connected to the bottom movable base via a hinge (see Figure 28b). The micromirror was erected by the external force field, and the left and right structures of the mirror plate were raised to form a bracket, thus, locking and keeping the micromirror perpendicular to the base. The movable base was connected to the micro-gear via a connecting rod (see Figure 28c). The transmission structure formed by the micro-gear and the crankshaft was Micromachinesconnected to2020 an, 11 in-plane, 214 thermal actuator via ratchet teeth. The actuator motion could drive21 ofthe 28 micro-gear to rotate in a specific direction and then drive the connecting rod. A displacement of 600 μm was generated with an open loop stepwise control of the gear system, producing a spectral loop stepwise control of the gear system, producing a spectral resolution of 8 cm 1. The complete resolution of 8 cm-1. The complete mirror cycle was performed with 225 steps. However,− the erection mirror cycle was performed with 225 steps. However, the erection of the components was done of the components was done manually rather than automatic self-assembly, causing high operating manually rather than automatic self-assembly, causing high operating complexity. Moreover, the thin complexity. Moreover, the thin mirror caused mirror warping after metallization, and the curvature mirror caused mirror warping after metallization, and the curvature was measured at 600 nm. This was measured at 600 nm. This actuator could only realize stepwise scanning, which limited its actuator could only realize stepwise scanning, which limited its application in the short wavelength. application in the short wavelength.

(a) (b) (c)

FigureFigure 28.28.A A three-dimensional three-dimensional miniature miniature FTS basedFTS onbased thermal on thermal actuator: actuator: (a) After assembly(a) After completion; assembly (completion;b) before assembly; (b) before (c) assembly; actuator structure. (c) actuator (Reprinted structure. with (Reprinted permission with from permission References from [52 References,53] SPIE). [52,53] SPIE). 3.3.2. Out-of-Plane Electrothermal MEMS Micromirror and FTS 3.3.2.Since Out-of-Plane the in-plane Electrothermal electrothermal MEMS MEMS Micromirror micromirrors and sustain FTS limited displacement, out-of-plane electrothermalSince the MEMSin-plane conﬁgurations electrothermal have MEMS been microm proposedirrors successively. sustain limited Wu etdisplacement, al. demonstrated out-of- a lateral-shift-freeplane electrothermal (LSF) MEMS large-vertical-displacement configurations have be (LVD)en proposed actuator successively. based electrothermal Wu et al. demonstrated micromirror in 2008 [55] (see Figure 29), whose mirror plate size was 0.8 mm 0.8 mm. The LSF-LVD actuator a lateral-shift-free (LSF) large-vertical-displacement (LVD) ×actuator based electrothermal comprisedmicromirror two in 2008 rigid [55] frames (see andFigure three 29), aluminum whose mirror (Al) /platesilicon size dioxide was 0.8 (SiO mm2) × bimorph 0.8 mm. beams,The LSF-LVD which couldactuator compensate comprised the two lateral rigid frames shift. It and could three generate aluminum a maximum (Al)/silicon vertical dioxide displacement (SiO2) bimorph of 620 beams,µm underMicromachineswhich 5.3could V 2020 driving compensate, 11, xvoltage the in lateral an FTS. shift. The It scanning could generate tilting of a the maximum micromirror vertical was displacement as high as 0.7 20◦of, dueof620 26 toμm the under response 5.3 V didrivingﬀerence voltage of each in drivingan FTS. arm.The scanning The displacement tilting of the was micromirror increased to was 1 mm as high in the as later0.7°, experimentsdue to the response [56], but difference the usable of scan each range driving was ar onlym. The 70 µ displacementm with tilting was<0.06 increased◦. to 1 mm in the later experiments [56], but the usable scan range was only 70 μm with tilting <0.06°.

Figure 29. 29. AnAn out-of-plane out-of-plane electrothermal electrothermal MEMS MEMS mirror. mirror. (Reprinted (Reprinted with permission with permission from Reference from Reference[55] Elsevier). [55] Elsevier).

