(12) United States Patent
`Ford et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`Us 6,178,033 B1
`Jan. 23, 2001
`
`US006178033B1
`
`(54) MICROMECHANICAL MEMBRANE TILT-
`MIRROR SWITCH
`
`(55)
`
`References Cited
`u.s. PATENT DOCUMENTS
`
`(75)
`
`Inventors: Joseph E. Ford; James A. Walker,
`both of Monmouth, NJ (US)
`
`(73) Assignee: Luoent Technologies, Murray Hill, NJ
`(US)
`
`5,579,151 * 1l!1996 Che
`5,917,647 "
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`5,978,128 “ 11.51999 Yoon
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`
`
`359,091
`..... .. 3593298
`359.3298
`
`“‘ cited by examiner
`Primary Exrtminer—Huy Mai
`
`(*) Notice:
`
`Under 35 U.S.C. 154(b), the term of this
`patent shall be extended for 0 days.
`
`ABSTRACT
`(57)
`A micromechanical tilt mirror device that includes a mem-
`
`M3“ 23» 1999
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`(52) U.S. Cl.
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`(58) Field of Search
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`359x241; 359,254; 359x290;
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`3591295, 298, 246, 251, 254, 247
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`on the membrane whereby it is tilted when the membrane is
`deformed by electrostatic forces. This mirror tilt is used to
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 1 of4
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`US 6,178,033 B1
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 2 of 4
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`US 6,178,033 B1
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`Jan. 23, 2001
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`Sheet 3 of4
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`U.S. Patent
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`Jan. 23, 2001
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`Sheet 4 of4
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`US 6,178,033 B1
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`1
`MICROMECHANICAL MEMBRANE TI1Il‘-
`MIRROR SWITCH
`
`FIELD OF THE INVENTION
`
`This invention relates to a tilt-mirror switch for use in
`
`steering an optical beam and more particularly to such a
`switch that uses as the mirror a coating on a thin membrane
`that is suspended and is subject to deflection by electrostatic
`forces.
`
`BACKGROUND OF THE INVENTION
`
`Tilt-rnirror switch arrays are becoming of increasing
`interest in systems that use optical beams either for trans-
`mission of information or for its control.
`
`The most common form of tilt-mirror in such arrays
`includes a substrate of which the top surface is mirrored to
`be highly reflective and the back surface is conductive to
`serve as an electrostatic plate. The substrate is suspended so
`that its center is supported on a fulcrum about which the
`substrate can pivot. Pairs of electrodes positioned on oppo-
`site sides of the fulcrum are used to create electrostatic
`
`forces that pivot the mirror between two stable positions,
`such that an incident beam can be reflected into a selected
`one of two difierent directions, depending on t.he voltage
`applied. By applying a control voltage to a selected pair of
`electrodes to have it attract by electrostatic forces the
`associated half of the substrate, the mirror can be tilted
`between the two reflective states. A major problem with such
`minors is the tendency of the mirrors, which are minute is
`size, to curl, which afiects both the direction in which the
`incident beam is reflected and the optical quality of the
`reflected beam.
`Another form of ruicroniirror for use as a variable reflec-
`tor in mirror arrays that is of current interest is one that
`involves a change in attenuation of an incident optical beam
`rather than a change in the direction of its refiection. Such
`a mirror is typically formed as a quarter-wave dielectric
`layer of a material, such as silicon nitride, and supported to
`act normally as a reflective mirror. Such a mirror is sym-
`metrically suspended over a conductive substrate, typically
`of doped silicon, by a fixed 1%: wavelength dielectric spacer,
`typically of a phosphosilicate glass (PSG). An electrode
`partially covers the membrane, leaving uncovered but sur-
`rounding a coated central portion that serves as the mirror.
`A voltage applied between the electrode and the underlying
`substrate creates an electrostatic force that, until eliminated,
`attracts the membrane symmetrically closer to the substrate.
`The membrane tension provides a linear restoring force
`when the electrostatic force is eliminated. When the mem-
`
`brane gap is reduced to about a half wavelength by the
`electrostatic force, the layer becomes an essentially antire-
`flective coating with close to zero reflectivity. The typically
`0.4 micrometer vertical deflection of the central portion is
`small compared to the typically 200-500 micrometer wide
`membrane. Mechanically,
`the device moves by elastic
`deformation, similar to a tuning fork. Electrically, the device
`behaves as a tiny capacitor with essentially zero-static power
`dissipation regardless of the reflectivity state.
