Thermal diffusivity-based temperature sensors in 65nm CMOS

Abstract

Integrated temperature sensors are widely used in various applications, including thermal monitoring, medical devices, and voltage/frequency references. An emerging class of temperature sensors operates by measuring the thermal diffusivity (TD) of silicon. This can be done by measuring the time it takes for heat to diffuse between a heater and a relative temperature sensor, both implemented in a silicon substrate. The key advantage of such TD-based sensors is that their accuracy improves linearly with lithographic precision.

Increasing the length of the heat path and/or moving to a more advanced technology node will enhance measurement accuracy. However, since silicon is an excellent heat conductor, the output of such sensors is at the millivolt level, which makes the design of accurate readout circuitry quite challenging.

In this work, an accurate readout circuit was designed for TD-based sensors realized in a 65nm process. It consists of a low-noise pre-amplifier, followed by a Sigma-Delta ADC. Based on simulation results, the designed circuit achieves a 23 mK (3σ) inaccuracy at 27◦C, which is negligible compared to the expected inaccuracy of the TD-based sensors.