Patent Publication Number: US-10768411-B2

Title: Hybrid MEMS scanning module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/622,084, filed Jun. 14, 2017, which is a continuation of U.S. patent application Ser. No. 14/622,942, filed Feb. 16, 2015 (now U.S. Pat. No. 9,798,135), which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to micro-mechanical systems, and particularly to optical scanning using such systems. 
     BACKGROUND 
     Microelectromechanical systems (MEMS) are very small machines (on the scale of a few micrometers to about ten millimeters) that are produced using modified semiconductor device fabrication technologies, such as photolithography, etching, and thin film deposition. Current MEMS devices include, inter alia, miniature scanning mirrors for use in optical projection and sensing. 
     For example, U.S. Pat. No. 7,952,781, whose disclosure is incorporated herein by reference, describes a method of scanning a light beam and a method of manufacturing a microelectromechanical system (MEMS), which can be incorporated in a scanning device. Other methods for fabrication of MEMS scanning devices are described in PCT International Publication WO 2014/064606, which is also incorporated herein by reference. 
     U.S. Patent Application Publication 2013/0207970, whose disclosure is incorporated herein by reference, describes a scanning depth engine, which includes a transmitter, which emits a beam comprising pulses of light, and a scanner, which is configured to scan the beam, within a predefined scan range, over a scene. The scanner may comprise a micromirror produced using MEMS technology. A receiver receives the light reflected from the scene and generates an output indicative of the time of flight of the pulses to and from points in the scene. A processor is coupled to control the scanner and to process the output of the receiver so as to generate a 3D map of the scene. 
     Another time-of-flight scanner using MEMS technology is the Lamda scanner module produced by the Fraunhofer Institute for Photonic Microsystems IPMS (Dresden, Germany). The Lamda module is constructed based on a segmented MEMS scanner device consisting of identical scanning mirror elements. A single scanning mirror of the collimated transmit beam oscillates parallel to a segmented scanning mirror device of the receiver optics. 
     PCT International Publication WO 2014/016794, whose disclosure is incorporated herein by reference, describes dual-axis MEMS scanning mirrors and magnetic driving arrangements for such mirrors. In the disclosed embodiments, a micromirror is mounted on a miniature gimbaled base, so that the base rotates relative to a support structure in the low-frequency (slow) scan direction, while the micromirror itself rotates relative to the base in the high-frequency (fast) scan direction. The micromirror assembly is produced by suitably etching a semiconductor substrate to separate the micromirror from the base and to separate the base from the remaining substrate, which serves as the support structure. The same magnetic drive can be used to power both the fast and slow scans. 
     U.S. Patent Application Publication 2014/0153001, whose disclosure is incorporated herein by reference, describes a gimbaled scanning mirror array, in which a substrate is etched to define an array of two or more parallel micromirrors and a support surrounding the micromirrors. Respective spindles connect the micromirrors to the support, thereby defining respective parallel axes of rotation of the micromirrors relative to the support. One or more flexible coupling members are connected to the micromirrors so as to synchronize an oscillation of the micromirrors about the respective axes. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved MEMS-based scanning modules. 
     There is therefore provided, in accordance with an embodiment of the present invention, a scanning device, which includes a base containing one or more rotational bearings disposed along a gimbal axis. A gimbal includes a shaft that fits into the rotational bearings so that the gimbal rotates about the gimbal axis relative to the base. A mirror assembly includes a semiconductor substrate, which has been etched and coated to define a support, which is fixed to the gimbal, at least one mirror, contained within the support, and a connecting member connecting the at least one mirror to the support and defining at least one mirror axis, about which the at least one mirror rotates relative to the support. 
     In a disclosed embodiment, the bearings include rolling-element bearings, including an outer race that is fixed to the base and an inner race that is fixed to the shaft of the gimbal. Typically, the one or more rotational bearings include a pair of bearings disposed at opposing ends of the gimbal axis, wherein the at least one mirror axis is perpendicular to the gimbal axis. 
