WO2009044324A2 - Mems scanning micromirror manufacturing method - Google Patents

Abstract

A MEMS scanning micromirror manufacturing method with a method for manufacturing a MEMS scanning micromirror having a cantilever beam including providing a silicon on insulator (SOI) wafer 200 having a first silicon layer 202, a second silicon layer 206, and an insulating layer 204 between the first silicon layer 202 and the second silicon layer 206, the thickness of the first silicon layer 202 being a final thickness of the cantilever beam 72; and fashioning the cantilever beam 72 from the first silicon layer 202 while maintaining the final thickness of the cantilever beam.

Description

MEMS scanning micromirror manufacturing method

FIELD OF THE INVENTION

This application claims priority from U.S. provisional application no. 60/977,717, filed October 5, 2007. U.S. provisional no. 60/977,721, filed October 5, 2007 (Applicants' docket no. PH009138) and U.S. provisional no. 60/977,713, filed October 5, 2007 (Applicants' docket no. PH009046) are related applications.

The technical field of this disclosure is Micro Electro Mechanical Systems (MEMS), particularly, manufacturing methods for MEMS scanning micromirrors.

SUMMARY OF THE INVENTION MEMS scanning micromirrors have been developed for the display of visual information. The MEMS scanning micromirror oscillates in one or two dimensions and a laser or other light beam reflects from the mirror surface. Varying the angle and timing of the beam incident on the mirror surface generates a visual image on a screen or other surface, such as a two dimensional display matrix. Different numbers of MEMS scanning micromirrors and lasers are used to produce images of different detail and colors. Exemplary uses for the MEMS scanning micromirrors are head up displays for automotive applications, wearable displays, projection displays, mobile phone and hand-held displays, and barcode scanners.

The present generation of MEMS scanning micromirrors includes a mirror plate attached to a frame by two collinear torsion beams, which create a scanning axis about which the mirror plate rotates. The torsion beams both support the mirror plate and provide the required torsional stiffness during rotation. The torsion beams are the only point of attachment between the mirror plate and the frame, and determine the resonant frequency of the MEMS scanning micromirror. The MEMS scanning micromirror also includes a driver to magnetically or electrically apply a torque to the mirror plate about the scanning axis without physical contact with the mirror plate. The driver typically drives the mirror plate at the resonant frequency. MEMS scanning micromirrors are made from single crystal silicon or polysilicon material using photolithography. Problems arise in manufacturing because the MEMS scanning micromirrors are fashioned from only the top silicon layer of a silicon on insulator (SOI) wafer. The MEMS micromirrors for imaging applications usually have significant thickness (80 -120 μm for 1-1.5mm micromirror diameters) to reduce the micromirror dynamic deformation. The components of the MEMS scanning micromirrors are formed by an etching process, such as deep reactive ion etching (DRIE) from the top side of the SOI wafer. Because the MEMS scanning micromirror is formed from a single side of the SOI wafer, the etching must proceed through the whole thickness of the top silicon layer of the SOI wafer to form the relatively inaccurate device outline and the components with critical dimension in the same time. This poses requirement for higher accuracy on the whole etch process and results in long etching process times, increasing manufacturing costs. Etching over the whole device thickness from one side also results in poor surface quality in the vertical walls and an increased risk of crack propagation when the vertical walls are exposed to high stress levels during operation, as in torsion beam suspended micromirrors. Manufacturing micromirror devices over the whole thickness of the top layer of a SOI wafer also requires different sets of masks to achieve different resonance frequencies for different MEMS scanning micromirrors, since the dynamic characteristics are controlled predominantly by the width of their components.

Another problem with fashioning the MEMS scanning micromirrors from only the top silicon layer of a SOI wafer is the need to provide a starting electrode for actuators using aligned combs. The actuator provides torque to drive oscillations of the MEMS scanning micromirror. FIGS. 1A-1C are side views of aligned comb actuators, angular comb actuators, and staggered comb actuators, respectively, for MEMS scanning micromirrors. Comb actuators use a number of interleaved frame combs and mirror combs. The starting torque for the MEMS scanning micromirrors is achieved by applying an electrical potential difference between the frame combs 26 and the mirror combs 28, which are misaligned in the angular comb actuators 22 and staggered comb actuators 24. The aligned comb actuators 20 do not have the vertical misalignment between the frame combs 26 and the mirror combs 28, so a starting electrode 29 must be applied to the frame combs 26 to establish the potential difference and a driving force between the frame combs 26 and the mirror combs 28. Because of the pulling function of the comb drives, for aligned combdrive electrodes the driving torque could be applied only during half of the oscillation cycle. MEMS scanning micromirrors with aligned comb drives are typically made by anisotropic backside wet etch followed by DRIE from the topside of the SOI wafers. In general, making the aligned electrodes is easier than making angular or staggered combdrive electrodes but the need of starting electrode adds to manufacturing time and cost.

