Microelectromechanical switch

Abstract

Microelectromechanical system (MEMS) switch (20) comprising: a substrate (22); a beam electrode (26) arranged on the substrate (22); a beam (24) comprising at least one anchoring section (46) coupled to the beam electrode (26), a first beam section (48) extending from the at least one anchoring section (46) along a first direction, and a second beam section (50) extending from the at least one anchoring section (46) along a second direction opposite to the first direction; a first control electrode (28) which is arranged on the substrate (22) facing the first beam section (48); a first contact electrode (32) which is arranged on the substrate (22) facing the first beam section (48); a second control electrode (30) which is arranged on the substrate (22) facing the second beam section (50), wherein the first control electrode (28) and the second control electrode (30) are coupled to each other to form a gate electrode (58); and a second contact electrode (34) which is arranged on the substrate (22) facing the second beam section (50); characterized by an electrical connection between the first and second contact electrode (32, 34) and the beam electrode (26), wherein the electrical connection comprises one or more passive components selected from resistors, coils and diodes.

Description

GENERAL STATE OF THE ART

[0001] Microelectromechanical system (MEMS) devices have a wide range of applications and are widely used in commercial products. One type of MEMS device is a MEMS switch. A typical MEMS switch contains one or more MEMS switches arranged in an array. MEMS switches are well-suited for applications including mobile phones, wireless networks, communication systems, and radar systems. In wireless devices, MEMS switches can be used as antenna switches, mode switches, transmit / receive switches, and the like.

[0002] Typical MEMS switches use a cantilever made of electroplated metal supported at one end and an electrical contact located at the other end of the metal cantilever. A control electrode is positioned beneath the metal cantilever. A direct current ("DC") actuation voltage is applied to the metal cantilever via the control electrode, forcing the metal cantilever to bend downward and make electrical contact with a lower signal trace. Once electrical contact is established, the circuit is closed, and an electrical signal can pass through the metal cantilever to the lower signal trace.

[0003] One type of MEMS device is a MEMS radio-frequency (RF) switch. MEMS RF switches are used for wireless devices due to their low drive power characteristics and their ability to operate in high-frequency ranges. However, a common problem within MEMS RF switches is when a significant RF voltage is applied between a beam electrode and a contact electrode. Such a voltage can couple to the control electrode and cause the switch to self-actuate. In other words, these MEMS switches commonly experience a problem in which the cantilevered beam within the switch can be actuated in the "OFF" state (self-actuating) due to the high-voltage RF signal. The high-voltage RF signal thus generates sufficient electrostatic force to pull the switch beam downward and cause failure.

[0004] US 2009 / 0 159 409 A1 discloses a microelectromechanical system switch (MEMS switch) having the features of the preamble of independent patent claim 1.

[0005] From US 7 602 187 B2 the use of a MEMS switch in a magnetic resonance imaging system for decoupling a radio frequency receiving coil in the transmitting mode is known. SUMMARY

[0006] According to the present invention, a microelectromechanical system (MEMS) switch is disclosed. The switch comprises a beam electrode arranged on a substrate. A beam includes at least one anchoring portion coupled to the beam electrode, a first beam portion extending from the at least one anchoring portion along a first direction, and a second beam portion extending from the at least one anchoring portion along a second direction opposite the first direction. A first control electrode is arranged on the substrate facing the first beam portion. A first contact electrode is arranged on the substrate facing the first beam portion. A second control electrode is arranged on the substrate facing the second beam portion. The first control electrode and the second control electrode are coupled together to form a gate electrode.A second contact electrode is arranged on the substrate facing the second beam section. An electrical connection is provided between the first and second contact electrodes and the beam electrode, wherein the electrical connection comprises one or more passive components selected from resistors, coils, and diodes. DRAWINGS

