EP1093142A2 - Dual motion electrostatic actuator design for mems micro-relay - Google Patents

Abstract

A dual micro-electromechanical actuator for providing dual actuation to amicromachine. The actuator includes a first substrate upon which a first actuatingelectrode is formed. A second substrate located proximate to but spaced from the firstsubstrate supports a second actuating electrode. A micromachine device, such as a thinmembrane, plate or cantilever, is disposed between the actuating electrodes. When oneof the actuating electrodes is selectively activated by applying a voltage thereto, anelectrostatic attraction force is produced which causes movement of the micromachine inthe direction of the activated actuating electrode.

Description

Field Of The Invention
• The present invention relates to micro-electromechanical systems (MEMS)and, more particularly, to electrostatic actuation of MEMS devices in flip-chip bondedgeometry.
Description Of The Related Art
• Micro-electro-mechanical Systems (MEMS) have widespread uses incommunications systems for performing, among other things, switching, relaying andwavelength routing functions. Electrostatic actuation is used to impart relativemovement to thin membranes used in such devices. When thin membranes are broughtinto contact over a large area, the membranes tend to stick to each other due tosurface-related attractive forces. Once this happens, it is difficult to separate themembranes due to the large surface-to-volume ratio, and the flexibility of the thinmembranes. In surface micromachined devices where the mechanical devices aretypically thin plates of structural materials like poly-crystalline silicon, thisphenomenon is referred to as stiction. Since electrostatic actuation can only provide anattractive force, it is usually impossible to recover, e.g. to reset a device, once amicromachine sticks to the substrate. The only known way to recover is tomechanically detach the micromachine from the substrate with a needle probe.
• Known MEMS relay devices use electrostatic actuation for moving twomembranes into contact with each other to establish an electrical connection. Typicalrelays depend on mechanical restoration or spring forces to separate the two surfaceswhen electrical isolation between the two surfaces is necessary. For example, a knownMEMS device 10 is shown in FIG. 1. The device includes asubstrate 12 and a mobilemicromachine, such as acantilever 14 having amovable end 18 and a fixedend 16secured tosubstrate 12. Thecantilever 14 is controlled by an actuatingelectrode plate 20 which provides an electrostatic pulling force oncantilever 14 for movingedge 18downward tosubstrate 12 when a voltage is applied between thecantilever 14 and theactuatingelectrode 20. Acontact electrode 22 is disposed underneath thecantilever 14and serves as a switch contact for closing a switch when thecantilever end 18 is in afirst position (an "on" condition) and for opening the switch when thecantilever end 18is in a second position (an "off" condition). To maintain thedevice 10 in the onposition, a voltage must be continuously applied to the actuation electrode. When thevoltage is no longer applied, (i.e. the switch is to be turned off), spring force in thecantilever causesend 18 to return to the second position.
• When two parallel metallic plates (e.g. membranes) of areaA separated by adistanced are placed in a vacuum (or air), the capacitanceC between the two plates isgiven by

  

  whereε₀ is the electrical permitivity of the vacuum, in the limitA>>d².When a voltagedifference ofV is applied between these two plates, there is an attractive forceF of

  

