The present invention relates to a micro-electro-mechanical device utilizing a comb actuator, and in particular to an array of optical micro-mirrors that are tillable under the control of an angular vertical comb drive.
Major challenges in electro-statically actuated optical micro-electro-mechanical (MEMS) devices is to achieve a relatively high angle of rotation and to lower the actuation voltage, especially in the switching axis, i.e. the “piano” or Y axis. Conventional biaxial MEMS devices, such as those disclosed in U.S. Pat. Nos. 6,934,439 issued Aug. 23, 2005 in the name of Miller et al, and 7,095,546 issued Aug. 22, 2006 in the name of Mala et al include two sets of parallel plate electro-static electrodes for both tilt (Y-axis) and roll (X-axis) movements requiring complicated electrode and supply configurations, such as the ones disclosed in U.S. Pat. Nos. 6,968,101 issued Nov. 22, 2005, and 7,010,188 issued Mar. 7, 2006 both in the name of Miller et al. providing limited tilt angle (Y axis) range and control.
Parallel plates (PP) electro-static electrodes suffer from pull-in instability, which limits useable angular range; accordingly, parallel plate electrodes for both piano tilt and roll does not provide sufficient range for next generation devices.
A vertical comb drive is a type of MEMS actuator capable of relatively high actuator power using electrostatic principals of operation, and can be fabricated using standard materials and scalable processes developed in the semiconductor industry. Vertical comb drives can be advantageously used to control high-speed, high-resolution micro-mirrors in a variety of optical applications including optical scanning, optical switching, free-space optical communications, optical phased arrays, optical filters, external cavity lasers, adaptive optics and oilier applications.
The actuation principle of a typical vertical comb drive is electrostatic, wherein a potential difference is applied between two comb structures, a movable comb (or a rotor), and a stationary comb (or a stator). When a voltage is applied between them, a torque is developed from the electrostatic filed causing the movable comb to rotate about supporting hinges toward the stationary comb until the electrostatic torque is balanced by the restoring mechanical torque of the hinge springs. Different types of vertical comb drive devices are described in further detail, for example, in U.S. Pat. No. 6,612,029 issued to Benin et al, which is incorporated herein by reference.
Conventional vertical comb drives are relatively efficient compared to parallel plate electro-static electrode actuators, and may be designed to avoid the pull-in phenomenon in the actuation direction associated with parallel plate electrodes. However, a major challenge with vertical comb drives is the sub-micron comb finger alignment accuracy that is required for the stability of the actuator.
One type of comb actuator is a staggered vertical comb (SVC) drive in which the rotor and stator combs are fabricated in different layers. A typical prior art process flow invokes creating the moving comb assembly by etching one silicon-on-insulator (SOI) wafer, and creating the stationary comb assembly by etching another SOI wafer, and then assembling the two etched wafers together to form the vertical comb drive. Different versions of such process are described in U.S. Pat. Nos. 6,925,710 and 7,079,299. However, stringent alignment requirements between the two wafers from which the two comb assemblies are formed can considerably complicate the device processing and negatively affect the device yield. A self-aligned mask process has been developed to overcome this issue disclosed in U.S. patent application Ser. No. 11/733,821 filled Apr. 11, 2007 in the name of Moffat et al, which is incorporated herein by reference, although such a self-aligned SVC process is relatively complex.
An object of the present invention is to provide a micro-mirror pivotable with an actuator array structure using angled combs in the switching axis (Y) to obtain a relatively large tilt angle and or to reduce the required voltage.
Accordingly, the present invention relates to a MEMs micro-mirror device comprising:
a substrate;
a mirrored platform pivotally mounted above the substrate for rotation about a first axis;
a first hinge enabling the mirrored platform to rotate about the first axis;
a stator comb actuator extending from the substrate; and
a rotor comb actuator, for interleaving with the stator comb actuator, extending at an acute angle from the mirrored platform for rotating the mirrored platform about the first axis.
