66 research outputs found

    Microtechnologies for Discharge-based Sensors.

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    Microdischarge-based sensors are known to offer advantages such as the ability to operate at temperature extremes and to provide large output signals that do not require local amplification. This work is primarily directed at the design and microfabrication of pressure sensors that use differential microdischarge currents. Two approaches are evaluated. The first uses a common anode and reference cathode located on a glass substrate, whereas a sensing cathode is located on an opposing silicon diaphragm that is deflected by applied pressure. Leads are transferred by electroplated through-glass vias. The second uses a common cathode and reference anode located on a silicon substrate, whereas a sensing anode is located on a thin film diaphragm that deflects under applied pressure. Leads are transferred by through-wafer isolated bulk-silicon lead transfer (TWIST). Fabricated sensors with 200-µm diameter have footprints as small as 300×300 µm2, and volume of ≈0.01 mm3, which is 150× smaller than prior work. The fractional differential current (I1-I2)/(I1+I2) increases monotonically from -0.7 to 0.2 as external pressure increases from 1 atm to 8 atm. The TWIST process can also be used to fabricate ultra-miniature capacitive pressure sensors with backside contacts that minimize the form factor and allow stacking of the sensor on interface electronics. A sensor with a 100-µm diameter diaphragm measures 150×150 µm2 in size. Fabricated sensors with thicknesses of 3 µm (C100t3) and 5 µm (C100t5) have dynamic ranges of 20 MPa and 50 MPa, respectively. Pressure responses in the non-contact mode and the contact mode are 3.1 fF/MPa, 5.3 fF/MPa for C100t3, and 1.6 fF/MPa, 1.6 fF/Ma for C100t5, respectively. This thesis also describes a preliminary exploration of options to initiate microdischarges using scavenged energy – in this case from mechanical impact. A miniature high voltage generator is formed by connecting multiple electrode pairs in series on a single PZT element. This strategy amplifies voltage roughly in proportion to the electrode pair count; a three electrode-pair device is used to successfully initiate microdischarges with peak voltages exceeding 1.35 kV.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111467/1/xinluo_1.pd

    Above-IC RF MEMS devices for communication applications

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    Wireless communications are showing an explosive growth in emerging consumer and military applications of radiofrequency (RF), microwave, and millimeter-wave circuits and systems. Applications include wireless personal connectivity (Bluetooth), wireless local area networks (WLAN), mobile communication systems (GSM, GPRS, UMTS, CDMA), satellite communications and automotive electronics. Future cell phones and ground communication systems as well as communication satellites will require more and more sophisticated technologies. The increasing demand for size and weight reduction, cost savings, low power consumption, increased frequency and higher functionality and reconfigurability as part of multiband and multistandard operation is necessitating the use of highly integrated RF front-end circuits. Chip scaling has made a major contribution to this goal, but today a situation has been reached where the presence of numerous off-chip passive RF components imposes a critical bottleneck to further integration and miniaturization of wireless transceivers. Microelectromechanical systems (MEMS) technology is a rapidly emerging enabling technology that is intended to replace the discrete passives by their integrated counterparts. In this thesis, an original metal surface micromachining process, which is compatible with CMOS post-processing, for above-IC integration of RF MEMS tunable capacitors and suspended inductors is presented. A detailed study on SF6 inductively coupled plasma (ICP) releasing has been performed in order to ascertain the optimal process parameters. This study has emphasized the fact that temperature plays an important role in this process by limiting silicon dioxide etching. Moreover, the optimized recipe has been found to be independent of the sacrificial layer used (amorphous or polycrystalline silicon) and its thickness. Using this recipe, 15.6 µm/min Si underetch rate with high Si: SiO2 selectivity (> 20000: 1) has been obtained. Single-air-gap and double-air-gap parallel-plate MEMS tunable capacitors have been designed, fabricated and characterized in the pF range, from 1 MHz to 13.5 GHz. It has been shown that an optimized design of the suspended membrane and direct symmetrical current feed at both ports can significantly improve the quality factor and increase the self-resonant frequency, pushing it to 12 GHz and beyond. The maximum capacitance tuning range obtained for a single-air-gap capacitor is 29% for a bias voltage of 20 V. The maximum capacitance tuning range obtained for a double-air-gap capacitor is 207% for a bias voltage of 70 V. The post-processing of X-FAB BiCMOS wafers has been successfully demonstrated to fabricate monolithically integrated VCOs with above-IC MEMS LC tank. Comparing a suspended inductor and the X-FAB inductor with the same design, it has been shown that increasing the thickness of the spiral from 2.3 to 4 µm and having the spiral suspended 3 µm above the passivation layers lead to an improvement factor of 2 for the peak quality factor and a shift of the self-resonant frequency beyond 15 GHz. No significant variation on bipolar and MOS transistors characteristics due to the post-processing has been observed and we conclude that the variation due to post-processing is in the same range as the wafer-to-wafer variation. Based on our metal surface micromachining process, coplanar waveguide (CPW) MEMS shunt capacitive switches and variable true-time delay lines (V-TTDLs) have been designed, fabricated and characterized in the 1 - 20 GHz range. A novel MEMS device architecture: the SG-MOSFET, which combines a solid-state MOS transistor and a metal suspended gate has been proposed as DC current switch. The corresponding fabrication process using polysilicon as a sacrificial layer has been developed to release metal gate suspended over gate oxide by SF6 plasma. Very abrupt current switches have been demonstrated with subthreshold slope better than 10 mV/decade (better than the theoretical solid-state bulk or SOI MOSFET limit of 60 mV/decade) and ultra-low gate leakage (less than 0.001 pA/µm2) due to the air-gap

