Microelectromechanical RF switch

Abstract
A MEMS switch with a bridge having three symmetric arms each having one end connected to a support arrangement and another end integral with a common central bridge portion. First and second conductors are deposited on a substrate, with the first conductor having an end with an open area which encompasses a pull down electrode which is also on the substrate, and of a height less than that of the conductor. A control voltage applied to the pull down electrode causes downward movement of the bridge, to present a relatively low impedance, thereby allowing a signal to propagate between the first and second conductors, without the bridge touching the pull down electrode. Each of the arms is slotted to reduce curl-induced stiffness.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




The invention in general relates to miniature switches, and more particularly, to a MEMS switch useful in radar and other microwave applications.




2. Description of Related Art




A variety of MEMS (microelectromechanical systems) switches are in use, or proposed for use, in radar and communication systems, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they can have a relatively high off impedance, with a low off capacitance, and a relatively low on impedance, with a high on capacitance, leading to desirable high cutoff frequencies and wide bandwidth operation. Additionally, the MEMS switches have a small footprint, can operate at high RF voltages and are compatible with conventional integrated circuit fabrication techniques.




Many of these MEMS switches generally have electrostatic elements, such as opposed electrodes, which are attracted to one another upon application of a DC pull down control voltage. At least one of these DC pull down electrodes is on a substrate and an opposing electrode is defined on the underside of a moveable bridge above the substrate. Upon application of the DC pull down control voltage, the bridge is deflected down and the electrical impedance is severely reduced (either by capacitive coupling or by direct ohmic contact), between first and second spaced apart RF conductors on the substrate.




In some MEMS switches the particular bridge design creates asymmetric transverse and longitudinal vibration modes during operation. Switching between on and off states moves the bridge and excites vibration modes, which can lead to undesirable electrical impedance modulation. This impedance modulation is further increased with bridge designs that are laterally asymmetric, causing twisting modes to occur.




In addition, the bridge may be fabricated from different layers. Internal stresses in the bridge's arms can cause the bridge arms to curl and thereby stiffen. This stiffening due to stress-induced curling can increase the pull down voltage requirement by more than 100%. This is undesirable from an integrated circuit operating point of view.




In conventional capacitive type MEMS switches the portion of the conductor below the bridge is covered with a dielectric layer. Repeated application of the DC pull down voltage between the bridge and pull down conductor causes a charge build up in the dielectric. This charge build up in the dielectric may cause the bridge to stick and remain attracted to the conductor in an on condition, even after removal of the pull down voltage.




It is an object of the present invention to provide a MEMS switch which reduces or eliminates the undesired asymmetric transverse and longitudinal vibration modes in the bridge structure.




It is a further object to eliminate the sticking problem in a capacitive type MEMS switch produced by charge built up in the dielectric.




SUMMARY OF THE INVENTION




A MEMS switch is provided which includes a substrate member with first and second spaced-apart RF conductors deposited on the substrate. A bridge member having at least three radially disposed arms of equal length is connected to a support arrangement on the substrate, with each arm having one end connected to the support arrangement and a second end integral with a common central bridge portion having an undersurface. At least one of the arms is electrically connected to the second conductor. The first conductor has an end portion facing the undersurface of the central bridge portion, with the end portion of the first conductor being constructed and arranged to define an open area. A pull down electrode is disposed within the open area of the first conductor and is electrically isolated from the conductor. The height of the pull down electrode is less than that of the end portion. The central bridge portion is drawn toward the first conductor upon application of a control voltage to the pull down electrode, to vary the electrical impedance between the first and second conductors. The impedance is varied from a high value (off state) to a low value (on state) relative to the impedance of the conductors, thus allowing a signal to propagate between the first and second conductors.




Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while disclosing the preferred embodiment of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art, from the detailed description.











BRIEF DESCRIPTION OF THE DRAWINGS




The present invention will become more fully understood from the detailed description provided hereinafter and the accompanying drawings, which are not necessarily to scale, and are given by way of illustration only. In addition, the use of spatial terms such as top, bottom, above, below etc. is for ease of explanation and not as structural or orientation limitations.





FIG. 1

is a plan view of a MEMS switch in accordance with one embodiment of the present invention.





FIG. 2

is a view along line


2


ā€”


2


of FIG.


1


.





FIG. 3

is an isometric exploded view of the switch of FIG.


1


.





FIG. 4

is a partial view of an arm of a prior art MEMS switch.





FIG. 5

is a partial view of one of the arms of the bridge of the switch of FIG.


1


.











