System integration of hybrid assembled large aperture Micro Scanner Array for fast scanning LiDAR sensors

Abstract. We present the design and system integration of a hybrid MEMS scanning mirror (MSM) array developed for real-time three-dimensional imaging with a panoramic optical field of view (FOV) of 360  deg  ×  60  deg (horizontal  ×  vertical). The pulsed time-of-flight light detection and ranging (LiDAR) system targets a distance measurement range of 100 m with a video-like frame rate of 10 Hz. The fast vertical scan axis is realized by a synchronous scanning MSM array with large receiver aperture. It increases the scanning rate to 3200 Hz, which is four times faster in comparison with state-of-the-art fast macroscopic polygon scanning systems used in current LiDAR systems. A hybrid assembly of frequency selected scanner elements was chosen instead of a monolithic MEMS array to guaranty high yield of MEMS fabrication and a synchronous operation of all resonant MEMS elements at 1600 Hz with large FOV of 60 deg. The hybrid MSM array consists of a separate emitting mirror for laser scanning of the target and 22 reception elements resulting in a large reception aperture of Deff  =  23  mm. All MSM are driven in parametric resonance to enable a fully synchronized operation of all individual MEMS scanner elements. Therefore, piezoresistive position sensors are integrated inside the MEMS chip, used for position feedback of the driving control. We focus on the MEMS system integration including the microassembly of multiple MEMS scanning elements using micromechanical self-alignment. We present technical details to meet the narrow tolerance budgets for (i) microassembly and (ii) synchronous driving of multiple MEMS scanner elements.


Introduction
Laser scanners are widely used for three-dimensional (3D) optical distance measurement systems. 1,2,3 The principle of optical distance measurement is a time-of-flight (ToF) measurement of light, also known as light detection and ranging (LiDAR), which is based on the well-defined velocity of light with c 0 ¼ 2.99792458 · 10 8 m∕s in vacuum. LiDAR systems measure the travelling time of light resulting from emission by a modulated light source (typically a collimated laser beam) scattering at the target, and time-resolved detection with a fast photodetector. Basis of all LiDAR techniques is the precise measurement of light travelling time Δt to and from the measuring object, where the distance z results for a measurement environment of refractive index n E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 1 ; 1 1 6 ; 1 3 8 z ¼ c 0 · Δt 2 · n : *Address all correspondence to Thilo Sandner, thilo.sandner@ipms.fraunhofer.de LiDAR techniques can be divided into pulsed and phase-modulated ToF measurement techniques and frequency-modulated continuous-wave techniques. 1 For a phase-modulated ToF technique, the light intensity of the emitter is modulated with a high frequency f mod . The distance z of the measured object results from the phase shift φ between emitted and detected light E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 2 ; 1 1 6 ; 6 8 7 z ¼ c · φ 4πf mod · n : (2) The phase φ is only uniquely measureable within the interval 0; : : : ; 2π (corresponds, e.g., to a distance measurement range of 1 m at f mod ¼ 150 MHz 2 ). To extend the uniqueness range of distance measurement, multiple modulation frequencies are also used. This results in a significantly smaller but higher resolution distance measurement range for a phase-modulated ToF distance measurement system (also with the advantage of simplified system complexity) in comparison with a pulsed ToF technique, which is typically used for long distance measurement ranges up to several 2 100 m.
