Radio frequency (RF) switch comprising an electromagnet formed on a magnet, a transmission line formed on the electromagnet and having a ground line and a signal line, and a movable contact connected to either the ground line or the signal line and capable of electrically coupling the ground line with the signal line. The transmission line is capable of propagating a RF signal if the ground line is electrically decoupled from the signal line. Conversely, the transmission line is incapable of propagating a RF signal if the ground line is electrically coupled with the signal line. The electromagnet can comprise an electromagnetic coil, formed in a layer of dielectric material, electrically coupled to a current source. Preferably, the movable contact is capable of being magnetically actuated, and is latchable. The magnet, the electromagnet, the transmission line, and the movable contact can have dimensions at a micron order of magnitude. The transmission line can be a coplanar waveguide. The coplanar waveguide can include a ground plane positioned between a layer of dielectric material and the electromagnet. The coplanar waveguide can also include means to electrically couple the ground plane to the ground line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to radio frequency switches. More specifically, the present invention relates to a latchable, magnetically actuated, ground plane-isolated radio frequency microswitch.

2. Background Art

Switches are typically electrically controlled two-state devices that open and close contacts to effect operation of devices in an electrical or optical circuit. Relays, for example, typically function as switches that activate or de-activate portions of electrical, optical or other devices. Relays are commonly used in many applications including telecommunications, radio frequency (RF) communications, portable electronics, consumer and industrial electronics, aerospace, and other systems. More recently, optical switches (also referred to as “optical relays” or simply “relays” herein) have been used to switch optical signals (such as those in optical communication systems) from one path to another.

Although the earliest relays were mechanical or solid-state devices, recent developments in micro-electro-mechanical systems (MEMS) technologies and microelectronics manufacturing have made micro-electrostatic and micro-magnetic relays possible. Such micro-magnetic relays typically include an electromagnet that, when energized, causes a cantilever to make or break an electrical contact. When the magnet is de-energized, a spring or other mechanical force typically restores the cantilever to a quiescent position. Such relays typically exhibit a number of marked disadvantages, however, in that they generally exhibit only a single stable output (i.e., the quiescent state) and they are not latching (i.e., they do not retain a constant output as power is removed from the relay). Moreover, the spring required by conventional micro-magnetic relays may degrade or break over time.

Non-latching relays are known. The relay includes a permanent magnet and an electromagnet for generating a magnetic field that intermittently opposes the field generated by the permanent magnet. This relay must consume power in the electromagnet to maintain at least one of the output states. Moreover, the power required to generate the opposing field would be significant, thus making the relay less desirable for use in space, portable electronics, and other applications that demand low power consumption.

Furthermore, microwave switches have been realized in mechanical or semiconductor technologies. While mechanical switches are characterized by low signal loss and good isolation, they have slow switching speeds, consume considerable power, and are bulky. Conversely, while semiconductor switches (e.g., Field Effect Transistors, Positive-Intrinsic-Negative diodes, etc.) enjoy high switching speeds, low power consumption, and compactness, in their ON states they contribute to signal loss, and in their OFF positions they suffer from inferior isolation. They also have limited switching current capacities. Although developed for microwave frequencies, these switches can be used throughout the radio frequency (RF) spectrum.

However, the development of MEMS has yielded an opportunity to realize RF switches that capitalize on the desirable features of both mechanical and semiconductor switches, while limiting the unwanted characteristics of these earlier technologies. Particularly, a bi-stable, latching switch that does not require power to hold the states is desired. Such a switch should also be reliable, simple in design, low-cost and easy to manufacture, and should be useful in RF, optical, and/or electrical environments.

BRIEF SUMMARY OF THE INVENTION

The radio frequency (RF) switch of the present invention comprises an electromagnet formed on a magnet, a transmission line formed on the electromagnet and having a ground line and a signal line, and a movable contact connected to either the ground line or the signal line and capable of electrically coupling the ground line with the signal line. The transmission line is capable of propagating a RF signal if the ground line is electrically decoupled from the signal line. Conversely, the transmission line is incapable of propagating a RF signal if the ground line is electrically coupled with the signal line. The electromagnet can comprise an electromagnetic coil, formed in a layer of dielectric material, electrically coupled to a current source.

