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 spectroscopy showed that the radiation absorbing properties of the CNT adlayer significantly enhanced the absorptivity, compared with the uncoated thermopile, across the IR spectrum (3 μm–15.5 μm). This led to a four-fold amplification of the detected infrared signal (4.26 μm) in a CO2 non-dispersive-IR gas sensor system. The presence of the CNT layer was shown not to degrade the robustness of the uncoated devices, whilst the 50% modulation depth of the detector was only marginally reduced by 1.5 Hz. Moreover, we find that the 50% normalized absorption angular profile is subsequently more collimated by 8°. Our results demonstrate the viability of a CNT-based SOI CMOS IR sensor for low cost air quality monitoring.This work was partly supported through the EU FP7 project SOI-HITS (No. 288481). MTC thanks the Oppenheimer Trust and the EPSRC IAA for their generous financial support.This is the author accepted manuscript. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/apl/106/19/10.1063/1.4921170

Semiconductor Gas Sensors: Materials, Technology, Design, and Application

This paper presents an overview of semiconductor materials used in gas sensors, their technology, design, and application. Semiconductor materials include metal oxides, conducting polymers, carbon nanotubes, and 2D materials. Metal oxides are most often the first choice due to their ease of fabrication, low cost, high sensitivity, and stability. Some of their disadvantages are low selectivity and high operating temperature. Conducting polymers have the advantage of a low operating temperature and can detect many organic vapors. They are flexible but affected by humidity. Carbon nanotubes are chemically and mechanically stable and are sensitive towards NO and NH3, but need dopants or modifications to sense other gases. Graphene, transition metal chalcogenides, boron nitride, transition metal carbides/nitrides, metal organic frameworks, and metal oxide nanosheets as 2D materials represent gas-sensing materials of the future, especially in medical devices, such as breath sensing. This overview covers the most used semiconducting materials in gas sensing, their synthesis methods and morphology, especially oxide nanostructures, heterostructures, and 2D materials, as well as sensor technology and design, application in advance electronic circuits and systems, and research challenges from the perspective of emerging technologies. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Thermal characterisation of miniature hotplates used in gas sensing technology

The reliability of micro-electronic devices depends on the device operating temperature and therefore self-heating can have an adverse effect on the performance and reliability of these devices. Hence, thermal measurement is crucial including accurate maximum operating temperature measurements to ensure optimum reliability and good electrical performance. In the research presented in this thesis, the high temperature thermal characterisation of novel micro-electro-mechanical systems (MEMS) infra-red (IR) emitter chips for use in gas sensing technology for stable long-term operation were studied, using both IR and a novel thermo-incandescence microscopy. The IR emitters were fabricated using complementary-metal-oxide semiconductor (CMOS) based processing technology and consisted of a miniature micro-heater, fabricated using tungsten metallisation. There is a commercial drive to include MEMS micro-heaters in portable electronic applications including gas sensors and miniaturised IR spectrometers where low power consumption is required. IR thermal microscopy was used to thermally characterise these miniature MEMS micro-heaters to temperatures approaching 700 °C. The research work has also enabled further development of novel thermal measurement techniques, using carbon microparticle infra-red sensors (MPIRS) with the IR thermal microscopy. These microparticle sensors, for the first time, have been used to make more accurate high temperature (approaching 700 °C) spot measurements on the IR transparent semiconductor membrane of the micro-heater. To substantially extend the temperature measurement range of the IR thermal microscope, and to obtain the thermal profiles at elevated temperatures (> 700 °C), a novel thermal measurement approach has been developed by calibrating emitted incandescence radiation in the optical region as a function of temperature. The calibration was carried out using the known melting point (MP) of metal microparticles. The method has been utilised to obtain the high temperature thermo-optical characterisation of the MEMS micro-heaters to temperatures in excess of 1200 °C. The measured temperature results using thermo-incandescence microscopy were compared with calculated electrical temperature results. The results indicated the thermo-incandescence measurements are in reasonable agreement (± 3.5 %) with the electrical temperature approach. Thus, the measurement technique using optical incandescent radiation extends the range of conventional IR microscopy and shows a great potential for making very high temperature spot measurements on electronic devices. The high power (> 500mW) electrical characterisation of the MEMS micro-heaters were also analysed to assess the reliability. The electrical performance results on the MEMS micro-heaters indicated failures at temperatures greater than 1300 °C and Scanning Electron Microscope (SEM) was used to analyse the failure modes

Modelling and Design Optimisations of CMOS MEMS Single Membrane Thermopile Detector Arrays