  that pulls the two plates toward each other. This force is widely used to actuate metallicmicromachines, since it is very simple to implement and the force can conveniently beprovided by application of a voltage. Furthermore, no power is dissipated as long as themicromachine is not moving, since no current actually flows through the device.
• Although this actuation mechanism has many advantages, it suffers from a fewlimitations. The most significant is that it can only provide an attractive force, and theresulting motion is thus in or toward a single direction. Another limitation is thatwhen the contact material wears, the surface attraction force tends to increase. At thesame time, and in particular for the device depicted in FIG. 10, as the mechanicalcantilever fatigues, the restorative force tends to decrease. The combined effect causesthe micro-relay to stick, resulting in device operation failure. Such failures are common in many MEMS devices where stiction destroys the mobility of themechanical parts.
Summary Of The Invention
• A dual motion micro-electromechanical actuator is disclosed for impartingcontrolled motion in both first and second directions to a micromachine, such as adiaphragm or cantilever. The actuator includes a first substrate upon which a firstactuating electrode is formed, and a second substrate spatially separated from the firstsubstrate, upon which a second actuating electrode is formed. A micromachine isdisposed between the first and second actuating electrodes. When one of the actuationelectrodes is selectively activated, such as by the application of a voltage, an electrostaticattraction force is produced between the micromachine and the activated electrode formoving the micromachine in the direction of the activated electrode, i.e. toward eitherthe first substrate or the second substrate.
• Such actuation mechanism can be used in a MEMS relay. In a preferredembodiment of the relay, the micromachine is configured as a cantilever having one endfixed to the first substrate, with the other end moveable between the first and secondsubstrates. A pair of contact electrodes are also included. One of the contact electrodesis supported by the second substrate and the other contact electrode is supported by themicromachine. When the micromachine is moved toward the second substrate, electricalcontact occurs between the contact electrodes, and when the micromachine is movedtoward the first substrate, electrical contact is broken between the contact electrodes.
• Other objects and features of the present invention will become apparent fromthe following detailed description considered in conjunction with the accompanyingdrawings. It is to be understood, however, that the drawings are designed solely forpurposes of illustration and not as a definition of the limits of the invention, for whichreference should be made to the appended claims.
Brief Description Of The Drawings
• In the drawings, wherein like reference numerals denote similar elementsthroughout the views:
• FIG. 1 is a schematic representation of a prior art micro-electromechanicalrelay.
• FIG. 2 depicts a dual electrostatic actuator in accordance with the presentinvention;
• FIGs. 3a and 3b are schematic representations of the portions of a dualactuator section for use with a presently preferred embodiment of the presentinvention;
• FIG. 4 schematically illustrates a dual actuator micro-electromechanical relayin accordance with a preferred embodiment of the present invention;
• FIG. 5 is a cross-sectional view of the device of FIG. 4 taken along the line A--A'thereof; and
• FIG. 6 is a cross-sectional view of the device of FIG. 4 taken along the line B--B'thereof.
Detailed Description Of The Currently Preferred Embodiments
• With reference now to FIG. 2, the general concept of the inventive dualelectrostatic actuator is demonstrated by an arrangement that includes three spaced-apartand parallel arranged conduction plates, namely an upperactuation electrode plate 42, aloweractuation electrode plate 44 and amiddle electrode plate 46. The upper and lowerelectrode plates are attached to separate substrates (not shown) and the middle plate ismechanically mobile relative to the top and bottom plates. The middle plate is referredto as a micromachine and may be configured, for example, as a diaphragm, a cantilever,or other movable type component. A first voltage source applies a voltage V₁ between thetop plate 42 andmicromachine 46 and a second voltage source applies a voltage V₂between thebottom plate 44 andmicromachine 46.
• To impart motion tomicromachine 46 in an upward (in the drawing) directiontowardupper plate 42, a positive (or negative) voltage V₁ is applied while V₂ is 0. Tomove themicromachine 46 in the illustrated downward direction towardlower plate 44,a positive (or negative) voltage is applied for V₂ while voltage V₁ is 0. In this manner,a positive actuation force may be imparted to the micromachine in either selecteddirection (upward or downward) depending on the desired direction of movementthereof.
• In the preferred embodiment, the sizes of theplates 42, 44 and 46 range fromabout 1 to 10,000 microns per side, with a thickness of between about 0.01 to 10microns. The spacing between the plates is between about 0.5 and 500 microns. In thisrange, the values for the voltages V₁ and V₂ are between about 0.1 and 500 V.
• FIGs. 3a and 3b show a presently preferred embodiment of a MEMS relay. Thedevice, like the prior art of FIG. 1, includes asubstrate 12 and a movable micromachine,such as acantilever 14 having amovable end 18 and a fixedend 16 secured tosubstrate12. Thecantilever 14 is controlled by an actuatingelectrode plate 20 which provides anelectrostatic pulling force oncantilever 14 for movingedge 18 downward tosubstrate 12when a voltage is applied between thecantilever 14 and the actuatingelectrode 20. Acontact electrode 22 is disposed on thecantilever 14 and serves as a switch contact. Asecond actuator section 50 for use with thesingle actuator 10 of FIG. 3a is depicted inFIG. 3b and includes asecond substrate 52, asecond actuating electrode 54 and asecondcontact electrode 56 having anelectrode head 58. The second activating electrodefunctions as theupper plate 42 of FIG. 2 in that when a voltage is applied, amobilemicromachine 14 disposed in proximity to thesecond actuating electrode 54 will undergoan electrostatic pulling force. When thesecond actuator section 50 is combined with thesingle actuator 10 of FIG. 3a, adual actuator device 60 is formed, as shown in FIGs. 4-6.For ease of illustration,second substrate 52 is not depicted in FIG. 4.