     In some embodiments, the connecting member includes hinges arranged along the at least one mirror axis. In one embodiment, the at least one mirror includes an array of two or more mirrors contained within the support, and the hinges include two or more pairs of hinges, which are connected respectively to the two or more mirrors and define respective, mutually parallel mirror axes about which the mirrors rotate. Typically, the mirrors in the array are coupled together with a coupling strength sufficient to synchronize an oscillation of the mirrors about the respective mirror axes. 
     In some embodiments, the device includes a drive, which is coupled to the mirror assembly and the gimbal so as to cause the at least one mirror to rotate about the at least one mirror axis at a first frequency, while causing the gimbal to rotate about the gimbal axis at a second frequency, which is lower than the first frequency. Typically, the first frequency is a resonant frequency of rotation of the at least one mirror within the support. 
     In a disclosed embodiment, the drive is an electromagnetic drive, which includes a stator assembly, which is fixed to the base and includes at least one core containing an air gap and one or more coils including conductive wire wound on the at least one core so as to cause the at least one core to generate a magnetic field in the air gap in response to an electrical current flowing in the conductive wire. At least one rotor includes one or more permanent magnets, which are fixed to the shaft of the gimbal and which are positioned in the air gap so as to rotate in response to the magnetic field. Drive circuitry is coupled to apply the electrical current to the one or more coils. 
     In one embodiment, the at least one rotor includes one or more further permanent magnets, which are fixed to the at least one mirror, and the stator assembly includes a further core positioned in proximity to the at least one mirror and wound with one or more further coils. The drive circuitry is coupled to apply the electrical current at the first frequency to the one or more further coils and at the second frequency to the one or more coils wound on the at least one core containing the air gap. 
     There is also provided, in accordance with an embodiment of the present invention, a method for producing a scanning device. The method includes etching and coating a semiconductor substrate so as to define at least one mirror, a support surrounding the at least one mirror, and a connecting member connecting the at least one mirror to the support and defining at least one mirror axis, about which the at least one mirror rotates relative to the support. The support is fixed to a gimbal having a shaft. The shaft is fitted into one or more rotational bearings that are held by a base on a gimbal axis, so that the gimbal rotates about the gimbal axis relative to the base. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of a scanning device comprising a hybrid scanning module, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic rear view of the scanning module of  FIG. 1 ; 
         FIG. 3  is a schematic exploded view of the scanning module of  FIG. 1 ; and 
         FIG. 4  is a schematic, pictorial illustration of a hybrid scanning module, in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     MEMS scanning modules, of the type described in the above-mentioned PCT International Publication WO 2014/016794, for example, have advantages of small size, light weight, and low energy consumption. The energy consumption is particularly low when the scan frequency is close to the resonant frequency of rotation of the scanning elements about their hinges, but can rise sharply at other frequencies. In the sorts of scanner designs and applications that are described in WO 2014/016794, resonant scanning is implemented in the “fast axis” scan of the mirror relative to the gimbal, but the “slow axis” scan of the gimbal itself is driven in a quasi-static mode, at a scan frequency far below the resonant frequency. 
     To reduce energy consumption, particularly in the non-resonant scan, it is desirable that the hinges be made as flexible as possible in torsion. For MEMS structures produced by etching a silicon wafer, greater flexibility is generally achieved by making the hinges thinner. Thin hinges, however, also have reduced resistance to linear displacement, which makes them more prone to breaking when subjected to mechanical shock and vibration. Thus, there is a difficult tradeoff in the design of MEMS scanners between the durability of the hinges on which the gimbal rotates and the energy needed to drive the rotation. 
