Micro-electro-mechanical optical device

A micro-electro-mechanical (MEM) optical device having a reduced footprint for increasing yield on a substrate. The MEM device includes an optical element having an outer edge and supported by a support structure disposed on a substrate. The support structure is mechanically connected to the substrate through first and second pairs of beams which move the structure to an active position for elevating the optic device above the substrate. When in an elevated position, the optical device can be selectively tilted for deflecting optic signals. The beams are connected at one end to the support structure, at the other end to the substrate and are disposed so that the first and second beam ends are located proximate the optical device outer edge. In a preferred embodiment, a stiction force reducing element is included on the outer edge of the optical device for reducing the contact area between the optic device edge and the substrate.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to optical communication systems
 and, more particularly, to micro-electro-mechanical optical devices.
 2. Description of the Related Art
 Optical communication systems typically include a variety of optical
 devices (e. g., light sources, photodetectors, switches, attenuators,
 mirrors, amplifiers, and filters). The optical devices transmit optical
 signals in the optical communications systems. Some optical devices are
 coupled to electro-mechanical structures (e. g., thermal actuators)
 forming an electro-mechanical optical device. The term electro-mechanical
 structure as used in this disclosure refers to a structure which moves
 mechanically under the control of an electrical signal.
 Some electro-mechanical structures move the optical devices from a
 predetermined first position to a predetermined second position. Cowan,
 William D., et al., "Vertical Thermal Actuators for
 Micro-Opto-Electro-Mechanical Systems", SPIE, Vol. 3226, pp. 137-146
 (1997), describes one such electro-mechanical structure useful for moving
 optical devices from predetermined first positions to predetermined second
 positions.
 In Cowan et al., the electro-mechanical structure is a thermal actuator.
 The thermal actuator is coupled to an optical mirror. Both the thermal
 actuator and the optical mirror are disposed on a surface of a substrate.
 The thermal actuator has two beams. A first end of each beam is coupled to
 the optical mirror. A second end of each beam is attached to the substrate
 surface.
 Each beam of the thermal actuator has two material layers stacked one upon
 the other. The stacked material layers each have a different coefficient
 of thermal expansion, with the topmost material layer of each beam having
 a coefficient of thermal expansion larger than that of the other material
 layer.
 The thermal actuator mechanically moves the optical mirror in response to
 an electrical signal being applied to the beams. Applying the electrical
 signal to the beams heats the stacked material layers. Thereafter, upon
 removal of the electrical signal, the stacked material layers cool. Since
 the topmost layer of each beam has the larger coefficient of thermal
 expansion, it contracts faster than the underlying material layer when
 cooled. As the topmost material layer contracts, it lifts the first end of
 each beam as well as the optical mirror coupled thereto a predetermined
 height above the plane of the substrate surface. Additional heating and
 cooling of the beams does not change the height of the optical device with
 respect to the plane of the substrate surface. As such, the usefulness of
 thermal actuators is limited to one-time setup or positioning
 applications.
 Thus, electro-mechanical structures suitable for controlling the movement
 of optical devices continue to be sought.
 SUMMARY OF THE INVENTION
 A micro-electro-mechanical (MEM) optical device having a reduced footprint
 is disclosed. The MEM optical device is disposed on a substrate and
 includes a support structure for supporting an optical element having an
 outer edge. The optical element is used to deflect light at a selected
 angle based upon an amount of tilt imparted to the optical element. The
 support structure is supported by first and second pairs of beams
 connected at a first end to the support structure and fixed at a second
 end to the substrate. The beam pairs are arranged, relative to the optical
 device, so that the first and second ends of each beam are disposed in
 close proximity to the optical device. This reduces the amount of
 substrate surface area required for supporting the MEM optical device. As
 such, an array having an increased number and yield of devices can be
 produced in a given substrate size.
 In a preferred embodiment, the second ends of the beams in each beam pair
 are located in close proximity to each other to provide for efficient
 electrical connection of activation leads to the beams, thereby reducing
 the amount of substrate surface area required for mapping the leads to the
 beams.
 In another preferred embodiment a stiction force reduction element is
 included on the optical element as well as on an inner frame used for
 supporting the optical element. The stiction force reduction element is
 configured as a plurality of protrusions radially disposed on an outer
 edge of the optical element and on an outer edge of the inner frame for
 reducing a region of contact between the substrate and either or both of
 the inner frame and optical element.