• Thedual actuator device 60 is a mechanical relay employing acantilever 14 asthe mobile micromachine actuating member. The cantilever may be fabricated of aconductive material or of an insulating material upon which a conductive material isdeposited.End 16 ofcantilever 14 is fixed to thelower substrate 12 and the firstactuating electrode is disposed between the lower substrate and the cantilever. Acontactelectrode 22 is formed on the cantilever for functioning, in this particular describedembodiment, as a section of a relay switch. As explained above, when a voltage isapplied to actuatingelectrode 20, an attraction force is produced for causingcantilever14 to pivot about fixedend 16 so that movingend 18 is pulled down towardlowersubstrate 12.
• Theadditional actuator section 50 and, in particular, thesecond actuatingelectrode 54 havingsecond contact electrode 56 disposed thereon, is spatially disposedabovecantilever 14 as shown in FIG. 4. To cause electrical connection between thecontact electrodes 22, 56, a positive voltage is applied between actuatingelectrode 54and themovable cantilever 14 which responsively generates an attraction electrostaticforce for pullingcantilever 14 towardsecond substrate 52. This, in turn, causescontactelectrode 22 disposed oncantilever 14 to become electrically connected to contactelectrode 56 without relying on an inherent spring force of thecantilever 14. Tomaintain electrical contact, a positive voltage will continue to be applied tosecondactuating electrode 54. When electrical contact betweenelectrodes 22 and 56 isthereafter no longer desired, the voltage will no longer be applied tosecond actuatingelectrode 54 but will, instead, be applied between first actuatingelectrode 20 and themovable cantilever 14 for causingcantilever 14 to move toward thefirst substrate 12,thereby causing physical separation between thecontact electrodes 22, 56.
• Another desired characteristic for a relay is the high open-state maximumvoltage characteristic. This is the voltage that can be applied across the contactelectrodes of the relay when the switch is open, without changing the relay status ordamaging the relay itself. For electrostatic actuators, a voltage across the contactelectrodes themselves can act as an actuation force, that is, a voltage applied between the two contact electrodes will pull the contact electrodes together and tend to close theswitch. Therefore, any design optimization to reduce the actuation voltage will also, ingeneral, decrease the open-state maximum voltage in electrostatically actuated MEMSrelays. One added advantage of the inventivedual actuator design 60 is that the secondactuation electrode 54 (used to actively open the switch) can be actively biased duringthe open state to counteract the force generated by large voltage difference across thecontact electrodes. This effectively increases the open-state maximum voltage.
• The preferred embodiment for the dualelectrostatic actuator 60 utilizes a flip-chipbonded geometry. In this geometry, thelower actuation electrode 20 and themobile micromachine 14 are fabricated on a single substrate (e.g., substrate 12) usingsurface micromachining technology, while theupper actuation electrode 54 is fabricatedonsecond substrate 52. For proper operation, theupper actuation electrode 54 is to beassembled in an appropriate location with respect to themobile micromachine 14 and thelower actuation electrode 20 by means of flip-chip bonding so that the producedelectrostatic actuation force will be properly directed. Spacers (not shown) of accuratethickness can be disposed between the upper and lower substrates to control the gap orspacing between themobile micromachine 14 andupper actuation electrode 54; thespacing or gap between themobile micromachine 14 andupper actuation electrode 54determines the amount of force produced by the upper actuation electrode for a givenvoltage.
• To assure electrical isolation between the various conductive components, thepreferred embodiment includes insulatinglayers 64, 65 and 66. The insulating layers arepreferably formed of silicon dioxide or silicon nitride. As shown in FIGs. 5 and 6,insulatinglayer 64 electrically isolatessecond substrate 52 fromsecond actuatingelectrode 54, insulatinglayer 65 electrically isolates cantilever 14 fromcontact electrode22, and insulatinglayer 66 electrically isolatesfirst substrate 12 from thefirst actuatingelectrode 20.
• Also in the preferred embodiment, electrical shorting between the actuatingelectrodes 20, 54 and themobile micromachine cantilever 14 should be avoided because the high voltages required for actuation can cause spark welding of delicatemicromachined components. Electrical shorting can be avoided by imbedding theactuation electrodes under an insulatinglayer 68, 69, also preferably formed of silicondioxide or silicon nitride, or by shaping the actuation electrodes in such a manner thatany regions of contact between the actuator electrodes and the micromachine have apotential difference of zero.
• Thus, while there have shown and described and pointed out fundamentalnovel features of the invention as applied to preferred embodiments thereof, it will beunderstood that various omissions and substitutions and changes in the form anddetails of the devices illustrated, and in their operation, may be made by those skilledin the art without departing from the spirit of the invention. For example, it isexpressly intended that all combinations of those elements which perform substantiallythe same function in substantially the same way to achieve the same results are withinthe scope of the invention. Moreover, it should be recognized that structures and/orelements shown and/or described in connection with any disclosed form orembodiment of the invention may be incorporated in any other disclosed or describedor suggested form or embodiment as a general matter of design choice. It is theintention, therefore, to be limited only as indicated by the scope of the claimsappended hereto.

