Surface acoustic waves and nanoscale imaging

Background on SAW devices, their nanofabrication, and imaging GHz acoustic waves with microwave impedance microscopy.

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Background · Fabrication · TMIM

This page gives the background for my work on nanoscale surface acoustic wave devices: what these waves are, how they are generated and fabricated on chip, and how we image them.

What is a surface acoustic wave?

Bulk and surface acoustic waves, and the Rayleigh, Love and Lamb wave modes
Bulk versus surface acoustic waves, and common surface-guided modes. Adapted from J. Micromech. Microeng. 31, 054003 (2021).

Mechanical vibrations in a solid travel as elastic waves: strain and stress coupled through the material’s elasticity. In the bulk, these waves fill the whole volume. At a free surface, the boundary conditions allow a different class of solutions: surface acoustic waves (SAWs), whose energy is confined within roughly one wavelength of the surface.

Depending on the stack and polarization, several surface-guided modes exist:

  • Rayleigh waves, with elliptical particle motion in the sagittal plane.
  • Love waves, which are shear-horizontal waves guided by a slower surface layer.
  • Lamb waves, which are plate modes in thin membranes.

In a piezoelectric material, each of these mechanical waves carries an electric potential with it. This coupling is what lets us launch, detect, and image them electrically.

SAW in MEMS: interdigital transducers

Optical image of an interdigital transducer and a schematic of SAW generation
An interdigital transducer (IDT) and a schematic of how it launches a surface acoustic wave.

The standard way to generate a SAW on chip is an interdigital transducer (IDT): interleaved metal fingers patterned on a piezoelectric film. An RF voltage across the fingers creates a periodic strain through the piezoelectric effect. When the drive frequency matches the finger period (λ = 2d in the schematic), the strain adds up coherently into a propagating surface wave.

Because SAW velocities are only a few kilometers per second, a micrometer-scale wavelength corresponds to gigahertz frequencies. This makes SAW devices compact RF components: they are the filters in every mobile phone. They are also a platform for sensing, acoustofluidics, and quantum acoustics.

Fabrication of nano-SAW devices

Device development loop and schematics of lithography, metal deposition and etching
The device development loop, and the lithography, metal deposition, and etching steps used in nanofabrication.

Building a device is a loop: simulation → CAD → fabrication → characterization, with each measurement feeding back into the next design. On the fabrication side, I patterned features using:

  • Electron-beam lithography: spin-coat resist, expose, and develop.
  • Metal deposition and lift-off to form electrodes.
  • Etching to transfer patterns into the piezoelectric layer. Physical sputtering, plasma etching, and reactive-ion etching differ in anisotropy and selectivity.

Conventional characterization

A vector network analyzer and an S21 transmission spectrum of a SAW device
A vector network analyzer and an S-parameter (S21) measurement of a SAW device.

The workhorse measurement is a vector network analyzer (VNA). It records S-parameters, the reflection and transmission between ports, as a function of frequency. This is precise and fast, but it only sees the device from its electrical ports and does not reveal where the energy goes.

Stroboscopic X-ray and laser interferometry images of surface acoustic waves
Imaging SAWs with stroboscopic X-ray and laser interferometry. From IEEE Microwave Magazine 21, 60 (2020).

Optical and X-ray methods, such as laser interferometry and stroboscopic X-ray imaging, can map the wave field. However, they are limited in spatial resolution, frequency range, or accessibility for nanoscale gigahertz devices.

Transmission-mode microwave impedance microscopy

Schematic of transmission-mode microwave impedance microscopy with channel 1 and channel 2 SAW images and line profiles
Transmission-mode microwave impedance microscopy (TMIM): setup, demodulation, and channel 1 / channel 2 images of a propagating SAW. Adapted from Phys. Rev. Applied 9, 061002 (2018).

Transmission-mode microwave impedance microscopy (TMIM) puts the detector on a scanning tip:

  1. The IDT launches the acoustic wave.
  2. A shielded cantilever probe picks up the oscillating piezoelectric potential at its position.
  3. The signal is demodulated against the source reference, giving two channels proportional to cos(kx + φ) and sin(kx + φ). Together they give the local amplitude and phase of the wave.

TMIM works up to about 10 GHz, with roughly 100 nm spatial resolution and a displacement sensitivity of about 1 fm/√Hz. That is enough to image wavefronts, standing waves, and leakage directly inside a working device.

In my work, I built a microwave impedance microscopy setup and developed Python tools to align and stitch scans, which made it possible to map large device areas.