The light from the emitter is scattered at the target and requires a large aperture of the optical receiver, requiring also optical signal detection of high dynamic range, because the intensity at the detector is inversely proportional to the square of the target distance. Furthermore, pulsed ToF techniques also have the benefit that higher light intensities can be used at the target due to eye safety reasons (depending on the wavelength, where λ ¼ 1550 nm has the advantage that light cannot damage the human retina)-advantageous to detect the laser signal from the optical background noise. A classical laser scanner combines an optical ToF distance measurement technique with a scanning mirror optic-using a synchronously scanning mirror element for both emitting and receiving optical channels-allowing light to be detected from only a small area around the actual illuminated measurement point. A 3D imaging is achieved by sequential twodimensional (2D) geometrical scanning in x and y combined with a ToF distance measurement in z. Typically, classical laser scanners are not used for real-time 3D imaging due to the limited scanning speed of conventional scanning mechanics. For real-time 3D imaging, alternative techniques exist. Flash LiDAR systems use a parallel ToF detection with 2D detector arrays and a simultaneous illumination of the entire scene. 4,5 3D ToF cameras also exist with parallel ToF detection with 2D array detectors [e.g. complementary metal-oxide semiconductor-singlephoton avalanche diode arrays] and nonscanned receiver optics combined with a single MEMS scanning mirror (MSM) for scanned local object illumination. [6][7][8] These MEMS-based 3D ToF systems are promising for actual developments of LiDAR systems for autonomous driving due to their potential for cost and size reduction. 8-10 Some MEMS-based LiDAR systems are also reported in Ref. 11. In Hofmann et al., 12 it is reported that a resonant 550 Hz 2D scanner with 7 mm aperture is used for 360 deg [one-dimensional (1D)] scanning of a conceptual pulsed 3D ToF automotive sensor by means of a circular scanning trajectory in combination with an omnidirectional scan lens. In Ye et al., 13 a new electromagnetically driven 1D resonant MEMS mirror made of Ti-Al instead of c-Si was published for a future LiDAR application. This resonant 1D MEMS scanner has a large circular aperture of 12 mm and achieves an optical scan range of 26 deg at a resonance frequency of 1240 Hz. In Senger et al., 14 a piezoelectric resonant driven 2D MEMS scanner with 20 mm mirror aperture was presented, resulting in a circular scan pattern at about 1445 Hz with only 0.1 deg optical scan range. In Schwarz et al., 15 a piezoelectrically driven resonant 1D MEMS mirror with an elliptical aperture of 2 mm × 4 mm and a scan frequency of about 1.5 kHz for automotive LiDAR were presented, enabling a total optical scan angle of 180 deg. Beside LiDAR, MEMS scanners 16 are also interesting for further 2D/3D measuring techniques that require fast optical scanning, e.g., for high-speed imaging of unsteady spray flows using frequency-modulated Doppler global velocimetry. 17 In spite of the alternative 3D ToF techniques, classical laser scanners have the advantage of higher measurement accuracy (e.g., in comparison with focal plane array-based 3D ToF cameras) due to the synchronous scanning principle, because only single measuring points within the scanned field of view (FOV) are sequentially illuminated. Hence, the LiDAR detector collects light from signal and background (noise) in a short time only from a small measured area of the target, which significantly reduces the influence of the background (noise). Also, a nearly unlimited number of 3D data (voxels) can be measured with high measuring rates of typically up to 1 × 10 6 voxel∕s and low measurement uncertainty. On the other hand, conventional laser scanners for 3D distance measurement involve expensive, heavy, and large rotating or vibrating mirrors for light deflection of the scanning ToF distance measurement. 2 Typically, the precision of the ToF distance measurement is limited by the amount of signal light available at the detector. Hence, a scanning mirror with a large aperture is required for the optical receiver of the LiDAR system to detect small amounts of light reflected or scattered by the measured target. Traditionally, polygon scanners are used, but for high scanning speeds (e.g., >30;000 rpm) great efforts (e.g., air bearings) are required to reduce abrasive wear, resulting in heavy (>10 kg) and costly scanning units. Replacing them with micromechanical scanning mirrors, which have the benefits of high scanning speed, low weight, high mechanical reliability (e.g., no friction and high shock resistance), is not straightforward, since a large mirror aperture of the receiver optics must also be guaranteed in addition to sufficiently large optical scan angles (>40 deg) and high scan frequencies of more than 100 Hz. Contrary to this, the aperture of a single MSM is limited to small values of typically 1; : : : ; 5 mm diameter due to dynamic mirror deformation. 16,18

MEMS-Based ToF LaserScanner
To overcome the aforementioned problems of a classical laser scanner, Fraunhofer developed the concept for a MEMS-based LiDAR laser scanner in 2008, 19 which uses an array of identical MEMS elements driven in a synchronized mode. This concept was motivated for industrial high-speed 3D LiDAR systems used, e.g., in railroad inspection. A first prototype LAMDA (i.e., Large Aperture MEMS scanner module for 3D Data Acquisition) 20 -developed for a phase-shifting distance measuring system-was limited to resonant 1D scanning at 250 Hz and 60 deg FOV using a monolithic MSM array with 80% optical filling factor and a total aperture of 334.2 mm 2 . It consists of 2 × 7 MEMS mirrors driven synchronized to a separate sending mirror [see Fig. 1(b)]. For mirror synchronization, the LAMDA system uses hybrid integrated optical position detectors. 20 In Sandner et al., 21 this MEMS-based LiDAR was extended by the concept of the adaptive 3D ToF laser camera TACO (i.e., Three-dimensional Adaptive Camera with Object detection and foveation) with foveation properties (see Fig. 2), e.g., to allow future autonomous robots to better interact with their environment. This 3D sensor concept of foveation-i.e., capturing distance images at video-like frame rates with coarse spatial resolution, rapidly detecting regions of interest (ROIs), and then concentrating further image acquisition on these ROIs using adaptive scanning-required a sophisticated 2D scanning device with quasi-static actuation, large effective aperture ≥ 5 mm, high scanning rates >1000 Hz, and a large optical scan range of FOV ≥60 deg.
