S. Irmer

Stress investigation of PECVD dielectric layers for advanced optical MEMS

Detailed stress investigations of silicon nitride and silicon dioxide defined by PECVD are presented. The spatial variation of the stress of the dielectric material is evaluated by both, laser induced diffraction imaging method (macroscopically averaged stress) and by a large number of laterally distributed MEMS structures on the wafer (microscopically detected stress). By changing the duty cycle of two different plasma excitation frequencies during the deposition, the stress of silicon nitride (deposited at 300 °C) is controlled in a wide range from +850 MPa (compressive) to −300 MPa (tensile). A similar dependence is also observed for silicon nitride deposited at 60 °C. In contrast, silicon dioxide shows in both cases no strong frequency dependence. The microscopically detecting involves a low cost MEMS technology based on photoresist as sacrificial layer. Applying this technology, differently shaped Fabry–Perot filter membranes with various stress values are implemented. The cavity length of the filters and the radii of curvatures of the upper DBR membranes are also varied. Concave, convex and flat membranes with radii of curvature of −0.31 mm, 0.19 mm and −184.71 mm, respectively, are produced.

Ultralow biased widely continuously tunable fabry-Perot filter

Optical filters capable of single control parameter-based wide tuning are implemented and studied. The surface micromachined Fabry-Perot filter consists of two InP-air-gap distributed Bragg reflectors and shows a wavelength tuning of more than 140 nm using only a single voltage of up to 3.2 V at currents below 0.2 mA. The membrane-based filter is designed to block all wavelengths in the whole range of 1250-1800 nm apart from its transmission wavelength.

Modeling of ultrawidely tunable vertical cavity air-gap filters and VCSELs

Tunable vertical cavity devices including an air-gap integrated in the cavity have been designed, fabricated, and investigated. The ultrawide wavelength tuning is realized by micromechanical actuation of Bragg mirror membranes. Based on optical and mechanical model calculations, the air-gap filters and vertical cavity surface emitting lasers (VCSELs) are designed for investigating mainly the optical tuning efficiency. In our research, we focus on two different mirror material systems, dielectric Si/sub 3/N/sub 4//SiO/sub 2/ and InP/air-gap Bragg mirrors and on two tuning concepts, respectively. For the dielectric mirrors, continuous tuning is achieved by thermal actuation of the Si/sub 3/N/sub 4//SiO/sub 2/ mirror membranes, and for InP/air-gap mirrors, electrostatic actuation of the InP membranes is used. To verify the optical and mechanical simulations, InP/air-gap filters are characterized by measuring reflectance spectra and the tuning behavior. The measured results agree with the simulations used to optimize the micromechanical and optical characteristics of air-gap filters and VCSELs for optical communication applications.

Surface micromachined optical low-cost all-air-gap filters based on stress-optimized Si3N4 layers

A new surface micromachining approach based on a multiple Si3N4- and silicon-layer stack is presented. The fabrication process is implemented by plasma-enhanced chemical vapour deposition of stress-optimized films, reactive ion etching using SF6/CHF3/Ar, wet chemical etching of the sacrificial silicon layers by KOH and critical point drying. Using this approach, the fabrication of an optical all-air-gap vertical-cavity Fabry–Perot filter is demonstrated. The surface micromachined filter consists of two DBR mirrors, each having five 590 nm thick Si3N4 membranes separated by 390 nm wide air gaps. The distance between the mirrors (cavity) is 710 nm. The optical characterization and a white light interferometer measurement document the accuracy of the layer positioning and the performance of this low-cost approach. The filter shows the designed filter dip at 1490 nm, the full width at half maximum (FWHM) of the filter is 1.5 nm and the insertion loss is just 1.3 dB. The process is compatible with a variety of materials, e.g. III–V compounds, silicon, as well as organic materials, facilitating a huge application spectrum for sensors.

A novel low-cost tunable dielectric air-gap filter

Dense wavelength division multiplex (DWDM) systems is a promising technology for long-haul networks using the established fiber networks. Tunable devices such as optical filters, highly selective photodetectors, as well as lasers are considered to be key components for dynamic WDM systems. A novel low-cost tunable dielectric filter consisting of an air-gap cavity embedded by two DBRs is presented. A FWHM of 8 nm and a tunability of 15 nm/mA at 2 k/spl Omega/ heating resistance is obtained.

Record wavelength tuning of 127 nm for vertical cavity Fabry-Perot filter

Demonstrated micromechanically tunable Fabry-Perot filters with record electromechanical and optical performance. Wavelength tuning of 127nm applying only 7.3V is achieved. The stop-band of these filters is extremely wide (1250nm - 1800nm). These results open new application fields for micromechanically tunable filters, such as spectrometry for environmental and process analysis, medical diagnostics and colorimetry.

Multiple air-gap InP-based VCSELs and filters with ultra-wide wavelength tuning-flexibility and shape of the membranes

Vertical cavity devices that comprise of multiple air-gaps and extremely thin InP membranes implemented by surface micromachining is presented. Both, for the tunable cavity and for the distributed Bragg reflector (DBR) mirrors the material module InP/air is applied. This design facilitates improved opto-mechanical device performance. With the same technological concept, a 1.55 /spl mu/m micromachined optical filter with an ultracontinuous wavelength tuning is fabricated and characterized. An 8-air-gap 1.55 /spl mu/m VCSEL structure is also implemented.