Wafer-Level Vacuum-Packaged Translatory MEMS Actuator with Large Stroke for NIR-FT Spectrometers

micromachines

Article Wafer-Level Vacuum-Packaged Translatory MEMS Actuator with Large Stroke for NIR-FT Spectrometers

Thilo Sandner 1,*, Eric Gaumont 1, Thomas Graßhoﬀ 1, Andreas Rieck 1, Tobias Seifert 2 , Gerald Auböck 3 and Jan Grahmann 1

1 Fraunhofer Institute for Photonic Microsystems (FhG-IPMS), 01109 Dresden, Germany; [email protected] (E.G.); thomas.grasshoﬀ@ipms.fraunhofer.de (T.G.); [email protected] (A.R.); [email protected] (J.G.) 2 Fraunhofer Institute for Electronic Nanosystems (FhG-ENAS), 09126 Chemnitz, Germany; [email protected] 3 Silicon Austria Labs (SAL), 9524 Villach, Austria; [email protected] * Correspondence: [email protected]; Tel.: +49-351-8823-152

Received: 2 June 2020; Accepted: 16 September 2020; Published: 23 September 2020

Abstract: We present a wafer-level vacuum-packaged (WLVP) translatory micro-electro-mechanical system (MEMS) actuator developed for a compact near-infrared-Fourier transform spectrometer (NIR-FTS) with 800–2500 nm spectral bandwidth and signal-nose-ratio (SNR) > 1000 in the smaller bandwidth range (1200–2500 nm) for 1 s measuring time. Although monolithic, highly miniaturized MEMS NIR-FTSs exist today, we follow a classical optical FT instrumentation using a resonant MEMS mirror of 5 mm diameter with precise out-of-plane translatory oscillation for optical path-length modulation. Compared to highly miniaturized MEMS NIR-FTS, the present concept features higher optical throughput and resolution, as well as mechanical robustness and insensitivity to vibration and mechanical shock, compared to conventional FTS mirror drives. The large-stroke MEMS design uses a fully symmetrical four-pantograph suspension, avoiding problems with tilting and parasitic modes. Due to significant gas damping, a permanent vacuum of 3.21 Pa is required. Therefore, an MEMS ≤ design with WLVP optimization for the NIR spectral range with minimized static and dynamic mirror deformation of 100 nm was developed. For hermetic sealing, glass-frit bonding at elevated process ≤ temperatures of 430–440 ◦C was used to ensure compatibility with a qualified MEMS processes. Finally, a WLVP MEMS with a vacuum pressure of 0.15 Pa and Q 38,600 was realized, resulting in ≤ ≥ a stroke of 700 µm at 267 Hz for driving at 4 V in parametric resonance. The long-term stability of the 0.2 Pa interior vacuum was successfully tested using a Ne fine-leakage test and resulted in an estimated lifetime of >10 years. This meets the requirements of a compact NIR-FTS.

Keywords: micro-opto-electro-mechanical system (MOEMS); translatory micro mirror; optical wafer- level vacuum package (WLVP); NIR Fourier-transform spectrometer; glass-frit wafer bonding

1. Introduction Near-infrared spectroscopy (NIRS) with spectral wideband light sources is an extremely versatile method used for quality control in chemical production processes. As absorption bands in the near-infrared spectral range (800–2500 nm) are relatively weak, the method can be used for an analysis of a wide range of materials in very diverse states (gas, liquid, and solid, as well as heterogeneous multicomponent systems) [1]. NIRS is used, e.g., to determine water in plastics or for rapid, nondestructive moisture measurement in food quality control or for physical parameters such as viscosity or grain size. Practically, quantitative chemical analysis using NIRS relies on multivariate statistical analysis using a prediction model built from spectra of calibration samples for which the

Micromachines 2020, 11, 883; doi:10.3390/mi11100883 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 883 2 of 28 analyte is known from primary reference measurements (such as Karl Fischer titration for the case of water [2]). Typically, NIR spectra in condensed phases contain very broad, overlapping bands. In order to capture a suﬃcient number of spectral bands for a reliable analytical result, NIR spectrometers are, thus, equipped with wideband sources. Such sources are realized by black-body emitters such as tungsten halogen lamps. As they have a low brightness due to the undirected thermal emission, the design of an NIR spectrometer involves optimizing the coupling of the source into the spectrometer and its optical throughput.

1.1. MEMS-Based Fourier-Transform (FT) Spectrometer As the Fourier-transform (FT) spectrometer based on the Michelson interferometer offers the highest throughput due to the multiplexing and etendue advantage over dispersive spectrometers [1,3], it has found numerous industrial applications so far. Conventional FT spectrometers do, however, suffer from slow acquisition (compared to diode-array-based dispersive spectrometers [4]) due to the inertia of the path-length scanning optics. For rough environments (in terms of vibration and shock) typically encountered in chemical process applications, the required mechanical stability implies that the scanning mechanics of an FT spectrometer is relatively bulky compared to diode-array spectrometers. MOEMS (micro-opto-electromechanical systems) have been investigated during the last decade for robust and fast optical path-length modulation, miniaturization, and cost reduction of FT spectrometers. They have the potential of making FT-NIRS a candidate technology for efficient, cost-effective spectroscopy solutions for industrial chemical analytics. Various MEMS concepts for optical path-length scanning have been reported, typically targeting an out-of-plane translation with large stroke required for the classical dual-beam Michelson interferometer FTS set-up, consisting of fixed and moving mirrors (here realized by an MOEMS) and an optical beam splitter. For large-stroke out-of-plane translation of the MEMS mirror, different actuation principles have been investigated so far: electrostatic [5–8], piezoelectric [9–11], magnetic [12–15], and electrothermal [16–22], typically using resonant operation for larger strokes, e.g., [7,8,11], but also using quasi-static actuation, e.g., [10,17]. Most of these different MOEMS actuation mechanisms suffer from limitations in spectral resolution due to parasitic effects of MEMS-based path-length modulation: first of all, mirror tilt [11,17,19], deformation of the mirror due to dynamically (owing to inertia) or statically (due to mirror suspension or optical coating) induced mechanical stress. In addition to the actuation mechanism, MEMS-based FTS can be subclassified by the principle of optical FTS instrumentation: (i) Michelson interferometer-based FTS, e.g., [7,12,23–27], or (ii) lamellar grating-based FTS, enabling a simplified interferometer set-up without optical beam splitter [6,8,28–32]. MEMS technology was also used to simplify the optical system integration of FTS; early approaches using bulk [5,6,33] and surface micromachining [34] were reported. During the last decade, monolithic integration of the interferometer optical bench into silicon was realized for a higher miniaturization and cost reduction of MEMS-based FTS, e.g., [35–38]. Although full on-chip integration of FT interferometers (including the translatory MOEMS) is limited to small mirror apertures and imperfect surface quality, these MEMS-FT spectrometers are starting to enter the market [38] for NIR applications and are used in cases with moderate requirements of spectral resolution due to their advantages in size and cost. Our work on compact MEMS IR-FTS is based on a classical Michelson FTS instrumentation, but using electrostatic resonant MEMS mirrors with large strokes for fast optical path-length modulation [7,23–27]. This MEMS-FTS approach, combining a classical optical instrumentation with a resonant MEMS (see Figure1a), enables larger mirror apertures (required for high optical throughput), a larger stroke of translatory oscillation of up to 1.4 mm [25], and an extreme repeatability and stability of the optical path modulation due to high Q resonant operation. These are advantages over monolithic MEMS-FTS and enable robust compact FT spectrometers with improved spectral resolution and SNR (signal-noise-ration). On the other hand, due to the significant gas damping [25], the resonant MEMS mirror has to operate in vacuum to achieve large strokes. In the past, we developed translatory MEMS mirrors with large strokes for a broadband MIR-FTS with a spectral range of Micromachines 2020, 11, 883 3 of 28

λMicromachines= (2–16) µ 20m.20,In 11, [x26 FOR], a PEER spectral REVIEW resolution of ∆ν = 12 cm 1 (4.5 nm @ 2 µm) and an SNR 3 of1000:1 27 − ≥ (for 200 ms measuring time and co-addition of spectra) were veriﬁed by experiment. In a single scan, ≥ anand SNR co-addition of 48 was of achieved,spectra) were while verified averaging by experiment. 500 scans In improved a single scan the, SNR an SNR to 1607. of 48 Thewas translatoryachieved, MEMSwhile averaging device uses 500 a point-symmetricscans improved pantographthe SNR to 1607. suspension The translatory of the 5 mm MEMS mirror device [25]. uses To achieve a point the- symmetric pantograph suspension of the 5 mm mirror [25]. To achieve the full scan amplitude of 500 full scan amplitude of 500 µm at 500 Hz, a vacuum pressure of 10 Pa was required. The MIR-FTS μm at 500 Hz, a vacuum pressure of ≤10 Pa was required. The MIR≤-FTS previously reported in [26,27] previously reported in [26,27] required a high optical MIR transmission without spectral gaps over required a high optical MIR transmission without spectral gaps over the entire MIR spectral range. For the entire MIR spectral range. For that reason, a costly hybrid assembly with a broadband ARC-IR that reason, a costly hybrid assembly with a broadband ARC-IR window of ZnSe had to be chosen for window of ZnSe had to be chosen for MEMS vacuum packaging (see Figure1b, [ 25]), instead of a MEMS vacuum packaging (see Figure 1b, [25]), instead of a cost-effective wafer-level vacuum package, cost-eﬀective wafer-level vacuum package, e.g., using a silicon or glass window. e.g., using a silicon or glass window.

FigureFigure 1. 1. MEMS-basedMEMS-based IRIR-FTS-FTS using a large-strokelarge-stroke four four-pantograph-pantograph MEMS MEMS mirror mirror:: (a ()a ) schematic schematic opticaloptical FTS FTS set-up set-up designed designed for for a a broadbroad MIR MIR (mid-infrared)(mid-infrared) spectral range of 2.5…...1616 µmµm using using a a translatorytranslatory MEMSMEMS [27]];; (b) hybrid optical optical vacuum vacuum package package of of MEMS MEMS using using a aZnSe ZnSe window window due due to to broadbroad spectralspectral MIRMIR rangerange [25]:]: 1 1,, MEMS MEMS chip chip;; 2 2,, cover cover spacer spacer;; 3 3,, ceramic ceramic board board;; 4, 4, MEMS MEMS chip chip soldered soldered onon ceramicceramic board;board; 5 5,, Al Al wire wire bonds bonds;; 6 6,, ZnSe ZnSe window window with with broad broad-band-band anti anti-reﬂex-reflex coating coating (BB- (BB-ARC);ARC); 7, 7,final ﬁnal hybrid hybrid vacuum vacuum package; package; BW BW,, bottom bottom wafer wafer;; DW, DW, device device wafer wafer;; TSW, TSW, top spacer top spacer wafer wafer;; TGW, TGW,top topglass glass wafer wafer);); (c) wafer (c) wafer-level-level vacuum vacuum package package (WLVP) (WLVP) of MEMS of MEMS for NIR for NIR-FTS-FTS with with a spectral a spectral range range of of0.8 0.8–2.5–2.5 µm.µm.

1.2.1.2. MotivationMotivation of This Study InIn thisthis article,article, wewe present a wafer wafer-level-level vacuum vacuum-packaged-packaged translatory translatory MOEMS MOEMS device, device, especially especially developeddeveloped forfor aa compactcompact NIR-FTNIR-FT spectrometer operating operating in in the the NIR NIR spectral spectral range range λλ = =(800(800–2500)–2500) nm. nm. TheThe MEMS-based MEMS-based NIR-FTSNIR-FTS targetstargets aa spectralspectral resolutionresolution ≤1155 cm −1 1andand SNR SNR >1000>1000 for for a ameasuring measuring time time ≤ − ofof 11 s. s. ForFor aa fastfast opticaloptical path-lengthpath-length modulation,modulation, it it requires requires a a highly highly precise precise (tilt (tilt-free)-free) out out-of-plane-of-plane MEMSMEMS translation translation with with 350 350µm µmamplitude amplitude (700 µ (700m stroke) µm stroke) and only and 80 nm only dynamic 80 nm mirror dynamic deformation. mirror Althoughdeformation. we developed Although awe translatory developed (pantograph) a translatory MEMS (pantograph) with 500 MEMSµm scan with amplitude 500 µm scan for MIR-FTS amplitude [25 ], for MIR-FTS [25], it is not suitable for an NIR-FTS with λmin = 800 nm due to the significantly too large it is not suitable for an NIR-FTS with λmin = 800 nm due to the significantly too large dynamic mirror dynamic mirror deformation of δp-p = 433 nm. Even for a reduced amplitude of 350 µm, this MEMS has deformation of δp-p = 433 nm. Even for a reduced amplitude of 350 µm, this MEMS has a too large a too large dynamic mirror deformation of δp-p = 303.1 nm (λmin/2.6), which exceeds the specified value dynamic mirror deformation of δ = 303.1 nm (λ /2.6), which exceeds the specified value of 80 nm of 80 nm by a factor of 3.8. Hence,p-p no pantograph MEMSmin for an NIR-FTS exists so far (see Table 1). To by a factor of 3.8. Hence, no pantograph MEMS for an NIR-FTS exists so far (see Table1). To achieve achieve the specified amplitude of 350 µm, the translatory MEMS has to be encapsulated in an optical the specified amplitude of 350 µm, the translatory MEMS has to be encapsulated in an optical vacuum vacuum package as well. Due to the NIR spectral range, a glass window with broadband antireflective packagecoating (BB as well.-ARC) Due is applicable to the NIR instead spectral of range,the zinc a glassselenide window (ZnSe) with used broadband in [25]. Now antireflective, vacuum MEMS coating (BB-ARC)packaging is can applicable be performed instead at of wafer the zinc level selenide instead (ZnSe) of the previous used in [25 cost]. Now,-intensive vacuum hybrid MEMS package. packaging can be performed at wafer level instead of the previous cost-intensive hybrid package.

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TableTable 1.1. SpeciﬁcationSpecification of NIR-FTSNIR-FTS and boundary conditions for for MEMS MEMS device. device.

