Researchers at the National
Institute of Standards and Technology (NIST) have developed a new calibration
technique that will improve the reliability and stability of one of NIST's
most versatile technologies, the microhotplate. The novel NIST device is being
developed as the foundation for miniature yet highly accurate gas sensors that
can detect chemical and biological agents, industrial leaks and even signs of
extraterrestrial life from aboard a planetary probe.
The NIST microhotplate uses its thermal efficiency in conjunction with a thermocouple to form a self-test temperature sensing system. Four microhotplates (left image) are seen with a strip of rhodium film (marked by an arrow) crossing the bottom right microhotplate. This strip makes contact with the platinum in the microhotplate structure (seen in the closeup image on the right) to form a stable thermocouple for measuring temperature. Credit: M. Afridi, NIST
The tiny microhotplates-no wider than a human hair-are programmed
to cycle through a range of temperatures. They can be coated with metal oxide
films tailored to detect specific gas species. Airborne chemicals attach to
the surface of the detector depending on the type of film and the temperature
of the surface, changing the flow of electricity through the device, which serves
as the "signature" for identifying both the type and concentration
of the gas in the ambient air.
Accurate microhotplate temperature measurements are crucial for the discrimination
and quantification of gas species, while reliable, long-term operation demands
that the microhotplate's temperature sensors be either highly stable or
able to sense when they've drifted, a functionality known as a "built-in
self test" (BIST). As demonstrated for the first time in a paper in an
upcoming issue of IEEE Electron Device Letters,* the new calibration method
satisfies both requirements.
A portion of the polysilicon heater making up the microhotplate originally
served as the device's temperature sensor. However, this sensor would
slowly drift over time from its initial calibration. Within three months, the
temperature readings were off by as much as 25 degrees Celsius at high temperatures.
The NIST engineers overcame this shortcoming by using data from two additional
temperature sensors-a highly stable, thin-film platinum/rhodium thermocouple
integrated in the microhotplate structure for one sensor and the thermal efficiency
of the structure itself for the other. Comparing the temperatures reported by
these two sensors provides the microhotplate with its internal monitoring system.
As long as the absolute value of the difference between the reported temperatures
remains below a specified threshold value, the average of the two readings is
considered reliable. Should the difference exceed the threshold, the system
reports an error.
The original polysilicon sensor still provides the microhotplate's initial
temperature measurement, which is used to calibrate the other two sensors. With
the complete "check and balance" system in place, temperature measurements
are accurate to within 1.5 degrees Celsius.
Having successfully demonstrated the new temperature calibration system for
their microhotplate, the NIST researchers are working on additional advancements
for the technology. Next in line is the development of a built-in system for
sensing contamination of the metal oxide films critical to the microhotplate's
use in gas detection.
* M. Afridi, C. Montgomery, E. Cooper-Balis, S. Semancik, K.G. Kreider and
J. Geist. Analog BIST functionality for microhotplate temperature sensors. IEEE
Electron Devices, Volume 30, No. 9 (September 2009).