`A more detailed description of such a device is found in
`our prior paper entitled, “Dynamic Spectral Power Equal-
`ization Using Micro-Optic Mechanics,” IEEE Photonics
`Technology Letters, Vol. 10, No. 10, October 1998, pps.
`1040-1042. In this device, the mirror coating to define the
`mirror area is centrally located on the membrane, and largely
`surrounded by an electrode so that the electrostatic force
`acting is relatively uniform over the surface of the mem-
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`brane. The change in spacing between the mirror coating and
`the substrate is relatively uniform over the entire area of the
`mirror coating so that there is little tilt in the mirror area. In
`this prior paper, we describe a wavelength division multi-
`plexer equalizer that utilizes such a device. Such an equal-
`izer depends primarily on control of the attenuation of the
`incident light.
`There are other mirror applications in which, instead of
`attenuation, defiecting or steering of the incident light is
`desired. To that end, it is desirable that the incident light be
`controllably steered, as by tilting by a prescribed amount,
`the mirror area on which the light is incident for reflection
`and possible redirection along a desired path with little
`attenuation. The present invention is primarily directed at a
`mirror for use in steering an incident beam.
`SUMMARY OF THE INVENTION
`
`In the present invention a membrane including a mirror
`area is suspended at its two ends over a substrate as in the
`prior art variable reflector discussed above, but modified for
`use as a tilt-mirror. To this end the mirror area is positioned
`asymmetrically on the membrane between the regions of
`suspension so that it is tilted a prescribed amount as the
`membrane is attracted and deformed asymmetrically by the
`electrostatic force between it and the substrate. Additionally,
`the electrode that overlies the membrane advantageously
`does not surround the mirror area but is positioned to
`augment
`the deformation tilt experienced by the mirror
`coating as the membrane is attracted.
`In particular
`embodiments, the angular tilt of the mirror can he further
`facilitated for a given electrostatic force, if desired, by
`appropriate thinning of the membrane at selected regions.
`Also in particular embodiments, the membrane is selectively
`braced to reduce the potential for curling of the mirror area
`during its deformation.
`The invention will be better understood from the follow-
`ing more detailed description taken in conjunction with the
`accompanying drawing.
`BRIEF DESCRIPTION OF THE DRAWING
`
`FIGS. 1A and 1B show in cross section the basic form of
`a single mirror device of the varied reflector prior art form
`in its two operating states, refiective and non-refiective.
`respectively.
`FIG. 2 shows in a similar cross section, the basic form of
`a single mirror device in accordance with the present inven-
`tion.
`FIGS. 3-5 show other tilt mirror embodiments of the
`invention.
`
`FIG. 6 shows schematically a two-dimensional tilt-mirror
`arrangement in accordance with the invention.
`DETAILED DESCRIPTION
`
`the prior art mirror
`With reference now to FIG. 1A,
`assembly 10 includes a substrate 12, typically of silicon
`doped to be conductive, dielectric spacers 14, three quarters
`of a wavelength thick, typically of a phosphosilicate glass
`(PSG), over which is suspended a thin membrane 16,
`typically of silicon nitride of a quarter wavelength thickness,
`that acts as a reflective dielectric mirror in its normal states,
`with a reflectivity, for example, of about 70 per cent. An
`electrode 18 largely overlies the membrane 14, leaving
`exposed only a small central enclosed portion 16A that acts
`as the mirror area.
`
`Upon the application of a. suitable d-c voltage, such as 30
`volts, between the electrode and the substrate, the central
`
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`US 6,178,033 B1
`
`3
`membrane area 16A is deformed uniformly by the electro-
`static forces created, emntially as shown in FIG. 1B. As
`discussed, the resulting change in the spacing between the
`substrate and the central minor area portion 16A of the
`membrane transforms the role of the central portion from
`that of a reflective minor coating to that of an antireflection
`coating in which state little incident light is refiected back.