DESCRIPTION OF THE PREFERRED EMBODIMENT




Referring now to

FIGS. 1

to


3


, the improved MEMS switch


10


includes first and second spaced apart RF conductors


12


and


13


, typically


50


ohm microstrips for carrying and propagating microwave signals, and deposited on a substrate


14


(generally over an oxide or other insulator). Typical substrates include gallium arsenide, silicon, alumina or sapphire, by way of example.




Switch


10


includes a bridge member


16


having at least three radially symmetrically disposed arms


18




a,




18




b,


and


18




c


of equal length. For a three arm embodiment as illustrated, the arms would be 120Ā° apart. Each arm includes a respective first, or distal end,


20




a,




20




b,


and


20




c,


as well as a respective second, or proximate end,


21




a,




21




b


and


21




c,


with these second ends being integral with a common central bridge portion


22


. This bridge design reduces twisting and radially asymmetrical vibration modes.




As best seen in

FIG. 3

, each of the first ends


20




a,




20




b,


and


20




c


of arms


18




a,




18




b,


and


18




c


is connected to a metallic or non-metallic support arrangement


26


positioned on substrate


14


(generally over an oxide or other insulator). Support arrangement


26


extends, by way of example, in a generally ā€œCā€ shaped orientation, from end


20




c


to end


20




b


so as to support the bridge


16


over substrate


14


, with common central bridge portion


22


being positioned over an end


30


of first conductor


12


. The support arrangement


26


includes an opening


32


, for a purpose to be described hereinafter.




Conductive bridge segments may be added, electrically connecting arm


18




c


with arm


18




a


via segment


34


, and electrically connecting arm


18




b


with arm


18




a


via segment


35


. If the support arrangement


26


is of a non-conductive material, a conductive segment


36


is added to complete the electrical connection with second conductor


13


. It is to be noted that the added current path length through segment


34


or


35


is small relative to the wavelength of the microwave signal being switched.




Although a generally C-shaped support arrangement is illustrated by way of example, other arrangements are possible. For example, support segments could extend linearly between the distal ends of the arms, instead of curving. Further, the support arrangement could be comprised of individual support posts, one under each of the distal ends of the bridge arms. In the latter case, segments


34


and


35


would be eliminated.





FIG. 4

illustrates a segment of a typical prior art bridge arm


40


. Bridge fabrication and/or a multilayer structure, produces stresses in the metal arm


40


and may cause the arm to curl, as indicated by the curl radius R, and thus stiffen to an objectionable degree. Controlling the internal stresses is difficult and the stiffening due to stress-induced curling can significantly increase the pull down voltage required for on/off switch operation. It may be shown that the degree of arm stiffening is directly related to the moment of inertia of the arm, and that curling increases this moment of inertia.




The present invention substantially reduces the effects of arm (and therefore, bridge) stiffening due to stress, and to this end reference is additionally made to

FIG. 5

which illustrates a sectional view of a portion of arm


18




a,


exemplary of all three arms.




The arm


18




a


includes a longitudinal slot


42




a


which lies along an axis A and extends generally from the support


26


to the common central bridge portion


22


. If curling of the arm occurs, the provision of slot


42




a


significantly reduces the effect of curling-induced stiffening, thus allowing for reduced pull down voltage requirements.




Curling however can not occur where the arm initially meets and is joined to the support


26


, as indicated by dotted line juncture


44




a.


Since there is no stiffening due to curling, arm


18




a


is accordingly weaker at this position and can potentially exceed its elastic limit during fabrication and/or continued operation of the switch. To prevent potential permanent deformation of the arm at this juncture


44




a,


there is provided individual stiffener elements


48




a


and


49




a,


each of which straddles the juncture


44




a


on either side of axis A, thereby eliminating the weak regions.




Referring once again to

FIGS. 1

to


3


, end portion


30


of first conductor


12


is constructed and arranged to define an open area


56


. Disposed within the open area


56


is a pull down electrode


58


of a height less than that of the end portion


30


and is electrically insulated from conductor


12


. A pad


60


, to which the pull down voltage is applied, is connected to the pull down electrode


58


via a thin film resistor


62


which passes through the opening


32


in support


26


and through an opening


64


in end portion


30


. The resistor


62


is intended to essentially eliminate loading on the microwave signals and should be of a relatively high impedance value with respect to the 50 ohm conductor impedance. If desired, the switch may be fabricated such that resistor


62


tunnels under support


26


and end portion


30


, thus eliminating openings


32


and


64


.




If the switch


10


is a capacitive type MEMS switch, then a dielectric layer


66


is deposited over the end portion


30


, but not over the open area


56


. When a pull down voltage is applied to pull down electrode


58


there is an electrostatic attraction with the undersurface


70


of the common central bridge portion


22


drawing it down to make contact with the dielectric layer


66


, acting as a mechanical stop. When contact is made, a capacitive electrical connection is made between the first and second conductors


12


and


13


.