It would be desirable to have a MEMS scanning micromirror manufacturing method that would overcome the above disadvantages. One aspect of the present invention provides a method for manufacturing a

MEMS scanning micromirror having a cantilever beam including providing a silicon on insulator (SOI) wafer having a first silicon layer, a second silicon layer, and an insulating layer between the first silicon layer and the second silicon layer, the thickness of the first silicon layer being a final thickness of the cantilever beam; and fashioning the cantilever beam from the first silicon layer while maintaining the final thickness of the cantilever beam. Another aspect of the present invention provides a method for manufacturing a MEMS scanning micromirror including providing a silicon on insulator (SOI) wafer having a first silicon layer, a second silicon layer, and an insulating layer between the first silicon layer and the second silicon layer; and etching an aligned comb actuator in the SOI wafer, the aligned comb actuator having interleaved mirror combs and frame combs. First electrical portions of the mirror combs and first electrical portions of the frame combs are in the first silicon layer and second electrical portions of the mirror combs and second electrical portions of the frame combs are in the second silicon layer.

Another aspect of the present invention provides a method for manufacturing a MEMS scanning micromirror including providing a silicon on insulator (SOI) wafer having a first silicon layer, a second silicon layer, and an insulating layer between the first silicon layer and the second silicon layer; etching a mirror body in the SOI wafer, the mirror body having a rotation axis with a first extension bar and a second extension bar parallel to the rotation axis; etching a frame in the SOI wafer, the frame having a mirror recess with a recess periphery, the frame having a first opposed frame bar and a second opposed frame bar on the recess periphery along the rotation axis; etching a first cantilever beam in the SOI wafer, the first cantilever beam being fixed to the first opposed frame bar perpendicular to the rotation axis and coupled to a first end of the first extension bar; etching a second cantilever beam in the SOI wafer, the second cantilever beam being fixed to the first opposed frame bar perpendicular to the rotation axis and coupled to a first end of the second extension bar; etching a third cantilever beam in the SOI wafer, the third cantilever beam being fixed to the second opposed frame bar perpendicular to the rotation axis and coupled to a second end of the first extension bar; etching a fourth cantilever beam in the SOI wafer, the fourth cantilever beam being fixed to the second opposed frame bar perpendicular to the rotation axis and coupled to a second end of the second extension bar; etching a first vertical support beam in the SOI wafer, the first vertical support beam being connected between the first frame opposed bar and the mirror body along the rotation axis; and etching a second vertical support beam in the SOI wafer, the second vertical support beam being connected between the second opposed frame bar and the mirror body along the rotation axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention, rather than limiting the scope of the invention being defined by the appended claims and equivalents thereof.

FIGS. IA- 1C are side views of aligned comb actuators, angular comb actuators, and staggered comb actuators, respectively, for MEMS scanning micromirrors; FIGS. 2A-2B are a top and cross section view, respectively, of a MEMS scanning micromirror made in accordance with the present invention;

FIGS. 3A-E are detailed views of comb fingers for a MEMS scanning micromirror made in accordance with the present invention;

FIGS. 4A-4B are top views of other embodiments of a MEMS scanning micromirror made in accordance with the present invention;

FIGS. 5A-5C are detailed cross section views of leaf springs for a MEMS scanning micromirror made in accordance with the present invention;

FIGS. 6A-6B are cross section views of a MEMS scanning micromirror made in accordance with the present invention; FIGS. 7A-7B are cross section views of the etch levels for a MEMS scanning micromirror made in accordance with the present invention;

FIG. 8 is a cross section view of mask patterns for a MEMS scanning micromirror made in accordance with the present invention;

FIGS. 9A-9C are cross section views of the bottom side etching sequence for a MEMS scanning micromirror made in accordance with the present invention;

FIGS. 10A- 1OD are cross section views of the top side etching sequence for a MEMS scanning micromirror made in accordance with the present invention; and

FIGS. 1 IA-I IB are cross section views of the finishing sequence for a MEMS scanning micromirror made in accordance with the present invention. FIGS. 12A-12E are detailed top views of flexible links of a mirror body for a MEMS scanning micromirror in accordance with the present invention; and

FIG. 13 is a detailed perspective view illustrating dimensions of a mirror body for a MEMS scanning micromirror in accordance with the present invention. FIG. 14 is a plan view of a picobeamer micromirror in an embodiment of the present invention, showing electrical connections for device actuation.

FIG. 15 is a detailed cross section view of wafer layers and points of application of driving potentials in an embodiment of the present invention.

FIG. 16 is a perspective view of flexible links of a mirror body of a picobeamer micromirror in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 2A -2B, in which like elements share like reference numbers, are a top and side view, respectively, of a MEMS scanning micromirror in accordance with the present invention. FIG. 2B is a cross section along section A-A of FIG. 2A with the mirror body tilted about the rotation axis. The MEMS scanning micromirror uses a pair of cantilever beam assemblies coupled to a mirror body at its outer corners to set the torsional stiffness about the rotation axis. A pair of vertical support beams supports the mirror body vertically at the rotation axis, but have a negligible effect on the torsional stiffness, so that the natural frequency of the mirror body is substantially determined by the cantilever beam assemblies. The natural frequency is substantially independent of the vertical support beams. The natural frequency as defined herein is the undamped frequency of the mirror body about its rotation axis. The vertical support beams define the out-of-plane rocking and vertical mode stiffness for the corresponding mode resonant frequencies. The torsional stiffness can be decoupled from the out-of-plane rocking and vertical mode stiffness so that the out-of-plane rocking and vertical mode frequencies can be set to desired values, such as higher values, without influencing the torsional mode stiffness and resonant frequency. As defined herein, the Y axis is along the rotation axis, the X axis is perpendicular the Y axis on the mirror plane when the mirror is at rest, and the Z axis is perpendicular to and out of the mirror plane when the mirror is at rest.

