Characterisation of the Etching Quality in Micro-Electro-Mechanical Systems by Thermal Transient Methodology

Our paper presents a non-destructive thermal transient measurement method that is able to reveal differences even in the micron size range of MEMS structures. Devices of the same design can have differences in their sacrificial layers as consequence of the differences in their manufacturing processes e.g. different etching times. We have made simulations examining how the etching quality reflects in the thermal behaviour of devices. These simulations predicted change in the thermal behaviour of MEMS structures having differences in their sacrificial layers. The theory was tested with measurements of similar MEMS devices prepared with different etching times. In the measurements we used the T3Ster thermal transient tester equipment. The results show that deviations in the devices, as consequence of the different etching times, result in different temperature elevations and manifest also as shift in time in the relevant temperature transient curves.Comment: Submitted on behalf of TIMA Editions (http://irevues.inist.fr/tima-editions

Gallium-based Solid Liquid Interdiffusion Bonding of Semiconductor Substrates near room temperature

Within this work, bonding technologies based upon the alloying of gallium with other metals to assemble semiconductor substrates for the possible application of encapsulation and 3D-integration of micro systems and devices have been researched. Motivated by the important demand to achieve low temperature processes, methods with bonding temperatures below 200°C were investigated. Necessary technologies like the deposition of gallium as thin film and subsequent micro structuring have been developed. The alloying between gallium and gold as well as gallium and copper was analysed in detail. A good correlation between the elemental composition of the interface and its mechanical and electrical parameters was established, particularly regarding its thermal dependence. It emerged that in case of combination Au/Ga Kirkendall void are extensively formed whereby serious problems with mechanical strength as well as hermeticity emerged. In case of Cu/Ga, this problem is existent to a much lesser degree; it was possible to create hermetic tight bonds. For the necessary pre-treatment of copper, several methods could be successfully demonstrated. In summary, the development of bonding technologies based upon metallic interfaces that exhibit electric conductance, high strength and hermetic seal could be demonstrated.In dieser Arbeit werden Bondverfahren zum Fügen von Halbleitersubstraten für mögliche Anwendungen für die Verkapselung und 3D-Integration von Bauelementen der Mikrosystemtechnik erforscht, die auf der Legierungsbildung von Gallium mit anderen Metallen beruhen. Motiviert von der zentralen Anforderung an niedrige Prozesstemperaturen wurden Methoden mit Fügetemperaturen deutlich unter 200°C untersucht. Dafür nötige Technologien zum Abscheiden von Gallium als Dünnschicht und das anschließende Mikrostrukturieren wurden entwickelt. Die Legierungsbildung zwischen Gallium und Gold sowie zwischen Gallium und Kupfer wurde im experimentell im Detail analysiert. Dabei konnte eine gute Korrelation zwischen der stofflichen Zusammensetzung und den mechanischen bzw. elektrischen Parametern der Zwischenschicht, auch und insbesondere hinsichtlich ihrer Temperaturabhängigkeit gefunden werden. Es stellte sich heraus, dass im Falle der Kombination Au/Ga Kirkendall Hohlräume in einer Menge entstehen, die zu erheblichen Problemen bezüglich mechanischer Festigkeit und Dichtheit der Fügeverbindung führen. Bei der Materialkombination Cu/Ga hingegen trat dieses Problem nur begrenzt auf; es war möglich hermetisch dichte Verbindungen herzustellen. Für die bei Kupfer nötige Vorbehandlung wurden mehrere Methoden erfolgreich getestet. Insgesamt konnte die Entwicklung von Fügetechnologien gezeigt werden, die metallische Zwischenschichten verwenden, elektrisch leitfähig sind, sehr gute Festigkeiten aufweisen und hermetisch dicht sind

Smart Platform for Low-Cost MEMS Sensors – Pressure, Flow and Thermal Conductivity

