# Patent

## Title
MEMS scanning mirror with tunable natural frequency

## Abstract

In one embodiment of the invention, a MEMS structure includes a first electrode, a second electrode, and a mobile element. The first electrode is coupled to a first voltage source. The second electrode is coupled to a second voltage source. The mobile element includes a third electrode coupled to a third voltage source. A steady voltage difference between the first electrode and the third electrode is used to tune the natural frequency of the structure to a scanning frequency of an application. An oscillating voltage difference between the second electrode and the third electrode at the scanning frequency of the application is used to oscillate the mobile element. In one embodiment, the mobile unit is a mirror.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of application Ser. No. 10/302,413, filed Nov. 22, 2002, now U.S. Pat. No. 7,034,370, which is incorporated herein by reference.


FIELD OF THE INVENTION
This invention relates to micro-electro-mechanical systems (MEMS), and more particularly to MEMS scanning mirrors.
DESCRIPTION OF RELATED ART
Various electrostatic designs for MEMS scanning mirrors have been proposed. Their applications include barcode readers, laser printers, confocal microscopes, fiber-optical network components, projection displays for projectors, rear projection TVs, wearable displays, and military laser tracking and guidance systems. Typically a MEMS scanning mirror is driven at its main resonance to achieve a high scan angle. Invariably the manufacturing processes produce MEMS scanning mirrors with dimensional inconsistencies that vary the natural frequencies of the individual devices. If the main natural frequency of a minority of the MEMS scanning mirrors is too low or too high, the minority devices will not produce the proper scan speed and the proper scan angle under an alternating current (AC) drive voltage selected for a majority of the MEMS scanning mirrors. Thus, an apparatus and a method are needed to tune the main natural frequency of the MEMS scanning mirrors to improve the manufacturing yield of these devices.
SUMMARY OF THE INVENTION
In one embodiment of the invention, a MEMS structure includes a first electrode, a second electrode, and a mobile element. The first electrode is coupled to a first voltage source. The second electrode is coupled to a second voltage source. The mobile element includes a third electrode coupled to a third voltage source (e.g., an electrical ground). A steady voltage difference between the first electrode and the third electrode is used to tune the natural frequency of the structure to a scanning frequency of an application. An oscillating voltage difference between the second electrode and the third electrode at the scanning frequency of the application is used to oscillate the mobile element. In one embodiment, the mobile element is a mirror.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B respectively illustrate assembled and exploded views of a MEMS structure 100 in one embodiment.
FIGS. 1C, 1D, and 1E illustrate top views of the layers in MEMS structure 100 in one embodiment.
FIG. 1F illustrates a method for configuring and operating MEMS structure 100 in one embodiment of the invention.
FIGS. 1G, 1H, 1I, and 1J illustrate top views of the various layers in MEMS structure 100 in different embodiments.
FIGS. 2A and 2B respectively illustrate assembled and exploded views of a MEMS structure 200 in one embodiment.
FIGS. 2C and 2D illustrate top views of the layers in MEMS structure 200 in one embodiment.
FIGS. 3A and 3B respectively illustrate assembled and exploded views of a MEMS structure 300 in one embodiment.
FIGS. 3C, 3D, 3E, 3F, and 3G illustrate top views of the layers in MEMS structure 300 in one embodiment.
FIG. 4 illustrates a MEMS system in one embodiment of the invention.
FIG. 5 illustrates DC and AC voltages used to oscillate a MEMS structure in one embodiment of the invention.