The MEMS scanning micromirror 30 includes a mirror body 50, a frame 60, cantilever beam assemblies 70, and vertical support beams 40. The mirror body 50 has a mirror 52 on a mirror support 54, and extension bars 56. In one embodiment, the mirror 52 is formed on the mirror support 54. In another embodiment, the mirror 52 is attached to the mirror support 54. The mirror body 50 can be square, rectangular, circular, elliptical, or any other planar shape desired for a particular application. The face of the mirror defines a mirror plane of the mirror support 54. Those skilled in that art will appreciate that the shape of the mirror 52 and the mirror support 54 are independent and can be any shape desired for a particular application, e.g., a circle, ellipse, square, rectangle, or other shape as desired. The extension bars 56 are parallel to rotation axis 58 of the mirror body 50, which is the rotation axis for the MEMS scanning micro mirror 30. The mirror body 50 is disposed within a mirror recess 62 of the frame 60.

The frame 60 forms the mirror recess 62 with a recess periphery 64. Opposed frame bars 66 are located on the recess periphery 64 along the rotation axis 58 and provide the connection points for the cantilever beam assemblies 70 and the vertical support beams 40.

The cantilever beam assemblies 70 include cantilever beams 72 fixed to the opposed frame bars 66 perpendicular to the rotation axis 58. The cantilever beam assemblies 70 provide torsional stiffness to the micro mirror body 50 about the rotation axis 58. The cantilever beams 72 are also flexibly or compliantly coupled to the end of the extension bars 56 of the mirror body 50 with flexible links 74. The flexible links 74 have low torsional stiffness around their axes parallel to the rotation axis 58 (around the Y axis) and reduced stiffness perpendicular to the rotation axis 58 (the X axis), which allows the mirror body 50 to rotate around the vertical support beams 40 relative to the rotation axis 58. The attachment of the mirror body 50 to the four points away from the rotation axis 58 reduces dynamic deformation in the mirror body 50. The torsional stiffness for rotation of the mirror around the Y axis is defined by the length, width, and most importantly the thickness of the cantilever beams 72 and the distance between flexible links 74 for the pair of cantilever beams 72 in a cantilever beam assembly 70. The combined stiffness in X direction of the vertical support beams 40 and the flexible links 74 prevent the movement of the mirror body 50 perpendicular to the rotation axis 58 (in the X direction) during operation. More detail on the flexible links 74 is provided below for FIG. 12.

The vertical support beams 40 are connected between the opposed frame bars 66 and the mirror body 50 along the rotation axis 58 to support the micromirror body 50 in the frame 60. In one embodiment, the vertical support beams 40 have narrow rectangular cross sections perpendicular to the rotation axis 58, with the long axis of the rectangle perpendicular to the face of the mirror 52 and the mirror body 50, and the short axis of the rectangle parallel to the face of the mirror 52. The torsional stiffness of the MEMS scanning micromirror 30 is provided by the cantilever beam assemblies 70, so the vertical support beams 40 are only required for support of the mirror body 50 and have a negligible effect on the torsional stiffness. The torsional stiffness of the vertical support beams 40 is as low as possible so that the torsional stiffness of the micromirror body rocking movement about the vertical support beams 40 relative to the rotation axis 58 is dominated by the stiffness of the cantilever beams 72. The vertical support beams 40 are sized so that the stiffness against vertical displacement of the mirror body 50 and against its rocking movement perpendicular to the rotation axis 58 (around the X axis) is as high as possible.

The MEMS scanning micromirror 30 can also include actuator 80 to provide torque to drive the mirror body 50 about the rotation axis 58. In one embodiment, the actuator 80 includes mirror combs 82 attached to the extension bars 56 interleaved with frame combs 84 attached to the frame 60. Applying a difference in electrical potential between an interleaved mirror comb 82 and frame comb 84 creates a driving force between the mirror combs 82 and the frame combs 84, which creates a torque on the mirror body 50 about the rotation axis 58. An oscillating electrical potential can be applied to drive the MEMS scanning micromirror 30 at its natural frequency. Other exemplary actuation methods include electromagnetic actuation and piezoelectric actuators. In electromagnetic actuation, the micromirror is "immersed" in a magnetic field and an alternating electric current through the conductive paths creates the required oscillating torque around the rotation axis 58. Piezoelectric actuators can be integrated in the cantilever beams or the cantilever beams can be made of piezoelectric material to produce alternating beam bending forces in response to an electrical signal and generate the required oscillation torque.

