US20070018065A1 - Electrically controlled tiltable microstructures - Google Patents
Electrically controlled tiltable microstructures Download PDFInfo
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- US20070018065A1 US20070018065A1 US11/187,054 US18705405A US2007018065A1 US 20070018065 A1 US20070018065 A1 US 20070018065A1 US 18705405 A US18705405 A US 18705405A US 2007018065 A1 US2007018065 A1 US 2007018065A1
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
- G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
- G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
- G02B26/0841—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD the reflecting element being moved or deformed by electrostatic means
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B3/00—Devices comprising flexible or deformable elements, e.g. comprising elastic tongues or membranes
- B81B3/0062—Devices moving in two or more dimensions, i.e. having special features which allow movement in more than one dimension
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B2201/00—Specific applications of microelectromechanical systems
- B81B2201/03—Microengines and actuators
- B81B2201/033—Comb drives
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B2201/00—Specific applications of microelectromechanical systems
- B81B2201/04—Optical MEMS
- B81B2201/042—Micromirrors, not used as optical switches
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B2203/00—Basic microelectromechanical structures
- B81B2203/05—Type of movement
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B81—MICROSTRUCTURAL TECHNOLOGY
- B81B—MICROSTRUCTURAL DEVICES OR SYSTEMS, e.g. MICROMECHANICAL DEVICES
- B81B2203/00—Basic microelectromechanical structures
- B81B2203/05—Type of movement
- B81B2203/058—Rotation out of a plane parallel to the substrate
Definitions
- This invention relates to electrically controlled microstructures with tiltable platforms, and has particular application to micromirrors.
- micromirror arrays with tilting capabilities such as optical cross-couplers, projection displays, optical attenuators and laser beam scanning and pointing systems. It is desirable that the micromirrors have a large tilt angle, to maximize the contrast between “on” and “off” states, and good mirror surface quality to reduce non-specular scatter that degrades image contrast. Achieving both qualities in micromachined devices has proven to be very difficult.
- micromirrors formed from thin metal films that are monolithically integrated onto the control circuitry have been used for projector displays.
- the mirrors are co-fabricated with the circuitry, leading to temperature and material limitations such as difficulty in achieving a flat mirror surface, especially when coatings have applied to it, scaling the mirrors in size, optical power and reflectivity limitations, and the proximity of the mirrors to the supporting substrate surface which limits their tilt angle.
- Another approach employs micromirrors that are fabricated on a stressed support structure, which releases and allows the mirror to pop up after fabrication has been completed to increase the tilt angle.
- all connections are made along the side of the device, which limits its scalability to large scale 2-dimensional arrays, and it has a low fill factor (percentage of total array area occupied by mirror surfaces). While the mirrors can be used to point a beam among different reception optical fibers, the relatively large spacing between mirrors makes the array unusable for quality display purposes.
- a different approach employs a single crystal silicon mirror with a polysilicon actuator bonded to it that enables a piston-like up and down motion, but not a tilting motion. Only an individual device is disclosed, which is generally not scalable to a full array.
- the present invention provides an electrically controlled microstructure that can be capable of relatively large angle tilts, with a smooth and sturdy surface for mirror or other applications, a capacity for a large fill factor, and applications for tilting, tilting and tipping (tilting about two different axes), and tilting combined with a piston motion.
- a support structure extends upwards from a substrate and supports a tiltable platform, the upper surface of which can be a mirror, by means of spaced flexible couplings that enable the platform to tilt relative to the support structure.
- Respective electrodes associated with the substrate and platform control the platform tilt in response to applied electrical signals.
- the platform electrodes are spaced below and tilt with the platform, with the platform extending laterally from the support structure further than the platform electrodes. This makes it possible to achieve a desired balance between tilt angle and the voltage magnitudes required to operate the device.
- the platform is preferably bulk micromachined, and the support structure surface micromachined.
- the flexible couplings and electrodes can be designed to provide combined tip/tilt and tilt/piston movements in a variety of applications.
- FIGS. 1 and 2 are perspective views illustrating one embodiment of the invention
- FIG. 3 is a fragmentary perspective view of an electrode support mechanism for the embodiment of FIGS. 1 and 2 ;
- FIG. 4 is a plan view of segmented lower electrodes that can be used with the embodiment of FIGS. 1 and 2 ;
- FIGS. 5-7 are sectional views illustrating successive stages in the fabrication of a tiltable mirror
- FIGS. 8 a and 8 b are perspective views of the device illustrated in FIGS. 1 and 2 performing left and right tilts;
- FIG. 8 c is a perspective view of the device illustrated in FIGS. 1, 2 and 4 performing a backward tip;
- FIG. 9 is a graph of platform tilt angle as a function of applied voltage between the platform and substrate electrodes for the device shown in FIG. 3 ;
- FIG. 10 is an exploded perspective view of a micromirror array conceptually illustrating flip-chip bump connections to underlying addressing circuitry
- FIG. 11 is a perspective view of a vertical comb drive embodiment of the invention.
- FIG. 12 is a fragmentary perspective view of an electrode support mechanism for the embodiment of FIG. 11 ;
- FIGS. 13, 14 , 15 and 16 are respectively block diagrams of cross-coupler, projector, optical attenuator and atmospheric compensation applications for the invention.
