Compact Thermal-Diffusivity-Based Temperature Sensors in 40-nm CMOS for SoC Thermal Monitoring

An array of temperature sensors based on the thermal diffusivity (TD) of bulk silicon has been realized in a standard 40-nm CMOS process. In each TD sensor, a highly digital voltage-controlled oscillator-based Sigma Delta ADC digitizes the temperature-dependent phase shift of an electrothermal filter (ETF). A phase calibration scheme is used to cancel the ADC's phase offset. Two types of ETF were realized, one optimized for accuracy and one optimized for resolution. Sensors based on the accuracy-optimized ETF achieved a resolution of 0.36 degrees C (rms) at 1 kSa/s, and inaccuracies of +/- 1.4 degrees C (3 sigma, uncalibrated) and +/- 0.75 degrees C (3 sigma, room-temperature calibrated) from -40 degrees C to 125 degrees C. Sensors based on the resolution-optimized ETFs achieved an improved resolution of 0.21 degrees C (rms), and inaccuracies of +/- 2.3 degrees C (3 sigma, uncalibrated) and +/- 1.05 degrees C (3 sigma, room-temperature calibrated). The sensors draw 2.8 mA from supply voltages as low as 0.9 V, and occupy only 1650 mu m(2), making them some of the smallest smart temperature sensors reported to date, and well suited for thermal monitoring applications in systems-on-chip.