Traditional electromechanical relays (EMRs) have been widely used for over a century since their invention. However, with the rapid development of microelectromechanical systems (MEMS) switching technology in recent decades, MEMS switches have become excellent alternatives in RF test instruments, meters, and RF switching applications due to their ease of use, small size, and ability to reliably transmit signals from 0Hz/dc to hundreds of GHz with minimal loss. They are also changing the way electronic systems are implemented.
Many companies are attempting to develop MEMS switch technology, but they all face the challenge of large-scale production and delivering reliable products in large quantities. Among them, Analog Devices (ADI) has actively invested in MEMS switch projects and built its own advanced MEMS switch manufacturing facilities to meet the industry's demand for mass production.
Basic principles
The key to ADIMEMS' switching technology is the concept of an electrostatically driven, micro-machined cantilever beam switch assembly. Essentially, it can be viewed as a micrometer-scale mechanical switch with metal-to-metal contacts driven by electrostatics.
The switch uses a three-terminal configuration for connection. Functionally, these terminals can be considered as the source, gate, and drain. The diagram below is a simplified schematic of the switch, with case A indicating that the switch is in the off position.
When a DC voltage is applied to the gate, an electrostatic pull is generated on the switching beam. This electrostatic force is the same as the attractive force between the positive and negative charged plates of a parallel-plate capacitor. When the gate voltage ramps up to a sufficiently high value, it generates a sufficiently strong attractive force (red arrow) to overcome the spring resistance of the switching beam, causing the beam to move downwards until the contacts reach the drain. The process is shown in case B of the diagram below.
At this point, the circuit between the source and drain is closed, and the switch is on. The actual force required to pull down the switch beam depends on the spring constant of the cantilever beam and its resistance to motion. Note: Even in the on position, the switch beam still has a spring force pulling the switch up (blue arrow), but as long as the electrostatic force pulling down (red arrow) is greater, the switch will remain on.
Finally, when the gate voltage is removed (case C in the diagram below), i.e., when the gate electrode is 0V, the electrostatic attraction disappears, and the switch beam, acting as a spring, has a sufficiently large restoring force (blue arrow) to disconnect the connection between the source and drain, and then returns to the original off position.
Figure 1 below is an enlarged view of four MEMS switches configured using a single-pole four-throw (ST4T) multiplexer. Each switch beam has five parallel resistive contacts to reduce resistance when the switch is closed and to improve power handling capability.
Figure 1. Four MEMS cantilever switch beams (SP4T configuration)
MEMS switches require high DC drive voltages to electrostatically drive the switches. To make the devices as easy to use as possible and further ensure performance, Analog Devices (ADI) designed a matching driver integrated circuit (IC) to generate the high DC voltage, which is co-packaged with the MEMS switch in a QFN form factor. Furthermore, the generated high drive voltage is applied to the switch's gate electrode in a controlled manner. It ramps up to a high voltage in microseconds. This ramp helps control the pull-up and pull-down of the switch beam, improving the switch's operating performance, reliability, and lifespan. Figure 2 below shows an example of a driver IC and MEMS chip in a QFN package. The driver IC requires only a low-voltage, low-current supply and is compatible with standard CMOS logic drive voltages. This co-packaged driver makes the switch very easy to use, and its power consumption requirements are very low, approximately in the range of 10mW to 20mW.
Figure 2 shows the driver IC (left) and MEMS switch chip (right) mounted and wire-soldered on a metal lead frame.
performance advantages
Taking the ADGM1004/ADGM1304SP4T series as an example, its parameters have many obvious advantages compared with traditional electromechanical relays (Figure 3).
Figure 3. Comparison of ADGM1004/ADGM1304MEMS with traditional electromechanical relays
The ADGM1004/ADGM1304SP4T includes an integrated driver and is suitable for relay replacements, RF test instruments, and RF switching. For product specifications and related evaluation board EVAL-ADGM1004EBZ, please visit the Digi-Key product page.
Application Examples
Previously, implementing DC/RF switching functionality in ATE test equipment required the use of EMR switches. However, using relays could limit system performance due to the following issues:
Relay switches are large in size and must comply with "no-go" design rules, which means they take up a lot of space and lack test scalability.
Relay switches have a limited lifespan, lasting only a few million cycles.
Multiple relays must be cascaded to achieve the required switching configuration (for example, the SP4T configuration requires three SPDT relays).
When using relays, PCB assembly problems may occur, which usually leads to a high PCB rework rate.
Achieving full bandwidth performance can be very difficult due to wiring limitations and relay performance limitations.
The relay drive speed is slow, on the order of milliseconds, which limits the testing speed.
Taking a typical DC/RF switch fan-out 16:1 multiplexing function as an example (Figure 4), nine DPDTMR relays and one relay driver IC are needed to achieve an 18:1 multiplexing function (eight DPDT relays can only produce a 14:1 multiplexing function). Figure 5 shows the same fan-out switching function, simplified by using only five ADGM1304 or ADGM1004SP4TMEMS switches.
Figure 6 shows a photograph of the PCB for visual demonstration of these two schematics. The left side shows the physical relay solution, illustrating the area occupied by the relay solution, the difficulty of maintaining symmetry between wiring connections, and the requirements for the driver IC. The right side shows the reduced PCB area and decreased wiring complexity of the switching function. By area, MEMS switches reduce the footprint by more than 68%, and by volume, potentially by more than 95%.
The ADGM1304 and ADGM1004 MEMS switches incorporate low-voltage, independently controllable switch drivers; therefore, they do not require external driver ICs. Due to the small package height of the MEMS switches (0.95mm for the ADGM1304 and 1.45mm for the ADGM1004), the switches can be mounted on the reverse side of the PCB. This smaller package height increases the achievable channel density.
Figure 6. Visual comparison of DC/RF fan-out test boards: achieving 16:1 multiplexing using nine EMR switches (yellow on the left) and five MEMS switches (red on the right).
Summary of this article
Finally, small-size solutions are typically a critical requirement for any market. MEMS offers a compelling advantage in this regard. Figure 7 below compares the packaged ADISP4T (quad-switch) MEMS switch design with the dimensions of a typical DPDT (quad-switch) electromechanical relay using actual photographs. The MEMS switch saves significant space, with a volume equivalent to only 5% of a relay. This ultra-small size significantly saves PCB area, increasing the possibility of double-sided PCB development. This advantage is particularly valuable for automated test equipment manufacturers with a pressing need to increase channel density.
Figure 7. Size comparison between ADI leadframe chip-scale packaged MEMS switch (four switches) and typical electromechanical RF relay (four switches).