[0007] These and other features and aspects of embodiments of the present invention will become more apparent upon reading the following detailed description with reference to the accompanying drawings, in which like reference numerals represent like parts throughout the drawings, and in which: Fig. 1 is a diagrammatic representation of a microelectromechanical system (MEMS) device for decoupling one or more surface coils of a coil system according to an exemplary embodiment of the present invention; Fig. 2 is a sectional view of a MEMS device including a MEMS switch system according to an exemplary embodiment of the present invention; Fig. 3 is a graphical representation of a MEMS switch according to an embodiment of Fig. 2; and Fig. 4 a schematic representation of a MEMS switch according to an embodiment of Fig. 2. DETAILED DESCRIPTION

[0008] According to embodiments of the present invention, a microelectromechanical system (MEMS) switch is disclosed. The MEMS switch includes a beam electrode disposed on a substrate. A beam includes an anchoring portion coupled to the beam electrode. A first beam portion extends from the anchoring portion along a first direction; and a second beam portion extends from the anchoring portion along a second direction opposite the first direction. A first control electrode and a first contact electrode are disposed on the substrate facing the first beam portion. A second control electrode and a second contact electrode are disposed on the substrate facing the second beam portion.

[0009] Referring to Fig. 1, a microelectromechanical system (MEMS) device 10 is disclosed for decoupling one or more surface coils 12 of a coil system 14 in a radio frequency (RF) device 15, e.g., a magnetic resonance (MR) imaging system. It should be noted that while an MR imaging system is disclosed herein, the MEMS device 10 may be used for other applications in other embodiments. For example, in another embodiment, the device 15 may be a radar system. In the illustrated embodiment, the MEMS device 10 enables switching to isolate one or more surface coils 12, specifically radio frequency (RF) magnetic resonance coils. In one embodiment, the MEMS device 10 is operable during an MR imaging transmit operation to decouple the surface coils 12 configured as receive surface coils.In one embodiment, the MEMS device 10 is in an open state during a transmit operation to decouple the surface coil 12 (an RF receive coil) from the coil system 14. The MEMS device 10 is in a closed state during a receive operation so that the surface coil 12 resonates and couples with the received MR signals, thereby transmitting the received MR signals to an RF receiver 16. The MEMS device 10 is controlled by a switch controller 18, which switches the MEMS device 10 from the open state to the closed state and vice versa. In some embodiments, the MEMS device 10 is in a normally open state (decoupled state) when the coil system 14 is not biased. In other embodiments, however, the MEMS device 10 is in a normally closed state when the coil system 14 is not biased.

[0010] It should be noted herein that in other embodiments, the MEMS device 10 may be used in conjunction with various types of magnetic resonance surface coils (also referred to herein as "surface coils") operating at different frequencies. The surface coils may be single-frequency or dual-frequency (doubly tuned) RF coils. The dual-frequency RF coils in some embodiments include concentric coil elements tuned to resonate at different frequencies, e.g., one resonant for carbon and one resonant for protons, namely, to resonate at the Larmor frequencies of carbon and protons to induce Larmor precession in the carbon atoms and protons. It should be noted that the MEMS device 10 is not limited to coupling with only receive surface coils.For example, the MEMS device 10 may be coupled to a transmitting coil only or a combination transmitting / receiving coil.

[0011] Various embodiments of the MEMS device 10 may be provided as part of a single-modality or multi-modality magnetic resonance imaging system. The MR imaging system may be combined with various types of medical imaging systems, such as a computed tomography (CT) system, a positron emission tomography (PET) system, a single-photon emission computed tomography (SPECT) system, an ultrasound system, or any other system capable of producing images, particularly of a human. Furthermore, the various embodiments are not limited to medical imaging systems for imaging human subjects, but may also include veterinary or non-medical systems for imaging non-human subjects, luggage, etc.

[0012] The MEMS device 10 may be coupled to one or more surface coils 12, e.g., one or more receiving surface coils. In one embodiment, a single MEMS device 10 may be coupled to each surface coil 12. In another embodiment, a single MEMS device 10 may be coupled to multiple surface coils 12. In a specific embodiment, a separate MEMS device 10 may be coupled to each of the surface coils 12. Furthermore, the MEMS device 10 may be configured to decouple all of the surface coils 12 or a selection of individual surface coils 12. Although the surface coils 12 are illustrated in a specific arrangement, such aswith an inner coil element and an outer element forming a pair of loop coils (dual-frequency or double-tuned RF coil element), the MEMS device 10 can also be used to control the decoupling of any type of MR imaging coil, in particular any type of magnetic resonance receive surface coil or transmit surface coil. It should be noted that the MEMS device 10 is not limited to coupling only with receive surface coils. In one embodiment, the MEMS device 10 can be coupled to a transmit-only coil or a transmit / receive coil combination.

