Shamshad Ali

CMOS integration of inkjet-printed graphene for humidity sensing

We report on the integration of inkjet-printed graphene with a CMOS micro-electro-mechanical-system (MEMS) microhotplate for humidity sensing. The graphene ink is produced via ultrasonic assisted liquid phase exfoliation in isopropyl alcohol (IPA) using polyvinyl pyrrolidone (PVP) polymer as the stabilizer. We formulate inks with different graphene concentrations, which are then deposited through inkjet printing over predefined interdigitated gold electrodes on a CMOS microhotplate. The graphene flakes form a percolating network to render the resultant graphene-PVP thin film conductive, which varies in presence of humidity due to swelling of the hygroscopic PVP host. When the sensors are exposed to relative humidity ranging from 10–80%, we observe significant changes in resistance with increasing sensitivity from the amount of graphene in the inks. Our sensors show excellent repeatability and stability, over a period of several weeks. The location specific deposition of functional graphene ink onto a low cost CMOS platform has the potential for high volume, economic manufacturing and application as a new generation of miniature, low power humidity sensors for the internet of things.

Dip pen nanolithography-deposited zinc oxide nanorods on a CMOS MEMS platform for ethanol sensing

This paper reports on the novel deposition of zinc oxide (ZnO) nanorods using a dip pen nanolithographic (DPN) technique on SOI (silicon on insulator) CMOS MEMS (micro electro mechanical system) micro-hotplates (MHP) and their characterisation as a low-cost, low-power ethanol sensor. The ZnO nanorods were synthesized hydrothermally and deposited on the MHP that comprise a tungsten micro-heater embedded in a dielectric membrane with gold interdigitated electrodes (IDEs) on top of an oxide passivation layer. The micro-heater and IDEs were used to heat up the sensing layer and measure its resistance, respectively. The sensor device is extremely power efficient because of the thin SOI membrane. The electro-thermal efficiency of the MHP was found to be 8.2 °C mW−1, which results in only 42.7 mW power at an operating temperature of 350 °C. The CMOS MHP devices with ZnO nanorods were exposed to PPM levels of ethanol in humid air. The sensitivity achieved from the sensor was found to be 5.8% ppm−1 to 0.39% ppm−1 for the ethanol concentration range 25–1000 ppm. The ZnO nanorods showed an optimum response at 350 °C. The CMOS sensor was found to have a humidity dependence that needs consideration in real-world application. The sensors were also found to be selective towards ethanol when tested in the presence of toluene and acetone. We believe that the integration of ZnO nanorods using DPN lithography with a CMOS MEMS substrate offers a low cost, low power, smart ethanol sensor that could be exploited in consumer electronics.

Enhanced spectroscopic gas sensors using in-situ grown carbon nanotubes

In this letter, we present a fully complementary-metal-oxide-semiconductor (CMOS) compatible microelectromechanical system thermopile infrared (IR) detector employing vertically aligned multi-walled carbon nanotubes (CNT) as an advanced nano-engineered radiation absorbing material. The detector was fabricated using a commercial silicon-on-insulator (SOI) process with tungsten metallization, comprising a silicon thermopile and a tungsten resistive micro-heater, both embedded within a dielectric membrane formed by a deep-reactive ion etch following CMOS processing. In-situ CNT growth on the device was achieved by direct thermal chemical vapour deposition using the integrated micro-heater as a micro-reactor. The growth of the CNT absorption layer was verified through scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The functional effects of the nanostructured ad-layer were assessed by comparing CNT-coated thermopiles to uncoated thermopiles. Fourier transform IR spectro...