A mirror-tilt-insensitive (MTI) FTS based on this MEMS was presented subsequently by Wu et al. in 2009 (see Figure 3030a)a) [[57].57]. It comprised one corner-cube retroreflector retroreflector and one fixedfixed mirror in each arm of thethe MichelsonMichelson interferometer. Both lightlight beams from beam splitter were directed to the dual-reflectivedual-reflective MEMS mirror, whichwhich couldcould compensatecompensate thethe tiltingtilting ofof thethe scanningscanning mirror. The actual OPD of this MIT FTS was four times of the the physical physical vertical vertical displacement; displacement; thus, the spectral resolution could be enhanced with the increase of the OPD. The whole device size was about 12 cm 5 cm 5 cm could be enhanced with the increase of the OPD. The whole device size was about 12 ×cm × 5 ×cm × 5 (seecm (see Figure Figure 30b), 30b), which which could could be decreased be decreased by reducing by reducing the size the of opticalsize of components.optical components. It could It achieve could 1 1 spectralachieve resolutionsspectral resolutions of 19.2 cm −of and19.2 8.1 cm cm−1 −and, which 8.1 werecm−1, obtainedwhich were by the obtained vertical displacementby the vertical of 131displacementµm and 308 of µ131m, μ respectively.m and 308 μm, respectively.

(a) (b)

Figure 30. Dual-reflective MEMS mirror-based FTS: (a) System Schematic; (b) a picture of mirror-tilt- insensitive (MTI)-FTS demonstration setup. CCR: Corner-cube retroreflector. PD: Photodetector. BS: Beam splitter. MM: MEMS micromirror. FM: Fixed mirror. LS: Light source. (Reprinted with permission from Reference [57] IEEE).

Wang et al. designed an extended-beam LSF electrothermal MEMS mirror to realize large displacement and small tilting in 2016 (see Figure 31a) [58]. The MEMS device was fabricated using SOI wafers via bulk and surface micromachining. The bimorph was made of Al/SiO2, and a platinum (Pt) layer was embedded into the bimorph as a heater. The size of the MEMS device was about 4.3 mm × 3.1 mm, where the mirror plate was about 1.1 mm × 1.1 mm. The maximum vertical displacement could reach 550 μm under 7 V DC driving voltage. The tilt angle was about 0.15° at 3.5 V, which was controlled at ± 0.002° under closed-loop control. The H-shaped electrothermal MEMS mirror based FTS system (see Figure 31b) could generate 860 μm OPD. The measured spectral resolution was about 19.4 cm−1.

Micromachines 2020, 11, x 20 of 26 due to the response difference of each driving arm. The displacement was increased to 1 mm in the later experiments [56], but the usable scan range was only 70 μm with tilting <0.06°.

Figure 29. An out-of-plane electrothermal MEMS mirror. (Reprinted with permission from Reference [55] Elsevier).

A mirror-tilt-insensitive (MTI) FTS based on this MEMS was presented subsequently by Wu et al. in 2009 (see Figure 30a) [57]. It comprised one corner-cube retroreflector and one fixed mirror in each arm of the Michelson interferometer. Both light beams from beam splitter were directed to the dual-reflective MEMS mirror, which could compensate the tilting of the scanning mirror. The actual OPD of this MIT FTS was four times of the physical vertical displacement; thus, the spectral resolution could be enhanced with the increase of the OPD. The whole device size was about 12 cm × 5 cm × 5 cm (see Figure 30b), which could be decreased by reducing the size of optical components. It could Micromachinesachieve spectral2020, 11 ,resolutions 214 of 19.2 cm−1 and 8.1 cm−1, which were obtained by the vertical22 of 28 displacement of 131 μm and 308 μm, respectively.