- FIGS. 1 and 2 An illustrative microstructure device which supports a tiltable platform in accordance with the invention is illustrated in FIGS. 1 and 2 .
- the term “platform” includes a mirror but is not limited to mirror applications; it can also provide a base for other functions such as filters, gratings, and non-optical applications.
- the fabrication preferably employs a hybrid micromachining approach that combines bulk and surface micromachine techniques.
- This approach to forming a microstructure is disclosed in U.S. Pat. No. 6,587,613 by the present inventor, issued Jul. 1, 2003, the contents of which are incorporated by reference herein. It involves the formation of a support structure for a bulk micromachined element by fabricating the support structure on the element using surface micromachining techniques.
- One implementation uses a 5-level surface micromachining technology that allows for the fabrication of complex movable components on translatable stages that can engage and interact with other subassemblies. This technology is commonly referred to as the Sandia Ultra-planar Multi-level MEMS Technology V (SUMMiT V). [M. S.
- micromirror In the context of a micromirror, this approach reverses the traditional technique of first fabricating a support structure, and then emplacing a mirror element on that structure. Instead, a relatively thick and sturdy bulk micromachined mirror element has a support structure built up on one surface by surface micromachining, forming layers of active support structure and sacrificial material on the mirror element, and then dissolving away the layers of sacrificial material.
- MEMS micro electromechanical system
- the mirror element is supported in a manner that allows it to move in a piston motion relative to a substrate for the support structure.
- the mirror element is preferably implemented as the device layer of a silicon-on-insulator (SOI) structure, with a doped silicon mirror supported by an insulative layer and thick handle layer that are removed towards the end of the fabrication sequence.
- SOI silicon-on-insulator
- the support structure is preferably formed from polysilicon together with sacrificial oxide material.
- FIG. 1 A microstructure device in accordance with the invention is shown in FIG. 1 with a tiltable mirror removed, and in FIG. 2 with the mirror 2 in place.
- the structure is preferably fabricated with a hybrid micromachining approach as described above.
- the mirror 2 is fabricated by bulk micromachining, while the remainder of the structure is fabricated upon the mirror with a surface micromachining technique such as SUMMiT V.
- SUMMiT V a surface micromachining technique
- the device can actually be used in any desired orientation.
- Successive layers are surface micromachined, beginning from the bulk micromachined mirror 2 .
- the first layer from the mirror seen only in FIG. 1 , is patterned into a pair of upstanding posts 6 a and 6 b which support the mirror and space it above the next layer.
- This layer includes a pair of polysilicon upper electrodes 8 a and 8 b on opposite sides of a central hollow, generally rectangular frame consisting of a pair of stiffly flexible beams 10 a and 10 b , and frame ends 12 a and 12 b connected the ends of beams 10 a and 10 b .
- Electrodes 8 a and 8 b are parallel to beams 10 a and 10 b , with electrode 8 a connected to and spaced from beam 10 a by a central torsion arm 14 a , and electrode 8 b connected to and spaced from beam 10 b by a similar central torsion arm 14 b .
- Posts 6 a and 6 b sit upon torsion arms 14 a and 14 b , respectively.
- the upper electrodes 8 a and 8 b are spaced above and separated from a corresponding pair of lower electrodes 16 a and 16 b , respectively, by two additional SUMMiT V layers 18 and 20 which extend between the frame ends 12 a , 12 b and a central portion 16 c of the lower electrode layer between lower electrodes 16 a and 16 b.
- a framework linking structure 22 may be provided on the underside of the lower electrode layer in a manner similar to that described in U.S. Pat. No. 6,587,613. This is a continuous structure across an array of MEMs devices and is built up on the devices after they have been individually fabricated. The structure 22 links the individual MEMs devices in an array. Electrical contacts to the MEMs devices can be deposited on the linking framework structure 22 using known metallization processes, or on the lower electrode layer if no linking framework is provided.
- FIG. 3 provides an enlarged view illustrating the relationship between upper electrode 8 a , beam 10 a and torsion arm 14 a .
- upper electrode 8 b There is a similar relationship between upper electrode 8 b , beam 10 b and torsion arm 14 b .
- This relationship allows for three degrees of freedom in the movement of electrode 8 a in response to an applied electrostatic voltage between the upper and lower electrodes 8 a and 16 a , which are aligned with each other on one side of the device.
- an electrostatic attraction or repulsion between upper and lower electrodes 8 a and 16 a can cause beam 10 a to flex downward or upward, respectively, as upper electrode 8 a is drawn down toward or repulsed up away from the lower electrode 16 a .
- This is indicated by the flex axis 24 for the beam and by the (exaggerated) upwardly flexed beam position indicated by dashed lines 10 a′.
- torsion arm 14 a can undergo a cantilever flexing that allows upper electrode 8 a to move somewhat up or down in response to electrostatic repulsion or attraction, respectively, between the upper and lower electrodes 8 a and 16 a . This is indicated by the flex arrow 26 .
- a torsional twisting force can be applied to arm 14 a by applying an electrostatic attraction or repulsion force between the upper and lower forward electrodes and an opposite electrostatic force between the upper and lower rear electrodes.
