Abstract
This paper presents a CMOS temperature sensor based on the thermal dependencies of the leakage currents targeting the 65 nm node. To compensate for the effect of process fluctuations, the proposed sensor realizes the ratio of two measures of the time it takes a capacitor to discharge through a transistor in the subthreshold regime. Furthermore, a novel charging mechanism for the capacitor is proposed to further increase the robustness against fabrication variability. The sensor, including digitization and interfacing, occupies 0.0016 mm2 and has an energy consumption of 47.7–633 pJ per sample. The resolution of the sensor is 0.28 °C, and the 3σ inaccuracy over the range 40–110 °C is 1.17 °C.
Highlights
The same technical advances that have permitted the aggressive technology scaling we have and continue to witness, such as sub-wavelength lithography, have increased the significance and complexity of process variations
The sensor displays an effective resolution of ±0.20 ◦ C. Note that these figures do not include the effect of the thermal noise present at the floating capacitor; in the worst case, for the maximum temperature, 110 ◦ C, the root mean square (RMS) value of this noise is approximately 210 μV, which has a negligible impact compared to the non-linearities of the sensor response
The power consumed by the sensing part is negligible when compared to the digitization part; one might think that the voltage divider—formed by a chain of PMOS devices connecting VDD and GND—could represent a significant contribution to the power of the whole system; this divider is only active during the duration of the second measure and, in any case, is very small compared to the digitization circuitry
Summary
The same technical advances that have permitted the aggressive technology scaling we have and continue to witness, such as sub-wavelength lithography, have increased the significance and complexity of process variations. Concerning power issues, increasing power densities worsen the thermal impact on reliability and performance, while decreasing supply voltages worsen leakage currents and noise; leakage power varies exponentially with key process parameters, such as gate length, oxide thickness and threshold voltage. In this context, temperature is by itself a source of new variations—many aging process are tightly coupled with thermal gradients and stresses—but, a victim of process uncertainties. The varying pulse width is generated employing the thermal dependencies of an on-chip resistor These sensors have been employed to monitor the temperature of mobile DRAM (Dynamic Random Access Memory), as described in [5].
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