Common sensor

A low-cost sensor developed by a Cambridge University spin-out could lead to inexpensive handheld gas monitors.

The devices could be used to detect toxic and combustible gases in automotive, environmental and other applications.

The sensor, from Cambridge CMOS Sensors, is based on a tungsten micro-hotplate with integrated electronics. The electronics are fabricated using the same silicon-on-insulator CMOS process used to make integrated circuits.

The developers claim that their design overcomes previous technical challenges that prevented resistive gas sensors from being used in portable, battery-powered instruments and low-power automotive units.

Through laboratory tests, the sensor has shown it can operate in temperatures up to 500°C, using no more than 5-10MW in continuous operation. Cambridge CMOS Sensors claims that existing sensors use around 1W and have a much slower response time. According to Julian Gardner, co-founder of the company, the thermal response time of the sensor is in milliseconds, compared to seconds for existing sensors.

‘We are able to use thermal modulation techniques to extract more chemical information,’ he said. ‘We have shown that one sensor can detect three different gases, thus improving selectivity and reducing cost for multi-gas sensing.’

Resistive gas sensors consist of a gas sensing material such as doped tin oxide heated by a micro-hotplate, which is a heater isolated on a membrane.

The gas concentration is determined by the change in resistance of the sensing material. Micro-calorimeters use a micro-hotplate with a catalyst such as palladium to catalyse the combustion of a flammable gas. The temperature rise caused by the combustion is directly proportional to the gas concentration.

Gardner, who heads the Sensors Research Laboratory at Warwick University, joined forces with Cambridge CMOS Sensors co-founder Florin Udrea from Cambridge University, several years ago to develop a kind of resistive gas sensor. One of their ideas for improvement focused on the sensing material used to detect the gas concentrations.

They initially considered using carbon nanotubes, zinc-oxide nanowires and mesoporous tungsten oxide.

‘We found that metal-oxide nanowires are better than carbon nanotubes,’ he said. ‘We are still working on the mesoporous films, which we are now testing for hydrogen sensitivity.’

Other researchers have attempted to create silicon-based micro-hotplate designs for resistive gas sensors. However, Gardner said he and Udrea are the first to develop a silicon-based micro-hotplate that can be manufactured through low-cost CMOS fabrication. ‘The main challenge was to design isothermal hotplates with good thermal stress management,’ he said. Gardner and Udrea developed the idea to use tungsten for the micro-hotplate’s heater and circuit interconnects.

Cambridge CMOS Sensors’ third founder, Bill Milne, said that tungsten is already used for circuit interconnects but Garden and Florin believed it could also be applied to other areas of their micro-hotplate design.

‘You can go to higher temperatures with tungsten than aluminium,’ he said, referring to the metal many other researchers used in their silicon-based micro-hotplate designs.

Some possible applications for the sensors include detecting air quality on aircraft for gases including carbon monoxide or it could be developed for breathalysers.

Gardner said that Cambridge CMOS Sensors is working with AlphaSense and other gas sensor companies to commercialise the technology.

‘We are also developing new markets for micro-hotplates outside the field of gas sensing,’ he added.

Siobhan Wagner