The MEMS scanning micromirror 30 can be manufactured from single crystal silicon or polysilicon material using photolithography and DRIE techniques. FIG. 3 A, in which like elements share like reference numbers with FIG. 2, is a detailed perspective view of comb fingers for a MEMS scanning micromirror in accordance with the present invention. The comb fingers 100 of the mirror comb 82 are interleaved with the comb fingers 110 of the frame comb 84. In one embodiment, the MEMS scanning micromirror can be manufactured from a silicon-on- insulator (SOI) wafer having an upper silicon layer and a lower silicon layer, with an insulating layer between the upper silicon layer and the lower silicon layer. In one embodiment, the mirror comb 82 and the frame comb 84 can be fabricated so that the insulating layer divides the combs parallel to the mirror, producing electrically isolated upper electrical portions and lower electrical portions in each of the comb fingers. The comb fingers 100 of the mirror comb 82 include first electrical portions 102 and second electrical portions 104 separated by insulating layer 106. The comb fingers 110 of the frame comb 84 include first electrical portions 112 and second electrical portions 114 separated by insulating layer 116. Applying a difference in electrical potential between the upper electrical portions in the mirror comb 82 and the lower electrical portions in the frame comb 84, or vice versa, can be used to generate an initial driving force between the mirror combs 82 and the frame combs 84 when the mirror combs 82 and the frame combs 84 are aligned and the mirror body 50 is at rest. The separation of the comb fingers in two (top and bottom) parts allows, by switching between opposed layers, to apply the driving potential (and eventually torque, depending on the comb fingers geometry) for duration greater then half oscillation period per oscillation cycle. In one embodiment, the cantilever beam assemblies can be fabricated in the upper silicon layer of the silicon on insulator wafer and can bring the electrical potential to the top first electrical portions 102 of the mirror combs 82. In another embodiment, the vertical support beams can be fabricated in the lower silicon layer of the silicon on insulator wafer and can bring the electrical potential to the bottom second electrical portions 104 of the mirror combs 82.

FIGS. 3B-3E illustrate the action of the mirror combs 82. Referring to FIG. 3B, the mirror is oscillating, with the mirror comb fingers away from the frame fingers and moving towards them. 102 and 104 connected to ground potential. Driving potential is applied to 112 and 114 from max. amplitude to aligned position of the comb fingers. Referring to FIG. 3C, the mirror is oscillating in the clockwise direction, when comb fingers are in aligned position. The driving cycle for the traditional monolith comb fingers stops here. T he driving potential must be switched off at aligned comb fingers position if there is no split of the comb fingers in top and bottom electric parts. For split comb fingers of the present invention: Potential is applied between 102 (grounded) and 114 until 102 became aligned with 114, so the split comb fingers inject more energy per oscillation cycle for equal other conditions.

Referring to FIG. 3D, the mirror is oscillating. When the opposing layers 102 and 114 of split comb fingers are in aligned position (no torque created); the driving potential between them must be switched off. Referring to FIG. 3D, the mirror is oscillating at extreme (FIG. 3B) position.

For rotation in opposite direction the driving potentials are switched in "mirrored order" to the given in FIGS. 4B-4D:

102, 104 grounded and 112,114 at driving potential; Driving potential between 104 (grounded) and 112; Driving potentials switched off.

FIGS. 4A-4B, in which like elements share like reference numbers with each other and with FIG. 2, are top views of other embodiments of a MEMS scanning micromirror in accordance with the present invention. In these embodiments, leaf springs flexibly coupled between the cantilever beams of the cantilever beam assemblies and the mirror body can be used to stiffen the mirror body of the MEMS scanning micromirror against in-plane disturbances and increase in-plane slide and rotation stiffness of the mirror suspension. The leaf springs springily couple the micromirror body to the cantilever beam assemblies.

The leaf springs can be positioned along the cantilever beams as desired for a particular application. Referring to FIG. 4A, leaf springs 90 are flexibly coupled between the cantilever beams 72 of the cantilever beam assemblies 70 and the mirror body 50. In this example, the leaf springs 90 are near the vertical support beams 40 and the opposed frame bars 66. Referring to FIG. 4B, the leaf springs 90 are flexibly coupled between the cantilever beams 72 of the cantilever beam assemblies 70 and the mirror body 50. In this example, the leaf springs 90 are near the flexible links 74.

FIGS. 5A-5C, in which like elements share like reference numbers with each other and with FIG. 4, are detailed cross section views along section B-B of FIG. 4A of leaf springs for a MEMS scanning micromirror in accordance with the present invention. The leaf springs 90 have different shapes to provide different stiffness. Referring to FIGS. 5A, 5B, and 5C, the leaf springs 90 are L shaped, V shaped, and flat, respectively. The leaf springs 90 can be placed at about the same height relative to the mirror (in the Z direction) as the vertical support beams 40. In one embodiment, the lower leg of the L in the L shaped leaf spring is at the same height as the rotation axis 58. In one embodiment, the lower tip of the V in the V shaped leaf spring is at the same height as the rotation axis 58. In one embodiment, the flat leaf spring is at the same height as the rotation axis 58.

FIGS. 6A-6B, in which like elements share like reference numbers with each other and with FIGS. 2A-2B, are cross section views of a MEMS scanning micromirror made in accordance with the present invention. FIGS. 6A-6B are views along sections A-A and B- B, respectively, of FIG. 2A. The vertical position of the components of the MEMS scanning micromirror is arranged so that some of the components are in the first silicon layer, some are in the second silicon layer, and some are in both the first silicon layer and the second silicon layer with the insulating layer dividing those elements horizontally.