Reliable characteristics and stabilization of on-membrane SOI MOSFET-based components heated up to 335??C

© 2016 IOP Publishing Ltd. In this work we investigate the characteristics and critical operating temperatures of on-membrane embedded MOSFETs from an experimental and analytical point of view. This study permits us to conclude the possibility of integrating electronic circuitry in the close vicinity of micro-heaters and hot operation transducers. A series of calibrations and measurements has been performed to examine the behaviors of transistors, inverters and diodes, actuated at high temperature, on a membrane equipped with an on-chip integrated micro-heater. The studied n- and p-channel body-tied partially-depleted MOSFETs and CMOS inverter are embedded in a 5 μm-thick membrane fabricated by back-side MEMS micromachining using SOI technology. It has been noted that a pre-stabilization step after the harsh post-CMOS processing, through an in situ high-temperature annealing using the micro-heater, is mandatory in order to stabilize the MOSFETs characteristics. The electrical characteristics and performance of the on-membrane MOS components are discussed when heated up to 335°C. This study supports the possibility of extending the potential of the micro-hotplate concept, under certain conditions, by embedding more electronic functionalities on the interface of on-membrane-based sensors leading to better sensing and actuation performances and a total area reduction, particularly for environmental or industrial applications.

Low power NDIR CO 2 sensor based on CMOS IR emitter for boiler applications

In this paper we demonstrate the use of a CMOS infra-red emitter in a low power Non Dispersive Infra Red (NDIR) based carbon dioxide sensor for application in domestic boilers. Compared to conventional micro-bulbs as IR wideband sources, CMOS IR emitters offer several advantages: They are faster, smaller, have lower power consumption and can have integrated circuitry. The emitter is a 1.16 mm × 1.06 mm chip with an integrated FET drive and consists of a tungsten heater fabricated in a CMOS process followed by Deep Reactive Ion Etching (DRIE) to form a thin membrane to reduce power consumption. The NDIR sensor consists of the emitter and a commercial detector placed 5 mm apart in a simple tube. Operating the emitter at 10 Hz with a power consumption of only 40 mW, the sensor was measured in the range of 6-14% by volume of CO2, showing a resolution of 0.5%, a response time of 20 s, and low cross-sensitivity to humidity.

In-Situ grown carbon nanotubes for enhanced CO 2 detection in non-dispersive-infra-red system

Non-dispersive-infra-red (NDIR) sensors are believed to be one of the most selective and robust solutions for CO2 detection, though cost prohibits their broader integration. In this paper we propose a commercially viable silicon-on-insulator (SOI) complementary metal-oxide (CMOS) micro-electro-mechanical (MEMS) technology for an IR thermal emitter. For the first time, vertically aligned multi walled carbon nanotubes (VA-MWCNTs) are suggested as a possible coating for the enhancement of the emission intensity of the optical source of a NDIR system. VA-MWCNTs have been grown in situ by chemical vapour deposition (CVD) exclusively on the heater area. Optical microscopy, scanning electron microscopy and Raman spectroscopy have been used to verify the quality of the VA-MWCNTs growth. The CNT-coated emitter demonstrated an increased response to CO2 of approx. 60%. Furthermore, we show that the VA-MWCNTs are stable up to temperatures of 500 °C for up to 100 hours.

On the reproducibility of CMOS plasmonic mid-IR thermal emitters

In this paper we report about the reproducibility of CMOS plasmonic mid-IR thermal emitters. The electro-thermal transduction efficiency of the devices is shown not to depend on the plasmonic structure geometry and to be extremely reproducible from device to device, wafer to wafer and lot to lot. The small variation in terms of optical performance is investigated through numerical simulation and attributed to variations in planar and cross sectional dimensions of the fabricated structures.

3D modelling of a thermopile-based SOI CMOS thermal wall shear stress sensor

This paper presents a multiphysic 3-D model of an SOI CMOS MEMS thermal wall shear stress sensor obtained using “Comsol Multiphysics”. It includes all the physical domains involved and their interaction. After a brief introduction, the device is presented and its working principle explained. The numerical model and the validation process are then described.

A geometry dependent predictive FEM model of a high temperature closed membrane SOI CMOS MEMS thermal conductivity sensor

This paper presents an experimentally verified geometry dependent predictive FEM model of a high temperature closed membrane thermal conductivity sensor. The sensor was developed using SOI CMOS MEMS technology. The FEM model presents a systematic approach to design thermal conductivity sensors by understanding heat transfer mechanisms between the sensor and the analyte environment. It also discusses how response time and sensor sensitivity are influenced by geometry of the sensor. The sensor was fabricated at a commercial foundry using a 1 μm process and measures only 1×1 mm2. The model establishes that a tradeoff is required between power consumption, response time and sensitivity based on the end user application.