LAMINATED HIGH-ASPECT-RATIO MICROSTRUCTURES IN A CONVENTIONAL CMOS PROCESS

Electrostatically actuated microstructures with high-aspect-ratio lami nated-beam suspensions have been fabricated using a 0.8 mu m three-met al CMOS process followed by a sequence of three maskless dry-etching s teps. Laminated structures are etched out of the CMOS silicon oxide, s ilicon nitride, and aluminum layers. The key to the process is the use of the CMOS metallization as an etch-resistant mask to define the mic rostructures. A minimum beam width of 1.2 mu m, gap of 1.2 mu m, and m aximum beam thickness of 4.8 mu m are obtained. These structural featu res will scale in size as the CMOS technology improves. The laminated material has an effective Young's modulus of 61 GPa, an effective resi dual stress of 69 MPa, and a residual strain gradient of 2 x 10(-4) mu m(-1). Multi-conductor electrostatic micromechanisms, such as self-ac tuating springs, x-y microstages, and nested comb-drive lateral resona tors, are successfully produced. A self-actuating spring is a lateral electrostatic microactuator without a stator that is insensitive to ou t-of-plane curl. A spring 107 mu m wide by 109 mu m long excited by an 11 V a.c. signal has a measured resonance amplitude of 1 mu m at 14.9 kHz. Finite-element simulation using the extracted value for Young's modulus predicts the resonance frequencies of the springs to within 7% of the measured values.