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John O | February 2017

Researchers using polymers to stabilize micro-hotplates for smart gas sensor


a team of researchers from the department of electrical and electronic information engineering at toyohashi university of technology (japan) have incorporated su-8, a polymer with mechanical stability and low thermal conductivity, as a supporting material to improve micro-hotplates in gas sensors.

 

gas_sensor_600

the researchers found that a thick layer of su-8 provided the necessary thermal isolation
in gas sensors. (toyohashi university)

 

according to a press release from the university, gas sensors for leakage alerts are common and important air quality monitors and there is a movement towards integrated chips that can provide sensing, signal processing, and communications. this would make the systems smaller and more cost-effective.

 

the article explained, “semiconductor gas sensors, which are the most widely used gas sensors, require a sensor material to be heated to several hundreds of degree celsius. therefore, in order to integrate these gas sensors with electronic circuits, a micro-hotplate (mhp), which is a mems-based heating structure, is required to thermally isolate the sensor and the circuits.”

 

su-8 is commonly used in microelectromechanical systems (mems), so adding the thick polymer to the micro-hotplates allows them to maintain mechanical stability in the high heat environment that the sensors require.

 

the mhp that the researchers developed had an area of 140 μm ´ 140 μm, and a 33-μm-thick su-8 layer deposited on its bridges. the mhp reached 500°c at five volts and consumed 13.9 mw for heating to 300°c. it maintain a constant operation under constant voltage for 100 minutes.

 

“owing to the thick su-8 layer, the mhp does not need the strict control of the stress that occurs inside the membrane during the fabrication process,” the press release explained. “this feature, together with the good thermal isolation property, enables the flexible layout design of the chip, and therefore, the mhp is beneficial to a miniature smart gas sensor chip. the researchers will advance their study to realize such smart gas sensors.”

 

the research was published in the journal of micromechanics and microengineering. the abstract read:

 

“in this study, bridge-type micro-hotplates (mhp) with an su-8 supporting layer were proposed for smart gas sensor applications. the proposed mhp consisted of a heating membrane with an area of 140 µm  ×  140 µm, and a 33 µm-thick su-8 layer deposited on its bridges. finite element method based simulation confirmed that the proposed mhp displayed good thermal isolation properties.

 

“the proposed mhp was successfully fabricated, and the properties of the mhp were characterized. current–voltage characteristics revealed that the mhp temperature can reach 550 °c at 5 v. the temperature of the mhp was calculated from changes in the resistance of the heater. power consumption of the mhp approximately corresponded to 13.9 mw for heating to 300 °c.

 

“this was comparable to the power consumption reported in the previous studies. furthermore, a stable operation under a constant voltage was observed for 100 min. the properties of the mhp indicated that it could potentially be utilized for applications related to integrated gas sensors.”

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