In a technological world that is trending towards smart and autonomous engineering, the collection of quality data is of unrivalled importance. This has led to a huge market demand for the development of low-cost, small and accurate sensors and thus has resulted in significant research into sensors, with the aim of advancing the price/performance ratio in commercial solutions. Micro Electro Mechanical Systems (MEMS) have recently offered an attractive solution to miniaturise and drastically improve the performance of sensors. In this thesis, MEMS technology is exploited to create a multi-sensor technology platform that is used to fabricate several sensing technologies. Piezo-resistive and piezo-electronic pressure sensors are designed, fabricated and tested. Different doping profiles, stress-engineered structures and electronic devices for pressure transduction are investigated, with focus on their sensitivity and non-linearity. A ring is fabricated in the metal layer around the circumference of the membrane that alleviates the effects of over/under etching. This is achieved by creating a new rigid edge of the membrane in the metal layer, which has tighter fabrication tolerances. A piezo-MOSFET is developed and shown to have greater sensitivity than similar state-of-the-art devices. Flow sensors based on a heated tungsten wire are designed, fabricated, tested and substantiated with numerical modelling. Calorimetric and anemometric driving modes are optimised with regards to device structure. Thermodiodes are also used as the temperature transduction devices and are compared to the traditional resistor method and showed to be preferable when further miniaturising the sensor. Thermal conductivity gas sensors based on a heated tungsten resistor are designed, tested and substantiated with numerical modelling. Holes through the membrane are used to improve the sensitivity to measuring carbon dioxide by 270%. Asymmetric holes are utilised to prove a novel method of measuring thermal conductivity in a calorimetric method. Designs improving this new concept are outlined and substantiated with analytical and numerical models. Linear statistical methods and artificial neural networks are used to differentiate flow rate and gas concentration using three on-membrane resistors. With membrane holes, the discrimination between gases in the presence of flow is improved. Neural networks provide a viable solution and show an increase in the accuracy of both flow rate and gas concentration. The main objective of the work in this thesis was to develop low-cost, low-power, small devices capable of high-volume production and monolithic integration using a single smart technology platform for fabrication. The smart technology platform was used to create pressure sensors, flow sensors and thermal conductivity gas sensors. Within each sensing technology, proof-of-concepts and optimisations have been carried out in order to maximise performance whilst using the low-cost, high-volume fabrication process, ultimately helping towards smart and autonomous engineering solutions driven by data

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

Nanowires fabricated by Focused Ion Beam

This thesis reports research on nanowires fabricated by FIB lithography with experiments to understand their mechanical, electrical and hydrodynamic properties. Au nanowires fabricated on Si$_3$N$_4$ membranes with width below 50nm exhibit liquid like instabilities and below $\sim$20nm the instabilities grow destroying the nanowires due to the Rayleigh- Plateau instability. Stability is better in the case for Si substrates than for the insulators Si0$_2$ and Si$_3$N$_4$. A series of 4-terminal resistance measurements were carried out on a "platinum" nanowire grown by FIB-induced decomposition of an organometallic precursor. Such nanowires are found to be a two phase percolating system, containing up to 70% by volume carbon. They have unexpected temperature behaviour which is explained using a percolation model with Kirkpatrick conduction in the presence of temperature induced strain. Au nanowire bridges of very small diameter were probed using AFM to investigate their deformation and fracture strength. Below a diameter $\sim$50nm, the mechanical properties are consistent with liquid-like behaviour. After reaching the fracture, the gold molecules from the bridge retract towards the fixed ends; rebinding of the gold causing reforming of the nanowire bridge can occur. FIB fabrication was also used to form a thermal bimorph MEMS cantilever which was investigated by AFM during actuation

Nagy megbízhatóságú integrált mikro- és nanorendszerek új tesztelési és vizsgálati módszerei, különös tekintettel az ambient intelligence kihívásaira = New design verification and examination principles and methods for high reliability integrated micro-nano-systems with special attention to the challenges of ambient intelligence

