NAPA, Calif.—The world's first gas sensor small enough for any smartphone was shown at the MEMS Executive Congress 2015 (held here, Nov. 4-6). Manufactured by Cambridge CMOS Sensors Ltd. (U.K.), the tiny 1 millimeter square MEMS-CMOS die are small and cheap enough to become ubiquitous—for the first time beating Apple in new types of MEMS sensors in smartphones.
Cambridge also announced its first design-win in K-Free Wireless Ltd. (Shenzhen, China). Carriers apply their own name to K-Free's white-box smartphones.
Cambridge CMOS Sensors' design win at K-Free one-ups Apple with its new all-digital construction of a MEMS-CMOS CCS811 sensor. The sensor can be configured to sense volatile organic compounds (VOC) such as carbon monoxide from cheap heaters, formaldehyde in cheap furniture, or even used as a breathalyzer since driving-drunk is a jail-sentence offense in China, according to Brown.
In the U.K., a relatively new type of sealed house construction practice—putting plastic under the entire house, including the actual foundation—has opened a market for sensing noxious gas build-ups inside houses where there is no ongoing ventilation.
For other worldwide markets, the all-digital CCS811 sensor can be configured with different top-metal oxides or filters to detect only CO2, only ethanol, or nearly any other noxious gas. It can also be configured to measure the outside air quality, including nitrogen dioxide [NO2], in cities from Beijing to Los Angeles.
"It can tell you when to open window for ventilation while inside where very low levels of VOCs have been found to make people's minds 61 percent less efficient in decision making," claimed Brown.
But the biggest benefit of the MEMS-CMOS gas sensor, according to Brown, is that it can be scaled continually as the International Technology Roadmap for Semiconductors (ITRS) scaled silicon chips to smaller and smaller sizes.
The very top layer of the chip has a metal-oxide layer (such as tin-oxide for VOCs) with gold contacts beneath it that measure the resistance of the metal oxide. Underneath the gold interconnections is the tiny hot plate that heats the whole assembly, thereby changing the resistance of the top layer in direct proportion to the gas coming in the package (through a hole).
The newest digital version then uses a Silicon Labs 8051 microcontroller (housed in the same package as the MEMS-CMOS die) to measure the resistance between the gold electrodes and buffer its changes in real time. When it passes a given level, an alarm interrupt is sent to the application processor in the smartphone (or the application processor polls the 8051 periodically to empty its buffer and draw a parts-per-million--PPM--graph of air quality, for instance, depending upon the app being run).
Five-year battery lifetimes are easy to attain with small lithium batteries with the current model. The package size today is 2.7-by-4.0-by-0.6 millimeters to hold both the CCS811 and 8051 in a land-grid array package and I2S interface using less than 1.2 milliwatts when active and less than 6-microwatts in idle mode.
Cambridge previously had an analog version that required the OEM to add its own microcontroller or, alternatively, to waste valuable application processor cycles turning on the micro hotplate, measuring the resistance between the gold electrodes and analyzing the results. The new CCS811 digital model, however, is expected to allow the gas sensor to penetrate the billion unit mass market for smartphones, wearables and Internet of Things (IoT) devices.
Founded in 2008, Cambridge CMOS Sensors took six years to develop its analog version, but only a year to follow up with digital. Next they are going to house multiple CCS811 die in the same package to sense a whole variety of gases to serve consumer and industrial sensor networks, as well as other mass markets that need small ultra-low-power gas sensors.