- gaps 24 may be left in lower electrode 16 a and 16 b to divide them each into two sections. This is illustrated in FIG. 4 , which shows only the lower electrode layer for this embodiment.
- FIG. 6 shows the device with the handle layer 32 removed after the device has been flip-chip bonded to substrate 40 . It can be removed mechanically, such as by grinding, or the oxide layer 34 can be dissolved by chemical etching to release the handle layer. In FIG. 7 the remaining oxide sacrificial material has been dissolved by a chemical etch.
- Separate control signals would be applied to the contacts 38 for each of the lower electrodes 16 a and 16 b , while the upper electrodes 8 a and 8 b would preferably be grounded.
- the silicon preferred for the structure would normally be sufficiently conductive to transmit these signals, although if desired it could be doped to increase its conductivity.
- the mirror 2 is shown at its maximum tilt angle, with the left mirror electrode 8 a bottomed out upon the left substrate 16 a electrode; this defines the maximum tilt angle.
- a voltage has been applied across the right hand mirror and substrate electrodes, 8 b , 16 b , causing them to move together and thus tilt the mirror 2 to the right.
- Left hand beam 10 a flexes upward, and right hand beam 10 b downward.
- a degree of up/down piston action for the mirror 2 can also be achieved by applying a common voltage signal to both of the lower electrodes 16 a and 16 b , so that both of the upper electrodes 8 a and 8 b are either attracted to or repulsed from the lower electrodes.
- the amount of piston movement available depends primarily upon the flexibility of the beams 10 a and 10 b.
- FIG. 9 illustrates the characteristics of one microstructure, showing that the required voltage to produce a given tilt angle increases at a greater rate than the angle. The results shown in FIG. 9 were achieved with a mirror that was 100 microns long and 5 microns thick, with a gap of 3.75 microns between the mirror and the mirror electrodes.
- the mirror electrodes 8 a , 8 b below the bottom surface of the mirror element 2 reduces the spacing between the mirror and substrate electrodes, thus enabling the application of lower voltages, especially for higher tilt angles. Since the mirror 2 itself extends laterally away from the support structure by a greater distance than the mirror electrodes, 8 a , 8 b , the mirror electrodes will not bottom out until a greater tilt angle has been reached than would be the case if the electrodes extended the full extent of the mirror length. Thus, in addition to lower operating voltages, the illustrated structure enables greater tilt angles.
- FIG. 10 illustrates an array of micromirrors 42 of the type shown in FIGS. 1 and 2 , connected to electronic drive circuitry 44 , typically an addressing ASIC, by means of flip-chip mounting in which indium “bumps” or solder balls 46 on the upper surface of the addressing electronics and the lower surface of the array provide electrical and mechanical connections between the array and the electronics substrate (the array bumps are not shown).
- indium “bumps” or solder balls 46 on the upper surface of the addressing electronics and the lower surface of the array provide electrical and mechanical connections between the array and the electronics substrate (the array bumps are not shown).
- a common ground bump would be used for all of the substrate electrodes, and a separate bump for each element electrode. This enables the entire array to be transferred to pixel-level drive electronics, eliminating planar interconnects that would otherwise be required.
- the resulting array has a large fill factor which provides high optical quality, is scalable to large array formats, exhibits system level simplicity and uses the mature and reliable bump contact technology.
- the array can be fabricated on a large scale wafer which is then segmented into individual pixels by a deep anisotropic etch. Such bonding can be accomplished by a range of different processes known to those skilled in the art, such as indium, solder or Au/Au thermocompression bonding.
- microstructures would preferably be flip-chip mounted to a drive circuit which multiplexes the actuation signals in order to address desired pixels or sets of pixels.
- each pixel would correspond to a unique combination of addressing rows and columns, with a particular pixel or set of pixels addressed by activating its corresponding row and column via the multiplexer.
- the invention offers a highly flexible approach to the manipulation of light, including adaptive optics, beam steering, projection displays and fiber switching, with a high fill factor.
- the device structure of FIGS. 1 and 2 can be adapted to provide 2-axis tip/tilt operation with the mechanical arms or flexors 14 a , 14 b to accommodate torsional deformation about the second axis and, by segmenting the two substrate electrodes into four, allowing a second axis tilt to be accomplished.
- Other designs to implement 2-axis tilt using the hybrid micromachining process will be evident to those skilled in the art, and are within the spirit of the invention. This also enables piston motion with all electrodes actuated.
- FIG. 11 is similar to the embodiment of FIG. 1 but in which interdigitated upper 48 a , 48 b and lower 50 a , 50 b “comb” electrodes are substituted for the planar electrodes 8 a , 8 b , 16 a , 16 b of FIG. 1 .
- FIG. 12 is an enlarged fragmentary view of the upper electrode 48 a ; the other upper electrode 48 b has a mirror image structure.
- an electrostatic attraction or repulsion between the upper and lower electrodes 48 a and 50 a causes beam 54 a to flex downward or upward. This is indicated by the flex axis 62 for the beam and by the (exaggerated) upwardly flexed beam position indicated by dashed lines 54 a ′.
