Low-power silicon planar micro-calorimeter employing nanostructured catalyst

This thesis describes the development of silicon planar micro-calorimetric gas sensors employing a nanostructured palladium (Pd) catalyst. Present commercial, bead-type calorimetric sensors have been manufactured for nearly forty years and are used in many applications, such as mining, water treatment and emergency services, with an estimated European market value of €221M by 2004. However, recent advances in both silicon micro-machining and nano materials have created the technologies necessary to transform the present labour-intensive fabrication process in to a new low-cost batch production. In addition, a reduction in power consumption, improved sensitivity and increased poisoning resistance of the sensor can also be achieved. Here, two generations of micro-calorimeter have been designed and fabricated comprising a silicon membrane structured micro-hotplate that can reach up to a temperature of 870'C without failure and an ultra-high surface area nanoporous Pd catalyst (about 20 m2/g), typically 25 run thick, deposited electrochemically on top of a gold electrode above the micro-heater. The exothermic reaction caused by the target gas (e.g. methane) interacting with the Pd catalyst results in an increase in the temperature and so resistance of the micro-heater. A Wheatstone bridge interface circuit is normally used to detect and measure the fractional resistance change. Full 3-D thermo-mechanical simulations have been performed employing experimental data in order to establish a simulation database for future developments. The differences between simulated and experimental results were found to be as low as 4.6%. The response of the sensors has been characterised in both continuous powering mode and pulse modulation powering mode. Device power consumption is only 50mW at 500'C in continuous mode, which is up to 100mW lower than that for commercial sensors. Typical response times of 2ms have been measured and so further power saving can be achieved when the sensors are operated in a pulse mode, e.g. 50% duty-cycle at 10Hz. Hence, an overall power saving of 75% could be achieved compared to commercial product. Infrared thermography revealed that a centre hot spot, commonly found with meander style micro-heaters, has been eliminated by the new drive-wheel micro-heater design. The sensitivity of the sensors has also been improved, up to a factor of 4 at 500'C ((60 mV/mm2)/%CH4), by the nanoporous catalyst and by heating it more isothermally. Furthermore, improvements have also been found on the poisoning resistance. Therefore, the potential commercialisation of the micro-calorimeter is very promising

Low power MEMS heating membrane

Cílem této práce je realizovat MEMS topnou membránu s nízkým příkonem. Práce obsahuje základní body návrhu a přehled technologií, které jsou určeny pro výrobu těchto topných elementů. Jako výchozí struktura pro realizaci topné membrány byl vybrán typ závěsné membrány. Na závěr této práce je uveden detailní technologický postup, který byl použit při výrobě finálních struktur a experimentální ověření odolnosti proti teplotnímu namáhání.The aim of this thesis is to create the MEMS heating membrane with low power consumption. This work includes the main of design and fabrication processes of these heating elements. It was decided that the final sample will be created as suspended membrane type of the structure. It the end of the thesis there is detailed overview of the whole fabrication processes used for fabrication of final samples and the experiment to prove the resistance due to thermal stress.

Micro-hotplate based CMOS sensor for smart gas and odour detection

Low cost, highly sensitive, miniature CMOS micro-hotplate based gas sensors have received great attention recently. The global sensor market is expanding rapidly with an expected increase of 5 ~ 8% grow thin the next five years. The application areas for a gas sensor include but are not limited to, air quality monitoring, industrial and laboratory conditions, military, and biomedical sectors. It is the key hardware component of an electronic nose, as well as the signal processing on the software side. In this thesis, both aspects of such a system were studied with new sensor technologies and improved signal processing algorithms. In addition, this thesis also described different applications and research projects using these sensor technologies and algorithms. A novel plasmonic structure was employed as an infrared source for anon- dispersive infrared gas sensor. This structure was based on a CMOS micro hot plate with three metal layers and periodic cylindrical dots to induce plasmon resonance, that allowed a tunable narrow band infrared radiation with high sensitivity and selectivity. Five gases were studied as target gases, namely, carbon monoxide, carbon dioxide, acetone, ammonia and hydrogen sulfide. These emitter sources were fabricated and characterised with a gascell, optical filters and commercial detectors under different gas concentrations and humidity levels. The results were promising with the lowest detection limit for ammonia at 10 ppm with 5 ppm resolution. On the data processing side, various signal processing methods were explored both on-board and on-board. Temperature modulation was the on-board method by switching the operating temperatures of a micro hotplate. This technique was proven to over come and reduce some typical sensor issues, such as drift, slow re-sponse/recovery speed (from tens of seconds to a few seconds) and even cross sensitivities. Off-board post processing methods were also studied, including principal component analysis, k-nearest neighbours, self-organising maps and shallow/deep neural networks. The results from these algorithms were compared and overall an 85% or higher classification accuracy could be achieved. This work showed the potential to discriminate gases/odours, which could lead to the development of a real-time discrimination algorithm for low cost wearable devices