FTSFTS Speciﬁcation Specification ValueValue MEMS MEMS Boundary Boundary Conditions Conditions Value Value Mirror aperture 5 mm Maximal mechanical stress s 1.5 GPa Mirror aperture 5 mm Maximal mechanical stress s11 ≤1.5 GPa≤ Resonance frequency 250–300 Hz Maximal shock acceleration 2500 g Resonance frequency 250–300 Hz Maximal shock acceleration 2500 g Mechanical amplitude 350 µm Mirror deformation δpp 80 nm SpectralMechanical range [λmin, amplitudeλmax] 800–2500350 µm nm Mirror Reﬂectance deformation of mirror δpp 80 nm 95% 1 ≥ Spectral resolution ∆ν 15 cm− Parasitic tilt angle 20” Spectral range [λmin, λmax] 800≤ –2500 nm Reflectance of mirror ≥95% Lifetime 10 years Vacuum pressure in WLVP 1 Pa Spectral resolution ∆ν ≥ ≤15 cm−1 Parasitic tilt angle 20” ≤ Lifetime ≥10 years Vacuum pressure in WLVP ≤1 Pa In this work, we contribute new results concerning the following three main topics:

NIR-FTS-specificIn this work, we contribute design of new a resonant results large-strokeconcerning the pantograph following MEMSthree main device topics: with minimized • dynamic mirror deformation of δ = 80 nm at 350 µm scan amplitude,  NIR-FTS-specific design of a resonantp-p large-stroke pantograph MEMS device with minimized development of a cost-effective optical (NIR) wafer-level vacuum package with process • dynamic mirror deformation of δp-p = 80 nm at 350 µm scan amplitude,  compatibilitydevelopment to of the a existing cost-effective qualified optical MEMS (NIR) scanner wafer process-level AME75 vacuum of packageFraunhofer with IPMS, process detailed characterization of new translatory MEMS devices with WLVP, investigation and • compatibility to the existing qualified MEMS scanner process AME75 of Fraunhofer IPMS,  eliminationdetailed characterization of failure mechanisms of new translatory of WLVP (e.g., MEMS influence devices of with process WLVP, temperature investigation on mirror and planarity),elimination and of failurelong-term mechanisms stability of of the WLVP desired (e.g. vacuum, influence pressure of process for 10 temperature years lifetime. on mirror ≥ planarity), and long-term stability of the desired vacuum pressure for ≥10 years lifetime. A broad variety of wafer-level vacuum-packaging technologies and wafer bond techniques existsA for broad hermetic variety sealing of wafer of-level smart vacuum MEMS-packaging sensors [39 technologies] or MOEMS and devices, wafer bond e.g., techniques torsional MEMS exists mirrorsfor hermetic [40– 43sealing]. Two of examples smart MEMS of wafer-level sensors [39 MEMS] or MOEMS vacuum devices, packages e.g. are, torsional exemplarily MEMS shown mirrors in Figure[40–432].. Two Typically, examples MEMS of WLVPswafer-level have MEMS a closed vacuum sealing packages area with are low exemplarily topography shown for hermetic in Figure wafer 2. bonding.Typically, VerticalMEMS WLVP feedthroughss have a areclosed also sealing often usedarea with for electrical low topography signal transfer for hermetic from thewafer inner bonding. cavity ofVertical WLVP. feedthroughs are also often used for electrical signal transfer from the inner cavity of WLVP.

Figure 2. 2. Examples of state-of-the-artstate-of-the-art wafer wafer-level-level MEMS MEMS vacuum vacuum packages packages:: (a (a) ) WLVP WLVP of of a a MEMS resonatorresonator using alternating layers of of evaporated Au Au and and Sn Sn [ 39]];; ( b) schematic set set-up-up of wafer wafer-level-level vacuum-packagedvacuum-packaged micro scanning mirrors using anodic bonding bonding of of a a glass glass cover cover wafer wafer to to a a polished Epipoly sealing frame and eutectic AuSn AuSn bonding bonding of of the the bottom bottom wafer wafer to to the the MEMS MEMS backside backside [40 [40]. ].

Concerning the the wafer wafer-level-packaging-level-packaging of of micro micro scanning scanning mirrors mirrors,, we we mention mention the thestate state of the of theart artdescribed described in [4 in0–43 [40].– Here,43]. Here,a micro a molded micro molded glass cap glass wafer cap with wafer 400– with900 µm 400–900 cavityµ heightm cavity is anodic heightally is anodicallybonded to bondedthe polished to the surface polished of an surface epi-polysilicon of an epi-polysilicon MEMS device MEMS [40,4 device1]. The [40MEMS,41]. Thebackside MEMS is backsidehermetically is hermetically sealed by eutectic sealed byAu eutectic bonding Au [44 bonding] using [ 44a ]3 usingµm thick a 3 µ electroplatedm thick electroplated Au layer. Au Using layer. Usinggetters, getters, a vacuum a vacuum pressure pressure of 0.1 mbar of 0.1 (10 mbar Pa) (10was Pa) achieved was achieved in [40]. In in [ [4402]]., a In modified [42], a modified WLVP process WLVP processof micro of scanning micro scanning mirrors mirrorswas presented was presented,, which allows which tilted allows glass tilted windows glass windows on a wafer on level a wafer (instead level (insteadof a parallel of a parallelwindow). window). For hermetic For hermetic sealing sealingof the top of theglass top wafer glass, wafer,glass-frit glass-frit bonding bonding was now was used now usedinstead instead of anodic of anodic bonding bonding (enabling (enabling a free choice a free of choice glass material of glass and material avoiding and high avoiding process high voltages). process voltages).In [42], an In inner [42], an vacuum inner vacuum pressure pressure of 0.1 of Pa 0.1 was Pa was reported reported using using a titanium a titanium thin thin-film-film getter. getter. Furthermore,Furthermore, a modular packaging concept for MOEMS was presented in in [ 4433]] allowing allowing also also vacuum vacuum packagespackages with integrated vertical electrical feedthrough feedthroughss ( (through-glassthrough-glass via via,, TGA). TGA). The concept of the MEMS WLVP reported in this work (see Figure 11c)c) waswas determineddetermined byby thethe followingfollowing specificsspecifics and limitations of of the the qualified qualified MEMS MEMS scanner scanner process process AME75 AME75 [45 [45]:]:  AlSiCu, which is used for metal signal lines and bond island45s (at the outer chip frame),

Micromachines 2020, 11, 883 5 of 28

AlSiCu, which is used for metal signal lines and bond island45s (at the outer chip frame), • high topography ( 2 µm due to metal lines) within the areas needed for hermetic sealing, • ≥ ultrasonic Al wire bonding required for good electrical contact of AlSiCu, • use of filled isolation trenches for electrostatic comb drives (also adding surface topography), • CMOS compatibility of all in-line processes used for fabrication of MEMS device wafers. • Due to the fixed MEMS process, the following consequences result for the NIR-WLVP: (i) no area with low topology exists on the MEMS surface (sealing frame is crossed by metal lines, see Figure1c) and (ii) vertical signal feedthroughs are not applicable due to the poor electrical contact to the AlSiCu surface. Hence, a hermetic bond technique is required which can tolerate high surface topologies of 2 µm. For this NIR-WLVP application, glass-frit bonding (which requires elevated ≥ process temperatures of 430–440 ◦C) was selected to be the best sealing method [46–52] in order to remain compatible with the fixed MEMS scanner process (see Table2 for a comparison of alternative hermetic bonding techniques). Earlier work on glass-frit-bonded WLVP of AME75 processed MEMS mirrors resulted in 500 Pa internal pressure [52] (without getter) (in this work, <1 Pa was required). For similar MEMS mirrors, a significant degradation of mirror planarity was observed at elevated process temperatures. The radius of mirror curvature decreased below R < 2 m at temperatures above 350 ◦C[53] (equal to a mirror deformation of δpp > 1.56 µm assuming a 5 mm mirror diameter), which was not acceptable for this work. An initial concept for NIR-MEMS WLVP using glass-frit bonding and integration of a state-of-the-art Zr-based getter for long-term stabilization of the internal pressure below <1 Pa was presented in [54]. Very first results demonstrate exemplarily a sufficient inner vacuum pressure of 0.25 Pa using only a single WLVP stack with simplified set-up (without Au mirror coating, no BB-ARC). New experimental results using a complete WLVP set-up (with Au mirror coating and BB-ARC) show that the initial NIR-WLVP concept [54] failed due to reliability issues. The main failure mechanism is unknown (e.g., inner outgassing sources, degradation of the Au coating).

Table 2. Qualitative comparison of wafer bonding approaches for applicability on MOEMS-WLVP.

Bonding Bonding Topo-Graphy Main Advantage Main Disadvantage Ref. Approach Temperature (◦C) Tolerance Good for silicon–glass High process voltage [46] Anodic 300–450 Low - Traditional Poor CMOS compatibility [46] Direct >800 Very low Excellent hermetic sealing, approaches Glass-frit 430–450 High possibility of incorporating Large seal area [46–48] metallic feedthroughs >200 Medium Ductile seal [39,46] Eutectic Complex deposition >200 Medium Re-melting temperature is [39,46] Metal Bonding SLID (solid-liquid- Lack of ductility in seal greater than bonding Approaches inter-diﬀusion) No collapse layer temperature Metal–to-metal to absorb topology >200 Low Mature process [46] Adhesive BCB >150 High Ductile seal Non-hermetic [46]

In this article, we carefully investigated the influence of technological factors (e.g., elevated process temperature during glass-frit bonding) on the inner vacuum pressure, optical performance (e.g., static mirror deformation), and long-term stability of the MEMS WLVP. The degradation of Au mirror planarity at elevated process temperatures (up to 440 ◦C) was observed as the main failure mechanism. Potential failure sources for outgassing and degradation of the long-term stability of the inner vacuum pressure were also investigated. These reliability issues were eliminated within the modified WLVP process, e.g., by using an additional Al2O3 diffusion barrier layer to prevent thermally induced degradation of the Au mirror coating. Finally, a translatory MEMS WLVP with vacuum pressure 0.25 Pa and Q > 18,000 was realized, resulting in a stroke of 700 µm at 267 Hz for driving ≤ at 4 V in parametric resonance. The long-term stability of the 0.2 Pa inner vacuum was successfully tested, which was estimated to be >10 years using a Ne fine-leakage test. Parasitic tilt of 20 arcsec was measured for selected MEMS devices over the full 700 µm scan. These meet the requirements of a compact NIR-FTS with high SNR > 1000 and a spectral resolution of ∆ν 15 cm 1 for a spectral ≤ − bandwidth of 1200–2500 nm. Micromachines 2020, 11, 883 6 of 28

In this article, we show that glass-frit bonding can be used for a relatively simple and cost-eﬀective WLVP process of optical MEMS, while avoiding degradation of optical performance even at high process temperatures of up to 440 ◦C. In addition to the NIR-FTS application, our experimental results and the observed failure mechanism of the glass-frit based WLVP process reported in this work could be helpful to a broader community using glass-frit bonding for low-cost WLVP of various smart MEMS sensors or MOEMS applications.

2. Materials and Methods This translatory MEMS actuator with optical WLVP was especially developed for fast optical path-length modulation in a compact NIR-FT spectrometer for NIR spectral region λ = 800–2500 nm. The general speciﬁcation of NIR-FTS and boundary conditions (considered for the MEMS design) are summarized in Table2. Additional requirements follow from the used MEMS processes. In this work, we reduced the scan amplitude of a 5 mm mirror aperture from 500 µm to 350 µm in comparison to the previous MEMS-based MIR-FTS [25]. With a stroke of 700 µm (equal to twice 1 the amplitude), this MEMS provides a spectral resolution of ∆ν = 14.2 cm− for a similar FTS instrumentation (shown in Figure1a). This is useful, because requirements on parasitic MEMS properties, e.g., parasitic tilt and static and dynamic mirror deformation, are signiﬁcantly higher due to the smaller wavelength. This can be seen in comparison to the requirements for a conservative NIR-FTS 1 design, targeting a high spectral resolution of ∆ν = 8 cm− . A very small value of only 2” follows for the parasitic tilt angle, which has to be guaranteed over the entire stroke of 1.25 mm; this would be highly risky. Moreover, a small mirror deformation of 80 nm = 1/10 λmin (p-p value) results from the minimal wavelength of 800 nm. Both (i) parasitic mirror tilt and (ii) static mirror deformation are the main challenges for the MEMS and WLVP process due to (i) geometrical tolerances of narrow spring geometries caused by the deep reactive ion etching (DRIE) process, and (ii) static mirror deformation resulting from thermal stress on the optical coating induced by high WLVP process temperatures of up to 430–440 ◦C, required for hermetic glass-frit bonding.

2.1. Translatory MEMS Design During the MEMS design process, the following technological constraints were ﬁxed by the MEMS scanner technology, available at FhG-IPMS [45]:

electrostatic resonant driving using vertical comb drives, • use of 75 µm thick SOI (silicon-on-isolator) layer of monocrystalline silicon, • no additional stiffening structures at mirrors backside. • The main challenges for this MEMS are (i) to enable a tilt-free large stroke with (ii) reduced static or dynamic mirror deformation and (iii) high mechanical reliability. An early design approach for translatory MEMS with 1.65 mm2 aperture was based on a mirror suspension using two folded bending springs, resulting in a limited amplitude of 100 µm and significant dynamic mirror deformations caused by the mirror suspension itself [7]. Next, in [23], a 1 kHz translatory 3 mm mirror with two pantograph suspensions was tested to increase the scan amplitude to 300 µm. Here, the pantograph suspensions use torsional springs as deflectable elements instead of bending springs. This has the potential for larger deflection and reduced mechanical stresses coupled into the mirror plate at the same time. Unfortunately, due to superimposed parasitic torsional modes, only an amplitude of 140 µm could be measured for the two-pantograph MEMS device, which is not suitable for an FTS. The problem of mode separation was fixed in [25] for a translatory MEMS device with 500 Hz and 5 mm diameter mirror using a fully symmetric mechanical design of four pantograph suspensions, enabling large strokes of up to 1.4 mm and avoiding problems with parasitic modes and tilting. Micromachines 2020, 11, 883 7 of 28

2.1.1. Pantograph Mirror Suspension for Large Stroke The actual NIR-FTS optimized translatory MEMS device uses also a point-symmetric configuration Micromachinesof four pantograph 2020, 11, x suspensions FOR PEER REVIEW of a 5 mm mirror aperture to guarantee a tilt-free out-of-plane translation7 of 27 (seeMicromachines Figure3 20b).20 One, 11, x singleFOR PEER pantograph REVIEW consists of six torsional springs (see Figure3a,c): two springs7 of 27 3arrangeda,c): two on springs the same arranged axis and on connected the same by axis stiff andlevers connected (SEM photographs by stiff levers of mechanical (SEM photographs pre-deflected of 3a,c): two springs arranged on the same axis and connected by stiff levers (SEM photographs of mechanicalMEMS samples pre-deflected are presented MEMS insamples Figure are4). presented Due to the in Figure orthogonal–anisotropic 4). Due to the orthogonal elastic– modulianisotropic of mechanical pre-deflected MEMS samples are presented in Figure 4). Due to the orthogonal–anisotropic elasticmonocrystallineelastic moduli moduli of of silicon,monocrystalline monocrystalline a design with silicon, four a pantographsdesign withwith fourfour instead pantogra pantogra of threephsphs was instead instead chosen of of three tothree be was more was chosen robustchosen tototo be fabricationbe more more robust robust tolerances to to fabrication fabrication and less tolerancestolerances sensitive and to parasitic lessless sensitivesensitive mirror to to tilt.parasitic parasitic Details mirror mirror of the tilt. tilt. three Details Details diff oferent of the the springthree three differentaxesdifferent are shownspring spring in axesaxes Figures areare shown3shown and5 . in In Figure addition 3 and to the FigureFigure torsional 5.5. InIn springs, additionaddition other to to the the mechanical torsional torsional springs, structures springs, other other are mechanicalvisible,mechanical used s structures fortructures limiting areare maximal visible,visible, usedused out-of-plane for limiting translation. maximalmaximal out out--ofof-plane-plane translation. translation.

FigureFigureFigure 3. 3.3. Pantograph PantographPantograph mirror mirror suspensionsuspension for for large large-stroke--strokestroke translatory translatory MEMS: MEMS: ( a (a) ))SEM SEMSEM detail detaildetail of ofof single singlesingle pantographpantographpantograph (pre (pre-deﬂected,(pre--deflected,deflected, three three parallel parallel torsional torsional spring spring ax axesaxeess are are are obvious) obvious);obvious); ;( (b(bb)) ) translatory translatory translatory MEMS MEMS MEMS devicedevicedevice designed designed for for for NIR NIR NIR withwith with fullfull full symmetric symmetric suspension suspension ofof 5 5 ofm mm 5m mmmirror mirror mirror using using usingfour four pantographs fourpantographs pantographs (FEA (FEA geometry(FEAgeometry geometry model model model of of movable movable of movable elements)elements) elements);; (c) details (c) details ofof pantographpantograph of pantograph suspension suspension suspension of of NIR NIR of- NIR-MEMSMEMS-MEMS design design design.. .