`In FIGS. 2A and 2B are shown in the quiescent and
`deformed states, respectively, cross sections of the basic
`form of a tilt mirror device 20, in accordance with the
`present invention,
`that involves a change in direction of
`reflection of an incident optical beam. The mirror device
`comprises a substrate 22, typically of silicon, a dielectric
`spacer 24, a membrane 26 that includes a coated portion 26A
`that serves as the mirror, and a top electrode 28 that only
`partially overlies the membrane 26. In this device, however,
`the coated region 26A of the membrane is not centrally
`located, as in the prior art, but is asymmetrically located near
`the left edge. Moreover. the electrode 28 is located on the
`membrane 26 only in a region right of the mirror portion
`26A. As a result, as seen in FIG. 2B, the mirror 26A, when
`deformed, is tilted from the horizontal, such that incident
`light would be reflected at an angle different from the normal
`in a new direction is as seen in FIG. 2B, and such deflected
`light can be selectively captured for utilization.
`In the
`absence of a deforming force, as in the case of FIG. 2A, the
`refiected beam is normal to the mirror coating and so it is
`refiected back along the direction of incidence.
`In actual practice, a simple suspended membrane under-
`going deflection will tend to have a slight curvature over the
`deflected length including the mirror rather than the flat
`surface that would he more desirable for controlled deflec-
`
`tion. Also differential then'nal expansions of the different
`materials used can lead to curling with variations in the
`operating temperature. Several techniques may be employed
`to keep flatter the mirror portion. In particular a bossed area
`surrounding the mirror portion of the membrane can help to
`keep flat the mirror area. Also by thinning the membrane
`appropriately, the flatness of the mirror area during defor-
`mation can be improved. A structure with a balanced mate-
`rial configuration on top and bottom of the membrane can
`help avoid curling due to temperature changes. Moreover, by
`isolating the mirror region physically from the rest of the
`membrane, curling can be minimized.
`The foregoing techniques will be described with reference
`to the remaining figures but it will be helpful first to describe
`a typical process for forming the basic device. The typical
`processing involves techniques that are now well established
`for the preparation of micromechanical devices and largely
`involve technologies developed originally for use in the
`manufacture of integrated circuits.
`The mirror devices are formed in large arrays typically by
`first coating a wafer, preferably of doped silicon, or of
`undoped silicon including a conductive coating over its top
`surface, with a layer of a dielectric material that can be easily
`etched, such as a phosphosilicate glass (PSG), of appropriate
`thickness to provide the spacers, and this PSG layer is
`covered in turn with a film suitable to serve as the
`membrane, typically of a material such as silicon nitride,
`undoped polysilicon or a silicon nitride-polysilicon compos-
`ite.
`
`This film is then patterned both to form the desired
`geometry of the mirror devices and to provide access holes
`in the film that will permit attack of the underlying PSG
`layer by a wet etch to leave a membrane suspended at its
`ends between pairs of PSG spacers. Then the mirror and
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`electrode matings are deposited and appropriately patterned
`on top of the structural film. Finally the wafer is immersed
`in the wet etch, typically hydrofluoric acid (HF), which
`selectively removes the PSG, to allow isotropic undercutting
`of the mechanically active membrane regions, thereby form-
`ing the tilt mirror array. Typically the mirror and electrode
`coatings are thin layers of gold to provide both the desired
`physical properties and to be resistant to the HF etch. As a
`possible modification, the deposition and patterning of the
`mirrors and electrodes may occur afier the sacrificial wet
`etch, if there is potential incompatibility between the metals
`to be used for the coatings and the wet etch.
