Taking the heat

29 August 2013

Silica aerogel

Silica aerogel is a very porous material and it was a challenge to create a good quality thin film

Aerogel could be used as an alternative insulator

The use of aerogel as an alternative insulator to an air gap will allow higher density sensor arrays to be fabricated

A team at the University of Louisiana at Lafayette in the US have created a thin film of silica aerogel on a silicon wafer and have demonstrated its potential to make more efficient metal oxide semiconductor (MOX) gas sensors.

Showing resistance

MOX gas sensors have applications in many areas, such as the detection of flammable or hazardous gases, and the aging of food stored in warehouses or refrigerators.

Using materials such as tin oxide, zinc oxide and titanium oxide, they work by detecting the change in resistance of the MOX material when a gaseous substance passes over the sensor. An array made of several MOX materials (out of a total of twenty-one) can detect almost any gaseous substance.

To enable the changes in resistance of the MOX materials, the sensor elements need to be maintained at a temperature of 300-500oC depending on the sensing material. They are fabricated onto a microhotplate with a heater to reach and maintain the temperature.

For the sensor to have a fast response, the heater must be able to bring the elements to the operating temperature quickly and evenly, while using as little power as possible. The heater therefore needs to be insulated from the substrate and usually an air pit is micromachined into the substrate, and the heater and sensing elements are suspended on a narrow bridge above the air gap.

Mind the gap

There are several disadvantages to the air gap approach: the number of sensor elements that can be included is reduced and each needs their own heater; the micromachining process is time-consuming and complicated; and the delicate microhotplate bridges can be damaged by post-processing procedures, resulting in fewer perfect sensors.

The researchers at the University of Louisiana at Lafayette have been studying silica aerogel as an alternative and the work presented in their Letter in this issue of Electronics Letters demonstrates the fabrication of a good quality thin film of silica aerogel and a first demonstration of its suitability for use in MOX sensors.

“Silica aerogel is a highly porous nanomaterial that has amazing heat insulation characteristics with a thermal conductivity less than that of the air,” said the first author Mohammad Seyed Jalali.

“Fabricating and processing a thin film of silica aerogel with high porosity was the most challenging part of our work. There were so many factors affecting the quality of aerogel thin film such as the spin coating environment, temperature and pressure control, solvent extraction, and supercritical drying. Furthermore, the resulting thin film aerogel needs to be crack-free and uniform to allow CMOS compatible processing.”

Through thick and thin

In their work, the researchers processed a 0.8µm thick film on a silicon wafer, and also successfully deposited a heating element on top of the aerogel. They achieved a temperature of 150oC by applying the relatively low power of 1.8 µW/µm2; a result that was in good agreement with their simulations.

They are now trying to develop thick layer aerogel processing so as to reduce the power consumption further. Their simulations have already shown that this will enable ultra-low power consuming gas sensors.

“Thick layer insulating aerogel can be achieved by either multilayer processing of aerogel with spin coating multiple times or creating deep cavities inside the silicon substrate and filling them with aerogel,” said Dr Mohammad Madani, the group leader for this project. “We have been studying the feasibility of processing recessed aerogel in the substrate.”

Ultimately the team would like to realise ultra-dense ultra-low power sensor arrays compatible with CMOS processing using silica aerogel. They would like to develop the processing techniques further so that any efficient heater design could be deposited on the thin-film aerogel, and that aerogel masking could be used to provide heat insulation only in the areas needed, and not over the whole substrate.

Further reading

This article is based on the Letter: Ultra dense and ultra low power microhotplates using silica aerogel (new window).

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, Volume 49, Issue 25

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