TheTheThe pantograph pantographpantograph geometry geometry was was optim optimizedized toto to achieveachieve (i) (i) a a lower lowerlower frequency frequencyfrequency for forfor an anan out out-of-planeout-of-of-plane-plane translationtranslationtranslation oscillation oscillationoscillation mode modemode of of 250 250 HzHz and and (ii) (ii) reducedreduced viscous viscous gas gas damping dampingdamping and andand demands demandsdemands on onon vacuum vacuumvacuum pressurepressurepressure using using pantograph pantographpantograph levers leverslevers compared compared to to previous previousprevious MEMS. MEMS.MEMS. In In general,In general, general, the the ﬁnalthe final final MEMS MEMS MEMS design design design was wasdevelopedwas de developedveloped within within within an iterative anan iterativeiterative design design process process using usingusing ﬁnite finite elementfinite elementelement analysis analys analys (FEA)isis (FEA) simulations(FEA) simulations simulations of single of of TM TM singleandsingle coupled and and coupled coupled physical physicalphysical domains domainsdomains with ANSYS with ANSYSMultiphysics,TM Multiphysics,Multiphysics, as well asas as simulation well well as as simulation simulation of dynamic of of dynamic transients dynamic transients(e.g.,transients voltage-dependent (e.g. (e.g., , voltagevoltage--dependent frequency–responsedependent frequency curves––responseresponse of parametric curves curves of of resonance) p parametricarametric using resonance) resonance) reduced-order using using reduced-order models (ROMs). reducedmodels- (ROMs).order models (ROMs).

(a) (b) (a) (b) Figure 4. SEM details of single pantograph suspension: mirror moved (a) up and (b) down. Figure 4. SEM detai detailsls of single pantograph suspension suspension:: mirror moved ( a) up and ( b) down.

Figure 5. SEM pantograph details of deflectable spring system: (a) spring 1; (b) spring 2; (c) spring 3. Figure 5. SEM pantograph details of deflectable spring system: (a) spring 1; (b) spring 2; (c) spring 3.

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3a,c): two springs arranged on the same axis and connected by stiff levers (SEM photographs of mechanical pre-deflected MEMS samples are presented in Figure 4). Due to the orthogonal–anisotropic elastic moduli of monocrystalline silicon, a design with four pantographs instead of three was chosen to be more robust to fabrication tolerances and less sensitive to parasitic mirror tilt. Details of the three different spring axes are shown in Figure 3 and Figure 5. In addition to the torsional springs, other mechanical structures are visible, used for limiting maximal out-of-plane translation.

Figure 3. Pantograph mirror suspension for large-stroke translatory MEMS: (a) SEM detail of single pantograph (pre-deflected, three parallel torsional spring axes are obvious); (b) translatory MEMS device designed for NIR with full symmetric suspension of 5 mm mirror using four pantographs (FEA geometry model of movable elements); (c) details of pantograph suspension of NIR-MEMS design.

The pantograph geometry was optimized to achieve (i) a lower frequency for an out-of-plane translation oscillation mode of 250 Hz and (ii) reduced viscous gas damping and demands on vacuum pressure using pantograph levers compared to previous MEMS. In general, the final MEMS design was developed within an iterative design process using finite element analysis (FEA) simulations of single and coupled physical domains with ANSYSTM Multiphysics, as well as simulation of dynamic transients (e.g., voltage-dependent frequency–response curves of parametric resonance) using reduced-order models (ROMs).

(a) (b) Micromachines 2020, 11, 883 8 of 28 Figure 4. SEM details of single pantograph suspension: mirror moved (a) up and (b) down.

Micromachines 2020, 11, x FOR PEER REVIEW 8 of 27 FigureFigure 5.5. SEMSEM pantographpantograph details of deflectable deflectable spring spring system: system: (a ()a )spring spring 1; 1; (b ()b spring) spring 2; 2;(c) ( cspring) spring 3. 3. 2.1.2.2.1.2. Reduction Reduction of of Dynamic Dynamic Mirror Mirror Deformation Deformation ToTo reduce reduce the the dynamic dynamic mirror mirror deformation deformation of of the the 75 75 µmµm thick thick silicon silicon mirror mirror plate plate to to the the NIR NIR-FTS-FTS specifiedspecified value value for for dynamic dynamic deformation deformation of of δδpppp ≤ 8080 nm nm == 1/101/10 λλmin, ,the the best best design design compromise compromise was was ≤ min foundfound by by reducing reducing the the resonance resonance frequency frequency of of the the used used translation translation mode mode to ~250~250 Hz. The The surface topologytopology of of the the dynamically dynamically deformed deformed mirror mirror plate, plate, occurring occurring for for a a harmonic harmonic mirror mirror oscillation oscillation of of 256.6256.6 Hz Hz at at a amaximal maximal z-zdeflection-deflection of of 350 350 µmµm,, is is shown shown in in Figu Figurere 66a.a. AA satisfactorysatisfactory dynamicdynamic mirrormirror deformationdeformation of of δppδ pp= 84= 84nm nm = λ=minλ/9.5min/ 9.5and and δRMSδ RMS= 24 =nm24 was nm wassimulated. simulated.

(a) (b)

FigureFigure 6. 6.ReductionReduction of of dynamic dynamic mirror mirror deformation: deformation: (a ()a FEA) FEA results results of of topology topology at at 350 350 µmµm deflection deﬂection;; (b()b SEM) SEM photograph photograph of of outer outer ring ring-shaped-shaped support support structure structure with with mechanical mechanical decoupling decoupling springs. springs.

WeWe have have to to mention mention here thatthat thethe mirror mirror deformation deformation could could not not be minimizedbe minimized to the to specifiedthe specified limit limitonly only by reducing by reducing the oscillation the oscillation frequency. frequency. On the one Onhand, the one the MEMS hand, devicethe MEMS becomes device mechanically becomes mechanicalfragile andly sensitivefragile and to mechanicalsensitive to shocksmechanical (also shock crucials (also for MEMS crucial fabrication). for MEMS fabrication) On the other. O hand,n the othera direct hand mechanical, a direct mechanical coupling of coupling pantograph of pantograph suspension andsuspension mirror plate and mirror significantly plate significantly deforms the deformmirrors inthe addition mirror in to addition the dynamic to the forces dynamic caused forces by inertiacaused (see by inertia Figures (see7a and Figure8a).s 7a and 8a). To avoid the additional deformation, the circular mirror aperture of 5 mm diameter is kept flexible by narrow radial springs within an outer ring-like support structure attached to the pantograph suspensions (see Figure6b). The additional soft spring elements enable the mechanical decoupling of the mirror plate from pantograph suspension. This makes the dynamic deformation profile almost rotationally symmetric, like a free circular plate with translational oscillation in z. In the past, several options to reduce dynamic mirror deformation for large-stroke translatory MEMS were investigated (results not yet published). We have to point out that the 75 µm SOI thickness of the mirror plate was not changeable due to the MEMS process AME75. Three variants of large-stroke pantograph MEMS designs were tested: (i) initial design with direct coupling of pantographs and mirror plate (see Figure7a, [25]), (ii) ring-like support structure with mechanical decoupling of inner (a) (b) mirror (Figure6b; variant used also within this work), and (iii) a conceptual design for dynamic Figure 7. Further options to minimize the mirror deformation of pantograph MEMS: (a) initial design with direct coupling of pantographs and mirror plate; (b) conceptual design for dynamic self- compensation of dynamic mirror deformation by means of additional outer inert masses and locally thinned mirror membrane.

To avoid the additional deformation, the circular mirror aperture of 5 mm diameter is kept flexible by narrow radial springs within an outer ring-like support structure attached to the pantograph suspensions (see Figure 6b). The additional soft spring elements enable the mechanical decoupling of the mirror plate from pantograph suspension. This makes the dynamic deformation profile almost rotationally symmetric, like a free circular plate with translational oscillation in z. In the past, several options to reduce dynamic mirror deformation for large-stroke translatory MEMS were investigated (results not yet published). We have to point out that the 75 µm SOI thickness

Micromachines 2020, 11, x FOR PEER REVIEW 8 of 27

2.1.2. Reduction of Dynamic Mirror Deformation To reduce the dynamic mirror deformation of the 75 µm thick silicon mirror plate to the NIR-FTS specified value for dynamic deformation of δpp ≤ 80 nm = 1/10 λmin, the best design compromise was found by reducing the resonance frequency of the used translation mode to ~250 Hz. The surface topology of the dynamically deformed mirror plate, occurring for a harmonic mirror oscillation of 256.6 Hz at a maximal z-deflection of 350 µm, is shown in Figure 6a. A satisfactory dynamic mirror deformation of δpp = 84 nm = λmin/9.5 and δRMS = 24 nm was simulated.

(a) (b)

Figure 6. Reduction of dynamic mirror deformation: (a) FEA results of topology at 350 µm deflection; Micromachines(b) SEM2020 photograph, 11, 883 of outer ring-shaped support structure with mechanical decoupling springs. 9 of 28

We have to mention here that the mirror deformation could not be minimized to the specified self-compensationlimit only by reducing of mirror the deformation oscillation frequency.by means of On additional the one outer hand inert, the masses MEMS of device a locally becomes thinned mirrormechanical membranely fragile (Figure and 7sensitiveb). Theresulting to mechanical dynamic shock mirrors (also deformations crucial for MEMS of these fabrication) variants are. O shownn the inother Figure hand8., Fora direct better mechanical comparison, coupling all variants of pantograph were simulated suspension with and 5 mm mirror mirror plate diameter significantly and identicaldeforms the operation mirror pointin addition (MIR-FTS: to the 500 dynamicµm amplitude, forces caused 500 Hz,by inertiaλmin = (see2.5 µFigurem). s 7a and 8a).

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of the mirror plate was not changeable due to the MEMS process AME75. Three variants of large- stroke pantograph MEMS designs were tested: (i) initial design with direct coupling of pantographs and mirror plate (see Figure7a, [25]), (ii) ring-like support structure with mechanical decoupling of (a) (b) inner mirror (Figure 6b; variant used also within this work), and (iii) a conceptual design for dynamic self-Figurecompensation 7. 7. FurtherFurther of options mirror options to deformation minimize to minimize the by mirror the means mirror deformation of deformationadditional of pantograph outer of pantograph inert MEMS: masses MEMS: ( ofa) initiala locally (a) design initial thinned mirrordesignwith membrane direct with coupling direct (Figure coupling of 7 b). pantographs The of pantographs resulting and dynamic mirror and mirror plate mirror; plate;(b )deformation conceptual (b) conceptual s design of these design for variants dynamic for dynamic are self shown- in Figureself-compensationcompensation 8. For of better dynamic of dynamiccomparison mirror mirror deformation, all deformation variants by means were by means simulatedof additional of additional with outer 5 inertmm outer massesmirror inert massesand diameter locally and and identicallocallythinned operation thinned mirror membrane. mirror point membrane.(MIR -FTS: 500 µm amplitude, 500 Hz, λmin = 2.5 µm).

FigureFigure 8.8.FEA FEA resultsresults ofof minimizationminimization of mirrormirror deformation of 5 mm pantograph mirrors mirrors of of 75 75 µmµm thickthick c-Si; c-Si; translatory translatory MEMS MEMS designsdesigns werewere developeddeveloped forfor MIR-FTSMIR-FTS for 500 µmµm amplitude at at 500 500 Hz Hz and and λ µ λminmin = 2.52.5 µm:m: (a (a) )initial initial MEMS MEMS design design with with dir directect mechanical mechanical coupling coupling of pant of pantographograph susp suspensionsensions and andmirror mirror plate plate;; (b) (bwith) with ring ring-shaped-shaped pantograph pantograph support support structure structure and and mirror mirror h heldeld by by mechanical mechanical decouplingdecoupling structure;structure; ((cc)) dynamicdynamic self-compensationself-compensation by means of additional outer outer inert inert masses masses and and locallylocally thinned thinned mirrormirror membrane.membrane.

λ ItIt is is obviousobvious for this study study that that the the initial initial variant variant results results in a in poor a poor mirror mirror planarity planarity of λmin of/3.4min (see/3.4 (seeFigure Figure 8a),8 whereasa), whereas the thevariant variant with with ring- ring-shapedshaped supp supportort reduce reducess the mirror the mirror deformation deformation by a factor by a factorof 1.7 of(Figure 1.7 (Figure 8b). A8 minimalb). A minimal dynamic dynamic deformation deformation of only λ ofmin/14.7 only wasλmin simulated/14.7 was for simulated the conceptual for the conceptualdesign with design dynamic with self dynamic-compensation self-compensation (Figure 8c). On (Figure the other8c). Onhand the, the other conceptual hand, the design conceptual results designin highe resultsr technological in higher complexity technological (local complexity thinning of (localthe mirror thinning backside of the by deep mirror reactive backside ion etching, by deep reactiveDRIE), resulting ion etching, also DRIE), in higher resulting risks for also asymmetries, in higher riskstilting for, and asymmetries, sensitivity to tilting, mechanical and sensitivity shock. to mechanicalTherefore, shock. we used in this work the ring-shaped pantograph support structure and 250 Hz resonanceTherefore, frequency, we used resulting in this in work a dynamic the ring-shaped mirror deformation pantograph of δ supportpp = 84 nm structure = λmin/9.5 and, which 250 Hzis resonancesufficient for frequency, the NIR- resultingFTS application in a dynamics (Figure mirror 6). deformation of δpp = 84 nm = λmin/9.5, which is suﬃcient for the NIR-FTS applications (Figure6). 2.1.3. Modal Analysis 2.1.3. Modal Analysis The FEA results of the linear modal analysis are summarized in Figure 9 for the first to 16th eigenmodes.The FEA Here, results a good of the mode linear separation modal analysis of the used are translation summarized mode in 1 Figure at 2579 Hz for to the the ﬁrst next to higher 16th eigenmodes.parasitic (tilting) Here, mode a good 2 at mode 1200 separation Hz is obvious. of the Higher used translation modes are modeonly of 1 atpractical 257 Hz rel toevance the next during higher parasiticthe wire- (tilting)bonding mode process 2 at or 1200 when Hz used is obvious. in mechanically Higher rough modes environments, are only of practical if they can relevance be excited during by high-frequency external vibrations. To avoid parasitic excitations, integral multiples of mode 1 must avoided during the MEMS design, which is guaranteed for this translatory MEMS device.

