Effects of Softbake Parameters on a Benzocyclobutene (BCB) Adhesive Wafer Bond
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Effects of Softbake Parameters on a Benzocyclobutene (BCB) Adhesive Wafer Bond Daniel N. Pascual North America Applications Center SUSS MicroTec 228 Suss Drive Waterbury Center, VT 05677 U.S.A. ABSTRACT This study explores the effects of softbake parameters including temperature and duration on the homogeneity and toughness of a Benzocyclobutene (BCB) adhesive bond. Experiments were initially performed on silicon wafer pieces and then verified using quartered wafers bonded on a different machine. A four-point bend delamination test was employed to quantitatively measure interface toughness while uniformity was analyzed visually. A softbake temperature of 150 °C for 10 minutes yielded strong void-free bonds with an interface toughness of 23 J/m2. INTRODUCTION Many bonding methods exist for assembling and packaging MEMS (micro electromechanical systems). Some of the most familiar types are anodic, direct, and intermediate-layer bonding [1]. The latter method, which includes adhesive bonding, is perhaps the most flexible technique due to the large selection of bonding materials available. Benzocyclobutene (BCB) is a material traditionally employed in the integrated circuit industry as a dielectric layer for input/output redistribution but has found new use as an adhesive [2][7][8]. Some advantages of using BCB are precise thickness control, chemical inertness, low temperature curing, and it can be patterned using standard photolithography [3][4]. One drawback is that BCB can not provide the hermetic seal necessary for some applications. Implementing BCB involves spin coating it onto a substrate. Then it must be heated or 'softbaked' prior to bonding [5]. Softbaking is a critical step because it removes a substantial amount of solvents that would otherwise evaporate during thermal curing. Observations within this study show how insufficient heating during softbake creates voids of trapped gases within the BCB layer. Conversely, overheating during this step causes decreased interface toughness. This makes implementing the proper parameters for softbake critical for bonds that are both strong and void free. Empirically determining the optimal softbake parameters typically involves processing many samples. So in an effort to minimize waste, initial testing was performed using small samples requiring less material. Once optimal settings for softbake were obtained, the process was verified on a larger scale. EXPERIMENTAL DETAILS The experiments in this study are designed to assess the effects of softbake time and temperature on the homogeneity and toughness of BCB bonds. In order to facilitate the experimental process, samples were constructed so that they could be softbaked, bonded, and tested in the same machine.
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Sample preparation and bonding Samples were prepared from silicon wafers (Montco Wafers, Pennsylvania) 6 inches in diameter, 690 µm thick, and polished on one side. A commercially available automatic coating system (SUSS MicroTec, ACS 200) with an optional ultraviolet-ozone cleaner module was u
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