Patent Application
20240071694
This description relates to a microelectromechanical systems (MEMS) switch implemented with a coplanar waveguide.
Microelectromechanical systems (MEMS) describes a manufacturing technology used to create microscale integrated devices or systems that combine mechanical and electrical components. These devices and systems have the ability to sense, control and actuate on the micro scale, and generate effects on the macro scale.
A coplanar waveguide is a type of electrical planar transmission line. In some examples, a coplanar waveguide is fabricated using printed circuit board technology, and is used to convey microwave-frequency signals. Additionally or alternatively, on a smaller scale, coplanar waveguide transmission lines are also built into monolithic microwave integrated circuits (MIMICs). In general, a coplanar waveguide is formed with a median metallic strip separated by two narrow slits from a ground plane.
A first example relates to a microelectromechanical system (MEMS) switch that is implemented with a coplanar waveguide. The MEMS switch includes an input terminal, and an output terminal, spaced apart from the input terminal. The MEMS switch also includes a beam extending between the input terminal and the output terminal of the MEMS switch. The beam includes a first edge and a second edge coupled to a gate of the MEMS switch. The beam includes a third edge proximate the input terminal, the first edge includes a first set of finger contacts proximate a first corner of the beam and a second set of finger contacts proximate a second corner of the beam. The beam also includes a fourth edge proximate the output terminal, the fourth edge opposing the third edge. The MEMS switch includes a first anchor coupled to the input terminal. The first anchor has a first segment extending from a region proximate the input terminal to a region overlying the first set of finger contacts of the beam. The first anchor has a second segment spaced apart from the first segment by an aperture in the first anchor, the second segment extending from the region proximate the input terminal to a region overlying the second set of finger contacts of the beam. A second anchor of the MEMS switch waveguide is coupled to the output terminal of the MEMS switch and to the second edge of the beam.
A second example relates to a MEMS switch that includes an input terminal. The input terminal is coupled to an anchor having an aperture that separates a first segment of the anchor and a second segment of the anchor. The MEMS switch also includes an output terminal, spaced apart from the input terminal and a beam coupled to the output terminal and a gate, the beam extending from a region proximate the output terminal to a region proximate the input terminal. The beam is configured to responsive to assertion of a control signal at a gate of the MEMS switches, contact the anchor to establish a current path between the input terminal and the output terminal. The beam is also configured to responsive to deassertion of the control signal at the gate, disconnect from the anchor to galvanically isolate the input terminal from the output terminal.
A third example relates to a system including a single pole double throw (SPDT) switch. The system includes a MEMS switch having a first anchor, a first beam, a first gate, a first input terminal and a first output terminal, in which the first anchor has a first aperture between a first segment of the first anchor and a second segment of the first anchor, the first input terminal is coupled to the first anchor. The first beam is configured to responsive to assertion of a first control signal at the first gate, contact the first anchor to establish a first current path between the first input terminal and the first output terminal. The first beam is also configured to responsive to deassertion of the first control signal at the first gate, disconnect from the first anchor to galvanically isolate the first input terminal from the first output terminal. The system additionally includes a second MEMS switch having a second anchor, a second beam, a second gate, a second input terminal and a second output terminal, in which the second anchor has a second aperture between a first segment of the second anchor and a second segment of the second anchor, the second input terminal is coupled to the second anchor. The second beam is configured to responsive to assertion of a second control signal at the second gate, contact the second anchor to establish a second current path between the second input terminal and the second output terminal. The second beam is also configured to responsive to deassertion of the second control signal at the second gate, disconnect from the second anchor to galvanically isolate the second input terminal from the second output terminal. The system additionally includes a receiver coupled to the first output terminal, a transmitter coupled to the second input terminal. The system further includes an antenna coupled to the first input terminal and to the second output terminal and a controller configured to provide the first control signal and the second control signal.
This description relates to a microelectromechanical systems (MEMS) switch implemented with a coplanar waveguide. The coplanar waveguide of the MEMS switch includes an input terminal and an output terminal. The input terminal is configured to be coupled to a signal source, and the output terminal is configured to be coupled to a load. In this description, the term “coupled”, or “couples” means either an indirect or a direct connection. As one example, the input terminal is coupled to a radio frequency (RF) transmitter (e.g., a signal source) and the output terminal is coupled to an antenna (e.g., a load). In some such examples, the MEMS switch is a first MEMS switch that operates in concert with the second MEMS switch to form a single pole double throw (SPDT) switch, wherein the input terminal of the second MEMS switch is coupled to the antenna (e.g., a signal source) and the output terminal of the second MEMS switch is coupled to an RF receiver (e.g., a load).
