InP based Micromechanics for Vertical-Cavity Micro-Opto-Electro-Mechanical Systems

Abstract: During the past decade, the amount of transmitted data hasbeen boosted due to an increasing use of the World Wide Web(WWW), video and audio transmissions. In the late 90s, thetraditional technology using twisted copper cables was nolonger capable to provide enough transmission capacity for theconstantly increasing data traffic. Long-haul optical fibernetworks have been installed to meet the rising demand.Wavelength division multiplexing (WDM) systems offer thepossibility to increase the transmission capacity of thesefiber networks even further by carrying several opticalchannels simultaneously. Reconfiguration of the network and addand drop operations are considered to be an important featureto achieve high flexibility. Hence, new wavelength tunabledevices are of high interest and the fabrication of tunablevertical-cavity devices is particularly interesting. Theycombine several advantages such as wafer level fabrication andcharacterization, and easy fiber coupling.In this work, InP-based micromechanics has been studied andapplied to fabricate wavelength tunable vertical-cavitymicromechanical devices for WDM systems. The investigatedmicromechanical devices rely on released InP membranes formingvertical air-gap structures. Deformation due to internal strainturned out to be a major issue. An unintentional gradientstrain is caused by a gradient arsenic distribution in thestructural layers and makes a reliable device productiondifficult. Several solutions to reduce the mechanicaldeformation of the released membranes are discussed like straincompensating layers, the use of tensile pre-strainedquarternary material as structural layer, and changed growthconditions for the epitaxial heterostructure.Novel device designs based on InP/air-gap DBRs combiningoptical active parts with electromechanical transducers formingso-called Micro-Opto-Electro-Mechanical-Systems (MOEMS) havebeen realized. Using optimized growth conditions, tunablefilters with record wide tuning range have been fabricated.Constant filter performance during actuation has been achievedby applying a stable cavity design. Photopumped vertical-cavitysurface-emitting lasers (VCSELs), potentially tunable resonantcavity detectors, and photopumped tunable resonant cavity lightemitting diodes (RCLED) have been manufactured. Anotherapproach relying on a two-chip concept has been pursued inparallel to fabricate selective detectors. First investigationson the fabrication of photonic crystals on InP membranes havebeen made to determine the technological feasibility.Keywords:MOEMS, vertical-cavity, InP, DBR, epitaxy,semiconductor processing, micromechanics, wavelength tunabledevices, long-wavelength