- the beam 54 a will tend to be somewhat less flexible than beam 10 a in the planar electrode embodiment because of the electrode fingers 52 a distributed along the length of beam 54 a.
- the invention is capable of at least +/ ⁇ 10° of tilt, and actuation speeds in excess of 50 kHz. Its preferred embodiment is highly manufacturable due to its use of a mature polysilicon processing technology, while the employment of a hybrid bulk and surface micromachining assembly process enables flexibility in its attachment to high voltage drive circuitry. With the drive circuitry provided directly below the MEMS device as illustrated, very compact systems integrating the electronics and MEMS are possible. The use of a thick, preferably dielectric platform is particularly useful for high optical power applications such as projectors.
- FIG. 13 An optical cross-coupler, is illustrated conceptually in FIG. 13 .
- Optical signals propagated through an array of input optical fibers 64 are directed in a desired pattern to corresponding fibers 66 in an output fiber array.
- the input light signals are processed through whatever input optics 68 may be required to direct them onto respective tiltable mirrors in a mirror array 70 constructed in accordance with the invention.
- the tilt angles, and also the tip angles if desired, of individual mirrors within the array 70 are controlled by a drive signal 72 so that each input signal is delivered to a desired fiber in the output array 66 .
- An array of lenses 74 will generally be provided, with one lens for each output fiber, to focus the signals onto the desired fibers. By adjusting the angle of the individual micromirrors, the input light signals can be distributed among the output fibers in a desired fashion.
- FIG. 14 illustrates the application of the invention to a color projector.
- a lamp 76 which includes appropriate optics, directs a white light beam onto a tiltable mirror array 78 in accordance with the invention.
- the individual mirrors within the array are controlled by a drive signal 80 so that they are directed onto desired portions of a color wheel 82 , with a timing signal input 84 applied to the mirror array and color wheel to keep them in synchronism.
- the resulting color light array is processed by projection optics 86 which output a projected image.
- FIG. 15 An optical attenuator embodiment is illustrated in FIG. 15 .
- Incoming light in an input optical fiber 88 is directed onto a mirror array 90 by appropriate optics 92 .
- An output optical fiber 94 is provided downstream from the mirror array, with appropriate optics 96 such as a lens array interfacing between the mirror array 90 and output fiber 94 .
- a drive and control signal 98 is applied to the mirror array 90 to steer the output optical signal array so that a desired portion of the light is incident on the output optics 96 and reaches the output fiber 94 , with the remainder of the light lost to the system.
- the mirrors will commonly be operated in tandem with each other, but this is not necessary.
- the degree of attenuation is controlled by the drive and control signal 98 , which controls how much of the input light reaches the output fiber 94 .
Abstract
Description
- 1. Field of the Invention
- This invention relates to electrically controlled microstructures with tiltable platforms, and has particular application to micromirrors.
- 2. Description of the Related Art
- There are various applications for micromirror arrays with tilting capabilities, such as optical cross-couplers, projection displays, optical attenuators and laser beam scanning and pointing systems. It is desirable that the micromirrors have a large tilt angle, to maximize the contrast between “on” and “off” states, and good mirror surface quality to reduce non-specular scatter that degrades image contrast. Achieving both qualities in micromachined devices has proven to be very difficult.
- In one approach, micromirrors formed from thin metal films that are monolithically integrated onto the control circuitry have been used for projector displays. The mirrors are co-fabricated with the circuitry, leading to temperature and material limitations such as difficulty in achieving a flat mirror surface, especially when coatings have applied to it, scaling the mirrors in size, optical power and reflectivity limitations, and the proximity of the mirrors to the supporting substrate surface which limits their tilt angle.
- Another approach employs micromirrors that are fabricated on a stressed support structure, which releases and allows the mirror to pop up after fabrication has been completed to increase the tilt angle. However, all connections are made along the side of the device, which limits its scalability to large scale 2-dimensional arrays, and it has a low fill factor (percentage of total array area occupied by mirror surfaces). While the mirrors can be used to point a beam among different reception optical fibers, the relatively large spacing between mirrors makes the array unusable for quality display purposes.
- A different approach employs a single crystal silicon mirror with a polysilicon actuator bonded to it that enables a piston-like up and down motion, but not a tilting motion. Only an individual device is disclosed, which is generally not scalable to a full array.
- The present invention provides an electrically controlled microstructure that can be capable of relatively large angle tilts, with a smooth and sturdy surface for mirror or other applications, a capacity for a large fill factor, and applications for tilting, tilting and tipping (tilting about two different axes), and tilting combined with a piston motion.
- In one aspect of the invention, a support structure extends upwards from a substrate and supports a tiltable platform, the upper surface of which can be a mirror, by means of spaced flexible couplings that enable the platform to tilt relative to the support structure. Respective electrodes associated with the substrate and platform control the platform tilt in response to applied electrical signals.
- In one embodiment, the platform electrodes are spaced below and tilt with the platform, with the platform extending laterally from the support structure further than the platform electrodes. This makes it possible to achieve a desired balance between tilt angle and the voltage magnitudes required to operate the device.
- The platform is preferably bulk micromachined, and the support structure surface micromachined. The flexible couplings and electrodes can be designed to provide combined tip/tilt and tilt/piston movements in a variety of applications.
