Post-Fabrication Melting Procedure With I-Shaped Beams for Stiction-Free Release of 2-D Surface-Micromachined Micromirrors Equipped With Repulsive-Force Actuators

This paper presents a post-fabrication melting procedure with an I-shaped beam design to solve the problem of stiction in surface-micromachined 2-D micromirrors equipped with repulsive-force actuators. This design adds extra I-shaped beams to the free-standing mirror plate to increase its stiffness about one thousandfold and allow it to survive the wet release process. Then, those beams are melted and broken electrically on-chip after fabrication to restore the original stiffness. With the aid of repulsive force, the I-shaped beam design can use a low current for melting. A high current was used in the previously presented T-shaped fusible tether beam design, but it causes splattering and residue. The I-shaped beam design has been successfully applied to a repulsive-force 2D micromirror that is integrated with head-up display and stiction is eliminated. The modeling, simulation, and prototyping processes are presented, and the ways in which it can be used in a general surface-micromachined free-standing structure to eliminate stiction are discussed.

I. Introduction

Surface micromachining technology [1], [2] has been used to fabricate microelectromechanical systems (MEMS), such as micro accelerometers [3], [4] and micromirrors [5]–[8], for decades due to its simplicity, maturity, low cost and high compatibility with CMOS process. In the last step of the surface micromachining process, the MEMS device is normally wet released, at which point the free-standing structure, especially if it is large size (i.e. 100s to 1 mm) structure, could easily stick to the substrate due to the capillary effect and electrostatic and chemical forces [9]–[11]. This is called stiction, and it is a fundamental problem with surface micromachining processes, as it causes low yield.