Referring to FIGS. 6A-6B, the silicon on insulator (SOI) wafer 200 from which the MEMS scanning micromirror is fashioned is illustrated by the dashed lines. The SOI wafer 200 includes a first silicon layer 202, a second silicon layer 206, and an insulating layer 204 between the first silicon layer 202 and the second silicon layer 206. The first silicon layer 202 and the second silicon layer 206 are electrically insulated from each other by the insulating layer 204, such as a buried oxide (BOX) layer, so components formed in the first silicon layer 202 are electrically insulated from components formed in the second silicon layer 206. The actuator 80 is an aligned comb actuator.

In this example, the cantilever beams 72 are formed in the first silicon layer 202; the vertical support beams 40, and opposed frame bars 66 are formed in the second silicon layer 206; and the extension bars 56, the frame 60 and actuator 80 are formed with portions in the first silicon layer 202 and in the second silicon layer 206. The mirror combs 82 and the frame combs 84 each have first and second electrical portions, so that applying a difference in electrical potential between the upper electrical portions in the mirror comb 82 and the lower electrical portions in the frame comb 84, or vice versa, can be used to generate an initial driving force between the mirror combs 82 and the frame combs 84 when the mirror combs 82 and the frame combs 84 are aligned and the mirror body 50 is at rest. Bottom cavities 208 are etched into the second silicon layer 206 to reduce the mass and mass inertia moment of the mirror body 50 for higher frequency applications.

The isolation of the components between the first silicon layer 202 and the second silicon layer 206 can be used to bring electrical potential to different components in the different layers. For example, the cantilever beams 72 formed in the first silicon layer 202 can be electrically connected to bring electrical potential to the upper electrical portions of the mirror comb 82. In another example, the vertical support beams 40 formed in the second silicon layer 206 can be electrically connected to bring electrical potential to the lower electrical portions of the mirror comb 82. Those skilled in the art will appreciate that the positioning of components in the first silicon layer 202 and/or second silicon layer 206 can be selected as desired for a particular application. For example, the vertical support beams 40 can be located in the first silicon layer 202 or the second silicon layer 206.

FIGS. 7A-7B, in which like elements share like reference numbers with each other and with FIGS. 2A-2B and 6A-6B, are cross section views of the etch levels for a MEMS scanning micromirror made in accordance with the present invention. The components of the MEMS scanning micromirrors are formed by an etching process, such as deep reactive ion etching (DRIE). The silicon on insulator (SOI) wafer 200 from which the MEMS scanning micromirror is fashioned is illustrated by the dashed lines. The levels can be realized with two top DRIE etch masks and three bottom DRIE etch masks.

The thickness of the first silicon layer 202 of the SOI wafer 200 at the cantilever beam 72 does not change during etching, so the thickness of the first silicon layer 202 of the SOI wafer 200 determines the final thickness of the cantilever beam 72. The first silicon layer 202 of the SOI wafer 200 can be formed to the final thickness of the cantilever beam 72 before fashioning the MEMS scanning micromirror from the SOI wafer 200. The first silicon layer 202 of the SOI wafer 200 can be formed by machining, etching, growing and polishing, or the like to achieve an accurate and uniform final thickness. Forming of the first silicon layer 202 accounts for uncertainties in the first silicon layer 202 of the SOI wafer 200 to provide a cantilever beam 72 having the desired uniform thickness, providing the desired performance characteristics in operation. Thickness, accuracy, and uniformity in the range of 0.3-0.5μm are desirable and achievable by grinding and polishing after bonding of the SOI wafer. The etch levels from the bottom side of the SOI wafer 200 are as follows:

BO - bottom of the SOI wafer 200, starting level of the bottom etch;

Bl - bottom of the MEMS scanning micromirror structure, including the frame 60 and opposed frame bars 66;

B2 -level defining the height of the comb drive fingers for the mirror combs 82 and the frame combs 84, and the bottom level of the vertical support beams 40; and

B3 - level of the bottom cavities 208 under the mirror surface, bottom side of the cantilever beams 72 and the insulating trenches for the bottom frame combs 114, with the insulating layer 204 being the etch stop for this level.

The etch levels from the top side of the SOI wafer 200 are as follows:

Tl - insulating layer 204 as the etch stop for this level, used to define the profile of the top side of the MEMS scanning micromirror structure, including cantilevers 72, flexible links 74 and insulation trenches 210;

T2 - level of the bottom of the insulating layer 204 reached from the top side of the SOI wafer 200, oxide etched to remove the insulating layer 204 for forming the comb drive finger parts 104 and 114 under the BOX layer of the mirror combs 82 and the frame combs 84, and the vertical support beams 40; T3 - level below the insulating layer 204 of the bottom of the comb drive fingers for the mirror combs 82 and the frame combs 84, and the vertical support beams 40; and

TO - level of the top surface of the SOI wafer 200, which is maintained at the initial level of the SOI wafer 200 and to which a reflective Al layer is deposited to form the reflective surface of the MEMS scanning micromirror.

