Roland Mueller-Fiedler

High-performance low-cost fabrication method for integrated polymer optical devices

For all optical path networks there is a need for optical crossconnect systems and switching matrices. Using polymer materials and technologies, not only low cost components but low power consumption devices can be fabricated. We optimize ORMOCERs, a new class of polymer materials, which combine the specific advantages of organic polymers and inorganic glasses, with respect to the need of photonic applications. They exhibit a high thermal stability of up to 250 degrees C and a low optical attenuation of 0.2 dB/cm at 1.3 micrometers and 0.4 dB/cm at 1.55 micrometers .

Monolithic integration of RF-MEMS and semiconductor devices for the K-band

In this paper a process for complete monolithic integration of semiconductor devices and radio-frequency micro-electro-mechanical systems (RF-MEMS) on a single substrate is presented. Our attempt was to combine RF-Schottky-Diodes to form sub-harmonic mixer and RF-MEMS-phaseshifters on a single chip. The diodes were etched from a molecular beam epitaxy grown silicon stack using two mesa etching steps. Nickel forms a nickel-silicon alloy (nickel silicide) during a rapid thermal processing step acting as Schottky-metallisation. On this stack, the RF-MEMS-fabrication starts with its metallisation layers as a back-end process. To insulate the relatively high actuation voltage (20-40 V) from the RF circuitry, a new concept for bias decoupling is presented. To demonstrate the functionality of the semiconductor integration approach, a mixer for 24 GHz has been designed in coplanar waveguide technology, the local oscillator frequency is at 12 GHz. Fabricated within the same run, switched line phaseshifters are used to show the MEMS capabilities. First tests of diodes revealed good results in their DC- and RF characteristics, the conversion loss of the subharmonic zero biased mixer reached 20 dB for 6 dBm power of the local oscillator. Fabricated teststructures of the phaseshifters achieved good results showing that transmission losses lower than 3 dB at a phaseshift of 180° can be reached.

Enhanced performance of polymer waveguides by replication and UV patterning in ORMOCER

Recent results on waveguide device fabrication by replication of inorganic-organic copolymers (ORMOCERs) are presented. The use of optimized ORMOCER resins offers advantages over conventional organic polymers. The organic as well as inorganic crosslinking is responsible to high thermal and chemical stability and thus an improved stability of the waveguide devices. Fluorination of ORMOCER side chains reduces the NIR absorption to < 0.4 dB/cm at 1.55 micrometers and < 0.3 dB/cm at 1.3 micrometers . Furthermore, the synthesis of purely inorganically crosslinked CH-free ORMOCER is possible. The refractive index can be tuned so that highly fluorinated core material can be combined with low-cost unfluorinated cladding material. The influence of the sidewall roughness of replication tools on scattering losses is investigated, and methods to fabricate smooth original structures have been developed leading to an additional scattering loss < 0.1 dB/cm in single-mode strip waveguides even at visible wavelengths. Furthermore, an improved deforming behavior is achieved. UV-patterning by UV-induced crosslinking is a second waveguide fabrication method used in ORMOCER. Applications of ORMOCERs in thermo- optical switching nodes are discussed.

Cross talk reduction in switching networks by asymmetrical off-on switches

A new off-on switching scheme is introduced which blocks a waveguide path in the passive off-state and transmits the signal in the active on-state. The operating principle is based on the self-diffraction of a narrow guided beam when it escapes from a waveguide with two dimensional confinement into a region of appropriate length with at most one dimensional confinement. In particular a remaining interface of the initial waveguide superimposes reflection, which in sum results in a very efficient asymmetrical blow out of the guided power. In the active on-state, low-loss waveguiding is sustained when an electrode causes an appropriate refractive index change, e.g. due to the thermo-optical effect. Thus the signal is received in the output waveguide, an identical pendant of the input guide. The switching behavior is almost digital, and the wavelength dependence is minimal only. This makes the device useful for switching and modulation in a multi-wavelength optical network. Calculations show on-off signal ratios of better than 30 dB e.g. in polymeric waveguide configuration of 3 mm length. This is nicely adapted to the demands of crosstalk reduction in switching networks. Experimental results from a thermo-optic polymer waveguide device with 32 dB off-state attenuation and 0.8 dB on-state excess loss fit well to the data obtained from the numerical modeling.