[0013] In Fig. Referring now to Figure 2, the MEMS device 10 is shown. In the illustrated embodiment, the MEMS device 10 includes a MEMS switch 20. The MEMS device 10 includes a substrate 22, a beam 24, a beam electrode 26, first and second control electrodes 28, 30, and first and second contact electrodes 32, 34. In some embodiments, more than one substrate may be used. This back-to-back configuration can be implemented as either one substrate or multiple substrates.

[0014] In the illustrated embodiment, a first intermediate layer 36 is disposed on the substrate 22. The first control electrode 28 is disposed on the first intermediate layer 36 via a second intermediate layer 38. The second control electrode 30 is disposed on the first intermediate layer 36 via a third intermediate layer 40. The first contact electrode 32 is disposed on the first intermediate layer 36 via a fourth intermediate layer 42. The second contact electrode 34 is disposed on the first intermediate layer 36 via a fifth intermediate layer 44. The beam electrode 26 is disposed on the first intermediate layer 36 via a sixth intermediate layer 37. It should be noted herein that the number of intermediate layers may vary depending on the application.

[0015] The beam 24 includes an anchoring portion 46, a first beam portion 48, and a second beam portion 50. In some embodiments, the beam may have more than one anchoring portion, where the anchoring portions are electrically coupled to each other. In the illustrated embodiment, the anchoring portion 46 is coupled to the beam electrode 26 via a seventh interlayer 52. The first beam portion 48 extends from the anchoring portion 46 along a first direction 54, and the second beam portion 50 extends from the anchoring portion 46 along a second direction 56 opposite the first direction 54. The first control electrode 28 and the first contact electrode 32 are arranged facing the first beam portion 48. The second control electrode 30 and the second contact electrode 34 are arranged facing the second beam portion 50.In the illustrated embodiment, the first control electrode 28 and the second control electrode 30 are coupled to form a gate electrode 58. The gate electrode 58 is any type of voltage source, e.g., a square-wave voltage source, that can drive or bias the MEMS switches 20 to cause the beam 24 in the MEMS switch 20 to bend or deflect, thus providing an electrical path through the MEMS switch 20 (i.e., a closed state of the MEMS switch 20). A seed layer 60 is formed on the beam 24, facing the beam electrode 26, the first and second control electrodes 28, 30, the first and second contact electrodes 32, 34, and the first interlayer 36.

[0016] Beam 24 may be made of various materials. For example, beam 24 may be made of one or more different metals, such as gold, gold alloy, nickel, nickel alloy, tungsten, or the like. Substrate 22 may include silicon, silicon dioxide, quartz, or the like, and the intermediate layers may include silicon nitride, silicon oxide, adhesion layers, or the like. Electrodes 26, 28, 30, 32, 34 may include metals such as gold, platinum, tantalum, or the like. In a specific embodiment, electrodes 26, 28, 30, 32, 34 may include metal oxides. It should be noted that the composition of beam 24, substrate 22, and electrodes 26, 28, 30, 32, 34 disclosed herein is not all-inclusive and may vary depending on the application.The MEMS switch 20 may be fabricated using techniques including deposition, anodizing, patterning, etching, or the like.

[0017] The dimensions of the beam 24 can be varied, for example, based on specific bending or deflection requirements, such as how much force is needed to bend or deflect the beam 24. The dimensions and configuration of the beam 24 can also be based on the voltage applied between the gate electrode 58 and the beam electrode 26, which is used to deflect the beam 24. The dimensions and configuration of the beam 24 can also be based on the voltage of the gate electrode 58 used to deflect the beam 24. It should be noted herein that the MEMS switch 20 can be manufactured from different materials and using different processes, for example, based on the particular application for the MEMS device 20 (e.g., application in an MR imaging system), to ensure that the device functions properly in a specific environment without adversely affecting the environment.

