Interfacial Adhesion of Spin-Coated Thin Adhesive Film on Silicon Substrate for the Fabrication of Polymer Optical Waveguide M.A. UDDIN, 1 W.F. HO, 1 C.K. CHOW, 1 and H.P. CHAN 1,2 1.—Optoelectronics Research Center & Department of Electronic Engineering City University of Hong Kong, Hong Kong. 2.—E-mail: eehpchan@cityu.edu.hk An investigation was carried out to study the interfacial adhesion of spin-coated polymeric adhesive thin film on a silicon substrate for fabrication of a polymeric optical waveguide. An adhesive shear button was made on a silicon substrate by using a photolithography process, and interfacial adhesion was measured with a Dage D2400 shear tester. Different adhesion strengths were found at different portions of the same sample. Higher adhesion strength was observed at the center of the substrate than at other locations of spin- coated adhesive films. Adhesion strength was also measured after heat expo- sure of the deposited and cured adhesive layer to evaluate the heat resistance of the adhesive film. After heat exposure, adhesion strength decreased sub- stantially from all locations of the substrate due to the thermal degradation. Again, the adhesion was measured for different plasma-treated substrate con- ditions. The surface morphology of plasma-treated and untreated silicon sub- strates before deposition were characterized by atomic force microscopy. Lower adhesion strength was unexpectedly observed for all plasma-treated substrates, even for higher surface roughness. The fracture surfaces after shear testing were also characterized by optical microscopy. The complete study provides important indications for the fabrication of better-performing polymeric optical waveguides for photonic devices. Key words: Polymer material, optical waveguide, interfacial adhesion, plasma surface treatment, heat treatment INTRODUCTION In contrast to inorganic materials, electro-optic (EO) polymers are very attractive for optical devices due to their advantages, such as large optical non- linear coefficients, fast response times, low dielec- tric constants, simple fabrication processes, and easy fabrication of multilayer structures. For ex- ample, polymeric epoxy adhesive can be used as a cladding material for optical waveguides, offering integratable photonic devices to satisfy a significant industry need. 1 Figure 1 shows the typical sche- matic cross-sectional diagram of a single inverted rib polymeric optical waveguide. The figure shows that the key requirement for polymeric optical waveguide fabrication includes a simple and straight- forward thin-film deposition process. The film can be deposited by a variety of techniques with differ- ent complexities and applicabilities. The choice of deposition technique depends upon the physico- chemical properties of the material, the film quality requirements, and the substrate being coated. The final properties of these films also depend on their morphologies, which are largely affected by the pol- ymer chain orientation and the state of aggregates. One of the simplest and most common techniques of applying thin films onto wafers is spin coating. This technique is assumed to lead to very uniform films with well-controlled thickness. 2 Due to the complex- ity of common spin coating solutions, the behavior of mixed solutions should be understood for the condi- tions of rapid fluid flow and convectively driven evaporation that occur during spin coating. 3 The critical issues of spin-coated adhesives in these applications are film thickness and adhesion strength. Many studies have been directed to pre- dict the final film thickness, but no attempt has been paid to the interfacial adhesion induced by the spinning process. High adhesion strength is a (Received January 8, 2006; accepted February 22, 2006) Journal of ELECTRONIC MATERIALS, Vol. 35, No. 7, 2006 Regular Issue Paper 1558