Micromachines 2020, 11, 883 10 of 28 the wire-bonding process or when used in mechanically rough environments, if they can be excited by high-frequency external vibrations. To avoid parasitic excitations, integral multiples of mode 1 must Micromachinesavoided during 2020, 1 the1, x FOR MEMS PEER design, REVIEW which is guaranteed for this translatory MEMS device. 10 of 27

FigureFigure 9. 9. ResultsResults of of FEA FEA modal modal analysis analysis:: eigenmodes eigenmodes and and separation separation to used to used translation translation mode mode 1 at 1 257 at Hz.257 Hz. 2.1.4. Electrostatic Comb Drives and Mechanical Reliability 2.1.4. Electrostatic Comb Drives and Mechanical Reliability This MEMS device is actuated by electrostatic comb drives, driven in parametric resonance [25,55]. This MEMS device is actuated by electrostatic comb drives, driven in parametric resonance Two versions of comb drive variants were investigated: (i) basic comb drives attached to the ring-like [25,55]. Two versions of comb drive variants were investigated: (i) basic comb drives attached to the support structure (see Figures3b and 10a), and (ii) additional comb drives at the pantograph levers ring-like support structure (see Figures 3b and 10a), and (ii) additional comb drives at the pantograph (Figures4 and 10b) to increase available frequency bandwidth at 350 µm amplitude. The results of levers (Figure 4 and Figure 10b) to increase available frequency bandwidth at 350 µm amplitude. The this article are based on MEMS devices with basic comb variants only (see Figure 10a), using four results of this article are based on MEMS devices with basic comb variants only (see Figure 10a), using comb drives symmetrically arranged at the support ring. Using ROM simulations of viscous gas four comb drives symmetrically arranged at the support ring. Using ROM simulations of viscous gas damping [56] and modifying the model parameters to a reduced vacuum pressure, a driving voltage damping [56] and modifying the model parameters to a reduced vacuum pressure, a driving voltage of 44.3 V was simulated to reach an amplitude of 350 µm at an assumed vacuum pressure of 10 Pa. of 44.3 V was simulated to reach an amplitude of 350 µm at an assumed vacuum pressure of 10 Pa. This is slightly below the simulated electrostatic stability (pull-in) voltage of Upull-in = 46.5 V. Hence, This is slightly below the simulated electrostatic stability (pull-in) voltage of Upull-in = 46.5 V. Hence, a a WLVP with <10 Pa inner vacuum pressure is required. WLVP with <10 Pa inner vacuum pressure is required. Concerning mechanical reliability, a mechanical stress of σ1 = 0.5 GPa at a maximal deflection of 350 µm and σ = 1.47 GPa at 2500 g equivalent shock acceleration were simulated using 1,eq × nonlinear FEA simulations. Both stress values were below the design limit of 1.5 GPa, required for ≤ high mechanical reliability. To enhance the robustness to mechanical shocks, additional mechanical structures were designed, which are arranged parallel to the torsion springs (see Figure5). In the event of mechanical contact due to mechanical shock, they cause the springs to stiffen and limit the out-of-plane translation. This stiffening concept was first demonstrated in [24] (see Figure4 on pp. 6).

(a) (b)

Figure 10. SEM photographs of electrostatic comb drives (visible are also the filled isolation trenches): (a) basic comb drives at ring-like support structure; (b) comb drive variant at pantograph levers.

Concerning mechanical reliability, a mechanical stress of σ1 = 0.5 GPa at a maximal deflection of 350 µm and σ1,eq = 1.47 GPa at 2500× g equivalent shock acceleration were simulated using nonlinear

Micromachines 2020, 11, x FOR PEER REVIEW 10 of 27

Figure 9. Results of FEA modal analysis: eigenmodes and separation to used translation mode 1 at 257 Hz.

2.1.4. Electrostatic Comb Drives and Mechanical Reliability This MEMS device is actuated by electrostatic comb drives, driven in parametric resonance [25,55]. Two versions of comb drive variants were investigated: (i) basic comb drives attached to the ring-like support structure (see Figures 3b and 10a), and (ii) additional comb drives at the pantograph levers (Figure 4 and Figure 10b) to increase available frequency bandwidth at 350 µm amplitude. The results of this article are based on MEMS devices with basic comb variants only (see Figure 10a), using four comb drives symmetrically arranged at the support ring. Using ROM simulations of viscous gas damping [56] and modifying the model parameters to a reduced vacuum pressure, a driving voltage of 44.3 V was simulated to reach an amplitude of 350 µm at an assumed vacuum pressure of 10 Pa. ThisMicromachines is slightly2020 below, 11, 883 the simulated electrostatic stability (pull-in) voltage of Upull-in = 46.5 V. Hence,11 of 28 a WLVP with <10 Pa inner vacuum pressure is required.

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FEA simulations. Both stress(a) values were below the design limit of ≤1.5 (GPab) , required for high mechanical reliability. To enhance the robustness to mechanical shocks, additional mechanical FigureFigure 10. 10. SEMSEM photographs photographs of of electrostatic electrostatic comb comb drives drives (visible are also the ﬁlledfilled isolation trenches):trenches): structures were designed, which are arranged parallel to the torsion springs (see Figure 5). In the event ((aa)) basic basic comb comb drives drives at at ring ring-like-like support support structure structure;; ( (bb)) comb comb drive drive variant variant at at pantograph pantograph levers. levers. of mechanical contact due to mechanical shock, they cause the springs to stiffen and limit the out-of- plane2.2. translation. MEMS Device This Wafer stiffening Fabrication concept was first demonstrated in [24] (see Figure 4 on pp. 6). Concerning mechanical reliability, a mechanical stress of σ1 = 0.5 GPa at a maximal deflection of 350 µmThe and MOEMS σ1,eq = 1.47 devices GPa at were 2500× fabricated g equivalent by shock the qualiﬁed acceleration IPMS were process simulated AME75, using developed nonlinear 2.2. MEMS Device Wafer Fabrication for electrostatic comb-driven micro scanning mirrors (Figure 11). They use 6” BSOI (bonded

silicon-on-isolator)The MOEMS devices substrates were with fabricated a 75 µ mby thick, the highlyqualified p-doped IPMS SOIprocess device AME75 layer,, 1 developedµm buried oxide for electrostatic(BOX) layer, comb and-driven 400 µmicrom handling scanning layer. mirrors Details (Figure of the 11) MEMS. They use process 6” BSOI can be(bonded found, silicon e.g., in-on [45- ]. isolator)For this substrates work we with have a to75 pointµm thick out the, highly following p-doped specifics SOI device of the devicelayer, 1 wafer µm buried (DW) relevantoxide (BOX for the) layerMEMS, and 400 WLVP: µm handling layer. Details of the MEMS process can be found, e.g., in [45]. For this work we have to point out the following specifics of the device wafer (DW) relevant for the MEMS WLVP: Use of field isolation trenches (formed from DRIE-etched open trenches by thermal oxidation and •  Userefilling of field isolation with polysilicon) trenches (formed to electrically from DRIE isolate-etched areas open of diff trencherentes electrical by thermal potential oxidation within and the refillingsame with SOI layerpolysilicon) needed to defineelectrical thely comb isolat drivese areas (Figure of different 10), electrical potential within the sameUse SOI of layer AlSiCu needed metal to lines define for the electrical comb drives signal ( transmissionFigure 10), from the bond islands at outer chip •  Useframe of AlSiCu to the metal inner lines comb for drive electrical actuator signal (no VIAtransmission (vertical interconnectfrom the bond access) islands exists), at outer chip frameUse to of the a thininner protected comb drive aluminum actuator layer (no VIA as standard (vertical optical interconnect coating, access) exists),  •Use of a thin protected aluminum layer as standard optical coating, CMOS (complementary metal–oxide–semiconductor) compatibility of all inline processes due to  •CMOS (complementary metal–oxide–semiconductor) compatibility of all inline processes due to restrictions caused by in-house CMOS processes for highly integrated micro mirror arrays [57]. restrictions caused by in-house CMOS processes for highly integrated micro mirror arrays [57].

(a) (b)

FigureFigure 11. Fabrication 11. Fabrication of device of device wafer wafer:: (a) MEMS (a) MEMS process process AM75 AM75;; (b) (backendb) backend integration integration of Au of Au coating. coating.

TheThe standard standard Al coating Al coating is not is notpossible possible to meet to meet the the high high reflectance reﬂectance of ofR > R 95%> 95% required required for for the the NIRNIR-FTS.-FTS. Therefore, Therefore, we weused used Au Aufor forreflection reﬂection coating coating and and a symmetric a symmetric coating coating design design for for thermal thermal compensationcompensation to guarantee to guarantee a small a small static staticmirror mirror deformation deformation of ≤λmin/10 of λ minafter/10 the after WLVP the WLVPprocess process.. Due ≤ to CMOSDue to compatibility, CMOS compatibility, in a backend in a backend process, process, identical identical Au coatings Au coatings were deposited were deposited on the onfront the and front rearand faces rear of facesthe silicon of the mirrors silicon mirrorsusing shadow using shadow masks for masks lateral for patterning lateral patterning of the Au of coatings. the Au coatings.

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2.3. Wafer-Level Vacuum Package of Optical MEMS In the previous MIR-FTS development, a WLVP was not applicable due to (i) the need to use a ZnSe window, and (ii) the limitation to use an open trench isolation [25], because a field trench isolation was not available for 75 µm SOI at the time. In addition to the ZnSe window, this open trench insolation (needed also around bond islands located outside the vacuum cavity) prevented any hermetic vacuum sealing on the wafer level. In this work, the challenge for hermetic sealing of WLVPs results from the topology and roughness of the DW within the areas needed for the future bonding frames. These bonding areas are crossed by the metal lines required to contact the outer bond islands. Profilometer measurements showed a significant topology of ~1.8 µm (pp) [52] on these DW locations. Due to the significant surface topology, most bonding methods which could potentially be used for vacuum-packaging (see Table2), such as metallic thermo-compression bonding, solid-liquid-inter-diffusion (SLID), eutectic AuSi bonding [39,44], and anodic bonding [46], are not Micromachines 2020, 11, x FOR PEER REVIEW 12 of 27 applicable to our WLVP process. For this work work,, glass glass-frit-frit bonding [ 47––5252]] is is the the most most reliabl reliablee bonding bonding method method,, allowing allowing hermetic hermetic sealing over surface topologies several µmµm high.high. Th Thisis bonding approach allow allowss realiz realizinging hermetic hermetic electricalelectrical interconnects using existing existing metal metal lines lines without without complex complex technological technological changes changes for for the the DW DW.. Only the mask layout was designed to form a closed tetra tetra-etyhl-ortho-silikat-etyhl-ortho-silikat (TEOS) (TEOS) oxide oxide frame frame on on DW within within the the areas areas reserved reserved for for glass glass-frit-frit bonding bonding (see (see Figure Figure 12a). 12 Firsta). results First results on WLVP on WLVP using glass using- glass-fritfrit bonding bonding for hermetic for hermetic vacuum vacuum sealing sealing of electrostaticof electrostatic resonant resonant tilting tilting MEMS MEMS scanners scanners were were reported inin [[5252].]. An inner vacuum pressurepressure ofof 2–202–20 mbarmbar (200–2000(200–2000 Pa)Pa) was was estimated estimated without without using using a getter.a getter. This This is is insu insufficientfficient for for this this translatory translatory MEMS,MEMS, requiringrequiring an internal pressure pressure of of at at most most 10 10 Pa. Pa. Hence, a thin film film getter [ [5858––6060]] is is needed needed for for this this WLVP WLVP,, using using a a high highlyly eff effiicientcient zirconium zirconium (Zr) (Zr)-based-based thin-filmthin-film getter from SAES Getters Getters (SA (SAESES Getters Getters S.p.A., S.p.A., Lainate Lainate (MI), (MI), Italy). Italy).

Figure 12. Translatory MEMS device after fabrication of of device wafer: ( (aa)) chip chip photograph photograph of of MEMS MEMS chip without WLVP; (b) microscopic detail of of pantograph suspension suspension and and comb comb drive; drive; ( (cc)) MEMS MEMS chip chip topography measured with WLI WLI (white (white light light interferometry); interferometry); metal metal lines lines add add significant signiﬁcant height height profile. proﬁle.

2.3.1. Concept of of WLVP WLVP for for NIR NIR-FTS-FTS The schematic setset-up-up of the optical MEMS wafer wafer-level-level vacuum vacuum packa packagege ( (WLVP)WLVP) is is shown shown in in Figure 1313a.a. The WLVPWLVP consistsconsists of of four four pre-fabricated pre-fabricated substrates, substrates, sequentially sequentially bonded bonded by by three three glass-frit glass- wafer bonding processes to form the ﬁnal WLVP stack of 3476 µm total thickness in addition to 3 the frit wafer bonding processes to form the final WLVP stack of 3476 μm total thickness in addition to× 3× layerthe layer thickness thickness of the of the glass-frit glass-frit bonding bonding layer. layer.

(b)

(a) (c)

Figure 13. Schematic set-up of the MEMS WLVP optimized for NIR-FTS: (a) schematic cross-section of WLVP stack; total thickness of WLVP stack is 3476 μm + thicknesses of glass-frit layers; (b) overall dimensions of encapsulated cavity; (c) MEMS WLVP chip.

Micromachines 2020, 11, x FOR PEER REVIEW 12 of 27

For this work, glass-frit bonding [47–52] is the most reliable bonding method, allowing hermetic sealing over surface topologies several µm high. This bonding approach allows realizing hermetic electrical interconnects using existing metal lines without complex technological changes for the DW. Only the mask layout was designed to form a closed tetra-etyhl-ortho-silikat (TEOS) oxide frame on DW within the areas reserved for glass-frit bonding (see Figure 12a). First results on WLVP using glass- frit bonding for hermetic vacuum sealing of electrostatic resonant tilting MEMS scanners were reported in [52]. An inner vacuum pressure of 2–20 mbar (200–2000 Pa) was estimated without using a getter. This is insufficient for this translatory MEMS, requiring an internal pressure of at most 10 Pa. Hence, a thin film getter [58–60] is needed for this WLVP, using a highly efficient zirconium (Zr)-based thin-film getter from SAES Getters (SAES Getters S.p.A., Lainate (MI), Italy).

Figure 12. Translatory MEMS device after fabrication of device wafer: (a) chip photograph of MEMS chip without WLVP; (b) microscopic detail of pantograph suspension and comb drive; (c) MEMS chip topography measured with WLI (white light interferometry); metal lines add significant height profile.

2.3.1. Concept of WLVP for NIR-FTS The schematic set-up of the optical MEMS wafer-level vacuum package (WLVP) is shown in Figure 13a. The WLVP consists of four pre-fabricated substrates, sequentially bonded by three glass- frit wafer bonding processes to form the final WLVP stack of 3476 μm total thickness in addition to 3× Micromachines 2020, 11, 883 13 of 28 the layer thickness of the glass-frit bonding layer.

(b)

(a) (c)

FigureFigure 13.13. Schematic set-upset-up ofof thethe MEMSMEMS WLVPWLVP optimized optimized for for NIR-FTS: NIR-FTS: ( a()a schematic) schematic cross-section cross-section of of WLVPWLVP stack;stack; totaltotal thicknessthickness ofof WLVPWLVP stack stack is is 3476 3476µ mμm+ +thicknesses thicknesses of of glass-frit glass-frit layers; layers; (b ()b overall) overall dimensionsdimensions ofof encapsulatedencapsulated cavity;cavity; ((cc)) MEMSMEMS WLVPWLVP chip.chip.

In addition to the 476 µm thick SOI device wafer (DW), containing all active (movable) MEMS structures, the WLVP stack consists of a 1000 µm thick top glass wafer (TGW, used as optical window), a 1000 µm thick (100) Si top spacer wafer (TSW), and a 1000 µm thick bottom (100) Si spacer wafer (BW). A double-sided polished borosilicate-glass wafer with high NIR transmission is used as the top glass wafer (TGW). For technological simplification of WLVP, an ordinary parallel configuration of the optical glass window was chosen instead of wedged optical windows. Parasitic etalon modulations on the measured NIR spectrum arising from multiple reflections between the TGW interior surface and the mirror surface were reduced by means of (i) a broadband antireflective coating (BB-ARC) deposited on the interior surface of the TGW, and (ii) a TSW of 1000 µm thickness, which always guarantees a minimum distance 650 µm between the TGW interior surface and the oscillating mirror, ≥ 1 resulting in an etalon-free spectral range slightly smaller than a spectral resolution of ∆ν = 8 cm− . The bottom wafer (BW), which finally hermetically seals the backside of the WLVP stack, contains a 600 µm deep cavity (TMAH (tetra-methyl-ammonium-hydroxide) etched) with a patterned Zr-based thin-film getter. For electrical characterization, the diced WLVP chips were glued and wire-bonded to a PCB (printed circuit board) using an additional 1 mm thick glass substrate on the backside of WLVP for simplified handling.