 Other objects and features of the present invention will become apparent
 from the following detailed description considered in conjunction with the
 accompanying drawings. It is to be understood, however, that the drawings
 are designed solely for purposes of illustration and not as a definition
 of the limits of the invention, for which reference should be made to the
 appended claims. It should be further understood that the drawings are not
 necessarily drawn to scale and that, unless otherwise indicated, they are
 merely intended to conceptually illustrate the structures and procedures
 described herein.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
 The present invention is directed to a micro-electro-mechanical optical
 device suitable for use in optical communication systems. Referring to
 FIG. 1, the micro-electro-mechanical optical device includes a
 micro-electro-mechanical structure 15 and an optical device 17 disposed on
 a surface of a substrate 13. The micro-electro-mechanical structure 15 is
 coupled to the optical device 17. For example, micro-electro-mechanical
 structure 15 is coupled to the optical device 17 with springs 14.
 The electro-mechanical structure 15 includes a plurality of beams 19, 26. A
 first end 22 of beams 19 is coupled to a plate 20 in hinged attachment
 with the substrate surface 13. The hinged plate includes a v-shaped notch.
 The hinged plate 20 is coupled to an engagement plate 25. The engagement
 plate 25 is also coupled with the optical device 17. A first end of beams
 26 is coupled to the engagement plate 25. A second end of beams 26 is
 coupled to the substrate surface 13. When unassembled the beams 19, 26,
 the hinged plate 20, and the engagement plate 25 lie flat on the substrate
 surface 13.
 The engagement plate 25 has a pair of v-shaped notches located at opposite
 ends thereof. Each pair of v-shaped notches on the engagement plate 25 is
 located within the region of the v-shaped notch on the hinged plate 20.
 Referring to FIG. 2, the first ends 22 of the beams 19 lift in an upward
 direction, substantially in an arc, above the plane of the substrate
 surface in response to the application of an activation force. As the
 first ends 22 of the beams 19 are lifted above the plane of the substrate
 surface, they rotate the hinged plates 20 out of the plane of the
 substrate.
 When the hinged plates 20 are rotated out of the plane of the substrate,
 the beams 26 lift the engagement plate 25 as well as the optical device
 above the plane of the substrate. As the engagement plate is lifted above
 the plane of the substrate, the pair of v-shaped notches on the engagement
 plate 25 slide into the v-shaped notch on the hinged plate 20. When the
 engagement plate 25 is lifted, it completes the rotation of the hinged
 plate 20 started by the beams 19 so that the hinged plates are about
 ninety degrees out of the plane of the substrate. The height of the
 v-shaped notch on the hinged plate 20 limits the height to which the
 optical device is lifted and holds it in a fixed, well-defined position.
 A variety of activation forces can be applied to the electro-mechanical
 structure to lift the first ends 22 of the beams. Referring to FIG. 3,
 when the activation force applied to the electro-mechanical structure is
 based on thermal contraction of the beams, each beam 19, 26 includes two
 or more material layers 31, 32 stacked one upon the other. The stacked
 material layers 31, 32 each have a different coefficient of thermal
 expansion.
 In one embodiment, the topmost material layer 31 of each beam 19, 26 has a
 coefficient of thermal expansion larger than that of the other material
 layer 32.
 Applying a current to the beams 19, 26 heats the stacked material layers
 31, 32. The current is applied to the beams 19, 26 from a current source
 (not shown). Referring to FIG. 4, when the stacked material layers 31, 32
 are heated they curl up, lifting the first end of each beam 19, 26 as well
 as the optical mirror (not shown) coupled thereto out of the plane of the
 substrate surface 13. The height that the end of each beam lifts out of
 the plane of the substrate surface depends on the length of the beams as
 well as the composition of the material layers used to form the beams.
 However, the height that the end of each beam 26 lifts out of the plane of
 the substrate surface is preferably sufficient to lift the engagement
 plate 25 and rotate the hinged plate about 90 degrees relative to the
 substrate surface.