[0018] In some embodiments, the MEMS device 10 may include multiple MEMS switches 20 that operate in either an open or a closed state, e.g., based on whether the imaging system (e.g., MR imaging system) is operating in a transmit or receive mode of operation, respectively, when coupled to the surface coil. In some embodiments, the MEMS switches 20 may be coupled in series to form a group. In certain embodiments, a set or group of MEMS switches 20 may be coupled in parallel.

[0019] When no actuation voltage is applied between the gate 58 and the bar electrode 26, the first bar portion 48 and the second bar portion 50 are arranged at a first position such that a first bar contact portion 62 of the first bar portion 48 and a second bar contact portion 64 of the second bar portion 50 are spaced from the first contact electrode 32 and the second contact electrode 34, respectively, which is referred to as an "open state." When an actuation voltage is applied between the gate 58 and the bar electrode 26, the first bar portion 48 and the second bar portion 50 are biased from the first position to a second position such that the first bar contact portion 62 and the second bar contact portion 64 are in contact with the first contact electrode 32 and the second contact electrode 34, respectively.the second contact electrode 34, allowing current to flow from the first and second beam contact portions 62, 64 to the first and second contact electrodes 32, 34, which is referred to as a "closed state".

[0020] As previously discussed, MEMS RF switches are used for wireless devices due to their low-power characteristics and high-frequency operation. However, when a conventional three-terminal MEMS switch is inserted into an RF-blocking path, a voltage is generated between a contact electrode and a gate electrode when the switch is open. This voltage is generated because the capacitance between the contact electrode and the bar electrode is of the same order of magnitude as the capacitance between the contact electrode and the gate electrode. This voltage may not be suitable if the switch blocks a relatively low voltage compared to a gate drive voltage of the switch.However, as the RF voltage between the contact electrode and the bar electrode increases, more voltage is generated at the control electrode, increasing the risk of self-actuation of the switch, which leads to damage of the MEMS switch.

[0021] According to embodiments of the present invention, two control electrodes, namely the first control electrode 28 and the second control electrode 30, are coupled to form a gate electrode 58. The first control electrode 28 and the second control electrode 30 are configured such that when an actuation voltage is applied between the gate electrode 58 and the beam electrode 26, the actuation voltage is applied equally to the first control electrode 28 and the second control electrode 30. This enables actuation of the first beam portion 48 and the second beam portion 50 using the same gate drive signal.

[0022] In Fig. Referring now to Figure 3, the MEMS switch 20 is shown with a back-to-back alignment according to an embodiment of Fig. 2. In the illustrated embodiment, the MEMS switch 20 has a symmetrical arrangement modeled as two triangles 66, 68, each triangle having three capacitors coupled to the contact electrodes 32, 34. The triangle 66 has a first capacitor 70 representing a capacitance between the gate electrode 58 and the first beam portion 48, a second capacitor 72 representing a capacitance between the gate electrode 58 and the first contact electrode 32, and a third capacitor 74 representing a capacitance between the first beam portion 48 and the first contact electrode 32.The triangle 68 has a fourth capacitor 76 indicating a capacitance between the gate electrode 58 and the second beam portion 50, a fifth capacitor 78 indicating a capacitance between the gate electrode 58 and the second contact electrode 34, and a sixth capacitor 80 indicating a capacitance between the second beam portion 50 and the second contact electrode 34.

[0023] In Fig. 4, the MEMS switch 20 includes a back-to-back alignment according to an embodiment of Fig. 2. In the illustrated embodiment, the MEMS switch 20 has a similar arrangement as in Fig. 3. In addition, the switch 20 is modeled with a capacitor 82, which represents a capacitance between the gate electrode 58 and the bar electrode 26.