A kutatás eredményeit mintegy 15 új publikációban fejtettük ki részletesen. Az elért eredményekben a következőkben foglalhatók össze: - Kutató és fejlesztő munkát végeztünk energiaforrással egybeintegrált érzékelők fejlesztése területén. - Kutató és fejlesztő munkákat végeztünk MEMS struktúrákba építhető tesztelő és öntesztelő struktúrák kifejlesztésére. Ezt a munkát a PATENT kiválósági hálózat nemzetközi projekt keretében különböző ad-hoc konzorciumokatban nemzetközi kooperációban végeztük. - Jelentős lépést értünk el a MEMS tokozások megbízhatósági vizsgálatában a termikus tranziens mérési módszer alkalmazásával. A kifejlesztett módszer több nemzetközi publikációban mutattuk be és a munka eredményeiből PhD disszertáció is készült. - Igen jelentős eredményeket értünk el a termikus anyagparaméterek pontosabb meghatározása területén. A témával foglalkozó publikációnkat az IEEE CPMT Best Paper díj adományozásával értékelte. - Jelentős eredményeket értünk el a mikrocsatornás hűtőszerkezetek hűtési tulajdonságainak minősítése területén. Az elért eredményeket 5 különböző nemzetközi kooperációban készült cikkben mutattuk be. | The research results are presented in detail in about 15 new publications. The achieved results can be summarized as follows: - Research and development work was accomplished in order to realize energy sources and sensors on the same chip. - Research and development work was carried out in order to develop new test and self test structures for the characterizations of MEMS systems. This work was done in the Framework of the PATENT Network of Excellence in different ad-hoc consortia. - Significant results were achieved in the field of reliability testing methods of MEMS packaging and MEMS etching with the help of the application of thermal transient testing. The newly developed methodology was presented in several international publications and it forms the basis of one PhD Thesis work. - Significant results have been achieved in the more accurate measurement of thermal material parameters. A publication dealing with this subject was given the IEEE CPMT Best Paper Award in 2006. - Significant results have been achieved in the qualification of the cooling properties of microchannel coolers. The results have been presented in 5 papers made in international cooperation

Integrated sensors for process monitoring and health monitoring in microsystems

This thesis presents the development of integrated sensors for health monitoring in Microsystems, which is an emerging method for early diagnostics of status or “health” of electronic systems and devices under operation based on embedded tests. Thin film meander temperature sensors have been designed with a minimum footprint of 240 m × 250 m. A microsensor array has been used successfully for accurate temperature monitoring of laser assisted polymer bonding for MEMS packaging. Using a frame-shaped beam, the temperature at centre of bottom substrate was obtained to be ~50 ºC lower than that obtained using a top-hat beam. This is highly beneficial for packaging of temperature sensitive MEMS devices. Polymer based surface acoustic wave humidity sensors were designed and successfully fabricated on 128° cut lithium niobate substrates. Based on reflection signals, a sensitivity of 0.26 dB/RH% was achieved between 8.6 %RH and 90.6 %RH. Fabricated piezoresistive pressure sensors have also been hybrid integrated and electrically contacted using a wire bonding method. Integrated sensors based on both LiNbO3 and ZnO/Si substrates are proposed. Integrated sensors were successfully fabricated on a LiNbO3 substrate with a footprint of 13 mm × 12 mm, having multi monitoring functions for simultaneous temperature, measurement of humidity and pressure in the health monitoring applications