For the TACO sensor, 21 the best technical compromise for building a fast adaptive scanning unit was found in a quasi-static/resonant raster-scanning MEMS mirror. Using vertical out-of-  plane comb drives-based on the quasi-static LinScan driving principle 22,23 -a quasi-static mechanical scan angle (MSA) of AE10 deg as well as a linearized or adaptive scanning 24,25 with partial foveation in the vertical direction was realized, enabling also a variable frame rate of 1; : : : ; 100 Hz (10 Hz typically). The TACO system uses a synchronized MSM array consisting of five hybrid assembled and synchronously driven 2D MEMS mirrors-selected by frequency of the fast horizontal scan axis with 3200 Hz bidirectional scanning rate. The TACO system uses a fiber amplified pulsed ToF distance measurement system at λ ¼ 1550 nm with 1 MVoxel 3D measurement rate and 5 mm distance uncertainty at 7.5 m measuring range.
For TACO, the hybrid MEMS array is built in a symmetric coaxial assembly of four peripheral receiving mirrors around the centered sending mirror, 21 This paper focuses on a fast MSM array with large aperture developed for the 3D laser scanner Fovea3D (i.e., adaptive 3D sensing with foveation) with panoramic FOV (60 deg × 360 deg). This paper is an extended work of Ref. 27, where we publish now several details of the MEMS system integration for the first time, e.g., (i) a self-aligned microassembly technique of the hybrid MSM array using wet-etched pyramidal self-alignment structures, and (ii) self-compensation structures for reducing deep reactive-ion etching (DRIE) caused tolerances of the resonance frequency. In Sec. 2, the concept of the 3D ToF camera Fovea3D is introduced. Section 3 presents the MEMS design and fabrication. In Sec. 4, experimental results of single MEMS scanner elements are shown. The system integration of the hybrid assembled MEMS scanner array is discussed in Sec. 5. Finally, this paper is concluded in Sec. 6, and an outlook on future applications of the presented MEMS technology is given in Sec. 7.

Concept of the 3D TOF Camera Fovea3D
In Sandner et al., 27 we extend the basic concept of the MEMS-based adaptive 3D ToF laser camera with foveation properties to longer measuring distances. It targets on a measuring range of up to 100 m, needed for outdoor security applications (e.g., observation of industrial objects, airports, etc.). The 3D ToF camera Fovea3D is based on the same pulsed ToF laser distance measurement system-developed for the TACO sensor 21 -and also uses a pulsed fiber amplified Fig. 2 Concept of adaptive MEMS-based 3D laser camera TACO 21 : schematic optical setup using a synchronized 2D raster MSM array to provide sufficient reception aperture (a) and 2D raster scanning mirror with adaptive vertical scanning enabling partial foveation with higher resolution in ROIs (b).
laser system with λ ¼ 1550 nm, P pulse ¼ 1.5 kW, P average ¼ 1.5 W, and 1 MVoxel measuring rate. To enable an enlarged distance measuring range of 100 m, the reception aperture had to be significantly increased to an effective aperture of D eff ¼ 23 mm in comparison with only D eff ¼ 5 mm used for the TACO sensor (with a limited distance measuring range of only 10 m). The target specifications of the 3D camera Fovea3D are summarized in Table 1 in comparison with the previous TACO system. Table 1 shows the following improvements for the Fovea3D system: 1. larger effective aperture D eff ¼ 23 mm, 2. vertical optical scan range FOV ¼ 60 deg, 3. horizontal optical scan range FOV ¼ 360 deg (rotational), 4. rotational frequency up to 600 rpm (10 Hz frame rate) with horizontal foveation.