If the switch is of the ohmic type, then no dielectric layer is present and the common central bridge portion


22


makes direct ohmic contact with the end


30


to complete an ohmic electrical connection between the first and second conductors


12


and


13


.




With a capacitive type MEMS switch, as illustrated by way of example in

FIGS. 1

to


3


, no pull down voltage is applied directly to the end


30


of first conductor


12


and accordingly, the pull down field is only between the common central bridge portion


22


and the pull down electrode


58


. Thus, there can be no electrical charge build up since no dielectric is deposited over pull down electrode


58


, or on the underside common area of the bridge


16


. Hence, the bridge


16


will not remain deflected after removal of the pull down voltage.




Although the end


30


of first conductor


12


is illustrated as being hexagonal, any design wherein the end of the conductor encompasses the pull down electrode is contemplated, including total or substantially total envelopment. Further, in order to ensure that the common central bridge portion


22


makes uniform contact with the end


30


, and does not experience any deflection, a stiffener


72


may be applied to the top surface of the common central bridge portion


22


.




Typical MEMS switches are generally made utilizing conventional well-known integrated circuit fabrication techniques. During the switch fabrication process, certain solvents are used to remove unwanted material. Surface tension effects, as a result of the solvents, can force the arms


18




a,




18




b


and


18




c


toward the substrate


14


to a degree where the elastic limit of the arms may be exceeded, thereby causing permanent deformation. To obviate this possibility, switch


10


is fabricated to include bumpers


74


positioned below respective arms


18




a,




18




b


and


18




c


to limit downward travel of the arms during the fabrication process.




The foregoing detailed description merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.



Claims
  • 1. A MEMS switch comprising:a) a substrate member; b) first and second spaced-apart RF conductors deposited on said substrate; c) a support arrangement on said substrate; d) a bridge member having at least three radially disposed arms of equal length, each said arm having one end connected to said support arrangement and a second end integral with a common central bridge portion having an undersurface; e) at least one of said arms being electrically connected to said second conductor; f) said first conductor having an end portion facing said undersurface of said central bridge portion; g) said end portion of said first conductor being constructed and arranged to define an open area; h) a pull down electrode disposed within said open area of, and electrically isolated from, said first conductor, and being of a height less than that of said end portion of said first conductor; i) said central bridge portion being drawn toward said first conductor upon application of a control voltage to said pull down electrode, to present a relatively low impedance, allowing a signal to propagate between said first and second conductors.
  • 2. A MEMS switch according to claim 1 wherein:said bridge member (d) has three arms spaced 120° apart.
  • 3. A MEMS switch according to claim 2 wherein:said support arrangement (c) extends between a first and second of said arms, and between said first and a third of said arms of said bridge member (d).
  • 4. A MEMS switch according to claim 3 and further comprising:m) an electrical path which extends between said first and said second arms, and between said first and said third arms of said bridge member (d).
  • 5. A MEMS switch according to claim 1 wherein:each said arm of said bridge member (d) lies along a longitudinal axis; and, each said arm includes a slot positioned along said longitudinal axis to reduce arm curling-induced stiffness.
  • 6. A MEMS switch according to claim 1 wherein:each said arm of said bridge member (d) includes at least one stiffener member positioned on the arm where it connects to said support arrangement.
  • 7. A MEMS switch according to claim 6 wherein:each said arm of said bridge member (d) lies along a longitudinal axis; and, each said arm includes two said stiffener members, one on either side of said longitudinal axis.
  • 8. A MEMS switch according to claim 1 and further comprising:j) a stiffener member positioned on said common central bridge portion of said bridge member (d).
  • 9. A MEMS switch according to claim 1 and further comprising:k) a dielectric layer disposed on said end portion of said first conductor, of said first and second spaced-apart RF conductors (b), such that a capacitive connection is made between said first and second conductors upon application of said control voltage.
  • 10. A MEMS switch according to claim 1 and further comprising:l) a pad deposited on said substrate member (a) for receiving said control voltage; and m) a thin film resistor connecting said pad (l) with said pull down electrode (h).
  • 11. A MEMS switch according to claim 1 and further comprising:o) bumper members positioned below said arms to limit downward travel thereof so as to prevent each said arm from exceeding its elastic limit during switch fabrication.
CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related in subject matter to patent application Ser. No. 10/157,935, filed May 31, 2002, and to patent application Ser. No. 10/322,728, filed Dec. 19, 2002 {NGC case 000251-078}, filed concurrently herewith, all of which are assigned to the same assignee as the present invention.

US Referenced Citations (1)
Number Name Date Kind
6384353 Huang et al. May 2002 B1