The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:
a is an isometric view of a micro-mirror structure in accordance with the present invention;
b is an isometric view of the internal platform and the external deck of the micro-mirror structures of
c is an isometric view of the substrate of the micro-mirror structures of
a is an isometric view of the internal platform and the external deck of the micro-mirror structures of
b is an top view of the internal platform and the external deck of the micro-mirror structures of
a to 8d illustrate a first method of manufacture in accordance with the present invention;
a to 9e illustrate a second method of manufacture in accordance with the present invention;
With reference to
An internal frame structure 4 is pivotable about the Y-axis, and in the ease of an AVC, includes bi-material cantilevered beams 5a and 5b on opposite ends thereof, with rotor combs 6a and 6b, respectively, extending therefrom perpendicular to the Y axis. A single cantilevered beam and rotor comb can be used is certain circumstances. Rectangular skeletal frames 7a and 7b extend from the cantilevered beams 5a and 5b, respectively, (or from the internal frame structure 4) surrounding and enclosing the rotor combs 6a and 6b, respectively. The external platform 8, pivotable about both the X and Y axes, surrounds the internal frame structure 4, and includes the reflective surface 3, a wing section 9a, a tail section 10, and a rectangular frame arm 18 extending between the reflective surface 3 and the tail section 10 on an opposite side of the external deck 8 to the wing section 9a. The mirrored section 3 is coated with a reflective coating, e.g. gold, for reflecting beams of light, while the tale section 10 provides an offsetting weight for balance.
The roll rotation about the X axis is achieved by a conventional parallel plate electro-static actuator including an X-electrode 9b mounted on the substrate 2 for attracting the underside of the wing section 9a of the external platform 8 acting as a matching electrode. The roll rotation doesn't affect the alignment of the comb drive due to fact that the internal frame structure 4 with the rotor combs 6a and 6b is arranged internal to the X-hinge (
With reference to
The mirrored sections 3 can be interleaved with mirrored sections from adjacent mirror platforms, which extend in the opposite direction, as illustrated in
With particular reference to
Along with the rotor combs 6a and 6b, the comb drive includes two sets of longitudinally extending fixed (stator) combs 26a and 26b mounted on the stator anchors 16a and 16b having fingers interleaved with the fingers of the rotor combs 6a and 6b, respectively. A rectangular cap 27 is formed in the middle of the internal platform 4 for bonding to the top end of the anchor post 14. Third and fourth torsional hinges 28 and 29, best seen in
The moving (rotor) combs 6a and 6b are in between the first and second torsional hinges 21 and 22, whereby the fingers of the moving combs 6a and 6b are free to rotate along with the external platform 8 about the Y-axis (piano) via the third and fourth torsional hinges 28 and 29, but are independent of the external platform 8 and remain stationary when the external platform 8 rotates about the longitudinal X axis via first and second torsional hinges 21 and 22.
The Y-axis torsional hinges 28 and 29, are preferably serpentine torsion springs having a length smaller than a width of the mirror platform 3 or the tail sections 10, whereby the mirrored platforms 3 can be closely packed together with only a small air gap therebetween, and are fixed to the central post 14, which in turn is mixed to the substrate 2.
A key advantage of the angular vertical comb drive in this embodiment is that the comb fingers for both the rotor and, stator combs 6a, 6b 26a and 26b can be processed simultaneously in the same layer, providing self-alignment, and then the moving rotor combs 6a and 6b can be tilted at an acute angle to the substrate 2, providing an angular vertical comb drive, upon release during manufacture, by means of the pre-stressed BI-material, e.g. silicon-silicon dioxide, cantilevered beams 5a and 5b. Accordingly, no post-fabrication tilting process or mechanisms are required.
The rotor and stator fingers for the rotor and stator combs 6a, 6b, 26a and 26b need to be vertically offset from each other to produce the electrostatic force in the vertical direction as a voltage is applied. If the rotor and stator fingers are in same plane, there is no force in the vertical direction hence no torque for mirror tilt. According to the present invention, an angular vertical offset (pre-tilt) is easier to realize in one layer by curling the cantilevered beams 5a and 5b during release in order to pre-tilt the rotor combs 6a and 6b. With limited space available, it is practically impossible to make all of the comb fingers in one layer and then offset the rotor or stator combs 6a, 6b 26a and 26b linearly in the vertical direction.
Accordingly, with reference to
Alternatively, the pre-stressed layer 32 may comprise a compressive poly-silicon layer deposited underneath the structural layer 31 or a tensile poly-silicon layer 32 on top of the thin structural layer 31. Like thermal silicon oxide, poly-silicon is expected to be stable at operational temperatures.