Claims (11)

1. A dual micro-electromechanical actuator, comprising:

   a first substrate;

   a first actuating electrode formed on said first substrate;

   a second substrate spaced from said first substrate;

   a second actuating electrode formed on said second substrate in spaced relation tosaid first electrode; and

   a micromachine disposed between said first and said second actuating electrodes,said first actuating electrode imparting an electrostatic attraction force to saidmicromachine in response to a voltage selectively applied to said first actuating electrodefor moving said micromachine to a first position in a direction toward said first substrate,and said second actuating electrode imparting an attraction force to said micromachine inresponse to a voltage selectively applied to said second actuating electrode for movingsaid micromachine to a second position in a direction toward said second substrate.
2. The dual micro-electromechanical actuator of claim 1, further comprising a firstcontact electrode supported by said micromachine and a second contact electrodesupported by said second substrate, said micromachine causing electrical contact betweensaid first and said second contact electrodes when moved to said second position, andcausing electrical isolation between said first and said second contact electrodes whenmoved to said first position.
3. The dual micro-electromechanical actuator of claim 1, wherein saidmicromachine comprises a diaphragm.
4. The dual micro-electromechanical actuator of claim 1, wherein saidmicromachine comprises a cantilever having a fixed end supported by said first substrateand a moveable end moveable between said first and said second positions.
5. The dual micro-electromechanical actuator of claim 2, wherein saidmicromachine comprises a cantilever having a fixed end supported by said first substrateand a moveable end moveable between said first and said second positions.
6. The dual micro-electromechanical actuator of claim 1, further comprising aninsulating material disposed between said first substrate and said first actuating electrode,and between said second substrate and said second actuating electrode.
7. The dual micro-electromechanical actuator of claim 2, further comprising aninsulating material disposed between said first substrate and said first actuating electrode,between said second substrate and said second actuating electrode, and between saidmicromachine and said first contact electrode.
8. The dual micro-electromechanical actuator of claim 1, further comprising meansfor preventing direct physical contact between said micromachine and said first actuatingelectrode, and between said micromachine and said second actuating electrode.
9. The dual micro-electromechanical actuator of claim 8, wherein said preventingmeans comprises an insulating material disposed between said micromachine and saidfirst actuating electrode, and between said micromachine and said second actuatingelectrode.
10. The dual micro-electromechanical actuator of claim 1, wherein saidmicromachine and said first actuating electrode are formed on said first substrate.
11. The dual micro-electromechanical actuator of claim 1, wherein said secondactuating electrode and said second substrate are flip-chip bonded to said first substrateand said first actuating electrode.

Images