- These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.
-
FIGS. 1 and 2 are perspective views illustrating one embodiment of the invention; -
FIG. 3 is a fragmentary perspective view of an electrode support mechanism for the embodiment ofFIGS. 1 and 2 ; -
FIG. 4 is a plan view of segmented lower electrodes that can be used with the embodiment ofFIGS. 1 and 2 ; -
FIGS. 5-7 are sectional views illustrating successive stages in the fabrication of a tiltable mirror; -
FIGS. 8 a and 8 b are perspective views of the device illustrated inFIGS. 1 and 2 performing left and right tilts; -
FIG. 8 c is a perspective view of the device illustrated inFIGS. 1, 2 and 4 performing a backward tip; -
FIG. 9 is a graph of platform tilt angle as a function of applied voltage between the platform and substrate electrodes for the device shown inFIG. 3 ; -
FIG. 10 is an exploded perspective view of a micromirror array conceptually illustrating flip-chip bump connections to underlying addressing circuitry; -
FIG. 11 is a perspective view of a vertical comb drive embodiment of the invention; -
FIG. 12 is a fragmentary perspective view of an electrode support mechanism for the embodiment ofFIG. 11 ; and -
FIGS. 13, 14 , 15 and 16 are respectively block diagrams of cross-coupler, projector, optical attenuator and atmospheric compensation applications for the invention. - An illustrative microstructure device which supports a tiltable platform in accordance with the invention is illustrated in
FIGS. 1 and 2 . As used herein, the term “platform” includes a mirror but is not limited to mirror applications; it can also provide a base for other functions such as filters, gratings, and non-optical applications. - Although not required by the invention, the fabrication preferably employs a hybrid micromachining approach that combines bulk and surface micromachine techniques. This approach to forming a microstructure is disclosed in U.S. Pat. No. 6,587,613 by the present inventor, issued Jul. 1, 2003, the contents of which are incorporated by reference herein. It involves the formation of a support structure for a bulk micromachined element by fabricating the support structure on the element using surface micromachining techniques. One implementation uses a 5-level surface micromachining technology that allows for the fabrication of complex movable components on translatable stages that can engage and interact with other subassemblies. This technology is commonly referred to as the Sandia Ultra-planar Multi-level MEMS Technology V (SUMMiT V). [M. S. Rogers and J. J. Snigegowski, Designing Microelectromechanical Systems-On-AChip in a 5-level Surface Micromachine Technology, 2nd Annual Int. Conf. on Engineering Design and Automation (August 1998), and M. S. Rogers and J. J. Sniegowski, 5-Level Polysilicon Surface Micromachining Technology Application to Complex Mechanical Systems, Proc. 1998 Solid State Sensor and Actuator Workshop, Pg. 144 (June 1998, Hilton Head, S.C.)]. Other multi-level MEMS surface micromachining processes may also be used within the scope of the invention.
- In the context of a micromirror, this approach reverses the traditional technique of first fabricating a support structure, and then emplacing a mirror element on that structure. Instead, a relatively thick and sturdy bulk micromachined mirror element has a support structure built up on one surface by surface micromachining, forming layers of active support structure and sacrificial material on the mirror element, and then dissolving away the layers of sacrificial material. The resulting micro electromechanical system (MEMS) device is capable of high quality optical surfaces and complex support structures. In the U.S. Pat. No. 6,587,613 patent, the mirror element is supported in a manner that allows it to move in a piston motion relative to a substrate for the support structure. A voltage is applied across a central electrode below the mirror element and the mirror element itself, to produce an electrostatic attraction between the two. The mirror element is preferably implemented as the device layer of a silicon-on-insulator (SOI) structure, with a doped silicon mirror supported by an insulative layer and thick handle layer that are removed towards the end of the fabrication sequence. The support structure is preferably formed from polysilicon together with sacrificial oxide material.