(a) (b)

Figure 30. 30. Dual-reflectiveDual-reﬂective MEMS MEMS mirror-based mirror-based FTS: ( FTS:a) System (a) System Schematic; Schematic; (b) a picture (b) of a mirror-tilt- picture of mirror-tilt-insensitiveinsensitive (MTI)-FTS demonstration (MTI)-FTS demonstration setup. CCR: Co setup.rner-cube CCR: retroreflector. Corner-cube PD: retroreﬂector. Photodetector. PD:BS: Photodetector.Beam splitter. MM: BS: Beam MEMS splitter. microm MM:irror. MEMS FM: micromirror.Fixed mirror. FM:LS: FixedLight mirror.source. LS:(Reprinted Light source. with (Reprintedpermission withfrom permission Reference [57] from IEEE). Reference [57] IEEE).

Wang etet al.al. designed designed an an extended-beam LSF LSF el electrothermalectrothermal MEMS MEMS mirror to realize large displacement and small tilting in 2016 (see Figure 3131a)a) [[58].58]. The MEMS device was fabricated using SOI wafers via bulk and surface micromachining. The bimorph was made of AlAl/SiO/SiO22, and a platinum (Pt) layerlayer waswas embeddedembedded into into the the bimorph bimorph as as a heater.a heater. The The size size of theof the MEMS MEMS device device was was about about 4.3 mm 4.3 3.1 mm, where the mirror plate was about 1.1 mm 1.1 mm. The maximum vertical displacement ×mm × 3.1 mm, where the mirror plate was about× 1.1 mm × 1.1 mm. The maximum vertical coulddisplacement reach 550 couldµm underreach 550 7 V μ DCm under driving 7 voltage.V DC driving The tilt voltage. angle wasThe abouttilt angle 0.15 was◦ at about 3.5 V, which0.15° at was 3.5 controlled at 0.002 under closed-loop control. The H-shaped electrothermal MEMS mirror based V, which was± controlled◦ at ± 0.002° under closed-loop control. The H-shaped electrothermal MEMS FTSmirror system based (see FTS Figure system 31b) (see could Fi generategure 31b) 860 couldµm OPD.generate The measured860 μm OPD. spectral The resolution measured was spectral about 1 19.4Micromachinesresolution cm− . was 2020 ,about 11, x 19.4 cm−1. 21 of 26

(a) (b)

Figure 31. H-shaped LSF electrothermal MEMSMEMS basedbased FTS: FTS: ( a()a) A A SEM SEM of of MEMS MEMS mirror; mirror; (b ()b a) picturea picture of ofFTS FTS system. system. (Reprinted (Reprinted with with permission permission from from Reference Reference [58 [58]] MDPI). MDPI).

Samuelson etet al.al. reportedreported aa folded-dual-S-shaped-bimorphfolded-dual-S-shaped-bimorph (FDSB) (FDSB) electrothermal electrothermal micromirror micromirror in in2014 2014 [59 [59].]. This This micromirror micromirror had had a footprint a footprint of 1.9of 1.9 mm mm1.9 × 1.9 mm mm with with a mirror a mirror aperture aperture of 1 of mm 1 mm (see × (seeFigure Figure 32). The32). FDSBThe FDSB design design consisted consisted of two of separate two separate inverted inverted series series connected connected (ISC) actuators,(ISC) actuators, which werewhich connected were connected by a joint by a or joint a hinge. or a hinge. Each S-shaped-bimorphEach S-shaped-bimorph was composed was composed of three of segmentsthree segments made madeof Al/ SiOof Al/SiO2, SiO22,/ AlSiO/SiO2/Al/SiO2, and2, SiO and2 /SiOAl,2 respectively./Al, respectively. A Pt A layer Pt layer was embeddedwas embedded into into the bimorphthe bimorph as a asheater. a heater. Six pairsSix pairs of FDSB of FDSB actuators actuators were were placed placed on the on opposingthe opposing sides sides of the of centralthe central mirror mirror plate. plate. The Themulti-actuators multi-actuators were were applied applied to compensate to compensate for the fo responser the response diﬀerence difference of each actuatorof each actuator and decrease and decreasethe scan tiltingthe scan angle. tilting The angle. FDSB The micromirror FDSB microm was fabricatedirror was onfabricated SOI wafers on SOI via awafers combined via a surface combined and surfacebulk micromachining and bulk micromachining process. The proces maximums. The displacement maximum displacement of 90 µm was of generated 90 μm was with generated<0.25◦ tiltwith at 1.2<0.25° V DC. tilt at 1.2 V DC.