     Embodiments of the present invention that are described herein address this difficulty using a hybrid design, which combines the advantages of MEMS hinges and mechanical bearings in a single scanning device. In the disclosed embodiments, a mirror assembly is produced by etching and coating a semiconductor substrate to define one or more mirrors, a surrounding support, and a connecting member, such as hinges connecting the mirrors to the support. The support, in turn, is fixed to a gimbal, which is typically not a MEMS device, but rather has a shaft that fits into and rotates within one or more rotational bearings in a base of the scanning device. Each mirror thus rotates, relative to the support and gimbal, about a mirror axis defined by the etched hinges, while the support and gimbal rotate relative to the base about a gimbal axis defined by the bearings. The mirror axis (or axes) and gimbal axis are typically (but not necessarily) mutually perpendicular, in order to give full, dual-axis scanning. 
     In this sort of hybrid device, the MEMS assembly can be designed so that the mirror or mirrors have low inertia and oscillate about the hinges at substantially any desired frequency, including resonant oscillation with a high quality factor, thus resulting in low power consumption. The gimbal, on the other hand, can be molded or machined with a stiff shaft, which is resistant to mechanical stresses, while requiring only minimal energy input to drive its rotation in the low-friction mechanical bearings. The hybrid device thus offers greater robustness and versatility, as well as lower energy consumption, than could be achieved by either all-MEMS or all-mechanical designs. 
     Reference is now made to  FIGS. 1-3 , which schematically illustrate a scanning device  20 , in accordance with an embodiment of the present invention.  FIG. 1  is a pictorial illustration of the device, while  FIG. 2  provides a rear view and  FIG. 3  is an exploded view. Device  20  comprises a scanning module  22  that includes an array of two mirrors  24 , which oscillate about their respective axes in mutual synchronization, as described below. Alternatively, however, the principles embodied in device  20  may be applied in scanning devices comprising only a single mirror (as illustrated in  FIG. 4  and described below), as well as in scanning devices containing arrays of three or more mirrors. 
     As can be seen most clearly in  FIG. 3 , scanning module  22  comprises a mirror assembly  60 , produced using MEMS techniques, which is mounted in a mechanical gimbal assembly  62 . Gimbal assembly  62  comprises a gimbal  30 , having a shaft  70  that fits into a pair of rotational bearings  32  at opposing ends of the shaft, thus defining a gimbal axis  72 . In an alternative embodiment (not shown in the figures), the shaft of the gimbal may be held in a single bearing or in a pair of bearings that are both on the same side of the gimbal, rather than on opposite sides as shown in the figures. Bearings  32  are contained in and held by a base  44 , which may be of any suitable type and form (and is drawn in dashed lines for clarity of illustration). A stator assembly  64 , fixed to base  44 , drives the rotation of gimbal  30  and mirrors  24  about their respective axes. 
     Mirrors  24  in assembly  60  are produced using MEMS techniques, such as the techniques described in the references cited above in the Background section, for example. A semiconductor wafer (typically single-crystal silicon) is etched to separate mirrors  24  from a surrounding support  26 , and likewise to produce pairs of hinges  28 , which connect the mirrors to the support along the mirror axes. Hinges  28  thus define mirror axes  68 , which are typically parallel to one another and perpendicular (or at least non-parallel) to gimbal axis  72 . A reflective coating, typically metallic, is formed on the surfaces of mirrors  24 , using thin-film deposition techniques that are known in the art. Hinges  28  are one example of the sort of connecting member that may connect the mirror or mirrors to the support and enable the mirrors to rotate; but in alternative embodiments (not shown in the figures), other sorts of connecting members may be etched from the wafer, such as cantilevered or tilting structures, as are known in the art. 
     Typically, mirrors  24  have a reflective area that can be anywhere in the range of 5-250 mm 2 , depending on application requirements, while assembly  60  has a wafer thickness that is less than 1 mm. Given these parameters, mirrors  24  can be made to rotate about axes  68  over large angles (typically ±10-25°) at high frequencies (typically 2-25 kHz). Assembly  60  may be sealed within a transparent cover (not shown in the figures), which may even be evacuated to reduce air resistance and eliminate acoustic noise as mirrors  24  rotate. In some embodiments, assembly  60  is designed so that mirrors  24  rotate resonantly about axes  68  at the desired scan frequency. The resonant frequency is determined by the inertia, material properties (elasticity and stiffness), and hinge geometry of the assembly. 