DETAILED DESCRIPTION
FIG. 4 illustrates a MEMS system 400 in one embodiment of the invention. MEMS system 400 includes a MEMS structure (e.g., MEMS structure 100, 200, or 300) with a mobile element that is electrostatically moved under voltages supplied by a voltage source 402. Voltage source 402 provides a voltage difference between a stationary electrode and a moving electrode of the mobile element to adjust the natural frequency of MEMS structure 100 to a desired scanning frequency. Voltage source 402 also provides an AC voltage difference between another stationary electrode and the moving electrode of the mobile element at the desired scanning frequency to oscillate the mobile element with a desired scanning angle.
The movement of the mobile element (e.g., the scanning frequency and the scanning angle) is measured by a sensor 404 and fed back to a controller 406. Controller 406 compares the measured movement to a desired movement of the mobile element and then instructs voltage source 402 to provide the appropriate voltages to achieve the desired movement. Although shown as individual components, MEMS structure 100, voltage source 402, sensor 404, and controller 406 can be build on the same chip or on different chips.
FIGS. 1A and 1B respectively illustrate assembled and exploded views of a MEMS structure 100 in one embodiment. MEMS structure 100 can be used in any application that requires a single axis of motion (e.g., a unidirectional scanning mirror). MEMS structure 100 includes a conductive layer 105, an insulating layer 107, and a conductive layer 109. In one embodiment, conductive layers 105 and 109 are made of doped silicon while insulating layer 107 is made of silicon dioxide (SiO2). Insulating layer 107 electrically insulates components on conductive layers 105 and 109. Insulating layer 107 is also used to physically bond conductive layers 105 and 109.
FIG. 1C illustrates a top view of one embodiment of conductive layer 105. Conductive layer 105 includes a scanning mirror 101 and a bias pad 112. Scanning mirror 101 includes a reflective area 124 that is connected by torsion hinges 102A and 102B to anchors 108A and 108B, respectively. Mirror 101 rotates about an axis 122.
In one embodiment, torsion hinges 102A and 102B include internal holes 114 to lower the rotational modal frequency of structure 100. The rotational modal frequency is the lowest of the modal frequencies to ensure scanning mirror 101 rotates about the axis 122 without coupling with other unwanted rotational and translational structural vibrations.
Mirror 101 includes moving teeth 104A and 104B (collectively “moving teeth 104”) on different sides of rotational axis 122. Moving teeth 104A and 104B extend from bars 106A and 106B, respectively. Bars 106A and 106B are connected to reflective area 124 and run parallel to torsion hinges 102A and 102B.
Bias pad 112 includes stationary teeth 103A and 103B (collectively “stationary teeth 103”) on the different sides of rotational axis 122. Stationary teeth 103A and 103B are respectively interdigitated with moving teeth 104A and 104B when bias pad 112 and mirror 101 are in the same plane (e.g., when mirror 101 is not rotated).
In one embodiment, anchor 108A is coupled to ground 116 and bias pad 112 is coupled to a direct current (DC) voltage source 118. DC voltage source 118 provides a DC bias voltage to bias pad 112. The DC bias voltage creates a steady voltage difference between stationary teeth 103 and moving teeth 104. The steady voltage difference between stationary teeth 103 and moving teeth 104 creates an electrostatic torque that rotates mirror 101 until the electrostatic torque is equal to the restoring torque in an equilibrium position. In effect, the steady voltage difference between stationary teeth 103 and moving teeth 104 creates a nonlinear electrostatic system that changes the natural frequency of MEMS structure 100. Accordingly, the natural frequency of MEMS structure 100 can be adjusted (e.g., tuned) by increasing or decreasing the steady voltage difference between stationary teeth 103 and moving teeth 104.
In one embodiment, DC voltage source 118 is built on the same chip as structure 100. Alternatively, DC voltage source 118 is built on a chip separate from structure 100. In one embodiment, DC voltage source 118 is servo-controlled during operation to generate a DC bias voltage value that produces the desired natural frequency of structure 100.
FIG. 1D illustrates a top view of one embodiment of intermediate layer 107. Insulating layer 107 has the same shape as conductive layer 105 but for mirror 101 in order to electrically insulate the components on layer 101. Insulating layer 107 defines a cross-shaped opening 121 for the scanning motion of mirror 101.
FIG. 1E illustrates a top view of one embodiment of conductive layer 109. Conductive layer 109 includes a drive pad 126 that defines a cross-shaped opening 111. Drive pad 126 includes stationary teeth 110A and 110B (collectively “stationary teeth 110”) on the different sides of rotational axis 122. Like opening 121, opening 111 is a free space reserved for the scanning motion of mirror 101. Stationary teeth 110A are interdigitated with moving teeth 104A when mirror 101 is rotated in a first direction (e.g., clockwise), and stationary teeth 110B are interdigitated with moving teeth 104B when mirror 101 is rotated in a second direction (e.g., counterclockwise). Teeth 110A and 110B are electrically connected. When an AC drive voltage is applied between pads 112 and 126, a translational resultant force is generated initially if the moving structure is symmetrical with respect to axis 122. This translational resultant force is not useful for the rotational motion. In reality, the structure is not completely symmetrical due to the manufacturing tolerance and will start to oscillate. Once the structure starts to oscillate, the torque increases and the translational resultant force decreases. This potential issue of small initial torque can be solved by making the force or the structure slightly asymmetric with respect to axis 122. For example, the length of teeth 110A and 110B can be made slightly different to generate relatively large initial torque. The mirror shape can be made slightly asymmetric with respect to axis 122 to produce the same effect.
