Thermal diffusivity sensing: A new temperature sensing paradigm

This paper presents an overview of recently developed absolute temperature sensors based on the measurement of the thermal diffusivity of silicon, D. Such sensors make use of the fact that, in IC-grade silicon, D has a well-defined temperature dependence and is insensitive to process spread. D can be determined by measuring the thermal delay between an on-chip heater and an on-chip relative temperature sensor This delay can then be digitized or used to define the output frequency of an oscillator. Proof-of-concept thermal diffusivity (TD) sensors were fabricated in 0.7μm and 0.18μm bulk CMOS, and in 0.5μm SOI technology. Their outputs are well-defined functions of temperature and they work over a wide temperature range from -70°C to 170°C. It was found that the inaccuracy of TD sensors scales with process technology and is insensitive to mechanical stress. An implementation in 0.18μm bulk CMOS achieved an untrimmed inaccuracy of ±0.2°C (3σ) from -55°C to 125°C.