Preferably, the movable contact is capable of being magnetically actuated, and is latchable. However, non-latching embodiments are envisioned. Also, the magnet, the electromagnet, the transmission line, and the movable contact can have dimensions at a micron order of magnitude.

The transmission line can be a coplanar waveguide. The coplanar waveguide can comprise a layer of dielectric material formed on the electromagnet. The signal line, a first ground line, and a second ground line can be formed on the layer of dielectric material. The signal line can be positioned between the first ground line and the second ground line. The signal line can be separated from the first ground line and the second ground line. The coplanar waveguide can further comprise a ground plane positioned between the layer of dielectric material and the electromagnet. The coplanar waveguide can also comprise means to electrically couple the ground plane to at least one of the first ground line and the second ground line.

The movable contact can be a cantilever, comprising a magnet formed on a layer of conducting material, wherein the magnet is made of a permalloy. The cantilever can be elastic. The cantilever can be supported by lateral torsion flexures. At least one of the lateral torsion flexures can be capable of electrically coupling the cantilever with the first ground line or signal line to which the movable contact is connected. The cantilever can be supported by a post, which can also be elastic. The post can be capable of electrically coupling the cantilever with the first ground line or signal line to which the movable contact is connected.

The present invention also comprises a method for propagating a RF signal from a first transmission line to a second transmission line. The method comprises the steps of: (1) connecting a latchable, magnetically actuated RF switch between the first and the second transmission lines, and (2) electrically decoupling a ground line of the latchable, magnetically actuated RF switch from a signal line of the latchable, magnetically actuated RF switch.

The present invention also comprises a method of making a latchable, magnetically actuated RF microswitch. The method comprises the steps of: (1) forming a magnet on a substrate; (2) forming an electromagnet on the magnet; (3) forming, on the electromagnet, a transmission line with a ground line and a signal line; and (4) forming a movable contact connected to one of the ground line and the signal line.

The electromagnet can be formed by: (1) depositing a first layer of a dielectric material onto the magnet; (2) depositing a layer of a sacrificial material onto the first layer of the dielectric material; (3) forming a hole in the layer of the sacrificial material, such that the hole defines the electromagnet;and (4) depositing a conductive material in the hole. The sacrificial material can be removed and a second layer of the dielectric material can be deposited onto the first layer and the conductive material.

The transmission line can be formed by: (1) depositing a layer of a dielectric material onto the electromagnet; (2) depositing a sacrificial layer onto the layer of the dielectric material; (3) forming a first hole, a second hole, and a third hole in the layer of the sacrificial material, such that the first hole defines the ground line, the second hole defines the signal line, and the third hole defines a second ground line; and (4) depositing a conductive material in the first hole, the second hole, and the third hole, thereby forming the transmission line with the ground line, the signal line, and the second ground line. A layer of conductive material can be deposited onto the electromagnet before depositing the layer of dielectric material. The layer of dielectric material can then deposited onto the layer of conductive material. A fourth hole can be formed in the layer of the dielectric material. The fourth hole can be aligned with at least one of the first hole and the third hole. The conductive material can also deposited into the fourth hole. The sacrificial material can be removed.

The magnetically actuated, ground plane-isolated radio frequency (RF) microswitch of the present invention can be used in a wide range of RF products. The present invention has the advantages of compactness, low RF signal loss, high switching speed, low power consumption, and excellent isolation performance.

The preferred embodiment of the invention is described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digit of each reference number identify the figure in which the reference number is first used.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown and described herein are examples of the invention and are not intended to otherwise limit the scope of the present invention in any way. Indeed, for the sake of brevity, conventional electronics, manufacturing, microelectromechanical systems (MEMS) technologies and other functional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail herein. Furthermore, for purposes of brevity, the invention is frequently described herein as pertaining to a microelectronically-machined relay for use in electrical or electronic systems. It should be appreciated that many other manufacturing techniques could be used to create the relays described herein, and that the techniques described herein could be used in mechanical relays, optical relays or any other switching device. Further, the techniques would be suitable for application in electrical systems, optical systems, consumer electronics, industrial electronics, wireless systems, space applications, or any other application. Moreover, it should be understood that the spatial descriptions (e.g. “above”, “below”, “up”, “down”, etc.) made herein are for purposes of illustration only, and that practical latching relays may be spatially arranged in any orientation or manner. Arrays of these relays can also be formed by connecting them in appropriate ways and with appropriate devices.