In one embodiment, stationary teeth 110 and moving teeth 104 form an electrostatic actuator (e.g., a vertical comb drive actuator) that oscillates scanning mirror 101. In this embodiment, drive pad 126 is coupled to an AC voltage source 120 and anchor 108A is coupled to ground 116. When activated, AC voltage source 120 provides an AC drive voltage to drive pad 126 that creates an oscillating voltage difference between stationary teeth 110 and moving teeth 104. Typically AC drive voltage has a frequency equal to the natural frequency of structure 100 to achieve the maximum scan angle. The oscillating voltage difference between teeth 110 and 104 causes electrostatic torques that create the scanning motion of mirror 101.
In one embodiment, AC voltage source 120 is built on the same chip as structure 100. Alternatively, AC voltage source 120 is built on a chip separate from structure 100. In one embodiment, AC voltage source 120 is servo-controlled during operation to generate an AC drive voltage that produces the desired scanning speed and scanning angle.
FIG. 1F illustrates a method 150 for configuring and operating a MEMS structure 100 in one embodiment. Structure 100 is generally a device from a batch of mass produced structures 100. Described below, actions 151 and 152 occur during the manufacturing of structure 100, and actions 153, 154, 156, and 160 occur during the use of structure 100.
In action 151, a designer determines the scanning frequency and the scanning angle of an application (e.g., 1 kHz and 5-10 degrees for a barcode reader) and modifies the basic design of structure 100 to achieve a specific natural frequency equal to the scanning frequency. The designer modifies the design by changing the stiffness of the hinges (e.g., the geometry of the hinges) or changing the inertia of the structure (e.g., the geometry of the mirror). Action 152 is followed by action 152.
In action 152, the designer presets the characteristics of the DC voltage difference and the AC voltage difference for this structure 100. The designer presets the amplitude of the DC bias voltage (FIG. 5) to tune the natural frequency of this structure 100 to the scanning frequency of the application. The designer presets the amplitude and the frequency of the AC drive voltage (FIG. 5) to achieve the desired scan angle for this structure 100. The designer can also preset the vertical offset of the AC drive voltage (FIG. 5) to achieve the desired neutral scanning position about which the oscillation occurs. These steps are necessary because each structure 100 is somewhat different from the others due to manufacturing inconsistencies. These characteristics are then stored into controller 406 for this structure 100 as the initial/default characteristics for the DC bias voltage and the AC drive voltage.
In action 153, an end user may store different characteristics for the DC bias voltage and the AC drive voltage in controller 406. The end user may wish to do so to change the desired scanning frequency, the desired scanning angle, and the desired neutral scanning position.
In action 154, controller 406 instructs voltage source 402 to apply the DC bias voltage and the AC drive voltage. Voltage source 402 represents the various DC and AC voltage sources (e.g., DC voltage source 118 and AC voltage source 120).
The DC bias voltage is initiated with the default values stored in controller 406 and then servo-controlled to ensure the rotational natural frequency is the scanning frequency. Servo-control of the DC bias voltage is necessary in the operational stage because the natural frequency of structure 100 may drift away from the desired value due to temperature changes, material aging, or any other reasons.
The AC drive voltage is initiated with the default values stored in controller 406 and then servo-controlled to ensure the desired scanning frequency and the scanning angle are achieved. Servo-control of the AC drive voltage is necessary in the operational stage because the scanning frequency, the scanning angle, and the neutral scanning position may drift away from the desired values due to temperature changes, material aging, or any other reasons. Action 154 is followed by action 158.
In action 158, sensor 404 is used to monitor the motion of the scanning mirror (e.g., the scanning frequency, the scanning angle, and the scanning neutral position) and the measured information is outputted to controller 406. Action 158 is followed by action 160.