Selective Resistive Sintering: A Novel Additive Manufacturing Process

Selective laser sintering (SLS) is one of the most popular 3D printing methods that uses a laser to pattern energy and selectively sinter powder particles to build 3D geometries. However, this printing method is plagued by slow printing speeds, high power consumption, difficulty to scale, and high overhead expense. In this research, a new 3D printing method is proposed to overcome these limitations of SLS. Instead of using a laser to pattern energy, this new method, termed selective resistive sintering (SRS), uses an array of microheaters to pattern heat for selectively sintering materials. Using microheaters offers significant power savings, significantly reduced overhead cost, and increased printing speed scalability. The objective of this thesis is to obtain a proof of concept of this new method. To achieve this objective, we first designed a microheater to operate at temperatures of 600⁰C, with a thermal response time of ~1 ms, and even heat distribution. A packaging device with electrical interconnects was also designed, fabricated, and assembled with necessary electrical components. Finally, a z-stage was designed to control the airgap between the printhead and the powder particles. The whole system was tested using two different scenarios. Simulations were also conducted to determine the feasibility of the printing method. We were able to successfully operate the fabricated microheater array at a power consumption of 1.1W providing significant power savings over lasers. Experimental proof of concept was unsuccessful due to the lack of precise control of the experimental conditions, but simulation results suggested that selectivity sintering nanoparticles with the microheater array was a viable process. Based on our current results that the microheater can be operated at ~1ms timescale to sinter powder particles, it is believed this new process can potentially be significantly quicker than selective laser sintering by increasing the number of microheater elements in the array. The low cost of a microheater array printhead will also make this new process affordable. This thesis presented a pioneering study on the feasibility of the proposed SRS process, which could potentially enable the development of a much more affordable and efficient alternative to SLS

Microheater Array Powder Sintering (MAPS) for Printing Flexible Electronics

Microheater array powder sintering (MAPS) is a novel additive manufacturing process that uses an array of microheaters to selectively sinter powder particles. MAPS shows great promise as a new method of printing flexible electronics by enabling digital curing of conductive inks on a variety of substrates. MAPS operation relies on establishing a precision air gap of a few microns between an array of microheaters, which can reach temperatures of 600°C, and a layer of conductive ink which can be deposited onto a flexible substrate. This system presents challenges, being: the fabrication of a microheater that can reach suitable temperatures in an acceptable time frame and is reliable, electronic control of a single microheater, electronic control of an array of microheaters, and precise control of the position of the array of microheaters relative to the substrate. This work describes the design and fabrication of a printer which uses this novel technology to print flexible circuit boards. Various simulations are discussed which are used to explore the parameters affecting the MAPS printing process. Then, a small microheater array is fabricated and controlled using an electronic circuit using a PID feedback loop. This microheater array is used in an experimental proof of concept machine to print conductive lines onto a flexible substrate. Finally, a prototype MAPS printer is developed which is capable of using an improved microheater array to print simple circuits onto flexible substrates

Design and optimisation of a high-temperature silicon micro-hotplate for nanoporous palladium pellistors

The conventional design of the heater in a silicon micro-hotplate employ a simple meandering resistive track to form a square element. We show that this heater structure produces an uneven thermal profile characterised by a central hot spot with a significant variation in temperature of some 50degreesC across the plate at an average temperature of 500degreesC. Four novel micro-heater designs are reported here and fabricated on hotplates with an active area that ranges from (200 X 200) muM(2) to (570 X 570) muM(2) in order to vary systematically the ratio of membrane to heater length from a value of 5.0-2.7, respectively. All the designs have been simulated using a 3D electro-thermo-mechanical finite element model and results agree well with thermal profiles taken using an infrared microscope. One of the designs, referred to here as 'drive-wheel' structure, performs best and reduces the lateral variation in temperature to only +/-10degreesC. The different resistive micro-heaters have been calibrated with the lowest power consumption being 50 mW at 500degreesC, which is well below the power consumption of any commercial pellistor; the maximum temperature before rupture being 870degreesC. The micro-hotplates were electrochemically coated with a 20 nm thick mesoporous palladium catalyst and the pellistors' response tested to 2.5% methane in air. The micro-heaters were observed to be stable for a period of 1000 h and should provide a good platform for exploitation in commercial catalytic pellistors. (C) 2002 Elsevier Science Ltd. All rights reserved