- A microstructure device in accordance with the invention is shown in
FIG. 1 with a tiltable mirror removed, and inFIG. 2 with themirror 2 in place. The structure is preferably fabricated with a hybrid micromachining approach as described above. In practice themirror 2 is fabricated by bulk micromachining, while the remainder of the structure is fabricated upon the mirror with a surface micromachining technique such as SUMMiT V. Thus, although inFIG. 2 themirror 2 is shown at the top of the structure, the structure is actually constructed using the bulk mirror as a starting point and fabricating successive layers in the downward direction from the mirror as seen inFIG. 2 . - Although the mirror is typically supported in an upright position, the device can actually be used in any desired orientation. Successive layers are surface micromachined, beginning from the bulk
micromachined mirror 2. The first layer from the mirror, seen only inFIG. 1 , is patterned into a pair ofupstanding posts upper electrodes flexible beams beams Electrodes beams electrode 8 a connected to and spaced frombeam 10 a by acentral torsion arm 14 a, andelectrode 8 b connected to and spaced frombeam 10 b by a similarcentral torsion arm 14 b.Posts torsion arms - The
upper electrodes lower electrodes central portion 16 c of the lower electrode layer betweenlower electrodes - A
framework linking structure 22 may be provided on the underside of the lower electrode layer in a manner similar to that described in U.S. Pat. No. 6,587,613. This is a continuous structure across an array of MEMs devices and is built up on the devices after they have been individually fabricated. Thestructure 22 links the individual MEMs devices in an array. Electrical contacts to the MEMs devices can be deposited on the linkingframework structure 22 using known metallization processes, or on the lower electrode layer if no linking framework is provided. -
FIG. 3 provides an enlarged view illustrating the relationship betweenupper electrode 8 a,beam 10 a andtorsion arm 14 a. There is a similar relationship betweenupper electrode 8 b,beam 10 b andtorsion arm 14 b. This relationship allows for three degrees of freedom in the movement ofelectrode 8 a in response to an applied electrostatic voltage between the upper andlower electrodes lower electrodes beam 10 a to flex downward or upward, respectively, asupper electrode 8 a is drawn down toward or repulsed up away from thelower electrode 16 a. This is indicated by theflex axis 24 for the beam and by the (exaggerated) upwardly flexed beam position indicated by dashedlines 10 a′. - Second,
torsion arm 14 a can undergo a cantilever flexing that allowsupper electrode 8 a to move somewhat up or down in response to electrostatic repulsion or attraction, respectively, between the upper andlower electrodes flex arrow 26. - Third, if either the upper or the lower electrodes, or both, are segmented into forward and rear sections that are electrically isolated from each other, a torsional twisting force can be applied to
arm 14 a by applying an electrostatic attraction or repulsion force between the upper and lower forward electrodes and an opposite electrostatic force between the upper and lower rear electrodes. This causes theupper electrode 8 a to pivot forward or rearward aroundtorsion arm 14 a, as indicated bypivot arrow 28. It is normally easier to divide the lower electrodes into electrically isolated forward and rear sections, than to divide the upper electrodes. For example,gaps 24 may be left inlower electrode FIG. 4 , which shows only the lower electrode layer for this embodiment.Lower electrodes - Referring back to
FIGS. 1 and 2 , it can be seen that, sincemirror 2 rides onposts torsion arms -
FIGS. 5-7 illustrate successive stages in the preferred fabrication technique for the device ofFIGS. 1-4 . Since the general fabrication technique is generally similar to that described in incorporated U.S. Pat. No. 6,587,613, it will not be described in great detail. InFIG. 5 an SOI (silicon on insulator) wafer 30 includes a relativethin silicon layer 2 anchored to ahandle layer 32 by anoxide layer 34. Thehandle layer 32 is generally thicker than thesilicon layer 2, with a suitable thickness of about 500 microns. Thesilicon layer 2 serves as the MEMs device's micromirror and is preferably on the order of 100 microns thick, although other thicknesses can also work. If it is too thick its movement can become too slow, while if it is too thin it can lose rigidity. The surface micromachined layers fabricated on themirror layer 2 are preferably on the order of 5 microns thick silicon. - The
handle layer 32 provides a convenient mechanism for holding and flipping the device during fabrication, and is then removed. The surface machined elements of the device, indicated by the same reference numbers as inFIGS. 1-3 , are formed from polysilicon structural material and are built up with oxide sacrificial material 36 (indicated by stippling) occupying areas that will become voids in later stages of the fabrication.Electrical contacts 38 are provided on the underside of theframework linking structure 22. The device can be flipped over once the framework linking structure has been emplaced, and electrically and mechanically bonded to asubstrate 40 via flip-chip bond contacts 38. -
FIG. 6 shows the device with thehandle layer 32 removed after the device has been flip-chip bonded tosubstrate 40. It can be removed mechanically, such as by grinding, or theoxide layer 34 can be dissolved by chemical etching to release the handle layer. InFIG. 7 the remaining oxide sacrificial material has been dissolved by a chemical etch. Separate control signals would be applied to thecontacts 38 for each of thelower electrodes upper electrodes -
FIGS. 8 a and 8 b are simplified perspective views illustrating the structure ofFIGS. 1 and 2 performing left and right tilts. The substrate upon which the support structure is mounted is not shown for simplification. InFIG. 8 a, a voltage has been applied across the left hand upper (mirror) and lower (substrate)electrodes mirror 2, which moves with themirror electrodes hand flex beam 10 a flexes downward, while theright hand beam 10 b flexes upward. In the figure themirror 2 is shown at its maximum tilt angle, with theleft mirror electrode 8 a bottomed out upon theleft substrate 16 a electrode; this defines the maximum tilt angle. Similarly, inFIG. 8 b a voltage has been applied across the right hand mirror and substrate electrodes, 8 b, 16 b, causing them to move together and thus tilt themirror 2 to the right.Left hand beam 10 a flexes upward, andright hand beam 10 b downward. -
FIG. 8 c illustrates the 2-axis movement capability of the segmented electrode embodiment illustrated inFIG. 4 . In response to an electrostatic repulsion between the forward electrode segments and/or an electrostatic attraction between the rear electrode segments, the mirror tips backward as shown. This version is also capable of left and right tilting. - A degree of up/down piston action for the
mirror 2 can also be achieved by applying a common voltage signal to both of thelower electrodes upper electrodes beams - The electrostatic force of attraction between the mirror and substrate electrodes is a function of the voltage applied across these electrodes. However, the tilt angle-voltage relationship is not linear.