Figure 32. SEM of an actuator array with an elevated mirror plate. (Reprinted with permission from Reference [59] IEEE).

Chai et al. presented a portable FTS using an H-shaped electrothermal MEMS mirror [9]. The mirror plate was connected to a unique H-shaped frame supported by symmetrically distributed thirty-two pairs of innovative three-level-ladder FDSB actuators (see Figure 33a). Each bimorph was S-shaped, including three segments: Al/SiO2 film layer, SiO2/Al/SiO2 film layer, and SiO2/Al film layer. titanium (Ti) layer was embedded into the S-shaped bimorph actuator as a resistor. The MEMS mirror (Entire chip: 3.65 mm × 11.4 mm; Mirror plate: 1.4 mm × 1.2 mm) could generate about 200 μm displacement at 5 Hz with only 5 Vpp and maintain a very small tilting of 0.029° without using any complex compensation or closed-loop control. An InGaAs photodetector was applied to record the interferogram, and a 1310 nm laser was used to monitor the MEMS motion. This FTS could cover a wide spectral range of 1000–2500 nm. A spectral resolution of 64.1 cm−1 was achieved for this MEMS FTS (see Figure 33b).

Micromachines 2020, 11, x 21 of 26

(a) (b)

Figure 31. H-shaped LSF electrothermal MEMS based FTS: (a) A SEM of MEMS mirror; (b) a picture of FTS system. (Reprinted with permission from Reference [58] MDPI).

Samuelson et al. reported a folded-dual-S-shaped-bimorph (FDSB) electrothermal micromirror in 2014 [59]. This micromirror had a footprint of 1.9 mm × 1.9 mm with a mirror aperture of 1 mm (see Figure 32). The FDSB design consisted of two separate inverted series connected (ISC) actuators, which were connected by a joint or a hinge. Each S-shaped-bimorph was composed of three segments made of Al/SiO2, SiO2/Al/SiO2, and SiO2/Al, respectively. A Pt layer was embedded into the bimorph as a heater. Six pairs of FDSB actuators were placed on the opposing sides of the central mirror plate. The multi-actuators were applied to compensate for the response difference of each actuator and decrease the scan tilting angle. The FDSB micromirror was fabricated on SOI wafers via a combined Micromachinessurface and2020 bulk, 11 micromachining, 214 process. The maximum displacement of 90 μm was generated23 ofwith 28 <0.25° tilt at 1.2 V DC.

FigureFigure 32. 32.SEM SEM ofof anan actuator actuator array array with with an an elevated elevated mirror mirror plate. plate.(Reprinted (Reprintedwith with permission permissionfrom from ReferenceReference [ 59[59]] IEEE). IEEE).