     In the pictured embodiment, the individual rotations of mirrors  24  are coupled together so that the oscillations of the mirrors about the respective axes are synchronized. For this purpose, the ends of hinges  28  are connected to transverse beams  66 , which are likewise etched from the wafer and deform as hinges  28 . The structure of flexible, elastic support  26 , including beams  66 , is chosen so that the support itself couples together the motion of mirrors  24  with a coupling strength sufficient to synchronize their oscillations about their respective axes of rotation. For this purpose, the components of assembly  60  are designed and etched so as to define a resonant mode of overall oscillation of the assembly, including support  26  and mirrors  24  together, having a quality factor sufficiently high so that the mirrors rotate in synchronization at the resonant frequency. Support  26  is mounted in gimbal  30  along its edges so that this resonant mode of oscillation is only minimally damped by the rigid gimbal. 
     Alternatively, the necessary coupling between mirrors  24  may be provided by flexible coupling members, for example, as described in the above-mentioned U.S. Patent Application Publication 2014/0153001, or by electrical or magnetic coupling between the mirrors. 
     In contrast to mirror assembly  60 , gimbal  30  is typically produced from a plastic, metal or composite material, by a molding or machining process for example. In the pictured embodiment, bearings  32  at the ends of shaft  70  are rolling-element bearings, such as ball bearings. Inner races  75  of bearings  32  are thus fixed to shaft  70 , while outer races  78  are fixed to base  44 , with balls  79  rolling between the races, as is known in the art. Alternatively, any other suitable sort of low-friction bearings may be used in device  20 . One example of bearings suitable for mirror motion are flexure pivot bearings, which have the required range of motion, with a restoring spring to stabilize the system dynamics. In other configurations, sliding bearings may be used. 
     The rotations of mirrors  24  and gimbal  30  are driven by an electromagnetic drive, comprising stator assembly  64  and rotors  34  and  50 , which are fixed respectively to shaft  70  of the gimbal and to the back side of mirror  24  (as seen in  FIG. 2 ), along with drive circuitry  40  connected to the stator assembly. The principles of this sort of electromagnetic drive are described in detail in the above-mentioned PCT International Publication WO 2014/016794. These principles are applied in device  20  to drive mirrors  24  to rotate about axes  68  at one frequency—typically a high (and possibly resonant) frequency—while causing gimbal  30  to rotate about axis  72  at a lower frequency. 
     In optical applications, the frequencies and amplitudes of rotation of mirrors  24  and gimbal  30  are typically chosen so as to define a raster scan over an angular range of interest. Optics  42  (such as the optical and optoelectronic components described in WO 2014/016794) direct a beam of radiation toward mirrors  24 , which scan the beam over a scene or area of interest, and/or receive radiation reflected from the scene or area via the mirrors. Both of mirrors  24  may be used for both transmission and reception, assuming optics  42  comprise a suitable beam-combiner (not shown), or alternatively, one mirror may be used for transmission and the other for reception. Further alternatively, depending on the application in which device  20  is used, optics  42  may only transmit radiation or only receive radiation via scanning module  22 . Depending on the application, the scanning module may comprise smaller or larger numbers of mirrors, of any suitable shape and geometric arrangement. All such alternative implementations and applications are considered to be within the scope of the present invention. 
     To drive the rotation of gimbal  30 , stator assembly  64  comprises a pair of cores  36 , comprising a suitable magnetic material, which are fixed to base  44 . Coils  38  of conductive wire are wound on cores  36 . Cores  36  contain an air gap  76 , in which rotors  34  are positioned. Rotors  34  comprise permanent magnets  74 , which are fixed at both ends of shaft  70 . (Alternatively, the rotation of gimbal  30  in bearings  32  could be driven by a single core and a single magnetic rotor at one end of shaft  70 .) 