A new SiC/HfB2 based micro hotplate for metal oxide gassensors

Abstract Solzbacher, Florian A new SiC/HfB2 based modular concept of micro hotplates for metal oxide gassensors Im Rahmen dieser Arbeit wurde ein neuer SiC/HfB2-basierter Mikroheizer mit niedrigster Leistungsaufnahme für die Anwendung in Metalloxid-Gassensoren entwickelt und demonstriert. Erstmals wurden Siliziumkarbid (SiC) und Hafniumdiborid (HfB2) als Werkstoffe für einen Mikroheizer eingesetzt. Durch geringe Modifikation der Herstellungsprozesse lässt sich der Heizer so variieren, dass der Einsatz sowohl für den automobilen Anwendungsbereich (12V- 24V) als auch für tragbare Geräte (1V-2V) für eine Vielzahl unterschiedlicher Messgase möglich ist. Es ist der erste Mikroheizer für Gassensoren überhaupt, der den Batteriebetrieb bei nur 1-2 V erlaubt. Der modulare Fertigungsansatz ermöglicht die Reduzierung der Entwicklungs- und Fertigungskosten für die unterschiedlichen Anwendungsbereiche. Aus der Marktentwicklung in der Sensorik, den industriellen Anforderungen und den zu den Metalloxid-Gassensoren im Wettbewerb stehenden alternativen Technologien ergeben sich das Anforderungsprofil des Sensors. Die Wahl der Materialien spielt eine Schlüsselrolle für die Heizereigenschaften. Der Mikroheizer besteht aus einer 1 (m dicken, an 150 (m langen und 10 bis 40 (m breiten Stegen aufgehängten Membran mit Außenmaßen von 100 (m x 100 (m. Alternativ kommen eine HfB2 - Dünnfilm-Widerstandsheizung oder ein dotierter SiC-Heizer zum Einsatz. Mit Leistungsaufnahmen von 32 mW werden Temperaturen von 600°C erreicht, was einer Effizienz von ca. 19 K/mW entspricht. Die verwendeten hexagonalen Strukturen ermöglichen dichtes Packen der Sensoren in Arrays bei hoher mechanischer Stabilität. Erste NO2 Sensoren mit gassensitiver In2O3 Schicht konnten gezeigt werden.A new SiC/HfB2-based micro hotplate with ultra low power consumption for the application in metal oxide micro gas sensors is developed and demonstrated. For the first time, silicon carbide (SiC) and Hafniumdiboride (HfB2) are used as materials for a micro hotplate structure. Using only slight modifications of the fabrication process, the device can be used either for automotive applications with operating voltages of 12V-24V or for battery operated handheld detectors with operating voltages of 1V-2V for a variety of different gases. It is the first micro hotplate device ever designed to work for low battery voltages of 1V-2V. The modular approach towards the processing allows easy modification for a variety of application fields and thus also reduces market entrance barriers. Based on the market development of micro sensors, the industrial requirements, and competing metal oxide gas sensors using alternative technologies, technical specifications for the hotplate as well as the state of the art's limits are determined. The new material choice plays a key role in the device properties. The micro hotplate consists of a 100 ?m x 100 ?m membrane supported by thin beams of 1 ?m thickness, 150 ?m length and 10 to 40 ?m width. Alternatively, an HfB2 ? thin film resistive heater or a doped SiC heater are used. Temperatures of 600°C are achieved using a power consumption of only 32 mW resulting in a thermal heater efficiency of ~19 K/mW. The hexagonal geometry allows close packing of the hotplates in array structures with high mechanical strength. NO2 sensors with gas sensitive In2O3 layer are presented

Light-assisted domain engineering, waveguide fabrication and microstructuring of lithium niobate

The thesis is focussing on the interaction of lithium niobate with UV and ultrafast laser radiation to achieve 1) ferroelectric domain inversion, 2) waveguide fabrication, and 3) surface microstructuring. Preferential ferroelectric domain inversion has been demonstrated by 'latent light-assisted poling' and 'inhibition of poling' using ultrafast laser irradiation at 400 nm and CW highly absorbed UV radiation (305..244 nm) respectively. The characteristics of the resultant domains have been experimentally investigated as a function of the fabrication conditions and a theoretical model have been proposed to explain the experimental observations. UV radiation in the 305 nm to 244 nm range have been used for the fabrication of optical waveguides in lithium niobate. The waveguiding characteristics and electro-optic response of the UV written optical channel waveguides have been investigated experimentally. Inhibition of poling and post processing has been used for the fabrication of ridge waveguide structures with enhanced refractive index change. Finally, a method for the fabrication of ultra-smooth lithium niobate single crystal photonic microstructures has been proposed. The method is based on surface tension reshaping of surface microstructures which are produced by preferential poling and subsequent etching. Whispering gallery mode resonators have been fabricated and characterised here

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