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

Laser-based packaging of micro-devices

In this PhD thesis the development of laser-based processes for packaging applications in microsystems technologies is investigated. Packaging is one of the major challenges in the fabrication of micro-electro-mechanical systems (MEMS) and other micro-devices. A range of bonding processes have become established in industry, however, in many or even most cases heating of the entire package to the bonding temperature is required to effect efficient and reliable bonding. The high process temperatures of up to 1100°C involved severely limit the application areas of these techniques for packaging of temperature sensitive materials. As an alternative production method, two localised heating processes using a laser were developed where also the heat is restricted to the joining area only by active cooling. Silicon to glass joining with a Benzocyclobutene adhesive layer was demonstrated which is a typical MEMS application. In this laser-based process the temperature in the centre of the device was kept at least 120°C lower than in the bonding area. For chip-level packaging shear forces as high as 290 N were achieved which is comparable and some cases even higher than results obtained using conventional bonding techniques. Furthermore, a considerable reduction of the bonding time from typically 20 minutes down to 8 s was achieved. A further development of this process to wafer-level packaging was demonstrated. For a simplified pattern of 5 samples the same quality of the seal could be achieved as for chip-level packaging. Packaging of a more densely packed pattern of 9 was also investigated. Successful sealing of all nine samples on the same wafer was demonstrated proving the feasibility of wafer-level packaging using this localised heating bonding process. The development of full hermetic glass frit packaging processes of Leadless Chip Carrier (LCC) devices in both air and vacuum is presented. In these laser-based processes the temperature in the centre of the device was kept at least 230°C below the temperature in the joining region (375°C to 440°C). Testing according to MIL-STD-883G showed that hermetic seals were achieved in high yield processes (>90%) and the packages did withstand shear forces in excess of 1 kN. Residual gas analysis has shown that a moderate vacuum of around 5 mbar was achieved inside the vacuum packaged LCC devices. A localised heating glass frit packaging process was developed without any negative effect of the thermal management on the quality of the seal

A 5 meter range non-planar CMUT array for Automotive Collision Avoidance

A discretized hyperbolic paraboloid geometry capacitive micromachined ultrasonic transducer (CMUT) array has been designed and fabricated for automotive collision avoidance. The array is designed to operate at 40 kHz, beamwidth of 40° with a maximum sidelobe intensity of -10dB. An SOI based fabrication technology has been used for the 5x5 array with 5 sensing surfaces along each x and y axis and 7 elevation levels. An assembly and packaging technique has been developed to realize the non-planar geometry in a PGA-68 package. A highly accurate mathematical method has been presented for analytical characterization of capacitive micromachined ultrasonic transducers (CMUTs) built with square diaphragms. The method uses a new two-dimensional polynomial function to more accurately predict the deflection curve of a multilayer square diaphragm subject to both mechanical and electrostatic pressure and a new capacitance model that takes into account the contribution of the fringing field capacitances

Semiconductor Packaging

In semiconductor manufacturing, understanding how various materials behave and interact is critical to making a reliable and robust semiconductor package. Semiconductor Packaging: Materials Interaction and Reliability provides a fundamental understanding of the underlying physical properties of the materials used in a semiconductor package. By tying together the disparate elements essential to a semiconductor package, the authors show how all the parts fit and work together to provide durable protection for the integrated circuit chip within as well as a means for the chip to communicate with the outside world. The text also covers packaging materials for MEMS, solar technology, and LEDs and explores future trends in semiconductor packages

Advances in electronic packaging technologies by ultra-small microvias, super-fine interconnections and low loss polymer dielectrics