`In FIG. 3, there is shown a single mirror device 30, in
`accordance with the invention, that utilizes a bossed mirror
`area. Generally, as discussed earlier, such devices will be
`assembled in large arrays, either one-dimensional or two-
`dimensional arrays. The device 30 includes a substrate 31,
`typically of silicon about 20 mils thick. The silicon advan-
`tageously is doped to be conductive to serve as one electrode
`of the capacitor that is to be formed. Alternatively, it can
`include a conductive coating over its top surface. Dielectric
`spacers 32A, 32B at opposite ends of the substrate support
`the membrane 33 over the substrate. As described earlier, the
`dielectric supports typically are of PSG, advantageously
`deposited by low pressure chemical vapor deposition
`(LPCVD), of appropriate thickness, typically less than 20
`microns thick. The membrane may typically be of silicon
`nitride polysilicon or a silicon nitride-polysilicon composite,
`of appropriate thickness (0.1—4 microns thick), and gener-
`ally a fraction of the PSG thickness. The membrane thick-
`ness needs to be sufficient to maintain the membrane essen-
`tially rigid with little sag between the end spacers in the
`absence of an applied electrostatic force designed to deflect
`it. The membrane 33 supports asymmetrically, near one of
`its edges, a mirror area 35, advantageously defined either by
`a dielectric multilayer reflector or by a metal coating, as of
`gold several hundred Angstroms thick, that will be highly
`reflective of the incident light. Typically the mirror area will
`be about 213-200 microns on a side. To minimize curling of
`the mirror 35,
`it
`is enclosed within a bossed frame 36,
`typically provided by a patterned layer of polysilicon
`between one and three micrometers thick to be of sufficient
`rigidity to serve the intended stiffening role.
`The bossed frame 36 is shown here as overlying the
`membrane 30; however it could also be positioned to
`underlie the membrane. The latter position would be
`advantageous,
`if chemical mechanical polishing (CMP)
`were to be used to provide a flat mirror.
`The polysilicon frame 36 should surround the mirror-
`coated area as closely as is feasible with available technol-
`ogy. The membrane 33 also supports a patterned electrode
`coating 3? of a conductive material, also such as gold up to
`a few microns thick, to serve as the top plate of a capacitor
`with the conductive substrate 31 serving as the bottom plate.
`This electrode 3'? advantageously is positioned along the
`membrane 33 such that, when an appropriate voltage is
`applied between the two plates of the capacitor, the mem-
`brane 33 is deflected by the force concentrated at the
`electrode, and the mirror area 35 is tilted enough, typically
`a few degrees from the horizontal is sulficient, such that an
`incident light beam is steered sufiiciently away from the
`normal direction to be readily distinguished from a beam
`that is refiected when the membrane 33 is not deflected. If
`
`desired, the angular defiection or tilt of the mirror area 35
`can be increased, either by having the electrode 37 extend
`over a larger percentage of the length of the unsupported
`membrane or by asymmetrically locating the electrode 37
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`US 6,178,033 B1
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`5
`closer to where the minor area 35 is located. For a 100
`micron long, 50 micrometer wide, structural bean:t formed
`by the membrane between the two spaced supports, there is
`readily achievable an angular deflection of about 2 degrees
`of a 10-micrometer long mirror located 5 micrometers in
`from the left end with only a 5,000 Angstroms deflection
`from the quiescent undeflected state.
`The amount of deflection for a given force can be
`enhanced,
`if desired, by the addition of etched regions
`appropriately located in the membrane to reduce its thick-
`ness there. In FIG. 4, there is shown the tilt mirror device 40
`that in most respects resembles the tilt mirror device 30
`shown in FIG. 3. It includes the substrate 41 and spacers 42A
`and 42B. It diflfers only in the elimination of the frame layer
`36 of polysilicon and its replacement by notched, or thinned,
`regions 44A, 44B in membrane 44 on opposite sides of the
`coated rnirror area 45. Typically these notched regions can
`be about 10 micrometers wide, can extend across the full
`width of the membrane 44 as shown, and can serve to thin
`etfectively the thickness of the membrane 44 to a fraction,
`for example, about one half its original thickness. The use of
`the notches 44A, 44B should help in concentrating the
`bending action to the mirror region between the notches. An
`electrode 47 is used to control the bending, as before.
`Another technique that can be used to minimize curling of
`the mirror area is illustrated by the tilt-mirror device 50
`shown in FIG. 5.
`It
`includes the silicon substrate 51,
`dielectric supports 52A, 52B, and a membrane 55. In this
`embodiment, polysilicon layers 54A, 54B are provided on
`the lower and upper surfaces, respectively, of the membrane
`55 between its notched areas 55A and 55B. The mirror
`coating 57 is provided over a portion of the top layer 54A of
`the dual polysilicon layers. Again the dual layers and the
`mirror coating advantageously are asymmetrically located
`near one end of the beam formed by the suspended mem-
`brane 55. As before, an electrode 58 is provided asymmetri-
`cally over the membrane to serve as the upper plate of the
`capacitor fonned with the conductive substrate 51.