The output terminal of the MEMS switch is spaced apart from the input terminal, and a beam extends in a region between the input terminal and the output terminal of the coplanar waveguide. In some examples, the beam is formed of an aluminum mesh or other conductive material. The beam includes a first edge and a second edge coupled to a gate of the MEMS switch. The beam has a third edge proximate the input terminal. The third edge includes a first set of finger contacts proximate a first corner of the beam and a second set of finger contacts proximate a second corner of the beam. The beam also has a fourth edge proximate the output terminal.
A first anchor is coupled to the input terminal. A second anchor is coupled to the output terminal of the coplanar waveguide and to the second edge of the beam. The first anchor has a first segment extending from a region proximate the input terminal to a region overlying the first set of finger contacts of the beam and a second segment spaced apart from the first segment, the second segment extending from the region proximate the input terminal to a region overlying the second set of finger contacts of the beam. Further, the first anchor has a dovetail shaped (e.g., trapezoidal shaped) aperture that separates the first segment from the second segment of the first anchor. Accordingly, the first segment and the second segment extend from the input terminal toward the third edge of beam at complementary angles (e.g., opposite angles).
The MEMS switch is normally opened, and electrically controllable. More particularly, a state of the MEMS switch is controllable with a control signal provided to the gate of the MEMS switch. In some examples, the control signal is provided by a controller (e.g., a microcontroller). In other examples, other devices provide the control signal. The MEMS switch is a normally open switch. In an open state, the input terminal and the output terminal of the MEMS switch are galvanically isolated. Assertion (such as a high logic state) of the control signal applies a bias voltage (e.g., about 40 volts (V) or more) to the gate of the MEMS switch. The bias voltage causes the beam to move such that the first and second set of finger contacts contact the first segment and the second segment of the first anchor, thereby establishing a current path between the input terminal and the output terminal.
As described herein, the first anchor of the MEMS switch includes the aperture (e.g., having a dovetail shape) separating the first segment from the second segment. This aperture reduces the surface area of the first anchor that overlies the beam. Instead, the ends of the first segment and the second segment (which also include finger contacts) overlay the beam. Accordingly, inclusion of the first aperture reduces a parasitic capacitance between the first anchor and the beam, thereby improving isolation between the input terminal and the output terminal during intervals of time that the MEMS switch is in the open state. This isolation degrades as a function of frequency. However, inclusion of the aperture curtails this degradation, such that at frequencies of about 40 gigahertz (GHz) or more, inclusion of the aperture improves the isolation by about 1 decibels (dB) or more. Further, analysis of a surface current (Jsurf) on the first anchor during intervals where the MEMS switch is in the closed state reveals that most of the current flows along a periphery of the first anchor (whether or not the first anchor includes the aperture). Therefore, inclusion of the aperture does not significantly impact an insertion loss of the MEMS switch. More particularly, at the higher frequencies of 40 GHz, inclusion of the aperture adds an additional insertion loss of about 0.06 dB. Thus, for a relatively small increase in insertion loss (e.g., about 0.6 dB), a significant improvement to the isolation (e.g., about 1 dB or more) of the MEMS switch is achieved by including the aperture.
The coplanar waveguide 104 includes an input terminal 124 and an output terminal 128. The input terminal 124 is configured to be coupled to a signal source, such as a transmitter or an antenna. The output terminal 128 is configured to be coupled to a load, such as a receiver or an antenna. The input terminal 124 and the output terminal 128 are spaced apart from each other.
A beam 132 is situated in the region between the input terminal 124 and the output terminal 128. The beam 132 is formed of a mesh of conductive material, such as aluminum. The 132 has a rectangular shape, with a first edge 136, a second edge 140, a third edge 144 and a fourth edge 148. The first edge 136 is proximate the input terminal 124 and the second edge 140, which opposes the first edge 136 is proximate the output terminal 128. The third edge 144 and the fourth edge 148 oppose each other, and extend between the input terminal 124 and the output terminal 128. A first set of finger contacts 152 is formed at a corner of the first edge 136 and the third edge 144 of the beam 132. A second set of finger contacts 156 is formed at a corner of the first edge 136 and the fourth edge 148. In some examples, there are four (4) or more finger contacts in the first set of finger contacts 152 and the second set of finger contacts 156. In other examples, there are more or less finger contacts in the first set of finger contacts 152 and the second set of finger contacts 156.
The input terminal 124 includes a via 160 that couples the input terminal 124 to a first anchor 164. The output terminal 128 also includes a via 168 that couples the output terminal 128 to a second anchor 172. The second anchor 172 is coupled to the second edge 140 of the beam 132. Moreover, the second anchor 172 has a parallelogram shape that tapers from the output terminal 128 toward the second edge 140 of the beam 132.
