Development of InP-based test structures for optical MEMS applications

III-V semiconductors such as InP have a direct bandgap and thus allow active optical devices such as lasers and optical amplifiers to be realized. The monolithic integration of InP based active optical devices with MEMS actuators will enable a new generation of versatile wavelength division multiplexed (WDM) devices from lossless switches to tunable lasers and optical filters to be realized at the 1.55 μm communications wavelength. While the electro-mechanical properties of silicon are well known,, those of InP must first be studied before InP-based optical micro-electro-mechanical systems (MEMS) can be realized. This can be done by the M-Test, which utilizes electrostatically actuated beams to obtain bending and stress parameters of thin films. An applied voltage between a beam and substrate causes the beam to bend down toward the substrate. The electrostatic force increases as 1/g² where g is the beam-to-substrate gap. The mechanical bending force of the beam is proportional to k(Δg) where Δg is the change in beam to substrate gap and k is the spring constant of the beam. The spring constant k is dependent on both material properties as well as beam geometry. Because the electrostatic and mechanical forces increase at different rates, it should be clear that an instability point exists at which the two forces no longer balance and the beam is pulled in to the substrate. This instability is known to occur at 2/3 the original beam to substrate gap. By measuring the pull in voltage for various beam geometries, Young´s modulus and intrinsic stress of the beam material can be found. Both properties are fundamental for MEMS design