FIGS. 8-11, in which like elements share like reference numbers with each other and with FIG. 7, illustrate the steps in fashioning the MEMS scanning micromirror. The left portion of FIGS. 8-11 illustrates fashioning an aligned comb actuator, the center portion illustrates fashioning a mirror support, and the right portion illustrates fashioning a vertical support beam. The figures are not illustrated to scale or with the components located in the actual position relative to each other, but illustrate formation of the relative vertical levels of the components of the MEMS scanning micromirror. FIG. 8 is a cross section view of mask patterns for a MEMS scanning micromirror made in accordance with the present invention. The masks are applied to the top and bottom surfaces of the SOI wafer 200. The masks include an aluminum mask at the comb drive fingers 220, low temperature oxide (LTO) silicon dioxide masks 222, aluminum layer masks 224, and resist masks 226. Those skilled in the art will appreciate that the particular materials for the masks can be selected for the particular etching process applied. In one embodiment, the etching process is deep reactive ion etching (DRIE).

FIGS. 9A-9C are cross section views of the bottom side etching sequence for a MEMS scanning micromirror made in accordance with the present invention.

Referring to FIG. 9 A, the first etch step is performed with the deposited resist mask 226 at the bottom, which defines the profile at level B3 as shown in FIG. 7. The depth of this etch step is time controlled to remove material equal to the difference between the depth of the levels B3 and B2 as shown in FIG. 7.

Referring to FIG. 9B, the resist mask 226 is stripped-off after the first etch step to expose the pattern for the second etch step with the aluminum layer mask 224. The depth of the second etch step is equal to the difference between the height of the levels Bl and B2 as shown in FIG. 7.

Referring to FIG. 9B, the aluminum layer mask 224 is removed before the third etch step and the LTO silicon dioxide mask 222 defines the profile of the level Bl as shown in FIG. 7. After the bottom etching, the first etch depth reaches the insulating layer 204, which acts as an etch stop at level B3 as shown in FIG. 7. The second etch depth reaches the bottom level of the comb drive fingers and the bottom level of the vertical support beam at level B2 as shown in FIG. 7. The third etch depth is stopped at the bottom surface of the MEMS scanning micromirror at level Bl as shown in FIG. 7. The insulating layer 204 determines the height of level B3, but levels Bl and B2 are controlled by the etching time.

FIGS. 10A- 1OD are cross section views of the top side etching sequence for a MEMS scanning micromirror made in accordance with the present invention. The top side of the SOI wafer 200 is completely protected with resist mask 226 during the bottom etching, which is removed to begin the top etching.

Referring to FIG. 1OA, a new resist mask 226 is patterned on top of the wafer over the mirror 52. The first etch step with aluminum layer masks 220, 224 continues until the etch depth reaches the insulating layer 204 at level Tl as shown in FIG. 7. Referring to FIG. 1OB, the first etch step forms the comb fingers 230 above the insulating layer 204. Referring to FIG. 1OC, the second etch step is an oxide reactive ion etching

(RIE) used to remove the exposed insulating layer 204 at the comb fingers 230 and other areas to reach level T2 as shown in FIG. 7. The oxide RIE also removes the LTO silicon dioxide masks 222 at the top of the SOI wafer 200 to reach level TO as shown in FIG. 7. Referring to FIG. 10D, the third etch step is a DRIE with aluminum layer masks 220, 224 for the comb fingers 203 and all other exposed areas that reaches levels Tl or T3 as shown in FIG. 7 depending on the level on initiating the third etch step.

FIGS. 1 IA-I IB are cross section views of the finishing sequence for a MEMS scanning micromirror made in accordance with the present invention. Referring to FIG. 1 IA, the aluminum layer masks 220, 224 have been removed, except under the resist mask 226 where the reflective aluminum layer of the mirror 52 remains. Referring to FIG. 1 IB, all masks have been removed and the structure of the MEMS scanning micromirror has been released. Those skilled in the art will appreciate that dual thickness resist, dual thickness oxide, or other non-metal mask materials can be used in place of the aluminum layer masks 220, 224 combined with oxide and resist masks, when aluminum masks are not desirable. FIGS. 12A-12E are detailed top views of flexible links of a mirror body for a

MEMS scanning micromirror in accordance with the present invention. In FIG. 12 A, the flexible link 74 is a high aspect ratio flexure connected to the cantilever beam at both ends and in the middle to the extension bars. Typical dimensions for 1 mm micromirror device are: Width 2,5-4μm, length 60-80μm, the height is the same as the thickness of the cantilever beams. The width at the connection points is around lOμm. In FIG. 12B, the flexible link 74 includes additional flexure elements allowing small X-axis translations. In FIG. 12C-12D, the flexible link 74 allows a greater deformation in X direction while keeping a high bending stiffness in vertical direction and a high stiffness against in-plane rotation of the micromirror. The flexible links length is increased to reduce the stress caused by translation in X direction. In FIG. 12C, the flexible link is made in the same layer as the cantilever beams, so the stiffness of the flexure is limited by the thickness of the cantilevers. In FIG. 12D, the flexible link 74 has increased stiffness in the vertical direction and against bending in the Y-Z plane achieved by fabricating them with increased height in the bulk single crystal silicon material under the insulating layer. In this case, additional structure as an electrical connection is needed to provide the potential to the top part of the moveable comb fingers. In FIG. 12E, the flexible link 74 the L shaped flexible links (rotated at 45 degrees) connecting the extension bar and the cantilever beam.