ChaiChai etet al.al. presentedpresented aa portableportable FTSFTS usingusing anan H-shaped H-shaped electrothermal electrothermalMEMS MEMS mirror mirror [ 9[9].]. TheThe mirrormirror plateplate waswas connectedconnected toto aa uniqueunique H-shapedH-shaped frameframe supportedsupported byby symmetricallysymmetrically distributeddistributed thirty-twothirty-two pairs pairs of of innovative innovative three-level-ladder three-level-ladder FDSB FDSB actuators actuators (see (see Figure Figure 33 33a).a). EachEach bimorphbimorph waswas S-shaped,S-shaped, includingincluding three segments: segments: Al/SiO Al/SiO2 film2 film layer, layer, SiO SiO2/Al/SiO2/Al/SiO2 film2 film layer, layer, and andSiO2/Al SiO film2/Al layer. film layer.titanium titanium (Ti) layer (Ti) layerwas embedded was embedded into the into S-shaped the S-shaped bimorph bimorph actuator actuator as a resistor. as a resistor. The MEMS The MEMS mirror mirror(Entire (Entire chip: chip:3.65 mm 3.65 × mm 11.4 11.4mm; mm; Mirror Mirror plate: plate: 1.4 1.4mm mm × 1.21.2 mm) mm) could could generate generate about about 200200 µμmm × × displacementdisplacement atat 55 HzHz withwith onlyonly 55 VppVpp andand maintainmaintaina avery verysmall smalltilting tiltingof of 0.029 0.029°◦ without without usingusing anyany complexcomplex compensationcompensation oror closed-loopclosed-loop control.control. AnAn InGaAsInGaAs photodetectorphotodetector waswas appliedapplied toto recordrecord thethe interferogram,interferogram, andand aa 1310 1310 nm nm laser laser was was used used to to monitor monitor the the MEMS MEMS motion. motion. ThisThis FTSFTS could could cover cover a a 1 widewide spectral spectral range range of of 1000–2500 1000–2500 nm. nm. A A spectral spectral resolution resolution of of 64.1 64.1 cm cm−−1 waswas achievedachieved forfor thisthisMEMS MEMS FTSMicromachinesFTS (see(see Figure Figure 2020 , 33 1133b).b)., x 22 of 26

(a) (b)

Figure 33. The scan stability H-shaped electrothermal MEMS based FTS: ( a) MEMS mirror; (b) FTS system. (Reprinted with permission from Reference [[9]9] IOP Publishing, Ltd.).Ltd.).

3.3.3. Summary Table3 3 summarizessummarizes several para parametersmeters of of electrothermal electrothermal MEMS MEMS micromirrors micromirrors and and FTS. FTS. The Theelectrothermal electrothermal actuators actuators have have a relatively a relatively larg largee range range of of movement, movement, compared compared to to electrostaticelectrostatic actuators and electromagnetic actuators. For comparable displacement, much smaller footprints are used inin electrothermalelectrothermal actuators. actuators. On On the the other other hand, hand, electrothermal electrothermal actuators actuators require require moderate moderate power aspower electrical as electrical current current is applied is applied to generate to generate Joule heating, Joule heating, and they and have they relatively have relatively low response low response speed sincespeed the since time the constant time constant is controlled is contro bylled thermal by thermal heating heating and dissipation. and dissipation.

Table 3. Comparison of electrothermal MEMS micromirrors and FTS.

Actuation Core of Work Authors Institution Displacement Device Size Reference Type FTS Condition Entire FTS: 10 × 10 mm2 Sin et al. UTA In-plane MI 30 μm 22 V [49] (Mirror: 0.5 × 0.45 mm2) Entire FTS: 10 × 10 mm2 Das et al. UTA In-plane MI 45 μm 45 V [50,51] (Mirror: 1 × 0.8 mm2) Reyes et al. BML In-plane MI 600 nm − − [52,53] Out-of- Wu et al. UF − 620 μm 5.3 V. − [55] plane Out-of- 131 μm and Entire FTS: 12 × Wu et al. UF MI − [57] plane 308 μm 5 × 5 cm3 MEMS chip: 4.3 SJTU and Out-of- × 3.1 mm2 Wang et al. MI 550 μm 7 V DC [58] UF plane (Mirror: 1.1 × 1.1 mm2) MEMS chip: 1.9 × 1.9 mm2 Samuelson Out-of- UF − 90 μm 1.2 V DC (Mirror [59] et al. plane Aperture: 1mm) MEMS chip: USST, WiO Out-of- 5 Vpp @ 5 3.65 × 11.4 mm2 Chai et al. Tech, and MI 200 μm [9] plane Hz (Mirror: 1.4 × UF 1.2 mm2) (UTA: The University of Texas at Arlington; BML: Block MEMS LCC; UF: University of Florida; SJTU: Shanghai Jiao Tong University; USST: The University of Shanghai for Science and Technology; WiO Tech: Wuxi WiO Technologies Co., Ltd.).

Micromachines 2020, 11, 214 24 of 28

Table 3. Comparison of electrothermal MEMS micromirrors and FTS.