The Fovea3D system should also acquire 3D distance images at video-like frame rates with coarse spatial resolution, rapidly detecting ROIs and then concentrating further image acquisition on these ROIs with adaptive scanning in the horizontal direction. From the optical setup of the previous TACO system (see Fig. 3), it is obvious that the 2D MEMS approach is not scalable to meet the specifications required for the Fovea3D system. This holds particularly true for the enlarged aperture of D eff ¼ 23 mm.
To enable the challenging specification of the 3D camera Fovea3D, the following system concept was used (see Fig. 4):  . They are hybrid assembled around the coaxial sending mirror optimized for reduced dynamic mirror deformation and low beam divergence of 1 mrad at λ ¼ 1500 nm. The hybrid assembly of fast scanning 1D MEMS array (using a self-alignment microassembly technique) was chosen because of the small frequency bandwidth ≤10 Hz available for synchronized operation of all individual scanner elements. The approach of hybrid assembled MEMS array enables: 1. increased MEMS yield and reduced overall costs using frequency selected scanner chips, 2. simplified rework of a damaged scan element. To provide the effective reception aperture of 23 mm, all receiving MEMS mirrors are precisely synchronized in respect to the oscillating sending mirror (see Fig. 5). In comparison with state-ofthe-art 360-deg laser scanners of the Velodyne company, e.g., the panoramic 3D camera HDL-64E, 28 which uses a rotational motor combined with 64 parallel and multiplexed single-channel ToF detectors over 26.9 deg vertical FOV, the Fovea3D camera can provide a larger vertical FOV of 60 deg with significantly increased vertical angular resolution due to MEMS scanning (limited only by the 1 mrad optical divergence of the collimated pulse laser beam).

MEMS Design and Fabrication
The resonant 1D MEMS scanners presented in this paper were especially designed for the system concept of the adaptive 3D ToF camera Fovea3D (shown in Figs. 4 and 5). Two separate MEMS scanner designs were developed and optimized for the use as sending and receiving mirrors. The MEMS design targets on the following items: 1. enhancement of frequency bandwidth of a single scanning mirror to guarantee robust synchronization of the MSM array, 2. enlargement of single receiver aperture (2.4 mm × 8.4 mm) to reduce the number of receiver elements required for full receiver aperture of D eff ¼ 23 mm, 3. guarantee a low beam divergence of 1 mrad and minimization of dynamic mirror deformation of sending mirror ≤155 nm (≤λ∕10 at λ ¼ 1550 nm).
Both scanning mirror designs were based on the electrostatic resonant driving principle of Fraunhofer Institute for Photonic Microsystems (IPMS). 18 Typically, these resonant microscanning mirrors are driven in parametric resonance. 29 For excitation in parametric resonance, a pulsed driving voltage U Drive with f pulse ¼ 2 · f oscillation pulse driving frequency and 50% duty cycle are used. A MEMS oscillation occurs only in a limited parameter range of U Drive and f pulse , the so called electrostatic stability region, where the maximal usable driving voltage is limited by the pull-in voltage U pull-in of the MSM. Typically, the resonant MEMS scanner starts with a frequency down-sweep using a fixed driving voltage. The MEMS oscillation occurs when the limit of the stability region of the parametric resonance regime is exceeded. The correlation between electrostatic stability region and voltage-dependent amplitude-frequency characteristics is exemplarily shown in Fig. 6 for the resonant 1600 Hz inner axis of the 2D MEMS scanner used in the TACO system (shown in Fig. 2), which is the reference for the Fovea3D design.
The specifications of sending and receiving mirrors are summarized in Table 2. In Fig. 7, the finite element analysis (FEA) models of both 1D MEMS scanner designs are exemplarily shown for the first eigenmode (used for scanning). The FEA simulation results of the dynamic mirror deformation are shown in Fig. 8 for nominal deflection at MSA ¼ AE15 deg and 1600 Hz. For the sending mirror, a distributed spring suspension 18 of the elliptic 3.3 mm × 3.6 mm mirror plate was used to minimize the dynamic deformation to δ PP ¼ 106.0 nm < λ∕10 and δ RMS ¼ 25.0 nm, respectively, enabling high optical resolution of the scanned laser spot at the target. For the receiver MSM, a larger deformation exists, which influences the coupling efficiency of the fiber-coupled photodetector only slightly.