2.3.2. Process-Flow for NIR-WLVP The schematic process flow of the WLVP is shown in Figure 14. For fabrication of the top spacer wafer (TSW), a double-sided, polished, 1000 µm thick, (100)-oriented Si wafer was used. The free optical apertures were realized by TMAH wet etching simultaneously from either side (using a SiO2 hard mask). Then, a first, 20 µm thick glass-frit bond layer (Ferro FX-11 036) was screen-printed (patterned by the sieve) on the top spacer wafer. For all glass-frit bonding frames, a width of 490–550 µm was chosen to avoid defects during wafer dicing. After screen printing, the glass-frit layers were first dried at 120 ◦C, followed by a glazing process at 425 ◦C for hardening and out-gassing of the glass-frit frame before bonding (see Figure 15). This was followed by the first bonding process to bond the top glass wafer (TGW) and top spacer wafer (TSW) at 440 ◦C (equal to the glass temperature of the glass-frit layer). For simplicity, the bond interface is located on top of the BB-ARC (uniformly deposited on the inner side of the TGW). Here, the TGW and TSW were aligned flat to flat. Next, the second glass-frit bond layer was screen-printed on the opposite side of the top spacer wafer, using identical processes for drying and pre-baking. Then, the Micromachines 2020, 11, x FOR PEER REVIEW 13 of 27

In addition to the 476 µm thick SOI device wafer (DW), containing all active (movable) MEMS structures, the WLVP stack consists of a 1000 µm thick top glass wafer (TGW, used as optical window), a 1000 µm thick (100) Si top spacer wafer (TSW), and a 1000 µm thick bottom (100) Si spacer wafer (BW). A double-sided polished borosilicate-glass wafer with high NIR transmission is used as the top glass wafer (TGW). For technological simplification of WLVP, an ordinary parallel configuration of the optical glass window was chosen instead of wedged optical windows. Parasitic etalon modulations on the measured NIR spectrum arising from multiple reflections between the TGW interior surface and the mirror surface were reduced by means of (i) a broadband antireflective coating (BB-ARC) deposited on the interior surface of the TGW, and (ii) a TSW of 1000 µm thickness, which always guarantees a minimum distance ≥650 µm between the TGW interior surface and the oscillating mirror, resulting in an etalon-free spectral range slightly smaller than a spectral resolution of ∆ν = 8 cm−1. The bottom wafer (BW), which finally hermetically seals the backside of the WLVP stack, contains a 600 µm deep cavity (TMAH (tetra-methyl-ammonium-hydroxide) etched) with a patterned Zr-based thin- film getter. For electrical characterization, the diced WLVP chips were glued and wire-bonded to a PCBMicromachines (printed2020 circuit, 11, 883 board) using an additional 1 mm thick glass substrate on the backside of 14WLVP of 28 for simplified handling.

2.3.2.stack Process of TGW-Flow and TSW for NIR was-WLVP glass-frit bonded to the front side of the device wafer (DW) using a slightly reduced bond temperature of 435 ◦C, in order not to inﬂuence the ﬁrst bond interface. The schematic process flow of the WLVP is shown in Figure 14.

Micromachines 2020, 11, x FOR PEER REVIEW 14 of 27 followed by the first bonding process to bond the top glass wafer (TGW) and top spacer wafer (TSW) at 440 °C (equal to the glass temperature of the glass-frit layer). For simplicity, the bond interface is located on top of the BB-ARC (uniformly deposited on the inner side of the TGW). Here, the TGW and TSW were aligned flat to flat. Next, the second glass-frit bond layer was screen-printed on the opposite side of the top spacer wafer, using identical processes for drying and pre-baking. Then, the stack of

TGW and TSW was glass-frit bonded to the front side of the device wafer (DW) using a slightly reducedFigureFigure bond 14 14. .temperature SchematicSchematic process process of 435 flow flow°C, of in ofwafer order wafer-level-level not vacuum to vacuum influence package package the for first fortranslatory translatorybond interface. MEMS: MEMS: ( a) 1 ( mma) 1 mmthick Theborofloatthick third borofloat-glass- glassglass wafer-frit bond waferwith NIRlayer with BB NIR (used-ARC BB-ARC foron interfinal onior interior hermetic surface; surface; (vacuumb) screen (b) screen sealingprinting printing ofand WLVP) andpre- pre-bakebake was of firstscreen of - printedglassfirst on- glass-fritfrittop layer of the layeron BW TSW on before (top TSW spacer (topthe spacergetter wafer) wafer)deposition front-side; front-side; (processc) device (c) device(see wafer Figure ( waferDW) 15with (DW)b). symmetric T withhis was symmetric Auused coating in Au order to achieveoncoating Si amirror thermal on Si plate; mirror decoupling (d) plate;screen ( dprinting of) screen getter and printing material pre-bake and from of pre-bake third the highglass of third -processfrit layer glass-frit temperature on bottom layer onwafer bottomneeded (BW) wafer forfront glass- - (BW) front-side; (e) first bonding of top glass wafer (TGW)–TSW stack using flat-to-flat alignment; frit preside;-bake. (e) Otherwise first bonding the of Zr top-based glass getter wafer layer (TGW would)–TSW be stack completely using flat activated-to-flat alignment; and already (f) saturated screen (f) screen printing and pre-bake of second glass-frit layer on TSW backside; (g) second bonding of in the printingambient and atmosphere pre-bake of and second would glass have-frit layerno effect on TSW within backside the WLVP.; (g) second The bonding Zr-based of thinTGW-film–TSW getter– TGW–TSW–DW stack; (h) deposition of Zr-based thin-film getter, using the extern PageWafer process was externallyDW stack; deposited, (h) deposition using of theZr-based PageWafer thin-film® process getter, usingof SAES the Getterextern s.PageWafer process of SAES of SAES Getters; (i) third bonding for final hermetic vacuum sealing of WLVP. Getters; (i) third bonding for final hermetic vacuum sealing of WLVP.

For fabrication of the top spacer wafer (TSW), a double-sided, polished, 1000 µm thick, (100)- oriented Si wafer was used. The free optical apertures were realized by TMAH wet etching simultaneously from either side (using a SiO2 hard mask). Then, a first, 20 µm thick glass-frit bond layer (Ferro FX-11 036) was screen-printed (patterned by the sieve) on the top spacer wafer. For all glass-frit bonding frames, a width of 490–550 µm was chosen to avoid defects during wafer dicing. After screen printing, the glass-frit layers were first dried at 120 °C, followed by a glazing process at 425 °C for hardening and out-gassing of the glass-frit frame before bonding (see Figure 15). This was

Figure 15. Screen-printed glass-frit bonding frames: (a) first glass-frit layer on TSW front-side; Figure 15. Screen-printed glass-frit bonding frames: (a) first glass-frit layer on TSW front-side; (b) after (b) after drying process at 120 C; (c) after glazing process at 425 C; (d) third glass-frit layer on BW drying process at 120 °C ; (c) after◦ glazing process at 425 °C ; (d) third◦ glass-frit layer on BW alignment; alignment; (e) bond frame after drying at 120 ◦C; (f) after getter deposition, using PageWafer process of (e) bond frame after drying at 120 °C ; (f) after getter deposition, using PageWafer process of SAES SAES Getters. Getters. The third glass-frit bond layer (used for final hermetic vacuum sealing of WLVP) was screen-printed onThe top offinal the hermetic BW before vacuum the getter sealing deposition process consists process (seeof two Figure main 15 steps:b). This (i) pre was-heatin usedg in to order 200 °C to forachieve 2 h before a thermal contacting decoupling the wafer of getter stack, material still under from vacuum the high pumping process temperature at process pressure, needed for in glass-fritorder to degaspre-bake. the glass Otherwise-frit layer the Zr-basedand avoid getter early layer getter would activation be completely in step 1, activated and (ii) andfinal already hermetic saturated vacuum in sealingthe ambient of the atmosphereWLVP stack and at 430 would °C an haved final no ebondingffect within pressure. the WLVP. Here The, the Zr-based getter material thin-film is getteractivated was duringexternally heating deposited, when temperature using the PageWafer increases® process to T > 300 of SAES °C. In Getters. this study, we also investigated the influence of the vacuum process pressure (using 0.1, 1, and 25 Pa) on the inner vacuum pressure of WLVP. The WLVP bonding processes were performed using a bond aligner SÜ SS BA8 (SÜ SS MicroTec SE, Garching, Germany) and an SÜ SS SB8 wafer bonder (SÜ SS MicroTec SE, Garching, Germany). The final wafer level vacuum package (WLVP) of translatory MEMS is shown exemplarily in Figure 16 before wafer dicing. The details of infrared and microscopic images show a good homogeneity of the bonding frame. Finally, the 3476 µm thick WLVP stack was separated by wafer dicing into individual chips. Moreover, the bond pads were opened for wire bonding using a three-step sawing process. For characterization, the MEMS WLVP chips are mounted and wire-bonded onto a dedicated PCB.

Micromachines 2020, 11, 883 15 of 28

The final hermetic vacuum sealing process consists of two main steps: (i) pre-heating to 200 ◦C for 2 h before contacting the wafer stack, still under vacuum pumping at process pressure, in order to degas the glass-frit layer and avoid early getter activation in step 1, and (ii) final hermetic vacuum sealing of the WLVP stack at 430 ◦C and final bonding pressure. Here, the getter material is activated during heating when temperature increases to T > 300 ◦C. In this study, we also investigated the influence of the vacuum process pressure (using 0.1, 1, and 25 Pa) on the inner vacuum pressure of WLVP. The WLVP bonding processes were performed using a bond aligner SÜSS BA8 (SÜSS MicroTec SE, Garching, Germany) and an SÜSS SB8 wafer bonder (SÜSS MicroTec SE, Garching, Germany). The final wafer level vacuum package (WLVP) of translatory MEMS is shown exemplarily in Figure 16 before wafer dicing. The details of infrared and microscopic images show a good homogeneity of the bonding frame. Finally, the 3476 µm thick WLVP stack was separated by wafer dicing into individual chips. Moreover, the bond pads were opened for wire bonding using a three-step sawing process. MicromachinesFor characterization, 2020, 11, x FOR the PEER MEMS REVIEW WLVP chips are mounted and wire-bonded onto a dedicated15 PCB. of 27

FigureFigure 16. 16. FinalFinal wafer wafer-level-level vacuum vacuum package package (WLVP) (WLVP) of of translatory translatory MEMS: MEMS: ( (aa)) 6” 6” wafer of MEMS WLVPWLVP before before dicing dicing;; (b (b) )infrared infrared microscopic microscopic image image of of bond bond interface; interface; details details of of bonding bonding frame before andand after after third third bonding bonding process process used used for for final ﬁnal hermetic hermetic vacuum vacuum sealing sealing of of WLVP; WLVP; a a good good homogeneity homogeneity ofof the the bond bond is isevident evident;; (c ()c details) details of of bond bond frame frame (bond (bond islands islands are are still still encapsulated). encapsulated). 3. Results and Discussion 3. Results and Discussion 3.1. Basic MEMS Characteristics without WLVP 3.1. Basic MEMS Characteristics without WLVP Initially, we studied the amplitude–frequency behavior depending on vacuum pressure and drivingInitially, voltage weusing studied MEMS the devices amplitude without–frequency WLVP. behavior This is a commonly depending used on vacuummethod to pressure estimate and the drivingvacuum voltage inﬂuence using on MEMS MEMS devices performance, without e.g., WLVP. [40, 61This], helpful is a commonly to determine used method the minimum to estimate vacuum the vacuumrequirements influence for on further MEMS WLVP performance, development. e.g., [4 Therefore,0,61], helpful the to MEMS determine samples the wereminimum placed vacuum into a requirementssmall vacuum for chamber further WLVP of 67 cm development3 inner volume,. Therefore, using a vacuumthe MEMS turbo samples pump were for external placed into evacuation. a small 3 vacuumThe vacuum chamber pressure of 67 cm inside inner the volume, cavity couldusing a be vacuum varied turbo from pump ambient for pressure external downevacuation. to 0.1 The Pa. vacuumThe amplitude pressure of inside translatory the cavityoscillation could was be measured varied from through ambient an optical pressure window down using to a0.1 Michelson Pa. The amplitudeinterferometer of translatory set-up with oscillation a helium–neon was measured (He–Ne) laser through operating an optical at 632.8 window nm wavelength. using a TheMichelson MEMS interferometerdevices are driven set-up in with parametric a helium resonance,–neon (He electrically–Ne) laser operating excited with at 632.8 a pulsed nm wavelength driving voltage. The MEMS of 50% devicesduty cycle are anddriven pulse in frequencyparametric of resonance twice the, mechanical electrically oscillation excited with frequency. a pulsed A driving frequency voltage down-sweep of 50% duty cycle and pulse frequency of twice the mechanical oscillation frequency. A frequency down- is required to start the oscillation. Before this test, the stability (pull-in) voltage of Upull-in = 45 V was sweepmeasured, is required limiting to start the maximal the oscillation. driving Before voltage. this Thetest, dependencethe stability of(pull the-in) amplitude voltage of on U pressurepull-in = 45 V is wasexemplarily measured, shown limiting in Figurethe maximal 17a for driving 12 V driving voltage. voltage. The dependence of the amplitude on pressure is exemplarily shown in Figure 17a for 12 V driving voltage.

Figure 17. Frequency–amplitude characteristics (experiment without WLVP using external vacuum chamber): (a) varied pressure of 4–100 Pa @ 12 V; (b) voltage dependency at 0.1 Pa; (c, insert) at 500 Pa.

It is obvious that a vacuum of below 4 Pa is needed to guarantee a stable parametric resonance oscillation at 575.5 Hz (pulse frequency) with 350 µm amplitude; this operation frequency is only 0.2 Hz above the instable resonance point [45,55]. The voltage dependence of the parametric resonance curves is shown for two relevant vacuum levels: at 0.1 Pa (target for this WLVP using getter, see Figure 17b) and 500 Pa (see Figure 17c, assuming a glass-frit bonded WLVP without getter, realized

Micromachines 2020, 11, x FOR PEER REVIEW 15 of 27

Figure 16. Final wafer-level vacuum package (WLVP) of translatory MEMS: (a) 6” wafer of MEMS WLVP before dicing; (b) infrared microscopic image of bond interface; details of bonding frame before and after third bonding process used for final hermetic vacuum sealing of WLVP; a good homogeneity of the bond is evident; (c) details of bond frame (bond islands are still encapsulated).

3. Results and Discussion

3.1. Basic MEMS Characteristics without WLVP Initially, we studied the amplitude–frequency behavior depending on vacuum pressure and driving voltage using MEMS devices without WLVP. This is a commonly used method to estimate the vacuum influence on MEMS performance, e.g., [40,61], helpful to determine the minimum vacuum requirements for further WLVP development. Therefore, the MEMS samples were placed into a small vacuum chamber of 67 cm3 inner volume, using a vacuum turbo pump for external evacuation. The vacuum pressure inside the cavity could be varied from ambient pressure down to 0.1 Pa. The amplitude of translatory oscillation was measured through an optical window using a Michelson interferometer set-up with a helium–neon (He–Ne) laser operating at 632.8 nm wavelength. The MEMS devices are driven in parametric resonance, electrically excited with a pulsed driving voltage of 50% duty cycle and pulse frequency of twice the mechanical oscillation frequency. A frequency down- sweep is required to start the oscillation. Before this test, the stability (pull-in) voltage of Upull-in = 45 V Micromachineswas measured,2020, 11limiting, 883 the maximal driving voltage. The dependence of the amplitude on pressure16 of is 28 exemplarily shown in Figure 17a for 12 V driving voltage.

