Laminated high-aspect-ratio microstructures in a conventional CMOS process

Abstract

Electrostatically actuated microstructures with high-aspect-ratio laminated-beam suspensions have been fabricated using a 0.8 μm three-metal CMOS process followed by a sequence of three maskless dry-etching steps. Laminated structures are etched of the CMOS silicon oxide, silicon nitride, and aluminum layers. The key to the process is the use of the CMOS metallization as an etch-resistant mask to define the microstructures. A minimum beam width of 1.2 μm, gap of 1.2 μm, and maximum beam thickness of 4.8 μm are obtained. These structural features will scale in size as the CMOS technology improves. The laminated material has an effective Young's modulus of 61 GPa, an effective residual stress of 69 MPa, and a residual strain gradient of 2 × 10 −4 μm −1. Multi-conductor electrostatic micromechanisms, such as self-actuating springs, x−y microstages, and nested comb-drive lateral resonators, are successfully produced. A self-actuating spring is a lateral electrostatic microactuator without a stator that is insensitive to out-of-plane curl. A spring 107 μm wide by 109 μm long excited by an 11 V a.c. signal has a measured resonance amplitude of 1 μm at 14.9 kHz. Finite-element simulation using the extracted value of Young's modulus predicts the resonance frequencies of the springs to within 7% of the measured values.