The first anchor 164 has a first segment 176 and a second segment 180. An aperture 182 in the first anchor 164 separates the first segment 176 from the second segment 180. The first segment 176 of the first anchor 164 extends at a first angle from the input terminal 124 to a region that overhangs the first set of finger contacts 152 of the beam 132. Similarly, the second segment 180 extends at a second angle from the input terminal 124 to a region that overhangs the second set of finger contacts 156, and the second angle is a complement (opposite) of the first angle. Accordingly, the aperture 182 has a dovetail shape in the illustrated example. Further, the first segment 176 includes a third set of finger contacts 184 that overhang the first set of finger contacts 152 of the beam 132, and the second segment 180 includes a fourth set of finger contacts 186 that overhang the second set of finger contacts 156. In some examples, there are an equal number of finger contacts in the first set of finger contacts 152, the second set of finger contacts 156, the third set of finger contacts 184 and the fourth set of finger contacts 186.
The third edge 144 and the fourth edge 148 of the beam 132 are coupled to a gate 190 for the MEMS switch 100. The gate 190 has two (2) terminals. Application of a bias voltage (e.g., about 40 volts (V) or more) across the gate 190 causes the beam 132 to move in a direction indicated by an arrow 192. Removal of the bias voltage causes the beam 132 to move in a direction indicated by an arrow 194 to decouple the beam 132 from the first anchor 164. Stated differently, applying the bias voltage applied to the gate 190 causes the beam 132 to move from a first position to a second position. The input terminal 124 and the output terminal 128 are galvanically isolated in the first position, and a current path is provided between the input terminal 124 and the output terminal 128 in the second position, such that the MEMS switch 100 is in an open state in the first position and the MEMS switch 100 is in a closed state in the second position.
In operation, the MEMS switch 100 is a normally opened switch such that the MEMS switch 100 is in the open state in situations where no bias voltage is applied across the gate 190. In the open state, the first set of finger contacts 152 and the second set of finger contacts 156 of the beam 132 are spaced apart from, and galvanically isolated from the third set of finger contacts 184 and the fourth set of finger contacts 186 of the first segment 176 and the second segment 180, respectively, of the first anchor 164. Accordingly, during time intervals where the MEMS switch 100 is in the open state, there is no current path between the input terminal 124 and the output terminal 128.
Conversely, during time intervals that the vias voltage is applied across the gate 190, the MEMS switch 100 is in a closed state. More specifically, as described herein, application of the bias voltage to the gate 190 causes the beam 132 to move in the direction indicated by the arrow 192. Moving the beam 132 in the direction indicated by the arrow 192 causes the first set of finger contacts 156 of the beam 132 to contact the third set of finger contacts 184 of the first segment 176 of the first anchor 164 and causes the second set of finger contacts 156 to contact the fourth set of finger contacts 186 of the second segment 180. Accordingly, application of the bias voltage across the gate 190 causes the first anchor 164 to contact the beam 132 to provide a current path between the input terminal 124 and the output terminal 128 of the MEMS switch 100.
Inclusion of the aperture 182 curtails parasitic capacitance between the beam 132 and the first anchor 164 during intervals where the MEMS switch 100 is in the open state. For example, the aperture 182 reduces a surface area between the first anchor 164 and the region of the beam 132 proximate the first edge 136 of the beam 132. Reducing this surface area reduces parasitic capacitance between the beam 132 and the first anchor 164 in the open state.
The first heat map 200 characterizes a surface current density (Jsurf) in amperes per meter (A/m) of the baseline anchor 204 where the MEMS switch is in a closed state. The first heat map 200 demonstrates that in situations where the aperture 182 is omitted, a periphery of the baseline anchor 204 has a greatest surface current density. Conversely, an interior region 208 of the baseline anchor 204 characterized by the first heat map 200 has a relatively low surface current density.
The second heat map 220 characterizes a surface current density (Jsurf) in A/m of the modified anchor 224 that includes the aperture 228. As is demonstrated, the first segment 232 and the second segment 236 have relatively high surface current densities. Comparing the first heat map 200 with the second heat map 220, inclusion of the aperture 228 (in the modified anchor 224 of the second heat map 220) does not greatly impact the overall surface current density. Accordingly, because the surface current density is low in the interior region 208 of the baseline anchor 204 corresponding to the first heat map 200, the removal of the interior region 208, thereby forming the modified anchor 224 with the aperture 228 has a relatively small impact on a resistance (which corresponds to insertion loss in the closed state) in a MEMS switch employing the modified anchor 224. For example, there are multiple metal layers in the MEMS switch employing the baseline anchor 204 or the modified anchor 224 that produce a parasitic parallel-plate capacitance effect. For example, the capacitance between the beam 212 and the baseline anchor 204 (in the open state) is greater than the capacitance between the beam 240 and the modified anchor 224 (in the open state) due to the inclusion of the aperture 228.