[0024] As explained above, high-frequency signal blocking is performed when the MEMS switch 20 is in an open state in which the first and second beam portions 48, 50 are separated from the first and second contact electrodes 32, 34, respectively. The voltage generated across the MEMS switch 20 contains high-frequency signals, causing capacitive coupling across each of the capacitances across the MEMS switch 20. Consequently, in such a configuration, the voltage at the beam electrode 26 is equal to half the voltage across the first and second contact electrodes 32, 34. The voltage at the gate electrode 58 is also equal to half the voltage across the first and second contact electrodes 32, 34, provided the capacitances are equal. As a result of such a configuration, self-assertion of the switch 20 is prevented.

[0025] The back-to-back configuration of the MEMS switch 20 allows electrical communication between the two control electrodes 28, 30 (in Fig.2). In one embodiment, this electrical communication occurs via a resistor, and in other embodiments, this electrical communication occurs passively via capacitors and / or inductors. In certain other embodiments, the electrical communication occurs actively using control logic. This electrical communication connection results in the same voltage at both control electrodes, and the voltage at the gate electrode is the same as the voltage at the beam. Under conditions where the capacitances across the switch 20 are equal, a voltage developed between the beam electrode and the gate electrode is nearly zero, even in the presence of much higher radio frequency signals. The exemplary MEMS switch 20 has a blocking voltage greater than 300 volts to prevent self-actuation of the switch 20 when the MEMS switch 20 is in an open state.

[0026] According to certain embodiments of the present invention, a capacitance between the first beam portion 48 and the first contact electrode 32 and between the second beam portion 50 and the second contact electrode 34 is equal. In some embodiments, the capacitance between the first contact electrode 32 and the first control electrode 28 and between the second contact electrode 34 and the second control electrode 30 is equal. In a specific embodiment, a capacitance between the beam 24 and the gate electrode 58 is greater than at least twice a capacitance between the first control electrode 28 and the first contact electrode 32.

[0027] The symmetry of the back-to-back configuration of switch 20 is based on a layout, process variability, and assembly configuration of the switch. One or more of the elements added to the switch may create an asymmetric configuration that causes a residual voltage to be generated between the gate 58 and the beam electrode 26 of the switch 20. In one embodiment, this residual voltage may be passively mitigated using a capacitor between the gate 58 and the beam electrode 26. In another embodiment, the residual voltage may be actively mitigated using control logic. As discussed above, the exemplary switch may include one or more substrates.

[0028] It should be noted here that the lifetime of the MEMS switch may be based on the magnitude of a residual voltage generated across the contact electrodes when the MEMS switch is in a closed state. Such a voltage may typically be referred to as a "hot-switch voltage." In applications where the RF voltage is removed before the switch is actuated, there is a possibility that a residual low-frequency or DC voltage may still remain across the switch due to the low open-state capacitance and low leakage current. According to the present invention, such an effect is mitigated by allowing electrical communication between the contact electrodes 32, 34 and the beam electrode 26 in the switch 20. This electrical communication connection is established via passive components such as resistors, inductors, and diodes.Such an electrical communication connection allows low-frequency signal components to pass through the open switch while maintaining the required high-frequency rejection.

[0029] In some embodiments, the lifetime of the MEMS switch 20 can be improved by providing the plurality of capacitors in series with the first and second contact electrodes 32, 34 of the switch 20. These capacitors make it possible to minimize both the hot-switch voltage and the hot-switch energy (i.e., the total charge transferred when the switch is closed). This implementation is particularly advantageous when the switch 20 is isolated from the influence of the gate control logic.

[0030] In some embodiments, an array having a back-to-back configuration of exemplary switches 20 is provided. In such embodiments, a single gate electrode 58 is used to actuate an array of switches 20 in series, allowing doubling of the gate voltage without increasing the need for additional gate electrodes. The number of MEMS switches 20 can be varied depending on the particular application, e.g., the environment in which the MEMS switch 20 operates. For example, in a magnetic environment or an RF environment, the number of MEMS switches 20 can be determined based on the voltage pulse effect such that a reverse bias is overcome. In particular, a number and configuration of MEMS switches 20 can be varied based on an RF reverse bias such that self-actuation by the RF signals is prevented.

