2016

GHz ultrasound MEMS transducer

Each black wire on the four sides has a width of  31.75 um

As a winner of Cornell Engineering Learning Initiatives (ELI) Undergraduate Research Award 2016, I was fully funded for a research project to study a MEMS GHz ultrasonic transducer developed by SonicMEMS lab. During my research, I was trained at Cornell NanoScale Science & Technology Facility (CNF) on microfabrication, thin film deposition and characterization.

This phased array has a 4-by-4 array of AlN piezoelectric elements, which can emit ultrasound at GHz frequency when pulses of RF signals are applied.

My goal was to study the steering and focusing of ultrasound waves through phasing, i.e. when the signals applied to the elements are given the appropriate lag/lead in their phases, interference between the sound waves emitted by different elements can cause the resulting sound wave to be steered and focused in a controlled manner.

My contribution includes designing a PCB for testing the given device and scripting MATLAB code to control an RF source, a function generator, and a GHz oscilloscope so as to generate the right pulses to be applied to each piezoelectric element.

 

I also researched on potential quarter wave length matching layers to be used for reducing acoustic impedance between this device and human skins.

The following diagram shows the PCB I designed for testing the ultrasonic transducer. It has 24 channels where 16 are used to send pulses to the 4-by-4 array of elements, while the remaining 8 are meant to be wire-bonded to 8 elements besides the array for receiving sound waves. Note that some traces looks convoluted because each trace must have the same length so that signal phases are not affected.

custom PCB for GHz ultrasonic transducer testing

Fabricated PCB with top layer in silver for ease of wire-bonding

The transducer is then bonded to this PCB with an ultrasound wire bonder as shown below.

Top and bottom of the transducer wire bonded to custom PCB

The diagram below shows the operating principle of this device. As each element emits ultrasound into silicon, ultrasound gets reflected and received by the piezoelectric elements.

Ultrasound gets reflected and received by the element [1]

When tested with a single channel and the device shows the following results. When no isopropyl alcohol (IPA) is applied on the backside of this device, we can see from the diagram on the left that the reflected signal strength decreases as sound wave dissipates in silicon. However, since the acoustic impedance of sound travelling from silicon to air is higher than that from silicon to IPA, when IPA is applied on the backside, the reflected signal decreases as sound wave travels into IPA and gets dissipated even more.

Then I tested multi-channel and phasing. Based on the picture below, I fixed the pulses applied to element 1 and made a phase sweep on element 2. By receiving from element 3 and 4, I was able to observe reasonable trends in the signal strengths.

[1] Hoople, J.T., Kuo, J.C., Abdel-Moneum, M., & Lal, A. (2015). Chipscale GHz ultrasonic channels for fingerprint scanning. 2015 IEEE International Ultrasonics Symposium (IUS), 1-4.

GHz ultrasound MEMS transducer with emitters in red and receivers in green