`FIG. 6 shows schematically a top view of a tilt-mirror in
`which the tilt can be in either of two essentially orthogonal
`dimensions. In this arrangement,
`the membrane 62 that
`supports the mirror 64 is held suspended over the substrate
`at four edges by two pairs of dielectric supports, disposed
`orthogonally with respect
`to one another, and separate
`electrodes 66, 68 are provided for separate control of each
`of the two possible orthogonal directions of deflection of the
`membrane. The mirror curl may be controlled in any of the
`ways discussed with reference to FIGS. 2-5.
`It is to be understood that the specific embodiments
`described are merely illustrative of the general principles of
`the invention and that a worker in the art could devise
`
`alternative embodiments without departing from the spirit
`and scope of the invention. In particular, tilt-mirror arrays of
`the kind described can find use in a wide variety of apparatus
`for use with optical signals, including Add,r‘Drop apparatus
`for use in WDM optical systems.
`What is claimed:
`
`1. A micromechanical tilt mirror device comprising:
`a conductive substrate;
`an insulative membrane including a mirror area;
`insulative spacers supporting the membrane over the
`conductive substrate normally in essentially a parallel
`relationship;
`an electrode on the conductive substrate for use with the
`conductive substrate in establishing an electrostatic
`force in response to an applied voltage for deforming
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`the membrane and tilting the mirror area of the mem-
`brane is assymetrically positioned between the ends of
`the membrane whereby the deformation of the mirror
`tilts the mirror area such that a light beam incident on
`the tilted mirror area is reflected in a predetermined
`direction;
`and means surrounding the mirror area for stiffening the
`flatness of the mirror area when tilted and reducing any
`curling of the mirror area.
`2. A micromechanical tilt-mirror device in accordance
`with claim 1 in which the stilfening means is positioned on
`the top surface of the membrane.
`3. A micromechanical tilt-mirror device in accordance
`with claim 2 in which the stiffening means is positioned on
`the bottom surface of the membrane.
`4. A micromechanical tilt mirror device in accordance
`with claim 1 in which the mirror area is located in the
`membrane between notched regiom of thinned membrane
`thickness.
`5. A micromechanical tilt-mirror device in accordance
`with claim 1 in which stilfening means are provided on both
`the top and bottom membrane surfaces.
`6. A micromechanical tilt mirror device in accordance
`
`with claim 1 in which the mirror area is defined by a
`reflective coating on the membrane.
`7. A electromechanical tilt-mirror device in accordance
`with claim 6 including means for providing stiffening to the
`mirror area to reduce curling of the mirror area.
`8.Amicromechanical tilt-mirror in accordance with claim
`1 in which the membrane thickness is between about 0.1 and
`4.0 microns.
`9. Amicromechanical tilt-mirror in accordance with claim
`8 in which the membrane is supported above the substrate a
`distance of between about 0.3 and 20 microns.
`
`10. An array of electromechanical tilt-mirrors on a com-
`mon substrate, each in accordance with the electromechani-
`cal tilt-mirror of claim 1.
`11. A electromechanical tilt-mirror device that can be
`tilted in either of two orthogonal directions comprising:
`a conductive substrate,
`a membrane including a mirror area asymmetrically dis-
`posed in two orthogonal directions along the
`membrane,
`two pairs of dielectric spacers disposed orthogonally for
`supporting the membrane along four edges defining the
`two orthogonal directions,
`and means for deforming the membrane and tilting the
`mirror selectively in either of the two directions.
`12. An electromechanical tilt-mirror device that can be
`
`tilted in either of two essentially orthogonal directions,
`comprising
`a conductive substrate,
`a membrane including a minor area,
`two pairs of dielectric spacers orthogonally disposed for
`supporting the membrane,
`separate pairs of electrodes for each of the two tilt
`directions, wherein the mirror area of the membrane is
`asymmetrically positioned on the membrane to provide
`two orthogonally different tilt directions.
`13. A electromechanical tilt-mirror in accordance with
`claim 12 in which the membrane includes means for main-
`
`taining the flatness of the mirror area.
`14. An array of electromechanical tilt-mirrors on a com-
`mon substrate each in accordance with the electromechani-
`cal tilt-mirror of claim 12.
`
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