Reed relays: from voiceband calls to GHz ATE, now facing MEMS disruption part 3

Part 1 covered the basics, Part 2 explored the reed relay and its applications in ATE, while this final section examines MEMS-based relays and their role in ATE applications.

Disruption: MEMS-based GHz relays

The virtues and capabilities of the magnetically actuated reed relay are clear, so why change? It’s the usual story: technology moves on and often adapts associated technologies to existing applications.

This is a situation with relays that require tangible physical contact. Microelectromechanical systems (MEMS) technology has evolved since its commercialization in the 1990s, initially serving as on/off accelerometers for triggering airbags, and subsequently as accelerometers and gyroscopes for navigation. MEMS technology is now also used for situations where physical circuit contact switching is needed.

MEMS switch technology is based on an electrostatically actuated, micromachined cantilever beam that serves as a switching element. It is a micrometer-scale mechanical relay, with metal-to-metal contacts that are actuated via electrostatics, not magnetics as in a reed relay or conventional armature relay.

The switch is connected in a three-terminal configuration. Figure 1 is a simplified graphic representation of the switch, with Case A showing the switch in the off position. When a DC voltage is applied to the gate, an electrostatic pull-down force is generated on the switch beam. This is the same electrostatic force as would be seen in a parallel-plate capacitor, having positive and negative charged plates that attract each other.

When the gate voltage reaches a sufficiently high value, it generates a sufficient attraction force (red arrow) to overcome the resistive spring force of the switch beam, causing the beam to move down until the contacts touch the drain, as shown in Case B2. This completes the circuit between the source and the drain, and the switch is now ON.

The actual force it takes to pull the switch beam down is related to the spring constant of the cantilever beam and its resistance to movement. Notice that 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 (red arrow) pulling down is larger, the switch will remain on.

Finally, when the gate voltage is removed (Case C), with 0 volts on the gate electrode, the electrostatic attraction force disappears. The switch beam acts as a spring with sufficient restoring force (blue arrow) to open the connection between the source and the drain, and then returns to the original off position.

This seems like a simple concept, but actual fabrication is a huge challenge requiring many critical CMOS IC steps, including sputtering, layering, etching, and more. Furthermore, to enhance reliability, a high-resistivity silicon cap is bonded to the MEMS die, forming a hermetic protective housing around the MEMS switch core. By hermetically enclosing the switch in this manner, the environmental robustness and cycle lifetime of the switch are enhanced, regardless of the external package technology used.

Note that MEMS switches are not limited to basic SPST configurations, as seen in Figure 2, where four MEMS switches are arranged in a single-pole four-throw (SP4T) multiplexer configuration. Each switch beam has five ohmic contacts in parallel to reduce resistance and increase power handling when the switch is closed.

Suitable drive circuitry is another major difference between a reed switch and a MEMS-based switch. A reed switch requires current drive for the actuation coil, whereas a MEMS switch requires a high voltage to create the electrostatic field. To mitigate the high-voltage challenge, some vendors of MEMS-based switches co-package a high-voltage drive chip with the MEMS device, allowing the user to apply only a low voltage (typically 5 volts) at modest power (10 to 20 mW) for actuation.

Note that MEMS-based switches do have their drawbacks compared to reed-relay based devices. One important difference is that MEMS devices are not compatible with “hot swapping,” where the device or its board is inserted or removed while the power is on. The system or chassis must instead be powered off, the device or board inserted or removed, and then power is applied.

MEMS-based relays can easily handle signals in the low-gigahertz range. Recognizing the potential of these relays, vendors of reed relays and ATE switching systems, such as Pickering, have partnered with MESM vendors to provide systems using this newer technology.

For example, Pickering has worked with Menlo Microsystems — a technology company that specializes in developing advanced MEMS technology for a variety of applications — to develop and market a MEMS-based PXI & PXIe RF-multiplexer product line. Menlo Micro’s Ideal Switch offers excellent RF characteristics up to 4 GHz and an operational life exceeding 3 billion operations, a significant improvement over the maximum 10 million operations typically offered by EMR-based solutions.

While these extreme life-cycle numbers seem unnecessary, life testing of some devices does require this level of intense operation. The Pickering 40/42-878 PXI & PXIe MEMS-based RF Multiplexers of Figure 3 handle up to 25 watts and have a 50-microsecond operating time, far faster than a conventional reed switch.

Pickering and Menlo Microsystems are not the only vendors of reed and MEMS relays. Some other sources include Coto Technology, Cynergy3/Sensata Technologies, Littelfuse, Toward Technologies, and Standex Electronics.

Conclusion

Relays that provide a physical, ohmic, mechanical contact between conductors are still widely needed. The legacy armature-based relay was joined decades ago by the reed relay, which has evolved from an audio-band-only capability to gigahertz capability. In turn, the reed relay is being challenged by MEMS-based relays, which offer comparable performance in many ways, while also providing unique attributes.

References

Pickering Electronics’ Miniature HV Reed Relay at the Heart of IC Test System for On Semiconductor, Pickering Electronics press release
Miniature High Voltage Reed Relays | ON Semiconductor Success Story, Pickering Electronics YouTube
Reed Relay Basics, Pickering Electronics
Comprehensive Guide to PXI RF Switching: MEMS vs. EMR and Solid State, Pickering Electronics
What is a Reed Relay, Pickering Electronics YouTube
Demo Compares MEMS Relay to Solid-State Device (Electronica), Menlo Microsystems
The Fundamentals of Analog Devices’ Revolutionary MEMS Switch Technology, Analog Devices
Reed Relay, Wikipedia
Reed Switch, Wikipedia.
Reed Relay & Reed Switch, Electronics Notes
Electromagnetic Switch, United States Patent Office, Patent 2,264,746, Dec. 2, 1941
The Resilience of the Reed Relay, IEEE Spectrum.

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