 Alternatively, when the activation force applied to the electro-mechanical
 structure is based on beam contraction due to intrinsic stress, each beam
 19, 26 includes two or more material layers 41, 42 stacked one upon the
 other on the substrate surface 13, as shown in FIG. 5. The topmost
 material layer 41 has an intrinsic stress. The topmost material layer
 optionally has a stress gradient therein. The bottom material layer 42 is
 a sacrificial layer.
 When the sacrificial material layer 42 is removed (e. g., by etching), the
 two or more topmost layers 41 lift the first end of each beam 19, 26 as
 well as the hinged plate (not shown) coupled thereto above the plane of
 the substrate surface 13. The height that the end of each beam lifts out
 of the plane of the substrate surface depends on the length of the beams
 as well as the composition of the material layers used to form the beams.
 However, the height that each beam 26 lifts out of the plane of the
 substrate surface is preferably sufficient to lift the engagement plate 25
 and rotate the hinged plate about 90 degrees relative to the substrate
 surface.
 Other suitable activation forces include scratch drives, and
 electromagnetic forces. Illustrative electro-mechanical structures based
 on scratch drives are discussed in Akiyam, T. et al., "A Quantitative
 Analysis of Scratch Drive Actuator Using Buckling Motion", Proc. 8.sup.th
 IEEE International MEMS Workshop, pp. 310-315 (1995) and
 electro-mechanical structures based on electromagnetic forces are
 discussed in Busch-Vishniac, I. J., "The Case for Magnetically Driven
 Microactuators", Sensors and Actuators A, A33, pp. 207-220 (1992).
 After the electro-mechanical structure 15 lifts the optical device 17 above
 the plane of the substrate, the lifted optical device 17 is moveable in
 response to an electrostatic field generated between such optical device
 17 and the substrate surface 13. The electrostatic field is generated by
 applying a bias voltage between the optical device 17 and the substrate
 surface 13.
 Referring to FIGS. 1 and 2, when the electrostatic field is generated
 between the optical device 17 and the substrate 13 (or electrode 27), such
 electrostatic field can rotate such optical device 17 around an axis 1-1'.
 The angle that the optical device 17 is rotated around axis 1-1' depends
 on the magnitude of the electrostatic field generated between the optical
 device 17 and the substrate 13 (or electrode 27). The magnitude of the
 electrostatic field depends on the amount of the applied bias voltage.
 Alternatively, the optical device can be made to deflect substantially
 towards the substrate or move using both rotation and deflection. Such
 motion depends on the electrode 27 geometry as well as the coupling
 between the optical device and the engagement plate.
 Both the optical device and the substrate are preferably conductive so that
 the bias voltage may be applied across them to generate the electrostatic
 field. When either of the optical device or the substrate are
 insufficiently conductive to generate the electrostatic field, conductive
 layers (electrodes) 27, 28 are optionally formed on regions thereof as
 shown in FIG. 1.
 The micro-electro-opto-mechanical device of the present invention is
 fabricated by providing a substrate that is suitably prepared (i.e.,
 doped, as appropriate) and cleaned. Suitable substrate materials include
 silicon, gallium arsenide, indium phosphide, germanium or indium tin oxide
 (ITO) coated glass.
 A plurality of material layers are formed in a planar arrangement on a
 surface of the substrate. Examples of suitable material layers include
 polysilicon, silicon nitride, and silicon dioxide.
 After each layer of the plurality of material layers are formed on the
 substrate, each layer is patterned to form a micro-electro-mechanical
 structure as well as an optical device. For example, the
 electro-opto-mechanical device shown in FIG. 1, including a mirror 17 as
 well as beams 19, 26, is fabricated using a Multi-User MEMS Process
 (MUMPS) provided by the MCNC MEMS Technology Applications Center, MCNC,
 Research Triangle Park, North Carolina (see SmartMUMPs Design Handbook at
 mems.mcnc.org).
 In the MUMPS process the micro-electro-mechanical structure and the optical
 mirror are formed in polysilicon layers, oxide layers (e.g.,
 phosphosilicon glass) provide sacrificial layers, and silicon nitride
 electrically isolates the micro-electro-mechanical structure and optical
 mirror from the substrate. The micro-electro-mechanical structure and
 optical mirror are formed in the polysilicon layers with multiple
 photolithography steps.