In action 160, controller 406 receives the motion information from sensor 404. Controller 406 computes and provides the needed DC bias voltage and the needed AC drive voltage to voltage source 402. The servo-control of the DC bias voltage is accomplished by perturbing the amplitude of the DC bias voltage and sensing the change in the scanning angle. If the DC bias voltage is increased and the scanning angle is also increased at the same time, then the natural frequency is approaching the scanning frequency, and vice versa. It is generally more effective to maintain the scanning amplitude by controlling the natural frequency with DC bias voltage change if the Bode plot shows a high Q factor of the main natural frequency.
The servo-control of the AC drive voltage is accomplished by perturbing the amplitude, the frequency, and the vertical offset of the AC drive voltage and sensing the change in the scanning angle, the scanning frequency, and the scanning neutral position. The amplitude of the AC drive voltage is increased to increase the angle of rotation, and vice versa. The frequency of the AC drive voltage is increased to increase the scanning frequency, and vice versa. The vertical offset of the AC drive voltage is changed to optimize the scanning neutral position. Action 160 is followed by action 154 and the method continues in a feedback loop.
FIG. 1G illustrates a top view of another embodiment of conductive layer 105 of structure 100. Same or similar parts between FIGS. 1C and 1G are indicated by the same reference numerals. In this embodiment, reflective area 124 is connected to bars 128A and 128B. Moving teeth 104A and 104B extend from opposite edges of bars 128A and 128B. The ends of bars 128A and 128B are connected by torsion hinges 130A and 130B to anchors 108A and 108B, respectively. Each of torsion hinges 130A and 130B has a serpentine shape that increases translational stiffness but maintains the torsional flexibility of hinges 102A and 102B. Similarly described above, DC voltage source 118 is coupled to bias pad 112 and ground 116 is coupled to anchor 108A. Method 150 described above can be used to configure and operate a structure 100 with conductive layer 105 of FIG. 1G.
FIG. 1H illustrates a top view of another embodiment of conductive layer 109. Same or similar parts between FIGS. 1E and 1H are indicated by the same reference numerals. In this embodiment, drive pad 126 only includes stationary teeth 110B. This configuration provides a large initial torque to excite the mirror rotational oscillation. The oscillating voltage difference between stationary teeth 110B and moving teeth 104B alone creates the scanning motion of mirror 101. However, the oscillating voltage difference may be increased to match the response amplitude of the above embodiment in FIG. 1E because layer 109 in this embodiment exerts a force with stationary teeth 110 on only one of the opposing sides. Method 150 described above can be used to configure and operate a structure 100 with conductive layer 109 of FIG. 1H.
FIG. 1I illustrates a top view of another embodiment of conductive layer 109. Same or similar parts between FIGS. 1E and 1I are indicated by the same reference numerals. In this embodiment, conductive layer 109 is divided into two drive pads 132A and 132B (collectively “drive pads 132”) that together define opening 121. Stationary teeth 110A and 110B extend from opposing edges of drive pads 132A and 132B, respectively. Drive pad 132A is coupled to an AC voltage source 134A while drive pad 132B is coupled to another AC voltage source 134B. AC voltage sources 134A and 134B have the same frequency but a phase difference of 180 degrees to provide the highest torsional actuation force and initial excitation torque. Thus, the oscillating voltage difference between stationary teeth 110 and moving teeth 104 creates the scanning motion of mirror 101. Method 150 described above can be used to configure and operate a structure 100 with conductive layer 109 of FIG. 1I.
FIG. 1J illustrates a top view of an additional layer 136 below conductive layer 109 that electrically insulate drive pads 132A and 132B. In one embodiment, insulating layer 136 is made of intrinsic silicon. Insulating layer 136 may include a free space reserved for the scanning motion of mirror 101.
FIGS. 2A and 2B respectively illustrate assembled and exploded views of a MEMS structure 200 in one embodiment. Like MEMS structure 100, MEMS structure 200 can be used in any application that requires a single axis scanning mirror. MEMS structure 200 includes a conductive layer 205, an isolative and bonding layer 207, and a structure anchoring layer 209. In one embodiment, conductive layer 205 is made of doped silicon while isolative layer 207 is made of SiO2 to electrically insulate elements of conductive layer 205. Layer 209 provides a support structure for the two upper layers. If layer 209 is made of non-conductive intrinsic silicon, layer 207 will only be used as a bonding layer and may be optional for this configuration.
FIG. 2C illustrates a top view of one embodiment of conductive layer 205. Conductive layer 205 includes a scanning mirror 201, bias pad 212, and drive pads 232A and 232B. Similar to mirror 101, mirror 201 includes a reflective area 224 that is connected by torsion hinges 202A and 202B to anchors 208A and 208B, respectively. Mirror 201 rotates about an axis 222.
In one embodiment, torsion hinges 202A and 202B include internal holes 214 to lower the rotational modal frequency. Mirror 201 also includes a set of moving teeth 204A and 204B (collectively “moving teeth 204”). Moving teeth 204A and 204B extend from bars 206A and 206B, which are on different sides of axis 222. Bars 206A and 206B are connected to reflective area 224 and run parallel to torsion hinges 202A and 202B.
