Two self-aligend plastic deformation processes are devleoped to fabricate silicon microstructures, including angular vertical comb-drives, scanning micromirrors, variable capacitors, and two-axial actuators based on global-furnace and localized-Joule heating methods. As a proof-of-concept demonstration, an ultrasonic packaging procedure that generates local plastic deformation of metallic bonding materials is also established for MEMS packaging applications.
Plastic defomation of silicon occurs at elevated temperature when silicon is stresses to a level higher than the reduced yield stress. The self-aligned plastic deformation process developed in this work utilizes the "key and key-slots" design between a lid and device substrate for self-alignment and micropillars are constructed on the lid substrate to provide mechanical stress on silicon microstructures. Vertically driven electrostatic microactuators are designed and fabricated by the aforementioned processes, eliminating the difficulties in alignment, wafer bonding and multi-masks processes.
First angular vertical comb-drives with self-aligned comb fingers are fabricated by the global plastic deformation process where the whole silicon substrate in annealed at high temperature. The fabricated scanning mirror actuators resonate at frequencies between 1.90 and 5.33 kHz achieving optical scanning angles up to 19.2 degrees with driving voltages of 40Vdc plus 13Vpp and a quality factor of 120. A small amount of elastic recovery is observed after the plastic deformation process and is experimentally characterized to give a design guideline. After continuous testing of 5 billion cycles at the maximum scanning angle on one fabricated scanning mirror, no clear degradation or sign of fatigue was observed.
Second the localized plastic deformation process is applied to fabricate torsional scanning micromirrors with special torsional spring designs to eliminate the buckling effect due to thermal expansion of plastically deformable fixed-fixed beams during the localized heating and plastic deformation process. Improved actuator design with double-sided symmetric angular vertical comb-drives achieved 50.9 degrees of optical scanning angle at a resonant frequency of 4.13 kHz with a quality factors of 202 under driving voltages of 30Vdc plus 14Vpp. The extended reliability tests of 20 billion cycles reveal no observable degradations.
Applications of microstructures fabricated by the self-aligned plastic deformation processes are further illustrated in variable capacitors, two-axial actuators and vertically driven linear actuators. Design and process variations of the self-aligned plastic deformation could lead to new device architectures for microsystems.