Principle of Operation

The basic structure of the microswitch is illustrated inFIGS. 1A and 1B, which include a top view and a cross sectional view, respectively. The device (i.e., switch) comprises a cantilever102, a planar coil104, a permanent magnet106, and plural electrical contacts108/110. The cantilever102is a multi-layer composite consisting, for example, of a soft magnetic material (e.g., NiFe permalloy) on its topside and a highly conductive material, such as Au, on the bottom surface. The cantilever102can comprise additional layers, and can have various shapes. The coil104is formed in an insulative layer112, on a substrate114.

In one configuration, the cantilever102is supported by lateral torsion flexures116(seeFIGS. 1 and 2, for example). The flexures116can be electrically conductive and form part of the conduction path when the switch is closed. According to another design configuration, a more conventional structure comprises the cantilever fixed at one end while the other end remains free to deflect (i.e., a cantilever). The contact end (e.g., the right side of the cantilever) can be deflected up or down by applying a temporary current through the coil. When it is in the “down” position, the cantilever makes electrical contact with the bottom conductor, and the switch is “on” (also called the “closed” state). When the contact end is “up”, the switch is “off” (also called the “open” state). The permanent magnet holds the cantilever in either the “up” or the “down” position after switching, making the device a latching relay. A current is passed through the coil (e.g., the coil is energized) only during a brief period of time to transistion between the two states.

(i) Method to Produce Bi-Stability

The method by which bi-stability is produced is illustrated with reference toFIG. 3. When the length L of a permalloy cantilever102is much larger than its thickness t and width (w, not shown), the direction along its long axis L becomes the preferred direction for magnetization (also called the “easy axis”). When such a cantilever is placed in a uniform permanent magnetic field, a torque is exerted on the cantilever. The torque can be either clockwise or counterclockwise, depending on the initial orientation of the cantilever with respect to the magnetic field. When the angle (α) between the cantilever axis (ξ) and the external field (H0) is smaller than 90°, the torque is counterclockwise; and when a is larger than 90°, the torque is clockwise. The bi-directional torque arises because of the bi-directional magnetization (by H0) of the cantilever (from left to right when α<90°, and from right to left when α>90°). Due to the torque, the cantilever tends to align with the external magnetic field (H0). However, when a mechanical force (such as the elastic torque of the cantilever, a physical stopper, etc.) preempts to the total realignment with H0, two stable positions (“up” and “down”) are available, which forms the basis of latching in the switch.

(ii) Electrical Switching

If the bi-directional magnetization along the easy axis of the cantilever arising from H0can be momentarily reversed by applying a second magnetic field to overcome the influence of (H0), then it is possible to achieve a switchable latching relay. This scenario is realized by situating a planar coil under or over the cantilever to produce the required temporary switching field. The planar coil geometry was chosen because it is relatively simple to fabricate, though other structures (such as a wrap-around, three dimensional type) are also possible. The magnetic field (Hcoil) lines generated by a short current pulse loop around the coil. It is mainly the ξ-component (along the cantilever, seeFIG. 3) of this field that is used to reorient the magnetization in the cantilever. The direction of the coil current determines whether a positive or a negative ξ-field component is generated. Plural coils can be used. After switching, the permanent magnetic field holds the cantilever in this state until the next switching event is encountered. Since the ξ-component of the coil-generated field (Hcoil-ξ) only needs to be momentarily larger than the ξ-component (H0ξ˜H0cos(α)=H0sin(φ), where α=90°−φ) of the permanent magnetic field and (p is typically very small (e.g., (φ.5°), switching current and power can be very low, which is an important consideration in micro relay design.

The operation principle can be summarized as follows: A permalloy cantilever in a uniform (in practice, the field can be just approximately uniform) magnetic field can have a clockwise or a counterclockwise torque depending on the angle between its long axis (easy axis, L) and the field. Two bi-stable states are possible when other forces can balance the torque. A coil can generate a momentary magnetic field to switch the orientation of magnetization along the cantilever and thus switch the cantilever between the two states.