As illustrated by the first bar graph 300, inclusion of an aperture, such as the aperture 228 of
As illustrated by the second bar graph 320, inclusion of an aperture in the modified anchor improves isolation by about 1.59 dB at 20 GHz (e.g., 22.65 dB for the baseline anchor compared to 24.24 dB for the modified anchor) and improves isolation by about 1.44 dB at 40 GHz (e.g., 16.82 dB for the baseline anchor compared to 18.26 dB for the modified anchor). As illustrated in the second bar graph 320, as the frequency increases, the isolation degrades. However, employment of the modified anchor curtails this degradation.
Accordingly, the first bar graph 300 and the second bar graph 320 demonstrate that at a cost of about 0.03 dB of insertion loss at 20 GHz, isolation is improved by about 1.59 dB. Also, the first bar graph 300 and the second bar graph 320 demonstrate that at a cost of about 0.06 dB in insertion loss at 40 GHz, isolation is improved by 1.44 dB. Thus, at both frequencies, 20 GHz and 40 GHz, the isolation of the MEMS switched is improved at a nearly negligible cost in insertion loss.
Referring back to
In the example illustrated, the system 400 is employable to implement a radio frequency (RF) transceiver that includes an RF receiver 416 and an RF transmitter 420 that communicate with an antenna 424 (e.g., a load, more generally). However, the SPDT switch 404 is employable in nearly any system where high isolation is needed. In some examples, the RF receiver 416 is configured to receive an RF signal from the antenna 424 that has a frequency of about 50 GHz to about 80 GHz. Similarly, the RF transmitter 420 is configured to transmit a signal to the antenna 424 that has a frequency of about 50 GHz to about 80 GHz. In other examples, other frequencies are employable.
In the example illustrated, the antenna 424 is coupled to an input terminal 425 of the first MEMS switch 408 and the RF receiver 416 is coupled to an output terminal 426 of the first MEMS switch 408. Also, the RF transmitter 420 is coupled to an input terminal 428 of the second MEMS switch 412 and the antenna 424 is coupled to an output terminal 429 of the second MEMS switch 412.
The SPDT switch 404 has two modes of operation, namely a receive mode and a transmit mode. A controller 430 (e.g., a microcontroller) controls the mode of operation of the SPDT switch 404. In some examples, the controller 430 is implemented in an IC package that communicates with the SPDT switch 404. In other examples, the controller 430 is integrated with the SPDT switch 404. The controller 430 includes embedded instructions for controlling a state of the SPDT switch 404. More particularly, the controller 430 provides a first control signal 432 to a gate 434 of the first MEMS switch 408 that controls a state of the first MEMS switch 408. Assertion of the first control signal 432 applies a bias voltage (e.g., a DC voltage of about 40 V or more) across a gate (e.g., the gate 190 of
The SPDT switch 404 is configured such that in the receive mode, the first control signal 432 is asserted, and the second control signal 436 is deasserted, such that the first MEMS switch 408 is in the closed state and the second MEMS switch 412 is in the open state. Also, the SPDT switch 404 is configured such that in the transmit mode, the first control signal 432 is deasserted and the second control signal 436 is asserted.
In the receive mode, an RF signal received by the antenna 424 flows along a receive path 440 from the antenna 424 through the first MEMS switch 408 (in the closed state) and to the RF receiver 416. Also, in the receive mode, the second MEMS switch 412 is in the open state, such that current does not flow across the second MEMS switch 412. In the transmit mode, an RF signal provided from the RF transmitter 420 flows along a transmit path 444 from the RF transmitter 420 through the second MEMS switch 412 (in the closed state) and to the antenna 424. Also, in the transmit mode, the first MEMS switch 408 is in the open state, such that current does not flow across the first MEMS switch 408.
In operation, because the SPDT switch 404 employs the first MEMS switch 408 and the second MEMS switch 412, high isolation between the RF receiver 416 and the RF transmitter 420 is achieved. More particularly, during intervals of time that the SPDT switch 404 is in the receive mode, such that the receive path 440 is active, received RF signals are prevented from reaching the RF transmitter 420, which prevents a loss of gain for the MEMS switch 100. Also, during intervals of time that the SPDT switch 404 is in the transmit mode, such that the transmit path 444 is active, signals provided by the RF transmitter 420 are prevented from flowing to the RF receiver 416 avoiding corruption of measurements of the transmitted signal. Moreover, as demonstrated by the second bar graph 320 of
In this description, unless otherwise stated, “about,” “approximately” or “substantially” preceding a parameter means being within +/−10 percent of that parameter. Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.
