PHYSICAL AND CHEMICAL PROPERTIES OF POLYCYANURATE ESTER RESINS FOR USE IN INTEGRATED OPTICS

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1 PHYSICAL AD CHEMICAL PROPERTIES O POLYCYAURATE ESTER RESIS OR USE I ITEGRATED OPTICS Christian Dreyer, Prof. Dr. Monika Bauer, Dr. Jörg Bauer raunhofer Institute of Reliability and Microintegration Branch Lab Polymeric Materials and Composites Kantstr. 55, D Teltow, Germany Phone: + 49 (0) 3328/ ax: + 49 (0) 3328/ dreyer@epc.izm.fhg.de, mbauer@epc.izm.fhg.de, jbauer@epc.izm.fhg.de orbert Keil, Prof. Dr. Huihai Yao, Crispin Zawadzki Heinrich-Hertz Institut für achrichtentechnik, Berlin GmbH Einsteinufer 37, D Berlin, Germany Phone: + 49 (0) 30/ ax: + 49 (0) 30/ keil@hhi.de, yao@hhi.de, zawadzki@hhi.de Introduction Today, in the era of telecommunication, the number of people that have access to the internet redoubles every six month. The data amount transferred between Europe and the United States even redoubles in less than sixty days. Therefore, the current infrastructure of the communication must be expanded rapidly. Because of its great bandwidth with more than 50 Tbps, its low loss and its insensitivity against electromagnetic fields, the glass fibre has almost completely replaced the copper wire for long-distance transmission and its applications are now extending more and more to short distance systems. or short distances the glass fibre has especially in the automotive and aeronautic field a lot of advantages. Because of its immense bandwidth, it saves a lot of material and weight, by name more than 30 kg of copper for a luxury car (like the Mercedes S-Klasse). In the aeronautic area not only the weight reduction is a great advantage, also the transferred data are not influenced by external electromagnetic fields, as they are produced e.g. by cellulars. The optical network technology not only requires glass fibre, but also a broad range of optical components, such as multiplexers, optical switches, splitters and combiners. Actually the most optical components are realised by using silica-based optical waveguides. The main disadvantages of these silica-based components are their high switching power, the low wavelength tuning range and last but not least the high production costs. or the above mentioned components, polymeric materials are very attractive, because of the potentially low production costs and the lower power consumption in optical switches. urthermore, polymeric multiplexers can be thermally tuned over a wide range because of their large thermo-optical coefficients. The use of reactive resins and the resulting thermosets permit cheap production of optical waveguides with procedures which are established in semiconductor-technology, such as photolithography and reactive ion etching (RIE). The Polycyanurate ester resins are a relatively new class of reactive polymers with a high thermal stability (Tg C), low optical loss, low birefringence, low dielectric constant, good adhesion properties on different surfaces, excellent resistance against most common solvents and amazing mechanical properties [2]. The glass fibre has two important so-called telecommunication-windows in the wavelength regions around 1310 and 1550 nm. In these regions selected polycyanurate waveguides show optical losses below 0,4 db/cm at 1550 nm and even less at 1310 nm (igure 4). Polycyanurates are used successfully for some years as waveguide-materials [3 7] by IZM and HHI and optical components were produced as well [4]. Thus, polycyanurates have a great potential

2 to meet the high requirements of Integrated Optics, because of their outstanding physical, chemical and thermo-optical properties. Experimental The monomers are obtained by the cyanogen-halogen-method in industrial scale (igure 1). HO OH (C 2 H 5 ) Hal C + 2 [(C 2 H 5 ) 3 H]Hal igure 1. Synthesis of Dicyanatobisphenol A using the cyanogen-halogen-method (Hal can be chlorine or bromine). The two dicyanates DCBA and DCHBA (igure 2) were purchased from Lonza AG, Switzerland and used without further purification. The other mono- and difunctional cyanates (igure 2) were synthesized using the cyanogen-bromide method (igure 1) [1]. 3C C3 Dicyanatobisphenol A (DCBA) Dicyanatohexafluorobisphenol A (DCHBA) Dicyanatododecafluorooctane (MCy2) H H H H Resorcindicyanate (RCDC) igure 2. Examples of mono- and difunctional cyanate-monomers. 4-luorophenylcyanate (PCy) The basic reaction to obtain polycyanurates is the cyclotrimerisation of aromatic or fluoroaliphatic cyanate esters (igure 3). This reaction has the advantage that no by-products, neither gaseous, nor liquid are formed during the reaction. The synthesis of the (pre)-polymers was performed by polymerisation in bulk of difunctional cyanates in sealed vials at temperatures between 140 and 190 C. The reaction was stopped before gelation rapidly with liquid nitrogen. The prepolymers were solved in 2-EEA and the solutions (45-65wt.-%) were filtered under cleanroom conditions. The thin layers (about 10 µm) were obtained by spincoating and thermal curing ( C). The curing can be followed by IR-Spectroscopy. To produce single-mode optical waveguides from polycyanurates, several requirements must be met. The first is to adjust the refractive index. This can be achieved very easily with the polycyanurates. Almost all monomers can be combined with each other, so the refractive index can be adjusted in a broad range almost continuously. To lower the optical loss at 1550 nm it is strongly recommended to have no absorbing groups in the system. Especially OH- and H-groups, but also C-H-groups (aliphatic more than aromatic) have strong absorption in the 1550 nm region. Thus it makes sense to replace the hydrogen against fluorine.