The above-described latching micro-magnetic switch is further described in international patent publications WO0157899 (titled Electronically Switching Latching Micro-magnetic Relay And Method of Operating Same), and WO0184211 (titled Electronically Latching Micro-magnetic Switches and Method of Operating Same), to Shen et al. These patent publications provide a thorough background on latching micro-magnetic switches and are incorporated herein by reference in their entirety. Moreover the details of the switches disclosed in WO0157899 and WO0184211 are applicable to implement the switch of the present invention as described below.

RF Switch

The latchable, magnetically actuated, ground plane-isolated radio frequency (RF) microswitch of the present invention can be used in a wide range of RF products. The present invention has the advantages of compactness, low RF signal loss, high switching speed, low power consumption, and excellent isolation performance.

High frequencies, and their concomitant large bandwidths, make microwave signals desirable for many applications. However, because they are characterized by short wavelengths, microwave signals are not readily processed by standard electronic, lumped circuit elements (e.g., resistors, capacitors, inductors, etc.). Rather than conveying signals along a conductor, microwave transmission lines (including waveguides) function by propagating waves through a dielectric. Often the propagated wave is shaped or constrained by one or more conductors within the dielectric or on its surface. When RF signals propagating along a transmission line encounter discontinuities, mode conversion—changing the waveform from one mode to another—typically occurs. Undesirably, mode conversion can lead to RF signal loss.

Traditionally, microwave switches have been realized in mechanical or semiconductor technologies. While mechanical switches are characterized by low signal loss and good isolation, they have slow switching speeds, consume considerable power, and are bulky. Conversely, while semiconductor switches (e.g., Field Effect Transistors, Positive-Intrinsic-Negative diodes, etc.) enjoy high switching speeds, low power consumption, and compactness, in their ON positions they contribute to signal loss, and in their OFF positions they suffer from inferior isolation. They also have limited switching current capacities. Although developed for microwave frequencies, these switches can be used throughout the RF spectrum.

The development of micromechanical devices and the integration of these with microelectronics to form microelectromechanical systems, or MEMS, has yielded an opportunity to realize RF switches that capitalize on the desirable features of both mechanical and semiconductor switches, while limiting the unwanted characteristics of these earlier technologies.

FIG. 4shows a cross sectional view of an embodiment of a RF switch400according to the present invention. RF switch400comprises a magnet402, an electromagnet404, a transmission line406, and a movable contact408. InFIG. 4, the x- and y-axes are as shown, while the z-axis (not shown) extends into the page and is perpendicular to both the x- and y-axes.

Electromagnet404is formed on magnet402. Transmission line406is formed on electromagnet404and has a first ground line410and a signal line412. Movable contact408is connected to either first ground line410or signal line412and can couple first ground line410with signal line412. Transmission line406can propagate a RF signal if first ground line410is electrically decoupled from signal line412. Conversely, transmission line406cannot propagate a RF signal if first ground line410is electrically coupled from signal line412.

Movable contact408can be magnetically actuated. Movable contact408can also be latchable. Magnet402, electromagnet404, transmission line406, and movable contact408can have dimensions at a micron order of magnitude such that RF switch400is a RF microswitch. Magnet400can be formed on a substrate414, and can be made of permalloy.

In an embodiment of the present invention, electromagnet404comprises an electromagnetic coil416formed in a layer of dielectric material418. Via holes in the layer of dielectric material418enable a conductor (not shown) to electrically couple electromagnetic coil416to a current source (not shown). The skilled artisan will appreciate other means by which electromagnetic coil416can be coupled to the current source.

