Monolithic Three-Dimensional Integration of Carbon Nanotube Circuits and Sensors for Smart Sensing Chips

Semiconducting carbon nanotube (CNT) film is a promisingmaterialfor constructing high-performance complementary metal-oxide-semiconductor(CMOS) integrated circuits (ICs) and highly sensitive field-effecttransistor (FET) bio/chemical sensors. Moreover, CNT logic transistorsand sensors can be integrated through a compatible low-temperaturefabrication process, providing enough thermal budget to constructmonolithic three-dimensional (M3D) systems for smart sensors. However,an M3D sensing chip based on CNT film has not yet been demonstrated.In this work, we develop M3D technology to fabricate CNT CMOS ICsand CNT sensor arrays in two different layers; then, we demonstratea preliminary M3D sensing system comprising CNT CMOS interfacing ICsin the bottom layer and CNT sensors in the upper layer through interlayervias as links. As a typical example, a highly sensitive hydrogen sensingIC has been demonstrated to perform in situ sensingand processing functions through upper-layer FET-based hydrogen sensorsexposed to the environment and bottom-layer CNT CMOS voltage-controlledoscillator (VCO) interfacing circuits. The M3D CNT sensing ICs converthydrogen concentration information (8-128 ppm) to digital frequencyinformation (0.78-1.11 GHz) with a sensitivity of 2.75 MHz/ppm.M3D sensing technology is expected to provide a universal sensingsystem for future smart sensing chips, including multitarget detectionand ultralow power sensors.