Thermal imaging devices based on Complementary Metal-Oxide-Semiconductor (CMOS) and Micro-Electro-Mechanical System (MEMS) technology are widely used across consumer and industrial applications. The combination of CMOS and MEMS technologies allows for the production of devices with high performance, good reliability and consistent reproducibility. Additionally, these technologies allow devices to be manufactured at low cost and a high volume. There are several types of thermal sensing technologies, however, this thesis mainly focuses on 8×8 thermopile based Focal Plane Arrays (FPAs). The core principles governing the function of thermopiles are based on the Seebeck effect. In this thesis, the structure and fabrication process of thermopile FPAs are described and discussed. The thesis describes the functionality of the array chip and introduces a new experimental technique, called the bi-directional electrical biasing method, which was applied to obtain the device’s responsivity and crosstalk measurements. Compared to traditional measurement approaches using laser sources, this novel method significantly reduces the complexity of the experimental setup, as no external laser source is required. The crosstalk of the 8×8 array is ~2.69% and the responsivity is ~73.1 V/W. A detecting system using a larger array chip was designed, created and successfully applied in a series of experiments that involved gesture recognition and people counting. In order to enhance the performance of the current array device, a 3D simulation model based on the Finite Element Method (FEM) was built using the COMSOL Multiphysics simulation tool. The numerical model was validated by comparing the model’s simulated values for responsivity, crosstalk and temperature distribution with experimental results. The difference between the simulations and experimental results was 90 V/W in the model with tungsten tracks. A 32×32 array design demonstrates the smallest pixel size that can be achieved based on this thermopile array design. The 32×32 array design increased responsivity to ~77.18 V/W and crosstalk remained 6% when the pixel size was reduced further in a 64×64 array design, at this level of crosstalk, image quality is likely to be significantly affected. Future work may focus on the implementation of carbon nanotubes or novel 3D thermopile designs. Carbon nanotubes, when deposited over the array chip, could enhance the absorption of IR radiation. While new thermopiles employing a 3D design could dramatically reduce array size and potentially achieve a fill factor of 100%

Fundamental Characterization of Low Dimensional Carbon Nanomaterials for 3D Electronics Packaging

Transistor miniaturization has over the last half century paved the way for higher value electronics every year along an exponential pace known as \u27Moore\u27s law\u27. Now, as the industry is reaching transistor features that no longer makes economic sense, this way of developing integrated circuits (ICs) is coming to its definitive end. As a solution to this problem, the industry is moving toward higher hanging fruits that can enable larger sets of functionalities and ensuring a sustained performance increase to continue delivering more cost-effective ICs every product cycle. These design strategies beyond Moore\u27s law put emphasis on 3D stacking and heterogeneous integration, which if implemented correctly, will deliver a continued development of ICs for a foreseeable future. However, this way of building semiconductor systems does bring new issues to the table as this generation of devices will place additional demands on materials to be successful. The international roadmap of devices and systems (IRDS) highlights the need for improved materials to remove bottlenecks in contemporary as well as future systems in terms of thermal dissipation and interconnect performance. For this very purpose, low dimensional carbon nanomaterials such as graphene and carbon nanotubes (CNTs) are suggested as potential candidates due to their superior thermal, electrical and mechanical properties. Therefore, a successful implementation of these materials will ensure a continued performance to cost development of IC devices.This thesis presents a research study on some fundamental materials growth and reliability aspects of low dimensional carbon based thermal interface materials (TIMs) and interconnects for electronics packaging applications. Novel TIMs and interconnects based on CNT arrays and graphene are fabricated and investigated for their thermal resistance contributions as well electrical performance. The materials are studied and optimized with the support of chemical and structural characterization. Furthermore, a reliability study was performed which found delamination issues in CNT array TIMs due to high strains from thermal expansion mismatches. This study concludes that CNT length is an important factor when designing CNT based systems and the results show that by further interface engineering, reliability can be substantially improved with maintained thermal dissipation and electrical performance. Additionally, a heat treatment study was made that enables improvement of the bulk crystallinity of the materials which will enable even better performance in future applications

A highly stable, nanotube-enhanced, CMOS-MEMS thermal emitter for mid-IR gas sensing

