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Integration of PVDF (Polyvinylidene Fluoride) in MEMS Production
The combination of materials with new processing setups often leads to new opportunities in well-known technologies. Polyvinylidene fluoride (PVDF) is widely used because of its chemical resistance and thermal properties. Usually, it is shaped by spray casting to produce plumber components. PVDF is also known for its piezoelectric behavior, which can be measured in PVDF films, like it was shown by Kawai [1] and others long ago. Nevertheless, piezoelectric materials are used in MEMS production, PVDF is not. Therefore, the main idea is to evaluate the possibilities and afterwards creating a way for the integration of PVDF in MEMS production for example as membrane for flexible parts with piezoelectric behavior.
Solubility and Spin coating
As a substrate with a high chemical resistance, PVDF is only soluble in a few solvents. However, to use PVDF films does not seem promising, because of the inclosed gas bubbles, the difficulties to produce thin films and the need to reach a bond between film and wafer.
Spin coating was chosen to get PVDF films on wafer substrates. To obtain a photoresist like solution of PVDF, three different solvents (DMF, DMAc, NMP) were tested. Each showed good solution result, even though it took up to several days to get a homogeneous solution.
Spin coating was performed with each solution. Afterwards the reached thickness of the PVDF layer was measured. A connection between amounts of PVDF in solution and thickness could be found as well as between the different solvents used for the solutions.
Measurement and cumulation of the piezoelectric behavior
For the following experiments, several wafers were prepared with PVDF by spin coating. Some were dried under standard conditions (95°C, 5 min). The others were dried in an electric field, to reach an increasing in the piezoelectric behaviour. These PVDF film were compared to PVDF films that were exposed to an electric field in solid state. The setup for that is shown in picture A.
The setup for the exposure of the liquid PVDF is far more complex because a defined air gap is needed during the baking process. Therefore, the setup was changed. The wafer is placed on the heating plate, which is used as on electrode. The other electrode is located parallel to the wafer with a small air gap of at least 4 mm.
A relation between electric field and piezoelectric behavior could be shown. Therefore the method seems promising. Picture B shows the results of the first spin coating tests with PVDF as membrane material for flexible parts.
[1] H. Kawai, “Piezoelectric Effect in Polarized Poly (vinylidene Fluoride)”, Japan. J. Appl. Phys., 8, 975, 1969