Figure 17. Frequency–amplitude characteristics (experiment without WLVP using external vacuum Figure 17. Frequency–amplitude characteristics (experiment without WLVP using external vacuum chamber): (a) varied pressure of 4–100 Pa @ 12 V; (b) voltage dependency at 0.1 Pa; (c, insert) at 500 Pa. chamber): (a) varied pressure of 4–100 Pa @ 12 V; (b) voltage dependency at 0.1 Pa; (c, insert) at 500 Pa.

ItIt is is obvious obvious that that a a vacuum vacuum of of below below 44 PaPa isis neededneeded toto guaranteeguarantee a stable parametric parametric resonance resonance oscillationoscillation at at 575.5 575.5 Hz Hz (pulse (pulse frequency) frequency) with with 350 350µm µm amplitude; amplitude; this this operation operation frequency frequency is onlyis only 0.2 0.2 Hz Hz above the instable resonance point [45,55]. The voltage dependence of the parametric resonance above the instable resonance point [45,55]. The voltage dependence of the parametric resonance curves curves is shown for two relevant vacuum levels: at 0.1 Pa (target for this WLVP using getter, see Figure is shown for two relevant vacuum levels: at 0.1 Pa (target for this WLVP using getter, see Figure 17b) 17b) and 500 Pa (see Figure 17c, assuming a glass-frit bonded WLVP without getter, realized and 500 Pa (see Figure 17c, assuming a glass-frit bonded WLVP without getter, realized previously in [52]). To achieve the full amplitude of 350 µm, a driving voltage of 30 V is needed for 500 Pa, which can be significantly reduced in this study to 3 V at 0.1 Pa. It was observed for this translatory MEMS that scan amplitude cannot be set independently by driving voltage and frequency (see Figure 17). It is obvious that all parametric resonance curves lie directly on top of each another. They do not form a parametric family of curves (as is usually the case for torsional MEMS scanners [45]), i.e., there exist no amplitude–frequency curves that can be displaced relative to each other via the driving voltage. On the other hand, the increase in driving voltage only causes an increase in maximum scan amplitude, accompanied by a decreasing frequency of the (unstable) resonance point. This behavior (lack of additional parametric response curves, displaced by a parameterized driving voltage) is explained by (i) the small electrostatic actuation range in z of comb drives (see Figure 10a) and (ii) the degressive spring characteristic [62]. Further experiments show a non-negligible influence of the volume of encapsulated vacuum cavity on the resulting pressure dependency of the MEMS characteristics (see Figure 18a). We modified the experimental set-up (i) to determine realistic values of the minimum vacuum requirements for the development of the WLVP, and (ii) to allow also an indirect measurement of the internal vacuum pressure inside the WLVP using a pressure-calibrated MEMS characteristic as a reference. Therefore, we encapsulated the MEMS within an additional cavity (similar in size and volume to the real WLVP), both evacuated inside the external vacuum chamber (see Figure 18c). We needed two iterations of this modified set-up to eliminate size effects. For reference, the pressure-dependent MEMS characteristics were first verified using the Michelson interferometer set-up. Finally, the vacuum pressure dependencies of scan amplitude and Q factor were measured with a laser vibrometer (Polytec MSA 500). Here, Q factors were calculated from the freely damped MEMS oscillation (see Figure 18b). We have to point out that this method is only applicable for large oscillation amplitudes >10 µm. Using a calibration of the internal pressure in terms of the measured Q factor, the minimum requirements for MEMS WLVP (defined for meeting the full 350 µm amplitude at 8 V) were determined as follows: ≤ maximal vacuum pressure: pmax = 3.21 Pa, • minimum Q factor: Q = 1177, • min critical values (lower limits): Q = 600 and pmax = 5.5 Pa. • Micromachines 2020, 11, x FOR PEER REVIEW 16 of 27 previously in [52]). To achieve the full amplitude of 350 µm, a driving voltage of 30 V is needed for 500 Pa, which can be significantly reduced in this study to 3 V at 0.1 Pa. It was observed for this translatory MEMS that scan amplitude cannot be set independently by driving voltage and frequency (see Figure 17). It is obvious that all parametric resonance curves lie directly on top of each another. They do not form a parametric family of curves (as is usually the case for torsional MEMS scanners [45]), i.e., there exist no amplitude–frequency curves that can be displaced relative to each other via the driving voltage. On the other hand, the increase in driving voltage only causes an increase in maximum scan amplitude, accompanied by a decreasing frequency of the (unstable) resonance point. This behavior (lack of additional parametric response curves, displaced by a parameterized driving voltage) is explained by (i) the small electrostatic actuation range in z of comb drives (see Figure 10a) and (ii) the degressive spring characteristic [62]. Further experiments show a non-negligible influence of the volume of encapsulated vacuum cavity on the resulting pressure dependency of the MEMS characteristics (see Figure 18a). We modified the experimental set-up (i) to determine realistic values of the minimum vacuum requirements for the development of the WLVP, and (ii) to allow also an indirect measurement of the internal vacuum pressure inside the WLVP using a pressure-calibrated MEMS characteristic as a reference. Therefore, we encapsulated the MEMS within an additional cavity (similar in size and volume to the real WLVP), both evacuated inside the external vacuum chamber (see Figure 18c). We needed two iterations of this modified set-up to eliminate size effects. For reference, the pressure-dependent MEMS characteristics were first verified using the Michelson interferometer set-up. Finally, the vacuum pressure dependencies of scan amplitude and Q factor were measured with a laser vibrometer (Polytec MSA 500). Here, Q factors were calculated from the freely damped MEMS oscillation (see Figure 18b). We have to point out that this method is only applicable for large oscillation amplitudes >10 µm. Using a calibration of the internal pressure in terms of the measured Q factor, the minimum requirements for MEMS WLVP (defined for meeting the full 350 μm amplitude at ≤8 V) were determined as follows:

 maximal vacuum pressure: pmax = 3.21 Pa, Micromachines 2020, 11, 883 17 of 28  minimum Q factor: Qmin = 1177,  critical values (lower limits): Q = 600 and pmax = 5.5 Pa.

(b)

(a) (c)

FigureFigure 18. 18. ExperimentalExperimental determination determination of minimum of minimum requirements requirements for for vacuum vacuum pressure pressure inside inside the the WLWLVPVP cavity: cavity: (a) (resultsa) results on onvacuum vacuum pressure pressure dependency dependency of Q of factorQ factor;; obvious obvious is the is thefinal ﬁnal calibration calibration characteristiccharacteristic for fordetermination determination of the of the vacuum vacuum pressure pressure inside inside the theWL WLVP;VP; shown shown are arealso also intermediate intermediate experimentalexperimental results results on onthe the influence inﬂuence of cavity of cavity size; size; (b) (bexemplary) exemplary result result of determination of determination of the of theQ Q factorfactor from from free freelyly damped damped oscillation oscillation measured measured with with laser laser vibrometer vibrometer;; (c) (newc) new setset-upup with with small small cavity cavity andand 1 mm 1 mm gap. gap.

3.2. Characteristics of Initial MEMS WLVP Run In the initial WLVP fabrication run, four wafer stacks, i.e., (i) two stacks without getter and gold coating and (ii) two stacks with zirconium (Zr)-based getter and gold coating, were vacuum-bonded at 25 Pa process pressure. Using the calibrated characteristic of Q vs. vacuum pressure, it was possible to measure the inner vacuum pressure within the cavity of the WLVP.

3.2.1. MEMS Characteristics with WLVP For the two WLVP stacks without getter, an insufficiently high internal vacuum pressure of 139 Pa (Q = 18.6) and 62 Pa (Q = 45.3) was measured below the critical values. On the other hand, for the WLVP stacks with getter, a sufficiently high Q factor of 18,000 (equal to 0.25 Pa) was achieved for the WLVP stack 03. The second stack 04 failed (p = 46.8 Pa and Q = 61.7) for an unknown reason. The frequency amplitude of a functional MEMS WLVP (sample of stack 03 measured with the Michelson interferometer set-up) is exemplarily shown in Figure 19b. At 0.25 Pa inner vacuum pressure, a driving voltage of 4 V is needed for an amplitude of 350 µm. The long-term stability of the inner vacuum was initially verified for WLVP stack 03 up to >10 years [54] using an ultrafine Ne vacuum leakage test [63], with an estimated mean time to getter saturation of 6.34 years. The long-term stability of the inner vacuum of stack 03 was experimentally verified after 32 months again, resulting in a vacuum of 0.47 Pa and a new estimated (residual) mean time-to-getter-saturation of 7.23 1.54 years. ± These results clearly demonstrate the potential of the present WLVP strategy for a sufficiently long lifetime >10 years. Micromachines 2020, 11, x FOR PEER REVIEW 17 of 27

3.2.1. MEMS Characteristics with WLVP For the two WLVP stacks without getter, an insufficiently high internal vacuum pressure of 139 Pa (Q = 18.6) and 62 Pa (Q = 45.3) was measured below the critical values. On the other hand, for the WLVP stacks with getter, a sufficiently high Q factor of 18,000 (equal to 0.25 Pa) was achieved for the WLVP stack 03. The second stack 04 failed (p = 46.8 Pa and Q = 61.7) for an unknown reason. The frequency amplitude of a functional MEMS WLVP (sample of stack 03 measured with the Michelson interferometer set-up) is exemplarily shown in Figure 19b. At 0.25 Pa inner vacuum pressure, a driving voltage of 4 V is needed for an amplitude of 350 µm. The long-term stability of the inner vacuum was initially verified for WLVP stack 03 up to >10 years [54] using an ultrafine Ne vacuum leakage test [63], with an estimated mean time to getter saturation of 6.34 years. The long- term stability of the inner vacuum of stack 03 was experimentally verified after 32 months again, resulting in a vacuum of 0.47 Pa and a new estimated (residual) mean time-to-getter-saturation of 7.23 Micromachines± 1.54 years2020. These, 11, 883 results clearly demonstrate the potential of the present WLVP strategy 18 for of 28a sufficiently long lifetime >10 years.

(a) (b)

Figure 19. Experimental results results of of translatory translatory MEMS MEMS with with WLVP: WLVP: ( (aa)) MEMS MEMS sample sample with with WLVP WLVP;; ( (bb)) frequency–amplitudefrequency–amplitude characteristic in in parametric parametric resonance resonance (sample (sample of of stack stack 03; 03; initial initial WLVP WLVP run). run).

3.2.2.3.2.2. Influence Influence of Process Process Temperature Temperature on on Optical Optical Coating Coating During the initial WLVP process,process, the MEMS devices have to withstand two two process process steps steps at at elevatedelevated process process temperatures temperatures of of435 435 and and 440 440 °C due◦C dueto hermetic to hermetic glassglass-fritfrit bonding. bonding. These high These process high processtemperatures temperatures can be can crucial be crucial for the for static the static planarity planarity of the of theAu Au-coated-coated mirrors. mirrors. To To investigate investigate the the influenceinfluence of of process process temperature temperature on optical on optical mirror mirror coating, coating the static, the mirror static deformation mirror deformation was measured was formeasured WLVP stacks for WLV 03 andP stack 04 befores 03 andand after 04 before the WLVP and process after the using WLVP white-light process interferometry using white (WLI).-light Diinterferometryfferent commercial (WLI). Au Different coatings commercial were used: Au Au-variant coatings 1:10were nm used Cr: +Au70-variant nm Au 1: coatings10 nm Cr for + stack 70 nm 04 (identicalAu coatings for for the stack front- 04 and(identical backside) for the and front Au-variant- and backside) 2:7 nm and Cr + Au70-variant nm Au 2: coatings7 nm Cr (identical + 70 nm Au for thecoatings front- (identical and back for surfaces) the front for- stackand back 03. The surfaces) WLI results for stack for stack 03. The 04 areWLI summarized results for sta inck Figure 04 are 20 ; significantsummarized defects in Figure caused 20;by significant the diffusion defects of Aucaused into theby the silicon diffusion substrate of Au are into obvious. the silicon These substrate diffusion Auare defectsobvious. are These less diffusion frequent Au at thedefects front are surface less frequent but aff ectedat the 100%front surface of the samples but affected on the 100% backside of the 1 (see Figure 20a). In consequence, a clearly too large static mirror curvature of 1/R = 0.50–0.85 m− 1 (equal to δpp = 1.5–2.5 µm and compared to the small initial curvature of 1/R = 0.0014–0.0454 m− with δpp = 4.5–142 nm static mirror deformation) was measured with WLI for stack 04. The results of stack 03 with Au coatings of variant 2 had fewer defects and a better performance, with final curvature 1 1/R = 0.13–0.19 m− (equal to δpp = 395–440 nm) after WLVP process in comparison to the initial values 1 in the range 1/R = 0.10 to 0.14 m (δpp= 309–440 nm). − − − Micromachines 2020, 11, x FOR PEER REVIEW 18 of 27 samples on the backside (see Figure 20a). In consequence, a clearly too large static mirror curvature of 1/R = 0.50–0.85 m−1 (equal to δpp = 1.5–2.5 µm and compared to the small initial curvature of 1/R = 0.0014–0.0454 m−1 with δpp = 4.5–142 nm static mirror deformation) was measured with WLI for stack 04. The results of stack 03 with Au coatings of variant 2 had fewer defects and a better performance, with final curvature 1/R = 0.13–0.19 m−1 (equal to δpp = 395–440 nm) after WLVP process in comparison Micromachines 2020, 11, 883 19 of 28 to the initial values in the range 1/R = −0.10 to −0.14 m−1 (δpp= 309–440 nm).

(a) (b) (c)

FigureFigure 20. 20. ExperimentalExperimental results results on on influence influence of of process process temperature on Au mirror coating:coating: (aa)) defectdefect imagesimages of of mirror mirror coating coating on on wafer wafer backside backside of of stack stack 04 04 (before (before final final vacuum bonding) bonding);; ((b)) comparison ofof diffusion diffusion defects defects (white (white-light-interferometry-light-interferometry (WLI (WLI),), microscopy) microscopy) onon mirrors mirrors front front-- and and backside backside;; (c) comp(c) comparisonarison of static of static mirror mirror deformation deformation before before and andafter after the WLVP the WLVP process process..

