# Glass Microbubble Encapsulation for Improving the Lifetime of a Ferrofluid-Based Magnetometer

## TL;DR

This paper shows how sealing a ferrofluid-based magnetometer in a glass microbubble significantly improves its lifetime by preventing fluid evaporation.

## Contribution

The novel use of glass microbubble encapsulation to enhance the longevity of a ferrofluid-based magnetometer is introduced.

## Key findings

- Glass microbubble encapsulation extended the device's lifetime from a few hours to over 50 days.
- The magnetometer achieved a noise equivalent magnetic field of 600 nT/√Hz at 0.5 Hz with a Metglas flux concentrator.
- The device can sense magnetic fields up to a modulation frequency of 40 Hz.

## Abstract

In this paper, we explore the use of chip-scale blown glass microbubble structures for MEMS packaging applications. Specifically, we demonstrate the efficacy of this method of packaging for the improvement of the lifetime of a ferrofluid-based magnetoviscous magnetometer. We have previously reported on the novel concept of a ferrofluid based magnetometer in which the viscoelastic response of a ferrofluid interfacial layer on a high frequency shear wave quartz resonator is sensitively monitored as a function of applied magnetic field. The quantification of the magnetic field is accomplished by monitoring the at-resonance admittance characteristics of the ferrofluid-loaded resonator. While the proof-of-concept measurements of the device have been successfully made, under open conditions, the evaporation of the carrier fluid of the ferrofluid continuously changes its viscoelastic properties and compromises the longevity of the magnetometer. To prevent the evaporation of the ferrofluid, here, we seal the ferrofluid on top of the micromachined quartz resonator within a blown glass hemispherical microbubble attached to it using epoxy. The magnetometer design used a bowtie-shaped thin film Metglas (Fe85B5Si10) magnetic flux concentrator on the resonator chip. A four-times smaller noise equivalent, a magnetic field of 600 nT/√Hz at 0.5 Hz was obtained for the magnetometer using the Metglas flux concentrator. The ferrofluid-based magnetometer is capable of sensing magnetic fields up to a modulation frequency of 40 Hz. Compared with the unsealed ferrofluid device, the lifetime of the glass microbubble integrated chip packaged device improved significantly from only a few hours to over 50 days and continued.