Flip chip bonding technologies for hybrid integration

Hybrid integration and especially the packaging of microelectromechanical systems (i.e. MEMS) cannot rely on standardised packaging solutions due to the diversity of microsystems. As an example, the packaging requirements of a pressure sensor are different from that of an accelerometer. Usually pressure sensors do not require a special sealing, whereas accelerometers need to be hermetically packaged. On the other hand, diaphragm-based (i.e. membrane) pressure sensors ask for special precautions in the die-attach process: Mismatches of the coefficient of thermal expansion between the components of the package may induce stress and strain in the membrane. Since the output signal of a pressure sensor is a measure of the static deformation of the membrane, temperature variations of the environment may render an unproperly attached pressure sensor useless. Moreover, an inappropriate bonding process may bias the output signal or the bonding process may render the pressure sensor even useless. In the present work several flip chip bonding processes with the focus on electrical properties and applicability were evaluated. Specifically, a bonding process based on anisotropic conductive adhesives (ACA) suitable for diaphragm-based pressure sensors was developed. The flip chip bonded sensors respond linearly to the applied pressure. The sensitivity to pressure and the offset shift due to temperature changes are within the tolerances of the sensor die according to the data sheet. The developed ACA-bonding process provides electrical and mechanical connection as well as fluidic sealing of the membrane to the outside world. Small changes of the bonding process allowed to adapt the process to plastic substrates with a low glass transition temperature Tg. Such substrates are usually employed in microfluidic applications because they are low-cost, easy to machine, resistant to acids and lyes, and compatible with most chemical and biological materials. Specifically, it was shown that the process is capable of bonding Si-chips onto PMMA substrates with a Tg<7sub> of 100°C. The process is especially suited for semiconductor chips which are too small for a standard microgasket approach. The results of the process development were applied to two novel cases : i) A pressure strip was developed which provides a non-destructive way to measure flow and pressure distributions and arbitrarily shaped bodies. The pressure strip consists of three components: A mechanically rigid carrier with an array of pressure sensors which transform the measured pressure into an electrical signal, a mechanically flexible fluidic strip which comprises openings and microfluidic channels which detect and guide the pressure signal to the pressure sensors, and a flexible electronic strip which processes the electronic signals and provides electrical connection to the outside world. Functionality was proven in a wind tunnel. Furthermore, the sensitivity to pressure and temperature was measured and are within the tolerances of the pressure sensors specifications which proves the quality of the developed bonding process ii) A novel flow sensor to measure very low-rate flows in the range of 1 μl/s was designed and implemented by using the previously developed flip chip ACA-bonding process. The concept is based on the measurement of pressures of a fluid at the inlet and outlet of of a microchannel. These pressures are guided to the front and back side of a differential pressure sensor. The pressure sensor is hence exposed only to the relatively small pressure difference between inlet and outlet. The system pressure has no impact on the pressure sensor. Existing flow sensors with comparable flow rate ranges either have longer response times or are sensitive to pressure peaks


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