Upon release, the stress in the pre-stressed layers 32 cause the cantilevered beams 5a and 5b to bend upwardly, and the rotor combs 6a and 6b to tilt at an acute angle to the substrate 2 and or the external deck 8, as shown in
Alternatively, as shown in
When a potential difference is applied between the grounded rotor combs 6a and 6b the internal frame structure 4 and the live stator combs 26a and 26b fixed to the substrate 2, a torque is produced by the electrostatic force in the vertical direction between the rotor and stator comb pairs 6a and 26a, 6b and 26b, and results in the tilting of the entire mirror device 1, i.e. the internal frame structure 4 and the external platform X, about the third and fourth (Y-axis) hinges 28 and 29.
The external platform 8 rotates about the X-axis when a potential difference is applied between the wing section 9a, and the X-electrode 9b patterned on the substrate 2, due to the electrostatic force of attraction between the wing section 9a and the X-electrode 9b. When the external platform 8 tilts in the roll or X-direction, the rotor combs 6a and 6b are stationary since they are arranged internal to the first and second X-axis hinges 21 and 22.
The present invention may be realized by starting with a double silicon on insulator (DSOI) structure or a single SOI structure, as shown in
The design may also be realized simply by a single SOI, as shown in
A staggered vertical comb drive embodiment illustrated for comparison purposes in
An internal rectangular or square skeletal frame structure 104 is pivotable about the Y-axis, with rotor combs 106a and 106b, respectively, extending therefrom parallel to the X axis. The rectangular frame 104 includes rectangular skeletal arms 107a and 107b extending from opposite ends sides thereof, surrounding and enclosing the rotor combs 106a and 106b, respectively. An external deck 108, pivotable about both the X and Y axes, substantially surrounds the internal frame structure 104 (at least three sides thereof), and includes the reflective surfaces 103a and 103b, and a wing section 109a. The mirrored sections 103a and 103b are coated with a reflective coating, e.g. gold, for reflecting beams of light.
The roll rotation about the X axis is achieved by a parallel plate electro-static actuator including an X-electrode 109b mounted on the substrate 102 for attracting the underside of the wing section 109a of the external deck 108 acting as a matching electrode. The roll rotation doesn't affect the alignment of the comb drive due to fact that the internal frame structure 104 with the rotor combs 106a and 106b is arranged internal to the X-hinge (
With reference to
With particular reference to
Along with the rotor combs 106a and 106b, the comb drive includes at least one set of longitudinally extending fixed (stator) combs 126 mounted on the substrate 102 having fingers interleaved with the fingers of the rotor combs 106a and/or 106b. The fixed combs 126 extend parallel to the rotor combs 106a and 106b, respectively, but in a parallel plane, i.e. the rotor combs 106a and 106b extend in a first plane including the outer deck 108, which is superposed above a second plane including the fixed combs 126 extending therein. A rectangular cap 127 is formed in the middle of the internal frame structure 104 for bonding to the top end of the anchor post 114. Third and fourth torsional hinges 128 and 129, best seen in
When a potential difference is applied between the grounded rotor combs 106b on the mirror 101, and live stator combs 126 fixed to the substrate 102, a torque is produced by the electrostatic force in the vertical direction between the rotor and stator comb pairs causing the rotor combs 106b to rotate at an acute angle to the stator comb 126, and results in the tilling of the entire mirror structure 101 about the Y-axis torsional hinges 128 and 129. The Y-axis torsional hinges 128 and 129, are preferably positioned between the first and second reflective surfaces 103a and 103b, and comprise serpentine torsion springs having a length smaller than a width of the mirror platform 103a, whereby reflective surfaces 103a and 103b or adjacent mirrors can be closely packed together with only a small air gap therebetween. The inner ends of the torsional hinges 128 and 129 are fixed to the cap 127, which is mounted on the central post 114, which in turn extends from the substrate 102.
The moving (rotor) comb fingers 106a and 106b are in between the first and second torsional hinges 121 and 122, whereby the moving comb fingers 106a and 106b are free to rotate along with the outer deck 108 about the Y-axis (piano) via the third and fourth torsional hinges 128 and 129, but are independent of the outer deck 108 and remain stationary when the outer deck 108 rotates about the longitudinal X axis via first and second torsional hinges 121 and 122.
The design of the embodiment of
The present invention claims priority from U.S. patent application No. 60/945,364 filed Jun. 21, 2007, which is incorporated herein by reference.
Number | Date | Country | |
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60945364 | Jun 2007 | US |