In an embodiment, transmission line406is a coplanar waveguide420. Coplanar waveguide420comprises a layer of dielectric material422formed on electromagnet404. Signal line412, first ground line410, and a second ground line424are formed on the layer of dielectric material422. Signal line412is positioned between first ground line410and second ground line424. Signal line412is separated from each of first ground line410and second ground line424. Typically, signal line412has a width “S” and is separated from each of first ground line410and second ground line424by a distance “W”. The total separation between first ground line410and second ground line424is a separation “2b”. Traditionally, width S is expressed as a width “2a”. The characteristic impedance of coplanar waveguide420can be expressed as shown in Eq. (1):
Z0={30π/(εeff)1/2}{(K(k0′)/K(k0)}
where:K is the elliptic integral function,k0=S/(S+2W),k0′=(1−k02)1/2, andεeffis expressed as shown in Eq. (2):
εeff=(1+{(εr−1)K(k1)K(k0′)}/{2K(k1′)K(k0)}),
where:εris the relative permittivity of the dielectric material of the layer of dielectric material422,k1=sin h(πS/2h)/sin h{π(S+2W)/4h}, andk1′=(1−k12)1/2.

In another embodiment, coplanar waveguide420further comprises a ground plane426positioned between the layer of dielectric material422and electromagnet404.FIG. 5shows an angled view of an electric field distribution of a wave signal propagating in coplanar waveguide420with ground plane426. The wave signal propagates along the z-axis. Returning toFIG. 4, the characteristic impedance of coplanar waveguide420with ground plane426can be expressed as shown in Eq. (3):
Z0gp={60π/(εeffgp)1/2}(1/{(K(k)/K(k′)+K(k3)/K(k3′)}),
where:k=a/b,k3=tan h(πa/2h)/tan h(πb/2h),k′=(1−k2)1/2,k3′=(1−k32)1/2, andεeffgpis expressed as shown in Eq. (4):
εeffgp={1+εrK(k′)K(k3)/K(k)K(k3′)}/{1+K(k′)K(k3)/K)(k)K(k3′) }.
A comparison of Eq. (1) with Eq. (3) shows that, for the same values of S and W, coplanar waveguide420with ground plane426has a lower characteristic impedance than coplanar waveguide420without ground plane426. RF signal loss associated with mode conversion can occur where RF signals are transferred from one transmission line to another. In this situation, RF signal loss is a function of the difference between the characteristic impedances of the two transmission lines. Thus, for given values of S and W, it is desirable to have low values of characteristic impedance. This will likely limit the difference between the characteristic impedances of two connected transmission lines and, consequently, reduce RF signal loss.

Coplanar waveguide420with ground plane426can further comprise via holes in the layer of dielectric material422enable a conductor (not shown) to electrically couple ground plane426to at least one of first ground line410and second ground line424. The skilled artisan will appreciate other means by which ground plane426can be coupled to at least one of first ground line410and second ground line424. Experiments by the inventors have shown that electrically coupling ground plane426to at least one of first ground line410and second ground line424acts to reduce rectangular waveguide modes, spurious parallel plate modes, and mode conversion.

In an embodiment, movable contact408is a cantilever428. Cantilever428comprises a magnet (described above) formed on a layer of conducting material (described above). Preferably, the magnet is made of permalloy. Cantilever428can be elastic. Cantilever428can be supported by lateral torsion flexures (described above). At least one of the lateral torsion flexures is capable of electrically coupling cantilever428with first ground line410or signal line412to which movable contact408is connected. Alternatively, cantilever428is supported by a post430. Post430can be elastic. Post430is capable of electrically coupling cantilever428with first ground line410or signal line412to which movable contact408is connected.

Advantageously, the present invention allows RF switch400to be directly coupled to input and output transmission lines without discontinuities. For example, because electromagnet404is positioned beneath movable contact408, transmission line406of RF switch400can be directly coupled to input and output transmission lines without discontinuities. In contrast, if electromagnet404was positioned so as to surround movable contact408, there would be discontinuities between transmission line406of RF400switch and an input transmission line, and between transmission line406of RF400switch and an output transmission line. Such discontinuities might require the use of bonding wires or other means to bridge a propagating RF signal from the input transmission line across electromagnet404to transmission line406of RF400switch, and from transmission line406of RF400switch across electromagnet404to the output transmission line. Because such a configuration requires changes in the transmission media of the RF signal, the configuration could cause mode conversions and their associated RF signal losses. The present invention avoids the possibility of RF signal losses due to mode conversions.