In Fig. 9, the electrostatic stability region (i.e., the parameter range of the MEMS driving in parametric resonance) is exemplarily shown for the first, second, and third eigenmodes of the sending MEMS mirror as well as the first and third subharmonic modes. The tilting mode used is   the first mode f 1 ¼ 1610 Hz. For precise rotational MEMS oscillation, any overlapping of the electrostatic stability regions of the first mode used with any parasitic or subharmonic modes has to be avoided by the MEMS design within the practically relevant range of driving parameters. In Fig. 9, a good mode separation is obvious for the sending mirror for all relevant driving voltages ≤180 V. Hence, a precise MEMS rotation in x (for the sending mirror shown) is guaranteed by the MEMS design. For MEMS fabrication, bonded silicon-on-insulator (BSOI) substrates with highly p-doped 75-μm thick device (SOI) layers were utilized. Details of the used qualified bulk micromachining MEMS scanner process (AME75) are found in Ref. 18. In contrast to the previous 1D MSM array LAMDA, 20 we included monolithically integrated piezoresistive (PZR) position sensors 30 for the design of the Fovea scanner to simplify the efforts for MEMS system integration. Photographs of the fabricated MSMs are shown in Fig. 10.
Details of the reception MSM are shown in Fig. 11. A challenge for the MEMS manufacturing was to ensure low tolerances of the resonant frequency due to the small frequency bandwidth <8 Hz at U max ¼ 180 V available for synchronous driving of multiple MEMS elements at nominal deflection (MSA ¼ 15 deg). To reduce the DRIE fabrication tolerances on the resonance frequency, we use latticed self-compensation structures located on the outer edge of the mirror plate [see Figs. 11(b) and 11(c)]. The self-compensation effect on the resonance frequency is enabled by reducing the moment of mirror inertia (by means of the latticed structures) in case the DRIE-etched trenches become too wide (resulting in a reduced spring constant and lower  resonance frequency, respectively). In the opposite case of DRIE-etched trenches that are too narrow, the spring constants increases, but the increases in resonance frequencies are selfcompensated by the increased moment of mirror inertia caused by the latticed compensation structures.

Experimental Results
The voltage-dependent amplitude frequency response characteristics of a receiving mirror are exemplarily shown in Fig. 12. They were measured in parametric resonance for varied pulse voltages (up to 180 V) and 50% duty cycle using an optical deflectometry setup with discrete photodiodes to measure the scan amplitude. A significantly wider frequency bandwidth of 7.5 Hz at MSA ¼ 15 deg is obvious for the new Fovea receiver mirror compared with the  The increased frequency bandwidth of 7.5 Hz of the Fovea3D scanner now enables sufficient margin for adequate MEMS yield and robustness to environmentally influenced changes in resonance frequency (e.g., temperature and ambient pressure). By means of the latticed selfcompensation structures, lower frequency tolerances were achieved. The experimental data of the statistical distribution of the eigenfrequency are shown in Fig. 12(b) (measured for a single 6″ wafer). A standard deviation of STD ¼ 3.8 Hz was measured for a batch of five 6″ wafers. The STD is smaller than the frequency bandwidth of 7.5 Hz. Hence, a small monolithic MSM array with increased optical fill factor (e.g., consisting of 1 × 3 or 2 × 3 scanner elements) could in principle be realized without reducing yield. However, this was out of the scope of this project.
For prospective real-time feedback control, PZR position sensors were integrated on the chip designed as a full Wheatstone bridge. 30 The PZR sensor characteristics are shown in Fig. 13. A sufficient sensitivity of S ¼ 320 mV∕V (620 mV∕V) was measured for the receiver (sending) mirror design at a nominal deflection of MSA ¼ 15 deg. A linearity error of the PZR sensors <1% was measured for the full scanning range of MSA ¼ 0; : : : ; 15 deg and <0.1% for the reduced range of MSA ¼ 0; : : : ; 13 deg.