FIG. 9 illustrates the characteristics of one microstructure, showing that the required voltage to produce a given tilt angle increases at a greater rate than the angle. The results shown inFIG. 9 were achieved with a mirror that was 100 microns long and 5 microns thick, with a gap of 3.75 microns between the mirror and the mirror electrodes. - Spacing the
mirror electrodes mirror element 2, rather than forming them directly on that surface, reduces the spacing between the mirror and substrate electrodes, thus enabling the application of lower voltages, especially for higher tilt angles. Since themirror 2 itself extends laterally away from the support structure by a greater distance than the mirror electrodes, 8 a, 8 b, the mirror electrodes will not bottom out until a greater tilt angle has been reached than would be the case if the electrodes extended the full extent of the mirror length. Thus, in addition to lower operating voltages, the illustrated structure enables greater tilt angles. -
FIG. 10 illustrates an array ofmicromirrors 42 of the type shown inFIGS. 1 and 2 , connected toelectronic drive circuitry 44, typically an addressing ASIC, by means of flip-chip mounting in which indium “bumps” orsolder balls 46 on the upper surface of the addressing electronics and the lower surface of the array provide electrical and mechanical connections between the array and the electronics substrate (the array bumps are not shown). Although only a single bump is illustrated on the addressing circuit for each microelement, in practice a common ground bump would be used for all of the substrate electrodes, and a separate bump for each element electrode. This enables the entire array to be transferred to pixel-level drive electronics, eliminating planar interconnects that would otherwise be required. The resulting array has a large fill factor which provides high optical quality, is scalable to large array formats, exhibits system level simplicity and uses the mature and reliable bump contact technology. If desired, the array can be fabricated on a large scale wafer which is then segmented into individual pixels by a deep anisotropic etch. Such bonding can be accomplished by a range of different processes known to those skilled in the art, such as indium, solder or Au/Au thermocompression bonding. - The microstructures would preferably be flip-chip mounted to a drive circuit which multiplexes the actuation signals in order to address desired pixels or sets of pixels. Typically, each pixel would correspond to a unique combination of addressing rows and columns, with a particular pixel or set of pixels addressed by activating its corresponding row and column via the multiplexer.
- In the context of micromirror arrays, the invention offers a highly flexible approach to the manipulation of light, including adaptive optics, beam steering, projection displays and fiber switching, with a high fill factor. The device structure of
FIGS. 1 and 2 can be adapted to provide 2-axis tip/tilt operation with the mechanical arms orflexors - Similarly, the flexibility afforded by the hybrid micromachining process enables designs incorporating other electrostatic electrode concepts, such as vertical comb drivers. Such an approach is illustrated in
FIG. 11 , which is similar to the embodiment ofFIG. 1 but in which interdigitated upper 48 a, 48 b and lower 50 a, 50 b “comb” electrodes are substituted for theplanar electrodes FIG. 1 . Theupper electrode fingers flexible beams lower electrode fingers framework linking structure 22. The mirror 2 (not shown) is supported onrespective posts beams -
FIG. 12 is an enlarged fragmentary view of theupper electrode 48 a; the otherupper electrode 48 b has a mirror image structure. As with the planar electrode embodiment, an electrostatic attraction or repulsion between the upper andlower electrodes causes beam 54 a to flex downward or upward. This is indicated by theflex axis 62 for the beam and by the (exaggerated) upwardly flexed beam position indicated by dashedlines 54 a′. Thebeam 54 a will tend to be somewhat less flexible thanbeam 10 a in the planar electrode embodiment because of theelectrode fingers 52 a distributed along the length ofbeam 54 a. - An interdigitated comb structures as in
FIGS. 11 and 12 tends to provide a greater degree of electrostatic stability than does the planar electrode structure described above. The planar electrodes tend to snap together once the gap between the upper and lower electrodes has been closed by more than roughly ⅓, but this does not occur with an interdigitated comb structure. On the other hand, the illustrated comb structure tends to have a somewhat lesser piston movement capability than the planar electrode version, due to the reduced flexibility of its flex beams 54 a and 54 b. While the upper and lower combs overlap somewhat, electrostatic attraction and repulsion forces are still generated because the center of mass of the upper comb will remain above that of the lower comb. - The invention is capable of at least +/−10° of tilt, and actuation speeds in excess of 50 kHz. Its preferred embodiment is highly manufacturable due to its use of a mature polysilicon processing technology, while the employment of a hybrid bulk and surface micromachining assembly process enables flexibility in its attachment to high voltage drive circuitry. With the drive circuitry provided directly below the MEMS device as illustrated, very compact systems integrating the electronics and MEMS are possible. The use of a thick, preferably dielectric platform is particularly useful for high optical power applications such as projectors.
