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

Abstract

This work demonstrates the fabrication of parylene diaphragms with three-dimensional configurations using deep-reactive ion etching (D-RIE, or ion-coupled plasma etching using the Bosch process) and assessed the characteristics of the resulting products. Parylene HT, which has superior temperature resistance up to 350°C, was used due to the significant heat generated in the D-RIE process. Parylene was first deposited on a silicon substrate and a diaphragm configuration was etched on the backside of this substrate. In contrast to wet etching, D-RIE was able to fabricate any outline of diaphragms with fine projections, such that could be employed in a microelectromechanical system (MEMS). Planar diaphragms were produced to ascertain the heat resistance of the parylene, and diaphragms (5mm in diameter, 10μm thick) with corrugated rectangular grooves (30–50μm deep) were also fabricated. Scanning electron microscopy (SEM) identified some defects in the corners of grooves when employing a depth of 50μm. Such grooves were eventually fabricated without defects by converting the rectangular corners to rounded shapes and by depositing aluminum on the parylene layer. It was found to be necessary to carefully assess the thermal stress in the vicinities of corners, and that this stress was primarily affected by the groove depth. Surface roughness and mechanical properties of the parylene layers, including Young’s modulus and yield stress, were assessed after the D-RIE process.