     Drive circuitry  40  applies a time-varying electrical current to coils  38 , which causes cores  36  to generate a time-varying magnetic field in air gaps  76 . Typically, drive circuitry  40  comprises a frequency generator, which generates electrical signals at the desired frequency or frequencies, along with suitable amplifiers to provide the desired current levels to the coils. The magnetic field generated in air gaps  76  interacts with the magnetization of rotors  34  and thus causes the rotors to rotate back and forth within air gaps  76  at a rotational frequency determined by the alternation frequency of the current applied by circuitry  40 . 
     Stator assembly  64  comprises an additional pair of cores  52 , positioned with their front ends in proximity to rotors  50  on the back sides of mirrors  24 . Rotors  50  typically comprise permanent magnets, which may be recessed into mirrors  24  so that the centers of mass of the mirrors are located along, or at least closer to, axes  68 . Cores  52  are wound with coils  54  of conductive wire, which are also driven with a time-varying current by drive circuitry  40 . In this case, the drive current alternates at the desired (higher) frequency of rotation of mirrors  24  about axes  68 . The resulting time-varying magnetic field emanating from the front ends of cores  52  causes rotors  50 , and hence mirrors  24 , to oscillate back and forth about hinges  28 . Alternatively, a single wire-wound core may be used to drive the rotations of both mirrors. 
     The shapes and geometries of cores  36  and  52  and of the corresponding coils  38  and  54  and rotors  34  and  50  are shown by way of example. Other suitable arrangements and configurations, using smaller or greater numbers of cores, coils and rotors, of different shapes and/or different locations, will be apparent to those skilled in the art after reading the present disclosure and the references cited herein, and are considered to be within the scope of the present invention. 
     Furthermore, the rotations of mirror assembly  60  and gimbal assembly  62  about the respective axes may be driven at the desired frequencies by other means than the particular magnetic drive configurations that are shown in the figures. For example, the drive currents that are applied to coils  38  may include a differential high-frequency component so that the magnetic fields emanating from cores  36  drive the rotations of rotors  50 , as well, at the high frequency. As another example, mirrors  24  may be weighted asymmetrically about axes  68  so as to mechanically couple the rotation of the mirrors about these axes to the rotation of gimbal  30  about axis  72 . Consequently, as gimbal  30  is driven to rotate, some of the rotational energy will be coupled into resonant rotation of mirrors  24  about their axes. Alternative drive modes of these sorts are described in the above-mentioned PCT International Publication WO 2014/016794. 
     As yet another example, other types of drives, such as an electrostatic drive, may be used in device  20 , particularly for driving the rotations of mirrors  24 . For example, the actuators described by Hah et al., in “Theory and Experiments of Angular Vertical Comb-Drive Actuators for Scanning Micromirrors,”  IEEE Journal of Selected Topics in Quantum Electronics  10:3 (May/June, 2004), pages 505-513, may be modified to drive mirrors  24  in device  20 . Alternatively, the drive may be based on voice coil actuation, piezoelectric actuation, or any other suitable method that is known in the art. 
       FIG. 4  is a schematic, pictorial illustration of a hybrid scanning module  80 , in accordance with another embodiment of the present invention. Module  80  is similar in construction and operation to module  22 , except that module  80  comprises only a single scanning mirror  82 . Mirror  82  is attached to a support  84  by hinges  86 , produced from a semiconductor wafer using the sort of MEMS processing that is described above. Support  84  is fixed to gimbal  30 , which is substantially identical to the gimbal shown in the preceding figures, as are the corresponding drive components. The rotation of mirror  82  about hinges  86  may be driven by a magnetic drive (similar to that shown in  FIG. 2 ), or by any of the other means that are noted above. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.