Capacitive Sensing for Microfluidic Systems

The development of two types of microelectromechanical systems (MEMS) using a capacitive detection method in a microfluidic environment is discussed. Capacitive sensors are available in many shapes and sizes and offer several benefits compared to other technologies (e.g. (piezo)resistive or optical sensing) such as low power consumption, excellent temperature stability and a wide range of detectable parameters, either directly or indirectly. Over the past decades, microfluidics - or the science and technology concerning applications of fluids at the microscale - has infiltrated many areas of research, industry and daily life. One typical example is the biological analysis using microdroplet manipulation in so-called labs-on-a-chip, but microfluidic systems can also be identified in many other applications such as inkjet printing nozzles and transdermal drug delivery devices. Moreover, characterization of complex molecules and interfaces at the microscale has significantly increased our understanding of fluids such as lung surfactants and foam on beer. The first chapter of the dissertation introduces some basic properties and applications/examples of capacitive sensors and microfluidic systems. Important materials, e.g. SU-8 and metals, and microfabrication steps, e.g. anodic bonding and critical point drying, used throughout the work are presented in chapter two. The ability to influence internal stress in thin metal films based on physical vapor deposition pressure is applied to create out-of-plane bending molybdenum cantilevers. The stress in copper is also characterized in order to obtain a quasi-neutral layer which can be used for the metallization of polymer MEMS. The complex, frequency-dependent permittivity of eight different polymers is characterized in the 1 kHz to 100 kHz range by means of a microcapacitor array and impedance measurements. While epoxy-based polymers such as SU-8, EpoCore and EpoClad have the higher dielectric constants, they are also shown to be more prone to conductivity losses at higher frequency, compared to polyimide and Parylene. Chapter three treats the development of an integrated, capacitive void fraction sensor for microchannels. This sensor, originally developed to detect boiling in a two-phase cooling application for high-performance integrated circuits, exploits the changing permittivity of the fluid confined in a microchannel of 100 µm by 500 µm to detect the void fraction in the mixture. The read-out circuit and signal processing algorithm achieve a sensitivity of 12.5 mV per percent void fraction. The main topic of chapters four and five is the design of a microtensiometer for surface pressure measurements at fluid-fluid interfaces. The first of these two chapters presents an in-depth analysis of a semi-flexible polymer microtensiometer which deflects under the surface pressure due to an insoluble surfactant forming a Langmuir monolayer around the structure. The read-out of this device is based on micrograph processing. A sensitivity of 2.03 µm deflection per mN/m surface pressure is achieved, i.e. a maximum resolution of +/-0.02 mN/m, taking into account the image capturing and processing parameters. Chapter five couples the semi-flexible polymer tensiometer to a capacitive read-out mechanism by integrating a metallized comb structure in the center of tensiometer body. Finite element simulations are used to optimize the device geometry. Preliminary prototype experiments provide a general proof-of-concept, while also revealing the necessity for improvements to increase the process yield as well as the robustness of the integration method. The sixth and final chapter of the dissertation presents the main conclusions of this work and provides an overview of the main possibilities for future work.nrpages: 196status: publishe