Abstract:
A Micro-Electro-Mechanical Systems (MEMS) device for actuating a gimbaled element, the device comprising a symmetric electromagnetic actuator for actuating one degree of freedom (DOF) and a symmetric electrostatic actuator for actuating the second DOF.

Description:
FIELD OF THE INVENTION 
     The present invention related to the field of micro-electro-mechanical systems (MEMS) electromagnetic and electrostatic actuators and more particularly the present invention relates to actuation schemes and architectures for scanning micro-mirror devices placed on a gimbaled element with a symmetric internal electro-static actuator and a symmetric external electro-magnetic actuator. 
     BACKGROUND OF THE INVENTION AND PRIOR ART 
     Many MEMS applications require tilting motion of reflecting surfaces (i.e., micro-mirrors). In particular, there are applications with the need for tilting motions in two directions simultaneously, i.e., a mechanism having two degrees-of-freedom (DOF). One of such application is a scanning micro-mirror device for the use of displaying images. 
     Micro-mirrors offer numerous advantages in realizing optical scanning systems. Their small size, low cost and low power consumption provide a compelling solution for image creation and display systems. However, current state-of-the-art design still fall short on achieving the required performance (i.e., resolution, scan range, repeatability, scan linearity and power) which is required to making micro-mirrors based displays competitive to other display technologies. 
     The actuation of micro-mirrors in two DOF is essential for the functionality of the device. One way to implement actuation of an element in two DOF is with two different elements, each of which moves simultaneously in orthogonal directions. One way to implement actuation of an element in two DOF is by actuating a single gimbaled element having two DOF. The preferred architecture for micro-mirror scanners is the gimbaled design, where a single mirror is manipulated across two DOF. This architecture utilizes only one mirror for the two dimensional scan, thus reducing the chip size and simplifying the optical system design. The mirror is manipulated across both axes by using an actuation mechanism. The scan across one axis (horizontal axis) is done at a relative high frequency, typically a few KHz, while the scan across the second axis (vertical axis) is done at a relative lower frequency, typically a few tens of Hz. 
     Actuation Mechanisms 
     The prevalent actuation mechanisms are:
         a) Electrostatic, where capacitance change induces an electrostatic force to move the mirror about an axis. Typically, comb drive actuators are used to create this movement.   b) Electromagnetic, where alternating current in a magnetic field induces a magnetic force to move the mirror. Most commonly, the mirror has current carrying coils, and is positioned inside a magnetic flux created by fixed magnets which are placed beside the mirror and coil unit.   c) Piezoelectric, where a piezoelectric material is used to translate voltage into mechanical force and consequently, mirror movement.
 
Electrostatic Mechanisms
       

     Reference is made to  FIG. 2  (Prior art), which illustrates a typical prior art electrostatic actuation mechanism  100 . A mirror  110  is affixed to moving element  120  (rotor) having an axis  122 . Two electrodes  130  (stator) are place below each end of element  120  and when a different electrical potential is introduced between element  120  and an electrode  130   a , a force F is created, attracting element  120  to electrode  130   a , thereby creating a movement of element  120  about axis  122 . When movement is required in the opposite direction, an electrical potential different is introduced between element  120  and the other electrode  130   b . The electrostatic actuation mechanism  100  also creates some force f on axis  122 , which typically, in MEMS technology, is flexible, and thus creating an unwarranted movement of axis  122  in the direction of force f. The unwarranted movement of axis  122  is a result of the electrostatic actuator  100  being non-symmetric. Furthermore, the usage of electrostatic actuators  100  in two DOF introduces more problems. Typically, micro-mirrors  110  are designed to operate at their resonant frequency (i.e. the frequency at which the mechanical structure oscillates). However, the scan linearity and repeatability in display applications is greatly affected, which causes pixel and thereby image blurring and distortion. Moreover, in most of the prior art work, a single actuator is used to excite motion in both scanning axes. As a result, there is a mechanical coupling of the two DOF (i.e., actuation of one DOF also induces some residual actuation force on the other DOF), which directly affects the scan linearity and the image sharpness and reduces the elements operation quality and efficiency. Various solutions have been proposed to this problem; however none provides a suitable solution to the problem of attaining a linear scan at low power consumption. U.S. patent application 2004223195, by Nomura, is an example of a gimbaled mechanism with two DOF using electrostatic actuators. 