FIG. 13 is a detailed perspective view illustrating dimensions of a mirror body for a MEMS scanning micromirror in accordance with the present invention. In one exemplary embodiment, the dimensions of the cantilever beams are:

Cantilever beam length l_b=420μm

Cantilever beam width W_b=100μm

Cantilever beam thickness t_b=17.5μm Distance between opposite suspension points a=950μm.

In one exemplary embodiment, the dimensions of the vertical support beams are:

Vertical support beam width w_vs=6.2μm

Vertical support beam height h_vs=36μm Vertical support beam length l_vs=62μm

In another exemplary embodiment, the dimensions of the vertical support beams providing a combined stiffness of the links in X direction is 1.25x 10 N/m are:

Vertical support beam width w_vs=6.2μm

Vertical support beam height h_vs=36μm Vertical support beam length l_vs=62μm

The combined stiffness of the vertical beams in X direction is l.Ox 10⁴N/m. The beam stiffness dominates the horizontal slide natural frequency. The links and cantilever beam stiffness in Y direction define the in-plane rotation mode resonance frequency. The oscillation frequency of the micro mirror scanners depends from the torsional stiffness of the suspension and its mass inertia moment around the tilt axis. The torsional stiffness contributed by the cantilever beams bending stiffness dominates the fundamental mode resonant frequency of the micromirror. For small oscillation angles, this stiffness can be found from the following formula:

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For a 1 mm micromirror, the torsional stiffness contributed by the cantilever beams is 2.3xlO^~4Nm/rad.

The torsional stiffness from the vertical support beams with the above given dimensions is 4.6x10^~6Nm/rad, which is 50 times less than the delivered by the cantilever beams torsional stiffness. The vertical support beams influence with less than 1% the 18.7 kHz fundamental mode resonant frequency of our 1 mm micromirror design. The mass inertia moment is about 1.7xlO^"14kgm².

The flexible links 74 with combined torsional stiffness around 1x10 ⁶ Nm/rad contribute even less to the increase of the micromirror fundamental mode frequency.

In the same design, the cantilever beams contribute with around l.lxlO³N/m bending stiffness to the out-of-plane oscillation mode, while the stiffness of the vertical support beams is 6.1xlO⁵N/m.

The cantilever beams dominate the torsional stiffness of the micromirror. The vertical support beams dominate the stiffness for the out-of-plane oscillation modes, which have great impact on the image quality. The possibility to define the fundamental mode and the higher order resonance frequencies with a greater flexibility makes it easier to design better quality scanning systems. FEM Simulations showed that the combined suspension has advantages in preventing parasitic mode oscillations, as vertical and out-of-plain rocking, by increasing their resonance frequencies to greater values compared to torsion-beam suspended micromirrors.

The table below shows the simulation results for optimized geometries of two torsion-beam suspended micromirrors and a micromirror with combined suspension (having the same micromirror size; similar tilt stiffness, mass inertia moments, stress in the suspension elements and surface deformation):

FIG. 14 is a plan view of a picobeamer micromirror in an embodiment of the present invention. Electrical connections for device actuation are at a wafer handle layer and bottom part of a micromirror 30, a top half of moveable comb fingers 117, a top-half of stator comb fingers 118 and a bottom half of stator comb fingers 119. An area of the micromirror 30 is left as "free real estate" 121. Potentials VO at the wafer handle layer and bottom part of the micromirror 30, Vl at the top half of the moveable comb fingers 117, V2 at the top-half of stator comb fingers 118 and V3 at the bottom half of stator combfmgers 119 may be applied during operation of a display. Driving potentials used may be, for example: during oscillation launch VO grounded, V2 driven with square pulses. During steady state oscillation V0=Vl=grounded and V2=V3. The device layer around the micro-mirror allows integrating of control electronics.

FIG. 15 is a detailed cross section view of wafer layers and points of application of driving potentials in an embodiment of the present invention. A picobeamer micromirror has a silicon top/device layer 231 and a silicon bottom/handle layer 232. The silicon top/device layer 231 has aluminum bonding pads and wirebonds 233 and vias 234, 235 extending to the silicon bottom/handle layer 232 through a buried oxide (BOX) layer 236. The picobeamer micromirror has moveable 237 and stationary 238 comb fingers and vertical support beam 239. VO. Vl, V2 and V3 are applied as shown. Driving potentials used may be, for example: during oscillation launch VO grounded, V2 driven with square pulses. During steady state oscillation, V0=Vl=grounded and V2=V3.

FIG. 16 is a perspective view of flexible links of a mirror body of a picobeamer micromirror in accordance with an embodiment of the present invention. Flexible link 74 is fabricated in second silicon layer 206. Additional electrical connections 240 are provided to link 74 and an electrical portion 102.

While the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the scope of the invention. The scope of the invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.