Inner moving teeth 204B are closer to reflective area 224 and are interdigitated with stationary teeth 210A and 210B (described later). Outer moving teeth 204A are farther from reflective area 224 and are interdigitated with stationary teeth 203A and 203B (described later).
In one embodiment, mirror 201 is asymmetric because it generally has a square shape with one or more corners removed. Thus, the center of gravity of mirror 201 is shifted to one side of the axis 222. Such a design may be preferred when an application requires mirror 201 to start at some initial rotational position or to reach some initial rotational position quickly.
Bias pad 212 includes stationary teeth 203A and 203B (collectively “stationary teeth 203”) on the different sides of axis 222. Stationary teeth 203A and 203B are respectively interdigitated with outer moving teeth 204A when bias pad 212 and mirror 201 are in the same plane (e.g., when mirror 201 is not rotated).
Drive pads 232A and 232B (collectively “drive pads 232”) respectively include stationary teeth 210A and 210B (collectively “stationary teeth 210”). Stationary teeth 210A and 210B are interdigitated with inner moving teeth 204B when drive pads 232 and mirror 201 are in the same plane.
In one embodiment, anchor 208A is coupled to ground 216 and bias pad 212 is coupled to a DC voltage source 218. DC voltage source 218 provides a DC bias voltage to bias pad 212 that creates a steady voltage difference between stationary teeth 203 and outer moving teeth 204A. Similarly described above, the steady voltage difference between stationary teeth 203 and moving teeth 204A creates an electrostatic force that changes the natural frequency of structure 200. Accordingly, the natural frequency of MEMS structure 200 can be tuned by changing the steady voltage difference between stationary teeth 203 and moving teeth 204A.
In one embodiment, stationary teeth 210 and moving teeth 204B form an electrostatic actuator (e.g., a comb drive actuator) that oscillates scanning mirror 201. In this embodiment, drive pads 232 are coupled to an AC voltage source 220. When activated, AC voltage source 220 provides an AC drive voltage to drive pads 232 that creates an oscillating voltage difference between stationary teeth 210 and inner moving teeth 204B. The oscillating voltage difference between stationary teeth 210 and inner moving teeth 204B causes electrostatic torque that creates the scanning motion of mirror 201.
Similarly described above, in one embodiment DC voltage source 218 and AC voltage source 220 are built on the same chip as structure 200. Alternatively, voltage sources 218 and 220 are built on one or more chips separate from structure 200. These one or more chips are then coupled to bias pad 212 and drive pads 232 via wires. In one embodiment, DC voltage source 218 is servo-controlled during operation to generate a DC bias voltage value that produces the desired natural frequency of structure 100, and AC voltage source 220 is servo-controlled during operation to generate an AC drive voltage that produces the desired scanning speed and scanning angle.
FIG. 2D illustrates a top view of one embodiment of isolative layer 207. Isolative layer 207 defines a cross-shaped opening 221. Similar to opening 121, opening 221 is a free space reserved for the scanning motion of mirror 201.
Method 150 (FIG. 1F) described above can be applied to operate structure 200.
FIGS. 3A to 3B respectively illustrate assembled and exploded views of a MEMS structure 300 in one embodiment. MEMS structure 300 can be used in any application that requires rotational motion with respect to two rotational axes (e.g., a bidirectional scanning mirror). MEMS structure 300 includes a structure anchoring layer 301, an insulating layer 304, a conductive layer 302, an insulating layer 305, and a conductive layer 303. In one embodiment, layer 301 is made of intrinsic silicon or doped silicon, conductive layers 302 and 303 are made of doped silicon, and insulating layers 304 and 305 are made of silicon dioxide (SiO2). Insulating layers 304 and 305 electrically insulate components on layers 301, 302, and 303. Insulating layer 304 is also used to physically bond layers 301 and 302. Similarly, insulating layer 305 is also used to physically bond conductive layers 302 and 303.
FIG. 3C illustrates a top view of one embodiment of conductive layer 303. Conductive layer 303 includes a scanning mirror 316, drive pads 306 and 309, ground pad 307, and bias pad 308. Scanning mirror 316 includes a reflective area 352 that is connected by serpentine torsion hinges 315A and 315B to anchors 328 and 329, respectively. Mirror 316 rotates about the Y-axis via hinges 315A and 315B. Hinges 315A and 315B determine the mirror scanning frequency/speed in the Y-axis.
Mirror 316 includes moving teeth 314A and 314B (collectively “moving teeth 314”) on different sides of the Y-axis. Drive pad 306 is connected by a serpentine torsion hinge 324 to a comb 388. Comb 388 has stationary teeth 313 that are interdigitated with some of moving teeth 314A when comb 388 and mirror 316 are in the same plane (e.g., when mirror 316 is not rotated about the Y-axis). Similarly, drive pad 309 is connected by a serpentine torsion hinge 326 to a comb 390. Comb 390 has stationary teeth 311 that are interdigitated with some of moving teeth 314B when mirror 316 is not rotated about the Y-axis.