    Micromachining

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    To present their work in the field of micromachining, researchers from distant parts of the world have joined their efforts and contributed their ideas according to their interest and engagement. Their articles will give you the opportunity to understand the concepts of micromachining of advanced materials. Surface texturing using pico- and femto-second laser micromachining is presented, as well as the silicon-based micromachining process for flexible electronics. You can learn about the CMOS compatible wet bulk micromachining process for MEMS applications and the physical process and plasma parameters in a radio frequency hybrid plasma system for thin-film production with ion assistance. Last but not least, study on the specific coefficient in the micromachining process and multiscale simulation of influence of surface defects on nanoindentation using quasi-continuum method provides us with an insight in modelling and the simulation of micromachining processes. The editors hope that this book will allow both professionals and readers not involved in the immediate field to understand and enjoy the topic

    Si-Micromachined Knudsen Pumps for High Compression Ratio and High Flow Rate.

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    This dissertation focuses on Si-micromachined Knudsen pumps. Knudsen pumps exploit thermal transpiration that results from the free-molecular flow in non-isothermal channels. The absence of moving parts, without frictional loss and mechanical failure, provides significantly higher reliability. For a high compression ratio, 48 stages are cascaded in series in a single chip of 10.35 × 11.45 mm2 area. A five-mask, single-wafer process is used for monolithic integration of the designed Knudsen pump. The pressure levels of each stage are measured by integrated Pirani gauges. Using 1.35 W, the fabricated pumps evacuates the encapsulated cavities from 760 to 50 Torr and from 250 to 5 Torr. Multistage Knudsen pumps are further explored using a two-part architecture. To increase the compression ratio, 162 stages are serially cascaded. The two-part architecture uses 54 stages designed for the pressure range of 760-50 Torr, and 108 stages designed for lower pressures. This approach provides greater compression ratio and speed than using a uniform design for each stage in the 48-stage Knudsen pump. The design has a footprint of 12 × 15 mm2. Using 0.39 W, the evacuated chamber is reduced from 760 to 0.9 Torr, resulting in a compression ratio of 844. The vacuum levels were sustained beyond 37 days of continuous operation. The dynamic calibration of microfabricated Pirani gauges is explored for increasing pressure measurement accuracy in the 162-stage Knudsen pump. Test results demonstrated that dynamic calibration can be significantly more accurate than conventional static calibration when Pirani gauges are embedded deep within a microfluidic pathway. Si-micromachined single-stage Knudsen pumps are explored for generating high-flow rates. A high density of thermal transpiration flow channels is arrayed in parallel for combined pumping operation. A design with 0.4 × 106 parallel channels in a footprint of 16 × 20 mm2 generates a measured 211 sccm air flow at a pressure difference of 92 Pa, using 37.2 W. The low-temperature atomic layer deposition (ALD) of Al2O3 is investigated for vacuum seals in wafer-level vacuum packaging applications. The conformal coverage provided by ALD Al2O3 is shown to seal micromachined cavities. Lifetime tests extending out to 19 months are reported.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102498/1/sdan_1.pd

