Abstract
Carbon nanotubes (CNTs) can be grown locally on custom-designed CMOS microstructures to use them as a sensing material for manufacturing low-cost gas sensors, where CMOS readout circuits are directly integrated. Such a local CNT synthesis process using thermal chemical vapor deposition (CVD) requires temperatures near 900 °C, which is destructive for CMOS circuits. Therefore, it is necessary to ensure a high thermal gradient around the CNT growth structures to maintain CMOS-compatible temperature (below 300 °C) on the bulk part of the chip, where readout circuits are placed. This paper presents several promising designs of CNT growth microstructures and their thermomechanical analyses (by ANSYS Multiphysics software) to check the feasibility of local CNT synthesis in CMOS. Standard CMOS processes have several conductive interconnecting metal and polysilicon layers, both being suitable to serve as microheaters for local resistive heating to achieve the CNT growth temperature. Most of these microheaters need to be partially or fully suspended to produce the required thermal isolation for CMOS compatibility. Necessary CMOS post-processing steps to realize CNT growth structures are discussed. Layout designs of the microstructures, along with some of the microstructures fabricated in a standard AMS 350 nm CMOS process, are also presented in this paper.
Highlights
Nanomaterials are a popular research topic as sensing materials for various sensing applications [1,2,3], including gas detection [4,5,6,7,8], due to their compact size [9], high sensitivity [10], low operating temperatures [11], low power consumption [12], etc
Local carbon nanotubes (CNTs) synthesis ensures CNTs with excellent structural quality, but the CMOS-compatible temperature is the main restriction in this integration process [32], and that is what we aim to solve in our approach of CMOS–CNT integration
The simulation results presented in this paper show the feasibility of reaching CNT synthesis temperature by local resistive heating, while keeping safe temperatures on the chip for the CMOS circuits
Summary
Nanomaterials are a popular research topic as sensing materials for various sensing applications [1,2,3], including gas detection [4,5,6,7,8], due to their compact size [9], high sensitivity [10], low operating temperatures [11], low power consumption [12], etc. Electrical properties of CNTs are influenced during interaction with different gas molecules [10], making these carbon-based nanomaterials an excellent candidate for ultra-sensitive gas sensing. Functional materials such as Pd and Pt can be deposited on CNTs [22] to enhance their sensitivity and selectivity to various gases, such as NO2 , NH3 , CO and O2 [23,24]. A CNT-based gas sensor can have small form factor, while it can sense gases even at low concentration exposure at room temperature Such a sensor is beneficial to numerous applications, including in the food-packaging industry, where the freshness of the food can be monitored by sensing released gases due to food degradation
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