Funder: National Physical Laboratory; doi: http://dx.doi.org/10.13039/501100007851Funder: Royal Society Dorothy Hodgkin Research FellowshipThe gas sensor market is growing fast, driven by many socioeconomic and industrial factors. Mid-infrared (MIR) gas sensors offer excellent performance for an increasing number of sensing applications in healthcare, smart homes, and the automotive sector. Having access to low-cost, miniaturized, energy efficient light sources is of critical importance for the monolithic integration of MIR sensors. Here, we present an on-chip broadband thermal MIR source fabricated by combining a complementary metal oxide semiconductor (CMOS) micro-hotplate with a dielectric-encapsulated carbon nanotube (CNT) blackbody layer. The micro-hotplate was used during fabrication as a micro-reactor to facilitate high temperature (>700 ∘C) growth of the CNT layer and also for post-growth thermal annealing. We demonstrate, for the first time, stable extended operation in air of devices with a dielectric-encapsulated CNT layer at heater temperatures above 600 ∘C. The demonstrated devices exhibit almost unitary emissivity across the entire MIR spectrum, offering an ideal solution for low-cost, highly-integrated MIR spectroscopy for the Internet of Things

Towards Integrated Mid-Infrared Gas Sensors.

Optical gas sensors play an increasingly important role in many applications. Sensing techniques based on mid-infrared absorption spectroscopy offer excellent stability, selectivity and sensitivity, for numerous possibilities expected for sensors integrated into mobile and wearable devices. Here we review recent progress towards the miniaturization and integration of optical gas sensors, with a focus on low-cost and low-power consumption devices

Local Synthesis and Direct Integration of Carbon Nanotubes into Microsystems for Sensor Applications

Carbon nanotubes (CNTs) have been intensively studied since their discovery more than two decades ago. A lot of research has exploited their extraordinary properties and various applications, meanwhile, revealed the challenges of fabricating the CNT-based device. Major challenges concern the high temperature required for the CNT growth, and the difficulty in handling and maneuvering the CNTs. An innovative approach to overcome these challenges is to locally synthesize and directly assemble CNTs into devices. Following such approach, this thesis developed a fabrication process with a high simplicity, a high controllability, and a CMOS/MEMS compatibility for the local synthesis and direct integration of CNTs into Si microsystems. This thesis covers the total process chain: from synthesis and integration of CNTs, to characterization, and to testing of a proof-of-principle gas sensor. The first key finding of this thesis is a simple and robust method to control the temperature for the growth of CNTs by using only electrical signals. During the growth process, a localized hot region for the growth of CNTs is created by locally heating a Si microelectrode (Joule heating). The induced temperature is monitored through in-situ measurements of the electrical resistance of the Si electrode. The measured resistance provides feedback to control the input power for heating the Si electrode. This pure electrical control enables a simple, automated and parallel process to synthesize locally and integrate CNTs directly into microsystems. The second key finding of this thesis is the diameter dependency for the effect of an applied electric field on the growth orientation of CNTs. A statistical analysis of 1100 CNTs showed that small-diameter CNTs (d 10 nm) were curved and did not align. In the transition regime, CNTs were moderately curved, but the average direction was at small angle with the electric field direction. The third key finding of this thesis is the correlation between local temperature and resulting characteristics of CNTs. A high gradient of temperature along the Si microelectrode due to Joule heating allowed for studying the effect of temperature. At the region where the temperature is highest ( 900oC), the nanostructure of CNTs had the highest degree of order, and the average diameter of CNT was smallest. At regions with lower temperatures, CNTs had a higher degree of defects and disorder, and a lower average diameter. The density of CNTs, however, was highest at the moderate-temperature region ( 850oC). The other contribution of this thesis is preliminary results on the development of CNT-based microsystems towards sensor applications. The preliminary results suggest that: (i) contact resistance at the CNT-Si interface could be reduced by both techniques of local annealing and local deposition of Platinum onto the CNT-Si contacts; (ii) thermal evaporation of metals could be used to functionalize the CNTs in a microsystem where CNTs are suspended and span two microelectrodes

Facile integration of ordered nanowires in functional devices

The integration of one-dimensional (1D) nanostructures of non-industry-standard semiconductors infunctional devices following bottom-up approaches is still an open challenge that hampers the exploita-tion of all their potential. Here, we present a simple approach to integrate metal oxide nanowires inelectronic devices based on controlled dielectrophoretic positioning together with proof of conceptdevices that corroborate their functionality. The method is flexible enough to manipulate nanowiresof different sizes and compositions exclusively using macroscopic solution-based techniques in conven-tional electrode designs. Our results show that fully functional devices, which display all the advantagesof single-nanowire gas sensors, photodetectors, and even field-effect transistors, are thus obtained rightafter a direct assembly step without subsequent metallization processing. This paves the way to lowcost, high throughput manufacturing of general-purpose electronic devices based on non-conventionaland high quality 1D nanostructures driving up many options for high performance and new low energyconsumption devices