    Optical Waveguides for Infrared Spectroscopic Detection of Molecular Gases

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    Fields like medical diagnostics, urban and industrial environmental monitoring or basic microbiological research greatly benefit from advances in chemical and biological sensing. These applications require rapid sample analysis, reduced needs for sample handling, or good sensor network. Such demands can be met with miniaturised sensors utilising methods which secure sufficient sensitivity and selectivity such laser absorption spectroscopy. However, such instruments are nowadays bulky, expensive, and require large sample volumes. Optical waveguides, a cornerstone of integrated opto-chemical sensors, are aiming at replacing current bulky and costly instrumentation based on free-space optics. They can realize large optical interaction pathlengths as well as provide simple functions of beam splitting or combining on a compact photonic chip, thus substituting e.g., multi-pass cells, Fabry-Perot cavities, or free-space interferometers such as those in FTIR instruments. To achieve competitive sensitivities, however, the waveguide device needs to meet two criteria: Low loss to allow long interaction paths, and large light–analyte interaction per unit length. This thesis presents the analysis, fabrication methods, and characterisation of optical waveguides for infrared tuneable diode laser absorption spectroscopy. A free standing waveguide for use in the mid-infrared spectral domain was developed to tackle the challenges above. Moreover, the waveguide features negligible etalon fringes in transmission, which otherwise interfere with measured spectral signatures. Compared to a free space beam, an outstanding 7 % stronger light-analyte interaction strength was measured with the waveguide

    Fabrication of SOI micromechanical devices

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    This work reports on studies and the fabrication process development of micromechanical silicon-on-insulator (SOI) devices. SOI is a promising starting material for fabrication of single crystal silicon micromechanical devices and basis for monolithic integration of sensors and integrated circuits. The buried oxide layer of an SOI wafer offers an excellent etch stop layer for silicon etching and sacrificial layer for fabrication of capacitive sensors. Deep silicon etching is studied and the aspect ratio dependency of the etch rate and loading effects are described and modeled. The etch rate of the deep silicon etching process is modeled with a simple flow conductance model, which takes into account only the initial etch rate and reaction probability and flow resistance of the etched feature. The used model predicts qualitatively the aspect-ratio-dependent etch rate for varying trench widths and rectangular shapes. The design related loading can be modeled and the effects of the loading can be minimized with proper etch mask design. The basic SOI micromechanics process is described and the drawbacks and limitations of the process are discussed. Improvements to the process are introduced as well as IR microscopy as a new method to inspect the sacrificial etch length of the SOI structure. A new fabrication process for SOI micromechanics has been developed that alleviates metallization problems during the wet etching of the sacrificial layer. The process is based on forming closed cavities under the structure layer of SOI with the help of a semi-permeable polysilicon film. Prototype SOI device fabrication results are presented. High Q single crystal silicon micro resonators have potential for replacing bulky quartz resonators in clock circuits. Monolithic integration of micromechanical devices and an integrated circuit has been demonstrated with the developed process using the embedded vacuum cavities.reviewe

