Nanoscale MEMS Resonator Structure Yields Controllable MHz-Range Filter, Isolator: Page 2 of 3

February 22, 2019 //By Bill Schweber
Nanoscale MEMS Resonator Structure Yields Controllable MHz-Range Filter, Isolator
By fabricating a nano-MEMS honeycomb lattice, researchers developed a solid-state device that can function as a voltage-tunable filter or unidirectional energy coupler for electrical energy in the 10 to 20-MHz range that’s been converted into acoustic-like phonons.

The neighbouring membranes are overlapped to create mechanical coupling. By changing the distance between etch holes, the researchers were able to control the amount of lattice coupling. Their work has been theoretical, beginning with a discrete mass-spring model and the many forces on each element (see figure 2).

2. Shown is the discrete-element model of the NEML
for analytical calculation of frequency dispersion.
At the top of (a) is the false-color scanning electron
microscope image showing three unit cells with the
deposited electrodes; at the bottom is a schematic
diagram of the mass-spring systems used to solve
the dynamic model. A close-up view of a half-unit cell
(b) shows the two rotational springs connected by a rigid
bar, and the elastic foundation, showing all relevant
parameters and degrees of freedom. (Source: Caltech)

They then proceeded to in-depth, deep-dive physics analysis of the lattice’s electromechanical and related properties, as well as its responses. Finally, they went beyond the theory and its postulated performance by fabricating and evaluating several devices. By applying a dc gate voltage VT to create a voltage-dependent electric field, they were able to significantly shift the frequency bands of the device (see figures 3 and 4).

It’s desirable that a physical channel being used as a waveguide be both stable and defect-free for reliable, consistent performance. However, they noted that energy transport in high-frequency mechanical systems, such as these microscale phononic devices, is particularly sensitive to defects and sharp turns because of backscattering and losses.

Frequency response of the lattices with 120 unit-cells.
The upper panel shows the response of the device
without electrodes, while the lower panel is with
electrodes; the shaded area indicates the frequency
stop bands. (Source: Caltech).

Since actual devices aren’t perfect, they also investigated the influence of possible fabrication errors that happen during the deposition process. In this case, the non-uniformity in the film thickness causes disorder in the mass matrix parameters used in the many equations of the dynamics.

4. Position of the bandgap center and size as a function of VT (left). The red and blue squares (dots) show the experimental (numerical) data for the bandgap center and bandgap frequencies, respectively. Averaged Q (quality) factor as a function of VT (right). Q is calculated by averaging f0/Δf of the individual peaks in the frequency spectra for different values of VT; f0 and Δf are the center frequency and full-width at half maximum of a resonance peak, respectively, while error bars represent the standard deviation of the resonance peaks for each VT. (Source: Caltech)

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