Fabrication of three-dimensional parylene HT diaphragms using D-RIE with a Si substrate

This work presents the fabrication of parylene diaphragms through deep-reactive ion etching. Poly-para-xylylene or parylene possess superior mechanical strength, electrical insulation, and high chemical resistance. Deep-reactive ion etching is a preferable method to fabricate the polymer considering that it yields a three-dimensional configuration, which becomes an upside over other polymers prepared by liquid coating. It also makes it possible to produce an arbitrary outline for a diaphragm with a fine surface.

Despite the wide applications of parylene in diaphragms of various sensors, actuators and a wide variety of protective layers, little attention has been put towards investigating potential fabrication methods using deep-reactive ion etching.

Dr. Satomitsu Imai from Nihon University in Japan developed a method for synthesizing three-dimensional configurations for application in microelectrochemical systems. He adopted the deep-reactive ion etching method and demonstrated the fabrication of a diaphragm through the same approach. Since the method cannot be used for parylene C that has low heat-resistivity, he settled for parylene HT. His work is now published in peer-reviewed journal, Sensors and Actuators A.

Professor Satomitsu Imai adopted parylene HT for its high heat resistivity. The polymer could endure the high heat produced during the deep-reactive ion etching. First, he deposited the polymer on a silicon substrate by vapor deposition and etched a diaphragm configuration on the back of the substrate. He prepared two samples for the experiment. One sample had a diaphragm with corrugated grooves and the other had a planar diaphragm. He selected the corrugated grooves to be the three-dimensional configurations since these indentations could be used to make diaphragms that will finally experience more deformations in microelectrochemical systems.

The author investigated the temperature produced when etching the polymer on the silicon substrate. He chose a heat label for temperature measurement since the test was done in a vacuum. The color of label turned black when the temperature at the point where it was fixed reached an indication point. When analyzing a photographic image of the label, the author realized that the maximum temperature produced during the process was about 149⁰C.

The author observed some defects in a few 50um grooves. Therefore, he developed two methods of preventing the defects. In the first method, he had to modify the rectangular corners to rounded edges. In the second method, he had to deposit a layer of aluminum on the parylene layer. Employing the two methods would enable him to fabricate rectangular 50 um deep grooves without defects. Above all, the author realized in the course of the study, that is was paramount to stringently assess thermal stresses developed near the corners, which were primarily influenced by groove depths.

The results of the study indicated that taking into account thermal stresses developed around the corners was important. The author analyzed the mechanical properties and the surface roughness of the diaphragms. He observed that the yield stress as well as the Young’s modulus increased by about 30%.