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Chip-Based Sensors with High Sensitivity Developed

02 November 2017

This is a chip-scale glass microspherical shell sensor array blown on a silicon substrate. Insert is a near-perfect spherical shell. Source: Tadigadapa Lab/Penn StateThis is a chip-scale glass microspherical shell sensor array blown on a silicon substrate. Insert is a near-perfect spherical shell. Source: Tadigadapa Lab/Penn State

In St. Paul’s Cathedral in London, a whisper can be heard across the circular whispering gallery. Electrical engineers have developed an optical whispering gallery mode resonator that can spin light around the circumference of a tiny sphere millions of times. This creates an ultrasensitive microchip-based sensor for multiple applications.

"Whispering gallery mode resonators, which are basically optical resonators, have been intensely studied for at least 20 years," said Srinivas Tadigadapa, professor of electrical engineering. "What people have done is to take an optical fiber and touch the end with a blow torch. When the melted fiber re-condenses, it forms a sphere at the tip. This can be coupled to a light source to make a sensor."

This type of sensor consists of solid spheres and is not compatible with microfabrication methods. But recently Tadigadapa and his team developed an innovative way to grow on-chip glass microspherical shells with incredible sensitivities that potentially can be used for motion, temperature, pressure or biochemical sensing.

The hollow borosilicate glass spheres are blown from sealed and pressurized cylindrical cavities etched into a silicon substrate. Using a glassblowing technique, the thin glass wafer, under high heat and external vacuum pressure, forms an almost perfect bubble. The researchers grew assortments of spheres from 230 microns to 1.2 millimeters in diameter, with a wall thickness between 300 nanometers and 10 micrometers.

"The bottom of the sphere is thinned until it is basically a hole," Tadigadapa said. "You can put the light on the outside of the sphere but do all the chemistry on the inner face of the shell. You can bring in any analyte that you want to identify, but it goes on the inner surface. That brings in a lot of possibilities. You can do chemical sensing, vapor sensing, biophysical sensing, pressure sensing and really outstanding temperature sensing."

After a few failed attempts, a team discovered that the key to making a high-quality sensor lies in making sure the equatorial plane of the sphere, its center, is above the surface of the chip.

To get an understanding of the quality of their spheres, Tadigadapa’s doctoral student Chenchen Zhang and recent doctoral graduate Eugene Freeman worked with Alexander Cocking, a doctoral student in the lab of Penn State laser expert Zhiwen Liu, professor of electrical engineering.

"The bottom of the sphere is thinned until it is basically a hole," Tadigadapa said. "You can put the light on the outside of the sphere but do all the chemistry on the inner face of the shell. You can bring in any analyte that you want to identify, but it goes on the inner surface. That brings in a lot of possibilities. You can do chemical sensing, vapor sensing, biophysical sensing, pressure sensing and really outstanding temperature sensing."

The paper on this research was published in the journal Scientific Reports.

To contact the author of this article, email Siobhan.Treacy@ieeeglobalspec.com


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