[0031] Although only certain features of the invention are illustrated and described herein, many modifications and changes will occur to those skilled in the art.

Claims

[1] Microelectromechanical system (MEMS) switch (20) comprising: a substrate (22); a bar electrode (26) arranged on the substrate (22); a beam (24) having at least one anchoring portion (46) coupled to the beam electrode (26), a first beam portion (48) extending from the at least one anchoring portion (46) along a first direction, and a second beam portion (50) extending from the at least one anchoring portion (46) along a second direction opposite to the first direction; a first control electrode (28) arranged on the substrate (22) facing the first beam portion (48); a first contact electrode (32) arranged on the substrate (22) facing the first beam portion (48); a second control electrode (30) disposed on the substrate (22) facing the second beam portion (50), wherein the first control electrode (28) and the second control electrode (30) are coupled together to form a gate electrode (58); and a second contact electrode (34) arranged on the substrate (22) facing the second beam portion (50); characterized by an electrical connection between the first and second contact electrodes (32, 34) and the bar electrode (26), the electrical connection comprising one or more passive components selected from resistors, coils and diodes. [2] The MEMS switch (20) of claim 1, wherein the first beam portion (48) includes a first beam contact portion (62). [3] The MEMS switch (20) of claim 2, wherein the second beam portion (50) includes a second beam contact portion (64). [4] The MEMS switch (20) of claim 3, wherein the first beam portion (48) and the second beam portion (50) are arranged at a first position such that the first beam contact portion (62) and the second beam contact portion (64) are spaced from the first contact electrode (32) and the second contact electrode (34), respectively, when no actuation voltage is applied between the gate electrode (58) and the beam electrode (26). [5] The MEMS switch (20) of claim 4, wherein the first beam portion (48) and the second beam portion (50) are biased from the first position to a second position such that the first beam contact portion (62) and the second beam contact portion (64) are in contact with the first contact electrode (32) and the second contact electrode (34), respectively, when the actuation voltage is applied between the gate electrode (58) and the beam electrode (26). [6] The MEMS switch (20) of claim 1, wherein the first control electrode (28) and the second control electrode (30) are configured to apply the control voltage equally to the first control electrode (28) and the second control electrode (30). [7] The MEMS switch (20) of claim 1, wherein the MEMS switch (20) has a blocking voltage greater than 300 volts when the MEMS switch (20) is in an open state. [8] The MEMS switch (20) of claim 1, wherein a capacitance between the first beam portion (48) and the first contact electrode (32) and between the second beam portion (50) and the second contact electrode (34) is the same. [9] The MEMS switch (20) of claim 1, wherein a capacitance between the first contact electrode (32) and the first control electrode (28) and between the second contact electrode (34) and the second control electrode (30) is the same. [10] The MEMS switch (20) of claim 1, wherein a capacitance between the beam (24) and the gate electrode (58) is greater than at least twice a capacitance between the first control electrode (28) and the first contact electrode (32). [11] The MEMS switch (20) of claim 1, further comprising a plurality of capacitors (72, 74, 78, 80) connected to either the first contact electrode or the second contact electrode. [12] The MEMS switch (20) of claim 1, wherein the MEMS switch (20) forms a MEMS radio frequency switch. [13] The MEMS switch (20) of claim 1, wherein the MEMS switch (20) is arranged in a device (10) configured for operation in a high frequency range. [14] The MEMS switch (20) of claim 13, wherein the device (10) comprises a magnetic resonance imaging system having a single-modality imaging system or a multi-modality imaging system. [15] MEMS switch (20) according to claim 14, wherein the MEMS switch (20) is arranged for coupling and decoupling one or more radio frequency receive surface coils (12) or radio frequency transmit surface coils of the magnetic resonance imaging system. [16] The MEMS switch (20) of claim 15, wherein the one or more radio frequency receive surface coils (12) and radio frequency transmit surface coils comprise either one or more single frequency coils or one or more dual frequency coils.

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