- There are numerous applications for the new microstructure, particularly in its micromirror array form. One such application, an optical cross-coupler, is illustrated conceptually in
FIG. 13 . Optical signals propagated through an array of inputoptical fibers 64 are directed in a desired pattern to correspondingfibers 66 in an output fiber array. The input light signals are processed through whateverinput optics 68 may be required to direct them onto respective tiltable mirrors in amirror array 70 constructed in accordance with the invention. The tilt angles, and also the tip angles if desired, of individual mirrors within thearray 70 are controlled by adrive signal 72 so that each input signal is delivered to a desired fiber in theoutput array 66. An array oflenses 74 will generally be provided, with one lens for each output fiber, to focus the signals onto the desired fibers. By adjusting the angle of the individual micromirrors, the input light signals can be distributed among the output fibers in a desired fashion. -
FIG. 14 illustrates the application of the invention to a color projector. Alamp 76, which includes appropriate optics, directs a white light beam onto atiltable mirror array 78 in accordance with the invention. The individual mirrors within the array are controlled by adrive signal 80 so that they are directed onto desired portions of acolor wheel 82, with atiming signal input 84 applied to the mirror array and color wheel to keep them in synchronism. The resulting color light array is processed byprojection optics 86 which output a projected image. - An optical attenuator embodiment is illustrated in
FIG. 15 . Incoming light in an inputoptical fiber 88 is directed onto amirror array 90 byappropriate optics 92. An outputoptical fiber 94 is provided downstream from the mirror array, withappropriate optics 96 such as a lens array interfacing between themirror array 90 andoutput fiber 94. A drive and controlsignal 98 is applied to themirror array 90 to steer the output optical signal array so that a desired portion of the light is incident on theoutput optics 96 and reaches theoutput fiber 94, with the remainder of the light lost to the system. In this application the mirrors will commonly be operated in tandem with each other, but this is not necessary. The degree of attenuation is controlled by the drive and controlsignal 98, which controls how much of the input light reaches theoutput fiber 94. -
FIG. 16 illustrates a system which compensates for atmospheric interference to a light signal. An inputoptical wavefront 100 which has been transmitted through a portion of the atmosphere is processed byinput optics 102, which input the signal onto amirror array 104 in accordance with the invention. The optical beam array reflected from the mirror array is transmitted through anoptical sampler 106 tooutput optics 108, which form the light signals into animage 110. The signal samples obtained bysampler 106 are delivered to awavefront sensor 112, which stores an impression of how a wavefront would appear in the absence of atmospheric interference, and compares it with the beam samples to determine the degree and nature of the atmospheric aberrations experienced by the input wavefront. Thewavefront sensor 112 produces a drive andcontrol output signal 114 that is transmitted to themirror array 104 and adjusts the mirror angular orientations to compensate for the atmospheric interference. Themirror array 104,sampler 106 andwavefront sensor 112 form an active feedback loop that continually updates the compensation as the atmospheric interference changes. - Numerous other applications for the invention can also be visualized. The term “light” as used above is not limited to visible light, but rather covers all regions of the electromagnetic spectrum capable of being directed by a mirror array as discussed herein. While particular embodiments of the invention have been shown and described, numerous alternate embodiments will be apparent to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.
Claims (31)
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
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US11/187,054 US20070018065A1 (en) | 2005-07-21 | 2005-07-21 | Electrically controlled tiltable microstructures |
PCT/US2006/027353 WO2007015747A2 (en) | 2005-07-21 | 2006-07-13 | Electrically controlled tiltable microstructures |
TW095126544A TW200716477A (en) | 2005-07-21 | 2006-07-20 | Electrically controlled tiltable microstructures |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US11/187,054 US20070018065A1 (en) | 2005-07-21 | 2005-07-21 | Electrically controlled tiltable microstructures |
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US20070018065A1 true US20070018065A1 (en) | 2007-01-25 |
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US11/187,054 Abandoned US20070018065A1 (en) | 2005-07-21 | 2005-07-21 | Electrically controlled tiltable microstructures |
Country Status (3)
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US (1) | US20070018065A1 (en) |
TW (1) | TW200716477A (en) |
WO (1) | WO2007015747A2 (en) |
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US20080030840A1 (en) * | 2006-08-02 | 2008-02-07 | Texas Instruments Incorporated | Sloped cantilever beam electrode for a MEMS device |
WO2010040897A1 (en) | 2008-10-08 | 2010-04-15 | Teknillinen Korkeakoulu | Method and system for producing hydrogen, and electricity generation system |
WO2019172767A1 (en) * | 2018-03-09 | 2019-09-12 | Scinvivo B.V. | Mems mirror for oct probe and method of manufacturing such mems mirror |
CN112787540A (en) * | 2019-11-07 | 2021-05-11 | 躍旺创新股份有限公司 | Out-of-plane motion engine for carrying reflectors and method of making same |
US11036030B2 (en) * | 2018-06-15 | 2021-06-15 | Silicon Light Machines Corporation | MEMS posting for increased thermal dissipation |
WO2024052172A1 (en) * | 2022-09-09 | 2024-03-14 | Robert Bosch Gmbh | Micromirror array with resiliently mounted individual mirror elements |
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CN111552072B (en) * | 2020-04-28 | 2022-07-12 | 安徽中科米微电子技术有限公司 | Large-size MEMS vertical comb micro-mirror and preparation method thereof |
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Also Published As
Publication number | Publication date |
---|---|
WO2007015747A3 (en) | 2007-04-05 |
WO2007015747A2 (en) | 2007-02-08 |
TW200716477A (en) | 2007-05-01 |
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