     Electromagnetic Mechanisms 
     Reference is made to  FIG. 3  (Prior art), which illustrates a typical prior art electromagnetic actuation mechanism  200 , including a magnet  210  and an element  220  having an axis  222  is wound with a coil  224 . When a DC electric current is introduced into coil  224 , a repelling/attracting force  226  is induced relatively to the magnetic field of static magnets  210  and the DC electric current, thereby creating a movement of element  220  about axis  222  in the direction of the repelling/attracting force  226 . When movement is required in the opposite direction, the polarity of the alternating electric current is introduced into coil  224  is changed, thereby inducing force in the opposite direction. 
     The main advantage of the electromagnetic actuation is the high force density, resulting in a device that can operate in protective environment without the need for vacuum. However, it is not trivial to use electromagnetic actuation for inner gimbaled moving elements. Therefore, it is prevalent to use electrostatic actuation for the above. Although a method that can simultaneously actuate a gimbaled element in two DOF, while using two different actuators, is more robust and less sensitive to mechanical coupling, but is not trivial for implementation. 
     Symmetric Electrostatic Mechanisms 
     To overcome the asymmetry of electrostatic actuation mechanism  100 , a different electrostatic actuation mechanism was introduced in U.S. Pat. No. 6,595,055 (U.S. &#39;055), given to Schenk et al. U.S. &#39;055 provided a scissors-like mechanism that introduced an electrostatic actuation mechanism with a pure torque applied to the axis of movement of the rotor, not giving raise to unwarranted force on the axis of rotation. 
     Reference is made to  FIG. 4  (Prior art), which illustrates a symmetric prior art electrostatic actuation, with scissors-like mechanism  150 . A mirror  160  is affixed to moving element  170  (rotor) having an axis  172 . Electrostatic actuation mechanism  150  also includes a stator element  180 , whereas there is some angle θ 0  between stator  180  and rotor  170 , when there is no electrical potential different between stator  180  and rotor  170 , i.e. V 1 (t)=V 2 (t). When a difference in electrical potential is introduced between stator  180  and rotor  170 , a force F is created, attracting rotor  170  to stator  180 , thereby creating a movement of rotor  170  about axis  172 . In this embodiment no residual forces are applied to axis  172 . However the mechanism introduced by U.S. &#39;055 has manufacturing difficulty as both stator  180  and rotor  170  are created from the same layer of silicon, which raises the problem of applying V 1 (t)≠V 2 (t) in the same layer of Silicon. U.S. &#39;055 provides a solution, which is difficult to manufacture, where the stator layer includes two additional sub-layers: an insulating sub-layer and a metal layer to which V 1 (t) is applied. 
     Feedback Control 
     A critical parameter in micro-mirror design is the attainable scan angle, which determines the optical system design and resulting size of the display. One of the main limitations in all actuation mechanisms is the maximum attainable scan angle since current or voltage, at the micro-mirror are limited. 
     To provide repeatability and linearity, a feedback mechanism is incorporated in the mirror design. The feedback mechanism however is susceptible to interference from the drive signals which are typically orders of magnitude stronger. Furthermore, the feedback control of existing scanners falls short of the required linearity and repeatability and typically sense one DOF. 
     Conclusion 
     Thus, there is a need for and it would be advantageous for applications using micro-mirrors architecture to have a system that can meet one or more of the following challenges: 
     a) Eliminating the coupling/interference/crosstalk between the two axes of motion; 
     b) Achieving low drive power while maintaining a linear and repeatable scan; 
     c) Increasing available drive force to increase scan angle: 
     d) Improving the feedback sensors to increase the resolution; and/or 
     e) Optimizing feedback algorithms to provide the required repeatability and linearity. 
