for integrated optical circuits

by R. Buestrich, F. Kahlenberg, M. Popall1, P. Dannberg2, R. Müller-Fiedler, O. Rösch,3 
1 Fraunhofer-Institut für Silicatforschung ISC, Würzburg -Germany
Fraunhofer Institut für Angewandte Optik und Feinmechanik IOF, Jena-Germany
3 Robert Bosch GmbH, FV/FLD, Stuttgart-Germany

We present a new photopatternable ORMOCER® material for the fabrication of integrated optical and opto-electronic devices. They exhibit low optical losses in the NIR range, especially at the most important wavelengths for optical communications (0.3 dB/cm at 1320 nm, 0.6 dB/cm at 1550 nm, without fluorination). 
The refractive index is easily and reproducibly tunable by mixing with an appropriate resin of different index. Moreover, process parameters have been optimized to achieve low cost packaging even in higher layer thicknesses (at present 100 mm within one step). In addition to a mechanistic study of the initial polycondensation reaction, examples for the applicability of the mentioned materials are given. Embedded channel waveguides have been produced as well as active devices like digital optical switches.

ORMOCER®s in general are a class of hybrid inorganic/organic materials which amongst other utilizations prove highly valuable for application in packaging of integrated circuits. They show good optical and dielectric properties as well as good thermal stability (decomposition not below 270 °C). Furthermore, their chemical synthesis meets the requirements of microoptics and microelectronics industry since it enables highly reproducible low cost production of ORMOCER® micro structures.

ORMOCER®s are manufactured in a two-step process, the first of which consists of the hydrolysis and polycondensation reaction of organically functionalized alkoxysilanes. The following reaction scheme (1) depicts the modified synthesis of the presented materials which focuses on alkoxylation. Diphenylsilanediol is reacted with  3-methacryloxypropyltrimethoxysilane under addition of a suitable condensation catalyst:




This initial step leads to storage stable resins and laquers with adjustable viscosities which can easily be applied to the respective substrates, e.g. silicon wafers or polymer substrates. The coating itself can be performed by means of typical processing methods such as spin-on techniques, dip-coating or curtain-coating. Due to the presence of organically crosslinkable functionalities (methacrylic substituent) the resulting coatings show negative resist behavior which makes them photopatternable upon exposure to UV-irradiation that is referred to as the second step in ORMOCER® synthesis. Negative resist in this context means that the exposed area is hardened and the resin covered with the mask can be washed away with a suitable solvent mixture.

Fig. 1 portrays the clean room processing of an ORMOCER® resin exemplified by the manufacture of optical chips containing a digital optical switching device. Such a device consists of an embedded optical waveguide, i.e. three ORMOCER® layers, buffer (lower refractive index), core (higher refractive index), and cladding (lower refractive index), and a heating device attached to the two branches of the Y-shaped waveguide core. The heater exploits the thermo-optical effect to reduce the index of the waveguide core upon heating and thus blocking the according branch from signal transmission. Therefore, the optical signal can only travel through the non-heated branch and the switch is realized. The separate layers are produced by spin-on, UV-exposure, development and postbake as illustrated.


Figure 1. UV-direct-patterning for integrated optics.


For the optical characterization of the presented ORMOCER®s, embedded channel waveguides were produced (Fig. 2), 7.1 cm long, with a cross section of 6x6 mm2. The refractive index difference between core and buffer/cladding layers was 0.005 at 1550 nm. Optical loss values were determined utilizing the cut back method. Intrinsic losses were found as 0.3 dB/cm at 1320 nm and 0.6 dB/cm at 1550 nm wavelength. 

Fig. 2 proves the smooth surface of the cladding layer due to planarization effects typical of the ORMOCER®.



Figure 2. Embedded ORMOCER® strip waveguide.


Read also : J. Sol-Gel Sci. Technol. 20, 181-186 (2001).

For more information contact: 

Dr. Michael Popall
Fraunhofer-Institut für Silicatforschung ISC
Neunerplatz 2
D-97082 Würzburg
Tel.:    ++49 (0) 9 31/41 00-5 22
Fax.:   ++49 (0) 9 31/41 00-5 59

Email: popall@isc.fhg.de
Web:  http://www.isc.fhg.de/gb/ormocere/o4.html


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