    Transfer printing based microassembly and colloidal quantum dot film integration

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    Micro / nanoscale manufacturing requires unique approaches to accommodate the immensely different characteristics of the miniscule objects due to their high surface area to volume ratio when compared with macroscale objects. Therefore, surface forces are much more dominating than body forces, which causes the significant difficulty of miniscule object manipulation. Because of this challenge, monolithic microfabrication relying on photolithography has been the primary method to manufacture micro / nanoscale structures and devices in place of microassembly. However, by virtue of the two-dimensional (2D) nature of photolithography, formation of complex 3D shape architectures via monolithic microfabrication is inherently limited, which would otherwise enable improvements in performance and novel functionalities of devices. Furthermore, monolithic microfabrication is compatible only with materials which survive in a wet condition during photolithography. Delicate nanomaterials such as colloidal quantum dots cannot be processed via monolithic microfabrication. In this context, transfer printing has emerged as a method to transfer heterogeneous material pieces from their mother substrates to a foreign substrate utilizing a polymeric stamp in a dry condition. In this thesis, advanced modes of transfer printing are studied and optimized to enable a 3D microassembly called ‘micro-Lego’ and a novel strategy of quantum dot film integration. Micro-Lego involves transfer printing for material piece pick-and-place and thermal joining for irreversible permanent bonding of placed material pieces. A microtip elastomeric stamp is designed to advance transfer printing and thermal joining processes are optimized to ensure subsequent material bonding. The mechanical joining strength between material pieces assembled by micro-Lego are characterized by means of blister tests and the nanoindentation. Moreover, the electrical contact between two conducting materials formed by micro-Lego are examined. Lastly, inspired from the subtractive transfer printing technique, protocols of quantum dot film patterning using polymeric stamps made of a shape memory polymer as well as a photoresist are established for the convenient integration of quantum dots in various geometries and configurations as desired. Transfer printing-based micro / nanoscale manufacturing presented in this thesis opens up new pathways to manufacture not only complex 3D functional micro devices but also high resolution nano devices for unparalleled performance or for an unusual functionality, which are unattainable through monolithic microfabrication

    Integrated Micro Gas Chromatographs with High-Flow Knudsen Pumps.

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    Environmental gas sensing typically requires both sensitivity and specificity; target vapor species must not only be detected and quantified, but also differentiated from interferents. This mission can be accomplished by micro gas chromatographs (μGCs), which allow preconcentration of samples and subsequent separation of complex vapor mixtures into individual constituents by their specific retention times. This thesis focuses on the system-level design, fabrication, and integration of μGCs, with the ultimate goal of fully microfabricated systems that can be easily manufactured and distributed to end-users. This thesis also explores the optimization of a micro gas pump – a critical μGC component, and generally recognized as a challenge for microsystems. Three generations of integrated µGC systems have been designed, fabricated, and evaluated. The iGC1 system demonstrates the feasibility of a low-cost three-mask fabrication approach for a µGC including a Knudsen pump, a preconcentrator, a separation column and a microdischarge-based detector, which are integrated in a 4-cc stack. The iGC2 system demonstrates a valveless µGC architecture, in which a bi-directional Knudsen pump provides reversible gas flow for (multi-stage) preconcentrators, which is essential for quantitative analysis. The iGC3 system replaces the microdischarge-based detectors in iGC1 and iGC2 with complementary capacitive detectors, facilitating a purely electronic interface for the fluidics. Additionally, it is compatible with the use of room air as the carrier gas. The quantitative analysis of 19 chemicals with concentration levels of well below 100 ppb is demonstrated, showing the promise of automated, continuous monitoring of indoor air pollutants. The pumps used in the iGCx systems are Knudsen pumps that use thermal transpiration provided by nanoporous media and have no moving parts. This thesis also describes an exploratory effort in which lithographically fabricated channels in silicon substrates provide the thermal transpiration. The Si-micromachined Knudsen pumps demonstrate >200 sccm flow rate. To increase the output pressure head, these pumps are arrayed in series, using both a stacked configuration and a planar one. The results show that the pressure and flow characteristics can be tailored over a wide performance range, extending the possible applications beyond µGC systems.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113581/1/yutaoqin_1.pd

    MEMS Technologies for Energy Harvesting

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    The objective of this chapter is to introduce the technology of Microelectromechanical Systems, MEMS, and their application to emerging energy harvesting devices. The chapter begins with a general introduction to the most common MEMS fabrication processes. This is followed with a survey of design mechanisms implemented in MEMS energy harvesters to provide nonlinear mechanical actuations. Mechanisms to produce bistable potential will be studied, such as introducing fixed magnets, buckling of beams or using slightly slanted clamped-clamped beams. Other nonlinear mechanisms are studied such as impact energy transfer, or the design of nonlinear springs. Finally, due to their importance in the field of MEMS and their application to energy harvesters, an introduction to actuation using piezoelectric materials is given. Examples of energy harvesters found in the literature using this actuation principle are also presented
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