Kristof Wouters

Diffusing and swelling in SU-8: insight in material properties and processing

The swelling behavior of SU-8 is studied. It is well known that negative resist swells under the influence of common processing liquids, developers, solvents, etc. However, when SU-8 is used as a construction material for plating molds, or for permanent structures, for instance in microfluidics or as active mechanical components in the MEM structure, a more quantitative investigation is needed. In this paper, the volume change is measured when the SU-8 epoxy is submersed in common processing liquids. An analytical model is derived to link the diffusion mechanism to the mechanical response of the SU-8 material. Using the obtained model, the diffusion constants are calculated from the mechanical displacement measurements. Also, the derived model can be used for the prediction of the mechanical behavior of SU-8 structures in the future. The swelling behavior is also correlated with the internal stresses that exist in the SU-8 film. This helps to understand crack formation and delamination of SU-8 patterns during processing. The results show that the built-in stress in the SU-8 epoxy is strongly dependent on the ambient or submersion liquid and that this effect in itself is strongly dependent on the softbake procedure of the polymer. The built-in stress in SU-8 was found to be maximum when submersed in propylene glycol methyl ether acetate and in isopropyl alcohol, both liquids that are used during the development, and found to be very low in water.

A Low-Cost and Highly Integrated Fiber Optical Pressure Sensor System

This paper presents a simple fiber-optic based pressure measurement system, in which both sensor and readout parts are constructed using batch micromachining techniques, supplemented with some straightforward assembly steps. Prototypes of relative and absolute pressure sensors parts and a readout part have been realized. Goal of this research is to enable deployment of such harsh environment sensors in high-volume, low-cost applications.

Comparison of methods for the mechanical characterization of polymers for MEMS applications

This paper gives an overview of a series of experiments conducted in order to characterize the mechanical behavior of two photodefinable epoxies. In particular, it focuses on the stiffness of the materials. The (complex) modulus is measured using five different methods: conventional nanoindentation, continuous stiffness measurement nanoindentation, nanoindentation on MEM structures, dynamic tensile tests and static tensile tests. The measured moduli range from 0.9 to 7.4 GPa for SU-8 and from 1 to 4.1 GPa for Epoclad, depending on the type of experiment and the loading speeds used in the experiments. Results reveal that conventional nanoindentation is not well optimized for polymer characterization and that there is a need for stiffness measurements using more fundamental approaches like a (dynamic) tensile test.

An in-plane SiGe differential capacitive accelerometer for above-IC integration

MEMS above-IC monolithic integration can yield a very compact device with good cost-effectiveness. One of the major challenges for this technology is to protect the CMOS from the heat introduced by the MEMS fabrication. In this paper, we present the design and fabrication of a novel lateral capacitive accelerometer, utilizing a low thermal budget SiGe MEMS technology. The accelerometer features a 4 µm SiGe structural layer thickness with a shock protector gap of 500 nm. Benefiting from the low temperature (~450 °C) SiGe MEMS technology, this inertial device demonstrates the achievability of fabricating above-IC mechanical sensors by 3D stacking. In this paper, the accelerometer design will be introduced first, followed by the introduction of the low thermal budget SiGe MEMS fabrication process. The fabricated devices have been characterized with a network/spectrum analyzer. Both a frequency sweep and a dc voltage sweep have been conducted. These electrostatic characterization results will be analyzed and compared with the design model.

Determining the physical properties of EpoClad negative photoresist for use in MEMS applications

In this paper, the internal stress and Young modulus of EpoClad are investigated using different methods. The swelling behavior when submerged in common processing liquids is also measured and reported. Different on-wafer micro electromechanical structures are used, together with nanoindentation measurements. The experimental results yield an internal stress that is strongly dependent on the processing conditions, the ambient and the submersion liquid. Youngs modulus is found to be 4.4 ± 0.2 GPa.

Design and measurement of stress indicator structures for the characterization of Epoclad negative photoresist

Using new materials for the structural or active layers in MEMS requires the knowledge of the material properties. In this paper, the internal stress and the coefficient of thermal expansion of Epoclad negative photoresist are measured. Despite being a photoresist, the epoxy-based material has very good mechanical properties. It enables the creation of microscale mechanical structures using this material. The internal stress and the coefficient of thermal expansion were measured using on-wafer stress indicator structures. The structures were designed to give a response caused by internal stress measurable using an optical microscope. The coefficient of thermal expansion is measured via the internal stress at different temperatures.