Self-Aligned Microassembly of MEMS Scanner Array
To enable synchronized operation of multiple MEMS elements with reasonable MEMS yield and costs, a hybrid assembly was chosen (for risk management reasons) to build up the hybrid MSM array instead of the monolithic MSM array originally used in Ref. 20. Therefore 23 frequency-selected MSM elements (required for the full reception aperture of D eff ¼ 23 mm) had to be precisely mounted using a newly developed microassembly technique with self-alignment. The geometrical self-alignment of the MEMS scanners was achieved by (i) a base plate with pyramidal MESA structures (see Fig. 14) and (ii) corresponding inverse pyramidal cavities etched inside the MEMS frame (see Fig. 15).
The pyramidal alignment structures were well defined by anisotropic wet-chemical etching of h100 oriented monocrystalline silicon using tetramethylammonium hydroxide (TMAH). The pyramidal alignment structures were arranged in orthogonal lines to precisely define the horizontal and vertical MEMS positions (see Fig. 15). The pyramidal MESA structures were deliberately over-etched at the convex corners to obtain a complete resolution of the etching compensation structures to guarantee a precise form-fit connection of MESA structure and MEMS scanner.
The silicon base plate (consisting of two separate parts) was glued to an aluminum base plate with epoxy (see Fig. 17). Next, the MEMS scanner elements were manually placed and bonded to the silicon base plate with epoxy resin. The 3D topology of the microassembled MSM arraymeasured with a white light interferometer (WLI)-is shown in Fig. 16. Although only a manual assembly was used, the hybrid assembled MSM array achieved small angular tolerances <2 mrad (within the optically active apertures) and small lateral tolerances <10 μm of the individual MEMS scanner elements with respect to each other. The resulting range of angular tilt of   the individual scanners of ∼2 mrad (optical 0.12 deg) is mainly caused by the mounting accuracy of the silicon base plate and the flatness of the aluminum support structure. The self-aligned microassembly on the MESA structures, on the other hand, did not show any significant gain in the spread of the presteering of the scan angles.

MEMS Scanning Head Fovea3D
Details of the optical MEMS scan head, including the MEMS driver electronics and hybrid MEMS scanner array, are shown in Fig. 17   containing the movable MEMS elements within the outer chip frame coupled to the environment. This reduces by design the temperature induced changes of resonance frequency by a factor of 10. Details of the mechanical decoupling structures consisting of two degree of freedom springs are shown in Fig. 14(b). The MEMS scanning head Fovea3D is rotated by an electromagnetic motor. For electrical connection of the rotating MEMS scanning head to the LiDAR system, six electrical slide contacts are used (see Fig. 4).

ToF Camera Fovea3D
The LiDAR system Fovea3D and the final MEMS scan head are shown in Fig. 19. The pulsed ToF distance measuring system uses a fiber amplified laser with P average ¼ 1.5 W, P pulse ¼ 1500 W, λ ¼ 1550 nm, and 1 MHz pulse repetition rate. This pulsed ToF distance measuring module [developed by Fraunhofer Institute for Physical Measurement Techniques (IPM)] achieves a technical performance that exceeds commercially available state-of-the-art systems in several criteria. Pulse repetition rates of up to 2 MHz, pulse generation times of a few picoseconds, and standard deviations in ToF measurements of about 45 picoseconds (equal to 6 mm distance uncertainty) were realized. The latter was caused by about 25 picoseconds attributable to the properties of the commercial timing module used, and 20 picoseconds to the entire signal chain of pulse generation, amplification, analog receiver, and pulse discrimination.
The analog receiver circuit has a wide dynamic range of >40 dB (optical power) and >80 dB (voltage). This results in an uncertainty of the ToF distance measurement of about 6 mm for 30-m distance of a target with 10% reflectance. These are good values that are unique even in their combination. Hence, the Fovea3D camera can capture 3D images at 1 Mpixel∕s or 10 frames at 100 kpixels∕s over a 120 deg ×60 deg FOV for horizontal oscillation or potentially 360 deg ×60 deg FOV for horizontal rotation using a typical distance measuring rate of 1 MVoxel∕s. First 3D images of the 3D ToF camera Fovea3D (using a preliminary scanning system with reduced receiver aperture of 10 mm instead of the full effective aperture of 23 mm) are shown in Fig. 20. Visible is a 3D point cloud with a foveated region within the ROI demonstrating the concept of horizontal foveation used in the Fovea3D system. In this test, both a small ball-pen (∅ < 1 cm) and a horizontal wire only 0.5-mm thick could be clearly detected at a distance of 5 m within the foveated ROI.

