Thermal Effects in Monolithic 3D ICs: Stacking and interconnecting a number of chips is one way to build a 3D integrated circuit, but another way is to build a 3D IC monolithically. Monolithic 3D ICs are fabricated step-by-step with sequential placement and connection of devices located in different layers of the same chip. Heat dissipation is an issue with the monolithic approach, because heat can negatively affect transistor electrical characteristics, performance and reliability. A particular area of concern is the need to understand the thermal effect of the second layer on the performance of devices in the first layer. A research team led by CEA-Leti investigated this in 3D ICs made from SOI wafers with 7nm-thick Si layers, with a base oxide thickness of 145nm. They used multiple p- and n-channel MOSFET configurations, heater-sensor arrangements and thermometry techniques. They found that the self-heating of individual transistors is more significant than thermal coupling between the layers. Moreover, for the first time they experimentally validated that the increase in a device’s channel temperature – as measured by gate-resistance thermometry – matches the value mathematically derived from the subthreshold slope. That is an important advance for monolithic 3D IC technology, because it can be used to anticipate and manage thermal effects in both logic and analog applications.
The top image is an electron microscope view of two stacked transistors fabricated in 3D sequential integration technology.
In the middle is a sequence of schematics showing the different configurations studied by the authors. C1 corresponds to a reference device fabricated with a standard hot process, whereas C2-C6 are 3D sequentially integrated. In C5 & C6 the two stacked transistors are connected by either the source (C5) or the gate (C6).
The bottom image is a schematic of the heater-sensor thermometry technique described by the authors. In this configuration a temperature change can be measured by either the gate resistance or by the subthreshold slope (SS) of the sensor.
(Paper 7.6, “Thermal Effects in 3D Sequential Technology,” K. Triantopoulos et al, CEA-Leti/Univ. Grenoble Alpes/IMEP-LAHC)