The fundamental motivation for this dissertation is to address the widening interconnect gap between integrated circuit (IC) demands and package substrates specifically for high frequency digital-RF systems applications. Moore's law for CMOS ICs predicts that transistor density on ICs will double approximately every 18 months. The current state-of-the-art in IC package substrates is at 20µm lines/spaces and 50-60µm microvia diameter using epoxy dielectrics with loss tangent above 0.01. The research targets are to overcome the barriers of current technologies and demonstrate a set of advanced materials and process technologies capable of 5-10µm lines and spaces, and 10-30µm diameter microvias in a multilayer 3-D wiring substrate using 10-25µm thin film dielectrics with loss tangent in the <0.005. The research elements are organized as follows with a clear focus on understanding and characterization of fundamental materials structure-processing-property relationships and interfaces to achieve the next generation targets. (a) Low CTE Core Substrate, (b) Low Loss Dielectrics with 25µm and smaller microvias, (c) Sub-10µm Width Cu Conductors, and (d) Integration of the various dielectric and conductor processes.Ph.D.Committee Chair: Tummala, Rao; Committee Member: Iyer, Mahadevan; Committee Member: Saxena, Ashok; Committee Member: Swaminathan, Madhavan; Committee Member: Wong, Chingpin

Integration of Benzocyclobutene Polymers and Silicon Micromachined Structures Fabricated with Anisotropic Wet Etching

Integration of low dielectric constant Benzocyclobutene (BCB) film with deep etched structures in silicon allows the fabrication of MEMS devices with low parasitic loss. A fabrication process is developed for integration of thin BCB film and deep anisotropically-etched grooves in silicon using potassium hydroxide (KOH). Gold (Au) is used as an etch mask to protect the low-k film during the highly-corrosive, long, and high-temperature KOH etching process. Metal/BCB adhesion is a key parameter in this masking design. Adhesion of the BCB and metal mask was improved by cure management of the BCB before and after metallization, surface treatment of the BCB before metallization, and high-temperature metallization. Test structures were fabricated to demonstrate the feasibility of this fabrication process. Adhesion improvement was successfully verified by studying BCB/metal interface using time-of-flight secondary ion mass spectroscopy and Auger electron spectroscopy. This study enables the development of the next generation micromotors/microgenerators

Semiconductor Packaging

In semiconductor manufacturing, understanding how various materials behave and interact is critical to making a reliable and robust semiconductor package. Semiconductor Packaging: Materials Interaction and Reliability provides a fundamental understanding of the underlying physical properties of the materials used in a semiconductor package. By tying together the disparate elements essential to a semiconductor package, the authors show how all the parts fit and work together to provide durable protection for the integrated circuit chip within as well as a means for the chip to communicate with the outside world. The text also covers packaging materials for MEMS, solar technology, and LEDs and explores future trends in semiconductor packages

A BCB Diaphragm Based Adhesive Wafer Bonded CMUT Probe for Biomedical Application

This dissertation presents the design methodology, fabrication procedure, and key experimental characterization results of a linear array of capacitive micromachined ultrasonic transducers (CMUT) for possible ophthalmic anterior segment imaging application. The design methodology involves analytical, 3-D electromechanical finite element analysis, and Verasonics Vantage 128 ultrasonic research platform based diagnostic imaging simulations to develop a technique that minimizes electrical charging and center frequency drift while improving the transduction efficiency. In the design, Bisbenzocyclobutene (BCB), a low K polymer from Dow Chemical Company, has been innovatively used for the first time to fabricate the structural layer of the CMUT diaphragm, realize the interelectrode dielectric spacer, and to act as a low temperature adhesive bonding agent. Additionally, the top CMUT electrode has been placed at the bottom of the diaphragm to affect higher capacitance change that increases sensitivity and provides additional decoupling of the electrical charging effects. Several arrays with element count ranging from 8 to 128 elements and a center frequency range of 5 MHz to 40 MHz have been designed and fabricated. Due to an unforeseen adhesion issue during wirebonding, a 32 channel 40 MHz CMUT array has been packaged manually to validate the fabrication process and CMUT operation. Extensive SEM inspections of the CMUT cross-sections show good agreement with the design specifications. Static and dynamic measurements using a Polytec laser Doppler vibrometer, impedance measurement using an Agilent vector network analyzer, and LCR measurement results are in excellent agreement with analytical and FEA analysis using IntelliSuite. The frequency analysis exhibits high electromechanical coupling coefficient of 0.66 at a low bias voltage of 20 V and high uniformity. A successful measurement of the lower drift of the center frequency 0.32% and higher coupling coefficient verifies the hypothesis that the excellent electrical, structural, and processing characteristics of BCB is a viable option to mitigate the dielectric charging and improve the transduction efficiency of CMUTs