Bias pad 308 is connected by a serpentine torsion hinge 325 to a comb 323B. Comb 323B is connected by a bar 330A to a comb 323A. Combs 323A and 323B respectively have stationary teeth 310A and 310B (collectively “stationary teeth 310”). Stationary teeth 310A and 310B are respectively interdigitated with some of moving teeth 314A and 314B when mirror 316 is not rotated about the Y-axis.
Ground pad 307 is connected by a serpentine torsion hinge 327 to an L-shaped bar 330B. Bar 330B is connected to anchor 329. Thus, ground pad 307 is connected to mirror 316 and moving teeth 314.
In one embodiment, ground pad 307 is coupled to ground 354 and bias pad 308 is coupled to a DC voltage source 356. DC voltage source 356 provides a DC bias voltage to bias pad 308. The DC bias voltage creates a steady voltage difference between stationary teeth 310 and moving teeth 314. As similarly described above, the steady voltage difference between stationary teeth 310 and moving teeth 314 creates a nonlinear electrostatic system that changes the natural frequency of MEMS structure 300 about the Y-axis. Accordingly, the natural frequency of MEMS structure 300 about the Y-axis can be changed (e.g., tuned) by changing the steady voltage difference between stationary teeth 310 and moving teeth 314.
As similarly described above, DC voltage source 356 can be built on the same chip as structure 300. Alternatively, DC voltage source 356 can be built on a chip separate from structure 300. In one embodiment, DC voltage source 356 is servo-controlled during operation to generate a DC bias voltage value that produces the desired natural frequency of structure 300 about the Y-axis.
In one embodiment, (1) stationary teeth 311 and moving teeth 314B and (2) stationary teeth 313 and moving teeth 314A form two electrostatic actuators (e.g., comb drive actuators) that oscillate scanning mirror 316 about the Y-axis. In this embodiment, drive pads 306 and 309 are coupled to an AC voltage source 360, and ground pad 307 is coupled to ground 354. When activated, AC voltage source 360 creates an oscillating voltage difference (1) between stationary teeth 311 and moving teeth 314B, and (2) between stationary teeth 313 and moving teeth 314A. Typically AC drive voltage has a frequency equal to the natural frequency of structure 300 to achieve the maximum scan angle. The oscillating voltage difference between the teeth causes electrostatic torques that create the scanning motion of mirror 316 about the Y-axis.
As similarly described above, in one embodiment AC voltage source 360 is built on the same chip as structure 300. Alternatively, AC voltage source 360 is built a chip separate from structure 300. In one embodiment, AC voltage source 360 is servo-controlled during operation to generate an AC drive voltage that produces the desired scanning speed and scanning angle about the Y-axis.
In one embodiment, conductive layer 303 further includes drive pads/combs 317A and 317B located on different sides of the X-axis. Combs 317A and 317B include stationary teeth 318A and 318B, respectively. Stationary teeth 318A and 318B are used to rotate mirror 316 about the X-axis (described later in reference to layer 302). Combs 317A and 317B are coupled to an AC voltage source 374 (described later).
FIG. 3D illustrates a top view of one embodiment of insulating layer 305. Insulating layer 305 has the same shape as conductive layer 303 but for mirror 316 in order to electrically insulate the components on layer 303. Insulating layer 305 defines an opening 358 reserved for the scanning motion of mirror 316.
FIG. 3E illustrates a top view of one embodiment of conductive layer 302. Conductive layer 302 includes rotational frame 364 and bias pads/combs 319A and 319B. Rotational frame 364 defines an opening 358 for the scanning motion of mirror 316. Rotational frame 364 includes combs 322A and 322B on different sides of the X-axis. Rotational frame 364 is connected by serpentine torsion hinges 332A and 332B to grounding pads/anchors 331A and 331B, respectively. Rotational frame 364 can rotate about the X-axis via hinges 332A and 332B. Mirror 316 is mounted atop rotational frame 364. Specifically, anchors 328 and 329 of mirror 316 are respectively mounted atop of anchor mounts 366 and 367 of rotational frame 364. This allows mirror 316 to rotate about the Y-axis using hinges 315A and 315B, and about the X-axis using hinges 332A and 332B.
Combs 322A and 322B respectively include moving teeth 321A and 321B (collectively “moving teeth 321”). Combs 319A and 319B respectively include stationary teeth 320A and 320B (collectively “stationary teeth 320”). Stationary teeth 320A and 320B are respectively interdigitated with moving teeth 321A and 321B when combs 322A, combs 322B, and rotational frame 364 are in the same plane (e.g., when rotational frame 364 is not rotated about the X-axis).
In one embodiment, anchor 331A is coupled to ground 368, combs 319A and 319B are coupled to a DC voltage source 370. DC voltage source 370 provides DC bias voltages to comb 319A and 319B. The DC bias voltages create a steady voltage difference between stationary teeth 320 and moving teeth 321. As similarly described above, the steady voltage difference between stationary teeth 320 and moving teeth 321 creates a nonlinear electrostatic system that changes the natural frequency of MEMS structure 300 about the X-axis. Accordingly, the natural frequency of MEMS structure 300 about the X-axis can be changed (e.g., tuned) by changing the steady voltage difference between stationary teeth 320 and moving teeth 321.
