Manufacturing advances pave way for CMOS gas sensors

Article By : David Pugh

New methods of manufacture and deposition of materials by flame pyrolysis mean that millions of sensors can be made daily in a foundry.

As advances in miniaturisation and manufacturing technologies open new markets to players in the MEMS sensing industry, foundries are now forecast to produce millions of gas sensors in the coming years.

Metal oxide (MOS) gas sensors were developed in the 1960s through research in Kobe, Japan. The sensing devices consisted of a screen-printed metal oxide layer (usually a modified version of SnO2, CuO or ZnO) heated to a high operating temperature (300-500°C). At such high temperatures, oxygen adsorbs to the surface and adjusts the conducting properties of the material, when gases in the atmosphere subsequently react with the adsorbed oxygen, the conducting properties of the material are further adjusted.

These discoveries created an industrial safety sector worth billions of dollars in commercial sensors for detecting toxic gases and monitoring carbon monoxide, partially driven by regulations in several countries. However, the large form factor and power requirements of such sensors prevented them from being used in more commercial fields until now.

Microelectromechanical system (MEMS) manufacturing processes have enabled MOS sensors to be manufactured in much higher volumes. Previously it was possible to print round 200 sensors simultaneously, requiring multiple layers of printed materials in a lab. New methods of manufacture and deposition of materials by flame pyrolysis mean that millions of sensors can be made daily in a foundry. Power requirements have been reduced by an order of magnitude, with sensors now consuming less than 10mW thanks to using a micro hotplate, as opposed to heating through platinum wire and using complex circuitry.

This new generation of chemical sensors collectively known as MEMS or CMOS gas sensors has allowed MOS sensing principles to be scaled down to a component level device. They are suitable for integration into many handheld, low-power devices such as phones and tablets as well as wearable badges and jewellery. Such sensors are poised to make a big impact as environmental gas sensors with the ability to detect pollutants in the air, expanding a market that IDTechEx expects will be worth $3 billion by 2027.

This revolution in the manufacturing process has led to several acquisitions in the gas sensor industry. Most notably AMS, one of the world’s largest sensor manufacturers, purchased Cambridge CMOS Sensors, a spin out from the universities of Cambridge and Warwick.

Cambridge CMOS grew from an early stage spin out to a world leader in gas sensor technology in less than six years thanks to its key position in manufacturing sensors using such innovative packaging techniques. AMS has subsequently seen large uptake in the Chinese market for components in smart phones and fitness trackers used as environmental monitors and breathalysers to monitor alcohol content.

__Figure 1:__ *The Cambridge CMOS CCS811 indoor quality monitor is one example of the new chemical gas sensors. (Source: IDTechEx)*

Manufacturers of more traditional MOS gas sensors are rushing to catch up with these emerging players. Figaro Engineering in Japan launched a range of miniaturised sensors to accompany its portfolio of sensors that are primarily used for carbon monoxide monitoring and are found in most homes in the U.S. and Europe.

Sensirion recently sold its entire barometric pressure sensor division to focus its resources on the growing market for environmental gas sensors. Its first gas sensor will be launched in 2017 and uses multiple metal oxide materials on a single micro hotplate to measure multiple gases on a sensor that measures just 2.45mm x 2.45mm x 0.9mm.

Such advances are driving gas sensors into consumer electronics devices with large uptake expected over the next five years. Expect to see these components in cell phones and wearable devices as well as indoor monitoring devices to monitor and combat the growing air pollution crisis in cities around the world.

This article first appeared on EE Times U.S.

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