 Photolithography is a process which includes the coating of one or more of
 the polysilicon layers and phosphosilicon glass layers with a photoresist
 (i. e., an energy sensitive material), exposure of the photoresist with an
 appropriate mask, and developing of the exposed photoresist to create the
 desired etch mask for subsequent pattern transfer into the one or more
 underlying polysilicon layers and phosphosilicon glass layers. The pattern
 defined in the photoresist is transferred into the one or more underlying
 polysilicon layers and phosphosilicon glass layers by etching for example
 in a reactive ion etch (RIE) system.
 The following example is provided to illustrate a specific embodiment of
 the present invention.
 EXAMPLE 1
 A micro-electro-mechanical optical device having the structure depicted in
 FIG. 1 was obtained from the MEMS Technology Application Center, MCNC,
 Research Triangle Park, North Carolina. The micro-electro-mechanical
 optical device was disposed on a surface of a silicon substrate. The
 silicon substrate had a resistivity of about 1-2 ohm-cm. A multi-layered
 planar arrangement of alternating polysilicon layers (POLY0, POLY1 and
 POLY 2) and phosphosilicon glass layers (OX1 and OX2), formed over a 600
 nm (nanometer) thick silicon nitride layer, was formed on the silicon
 substrate.
 The polysilicon layers POLY0, POLY1 and POLY 2 had thicknesses of about 0.5
 .mu.m (micrometers), 2.0 .mu.m, and 1.5 .mu.m, respectively. The
 phosphosilicon glass layers OX1 and OX2 had thicknesses of about 2 .mu.m
 and 0.75 .mu.m, respectively. A 0.5 .mu.m layer of Cr/Au was formed on the
 POLY2 layer.
 The silicon nitride layer, the polysilicon layers (POLY0, POLY1 and POLY2),
 the phosphosilicon glass layers (OX1 and OX2), and the Cr/Au layer were
 formed on the silicon substrate using low pressure evaporation techniques.
 Referring to FIG. 1, the electro-mechanical structure 15 and the optical
 device 17 were defined in the multi-layered planar arrangement using
 photolithographic techniques. The electromechanical structure included two
 beams 19, 26 each coupled at one edge to a plate 20 in hinged attachment
 with the substrate. The beams 19 each had a width of about 50 .mu.m and a
 length of about 300 .mu.m. The beams 26 had a width of about 100 .mu.m and
 a length of about 500 pim. The beams were defined in the POLY1 and POLY2
 layers. Beams 19, 26 also had a Cr/Au layer deposited thereon to create
 intrinsic stresses, making them curl and move the structure.
 The hinged plates 20 had a width of about 300 .mu.m and a height of about
 70 .mu.m. The v-shaped notch had a notch height of about 50 .mu.m. The
 hinged plates 20 were defined in the POLY2 and POLY1 layers.
 The engagement plate 25 had a length of about 400 .mu.m and a width of
 about 150 .mu.m. Each v-shape notch had a notch height of about 70 .mu.m.
 The engagement plate 25 was defined in the POLY1 and POLY2 layers.
 The optical device was a mirror having dimensions of about 300
 .mu.m.times.300 .mu.m. The optical device was defined in the POLY1, POLY2,
 and Cr/Au layers.
 An edge of the optical device was coupled to the engagement plate with a
 spring 14. The spring is defined only in the POLY1 layer.
 The electrodes 27 were about 300 .mu.m long and about 200 .mu.m wide and
 were formed using the POLY0 layer.
 Each fabrication step mentioned above was performed at the MEMS Technology
 Application Center, MCNC, Research Triangle Park, North Carolina.
 After the electro-mechanical structure and the optical device were defined
 in the POLY0, POLY1, POLY2, OX1, OX2, and Cr/Au layers, the
 electro-mechanical structure and the optical device were released from the
 surface of the silicon substrate by etching the phosphosilicon glass
 layers in a bath of 49% HF at room temperature for about 1-2 minutes.
 When the phosphosilicon glass layers were then removed, the beams rotated
 the hinged plate and lifted the engagement plate as well as the optical
 device off of the substrate surface. The optical device was lifted to a
 height of about 50 .mu.m above the substrate surface.
 A voltage of about 100 V was applied between the optical device and pad 27
 on the substrate surface. After the voltage was applied between the
 optical device and pad 27, the optical device pivoted about axis 1-1'
 (FIG. 1), so that the optical device was at an angle of about 5.degree.
 with respect to the substrate surface.