Actuation Work Authors Institution Core of FTS Displacement Device Size Reference Type Condition Entire FTS: 10 10 × Sin et al. UTA In-plane MI 30 µm 22 V mm2 (Mirror: 0.5 [49] × 0.45 mm2) Entire FTS: 10 10 × Das et al. UTA In-plane MI 45 µm 45 V mm2 (Mirror: 1 [50,51] × 0.8 mm2) Reyes et al. BML In-plane MI 600 nm [52,53] − − Wu et al. UF Out-of-plane 620 µm 5.3 V. [55] − − 131 µm and Entire FTS: 12 5 Wu et al. UF Out-of-plane MI × × [57] 308 µm − 5 cm3 MEMS chip: 4.3 × Wang et al. SJTU and UF Out-of-plane MI 550 µm 7 V DC 3.1 mm2 (Mirror: 1.1 [58] 1.1 mm2) × MEMS chip: 1.9 Samuelson × UF Out-of-plane 90 µm 1.2 V DC 1.9 mm2 (Mirror [59] et al. − Aperture: 1mm) MEMS chip: 3.65 USST, WiO 5 Vpp @ 5 × Chai et al. Out-of-plane MI 200 µm 11.4 mm2 (Mirror: [9] Tech, and UF Hz 1.4 1.2 mm2) × (UTA: The University of Texas at Arlington; BML: Block MEMS LCC; UF: University of Florida; SJTU: Shanghai Jiao Tong University; USST: The University of Shanghai for Science and Technology; WiO Tech: Wuxi WiO Technologies Co., Ltd.).

3.4. Summary of Electrostatic, Electromagnetic, and Electrothermal Actuators for MEMS FTS As discussed in this section, electrostatic, electromagnetic, and electrothermal actuators have been applied in the miniaturization of FTS devices. Relative advantages and disadvantages of an electrostatic actuator, electromagnetic actuator, and electrothermal actuator are summarized in Table4.

Table 4. Comparison of various actuators.

Various Actuators Advantages Disadvantages Vacuum package and resonance Fast response operation Electrostatic actuators Limited displacement Low power consumption Pull-in behavior External magnetic ﬁeld needed Electromagnetic actuators Moderately large displacement Large size Large displacement Large power consumption Electrothermal actuators Sensitivity to environmental Moderately fast response temperature changes

Electrostatic actuators have high speed and low power consumption, but it is diﬃcult to obtain large scan displacement. Even though there are reports about large piston displacement with electrostatic actuation, those designs require vacuum packaging and resonance operation, which dramatically increase the cost and overall device size. Electromagnetic actuators usually need an external magnetic ﬁeld, which is not conducive to system integration and may cause electromagnetic interference problems. Electrothermal actuators can reach about 1 mm scan range at a low voltage without the need of a resonant operation, which is highly preferred for FTS applications. Promising results have been achieved from electrothermally-actuated MEMS FTS. Micromachines 2020, 11, 214 25 of 28

For an FTS application, the accuracy and repeatability of the acquired spectra are of signiﬁcance too, which will have implications on the stability of the response and transferability of the calibration models between instruments.

4. Conclusions MEMS presents a huge potential in the miniaturization of FTS devices. With the markets of MEMS based FTS growing rapidly, more demands for new applications are continually emerging. A majority of MEMS based devices have become commercial products. In this article, we have summarized MEMS based FTS systems for spectral analysis and material detection. The basics and common applications of FTS are presented. Three types of MEMS FTS, including electrostatic, electromagnetic and electrothermal, are introduced and shown with examples. The spectral resolution is a key parameter used to evaluate the performance of a MEMS FTS. Besides that, the repeatability and reliability are also two main issues for the commercialization of MEMS FTS to be further mature.

Author Contributions: Investigation, J.C., T.C. and Y.L.; resources, J.C., K.Z. and Y.X.; writing—original draft preparation, J.C.; writing—review and editing, Y.X. and K.Z.; supervision, W.L. and G.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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