     The invention described henceforth, presents a new paradigm in actuation schemes and architecture of gimbaled elements, which eliminates the mechanical coupling of the two DOFs. This invention enables a simple implementation and sufficient power for high quality performances typically required in such devices. 
     BRIEF SUMMARY OF THE INVENTION 
     The term “gimbaled element” as used herein refers to an element with two angular degrees of freedom, capable of moving about two axes simultaneously, the angles rotating about axes which are substantially mutually orthogonal and coplanar. Reference is made to  FIG. 1  (Prior art), which illustrates a gimbaled element  10 . Gimbaled element  10  includes an inner element  30  that can rotate about axis  32  and outer element  20  that can rotate about axis  22 , whereas the two axes  22  and  32  provide gimbaled element  10  the two DOF. 
     According to the present invention there is provided a micro-electro-mechanical system (MEMS) device for actuating a gimbaled element. The MEMS device includes an electromagnetic actuator for actuating a first angular degree of freedom (DOF) of rotation about the vertical axis; and an electrostatic actuator for actuating the second angular DOF of rotation about the horizontal axis, wherein the horizontal axis and the vertical axis are orthogonal and coplanar. The electromagnetic actuator has a symmetric structure, thereby actuating forces produced by the electromagnetic actuator, create only a rotational movement of the first DOF, about the horizontal axis. The electromagnetic actuator excites only negligible residual actuation force on the second DOF. The electrostatic actuator uses electrostatic fringing fields, thereby creating an actuation force, and since the electrostatic actuator has a symmetric structure, the actuating forces create only a rotational movement of the second DOF about the vertical axis. The electrostatic actuator excites only negligible residual actuation force on the first DOF. Both the rotor and the stator of the electrostatic actuator are created from the same layer and there is no electrical potential difference between both sides of the layer of the electrostatic actuator. 
     The electrostatic actuator actuates the second degree of freedom in the horizontal scan direction and the electromagnetic actuator, actuates the first degree of freedom in the vertical scan direction. The electromagnetic actuator includes external fixed electromagnet coils and internal rotating magnets, wherein the rotating magnets are affixed to said horizontal axis and actuated by the electromagnet coils. One or more electromagnetic actuators can be used on each side of the horizontal axis. 
     In embodiments of the present invention the electrostatic actuator include a frequency sensor with high signal to noise ratio. 
     The MEMS device of the present invention is preferably manufactured using a 4-masks Silicon-On-Insulator (SOI) fabrication process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become fully understood from the detailed description given herein below and the accompanying drawings, which are given by way of illustration and example only and thus not limitative of the present invention. 
         FIG. 1  (Prior art) illustrates a gimbaled element with two degrees of freedom; 
         FIG. 2  (Prior art) illustrates a typical prior art electrostatic actuation mechanism; 
         FIG. 3  (Prior art) illustrates a typical prior art electromagnetic actuation mechanism; 
         FIG. 4  (Prior art) illustrates a symmetric prior art electrostatic actuation, with scissors-like mechanism; 
         FIG. 5  is a top perspective view illustration of a gimbaled subsystem, according to embodiments of the present invention; 
         FIG. 6  is a top perspective view illustration of the gimbaled elements (the mirror and annular element with the 2 nd  degree of freedom) of a gimbaled subsystem, according to embodiments of the present invention 
         FIG. 7  is a schematic top perspective view of an electromagnetic actuator of a gimbaled subsystem, according to embodiments of the present invention; 
         FIG. 8   a  is a schematic front view of an electrostatic actuator of a gimbaled subsystem, according to embodiments of the present invention; 
         FIG. 8   b  is a schematic top view of the electrostatic actuator shown in  FIG. 8   a;    
         FIG. 9  depicts a top view of the electrostatic actuator of a gimbaled subsystem, according to embodiments of the present invention; 
         FIG. 9   a  is an enlargement of a portion of the electrostatic actuator shown in  FIG. 9 ; and 
         FIG. 10  depicts a side view of an electronic scanned image of a tooth of the rotor of an electrostatic actuator, according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is of a MEMS actuation scheme and architectures for scanning micro-mirror devices placed on a gimbaled element with a symmetric internal electro-static actuator and a symmetric external electromagnetic actuator. The external electromagnets for inducing magnetic flux are static, having the fixed magnets deposited on the mirror itself. The present invention actuator design enables a planar assembly of the micro-mirror and electromagnet and there are no mechanical limitations on the travel of the mirror. Furthermore, the gimbaled element of the present invention provides better electromagnetic conversion efficiency with no power limits. This innovative architecture of external electromagnets enables to implement a very powerful actuator which is symmetric to the scanner vertical rotation axis. Excitation of secondary DOF and image blurring are eliminated. The innovative architecture and design of the gimbaled element enables a standard 4-masks Silicon-On-Insulator (SOI) fabrication process. 
     Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     Referring now to the drawings,  FIG. 5  is a top perspective view illustration of a gimbaled subsystem  500 , and  FIG. 6  is a top perspective view illustration of the gimbaled elements (inner element with mirror  510  and external annular element  525 ) of a gimbaled subsystem  500  according to embodiments of the present invention. Gimbaled subsystem  500  includes a mirror  510  generally internal and at the center; mirror  510  is mounted on an element which serves as the rotor of the electrostatic actuator  520  that moves mirror  510  about axis  522 . Gimbaled subsystem  500  also includes annular element  525 , which also serves as the stator of electrostatic actuator  520 , can move about axis  532 , and is actuated by electromagnetic actuators  530 , that are positioned on axis  532 . Magnets  534  of electromagnetic actuators  530  are positioned adjacent to axis  532 , and electromagnets  536  are positioned outside of magnets  534 . Magnets  534  have a polarity (north-south) in the vertical direction. 
     The Electromagnetic Mechanisms 
     Referring now made to  FIG. 7 , which is a schematic top perspective view of electromagnetic actuator  530  of gimbaled subsystem  500 , according to embodiments of the present invention.  FIG. 7  schematically illustrates electromagnetic actuator  530  which includes internal magnets  534  rotating about axis  532  and fixed external electromagnets  536 . Rotating magnets  534  are also shown in  FIG. 7  where magnet  534   a  is attached symmetrically outside axis  532   a  and magnet  534   b  is positioned symmetrically outside axis  532   b . It should be noted that one electromagnetic actuator  530  is enough to actuate gimbaled subsystem  500 , and that more than one electromagnetic actuator  530  can be placed on each side of axis  532 . Electromagnetic actuator  530  has a symmetrical structure and hence the actuating force produced, creates only a rotational movement of annular element  525  about axis  532 , with no excitation of the inner element with mirror  510 . Electromagnetic actuator  530  applies no unwarranted forces on axis  532 , which typically, in MEMS technology, is flexible. Due to the symmetrical structure and lack of unwarranted forces on axis  532 , electromagnetic actuator  530  provides a linear electromechanical response. 
     When DC electric current is introduced into the coils of electromagnets  536 , magnetic flux is formed, thereby creating a repelling/attracting force rotating magnets  534  which is attached to axis  532 , and thereby creating a rotational movement of annular element  525  about axis  532 , in the direction of the repelling/attracting force. When movement is required in the opposite direction, the polarity of the DC electric current is introduced into the coils of electromagnets  536  is changed, thereby creating magnetic flux in the opposite direction. Electromagnet actuator  530  actuates outer, annular element  525  of gimbaled subsystem  500 , providing the scan across the vertical axis, which is done at a relative low frequency, typically a few tens of Hz. 
     The design of electromagnetic actuator  530  enables a planar assembly of micro-mirror  510  and electromagnet  530 . The design does not suffer from mechanical limitations on the travel of the mirror and provides good electromagnetic conversion efficiency with no power limits. 