Reliability of platinum electrodes and heating elements on SiO 2 insulation layers and membranes

In this work, failure mechanisms of Pt electrodes including adhesion problems, material migration due to thermally induced compressive stress and electromigration that could occur in the platinum electrodes and heater structures at temperatures above 600 °C have been systematically studied, after the deposition. Lifetime determination, scanning electron microscopy and XRD analysis have been applied for samples which have experienced different loading conditions in order to qualitatively and quantitatively understand the phenomena. Electromigration testing is performed with the aim to enable time-to-failure prediction for sensor elements and compare different platinum layers in terms of their stability. Dedicated, application-related test structures are used so that the results are applicable to sensor lifetime estimations. Furthermore, a method for the determination of thermal conductivity of thin insulating films has been adapted for the characterization of plasma-enhanced chemical vapor deposition (PECVD) silicon oxide and successfully applied on two materials with different deposition recipes. These two materials are used for the fabrication of platinum-based heating elements with PECVD SiO2 as insulation or membrane layer. The results for the two recipes are similar but with a significant difference. A slight increase of the conductivities has been observed due to a thermal anneal of the test structures at temperatures above 700 °C.

Reliability characterization of a soot particle sensor: Analysis of stress- and electromigration in thin-film platinum

In this work we present a systematic investigation of failure mechanisms for thin-film platinum heater structures and interdigitated electrodes as components of a resistive type soot particle sensor. We study stress-migration and electromigration and effects of their interaction. Lifetime determination and SEM imaging are applied for samples which have experienced different load conditions to quantitatively and qualitatively understand the phenomena. We use dedicated, application-related test structures to ensure that the results are transferable to sensor lifetime estimations.

Optical receiver module for very high bit rates

A microoptical receiver module for very high bitrates was designed and realized. We employed well established silicon technologies for integrating the optical components of the microoptical system. For reducing the optical spot size on the photo diode due to the beam divergence of the transmission fiber a monolithically integrated silicon microlens was used. The microlenses were fabricated by melting of photoresist and transferring the preforms into the silicon substrate. We characterized the lenses by lateral shearing interferometry in transmission. The beam diameter on the photodetector (l/e2) could be reduced by about one order of magnitude to less than 10 micrometers . The sensitivity of the modules was about 0.5 A/W at 1310 nm and 0.6 A/W at 1550 nm.

Short amplifying planar waveguides by new glass processing technology

Short amplifying waveguides in which the dopand is confined to the waveguide core were realized by a new glass processing technology with high demands to the optical and thermal properties of the core, cladding and substrate glass. By the improvement of the etching procedures primary structures with a roughness of less than 50 nm were obtained. Simulations, which showed that in a 12 cm waveguide 20 dB gain can be achieved at 1549 nm, were made to evaluate the potential of the amplifier. Gain measurements were performed in waveguides with a dopand concentration of 3 wt.% erbium and an internal gain of more 3.7 dB was achieved in a 1.8 mm glass waveguide at a wavelength of 1549 nm.

Assembly and interconnection technologies for electrical and optical microsystems

Silicon micromachining, thin-film-multilayer-technology, integrated optical circuits and micro-optics were combined to realize optoelectronic microsystems. The potential of silica based inorganic waveguide materials and optical polymers were investigated and compared with respect to their process compatibility and their ability for integration with electronic, optoelectronic and micromechanical functions. An integrated optical modulator, an optical 2 by 2-switch and a micro-optical receiver were realized as test devices. The results reveal advantages of polymeric materials with respect to the optical functions of modulating or switching of light on the basis of thermo- optical phase shifting. However, the integration of the various process steps into a complete microsystem could only be accomplished with silica based optical circuits.