3.2.3.3.2.3. Discussion Discussion on on Improvements Improvements of of MEMS MEMS WLVP WLVP TheThe significant significant deg degradationradation of of mirror mirror planarity planarity was was caused caused by by diffusion diffusion of gold into thethe siliconsilicon mirrormirror plate plate,, resulting resulting in in higher higher mechanical mechanical stresses. stresses. It also It also disturb disturbeded the theoriginal original symmetry symmetry of the of mirrorthe mirror coating coating required required for temperature for temperature compensation. compensation. The higher The higherdefect defectdensity density on the onrear the mirror rear surfacemirror surfacecan be can explained be explained by its byhigher its higher thermal thermal load load in thein the bonding bonding tool. tool. As As a a consequence, consequence, the the chromchromiumium adhesion adhesion layer layer could could not not safely safely prevent prevent the the Au Au diffusion diffusion.. In In order order t too pre preventvent Au diffusion diffusion,, wewe tested tested an an additional, additional, diffusion diffusion-tight-tight barrier barrier layer layer,, which was deposited on the bare silicon mirrormirror surface before the Au coatings were deposited. A 40 nm thick barrier layer of Al O was homogenously surface before the Au coatings were deposited. A 40 nm thick barrier layer of Al22O3 was homogenously depositeddeposited with with atomic atomic layer layer deposition deposition (ALD) (ALD) onto onto t thehe entire entire DW. DW. First First,, we simulated the influenceinfluence ofof thethe process process temperature temperature on on the the (i) (i) mirror mirror planarity planarity and and (ii) (ii) the the defect density of the Au coatingcoating afterafter thethe WL WLVPVP process process using using MEMS MEMS dummy dummy wafers wafers ( (withoutwithout electrical function). Two groups were testedtested (i) with and (ii) without Al O diffusion barrier layer. We also compared the previously tested variants (i) with and (ii) without Al22O3 3diffusion barrier layer. We also compared the previously tested variants ofof Au Au coatings coatings of of two two different different commercial commercial suppliers. suppliers. ForFor the the experimental experimental simulation simulation of of the the WL WLVPVP process process,, the samples were exposed to thethe samesame temperaturetemperature cycles cycles and and thermal thermal budget budget as as in in the the real process. In In Figure Figure 2121 the results on on mirror mirror curvaturecurvature are displayed displayed beforebefore and and after after the the simulated simulated WLVP WL processVP process as boxplots as boxplots for all tested for all variants. tested variants.It is obvious It is obvious that the th Auat coatingthe Au variantcoating 2variant with an2 with ALD an barrier ALD layerbarrier achieved layer achieved the smallest the smallest mirror 1 curvature of 1/R = 0.02 0.015 m corresponding−1 to a static mirror deformation of δpp = 55.1 27.9 nm. mirror curvature of 1/R± = 0.02 ± −0.015 m corresponding to a static mirror deformation of± δpp = 55.1 ±27.9Without nm. theWithout ALD the barrier ALD layer,barrier small layer Au, small diff usionAu diffusion defects defects on the on backside the backside of the of wafer the wafer result result in a larger curvature and a broader variation range. In all samples with the Al O diffusion barrier layer, in a larger curvature and a broader variation range. In all samples with the2 3 Al2O3 diffusion barrier layerno di, ffnousion diffusion defects defects occurred occurred after theafter simulated the simulated thermal thermal process process load. load. TheThe reason reason for for the the poor poor vacuum vacuum of of the the defective defective WLVP WLVP stack stack 04 04(with (with p =p 46.8= 46.8 Pa, Pa,although although a Zr a- basedZr-based getter getter was wasused) used) was suspected was suspected to originate to originate in local in defects local defects inside insidethe glass the-frit glass-frit bonding bonding frame. Afterframe. external After externalgetter deposition getter deposition at the SAES at the Getters, SAES loc Getters,al defects local of defects cracks ofand cracks spalling and were spalling observed were (seeobserved Figure (see 22a, Figure bond 22 framea, bond before frame hermetic before hermetic vacuum vacuum sealing), sealing), which which could could probably probably increase increase the leakagethe leakage rate. rate.The receiving The receiving inspection inspection at SAES at SAESalso showed also showed small cracks small crackswithin withinthe previously the previously defect- freedefect-free glass-frit glass-frit bond frames. bond frames. The hypothesis The hypothesis of an ofincreased an increased leakage leakage rate ratefor WLVP for WLVP stack stack 04 04due due to cracksto cracks was was checked checked by means by means of an of ultrafine an ultrafine neon neon vacuum vacuum leakage leakage test test [63]. [63 ].

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Figure 21. Improvement of the Au mirror coating and reduction of the influence of the process temperature on the Au mirror coating; experimental results of mirror planarity of simulated WLVP processes using MEMS dummy wafers, comparison of samples with and without diffusion barrier

layer of 40 nm thick Al2O3 deposited by ALD. Kx and Ky denotes the curvature in x- and y-directions, respectively.

However, the test showed a low leakage rate identical to the good samples. In other words, the glass-frit bond frames were hermetically sealed. The cause of the error had to be inside the vacuum cavity. By means of a comparative test using residual gas analysis (RGA) at SAES Getters, a high pressure of 4.4 mbar (440 Pa) and an unusual residual gas atmosphere was measured, containing 97% hydrogen and methane (inside the cavity) only for tested WLVP samples of the affected stack 04. This indicates an internal source of outgassing. Microscopic investigations and correlation with further SEM inspections revealed local defects of opened voids within the filled isolation trenches (see Figure 22c). The microscopic inspections found that 36.4% of all device wafers of this study were affected by Figure 21. ImprovementImprovement of of the the Au mirror coating coating and and reduction reduction of of the the inﬂuence influence of of the the process process these defects, observed also in wafer stack 04 but not for stack 03. It is suspected that these voids temperature on on the the Au Au mirror mirror coating; coating; experimental experimental results results of mirror of mirror planarity planarity of simulated of simulated WLVP contained polymer residuals from the DRIE passivation process. In the improved WLVP process, the processesWLVP processes using MEMS using MEMS dummy dummy wafers, wafers, comparison comparison of samples of samples with and with without and without diffusion diﬀ usion barrier 40 nm thickbarrier Al layer2O3 barrier of 40 nm layer, thick deposited Al O deposited on the byentire ALD. DWKx andby atomicKy denotes layer the deposition curvature (ALD) in x- and, may layer of 40 nm thick Al2O3 deposited2 3 by ALD. Kx and Ky denotes the curvature in x- and y-directions, help to encapsulate these void defects. Afterward, failure (observed at stack 04) no longer occurred. respectively.y-directions, respectively.

However, the test showed a low leakage rate identical to the good samples. In other words, the glass-frit bond frames were hermetically sealed. The cause of the error had to be inside the vacuum cavity. By means of a comparative test using residual gas analysis (RGA) at SAES Getters, a high pressure of 4.4 mbar (440 Pa) and an unusual residual gas atmosphere was measured, containing 97% hydrogen and methane (inside the cavity) only for tested WLVP samples of the affected stack 04. This indicates an internal source of outgassing. Microscopic investigations and correlation with further SEM inspections revealed local defects of opened voids within the filled isolation trenches (see Figure 22c). The microscopi c inspections found that 36.4% of all device wafers of this study were affected by these defects(a) , observed also in wafer stack(b) 04 but not for stack 03. It is suspected(c) that these voids contained polymer residuals from the DRIE passivation process. In the improved WLVP process, the FigureFigure 22. 22. DefectDefect images images of of potentially potentially parasitic parasitic e ﬀ effectsects on onyield yield and long-termand long- stabilityterm stability of inner of vacuum: inner 40 nm thick Al2O3 barrier layer, deposited on the entire DW by atomic layer deposition (ALD), may vacuum:microscopic microscopic images of images (a) local of cracks (a) local and spalling cracks and of BW spalling glass-frit of frame BW afterglass getter-frit frame deposition after at getter SEAS; help to encapsulate these void defects. Afterward, failure (observed at stack 04) no longer occurred. deposition(b) small at cracks SEAS inside; (b) small glass-frit cracks bonding inside glass frame-frit of BWbonding before frame getter of deposition; BW before (getterc) SEM deposition photograph; (c) of SEMopen photograph void inside of ﬁlledopen trenchvoid inside isolation. filled trench isolation.

ToHowever, reduce the the risks test for showed hermeticity a low of leakageWLVP, the rate final identical glass-frit to thebonding good process samples. used In for other hermetic words, vacuumthe glass-frit sealing bond of WLVP frames was were changed. hermetically To avoidsealed. a Theny cracks cause of or the spalling error had inside to be the inside final the glass vacuum-frit bondingcavity. frame By means (caused of aduring comparative the external test usinggetter residualdeposition) gas, analysiswe decoupled (RGA) the at getter SAES Getters,deposition a high on thepressure BW. For of this 4.4 mbar purpose, (440 wePa) andscreen an-printed unusual the residual final gas glass atmosphere-frit bonding was frame measured, (used containing for hermetic 97% hydrogen and methane (inside the cavity) only for tested WLVP samples of the affected stack 04. This indicates an internal source of outgassing. Microscopic investigations and correlation with further SEM inspections revealed local defects of opened voids within the filled isolation trenches (see Figure 22c). The microscopic(a) inspections found that 36.4%(b of) all device wafers of this study were(c) affected by these defects, observed also in wafer stack 04 but not for stack 03. It is suspected that these voids contained polymerFigure residuals 22. Defect from images the DRIE of potentially passivation parasitic process. effects In on the yield improved and long WLVP-term stabilityprocess, of the inner 40 nm vacuum: microscopic images of (a) local cracks and spalling of BW glass-frit frame after getter thick Al2O3 barrier layer, deposited on the entire DW by atomic layer deposition (ALD), may help to deposition at SEAS; (b) small cracks inside glass-frit bonding frame of BW before getter deposition; (c) encapsulate these void defects. Afterward, failure (observed at stack 04) no longer occurred. SEM photograph of open void inside filled trench isolation. To reduce the risks for hermeticity of WLVP, the final glass-frit bonding process used for hermetic vacuum sealing of WLVP was changed. To avoid any cracks or spalling inside the final glass-frit To reduce the risks for hermeticity of WLVP, the final glass-frit bonding process used for hermetic bonding frame (caused during the external getter deposition), we decoupled the getter deposition vacuum sealing of WLVP was changed. To avoid any cracks or spalling inside the final glass-frit on the BW. For this purpose, we screen-printed the final glass-frit bonding frame (used for hermetic bonding frame (caused during the external getter deposition), we decoupled the getter deposition on the BW. For this purpose, we screen-printed the final glass-frit bonding frame (used for hermetic

Micromachines 2020, 11, 883 21 of 28 MicromachinesMicromachines 20202020,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 2020 ofof 2727 vacuumvacuum s sealingsealingealing of of WLVP) WLVP) onon thethethe backsidebacksidebackside ofofof thethethe DW. DW.DW. The TheThe thermal thermalthermal inﬂuence influenceinfluence of ofof this thisthis additional additionaladditional third thirdthird glassglass-fritglass--fritfrit bonding bonding process process (affecting (a(affectingﬀecting thethe mirrormirror planarity)planarity) waswas testedtested tototo bebebe tolerable.tolerable.tolerable.

3.3.3.3. FinalFinal CharacteristicsCharacteristics of of Improved Improved MEMS MEMS WLVP WLVP Finally,Finally, fourfourfour WLVP WLVPWLVP stacks stacksstacks were werwere fabricatede fabricatedfabricated with withwith the thethe improved improvedimproved process proproccessess (exchange (exchange(exchange of third ofof thirdthird glass-frit glassglass-- fritlayerfrit layerlayer to DW toto DWDW backside, backside,backside, see seesee Figure FigureFigure 23 a),2323a)a) using,, usingusing also alsoalso a reducedaa reducedreduced vacuum vacuumvacuum process processprocess pressure pressurepressure to toto enhance enhanceenhance thethethe effective eeffectiveﬀective gettergetter capacitycapacity capacity ofof of WLVP.WLVP. WLVP. TwoTwo Two samplessamples samples eacheach each werewere were vacuumvacuum vacuum-bonded--bondedbonded atat at 55 5PaPa Pa (#8,(#8, (#8, #9)#9) #9) andand and 0.10.1 Pa0.1Pa (#10, Pa(#10, (#10, #11).#11). #11). TheThe The innerinner inner vacuumvacuum vacuum pressurepressure pressure waswas was measuredmeasured measured forfor for individualindividual individual chips chips withwith aaa laser laserlaser scanning scanningscanning vibrometervibrometer (Polytec (Polytec MSA500)MSA500) usingusing thethethe calibration calibrationcalibration method methodmethod shown shownshown in ininFigure FigureFigure 18 1818... In InIn comparison comparisoncomparison to toto the thethe initialinitialinitial results resultsresults ( (see(seesee FigureFigure 1919b19bb::: 25 25 PaPa processprocess pressure,pressupressurere,, resultingresultingresulting ininin 0.250.250.25 Pa PaPa of ofof WLVP), WLVPWLVP)), we, wewe achieved achievedachieved a aa slightlyslightly reduced reduced medianmedian vacuumvvacuumacuum pressurepressurepressure of ofof 0.155 0.0.155155 ±± 0.019 0.0.001919 Pa. Pa.Pa. The TheThe boxplot boxplotboxplot diagram diagramdiagram of ofof inner innerinner vacuum vacuumvacuum ± pressurepressure is is is shown shown shown in in Figurein FigureFigure 23b. 23 23 Web.b. onlyWeWe only foundonly foundfound a neglectable a a neglectable neglectable dependency dependenc dependenc on vacuumyy onon process vacuum vacuum pressure, process process pressureresultingpressure,, resultingresulting in 0.170 inin 0.013 0.0.117070 ±Pa± 0.0.0 inside01133 PaPa inside WLVPinside WLVPWLVP for 5 Pa forfor and 55 PaPa 0.114 andand 0.0.1141140.025 ±± 0.0.00 Pa2525 Pa forPa forfor 0.1 0.10.1 Pa. PaPa. Furthermore,. Furthermore,Furthermore, ± ± higherhigher QQ factorsfactorsfactors werewere measured, measured, typically typically inin thethe rangerange ofof QQ == 3838,600–48,500.38,,600600––4848,,500500..

((aa)) ((bb))

FigureFigure 23. 23. FinalFinal results results of of WLVP: WLVP: ((aa)) processprocess changechangechange ofofof third thirdthird glass-frit glassglass--fritfrit bond bondbond frame frameframe on onon backside backsidebackside of ofof DW; DW;DW; (((bb))) boxplot boxplot ofof innerinner vacuumvacuum pressurepressure forforfor WLVPWLVPWLVP stacks stacksstacks (left) (left)(left) and andand normalized normalizednormalized conductance conductanceconductance of ofof leakage leakageleakage channelchannel measured measuredmeasured for for WLVP WLVP stackstack #10.#10.

TheThe influence influenceinfluence ofof aa higherhigher QQ factorfactorfactor ononon thethethe resulting resultingresulting frequency frequencyfrequency characteristic characteristiccharacteristic is isis exemplarily exemplarilyexemplarily shown shownshown ininin Figure Figure 2424aa forfor twotwo samplessamplessamples ofofof the thethe initial initialinitial ( (Q(QQ= == 18,000) 1818,,000)000) and andand final finalfinal ( Q((QQ= == 46,598) 4646,,598)598) WLVP WLVPWLVP run. run.run. The TheThe rise riserise of ofof thethethe frequency frequencyfrequency responseresponse curvescurvescurves is isis fairly fairlyfairly identical. identical.identical. No NoNo steeper steepersteeper characteristic characteristiccharacteristic (as (as(asanticipated) anticipated)anticipated) is isis visible visiblevisible for forfor thethethe sample samplesample with with higher higher QQ value.vvalue.alue. OnOn thethe contrary,contrary, it itit is isis somewhat somewhatsomewhat flatter flatterflatter due duedue to toto the thethe used usedused higher higherhigher driving drivingdriving voltagevoltage of of 88 VV (preferred(preferred(preferred ininin the thethe meantime meantimemeantime for forfor FTS FTSFTSintegration). integration).integration). Furthermore, FurthermoreFurthermore,, in inin Figure FigureFigure 24 2424b,bb,a, aa good goodgood reproducibilityreproducibility of ofof frequency frequency characteristicscharacteristiccharacteristicss isis shown.shownshown.. FromFrom bothbbothoth diagrams,diagramsdiagrams,, aaa robust robrobustust WLVP WLVPWLVP process processprocess windowwindow for for inner inner vacuum vacuum pressurepressure andand frequencyfrequency characteristiccharacteristic isisis obvious.obviousobvious..