 The micro-electro-mechanical optical device 12 is typically produced in
 bulk on a common substrate 13 and may be used, for example, in a
 wavelength routing device for directing multiple wavelengths among various
 optical fibers and devices. For such use, the multiple devices 12 are
 arranged in an array 80 on the substrate 13, as for example shown in FIG.
 9. As discussed above, each device 12 includes the
 micro-electro-mechanical structure 15 and the optical device 17 and is
 operated and maneuvered by applying appropriate electrical signals through
 electrodes 50 for activating the beams 19, 26 and elevating the optical
 device 17 away from the substrate 13. Springs 30 and electrodes 50, 52 are
 then used to provide tilt to the optical devices 17 with respect to the
 plane containing the substrate 13. In this manner, an array 80 of optical
 devices 17 can receive multiple optical channels from, for example, a
 demultiplexed signal provided on an optic fiber, and deflect each optical
 signal to a distinct output optic-fiber through angular displacement and
 adjustment of the individual optical devices 17.
 With reference now to FIG. 7, a preferred embodiment of the
 micro-electro-mechanical device 12 supported by structure 15 is depicted.
 Structure 15 has two pairs of beams 19 and 26, with each beam connected at
 its respective end 22 to a different plate 20 relative to the other beam
 in the corresponding pair. For example, plate 20a is moveable to an
 engagement position for raising device 17 out of the plane of the
 substrate 13 by beams 19a and 26a connected at their ends 22 to plate 20a.
 Similarly, plate 20b is moveable via the beams 19b and 26b connected
 thereto. The remaining beam ends 23 are fixed to substrate 13 and provide
 leverage for raising optical device 17. As explained in detail above, the
 beams are activated by applying electrical signals thereto, such as
 through a plurality of activation leads 54. For increased mobility and
 angular displacement or tilt, structure 15 includes an inner frame 16
 having an outer edge 91. Frame 16 is pivotally secured to plates 20a, 20b
 by springs 30 to provide for tilt of frame 16 along an axis parallel to
 directional axis X. A second pair of springs 32 pivotally connects optical
 device 17 to the inner frame 16 to provide tilt of device 17 about an axis
 parallel to directional axis Y.
 When an array of devices 12 are formed on a common substrate, as shown in
 FIG. 9, the beam arrangement of the device 12 shown in FIG. 7 limits the
 distance between adjacent devices because a fixed amount of substrate
 surface area is required to accommodate the beam pairs 19 and 26. In
 addition, since the substrate-fixed ends 23 of the beams in each pair are
 distally located from each other for a subject structure 15 (e.g., the
 ends 23 of beams 19a and 19b), the activation leads 54 are, likewise,
 distally spaced. To activate the beams, leads 54 must be gathered and
 provided to an edge of the substrate so that electrical signals can be
 applied thereto. Such an arrangement dictates the need for spaces or
 arteries 160, 162 formed on substrate 13 to accommodate mapping of the
 activation leads 54 to a location, such as at or proximate an edge of
 substrate 13, where the signals can be applied to such leads. The arteries
 160, 162 shown in the micro-electro-mechanical optical device array of
 FIG. 9 are configured as rectangular regions having widths w and w',
 respectively, which divide the array into four separate regions.
 The need for arteries 160, 162 to accommodate mapping of the activation
 leads 54, which is dictated by the layout or arrangement of the beam pairs
 19, 26, reduces the amount of substrate surface area available for
 supporting additional devices 12. Thus, and in accordance with another
 embodiment of the present invention, a modified micro-electro-mechanical
 optical device 112 having a form-fitting beam structure is provided and
 shown in FIG. 8. Device 112, like the device 12 described above, is formed
 on a substrate 113 and includes a micro-electro-mechanical structure 150
 for supporting and elevating an optical device 117 away from the
 substrate. For illustrative variation, optical device 117 is shown having
 an oval shape as opposed to the round shape of FIG. 7 or the square shape
 of FIG. 1. It will nevertheless be understood that any shape for device
 117 can be used without departing from the intended scope and
 contemplation of the present invention.