Furthermore, where movable contact408consumes a given area of substrate414in the x-z plane, having electromagnet404positioned in a configuration that surrounds movable contact408would cause RF switch400overall to consume a larger area of substrate414. In contrast, by positioning electromagnet404beneath movable contact408, the present invention limits the area of substrate414consumed by RF switch400to be comparable to that of movable contact408. Where electromagnet404is realized as electromagnetic coil416, electromagnetic coil416can be shorter in length. A shorter length electromagnetic coil416can consume less power.

Additionally, ground plane426isolates the propagating RF signal from electromagnet404, magnet402, and substrate414. Particularly, where RF switch400is latchable, electromagnet404is energized only to change the position of movable contact408. This limits the strength of the magnetic field to which the propagating RF signal is exposed and limits the power consumed by RF switch400.

(i) Method for Coupling a RF Signal Between Transmission Lines

FIG. 6shows a flow chart of a method600for coupling a RF signal from a first transmission line to a second transmission line. In method600, at a step602, a latchable, magnetically actuated RF switch is connected between the first and the second transmission lines. At a step604, a ground line of the latchable, magnetically actuated RF switch is electrically decoupled from a signal line of the latchable, magnetically actuated RF switch.

(ii) Method of Making a Latchable, Magnetically Actuated RF Microswitch

FIG. 7shows a flow chart of a method700of making a latchable, magnetically actuated RF microswitch. In method700, at a step702, a magnet is formed on a substrate. The magnet can be formed by depositing a soft magnetic material onto the substrate. At a step704, an electromagnet is formed on the magnet. At a step706, a transmission line, with a ground line and a signal line, is formed on the electromagnet. At a step708, a movable contact, connected to the ground line or the signal line, is formed.

To further explain step704,FIG. 8shows a flow chart of a preferred method of forming the electromagnet. At a step802, a first layer of a dielectric material is deposited onto the magnet. At a step804, a layer of a sacrificial material is deposited onto the first layer of the dielectric material. At a step806, a hole is formed in the layer of the sacrificial material, such that the hole defines the electromagnet. At a step808, a conductive material is deposited in the hole. Optionally, at a step810, the sacrificial material is removed. Optionally, at a step812, a second layer of the dielectric material is deposited onto the first layer and the conductive material.

To further explain step706,FIG. 9shows a flow chart of a preferred method of forming the transmission line. Optionally, at a step902, a layer of conductive material is deposited onto the electromagnet. At a step904, a layer of a dielectric material is deposited onto the electromagnet or the layer of conductive material. At a step906, a sacrificial layer is deposited onto the layer of the dielectric material. At a step908, a first hole, a second hole, and a third hole are formed in the layer of the sacrificial material, such that the first hole defines the ground line, the second hole defines the signal line, and the third hole defines a second ground line. At a step910, a conductive material is deposited in the first hole, the second hole, and the third hole, thereby forming the transmission line with the ground line, the signal line, and the second ground line. In one embodiment, at a step912, a fourth hole is formed in the layer of the dielectric material. The fourth hole is aligned with at least one of the first hole and the third hole. In this embodiment, at a step914, the conductive material is deposited into the fourth hole. Optionally, at a step916, the sacrificial material is removed.

CONCLUSION

Although the present invention is described in relation to RF switches realized with coplanar waveguides, the skilled artisan will appreciate that the teachings of the present invention are not limited to this embodiment. The present invention can also be realized using other RF waveguide and transmission line technologies such as, but not limited to, open microstrips, covered microstrips, inverted microstrips, trapped inverted microstrips, striplines, suspended striplines, coplanar striplines, slotlines, grounded dielectric slabs, coaxial lines, two wire lines, parallel plate waveguides, rectangular waveguides, circular waveguides, ridge waveguides, dielectric waveguides, microshield lines, and coplanar waveguides suspended by membranes over micromachined grooves. Therefore, the present invention is not limited to coplanar waveguide RF switch embodiments.

Furthermore, the corresponding structures, materials, acts and equivalents of all elements in the claims below are intended to include any structure, material or acts for performing the functions in combination with other claimed elements as specifically claimed. Moreover, the steps recited in any method claims may be executed in any order. The scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given above. Finally, it should be emphasized that none of the elements or components described above are essential or critical to the practice of the invention, except as specifically noted herein.