((aa)) ((bb))

FigureFigure 24. 24. FrFrFrequencyequencyequency amplitude amplitude characteristiccharacteristicscharacteristicss inin parametricparametric resonance resonanceresonanceat atat 4 44 V: VV: (: a((a)a)) influence influenceinfluence of ofofQ QQfactor factorfactor onon frequency frequency characteristic, characteristic, shownshown areare WLVPWLVP MEMS MEMSMEMS of ofof the thethe initial initialinitial and andand final finalfinal WLVP WLVPWLVP run; runrun; (; b ((b)b) exemplary) exemplaryexemplary comparisoncomparison of ofof two twotwo WLVP WLVP MEMSMEMS devicesdevices (from(from didifferentdifferentfferent wafers); waferwaferss));; good goodgood reproducibility reproducibilityreproducibility is isis obvious. obvious.obvious.

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Furthermore, the time until complete getter saturation (afterward, the inner vacuum pressure Furthermore, the time until complete getter saturation (afterward, the inner vacuum pressure starts to increase) was estimated using several time-shifted vibrometer-based control measurements starts to increase) was estimated using several time-shifted vibrometer-based control measurements of of the internal pressure using individual chips of stack #10. Therefore, a normalized conductance of the internal pressure using individual chips of stack #10. Therefore, a normalized conductance of the the leakage channel to the inner WLVP cavity of 0.166 cm3 volume was calculated for stack #10 (Figure leakage channel to the inner WLVP cavity of 0.166 cm3 volume was calculated for stack #10 (Figure 23b) 23b) as follows: as follows: r 16 18 L퐿 푔g 퐿̃eL==(22.5.5 10 10−−16 ± 22.3.3 10 10−18) √ @ 2020°◦퐶C.. (1) (1) ± s푠 푚표푙mol..퐾K UsingUsing this this value value and and the the specification speciﬁcation of of the the used used PageWafer PageWafer®® thinthin-ﬁlm-film getter getter,, we we calculated calculated the the timetime until until complete complete getter getter saturation saturation (after (after which which the the internal internal pressure pressure of of WL WLVPVP starts starts to to increase). increase). ThisThis calculation calculation is is based based on on the the assumption assumption of of a atemporarily temporarily constant constant conductance conductance of of the the leak leak channel channel andand without without internal internal sources sources of of o outgassing.utgassing. A A mean mean time time to to getter getter saturation saturation of of 7.91 7.91 ± 0.080.08 years years was was ± calculatedcalculated for for stack stack #10. #10. TheThe parasitic parasitic tilt tilt angles angles in in x xandand yy, ,occurring occurring during during a a translatory translatory oscillation oscillation in in zz withwith 350 350 µmµm amplitude,amplitude, we werere also also measured measured for for individual individual samples samples with with laser laser scanning scanning vibromet vibrometryry (Polytec (Polytec MSA MSA 500).500). We We observed observed parasitic parasitic tilt tilt angles angles in in the the range range of of 20 20–80–80 arcsec arcsec,, where where a ann average average of of 60.2 ± 15.615.6 ± arcsecarcsec was was achieved. achieved. A A result result of of a asample sample with with aa small small parasitic parasitic mirror mirror tilt tilt of of ±2020 arcsec arcsec is is exemplarily exemplarily ± shownshown in in Figure Figure 25 25a.a.

(a) (b)

FigureFigure 25. 25. ExemplaryExemplary results results of of (a ()a parasitic) parasitic mirror mirror tilt tilt within within a afull full translational translational oscillation oscillation of of 350 350 µmµm amplitudeamplitude,, and and (b ()b static) static mirror mirror deformation deformation of of the the Au Au-coated-coated mirror mirror plate plate after after WLVP WLVP process. process.

TheThe resulting resulting static static mirror mirror deformation deformation of of the the final ﬁnal WLVP WLVP (measured (measured with with whitewhite-light-light interferometry)interferometry) is is exemplary exemplary shown inin FigureFigure 25 25b.b. A A mean mean static static mirror mirror deformation deformation of δ pp_xof δpp_= x100.1 = 100.1 nm nmand andδpp δ_ppy_=y = −887.37.3 nmnm waswas measuredmeasured inin thethe x-x- andand yy-directions.-directions. While the absolute values values of of the the − curvaturescurvatures along along the the x-x- andand yy-directions-directions are are within within the the permissible permissible limits, limits, they they now now have have opposite opposite signs,signs, in in contrast contrast to to the the previous previous results results of of Figure Figure 21 21. .This This saddle saddle deformation deformation arises arises from from an an incorrect incorrect alignmentalignment of of the the shadow shadow masks masks used used for for the the patterning patterning of of front front and and back back side side Au Au coatings coatings..

4.4. Conclusions Conclusions InIn this this article article,, we we presented presented a awafer wafer-level-level vacuum vacuum-packaged-packaged (WLVP) (WLVP) translatory translatory MEMS MEMS actuator actuator developeddeveloped for for a compact a compact NIR NIR-FT-FT spectrometer spectrometer with with high high SNR SNR >1000> 1000in the in spectral the spectral bandwidth bandwidth of 1200 of– 25001200–2500 nm. For nm. this purpose, For this purpose,two objectives two objectives had to be hadsolved to which be solved were which not available were not with available the current with statethe of current the art: state (i) design of the of art: a large (i) design-stroke of pantograph a large-stroke MEMS pantograph device with MEMS minimized device withdynamic minimized mirror deformationdynamic mirror of ≤80 deformation nm and (ii) of development80 nm and (ii)of a development glass-frit-based of a optical glass-frit-based WLVP (to optical be compatible WLVP (to to be ≤ thecompatible fixed MEMS to the process) ﬁxed MEMS, avoiding process), degradation avoiding degradation of the optical of the mirror optical quality mirror at quality high process at high temperatures.process temperatures. TheThe large large-stroke-stroke resonant resonant MEMS MEMS design design uses uses a a full fullyy symmetric symmetricalal four four-pantograph-pantograph mirror mirror suspension,suspension, avoiding avoiding pro problemsblems with with tilting tilting and and parasitic parasitic modes. modes. To To minimize minimize dynamic dynamic mirror mirror deformationdeformation,, the the best best design design compromise compromise was was found found in in (a) (a) reduction reduction of of the the resonance resonance frequency frequency of of out-of-plane translation to <270 Hz and (b) mechanical decoupling of the 5 mm mirror plate from the pantograph suspension using a ring-shaped support structure with additional radial decoupling

Micromachines 2020, 11, 883 23 of 28 out-of-plane translation to <270 Hz and (b) mechanical decoupling of the 5 mm mirror plate from the pantograph suspension using a ring-shaped support structure with additional radial decoupling springs. The use of additional stiffening structures at the mirror backside [64] was avoided to simplify the MEMS process. This MEMS design approach results in a sufficiently small dynamic mirror deformation of δpp = 84 nm = λmin/9.5 at 350 µm amplitude and 267 Hz out-of-plane translation, driven electrostatically in parametric resonance. Due to significant gas damping, the MEMS device has to be operated in vacuum. In a first step, we experimentally studied the influence of vacuum pressure and cavity size on the MEMS behavior. The minimum requirements of 3.21 Pa and Q = 1177 inside ≤ the WLVP cavity of 0.166 cm3 size were determined by laser vibrometer experiments. From these experiments, we also developed an experimentally verified calibration model for the Q-factor-based determination of the inner vacuum pressure inside the MEMS WLVP. The challenge for the hermetic sealing of this MEMS is the significant surface topology 2 µm ≥ caused by the AlSiCu metal lines crossing the sealing area. For the hermetic sealing of NIR-WLVP, we selected the glass-frit bonding to be best suited and technologically compatible with our in-house MEMS scanner process AME75, resulting in lower development efforts. In contrast to alternative bonding approaches (see Table2), the highly ductile 25 µm thick glass-frit bond layer safely seals the WLVP hermetically and simultaneously forms embedded lateral signal feedthroughs to outer bond islands. On the other hand, glass-frit bonding requires high process temperatures of 430–440 ◦C, which the MEMS device has to withstand without compromising its optical performance. In comparison to eutectic alloy bonding (e.g., Au0.80Sn0.20 with a eutectic temperature of 280 ◦C[39]), glass-frit-based WLVP also requires a broader sealing frame, resulting in a larger chip size and higher costs. In our case, the 530 µm wide sealing frame is fully acceptable compared to the large overall MEMS chip size of 12.4 12.4 mm2. The NIR-FTS optimized WLVP was developed in two iterations to investigate × failure modes and optimize reliability. For a high reflectance 95% in NIR, we had to use Au for ≥ optical coating. To enable thermal compensation of the stresses induced by the bonding processes, we used a symmetric coating design [65], depositing identical Au coatings on the front- and backside of the mirror. A small static mirror deformation of 100 nm was achieved after the WLVP process. ≤ To guarantee a long-term stable inner vacuum pressure of 0.2 Pa, we applied a Zr-based thin-film getter using the external PageWafer process from SAES Getters. In the initial WLVP run, we achieved 0.25 Pa inner vacuum pressure of the WLVP and a Q factor of 18,000 at 25 Pa process pressure, resulting in a driving voltage of only 4 V to meet the required amplitude of 350 µm. After 32 months, the remaining mean time-to-getter saturation was estimated to be 7.2 1.5 years, which demonstrates the potential of ± the WLVP for a sufficiently long lifetime >10 years. On the other hand, several serious problems were observed for the original WLVP concept [54]. Initially, the 70 nm thick Au coatings with 7–10 nm Cr adhesion layer were deposited directly onto the silicon mirror plate. After the WLVP process, significant mirror defects and unacceptable large mirror deformations caused by Au diffusion into silicon were observed. Via ALD deposition of an additional 40 nm thick Al2O3 diffusion barrier layer (which was homogeneously deposited on the entire MEMS device wafer prior to Au deposition), this problem could be completely eliminated. In addition to fixing the Au–Si reaction (not prevented by a Cr layer only), the conformed Al2O3 layer is also advantageous to encapsulate inner sources of out-gassing contained within the DW (e.g., voids of the filled trenches). In this work, another potential reliability problem for the final glass-frit bond was observed. In the initial WLVP run, the final glass-frit bond layer (containing also the thin-film getter) was deposited on the BW before getter deposition using the PageWafer process from SAES Getters. After getter deposition, small cracks or spallings were observed inside the final glass-frit bonding frame. To avoid this potential risk for WLVP hermeticity, we finally switched the third glass-frit layer from BW to DW backside in order to decouple the final hermetic vacuum sealing from the getter deposition. In the final WLVP process, the inner vacuum pressure was reduced to 0.15 0.02 Pa and higher Q ± factors 38,600–48,500 were measured due to a reduced process pressure of 0.1–5 Pa. However, it was shown that the process pressure has a minor influence on the internal vacuum pressure. The achieved Micromachines 2020, 11, 883 24 of 28 inner vacuum pressure is comparable to the state-of-the-art WLVP of MOEMS [42], but avoids the thermal degradation of mirror planarity observed for glass-frit bonded micro scanners in [52,53]. Finally, static mirror deformations of 100 nm were measured for WLVP samples with symmetric Au ≤ coatings and additional 40 nm thick Al2O3 diffusion barrier layers. The mean time-to-getter saturation was estimated to be 7.9 0.1 years. The residual dynamic tilt angle (which occurs during a full ± translational oscillation with 350 µm amplitude) was measured to be in the range of 20–80 arcsec. For NIR-FTS system integration, MEMS devices with small tilt must be selected to guarantee an SNR >1000 in the spectral bandwidth 1200–2500 nm. Finally, we compared the performance of this NIR-FTS-specific pantograph MEMS device (350 µm amplitude, resonant operation at 267 Hz, dynamic mirror deformation of δpp = 84 nm) with the latest state of the art for MEMS-based NIR-FTS. In [64], a MEMS-based NIR-FTS with 7 nm spectral resolution was reported using a translatory MEMS with 3 mm mirror diameter, driven electrostatically in resonance at 265.5 Hz, resulting in an amplitude of 125 µm in normal ambient conditions. At 125 µm amplitude, a parasitic tilt of 2.2/1000◦ and a mirror deformation of 100 nm were measured. Compared to [64], our pantograph MEMS has a higher optical throughput and a 2.8-foldhigher amplitude. Hence, it has the potential for increased spectral resolution and higher SNR.

5. Summary and Outlook Although monolithic, highly miniaturized MEMS-based NIR-FTSs exist today, we follow a classical optical FT instrumentation using a resonant MEMS with precise out-of-plane translatory oscillation of a 5 mm diameter mirror for optical path-length modulation. Our advantages are a higher optical throughput and resolution in comparison to highly miniaturized systems, as well as mechanical robustness and insensitivity to vibration and mechanical shock, compared to conventional FTS mirror drives. The new vacuum WL-packaged translatory MEMS devices are very promising for compact FTSs, potentially allowing to replace expensive and complex conventional mirror drives. The versatility, high acquisition rate, and robustness of an MOEMS-based FTS makes it ideal for process control and applications in harsh environments (e.g., surveillance of fast chemical reactions). This potentially enables a new family of compact FT analyzers for the NIR spectral region λ = 800–2500 nm with a spectral resolution of 15 cm 1, 500 scans/s, and SNR > 1000 within an acquisition time of 1 s (with ≤ − co-addition of spectra). It should lead to a sensitive, reliable, and easy-to-use stand-alone NIR-FT spectrometer qualified for industrial applications in harsh environments, e.g., applied for ad hoc inspection of food quality or environmental parameters. The results of the final system integration into a miniaturized FT-NIR spectrometer (using selected MEMS devices with minimal parasitic tilt) will be published elsewhere. For further developments of NIR-FTS systems, new developments should also be considered [64,66–68]. Compared to alternative WLVPs, the glass-frit bonding process offers long-term hermeticity along with good sealing properties and tolerates the topography of the joining partners by a planarizing effect of the softened glass-frit during the bonding process. In parallel, conductive tracks or metallic lead-throughs for device signaling can be covered within the softened glass, preventing the need for processing complex and obviously more expensive vertical device signaling [48,49], with new questions arising related to hermeticity and device design and compatibility. Additionally, glass-frit bonding offers process flexibility due to a wide variety of suitable substrates including CMOS compatibility [50]. By application-related benchmarking of glass-frit bonding with alternative WLVP, glass-frit bonding can offer a good compromise of challenges, advantages, and disadvantages with respect to the specific features of the final application [39], required tooling, and the involved manufacturing processes. Additionally, glass-frit bonding still offers potential in terms of reducing the bonding temperature below (already available) 380 ◦C with ongoing glass-frit material development, substituting lead-based with lead-free glass-frit materials and using alternative or improved glass-frit deposition methods. In order to minimize outgassing behavior, the tuning of the thermal conditioning of the glass frit and the involved bonding procedures could be investigated, along with the reduction of the size and Micromachines 2020, 11, 883 25 of 28 volume of the printed glass-frit bonding interface and its influence on the resulting hermeticity and reliability of the device.

Author Contributions: Conceptualization, T.S. (Thilo Sandner), G.A. and T.G.; methodology, T.G., T.S. (Tobias Seifert) and G.A.; investigation, E.G., A.R. and T.S. (Tobias Seifert); writing—original draft preparation, T.S. (Thilo Sandner); writing—review and editing, T.S. (Thilo Sandner), T.G., G.A. and T.S. (Tobias Seifert); supervision, T.S. (Thilo Sandner) and E.G.; project administration, T.S. (Thilo Sandner) and T.G.; Funding acquisition, T.S. (Thilo Sandner) and J.G. All authors read and agreed to the published version of the manuscript. Funding: G. Auböck acknowledges funding by the Austrian ‘ASSIC’ COMET program, co-funded by the BMVIT, the BMWFW, and the Federal Provinces of Carinthia and Styria. Acknowledgments: The authors would like to acknowledge Thomas Gisler (T.G.) for his substantial contributions. We also acknowledge the support of Christian Drabe on MEMS technology, Renate Werner on sample preparation for MEMS characterization, and Wolfram Pufe for preparing SEM photographs. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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