As similarly described above, in one embodiment DC voltage source 370 is built on the same chip as structure 300. Alternatively, DC voltage source 370 is built on a chip separate from structure 300. In one embodiment, DC voltage source 370 is servo-controlled during operation to generate a DC bias voltage value that produces the desired natural frequency of structure 300 about the X-axis.
As described above, comb 317A and 317B (FIG. 3C) respectively have stationary teeth 318A and 318B (FIG. 3C). Moving teeth 321A (FIG. 3E) of rotational frame 364 (FIG. 3E) are interdigitated with stationary teeth 318A when mirror 316 (FIG. 3C) is rotated in a first direction, and moving teeth 321B (FIG. 3E) of rotational frame 364 are interdigitated with stationary teeth 318B when mirror 316 is rotated in the opposite direction.
In one embodiment, (1) stationary teeth 318A and moving teeth 321A and (2) stationary teeth 318B and moving teeth 321B form two electrostatic actuators (e.g., a comb drive actuators) that oscillate scanning mirror 316 about the X-axis. In this embodiment, combs 317A and 317B are coupled to an AC voltage source 374 (FIG. 3C) and ground pad 331A (FIG. 3E) is coupled to ground 368 (FIG. 3E). When activated, AC voltage source 374 creates an oscillating voltage difference between stationary teeth 318A and moving teeth 321A, and between stationary teeth 318B and moving teeth 321B. Typically the AC drive voltage has a frequency equal to the natural frequency of structure 300 to achieve the maximum scan angle. The oscillating voltage difference between the teeth causes electrostatic torques that create the scanning motion of mirror 316 about the X-axis.
As similarly described above, in one embodiment AC voltage source 374 is built on the same chip as structure 300. Alternatively, AC voltage source 374 is built on a chip separate from structure 300. In one embodiment, AC voltage source 374 is servo-controlled during operation to generate an AC drive voltage that produces the desired scanning speed and scanning angle about the X-axis.
FIG. 3F illustrates a top view of one embodiment of insulating layer 304 Insulating layer 304 has the same shape as conductive layer 302, but for rotational frame 364, in order to electrically insulate the components on layer 302. Insulating layer 304 defines opening 358 reserved for the scanning motion of mirror 316 and rotational frame 364.
FIG. 3G illustrates a top view of one embodiment of structure anchoring layer 301. Layer 301 includes a frame 378 that defines opening 358 for the scanning motion of mirror 316 and rotational frame 364. Rotational frame 364 is mounted atop frame 378. Specifically, anchors 331A and 331B of rotational frame 364 are respectively mounted atop of anchor mounts 380 and 382 of frame 378. Combs 319A and 319B of conductive layer 302 are respectively mounted atop of comb mounts 384 and 386.
Method 150 (FIG. 1F) described above can be modified to configure and operate a MEMS structure 300 in one embodiment. Structure 300 is generally a device from a batch of mass produced structures 300.
In action 151, a designer determines the scanning frequencies and the scanning angles for both axes of rotation of an application and modifies the basic design of structure 300 to achieve specific natural frequencies equal to the scanning frequencies. The designer modifies the design by changing the stiffness of the hinges (e.g., the geometry of the hinges) or changing the inertia of the structure (e.g., the geometry of the mirror). Action 152 is followed by action 152.
In action 152, the designer presets the characteristics of the DC voltage differences for both axes of rotation to tune the natural frequencies of this structure 300 to the scanning frequencies. The designer also presets the characteristics of the AC voltage differences for both axes of rotation to achieve the desired scan angles and the desired neutral scanning positions about which the oscillation occurs. These characteristics are then stored into controller 406 for this structure 300 as the initial/default characteristics for the DC bias voltages and the AC drive voltages.
In action 153, an end user may store different characteristics for the DC bias voltages and the AC drive voltages in controller 406. The end user may wish to do so to change the desired scanning frequencies, the desired scanning angles, and the desired neutral scanning positions.
In action 154, controller 406 instructs voltage source 402 to apply the DC bias voltage and the AC drive voltage. Voltage source 402 represents the various DC and AC voltage sources (e.g., DC voltage sources 356 and 370, and AC voltage sources 360 and 374).
The DC bias voltages are initiated with the default values stored in controller 406 and then servo-controlled to ensure the rotational natural frequencies are the scanning frequencies.
The AC drive voltages are initiated with the default values stored in controller 406 and then servo-controlled to ensure the desired scanning frequencies, the desired scanning angles, and the desired scanning neutral positions are achieved. Action 154 is followed by action 158.
In action 158, sensor 404 is used to monitor the motion of the scanning mirror and the measured information is outputted to controller 406. Action 158 is followed by action 160.
In action 160, controller 406 receives the scanning frequencies and angles information from sensor 404. Controller 406 computes and provides the needed DC bias voltages and the needed AC drive voltages to voltage source 402. Action 160 is followed by action 154 and the method continues in a feedback loop.
Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.