     The Electrostatic Mechanisms 
     The present invention overcomes the complexity of the implementation of electrostatic actuation of dual-gimbaled scanners, with a unique design based on a standard 4-masks SOI fabrication process. The architecture uses a symmetric structure and a novel grounding scheme. The grounding scheme (as described in  FIGS. 8   a  and  8   b ) enables to apply the same electric potential on all the structural matter of the device, thus simplifies significantly the implementation of the actuator. The structure provides highly dense actuation forces in a relatively small chip area, and the symmetry of the actuator  520  ensures no mechanical coupling between the two axes. 
     Referring now made to  FIGS. 8   a  and  8   b .  FIG. 8   a  is a schematic front view of a comb like structured electrostatic actuator  520  of a gimbaled subsystem  500 , according to embodiments of the present invention, and  FIG. 8   b  is a schematic top view of the electrostatic actuator  520  shown in  FIG. 8   a . The actuation scheme utilizes fringing fields of electrostatic force fields  528  between an electrode  524 , placed on top of stator  523 , and rotor  521 . Both stator  523  and rotor  521  are in the same electric potential and manufactured from the same Si layer in the same process. Reference is also made to  FIG. 10 , which is a side view of an electronic scanned image of a tooth of stator  523  of an electrostatic actuator  520 , according to embodiments of the present invention. The stator  523  tooth is made of Si and has a thin layer of insulator  526  and then a thin metal layer  524 . When an electric potential difference is introduce between the Si layer of rotor  521  and the thin metal layer  524  of stator  523 , a force  528  created from the fringing electrostatic fields causes rotor  521  to rotate about rotor  521  axis. 
     Reference is also made to  FIG. 9 , which depicts a top view of the electrostatic actuator  520  of a gimbaled subsystem  500 , according to embodiments of the present invention, and FIG.  9   a , which is an enlargement of a portion of the electrostatic actuator  520  shown in  FIG. 9 . The comb structure of electrostatic actuator  520  can be observed, including the multiple teeth of rotor  521  and respective multiple teeth of stator  523 . The multiple teeth of rotor  521  are affixed to the inner element of gimbaled subsystem  500  with mirror  510 , and the multiple teeth of stator  523  are affixed to external annular element  525 . Hence, electrostatic actuator  520  actuates inner element of gimbaled subsystem  500 , providing the scan across the horizontal axis, which is done at a relative high frequency, typically a few KHz. 
     Electrostatic actuator  520  has a symmetrical structure and hence the actuating force produced, creates only a rotational movement of inner element with mirror  510  about axis  522 , with no excitation of annular element  525  about axis  532 . Electrostatic actuator  520  applies no unwarranted forces on axis  522 , which typically, in MEMS technology, is flexible. Due to the symmetrical structure and lack of unwarranted forces on axis  522 , electrostatic actuator  520  provides electromechanical response around its rotation axis only. 
     Feedback Control Architecture 
     In order to operate the actuators in a closed loop format, feedback sensors are required. These sensors can be utilized either in the structure itself, or by external sensors (e.g., position sensing detector). 
     The present invention uses a combination of frequency and position sensing control schemes, to achieve a more precise and optimized operation of the mirror. The electrostatic drive actuation includes an integrated frequency sensor to obtain high signal-to-noise ratio and the electromagnetic drive actuation includes a position feedback design. The integration of the frequency sensor and the position feedback design provides true raster scanning. The frequency sensing of electrostatic actuator  520 , can utilize the comb like fingers of stator  523  and rotor  521 . The electromagnetic drive actuation includes position sensing detectors, which can be implemented as internal sensing in the design of the drive or external sensing element. 
     The Gimbaled Subsystem 
     Electrostatic actuator  520  actuates inner element of gimbaled subsystem  500 , providing the scan across the horizontal axis, which is done at a relative high frequency, typically a few KHz. Electromagnet actuator  530  actuates outer annular element  525  of gimbaled subsystem  500 , providing the scan across the vertical axis, which is done at a relative lower frequency, typically a few tens of Hz. 
     There is no or negligible mechanical coupling of the two degrees of freedom, i.e. electrostatic actuator  520  actuates only the inner element of gimbaled subsystem  500 , and electromagnet actuator  530  actuates only the outer, annular element  525  of gimbaled subsystem  500 . 
     Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact design and operation shown and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.