Paper MN+AS+SS-MoM3
Meso Scale MEMS Inertial Switch Fabricated using Electroplated Metal on Insulator (MOI) Technique

Monday, October 28, 2013, 9:00 am, Room 102 A

Micro switches triggered by inertia are widely used as safety and protection devices in airbags, arming and firing systems. These devices are typically fabricated of silicon and incorporate a movable proof mass suspended on flexure-type springs. When a sufficient acceleration is applied, the mass moves towards the fixed electrode resulting in an electrical path that triggers an electric circuit. Electrodeposited metallic devices offer an attractive alternative to silicon in the fabrication of high aspect ratio devices. Nickel is one of the most common materials used for this purpose. The Young’s modulus of nickel is close to that of silicon though its density is nearly four times higher and the electric conductivity is five orders of magnitude higher than of highly doped silicon. Nickel is also exceptionally resistant to wet and dry chemical etching, aggressive chemicals and corrosion.

In this work, we report on a novel approach for the fabrication of high aspect ratio electrodeposited nickel MEMS devices. The two mask process is distinguished by its simplicity and does not require formation of anchors/vias for the attachment of the device to the substrate. In this context, similarity between this process and common silicon on insulator (SOI) fabrication paradigm can be mentioned. KMPR negative photoresist is used as a mold due to its ability to yield high aspect ratio structures (>5:1) with vertical sidewalls as well as the relative ease of removal. The devices are fabricated on a 2” single side polished wafers with 4 µm of thermally grown silicon dioxide (TOX). First, a lift-off metallization is performed to define a patterned Cr/Cu seed layer. At the second stage, a 40 µm thick KMPR 1050 negative photoresist is spun on top of the seed layer followed by electrodeposition of a 35 µm thick nickel layer. Next, the stripping of the KMPR mold is performed by ultrasonication bath of remover PG followed by etching with O₂ plasma to remove the resist leftovers. Finally, the wafer is diced into 3mm × 3mm chips and the devices are released first by dipping in a HF to etch the sacrificial oxide and then by etching the copper and chrome. The HF etch time is tailored in such a way that the anchors remain unreleased whereas the free standing elements are released by undercut. The fabricated devices were mounted in a ceramic enclosure and characterized using a drop tester. The triggering event was captured by registered the steep decrease of the resistance down to less than 10 Ω value and functionality of the device was demonstrated in the experiment. Good agreement between the designed values of the triggering time and the experimental data was observed.