 A primary difference between the structure 15 of FIG. 7 and the structure
 115 of FIG. 8 lies in the configuration and arrangement of the beams 119
 and 126. In the embodiment of FIG. 8, beams 119 and 126 are disposed
 relative to the optic device 117 so that both ends of each beam (i.e. ends
 122, 123) are located proximate to the optic device 117. In other words,
 both ends are located proximate to an outer edge 190 of the device 117.
 This is contrary to the beam arrangement of FIG. 7 wherein only the first
 beam ends 22 are located proximate outer edge 90 of optic device 17. The
 form-fitting beam arrangement of FIG. 8 includes two pairs of form-fitting
 beams, namely a first pair formed of beams 119a, 119b and a second pair
 formed of beams 126a, 126b, with each beam being connected at a first end
 122 to an end of a respective plate 120a, 120b for rotating the plates and
 elevating the optic device 117, as discussed above. The second ends 123 of
 each beam in each beam pair are fixed to substrate 113 and are positioned
 in close proximity to the outer edge 190 of optical device 117. The second
 ends 123 of the beams in each pair (e.g., beams 119a, 119b) are also,
 preferably, positioned proximate to each other as shown. This form-fitting
 beam arrangement results in an advantageous reduction in the overall
 footprint or occupied substrate area for the modified
 micro-electro-mechanical device 112.
 With reference now to FIG. 10, an array of the form-fitting
 micro-electro-mechanical optical devices is there depicted. As shown, the
 array includes a plurality of devices 112 employing the form-fitting beam
 arrangement of FIG. 8. The locations of the beam ends 123 for each beam
 pair 119, 126 allow for a more economical layout of the individual devices
 112 and mapping arrangement for the activation leads 154. As a result, the
 need for the arteries 160, 162 shown in FIG. 9 is eliminated and the
 physical separation between adjacent devices 112 is reduced as compared to
 the necessary separation between adjacent devices in the array of FIG. 9.
 This, as will be appreciated, allows for an increase in the number of
 devices 112 accommodated by a single substrate 113.
 During operation of device 112, a signal may be applied to one or both
 spring pairs 130, 132 to cause tilt of the inner frame 116 and/or optic
 element 117. The amount of tilt is proportional to the strength of the
 signal applied to the springs. For a signal of particular strength, the
 outer edge 191 of the inner frame 116 or the outer edge 90 of the optic
 device may come into contact with the surface of substrate 113. When this
 occurs, an attraction or stiction force between the substrate and frame
 edge 91 or between the substrate and optic device edge 90 will retard or
 obstruct further movement of the frame or optic element, such as when the
 optic element 117 is to be moved back or returned to a horizontal position
 relative to the substrate 113.
 It has been discovered that the level or amount of stiction force present
 between two items is related to the amount of surface area in contact
 between the two items. Using this discovery, a preferred embodiment of the
 present invention includes a contouring element for limiting the contact
 surface area between the substrate and the outer edges of the inner frame
 and/or the optic device 117. This feature is shown in FIG. 11, which
 depicts a section of the optic device 117 and inner support 116. As shown,
 the leading edge 90 of the optic device 117 includes a plurality of
 radially-spaced protrusions or fingers 200 which extend outward away from
 the device 117. The leading edge 91 of inner frame 116 also includes a
 plurality of protrusions radially arranged and oriented in a direction
 away from the inner frame 116. When either the inner frame edge 91 or the
 optic element edge 90 comes into contact with substrate 113, the region of
 contact is reduced and occurs only between the substrate and the tips of
 the protrusions 200. This minimizes the stiction force and provides for
 increased maneuverability of the micro-electro-mechanical structures 115.
 Thus, while there have shown and described and pointed out fundamental
 novel features of the invention as applied to preferred embodiments
 thereof, it will be understood that various omissions and substitutions
 and changes in the form and details of the devices illustrated, and in
 their operation, may be made by those skilled in the art without departing
 from the spirit of the invention. For example, it is expressly intended
 that all combinations of those elements which perform substantially the
 same function in substantially the same way to to achieve the same results
 are within the scope of the invention. Moreover, it should be recognized
 that structures and/or elements shown and/or described in connection with
 any disclosed form or embodiment of the invention may be incorporated in
 any other disclosed or described or suggested form or embodiment as a
 general matter of design choice. It is the intention, therefore, to be
 limited only as indicated by the scope of the claims appended hereto.