Thermal Diffusivity Sensors: Temperature Sensors that Scale

SAN FRANCISCO—Thermal diffusivity based sensors are the future, according to professor Kofi Makinwa of Delft University of Technology, speaking at the International Solid States Circuits Conference here Monday (Feb. 20).

Makinwa said he has been working on the technology for six years and that their importance was increasing owing to the fact thermal sensors are to be found in practically any computer system.

“Systems would self-destruct if you let them have their own way,” he noted, explaining the need for a more practical heatsink.

With systems becoming increasingly multi-core, hot spots throughout the processor are increasing, said Makinwa. The problem is becoming progressively complex because the hot spots move dynamically based on various processes, he said.

“SoC monitoring needs a fast conversion rate since thermal transients can have milisecond time constraints,” said Makinwa, adding that they also needed to be very small and consume as little power as possible, though they didn’t necessarily need to be highly accurate.

Since most properties of silicon are temperature dependent, Makinwa said band gap temperature sensors were simply not good enough and that a different approach was needed.

“Heat diffusion is a mechanical process,” said Makinwa, noting that this needed to be more about phonons than electrons.

The system works by measuring the thermal delay between an on-chip heater and an on-chip relative temperature sensor. That delay can then be digitized or used to define the output frequency of an oscillator.

“Thermal diffusivity of silicon is strongly temperature dependent,” said Makinwa, noting that thermal diffusivity based sensors were fully CMOS compatible, were insensitive to doping variations, packaging stress and could operate at higher temperatures, even up to 250 degrees centigrade. They also have performance accuracy and speed that scales with process, said Makinwa.

Makinwa said proof-of-concept thermal diffusivity sensors have already been fabricated in 0.7μm and 0.18μm bulk CMOS, as well as in 0.5μm SOI technology and were found to work over a wide temperature range from -70°C to 170°C.

An implementation in 0.18μm bulk CMOS achieved an untrimmed inaccuracy of ±0.2°C (3σ) from -55°C to 125°C, said Makinwa, concluding that therefore, TD sensors were the best solution for thermal management in nanometer CMOS.
Thermal Diffusivity Sensors: Temperature Sensors that Scale

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