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Thermal measurement and management is crucial in three dimensional integrated circuits (3DICs) technology because increasing temperature stress is one of the main challenges due to high power density. Because of the physical adjacency and use of Through Silicon Vias (TSVs) as thermal exchangers between the stacked layers, the thermal profiles of the layers are highly correlated with each other. Any planar hotspot in a layer in a 3DIC is converted to a volumetric spatial hotspot. Run time thermal management in 3DICs requires proper monitoring and measurement of these spatial hotspots inside the chip. Having spatial hotspots and high thermal correlation between layers is a motivation for designing 3D thermal sensors. A new ring oscillator based 3D thermal sensor is proposed in this paper. Use of this sensor will reduce total number of needed sensors to monitor a typical whole 3DIC by 48% with same reading error in comparison with use of conventional 2D sensors.