Design and Fabrication of CMOS Microstructures to Locally Synthesize Carbon Nanotubes for Gas Sensing

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.

Post-CMOS processing challenges and design developments of CMOS-MEMS microheaters for local CNT synthesis

“…Although accurate microheater temperatures were not determined due to fundamental resolution limitations, the results indicated high thermal gradient around the heaters 39 . Thermal characterization can be done more accurately on the latest generation CMOS chips due to the inclusion of the larger microstructures.…”

“…Considering the boundary conditions, the microheaters were designed within a varied length ranging mostly from 6 µm to 15 µm. Heater dimension considerations have previously been analysed based on simulation results 39,40 . A minimum distance of 15 µm is considered between two contact pads for the ease of wire bonding.…”

“…Measuring the microheater temperature during joule heating was another challenge. The width of the microheaters is too small to get reliable thermal measurements even with a highperformance mid-wave infrared (MWIR) microscope 39 , the operating wavelength used in such microscopes being 3-5 times the width of the microheaters.…”

Special Issue “Advanced Nanomaterials Based Gas Sensors”

“…Working at room temperature, the sensor was sensitive to acetone, ethanol, isopropanol and isopropene, with a low detection limit below 10 ppm and a sensitivity of 0,0006 ppm −1 . The contributions in this Special Issue based on CNTs are not only focused on different ways of doping/decorating the CNTs to enhance their sensitivity to specific gases but also include an innovative way to locally grow the CNTs on a silicon substrate in a way that is compatible with CMOS technology [8]. Thermally isolating the CNTs' growing zones from the rest of the substrate allows for reaching the high temperatures required for the nanotubes' growth without affecting the CMOS circuitry, which can be degraded when heated above 300-400 • C.…”