MEM Relay Simulations

Hello, this is Ben Bracker, HMC class ‘22, engineering major. I am working with Prof. Spencer’s ACE lab to study MEM relays. This post shows how these devices work and what we have been up to now that we are working remotely.

A relay is a type of electrical switch that opens and closes automatically in response to an electrical control signal.  A Micro Electro-Mechanical (MEM) relay is a micrometer-scale electrical switch whose mechanical motion is controlled by electrostatic forces.  All electrical switches connect or disconnect at least two wires. In the case of MEM relays, wires are connected when the switch closes causing the two wires to mechanically touch. When the relay opens, the two wires mechanically separate, which breaks their electrical connection.  The pair of wires that a MEM relay connects or disconnects are called the source and the drain. The pair of wires that carry the control signal are called the gate and the body. (If these names seem familiar, it is because they are borrowed from MOSFET transistors.)

Switch and Relay Symbols

MEM relays convert the control signal to mechanical motion using the electrostatic force.  (This differs from macroscopic relays, which use an electromagnet to generate a magnetic force.) When a non-zero voltage is applied between the gate and the body, the gate and the body build up charge because they form a capacitor. From the charge buildup comes the electrostatic force, which causes the arms surrounding the gate to bend. The gate moves downward, and once there is sufficiently large control voltage, the source, which is attached underneath the gate, touches the drain which causes the mechanical and electrical connection between source and drain. When 0V is applied between the gate and the body, the electrostatic force vanishes, and the spring force in the arms lifts the gate and disconnects the source from the drain.

SEM Image of a MEM Relay

X. Hu, S. F. Almeida, Z. Alice Ye, and T.-J. K. Liu, “Ultra-Low-Voltage Operation of MEM Relays for Cryogenic Logic Applications,” in 2019 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, Dec. 2019, pp. 34.2.3

Cross-sectional view along D-D’ of relay in: (a) in open state (b) closed state

X. Hu, S. F. Almeida, Z. Alice Ye, and T.-J. K. Liu, “Ultra-Low-Voltage Operation of MEM Relays for Cryogenic Logic Applications,” in 2019 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, Dec. 2019, pp. 34.2.3

Over the past couple years, Ethan Falicov ‘21 has used our probe station to measure the electrical characteristics of relays fabricated at UC Berkeley’s Marvell Nanofabrication Laboratory. In addition to electrical probes, we recently set up a laser doppler vibrometer (LDV), which uses the doppler effect to determine a relay’s speed based on laser light reflected off the top of its gate. This allowed us to experimentally observe the relays’ mechanical properties as well — something most relay research teams have not had the chance to do. Our preliminary results show that the movement is detectable, reaching at least 600µm/s for a control voltage sweep on the order of 10’s of µs.

Lab Setup

Now, during Summer 2020, we are simulating the relays in HSPICE using both electrical and mechanical modeling encoded in Verilog-A. We hope to generate simulated waveforms which we can compare to the LDV data we collected during the Spring semester. Once we adjust our model to reflect the experimental evidence, we can tweak the parameters to predict the performance of relays not yet fabricated. 

Ben Bracker (bbracker@hmc.edu)

Thanks to Prof. Spencer for editing help.