Conclusion
A compact and fast 1D MEMS scanning module with large aperture was developed and integrated into the panoramic adaptive 3D ToF laser camera Fovea3D for an increased distance measuring range of up to 100m. The adaptive linearized scanning is realized in the horizontal direction by means of a conventional electromagnetic drive, enabling a 3D ToF camera with video-like image rate and partial foveation properties as required for future outdoor security applications (e.g., monitoring/surveillance of industrial plants and constructions, airports, etc.). Resonant vertical scanning at 1600 Hz oscillation frequency guarantees the fast bidirectional scanning rate for video-like 3D imaging and a large vertical optical scan range of up to 60 deg.
To provide the full effective reception aperture of 23 mm (required for the long-distance ToF camera Fovea3D), 23 single 1D MSM elements, selected by eigenfrequency, were hybrid integrated using a new microassembly technique with self-alignment. A FPGA-based MEMS driving electronics were developed for synchronous operation of all individual receiver MSM elements with respect to the coaxial sending mirror. For the industry-ready resonant MEMS scanner technology of Fraunhofer IPMS, the hybrid concept of Fovea3D allows the best technical compromise of opposite requirements: partial foveation, fast scanning speed, large scan range, enlarged effective aperture of D eff ¼ 23 mm, high yield, and moderate system costs. The adaptive 3D ToF camera Fovea3D provides a distance measuring rate of 1 MVoxel∕s and a ToF distance measurement uncertainty of 6 mm at 30-m distance measuring range. This allows, e.g., 3D images at 1 Mpixel∕s or 10 frames at 100 kpixels∕s over a 120 deg ×60 deg (for horizontal oscillation) or (potentially) 360 deg ×60 deg (for rotation) large FOV.

Outlook
The use of MSMs for 3D ToF LiDAR systems is highly topical. 11 Nowadays, there are many R&D activities and industrial product developments focusing on new applications, especially lower priced miniaturized 3D LiDAR sensors required for high-volume markets, most notably/in particular for automotive applications, such as autonomous driving. [8][9][10] To meet the high requirements of automotive applications in terms of cost, miniaturization, and reliability, 32 MSMs are typically considered only for the laser emitting path of the LiDAR sensor, whereas the receiver optics are realized by nonscanning optics. Frequently, line scanning in combination with optical array detectors is preferred 9,33 to 2D single point scanning. Nowadays, solid-state LiDAR systems [34][35][36][37] have to be considered in addition to MEMS-based LiDAR. Nevertheless, the approach of fast scanning 1D MSM arrays for ToF receiver optics presented in this paper is very interesting for sophisticated LiDAR measuring systems (pushbroom scanning ToF systems, e.g., for railroad safety inspection), which require very high scanning speeds. Especially, for highly mobile, movable LiDAR platforms, lightweight, fast scanning systems for scanning receiver optics do not exist. The hybrid assembled MEMS scanner array presented in this paper will be able to increase the scanning speed by a factor of four in comparison with state-of-the-art LiDAR systems used, e.g., for mobile 3D road inspection systems. 2 Thilo Sandner studied electrical engineering at the Technical University of Dresden, Germany, where he received his doctorate in 2003. Since 2003, he has been working as a scientist at the Fraunhofer IPMS, where he headed the R&D group for MSMs for more than 10 years. Currently, he works as a project manager and key researcher for development of MOEMS components, system design, and new applications of photonic microsystems such as MEMS-based LiDAR.
Thomas Graßhoff studied microsystem technology at the University of Applied Sciences Berlin, Germany. He has been working as a MEMS designer at the Fraunhofer IPMS since his graduation in 2005. In 2014, he received his MSc degree in computational engineering from Beuth University of Applied Sciences Berlin. His field of work as a scientist includes the component design of MEMS devices with a focus on FEA simulation of mechanical and multiphysical domains.
Wolf-Dietrich Owe studied electrical engineering at the University of Applied Sciences Dresden, Germany. He has been working as a scientist at the Fraunhofer IPMS since 1992. His work focuses on the lithographic mask layout and reliability tests of microscanning devices.
Andreas Herrmann studied microsystem technology at the University of Applied Sciences Mittweida, Germany. Since 2002, he has been working as a scientist at the Fraunhofer