3 O R R O O O 3n C O R O C O R O igure 3. Polycyclotrimerisation of cyanates. n igure 5 shows the UV-VIS-IR-Spectra of two different cyanate monomers: Dicyanatobisphenol A, which is containing no fluorine and Hexafluorobutylcyanate, with 55.1wt.-% of fluorine. The Dicyanatobisphenol A shows an absorption from about 0.6 db/cm at 1550 nm but the highly fluorinated monomer only about 0.2 db/cm. This results in synthesis of binary and ternary Polycyanurate-systems. igure 4 shows the strong differences of the optical loss comparing a non-fluorinated polycyanurate based slab-waveguide (more than 1.2 db/cm@1550 nm) with a waveguide containing more than 40wt.-% of flourine (0.32 db/cm@1550 nm). -6 optical loss non-fluorinated cured under nitrogen slope = -1,31 db/cm fluorinated cured under nitrogen slope = -0,32 db/cm fluorinated cured under air slope = -0,44 db/cm propagation length [mm] igure 4. Propagation length dependence of optical nm of fluorinated and non-fluorinated polycyanurate based slab-waveguides measured by the sliding prism method. The refractive index can be adjusted over a wide range by using different comonomers. Rising the fluorine content decreases the refractive index, whereas introducing chlorine or even bromine increases the refractive index. igure 6 shows the resulting refractive indices of five different copolymer-systems with different monomer contents.

4 7,5 Hexafluorobutylcyanate Dicyanatobisphenol A Absorption [db/cm] 5,0 2,5 1550nm 0, Wavelength [nm] igure 5. UV-VIS-IR-Spectra of Hexafluorobutylcyanate and Dicyanatobisphenol A. refractive 1,60 1,58 1,56 1,54 1,52 1,50 1,48 1, Comonomer (mol%) DCBA/BrMC DCBA/MC3 DCHBA/BrMC DCHBA/MC1 DCHBA/MC2 igure 6. Refractive indices of five different binary copolymer-systems with different comonomer contents. The absolute refractive index as well as the birefringence of the cured layer is depending on the composition of the copolymers, as well as considerably on the used substrate, as shown in igures 7a and 7b. 0,0030 nte - ntm Refractive 1,4720 1,4710 1,4700 1,4690 1,4680 1,4670 1,4660 1,4650 1,4640 1,4630 Quartz 1550nm 1550 nm Silicon Polymer 1 Substrate Polymer 2 Polymer nm 1550nm Birefringence (nte- 0,0025 0,0020 0,0015 0,0010 0,0005 0,0000 Quarz Silicon Polymer 1 Substrate Polymer 2 Polymer 3 nte - ntm igure 7a. Refractive index of Polycyanurate- Monolayers on different substrates (cured at 200 C). igure 7b. Birefringence of Polycyanurate-Monolayers on different substrates (cured at 200 C). The single-mode optical waveguides were fabricated by conventional photolithographic processes and reactive ion etching (RIE) as shown in igure 8. The obtained waveguides have very smooth surfaces as shown by SEM (igure 9). The further achieved waveguides were covered with a buffer

5 layer (thickness about 15 to 25 µm). The resulting embedded waveguides show a good rectangular shape and reveal an optical loss about 0.1 to 0.2 db/cm higher than the planar waveguide before the structuring process. Waveguide abrication process Substrate buffer layer guiding layer photoresist UV-radiation developing photoresist sputtering Al-mask lift-off RIE structuring guiding layer removing Al-mask cladding layer formation metal electrode igure 8. Waveguide fabrication process. igure 9. SEM-micrograph of a RIE-structurated polycyanurate waveguide (width: 6 µm). We have improved the optical properties of the polycyanurate ester resins within the last years, especially the optical loss at 1550 nm. urther we optimised the synthesis of monomers and prepolymers. Last but not least we enhanced the processing properties of the materials, like layer quality, solubility etc.. The actual work is focused on the elucidation of structure-properties relationships of different monomers, prepolymers and fully cured thermosets and last but not least the design and production of integrated optical components.

6 igure 10. Cross-section photograph of an embedded polycyanurate waveguide of 6.1x7.0µm size. References [1] D. Martin, M. Bauer, Organic Synthesis 1983, 61, 35. [2] I. Hamerton, (ed.) "Chemistry and Technology of Cyanate Ester Resins", Chapman & Hall, Glasgow, [3] W. Wirges,. Keil, H.H. Yao, S. Yilmaz, C. Zawadzki, M. Bauer, J. Bauer, C. Dreyer Micro System Technologies, Potsdam, 1998, [4]. Keil, W. Wirges, H. Yao, S. Yilmaz, C. Zawadzki, M. Bauer, J.Bauer, C. Dreyer PO'99, Chiba, Japan, 1999, [5] J. Bauer, C. Dreyer, M. Bauer, C. Zawadzki, S. Yilmaz, W. Wirges, H. Yao,. Keil Polymer Preprints (2), [6] C. Dreyer, M. Bauer, J.Bauer,. Keil, W. Wirges, H. Yao, C. Zawadzki React 2000, Dresden, Germany, [7] C. Dreyer, M. Bauer, J.Bauer,. Keil, W. Wirges, H. Yao, C. Zawadzki Berliner Polymerentage 2000, Berlin, Germany, [8] C. Dreyer, M. Bauer, J.Bauer,. Keil, W. Wirges, H. Yao, C. Zawadzki luoropolymer 2000, Savannah, GA, USA, 2000.