## Claims

1. A method for controlling a MEMS structure having a mobile element, comprising:
determining an amplitude of a DC voltage difference between a first stationary electrode and a moving electrode of the mobile element, the DC voltage difference causing a natural frequency of the structure to be at least approximately equal to a scanning frequency of an application;
recording the amplitude of the DC voltage difference for use in the application; and
recording the scanning frequency as a frequency of an AC voltage difference between a second stationary electrode and the moving electrode of the mobile element for use in the application, the AC voltage difference causing an oscillation of the mobile element.

2. The method of claim 1, wherein said determining an amplitude of a DC voltage difference comprises:
applying a DC bias voltage to the first stationary electrode;
grounding the moving electrode of the mobile element; and
adjusting an amplitude of the DC bias voltage until the natural frequency of the structure is at least approximately equal to the scanning frequency.

3. The method of claim 2, wherein said recording the amplitude of the DC voltage difference comprises programming the amplitude of the DC bias voltage into a controller for operating the MEMS structure.

4. The method of claim 3, wherein said recording the scanning frequency as a frequency of an AC voltage difference comprises programming the scanning frequency into the controller.

5. The method of claim 1, further comprising:
grounding the moving electrode;
applying a DC bias voltage to the first stationary electrode to cause the DC voltage difference between the first stationary electrode and the moving electrode; and
applying an AC drive voltage to the second stationary electrode to cause the AC voltage difference between the second stationary electrode and the moving electrode.

6. The method of claim 5, further comprising:
recording the scanning frequency as a frequency of a second AC voltage difference between a third stationary electrode and the moving electrode of the mobile element for use in the application, the second AC voltage difference being out of phase with the first AC voltage difference; and
applying a second AC drive voltage to the third stationary electrode to cause the AC voltage difference between the third stationary electrode and the moving electrode.

7. A method for controlling a MEMS structure having a mobile element, comprising:
applying a DC voltage difference between a first stationary electrode and a moving electrode of the mobile element, the DC voltage difference causing a natural frequency of the structure to be at least approximately equal to a desired scanning frequency of an application; and
applying an AC voltage difference at the desired scanning frequency between a second stationary electrode and the moving electrode, the AC voltage difference causing an oscillation of the mobile element.

8. The method of claim 7, further comprising:
measuring a scanning frequency and a scanning angle of the mobile element;
comparing the measured scanning frequency and the measured scanning angle with the desired scanning frequency and a desired scanning angle; and
adjusting an amplitude of the DC voltage difference to change the natural frequency of the structure to be at least approximately equal to the desired scanning frequency of the application.

9. The method of claim 8, further comprising:
adjusting at least one of an amplitude, a frequency, and a vertical offset of the AC voltage difference to change at least one of the scanning frequency, the scanning angle, and a neutral scanning position.

10. The method of claim 7, wherein said applying a DC voltage difference comprises:
applying a DC bias voltage to the first stationary electrode; and
grounding the moving electrode.

11. The method of claim 10, wherein said applying an AC voltage difference comprises applying an AC voltage to the second stationary electrode.