Claims

CLAIMS:

1. A method for manufacturing a MEMS scanning micromirror having a cantilever beam comprising: providing a silicon on insulator (SOI) wafer 200 having a first silicon layer 202, a second silicon layer 206, and an insulating layer 204 between the first silicon layer 202 and the second silicon layer 206, the thickness of the first silicon layer 202 being a final thickness of the cantilever beam 72; fashioning the cantilever beam 72 from the first silicon layer 202 while maintaining the final thickness of the cantilever beam.

2. The method of claim 1 wherein the providing comprises machining the first silicon layer 202 to the final thickness.

3. The method of claim 1 wherein the providing comprises etching the first silicon layer 202 to the final thickness.

4. The method of claim 1 wherein the providing comprises growing the first silicon layer 202 to the final thickness.

5. The method of claim 1 wherein the growing comprises growing the first silicon layer 202 to a preliminary thickness and machining the first silicon layer 202 from the preliminary thickness to the final thickness.

6. A method for manufacturing a MEMS scanning micromirror comprising: providing a silicon on insulator (SOI) wafer 200 having a first silicon layer 202, a second silicon layer 206, and an insulating layer 204 between the first silicon layer 202 and the second silicon layer 206; and etching an aligned comb actuator in the SOI wafer 200, the aligned comb actuator having interleaved mirror combs 82 and frame combs 84; wherein first electrical portions 102 of the mirror combs 82 and first electrical portions 112 of the frame combs 84 are in the first silicon layer 202, and second electrical portions 104 of the mirror combs 82 and second electrical portions 114 of the frame combs 84 are in the second silicon layer 206.

7. The method of claim 6 further comprising etching cantilever beams 72 in the first silicon layer 202, the cantilever beams 72 being electrically connected to the first electrical portions 102 of the mirror combs 82.

8. The method of claim 6 further comprising etching vertical support beams 40 in the second silicon layer 206, the vertical support beams 40 being electrically connected to the second electrical portions 104 of the mirror combs 82.

9. A method for manufacturing a MEMS scanning micromirror comprising: providing a silicon on insulator (SOI) wafer 200 having a first silicon layer 202, a second silicon layer 206, and an insulating layer 204 between the first silicon layer 202 and the second silicon layer 206; etching a mirror body 50 in the SOI wafer 200, the mirror body 50 having a rotation axis 58 with a first extension bar 56 and a second extension bar 56 parallel to the rotation axis 58; etching a frame 60 in the SOI wafer 200, the frame 60 having a mirror recess

62 with a recess periphery 64, the frame 60 having a first opposed frame bar 66 and a second opposed frame bar 66 on the recess periphery 64 along the rotation axis 58; etching a first cantilever beam 72 in the SOI wafer 200, the first cantilever beam 72 being fixed to the first opposed frame bar 66 perpendicular to the rotation axis 58 and coupled to a first end of the first extension bar 56; etching a second cantilever beam 72 in the SOI wafer 200, the second cantilever beam 72 being fixed to the first opposed frame bar 66 perpendicular to the rotation axis 58 and coupled to a first end of the second extension bar 56; etching a third cantilever beam 72 in the SOI wafer 200, the third cantilever beam 72 being fixed to the second opposed frame bar 66 perpendicular to the rotation axis 58 and coupled to a second end of the first extension bar 56; etching a fourth cantilever beam 72 in the SOI wafer 200, the fourth cantilever beam 72 being fixed to the second opposed frame bar 66 perpendicular to the rotation axis 58 and coupled to a second end of the second extension bar 56; etching a first vertical support beam 40 in the SOI wafer 200, the first vertical support beam 40 being connected between the first frame opposed bar 66 and the mirror body 50 along the rotation axis 58; and etching a second vertical support beam 40 in the SOI wafer 200, the second vertical support beam 40 being connected between the second opposed frame bar 66 and the mirror body 50 along the rotation axis 58.

10. The method of claim 9 wherein the providing comprises providing a silicon on insulator (SOI) wafer 200 having a first silicon layer 202 with a final thickness of the first cantilever beam 72, and the etching a first cantilever beam 72 in the SOI wafer 200 comprises etching a first cantilever beam 72 in the first silicon layer 202 of the SOI wafer 200 and maintaining the final thickness of the first cantilever beam 72.

11. The method of claim 9 further comprising etching an actuator 80 in the SOI wafer 200, the actuator 80 operably connected to the mirror body 50 to provide torque about the rotation axis 58.

12. The method of claim 11 wherein the actuator 80 comprises: a first mirror comb 82 attached to the first extension bar 56; a second mirror comb 82 attached to the second extension bar 56; and a first frame comb 84 and a second frame comb 84 attached to the frame 60; wherein comb fingers of the first mirror comb 82 are interleaved with comb fingers of the first frame comb 84, and comb fingers of the second mirror comb 82 are interleaved with comb fingers of the second frame comb 84.

13. The method of claim 12 wherein the first mirror comb 82 has a first electrical portion 102 in the first silicon layer 202 and a second electrical portion 104 in the second silicon layer 206, the first electrical portion 102 and the second electrical portion 104 being separated by the insulating layer 204.