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A tip-tilt-piston micromirror array based on electrothermal bimorph actuation is presented. The micromirror uses a compactly folded actuator design that can realize high fill-factor with a simple fabrication process. A 4 × 4 micromirror array with sub-apertures of 0.9 mm and a fill-factor of 54% is demonstrated. A piston actuation of about 200 μm and tip-tilt scanning of ±18° optical angles are obtained at a driving voltage as small as 4.5 Vdc. The mirror's tip-tilt steering capability and piston control make it promising for optical phased array applications. The phased array concept is demonstrated by phasing two adjacent mirrors on the mirror array. Other device characterizations including frequency, transient response, and mirror surface quality are also reported.